Claims
- 1. In a one-step process for rotational molding articles having a foamed interior and non-foamed exterior skin wherein a molding composition comprised of neat polymer particles and polymer particles containing a chemical foaming agent are combined and heated in a mold at a temperature above the melt temperature of the polymers and above the decomposition temperature of the foaming agent while biaxially rotating the mold, to produce an article which is uniformly foamed with an exterior skin which is smooth and substantially free of surface defects, the improvement wherein the molding composition comprises:
- (a) 25 to 75 percent by weight, based on the weight of the total composition, ethylene polymer pellets ranging in size from 1/16 inch to 3/16 inch in diameter and containing 0.25 to 7.5 weight percent chemical foaming agent, based on the weight of the ethylene polymer, said ethylene polymer having a melt index from 0.25 g/10 mins. up to 25 g/10 mins., and
- (b) 75 to 25 percent by weight, based on the weight of the total composition, ethylene polymer powder containing:
- (i) a major proportion of fractional melt index ethylene polymer powder wherein 80 percent or more of the powder particles are greater than 250 microns in size and
- (ii) a minor proportion of ethylene polymer powder wherein 80 percent or more of the powder particles are less than 250 microns in size and the ethylene polymer has a melt index greater than 1 g/10 mins., and with the proviso that the mean particle size of (b)(i) and (b)(ii) differ by at least 100 microns.
- 2. The process of claim 1 wherein (a) comprises from 30 to 60 weight percent of the total composition and (b) comprises from 40 to 70 weight percent of the total composition.
- 3. The process of claim 1 wherein the foaming agent is selected from the group consisting of azodicarbonamide, modified azodicarbonamide, sodium bicarbonate and sodium bicarbonate/citric acid.
- 4. The process of claim 3 wherein the foaming agent is present in an amount from 0.5 to 5 weight percent based on the weight of the ethylene polymer.
- 5. The process of claim 4 wherein the foaming agent is azodicarbonamide or modified azodicarbonamide.
- 6. The process of claim 1 wherein (a) is a high density ethylene homopolymer or copolymer of ethylene and C.sub.3-8 .alpha.-olefin having a melt index from 1 to 10 g/10 mins.
- 7. The process of claim 6 wherein (a) has a density from 0.941 to 0.970 g/cm.sup.3.
- 8. The process of claim 7 wherein (a) is a copolymer of ethylene with butene-1 or hexene-1 having a density of 0.941 to 0.959 g/cm.sup.3.
- 9. The process of claim 1 wherein (a) also contains 0.01 to 1 weight percent, based on the weight of the ethylene polymer, of an additive selected from the group consisting of calcium carbonate, zinc oxide, talc and calcium stearate.
- 10. The process of claim 1 wherein (b)(i) constitutes from 75 to 97.5 percent of the total powder mixture and (b)(ii) constitutes from 2.5 to 25 percent of the total powder mixture.
- 11. The process of claim 10 wherein (b)(i) has a particle size form 250 to 1000 microns and is a low density ethylene homopolymer having a melt index from 0.1 to 0.8 g/10 mins.
- 12. The process of claim 11 wherein (b)(i) is a low density polyethylene having a density from 0.915 to 0.930 g/cm.sup.3.
- 13. The process of claim 12 wherein (b)(i) has a particle size from 250 to 600 microns and a melt index from 0.2 to 0.5 g/10 mins.
- 14. The process of claim 10 wherein (b)(ii) has a particle size from 10 to 250 microns and is a low density copolymer of ethylene with butene-1, hexene-1 or octene-1 having a melt index from 1 to 10 g/10 mins.
- 15. The process of claim 14 wherein (b)(ii) is a linear low density polyethylene having a density from 0.915 to 0.940 g/cm.sup.3.
- 16. The process of claim 15 wherein (b)(ii) has a particle size from 20 to 225 microns and a melt index from 2 to 7 g/10 mins.
- 17. The process of claim 1 conducted at a temperature from 200.degree. C. to 350.degree. C. with a rate of rotation about the outer axis of 12 to 25 rpm and rate of rotation about the inner axis of 7 to 20 rpm.
- 18. The process of claim 1 wherein (a) comprises from 30 to 60 weight percent, based on the weight of the total composition, high density copolymer of ethylene and butene-1 or hexene-1 having a density from 0.941 to 0.959 g/cm.sup.3 and a melt index from 1 to 10 and containing from 0.5 to 5 weight percent, based on the weight of the high density copolymer, of a foaming agent selected from the group consisting of azodicarbonamide and modified azodicarbonamide and (b) comprises from 40 to 70 weight percent based on the weight of the total composition, ethylene polymer powder mixture containing from 75 to 97.5 weight percent, based on the total powder mixture, low density polyethylene powder having a density from 0.915 to 0.930 g/cm.sup.3 , melt index from 0.2 to 0.5 g/10 mins. and particle size from 250 to 1000 microns and 2.5 to 25 weight percent, based on the total powder mixture, linear low density polyethylene powder having a density from 0.915 to 0.940 g/cm.sup.3 , melt index from 2 to 7 g/10 mins. and particle size from 10 to 250 microns.
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of application Ser. No. 08/842,777 filed on Apr. 17, 1997, now U.S. Pat. No. 5,783,611.
US Referenced Citations (9)
Divisions (1)
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Number |
Date |
Country |
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842777 |
Apr 1997 |
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