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Tuesday, 7 March 2017
Sustainable wood-plastic composites from bio-based polyamide 11 and chemically modified beech fibers
Published Date
Sustainable Materials and Technologies December 2015, Vol.6:6–14, doi:10.1016/j.susmat.2015.10.001 Open Access, Creative Commons license
Author
Patrick Zierdt a,,
Torsten Theumer a
Gaurav Kulkarni a
Veronika Däumlich a
Jessica Klehm a
Ulrike Hirsch a
Andreas Weber b
aFraunhofer Institute for Mechanics of Materials IWM, Walter-Hülse-Straße 1, 06120 Halle (Saale), Germany
bInstitut für Holztechnologie Dresden IHD, Zellescher Weg 24, 01217 Dresden, Germany
Received 5 August 2015. Revised 14 October 2015. Accepted 14 October 2015. Available online 26 October 2015.
Abstract
Wood-plastic composites from bio-based polymers and wood fibers (bio-WPC) provide an improved sustainability and carbon footprint compared to conventional composites. Actually, the implementation of this approach into industrial applications is hindered by the missing knowledge on the mechanical and thermo-mechanical properties of such bio-WPC. In this study, the properties of a bio-WPC from bio-based polyamide 11 (PA 11) and chemically modified beech fibers were investigated. The chemical modification of the beech fibers by an alkaline treatment with an aqueous solution of sodium hydroxide (NaOH) was done to support the melt processing and adhesion to the PA 11 matrix. Analysis of the modified fibers by Thermogravimetric Analysis (TGA) proved an increased thermal stability, as identified by an increase of the extrapolated TGA onset temperature from 290 to 330 °C. This improvement resulted from hemicellulose removal, as confirmed through Attenuated Total Reflection Infrared Spectroscopy (ATR-FTIR). Consequently, mechanical and thermo-mechanical analysis of the processed bio-WPC showed an increase in elastic modulus and storage modulus of the composites by the chemical treatment of the fibers. This effect was attributed to an increased number of hydrogen bonds between the modified beech fibers and the PA 11 matrix. The overall mechanical properties of the investigated bio-WPCs support their use as sustainable construction material for technical applications.
Keywords
Sustainable WPC
Bio-WPC
Polyamide 11
Beech Fiber
Chemical Modification
1 Introduction
Wood-plastic composites (WPC) have been used as construction materials for many years due to their beneficial properties in comparison to synthetic fiber composites, like decreased density and lower costs [1]. The raw materials used to process WPC are mainly polyolefin thermoplastics, such as polyethylene (PE), polypropylene (PP) or polyvinylchloride (PVC), and wood flour or fibers mainly from softwood like spruce or pine [2], [3] and [4]. Typical applications include extruded decking and profiles in the building and construction sector and also complex interior parts for automotive applications, produced by injection molding [5] and [6]. The next generation of these materials is based on the implementation of biogenic matrix polymers and thus being processed from completely regenerative raw materials [7]. The idea behind the approach of such completely biogenic wood-plastic composites (bio-WPC) is that the amount of CO2 generated by the processing of the composite material is partially or completely compensated by the CO2 consumed in the growth phase of the plants used as raw materials [8]. Thus, construction materials with a significantly improved sustainability and carbon footprint could be produced. Despite the ecological advantages, economic interests like decreased dependency in fossil resources and the support of the development of new and sustainable technologies are also addressed with this approach.
Biopolymers can be divided into biodegradables and non-biodegradables, the latter being of particular interest for the use as matrix polymers in technical composite materials since they can be seen as drop-in solutions for already existing conventional plastics [9]. In recent years, bio-based polyethylene from sugar cane and bio-based polyamides from castor oil have become commercially available and represent the most interesting alternatives to their fossil-based counterparts [10]. The main obstacle in producing WPC based on polyamides is their relatively high melting temperature which promotes thermal degradation of the wood fibers during processing. Despite this, conventional polyamide 6 (PA 6) has been studied as matrix material for natural fiber-reinforced composites because of its beneficial thermo-mechanical properties [11], [12], [13], [14], [15], [16], [17] and [18]. Bio-based polyamides, like polyamide 11 (PA 11), polyamide 10.10 (PA 10.10) and polyamide 6.10 (PA 6.10), have generally lower melting temperatures than conventional PA 6 [19]. This enables the melt processing of wood fiber reinforced composites before the start of thermal degradation of the wood fibers, which has been reported to be at around 220 °C [20]. Accordingly, there is a great potential to develop sophisticated and sustainable bio-WPCs with these bio-based polyamides. However, actual market prices of bio-based polyamides are up to five times higher than conventional PA 6 due to lower production capacities [21].
