"The higher the ratio of pot-life to cure, the faster the rework process becomes without sacrificing capability or introducing new risks. We need to give the workers enough time to make the repair. With the SikaBiresin® CR910 we have found the best solution on the market. " Roger Schütt
TPI Composites, the world’s leading independent blade manufacturer, knows better than most the technical challenges associated with wind turbine blade rework and repair. To reduce cycle times in blade production and uptower repairs, Global Technical Program Director Roger Schütt turned to Sika’s latest hand lamination system to enable faster curing than their previous epoxy resin. The SikaBiresin® CR910 system, developed, tested and qualified together with Sika, is now in the first phases of serial production.
Could you share how you discovered the product and what prompted your interest?
Roger Schütt: The initial trigger for TPI to begin looking for solutions in the market is usually efficiency, cost optimization or both. The overall goal we set out for this project was to enable more consistent cycle time in serial production through faster reworks. As wind blades have become the giants we know today, and considering the complexity and high amount of manual labor any serial production faces, occasional defects requiring reworks will arise. The majority of reworks in serial production are small in size but still need completion to resume progress in the mold. A very significant element of the rework process is curing time which takes multiple hours to complete – impacting cycle time. In our mind, this posed a significant potential to tap into. We set out to identify a resin system enabling faster cure time while still allowing enough pot-life to wet the fabrics in a capable and secure manner.
What kind of defects are we talking about? And what rework materials were you using before?
RS: Cycle time is something special for TPI. We are one of the very few companies who can consistently build modern blades in an average of 24 hours or less. We have been looking at process steps on which defects impact cycle time the most. On our sites the shell molds are the drivers for cycle time. A statistical analysis showed that most defects are of small size and would be found after infusion. As a result, the sanding and lamination work do not take a long time. Curing, on the other hand, is nearly unimpacted by defect size. Speeding up cure time for those defects would impact cycle time in a way that would drive consistency by reducing the time influence of rework.
The resin systems we typically apply for rework are the same epoxy systems we use to build blades. There are options for faster hardeners, but usually those also reduce pot-life significantly, introducing time constraints to mixing, exotherm reaction and lamination.
We have been looking for a resin system enabling significant improvements in cure while retaining a reasonable pot-life – increasing the ratio between pot-life and cure time, if you will.
Our expectation in finding a system with a better ratio would be outside of epoxy chemistry, so we screened and in-house-tested all systems on the market.
In the end we were very surprised to find a winning combination in the properties of the SikaBiresin® CR910 – an epoxy system.
Despite the low expectations towards an epoxy system, you actually found one that had the best performance. Why is the ratio between curing time and open time crucial for this job?
RS: A good hand lamination takes time, depending on the size of the defect. In order to retain process capability while making sure we do the reworks the right way, there needs to be enough pot-life to mix and apply the material. Enabling faster cure time, on the other hand, is safe and reduces cycle time impact.
The higher the ratio of pot-life to cure, the faster the rework process becomes without sacrificing capability or introducing new risks. We need to give the workers enough time to make the repair. With the SikaBiresin® CR910 we have found the best solution on the market.
During the testing phase TPI Composites really tried to test the behavior of the material in all applications. For example, TPI Composites was the first customer doing infusion rework with this system, which was originally designed only for hand lamination rework. How did that go?
RS: The challenge for infusion is primarily dominated by flow rate, which is affected by permeability and viscosity. As stated earlier, the majority of the defects are small in size, so we figured that, by using enough flow media, we would be able to enable infusion even with the higher viscosity of the SikaBiresin® CR910. It worked.
So this means that the new material would allow you to do all three targeted resin applications: hand lamination and infusion rework, both in service and in the factory, as well as adhesive joint overlaminations, with one material?
RS: Yes, that’s true, as it can be used for all these applications. In production we use it with both hand lamination and infusion, depending on defect location and size. However, its strongest feature is most certainly hand lamination application.
I totally agree. Infusion is complex in contrast to hand lamination. But during the material qualification at TPI Composites you for sure also had some obstacles to tackle.Can you give an example?
RS: There were no obstacles in terms of hand lamination, dripping, wetting, or mixing of the product. Everything really worked well. To enable a wider range of application we explored the mix of the faster and slower hardener types. We found the fast hardener is ideal for small hand lamination rework, and a mix of both hardeners is a good fit for small infusion rework.
How did Sika support you with this inquiry then?
"As we wanted to have the benefit of lower viscosity for infusion application, which is provided by the slower hardener, while also keeping a fast cure time, we explored the options for mixing hardeners. As rheology and interaction between different hardeners can be complex, we reached out to Sika to ensure we achieve the right mixing rate of all 3 components. Your team responded to our inquiry quickly and with clear guidelines. This was really helpful in order for us to continue the testing phase. And finally, we could conclude that the quality and results were in line with our expectations. During this testing phase, we also very much appreciated that Sika, as a big global player, was able to deliver material easily to our production and test sites all over the world." Roger Schütt
Nice to hear that you had a really good collaboration between the teams across the globe. We also highly appreciated your efforts. You mentioned in the beginning that the ratio between pot-life and cure speed was the major reason for the qualification. Were you able to justify a decreased cure time in the field after introduction of the material as well? And if so, how much was it?
RS: We are still in the implementation phase for serial production. But when we mentioned we had a product in the pipeline for standard rework, our production sites were excited as it enabled more consistent and lower cycle time. Initial feedback from the sites is very positive. The resin provides enough time for the application while accelerating the cure process.
In summary, would you recommend this new resin for others? And if so, who would benefit most?
RS: Well, I think anyone interested in a fast-curing, versatile system with the potential to reduce cycle times for rework application in serial production should have a look.
Thank you, Roger, for the talk. Sika wishes you all the best and is looking forward to the next development in collaboration with TPI Composites.
RS: Thank you. TPI Composites is looking forward to continuing the good collaboration as well.