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Use of preimpregnated sisal yarn in woven reinforced polypropylene sheets: thermoformability and mechanical properties.

INTRODUCTION

Cellulosic natural fibers are being incorporated into polymeric matrices to reduce cost and to improve mechanical properties. This trend is being driven mainly by government regulations on waste dumping that promote the use of renewable and recyclable materials. Natural sisal, jute, flax, hemp, aspen, coconut, or banana fibers cannot match the mechanical properties of synthetic glass, carbon, or Kevlar in high performance applications. However, there are applications for which the mechanical requirements exceed the properties of the unfilled plastics and in which the use of synthetic fiber is not a cost-efficient alternative; then thermoplastics reinforced with natural fibers become a cost-efficient option.

In previous work, we have studied the injection molding process for manufacturing composites made out of polypropylene (PP) and sisal fiber (SF), compatibilized with maleic anhydride grafted polypropylene (MA-g-PP), by using a thermoplastic impregnation process to obtain a continuous SF/PP rod. Pellets containing long fibers (~13 mm), obtained by cutting the impregnated pultruded rods, were dry-blended with regular PP pellets at the injection machine hopper, and SF/PP composites with excellent mechanical properties were directly obtained by injection molding [1].

The automotive industry's interest in woven, natural fabric-reinforced thermoplastics (WNFRT) is increasing due to their excellent mechanical-specific properties and competitive raw materials cost. However, production costs may become high because of technological requirements; to obtain acceptable composites properties, good fibers impregnation and matrix-fiber adhesion are needed. Currently available processing technologies that render good fiber impregnation and matrix-fiber adhesion are still expensive. The high viscosity of molten polyolefins and the lack of interfacial adhesion between the nonpolar polyolefins and the hydrophilic sisal fiber surfaces cause poor fiber impregnation and weak interfacial adhesion. The high viscosity of PP is assumed to be responsible for poor penetration of the liquid PP into the fibers bundles, thus diminishing the SF/PP contact area. Much better fiber surface wetting is observed when MA-g-PP is added to the composite matrix [2]. This coupling agent supplies polar functional groups that increase the fiber-matrix adhesion by an esterification reaction between cellulosic fiber hydroxyl groups and the anhydride functionality of MA-g-PP [2-9]. Besides, the MA-g-PP displays a much lower viscosity that improves the liquid matrix penetration into the fibers bundles. A major drawback for the use of MA-g-PP is its high cost, which may reach about four times the price of polyolefin matrix; therefore, every effort made to reduce the amount of MA-g-PP used in composites will significantly reduce the final cost.

In this work, a low-cost scheme is shown to produce a continuous sisal fiber fabric-polypropylene composite with excellent mechanical properties that may be thermoformed to an acceptable level. A single-screw extruder and a specially designed die were used to impregnate a continuous SF yarn with a blend of a low-viscosity PP and MA-g-PP. The expectation is to achieve good SF yarn impregnation quality, improve the fiber bonding, and reduce the total mass fraction of expensive MA-g-PP in the final composite [1,10]. The yarn thus obtained was knitted to make impregnated SF fabrics, and compression-molded between two PP sheets to obtain a sheet of SF/PP-reinforced compound. Deep cups were thermoformed from the WNFRT sheets. Excellent quality cups were obtained for adequate thermoforming processing conditions. The fiber content of the composites was measured after polyolefin extraction in a high-temperature Soxhlet. The composite's mechanical properties were determined. Formability of the sheet-molded compound was studied by using a punch-and-ring system and a universal testing machine.

EXPERIMENTAL

Materials

SF (Agave Sisalana) yarn with a fiber diameter of about 100-200 [micro]m was obtained from Brascorda Co. (Brazil). The fibers were used for this work as received, without any surface modification or chemical treatment. Under this condition, a single fiber has a tensile strength of 628 MPa, and a tensile modulus of 16.142 GPa (ASTM D3379-75).

MA-g-PP (1.0 wt% MA) (Polybond 3200, produced by Crompton Europe) was used as a coupling agent.

Commercial PP (Cuyolen-1102KX) was supplied by Petroquimica Cuyo.

