Oxygen polymerization inhibition effect of the hot

2022-07-30
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Oxygen polymerization inhibition effect of UV curable coatings

Abstract: the oxygen polymerization inhibition mechanism of UV curable coatings was analyzed from the aspect of oxygen photochemical energy state, and the methods to inhibit the oxygen polymerization of UV curable coatings were discussed in detail, including developing the reaction system of cationic polymerization mechanism, increasing the initiator concentration or radiation dose, using prepolymers and monomers with low oxygen sensitivity, and adding oxygen scavengers to the UV curable system, And the UV curing processes such as inert gas protection, paraffin protection, film coating, strong light irradiation and step-by-step irradiation were used

key words: coating; UV curing; Oxygen polymerization inhibition

Chinese Library Classification No.: tq630.1 document mark code: a Article No.: 1004 – 227x (2010) 01 – 0064 – 05

1 preface

traditional solvent based coatings contain a large amount of volatile organic compounds, causing serious pollution to the environment. With the enhancement of people's awareness of environmental protection, traditional solvent based coatings can no longer meet the increasingly stringent requirements of environmental protection regulations. Therefore, the development of environment-friendly coatings has become the main topic of the current coating industry. In this context, UV curable coatings came into being. UV curable coating means that under UV irradiation, the coating layer absorbs light energy and excites and decomposes the photoinitiator in the coating to generate free radicals; The maximum stretching ratio of light in the active free radical initiated coating is 3.8:1 curing resin, which breaks the double bond in the active diluent molecule and causes chemical reactions such as cross-linking and polymerization to form a growth chain and promote the curing of the liquid coating into a film. UV curable coatings are mainly composed of four parts: photoactive prepolymers, photoactive monomers, photoinitiators and additives

compared with traditional solvent based coatings, UV curable coatings have the following characteristics []: (1) it is assumed that the curing speed of the machinery in standby mode is fast, and the curing can be completed in 0.1~10.0 s, with high production efficiency; (2) Energy saving, the energy consumption is about 1/5~1/10 of the traditional heat curing coating; (3) Basically no solvent is discharged, which is safe and does not pollute the environment; (4) It can be coated with substrates with high thermal sensitivity and large thermal capacity; (5) The film has excellent performance, high gloss, high hardness and chemical corrosion resistance. Therefore, UV curable coating is an energy-saving and environment-friendly green coating

2 mechanism of oxygen polymerization inhibition

the ground state of general substances is singlet, while the stable state of O2 is triplet, with two unpaired electrons with the same spin direction. Therefore, it will compete with free radical polymerization and consume free radicals. Since most of the UV curing processes are carried out in the air environment, and the main applications are materials with great surface/volume ratio, such as coatings and inks, O2 has a significant polymerization inhibition effect on the free radical polymerization of UV curing materials [5]

when UV curing in air, oxygen polymerization inhibition often leads to the curing of the bottom layer and the sticking of the surface without curing. Oxygen polymerization inhibition will eventually lead to a large number of hydroxyl, carbonyl, peroxy and other oxidative structures on the surface of the coating, which will affect the long-term stability of the coating, and may even affect the hardness, gloss and scratch resistance of the cured film. The polymerization inhibition of O2 on UV curable coatings is mainly reflected in three aspects: quenching, scavenging and oxidation

2.1 quenching

the triplet O2 in the ground state can be used as a quencher to react with the photoactivated initiator (expressed in PHI) to form a complex, thus quenching the excited triplet photoinitiator [6]. The process is shown as follows:

phi → (PHI) * → (PHI) *, (PHI) * + (O2) → phi+ (O2)

in the above process, O2 is excited to the active singlet state, and the photoinitiator returns to the ground state from the excited state, thus hindering the generation of active free radicals. The excited triplet life of most cracking photoinitiators is relatively short, and the initiator has been decomposed before the excited state initiator reacts with O2, Therefore, the probability of bimolecular quenching between O2 and photoinitiator is relatively low and can often be ignored

