BORBÉLY-KUSZMANN, A.

NAGY, J.

PÁRKÁNYI, L.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PREPARATION OF SIJLPHUR, SULPHUR AND CARBON CONTAINING POLYSILOXANE ELASTOMERS By J. NAGY, L. P.{RIdNYI and A. BORBELy-KUSZMANN Department of Inorganic Chemistry, Technical U lliversity, Budapest (Received April 29, 1970) Introduction The skeleton of the silicone elaston:.ers is usually formed by a polysiloxane chain containing methyl groups on the free valencies of the silicon atoms. There are two possibilities of vulcanization, depending on the feature of tbe terminal groups of the polysiloxane chain. If these are methyl or vinyl groups, the crosslinking can be achieved by so-called hot vulcanization, using different organic peroxides, sulphur or active sulphur compounds (Se, Te, dithiocarha· mates, guanidines etc.) and pressure. In this case the silicone rubber contains the sulphur in the crosslinks [1,2,3]. If the terminal groups of the poly-siloxane chains are more reactive (e.g. OH, alkoxy or acetoxy) the silicone ruhber can be synthesized by means of thiurames or mercaptans in the pre-senc: of catalysts (e.g. dibutyl-tin-dilaureate) at room temperature [4]. The first silicone polymer, containing polysulphide links in the siloxane chain was synthesized by NASIAK and POST [5], who did not expose this polymer to vulcanization. PL UDDEMANN [6] built up a crosslinked polysulphid elastomer by the help of thc branched groups of the polynlf>r, hydrolysed by the moisture of air. In our own experiments the synthesis of silicone dastomers, containing sulphur as well as sulphur and carbon in the polysiloxane chain in distinct sequencies, had heen aimed at. The synthesized elastomers were characterized by their average molecular weight, their intrinsic viscosity and by their ter-minal group analysis. Thereafter vulcanization was done at room temperature, and the mechanical properties of the resulting ruhhers were tested. The oh-tained results ,,-ere compared with test data on pure polysiloxane elastomer, vulcanized undrr similar conditions. E ,(perimental part In our experiments elastomers had been synthetized c~ntaining 1. -Si S -Si-2. -Si C-S Si-3. Si-C-Si-S Si- and 4. Si-C Si-26 J . .'iAGY et al. units. The ratio between these units (£) and the silicon atoms of the polysiloxane chain were chosen as E/Si 1/20; 1/50; 1/100 and 1/150. The hetero units mentioned above were built in into the polysiloxane chain by the following compounds: 1) R R "" ~ ./Si /"" R S S R ",I 1/ Si Si / ", / "" R S R HexamethyI-cyclotrisilthiane (R = CH3) Bp3S: 151,5-153,5' 2) 2.2.5.5-Tetramethyl-2,5 -disila-1 ,4-dithia-cyclohexane (R = CH3) m.p.: 82-83° 3) (CI-CH 2-Si(CH3}z)zS Bis-(chloromethyl-dimethylsilyl)-sulphide BplO: 80- 81 c 4) Cl- CH z- Si(CH3)2- Cl Chloromethyl-dimethyl-chlor;.ilane. For the synthesis of the hetero-siloxane elastomers two routes can be used: A) Starting from the heteroatom containing silicon compound an oligomer has to be produced first (e.g. by the way of Grignard reaction, addition etc.) and tbis has to be copoly-merized with cyclic siloxanes in the presence of catalysts [7, 8]. B) Starting from cyclosiloxane a prepolymer is to be made and this has to be joined with a heteroatom contaiuing silicon compound in the presence of a catalyst. This method is used in the industrial production too [9]. In the synthesis of the silicone elastomers containing heteroatoms we used always this second method. Starting from octamethyltetracyclosiloxane a prepolymer was formed in the presence of 3% concentrated sulphuric acid as catalyst, then the appropriate heteroatom con-taining silicon compound was added in the required ratio, and the reaction mixture was stirred at