The organic peroxide used to lower the PP average molecular weight and melt viscosity was 2,5-di (tert-butylperoxy)-hexane, with [T.sub.1/2] = 4 minutes at 170[degrees]C, supplied by Akzo Nobel Quimica S.A.

Low Viscosity PP and Blended Pellet Preparation

The PP pellets (Cuyolen-1102KX) were impregnated with 9000 ppm of 2,5-di(tert-butylperoxy)-hexane. The peroxide was previously dissolved in enough hexane, which was later removed by evaporation.

The peroxide-impregnated PP was melted, mixed, extruded, cooled, and pelletized using a single-screw extruder at 180 & C and with a residence time of 1.5 minutes.

Melt modified PP (mPP) and MA-g-PP were mixed and pelletized to obtain a MA-g-PP/mPP blend containing 20 wt% of MA-g-PP.

Samples Preparation

Continuous sisal yarns were pulled through a melt impregnation die especially designed for this work. The extrusion die consists of a single rotating disk, powered by an external electric motor, rotating into a polymer melt pool. The action of the single powered disk impregnates the sisal yarn with molten polymer supplied by a single-screw extruder (21 mm diameter, L/D = 24) at low pressure (about 0.2-0.4 MPa). The low-pressure operation significantly reduces fiber breakage, and allows a fast yarn impregnation speed (between 10 and 20 m/min). A simple schematic of the process is shown in Fig. 1. The thermoplastic pultrusion line was operated with an extruder mass flow of 0.55 Kg/h, the line speed of the sisal yarn was between 10 and 20 m/min, and the rotating speed of the disk was 250 rpm.

From the impregnation process, continuous SF yarns, impregnated either with mPP or with MA-g-PP/mPP blend, were obtained. These impregnated yarns were used to make a woven structure in the form of an orthotropic, bidirectional fabric. These fabrics were hot compression-molded between thin sheets of unmodified PP. Tensile specimens, flexural specimens, and circular specimens for thermoforming were cut and machined from the woven SF/PP composite sheet (W) and from the woven SF/PP/MA-g-PP composite sheet (W-MA). Tensile and flexural specimens were cut, always with fibers aligned parallel to the main stress direction.

A blend of 60 wt% of long SF and 40 wt% of PP, and a blend of 60 wt% of long SF, 35.75 wt% PP, and 4.25 wt% MA-g-PP were prepared in a Brabender mixer. The fibers were added to a PP melt at 180[degrees]C and blended at a rotor speed of 50 rpm for 10 min. The blends ware taken from the mixer while hot, and then compression molded. Tensile specimens, flexural specimens, and circular specimens were cut and machined from the nonwoven SF/PP composite sheet (NW) and the nonwoven SF/PP/MA-g-PP composite sheet (NW-MA).

Fiber Content Measurement

The fiber content of the impregnated yarn and for the molded composite sheet were measured. The polyolefins were dissolved in a high-temperature Soxhlet extraction system, with boiling hot xylene acting for several hours. The fibers were then recovered by filtration, washed, dried, and weighted.

[FIGURE 1 OMITTED]

Characterization of the Interface Adhesion

Mechanical properties, infrared spectroscopy, and optical observation of hot compression-molded specimens were used to study the adhesion between fibers and the matrix.

Tensile and flexural specimens were cut from the hot compression-molded sheets. Stress-strain curves were calculated from the force-displacement graphs measured in an Instron 4467 Universal Mechanical Testing Machine. Tests were conducted under ASTM D 638 and ASTM 790 specifications.

Fourier transform infrared (FT-IR) spectra of SF before and after the three-step (extrusion-knitting-compression molding) process were measured with a Mattson FT-IR unit, Model Genesis II. The sisal fibers were recovered from the compression-molded sheets by dissolving the polymeric matrix in boiling hot toluene for several hours. The absorbance IR spectra were recorded in the 2000-1500 [cm.sup.-1] range with a resolution of 2 [cm.sup.-1], with 100 scans for each spectrum. FT-IR has been also used by other authors to study the interfacial adhesion between natural fibers and MA-g-PP [5,9].

Scanning electron microscopy (SEM) was used to study the break surfaces after mechanical testing.

