Abstract
Utilization of cellulose nanocrystals as an additive in the formulation of biocomposites made with Acrodur® resin is presented. Natural fibers/polyethylene terephthalate mats were impregnated with Acrodur® and hot-pressed into the final thickness of 3 mm after drying. Biocomposites with 2 wt.% and 5 wt.% cellulose nanocrystal (dry-basis) were also produced. The produced biocomposite panels were then tested to determine the flexural strength, flexural modulus and Izod impact strength. The results revealed that adding cellulose nanocrystal to the composite formulation increased flexural modulus significantly up to 970 MPa (17.5% increase) at a panel density of 0.5 g/cm3, while it did not significantly affect flexural strength values. A slight reduction was observed in the impact strength of the samples by adding cellulose nanocrystal. The fractured samples of impact test were observed under a scanning electron microscope. It was shown that in all cases, the fracture happened due to the failure of the fibrous system and in particular natural fibers. Thermal stability of the composites was also investigated using thermo-gravimetric analysis. It was found that adding cellulose nanocrystal slightly reduced the thermal stability of the biocomposites. Potential compatibility of cellulose nanocrystal particles with Acrodur® resin is promising and the improvement in flexural modulus can lead to the design of lighter parts for automotive applications such as door panels, headliners, and underbody shields.
Introduction
The term biocomposite is commonly used to describe fiber-reinforced polymer composites that are partly or completely derived from renewable resources. Biocomposites have provided beneficial promises to the environment being used as green replacements for petroleum-based products. In comparison to petroleum-based composites, biobased composites offer better recyclability, more efficient energy recovery during incineration, lower carbon footprints, and in many cases better specific mechanical properties.1–5
Thermosetting resins are commonly used for the production of natural fiber-reinforced composites due to their decent compatibility with hydrophilic surfaces of natural fibers. One of the major concerns associated with conventional thermosetting matrices widely utilized for biocomposites is the emission of possible carcinogens such as styrene in polyester and vinyl ester resins, and formaldehyde in phenol- and urea-formaldehydes, not only during production processes but also from the finished products. 3 Therefore scientists worldwide are actively seeking alternatives to common thermosetting resins. Acrodur® is an acrylic-based resin first introduced by BASF as a formaldehyde-free binder for ecofriendly binding applications. The ability of Acrodur® to provide adequate adhesion to natural fibers without any surface treatments is highly beneficial for the processing cost reduction. 2 Acrodur® is basically a polyester of polycarboxylic acid and polyalcohol in an aqueous binder system with no styrene emission where the sole byproduct upon curing is water. Acrodur® exhibits thermoplastic properties below its curing temperature, which makes it an ideal candidate for semi-finished products and facilitates further assembly and processing. An esterification reaction is believed to occur between the acid groups of Acrodur® and hydroxyl groups of cellulosic fibers upon curing in presence of moisture 2 giving the composite strength and stiffness.
Liang et al. 5 compared the mechanical and physical properties of three different biocomposites, including kenaf fiber/soy protein, kenaf fiber/Acrodur® DS 3530, and kenaf fiber/soy protein/Acrodur® DS 3530. They found that the kenaf fiber/Acrodur® DS 3530 composites had the highest flexural modulus and flexural strength of all. The hybrid of kenaf fiber/soy protein/Acrodur® DS 3530 was ranked second and the kenaf fiber/soy protein composites showed the lowest modulus of elasticity (MOE) and modulus of rupture (MOR). It was also revealed that the pressing time had a significant influence over the flexural properties and densities of the produced biocomposites. Acrodur resin showed to reduce the press cycle substantially, which is very favorable to the industry. Using Acrodur® resin as a binder also brought about a considerable improvement in the sorptive properties of the produced composites by reducing their thickness swelling and water absorption. In another study, the optimization of some processing variables such as mixing ratio, relative humidity, curing time, and temperature was done for the production of flax fiber-reinforced Acrodur biocomosites. 3 The optimized composite showed to have relatively high tensile modulus and strength, thermal stability, and hydrophobicity at a favorably low density. Khalfallah et al. 2 worked on the development and characterization of Acrodur® biocomposites reinforced by flax tapes. The flax tapes consisted of long flax fibers unidirectionally arranged without any twist. They investigated the relationship among the main processing parameters including drying, fiber volume fraction, densification, and curing time and temperature. It was found that the optimal biocomposite could be obtained with 55 wt.% fibers at a density of 0.93 g/cm3 by drying the prepregs at 100℃ for 20 min under vacuum and curing the resin at 170℃ for 3 min. In another study, a statistical response surface methodology (RSM) was used to find an optimal processing condition (moisture content, curing temperature, and time) to obtain the maximum flexural modulus and strength values for nonwoven flax fiber reinforced Acrodur® biocomposites. 4 Salim et al. 6 studied the influence of different fiber treatments on the interfacial and mechanical properties of kenaf fiber reinforced Acrodur® composites. They used alkali treatments at room temperature and at 60℃ along with a heat treatment at 140℃ for 10 h applied to kenaf fibers. It was concluded that alkali-treated kenaf fibers had smaller diameters but higher density. Alkali-treated fibers also showed to have better wettability by the Acrodur® resin that imparted better flexural behavior and dynamic mechanical properties to the composite. However, alkali treatments negatively impacted the composite's fracture toughness.
