Electrospun Polybutylene Succinate Coatings for Sustainable Cardboard Packaging: Structure‐Property Relationships

Abstract

Producing thin biopolymer‐coated paper and cardboard is essential for a large range of applications, such as food packaging, as it allows to maintain flexibility while reducing the environmental impact and enhancing barrier properties. This study investigates electrospinning to generate polybutylene succinate (PBS)‐coated cardboards with contrasted structures (i.e., different thicknesses of the individual layers). Four contrasted structures of PBS‐coated cardboards are produced, presenting low polymer thickness remaining on top of the substrate (10 to 25 µm) but differing in the thickness of their characteristic layers (including the impregnated layer). The effect of the electrospinning parameters (deposition time, method (direct vs indirect)) and annealing parameters on polymer‐coated cardboards’ structures are investigated. The annealing pressure only reduced the coated cardboard thickness, while the electrospinning method influenced the overall structure by affecting both the thickness of the impregnated layer and of the remaining layer of cardboard. The mechanical properties tested by tensile test are maintained or enhanced after coating while oxygen barrier properties are largely enhanced for some samples. Decreasing the impregnated layer thickness resulted in a Young's modulus increase (+3 to +23%) and a decrease in stress at break (−40 to −70%), highlighting the impact of polymer‐coated cardboards's structures on their mechanical properties.

The electrospinning technique is used to produce different PBS‐coated cardboard structures, which are characterized and discussed in terms of morphology, oxygen barrier, and mechanical properties.

1. Introduction

As packaging materials, paper and cardboard are limited to a low range of applications due to their poor barrier properties (ex: gas, liquid, grease), especially for food packaging applications. To overcome this issue, emerging materials such as polymer‐coated papers and cardboard are being developed as an interesting alternative^{[ 1} , ^{2 ]} with enhanced overall barrier properties.^{[ 3} , ^{4 ]} Among investigated polymer coatings, bio‐based and biodegradable polymers present the advantages of ensuring the overall biodegradability of these materials.^{[ 5} , ⁶ , ⁷ , ⁸ , ^{9 ]} However, a thick coating of polymer may induce a rise in costs as well as further processability issues, therefore the interest is focused on the application of low coating weights.^{[ 10 ]}

Achieving the sought barrier properties with thin polymer layers requires the use of specific techniques allowing the continuity of thin coating layer. Among these techniques, electrospinning has emerged as a prominent method, capable of producing ultra‐thin, uniform polymer films, from several nanometers to few microns.^{[ 11} , ¹² , ^{13 ]} Electrospinning involves the production of fine fibers from a polymer solution, using an electric field, which allows a precise control over fiber morphology.

Different parameters can impact the production of polymer films using electrospinning, such as the polymer solution properties (viscosity, surface tension, and conductivity) but also the applied voltage and the tip‐to‐collector distance, all known to have an impact on the size and the morphology of the produced fibers as already evidenced in various studies.^{[ 14} , ¹⁵ , ¹⁶ , ¹⁷ , ^{18 ]} Depending on the electrospinning conditions, different fiber morphologies, such as beads, beaded‐fibers, and varying fiber thicknesses, can be obtained through electrospinning, each with distinct effects on the final film's characteristics.^{[ 15} , ¹⁷ , ^{18 ]} To achieve a continuous and homogeneous polymer film, electrospinning must be followed by an annealing process involving the application of low temperature and low or without pressure to induce interfibers coalescence and rearrangement of the nanofibers, aiming to reduce the surface tension.^{[ 19 ]} This technique is particularly advantageous for producing polymer‐coated cardboards with improved mechanical and barrier properties. One main advantage being that when using electrospinning, the solvent does not enter in contact with the cellulosic substrate, therefore it does not compromise the substrate's structure, i.e., inducing swelling or compressing as for solvent casting and thermocompression, respectively.^{[ 20} , ²¹ , ^{22 ]} Also, as electrospinning usually takes place at room temperature, it is adequate for the processing of biopolymers.^{[ 12} , ¹³ , ¹⁸ , ²³ , ^{24 ]}

Among electrospun biopolymers, poly(butylene succinate) (PBS) is widely used for a large range of applications, including the production of membranes and scaffolds.^{[ 11} , ¹⁷ , ²⁵ , ²⁶ , ^{27 ]} Indeed, PBS is a biodegradable crystalline thermoplastic polymer that can be bio‐based, produced by renewable resources.^{[ 28} , ^{29 ]} It possesses the advantages of good thermal stability, high flexibility and great mechanical properties and processability.^{[ 29 ]} While some studies use PBS as biopolymer coating on paper and cardboard,^{[ 30} , ^{31 ]} based on our current knowledge, no study using electrospun PBS as polymer coating has been found.

The influence of production processes on the structure of polymer‐coated papers and cardboards have already been highlighted in a previous study, to produce papers and cardboards coated with poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) using thermocompression.^{[ 32 ]} However, to the best of our knowledge, no study investigates the impact of the electrospinning and annealing conditions on the structure of intrinsic structure of polymer‐coated papers and cardboards and their properties.

In this context, this study aims to produce polymer‐coated cardboards with contrasted structures and thin polymer coating layer, using different electrospinning and annealing conditions. PBS will be used as polymer coating while a blotting paper will be used as cellulosic substrate to favor the impregnation of the polymer coating. First, conditions and parameters of electrospinning and annealing to achieve a continuous and homogeneous polymer layer will be investigated. Then, PBS‐coated cardboards exhibiting contrasted structures will be produced, highlighting the impact of the coating process on the characteristic layer's thicknesses. Finally, the impact of the PBS‐coated cardboards structures on their tensile and barrier properties will be investigated.

