Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier

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1 Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier Mazatusziha Ahmad, Abdul Razak Rahmat and Azman Hassan* Department of Polymer Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, UTM Skudai, Johor, Malaysia Received: 30 May 2009, Accepted: 25 June 2010 Summary The objective of this study was to investigate the effects of rice husk and acrylic impact modifiers on the mechanical properties of unplasticised poly(vinyl chloride) (PVC-U) composites. The composites were prepared using a two-roll mill at temperature 165 C before being hot pressed at 185 C. The incorporation of rice husk (RH) fillers from 10 to 40 per hundred resin (phr) has increased the flexural and tensile modulus of the unmodified and modified (8 phr impact modifier) PVC-U composite. The flexural strength for both unmodified and modified PVC-U composite was observed to increase until RH loading of 20 phr. However, the tensile and impact strength of PVC-U composite decreased with RH loading. The scanning electron microscopy (SEM) showed that the rice husk fillers agglomerated and unevenly distributed throughout the matrix. The result showed that the impact strength of the filled PVC-U composites (20 phr filler) increased but the tensile and flexural properties decreased with increasing impact modifier content. The formulation containing 8 phr of acrylic impact modifier and 20 phr of RH loading showed the best balance of stiffness and toughness properties. 1. INTRODUCTION The utilization of natural fillers in thermoplastic composites has grown rapidly during the past few years. The fillers derived from agricultural sources such as empty fruit bunch, pineapple leaf fibre, wood flour, jute and rice husk (RH) offer many advantages which include being easily obtained from natural sources, relatively cheap, low in density and nonabrasive 1-2. Many researchers have investigated the use of RH, a by-product of rice milling process, and rice husk ash (RHA) as fillers in thermoplastic composites The focus of the studies was the effects of fibre content, surface treatments and coupling agents on the mechanical properties of the composites. It was reported that with incorporation of the fillers, modulus of the composite increased, while elongation at break, tensile strength and Izod impact strength decreased 1,2,5-7. The polymer matrices studied in RH or RHA filled thermoplastics composites are polyethylene 4,8,9, polypropylene 2,3,5,6 and polyurethane 7. The application of RHA unplasticized PVC (PVC-U0 composite has been reported by Hassan and Sivaneswaran 10. Recently Crespo et al. 11 reported their study on RH-filled plasticized PVC. They observed that RH-filled plasticized PVC decreased with filler loadings and plasticizer concentrations regardless of particle size. In contrast to tensile strength, tensile modulus increased as the filler amount was increased. The increased rigidity of the composite was also accompanied by an increase in hardness. PVC-U is a tough and durable material with many applications where its basic properties effectively meet the demands of service and use. A major application for PVC-U is in extrusion products such as pipe, gutters, conduit, sheet and a wide range of complicated profiles such as window frames. The usefulness of PVC-U can be increased by physically blending various modifiers with the polymer prior to use for plastics products. Impact modifier is incorporated into PVC to improve its robustness in order to use over a wide temperature range and extends the limits of ductility usually associated with conventional rigid PVC compounds. Thus, it gives more impact resistance and better properties to meet the requirements of extruded profiles. Although a study on the effects of RH on plasticized PVC has been reported (Crespo et al. 11 ), no publications have been found on RHfilled PVC-U composites. Corresponding author*: azmanh@fkkksa.utm.my Smithers Rapra Technology, 2010 Polymers & Polymer Composites, Vol. 18, No. 9,

