Home-made single screw extruder was designed and constructed with screw

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1 Iranian Polymer Journal 16 (10), 2007, Available online at: Tensile Properties o Extruded Short Glass Fibre/Low Density Polyethylene Composites Shadi Sawalha*, Amjed Mosleh, and Abdulah Manasrah Chemical Engineering Department, An-Najah National University, Nablus, P.O. Box: 7, Palestine ABSTRACT Received 15 July 2007; accepted 31 October 2007 Home-made single screw extruder was designed and constructed with screw length to diameter ratio 20:1 operating at dierent screw speeds ranging rom 7-28 rpm and at dierent temperatures. The extruder was used to produce rod composite samples composed o low density polyethylene and short E-glass ibre. The eects o ibre contents and extruder operating variables on the tensile modulus and ductility were investigated. The tensile modulus increased with ibre weight percent up to wt% as residual ibre content (20 wt% prepared sample beore processing) and had a value o 827 MPa, then slightly decreased to lower values due to bad wettability and poor distribution/dispersion o ibres in the LDPE matrix. The clear decrease in ibre content ater processing (residual ibre content) which is about 40-55% is due to the sticking o ibres in hopper or in the dierent extruder zones. On the other hand, ductility as percentage elongation-at-break decreased signiicantly at low ibre concentration then decreased in lower rate at higher ibre contents. Operating variables o the extruder were ound to aect the tensile properties o LDPE/glass ibre composites by aecting the interacial adhesion between the two components. This eect was clear rom the analyzed data o the modulus eiciency actor K E. Key Words: glass reinorced composites; tensile modulus; modulus eiciency actor; ductility; residual ibre content. ( * ) To whom correspondence to be addressed. engsawalha@najah.edu INTRODUCTION Low density polyethylene is a nontoxic thermoplastic polymer used in polymer industry and is one o the most versatile polymers due to its low density, low cost, and high processability. However, its use is restricted because o several drawbacks, such as low strength, low tensile modulus, and poor heat resistance [1-4]. Mechanical properties o polyethylene can be modiied and enhanced by adding dierent modiied additives such as glass beads, calcium carbonate, talc, mica, wood ibres, sisal, kena, pineapple, etc. Most o the

2 Tensile Properties o Extruded Short Glass Fibre... Sawalha S. et al. incorporation o short glass ibres which increases the stiness, strength, and thermal stability o polyethylene [3,4]. Short ibre reinorced thermoplastics are very important commercial materials. Where a good processability is typical o short ibre composites, the mechanical properties o inal products mainly depend on several actors such as ibre content, inherent properties o ibres and polymer matrices, ibre surace treatment and impregnation, adhesion and interaction o ibres into the polymeric matrix, residual ibre length, and orientation and distribution o ibres within the polymeric matrix. Some o these actors are aected by the processing technique used to manuacture the reinorced thermoplastic polymers [5-7]. Extrusion has become the most popular process or compounding the dispersed or short ibre reinorcement with a thermoplastic mainly due to its economic and continuous operation. Single screw extruders with a screw length to diameter ratio o 20:1 or above ind application in blending chopped glass ibres with thermoplastic polymers [8]. Tensile properties o short ibre reinorced composites depend on the orientation o ibres within the matrix (i.e., longitudinal or random) and can be estimated theoretically by equations which are derived rom Halpin Tsai equations [9]. The equations which are applied to predict the modulus o the composite are as the ollows: longitudinal modulus: E T where, and ( ) 1+ 2 L / d ηlv EL = V 1 ηlv transverse modulus: η η L T 1+ 2ηTV = 1 η V = = T m ( E / Em ) 1 ( E / E ) + 2( L / d ) m ( E / Em ) 1 ( E / E ) + 2 m (1) (2) (3) (4) where, L, d, V m, V, E m, E are ibre length, ibre diameter, volume percent o matrix, volume percent o ibre, tensile modulus o matrix, and tensile modulus o ibre, respectively. Also Tsai developed a method o calculating the tensile modulus o composite which are randomly oriented in 2-D. 3 5 E 2D = EL + ET (5) 8 8 Fibres may also be oriented randomly in three dimensions to give truly isotropic materials. An approximate equation or modulus in this case is given by eqn (6) [10] 1 4 E D = EL ET (6) where, E L and E T are estimated rom eqns (1) - (4). Tensile modulus depends on the interaction between ibres and matrix and it is also aected by ibre aspect ratio and