RUBBER MODIFICATION STRATEGIES FOR MITIGATING SURFACE DEFECTS IN METALLIC POLYPROPYLENE COMPOSITES: A COMPARATIVE STUDY OF EPR AND EPDM

Abstract

Weld line visibility represents a critical barrier to broader industrial adoption of mold-in-color metallic polypropylene technology. This investigation systematically evaluates rubber modification as a novel mitigation strategy, comparing ethylene-propylene rubber (EPR) and ethylene-propylene-diene monomer (EPDM) as elastomeric modifiers at 5 and 10 wt% loadings in aluminum flake-pigmented polypropylene (2 wt% Al 27). Comprehensive characterization encompassing gloss measurements, spectral reflectance analysis, optical microscopy, and flow mark quantification revealed that both rubber types effectively reduce weld line visibility through altered melt rheology and disrupted pigment segregation at flow front convergence zones. EPR modification produced moderate gloss reductions (10.4% and 19.2% at 5% and 10% loadings respectively) while maintaining acceptable metallic appearance (38.77% and 34.97% gloss) without introducing flow marks. EPDM exhibited stronger optical property suppression (13.9% and 24.7% gloss reductions) but critically generated severe flow mark defects (7.00 and 13.00 marks per specimen at 5% and 10% loadings versus 0.67 for control), attributed to enhanced melt elasticity and flow instabilities. Statistical analysis (ANOVA, p < 0.001) confirmed significance of all observed effects. Microstructural examination revealed reduced aluminum particle accumulation and improved distribution uniformity at weld regions in rubber-modified systems. EPR at 5 wt% loading emerges as optimal formulation, balancing substantial weld line reduction with maintained surface quality, enabling broader mold-in-color metallic technology adoption in automotive and consumer applications.