This article talks about adhesives, looks at various polymer chemistries and curing options available, and provides information.
One emerging market in PSAs is UV-curable formulations that can be applied at room temperature. These compounds are liquid at room temperature and usually consist of acrylated monomers and oligomers along with a photoinitiator (PI) that activates when exposed to UV light to polymerize the material. The intensity, wavelength, and duration of UV exposure determines the shear and peel strength of the adhesive. This creates the opportunity to custom-design an adhesive for a given application but also means process control is critical to achieving the desired result. The printing method used to apply UV-curable PSAs can also affect peel. Flexographic printing is often recommended, since screen printing produces a rougher coating with lower peel strength.
One advantage of UV-curable PSAs is the ability to process in-line while printing the label, since the equipment is the same as that used for UV-curable inks. Since the products are 100 % solids, there is no need for the drying ovens that are required for water- or solvent-based acrylics. Also, the humidity controls necessary for proper curing rates of water-based products are not an issue. Cure speeds vary with thickness. A thin coating, 25 µm or less cures relatively quickly, but thicker coatings introduce delay times that impede the efficiency of the overall label manufacturing process.
UV-curable products can be environmentally friendly alternative to solvent-based adhesives in certain applications. Some customers prefer them in cases where the adhesive adheres well to the substrate, the substrate is sufficiently heat resistant to withstand UV-curing, and the product will be used in a benign environment that does not experience high heat or humidity. One caveat is that exposure to sunlight will activate the PI, causing further crosslinking of the polymer and hardening the material. Some formulations, however, can still retain adhesion after exposure to light, although their pressure-sensitive characteristics may be compromised. Substrates with internal UV-curing inhibitors can also extend the use of UV-curable PSAs to applications such as transparent labels exposed to outdoor environments.
Many in the adhesives industry are not convinced that room temperature UV-curable PSAs are the wave of the future, although they may be a solution for manufacturers who regularly use UV-curable inks and want to streamline their production processes. There is concern that limited temperature resistance and relatively low bond strengths make it difficult for today’s UV-curable adhesives to compete with solvent-based acrylics or UV-curable hot melts. Costs are also higher than for competing products, and shelf life is shorter. Some suppliers have removed UV-curable adhesives from the market because of lack of demand. It appears that improvements in both adhesive chemistry and curing equipment may be needed in order to achieve more widespread adoption.
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