2. Experimental Section

2.1. Materials

2.1.1. Cellulose Substrate

Untreated blotting paper was used as a cellulose substrate and was supplied by the Centre Technique du Papier (Grenoble, France), with a basis weight of 256 ± 7 g.m⁻² and a thickness of 472 ± 20 µm. This cellulosic substrate was used to emphasize polymer coating impregnation.

2.1.2. Polymers

Polybutylene succinate (PBS) was used as biopolymer coating, with a density of 1.26 gcm⁻³, and was purchased in the form of pellets under the reference BioPBS (FZ91PM / FZ91PB, Mitsubishi Chemical Corporation, Tokyo, Japan).

2.1.3. Solvent

Pure trifluoroethanol (ref. 8.08259, Sigma–Aldrich, Merck, Darmstadt, Germany) was used as solvent to dissolve PBS pellets before electrospinning.

2.2. Preparation of the Polymer Solutions

Before electrospinning, a PBS solution was prepared by dissolving PBS at concentrations of 3, 7, and 15 wt% in trifluoroethanol. The prepared solutions were hermetically closed, stirred for 1 h at 50 °C, and cooled to room temperature until further use. The biopolymer solutions were characterized in terms of their viscosity, surface tension, and conductivity before electrospinning. Viscosity was determined using a rotational viscometer (ROTAVISC lo‐vi, IKA‐Werke GmbH & Co. KG, Staufen, Germany). Surface tension was determined using the Wilhelmy plate method with an Easy Dyne K20 tensiometer (Krüss GmbH, Hamburg, Germany). Finally, the conductivity was measured using a conductivity meter, Xs Con6 (Hanna Instruments, Woonsocket, RI, USA). The properties of the different PBS solutions are listed in Table  1 .

Table 1.

Properties of the different PBS solutions.

PBS concentration in TFE (%w/w)	Conductivity (µS.cm−1)	Surface tension (mN.m−1)	Viscosity (cP)
15	0.18 a)	24.2 ± 0.1	1035 ± 32
7	0.68 a)	21.8 ± 0.1	149 ± 6
5	0.67 a)	21.5 ± 0.1	83 ± 1
3	0.69 a)	21.0 ± 0.1	47 ± 1

^a) for conductivity measurements, experimental errors are < 0.05%.

2.3. Materials Production by Electrospinning and Annealing

2.3.1. Electrospinning

Before processing, the solutions were stirred for one hour at room temperature to ensure their homogeneity. The solutions were then electrospun using a Fluidnatek LE‐500 apparatus (Bioinicia S.L., Valencia, Spain) under a controlled atmosphere of 27 °C and 30% RH equipped with a single 22G needle. The tip‐to‐collector distance was 21 cm, the applied tension was of 9.5 and −1 kV on the top and bottom, respectively, and the solution's flow rate was set at 2000 µL.h⁻¹.

To produce self‐supported PBS films, fibers produced by electrospinning of PBS solutions were collected on a metallic plate covered by a conductive polyethylene foil to allow better detachment of the fiber mat.

Two strategies were developed to produce the PBS‐coated cardboards:

Direct Electrospinning

Direct collection of electrospun fibers on cardboard (referred to as direct electrospinning). Direct electrospinning was performed on an uncoated cardboard (20 × 8 cm²), which was used as a substrate and placed on the collector. A large uncoated cardboard was necessary to ensure good collection of fibers on top of the substrate. Once the fibers were collected onto the cardboard, four squares (4.5 × 4.5 cm²) were cut at the center of the material and were then annealed. Prior to use, uncoated cardboard was weighed twice, their surface area was assessed, and their total thickness was measured at 20 different locations spread evenly on the surface of the material (see the corresponding paragraphs for more information regarding each technique).

In this method, when further assessing the coated‐cardboards’ structures (4.5 × 4.5 cm²), requiring the thickness and mass of the uncoated cardboard having the same dimension as the coated cardboard, the thickness used was the mean thickness of the 20 measurements made on the uncoated cardboard (20 × 8 cm²). The basis weight of uncoated cardboard (20 × 8 cm²) was assessed and was considered homogeneous. Therefore, the mass of the uncoated cardboard (4.5 × 4.5 cm²) was calculated using the basis weight of the total uncoated cardboard, divided by the surface of the coated cardboard.

Indirect Electrospinning

For indirect electrospinning, the electrospun fibers were first collected on a conductive polyethylene foil. Then, they were cut into 4.7 × 4.7 cm² squares (a little larger than the uncoated cardboard to ensure its full coverage) and deposited on the uncoated cardboards, previously cut at 4.5 × 4.5 cm², at the annealing step.

