Organic–Inorganic Hybrid Materials from Vegetable Oils

Abstract

The production of materials based on fossil resources is yielding more sustainable and ecologically beneficial methods. Vegetable oils (VO) are one example of base materials whose derivatives rival the properties of their petro‐based counterparts. Gaps exist however and one way to fill them is by employing sol–gel processes to synthesize organic–inorganic hybrid materials, often derived from silane/siloxane compounds. Creating Si─O─Si inorganic networks in the organic VO matrix permits the attainment of necessary strength, among other property enhancements. Consequently, many efforts have been directed to optimally achieve organic–inorganic hybrid materials with VOs. However, compatibilization is challenging, and desirable conditions for matching the inorganic filler in the organic matrix remain a key stumbling block toward wider application. Therefore, this review aims to detail recent progress on these new hybrids, focusing on the main strategies to polymerize and functionalize the raw VO, followed by routes highlighting the addition of the inorganic fillers to obtain desirable composites.

Vegetable oils (VOs) have emerged as viable alternatives to petro‐based feedstocks for polymeric materials but often require adjustment of properties via composite formation with inorganic compounds. VO functionalities are advantageous for compatibilization routes and strategies on hybridization strategies are reviewed. Future VO/hybrid research directions are suggested, such as life cycle assessment and improving recyclability, to make such materials more sustainable.

1. Introduction

The worldwide consumption of plastic keeps increasing, despite calls for reducing wasteful polymer processes. In 2019, 460 million tons (Mt) of plastic were produced with estimates suggesting this amount could triple by 2060.^{[ 1 ]} Most production comes from non‐sustainable fossil‐based resources and is used in omnipresent applications such as packaging, medicine, and goods in the construction industry.^{[ 2 ]} The dependence on plastics comes from their diverse properties, such as durability, malleability, relatively low density, and affordability. However, these properties come with the cost of high waste production and significant greenhouse gas emissions.^{[ 3 ]} It is now urgent to find new solutions to reduce the use of fossil‐based materials and to use polymers more sustainably. One solution could be to use renewable resources, for example.^{[ 4 ]} However, in 2021, only 1% of overall plastic production came from renewable resources.^{[ 5} , ^{6 ]} This low fraction is due to many factors. Resistance to change from petrochemically‐based plastics is high as such materials have a long history of development and successful implementation across many applications. Furthermore, bio‐based plastics are often less effective and often contain impurities or a range of ill‐defined structures that thwart attainment of the properties observed from their non‐renewable counterparts. Still, research is continuing, and some synthesized alternative bio‐based materials have achieved comparable or better properties relative to petrochemically‐based analogs.^{[ 7 ]} Starch, cellulose, proteins, chitin, and lignin are a few examples of renewable resources that can be used as building blocks.^{[ 8 ]} For example, bio‐poly(ethylene furanoate) (bio‐PEF) has been synthesized as an alternative to poly(ethylene terephthalate) (PET) in packaging applications. This biomaterial is synthesized from 2,5‐furandicarboxylic acid which can be generated via sugars derived from cellulose.^{[ 5} , ⁹ , ^{10 ]} Greenhouse gas emissions can be reduced from ≈45–50% while producing PEF instead of traditional petroleum‐based PET.^{[ 11 ]} Superior performance properties compared to the PET benchmark have been observed. For example, 10 times lower O₂ permeability and 20 times lower CO₂ permeability were measured, giving it extremely good barrier properties compared to PET.^{[ 12 ]} Along with excellent barrier properties, this polyester has enhanced thermal and mechanical properties, which are obvious benefits to food industry applications. Comparing the two materials, PEF has ≈10 °C higher glass transition temperature (T_g) than PET and can be processed at lower temperatures as its melting point is ≈30 °C lower, and it offers higher stiffness and strength, (e.g., higher Young's modulus – 2.0 versus 1.3 GPa).^{[ 13} , ^{14 ]}

Vegetable oils (VO) are a particularly interesting renewable resource available to synthesize sustainable polymeric materials due to their abundance and relatively low cost.^{[ 8 ]} For example, the worldwide production of linseed oil in 2019 was 3.41 Mt (29.5% produced by Kazakhstan and 14% produced by Canada) which increased by almost 30% in only 2 years.^{[ 15 ]} Due to the presence of reactive moieties (e.g., double bonds, esters) they are suitable building blocks for several applications. Many polymers have been synthesized with VO such as poly(urethane)s, poly(hydroxyurethane)s, and poly(acrylate)s.^{[ 16} , ¹⁷ , ^{18 ]} Unmodified oil can be used directly but often needs to be functionalized to diversify the number of reactive functionalities required. Further enhancements, particularly for mechanical properties, are possible via the incorporation of inorganic fillers. Such organic–inorganic hybrids are suitable for coatings and adhesives, for example, see ref. [19]. In such hybrids, inorganic particles are mixed in an organic network, typically by a sol–gel procedure, resulting in materials with improved thermal or mechanical properties. Silica is valued as a filler, leading to interesting composite materials. It is also possible to create covalent bonds between the filler and the matrix, if the filler has a specific functionality that can react with the polymeric matrix. Furthermore, the formation of the Si─O─Si network via sol–gel mechanisms is often easily initiated by the moisture in the air.

The combination of VO‐based hybrids offers distinct advantages and deserves a perspective based on the rich chemistry and possible adoption of greener and commercially feasible hybrid materials.^{[ 20} , ²¹ , ²² , ^{23 ]} As detailed earlier, vegetable oils have been established as economically attractive bio‐based feedstocks for polymers and related additives with versatile functionalities while silanes are widely adopted for crucial property adjustments (e.g., curing, moisture‐repellency) in many formulations. The marriage of these two concepts warrants review to provide viable design options to meet the environmental and broad performance demands of industries relying on coating and sealants. First, a precise description of the type of oils to be considered is revealed, and the sol–gel procedure using silanes to achieve the necessary curing behavior is detailed. Then, the review will address the types of polymers synthesized to obtain the moisture‐cured materials derived from VOs. Initially, the focus will be on the formation of a hybrid material directly from the raw VO without any polymerization involved. Second, an examination of the Organic–Inorganic materials obtained from the polymerization of VO will follow and finally we will consider the composites made from modifying the filler involving a sol–gel process that is compatibilized in a VO matrix.

1.1. Vegetable Oil

It has been established that biomass can be used for industrial utilization without compromising the food supply.^{[ 24 ]} Vegetable oils and fats consumption are shared between food, feed, and biomass. It is important to be aware that competition exists between utilizing vegetable oils for biomass production versus animal feed and human food. This ethical debate is known as the “food for fuel” debate.^{[ 25 ]}

Vegetable and animal oils and fats are nowadays the most important renewable feedstock of the chemical industry. The versatility in conjunction with the well‐established modification routes for these kinds of oils makes them excellent candidates to replace petroleum‐based feedstocks.^{[ 26 ]} Furthermore, they are universally available, they have low toxicity, their prices are relatively low, and they meet at least seven of the 12 green chemistry principles.^{[ 27 ]}

VOs are extracted from plants, generally from seeds (e.g., soybean, linseed, grape seed oil), but also from other parts of plants (e.g., olive oils). They are composed of triglycerides which are esters of glycerol with three fatty acid side chains, as displayed in Figure  1 .

Figure 1.

General example of a VO structure made of triglyceride.

The differences between the VOs come from the structure of the fatty acid side chains which can differ in their length from 8 to 24 carbon atoms, the concentration of unsaturated sites (between 0 and 7 C═C double bonds), and the presence of other moieties (epoxy or hydroxyl functional groups, for example). Table 1 provides examples of structures of most common fatty acids side chains founds in VOs.

Table 1.

List of common fatty acids with their chemical structures.

Fatty Acids	Formula	Structure
Caprylic acid	C8H16O2
Myristic acid	C14H28O2
Palmitic acid	C16H32O2
Stearic acid	C18H36O2
Oleic acid	C18H34O2
Linoleic acid	C18H32O2
Linolenic acid	C18H30O2
Ricinoleic acid	C18H34O3
α‐Eleostearic acid	C18H30O2
Vernolic acid	C18H32O3
Licanic acid	C18H28O3

The composition of the fatty acids in the VO can be different from one batch to another because it varies according to the plant but also with the growing conditions and season.^{[ 28 ]} The physical state of the VO depends on their fatty acid side chain composition. Usually, VOs with high carbon atom contents but low number of double bonds will have a higher boiling point.^{[ 8 ]} The number of unsaturated sites can be measured with the iodine value (IV) which represents the mass in mg of iodine that reacts with the double bonds of 100 g of VO. The higher IV directly relates to the higher number of unsaturated sites in the VO tested. Table 2 summarizes IV and compositions of the most common VOs.

Table 2.

Composition of VOs.^{[ 29} , ³⁰ , ^{31 ]}

Vegetable Oil	Average number of double bonds	Iodine value/mg per 100 g	Fatty acids [%]
			Palmitic (C16:0)	Stearic (C18:0)	Oleic (C18:1)	Linoleic (C18:2)	Linolenic (C18:3)
Canola	3.9	100–115	4.0	1.8	60.9	21.0	8.8
Castor	3.0	81–91	2.0	1.0	7.0	3.0	0.5
Corn	4.5	102–130	10.9	2.0	25.4	59.6	1.2
Cottonseed	3.9	90–119	21.6	2.6	18.6	54.4	0.7
Linseed	6.6	168–204	5.5	3.5	19.1	15.3	56.6
Olive	2.8	75–94	13.7	2.5	71.1	10.0	0.6
Palm	1.7	44–58	42.8	4.2	40.5	10.1	/
Peanut	3.4	84–100	11.1	2.4	46.7	32.0	/
Rapeseed	3.8	94–120	4.0	2.0	56.0	26.0	10.0
Sesame	3.9	103–116	9.0	6.0	41.0	43.0	1.0
Soybean	4.6	117–143	11.0	4.0	23.4	53.3	7.8
Sunflower	4.7	110–143	5.2	2.7	37.2	53.8	1.0

The production of some of these vegetable oils has been published in the Yearbook Table of the USDA for the years 2023/2024. Figure 2 presents the data for six vegetable oils, showing that soybean oil is the one with the highest world production, with almost 400 000 Mt produced in 2023/2024.

