Adhesives Magazine

Formulating UV-Curing Liquid Pressure-Sensitive Adhesives

November 1, 2008
This article focuses on the many formulation factors that influence the performance of UV-curing pressure-sensitive adhesives (PSAs).



Many formulation factors can influence the performance of UV-curing pressure-sensitive adhesives (PSAs). Several basic material fundamentals should be considered, including polymer rheology, molecular weight, functionality and crosslink density. These topics will be addressed first to provide insight into the behavior of polymeric materials in PSA applications. The various raw materials that are used in practical UV formulations will then be reviewed, as well as formulations for certain applications.

Most commercially available UV-curing PSA systems are based on free-radical curing liquid systems, therefore emphasis will be on this technology.

Materials Fundamentals

An understanding of the general fundamental material properties required of PSAs and, specifically, UV-curing PSAs will provide the formulator with the tools required to minimize trial-and-error approaches and speed development time. The most important fundamental materials properties for PSA applications are:
  • Rheology
  • Molecular weight
  • Functionality
Rheology is the study of the change in form and flow of a matter. It is generally applied to viscoelastic materials. The rheological properties of the uncured adhesive are important with regard to application and coating. However, they are also important in the cured state, as they must be capable of a degree of flow to provide wetting and tack, yet have sufficient resistance to stress to provide for high adhesive strength. The correct rheological properties for a PSA require a careful balancing of these properties.

To counteract the viscous flow, PSAs are based on very high-molecular-weight rubber polymers. These polymers rely on the entanglement of molecules to restrict flow. When high strength, heat resistance and chemical resistance are required, the entanglements themselves are not sufficient to restrict flow due to service stress. In these cases, the molecules are chemically crosslinked to provide for a three-dimensional network structure. This is the function of UV-curing mechanisms.

In PSAs, the crosslink density or the molecular weight between crosslinks provides a measure of the balance that can be achieved between holding power and viscous flow. This crosslink density can also be measured by the glass-transition temperature of the adhesive. The higher the glass-transition temperature (Tg) for a specific adhesive, the higher the crosslink density or the lower the molecular weight between crosslinks.

Functionality plays an important role in determining crosslink density. The functionality of a polymer is the number of reactive sites contained in the polymer molecule. The reactive sites are the connecting points for crosslinking to take place. Therefore, the higher the functionality, the higher the crosslink density, holding other factors constant.

The discussion above considers the base polymer in the UV-curing PSA formulation. In order to further modify the system to provide for the breadth of properties required for a practical adhesive, many additives and modifiers are also required. Thus, the adhesive formulator has many tools at his disposal. In fact, there are more tools than in conventional PSA formulation, due primarily to the effect of UV dosage and possible oxygen inhibition on crosslink density.

Table 1.

Formulation

The conventional liquid UV-curable PSA is comprised of four essential components: oligomers, monomers, photoinitiator and additives. A typical formulation for a radiation-cured adhesive is provided in Table 1. The wide choice of raw materials available allows maximum latitude to achieve the desired properties.

With conventional adhesives, the final performance properties are achieved during the resin polymerization process in a reactor. With UV technology, the polymerization takes place during the curing process. In effect, radiation-curable adhesives are a self-contained polymer factory of sorts.

Table 2.

Oligomers

Oligomers determine the overall properties of any adhesive crosslinked by radiant energy. Oligomers are moderately low-molecular-weight polymers, most of which are based on the acrylation of different structures. The acrylation imparts the unsaturation or C=C group to the ends of the oligomer; this serves as the functionality. The oligomer used in PSA applications is generally a multi-functional elastomeric polymer, such as an aliphatic urethane acrylate.

Oligomers provide much of the shear strength in the UV PSA formulation. However, selection of the oligomer will also affect more viscous properties, such as tack and peel strength. The high-molecular-weights and glass-transition temperatures are generally well below room temperature to allow the oligomer to offer elastic properties at room temperature. This provides the viscoelasticity required for good tack and adhesion. Other factors that are affected by choice of oligomer include: reactivity, creep resistance, heat and chemical resistance, and color retention. Of course, cost is also an important factor as oligomers often have the greatest weight concentration in an adhesive formulation.

In the acrylate family, there are several possible UV-curing oligomers that can be used in PSA formulations. Each of these has certain advantages and disadvantages.

Epoxy acrylates are one of the dominant oligomers in the radiation-curable coatings market. In most cases, epoxy acrylates do not have any free epoxy groups left from their synthesis but react through their unsaturation.

