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Improvement of PVC wearability by addition of additives
Powder Technology 103 Ž1999. 182–188
Improvement of PVC wearability by addition of additives Fenglin Yang ) , Vladimir Hlavacek Laboratory for Ceramic and Reaction Engineering, Department of Chemical Engineering, State UniÕersity of New York at Buffalo, Amherst, NY 14260, USA Received 26 June 1998; received in revised form 22 December 1998
Abstract The wear of polyvinyl chloride ŽPVC. filled with a variety of inorganic and organic additives in different proportions and combinations have been investigated. Wear of pure PVC was tested for comparison. Grade 600 silicon carbide abrasive paper was used as abrading counterface. Results showed that the additives Žfillers. have different effects on the wear resistance of PVC composites according to the properties of the additives and the amount added. SiC and Al 2 O 3 as filler improved the wear resistance of PVC significantly. Si and wollastonite also enhanced the wear resistance. Fly ash and B 4C reduced wear of PVC only when over 10% of fly ash or 7% of B 4C was added. Surprisingly, PVC filled with CaCO 3 or SiO 2 , the most commonly used fillers in industrial PVC materials, gave rise to a very low wear resistance. Based on experimental observations, characteristics of wear of PVC composites can be classified into four categories: Ža. worn material in the form of fragmented thin and coherent transfer film; Žb. fairly thick transfer film loosely bonded on to the sliding track; Žc. poorly adhering debris and particles left around the sliding track; Žd. little worn material was observed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: PVC; Wear; Additives; Abrading counterface; Silicon carbide
1. Introduction Polyvinyl chloride ŽPVC. is now the second most commonly used plastic material in terms of volume. Great achievements have been made in modifying its structure, behavior, and properties by including miscellaneous additives. Aspects of mechanical, thermal and electrical properties of PVC composites were intensively studied. However, knowledge of the effects of fillers on wear resistance of PVC composite materials are limited and wear data are not available. Wear is generally understood as progressive loss of material from the surface of a solid body caused by a mechanical action, i.e., the contact and relative motion with a solid, liquid, or gaseous phase. Since at least two components of a system are involved in wear, it is not a pure material characteristic, but only a system characteristic. For rigid polymers, the simplest physical concept of abrasive wear is that asperities of the harder surface penetrate the polymer and remove material by shearing or
)
Corresponding author
cutting w1x. Later studies w2x showed that the roughness of the counterface has a profound effect on the wear of some polymers. Polymers usually have poor resistance to abrasive sliding attack because of their relatively low levels of hardness and strength, high plasticity, and low thermal conductivity. Fillers will certainly improve these properties therefore promote the wear resistance. The mechanisms revealing how fillers reduce wear are not well established. At present, there appear to be two broad explanations. One of them stems from the observation that excess filler concentration is noticed on the composite surface after prolonged sliding. Based on this observation the bulk of the load is supported by the concentrated filler resulting in increased wear resistance of the composite w3x. The second suggestion is that fillers may improve the adhesion of the transfer film to the counterface and thereby suppress the wear process. While the first explanation might be accepted easily by common sense, the second is complicated. In a broad sense, it is considered to be either physical or chemical in nature. Physical interaction involves van der Waals forces and is comparable in strength with the forces between
0032-5910r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 9 9 . 0 0 0 2 6 - 1
F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
molecular chains within the polymer itself. The chemical interaction, which is promoted by the high temperature generated at the sliding interface, may increase the adhesion of the transfer film. Some studies proved that there exists oxidation or chemical decomposition of unfilled and filled polymers in sliding contact w4x. However, the evidence for chemical effects in improving the adhesion of transfer film to the counterface is as yet inconclusive. Since a large number of fillers are abrasive, they tend to abrade the counterface. The effect of the fillers to the resistance of wear depends on many factors, such as the shape, size, relative hardness and aspect ratio of the filler particles, the content of filler in the composite, the sliding speed of the specimen on the counterface, the load exerted on the specimen, the environmental condition and so on w5,6x. The initial counterface roughness also seems to affect the transfer film and so the wear rate. It has been stated in the literature w7x that there is an optimum roughness for minimum wear. A direct correlation between these variables and the wear rate is often difficult to establish because of the breakdown and accumulation of the filler particles in the contact zone w8,9x and the work hardening of the counterface w10x. Many composites exhibit pronounced environmental sensitivity, which is attributed to specific filler–environment interactions w11,12x. A collection of data abstracted from the literature by Arkles et al. w13x shows no discernible pattern of behavior in this respect. The objective of this work is not to correlate these variables but to investigate the wear behavior of PVC filled with different materials in varying proportions and combinations, and to explore a practical means of obtaining high wear-resistant PVC composites. The chosen fillers for this study include inorganic materials such as Si, SiC, Al 2 O 3 , CaCO 3 , B 4 C, wollastonite fiber, fly ash, and organic materials such as carbon black, graphite, ABS, PTFE, etc.
