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Abstract: The adhesion of films and coatings to rigid substrates is often measured using blister geometries, which are loaded either by an applied pressure or a central shaft. The measurement will be affected if there are residual stresses that make a contribution to the energy release rate. This effect is investigated using analytical solutions based on the principle of virtual displacements. A geometrically nonlinear finite element analysis is conducted for comparison. Furthermore, the relationships among strain energy release rate, load, deflection, and fracture radius are discussed in detail. Both analytical solutions and numerical results reveal that uniform tensile residual stresses reduce a specimen’s deflection if it experiences plate behavior under small loads. However, this effect becomes negligible when membrane stresses induced by the loading become dominant. [Copyright &y& Elsevier]

• Two PET-based backsheets were exposed to indoor UV accelerated weathering. • Single cantilever beam (SCB) tests were used to measure the adhesion in the backsheets. • Both backsheets showed high initial (unweathered) adhesion energies. • After moderate exposure, the polyurethane-based adhesives showed significant adhesion loss. • After high exposure, the PET outer layer showed appreciable UV-induced surface degradation. Delamination between layers in photovoltaic (PV) backsheets is often reported in the literature, causing voids that can collect moisture, diminish module backside heat dissipation, and reduce the backsheet's effectiveness as a physical barrier. While backsheets with weathering-resistant fluoropolymer outer layers have traditionally been used in modules, more recent backsheets using non-fluoropolymer outer layers, such as polyethylene terephthalate (PET), have been developed. These backsheets have shown signs of premature degradation, and their adhesion degradation, in particular, has not been widely studied. In the present work, the single cantilever beam (SCB) adhesion test was used to quantify the adhesion energy in two commercially available PET-based backsheets. To study the effect of minor changes in formulation, the backsheets were obtained from same manufacturer and product line but during different years. To study the effect of environmental variables on adhesion degradation, the backsheets were subjected to artificial weathering at controlled temperature, humidity, and ultraviolet (UV) radiation in an indoor weathering chamber, and the adhesion energy was quantified at several intervals of exposure time. Layering structure, composition, and adhesion failure mode were compared between the backsheets, using Raman and infrared spectroscopy and thermogravimetric analysis. The results show a large difference in initial (unexposed) adhesion energy between the backsheets, despite very similar structures and compositions. Following exposure, adhesion energy dropped significantly, primarily due to thermo-hydrolytic degradation of the polyurethane (PU)-based adhesive layers. Significant UV-induced adhesion degradation of the PET outer layer surface was also observed. The study represents an important step in understanding adhesion degradation in PET-based backsheets, suggesting ways in which adhesion integrity – and, correspondingly, module service life – can be improved. [ABSTRACT FROM AUTHOR]

High-resolution reflection interference contrast microscopy (HR-RICM) was developed for probing the deformation and adhesion of phospholipid vesicles induced by colloidal forces on solid surfaces. The new technique raised the upper limit of the measured membrane–substrate separation from 1 to 4.5 μm and improved the spatial resolution of the heterogeneous contact zones. It was applied to elucidate the effects of wall thickness, pH and osmotic stress on the non-specific adhesion of giant unilamellar vesicles (ULV) and multilamellar vesicles (MLV) on fused silica substrates. By simultaneous cross-polarization light microscopy and HR-RICM measurements, it was observed that ULV with the wall thickness of a single bilayer would be significantly deformed in its equilibrium state on the substrate as the dimension of its adhesive–cohesive zone was 29% higher than the theoretical value of a rigid sphere with the same diameter. Besides, electrostatic interaction was shown as a significant driving force for vesicle adhesions since the reduction in pH significantly increased the degree of deformation of adhering ULV and heterogeneity of the adhesion discs. The degree of MLV deformation on the solid surfaces was significantly less than that of ULV. When the wall thickness of vesicle increased, the dimension of contact zone was reduced dramatically due to the increase of membrane bending modulus. Most important, the adhesion strength of colloidal adhesion approached that of specific adhesion. Finally, the increase of osmotic stress led to the collapse of adhering vesicles on the non-deformable substrate and raised the area of adhesive contact zone. To interpret these results better, the equilibrium deformation of adhering vesicle was modeled as a truncated sphere and the adhesion energy was calculated with a new theory. [Copyright &y& Elsevier]

A hyperelastic shell with spherical cap geometry is compressed to a rigid planar substrate. Using Maugis' graphical method and a numerical approach, the quasi-static adhesion-detachment trajectory, the interrelation between applied load, approach distance and contact radius, and the critical pull-off parameters are determined for ranges of materials stiffness, shell thickness, shell depth and interfacial adhesion energy. [ABSTRACT FROM AUTHOR]