orientation which can be related to modulus eiciency actor K E estimated rom the modiied rule o mixture [11-12] EC = K EV E + V mem (7) modulus eiciency actor can be related to Cox s ibre length eiciency actor η l and orientation eiciency actor η 0 [13]. Values o K E and the degree o discrepancy between the theoretical and experimental tensile moduli indicate the compatibility and adhesion between ibres and polymeric matrix. Another property which is important to be estimated is ductility. Ductility can be deined as the ability o material to withstand plastic deormation up to racture and is estimated in terms o percentage elongation-at-break as ollows [14]: l lo Ductility ( El %) = 100% l o (8) where, l o and l are initial length and inal length o sample at break, respectively. In this work, home-made single screw extruder is used to process a composite material composed rom low density polyethylene as a polymeric matrix and chopped strand short E-glass ibre as reinorcement. 720 Iranian Polymer Journal / Volume 16 Number 10 (2007)

3 Sawalha S. et al. Tensile Properties o Extruded Short Glass Fibre... The eects o ibre content on modulus and ductility have been observed. The extruder was operated at dierent temperatures and screw speeds to determine their eects on the orientation o ibres, wettability, and aspect ratio o the reinorcement which have a direct eect on the tensile modulus and ductility o composite materials. Modulus eiciency actor and theoretical values o tensile moduli were estimated to indicate the compatibility and adhesion between ibres and LDPE matrix. Thus, the novelty o this work is the concept o modulus eiciency actor and its dependency on the above parameters and actors that aect the mechanical properties o the composites. On the other hand, examining the eect o mixing technique used prior to the extruder on the ibre content is one o the objectives o this work. EXPERIMENTAL Materials The material used in this research was low density polyethylene. IPethen 323 LDPE supplied by Carmel oleins Company with melt low reerence (190; 2.16) o 2g/10min. Short E-glass ibres in the orm o chopped strands o about 38 mm in length and 10.6 μm in diameter or each ibre supplied by VOSS- CHEMIE were used at dierent concentrations. The ibre length was reduced to 8 mm beore processing. Processing Low density polyethylene and short glass ibres were mixed together at dierent weight percentages (5, 10, 15, 20, 25, 30, 35, 40 and 45) by using a simple tumbling device. The blends were introduced into the eed zone o the extruder. The extruder had a L/D ratio o 20 with three distinct zones (eed, compression, and metering). The light depth in the metering zone was 2.25 mm, helix angle 17.7º, which could be operated at dierent rotating speeds varied rom about 7 to 28 rpm. The extruder was attached with a capillary die o 10 mm in diameter. The prepared samples were extruded at the same temperature 200ºC and the same screw speed o 17 rpm. The samples which emerged rom the die were cooled to room temperature by a water path. A 20 wt% sample has been processed at dierent temperatures (180, 200, and 220ºC) and at dierent screw rotation speeds ( 7, 17, and 28 rpm) to examine the eect o processing variables on the mechanical properties o the composite. Testing Residual Fibre Content This test was carried out examine the eect o processing technique used on the residual ibre content in the polymeric matrix. Five samples rom each prepared composites (5, 10, 15, 20,, 45 wt%) were placed in a urnace at 450ºC or 5 h [5] to digest the LDPE matrix and leave ibres alone. The remaining ibre was weighed and the residual ibre weight percentage was calculated. Tensile Test Tensile test was carried out by using Gunt Hamburg apparatus WP 310 machine at constant speed 5mm/min and at room temperature. Five trials o each sample were taken. The samples were 100 mm gauge length and 6 mm gauge diameter. Tensile test was done to measure the tensile modulus and ductility (as percentage elongation). Morphological Test This test was perormed by using an optical microscope BX 60 M system Microscope Olympus. It was done to observe the break down o the ibres and measure the residual ibre length ater processing, or that the matrix should be digested at 450 C or about 5 h to burn the low density polyethylene matrix and just leave the reinorcement component. The residual ibre length is measured under the microscope by using an attached micrometer, the length is reported as the average length o ten segments taken randomly rom the digested samples. RESULTS AND DISCUSSION Residual Fibre Content Fibre content ound to be decreased ater processing with an average weight percentages (2.33, 4.57, 7.35, 10.52, 14.44, 17.82, 23.25, 28.35, and wt%). This decrease is due to the sticking o ibres in the hopper or remaining in the extruder zones, because o the ineicient method o mixing used prior to the Iranian Polymer Journal / Volume 16 Number 10 (2007) 721

