EVA Crosslinking Degree Testing System

EVA Crosslinking Degree Testing System EVA Crosslinking Degree Testing System [Shanghai Taitest De] 1. Preface: The adhesive film for solar cell packaging is made of EVA as the base material, supplemented with several modifiers, and hot-rolled into thin film products by film forming equipment

1. Preface
The adhesive film for solar cell packaging is made of EVA as the base material, supplemented with several modifiers, and hot-rolled into thin film products by film forming equipment. This product undergoes cross-linking reaction when heated during the packaging process of solar cells, and belongs to thermosetting hot melt adhesive film. The cured adhesive film has a relatively high light transmittance, adhesive strength, thermal stability, airtightness, and aging resistance.
Linear or mildly branched polymers are transformed into a three-dimensional network structure through the action of crosslinking agents. It can significantly improve the cohesive strength, thermal stability, and other properties of polymers. In order to understand the relationship between the physical properties and crosslinking degree of EVA adhesives, as well as the relationship between crosslinking degree and crosslinking agent dosage, it is necessary to correctly determine the crosslinking degree. The determination of crosslinking degree is crucial for controlling production processes, selecting raw materials, and ensuring product quality.

2、 Experimental section
（1） Principle
EVA adhesive, due to heating, bonding and curing in the application process, part of EVA is cross-linked into gel. The solvent xylene is used to extract the uncrosslinked portion of the sample, allowing for the determination of crosslinking degree.
（2） Instruments and reagents
1. Instruments
(1) Large mouth round bottom flask and stopper. Connect with a grinding mouth or cork stopper. If testing 1-2 samples, it is advisable to use 500ml; It is advisable to use 1000ml for the determination of 2-3 samples; If several but not more than 6 are used, a 2000ml flask is more suitable.
(2) Heating sleeve or constant temperature oil bath. Require sufficient heat capacity to add boiling xylene (boiling point of 138-144 ℃).
(3) Reflux condenser tube. Connect the condenser tube with a grinding mouth or a cork to the flask. The grinding mouth size of the condenser tube should be consistent with that of the round bottom flask, and the length should be appropriate.
(4) Bracket and clip. Used for fixing flasks and condenser tubes.
(5) Vacuum oven (or ordinary oven), equipped with a vacuum source capable of producing a vacuum degree of 0.87 MPa and a thermocouple capable of measuring 150 ℃.
(6) 120 mesh stainless steel mesh.
(7) A tray electronic analytical balance with an accuracy of 1/10000 (usually tested in the laboratory, an analytical balance with an accuracy of 1/1000 can also be used, but to obtain very accurate data, an analytical balance with an accuracy of 1/10000 must be used).
(8) Drying machine.
Extraction device as shown in Figure 1
A - Copper wire for labels and hanging samples
B - Reflux condenser tube
C-Ring clamp
D - Inlet pipe
E - Grinding type cork stopper
F-Large mouth round bottom flask
G - Transformer
H - Water outlet pipe
I - Bracket
J-Xylene
K - Heating sleeve
L-120 mesh stainless steel mesh sample bag
2. Reagents
(1) Xylene A.R. can be analytical pure or chemically pure.
(2) 2,6-di-tert-butyl-4-methylphenol (i.e. antioxidant 264).

3、 Experimental methods
1. Analysis steps
The production of stainless steel mesh bags:
First, clean the stainless steel mesh, air dry it, and place it in an oven at around 100 ℃ for drying. After cooling, cut 35 × 90mm and fold it in half into a rectangular shape of 35mm × 45mm. Fold both sides into 7.5mm and nail them together to make a bag with an open top (approximately 20 × 45mm in size). Make sure that the folded sides can be nailed by a nail needle, and the prepared sample bag should be able to fit into a flask, with a size that is not too large. Weigh (W1).
Analysis operation:
Cut the cross-linked EVA adhesive samples (taken from different parts) into small pieces with scissors and store them at 1/10000 electron density
Accurately weigh 0.1450-0.1500g (accurate to 0.0001g) of the sample on a balance, place the sample in a 120 mesh stainless steel mesh bag of known weight, and weigh the bag and sample (W2).
Pin the folded edge of the bag to form a sample package, and weigh the bag and sample (W3).
The sample package is suspended in a flask with a reflux device using copper wire, and xylene is used as the solvent (the solvent should be sufficient, and a 2000ml flask should contain 1000g of solvent); A 1000ml flask should contain 500g of solvent. Boil and reflux for 5 hours, with a typical reflux rate of 20-40 drops/min. To prevent re crosslinking of the sample, about 1% antioxidant is added. After cooling, the sample bag is taken out (hung up to remove excess solvent) and then placed in a vacuum oven (temperature 141 ± 20 ℃, vacuum degree 0.8Mpa) for 3 hours. The sample bag is taken out and cooled in a dryer for 15 minutes, and then weighed (W4).
2. Calculation

W1: The weight of a bag that is sealed on three sides and opened on one side. （g）
W2: Weight of the bag containing the sample (bag with three sides sealed and one side open).
W3: The weight of the bag containing the sample seal.
W4: Sample bag weight after extraction and drying.

（4） Analysis and Results
Through multiple experiments, the results show that stainless steel mesh bags and uncrosslinked EVA resin do not affect the accuracy of this analysis method.
After 5 hours of extraction, there was little change in crosslinking degree.
As the amount of crosslinking agent increases, the crosslinking degree of EVA will also increase, which is consistent with the actual process.

（5） Discussion
1. This testing method belongs to solvent extraction method. The sample should be taken from representative different parts. The extractant should reach a certain boiling point to achieve stirring effect. The bottom edge of the suspended sample bag should be as close as possible to the bottom of the bottle to ensure dissolution.
2. The extractant xylene (containing antioxidant) can be reused until it becomes turbid. The turbid extractant does not need to be poured out and can be recycled for reuse, with a recovery rate of 80-90%.
Xylene is a toxic and flammable solvent that can only be used in fume hoods. Wear protective equipment and handle with caution.
3. Auxiliary equipment:
Copper wire, staples, fine copper wire, rubber tube, scissors, tweezers, pliers, needles, rubber gloves, cotton gloves, thermometer for 0-200 ℃, plate.

Crosslinking degree test

(1) Reagent: Xylene

(2) Sample preparation

Cut the cross-linked adhesive film into small pieces for later use

(3) Inspection steps

Wash, dry, and weigh the stainless steel mesh bag to W1 (accurate to 0.01g)

Take 0.5g ± 0.01g of the sample, place it in a stainless steel mesh bag, and weigh it as W2 (accurate to 0.01g)

Seal the bag opening to make a sample package, and weigh it to W3 (accurate to 0.01g)

The sample package is suspended in a flask with a fine wire under a reflux condenser tube. A 1/2 xylene solvent is added to the flask, heated to around 140 ℃, and refluxed for 5-6 hours at a reflux rate of 20 to 40 drops per minute

Cool down and remove the sample package, hang it to remove solvent droplets, then place it in a vacuum oven with a temperature controlled at 140 ℃ and a vacuum degree of 0.08Mpa. Dry for 3 hours to completely remove the solvent

Remove the sample package from the vacuum oven, cool it in a dryer for 20 minutes, and weigh it as W4 (accurate to 0.01g)

Calculation of results

C=[1-（W3-W4）/（W2-W1）]×100