The chemical modification of wood fibers has been studied in the past mainly to improve their adhesion to thermoplastic polymers or coupling agents and in consequence to improve the mechanical properties of the subsequently processed wood-plastic composites. One widely used chemical treatment is mercerization, an alkaline treatment with aqueous sodium hydroxide (NaOH) [22]. The main effect is the disruption of the hydrogen bonds on the fiber surface, leading to a higher roughness of the surface by removal of certain amounts of lignin, waxes, oils and hemicelluloses. In consequence, this increases the amount of hydroxyl groups on the fiber surface and improves wetting and adhesion with thermoplastic polymers. Additionally, the thermal stability of the wood fibers can be enhanced. Improvement of the thermal stability of wood fibers can also be done by physical modification techniques, especially by heat treatment [23]. The general mechanism of both treatments is the selective decomposition of the hemicellulose components, which display the least thermally stable components of wood fibers.
Although non-biodegradable biopolymers, such as bio-based polyamides, are gradually entering the market, there is no extensive knowledge about their potential as matrix polymers in composite materials for technical applications. There is a multitude of studies available on wood-plastic composites based on conventional polyolefins, but only a few studies discuss the approach of bio-WPC from non-biodegradable biopolymers as sustainable composite materials for technical applications. Furthermore, the mechanism of improving the thermal stability of wood fibers by chemical modification is already discussed but not systematically applied to process wood-plastic composites from highly melting polymers like polyamides in literature. In this study, the processing and the resulting properties of a bio-WPC from bio-based polyamide 11 (PA 11) and chemically modified pulped beech fibers were therefore investigated. In the first part, modification of the morphology and the thermal stability of the beech fibers was done by an alkaline treatment to improve the processability with the PA 11. In the second part, characterization of the morphological, mechanical and thermo-mechanical properties of the subsequently processed bio-WPC was done. The goal of these investigations was to quantify the potential of bio-WPCs based on bio-polyamides and further to support their implementation as sustainable drop-in materials for conventional WPCs in already existing technical applications.
2 Materials and methods
2.1 Materials
2.1.1 Bio-based polyamide 11
The matrix polymer used to produce the bio-WPC was a bio-based polyamide 11 (PA 11, Rilsan BESNO TL) from Arkema Company. The raw material for this polymer is castor oil, which can be converted into 11-aminoundecanoic acid and subsequently polycondensed into PA 11 with a bio-based carbon content of > 98%. The melting temperature of the used grade is 186 °C and the density is 1.02 g/cm3. The material was used as delivered in spherical pellet form and dried at 80 °C for 8 h before processing.
2.1.2 Chemically modified beech fibers
The beech fibers (BF) used to process the bio-WPC were derived by a thermo-mechanical pulping process from beech wood (Fagus sylvatica). In the first step, a hydro-thermal pretreatment of the beech wood chips was done at 80 °C. Secondly, the pretreated wood chips were transported into a digester, where they were plasticized over 4 min at a temperature of 170 °C and a pressure of 9 bar. Given this, the plasticized wood material was conveyed into a twin disk refiner with a gap of 0.1 mm. Finally, the obtained beech fibers were dried at 80 to 100 °C to a water content of approximately 6.5% as measured with the Soxhlet method. The chemical modification of the beech fibers was done separately after the pulping process with an alkaline treatment (mercerization). For this, the fibers were soaked in distilled water and then washed in an aqueous solution of sodium hydroxide (NaOH) with a concentration of 10 g/l at room temperature for 60 min. Afterwards, the fibers were removed and washed again with distilled water until pH 7 was achieved. Drying of the modified beech fibers (BFmod) was done at 100 °C for 5 h in a convection oven to a moisture content of approximately 2.8% as measured with the Soxhlet method.
