Ethylene Vinyl Acetate Encapsulant Research Articles

Hydrophobic deep eutectic solvents as novel media for the recycling of waste photovoltaic modules

Disposing of end-of-life photovoltaic (PV) modules has posed a significant environmental challenge due to the absence of proficient and sustainable recycling technologies. Separating PV modules, which are ethylene–vinyl acetate (EVA)-bonded layers, including glass, solar cells (i.e., silicon wafers), and backsheets, establishes a foundation for effectively reclaiming valuable resources from PV modules. Removing EVA encapsulation is critical for disassembling PV modules; nonetheless, effective and feasible technology is currently limited. Conventional EVA dissolving agents (e.g., benzene and trichloroethylene) are highly toxic and effective only for non-cross-linked segments of EVA. Thus, developing efficient and environmentally benign reagents for PV waste recycling is urgently needed. Herein, a novel recycling route employing deep eutectic solvents (DESs) with switchable hydrophilicity has achieved successful separation of the component layers within PV modules. Complete separation of the glass and backsheet bonded by the EVA film was achieved under atmospheric pressure and moderate temperature conditions (80 °C for 2 h) by hydrophobic DESs composed of menthol and decanoic acid ([Men][DecA] (1:1)). [Men][DecA] effectively induced EVA swelling, concurrently accelerating EVA dissolution by substituting the acetate groups in the side chains of EVA with the hydroxyl groups of the DES. The superior PV module disintegration can be attributed to the exceptional EVA dissolution ability of the [Men][DecA] framework, and the achieved high solid–liquid ratio (40 g/L) for EVA in [Men][DecA] is beneficial for industrial processes. Furthermore, the reusability of [Men][DecA], which is endorsed by its switchable hydrophilicity, introduces a sustainable platform for PV module disintegration, thereby facilitating the large-scale recycling of PV waste.

Assessing Field Degradation of Photovoltaic Modules by Near‐Infrared Absorption Spectroscopy of Ethylene Vinyl Acetate Encapsulant

The degradation status of polymer encapsulants in photovoltaic (PV) modules with backsheets of various compositions is evaluated by near‐infrared absorption (NIRA) spectroscopy. The method is used to identify backsheet types and to assess the content of carbonyl species in the ethylene vinyl acetate copolymer encapsulant expressed as a carbonyl index (CI). The CI distributions are collected for PV modules of different ages and in different climatic conditions from twelve PV installations in Europe (EU) and South Africa (ZA) and the peak of the index distribution is found to be related exponentially to the total dose of solar irradiation. The CI growth rate with the irradiation dose in EU is around two times larger than in ZA, most probably due to more massive moisture ingress into PV modules installed in a more humid EU climate. For similar total solar exposures, the CI is found to depend on the backsheet structure. Repeated measurements on the same plant after an additional 2 years of module operation confirm the capacity of NIRA spectroscopy to track the temporal progression of encapsulant degradation using CI as a quantifier depending on the encapsulant‐backsheet bill of materials, plant age, and climatic zone.

Advanced analysis of ethylene vinyl acetate copolymer materials for photovoltaic modules

Ethylene vinyl acetate (EVA) copolymers are commonly used as encapsulation material and as adhesive layer for backsheet laminates of photovoltaic (PV) modules. While Fourier-Transform Infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) are well established to analyse EVA encapsulants in PV modules, X-ray Photoelectron Spectroscopy (XPS) is seldomly used. Hence, the objective of this paper was to evaluate the potential and limitations of XPS for advanced analysis and testing of EVA encapsulants. Consequently, EVA grades of different vinyl acetate (VA) comonomer content were characterized and evaluated systematically by XPS, FTIR and DSC. Independent on the analytic technique linear relationships between the obtained values and VA contents were confirmed. Slightly higher VA contents on the surface deduced by XPS were attributed to a surface layer rich of amorphous phase material. In addition, XPS analysis of peroxide modified encapsulants with a VA comonomer content of 30.2 wt% revealed a higher carbon content on the surface after crosslinking. Hence, it is proposed that peroxide crosslinking is associated with deacetylation and the subsequent release of low molar mass decomposition products. Overall, the exceptional surface sensitivity of XPS was clearly confirmed for analysis and testing of EVA grades, highlighting the potential of the method for further degradation and interaction studies of components of PV modules.