Figure 2.

World production of vegetable oils obtained from USDA, Oil crops Data: Yearbook Table, for the years 2023/2024.

VOs contain several reactive functional groups which can be used to add functionalities necessary to enhance their performance. These active groups are highlighted in Figure  3 below, which shows the double bonds (a), the allyl carbon atoms (b), the ester groups (c), α‐carbonyl carbons (d), and ω‐carbon atoms (e).^{[ 32 ]}

Figure 3.

Example of actives groups present on VOs.

The double bonds are one of the main functional groups that can be easily modified via oxidation, carbonylation, metathesis, hydrogenation, or epoxidation, etc.^{[ 33 ]} Among these modifications, epoxidation is well‐established and many such epoxidized oils are commercially available. Epoxidized VOs are useful building blocks and can be used without any further modification to synthesize polymers (cf. Section 3), but since oxirane rings are highly reactive it is possible to transform the EVOs into desired functionalities such as hydroxyl functions or carbonate moieties. Consequently, (epoxidized) VOs are widely used for developing new green thermoplastics and thermosets for polyamides, polyacrylates, polyesters, polyurethanes, and the latter's offshoot, polyhydroxyurethanes.^{[ 16} , ¹⁷ , ²⁶ , ³¹ , ³³ , ³⁴ , ^{35 ]} To enhance the properties of these polymers, sometimes reinforcement is necessary, which can be obtained by synthesizing organic–inorganic hybrid materials and VO functionalities are well‐suited to combine with toughening inorganic fillers.

However, possessing multiple functionalities such as that shown in Figure 3 can limit wider application. For example, if linear polymerization is desired via chain polymerization (e.g., free radical), the multiple double bonds will invariably lead to branched or cross‐linked products. Acrylated epoxidized soybean oils are available commercially and used in the coatings industry, but applications requiring more control of the polymer microstructure may be problematic. Nevertheless, related monomers have been derived from fatty acids and vegetable oils and used in radical polymerization processes, with many examples of controlled microstructure polymers obtained via reversible deactivation radical polymerization.^{[ 36} , ^{37 ]} Future directions may involve processes to modify the monomer precursor structure, like the tall oil fatty acids (TOFA), which possess free acids that negatively affect resin stability. Subsequent conversion via electrochemical non‐Kolbe decarboxylation into alkenes, which were then treated with oxone yielded low viscosity epoxies. These were then reacted with anhydrides to give thermoset materials.^{[ 38 ]} This work is but one example that suggests possibilities to more carefully tailor the starting feedstock to give improved products.

1.2. Hybrid Organic–Inorganic Materials

Combining an organic matrix with inorganic filler can be done in various ways.^{[ 39} , ^{40 ]} The most common route to synthesize hybrid organic–inorganic materials is the sol–gel process. This method enables solid, cured material from a solution (i.e., the sol); predominantly when the curing process is triggered by moisture. Sol–gel processes can be divided into two main mechanisms. In one case, ambient moisture hydrolyzes the substrate, which is condensed in a second step to form the oxide framework. This method is widely used since the conditions are mild and it is possible to control the morphology and the properties of the materials by changing the reactants, catalysts, and reaction conditions (e.g., relative humidity, temperature).^{[ 41 ]}

One of the most common inorganic moieties used with VOs is based on silicon, generally silica, silane, or siloxane (a network formed with Si─O─Si bonds).^{[ 41 ]} These silicon‐based materials offer hydrophobicity, thermal resistance, flexibility, and gas permeation to the host material. For hybrids based on silica incorporation, the process involves the hydrolysis of the silane, creating silanols, followed by their condensation leading to a Si─O─Si network, as described in Figure  4 .

Figure 4.

Schematic of the sol–gel process involving two steps. First, the catalyzed hydrolysis occurs followed by the condensation to form the network.

The hydrolysis is triggered by moist air and can often take several days to complete. The relative humidity can play a major role in the time taken to complete the process and vary seasonally – control of the humidity is essential toward more reproducible and consistent processes. The hydrolysis can be accelerated by temperature or by the addition of a catalyst where hydrolysis speeds up with acids and condensation is augmented by bases.^{[ 42 ]} Nevertheless, the most efficient process involves a compromise using a weak acid like acetic acid (HAc, pKa = 4.75) which can hasten both reactions.^{[ 43 ]}

Silane end‐capped polymers are another way to produce organic–inorganic hybrids in a simple manner. In silyl modified polymers, covalent bonds form directly between the inorganic filler (i.e., the silane) and the organic matrix (i.e., the polymer), which will enhance even more the mechanical properties. The formation of this type of network is shown in Figure  5 . Due to the presence of the silane moieties, the sol–gel process with moisture curing can efficiently produce a densely cross‐linked coating.

Figure 5.

Formation of the Si─O─Si network from a silylated polymer. (In blue: polymer, orange triangle: silane functions).

The sol–gel process is also used with non‐silicate inorganic alkoxides, such as metal alkoxides derived from zirconium, titanium, aluminum, or tin. Typically, the reaction with alkoxysilanes is done under mild conditions, making them the most extensively studied sol–gel materials. Tetra‐alkoxy silicates (Si(OR)₄) and modified silicates are the most widely used of such components and some are identified in Figure  6 .

Figure 6.

Structure of some commonly used silanes.

Adhesive and sealants are generated from this method, which when compared to their counterparts synthesized from polyurethanes, are less harmful. Indeed, no toxic chemicals like isocyanates are needed, and the overall formation of material from the sol–gel process is greener.^{[ 43} , ^{44 ]} Furthermore, the adhesion to the metal surface is often enhanced through the direct creation of bonds between the hydroxyl group on the metal surface and the Si‐OH moieties from the silane, which will be described in detail in Section 5.

Modification of VO with silanes has been broadly applied to combine the sustainable aspects offered by the VO feedstock with the property enhancement enabled by silanization. There are several sub‐routes possible. Figure  7 delineates the three ways to modify VOs with inorganic moieties. First, we will strictly describe the direct modification of VO with silane either from the raw oil or from the related fatty acids. Second, the synthesis of polymers from VO is possible, such as a conventional polyurethane first formed by reacting hydroxyl groups of the oil with diisocyanates. The products are then modified with a silane to permit moisture curing and obtain crosslinked materials. Finally, the incorporation of filler into the VO matrix without necessarily using a covalent linkage between the oil and the filler will be addressed. The silane will be used to modify the surface of the filler, as there are OH groups already present on the surface of the particles, allowing the creation of Si─O─Si bonds between the particle and the silane. Using the most convenient silane with the required function will help the compatibilization between the fillers with the vegetable matrix. However, it should be noted that not all silanes are easy to apply. In hybrid non‐isocyanate poly(urethane)s, our group attempted to modify the mechanical properties as the NIPUs fell short compared to conventional poly(urethanes). One approach is the use of fillers, and we found that applying an epoxy or isocyanate functional silane was effective,^{[ 45 ]} but the handling of these should be scrutinized in the future as the use of epoxy silanes (e.g., GLYMO) that have been used traditionally as adhesion promoters have recently been classified as having genotoxic potential.^{[ 46 ]}

Figure 7.

Schematic of the different organic–inorganic hybrids described in this review. (Left) organic–inorganic hybrid of raw VO, described in Section 3. (Middle) organic–inorganic hybrid of polymerized VO, described in Section 4. (Right) Formation of composites, described in Section 5.

2. Modification of Neat Vegetable Oil

As mentioned in the introduction, VOs have a complex structure based on a triglyceride of fatty acids. For some applications, this motif is effective and there is no need to polymerize the VO. The different reactive functionalities already present on the VOs are directly reacted with silanes.

2.1. Epoxidized Vegetable Oil

As previously described, epoxidized vegetable oils (EVO) are commercially available and are one of the most prominent industrial forms of VOs used. The conventional epoxidation process is done via Prileshajev‐epoxidation or by peroxy acids^{[ 47} , ^{48 ]} (cf. Figure  8 ). Soybean and linseed oil are commonly applied as they can produce EVOs with a high number of oxirane groups on the backbone which will create interesting materials, but adding the silane coupling agent will further enhance the mechanical properties.

Figure 8.

Reaction path for epoxidation of a double bond.

An analogy can be made between the EVOs and epoxy resins. Traditionally epoxy resins are petroleum‐based, but recently bio‐based epoxies synthesized from vegetable oil or plant‐based oil exhibited performance comparable to traditional epoxies.^{[ 49 ]} The main difference between the epoxy resin and the EVOs used for our system comes from the curing agent used to obtain the final material. This curing agent will have two or more functional groups (most commonly an amine‐terminated agent, ‐NH₂) that can react with the epoxy groups resulting in a crosslinked material. Different processes can be used to perform the reaction, for example, through light, UV, or heat curing.^{[ 50 ]} For the epoxy resin no sol–gel process is needed and a completely bio‐based curing agent could be used. Nevertheless, it is sometimes necessary to add reinforcement filler such as bio‐based fillers (e.g., from cellulose, lignin, or starch) or other inorganic fillers such as carbon nanotubes or graphene oxide. The difference between the two systems is how the network is created; no inorganic network is created in a conventional epoxy thermoset while the materials obtained through the sol–gel process inherently create one.

Comparison between raw VOs and EVOs showed that epoxidation has a dramatic impact on the properties of the created material. De Luca's group synthesized a series of inorganic‐organic coatings from castor oil and tetraethyl orthosilicate (TEOS).^{[ 51 ]} They explored the variations obtained from pure castor oil and epoxidized castor oil when combined with TEOS. The incorporation of TEOS was more effective when the VO was epoxidized, showing microphase separation even with low fractions of pure castor oil. Hardness and tensile strength increased with higher ratios of TEOS for all samples due to the formation of the Si─O─Si network.

Barbieri's group developed new hybrid films using epoxidized castor oil combined with 3‐glycidoxypropyl trimethoxysilane (GPTMS) and TEOS.^{[ 52 ]} To facilitate the sol–gel process between epoxidized castor oil and the silanes, the oil was acidified beforehand to promote the cleavage of the oxirane groups. After completing the sol–gel process and creating the Si─O─Si network, different films were mechanically tested. The best hybrid system was obtained with a combination of GPTMS reacting with 25% of the epoxy rings of the VO and 10% of TEOS.

