Conifer needles as thermoplastic composite fillers: Structure and properties

Barton-Pudlik, J., and Czaja, K. (2016). "Conifer needles as thermoplastic composite fillers: Structure and properties," BioRes. 11(3), 6211-6231.

Abstract

This study describes the properties of thermoplastic polymer composites based on polyethylene (of low and high density) and ethylene-propylene copolymers using various types of conifer needles (pine, spruce, fir, and cedar) as fillers. For the needles, thermogravimetric analysis (TGA) and TGA/Fourier transform infrared spectroscopy (TGA/FTIR) were performed to investigate their structures and thermal resistance, as required for the composite processing methods. Moreover, structural differences were studied for the analyzed fillers and composite materials (FTIR). The results were compared with the values obtained for composites with conifer wood flour. Composites with conifer needles (pine) had increased water absorption and similar strength properties. However, irrespective of the degree of filling, composites with pine needles were positively characterized by the highest melt mass flow rate (MFR) values and showed a slightly better impact resistance than composites filled with other flours. Thus, shredded coniferous needles with sufficient thermal resistance could be successfully used as fillers in composites. This conclusion was based on thermoplastic polymers as an alternative and/or supplement to the wood flour used in the manufacture of wood-polymer composites.

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Conifer Needles as Thermoplastic Composite Fillers: Structure and Properties

Joanna Barton-Pudlik * and Krystyna Czaja

Keywords: Conifer needles; Biocomposites; Thermoplastic matrices; Structure properties; Mechanical properties

Contact information: Faculty of Chemistry, Opole University, Oleska 48, 45-052 Opole, Poland;

* Corresponding author: jbarton@uni.opole.pl

INTRODUCTION

Recently there have been increased quantities of composite materials utilizing renewable raw materials, especially plant-based ones. The advantages of developing new composites with plant fillers are the relatively low prices of plant materials and their virtually unlimited availability (renewable resources). Partial replacement of a polymer matrix with a plant filler results in materials with desirable functional properties at a significant reduction in costs, with the possibility to employ the typical processing methods used for polymeric materials. Additionally, there are no problems in the disposal and/or recycling of products derived from these materials (Farouk 2014). Careful qualitative selection (type, size, and shape of the filler particles; type of the polymer matrix; presence of a compatibilizer; and other excipients) and quantitative composition of the composites thus gives great opportunities for the preparation of materials with the desired properties for the planned fields of use. Plant materials that are most commonly used in polymer composites include kenaf, sisal, jute, flax, hemp, wood flour, ramie, bamboo, cotton, and coconut fibers (Wypych 2008). The availability of these raw materials largely depends on the geographical location and climate. Hence, wood flour (wood-polymer composites, WPC) has a crucial role in the European and American polymer composite markets (Vogt 2006; Li and Wu 2013; Ramamoorthy et al. 2015). Wood fillers are derived from various types of trees that can be divided into hardwood (beech, oak), softwood (pine, spruce), and fast-growing species (poplar, acacia, willow).

The total production of industrial roundwood amounts to 1424.4 million m³, including 852.3 million m³ (approx. 60%) of coniferous wood and 572.1 million m³ of non-coniferous wood (Wolf-Crowther et al. 2011). An analysis of the European market indicated that the total production of industrial roundwood amounted to 469 million m³, including 358.7 million m³(approx. 76%) of conifers. The data indicated that coniferous wood is very important for the industry and economy, both on the global and European scale. It results from the use of wood and wood products as a source of fuel, energy, paper and pulp, and raw materials for the preparation of various types of construction and furniture. Given the volume of the coniferous material consumed, waste products from conifer processing, such as leaves and needles, should be effectively utilized.

Moreover, an important factor that influences the need for wood waste management (derived mainly from coniferous trees) is the high risk of fire. The causes of the most recent relevant forest fires in the EU, particularly in South European countries (France, Greece, Italy, Portugal, and Spain) are varied (Bassi and Kettunen 2008). Pine forests, occurring in large numbers in Europe, are especially vulnerable to fires because they are highly flammable. Moreover, pine needles disintegrate slowly and create large quantities of highly flammable biomass. This is mainly due to their high content of resin and essential oils, which evaporate at high temperatures and feed the fire (Bassi and Kettunen 2008).

