Low Profile Additives (LPA)

by Nagesh Potluri, Ph.D in Core Competencies Posted on 10/22/2020 11:36 AM

There is a growing customer consciousness for eco-friendly products with a lower carbon footprint. The commercial transportation industry requires class-A finish for the vehicle's exterior surface or body. Sheet molding compounds (SMC) and Bulk molding Compound (BMC) are used in making components that include truck hoods, doors, headlamp protectors, tools, and electrical appliances. The lower cost of unsaturated polyester (UPE) makes it the composite industry's materials of choice for cost-sensitive applications. However, the use of UPE causes more problems as compared to their benefits in the industry. These problems include poor appearance, fiber patterns showing at the surface, cracks and voids in thick sections, and shrinkage. UPE is known to shrink by about 7-12 percent during the fabrication of the composite parts. This UPE resin shrinkage does not make it fit for applications that require thickness tolerance and high surface finish. Increasing UPE resin's viscosity is one solution to decrease the shrinkage, but this will reduce the finished product's mechanical properties. Shrinkage also makes it hard to produce fiber-reinforced UPE composites suitable for applications using Resin Transfer Molding and pultrusion process. Furthermore, UPE mixed with pigment shows an unacceptable darkening of pigment color due to the refractive index difference between air pockets and the resin.

Low Profile Additives (LPA) are added to the resin to compensate for the shrinkage of the UPE. Commonly used LPA are thermoplastics materials such as PVA, PMMA, PU, Polystyrene (PS). The mechanism of reducing shrinkage of UPE involves - strain relief through stress cracking. Shrinkage of resin leads to residual stress at the interface between LPA and UPE resin. When stress cracking initiates, it creates voids and space between the UPE and LPA domains. This space or gap compensates for the shrinkage of UPE. The use of LPAs can result in two types of molecular arrangements. A small difference in polarity between the constituent (i.e., PVAc/UPE and PU/UPE systems) results in a co-continuous globule microstructure with the LPA phase surround the crosslinked UP microgel particles. On the other hand, large polarity difference between the constituents MMA/UPE and PS/UPE systems) results in a microstructure with two distinct LPA and UP phases. Better shrinkage control and higher impact and tensile strength are obtained when the LPA and UPE are compatible. In contrast, due to lower cross-linking density in the resin phase, a higher Youngs modulus is attainable for a less compatible LPA and UPE system.

PVA (Poly Vinyl Acetate) is the fastest-growing type of LPA, and in general terms of volume and value and market is projected to grow from USD 381 million in 2018 to USD 664 million by 2023, at a CAGR of 11.8%. To address current trade-offs between surface finish and mechanical properties requires new LPAs that don't compromise mechanical properties over the surface finish and vice-versa. Therefore, there is a growing demand for creating new LPAs that can overcome some of the aforementioned issues. ChemPrise is actively pursuing alternative strategies for increasing the systems' free volume to compensate for the shrinkage and, at the same time, create acceptable internal pigment ability for the molded parts.

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