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Natural fibers can be used as reinforcements in non-structural applications of thermoplastic composites (TPC). Commingling provides an intimate blend of matrix and reinforcing fibers. In this study, polypropylene (PP) and textile cotton fibers are commingled and fabricated to composite laminates. Process variables like temperature, pressure and holding time were found to affect the laminate qualities and also the mechanical properties. Fiber content, number of plies in a laminate and winding pattern or fiber orientation of a particular laminate is also important for the optimization of the mechanical properties. The modification of the interface by potassium permanganate, dicumyl peroxide and maleic anhydride modified PP enhanced the tensile properties of cotton fiber reinforced polypropylene commingled composite systems.
Thermoplastic matrix composites (TPC) possess many advantages compared to thermoset matrix composites (TSC). Some of them are their higher fracture toughness, unlimited shelf life, good solvent resistance, recyclability and fast, clean and automated processability. But the higher viscosity of the molten thermoplastics (500-5000 Pa.s) compared to thermosets (100Pa.s) make some difficulties during the processing steps . This affects the wetting and impregnation of the reinforcing fibers by the molten matrix and hence lowers the consolidation quality of a composite laminate. So some partially impregnated intermediate materials are required for the more efficient manufacturing of TPCs . One way to get good impregnation is to use commingled yarns, which gives close contact between the reinforcement and matrix fibers . This results in smaller flow paths for the molten TP matrix during the processing stage. Commingling can be done either by the intimate mixing or by the side by side mixing of matrix and reinforcing fibers. Better impregnation can be attained by selecting thermoplastic matrix fibers and reinforcement fibers having comparable diameters .
The consolidation consists of three important stages: coalescence of molten matrix, flow of matrix within and out of the reinforcing tows and void reduction by the application of pressure. The entrapped gases dissolve into the matrix in the void reduction phase .
According to Long et al.  the main phases during the consolidation of commingled yarns are:
1. Coalescence of the matrix as the thermoplastic fibers melts
2. Application of pressure, forcing the individual plies into intimate contact at the matrix/matrix interface
3. Compaction of the tows, causing longitudinal flow for matrix present within a tow and transverse flow for matrix in the inter-tow spaces and
4. The void reduction phase, the mechanism of which is a combination of void volume reduction and dissolution of entrapped gases into the matrix.
They also studied the effect of compaction rate, temperature and pressure on the consolidation of the commingled fabric made up of glass and polypropylene. An increase in compaction rate increased the amount of void at the end of the consolidation phase but the application of pressure during cooling resulted in a dramatic reduction in void content.
Wakeman et al.  experimentally studied the effect of compression moulding parameters on the consolidation quality of commingled composite laminates made up of glass and polypropylene fibers and found that the preheat temperature had the greatest effect on the macro and micro mechanical properties of the laminate. Ye and Friedrich  studied the effect of processing conditions on the mode I and mode II interlaminar fracture toughness of the commingled yarn based glass and polypropylene composites and proved that rapid cooling process resulted an improvement in the values of toughness. Low cooling rate results in coarse spherulites and large voids in the resin rich areas. But rapid cooling conditions results in a lower degree of crystallinity and smaller spherulitic size. Several models have been developed to explain the impregnation and consolidation of commingled yarn composites. They can be used to predict the time required to completely wet the reinforcing fibers and to remove the void present in the laminate. Ye et al.  used the following assumptions for modeling the impregnation behavior:
-Low shear rate gives Newtonian flow and flow is laminar,
-Darcy's law governs the impregnation into fiber tows and fiber permeability is a function of fiber volume fraction,
-Flow takes place in the plane of the cross section of the reinforcing fibers, and
-Capillary and gravity effects have no significant role in the flow of molten matrix.
Natural fibers offer good opportunities as reinforcement materials for composites as they have good mechanical properties. These renewable raw materials have low density and possess high specific strength and modulus. They have ecological and environmental advantages over man-made reinforcements. Low cost, biodegradability, non-abrasiveness, generation of rural/ agricultural economy etc. are the added advantages of natural fibers in the composite manufacturing industry [11-13]. Need of low processing temperatures, possibility for moisture absorption and poor wettability and adhesion with synthetic resins are their disadvantages. These factors may lower their strength, stiffness and environmental resistance. Various treatments and modifications are possible to increase the strength and environmental resistance of natural fiber reinforced plastic materials [14-16].
