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Physical and chemical methods for inhibiting oxygen inhibition
Dec 17, 2018

Surface oxygen inhibition during ultraviolet (UV) curing is a problem that has been plaguing people:

When photocured in air, oxygen inhibition often causes the undercoat layer to solidify and the surface to be uncured and tacky.

Oxygen inhibition can lead to a large number of oxidizing structures such as hydroxyl, carbonyl and peroxy groups in the surface of the coating, which may affect the long-term stability of the coating, and may even affect the hardness, gloss and scratch resistance of the cured film. performance.

why?

The ground state of a general substance is a singlet state, and the stable state of O2 is a triplet state, and there are two unpaired electrons having the same spin direction. Therefore, it competes with the polymerization of free radicals to consume free radicals.

Since most photocuring processes are carried out in an air environment, and the main applications are materials with extremely large surface/volume ratios such as coatings and inks, O2 has a resistance to free radical polymerization of photocurable materials. Gathering.

Especially when the film thickness is thin, the concentration of oxygen in the oily organic system is usually less than or equal to 2×10-3 mol/L, which not only inhibits the polymerization of dissolved oxygen molecules in the formulation system, but also in the curing process during the photoinitiation process. The consumption of oxygen molecules, the oxygen in the air on the surface of the coating can also quickly diffuse into the cured coating, continuing to hinder the polymerization. The original dissolved oxygen concentration in the system is very low and is relatively easy to consume. For a closed system, the process by which the primary living radicals consume dissolved oxygen is substantially equivalent to the polymerization induction period. Relatively speaking, oxygen that diffuses from the outside to the inside of the coating is the main reason for hindering polymerization. Oxygen inhibition is also most likely to occur in the superficial layers of the coating or in the entire thinner coating, as oxygen molecules in the environment diffuse more easily in these areas.

Oxygen destroys the surface mode - quenching, scavenging and oxidation. The specific mechanism is as follows:

Annihilation

The triplet O2 in the ground state can be reacted as a quencher with a photoactivated initiator (indicated by Phi) to form a complex, thereby quenching the photoinitiator that excites the triplet state. The process is expressed 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 from the excited state to the ground state, thereby hindering the generation of active radicals. Most of the pyrolysis photoinitiators have a short lifetime of excited triplet. Before the excited initiator reacts with O2, the initiator has been decomposed, so the probability of bimetallic quenching of O2 and photoinitiator is relatively low. Can often be ignored.

Clear

The ground state O2 is essentially a double radical, so it has a strong addition activity to the active radicals generated during photoinitiation [k>109/(mol·s)], forming a relatively stable peroxidation radical. This process has a faster rate and can compete with the addition reaction of living radicals to the monomer, and has the most significant hindrance to the polymerization process. It can be divided into the following 2 steps:

The living radical initiates polymerization of the monomer.

R·+CH2═CXY→R—CH2—C·XY+ monomer→polymer

Active free radicals are added to O2.

R·+O2→R─O─O·(peroxy radical)

R-CH2-C·XY+O2→R-CH2-CXY-O-O·

Oxidation

 

Oxygen molecules can also oxidize free radicals that have been polymerized with monomers to peroxides, preventing polymerization of the monomers.

 

Obviously, in all three cases, the rate of polymerization will decrease and the formation of peroxide will affect the performance of the cured coating. It should be noted that the reaction rate constant of radical R· and O 2 is 104 to 105 times larger than the reaction rate constant of the monomer molecule, so even if only a small amount of oxygen is present in the coating, the reaction between R· and O 2 cannot be ignored. When the peroxide radical ROO· is generated. Since ROO· is very stable and has no ability to initiate polymerization, the presence of O2 consumes the active radical R·, causing the reaction polymerization rate to decrease and exhibiting an induction period. Therefore, O2 is a polymerization inhibitor for free radical polymerization of a photocuring system at normal temperature.

 

Existing oxygen inhibition polymerization method

Physical methods: inert gas protection method, floating wax, film, strong light irradiation, distributed irradiation

Chemical method: Add substances that provide active hydrogen - thiol, amine, ether acrylate (acrylate can be integrated with the coating to prevent surface cracking, but also reduce odor); provide hydrogen atom capacity under the same conditions: thiol Class>Amines>Ethers

Taking an amine as an example, the reaction mechanism is as follows: there are 6 active hydrogens on the amine, which can consume 6 oxygen.

 

After a series of experiments on these methods, we came to the following conclusions:

 

Regardless of the high or low curing equipment, as long as the thiol, ammonia or ether modified acrylate can improve the surface reactivity;

The surface reactivity increases as the concentration of the modified acrylate increases.

Sulfhydryl groups can work synergistically with polyether acrylates or highly reactive structures;

Changing the formulation or thickness of the coating can also provide surface reactivity. Shorten the distance that low energy is applied to the substrate to prevent surface cure from being destroyed.