Troubleshooting sealing problems
Most of the time my lamination performs fine, however, occasionally there are seal problems. With everything remaining the same from run to run, what might be causing this to happen?
Answer
The most obvious thing to check first would be the consistency of your sealant layer. Variations in gauge, chemical composition and/or slip level can have an effect on sealability. However, if you are using a film supplier with good process and quality control systems in place, these possible causes may be ruled out very quickly. Sealing problems can also be caused by corona treatment on the sealant surface. This treatment on the backside of the film could be a problem from the film supplier or could occur if the film is retreated in line during converting. Backside treat will weaken seals and cause other issues such as blocking.
Work closely with your film supplier to determine if this might be your problem. Silicone is another potential cause of sealing problems that should be avoided. Silicone may be applied to a piece of equipment to get it to move faster or prevent sticking issues. Even the smallest amount of Silicone on film can dramatically affect the sealability. Additives in the film or the adhesive can also have an effect on sealability. Metal stearates for example, in particular calcium stearate can be particularly troublesome. In addition, high levels of antioxidants, UV Blockers, and/or UV Stabilizers can bloom to the surface and reduce seal strength. Generally problems may be seen with antioxidants when the concentration is 2,000 ppm or greater.
UV Stabilizers are less migratory because of their larger molecular size so levels might be much higher than 2,000 ppm before issues surrounding surface bloom occur. In general, the thicker the film, the more potential material to bloom to the surface and the greater the effect on sealability. Another sealing issue that is fairly common is called the “anti-sealing effect”. The anti-sealing effect is seen when the converter is using a two-part urethane adhesive system on the laminator. One of the components in the urethane adhesive can migrate quickly through the sealant layer before it actually reacts with the second component. It migrates through the film to the surface, thus contaminating the sealing interface, making the film virtually impossible to seal at any temperature. (For an excellent article on the anti-sealing effect see Anti-Sealing Effect in Adhesive Lamination Structures presented by a team from Exxon and Morton consisting of Achiel Van Loon, R.Dickmann, O. Lacoste, F. Vermeiren 1992.)
If you have any questions about the adhesive systems that you are using and how they might affect the seal performance of your polyethylene film, contact your vendor and be as specific as possible. The more information they have about your concerns and experiences the easier it will be for them to help you.