Thermoforming Capability Evaluation

An evaluation of the hot formability of the sheets was performed by stamping a cup on a circular blank. The apparatus consists of a two-half holder ring and a concentric punch mounted on an INSTRON 4467 testing machine (Fig. 2). A similar instrument has been used by Bhattacharyya et al. [11] for the same purpose.

Circular specimens cut from the compression molded sheets were placed between the holder ring halves. This assembly was then heated in an oven. Once the desired temperature was achieved, the assembly was taken from the oven and placed at the testing machine, centering the holder ring with the punch. The sheet material was forced through the die using the punch at a crosshead speed of 200 mm/min. The punch force and displacement were recorded, and the forming energy was calculated.

RESULTS AND DISCUSSION

Fiber Content Determination

Results from high-temperature Soxhlet extraction indicate that the SF content for the yarn impregnated with molten polymer is 74.35 wt%. The MA-g-PP content corresponding to the extracted 25.65 wt% polymer is 5.13 wt%.

Table 1 shows the SF content obtained by Soxhlet extraction technique for all the composites used for this work. For NW-MA and W-MA, the MA-g-PP contents were calculated from the SF contents in the composites and the estimated MA-g-PP content in the impregnated sisal yarn. For NW and NW-MA, the lengths of the extracted fibers were measured and the average value obtained was 35.6 mm.

Tensile and Flexural Properties

Table 1 also shows the tensile and flexural properties for all the composites used for this work, and also for unfilled PP. The incorporation of 61 wt% of SF into the PP matrix increases the composite flexural modulus, tensile modulus, and the tensile strength. Large differences are observed between the woven and the nonwoven composites and between the SF/PP composites compatibilized with MA-g-PP and the SF/PP composites not compatibilized with MA-g-PP.

[FIGURE 2 OMITTED]

Figure 3 shows stress-strain data from three-point-bending tests for all the composites and the PP used for this work. At a first glance we can observe that for nonwoven and also for woven composites, the addition of 4.2% of MA-g-PP causes large increases in modulus and strength, and also restricts the maximum attainable strain before failure.

For the case of nonwoven composites without MA-g-PP, the incorporation of SF into the PP matrix increases the tensile modulus by 31%, increases the flexural modulus by 32%, and decreases the tensile strength by 44% compared with unfilled PP. When the coupling agent is not used, the mechanical properties are not improved, and the SF acts only as a filler to reduce raw material cost by about 80%, because the PP is approximately three times more expensive than SF. The addition of 4.20 wt% of MA-g-PP to the nonwoven composites increases the raw materials costs by about 75%, but compared with unfilled PP the tensile modulus increases by 137%, the flexural modulus increases by 160%, the tensile strength increases by 77%, and the raw material cost is still close to unfilled PP.

For the woven SF/PP composites, the incorporation of woven SF into the PP matrix increases the tensile modulus by 157%, increases the flexural modulus by 216%, and increases the tensile strength by 77% when compared with unfilled PP. The addition of 4.20 wt% of MA-g-PP to the woven composites also increases the cost of raw materials by about 75% but the tensile modulus increases by 477%, the flexural modulus increases by 600%, the tensile strength increases by 300%, and the raw material cost is still close to unfilled PP. The woven fabric orthogonal structure provides a much larger fraction of fibers aligned parallel to the testing direction.

[FIGURE 3 OMITTED]

For rigid component applications, the best improvement in mechanical properties is obtained for the case in which a woven SF, impregnated with a small quantity of MA-g-PP, is used. The use of SF well-impregnated with PP is also a very attractive, low-cost way to improve the mechanical properties of PP.

Interfacial Adhesion Characterization

Figure 4 shows the FT-IR spectra at the 2000-1500 [cm.sup.-1] region, for the SF before the extrusion impregnation process, and for the SF extracted from W-MA specimens. The characteristic peak at 1742 [cm.sup.-1] found for the SF extracted from W-MA corresponds to ester bonds. The esterification of the maleic anhydride groups of the MA-g-PP with the cellulosic hydroxyl groups is responsible for a better fiber/matrix adhesion, and allows a better stress transfer and distribution inside the composite. This is the reason for the improvements of the tensile and flexural properties for SF/PP composites when MA-g-PP is added.

