GB1560630A

GB1560630A - Manufacture of foam mouldings from olefin polymers

Info

Publication number: GB1560630A
Application number: GB39491/76A
Authority: GB
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1975-09-24
Filing date: 1976-09-23
Publication date: 1980-02-06
Also published as: FR2325489A1; AT352404B; FR2325489B1; IT1077006B; DE2542452C3; DE2542452A1; DE2542452B2; CH598938A5; ATA707576A; SE7610474L; DK429776A; BE846442A; SE420840B

Description

(54) MANUFACTURE OF FOAM MOLDINGS FROM OLEFIN POLYMERS (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 manufacture of a foam molding from an olefin polymer by compressing foam particles of the olefin polymer with gas, filling the compressed particles into a closed mold at a pressure such that the compressed particles occupy from 20 to 80% of their original bulk volume, releasing the gas pressure in the interior of the mold and heating and welding the particles.

Moldings of olefin polymer foams are manufactured, in the prior art, by loosely filling foam particles into molds, heating them by means of a heating medium which flows through their interspaces and sintering them by an externally applied mechanical pressure (cf.

German Published Applications DAS 1,629,295 and 1,629,281). This process requires molds of expensive mechanical construction to permit moving a part of the inner wall of the mold in order to apply the requisite pressure. It is therefore restricted to moldings of simple geometry.

It has also been proposed to mix finely divided olefin polymer foams with binders, compress the mixture and cure it under pressure (cf. German Laid-Open Application DOS 1,66 ,648). Since the binder has a different chemical composition from the foam, this process gives products with altered properties, eg. altered resistance to chemicals or environmental factors.

Finally, our UK Patent Specification No. 1,201,800 discloses a process for the manufacture of an expanded plastics (foam) molding wherein particles of a closed-cell expanded olefin polymer are compressed in a container by forcing a gas into the container and the pressure on the compressed particles is reduced in a mold, which may be the container in which compresson of the particles occurred or may be a mold into which particles precompressed in a separate container are introduced under pressure, so that the compressed particles expand as a result of the excess pressure within the closed cells and bond together to form the molding. The bonding may be achieved by use of an adhesive to stick the particles together or by fusing the particles together. In the latter embodiment the surfaces of the particles are at a temperature above the crystallite melting point of the polymer during the expansion in the mold, for example by heating them before introducing them into the container or mold or by heating them with heated gas (hot air) in the container or mold, e.g. if the particles have been poured in loosely. The procedure described in that specification frequently requires long heating times and it is very difficult to produce moldings exhibiting uniformly good welding.

The present invention seeks to provide a process for the manufacture of foam moldings without incurring the above-mentioned disadvantages.

According to the present invention there is provided a process of the type defined at the outset, wherein, after release of the gas pressure, steam is introduced to heat the particles, the steam being introduced in an amount to provide a steam pressure which is equal to or greater than the effective pressure in the mold during introduction of the foam particles, and the steam pressure in the interior of the mold is then released to cause the particles to expand and weld together.

Preferred olefin polymers for the purposes of the present invention are polymers of olefins of 2 to 4 carbon atoms. Olefin polymers having an X-ray crystallinity of more than 20% at 25"C are particularly suitable for the process. For example, homopolymers of ethylene or propylene or copolymers of these monomers may be used. Copolymers of ethylene with other ethylenically unsaturated monomers, in which copolymers at least 50 per cent by weight is constituted by copolymerized units of ethylene are particularly suitable for the process. Copolymers of ethylene with from 5 to 30 per cent by weight of esters of acrylic acid or methacrylic acid or vinylcarboxylic acid esters are particularly suitable.

Amongst the comonomers, n-butyl acrylate, t-butyl acrylate and vinyl acetate are particularly important. Mixtures of the olefin polymers with one another or with other polymers may also be used.

The process of the invention involves the manufacture of foam moldings of olefin polymers by compressing foam particles with gas, filling the compressed particles into a mold and releasing the gas pressure in the interior (cavity) of the mold. In a preferred embodiment polyolefin foam particles, which optionally are partially crosslinked and which preferably have a gel fraction of from 20 to 80% by weight, are compressed with gas and filled pneumatically, against a back-pressure (A), into a mold which does not close gas-tight and which possesses perforations and is surrounded by pressure-tight chambers. The back-pressure is advantageously from 1.5 to 3.5 bars. As a result of the pressure prevailing in this filling step, the olefin polymer particles are reduced in volume to from 20 to 80% of their original bulk volume, (i.e. bulk volume prior to compression), the reduction in volume in the initial compression being possibly even greater as some re-expansion occurs when the back pressure (A) is not as great as the initial compression. Therefore, small filling orifices, and small mold cross-sections, can be used.

