Some Like it Hot Author: Graham Budden Company: Dow Corning Limited Cardiff Road Barry CF63 2YL U.K. Tel: +44-1446-732350 Fax: +44-1446-730495 1
Introduction licone elastomers have been used in extreme temperature environments virtually since their invention. Even from their early commercialisation during World War 2 they were put to use in the temperature range -50 o C to +300 o C for limited exposure in military applications, where they provided unrivalled utility in, for instance, aircraft engine gaskets and greases. Today, the applications for silicone elastomers are very much more diverse and are not limited to extreme environments. The possibility to fine tune their aesthetic and mechanical properties means that their applications are now very wide ranging. In the textile industry their use extends from fashion wear to technically demanding applications such as automotive airbags. In all cases the silicone used is the same; the elastomer found on the decorative lace of hold up stockings and that found in industrial compensators share about 70% common materials. So it is indeed possible to have a crystal clear, soft and colourless silicone elastomer with high elongation on the one hand as well as one that is hard, abrasion resistant and flame retardant on the other. To further illustrate this, some typical uses for silicone elastomer coatings on fabrics and a summary of their particular requirements are shown in Table 1. Table 1. Typical Uses of licone Rubber Coatings Hold up stockings Conveyor belts Compensator Medical protective wear utdoor clothing and tents Airbags Crystal clear, non-slip, high elongation, soft, easy processing Thermally stable, food grade, non-slip, adhesion Chemical/thermal stability, abrasion resistant, adhesion Hydrophobic, autoclavable, adhesion Hyrophobic, flexible, thermal stability, adhesion, colourless Thermal stability, age stability, adhesion, strength, slip In many cases the requirement is for some kind of thermal stability to high temperature (compensator) or low (outdoor) temperature or over a very prolonged period of temperature fluctuation above and below room temperature (airbags). licones are stable to these wide ranging variations in temperatures more than any other elastomer and the reasons for this stability together with an understanding of some basic silicone properties are the subject of this paper. So why are silicones so stable? licone Characteristics 2
The performance of silicone elastomers in extreme temperatures can be attributed to some very basic properties of the polymer backbone as well as to the composition of the formulated product. Considering polydimethyl siloxane (PDMS) as the fundamental polymer unit used to formulate silicone elastomers, the properties shown in Table 2, are absolutely characteristic. Table 2. loxane Characteristics loxane property High -C and - bond strength(kj/mol) -C =363 - = 451 cf. C-C = 347 Low association of side groups Very low energy barrier to - bond rotation Effect on silicone elastomer Resistance to oxidative and chemical degradation. Low glass transition temp. Hydrophobicity. Low surface energy. Low glass transition temp. Hydrophobicity. Very flexible. Low surface energy. Apart from the elastomer characteristics broadly determined by the silicone polymer and fillers, other components such as catalysts and additives can also influence the heat resistance. Environmental conditions are another factor to consider - high humidity, for example, causes faster degradation of the elastomer at high temperature (>150 o C) than would occur at the same temperature in dry air. Turning up the Heat The degradation of silicones in air has been studied over many years and with broad agreement of the mechanisms involved. At high temperature (>150 o C), cleavage of the -C bond as well as depolymerisation of the siloxane chain occurs. The nature of the side group has a major influence on thermal stability with aromatic groups giving greater stability than alkyl groups. For example, the stability increases in the order: -C 2 H 5 < - < - C 6 H 5 The effect of side group oxidation in elastomers is to increase crosslink density so that mechanical properties of a silicone elastomer alter. The expected result is reduced elongation, reduced tensile strength, and increased hardness, but countering this is the effect of degradation of the polymer backbone which tends to reduce crosslink density. Thus the overall effect is dependant on the relative extent of each of the degradation mechanisms. A Closer Look at Degradation 3