2.2 Methods
2.2.1 Attenuated total reflection infrared spectroscopy (ATR-FTIR) of beech fibers
The ATR-FTIR measurements were conducted with a Bruker Equinox 55 device with an ATR unit. The unit is equipped with a diamond ATR crystal with a total measurement area of 4 mm2. The measuring chamber was continuously purged with gaseous N2. For data collection and processing, the software OPUS 6.5 was used. All spectra were collected over the wave number span of 650–4000 cm− 1 at a resolution of 4 cm− 1. Before the specimen measurement, a reference spectrum of the gaseous environment was collected. Then, compacted pellets of the wood fibers were placed onto the ATR crystal and secured with a screw. For each sample, three spectra were collected and converted into a mean spectrum. The baselines of the spectra were corrected using the concave rubber band method with 10 iterations.
2.2.2 Thermogravimetric analysis (TGA) of beech fibers
The characterization of the thermal stability of the beech fibers was done by TGA with a Netzsch STA 449 F1 device. Compacted pellets of approximately 4.5 mg of wood fibers encased in aluminum pans were placed into the device and the weight loss from room temperature to 600 °C was measured in dynamic mode with a heating rate of 10 K/min. During the measurements, the measuring chamber was continuously purged with gaseous N2. Analysis of the obtained data was done with the software Proteus Analysis.
2.2.3 Preparation of bio-WPC and test specimens
The preparation of the bio-WPC was carried out by discontinuous mixing in a two-roll internal mixer (PolyLab, Thermo Scientific). Compounds of the PA 11 and the modified as well as the unmodified beech fibers were processed with fiber contents of 30, 40 and 50 wt.%. The processing conditions were kept constant for all the composites at 196 °C melt temperature and 50 rpm rotation speed. After 3 min melting of the PA 11, the beech fibers were added and the components were mixed for 5 min. Subsequently, the composites were removed from the mixing chamber and directly injection molded into standard test specimens by a lab scale plunger injection molding machine (MiniJet, Thermo Scientific). Processing parameters for the preparation of the test specimens were kept constant at 220 °C melting temperature, 110 °C mold temperature, 3.5 min melting time, 900 bar injection pressure and 460 bar hold pressure. The processed test specimens were immediately sealed in multilayered aluminum foil to prevent water uptake (fresh as-molded).
2.2.4 Morphological characterization of beech fibers and bio-WPC
The morphological properties of the beech fibers were determined by manual statistical analysis of the length and width of the unmodified and modified fibers. For this, random fiber samples were individually arranged between two transparent polypropylene films and optical scans were made. Picture analysis was subsequently done manually with an evaluation program (CellF) for approximately 2500 single fibers. In result, the average fiber length and fiber width were obtained. Additionally, the surfaces of the unmodified and modified beech fibers as well as the fracture surfaces of Charpy notched impact test specimens of the bio-WPC were analyzed by Scanning Electron Microscopy (SEM), with a FEI Quanta 3D FEG device at an acceleration voltage of 5 kV. To avoid charging, the fiber samples and test specimens were coated with Platinum (Pt) beforehand.
2.2.5 Mechanical characterization of bio-WPC
Testing of the mechanical properties of the processed bio-WPC was done by tensile testing according to ISO 527 with a universal testing machine (Z050 AllroundLine, Zwick) and Charpy notched impact testing according to ISO 179eA with a notch test apparatus (CEAST Resil Impactor Junior, Instron). For each composite, 6 specimens were tested and mean values of the mechanical parameters were calculated. The testing was done on fresh as-molded (sealed) and conditioned specimens to investigate the impact of water uptake on the mechanical properties. The conditioning of each specimen was done in a climate chamber at 70 ± 1 °C and 62 ± 1% relative humidity until the weight gain in three successive measurements was less than 0,1% according to ISO 1110. The resulting water content of the conditioned test specimens was calculated from the relative weight gain compared to the fresh as-molded test specimens.
