GB1590027A - Continuous manufacture of foamed strands slabs or sheets of crosslinked olefin polymer - Google Patents

Description

(54) CONTINUOUS MANUFACTURE OF FOAMED STRANDS, SLABS OR SHEETS OF CROSSLINKED OLEFIN POLYMER (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, 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 present invention relates to a process for the continuous manufacture of foamed profiles (i e. strands, slabs or sheets) of crosslinked olefin polymers.

Various processes for the manufacture of crosslinked polyolefin foams have been disclosed, and some are employed in practice.

According to one process, a mixture of a polyolefin, a blowing agent which eliminates gas and a peroxide is molded and then heated in a heating tunnel by means of hot air under atmospheric pressure, so that crosslinking due to the peroxide and foaming due to the blowing agent can take place, It is a disadvantage of this process that the foam profiles obtained have coarse cells and the thickness of the proW files is restricted to about 25 mm, since, at greater thicknesses, the temperature required for foaming is only achieved, in the body of the material, after a very long time, due to the insulating action of the foamed surface zones.

Another process employs high energy radiation, in place of organic peroxides, to cause crosslinking. However, crosslinking by the electron beams employed decreases greatly as the depth of penetration increases, so that even if the material is irradiated from both sides, the thickness of the foamable matrix is restricted to about 3 mm. These products, again, have relatively coarse cells.

The above disadvantages do not arise in another process, where foaming is carried out under pressure, for example in a press, whlch is only opened after crosslinking, by means of a peroxide, has been concluded and the blowing agent has been completely decomposed. This process can however only be carried out batchwise and furthermore presents mold release problems.

The present invention seeks to provide an improved process which can be used for the manufacture of very fine-celled crosslinked polyolefin foams of substantial thickness.

According to the present invention there is provided a process for the continuous manufacture if a foamed strand, slab or sheet from 1 to 200 mm thick, of an olefin polymer, which process comprises A) homogenizing, at a temperature above the melting point of the polymer but below the decomposition point of the components a) to d) below, a mixture of a) an olefin polymer, b) from 1 to 30 per cent by weight based on the olefin polymer of a solid organic blow ing agent which on heating liberates an inorganic gas, and c) from 0.1 to 10 per cent by weight based on the olefin polymer of an organic per-com pound as crosslinking agent, with or without d) from 0.1 to 10 per cent by weight, based on the olefin polymer, of one or more foaming assistants, crosslinking promoters and/or other additives, B) crosslinking and foaming the olefin polymer at a mean polymer temperature at from 1600 to 280"C in a foaming zone, and C) forcing the foam out of the foaming zone, whilst cooling it and causing it to solidify, in which process the average pressure in the foaming zone is from 5 to 150 bars and the ratio of the length (as herein defined) of the foaming zone to its maximum width (as herein defined) is from 100:1 to 2,000:1.

Olefin polymers which may be used for the process include high pressure, medium pressure and low pressure polyethylene, polypropylene, polybutene, copolymers of ethylene with up to 50 per cent by weight of propylene, I-buten butadiene, vinyl acetate or acrylic esters, and mixtures of two or more of these polymers, as well as mixtures of polyolefins with up to 40 per cent by weight of polymers which, because of possessing unsaturaied main chains or side chains, are particularly capable of crosslinking reactions, e.g. 1,4-polybutadiene, 1,2-polybutadiene, natural rubber, polyisoprene and styrene/butadiene copolymers. The use of polyethylene is preferred.

