Commingled composites of polypropylene/coir-sisal yarn: effect of chemical treatments on thermal and tensile properties

2.1 Materials

The matrix used for the study is PP, which was supplied by Superfil Products Ltd. (Chennai, India), in monofilament form and has the following properties (average values are given): diameter, 0.500 mm; denier, 1609; and percentage of elongation, 25%. The monofilament form of PP is preferred here as we used commingling method for the fabrication of the composite, which requires both the matrix and reinforcement to be in the fibrous form. Moreover, when we used fibrous form of PP rather than granules, the melt flow distance of PP can be reduced as fibers were uniformly distributed throughout the composite. Coir/sisal blended yarn was used as the reinforcement. The sample was collected from Central Coir Research Institute, Alleppey, India. The given sample which was a blend of coir and sisal comprises 80% coir (by weight) and 20% sisal and has 968 m/kg and 10.40% runnage and elongation, respectively. Potassium permanganate (KMnO₄, Merck, Mumbai, India), toluene diisocyanate (TDI, Merck Mumbai, India), maleic anhydride modified PP (MAPP, Sigma Aldrich, US), and vinyltrimethoxy silane (VTMO, Acros Organics Geel, Belgium) of AR grade were used for the chemical treatments.

2.2 Extraction of fibers

The coir research institute, Alleppey, developed a pollution-free technology called castor oil treatment, to process coir fiber. This method is based on vegetable oil-in-water emulsion that converts the mechanically separated coir fiber into a yarn. A thin uniform layer of cleaned coir fibers was treated with an emulsion made from castor oil, water, and non-ionic detergent stabilized by urea. It was then covered with plastic sheet for 24 h and then spun into yarn. The advantage of this method over traditional method is that it does not require soaking/retting and there is no weight loss of bit fiber. It is a zero effluent process and causes no harm to aquatic life (4).

Sisal fibers can be extracted by a method known as decortication. In this method, the leaves are crushed and beaten by a rotating wheel set with blunt knives. The fiber is washed with water then dried, brushed and baled.

2.3 Fabrication of composites

Commingled composite fabrication was done by using a specially designed winding machine. The PP and coir-sisal blended yarn was wounded on a metal plate in a particular pattern, and the plate was then compression molded. There are certain processing parameters that are to be considered while fabricating the laminate. Temperature, pressure, and holding time are optimized as 210°C, 0.5 MPa, and 9 min, respectively. The resulting material was cooled at room temperature, and samples were taken for thermal and tensile studies. The size of the laminate is 15 cm×15 cm with a thickness of approximately around 2–3 mm. The fiber contents used are 13.06 wt%, 20.89 wt%, and 31.20 wt% of coir-sisal yarn. A sketch of the winding machine and the winding pattern of the laminate adopted for the study are illustrated in Figure 1.

Figure 1:

(A) Sketch of the winding machine; (B) winding pattern of the laminate.

2.4 Fiber surface treatments

Various surface treatments have been done to improve the compatibility between the polymer matrix and the natural fiber reinforcement, as the matrix usually is hydrophobic, while the cellulosic reinforcement is hydrophilic; i.e., the former is non-polar and the latter is polar. This incompatibility may lead to deterioration in the overall properties of the composite. In order to overcome this demerit, various chemical treatments such as with KMnO₄, MAPP, VTMO, and TDI were used. Chemical treatments improve the adhesion between the matrix and reinforcement (12, 13). All the treatments were done during the winding process. KMnO₄ treatment was done with 0.5% solution of KMnO₄ in acetone. Two percent solution of vinyltrimethoxy silane in water-ethanol (4:6 volume ratio) mixture was used for silane treatment, a 2% solution of MAPP in acetone was used for MAPP treatment, and a 2% TDI in chloroform was used for TDI treatment. After each treatment the sample was placed in a hot air oven for 1–3 h and then compression molded.

2.5 Thermogravimetric studies and tensile properties

The thermal stability of the commingled composite was studied using thermogravimetric analysis (TGA). Both treated and untreated samples were analyzed. The instrument used is Perkin Elmer (STA 6000) TG Analyzer. TGA was done according to American Society for Testing and Materials (ASTM) D2288, under nitrogen atmosphere with samples weighing 10–20 mg. The samples were placed in crucibles, and the test was done over a temperature range of 40–750°C at a scan rate of 10°C/min. The tensile test was done using Tinius Olsen H 25K-5 UTM according to ASTM D3039. The selected cross head speed was 2mm/min and with a preload of 0.5 N. Rectangular specimens having 10 cm length and 1.5 cm width were used for the analysis. An average of five tensile values were used to plot the graph.

