Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting: Part II

doi:10.1016/S0300-9440(03)00149-8

Progress in Organic Coatings

Volume 48, Issue 1, November 2003, Pages 101-111

https://doi.org/10.1016/S0300-9440(03)00149-8 Get rights and content

Abstract

The photoinitiated radical polymerization of acrylate resins has been shown to proceed more rapidly and extensively in a carbon dioxide atmosphere than in the presence of air. Polymerization profiles were recorded by real-time infrared spectroscopy for a few micron thick coatings exposed for 1 s to UV radiation. The importance of O₂ inhibition was shown to depend on a number of factors, such as the nature and concentration of the photoinitiator, the reactivity and viscosity of the acrylate monomer, and the wavelength and intensity of the UV radiation. CO₂ inerting was required for achieving an effective surface cure of poorly reactive formulations exposed to UV light of low intensity.

Introduction

Highly cross-linked polymer networks can be produced quasi instantly at ambient temperature by photoinitiated polymerization of multifunctional monomers and oligomers [1], [2]. This technology has found its major applications in the coating industry for the surface protection of various materials (plastics, metals, wood, paper) by fast drying varnishes [3]. UV-cured coatings are usually made of acrylate-based resins, known for their high reactivity, which undergo a radical-type polymerization upon UV exposure in the presence of a photoinitiator. This chain reaction is yet strongly inhibited by atmospheric oxygen, which scavenges very efficiently both the initiating and the polymer radicals, as shown in Scheme 1. The rate of polymerization was shown to be the most sensitive parameter to quantify oxygen sensitivity in UV-curable coatings [4], [5]. For thin films in contact with air, it is necessary to perform the UV-curing under intense illumination or by using an important amount of photoinitiator in order to produce a large excess of initiator radicals which will consume the oxygen dissolved in the resin.

The most effective way to overcome oxygen inhibition is to work in an inert atmosphere, by flushing the UV oven with nitrogen [6], [7] or carbon dioxide [8]. The latter gas being heavier than air, it can be easily maintained in a container. By using a pool-type photoreactor 3D objects have been readily UV cured by this process [9]. As the curing step will be achieved faster and more completely in an O₂-free environment, the resulting UV-cured coatings will exhibit improved surface properties, in particular a higher gloss and a better scratch resistance than coatings cured in the presence of air.

In a previous kinetic study of the photopolymerization of acrylic resins [10], we have shown by real-time infrared spectroscopy that the diffusion of atmospheric oxygen into the sample is depending on the O₂ concentration, the temperature, the formulation viscosity and the film thickness; all these parameters determine the oxygen diffusivity into the coating. Inerting by carbon dioxide becomes thus mandatory when thin films made of highly fluid monomers are to be cured at above ambient temperature. Another critical factor with respect to the inhibitory effect of oxygen is the reactivity of the resin, as well as the exposure time. The lower is the formulation reactivity, the longer is the UV exposure and the stronger is the oxygen inhibitory effect. In the present work, we have quantified the beneficial effect of replacing air by carbon dioxide when the reaction time was increased as a result of a lower formulation reactivity and unfavorable irradiation conditions. The influence of the following parameters on the polymerization kinetics has been examined for acrylate-based resins: the type of photoinitiator and its concentration, the reactivity of the monomer and of the functional oligomer, the light intensity and the radiation wavelength. Polymerizations were typically performed in a 1 s timescale in order to be as close as possible to the conditions found in most industrial UV-curing applications.

Section snippets

Experimental conditions

Photopolymerization experiments have been carried out with a polyurethane acrylate (PUA) resin (Laromer^® 8987 from BASF), containing 3 wt.% Irgacure^® 2959 (from Ciba SC), which was exposed to a UV light at a typical intensity of 30 mW cm⁻² at room temperature. Scheme 2 shows the chemical formulas of the various photoinitiators used in this study, all from Ciba SC, except Lucirin TPO from BASF. The experimental procedure has been detailed in Part I and [11]. The UV-curable formulation was applied

Results and discussion

Previous studies have shown that the inhibitory effect of oxygen in the photopolymerization of acrylate-based resins depends on the type of photoinitiator selected, its concentration, the resin reactivity and the light intensity [12], [13]. We have tried to quantify the importance of these various factors on the polymerization kinetics.

Conclusion

Radical-type polymerizations are known for their high sensitivity toward molecular oxygen, which has a detrimental effect on the course of the reaction. It is particularly pronounced when the polymerization is carried out in thin films, like for UV-curable coatings. The most efficient way to overcome O₂ inhibition in such systems is obviously by performing the UV irradiation in an inert atmosphere. A number of parameters appear to play a key role in determining the reactivity of the system and

Acknowledgements

The authors K. Studer and C. Decker wish to thank BASF (Ludwigshafen) for a research grant.