Preferred organic, solid blowing agents which liberate inorganic gases when heated are those having a decomposition temperature above 1600C and in particular above 1800C. Examples of suitable components are nitroso compounds, e ,g, dinitrosopentamethylenetetramine, N,N' dimethyl-N ,N'-dinitroso'terephthalamide and p ,p'-hydroxy-bis.(N-Mtroso-N-methyl-benzene) sulfonamide, hydrazides, e.g. benzene-l, 3-disulfonic acid dihydrazide, diphenylsulfone-3, 3'- disulf-hydrazide, p ,p'-hydroxy-bis-benzenesulf- onyl-hydrazide, P-naphthalenesulfonic acid hydrazide and benzenesulfonic acid N-phenylhydrazide, as well as p,p'-hydroxy-bis-bensene sulfonysemicarbazide and diphenyl-4 ,4'-disul- fonylazide. However, for economic reasons, azodicargoxamide (decomposition 1900C) is preferred. The amounts of blowing agent employed depend on the desired degree of foaming, and are from 1 to 30 per cent by weight, preferably from 5 to 20 per cent by weight, based on the olefin polymer. To achieve special effects, mixtures of two or more blowing agents may also be employed.

The gas-forming characteristics and the decomposition point of the blowing agents can be varied within wide limits by adding certain foaming assistants, e.g. metal oxides, fatty acid salts, and sulfides and oxides of zinc, lead, cadmium or tin. Free fatty acids, e.g. stearic acid, act similarly. Depending on the desired decomposition point, from 0.1 to 5 per cent by weight, based on the olefin polymer, of the above compounds may be added, Preferably from 0,5 to 3 per cent by weight of zinc oxide or zinc stearate are added.

Particularly suitable per compounds for use as crosslinking agents are those per compounds which have such a high decomposition point that when they are processed together with olefin polymers in conventional industrial mixing equipment, no significant decomposition of the peroxide occurs. Examples of conventional per compounds are: tert.-butyl permaleate, 2,5dimethylhexane 2,5-diperbenzoate, tert .butyl peracetate, tert.-butyl perisononate, di-tert. butyl diperphthalate, tert .-butyl perbenzoate, 2,5-dimethylhexane ,fi-diperisononate, tert. butyl per-s, 5, 5-trimethylhexoate, 2 ,2-bis-(butyl- peroxy)-butane, dicumyl oxide, tert.-butyl cumyl peroxide, 2f 5=dimethylc2ss5-bis4te. t.-butylper- oxy)-hexane, 2,5-dimethyl-2,5-bis-(tert.-butyl; peroxy)-hexyne, 1 ,3-bis-(tert.-butylperoxyisopropyl)-benzene, di-tert.-butyl peroxide, tert.-blityl hydroperoxide, cumene hydroper oxide, methyl ethyl ketone peroxide, acetylacetone peroxide, cyclohexanone peroxide and 3,5 ,5-trimethylcyclohexanone perketal.

The most suitable per compound for any particular case depends on the nature of the olefin polymer employed.

The amount of per compound added is from 0.1 to 10 per cent by weight, preferably from 0.25 to 5 per cent by weight, based on the polymer. The amounts should preferably be such that the olefin polymer is crosslinked to a gel content of 10-80 per cent by weight, especially 30-60 per cent by weight.

The crosslinking effect can be increased by adding a crosslinking promoter to the mixture.

This is an organic compound which contains two or more olefinically unsaturated double bonds which participate directly in the formation of bridges between the high molecular weight olefin polymer chains. Examples of effective promoters are dialkyl phthalates, maleates and fumarates, triallyl isocyanurate, triallyl phosphate, divinylbenzene, ethylene glycol diacrylate, and ethylene glycol dimethacrylate.

Such crosslinking promoters display their optimum efficiency when used in amounts of from 0.2 to 5 per cent by weight, but concentrations of from 1 to 3 per cent by weight, based on the olefin polymer, are preferred.

The properties of the foams obtained can be modified as desired by incorporating other conventional additives, e.g. anti-oxidants, light stabilizers, flameproofing agents, dyes or other conventional polymer auxiliaries The mixture of olefin polymer, blowing agent and crosslinking agent (per compound), with or without further additives, is homogenized above the melting range of the polymer but below the decomposition point of the blowing agent and peroxide; this homogenization may be carried out on conventional mixing equipment, e.g. extruders, kneaders or mills, preferably at from 110 to 1700C, especially from 120 to 140 C.