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Abrasive, Niagara Falls, NY. was used as the counterface. SEM ŽModel S-800, Hitachi, Gaithersburg, MD. characterized morphologies of the sliding track on the counterface. 2.2. Experimental procedure General experimental procedures include the following three major steps. 2.2.1. Mixing of powders Commercial unplasticized PVC powder and miscellaneous fillers were procured from the manufacturers. Specifications of PVC and the fillers are listed in Table 1. Mixing of PVC powder with filler is critical for obtaining a specimen of uniform physical properties. To achieve uniformity, the following steps were taken. First, the PVC powder and filler were weighed according to designated proportions. A typical dose was 20 g. The weighed samples together with 100 milling balls Ž4 mm in diameter. were put into a plastic bottle Ždiameters 50 mm, heights 80 mm.. The bottle was then fixed to the Turbula Shaker– Mixers and was rotated and shaken for 2 h to get a well-mixed mixture. The proportions of additives are measured by weight percentage. 2.2.2. Hot molding The principle of specimen molding process is to heat the PVCrfiller powder to produce a PVC melt while exerting high pressure so that the filler may be embedded into a PVC matrix. The die was designed to make a specimen in the form of 12 mm Ždiameter. by 25 mm Žlength. cylinder. The molding process occurs as follows. As shown in Fig. 1, a powder mixture of 5 g was loaded into the die, a load of 1 ton was applied onto it through a punch, and heating was begun. When temperature of 1008C was reached, it was maintained for five min, and then
2. Experimental set-up and procedure 2.1. Supporting equipment An analytical balance ŽModel 200, maximum sensitivity 0.0001 g, Sartorius, Long Island, NY. was used for the sample mass measurement. PVC and filler powders were mixed by a Turbula Shaker–Mixers ŽType T2C, Impandex, NJ.. Specimens of the composites were molded by a press ŽLaboratory Press, Fred S. Carver. and a die made of steel. Electrical heating tape was wound on the outside surface of the die to supply heat for melting the powder mixture. The heating temperature was controlled by a temperature controller ŽType BS5001 K2, 10008C, OMEGA Engineering.. Wear tests were performed in a pin-on-disc machine. Grade 600 silicon carbide abrasive paper ŽCarborundum
Table 1 Specifications of PVC and fillers Name
Particle size
PVC Al 2 O 3 SiC Si SiO 2 Fly ash Žinorganic part. B4C CaCO 3 Wollastonite fiber ABS PTFE Graphite Carbon black
Unplasticized, y100 mesh Fused, y325 mesh y400 mesh y325 mesh y325 mesh y325 mesh y270 mesh y325 mesh NYAD G y120 mesh 12 mm y400 mesh 14 mm
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F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
Ti-coupling agent is considered to have two active parts. One part is compatible with the inorganic filler and the other part with polymer w14x. The amount of this agent added was 1% Žby weight. of the filler. To attain uniform distribution of the coupling agent, cyclohexane was used as a solvent to dissolve the coupling agent and then the filler was immersed into the solution. The slurry was heated and stirred until a complete evaporation of the solvent occurred. The filler coated by coupling agent was then mixed with PVC powder and was hot molded. Experimental results indicate that this coupling agent improved essentially the adhesion between certain inorganic filler and PVC.
Fig. 1. Hot molding set-up: Ž1. temperature controller, Ž2. press, Ž3. plug, Ž4. wire, Ž5. electric heating tape, Ž6. punch, Ž7. die and Ž8. specimen.
