Chemical knowledge-solve the problem of weather resistance of powder coating film

With the rapid development of national economy, the application of powder coating in outdoor is more and more common Thus, as a protective and decorative powder coating, its weather resistance and durability have also attracted more and more attention, especially for the coating of indoor and outdoor articles such as ceiling, curtain wall board, water dispenser, air conditioner, washing machine, aluminum profile, etc., which will have a series of changes in the natural environment, specifically, the gloss will decline, the color will be dimmed, and rust spots or cracks will appear on the surface of the coating , begin to slightly pulverize With the extension of time, the phenomenon that the performance of powder coating decreases gradually is called aging; in the natural environment, the aging phenomenon of powder coating is called natural aging because of the effect of surrounding natural factors such as sunshine, rain, oxidation, cold and heat changes and microorganisms The ability of powder coating to inhibit or delay natural aging is called natural aging resistance, which is also called weather resistance There are many factors affecting the weatherability of powder coatings, including internal factors such as resin, curing agent, pigments and fillers, as well as the structure and performance of other additives; as well as natural factors (external factors) such as the role of sunlight (mainly UV), atmospheric composition (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt fog, etc.), temperature change, etc Ultraviolet radiation is the main reason for the natural aging of powder coatings, and oxygen in the atmosphere is an important factor to promote the natural aging Under the action of ultraviolet and oxygen, the powder coating will produce automatic oxidation reaction, i.e oxidation chain reaction, which will degrade the powder coating The reaction was accelerated by water and heat, which promoted the photooxidation The polluted atmosphere contains polycyclic aromatic compounds (i.e the compounds containing more than one benzene ring in the molecule) and trace impurities in the raw materials of powder, which are the sensitive agents for the photo oxidative degradation of powder coatings The combination of these reactions and the degree of influence on the aging of powder coatings vary with different environments, as well as the differences in chemical composition and physical structure of powder coatings, resulting in great differences in the weatherability of powder coatings Therefore, the influence of various factors can not be ignored Only by understanding how various factors work on the powder coating and grasping the main contradictions, can we find out the countermeasures to improve its weatherability 1 Types and functions of light stabilizers 1.1 definition of light stabilizers Light stabilizers are substances to improve the stability of molecular materials, which can prevent photooxidation aging of polymer materials and greatly improve their service life Light stabilizers can inhibit or weaken the photodegradation and improve the light resistance of polymers and composites Because most light stabilizers absorb UV light, they are also commonly called UV absorbers Light stabilizers are very effective in preventing light aging and prolonging the service life of composite materials, and the dosage is very small, generally only 0.01% - 0.5% of polymer mass 1.2 types and functions of light stabilizers light stabilizers can be roughly divided into four categories, namely, UV absorbers, quenching agents, free radical spreading agents and light shielding agents 1.2.1 UV absorbers: before the UV light endangers the powder coating, they can strongly and selectively preferentially absorb the high-energy UV light which is harmful to the powder coating, and convert it into harmless low-energy radiation to prevent the degradation of the UV light on the powder coating It can absorb the radiation energy of ultraviolet ray before the resin component Through the rearrangement of the electronic structure in its own atoms, it can convert the radiation energy into heat energy and dissipate it in the atmosphere Once the heat energy atom is released, it will return to the original state 1.2.2 quenching agent (energy transfer agent) has the function of making the heat energy released not damage the powder coating It does not directly absorb the incident ultraviolet rays, only when the powder coating is stored in a certain amount of energy, it will absorb the excessive energy that will cause the degradation of the powder coating, and transfer this energy to the surrounding atmosphere in the form of heat 1.2.3 free radical trapping agent this kind of light stabilizer can capture the active free radicals generated in the polymer, so as to inhibit the photooxidation process and achieve the purpose of light stability When the coating of powder coating absorbs ultraviolet energy and decomposes to produce active radicals which lead to automatic oxidation reaction, the radicals can be captured by the free radical trapping agent, which can be converted into stable polymer compounds through chemical reaction, so that the photooxidation reaction is terminated 1.2.4 the light shielding agent can reduce the transmission effect of ultraviolet on the material, which is the most economical and practical additive material 2 The way to improve the weather resistance of powder and the mechanism of light protection Because the curing temperature of powder coating is about 200 ℃, in the natural environment, the general temperature will not cause the thermal aging of powder coating Sunlight can cause different degrees of damage to powder coating and other polymer materials, resulting in degradation of coating film, resulting in deterioration of appearance and performance of products The most harmful to the powder coating is the ultraviolet light of 290nm ~ 400nm The degradation of the powder coating by sunlight is the combined effect of light and oxygen, which is usually called photooxidation degradation or photooxidation aging Whether the coating is stable, broken or oxidized depends on the energy absorbed by the material chosen by the powder coating If the absorbed light energy is greater than the bond energy, the chain