Process for the controlled hardening of unsaturated polyester resins

#This invention relates to a process for the controlled hardening of unsaturated polyester resins at temperatures of -5.degree. C. to +150.degree. C. using organic peroxide initiators.

#As known, organic peroxides are utilized in the cross-linking of unsaturated polyester resins. These compounds decompose into radicals upon the effect of heat or accelerators and act as initiators to start copolymerization. During the progress of the copolymerization process the initially liquid polyester resin converts into a solid, insoluble substance. The use of colourless organic peroxides in these cross-linking processes has two disadvantages: it cannot be observed visually whether the initiator has been mixed with the polyester resin at all, and the progress of hardening cannot be visually controlled but only by mechanical hardness tests. The hardness observed allows conclusions to be drawn on the extent of cross-linking in the polyester product.

#On objects with complicated shape and complex curved surfaces such tests can be performed, if at all, only at some parts. Therefore a method is required which enables one to draw conclusions on the extent of hardening also in such instances. The invention aims at the elaboration of a new method capable of solving this problem.

#20 g of cobalt naphthenate accelerator (cobalt content: 1%) are admixed with 2 kg of an unsaturated polyester resin. 0.1 g of methylene blue are admixed with 20 g of acetylacetone peroxide initiator, and the resulting solution is added to the resin. One third of the resulting mixture is applied with a brush to a glass sheet equipped with a shape separator, and the resin is distributed evenly on a surface of 1 m.sup.2. A 1 m.sup.2 fibreglass quilt with a specific gravity of 450 g/m.sup.2 is placed onto the resin film, and the quilt is impregnated with half of the remaining resin mixture. Thereafter a second fibreglass quilt of the same specific gravity is placed onto the first, and the remainder of the resin is applied with a brush onto the quilt. The impregnated semi-finished product has an intense blue colour. In the cross-linking process of the resin gelling sets in 30 minutes after admixing, whereupon the colour starts to fade, and, when operating at 22.degree. C., the resin becomes completely colourless 2 hours after admixing. This indicates that the hardening of the polyester is complete, and thus the fibreglass-reinforced polyester sheet can be removed from the glass sheet.

#This example relates to the production of fibreglass-reinforced corrugated boards. A polyester resin is fed by an injector pump onto a cellophane foil, drawn continuously by the machine, in an amount sufficient to impregnate the fibreglass quilt forwarded in the next step continuously by the same machine. 10 g of cobalt naphthenate accelerator, furthermore a mixture of 5 g of tert.-butyl peroctoate, 2.5 g of cyclohexanone peroxide, 2.5 g of triethyl phosphate and 0.05 g of brillantcresyl blue, each calculated for 1 kg of the resin, are added to the resin by the injector pump. When passing the impregnating stage of the machine the fibreglass quilt is impregnated with the liquid resin and becomes vivid blue.

#The machine forwards the impregnated quilt between cellophane foils to the hardening stage, where the resin is cross-linked at 80.degree. C. If cross-linking proceeds to the required extent, the final board leaving the hardening stage is completely colourless. Thereby the quality of the board obtained in the hardening stage can be controlled continuously.