2 Mazatusziha Ahmad, Abdul Razak Rahmat and Azman Hassan 2. EXPERIMENTAL 2.1 Materials The unplasticised PVC used in this study was supplied by Industrial Resins Malaysia, Sdn. Bhd. (IRM). It is a homopolymer PVC powder with a bulk density of 0.5 g/cm 3 and a solution viscosity of K-value 66. Rice husk reinforcing filler was collected from Bernas Perdana Sdn. Bhd., Penang, Malaysia. Acrylic impact modifier (D-200) was supplied by Elf Atochem. Other ingredients in the formulations were calcium and acid stearate (SAK- Cs, Sun Ace Koh) as lubricants, liquid tin stabilizer (Thermolite T890, Elf Atochem) as heat stabilizer, acrylic polymer (PA 20, Kaneka) as processing aids and titanium oxide (TR 92, Tioxide) as pigment. 2.2 Preparation of Rice Husk Filled Impact Modified PVC-U Composite The rice husk powder received was sieved to the size of less than and equal to 75 mm using a Restsch Shaker, Germany. It was then dried in an air oven at 105 C for a period of 24 hours to expel moisture prior to blending. Later, the dry blending of PVC-U, rice husk and other additives was carried out in a heavy-duty laboratory mixer for 10 minutes. The blend formulations, which were based on typical commercial PVC window, are summarized in Table 1. Once mixed, the dry blended compounds were directly fed into a laboratory scale two-roll mill to form a sheet. The nip setting of the mill was set at 2 mm and the temperature of the roll was 165 C. The time for two-roll milling was 10 minutes and the speed was 10 rotations per minute. The composite sheets produced were compression moulded into panels to produce the specimens for mechanical property testing. Compression moulding of the specimens was performed in a hydraulic preheated press at 185 C and 120 kg/ m 2 for 5 minutes. The mould was then cooled to room temperature under pressure by circulating cold water in the press. From these panels, the dogbone shaped specimens (ASTM D638) were made for tensile testing (1 mm thick). Rectangular samples with dimensions of 12.7 mm x 63.5 mm and 125 mm x 13 mm (both at 3 mm thick) were cut from the panel sheets for impact and flexural testing, respectively. 2.3 Mechanical Properties Testing Tensile testing was carried out according to the ASTM standard D638 on a Lloyd machine at a crosshead speed of 20 mm/min. Three tensile properties were determined from the stress-strain curves: i) the tensile strength, ii) the Young s modulus, and iii) the elongation at break. All evaluations were made at room temperature (25 C). Five specimens were tested for each formulation. The notched Izod impact strength of the samples was performed according to the ASTM standard D256. The test Table 1. Formulation used in Rice husk filled PVC-U/Composite Ingredients Concentrations (phr) PVC (K value =66) 100 Tin stabilizer 2 Calcium stearate 0.5 Processing aid (acryclic polymers) 1.5 Titanium oxide 4 Stearic acid 0.6 Acrylic impact modifier 0, 4, 8, 12 Rice husk powder 10, 20, 30 and 40 was conducted using IMPats 15 Izod impact tester manufactured by ATS FAAR, Italy. Notching was carried out using the notching tool with a notch depth fixed at 2.6±0.2 mm. Flexural properties, which cover flexural strength and modulus, were determined using a Lloyd machine according to the ASTM standard D790, three-point bending system. A cross head speed of 3 mm/min was used and the test performed at a temperature of 25 C. To retain consistency, a jig that allowed a span of 50 mm was used. Minimum of five samples were tested for each formulation. 3. Results and Discussion 3.1 Effect of Acrylic Impact Modifier Content on Impact Strength of Unfilled and RHfilled PVC-U Figure 1 shows the effect of different impact modifier content on the unfilled and filled PVC-U composite. It was found that for the unfilled samples, the impact strength increased with increasing impact modifiers content from 0 phr (unmodified) to 12 phr. A sharp increased (88%) in impact strength values, which indicates ductile behaviour was observed as the impact modifier loading was increased from 8 to 12 phr. Ductile failure means a large energy is being absorbed before failure occurs. On the other hand, brittle failure entails low impact energy being absorbed with no gross deformation before failure. The acrylic impact modifier is a rubbery core structure surrounded by a shell of higher glass transition temperature. The major function of core shell impact modifier besides improving the impact resistance of a plastic is to act as stress concentrators throughout the polymer matrix. The stress concentrations are produced by the large difference that exists between the modulus of the rubbery impact modifier particles and the polymer matrix. 528 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