After completely filling the mold cavity, the gas pressure is released preferably down to atmospheric pressure. The particles expand because of the compressed air contained in them and as a result substantially expel the air present between the foam particles, without the latter welding to one another.

In the next step of the process, steam which is preferably at a temperature of from 112 to 145"C is passed into the cavities until it reaches a predetermined back-pressure (B), which is equal to or greater than the filling back-pressure (A), i.e. from 1.5 to 3.5 bars in the above embodiment. This process takes place in a few seconds, preferably in less than 20 seconds, which is important in order to achieve a faithful foam molding. After reaching the pre-set back-pressure (B) the latter can, if required, be maintained for a short time, preferably from 0.5 to 5 seconds. The steam pressure can for example be up to 10 bars and preferably up to 5 bars. As the next pressure step, the steam pressure in the mold chambers if released.

preferably down to atmospheric pressure. This causes the particles to expand again, fit tightly against one another and weld at the heated surfaces. To achieve rapid cooling, water or cold air can now be passed through the mold chambers or spray jets in the mold chambers can be brought into action.

It can also be advantageous to flush out the air contained in the mold chambers by means of steam at 100"C before building up the pressure with steam.

The process according to the invention gives foam moldings which correspond to the predetermined mold cavity, the welding of the foam particles being almost complete. In a fracture test, more than 90% of the fracture occurs through the foam, that is to say not along the particle interfaces.

Example 1 1.5 liters of compressed round particles of polyethylene foam, which had a diameter of 6 mm and a bulk density of 0.0123 g/cm3 and which no longer contained any blowing agent, were filled into a mold of 0.75 liter capacity which had small slit-like perforations and was surrounded by 2 chambers. The gel fraction of the foam particles was 48% by weight. The foam particles were introduced from a pressure vessel, in which they had been precompressed, via a hose line through a sealable orifice in the mold wall. The pressure vessel was under an adjustable pressure of 3.0 bars. During the filling process, relief valves in the chambers set up a back-pressure (A) in the mold of 2 bars so that the foamed particles in the mold, occupied about 50% of their initial bulk volume prior to compression. After filling the mold, the filling orifice was closed gas-tight and the pressure in the chambers was released. The steam chambers were then flushed with a stream of steam, with the valves open. After closing the valves, the steam pressure in the chambers built up to 2.3 bars.

After reaching this pressure, the steam supply was shut off and the pressure immediately released from the chambers. Water for cooling was then fed into the chambers through spray jets. After the molding had cooled sufficiently, it was removed from the mold.

The total working cycle was made up as follows: Closing the mold 3 secs Pressure build-up in the mold chambers 3 secs Filling 12 secs Releasing the pressure 4 secs Flushing the mold chambers 10 secs Steam pressure build-up 4 secs Releasing the pressure 1 sec Cooling with water 120 secs Opening the mold 3 secs Total cycle 160 secs Example 2 Using the apparatus described in Example 1, 1.8 liters of precompressed polyethylene foam particles which had a bulk density of 0.0136 g/cm3, a diameter of 8 mm and a gel fraction of 60% by weight were introduced under a back-pressure (A) of 2.2 bars (causing the particles to occupy about 60% of their original bulk volume prior to compression).

After releasing the pressure, the chambers were flushed with steam and a steam pressure (B) of 2.8 bars was built up. This pressure was maintained for 5 seconds. After releasing the pressure, the mold was cooled by passing air through the chambers. The molding had a well-welded surface. In a fracture test, more than 90% of the fracture occurs through the cells.

COMPARATIVE EXPERIMENTS Using a box mold of 4.1 liters capacity, 9.0 liters of polyethylene foam beads having a bulk density of 0.013 g/cm3 and a diameter of 6 mm were filled into the mold under a back-pressure of 2.0 bars in the mold, and a filling pressure of 2.6 bars; these conditions were the same in each experiment.

In the first experiment, the welding of the particles was effected with the aid of steam and in the second experiment with the aid of hot air. The times (in seconds) required for the individual steps are made up as follows: Comparative experiments 1 2 steam hot air (350"C) max. 2.6 bars mold wall tempera ture about 145"C Closing the mold 5 5 Pressure build-up with air 2 2 (2.0 bars) Filling the mold 20 20 Releasing the pressure 3 from the mold Flushing the mold chambers with steam or pre-heating 5 400 the mold chambers with air Steam pressure build-up or passage of hot air (2.6 bars) 12 60 Pressure release 20 20 Cooling with water 90 180 Waiting time 10 10 Opening the mold 3 3 Total time in secs 170 700 When using steam, the foam moldings obtained show very good fusion at all points, and are true to the mold. When using hot air, inadequate welding of the foam particles is observed along the edges of the molding, so that the foam particles crumble away when the exterior of the molding is touched.

