DE1904860A1 - Process for the production of polyolefin foams - Google Patents

Description

*** PATENT ANVaITE '

DR. E. WIEGAND DIPL-ING. W.NIEMANN 1904860

DR. M. KÖHLER DIPL-ING. C. GERNHARDT

MÖNCHEN HAMORG TELEPHONE 55547ί, 8000 MONKS 15, TELEGRAMMEi KARPATENT NUSSBAUMSTRASSE 10

January 31, 1969

W. 14 103/69

Mitsubishi Petrochemical Co., Ltd. Tokyo (Japan)

Process for the production of polyolefin foams

The invention relates to a method for producing crosslinked polyolefin foams. In particular The invention relates to a method for producing a polyolefin foam having a very small and uniform closed pore structure within a short period of time.

It was known in the art that a foamed thermoplastic resin can be obtained by first a mixture of the thermoplastic resin with

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one of these "incorporated propellants" into a flowable one State by 'heating brings, then the liquid or flowable Resin by the action of the propellant gas generated from the propellant at this temperature, with formation foamed a porous structure. To obtain eties thermoplastic Resin foam of good quality, this heretofore known method must generally be carried out so that the arz to be foamed is in the liquid or flowable state, with the escape of the gas produced ' should be prevented from the system at the foaming temperature. With polyolefin, especially with polyethylene, however, is the optimum temperature range at which the flowable fluid suitable for carrying out the foaming State is so tight that it is extremely difficult to carry out a practical foaming process drive to maintain such a temperature. It was

therefore not possible to make a good one from ordinary polyethylene Obtain good and low density foam.

This difficulty in foaming polyethylene is mainly due to its poor crystallinity and low glass transition temperature. When heated, the viscosity of polyethylene suddenly decreases and the polyethylene becomes considerably fluid at a temperature above its melting point. To such a sudden To moderate the decrease in viscosity on heating, it is therefore expedient and desirable to use intermolecular crosslinking bring about in the polyethylene to be foamed. Recently, the technical production of poly-

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ethylene foams largely a modification of polyethylene by means of such crosslinking procedures applied and a mixture, the polyethylene together with a blowing agent or foaming agent and a Contains organic peroxide in admixture, is usually used as foamable polyethylene beads.

With these foamable beads, however, this is the case Problem of the difficulty in regulating the flowable or liquid state of polyethylene in Foaming has not yet been fully resolved. Despite the fact that the crosslinking reaction caused by the Decomposition of the organic peroxide is initiated, the diffusion of gas from the propellant or foaming agent must go ahead, the practical foaming process is inevitable accompanied by premature foaming, which leads to the production of foams with irregular, and structures containing coarse open pores is brought about.

To overcome the difficulties outlined above has already 'proposed an improved method in which an organic peroxide with one of the The point of decomposition of the blowing or foaming agent is different Decomposition point applied. For example, a propellant is used with foaming agent a higher decomposition point than that of the organic peroxide, and the latter is used first at one temperature lower than the decomposition point of the blowing agent or foaming agent that was used,

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in order to bring about a crosslinking in the polyethylene in such a way that the flowable or liquid state is regulated is carried out, after which the foaming is carried out by heating to a temperature above the decomposition point of the blowing agent will. "■.

However, this procedure still has many disadvantages, for example due to the limited selection of the blowing or foaming agent to be used, the low one Speed of the crosslinking reaction, the need for a large amount of the expensive organic peroxide and the extended reaction time.

Another measure that has been taken so far to bring about a Such crosslinking has been proposed involves irradiation using high radioactive rays Energy. However, this method of operation is very expensive and requires the use of a generator that is rarely available for radioactive rays, thereby causing an inevitable increase in the manufacturing cost.

A much more serious problem inherent in the conventional working methods is that the foaming of isotactic polyprobes is extraordinary is difficult. ·

The object of the invention is therefore to create a method for producing a polyolefin foam with one with very small and uniform pores

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Structure within a short period of time, with all the difficulties noted above have been overcome.

The purposes according to the invention are now achieved by carrying out the crosslinking under high pressure.

In particular, the method according to the invention consists in that a polyolefin approach, which is an organic Per oxy d and a propellant contains, heated under a 'pressure of at least 500 kg / cm, thereby at least one To decompose part of the incorporated organic peroxide and to bring about an intermolecular crosslinking of the polyolefin, and then releases the pressure and the foaming by the action of the gas generated from the propellant causes.