3 Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier However, the addition of RH has lowered the impact strength of RHfilled impact modified PVC-U. As shown in Figure 2 it can be observed that the impact strength of 20 phr RH-filled impact modified PVC-U decreased at all levels of impact modifier compared to the unfilled modified PVC-U (Figure 1). As the impact modifier content increased from 0 to 8 phr, the impact strength increased by 42%. However, the increase did not result in a change from brittle to ductile failure. This is consistent with the SEM micrograph of sample containing 20 phr RH and 8phr impact modifier (Figure 13b) which still showed a typical brittle fracture. Acrylic impact modifier had a capability to compensate for the detrimental effect caused by the filler by lowering the yield stress of PVC-U matrix and allowing shear yielding rather than fracture when the samples were subjected to the sudden load 12. A similar trend was also observed by Aznizam Abu Bakar and Azman Hassan 13 who investigated the effect of acrylic impact modifier on Empty Fruit Bunch (EFB) filled PVC composites. They found that at 20 phr EFB, the impact strength increased from 6.79 to 7.22 kj/m 2 as the impact modifier increased from 0 to 9 phr. However, this increase in impact strength (about 6%) was lower than the present finding (42%). Therefore, to achieve a good combination of mechanical properties, 8 phr of acrylic impact modifier content was chosen to be used in the RH-filled modified PVC-U composites in further formulation. Thus, its concentration in the PVC compound will be as low as possible, consistent with toughness requirements and minimize alterations in the desirable properties of PVC. in Figure 3. It was found that the flexural modulus of both unfilled and RH-filled PVC-U composites (20 phr) decreased with increasing impact modifier content. The results also showed that at all impact modifier loadings, the flexural modulus for the unfilled was lower than for the RHfilled PVC-U composites. The drop of flexural modulus as the impact modifier increased from 0 to 12 phr, were 32% and 10% for the unfilled and RH-filled PVC-U composites respectively. The decrease in flexural modulus indicates that the composite has become less stiff. Impact modifiers, due to the rubbery nature, are expected to reduce the stiffness of the polymer. The improvement of impact strength but adversely affecting the flexural properties with increasing impact modifier content is agreed by other researchers 14. By adding impact modifier, the samples showed a rubbery behaviour, which result in excellent improved in elasticity and toughness. Due to that, less stress was used during deformation and therefore the flexural modulus decreased. The addition of RH increased the flexural modulus of PVC-U at all levels of impact modifier content relative to the unfilled sample. This is because the stiffness of RH is greater than that of the PVC matrix. Furthermore the addition of fillers Figure 1. Effect of acrylic impact modifier content on impact strength of unfilled PVC-U Figure 2. Effect of acrylic impact modifier content on impact strength of RH-filled PVC-U (20 phr) 3.2 Effect of Acrylic Impact Modifier Content on Flexural Properties of Unfilled and RHfilled PVC-U The effects of impact modifier content on flexural properties are shown Polymers & Polymer Composites, Vol. 18, No. 9,

4 Mazatusziha Ahmad, Abdul Razak Rahmat and Azman Hassan Figure 3. Effect of acrylic impact modifier content on flexural modulus of unfilled and RH-filled PVC-U Figure 4. Effect of acrylic impact modifier content on flexural strength of unfilled and RH-filled PVC-U (20 phr) Figure 5. Strong interaction of RH with PVC matrix restricts the chain mobility which made the polymers stiffer. Similar trend was found in flexural strength with the addition of acrylic impact modifier as shown in Figure 4. Flexural strength decreased with increasing impact modifier loading from 0 to 12 phr. The reduction was about 28% and 19% for the unfilled and RH-filled PVC-U composites, respectively. With the incorporation of RH filler, the flexural strength of 12 phr impact modified PVC-U samples was higher than that of the unfilled samples by about 16%. This could be due to good interaction between the surface of RH and the matrix. Figure 5 shows the interaction of RH with PVC matrix. 3.3 Effect of Acrylic Impact Modifier Content on Tensile Properties of Unfilled and RHfilled PVC-U The effect of impact modifier on tensile strength and Young modulus are shown in Figures 6 and 7, respectively. In the presence of acrylic impact modifier in RH-filled PVC composites (20 phr RH), it decreased the tensile properties (tensile strength and Young s modulus). Tensile strength of the samples decreased up to 18% for the RH-filled and 28% for the unfilled impact modified PVC-U samples with the addition of 4 to 12 phr loading of acrylic impact modifier (Figure 5). The impact modifier acted as a yield promoter because it lowered the yield point (stress), allowing the PVC to yield or deform rather than fracture. Incorporation of RH filler decreased the tensile strength of RH-filled PVC-U composites at all impact modifier contents. The trend of flexural strength results differ from tensile strength because in flexural strength the RH-filled is higher and in tensile strength the unfilled samples is higher. One possible explanation is that in flexural tests the RH is able to support the stress transfer from the matrix and contributes to the strength of the PVC composite. 530 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