temperature was set to 1808C and the load increased to 2 tons. After 1808C was attained, the temperature was held for 2 min and heating was discontinued. During the 2 min, the load was released to zero and reapplied to 2 tons three times to release bubbles inside the specimen. The die, with the specimen in it, was cooled to room temperature under 2 tons of load. The specimen was then removed from the die. The control of temperature and the heating time is the most important step in the hot molding process. If the temperature is too low and the heating time is not sufficient, PVC will not melt completely and the specimen will be a semi-powder stick with very low strength. If over heated, PVC will ‘burn’ and will lose its strength also. Thermal dehydrochlorination of PVC starts at 1008C w14x. For this reason, the process is designed to achieve the melting of PVC with minimum degradation by first heating the sample above glass transition temperature, Tg Ž70– 808C. but below melting point Tm Ž165–2108C., and then completing the melting process at 1808C for a fairly short time. The bubble removal step guarantees the specimen with uniform structure and free of void. It was observed during the experiments that some inorganic filler was incompatible with PVC, making the fabricated specimen fragile. To overcome this, the following two methods were applied. Ža. A total of 10% ABS powder was added to the PVCrfiller powder mixture. ABS is highly compatible with PVC and is widely used as an impact modifier of PVC to enhance its toughness. The blend of PVCrABS forms a copolymer or graft during the melting process. By blending of ABS, the mechanical properties of the polymer matrix are substantially improved and the strength of the composite is ameliorated. Žb. Titanium butoxide was introduced as a coupling agent for some of the composites in order to improve the adhesion and wetting between PVC and the filler. This
2.2.3. Wear measurement The device for wear test is shown in Fig. 2. The pin-on-disc machine consisted of a stationary, cylindrical pin Žspecimen. resting on the surface of a flat disc which rotates in a horizontal plane. Grade 600 silicon carbide abrasive paper fixed onto the disc was used as the abrading counterface. Load can be added on top of the pin. The abrasive paper from the same batch was used throughout to avoid deviation of the measurement. The wear measurement was performed under the following conditions: Load Wear time Sliding speed Counterface
4N 10 min 0.785 mrs Grade 600 silicon carbide abrasive paper
The pin specimen was first pre-rubbed under the same experimental conditions before the wear test until even contact with the counterface was established, then the abrasive paper on the disc was replaced with a fresh piece and the wear test started. The wear of the specimen was measured by weighing the pin specimen before and after
Fig. 2. Pin-on-disc machine: Ž1. disc, Ž2. abrasive paper, Ž3. sample holder, Ž4. load, Ž5. pivot, Ž6. specimen, Ž7. motor and Ž8. speed controller.
F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
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the wear test Žthe pin specimen was cleaned before weighing.. 3. Results and discussions 3.1. Loss of material after wear Mass loss of pure PVC after wear under the above mentioned testing conditions was 0.0190 g. The wear of all the PVC composites was expressed as a relative mass loss, which is calculated by the following equation: WC Xs = 100% WPVC where, X is the relative mass loss, WC is the mass loss of composite and WPVC is the mass loss of pure PVC. All wear results were plotted in Fig. 3Ža. to Žc.. It is seen from Fig. 3 that the composites of PVCr Al 2 O 3rcoupling agent ŽFig. 3Ža.. and PVCrSiCrABS ŽFig. 3Žb.. have a sharply reduced wear rate. Their relative mass loss after wear is 18.42% and 12.11% respectively with only 5% filler added. Si as a filler ŽFig. 1Ža.. causes an obvious reduction of wear rate and the reduction is proportional to the Si content. The combination of PVCrwollastoniterABS ŽFig. 3Žb.. reduced wear rate only when the wollastonite content was over 20%, but when the coupling agent was used, the improvement of wear was obvious, B 4 C and fly ash ŽFig. 3Žc.. filled to PVCrABS blend improved wear resistance only when over 10% of fly ash or 7% of B 4 C was added. Surprisingly, CaCO 3 ŽFig. 3Ža.,Žb.. and SiO 2 ŽFig. 3Žc.., the most commonly used fillers for industrial PVC materials, had a negative effect on wear resistance. The wear of PVCrABSrCaCO3 was approximately 2.4 times that of pure PVC when the content of CaCO 3 was in the range of 5%–30%. PVCrABSrSiO2 has a wear weight loss close to that of PVCrABSrCaCO3 when 5–10% of SiO 2 was added. Though the wear rate decreased as more SiO 2 was included, it was still 1.3 times that of pure PVC even with 30% of SiO 2 contained. The blending of ABS into PVCrfiller afforded better strength to the specimen, but ABS alone ŽFig. 3Žc.. has no positive effect on the wear resistance. Molding of PVC and PTFE mixture was attempted. Fabricated samples were brittle even at a low composition of 2% PTFE. This probably was attributed to the non-compatibility of these two polymers resulted from their different structures and physical properties. Carbon black and graphite are not compatible with PVC. Attempts in preparing samples were futile even with 5% of carbon or graphite added. 3.2. Morphologies of the sliding track Morphologies of four kinds of typical sliding tracks on the counterface were recorded by SEM. Figs. 4Ža. –7Ža.
Fig. 3. Ža. – Žc. Relative wear mass loss of PVC composites.
show the sliding tracks amplified 200 times. These clearly reveal the transfer film on the counterface. In these figures, the rough background is the surface of the abrasive paper and the smooth areas reflect the transfer film formed. Figs. 4Žb. –7Žb. are the tracks amplified 350 times giving a closer look at how the worn materials were bonded onto the counterface.
