QUV (UVA) is used in pure polyester powder coatings

Guo Zhichao 1, Wu Xueshing 1, Chen Yubin 2, Zhang Jianfei 2
1. Tangshan Changhong Coatings Co., Ltd.
2. Northern Coatings Industry Research and Design Institute
Guide Reading
Using QUV (UVA) laboratory artificial acceleration of polyester/TGIC powder coating aging resistance testing, studied the composition of polyester resin structure and anti-aging properties of the association between the effects of fillers and additives.
Introduction
Powder coating in powder form, by high temperature curing to form a coating, with resource-saving, energy-saving, pollution-free, high labor productivity and easy to achieve automation and other characteristics, has become a rapid development of new coating products, is expanding and replacing solvent-based coating applications.
With the development of outdoor home appliances, furniture and equipment and other powder industry, the application of weather-resistant powder coatings has attracted more and more attention, and now weather-resistant powder coatings have a huge market and business opportunities, the main markets are: air conditioning, aluminum profiles, automotive industry, highway construction, outdoor furniture and lamps and other .
weather-resistant powder coatings mainly have pure polyester, polyurethane and acrylic powder coatings and other categories, of which pure polyester powder coatings have low cost, no toxic substances when cured, the coating has good decoration and weather resistance.
is particularly suitable for outdoor UV-resistant places, so by the powder industry colleagues care, is considered to be the ideal outdoor powder coating, rapid development.
the second largest production of thermostate powder coatings in China after epoxy polyester powder coatings, and its application is more prominent.
coating film in natural exposure conditions for aging, it takes a long time, can not quickly compare its performance, the use of QUV (UVA) artificial accelerated aging test, you can quickly understand the anti-aging properties of coatings, assess the intrinsic quality of powder coatings

pure polyester powder coating according to the use of curing agents are different, this paper to the domestic market has an absolute advantage of weather-resistant powder coatings, isocyanate triglycerides (TGIC) as curing agent polyester powder coatings to test.
1. Experiment
1.1 Raw materials
polyester resin, TGIC, R-215 titanium dioxide, vanadium sulfate, leveling agent, brightener, UV absorber, sabbath and antioxidants, etc.
1.2 powdering equipment
MIX-5 premixer, EX30 twin screw extruder, air-cooled track-type chip crusher, ACM-2 vertical grinder (ACM grinding), high-voltage electrostature spray equipment, etc.
1.3 Production process
using the general powder coating preparation process: premixed, high-temperature melt extrusion, crushing and screening, high-pressure electrostitive spraying, 190 degrees C/15 min baking plate.
model is chromate-treated aluminum, 70 cm × 150 cm, white coating. TGIC is selected with polyester resin as the main film-forming substance, and the reaction is shown in Figure 1.
of polyester resin specifications and prepared powder coatings are tested in Table 1.
table 1 polyester resin specifications and prepared powder coating routine performance testing
IPA-1, IPA-2 is a polyester resin based on methylphenidate and new glycol, and TPA-3, TPA-4 is a polyester resin based on benzoic acid and new glycol. IPA-1, TPA-3 formulations contain the right amount of UV absorbers and antioxidants.
1.4 Laboratory Accelerated Aging Principles and Accelerated Test Conditions
A QUV artificial accelerated aging test box that quickly and truthfully reproduces damage to materials such as sunlight, rain, and dew: it only takes days or weeks;
QUVs reproduce damage that can take months or years outdoors: including aging, gloss drop, powdering, cracking, blurring, brittleness, mechanical strength loss, and oxidation.
QUV uses a UVA-340 light source, as the UVA-340 lamp simulates the ultraviolet short-band light of the sun's light optimally. The spectral energy distribution (SPD) of UVA-340 is well aligned with the solar spectrum from the cut-off point of sunlight to approximately 360nm (see Figure 2).