2.2.6 Dynamic mechanical thermal analysis (DMTA) of bio-WPC
The thermo-mechanical properties of the processed bio-WPC were analyzed by DMTA between − 70 and 120 °C on fresh as-molded specimens (60 × 10 × 4 mm). The experimental set-up used was a force controlled oscillating three-point bending mode (ISO 6721), the testing device was a DMA Q800 from TA Instruments. The linear viscoelastic regime of each composite was identified by amplitude sweeps from 0.1 to 3 MPa with a frequency of 1 Hz at a temperature of − 70, 25, and 120 °C. Ultimately, the DMTA measurements were done at an amplitude of 0.3 MPa, a frequency of 1 Hz and a heating rate of 3 K/min in the temperature range of − 70 to 120 °C for all composites. During all measurements, the measuring chamber was continuously purged with gaseous N2. Values of the storage modulus (E′), loss modulus (E″), and loss factor (tan δ) of the composites were obtained and analyzed as a function of temperature.
3 Results and discussion
3.1 Chemical modification of beech fibers
The surface of the unmodified (BF) and modified beech fibers (BFmod) was analyzed by ATR-FTIR. The obtained spectra in the fingerprint region from 2000 to 800 cm− 1 of the two fiber types are shown in Fig. 1. An overview of the characteristic bands and their assignments is given in Table 1. The first significant difference in the spectra is the disappearing band at around 1729 cm− 1 for the modified fibers. This band can be assigned to the stretching of carbonyl groups (CO) in the unmodified beech fibers, which are mainly occurring in the branched xylan type polymers of the hemicellulose but also in the cellulose and lignin [24]. Interestingly, the characteristic peak for the stretching of the carbonyl groups for pristine beech wood is reported to be at 1740 cm− 1[25]. This indicates that the hydro-thermal treatment of the beech wood during the thermo-mechanical pulping process influenced the chemical composition of the beech wood in terms of an increase in the relative concentration of lignin by a dissolution of cellulose or hemicellulose components, which shifted the peak position to a lower wavenumber [26].
Fig. 1. ATR-FTIR spectra of unmodified (BF) and chemically modified beech fibers (BFmod).
Table 1. Assigned ATR-FTIR peaks in the fingerprint region from 2000 cm− 1 to 800 cm− 1 of unmodified (BF) and chemically modified beech fibers (BFmod).
The second significant difference in the spectra in Fig. 1 is the disappearing band at around 1236 cm− 1 and the emerge of two smaller peaks at 1266 and 1224 cm− 1 for the alkaline modified fibers. The strong peak at 1236 cm− 1 can be assigned to the stretching of the C–O bonds in the ester groups of the xylan based hemicellulose polymers, while the peaks at 1266 and 1224 cm− 1 belong to the C–O stretching in the guaiacyl and syringyl polymers of lignin, respectively [27]. Again, these peaks are slightly shifted to lower wavenumbers as reported in literature for hardwood lignin [28]. From these findings can be concluded that the alkaline treatment of the pulped beech fibers significantly altered the chemical surface properties by partial removal of the hemicellulose, while the cellulosic and lignin components were retained. The alkaline conditions caused the de-esterification of the carboxylic groups in the xylan polymers and the release of hemicellulose into solution by formation of acetic acids. Comparable findings on alkaline modification of natural fibers have been reported from Gregorova et al. [29] for beech wood flour, Pranovich et al. [30] for pulped spruce fibers and Mwaikambo et al. [31] for plant fibers from hemp, sisal, jute and kapok.
