Production of expanded polyolefin particles

Details Production of expanded polyolefin particles Production of expanded polyolefin particles

1997-11-24

1999-07-20

Foelak, Morton

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

521 56, 521 59, 521 94, C08J 910, C08J 918

Patent

active

059256869

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for producing expanded polyolefin (EPO) particles by impregnation of polyolefin granules with a volatile blowing agent in suspension under pressure at elevated temperature.
Said process is known per se and is operated on a large industrial scale. EP-A 53 333 and EP-A 123 144 disclose the use of organic compounds such as halogenated hydrocarbons, e.g. dichlorodifluoromethane, or hydrocarbons, e.g. butane, as blowing agents. However, such organic blowing agents are not unproblematical for various reasons, e.g. owing to their low environmental compatibility, their combustibility or their toxicity.
EP-A 113 903 and EP-A 168 954 therefore propose carbon dioxide as blowing agent. However, when CO.sub.2 is used the process has to be carried out under relatively high pressure, which is disadvantageous in many cases.
It is an object of the present invention to provide an alternative volatile blowing agent for EPO production.
We have found that this object is achieved by using ammonia as blowing agent.
The invention accordingly provides a process for producing EPO particles by impregnation of polyolefin granules with at least 1% by weight of ammonia in suspension under pressure at elevated temperature and subsequent decompression.
DE-A 38 00 650 and 39 23 913 describe the production of foams, wherein carbon dioxide or ammonia in liquid form is injected into a molten thermoplastic, e.g. a polyolefin or polystyrene, and the melt is extruded with foaming. This process is a fundamentally different foaming process from that of the present invention. This extrusion-foaming process has the fundamental disadvantage that modified polymers have to be used (EP-A 0 588 321), the space-time yield is low and the bulk density can be varied only within a narrow range. The foam particles obtainable by the extrusion technique also have a narrow processing range and give foam moldings having an unsatisfactory mechanical property profile.
In the process of the invention, the use of ammonia in combination with other blowing agents should not be excluded. Auxiliary blowing agents which can be used are, for example, nitrogen, noble gases, carbon dioxide and, in small amounts, hydrocarbons such as propane, butanes, pentanes, heptanes, octanes, cyclopentane and cyclohexane.
For the purposes of the present invention, polyolefins are to 12, % by weight of ethylene and/or a C.sub.4 -C.sub.10 -.alpha.-olefin, preferably a copolymer of propylene with from 0.5 to 6% by weight of ethylene or with from 0.5 to 15% by weight of 1-butene or a terpolymer of propylene, from 0.5 to 6% by weight of ethylene and from 0.5 to 6% by weight of 1-butene, or weight of a polyolefin elastomer, e.g. an ethylene/propylene block copolymer containing from 30 to 70% by weight of propylene, addition of compatibilizers).
The crystallite melting point (DSC maximum) of the polyolefins listed under a) to e) is generally between 90 and 170.degree. C. Their heat of fusion, determined by the DSC method, is preferably from 20 to 300 J/g, the melt flow index MFI (230.degree. C., 2.16 kp for propylene polymers and 190.degree. C., 2.16 kp for ethylene polymers) in accordance with DIN 53 735 is from 0.1 to 100 g/10 min.
The production of the EPO particles starts out from polyolefin granules which preferably have a mean diameter of from 0.2 to 10 mm, in particular from 0.5 to 5 mm. In a stirred reactor, 100 parts by weight of these granules are dispersed in from 100 to 500 parts by weight of liquid, preferably water, together with from 0.1 to 10 parts by weight, preferably from 1 to 7 parts by weight, of a suspension aid. Examples of suitable suspension aids are tricalcium phosphate, calcium carbonate, siliceous earth and aluminum oxide. The suspension liquid used can also be an alcohol such as ethanol or a mixture of alcohols with water.
The blowing agent is then injected in amounts of at least 1, preferably from 2 to 50, parts by weight, in particular from 10 to 30 parts by weight, based on 100 parts by weight of polymer, and the contents of the

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