Predicting encapsulant delamination in photovoltaic modules bridging photochemical reaction kinetics and fracture mechanics

AbstractPhotovoltaic (PV) modules are subjected to environmental stressors (UV exposure, temperature, and humidity) that cause degradation within the encapsulant and its interfaces with adjacent glass and cell substrates. To save experimental time and to enable long‐term assessment with intensive degradation only taking place after many years, the development of predictive models is indispensable. Previous works have modeled the delamination of the ethylene vinyl acetate (EVA) encapsulant/glass and encapsulant/cell interfaces under field aging conditions with fundamental photochemical degradation reactions that lead to molecular scission and loss of interfacial adhesion, characterized by the fracture resistance, Gc. However, these models were fundamentally limited in that the following aspects were not incorporated: (i) molecular crosslinking in the field, (ii) synergistic autocatalytic interactions of degradation mechanisms, (iii) connection between degraded encapsulant structure and its mechanical properties, and (iv) rigorous treatment of the plasticity contribution to Gc with finite element models. Here, we present a time‐dependent multiscale model that addresses these limitations and is applicable to a wide range of encapsulants and interfaces. For the reference EVA encapsulant and its interfaces with the glass and cell, the presented model predicts an initial rise in Gc in the first 3 years of field aging from crosslinking, then a subsequent sharp decline from degradation mechanisms. We used nanoindentation to measure the changes in EVA mechanical properties over exposure time to tune the model parameters. The model predictions of Gc and mechanical properties match with experimental data and show an improvement compared to previous models. The model can even predict switches in failure interfaces, such as the observed EVA/cell to EVA/glass transition. We also conducted a sensitivity analysis study by varying the degradation and crosslinking kinetic parameters to demonstrate their effects on Gc. Model extensions to polyolefin elastomer‐ and silicone‐encapsulants and their interfaces are also demonstrated.

Improvement of solder interconnections applied on back contact solar cells with low-temperature copper paste busbars

This work addresses the solderability and the reliability of n-type IBC ZEBRA cells with screen printed copper paste busbars. Improvement of the solderability by Sn60Pb40 solder alloy coated PV ribbon using an industrial automated IR stringer is reported. For qualification of the module reliability, climate chamber thermal cycling (TC) and damp heat tests (DH) were performed on mini modules with variations in material combinations: low temperature silver and copper pastes, each of them demanding dedicated soldering process parameters and two different encapsulation materials: polyolefin elastomers (POE) and ethylene vinyl acetate (EVA). All tested modules passed 3 x IEC TC and 2 x IEC DH, while the modules with EVA encapsulant passed DH 3000 as well. The copper paste metallized modules with EVA encapsulant showed no degradation in pseudo fill factor (pFF) and open circuit voltage (VOC), proving the statement that no copper diffuses into the silicon bulk.

An insight into a combined effect of backsheet and EVA encapsulant on field degradation of PV modules

AbstractPolymer components, in particular, backsheets (BSs) and ethylene‐vinyl acetate (EVA) encapsulants, play a crucial role in the degradation of commercial silicon photovoltaic (PV) modules. Degradation mechanisms depend in a complex way on the composition and properties of all involved polymers. In this study, we demonstrate that differences in the composition of EVA encapsulants, in particular, the presence of additives, such as UV blockers, affect the degradative processes in a decisive way, even for modules with identical BSs. A combination of spectral, imaging, and electrical characterization techniques applied in a mixed lab/field study showed that the field aging of PV modules with the same BSs and different encapsulant components results in the development of different degradation signs, such as insulation resistance loss, corrosion of metal interconnects, water ingress, oxidation depth in the encapsulant, and potential‐induced degradation, depending on the presence of additives in the encapsulant. The present study provides an outlook on the complex influence of polymer components on the lifetime and performance of silicon PV modules.