In a preferred embodiment, the homo geniied mixture is provided with a lubricant before entering the foaming zone, so that a film of lubricant forms between the foam strand and the wall of the foaming tunnel. For this purpose, conventional lubricants which are liquid at the processing temperature and preferably do not cause surface dissolution of the olefin polymer may be employed; examples include silicone oils, glycols, and esters and amides of higher fatty acids. Up to 10 per cent by weight, preferably from 0.2 to 5 per cent by weight, of lubricant may be added to the mixture.

The mixture is then crosslinked and foamed in a foaming zone at from 160 to 2800bpre- ferably from 200 to 2500C. Below 160 C, no foaming occurs if the conventional blowing agents are used, whilst above 280"C there is the danger that the olefin polymer may suffer i:lermal degradation. The stated terrpcratures relate to the mean temperature of the foaming mixture in the foaming zone. The foaming tunnel is advantageously heated externally where the mixture enters, in order to initiate the decomposition of the peroxide and the blowing agent. The peroxide, the blowing agent and the additives are advantageously chosen in such a way that crosslinking takes place first, followed by foaming. It is also advantageous if the decomposition of the peroxide and of the blowing agent are exothermic reactions. lit order to solidify the edge zone of the foam web formed, it is at times advantageous to cool the wall of the foaming tunnel where the foam issues.

It is essential that the foaming takes place under superatmospheric pressure. The pressure in the foaming zone is required according to the invention to be from 5 to 150 bars, preferably from 20 to 80 bars. This pressure is generated by the gases formed on decomposition of the blowing agent. The stated pressures are average values. The pressure is highest at the beginning of the foaming zone; it decreases slightly due to friction against the walls, and becomes atmospheric pressure when the foam leaves the foaming tunnel. The value of the pressure, and the progressive reduction in pressure, can be influenced by the length and shape of the foaming tunnel and by the wall friction. The use of superatmospheric pressure in the foaming zone results in a very fine-celled foam of uniform foam structure.

It is also essential that the ratio of the length of the foaming zone to its maximum width is from 100:1 to 2,000:1, preferably from 200:1 to 500:1, IFthe foaming tunnel is too short, the core of the foam product, after issuing from the foaming tunnel, still contains undecomposed blowing agent, which can then expand and cause the foam product to split open.

The foaming zone comprises the entire pressureassisted zone downstream from where the mixture leaves the homogenizing zone, so that the length of the foaming zone is defined as the distance between the point at which the mixture leaves the homogenizing zone and the point at which the foam leaves the zone of elevated pressure (5 to 150 bars).

The foaming zone may be in the form of a foaming tunnel of uniform cross-section over its entire length. However, the cross-section can also increase stepwise or continuously, in the direction of product flow. In that case, the maximum width of the foaming zone is defined as the internal diameter of the foaming tunnel at the point at which the foam issues, In the case of rectangular profiles, the maximum width is defined as the length of the short side of the rectangular profile. The dimensions of the foam product approximately correspond to the dimensions of the foaming tunnel at the point at which the foam leaves the tunnel.

After leaving the foaming zone, the foam product is cooled by the surrounding air, and solidifies. Depending on the form of the outlet orifice of the foaming tunnel, the foam strand, slab or sheet may be of a ry desired shape, but a round or rectangular cross-section is preferred and the thickness may vary within wide limits, which are from 1 to 200 mm, preferably from 3 to 120 mm; the density of the foam is preferably from 20 to 200 g/l.

The foam products in the form of strands, slabs or sheets may be used above all as insulating materials.

In the Examples, parts and percentages are by weight.

EXAMPLE 1 93.2 parts of high pressure polyethylene (density 0.92 g/cm ,melt index 1.5 g/10 min.), 5 parts of azodicarbonamide to act as a blowing agent, 1.6 parts of a 40 per cent strength 1,3bis-(t-butyl-peroxyisopropyl)-benzene (on kieselguhr), to act as a crosslinking agent, and 0.2 part of a phenolic anti-oxidant are fused, and thoroughly mixed, in an extruder at 1260C.

