PSA Problem Solving: Analytical Tests Augment Conventional Data
By: Gary A. Avalon and Michael A. Bradshaw
Chemsultants International Network
When dealing with complex problems it is 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 a number of materials are typically 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 top coated and primed, and finally all of these components are married in a lamination process. Afterwards, 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 is up to the investigator to isolate the root cause to 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 solve the problem.
Coupled with common physical tests, certain analytical techniques can be run to form a “roadmap” to the root cause identification of quality issues. Two of the many possible techniques that can be used are presented here: Thermo-gravimetric analysis (TGA) and Infrared spectroscopy (IR). These techniques are good starting points for analytical testing.
Thermo-gravimetric Analysis (TGA)
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 continually monitored 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 careful adherence to parameters for comparison to user-created databases.
Molecules above absolute zero temperature (- 296 °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 infrared 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 approximate 3 – 5% by weight.
Infrared Spectroscopy (IR)
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.
A powerful feature of IR is that it is 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 presence or lack thereof 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 x 10-6 meters 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 is then 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 detect presence or lack thereof 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.
Conclusion
The two techniques described herein offer different types of information about pressure sensitive products and materials. There are many more advanced analytical techniques available to study PSA’s and coatings. These two techniques offer a good starting point for coupling to physical data. It is 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.
Gary A. Avalon
Gary Avalon graduated from the Fenn College of Engineering at Cleveland State University with a B.S. in Bioengineering. He began his career in the Polymer Application Lab of Diamond Shamrock Plastics Corporation where he spent 7 years working with PVC and PP formulations for extrusion and molding applications. He then joined the Avery Dennison Specialty Tape Division where he spent over 19 years in numerous positions including roles as their Technical Director and Global Business Director. He has extensive background in the development and commercialization of numerous PSA-based materials for Consumer Healthcare and Medical applications. In 2001 he joined the Chemsultants International Network as Group Marketing Director for all three divisions of the company.
Michael A. Bradshaw
Michael Bradshaw graduated with an American Chemical Society Approved B. S. in Chemistry from Bloomsburg University in Bloomsburg, PA. He completed some graduate studies in Physical Chemistry at Ohio State University and spent an Internship at the Center for Photochemical Sciences at Bowling Green State University. Mike is a Research Chemist with Chemsultants Inc. in their Contract Research Laboratory. His analytical experience includes background in UV-VIS, FTIR, Fluorescence, and NMR spectroscopies; GC-MS, HPLC, TGA and DSC as well as photochemistry (time-resolved absorption/emission of photo-chemically active molecules in solvent, micelluar, and hydrogel environments). He has applied these skills in the area of emulsion-based, solvent-based, hot melt and UV-curable resins and adhesives.
Chemsultants International, Inc. 9079 Tyler Blvd. Mentor, OH 44060 Phone: 440.974.3080 Fax: 440.974.3081
About Us Contact Us Site Map Career Opportunities Locations
