LU86529A1 - PROCESS FOR PRODUCING CROSSLINKED POLYOLEFIN FOAM AND FOAM THUS OBTAINED - Google Patents

Description

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PROCESS FOR PRODUCING CROSSLINKED POLYOLEFIN FOAM AND

FOAMS OBTAINED

The present invention relates to a process for producing foam of crosslinked or non-crosslinked polyolefins. More particularly, the invention relates to a process for the production of crosslinked or non-crosslinked polyethylene foam having improved mechanical properties and small cells.

Generally, crosslinked polyolefin foams, and in particular those of crosslinked polyethylene, are produced in two stages. During the first step, a composition which comprises the polyolefin and a few additives is introduced into an extruder, the mixture is heated so as to soften the polyolefin and then introduced into the extruder, so as to incorporate it perfectly into the polyolefin, the required amount of blowing agent, most often taken in solid form, and crosslinking agent. This mixture is extruded in the form of a sheet, which during the second stage will be introduced into an oven at high temperature, so as to achieve the expansion, by decomposition of the swelling agent, and the chemical crosslinking, by l action of the crosslinking agent, most often a peroxide.

It will be readily understood that in such a type of process, neither the swelling agent nor the crosslinking agent can decompose during the extrusion of the first step.

This implies that the extrusion of the first step i - 2 - must be carried out at low temperature and in particular at a temperature below that of decomposition of the swelling agent or of the crosslinking agent. Most often, it must be extruded at a temperature below about 135 ° C.

However, it is well known in the field of polyethylene; crosslinked, that linear low density polyethylene (hereinafter LLDPE) provides an improvement in the mechanical properties of the foams obtained, as moreover described in Japanese patent application 59135236.

It would therefore be interesting to use this type of polyolefin to manufacture crosslinked polyethylene foams; but in general it is difficult to extrude at temperatures below 135 ° C.

To overcome this drawback due to heat-sensitive agents, it has already been proposed to carry out crosslinking by irradiation; however, this process is extremely expensive. In addition, it has been found that with an irradiation process, it is limited to small thicknesses of foam to be treated, because there is a crosslinking gradient.

It is also known to use 0.5 to 5% of a fluoropolymer, such as teflon, to facilitate the extrudability of LLDPE. However, this has proven to be completely ineffective at temperatures as low as 135 ° C.

It is therefore necessary to be able to have a low temperature extrusion process enabling chemical crosslinking to be used. This is also important for the production of foams of greater thickness, which cannot be crosslinked by irradiation.

It is also important to be able to obtain foams having small cells, because they promote better mechanical properties including compressive strength, residual crushing. They promote

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- 3 - ** 1 also lower thermal conductivity. They also have an attractive visual appearance.

The object of the present invention is to remedy the drawbacks mentioned above.

The present invention relates to a process for r extruding at low temperature a composition intended to produce a polyethylene foam crosslinked or not, without there being any risk of decomposition of the heat-sensitive agents.

Another object of the present invention is to obtain, by means of the process of the invention, a crosslinked or non-crosslinked polyethylene foam which has improved mechanical properties, as well as small cells.

The process of the present invention for producing cross-linked or non-crosslinked polyethylene foams in two stages comprising the extrusion of a composition containing the polyethylene and the heat-sensitive additives to form a sheet of homogeneous composition, and the unwinding of this sheet in an oven at high temperature in order to allow expansion and crosslinking of the polyethylene, is characterized in that, during the first step, - a linear low density polyethylene containing 0.1 to 10 parts of a polyether is introduced into an extruder per 100 parts of polyethylene; - 2.5 to 25 parts of swelling agent and 0 to 2 parts of crosslinking agent are introduced simultaneously into the extruder, so as to mix it intimately with the polyethylene composition which circulates therein, from 0.05 to 1 , 5 parts of pigment and 10 to 15 parts of flame retardant (calculated per 100 parts of polyethylene); - This mixture of polyethylene and polyether is heated to a temperature sufficient to soften it, without however exceeding 1 3 5 0 C; - A sheet of homogeneous composition is extruded at a temperature which does not exceed 135 ° C; - the sheet thus obtained is rolled up to transfer it; and in the second step,

- the sheet obtained in the first step is unrolled, and it is passed through an oven according to a temperature profile ranging between 140 and 260 ° C.

