Bonding principle of EVA photovoltaic packaging film

The function of the film in the photovoltaic module is to stick the photovoltaic glass, the battery and the backplane together, and at the same time play a role of protecting the battery and isolating the air. The cost of the module is 3%-4%.
The types of PV film include EVA film and POE film. EVA film includes traditional transparent EVA film, white EVA film and others (such as anti-PID film, etc.), and POE film includes cross-linked, thermoplastic, white, co-extruded and many other types. Among them, EVA photovoltaic film occupies more than 80% of the market, and is a deservedly mainstream product.
This article will take you to understand the principle of EVA photovoltaic encapsulation film to achieve bonding, as well as the main factors affecting bond strength ~

Explanation of bonded package principle
The raw materials of EVA photovoltaic film are EVA resin, cross-linking agent, silane coupling agent and other modifiers, etc..
After stirring in the mixer, mixed in a closed container for a period of time, extract the mixture in the film production line, cast extrusion into molten film. After the molten film is cooled by several cooling rollers, it is cut and wound into rolls; it is tack-free at room temperature, which is easy to operate, and when the components are laminated, EVA is heated to a certain temperature and is in a molten state to achieve bonding.
High mobility
Better fluidity can ensure that EVA film is laid flat on the substrate, increasing the contact area with the substrate, relying on its own polarity and the performance of intermolecular forces between the substrate, the formation of physical bonding points to achieve bonding.
Silane coupling agent
Since PV glass is an inorganic material with smooth surface and PV backsheet is a fluorine containing material with difficult surface bonding, EVA adhesive film usually contains silane coupling agent to modify EVA to enhance its polarity. Through the heating and curing process, the silane coupling agent reacts with water and forms a covalent bond with the substrate and EVA, connecting the inorganic glass to the polymer EVA like a bridge, thus realizing the bonding effect.
Bond strength influencing factors
EVA’s own performance influence
Raw material EVA in VA content less, then heat resistance is good, but poor polarity, resulting in adhesive film bonding and low-temperature flexibility is poor; VA content is more, there is better low-temperature flexibility and bonding. In addition, the greater the melt index, the better the EVA mobility, good flatness, the more physical bonding points, the greater the peel strength; but the melt index to a certain extent, EVA will appear lower polymerization, resulting in their own strength is reduced, the bonding force is reduced instead. In order to maintain good bonding, the general use of VA content 28-33%, MI 10-400 resin.
Effect of EVA cross-linking degree
Cross-linking degree refers to the quality ratio of EVA film cross-linked into a network of molecules when heated. Component lamination, when the heating temperature of the laminator reaches the decomposition temperature of the cross-linker, the peroxide bond in the cross-linker breaks to form peroxide radical RO-, which is very easy to combine with the alkyl group of EVA branched chain H, two alkyl reactive groups combined to form cross-linked EVA
The degree of crosslinking determines the cohesive strength of EVA film, which affects the EVA film and backsheet and glass peel strength, which determines the quality of photovoltaic cell components.

Cross-linking agent added more, although the degree of cross-linking is high, but too much prone to aging yellowing; too little cross-linking agent, cross-linking is too low, bond strength and aging resistance is also affected, experimental analysis has proved that the degree of cross-linking in 75%-80% for the best.
In addition, the curing temperature, curing time, crosslinker content, the compatibility of the entire formulation system, glass and backing surface properties will directly or indirectly affect the EVA adhesive film peel strength.

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