Abstract: Adhesion of a pre-stressed silicone rubber film to a planar graphite surface was investigated by a new cylindrical punch method. A homemade apparatus was constructed to meet force and displacement resolutions of 0.1μN and 10nm. When the punch approached the intersurface force range across the punch-film gap, the film jumped into contact at “pull-in”. Upon unloading, once the tensile load reached a threshold, a spontaneous delamination occurred at “pull-off” with a non-zero contact circle. A theoretical model was constructed based a simple energy balance. The new method can be used to characterize an adhesion interface between a pre-stressed free-hanging film and a rigid substrate. [Copyright &y& Elsevier]

Abstract: The elastic properties and adhesion of a thin silicone rubber membrane have been characterized. An effective instrumentation embodying a video-enhanced microscope, which meets force and displacement resolution of and 10nm, was developed. It provides the capability of simultaneously measuring both the applied force and the resultant displacement of the thin polymer membrane. A linear theoretical elastic solution was applied to quantitatively interpret the measured central deflection of the membrane under a circular concentrical load. Young''s modulus of the membrane can be easily determined once the applied force and the central deflection, together with the essential dimensions, are known. The membrane jumped into an adhesion contact when the punch approached the range of the inter-surface force across the punch-membrane gap. A pull-off event was observed at a nonzero contact circle when the tensile load reached a critical threshold. The experiment is basically consistent with a theoretical model based on linear elasticity and an energy balance. The new method can be used to measure mechanical behavior of prestressed ultra-thin biomembranes and thin walled biocapsules with a residual stress. [Copyright &y& Elsevier]

Photovoltaic backsheets are exposed to harsh outdoor weathering conditions throughout their service lives that can compromise their protective function, through adhesive debonding between their constituent layers and between the backsheet and the module. A large-scale study on adhesive degradation was conducted on 37 field-exposed modules, that spanned 19 different module manufacturers, that were deployed among six Köppen–Geiger climatic zones, and that were fielded between 0 and 28 years. Six outer layer polymer classes were identified among the backsheets: polyamide (PA), polyethylene terephthalate (PET), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), fluoroethylene vinyl ether copolymer (FEVE), and tetrafluoroethylene hexafluoropropylene vinylidene fluoride copolymer (THV). Two adhesion tests were used to measure the adhesive strength: the wedge test and the width-tapered single-cantilever beam (SCB) test. Adhesion energies were compared across exposure time and Köppen–Geiger climatic zone. Many of the PET-, PVF-, and PVDF-based backsheets experienced degradation of the adhesive layer between the backsheet outer and core layers. A consistent trend of decreasing adhesion energy with exposure time was observed in five out of the six backsheet types. Trends among adhesion energy and climatic zone, while expected, were not observed, possibly due to broad ranges in temperature, humidity, and precipitation defining Köppen–Geiger climatic zones. The cantilever beam measurements produced an upper cutoff of approximately 100 J/m2, above which no modules exhibited field delamination. The results are part of a two-part study quantifying adhesion in both field-weathered and indoor-exposed PV backsheets. • Thirty-seven (37) PV modules were retrieved from various regions around the world • Two adhesion tests were used to characterize backsheet adhesion • Adhesion energy decreased with number of years of field exposure • Backsheets with adhesive tie layers often showed failure in an adhesive layer • Modules with adhesion energies greater than 100 J/m2 showed no field delamination [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) backsheets provide critical moisture, mechanical, and electrical insulation to the backside of PV modules, but their continued functionality depends upon their ability to remain well adhered over years of harsh environmental exposure. A study of adhesive strength was conducted on several PV backsheet types exposed to indoor accelerated weathering. Two adhesion tests – the wedge test and single cantilever beam test – were used to measure adhesion energy in four backsheets: two with fluoropolymer-based outer (airside) layers – polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF) – and two with non-fluoropolymer-based outer layers–polyamide (PA) and polyethylene terephthalate (PET). The effects of changes in temperature, ultraviolet (UV) irradiance, relative humidity, and a periodic water spray were studied. The PET-, PVF- and PVDF-based backsheets all showed instances of debonding in an adhesive layer, which is commonly reported. For the PA-, PET-, and PVDF-based backsheets, adhesion energy dropped fairly rapidly with exposure, reaching very low levels by 4000 h. The PVF-based backsheet was relatively robust to weathering. Pull-off of an outer chalking layer in the PA- and PET-based backsheets suggested a sensitivity to UV. Adhesion in these two backsheets was also highly sensitive to changes in moisture level. Changes in UV irradiance and temperature did not show a statistically significant effect on adhesion loss for the exposure levels used. The present work is part of a two-part adhesion study on both field-weathered and indoor-exposed backsheets, and forms a basis for understanding adhesion degradation across a variety of backsheet types and degradation factors. • Four indoor-exposed backsheet types were adhesion-tested using two different methods. • Adhesion in the PVF-based backsheet was relatively stable. • The PA-, PET- and PVDF-based backsheets exhibited substantial decrease after 4000 hours. • The PA- and PET-based backsheets exhibited sensitivity to changes in moisture level. • The PET-, PVF-, and PVDF-based backsheets all exhibited debonding in an adhesive layer. [ABSTRACT FROM AUTHOR]