Jl. Rubb. Res. Inst. Sri Lanka (I98S) 64, 29 -32 A LINEAR GEL/POLYMER SYSTEM BASED O N RADIATION PREVULCANIZED NATURAL RUBBER LATEX By S. W . K A R U N A R A T N E A N D W . S . E. F E R N A N D O I N T R O D U C T I O N Stable cross linked natural rubber (NR) latices were developed initially by Schidrovitz in 1923 and its use as a processing aid was first patented by Philpott in 1958. This patent covers the production of superior processing (SP) rubbers by blending a NR. latex containing prevulcanlsed rubber particles with excess vul­ canising agents removed by centrifugation with an ordinary unvulcanised latex or raw latex and coagulating them together. It is now well known thatthe processing properties of raw rubber could be Impro­ ved by incorporating cross linked gel into the essentially linear polymer.These gel containing compounds' extrude faster with lower and more uniform die swell greater die definition with a smoother surface and better shape retention. They also calender well giving smooth surfaces with lower shrinkage factor (Mooney, 1985; Makelyery, 1962; Middleman and Gavis, 1965; Gavis and Gill, 1956). Die swell is caused by the recovery of elastic deformations imposed by the restricted flow through a die cavity. The presence of gel polymer reduces the amount of material that is elastically deformed during flow since the gsl particles being cross linked material undergoes relatively little deformation. This, again,-will be deter­ mined by the extent of cross linking in the gel particles. In addition to the sol gel ratio in the compound the extent of cross linking in the gel fraction or the relative tightness of the gel could also influence the rheological properties. A convenient method to study this effect is to blend irradiated latex with raw latex. . This paper describes the rheological properties with special emphasis on ex­ trusion characteristics, of superior processing rubber prepared by preblending radiation prevulcanized NR latex with raw latex. MATERIALS A N D M E T H O D S Preparation of linear gel/polymer systems A commercial grade of LA centrifuged latex was diluted to 40% dry rubber content (DRC) and irradiated using a C 0 60 source ( gamma—rays). A dose of 3 megarads was given over a period of 5 hours. A sensitiser, C C I 4 was used at 3% on the latex, in order to bring about efficient cross linking at a reduced dose level. 29 S. W . K A R U N A R A T N E & W . S . E. F E R N A N D O The Irradiated latex was diluted to 15% D R C and mixed with field latex adjus­ ted to the same D R C . 2% Formic acid was used as the coagulant. After coagula­ tion the rubber was milled Into a crepe, and dried in a drying tower at 35°C. Post extrusion Swelling of a Linear gel/polymer system based on Radiation Pre- vulcanised Latex (RPVL) was studied using the formulation given In Table I . This was compared with a formulation without the gel component but with the incor­ poration of factice (similar quantity to gel fraction). Table I. Formulations for extrusion of gum compounds (0 (2) RSS 3 67.0 100.0 IPVR/NR blend 66.0 — (50 : 50) 33.0 Factice — 33.0 ZnO 5.0 5.0 Stearic acid 2.0 2.0 Flectol H 1.5 1.5 MBTS 0.9 0.9 TMTD 0.2 0.2 Sulphur 2.5 2.5 Compounding procedure Is given in Table 2. A laboratory intermix.was used In the Investigations. The rotor and chamber temperature in the intermix was 60°C. Table 2. Compounding procedure using the laboratory Intermix Action Time (Mlns) Add rubber 0 + Factice/(NR/IPVL) blend 3/4 Add ZnO, stearic acid I 3/4 Flectrol H MBTS+TMTD 3 J Dump 4- Sulphur was added on the two roll mill In 5 minutes (') (2) Compound Mooney (ML I + 4 ) — I00«C 26 18 Mooney scorch time at I20°C mins 18.0 16.0 Compound density, g/cc 0.903 0.926 30 A L INEAR GEL /POLYMER SYSTEM BASED O N R A D I A T I O N P R E V U L C A N I Z E D N R L A T E X Where D„ Is the die diameter and D is the diameter of the extrudate. Diameter of the extrudate was calculated using the formula, 2R = 2 I M G A V Where M is the wt per cm of the extrudate. and G is the compound density. D I S C U S S I O N O F RESULTS General properties of linear gel/polymer systems Due to their two-phase structure they would normally increase the stiffness and Mooney viscosity of the rubber mix and at the same time improve its flow be­ haviour under factory processing conditions as in extrusion. Unlike most proces­ sing aids SP rubber does not impair the excellent physical properties, such as ten­ sile strength (TS) of NR vulcanizates. Extrusion rates and the percentage swell were measured at 3 different screw speeds (20,40 and 80 rpm). At all 3 speeds the rate of extrusion was faster with the linear gel/polymer system and as the screw speed was Increased the difference In. rate of extrusion was also magnified (Table 3). Table 3. Extrusion results Compound Compound (') Extrusion rate cm/min at 20 RPM 44.0 34.0 40 RPM 75.0 60.0 80 RPM 155.0 