- October 01, 2000, David J. Bentley Jr., RBS Technologies
Heat seal coatings are formulated materials that are applied to a substrate on a coater and then wound into a roll. In a subsequent operation, the coated material is unwound and the heat seal coating is adhered to another surface with heat and pressure. The heat seal coating functions as an adhesive that bonds the two surfaces together. An advantage of this type of adhesive is that it does not have to bond immediately as with a laminating adhesive. Instead, the bonding operation is a distinctly separate operation from the coating operation.
A problem that can occur with heat seal coatings is that they can block when wound into a roll. This is essentially premature adhesion and causes the roll of heat seal coated material to form a log that cannot be unwound. Obviously, the roll is completely unusable.
How can you prevent such blocking? The first step is to understand its causes. This month and next, we will focus on some of the common causes of blocking in heat seal coatings. This month's column addresses those causes that relate to the application process.
The first point to consider to prevent blocking is the deposition weight of the heat seal coating. All heat seal coatings have a recommended coating thickness. This will depend on the particular coating, the substrates, the sealing conditions, etc. You must follow information about coating weight suggestions from the coating supplier religiously. Insufficient coating deposition obviously will cause poor or zero adhesion. Excessive coating deposition easily can lead to blocking in the roll. Too much coating simply provides an opportunity for the material to squeeze out from the roll.
Drying conditions for those heat seal coatings that contain solvents or water are extremely critical to prevent blocking. Drying improperly is probably the most common cause of blocking. Many blocking difficulties, in fact, are the direct result of conditions that the coating encounters during drying.
Insufficient drying is a common cause of blocking. When all volatile components are not removed from a coating, the remaining solvent or water can act as a plasticizer or softener in the coating. Because the resulting material is softer than it should be, it can block very easily. The insufficient drying can result from excessively low temperature, insufficient residence time in the oven, or unsatisfactory removal of the volatiles from the oven through air exchange.
The opposite end of the drying spectrum—too much drying—is equally bad and also can cause blocking problems.
Excessive drying resulting from temperatures that are too high or residence time that is too long can cause any low molecular weight materials or other soft ingredients that are present in the coating to bloom to the surface. Having these soft materials at the surface as the coating is wound obviously will contribute to blocking.
Another problem with excessive drying is that the dried material will exit from the oven at an especially high temperature. When wound into a roll, the heat is retained and permeates through the entire roll. If too much heat is present, the coated roll will block over time.
The temperature profile in the drying oven is very important when evaporating the solvent or water from a heat seal coating. Excessively hot air impinging on the surface of the wet coating as it enters the oven will cause the top surface of the coating to dry or skin over.
Solvent or water remaining in the lower part of the coating will be unable to penetrate through this skin during the remainder of the trip through the oven. The retained solvent or water will act as a plasticizer to soften the coating after winding, exactly as explained above for the case of insufficient drying.
Another processing variable that can cause blocking is winding the coating into a roll with excessive pressure. Even properly dried heat seal coatings usually will contain some heat when they are wound after the coating operation. This amount of heat with excessive pressure will soften the coating and cause it to block. Rolls of coated heat seal therefore should be wound neatly but not excessively tight.
David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at email@example.com.