3.2 Thermal stability of beech fibers
The thermal stabilities of the unmodified and modified beech fibers were analyzed by TGA measurements. The resulting thermograms of the mass loss and of the first derivative in relation to temperature are shown in Fig. 2 (a) and (b), respectively. An overview of the identified thermogravimetric characteristics of the beech fibers is given in Table 2. Three stages of mass loss were found for both types of beech fibers. The first stage can be related to the release of water and other volatiles from the fibers. This stage corresponds to a mass loss of approximately 4% for the unmodified and modified fibers. The second stage can be attributed to the decomposition of the hemicellulose, lignin and cellulose components of the beech fibers. The onset of this second decomposition stage was found to be significantly shifted to a higher temperature for the modified beech fibers, presumably due to the removal of hemicellulose from the fiber surface as identified by the ATR-FTIR measurements. While for the unmodified fibers an extrapolated TGA onset temperature (Tonset) of 290 °C was found, an extrapolated TGA onset temperature of 330 °C was identified for the modified fibers. The third and final stage of thermal decomposition extends to the ending test temperature with a total mass loss (ML600 °C) of approximately 87% for the unmodified and modified beech fibers.
Fig. 2. TGA thermograms of unmodified (BF) and chemically modified beech fibers (BFmod); (a) TGA curves and (b) DTG curves.
Table 2. Thermogravimetric characteristics of unmodified (BF) and chemically modified beech fibers (BFmod).
Material
Tonset
[°C]
T25
[°C]
T50
[°C]
ML600 °C
[%]
BF
290
313
353
87
BFmod
330
333
363
88
Tonset — onset temperature of second decomposition stage.
T25 — temperature at 25% mass loss.
T50 — temperature at 50% mass loss.
ML600 °C — mass loss at 600 °C.
The results of the TGA investigations showed that the applied alkaline treatment effectively enhanced the thermal stability of the beech fibers. Corresponding results on the thermal stability of unmodified and modified wood fibers have been reported in literature by Nguyen et al. [20] and [35]. From the main components in beech wood, hemicellulose is the least thermally stable, followed by lignin and cellulose [36]. Accordingly, an improvement of the thermal stability of the wood fibers is mainly effected by a selective dissolution of the hemicellulose components. These conclusions correlate to the findings of the ATR-FTIR measurements and prove the effectiveness of the applied chemical modification of the beech fibers.
3.3 Morphology of beech fibers and bio-WPC
The fiber length distribution and fiber width distribution of the unmodified and chemically modified beech fibers as obtained by manual statistical analysis of optical scans are shown in Fig. 3 (a) and (b), respectively. The resulting morphological characteristics are presented in Table 3. It can be seen that the chemical treatment of the beech fibers had an effect on the medium fiber length but not on the medium fiber width. While for the unmodified fibers a medium fiber length of 1.72 mm was identified, this value increased to 2.12 mm for the chemically modified fibers. This effect can be attributed to the washing out of smaller fibers and flour particles by the chemical treatment of the beech fibers with the aqueous solution of sodium hydroxide. On the other hand, both fiber types had comparable medium fiber widths, identified as 189 μm for the unmodified beech fibers and 194 μm for the chemically modified beech fibers. Accordingly, the aspect ratio of the two fiber types varied, resulting in 9.1 for the unmodified fibers and 10.9 for the chemically modified fibers.
Fig. 3. Morphological characteristics of unmodified (BF) and chemically modified beech fibers (BFmod); (a) fiber length distribution and (b) fiber width distribution.
Table 3. Morphological characteristics of unmodified (BF) and chemically modified beech fibers (BFmod).
Material
Fiber length
[mm]
Fiber width
[μm]
Aspect ratio
BF
1.72 ± 0.66
189 ± 53
9.1
BFmod
2.12 ± 0.92
194 ± 49
10.9
However, the obtained morphological characteristics of the investigated pulped beech fibers are in good agreement with morphological data of pulped beech fibers recently published by Schirp et al. [37]. The processing parameters of the refining process have a strong influence on the resulting fiber length and width. Length and width of the resulting fibers especially increase with refiner disc distance. Schirp et al. [37] found a fiber length of 2.36 mm and a fiber width of 185 μm, when a disc distance of 0.15 mm was used. These values are comparable to the obtained values in this study, where a disc distance of 0.1 mm was used.