2.6 Fourier transform infrared spectroscopy and scanning electron microscopy

Perkin-Elmer 400 IR spectrophotometer was used for recording the Fourier transform infrared (FTIR) spectra. Both treated and untreated samples were analyzed according to ASTM E1252. ATR technique was used, as the samples can be examined in the solid state without any preparation. The spectra were recorded from 450 to 4000 cm^-1. JEOL JSM–6390 scanning electron microscope was used for recording the scanning electron microscopy (SEM) images of the sample. The fractured surface of both treated and untreated samples and the surface morphologies of treated fibers were recorded.

3.1 Thermogravimetric analysis

Composite fabrication by commingling technique is much easier and effective compared to other fabrication methods. As mentioned in Section 2.3, there are certain parameters which are to be considered especially at the time of compression molding. Temperature is one such parameter. A temperature below 210°C was found to be insufficient for the proper wetting of the natural fiber by the molten matrix, and above this temperature there is a possibility for the natural fibers to get decomposed. Holding time is another important parameter, and we found that a low holding time is insufficient for the proper flow and wetting of the natural fiber. Long holding time can lead to degradation of fibers. Another important parameter is the pressure. At low pressures, air can be trapped inside the laminate creating voids, and at a higher pressure, flow of matrix from the plates occurs and fiber orientation may get altered. In this study we found out that the optimum processing conditions required are 210°C, 0.5 MPa pressure, and a holding time of 9 min. The laminate obtained after cooling was subjected to various studies, and the results are discussed below.

The thermal stabilities of both treated and untreated composites were estimated by TGA studies. The TGA curves in Figure 2 clearly show that the thermal stability of neat PP is higher than that of coir-sisal blended yarn. Chemically treated composites show thermal stability between that of PP and coir-sisal blended yarn. Among them MAPP treated composites has much higher thermal stability than other treated ones. The single-stage decomposition of PP, shown in Figure 2, is due to the breaking of saturated and unsaturated carbon bonds present in it (14). All the composite and coir-sisal blended yarn shows a three-stage decomposition. Natural fibers mainly contain cellulose as its major constituent. The thermal stability of cellulose is due to hydrogen bonding, which can distribute the thermal energy over many bonds. At a temperature range of 75–175°C, dehydration as well as degradation of lignin occurs. In the second stage, i.e., at 325°C, a gradual degradation occurs that may be due to depolymerization of hemicelluloses and cleavage of glycosidic linkages of cellulose. In the third stage, i.e., 360°C and above, rapid volatilization happens, and products formed are levoglucosan and char (15).

Figure 2:

The (A) TGA and (B) DTG curves of coir-sisal yarn, PP, treated and untreated composite. TGA and DTG curves of (a) coir-sisal yarn, (b) untreated composite, (c) KMnO₄ composite, (d) VTMO treated composite, (e) TDI treated composite, (f) MAPP treated composite, and (g) PP.

Chemical treatments can influence the thermal stability of composites to a considerable extent. This is mainly due to the increased interfacial adhesion between the matrix and the reinforcement (16). The thermogram clearly shows that MAPP treated composite is more stable than the other treated composites. The compatible blend formed between the PP segment of MAPP and the bulk PP present in the composite reacts with coir-sisal yarn, thereby forming covalent bonds across the interface between the coir-sisal yarn and PP surface which in turn increases the interfacial adhesion between them (17, 18).

The DTG curves and the temperatures corresponding to major weight loss are given in Figure 2. DTG curves show a broad peak in the region 300–450°C, which is due to the degradation of hemicellulose. Figure 2 reveals that the thermal stability of treated composite is improved significantly, which may be attributed to increased interfacial adhesion of fiber and matrix.

3.2 Tensile properties

3.2.1 Effect of fiber content

The tensile strength and tensile modulus obtained for composite having various fiber content is shown in Figure 3(A). From the figure it is clear that as the fiber content increases both tensile strength and tensile modulus increase. The tensile strength obtained for unreinforced PP laminate was 23.5 MPa, whereas with 13.06 wt% fiber content the tensile strength increased to 26.9 MPa. Further increase in fiber content to, 20.89% and 31.2% increases the tensile strength to 27.2 and 27.5 MPa, respectively. The increase in tensile strength is due to the high strength and reinforcing ability of coir-sisal yarn present in the composite.

