Porous polymer particles

Details Porous polymer particles Porous polymer particles

2001-06-28

2003-03-25

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...

C521S060000, C264SDIG009

Reexamination Certificate

active

06538042

ABSTRACT:

The present invention relates to porous polymer particles and a process for their preparation. In particular, the invention relates to porous polyvinylarene particles, which can be expanded to foamed articles.
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 blowing agent, 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 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 the volatile organic blowing agent 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 organic blowing agents 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 blowing agent 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.
It has therefore become an objective of this invention to prepare polyvinylarene particles that can be expanded but do not have the safety, environmental and foam-related problems.
From GB patent No. 1,106,143 and PCT application No. WO 98/01489 expandable particles are known in which water is used as blowing agent. Although such particles circumvent some of the problems mentioned, one of the drawbacks of such particles is that water is less volatile than most conventional blowing agents. Therefore, specific measures need to be taken to stimulate the evaporation of water in order to arrive at a suitable expandability. In GB patent No. 1,106,143 specific reference is made to the fact that an additional volatile organic blowing agent needs to be added to obtain a satisfactory expandability.
So the inventive concept came up to prepare high-density expandable polyvinylarene particles, which could then be expanded using an innocuous blowing agent. However, the known high-density particles still contain a relatively high amount of volatile organic blowing agent. In this respect reference is made to U.S. Pat. No. 3,973,884 describing polymer beads with a relatively high density. The patent specifically describes that for the production of polystyrene 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, the patent continues that the difference is small and that in particles for low-density applications the pentane content may be 6-7.2% by weight, whereas for high-density applications polystyrene particles contain 5.8 to 7.0% by weight pentane. When these particles are expanded to relatively high densities, only a small amount of pentane will be needed for the expansion. Consequently, the resulting high density particles will still contain about 4.3 to 5.5% by weight pentane.
The current invention now provides a particle that avoids all drawbacks mentioned above. Accordingly, the present invention provides porous polyvinylarene particles having an apparent density d
o
of 600 to 200 kg/m
3
, which particles contain a nucleating agent and 2.0% by weight or less, based on the amount of polyvinylarene, of a volatile organic blowing agent.
It will be evident that these particles have eradicated all above-mentioned problems. Because the amount of volatile organic blowing agent is 2.0% by weight or less, the majority, if not all, thereof is dissolved in the polymer matrix, so that it does not readily evaporates.
The porous polyvinylarene particles according to the present invention are typically pre-expanded particles. That entails the advantage that the particle already has a certain pore (cell) structure. Because the pre-expansion has been conducted such that the apparent density ranges from 600 to 200 kg/m
3
, the pores are relatively small and the volume increase compared with the unexpanded particle is very small so that the transportation costs have not become prohibitive. Suitably, the apparent density d
o
ranges from 530 to 250 kg/m
3
. Typically, this means that the volume of the pre-expanded particle may have increased by from about 1.5 to less than 3 times compared to the volume of the original unexpanded particle. This way, the volume increase has not become such that the transportation costs outweigh the advantages. Most suitably, the apparent density d
o
ranges from 450 to 350 kg/m
3
.
The pores are suitably such that the average pore size is from 5 to 100 &mgr;m, preferably from 5 to 60, most preferably 10 to 40 &mgr;m. The average pore size is measured by cutting the particles half way through and imaging the samples with a JEOL JSM T220A Scanning Electron Microscope, using 10 keV beam energy, 8-38 mm continuously working distance, secondary electron imaging, and 5 nm resolution (JEOL is a trademark). The presence of a certain pore structure in the particles of the present invention allows them to be impregnated by an innocuous blowing agent.
The particles of the present invention contain a nucleating agent, which is a compound that promotes the formation of cells. Nucleating agents suitably exist as a separate phase in the polystyrene/blowing agent
ucleating agent system at the glass transition temperature of the polystyrene/blowing agent mixture. Advantageously, they are homogeneously distributed throughout the polyvinylarene matrix. Preferably, a nucleating agent has an average particle size from 100 nm to 10 &mgr;m, as measured by electronic microscopy, more preferably from 120 nm to 5 &mgr;m, most preferably from 140 nm to 1 &mgr;m. Nucleating agents are suitably used in an amount of 0.01 to 3% by weight, based on vinylarene, preferably in an amount of 0.05 to 2% by weight.
Examples of nucleating agents are finely dispersed inorganic compounds, organic compounds and polymer particles. Examples are carbonate compounds such as calcium carbonate, sulphate compounds such as barium sulphate and calcium sulphate, silicate compounds such as

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