Method of production of particulate polymers

Details Method of production of particulate polymers Method of production of particulate polymers

1998-05-05

2001-05-29

Teskin, Fred (Department: 1713)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C526S346000, C526S909000, C523S205000, C521S056000, C521S146000

Reexamination Certificate

active

06239224

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to improvements in the production of particulate polymers, in particular expandable polystyrene (EPS).
BACKGROUND OF THE INVENTION
Millions of tonnes of expandable polystyrene 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
3-6
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 EPS 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 now 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.
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.
SUMMARY OF THE INVENTION
Viewed from one aspect the invention thus provides the use of substantially monodisperse polymeric particles as polymerization seeds in a suspension polymerization preparation of a particulate polymer having a mode particle size of at least 200 &mgr;m, preferably at least 300 &mgr;m.
Viewed from a further aspect the invention provides a process for the preparation of polymer particles of a first mode size which is at least 200 &mgr;m, preferably at least 300 &mgr;m, wherein a suspension polymerization is effected using a suspension containing polymer seed of a second mode size which is smaller than said first mode size, characterized in that as said polymer seed is used a substantially monodisperse particulate polymer.
Viewed from a further aspect the invention provides a particulate polymer material, e.g. a suspension polymerization product, having a mode particle size of between 60 and 3000 &mgr;m, especially between 100 and 2500 &mgr;m, in particular between 150 and 2000 &mgr;m, more particularly 200 to 1500 &mgr;m, e.g. 250 to 1000 &mgr;m and more especially 300 to 700 &mgr;m, and a CV of less than 12% (preferably no more than 10% and more preferably no more than 5%), with the proviso that where the mode particle size is above 700 &mgr;m the CV is below 10% (and more preferably below 7.5%), and optionally with the proviso that where the mode particle size is above 200 &mgr;m the polymer material is a styrenic material, e.g. a polystyrene.
Viewed from a yet further aspect the invention provides a process for the preparation of polymer particles of a first mode size which is in the range 200 to 300 &mgr;m, which process comprises:
(a) obtaining a first substantially monodisperse particulate polymer seed material having a mode size of no more than 100 &mgr;m, preferably no more than 60 &mgr;m, more preferably no more than 50 &mgr;m;
(b) using said first seed material, performing a suspension polymerization to yield a second substantially monodisperse particulate polymer seed material having a mode size greater than that of said first seed material and not more than 150 &mgr;m, preferably not more than 100 &mgr;m and more preferably not more than 10 &mgr;m;
(c) using said second seed material, performing a suspension polymerization to yield a third substantially monodisperse particulate polymer seed material having a mode size greater than that of said second seed material and not more than 1000 &mgr;m;
(d) using said third seed material or a seed material derived therefrom, performing a suspension polymerization to yield polymer particles of said first mode size; and
(e) optionally impregnating and/or heat treating and/or surface modifying the particulate product of step (d).
In this process, one or more of steps (b), (c) and (d) may, but preferably do not, involve removal of over- or under-sized particles from t

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