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Mechanical and Morphological Characterization of PVC Plastisol Composites With Almond Husk Fillers

J.E. Crespo, L. Sanchez, F. Parres, J. Lo´pez Mechanical and Materials Engineering Department, Polytechnic University of Valencia, Valencia, Spain

The present study develops new composite materials derived from environmentally friendly material, based on lignocellulosic fillers combined with a thermoplastic matrix. Almond husk, obtained as a by-product of the agrifood industry, has been used as a filler, combined with PVC thermoplastic matrix. This composite type (lignocellulosic material/thermoplastic matrix) is the object of this work for the advantages that it offers in environmental protection terms. With a view to identifying the degree of influence of filler amount, plasticizer concentration, and filler particle size on the properties of this new composite, we tested its mechanical properties and analyzed tensile fracture surfaces using scanning electron microscopy. POLYM. COMPOS., 28:71–77, 2007. © 2007 Society of Plastics Engineers

INTRODUCTION In recent years, an increasing number of research papers have dealt with thermoplastic matrix composites containing natural cellulose fillers such as wood flour [1, 2], hemp [3], jute [4, 5], kenaf [6], and other natural fibers [7–9]. Apart from the fact that these natural fillers are inexpensive, they are also of particular interest because their biodegradability properties help to reduce environmental degradation [9 –11]. Composites filled with natural fibers, moreover, have a number of advantages over composites containing inorganic fillers. Firstly, natural fibers are generally stiffer, more resistant, and less [12] dense than commonly used inorganic reinforcing fillers such as fiberglass, calcium carbonate, and mica [13–15]. Secondly, their availability, abundance, versatility, and recyclability [16] make them interesting candidates for low-cost filling applications. Moreover, the fact that natural fillers have low hardness values also reduces wear and tear on processing equipment. Current social and political concerns for the environment

Correspondence to: J.E. Crespo Amoro´s; e-mail: [email protected] Contract grant sponsor: CICYT, Ministry of Science and Technology; contract grant number: MAT 2003– 05511; Contract grant sponsor: Generalitat Valenciana; contract grant number: GV06/051. DOI 10.1002/pc.20256 Published online in Wiley InterScience (www.interscience.wiley.com). © 2007 Society of Plastics Engineers

POLYMER COMPOSITES—2007

have led to increased protection of forest resources, and this is why combinations of natural fillers and synthetic polymers have found a wide number of applications as inexpensive substitutes for traditional wood products [17]. Certain synthetic polymers are currently being combined with reinforcing fillers in an attempt to improve the mechanical properties of the resulting composites and equip them with the characteristics required by their intended applications. A number of studies have focused specifically on ways of using lignocellulosic rather than synthetic fibers as reinforcing elements [18 –24]. A number of studies in the plastics processing industry have revealed that demand for natural composite fillers in automotive applications has grown by 60%, and that there is an increasing demand for natural fiber–PVC composites in building applications and applications where replacements are required for traditional wood products [10, 25–27]. Natural fillers that facilitate design innovations (in geometry, aesthetics, and surface finishes) are also in demand in sectors such as interior design (furniture, wood), household, footwear, and most particularly, the toy sector, where rotational molding processes are used to produce imitationwood products. The main aim of this work is the development of new composite materials derived from environmentally friendly materials, based on the addition of lignocellulosic fillers combined with a thermoplastic matrix. EXPERIMENTAL Materials PVC, Lacovyl PB1172H, supplied by Atofina S.A. (Atofina UK Ltd., Midlands, UK) with a K value of 75, was used as a polymer matrix and in rotational molding applications. Carboxylate plasticizer, di-isononyl-cyclohexane-1,2 dicarboxylate (trade name Hexamoll® Dinch), supplied by BASF S.A. (BASF Ltd., Cheshire, UK) was combined with the PVC resin to produce the plastisol. In addition, 2 phr of heat stabilizer based on Ca-Zn (Vinstab H-675), supplied by Hebron S.A (Hebron S.A. Industrias Quı´micas y Farmaceú-

TABLE 1.

Formulations used in the PVC/almond husk composites.