2.3.2. Annealing

After the electrospinning, a thermal post treatment so called “annealing” was applied to the 4.5 × 4.5 cm² samples during a short duration (30 s). The aim of this thermal treatment is to lead to a continuous polymer film by so‐called interfiber coalescence process, resulting in film with enhanced properties (adhesive, flexibility, barrier, and optical).^{[ 12} , ^{33 ]} This thermal treatment was performed using a 4122 hot‐plate press (Carver Inc., Wabash, IN, USA). To do so, the sample, that is direct PBS‐coated cardboards or indirect PBS‐coated cardboards, was placed between two Teflon foils under an optimal temperature and a desired pressure. The optimal temperature corresponds to the lowest temperature below the polymer melting point, leading to a continuous polymer film by the interfiber coalescence process. The applied pressure ranged from 9.8 to 58.5 bar, as measured using the 3‐inch diameter ram of the hydraulic unit, corresponding to loads of 0.5 to 3 metric tons and pressures of 22.0 to 131.8 bar on the 4.5 × 4.5 cm² samples. These conditions do not lead to sample deterioration as highlighted in previous studies.^{[ 21} , ^{32 ]}

The control cardboards are uncoated cardboards that undergo a cycle of annealing at the same temperature, deposition time, and pressure as their corresponding PBS‐coated cardboards. In the same way, control polymer films, which were self‐supported PBS films, were produced under the same conditions as those for control cardboards.

The samples were stored in desiccators at ambient temperature and protected from direct light before further characterization.

Eight samples were prepared for each set of conditions.

2.3.3. Nomenclature

In this study, the different materials are referred to as “Sj_X”, with X being the type of material, X = {CC,C,F} for the coated cardboard, the control cardboard, and the control polymer film respectively, and j indicating the number of the structure (S): j = {1:10}, each structure having specific process conditions.

2.4. Materials Characterization

2.4.1. Surface Area

The surface areas of the samples, $S$, were determined by image analysis, using Fiji software.^{[ 34 ]} The uncoated cardboards (i.e., which serve as cellulosic substrates for the coated cardboard) and control cardboards were scanned using an office scanner, with a scale integrated into the scan. The image was transformed into 8‐bit (black and white) images, the scale was set, and a manual threshold was applied until the surface of the sample was clearly identified. The surface area was then automatically calculated using Fiji software.

2.4.2. Mass, Basis Weight, and Coating Weight

The mass of the samples, $m$ (g) was determined using a 4‐digit balance (LX 220 A, BALCO, Switzerland). All samples were systematically weighted twice, and the mean value was taken.

The basis weight, $bw$, (g.m⁻²) of the samples was determined as follows (Equation 1):

$$bw=\frac{m}{S}$$ (1)

with $S$, the surface area of the sample (m²) and $m$ its mass (g).

The coating weight, $cw$, (g.m⁻²) of the polymer coating was determined using Equation (2).

$$cw=b{w}_{cc}-b{w}_{uc}$$ (2)

with $b{w}_{cc}$ and $b{w}_{uc}$ being the basis weights of the polymer‐coated cardboard and of the uncoated cardboard, respectively, in g.m⁻².

2.4.3. Total Thickness

For PBS‐coated cardboards, corresponding controls, and uncoated cardboards, the total thickness of the materials was measured at ten points equally distributed on the sample using a handheld micrometer with a resolution of 1 µm (Absolute Digimatic Thickness Gauge, Mitutoyo Corporation, Kawasaki, Japan).

2.4.4. Multilayer Structure: Thickness of Each Layer

The structure of the polymer‐coated cardboards, i.e., the thickness of each layer constituting the material, was determined using a methodology developed in a previous study.^{[ 21 ]}

The characteristic layers (Figure 1 ) and their corresponding thicknesses are as follows:

• The free polymer layer, thickness $l_{fp}$ (i.e., polymer coating layer remaining on top of the cellulosic substrate)

• The impregnated layer, thickness $l_i$ (i.e., the layer composed of polymer that impregnated through the cellulosic substrate)

• The free cardboard layer, thickness $l_{fc}$ (i.e., the cardboard layer which has not been impregnated by the polymer)

Figure 1.

Representation of the characteristic layers on a PBS‐coated cardboard produced from indirect electrospinning.

Eight coated cardboards were produced for each set of conditions, i.e., for each structure. The presented structures represent the mean structures of eight samples.

2.4.5. Optical Properties

Optical properties of the PBS films annealed at different temperatures were determined on 50 × 30 mm specimens by assessing the light absorption for a defined range of wavelengths, from 400 to 800 nm. Measurements were performed using a UV–vis spectrophotometer (VIS3000, Dinko Instruments, Barcelona, Spain).

Transparency (mm⁻¹) of the PBS films were assessed using Equation (3).^{[ 35 ]}

$$\mathrm{Transparency}=\frac{A_{600}}{l}$$ (3)

with $A_{600}$ the absorbance of the sample for a wavelength of 600 nm, and $l$ the thickness of the PBS film (mm). Three replicates were performed for each annealing temperature.

2.4.6. Scanning Electron Microscopy (SEM)

Surface and cross‐sectional images of the PBS‐coated cardboards and polymer films used to assess the optimal annealing temperature were obtained using scanning electron microscopy at an acceleration voltage of 10 kV (S‐4800, Hitachi, Tokyo, Japan). Prior to observation, the samples (≈5 × 5 mm²) were fixed to holders using carbon double‐sided adhesive tape and then sputtered with a gold‐palladium mixture under vacuum for 2 min using a Polaron sputter‐coated (Quarum Technologies, Kent, UK). For cross‐sectional observations, PBS films were cut by cryofracture (films were immersed for ≈30 s in liquid nitrogen and sharply bent).

2.4.7. Tensile Properties

Samples were cut in dog bone shaped specimens with a gauge length and a width of 25 and 5 mm respectively. The samples were conditioned at 23 °C and 0% RH before characterization. Tensile tests were performed using a universal testing machine (AGS‐X 500N, Shimatzu, Kyoto, Japan) at room temperature, and according to the ASTM D638 (Type IV) standard. The samples were tested under monoaxial tension with a crosshead speed of 10 mm·min⁻¹. At least six replicates were performed and five thickness measurement points were taken from the sample.