Within this group of oligomers, there are several major subclassifications: aromatic difunctional epoxy acrylates, acrylated oil epoxy acrylates, novolac epoxy acrylate, aliphatic epoxy acrylate and miscellaneous epoxy acrylates. Table 2 provides general descriptions of several epoxy acrylate oligomers.

Urethane acrylates are produced by reacting polyisocyanates with hydroxyl alky acrylates, usually along with hydroxyl compounds, to produce the desired set of properties. Urethane acrylates are the most expensive of the acrylates. There are many different types of urethane acrylate oligomers that feature variations in the following parameters.

Table 3.

The major characteristics that differentiate urethane acrylates are presented in Table 3. As would be expected of a urethane, these properties are primarily determined by the isocyanate and polyol used in its manufacture.

A variety of polyester acrylates is available, which enables a range of properties. They are generally low-viscosity resins that require no reactive diluents. Polyester acrylates provide performance properties between those of urethane acrylates and epoxy acrylates. A disadvantage of some types of polyester acrylate is their irritancy. This is particularly true for low-molecular-weight, highly reactive resins.

Polyester acrylates vary in functionality, chemical backbone and molecular weight. The influence of the functionality is similar to that for the urethane acrylates. The chemical backbone has a large influence on properties such as reactivity, color stability, hardness, reactivity, etc. Typically, the higher the molecular weight, the higher the flexibility and viscosity and the lower the reactivity.

Polyether acrylates have the lowest viscosity of the acrylate resins and are typically used with very little monomer or reactive diluents. They generally have high flexibility but relatively poor water and chemical resistance. To overcome these drawbacks, polyether acrylates are mostly used in combinations with other oligomers or monomers. An interesting property of some polyether acrylates is that they are compatible with water and can be used in water dilutable systems.

Acrylic acrylates, like urethane acrylates, have a very versatile chemistry, and there are many variations available to the formulator. These resins are often used because of their good adhesion to difficult substrates, such as low-surface-energy plastics.

Miscellaneous oligomers are generally specialty products that typically comprise melamine acrylates, silicone acrylates, etc. Other types of radiation-curable resins include unsaturated polyesters dissolved in styrene or acrylics. More recently, polyester resins have appeared on the market in the form of non-acrylic vinyl ether blends.

Table 4.

Monomers

Monomers are primarily used to lower the viscosity of the uncured material and to facilitate application. However, they are also used to make adjustments of the formulation, such as improved surface wetting, leveling, and physical properties. Since most oligomers are too viscous to be applied with conventional coating equipment, most radiation-cured formulations are diluted down to a viscosity of 100-10,000 cps by adding a lower-molecular-weight monomer.

There are primarily two types of monomers: monofunctional, which are used primarily as a diluent; and multifunctional, which can be used as a diluent and crosslinker. Multifunctional monomers can be di-, tri-, and polyfunctional. The greater the functionality, the greater the crosslinking potential of the monomer. In this way, the functional monomers can be used to adjust properties of the final adhesive as well as to reduce viscosity. The characteristics provided by functionality are summarized in Table 4.

Monomer chemistry also has an influence on the polymerization process and physical properties of the final adhesive. Increasing the monomer functionality leads to higher cure speed, higher Tg, higher crosslink density, higher shear strength, and greater chemical and thermal resistance, but lower flexibility and low conversion. A balance is generally required between adhesive strength and rigidity. Rigid adhesives have high shear strength and chemical/thermal resistance but exhibit low peel strength. More flexible adhesives have high peel and impact strength and better adhesion to plastic substrates, but they do not have the heat and chemical resistance of their more densely crosslinked (more rigid) counterparts.

The monomer used as a reactive diluent in a UV-curable resin plays a key role: it affects both the cure speed and the polymerization extent, as well as the properties of the final product. An increase in monomer functionality generally accelerates the curing process, but at the expense of the overall monomer conversion. Poor conversion leads to a crosslinked polymer, which contains a substantial amount of residual unsaturation. As a result of increased crosslink density, UV adhesives become more rigid and more resistant to chemicals, temperatures, and abrasion. However, they become less flexible and less resistant to impact and thermal cycling.

