Chemically embossed metallocene polyethylene foam

Details Chemically embossed metallocene polyethylene foam Chemically embossed metallocene polyethylene foam

2000-06-07

2001-02-13

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C106S001150, C106S001150, C524S091000, C524S104000

Reexamination Certificate

active

06187840

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to making a chemically embossed metallocene polyethylene (MPE) foam using a highly co-activated blowing agent of azodicarbonamide and an alkyl functionalized triazole derivative as an inhibitor. The alkylamino group of this liquid azole makes it readily dissolvable in non-polar inks which gives good adhesion to MPE surfaces. The compatibility of the hydrocarbon structure also increases permeation of the inhibitor into the polyethylene composition. The initial effort showed normally activated azodicarbonamides were not able to blow MPE. Use of a highly co-activated azodicarbonamide and a liquid azole compound in the present invention yield an unusually excellent textured MPE foam.
BACKGROUND OF THE INVENTION
Sheets of cellular resinous composition have been widely used as decorative surface coverings. It is well known to those skilled in the art that foamed polymer sheets may be textured by the process commonly referred to as “chemical embossing”. Such chemical embossing techniques have normally involved coating/calendering a base material, such as a paper felt with a foamable polymer layer., A chemical blowing agent is normally uniformly dispersed in the foamable layer and is adapted, upon subsequent heating to a sufficiently elevated temperature, to decompose and to liberate gaseous decomposition products to expand and create the foamed product. For chemically embossed sheets, the surface of a foamable polymer is printed with an ink composition containing an agent which inhibits foaming in the printed areas when the foamable polymer composition is subsequently subjected to a heat treatment. The areas which have not been printed over thus expand normally on heating while expansion in the printed areas containing the inhibitor is suppressed, resulting in a textured surface with depressions in those areas printed with the inhibiting ink.
The most frequently used polymers in the preparation of resilient sheet flooring are polyvinyl chloride (PVC) and vinyl chloride copolymers. Each layer in a composite sheet structure provides unique functions for an overall performance. A foamable layer made of PVC has certain disadvantages such as poor strength and low elastic recovery, and there is accordingly need for an alternate foamable layer. It has now been found that a particular class of polyethylene-metallocene polyethylene (single site catalyst-based polyolefin resin) can provide excellent features for a foamable layer for flooring. It would be desirable to provide an economical way to produce an embossed MPE foam layer for commercial production.
The structure of cellular gas-filled polymers can be formed either by foaming a polymer system, by introducing gas-filled microspheres (microballons) into a system, or by extracting material by a post-treatment, resulting in the formation of cells or pores. The method of foaming is the one most generally employed for making polymers with a cellular structure. Based on the mechanism by which gas is liberated, the compounds used for foaming polymers may be classified as chemical and physical blowing agents. Chemical blowing agents (CBA) are individual solid compounds or mixtures of solid compounds that liberate gas as a result of chemical reactions, including thermal decomposition, or as a result of chemical reactions of chemical blowing agents. Physical blowing agents are liquid compounds that gasify as a result of physical process (evaporation, desorption) at elevated temperatures or reduced pressures.
A chemically embossed foam utilizes a chemical blowing agent. The blowing agents which have found the most wide-spread use are those compounds having the >N—N< or —N=N— linkages which decompose at elevated temperature to yield an inert gas high in nitrogen. Hereinafter, such blowing agents shall be referred to as “nitrogen containing blowing agents”. One example of a nitrogen containing blowing agent is azodicarbonamide (or “azo”). Nitrogen has a low permeability in polymers. Decomposition is a function of chemical activation, particle size and temperature. It is common practice to add accelerators into the compositions to accelerate the decomposition of the blowing agents, reduce the decomposition temperature and/or narrow the decomposition temperature range. Typical activators for azo blowing agents include acids, bases, metal organic salts, oxides, amines and urea, etc.
One critical requirement to be taken into account when selecting a chemical blowing agent is that the temperature of decomposition of the CBA must be close to the melting point and the hardening temperature of polymer. It would be desirable to find a suitable blowing agent and activators for a metallocene polyethylene, which has a low melting/softening temperature around 60° C. (lower than 100° C.).
The inhibitor for the blowing agent is applied to one surface of the resinous polymer composition in any desired area. The inhibitor is preferably applied in a liquid carrier which allows better control of the concentration of inhibitor applied. The inhibitor composition is conveniently formulated in the manner of a printing ink. The effectiveness of inhibition depends on the permeability (solubility and diffusion) of the inhibitor in a printing ink to the foam substrate. Polyethylene is not permeable to many solids and liquid organic and inorganic materials.
A wide range of compounds have been claimed to act as inhibitors for chemical embossing foam of floor and wall covering surfaces. The choice of an inhibitor for the blowing agent will depend on the particular blowing agent utilized in the system. Triazole compounds such as benzotriazole (ETA) and tolyltriazole (TTA) are widely used in solvent-based inks for chemically embossing foam made by azo blowing agents. However, their use in non-polar inks which give good adhesion to polyethylene surfaces, is limited by a lack of solubility. Therefore, these widely used inhibitors are of limited utility in polyethylene systems. It would be desirable to provide an inhibitor which is soluble or compatible in a non-polar ink, and has a high penetration rate into polyethylene for producing chemical embossing foam of MPE.
SUMMARY OF THE INVENTION
According to the present invention, there is a novel activated chemical blowing agent for use in making a metallocene polyethylene foam sheet suitable as a floor covering. The novel chemical blowing agent is a highly co-activated blowing agent comprising a nitrogen containing blowing agent, preferably azodicarbonamide, and at least two activators. Activators comprise citric acid, oxalic acid, p-toluene sulfonic acid, phosphoric acid, potassium carbonate, borax, triethanol amine, zinc chloride, zinc acetate, zinc oxide, zinc stearate, barium stearate, calcium stearate, urea, and polyethylene glycol. The preferred co-activators for this invention are zinc oxide and urea. The compositions of the activators are generally from about 10% to 70% by weight of the azodicarbonamide, preferably from 20% to 50%, and more preferably from about 35% to 45%. Preferably, the co-activator has the composition ratio for co-activators of 2:1 to 1:2 when two activators are used, and most preferably about 1:1, although ratios of 10:1 to 1:10 can also be useful.
In accordance with the invention, a process and composition are provided for making a foam-growth-controlling printing ink composition for use in chemical embossing of foamable metallocene polyethylene sheets. The printing ink composition comprises a thermoplastic binder resin, a non-polar solvent, an optional pigment, and an inhibitor package for preventing the foaming of the foamable materials containing the nitrogen containing blowing agent. The inhibitor package is a triazole compound of the formula:
where R can be hydrogen, one or more aliphatic moieties containing up to 20 carbon atoms, one or more cycloaliphatic moieties containing from 3 to 20 carbon atoms, or one or more aryl or substituted aryl groups containing from 6 to 30 carbon atoms and attached by a single bond to any or all of the atoms

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