Studies on the Use of Cardanol as Plasticizer for Silica- Filled Nitrile Rubber

Transcription

1 Studies on the Use of Cardanol as Plasticizer for Silica- Filled Nitrile Rubber Mary Alexander 1,2, Beena T. Abraham 3 and Eby Thomas Thachil 1* 1 Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi, Pin: , Kerala, India 2 Department of Chemistry, Union Christian College, Aluva, Pin: , Kerala, India 3 Department of Chemistry, SNM College, Maliankara, Pin: , Kerala, India Received: 6 March 2007, Accepted: 5 February 2008 ABSTRACT Cardanol is the main ingredient of cashew nut shell liquid (CNSL), an agrobyproduct. Its comparative merits for plasticizing silica-filled nitrile rubber (NBR) is the subject of this paper. Esters like dioctyl phthalate (DOP), polyesters, polyester ethers, polyester polyethers, etc are the conventional plasticizers employed for such compounding requirements. In this study, the mechanical properties, ageing behavior and torque time curves during cure of NBR plasticized by both these materials are compared under identical conditions. It has been established that cardanol when used as plasticizer, gives rise to similar mechanical properties, cure times and superior thermal stability in NBR vulcanizates. INTRODUCTION Cardanol is the main ingredient of cashew nut shell liquid (CNSL), a by-product of the cashew industry. It is a naturally occurring substituted phenol with a long side chain in the meta position which can take part in a variety of reactions (1). The structure of cardanol is given in Figure 1. As an agro-byproduct it has the advantages of low cost and renewable supply (2-5) and can replace phenol (6) in many applications. Cardanol is obtained by vacuum distillation of commercial grade CNSL conforming to Indian Standard, I S: In an earlier publication (8) we have reported the results of a study for evaluating the performance of cardanol for plasticizing silica-filled natural rubber. *Author to whom correspondence should be addressed: ethachil@cusat.ac.in Phone No , Fax No Smithers Rapra Technology,

2 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil Figure 1. Structure of cardanol Plasticizers are generally used in NBR compounds for ease of processing and to improve low temperature properties. Epoxidized rubber seed oil has been used as plasticizer for the processing of NBR compounds (9). In the present work, cardanol has been used in place of DOP, a conventional plasticizer for plasticizing NBR. The properties of the vulcanizate and the torque-time behavior of the compounds during cure have been studied to make a comparative evaluation of DOP and cardanol performance. Emphasis is placed on the ageing characteristics of both vulcanizates. No attempt has been made to elucidate chemical interactions if any beween cardanol and other ingredients. Many workers have highlighted the potential for application of cardanol in rubber processing from time to time. The effect of cardanol-novolak on the properties of natural rubber has been investigated (10). A concentration of phr of the resin showed the greatest improvement in tensile strength. The hardness and abrasion resistance of the composite increased while the elongation at break decreased with increasing cardanol formaldehyde content. The effect of cardanol on the processing, curing, physical and mechanical properties and ageing of SBR has been studied (11). Compounding of natural rubber containing cardanol in the presence of black filler and subsequent vulcanization using various accelerator systems have been the subject of another study (12). The abrasion resistance, elongation and tear strength were higher compared to compounds not containing cardanol. The properties of natural rubber modified with phosphorylated cashew nut shell liquid (PCNSL) have been compared based on another plasticizer, 2-ethyl hexyl diphenyl phosphate (Santicizer 141) 13 by Pillai et al. Phosphorylated cashew nut shell liquid modified NR vulcanizates showed better tensile properties and resistance to thermo-oxidative decomposition and flame compared to those containing similar dosages of Santicizer 141. Modification of rubber by cardanol formaldehyde resins and epoxidised cardanol (14) has attracted the attention of another group. The adhesive properties of blends of cardanol resole resin with polychloroprene is another 76