120.0 Weight of extrudate (as above) In gms/min 20 RPM 84.0 84.0 40 RPM 146.2 155.1 80 RPM 288.0 310.2 Diameter of extrudate/ln cm 20 RPM 1.64 1.84 40 RPM 1.65 1.88 80 RPM 1.61 1.89 Extrudate swell 20 RPM 41.3 58.6 40 RPM 42.2 62.0 80 RPM 38.8 62.9 31 . Extrusion Swell (ES) was calculated using the formula. D - D 0 E S = x 100 S. W . K A R U N A R A T N E ft W . S . E. F E R N A N D O The unfilled compound remains least affected by irradiation pre vulcanized rubber (IPVR) and the rate of vulcanization remains relatively unaltered compa­ red to the control. The compound containing factice can be easily identified due to its lower Mooney viscosity. The effect of SP rubber (as linear gel/polymer system based on RPVL) in filled compounds was determined using the formulation given in Table 4. The com­ pounding procedure is given in Table 5. The extrusion characteristics and cure characteristics are given In Tables 5, 6 & 7. Table 4. JPVL/NR blends in extrusion Formulations with filler RSS 3 100.0 100.0 SP Rubber (as IPVL/NR blend) 50 : 50 25.0 ZnO 5.0 Stearic acid 2.5 5.0 2.5 1.0 0.3 2.5 1.0 25.0 25.0 3.5 1.0 MBTS 1.0 TMTD 0.3 Sulphur 2.5 Flectol H 1.0 Clay 25.0 Whiting 25.0 Dutrex R 3.5 Paraffin wax 1.0 Table 5. Compounding procedure Intermix rotor and chamber at 60°C Action Time (Mins) Add rubber (+SP rubber) 0.0 Add ZnO + stearic acid Antioxidant + wax 1.0 Add whiting + clay + oil I 3/4 Dump 4.0 Accelerator and sulphur were added on the two roll mill in 5 minutes. 32 A L INEAR G E L / P O L Y M E R SYSTEM BASED O N R A D I A T I O N P R E V U L C A N I Z E D N R L A T E X Table 6. Extrusion characteristics , Compound Compound with SP without SP Compound density g/cc I.I 10 I.I 13 Extrusion rate/cm/min 20 RPM 53.0 44.0 40 RPM 95.0 77.0 80 RPM 181.0 157.0 Weight of compound extruded (as above) in gms per minute 20 RPM 106.4 111.0 40 RPM 190.6 189.8 80 RPM 366.2 374.6 Table 7. Vu/conJzot/on characteristics. With SP Without SP Compound Mooney ML 1 + 4 at I00°C 33 29 Mooney scorch time mlns at I20°C 21 22£ Monsanto rheometer at I50°C Minimum torque (N.m.) 7 I I Scorch time T s 2 (mins) 3.5 5 Maximum torque, (N . m.) 86 79 Time for 90% cure (mlns) 6 8 Physical properties of vulcanizates (Vulcanization time, 20 mins at I40°C) Tensile strength MPa 24.9 21.5 E B % 625 610 Modulus at 300% MPa 4.7 5.0 Modulus at 100% MPa 2.05 1.96 Hardness, IRHO 53 50 The physical properties of vulcanizates before and after ageing at I00°c for 3 days is given in Table 8. Table 8. Physical properties after ageing at lOOfC for 3 days. With SP Without SP Tensile strength (MPa) 11.3 7.3 • Modulus at 300% (MPa) 5.9 5.5 Modulus at 100% (MPa) 2.4 2.5 E B % 415 290 Change in hardness IRHD + 2 +3 33 S. W . K A R U N A R A T N E & W , S; E. F E R N A N D O REFERENCES G A V I S , J., and GILL, J. (1956); }. Polymer Sci. 21. 353, M A K E L V E R Y , J. M. (1962) Polymer Processing, Wiley, New York, Chap. 3. M I D D L E M A N , S . , and GAVIS , J. (1965). Phys. Fluids, 4. 335. M O O N E Y , M. (1958) In Rheology, Vol . II.' F.R. E IR ICH, Ed., Academic Press, New York, Chap. 5. R O S E N , D . L., and R O D R I G I E Z , F., (1965) Flow Properties of a linear—gel polymer system. J Appl. Polymer Science. 9, 1601—1613. 34 Even in the filled compound there is a substantial reduction indie swell and also there is a substantial improvement in the extrusion rate, when SP rubber (linear gel/Polymer system) is incorporated in to the formulation. The percentage ret­ ention of physical properties after ageing at I00°C for 3 days is mush better in for mulations containing a linear gel polymer system based on RPVL. C O N C L U S I O N A linear gel-polymer system based on RPVL remains substantially altered in structure from normal rubber and it is necessary to make a few compounding adjustments to secure optimum properties. In gum vulcanizates the improvement in processability with the addition of cross linked polymer is.much more evident than in filled vulcanizates. This is to be expected as the filler incorporation reduces the nerviness of the rubber and results in the suppresion of the die swell during extrusion. The cross linked poly­ mer in the gel-polymer system behaves as a highly structured filler capable of impro­ ving the processability of.the system far in excessof the proportion of the gel-poly­ mer system incorporated in the compound. The amount of deformation of an extrudate is determined also by the extent of cross linking in the gel fraction as the extent of deformation is controlled by the tightness of the gel and its ability to restrict elastic deformation during the motion within the extruder. By increa­ sing the irradiation dose, the extent of cross linking within the latex particles can be increased up to an optimum value, and this system can be used to carry out a more complete investigation on shear stress/shear rate characteristics in polymers.