Application of the hottest quvuva in pure polyeste

Preliminary study on the application of QUV (UVA) in pure polyester powder coating

preliminary study on the application of QUV (UVA) in pure polyester powder coating

December 31, 2020

Guo Zhichao 1, Wu Xueshen 1, Chen Yubin 2, Zhang Jianfei 2

1 Tangshan Changhong coating Co., Ltd.

2 Northern coating industry research and Design Institute

guide

the aging resistance of polyester/tgic powder coating was tested by QUV (UVA) laboratory. The correlation between the composition and structure of polyester resin and aging resistance was studied. At the same time, the effects of pigments, fillers, additives and other factors on aging resistance were also introduced

introduction

powder coating is coated in powder form and solidified at high temperature to form a coating. It has the characteristics of saving resources, energy, pollution-free, high labor production efficiency and easy to realize automation. It has become a new coating product with rapid development and is expanding and replacing the application field of solvent-based coating

with the development of outdoor household appliances, furniture and equipment and other powder industries, the application of weather resistant powder coatings has attracted more and more attention. Now, weather resistant powder coatings have huge markets and business opportunities. The main markets are: air conditioning, aluminum profiles, automotive industry, highway construction, outdoor furniture and lamps and other coatings

weather resistant powder coatings mainly include pure polyester, polyurethane and acrylic powder coatings, among which pure polyester 2 and relative humidity are less than 80%; Powder coating has low cost, no toxic substances are emitted when curing, and the coating has good decoration and weather resistance

it is especially suitable for outdoor places with high requirements for UV resistance, so it has received the care of colleagues in the powder industry and is considered to be the most ideal outdoor powder coating, which has developed rapidly

among the thermosetting powder coatings in China, its output is second only to epoxy polyester powder coatings, and its application is more prominent

it takes a long time for the film to age under natural exposure, and its performance cannot be compared quickly. By using QUV (UVA) artificial accelerated aging test, we can quickly understand the aging resistance of the coating and evaluate the internal quality of powder coating

adjust the proportion of chemical raw materials according to its performance, design and improve products, and provide a theoretical basis for the production of powder coatings with excellent comprehensive performance

there are many kinds of pure polyester powder coatings according to the different curing agents. This paper tests the polyester powder coatings with TGIC as the curing agent, which is the weather resistant powder coating that has the absolute advantage in the domestic market

1. Experiment

1.1 raw materials

polyester resin, TGIC, R-215 titanium dioxide, barium sulfate, leveling agent, brightener, UV absorber, benzoin and antioxidant, etc

1.2 pulverizing equipment

mix- 5 premix machine, ex30 twin-screw extruder, air-cooled crawler tablet crusher, acm-2 vertical mill (ACM mill), high-voltage electrostatic spraying equipment, etc

1.3 production process

general powder coating preparation process is adopted: premixing, high-temperature melt extrusion, crushing and screening, high-voltage electrostatic spraying, 190 ℃/15 min baking plate

the sample is chromated aluminum, 70 cm × 150cm, white coating. TGIC with polyester resin is selected as the main film-forming material, and the reaction is shown in Figure 1

see Table 1 for the specification of polyester resin and the routine performance test of the prepared powder coating

Table 1 polyester resin specification and routine performance test of prepared powder coating

ipa- 1 and ipa- 2 are polyester resins mainly made of isophthalic acid and neopentyl glycol; Tpa-3 and tpa-4 are polyester resins mainly made of terephthalic acid and neopentyl glycol. Ipa-1 and tpa-3 formulas contain an appropriate amount of UV absorbers and antioxidants

1.4 accelerated aging principle and accelerated test conditions in the laboratory

the QUV artificial accelerated aging test box can quickly and truly reproduce the damage of sunlight, rain and dew to materials: it only takes a few days or weeks

quv can reproduce the damage that can take months or years to produce outdoors: including fading, gloss decline, chalking, cracking, blurring, embrittlement, mechanical strength decline, oxidation and other film aging phenomena

quv adopts UVA-340 light source, because UVA-340 lamp tube has the best simulation effect on ultraviolet short band light of sunlight. The spectral energy distribution (SPD) of UVA-340 is in good agreement with the solar spectrum from the cut-off point of sunlight to about 360nm (see Figure 2)

quv (UVA) test procedure: Standard astmg154-2006 test procedure 6; Cycle 8 h; Condensation under ultraviolet light for 4 hours; Lamp model uva- 340; Light intensity 1.55 w (M2 · nm) - 1 · @340 nm; The blackboard temperature is 50 ~ 60 ℃

according to the above test conditions, expose the samples respectively, and take out the samples at 100 h, 200 h, 300 h, 400 h and 500 h to test their gloss and color difference

1.5 parameter measurement and calculation

four self-made pure polyester powder coating samples, numbered ipa-1, ipa-2, tpa-3 and tpa-4, are used. There are three parallel samples in each group of tests. Each sample is measured at the same position in the self-made mold, and the average value of the three samples is taken as the test result of this kind of sample

before the exposure of the sample, the color and specular gloss shall be measured respectively. After the specified exposure period, the sample shall be taken. After the deionized water is washed and dried, the color and gloss shall be measured, and the gloss retention rate and color difference shall be calculated. The calculation method is shown in formula (1) and (2)

where: l*, a*, b* are the initial color values of the template; 50. A and B are the color values after the sample test [4]