This mixture is forced at a pressure of 70 bars, by means of a twin-screw extruder with a throughput of 8 kg/h, through a 6.4 m long cylindrical foaming tunnel of 17 mm internal diameter, the wall temperature being 260 C and the mean foam temperature slightly less at 2400C. 1 part of silicone oil, to act as a lubricant, is pumped in through a sintered metal ring at the beginning of the foaming tunnel. A foam strand having a density of 0.086 g/cm3 and possessing a uniform cell structure with more than 10 cells/mm leaves the end of the foaming tunnel.

EXAMPLE 2 This is carried out as described in Example 1, but with 88.2 parts of polyethylene and 10 parts of azodicarbonamide. The foam strand obtained has a density of 0.043 g/cm3, and a uniform cell structure with more than 15 cells/mm.

COMPARA TIVE EXAMPLE This is carried out as described in Example 1, but with a 1 m long foaming tunnel. A completely split foam strand Is obtained.

EXAMPLE 3 89.5 parts of hiNh pressure polyethylene (density 0.92 g/cm melt index 1.5 g/10 min., 10 parts of azodicarbonamide to acts as a blowing agent and 0.5 part of di-t-butyl peroxide to act as a crosslinking agent are continuously fused at 130 C, and thoroughly mixed, in a twin-screw extruder, with a total throughput of 28 kg/h. This mixture is forced by the extruder through a cylindrical foaming tunnel which increases stepwise, section by section, from a width of 17 mm to a width of 46 mm.

Section 1, of 17 mm internal diameter, is 4 m long, section 2, of 24 mm internal diameter, is 2 m long, section 3, of 30 mm internal diameter, is 2 m long and section 4, of 46 mm internal diameter, is 2 m long. 50 mm long conical adapters are used as connectors between the sections. The total length is 10.15 m. 0.4 part of glycol, to act as a lubricant, are pumped in through four holes, distributed uniformly ove the etinhery, at the beginning of section 1 of the foaming tunnel. The wall temperature is 220 C in the tunnel section 1,220"C in section 2, 1800C in section 3 and 100"C in section 4, the mean temperature of the foam being 2000C. Using a pressure of 50 bars at the beginning of tunnel section 1, a foam strand of density 0.040 g/cm3 leaves the end of the foaming tunnel; the strand has a diameter of 46 mm, a smooth surface, and a uniform cell structure with more than 15 cells/mm. The gel content is 46%.

WHAT WE CLAIM IS: 1. A process for the continuous manufacture of a foamed strand, slab or sheet, from 1 to 200 mm thick, of an olefin polymer, which process comprises A) homogenizing, at a temperature above the melting point of the polymer but below the decomposition point of the components a) to d) below, a mixture of a) an olefin polymer, b) from 1 to 30 per cent by weight based on the olefin polymer of a solid organic blow ing agent which on heating liberates an inorganic gas, and c) from 0.1 to 10 per cent by weight based on the olefin polymer of an organic per-com pound as crosslinking agent, with or without d) from 0.1 to 10 per cent by weight based on the olefin polymer of one or more foaming assistants, crosslinking promoters and/or other additives, B) crosslinking and foaming the olefin polymer at a mean polymer temperature of from 160 to 2800C in a foaming zone, and C) forcing the foam out of the foaming zone, whilst cooling it and causing it to solidify, wherein the average pressure in the foaming zone is from 5 to 150 bars and the ratio of the length (as herein defined) of the foaming zone to its maximum width (as herein defined) is from 100:1 to 2,000:1.

2. A process as claimed in Claim 1, wherein the homogenized mixture from stage A is provided with a lubricant before entering the foaming zone.

3. A process as claimed in Claim 1 or 2, wherein the olefin polymer is polyethylene.

4. A process as claimed in any of Claims 1 to 3, wherein the blowing agent has a decomposition temperature above 1800C.

5. A process as claimed in any of Claims 1 to 4, wherein from 0.5 to 3 per cent by weight, based on the olefin polymer, of zinc oxide or zinc stearate is present as foaming assistant (d).

6. A process as claimed in any of Claims 1 to 5, wherein an amount of organic per-compound sufficient to crosslink the olefin polymer to a gel content of 30 to 60 per cent by weight is employed.

