MAPP Compatibilized Recycled Woodchips Reinforced Polypropylene Composites with Exceptionally High Strength and Stability

Abstract

Wood chips were used in their original form without any physical or chemical treatment as reinforcement for polypropylene to develop composites as potential replacement for medium density fiber (MDF) boards, gypsum based false ceiling and other building materials. Wood chips are generated as byproducts and have limited and low value applications. Composites with up to 90% wood chips were developed through compression molding and the mechanical, acoustic and thermal properties were studied. Further, maleated polypropylene (MAPP) was used (1–5% w/w based on woodchips used) as compatibilizer and changes in properties were recorded. Up to 300% increase in tensile properties were observed when 5% compatibilizer was present. Tensile properties of the composites containing MAPP were higher than that of commercially available medium density plywood boards and also gypsum based ceiling tiles. Addition of MAPP did not change thermal conductivity but decreased sound absorption. Wood chips reinforced PP composites containing MAPP show exceedingly high properties and could replace particle, fiber boards and other building materials in current use. Utilizing the wood waste also results in environmentally friendly, sustainable and low cost building materials.

Graphical Abstract

Statement of Novelty

We report the use of woodchips to develop composite with properties better than currently used medium density boards. For the first time, we demonstrate that discarded wood chips can be used in their native form without any chemical treatment to develop biocomposites.

Introduction

Biobased composites developed using agricultural residues and coproducts have gained major prominence in the last few years. Large availability, low cost and unique properties make biobased materials ideal for use as both reinforcement and matrix in composites. In addition, the need to replace synthetic polymer based materials and reduce the environmental impact have put an impetus on developing biobased products. It has been estimated that the global market for biobased products and materials is expected to reach about USD 86 billion by 2026. In addition to the large opportunity, it is also inevitable to ensure that natural resources are utilized to the maximum.

Wood and timber are one of the largest commodity products both in terms of volume and value. Currently, it has been estimated that the global wood product market will have a steady growth and reach to about 900 billion by 2025 (https://www.researchandmarkets.com/reports/5238003/wood-products-global-market-report-2021-covid-19). Among the various types and applications of wood, sawn wood occupies a prominent position. For instance, the global production of sawn wood was about 463 million m³ in 2016 and reached about 473 m³ in 2020 and the growth is expected to increase further. Wood sawing generates considerable amount of waste generally called wood chips. About 20–30% of wood gets removed as chips which have limited applications. Typically, wood chips are used as fuel, mulching, to prepare pulp and also as reinforcement for composites. Recycling wood chips are useful for capture carbon and an attractive from a sustainability point of view.

To prepare composites, wood chips have been combined with various matrices and subject to injection, compression or other molding techniques. Wood chips formed in carpentry units were collected and combined with Expanded Polyester Waste (EPW) dissolved in acetone and made into composites using injection molding [1]. The products developed were suggested to be useful for carpentry, building and other applications. In another study, low density polyethylene waste was combined with pine wood waste in the presence of maleic anhydride grafted polyethylene as a coupling agent. Tensile strength of the samples was between 10 and 1.25 MPa. Similarly, pinewood and polypropylene were combined together with MAPP as the compatibilizer and made into composites through melt compounding and later hot-press (compression) molding. Good interfacial interaction leading to uniform and better stress transfer was observed due to the presence of the compatibilizer [2]. Novel biocomposite panels were developed by combining wood waste available with hydrated natural lime as binder and metakaolin as additive. The biocomposites formed had compressive strength of 250 kPa and flexural strength of 150 kPa. Interestingly, the composites had thermal conductivity between 0.04 and 0.07 W/mK which satisfied the requirement (ASTM standard) for wall insulation [3]. Further improvements in wood polymer composites have been made by using compatibilizers, particularly isocyanates [4]. Similar to using wood chips, several agricultural residues have been used as reinforcement to develop composites that meet the specified standards for construction, interiors (false ceilings, partition panels) etc. due to their large availability, low cost and unique properties [5, 6]. A few agricultural coproducts such as sheep wool have inherent flame resistance, high acoustic and noise insulation properties that are highly suitable for building applications [7].

