Process for the preparation of expanded polyvinylarene...

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

2001-06-28

2002-09-24

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, C264S051000, C264S053000, C264SDIG009

Reexamination Certificate

active

06455599

ABSTRACT:

The present invention relates to a process for the preparation of expanded polyvinylarene particles and to a process for the preparation of foamed articles from these expanded particles.
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% by weight of such liquid hydrocarbon, such as butane, n-pentane or mixtures of pentanes. Also halogenated hydrocarbons have been used for this purpose. These particles can then be expanded to particles with a reduced density. Apparent densities for packaging particles typically are 20 to 60 g/l. 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 1080 kg/m
3
corresponds to an apparent density of around 720 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, e.g. 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.
The present invention intends to eradicate the above drawbacks by providing a process for the preparation of expanded polyvinylarene particles in which polyvinylarene particles pre-expanded to an apparent density do ranging between 600 and 200 kg/m
3
are impregnated by an inorganic gas, and the impregnated particles thus obtained are expanded to an apparent density of at least three times lower than d
o
.
The advantages of the present invention are enormous. At the facilities of the polyvinylarene manufacturer, unexpanded polyvinylarene polymer particles containing a blowing agent are prepared and the latter particles are pre-expanded to a reduced apparent density in the range from 600 to 200 g/l. If this density reduction is obtained by the use of a hydrocarbon blowing agent, the emitted blowing agent can be collected and handled further, (e.g. re-used) in the plant of the polyvinyl-arene manufacturer.
Secondly, the polyvinylarene manufacturer has complete control over the product quality in the period between production and use by the customer. Using the process of the present invention, the polyvinylarene manufacturer will be in a position to extract at least the blowing agent from the voids. If he has done so, there will be no loss of blowing agent during transport and/or storage of the pre-expanded particles. This will ensure that the customer will always obtain pre-expanded particles with a consistent cell structure and with consistent foaming properties.
Because the pre-expansion has been conducted such that the apparent density d
o
ranges from 600 to 200 kg/m
3
, the volume increase compared to the volume of the original unexpanded particle is from about 1.5 to less than 3 times. This increase is so small that the transportation costs do not become such that they outweigh the advantages. On the other hand, the volume increase does ensure that pores are present in the pre-expanded particle having such a size that the customer can impregnate a reasonable amount of inorganic gas into them, in order to render the particles sufficiently expandable.
The pre-expanded polyvinylarene particles of the present invention have an apparent density do of 600-200 kg/m
3
. Preferably, the apparent density do ranges from 530 to 250 kg/m
3
, more preferably from 500 to 300 kg/m
3
and most preferably from 450 to 350 kg/m
3
.
The pre-expanded particles can be obtained by pre-expansion of any unexpanded polyvinylarene particle containing a sufficient amount of blowing agent to arrive at the apparent densities as defined. The unexpanded polyvinylarene particles may be prepared by various methods, including solution polymerisation, bulk polymerisation, suspension polymerisation or mixtures of these methods. The blowing agent may be added after the polymerisation, as described in U.S. Pat. No. 2,681,321. It is also possible to add the blowing agent during the polymerisation of the vinylarene monomers or to add it before the polymerisation to the monomers. Preferably, the blowing agent is added during the polymerisation of the vinylarene monomers, the polymerisation advantageously being conducted in suspension. The unexpanded polyvinylarene particles suitably have an average particle size of 0.2-3 mm.
Suitable particles for use in the present invention are e.g. described in U.S. Pat. No. 3,973,884, which describes polymer particles with a relatively high density. These high density polymer particles are obtained by pre-expansion of unexpanded polymer particles containing 5.8-7.0% by weight pentane. From GB patent No. 1 106 143 and PCT application No. WO 98/01489 it is known that water may also be used as blowing agent. Apart from hydrocarbons, e.g. C
2
-C
6
hydrocarbons or halogenated hydrocarbons, or water, other blowing agents may be used as well. Examples are inorganic blowing agents (carbon dioxide) or so-called chemical blowing agents, i.e. compounds that release gaseous components upon heating. The latter compounds are usually solid and are particularly used in bulk polymerisation techniques. Examples of chemical blowing agents are carbon dioxide-or nitrogen-liberating solid compounds like azodicarbonamide.
U.S. Pat. No. 3,973,884, mentioned in the previous paragraph, specifically describes that for the production of particles for higher-density applications it is possible to use less of pentane or other blowing agent than has been used in particles for low-density applications. However, it continues by saying that the difference is small and that in particles for low-density applications the pentane content may be 6-7.2% wt, which is hardly more than the 5.8 to 7.0% by weight pentane content of the particles for high-density applications.
It has now been found that smaller amounts of hydrocarbon blowing agent also enable a density reduction to a level of between 600 and 200 kg/m
3
. The resulting pre-expanded particles are extra advantageous in view of environmental and safety concerns during transport and storage. Therefore, it is preferred to prepare the pre-expanded

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