Sticking With It | Overview of Radiation Curable Pressure-Sensitive Adhesives
- Published: June 15, 2017, By Ingrid Brase
Adhesives expert Ingrid Brase considers the benefits and drawbacks of radiation curable products.
In several of my past columns, I have shared some thoughts on how to select the right pressure-sensitive adhesive (PSA) for different end use markets. This installment will change direction slightly to provide some insight into one of the “emerging” adhesives technologies in the market: radiation-curable adhesives.
I have purposely put the word “emerging” in quotes as this technology has been around for more than 20 years! Over the past five, it finally seems to be attracting commercial interest on a broader scale in North America.
So, let’s explore what radiation-curable PSAs are, how they compare to other technologies available, the benefits and watch–outs, and finally some thoughts on their future.
definition
Radiation-curable PSAs are a class of products in which ultraviolet (UV) light or electron beam (EB) is used to “cure” or build the molecular weight and/or cross-link the adhesive. If UV light is used as the energy source, the adhesive base either contains a photoinitiator or one can be added as a formulation ingredient.
When the adhesive is exposed to UV light, the photoinitiator begins the cross-linking process. For e-beam, no photoinitiator is needed, and when the adhesive is exposed to the energy of the electron beam, cross-linking is initiated.
All the typical chemistries used to make PSAs, i.e., styrene block copolymers, acrylics, can be cross-linked in this manner to add cohesive strength to the final product. For this discussion, I will focus on UV-curable acrylic pressure-sensitives as this is one of the more common chemistries. This is also where there has been a considerable growth in commercial interest over the past number of years.
evolution
First, let’s look at the evolution of the radiation-curable chemistry and its use in PSAs. First generation products, what I will refer to as the monomer syrup approach, used the light curing process to build the PSA backbone. A variety of monomers are first polymerized by conventional means to create very short-chained “polymers.” Photoinitiators are added to the syrupy mixture. The mixture is coated to liner or substrate and exposed to UV light. This is done in an inert (i.e., no oxygen) environment, and the process must be run at very slow speeds to build polymers of sufficient molecular weight to be effective PSAs.
The next generation of products, the oligomer approach, is like technology used to create UV-curable inks. Here short-chained polymers or oligomers are mixed with reactive monomers and photoinitiators to create the syrup. This is again coated to a liner or substrate and exposed to UV light to create the adhesive. The advantage of this system versus the monomer approach is that it can be processed without inerting.
Both approaches are still used in pressure-sensitive applications but have limitations. A major drawback is that they are usually expensive due to process time. It is also challenging to build the balance of adhesion–cohesion needed in many demanding tape applications.
Polymer-based systems
And now to the main topic of my discussion: polymer-based systems. These UV-curable acrylic pressure-sensitives are available in all the adhesive forms, that is, solvent-based, water-based, and 100% solids. The 100% solids form is one of the most popular as it has several potential advantages over the other product forms. The 100% solids products are generally polymerized by the adhesive manufacturer in solvent, which is then removed to create the final 100% solids hot melt product.
Similar to conventional hot melts, the materials are solid at room temperature and become liquid when heated. Typical melt temperatures are much lower than for rubber-based hot melts, typically around 270 deg F for these acrylics versus 325–350 deg F for rubber-based products.
The photoinitiators are either polymerized into the backbone of the acrylic polymer base or added during formulation. Various types of photoinitiators can be used and fall into two broad categories. Free radical cure systems are fully cured under the lights, while with cationic or “dark-cure” systems, UV light initiates the cure mechanism, and cure is completed after exposure or “in the dark.”
So how does this technology approach compare to the more conventional forms of PSAs? These UV acrylics are like rubber-based hot melts in that they can be easily coated as a 100% solids product with no need to worry about solvent or water removal. They are generally also formulated systems containing tackifiers and photoinitiators (if they are not polymerized into the backbone of the polymer).
Like water-based and solvent-based acrylics, they can be designed to meet a wide variety of end use performance requirements. They have many of the same advantages of solvent-based acrylics, namely, resistance to solvents and cleaners, UV stability once fully cured, and a good balance of adhesion with cohesive strength. Like hot melts, they do not require ovens to dry, do not require special storage, have no appreciable VOCs, and can be coated at reasonable speeds.
And how are they different? The polymer structure is built differently as the polymer backbone is generally lower molecular weight than typical solvent-based systems. While solvent-based and some water-based systems use chelates to develop cross-linking and cohesive strength, these systems build cohesion through the UV process, which cross-links and builds the molecular weight of the polymers by chain extension.
The biggest difference is in the actual manufacturing process of creating the final PSA product. When conventional solvent-based acrylics are coated and dried, the cross-linking mechanism is activated during the drying process with the removal of solvent or via the addition of a cross-linking agent like isocyanate. Water-based acrylics also will reach their final performance once water is removed. Thus, if the adhesives are properly dried and the specified performance properties are not reached, it is most likely due to an issue in the adhesive formulation. By contrast, UV-curable acrylics require more care and attention during the coating process. Since full cure only occurs when the appropriate dose of UV light to cause the photoinitiator to activate is delivered, proper processing becomes more critical. Line speed, coat weight, and UV bulb type and strength will all impact the dose.
Let’s look at each of these in a bit more depth to understand their importance and inter-relationship.