Gu and Liyan  analysed the tensile strength and the failure mechanism of flax yarn and PP composites fabricated from their commingled yarn developed by twisting the filaments in a fancy twister. Sherely et al. of our group used a periodical method to estimate the thermophysical properties like thermal conductivity, thermal diffusivity and specific heat of polypropylene (PP)/banana fiber commingled composites at room temperature as a function of the banana fiber loading and for different chemical treatments given to the banana fiber. Reich et al.  used various ways to improve the properties associated with polypropylene/sisal fibers composite through different modifications to fiber. Modification was aimed to improve the adhesion of matrix to fiber by overcoming the hydrophilic nature of the fiber was attained by removing the hydroxyl group present mainly in the cellulose component in the fibers. MartÄ±´nez et al.  used methods to compatiblise commingled plastics with maleic anhydride modified polyethylenes and by ultraviolet preirradiation. Sui et al. fabricated polypropylene (PP) based composites using a novel plant fiber, sunflower hull sanding dust using a twin-screw extruder and characterized the thermal and mechanical properties.
In this study textile cotton yarn was used to develop hybrid yarns with PP and hence to fabricate commingled composite laminates. Cotton plant (Gossypium) is a shrub native to tropical and subtropical regions around the world including the Americas, India and Africa. The fiber is most often spun into thread and used to make soft textile clothes. When processed to remove seeds and traces of wax, proteins etc., the cotton consists almost entirely of pure cellulose. The cellulose is arranged in such a way that it gives a high degree of strength and durability to cotton fibers. Each fiber is made up of 20-30 layers of cellulose units coiled in a series. The kinking and interlocking makes them ideal for spinning.
The aim of this work is to analyze the tensile properties of PP/cotton commingled composites with reference to processing variables like temperature, pressure and time, reinforcing fiber content and their orientation and chemical modification with benzoyl peroxide, potassium permanganate and maleic anhydride modified PP. To the best of our knowledge not many studies have been performed on the commingled thermoplastic composites reinforced with natural fibers.
Sanghi Polyesters Limited, Andhra Pradesh, India, kindly supplied the Polypropylene (PP) fibers used for this study. They have their denier value in the range 790-800, percentage elongation of 26.6 and tenacity of 3.33 gpd. The number average molecular weight of polypropylene was 5.2x104 g/mol and weight average molecular weight was 3.1x105 g/mol. Commercially available 'Vardhman' cotton threads (ART A-501) of Mahavir Spinning Mills, Ludhiyana, India were used as reinforcement fibers. They have a specific density of 1.520, a breaking strength of 3.0-4.9 g/denier and breaking elongation of 8-10 %. Other reagents used were of analytical grade.
Fabrication of composite laminates and study of the effect of process variables
To meet the difficulty of mingling the natural fibers and PP with proper fiber alignment, we designed and developed a winding machine (Fig.1) to commingle and to wind them on a metal plate of 1.25 mm thickness. Initially the spools of the two different fibers were placed on the holders of the machine. They were then allowed to mingle at a common point and by rotating the handle of the machine it was wound on the metal plate. When the winding reaches at one end of the plate, the lever is used to change the direction of the winding. The square shaped plate can be put in 90 0 opposite direction to the initial winding. For tensile analysis, a [04/904] winding pattern with cotton fiber weight content of 19.2 % was adopted. It means the first four layers are in one direction and next four layers are perpendicular to it. The plate with the windings of mingled cotton and PP was then compression molded under the specified conditions. It was then taken out and cooled under normal room conditions and cut into laminates. The molding temperatures opted for the studies were 180, 195, 210 and 220 0C at a holding pressure of 0.6 MPa for 6 minutes.
