From：Matrix ballast aluminum alloy pv bracket system Release time：2018-11-07
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As a key component of the crystalline silicon solar module, the backplate plays an important role in its safety, service life and reduction of power attenuation. To protect the battery, the backplane must have good mechanical strength and toughness, weather resistance, insulation, water vapor barrier, chemical corrosion resistance and other balanced performance. The backboard can be divided into two categories of materials: fluorine-containing backboard and non-fluorine-containing backboard. There is a consensus in the industry that fluorine-containing films and fluorine-containing coatings are used for backboard materials. At present, only fluorine-containing materials have been verified through long-term outdoor experiments, and the market share of fluorine-containing backboard is over 90%. With the fast pace of photovoltaic affordable access to the Internet, there is great pressure to reduce costs and improve efficiency in every link of the industrial chain, as well as the photovoltaic backplate, the most important packaging material for solar modules. In order to further reduce the cost of backplane, some enterprises try to use PET materials to replace the fluorine film or fluorine coating as the outermost layer of backplane, saving the cost of fluorine film or fluorine coating, so as to lower the price. But does the fluorine-free PET backplate still meet the performance requirements of the components?
1. Structure and changes of PET backplate
At present, the PET backplate mainly has the following two structures: the first one is a three-layer structure, which is in order to strengthen PET(50um), ordinary PET(150um) and E film adhesion layer. Adhesive adhesion is used between layers, which needs to be made by two composite processes. Authors in order to further meet customer demand, only around the PET, the structure of the A/B PET was born, the structure of the A/B PET with ordinary PET differs itself has relatively clear two layer structure, the general structure of A/B of the total thickness of PET in about 160 um, lateral thin accounted for 10% of the total thickness of UV resistant layer, the inner layer for the common PET, A/B structure are formed by co-extrusion, PET inside another E composite film or PO film adhesive layer deformation into two layers of structure of PET back, its A three layers structure of PET back process more simple and more thin, therefore less cost. In addition, the total thickness of PET, especially the UV resistant layer, decreased significantly, and the reliability was also significantly reduced.
Figure 1: different PET structure backplane
The aging mechanism of PET
As a kind of polymer material, PET is prone to hydrolysis, ultraviolet photoaging degradation and thermal aging degradation due to its own structure. Its internal structure is characterized by molecular chain oxidation, fracture and molecular weight reduction. When used in the outermost layer of the back plate, it is prone to surface yellowing, pulverization, embrittlement and even cracking. In order to improve its weather resistance, titanium dioxide inorganic pigments are usually added to the outermost PET to prevent the invasion of ultraviolet rays. However, in the process of outdoor use, due to the combined action of light and water, the surface of PET is easy to powder and whiten, resulting in the release of titanium dioxide, resulting in the decline of the performance of PET structure.
Wet and hot aging of PET
From the analysis of the failure mechanism of PET subjected to damp-heat aging, PET is very easy to hydrolyze under the condition of high temperature and high humidity, and H2O molecules attack ester bonds, as shown in figure 2, causing its ester bonds to undergo hydrolysis and fracture, generating oligomers with carboxyl groups, which can promote further hydrolysis reaction at the same time. The method of anti-hydrolysis of PET is to increase polymerization degree, increase molecular weight, decrease molecular weight distribution and increase crystallinity appropriately. With the continuous progress of technology, PET produced by this method has a higher degree of hydrolysis resistance at present. After PCT48h, there is no embrittlement, and some even can achieve PCT60h or more.
Figure 2:PET hydrolysis reaction
Ultraviolet aging of PET backplate
From the PET by ultraviolet aging failure mechanism analysis, ultraviolet aging reaction can be divided into PET photolysis and light oxidation reaction, in photolysis reactions, PET via radical rearrangement reaction, macromolecular main chain rupture, and produce the by-products, such as CO and CO2 gas change mechanical properties of PET material, such as tensile strength and elongation at break, as shown in figure 3. In the photooxidation reaction, hydrogen peroxide groups are generated on the aromatic ring of PET, and further hydroxyl or diphenol hydroxyl derivatives are generated. The yellowing of PET is also mainly caused by these fluorescent products, as shown in figure 4.
Figure 3: PET photolysis reaction figure 4: PET photooxidation reaction