GB1566056A

GB1566056A - Process for the cross-linking eextrusion and foaming of branched polyolefins

Info

Publication number: GB1566056A
Application number: GB7033/77A
Authority: GB
Original assignee: ST Dupont SA
Current assignee: ST Dupont SA
Priority date: 1976-02-26
Filing date: 1977-02-18
Publication date: 1980-04-30
Also published as: FR2342148A1; IT1071581B; NL7702105A; FR2342148B1; DE2707874A1; BE851850A

Description

(54) PROCESS FOR THE CROSS-LINKING, EXTRUSION AND FOAMING OF BRANCHED POLYOLEFINS (71) We, S. T. DUPONT, a French body corporate of Tour Maine Montparnasse, 33 Avenue du Maine, Paris 15e, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention concerns a process for the cross-linking, extrusion and foaming of a polyolefin, more particularly a branched polyolefin.

In the present description, and in the annexed claims, the term "branched polyolefin" designates a polyolefin selected from the following: -polypropylene, which has units of formula:

--polybutene-l, which has units of formula:

-polyisobutene, which has units of formula:

-the polymethylpentenes, which have units of formula: a) polymer of 2-methyl pent-l-ene

b) polymer of 4-methyl pent-l-ene

It is well known that branched polyolefins can be cross-linked using free radical initiators, of which the organic peroxides are the most frequently used.

However, the cross-linked polyolefins have such high viscosity that it is impossible to shape them by extrusion, so that they are generally shaped by injection molding.

In order to alleviate this disadvantage, it has already been proposed to simultaneously extrude and cross-link polyolefins. Thus, United States Patent 3,546,326 describes the production of shaped objects by extruding polybut-l-ene, comprising inserting particles of isotactic polybut-l-ene into an extruder, together with a cross-linking system comprising a mixture of sulphur and an organic peroxide, raising the temperature of the mixture sufficiently to bring it into a plastic condition and cause the organic peroxide to decompose, and extruding this mixture through the die of the extruder to produce an article of cross-linked polybut-l-ene.

However, the process described in this patent is applicable only to a specific polyolefin, i.e. polybut-l-ene, with a specific cross-linking agent (a mixture of sulphur and an organic peroxide).

Another process, described in United States Patent 3,640,919, comprises simultaneous cross-linking of polybutene in the presence of a mixture of sulphur, a compound comprising allyl groups and a peroxide, and expansion thereof using a nitrogen-containing blowing agent, by heating the mixture of polybutene, curing system and blowing agent, under pressure, to a temperature corresponding to the decomposition temperature of the peroxide, cooling the mixture and allowing it tot revert to atmospheric pressure. This patent specifies, however, that the viscosity of the mixture being processed should be fairly high in order to prevent gas from escaping from the cells by wall-penetration during expansion and, to this end, the mixture contains a compound containing a number of allyl groups.

It has also been proposed (in U.S. Patent 3,843,757) to simultaneously expand and extrude a linear polyolefin such as polyethylene, by injecting a blowing agent into a polyolefin in an extruder and extruding the mixture at a temperature such that the polyolefin expands on leaving the extruder die without bursting of cell walls of the resulting foam.

We have now developed a process for the cross-linking, extrusion and foaming of a branched polyolefin (as defined above).

The process according to the invention comprises: (a) heating a mixture of the polyolefin and a peroxide cross-linking agent, while the mixture is being forwarded through the body of an extruder, to a temperature not less than the melting temperature of the polyolefin and the decomposition temperature of the peroxide, the polyolefin being maintained at this temperature for a period of at least one half of the half-life of the peroxide, (b) then injecting a liquefied gas which is capable of acting both as a blowing agent and as a solvent for the polyolefin, under pressure into the mixture in the extruder body, (c) then cooling the mixture to a temperature which is above the solidification point of the polyolefin so that the polyolefin is still liquid, and (d) then extruding the mixture through an extrusion die to a zone at ambient pressure so that the extrudate expands in said zone.

The process according to the invention can be carried out using a conventional extruder, such as a twin-screw extruder. An example of a suitable such extruder is the Colombo LMP RC-91E extruder, as described in U.S. Patent 3,252,182. Such an extruder has two compression zones.

The liquefied gas (which is preferably a halogenated hydrocarbon, such as one marketed under the Trade Mark Freon, for example, one or more of Freons 11, 12, 114 and C318) is preferably injected between two such compression zones, whereby escape of gas from the inlet via the feed zones is opposed.

The process according to the invention may be carried out relatively inexpensively, and foams of low specific weight (lighter than conventional foams) may be obtained. Thus, it is possible to obtain foams with an expansion factor (that is, the ratio of the volume after expansion to the volume before expansion) of 30 to 100. Foams obtained according to the invention may be used, for example, for the absorption of liquefied gases, notably liquefied gaseous hydrocarbons.

