NO133106B - - Google Patents

Description

The object of the invention is a method for producing foamy ethylene copolymers.

It is known that the heat resistance of foam plastic can be increased

by cross-linking using peroxides or high-energy radiation.

The invention relates to a method for production

of cross-linked foam plastic based on thermoplastic ethylene-copoly-

mers containing 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 dicarboxylic 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, crosslinking agents and further auxiliaries at 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.

The method is characterized by the addition of 5-7%, calculated on the weight of the copolymer, of a thiuram disulfide or tetrasulfide, as well as preferably small amounts of metal oxide and/or fatty acid, as a crosslinking agent, after which

the homogeneous mass is expanded in a known manner.

From GB-PS 1 050 164 it is admittedly known to produce cross-linked foam plastic on the basis of ethylene copolymers, using organic peroxides as cross-linking agents. To enable the addition of peroxide at moderate temperatures, a small amount of solvent (blowing agent) was added for the polymer.

As peroxide cross-linked plastic is difficult to extrude, the extrusion was carried out so quickly that only part of the cross-linking took place in the extruder. The cross-linking was completed in a heating zone arranged after the extruder.

However, the applicant has found that even with these measures it is not possible to work with peroxides as cross-linking agents in a satisfactory manner, as the peroxide cross-linking tends to proceed unevenly, so that there are areas in the product that are very strongly cross-linked, and such that practically is not for the net The network share, calculated on the entire product, can then be low. Such a product can indeed be extruded, but in the extruded material there are constant disturbances due to the strongly cross-linked areas. The result is therefore not homogeneous, but a qualitatively inferior extrudate.

In contrast to the above, uniform crosslinking occurs with the method according to the present invention. Above all, the cross-linked olefin copolymers can be extruded without interference. Unlike the technique that uses peroxide,

it is here possible to also extrude highly cross-linked olefin copolymers (cf. e.g. example 7 below). This fact is connected with the different type of cross-linking, as the cross-linking of the mixture according to the invention takes place with the help of sulfur bridges. How this experimentally proven effect can be explained theoretically, one cannot say with certainty today.

Incidentally, the swelling agents used according to the British patent are partially consistent with the propellants used according to the present invention. As regards the vulcanisation aids, it is known that these increase the ability of metal oxides to crosslink vinyl chloride plastics, see GB-PS 1 015 413.

However, this is a completely different plastic, and there is no suggestion that it should be foamed.

One method which shows great similarity to the present invention, but — : comprises the addition of elemental sulphur,

is mentioned in NO-PS 131.548.

Hydrocarbons and halogenated hydrocarbons whose boiling point is 25 to 150°c below the melting point of the copolymer can be used as a propellant according to the invention. By melting point is meant here the crystallite melting point. Suitable propellants are

a) aliphatic hydrocarbons with 3 to 5 C atoms, such as propane, butane, pentane or propene, butene or pentene. Special

significance as a propellant is isobutane or hydrocarbons with 5 to 7 carbon atoms and at least 2 adjacent methyl groups, which have a boiling point between -10 and +60°C. Of these, isopentane, isohexane, 2,2-dimethylbutane can be mentioned. Of the chlorinated hydrocarbons, methyl chloride, ethyl chloride and dichloromethane should be mentioned above all.

Suitable chlorofluorocarbon compounds are dichlorodifluoromethane, fluorotrichloromethane, monofluorochloromethane, 1,2,2-trifluorotrichloroethane and 1,1,2,2-tetrafluorodichloroethane.

Amounts of propellants are used such that the mixtures contain 15 to 35 parts by weight of the propellant, calculated on 100 parts by weight 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.

b) Solid propellants which decompose at the foaming temperature, forming nitrogen or carbonic acid, e.g. azo compounds such as

azodicarbonamide or azoisobutyric acid dinitrile or aromatic sulfhydrazides. Furthermore, carbonates in mixture with acids such as mixtures of sodium carbonate and organic acids can be used.

The ethylene copolymers are mixed with the propellant in the presence

of 0.5-7, preferably 1-5, parts of thiuram disulfide and/or tetrasulphide, calculated on 100 parts of copolymer. Mixtures of di- and tetrasulphides can also be used. Particularly advantageous are tetramethylthiuram disulfide, tetraethylthiuram disulfide, dipentamethylthiuram tetrasulfide and dimethyldiphenylthiuram disulfide.

Foams with particularly advantageous properties, such as finely divided cell structure and low bulk density, are obtained when the ethylene copolymers are mixed together with thiuram di- or -tetrasulphides and fatty acids, as well as oxides of divalent metals.

