Adhesive formulations for lens applications

The work presented within this thesis was focused on the development of a novel polyurethane adhesive which could be used in the construction of sunglass lens. This adhesive must satisfy certain objectives: (i) the cured adhesive must be optically clear with the final haze value < 1.5% when laminated between two layers of plastic, (ii) the adhesive must bond cellulose triacetate, bisphenol-A polycarbonate and any other laminate combinations containing these plastics, (iii) each fully cured laminate must have a peel strength of ≥ 3 N mm-1 as determined by 180° T-peel testing and (iv) the fully cured adhesive must be free of thermal transitions within the window of -20°C to 100°C which would otherwise affect the in-use performance. To achieve these aims a series of aliphatic and aromatic polyurethane adhesive based on IPDI and MDI were formulated. Within these formulations a range of soft-segments were used which included PPG, PCD and PDEGA. Finally, less conventional chain-extenders were used to improve the final haze value of the adhesive. Cellulose triacetate was tested both as received and following saponification of the interface. Polycarbonate was used as received and following pre-treatment of the interface by an ethanolamine in isopropyl alcohol solution. The results within this thesis have shown that adhesion to multiple interfaces is a complex task. Using MDI-based adhesives it was shown that the compatibility with polycarbonate was high and surface pre-treatment by ethanolamine was not required. In most instances, peel strengths of ≥ 3 N mm-1 were obtained. Cellulose triacetate however, had a poor compatibility with MDI-based polyurethanes. In all instances the peel strength obtained was < 1 N mm-1. In order to obtain laminates of ≥ 3 N mm-1 saponification of the interface was required. Deacetylation of the interface leaves a regenerated cellulose surface and these labile hydroxyl groups able to react with the isocyanate groups within the prepolymer adhesive. Obtaining MDI adhesives of low haze was not straight forward. It was shown that using non-crystalline soft-segments such as PPG and PDEGA was the best approach but their application was problematic. Due to the requirement for surface treatment of cellulose triacetate and the difficulty of application, MDI-based polyurethanes were thus shown not to be appropriate. Following this IPDI-based adhesives were next tested. For these adhesives, noticeable differences in both the adhesion behaviour and the haze values were observed. Moving to an aliphatic isocyanate improved the compatibility with cellulose triacetate, whilst not reducing the performance with polycarbonate. When using either PPG or PCD, low peel strengths values where obtained with cellulose triacetate and like MDI-based adhesives saponification was required for high peel strength. When using the soft-segment PDEGA however, peel strength of ≥ 3 N mm-1 were obtained and this value was obtained regardless of surface treatment. It was shown that this improved adhesion was linked to IPDI-based adhesives having a phase-separated morphology which promotes adhesion by hydrogen bonding. This phase-separated morphology was also advantageous towards the haze, with values well inside the 1.5% obtained. Thus it was shown that in order to obtain an optically clear adhesive with a haze of < 1.5% that is void of thermal transition between -20°C to 100°C and can bond either untreated cellulose triacetate or polycarbonate with a peel strength of ≥ 3 N mm-1 was only possible using a polyurethane adhesive based on IPDI and PDEGA.The work presented within this thesis was focused on the development of a novel polyurethane adhesive which could be used in the construction of sunglass lens. This adhesive must satisfy certain objectives: (i) the cured adhesive must be optically clear with the final haze value < 1.5% when laminated between two layers of plastic, (ii) the adhesive must bond cellulose triacetate, bisphenol-A polycarbonate and any other laminate combinations containing these plastics, (iii) each fully cured laminate must have a peel strength of ≥ 3 N mm-1 as determined by 180° T-peel testing and (iv) the fully cured adhesive must be free of thermal transitions within the window of -20°C to 100°C which would otherwise affect the in-use performance. To achieve these aims a series of aliphatic and aromatic polyurethane adhesive based on IPDI and MDI were formulated. Within these formulations a range of soft-segments were used which included PPG, PCD and PDEGA. Finally, less conventional chain-extenders were used to improve the final haze value of the adhesive. Cellulose triacetate was tested both as received and following saponification of the interface. Polycarbonate was used as received and following pre-treatment of the interface by an ethanolamine in isopropyl alcohol solution. The results within this thesis have shown that adhesion to multiple interfaces is a complex task. Using MDI-based adhesives it was shown that the compatibility with polycarbonate was high and surface pre-treatment by ethanolamine was not required. In most instances, peel strengths of ≥ 3 N mm-1 were obtained. Cellulose triacetate however, had a poor compatibility with MDI-based polyurethanes. In all instances the peel strength obtained was < 1 N mm-1. In order to obtain laminates of ≥ 3 N mm-1 saponification of the interface was required. Deacetylation of the interface leaves a regenerated cellulose surface and these labile hydroxyl groups able to react with the isocyanate groups within the prepolymer adhesive. Obtaining MDI adhesives of low haze was not straight forward. It was shown that using non-crystalline soft-segments such as PPG and PDEGA was the best approach but their application was problematic. Due to the requirement for surface treatment of cellulose triacetate and the difficulty of application, MDI-based polyurethanes were thus shown not to be appropriate. Following this IPDI-based adhesives were next tested. For these adhesives, noticeable differences in both the adhesion behaviour and the haze values were observed. Moving to an aliphatic isocyanate improved the compatibility with cellulose triacetate, whilst not reducing the performance with polycarbonate. When using either PPG or PCD, low peel strengths values where obtained with cellulose triacetate and like MDI-based adhesives saponification was required for high peel strength. When using the soft-segment PDEGA however, peel strength of ≥ 3 N mm-1 were obtained and this value was obtained regardless of surface treatment. It was shown that this improved adhesion was linked to IPDI-based adhesives having a phase-separated morphology which promotes adhesion by hydrogen bonding. This phase-separated morphology was also advantageous towards the haze, with values well inside the 1.5% obtained. Thus it was shown that in order to obtain an optically clear adhesive with a haze of < 1.5% that is void of thermal transition between -20°C to 100°C and can bond either untreated cellulose triacetate or polycarbonate with a peel strength of ≥ 3 N mm-1 was only possible using a polyurethane adhesive based on IPDI and PDEGA