Multilayer foams

Details Multilayer foams Multilayer foams

1998-11-12

2002-08-27

Ball, Michael W. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S244110

Reexamination Certificate

active

06440241

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to foamed products. More Particularly, the present invention relates to foamed products comprising a plurality of layers of a foamed thermoplastic composition having a low density.
BACKGROUND OF THE INVENTION
Methods for producing foams comprising a plurality of polymer layers are known. U.S. Pat. No. 4,053,341 discloses a process for producing a laminate comprising a plurality of cross-linked polyethylene foam layers having an anisotropic pore structure in at least one internal layer. The internal layer comprises a cross-linking agent and a large amount of blowing agent, whereas the external layers comprise a cross-linking agent and a small amount of blowing agent. The layers are welded at 160° C. and heated at 190° C. to 250° C. to foam the structure. A disadvantage of this process is that it requires two steps: first welding and cross-linking the extruded plastic layers, then foaming at a higher temperature. Further, the process is limited to the production of a multi-layer foam sheet with limited cross-section.
U.S. Pat. Nos. 4,107,247 and 4,206,165 and Canadian Patent 1,100,726 describe a method for the manufacture of a plastic sandwich element in which a first extruder produces a foamed cellular plastic at a defined temperature. A second extruder produces a solid plastic at a temperature at least 14° C. (25° F.) hotter than the temperature of the foamed cellular core to promote foaming of the core. The resulting sandwich is extruded through an extrusion die. A drawback of this process is that there is only one single central foam layer confined between two solid plastic films. Further, the resulting foam has comparatively poor insulation and dimensional stability properties.
U.S. Pat. No. 4,773,448 discloses a process for producing a plastic pipe with a hard outer shell lined with an inner shell of soft resilient plastic foam with a smooth axial passageway adapted to be a conduit for liquid. An outer PVC shell is extruded and an inner polyethylene foam material is simultaneously extruded inside the PVC tube. A disadvantage of this process is that the maximum cross-section is the size of the tube. Further, because of the restrained foaming it is expected that the resulting foam exhibits a very high foam density.
JP 50-107067 discloses a process for producing a thermoplastic foam having a non-foamed skin layer by using co-extrusion. A polyethylene foam having a polypropylene skin layer and a polystyrene foam having a polyethylene skin are prepared. A disadvantage of this process is that there is only one single central foam layer confined between two solid skin layers.
Japanese Pat. No. 48-5100 discloses a process for producing a multi-layer polystyrene foam having different foam densities, for example, high density outside, low density inside, and a reinforcement material in between. The material is used as pillars in homes, exhibiting desired bending and strength properties. A disadvantage of this process is that only foams having a comparatively high density of 200 kg/m
3
can be manufactured.
Japanese Pat. No. 50-50473 discloses a laminated molded product composed of a foam layer and a non-expanded thermoplastic layer. Both layers are molded into a desired shape. The lamination can take place separately using two extruders and two dies, or simultaneously using two extruders and one multilayer die. A disadvantage of the production process described therein is that only products having a small cross-section and a high density are obtained.
Laid-open Japanese Patent Application H7-112480 discloses a rectangular molded resin product composed of a hollow multilayer resin filled with a resin foam inside the hollow multilayer. The material for the hollow multilayer is polypropylene. The foam is polypropylene or polyurethane. The disadvantage of this manufacturing process is that the molded resin product can have one single foam layer only. Moreover, only products exhibiting a restricted rectangular cross-section and a high foam density can be produced.
U.S. Pat. No. 3,694,292 describes an extrusion head for producing a multilayer blown tubular film.
Generally it has to be noted that considerable problems arise with regard to corrugation of single layers when producing multilayer foams. Particularly, when producing multilayer foams which exhibit three or more foam layers, the foaming of the center layer or layers takes place in a different environment as compared to the outer layers.
Thus, there is a great need to provide a multilayer foam having excellent insulating and dimensional stability properties at a low density. It would also be desirable to provide a method for producing a multilayer foam which is simple, inexpensive and environmentally acceptable and results in products having good physical properties.
SUMMARY OF THE INVENTION
In one aspect, the present invention thus pertains to a method for producing a polymer or copolymer multilayer foam, comprising the steps of: (a) providing at least one foamable composition comprising at least one polymer or copolymer and a blowing agent formulation, (b) extruding the composition through a die having a plurality of orifices, (c) foaming the extruded composition at a foaming temperature which is above the glass-transition temperature or the melting temperature of the polymer composition, (d) maintaining the foaming product at an elevated temperature for a sufficient period of time to obtain adhesion between individual foam layers, and (e) allowing the foamed product to cool, characterized in that a foamed product is obtained comprising a plurality of adherent foam layers, wherein at least one of these foam layers extends across the whole breadth of the foamed product.
In a further aspect, the invention relates to a polymer or copolymer multilayer foam, obtainable by the method as described above. This product has excellent properties with regard to thermal insulation, dimensional stability, water diffusion resistance and water vapor permeation resistance. Furthermore, the foam of the present invention has a smooth, uniform surface equivalent to current monolithic foams. This preferred appearance is obtained on both sides of the final foam product without any additional surface-modifying operations.
DETAILED DESCRIPTION OF THE INVENTION
The foamable composition comprises at least one polymer or copolymer. The composition may comprise a single polymer, a single copolymer, mixtures of polymers, mixtures of copolymers or mixtures of polymers and copolymers.
According to the present invention a multilayer foam is composed of a plurality of polymer or copolymer foam layers. These foam layers may be comprised of the same or of different foamable compositions. The multilayer foam comprises at least two and preferably at least three foam layers. At least one of these foam layers is a homogeneous foam layer, that is, a monolithic foam part extending across the whole breadth of the foam. This homogeneous or monolithic foam layer is obtained by extruding the foamable composition through an orifice which extends across the whole breadth of the die, for example, a slit, or through a continuous orifice, for example, a circle.
The present invention relates to such multilayer foams composed of a variety of such monolithic layers, but also to such foams comprising at least one layer extending across the whole breadth of the foam and at least one layer which is comprised of a plurality of individual coalescent foam strands.
By means of the process for the production of multilayer foams according to the invention, it becomes possible to obtain foams having a greater overall thickness, or to produce them in a simpler manner. Moreover, the resulting multilayer foams have an oriented structure and offer an improved thermal insulation performance at the same density or thickness compared to monolithic foams. Furthermore, they exhibit an excellent dimensional stability and strength with similar or, more preferably, substantially equivalent water permeation resistance and water absorption properties

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