Enhancement of mechanical, thermal and water uptake performance of TPU/jute fiber green composites via chemical treatments on fiber surface

2.1 Materials

The commercial thermoplastic polyurethane was purchased from FKA Merquinsa, Barcelona, Spain under a trade name of Pearlthane^® ECO D12T85. This eco-grade TPU consists of biomass with 46% content and it has a density of 1.15 g/cm³ according to producer. Bangladesh oriented jute fibers (JFs) used in this study and they were obtained from the local supplier in rowing form. JFs were chopped into 3-4 mm length before surface treatments and compounding steps. Reagent grade sodium hydroxide (NaOH) and ether solvent were supplied by Sigma Aldrich, Missouri, USA. Silane coupling agent was γ-aminopropyl triethoxy silane (APTES) and solvent was ethanol were purchased from Merck AG, Darmstadt, Germany. Eco-grade epoxy resin was supplied from Ferrer Dalmau SA, Barcelona, Spain under the trade name of Super Sap CPM. It has bio-content of 47.5% and a density of 1.09 g/cm³ indicated by producer.

2.2 Surface treatments of JF

JF were washed to remove impurities and dried at 80°C for 24 h using vacuum oven (FN 055/120, Nuve AS, Ankara, Turkey). The neat JF sample was named as JF. Alkaline treatment was applied to JF by continuous mixing in 2 wt% water solution of NaOH for 2 h. The excess of NaOH was eliminated after washings 3 times with water. This portion was dried at 80°C in vacuum oven overnight and named as Na-JF. Silanization of JF was applied using similar modification routes in literature (38, 39, 40). During silane treatment, Na-FF was mixed in 2 wt% solution of APTES/ethanol for 2 h. The excess of silane contamination was removed by washing 3 times with ethanol. Silane modified JF was named as Si-JF. Another portion of Na-JF was subjected to epoxy modification via mixing in 4 wt% solution of epoxy/ether for 1 h. After washings 3 times with ether, sample was air-dried at room temperature. Epoxy treated portion was named as EP-JF.

2.3 Preparation of composites

TPU and JF samples were dried at 100°C for a period of 8 h to avoid possible moisture before extrusion process. Composites were fabricated using lab-scale twin screw extruder (15 mL micro-compounder, DSM Xplore, Netherlands) at constant loading level of 30 wt%. Processing temperature, screw speed and mixing time parameters were 200°C, 100 rpm and 5 min, respectively. The unfilled TPU was also mixed under the same processing conditions. Test samples of composites were shaped using injection molding instrument (Micro-injector, Daca Instruments, UK) in which barrel temperature of 205°C and injection pressure of 5 bar were applied. Injection molded dog-bone shaped specimens with the dimensions of 7.4 × 2.1 × 80 mm³ according to ASTM D-638 standard (41) were obtained from injection molding process.

2.4 Characterization methods

Fourier transformed infrared spectroscopy (FTIR) measurements in attenuated total reflectance (ATR) mode were performed using IR-spectrometer (Bruker VERTEX 70, Massachusetts, USA). The measurements were done at a resolution of 2 cm^-1 with 32 scans from 600 cm^-1 to 3800 cm^-1 wavenumbers. Elemental analysis was studied with energy dispersive X-ray spectroscopy (EDX) technique during examination of selected SEM microphotographs (JSM-6400 Electron Microscope, JEOL Ltd, Tokyo, Japan) of JF samples. Tensile test measurements of TPU and TPU/JF composites were performed by using Lloyd LR 30 K (West Sussex, UK) universal tensile testing machine in accordance with the ASTM D-638 standard. 5 kN load cell and 5 cm‧min^-1 crosshead speed were applied during tensile test. Tensile strength, elongation at break and tensile modulus parameters were recorded as an average of five samples. Digital Shore hardness tester (Zwick R5LB041, Ulm, Germany) was utilized in order to evaluate A-type Shore hardness values of TPU and composites. Measurements were carried out according to ASTM D2240 standard (42). DMA analyses were done by conducting DMA 8000 (Perkin Elmer, Massachusetts, USA) dynamic mechanical thermal analyser at temperature range between -120°C and 120°C in dual cantilever bending mode at a constant frequency of 1 Hz and heating rate of 5°C‧min^-1. MFI values of TPU and its composites were measured by Coesfield Meltfixer LT, Dortmund, Germany. Test measurements were conducted under the standard specified load of 2.16 kg at process temperature of 200°C. Melt flow rate results represent an average value of at least ten weighted samples with standard deviations. The characterization of water uptake values of TPU and composites was performed by immersing samples in a water bath at room temperature according to ASTM D570 procedure (43). Test samples were periodically taken out from the water, wiped with tissue paper to avoid their surface water, weighed and put back into the water repeatedly during 30 days period. Morphological characterizations of composites were examined by JSM-6400 field emission scanning electron microscope, JEOL Ltd, Tokyo, Japan. Surfaces of cyro-fractured samples were coated with a thin layer of gold in order to obtain conductive surface. SEM micrographs were taken at various magnifications from ×250 to ×5000.