2.4.8. Oxygen Barrier Properties

Samples were cut into circles with a diameter of 2.52 cm, giving an exposed surface area of 5 cm², and were mounted between two metallic chambers using an Oxygen Permeation Analyzer M8001 (Systech Illinois, Tham, UK). Grease was used to ensure tightness between the two metallic chambers, and a rubber mask was screwed to the sample to tighten the sample and ensure no leakage. For coated cardboards, the uncoated side was oriented towards the oxygen flux, as recommended,^{[ 36 ]} to avoid edge effects. First, a nitrogen purge was performed in the cell to ensure the absence of residual oxygen. Subsequently, an oxygen flow of 10 mL.min⁻¹ was applied to the lower chamber. Measurements were performed at 23 °C and 50% RH and were stopped when the oxygen transmission rate (OTR) reached a constant value.

2.5. Statistical Analysis

Analysis of variance (ANOVA) was conducted using XLSTAT (Lumivero, Denver, USA). If significant differences were found, comparisons were made using the Tukey's Multiple Comparison Test (p ≤ 0.05) with confidence of 95%.

3. Results and Discussion

3.1. Electrospinning of PBS

3.1.1. Obtention of Different PBS Morphologies

Electrospinning was used to produce polymer‐coated cardboard with contrasted structures presenting low‐to‐no impregnation of the polymer, and with thinner free polymer coating, unreachable using thermocompression only.^{[ 32 ]} Before producing polymer‐coated cardboards based on untreated blotting paper and PBS, investigation regarding the obtention of different PBS morphologies, as well as the choice of optimal annealing parameters were carried out.

As different polymer morphologies (fibers, beads, or a mixture between fibers and beads) can be obtained using electrospinning,^{[ 18 ]} trials were performed to define the optimal parameters for the obtention of all three different morphologies of PBS (see part 1, Supporting Information).

To do so, different parameters were used to obtain those morphologies: PBS concentration in solution ([PBS]), solution's flow rate, tip‐to‐collector distance, and applied voltage (ΔV). The temperature and relative humidity were set at 27 °C and 30% RH respectively, and the electrospinning deposition time was set to 20 min. A limitation to the use of all PBS morphologies as coating for cardboards is the deposition time required to obtain a homogeneous mat of PBS when using a single emitter. Indeed, when PBS concentrations are low, it can take up to a few days to obtain a thick PBS film when a basic electrospinning lab tool is utilized. For this reason, in this seminal study only fibers obtained using 15%w/w of PBS were used for further coating on cardboard and characterization. Electrospinning parameters used are presented in Table 2 .

Table 2.

Parameters used for the obtention of PBS fibers, at 27 °C and 30%RH.

Parameters	Units	Values
[PBS]	%w/w	15
solution's flow rate	µL.h−1	2000
tip‐to‐collector distance	cm	21
ΔV	kV	10.5

3.1.2. Optimal Annealing Temperature

To produce polymer‐coated cardboards using electrospinning, once PBS films are obtained, either collected from electrospinning on a conductive polyethylene foil (indirect) or directly on the cardboard (direct), the annealing process is necessary to obtain a continuous polymer film (i.e., reducing and eliminating the interfiber porosity). It is therefore necessary to assess the optimal annealing temperatures, i.e., the lowest temperatures required to obtain by interfiber coalescence. It corresponds to the temperature for which continuous and homogeneous polymer films (for the PBS control films) and coating layers (for the PBS‐coated cardboards), without porosity, are obtained.

To assess the optimal annealing temperature, temperatures ranging from 100 to 140 °C were tested (below and above the melting temperature of PBS at 115 °C), with an annealing time of 30 s at the minimum pressure. These temperatures were tested both on indirectly‐electrospun PBS‐coated cardboards, and on the PBS fibers mats to ensure the relevance of the annealing temperature. Indirect deposition of the electrospun fiber was chosen over direct deposition as it represents the “worst case scenario” for adhesion. Indeed, electrospun fibers directly deposited on the cardboard already present better adherence to the substrate. Therefore, it is assumed that the temperature chosen for the PBS‐coated cardboards produced with indirect electrospinning is the most representative. An overview of resulting images is shown in Figure 2 while the complete figure with all temperatures is presented in Figure S3 (Supporting Information).

Figure 2.

Aspect of PBS‐coated cardboard (PBS_CC) by A) photography and B) SEM, and self‐supported PBS films (surface by C) photography and D) SEM, and E) cross‐section by SEM) depending on the applied annealing temperature. Only annealing temperatures of 105, 120, 130, and 140 °C are presented here, all temperatures are presented in Figure S3 (Supporting Information). Squared holes visible at the center of self‐supported PBS film photos, from 100 to 125 °C, represent areas of the sample used for the SEM analyses. Photos of PBS films and PBS_CC are taken on 4.5 × 4.5 cm samples. Annealing was performed at low pressure for 30 s. Raw images are available in the dataset corresponding to this article: https://doi.org/10.57745/MGVODT.