According to the method according to the invention, the crosslinking reaction carried out under a high pressure / so that the molecules in the polyolefin come very close to each other, provided the high. · Pressure at a temperature above the melting point is given to the crosslinking reaction easier to favor and the expansion of the gas generated from the at least partially decomposed propellant is suppressed, and the gas is uniformly dispersed throughout the polyolefin. Furthermore, since the If high pressure is exerted only for a short period of time, there is hardly any phase separation between the gas and the molten polyolefin instead. By releasing the pressure at this point, foaming can be prevented by the Expansion of the gas, which after its generation is ocular

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has been compressed once, and by the subsequent Generation and expansion of the gas from freshly decomposed propellant contained in the molten mixture had not been implemented.

According to the method according to the invention, as above. described, many advantages are achieved * which, for example, on a (l) reduction of the production time, (2) Saving in the required amount of organic peroxide, (3) lowering the reaction temperature, (4) Ease of making a highly expanded foam with a very small size: and uniform Structure having pores (regardless of whether the decomposition of the blowing agent in the course of the crosslinking reaction took place or not), (5) the realization of the production of foams from isotactic polypropylene based among others.

A method of making polyethylene foam using a press has been proposed in the art. However, the pressure actually applied to the resin itself by means of a press of a conventional type is ordinary less than 200 kg / cm and in particular in the range of 10 to 50 kg / cm. Such an operation is evident fundamentally different from the method according to the invention from a technical point of view.

The polyolefin resins to be used in the process according to the invention typically comprise polyethylenes according to any of the high pressure, medium pressure and low pressure processes, isotactic polypropylene, copolymers of olefins with the olefin as the main component contain, e.g. ethylene / vinyl acetate, mixed polymer, ethylene /

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Propylene composite polymer and mixtures thereof.

The organic peroxides to be used in the process according to the invention include those whose decomposition points are higher than the points at which the polyolefins used begin to become liquid, and are, for example, dicumyl peroxide (decomposition point about 166 ° C.), 2,5-dimethyl 2,5-di- (tert-butylperoxy) -hexane (decomposition point about 157 ⁰ C), di-tert-butyl perphthalate (decomposition point about C) and the like.

The blowing agents to be used according to the invention are those which are used to form blowing or blowing agents Gases are capable of decomposition at the temperature (or pressure) used. It can therefore be common Nitroso type, aromatic hydrazide type, azo type and similar organic and inorganic blowing agents Propellants, e.g., sodium bicarbonate, together if desired with a foaming agent.

obtaining a foam with a more uniform one and small-pore structure, it is necessary to mix the polyolefin with the organic peroxide, blowing agent and other additives, which may optionally be added, e.g. fillers, to mix thoroughly. Mixing is using from mixing rolls, Banbury mixers, extruders and presses Oedgl. executed to have a uniform dispersinn of each Ensure component in powder or molten state.

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The temperature during mixing should of course be lower than the decomposition point of the organic peroxide and the Propellants that are used, respectively, and in particular of the latter, be. '

The ratio of the added amount of the organic peroxide, the blowing agent and other additives varies largely depending on the types of used Additives, the types of polyolefin to be foamed and of the intended use of the foams to be produced; accordingly, the relationship becomes appropriate for each case chosen.

The polyolefin that incorporates the necessary materials the foaming treatment is then carried out in

Form of pellets or similar suitable shapes. The foaming operation is carried out using any device and means or force capable of applying a pressure of at least 500 kg / cm to the polyolefin mixture and a temperature high enough to cause decomposition of the added. organic peroxide and the foaming of the polyolefin by decomposition and expansion of the blowing agent after the pressure has been released, is suitable. When practiced on an industrial scale, the process according to the invention can be carried out batchwise or continuously. In continuous operation, the polyolefin, blowing agent and peroxide are fed continuously, for example to a heating cylinder equipped with a piston, then intermittently to a pressure above 500 kg / cm ²

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compressed and continuous by the movement of the piston released at atmospheric pressure to cause foaming.

The first stage of pressurization and heating will be • Mainly designed for the purpose of intermolecular crosslinking of the polyolefin through the decomposition of the to bring about incorporated organic peroxide. At this point, however, the decomposition of at least caused some of the incorporated propellant

The gas generated by the decomposition of the blowing agent is usually in the molten polyolefin dissolved state, although it can sometimes appear in the form of bubbles.