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F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
Fig. 4. SEM photos of the sliding track of PVCrSi amplified: Ža. 20 times and Žb. 350 times.
Fig. 5. SEM photos of the sliding track of PVCrABSrCaCO3 amplified: Ža. 20 times and Žb. 350 times.
F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
Fig. 6. SEM photos of the sliding track of pure PVC amplified: Ža. 20 times and Žb. 350 times.
Fig. 7. SEM photos of the sliding track of PVCrSiC amplified: Ža. 20 times and Žb. 350 times.
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3.3. Classification of the filled PVC composites By summarizing the experimental data and observations of the worn material on the counterface, the characteristics of the wear of PVC composites can be classified into four categories for the fillers studied. Ža. For the PVCrSi composite, the worn material was in the form of fragmented thin and coherent transfer film ŽFig. 4.. This transfer film was formed mainly by embedment of worn material into the crevices of the asperities on the abrasive paper. The presence of Si improved the adhesion of the transfer film. The asperities were covered by the transfer film so that the abrasion was diminished, which explained why Si-filled PVC reduces the wear rate. Žb. For composites such as PVCrABS, PVCrABSr wollastonite, PVCrABSrB4 C, and PVCrABSrCaCO3 a fairly thick transfer film was loosely bonded onto the sliding track ŽFig. 5.. It seems that the bonding between the transfer film and the counterface is primarily due to mechanical interlocking. Filler does not improve the adhesion significantly. Such a film is scraped off easily when subjected to the shearing Žor peeling. action produced when the composite material slides against it. A fresh film is continuously being formed as the old one is removed. Direct observation of the counterface during the wear test indicated the formation and removal of the transfer film by the continued accumulation of debris and particles near both edges of the sliding track. In this case, the wear was mainly due to the loss of material arising from the transfer followed by the scraping action and that is probably the reason why most of those composites showed insignificant improvement on the wear resistance and some even showed a negative effect. Žc. For pure PVC and PVCrCaCO3 composites, poorly adhering debris and particles were left on the counterface ŽFig. 6.. A coherent transfer film on the counterface was not detected. From Fig. 6a, only the crimping scraps are noticeable on the sliding track. This suggested that the heavy wear of the composite observed on the counterface was caused by the penetration of the asperities of the harder counterface. In other words, shearing or cutting of the composite and the easy removal of the loosely deposited particles and debris from the sliding track accounted for the wear loss of this composite material. Žd. PVCrAl 2 O 3 and PVCrABSrSiC composites demonstrated a strong wear resistance. Little worn material observed ŽFig. 7.. Direct inspection on the abraded surface of the specimen showed the presence of concentrated filler. The hardness of the filler and firm bonding between filler and polymer provided these composites with enhanced mechanical properties, which is equivalent to that of the
abrasive paper. The load is supported by the concentrated filler, which protects the specimen from being abraded, resulting in the increased wear resistance of the composite.
4. Conclusions The addition of various fillers to PVC matrix modifies the wear-resistance of the composites. SiC and Al 2 O 3 show the highest improvement on the wear resistance. Si and wollastonite enhance the wear resistance obviously. Fly ash and B 4 C reduced wear only when 10% fly ash or 7% of B 4 C was included. CaCO 3 and SiO 2 have a negative effect on wear resistance. Carbon, graphite, and PTFE are not compatible with PVC. Blending of ABS to the composite improves the strength of the composite. Titanium butoxide as a coupling agent enhances the adhesion and wetting between polymer and some inorganic fillers.
