1091657

Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber LatexEffect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex S. Palaty1*, P.V. Devi1, and J. Honey2 1Department of Chemistry, Bharata Mata College, Thrikkakara, Cochin 682 022, India 2Facultyof Applied Sciences, Indian Institute of Space Science and Technology, Thiruvananthapuram 695022, India Received: 2 August 2010, Accepted: 20 December 2010 SUMMARY This paper reports on the study of the prevulcanisation of natural rubber latex (NRL) at room temperature. NRL was prevulcanised at room temperature using a zinc butyl xanthate [Zn(bxt)2]–zinc diethyl dithiocarbamate [ZDC] accelerator system. High-temperature (55–60°C) prevulcanisation of NRL was also done using the conventional accelerator system. Effect of storage on the colloidal properties of room-temperature prevulcanised latex was studied up to 30 days, and these properties were compared with those of conventional high-temperature prevulcanised latex. The colloidal properties were found to be superior for the room-temperature prevulcanised latex. The mechanical stability time (MST) of room-temperature prevulcanised latex increases on storage. The room temperature prevulcanised latex is found to be stable for 30 days of storage. Keywords: prevulcanisation; colloidal properties; mechanical stability time; volatile fatty acid number; potassium hydroxide number INTRODUCTION Prevulcanisation is the period for which the compounded latex is stored after mixing prior to use in the production line. During prevulcanisation, crosslinking of the rubber molecules occurs inside discrete rubber particles dispersed in Corresponding author. ©Smithers Rapra Technology, 2011 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 201 S. Palaty, P.V. Devi, and J. Honey the aqueous phase of the latex. Prevulcanisation offers a means to control the extent of crosslinking needed in the final vulcanised film. Prevulcanised latex does not need further compounding by the product manufacturer and requires less energy for drying. The production of various dipped goods, the bulk of which is in the form of examination gloves, uses prevulcanised latex [1]. Natural rubber latex (NRL) particles were covered with some proteins and phospholipids, which gave colloidal stability to the NRL [2]. The study of colloidal stability of prevulcanised latex is very important in the latex dipped goods manufacturing industry. The colloidal stability of prevulcanised latex depends on many factors such as the nature of the latex, the amount of potassium hydroxide (KOH) and carboxylate soap, the dosage of vulcanising ingredients, and the prevulcanisation conditions such as time and temperature [3]. There are two opposing factors that affect the colloidal stability of prevulcanised latex. One is the presence of residual vulcanising ingredients such as zinc oxide (ZnO), which reduces the colloidal stability by ZnO thickening. The other is the addition of alkalis and carboxylate soaps, which increases the stability by increasing the negative charge on the surface of rubber particles and by increasing surface adsorption. The properties of sulphur prevulcanised latex may undergo changes during storage because of the presence of surface- active agents and residual vulcanising ingredients. During storage, the chemical composition of the latex changes significantly owing to the action of bacteria, enzymes and preservatives. These changes are reflected in the properties of the latex, particularly its mechanical stability time (MST), volatile fatty acid (VFA) number, and potassium hydroxide (KOH) number, and hence they have received most attention [4]. The temperature of vulcanisation is very important in determining the quality of the rubber products. Optimum properties are obtained when curing is done at the lowest possible temperature [5]. At present, prevulcanisation of natural rubber latex is done by heating it for 2–3 hours at 55–60°C, which affects its colloidal stability. Thus, the main drawback of high-temperature prevulcanised NRL is its low colloidal stability. Other disadvantages of this high-temperature vulcanisation include high energy consumption, degradation of rubber molecular chains, poorer safety, high insulation costs, less flexibility in designing the compound for each component at high temperature, inconvenience in the curing room, and the risk of overvulcanisation. However, low-temperature vulcanisation results in products of good quality and fine appearance. The effect of vulcanisation time and storage on the stability and physical properties of high-temperature prevulcanised NRL was reported earlier [3]. This paper reports on the study of the colloidal stability of room temperature prevulcanised latex stored for a period of 30 days. These properties were compared with those of high-temperature prevulcanised latex. The room- 202 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex temperature prevulcanisation of NR latex was done