Pine needle wastes (a frequent cause of fires) also need special management in coniferous forests in northwestern India. Pine needles can be used to produce polymer composites based on thermosetting matrices (Singha and Thakur 2008). These composites can be synthesized via matrices such as resorcinol-formaldehyde resin (Singha and Thakur 2008), urea-formaldehyde resin (Singha and Thakur 2009), phenol-formaldehyde resin (Singha and Thakur 2010), and urea-resorcinol-formaldehyde resin (Singha et al. 2011). In these studies, the needle particles ranged in size from 200 microns to 1 cm, and the percentage by weight was 10 to 50%. Composites were prepared by compression molding. The mechanical strength properties of composite materials (tensile strength, compressive strength, and flexural strength) containing up to 30% of pine needle particles were superior to composites without particles, but a higher percentage of the filler in the composite decreased the strength parameters. Better properties can be obtained for materials that contain smaller-size filler particles (Singha and Thakur 2008, 2009, 2010; Singha et al. 2011). In addition, composites can be made with pine needle particles modified by mercerisation or acetylation. The modified filler results in improved dispersion in the urea-resorcinol-formaldehyde resin, improved mechanical strength of the composite product, and improved impact resistance compared with unmodified composites containing simply pine needle particles (Singha and Jyoti 2013).

Pine needle particles have also been used in polyurethane composites based on an aromatic polyisocyanate (Chauhan et al. 2012, 2013). The multiple particles of pine needles with dimensions of about 2 mm were further modified by soaking in a 10 to 40% aqueous solution of urea phosphate, and the composites containing pine needle particles were prepared via a hydraulic press. The materials were described in terms of their biological resistance, flammability, dimensional stability, and thermo-acoustic properties (Chauhan et al. 2012, 2013).

Dry pine needle fibers can be used as a reinforcement material in polypropylene composites (Malkapuram et al. 2010). The filler known as treated pine needle fiber (TPNF) was processed prior to incorporation into the polymer matrix to give a cooked pine needle pulp. For this purpose, bundles of pine needles were fed into the hopper of the refiner to transform them into small fibers, and wet pine needle fibers were subjected to soda pulping in digesters to remove the lignin content. Thermal analysis of the obtained composites found that the thermal stability and melting temperature of polypropylene (PP) decreased with increased TPNF content (Malkapuram et al. 2010).

The above examples show that the properties of composites based on the thermosetting matrix (mainly various formaldehyde resins) have been widely tested. This is confirmed by the patent (Woopar Products Ltd 1978), which claims a method for preparing composite materials by mixing and pressing a thermosetting resin with shredded particles of such materials as wood chips, cut branches, bark, pine leaves and needles, and peat.

Only one work has been published on thermoplastic polypropylene composites reinforced by pine needle fibers (Malkapuram et al. 2010). This work, however, used delignified pine needle fibers processed into a pulp. However, there are no examples of the applicability of raw pine needles in thermoplastics, or other types of coniferous needles in any polymer matrices. Thus, it is reasonable to use needles as fillers in polymer composites, including those with thermoplastic matrices.

In summary, as shown by literature data, it is known that mainly pine needles are used as fillers in a thermosetting matrix. However, the possibility of using different, and what is important unmodified conifer needles in terms of processing of thermoplastics, has not been evaluated yet. This research program defined the characteristics of thermoplastic polymer composites based on polyethylene (PELD, PEHD) and its propylene copolymer, using up to 50 wt.% of various conifer needles as fillers (pine, spruce, fir, and cedar). These needle fillers were tested in terms of their structures and, above all, thermal resistance, which is required for their use in composites based on thermoplastic matrices. Moreover, the paper compares the properties of composites based on conifer needles with composites containing wood flour derived from coniferous trees.

EXPERIMENTAL

Materials

Conifer needles from pine (P), fir (F), spruce (SP), and cedar (Thuja – T) were shredded into flour (< 500 µm) or particles (> 500 µm). The commercial wood flour Lignocel C-120 (C120, 70-150 µm, a mixture of coniferous wood) was obtained from Rettenmaier & Söhne GmbH Co KG (Rosenberg, Germany). The natural, coniferous wood flour (S) was prepared from the pine material, and it was sieved to a diameter < 500 µm.

The following polyolefin matrices were used to prepare the test composites: PEHD (Hostalen GC 7260, MFI = 8 g/10 min (190 °C, 2.16 kg), density 0.960 g/cm³), PELD (Lupolen 1800S, MFI = 20 g/10 min (190 °C, 2.16 kg), density 0.917 g/cm³), and propylene-ethylene copolymer, co(E-P) (Moplen EP540P, MFI = 15 g/10 min (230 °C, 2.16 kg), density 0.900 g/cm³).

Composition Preparation

The fillers were initially dried at 70 °C for 12 h to remove moisture. To prepare the composite material, the polymer and the unmodified fillers were mixed at a specific ratio (Table 1) and compounded using a co-rotation twin-screw extruder (diameter 24 mm and L/D = 40). The 10 temperature zones of the extruder were controlled at 160 to 190 °C from the feeding zone to the die zone. The screw speed was maintained at 60 rpm. The extruded strands were cooled in air and pelletized.