Thermal behaviors of ethylene vinyl acetate encapsulants in fielded silicon photovoltaic modules

AbstractAging of silicon photovoltaic (PV) module packaging is one of the greatest limiters of PV module service lifetimes. Module characterization typically focuses on power degradation metrics, which do not convey the complexities of often simultaneous degradation mechanisms. In this work, PV modules with pristine references and known fielding histories were investigated by non‐destructive and destructive methods. Modules from Canadian Solar, Mission Solar, and Hanwha Q‐Cells were fielded for up to three years; select modules were removed from fielding each year for coring to allow for characterization of the encapsulant. Modules are commonly encapsulated with two protective layers of partially‐crystalline ethylene vinyl acetate (EVA) polymer that must undergo a crosslinking reaction to achieve desired properties. The extent of crystallinity of the encapsulants as studied by differential scanning calorimetry showed differences between manufacturers and over time. Some encapsulants showed different magnitudes of crystal sizes which changed after fielding; encapsulants with the monodisperse crystal sizes did not change with fielding. This is due to differences in thermal history. These results have implications for stress development during module aging, since EVA crystal melting and crosslinking reactions can result in encapsulant density changes.

UV resilient thermoplastic polyolefin encapsulant for photovoltaic module encapsulation

A new UV-365 acceleration test is used to study the reliability of the newly developed thermoplastic polyolefin (TPO) encapsulant and the most commonly used ethylene vinyl acetate (EVA). Two orders of magnitude higher rate of decrease in transmittance and the rate of increment in yellowness have been found for EVA as compared to TPO. Fluorescence images show the fluorophores generation in EVA laminate as the UV dose increases. Fluorescence image and spectra clearly showed the development of brown-yellow color fluorophores emission in the visible region for the EVA laminate. No significant fluorescence is observed in the TPO laminate. The Raman spectra of TPO laminate showed a lower fluorescence background at the center than the edge, whereas the EVA laminate shows the opposite trend. A 35% and 12% increment in the crystallinity has been observed for EVA and TPO encapsulants, respectively. One order of magnitude lower increment rate in the carbonyl index and ether index has been found in TPO compared to EVA. Eighteen days of the UV-365 test is sufficient to identify the EVA early discoloration/fluorophores generation. This method can be very beneficial for the PV industry and encapsulant manufacturers for quickly assessing the reliability of any encapsulant.

Thermal Lifetime Estimation of EVA Encapsulants from Activation Energy based Method

A rapid test method based on a logarithmic degradation model for the lifetime assessment of ethylene-vinyl acetate (EVA) used as encapsulants in Photovoltaic (PV) module is proposed. In general, encapsulants are used in widely varied conditions. However, factors such as voltage stress, irradiation, mechanical shock and vibration, environmental conditioning and chemical contamination should be evaluated. In the present study evaluation is carried on the dumbbell specimens of encapsulating material itself and is based on the percentage of reduction in the property i.e elongation percentage, which is a destructive test. Dumb-bell specimens as per ISO 37:2011(E) standard are employed. Adequate number of test specimens were subjected to thermal aging at three different temperatures 35 °C, 45 °C and 55 °C. The conditioned specimens after removing from oven were stored in decicator prior to testing i.e. while the specimens were attaining room temperature. The constant factors of the life time line a and b were calculated, and finally, the lifetime values were estimated.

A comparison of evolution of adhesion mechanisms and strength post damp-heat aging for a range of VA content in EVA encapsulant with photovoltaic backsheet

Ethylene vinyl acetate (EVA) is an effective encapsulant to adhere various layers in the photovoltaic (PV) module. In this study, we have fabricated EVA films containing different vinyl acetate (VA) content (18%, 24%, 33% and 40%) and laminated between two layers of backsheet (BS). These laminates have been subjected to damp-heat (DH) ageing at 85OC and 85% relative humidity (RH) for 1000 h to evaluate the change in adhesion strength at EVA/BS interface using the T-peel test. The adhesion strength for EVA containing 18% VA content has been found to improve but it deteriorates for EVA with 33 and 40% VA content after ageing. However, adhesion strength for EVA containing 24% VA content is found unaffected after ageing. The degree of cross-linking and VA content have been determined through gel content measurement and thermo-gravimetric analysis (TGA) respectively. The degree of cross-linking and VA content increases significantly in EVA18 but decreases in EVA40 due to DH ageing. The chemical changes have been examined by the Fourier transform infrared spectroscopy (FTIR) and the X-ray photoelectron spectroscopy (XPS). The failure mode (cohesive or delamination) at the interface and its correlation with the adhesion strength has been explored using XPS. The failure mode at the EVA18/BS interface alters from delamination type in pristine condition to the cohesive type due to DH ageing. However, the failure mode for EVA24 and EVA40 is found to be cohesive in both the pristine and aged condition.

