For years, zinc diacrylate (ZDA) has been used in numerous peroxide-cured rubber applications. These metal acrylate oligomers have not found much use in radiation-curing applications, such as coatings and inks, due to their lack of solubility in most monomers and oligomers.
Recently, Sartomer chemists developed a series of organic metal acrylate oligomers that are soluble in most monomers, leading to higher formulation capabilities. These new oligomers were studied in various applications. Due to their hydrophobic nature, the metal-containing oligomers can be used at high levels in standard radiation-curable formulations. They have also 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. Because of these physical properties, formulators will now be able to create coatings with excellent adhesion that are clear and flexible, and have anti-fungal properties.
Metal Acrylate Compositions
Sartomer patents1,2 describe 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. Sartomer has commercialized several soluble metal-containing oligomers, but the focus of this paper 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, Sartomer's chemists have studied the new metal-containing oligomers and their benefits in different application areas. Bailey's work discusses the benefits of reversible crosslinking in solventborne and hot-melt pressure-sensitive adhesives (PSAs).3,4 Ceska reported the use of the novel oligomers in radiation-curable coatings on several substrates.5,6 Also over the past several years, the metal-containing hybrid oligomers have been evaluated in radiation-curable PSAs. The study discussed in this paper focuses on radiation-curable coatings.
Ceska reported that polymers crosslinked with ZDA possess ionic crosslinks, which could result in enhanced properties when compared to more conventional covalent crosslinks.5 Elastomers crosslinked with ZDA possess low tan d values comparable to sulfur-crosslinked elastomers. But elastomers covalently crosslinked with standard multifunctional acrylates have a significantly higher tan d. The similarity of the sulfur and metallic crosslinks is relative to the mobile nature of these bonds compared to crosslinks formed by typical multivalent acrylic monomers.
UV-Curable Coating Results
Ceska reported the adhesion results for a UV-curable coating tested with 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 0 to 16 parts. The base formulation is shown in Table 1.These formulations were cured using two 300-watts-per-inch mercury vapor lamps with four passes of 750 mJ/cm2 energy for a 5.0- to 6.0-mil-thick coating. Crosshatch adhesion was then run using the 3M #610 tape adhesion test both before and after heating at 200 °C for 10 minutes. As expected, the adhesion significantly improved with baking the coating. The results of the adhesion study on aluminum are shown in Figure 4. The percent of coating remaining after the tape adhesion test is shown in Figure 4.
Using the same samples containing the CN2405, the same test was also run on tin-plated steel. The results are shown in Figure 5.
The same coating sample containing the CN2405 was also tested on cold-rolled steel. These results are shown in Figure 6.
And finally, the same coating sample containing the CN2405 was also tested on glass. These results are shown in Figure 7.
Though not shown in this work, in all cases water resistance of the cured films was excellent. Full adhesion was maintained after 24 hours in room-temperature tap water.
Conclusions
As shown in the test results, the use of the zinc-containing oligomers in UV-cured coatings promoted adhesion to different types of metal and glass substrates. In addition, post-baking these coatings yielded a much stronger bond to all the substrates tested.These zinc-containing oligomers represent a new raw material type for both peroxide- and radiation-curable systems, as well as crosslinking oligomers used in bulk and solution polymerization. When formulated with monofunctional monomers, the hybrid metal-containing oligomers can provide reversible crosslinking for UV hot melt adhesives, powder coatings and hot melt coating applications. Because of this, these oligomers, shown to promote adhesion to metal and glass substrates, may have many uses in various applications for coatings, adhesives, inks and electronics.
For more information, contact Sartomer Co., 502 Thomas Jones Way, Exton, PA 19341; phone 610/363.4100; fax 610/363.1498; or visit www.sartomer.com.
References
1 "Polyurethane Salts", M. Fan, et al, US Patent No. 6,380,278, April 30, 2002.2 "Oil Soluble Metal-Containing Compounds, Compositions, and Methods", G.W. Ceska, et al, US Patent No. 6,399,672, June 4, 2002.
3 "Novel Metal-Containing Crosslinkers for High Solids and Hot-Melt Acrylic Pressure Sensitive Adhesives - The Benefits of Reversible Crosslinking", M. Bailey, et al, TAPPI PLACE Conference, Orlando, FL, Aug. 4-7, 2003.
4 "Metal-Containing Oligomers Deliver the Benefits of Reversible Crosslinking to High Solids and Hot Melt Pressure Sensitive Adhesives", Sartomer's New Product Alert, Volume 4, Number 12, July 2004.
5 "Hybrid Oligomers - Organic/Inorganic Acrylics", G.W. Ceska, et al, RadTech e5 Conference, Charlotte, NC, May 3-5, 2004.
6 "Unique Metal-Containing Oligomers Impart Excellent Adhesion and Unlock New Potential Opportunities", Sartomer's New Product Alert, Volume 4, Number 9, June 2004.
