How to inhibit oxygen blocking?
Oxygen blocking is also called oxygen inhibition. Due to the high concentration of oxygen in the surface layer, the oxygen inhibition will lead to the lower layer has been cured, the surface is still not cured and sticky and not dry. Oxygen inhibition not only prolongs the curing time, but also may affect the performance of the cured surface layer, such as hardness, abrasion resistance, scratch resistance, etc. The curing reaction of almost all conventional photoinitiator materials is affected by oxygen.
To solve the problem of oxygen-blocked aggregation is mainly from the reaction mechanism, reaction rate, curing process to change three aspects to solve.
1. change the reaction mechanism: improve the photoinitiator system to suppress the phenomenon of surface oxygen blocking aggregation, such as formulation viscosity: in the lower viscosity formulations, the rate of oxygen diffusion will be faster, so the higher viscosity formulations oxygen blocking situation will be reduced. Since an increase in temperature reduces the viscosity of the system, curing at lower temperatures will result in less oxygen-blocking.
2. Change the reaction rate: increase the initiator concentration or increase the light intensity, because oxygen blocking is due to the reaction between oxygen and free radicals, then a high concentration of photoinitiator can generate more free radicals, which prevents the consumption of oxygen and prevent the diffusion of oxygen to the coating, and finally achieve the effect of overcoming oxygen blocking. Of course, the rate of free radical generation and the type of photoinitiator also has a great relationship; you can choose a modified curing resin, and then add oxygen-consuming monomers or groups; adding one or more oxygen scavengers to the light-curing system can alleviate the oxygen-blocking effect
3. dye sensitizer: In the presence of certain pigment sensitizers (such as 1,3-diphenylisobenzofuran), products with photoinitiator function (such as 1,2-dibenzoylbenzene) can be produced, thus alleviating the role of oxygen blocking. However, this method will bring some color to the formulation, so its use is limited.
4. Change the curing process: inert gas protection method; floating wax method; laminating method; strong light irradiation method; step-by-step irradiation method
But different coping methods have their pros and cons.
|Inert gas protection
|No negative impact on product performance
|Expensive; difficult to implement
|Floating wax method
|Affects product performance; takes time to complete migration
|A good solution when the membrane is part of the product
|Material cost and membrane removal
|Increase initiator concentration
|Easy to implement
|Increased residue or by-products can reduce product performance
|Intense light irradiation method
|No negative impact on the product
|Increases equipment costs
|Improves heat resistance; reduces water absorption; improves adhesion
|Inexpensive; can improve adhesion
|Yellowing problems after curing; sensitive to moisture
|Can be used in large quantities
|Reduced temperature resistance; may reduce water resistance