Expandable olefin bead polymers

Details Expandable olefin bead polymers Expandable olefin bead polymers

2000-09-26

2002-09-10

Foelak, Morton (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S056000, C521S059000

Reexamination Certificate

active

06448300

ABSTRACT:

The invention relates to expandable olefin bead polymers which comprise blowing agent and can be expanded to give a moldable foam. Moldable polyolefin foams are increasingly used to produce foam moldings in automotive construction, in packaging and in the leisure sector. However, moldable foams are much more voluminous than unfoamed expandable beads, e.g. those based on polystyrene (EPS beads), and this is disadvantageous during transport and in storage, since the space required is large.
EPS beads can, is as known, be prepared by impregnating polystyrene pellets with a volatile hydrocarbon blowing agent in aqueous suspension, cooling the suspension and isolating the impregnated beads. Since polystyrene has good capabilities for retaining hydrocarbons, these diffuse only very slowly, so that the beads comprising blowing agent can be stored for prolonged periods without loss of blowing agent.
However, in the case of polyolefins this is not readily possible, and unfoamed expandable polyolefin beads have not therefore hitherto been available. The known expanded polypropylene moldable foams (EPP) are produced on an industrial scale by impregnating polypropylene pellets with a volatile blowing agent in aqueous suspension under pressure and reducing the pressure, whereupon the impregnated beads foam. The blowing agents used in practice comprise butane, dichlorodifluoromethane and CO
2
. Since these blowing agents are lost again relatively rapidly by diffusion out of the polypropylene, polypropylene beads comprising blowing agent and prepared in this way have not been regarded as storable.
EP-A 540 271 describes a process for preparing expandable polyblend beads made from polyphenylene ether with a polyolefin. Here, minipellets made from a polyphenylene ether/polyolefin blend are impregnated in aqueous dispersion in a pressure vessel with a halogenated hydrocarbon blowing agent, preferably trichlorofluoromethane, the dispersion is cooled and the expandable beads are isolated. A disadvantage of the process is that it can apparently only be carried out with halogenated hydrocarbons as blowing agent. But these are environmentally hazardous. In addition, when the examples are repeated, very large-cell foams are obtained when the expandable beads are foamed.
It is an object of the present invention to provide unfoamed expandable beads made from olefin polymers, comprising a preferably halogen-free blowing agent and foamable to give small-cell foams. The olefin polymers must not comprise any admixed thermoplastics with a glass transition point above 180° C. (Polyphenylene ethers have a glass transition point at about 200° C.). It is preferable for there to be less than 5% by weight of thermoplastics of other types, and in particular for there to be none at all.
We have found that this object is achieved by expandable olefin bead polymers which may have been blended with up to 50% by weight, preferably less than 5% by weight, of a thermoplastic with a glass transition point below 180° C. and which comprise from 1 to 40% by weight of an organic blowing agent with a boiling point of from −5 to 150° C., based in each case on the weight of the olefin polymer.
The polymers are characterized by a bulk density above 400 g/l and in that they can be foamed to a bulk density below 200 g/l after storage for one hour at room temperature in free contact with the atmosphere by heating above 100° C. During this, the beads comprising blowing agent should melt at least to some extent, so that the foaming process can proceed to completion. In the case of the preferred propylene polymers, the ideal foaming temperature is from 130 to 160° C., preferably 150° C.
The invention further provides a process for preparing these expandable beads, in which polyolefin pellets are impregnated in suspension in a pressure vessel at elevated temperature with from 2 to 50% by weight of a preferably halogen-free, organic blowing agent, the batch is cooled below 100° C., and the impregnated pellets are isolated and washed.
EP-A 778 310 describes a process for preparing moldable polyolefin foams, in which a first step prepares partially foamed beads with a bulk density of from 120 to 400 g/l by extruding polyolefin comprising solid blowing agents, and a second step then uses steam to foam these further.
The novel expandable olefin bead polymers are practically unfoamed beads with a bulk density above 400 g/l, preferably above 500 g/l, and can be foamed to a bulk density below 200 g/l, preferably below 150 g/l and in particular below 100 g/l after storage for one hour, at room temperature in free contact with the atmosphere by heating above 100° C., and in the case of propylene polymers preferably at from 130 to 160° C., in particular at 150° C.
The first condition expresses the fact that the beads practically do not foam when the pressure on the impregnating batch is reduced. The olefin polymer pellets used as starting material have, depending on their composition and the shape of their particles, a bulk density of from 450 to 700 g/l. The second condition expresses the fact that, even after storage for one hour in free contact with the atmosphere, the beads still comprise sufficient stored blowing agent to be capable of good foaming. This storage for one hour at room temperature in free contact with the atmosphere is therefore significant in practice and is also realistic, since practical treatment and handling of the beads comprising blowing agent prior to their packing and after their removal from the packing prior to their foaming does not take more than one hour in total. Very little blowing agent should escape during this period. As the beads have generally been packed in sealed containers or in gas-tight film sacks when they are stored and transported, the amount of blowing agent which can escape during these stages is also insignificant.
The novel polyolefin beads comprising blowing agent can normally be stored for a number of days without escape of any substantial amount of blowing agent. Prolonged storage in the open should, however, be avoided.
For the purposes of the invention olefin polymers are
a) Homopolypropylene,
b) Random copolymers of propylene with from 0.1 to 15% by weight, preferably 0.5 to 12% by weight, of ethylene and/or a C
4
-C
10
-&agr;-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 made from propylene, from 0.5 to 6% by weight of ethylene and from 0.5 to 6% by weight of 1-butene, or
c) A mixture of a) or b) with from 0.1 to 75% by weight, preferably from 3 to 50% by weight, of a polyolefin elastomer, e.g. an ethylene-propylene block copolymer with from 30 to 70% by weight of propylene.
d) Polyethylene (LLDPE, LDPE, MDPE, HDPE) or
e) A mixture of the polyolefins mentioned under a) to d) (if desired with addition of compatibilizers).
Olefin polymers prepared using either Ziegler or metallocene catalysts are suitable.
The crystalline melting point (DSC maximum) of the polyolefins listed under a) to e) is generally from 90 to 170° C., their enthalpy of fusion, determined by DSC, is preferably from 20 to 300 J/g, and the melt index MFR (230° C., 2.16 kp for propylene polymers and 190° C., 2.16 kp for ethylene polymers) is preferably from 0.1 to 100 g/10 min to DIN 53 735.
Preferred polyolefins are homo- or copolymers of propylene with up to 15% by weight of ethylene and/or 1-butene, particularly preferably propylene-ethylene copolymers with from 1 to 5% by weight of ethylene. They have a melting point of from 130 to 160° C. and a density (at room temperature) of about 900 g/l.
The olefin polymer may have been blended with up to 50% of its weight of a thermoplastic of a different type and having a glass transition temperature (point of inflection in DSC curve) below 180° C. Examples of suitable thermoplastics are polyamides in amounts of from 5 to 40% by weight and conventional compatibilizers, e.g. block copolymers, such as Exxelor P 1015 (EXXON), may be added to the mixture here.
It has been foun

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