Predicting Early EVA Degradation in Photovoltaic Modules From Short Circuit Current Measurements

One of the common failures in photovoltaic modules is the degradation of the ethylene-vinyl acetate (EVA) encapsulant due to prolonged ultraviolet exposure and other environmental stress factors, such as temperature and humidity. Experimental studies have shown that significant reduction in the optical transmission due to EVA degradation leads to loss in the available power by more than 50%. In this article, a novel approach to predict the early degradation of EVA encapsulant is proposed by correlating EVA degradation with short-circuit current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ). An electrical circuit simulator, simulation program with integrated circuit emphasis (SPICE), is used to evaluate the short-circuit current obtained under varying optical transmission caused by EVA discoloration. The simulation follows three steps: simulation of the transmitted solar spectrum; simulation of the spectral short-circuit current density; and simulation of the current-voltage (I-V) curve to obtain short-circuit current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ), maximum power output (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ), open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) and fill factor. Results show that the reduction in short-circuit current due to EVA degradation differs from the reductions expected due to a spectrally-uniform reduction of intensity of the solar irradiance. Both types of variation are linear, however, the slope due to EVA degradation is larger than the slope obtained for normal intensity variations in the solar irradiance. This model, when applied in conjunction with solar irradiance measurements, can predict early onset of EVA encapsulant failure, thereby enabling preventative measures to be taken.

Inspection Data Collection Tool for Field Testing of Photovoltaic Modules in the Atacama Desert

The Atacama Desert receives the highest levels of solar irradiance in the world with an annual average of 2500 kWh/m2 for the global horizontal irradiance and 3500 kWh/m2 for the direct normal irradiance. One of the challenges is the large portion of ultraviolet light. This part of the spectrum be detrimental for the encapsulant materials, reducing their lifetime. To develop a module adapted to the Atacama Desert conditions, it is imperative to have standardized information from first-hand about the typical faults experienced by photovoltaic modules operating in the desert. This work reports on the design and implementation of the Inspection Data Collection Tool to evaluate crystalline silicon-based moules operating in desert climates. The tool brings together novel features of compatibility with current standards, efficient mobile-type instrumentation (equipment and tools), clear procedures/protocols for non-expert users and low development costs. A total of 95 modules were inspected to characterize failure/degradation issues. Three components of the solar modules were assessed: front cover glass, ethylene-vinyl acetate encapsulant and solar cells. Seven abnormalities were analyzed: Soiling, front cover glass discoloration, encapsulant delamination, hotspots, partial shading, cell fracture and faulty soldering. Soiling was the most common issue, showing correlation between dust deposition and location.

Damp heat resilient thermoplastic polyolefin encapsulant for photovoltaic module encapsulation

Thermoplastic polyolefin (TPO) is a non-crosslinking material used in photovoltaic module encapsulation. TPO thermal degradation and its weather stabilities are still not known. In this work, TPO laminates were prepared and subjected to extended damp-heat weather test. TPO encapsulant properties were studied and compared with the most commonly used encapsulant, Ethylene-Vinyl Acetate (EVA). Four times lower rate of transmittance loss and increment rate in the yellowness index values have been found for TPO compared to EVA laminate. Damp-heat induced degradation mechanism, and its components were studied using Raman and Fourier-transform-infrared (FTIR) spectroscopy. Raman spectra revealed that the carbonyl group loss rate at the edge is one order of magnitude higher than the center. FTIR has shown a higher rate of acetic acid and hydroxyl group formation in EVA as compared to the TPO. FTIR also demonstrated that TPO has a lower water absorption because of its nonpolar nature, whereas EVA shows more water absorption due to its polar nature. The rate of change in thermal properties of TPO and EVA encapsulants were studied through the heat-cool-heat cycle. A higher rate of increment in the degree of crystallinity is found for EVA as compared to the TPO encapsulant during the extended damp-heat acceleration test. This study helps to understand the TPO and EVA encapsulants degradation behavior, degradation components, and mechanism to the encapsulant researchers and PV module manufacturers when a PV module experiences the hot-humid conditions. This TPO may result in the industry adopting the different PV modules for fabricating.