It is possible to divide the degradation mechanisms into four categories (Table 3): Table 3. loxane Degradation Degradation Mechanism By-Products Thermal Homolytic bond cleavage not involving oxygen Low molecular weight siloxanes Thermo-oxidative Free radical oxidation of C 2, H 2, C, CH 2 etc the alkyl moiety involving oxygen Hydrolytic & chemical Water/ external catalyst /impurities Low molecular weight siloxanes Radiation Free radical cross linking Clearly the first two mechanisms are important in understanding the stability of silicone coating elastomers at high temperature. However, in most applications, the coating is exposed to humid air, so hydrolytic and chemical mechanisms are also important. So it is worthwhile to look at these three mechanisms in a little more detail: Proposed mechanisms for thermal degradation: 1. Unzipping depolymerisation 1 The presence of silanol groups is responsible for depolymerisation, particularly in the temperature range 120 o C to 275 o C. lanol may be an essential component in the formulation, or present as an impurity or formed by the oxidation of side groups. H 120C - 275C H + H3C + other cyclics 2.Bond exchange 2 4
This mechanism has been shown to be important at very high temperature, above 350 o C.Here there is random scission and bond formation with consequent increase in polydispersity and formation of cyclic species. Depolymerisation is promoted by impurities such as metals. > 350C C H 3 CH H 3 3 C + H3C + other cyclics and > 350C CH H3C 3 C H 3 Proposed mechanism for thermo-oxidative degradation: Alkyl attack 3 In this scheme there is direct attack of oxygen on the alkyl group, and it is only noticeable if the temperature is above 200 o C. Aromatic groups suppress the reaction, being less susceptible to radical attack, whereas vinyl and H substituents are more reactive. Larger alkyl side groups are also more susceptible. Cross-linking occurs through formation of alkyl and siloxane links but free radicals and oxygen do not effect the siloxane backbone to any great extent. Hence stabilisation of R- is a route to more oxidatively stable polymers. This mechanism is put to useful purpose in a controlled way in the formation of peroxide cured high consistency siloxane elastomers, where the peroxide radicals are caused to form alkyl links between polymer chains, thus forming a cross-linked 5
polymer network. Stability of the final elastomer is enhanced by using acid acceptors to neutralise further radical formation. In the extreme conditions where silicones are caused to burn, studies by cone calorimetry have shown significantly lower rate of heat release, production of carbon monoxide and smoke rate compared to organic systems. - +.. - > 200C H > 200C. H - 3 C + CH 2 + H - + H. Cross linking species: H C H 3 condensation. -. H - proton abstraction leading to alkyl cross link and free radical propagation Proposed mechanism for hydrolytic and chemical degradation: The influence of moisture 4 Studies have shown that cleavage of the backbone by moisture is the principal mode at lower temperatures 120 o C to 275 o C and is of most concern in normal operating environments. The presence of water on particulate fillers used in elastomers greatly 6
worsens stability. In addition, contamination by strong acids or bases together with water leads to silanolate formation and depolymerisation, again catalysed by metal ions 5. C H 3 C H 3 H - M CH 3 120C- 275C C H 3 + H - - M H3C + other cyclics Summary The degradation mechanisms which apply to various operating temperature ranges are summarised in Table 4 Table 4. Temperature Effect on Degradation Mechanism Temperature Range o C Mechanism Active species 120-275 Thermal - lanol depolymerisation 120-275 Hydrolytic & chemical Water & catalyst H-, M+ 200 + Thermo - oxidative xygen 350 + Bond exchange PDMS 7