using a zinc butyl xanthate [Zn(bxt)2]–zinc diethyl dithiocarbamate [ZDC] accelerator system [6-8]. The important colloidal properties of the prevulcanised latex studied are the total solids content (TSC), the dry rubber content (DRC), the ammonia content, the VFA number, the KOH number, the MST, and the coagulum content. The TSC of latex is the percentage by mass of the whole latex that is non-volatile under specified conditions of drying in an open atmosphere at an elevated temperature. The DRC is the percentage by mass of the latex that is coagulated under specified conditions of colloidal destabilisation. The ammonia content represents the free alkali content of the latex. The VFA number indicates the number of grams of KOH required for neutralising the volatile fatty acid in a latex sample containing 100 g of TSC. The KOH number represents the number of grams of KOH equivalent to the anions present as ammonium salts in a quantity of latex that contains 100 g of TSC. The mechanical stability of the latex is its ability to withstand colloidal destabilisation effects of mechanical influences such as shearing and agitation. The coagulum is the material that is retained by a mesh of arbitrary size under specified conditions of filtration [9]. EXPERIMENTAL Equipment Used The equipment used in this study included a Brookfield LVT viscometer (Brookfield Engineering Laboratories, Inc., Strongton, MA), mechanical stability apparatus (M/s Klaxon Signals, Oldham, UK), an MKVI digital pH meter (Systronics Naroda, Ahemedabad), a centrifuge (Elektorcraft [India] Pvt. Ltd, Mumbai, India), a hot air oven (Rotek RHO-98-HFSS; B&C Industries, Kerala, India), Markham-type still apparatus, a water bath, and a ball mill with a speed of 50 rpm. Materials Used High-ammonia (HA) centrifuged NRL conforming to the BIS 5430-1981 was purchased from Njavallil Rubber Latex Pvt. Ltd, Cochin, India, and was used for prevulcanisation. The compounding ingredients, such as zinc diethyl dithiocarbamate (ZDC), zinc oxide (ZnO), sulphur (S), and dispersal F (M/s Standard Chemicals Co. Pvt. Ltd, Chennai, India) were commercial grade. Acetic acid, hydrochloric acid, barium hydroxide [Ba(OH)2], ammonium hydroxide, KOH, and formaldehyde (supplied by E. Merck Ltd, Mumbai, Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 203 S. Palaty, P.V. Devi, and J. Honey India) were analytical grade and were used as supplied. For the preparation of Zn(bxt)2, commercial-grade n-butyl alcohol, KOH, zinc chloride (ZnCl2), and carbon disulphide (CS2) were used. Experimental Procedure The colloidal properties of the high-ammonia centrifuged latex were determined according to IS 9316-1987 and are reported in Table 1. Zn(bxt)2 was prepared in the laboratory as reported earlier [7, 8, 10]. Zinc butyl xanthate was prepared by reacting equimolar amounts of n-butyl alcohol, potassium hydroxide, carbon disulphide, and zinc chloride. 50% dispersions of the vulcanising agents were prepared using the required amount of vulcanising agents and distilled water by grinding in a ball mill under standard conditions. Dispersol F was used as the dispersing agent during ball milling to prevent the dispersed particles from reaggregation; chemically, it is sodium naphthalene formaldehyde sulphonate. NRL was deammoniated to 0.2% before compounding to avoid ZnO thickening. NRL was compounded as per the formulations given in Table 2. Table 1. Properties of the high-ammonia (HA) centrifuged NR latex used for prevulcanisation Properties Value BIS 5430-1981 Test methods requirements Dry rubber content (mass %) 59 60a IS 3708(part 1) 1985 Total solids content (mass %) 60 61.5a IS 3708(part 1) 1966 Non rubber solids (mass %) 1 2b IS 9316(part 4)1988 Ammonia content (mass %) 0.86 0.6a IS 3708(part 4)1985 Volatile fatty acid number 0.02 0.15b IS 3708(part 7) 1986 Potassium hydroxide number 0.33 1b IS 3708(part 5)1985 Mechanical stability time (s) 990 475a IS 3708(part 6)1985 Coagulum content (mass %) 0.01 0.05b IS 3708 (part 3)1987 a Minimum b maximum Prevulcanisation of compound A was performed by heating the latex mix at 55–60°C for 3 h. Compound B was kept at room temperature, with occasional stirring. The time required for its prevulcanisation at room temperature was optimised as 5 days [7]. The colloidal properties of the prevulcanised latex were determined after 5, 10, and 30 days of storage and compared with those of high-temperature prevulcanised latex; the results are given in Figures 1 to 204 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex Table 2. Formulation of latex mixes Ingredients Parts by weight (g) A B NR latex 167 167 10% KOH 1.5 1.5 10% potassium oleate 0.75 0.75 10%Vulcastabe VL 1.0 1.0 50% S 2.5 2.5 50% ZDC 1.5 1.5 50% ZnO 1.0 1.0 50% Zn(bxt)2 — 1.5 10. The important colloidal properties studied here are the TSC, the DRC, the non-rubber solids, the ammonia content, the VFA number, the KOH number, the MST, the Brookfield viscosity (cP), the coagulum content, and the pH. The