Lamination grade polycarbonate is a high quality optical grade polycarbonate sheet that is often laminated to glass using TPU. Applications include military transparent armor and making bullet resistant windows for commercial vehicles.
When used to make bullet resistant windshields for commercial vehicles, the quality has to be exceptionally high as any slight defect can cause driver problems. This blog post discusses two quality issues that are currently being experienced by lamination grade polycarbonate customers. These problems are not confined to only one supplier but extend across multiple suppliers for reasons that we will discuss. However, while it is on the suppliers to resolve the issues in the medium term, there are things that can be done to mitigate the issues in the short-term.
The problems – gel spots and surface contamination.
There are two problems that are affecting the industry, we will call them gel spots and surface contamination.
- Gel spot locations can be identified before the masking is removed. In the picture below there appears to be a small air bubble under the masking. When the masking is removed, if you look closely, you can see a small smudge on the sheet. This smudge is very easy to miss. These gel spots can occur on either the coated or uncoated side of a one side coated sheet. If the gel spots are not removed prior to lamination, particularly if they are on the uncoated side, will spread in the lamination process and become a larger defect in the finished sheet.
- Surface contamination is a small piece of debris under the masking. It is not part of the sheet but rests on the surface of the sheet. If this debris is not removed prior to lamination, it will cause a significant defect and indent after lamination.
The image above shows a sheet with the white masking on. A small air bubble can be seen. In some cases there is nothing under the masking, in which case it is likely that there will be a gel spot. In other cases there will be surface contamination.
The cause of the problems
Many of the large high quality polycarbonate sheet producers do not put the hard coat onto the sheet themselves. Coating is a complex operation and there are not many companies that are able to achieve the quality required for lamination grade polycarbonate. For that reason most of the large polycarbonate sheet producers use the same coating company to coat their products.
Only recently have sheet producers become aware of the gel spots and surface contamination issues. It is possible that these issues have been occurring for years and that customers have either only just become aware of them as quality requirements have increased, or have lived with the problem.
We recently spent time working with a customer to improve their lamination yield. During this work we not only identified both issues, but also identified that two major suppliers had the same issues with their premium products. We will not mention the suppliers in this article but they are two of the top three major global suppliers of lamination grade sheet.
Now the causes have not been fully identified but they are not part of the uncoated sheet. During coating the masking is removed from both sides of the sheet, the sheet is coated and then new masking is applied. It is suspected that the gel spots and surface contamination are either caused by the actual masking applied by the coater (such as plasticizer deposits), or something during the masking application process. The major coater is aware of the issues and work is ongoing to fully identify the cause and resolve it. The masking supplier is also aware of the issue.
The work done by the coater will hopefully be successful and resolve the issue for all of the polycarbonate sheet suppliers using them. Customers need to be aware that this is currently a multi-supplier issue and switching supplier is unlikely to make the issue go away. In the short-term, customers can easily mitigate the issue. How to mitigate the issue is the next thing that we will discuss.
Mitigating gel spots.
Gel spot locations are actually easier to identify prior to removing the masking. Small air bubbles in the masking are easy to see and show the location of the gel spots. Once the location of the gel spots is identified they can easily be removed with a lint free cloth and a small amount of Iso-Propyl alcohol. It is particularly important that gel spots on the uncoated side are remove as these will turn into a significant defect after lamination.
It is less clear at the moment whether or not it is important to remove the gel spots on the coated side before lamination. It may be possible that these can still be easily removed after lamination. More testing is required to confirm this hypothesis.
Another possibility is that the cleaning roller discussed in the next section may remove the gel spots, but we have not verified this method.
Mitigating surface contamination.
Most laminators remove the masking from the uncoated side and clean the surface of the sheet with something like a Teknek hand cleaning roller. Here is a link to the Teknek webpage. As a result the surface contamination on the uncoated side is not an issue.
However, different laminators have different practices for the coated side. It is highly recommended that masking is also removed on the coated side and the sheet is similarly cleaned before bagging with a press plate. For customers who take this approach the surface contamination is not an issue.
Some customers however, choose to leave the masking on the coated side and then add the press plate to the stack. In the lamination process, the press plate will then press any surface debris into the sheet causing a major defect. We have seen a reduction in yield of as much as 20% by not removing the coated side masking. We recommend that any customer that is not removing the masking on the coated side and then cleaning the sheet prior to adding the press plate should consider changing their operation.
Conclusion
At this stage it is not clear whether gel spots and surface contamination are an old issue or a new issue that has been identified as quality requirements increase. In either case it is an issue that is affecting many of the top suppliers.
It is certainly important that the supplier of high quality sheet and their coater work to resolve this issue. However, the final solution may take some time. In the short-term both issues are easily manageable by good operating practice. If you would like to discuss the issue further please contact us.