Figure 3:

Variation of tensile properties by (A) fiber content (B) various chemical treatments on PP/coir-sisal composites.

Tensile modulus is also dependent on the fiber content. With fiber contents of 13.06%, 20.89%, and 31.20% the tensile modulus varied as 1080, 1249, and 1260 MPa, respectively. This increase is attributed to the rigidity imparted by the reinforcing fiber.

3.3 Theoretical modeling

Researches in the area of composites have dealt with calculations of the tensile strength and Young’s modulus. Several models have been used for these calculations. A reliable and exact measurement of tensile strength of the reinforcing materials is an essential pre-requisite while using these models (20). The parameters like fiber composition, fiber dispersion, fiber geometry, and the degree of interfacial adhesion between fiber and matrix can influence the mechanical properties of the composite to a greater extent (21). There are a number of theories and equations in the literature that can explain the relationship between these parameters and mechanical properties of the composites.

3.3.1 Parallel and series models

In the parallel model, tensile strength of the composite is calculated using the equation

[1]Tc=TfVf+TmVm [1]

where V_f and V_m are the volume fractions of the fiber and matrix, respectively. T_c, T_f, and T_m are the tensile strength of composite, fiber, and matrix, respectively.

In the series model, the equation used for the calculation of tensile strength is given as

[2]Tc=TmTfTmVf+TfVm [2]

In the series model, stress was assumed uniform in both matrix and fiber, whereas in the parallel model, isostrain conditions exist for both matrix and fiber.

From Figure 4, it is clear that the theoretical values and experimental values are almost the same in the case of the series model, while the parallel model shows a positive deviation from the experimental value. This is due to the uneven distribution of stress between non-aggregated and aggregated fibers, and the assumption of uniform strain implied in the parallel model made it oversimplified (6).

Figure 4:

Comparison between experimental and theoretically predicted value of tensile strength of untreated composite as a function of fiber loading.

3.4 FTIR spectral analysis of coir-sisal yarn

The various fiber surface treatments have been characterized by FTIR spectroscopy. Figure 5 gives the FTIR spectra of untreated and chemically treated yarn.

Figure 5:

FTIR spectrum of untreated and chemically treated coir-sisal yarn.

The peak in the range 3100–3340 cm^-1 is due to the OH bond from the cellulose, hemicelluloses, and lignin present in coir and sisal. The peak at 1641 cm^-1 corresponds to the aldehydic group present in lignin. All the chemically treated composites show a similar trend in the infrared spectrum. The peak corresponding to OH group in cellulose disappeared/diminished indicating that the cellulosic OH reacts with various chemical reagents. As a result, the hydrophilicity of coir-sisal fiber is decreased largely, which in turn increases the interfacial adhesion between the matrix and the reinforcement. A peak at 1722 cm^-1 in MAPP treated composite is due to the anhydride and carboxylic acid group, and in KMnO₄ the peak corresponding to 1634 cm^-1 is due to the aldedyde group.

3.5 SEM analysis

Surface morphology of treated and untreated composite was studied using SEM images.

A uniform arrangement of microfibrils can be observed for untreated coir-sisal yarn [Figure 6(A)]. Chemical treatments remove waxes and other impurities present in the surface of coir-sisal yarn and thereby make the surface more rough compared to untreated one.

Figure 6:

SEM micrographs of (A) untreated, (B) MAPP treated, (C) KMnO₄ treated, (D) VTMO treated, and (E) TDI treated coir/sisal yarn.

The fractured surfaces of both treated and untreated PP/coir-sisal yarn composite were analyzed. From the image (Figure 7), it is clear that the compatibility between the matrix and reinforcement is less in the case of untreated composite. The fiber pullout reveals that there is no interfacial adhesion between the matrix and reinforcement. The SEM image of treated composite clearly shows a reduction in fiber pullout as chemical treatment improves the interfacial adhesion. Failure of treated samples occurs due to the breaking of reinforcement.

Figure 7:

SEM micrographs of (A) untreated PP/coir-sisal composite (B) VTMO treated composite.