Not only with polymers, wood chips have been used as reinforcement for concrete as well to form wood-cement composites. It was reported that alkali cooking of chips and addition of a silane coupling agent provided high flexural strength with properties suitable for various applications [8]. Wood particles generated from pine trunks were combined with Portland cement along with a super-plasticizer and were found to have the physical and mechanical properties required to meet standards [9, 10]. In another approach, olive wood scraps (chips) were extruded with poly(lactic acid) and the composite was fabricated into 3D models using the fused filament fabrication process. Adding up to 20% wood chips in PLA was found to be beneficial both from the product and environmental perspective [11].

Previous studies on utilizing wood chips for composites have mostly chosen the injection molding approach where the chips are made into powder, combined with a matrix and then converted into composites. Making the chips into powder decreases the aspect ratio which reduces mechanical properties and hence not ideal. We have reported a unique approach of using agricultural residues in their native form to form composites. In this approach, the reinforcement is made into a sandwich type pre-preg and later compression molded into composites. This system allows us to retain the original shape and size of the reinforcement and also avoids the need for mechanical or chemical processing. Based on our previous studies, we have used wood chips in their native form and combined them with polypropylene as matrix. Our intention is to develop biocomposites using woodchips in their native form without the need for physical or chemical treatments. This will ensure that the cost and commercial viability of the woodchip-PP composites will be high and also the inherent properties of the woodchips will be preserved. Various ratios of the matrix and reinforcement were used and the changes in mechanical and other properties were investigated. The compatibility and adhesion between the nonpolar PP and polar lignocellulosic reinforcement (wood) is poor resulting in low stress transfer inferior strength and other properties. Therefore, maleated polypropylene was used as a compatibilizer and the effect of MAPP on the tensile, thermal, acoustic resistance were studied between compatibilized and uncompatibilized composites [12, 13]. Further, the wood-PP composites were evaluated for commercial applications by comparing against medium density fiber (MDF) boards and gypsum tiles having similar density and currently sold on the market.

Materials and Methodology

Materials

Sal woodchips (Shorea robusta) were purchased from local mills processing wood. Wood chip size was determined by measuring the length, width, and thickness of 50 pieces manually. The wood chips had average length of 5–38 mm, width between 2 and 18 mm and thickness was between 0.2 and 2 mm. Initial moisture content (w/w%) of the wood chips was 28% and density was 0.72 gm/cm³ as per literature. Approximate composition of the wood chips was Cellulose—40–50%, Hemicellulose—15–25%, Lignin—15–30% and Ash—2–10%. Non-woven PP webs used in this study were purchased from KT international, Mumbai, India and had density of 0.9 gm/cm³, thickness of 80 μm and Melt Flow Index of 3.3 g/10 min at 230 °C. The compatibilizer, maleated polypropylene (MAPP) used had viscosity of 120 cps and specific gravity of about 0.1. MDF board used for comparison was of 0.61 g/cm³ density and thickness of 5 mm and length and width were 600 mm). Similarly, the commercially available gypsum tiles had density of 0.70 g/cm³, thickness between 8 and 10 mm and length and width was 600 mm.

Methods

Fabrication of the Composites

Composites were fabricated using wood chips as the reinforcement and polypropylene as the matrix. The required proportion of wood chips and polypropylene were weighed to obtain ratios of 60/40, 70/30, 80/20 and 90/10 (woodchips/PP w/w%). The wood chips were uniformly distributed over layers of the PP web to form a sandwich type pre-preg. The pre-preg was placed between two layers of aluminium foil and compression molded in a press at 180 °C for 5–8 min at 13.8 N/mm². After the desired compression, the mold was cooled to about 35 °C by running cold water and the composites formed were collected.

