Comparison of UV free radical and cationic curing
Ultraviolet (UV) curing coating is a new environmentally friendly coating first developed by Germany in the late 1960s, which has the advantages of high efficiency, energy saving, non-pollution, fast film formation and excellent coating performance, and thus has been rapidly developed. In 1994, China consumed 3,100-3,300 tons of various types of UV-curable coatings, and in 1998, the consumption amounted to 6,200-6,400 tons, with an average annual growth rate of more than 25%. UV curing can be divided into Free radical curing and cationic curing two. Domestic free-radical curing is now commonly used, cationic curing has also been reviewed in a number of literature. However, the specific use of cationic curing technology, domestic has not been reported; UV free radical and cationic curing in the use and performance of the differences between the two, but also have not seen the relevant literature.
1) UV radical curing and cationic curing mechanism of comparison
Under UV irradiation, the decomposition of different photoinitiators produce different results, some produce free radicals, some produce cations, free radicals or cations can trigger the corresponding zwitterionic and reactive diluents with reactive activity, polymerization reactions occur, the formation of three-dimensional network structure of the polymer.
In UV-initiated free radical polymerization, there is more chance of free radical chain deactivation or termination, and there is less possibility of continuing polymerization and curing when the light stops. Meanwhile, oxygen is also easy to react with free radicals to generate more stable peroxy radicals, so oxygen plays a role in blocking the polymerization. In the cationic polymerization process (there are also a small number of free radicals generated, but mainly cation-initiated curing), because the cations can not be coupled between the two will not react with oxygen. Even if the chain transfer reaction occurs, a new cationic active center will be generated, so that the cationic curing reaction continues
2) Free radical formulations and cationic formulations of curing speed comparison test
Whether on paper or aluminum, the free radical formulation cures faster than the cationic formulation. This is because.
cationic initiator by UV irradiation, resulting in super acid active center, due to the presence of alkaline impurities in the system, the active center is neutralized by alkali first, resulting in the speed of cationic polymerization mantle.
As a cationic curing initiator mostly aryl iodide (sulfur) ionium salt, UV irradiation, the cationic active center generated by the larger volume, attacking the epoxy group on the carbon atom, to bimolecular nucleophilic substitution (Sw2), the site resistance effect is larger, and free radical polymerization does not exist this site resistance effect, so the cationic poly table speed is slower than the free radical.
3) Comparison of the effect of oxygen on the curing speed of both
Oxygen significantly affects the speed of free radical curing, while the effect on the cation is very weak. The blocking effect of oxygen on free radical polymerization can be seen by the mechanism formula, because O2 is very easy to react with radical R- to produce peroxy radical ROO-, which is difficult to initiate free radical polymerization. The radical R- and O: reaction rate constant is 104 to 105 times larger than that of R- and monomer molecules. Therefore, if O2 is present in the coating, then R- will first react with O2 and be consumed, greatly slowing down the reaction rate. In addition, O2 has two more unpaired electrons with opposite spin directions and is a stable triplet state. However, under UV irradiation, it will become very active and can combine with the excited state of the photoinitiator, and then decompose into the ground state of the photoinitiator and single-linear state of O2. its reaction rate constant k, up to 109 orders of magnitude Kaw, thus reducing the efficiency of the photoinitiator. During the cationic curing process, O2 does not react with the strong acid active center produced by the initiator. Therefore, even if a trace amount of O2 is present in the coating, it will have a large blocking effect on free radical curing, while it has little effect on the cationic system.
4) Comparison of the effect of temperature on curing speed
The control of temperature is also an important factor. In order to examine the effect of temperature on the curing speed of both, the above two formulations were cured at different temperatures, and the curing speed of both free radical and cationic formulations tended to increase with the increase in temperature. This is because the photoinitiator has the smallest initiation rate in the photoinitiated polymerization process and is the slow step in controlling the reaction. Increased temperature is conducive to the initiator to obtain the activation energy required for decomposition and rapid generation of free radicals or cations, and the temperature is conducive to the opening of the n-bond or ring in the double bond of the polymerization system, triggering the polymerization reaction, so that the curing speed of the coating is accelerated. However, the initiator is easy to thermal decomposition, so the curing temperature is generally controlled below 80℃.
5) Comparison of the overall performance of the coating
Cationic curing system than free radical curing system adhesion is excellent, especially the cationic system in aluminum has reached 100% adhesion. The reason for this difference, because from the free radical curing mechanism and cationic curing mechanism can be seen in the free radical polymerization, monomer or zwitterionic distance from the van der Waals force distance before curing to the covalent bond distance after curing, and curing speed, so the volume shrinkage is obvious, resulting in high internal stress and poor adhesion. Although the same volume shrinkage caused by the distance between the van der Waals force action to the covalent bond after curing exists in the polymerization of epoxy compounds, on the other hand, when the epoxy monomer is polymerized, the ring in the monomer opens to form a chain structure unit larger than the monomer molecular structure, offsetting part of the volume shrinkage. As a result, the adhesion between the cationic cured film and the substrate is significantly enhanced compared to that of free radicals. Comparing the solvent resistance of free radical and cationic cured coatings, the difference is significant, and the solvent resistance of cationic cured coatings is greatly improved with time. The free radical reaction mechanism shows that in the free radical polymerization process, the solvent resistance does not change much with the extension of time because the free radical curing speed is fast and the coating can be dried inside and outside in a short period of time. Cationic polymerization is different, when the UV light source removed, the cationic active center in the system will not be two combined and disappear, even if there is a chain transfer reaction (see cationic curing mechanism formula), will also be in the chain termination at the same time, there will be a new cationic active center. Therefore, after UV irradiation, the first in a relatively short period of time to form a curing film on the surface of the coating, to achieve “surface dry”, after the coating leaves the UV light source, the inner coating film still exists in large quantities of cations, continue to open the ring reaction with epoxy compounds, from the surface and inside, the formation of a polymeric cross-linked whole, to dry. Therefore, with the extension of time, the solvent resistance of the cationic cured coating film is greatly improved.
UV free-radical curing and cationic curing curing speed with the increase in temperature, and the free-radical curing speed is greater than the cationic curing speed.
free radical curing speed, volume shrinkage, poor adhesion, cationic curing volume shrinkage is small, excellent adhesion.
Oxygen has a significant coalescence blocking effect on free radical curing. Cationic curing without oxygen blocking effect, but there is a “dark reaction”, with the extension of time, its solvent resistance greatly improved;.
Comparison between the two, free radical curing is suitable for adhesion requirements are not very high, but requires rapid curing of inks and coatings, cationic curing technology is suitable for high adhesion requirements of inks and coatings.
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