Materials Resin (Lacovyl PB1172H) (phr) Plasticizer (HEXAMOLL®DINCH) (phr) Stabilizer (Vistab H-675) (phr) Filler (wt%)

Formulation 100 40, 50, 60, 80 2 20, 30, 40, 50, 60

ticas, Barcelona, Spain), was added to prevent thermal decomposition of the polymer during the curing of the pastes. The cellulose filler was ground almond husk, supplied by Jesol Materials Primas (Valencia, Spain). The almond husk particles used were separated in different sizes using a sieve Cisa® Sieve Shaker model RP09 (Barcelona, Spain). The particle size used was 150, 500, and 1,000 ␮m. Sample Preparation The first step in the preparation of the PVC/almond husk composite was to prepare the plastisol using an appropriate blend of PVC, plasticizer, and stabilizer. Plasticizer concentrations ranged between 40 and 80 phr of PVC resin. The mixture was blended for 20 min in a 5KPMS K.A.P.L. mixer (St. Joseph, Michigan) while the rotation speed was 60 rpm at room temperature, the mixing time used was the same for all composite ranges since homogeneous mixtures were obtained. The cellulose filler was then added and the paste mixed until it had a smooth consistency. The amount of filler in the PVC matrix ranged from 20 – 60% by weight. Once mixed, the paste was kept in a vacuum for 30 min in a 001LC MCP vacuum chamber (HEK-GMBH, Lu¨beck, Germany) at a maximum of 1 bar below the atmospheric pressure to remove any air bubbles that might have formed in the composite. Three types of sample mixtures, one for each of the filler particle sizes to be analyzed (150, 500, and 1,000 ␮m), were prepared under the same conditions. The formulations used in these samples can be observed in Table 1. The Curing Process The composites were cured in a 2416CG Carbolite oven (Keison Products, Oregon) at a curing temperature of 200°C for 9 min on an aluminium mold measuring 190 ⫻ 140 ⫻ 4 mm3. The curing conditions considered in this work were obtained from previous works on which curing the temperature and time as a function of mechanical properties were optimized [17, 28]. When the curing process was completed, the samples were cut according to ISO 527 using a MEGA KCK-15-A hydraulic press (Melchor Gabilondo S.A., Vizcaya, Spain). Mechanical Testing The tensile properties of the PVC/almond husk composites were tested on an ELIB 30 universal testing machine 72


FIG. 1. Graphic correlation of tensile strength and filler amount in PVC/almond husk composite with 150 ␮m particle size.

(S.A.E. Iberstest, Madrid, Spain) according to ISO 527. Five specimens from each composition were tested with a load cell of 5 kN and a crosshead speed of 20 mm/min. Shore D hardness was measured using a Baxlo durometer (Baxlo, Barcelona, Spain) according to ISO 868. Five samples, each of a thickness of 3 mm, were tested for each formulation. Morphological Study The morphology of the tensile fracture surfaces of the samples was analyzed using a JEOL 6300 scanning electron microscope (JEOL USA, Peabody). The samples were first sputter coated with a fine layer of gold under vacuum for 60 s [9, 19, 29]. RESULTS AND DISCUSSION Our analysis of the different study variables and discussion of results is divided into the following points: influence of almond husk filler amount, influence of plasticizer concentration, and influence of filler particle size. Influence of Filler Amount Figure 1 shows the effect of different amounts of filler on the tensile strength of the 150 ␮m particle size composite. It can be seen that there is a difference between the tensile strength of the unfilled plasticized PVC and that of the plasticized PVC containing almond husk, with tensile strength values decreasing as the filler concentration increases. This decrease in strength is due to the effect of the concentration of stresses around the filler particles, attributable to the weak interfacial interaction between the filler and the PVC matrix as a result of poor adhesion [14, 30]. Increased plasticizer concentration is also characterized by decreased tensile strength in all of the PVC/almond husk DOI 10.1002/pc

FIG. 2. Graphic correlation of elastic modulus and filler content in PVC/almond husk composite with 150 ␮m particle size.