room tcnlpcrature until thc viscosity of the polymer remained constant. The elastomer was resolved in ether, the solution was deacidized by washing with sodium bicarbonate, dried (CaClz) and evaporated. Traces of the solvent and oligomers were removed by vacuum distil-lation. The remaining silicone elastomers had following properties: Table I elastomers Type of unit" Average Yiscosity )L weight Tenninal group S/5i rate OH SH SiS 35100 0.200 1 : 20 SiS 41100 0.222 1 : 50 SiS 50000 0.254 1 : 100 SiS 84000 0.363 1 : 150 CS 39400 0.216 1: 20 CS 147000 0.514 1 : 50 CS 182000 0.593 1 : 1C)0 CS 178000 0.584 1 : 150 C 9800 0.086 + 1 : 20 C 24000 0.156 , 1 : 150 CSiS 20000 0.136 , 1 : 20 CSiS 42000 0.225 + 1 : 150 NG 20500 0.140 0 * The abbreviations used in this table are: SiS = -Si-S-Si-: CS = -CHo-S-Si 1 = -Si-CHz-Si-; CSiS = -CHz-Si-S-Si-CHz-O-: and NG'=-SI-O-Si-. PREPARATIO:Y OF SCLPHc'R 27 Vulcanization of the elastomers The synthetized silicone elastomers, containing heteroatoms in their skeleton underwent vulcanization at room temperature, using tetraethoxy-silane as crosslinking agent and dibuthyl-tin-diacetate as initiator. The composition of the reaction mixture, used for the vulcanization was the following: 100 g elastomer, 2.24 . 10-2 mol tetraethoxy-silane and 9.25 . 10-4 mol dibuthyl-tin-diacetate. The elastomers of the SiS and CS type became rubbers after 24 hours with exception of the CS 1 : 20 type, which became just a gel. The same happened to the elasto-mers of the CSiS type, while the elastomers containing - CH 2- groups in the polysiloxane chain remained unchanged. The SiS and CS type rubbers were tested for elasticity and solvent resistance. Discussion The vulcanization experiments of the different polysiloxane elastomers showed this process as well as the features of the resulting products to be strongly terminal-group dependant. The reactivity of these terminal groups towards tetraethoxy-silane determines the vulcanization rate. For instance, if the terminal groups of the elastomer are hydroxyls as in the case of the SiS elastomer - the vulcanization process is the same as described for the normal silicone rubbers, vulcanized at room temperature [9]. Changing part of these terminal groups to - SH groups as in the CS type elastomer -the vulcanization rate decreases and if the ratio of SH to OH groups will reach 2 : 1, no rubber result. The fact that in the case of the CSiS type elasto-mer only a gel formed indicated the low reactivity of the chloromethyl terminal groups, insufficient for the elastomer to establish crosslinks with tetraethoxy-silane. The reduced reactivity of the chloromethyl groups is indicated even by the lower average molecular weight of the elastomer, proving that all the Table II Elasticity testing of silicone rubbers Type of rubber 0 ,0 CS,lOO C5-150 SiS .. 50 SiS,lOO SiS~150 :-iG Elasticity 40.3 39.5 79.2 74.2 74.6 76.5 ReI. hyster 59.7 60.5 20.8 25.8 25.4 23.5 Perm. set 39.5 38.5 4.0 9.5 6.5 9.5 3 hours at 150c Elasticity 32.2 26.0 67.1 81.0 75.6 72.2 76.5 ReI. hyster. 67.8 74.0 32.9 19.0 24..1 27.8 23.5 Perm. set 65.0 65.0 7.0 5.0 8.5 5.5 7.5 ---~ .. -"------~----~-- - --.~-~--6 hours at 150' Elasticity 81.0 75.6 75.0 ReI. hyster. 