Process for the preparation of polymer particles

Details Process for the preparation of polymer particles Process for the preparation of polymer particles

1998-12-30

2001-06-05

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S088000, C526S240000, C526S287000, C526S318600, C526S347000, C524S457000, C524S458000, C524S817000, C524S819000, C521S056000, C521S146000, C521S147000

Reexamination Certificate

active

06242540

ABSTRACT:

The present invention relates to a process for the preparation of polymer particles containing a polymer of a vinylarene monomer and a physical foaming agent, and to such polymer particles and foamed articles.
Particles that contain such a polymer and foaming agent are generally known as expandable polymer particles. A well-known type of expandable polymer particles is expandable polystyrene. Expandable polystyrene is produced on a commercial scale by suspension polymerisation. The foaming agent is usually a low-boiling hydrocarbon, such as a C
3
-C
8
hydrocarbon, in particular pentane isomers. The expandable polystyrene is used for making foamed articles that are produced by expanding the polystyrene particles. In the expansion process the hydrocarbon foaming agent is released and may be emitted into the environment. Such emissions are regarded undesirable and ways are sought to avoid such emissions. One way is to recover or burn the emitted hydrocarbon. Another way is to reduce the amount of hydrocarbon foaming agent in the expandable polymer particles.
In U.S. Pat. No. 5,096,931 expandable polystyrene is described which contains polystyrene, a small amount of a polar polymer, some water and a reduced amount of hydrocarbon foaming agent. Although the content of hydrocarbon foaming agent has been reduced such agent must still be present to achieve satisfactory expansion.
GB-A-1,106,143 discloses a process for preparing water-expandable polystyrene particles by mixing by vigorous mechanical agitation styrene monomer, water and an emulsifier with a free-radical initiator to obtain an emulsion containing small droplets of water. Subsequently, the emulsion is suspended in an aqueous phase and the suspension obtained is subjected to polymerisation. In order to achieve a satisfactory excansion certain amounts of organic foaming agents are included.
In experiments to verify the merits of the teaching of the above patent GB-A-1,106,143 it was found that the finely dispersed water droplets obtained in the first emulsion tend to coalesce and form bigger droplets during polymerisation. In an experiment in GB-A-1,106,143 it is confirmed that droplets bigger than 40 &mgr;m cause unsatisfactory foamed articles after expansion. Vigorous agitation is apparently necessary in this known process to create and maintain the finely dispersed water droplets. However, it is awkward to stir in commercial operation at such high energy inout.
Hence, it would be desirable if the tendency of the water droplets to coalesce could be reduced.
Surprisingly, it was found that the tendency for the water droplets to grow could be reduced by creating a viscous water-containing emulsion before completely polymerising the vinylarene monomer in suspension polymerisation. This makes it possible to stir less vigorously.
Accordingly, the present invention relates to a process in which a viscous water-containing emulsion of vinylarene monomer is pre-polymerised before being suspended in an aqueous medium. The present invention relates to a process in which the emulsifier is prepared in-situ. The patent case claiming priority of European application No. 96201904.8 (WO 96/10547) relates to a process in which emulsifier is added.
The present invention provides a process for the preparation of polymer particles containing a vinylarene polymer by suspension pohymerisation, which process comprises:
a) preparing a pre-polymerised mass based on a vinylarene monomer, a copolymerisable compound containing a polar moiety and a vinyl moIety and water emulsified therein, by reacting the vinylarene monomer together with the copolymerisable compound such that the vinylarene monomer is pre-polymerised to a conversion degree of 20 to 70%;
b) suspending the pre-polymerised mass in an aqueous medium to yield suspended droplets, and
c) polymerising the vinviarene monomer in the suspended droplets to complete monomer conversion to yield suspended polymer particles.
The present invention has the advantage that the in-situ prepared emulsifier has been found to be very effective. This makes that a substantial amount of water can be incorporated in the beads.
The present process is capable of yielding polymer particles with satisfactory expandability properties that do not contain an organic foaming agent. The process is therefore preferably conducted in the substantial absence of C
3
-C
6
hydrocarbon foaming agent. In the substantial absence means in an amount less than 0.5% wt based on the amount of vinylarene monomer, preferably less than 0.25% wt, more preferably in the complete absence of such foaming agents.
In a further embodiment of the invention the polymer particles obtained are separated from the aqueous mixture and, optionally, expanded to yield pre-expanded particles which are optionally treated further to yield foamed articles. The present invention furthermore relates to expandable polymer particles based on a vinylarene monomer and 0.001 to 5% wt, based on the amount of vinylarene monomer, of a copolymerisable compound containing a polar moiety and a vinyl moiety, which polymer particles further contain 1-20% wt based on he weight of vinylarene monomer, of water emulsified therein.
The present invention also relates to foamed articles based on a polymer of a vinvlarene monomer and 0.001 to 5% wt, based on the amount of vinyl arene monomer, of a copolymerisable compound containing a polar moiety and a vinyl moiety.
The creating of the viscous, pre-polymerised mass is preferably carried out by bulk polymerisation of the vinylarene monomer to the desired degree. The emulsifier can be prepared in-situ in various ways. Preferably, the vinylarene, copolymerisable compound and water are mixed together, and subsequently subjected to prepolymerisation.
The copolymerisable compound can be selected from any compound which combines the two functionalities: a polar moiety and a vinyl moiety. Suitable compounds are styrene compounds with a polar moiety. The polar moiety is preferably derived from an acidic moiety, more particularly from an inorganic acidic moiety. Suitable examples include acrylic acid or salts thereof. Other preferred examples are polar styrene derivatives such as styrene sulphonic acid or a salt thereof.
In order to prepare the emulsifier, the mixture of copolymerisable compound and vinylarene compound preferably contains a phase transfer catalyst which enables copolymerisation. The phase transfer catalyst can be selected from a wide range of known compounds. For anionic polar moieties, suitable phase transfer catalysts are quaternary ammonium and/or phosphonium compounds. Preferable compounds include tetra-alkyl (e.g. with 1 to 15 carbon atoms) ammonium or phosphonium halides, such as tetra butyl ammonium bromide, tetra methyl ammonium chloride or trioctyl methyl ammonium chloride.
The emulsification can be achieved easily by stirring of the water and the other components of the emulsifying mixture. Suitable stirring is already carried out at an energy input equivalent to or less than 500 rotations per minute for a 70 l reactor, even at an energy input equivalent to or less than 350 rotations per minute for a 70 l reactor.
The amount of copolymerisable compound to be used is to some extent dependent on the amount of water to be emulsified. Suitably, the amount of copolymerisable compound ranges from 0.001 to 5% wt, based on the amount of vinylarene monomer. Preferred ranges are from 0.05 to 3, more preferred from 0.1 to 1.5% wt.
The amount of water to be emulsified which to some extent determines the desired amount of copolymerisable compound, can be chosen between wide ranges. Suitably the amount of water ranges from 1 to 20% wt, based on the weight of the vinylarene monomer. Well-expandable particles can be obtained when from 3 to 15% wt of water is emulsified. Below 1% wt the expandability may be too low, whereas at very high water contents the particles yield expanded articles that may run the risk of collapsing.
In the water to be emulsified an electrolyte may be included. Suitable electrolytes are alkali and alka

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