Prefoaming of poly(meth)acrylimide particles for subsequent foam moulding in closed tools

Abstract

The invention relates to a process for the production of prefoamed poly(meth)acrylimide (P(M)I) particles which can be further processed to give foam mouldings or composites. A feature of this process is that a polymer granulate is first heated and thus prefoamed in an apparatus by means of IR radiation of a wavelength suitable for this purpose. Said granulate can be further processed in subsequent steps, e.g. in a press mould with foaming to give a moulding or a composite workpiece with foam core.

Claims

1. A process for the production of prefoamed poly(meth)acrylimide (P(M)I particles, comprising prefoaming unfoamed P(M)I particles by infrared radiation (IR) with an IR source, at least 80% of which has a wavelength of from 1.4 to 10.0 m, wherein the IR source used emits to an extent of at least 5% in a wavelength range from 5.0 to 9.0 m, and wherein the temperature of the IR source, calculated by Wien's method, is from 780 K to 1800 K.

2. The process according to claim 1, wherein the IR source used emits to an extent of at least 5% in the wavelength range from 5.3 to 6.5 m or from 7.8 to 8.9 m.

3. The process according to claim 1, wherein the temperature of the IR source, calculated by Wien's method, is from 800 to 1200 K.

4. The process according to claim 1, wherein the unfoamed P(M)I particles have a particle size of from 0.5 to 5.0 mm.

5. The process according to claim 1, wherein the prefoaming is carried out within at most 5 min.

6. The process according to claim 1, wherein the unfoamed P(M)I particles are transported in a single layer on a conveyor belt through a heater unit having sources of IR.

7. The process according to claim 1, wherein the unfoamed P(M)I particles are obtained as granulate from a semifinished P(M)I product by grinding.

8. The process according to claim 1, wherein the unfoamed P(M)I particles are a suspension polymer.

9. The process according to claim 1, wherein the maximum size of the prefoamed P(M)I particles is from 1.0 to 25 mm.

10. The process according to claim 1, wherein a bulk density of the prefoamed P(M)I particles is from 60 to 300 kg/m.

11. A process, comprising: prefoaming unfoamed P(M)I particles by infrared radiation (IR) with an IR source, at least 80% of which has a wavelength of from 1.4 to 10.0 m, wherein the IR source used emits to an extent of at least 5% in a wavelength range from 5.0 to 9.0 m, wherein the temperature of the IR source, calculated by Wien's method, is from 780 K to 1800 K, wherein the unfoamed P(M)I particles are transported in a single layer on a conveyor belt through a heater unit having sources of IR, and wherein after transport through the heating unit, the prefoamed P(M)I particles are transported directly into a shaping mould or into a storage container from which material is charged to at least one shaping mould.

Description

WORKING EXAMPLES

(1) Material used as PMI granulate is marketed as PMI foam with the product name ROHACELL RIMA by Evonik Industries. The granulate was produced by means of grinding by a RS3806 chopper mill from Getecha from a polymerized polymer sheet that had not been prefoamed. The maximum diameter of the resultant granulate was 5 mm at the largest point.

Comparative Example 1

Prefoaming by Means of Convection Oven

(2) The ground material that had not been prefoamed, from the mill, had an envelope density of about 1200 kg/m.sup.3 and a bulk density of about 600 to 700 kg/m.sup.3. These two densities are reduced by the prefoaming in an oven. This is achieved by variation of residence time, and also of the temperature. For this, the free-flowing ground material is distributed onto a metal sheet covered with release film. This should be achieved with maximum uniformity and, in order to guarantee homogeneous foaming, the layer thickness should not exceed the largest grain diameter. The sheet is then placed for by way of example 45 min. in the oven that has been preheated to prefoaming temperature.

(3) The bulk density can thus be reduced from about 600-700 kg/m.sup.3to about 360-460 kg/m.sup.3 in 30 minutes at a prefoaming temperature of 175 C.

Inventive Example 1

Prefoaming by Means of IR Chamber

(4) The sources used were from KRELUS Infrared AG, with the following properties:

(5) These are medium-wave metal foil sources with main wavelength 2.5 m (effective up to 9.6 m). 2.5 m here correspond to a temperature of 850 C. calculated by the Wien method. The support is a metal housing, and the metal foils serve as resistance material and are corrugated in order to provide a large emission surface.

(6) In the IR chamber there are sources arranged over the entire upper and lower surface (3*3 modules) with a nominal power rating of (3*3*2.5 kW): 22.5 kW total power rating. The sources have continuously variable control and do not have active cooling. The large-area source is composed as a module with a single-module size of 123248 mm, source height being 65 mm.

(7) The chamber equipped with the sources of IR radiation is operated for 1.5 h with large-area source switched on, with a resultant surface temperature of about 160 C. and a resultant underside temperature of about 135 C. The aim of this is to improve reproducibility of the results with respect to prefoaming that is carried out continuously.

(8) The prefoaming material is then distributed as described above on the preheated carrier, which is placed in the chamber. For the prefoaming process, the upper and lower source field is activated. Radiation sources used comprised a plurality of sources emitting at a wavelength maximum of from 1.4 to 3.0 m. Once the foaming time of 10 min. has expired, the sources are switched off, and the carrier with ground material is removed from the oven.

(9) Example of prefoaming parameters: With a prefoaming temperature of about 190 C., bulk density can be reduced from about 600-700 kg/m.sup.3 to about 130 kg/m.sup.3 in 2 minutes. The diameter of the particles used, in each case at the thickest point, was from 1 to 5 mm. The diameter of the prefoamed particles, in each case at the thickest point, was from 2 to 20 mm.

Inventive Example 2

(10) The method for Inventive Example 2 is analogous to that for Inventive Example 1, except that a different radiation source is useda source from OPTRON GmbH:

(11) These are short-wave sources emitting mainly at wavelength 1.2 m. 1.2 m here corresponds to a temperature of 2350 K calculated by the Wien method. The carrier is composed of aluminium profiles and metal sheets.

(12) Again, this radiation source is modular. The combination here is termed IR cartridge. The set-up in this case has a source field with 72.75 kW sources using what are known as twin sources backed by gold reflector and with ventilators for cooling. The total radiative power of this set up is therefore 19.25 kW. The size of the source field is 560500150 mm. That gives a heated area of 400420 mm. The distance is analogous to that in Inventive Example 1.

(13) With a set up of this type, results achieved were identical with those in Inventive Example 1 after as little as 5 min.

(14) As can be seen from comparison of Comparative Example 1 and Inventive Examples 1 and 2, it is possible to achieve markedly lower bulk densities, i.e. markedly greater degrees of prefoaming, in a markedly shorter time by the method of the invention.

(15) From Inventive Example 2 it is apparent that particularly efficient foaming is achieved when operations are carried out in the wavelength region of the maximum absorption of the PMI.