Research on Weatherability of Powder Coating Film

China Coatings Online News Information: The use of
coatings in the outdoor area is becoming more and more common. So as a protective and decorative powder coating coating, its weather resistance and durability has also received more and more attention, especially such as ceiling, curtain wall panels, water dispensers, air conditioners, washing machines, aluminum profiles and other indoor and outdoor supplies coating, in the natural environment will occur a series of changes, the specific performance of the gloss drop, color dim, coating surface rust or cracks, began to slightly powder ...
The coating of powder coating is gradually reduced over time, the phenomenon of gradually decreasing performance is called aging, in the natural environment, powder coating is affected by the surrounding natural factors such as sun, rain, oxidation, hot and cold changes and microorganisms, and the resulting aging phenomenon is called natural aging. The ability of powder coatings to inhibit or delay natural aging is called natural aging resistance, also known as weather resistance.
There are many factors that affect the weather resistance of powder coatings, including the structure and properties of resins, curing agents, pigments, and other additives, and internal factors such as coating processes, as well as the role of sunlight (mainly ultraviolet light), the composition of the atmosphere (oxygen, ozone, industrial fumes, etc.), humidity (including acid rain, salt spray, etc.), temperature changes and other natural factors (external factors). Ultraviolet radiation is the main cause of natural aging of powder coatings, and oxygen in the atmosphere is an important factor to promote natural aging. Under the effect of ultraviolet rays and oxygen, it causes the powder coating to produce an automatic oxidation reaction, i.e. an oxidation chain reaction, which degrades the powder coating. Water and heat accelerate this reaction and promote photooxidation. The contaminated atmosphere contains thick ring aromatic compounds (i.e. compounds containing multiple benzene rings in molecules) and trace impurities in powder raw materials, which are sensitive agents for photooxygenation of powder coatings. The combination of these reactions and the degree of influence on the aging of powder coatings vary with the environment, but also with the chemical composition and physical structure of powder coatings, resulting in very different weather resistance of powder coatings.1, the type and effect of light stabilizers 1.1 definition of light stabilizers
light stabilizers are a substance to improve the stability of molecular materials, it can prevent polymer materials from aging of light and oxygen, greatly improving their service life.
light stabilizers can inhibit or reduce photodegradation and improve the photoresistability of polymers and composites, which are often referred to as ultraviolet absorbers because most photostabilizers absorb ultraviolet light. Light stabilizers are very effective in preventing photoaging of composite materials and prolonging their service life, and their dosage is very small, generally only 0.01% to 0.5% of polymer quality.1.2 light stabilizer types and effects
light stabilizers can be broadly divided into four categories, namely UV absorbents, annihilation agents, free-based spreaders and light shields.
1.2.1 UV Absorbers
Before UV rays endanger powder coatings, they can strongly and selectively prioritize the absorption of high-energy UV rays that are extremely harmful to powder coatings and convert them into harmless low-energy radiation to prevent UV light from degrading powder coatings.
It can absorb ultraviolet radiation energy before the resin component, through the rearflow of the electron structure in its own atoms, the radiation energy into thermal energy and dissipate in the atmosphere, once the release of thermal energy atoms and return to its original state;
1.2.2 annihilator (energy transfer agent)
it has the function of making the released thermal energy do not damage the powder coating. It does not directly absorb the incoming ultraviolet light, but only when the powder coating is stored in a certain amount of energy, only to absorb the excess energy that will cause the powder coating degradation, this part of the energy in the form of heat transferred to the surrounding atmosphere.
1.2.3 free-form capture agent
such photostabilizers capture active free-agents produced in polymers, thereby inhibiting the photooxidation process for photostabilizing purposes. When the powder coating absorbs ultraviolet energy and decomposes the active freelances that cause automatic oxidation reactions, free-form capture agents capture these free fundamentals and chemically convert them into stable polymer compounds, thus terminating the photooxidation reaction.
