No thickening, no SMC
In the 1960s, Bayer Company of Germany discovered the thickening mechanism of the reaction of the carboxyl-terminated carboxyl group of the oxides and hydroxides of group IIA elements with unsaturated polyester resins, which pioneered the industrial production of SMC.
80% of SMC defects are caused by poor thickening
Resin, low shrinkage, grouping of fillers：
uneven surface gloss
Glass fiber flow is uneven
Problems with existing thickening methods:
The thickening method commonly used in the industry is mainly based on the reaction of MgO, CaO, Mg (OH) 2, Ca (OH) 2 and the terminal carboxyl group of the resin.
A large number of high-performance resins, such as vinyl ester resins, epoxy resins, and polyurethane resins, do not have carboxyl functional groups that can react with MgO, CaO, Mg (OH) 2, Ca (OH) 2, etc.
One of the existing solutions:
1.use the reaction of isocyanate and hydroxyl to thicken.
Only suitable for hydroxyl-containing resins, such as vinyl ester resins, do nothing against other resins;
The reaction rate is difficult to control and the thickening is unstable;
Isocyanate reacts easily with water to form bubbles;
The second existing solution:
modify the vinyl ester resin and attach the carboxyl functional group.
Even after the carboxyl functional group is attached, it is difficult to thicken the unsaturated polyester resin.
Increase the difficulty of synthesis and increase the cost.
Need to find a new thickening mode.
Composition of SMC
Glass fiber (20-30%)
Resin (60 parts, 60% non-volatile components + 40% styrene)
Low shrinkage agent (40 parts, 40% non-volatile components + 60% styrene)
Curing agent, release agent, etc.
Our new ideas (theoretical basis)
The ratio of unsaturated polyester to thermoplastic polymer is about 2: 1. The thermoplastic polymer used as a low-shrinkage additive is a high content component in SMC / BMC.
When the thermoplastic polymer molecular chain used as a low-shrinkage additive is connected to a carboxyl functional group, when adding thickeners such as MgO, CaO, Mg (OH) 2, Ca (OH) 2, these thermoplastic polymers will also It reacts with thickener to increase molecular weight, thereby increasing system viscosity.
The molecular weight of unsaturated polyester is generally 1000-3000; and the molecular weight of thermoplastic polymers in low-shrinkage additives is generally tens of thousands to hundreds of thousands. Therefore, after the thermoplastic polymer in the low shrinkage additive reacts with the thickener, the increase in viscosity of the entire system is more significant than that of the unsaturated polyester resin.
For commonly used low-shrinkage types, the following methods can be used to modify:
PS: By copolymerizing styrene with carboxyl functional group-containing monomers such as acrylic acid, methacrylic acid, fumaric acid, maleic anhydride, maleic acid and the like, a carboxyl functional group-containing polymer is obtained.
PMMA: By copolymerizing methyl methacrylate with carboxyl functional group-containing monomers such as acrylic acid, methacrylic acid, fumaric acid, maleic anhydride, maleic acid, etc., a polymer containing carboxyl functional groups is obtained.
PVAc: Copolymerization of vinyl acetate with monomers containing carboxyl functional groups such as acrylic acid, methacrylic acid, etc. to obtain polymers containing carboxyl functional groups.
Saturated polyester: By controlling the excess of carboxylic acid during the polycondensation reaction, or using the method of dibasic acid and polyacid end-capping, a polymer containing carboxyl functional groups is obtained.
By modifying the thermoplastic polymer in the low-shrinkage additive to enable it to achieve thickening requirements, the choice of resin can be more extensive. For example, other resins such as vinyl resins, single-component polyurethane resins, and ordinary unsaturated polyester resins can be freely selected without necessarily selecting special unsaturated polyester resins, which improves the performance and flexibility of SMC / BMC.
It can also be used with SMC special unsaturated polyester resin to enhance the thickening effect.