To study the effect of pressure on the tensile behavior of cotton / PP hybrid yarn system, [04/904] laminates were fabricated by applying pressures of 0.2, 0.6, 1.0, and 1.4 MPa with a mold temperature of 205 0C for 6 minutes. At a mold temperature 205 0C and at a pressure of 0.6 MPa, laminates were kept for compression molding for the time intervals of 3, 6,9,12 and 15 minutes to prepare specimen with 19.2 % cotton content for analyzing the effect of residence time on tensile properties.
Effect of fiber content, number of layers and layer pattern
To study the effect of fiber content on the tensile properties of [04/904] laminates of cotton/ PP composites, both fibers were mingled in the ratio as given in the Table 1. The corresponding mass fraction of cotton is also given.
Numbers of layers used to study its effect were varied from [03/903] to [07/907] by adding one more winding in 2:1 ratio in both directions. Various fiber orientation patterns were also adopted to fabricate eight layer composite laminates with 19.2 percent cotton. These laminates were then compression molded for a holding time of 6 minutes, under a pressure of 0.6 MPa at 205 0C.
Chemical treatments and modifications
Potassium permanganate(KMnO4) and benzoyl peroxide (BP) treatments
[04/904] layer pattern with 19.2 weight percentage of cotton was adopted to fabricate composite samples to study the effect of various treatments. During winding each layer is cleaned with acetone to remove impurities from the fiber surfaces. A 0.1% solution of KMnO4 in acetone was applied on each layer. The sample was put in a hot air oven at 40 0C for half an hour to remove acetone from the surface of the fibers. It was then processed at 210 0C under 0.6 MPa pressures for 6 minutes. The same method was followed for the treatment with BP.
Treatment with maleic anhydride grafted polypropylene(MAPP)
The number average molecular mass of maleic anhydride grafted polypropylene used for coupling was 7000 g/mol. 0.1 g of dicumyl peroxide (DCP) was dissolved in 100 ml of acetone and 2 g of MAPP was added and applied on each layer during their winding on metal plates.
Tensile testing of rectangular specimen of size 100 mm x 10 mm x 0.5 mm and gauge length 40 mm were carried out according to ASTM D-3039 standard using a Universal testing machine TNE series 9200 at a cross head speed of 2 mm per minute. The tensile modulus and elongation at break of the composite were also determined. At least five specimens were tested for each set of samples and the mean values were reported. Unreinforced PP specimen were also tested under the same test conditions. A Shimadzu IR-490 spectrophotometer was used to analyze the change in the chemical structure of the treated samples. Fracture surface of the treated and untreated samples were analyzed by a scanning electron microscope (JOEL 35C model) to observe the changes in the surface morphology.
Results and Discussion
Effect of process variables
Effect of temperature on [04/904] commingled composite laminate with 19.2 percentage weight of cotton was studied with an applied holding pressure of 0.6 MPa for 6 minutes. The values of tensile break stress and tensile modulus at various temperatures are given in Fig.2. A processing temperature below 190 0C is not sufficient to wet the fibers completely and showed very weak consolidation qualities and tensile properties. But further increase of temperature reduces the viscosity of the molten PP according to the equation 
η= η∞exp(C/T) (1)
where η∞ = 0.564 and C= 3315 K for PP.
The tensile break stress value of samples prepared at 180 and 195 0C were low and it denotes the lack of enough temperature to reduce the viscosity and to further impregnate the system well. But a higher temperature of 210 0C cause proper flow of molten PP and consolidation quality was good enough to increase the tensile break stress value to 41 MPa. Too high temperatures cause the degradation of natural fibers like cotton and also of PP. Tensile modulus also showed a similar trend with a maximum value of 1375 MPa at 210 0C
Figure 3 shows the effect of processing pressure on the tensile break stress values of cotton /PP commingled fiber composite system. From the graph it is clear that pressure is essential to provide a good consolidation quality to the laminate. Pressure removes the entrapped air and volatiles from the melt and forces the molten matrix resin to fill the voids and interfiber spaces. The maximum break stress value is obtained at a heating pressure of 0.6 MPa for the samples prepared at 210 0C for 6 minutes. A further increase of pressure causes the outward flow of molten PP from metal plates and paves way for a gross misalignment of fibers.
