In the booming photovoltaic industry, EVA has attracted much attention as a key photovoltaic material. The so-called EVA is a copolymer of ethylene and vinyl acetate, in which the VA content is in the range of 25% – 40%. It is essentially a hot melt adhesive. At room temperature, it is non-adhesive and anti-adhesive, which enables it to remain relatively stable when untreated, making it easy to store and transport. When heat-pressed under specific conditions, the magic happens: it melts and cross-links and cures, eventually transforming into a completely transparent substance. This cured EVA is tightly bonded to the glass, which greatly enhances the light transmittance of the glass, thus playing an indispensable role in improving the output transparency of solar modules. Its thickness is usually between 0.4mm and 0.6mm, with a flat surface and uniform thickness. It also contains a cross-linking agent inside, which can be successfully cross-linked at a curing temperature of 150°C, and a stable adhesive layer can be formed through the extrusion moulding process.
Historically, in the early days of photovoltaic materials, researchers experimented with various combinations of polymers, with EVA coming to the fore due to its unique properties. For example, EVA was first used in some of the early small-scale solar projects, and although optimisation of its performance was still in its infancy, it already showed potential for improving light transmission in combination with materials such as glass.
When analysed from a performance perspective, EVA has a number of outstanding properties. It is extremely flexible, just like rubber, and can bend to a certain extent without breaking, which allows it to protect the internal components in different installation environments and in the face of external impacts. Its impact resistance is also not to be underestimated, in the event of an external object impact can be absorbed and dispersed energy, to avoid serious damage to the solar module. Its elasticity allows it to recover quickly from minor deformations, ensuring the stability of the material. Optical transparency is an important advantage in the field of photovoltaics, enabling maximum light transmission, reducing light loss and improving photoelectric conversion efficiency. In low temperature environment, it can still maintain good flexibility, which is significant for some cold areas of solar energy facilities installation. Its adhesive properties allow it to bond with a wide range of materials to build a solid module structure. Environmental stress cracking resistance ensures that cracks will not easily appear and affect performance in complex and changing natural environments, such as wind and sand erosion and drastic temperature changes. Weather resistance enables it to withstand long periods of direct sunlight, rain, wind and snow, and other harsh climatic conditions. Chemical resistance ensures that when exposed to some chemical substances, no chemical reaction will occur and lead to performance degradation. Heat sealability facilitates encapsulation operations during the production process and improves production efficiency.
The properties of EVA are closely related to molecular weight, characterised by the melt index MI, and vinyl acetate content, expressed as VA. When the MI remains constant, an increase in VA content acts as an injection of more ‘oomph’ into the EVA, resulting in improved elasticity, flexibility, adhesion, compatibility and transparency. Conversely, if the VA content decreases, the EVA gradually converges to the performance characteristics of polyethylene. When the VA content is determined, a decrease in MI results in a lower softening point, improved processability and surface gloss, but a decrease in strength, although an increase in molecular weight enhances impact resistance and stress cracking.
In terms of VA content classification, EVAs in different content ranges have very different applications. For example, EVA with a VA content of 5% to 15% is widely used in agricultural films because of its relatively high hardness and flexibility, providing crops with good insulation and moisture retention while offering a certain degree of durability; in packaging films to protect products from external contamination and minor impacts; and in cable jackets to effectively insulate and protect the internal conductors of cables. When used in cable jacketing, it can effectively insulate and protect the conductor inside the cable. When the VA content is in the range of 15% to 40%, its flexibility and adhesion are further improved, so it is often used in the manufacture of shoe soles, providing comfortable foot feeling and good anti-slip performance; in the field of sealing strips, it can tightly fill the gaps and play the role of sealing and waterproofing, sound insulation, etc.; in the production of foam, it can make materials with good cushioning performance, and due to its good bonding performance with a number of materials, it can also be made into a variety of hot-melt materials, which can be used to make a variety of hot-melt materials. In the production of foam, it can produce materials with good cushioning performance, and because of its good bonding performance with many materials, it can also be made into a variety of hot melt adhesives used in the bonding process in industrial production, while EVA with a VA content of 40% to 70% is mainly used as a modifier for plastics processing, which can improve the performance of other plastics, such as increasing the toughness, improving the impact resistance, and so on. EVA with a VA content of 70% to 95% is sold as emulsions and is used in paint formulation to provide good adhesion and flexibility for coatings, and when used in paper and fabric coatings to enhance their water resistance, abrasion resistance and flexibility.
Temperature has a critical effect on the adhesion of EVA, which in turn has a direct impact on the performance and service life of the part. In the molten state, EVA bonds to crystalline silicon solar cell wafers, glass and TPT by both physical and chemical bonding mechanisms. Unmodified EVA has a transparent, soft appearance, hot-melt adhesion, low melt temperatures and good melt flow, all of which make it advantageous for initial applications. However, it also has obvious defects, poor heat resistance, easy to deform at high temperatures, large elongation and lack of elasticity, low cohesive strength, poor creep resistance. This results in the actual use of the process, easy due to thermal expansion and contraction phenomenon of the chip fragmentation, which in turn led to adhesive delamination and other serious problems, which will undoubtedly greatly reduce the performance and service life of solar modules.