References w1x J.K. Lancaster, Abrasive wear of polymers, Wear 14 Ž1969. 223–239. w2x D. Hollander, J.K. Lancaster, An application of topographical analysis to the wear of polymers, Wear 25 Ž1973. 155. w3x B. Arkles, J. Theberge, M. Schireson, Wear behavior of thermoplastic-filled PTFE composites, J. Am. Soc. Lubric. Eng. 33 Ž1977. 33–38. w4x D.H. Buckley, R.L. Johnson, NASA Report TN-D-2073, 1963. w5x J.K. Lancaster, Polymer-based bearing materials: the role of fillers and fiber reinforcement, Tribology 5 Ž1972. 249–255. w6x C.J. Speerschneider, C.H. Li, The role of filler geometrical shape in wear and friction of filled PTFE, Wear 5 Ž1962. 392–399. w7x J.P. Giltrow, J.K. Lancaster, The role of the counterface in the friction and wear of carbon fiber reinforced thermosetting resins, Wear 6 Ž1970. 359–374. w8x B.J. Briscoe, M.D. Steward, A.J. Groszek, The effect of carbon aspect ratio on the friction and wear of PTFE, Wear 42 Ž1977. 99–107. w9x K. Tanaka, Friction and wear of glass and carbon fiber-filled thermoplastic polymers, in: W.A. Glaeser, K.C. Ludema, S.K. Rhee ŽEds.., Wear of Materials 1977, ASME, New York, 1977, pp. 510–517. w10x J.K. Lancaster, Lubrication of carbon fiber-reinforced polymers, Wear 20 Ž1972. 315–333. w11x R.R. Hart, Performance of PTFE based piston rings in an unlubricated compressor, Proc. Inst. Mech. Eng. 181 Ž1996. 13–22, Part 1. w12x J.H. Fuchsluger, R.D. Taber, The effect of atmospheric environment on the wear properties of filled TFE materials, J. Lubric. Technol. 93 Ž1971. 423–429. w13x B.C. Arkles, S. Gerakaris, R. Goodhue, Wear characteristics of fluoropolymer composites, in: L.H. Lee ŽEd.., Advances in Polymer Friction and Wear, Polym. Sci. Technol. 5B Ž663. Ž1974. 663–688. w14x J.A. Brydson, Plastics Materials Ž5th edn.., Butterworth, London, 1988.
Improvement of PVC wearability by addition of additives Fenglin Yang ) , Vladimir Hlavacek Laboratory for Ceramic and Reaction Engineering, Department of Chemical Engineering, State UniÕersity of New York at Buffalo, Amherst, NY 14260, USA Received 26 June 1998; received in revised form 22 December 1998
Abstract The wear of polyvinyl chloride ŽPVC. filled with a variety of inorganic and organic additives in different proportions and combinations have been investigated. Wear of pure PVC was tested for comparison. Grade 600 silicon carbide abrasive paper was used as abrading counterface. Results showed that the additives Žfillers. have different effects on the wear resistance of PVC composites according to the properties of the additives and the amount added. SiC and Al 2 O 3 as filler improved the wear resistance of PVC significantly. Si and wollastonite also enhanced the wear resistance. Fly ash and B 4C reduced wear of PVC only when over 10% of fly ash or 7% of B 4C was added. Surprisingly, PVC filled with CaCO 3 or SiO 2 , the most commonly used fillers in industrial PVC materials, gave rise to a very low wear resistance. Based on experimental observations, characteristics of wear of PVC composites can be classified into four categories: Ža. worn material in the form of fragmented thin and coherent transfer film; Žb. fairly thick transfer film loosely bonded on to the sliding track; Žc. poorly adhering debris and particles left around the sliding track; Žd. little worn material was observed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: PVC; Wear; Additives; Abrading counterface; Silicon carbide
1. Introduction Polyvinyl chloride ŽPVC. is now the second most commonly used plastic material in terms of volume. Great achievements have been made in modifying its structure, behavior, and properties by including miscellaneous additives. Aspects of mechanical, thermal and electrical properties of PVC composites were intensively studied. However, knowledge of the effects of fillers on wear resistance of PVC composite materials are limited and wear data are not available. Wear is generally understood as progressive loss of material from the surface of a solid body caused by a mechanical action, i.e., the contact and relative motion with a solid, liquid, or gaseous phase. Since at least two components of a system are involved in wear, it is not a pure material characteristic, but only a system characteristic. For rigid polymers, the simplest physical concept of abrasive wear is that asperities of the harder surface penetrate the polymer and remove material by shearing or
)
Corresponding author
cutting w1x. Later studies w2x showed that the roughness of the counterface has a profound effect on the wear of some polymers. Polymers usually have poor resistance to abrasive sliding attack because of their relatively low levels of hardness and strength, high plasticity, and low thermal conductivity. Fillers will certainly improve these properties therefore promote the wear resistance. The mechanisms revealing how fillers reduce wear are not well established. At present, there appear to be two broad explanations. One of them stems from the observation that excess filler concentration is noticed on the composite surface after prolonged sliding. Based on this observation the bulk of the load is supported by the concentrated filler resulting in increased wear resistance of the composite w3x. The second suggestion is that fillers may improve the adhesion of the transfer film to the counterface and thereby suppress the wear process. While the first explanation might be accepted easily by common sense, the second is complicated. In a broad sense, it is considered to be either physical or chemical in nature. Physical interaction involves van der Waals forces and is comparable in strength with the forces between
0032-5910r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 9 9 . 0 0 0 2 6 - 1
F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
molecular chains within the polymer itself. The chemical interaction, which is promoted by the high temperature generated at the sliding interface, may increase the adhesion of the transfer film. Some studies proved that there exists oxidation or chemical decomposition of unfilled and filled polymers in sliding contact w4x. However, the evidence for chemical effects in improving the adhesion of transfer film to the counterface is as yet inconclusive. Since a large number of fillers are abrasive, they tend to abrade the counterface. The effect of the fillers to the resistance of wear depends on many factors, such as the shape, size, relative hardness and aspect ratio of the filler particles, the content of filler in the composite, the sliding speed of the specimen on the counterface, the load exerted on the specimen, the environmental condition and so on w5,6x. The initial counterface roughness also seems to affect the transfer film and so the wear rate. It has been stated in the literature w7x that there is an optimum roughness for minimum wear. A direct correlation between these variables and the wear rate is often difficult to establish because of the breakdown and accumulation of the filler particles in the contact zone w8,9x and the work hardening of the counterface w10x. Many composites exhibit pronounced environmental sensitivity, which is attributed to specific filler–environment interactions w11,12x. A collection of data abstracted from the literature by Arkles et al. w13x shows no discernible pattern of behavior in this respect. The objective of this work is not to correlate these variables but to investigate the wear behavior of PVC filled with different materials in varying proportions and combinations, and to explore a practical means of obtaining high wear-resistant PVC composites. The chosen fillers for this study include inorganic materials such as Si, SiC, Al 2 O 3 , CaCO 3 , B 4 C, wollastonite fiber, fly ash, and organic materials such as carbon black, graphite, ABS, PTFE, etc.