Many such examples exist for these “direct” hybridized systems. In one case, Gregório and co‐workers prepared organic–inorganic films using hydroxylated or epoxidized soybean oils as an organic matrix with TEOS as an inorganic filler (90:10 wt:wt).^{[ 53 ]} The hydroxyl groups in the organic soybean oil condensed with hydrolyzed TEOS, forming the organic–inorganic film. Different soybean polyols were analyzed for their OH values, and it was found that the results depended more on the number of OH functions than on the presence of other functions. The best result was obtained for the completely converted polyol with an OH value of 198 mg KOH/g.

It is also possible to compare the differences obtained by using diverse VOs. For example, Tsujimoto et al., synthesized green nanocomposites from epoxidized soybean and linseed oil with GPTMS.^{[ 54 ]} In the presence of a thermally latent cationic catalyst, they copolymerized the VO with the silane, forming the polymer matrix and siloxane network simultaneously. Seven concentrations were tested of GPTMS in soybean oil and two ratios with linseed oil. Higher values for hardness and tensile strength were obtained with a higher content of GPTMS, up to 75 wt%, without showing any limitations. Comparison between the two kinds of VOs revealed that epoxidized linseed oil had higher hardness hybrids than ones derived from epoxidized soybean oil, which was attributed to the higher content of epoxy rings available for crosslinking in the former. Luo et al transformed a ESO by converting the groups into acetoacetoxy groups and then crosslinking them with a suitable amine.^{[ 55 ]} This is striking for a few key reasons and cues future designs for incorporating recycling/durability to such hybrid materials. First, the soybean oil was reacted with 2‐mercaptoethanol via thiol‐ene clicking with UV light to yield hydroxyl functional soybean oil that was subsequently reacted with t‐butyl acetoacetate, giving the acetoacetylated soybean oil. Hybridization was then done using amino alkylsilanes, showing good mechanical properties. However, the authors did not elaborate that amino‐ketone coupling resulted in vinylogous urethane formation, one of the many types of coupling used extensively in dynamic covalent linkages for reversible thermosets.^{[ 56 ]}

Finally, Uyama's group synthesized nanocomposites from EVO‐based matrices (soybean oil, linseed oil, and fish oil) using silane coupling agents GPTMS and 2‐(3,4‐epoxycyclohexyl)‐ethyl trimethoxysilane (ECTMS) in the presence of a thermally latent catalyst.^{[ 57 ]} A series of 11 materials were tested, investigating the effects of i) the nature of the oil, ii) the silane coupling agent, and iii) the weight percentage of the silane. In every case, the higher content of the silane coupling agent yielded the highest tensile modulus, attributed to the increased crosslink density.

2.2. Modification of the Double Bond

Most VOs have reactive double bonds which can be exploited to react with a diverse range of functionalities, as alluded to in the previous section. Hence, the possibility of using a functionalized silane with these double bonds and their derivatives has been studied extensively. Bexell et al. have shown that mercapto silanes can be coupled with the double bonds of linseed oil via a photoinduced thiol–ene reaction.^{[ 58 ]} The structure of the silane used, and the synthetic pathway is displayed in Figure  9 . The increased hydrophocity of the surface, indicated through contact angle measurements, was sufficient proof that the double bonds of the linseed oil reacted with the thiols. Ultimately, the hydrolysis of the silane on the metal substrate formed a lubricating layer and reduced friction.

Figure 9.

Reaction between 3‐(trimethoxysilyl)‐1‐propanethiol and VO used by Bexell et al.^{[ 58 ]}

The presence of hydroxyl groups in the VOs is also frequently exploited as isocyanates can react easily with them to form urethane bonds without necessarily synthesizing a polyurethane. Kahraman describes this synthesis, where they functionalized hydroxyl groups of castor oil with an isocyanate‐functionalized silane.^{[ 59} , ^{60 ]} They also added norbornyl acrylate and additives to obtain a UV‐curable formulation, which subsequently enabled the synthesis of highly hydrophobic and considerably toughened coatings from this oligomer‐based method.

2.3. Fatty Acids Modification

To improve the reactivity of the VOs, sometimes it is necessary to convert them to fatty acids and use the VO derivative as a precursor for the reaction with silane. Szubert and co‐workers showed silylated rapeseed oil derivatives can be obtained by transesterification and saponification of the VO which is then reacted with 3‐chloropropyl trimethoxysilane (CPMS).^{[ 61 ]} The resulting silylated rapeseed oil fatty acid was used for the preparation of wood coatings using a sol–gel process to bond the coating to the wood surface by forming wood–O–Si linkages. Several studies have described silane coatings of wood, but it is not the only material that can be treated with a silane coating.^{[ 62 ]} This strategy is illustrated in Figure  10 .

Figure 10.

Condensation and curing process of a silane on a substrate, with available ‐OH functions.

In another study, Szubert and co‐workers described the use of fatty acid derivatives as coatings on concrete.^{[ 63 ]} The fatty acids were first esterified and then silylated with triethoxysilane. Analogously to the silane coating on wood, the sol–gel process allowed bonding of native hydroxyl groups in the concrete to the silane moieties of the coating. The obtained bio‐sourced coating reduced the water absorbability by about 95%.

Alam's group described the synthesis of an anticorrosive coating using successive modifications to obtain mechanically and chemically robust corrosion systems.^{[ 64 ]} Linseed oil was converted into a diol fatty amide, followed by the introduction of ester and urethane linkages along with the incorporation of an inorganic moiety (TEOS). The formed coating provided four times higher protective interference compared to the unprotected carbon steel, which was attributed to the combination of functional groups employed.

Functionalized VOs can also be used as ceramers, which are hybrid coatings made of organic and inorganic domains synthesized via the sol–gel process where metal‐oxo clusters are formed in the organic matrix. This class of coatings is promising, due to their ability to bond to different organic and inorganic substrates and has been highlighted in UV‐coatings described by the Soucek group.^{[ 65 ]} Consequently, Soucek and co‐workers combined raw VO with fatty acid derivatives to improve the global performance of the materials.^{[ 66 ]} They evaluated the modification of linseed fatty acids and tung oil to synthesize alkoxysilane‐functionalized alkyd coatings. They first modified linseed oil to obtain fatty acids, which were then functionalized with 3‐triethoxysilylpropyl isocyanate (IPTES), as displayed in Figure  11 , (reaction between NCO groups and OH groups, 15% of the OH groups are end‐capped).

Figure 11.

Structures of the modified linseed oil and tung oil used by Soucek's group.^{[ 59 ]} In orange is highlighted the end capped silane.

A contrasting family of compounds was prepared by modifying tung oil using a Diels–Alder reaction between the double bonds of the oil and methacryloxypropyl trimethoxysilane (silane with a methacrylate function). A series of 11 coatings were tested: the control was made only with modified linseed fatty acids; in the other ones, different ratios of either silane‐end capped linseed fatty acids or silane modified tung oil were incorporated. Among many properties measured, they tested the tensile and coatings properties but also the effect of humidity on the moisture curing at 25 or 75% of relative humidity. First, the addition of alkoxysilane enhanced the overall performance of the materials, especially when cured at high humidity, with improvement of modulus and tensile strength (greater improvement with the higher ratios). Nevertheless, when cured at low humidity, this adhesion was found to diminish when increasing the amount of alkoxysilane added (higher than 30 wt%). High humidity has a clear impact on the results. In summary, the silane modified oil provided faster curing and harder films, while silane modified fatty acids improved the adhesion and tensile properties.

Soucek's group continued examination of linseed oil modification by synthesizing an epoxidized cyclohexene derivatized linseed oil,^{[ 67 ]} as well as a cycloaliphatic epoxide adduct of linseed oil.^{[ 68 ]} To enhance the properties of the coatings, they incorporated TEOS oligomers (previously prepared by a sol–gel process of TEOS monomer), followed by UV‐curing. TEOS oligomer was chosen instead of monomer because their curing was done using UV irradiation instead of conventional moisture curing. The UV curing of the organic phase is faster than the moisture curing of the inorganic network, and thus using a preformed network permitted matching of the two curing timescales. Using TEOS oligomer probably caused the broad silica particle size distribution in the organic matrix (from 75 to 984 nm). Nevertheless, the overall properties of the material were enhanced by the presence of TEOS, where an optimum ratio was reached at 15 wt% of TEOS oligomer which best balanced the tensile properties, fracture toughness, hardness, adhesion, and abrasion.

Another interesting application where silylated VOs are used is for bio/medical materials, where several studies examined new gelatin‐based materials for wound dressings derived from VO. For example, the Agarwal group designed a non‐leachable antibacterial film made from castor oil.^{[ 69 ]} They first cleaved the VO to obtain an amino‐functionalized fatty acid that is then reacted with a silane to finally obtain a methoxy‐silane‐functionalized quaternary ammonium compound bearing a ricinoleic fatty acid from castor oil, as shown in Figure  12 . The inclusion of the quarternary ammonium group acted as the biocidal agent and the sol/gel process provided the mechanical integrity.

Figure 12.

Structure of the modified fatty amide obtained from castor oil used by Agarwal group to create an antibacterial film.^{[ 69 ]}

To form the final wound dressing, sol–gel hydrolysis and condensation were performed with poly(vinyl alcohol) (PVA) as the synthetic hydrogel. Covalent linkages between the methoxy‐silane groups of the biocidal agent and the hydroxyl groups present in the PVA gel were thus created, leading to the formation of a cross‐linked network. Furthermore, self‐condensation of the silane moieties was also happening. Optimal wound dressing materials were produced by generating inorganic domains in the organic network to give the desired mechanical strength. Additionally, castor oil and its major fatty acids like ricinoleic acid, are well suited for the biocidal agent. Longer chain molecules with backbones from 12 to 18 carbons possessed greater antibacterial efficiency.^{[ 70} , ⁷¹ , ^{72 ]}

Small molecule precursor materials from VOs may not have sufficient physical or mechanical properties and thus require polymerization. One effective option is to polymerize the oil before adding the curable silane moieties. Different backbones can be synthesized, leading to a broad variety of hybrid materials. The focus in succeeding section will introduce how the silane is incorporated in the polymer matrix (i.e., before or after polymerization).