In order to solve these problems, chemical cross-linking method came into being. Organic peroxide cross-linking agent is added to EVA, when EVA is heated to a specific temperature, the cross-linking agent will decompose to produce free radicals, these free radicals are like a ‘connection messenger’, triggering the combination of EVA molecules, and gradually forming a three-dimensional mesh structure, which ultimately leads to the cross-linking of the EVA adhesive layer and curing. When the degree of cross-linking reaches more than 60 per cent, EVA is able to withstand atmospheric changes better and the phenomenon of thermal expansion and contraction is effectively curbed. However, it should be noted that the degree of cross-linking is not the higher the better, according to theoretical studies and a lot of practical experience shows that the higher the degree of cross-linking, although the transmittance of EVA will be improved, the overall output power of the component will also be increased accordingly, after careful adjustment of the parameters of the laminating process, the degree of cross-linking of the EVA can reach a maximum of 95 – 98 per cent, but at this time in the application of the process of producing the risk of cracking will be sharply increased. On the other hand, EVA with a low degree of cross-linking is prone to delamination with glass and backsheets, which leads to a significant reduction in the mechanical properties of the internal circuits themselves. Currently, after much trial and error, manufacturers generally agree that a cross-linking level of around 85% is the optimum balance between performance and risk minimisation.
EVA also has a unique performance in terms of UV cut-off. The intensity of sunlight is distributed in a regular pattern, with 0.7nm – 280nm of light hardly reaching the earth, 280nm – 400nm in the UV region, 400nm – 750nm in the visible range, and 750nm – 3000nm in the infrared. Existing EVA products, such as Foster F406, have a low UV cut-off, while most EVAs produced by other manufacturers have a UV cut-off of 360nm – 380nm, which indicates that EVA itself has a certain UV cut-off capability. The UV cut-off relies on the UV absorbers inside the EVA, which absorb the UV light and convert it into heat to be emitted, thus protecting the solar module from excessive UV damage. However, there is a lack of detailed and accurate data on the lifespan of UV absorbers, which has become a mystery in the field of EVA materials research. Once the UV absorber fails, EVA may undergo property changes, such as yellowing, as a result of prolonged exposure to UV light.
The cross-linking reaction of EVA is a key part of its performance enhancement, as EVA film, as a thermosetting hot-melt adhesive, undergoes a cross-linking reaction during the heating process to form a thermosetting gel resin. Before lamination, the EVA film has a linear macromolecular structure. When heated, the cross-linking agent breaks down to form reactive free radicals, which trigger intermolecular reactions between the EVA molecules, gradually connecting the molecules to form a mesh structure. This web-like structure is like a solid ‘spider web’, which greatly improves the mechanical properties of EVA, making it more robust and durable; heat resistance has been significantly improved, enabling it to work stably at higher temperatures; solvent resistance has been enhanced, making it less susceptible to erosion by chemical solvents; and ageing resistance has been improved, enabling it to be used for long periods of time and The aging resistance has also been improved to maintain stable performance over long periods of time.
EVA films are made up of a number of components, including the EVA body, the crosslinking agent system (covering both the crosslinking initiator and the crosslinking agent), the polymerisation blocking agent, the heat stabiliser, the light stabiliser, the silane coupling agent and other components. These components work synergistically with each other to determine the performance of the EVA. For example, the cross-linking agent system is responsible for initiating the cross-linking reaction when heated, which builds up the mesh structure of EVA; the heat stabiliser protects EVA from excessive decomposition or deformation at high temperatures; the light stabiliser helps to protect EVA from damage caused by ultraviolet and other rays of light; and the silane coupling agent plays an important role in enhancing the strength of the bond between EVA and other materials.
In practice, EVA is subject to a number of failures. Yellowing is one of the more common problems, mainly caused by two factors. On the one hand, the additive system reacts with each other to trigger yellowing, which is like an internal ‘chemical reaction melee’, an undesired chemical reaction between different additives, thus changing the colour and performance of EVA; on the other hand, the EVA molecule is under the conditions of oxygen and light, and its own de-acetylation reaction leads to yellowing. Therefore, the design of EVA formulations is of paramount importance, as it directly determines the anti-yellowing performance of EVA. Bubbles should not be ignored, one of the internal components of the EVA bubbles generated by the failure to pump out in a timely manner, which is closely related to the EVA additive system, the degree of matching of other materials and EVA and laminating process and a variety of other factors; the other is a poor match between the materials in the lamination of the bubbles generated, which is just like two ‘personality’ partners forced together, which is the ‘personality’ of the partners. This is just like two partners with ‘incompatible personalities’ forcibly combined together, which will inevitably produce contradictions and problems. Delamination phenomenon also occurs from time to time, and the backplane delamination may be due to unqualified crosslinking degree or poor bonding strength with the backplane; and glass delamination may be silane coupling agent problems, glass surface is not clean or unqualified cross-linking degree and other reasons.
In summary, EVA as a photovoltaic material plays an extremely important role in solar modules, although it has many excellent properties, but also faces some challenges and problems. With the continuous progress of science and technology and in-depth research, it is believed that in the future, the performance of EVA will be further optimised, and its application in the photovoltaic field as well as other related fields will be more extensive and in-depth, contributing to the global energy transition and sustainable development. At the same time, research on EVA materials will continue to promote the development of the entire field of materials science, leading to the birth and application of more new materials.