1. Definition of UV monomer polymerization blocker
A substance that can completely terminate the free radical polymerization reaction of alkene monomers.
2. Role of polymerization inhibitors
In the unsaturated compound system, the polymerization inhibitor can preferentially interact with the free radicals in the system to form non-radicals, or form radicals with low activity, which are not sufficient for re-initiation, and can effectively block the chain polymerization of free radicals.
It can effectively block the chain polymerization of free radicals. It has great benefits for the stability, storage and transportation of resin.
Blocking mechanism: According to the role of inhibiting the polymerization reaction, the substances that can terminate each radical and stop the polymerization reaction until they are completely exhausted are called blockers or inhibitors;
And can only make the free radical activity weakened, slowing down the polymerization reaction, but can not terminate the reaction of substances called blockers.
3.UV monomer according to the mechanism of polymerization inhibitors classification
3.1. Phenolic polymerization inhibitors
a. Hydroquinone
More commonly used, low price, better effect at room temperature.
But sometimes it will cause the system color darkening, often not used.
b. Para-hydroxyanisole
Can give the resin good storage stability. The solubility of this product in organic solvents Z good, the product color Z light.
c.2,6-Di-tert-butyl-p-methylphenol
Widely used polymerization inhibitor, strong polymerization inhibiting ability, good heat resistance and stability, low price.
Toxicity is greater.
d.2,5-di-tert-butylhydroquinone
Can slowly react with free radicals over a long period of time to destroy the free radicals generated in resin storage.
It can improve the storage stability of the resin while having little effect on the gelation time.
e.2-tert-butylhydroquinone
It is an effective storage stabilizer for unsaturated polyester resins as well as a stabilizer for highly reactive resins.
Its function is comprehensive and can play a good role in a wide range of temperature. And only slightly prolong the curing of the resin at elevated temperatures. This product is often used in combination with other polymerization inhibitors.
Features:Widely used and effective. Must be dissolved in the system, and the presence of oxygen to show the effect of polymerization inhibition.
3.2. Quinone polymerization inhibitors
a. P-benzoquinone
UV monomer can still function in the absence of oxygen, suitable for etherification process protected by nitrogen or other inert gas.
Yellow color, has influence on resin color.
b.Methylhydroquinone (THQ)
Good effect, used in the production of high activity unsaturated polyester resin, commonly used in gel coat resin, SMC resin. This product has good solubility and good high temperature polymerization blocking effect.
c.Other quinone polymerization inhibitors
Features:Can play a role in preventing polymerization under anaerobic conditions.
Polycondensation blocking effect varies for different monomers
Example 1: p-benzoquinone is an effective polymerization inhibitor for styrene and vinyl acetate, but only acts as a retarder for methyl acrylate and methyl methacrylate.
Example 2: Tetrachlorobenzoquinone is an effective polymerization inhibitor for vinyl acetate, but has no polymerization inhibiting effect on acrylonitrile.
The mechanism of polymerization blocking:The mechanism of polymerization blocking of quinone is not completely clear, it may be that quinone and free radicals undergo addition or disproportionation reaction to generate quinone-type or semi-quinone-type free radicals, and then combine with active free radicals to get inactive products, which play a role in blocking polymerization.
3.3. Aromatic nitro compounds polymerization inhibitors
Commonly used are
Features:Aromatic nitro compounds are not as effective as phenols in blocking polymerization.
Only used for vinyl acetate, isoprene, butadiene, styrene, there is no blocking effect on acrylates and methacrylates.
Mechanism of polymerization inhibition:Nitrobenzene acts as a polymerization inhibitor by generating stable nitroxide radicals with free radicals.
3.4. Inorganic compounds polymerization inhibitors
Commonly used are ferric chloride, cuprous chloride, copper sulfate, titanium trichloride, sodium sulfate, ammonium thiocyanate.
