PSA Problem Solving

Published: February 01, 2003, By Gary A. Avalon and Michael A. Bradshaw, Chemsultants Inc.

When dealing with complex problems, it's best to take a step-by-step approach. This methodology can aid the investigation of quality issues commonly encountered with pressure-sensitive-adhesive (PSA) products. In the production of pressure-sensitive products, typically a number of materials are brought together. Adhesive components are blended or compounded together; release liners are cast and finished prior to being silicone coated; facestocks may be extruded or topcoated and primed; and finally all of these components are married in a lamination process. Afterward, the final base materials are finished or slit to size prior to packaging and shipping to a converter. Indeed, this is quite an involved process for a simple product such as a label or tape to be born.

Most often this process results in high quality materials. Occasionally, problems arise and the final product may not meet the required specification. In order to define the nature and extent of the problem, a series of tests must be performed. Routine adhesive testing such as coating weight/consistency, peel adhesion, tack, and/or shear can identify adhesive performance issues. However, these tests alone may not lead to the source and eventual resolution of the quality issue.

It's up to the investigator to isolate the root cause of a physical/surface-related issue (i.e., contamination) or a bulk adhesive chemistry problem (i.e., improper formulation) while using knowledge of the process and application to reach a solution.

Coupled with common physical tests, certain analytical techniques can be run to form a “roadmap” to the root cause identification of quality issues. Presented here are two of the many possible techniques that can be used: thermo-gravimetric analysis (TGA) and infrared spectroscopy (IR). These techniques are good starting points for analytical testing.

Thermo-Gravimetric Analysis
TGA examines the decomposition mechanism(s) of a sample by monitoring changes in mass with increasing temperature. This technique has been used to study decomposition as well as other mechanisms. TGA involves placing tens of milligrams of a sample on a specialized analytical balance housed inside a precise temperature-controlled oven. As the temperature is increased, components present in the sample may decompose, leading to decreases in sample mass. Via the aid of user-constructed databases, qualitative identification can be made based on the component's maximum rate of decomposition temperature and decomposition profile. Since the mass of the sample is monitored continually during the analysis, quantitative data is gathered in terms of weight percentage of a decomposed component. Obvious disadvantages of the technique are its destructive nature and the necessity for careful adherence to parameters for comparison to user-created databases.

TGA can be used to investigate the presence, or lack thereof, of components and/or impurities as well as to present quantitative information.

Infrared Analysis
Molecules above absolute zero temperature (-296 deg C) are in constant motion (i.e., vibrations, rotations, etc.). Molecular vibrations that result in a non-zero dipole moment occur at frequencies in the IR region of the electromagnetic spectrum. The exact vibrational frequencies depend upon the masses of the atoms involved and their bonding arrangement. These two criteria are commonly coupled together in the term “functionality.” Detection of what functionalities are present in a sample is possible by irradiating the sample with IR light and monitoring what frequencies are absorbed. Correlation charts of IR frequency and functionality have been painstakingly developed by spectroscopists and are widely available. IR is not a good quantitative tool, as detection limits are approximately 3%-5% by weight.

IR is one of the most frequently used analytical tools for quality analysis. Most applications are centered upon comparing lots of the same material or comparing unknown material to database spectra. An example to illustrate this involves comparison of IR spectra of a rubber-based adhesive to an acrylic adhesive.

As can be seen from the overlaid spectra, there are different frequencies of IR energy that are absorbed in a rubber-based adhesive opposed to an acrylic adhesive due to the different functionalities present.

A powerful feature of IR is that it's an additive technique. In other words, if a sample contains three IR-active components, the IR spectrum of the sample will appear like an overlay of the spectra of the three separate IR-active components. Therein lies the ability to ascertain the presence or lack of IR-active components or impurities in a sample. An attractive feature of IR spectroscopy for the adhesives/coatings industry is the ability to analyze the surface of a material (up to 2 × 10-⁶ m deep). The apparatus to make this possible involves a crystal with proper refractive index properties (zinc selenide is most common). The sample is placed onto the surface of the crystal with minimal pressure to ensure good contact. IR light then is used to probe the sample surface in one or multiple spots.

IR can be used for qualitative comparisons of different lots of the same material, unknown materials to databases, as well as to detect the presence or lack of components or impurities in a sample. Disadvantages of IR spectroscopy are detection limit, non-IR active components, and the possibility of samples possessing similar IR spectra. Examples of the last disadvantage include comparing IR spectra of n-octane to n-nonane or comparing spectra of different molecular weights of the same polymer.

The two techniques described here offer different types of information about p-s products and materials. There are many more advanced analytical techniques available to study PSAs and coatings. These two techniques offer a good starting point for coupling to physical data. It's easy to see how information relating to the structural makeup or physical proportions of your ingredients can point you to possible reasons for lack of product performance. While routine peel, tack, and shear can let you know you have a problem, it is the analytical work that tells you what may have gone wrong.