FIG. 4. Graphic correlation of tensile strength and plasticizer concentration in PVC/almond husk composite with 150 ␮m particle size.

composite formulations studied because of the high mobility of the polymer chains. This behavior is also observed for the 500 and 1,000 ␮m particle formulations. The results obtained for the elastic modulus (Fig. 2) of the composites containing 150 ␮m filler particles show that, in contrast to tensile strength, elastic modulus increases with filler concentration. In fact, the elongation at the break of the composite increases significantly with filler content and similar behavior is observed across the range of plasticizer concentrations used, although samples with 40 phr plasticizer and 60 wt% filler show a considerable decrease in elastic modulus. This situation can be explained if we take into account that there is certain filler amount that restricts the matrix capacity to aggregate the dispersed filler, and this situation is much more pronounced for lower plasticizer contents. This fact promotes a lack of cohesion among the matrix–particle interface. The increased stiffness of the

PVC/almond husk composite is accompanied by an increase in hardness, as can be seen in Fig. 3 (particle size of 150 ␮m). This behavior is similar across the range of plasticizer concentrations used [13, 31] and in both the 500 and 1,000 ␮m particle formulations.

FIG. 3. Graphic correlation of Shore D hardness and filler content in PVC/almond husk composite with 150 ␮m particle size.

FIG. 5. Graphic correlation of tensile modulus and plasticizer concentration in PVC/almond husk composite with 150 ␮m particle size.

DOI 10.1002/pc

Influence of Plasticizer Concentration The behavior of the PVC/almond husk composite with different plasticizer concentrations is similar to the unfilled plasticized PVC. Figure 4 shows the variation in tensile strength against plasticizer concentration for the 150 ␮m particle size; specifically, tensile strength decreases linearly with increased plasticizer concentration, a pattern which is repeated across the range of filler amounts and in both the 500 and 1,000 ␮m particle sizes.


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FIG. 6. Graphic correlation of Shore D hardness and plasticizer concentration in PVC/almond husk composite with 150 ␮m particle size.

Increased plasticizer concentration once again has a negative effect on the stiffness of the 150 ␮m particle size, as can be seen in Fig. 5, which shows the variation in elastic modulus with different plasticizer concentrations [18, 29, 32]. The stiffness of the composite decreases with increased plasticizer concentration, as does the hardness of the composite, as shown in Fig. 6, which displays the variation in hardness of the 150-␮m composite with different plasticizer concentrations. Composite hardness also decreases with increased filler content. It should be noted that increased plasticizer concentration has a similar effect on the filled and unfilled plasticized PVC, and that this behavior is observed across the range of filler contents used and in both the 500 and 100 ␮m particle sizes.

FIG. 7. Graphic correlation of tensile strength and filler content for different particle sizes in PVC/almond husk composite. Plasticizer concentration of 40 phr.

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FIG. 8. Graphic correlation of elastic modulus and filler content for different particle size. 150 ␮m almond husk particle size at different plasticizer concentrations.

Influence of Particle Size Particle size, like plasticizer concentration and filler loading, also influences the mechanical properties of the test composites, which was seen to vary considerably depending on the size of particle. Figure 7 shows the variation in tensile strength for each of the different-sized almond husk particle formulations at different filler amounts and a plasticizer concentration of 40 phr. Tensile strength exhibits a marked downward tendency as filler amount increases, and this pattern is repeated across the range of plasticizer concentrations studied. The tensile strength of the composite with a filler amount of 20 wt% is 35% less than that of the unfilled plasticizer. Loss of tensile strength is more gradual for filler amount above 20 wt%.

FIG. 9. Graphic correlation of Shore D hardness and filler content for different particle size. 150 ␮m almond husk particles at different plasticizer concentrations.

DOI 10.1002/pc

FIG. 10. SEM morphology of ground almond husk particle surface at different magnifications: (a) ⫻1,000; (b) ⫻5,000.

