NO133105B - - Google Patents

Description

NO-PS 131 548 describes a method for the production of cross-linked foam plastic based on thermoplastic ethylene-iopolymers, which contains a) 60-97% by weight ethylene,

b) 3-40% by weight of one or more vinyl esters of acids with 2-5

C atoms or alkyl esters of ethylenically unsaturated mono- or di-carboxylic acids with up to 5 C atoms, and optionally c) up to 20% by weight of acrylic or methacrylic acid, by mixing the ethylene copolymers with propellants, cross-linking agents and further auxiliaries by temperatures of 10-100°C above the crystallite melting point of the polymers and at pressures that are above the propellant pressure at the mixing temperature, and the method is characterized by the addition of the ethylene copolymers, as crosslinking agents and auxiliaries:

1) 0.5-5 parts sulphur,

2) 0.5-7 parts of an oxide of a divalent metal,

3) 0.2-5 parts vulcanization aid and

4) 0.2-2 parts of a fatty acid or resin acid, where all parts are parts by weight calculated on 100 parts by weight of copolymers,

after which the homogeneous mass is expanded in a known manner.

The present invention relates to a further development of the above method. The invention consequently provides a method as stated in NO-PS 131 548, which is characterized in that the homogenized mixture; before it expands, is cooled to a temperature between the crystallite melting point and a temperature 20°c lower than this.

Several methods are known by which foams of ethylene polymers with open cells can be obtained. These are comprised of such foamy ethylene polymers where the cells are not separated by closed cell membranes. Thus, it is e.g. known a method by which foamy ethylene polymers which have closed cells are subjected to strong pressure stresses. For example, webs of foamy ethylene polymers can be passed between rollers and pressed together, whereby the cell membranes are torn open.

In another known method, foamy ethylene polymers are treated with solvents. This method also destroys part of the cell membranes, so that the cells are in contact with each other.

Furthermore, it is known to mix ethylene polymers with solid, soluble substances and dissolve these substances from the mixtures. This method also produces foamy products with open cells. These methods, however, present technical difficulties and are cumbersome, so that they are not completely satisfactory. Moreover, it is partly not possible with these methods to obtain transparent foams of ethylene polymers with a low volumetric weight.

It has now been found that it is particularly advantageous to produce open-cell foam plastic from ethylene copolymers by working according to the above-mentioned method.

Copolymers of ethylene containing 5 to 20% acrylic acid and 5 to 20% acrylic acid t-butyl ester polymerized are particularly suitable for the production of soft, open-cell foam plastic.

The copolymers are mixed with the aforementioned cross-linking agents and auxiliaries and with low-boiling propellants. The details regarding the preferred cross-linking agents and auxiliaries can be found in NO-PS 131 548.

For the production of foams with a particularly fine cell structure, so-called nucleating agents can be added to the mixtures. Suitable is e.g. finely powdered silicates, e.g. talc, which it is most appropriate to use in amounts from 1 to 3 percent by weight, calculated on the copolymer. The use of such inert powder materials as nucleating agents in polystyrene foam extrusion is in and of itself known from the older priority Norwegian interpretation document no. 129 910. The solid propellants can also serve as nucleating agents, when they are added in small quantities next to a liquid -shaped propellant.

IN

The mixtures Jean besides -polymerizate, crosslinking agent

and propellant contain additional components, e.g. fill-

substances, dyes, flame retardants, antistatic agents, stabilizers, lubricants or other polymers.

These other polymers can, for example, be polyethylene, polypropylene or polyisobutylene, which can be added in quantities of up to 20% by weight, calculated on the total polymer.

The choice of propellant will also be the same as in NO-PS

131 548, and reference is made to the explanation there.

One uses such quantities of propellants that the mixtures

of thermoplastic ethylene copolymers and propellant contains 15 to 35 percent by weight of the propellant, calculated on 100 parts of copolymer. The amount of propellant depends on the desired volume weight of the foamy ethylene copolymer and on the pressure and temperature conditions during processing.

