so far the photo-initiated free radical polymerization system is the most widely used, but it also has its own insurmountable drawbacks that limit its application. in the process of free radical polymerization, there will be oxygen inhibition. besides, the reaction system has poor adhesion and high volume shrinkage, which may result in the reduction of dimensional accuracy of the product. the impact of stress may lead to material deformation, easy to break and poor adhesion on substrate.
research on cationic uv curing systems can be traced back to the last century, and cationic uv curing has a series of unique advantages. first of all, the biggest advantage of the cationic photopolymerization system is no oxygen inhibition. there is no need to carry out a reaction in an inert gas atmosphere, which not only saves the reaction cost, but also simplifies the reaction conditions. it also overcomes the problem that has always limited the free radical photopolymerization reaction. cationic photopolymerization is a living polymerization reaction. once the polymerization is initiated, the reaction will not stop even if the light source is removed. there is an obvious post-curing reaction, which makes the reaction more thorough. it is called "immortal polymerization" and suitable for thicker coatings. however, free radical polymerization is very dependent on the light source, because the reaction will stop immediately once it leaves the light source. the monomers or polymers reacted in the cationic uv curing system generally can not only undergo double-bond polymerization but also undergo ring-opening reaction, such as cyclic ethers, acetals, vinyl ethers, etc.. reactant types are increased and the toxicity is lower than that of acrylate monomers or polymers. after polymerization and ring-opening reaction, the volume of the cationic uv curing system will expand to a certain extent, which will compensate for the reduction of the volume in the polymerization process. it also helps greatly increase the adhesion to substrate and improve the mechanical properties and thermal stability of the uv-cured coating. commonly used monomers or oligomers in cationic uv curable systems are vinyl ethers, oxetanes and epoxies.
although the active monomer does not occupy the largest proportion in the uv curing reaction system, it is still considered as an important part. due to its relatively low viscosity, the active monomer has the function of dissolving the uv curable oligomer. the viscosity of the formulation system can be adjusted to a suitable state by adjusting the amount of the active monomer added. moreover, the active monomer has uv curing active group, which can participate in the polymerization reaction as a reactant and it will not remain in the curing system like a solvent. furthermore, it has a great contribution to the photopolymerization reaction rate and the properties of cured films. it also helps reduce the pollution of organic solvent on the environment. because of the insurmountable oxygen inhibition problem of free radical polymerization, we will focus on reactive monomers that can undergo cationic uv curing.
there are many kinds of reactive monomers. according to the number of functional groups, they can be divided into three types: mono-, bi- and multi-functional. the reactivity of reactive monomers is not only related to the number of functionalities, but also to the types of functional groups, and they are all positively related. the more functional groups they have, the higher the uv curing rate, conversion rate and cross-linking density of the surface film will be.
epoxy-based reactive monomers are a type of monomers which have been widely used now. these monomers will undergo a ring-opening reaction in the uv curing process and the volume shrinkage of the cured film is small. volume shrinkage may lead to a decrease in dimensional accuracy of the product. the stress generated by shrinkage may cause material deformation, cracking and poor adhesion on substrate. however, the volume expansion caused by uv curing and ring opening of epoxy-based reactive monomers just makes up for the volume shrinkage produced by polymerization, so it has excellent thermal stability and mechanical properties in addition to small volume shrinkage. the reaction rate of epoxy-based reactive monomers is lower than that of acrylate-based monomers with the same functional group. however, the reaction rate of cycloaliphatic epoxies is relatively fast and the polymer product has good weather resistance and electrical insulation, so it is widely used in cationic uv curing system.
the ring tension (107kj/mol) of oxetane-based reactive monomers is similar to that of cycloaliphatic epoxies. it is only slightly lower than the ring tension of the epoxy group of cycloaliphatic epoxies. however, the basicity of cycloaliphatic epoxies is stronger than that of the oxetane, so oxetane is more prone to cationic ring-opening polymerization and its reaction rate is higher. the reactivity of mono-functional oxetane is not lower than or even higher than that of bi-functional epoxy. furthermore, the viscosity, toxicity, volatility and volume shrinkage of oxetane are relatively low, which attributes to excellent mechanical properties and thermal stability. applications include uv coatings, inks, electrical encapsulation, dental resins, 3d printing and so on.
syna-epoxy 101 is a cationic, uv-curable, reactive and water-soluble epoxy diluent.
nantong synasia new material co., ltd. can provide a variety of oxetane-based reactive monomers such as s-101, s-221 and other grades. these products have been widely used in many industries. welcome all customers to inquire!