Schirp et al. [37] also showed that the processing of beech fibers with polyethylene to conventional WPC by lab scale kneading and injection molding leads to a significant decrease in fiber length in the composite. Similar studies on the resulting fiber length of beech fibers in polyamide 11 based WPC offer further scope for investigation. Conventional Soxhlet extraction of the wood fibers from the processed composites is not applicable due to the chemical resistance of PA 11, whereby more sophisticated techniques have to be used to obtain the fiber length in the composites.
A detailed view on the surface structure of the unmodified and chemically modified beech fibers is given by the SEM pictures in Fig. 4 (a) and (b), respectively. It can be seen that the chemical modification strongly affected the surface structure of the fibers. The modified fibers showed a much more textured surface, indicating an increase of the surface area by the chemical modification. Similar findings have been reported by Valadez-Gonzales et al. [38] for the alkaline modification of henequen fibers. They found that the modified fibers showed much more crevices on the fiber surface.
Fig. 4. SEM pictures of (a) unmodified beech fibers (BF), (b) chemically modified beech fibers (BFmod), (c) fracture surface of PA11/BF-50/50 and (d) fracture surface of PA11/BFmod-50/50.
The fracture surfaces of the Charpy notched impact strength test specimens of the bio-WPCs with the unmodified and chemically modified fibers are shown in Fig. 4 (c) and (d). Coverage of the wood fibers with the matrix polymer can clearly be seen in both composites, indicating a good fiber/matrix adhesion. However, the fracture surfaces of the bio-WPC with the unmodified fibers in Fig. 4 (c) also show partial debonding and crack propagation in the fiber/matrix interface, while this can hardly be seen for the bio-WPC with the modified fibers in Fig. 4 (d). This qualitatively indicates a better fiber/matrix adhesion due to the chemical treatment of the fibers.
3.4 Mechanical properties of bio-WPC
An overview of the obtained mechanical properties of the investigated bio-WPC from PA 11 and unmodified and chemically modified beech fibers is given in Fig. 5. A detailed listing of the mechanical values is given in Table 4. As can be seen in Fig. 5(a), a positive correlation of the elastic modulus (E-modulus) and the fiber content was found. The E-modulus increased with increasing fiber content and the values for the composites with the modified fibers were slightly higher compared to the composites with the unmodified fibers. The highest E-modulus of 5049 MPa was found for the composite with 50 wt.% of modified fibers. This corresponds to a four-fold value compared to the neat PA 11 and an increase of approximately 8% to the composite with 50 wt.% unmodified fibers. Water uptake due to conditioning resulted in a drop in the E-modulus for all composites, which can be attributed to matrix softening as can be seen on the values of the neat PA 11. Similar results have been found for the tensile strength as shown in Fig. 5 (b). Interestingly, no increase in tensile strength of the composites with the modified fibers compared to the composites with the unmodified fibers could be found. Again, the composites with the highest fiber content of 50 wt.% showed the highest values with 65 MPa. This significant increase in stiffness and strength caused an expectable decrease in toughness, as can be seen in Fig. 5 (c) and (d) for the strain at break and the Charpy notched impact strength, respectively. For these two values, a negative correlation with the fiber content was found, with the lowest values for the composites with the highest fiber content of 50 wt.%. Conditioning caused a slight increase in strain at break and impact strength for the composites, while for the neat PA 11 a decrease in strain at break and a significant increase in impact strength was found.
Fig. 5. Mechanical properties of bio-WPC from polyamide 11 and unmodified (PA11/BF) and chemically modified beech fibers (PA11/BFmod) as a function of fiber content; (a) E-modulus, (b) tensile strength, (c) strain at break and (d) Charpy notched impact strength.
Table 4. Mechanical values of bio-WPC from polyamide 11 and unmodified (PA11/BF) and chemically modified beech fibers (PA11/BFmod).
