Sticking With It | PSA Acrylics Primer
- Published: February 13, 2018, By Ingrid Brase
Adhesives expert Ingrid Brase peels back the curtain to show how chemists build performance into pressure-sensitive systems.
As I begin another year of articles related to pressure-sensitive adhesives (PSAs), I thought a good place to begin is to o go back to the beginning…the chemistry. Now before everyone who is not a chemist starts to panic, take heart! I will try and keep this at a level everyone can understand.
My objective is to provide some insight into why different chemistries—that is, acrylic, rubber, etc.—give the performance they do in end-use applications. If we peel back the curtain and take a look at how chemists build these adhesives, it provides valuable insight.
Building blocks
So, let’s start with some fundamentals. All acrylic PSAs are polymer-based. A polymer can be described as a series of small molecules called monomers that are linked together. A very simple analogy is to view monomers as beads and the polymer as a necklace. Different beads can be put together to achieve different patterns. In a similar fashion, chemists combine different monomers together to develop specific performance benefits.
Monomers can be classified by their Tg, or glass transition temperature, which is an indication of how stiff or flexible they are. Lower Tg monomers are flexible, while higher Tg monomers are stiffer.
Two common building blocks, 2-ethylhexyl acrylate and butyl acrylate, have low Tgs and are used because they give the polymer flexibility, which helps impart tackiness. Vinyl acetate and methyl methacrylate are high Tg monomers and thus add stiffness to the polymer.
In addition to considering monomers because of their Tg, chemists also can add functionality through careful monomer selection. For example, acrylic acid imparts polarity to the polymer and also can be a site for cross-linking. This aids the adhesion to polar surfaces like metals. The monomers are polymerized by adding heat and initiators, which will cause the monomers to link together to form polymer chains. The chains can be straight, or linear, or branched.
POLYMERIZATION
But adhesive performance is not simply a function of monomer selection. Degree of polymerization, the length of the polymer chain, will affect its properties. The degree of polymerization and whether the polymer is more branched or linear can impact coating performance of the final adhesive as well as its adhesive properties.
Most adhesives will be a mix of different molecular weight polymers. How broad or narrow the polymer mix will be can be controlled by the initiators and polymerization method. Most commonly used adhesives will contain a broader range of molecular weight polymers.
Branching of the polymer can be controlled by monomer selection as well as polymerization conditions. In general, polymers created in a solvent-based system tend to be more linear than emulsions created as water-based systems. In solution, branched polymers tend to have higher viscosities than linear because they have more surface area.
CROSS-LINKING
The third parameter of creating the polymer for use in an adhesive is cross-linking. This can be described as adding links or bridges between the monomer components within the polymer or to other polymer chains. Cross-linking adds stiffness and cohesive strength to the adhesive. The amount of cross-linking must be carefully controlled as too much cross-linking will severely reduce the adhesion of the polymer and thus the adhesive.
Cross-linking also helps to build the size of the polymer post polymerization. This is important as it helps the chemist create polymers that are reasonable to coat while the cross-linking adds the cohesive strength once the carrier, solvent, or water, is removed. In this fashion, the delicate balance of adhesion–cohesion can be achieved.
Cross-linking agents typically are molecules that have functional or active sites to provide the “hook” to the polymer on each end so they can create the link or bridge. They can be metal chelant systems containing zinc or aluminum, or organic molecules like isocyanates and melamine. The metal chelants are used in solvent-based adhesives referred to as “one-part.” The chelants remain stable as long as the specifically formulated solvents used to keep them inactive are present.
When the adhesive is coated, and the solvents are flashed off during the drying process, the chelant becomes active and the links to the polymer chains are formed. The organic molecules, like melamine and isocyanate, are very active and are generally added right before coating. This is referred to as a “two-part” system. Like the chelants, they create links between the polymer chains.
SOLVENT SELECTION
Chemists also have other tools to control the performance of the polymer-based systems they create. In fact, proper solvent selection helps the chemist during the polymerization process as some solvents help control that process. Most solvent-based acrylics are polymerized in a mix of solvents, which is adjusted to provide optimal coating performance after polymerization. The final solvent-based adhesive will contain a mix of solvents that have different boiling points.