183
Abrasive, Niagara Falls, NY. was used as the counterface. SEM ŽModel S-800, Hitachi, Gaithersburg, MD. characterized morphologies of the sliding track on the counterface. 2.2. Experimental procedure General experimental procedures include the following three major steps. 2.2.1. Mixing of powders Commercial unplasticized PVC powder and miscellaneous fillers were procured from the manufacturers. Specifications of PVC and the fillers are listed in Table 1. Mixing of PVC powder with filler is critical for obtaining a specimen of uniform physical properties. To achieve uniformity, the following steps were taken. First, the PVC powder and filler were weighed according to designated proportions. A typical dose was 20 g. The weighed samples together with 100 milling balls Ž4 mm in diameter. were put into a plastic bottle Ždiameters 50 mm, heights 80 mm.. The bottle was then fixed to the Turbula Shaker– Mixers and was rotated and shaken for 2 h to get a well-mixed mixture. The proportions of additives are measured by weight percentage. 2.2.2. Hot molding The principle of specimen molding process is to heat the PVCrfiller powder to produce a PVC melt while exerting high pressure so that the filler may be embedded into a PVC matrix. The die was designed to make a specimen in the form of 12 mm Ždiameter. by 25 mm Žlength. cylinder. The molding process occurs as follows. As shown in Fig. 1, a powder mixture of 5 g was loaded into the die, a load of 1 ton was applied onto it through a punch, and heating was begun. When temperature of 1008C was reached, it was maintained for five min, and then
2. Experimental set-up and procedure 2.1. Supporting equipment An analytical balance ŽModel 200, maximum sensitivity 0.0001 g, Sartorius, Long Island, NY. was used for the sample mass measurement. PVC and filler powders were mixed by a Turbula Shaker–Mixers ŽType T2C, Impandex, NJ.. Specimens of the composites were molded by a press ŽLaboratory Press, Fred S. Carver. and a die made of steel. Electrical heating tape was wound on the outside surface of the die to supply heat for melting the powder mixture. The heating temperature was controlled by a temperature controller ŽType BS5001 K2, 10008C, OMEGA Engineering.. Wear tests were performed in a pin-on-disc machine. Grade 600 silicon carbide abrasive paper ŽCarborundum
Table 1 Specifications of PVC and fillers Name
Particle size
PVC Al 2 O 3 SiC Si SiO 2 Fly ash Žinorganic part. B4C CaCO 3 Wollastonite fiber ABS PTFE Graphite Carbon black
Unplasticized, y100 mesh Fused, y325 mesh y400 mesh y325 mesh y325 mesh y325 mesh y270 mesh y325 mesh NYAD G y120 mesh 12 mm y400 mesh 14 mm
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F. Yang, V. HlaÕacekr Powder Technology 103 (1999) 182–188
Ti-coupling agent is considered to have two active parts. One part is compatible with the inorganic filler and the other part with polymer w14x. The amount of this agent added was 1% Žby weight. of the filler. To attain uniform distribution of the coupling agent, cyclohexane was used as a solvent to dissolve the coupling agent and then the filler was immersed into the solution. The slurry was heated and stirred until a complete evaporation of the solvent occurred. The filler coated by coupling agent was then mixed with PVC powder and was hot molded. Experimental results indicate that this coupling agent improved essentially the adhesion between certain inorganic filler and PVC.