Material
Fiber content
[wt.%]
E-modulus
[MPa]
Tensile strength
[MPa]
Strain at break
[%]
Impact strength
[kJ/m2]
PA11
0
1297 ± 28
48 ± 2
136.7 ± 9.5
12.8 ± 1
PA11/BF
30
2983 ± 75
53 ± 1
4.2 ± 1.3
2.8 ± 0.1
40
3764 ± 69
58 ± 1
2.9 ± 0.4
2.8 ± 0.4
50
4684 ± 184
65 ± 2
2.2 ± 0.2
2.4 ± 0.2
PA11/BFmod
30
3278 ± 85
57 ± 1
6.4 ± 1.1
3.0 ± 0.1
40
4012 ± 90
59 ± 2
3.4 ± 0.7
3.0 ± 0.1
50
5049 ± 108
65 ± 2
2.6 ± 0.2
2.9 ± 0.1
As expected, the water uptake during conditioning was found to be dependent on the fiber content. An increase in fiber content from 30 to 50 wt.% led to an increase in water content from 1.18 to 1.98% after conditioning for the composites with the unmodified fibers, respectively. The water uptake was found to be further increased for the composites with the modified fibers. Here, a maximum water content of 2.46% was found for the highest fiber content of 50 wt.%. This can be attributed to the increased number of hydroxyl groups on the surface of the modified fibers, which promote water uptake. However, this additional water uptake had no significant softening effect on mechanical properties compared to the composites with the unmodified fibers.
The results of the mechanical characterization of the bio-WPC show that the chemical modification of the beech fibers had a positive influence on the properties. This can be attributed to a better load transfer from the matrix to the fibers by an increased number of hydrogen bonds between the modified beech fibers and the PA 11 matrix. However, this effect becomes insignificant at high deformation rates and strains. According results have been reported by George et al. [39] and Kalia et al. [40] for alkaline pretreatments in natural fiber reinforced composites. It was also reported that alkaline treatments can influence the crystalline structure of the cellulose and the mechanical properties of the fibers. A positive influence of an alkaline treatment on the E-modulus of bio-WPC from poly(3-hydroxybutyrate) and beech wood fibers has been reported by Gregorova et al. [29]. However, higher values for E-modulus and tensile strength were achieved by a chemical treatment of the beech fibers with stearic acid.
3.5 Thermo-mechanical properties of bio-WPC
The thermo-mechanical properties of the processed bio-WPC were investigated by Dynamic Mechanical Thermal Analysis (DMTA). The storage modulus and loss modulus of the composites as a function of temperature are shown in Fig. 6 (a) and (b), respectively. A typical behavior of the storage modulus with three confined regions, the glassy region, the glass transition and the rubbery region, was found for all composites. Consistently, the storage modulus showed a positive correlation with the fiber content. The composites made from the chemically modified fibers had a higher storage modulus than the composites made from the unmodified fibers, as already observed via the E-moduli in the tensile tests. However, no such difference between modified and unmodified fiber based composites was found in the rubbery region. Similar results for the dynamic mechanical properties of wood and natural fiber reinforced polyamide 6 have been reported in literature [12], [13] and [14].
Fig. 6. Dynamic mechanical properties of bio-WPC from Polyamid 11 and unmodified (PA11/BF) and chemically modified beech fibers (PA11/BFmod) as a function of temperature; (a) storage modulus and (b) loss modulus.
In general a good dispersion of wood fibers in the polymer matrix leads to an even distribution of the applied stress throughout the composite, which results in an increasing storage modulus with increasing fiber content. Hosseinaei et al. [32]studied the dynamic mechanical properties of wood-plastic composites from polypropylene and hemicellulose-extracted wood fibers. It was found that the crystallinity of the matrix polymer was affected by the surface properties of the wood fibers, resulting in a higher crystallinity for the composites with the modified wood fibers, which in result lead to an increased storage modulus in the glassy region. Additionally, the extraction of the hemicellulose from wood fibers changed the mechanical properties of the fibers itself, which also affected the storage modulus of the composite. Similar results have been reported by Aydemir et al. [18] for composites from heat-treated wood and polyamide 6.
