Bulk molding compounds (BMC) and sheet molding compounds (SMC) are known for being lightweight and creating long-lasting products. Ongoing research is leading to new uses for these materials in more demanding environments and applications. Molding compounds can now be formulated for low-density applications, car exteriors and outdoor applications. Traditionally, these materials needed to be coated or painted when exposed to outdoor environments. But today, formulation design considerations can minimize or even eliminate the need for coating. This allows the physical properties and intrinsic color of the compounds to be used in products without the additional step and cost of coating.
Molding compounds are susceptible to many outside elements, including heat, freeze/thaw, moisture and UV light. These elements degrade the compound both cosmetically and physically. Cosmetic changes include chalking, color fading and fiber-glass bloom. These changes, while only cosmetic, can make the product appear inferior in applications such as tactile panels on sidewalks or electrical box covers. Physical changes can affect the tensile, flexural and impact properties of the product. To overcome these issues, outdoor exposure resistance needs to be included in choices made during the selection of raw materials and formulation of the BMC/SMC.
Formulation and Design
When formulating molding compounds for outdoor exposure, choosing the right components is the first step. However, it is important not to look solely at each component, but how they interact as a whole. Each component in BMC/SMC can exhibit different weathering resistance, especially the resin, filler, thickener, glass fiber and pigment.
Resin chemistry is a good place to start considering how your finished molding compound will perform outdoors. Isophthalic and maleate-based resins typically perform better in weathering than orthophthalic-based unsaturated polyester resins. Vinyl ester and aromatic epoxy resins generally are not known for their weathering resistance, but stabilizer packages can improve them. Aliphatic epoxies weather very well compared to their aromatic counterparts and are a good choice for outdoor applications. Resin producers also manufacture specialty resins with tailored chemistry that have superior UV resistance. Filler choice also contributes to or detracts from weathering resistance of a molded compound. Aluminum trihydrate, for example, is superior to calcium carbonate.
In addition, glass fiber selection also affects how BMC/SMC performs outside in large part because of the sizing on the glass fiber, which serves as the bonding interface between the compound and the glass. If the sizing degrades in weathering that interface becomes compromised, leading to poor performance. Bisphenol-A sizing chemistries exhibit poor resistance to weathering and can lead to detrimental performance of the BMC/SMC in outdoor applications.