For the compatibilization with MAPP, the desired amount of MAPP (1, 3 or 5% based on weight of the wood chips) was dispersed in water and the wood chips were added into the MAPP suspension. Later, excess water was drained and the MAPP treated wood chips were placed in an oven at 50 °C for 24 h. Once the MAPP treated wood chips were dry, they were used to prepare the pre-preg and then compression molded into composites as described above. Composites were prepared for a thickness of 10 mm (similar to the thickness of commercially available gypsum ceiling tiles) but with varying densities.

Testing of Composites

Wood chips-PP composites were tested for their mechanical properties (tensile and flexural) after conditioning the samples in an environmental chamber (Memmert CTC256) at 21 °C and 65% relative humidity for at least 24 h. The tests were conducted in a universal tensile tester (MTS Exceed E43, MTS Corporation, Minnesota, USA) equipped with 100 N load cells. Tensile tests were performed as per D 638-14 standard on samples measuring 165 mm in length and 19 mm at its widest section and flexural tests were done as per ASTM D790 with 3-point bending assembly using samples of 203 mm long and 76 mm wide. At least 15 samples from three different composites were tested for each condition and the average and ± one standard deviation are reported. Tensile and flexural tests were also done after conditioning the samples at different humidities and temperature to determine the resistance of the composites to moisture. Thermal conductivity of the samples (200 × 200 mm) were measured as per ISO 8301:1991 standard in an HFM 436 Lambda meter (Erich Netzsch Gmbh & Co. Holding Kg, Selb, Germany). Acoustic absorption of the composite samples of two different diameters (99.5 mm and 29.5 mm) was done as per ASTM E1052-12 in an impedance tube (Bruel and Kjaer) with frequency range between 2000 and 6000 Hz. Morphological analysis of the composites was based on the observations in a scanning electron microscope (Hitachi VP 3000 N) after coating the samples with gold–palladium. Percentage water sorption of the wood chip/PP composites was determined using ASTM D570-98 standards on samples measuring 60 × 60 mm. Samples of known weight were immersed in water at room temperature and the change in weight was recorded every 2 h until saturation was attained and the change in weight was used to determine the % water sorption. In all the above tests, the averages of at least three trials were reported.

Results and Discussion

Influence of Wood Chip Content on Tensile Properties

Wood chips act as a reinforcement and are expected to increase the strength of polypropylene. However, the tensile strength and modulus of the composites were observed to decrease with increasing amount of reinforcement (Fig. 1). Such decrease should mainly be due to the lack of sufficient PP to bind the chips together. Unlike fibrous reinforcement where the aspect ratio is high and the small fiber diameters allow penetration of the matrix, the chips are continuous pieces and the matrix is unable to penetrate through the chips. Hence, there will be lower binding between the chips and matrix and hence the tensile properties decrease with increasing wood chip content. Compared to 60% chips as reinforcement, the composites containing 90% chips had about 75% lower tensile strength and modulus. Although 60% reinforcement provided the highest strength and modulus, it is preferable to have the highest amount of wood chips possible to reduce cost and to make it more attractive for recycling/reuse. The extent of reinforcement and matrix could be chosen based on the specific requirements.

Fig. 1.

Tensile properties of wood chip reinforced PP composites having bulk density of 0.81 g/cm³ and compression molded at 180 °C for 10 min

Similar trend was also observed for the flexural strength and modulus with about 80% decrease in strength and 77% decrease in modulus when wood chip content was increased to 90% compared to the properties containing 10% wood chips. Unlike the tensile properties, both the 60 and 70% wood chip composites have similar properties and a decrease of 42–55% in modulus was observed when the proportion of chips was increased from 70 to 80% (Fig. 2). However, the strength of 12 MPa and modulus of 1.9 GPa at 80% wood chip content is considerably high compared to similar composites developed using various agricultural residues as reinforcement. For example, polypropylene composites reinforced with 60–90% sheep wool had flexural strength between 11 and 16 MPa and considerably low modulus between 450 and 600 MPa [14]. Similarly, PP reinforced with chicken feathers had strength between 6 and 11 MPa and modulus between 400 and 550 MPa [5]. In other studies, polypropylene composites reinforced with mulberry stems had flexural strength between 32 and 40 MPa and modulus between 8 and 9 GPa at similar densities [15]. Other agricultural residues such as coffee husk, ground nut shell-rice husk when used as reinforcement for PP have shown maximum strength of 37 MPa. Hence, the inherent properties of the reinforcement considerably influence the properties of the composites. Commercial gypsum boards used as false ceiling and with similar density have flexural strength of about 7 MPa, considerably lower than the strength of the wood chip reinforced PP composites obtained in this study.