Preparation of Samples for Analysis

The test specimens were obtained from pellets that were injection-molded into standard impact and tensile test specimens. The pellets were dried at 70 °C for 12 h; the specimens were obtained at injection and mold temperatures of 165 °C and 60 °C, respectively. Samples for structural testing by Fourier transform infrared spectroscopy (FTIR) were prepared by pressing the obtained pellets into thin films.

Table 1. Nomenclature and Formulation of Materials

Thermogravimetric Analysis (TGA)

A thermogravimetric analyzer TGA/DSC1 Mettler Toledo was employed to evaluate the decomposition temperatures of wood flours, PEHD, and WPC composites. The TGA was performed by heating a sample from room temperature to 600 °C at 10 °C min^-1under a continuously flowing nitrogen stream.

The qualitative analysis of gases released during thermal degradation of samples of the coniferous plant needles (fir, pine, spruce, and cedar) was performed under inert atmospheric conditions using TGA coupled with an FTIR spectrometer. The TGA/FTIR analysis was performed using a Mettler Toledo TGA/DSC1 instrument connected to a Nicolet 30 FTIR spectrometer (Greifensee, Switzerland). The sample of approximately 15 mg was placed in a ceramic crucible and heated from 25 to 900 °C in a nitrogen atmosphere (flow rate of 60 mL/min.). The heating rate was 10 °C/min. The purge gas carried the decomposition products from the TGA apparatus to the 70 mL KBr cuvette for FTIR detection. The pipes and the gas chamber were both heated to 200 °C to prevent condensation of the transferred gases. The spectra were recorded from 500 to 4000 cm^-1 with the spectral resolution of 4 cm^-1, and there were 16 scans on average per sample.

Functional Properties

The tensile tests were carried out under standard conditions (ISO 527-2 (1998)). Five specimens from each composition were tested using an Instron tensometer (Norwood, Massachusetts, USA) equipped with a load cell of maximum capacity of 1 kN, operating at a grip separation speed of 100 mm/min. The Charpy impact strength was measured using a Zwick/Roell HIT 50P (Ulm, Germany) under standard conditions (ISO 179-1 (2010)). The melt flow index (MFI) was measured using a Zwick/Roell Aflow extrusion plastometer type BMF-005 (Ulm, Germany), in accordance with ISO 1133-1 (2011).

Water Absorption

The composite samples for water absorption measurements were cut into 10 × 10 mm squares with a thickness of 1 mm. The samples were dried in a vacuum oven at 50 °C for 24 h, then cooled in a desiccator and weighed. The samples were immersed in distilled water at 23 °C for 14 days. During this period, samples were removed from water at 24-h intervals to be weighed.

Morphological Characterization

The samples were analyzed by scanning electron microscopy (SEM) with a Hitachi TM 3000 (Tokyo, Japan). Samples were cooled in liquid nitrogen, fractured, and coated with a thin film of gold in a sputter coater to eliminate electron charging. The coated samples were observed at different resolutions using the voltage of 15 kV.

Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectroscopy was performed using a Thermo Scientific Nicolet Nexus spectrometer (Waltham, MA, USA) with a Smart iTR accessory. The spectra were recorded from 700 to 4000 cm^-1 with a resolution of 2 cm^-1and 32 scans per sample.

RESULTS AND DISCUSSION

Thermal Resistance of Needle Thermal Decomposition Products

Fillers in thermoplastic composites require thermal stability, which depends on the established processing conditions. In polyolefin composites, the fillers should offer good thermal stability up to 200 °C. To confirm the possibility of using needles as fillers, a thermogravimetric analysis of the selected plant fillers was conducted. The obtained TGA-DTG patterns versus temperature for needle samples in N₂ are shown in Fig. 1.

The TG profile, as shown in Fig. 1(a), indicated five distinct weight loss stages. The first step of pine needles pyrolysis mainly involved water evaporation. Other peaks are associated with gas emissions. The thermograms obtained for pine and spruce needles were similar (Fig. 1(a) and (c)). Slightly larger differences were observed for the fir needles (Fig. 1(b)), while the biggest differences were noted in the thermogram for the cedar needles (Fig. 1(d)). The essential part of the decomposition of pine (approx. 54.1%), spruce (approx. 49.8%), and fir (approx. 52.4%) needles occurred between 340 and 350 °C. The thermal decomposition of the cedar needles was more complex, with the highest weight loss observed at approximately 270 °C (29.4%). A detailed thermal analysis is provided in Table 2.

