(123b) Promoting Adhesion between Immiscible Polymers
Most plastic products consist of two or more polymers. Dramatic property improvements of packaging films, coextruded profiles, engineering plastics, and soft feel over-molded items are achieved via combining immiscible polymers. Controlling adhesion between these polymers is critical to optimizing their performance. We show how interfacial entanglements control the adhesion of bare interfaces. For amorphous polymers adhesion developed by thermal or solvent welding can be predicted from the entanglement molecular weight of each polymer and their interaction parameter, Ï. Shear flow at high stresses disentangles the interface, reducing adhesion. Reactions between immiscible chains can form bonds across interfaces, greatly increasing adhesion. In fact in coextrusion this is the main approach used to âstickâ polymers together, often via a thin, third component tie layer. We have found that extensional flow, such as occurs in compounding mixers and extrusion dies, can greatly accelerate these reactions (AIChE Journal, 2013). We have also found that small levels of catalyst can further accelerate reactions, especially if the catalyst preferentially wets the interface (ACS Macro Letters, 2015). This creates opportunities to use less reactive functional groups. Addition of premade block copolymers in principle can bridge interfaces but they often localize in micelles. We have recent success in promoting adhesion with small amounts of tetrablock copolymers (Science, 2017). We will also discuss current research on the role of crystallization at interfaces on polymer-polymer adhesion.