- The expansion and crosslinking of the treated polyethylene sheet is carried out in the oven; - Recovering at the outlet of the oven the sheet of polyethylene foam crosslinked or not having cells with an average size less than 0.4 mm.

The size of the cells is determined by counting the number of cells within a given distance. This distance is then divided by the number of cells counted (ASTM).

The Applicant has found it quite unexpectedly that if one adds to a linear low density polyethylene a polyether at a rate of 0.1 to 10 parts per 100 parts of polyethylene, one could easily carry out the extrusion of the composition of polyethylene and heat-sensitive agents at a temperature below 135 ° C and even below 130 ° C.

In addition, the Applicant has also found that the foam produced with low density linear polyethylene with polyether additive, the foam not only had improved mechanical properties, but also small cells, the average size of which is less than 0.4 mm and most often less than 0.3 mm.

According to the method of the present invention, a low density linear polyethylene is used, the density of which can in particular be between 0.910 and 0.930, and the flow index of which can vary between 1 and 5 g / 10 min.

Generally, linear low density polyethylene is used which is a copolymer of ethylene and butene, although other copolymers can be used such as ethylene-hexene and ethylene-octene copolymers.

The Applicant has now found that in order to extrude a low density linear polyethylene having the above characteristics, at a temperature as low as 135 ° C., it was absolutely necessary to add a polyether to it. This polyether can be added either directly to the inlet of the extruder, or pumped in molten form into the barrel of the latter.

Although low density linear polyethylenes having a density of less than 0.910 can be extruded at low temperature, as described in another patent application filed today by the Applicant, it is understood that a polyether can be added to these linear polyethylenes low density density below 0.910, to facilitate their extrusion at low temperature.

As polyethers which are suitable for the process of the invention, mention may in particular be made of polyethylene glycol of molecular weight between 5,000 and 30,000, polypropylene glycol of molecular weight between 2,000 and 10,000, or alternatively polyoxy methylene.

The Applicant has also found that when polyethylene glycols were used, the most effective were those with the lowest molecular weight.

This is the reason why polyethylene glycols of molecular weight between 7,000 and 12,500 are preferably used.

The adequate amount of polyether has been found to vary between 0.1 and 10 parts per 100 parts of linear low density polyethylene. The amounts to be used also depend on the type of polyether used. Thus, when polyethylene glycol is used, the adequate amounts are between 0.1 and 5 parts per 100 parts, while when polypropylene glycol is used, the amounts. * 1 - ό - adequate are most often between 0.1 and 3 parts per 100 parts of low density polyethylene.

The Applicant has also found that the addition of a rubber of the polyisobutene type with a molecular weight of between 500 and 5,000 also had a beneficial effect on the extrudability. Generally, the amounts of polyisobutylene vary between 5 and 20 parts per 100 parts of linear low density polyethylene.

According to one embodiment of the process of the invention, linear low density polyethylene (LLDPE) is introduced into the feed hopper of the extruder, as well as 0.1 to 10 parts of a polyether, per 100 parts of LLDPE.

The extruder is provided with a heating system which gives a temperature profile between the inlet and the die ranging in particular from 105 - 110 - 120 to 125 ° C.

0 to 2 parts per 100 parts of polyethylene of a crosslinking agent and 2.5 to 25 parts per 100 parts of polyethylene of a blowing agent are also added to the extruder.

As examples of crosslinking agent, it is possible in particular to use peroxides whose half-life time at 120 ° C. is between 20 hours and 20 minutes. As typical examples of such peroxides, mention may be made of dicumyl peroxide, di-tert-butyl peroxide, tert-butyl perbenzoate, tert-butyl peracetate and di-tert-butyl peroxy-3,3, 5-trimethylcyclohexane.