4 Tensile Properties o Extruded Short Glass Fibre... Sawalha S. et al. extruder. To improve the eiciency o the designed extruder and/or prevent the sticking o ibres and the decrease in the residual ibre content; compounding machines (suitable mixers) may be used prior to the main operation. Compounding the ibres into the polymeric matrix would lead to well distributive/dispersive mixture and the properties o the inal composite are improved. The Eect o Fibre Content on Tensile Modulus and Ductility Elastic modulus was calculated rom the tangent at low stresses or the elastic region in the stress/strain curve. To examine the degree o compatibility between the ibres and the matrix, theoretical values o the composite tensile modulus were calculated in three dierent manners according to the orientation o ibres within the LDPE matrix; in one direction (longitudinal short ibre), randomly oriented in two dimensions, and randomly oriented in three dimensions. Theoretical tensile moduli were calculated, which were based on the residual ibre content and residual ibre length equal to 4 mm. Figures 1, 2 show the relation between ibre content as weight percentage o the experimental and theoretical tensile modulus, respectively. It is clear rom Figure 1 that the tensile modulus increases with ibre content up to 10.52% residual ibre content (20 wt% prepared sample) then decreases slightly to a lower Figure 1. Relation between experimental tensile modulus and residual glass ibre content. Figure 2. Relation between theoretical tensile modulus and residual glass ibre content. value and remains nearly constant. It is believed that this decrease is due to the decrease in wettability o the matrix to the ibres, where the matrix loses its ability to wet the ibres at high concentration and thereore, voids are ormed in the composite and ibres act alone and the matrix cannot transer the load between the ibres that are not impregnated. Another reason or this unexpected decrease ater such low ibre content may be due to poor distribution and/or dispersion o glass ibres within the low density polyethylene matrix which is related to the use o ineicient method o mixing. The increase o tensile modulus with dierent reinorcements or thermoplastic composites has been observed by several researchers [3, 15-17]. The values o tensile modulus are very low compared with the theoretical ones, this discrepancy may be due to the damage o ibres during processing, poor distribution /dispersion o ibres in the polymeric matrix, or it can be related to the poor adhesion between glass ibre and low density polyethylene. As we believe, the interacial adhesion between ibres and matrix can be improved by the use o appropriate coupling agents which must be compatible with both the reinorcement and the polymeric matrix such as a suitable silane or copolymer. Where, poly (ethylene-g-maleic anhydride) is a suitable copolymers used or this purpose as reported by other researchers [4]. Poor adhesion can be observed 722 Iranian Polymer Journal / Volume 16 Number 10 (2007)

5 Sawalha S. et al. Tensile Properties o Extruded Short Glass Fibre... behaviour may be attributed to the ormation o dense structures within the composite volume as the ibre concentration increases, leading to the production o anisotropic inal product. Gergopoulos et al. [3] and Gupta et. al. [16] have ound the same behaviour or other thermoplastic ibre composites. Also, Bkiaris et al. [4] have ound similar trend or glass ibre/ldpe composite beore adding silanes or copolymers. Figure 3. Eect o residual ibre content on modulus eiciency actor. by the estimation o the modulus eiciency actor K E rom the modiied rule o mixture. K E has very low values and decreases with increasing ibre contents as it is illustrated in Figure 3. Percentage o elongation-at-break value was estimated to observe the eect o ibre content on the ductility o the polymeric matrix. This eect is shown in Figure 4. It is obvious that Figure 4 exhibits a signiicant decrease in ductility at lower ibre concentration where, its value decreases at lower rate. This The Eect o Extruding Temperature on the Tensile Modulus and Glass Reinorced LDPE Processed at Constant Screw Speed (17 rpm) The eect o temperature on tensile modulus is illustrated in Figure 5. The 20 wt% composite samples were prepared at dierent temperatures (180, 200, and 220 C) to observe their impact on the properties o the inal product. Viscosity o most polymer melts decreases with increasing temperature [18]. This decrease in viscosity aects the wettability o the matrix to