Preparation of polymer particles

Details Preparation of polymer particles Preparation of polymer particles

2001-10-09

2003-05-06

Niland, Patrick D. (Department: 1714)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S458000, C524S460000, C526S080000, C526S087000

Reexamination Certificate

active

06559217

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is the national phase of International (PCT) Patent Application Serial No. PCT/GB00/01335, filed Apr. 10, 2000, published under PCT Article 21(2) in English, which claims priority to and the benefit of United Kingdom Patent Application No. 9908164.8, filed Apr. 9, 1999, and United Kingdom Patent Application No. 0007006.0, filed Mar. 22, 2000.
This invention relates to improvements in the production of particulate polymers, in particular polystyrene particles.
The process of the invention is especially suitable for the production of EPS beads, but the processes may be used for the preparation of any particulate polymer producible by suspension polymerization, in particular styrenic homo- and copolymers and vinyl homo- and copolymers. Examples of appropriate monomers include vinyl aliphatic monomers such as esters of acrylic and methacrylic acids, acrylonitrile, and vinyl aromatic monomers such as styrene and substituted styrene.
Millions of tonnes of expandable polystyrene (EPS) are produced each year. The EPS is produced in bead form and is used for the manufacture of a wide range of products ranging for example from thin-walled cups to packaging materials to large blocks used in construction.
The different end uses require EPS beads of different sizes, typically as follows:
200-600 &mgr;m—Cups and trays
400-800 &mgr;m—Thin-walled packaging materials
600-1100 &mgr;m—Normal packaging materials
900-2000 &mgr;m—Insulation boards and block materials.
EPS beads falling outside these target size ranges are typically considered to be “off-specification” material and command substantially lower prices.
EPS is normally produced by a suspension polymerization process in which styrene is polymerized in the presence of a free radical generator. The polystyrene (PS) beads produced in this way are impregnated with a blowing agent, typically a C
1-4
hydrocarbon such as pentane, to produce the expandable. polystyrene (EPS) beads.
Suspension polymerization however produces PS beads with a broad size distribution and the PS or EPS beads must be size-classified, i.e. separated into fractions having the appropriate particle size range for the desired end use.
Although variation of the suspension polymerization conditions may allow the EPS producer to optimize the fraction of polymer beads having the particular desired size, e.g. for thin-walled packaging materials, a large proportion of the total bead yield will still be in less desired grades or will be of off-specification sizes.
Sekisui, in GB-A-1416405, describe a process in which the suspension polymerization of styrene is effected in the presence of polystyrene seeds of a size smaller than the desired mode size of the end product.
Typically the Sekisui process may be performed as a two-stage suspension polymerization. The first stage is terminated when relatively small beads, e.g. having a mode size of about 900 &mgr;m, have been formed. The small beads are then graded to remove overly small and overly large fractions leaving PS seeds, e.g. having sizes of 400 to 1800 &mgr;m, and the seeds are then used in the second of the suspension polymerization stages to yield PS beads having a narrower size distribution about the desired particle size than is achieved in the conventional suspension polymerization. Even this process however yields a product which has a broad particle size distribution and the complexity of the grading procedure is increased since it must be carried out on the smaller particles produced in the first polymerization stage.
The grades of EPS produced by the conventional and Sekisui processes contain a range of particle sizes, e.g. particle diameters differing by several hundreds of micrometers. This has a deleterious effect on the processing of EPS beads into expanded polystyrene products.
In expanded polystyrene product formation, EPS beads are first pre-expanded to produce free flowing expanded particles, typically using steam at a temperature below 100° C., and then tempered in a silo through which air is passed, before being filled into a mould and steamed, typically at 110 to 120° C., to complete expansion and fusion of the particles.
The tempering period is required for the pre-expanded particles to develop the necessary resilience for the moulding process.
The particle size distribution in the EPI beads is not simply replicated in the pre-expanded beads, instead the size distribution is broadened and a broad density distribution is created as under the same conditions the smaller EPS beads will expand less and the larger beads more leading to the tempering silo containing a mixture which includes small high density particles and larger low density particles. Settling out of the smaller higher density particles occurs in the tempering silo resulting in non-uniformity of the pre-expanded particle mixture fed from the silo to the moulds and accordingly to variations in the final densities and strengths of the moulded products. Thus the broad particle size distribution for the graded EPS beads and hence of the pre-expanded particles leads to difficulties in production of expanded polystyrene products and makes the product quality inconsistent and difficult to control.
We have found that it is possible to produce substantially monodisperse (i.e. single sized) polymeric seed particles which can be used in suspension polymerization to generate larger but still substantially monodisperse polymer particles, for example substantially monodisperse EPS particles with a size suitable for one of EPS's typical end uses. This process for polymer particle production is described in WO99/19375. The disclosures of WO99/19375 are incorporated herein by reference.
Since the product is substantially monodisperse, mechanical grading is not required thereby leading to considerable savings in terms of production equipment and process duration. Moreover undesired grades are not produced thereby leading to a considerable increase in effective yield and a reduction in waste. Furthermore the problems associated with a broad particle size range within a graded EPS product are avoided or reduced.
The particle size expansion cycles in the polystyrene particle production process of WO99/19375 are relatively time consuming and we have now found that the process may be accelerated if in at least one expansion cycle the seeds are pre-swelled in a process somewhat similar to the Ugelstad polymerization process described in EP-B-3905 (Sintef) and U.S. Pat. No. 4,530,956 (Ugelstad) the disclosures of which are hereby incorporated by reference.
Polymer beads may be produced by diffusing a monomer and a polymerization initiator (or catalyst) into polymer seeds in an aqueous dispersion. The seeds swell and following initiation of polymerization, e.g. by heating to activate the initiator, larger polymer particles are produced. The maximum volume increase due to swelling and polymerization is normally about ×5 or less. The late Professor John Ugelstad found that the capacity of the seeds to swell could be increased to a volume increase of ×125 or even more if an organic compound with relatively low molecular weight and low water solubility is diffused into the seeds before the bulk of the monomer is used to swell the seeds. The effect is based on entropy rather than particularly on the chemical nature of the organic compound.
Conveniently the polymerization initiator may be used for this purpose. Organic solvents, e.g. acetone or a portion of the monomer, may be used to enhance diffusion of the organic compound into the seeds. This “Ugelstad polymerization process”, which is described for example in EP-B-3905 (Sintef) and U.S. Pat. No. 4,530,956 (Ugelstad), may be used to produce monodisperse particles, If necessary carrying out several swelling and polymerization stages to reach the desired particle size.
In a simplified version of the Ugelstad process the enhanced capacity for swelling may be achieved simply by the use of oligomeric seed particles, e.g. where the oligomer weight ave

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