To assess its possible fate under different environmental conditions, the release of Darocur from photocrosslinked silicone acrylates and its biodegradation were investigated. Leaching tests in water were conducted on thick coating and thin coated plastic films. Results showed no biodegradation of thick coatings and PI under anaerobic conditions. An inhibitory effect was even observed on the biodegradation of glucose used as a reference biodegradable molecule. Under aerobic conditions however, PI was totally biodegraded and used as a carbon source, unlike the silicone coating which was not biodegraded.
National and European regulations such as REACH Registration, Evaluation and Autorisation of Chemicals encourage industries to progressively replace potentially hazardous substances they might use by environmentally friendly substances. In coatings and ink industries, photo-induced crosslinking has become a mainstream technology for many applications because it provides products with better properties, sush as glossness, durability and abrasion resistance [ 1 ].
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One of the application fields for UV curing is silicone release coatings. These ones are commonly obtained by photopolymerization of silicone acrylate monomers in the presence of a radical photoinitiator such as Darocur 2-hydroxymethylphenylpropanone [ 4 , 5 , 6 , 7 ].
Consequently only part of the photoinitiator is actually consumed in the reactional process [ 7 , 8 ]. The coated products therefore retain most of the amounts of Darocur initially added [ 8 , 9 , 10 ]. Due to the small size and molecular weight Green has reported that Darocur can be readily extracted from cured coatings with various solvents in Soxhlet extraction [ 8 ]. The fate of silicone in the environment has been extensively studied in the s until the early s by Dow Corning Company. Indeed, following a very strong market development of these products, producers have widely communicated to reassure consumers about the non-toxicity of the degradation products for themselves and for the environment.
Since those years, very few studies on their end-of-life have been published [ 11 ] and to our knowledge no work has been yet published on silicone acrylates or Darocur Thus, special care has to be paid to potential emissions of photoinitiator and acrylate monomers and the fate of these substances in the environment.
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In fact, the disposal of solid silicone waste varies from one country to another, but in most cases they are landfilled or incinerated. Biotic and abiotic factors are interconnected. For example, microbial production of organic acids under anaerobic conditions can induce acidification of the medium [ 12 ]. Aerobic biodegradation of Darocur according to the safety data sheets of the molecule.
The objective of the present study was therefore to develop and apply an exploratory experimental approach to assess the potential physico-chemical and biological degradation of silicone acrylates under the effect of factors which they may be exposed to during the use of the coated products or at their end-of-life stages.
Leaching and hydrolysis of the silicone acrylate polymers was assessed by following the release of photoinitiator Darocur from thick coatings and thin coated plastic films at different pH. The biodegradability of these films and Darocur was also studied under aerobic and anaerobic conditions.
Furthermore, the effects of these phenomena on the biodegradation of a reference biodegradable molecule glucose were monitored under aerobic and anaerobic conditions in order to assess a potential inhibition of microbial activity. Chemical structure of Darocur a and silicone acrylate oil b. These fluorescent X-rays are captured by the detector and counted by a multi-channel analyzer. The X-ray analysis gives a signal proportional to the Si-atoms concentration in the coating. Calibration curves in X-ray units are used to calculate the actual silicone coat.
All experiments were performed in triplicates.
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Several types of blanks were also performed in triplicates in which flasks containing i de-ionized water alone or ii a solution of Darocur at a known concentration in de-ionized water at different pH, or iii uncoated LDPE pieces and de-ionized water, were treated in the same manner as the assays. The solutions were sampled at different times and analyzed for Darocur by UV spectrophotometry. A calibration range of Darocur was prepared in de-ionized water from 1.
The calibration curve was linear with optical density ranging from 0 to 0. Leaching solutions were diluted before analysis, if necessary, to be within this range of absorbance. Calibration was done between 0. The device used is illustrated in Fig. Experimental apparatus to assess aerobic a and anaerobic b biodegradation. Methane contents in biogas between two sampling times were estimated by linear interpolation.