Holding time is another factor that may affect the mechanical properties of the laminates. As evident from Fig. 4, a low holding time is insufficient for the proper flow, wetting and impregnation of the reinforcing fibers by the matrix resin. But too high holding times at a higher processing temperature cause the degradation of natural fibers like cotton. Surface yellowing and irritating odor were resulted by the degradation of fibers during the processing steps. In this particular study the optimum holding time was found to be in the range of 7-9 minutes at a processing temperature of 205 0C and at a holding pressure of 0.6 MPa.
Effect of fiber content, number of layers and layer pattern
The tensile strength at break of PP laminate fabricated from its own fibers was 22 MPa (Fig.5). When PP and cotton were mingled and winded in the ratio 3:1, corresponding to a cotton fiber weight percentage of 13.7, the break stress had increased to 30 MPa. When winded in 2:1 ratio (cotton weight percentage of 19.1) the value raised to 41 MPa. It again increased to 47 MPa when PP- cotton mixing ratio was 1:1 with the cotton weight percentage of 32.2. 1:2 ratio mingling and winding of PP and cotton gave a break stress of 50 MPa.
The tensile modulus, which is a measure of the rigidity of a composite system, is also dependent on the fiber content. Unreinforced PP has a tensile modulus of 820 MPa .When cotton fiber was introduced into it in the form of commingled yarn at the weight percentage of 19.2, the modulus value increased to 1660 MPa (Fig.5). Thus fiber imparts some rigidity to the system. On further weight increase to 32.2 and 48.8%, the tensile modulus value increased to 1750 and 1810 MPa. The tensile break stress value increases gradually and proportionally as the layer numbers has increased from [03/ 903] to [07/907] by adding one more cotton/ PP winding in a step (Fig.6). This may be due to the addition of more quantities of densier reinforcement into the system.
The winding pattern of an eight-layer laminate also influences the tensile properties of cotton/ PP commingled composite system. In this study tensile property was measured in the direction in which majority of the reinforcements align. Fig .7 gives the variation of tensile break stress with different winding patterns. Unidirectional fiber system gives the maximum tensile strength in the fiber direction. As the amount of transverse fibers increases, the property decreases. The eight layer laminate with alternate 0 and 90o winding of mingled cotton/ PP yields the minimum tensile break stress of 28 MPa. Many voids were clearly visible as this winding pattern creates more space for matrix filling. Presence of voids reduces the consolidation quality of a composite laminate.
Effect of chemical treatments
Interface has an important role in improving the fiber matrix adhesion. Fig.8 gives the variation of tensile break stress by different chemical treatments of the laminates with 19.2% weight of cotton. The treatment with KMnO4 enhances the tensile strength value from 41 MPa to 46 MPa. The mechanism of the permanganate induced grafting of polypropylene on cotton fiber is given in Scheme 1 .
Scheme 1: Mechanism of permanganate induced grafting of PP on cotton fiber
On treating with MAPP, the PP segment forms a compatible blend with the bulk PP through co-crystallization. The polar part, that is, the acidic anhydride group forms hydrogen bonds and chemical bonds with the hydroxyl group of the cellulose through esterification. Thus the coupling agent is strongly anchored on the fiber . The mechanism of the above is given in Schemes 2 and 3.
Scheme 2: The mechanism for the formation of maleic anhydride modified PP
Scheme 3: Model for the formation of the interphase between MAPP and hydroxyl group of cellulose present in a natural fiber.
The FTIR spectrum of MAPP treated cotton/PP commingled composite is given in Fig.9. The broad peak near to 3000 cm-1 gives the presence of unreacted cellulosic hydroxyl groups of cotton fiber. The other two peaks at 2913 and 2838 cm-1 are due to the stretching of -C-H- bonds present PP and maleic anhydride. The esterification reaction between the cellulosic hydroxyl group of cotton fiber and the anhydride part of MAPP gives the -C-O- stretching frequency at 1455 cm-1.The -C-H- deformation causes the peaks at 1375 cm-1.
Treatment with MAPP is also helpful in reducing the fiber hygroscopicity as it decreases the quantity of highly hydrophilic hydroxyl groups of the cellulose. Benzoyl peroxide and dicumyl peroxide are free radical generators and induces the formation of radicals on fiber and matrix and favors the coupling.