2. Results and discussion

Figure 3 is a comparison of the light retention of powder coating sample QUV (UVA-340)

it can be seen that the light retention of polyester powder coating mainly composed of isophthalic acid and neopentyl glycol has little change from beginning to end, and the light retention of 500 h is 81% to 89%; The light retention rate of polyester powder coating mainly composed of terephthalic acid and neopentyl glycol went down all the way, which was 29% - 38% after 500 hours

evaluated according to gb/t1766-1955, the light loss rate of ipa-1 and ipa-2 is 20%, and the very slight light loss is grade 1; Tpa-3 and tpa-4 have a light loss rate of more than 70%, which is a serious light loss and belongs to grade 4

ipa-1 is calculated by formula (2) after being tested by color difference meter Δ E=0.68，IPA -2 Δ E =0.79，TPA -3 Δ E =1.37，TPA -4 Δ E =1.53。

measure and calculate the color difference between samples before and after aging according to gb/t 11186.2 and 3. According to the color difference value, ipa-1, ipa-2, tpa-3 and tpa-4 have no color change, which is grade 0

in addition, there are no cracks, bubbles, rust spots and other paint diseases on the surface of the sample film after the test. The order of light retention is ipa-1> ipa-2> tpa-3> tpa-4. Obviously, the light retention of IPA series is much higher than that of tPA series

2.1 effect of polyester resin on weather resistance

according to the data, the light retention of the coating film of polyester resin mainly synthesized from isophthalic acid and neopentyl glycol is 2 ~ 3 times higher than that of polyester resin mainly synthesized from terephthalic acid and neopentyl glycol

the main reason is that terephthalic acid absorbs 297 nm short wave ultraviolet light, and carbonyl and ketone groups in the resin form free radicals due to absorption of ultraviolet light, and combine with oxygen to form peroxides

the thermal oxidation and photooxidation processes are superimposed together, causing the polymer chain to fracture, so the coating begins to age, resulting in the decline of the gloss of the coating film, the darkening of the color, and serious paint diseases such as cracks and chalking, which affect its decorative and mechanical properties

in contrast, the maximum absorption band of isophthalic acid is not in this band, the light energy absorbed by chemical groups is far less than the bond dissociation energy, and there is no or little molecular chain fracture, so it has good weather resistance

but the impact resistance and cupping test performance of the film are slightly worse. Generally speaking, resins with high TPA content have better mechanical properties, but poor weather resistance; The resin with higher IPA content has better weather resistance, but poor mechanical properties

in order to obtain polyester with excellent comprehensive properties, the proportion of IPA and TPA can be adjusted appropriately, and other polyols and polybasic acids can be used to balance the weather resistance and mechanical properties of polyester

due to the time relationship, no natural exposure test was carried out. In order to illustrate the impact of IPA and TPA on the weather resistance of the coating film in polyester, the natural exposure test results of powder coatings made by etens in the process of polyester synthesis, using IPA instead of tPA appropriately, are used as a reference, as shown in Figure 4

from the comparison, it can be seen that IPA polyester is indeed very helpful to improve the weather resistance, which also confirms the above judgment

in addition, whether using QUV (UVA lamp tube) to test the sample or outdoor exposure in Florida, the gloss of the sample is degraded, and the downward trend is consistent, which proves that accelerated laboratory testing can well predict the outdoor durability of pure polyester powder coating materials

2.2 influence of curing agent

at present, the total investment of pure polyester system in outdoor weather resistant powder coatings is more than 160billion yuan, mainly including polyester/tgic system and polyester/primid system. The outstanding advantages of polyester/tgic powder lie in its good weather resistance, no by-products during curing reaction, good yellowing resistance when baking the film, and good weather resistance of the film

at present, it has become a priority in the field of outdoor coating. Polyester/primid system is a substitute developed due to the irritation of TGIC to human body or skin

the system has the advantages of safety and non toxicity, stable reaction activity, easy electrification, good storage stability, low dosage, etc., but the disadvantages are easy to yellowing, easy to produce pinholes, and slightly poor film flatness

since TGIC weather resistant powder coating is the dominant one in the domestic market, polyester powder coating with TGIC as curing agent is also used for testing in this paper. TGIC is a white powder with epoxy equivalent value of 104 and melting point of 100.4 ~ 109.9 ℃

tgic's three epoxy functional groups provide sufficient activity, which is different from bisphenol A epoxy resin in that it does not contain benzene ring and ether bond. Therefore, the cured product has excellent UV resistance, good light and color retention, and the stable triazine ring structure also ensures that the coating has good heat and weather resistance

2.3 the role of UV absorbers and antioxidants

the harmful substances such as 290 ~ 400 nm UV in sunlight, followed by oxidation of air and acid in rainwater are more destructive to outdoor powders. For example, the well-known iPhone 5/5s

therefore, on the basis of selecting materials, an appropriate amount of UV absorbers, antioxidants and free radical trapping agents are added to the powder coating formula to slow down the aging of the coating. In particular, light colored coatings such as white can prevent discoloration and yellowing. The application of appropriate additives in the formula can indeed reduce this phenomenon

UV absorbers can absorb UV radiation energy before resin components. Through the rearrangement of the electronic structure in its own atoms, the radiation energy is converted into heat energy and dissipated in the atmosphere. Once the heat energy is released, the atoms return to their original state

antioxidants can inhibit or delay the oxidative degradation of polymers and greatly improve the service life of polymer materials [8]

the principle of using UV absorbers and antioxidants is to use less and have high efficiency, good compatibility with resin, no adverse reactions with resin and other additives, and low toxicity. In addition, some polyesters have been added with UV absorbers and antioxidants when leaving the factory. We should understand this clearly

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