What is temperature stability? (Lamination)

[says]

I'm doing some work on solar and I'm reading about properties of lamination materials, particularly EVA lamination.

Anyway, one of it's properties is 'temperature stability' but I can't find a clear definition of what this means.

EVA lamination is sandwiched in between a solar cell, so I can understand we wouldn't want the temperature of the EVA to get too hot, otherwise the heat might dissipate into the cell, making it less efficient.

 

[Chestermiller]

I'm not sure, but this might refer to the ability of the laminate to resist warping when its temperature changes. Laminates can warp because they cannot expand thermally (significantly) in the directions of the reinforcement.

 

[Kevin McHugh]

In polymers, thermal stability refers to the polymer's resistance to oxidative degradation upon exposure to elevated temperatures over time. Chain scission due to thermal degradation results in the formation of free radicals, which further react with the polymer chain. The polymer's molecular weight distribution changes to wider, flatter distribution, impacting its physical properties. Most polymers are protected from this phenomenon by the use of hindered phenolic antioxidants. The hindered phenols are free radical traps that form stable products after reaction with the free radical. Phosphites are used in conjuction with the hindered phenols as a secondary mechanism against free radicals. Typical concentrations of antioxidant are on the order 1000 ppm, and the ratio of primary to secondary antioxidant is usually 2:1. The stabilty of the EVA will depend on the vinyl acetate content of the copolymer. Low VA content is about 7%, and the highest content is 28%.To be honest, I've forgotten which way the relationship goes, but my gut says high VA content copolymers are less thermally stable than low VA content copolymers.

Thermal stability is usually determined by oven aging studies. The polymer is placed in an oven at 105 C for a period of time. The polymer is tested for yellowness index, impact resistance, and tensile and elongation. periodically. With this data, you can tell how long service life would be at a given temperature before failure.