PBS‐Coated Cardboards

First, different annealing temperatures were applied to PBS‐coated cardboards produced by indirect deposition of electrospun fibers followed by annealing. As shown in Figure S3, for PBS‐coated cardboards annealed at 100 °C, there was a lack of adhesion, visible to the naked eye, between the polymer film and the cardboard resulting in easy delamination of the two layers. The surface of the PBS‐coated cardboard seemed continuous and homogeneous by annealing at 105 °C. Annealing at higher temperatures (from 110 °C to 125 °C) resulted in the presence of holes. Holes appearing in the latter annealing temperature range could arise from reduced polymer viscosity in the vicinity of melting and its penetration into the cardboard. Indeed, the polymer impregnation within the cardboard's porosity leads to a lower quantity of PBS at its surface and therefore may result in the formation of a discontinuous polymer layer. This explanation is congruent with observation made from the self‐supported polymer film for the same range of temperature. When melting, the apparent volume occupied by the polymer decreases and thus leads to the formation of holes in the structure until reaching a homogeneous film where the apparent volume equals the polymer volume. For annealing temperatures above the polymer melting point, observation of defects on the surface of the samples suggests the degradation of the polymer, also observed for these temperatures on the self‐supported polymer film. It is worth noting that for an annealing temperature of 130 °C, the material seems to be more homogeneous than for lower and higher temperatures. However, this sample still presents some surface defects or cavities.

Therefore, from observation of the generated surface morphology of the PBS‐coated cardboards, the optimal minimal annealing temperature required to achieve the most homogeneous and continuous layer was selected at 105 °C.

PBS Control Films

Regarding the PBS control films, annealing between 125 and 130 °C resulted in the most homogeneous self‐supported PBS films. Indeed, increasing further the annealing temperature resulted in an apparent degradation of the polymer, highlighted by the presence of defects on the surface of the sample, visible on SEM images. When applying temperatures lower than 130 °C, the PBS films were not fully homogeneous due to the presence of some remaining fiber porosity (for 100 and 105 °C) and holes (for 110 and 115 °C). Figure 3 shows the evolution of the transparency of the self‐standing fiber mats with increasing annealing temperature. From this figure, a reduction of interfiber porosity resulting in high light transmission which occurred above 110 ^°C, is highlighted.

Figure 3.

Evolution of the transparency of the PBS films as a function of the annealing temperature. Three replicates were performed for each annealing temperature.

It is worth noting that the sample exhibiting the highest value of light transmission was for the sample annealed at 130 °C. This confirms the observation made from SEM images suggesting that the best homogeneity of the sample was reached at an annealing temperature of 130 °C.

By combining the observations of the PBS‐coated cardboards and of the self‐supported PBS films, it seems that two different annealing temperatures were required to yield the optimal continuous multilayer and monolayer morphologies, i.e., 105 and 130 °C respectively. Indeed, as shown, the temperature must be sufficient for fiber coalescence but prevent any defects and pinholes from appearing on the surface of the sample.

The reason for the lower annealing temperature required when coating paperboard is not yet fully deciphered. It could be due to a higher flexibility of the board as a fiber‐based substrate that facilitated the interfiber coalescence process of the PBS. In view of the above results, it was decided that further characterization of the materials would be better done at those two annealing different temperatures.

3.2. Production of PBS‐Coated Cardboards with Contrasted Structures

3.2.1. Investigated Parameters

Considering parameters as fixed for the obtention of PBS fibers (part 3.1.), other parameters were studied to obtain PBS‐coated cardboards with contrasted structures (Table 3 ).

Table 3.

Electrospinning and annealing parameters used to produce PBS‐coated cardboards.

Structure	Electrospinning		Annealing
	Direct or indirect	Deposition time [min]	T [°C]	Duration [s]	Pressure
S1	Direct	45	130	30	Low
S2	Indirect	45	130	30	Low
S3	Direct	90	130	30	Low
S4	Indirect	90	130	30	Low
S5	Direct	45	130	30	High
S6	Indirect	45	130	30	High
S7	Direct	90	130	30	High
S8	Indirect	90	130	30	High
S9	Indirect	90	105	30	Low
S10	Indirect	90	105	30	High

First, two methods for the collection of the electrospun PBS fibers were investigated, concretely direct and indirect electrospinning.

Then, it is worth noting that the thickness of PBS deposited on the conductive polyethylene foil (indirect electrospinning) or directly on the cardboard (direct electrospinning) is determined by the deposition time. Indeed, a greater deposition time, for the same solution's flow rate and concentration of PBS solution, will lead to a greater amount of polymer electrospun, and therefore collected.

Two annealing pressures were also investigated, the lower pressure (Plow) corresponds to the minimum pressure detectable by the apparatus, i.e., 9.8 bar, while the maximum pressure applied was 58.5 bar.

Finally, both optimal annealing temperatures determined earlier were also used.

3.2.2. Obtained Structures of PBS‐Coated Cardboards

After being produced according to the selected electrospinning and annealing parameters, the structure of the resulting polymer‐coated cardboards was assessed. Mean structures, obtained from eight replicates for each structure, are presented in Figure 4 .

Figure 4.

Mean structure of polymer‐coated cardboards as a function of their process parameters, obtained from eigth replicates. The structure is composed of the three characteristics layers: the free polymer, the impregnated, and the free cardboard layer.

For structures to be significantly different, they must possess characteristic layer thicknesses significantly different from the other structures. From the comparison of each of the layer's thicknesses, and the statistical analysis (see, Figure S4, Supporting Information), it is possible to evidence four contrasted structures, i.e., differing significantly by at least one layer thickness, and all characterized by a thin free polymer layer:

• contrasted structure 1 {PBS_S1, S3, and S9_CC} characterized by a medium impregnated layer and a thick free cardboard layer.