QUV (UVA) test procedure: standard ASTMG154-2006 test procedure 6; cycle 8 h; UV light 4 h condensation; lamp model UVA-340; light strength 1.55 W (m2.nm)-1.340 nm; blackboard temperature 50 to 60 degrees C.
the above test conditions, the samples were exposed to the samples and the samples were taken out during the 100 h, 200 h, 300 h, 400 h and 500 h periods to test their gloss and color difference.
1.5 parameter measurement and calculation
using homemade pure polyester powder coating model, a total of 4 kinds, numbered IPA-1, IPA-2, TPA-3 and TPA-4, each set of test parallel models are 3 pieces, each sample in the same position in the homemade mold measurement, take the average of 3 samples as the test results of this type of sample.
Before the sample is exposed, the color and mirror gloss are measured separately, sampled after the specified exposure cycle, after drying with deionized water, the color and gloss determination is carried out, the gloss hold rate and color difference are calculated, and the calculation method is shown as (1) and (2).
: L, a, b, is the initial color value of the model, and L, a, b is the color value of the model after the experiment.
2. Results and
Figure 3 is a comparison of the photon preservation rate of the powder coating model QUV (UVA-340).
it can be known that the preservation rate of polyester powder coatings, mainly benzoic acid and new glycol, has not changed much from beginning to end, the preservation rate of 500 h is 81% to 89%, and the preservation rate of polyester powder coatings based on benzoic acid and new glycol has been down all the way down, 500 h at 29% to 38%.
rated by GB/T1766-1955, IPA-1, IPA-2 loss rate of 20%, very slight loss of 1 level, TPA-3, TPA-4 loss rate of more than 70%, is a serious loss of light, level 4.
tested by a chroma differential meter and calculated by formula (2), IPA-1 e is 0.68, IPA -2 is E is 0.79, TPA -3 is E is 1.37, and TPA -4 is 1.53.
the color difference between the models before and after aging is measured and calculated by GB/T 11186.2, 3, and the color difference is rated IPA-1, IPA-2, TPA-3, TPA-4 as 0.
in addition, the model coating surface after the test is free of cracking, bubbles and rust spots and other paint diseases. The IPA-1> IPA-2 >TPA-3>TPA-4 is arranged in order, and it is clear that the IPA series has a much higher preservation rate than the TPA series.
2.1 Effect of polyester resin on weather resistance
According to the data, the polyester resin mainly synthesized by methylphenidate and new glycol and the polyester resin mainly composed of benzoic acid and new glycol increased the photo protection rate of the coating film by 2 to 3 times.
The main reason is the absorption of 297 nm of UV rays on benzoic acid, the resin of niobium and ketones because of the absorption of ultraviolet light and the generation of free fundamentals, and oxygen combined into peroxide free fundamentals
;
Compared with the largest absorption band of benzoic acid is not in this band, the chemical group absorbs light energy is much smaller than the bond-off energy, there is no or rare break in the molecular chain, so the weather resistance is good
Generally speaking, the resin with higher TPA content has better technical performance, but the weather resistance is poor, while the resin with higher IPA content has better weather resistance, but the performance is poor.
in order to obtain polyester with high overall performance, the ratio of IPA to TPA can be adjusted appropriately, and other polyols and polyic acids are used to balance the weather resistance and mechanical properties of polyester.
Due to the time relationship, no natural exposure test was done, in order to illustrate the effect of IPA and TPA on coating weather resistance in polyester, D. Maetens in the polyester synthesis process, the use of IPA appropriate alternative to TPA made natural exposure test results of powder coatings to make a reference, see Figure 4.