3. Silane‐Terminated Polymers from Vegetable Oil

The modification of the VOs by polymerization is sometimes necessary either to obtain better properties (physical and mechanical) but also to take advantage from the nature of the polymer backbone. One desirable option is to polymerize the oil before adding the silane moieties to cure the material. Different backbones can be synthesized, which are shown in Figure  13 .

Figure 13.

Possible ways to polymerize a VO: in pink from the epoxy function, in green from the hydroxyl function, and in blue from the double bond.

We will also focus on how the silane is incorporated in the polymer matrix (i.e., before or after polymerization) by first discussing how EVO is used most prominently in the formation of polymers derived from the epoxy, hydroxyl and double bonds to form polymers such as epoxy resins, polyurethane and related polyhydroxyurethanes, other types of polymeric backbones that can be used to synthesize silylated polymeric materials based on VOs.

3.1. Epoxy Based Polymer

Epoxy resins are formed from the oxirane rings of the EVOs reacting with an amine to form epoxy‐amine linkages. Most often, the amine is capped with a silane moiety to enable the sol–gel process and form the cured material. Martinelli's group investigated a series of organic–inorganic films using epoxidized castor oil as the organic matrix and 3‐aminopropyl triethoxysilane (APTES) with different ratios of TEOS as the inorganic moiety.^{[ 73 ]} The study aimed to understand the variations in the properties of these coatings by testing six different ratios (APTES reacted with 0, 10, 20, 50, 70, or 100% of epoxy groups present in the oil). Initially, all synthesized coatings were found to be homogeneous, with no phase separation observed in SEM images. Hardness results improved with a higher content of the inorganic network, stemming from both using APTES alone and the combination of APTES and TEOS.

Continuing this theme, the same group prepared similar organic–inorganic coatings using epoxidized castor oil as the organic matrix. They utilized the two silane precursors, APTES and TEOS, and introduced titanium(IV) isopropoxide (TIP) as an inorganic precursor to enhance the mechanical properties of the coatings.^{[ 74 ]} APTES and TEOS were chosen to produce Si─O─Si crosslinking, while TIP induced a Ti–O–Ti network via the sol–gel process. The key distinction lies in the observation that APTES can react with epoxy groups through its amine and not solely via the sol–gel process. The differences between the two silane precursors were investigated and it was seen that both combinations resulted in microscopically homogeneous films, confirming effective incorporation of the inorganic networks in the VO matrix. TEOS was found to enhance the hardness of the coatings, while APTES brought more flexibility. Additionally, TIP was shown to improve the curing time of the coatings.

Lapinte's group highlighted the preparation of Si‐coated linseed oil and showed for the first time a better understanding of the side reactions that can occur when a VO reacts with an amine.^{[ 75 ]} They employed the reaction of epoxidized linseed oil with 3‐aminopropyl trimethoxysilane (APTMS), and proved that the amino silane opened the epoxy rings and was also involved in the trans‐amidification of the triglyceride. In both cases, the modified VO was moisture‐cured via the sol–gel process, and viable coatings were produced. They showed that by changing the conditions of the reaction, they were able to control the yield of each route and thus dictate the final cross‐linking density. Figure  14 shows the possible structures obtained after the reaction of VO with amines.

Figure 14.

Structure of the two possible products that can be obtained from the reaction of VO with amine: 1) ring opening of epoxy groups and 2) trans‐amidation of the triglyceride.

3.2. Polyurethane‐Based Polymer

Polyurethanes (PUs) are versatile polymers used as coatings, adhesives, and foams, which have been used extensively since the 1950 s. They are commonly formed by urethane linkages which come from the reaction of suitable multi‐functional alcohol and isocyanate bearing monomers, and their repeat unit structure is given in Figure  15 . Different polyol and isocyanates can be used to tailor the properties and VOs can be readily modified to obtain several hydroxyl or isocyanate functions in their structures, and thus are good building blocks to impart into PUs.^{[ 34 ]}

Figure 15.

Reaction between a diol and a diisocyanate to synthesize a polyurethane, showing the urethane linkage in purple.

Adding silane moisture curing enhances the performance of the VO‐based PUs. We can distinguish three ways to add the silane into the PU network (Figure  16 ). First, the PU can be synthesized in a preliminary step and then the polymer can be end‐capped with a silane (part 1 of Figure 15). Second, a modified polyol can be formed by adding a silane function to it. The modified polyol will then react with an VO terminated isocyanate to form the PU and finally, the moisture curing is done via the previously attached silane on the polyol segments (part 2).^{[ 34 ]} Third, a silane functionalized isocyanate is synthesized and reacted subsequently with a VO's polyol to form the PU (part 3). The sol–gel curing is triggered by moisture after the synthesis of the PU.

Figure 16.

Explanation of the three ways to obtain PU‐based hybrid materials. 1. Synthesis of a PU in the first step, then the addition of the silane. 2. Synthesis of a polyol capped with a silane, used to form the PU in the second step. 3. Synthesis of an isocyanate capped with a silane, used to form the PU in the second step. At the end, all the VO‐based PU are end‐capped with a silane and can be moisture cured via the sol–gel process to fabricate hybrid materials.

3.2.1. Silane End‐Capped Polyurethane

The most common way to synthesize precursor PUs from VOs is to prepare a polyol from the raw oil. Castor oil is mainly composed of ricinoleic acid which already has a hydroxyl group in its structure, thus making it very convenient for the synthesis of PUs. Soybean oil is also often used, especially its epoxidized (commercially available) form where the oxirane groups can be opened to obtain hydroxyl functions. Indeed, any EVO can be employed in the same manner. With bio‐based polyurethanes it is possible to produce simple, inexpensive, and scalable processes for multifunctional coatings. Several groups have investigated novel routes to produce enhanced PU from renewable sources.

LaForest's group proposed a novel route to prepare polyols from soybean oil, which was then used to produce moisture‐curable coatings via polyurethane terminated silanes.^{[ 76 ]} They showed that it was possible to obtain high molecular weight polyol with narrow molecular weight distribution from the ring opening polymerization of epoxidized soybean oil using fatty acids such as tall oil fatty acids. They consequently prepared polyols and applied them with di or tri‐isocyanate to synthesize isocyanate terminated polyurethane. The obtained PUs were then end‐capped with amino silanes to produce moisture‐curable coatings.

Castor oil was used by the Mandal group to form PU/siloxane cross‐linked films.^{[ 77 ]} A hydrophobic film was created by reacting a castor oil‐derived PU with APTMS, forming urea linkages, and the product was then hydrolyzed and condensed to obtain a silicate‐crosslinked network. Several ratios of siloxane crosslinks were tested, and it was shown that the properties of the resulting films, such as hydrophobicity (higher contact angle) are enhanced by having more siloxane in the network. Indeed, higher siloxane ratios yielded higher contact angles (from 74° to 96.3°) and the overall surface free energy decreased. Scanning electron microscopy (SEM) images show microphase separation of the polysiloxane domains, while atomic force microscopy (AFM) revealed smoother surfaces with more siloxane, leading to reduced roughness and lower wettability.

Ren et al., developed a humidity‐insensitive pressure‐sensitive adhesive (PSA) from castor oil and siloxane.^{[ 78 ]} First, the castor oil was capped with an isocyanate which was then reacted with APTES. Expectedly, the thermal stability improved with the incorporation of APTES creating the siloxane network. Moreover, interesting properties such as holding force, tack, and peeling force increased. For example, the 180° peel force remained nearly constant, decreasing by only 3% with increased humidity.

Synthesis of VO‐derived polyurethanes in dispersed media is also an efficient way to produce coatings. Several groups reported the synthesis of polyurethane‐silane hybrids in emulsion, particularly ones that were waterborne, as illustrated in Figure  17 .

Figure 17.

Waterborne polyurethane dispersion condensed with moist air to create a film. Before moisture curing, the silane functions are not linked to each other. After the moisture curing Si─O─Si linkages are formed between the silane functions.

In a study by Gaddam et al., polyurethane‐silanol dispersions were synthesized from cottonseed oil.^{[ 79 ]} First, polyols were synthesized from pre‐made epoxidized cottonseed oil where the oxirane rings were opened and some of the created hydroxyl groups were functionalized with phosphoric acid to obtain phosphorylated polyol. Next, the polyols were condensed with excess isocyanate (isophorone diisocyanate) to form the actual PU‐terminated isocyanate. Finally, APTES was used to react with the rest of the isocyanate moieties creating the waterborne polyurethane‐silanol dispersion. Cast siloxane cross‐linked films were achieved by drying the dispersions at room temperature for one day and another day at 50 °C. Different ratios of OH functionality were tested, leading to variable cross‐link densities. It was indicated that the networks with a higher content of Si─O─Si linkages resulted in the best thermal and mechanical properties with good corrosion resistance due to the low surface energy of the siloxane matrix.

The same group used a similar process to synthesize anionic waterborne polyurethane‐silanol dispersions.^{[ 80 ]} The polyols were still synthesized from cottonseed oil but this time the oxirane groups were functionalized with different ratios of 4‐aminobenzoic acid, leading to a series of polyols with varying hydroxyl numbers. Following this, the reaction between the hydroxy group and isocyanate led to ionizable polyurethanes. The same silane compound was attached to the fragment of the isocyanate chain‐end and cast films were produced following a similar procedure. The same trend was observed, showing that the higher content of siloxane (meaning fewer anionic groups) gave better mechanical properties and higher chemical resistance.

Cheng et al. demonstrated that linseed‐oil‐derived waterborne polyurethane‐silica hybrid materials could be derived from linseed oil.^{[ 81 ]} They first transesterified linseed oil to obtain the monoglyceride which was then reacted with an excess of isocyanate to synthesize a PU terminated with ‐NCO groups. Then, different ratios of APTMS were used to create the hybrid material. After the reaction between the isocyanate and silanes, the alkoxy groups undergo hydrolysis and condensation reactions to create the crosslinked network. The Si─O─Si network created in the polyurethane matrix significantly improved the mechanical properties and hydrolysis resistance of the overall material. For example, comparing the film with only 5% of APTMS to the neat one, tensile strength increased from 18.04 to 30.50 MPa.