Fig. 1. Hot molding set-up: Ž1. temperature controller, Ž2. press, Ž3. plug, Ž4. wire, Ž5. electric heating tape, Ž6. punch, Ž7. die and Ž8. specimen.
temperature was set to 1808C and the load increased to 2 tons. After 1808C was attained, the temperature was held for 2 min and heating was discontinued. During the 2 min, the load was released to zero and reapplied to 2 tons three times to release bubbles inside the specimen. The die, with the specimen in it, was cooled to room temperature under 2 tons of load. The specimen was then removed from the die. The control of temperature and the heating time is the most important step in the hot molding process. If the temperature is too low and the heating time is not sufficient, PVC will not melt completely and the specimen will be a semi-powder stick with very low strength. If over heated, PVC will ‘burn’ and will lose its strength also. Thermal dehydrochlorination of PVC starts at 1008C w14x. For this reason, the process is designed to achieve the melting of PVC with minimum degradation by first heating the sample above glass transition temperature, Tg Ž70– 808C. but below melting point Tm Ž165–2108C., and then completing the melting process at 1808C for a fairly short time. The bubble removal step guarantees the specimen with uniform structure and free of void. It was observed during the experiments that some inorganic filler was incompatible with PVC, making the fabricated specimen fragile. To overcome this, the following two methods were applied. Ža. A total of 10% ABS powder was added to the PVCrfiller powder mixture. ABS is highly compatible with PVC and is widely used as an impact modifier of PVC to enhance its toughness. The blend of PVCrABS forms a copolymer or graft during the melting process. By blending of ABS, the mechanical properties of the polymer matrix are substantially improved and the strength of the composite is ameliorated. Žb. Titanium butoxide was introduced as a coupling agent for some of the composites in order to improve the adhesion and wetting between PVC and the filler. This
2.2.3. Wear measurement The device for wear test is shown in Fig. 2. The pin-on-disc machine consisted of a stationary, cylindrical pin Žspecimen. resting on the surface of a flat disc which rotates in a horizontal plane. Grade 600 silicon carbide abrasive paper fixed onto the disc was used as the abrading counterface. Load can be added on top of the pin. The abrasive paper from the same batch was used throughout to avoid deviation of the measurement. The wear measurement was performed under the following conditions: Load Wear time Sliding speed Counterface
4N 10 min 0.785 mrs Grade 600 silicon carbide abrasive paper
The pin specimen was first pre-rubbed under the same experimental conditions before the wear test until even contact with the counterface was established, then the abrasive paper on the disc was replaced with a fresh piece and the wear test started. The wear of the specimen was measured by weighing the pin specimen before and after
Fig. 2. Pin-on-disc machine: Ž1. disc, Ž2. abrasive paper, Ž3. sample holder, Ž4. load, Ž5. pivot, Ž6. specimen, Ž7. motor and Ž8. speed controller.
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the wear test Žthe pin specimen was cleaned before weighing.. 3. Results and discussions 3.1. Loss of material after wear Mass loss of pure PVC after wear under the above mentioned testing conditions was 0.0190 g. The wear of all the PVC composites was expressed as a relative mass loss, which is calculated by the following equation: WC Xs = 100% WPVC where, X is the relative mass loss, WC is the mass loss of composite and WPVC is the mass loss of pure PVC. All wear results were plotted in Fig. 3Ža. to Žc.. It is seen from Fig. 3 that the composites of PVCr Al 2 O 3rcoupling agent ŽFig. 3Ža.. and PVCrSiCrABS ŽFig. 3Žb.. have a sharply reduced wear rate. Their relative mass loss after wear is 18.42% and 12.11% respectively with only 5% filler added. Si as a filler ŽFig. 1Ža.. causes an obvious reduction of wear rate and the reduction is proportional to the Si content. The combination of PVCrwollastoniterABS ŽFig. 3Žb.. reduced wear rate only when the wollastonite content was over 20%, but when the coupling agent was used, the improvement of wear was obvious, B 4 C and fly ash ŽFig. 3Žc.. filled to PVCrABS blend improved wear resistance only when over 10% of fly ash or 7% of B 4 C was added. Surprisingly, CaCO 3 ŽFig. 3Ža.,Žb.. and SiO 2 ŽFig. 3Žc.., the most commonly used fillers for industrial PVC materials, had a negative effect on wear resistance. The wear of PVCrABSrCaCO3 was approximately 2.4 times that of pure PVC when the content of CaCO 3 was in the range of 5%–30%. PVCrABSrSiO2 has a wear weight loss close to that of PVCrABSrCaCO3 when 5–10% of SiO 2 was added. Though the wear rate decreased as more SiO 2 was included, it was still 1.3 times that of pure PVC even with 30% of SiO 2 contained. The blending of ABS into PVCrfiller afforded better strength to the specimen, but ABS alone ŽFig. 3Žc.. has no positive effect on the wear resistance. Molding of PVC and PTFE mixture was attempted. Fabricated samples were brittle even at a low composition of 2% PTFE. This probably was attributed to the non-compatibility of these two polymers resulted from their different structures and physical properties. Carbon black and graphite are not compatible with PVC. Attempts in preparing samples were futile even with 5% of carbon or graphite added. 3.2. Morphologies of the sliding track Morphologies of four kinds of typical sliding tracks on the counterface were recorded by SEM. Figs. 4Ža. –7Ža.
