TITLE EXPANDED ETHYLENE COPOLYMERS BACKGROUND OF THE INVENTION The present invention relates to ethylene copolymer compositions that are useful for fabricating films and molded articles. More particularly, the ethylene copolymer compositions of the present invention contain compounds that react with the copolymer to liberate a gas which forms bubbles. These bubbles, which are dispersed in the end product films or molded articles, render certain desirable aesthetic characteristics to the end products.

Ethylene copolymer compositions used in the present invention-minus the gas liberating compounds-are well known in the packaging industry. Examples of such uses are as sealants or shrink film in mono-or multilayer films for food packaging or bottle bundling or as injection molded stoppers for perfume bottle caps.

It is also well known how to enhance the properties of these copolymers by using additives to color them or to achieve certain effects such as filling them with minerals fillers and to create a'stone'like effect or similar.

In the case of beverage and fragrance packaging it is sometimes important to create the impression of freshness and coolness in order to improve the market acceptance of a product. This can be best done by the creation of bubbles in the article that is used as a packaging material. A possibility is to add water directly to the copolymer composition or on a carrier, though this can lead to a very random distribution of bubbles and to rather large bubbles in the molded parts.

Also it is very difficult to reproduce results as polymers are sometimes water sensitive.

The ethylene copolymer composition of the present invention overcomes the aforementioned problems, and yields films and molded articles having a reproducible, attractive dispersion of bubbles.

SUMMARY OF THE INVENTION The present invention provides a thermoplastic composition comprising a copolymer of ethylene and an a, p-unsaturated C3-Cg carboxylic acid and a compound which reacts with the acid group of the copolymer to liberate gas. The copolymer may further contain an acrylate, and may be partially neutralized.

Processing the composition, e. g. extruding or casting or blowing a film, or injection molding a molded article, liberates gas to yield a dispersion of bubbles in the film or molded article.

DETAILED DESCRIPTION OF THE INVENTION The thermoplastic copolymers used in the present invention are known. They are copolymers of ethylene and an a, p-unsaturated C3-C8 carboxylic acid, and optionally contain at least one softening comonomer that is copolymerizable with ethylene. Acrylic and methacrylic acids are preferred acid comonomers. The softening comonomer can be an alkyl acrylate selected from the group consisting of n-propyl-, n-butyl, n-octyl, 2-ethylhexyl-, and 2-methoxyethyl-acrylates. The preferred alkyl acrylates are n-butyl-, 2-ethylhexyl-, and 2-methoxyethyl- acrylates. The softening comonomer can also be an alkyl vinyl ether selected from the group consisting of n-butyl, n-hexyl, 2-ethylhexyl-, and 2-methoxyethyl- vinyl ether. The preferred alkyl vinyl ethers are n-butyl vinyl ether and n-hexyl vinyl ether.

The copolymer can be from about 10 to 99.5% neutralized with metal ions selected from groups la, Ib, IIa, IIIa, IVa, VIb, and VIII of the Periodic Table of Elements such as sodium, potassium zinc, calcium, magnesium, lithium, aluminum, nickel, and chrominum. Such neutralized ethylene acid copolymers

are known in the art as'ionomers'. Typically, neutralization will be from 10-70%. Preferably the copolymer has from about 35 to about 70% of the carboxylic acid groups ionized by neutralization with metal ions selected from the group consisting of sodium, potassium, zinc, calcium, and magnesium. Ionomers and their methods of manufacture are described in US Patent No. 3,264,272.

Suitable ionomers for use in the present invention are commercially available under the trademark SURLYN; suitable ethylene acid copolymer are available under the trademark NUCREL ; both from the DuPont Company. The copolymer will typically comprise from 95 to 99.95% by weight of the composition. If desired for certain applications, the composition can be, in turn, blended with other polyolefins prior to extrusion or molding. For example, blends of 8-100 weight % composition and 0-92 weight % polyolefins may produce satisfactory results whilst reducing overall costs.

The compound that reacts with the acid group of the copolymer is a metal carbonate with the metal ion chosen from the groups la, Ib, IIa of the periodic <BR> <BR> <BR> <BR> Table of Elements. A preferred such metal carbonate is sodium carbonate. The metal carbonate will typically comprise from 0.01 to 0.2% by weight of the composition.

In practice, the metal carbonate can be added to the composition in the form of a concentrated masterbatch of the metal carbonate and the copolymer. During the mixing of the masterbatch with the copolymer in order to form composition suitable for extrusion or casting or molding, the metal carbonate reacts with the carboxylic acid functionality of the copolymer to liberate, in a controlled fashion, carbon dioxide gas. Bubble distribution it the resulting article can be controlled by the degree of homogeneous mixing of the metal carbonate into the copolymer.

Bubble density and bubble diameter can be varied by varying the pressure of the system during formation of the bubbles. High pressure will result in few and fine bubbles. Low pressure will favor more and larger bubbles.

In addition, the melt flow index of the composition will also impact bubble size.

A composition with a high melt flow index will provide a low resistance against bubble formation and therefore create large, coarse bubbles, whereas a composition with a low melt flow index will favor the creation of smaller bubbles.

In addition, compositions according to the present invention can contain additives, processing agents as known in the art to impart various desirable characteristics.

