What’s Next In Backsheets

TaiyangNews: At the SNEC 2016 show in Shanghai, your booth had a banner, saying "World's No 1 Backsheet supplier." How did you manage to become a market leader in less than a decade?

Mark Wu: Cybrid started questioning the conventional wisdom and provided the PV industry with solutions based on technology innovations.
To understand this we need to look at the history
of backsheets. Since the early days of PV to a decade back, backsheets had a typical structure — UV protective layer / core PET layer / UV protective layer. The main backsheet configurations were Tedlar/Polyester/Tedlar (TPT) and Kynar/Polyester/ Kynar (KPK). However, the inner UV blocker film is not required, as the inner layer is exposed to much less UV. The cumulative UV exposure of the inner film is about 1/10 of the outer layer of the backsheet. But companies like DuPont, the supplier of Tedlar in case of TPT configuration and Arkema, the producer of Kynar in KPK, have been promoting this kind
of configuration as they can sell more films per backsheet. DuPont, in particular, never allowed its vendors to change the structure.

However, in 2005 to 2006, Madico came out with an innovative idea of replacing the inner fluoropolymer layer with a so called "E" film, which is made of polyethylene. They were the first one to realize that the inner layer has no big exposure to UV, thus it is not necessary to use an expensive fluoropolymer film. The inner layer has just one function – sticking to EVA -, which is very well taken care by the "E" layer. Polyethylene is a kind of relative of EVA, thus gels easily with encapsulant.

TaiyangNews: But why do you think did DuPont allow them to change the structure?

Mark Wu: That is mainly due to the shortage
of Tedlar. DuPont was focusing on supplying companies in Europe and Asia and shortened supplies to American companies. This gave Madico a chance to develop an alternative to TPT and came out with a TPE backsheet. Then, the idea has been copied by other backsheet suppliers. The approach also served as a platform for KPE backsheets.

TaiyangNews: But Cybrid came into the market with KAK, right?

Mark Wu: Actually, it could have been TPT. But actually we were rejected by DuPont. Thus we approached Arkema. Even there we were not lucky in the first place. When we approached Arkema, they also told us we were late. As a company policy, they have only three partners — and we would be the fourth. Upon request, they gave us the sample, but were clear that it is not official. They also needed a standby, because Arkema's Kynar is very difficult to laminate. The film has a special structure of a triple layer and the main reason is the very high fluorine content of 59% compared to 41% with Tedlar. On top, the skin layer is already 100% PVDF. The Kynar film is also shiny, thus any flaws are easy to detect. Making a backsheet with Kynar requires high grade adhesive.

We tried all commercially available adhesives but were not successful. Then we took around 6 months to develop our adhesive. Due to the superior final film quality as a result of our in-house developed adhesive, Arkema changed their policy and allowed Cybrid to be their fourth partner in China. Then, we were in fact the first one in China to commercialize a Kynar based backsheet. Only little later, in 2009, we started supplying backsheets to the top tier companies, like Canadian Solar and Suntech.

TaiyangNews: When you consider that using
a thick fluoropolymer film on the inside of the backsheet is not necessary, especially what Madico came out with is very innovative. What is the reason for not following Madico's path of using "E" layer?

Mark Wu: Very soon we realized the weakness of the "E" film. It is a thermoplastic material with a low melting point, and polyethylene is not strong against UV. Thus we started developing our so called fluorine skin "F" layer.

TaiyangNews: Eventually you also came up with a KPE product, a backsheet that contains an "E" layer. Why?

Mark Wu: But at that moment Cybrid was a small company and we had to make money. The Chinese module makers were rich at that time and the market was overseas, in Europe. The module producers were not believing in our concept of fluorine skin.

So we changed the strategy. We continued the development of our "F" layer in the background and developed the KPE backsheet in parallel.
At the same time we have also made improvements to the "E" layer. Before Cybrid launched its super KPE film, all "E" films were transparent, but we made it white. We put TiO2 into the compound of the "E" film to make it a UV blocker.

TaiyangNews: What is the issue of having a transparent film?

Mark Wu: As mentioned earlier, 1/10 of UV reaches the inner layer. If the inner film is transparent,
then UV passes through and reaches the PET
film, leading to yellowing of PET and adhesive.