licone Elastomers licone elastomers can be conveniently classified according to their cure mechanism. The principal types of elastomer are shown in Table 5, together with some typical applications. Table 5. Principal Types of licone Elastomer Elastomer type Cure mechanism loxane composition High consistency Peroxide, free Vinyl and/or rubber (HCR) radical cross link dimethyl Room temperature vulcanising (RTV) Liquid silicone rubber (LSR) Typical use Automotive hoses, moulded parts. functionality. Condensation lanol functional Sealants Some fabric coating Hydrosilylation Vinyl and hydride functionality Injection moulded parts. Fabric coating. In all cases the elastomers are compounded with fillers and additives to provide improved mechanical and physical properties. For instance, silica is commonly used as a reinforcing filler, as indeed it is used to reinforce organic elastomers, but unlike the small improvement in mechanical properties seen with organics when filling with silica, the tensile strength obtained when filling a silicone elastomer with silica typically improves by a factor of 40. Hence the choice of filler greatly effects the physical and mechanical properties of a silicone elastomer. Certain additives may also be used to improve stability, namely oxides of cerium and iron. In contrast to organic elastomers, silicones do not contain ester or hydrocarbon plasticisers or softeners, so are less prone to embrittlement through additive migration to the surface, evaporation and hardening at elevated temperatures. Additionally they remain flexible and rubber like down to a much lower temperature, typically - 100 o C. Service life has been reported as 30,000hr at 150 o C and 10,000hr at 200 o C in air 6. Figures 1-3 illustrate the effects found when formulating a LSR using different fillers. Figure 1. 8
Hardness on heat ageing at 200C % change 40 30 20 10 0-10 0 10 20 30 40 50-20 -30 Weeks at 200C Quartz lica Resin Figure 2 Tensile strength on heat ageing at 200C % change 0-20 0 10 20 30 40 50-40 -60-80 -100 Weeks at 200C Quartz lica Resin Figure 3 Elongation on heat ageing at 200C % change 0 0-20 10 20 30 40 50-40 -60-80 Quartz lica Resin -100 Weeks at 200C Stability of typical Pt curing LSR systems to heat ageing at 200 o C over 1 year (9000 hours) in air is shown. Changes in Durometer hardness, tensile strength and elongation were measured on test slabs prepared by press curing for 10 minutes at 150 o C followed by post curing for 2 hours at 150 o C. 9
As an example of the increased stability found on adding iron oxide, the thermal stability of an RTV curing system is shown in Table 6, with and without iron oxide: Table 6. Increasing stability using iron (III) oxide Duro Shore A Elongation % Tensile M.Pa Modulus 100% 7d@ RT +42d@ 230C 7d@ RT +42d@ 230C 7d@ RT +42d@ 230C 7d@ RT +42d@ 230C Base 27 >70 480 brittle 2.2 brittle 0.6 brittle Base + 25 16 600 560 2.8 1.3 0.5 0.3 Base+ = Base + 2% Fe 2 3 The widespread use of silica however has some drawbacks. nce silica has a very high surface area, it can act as a reactive surface and this is one reason why the silica is normally hydrophobed before or during compounding. Even so, residual surface - H, can cause cleavage of polymer chains 7,8. Subsequent bonding of cleaved silicone to silica results in an increase in crosslink density with loss of mechanical properties. Additionally silica often adsorbs significant amounts of water, so hydrolytic degradation is a further consequence. However, silica does redeem itself above 250 o C where it is reported to retard the unzipping depolymerisation by adsorption of H 7. licone Elastomers for Fabric Coating. As already indicated, the most prevalent type of silicone elastomer used for fabric coating is the LSR. It s popularity as a material for coating fabric is due to the fact that it possesses all the characteristics of an easy to use and versatile system: 2 part solventless system being easily mixed and easy to use. Viscosity from ca. 5000 mpa.s allowing it to be easily applied to the coating head and applied by knife over roller, or knife over air. Heat cured, typically requiring about 1-2 minutes cure at 160 o C. Long mixed bath life of at least 8 hours at room temperature. Good adhesion to glass, polyamide and polyester fabric. Appearance normally ranges from colourless and transparent to opaque white, and can be easily pigmented. Food grade possible. Typical properties of such LSR products are shown in Table 7. Table 7. Typical fabric coating LSR properties Mixed viscosity, mpa.s 15,000-200,000 Hardness, Durometer Shore A 15-70 Tensile strength, MPa (psi) 3.5 (500) - 9.0 (1300) Elongation % 100-800 Tear strength. kn/m (ppi) 5(28) - 40 (230) The data presented in Figures 1-3 is also valid for fabric coating LSR since formulations are similar. In a specific study of four representative types of fabric 10