From Fig. 10, it is clear that chemical treatments increase the tensile modulus of the cotton/ PP composite system compared to the untreated one. In untreated system the stiffer PP matrix surrounds the fiber but on treatment, the system becomes more rigid and hence the modulus of the composite increases. Introduction of cotton fiber in a weight percentage of 19.2 increases the modulus of virgin PP from 847 MPa to 1660 MPa. Treatment with potassium permanganate raises the value to 1820 MPa while that with MAPP puts into 1950 MPa for 19.2 percentage cotton reinforced PP composite system.
In MAPP, maleic anhydride groups are grafted on to the PP backbone. The increase in tensile strength is due to the esterification reaction between the cellulosic hydroxyl group of cotton fiber and the anhydride part of MAPP, which causes a reduction in the interfacial tension and an increase in the interfacial adhesion between the PP and cotton fiber. The increase in properties is due to the reduction in both the fiber pull out and the debonding of fiber and matrix. The SEM photos given in Fig. 11 (a) and (b) show reduction in fiber pull out by the chemical treatment with the MAPP compared to the untreated samples. The improved adhesion by fiber-matrix bonding transfers the stress efficiently from the matrix to fiber and leads to an improvement in the reinforcing effect.
Theoretical modeling of tensile properties
A number of theories and equations can be used to predict the tensile properties of a composite material. They are given below.
1. Modified rule of mixture
According to this rule, the ultimate strength of the composite,
where Tm is the matrix strength at the failure strain of the fiber, Tf is the ultimate strength of fiber, Vf is the fiber volume fraction and Vfe is the effective fiber volume fraction.
where p is the degradation parameter denoting the effectiveness of fiber volume fraction with value lying between 0 and 1 and also,
where ΔTc is the difference between the experimentally measured strength and the strength predicted by the rule of mixtures.
2. Parallel and series models
According to these models,
(Parallel model) (5)
(Series model) (6)
where Tc, Tm and Tf are the tensile strength of the composite, matrix and fiber respectively. These equations can also be used to predict the modulus values of the composite from the moduli and volume fractions of the fiber and matrix.
3. Hirsch model
It is a combination of parallel and series model. According to this,
where x is the parameter that determines the stress transfer between the fiber and the matrix. Modulus can also be predicted using this relation.
Figure 12 gives a comparison between the experimental and theoretical value of tensile strength of untreated composites as a function of fiber loading. On analyzing the figure, it is very clear that the theoretical values and experimental values are almost the same in the case of the modified rule of mixtures. This is because of incorporating the term p, the fiber degradation parameter, for the effective fiber volume fraction of the composite system. Series model shows a large negative deviation from the experimental values at high fiber loadings. The parallel model gives a small positive deviation while Hirsch model yields a small negative deviation. Since the models used to explain the tensile properties do not consider the factors like the presence of voids, fiber-fiber interactions, non-uniformity in the shape of reinforcement, growth of transcrystalline layer at the fiber-matrix interphase etc., the theoretical values may deviate from the experimental results. Further optimization of process variables can be done with the help of experimental designing.
Natural fibers can be used as a substitute for plastic reinforcements in non-structural applications of thermoplastic composite systems. Commingling of cotton fibers with PP fibers decreases the melt flow distance of the molten resin. Due to the chance for void reduction, a good consolidation quality for the fabricated laminate can be expected. The ultimate mechanical properties depend very much on the processing parameters like temparature, pressure and holding time. The study involves the optimization of the process variables of cotton fiber reinforced polypropylene based commingled composite systems. A temparature greater than 200 0C is essential for the proper flow and for the complete wetting of the fibers by the resin. The tensile strength was the highest for the laminate fabricated at 210 oC. Sufficient pressure should be applied during the compression molding to ensure the removal of entrapped air and other volatiles from the laminate. Processing time, fiber content, numbers of layers or plies in a laminate and winding pattern or fiber orientation are the other factors that affect the tensile strength during the fabrication. Of the various treatments, modification with maleic anhydride modified polypropylene was most effective in increasing the tensile strength of cotton/PP commingled composite system. Theoretical modeling was also used to predict the tensile properties.