Cellulose nanomaterials mostly derived from forest resources have attracted increasing interest owing to their outstanding characteristics such as biodegradability, high stiffness and strength, light weight, high surface area, and the ability of forming hydrogen bonds with most of biobased and petroleum-based polymers. 7 The two main forms of cellulose nanomaterials are cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC). While CNFs are mainly produced by mechanical disintegration of pulp fibers, the most common method to produce CNCs is acid hydrolysis. 8 Apart from the aforementioned properties, the capability of cellulose nanomaterials to build self- or direct-assembled hybrid structures has made them an excellent candidate for a wide range of applications, as binders, reinforcing agents, and filaments for instance.9–26 Incorporation of CNC as a reinforcing agent only at a few percent (typically 1–5%) loading in resin matrices has shown to improve the mechanical properties of the resultant composites mainly due to the formation of percolating networks and strong interaction between CNC and matrix.14,18–26
The aim of this study was to evaluate the contribution of adding CNC to the formulation of Acrodur® resin used to produce natural-fiber based biocomposites. A full array of experiments were carried out on individual components of the composites and results were analyzed in light of changes in the properties.
Materials and methods
Materials
Nonwoven mats of 70% natural fibers/30% polyethylene terephthalate (PET) and weight of 1000 g/m2 (gsm) were provided by Carver Non-woven Technologies, LLC (Fermont, Indiana, USA). Acrodur® DS 3515 suspension (60 wt.% solids) and Lutensol® TDA 10 were supplied by BASF Chemical Company (Ludwigshafen, Germany). Acrodur® DS 3515 is an aqueous dispersion composed of a styrene-acrylic polymer modified with a poly-carboxylic acid and a polyol cross-linker. 27 Lutensol TDA 10 is an active nonionic surfactant made up of a 10-mole ethylene oxide adduct of tridecyl alcohol. 28 The CNC was received in the form of a slurry of 12 wt.% CNC with no additional surface functionalization. The CNC was a product of the Forest Products Laboratory (Madison, Wisconsin, USA) distributed by the Process Development Center of the University of Maine and was the product of sulfuric acid hydrolysis of softwood dissolving pulp.
Composite production
Different formulations of resins used for biocomposite samples production.
CNC: cellulose nanocrystals.
Nonwoven mats were cut into 100 × 100 mm2 square pieces and impregnated by soaking in baths of different mixtures for 2 min to ensure that the mixture completely penetrated into the mat. Semi-finished products (prepregs) were then dried in an oven at 75℃ overnight. After drying process, prepregs were pressed into the final thickness of 3 mm (panel target density of 0.5 g/cm3) at 200℃ for 2 min using a hydraulic hot press (Carver, Inc., Wabash, IN, USA). Before pressing, two metal stops (thicknesses of 3 mm) were placed between the two press platens and on both sides and parallel to the opposite edges of prepregs to provide position control during pressing process. After pressing, composite panels were cut and trimmed into the desired shapes and sizes, based on the testing standards for further steps. Composite samples with 2 wt.% and 5 wt.% CNC (dry-basis) were produced. Control samples were prepared in the same way with no CNC added.