In-Plane Residual Stress Map for Solar PV Module: A Unified Approach Accounting the Manufacturing Process

The photovoltaic (PV) cell fracture reduces the efficiency of solar PV modules and the residual stresses generated in the silicon cells by various stages of manufacturing process are attributed as one of the primary reasons among others. In this study, a unified approach to investigate the residual stresses present across the PV module owing to differing material properties and thermal cycles during manufacturing process is attempted. In particular, an in-plane residual stress map for different layers of solar PV module and the stress jump across the layered structure arisen during the manufacturing process are quantified through finite element method by considering ethylene vinyl acetate encapsulant as a linear viscoelastic material. The maximum residual stress presence in the cell is observed during the encapsulant curing cycle of lamination process. The predictions on the stress pattern and magnitudes from the simulations collaborate well with the experimental observations reported in literature.

Degradation of copper‐plated silicon solar cells with damp heat stress

AbstractCrystalline silicon solar cells with copper‐plated contacts are fabricated, encapsulated in ethylene‐vinyl acetate (EVA), and subject to extended damp heat stress (85° C and 85% relative humidity). We source cell precursors from several different cell manufacturers and employ several different patterning methods of the silicon nitride layer and deposit a plated front contact stack of nickel, copper, and tin using light‐induced plating. Across different Cu‐plated samples, we find similar degradation that impacts both series resistance and diode quality of the cells, indicating that there is some degradation of the p‐n junction. The overall degradation is on the order of 15%–20% of maximum power (PMP), and roughly half of this degradation is attributable to degradation of the p‐n junction. Control samples with silver‐screenprinted contacts do not exhibit the same degradation, and p‐n junction degradation in copper‐plated samples is prevented by changing the encapsulant from EVA to a polyolefin. The degradation mode is hypothesized to be the diffusion of copper from the contact, followed by the transport of this copper into the silicon cell via some mechanism facilitated by the degraded EVA encapsulant.

Nondestructive characterization of polymeric components of silicon solar modules by near-infrared absorption spectroscopy (NIRA)

Degradation of the encapsulation of solar modules is a crucial problem and, accordingly, tools for assessing and understanding the according degradation is an important research topic. Here, we suggest near-infrared absorption spectroscopy (NIRA) as a tool that has a strong potential to determine non-invasively the composition of encapsulant and backsheet in typical commercial solar modules. Correct polymer identification by NIRA was supported by Raman spectral microscopy of the backsheet air sides and cross sections, with a hundred solar modules of different manufacturers analyzed by both spectroscopic methods. Furthermore, a specific NIRA measurement regime of the ethylene vinyl acetate encapsulant is presented, which potentially allows to probe the composition of the encapsulant non-destructively under lab conditions but also on silicon solar modules in the field installed outdoors. The present report shows NIRA to be a powerful method for non-destructive analysis of backsheets and encapsulants of photovoltaic silicon modules.

Bending fatigue damage reduction in indium tin oxide (ITO) by polyimide and ethylene vinyl acetate encapsulation for flexible solar cells

Hybrid organic-inorganic perovskites hold tremendous promise for next-generation portable power source applications. However, the device reliability under severe mechanical deformations is limited due to the use of brittle indium tin oxide (ITO). In the current study, we have attempted to enhance the bendability of the device by employing an encapsulation on the ITO coated polyethylene terephthalate (PET) substrate. The aim of this encapsulant buffer layer is to shift the location of the maximum bending stresses away from the ITO as well as provide barrier properties against the environment. Ethylene-vinyl acetate (EVA), as well as Polyimide (PI) films, have been used as encapsulating materials with the aim to shift the location of the neutral axis to the ITO. Finite element analysis has been done to evaluate the principal stresses generated in ITO during monotonic bending under different modes of bending, the thickness of encapsulant layers as well as radii of bending. The effect of the number of bending cycles, mode of bending as well as the bending radii on the conductivity of the ITO have been studied experimentally. A large change in resistance of ITO film has been used as a metric to determine the failure of the ITO film. Samples encapsulated with an optimum thickness of PI shows a significant reduction in percentage change in resistance as compared to raw samples for both cases of bending, while EVA encapsulated samples hardly show any improvement. Moreover, both the modulus and thickness of an encapsulant play an important role in enhancing the bendability.

