Carbon fiber is now found in some SMC formulations and can shows signs of degradation in weathering. The carbon fiber can be stabilized with pigments and stabilizers in paste that will allow it to perform well in weathering.
The performance of pigments in weathering is often one of the first thoughts since changes in the color of the product are so noticeable. Carbon blacks often mask weathering failure more readily than other types of pigments. Other colors – notably reds and yellows – typically show poorer levels of performance. It should be noted that high-quality pigments generally show better resistance to color shift during weathering. This advantage can be wasted if the resin choice is poor and begins to chalk or shift color.
Other ingredients in BMC/SMC formulations, such as low-profile additives, inhibitors and monomers, also affect resistance to weathering, especially moisture absorption and UV resistance. Poor performance of any of these can change the performance of the finished compound.
Weather resistance can be enhanced by adding specialty chemicals to molding compounds, including anti-oxidants, UV absorbers and radical scavengers. Each chemical has a different mechanism to aid the compound’s weathering resistance, though the addition of these chemicals could have positive or negative interactions with the weathering performance of the base materials in your compound. BMC/SMC raw material suppliers often will tailor a blend of specialty chemicals into the products themselves to enhance the product’s weathering performance.
Compound molding can affect how the product behaves in outdoor weathering. One of the more common failures is exposure of the glass as the top layer of resin degrades. Not only is this cosmetically unacceptable, but the physical properties of the product can be affected. The glass must be adequately wet out to keep good adhesion to the compound. Using the correct thickener and appropriate molding viscosity can be critical to achieving a robust resin-rich layer over the glass and ensuring weathering protection.
Finally, the texture of the molded part is often overlooked. The texture affects how much of the compound is exposed to the weather, how dirt/salt build up and how water will freeze and thaw, thus changing the degradation of the compound.
Testing Weathering Resistance
Modifying the formulation and molding to create a more outdoor-stable product requires testing to confirm. We don’t want to wait for 5, 10 or even 20 years of exposure to weathering from different locations for feedback. Many industries use accelerated weathering methods to predict and correlate direct outdoor exposure results. Accelerated weathering tests are primarily defined by the light source – fluorescent or Xenon-Arc (a specialized gas discharge lamp). Xenon-Arc light closely simulates sunlight and can be set up with different wavelength filters and water exposures.