- March 01, 2001, David J. Bentley Jr., RBS Technologies
Green is an important color in our everyday life. In traffic, it indicates “go.” In nature, it often designates life, as in the budding of trees after a period of dormancy. Green also has significance in the converting industry, where it means brand new, fresh, young, unaged, or uncured. Green strength, green bond, green COF, etc., are all common expressions.
Why measure properties in their green state? Many adhesives or coatings do not provide their ultimate properties immediately upon use. They are part of a dynamic process that provides an initial result but ages to a different final value. In some applications, an adhesive or coating may not change properties, but the total construction may be dynamic. Consider an example for each case.
A curing adhesive often consists of two components mixed together before application. A reaction begins then or upon application to a substrate. Sufficient bond exists initially to give adequate performance. After complete curing in a few days or weeks, the bond increases to its final level because of a chemical reaction that goes to completion.
A question always arises during preparation of a lamination using a curing adhesive: Will the bond cure to the final value that is necessary? You can compile a historical pattern by measuring the green or initial bond and the cured bond over a time that encompasses many runs of the adhesive in the particular construction. This indicates that a given green strength normally will cure to a certain final value. A converter then can measure the green strength and almost be certain that the final bond value will be acceptable.
For the other example, consider a slip coating. This material provides a certain degree of slip or COF after application to a substrate. Such coatings often work by incorporating a wax or other slip additive into the coating formulation. After application of the coating to the substrate, this slip additive slowly migrates through the coating and forms a layer on the surface that provides the necessary COF. Obviously, the green or initial coefficient is not the same as the final value after the coating has had the opportunity to age so the wax or slip additive can bloom to the surface. A converter that is using a slip coating will probably also establish a historical record to indicate that certain green strength values will age to corresponding final values.
Unfortunately, green measurements of properties by a converter do not always guarantee that the final, desired value will occur. One example will illustrate this possibility. For a two-component adhesive mixed in an incorrect ratio, the green bond may be exactly the same as that for the adhesive mixed properly. With the improper mixing ratio, a construction using the adhesive may never cure to the proper value. Green strength measurement, therefore, does not work in this case.
Because green values may not be entirely indicative of final values, converters may use other techniques in an attempt to predict the final properties of a material when first made. Again, the material at an elevated temperature in an oven for a period is one such technique that is popular.
For the example of a two-component adhesive cited above, the aging at elevated temperature forces the adhesive to cure to its final value. In the case of the slip coating mentioned earlier, the exposure to elevated temperature causes the slip additive to migrate through the coating to the surface to give a reading that should equal the final value after normal room temperature aging.
Although this aging eliminates the reliance on green measurements, running both a green test and an elevated temperature test can provide the most information.
Before using green measurements as a signal that a particular construction is “go,” converters must establish a history, make certain everything is normal and correct, and consider using other tests also.
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 firstname.lastname@example.org.