Some Causes of Blocking in Heat Seal CoatingsPart II

Last month this column covered some causes of blocking when using heat seal coatings. The problem primarily addressed concerned processing conditions involving coating weight, drying conditions, and pressure of the coated roll as it was wound. This month's column will discuss other common causes of blocking when using heat seal coatings.

Ambient conditions of temperature and humidity can play an important role in blocking of heat seal coatings. The first column discussed the results of excessive heat in an oven during drying of a heat seal coating and the fact that it definitely can contribute to blocking. Excessively high ambient air temperature during coating of a heat seal coating can have exactly the same effect.

Such high temperatures cause the coating (after the drying) to be at a higher than normal temperature as it enters the roll for winding. This allows excessive heat retention in the wound roll. This heat can lead to softening of the heat seal coating and subsequent blocking. High humidity also can lead to blocking by causing the coated heat seal adhesive to have a high level of moisture or solvent as it enters the roll for winding. This also acts as a plasticizer or softener that will cause blocking as the coating ages.

Converters using heat seal coatings sometimes opt to formulate materials they purchase from suppliers to achieve special properties that are proprietary to their product. This may work well, or it may cause serious problems. In the case of heat seal coatings, adding an ingredient that has a low molecular weight or is a softener or plasticizer definitely can contribute to the tendency of the resulting modified heat seal coating to block in the roll.

Before adding any formulating ingredients to a heat seal coating, the converter should be certain that no adverse conditions will arise either with blocking of the coating or with any other property.

In addition to formulating, converters often add solvent or water to a heat seal coating to dilute the material for proper coating weight application. Addition of excessive diluent or the wrong high boiling diluent can cause blocking because the oven conditions will not evaporate all the diluent during drying.

Blocking with a heat seal coating can simply be the result of using an improper coating for an application. A converter may use a particular heat seal coating for an application that gives perfect results. Just because it works for that application does not mean it will work for every other application.

Changing substrates, sealing conditions, or anything else may mean that the converter must evaluate various heat seal coatings to find the proper one to use for the new product. Failure to do so could contribute to blocking.

Substrates can play an important role in blocking with a heat seal coating. A heat seal coating might work on a particular substrate by providing exactly the adhesion required with absolutely no blocking. Changing to another substrate may cause the coating to block. How could this happen?

A frequent cause is that the new substrate contains low molecular weight ingredients or other incompatible or soft materials. These migrate from the substrate through the coating during the coating and drying operations and end up on the surface of the dried heat seal coating. This soft material on the surface causes blocking when the coating is wound into a roll.

Shipping and storage conditions are very common causes of blocking with heat seal coatings. Placing a roll of heat seal coated substrate in a truck or rail car that experiences very hot conditions during transportation will cause the coating to soften and block. Very hot warehouse or other storage facilities also will contribute to blocking by softening the coating in the roll.

Excessive humidity during shipping and storage also can cause a heat seal coating to take up water and become softer and block. Ideally, a heat seal coated material should be transported and stored at conditions as close as possible to room temperature and humidity.

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 dbentley@unm.edu.


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