Fig. 2.

Flexural properties of wood chip reinforced PP composites having bulk density of 0.81 g/cm³ and compression molded at 180 °C for 10 min

Influence of Compatibilizer on Tensile and Flexural Properties

Since wood chips are hydrophilic and polypropylene is hydrophobic, we considered the possibility of enhancing the properties of the composites by using a compatibilizer. Interestingly, substantial improvements were observed in both strength and modulus due to the addition of the compatibilizer and the extent of improvement was dependent on the amount of compatibilizer used. Even at 1% MAPP, the strength increased 232% as seen from Fig. 3 and further increase in MAPP content to 3% enhanced strength by 264% and further upto 311% with 5% MAPP. Interestingly, the extent of increase in properties was higher when the % of PP in the composites was higher. For example, composites with 20% PP and 80% wood chips had a 254% increase in strength with 5% MAPP compared to 311% for 30% PP composites. Although modulus also showed an increase with MAPP, the extent of increase was considerably less. A progressive increase of 191%, 200% and 233% was seen as the MAPP content was increased from 1 to 5% compared to composites without any MAPP and when the ratio of PP in the composites was 30%. Corresponding increase in modulus was 165%, 170% and 235% for the composites containing 20% PP (Fig. 4).

Fig. 3.

Tensile properties of wood chip reinforced PP composites, with and without MAPP, having bulk density of 0.81 g/cm³ and compression molded at 180 °C for 10 min in comparison to commercially available MDF board having bulk density of 0.61 g/cm³

Fig. 4.

Flexural properties of composites, with and without MAPP, made with wood chips and PP having bulk 0.81 g/cm³ density and compression molded at 180 °C for 10 min and with 1, 3 or 5% emulsion

Extent of improvement in flexural properties of the composites were also dependent on the amount of MAPP used. The highest increase in flexural strength was from 20 to 27 MPa (35% increase) and modulus was marginal by about 11%. However, unlike tensile properties, the improvement in flexural properties was higher for lower PP content. Composites containing 20% PP had strength increase from 12 to 18.6 MPa (55% increase) and modulus increased from 1.9 to 2.5 GPa (34% higher) as MAPP content was increased to 5%. Amount of MAPP directly influences the improvement in properties of the composites. Several reports have shown that addition of MAPP positively changes the interfacial adhesion and binding between the matrix and reinforcement [14]. A 200% increase in flexural strength and 139% increase in flexural modulus was reported when MAPP was used as a compatibilizer for oil palm fiber reinforced (75–80%) PP composites [16]. Composites containing MAPP had only fiber breakage than pullouts also suggesting the excellent interaction between the matrix and reinforcement. The OH groups in wood chips forms ester linkages with MAPP leading to better interactions. Such interactions have been reported for rice husk and MAPP also [15]. Compared to the MDF board, the sawdust-PP composites had nearly 200% higher flexural strength and 550% higher modulus. Such exceptional high strength is due to the relatively uniform distribution, fewer voids and inherently high strength of wood chips compared to the components in MDF. The properties of the composites obtained in this study were compared to a commercially available medium density fiber (MDF) board with similar density. As seen from Table 1, the woodchips-PP composites have higher strength and modulus compared to the MDF board. Hence, the woodchip PP composites could be suitable to replace the MDF board for various applications.

Table 1.