Of the fatty acids, stearic acid, oleic acid and palmitic acid are particularly relevant.

Of the metal oxides, barium oxide and zinc oxide are particularly common

suitable.

At the rate of 100 parts of copolymer, 0.2-2 parts, especially 0.3-1 part, of a fatty acid and 0.5-7 parts, especially 1-5

parts of the metal oxide.

For the production of foams with a particularly fine cell structure, so-called nucleating agents can be added. Particularly suitable are finely powdered silicates, e.g. talc, which is suitably used 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 priority older Norwegian patent document no. 129 910. The solid propellants can also serve as nucleating agents, when they are added in smaller quantities alongside a liquid propellant.

In addition to the components mentioned1, the mixtures may contain additional components, such as fillers, dyes, flame retardants, antistatic agents, stabilizing agents, slip agents or other polymers in the set, e.g. polyethylene, polypropylene and polyisobutylene. These other polymers can be present in quantities of up to 20% by weight, calculated on the copolymer.

The thermoplastic ethylene copolymers are mixed in a thermoplastic state with the propellants. As mentioned, you then 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 pressure that is above the propellant pressure, so that the propellant is always in a liquid state under the mixing conditions. The propellant pressure is understood as the vapor pressure of the propellant above the molten copolymer. The mixtures are expanded after the mixing process has ended in a homogeneous state. By this it is to be understood that the mixtures are brought into a zone where the pressure is below the propellant pressure at the mixture temperature. It is most appropriate to expand the mixtures in a zone with atmospheric pressure. Continuously working mixing devices are advantageously used for the preparation of the mixture. Among these, extruders (screw presses) in particular have proven suitable. The dimensions of the mixing

the direction must be chosen so that a homogeneous mixture of ethylene copolymer, the additives and the propellant is formed at the exit from the mixing chamber and entry into the expansion zone. The mixtures

can also be produced in discontinuously working devices, e.g. in pressure vessels with a stirring device. According to the method according to the invention, endless foam profiles can be obtained, e.g. plates, inflated foil tubes or pipes. The procedure is also suitable

for wrapping objects with foam plastic.

The procedure is particularly important in the manufacture

of finely divided foams. These can e.g. achieved by continuous pressing of the mixtures to be foamed, and division of the mixture before foaming. According to the method with liquid propellants, foam particles with diameters between 3 and 20 mm are obtained, whose volume weight can be above 15 g/l, preferably between

15 and 40 g/l.. With solid propellants foams with 300 to 600g/l volume weight are obtained. Such foam particles can e.g. used as padding material, fillers for cushions, insulation or as agents to make the ground (subsoil) porous. Plates and foils produced in accordance with the invention are useful for heat and sound insulation.

The foam substances obtained according to the method have a gel content which can be between 20 and 70 percent by weight. By gel content is to be understood the insoluble proportions which are, for example, determined by dissolving the polymer in decahydronaphthalene at temperatures of 140°c. The foams obtained have a thermal stability which is 20 to 30°c above the thermal stability of foams which are produced exclusively by using the copolymers. It was, as discussed above, surprising that the mixtures which according to the invention are to be foamed, despite the high gel content, can be squeezed out through nozzles and foamed up.

The parts mentioned in the examples are weight parts.

EXAMPLE 1

100 parts of a terpolymer of 82 parts ethylene, 8 parts acrylic acid and 10 parts acrylic acid t-butyl ester are mixed with 2 parts tetramethylthiuram disulphide, 0.5 part stearic acid and 3 parts talc. In a twin-screw extruder, 60 parts of this mixture per hour and mixed with 9 parts isobutane. The residence time in the extruder is 4 minutes for the mixture, and the temperature in the nozzle is 160°c. The homogeneous mixture is pressed out over a wide-slit die with dimensions 75 x 4.7 mm. Thereby, the mass foams up into an endless foam band with a cross section of 80 cm. the volume weight of the foam is 25 g/l, the gel content 48 percent by weight (the content is determined by dissolving the foam at 140°C in tetrahydronaphthalene. - The percentages indicate the foam's content of insoluble components). The foam is heat-resistant up to 100°C. It can e.g. used as cold and heat insulation.

A mixture extruded under the same conditions, which does not contain any additives, when using nozzles with the same opening, does not lead to any usable foam. If you reduce the nozzle cross-section to 30 x 4.7 mm, you can achieve a foam that has a cross-section of 25 cm^ and a heat resistance of up to 74°c. This foam

have more rough cells. structure.