However, in comparison to conventional wood-plastic composites predominantly based on petrochemical PE, PP and PVC the thermo-mechanical properties of the processed bio-WPC were improved. This can be attributed to the higher glass transition temperature of the matrix polymer PA 11. The influence of the fiber content and the chemical modification of the beech fibers on the glass transition temperature of the bio-WPC as derived by the peak position of the loss factor (tan δ) can be seen in Fig. 7 and Table 5, respectively. The glass transition temperature (Tg) of the neat PA 11 was found to be 67.4 °C. The addition of the beech fibers resulted in no significant change of the glass transition temperature in the composites, regardless of whether unmodified or modified fibers were used. In general, the tan δ peak decreased with increased fiber content, which indicates improved elastic properties by formation of network structures in the composites [41]. Interestingly, the peaks of the composites made from the unmodified beech fibers are generally lower than their counterparts made from the chemically modified fibers, which indicates that the crystallinity of these composites should be higher. This can be addressed to promoted nucleation by smaller fibers and flour particles, which were washed out by the wet chemical treatment. According investigations are the focus of further studies. In general, the identified glass transition temperatures of the investigated bio-WPC were significantly higher compared to conventional wood-plastic composites predominant based on polyolefins like PP [42]. In conclusion, this enables higher service temperatures for new applications with increased sustainability.
Fig. 7. Dynamic mechanical loss factor (tan δ) of bio-WPC from polyamide 11 and unmodified (PA11/BF) and chemically modified beech fibers (PA11/BFmod) as a function of temperature.
Table 5. Glass transition temperature (Tg) of bio-WPC from polyamide 11 and unmodified (PA11/BF) and chemically modified beech fibers (PA11/BFmod) as obtained by the peak analysis of tan δ.
Material
Fiber content
[wt.%]
Tg
[°C]
PA 11
0
67.42
PA 11/BF
30
63.44
40
68.79
50
66.84
PA 11/BFmod
30
66.31
40
66.78
50
66.94
4 Conclusions
Completely bio-based wood-plastic composites (bio-WPC) provide an approach to create sustainable construction materials for technical applications. In this study, the processing and resulting properties of such a bio-WPC from bio-based polyamide 11 (PA 11) and unmodified as well as chemically modified beech fibers were investigated. The chemical treatment of the beech fibers with an aqueous solution of sodium hydroxide resulted in an increase in thermal stability, as shown by an increased extrapolated TGA onset temperature of 330 °C. This effect can be attributed to the partial removal of less thermal stable hemicellulose components, as proven by the ATR-FTIR analysis.
Another positive effect of the chemical treatment was found in the rougher structured fiber surface, leading to an increased number of reactive hydroxyl groups available to promote fiber-matrix adhesion. In result, subsequent measurements of the tensile properties of the processed bio-WPC showed that the chemical treatment also had a beneficial effect on the E-modulus as measured by an increase of approximately 8% in comparison to the bio-WPC with the unmodified fibers. The overall mechanical and thermo-mechanical properties of the investigated composites proved that bio-WPCs based on bio-based polyamide 11 and chemically modified beech fibers provide a sustainable alternative to conventional wood-plastic composites based on petrochemical PE, PP or PVC.
From these findings it can be concluded that a systematically applied chemical modification of wood fibers, e.g. by an alkaline treatment with an aqueous solution of sodium hydroxide, can significantly improve the processability with high melting polymers like bio-based polyamides as well as the mechanical properties of the composite and seems therefore to be crucial for the implementation of sophisticated bio-WPC. However, major challenges for the broad use of such bio-WPCs derive by their economically competitiveness to conventional composite materials since bio-polyamides actually have higher market prices compared to petrochemical polymers.
Acknowledgment
Co-financing of the presented study by the German Federal Ministry of Education and Research is gratefully acknowledged (project number: 031A076).
O) in the unmodified beech fibers, which are mainly occurring in the branched xylan type polymers of the hemicellulose but also in the cellulose and lignin [24]. Interestingly, the characteristic peak for the stretching of the carbonyl groups for pristine beech wood is reported to be at 1740 cm− 1 [25]. This indicates that the hydro-thermal treatment of the beech wood during the thermo-mechanical pulping process influenced the chemical composition of the beech wood in terms of an increase in the relative concentration of lignin by a dissolution of cellulose or hemicellulose components, which shifted the peak position to a lower wavenumber [26].
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