3 recently explored subject (15). A phenol cardanol ratio of 80:20 is optimum for shear strength of Al-Al bonds, while 60:40 is optimum for both peel as well as shear strength. The copolymer based on phenol, cardanol and formaldehyde is a better choice for the resin than either of the individual condensation products of phenol or cardanol with formaldehyde. Rubber Plasticizers Plasticizers are low molecular weight non-volatile substances which improve the flexibility and processability of a polymeric material. Even small quantities of plasticizer markedly reduce the T g of the polymer. This effect is due to the reduction in cohesive forces of attraction between polymer chains. Plasticizer molecules penetrate the polymer matrix and establish polar attractive forces between the polymer chains and increase the segmental mobility, thereby reducing the T g value (16). A large number of plasticizers are used for rubber processing. Plasticizers are generally used in NBR compounds to improve processing and low temperature properties. Typically they are ester types, aromatic oils or polar derivatives and can be extractable or non extractable depending upon the end use applications (17). Examples are dibutyl phthalate, dibutyl sebacate, dioctyl phthalate, and trixylyl phosphate. Polyesters, polyester ethers, polyester polyethers, pentacrythritol ester and xylene formaldehyde resins are used. The viscosity, tackiness and processability of NBR compounds can be adjusted using plasticizers. They also influence the elasticity, low temperature flexibility and swelling resistance of the vulcanizates. In concentrations up to 30 phr, typical plasticizers based on ethers, esters or polythioethers are very effective in increasing the rebound elasticity and low temperature flexibility of vulcanizates (18). However all plasticizers negatively influence most mechanical properties and swell resistance of the vulcanizates and volatilize at high use temperatures. For high temperature resistance of NBR, plasticizers of low volatility, such as butyl carbitol formal and polyester polythioethers should be used (18). These plasticizers simultaneously improve low temperature flexibility. Several other substances having plasticizing ability are used for special situations. Often these materials bequeath other properties which make compounding easier or give improved vulcanizate properties. 2. RAW MATERIALS Acrylonitrile-butadiene rubber, Aparene N-50 with 44% acrylonitrile content and Mooney viscosity ML 80-95, was obtained from Apar Polymers Ltd., 77

4 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil Mumbai, India. Zinc Oxide and stearic acid were supplied by M/s Meta Zinc Ltd., Mumbai and Goderej Soaps Pvt Ltd., Mumbai respectively. Cyclo hexyl benzothiazil sulphenamide (CBS) and tetramethyl thiuram disulphide (TMTD) used in the present study were obtained from Polyolefine Industries, Mumbai. Sulphur was supplied by Standard Chemicals Co. Pvt. Ltd. Chennai. Precipitated silica used in the present study was supplied by Rub-Chem Industries, Mumbai. Dioctyl pthalate used in the present study was of commercial grade supplied by Rubo-Synth Impex Pvt. Ltd., Mumbai. The viscosity of the sample was 60 m Pa.s. Refined CNSL conforming to Indian Standard IS: 840(1964) was supplied by Pierce Leslie Limited, Cochin in 200 L barrels. The Indian standard specification is given in Table 1. Cardanol was separated from commercial grade CNSL by distillation under reduced pressure (1 mm Hg). The pale yellow fraction collecting at C is cardanol (19). Table 1. Indian Standards (IS: 840(1964) specification for CNSL Characteristic Requirement Specific gravity Viscosity at 30 C,cp(max) 550 Moisture,% by wt.(max) 1.0 Matter insoluble in toluene, % by wt.(max) 1.0 Loss in wt. on heating, % by wt.(max) 1.0 Ash, % by wt.(max) 1.0 Iodine value (max) a.) Wij s method b.) Catalytic method Polymerization Time, min(max) Viscosity at 30 C,cp(min) Viscosity after acid washing at 30 C,cp(min) EXPERIMENTAL 1) Compounding Mixes were prepared on a laboratory size two roll mixing mill (16 x 33 cm) at a friction ratio of 1:1.25 as per procedure given in ASTM D (2001) over a time period of 18 min. For this work only silica filler was employed as per compounding formulations given in Table 2. Each formulation was repeated with cardanol replacing aromatic oil. 78