1.2.4 light shield
reduces the transmission of ultraviolet light to the material, is the most economical and practical addition material.2, the way to improve powder weather resistance and optical protection
because the curing temperature of powder coating is about 200 degrees C, in the natural environment, the general temperature will not cause the powder coating to produce thermal aging. Sunlight on powder coatings and other polymer materials can cause varying degrees of damage, resulting in film degradation, making the appearance and performance of the product worse. The most harmful to powder coatings is 290nm to 400nm of ultraviolet light, sunlight on powder coating degradation is the combined effect of light and oxygen two substances, commonly referred to as photooxidation degradation, or photooxygenation aging. After the powder coating absorbs light from the sun, whether the coating is stable, broken or oxidized depends on the energy of the wavelength absorbed by the material chosen by the powder coating. If the absorbed light energy is greater than the key ionization energy, the chain break between the molecules of the powder coating will occur and the performance will be deteriorated, i.e. powdering, brittle cracking, color change and so on.
aging of powder coatings is a complex process. To inhibit the process, extend service life and improve its weather resistance, it can be solved from the following three aspects.2.1 Choose a good weather-resistant resin and curing agent
mainly choose a higher level of resin, such as: outdoor polyester resin, acrylic resin, polyurethane resin, fluorine resin and the corresponding curing agent.
2.1.1 Pure Polyester System
Pure Polyester System Currently used in outdoor weather-resistant powder coatings are mainly two kinds, one is polyester/tgic system, one is polyester/primid system.
advantage of polyester/tgic powders is their weather resistance. At present, in the field of outdoor coating, it has become a priority for people to develop products. It has many advantages, such as good weather resistance, good heat resistance and anti-yellowing. The most deadly disadvantages of polyester/tgic powder coatings are the high toxicity, environmental damage and genotoxicity of the curing agent tgic, followed by the appearance of less than polyurethane powder coatings.
polyester/primid system is a substitute 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.
2.1.2 acrylic system
acrylic powder coating main advantages: due to acrylic resin color light, can be made transparent and bright light color, and film bright, plump, high hardness, not easy to scratch, good pollution resistance, excellent color preservation, has a very beautiful appearance. Its main disadvantage is its high price, the impact performance of the coating film is poor, and other powder coating system solubility is poor. But its weather resistance level is almost the same as that of the polyester/tgic system.
2.1.3 Polyurethane System
Polyurethane is a polymer containing urethane bonds, polyurethane powder coating is composed of hydroxy polyester, closed polycyanide, pigments and additives, such as thermooenstic powder coating. Polyurethane powder coating has excellent quality coating film, excellent performance, has been in building materials external components and air conditioners and other home appliances have been widely used, in China, polyurethane powder coating performance is being recognized, the proportion will gradually increase.
its outdoor tanning and physical mechanical properties can be comparable to pure polyester/tgic powder coatings. At present, domestic powder coating manufacturers rarely choose this type of powder coating, in addition to the high price of curing agents, this type of powder coating also has the following shortcomings:
in the process of curing into the film, to release the closed agent, resulting in air pollution in the baking environment, smell bad;
2.1.4 Fluorine Resin System
Fluorine resin powder coating as an exterior decorative coating for high-rise and super-high-rise buildings, with excellent light and chemical resistance, not only anti-pollution, but also self-cleaning capacity, can be kept clean for several years as a new coating. The storage stability of the coating made of thermoset fluorine resin powder coating is good, the physical and physical properties of the coating film are good, and the flexibility, hardness, adhesion and other properties of the coating film can be balanced by reasonable matching of the various parts of the system.
the disadvantage is that the pigment dispersion in the production process is poor, the coating surface gloss is low, the need for high temperature baking, so the application is limited.
, the selection of high-grade resins and curing agent methods to produce weather-resistant powder coatings, there are certain limitations. For powder coatings, the addition of light stabilizers and antioxidants and more reasonable selection of materials, formulation design is the best solution.2.2 added light stabilizers and antioxidants
many light stabilizers have both photostability and antioxidant effects. Different types of photostabilizers have different action conditions and vary according to the formulation composition of powder coatings, and the behavior and characteristics of photodegradation vary. Therefore, the selection should be careful, and to pass the test verification. Through the analysis of the natural process and condition of powder coating, the following light stabilization measures can be taken.
2.2.1 Add UV absorbers
Note that the same UV absorber in different concentrations, film thickness when its ability to absorb UV rays is also different, the thicker the film UV absorber protection effect is better. Commonly used UV absorbers are xybenzene compounds, benzodiazole compounds.
added annihilation agent 2.2.2
mainly some two-priced organic nickel compounds. Due to the good performance of organic nickel light stabilizer, the new varieties in the 1980s were more open and lively, and in recent years, organic nickel complexes have been replaced by other non-toxic or low-toxic annihilation agents due to the toxicity of heavy metal ions.
free-agent catcher at 2.2.3
these light stabilizers are mainly blocked hals.