5 Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier The other possible explanation is the difference in the strain rate of both tests. The tensile test was done at higher strain rate and therefore the strength reduces in the filled samples due to the poor adhesion between the matrix and filler. Figure 6. Effect of acrylic impact modifier content on tensile strength of unfilled and RH-filled PVC-U (20 phr) 3.4 Effect of RH Content on Impact Strength of Unmodified and Impact Modified PVC-U Composite Figure 8 shows the effect of RH content on impact strength of both unmodified and impact modified PVC-U composites (8 phr impact modifier). It was found that the impact strength decreased with increasing RH fillers content for both blends. There was a relatively sharp decrease in impact strength for unmodified and modified PVC-U upon the addition of 10 phr RH followed by a gradual decrease upon increasing filler loading. In the filled composites as the filler loading increased the tendency for agglomeration was also increased. As filler agglomeration increased, interfacial adhesion became weaker leading to weaker interfacial regions 10. Therefore, a reduction in impact strength was observed. This result is expected for the filled systems and has been reported by Aznizam Abu Bakar and Azman Hassan 13, in their study on PVC-U composites. It is also interesting to observe that the addition of impact modifier has enhanced the impact strength of PVC-U samples at all levels of RH content. The effectiveness of the impact modifier however, decreased with increasing RH content. This result is in agreement with other researcher 14,15. Poor adhesion of the filler and matrix becomes dominant and weakens the stress transfer from the polymer matrix to the filler and deteriorated the impact strength at higher filler loading. As the filler content increased from 10 to 40 phr, it was observed that the impact strength reduced to about 45 % for the modified PVC-U. It was found Figure 7. Effect of acrylic impact modifier content on Youngs modulus of unfilled and RH-filled PVC-U (20 phr) Figure 8. Effect of RH content on impact strength of unmodified and impact modified PVC-U Polymers & Polymer Composites, Vol. 18, No. 9,

6 Mazatusziha Ahmad, Abdul Razak Rahmat and Azman Hassan that the difference in impact strength between unmodified and impact modified PVC-U composites became smaller as the RH content increased. This shows that as the amount of filler increased, the effectiveness of the impact modifier in enhancing the impact strength reduced. This can be clearly observed at 40 phr RH content whereby the impact strength differed by 2% between unmodified and modified composites. The reduction of impact strength could be due to two factors. The first factor could be due to the detrimental effect of filler because the volume they take up, thus affect on the impact performance. RH filler, unlike the matrix, was incapable of dissipating stress through the mechanism known as shear yielding prior to fracture. It might also hinder the local chain motions of the PVC molecules that enable the matrix to shear yield. As a result, the ability of filled composites to absorb energy during fracture propagation would be decreased. Secondly, it is known that composite materials with satisfactory mechanical properties can only be achieved if there is good dispersion and wetting of filler in the matrix that will give rise to a strong interfacial adhesion. However, this is not the case when RH fillers are used in PVC. Although the polarity of RH fillers capable of forming a physical interaction with the polar PVC, it is a relatively weak interaction compared to the fillerfiller interaction caused by hydrogen bonding. Therefore, RH fillers have a greater tendency to agglomerate, which consequently lowers the area of contact with the matrix. Meanwhile, the moisture absorbed may also interfere the physical bonding between RH fillers and PVC, and potentially acting as a lubricant to the filler-matrix interphase 16. These factors increase the weakness of interfacial adhesion between fillers and matrix, and become the potential sites crack growth as inability of fillers to support the stress transfer to the polymer matrix. All these reasons are likely to be responsible for the decrease in impact strength of the composites. 