• contrasted structure 2 {PBS_S2 and S4_CC} characterized by a thin impregnated layer and a thick free cardboard layer.

• contrasted structure 3 {PBS_S5 and S7_CC} characterized by thin free polymer and impregnated layer and a thin free cardboard layer.

• contrasted structures 4 {PBS_S6, S8 and S10_CC} characterized by a medium impregnated layer and a thin free cardboard layer.

Free Polymer Layer Thickness ($l_{fp}$)

First, it is possible to note that for all structures, thin free polymer layer thicknesses were obtained and seemingly neither of the investigated parameters had an influence on the free polymer layer thickness. Indeed, no change in $l_{fp}$ was noted between the PBS‐coated‐cardboards structures. Surprisingly, even if the thickness of coated PBS is thicker (i.e., ≈25 µm compared to ≈10 µm for thinner PBS films), the impact on the coated‐cardboard structures remains negligible. This can be explained by the variability in the measurement of the total thickness of coated‐cardboards, which is later used for the calculation of $l_{fp}$, for which the standard deviation from the measurement was in the same range as the thickness of the PBS film (i.e., from 10 to 30 µm). The variability of the thickness measurement was also evidenced for the uncoated cardboard, inferring that the nature and intrinsic inhomogeneity of the cellulosic substrate is the cause of such variability. It is worth keeping in mind that high standard deviations are noted when assessing the characteristics layers thicknesses, which can therefore prevent further conclusions to be drawn as regard of the impact of the electrospinning parameters on the PBS‐coated cardboards structures.

Impregnated Layer Thickness ($l_i$)

Then, different impregnation thicknesses were observed depending on the nature of the electrospinning (direct or indirect). Interestingly, when the lowest annealing pressure combined with the direct electrospinning was applied, the impregnated layer was thicker, whereas the contrary was observed for the highest annealing pressure. On the other hand, for indirect electrospinning, the higher impregnation occurred at the highest pressure. These apparent counterintuitive observations could be induced by the behavior of the fiber‐based board under the different pressure conditions applied. These latter depend on how the coating was carried out and add a significant level of complexity to this particular type of multilayers.

Free Cardboard Layer Thickness ($l_{fc}$)

Finally, regarding the free cardboard thickness, $l_{fc}$, a decrease in its thickness was noted when increasing the annealing pressure, leading to an overall decrease in total thickness of the PBS‐coated cardboard. This is easily explained by the compression of the free cardboard layer when applying greater pressure. Indeed, for uncoated cardboards, which are porous, with higher pressure a decrease in their thickness and in their global porosity was noted compared to uncoated cardboards annealed at lower pressures. Aside from the annealing pressure, an increase in free cardboard layer was noted as both thicknesses are related. It is congruent to what was observed for the impregnated layer thickness, i.e., a decrease for indirect electrospinning compared to direct electrospinning,

3.3. Impact of PBS Coated Cardboards Structures on their Physical Properties

3.3.1. Mechanical Properties

As no specific standard has been developed to date regarding the characterization of polymer‐coated papers and cardboards’ tensile properties, in the literature both standards used for papers and cardboards (ISO 1924, TAPPI T489 and 494, and ASTM D828)^{[ 37} , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ^{46 ]} or for polymer films (ASTM D638 and D882)^{[ 47} , ⁴⁸ , ^{49 ]} were used. In our study, only one standard (ASTM D638), usually used for polymer films, was used to be able to compare values between each other. However, it is worth noting that the measurement of tensile properties on these systems is complex and must be carefully considered. Thus, tensile properties (Young's modulus, stress at break, and strain at break) of PBS‐coated cardboards, and corresponding control cardboards and polymer films were assessed and compared in Figure S5.

Impact of the Polymer Coating

First, as compared to control cardboards, all considered tensile properties were enhanced or at least maintained when coating a polymer onto the cardboards (Figure S5), congruently with observations from the literature.^{[ 37} , ⁴¹ , ⁴² , ⁵⁰ , ^{51 ]} This could be related to the low rigidity of PBS films (Young's modulus twice as low as for the cardboard), in the same range of Young's modulus (≈1000 MPa) than for PBS films produced either using thermocompression^{[ 52 ]} or extrusion.^{[ 53 ]} However, it is worth noting that the impact of the polymer coating on the tensile properties is also highly dependent on the coating weight applied^{[ 37} , ⁵⁴ , ^{55 ]} and on its nature.^{[ 41} , ⁴² , ⁵⁰ , ^{56 ]} The stress at break was increased, here by a factor 1.3, except for PBS_S4. The increase in stress at break is only evidenced for an annealing temperature of 130 °C (PBS_S1 to S8_CC), as for 105 °C (PBS_S9 and S10_CC) no significant impact of the coating on the stress at break is highlighted. One explanation may be related to the structure of the free PBS layer on PBS‐coated cardboards. Indeed, as highlighted in Figure 2, when annealed at 105 °C, PBS films showed a porous structure which could lead to an uneven distribution of stress during the tensile test combined to a lesser compact structure. These characteristics would therefore lead to lower resistance to mechanical stress compared to the PBS layer of PBS_CC annealed at 130 °C for which continuous polymer films were obtained. Regarding the strain at break, a slight, but not significant, increase was observed when coating PBS onto cardboards.

Impact of the Individual Layer's Thicknesses

As the complex mechanical structure of biopolymer‐coated papers and cardboards is to the best of our knowledge not documented in the literature, no results on the impact on the individual layer's thicknesses were found to be compared to our own results.