Gurunathan and Chung studied a series of cationic waterborne polyurethane‐silica hybrid coatings.^{[ 82 ]} They used cottonseed oil to create polyurethane oligomers terminated with NCO functional groups which were then capped in a second step with APTMS. After crosslinking via Si─O─Si bond formation, the stability was enhanced, leading to a cured network that was an effective mass transfer barrier and thermal insulator. Even adding as low as 2wt% of APTMS greatly improved the performance of the material, as Young's modulus increased nearly eightfold from 11.97 to 98.74 MPa.

The same researchers also studied a biorenewable‐based coating made from castor oil which was first reacted with Tolonate, a partially biobased diisocyanate, to obtain a PU terminated isocyanate.^{[ 83 ]} Various APTMS ratios were then used to crosslink the PU prepolymer. The obtained polyurethane−siloxane nanocomposite exhibited better thermal stability compared to the PU without any siloxanes. Again, the formed Si─O─Si network acted as an effective thermal insulator and mass transfer barrier for the volatile compounds generated during the degradation process. The Young's modulus also increased significantly with only 5 wt % of APTMS added, about four times higher than the neat PU prepolymer (26.3 to 109.3 MPa).

The Larock group synthesized a waterborne PU from castor oil, which was then reacted with APTES to form a PU‐silica nanocomposite dispersion, resulting in a network where the silica particles are reinforcers that increase the subsequent coating's crosslink density.^{[ 84 ]} Transmission electron microscopy (TEM) showed that the silica nanoparticles were embedded in the polyurethane matrix, forming a core–shell structure. The formation of a core–shell microstructure was suggested to be due to the presence of carboxylate groups in the PU chains after adding water (shell) and the silicon alkoxides being positioned in the core. The addition of only 2 wt% Si content strikingly enhanced Young's modulus from 32.3 to 116 MPa, showing how effectively the stress was transferred from the silica to the polymer matrix, along with improved thermal stability. These different studies show how the utilization of biorenewable resources combined with silane or siloxane curing can generate environmentally friendly PU coatings, which are valuable for attaining high‐performance biomaterials, which are good candidates for cardiac tissue engineering.^{[ 85} , ^{86 ]}

Baheiraei et al., reinforced a PU synthesized from castor oil with siloxane domains, which were used as electrically conductive polyurethane/siloxane networks for cardiac tissue engineering applications.^{[ 87 ]} Here, the hydroxyl moieties of the castor oil were reacted with excess diisocyanate, resulting in a PU with terminal isocyanates. Then, an amino silane was reacted with this PU to form the castor oil‐based PU terminated silane. Next, to have electroactivity, an aniline tetramer was added to form the final conductive film, along with additional silane coupling agents to aid the sol–gel film formation. Thus, the synthesized films had excellent tensile properties, while incorporating conductivity and still being biodegradable.

A similar material was synthesized by Gharibi et al., to serve as a wound healing material.^{[ 88 ]} Castor oil‐based PU was cured with siloxane moieties to form a membrane suitable for wound dressing with, for example, proper tensile strength in both hydrated and the dry state. This shows how the sol–gel procedure with silane is versatile and can be used for several applications, especially in biomedical materials.

3.2.2. Silane End‐Capped Polyol

In this section, the second way to produce organic–inorganic hybrids PU is discussed (cf. Figure 16). where silane end‐capped polyols react with isocyanates to obtain PUs.

Ahmad et al., synthesized a coating from linseed oil for an anticorrosive material.^{[ 89 ]} They synthesized a linseed fatty amide diol first, followed by silylation with TEOS curing agent. The concentration of TEOS used in the first step was calculated to have sufficient free hydroxyl groups to form urethane linkages with isocyanates in the second step to yield a hybrid organic–inorganic PU film. All the inorganic domains made of Si─O─Si bonds are interconnected to an organic domain consisting of the organic PU polymer network. This kind of structure improved the mechanical and anticorrosive properties, with the material having a service temperature of up to 200 °C.

Zhou's group studied the structure‐property relationships of several polyurethane dispersions synthesized from castor oil and a bio‐based dimer fatty acid diisocyanate.^{[ 90 ]} The diisocyanate has a long fatty acid chain that provides poor mechanical strength but good flexibility. To overcome this limitation, the authors used different ratios of modified castor oil during the reaction by the addition of IPTES, which increased crosslinking density through a sol–gel process. Although the bio‐content was reduced somewhat, it remained at ≈80% while having improved tensile strength and Young's modulus with higher siloxane contents. For example, the prepolymer without any modified castor oil had a Young's Modulus less than 0.1 MPa whereas the prepolymer with almost 50 wt% of siloxane‐terminated PU had a Young's Modulus of 2.45 MPa. Despite the general trend of improved stability with more siloxane, TGA showed that the prepolymers with higher contents of siloxane were less stable, suggesting oxidative degradation caused the breakdown of the Si─O─Si network.

Ghosal et al. detailed the fabrication process of a silica hybrid polyurethane coating.^{[ 91 ]} They initiated the process with soybean oil, which underwent hydrolysis to yield monoglycerides with glycerol, forming a diol. Subsequently, this diol underwent a reaction with TEOS to produce a polyol enriched with silane moieties. This novel polyol was then further reacted with a diisocyanate to ultimately synthesize the PU. The researchers employed three different TEOS loadings during the PU synthesis, and the resulting coatings were systematically compared. Their findings highlighted that the incorporation of a Si─O─Si network significantly enhanced corrosion resistance, particularly in 3.5 wt% NaCl and 3.5 wt% HCl solutions.

The creation of bio‐based polyurethanes derived from Prosopis juliflora oil was documented by Jagtap and co‐workers.^{[ 92 ]} In the initial stage, they epoxidized the VO to enable the reaction with the amino group of APTMS, resulting in a silanized VO. The remaining hydroxyl groups then reacted with isophorone diisocyanate to yield the PU. Initially, all hydroxyl groups were reacted with the diisocyanate at a 1:1 molar ratio. Subsequently, the NCO functional group molar ratio was reduced incrementally from 0.8, down to 0.2, allowing free OH groups to remain within the matrix, which was employed for siloxane network formation. Materials synthesized with a higher NCO content exhibited higher crosslinking density, as confirmed by the gel content, leading to improved mechanical properties, such as hardness, adhesion, and impact resistance. For example, scratch hardness increased from 1.8 kg with 0.2 molar ratio (NCO:OH) up to 3.5 kg when the ratio is increased at NCO:OH was near unity.

Shaik et al. devised a unique diol with hydrolyzable –Si–OR groups as crosslinkers.^{[ 93 ]} They initiated the process with two silanes: APTMS and GPTMS; using a moisture‐free nitrogen atmosphere, the amine groups of APTMS underwent a dual reaction with the epoxy groups, resulting in the formation of a diol, as shown in Figure  18 . This formed diol was mixed with castor oil and reacted with a diisocyanate to produce a PU. Three diol ratios (5, 10, and 20 wt%) were investigated, maintaining a constant percentage of castor oil, and a slight excess of diisocyanate was employed. The resulting films underwent crosslinking through the sol–gel process under atmospheric moisture and laboratory humidity conditions for 40 days. Mechanical property (tensile strength and elongation at break) testing revealed that the weight percentage of the diol significantly influenced the results. Altering the diol ratio led to changes in hard segment content, crosslinking density, and intermolecular interactions between the hard segments. With the increasing ratio of diols, elongation decreases were observed. An elongation at a break of 16.81% with the higher ratio (20 wt%) was measured, compared to 54.23% with the lower ratio (5 wt%).

Figure 18.

Structure of the diol synthesized by Shaik's group from APTMS and GPTMS.^{[ 93 ]}

The same research group also conducted a study on modifying castor oil to introduce carboxylic acid moieties.^{[ 94 ]} The acid‐terminated castor oil was reacted with GPTMS to develop polyurethane/urea‐silica hybrid coatings. These coatings were examined for properties such as swelling and contact angle, demonstrating a direct correlation with the NCO:OH ratio used in the synthesis. Higher NCO:OH ratios resulted in greater hydrophobicity of the hybrid coating film. The chemical linkage of silica to the polymer matrix provided reinforcement and additional crosslink nodes in the system, as confirmed by thermomechanical properties. Indeed, T_g (obtained by DSC analysis), increased from 18.2 °C with the lowest ratio up to 56.4 °C with the higher ratio.

An alternative approach for synthesizing silanized castor oil was proposed by Shen and co‐workers.^{[ 95 ]} They utilized a thiol‐ene reaction between 3‐mercaptopropyl trimethoxy silane and the castor oil's double bonds. This process retained free hydroxyl groups on the castor oil, which then reacted with a diisocyanate to form a polyurethane. In a subsequent step, the silane moieties underwent a sol–gel process to cure the film. The introduction of silane groups in the polyurethane resulted in water‐repellent surfaces, along with enhanced thermal stability and mechanical properties, making it appealing for coating applications. A further step was taken to create an aqueous dispersion using the same system, resulting in a core–shell type morphology.^{[ 96 ]} Once again, this system exhibited enhancements in thermal stability as demonstrated by the increased T₁₀ from 259 to 304 °C for the samples without any addition of silane to one with 2.7 wt%. The surface morphology is also affected by the addition of silane as observed by AFM analysis (tapping mode) and SEM imaging, where surface roughness increased with silane content. Microphase separation was seen due to the inclusion of siloxane network domains. Furthermore, the introduction of silane also enhanced the water resistance of the hybrid films as the water contact angle increased from 84° to 90° with a higher ratio of silane.