These include: anti-blocking agents such as Si02, slip agents in the form of amides and colorants in amounts ranging from 0.1-2 weight % of the composition, or any other additives for performance enhancement of the film or molded article in the final application.

Molding Articles can be injection molded with aesthetically appealing centers having very fine bubbles of uniform diameter. In order to avoid surface defects in the articles, which are caused when bubbles come into contact with the mold surface and cause surface roughness (also known as'splash'), it is preferable to co-inject over the composition of the present invention a composition not containing a compound which reacts to liberate gas. This yields an article having a smooth exterior and an interior containing uniformly dispersed bubbles.

Films For the production of modified extruded film showing the same bubble effect it can be done by cast film or by blown film process. In both cases but in particular in the case of the blown film process where the film is made by blowing a tube of the liquid polymer it is preferable to co-extrude the composition of the present invention with a composition not containing a compound which reacts to liberate gas. This is necessary to assure a sufficient stability of the web and in particular to contain the over pressure within the tube during the blown film process.

The result is a surprisingly appealing looking film and a stable extrusion process due to the controlled gas release and stability brought to the structure by the unmodified co-extrusion layers. When extruded under specific conditions known in the art, e. g. as described in EP 0 537 080 B1, they show tear properties in transverse direction, which is desirable in various packaging applications. The resulting film provides for a packaging film giving its contents an appearance of <BR> <BR> <BR> <BR> 'freshness'and'coolness'while still having maintained directional tear properties for easy opening performance.

Examples Iniection Molding Materials used: Surlyn PCIOO Ethylene methacrylic acid ionomer, MFI =0.91 at 190 °C/2.16 kg Surlyn PC350 Ethylene methacrylic acid ionomer, MFI = 3.5 at 190 °C/2. 16 kg Nucrel 1207 Ethylene methacrylic acid copolymer, MFI = 7 at 190 °C/2.16 kg Liquicolor VL3576 Base liquid for liquid colors supplied by BASE saturated with water Na2C03 Masterbatch of ethylene methacrylic acid copolymer (10 weight % methacrylic acid, MFI = 500 at 190 °C/2.16 kg) containing 60 wt % Na2C03 All MFI given in g/10 min.

The examples described below are run on a Billion H1180/450 injection molding machine using Surlyn PC 100, Surlyn PC 350 and Nucrel 1207 as base polymers.

Two different types of foaming agent are used: Liquicolor VL3619, containing water until its saturation; From 0.1 to 1 weight % of Na2C03 masterbatch (= 0.06-0.6 wt % Na2C03).

As can be seen from the data below it is possible to produce different levels of foaming by changing concentration of foaming agent and pressure. The melt temperature is 213 °C.

The Surlyn PC grades have a superior esthetic appearance because of higher clarity compare to the Nucrel. Amongst the Surlyn grades, PCIOO seems to have the better surface appearance as the higher viscosity (lower MFI) prevents the bubbles from reaching the surface, thereby creating splash marks.

The liquid concentration samples on the other hand lead to very poor appearance and reproducibility and even dissolving Na2C03 in the liquid carriers gives very poor foaming behavior.

No Control I Control 2 1 2 3 4 5 Surlyn PCIOO 100 100 100 100 100 Liquicolor VL3576 0.5 Surlyn PC350 100 Nucrel 1207 100 Na2CO3 masterbatch 0 0 0.5 0.5 0.5 0.25 0.25 Hold up pressure (MPa) 100 100 100 100 7 Remarks: 1 2 3 4 5 6 7 1 no bubbles 2 large bubbles, poor distribution 3 fine bubbles 4 fine bubbles, few splashes 5 fine bubbles, many splashes 6 fine bubbles, lower injection pressure 7 fine bubbles, large number of bubbles It can be seen that Na2C03 masterbatch gives a bubble concentration which is controllable by concentration of Na2C03 as well as by the pressure and is also reproducible from one shot to the other due to fine us form bubble size. Higher concentrations of the masterbatch increase viscosity and increase injection pressure and vice versa.

Blown Film Materials used: Stamylan 3020H LDPE, MFI 190°C/2.16 kg = 1.7-2.2 commercially available from DSM Nucrel 0407 Ethylene methacrylic acid copolymer, MFI = 7 at 190°C/2.16 kg, 4% methacrylic acid Surlyn AD8455 Ethylene methacrylic acid ionomer, MFI 190 = 3 at °C/2.16 kg, Zn Ionomer Masterbatch Contains 60 wt % Na2C03 in ethylene methyacrylic acid (MFI = 500 at 190°C/2.16 kg, 10% methacrylic acid) For preparation of these films a Reifenhauser coextrusion line is used with a 3 layer Barmag die to produce the following structure: Example Control 1 Inside bubble layer/thickness Stamylan 3020H/ 6 11m Stamylan 3020H/14, um Tie layer/thickness Surlyn AD8455/44 Am Nucrel 0407/34 l lm Outside bubble layer/thickness Stamylan 3020H/611m Stamylan 3020H/12, um Masterbatch added to tie layer 0% 1 % Production stable stable Observations no bubbles bubbles Tear transverse direction transverse and machine directions The above example shows that it is possible to obtain an aesthetically looking bubble effect suggesting'freshness'in a film while at the same time this film will have low tear strength in transverse direction and will therefore be suitable for easy opening beverage bundle packaging applications.