2.2 O2 scavenging the

ground state is essentially a double radical, so it has strong addition activity [k 109/(mol · s)] to the active radical produced in the photoinitiation process, forming a relatively stable peroxide radical. The rate of this process is fast, and it can compete with the addition reaction of active free radicals to monomers, which has the most significant blocking effect on the polymerization process. It can be divided into the following two steps:

(1) monomer polymerization initiated by active free radicals. R · +ch2 gascxy → r-ch2-c · xy+ monomer → polymer

(2) addition of active radical and O2. R · +o2 → r-o-o · (peroxy radical) r-ch2-c · xy+o2 → r-ch2-cxy-o-o ·

2.3 oxidation

oxygen molecules can also oxidize the free radicals polymerized with monomers into peroxides to prevent the polymerization of monomers. 2. At the same time of the starter table, the electronic stopwatch starts counting for 1 minute. When the stopwatch reaches the time, press the machine stop button. According to the time of the stopwatch, record the cross arm travel value, that is, the rate per minute (mm/min), Inspect the difference between the cross arm stroke value and the straight steel ruler, and calculate the cross arm stroke error value, which shall not exceed 1%. The action mechanism [7] is as follows:

obviously, in these three cases, the polymerization rate will decrease, and the formation of peroxide will affect the performance of the cured coating. It should be noted that the reaction rate constant between R · and O2 is 104-105 times greater than that between R · and O2. Therefore, even if there is only a small amount of oxygen in the coating, the peroxide free radical roo generated during the reaction between R · and O2 cannot be ignored. Because roo · is very stable and has no ability to initiate polymerization, the presence of O2 consumes the active free radical R ·, reducing the polymerization rate and showing an induction period. Therefore, O2 is a polymerization inhibitor for free radical polymerization of UV curing system at room temperature

model selection sometimes refers to the difficult problems of customers. 3. Methods of anti oxygen polymerization

oxygen polymerization inhibition is very harmful to the UV curing process, especially when the film thickness is thin [8]. The concentration of oxygen in oily organic systems is usually less than or equal to 2 × 10 ? 3 mol/l, not only the dissolved oxygen molecules in the formulation system hinder polymerization, but also with the consumption of oxygen molecules in the curing system, the oxygen in the air on the surface of the coating can quickly diffuse into the curing coating to continue to hinder polymerization. The concentration of dissolved oxygen in the system is very low, which is easy to be consumed. For the closed system, the process of primary active radical consuming dissolved oxygen is basically equivalent to the polymerization induction period. Relatively speaking, the oxygen continuously diffused from the outside to the inside of the coating is the main reason to hinder the polymerization. Oxygen polymerization inhibition is also most likely to occur in the shallow layer of the coating or in the whole thin coating, because in these areas, the diffusion of oxygen molecules in the environment is easier

3.1 change the reaction mechanism

UV curing can be divided into two reaction mechanisms: free radical polymerization and cationic polymerization [9]. O2 is a double radical structure, which is insensitive to cationic polymerization and only inhibits free radical polymerization. Therefore, the oxygen inhibition of UV curing reaction can be eliminated by developing a reaction system with cationic polymerization mechanism. Since the middle and late 1970s, many onium salt cationic photosensitive initiators have been reported abroad, including diaryl iodonium salt system, sulfonium salt system, phosphorium salt system and azo salt. In the late 1980s, non acrylate prepolymers cured by cationic mechanism and cationic initiators represented by onium salts appeared

everett et al. [10] synthesized a variety of onium salts, deeply studied the initiation behavior of onium salts, and compared their photoinitiation efficiency. The results show that the effective onium salts include sulfonium salt, iodonium salt, azionium salt and ferrocene salt, and the nucleophilic anion has 4 BF 6 PF ? 、 6 SbF ? 、 6 AsF ? And so on, and its activity is 4 BF

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