test methods used are given in Table 1. The TSC and DRC values were determined using the conventional method of heating in a hot air oven. If m0 is the initial latex sample mass and m is the mass of the residue after evaporation (at 100°C for 2 h), then TSC = (m/ m0) 100. Also, if m0 is the initial latex sample mass and m is the mass of dry Figure 1. Effect of storage on the TSC of compound B and its comparison with compound A Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 205 S. Palaty, P.V. Devi, and J. Honey Figure 2. Effect of storage on the DRC of compound B and its comparison with compound A Figure 3. Effect of storage on the non-rubber solids of compound B and its comparison with compound A 206 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex Figure 4. Effect of storage on the ammonia content of compound B and its comparison with compound A Figure 5. Effect of storage on the VFA number of compound B and its comparison with compound A Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 207 S. Palaty, P.V. Devi, and J. Honey Figure 6. Effect of storage on the KOH number of compound B and its comparison with compound A Figure 7. Effect of storage on the Brookfield viscosity of compound B and its comparison with compound A 208 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex Figure 8. Effect of storage on the MST of compound B and its comparison with compound A Figure 9. Effect of storage on the coagulum content of compound B and its comparison with compound A Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 209 S. Palaty, P.V. Devi, and J. Honey Figure 10. Effect of storage on the pH of compound B and its comparison with compound A coagulum separated from it using acetic acid, then DRC = (m/m0) x 100. The difference between the percentage of TSC and DRC gives a measure of the percentage of non-rubber solids. The ammonia content was determined by titrating a certain amount of diluted latex with standard hydrochloric acid. The VFA number of prevulcanised latex was periodically determined using a Markham-type still apparatus. The VFA number was measured by the quantity of Ba(OH)2 that is required to neutralise the distilled-off short-chain fatty acids. The calculation of the VFA number is as follows: A × N × 561 VFA number = w × TS where A is the quantity of Ba(OH)2 solution (in mL) required for titration of the sample, N is the normality of the Ba(OH)2 solution, w is the mass of latex corresponding to 10 mL of acidified serum, and TS is the percentage of total solids in the latex. The potassium hydroxide number was determined by reducing the ammonia content of the test latex sample to a specified level by reacting with formaldehyde, diluting with water to a definite TSC, and then titrating with standard carbonate-free aqueous KOH solution. 210 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex The MST was determined using a mechanical stability apparatus. In this test, a specified amount of prevulcanised latex of specified TSC is subjected to mechanical agitation under specified conditions, and the time required to produce the first obvious sign of flocculation is recorded as the MST (in s). To determine the coagulum content, a known quantity of latex is diluted with a known volume of 5% potassium oleate solution and filtered through an 85 mesh sieve, and the coagulum retained on the sieve is dried and weighed. If m0 and m are the masses of latex test sample and the dried coagulum respectively, then: m Coagulum content =   × 100  m0  Viscosity measurements of the prevulcanised latex were carried out with a Brookfield LVT viscometer. The experiments were performed by varying the size and speed of the spindle in order to achieve a shear stress close to 100%. The pH of the latex was determined using a pH meter according to ISO standard 976-1986. RESULTS AND DISCUSSION Table 1 shows the colloidal properties of the high-ammonia centrifuged latex. The maximum and minimum limits are given. The test methods are also indicated. From Figures 1 to 3 it is clear that the TSC, DRC, and non-rubber solids of room-temperature prevulcanised latex remain almost constant during the entire period of storage of 30 days and are comparable with the corresponding characteristics of high-temperature prevulcanised latex. The ammonia content of the room-temperature prevulcanised latex decreases slightly on storage (Figure 4). The VFA number of the room-temperature prevulcanised latex increases on storage (Figure 5). However, it does not exceed the acceptable VFA number limit for the concentrated latex industry, i.e. 0.15. During storage, bacterial degradation of the latex constituents causes the formation of short-chain fatty acids (mostly of formic, acetic, and propionic acids), with a resultant decrease in the pH value of the latex (Figure 10) and a corresponding increase in the VFA number11. Thus, the VFA number is used as an important measure of the level of deterioration and stability of the latex, and it