But today, more chemists and formulators are turning to UV-cure technologies. In fact, a post-9/11 report by the Freedonia Group, Cleveland, predicts that radiation-cure adhesives will explode at a rate of 10% per year, reaching almost eight million pounds in 2005. The report also predicts that the fastest growth will occur in PSAs and other small-volume adhesives because of a gradual reduction in raw material costs.
EPA regulations have also helped ensure a healthy future for UV-cure PSAs. Recent regulations force manufacturers across a broad range of industries to switch to no- or low-VOC technologies.
Because these properties are critical to the adhesive's performance, formulators must be aware of any factors that can directly affect them - such as temperature, aging, film thickness, cure rate and post-cure parameters. They must also be aware of formulation variables - including oligomer selection, tackifier addition, monomer structure, molecular weight and glass transition - that directly impact tack, peel and creep.
Traditionally, PSAs have been developed around rubber-based formulations with the inclusion of rosin esters or C-5 and C-9 hydrocarbon resins as "tackifiers" - those resins that promote the "sticky" feeling of PSAs.
For example, UV-curing waste can be disposed of as ordinary solid waste, increasing environmental safety and offering significant savings. Additionally, UV PSAs are generally safer for formulators to handle than the solvent materials they replace, offering lower skin and eye irritation.
Environmental and safety benefits, coupled with the desire for fast-curing UV/EB systems, are creating a fast growth market for UV PSAs.
On top of these benefits, raw materials prices for UV-cure PSAs are gradually declining - and are expected to continue doing so in the next few years.
The selected monomer blend included ethoxylated, trifunctional and monofunctional acrylates incorporated at 40 percent into the base formulation. The acrylate monomers provided adhesives with different crosslink densities, solubilities, glass-transition temperatures and flow characteristics. Fifty percent CN-966, the highly flexible urethane acrylate, and 10% photoinitiator made up the balance of the formulation.
Application: PSAs were applied to Mylar films. Wire-wound drawdown bars applied the adhesive at 0.0005 to 0.006 inches to test properties vs. adhesive thickness.
Triacrylates: Rapidly increasing molecular weight and crosslink density causes the product to become tack-free, hard and solvent-resistant. These properties make triacrylates suitable for coatings. However, the high crosslink density makes high concentrations unsuitable for use in PSAs. Increasing the molecular weight and lowering the glass-transition temperature improve the rolling-ball tack at fast cure speeds. (See Table 3.) Rolling-ball tack decreases as product crosslink density increases, again, making triacrylates unsuitable for PSAs.
Photoinitiator selection was designed to accelerate post-cure phenomenon characteristics. As stated earlier, the property-forming bond is called tack and the tensional force required to remove the adhesive tape is called peel.
Again, the third property is flow resistance or creep. A broader sense would include the PSA's entire stress/strain behavior. Rolling-ball tack was measured using a 0.8-cm stainless steel ball rolled from a 6-cm height at a 20 deg elevation. The distance the ball rolls from the incline base to the point when forward motion is stopped was measured and recorded. Remember that rolling ball tack (RBT) does not measure adhesive bond strength. Rather, it measures adhesive flow and wetting characteristics.
When dealing with monomer structure, molecular weight significantly affects rolling-ball tack. Urethane acrylate concentration, tackifier and adhesive formulation thickness determine PSA properties.
Four monofunctional acrylates were tested extensively. The acrylates ranged from highly polar to non-polar, and lowest molecular weight of 188 to highest molecular weight of 450. Polyethylene glycol, pure alkane and aromatic-ring chemistry directly influenced PSA performance.
As shown in Table 4 (page 36), SR-495 or monohydroxy caprolactone acrylate yielded significantly higher viscosities. Molecular weight increase accounted for the change. The large monomer molecular weight resulted in less freedom of movement for all formulation ingredients, thereby increasing viscosity.
SR-395 cured the slowest, enhancing the cure rate vs. rolling-ball tack. RBT decreases when monomer level increases in the formulation. This is true for all the monofunctional monomers tested.
However, SR 395-based formulations decrease in RBT with increasing energy. SR-395's slow curing results in molecular weight increase with increased energy. Table 5 reveals the cohesive nature with large volumes of SR-395 or small energy applied to the adhesive.
The SR 395-based adhesive split leaves residue on both the test surface and Mylar film. It is important to note that a PSA should have adhesive failure. SR-256 and SR-495 failed adhesively to metal surfaces tested.