Another approach using a silylated castor oil for the formation of a hybrid polyurethane was done first by developing a precursor via reaction of VO hydroxyl groups with succinic anhydride to yield an acid‐terminated castor oil.^{[ 96 ]} This acid‐terminated castor oil then reacted with the epoxy functionality of GPTMS. The researchers intentionally left free hydroxy functional groups on the castor oil to provide reactive sites for isophorone diisocyanate in varying ratios, always in excess, to produce NCO‐terminated polyurethanes. The resulting films were then cured under ambient humidity and the direct bonding of the silane to the VO significantly improved mechanical and viscoelastic properties, acting as a reinforcement filler, and increased the cross‐link density of the castor oil, along with the material stiffness T_g. The reduction in chain mobility through the formation of the Si─O─Si network was cited as a contributing factor. Moreover, it was demonstrated that these properties all improved with higher NCO‐OH ratios, suggesting a correlation between hydrogen bonding and the formation of more urethane/urea segments.

Another variant of hybrid polyurethanes is from acrylated alkoxysilane castor oil, which was introduced by modifying castor oil with maleic anhydride followed by coupling with 3‐trimethoxysilylpropyl methacrylate.^{[ 97 ]} The resulting acrylated castor oil was then reacted with excess isophorone diisocyanate to provide the title NCO‐terminated polyurethane, which was subsequently cured as depicted in Figure  19 . The obtained hybrid films were tested for thermal, mechanical, and viscoelastic properties with improvements in each observed with increased acrylate‐silane moieties. This enhancement was attributed to the modification of castor oil acting as a reinforcement agent along with the higher crosslink density.

Figure 19.

Description of the synthetic route for organic–inorganic hybrid PU used by Shaik's group.^{[ 97 ]}

3.2.3. Silane End‐Capped Isocyanate

Isocyanates can be modified with a silane functionality, which after the PU is synthesized, can be moisture‐cured (see Figure 16). Meng et al., synthesized a series of organosilicon hybrid bio‐based polyurethanes from castor oil.^{[ 98 ]} They first prepared an isocyanate‐terminated trimethoxysilane by reacting 3‐trimethoxysilyl‐1‐propanethiol with a triisocyanate. This modified isocyanate, serving as the hard segment, was then used to form a polyurethane with castor oil as the soft segment. Poly(ethylene adipate) diol was employed as a chain extender, and the same silane was used as an end‐capping reagent. After moisture curing, different coatings were obtained, with testing revealing improved thermal stability attributed to the Si─O─Si network. For instance, tensile strength increased from 9.5 to 22.3 MPa as the content of 3‐trimethoxysilyl‐1‐propanethiol increased to 2 mol in the composition used for the preparation of the silane‐modified isocyanate. SEM imaging confirmed the reduced microphase separation between the hard and soft segments with increased siloxane content, indicating improved compatibilization of the two phases.

It is also possible to synthesize the organic–inorganic hybrid PU in dispersion. Ahmad's group reported the synthesis of a bio‐hybrid transparent coating based on linseed oil and 3‐isocyanatopropyl triethoxysilane (IPTES).^{[ 99 ]} They first reacted with linseed monoglyceride to synthesize a neopentyl glycol‐modified waterborne linseed alkyd via polycondensation. The resulting product was then employed to form films with urethane linkages by reacting the alkyd's hydroxyl functions with isocyanates from IPTES. Consequently, the silane moieties from the IPTES groups permitted a sol–gel process with acidic catalysis after the polyurethane formation to form the hybrid material. Various ratios of IPTES were tested, revealing that coatings became brittle with over 60 wt% IPTES content. Higher IPTES loading correlated with better mechanical properties, with TEM images confirming effective mixing, showcasing fine interconnected phases where IPTES acted as a bridge between organic and non‐organic segments, enhancing phase compatibilization.

Other works also employed IPTES to functionalize a castor oil to create a silane‐terminated VO.^{[ 100 ]} This modified VO was cross‐linked through the sol–gel process to produce solid microparticles through an emulsion of oil in water. To promote in situ oil cross‐linking and stabilize the modified oil, a gelling agent was employed. The design demonstrated potential as a drug delivery system, with the ability to entrap a model drug and release it under favorable conditions after 10 h. Cytocompatibility testing yielded positive results, although a notable drawback was identified in the failure to remove the catalyst, which was deemed toxic. The authors suggested the consideration of biocompatible catalysts to address this issue.

Some approaches have relied on the combination of different crosslinking systems. Indeed, moisture curing of the sol–gel process to create the siloxane network was applied, but several groups also incorporated another crosslinking trigger, by oxidation for example, as mentioned in the studies cited earlier from the Soucek and Pellegrene groups. These groups investigated the synthesis of polyurethane made from modified linseed oil alkyd combined with triethoxysilane functionalized isocyanate.^{[ 101 ]} This polyurethane was incorporated into unmodified linseed oil fatty acids resin at five ratios, ranging from 0 to 40%. The resulting coatings exhibited higher crosslinking density compared to the bare resin, contributing to enhanced durability and hardness. Two curing processes occurred in these materials: first, the curing of the organic phase through autoxidative crosslinking of the fatty acids, and second, the sol–gel process of the inorganic phase. SEM images highlighted the presence of silica aggregates in the coatings with higher ratios. Consequently, the most significant improvements in tensile and corrosion properties were observed in materials containing only 10 and 20 wt% of silane end‐capped linseed fatty acids.

3.3. Poly(hydroxyurethane) Based Polymer

More environmentally friendly PU alternatives can be derived from VOs. Traditional PU production often involves the use of potentially harmful isocyanates. To circumvent the use of isocyanates, alternative PU polymerization methods have been intensively investigated.^{[ 102 ]} These so‐called non‐isocyanate polyurethanes (NIPUs) are primarily formed through the reaction of cyclic carbonates and diamines, resulting in a hydroxy urethane linkage (see Figure  20 ). Researchers are exploring ways to enhance the mechanical properties of these poly(hydroxyurethane)s (PHU), and one avenue is the synthesis of hybrids, as mentioned by Ecochard et al.^{[ 103 ]} For example, one option for strengthening PHUs involves incorporating a silica network through the sol–gel procedure. Importantly, VOs can be easily substituted as a building block for the monomers used in the production of PHUs.

Figure 20.

Reaction between a cyclic carbonate and a diamine to synthesize polyhydroxyurethanes.

In the quest for synthesizing PHUs, the requisite carbonate functionalities are obtained through a direct and practical method: the carbonation of epoxy groups. As previously highlighted, the versatility of VOs makes them amenable to facile modification for increased epoxy group content. A noteworthy contribution comes from Gharibi et al., who elucidated the creation of an organic–inorganic coating from epoxidized soybean oil, specifically tailored for biomedical applications.^{[ 104 ]} The synthesis commenced with the carbonation of epoxy functionalities in soybean oil, followed by the addition of an amine (dimethylaminopropylamine) to form a hydroxyurethane linkage. Importantly, an excess of amine was judiciously introduced, yielding chain ends with tertiary amine groups. This PHU‐terminated amine then reacted with CPMS, culminating in the formation of a curable monocomponent methoxysilane. This hybrid entity exhibited methoxysilane, quaternary ammonium, and urethane components embedded within the soybean oil prepolymer. The final coating was prepared using a sol–gel process, where three different concentrations of HCl catalyst were employed. Impressively, the gel content reached 98%, achieved with the highest concentration of catalyst utilized. The material underwent a comprehensive evaluation, encompassing various physical and mechanical properties. Unsurprisingly, the coating with the highest gel content emerged as the optimal formulation, which was attributed to heightened sol–gel crosslinking facilitated by the urethane linkages, hydroxyl groups, and quaternary ammonium moieties placed into the material's structure. Beyond the improved mechanical properties, the material showcased its biocompatible character, being resilient against bacteria such as Escherichia coli and Bacillus subtilis. Importantly, the absence of leaching of toxic materials was observed, reinforcing the material's promise for biomedical applications using sustainable and biocompatible materials.

The same group extended their research into the modification of soybean oil to synthesize antimicrobial coatings.^{[ 105 ]} Here, they embarked on a two‐step process involving soybean oil. Initially, they carbonated epoxidized soybean oil using CO ₂ . Subsequently, they end‐capped the carbonated soybean oil by reacting the carbonate functionalities with the amine functional APTMS, resulting in the formation of hydroxyurethane bonds (cf. Figure  21 ). Concurrently, a fatty amide was synthesized from soybean oil and subsequently reacted with CPMS, yielding a methoxysilane‐functionalized quaternary ammonium‐containing fatty amide. Coatings were then prepared by blending the two modified components at three different ratios with the sol–gel process. This innovative approach showcases a comprehensive modification of soybean oil, harnessing its various functionalities to create a dual‐component system with enhanced antimicrobial properties. The coatings with a higher content of the quaternary ammonium‐containing compound demonstrated promising biocidal activity, primarily attributed to their elevated surface hydrophobicity. These coatings exhibited super‐hydrophobic surfaces, effectively preventing bacterial adhesion. Interestingly, the presence of hydroxyurethane bonds and the siloxane network led to a higher pendulum hardness (263 s ⁻¹ ) in coatings with lower quaternary ammonium content. However, an increase in the quaternary ammonium‐containing compound resulted in a decreased crosslink density.

Figure 21.

Structure of the end‐capped VO used by Gharibi et al.^{[ 98 ]}

In a related study, Deng's group detailed the preparation of a green polyhydroxyurethane synthesized from soybean oil and lignin.^{[ 106 ]} Initially, they created silane end‐capped soybean oil by reacting carbonated soybean oil with APTES, forming the hydroxyurethane linkage. Subsequently, three different ratios of lignin (20%, 35%, and 50%) were introduced, and the polymer was cured under two conditions: i) at ambient temperature and ii) at 60 °C. The procedure is outlined in Figure  22 . The tensile strength and elongation at break exhibited disparate outcomes based on the curing conditions. Higher tensile strength was observed when the material was cured at 60 °C, while elongation at break increased with lower curing temperatures. The authors explained these results by considering the role of lignin at different temperatures. At higher temperatures, lignin acted as a crosslinking agent, enhancing the overall material's rigidity. In contrast, at lower temperatures, lignin merely blended with the polymer without acting as a crosslinker. This resulted in more available silanols, enabling their reaction and forming the Si─O─Si network. Consequently, at higher temperatures, with a higher ratio of lignin, the tensile strength increased up to 1.4 MPa due to the heightened crosslink density of the lignin network. However, at lower temperatures, the elongation at break increased with a higher lignin ratio, attributed to steric hindrance limiting the formation of a 3‐D network, in a lower overall crosslinking density.