EXAMPLE 2

100 parts of a copolymer of 78 parts of ethylene and 22 parts of acrylic acid t-butyl ester are mixed dry with 1.8 parts of dimethyl

in

diphenylthiuram disulfide and 2 parts talc. 18 parts of this mixture are plasticized per hour in a single screw extruder and mixed with 3 parts tetrafluorodichloroethane. The residence time in the extruder is 3 minutes, the temperature in the nozzle is 165°C.

If you extrude through a round nozzle with a diameter of 4 mm and distribute the foaming strand using a hot-cutting device, particles with a diameter of 13 mm are obtained.

The foam particles have a gel content of 40% by weight and a heat stability of 90°C.

With foam from the copolymer without cross-linking additives, only heat stabilities of 75°C are achieved. An attempt to improve the thermal stability by adding 1 part benzoyl peroxide, depending on the temperature control of the extruder, led either to no noticeable cross-linking of the copolymers or to sudden, complete cross-linking and thereby to a standstill of the work process.

EXAMPLE 3

100 parts of a copolymer of 72 parts ethylene, 5 parts •acrylic acid-t-butyl ester and 17 parts acrylic acid are mixed dry with 3 parts dipentamethylene thiuram tetrasulphide and 3 parts talc. This mixture is plasticized in an extruder at a rate of 15 parts per hour and mixed with 3 parts of a hydrocarbon mixture of 60% by weight isobutane and 40% by weight n-butane. The homogeneous mixture is expanded over a foil blowing nozzle. A foam foil with closed cells of volume weight 65 g/l with a thickness of 1.2 mm is produced.

EXAMPLE 4

100 parts of an ethylene copolymer, which contains

12 parts vinyl acetate polymerized, mixed dry with 1.5 parts tetratylthiuram disulphide, 2 parts zinc oxide and 0.5 part azodicarbonamide as nucleating agent. The mixture is plasticized while adding 15 parts of isopentane per 100 parts of polymerizate in a single-screw extruder with pre-charged output extruder and extruded in the form of a round profile. The resulting foam strand has a gel content of 25%. A foam strand which was produced from the copolymer isate and the propellant without the additives according to the invention, had half the diameter and a coarser cell structure.

Claims (1)

xuoxuu EXAMPLE 5 100 parts of a copolymer of 88 parts ethylene and 12 parts vinyl acetate are dry mixed with 3.5 parts tetraethylthiuram disulfide and 3 parts talc. In a twin-screw extruder, the mixture is softened while adding 18 parts of a mixture of 60 parts isobutane and 40 parts n-butane. The residence time of the mixture in the extruder is 3.5 minutes and the peak temperature 156°C. The homogeneous mixture is extruded over a hole nozzle into foam strands. The volume weight of the manufactured foam strands is 35 g/l and the gel content 35%. EXAMPLE 6 100 parts of a copolymer of 94 parts ethylene and 6 parts maleic acid dimethyl ester are mixed dry with 2.5 parts tetramethylthiuram disulphide and 2 parts silicon dioxide as nucleating agent. The mixture is softened while adding 15 parts of neopentane in a twin screw extruder. The residence time for the mixture in the extruder is 4 minutes at a maximum temperature of 161°C. The homogeneous mixture is extruded through a wide slot nozzle into a foam belt. The volumetric weight of the produced foam is 40g/l, the gel content 32% and the thermal stability 95°C. EXAMPLE 7 100 parts of a copolymer of 82 parts ethylene and 18 parts methacrylic acid methyl ester are mixed dry with 3 parts tetraethylthiuram disulfide and 3 parts talc. In a twin-screw extruder, the mixture is softened while adding 17 parts of n-butane. The residence time for the mixture in the extruder is 3 minutes and the peak temperature 150°C. The homogeneous mixture is extruded into a foam strip through a wide gap nozzle. The volume weight of the foam is 45 g/l, the gel content 34% and the heat stability 93°C. Process for the production of cross-linked foam plastic based on thermoplastic ethylene copolymers containing 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 dicarboxylic 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, crosslinking agents and further auxiliaries at temperatures of 10-100°C above the crystallite melting point of the polymers and at pressure which is above the propellant pressure at the mixing temperature, characterized in that 0.5-7%, calculated on the weight of the copolymer, of a thiuram disulphide or -tetrasulphide, as well as preferably small amounts of metal oxide and/or fatty acid, are mixed as the fo me tning forged 1, whereupon the homogeneous mass is expanded in a known manner.