Other blanks were followed by incubating uncoated LDPE pieces. All assays and blanks were perfomed in triplicates. Assays using coated LDPE films were not carried out at the same time as with thick films, and therefore two series of blanks were performed blanks with no substrates nor glucose and blanks with glucose.
The experimental methodology developed in this study aimed at evaluating the potential release of the residual amounts of Darocur from the silicone films into the environment via transfer into water.
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Leaching tests were done in that purpose. It was also considered necessary to evaluate the potential biotic and abiotic degradation of the films since these phenomena may accelerate the release of Darocur. Leaching tests were therefore done at different pH to estimated potential acidic or alkaline hydrolyses of the films, and biodegradation tests were conducted. It was observed that the photo-initiator used in the formulation i.
This result suggested a recombination of the unreacted free radicals into the original molecule Darocur during silicone acrylates photocrosslinking.it.ynuhivasig.gq
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It indicated that Darocur was not strongly trapped within or bound to the silicone matrix. Leaching was faster when the thickness decreased. According to Green, [ 8 ], migration of Darocur can be influenced by other factors, including above all the polymer matrix itself, the entanglement of free photoinitiator in the polymer, the degree of cross-linking and the nature of the medium in which it is used.
Results obtained with thin films are shown in Fig. It can be seen that around 4.
The release of Darocur was therefore found to be much lower from thin coated films Fig. This observation was explained by the fact that photocrosslinking could be more effective within a thin film than thick ones during UV exposure. Indeed, it has been demonstrated that highly cross-linked polymers reduce the extraction of the photoinitiator by entanglement [ 8 ]. The thick films provided by the manufacturer were used in the present study to optimize the analytical methodology.
Given the very different thicknesses of the two types of coatings, the degree of crosslinking in each film is very probably quite different. Release of Darocur and silicon over 15 days of leaching in de-ionized water pH 6 , acidic pH 2 and alkaline pH 12 aqueous solutions of LDPE coated films. Blanks with uncoated LDPE films confirmed that under the conditions of the leaching tests these films did not release any compound which may have interferred with Darocur analysis.
Moreover, the stability of Darocur in aqueous solutions at the different pH was also verified. This phenomenon could explain a possible significant release of Darocur after silicone layer hydrolysis but whose analysis was limited by silicon accumulation in solution or physico—chemical interactions probably adsorption between silicone byproducts and Darocur Indeed, a high rate of silicon at pH 12 was not linked with a large release of Darocur Similar results by using different silicone elastomers were obtained by Laubie, , regarding catalyst concentrations, his findings are completely the opposite in all conditions pH 2, pH 6, pH 12 with a very high Si concentration at pH 12 but a very high concentration of catalyst at pH 2 indicating that catalyst is not strongly trapped by the silicone matrix no chemical or physical bonds [ 11 ].
Highlighting critical factors such as reactivity, cure speeds, and application details, Industrial Photoinitiators: A Technical Guide is a practical, accessible, industrially oriented text that explains the theory, describes the products, and outlines the practice in simple language for the novice and expert alike. It discusses the light absorption process and the excited states that are produced by the photoinitiator prior to the production of radicals. Next, the book examines the structure of the photoinitiator and the mechanism by which a reactive species is formed, along with the deactivating processes that inhibit the various photophysical and photochemical steps that occur as radicals are produced.
Commercial photoinitiatorsProviding a list of companies and suppliers, the author explores the distinct groups of commercial photoinitiators and the variations in their structure. He describes the factors that determine which photoinitiators should be chosen for a particular application and how the different types are used. He also explains the reasons why a cured coating may display yellowing and describes the use of stabilizers to reduce the yellowing effect.
Cationic curingExploring a small but growing area of interest in the UV curing industry, the book concludes by discussing cationic curing and examines the different scope of parameters and different chemistry involved.