Figure 22.

Procedure used to synthesize green polyhydroxyurethanes from soybean oil and lignin.^{[ 106 ]}

3.4. Additional Polymerizations

As explained in the introduction, VOs are highly versatile, with multiple functionalities available to form unique material combinations. The ensuing section will discuss modification of VOs that contrast with the approaches previously addressed. For example, polymerization of the double bonds in the VO is possible just by adding an initiator. Perdoch et al., recently studied silylation of linseed oil using an organosilicon compound containing a vinyl group, vinyltrimethoxysilane.^{[ 107 ]} The resulting silylated linseed oil coating exhibited remarkable long‐term stability. When applied to wood surfaces, the coating contributed to significantly improved resistance against water exposure, highlighting the potential of silylated linseed oil as a durable and water‐resistant coating.

Colak et al., synthesized an acrylated soybean oil, which was then reacted with APTES through a Michael addition (Figure  23 ).^{[ 108 ]} An excess of acrylated soybean oil was utilized, and the remaining acrylate functions were employed for either homopolymerization of the acrylated VO or copolymerization with styrene (50/50 weight %) via radical polymerization. Following polymerization, the material underwent sol–gel moisture curing. The resulting material exhibited suitable properties for pressure‐sensitive adhesives, with the adhesion force increasing eightfold after curing, attributed to the formation of Si─O─Si networks at the interface. However, when cured at 92% relative humidity, adhesive properties decreased by up to 15%. Comparing materials based on homopolymerization and copolymerization, the researchers observed improved mechanical properties with the incorporation of styrene, such as a 35% increase in storage modulus (up to 43 MPa) due to the increase in the crosslink density. Supporting this observation was the increase of T_g from 127 °C before moisture cure to 160 °C after curing. The effect of higher crosslink densities was clearly observed in the results of swelling tests in tetrachloromethane. The swelling ratio clearly decreased after moisture cure from 1.5 to 1.4, indicating a tighter network.

Figure 23.

Modified acrylated soybean oil used by Colak et al.^{[ 108 ]}

4. Fiber and Filler Incorporation

The sol–gel procedure can be an effective platform for synthesizing composites incorporating hybrid components. The performance of nanocomposites is keenly dependent on the dispersion of the fiber or filler into the matrix. Generally, functionalization is needed to obtain a better compatibilization with the matrix. Silylation of the fiber is a possible functionalization option as it could be an efficient coupling agent to form covalent bonds with the sol–gel procedure while also substantially reinforcing the composite.^{[ 109 ]} We will address the effect of fillers when combined with VO‐derived materials in terms of them being inorganic or organic in the following sections.

4.1. Inorganic Filler

A quite common reinforcement filler is silica (SiO₂), often in the form of a nanoparticle. Generally, the surface of the silica particle is modified with a silane bearing a desired function. The silane will react in the sol–gel process with the hydroxyl functions present at the surface of the silica nanoparticle by forming Si─O─Si bonds: e.g., a silica nanoparticle‐O‐Silane, as described in Figure  24 .

Figure 24.

Structure of modified silica nanoparticles. R₁ can be chosen to have the needed chemical function to be involved in the next step of the synthesis (i.e., carbonate for PHU synthesis, for example).

Yan's group modified nano‐SiO₂ particles with APTES through the sol–gel process, resulting in amino‐functionalized nano‐SiO₂.^{[ 110 ]} They incorporated these modified nano‐SiO₂ particles into a bio‐based polyurethane matrix derived from Sapium sebiferum‐based polyol and isophorone diisocyanate. Four different weight ratios of silica nanoparticles (1, 3, 5, and 7 wt%) were tested. The researchers reported improved thermal stability due to restricted polymer chain movement, with nano‐SiO₂ acting as a thermal cover layer. However, beyond a certain loading (3 wt% yielding optimal results), aggregation of nano‐SiO₂ particles weakened thermal stability and caused incompatibility with the matrix. Tensile strength reached its maximum (12.4 MPa) at 3 wt% incorporation, over 2.5 times higher than the neat PU.

Mandal's group also worked with amino‐functionalized silica nanoparticles. They used APTMS to modify silica nanoparticles' surface and synthesized polyurethane using castor oil.^{[ 111 ]} Like the Yan group's findings, the incorporation of amino‐functionalized silica nanoparticles improved mechanical and thermal properties until nanoparticle aggregation occurred, limiting further enhancement. They highlighted that amino‐functionalization increased crosslinking density, improving compatibility.

Ghosal et al. synthesized a hybrid polyurethane nanocomposite from linseed oil and silica.^{[ 112} , ^{113 ]} They added TEOS to linseed oil in different ratios and used toluene diisocyanate for polyurethane formation, along with a fumed silica dispersion. Optimal mechanical results were obtained with 0.5 mol of TEOS on linseed oil and 2 wt% of fumed silica incorporation. SEM imaging confirmed good interactions between the matrix and the inorganic phase. Notably, phase separation and aggregation occurred at higher TEOS ratios and silica content, emphasizing the importance of finding the right balance.

Güngör and co‐workers presented a straightforward method for the preparation of reinforced polyhydroxyurethane coatings using silica nanoparticles.^{[ 114 ]} First, they devised a facile technique to modify the surface of silica nanoparticles, imparting carbonate functionality. Simultaneously, carbonated soybean oil was prepared from epoxidized soybean oil at 110 °C under a CO₂ stream for 24 h, utilizing 5 wt% tetrabutylammonium bromide as a catalyst. In a subsequent step, varying ratios of carbonated silica nanoparticles and carbonated soybean oil were mixed with butylenediamine to synthesize poly(hydroxyurethane)s. The resulting coatings were cured at 75 °C in a Teflon mold for 24 h. Evaluation of the nanoparticle dispersion within the polyurethane matrix was conducted through SEM imaging. The initial results revealed aggregates and phase separation, indicating the need for improved dispersion. To address this, polypropyleneglycol diglycidylether (PPG), pre‐treated with carbonation in a preliminary step, was introduced into the preparation. Subsequent SEM images confirmed a more effective incorporation of silica nanoparticles into the matrix composed of a 50–50 wt% blend of carbonated soybean oil and carbonated CPPG. Ultimately, the reported mechanical and physical properties underscored that the superior coating was achieved with the dual matrix containing the higher weight percentage of modified silica. This methodology not only provides a versatile approach for reinforcing hydroxyurethanes coatings with silica nanoparticles but also highlights the importance of optimizing nanoparticle dispersion for enhanced material performance.

The same sol–gel process can be exploited with other fillers by modifying them with silanes. This time the bonds formed will be no longer Si─O─Si but Filler─O─Si, as silane moieties will react with the hydroxyl bonds present on the surface of the filler used (cf. Figure  25 ). A wide variety of fillers can be used, ranging from titanium dioxide to basalt fillers.

Figure 25.

Example of the surface modification of other fillers with silane moieties using the hydroxyl functional groups present on the surface of the fillers. In blue, a modified basalt fiber is shown, in yellow zinc oxide and in green titanium oxide. R₁ is chosen to be the desired functionality to help the compatibilization of the filler to the polymer matrix.

Wang et al. investigated soy‐based PU nanocomposites reinforced with palygorskite (Pal).^{[ 115 ]} They synthesized a polyol from soybean oil and reacted it with a diisocyanate to form the soy‐based PU. The nanocomposite was prepared using silylated Pal, achieved by hydrolyzing the silane and condensing the OH to obtain Si‐O‐Pal bonds and Si─O─Si bonds, creating a rigid polysiloxane network. Incorporating the silylated Pal into the PU matrices significantly improved tensile strength (303% improvement). Thermal stability is also enhanced, especially for the sample with a 12 wt% incorporation of silylated Pal where the initial decomposition temperature (temperature at 5% weight loss) is delayed by 5.7 °C compared to the neat PU.

PU prepolymers from monoglycerides derived from various VOs (castor, linseed, coconut, mustard, sunflower, and rice bran oils) were synthesized by reacting them with diisocyanate.^{[ 116 ]} Improvement of the coatings was attempted by adding TiO ₂ nanoparticles, whose surface was modified with vinyltriethoxysilane for better incorporation into the matrix via the sol–gel process. Despite successful production of PU coatings from all tested oils, the addition of modified TiO ₂ nanoparticles did not significantly enhance properties such as hardness or scratch resistance. In contrast, Raju's group prepared poly(ester amide urethane) siloxane‐modified ZnO hybrid coatings from Thevetia peruviana seed oil.^{[ 117 ]} They modified the oil by converting it to a polyester‐amide, which was then reacted with a diisocyanate to form the desired polyurethane prepolymer. ZnO particles were modified using APTMS, allowing their sol–gel linkage to the prepolymer. Thermal stability increased with up to 10 wt% of incorporated particles, after which it decreased, possibly due to particle aggregation. Anti‐bacterial tests against E. coli showed efficacy, with better results observed for 10 wt% ZnO, while higher concentrations were speculated to cause particle aggregation and impact activity negatively.

Modified basalt fibers using different silanes were integrated into epoxidized soybean or linseed oil matrices.^{[ 118} , ^{119 ]} Sol–gel processes between basalt fiber surfaces and silanes facilitated attachment. Amino‐silanes directly reacted with oxirane groups in the VO, while glycidyl‐silanes first reacted with free carboxylic acids and then with oxirane groups. In the first study, modifying the surface of basalt fibers enhanced mechanical properties, with a higher tensile strength (≈11% higher) observed when the fibers were treated with silanes. Notably, employing a glycidyl‐functional silane yielded better results compared to amino silane. The choice of VO also influenced the tensile strength, with linseed oil providing higher strength than soybean oil. This difference was attributed to the higher crosslinking achieved with linseed oil, which has a higher epoxy equivalent weight.