As examples of swelling agent, mention may in particular be made of para-toluene sulfonyl hydrazide, azodicarbonamide, and oxy bis (benzene sulfonyl hydrazide), aminopentamethyltetramine, used alone or in the presence of activators or agents of coréticu1 atiοn.

Extrusion can be carried out on a single screw or double screw system.

Although a mixture comprising low density linear polyethylene having the required properties, the heat-sensitive agents and the usual additives can be extruded, the Applicant has found that it is possible to extrude, under the recommended conditions, mixtures comprising from 70 to 100% by weight of linear low density polyethylene of the invention, from 0 to 30% by weight of a polymer chosen from low density polyethylenes having a density between 0.915 and 0.930, preferably between 0.918 and 0.923 and a flow index between 3 and 5 g / 10 min. and the ethylene-vinyl acetate, ethylene-propylene diene (EPDM) and ethylene propylene (EPM) copolymers.

On leaving the extruder, the sheets of homogeneous composition are rolled up and cooled before being transferred to the expansion line.

The rolls of low density linear polyethylene sheets containing the blowing and crosslinking agents as well as other additives such as dyes are transferred to the head of the expansion line. The sheet is unwound flat and introduced into an oven heated to a temperature of about 140 to 260 ° C and preferably to a temperature ranging between 190 and 240 ° C and in any case at a temperature sufficient to break down blowing agents and crosslinking agents.

The foam is expanded and crosslinked and a sheet of foam, crosslinked or not, is drawn from the oven, depending on whether or not a crosslinking agent is present.

In addition, it was found that with the linear low density polyethylene and the polyether of the invention, a foam was obtained which had cell dimensions of less than about 0.4 mm, while with other non-additive polyethylenes polyether, these dimensions never go below about 0.5 to 0.7 mm.

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The following examples are given in order to better illustrate the process of the invention.

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Example 1

70 parts by weight of a linear low density polyethylene having a density of 0.918 and a flow index of 2 g / 10 min. Were introduced into an extruder, and 30 parts of a low density polyethylene having a density of 0.923 and a flow index of 4 g / 10 min.

0.5 parts by weight of a polyethylene glycol of molecular weight 3500 and having a solidification point of between 54 and 58 ° C. are added simultaneously to the extruder, as well as 15 parts by weight of azodicarbonamide and 0.9 parts by weight of dicumyl peroxide.

This mixture is extruded according to the following temperature profile: 105 ° C (inlet) - 115 ° C - 120 ° C - 125 ° C (die). The extrusion went perfectly.

There was thus obtained a sheet with a thickness of 2.64 mm and 0.735 m in width. This sheet was then introduced into an oven to achieve the expansion and crosslinking of the polyethylene. The inlet temperature was 190 ° C and the outlet temperature was 242 ° C. The winding speed was 10 m / min.

There was thus obtained a foam with a density of 37 Kg / m3 and the cell size of which was 0.16 mm.

For comparison, an attempt was made to extrude the same mixture but which did not contain polyethylene glycol. We could not work at such a low temperature. It had to be extruded at 145 ° C. and a blockage of the die was observed after 5 minutes of operation.

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Example 2

The procedure described in Example 1 was repeated, but with various types of polyethylene glycol, themselves used at different concentrations.

The information relating to the polyethylene glycols used, as well as the size of the cells of the foam obtained, are indicated in Table 1.

Table 1

Molecular Weight Test Quantity Polyethylene cell size (parts in (in mm) weight) 2 A 4,000 1 0.30 2 B 4,000 3 0.32 2 C 8,000 0.5 0.19 2 D 8,000 1 0, 13 2 E 20,000 0.5 0.40 2F 30,000 0.5 0.50 4 * t - π -

Example 3

100 parts by weight of a linear low density polyethylene having a density of 0.918 and a flow index of 2 g / 10 min were introduced into an extruder.

1 part by weight of a polypropylene glycol of molecular weight 5,000 is added simultaneously to the extruder, as well as 4.6 parts by weight of azodicarbonamide and 0.9 parts by weight of dicumyl peroxide.