Coating thickness
First, let’s look at coating thickness. As has already been discussed, cure is affected by delivering UV light to the coating, activating the photoinitiator to begin curing or cross-linking/creating chain extension. Thus, the coating must be of sufficient clarity and be thin enough for light to completely penetrate the coating for free radical photoinitiators to be completely activated. Generally, the limit on coatweight for these systems is about 4 mils or 100 gsm. The adhesive manufacturers will carefully select formulation ingredients to minimize light absorption to allow for full penetration.
But what if a thicker coatweight is required? This is when the use of a cationic or “dark-cure” UV system is beneficial. The UV light does not need to fully penetrate the coating. However, it must be recognized that the dark-cure systems will take time after coating to fully be cured, generally at least 24 hours.
Line speed
Next, there is line speed. Line speed is a factor as it will directly impact the exposure of the coating to the UV light source. The cure dose will be controlled both by the lamp intensity and time of exposure. The adhesive supplier should provide a minimum UV dose needed to cure, defined in milli-joules per square inch (or centimeter) per thickness.
This can then be used to determine the line speed required to ensure this level of UV energy is delivered to the coating. In many cases, multiple sets of UV lamps are used to improve line speed.
Energy source
Finally, there is the energy source—the UV lights. There are three types of UV lights available:
- arc lamps
- microwave lamps
- LED lamps
Current UV acrylic pressure-sensitive systems are not designed to cure with LED lamps; they generally operate at longer wavelengths and produce a narrow output band.
Arc lamps are medium pressure lamps containing low levels of mercury. The bulbs can be manufactured to different lengths, up to 3 meters, giving even exposure across their width. Bulbs are generally a bit wider than the web width to ensure full cure out to the edges. The strength of the UV light produced by arc bulbs will begin to decay over time and must be monitored closely so that line speed can be adjusted to ensure an adequate dose of UV light is delivered so the coating will fully cure.
Microwave systems use medium pressure mercury-based bulbs in which the mercury vapor is excited by microwave radiation. The major benefit of microwave lamps is that when they begin degrading they completely die, so they are “on” or “off.” They are typically available as smaller units and must be properly installed across wider webs so no gaps in intensity are present. UV dose is easily measured using many available UV meters. Micro versions of UV intensity meters are available to fit into the normal production process.
Proper maintenance of the UV bulbs is also important. They must be cleaned periodically of splatter, dust, etc., from processing so that foreign materials do not interfere is delivering UV light to the coating.
Delivery systems
While we are discussing processing, there is also the adhesive delivery systems to keep in mind. The UV-curable acrylic hot melts are designed to be dispensed using conventional hot melt delivery systems, melt tanks, and drum unloaders.
Most products are provided in drum form, so I will focus on this delivery method. It is generally recommended to use a system where the drum unloader will deliver adhesive to a melt tank, which supplies a reservoir of melted adhesive to the coating head. Temperature management throughout the system is vital. As indicated earlier, target melt temperatures of 270 deg F are recommended to maximize product stability. Here higher temperatures are not a plus as they decrease stability and do not result in appreciable viscosity reduction, especially if compared to conventional rubber-based hot melts.
Since the acrylic polymers are not compatible with rubber-based hot melts, it is ideal to have two separate unloader systems to avoid contamination. However, with proper clean out, the UV-curable acrylics can be melted in the same system as rubber-based hot melts.
Uses and Limitations
So that was a long background to the true meat of what you probably want to know: Where can I use these products? Simply put, UV-curable acrylic hot melts can be designed to meet most of the applications where water-based and solvent-based acrylics are typically found. Adhesive suppliers have begun to offer a variety of these adhesives designed to meet various specific end use application criteria. There is some school of thought that UV-curable acrylics could eventually replace all solvent-based acrylics because of the advantages previously described.
For tapes, UV-curable acrylics are now beginning to replace solvent-based acrylics in many of the thicker film products. They offer significant advantages in line speed and efficiency. Compared to water-based acrylics, use of UV-curable acrylics provides better blush and moisture resistance.
For converters who already have hot melt coating equipment, the UV-curable acrylics are an avenue to higher end applications where conventional rubber-based hot melts are not suitable, including specialty labels and tapes. Even use in the medical tape segment is viable if the proper non-migratory ingredients are selected; currently there are several commercially available products that are suitable.
But are there limits? All good things have limits and this technology has its as well. As previously described, there is an upper end in coatweight thickness for free radical cured chemistries; currently this is the most widely available group of products.
On the other end of the thickness spectrum, the hot melt based UV-curable acrylics are not suitable for thin film optically clear applications; limit would be no thinner than 0.7 mil. These products are generally very viscous and so lower thickness coatings will have significant defects caused by the coating method, including streaking.
Outlook
So, what does the future hold for this technology approach? As I indicated in my introduction, UV-curable PSAs have been available for almost 20 years but seem to only have begun to show significant North American sales in the past five or so years. Why? Some of this may be infrastructure related; there was available capacity especially for solvent-based systems, so no need to invest in new assets.
As equipment ages and converters consider expansion or replacement options, exploring the move to UV-curable PSAs is prudent. It allows expansion without adding costly flammable storage required of solvent materials as well as a lower cost coater as there is no drying oven, or for solvent-based systems, thermal oxidizers. For some converters facing local concerns on expanding a solvent-based footprint, it may be the only option.
Since the coater footprint is smaller and these hot melt products are non-flammable, they also require less manpower to monitor and process.
In summary, UV-curable acrylics are now a viable option when considering what product to select to manufacture high performance pressure-sensitives. I hope this overview provides some insight into this exciting technology.
As always, if you have questions or comments, please forward them.
Until next time, 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.