The peroxide used according to the invention may be conventional, for example, a diacyl peroxide, a dialkyl peroxide or a peroxyester. An example of a particularly suitable peroxide is dicumyl peroxide. The peroxide is preferably used in an amount of 0.1 to I part per thousand, based on the weight of polyolefin. It is a particular feature of the present invention that such low amounts can be used as amounts of 0.5 to 5 Ó based on the weight of polyolefin (these being conventional amounts) tend to cause degradation of the polyolefin.

As mentioned above, the mixture of the molten polyolefin and the peroxide is maintained at a temperature above the decomposition temperature of the peroxide for at least one half of the half-life of the peroxide; this ensures adequate decomposition of the peroxide and consequent cross-linking. It is particularlypreferred to maintain the mixture at such a temperature for one to two half-lives of the peroxide.

It will be noted that the process according to the invention does not require the use of a chemical blowing agent, the blowing function being performed by the liquefied gas. However, a chemical blowing agent may also be added, generally in a small amount, if desired. In this case, the chemical blowing agent plays only an auxiliary role, enabling smaller, more uniformly distributed cells to be formed.

In order that the invention may be more fully understood, the following Examples are given by way of illustration only. In these examples, polypropylene and polybutene-l are used, respectively. Equivalent results have been obtained using polyisobutene and polymethylpentenes.

Example 1.

This example concerns polypropylene and covers three compositions which differ in that one of them contains no blowing agent other than Freon 11, while the two others contain a small amount of an additional blowing agent.

The compositions, by weight, of these three mixtures are given in Table I, below, where the names or trademarks under which the different constituents are sold, are also shown.

TABLE I

Components Composition 1 Composition 2 1 Composition 3 Polypropylene 10,000 10,000 10,000 Resin ("Napryl") Magnesium silicate powder 200 200 200 powder ("Mistron ZSC") Emulsifier 100 100 100 "Atmos 150" Dicumyl peroxide 5 5 5 (40% solution by weight of "Di-cup 40") Halogenated hydrocarbon 2,500 2,500 2,500 ("Freon 11") Additional blowing agent 60 0 30 ("Celogene AZ") A double twin screw extruder, the Colombo LMP RC--91E model, was used.

The extrusion profile (temperature of the processed mixture as a function of time, i.e. as the composition progresses through the body of the extruder) is shown on the annexed drawing.

In the first zone (Zone 1), the ingredients other than Freon were introduced individually into the extruder and mixed, compacted and heated until a liquid or paste phase was obtained (2200 C).

The mixture was maintained in the molten state for a period long enough to bring about the decomposition of the peroxide and cross-linking of the polypropylene (Zone 2).

The Freon was then injected, in the liquid state, at a pressure between 70 and 140 kg/cm2 (1,000 to 2,000 psi), which results in a sharp drop in the temperature of the mixture (Zone 3).

The temperature then rises again, slowly (Zone 4), but is maintained at a level just above the solidification temperature of the branched polyolefin (150 C) with simultaneous homogenization of the mixture, in order to dissolve the resin in the Freon and prevent the latter from leaking through the extrusion orifice.

Finally, the mixture was extruded at a temperature slightly greater than the solidification temperature of polypropylene to prevent the walls of the cells, which are formed simultaneously, from bursting under the pressure of gas released.

The foams 1, 2 and 3, obtained in this way, have specific gravities of 0.029, 0.035 and 0.039, respectively.

These foams can be used for the storage of liquefied fuel gases, in accordance with our French patent No. 71 32 946, and its two supplements, No. 7220346 and No. 73 10 590. This French patent concerns a process and appliances for storing, in liquid form, a liquefiable product (saturated hydrocarbon, such as propane, butane, pentane, or a mixture thereof), with a view to distributing it in gaseous form in an environment where the pressure is less than the storage pressure. The corresponding storage enclosure contains, in addition to the liquid product to be distributed, a polymer with respect to which the liquid behaves as a swelling agent.

The low-density polyolefin foams obtained according to the present invention are particularly suitable as the polymer because, when in contact with liquefied hydrocarbons, they absorb them, with swelling; this retention subsequently facilitating the distribution, in gaseous form, of the liquid which is not contaminated by the foam.

In order to illustrate this property and this application of the polypropylene foams obtained in the present example, their absorption index for liquefied butane when they are immersed into this hydrocarbon, has been obtained; i.e. the weight of butane in grams absorbed by each gram of foam. The results obtained are shown in Table II, below.

TABLE II

Foam 1 Foam 2 Foam 3 Absorption index 9.0 7.2 8.4 These results show that the foams prepared in accordance with the invention have very satisfactory absorption properties with respect to liquefied hydrocarbons, and that they can be applied with advantage for storing liquefied gases as described above.

Example 2.

This example concerns four formulations of polybutene-l, the composition by weight of which is given in Table III, below, where the commercial names (registered trademarks) of the different components are also indicated.