3.5 Effect of RH Content on Flexural Properties of Unmodified and Impact Modified PVC-U Composite Figure 9 shows the effect of RH loadings on flexural modulus of unmodified and Figure 9. Effect of RH content on flexural modulus of unmodified and impact modified PVC-U (8 phr impact modifier) impact modified PVC-U composites (8 phr impact modifier). It was found that the flexural modulus of the unmodified PVC-U samples was higher than the impact modified ones at all filler loadings. There was a considerable increase in flexural modulus with increasing RH content from 0 to 40 phr. The addition of 40 phr RH increased the flexural modulus of RH-filled impact modified PVC composites by 59% compared to the unfilled impact modified PVC, while flexural modulus of RH-filled unmodified PVC composites at 40 phr RH, was 46% higher than the unfilled unmodified PVC. As mentioned before, RH is able to impart a significant improvement in stiffness by hindering the movement of PVC molecules. The movement of PVC molecules became more difficult with increasing filler content. Figure 10 shows the result of flexural strength measurements for both types of composites. The flexural strength increased with RH loading up to 20 phr and slightly decreased at 30 to 40 phr RH for unmodified PVC-U composites. However, the flexural strength for impact modified PVC-U composites increased at 30 phr and decreased at 40 phr. The difference between both composites became smaller at 30 and 40 phr RH contents. The trend is similar with the study reported by Rozman et al. 7 on the mechanical and physical properties of polyurethane composites based on rice husk and polyethylene glycol, where flexural strength increased up to a certain extent of RH content, after which it decreased. The increase in the amount of RH has consequently reduced the ability of stress transfer, and therefore decreased the flexural strength of the composites. 3.6 Effect of RH Content on Tensile Properties of Unmodified and Impact Modified PVC-U Composite Figure 11 shows the effect of RH loading on the tensile strength of 532 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

7 Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier PVC-U composites. In general, tensile strength decreased with RH loadings. This is not surprising since other studies have also indicated that the incorporation of filler into thermoplastic matrix may not necessarily increase the tensile strength of the composites 6,11. Tensile strength for the unmodified PVC-U was higher compared to the impact modified PVC-U at all RH loadings. With the addition of 10 to 40 phr RH, tensile strength decreased to about 17 and 10% for the unmodified and impact modified PVC-U samples respectively. The tensile strength decrease as the filler loading increased may be attributed to the reduction in deformability of a rigid interface between filler and the matrix component 17. As the filler loading increased, it thereby increased the interfacial area. The worsening interfacial bonding between filler (hydrophilic) and matrix polymer (hydrophobic) might decrease the tensile strength. The deterioration of tensile properties indicates the incapability of filler particles to support the transfer of stress from the matrix to the filler. RH fillers, whose particles are irregularly-shaped, have poor capability to support stress transmitted from the thermoplastic matrix. This reduces the strength improvement of the composites. However, incorporation of RH into a PVC-U matrix has resulted in the improvement of Young modulus for both RH-filled unmodified and modified PVC-U composites. The effect of different loadings of RH on the Young s modulus is illustrated in Figure 12. At 40 phr RH content, Young modulus improved by 31% for impact modified PVC-U, while unmodified PVC-U improved about 27%. Similar effects were observed by other researchers who studied the incorporation of particulate fillers including RHA into plastics 17. The trend observed indicates that the ability of RH fillers to impart greater stiffness to the PVC-U composites. Figure 10. Effect of RH content on flexural strength of unmodified and impact modified PVC-U (8 phr impact modifier) Figure 11. Effect of RH content on tensile strength of unmodified and impact modified PVC-U Figure 12. Effect of RH content on Youngs modulus of unmodified and impact modified PVC-U Polymers & Polymer Composites, Vol. 18, No. 9,

