Method for Coating Fine Particulate Expandable Styrene Polymers

The following specific examples and control examples demonstrate the advantageous results obtained with respect to the process of the present invention over the state of the art.

The following dispersions were used:

(A) 325 parts by weight of fully desalted water (or potable water) were heated to 90.degree. C. While stirring vigorously, 175 parts by weight of a mixture of glycerin monostearate and distearate are introduced into the heated water. After stirring for 30 to 60 minutes, a fine-particulate dispersion with a solid content of about 35% by weight is obtained. The dispersion is used at a temperature of 80.degree. C.

(B) A dispersion with 20% by weight of solid content glycerin stearates is prepared under the same conditions, and used at a temperature of 75.degree. C.

(C) 100 Parts by weight of a commercial sorbitol palmitic acid ester are slowly introduced into a 60.degree. C. mixture of 110 parts by weight of fully desalted water and 110 parts by weight of methanol. Without heating but while stirring vigorously, 180 parts by weight of 15.degree. C. water are used to slowly dilute. The solid content of the dispersion is 20% by weight. The dispersion is used at a temperature of 30.degree. C.

(D) In a similar manner, a dispersion with a solid content of 35% by weight is prepared and used at a temperature of 45.degree. C.

(E) 400 Parts by weight of fully desalted water are heated to 90.degree. C. 75 Parts by weight of a commercial mixture of glycerin monostearate and distearate and 25 parts by weight of magnesium stearate are introduced while stirring vigorously. The mixture is allowed to cool to 70.degree. C. while being stirred vigorously. The dispersion is present in the form of a paste with a solid content of 20% by weight and is used at a temperature of 70.degree. C.

Dry, foamable polystyrene beads with grain diameters of

(.alpha.) 0.9 to 2.0 mm

(.beta.) 0.5 to 0.9 mm

are mixed at different times in a Nautamix type mixer having the designation MBXE--28 R and manufactured by Nautamix B. V., Haarlem, Netherlands, at room temperature with 0.1 parts by weight of a commercial mixture of glycerin monostearate and distearate. Upon completion of the mixting time, the proportion of "free" coating materials is determined by sifting.

The same control examples are repeated at temperatures of 20.degree. to 35.degree. C. As shown in Table I, less than 50% of the added coating materials adhere tightly to the bead surface.

Foamable polystyrene beads (0.5 to 0.9 mm in diameter) are provided with 0.3% of the same coating materials in the same mixer as in Example 1, these coating materials being sprayed in the form of a melt onto the beads during the mixing process within 4 to 9 minutes.

Result: about 82% of the coating materials are distributed unevenly on the head surface, about 13% are contained in the product in the form of dust and the losses amount to about 5% (wall deposits and dust losses).

Batch operation is substantially impossible due to frequent nozzle clogging.

Control Example 2 is repeated, except for the variation that a 10% aqueous dispersion is added during the mixing process to the beads present in the mixer. After a 15 minute mixing time, the water is evaporated by means of hot air. The deposition is satisfactorily uniform, as 92% of the bead surface tightly bonds the coating materials and as there is only 5% dust in the product with only slight deposits in the mixer. However, the water evaporation substantially encumbers the discontinuous mixing process, as on one hand the mixing times ought to be short, and on the other the air flows may not be large in order to prevent dusting. Analytic monitoring of the drying process constitutes an additional complexity.

The dispersions A-E are used. These dispersions are aspirated in a vacuum drier filled about 75% with 20 tons of undried, expandable polystyrene beads 0.4 to 3.0 mm in grain diameter. The pressure is 10 to 25 mbars. Mixing proceeds for about 4 to 5 hours at the stated pressure and drying is at 30.degree. C.

The results from the Control Examples 1-3 and specific Examples 1A-E are tabulated in Table II.

Not only do the coating means obtained in the examples adhere better substantially, they are also spread uniformly.

This examples illustrates (Table III) that the defective grain obtained in the coating process of the invention can be used again in the suspension polymerization without thereby incurring adverse effects.