What is it about?
In this study, we report an adhesion mechanism in which a polyurethane adhesive diffuses into polyethylene under moderate thermal processing conditions that are compatible with industrial practice. Although polyethylene is one of the most widely used plastics, its notoriously low surface energy has made reliable adhesion difficult. Our previously developed “nailing adhesion” technique enabled strong bonding to polyethylene, but required high processing temperatures close to the melting point of polyethylene. While such conditions are feasible for certain industrial applications, they inevitably limit broader use. In contrast, the newly developed method achieves robust adhesion at a significantly milder temperature of 80 °C. Beyond its immediate industrial relevance, the study also addresses the scientifically intriguing problem of polymer–polymer diffusion. Conventionally, dissimilar polymers are considered immiscible, and it has generally been assumed that no diffusion occurs between them. To overcome this conceptual barrier, we quantitatively demonstrate that the molecular weight—i.e., molecular size—of the polyurethane adhesive plays a crucial role.
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Why is it important?
The molecular weight of the polyurethane adhesive is central to this bonding mechanism. Although our previous work established design principles for polyurethanes capable of mixing with polyethylene (Macromolecules, 2023, 56, 2429. DOI: 10.1021/acs.macromol.2c02503), the influence of molecular weight remained unclear. Lower-molecular-weight chains possess smaller molecular sizes, which facilitates diffusion into polyethylene. However, reducing molecular weight also diminishes the solid-like character of the polyurethane and enhances its liquid-like behavior. As a result, the mechanical strength of the adhesive decreases, ultimately compromising adhesion performance. These competing effects suggest the existence of an optimal molecular weight that balances diffusion ability and mechanical robustness, and we quantitatively verify this hypothesis.
Perspectives
This research was initially motivated by an engineering challenge: developing a practical method for adhesion to polyethylene. At the same time, the technical development was firmly grounded in scientific investigation. The relationship between diffusion behavior and molecular weight was quantitatively elucidated using neutron-based analytical methods, providing insight of fundamental scientific value. The polyurethanes studied here consist of hydrogenated polybutadiene—structurally similar to polyethylene—decorated with highly polar urethane groups that are intrinsically incompatible with polyethylene. We show that the presence of these urethane groups is essential for enabling diffusion. Although this conclusion may appear counterintuitive, we demonstrate in the paper that it is thermodynamically reasonable. Through scientifically supported technological development, we were able to obtain results that are both practically impactful and scientifically intriguing.
Ph.D. Yoshihiko Shiraki
Tosoh Corporation
Read the Original
This page is a summary of: Adhesion to untreated polyethylene by diffusion: Effect of polyurethane adhesive molecular weight on polyethylene penetration, Polymer, May 2024, Elsevier,
DOI: 10.1016/j.polymer.2024.127073.
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