the ibres and it may also aect the dispersion o reinorcement within the polymeric matrix. All these aspects have direct eect on the mechanical properties o the reinorced polymers. As it is shown in Figures 5, tensile modulus at 180 C has lower values than those at 200 C. This can be attributed to a decrease in the viscosity o LDPE matrix and in this case liquid (matrix) will low over the reinorcement, covering greater areas o the rough surace while displacing more air. This means that at Figure 4. Eect o residual glass ibre content on ductility as percent elongation-at-break. Figure 5. Eect o processing temperature on tensile modulus o glass ibre/ldpe composite. Iranian Polymer Journal / Volume 16 Number 10 (2007) 723

6 Tensile Properties o Extruded Short Glass Fibre... Sawalha S. et al. Table 1. The eect o processing temperature on adhesion between glass ibre and low density polyethylene. Temperature ( C) Wettability Orientation K E Adhesion Bad Fair Bad Very low High Low Very poor Poor 200 C, there are considerable wettability and adhesion compared to 180 C. Also, it is valuable to notice that there is no change in the orientation and distribution o the ibres within the polymeric matrix. On the other hand, at 220 C tensile modulus decreases although there is an increase in wettability o the ibres and the matrix. This decrease is due to the loss o ibres longitudinal orientation and distribution within the low viscosity polymeric matrix which is resulted rom the large plastic low that occurs in the matrix (presented by Sanomura et al. [13]) or to some extent to the degradation o the matrix at high temperatures. Wettability and orientation are the actors which aect the degree o adhesion between glass ibre and LDPE which can be estimated by the modulus eiciency actor K E. For the composite samples prepared at dierent temperatures K E is shown in Figure 6. The eect o processing temperature on the adhesion between reinorcement and low density polyethylene is summarized in Table 1. The Eect o Screw Speed on the Tensile Modulus o Composite Processed at Constant Temperature (200ºC) Speed o rotation has a signiicant eect on both the polymeric matrix and the reinorcement. Shear rate applied on the processed material increases by increasing the speed o rotation. The matrix viscosity decreases because polymer melt acts as a shear thinning (pseudoplastic) luid [18]. Increasing the shear rate will damage the ibres, decrease their residual length, and change orientation rom longitudinal to 2- D or 3-D randomly oriented composite. All these parameters, viscosity, residual ibre length, and orientation aect the tensile modulus and ductility o the short glass reinorced LDPE. The eect o screw speed on tensile modulus o 20wt% prepared samples Figure 6. Eect o processing temperature on modulus eiciency actor. Figure 7. Eect o screw speed on tensile modulus o glass ibre/ldpe composite. 724 Iranian Polymer Journal / Volume 16 Number 10 (2007)

7 Sawalha S. et al. Tensile Properties o Extruded Short Glass Fibre... Table 2. The eect o screw speed on adhesion between glass ibre and low density polyethylene. Speed (rpm) Fibre length (mm) Wettability Orientation K E Adhesion Bad Fair Very bad Very low High Low Very poor Very poor is shown in Figure 7. Residual ibre length depends on the parameters o the processing techniques. In this work, it was ound that the residual ibre length decreased with speed o rotation. Beore processing, the average ibre length was about 8 mm which was decreased to 5.2, 4, and 2.8 mm at 8, 17, and 28 rpm, respectively. Eect o parameters o processing techniques as screw speed or extrusion rate were studied by other researchers and they have ound that screw speed has nearly the same eect on the ibre residual length [5,13,19-20]. It is obvious rom Figure 7 that the speed o rotation aects tensile modulus o reinorced LDPE. High values were obtained at moderate speed o 17 rpm. At low speed there is low shear rate which reduces the average ibre length rom 8 mm beore processing to 5.2 mm. This condition keeps ibres in good orientation and low shear rate would give high melt viscosity. Thereore, the matrix cannot wet the ibres which in spite o good ibres orientation it leads to bad and Figure 8. Eect o screw speed on modulus eiciency actor. poor adhesion o the LDPE and glass ibre. This poor adhesion produces low values o tensile modulus. On the other hand, at high speed o 28 rpm, it is noticed that tensile modulus and ductility decrease again. This phenomenon can be attributed to the great damages o the ibres where ibre length reduced rom 8 mm beore processing to about 2.8 mm and also ibres being oriented randomly. These results can be supported by the estimation o modulus eiciency actor, which depends directly