Investigation of newly developed thermoplastic polyolefin encapsulant principle properties for the c-Si PV module application

Ethylene-vinyl acetate (EVA) is the predominating material of choice for making the encapsulant film for photovoltaic (PV) modules. The easy accessibility, low cost, high transparency, long track record, widespread know-how on processability and performance, and to some extent, ignorance of the criticality of encapsulant film on the long-term performance of PV modules have made EVA, a dominant player in the PV industry. In parallel, due to economic reasons, the majority of encapsulant development has moved to a direction of compromising the quality to meet the cost target. In recent years, the PV industry has started recognizing, polyolefin-based encapsulants as technically superior when compared to EVA. Polyolefin-based encapsulant comes in either a crosslinked or thermo-plastic version. Thermoplastic polyolefin offers several advantages related to processability and performance over the crosslinked version. In the current study, we compare a newly developed thermoplastic polyolefin-based encapsulant and a state of the art EVA encapsulant from different aspects of fundamental material properties, like optical, thermal, mechanical, etc. and discuss their implication on the performance of a solar module.

Preparation and properties of nano zinc oxide doped ethylene vinyl acetate nanocomposites for solar cell encapsulation

Long term stability of the encapsulant is needed to resist heat, moisture, ultraviolet (UV) light and other weathering stresses. In this study, a new way of improving the properties of ethylene vinyl acetate (EVA) encapsulant was reported by doping a very small quantity (0.05 wt%, 0.1 wt% and 0.15 wt%) of nano Zinc Oxide (n-ZnO). EVA/n-ZnO composites were made by solvent casting process with addition of surface modified n-ZnO, followed by compression moulding to achieve thin films with thickness of 0.4 mm to 0.5 mm. Then, the intrinsic EVA and EVA/n-ZnO nano-composites were characterized to check the electrical, thermal, mechanical, and optical properties. The nano-composites exhibited superior electrical insulation than the intrinsic one. High temperature electrical insulation was remarkably good for the nano-composites. The effect of n-ZnO was also prominent in the mechanical behavior of encapsulants; tensile strength increased from 3.7 MPa (for intrinsic EVA encapsulant) to 15.3 MPa (for 0.1 wt% nano-composite). Elongation at break was also increased for nano-composites. EVA nano-composites were able to maintain low dielectric constant and low dissipation factor even at elevated temperature. Another one important criteria is light (UV and visible) transmission through the encapsulant. UV and visible light transmittance spectrum of EVA nano-composites were found better than the commercially available encapsulant. Therefore, n-ZnO was successfully doped on to the EVA matrix to provide better electrical, thermal, mechanical, and optical properties even at elevated temperature.

Tuning solar absorption spectra via carbon quantum dots/VAE composite layer and efficiency enhancement for crystalline Si solar module

AbstractIn this work, fluorescent carbon quantum dots (CQDs) composite transparent film with blue emission was prepared via hydrothermal fabrication, mixing with vinyl acetate‐ethylene (VAE) emulsion, coated and cross‐linked on the glass in sequence. This CQD/VAE film has good sticking ability and compatibility of sealing ethylene‐vinyl acetate (EVA) material and works well as luminescent downshifting (LDS) materials. Based on these features, we did a set of optical characteristics analysis for CQD/VAE film and inserted a CQD/VAE film between glass and EVA encapsulation in a mini metal wrap through (MWT) solar module. This LDS could convert UV part of solar spectra, which are normally blocked by EVA, into the blue range and improve the light absorption by solar cell. Enhancement in the performance of solar modules was proved, especially the improvement of current of short circuit (Isc), external quantum efficiency (EQE) of the UV wavelength and solar module efficiency. This material provides the chance for prompt application, since it simply solves the dispersion problem of LDS material inside encapsulation and keeps the transparency of sealing polymer materials.