During the drying process, solvent removal happens as the solvents “boil” out of the adhesive. Controlling the mixture allows for solvent removal without compromising the quality of the coating; if too much solvent “boils’ off at once, it can cause bubbles at the surface resulting in a blistered coating. Optimizing dryer zone temperatures, taking the solvent blend into account, creates smooth, defect-free coatings.
In the case of water-based acrylics, polymers are created in an entirely different manner. The same monomers are used, i.e., butyl acrylate, vinyl acetate, etc. None of these are miscible in water so the polymers are formed as discrete particles suspended in the water. Surfactants are used to keep these particles mixed in the water acting as “emulsifying“ agents.
In general, emulsion polymers will have a milky appearance since they are not soluble. Like solvent-based systems, degree of polymerization will be controlled by heat and the initiators used. The surfactant blend also will impact the polymer formation.
Emulsions are usually evaluated for particle size; this measures the distribution of the polymer particles. Particle size can have a very dramatic effect on coating performance. Pure emulsion systems generally do not coat well; to create the final adhesive system, other additives are mixed with the emulsion. These can include additional surfactants, thickeners to add viscosity, and defoamers.
ANALYSIS & CHARACTERIZATION
So how are polymers characterized to ensure consistency? There are any number of analytical measurements that can be carried out once the polymers are formed. The two simplest are percent solids and viscosity. These give a quick macro view of the polymer.
A more detailed view can be achieved via molecular weight analysis, infrared scans, and various rheological methods. As previously mentioned, emulsions also will be characterized by particle size. Rheology also will provide valuable information on adhesive performance. Residual monomers—monomers that did not react to form a polymer—are also monitored to verify that full polymerization has been achieved. This is also important to ensure that the adhesives will comply to various regulatory requirements.
Adhesive manufacturers will do quality control analysis of the adhesive components as an in-process control step. Solvent-based polymers will then be modified by adding solvent and cross-linking agents (for the one-part systems). Emulsions will be blended with other additives so they are coater ready.
In some cases, other additives can be blended with the polymers. For instance, if higher tack is required, tackifying resins can be included (more on tackifying resins in my next article on hot melts). Fire retardants, UV inhibitors, or dyes can be blended into the adhesives as needed to meet end-use requirements. Chemists must carefully consider the amount and how these additional materials will be blended into the adhesive as they will impact adhesive and coating properties.
So how does the difference in polymerization method impact adhesive performance? One take-away (hopefully!) from the descriptions of solvent-based acrylics versus emulsion acrylics is that the solvent-based systems contain less additives than the emulsion systems, In the case of solvent-based polymerization, there is only the solvent blend, monomers, and initiators. Emulsions will have monomer, water, and initiator, as well as surfactants. The coater-ready forms of emulsions will then have additional additives to assist in obtaining smooth, defect-free coatings. So the performance of solvent-based adhesives directly correlates to the performance that the chemist has built the polymer to deliver.
Hopefully this overview will provide some insight into the challenges chemists face to create acrylic-based polymers for pressure-sensitive adhesives. Next time we will delve into the world of rubber-based hot melt pressure-sensitives.
Until then…keep Sticking With It!
About the Author
Ingrid Brase is a technical market strategist recognized for her ability to translate technical needs into business solutions. Her understanding of pressure-sensitive adhesives and their use is complemented by her strengths in strategic marketing, project management, new product development, and key account management. She is available for consulting or contact assignments in these areas. Ingrid’s expertise is a result of more than 20 years of experience in the p-s adhesives business. She was most recently the market segment director for Henkel Corp., rising to that position after various assignments in the p-s business unit. She began her career as a research scientist then progressed to market-focused roles. Ingrid earned her MBA at Rider Univ. and holds a BS in chemistry from SUNY/Oneonta. She has served on the board of directors for TLMI and AIMCAL in addition to chairing technical teams for both trade associations. Ingrid is a well-known speaker and author on topics related to adhesive use. To learn more about Ingrid or contact her, visit www.ingridbrase.com, e-mail to This email address is being protected from spambots. You need JavaScript enabled to view it., or call her at 609-558-9760.