The thermoplastic ethylene copolymers are mixed in a thermoplastic state with the propellants. It is appropriate to work at temperatures that are 10 to 100°C, especially 30 to 90°C, above the crystallite melting point of the polymers. The mixtures are kept under a pressure that is above the propellant pressure at the mixing temperature, so that the propellants are constantly in a liquid state under the mixing conditions. Propellant pressure means the vapor pressure of the propellant above the molten copolymer.

As mentioned, the homogeneous mixtures of copolymer, propellant and crosslinking agent must be cooled before extrusion to temperatures that are 20 to 0°C below the crystallite melting point

• to the polymeric compound, i.e. immediately before the outlet opening, e.g. by continuously working mixing devices. Immediately before the nozzle, a zone is placed where the homogeneous mixtures are kept in the aforementioned temperature range. After the mixing process is finished, the mixtures are expanded in a homogeneous state. This means that the mixtures are brought into a zone where the pressure is below the propellant pressure. It is most appropriate to expand the mixtures in a zone with atmospheric pressure. It is advantageous to use continuously working mixing devices for preparing the mixture. Of these, extruders in particular have proven useful. The dimensions of the mixing device must be chosen so that a homogeneous mixture of ethylene copolymer, additives and propellant is formed at the outlet of the mixing chamber. The mixtures can also be produced in discontinuously working devices, e.g. in pressure vessels with

stirrer.

It has been found that the obtained foams have the more

open cells, the smaller the polymer's expansion during foaming. The expandability is reduced by the cross-linking, whereby it

a gel content in the mixture of over 20% is required to achieve foams with at least 20% open cells. At a gel content of 40 to 60%, up to 70 - 80% open cells were obtained

in the foams. Given the gel content in it. propellant-containing polymer melts, the number of open cells can be strongly influenced by changing the melt temperature at the moment of foaming. An increase in temperature corresponds to an increase in the number of open cells.

With the method according to the invention, foam plastic of different shapes can be obtained. Thus, it can e.g. continuous profiles are obtained, e.g. plates, blown foils or tubes. Moreover, the procedure is suitable. for wrapping objects with foam plastic. Finely divided foams can also be produced using the method. These can e.g. is obtained by continuous extruding and dividing the mixture before foaming. The method results in foam plastic with a volume weight above 15 g/litre, preferably between 15 and 40 g/litre, in connection with liquid propellants. Such foams can e.g. used for cushions, insulation, packaging or in finely divided form as a means of making the soil porous. Plates and foils produced by the method according to the invention are used for heat and sound insulation.

The foam substances obtained by the method according to the invention have a gel content which can be up to 70% and preferably lies between 30 and 60%. Gel content is to be understood as the insoluble portions such as e.g. can be determined by dissolving the polymer in decahydronaphthalene at 140°C. The obtained foams have as good thermal stability as the foams according to NO-PS 131 548.

It is a particular advantage that the open-celled, soft foam plastics such as. produced by the method according to the invention, does not shrink, as it e.g. is the case in the production of soft foams with closed cells from ethylene copolymers.

Depending on the percentage of open cells, calculated on the total cell content, the products can serve different purposes. Thus, e.g. foam with less than 40% open cells sufficiently high resistance in larger layer thicknesses against the penetration of gas and liquid components. The foams can therefore, like soft foams with closed cells, for example serve as sealing material, for packaging and for insulating containers. In addition, individual particles can be welded into block material at a later stage.

Foam with more than 40% open cells is softer than foam of the same composition with a lower percentage of open cells. These foams can be used as sponges, for filling pillows or for packaging purposes.

The parts given in the examples are parts by weight, the percentages are percentages by weight.