That means the inner layer must exhibit partial UV blocking properties. This is where our innovation came into picture — we put TiO2 into the "E" layer. It is Cybrid's invention and from then on, everybody followed us.

TaiyangNews: What are the other improvements you did to the "E" film?

Mark Wu: The other change we adapted with the "E" layer is that we did not use 100% polyethylene. Instead we used a special compound to increase the melting point — from 123 to 168 degrees Celsius. This is necessary, because polyethylene is a thermoplastic. The lamination process is typically accomplished at 145  degrees C,  which is higher than the melting point of polyethylene, and that causes a reduction of the thickness of the "E" layer as well as the backsheet
in the laminate. This directly impacts the electrical insulation properties of the backsheet negatively. That is why we increased the melting point of the "E" layer.

TaiyanNews: Despite the great improvements of the "E" layer you continued with the "F" layer development. Why?

Mark Wu: We still think "E" is a compromise. Though the UV exposure to the inner layer is not strong, we felt the necessity for a fluoropolymer. At the same time, using a complete fluoropolymer film is sort of over-engineering the backsheet. As an intermediate, in 2012, we launched our KPf, wherein F is a very thin fluorine polymer film.

TaiyangNews: Can you provide any details about your KPf?

Mark Wu: We developed the formulation by ourselves; we have a very special processing technology. We coat the fluorine polymer liquid on the PET directly and put it into a high temperature oven. At this high temperature, the fluorine liquid takes the shape of a film with a very high adhesion to PET. The catch is that we do not use any adhesive. This "F" layer is a thermostatic material without any dimensional changes, which eliminates the concern about the thickness change in the module lamination process. Then we added a special TiO2 into the fluorine polymer skin layer, which is why our "F" is also white and blocks UV. Now we've licensed our technology to Agfa.

In our KPf product we have changed a second
thing – and this is regarding the core PET. Fluorine polymer has excellent UV stability, but unfortunately has bad water-vapor barrier properties. One did
not pay attention to the anti-hydrolysis properties of the PET film in the past, the main focus was on the sandwiching layers. We thought that increasing the stability of the core PET layer is very important. As we saved cost with the "F" layer, we diverted some of the savings to upgrade to anti-hydrolysis grade PET film. With this move initiated by Cybrid, in the meantime nearly every Tier 1 module manufacturer has established a standard for anti hydrolysis grade. Finally, KPf is now our core product.

TaiyangNews: As cost is the keyword in the PV world, could you provide us some background on the cost side of your KPf?

Mark Wu: Our "F" layer as such is not more economic than the"E" layer. However, our solution

is less expensive when compared to the "E" layer
+ adhesive. In summary, the costs of KPf is lower than KPE, but the price is not. We have to earn
our rewards. But most important is that everyone has accepted our idea. Since 2013, all the major companies are using our KPf product and it has become a kind of a standard for backsheets in China.

TaiyangNews: What do you think about non- fluoropolymer based backsheets?

Mark Wu: Technically speaking our PPF back sheet is also a fluoro-backsheet using an inner fluorine skin, but practically it has a PET outer layer. This product range is mainly promoted for rooftop applications, where modules have no UV exposure on the backside. But for solar farm applications, we do not recommend our PET outer layer backsheets.

TaiyangNews: And what is your opinion on coatings?

Mark Wu: The coating has very bad water permeability. The coating polymer is very hard and there is a difference in thermal expansion coefficients for PET and coating. They are cheaper than KPf, but are still losing market share. Moreover, we think there is a potential for lowering the costs of KPf to the level of coatings, because KPf can be produced at high speed.

TaiyangNews: What comes next ?

Mark Wu: KPf+ comes with a reflective inner surface for high efficiency modules. We also have a solution for high humidity and high temperature conditions. All our backsheets have 1.7 to 1.9 g/m2d water vapor permeability. As this may not be suitable for high humidity and high temperature areas, we have developed a backsheet called KPX, which can bring down water vapor permeability to less than 1 g/m2d. It is a chemically different inner layer.

TaiyangNews: Do you have any further improvements planned for the core PET layer?

Mark Wu: We are developing a plastic alloy to replace PET. We will provide more details when it is ready.

TaiyangNews: Thank you.

The interview is published in TaiyangNews' Market Survey on Backsheets for Solar Modules 2017, which can downloaded free of charge here.