LU86528A1 - Opt. crosslinked polyethylene foam sheet - produced by extrusion of specified compsn. into sheet, wound and unwound in oven having temp. gradient - Google Patents

Abstract

2-step prodn. of opt. crosslinked polyethylene foam from a PE compsn. contg. heat sensitive additives comprises: (1) Introduc-ing LDPE of density below 0.910 and M.1. below 3 g/10min into an extruder simultaneously with 1.5-25 pts. blowing agent and 0-2pts. crosslinking agent (per 100 pts. PE) to form a mixt. heated at a temp. sufficient to soften the polymer without exceeding 135 deg.C extruding a sheet of homogenesus compsn. and winding the obtd. sheet, and (2) unwinding the sheet, passing it in an oven in which a temp. gradient of 140-260 deg.C is maintained to expand and crosslink the sheet and recover-ing the foamed sheet at the outlet of the oven, the sheet contg. pores of average size below 0.4 mm.

Description

i f- <Γ4-ί »SYN 10

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 some additives, the required quantity of swelling agent, most often taken in solid form, and of crosslinking agent, is introduced into an extruder, and the mixture is heated. softening the polyolefin and perfectly incorporating all the additives and other heat-sensitive agents into the polyolefin. 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 - 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, extrusion is required at a temperature below about 135 ° C.

However, it is well known in the field of crosslinked polyethylene, 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, after extrusion; 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 1350C.

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 get? foams with small cells, because they promote better mechanical properties including resistance to compression, residual crushing. They also promote * r - 3 - 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 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 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 of homogeneous composition, and the unwinding of this process. sheet in an oven at high temperature in order to allow the expansion and crosslinking of the polyethylene, is characterized in that, during the first stage, - a linear low density polyethylene having a density of less than 0.910 and a density is introduced into an extruder flow index less than 3 g / 10 min .; - 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 pigments and 10 to 15 parts of flame retardant (calculated per 100 parts of polyethylene); - This polyethylene is heated to a temperature sufficient to soften it, without exceeding 135 ° C; - A sheet of homogeneous composition is extruded at a temperature which does not exceed 135 ° C; c - 4 - - the sheet thus obtained is rolled up for transfer; and during 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 exit 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, quite unexpectedly, that if a linear low density polyethylene having well-defined density and flow index characteristics is used, it is easy to extrude 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 this particular type of linear low density polyethylene not only has improved mechanical properties, but also small cells, the average size of which is less than 0.4 mm and the most often less than 0.3 mm.

The Applicants have now found that to extrude a mixture of polyethylene with heat-sensitive agents, at a temperature below about 135 ° C., it was absolutely necessary to use a linear low density polyethylene whose density is less than 0.910 and whose index of flow is less than 3 gr / 10 min. (at 190 ° C under 2.16 Kg).

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

It has been found that with linear low density polyethylenes which do not meet the above criteria, it is not possible to extrude the composition at a temperature as low as 135 ° C.

It should be noted that when the crosslinking agent decomposes during the first extrusion, there is blockage of the screws of the extruder, which leads to a complete shutdown of the installation.

According to one embodiment of the process of the invention, linear low density polyethylene with a density of less than 0.910 and preferably less than 0.905 is introduced into the feed hopper of the extruder.

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

0 to 2 parts per 100 parts of polyethylene, a crosslinking agent, and 2.5 to 25 parts per 100 parts of polyethylene, 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), aminopentameth y11etramine, used alone or in the presence of activators or coretal agents 1 ation.

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

Although it is possible to extrude a mixture comprising - the heat-sensitive agents, the usual additives and one or more linear low density polyethylene having the required properties, 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 and from 0 to 30% by weight of a polyethylene chosen from low density polyethylenes having a density between 0.900 and 0.930 and preferably between 0.918 and 0.923 and a flow index of between 3 and 5 g / 10 min., and the ethylene-vinyl acetate, ethylene-propylene-diene (EPDM) and ethylene-propylene (EPM) copolymers.

At the exit of 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 and flame retardants are transferred to the top of the expansion line. The sheet is unwound flat and introduced into an oven heated to a temperature between 140 and 260 ° C, preferably between 190 and 240 ° C and in any event at a temperature sufficient to decompose the 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 has been found that with the linear low density polyethylene of the invention, a foam is obtained which has cell dimensions less than about 0.4 mm, while with other polyethylenes, these dimensions never fall below „from about 0.5 to 0.7 mm.

With the process of the invention, it has been found that thermoforming of deep pieces is more easily carried out, while creases or tears are obtained with foams crosslinked by irradiation.

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

"- 8 -t

Example e_1_

100 parts by weight of a linear low density polyethylene having a density of * 0.905 and a flow index of 2.6 g / 10 min are introduced into an extruder.

4.6 parts by weight of azodicarbonamide and 1 part by weight of dicumyl peroxide are added simultaneously to the extruder.

This mixture is extruded according to the following temperature profile: 105 ° C (inlet) - 110 ° C - 120 ° C - 128 ° C

(Faculty).

A sheet with a thickness of 1.64 mm and a width of 0.735 m is obtained, at a speed of 0.8 m / min. The low temperature extrusion went perfectly.

