- August 31, 2005, David J. Bentley Jr., Contributing Editor
When customers purchase meat or cheese from the full-service delicatessen at a typical supermarket, the clerk usually asks how thick they want the slices. Customers often have very specific ways they like their deli.
Converting presents a similar situation. Some applications require a heavy coating weight of an adhesive, coating, or primer, but others might require a lower coating weight. The choice depends on the particular adhesive, coating, or primer in use and the final application.
Converters should rely on the supplier of a material to determine the coating weight to apply. For example, a pressure-sensitive adhesive used for a decal might require a coating that is 1 mil thick. A laminating adhesive for a snack package would probably be approximately 3 lbs/ream. A heat-seal coating might require 3–5 lbs/ream for best results, but a primer on foil for use to accept an extrusion coating might need 2–3 lbs/ream.
The normal procedure a converter follows is to evaluate a product at the midpoint of any range specified by the supplier. After testing by the converter and the final customer, this original coating weight might need a slight adjustment up or down to achieve the necessary combination of properties. Once converter and customer agree on a certain thickness of adhesive, coating, or primer based on their evaluation, the converter should never change this without initiating another complete series of tests involving the end-user.
The first reason for the ban on changing the adhesive, coating, or primer thickness is one of simple economics: Choosing the material at the originally approved thickness resulted in a certain cost input to the final structure. Increasing the amount of material applied is going to increase the cost of that material. This move obviously will reflect negatively on the bottom line and decrease the profit the converter was hoping to achieve.
Unfortunately, converters also must avoid the temptation to make the bottom line look better and increase their profit by using less adhesive, coating, or primer. Taking steps to make any construction less expensive by reducing the thickness of a material requires very extensive evaluation. Failure to do so usually will result in problems with the performance of the finished material.
In addition to changing the financial picture, excessive or insufficient thickness both can cause performance problems. The primary reason for this relates to the combination of adhesive and cohesive properties built into such a material.
The adhesive properties of an adhesive, coating, or primer usually cause problems at low coating weights. As the deposition of the material goes lower and lower, eventually a point occurs where it simply may not adhere to a substrate any longer. Obviously this is unsatisfactory.
In the opposite case, where deposition goes higher and higher, eventually a point may occur when the mass of adhesive, coating, or primer is so large that the failure when applied to a substrate is cohesive. This may give a bond value that is too low for the intended application or a bond that fails cohesively when adhesive failure is desirable.
Certain medical packages and some packages that have features such as the ability for reclosing are good examples of cases where the proper type of failure of an adhesive, coating, or primer is critical. Medical packages often contain a variety of substrates held together with different adhesives and coatings. The selection of the particular material and its coating weight are such that opening the package during use results in a separation of two specific substrates. The same statement is true for a package that will be reclosed.
In the case of the medical package, unwanted separation at opening due to cohesive failure from excessive coating thickness between two substrates might expose a tacky adhesive that would make handling difficult.
For the package that would be reclosed, unwanted separation due to cohesive failure could expose a non-tacky material when the opposite is desired.
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.