Multi-scale stochastic study of glass fiber sizing effects in automotive structural sheet molding compounds

Three commercially available sizings A, B, and C were studied in this work based on their target use towards class-A, semi-structural, and general-purpose SMC formulations. Each chemical sizing treatment A, B, and C was compounded on a pilot-scale SMC line with target 55 wt% fiber content using the same paste formulation suitable for high fiber content. Compression molding and sample preparation was conducted systematically and consistently to minimize variance when comparing test results. Notable differences in wetting and sheet density of the compounded charges were observed. After compression molding one chemical sizing exhibited blister defects on the panel surfaces of approximately 1/3rd of the molded panels, indicating the entrapment of air during flow which was not observed in the other compounds. ANOVA revealed significant differences (17 %–20 %) in IFSS at the fiber-matrix interface resulting from chemical sizing. Despite these observed differences, ANOVA revealed no significant differences in quasi-static tension, shear, flexure, or compression properties of molded panels containing the different glass fiber sizings. This work concludes that chemical sizing significantly affects SMC compounding, molding, and microscale IFSS. Provided the composites can consolidate without the formation of blister defects, the overall structural performance is not observed to change statistically when considering inherent sources of variability stemming from localized effects of fiber volume and fiber orientation at the mesoscale. Outcomes of this work allow for greater confidence in producing semi-structural SMC components using generalized chemical sizing packages as long as consolidation of SMC parts is achieved through compounding and molding.