Fig. 1. Thermal stability of (a) pine, (b) fir, (c) spruce, and (d) cedar needles

Table 2. Thermal Analysis of Needle Test Samples

The analyzed temperature ranges were below 100 °C (water), 100 to 300 °C, 301 to 400 °C (major thermal decomposition products), and above 400 °C (residues). Pine, spruce, and fir needles were characterised by three major similar mass losses at 78 to 86 °C (4.6 to 5.1%), 340 to 354 °C (49.8 to 54.1%), and 403 to 410 °C (14.2 to 15.4%). The pyrolysis of hemicellulose, cellulose, and lignin occurs at 220 to 315 °C, 315 to 400 °C, and 150 to 900 °C, respectively (Yang et al. 2007). Thus, the main building blocks of Pine, spruce, and fir needles were cellulose and lignin, with low hemicellulose content.

For cedar needles, there were additional peaks at 151 °C, 218 °C, 254 °C, and 269 °C, and the total quantity of pyrolysis products was 49.9%. However, above 300 °C (335 °C), these needles generated only 9.8% volatile products. Based on the study by Yang et al. (2007), the main decomposition product of cedar needles is hemicellulose. In addition, cedar showed a shift of the last peak maximum (which corresponds to the residue content) to the temperature of 430 °C, which was more than 20 degrees above the final decomposition temperature of the other needles. Decomposition of the cedar needles began as early as above 150 °C, which shows lower thermal resistance of cedar needles compared with the other samples. This result indicates the limitations of using this material as a filler in polyolefin composites.

The gaseous products released during the thermal decomposition of coniferous samples were analyzed qualitatively under inert atmospheric conditions using the FTIR method coupled with a TGA instrument. The spectra were recorded for the temperatures 25 °C to 900 °C. Three-dimensional (3D) infrared diagrams were obtained (Fig. 2).

The TGA/FTIR spectra showed similar distribution profiles for particular samples. However, there were some differences in peaks occurring at different times, at different wavenumbers, and with different intensities. The obtained spectra were separated at the first stage, and the decomposition products were identified on the basis of their typical wavenumbers (Table 3). The major decomposition products of the conifer needles were water, carbon dioxide, carbon monoxide, alkanes (mainly methane), methanol, and acetic acid.

Fig. 2. 3D spectrograms of volatiles from pyrolytic samples of (a) pine, (b) fir, (c) spruce, and (d) cedar

Table 3. Dominant Volatiles from Different Conifer Needles

Additionally, analyzing the position of successive bands revealed shifts for each needle sample. The characteristics of the observed changes were analyzed in relation to the position of the bands in a spectrum of the pine needle sample. The presence of water molecules was observed at the wavenumber of 1554 cm^-1. For fir, spruce, and cedar, the water peaks were found at slightly lower wavenumbers of 1531 cm^-1, 1520 cm^-1, and 1540 cm^-1, respectively. A similar trend was observed for CO₂, which was located at 2353 cm^-1 for pine and a bit lower for fir (2349 cm^-1), spruce (2333 cm^-1), and cedar (2325 cm^-1). CO was observed at 2188 cm^-1 for pine and at 2179 cm^-1 for spruce and cedar. Further changes were observed in the peaks corresponding to methanol, acetic acid, and alkanes. For pine needles, methanol was assigned to the peak at 1005 cm^-1, which was shifted to higher wavenumbers in fir (1056 cm^-1), spruce (1096 cm^-1), and cedar (1028 cm^-1). The presence of acids (mainly acetic acid) and alkanes (primarily methane) was associated with two peaks. For pine needles, these wavenumbers were 1157 cm^-1 and 1755 cm^-1 for acetic acid and 2932 cm^-1 and 3024 cm^-1 for alkanes. Similar to methanol, an upward shift was observed for the remaining samples. For the fir needles, these were 1166 cm^-1 and 1765 cm^-1 (acetic acid), with no alkane peaks. For spruce and cedar needles, peaks situated at 1188 cm^-1 and 1773 cm^-1 and at 1174 cm^-1 and 1771 cm^-1, respectively, represented acetic acid, and the wavenumbers 2991 cm^-1 and 2953 cm^-1, respectively, indicated alkanes. In turn, the bands with maxima at 3015 cm^-1 and at 3014 cm^-1 were shifted towards lower values compared with the pine needle sample.

Fig. 3. Gram-Schmidt curves of pyrolytic samples from (a) pine, (b) fir, (c) spruce, and (d) cedar

Table 4. Thermal Decomposition Products of Conifer Needles

A Gram-Schmidt analysis of the curves indicated the order in which various gases were released (Fig. 3). The time of release for decomposition products is shown in Table 4. The most intensive absorbance band over all spectra was attributable to CO₂. Interestingly, the first decomposition products (after evaporation of water) were alcohol and acid; for the pine needle sample, these products were identified after 19.4 min. The remaining samples were distinguished by the presence of two retention times, the first of which was lower compared with the time specified for the pine needle sample. This result may indicate a slightly higher resistance of pine needles to thermal decomposition. Finally, the decomposition of cedar needles started earliest of all samples, which confirms the earlier conclusion that this plant product offers the lowest thermal resistance.