References (16)

C. Decker
Prog. Polym. Sci.
(1996)
M. Awokola et al.
Prog. Org. Coat.
(2002)
C. Decker
Polym. Int.
(2002)
S. Davidson, Exploring the Science, Technology and Applications of UV and EB-Curing, SITA Technology, London,...
C. Hoyle, M. Cole, W. Kuang, S. Jonsson, C. Nason, T. Ishijima, R. Kess, K. Viswanathan, T.Y. Lee, R. Ng, C. Miller,...
M. Kunz, R. Strobel, D. Gysau, in: Proceedings of the RadTech North America Conference, 1996, p....
R. Müller, in: Proceedings of the RadTech Europe Conference, 2001, p....
T. Henke, in: Proceedings of the RadTech Europe Conference, 2001, p....

There are more references available in the full text version of this article.

Cited by (98)

Self-matting waterborne polyurethane acrylate wood coating by 222 nm far-UVC irradiation in ambient air
2024, Progress in Organic Coatings
Self-matting coatings, featuring micro-winkles for low gloss and a soft tactile feel, were obtained by employing 222 nm far-UVC radiation emitted by a Krypton-Chloride excimer lamp applied to UV-curable waterborne polyurethane acrylate wood coatings. The 222 nm far-UVC penetrates the wet film's top layer (a few micrometers) to induce CC to ·C−C· biradical, participating in the free radical polymerization process and resulting in the top layer completing gradient curing first. Subsequently, the thicker lower layer begins gradient curing due to the radical produced by the photoinitiator, and volume cure shrinkage induces internal stress, resulting in the formation of micro-wrinkles. This study systematically investigated the effects of varied photoinitiator concentrations (2.0 wt% and 3.0 wt%), film thicknesses (60 μm and 110 μm), and curing temperature (low: 28.3 °C, high: 37.4 °C) on the coating films' properties. At low curing temperature, the optimal matte film is characterized by gloss values of 3.6 GU at 60° and 5.6 GU at 85°. When maintaining consistent coating thickness and curing temperature, gloss values increased with higher photoinitiator concentration, while roughness values decreased. Conversely, with the same photoinitiator concentration and curing temperature, gloss values decreased as coating film thickness increased, while roughness values increased. At high curing temperature, the coating film exhibits the best matte effect with gloss values of 2.8 GU at 60° and 3.6 GU at 85°. Film gloss values experienced a slight reduction with increased film thickness or photoinitiator concentration. Scanning electron microscope observations indicated that coating films cured at high temperature displayed higher micro-wrinkles with folding than films cured at low temperature. This study demonstrates that self-matting coating films exhibit favorable physical and mechanical properties, and the curing process can occur directly in ambient air. The utilization of 222 nm far-UVC irradiation holds the potential for the large-scale production of matte coatings for wooden products.
Self-healing plasma electrolytic oxidation (PEO) coating developed by an assembly of corrosion inhibitive layer and sol-gel sealing on AA2024
2023, Corrosion Science
The inherent porous structure of PEO coatings is regarded as a drawback for long-term protection. In this investigation, the presence of pores and defects in the PEO coating was exploited as reservoirs for corrosion inhibitors to generate self-healing properties on AA2024. Smart coating systems were fabricated on the PEO layer in which 8-hydroxyquinoline (8-HQ) and benzotriazole (BTA) were employed as corrosion inhibitive layers followed by a sol-gel sealing. The protective performance relies on the mechanical interlocking between the sol-gel coating and the sublayer plus the sealing ability of the sol-gel in the presence of the intermediate layer.
Microparticles by microfluidic lithography
2023, Materials Today
Microparticles have been drawing extensive interests over recent years due to their unique shapes, complex structures, and the capability of incorporating various functions within a single entity. They have been demonstrated to have promising prospects in diverse fields such as biological analysis and diagnosis, tissue engineering, anticounterfeiting, mechanical engineering, and structural materials. Compared with the traditional preparation methods, microfluidic lithography has opened up a new way for the preparation of high precision, good monodispersity, and high throughput microparticles. There is a great need for a most comprehensive review that systematically summarizes the research findings emerging in recent years and also provides guidance for future development in this area. Here, the recent progress of microfluidic lithography methods and the obtained microparticle morphologies are discussed based on a comprehensive summarization of the basic elements used in microfluidic lithography (i.e., microfluidic devices, precursors, masks, and UV light). The postprocessing techniques including self-assembly and sintering are presented to potentially bridge microparticles with practical applications. Furthermore, the application prospects of microparticles are analyzed from the aspects of cell manipulation, bioassay, and anticounterfeiting. Finally, the limitations of functional microparticles are summarized and their future advances are prospected, aiming to provide help for the controllable preparation and application of functional microparticles.
Material jetting for advanced applications: A state-of-the-art review, gaps and future directions
2022, Additive Manufacturing