Kessler's group investigated the influence of silanes on the interfacial adhesion of reinforced ring‐opening metathesis polymerization (ROMP)‐based bio‐renewable polymers.^{[ 120 ]} The polymer was derived from a linseed oil‐based monomer reacting with dicyclopentadiene. Glass fibers were incorporated into the matrix and modified with silanes bearing pendent norbornyl groups for better compatibility. Two silane coupling agents were tested, with silane usage enabling higher crosslinking, leading to increased stiffness and significantly improving interfacial adhesion. The interfacial shear strength increased by ≈50–150%, depending on the silane used. In a subsequent study, the group explored an automated pultrusion process for cost‐effective composite material production using the same high‐performing silane from the previous study.^{[ 121 ]} The researchers investigated the optimal concentration of the silane for this process and found that dipping heat‐cleaned glass fibers in a 3% silane solution for 3 min, followed by hydrolysis and curing at 120 °C for 10 h, yielded the best results in flexural test and for dynamic mechanical analysis.

Reinforced PHUs derived from soybean oil using a ZnO filler have been described.^{[ 122 ]} The approach involved synthesizing bio‐based polyhydroxyurethane from carbonated soybean oil and various amines. To reinforce the material, carbonated silica or carbonated ZnO nanoparticles were introduced. The carbonation of the filler was achieved through a sol–gel process, involving the hydrolysis/condensation of 3‐(glycidoxypropyl) trimethoxysilane with the surface of the filler, followed by the carbonation of the epoxy function of the silane. The fillers, possessing carbonate functions, could directly react with the matrix, establishing covalent linkages between the fillers and the PHU matrix. The authors delved into investigating the influence of the amine structure and the presence of the filler on the mechanical properties of the resulting material. They observed that Young's modulus of the PHU increased with the rigidity of the amine used, indicating a direct correlation between the rigidity of the amine and the stiffness of the PHU. Additionally, the authors reported that, across all formulations, Young's modulus increased with the presence of the filler (5 wt%). This enhancement was attributed to the increased crosslink density facilitated by the incorporation of fillers, signifying their role in reinforcing the polyhydroxyurethane matrix.

4.2. Organic Filler

Not only inorganic fillers can be incorporated into polymeric matrices, but organic ones can be also effective. Several groups have shown that it is possible to compatibilize organic fillers from vegetable sources into polymers. The sol–gel process is again a useful tool to have better incorporation of the filler in the matrix, while yielding a process that leads to green materials, which are often totally biodegradable.

One such example was provided by Wu et al., who proposed to incorporate a vegetable fiber (from sisal) into epoxidized soybean oil.^{[ 123 ]} To enhance the performance of the obtained materials, they modified the fibers with silane (either methacryloxypropyl trimethoxysilane or vinyl triethoxylsilane). Following the sol–gel process, some hydroxyl groups on the vegetable fiber reacted with the silanol groups. This pre‐treatment enhanced the incorporation of the fiber into the vegetable matrix as covalent bonds between the matrix and fibers were created, which was supported by the increase in measured interfacial adhesion. Furthermore, the overall material was shown to be biodegradable after soil burial tests.

Furthermore, vegetable fiber from oil palm empty fruit bunch is an intriguing organic filler, which is a biomass formed during the production of palm oil. This oil palm fiber can be used to produce composites when mixed in a polymeric matrix. Mohd Ishak et al. investigated the incorporation of the oil palm fiber into high‐density polyethylene.^{[ 124 ]} To enhance compatibilization, they modified the surface of the fiber with two different amino silanes, either APTES or APTMS. They showed modification enhances the performance of the final composite compared to the unmodified form, especially in terms of the material stiffness. Nevertheless, tensile strength had limited improvement even after treatment. The authors suggested the fibers were not well dispersed, and that bundles or aggregates were formed, which was confirmed by SEM, leading to the poor performance of the composite.

5. Conclusion

Much research is ongoing to produce novel bio‐based materials relying on VOs to reduce the reliance of feedstocks from petroleum‐based materials. This review provides a synopsis on the production of enhanced materials from VO combined with silane moisture curing. VOs have potential as green biomass, and their utilization could be an answer to produce more sustainable materials. We have seen that VOs are versatile and their intrinsic functionalities make them good building blocks. When combined with silanes to synthesize moisture‐cured materials via the sol–gel process, the obtained materials are competitive to their petro‐based analogs.

Different paths can be used to produce those green materials based on silylated VOs, as summarized in Figure  26 . First, the raw VO can be used without any modification with curing done via the sol–gel process by adding a silane in the medium. The silane can be either covalently linked (see Figure 26b) or added in a neat form to VOs (see Figure 26a). For example, we have seen the production of material from VOs mixed with TEOS which does not imply any covalent bonds between the oil and the silane. Second, it was straightforward to produce polymers from VOs due to the multiple functions present in the oils. Several kinds of polymerization are possible from the reaction of the epoxy rings with amines to the synthesis of poly(hydroxyurethane)s. Again, the silane can be either solely mixed into the polymeric matrix without forming covalent bonds (see Figure 26c) or it can form permanent linkages with the polymerized VO (see Figure 26d). Finally, the combination of VO and silane can be also applied in the synthesis of composite materials (see Figure 26e). All the materials produced from these different paths have hybrid organic–inorganic networks which are often the key to reach desirable properties.

Figure 26.

Summary of the different ways to obtain cured hybrid Organic–Inorganic materials from VOs employing a sol–gel process. (Left) From raw VO mixed with either neat silane: no formation of covalent bond, only a Si─O─Si network (in orange) or functional silane: formation of covalent bonds between the VO and the siloxane and formation of a Si─O─Si network; (Middle) From polymerized VOs with either neat or functional silanes (same explanation as in left panel); (Right) Composites: functionalization of the filler surface with a silane using the sol–gel method and then mixed into a VO.

Across all these hybrids, the main objective of producing high‐performance materials is to ensure the good incorporation of the inorganic network into the organic matrix. If the compatibilization is not effective, agglomerates form and are reflected in poor material properties. Indeed, most of the time, the mechanical properties will be enhanced, and the material will have a higher contact angle and a smoother surface due to the lower surface energy provided by the silane additives used. The key is thus to determine the optimal ratio of inorganic functionality to add to the organic matrix to improve the performance sufficiently to be competitive with the fossil‐based analog. Higher loadings generally lead to more significant property improvements, but this also leads to a greater risk of agglomeration and potentially greater cost. To prevent aggregation, one possible option is to use an inorganic filler bearing a functional group that could be used to create a covalent bond between the inorganic network directly with the organic matrix. This will aid better dispersion throughout the matrix, and it will also improve the performance of the material as two networks will be created. Indeed, it will have the usual Si─O─Si network from the moisture curing of the silane, but it will also have bonds between the silane and the organic matrix, leading to improved overall crosslink density and better adhesion.

Another crucial point to produce this kind of materials is their recyclability and their degradation. The introduction of inorganic moieties into the materials improves their physical and mechanical properties, but it can also have an impact on their ability to be recycled. Commonly these types of hybrid materials are treated as composite materials and their recycling process involves pyrolysis or chemical treatments to eliminate the organic network. Once separated, the recycling of the inorganic network can be done either by dissolution in alkaline solution, or by melting as performed in the glass industry.^{[ 47 ]}The silane‐based parts, once separated may be treated in the same manner as for pure silica gels, except that some side species (as organosilicates R_nSi(OH)_4−n) may be released.

This review inherently focused on the positive aspects of employing VOs in place of fossil fuel‐based analogs; this comes up short in truly assessing the sustainability of VO‐based composites. It is recognized now that any process should be assessed holistically, examining not only the materials sources, but other key inputs and their impact on the sustainability of the overall process. An example of guidance in using appropriate metrics is exemplified by that done for other claimed green materials, such as polymer composites containing natural or carbon fiber.^{[ 125} , ¹²⁶ , ^{127 ]} Indeed, reports have started to emerge examining changes to the entire process, where the acrylated soybean oil composites with amine‐functional siloxanes were examined without using catalyst, solvent, or high temperatures.^{[ 128 ]} Further work using this life cycle assessment (LCA) framework is encouraged to produce truly more sustainable VO/inorganic hybrids.

The review also noted a case where researchers adopted vitrimer chemistry via dynamic covalent bonds such as the vinylogous urethane linkage. This was not studied comprehensively, but it shows that such systems are readily attaining in VO/inorganic composites. The application of silane/siloxy chemistry^{[ 129 ]} is another kind of dynamic covalent that could be readily exploited in many of the systems cited. The extension of the product life by being able to use lower energy processes to recycle materials is a research path worth exploring.

In summary, hybrid organic–inorganic materials based on the combination of VOs with silanes are a promising route to compete with fossil‐based materials. This review focused only on VO, but many other biomass feedstocks could be used in the same manner, such as chitin and chitosan, peptides derived from the fractionation of the biomass, terpenes, cellulose, and other monomeric sugars coming from the extraction of biomass.^{[ 130 ]} Various materials could be obtained following the same process by introducing additional inorganic species with new functionalities leading to new types of linkages. The huge number of possible feedstocks from bioproducts, highlighted by the VO‐based materials in this review, when combined with the rich hybrid chemistries such as that found with silanes, are promising for adaptation of tailored products from biorefinery feedstocks.

Conflict of Interest

The authors declare no conflict of interest.

Biographies

Eline Laurent received her Ph.D. in Polymer Chemistry from the Institute Charles Sadron (ICS) of the University of Strasbourg in 2020, under the supervision of Dr. Jean‐François Lutz. She was working on the synthesis of sequence defined storing information at the molecular level. Then she joined McGill University in the team of Professor Milan Maric in 2022. Currently, her research interests mainly focus on the synthesis of greener materials from bio‐sourced raw materials, including innovative and bio‐based polyhydroxyurethanes as well as vitrimeric materials.

Milan Maric received his Ph.D. in Chemical Engineering from the University of Minnesota‐Twin Cities in 1999, working with Chris Macosko, focusing on the reactive compatibilization of poly(siloxanes) with thermoplastic matrices. Following a 4 year stay as a member of the Scale‐Up Engineering team at the Xerox Research Center of Canada, he joined McGill University in 2003, where his research program has included applications of nitroxide mediated polymerization, development of green plasticizers and additives, novel hybrid bio‐based non‐isocyanate poly(urethanes) and vitrimeric materials.

Laurent E., Maric M., Organic–Inorganic Hybrid Materials from Vegetable Oils. Macromol. Rapid Commun. 2024, 45, 2400408. 10.1002/marc.202400408