Regarding Young's modulus, an increase was observed (from +3 to +23%) when the free cardboard layer thickness ($l_{fc}$) increased, corresponding also to a decrease in impregnated layer thickness ($l_i$) (Figure 5 ). Such results can be expected as when the free cardboard thickness increases its contribution to the Young's modulus value increases too, and as the Young's modulus of control cardboards are lower than those of the control polymer films, it results in a decrease in the overall Young's modulus.

Figure 5.

Tensile properties of PBS‐coated cardboards as a function of their structure. a) Young's modulus, b) Stress at break, and c) Strain at break. Letters subscripts represent significative differences highlighted by the statistical analysis, i.e., data having the same letter subscripts are not significantly different. Six replicates were performed for each structure.

For the stress at break, a significative decrease of stress at break when increasing the free cardboard layer thickness ($l_{fc}$) was observed for all PBS‐coated cardboards, with factors ranging from 1.4 to 1.7. The explanation proposed for Young's modulus, i.e., an increase in contribution of the free cardboard layer, can also explain this tendency.

Finally, as regards to the strain at break, all values for PBS‐coated cardboards did not significantly differ depending on the structure of the coated cardboard even if slightly highest values could be obtained when $l_{fc}$ was greater. However, the strain at break of PBS control films was significantly higher than that of the control cardboards (Figure S5). Therefore, it is possible that the contribution of the increase in free cardboard layer thickness is not noticeable as the strain at break might be brought to the coated cardboard by the polymer at a larger scale than by the cardboard.

Impact of Other Parameters

From Figure 5, while the thickness of the free polymer layer seems not to have any impact on the tensile properties of the coated cardboards (PBS_S1_CC vs PBS_S3_CC), the method of electrospinning (direct or indirect) seems to have an impact on the properties. Indeed, PBS_S1_CC and PBS_S2_CC were compared to assess the impact of the electrospinning method on the properties. When proceeding to an indirect electrospinning of the fibers, it resulted in a higher value of Young's modulus (2161 ± 199 and 2805 ± 338 MPa, for PBS_S1_CC and PBS_S2_CC respectively). The impact of the coating process on the mechanical properties of coated paper and cardboard was already highlighted in the literature. Indeed, in their study Kumar et al. evidenced better mechanical properties for cardboards coated with cellulose nanofibers, produced by roll‐to‐roll coating than for those produced by batch processes (size press, bar coating, and dip coating).^{[ 57 ]} But such impact of the process on the tensile properties is not always highlighted, as for example Lavoine et al. did not evidence significant impact in mechanical properties using size press or bar coating for microfibrillated cellulose coated cardboards.^{[ 58 ]}

Finally, a slight decrease in Young's modulus and in stress at break was noted when decreasing the annealing temperature, from 130 °C (PBS_S2_CC) to 105 °C (PBS_S9_CC) with the other parameters remaining unchanged. This may be explained by the low annealing temperature used for PBS_S9_CC for which the fibers remained unmelt and more amorphous than molten and recrystallized fibers. This was already observed in the literature for electrospun PHBV for which the crystallinity of the fibers and of the low temperature annealed fibers formed into films were similar.^{[ 59 ]}

3.3.2. Oxygen Barrier Properties

For the oxygen barrier properties characterization, only four PBS_CC structures allowed to obtain measurable permeation properties without providing overrange values (i.e., outside the measuring range of the apparatus). These four PBS‐coated cardboards structures (S1, S2, S3, and S9) were all produced using the lower annealing pressures, suggesting that a higher annealing pressure might have led to morphologies with more pinholes or defects.

Oxygen transmission rate (OTR) measurements were assessed for all PBS‐coated cardboards and controls (Figure 6 ). OTR values of control cardboards provided overrange values (> 432 000 cm³ m⁻² day⁻¹) as expected.

Figure 6.

OTR results for the coated cardboards (OTR_CC) (●), the control films (OTR_F) (■), and the control carboards (OTR_C, obtained from a previous study) (▲), as a function of the polymer‐coated cardboard's structure. When two replicate values are significantly different, both are represented. The characterized control films were the same for all PBS_CC.

It is also worth noting that for the samples that provided measurable barrier data the repeatability was low, both for OTR values of PBS_F and PBS_CC. Indeed, some specimens tested also overrange, suggesting that at least lab samples of such multilayer materials based on highly rough substrates such as those of very thick fiber‐based boards remain a challenge. Moreover, the mass transfer of oxygen seemed to be directly affected by the free polymer layer, therefore when its thickness was too low, the coverage of the fibers was not continuous, hence leading to uneven barrier properties and would therefore explain the low number of measurable OTR values for PBS_CC and their low reproducibility. Such impact of a discontinuous polymer coating layer on the barrier properties has already been highlighted in the literature.^{[ 60 ]}

Effect of Polymer Coating

Even if the coating layers produced at the lab scale were not fully continuous with imperfect coverage of the board substrate, it was possible to achieve an improvement in barrier properties (i.e., a decrease in OTR value) for all the characterized PBS_CC. Indeed, OTR values for uncoated and control cardboards were above 4.3 $\times$ 10⁵ cm³.m⁻².day⁻¹ while those of PBS_CC were ranging from 70 to 2 $\times$ 10³ cm³.m⁻² .day⁻¹. Such improvements are within the range of OTR values reported in the literature for other similar systems.^{[ 41} , ⁵⁵ , ⁶¹ , ⁶² , ^{63 ]}

Effect of the Individual Layer's Thicknesses

The effect of each of the individual layers on oxygen barrier properties of PBS‐coated cardboards was investigated. While presenting low free polymer thickness, the OTR values of PBS_CC were in the same range as the control films, suggesting that barrier properties towards oxygen of the polymer were not significantly deteriorated after coating on a cellulosic substrate. The impact of the free polymer layer on the barrier properties has not been discussed as all PBS‐coated cardboards present similar thickness of free polymer layer. It is important to note that, compared to similar systems, produced materials exhibit good oxygen barrier properties relative to the amount of polymer coating applied. Indeed, Figure 7 shows OTR values reported in the literature for polymer‐coated cardboard as a function of coating thickness.

