Viscoelastic silicon rubber compositions
The invention provides for new viscoelastic silicone rubbers and compositions and methods for making and using them. The invention provides for viscoelastic silicone rubbers that are stiffer on short timescales than they are on long timescales. When subjected to brief stresses, they are relatively stiff and elastic, and they resist changing shapes. When subjected to sustain stresses, however, they are relatively soft and accommodating, and they gradually change shapes. When those stresses are removed, they gradually return to their original shapes. These viscoelastic silicone rubbers resist compression set and they are extremely resilient in response to sudden impacts. They can be dense rubbers, foam rubbers, and particles.
1. A viscoelastic silicone rubber composition comprising:
(a) at least one polyorganosiloxane;
(b) at least one permanent crosslinking agent present in an amount to provide sufficient permanent crosslinks to give the composition an equilibrium shape when cured;
(c) at least one temporary crosslinking agent present in an amount to provide sufficient temporary crosslinks to give the composition a stiffness when cured that is greater on short timescales than it is on long timescales; and
(d) at least one softening agent present in an amount sufficient to make the average lifetime of the temporary crosslink of shorter duration than the average lifetime of the temporary crosslink in the absence of the softening agent.
2. A viscoelastic silicone rubber composition of claim 1 comprising:
(a) at least one polyorganosiloxane comprising at least one ethylenically-unsaturated group.
3. A viscoelastic silicone rubber composition of claim 1 comprising:
(a) at least one branched polyorganosiloxane.
4. The viscoelastic silicone rubber composition of claim 3 , further comprising at least one linear polyorganosiloxane.
5. A viscoelastic silicone rubber composition of claim 1 , wherein:
(a) the polyorganosiloxane comprises a silanol-terminated polyorganosiloxane polymers of formula (I)
having a molecular weight ranging from 400 to 50,000 Dalton and a viscosity ranging from 10 to 10,000 cSt and preferably from about 15 to 2,000 cSt and wherein “m” is 1 or greater and represents the number of the repeating units in parentheses to give the molecular weight of the particular polymer;
(b) the permanent crosslinking agent is selected from a vinyltriacetoxysilane, vinyltrimethoxysilane, vinyltrichlorosilane, and vinyltriethoxysilane; and
(c) the temporary crosslinking agent is selected from a boron-containing compound, a titanium-containing compound, an aluminum-containing compound, or a mixture thereof.
6. The viscoelastic silicone rubber composition of claim 1 , wherein the polyorganosiloxane is silanol-terminated.
7. The viscoelastic silicone rubber composition of claim 1 , wherein the polyorganosiloxane is partially crosslinked and has at least three terminal silanols.
8. The viscoelastic silicone rubber composition of claim 1 , wherein the polyorganosiloxane is branched.
9. The viscoelastic silicone rubber composition of claim 1 , wherein the permanent crosslinking agent is a siloxane bond-forming crosslinking agent, a carbon-carbon bond-forming crosslinking agent, or a mixture thereof.
10. The viscoelastic silicone rubber composition of claim 1 , wherein the permanent crosslinks are formed using condensation-cure crosslinking, addition-cure crosslinking, peroxide cure crosslinking, or a mixture thereof.
11. The viscoelastic silicone rubber composition of claim 1 , wherein the permanent crosslinking agent is present in amount ranging from about 0.02 wt % to 10.0 wt %.
12. A viscoelastic silicone rubber composition of claim 1 , wherein the temporary crosslinking agent is selected from a boron-containing compound.
13. A viscoelastic silicone rubber composition of claim 12 , wherein the boron-containing compound is selected from boric acid, trimethyl borate, triethyl borate, and tri-isopropyl borate.
14. A viscoelastic silicone rubber composition of claim 1 , wherein the temporary crosslinking agent is present in amount ranging from about 0.01 wt % to 20.0 wt %.
15. A viscoelastic silicone rubber composition of claim 1 , wherein the softening agent is present in an amount sufficient to make the average lifetime of the temporary crosslink of shorter duration than the average lifetime of the temporary crosslink in the absence of the softening agent.
16. A viscoelastic silicone rubber composition of claim 1 , wherein the softening agent is present in an amount ranging from about 0.01 wt % to 5.0 wt %.
17. A viscoelastic silicone rubber composition of claim 1 , further comprising at least one filler.
18. A viscoelastic silicone rubber composition of claim 1 , further comprising at least one additive, or at least one catalyst, or at least one blowing agent, or at least one passivating agent.
19. A shaped article comprising a cured viscoelastic silicone rubber composition of claim 1 .
20. A viscoelastic silicone rubber composition of claim 1 , wherein the softening agent is selected from the group consisting of water, an alcohol, a polyol, an acid, and a base.
21. A viscoelastic silicone rubber composition of claim 1 , wherein the softening agent is selected from the group consisting of isostearyl alcohol, isostearic acid, oleic acid, and acetic acid.