The proven ability of additive manufacturing (AM, also known as 3D printing) to fabricate complex components with substantial reductions in material wastage and reduced lead times has made it a key enabling technology for numerous important industrial applications. Of current AM processes, material jetting (MJ) is demonstrating considerable potential for producing multi-material, intricate, 3D components and systems with integrated functionality, with the additional benefit that the process can be easily integrated with other manufacturing procedures. However, material jetting of functional materials to produce advanced applications still poses numerous technological challenges which hinder its full industrial exploitation. These extend from the limited range of high-performance materials with consistent properties usable for MJ, through the need to enhance the process itself by optimizing the droplet formation process, tuning waveforms for specific processes, modification of substrate and substrate/jetted material interactions, the complexities of curing and post-processing procedures, and the challenge of characterizing the 3D printed parts. In this context, this article attempts to provide a comprehensive discussion of the principles and characteristics of the most recent material jetting technology and reviews the state-of-the-art research and development being conducted. This review identifies existing gaps with regards to high-performance UV-curable inks, printing behavior of non-Newtonian fluids, optimum jetting and curing strategies and effective measures for achieving high-precision MJ. Future work should bridge the aforementioned gaps in order to improve the performance of the technology, thereby making it more attractive for large-scale adoption by industry and increasing market acceptance and penetration of material jetting for advanced applications.
Preparation of multifunctional silicone acrylate prepolymers with anti-oxygen inhibition ability for application in UV-curable anti-smudge coatings
2022, European Polymer Journal
Citation Excerpt :
The physical method involves the removal of oxygen or air from the system, so that the UV curing process can then be successfully carried out. Filling a system with an inert atmosphere or laminating a substrate with a transparent film is a simple and useful way to overcome the effect of oxygen inhibition [17,18], but this redundant operation will increase the production cost. The chemical method involves the addition of some active chemicals to absorb oxygen and produce active free radicals during the UV curing process, so as to continue to initiate the photopolymerization reaction [15].
UV curing technology is a new green technology developed in the 21st century, and has been widely used in daily life and industrial production. However, a major drawback encountered with UV curing is oxygen inhibition, especially in free radical photopolymerization. As reported herein, three multifunctional silicone acrylate prepolymers were obtained via the one-step hydrosilylation of dipentaerythritol hexaacrylate (DPHA) and hydride-terminated polydimethylsiloxane (H-PDMS). These prepolymers contain five acrylate groups at each end of the polysiloxane chain, and also has a high viscosity, all of which contribute to overcome oxygen inhibition, and thus they have efficient anti-oxygen inhibition abilities. After the addition of a few photoinitiators, each of the prepolymers could be rapidly cured in air to yield a transparent film. The transparency of these cured films is over 95% on the surface of an underlying polyethylene terephthalate (PET) substrate. They remain stable at temperatures reaching up to 352.2 to 364.8 °C, thus demonstrating that they have excellent heat resistance. Water droplets exhibit water contact angles between 103° and 105° on these films, and surface energies of the cured films are from 24.58 to 20.69 mN/m. Finally, the coatings have good anti-smudge and anti-fingerprint properties, and the overall results show that these films have potential applications as UV-curable anti-smudge coatings.
One-step synthesis of novel multifunctional silicone acrylate prepolymers for use in UV-curable coatings
2022, Progress in Organic Coatings
As reported herein, three multifunctional silicone acrylate prepolymers were obtained via the one-step hydrosilylation of pentaerythritol tetraacrylate (PETTA) and hydride-terminated polydimethylsiloxane (H-PDMS). The structures of these prepolymers were subsequently characterized by FT-IR and ¹H NMR spectroscopy techniques, which confirmed the successful synthesis of the target products. These prepolymers contain three acrylate groups at each end of the polysiloxane chain, which can participate in the UV curing process, and thus they have high UV curing activities. After the addition of a little photoinitiator, each of the prepolymers could be rapidly cured to yield a transparent film. The transparency of these cured films is over 95% on the surface of an underlying polyethylene terephthalate (PET) substrate. Water droplets exhibit water contact angles between 100° and 105° on this film, these films remain stable at temperatures reaching up to 356.0 to 386.8 °C, thus demonstrating that these films are hydrophobic and have excellent heat resistance. Finally, the coatings have good anti-smudge properties, and the overall results show that these films have potential applications as UV-curable anti-smudge coatings.

View all citing articles on Scopus

View full text