5 Table 2. NBR formulation for varying silica content Sample NR ZnO St.acid DEG Silica DOP CBS TMTD S ) Curing The cure characteristics of the mixes were determined using Rubber Processing Analyser RPA 2000 supplied by Alpha Technologies, USA, as per ASTM D The rubber compounds were vulcanized up to the optimum cure time at 150 C and 11.6 MPa. The mouldings were cooled in water at the end of the curing cycle and stored in a cool dark place for 24 h prior to physical testing. 3) Physical Testing (a) Tensile tests: The tensile properties (ASTM D 412) were determined using dumb bell-shaped specimens punched out from vulcanized sheets. Tear tests (ASTM D 624) were conducted using Type-C un-nicked test pieces with a 90 angle and tab ends. The measurements were carried out on a Shimadzu Universal Testing Machine (10KN) with a grip separation of 40 mm, at a crosshead speed of 500 mm/min. (b) Hardness: The hardness (Shore A) of the moulded samples was determined using a Zwick 3114 Hardness Tester in accordance with ASTM D The tests were performed on unstressed samples of 30 mm diameter and 6 mm thickness. The readings were taken after 10 s of indentation when firm contact had been established with the specimen. (c) Abrasion resistance: The abrasion resistance of the samples was determined using a DIN Abrader (DIN 53516) as per ASTM D Samples having a diameter of 15±0.2 mm and a thickness of 20 mm were kept on a rotating sample holder and 10N load was applied. Initially, a pre-run was given for the sample and the initial weight taken. The weight after final run was also noted. The abrasion loss in cc/h was calculated using the formula; Abrasion loss = (loss of wt. / sp.gr.) x 60/2.2 for an abrasion time of 2.2 minutes. 79

6 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil (d) Rebound resilience: Rebound resilience is the ratio of energy given up on recovery from deformation to the energy required to produce the deformation. It is expressed as a percentage and is measured using a vertical rebound resilience tester as per ASTM D A plunger weighing 28±0.5 g is dropped from a height of 40 cm to the sample of thickness 12.5 mm and the rebound height is measured. 4) Soxhlet extraction Known weights of different samples of vulcanized rubber were packed in Wattman 1 filter paper and extracted in a Soxhlet apparatus using toluene as solvent and the loss of weight (%) was noted. This test was done on freshly moulded samples as well as on those subjected to aging for 72 hrs at 100C in an air oven. 5) Crosslink density Crosslink density was determined on both DOP and cardanol-based samples loaded with 15 phr silica using toluene as per ASTM D ) Ageing tests Oxidative ageing tests were carried out on the samples for ten days in accordance with ASTM D using an air oven at 100 C. After another 24 hours of conditioning at ambient temperature, tensile properties were determined as per ASTM D RESULTS AND DISCUSSION Curing studies indicate that cure characteristics do not vary much with the plasticizer. Figure 2 is a comparison of typical cure curves for both cases. Similar torque values are attained for cardanol and DOP based samples during cure. The minimum torque values (Table 3), which indicate viscosity of the compound, are lower for cardanol-based samples. Hence, cardanol is a better plasticizer for NBR. The lower values of maximum torque attained during cure and subsequent to it by the cardanol-containing compound point to a lower shear modulus and a more effective plasticizing action. 80

7 Figure 2. Cure curves of both cardanol and DOP-based NBR at 15 phr silica Table 3. Minimum and maximum torque values during cure for cardanol and DOP based samples with varying filler content Sample Torque 5 phr 10 phr 15 phr 20 phr 25 phr (dnm) Cardanol minimum DOP minimum Cardanol maximum DOP maximum Figure 3 shows the variation of tensile strength of silica-reinforced nitrile rubber for both cases. The cardanol-based samples show better tensile values. About 20 phr silica appears to be the optimal filler content for silica filled NBR. Figure 4 shows the change in elongation-at -break on using cardanol in place of DOP. Cardanol shows somewhat higher elongation at break. Modulus values at 300% elongation (not shown) are almost similar for both samples. The tensile properties, in general, show that cardanol is the superior plasticizer. The tear strength of samples containing both plasticizers has been measured as a function of silica content (Figure 5). Although both cases show almost equivalent performance at 5 phr silica content, the cardanol-based sample has clear superiority at higher silica contents. This is an indication of better 81

8 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil Figure 3. Variation of tensile strength (MPa) with silica loading Figure 4. Variation of elongation-at-break (%) with silica loading Figure 5. Variation of tear strength (MPa) with silica loading 82

9 wetting of the filler by cardanol resulting in a more homogeneous product in comparison with DOP. The better wetting properties may be the result of lower viscosity, structural similarities with carbon black etc. Figure 6 shows the variation of abrasion loss with silica content. Abrasion loss is slightly less when cardanol is used as plasticizer. This again points to better homogeneity in the vulcanizate possibly due to more effective wetting action by cardanol. Surface hardness (Figure 7) of specimens employing both plasticizers shows similar behaviour in both cases. Figure 8 is a plot of the weight loss on Soxhlet extraction of freshly moulded NBR samples plasticized by the two different materials. In general, the amounts of toluene-soluble material in the vulcanizate for both cases are similar. Figure 6. Variation of abrasion loss (cc/hr) with silica loading Figure 7. Variation of hardness (Shore A) with silica loading 83