Flexural test
To investigate the flexural properties of the produced biocomposites, three-point bending tests were carried out on the specimens according to ASTM D 790-03 using an Instron 5966 universal testing machine (Instron, Norwood, MA, USA) with a 10 kN load cell capacity. The nominal dimensions of each specimen were 100 mm by 12.5 mm with the average thickness of 3 mm. The span length and cross-head speed were 40 mm and 3 mm/min, respectively. Five replicates of each biocomposite formulation were tested and all specimens were conditioned to ∼23℃ and 50% relative humidity for at least 72 h prior to testing. MOE and MOR were calculated form the flexural stress–strain curves. In addition, toughness values were calculated by integrating the areas under force–displacement curves.
Izod impact test
Izod impact tests were conducted to measure the impact resistance of the biocomposites using a CEAST pendulum impact tester (Model Resil 50B, CEAST®, Akron, OH, USA). Experiments were carried out on un-notched composite samples (five replications) with nominal sizes of 45 mm (length) × 12.5 mm (width) × 3 mm (thickness) in accordance with ASTM D 256-10. All samples were conditioned to ∼23℃ and 50% relative humidity for at least 72 h prior to testing. The pendulum initial angle, maximum energy, and speed were 150°, 2.75 J, and 3.46 m/s, respectively.
Scanning electron microscopy (SEM)
For a better understanding of impact failure in the biocomposites, fractured surfaces of the impact samples were observed under a Hitachi TM3000 tabletop scanning electron microscope (Hitachi, Ltd, Chiyoda, Tokyo, Japan). The tabletop SEM did not require sputter coating of samples prior to imaging.
Thermogravimetric analysis (TGA)
To assess the thermal stability of the mat, CNC, Acrodur®, and the resultant biocomposites, thermogravimetric analyses were done using a TGA Q500 (TA Instruments, Inc., New Castle, DE, USA). Experiments were conducted on 10–20 mg samples placed in platinum sample pans using a high-resolution-dynamic mode with the heating regime of ramping up to 600℃ at a rate of 10℃/min in a nitrogen atmosphere.
Differential scanning calorimetry (DSC)
Thermal characteristics of the mat, CNC, Acrodur®, and the derived biocomposites were investigated through differential scanning calorimetry using a DSC Q2000 (TA Instruments, Inc., New Castle, DE, USA). Lightweight vented aluminum pans and lids (Tzero Low-Mass) were used to allow for moisture removal from the samples. Samples of 3–7 mg were loaded to the pans covered and sealed with the lids using a Universal Crimper Press. An empty sealed pan was also used as a reference to balance the heat capacity of the sample-loaded pans. DSC runs started with equilibrating at 35℃ and then ramped up to 250℃ at a heating rate of 10℃/min. In order to examine whether or not the resin was fully cured, the DSC runs continued by decreasing temperature from 250℃ to 35℃ at the same rate (10℃/min), and then repeating the ramping-up stage.
Statistical analysis
Statistical analyses were carried out on the experimental data using IBM SPSS Statistics Version 23 (IBM Corp., Armonk, NY, USA). The mechanical properties were statistically analyzed using one-way ANOVA tests. Duncan's multiple range test was used to compare the group means. Comparisons were made based on a 95% confidence interval.
Results and discussion
Flexural properties
The flexural modulus and flexural strength of the biocomposites are depicted in Figure 1. As shown in this figure, adding 5% CNC (dry basis) to the composite formulation resulted in a roughly 17.5% increase in the flexural modulus of the produced composites compared to the control samples. The statistical analysis performed showed that this increase was significant at 95% confidence level. This can be attributed to the high elastic modulus of CNC,
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which helps reinforce the entire composite structure. A slight reduction in flexural modulus was observed when only 2% CNC was used but the decrease was not statistically significant. The addition of either 2% or 5% CNC to the Acrodur® composite formulation did not significantly change the flexural strength (MOR). Overall, it was found that it is possible to significantly improve the stiffness of Acrodur®-based biocomposites without negatively affecting their flexural strength.