Comparison of the flexural properties of composites made with sawdust and PP Composites with commercially available medium density fiber boards and gypsum based ceiling tiles of similar density

Emulsion %	Wood chips-PP (%W/W)	Strength (MPa)	Modulus (MPa)
0%	70/30	20.8 ± 2.1	2912 ± 188
	80/20	12.1 ± 1.6	1865 ± 257
1%	70/30	21.9 ± 3.5	2655 ± 414
	80/20	14.7 ± 2.1	1932 ± 313
3%	70/30	30.1 ± 1.6	3218 ± 146
	80/20	16.5 ± 1.4	2250 ± 133
5%	70/30	27.2 ± 1.6	3228 ± 134
	80/20	18.6 ± 1.3	2494 ± 263
Standard	MDF board	15.5 ± 2.3	590 ± 86
	Gypsum tiles	7.1 ± 1.4	470 ± 74

The composites were made using two different (70 and 80%) reinforcement and with MAPP content of 1, 3 or 5% based on weight of the wood chips

Performance of the Composites Under Different Humidities

Enhancement in the flexural and tensile properties of the composites due to the addition of MAPP were more evident at higher humidities (60 and 90%). As seen from Figs. 5 and 6, the strength and modulus of the composites with 3% MAPP increased by 67% when the humidity was increased from 30 to 60%. Further increase in humidity from 60 to 90% resulted in lower strength and also modulus compared to the properties at lower humidities. Woodchips are hydrophilic and absorb moisture at high humidity. Moisture in the wood enables relatively easier movement of the molecules resulting in lower strength and modulus. At 60% humidity, there is optimum moisture for the polymer chains to slide under stress and bear the load. Higher humidities cause free movement of the polymer chains and hence reduce the strength of the composites.

Fig. 5.

Tensile properties of composites, with and without MAPP, prepared by compression molding at 180 °C for 10 min and conditioned at different relative humidities

Fig. 6.

Flexural properties of composites, with and without MAPP, made with wood chips and PP. Composites at bulk density of 0.81 g/cm³ were prepared by compression molding at 180 °C for 10 min and conditioned at different relative humidities

Morphology of the Wood Chips and Composites

Untreated wood chips have a smooth but irregular surface as seen from Fig. 7a. Small fragments of wood are being held together and the fibrous (Fig. 7b) nature of the chips can also be observed. Nearly cylindrical fibers are neatly stacked and influence the strength and structure. The cross-section (Fig. 7c) shows the presence of voids and fewer voids are also observed between the fragments. Voids have diameters between 10 and 20 µm and appear to run parallel to a adjacent layer without any voids. Tight embedding of the chips within the PP matrix can be observed in Fig. 7d. However, the voids in the chips persist in the composites and probably help in improving the thermal and noise insulation. After treating with the emulsion, a uniform and relatively thick layer of deposit can be observed (Fig. 7c) and the emulsion covers most of the voids seen before treatment (Fig. 7d). Due to the filling up of the voids (Fig. 7e) and spaces, the mechanical properties increase but the thermal and noise insulation decrease as will be discussed later. However, morphological analysis of the cross-section of the fractured region shows that not all voids are covered and few with open structure still exist. Apparent pull-out and breaking of the wood chips during tensile test is observed indicating good interaction between the matrix and the reinforcement (Fig. 7f).

Fig. 7.

Morphological images of the surface of untreated wood chips (a); cross-section of untreated wood chips (b, c); surface (d) and cross-section (e) MAPP treated wood chips and (f) is the cross-section of the composite with MAPP after tensile failure showing craters or voids created due to the pull out or removal of wood chips

Interactions Between Matrix and Reinforcement

FTIR studies were done to observe the chemical changes and interactions between PP and wood chips with and without the compatibilizer. As seen from Fig. 8, the FTIR spectra of pure MAPP shows intense large peak at 3537 cm⁻¹ which is due to the presence of OH groups and also contributed by OH stretching. Another peak at 2922 cm⁻¹ is due to the symmetric –CH stretching [16]. These two major peaks decrease considerably in intensity in the composites developed due to the lower amounts of MAPP present. The reduction in the OH peak intensity suggests that hydrogen bonds were generated as ester linkages between the matrix and reinforcement [17]. Other typical peaks observed both in the composites and pure MAPP at 1457 cm⁻¹ and 1376 cm⁻¹ are from the –CH₂ and CH₃ bending vibration of PP, respectively [18]. We did not observe any major shifts in peaks seen in earlier studies after MAPP was used as compatibilizer which has also been observed by other researchers [17].

