Polystyrene Low Profile Additive (LPA-PS)
Key Features
- Controls UPR volumetric shrinkage — enables Class A surface in SMC/BMC molding
- Phase separation mechanism creates microvoids compensating for polyester cure shrinkage
- Most cost-effective LPA type for standard SMC production with CaCO3 filler
- High compatibility with orthophthalic and isophthalic UPR resin systems
- Used at 10–20 phr loading in compression and injection molded composites
Polystyrene Low Profile Additive (LPA-PS) is a thermoplastic polystyrene dissolved in styrene monomer, formulated for use in unsaturated polyester resin (UPR) sheet molding compounds (SMC) and bulk molding compounds (BMC) to control volumetric shrinkage during cure. Unsaturated polyesters typically shrink 5–8% by volume during free-radical cure due to the decrease in molecular volume as double bonds are converted to single bonds. In closed-mold compression and injection molding, this shrinkage causes surface defects including sink marks, fiber readthrough (fiber print), and surface waviness that are incompatible with Class A surface quality requirements.
The LPA-PS mechanism relies on phase separation during cure: polystyrene is miscible with the liquid UPR/styrene mixture before cure, but becomes thermodynamically incompatible as the UPR crosslinks during polymerization. Phase-separated polystyrene domains create internal microvoids that compensate for the bulk polyester shrinkage. The macroscopic result is near-zero volumetric change (low profile) and a smooth, void-free surface suitable for painting without additional surface preparation.
LPA-PS is the most widely used LPA type in North American and European SMC production due to its low cost, good compatibility with most UPR formulations, and consistent surface quality performance. It is used at 10–20 phr in SMC and BMC compounds. LPA-PS works best with calcium carbonate filler and produces Class A surface finish in automotive body panels, hoods, trunk lids, and fenders manufactured by compression molding.
Specifications
| Parameter | Value |
|---|---|
| Appearance | Clear to slightly hazy viscous liquid |
| Flash point | 31°C (due to styrene content) |
| Base polymer | Polystyrene (molecular weight 50,000–200,000 g/mol) |
| Solids content | 40–50% PS in styrene |
| Carrier solvent | Styrene monomer |
| Styrene content | 50–60% |
| Viscosity (25°C) | 500–5000 mPa·s |
Applications
FAQ
LPA-PS creates phase separation through a spinodal or nucleation-and-growth mechanism during cure. If the PS molecular weight is too high or the mixing with the UPR matrix is incomplete, coarser phase domains form, which can produce a slightly hazy or orange-peel surface texture rather than a mirror-flat Class A finish. PVAc LPA (polyvinyl acetate) typically produces finer phase separation and is better for the most demanding Class A surface requirements. LPA-PS is preferred when cost is the primary driver and surface requirements are met.
Direct Contact
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