Particle size also has a considerable effect on the elastic modulus of the composite, with stiffness values increasing in line with filler loading. Maximum stiffness values are achieved at filler contents of 40 and 50 wt%, as can be seen in Fig. 8, which shows the effect of filler content on the tensile modulus of the composite. This increase in stiffness is observed across the range of plasticizer concentrations studied, but peak values are obtained for the 150 ␮m particle formulation as larger particles tend to be aligned more evenly throughout polymer matrixes, which results in greater stiffness. This is due to a better distribution, inside matrix, of the particles of smaller size (150 ␮m), which provides materials with a bigger homogeneity than materials with particles with higher size. Figure 9 shows the variation in Shore D hardness for the different particle sizes at different filler amounts and a plasticizer concentration of 40 phr. Hardness values increase significantly as the concentration of almond husk filler increases. In general, the highest hardness values are obtained at a plasticizer concentration of 40 phr and high DOI 10.1002/pc

FIG. 11. SEM micrograph of fracture surface obtained from tensile test of plasticizer PVC with almond husk: (a) 40 phr, 20 wt%, 150 ␮m, ⫻500; (b) 40 phr, 20 wt%, 150 ␮m, ⫻1,500.

filler contents, although this behavior is also observed for the other formulations studied. Morphological Study of Fracture Surface Figure 10 shows the SEM micrographs of the surface morphology of ground almond husk at different magnifications. The fracture surface is highly characteristic of fillers of this type. The surface of the particle has a very rough perimeter, which is attributable to the crushing process and the considerable hardness of the filler. A more detailed analysis of the particle at higher magnifications reveals a highly rough surface formed by rounded nodules, which would facilitate adhesion between the particle and the matrix [23]. A morphological study of the tensile fracture surfaces of the PVC/almond husk composites in Figs. 11–13 shows the different types of morphologies formed on the fracture surfaces of samples with a 20 wt% filler loading taken as an POLYMER COMPOSITES—2007

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FIG. 12. SEM micrograph of fracture surface obtained from tensile test of plasticizer PVC with almond husk: (a) 40 phr, 20 wt%, 500 ␮m, ⫻750; (b) 40 phr, 20 wt%, 500 ␮m, ⫻1,500.

example. The surfaces are similar for all of the plasticizer concentrations regardless of particle size. All of the samples exhibit a markedly heterogeneous and rough fracture surface with rounded voids, or cavities, and highly pronounced crests, caused by the tensile drawing of material during the deformation of the plastic [25]. The typical surface morphology of the almond husk filler where it has bonded to the PVC matrix was also visible. The main difference observed in the SEM morphologies is a greater formation of cavities and well-defined crests in the plastic matrix at high plasticizer concentrations due to the high plastic deformation of the composite. A layer of matrix material is formed on the surface of the almond husk filler because of the high adherence properties of its rough, irregular surface, as described earlier. CONCLUSIONS The filler content levels influenced the PVC/almond husk composite regardless of particle size (150, 500, or 1,000 76


FIG. 13. SEM micrograph of fracture surface obtained from tensile test of plasticizer PVC with almond husk: (a) 80 phr, 20 wt%, 1,000 ␮m, ⫻350; (b) 80 phr, 20 wt%, 1,000 ␮m, ⫻1,500.

␮m). Although its tensile strength decreased, the resulting composite became stiffer with increased filler amounts in all of the particle sizes. The behavior of the composite at different plasticizer concentrations was very similar to that of plasticized PVC without filler: tensile strength values decreased as plasticizer concentration increased; since the material has a more elastic behavior with the plasticizer addition regardless of particle size or filler content, the tensile strength decreases and consequently, a considerable loss of stiffness occurs. Particle distribution had a considerable influence on the properties of the PVC/almond husk composite, and it was seen that smaller particles equipped the material with improved mechanical properties because of better dispersion and alignment of the particles within the PVC polymer matrix. This homogeneity of the elaborated material is mainly remarkable in the material stiffness and it can be observed for low plasticizer amounts, where the biggest DOI 10.1002/pc

values of stiffness are obtained for low plasticizers concentrations. ACKNOWLEDGMENTS The authors wish to thank R⫹D⫹i Linguistic Assistance Office at the Polytechnic University of Valencia for their help.

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