[FIGURE 4 OMITTED]

Figure 5a and b shows SEM micrographs of rupture surfaces corresponding to tensile nonwoven composite specimens. Figure 5a corresponds to NW (without MA-g-PP). Figure 5b corresponds to NW-MA (with MA-g-PP). The SF surface shown in Fig. 5a has no bonded PP, and the fibers have been pulled out from the PP matrix without fiber rupture. The SF surface shown in Fig. 5b shows some PP still bonded after the specimen rupture; this image shows a strong interfacial adhesion that in some cases causes the SF breakage during tensile test.

Thermoforming Evaluation

Figure 6 shows the force-displacement curves for W-MA composites thermoformed at different blank-forming temperatures using the INSTRON machine and the apparatus shown in Fig. 2. For all the testing conditions used, the maximum forming force is achieved just before the punch starts forming the cylindrical zone of the cup -8 mm of crosshead displacement. At blank-forming temperatures above the matrix melting point, the W-MA composites sheets can be easily thermoformed with low stress, the punch shape is copied exactly, and good surface finish is obtained. At temperatures below the matrix melting point, forming requires much more stress and energy, the punch shape is not copied well, and the surface finish obtained is poor.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Figure 7 shows that PP sheets can be successfully thermoformed at temperatures above 140[degrees]C, and NW-MA and W-MA sheets can be successfully thermoformed at temperatures above 170[degrees]C. Below these temperature ranges the sheets break or cannot copy exactly the shape of the punch.

Unfilled PP can be thermoformed at 25[degrees]C below the PP peak melting temperature, because at that temperature the crystals can easily be sheared at low stress. Above the PP melting point, the unfilled PP sheets flow into the heating oven before the action of the forming punch. This way the forming temperature range can be defined as 140-160[degrees]C. From 100[degrees]C and up to 140[degrees]C, unfilled PP can be thermoformed with severe surface damage.

[FIGURE 7 OMITTED]

W-MA and NW-MA composites can be thermoformed only above 150[degrees]C, because the real deformation applied to the polymer matrix is much larger for the composites than for the unfilled PP. The SF network restricts the free flow of the hot polymer matrix, and the sheets can be heated and conformed at higher temperatures with much better surface finish.

Thermoformed cups walls are 6.65% thinner than the original unfilled PP flat sheet. The thinning for W-MA is 2%, and for NW-MA it is 2.6%. This is related to the fact that the SF network (woven or nonwoven) restricts the elongation suffered by the unfilled PP sheets during the thermoforming process. The thinning reduction prevents excessively thin and weak areas in the final parts, and this fact is considered a distinctive advantage for the proposed method.

Figure 8 shows the forming mechanical energy (FME) for PP, NW-MA, and W-MA sheets at different blank-forming temperatures. The FME is calculated from the force-displacement curves. At temperatures below the PP melting range the FME is high because much of this energy is used to deform the polymer matrix sheet. Increasing the blank-forming temperature, the FME diminishes exponentially due to the much lower yield stress of the polymer matrix sheet. Entering the PP melting range the FME falls dramatically, because the PP matrix becomes a liquid and can flow and easily copy the shape of the punch.

CONCLUSION

In this work a thermoplastic pultrusion system was designed and developed. Continuous SF yarns (75 wt%) were impregnated to obtain continuous thin rods with a melt blend of polypropylene (20 wt%) and MA-g-PP (5 wt%) at high-speed production. These impregnated yarns were knitted to obtain a woven structure in the form of an orthotropic, bidirectional fabric, and compression molded between thin sheets of unmodified PP to obtain woven sisal fabric-reinforced PP sheets. Due to the presence of the coupling agent and the bidirectional array of the fibers, the flexural modulus increases 600%, the tensile modulus increases 475%, and the tensile strength increases 300% compared with unfilled PP. The composite sheets were successfully thermoformed at a temperature between 150-190[degrees]C, by stamping a cup on a circular blank to obtain a 3D shape. Above the matrix melting range, the woven composites were thermoformed with very low forming energy and excellent surface finish.

