Crosslinked elastic material formed from a linear block copolymer
View Patent ↗An elastic material for use in an absorbent article is provided. The elastic material contains a crosslinked network formed from a linear block copolymer having a monoalkenyl aromatic midblock positioned between conjugated diene endblocks (e.g., butadiene-styrene-butadiene (“B-S-B”) triblock copolymer). Prior to crosslinking, such linear block copolymers have a relatively low viscosity and thus may be readily formed into a precursor elastic material (e.g., film, strands, web, etc.) that is subsequently crosslinked to achieve the desired elastic and mechanical properties. Crosslinking is typically achieved through the formation of free radicals (unpaired electrons) that link together to form a plurality of carbon-carbon covalent bonds at the conjugated diene endblocks.
1. An elastic material comprising a crosslinked network, the crosslinked network containing a linear block copolymer having a monoalkenyl aromatic midblock positioned between conjugated diene endblocks, wherein the monoalkenyl aromatic midblock constitutes from 15 wt. % to about 30 wt. % of the copolymer and has a number average molecular weight of from about 5,000 to about 35,000 grams per mole, wherein the copolymer is a triblock copolymer, the monoalkenyl aromatic midblock includes styrene or a styrene derivative, the conjugated diene endblocks include butadiene, and from about 5 to about 25 mole percent of the butadiene units in the conjugated diene endblocks have 1,2 vinyl configuration;
wherein the elastic material is a melt-extruded film.
2. The elastic material of claim 1 , wherein the copolymer has a melt flow index of from about 1 to about 500 grams per 10 minutes, measured in accordance with ASTM Test Method D1238-E at 190° C.
3. The elastic material of claim 1 , wherein the copolymer has a melt flow index of from about 15 to about 500 grams per 10 minutes, measured in accordance with ASTM Test Method D1238-E at 190° C.
4. The elastic material of claim 1 , wherein the conjugated diene endblocks have a number average molecular weight of about 30,000 to about 120,000 grams per mole and the monoalkenyl aromatic midblock has a number average molecular weight of about 7,500 to about 30,000 grams per mole.
5. The elastic material of claim 1 , wherein the copolymer is hydrogenated.
6. The elastic material of claim 1 , wherein the copolymer constitutes about 10 wt. % or more of the elastic material.
7. The elastic material of claim 1 , wherein the copolymer constitutes about 50 wt. % or more of the elastic material.
8. The elastic material of claim 1 , wherein the monoalkenyl aromatic midblock has a number average molecular weight of from about 7,500 to about 30,000 grams per mole.
9. The elastic material of claim 1 , wherein the film is a cast or blown film.
10. The elastic material of claim 1 , wherein the film is stretched in the machine direction, cross-machine direction, or both.
11. The elastic material of claim 10 , wherein the film is microporous.
12. The elastic material of claim 1 , wherein the film is multi-layered and contains at least one base layer and at least one skin layer.
13. The elastic material of claim 1 , wherein the copolymer is not elastic prior to crosslinking.