The degree of crosslinking refers to the mass ratio of linear molecules cross-linked into network molecules when the EVA film is heated. When the solar module is laminated, heating to a certain temperature EVA is in a molten state, the cross-linking agent in the formulation decomposes to generate free radicals, and initiates the bonding of long-chain molecules of EVA, so that it can be combined with solar silicon wafer, ultra-white tempered glass, and TPT back. The board is bonded and cured, and the three layers of materials are integrated into one. The cured solar cell sheet no longer moves, and substantially does not cause heat shrinkage. The EVA cross-linking mechanism is as follows:
When the heating temperature of the solar panel laminator reaches the decomposition temperature of the crosslinking agent, the peroxygen bond in the crosslinking agent is broken to form a peroxy radical RO-, which is easily combined with the H of the EVA branch chain, two alkyl reactive groups combine to form crosslinked EVA.
There are two main reasons for affecting the degree of cross-linking of EVA: the amount of cross-linking agent added and the lamination temperature. At the crosslinking agent activation temperature, the degree of crosslinking increases with the addition of the crosslinking agent; under the quantitative crosslinking agent, the degree of crosslinking increases as the lamination temperature increases, but does not rise after reaching a limit, and vice versa. Also.
In order to improve the production efficiency of solar module manufacturers, the lamination process tends to be low-temperature lamination, short-time lamination, which mainly depends on the half-life of the cross-linking agent, active oxygen content, free-radical properties, etc. Currently, the industry generally uses laminating at 140 ° C. 10min lamination process; more cross-linking agent, although the degree of cross-linking is high, but too much aging yellowing; too few cross-linking agents, too low cross-linking, bond strength and anti-aging are also affected, Experimental analysis proves that the degree of crosslinking is optimal from 75% to 80%.
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The peel strength of the EVA film and the back sheet and glass determines the quality of the solar photovoltaic panel. EVA is non-tacky at normal temperature and is easy to handle. However, when it is heated to a certain temperature during the lamination process, physical and chemical changes occur, and the solar silicon wafer, tempered glass and back sheet are bonded. If the bonding is not strong, degumming can occur in a short period of time, and even the case where the glass falls off and the injured person appears.
The adhesion of EVA film is mainly determined by the EVA raw material and the coupling agent in the formulation. If the VA content of the raw material EVA is small, the heat resistance is good, but the adhesion and low-temperature flexibility are poor. When the VA content is large, the low-temperature flexibility and adhesion are good. In addition, the larger the melt index, the better the EVA fluidity, the better the tiling property, the more physical bonding points, the greater the peel strength; but when the melting index is large, the EVA will have a lower degree of polymerization, leading to itself the strength is reduced and the adhesion is reduced. In order to maintain good adhesion, a resin having a VA content of 28 to 33% and MI10-400 is generally selected.
EVA is a weakly polar material in polymers, and glass is an inorganic material with a smooth surface. It is difficult to bond together for a long time; TPT backsheet is a fluorine-containing material that is difficult to bond on the surface, so it needs to be formulated. A coupling agent is added to modify the EVA to enhance polarity.
A four-step model for the mechanism of action of silane coupling agent and resin is proposed. (1) Three Si-X groups connected to silicon are hydrolyzed to Si-OH; (2) Si-OH is dehydrated and condensed into Si-OH the oligosiloxane; (3) the Si-OH in the oligomer forms a hydrogen bond with the OH on the surface of the substrate (4) and forms a covalent bond with the substrate accompanying the water reaction during the heat curing. The coupling agent connects the inorganic glass to the polymer EVA like a bridge.
In addition, curing temperature, curing time, crosslinker content, compatibility of the entire formulation system, and surface properties of the glass and backsheet directly or indirectly affect the peel strength of the EVA film.
The bonding strength of the EVA film should be as reasonable as possible, not the bigger the better. The bonding strength is too low, and the adhesion to the back sheet and the glass is poor, and it is easy to peel off, failing to meet the technical standards and quality requirements of the product; the bonding strength is too high, the peeling strength is too large, the product is difficult to repair, and the solar cell sheet is easily caused damage. The durability of the bond strength is equally important. A good EVA film needs to maintain good bond strength after various aging tests.