is a good indicator of the state of preservation of the latex. The KOH number of the room-temperature prevulcanised latex increases slightly on storage (Figure 6). This indicates an increase in the concentration of acids, Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 211 S. Palaty, P.V. Devi, and J. Honey which are present as ammonium salts [12]. However, even after 1 month, the value is very much lower than the acceptable limit of 1. The increase in Brookfield viscosity during storage (Figure 7) is due to ZnO thickening of the latex and also to the slow liberation of Zn ions from the accelerator [1]. The zinc ions tend to destabilise the latex by neutralising some of the fatty acid anions and proteinate anions on the rubber particle surface. The coagulum content of the room-temperature prevulcanised latex remains almost constant during storage (Figure 9) and is comparable with that of high-temperature prevulcanised latex. The MST of the room-temperature prevulcanised latex increases on storage (Figure 8). On storage, the ammonia present in the latex, the alkali, and the potassium soap added during compounding slowly hydrolyse the proteins and phospholipids to fatty acid anions and other products. The liberated fatty acid anions are adsorbed at the particle interfaces and thus enhance the stability of the latex owing to a higher surface charge and therefore a higher repulsive energy between particles [4]. This accounts for the increase in MST of the prevulcanised latex. High-temperature prevulcanised latex has a low value of MST compared with room-temperature prevulcanised latex. Thus, one of the advantages of room-temperature prevulcanised latex is its high colloidal stability, which will be of significant technological importance to the rubber dipped goods manufacturing industry, providing a means for effecting better product quality control and improvement. Another important advantage is its non-consumption of energy during prevulcanisation, which will make the process highly economic. CONCLUSIONS The colloidal properties of room-temperature prevulcanised latex, prepared using a Zn(bxt)2–ZDC accelerator system, are found to be superior to those of conventional high-temperature prevulcanised NR latex. The mechanical stability time of room-temperature prevulcanised latex increases on storage and has a high value compared with that of high-temperature prevulcanised NR latex. Even after 1 month of storage, the values of the VFA number and KOH number of room-temperature prevulcanised latex are much lower than the acceptable limits of these values for the concentrated latex industry. Properties such as the TSC, DRC, non-rubber solids, and coagulum content of room-temperature prevulcanised latex have values comparable with those of high-temperature prevulcanised NR latex. Thus, room-temperature prevulcanisation of NR latex using a Zn(bxt)2–ZDC accelerator system leads to a high energy saving and makes the process more economic. The colloidal stability of the latex can be improved owing to the non-application of temperature. 212 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 Effect of Storage on the Colloidal Properties of Room-temperature Prevulcanised Natural Rubber Latex ACKNOWLEDGEMENT The authors would like to acknowledge KSCSTE and the University Grants Commission for financial support. REFERENCES 1. Ho C.C. and Khew M.C., Langmuir, 15(19) (1999), 6208–6219. 2. Ho C.C., Kondo T., Muramatsu N., and Ohshima H., J. Colloid Interface Sci., 178(2) (1996) 442–445. 3. Sasidharan K.K., Rani J., Palaty S., Gopalakrishnan K.S., and Rajammal G., J. Appl. Polym. Sci., 7(5) (2005) 1804–1811. 4. Claramma N.M., Varghese L., and Mathew N.M., Indian J. Natural Rubber Res., 8(1) (1995) 1–7. 5. Palaty S., Devi P.V., Honey J., and Rani J., Effect of low temperature prevulcanisation on the colloidal and mechanical properties of natural rubber latex, in Proceedings of the 7th International Conference on Advances in Polymer Technology, pp. 199–205 (2008). 6. Palaty S., Devi P.V., Mary K.J., and Rani J., Studies on the mechanical properties of room temperature prevulcanised natural rubber latex. in press J. Appl. Polym. Sci. 7. Palaty S., Devi P.V., and Honey J., Progress in Rubber, Plastics, and Recycling Technology, 25(3) (2009) 187–197. 8. Palaty S., Devi P.V., and Mary K.J., Progress in Rubber, Plastics, and Recycling Technology, 26(4) (2010) 199–213. 9. Blackley D.C., Polymer Latices Science and Technology: Fundamental Principles. Vol. 1, 2nd edition. Chapman and Hall, London, UK, pp. 417–535 (1997). 10. Palaty S. and Rani J., J. Appl. Polym. Sci., 78(10) (2000) 1769–1775. 11. Blackley D.C., High Polymer Latices. Vol. 1. Maclaren & Sons Ltd, London, UK, p. 231 (1966). 12 Blackley D.C., High Polymer Latices. Vol. 2. Applied Science Publishers, London, UK, p. 464 (1966). Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011 213 S. Palaty, P.V. Devi, and J. Honey 214 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011