The gaseous products generated during pyrolysis were derived from decomposition of the three main components of needles, i.e., cellulose (H₂O, CO₂), hemicellulose, and lignin (H₂O, CO₂, CO, CH₃OH, CH₃COOH) (Harun and Afzal 2014). TGA/FTIR analysis of torrefied lignocellulosic components indicates that torrefaction temperature and duration affects the breakdown of cellulose, xylans, and lignins extracted from spruce and poplar (Lv et al. 2015). The highest long-time weight loss was observed for cellulose. Thus, the tested types of needles with a cellulose content higher than the lignin content decompose slightly faster. This result was confirmed by the observed peaks as well as by the time of occurrence of the decomposition products. In both cases, the presence of CO₂ for the cedar sample was identified at the lowest wavenumber (2325 cm^-1), and the presence of H₂O and CO₂ was reported at the shortest retention times (6.5 min, 31.3 min). In the pine needles, the presence of H₂O and CO₂ at the highest wavenumbers (1554 cm^-1, 2353 cm^-1) and longest retention times (8.4 min, 32.5 min) reflected the highest thermal stability.

Based on the TGA and TGA/FTIR analysis, the needles derived from various conifers have slightly different chemical compositions. The lowest thermal resistance, and thus probably the greatest hemicellulose content, was found in cedar needles. Furthermore, the lowest residue content was observed in cedar needles (three times less than in the other samples). This result was also confirmed by the smallest lignin content (more thermally stable and more difficult to degrade). Moreover, pine and spruce needles had lower decomposition temperatures than fir needles, which may indicate higher resin contents. Based on these results, three (pine, spruce, and fir) of the four types of needles were selected because they displayed the appropriate thermal stability (up to 200 °C) for the production of thermoplastic polyolefin composites.

Fig. 4. Tensile strength for PELD and composites containing 10 wt.% or 30 wt.% of flour or particles of pine needles

Composition of Needle-Reinforced Polyolefin Composites

Pine, fir, and spruce needles were used as fillers in several polyolefin matrices (PELD, PEHD, co(E-P)). First, the impact of the filler particle size on the strength properties of the resulting composite was examined in PELD matrix composites, which contained flour or particles of pine needles (Fig. 4). The increased filler content resulted in a lower tensile strength of the composites, regardless of the size of the particles introduced (flour, particles), but especially at a higher filler content (30 wt.%). Regarding the mechanical properties, the samples containing a filler of larger dimensions showed slightly lower strength parameters, regardless of the filler content in the sample. In subsequent tests, therefore, the filler was always used in the form of shredded flour.

Fig. 5. Tensile strength of different composites with conifer needles

The tensile strength results for composites containing 30 wt.% of various needle flours, i.e., fir (F), pine (P), and spruce (SP), were compared with the corresponding values for neat polyolefins (PELD, PEHD, co(E-P)) processed in the same manner (Fig. 5). The type of introduced needles did not significantly affect the tensile strength. The composites based on the PEHD matrix demonstrated the highest tensile strength values, and so did the matrix itself. In this case, the use of the filler material in the form of needles did not reduce the tensile strength of the basic polymer. Moreover, the results were highly reproducible for each sample, which reflected good homogeneity of the materials. Thus, the PEHD-based composites contain pine needles flour were selected for further analysis (PEHD_P). Their properties were compared with those typical for WPC composites, which contained coniferous wood flour, i.e., commercial Lignocel C120 (PEHD_C120), and flour obtained from shredded pine-wood waste (PEHD_S).

Adding wood fillers to WPC composites improves the tensile strength when the filler content does not exceed 50 wt.% by weight (Bouafif et al. 2009). Any higher percentage of the wood filler is associated with increased fragility and thus deterioration of strength. Measurements of the tensile strength of the analyzed materials confirmed this correlation (Fig. 6).

The tensile strength value with respect to the polymer matrix (26.8 MPa) of composites with Lignocel C120 and pine-wood flour demonstrated a slight increase, and the composites with the flour of pine needles showed no change. Thus, all composites had strength parameters similar to PE even when there are filled up to 50% by weight.

Fig. 6. Tensile strength of PEHD and of various composites with 30 wt.% and 50 wt% pine needles, Lignocel C120, or pine flour

Fig. 7. Impact strength of PEHD and composites with 30 wt.% and 50 wt.% pine needles, Lignocel C120, or pine flour

The addition of various plant fillers to the polymer matrix reduces the impact strength of composites with an increase of the filler content in relation to the corresponding value for the polymer matrix (Safdari et al. 2011; Ayrilmis and Kaymakci 2013; Abu Bakar et al. 2016). Data on composites with coniferous fillers confirmed that correlation (Fig. 7). Moreover, the impact strength value decreased each time the filler content increased. When the characteristics were analyzed for each type of coniferous filler, the composites with conifer needles (pine) had a slightly better impact resistance than the corresponding composites filled with other flours; this effect was particularly noticeable with the composite containing the commercial flour Lignocel C120.