Fig. 8.

FTIR spectrum of MAPP and wood chip-PP composites containing 0–5% MAPP as compatibilizer

Changes to the Sound Absorption Properties

Composites containing MAPP showed lower noise absorption capabilities compared to those without the compatibilizer. A progressive reduction in noise absorption is seen as the amount of MAPP increases (Fig. 9). However, distinct noise absorption peaks are noticed at 1500 Hz for both the composites with and without the compatibilizer. This peak reduces as the amount of MAPP increases. A broader peak is between 2750 and 3250 Hz for the composites with MAPP and the composites without MAPP show a sharper and narrower peak in the same region (Fig. 8). Additionally, the composites without MAPP had another distinct and sharp peak at 4750 HZ. The overall sound absorption of the compatibilized wood chip composites was lower than the composites without throughout the frequency range which is because of the better compaction and interaction between the reinforcement and matrix. As the MAPP content increases, there is good interaction between matrix and reinforcement which reduces the number of voids and spaces and hence the sound absorption decreases. The maximum absorption coefficient obtained for the woodchip-PP composites was 0.275 which is low compared to composites developed using other agricultural residues as reinforcement. For examples, sheep wool reinforced PP composites had sound absorption co-efficient of up to 0.86 [19]. Similarly, hybrid composites made from sheep wool and chicken feathers also had higher sound absorption of 0.55 [5]. Extent of sound absorption in composites mainly depends on the voids and inherent properties of the matrix and reinforcement. Wood chips have relatively low void content and they are tightly packed in the composites with the matrix (PP) with minimum voids. Hence, there is lower noise absorption. Previous studies have reported that the functional groups in MAPP react well with the hydroxyl groups on wood chips and lead to lower porosity and hence lower sound absorption [19, 20].

Fig. 9.

Sound absorption coefficient of wood chip-PP composites without and with 3% MAPP

Thermal Conductivity of Samples

Addition of MAPP marginally increases the thermal conductivity of the composites (Table 2). The extent of increase in conductivity varied with the amount of MAPP. At 1% MAPP, only a marginal (8%) increase in conductivity was observed which increased to 11% and 12.7% when MAPP was content was increased to 3 and 5%, respectively. Composites used for building applications are suggested to have a thermal conductivity less than 0.1 W/mK. Both the composites with and without MAPP satisfy this requirement. Composites reinforced with different biomasses have shown conductivity ranging from 0.05 to 0.156 W/mK. Previous studies have shown that composites containing wood chips and ABS panel as core had thermal coefficients between 0.0564 and 0.0605 W/mK and that pure wood shaving had conductivity of 0.150 W/mK [21]. In another study, gypsum reinforced with 20% wood chips had thermal conductivity of 0.5 W/mK [22]. Thickness, density, void content and inherent properties of the samples affect the thermal conductivity. The proportion of wood chips, PP and density and thickness can be varied to obtain desired level of conductivity. Hence, the thermal conductivity obtained in this research are acceptable for most practical applications.

Table 2.