Example 1

An ethylene copolymer containing 10% acrylic acid

and 8% acrylic acid t-butyl ester, and which has a crystallite melting point of 90°C, is mixed with a mixture of 1.5% sulphur, 5% zinc oxide,

1% stearic acid, 0.8% dibenzothiazole disulphide and 3% talc, in each case based on the copolymer. The mixing takes place in an extruder. 18% isobutane is added to the mixture, again based on the copolymer. The mixture is kept at different temperatures in the mixing area of the extruder. The mixtures are always extruded at 85°C. The gel content/ as well as the proportion of open cells, as well as the elongation

before breaking (the elongation at break) of the foam obtained is determined. To determine the proportion of open cells, the foam is placed in alcohol. The amount of absorbed alcohol is determined. The determination of the elongation at break is carried out according to DIN 53 371.

The results are listed in Table 1.

As the results show, only open cell foams are obtained when the gel content is greater than 15%. It also turns out that at a given extruder temperature, the number of open cells depends on the gel content x the mixture.

Example 2

As in example 1, a copolymer of the composition described there is mixed with the aforementioned crosslinking agents and the propellant at a temperature of 168°c. A homogeneous, expandable mixture with a gel content of 30% is produced. The mixture thus obtained is cooled immediately before extruding it to different temperatures. The proportion of open cells is determined. The results are listed in Table 2.

As the results show, the proportion of open cells increases with increasing extrusion temperature.

Example 3

In a twin-screw extruder, a mixture of

100 parts of a copolymer (crystallite melting point 90°C), containing 85% ethylene, 9% acrylic acid, and 6% acrylic acid t-butyl ester, with 3 parts talc, 1.5 parts sulfur, 5 parts barium oxide, 1 part palmitic acid and 0.8 part tetramethylthiuram disulfide. The temperature in the mixing zone is 170°c. The average length of stay is between 2 and 30 minutes. The homogeneous, expandable

mixture is pressed through a wide slot nozzle. The temperature in the mixture immediately before extrusion is 90°c, i.e. exactly the crystallite melting point of the copolymer. A foam plastic tape is obtained which has a volume weight of 30 g/l. The band contains 50% gel shares, 75% of the cells are open.

Example 4

100 parts of a copolymer of 83% ethylene and 17% acrylic acid t-butyl ester, which has a crystallite melting point of 92°c, is mixed dry with 3 parts talc, 2.5 parts calcium oxide, 1 part sulphur, 0.5 part stearic acid and 0.4 part diphenylguanidine. In a twin-screw extruder, 15 parts of this mixture are mixed with 3 parts of a propellant mixture containing 50% isobutane and 50% normal butane. The homogeneous mixture is pressed through a round nozzle and divided into particles before expansion using a cutting device. Immediately before the division, the mixture has a temperature of 88°c. The cut-off particles expand to a diameter of approx. 10 mm and a volume weight of 0.22 g/cm 3. The particles can be sintered into a block.

Example 5

100 parts of an ethylene/vinyl acetate copolymer with a melting point of 95°C, containing 10% by weight of vinyl acetate,

mixed with 2 parts zinc oxide, 1 part sulphur, 0.5 part stearic acid,

0.5 parts of 2-mercaptobenzothiazole and 2 parts by weight of talc. In a twin-screw extruder, a propellant mixture consisting of 80% propane and 20% isobutane is added to this mixture. The temperature in the mixing zone amounts to 170°c. The resulting mixture is extruded through a wide-slot nozzle. It has a gel content of 25%. The temperature immediately before the extraction is 85°C. A foam plastic containing 15% open cells is obtained. If the mixture is pressed out at 70°c,

then a foam plastic with closed cells is obtained.

Example 6

As described in example 1, ethylene copolymers are mixed with the additives and propellants, and the homogeneous mixtures are extruded at specific temperatures. The composition of the copolymers, type and quantity of additives and propellants, as well as the temperature of the mixtures during extrusion, are indicated in the following table. The percentages refer to the amount of copolymer.

Claims (1)

Process for the production of cross-linked foam plastic based on thermoplastic ethylene copolymers, as stated in NO-PS 131 548, characterized in that the homogenized mixture, before it is expanded, is cooled to a temperature that lies between the crystallite melting point and 20°c lower than this.