1. Field of the Invention
This invention relates to a blowing agent concentrate comprising a chemical blowing agent and a thermoplastic vehicle therefor. More especially this invention relates to a blowing agent concentrate for use together with polyalkyleneterephthalates of high molecular weight to form structural foam products of such polyalkyleneterephthalates. This invention is especially concerned with blowing agent concentrates comprising a chemical blowing agent and a thermoplastic vehicle therefor, said thermoplastic vehicle being a polyester and/or copolyester having a crystalline melting point between 100.degree. C. and 220.degree. C. and a reduced viscosity between 0.4 dl/g and 1.6 dl/g.
2. Discussion of the Prior Art
Several methods are known for the incorporation of chemical blowing agents into the thermoplastic composition that is to be foamed. For example, the usually powdered blowing agent can be dusted onto the plastic granules in the required concentration of 0.1 to 5.0% by weight, as a rule, depending on the activity of the blowing agent. This method is still used widely, but it has a number of decided disadvantages. During the mixing procedure, dust is raised, sometimes to a considerable extent. This dust results in a contamination of the mixing units and of the surrounding area, and is furthermore harmful to health in some cases. Furthermore, and especially in the case of the addition of large amounts, separation and settling of ingredients can occur. The fluctuating contant of the blowing agent that results militates against uniform quality in the products made from such mixtures. In the case of conveyor transportation, settling and losses of the blowing agent occur. Pneumatic conveying over relatively long distances is almost impossible. In fabrication, settling and irregular distribution of powdered chemical blowing agents can take place in the supply hopper of the fabricating machine, resulting in quality variations. In the area of the machine where the materials are fed in, when the decomposition temperature of the blowing agent is exceeded, blowing gas losses can be produced by loss through the feed opening, which quite considerably limits the blowing gas yield.
For the avoidance of these disadvantages, provision has been made (cf., e.g., German "Offenlegungsschrift" No. 2,434,085) for incorporating the blowing agent powder directly into the plastic to be foamed. The blowing agent that is used is, in this process, first incorporated into the melt of the polymer at a rate which is in a fixed ratio to the polymer, and is an integral component of the granules produced. During fabrication to structural parts, the blowing agent contained in the granules is activated and permits the formation of the structural foam product. In this method of incorporating the blowing agent it is highly disadvantageous that a partial decomposition of the blowing agent takes place during the production of the granules containing it, since the fabrication temperature required for the achievement of a homogeneous melt is so close to the composition temperature of the blowing agent, depending on the blowing agent used, that the latter yields gas before the fabrication process begins, and therefore its further effect is greatly limited. The consequences are variations of density in the foam structure. In many cases, the blowing pressure, reduced by the premature gas losses, is no longer sufficient for a perfect formation of the structural foam piece. Another deficiency of this method is that such blowing-agent-containing plastics require a larger inventory, since batches with different blowing agent contents have to be prepared which are adapted to the flowing characteristics of the plastic to be foamed and to the dimensions of the moldings that are to be produced.
For the avoidance of these disadvantages, the blowing agent has been incorporated in concentrated form into a polymer which is to be prepared in the plastic state at a temperature below its decomposition temperature, and the mixture is blended with the plastic to be foamed, in the necessary ratio. In this manner the disadvantages which occur in the handling of powdered blowing agents are avoided. The preparation and stocking of a plurality of batches of different blowing agent contents is not necessary, since the desired blowing agent contents can be matched to the particular requirements immediately prior to the actual foaming process by varying the ratio of admixture of the blowing agent concentrate with the polymer that is to be foamed.
Blowing agent concentrates on the basis of polystyrene and polyolefins are known. These blowing agent concentrates, however, have the deficiency that they are not suitable for the manufacture of structural foam products from polyalkylene terephthalates of high molecular weight. When such blowing agent concentrates are used in conjunction with polyalkylene terephthalates, fabricated products are obtained which have reduced physical, mechanical and thermal characteristics. For this reason it has hitherto been considered advisable, for the production of structural foam products from polyalkylene terephthalates, to use the above-described disadvantageous methods for the incorporation of the blowing agent.