Thermal conductivity of the wood chip/PP composites, with and without MAPP

EM %	Thermal conductivity (W/mK)	Thermal resistance (m2 K/W)
0	0.062 ± 0.004	0.111 ± 0.001
1	0.067 ± 0.002	0.124 ± 0.003
3	0.071 ± 0.001	0.139 ± 0.002
5	0.079 ± 0.001	0.142 ± 0.001

Moisture Absorption

Addition of emulsion (MAPP) considerably decreased the water absorption as seen from Fig. 10. There is a progressive decrease in amount of water absorbed with increasing amount of MAPP and immersion time. Composites without any compatibilizer sorb up to 55% of water within 24 h and hence affect the strength and stability. Comparatively, the MAPP containing composites have considerably lower absorption. Even addition of 1% MAPP decreased water sorption to a maximum of 35% compared to 55% without any compatibilizer. Further increase in MAPP content to 3 and 5% decreased the water absorption to 5% even after 24 h of immersion in water. MAPP forms a hydrophobic layer on the hydrophilic woodchips that restricts movement of water. Also, there are fewer voids which decrease the transport of water and hence the overall water sorption decreased.

Fig. 10.

Effect of MAPP on the water absorption of the composites

Comparison of Properties of the Woodchips-PP Composites

The woodchips-PP composites developed in this study, particularly those compatibilized with MAPP, either meet or exceed the tensile and flexural strength and thermal and acoustic resistance compared to most other biocomposites. Although the woodchip composites have slightly higher density than the commercial medium density fiberboards (MDF), the tensile and flexural strength are up to 6 times higher (Table 3). Thermal resistance of the woodchip boards is also better than commercialy MDF whereas sound absorption co-efficients are similar. Woodchip-PP composites also have considerably higher strength compared to gypsum tiles used as false ceilings. Hence, we believe that MAPP compatibilized woodchip-PP composites have the potential to replace the commercial available medium density boards.

Table 3.

Comparison of the properties of biocomposites developed using various biobased resources and to commercially available medium density boards with the composites developed from woodchips in this study with and without using the compatibilizer (MAPP)

Biocomposite	Density (g/cm3)	Tensile strength (MPa)	Flexural strength (MPa)	Thermal conductivity (W/mK)	Sound absorption coefficient	References
Coir/GNS/Gypsum	932–1269	–	0.6–5.6	0.06–0.12	0.1–0.75	[6]
Coir/wool/Gypsum	420–1200	–	1.25–3.78	0.17–0.305	0.15–0.35	[23]
Cork-Gypsum	810–1074	–	1.82–8.12	0.120–0.190	~ 0.05–0.35	[22]
Commercial Gypsum Board	820	~ 9.6	7.16	0.170	0.42–0.56	[7]
GNS/RH/PP Hybrid	565–1080	~ 4–15.6	~ 17–37.6	0.156–0.270	0.11–0.48	[7]
SW/CF/PP	420–780	4–20	5–18	0.059–0.103	0.25–0.55	[23]
Woodwaste/Gypsum	702–1307	1.23–4.1	–	0.2–0.5	0.1–0.65	[21]
Commercial MDF Board	619	1.2–13.1	2.3–15.5	0.3	0.06–0.15	This study
Wood chips/PP	810	1.9–9.2	4.5–23.6	0.062	0.05–0.28	This study
Wood chips/PP/MAPP	810	7.4–16.8	14.7–30.1	0.079	0.02–0.15	This study

GNS groundnut shells; RH rice husk; SW sheep wool; CF chicken feathers

Conclusions

Wood chips reinforced polypropylene have excellent strength, stability to humidity and other properties required for commercial applications. Addition of even 3% MAPP as compatibilizer increased the tensile strength by 144% and flexural strength by 154%. Although thermal and acoustic resistance did not show any major changes, the values obtained were similar or better than that of common MDF boards and gypsum used for ceiling tiles. The wood chip-PP composites had considerably low moisture absorption of less than 5% even after immersing in water for 24 h. Morphological and FTIR analysis showed good interaction between the matrix and reinforcement. Utilizing wood chips considered as waste for developing value added composites will benefit the environment and also provide novel low cost, sustainable and green materials thereby promoting circular economy and green building initiatives.

Funding

No specific funding was received for this research.

Data Availability

Enquiries about data availability should be directed to the authors.

Declarations

Conflict of interest

Authors state that they do not have any conflict of interest.