A recent study in the journal Polymers identifies agents for regulating the shrinkage effect of UV curing to enable its use in precision equipment.
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The process of UV-curing coating is primarily a surface treatment process for protection and to increase the service life. UV curing could be described as a drying process with the occurrence of polymerization. However, UV curable coatings are not preferred for precision instruments due to the shrinkage of material resins.
This article focuses on the latest study on a novel technique that significantly reduces shrinkage in UV coatings. The advantage of UV curing is the shorter time compared to conventional techniques. The strong forces created by the polarization also increase the strength and lead to an enormous improvement in the mechanical and chemical properties. Technology has really revolutionized the future of UV curing.
Material composition and properties
Ultraviolet curing technology developed rapidly in the 1990s due to its various advantages over the classical methods.
The basic components of UV curing consist of acrylated urethane resins, acrylated oils, unsaturated polyesters, oligomers, diluents, photopolymerization initiators and other additives for process optimization such as pigments and stabilizers, etc. Acrylic polyesters are produced with variable viscosity values and have a considerably low molecular weight.
Epoxies are useful because their properties offer a good rate of cure. They are characterized by a relatively higher level of skin irritation and the high molecular weight of epoxy resins. These properties enable them to have excellent corrosion resistance properties. Urethanes are preferred because of their well-known good properties such as toughness, stability, temperature and chemical resistance, along with a faster cure rate.
Industrial treatments are necessary to lower the viscosity of diluents for effective operation. This is done by adding more monomers. The selection is based on essential properties such as financial constraints, volatility, solvation efficiency and odor.
Photoinitiators are light-sensitive materials and enable absorbed light to be used to polymerize the binder. The photoinitiator is selected critically taking into account the wavelength range of the light and the radiation properties. Therefore, all materials are carefully selected after careful consideration with regard to their properties and compatibility.
Industrial uses and restrictions
The UV curing technology is used by various industries such as the printing industry with screen and letterpress printing. The paper and ink industry uses it to increase the lifespan of their products. The plastics processing industry uses UV curing for special purposes such as plastic packaging.
The UV edge coating improves production and packaging effectiveness, resulting in higher profits. The distinctive process properties of UV curing enable it to be used for almost all substrates such as composite materials, magnetic tapes, leather, vinyl and even human teeth.
Even with such advantageous properties, UV curing has limitations in industrial use. It involves the use of acrylated polyesters and their use has one major disadvantage. Their low molecular weight leads to slow reaction rates and an increase in surface inhibition. Some other limitations are the lower penetration of UV curing and the high price tag along with the large volume of UV curing equipment.
However, newer devices such as the Photo-DSC 204 F1 Phoenix from NETZSCH are of a suitable size and enable efficient UV curing. It is also operational from -100 ° C to 200 ° C and allows the user to select the appropriate light intensity, temperature and exposure time. The biggest limitation, however, is the shrinkage that occurs in precision equipment from UV curing. The latest research by Mr. Y. Tao and his team has shown a successfully implemented UV curing model against shrinkage.
The latest study involves the integration of hollow polyurethane acrylate (PUA) microspheres with the mixture of tripropylene glycol diacrylate (TPGDA) coatings. The PUA is mixed with different fractions of the volume mixture with different diameters and thicknesses. In addition, an empirical formula is developed that relates various geometric parameters and shrinkage ratios.
The essential reagents include 2-hydroxy-2-methylpropiophenone (Photoinitiator 1173), 1,6-hexanediol diacrylate (HDDA), polyvinyl alcohol (PVA) and PUA resin. Microfluidic technology was used to create PUA microspheres.
To set the shrinkage ratio, an amalgam was produced from a TPGDA coating and a PUA mixture. Photoinitiator 1173 and TPGDA solution were generated in weight ratios of 3% and 97%, respectively. The hardening process leads to a polymerization which leads to the solidification of the resin phases. Thin films made of PUA material are subjected to mechanical experiments; Nanoindentation and in-situ tensile tests are carried out for experimental results and validation.
The anti-shrinkage model is developed by mixing TPGDA with PUA balls, whereby uniaxial tensile tests and finite element analysis (FEA) are initially used for the mechanical properties, followed by the development of empirical formulas.
Nanoindentation tests were performed to accurately measure the Young’s modulus of PUA films. The FEA analysis was carried out in Abaqus via a precise RVE model development. A system without microspheres is used as a reference control group for validation. The test results are first compared with the standard system and then with the developed empirical formula.
The study showed that the results are comparable and led to the successful development of the anti-shrink model for UV curing.
Impact and Limitations
The modern study developed a model that prevents resins from shrinking after UV curing.
This would improve the mechanical properties of substances after the UV curing process that convert intermolecular forces into intramolecular forces.
Reducing shrinkage leads to a decrease in stress cracking, tension and warpage of the coating, increasing the efficiency of UV curing and increasing the service life and mechanical properties of products from various industries such as the paper industry, plastics industry, pipe repair, composite industry and vinyl industry .
At the same time, a significant reduction in shrinkage would enable UV curing to be used in the precision instrument industry, leading to a revolution in the manufacturing industry.
However, while useful in all respects, this process has only been performed using acrylated urethane microspheres. The calculations for other polymers would be completely different, as would the resulting properties.
In short, the problems of UV shrinkage in precision instruments were the only reason for their limited use. However, this study along with several others focused on the purpose of reducing shrinkage was very useful and productive.
Implementation of this research in various industries, particularly precision instruments, would lead to increased applications of UV curing around the world.
AZO. (2021, October). Equipment details. From AZOM: https://www.azom.com/equipment-details.aspx?EquipID=407
Marx, P. & Wiesbrock, F. (2021). Expanding monomers as anti-shrinkage additives. Polymers, 13(5), 806. Retrieved October 24, 2021. https://www.mdpi.com/2073-4360/13/5/806
Tao, Y., Sun, G., Wei, Y., Liu, R. & Zhao, J. (2021). An anti-shrinkage model of an ultraviolet curing coating filled with hollow polyurethane-acrylate microspheres. Mechanics of Materials, 163. doi: https://doi.org/10.1016/j.mechmat.2021.104091
Xu, J., Jiang, Y., Zhang, T., Dai, Y., Yang, D., Qiu, F.,. . . Yang, P. (2018). Synthesis of a UV-curing, aqueous polyurethane-acrylate coating and its photopolymerization kinetics using FT-IR and Photo-DSC methods. Advances in Organic Coatings, 10-18. https://www.researchgate.net/publication/327371815_Synthesis_of_UV-curing_waterborne_polyurethan-acrylate_coating_and_its_photopolymerization_kinetics_using_FT-IR_and_photo-DSC_methods