3.1 EDX and FTIR analyses of JF surface

The elemental analysis data and SEM micrographs of fibers obtained from the SEM/EDX analysis are represented in the Table 1 and Figure 1, respectively. It can be seen from the Table 1 that oxygen content of unmodified JF increased by 9.58% after alkaline treatment due to formation of hydroxyl groups on the surface of Na-JF. Silicon content also rose with 3.46% at the surface of Si-JF attributed to the silane coupling agent. Carbon concentration of EP-JF observed as 6.13% higher compared to JF which indicates the presence of epoxy groups on the fiber surface. As can be seen from Figure 1, alkaline treatment caused fibrillation for untreated JF. Surface of JF became smoother after epoxy treatment.

Figure 1

SEM images of JF samples.

FTIR spectra of pristine JF and surface treated JF samples are given in Figure 2. The characteristic absorption bands from 400 cm^-1 to 1800 cm^-1 range display the oxygen functionalities, which owing to C–O stretching at broad peak between 900 cm^-1 and 1200 cm^-1, COO– asymmetric stretching and C=O stretching vibrations around 1620 cm^-1 and 1730 cm^-1, respectively (44,45). Increments on absorption of these oxygen related peaks for Na-JF were clearly observed. Intensities of these peaks were relatively lower in the case Si-JF which indicates the chemical modification of silane coupling agent with the surface hydroxyl groups. The intensity of band at 1420 cm^-1 which stem from COO– group slightly increases after alkaline treatment. The band at 1270 cm^-1 assigned to C–O and COO– groups of hemicellulose which was observed as remarkably more intense for untreated JF (46,47). Broad band that centered at 3300 cm^-1 is attributed the stretching hydroxyl group (48). Si-JF and EP-JF exhibited lower transmittance for that band due to the modification of hydroxyl groups of cellulose and lignin portions of JF with silane coupling agent and epoxy resin, respectively. The peak around 870 cm^-1 is stem from the characteristic Si-O vibration which is observed in the spectrum of Si-JF samples (49, 50, 51). These findings proved that chemical changes were taken place on the surface of JF during chemical treatments.

Figure 2

FTIR spectra of neat and modified JF samples.

3.2 Tensile properties

Tensile test data of composites before and after water immersion including tensile strength, percentage strain and tensile modulus are listed in Table 2 and representative strength-strain curves of TPU and its composites are shown in Figure 3. Tensile test results of samples water absorption test applied are indicated with (WA) in Figure 3. According to Table 2, additions of JF and Na-JF caused reduction for tensile strength of neat TPU with the contents of 4.8% and 3.2%, respectively. On the other hand, Si-JF and EP-JF inclusions yield slight improvements in tensile strength values by displaying 0.3% and 0.6% increase, respectively. Percent elongations of composites were found to be much lower than that of TPU. Almost identical tensile modulus values were obtained for Na-JF and unmodified JF filled composites with modulus of neat TPU. On the other hand, silane and epoxy treated JF containing composites showed remarkable increase in tensile modulus. Enhancements of tensile parameters by the inclusion of surface modified natural fiber to TPU matrix were also reported in the literature (38,40,52). It can be clearly seen from Figure 3 that, TPU/JF and TPU/Na-JF composites showed necking behavior.

