Process for the preparation of expandable polyvinylarene...

Details Process for the preparation of expandable polyvinylarene... Process for the preparation of expandable polyvinylarene...

2001-08-22

2003-06-03

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...

C264S050000, C264SDIG009, C521S060000

Reexamination Certificate

active

06573306

ABSTRACT:

The present invention relates to a process for the preparation of expandable polyvinylarene particles and to a process for expanding these particles and to the preparation of foamed articles from the expanded particles thus expanded.
For many years it has been known that particles of polyvinylarenes, such as polystyrene, can be rendered expandable and that the particles thus obtained can be used in the preparation of foamed articles. In this respect reference is made to, e.g., U.S. Pat. No. 2,681,321 which discloses a process in which polystyrene particles are exposed to liquid hydrocarbons and treated such that the liquid hydrocarbon is dispersed in the polystyrene particles. Particles thus prepared contain generally 4 to 8% wt of such liquid hydrocarbon, such as butane, n-pentane or mixtures of pentanes. These particles can then be expanded to beads with a reduced density. Apparent densities for packaging particles typically are 20 to 60 kg/m
3
. Once expanded, the particles are fused in a steam-heated mould to yield a foamed article of a desired shape.
One of the factors that influence the expansion of the polystyrene particles is the amount of hydrocarbon blowing agent. From Kirk Othmer, Encyclopedia of Chemical Technology, third edition, Volume 21, page 838, it can be read that the density of particles containing 5.7% wt n-pentane is typically 1080 kg/m
3
, compared to a value of 1050 kg/m
3
for pure polystyrene beads and compared with a calculated density of 1020 kg/m
3
for a simple mixture in which the n-pentane is dissolved in polystyrene. If all pentane would be in voids the calculated density would be 1120 kg/m
3
. Thus it has been suggested that part of the hydrocarbon blowing agent is present in little voids in the polystyrene. The skilled man will appreciate that the above densities are particle densities, which can be recalculated to apparent densities. A particle density of 1050 kg/m
3
corresponds to an apparent density of around 680 kg/m
3
.
A drawback of the present practice is that during the transport and storage of the unexpanded particles hydrocarbons may evaporate from the particles, in particular from the voids. When the particles are transported and/or stored at varying temperatures and/or duration, the amounts of e.g. pentane retained may vary significantly. Apart from extra safety measures that have to be taken during transport, like gas-tight packaging, it will be appreciated that such a variation may have an effect on the resulting foam obtained after expansion.
Furthermore, the expansion process itself also causes that hydrocarbons originally present in the unexpanded particles are emitted into the environment. In order to reduce the emissions, complicated equipment has been developed to collect the emitted hydrocarbons for further handling, i.e. combustion. This equipment is to be installed in the facilities of the end-user of the particles, i.e. the customer who produces the foamed articles. This requires additional expertise and investments with these customers.
U.S. Pat. No. 5,358,675 discloses a process in which a polymeric material, e.g. polystyrene, is combined with another second phase material, e.g. a rubber, in an extruder at elevated temperature. The mixture is impregnated with nitrogen at pressures ranging from 800 up to 2000 psi (5,500-13,800 kPa). The presence of the rubber is necessary to obtain a suitable cell size above 10 &mgr;m. If no rubber is added the cells obtained are too small. Although the process of this document overcomes the environmental problem, it requires excessive pressures. It has now been found that particles of polyvinylarene can be expanded sufficiently by avoiding these high pressures.
Accordingly, the present invention provides a process for the preparation of expandable polyvinylarene particles in which compact polyvinylarene particles are impregnated by an inorganic N
2
- and/or O
2
-containing gas at a temperature below 95° C. and at a pressure of 100 to 2,000 kPa gauge.
Compact polyvinylarene particles suitably have an apparent density of more than 600 kg/m
3
, more suitably more than 620 kg/m
3
. Generally, the apparent density of these particles will not exceed 700 kg/m
3
.
Typically, the compact polyvinylarene particles of the present invention are not expandable before they are impregnated with an N
2
- and/or O
2
-containing gas. In practice, this means that they contain less than 0.5% by weight, based on the amount of vinylarene, of volatile organic compounds, more preferably less than 0.1% by weight. Most preferably, the compact polyvinylarene particles do not contain any volatile organic compound at all. The amount of water in the compact polyvinylarene particles will suitably not exceed 1% by weight, based on the amount of vinylarene.
The compact polyvinylarene particles may be prepared by various methods, including bulk polymerization, solution polymerisation, and suspension polymerization or mixtures of these methods. Preferably the polymerization is carried out in a bulk polymerization or suspension polymerization process. In bulk techniques, the polymerisation is carried out in a conventional way to obtain small particles (nibs). Suspension polymerisation includes the suspending of vinylarene monomer in an aqueous phase and polymerising the suspended droplets. Both methods ensure that sufficient voids are present in the polymer particles into which the inorganic N
2
- and/or O
2
-containing gas can be impregnated.
Suspension polymerisation is suitably carried out in the presence of suspension stabilisers. Suitable suspension stabilisers are well known in the art and comprise poly(vinyl alcohol), gelatine, agar, polyvinyl pyrrolidine, polyacrylamide, inorganic stabilisers such as alumina, bentonite, magnesium silicate or phosphates, like tricalciumphosphate and/or disodiumhydrogen phosphate, optionally in combination with any of the stabilising compounds mentioned earlier. The amount of stabiliser may suitably vary from 0.1 to 0.9% wt, based on the weight of the aqueous phase.
The suspension polymerisation is suitably carried out at two temperature stages, in which the temperature in the first stage is from 85 to 110° C. and in the second stage is from 115 to 140° C.
The polymerization per se is well known in the art. It may be initiated thermally, via free-radical polymerization or via anionic polymerisation. Although all methods are equally possible, preference is given to free-radical polymerization. Suitable free-radical initiators can be selected from conventional initiators for free-radical polymerization. They include in particular organic peroxy compounds, such as peroxides, peroxy carbonates and peresters. Typical examples of these peroxy compounds are C
6-20
acyl peroxides, such as decanoyl peroxide, benzoyl peroxide, octanoyl peroxide, stearyl peroxide, peresters, such as t-butyl perbenzoate, t-butyl peracetate, t-butyl perisobutyrate, t-butyl-peroxy-(2-ethylhexyl)carbonate, hydroperoxides and dihydrocarbyl peroxides, such as those containing C
3-10
hydrocarbyl moieties, including di-isopropyl benzene hydroperoxide, di-t-butyl peroxide, dicumyl peroxide or combinations thereof. Other initiators different from peroxy compounds are also possible, e.g., &agr;,&agr;′-azobisisobutyronitrile.
The vinylarene comprised in the polymer of the present process consists preferably mainly of styrene. The polyvinylarene may contain up to 10% mole of another vinyl group containing monomer, such as acrylonitril, acrylic or methacrylic acid or esters, substituted styrene, such as chlorostyrene, or &agr;-methylstyrene, or divinylbenzene. However, preferably the vinylarene in the polyvinylarene consists for more than 99% mole of styrene. More preferably, the polyvinylarene is pure polystyrene.
It may be advantageous to polymerise the vinylarene monomers in the presence of other polymers such as polyphenylene oxide. Suitable polyphenylene oxides have been described in EP-A-350137, EP-A-403023 and EP-A-391499. The polyphenylene oxide is preferably present in an amount of between 1 and 30% wt,

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