Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-09-24
2001-11-27
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, C521S139000, C521S076000, C521S081000, C521S137000, C521S138000, C522S129000, C522S131000, C428S343000
Reexamination Certificate
active
06323251
ABSTRACT:
TECHNICAL FIELD
This invention relates to foams and foam articles comprising a discontinuous thermoset phase in a thermoplastic matrix.
BACKGROUND
Foamed polymeric materials have found use in many industrial applications. Foams are generally made of either thermoplastic or thermoset materials. Polyurethane foams comprise the most widely-known thermosetting polymer foams. Typically, thermoset foams are cured as foam formation occurs, and no post-foaming cure is possible or necessary. The largest industrial volumes of thermoplastic polymer foams include polystyrene and polyethylene foam.
Foamed structures have been prepared using both chemical and physical blowing agents. Chemical blowing agents are materials that decompose, typically upon heating, to produce a gas. The formation of gas bubbles within a polymer matrix produces a polymer foam. Physical blowing agents are materials that are typically gases at ambient temperature and pressure, which are mixed with a polymer to form a foam. Physical blowing agents are typically used in a foaming process in a liquid or supercritical form under high pressure. Liquid or supercritical carbon dioxide is an example of a physical blowing agent. Upon an increase in temperature or reduction in pressure, the blowing agent expands, producing foam in the polymer matrix in which it is mixed.
Foam tapes comprise an important industrial segment. Typical constructions include a foam layer that may have a backing layer and, on an opposite side, an adhesive layer, such as a pressure-sensitive adhesive layer. Alternatively, an adhesive layer may be coated on both sides of a foam layer to make, e.g., a mounting tape. Such a tape may be stiff or compliant, thick or thin, depending on the nature of the foam.
SUMMARY OF INVENTION
The present invention relates to thermoplastic foams with incorporated thermosettable materials and to the foams that result from curing the thermosettable materials. Foams of the invention can be prepared using a blowing agent that can be a swelling agent and plasticizer for the thermoplastic polymer matrix and can be a solvent that allows the thermosettable material to be introduced into the thermoplastic matrix.
The foams of the present invention can be made to possess useful characteristics such as variable modulus. This allows, for example, the ability to increase the modulus or stiffness of foam mounting tape in a specified pattern, such as, e.g., through image-wise curing of the polymer foam. This ability would constitute a distinct use advantage. The foams can also be made to provide encapsulated materials in the foam cells. This type of foam could be used to isolate reactive materials, which could be brought into contact, upon demand, by rupturing the foam cells. The method of the present invention enables the making of a thermoplastic foam comprising up to 45 weight % thermoset material.
One aspect of the invention features a foam comprising a discontinuous phase of at least one cationically-curable thermoset material in a continuous phase of at least one thermoplastic material. In another aspect, the foam further comprises at least one unactivated radiation-activatable curing agent for the thermoset material. The thermoset material may subsequently be partially or totally cured. It may also be selectively cured, for example, when used as a flexographic mounting tape. The thermoset material may be contained within the foam cells or may be entrained in the thermoplastic material. The thermoset material may comprise about 1 weight % to 45 weight % of the combined weight of the thermoplastic and thermoset materials.
Another aspect of the invention features articles comprising the foam material of the invention. The articles may include a flexographic printing plate construction or flexographic mounting tape. At least one surface of an article comprising the foam of the invention may also be covered with a coating such as a pressure sensitive adhesive. The article may also comprise a thermoplastic matrix encapsulating pockets of thermoset material.
Another aspect of the invention features a method for making a hybrid thermoplastic/thermoset film by mixing at least one thermoplastic material with at least one cationically curable thermoset material and at least one blowing agent at a temperature and pressure sufficient to form and maintain a uniformly dispersed mixture such that the thermoset forms a discontinuous phase in a continuous thermoplastic phase, shaping the mixture, and exposing the mixture to a reduced pressure to cause nucleation and cell formation. The blowing agent may be chemical or physical and the method may be performed as a batch or continuous process. Surfactants and stabilizers may also be added.
As used in this invention:
“thermoplastic/thermoset” means a material containing both thermoplastic and thermoset materials;
“supercritical fluid” means a substance, which is typically a gas at ambient temperature and pressure, compressed to a state where it has the density and solvation characteristics of a liquid, but the viscosity, permeability, and diffusivity of a gas; a supercritical fluid is a single phase material that exists above a critical point, which point is determined by a critical temperature, T
c
, and critical pressure, P
c
, which T
c
and P
c
depend on the particular gas (for example, the T
c
and P
c
for carbon dioxide are approximately 31° C. and 7.4 MPa (1078 psia), respectively);
“foam density” means the weight of a given volume of foam;
“polymer matrix” means the polymeric, or “non-cell,” areas of a foam; it includes the continuous thermoplastic phase and any thermoset material entrained in the thermoplastic material;
“functional group” means an atom or group of atoms, acting as a unit, that has replaced a hydrogen atom in a hydrocarbon molecule and whose presence imparts characteristic properties to the molecule;
“thermoset” and “thermosettable material” are used interchangeably and mean a material that solidifies irreversibly, due to crosslinking, when heated or subjected to irradiation;
“hybrid” means having two or more different characteristics or types of structures, e. g., a thermoplastic/thermoset material;
“cationically curable monomer” means a monomer capable of polymerization when exposed to a cationic moiety;
“polymerizable composition” or “curable composition” as used herein means a mixture of an initiator system and a cationically curable monomer; alcohols and adjuvants optionally can be present;
“cure” and “polymerize” are used interchangeably and mean to supply sufficient energy to a composition in the form of actinic radiation (e.g, heat, light) or electron beam in any order or combination to alter the physical state of the composition, to make it transform from a fluid to less fluid state, to go from a tacky to a non-tacky state, to go from a soluble to insoluble state, or to decrease the amount of polymerizable material by its consumption in a chemical reaction; and
“variable modulus” means exhibiting non-uniform compression modulus over a described surface area and/or through the cross section of a material, e.g., between two major parallel surfaces, typically induced by patterned thermoset cure.
An advantage of at least one embodiment of a polymeric foam of the present invention is the presence of fully or partially cured and uncured segments in the same article, providing variable modulus properties. The modulus may vary in a vertical and/or horizontal direction through a given section of foam. The modulus variation through a section of foam may have a gradient or may have distinct transitions. These properties can be employed advantageously in many applications, including flexographic printing.
An advantage of at least one embodiment of the present invention is the increased amount of thermoset monomer that can be introduced into a thermoplastic matrix. This allows the thermoplastic foam to have some of the favorable properties of the thermoset material, such as reinforcing properties. At least one aspect of the present invention can allow for the incorporation
Elsbernd Cheryl Lee Senger
Gehlsen Mark David
Gozum John Ekrem
Perez Mario Alberto
Waid Robert Dennis
Bagwell Melanie D.
Seidleck James J.
Zillig Kimberly S.
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