Low-Volatility Styrene Polyester Resin

In hand lay-up and spray-up molding processes, resin is typically applied in layers to an open mold. Especially during the spray-up process, the resin is atomized and sprayed, with some forming fine particles that deposit on the mold surface. However, before the resin fully cures, styrene continues to volatilize from it, leading to an increased concentration of styrene in the workshop air. This not only results in the loss of styrene but also poses a potential environmental hazard. In workshops with poor ventilation, the concentration of styrene in the air can become excessively high, potentially impacting the health of operators exposed to this environment over a long period. Therefore, various countries have established Threshold Limit Values (TLVs) for styrene concentration in the air, typically based on an 8-hour workday and a 40-hour workweek. For example, both the UK and the US have set the TLV for styrene at 100 g/m³, while Sweden has restricted it to 50 g/m³.

To ensure that the styrene concentration in workshop air remains below the specified TLV, it is necessary to enhance ventilation. However, relying solely on ventilation can cause indoor temperatures to drop in winter, increasing heating costs, making it crucial to reduce the volatilization of styrene in polyester resin.

Early low-volatility resins reduced styrene volatilization by adding a small amount of paraffin wax as a volatilization inhibitor. During the curing process, the paraffin forms a thin film on the resin surface, acting as an air barrier. However, the addition of paraffin could lead to delamination in laminated materials.

To improve this situation, subsequent formulations were developed that combined high- and low-melting-point paraffins with various polymers, such as poly(butylene succinate) and poly(butyl acrylate). Additionally, a combination of a volatilization inhibitor (like paraffin) and an adhesion promoter was used. The adhesion promoter could be hydrophobic ethers or esters containing two hydrocarbon groups and at least one double bond, as well as unsaturated isoprene and its derivatives, such as linseed oil, dipentene, and trimethylolpropane diallyl ether. The typical addition level of paraffin ranges from 0.05% to 0.5% (by mass), while the adhesion promoter is added at 0.1% to 2% (by mass).

In addition to adding inhibitors, the following methods can be used to reduce styrene volatilization:

1. Reducing Styrene Content: By lowering the styrene content in the formulation, the amount of styrene volatilizing during the curing process can be directly reduced. This is usually achieved by introducing other cross-linking monomers or reactive diluents to maintain resin performance.

2. End-Capping Techniques: Introducing low-volatility end-capping agents in the resin can reduce styrene volatilization. These agents chemically bind styrene within the polymer chain, thereby reducing its release.

3. High-Solids Resins: Increasing the proportion of solid components in the resin reduces the proportion of volatile components, thereby decreasing styrene volatilization. This approach typically requires improvements in resin production processes to ensure that high-solids resins still possess good application properties and final product quality.

4. Addition of Nanomaterials: Adding nanomaterials, such as nanosilica or nano calcium carbonate, to the resin can inhibit styrene volatilization by altering the resin's microstructure. These nanomaterials can increase resin viscosity and cross-linking density, thereby reducing styrene migration.

5. Improving Curing Processes: Optimizing curing processes, such as adopting lower temperatures and shorter curing times, can reduce styrene volatilization during curing. Additionally, using styrene-free UV-curing processes can effectively minimize styrene volatilization.

To further reduce styrene volatilization, process improvements are also being continuously advanced, with hand lay-up and spray-up molding processes gradually transitioning to closed mold technologies, such as Resin Transfer Molding (RTM).