break between the molecules of the powder coating will occur, which will lead to the deterioration of the performance, that is, the phenomenon of pulverization, embrittlement, discoloration and so on The natural aging of powder coating is a complex process In order to restrain this process, prolong its service life and improve its weatherability, the following three aspects can be solved 2.1 the selection of resin and curing agent with good weather resistance is mainly to select higher level resin, such as outdoor polyester resin, acrylic resin, polyurethane resin, fluorine resin and corresponding curing agent 2.1.1 pure polyester system at present, there are two kinds of powder coatings applied to outdoor weather resistance, one is polyester / TGIC system, the other is polyester / Primid system The outstanding advantage of polyester / TGIC powder is its weather resistance At present, in the field of outdoor painting, it has become a priority for people to develop products It has many advantages, such as good weather resistance, heat resistance and yellowing resistance The most fatal disadvantage of polyester / TGIC powder coating is the high toxicity, environmental damage and genotoxicity of curing agent TGIC, followed by the appearance inferior to polyurethane powder coating Polyester / primed system is a substitute developed because of the stimulation of TGIC to human body or skin The system has the advantages of safety, non toxicity, stable reaction activity, easy electrification, good storage stability, low consumption, etc 2.1.2 main advantages of acrylic system acrylic powder coating: due to the light color of acrylic resin, transparent and bright light color can be prepared, and the film is bright, plump, high hardness, not easy to scratch, good pollution resistance, excellent light and color retention, and has a very beautiful appearance Its main disadvantages are high price, poor impact performance and poor solubility with other powder coating systems But its weather resistance level is almost the same as polyester / TGIC system 2.1.3 polyurethane system polyurethane is a kind of polymer containing urethane bond Polyurethane powder coating is a thermosetting powder coating composed of hydroxypolyester, closed polycyanide, pigment, filler and assistant Polyurethane powder coating has good quality coating and excellent performance It has been widely used in building materials, external parts, air conditioners and other home appliances In China, the performance of polyurethane powder coating is being recognized by people, and its proportion will gradually increase Its outdoor insolation and physical mechanical properties are comparable to those of pure polyester / TGIC powder coating At present, the domestic powder coating manufacturers rarely choose this type of powder coating In addition to the expensive curing agent, this type of powder coating has the following disadvantages: in the process of curing and film forming, the sealer should be released, causing air pollution in the baking environment, and the smell is unpleasant; the film should not be too thick, or it is easy to cause pinholes, bubbles and other defects in the film; High curing temperature 2.1.4 fluororesin system fluororesin powder coating, as the coating for building exterior decoration, is used in high-rise and super high-rise buildings, with excellent light and chemical resistance, not only anti pollution, but also self-cleaning ability, which can keep clean and clean as new coating film for several years The coating made of thermosetting fluororesin powder coating has good storage stability, good physical and mechanical properties of the coating film It can balance the flexibility, hardness, adhesion and other properties of the coating film through the reasonable matching of each component in the system The coating film has excellent acid rain resistance, ozone resistance, UV resistance and chemical medicine performance, which is the best variety of the powder coating Its disadvantages are poor dispersion of pigment, low gloss of coating surface and high temperature baking, so its application is limited To sum up, there are some limitations in selecting high-grade resin and curing agent to produce weather resistant powder coating For powder coating, adding light stabilizer, antioxidant and more reasonable choice of materials, formula design is the best plan 2.2 many light stabilizers added with light stabilizers and antioxidants have double effects of light stability and oxidation resistance Different types of light stabilizers have different mechanism of action, and the photodegradation behavior and characteristics are different due to the composition of powder coatings Therefore, it is necessary to select carefully and pass the test verification By analyzing the natural process and mechanism of powder coating, the following light stabilization measures can be taken 2.2.1 add UV absorber Note that the same UV absorber has different UV absorption capacity at different concentration and film thickness The thicker the film, the better the protection effect of UV absorber The commonly used ultraviolet absorbers are benzophenone compounds and Benzotriazole Compounds 2.2.2 the addition of quenching agent is mainly some bivalent organic nickel compounds In recent years, organonickel complexes have been replaced by other non-toxic or low toxic quenching agents due to the toxicity of heavy metal ions 2.2.3 add free radical trapping agent, which is mainly hindered amine (HALS) 2.2.4 add optical shielding agent, which is a kind of pigments and fillers with high refractive index and strong absorption capacity They can reflect or absorb ultraviolet rays in powder coating, so that the light can not penetrate into the coating, thus reducing the harm of ultraviolet rays and playing a role of light shielding 2.2.5 the action mechanism and application characteristics of antioxidant In fact, antioxidant is a kind of substance that can inhibit or delay the oxidative degradation of polymer There are many kinds of antioxidants with different functions If they are classified according to their mechanism, they can be divided into two categories One is chain terminated antioxidants The growth of chain interruption of reaction between these antioxidants and free radicals produced in polymers is also called the main antioxidant