Problems with using old sheet
One question that we are often asked by customers is “can we use the old sheet that we have in stock and not have problems?”
In general, the answer to this question is yes, as long as the sheet is still masked and has been stored in good conditions. The properties of the sheet will not deteriorate over time. The only thing that you may need to do, depending upon the application, is dry the sheet before laminating or thermoforming (and we would recommend that you dry even new sheets for these applications).
However, we recently came across a very specific problem that one of our customers was having while using old sheet. The customer laminates polycarbonate sheet to glass using transparent polyurethane. There was some old sheet that the customer wanted to use in this application, the sheet had been produced about three years ago, but was fully masked and had been stored carefully. When the polycarbonate was bond to the polyurethane, the bond strength was measured and was found to be about 10% lower than when using new sheet. In ballistics laminates, this lower bond strength can potentially lead to some longer term problems with ballistics performance and de-lamination. An extensive study was therefore carried out to identify the cause of this lower bond strength.
The conclusion of the study was that over time, plasticizers from the masking had migrated to the surface of the polycarbonate sheet. These plasticizers then weakened the bonding of the polycarbonate to the polyurethane. As a result of these studies we are now recommending that polycarbonate used for ballistics laminates is used within 12 months of production.
In theory, this recommendation sounds simple to achieve. The customer would just need to look at the production date on the packaging and confirm that it is less than 12 months old. However, the customer needs to understand that the production date on the packaging and/or masking is probably the date that the hard coat was put onto the sheet. In practice, the actual production of the original sheet could be many months or even years before this date. The plasticizers would have started migrating to the surface of the sheet ever since the original production date of the sheet and the coating process would not have removed them.
Fortunately, in a world where most produces have quality systems in place, traceability of material is nearly always available. We would now recommend that all laminators of ballistics material ask their polycarbonate supplier to send them the material lot number, the coating date of the sheet (if coated) and the production date of the base sheet. If the production date of the base sheet is over 12 months old, it is likely to have lower bonding strength which may decrease the product performance. If the sheet supplier is unable or unwilling to supply these dates, we would recommend that adhesion testing is carried out or material is purchased from another supplier.