Optimisation of glass–PMMA adhesively bonded joints for optical applications in demanding environments

<p dir="ltr">The development of robust joining methods for optical assemblies with the use of adhesive bonding requires a degree of surface treatment of the optical materials employed, to optimise adhesive bond durability, especially when there are operational requirements for good performance in demanding environments. Several important surface physicochemical parameters have been identified as required to provide this optimisation, especially in demanding exposure conditions, namely: wettability, surface roughness, absence of contamination, suitable chemistry, UV (ultraviolet) and thermal cycling resistance.</p><p dir="ltr">In the present study, the effect of atmospheric pressure plasma treatment (APPT) on polymethyl methacrylate (PMMA) and glass substrates was investigated in terms of both the chemical and topographical changes introduced to the polymer surface and its influence on PMMA-to-glass adhesion. The use of a silane-based primer in this bonding system was also studied. The changes introduced to the PMMA and glass surfaces, as a result of plasma processing, were identified using a combination of: X-ray photoelectron spectroscopy (XPS); atomic force microscopy (AFM), and; contact angle analysis (CA). Single lap shear (SLS) testing of the as-bonded PMMA-to-glass structures was performed as a function of various PMMA surface treatment conditions, and, after exposure of the bonded joints to multiple temperatures in order to assess the effect of the surface treatment on the strength of these joints. It was found that APPT treatment, with various gas mixtures, lowers the water contact angle of PMMA and glass and increases their surface free energy. The plasma gases used were argon, helium and oxygen, either on their own or in combination. The chemical composition of the plasma modified PMMA and glass surfaces showed an increase in the level of oxygen present and a corresponding decrease in carbon content, as observed by XPS. Furthermore, AFM indicated a significant change in the topography of the PMMA surface after APPT exposure with a 100–200% increase in mean roughness values with optimised plasma conditions.</p><p dir="ltr">The above-mentioned physicochemical changes to the surfaces of the adherends led to much improved adhesion of the PMMA-to-glass. APPT treatment improved the strength of the SLS joints from 0.28 MPa to 0.58 MPa. In addition, plasma treated PMMA used in combination with a silane-based primer gave a significant further enhancement in observed adhesion levels with SLS values increasing up to 1.56 MPa. Moreover, the adhesion strength of the bonded samples remained stable after both high temperature exposure at 70 °C and temperature cycling with exposure of the bonded joints from −50 °C to 70 °C. This temperature range had negligible effect on the strength of the adhesive joints after 30 thermal cycles.</p><p dir="ltr">Preliminary studies were carried out to investigate the best performing adhesives for PMMA to glass bonding, able to pass a series of performance criteria mainly focused in the successful accommodation of thermal stresses due to mismatch of the coefficients of thermal expansion between the two adherents and to ensure good optical performance after prolonged exposure to UV radiation.</p><p dir="ltr">Assemblies of polymethyl methacrylate (PMMA) and glass bonded with a flexible silicone adhesive were subjected to a temperature gradient from +30°C to -40°C with humidity at 0% inside a climate chamber and their behaviour is recorded with a 3D digital image correlation (DIC) setup. The deformations and the thermally induced surface strain development due to thermal loading were recorded through the window of the climate chamber using a stereo camera system with two charge-coupled device (CCD) cameras. Narrow field measurements were performed near the edges of the joint where usually a high concentration of peeling and shear strains is expected. With the use of commercial DIC software, the thermal deformation of the structure and the developed surface strain fields (εxx, γxy, εyy) were analysed. Joints with different bond line thickness (0.5 mm, 2 mm, 3.2 mm) were investigated.</p><p dir="ltr">The continuous image capture with the CCD cameras allows the identification of the failure conditions and the exact moment of failure of the assembly. Additionally, the DIC experiment results are evaluated with the help of finite-element analysis (FEA) model results, which were in good agreement, showing that the DIC method has the potential to be successfully applied for the investigation of thermally induced strains in adhesively bonded assemblies and thus contribute to understanding of the underlying thermal/mechanical behaviour.</p><p dir="ltr">Furthermore, an investigation of the performance of adhesively bonded joints after exposure to UV radiation was conducted. UV exposure led to reduction of shear strength of the bonded joints. Without any prior surface treatment, the reduction of shear strength was: 73% and 80% after 563 and 1126 hours respectively. On the other hand, in the samples where a surface treatment (argon/0.5% oxygen plasma plus primer 1200) was employed; the reduction of shear strength was 32% after 563 hours and 37% after 1126 hours of exposure, showing a more stable behaviour.</p><p dir="ltr">FTIR data revealed a characteristic decrease in the ≡Si-O-Si≡ band intensity with increased exposure time to UV radiation. The strongest loss of absorbance in this region was observed in the samples that were exposed outdoors for 6 and 12 months. The reduction of the ≡Si-O-Si≡ signal indicates a chain scission that occurred within the PDMS network. The decrease in adhesion strength of the bonded joints can be explained by the efficacy of UV radiation to lead in scission of the main PDMS backbone. Increased accelerated UV exposure did not alter significantly the colour and transmittance in the visible of the PDMS samples. An excess yellowness and loss of transmittance was observed for the samples that were exposed to outdoors weathering which was mainly due to the organic and atmospheric contamination of the PDMS surface.</p><p dir="ltr">A combination of the above-mentioned studies provides a better understanding of how the aforementioned surface treatment methods relate to influence glass-PMMA adhesive joint durability after exposure to accelerated degradation conditions simulating demanding environments.</p>