MOE and MOR of the biocomposites. Common letters over bars indicate no significant difference at 95% confidence level.
Impact and toughness properties
Results of the Izod pendulum impact tests are illustrated in Figure 2. It can be seen that the addition of CNC (at both levels) led into a slight reduction in the impact resistance of composites. This might be attributable to the higher stiffness and less ductility of CNC compared to the neat Acrodur® that made the entire structure slightly brittle.
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Increasing CNC content to 5% did not further reduce impact resistance significantly as evidenced by the statistical analysis. Toughness values corresponding to each sample were also obtained by calculating the area under the force–displacement curve from the three-point bending test. Results indicated that the control samples had the highest toughness values, while the lowest toughness was observed in the 5% CNC add-on composites (Figure 2). The crystalline structure of cellulose nanoparticles is thought to restrict the polymer matrix chain mobility and lower the inelastic deformation, thus decreasing the effective toughness of the resulting composite.
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The same trend is almost seen for both impact resistance and toughness i.e. adding CNC to the formulation of biocomposites led to a reduction in both properties. A significant linear relationship was found between toughness obtained from flexural tests and impact strength determined from Izod testing (Figure 2 inset) indicating the possibility to obtain information about impact strength from the three-point bending data if impact data are not available.
Impact resistance and toughness of the biocomposites. Correlation between impact resistance and toughness (top right). Common letters over bars indicate no significant difference at 95% confidence level.
SEM micrographs (Figure 3(a) to (c)) of the fracture surfaces after impact test illustrates that in all cases, fracture happened because of the failure of the fibrous constituent, natural fibers in particular. Because natural fibers fail at lower stress values than the adhesive strength, failure mode was not found to be affected by the addition of CNC to the formulations.
SEM micrographs of the fractured surfaces: (a) control; (b) 2% CNC-added; (c) 5% CNC-added composites.
Thermal stability
Weight loss (TG) and derivative weight loss (DTG) thermograms of mat, neat CNC and neat Acrodur® as well as 5% CNC-added Acrodur® composite (as a selected formulation of the produced CNC-added Acrodur® biocomposites) are shown in Figure 4(a) and (b). There is a very slight weight loss shown for the neat Acrodur® around 100℃ corresponding to the moisture evaporation. In the range of 175–375℃, a higher weight loss is seen that is attributable to the decomposition of polyester and formation of some volatile products such as CH4, CO, CO2, C2H2, and C2H4. The highest weight loss for the Acrodur® showed to happen from almost 375℃ to 400℃.
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Thermograms of the composite constituents: (a) TG; (b) DTG.
Thermograms of the mat show a similar first weight loss around 100℃ attributed to the moisture evaporation from fibers. The second considerable change happened around 230℃ predominantly due to the initiation of decomposition of natural cellulosic fiber components such as hemicelluloses, pectin, and waxes. Within the range of 265–390℃, the degradation of glycosyl units in cellulose and lignin along with the degradation of PET fibers above 300℃ led into a significant weight loss shown in the mat thermograms.2,30
CNC showed to lose about 6% mass during the evaporation of residual moisture. There is a significant weight loss starting from 225℃ attributed to the beginning of depolymerization in cellulose, followed by the breakage of glycosidic linkages and production of residual char. 31 Overall, neat Acrodur® film showed a very interesting thermal stability. Although it showed to start degrading at lower temperature (>175℃) compared to CNC (>225℃) and mat (>230℃), it presented a better thermal stability performance. The first 20% weight loss occurred at around 325℃ for Acrodur®, however, this happened to CNC and mat at about 230℃ and 310℃, respectively. Therefore, Acrodur® helps improve the thermal stability of the derived biocomposite.
Figure 5(a) and (b) illustrate TG and DTG curves of the 2% and 5% CNC add-on composites along with the control composite (without CNC) samples. It can be seen that adding CNC in both cases (2% and 5%) to the formulation resulted in slight reductions in the thermal stability of the resultant biocomposites. However, these are not considerable reductions in the composites' thermal stability mainly attributed to the lower percentage of CNC in the formulations.
(a) TG and (b) DTG of the biocomposites.