[FIGURE 8 OMITTED]

The use of this nonconventional four-step process (impregnation-knitting-compression-molding-shaping) enables rapid automated production of 3D composite structures with excellent mechanical properties, excellent surface finish, short cycle time, and controlled raw material cost. The use of impregnated yarns followed by knitting and thermoforming (W-MA) results in products that show twice the Young's modulus and twice the tensile strength of similar composites in which the fibers are randomly oriented (NW-MA). The previous impregnation of the SF yarns allows excellent wetting of the fibers with excellent fiber/matrix adhesion, keeping the need for expensive MA-g-PP to a minimum. Knitting the previously impregnated yarns is necessary to obtain cheaper 3D thermoformed products with much better properties. The presence of the knitted reinforcement allows efficient thermoforming with very low wall-thickness reduction, thus improving the quality of the products.
TABLE 1. Measured properties for all composites.

Properties/composite PP NW NW-MA W W-MA

Sisal fiber content (%) 0 61.0 60.9 60.4 61.2
MA-g-PP content (%) 0 0 4.20 0 4.22
Flexural modulus (GPa.) 1.140 1.504 2.956 3.611 7.968
 ASTM D 790
Tensile strength (MPa) 24.287 13.578 42.987 50.361 96.57
 ASTM D 638
Tensile modulus (GPa) 1.286 1.686 3.054 3.301 7.423
 ASTM D 638


ACKNOWLEDGMENTS

We thank Dr. Jose Carella for useful discussions. C.J.P. thanks CONICET for a Post-Doctoral Scholarship.

Contract grant sponsor: Agencia Nacional de Promocion Cientifica y Tecnologia (ANPCYT); contract grant number: PICT 99-14-07247.

REFERENCES

1. L.M. Arzondo, A. Vazquez, J.M. Carella, and J.M. Pastor, Polym. Eng. Sci., 44, 1766 (2004).

2. K.L. Fung, R.K.Y. Li, and S.C. Tjong, J. Appl. Polym. Sci., 85, 169 (2002).

3. R. Gauthier, C. Joly, A.C. Coupas, H. Gauthier, and M. Escoubes, Polym. Compos., 19, 287 (1998).

4. R. Karnani, M. Krishnan, and R. Narayan, Polym. Eng. Sci., 37, 476 (1997).

5. M. Kazayawoko, J.J. Balatinecz, and L.M. Matuana, J. Mater. Sci., 34, 6189 (1999).

6. X.L. Xie, R.K.Y. Li, S.C. Tjong, and Y.W. Mai, Polym. Compos., 23, 319 (2002).

7. K.L. Fung, R.K.Y. Li, and S.C. Tjong, J. Appl. Polym. Sci., 85, 169 (2002).

8. S. Mohanty, S.K. Verma, S.K. Nayak, and S.S. Tripathy, J. Appl. Polym. Sci., 94, 1336 (2004).

9. J.M. Felix and P. Gatenholm, J. Appl. Polym. Sci., 42, 609 (1991).

10. K.L. Fung, X.S. Xing, R.K.Y. Li, S.C. Tjong, and Y.-W. Mai, Compos. Sci. Technol., 63, 1255 (2003).

11. D. Bhattacharyya, M. Bowis, and K. Jayaraman, Compos. Sci. Technol., 63, 353 (2003).

L.M. Arzondo

Instituto de Investigaciones en Ciencia y Tecnologia de Materiales (INTEMA) (UNMdP-CONICET); Departamento de Ingenieria en Materiales, Facultad de Ingenieria, Universidad Nacional de Mar del Plata, Juan B. Justo 4302, 7600 Mar del Plata, Republica Argentina

C.J. Perez

Instituto de Investigaciones en Ciencia y Tecnologia de Materiales (INTEMA) (UNMdP-CONICET), Facultad de Ingenieria, Universidad Nacional de Mar del Plata, Juan B. Justo 4302, 7600 Mar del Plata, Republica Argentina

Correspondence to: C.J. Perez; e-mail: cjperez@fi.mdp.edu.ar
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Author:Arzondo, L.M.; Perez, C.J.
Publication:Polymer Engineering and Science
Geographic Code:3ARGE
Date:Jul 1, 2005
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