The effect of monomer on glass-transition temperature (which is a result of crosslink density) is an important tool for the formulator since the mechanical properties of the adhesives are strongly influenced by the Tg. If the Tg of the adhesive is below the expected service temperature, the adhesive will generally exhibit some of the following properties.
  • Flexible with a high degree of elongation
  • High peel and impact strength
  • Good resistance to thermal cycling
  • High thermal expansion coefficient (well suited for plastic substrates)
  • High degree of creep when exposed to constant stress
  • Poor blocking resistance (tacky)
  • High moisture uptake
  • Good chemical and temperature resistance
On the other hand, if the Tg is above the expected service temperature, then the adhesive usually can be characterized as exhibiting some of the following qualities:
  • Rigid and to some extent brittle
  • Low impact and peel strength
  • Prone to crack propagation
  • Low thermal expansion coefficient
  • Poor resistance to thermal cycling
  • High shear strength
  • Low water uptake and swelling, and high barrier properties against chemicals and water
  • High temperature and chemical resistance.
The range of radiation-curable reactive monomers offered today is almost unmanageable. Formulators who have to provide optimum product performance at the best cost are sometimes overwhelmed by the vast array of choices. Because there are so many monomers available, it is important to keep in mind some general guidelines. There are four major parameters that contribute to the monomer’s characteristics: functionality, chemical backbone, chemical structure and molecular weight.

The type and molecular weight of the backbone chain in a monomer can be varied to provide lower skin irritation, better flexibility, and faster cure speeds. Monomers can also be tailored for water-dispersible, adhesion-promoting and pigment-dispensing applications. In addition to providing the functions noted above, monomers could be used as a chemical intermediate to produce copolymers that enhance performance properties.

Table 5.

Photoinitiators

Photoinitiators absorb light and are responsible for the production of free radicals. High-energy free radicals induce crosslinking between the unsaturation sites of monomer, oligomers and polymers. Arguably, the most important additive is a photoinitiator for UV-cured adhesives. Photoinitiators are not needed for electronic beam-cured systems because the electrons themselves are able to initiate crosslinking by virtue of their higher energy.

Table 5 is a selected listing of photoinitiator chemicals. A typical photoinitiator for a UV-curable acrylic system is based on an aromatic keto compound. Often more than one photoinitiator is employed to provide for cure with a specific radiation source. The photoinitiator package will also need to be optimized for a given adhesive thickness and UV dosage.

The photoinitiator determines not only how but where the cure will occur. A high-surface-cure photoinitiator, for example, tends to increase shear properties while destroying the tack of the system. A good through-cure product may leave the surface very tacky but exhibit poor cohesive strength due to the fact that the surfaces are not well crosslinked.

Some UV-curable adhesives contain a combination of UV and IR initiators to take advantage of the IR output that many UV lamps generate. At times, a photoactive crosslinking agent is used to improve cohesive strength without affecting tack and peel. Table 6 shows an example of a UV-curable formulation.

An essential requirement of UV curing is that the adhesive has to be transparent to UV light in order to be cured. Filled or pigmented adhesives may pose a curing challenge. Another disadvantage is that one transparent substrate is normally required, and a limited depth of cure can be achieved. These disadvantages have generally been overcome by the development of dual-cure adhesive systems. In these systems, two independent curing mechanisms are incorporated into a single formulation. The adhesives can be cured first to a chemically stable state by UV radiation and then advanced to a full cure by a second means such as thermal cure.

Additives

The most common additives in all UV-cured resins are stabilizers, which prevent gelation in storage and premature curing due to low levels of light exposure. Color pigments, dyes, defoamers, adhesion promoters, flatting agents, wetting agents and slip aids are examples of other additives.

Tackifiers are required in pressure-sensitive radiant-cured adhesives to improve the tack and pressure-sensitive nature or “stickiness” of the adhesive. Traditionally, these formulations have included tackifiers consisting of solid rosin esters of C-5 and C-9 hydrocarbon resins.

However, solid tackifing agents are difficult to incorporate into UV-curable oligomers and monomers without the use of a solvent and/or heat. This is often a time-consuming and expensive process. New low-viscosity oligomers have been developed that are said to provide excellent tack properties without the need of a solid resin additive.2

Oxygen scavengers may be required as oxygen inhibits the curing of acrylates. These act by quenching the photoinitiator or by scavenging free radicals. Scavenging produces stable species that slow down the cure rate but also degrade the properties of the cured adhesive. Other methods of oxygen inhibition occur via nitrogen blanketing, the use of high intensity lamps, and by varying the initiator type and concentration.

Other additives used to improve the performance of radiant-cured adhesives are similar to those that might be found in more conventional adhesives. These include adhesion promoters, fillers, antioxidants and plasticizers.

Photoinitiators, sensitizers and other radiation-sensitive additives may have an effect on the adhesive properties and especially on processing of the adhesive. Thus, all additives need to be tested with regard to storage and processing properties, as well as with regard to their adhesive properties.