The Secret of Crosslinking

The Background

The relevant PV (Cable) Standards require the polymeric materials used to be crosslinked. Before we go into different methods of crosslinking and its (dis)advantages, let′s have an understanding of what crosslinking really means.

The molecules chains of polymeric materials are "free elements" and if they are not bonded together, they tend to lose its shape (form) when exposed to some sort of stress (typically heat).

For that reason, we need to ensure that there will be a bond created between the molecular chains of the polymers. This can be achieved either chemically or physically. Crosslinked polymers are thermally, mechanically stable, and insoluble due to the strong bonds. Once those crosslinks are formed, it is difficult to break them apart. The final products are close to impossible to re-process. The PV industry is using crosslinked materials (not only cables), as it is required for the polymers to withstand a long-term heat exposure without the risk of damaging (melting, reshaping) its structure.

The left-side picture shows the molecules independently flowing, while the right-sided picture indicates the bonds (red dots) of the molecules after the crosslinking procedure.

Advantages of Crosslinking (Effects on materials)

1. Thermal Properties

Among all, thermal properties show the most significant effect after crosslinking. The following are the most significant ones:

• dimensional stability
• flame resistance
• heat distortion temperature
• creep
• glow wire resistance

2. Mechanical Properties

Improvement of physical/mechanical characteristics (without any noticeable dimensional change) can be observed on various types of polymeric materials after crosslinking. The following changes can be observed:

• improved impact resistance
• improved creep resistance
• improved resistance to stress cracking
• improved resistance to abrasion

3. Chemical Properties

Crosslinked plastics typically show an improved resistance to:

• Aggressive chemical agents, such as salt haze, grease, ammonia, or oil
• Hydrolysis
• Solvents/acids

How to crosslink the materials?

The most common way is to use chemical additives. This can be done by nearly every extruder/plastics-processing company. No special equipment is needed, no special know-how is required. Quick′n dirty.

As an alternative to adding chemical compounds, there is a possibility to use a radiation crosslinking. This one is based on the effect of high-speed electron flows (typically beta or gamma rays). It requires a highly sophisticated, and very expensive equipment, which is typically installed in specially designated shielded buildings (similar requirements as for areas with high x-ray radiation). It is not only the equipment itself - gaining the experience, going through the learning phase and creating own know-how for the technology is a true treasure for the companies with such equipment, and typically it takes years before gaining the truly top-class performance. The high level of complexity combined with enormous investment costs are the reason, why there are only very few cable makers, who are able to provide electron-beam crosslinking service.

I can already imagine your thoughts by comparing both methods - "the electron-beam crosslinking must be so expensive due to the massive investment needed". Actually it is, but....

...let′s summarize!

The chemical additives used for crosslinking trigger a chemical reaction which never stops. The crosslinking process continues through the whole lifetime of the polymeric product. The more bonds are created, the harder and stiffer the material gets. In case of the PV cables, this is something what you wish to avoid, because the chemically crosslinked cable will have the tendency to get more & more rigid, despite it shall remain its flexibility. The more rigid the cable gets, the more it tends to have (micro)cracks on the surface and the more it can lead into dangerous situations due to the insulation fragility. What happens afterwards, I have already demonstrated in previous articles.

Opposite to that - if the cable surface is treated with the electron-beam crosslinking, the effect of bonding the molecules is happening during the exposure time only. The exposure time, as well as the radiation intensity is precisely calculated and tested to achieve the most suitable properties. Once the exposure is over, the crosslinking process within the polymeric material is finished. Once it stops, it will remain it′s elasticity & flexibility over the whole lifetime. The physical properties of the cable will remain unchanged, despite the resistance of the cable against thermal, mechanical and chemical stress.

The difference between chemical and radiation crosslinking can be so significant, that several countries and its regulatory authorities related to the PV industry have clearly defined the requirement for electron-beam crosslinking only. It is simply worth it.

And perhaps a small bonus on top of it! The chemical crosslinking additives have a characteristic smell, which is attracting animals who enjoy chewing the cables. It would be dare to say that electron-beam crosslinking can substitute anti-rodent properties, however, if the rat would have the choice of chemically crosslinked vs. electron-beam crosslinked cable, its attraction over the chemicals will be quite obvious.

Lesson learned:

GET YOUR CABLES RADIATED!