For years, Sartomer Co. has produced zinc diacrylate (ZDA), which is primarily used in peroxide-cured rubber applications. These metal acrylates have not found much use in radiation-curing applications due to their lack of solubility in most monomers and oligomers.

Recently, we developed a series of organic metal acrylates that are soluble in most monomers, which now lead to higher formulation capabilities. The new oligomers have been studied by Sartomer chemists in various applications.1-4 Due to their hydrophobic nature, the metal-containing oligomers can be used at high levels in standard radiation-curable formulations. The metal-containing oligomers described in this article have been shown to promote adhesion to metal and glass. It is anticipated that these products and products based on other metals will demonstrate high value performance, such as conductivity, high refractive index, anti-fungal properties, and more.

Metal Acrylate Compositions

Sartomer patents5, 6describe the synthesis of two types of soluble metal oligomers. Figure 1 shows the reaction for obtaining a polyester metal acrylate, and Figure 2 shows a urethane metal acrylate utilizing zinc oxide to provide the metal content in the synthesized oligomer.

We have commercialized several soluble metal-containing oligomers, but the focus of this article is on CN2404, a polyester zinc acrylate oligomer, and CN2405, a urethane zinc acrylate oligomer. Figure 3 shows the structures of these two oligomers.

Discussion

For the past few years, our chemists have studied the new metal-containing oligomers and their benefits in different application areas. Bailey's work1,4discusses the benefits of reversible crosslinking in solvent-based and hot-melt pressure-sensitive adhesives (PSAs). Ceska reported2, 3on the use of the novel oligomers in radiation-curable coatings on several substrates. Also, the metal-containing hybrid oligomers have been evaluated in radiation-curable PSAs over the past four years. Each of these application areas will be covered in detail.

Solvent-Based PSAs

Bailey evaluated the new metal-containing oligomers in a solvent-based PSA formulation. He used CN2404 as a co-reactant in the preparation of a solvent-based PSA. Bailey found that the CN2404 improves adhesion to a broad range of substrates with excellent durability on exposure to moisture, heat, and ultraviolet light. Table 1 shows the sample formulations for a solvent-based PSA along with DSC and adhesive testing results. The metal-containing solvent-based PSA yielded higher tack and peel than the PSA not containing the metal.

Hot-Melt PSAs

Bailey continued his studies using the metal acrylates in hot-melt pressure-sensitive adhesives (HM PSA). He found that the new oligomer introduced ionic crosslinks into the backbone. The ionic crosslinks yielded thermoset behavior and enhanced the adhesive properties at ambient temperature. At elevated temperatures, the crosslinks broke to yield thermoplastic behavior.

Crosslinked HM PSAs based on 2-ethylhexyl acrylate acrylic acid, and the metal-containing oligomers were prepared by solution polymerization with an azonitrile initiator and a hydrocarbon resin. The formulation for the test HM PSAs is shown in Table 2. The crosslinked HM PSAs exhibit a similar melt viscosity profile as an uncrosslinked polymer and have enhanced adhesion to cold rolled steel (see Table 2).

UV-Curable Coating

Ceska2, 3reported adhesion results for a UV-curable coating comparing the CN2404 and the CN2405. In this study, a base monomer, oligomer, and photoinitiator blend was prepared in which the zinc-containing oligomer was varied from 1 to 16 parts. The base formulation is shown in Table 3.

These formulations were cured using two 300 watts per inch mercury vapor lamps with four passes of 750 mJ/cm2energy for a 5.0-6.0 mil thick coating. Crosshatch adhesion was then run using the 610 tape adhesion test both before and after heating at 200ºC for 10 minutes. As expected, the adhesion significantly improves with baking the coating. The results of the adhesion study on aluminum are shown in Figure 4.

Using the same samples containing the CN2405, the same test was also run on tin-plated steel. These results are shown in Figure 5. The CN2405 was also tested on cold rolled steel and glass. Similar results were obtained with the other two substrates.

UV-Curable PSA

Both the CN2404 and the CN2405 were evaluated in UV-curable PSA formulations. A base formulation was made and then 2 and 6 parts of the metal acrylates were added to evaluate adhesion and tack of the PSA. The UV-curable PSA formulation is shown in Table 4.

These PSA blends were then evaluated at two different adhesive thicknesses by curing with two 300 watts per inch mercury vapor lamps measuring 750 mJ/cm2energy. The results are shown in Figure 6. The control sample does not contain any of the zinc-containing oligomers. At low adhesive thicknesses (0.4 +/- 0.1 mil), the control has a 180º peel adhesion of around 2.1 pounds-force. All of the zinc-containing PSAs have much better 180º peel adhesion (2.7-3.6 pounds-force) than the control. When the adhesive thickness is increased to 1.4 +/- 0.1 mils, the control has slightly better 180º peel adhesion (3.8 pounds-force) when compared to the rest of the thicker zinc-containing PSAs (3.1 to 3.8 pounds-force).

The tack was also evaluated on all of the UV-cured PSAs. The results are shown in Figure 7. For both adhesive thicknesses, the tack for the control is lower than the tack for all the other PSAs, which all contain the zinc oligomers. The only tack that didn't seem much better than the control was the sample that contained 6% CN2404. If a thin PSA is desirable, then a small addition of one of these zinc-containing oligomers would be suggested to improve both the 180º peel adhesion and the tack.

Conclusion

This article demonstrates the many different applications in which the new zinc-containing oligomers have been evaluated. The metal-containing crosslinking oligomers can be prepared and used as co-reactants for acrylic pressure-sensitive and hot-melt adhesives. The polymerizations can be done by both bulk and solution polymerization and still result in a free flossing tacky solid or solution. The use of zinc-containing oligomers in UV-cured coatings promoted adhesion to different metal and glass substrates. And when the zinc-containing oligomers were added into a base UV-curable PSA, both the 180º peel adhesion and tack were improved.

These products represent a new raw material type for both peroxide- and radiation-curable systems, as well as crosslinking oligomers used in bulk and solution polymerization. The performance properties were improved in all applications. Additional work in both the application areas and new metal-containing molecules are active projects at Sartomer.

For more information on oligomers, contact Sartomer Co., 502 Thomas Jones Way, Exton, PA 19341; phone (610) 363-4100 or (800) SARTOMER (727-8663); fax (610) 363-4140; or visit http://www.sartomer.com .