The sheet obtained in the extruder was rolled up, which was unrolled in an oven whose inlet temperature was 182 ° C and outlet temperature of 241 ° C.

The expansion and the crosslinking of the linear low density polyethylene were thus carried out, the foam obtained had the following characteristics:

Thickness: 3.5mm Width: 1.5m

Volumic mass ; 140 Kg / m3

Cell size: 0.3mm

Tensile at break perpendicular direction: 160 N / c m 2 at 300% elongation parallel direction: 175 N / cm2 at 290% elongation

Compression at 40%: 10.6 N / cm2

Tear test perpendicular direction: 165 N / cm parallel direction: 160 N / cm.

We notice that there is uniformity of results in -r * - 9 - in both directions.

By way of comparison, a linear low density polyethylene was used which did not comply with the conditions of the invention; indeed this LLDPE had a density of 0.920 and a flow index of 2.

100 parts by weight thereof were introduced into the extruder simultaneously with 4.6 parts of azodicarbonamide and 1 part of dicumyl peroxide.

We note that we cannot work at such low temperatures, and we had to extrude at 145 ° C; however after 5 minutes, the die was completely clogged.

By way of comparison, a low density polyethylene having a density of 0.923 and a flow index of 4 g / 10 min was used in place of the LLDPE of the invention. The mixture of low density polyethylene, azodicarbonamide and dicumyl peroxide was extruded according to the same temperature profile as that described above.

The sheet obtained by extrusion was then expanded in the oven under the same conditions as those mentioned above.

The foam obtained had 0.7 mm cells of medium size.

i 4 - 10 -Example 2

70 parts by weight of low density linear polyethylene (LLDPE) having a density of 0.905 and a flow index of 2.6 g / 10 min. And 30 parts by weight of a low polyethylene were introduced into an extruder. density having a density of 0.923 and a flow index of 4 g / 10 min.

15 parts by weight of azodicarbonamide and 1 part by weight of dedicumyl peroxide are added simultaneously to the extruder.

This mixture is extruded according to the following temperature profile. · 105 ° C (inlet) -110 ° C - 120 eC - 125 ° C (die).

A sheet with a thickness of 2.65 mm and 0.735 m wide is obtained. The low temperature extrusion went perfectly.

The sheet was then subjected to expansion and crosslinking by passage through an oven with an inlet temperature of 187 ° C and an outlet temperature of 232 ° C.

The foam obtained had the following properties: Thickness: 7.5 mm The ranger: 1.5 m

Density: 37 Kg / m3 Traction at break: perpendicular direction: 51 N / c m 2 at 200% elongation parallel direction: 56 N / cm2 at 210% elongation

Tear test: perpendicular direction: 43 N / cm parallel direction: 39 N / cm

Cell size: 0.28 mm = 0.042 Watt / m. ° K

With the foam obtained, objects having an H / D (height / diameter) of 2 were thermoformed without any folds, while for the same object obtained from a crosslinked foam by physical means there is the appearance of folds.

Ja ’-11- For comparison, 100 parts by weight of an LLDPE was used, having a density of 0.923 and a flow index of 4 g / 10 min.

„The foam obtained from this polyethylene had the following properties:

Tensile at break: perpendicular direction: 22 N / cm2 at 100% elongation parallel direction: 31 N / cm2 at 91% elongation

Tear test: perpendicular direction: 40 N / cm parallel direction: 25 N / cm.

Cell size: 0.7 mm average = 0.046 Watt / m. ° K

This comparative example clearly shows that the desired cell size and good mechanical properties are not obtained. In addition, the is higher. .

Example 1

The procedure described in Example 2 was repeated, but this time using 70 parts by weight of an LLDPE, having a density of 0.895 and a flow index of 2 g / 10 min. The rest of the conditions described in Example 2 were maintained. The extrusion went perfectly.

The foam obtained after passage through the oven, under the same conditions as those of Example 2, had the following properties:

Density: 40 Kg / m3 ^ Cell size: 0.19 mm.

Claims (4)

1. 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 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, characterized in that, during the first step, - a linear low density polyethylene having a density less than 0.910 and a lower flow index is introduced into an extruder at 3 g / 10 min .; - 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 polyethylene is heated to a temperature sufficient to soften it, without exceeding 135 ° 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; - Recovering from the oven the sheet of polyethylene foam crosslinked or not, having cells of an average size less than 0.4 mm. 2. Method according to claim 1, characterized in that ft - 13 - ♦ a low density linear polyethylene is used having a density less than 0.905 and a flow index less than 3 g / 10 min. 3. Method according to claims 1 and 2, characterized in that a polyethylene mixture is used comprising from 70 to 100% by weight of a linear low density polyethylene having a density less than 0.910 and a lower flow index at 3 g / min., and from 0 to 30% by weight of a polyethylene chosen from low density polyethylenes having a density between 0.915 and 0.930 and a flow index between 3 and 5 g / 10 min., ethylene-vinyl acetate, ethylene-propylene-diene (EPDM), ethylene-propylene (EPDM) copolymers. 4. Polyethylene foam crosslinked or not obtained according to the process described in any one of claims 1 to 3.