2.2.4 to the
, which is a type of pigment with a large refractive index and strong absorption capacity. They reflect or absorb UV rays in powder coatings, preventing light from entering the inside of the powder coating, thus reducing UV hazards and acting as a light shield.
2.2.5 antioxidant's acting agent and application characteristics
in fact, antioxidant is a substance that inhibits or delays the oxidation and degradation of polymers. There are many varieties of antioxidants, the efficacy is not the same, if classified according to its structure can be divided into two categories, a class called chain termination antioxidants, such antioxidants and polymers produced in the growth of the free-based reaction interruption chain is also known as the main antioxidant. The main antioxidant has blocked phenol typical brand number 1010/1076, Zhongfangamine class, another type is called preventive antioxidant, this kind of antioxidant can inhibit or slow the production of free agents in the trigger process, also known as auxiliary antioxidants. Preventive antioxidants are phosphates, representing 168, desulfurized amino acid metal salts and so on.
2.2.6 antioxidants
(1) color change. The color change of antioxidants is an important factor for us to consider. In general, phenolic antioxidants are non-polluting and can be used in colorless or light-colored systems, while aramids have strong chromic and polluting properties.
(2) compatible. The ideal antioxidant is good compatible in powder coating polymers, depending on the structure, type and temperature of the antioxidant.
structure consisting of (3). Different powder coating composition structures have different antioxidant properties, which must be taken into account when selecting antioxidants.
(4) heat. The effects of heat are extremely important, with the rate of oxidation approximately doubles for every 10 degrees C increase in temperature. At 100 degrees C, the oxidation rate will be 256 times that of room temperature (20 degrees C). Therefore, oxidation at high temperatures is a very important factor. Antioxidants with good high temperature resistance must be selected. For example, hydroquinine has special use value at high temperatures.
(5) metal ions. The presence of trace elements such as copper, iron and manganese, which are variable-priced metal ions, accelerates the oxidation of polymers and can therefore be resisted with metal ion passivants.
2.2.7 antioxidant dosage
the amount of antioxidants depends on the type of powder coating. Crosslinking system, efficiency of antioxidants, synergies and cost factors. Most antioxidants have a suitable concentration value, within the appropriate concentration to increase the amount of antioxidant capacity increased, more than the appropriate concentration has adverse effects, in addition to the antioxidant volatility, extraction, oxidation loss and other factors should be considered. 3, carefully carried out powder coating formulation design
(1) in the selection of raw materials should avoid the selection of free-forming compounds, significantly promote oxidation reaction of metal compounds and photosensitive substances, as far as possible to choose raw materials with light shielding. The following are raw materials with light shielding:
(1) carbon black absorbent capacity is strong, all powder coatings can provide the best weather resistance. Among them, easy to process the groove method carbon black effect is the best, paint with pigment carbon black more use of this method of production. This carbon black is acidic, with an oxygen content of 3-5% and a particle size of 30-35nm. The concentration of less than 2% has a significant effect. More than 5% of powder coatings have reduced physical and mechanical properties. It has synergy with acid antioxidants, but it has a counter-effect when used with amine antioxidants.
(2) titanium dioxide reflects ultraviolet light at wavelengths of 340 to 360 nm. The refractive index of the redstone type titanium white powder is 2.72, and its shielding efficiency is greater than that of the sharp titanium type titanium white powder (refractive index 2.55). And can absorb less than 410nm of ultraviolet light.
(3) zinc oxide refractive index 2.03, is an effective and inexpensive light shielding agent. Especially for 240 to 380nm range of ultraviolet light has a better protection capacity, particle size of 0.11 m when the best effect. Dosing 5 to 10%. If used in the same time as antioxidants (e.g. 1% zinc ddethyldythione for zinc amino acid), the dosage can be reduced to 2%.
(4) calcium carbonate, radon sulfate refractive index 1.5-1.7, in the wavelength range of 300 to 400nm, there is also a high reflectivity.
addition, indisin, green and other organic pigments also have an effect. Iron oxide red granules are also effective, but at high temperatures they degrade PVC and some polymers.
(1) when using light shield