Features: high efficiency of polymerization, and can be used as an aqueous phase polymerization inhibitor
Mechanism of coalescence blocking: coalescence blocking by charge transfer
3.5.Oxygen polymerization blocking effect
Molecular oxygen has two unpaired electrons, which can play a dual role of polymerization inhibition and initiation.
Blocking mechanism:R-+O2 →ROO –
Oxygen and macromolecular chain radicals generated by the peroxide radicals are more inactive, at room temperature or slightly higher temperature can not trigger the co-polymerization reaction, this oxygen blocking effect makes unsaturated polyester resin and air contact surface curing incomplete and sticky.
But at high temperature oxygen and free radicals generated peroxide radicals can be decomposed into reactive radicals, thus triggering the polymerization reaction.
4, other classifications of polymerization inhibitors
4.1. Classified by temperature
4.2. Classified by principle
4.3. Classified by composition
5、Selective method of polymerization inhibitor
The main requirement for choosing a polymerization inhibitor is to have a high polymerization blocking efficiency, it should also consider its solubility in the system, and the adaptability of the resin.
Some of the monomers used in non-light-curing system also requires the monomer in the resist can be easily removed by distillation or chemical methods, or both at room temperature to play a role in the resist, but also in the reaction temperature when the rapid decomposition.
5.1. Good miscibility with monomers and resins, only miscible can play a role in blocking.
5.2. Can effectively prevent the occurrence of polymerization reaction, so that monomer, resin, emulsion or adhesive has sufficient storage period.
5.3. The polymerization inhibitor in the monomer is easy to remove or does not affect the polymerization activity. z good choice of room temperature is an effective inhibitor, and at a suitably high temperature to lose the inhibitor, so that you do not have to remove the inhibitor before use.
For example, tert-butyl catechol, p-phenol monobutyl ether is this type of polymerization inhibitor.
5.4.UV monomer does not affect the appearance of the Z end product.For example, the polymerization inhibitor in the preparation of adhesives in the process of oxidation due to high temperature discoloration and affect the appearance of the product.
5.5. Several polymerization inhibitors used in conjunction with each other can significantly improve the effect of polymerization.
Example 1: unsaturated polyester resin by adding hydroquinone, tert-butyl catechol and copper naphthenate three kinds of inhibitors.Z strong activity of hydroquinone, in miscible with styrene and polyester can withstand high temperatures of about 130 ℃, within 1min does not have a copolymerization effect, can be safely mixed dilution.
Tert-butyl catechol at high temperatures, the blocking effect is very poor, but at a slightly lower temperature (such as 60 ℃ when), the blocking effect is 25 times higher than hydroquinone, can have a longer storage period, copper naphthenate at room temperature to play a blocking effect, and high temperature and promote the role.
Example 2: In the presence of oxygen. The blocking effect of p-tert-butylcatechol mixed with phenothiazine, hydroquinone and diphenylamine is about 300 times higher than either one alone.
5.6.UV monomer polymerization inhibitor dosage is appropriate, more is harmful.
For example, iodine dosage of 10-4 mol / L, is an effective polymerization inhibitor, but more than this amount will trigger the polymerization reaction. Iodine is generally not used alone, need to add a small amount of potassium iodide to increase solubility and improve the efficiency of polymerization.
5.7.UV monomer is non-toxic, harmless, no environmental pollution
5.8. Stable performance, inexpensive and easy to obtain
6. Think
What is the effect of initiator, reducing agent and polymerization inhibitor content on resin curing and performance?
Example: UV monomer experimental materials
Experimental method: UV monomer respectively change the content of initiator, amine and reductant to test and compare the double bond conversion, mechanical properties and curing rate of the resin.
The table of different contents of initiator, amine and polymerization inhibitor added to the resin is as follows.
Experimental results.
Conclusion.
Within a certain range, the degree of conversion and mechanical properties of UV monomer resins were positively correlated with the BPO and DEPT contents and negatively correlated with the resist content.
Increasing the content of BPO and DEPT can increase the curing rate of the resin, and increasing the content of the resist will decrease the curing rate of the resin.