Research On The Manufacture Of Films Of Low Pressure Sheet Molding Compound

LPMC, a new generation of molding compounds developing in 1990s, has outstanding properties such as low molding pressure and temperature, greatly saves great of the investment in the equipments and mould, and in favor of the development of molding products, which means a wide application field.The crystalline unsaturated polyesters were synthesized by applying the method of "gradient rising temperature" . The melt points of these crystalline unsaturated polyesters vary from 45℃ to 89℃. They have high crystallinity, excellent heat stability and nice physical and chemical properties. These crystalline unsaturated polyesters were suitable to be used as physical thickener in low pressure sheet molding compound. The melt point of crystalline polyester could be controlled by regulating the mol ratio of the two glycols, saturated diacid and unsaturated diacid, and when the proportion of neopentyl glycol in the system was increased, the values of mechanic properties of crystalline unsaturated polyester improved first, and then dropped.The thickening effect of resin paste was investigated by varying the amount of crystalline polyester resin and the thickening mechanism of crystalline polyesters using in low pressure sheet molding compounds was studied, too. Results show that the thickening effect is excellent when the amount of crystalline polyester resin using in resin paste is 10%, and there are three kinds of functions acting in the processing of thickening. That is: swelling, hydrogen bonds and induction crystallization. The research supports the producing processing for low pressure SMC with theory basement.Several single thickening systems were studied in this paper. The effect of the using amount of each thickener on the thickening property of unsaturated polyester resin were discussed and the superiorities and inferiorities of each thickener in the process of thickening were analyzed and compared, too. By means of using combined thickeners, the earth oxide thickening agent was combined with crystalline polyester to thicken the UP resin system, the thickening effect was improved. The results showed that the thickening effect of combined thickeners which were well suitablefor viscosity conditions of low pressure molding and low cost conditions of industrialized production, was better than single thickener.A serial of single-initiator systems and combined-initiator systems using in unsaturated polyester resin were studied in this paper, too. It was discovered that the gel time and the curing time of resin can be shortened by using combined-initiator systems, compared with single-initiator systems. Besides, the exothermic peak of the curing system of resin can be varied and the productivity can be proved, too. In the same combined-initiator system, the changing amount of low temperature initiator or high temperature initiator has little affection on the storing period of resin. Among all the combined-initiator systems, the system of l,l-Bis( tert-butyl peroxy) cyclohexane / Tert-butyl peroxy-2-ethyl hexanoate is the better. By means of using this system, the resin can be well cured and the apparent properties of products are better than that with single-initiator systems in the same molding temperature. Further more, it makes the products have a good mechanical property. In a word, l,l-Bis( tert-butyl peroxy) cyclohexane / Tert-butyl peroxy-2-ethyl hexanoate is a suitable moderate temperature curing system for low pressure sheet molding compound.In order to ensure the physical thickening effect of crystalline polyester, several rolls in the machine must be heated up, then the grain crystalline polyesters turn into liquid state and flow together, which drops the viscosity of resin paste to better swell fibers. In the end of rolls, crystalline polyesters turn to solid state again, and the viscosity of resin paste rises to a level to be molded, and then the aim of thickening is attained.