7. A process as claimed in any of Claims 1 to 6, wherein from 1 to 3 per cent by weight, based on the olefin polymer, of a crosslinking promoter is employed.

8. A process as claimed in any of Claims 1 to 7, wherein the ingredients and conditions are selected such that crosslinking of the olefin polymer precedes foaming.

9. A process as claimed in any of Claims 1 to 8, wherein the average pressure in the foam'..

ing zone is 20 to 80 bars.

10. A process as claimed in any of Claims 1 to 9, wherein the foaming zone has the form of a cylindrical tunnel of uniform cross section.

11. A process for the continuous manufacture of a foamed strand, slab or sheet carried out substantially as described in any of the foregoing Examples 1 to 3.

12. A foamed strand, slab or sheet when manufactured by a process as claimed in any of Claims 1 to 11.

13. A foamed strand, slab or sheet as claim ed in Claim 12 when used as an insulating material.

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

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. being 2000C. Using a pressure of 50 bars at the beginning of tunnel section 1, a foam strand of density 0.040 g/cm3 leaves the end of the foaming tunnel; the strand has a diameter of 46 mm, a smooth surface, and a uniform cell structure with more than 15 cells/mm. The gel content is 46%. WHAT WE CLAIM IS:

1. A process for the continuous manufacture of a foamed strand, slab or sheet, from 1 to 200 mm thick, of an olefin polymer, which process comprises A) homogenizing, at a temperature above the melting point of the polymer but below the decomposition point of the components a) to d) below, a mixture of a) an olefin polymer, b) from 1 to 30 per cent by weight based on the olefin polymer of a solid organic blow ing agent which on heating liberates an inorganic gas, and c) from 0.1 to 10 per cent by weight based on the olefin polymer of an organic per-com pound as crosslinking agent, with or without d) from 0.1 to 10 per cent by weight based on the olefin polymer of one or more foaming assistants, crosslinking promoters and/or other additives, B) crosslinking and foaming the olefin polymer at a mean polymer temperature of from 160 to 2800C in a foaming zone, and C) forcing the foam out of the foaming zone, whilst cooling it and causing it to solidify, wherein the average pressure in the foaming zone is from 5 to 150 bars and the ratio of the length (as herein defined) of the foaming zone to its maximum width (as herein defined) is from 100:1 to 2,000:1.

2. A process as claimed in Claim 1, wherein the homogenized mixture from stage A is provided with a lubricant before entering the foaming zone.

3. A process as claimed in Claim 1 or 2, wherein the olefin polymer is polyethylene.

4. A process as claimed in any of Claims 1 to 3, wherein the blowing agent has a decomposition temperature above 1800C.

5. A process as claimed in any of Claims 1 to 4, wherein from 0.5 to 3 per cent by weight, based on the olefin polymer, of zinc oxide or zinc stearate is present as foaming assistant (d).

6. A process as claimed in any of Claims 1 to 5, wherein an amount of organic per-compound sufficient to crosslink the olefin polymer to a gel content of 30 to 60 per cent by weight is employed.

7. A process as claimed in any of Claims 1 to 6, wherein from 1 to 3 per cent by weight, based on the olefin polymer, of a crosslinking promoter is employed.

8. A process as claimed in any of Claims 1 to 7, wherein the ingredients and conditions are selected such that crosslinking of the olefin polymer precedes foaming.

9. A process as claimed in any of Claims 1 to 8, wherein the average pressure in the foam'..

ing zone is 20 to 80 bars.

10. A process as claimed in any of Claims 1 to 9, wherein the foaming zone has the form of a cylindrical tunnel of uniform cross section.

11. A process for the continuous manufacture of a foamed strand, slab or sheet carried out substantially as described in any of the foregoing Examples 1 to 3.

12. A foamed strand, slab or sheet when manufactured by a process as claimed in any of Claims 1 to 11.

13. A foamed strand, slab or sheet as claim ed in Claim 12 when used as an insulating material.