Polyolefin (PO) Encapsulants: To Crosslink or not to Crosslink, That is the Question

The famous Shakespeare sentence “To be or not to be, that is the question” recalls us that we should continuously deal between, either suffering the problems as they come from the “outrageous Fortune” or “taking arms against a sea of troubles and by opposing end with them”.

This enquire is a general, timeless and worldwide question. In fact we are continuously answering questions everyday, and finally deciding between to leave things as they are or by the contrary, taking actions and trying to control our destiny.

The Photovoltaic Encapsulants do not escape to this and bring us in front of deals of this type…. But let’s go to the point, and let me explain myself.

Until now Photovoltaic EVA Encapsulants have been conveniently transformed from thermoplastics into thermosets taking a critical part of the PV modules. Before the lamination takes place the EVA is a thermoplastic polymer material. In a practical way, this means that it is a polymer that becomes pliable or moldable above a determined temperature that it solidifies upon cooling. But, during the lamination process, the EVA’s molecules suffer a series of chemical changes that bound the molecules together and convert the EVA into a different structured material, known as thermoset. As all thermosets it behaves as a solid, infusible and insoluble polymer web.

All thermoplastic polymers are “easy going” materials that can be melted and solidified as many times as needed. This job is an up and down temperature excercise. It is just a matter of reaching their soften temperature and later cooling down. The thermosets are a bit more difficult processing materials because to reach the final objective properties, they have to be processed accurate and precisely, following a recipe. 

Recently some new materials have entered the “arena at PV market”. They have been generically called and identified by the name of PolyOlefins (PO). But in fact these new Encapsulants belong to the same family of polymers as EVA. Turning around that, it means that the Ethylene VinylAcetate (EVA) is also a polyolefin, that historically has been named and known by its acronym, EVA.

Our Question 

The polyolefins (POE) have some important differences when compared to EVAs that make them a much better Encapsulants with improved dielectric properties. This great advantage has been disclosed in detail in a different article in our Web that we strongly recommend you. Unfortunately the same properties that make them excellent dielectric materials, are at the same time the responsable of their poor chemical reactivity. That means that they are very difficult materials to reticulate/crosslink and to convert into thermosets. Knowing the technical difficulties one might consider, as a film manufacturer, to produce a non crosslinkable PO, which is the easy option. But is it worth doing that from the module point of view?. So our question was: Should our PO Crosslink or not to Crosslink?. To take a decision, we analyzed what PO crosslinking/ non crosslinking would mean for the yield and life of the modules. 

Before going to the details, first of all, we need to recall that all polymers, including the polyolefins (EVA, PO, PP, PE…), might have some chain parts in their structure and not so regular ones. The regular chain sectors when cooling down the polymer might end up forming crystalline domains (organized and folded microstructures). These organized domains are responsible for the turbidity (haze) in the films.

See at the top part of the picture below a representative image of organized and non organized molecular parts of a standard PO. After lamination, if there is no crosslinking the polymer structure might form new crystalline domains than the original one had (bottom left structure). The structure is rebuilt during heating and cooling generating a similar one but not the same one. But if crosslinking takes place during lamination, then the possibility of internal molecular organization is minimal or impossible (bottom right structure), and a completely new polymer structure gets formed.

Knowing that, if we compare two photovoltaic modules made with the same type of Encapsulant but with the only difference that one it is not crosslinked and the other one it is crosslinked, then we will realize the following differences between them:

1.- Mechanical resistance. The crosslinked PO is going to keep its molecular structure and no changes will take place on it with ambient temperature changes (that can go from very low to 80ºC in hot areas). By the contrary the non crosslinking PO will suffer structural changes with day to day temperatures fluctuation affecting its mechanical resistance and reliability. Their elastic modulus and stress-strain behaviour will be out of control.

2.- Creep. In the case of non crosslinked PO the molecules are tied up and might suffer spatial displacements at high temperatures of operation. These internal movements are more intensive as the temperature increases. If the Encapsulant reaches the temperatures in the range of 70-80ºC then the softening of the material might force the displacement of the front from the back side of the module, forcing the creep effect.

3.- Optics. The optical properties (transmittance and haze) of the non crosslinked PO will be always lower than the crosslinked ones because of the presence of the crystalline domains. But the most dangerous thing is that because of the fact that the non crosslinked materials are not “frozen” the will change along the life of the module without any control. That means that the optics might fluctuate along the time, just depending on the weather conditions.

4.- Weatherability. In the case of non crosslinked PO, since the molecules have some freedom, the adhesion of the Encapsulant to the different type of substrates in a module might weaken, generating tensions between the different materials. These tensions facilitate moisture penetration inside the modules, degrading  electrical parts. 

5.- Dielectric properties. The isolation properties are also affected by the level of crosslinking of the PO. During operation, the structural consistency of the Encapsulant, being temperature dependent, might suffer a decrease of its dielectrical properties. In the case of the non crosslinked PO, the softening affects negatively to the dielectric properties of the materials.

When we considered all these important differences depending on providing a non crosslink or a crosslink PO, we decided to go for a Crosslinkable PolyOlefin Encapsulant. That was regardless of the hard effort that this decision represented to our Team. It was hard job, but finally at present we manufacture the NovoSolar® PL in Europe. This is the product that meets all mentioned properties and values critical for a module, that means the best Encapsulant that might be used to produce photovoltaic modules.

So, if you look around for a PolyOlefin (PO) type of Encapsulant, we suggest you not to choose a PO labeled with a T (a TPO Thermoplastic PolyOlefin type). Look better for a X label PO (a XPO Crosslink PolyOlefin type), or a POE type (PolyOlefin Elastomers) like others like to call XPOs. Don’t get mislead with letters…. 

Your Question  

And now is your Shakespeare type of question in turn: Should I use a Crosslinkable or a non Crosslinkable PO to build my modules?

What to do? Should you leave the Encapsulant to the “outrageous Fortune”, or should it be “frozen” for ever during lamination?……. at the end this is up to you depending on your Quality standards. You choose…. but don’t forget to look also at other properties…. like their optical, adhesion and dielectric properties. 

Your Suggestions and our Compromise   

If you have any comments about this subject we are eager to hear them from you. And be sure that if they bring good ones to us that help us to improve, you will be entitled for a valuable discount in your next NovoSolar® PLX+ purchase order…

THECHNICAL LIBRARY