As can be seen from the comparison, IPA polyester is indeed helpful for improved weather resistance, which confirms the above judgment;
in addition, whether using QUV (UVA lamp) to test the model or Florida outdoor exposure, the gloss of the model has degraded, the downward trend is consistent, which proves that laboratory accelerated testing can be a good predictor of the outdoor durability of pure polyester powder coating materials.
effects of
2.2 curing agents
pure polyester systems are currently used in outdoor weather-resistant powder coatings mainly polyester/TGIC systems and polyester/primid systems. The outstanding advantage of polyester/TGIC powder is that it has good weather resistance, no by-products are produced when curing reaction, good yellowing resistance when coating film baking, and good comprehensive properties such as coating film weather resistance.
is currently in the field of outdoor coating, it has become a priority development of varieties. Polyester/primid systems are alternatives developed due to the presence of TGIC irritation to the human body or skin.
the system has the advantages of safe and non-toxic, stable reaction activity, easy to charge, good storage stability, low dosage and so on, the disadvantage is easy to yellow, easy to produce pinholes, coating film flatness slightly worse.
the absolute advantage in the domestic market is TGIC weather-resistant powder coatings, this paper also uses TGIC as a curing agent polyester powder coatings for testing. TGIC is a white powder with epoxy equivalents of 104, melting point of 100.4 to 109.9 degrees C.
TGIC's 3 epoxy groups provide sufficient activity, it differs from bisphenol A epoxy resin in that it does not contain benzene ring and ether bonds, so the curing product has excellent UV resistance, good light preservation, color preservation, and a stable trioenium ring structure also ensures that the coating film has good heat and weather resistance.
2.3 Effects of UV absorbers and antioxidants
the most damaging to outdoor powders is mainly 290 to 400 nm of UV rays in sunlight, followed by harmful substances such as air oxidation and acid in rainwater.
therefore, on the basis of the selection of materials, powder coating formulations are added to the appropriate amount of ultraviolet absorbents, antioxidants and free-based catchers to slow the aging of the coating. In particular, white and other light-colored coatings, can prevent color change yellowing, in the formulation of suitable additive applications can indeed reduce the occurrence of this phenomenon.
Ultraviolet light absorbers can absorb ultraviolet radiation energy before the resin components, through the re-discharge of the electron structure in its own atoms, the radiation energy into thermal energy and dissipated in the atmosphere, once the release of thermal energy, the atoms return to their original state
;
the principle of using ultraviolet light absorbers and antioxidants is that the amount is small and the efficiency is high, the compatible with resin is good, does not have adverse reactions with resins and other additives, and the toxicity is small. In addition, some polyesters have been factory added UV absorbents and antioxidants, to understand this.
2.4 Effect of pigment fillers
The efficiency of goldstone-type titanium dioxide absorbing ultraviolet light is 5 to 7 times that of sharp titanium type, and a sufficient amount of gold-redstone-type titanium dioxide can effectively absorb ultraviolet light, thus preventing the polymer from aging due to ultraviolet radiation, is the main component of light powder coating;
it is not suitable for the use of fillers such as sharp titanium-type titanium dioxide powder with poor weather resistance and calcium carbonate, which is easy to powder, and studies have shown that powder coating powdering mainly starts with pigment particles, especially where pigments are synths, which is a photochemical process.
In terms of pigment varieties, in addition to considering heat resistance, it is also required that light resistance and weather resistance are good, light resistance should reach 7 to 8 levels (8 levels of the best), weather resistance should reach 4 to 5 levels (5 levels of the best).
3. Conclusion
1) QUV (UVA) aging test machine can predict the outdoor durability of polyester/TGIC coating film, performance and outdoor exposure have a certain correlation, the trend is consistent.
2) The structural properties of polyester resins and curing agents play a crucial role in the weather resistance of powder coatings, which is more long-lasting than benzoic acid, and pigments and additives.
There are many factors that affect the weather resistance of powder coatings, including internal factors such as the structural properties of resins, curing agents, pigments and other additives, as well as the role of sunlight (mainly ultraviolet light), the composition of the atmosphere (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt spray, etc.), temperature changes and other natural factors (external factors).
, natural exposure and artificial accelerated aging test should be combined to draw more reliable conclusions when judging the weather resistance of coating film.
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