Designing Soft and Transparent Films Based on Multi-Phase Polypropylene Copolymers and Styrene Block Copolymers

Concerns about the environmental and health impacts of plasticized poly (vinyl chloride) (PVC), from plasticizer loss to microplastic formation, have created a clear demand to find alternative packaging materials for medical and pharmaceutical use. As a possible polyolefin-based alternative, we blended polypropylene-ethylene copolymers with different ethylene content-controlled phase structures with styrene-ethylene/butylene-styrene block copolymer (SEBS), as modifier. SEBS is elastomeric and performs mechanically like a cross-linked rubber due to its unique microphase-separated morphology of hard spherical polystyrene (PS) domains dispersed in the soft elastomeric ethylene-butylene copolymer (EB) phase. Tests with injection-molded samples and cast films demonstrated promising combinations of flexibility, durability, and transparency-qualities essential for soft medical packaging like infusion pouches and blow-fill-seal bottles. For the desired level of flexibility (reflected by a flexural modulus of 150-250 MPa), blends with two random-heterophasic (RAHECO) copolymers achieved the lower limit with only 15-25 wt.-% SEBS, compared to the 37 wt.-% needed for a single-phase random copolymer (RACO). These blends also exhibited greater toughness and excellent transparency. In contrast, a standard impact copolymer (HECO), with its more crystalline matrix, required a higher modifier content of 45 wt.-% SEBS. Film morphology analysis indicated a gradual shift in disperse phase structure and orientation, leading to phase inversion at the highest SEBS content without negatively affecting transparency.