This mixture is extruded according to the following temperature profile: 105 ° C (inlet) - 115 ° C - 120 ° C - 125 ° C (flask). The extrusion went perfectly.

This gave a sheet with a thickness of 1.72 mm and 0.735 m in width.

This sheet was then introduced into an oven to achieve the expansion and crosslinking of the polyethylene. The inlet temperature was 190 ° C and the outlet temperature was 242 ° C. The winding speed was 3 m / min.

There was thus obtained a foam with a thickness of 3.75 mm, a density of 123 kg / m 3 and the size of the cells of which was 0.3 mm.

This foam had the following properties:

Tensile at break: perpendicular direction: 157 N / cm2, 230% elongation parallel direction: 146 N / cm2, 225% elongation

Tear test: perpendicular direction: 192 N / cm parallel direction: 174 N / cm

Compression at 40% 8.75 N / cm2

Claims (8)

1. A method for producing foams of crosslinked or non-crosslinked polyethylene, in two stages comprising the extrusion of a composition containing the polyethylene and the heat-sensitive additives to form a sheet in an oven at high temperature in order to allow expansion and crosslinking polyethylene, characterized in that, during the first step, - a low density linear polyethylene containing 0.1 to 10 parts of a polyether per 100 parts of polyethylene is introduced into an extruder; - 2.5 to 25 parts of swelling agent and 0 to 2 parts of crosslinking agent, from 0.05 to 1, are introduced simultaneously into the extruder, so as to mix it intimately with the polyethylene composition which circulates therein; 5 parts of pigment and 10 to 15 parts of flame retardant (calculated per 100 parts of polyethylene); - This mixture of polyethylene and polyether is heated to a temperature sufficient to soften it, without however exceeding 135 e C; - A sheet of homogeneous composition is extruded at a temperature which does not exceed 135 ° C; - the sheet thus obtained is rolled up to transfer it; and during the second step, - the sheet obtained in the first step is unwound, and it is passed through an oven according to a temperature profile ranging between 140 and 260 ° C; - The expansion and crosslinking of the treated polyethylene sheet is carried out in the oven; - The sheet of crosslinked or non-crosslinked polyethylene foam having cells with an average size of less than 0.4 mm is recovered at the outlet of the oven. 2. Method according to claim 1, characterized in that one uses a polyether chosen from polyethylene glycol, polypropylene glycol and po1 yoxyméthy1ène. , -¾ "a" 3. Method according to claim 2, characterized in that a polyethylene glycol of molecular weight between 5,000 and 30,000, preferably between 7,000 and 12,500, is used. at a rate of 0.1 to 5 parts per 100 parts of polyethylene. 4. Method according to claim 2, characterized in that a polypropylene glycol of molecular weight between 2,000 and 10,000 is used, in a proportion of 0.1 to 3 parts per 100 parts of polyethylene. 5. Method according to any one of claims 1 to 4, characterized in that one adds to the mixture of linear low density polyethylene, and heat-sensitive additives, a polyisobutene of molecular weight between 500 and 5,000 at a rate of 5 20 parts per 100 parts of linear low density polyethylene. 6. Method according to any one of claims 1 to 5, characterized in that one uses a low density linear polyethylene having a density between 0.910 and 0.930 and a flow index between 1 and 5 g / 10 min . 7. Method according to any one of claims 1 to 6, characterized in that a polyethylene mixture is used comprising from 70 to 100% by weight of a low density linear polyethylene having a density between 0.910 and 0.930 and a flow index between 1 and 5 g / min., and from 0 to 30% by weight 11 * - 14 - of a polyethylene chosen from low density polyethylenes having a density between 0.915 and 0.930 and an index d flow between 3 and 5 g / 10 min., ethylene-vinyl acetate, ethylene-propylene-diene (EPDM) and ethylene-propylene (EPM) copolymers. 8. Crosslinked polyethylene foam or not obtained according to the process described in any one of claims 1 to 7.