Study of catalysts for the synthesis of polyester resins for powder coatings

Abstract: The effects of non-butyltin catalysts and monobutyltin catalysts and their dosages on the catalytic esterification reaction activity of polyester resins for powder coatings, the relative molecular masses of polyester resins and their distributions were investigated.

The effects of non-butyltin catalysts and their dosages on the heat resistance and aging properties of powder coatings prepared from synthetic polyester resins were focused on, and then the suitable non-butyltin catalyst species and their dosages were determined.

At present, manufacturers of polyester resins for powder coatings around the world mainly choose butyltin compounds as catalysts for polyester resin esterification reactions. After the catalysts are involved in the catalytic reactions, they will remain permanently in the molecular structure of polyester resins and gradually precipitate out to the surface with time migration, which has mutagenic effects on the environment and organisms.

With the increasing pressure of environmental protection in China, the choice of catalysts with no or low pollution to the environment in the reaction of synthetic resins has become an urgent problem.

At present, the main choice of titanium catalysts and non-butyltin catalysts, including titanium catalysts are easy to hydrolysis, poor stability, used to synthesize polyester resin reaction time is long, yellow color problems, there are still many technical problems to be solved, can not be used on a large scale.

The non-butyltin catalysts mainly change the structure and type of hydrocarbon group, which can reduce the effect of environmental hazards.

 

On the other hand, China’s larger foreign trade market, the EU region, introduced in 2010 (EU) 276/2010 (REACH) regulations, the use of organic tin compounds put forward new restrictions on the requirements, the use of no more than 0.1%.

 

In order to adapt to this new change, the domestic powder polyester resin industry has now been studying the production of catalyst powder coating resins without butyltin.

Organotin compounds are metal organic compounds formed by the direct combination of tin and carbon 2 elements, the general formula is RnSnX4-n, in the general formula, n = 3 organotin compounds toxic, n = 1 and n = 2 organotin compounds of the second most toxic, n = 4 organotin compounds of very low toxicity or non-toxic.

The nature of the R group in the general formula has a great influence on the toxicity of the compound, and when R is propyl or butyl, it is biologically active and has a large effect. The catalyst C-01 studied in this paper is highly toxic, followed by C-02, C-03 and C-04.

Purified terephthalic acid, isophthalic acid, neopentyl glycol, solid butyltin catalyst C-01, non-butyltin catalysts C-02, C-03, C-04, TGIC, titanium dioxide, barium sulfate, leveling agent, benzoin, are all industrial products.

The test was conducted with 8L small stainless steel reactor; small powder making equipment such as φ30 twin-screw extruder; small electrostatic spraying equipment; coating performance testing equipment such as spectrophotometer.

Add polyol, polyacid and catalyst into 8L reaction kettle according to the recipe amount and stir well. Under the protection of N2, gradually increase the temperature to 180~250℃ and react for 12~16h, and get the product with acid value, viscosity and other requirements after vacuum condensation.

According to the basic formula in Table 1 to prepare powder coatings, the process flow is: ingredients → premixing → extrusion → pressing → crushing → sieving → product, the resulting powder coating with electrostatic spraying, and curing in a certain curing conditions to obtain the coating, the coating and coating performance testing.

 

GPC test conditions: column filling material is styrene-divinylbenzenegels; mobile phase is THF; column length is 300mm; column diameter is 8mm; flow rate reaches 1.0mL/min; column pressure is 0.5~0.6MPa; detection with IR detector at 30~34℃; injection volume is 20μL.

 

The measurement was carried out using a UniversalV1.717TAInstrumentsDSC2910 thermal analyzer with a ramp-up rate of 20℃/min, a test temperature range of -80~200℃, and an ambient atmosphere of N2.

 

HH-S digital display thermostatic water bath is used, the distilled water inside the pot is in boiling state, the sample plate is suspended in the water, ensure that 4/5 of the plate is in the water, do not touch the inner wall of the water bath;

At the same time, there is a certain gap between each test iron plate to prevent the iron plate from contacting each other to affect the test. After boiling for 2h, remove the sample plate and measure the color difference ΔL1, Δa1, Δb1 and gloss G1.

 

Adopt precision heating oven, set the temperature to 230℃, and put the test specimen into it when the temperature reaches. After that, when the display temperature reached 230℃, the time was started, and after 60min baking, the color difference ΔL1, Δa1, Δb1 and gloss G1 of the sample were measured.

Do QUV-B fluorescent ultraviolet lamp exposure test on the test specimen, according to GB/T14522 “artificial climate accelerated test method for plastic, coating and rubber materials for mechanical industrial products” (UVB313 lamp, irradiance 0.68W (m2-nm), light 60℃/4h, condensation 50℃/4h for). Measure the gloss according to GB/T9754, and assess the degree of discoloration according to GB/T1766.