Monoacrylate molecular weight itself affects RBT to a lesser degree. Increasing monomer molecular weight from SR-256 (molecular weight 188) through SR-395 (molecular weight 212) then up to SR-495 (molecular weight 344) showed a slight improvement in RBT using tackifier resins. Increasing monoacrylate molecular weight showed a trend when increasing RBT up to that of SR-495. Lower values indicate better RBT. The non-polar nonyl carbons and phenyl rings produced the hard segment. This hardness resulted in less RBT.
Acrylate homopolymers resulted in a highly polar, flexible block. The non-polar nonyl carbons and phenyl rings produced the hard segment. This hardness also resulted in less RBT.
Film thickness is critical when testing UV-cured PSAs. Film thickness applied from 0.0001 to 0.003 inches (25 to 75 microns) minimally affected PSA properties. However, there was a notable decrease in RBT with increasing adhesive-film thickness.
The adhesive tape was then peeled at a 180 deg angle at one-inch-per-second (2.54 cm/sec) peak peel with average peel data recorded. Peel strength depends on cure rate, monomer molecular weight/structure, tackifier resin and oligomer level.
When increasing SR-395 [or decreasing cure rate to 16 m/min (50 ft/min)], cohesive failure resulted. In addition, SR-395 odor remained in the finished adhesives.
Monoacrylate molecular weight continued to play an active role in peak peel strength. SR-256 is a highly polar monomer particularly suited to Norsolene S-95 (C9 hydrocarbon resin).
The C9/SR-256 PSA yielded 400+ gm/inch peel strength. This is similar to desk tape with 400 gm/inch peel strength. The SR-256 adhesives similarly had rolling-ball tack and creep resistance with the same performance.
The non-polar alkane structure, SR-395, yielded exceptional peel strength formulated with the non-polar CN-966, or the pentaerythritol-based rosin ester. It was noted that
S-95, a C9 resin, did not increase the physical properties. A proposed explanation is the difference in structure and polarity. The SR-395 did exhibit residual odor despite the excellent peel strength.
The caprolactone acrylate did not show good compatibility with any tackifier. The hydroxy group provided polarity, and the caprolactone did not offer solvency characteristics. The SR-495 would provide good weathering and a hydroxy group for dual-cure mechanisms. Reacting SR-495 with an anhydride, or isocyanate, through post-cure would make a good UV-curable PSA, forming laminating-adhesive bonds.
Highly flexible, urethane diacrylate CN-966 did not significantly affect the peak peel strength. It was found that SR-395 did go through an optimization point at 50%. The high peel strength (1,240 gm/cm) did not enhance the creep resistance. The SR-395 creep resistance did improve with increased urethane acrylate.
SR-395's peak peel increased by adding 12.5% tackifiers. Increasing tackifier to 25% did not further enhance properties. It was found that increasing CN-966 urethane diacrylate significantly increases peak peel strength over the tackifier additions.
Rolling-ball tack and peak peel strength depend on monomer structure/molecular weight, concentrations, tackifiers, cure, and adhesive thickness. Additional perspective examines rolling-ball tack vs. peak peel strength.
Testing found that high rolling-ball tack (low distance in cm) yielded lower peak peel strength. Tack decreased (increase in RBT distance in cm) while peak peel strength increased. The four data points are noted as the four monofunctional monomers tested. S-95 increased rapidly, peaking in the SR-395.
Peak peel was very dramatic. The data is the same as was found above, however, making the monofunctional monomer the important variable.
The SR-495 or caprolactone acrylate consistently yielded high RBT and moderate peak peel strength. SR-256 or ethoxy-ethoxyethyl acrylate had high RBT and good peel strength. The performance properties of SR-256-based adhesives are similar to desk tape. Performance properties excelled that of high peel strength, high tack and cellophane packaging tape.
The 52.5% SR 395-based formulation developed little cystallinity, or creep resistance, despite the high cure energy. Note the steady improvement with increasing CN-966 urethane diacrylate content and speed.
The SR-495, and SR-256 passed the eight-day creep-resistance test. Incorporating 12.5% S-95 tackifying resin dramatically improved creep resistance based on SR-395 PSA. The tackifier resins increased the glass transition of the -36 deg C SR-395 monofunctional monomer.