Fig. 3. Ža. – Žc. Relative wear mass loss of PVC composites.
show the sliding tracks amplified 200 times. These clearly reveal the transfer film on the counterface. In these figures, the rough background is the surface of the abrasive paper and the smooth areas reflect the transfer film formed. Figs. 4Žb. –7Žb. are the tracks amplified 350 times giving a closer look at how the worn materials were bonded onto the counterface.
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Fig. 4. SEM photos of the sliding track of PVCrSi amplified: Ža. 20 times and Žb. 350 times.
Fig. 5. SEM photos of the sliding track of PVCrABSrCaCO3 amplified: Ža. 20 times and Žb. 350 times.
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Fig. 6. SEM photos of the sliding track of pure PVC amplified: Ža. 20 times and Žb. 350 times.
Fig. 7. SEM photos of the sliding track of PVCrSiC amplified: Ža. 20 times and Žb. 350 times.
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3.3. Classification of the filled PVC composites By summarizing the experimental data and observations of the worn material on the counterface, the characteristics of the wear of PVC composites can be classified into four categories for the fillers studied. Ža. For the PVCrSi composite, the worn material was in the form of fragmented thin and coherent transfer film ŽFig. 4.. This transfer film was formed mainly by embedment of worn material into the crevices of the asperities on the abrasive paper. The presence of Si improved the adhesion of the transfer film. The asperities were covered by the transfer film so that the abrasion was diminished, which explained why Si-filled PVC reduces the wear rate. Žb. For composites such as PVCrABS, PVCrABSr wollastonite, PVCrABSrB4 C, and PVCrABSrCaCO3 a fairly thick transfer film was loosely bonded onto the sliding track ŽFig. 5.. It seems that the bonding between the transfer film and the counterface is primarily due to mechanical interlocking. Filler does not improve the adhesion significantly. Such a film is scraped off easily when subjected to the shearing Žor peeling. action produced when the composite material slides against it. A fresh film is continuously being formed as the old one is removed. Direct observation of the counterface during the wear test indicated the formation and removal of the transfer film by the continued accumulation of debris and particles near both edges of the sliding track. In this case, the wear was mainly due to the loss of material arising from the transfer followed by the scraping action and that is probably the reason why most of those composites showed insignificant improvement on the wear resistance and some even showed a negative effect. Žc. For pure PVC and PVCrCaCO3 composites, poorly adhering debris and particles were left on the counterface ŽFig. 6.. A coherent transfer film on the counterface was not detected. From Fig. 6a, only the crimping scraps are noticeable on the sliding track. This suggested that the heavy wear of the composite observed on the counterface was caused by the penetration of the asperities of the harder counterface. In other words, shearing or cutting of the composite and the easy removal of the loosely deposited particles and debris from the sliding track accounted for the wear loss of this composite material. Žd. PVCrAl 2 O 3 and PVCrABSrSiC composites demonstrated a strong wear resistance. Little worn material observed ŽFig. 7.. Direct inspection on the abraded surface of the specimen showed the presence of concentrated filler. The hardness of the filler and firm bonding between filler and polymer provided these composites with enhanced mechanical properties, which is equivalent to that of the
abrasive paper. The load is supported by the concentrated filler, which protects the specimen from being abraded, resulting in the increased wear resistance of the composite.
4. Conclusions The addition of various fillers to PVC matrix modifies the wear-resistance of the composites. SiC and Al 2 O 3 show the highest improvement on the wear resistance. Si and wollastonite enhance the wear resistance obviously. Fly ash and B 4 C reduced wear only when 10% fly ash or 7% of B 4 C was included. CaCO 3 and SiO 2 have a negative effect on wear resistance. Carbon, graphite, and PTFE are not compatible with PVC. Blending of ABS to the composite improves the strength of the composite. Titanium butoxide as a coupling agent enhances the adhesion and wetting between polymer and some inorganic fillers.
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