DSC analysis
Results of DSC analyses on the mat, pure CNC, neat Acrodur®, Acrodur®/CNC mixture (with the same mixing ratio used for the 5% CNC-added composites), and 5% CNC-added Acrodur® composites are presented in Figure 6(a) to (d). As shown in typical DSC scan (Scan 1) of the mat (Figure 6(a)), a broad endothermic peak is observed in the range of 35–140℃ attributable to the moisture evaporation. Beyond 230℃, another peak starts to happen corresponding to the decomposition of natural cellulosic fibers, which is in accordance with the results of thermogravimetric analysis. In the second scan (Scan 2), no endothermic peak is observed around 100℃ because of the moisture removal in the previous heating step. The curve starts decreasing after above 200℃ due to the degradation of the rest of cellulosic fiber components including the rest of hemicelluloses, pectin, and waxes. The PET present in the mat formulation was not expected to show a melting peak in the temperature range used in this test.
DSC curves of (a) mat, (b) pure CNC, (c) neat Acrodur®, (d) Acrodur®/CNC mixture, and (e) derived composite.
The first DSC scan of CNC illustrates a quite wide peak in the range of 35–190℃ attributed to the moisture evaporation, followed by a slight decrease starting above 225℃ due to the decomposition of cellulose. The second scan of the CNC film does not show any considerable changes in the thermal curve within the range of scanning i.e. 35–250℃. Generally, the DSC curves of CNC and natural fiber mat are very similar.
The DSC first scan of the neat Acrodur® shows a slight decrease around 100℃ due to the evaporation of residual moisture content. A quite sharp peak is seen in the range of 175–250℃, which can be attributed to the Acrodur curing. In the second scan of the Acrodur, no peak is observed within the aforementioned temperature range, which verifies the completion of curing in the previous heating stage. CNC incorporation did not affect the curing behavior of the Acrodur®, as the corresponding endothermic peak of curing was showed to happen almost within the same range occurring for the neat Acrodur® samples (Figure 6(d)). There was broad peak within the range of 35–125℃ shown in the first scan of the 5% CNC-added Acrodur® composite, which is attributable to the evaporation of the residual moisture. There is a slight change in the curve right above 200℃ corresponding to the post-curing of the Acrodur® matrix. No such changes are observed in the second DSC scan of the composites (Figure 6(e)), which confirms that no moisture was left in the sample and a full resin curing happened in the previous heating stage.
Conclusions
CNC was utilized as an additive in the formulation of natural fiber/PET reinforced Acrodur® biocomposites. Composite samples with 2% and 5% (pph of the total dry weight) along with control samples were produced. Mechanical properties of the produced composites were investigated through flexural and Izod impact tests. Results showed that the addition of CNC helped increase the flexural modulus of the derived composites while maintaining the flexural strength. However, the presence of CNC in the system caused the impact resistance and the toughness of the resultant composites to slightly decrease. Tensile properties were not evaluated in this study, but future work can include tensile tests to have a better idea of the material performance in different modes of deformation. The thermal behavior of the composites and their constituents were also assessed using TGA and DSC analyses. It was revealed that the thermal stability of the biocomposites was marginally reduced by adding CNC but the reduction was not substantial. Overall, the potential compatibility of CNC with Acrodur® resin and natural fibers as well as the CNC's positive contribution to the stiffness and strength development are promising for designing a new generation of high-strength and lightweight automotive parts. Furthermore, using natural fibers and Acrodur® resin in the formulation of sustainable composites brings about better recycling and substantial cost savings. The method used in the current study has also a significant potential to be utilized for sustainable composite manufacturing. Future studies should focus on evaluating a wider range of resin formulations and the evaluation of the compatibility between resin components and fibers as well as dispersion of the CNC particles in the resin matrix.
Footnotes
Acknowledgements
The authors would like to thank BASF for providing the Acrodur resin and other chemicals used in this study. They also thank Carver Non-woven Technologies for supplying natural fiber mats.
Authors' contribution
E.A. and M.T. conceptualized the idea, E.A. carried out sample production and testing, and E.A. and M.T. wrote the manuscript and analyzed the data.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was supported by the USDA National Institute of Food and Agriculture, McIntire-Stennis project number #ME041616 through the Maine Agricultural & Forest Experiment Station.