2.1 Catalyst and esterification reaction time

It can be seen that in the esterification reaction stage, with the increase of catalyst use ratio, the reaction time of the whole process becomes shorter, and the activation energy of esterification and dehydration reaction between hydroxyl group of polyol and carboxyl group of polyacid gradually decreases with the increase of catalyst dosage, the easier the reaction is carried out.

 

Through cross-sectional comparison, among the four catalysts, the catalytic reaction effect of C-03 is good, and its catalytic reaction time at different ratios is shorter among these four catalysts, and the reaction time to reach the same reaction degree at 0.10% for C-01 and C-03 and 0.15% for C-04 is basically comparable, so it can be considered that their activation energies are basically the same.

The corresponding catalytic efficiencies are also basically comparable, but since C-01 contains butyltin, the use of non-butyl catalyst C-03 can be given priority in the actual synthesis.

In summary, in the synthesis of polyester resin for powder coatings, the amount of catalyst is suitable at 0.10%, and a high catalytic efficiency is obtained with less addition.

The catalytic reaction time of non-butyltin catalyst C-03 is basically the same as that of butyltin catalyst C-01 when the addition ratio is about 0.10%, and the esterification reaction proceeds smoothly and the process is controllable. As C-03 does not contain butyltin, it is less polluting to the environment and can be a substitute for C-01.

2.2 Relative molecular masses of synthetic polyester resins and their distribution

The relative molecular masses and their distributions of synthetic polyester resins are shown in Table 2. It can be seen that the number average relative molecular weight Mn of the synthesized polyester resin is between 3100 and 4000, the heavy average relative molecular mass Mw is between 7000 and 9000, and the relative molecular mass distribution D is between 2.10 and 2.30.

The relative molecular mass distribution tends to be balanced with the increase of catalyst dosage, and basically stabilized at about 2.12.

From the distribution curves of relative molecular masses, it can be seen that the distribution of relative molecular masses basically conforms to normal distribution, which indicates that the synthesized polyester resins have uniform distribution of relative molecular masses and less side reactions during the polycondensation reaction. Table 2 GPC analysis of some catalyst synthesized resin samples observing Mn and Mw when the amount of non-butyltin type catalyst C-03 is 0.15%, comparing with butyltin type catalyst C-01, it can be found that the difference between Mn prepared by C-03 and non-butyltin catalyst C-01 is not much, Mw is slightly smaller than C-01, but the relative molecular mass distribution is slightly narrower, and the synthesis reaction results are more satisfactory and reach the expected The results of the synthesis reaction were more satisfactory and reached the expected target, as detailed in Table 2.

2.3 DSC characterization of the synthesized polyesters

Tg is the transformation temperature of the amorphous polymer from the glassy state to the highly elastic state (or vice versa), which is an important indicator of the heat resistance of the polymer in use, and thus affects the stability of the powder coating storage, Tg of polyester resin is one of the important parameters of the resin.

This test was conducted to determine the Tg of some synthetic polyester resins according to the method shown in 1.5.2, and the results are listed in Table 3. It can be seen that the Tg variation regions of several ratios of catalysts are close to each other, and the Tg is all around 68°C.

The general industry requirement for polyester resins for powder coatings is Tg>50°C, while Tg>65°C is preferred. Therefore, the Tg of polyester resins of the synthetic samples in this test can all meet the requirements of use.

And the Tg of the resins synthesized by different ratios of C-01 and C-032 catalysts are not too different and meet the use requirements.

2.4 Effect of catalyst on boiling resistance

The standard test specimens were determined according to the method listed in 1.5.3. The changes of gloss and color difference after the test are shown in Figures 3 and 4.

The color difference ΔE of the samples after the test, except for the deviation of C-04 at the dosage of 0.15%, there is a very slight discoloration, the color difference ΔE of other samples ≤ 0.9, the color difference before and after boiling is not much, the performance of synthetic resin catalyzed by non-butyl catalysts and butyl catalysts before and after boiling are not much different, the impact is small.

From the test data, it can be seen that when Δb deviates more, the color difference ΔE then becomes larger, thus it can be concluded that the change of color difference ΔE in the boiling test is mainly caused by the yellow edge change Δb.

2.5 Effect of catalyst dosage on heat resistance performance

After continuous baking at 230℃ for 60min, the surface of the coating film of some test samples started to decompose, which caused the gloss to become higher instead.

From the test results, for both the use of butyltin type and