In an analysis of composite processability (Fig. 8), plant fillers lowered the MFR value, which indicated reduced flow properties. This effect was additionally enhanced by increased filler content, as previously noted (Jam and Behravesh 2007; Jafarian and Behravesh 2009). Regardless of the degree of filling, the composites with pine needles were positively characterized by the highest MFR value within the group of tested composites. The beneficial qualities (impact strength and processability) of pine needles flour composites compared with WPC composites (filled with commercial Lignocel C120 and with the flour obtained from shredded pine waste) probably resulted from the highest resin content in the filler, acting as a specific internal sliding agent (i.e., lubricant).

Fig. 8. MFR for PEHD and for different composites with 30 wt.% and 50 wt.% pine needles, Lignocel C120, or pine flour

Water Absorption

One significant parameter affecting the stability of materials, including those with hydrophilic raw material, is their vulnerability to water absorption. Water absorption tests of the developed composites confirmed that the plant filler was responsible for water absorption by the composites (Fig. 9). From nearly zero absorbance in the hydrophobic polymer matrix, the water absorption increased with increasing plant filler content (form 30 wt.% to 50 wt.%) in composites. Furthermore, the composites containing the pine needles flour demonstrated higher water absorption than those filled with wood flour (Lignocel C120 and pine-wood S). In composites with 30 wt.% filler content, these differences become more obvious during extensive research periods. For the materials containing 50 wt.% filler, the differences were manifested mainly in the initial period of the study (72 h). After that time, the composites with pine needles flour seemed to be saturated, while WPCs continued absorbing water. The observed differences indicate the increased hydrophilicity of the needle flours with respect to the wood flours (S and C120), which probably resulted from a higher content of resins in the needles, including the number of OH groups, alcohols, and phenols.

Fig. 9. Dependence between water absorption capacity of PEHD composites containing 30 wt.% or 50 wt% of plant fillers and the incubation time

Microscopic Analysis

To observe superficial and structural differences in the analyzed materials, microscopic images of the surfaces of composite samples and cross-sections of their fractures were taken with a scanning electron microscope (Fig. 10).

Fig. 10. Photomicrographs of composites surfaces (PEHD_P/30 – 1 a, PEHD_C120/30-2a, PEHD_S/30-3a) and their fractures (1b, 2b and 3b respectively)

The surface images indicated that composites with pine needles flour had a much lower density. The filler inclusions are visible together with the delaminated surface (Fig. 10.1a). For composites containing Lignocel C-120 (Fig. 10.2a) and the pine-wood flour (Fig. 10.3a), the above characteristics were observed also, but the size and extent were much lower. These results correlated with the results of water absorption tests. Higher water absorption was observed during the first h of experimentation, demonstrated by the materials with the pine needles filler (especially those of the filling of 50 wt.%), which may result from higher surface delamination. The weight gain of composites with Lignocel and with pine flour was much less rapid due to greater coherence at the surface. This effect was attributed to the higher hydrophilicity of the needles, which resulted in the inferior compatibility with the hydrophobic polymer matrix. However, after reaching a certain level of saturation, the composites with the pine needles flour demonstrated a significantly lower weight gain associated with water absorption (water absorption inhibition). Meanwhile, a steady and slow increase in the mass was observed for the remaining samples. This difference was attributed to the more compact internal structure of the composite with needles (Fig. 10.1b) compared with the other materials (Fig. 10.2b and 3b).

Structural Analysis

To assess the structural changes in the composite materials and to clarify the differences in the above-mentioned properties, thin films were subjected to the infrared FTIR analysis (Fig. 11).

Fig. 11. FTIR spectra of PEHD (blue line) and PEHD composites with 50 wt.% pine needle flour (red line), Lignocel C120 (green line), and pine flour (black line) from 4000 to 700 cm^-1. The most significant bands were marked, and their assignments are shown in Table 5.

Fig. 12. FTIR spectra of PEHD composites with 50 wt.% pine needle flour (red line), Lignocel C120 (green line), and pine flour (black line) from 1830 to 700 cm^-1

The comparative analysis of these spectra and the previously presented spectra of fillers allowed identification of the bands derived from plant fillers. There were some differences in the chemical structures of the analyzed composites, which are associated with the three different plant materials. Both qualitative and quantitative (changes in band intensities) changes were observed. For example, the band at about 3336 cm^-1, which is typical for the O-H stretching vibrations (hydrogen bonded), was not visible in the spectrum of the PEHD matrix. The fingerprint area (1800 to 850 cm^-1) of the vibrations derived from the building blocks of lignocellulosic biomass, i.e. lignin, hemicellulose, and/or cellulose, was especially interesting (Figs. 11 and 12). Table 5 describes the bands for the composite with pine needles (PEHD_P/50). Table 6 presents the collected data on the intensity of peaks in the spectra of all three composites.