Figure 7.

Comparison of OTR results obtained in this study to values obtained in different articles from the literature.^{[ 56} , ⁶⁴ , ⁶⁵ , ^{66 ]} If necessary, OTR values were converted to have a uniform unit between values. Error bars are represented for both axes (OTR and coating thickness), if mentioned in the corresponding article, but they may not be visible due to the scale used in this figure.

For our study as well as for Hult et al.,^{[ 56 ]} Bedane et al.^{[ 64 ]} and Poulose et al.,^{[ 66 ]} the free polymer thickness is considered whereas for Tyagi et al.,^{[ 65 ]} the coating thickness considered is the thickness of the polymer film deposited onto the cardboard, without considering any impregnation. Therefore, it is possible to assume that the coating thickness of materials obtained by Tyagi et al.^{[ 65 ]} is lower due to the polymer impregnation within the substrate's porosity. As mentioned in a previous study, from the information available in articles, it is sometimes not possible to assess the free polymer coating thickness.^{[ 67 ]}

The best oxygen barrier performance was achieved by Bedane et al. ^{[ 64 ]} at high coating thicknesses (>25 µm), whereas Hult et al.^{[ 56 ]} reported the lowest performance (i.e., highest OTR) for very thin coatings. In this context, produced electrospun PBS‐ coated cardboards seem to offer a good balance between coating thickness (ranging from 2 to 20 µm) and oxygen barrier performance. Nevertheless, ensuring measurement reproducibility and the absence of defects is crucial to confirm this observation.

Effect of the Electrospinning and Annealing Parameters

The impact of the electrospinning and annealing parameters were difficult to evidence due to the lack of reproducibility of the measurements, therefore impeding any other conclusions to be drawn on the subject.

4. Conclusion

This study provided significant new insights regarding the impact of applying low free biopolymer layer thickness on paperboard. Thus, applying a thin layer of annealed electrospun PBS onto the cellulosic substrate studied in the conditions carried out in this study did not allow to provide a fully coherent material with good reproducibility in terms of oxygen barrier properties.

To be able to obtain homogeneous and continuous PBS films and coating, preliminary studies were performed, which led to the obtention of three PBS morphologies: fibers, beaded fibers, and beads. However, to produce PBS‐coated cardboards, only fibers were used due to the too long deposition time necessary to obtain beads or beaded fibers mixtures at the lab scale with a single emitter. Then, an investigation of the optimal annealing temperature (i.e., the lowest annealing temperature allowing a homogeneous and continuous polymer layer/film) evidenced that depending on the materials, the optimal annealing temperature could vary. Indeed, the optimal annealing temperature for the PBS‐coated cardboards (105 °C) did not allow the obtention of continuous and homogeneous self‐supported PBS films. Therefore, two annealing temperatures were used throughout the study.

Four contrasted structures of PBS‐coated cardboards, varying in free cardboard, impregnated, and total thicknesses, were obtained from electrospinning combined with annealing. Various parameters were used and changed for the obtention of these structures: electrospinning (deposition time, nature of the electrospinning: direct/indirect) and annealing parameters (temperature, pressure). All the produced coated cardboards exhibited a thin polymer coating layer, but no significant difference was noted depending on the electrospinning time. Two main parameters impacting the structure of PBS‐coated cardboards were 1) the annealing pressure, decreasing significantly the free cardboard thickness due to the compression, and 2) the method of electrospinning, i.e., whether PBS‐fibers were directly collected on the cardboard (direct) or first collected on a conductive polyethylene foil and then deposited and coated onto the cardboard (indirect).

Finally, the impact of the multilayer structure and of the process on the tensile properties were assessed. An increase in Young's modulus and stress at break was noted when increasing the free cardboard layer, as observed for PHBV_CC. Regarding the process, only the method of electrospinning (direct or indirect) and the annealing temperature were evidenced as having an impact on the Young's modulus. Indeed, an indirect electrospinning seems to increase the Young's modulus, while a decrease in annealing temperature slightly decreased the Young's modulus.

Further investigation into the relationship between electrospun morphology, scalability, deposition thickness, and post‐processing parameters and methods in the production of cardboard multilayers with bio‐based polymers is still needed. This understanding is essential to develop more homogeneous and enhanced barrier materials required by the packaging industry, which is the one of the focus of ongoing studies in our research groups.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Vercasson A., Prieto C., Gaucel S., Gontard N., Guillard V., Angellier‐Coussy H., Lagaron J. M., Electrospun Polybutylene Succinate Coatings for Sustainable Cardboard Packaging: Structure‐Property Relationships. Macromol. Rapid Commun. 2025, 46, 2500048. 10.1002/marc.202500048

Data Availability Statement

Data used in this manuscript are available in the dataset corresponding to this article: https://doi.org/10.57745/MGVODT.