Figure 3

Stress-strain curves of TPU and composites.

In the case of tensile test data obtained after water uptake test, the immersion of unfilled TPU into water caused no remarkable change in its mechanical properties. On the other hand, relatively lower strength values were observed for composites after water immersion. This reduction trend may due to the hydrophilic nature of JF which is common for all of the natural fibers (53,54). Percentage strain of composites displayed increase by water immersion which indicates the plasticizing effect. Based on the comparison of tensile test parameters before and after water ageing, silane treated JF containing composite exhibited the lowest changes thanks to JF gained hydrophobic character after silanization process (55).

3.3 Shore hardness test

Shore hardness is a characteristic parameter for elastomers and related composites. According to the A-type Shore hardness values displayed in Table 3, all of the composites gave around 10 points higher values than that of unfilled TPU. Si-JF addition led to the maximum improvement for shore hardness values among all of the composites by exhibiting 12 points higher value. The similar finding was obtained from the recent study in the literature that surface modification of natural additive led to increase in Shore hardness of TPU (56).

3.4 Thermo-mechanical performance

Storage modulus and Tan δ curves as a function of temperature were shown in Figures 4a and 4b, respectively. Storage modulus curve of TPU became broadened after Si-JF and EP-JF additions. Relatively lower storage modulus values were observed for Na-JF and JF containing composites. Silane treated JF containing composite gave the highest storage modulus stem from the restriction of chain motions after inclusion of Si-JF into polymer phase (57, 58, 59, 60). The peak point of Tan δ curve corresponds to glass transition temperature (T_g) of polymer. According to Figure 4b, incorporations of JF caused increase in the peak value of Tan δ regardless of treatment types. As a similar with previous results, silane and epoxy treated JF containing composites exhibited enhancement of T_g values. As indicated in Figure 4b, T_g of unfilled TPU shifted to 17°C higher after Si-JF inclusion.

Figure 4

DMA curves of TPU and composites.

3.5 Melt-flow behaviors

MFI parameter provide processing related information of polymeric material in the case of melt blending technique. MFI values of TPU and relevant composites shown in Figure 5. All of treatments resulted in increase for MFI value of neat TPU. The highest improvement was observed on TPU/Si-JF composites with 8.42 points increase since enhanced surface interactions between silane covered surface of JF and TPU were established by the help of the treatment process (61, 62, 63).

Figure 5

MFI values of TPU and composites.

3.6 Water absorption measurements

The water absorption test was conducted for composite samples in time interval of 30 days in order to evaluate their potential use for out-door applications. Water uptake values of TPU and composites can be seen in Figure 6. Nearly 1% water uptake was achieved for unfilled TPU within a few days and it remained constant throughout the test. This result is found to be in accordance with the single phase diffusion model described by Fick’s law since the diffusion of water yield weight gain of material (64,65). The pristine JF loaded composite sample reached nearly 7% water absorption value at the end of the test. Additions of alkaline and epoxy treated JF yield slightly lower water uptake values compared to unmodified JF. On the other hand, the reduction of water uptake for silane modified JF containing sample was found to be the most distinctive among composites. Silane layer donated the hydrophobic character on the JF surface (66,67). For this reason, higher water repellency was obtained for TPU/Si-JF with respect to other composites.

Figure 6

Water uptake curves of TPU and composites.

3.7 Morphological characterization

Morphological properties of composites were examined by representative SEM micrographs of selected samples which are shown in Figure 7. According to SEM micrograph of neat JF filled composite, large gaps were formed between JF and TPU phase due to their incompatibility. Alkaline treated JF showed fibrillation and relatively better adhesion to polymer matrix. It can be clearly seen from Figure 7 that silane and epoxy treated JF surfaces were covered by TPU because of the improved adhesion. Si-JF displayed excellent adhesion to polymer matrix thanks to the increase of interfacial interaction between two phases after treatment process. Improvement adhesion between natural fiber and polymer matrix after surface treatments have been indicated in similar studies in the literature (37,52,60,67,68). These findings proved the enhancement for related properties of composites described in earlier sections.

Figure 7

SEM micro-images of composites.