Table 5. FTIR Analysis of the Fingerprint Area (1800 to 850 cm^-1) for PEHD_P/50

Note: assignments based on Müller et al. (2009), Emandi et al. (2011); Xu et al. (2013), and Catto et al. (2015)

Table 6. Relative Peak Height in the Range of 1750 to 1000 cm^-1

The band at 1735 cm^-1 represents hemicellulose. PEHD_P/50 had the highest relative peak height in relation to the internal standard peak (2020 cm^-1) at this wavenumber. However, a much higher peak intensity around 1510 cm^-1, typical for lignin, was demonstrated in composites with wood flours (PEHD_C120/50 and PEHD_S/50). However, it was impossible on that basis to unambiguously conclude that these two composites contained more lignin than the composite with the needles. The peak intensity depends on the amount of extractives, which may exhibit absorbance at around 1510 cm^-1. An example is benzoic acid, which is present in wood tannin and has an aromatic ring in its structure.

Additional bands were recorded at approximately 1250 cm^-1, which corresponds to the aromatic ring vibration in guaiacyl and syringyl residues of lignin (Fig. 12). For the PEHD_P/50 composite, the relative height of the peak at the wavenumber of 1265 cm^-1 (0.778), corresponding to the guaiacyl residues, was greater than the height of the peak at 1240 cm^-1 (0.739). It is similar for the PEHD_C120/50 composite, where the values amounted to 1.368 and 1.060. For PEHD_S/50, they amount to 0.682 and 0.498, respectively. The results confirmed that fillers obtained from conifers showed higher contents of guaiacyl residues (Müller 2009; Emandi 2011; Tomak 2014).

The spectra of the three composites also indicated the highest differences in the peaks between 1500 and 1750 cm^-1 (Fig. 13). From 1590 to 1650 cm^-1, four very weak peaks were observed in the spectra from composites with the commercial wood flour (PEHD_C120/50, green line) and shredded natural pine flour (PEHD_S/50, black line). In the spectrum of the needle composite (PEHD_P/50, red line), two much higher bands were observed. The higher intensity of the band in the range of 1630 to 1660 cm^-1, representing adsorbed OH groups and conjugated C-O groups resulted from the more hydrophilic nature of this raw material.

Fig. 13. FTIR spectra of PEHD composites with 50% pine needle flour (black line), Lignocel C120 (orange line), and pine flour (pink line) in the wavenumber range of 1850 to 1500 cm^-1

At the wavenumber of 1695 cm^-1, the PEHD_S/50 composite showed a band that was much less intense than in the PEHD_P/50 needle composite, and it practically did not exist in the sample containing commercial flour. Thus, the intensity (height) of the peak for PEHD_S/50, compared with the peak at the wavenumber of 1727 cm^-1, was nearly twice as high (1.54) as the corresponding figure for PEHD_P/50 (0.87) (Table 6). The peak at 1695 cm^-1 is associated with acid groups (carboxylic) held together by hydrogen bonds. This result indicated that their origin is not linked to the structure of cellulose or lignin, and it most likely resulted from other additives, e.g., natural resin (terpenoids and their derivatives) and phenolic compounds (phenolic acids and phenols). Pine resin is predominant in the resin canals of pine wood and needles. It consists of high fragility rosin (resin acids) and turpentine (pinene). The lack of this band in the PEHD_C120/50 sample may result from the elimination of these substances from the wood flour during industrial processing. The presence of resins in the applied plant fillers accounted for the previously diagnosed improvement in processing properties, i.e., MFR, and the percentage of acid components of the resin (fragility) explains the favourable increase in impact resistance.

A quantitative analysis was performed by measuring the relative areas of particular bands. For this purpose, the total area from 1830 to 842 cm^-1 was measured (Fig. 12; Table 7). The same method was used for measuring the surface area of peaks representing carbonyls (1807 to 1680 cm^-1), unsaturated groups (1680 to 1550 cm^-1), lignin (1510 to 1500 cm^-1), and cellulose (1373 to 1663 cm^-1).