The reduced heating time and cooling time when using steam is technically and economically very advantageous.

WHAT WE CLAIMS IS: 1. A process for the manufacture of a foam molding from an olefin polymer by

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The total working cycle was made up as follows: Closing the mold 3 secs Pressure build-up in the mold chambers 3 secs Filling 12 secs Releasing the pressure 4 secs Flushing the mold chambers 10 secs Steam pressure build-up 4 secs Releasing the pressure 1 sec Cooling with water 120 secs Opening the mold 3 secs Total cycle 160 secs Example 2 Using the apparatus described in Example 1, 1.8 liters of precompressed polyethylene foam particles which had a bulk density of 0.0136 g/cm3, a diameter of 8 mm and a gel fraction of 60% by weight were introduced under a back-pressure (A) of 2.2 bars (causing the particles to occupy about 60% of their original bulk volume prior to compression). After releasing the pressure, the chambers were flushed with steam and a steam pressure (B) of 2.8 bars was built up. This pressure was maintained for 5 seconds. After releasing the pressure, the mold was cooled by passing air through the chambers. The molding had a well-welded surface. In a fracture test, more than 90% of the fracture occurs through the cells. COMPARATIVE EXPERIMENTS Using a box mold of 4.1 liters capacity, 9.0 liters of polyethylene foam beads having a bulk density of 0.013 g/cm3 and a diameter of 6 mm were filled into the mold under a back-pressure of 2.0 bars in the mold, and a filling pressure of 2.6 bars; these conditions were the same in each experiment. In the first experiment, the welding of the particles was effected with the aid of steam and in the second experiment with the aid of hot air. The times (in seconds) required for the individual steps are made up as follows: Comparative experiments 1 2 steam hot air (350"C) max. 2.6 bars mold wall tempera ture about 145"C Closing the mold 5 5 Pressure build-up with air 2 2 (2.0 bars) Filling the mold 20 20 Releasing the pressure 3 from the mold Flushing the mold chambers with steam or pre-heating 5 400 the mold chambers with air Steam pressure build-up or passage of hot air (2.6 bars) 12 60 Pressure release 20 20 Cooling with water 90 180 Waiting time 10 10 Opening the mold 3 3 Total time in secs 170 700 When using steam, the foam moldings obtained show very good fusion at all points, and are true to the mold. When using hot air, inadequate welding of the foam particles is observed along the edges of the molding, so that the foam particles crumble away when the exterior of the molding is touched. The reduced heating time and cooling time when using steam is technically and economically very advantageous. WHAT WE CLAIMS IS:

1. A process for the manufacture of a foam molding from an olefin polymer by

compressing foam particles of the olefin polymer with gas, filling the compressed particles into a closed mold at a pressure such that the compressed particles occupy from 20 to 80% of their original bulk volume, releasing the gas pressure in the interior of the mold, and heating and welding the particles, wherein, after release of the gas pressure, steam is introduced to heat the particles, the steam being introduced in an amount to provide a steam pressure which is equal to or greater than the effective pressure in the mold during introduction of the foam particles, and the steam pressure in the interior of the mold is then released to cause the particles to expand and weld together.

2. A process as claimed in claim 1, wherein polyethylene, polypropylene or a copolymer of ethylene with up to 50% by weight of one or more other ethylenically unsaturated monomers is employed as olefin polymer.

3. A process as claimed in claim 1, wherein the olefin polymer is polyethylene.

4. A process as claimed in any of claims 1 to 3, wherein the olefin polymer used is partially crosslinked and has a gel fraction of from 20 to 80% by weight.

5. A process as claimed in any of claims 1 to 4, wherein the steam is introduced at from 112 to 145"C.

6. A process as claimed in any of claims 1 to 5, wherein the steam is introduced over a period of less than 20 seconds and the steam pressure is maintained for from 0.5 to 5 seconds prior to release.

7. A process as claimed in any of claims 1 to 6, wherein the effective pressure in the mold during introduction of the foam particles is from 1.5 to 3.6 bars and the steam pressure is not more than 10 bars.

8. A process for the manufacture of a foam molding carried out substantially as described in either of the fore-going Examples 1 and 2.

9. Foam moldings of olefin polymers when manufactured by a process as claimed in any of claims 1 to 8.