Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-06-23
2003-03-04
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S135000, C521S096000, C521S093000, C036S043000
Reexamination Certificate
active
06528550
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to foams of ethylene vinyl acetate. This invention specifically relates to microcellular foams of ethylene vinyl acetate and ethylene/acrylic acid copolymer as well as improved methods of production.
BACKGROUND OF THE INVENTION
Ethylene and vinyl acetate (EVA) copolymers are well known items of commerce having a wide variety of applications. Ethylene vinyl acetate polymers are produced by copolymerizing ethylene and vinyl acetate monomer. As the bulky acetoxy group content increases, the polymer becomes more amorphous, and possesses increased flexibility, rubberiness, low temperature properties, tackiness and heat sealability. This thermoplastic material is therefore widely used in flexible packaging, hot melt adhesives, electrical, medical and many other applications.
EVA resins can be conveniently crosslinked by both peroxide or irradiation to enhance mechanical properties and heat resistance. It is this crosslinking, coupled with the inherent rubbery nature of the polymer which makes EVA suitable for production of tough and abrasion resistant foams, especially suitable for footwear applications.
Crosslinked EVA foams can be manufactured by two methods—the ionizing radiation method and the chemical crosslinking method. The ionizing method, however, is restricted to pieces less than ¼ inches thick, and thus is of limited use. The chemical crosslinking method has found more commercial applicability. By crosslinking, the viscosity of EVA at high temperatures is increased and the individual cell is kept in a stable condition without rupture or agglomeration. Low-density microcellular foam can be obtained. By selecting the vinyl acetate (VA) content, the EVA foam is flexible and highly resilient with easy coloring and adherent to other materials. The application is used widely in shoe soles, sandals and cushion materials.
The prevalent chemical crosslinking method for producing microcellular cross-linked EVA foams is the compression molding process. Although this process produces high quality foam, it requires long processing time and generates a high level of scrap. A press-molding foam process involves many steps: compounding, press molding into foam, cutting into shape, then press molding into a final product. The process is long, tedious and labor intensive. Improving productivity would be highly desirable, as well as improving foam performance to achieve lighter density and improved properties.
The injection molding process was developed more than ten years ago to overcome the drawbacks of compression molding. This process has not been widely accepted due to the lack of high quality consistent compounds to reproduce exactly the same size from shot to shot over production cyles lasting several days. In such processes, achieving a balance between foam density and performance properties (e.g. compression set) can be difficult to attain. See, e.g., “Microcellular Crosslinked EVA Foam By Injection Molding Process”, John Lee, 2060/ANTEC, 97.
There is a continued need to improve EVA foam properties, especially to improve compression set resistance for lower density EVA foams. A need also exists to develop EVA compounds that can enable shorter, more cost-effective processes to produce EVA foam.
REFERENCES:
patent: 4073844 (1978-02-01), Wada et al.
patent: 4314035 (1982-02-01), Hobes et al.
patent: 4331779 (1982-05-01), Park
patent: 4364565 (1982-12-01), Tomar
patent: 4480054 (1984-10-01), Enderle
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John (Jeong) Lee, Microcellular Cross-Linked EVA Foam by Injection Molding Process,ANTEC '97, 823, 2060-2064, '97.
PCT International Search Report for International application No. PCT/US00/17774, dated Nov. 13, 2000.
Chou Richard T.
Hsu Chin-Fu
Ou Yun Tong
Whelchel Wayne C.
Bissett Melanie
E. I. du Pont de Nemours and Company
Seidleck James J.
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