Table 7. Relative Areas of the Various Molecules and the Total Fingerprint Area

As shown in Table 7, the size of the carbonyl area increased in the following sequence: PEHD_C120/50 < PEHD_S/50 < PEHD_P/50; the area of unsaturated groups increased as follows: PEHD_S/50 < PEHD_C120/50 < PEHD_P/50. Thus, the composite with pine needle flour contained about twice as many carbonyl groups as the other two composites, and many more unsaturated groups (approximately twice as many as PEHD_C120/50 and even three times more than PEHD_S/50). The carbonyl band is mainly associated with hemicellulose or other compounds containing acid groups (Table 5, band at 1695 cm^-1), which are derived from resin compounds in the plant filler. The area of unsaturated groups was primarily associated with lignin.

Table 8 shows the percentage of surface area corresponding to lignin and cellulose in the analyzed fingerprint area. The lignin content increased in the following sequence: PEHD_C120/50 < PEHD_S/50 < PEHD_P/50, and the content of cellulose/hemicellulose increased as follows: PEHD_P/50 < PEHD_C120/50 < PEHD_S/50. To accurately determine the proportion of cellulose and lignin, the cellulose and hemicellulose/lignin ratio (R^FTIR) (Emandi, 2011) was determined, taking the surface area from the range of 1373 to 1363 cm^-1 as the value corresponding to the cellulose content, and the surface area from the range 1510 to 1500 cm^-1 for the lignin, which for the PEHD_P/50 composite amounted to 1.165. As shown in Table 8, this ratio decreased in the following sequence: PEHD_C120/50 > PEHD_S/50 > PEHD_P/50. Thus, the composites containing Lignocel C120 had the highest content of cellulose/hemicellulose in proportion to the amount of lignin. The highest amount of lignin in relation to cellulose was found in the composites with pine needle flour.

Cellulose has very good strength properties, and therefore the aim of introducing plant fillers into a polymer matrix is to improve this parameter (Lao et al. 2014). Analysis of the mechanical properties showed that the PEHD_S and PEHD_C120 composites demonstrated similar tensile strength, which was higher than the PEHD_P composites. Furthermore, PEHD_S and PEHD_C120 contained more cellulosic material than PEHD_P, and the cellulose/hemicellulose-to-lignin ratio was also higher in these composites (2.196 and 2.457) than in PEHD_P/50 (1.165). This data could explain the observed differences in the strength parameters. Summarized obtained quantitative data became the basis for the explanation of the differences in the functional properties of composites under comparison.

Table 8. Lignin and Cellulose Contents and the R^FTIR Index Value

CONCLUSIONS

Coniferous needles from pine, spruce, fir, and cedar were evaluated as fillers in composites based on thermoplastic matrices. TGA/FTIR thermal analysis showed that the major decomposition products of the analyzed plant materials include water, carbon dioxide, carbon monoxide, alkanes (mainly methane), methanol, and acetic acid. Based on the results, pine, spruce, and fir needles were selected to be used in composites. Cedar needles were rejected because of insufficient heat resistance, which prevents effective application in processing with the use of a thermoplastic polymer.
The different types of needles were not much different as fillers in a polymer matrix (PELD, PEHD, co(E-P)), and the most favorable form was fine particles (flour).
A comparative study of the composites based on the PEHD matrix containing 30% or 50% of the pine needle flour with the composites containing various types of coniferous fillers, i.e., commercial Lignocel C120 (a mixture of coniferous trees) or pine-wood flour (obtained from pine waste products), indicated that the composites of conifer needles (pine) had increasing water absorption, and similar strength properties. However, they showed a slightly better impact resistance than the corresponding composites filled with other flours. Regardless of the degree of filling, the composites with pine needles were positively characterized by the highest MFR value among the group of composites tested.
The beneficial qualities (impact strength and processability) of the composites with the pine needle flour compared to two types of WPCs (filled with commercial Lignocel C120 and the flour obtained from shredded pine waste) probably resulted from the highest content of resins in the filler, which acted as a specific, internal sliding agent (i.e., internal lubricant). This conclusion was confirmed by the FTIR analysis. Furthermore, the analysis indicated qualitative differences in the structures of tested composite fillers relating to their contents of cellulose and lignin. The obtained quantitative data became the basis for the explanation of the differences in the functional properties of composites under comparison.
The results of the study indicated that the shredded, unmodified coniferous needles with the sufficient thermal resistance (pine, spruce, fir) can be successfully used as fillers in composites based on thermoplastic polymers as an alternative and/or supplement to the wood flour used in the manufacture of WPCs.
The results indicate a new area of use of wood waste (in the form of needles of coniferous trees), which is development of various types of high-performance materials.

ACKNOWLEDGMENTS

The authors thank Katarzyna Merkel, Ph.D., and Joanna Lenża, Ph.D., from the Department of Material Engineering of the Central Mining Institute in Katowice for conducting the TGA/FTIR analysis.

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Article submitted: March 12, 2016; Peer review completed: May 2, 2016; Revisions accepted: May 18, 2016; Published: June 2, 2016.

DOI: 10.15376/biores.11.3.6211-6231