on wettability, orientation, and aspect ratio. This is an indication o the degree o adhesion between LDPE matrix and short glass ibres. The eect o screw speed on modulus eiciency actor is shown in Figure 8 and its eect on adhesion is summarized in Table 2. CONCLUSION Home-made single screw extruder with designed parameters can be used to process short glass reinorced thermoplastics. Optimum parameters o operating conditions; temperature, and screw rotation speed led to high wettability, longitudinal orientation o the ibres within the matrix, and minimum damage o the ibres. All these parameters were related to modulus eiciency actor K E which is a direct indication o the adhesion between ibres and the matrix. It was concluded that i the extruder operated at 200 C and its screw rotated at moderate speed, better adhesion would be obtained and the best mechanical properties o the inal product are achieved. It is noticed rom the previous results that tensile modulus o LDPE-short glass ibre composite processed through the designed extruder increases up to wt% residual ibre content (20 wt% prepared sample). This is then decreased to relatively lower value due to the Iranian Polymer Journal / Volume 16 Number 10 (2007) 725

8 Tensile Properties o Extruded Short Glass Fibre... Sawalha S. et al. ormation o voids and the decrease in wettability between the ibres and matrix which lead to the decreased adhesion. It is clear that there is a great discrepancy between the values o experimental and theoretical tensile moduli which may be explained by the damage o ibres, poor distribution/dispersion o ibres in LDPE matrix, and poor adhesion between reinorcement and matrix. From the values o the residual ibre weight percentages (ibre content ater processing) it is concluded that the mixing method used prior to the extruder is ineicient and leads to the decrease in ibre content and the poor distribution /dispersion o ibres within the polymeric matrix. REFERENCES 1. Yang H.S., Wolcott M.P., Kim H.S., Kim S., Kim H.J., Eect o dierent compatibilizing agents on the mechanical properties o lignocellulosic materials illed polyethylene bio-composites, Comp. Struct., 79, , Kontou E., Niaounakis M., Thermo-mechanical properties o LLDPE/SiO 2 nanocomposites, Polymer, 47, , Georgopoulos S. Th., Tarantili P.A., Avergerinos E., Adreopoulos A.G., Koukios E.G., Thermoplastic polymers reinorced with ibrous agricultural residues, Polym. Deg. Stab., 90, , Bkiaris D., Matzinos P., Prinos J., Flaris V., Larena A., Panayotou C., Use o silanes and copolymers as adhesion promoters in glass iber/polyethylene composites, J. Appl. Polym. Sci., 80, , Barbosa S.E., Kenny J.M., Processing o shortiber reinorced polypropylene-inluence o processing conditions on the morphology o extruded ilaments, Polym. Eng. Sci., 40, 11-22, Qiu W., Mai K., Zeng H., Eect o macromolecular coupling agent on the property o PP/GF composites, J. Appl. Polym. Sci., 71, , Dyer S.R., Lassila L.V.J., Jokinen M., Vallitu P.K., Eect o iber position and orientation on racture load o iber reinorced composite, Dent. Mater., 20, , Berlin A.A., Volson S.A., Enikolopian N.S., Negmatov S.S., Principles o Polymer Composites, Springer-Verlag, Berlin Heidelberg, Ch. 3, Mallik P.K., Fiber-Reinorced Composites, Marcel & Dekker, New York, Ch. 3, Nielsen L.E., Landel R.F., Mechanical Properties o Polymers and Composites, Marcel Dekker, New York, Ch Bigg D.M., Hiscock D.F, Preston J.R., Bradbury E.J., Thermoplastic matrix sheet composites, Polym. Comp., 9, , Bigg D.M., Bradbury E.J., The impact perormance o sheet plastic composites, Polym. Eng. Sci., 32, , Sanomura Y., Hayakawa K., Mizuno M., Kawamura M., Eect o process conditions on young s modulus and strength o extrudate in short-iber-reinorced polypropylene, Polym. Comp., 28, 29-35, William D., Callister Jr., Materials Science and Engineering: An Introduction, John Wiley, New York, Ch. 6, Kalaprasad G., Kuruvilla J., Sabu T., Inluence o short iber addition on the mechanical properties o sisal reinorced low density polyethylene composites, J. Comp. Mater., 31, , Gupta A.P., Saroop U.K., Verma M., Studies o mechanical and thermal properties o polypropylene/lldpe-copolymer blends and its glass iber compositions, Polym.-Plast. Tech. Eng., 43, , Lee N.-J., Jang J., The eect o ibre content on the mechanical properties o glass ibre mat/polypropy lene composites, Comp. Part A. Appl. Sci. Manu., 30, , Morton-Jones D.H., Polymer Processing, Chapman & Hall, London, Ch. 2, Yukio S., Kawamura M., Fiber orientation control o short-iber reinorced thermoplastic by ram extrusion, Polym. Comp., 24, , Wall D., Processing o iber reinorced thermoplastic using co-rotating twin screw extruder, Polym. Comp., 10, , Iranian Polymer Journal / Volume 16 Number 10 (2007)