TABLE III

1 2 3 4 Polybutene-1 10,000 10,000 10.000 10,000 ("Witron 12 005") Magnesium silicate powder 200 200 200 200 ("Mistron ZSC") .

Emulsifier 100 100 100 100 ("Atmos 50") Dicumyl peroxide (40% 10 5 10 10 solution of "Di-cup 40") Halogenated hydrocarbon 2.000 2,000 2.000 2.500 ("Freon 12") Additional blowing agent 20 20 10 0 ("Celogene OT") These compositions were processed in the same extruder as for Example 1 and the extrusion profile is shown on the annexed drawing. The operations carried out in the Zones 1, 2, 3 and 4 were the same as in Example 1.

The four polybutene-l foams obtained in this way had densities of 0.014, 0.014, 0.020 and 0.028 g/cm3, respectively.

The absorption properties of these foams, tested in liquefied butane, are shown in Table IV, below.

TABLE IV

Foam 1 Foam 2 Foam 3 Foam4 Absorption index 13.68 13.54 9.68 8.32 The foams prepared in this example, like those of Example 1. thus have excellent absorption properties for liquefied gaseous hydrocarbons.

WHAT WE CLAIM IS: 1. A process for the cross-linking, foaming and extrusion of a branched polyolefin (as defined herein), which comprises: (a) heating a mixture of the polyolefin and a peroxide cross-linking agent, while the mixture is being forwarded through the body of an extruder, to a temperature not less than the melting temperature of the polyolefin and the decomposition temperature of the peroxide, the polyolefin being maintained at this temperature for a period of at least one half of the half-life of the peroxide.

(b) then injecting a liquefied gas which is capable of acting both as a blowing agent, and as a solvent for the polyolefin, under pressure into the mixture in the extruder body, (c) then cooling the mixture to a temperature which is above the solidification point of the polyolefin so that the polyolefin is still liquid, and (d) then extruding the mixture through an extrusion die to a zone at ambient pressure so that the extrudate expands in said zone.

2. A process according to claim 1, in which the peroxide is a dialkyl peroxide.

3. A process according to claim 2, in which the peroxide is dicumyl peroxide.

4. A process according to any of claims 1 to 3, in which the amount of peroxide used is from 0.1 to 1 part per thousand of the weight of the polyolefin.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE III 1 2 3 4 Polybutene-1 10,000 10,000 10.000 10,000 ("Witron 12 005") Magnesium silicate powder 200 200 200 200 ("Mistron ZSC") . Emulsifier 100 100 100 100 ("Atmos 50") Dicumyl peroxide (40% 10 5 10 10 solution of "Di-cup 40") Halogenated hydrocarbon 2.000 2,000 2.000 2.500 ("Freon 12") Additional blowing agent 20 20 10 0 ("Celogene OT") These compositions were processed in the same extruder as for Example 1 and the extrusion profile is shown on the annexed drawing. The operations carried out in the Zones 1, 2, 3 and 4 were the same as in Example 1. The four polybutene-l foams obtained in this way had densities of 0.014, 0.014, 0.020 and 0.028 g/cm3, respectively. The absorption properties of these foams, tested in liquefied butane, are shown in Table IV, below. TABLE IV Foam 1 Foam 2 Foam 3 Foam4 Absorption index 13.68 13.54 9.68 8.32 The foams prepared in this example, like those of Example 1. thus have excellent absorption properties for liquefied gaseous hydrocarbons. WHAT WE CLAIM IS:

1. A process for the cross-linking, foaming and extrusion of a branched polyolefin (as defined herein), which comprises: (a) heating a mixture of the polyolefin and a peroxide cross-linking agent, while the mixture is being forwarded through the body of an extruder, to a temperature not less than the melting temperature of the polyolefin and the decomposition temperature of the peroxide, the polyolefin being maintained at this temperature for a period of at least one half of the half-life of the peroxide.

2. A process according to claim 1, in which the peroxide is a dialkyl peroxide.

3. A process according to claim 2, in which the peroxide is dicumyl peroxide.

5. A process according to any of claims I to 4, in which said period is one to

two of said half-lives.

6. A process according to any of claims 1 to 5, in which the liquefied gas is an halogenated hydrocarbon.

7. A process according to any of claims 1 to 6, in which the liquefied gas is injected into the extruder between two compression zones.

8. A process according to any of claims 1 to 7, in which the amount of liquefied gas injected is such that the expansion factor of the foam, at the extruder die, is between 30 and 100.

9. A process according to any of claims 1 to 8, in which a chemical blowing agent is introduced into the extruder in addition to the liquefied gas.

10. A process for the cross-linking, extrusion and foaming of a branched polyolefin (as defined herein), substantially as herein described in the production of Foam 1, 2 or 3 in Example 1 or in the production of any of Foams 1 to 4 in Example 2.

11. An extruded cross-linked branched polyolefin foam when produced by the process claimed in any of the preceding claims.