coatings, formulations were monitored for their thermal and ageing properties. The study shows elastomer properties: Degradation profile Figures 4 to 7 are thermal gravimetric analyses (TGA) of four typical LSR formulations for fabric coating. By this method, the precise formulation can be seen to have a significant effect on the thermal degradation profile of the LSR. Figure 4 lica (X) filled Figure 5 Non silaceous filler Figure 6 Quartz/resin filled 11
Figure 7 lica (Y) filled These TGAs were recorded in a nitrogen atmosphere so the thermal degradation mechanism dominates. As a comparison, and to show the effect of red iron oxide on the thermal properties, the LSR in Figure 4 was also analysed by TGA in an air atmosphere. Figure 8 shows the comparison. Figure 8 TGA of an LSR in nitrogen and air atmosphere 12
Stability decrease due to oxidation Volatiles Air/RI Air Degraded pdms N 2 /RI N 2 2% RI lica filler The Figure shows the reduction in stability in air compared to nitrogen, in this case by about 100 o C. Additionally, the Figure suggests that since, in air, the total weight loss is greater without red iron oxide and the rates of weight loss are very similar, then the increase in stability caused by red iron oxide is by catalysis of PDMS to silica rather than by significantly inhibiting volatile production. Specific heat capacity up to 1.7 J/g. o C Measurement of specific heat capacity using differential scanning calorimetry (DSC) gives results varying between 1.61 and 1.76 J/g o C at 200 o C, with additives increasing or decreasing the values depending on their own heat capacities. Thermal conductivity typically 0.2 to 0.4 W/m.K Thermal conductivity measurements show that conductivity is a function of filler type and filler amount, as would be expected. Conductivity in an LSR can be as low as ca. 0.15 W/m.K and as high as ca. 1.0 W/m.K Stable adhesion 13
Carefully formulated LSR formulations have been tested for their adhesion to polyamide fabric using the scrub test (IS 5981). Tests were done before and after an ageing process involving cycling the temperature from -40 o C to + 125 o C, and relative humidity of 95% over a period of ca. 3 weeks. Excellent adhesion throughout the cycle with no loss in adhesion is achievable. Failure mode can vary depending on the formulation and in many cases can be due only to abrasive wear. In summary.. licone elastomers are now well accepted for their unique range of properties which stem from their molecular characteristics. In particular their high and low temperature stability gives them a wider operating range than any other elastomer. Figure 9 shows general limits of applicability of various elastomers and it can be seen that only fluoro polymers can exceed silicones to reach even higher temperature limits. Care must be taken in formulating the silicone elastomer as the properties can be further modified by the compounds they are formulated with and by contaminants remaining in the elastomer after manufacture which improve or reduce the temperature stability. Figure 9. Usable temperature range of various elastomers 200 150 Temperature, oc 100 50 0-50 Natural isoprene Butadiene Nitrile butadiene Chloroprene Urethane licone Fluoro copolymer -100 Despite their superior thermal stability, silicone elastomers cannot compete with traditional elastomers when physical properties alone are important. Tensile and tear strength may be about a quarter of that for butyl rubber for instance and hardness may be about one half, but their elongation is normally higher and they can be formulated for excellent transparency. Processing silicone is very much easier than with organic rubbers. For LSR s and RTV s there is no solvent to handle, they are normally only one or two parts and cure is rapid. If these properties are important, there is indeed no competition. References 14
1 Yu.A.Aleksandrova et al., Vysokomol.Soed., A10, 1078, (1968) 2 T.H.Thomas, T.C.Kendrick, J.Polym. Sci., Part A-2, 7, 537, (1969) 3 K.A.Andrianov, Metalorganic Polymers, Interscience, New York, 50,(1965) 4 D.K. Thomas, Polymer, 7, 99, (1966) 5 N.Grassie, I.G.Macfarlane, Euro. Pol. J., 14, 875, (1978) 6 D.K.Thomas, Polymer, 13, 479, (1972) 7 R.M. Aseyeva,et.al., Polym,Sci, U.S.S.R. A 15, 2, 2104 (1973) 8 S.Ross, G.Nishioka, J.Colloid and Interf.Sci.,65, 216, (1978) 15