In the manufacturing process of photovoltaic cells, the cross-linking degree of the EVA layer is an important factor in determining its performance. As a packaging material, the EVA layer not only protects the battery from the external environment, but also ensures the mechanical stability of the entire photovoltaic module. The advantages of EVA layer in adhesion, durability, optical properties, etc. make it more and more widely used in module and various optical products. The Degree of crosslinking Test System from Millennial Solar accurately measures the gel content of the sample through the xylene extraction method, thereby accurately calculating the cross-linking degree of EVA.

What is EVA material?

Among solar cell packaging materials, EVA is the most important material. It is a hot-melt adhesive film that is non-sticky and anti-adhesive at room temperature. After being hot-pressed under certain conditions, it will melt, bond, and cross-link and solidify to encapsulate solar cells. On the one hand, the cured EVA can withstand atmospheric changes and is elastic, and is bonded to the upper protective material glass and the lower protective material backing plate TPT; On the other hand, after being bonded to glass, it can increase the light transmittance of the glass, play the role of anti-reflection, and gain the output of the module. It has achieved quite satisfactory results in solar cell packaging and outdoor use.

What is EVA cross-linking degree?

The degree of cross-linking refers to the mass ratio of linear molecules cross-linked into network molecules when the EVA film is heated. When laminating photovoltaic modules, when the heating temperature of the laminator reaches the decomposition temperature of the cross-linking agent, the peroxy bonds in the cross-linking agent break to form peroxy free radicals RO -, which can easily interact with the H of the alkyl groups on the EVA branch chains. When two alkyl reactive groups are combined, cross-linked EVA is formed. The mechanism is as follows:

EVA cross-linking mechanism diagram

The cross-linking degree determines the cohesive strength of the EVA film and affects the peeling strength of the EVA film and the back glass. The cross-linking degree detection data will directly reflect the reliability of the photovoltaic module packaging, thereby determining the quality of the photovoltaic module.

What are the factors that affect the cross-linking degree of EVA?

Amount of cross-linking agent added: An increase in the amount of cross-linking agent added will increase the degree of cross-linking, but too much will easily cause aging yellowing; if the amount added is too little, the cross-linking will be too low. Bonding strength and anti-aging ability will also be affected, and experiments have proven that 75%-80% is optimal.

Lamination temperature: Lamination temperature directly affects the cross-linking degree of EVA. If the lamination temperature is too low, the degree of cross-linking may be insufficient, which may cause the cells to shift or even break. On the contrary, if the lamination temperature is too high, the EVA will age and turn yellow, affecting the service life of the components.

Six types of EVA films (labeled 1-6 respectively) were laminated at lamination temperatures of 140°C and 145°C. The prepared samples were tested for cross-linking degree according to chemical methods. The experimental results are as shown below.

Lamination retention time: Lamination retention time will also affect the cross-linking degree of EVA. If the holding time is too short, the degree of cross-linking may be insufficient; if the holding time is too long, the degree of cross-linking may be too high.

Cross-linking degree versus lamination time graph

 As shown, after approximately 3-4 minutes, EVA composites exhibit >80% gel content for lamination times >4 minutes, with an expected maximum gel content of 90±1%; As the lamination time increases, the stability decreases significantly: the sample with a lamination time of 2 minutes shows high variability, indicating the inhomogeneity of cross-linking in the early state of the reaction, which levels off as the reaction approaches completion.

How to conduct xylene extraction test?

1. Make a stainless steel mesh bag: Make a 120-mesh stainless steel mesh bag with an opening at the top, and weigh it (W1).

2. Sampling: Accurately weigh the cross-linked EVA gel sample on a 1/10000 electronic balance, put the sample into a stainless steel mesh bag of known weight, and weigh it (W2).

3. Extraction: Weigh the sample package to a weight of (W3) and suspend it in the reaction bottle. Use xylene as the solvent and heat to about 140°C, then boil and reflux for 3 to 5 hours. The general reflux rate is 20 to 40 drops/min.

4. Calculation: After cooling and taking out, place it in a vacuum oven for 3 hours to completely remove the solvent, place it in a drying dish to cool for 10~20 minutes, take it out and weigh it (W4).

Cross-linking degree C(%)=[1-(W3-W4)1(W2-W1)]×100%

W1: Weight of empty bag with three sides sealed and one side open

W2: Weight of the bag containing the sample (a bag with three sides sealed and one open)

W3: Weight of bag with sample seal

W4: Weight of sample bag after solvent extraction and drying

This test method is a solvent extraction method. Samples should be taken from different representative locations. The extraction agent must reach a certain degree of boiling and serve as a stirring agent. The bottom edge of the hanging sample bag should be close to the bottom of the bottle to ensure dissolution.

Millennial Degree of crosslinking Test System

Degree of crosslinking Test System includes standard configurations such as all-steel fume hoods, vacuum drying ovens, electronic balances, digital temperature control heating jackets, flasks, and condensers. In addition to testing the EVA cross-linking degree of photovoltaic module packaging, it can also be used to test other materials such as polyethylene cross-linking (PE-X), natural polymer ion cross-linking and polymer crystallinity.

E-mail: market@millennialsolar.com

Introduction:

Millennial Degree of crosslinking Test System determines the degree of crosslinking by measuring the gel content of the product, extracting the sample in the selected solvent for the specified time and weighing its mass before and after extraction to ensure that it has not been dissolved after extraction. The mass percentage of the residue (i.e. gel content) is taken as the cross-linking degree of the sample.

Fulfill the standard:

IEC 62788-1-6:2017, GB/T 29848-2018, CQC 3307-2014

Features:

•Heating power proportion is adjustable

•Has good corrosion resistance and chemical resistance.

•The temperature rises quickly, the temperature control is precise, and the operation is simple. It can measure 1 to 6 samples at the same time.

•The intelligent temperature controller has the functions of setting and measuring temperature, dual digital display, PID self-tuning, and accurate and reliable temperature control.

EVA Degree of crosslinking Test System is widely used in the quality control and testing links of production lines to ensure that products meet standard requirements through comprehensive testing of photovoltaic modules. At the same time, it can also help researchers gain an in-depth understanding of the manufacturing process and performance characteristics of photovoltaic modules, providing an important reference for the sustainable development of the industry. In the future, as photovoltaic modules develop towards higher efficiency and longer lifespan, Millennial Solar will help the solar energy industry move towards a brighter future.