10 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil Figure 9 shows the variation of crosslink density with silica content. Comparison of the two shows that cardanol-based samples have slightly lower crosslink density. This has not led to any reduction in mechanical properties as seen already. Figures indicate the ageing behavior of samples employing each plasticizer. The mechanical properties have been estimated after ageing. The behavior of the two sets of samples shows similarity in the case of tensile strength, tear strength, and elongation at break. These properties show a more rapid fall after the first day of ageing in the case of cardanol-based vulcanizate. Figure 8. Variation of weight loss after Soxhlet extraction of NBR vulcanizate with 5-25 phr silica, using toluene Figure 9. Variation of crosslink density with silica content 84

11 Figure 10. Ageing at 100 C: Variation of tensile strength of NBR (silica loading 15 phr) with number of days on using cardanol / DOP as plasticizer Figure 11. Ageing at 100 C: variation of elongation at break (%) of NBR (silica loading 15 phr) with number of days on using cardanol / DOP as plasticizer Figure 12. Ageing at 100 C: variation of tear strength of NBR (silica loading 15 phr) with number of days on using cardanol / DOP as plasticizer 85

12 Mary Alexander, Beena T. Abraham and Eby Thomas Thachil For the rest of the ageing period (3-10 days), the properties of cardanol and DOP based rubber show almost steady values. The elongation-at-break (Figure 12) is slightly higher for cardanol-based vulcanizate in the early stages of ageing. Hence embrittlement during ageing is less in the case of cardanol. Analysis of free sulphur content (Figure 13) shows that the amount of free sulphur in the case of cardanol goes up after one day of aging. One possible scenario for this involves vulcanization reactions between cardanol and sulphur (20). The sulphur cardanol linkages may be sensitive to the prolonged aging process like other vulcanized vegetable oils (factices) which are generally thermally unstable (21). This will result in liberation of sulphur after 24 h of ageing. This sulphur presumably gets utilized by the rubber. A similar behaviour has also been noticed in the case of natural rubber plasticized by cardanol (20). Figure 14 shows comparative weight loss of the vulcanizates on Soxhlet extraction after aging for 72 hrs. The amount of extractables after aging is substantially lower in the case of cardanol. Presumably cardanol remains unextractable even after breakage of the cardanol sulphur bond. This shows that cardanol is a more suitable candidate where thermal stability and extractability are of concern. The increase in extractables in the case of DOP might mean that the ester offers less protection to NBR from chain scission reactions leading to the formation of extractable oligomers. Figure 13. Variation of free sulphur content with ageing 86

13 Figure 14. Weight loss on Soxhlet extraction after ageing for 72 hr CONCLUSIONS Cardanol has considerable plasticizing effect on NBR. From a comparison of tensile strength, elongation at break, modulus and tear strength of the vulcanizates, it is seen that cardanol is equivalent to DOP as plasticizers for silica-filled NBR. Other properties like resilience, abrasion loss and surface hardness are also comparable for both plasticizers. Incorporation of cardanol also leads to similar cure times. Tensile strength and tear strength values after ageing establish that cardanol leads to a vulcanizate of similar thermal stability. Free sulphur analysis and extraction using toluene suggest that cardanol undergoes chemical changes during cure / ageing which are generally beneficial. Cardanol can hence be considered as superior to DOP as a plasticizer for NBR containing silica. REFERENCES 1. Mary C. Lubi and Eby Thomas Thachil, Cashew nut shell liquid (CNSL) a versatile monomer for polymer synthesis, Designed Monomers and Polymers, 3 (2000) 2, Wassermann, and C.R. Dawson, Ind. Eng.Chem., 37 (1945) Carraher C.E. and Sperling I.H., Polymer Applications of Renewable Resource Materials, Plenum press, NewYork (1981). 4. Roy S.S., Kundu A.K., and MaMaili S., J. Applied Polymer Sci., 36 (1988)

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15 20. Mary Alexander and Eby Thomas Thachil, A comparative study of cardanol and aromatic oil as plasticizers for carbon black filled natural rubber, J. Applied Polymer Sci.,102 (2006) 5, Werner Hofmann, 21. Rubber Technology Hand Book, p 306, Hanser publisher, New York (1989). 89