This process is followed by a pressure release stage during which the foaming is essentially completed is then cooled to a temperature below the pour point of the polyolefin.

The ^E rfindung is apparent in the following explained in more detail by examples.

example 1

To 100 parts by weight of high pressure polyethylene (spez.Gew. = 0.920, MI = 4) were admixed at einer'Walzentemperatur of 115 ⁰ C. 0.5 parts by weight of dicumyl peroxide and 10 parts by weight of azodicarbonamide, to a uniformly dispersed Mixture, which then becomes grains or granules

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was molded. Using this mixture, foam was generated under varying pressures, with a so improved Koka flow tester (Koka type flow tester) was applied to generate high pressure. After heating at a given pressure under the condition of 190 ° C./5 min., the pressure was released to complete the expansion and then cooling was carried out.

The results of the tests are listed in Table I below.

Table I. Bulk density (g / era- ⁵ ) Attempt no Pressure (kg / cm) 0.12 1 O 0.082 2 50 0.055 3 500 0.047 4th 2000 0.045 5 5000

The foamed obtained in experiments nos. 1 and 2 Products had an irregular structure with a number of visibly large pores.

The products obtained in experiments 3, 4 and 5, i.e. the foam products obtained by the method according to the invention were uniform porous structure without visible pores.

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Example 2

To 100 parts by weight of a powdered is ο tactical polypropylene (MI = k) , 3 parts by weight of dicumyl peroxide and 100 parts by weight of toluenesulfonyl hydrazide were added and dispersed using a ribbon blender to give a compounded mixture. The mixture was heated under the condition of 240 ° C / 8 min. At a given pressure, then expanded and cooled by releasing pressure. The results of the experiments are summarized in Table II below.

Table II Bulk density (κ / cnr ⁵ ) Attempt no. • pressure (kß / cm) 0.43 6th 0 $ 0.3 7th 50 0.15 8th 500 0.12 9 2000 0.11 10 5000

The foam products obtained in experiments 6 and 7 had irregular structures with very large pores.

The foams obtained in Runs 8, 9 and 10 had uniform and very small pores.

. Example 3

To 100 parts by weight of a pulverulent block copolymer of propylene / ethylene with a content of 17 #

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Ethylene was 5 parts by weight of 2,5-dimethyl-2,5-di- (tert-butylperoxy ) -hexane and 10 parts by weight of nitrosopentamethylenetetramine added and dispersed using a ribbon blender to obtain a mixture. .

This mixture was heated under the condition of 240 ° C / 8 min. At a given pressure, followed by the pressure has been released to complete the expansion; then it was cooled. At a pressure above 500 kg / The foam products obtained had uniform fine-pored structures with no visible pores.

Example 4

The test was carried out using a pressure-resistant cylinder fitted with a reciprocating piston connected to a crank mechanism of the press, and also on the lower part with one that can be moved slightly vertically by the action of a spring Arm or bracket (bracket) was fitted.

A mixture of 100 parts by weight of high-density polyethylene (MI = 0.5), 0.5 parts by weight of di-tert-butyl peroxide and 3 parts by weight of azobiscarbonamide was continuously fed to the cylinder described above, which was fed to a Internal temperature of about 200 ⁰ C was heated, and to a given pressure at 240 cycles / min. compressed. The contents were continuously discharged into the atmosphere from the lower outlet of the cylinder to cause foaming.

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The foam obtained at a pressure of 2000 kg / cm Product had a density of 0.15 g / cnr and a fine and uniform pore structure,

while the product obtained at a pressure below 500 kg / cm has a density as high as 0.50 to. 0.60 g / cnr. '^;

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Claims (3)

- 14 claims Process for the production of polyolefin foams, characterized in that one is compounded Polyolefin mixture containing an organic peroxide and a blowing agent incorporated under a pressure of heated at least 500 kg / cm and thereby an intermolecular crosslinking of the polyolefin by the decomposition of at least a part of the incorporated organic peroxide and then releases the pressure to cause foaming to effect by the propellant. 2. The method according to claim 1, characterized in that the compounded polyolefin mixture under pressure partially heated. 3. The method according to claim 1, characterized in that that you have a compounded polyolefin mixture continuously in a pressure-resistant cylinder attached to its one part is equipped with a reciprocating piston and on its other part with a bracket or arm, feeds and heated under pressure and released continuously. 9Q9837 / 1543