Cationic UV coatings and inks are used for decorating difficult substrates, such as plastics and metals, because of their excellent adhesion, which has been attributed to the low shrinkage of the epoxide ring-opening polymerization (see Figure 1).
Cationic UV adhesives typically contain epoxides, polyols and cationic photoinitiator. The cationic photoinitiator releases an acid when exposed to UV light. The acid catalyzes epoxide polymerization, and hydroxyls react with polymerizing epoxide chains (see Figure 1).1 The acid remains active for some time after it is released by the photoinitiator, allowing cure to continue and properties to improve with time. This phenomenon of continuing cure is known as the “dark cure” effect.
Films such as clear polyester, white films, metallized films and aluminum foil limit or prevent UV-light transmission. Figure 3 shows the UV transmission of some clear films for comparison. Almost all the UV light passed through the clear polyethylene (PE) and oriented polypropylene (OPP) films. The PE and OPP may have contained very small amounts of or no chromophores that absorbed UV light.
On the other hand, the clear polyester (PET) prevented transmission of about half of the UV light (see Figure 3). The PET (poly[ethylene terephthalate]) polymeric backbone contains aromatic terephthalate chromophores that absorb UV light. Thicker PET would absorb more UV light because the light would have to pass through more chromophores. White films, metallized films and aluminum foil would prevent transmission of all of the UV light.
In order to deliver sufficient UV light to the adhesive when using UV to laminate opaque substrates, the UV cure must occur before laminating, as shown in the press schematic in Figure 2a. Figure 2b is a press schematic for UV curing after laminating, which can be used for films that are transparent to UV light.
A properly formulated cationic adhesive intended to be UV-cured before nipping must be tacky just before nipping, and the properties must build after nipping. Certain ingredients can be used in cationic adhesives to achieve this balance. The relative amounts of epoxides and polyols and their chemical compositions affect both cure rate and adhesive properties.2 Cycloaliphatic epoxides polymerize faster than aliphatic epoxides and glycidyl ethers in the presence of acid catalyst.
Cycloaliphatic epoxides are the preferred materials for cationic UV applications because of their fast cure, low viscosity and low odor. However, cycloaliphatic epoxides cure too fast to be used in adhesives intended to be UV-cured before nipping unless cure retardants are included in the formulations. Polyurethane and polyether polyols and some nitrogen-containing polymers retard cationic cure because they contain nucleophilic groups that decrease the activity of the acid catalyst. Such materials can be used in cationic adhesives that are to be UV-cured before nipping.3,4,5
Since cationic coatings containing epoxidized polybutadiene typically have good adhesion to plastics, the polyurethane diol in A was replaced on a weight basis with epoxidized polybutadiene to give Adhesive B in Table I. Adhesive B cured too fast to be UV-cured before laminating in the lab because the epoxidized polybutadiene did not retard cure. Therefore, B was laminated before UV curing in the lab according to the press schematic depicted in Figure 2b. The T-peel strength of B was excellent using OPP as shown in Figure 5.
Figure 7 shows that D, which had an R value of 2, had greater T-peel strength than C and E, which had R values of 1 and 4, respectively, in PE laminates. Adhesives C, D and E were also low-viscosity (see Table II), which was beneficial for applying them at room temperature. The adhesives were odorless and contained almost no VOCs. The data in Figure 7 further demonstrates the cationic dark-cure effect on T-peel strength.
The flexible cycloaliphatic epoxide resin, UVR 6128, in D was replaced with a harder, less flexible cycloaliphatic epoxide, UVR 6110 (see Figure 6), at the same R value (not the same weight percent) while using the same polyol and designated F in Table II. Adhesive F, which contained UVR 6110, had poorer T-peel strength after 24 hours than D, which contained UVR 6128, as shown in Figure 7, when the laminates contained PE and were UV-cured before laminating.
The stiffer UVR 6110 probably decreased the ability of the adhesive to dissipate energy during the T-peel test resulting in earlier failure. Formulation optimization could possibly improve the T-peel strength using UVR 6110 and the polyether triol.
The effects of other ingredients on T-peel strength were also determined using PE film. Adhesive G, containing UVR 6128 and a vinyl resin (see Table II), had good T-peel strength as shown in Figure 7. The vinyl resin also increased tack after UV-curing and before laminating.
Formulations containing two glycidyl ether epoxides, a conventional liquid bisphenol A diglycidyl ether (H) and dipropylene glycol diglycidyl ether (I), both of which retarded cure, gave poor T-peel strength when used with UVR 6128 (see Figure 7) and PE film. Perhaps the formulations cured too slowly because of the high glycidyl ether epoxide resin concentrations. Adhesives H and I had very little tack after UV cure. Further optimization may uncover formulations containing the glycidyl ether epoxide resins that would improve tack and T-peel strength.
In some cases, the T-peel strength decreased with time (see Figure 7). Decreasing T-peel strengths could indicate that the adhesives became too brittle as they dark cured. This points out the importance of both optimizing the formulations and measuring the properties as a function of time when designing cationic adhesives. (Previously, an experimental design was demonstrated to be a useful timesaving formulating tool for maximizing cationic adhesive T-peel strength.2 Variable T-peel strengths in Figure 7 were probably the result of experimental error because the standard deviations of the T-peel measurements were typically large relative to the results.)
The adhesives were applied to 6-in x 10-in pieces of film in the direction of film orientation (machine direction) using a 2.5 wire-wound bar. The adhesives were UV-cured by passing the samples once through the conveyorized UV-cure unit using the conditions described.
In an attempt to mimic UV-curing the adhesives before nipping on a press, adhesives were applied to films and the samples were passed through the UV unit. Immediately after exiting the UV unit, 6-in x 10-in pieces of film were placed on the adhesives, and the laminate samples were pressed together using a rubber hand-brayer. To simulate UV curing after nipping the films on a press, the laminates were constructed, pressed together with the brayer and then passed through the UV unit.
Samples 1 in x 9 in were cut from the laminates at different times after UV curing, and T-peel strengths were measured. A computerized Instron Model 1122 instrument was used to measure T-peel adhesive strengths following ASTM method D1876-95.6
The samples were pulled at a rate of 10 in/min. Four or five samples were tested for each set of conditions, and average peel strengths were reported.
For additional information on CYRACURE Resins and Photoinitiators, contact Union Carbide Corp., 39 Old Ridgebury Road, Danbury, CT 06817, or call 800-568-4000.
For more information on the conveyorized UV-cure unit, contact Fusion UV Systems, Inc., 910 Clopper Rd. Gaithersburg, MD 20878; 301-527-2660; fax 301-527-2661.
Information on the IL 390B Light Bug radiometer is available from International Light, Inc., 17 Graf Rd., Newburyport, MA 01950; 978-465-5923; fax 978-462-0759.
To learn more about the computerized Instron Model 1122 instrument used to measure T-peel adhesive strengths, contact Instron Corp., 100 Royall St., Canton, MA 02021; phone 781-575-5000.