19.0 24.4 21.8 25.0 Perm. set 5.0 8.5 7.5 7.5 28 J. SAGY et al. chloromethyl groups were present at terminal positions, preyenting the poly-siloxane chains from further condensation or crosslinking. Comparative study of the SiS and CS types, and. the pure polysiloxane rubber proved that the sulphur content of the Si - S - Si units had a favourable influence on elasticity. The elastic properties of the different rubbers were investigated before and after being heated to 150 QC for 3 and 6 hours, respec-tively. The data proved the benefit of having built in sulphur into the silicone rubber, as in the series of the SiS type ruhbers the elasticity increased parallel to the sulphur content. On the other hand, the same inyestigations on the series of the CS type ruhbers showed that the Si-CH~-S Si- units signif-icantly diminished thc clasticity of the polymers after a treatment at 150 QC for 3 and 6 hours, respectivcly. Time (hours) Table III Swelling of the rubbers (~~) in ethanol Type of t he rubber CS·150 SiS-20 SiS<iO SiS·IOO SiS-150 );G .----"--~------2 1 -') .;)~ LlS 1.96 1.95 2.26 lAS 24 2.66 2.42 2.04 1.9S 2.52 3.U 3.13 2.44 96 1.S2 2.43 1.63 0.05 1.78 2.40 2.50 1.65 170 1.21 2.55 0.S2 0.49 Ll8 1.69 2.35 0.86 Table IV Swelling of the rubbers (° 0) in acetone Time Type of the rubber (hollr,) -----~--- _._---CS·50 CS·IOO CS·150 SiS-20 SiS-50 SiS·IOO SiS·150 NG 2 2.'5.7 25.9 22.9 20.4 23.0 23.1 23.2 18.5 2·1 28.0 2-1,.5 24.3 22.4 24.5 26.3 2-1,.8 24..4 96 24.7 20.0 21.2 16.4 20.0 22.·j. 22.5 20.6 170 22.7 16.5 17.7 15.2 1:1.0 19.5 18.5 16.0 Table V Swelling of the rubbers (': 0) in toluene Time (hours) CS':jO CS· 100 CS·150 SiS-20 SiS-50 SiS-lOO SiS-150 );G 2 278 236 213 144 125 HI 150 127 2-1, 739 725 650 309 297 339 335 292 96 956 786 319 324 3-1,4 338 317 170 1074 794 295 316 361 319 302 PREPARATIOS OF SULPHCR 29 The solvent resistance of the rubbers was studied for three solvents: ethanol, acetone and toluene. All of them can have two simultaneous effects: partly they will swell the rubber by penetration, and partly they will dissolve the polymers of lower molecular weight. As this two effects entrain opposite changes in the weight, the mass change of the test pieces will be their result-ant. If the swelling surpasses the dissolution, there will be a positive change in the weight, and in opposite case there will be a loss of it. Evaluation of these data led to the conclusion, that the heteroatoms huilt into the polysiloxane chains did not significantly alter the solvent resist-ance, as against the -Si-CH!-S Si- unit, ,,,'hich extremely reduced the toluene resistance of the rubber. Summary The synthesis of clastomers, containing sulphur as well as sulphur and carbon as hetero-atoms in the polysiloxane chains is described. The different elastomers ,,'ere vulcanized in the presence of tetra-ethoxy-silane as crosslinking agent, and dibuthyl-tin-diacetate as ini-tiator. The elasticity an:! the solycat re3istance of the resulting rubbers to ethanol, acetone and toluene were iIlyestigated. A correlation was established between the sulphur content and the properties of the diffe rent rubbers. References 1. POL:\U:S-TEER, 1.. KEITH. E .. KOCH, R. J.: Ind. Eng. Chem. 49, 49 (1957). 2. KOCHEV, Y. ::'\., Fo~mcHEvA, ::\1. 1\1.: Kautshuk is Resina 18, 43 (1959). 3. l\IORTO:S-, M .. DEISZ, M. A.: U. S. Pat. 2,960.492 (1965). 4. l\"IETZSCHE, S., WICK. ::\1.: n. S. Pat. 3.050,485 (1962). 5. K-I.SIAK, L. D., POST, H.: J. Org. Chem. 24, 492 (1958). 6. PU.'DDE:\lA:S-:S-, E. P.: U. S. Pat. 3,117.461 (1963). 7. BLUESTEIl'i, B. A.: J. Am. Chem. Soc. 70, 3068 (1948). 8. GooD,n:s-, J. T. JR.: DBP ~o 8·H.901 (1947), 9. ::'\OLL, W.: Chernie und Technologie der Silicone. Yerlag Chemie GmhH. \"einheirn, (1968) p. 315-318 and 339. Prof. Dr. J6zsef NAGY 1 t~~z_~n:~,(~K~~:~LY_KlCSZ)IANN f Budapest XI., Gellert ter 4., Hungary 

PREPARATION OF SULPHUR, SULPHUR AND CARBON CONTAINING POLYSILOXANE ELASTOMERS

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PREPARATION OF SULPHUR, SULPHUR AND CARBON CONTAINING POLYSILOXANE ELASTOMERS

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Periodica Polytechnica (Budapest University of Technology and Economics)