05/16/2024
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation Proposal defense by Matthew Drew on "Optical and Adhesion Properties of Polymer/Sapphire Composites."

Candidate Name: Matthew Drew
Degree: Doctoral
Defense Date: Tuesday, May, 21, 2024
Time: 10 a.m. to noon
Location: Perry 215

Committee:

  • Advisor: Davide Masato, Assistant Professor, Plastics Engineering, UMass Lowell
  • Stephen Johnston, Professor, Plastics Engineering, UMass Lowell
  • Alireza Amirkhizi, Professor, Mechanical Engineering, UMass Lowell

Brief Abstract:
Sapphire, renowned for its transparency and durability, is an excellent choice for high-impact applications requiring protection. Its remarkable stiffness and scratch resistance ensure resilience against impacts while preserving clarity, making it an appealing option for protective eyewear. However, its high density can lead to designs that are too heavy for military or civil applications.
To address weight limitations and improve ballistic performance, lamination with a polycarbonate backing layer was proposed in the literature. For eyewear applications, sapphire is ground and polished to a thin layer to minimize the overall weight. The sapphire striking layer maintains the high impact resistance, while the polycarbonate serves as a spalling layer and dampens the impact. The composites can be laminated in different ways using an intermediate adhesive layer. Hence, the optical and mechanical performance of the laminated composite depends on the adhesive material and the adhesion obtained from the joining process.

This work investigates the design, manufacturing, and characterization of curved polycarbonate/sapphire composites through experiments and simulation. The design focuses on strategies to minimize optical distortions through lens profile design. The manufacturing of polycarbonate lenses relies first on injection molding and its ability to produce dimensionally and optically stable parts. Then, compression molding is used for lamination with a thermoplastic polyurethane adhesive layer. Numerical simulations support the design for manufacturing, facilitating design iterations and process design.

The curved lens designs proposed in this project are characterized by non-uniform wall thickness to correct for optical distortions around the field of view. However, the part thickness variation can lead to increased residual stresses and differential shrinkage, thus negatively affecting the optics. The flow-induced residual stresses and thermal stresses caused by the processes can induce birefringence, negatively affecting the optical properties. The research investigates mold nickel plating to modify the surface mechanical and thermal properties, thus engineering the polymer/mold interactions towards better flow and resulting optical properties. Injection molding experiments consider different lens designs, mold surface properties, and processing conditions. The numerical simulations study thermal contact resistance and its optimization. The results guide the manufacturing of polycarbonate lenses characterized by high light transmission, minimized residual stresses, and the required dimensional stability.

The lamination of polycarbonate onto thin curved sapphire lenses is studied, focusing on the adhesion layer and processing. A compression molding approach laminates the two materials under proper pressure and temperature conditions. The adhesion between the two materials is measured experimentally using a custom-designed double lap shear joint test sample. The samples are produced using different adhesive layer thicknesses. The processing window is studied and defined before transferring to the curved lenses. These are characterized for transparency, optical distortion, and impact resistance.

The research results are expected to advance the knowledge of laminated composites for lightweight, protective eyewear. The process development research will enable the application of the design approach to other material systems and applications, both military and civil. The scientific novelty of the work lies in the studies of the polymer/mold interface interactions in injection molding and the sapphire/TPU/polycarbonate adhesion strength developed in compression molding.