8 Mazatusziha Ahmad, Abdul Razak Rahmat and Azman Hassan Figure 13. SEM micrograph of impact fracture surfaces of unfilled unmodified PVC-U 3.7 Scanning Electron Microscopy Analysis Scanning Electron Microscopy (SEM) analysis was employed to obtain some qualitative evidence on the bonding quality between the RH and PVC, the dispersion of RH in the PVC matrix and the correlation between fracture surface and energy absorbed. Figure 13 shows the SEM micrograph of the PVC-U compound sample with impact strength of 4.85kJ/m 2. The fracture surface shows a jagged appearance. However, with an incorporation of 10 phr RH a rougher surface is observed compared to PVC-U compound as shown in Figure 14a. Figure 14a. SEM micrograph of impact fracture surfaces of RH-filled impact modified PVC-U composites at 10 phr RH content Figure 14b. SEM micrograph of impact fracture surfaces of RH-filled impact modified PVC-U composites at 20 phr RH content Figure 14c. SEM micrograph of impact fracture surfaces of RH-filled impact modified PVC-U composites at 30 phr RH content Figure 14d. SEM micrograph of impact fracture surfaces of RH-filled impact modified PVC-U composites at 40 phr RH content 534 Polymers & Polymer Composites, Vol. 18, No. 9, 2010

9 Mechanical Properties Of Unplasticised PVC (PVC-U) Containing Rice Husk and an Impact Modifier The impact strength decreases to 3.65kJ/m 2. Figures 13 and 14a to 14d clearly show that the fracture surface becomes rough as the filler content increases from 0 to 40 phr. The RH filler can be observed by SEM to be distributed unevenly throughout the matrix. Due to hydrogen bonding, this filler had greater tendency to agglomerate. The presence of filler agglomeration is expected to give a detrimental contribution to the impact strength of the PVC-U composites. The impact strength for composites at 20 phr RH filler decreased to 3.59kJ/m 2 and a further increment in RH filler content to 30 phr, reduced the impact strength to 3.15 kj/m 2 ; at 40 phr, the impact strength reduces to 3.05 kj/m 2. In other words, the ductile portion contributed by PVC-U matrix is reduced, and the failure mode became more brittle as the RH filler content is increased. Fillers unlike the matrix are incapable to dissipate stress through the mechanism known as shear yielding prior to fracture. Therefore, the total ability of the material to absorb energy is decreased. Thus, the impact strength tends to decrease with increasing RH filler content. Figure 14a also shows some holes that indicate the filler pullout. This may be due to poor interfacial bonding between filler and matrix. Further evidence for the poor bonding can be observed from Figures 14b to 14d. Figure 15a. SEM micrograph of impact fracture surfaces of unfilled impact modified PVC-U composites at 8 phr impact modifier content Figure 15b. SEM micrograph of impact fracture surfaces of unfilled impact modified PVC-U composites at 12 phr impact modifier content Figure 15a shows a micrograph of unfilled impact modified PVC-U sample at 8 phr impact modifier content with impact strength kj/m 2. The fracture surface appearance was different from the unmodified unfilled PVC-U. The fracture surface shows the appearance of ductile yielding. A smooth surface was observed as shown in Figure 15b for sample at higher impact modifier content of 12 phr with high impact strength, kj/ m 2. The micrograph clearly showed ductile yielding where high impact energy was absorbed. 4. Conclusion The effects of rice husk and acrylic impact modifiers on the mechanical properties of unplasticised poly(vinyl chloride) (PVC-U) composites were investigated. The flexural and tensile modulus of unmodified and modified (8 phr impact modifier) PVC-U composites increased with increasing RH content. The flexural strength for both unmodified and modified PVC-U composite was observed to increase up to an RH loading of 20 phr. However, the tensile and impact strength of PVC-U composite decreased with RH loading. Scanning electron microscopy (SEM) showed that the rice husk fillers agglomerated and unevenly distributed throughout the matrix. The impact strength of the filled PVC-U composites with 20 phr filler content increased with increasing impact modifier content from 0 to 8 phr. Polymers & Polymer Composites, Vol. 18, No. 9,

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