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

With the rapid development of national economy, powder coating is more and more widely used in outdoorAs a result, the weather resistance and durability of powder coating as protection and decoration have been paid more and more attention, especially the coating of indoor and outdoor articles such as ceiling, curtain wall board, water dispenser, air conditioner, washing machine, aluminum profile and so on, which will have a series of changes in the natural environment, such as the decline of luster, dim color, rust spots or cracks on the surface of the coating, Start to pulverize slightly< br / > the phenomenon that the coating performance of powder coating decreases gradually with the extension of time is called aging; in the natural environment, the aging phenomenon of powder coating is called natural aging due to the effect of surrounding natural factors such as sunshine, rain, oxidation, cold and heat changes and microorganismsThe ability of powder coating to inhibit or delay natural aging is called natural aging resistance, which is also called weather resistance< br / >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 agingUnder the action of ultraviolet and oxygen, the powder coating will produce automatic oxidation reaction, i.eoxidation chain reaction, which will degrade the powder coatingThe reaction was accelerated by water and heat, which promoted the photooxidationThe polluted atmosphere contains polycyclic aromatic compounds (i.ethe 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 coatingsThe 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 coatingsTherefore, the influence of various factors can not be ignoredOnly 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< br / > < br / > 1 type and function of light stabilizers < br / > < br / > 1.1 definition of light stabilizers < br / > < br / > light stabilizers is a kind of substance to improve the stability of molecular materials, which can prevent the photooxidation aging of polymer materials and greatly improve their service life< br / >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 < br / > < br / > 1.2 types and functions of light stabilizers < br / > < br / > light stabilizers can be roughly divided into four categories, i.e UV absorbers, quenching agents, free radical spreading agents and light shielding agents < br / > < br / > 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 < br / > < br / > 1.2.3 free radical trapping agent < br / > < br / > 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 < br / > < br / > 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 < br / > < br / > 2.1 the selection of resin and curing agent with good weather resistance < br / > < br / > mainly refers to the selection of higher grade resins, such as outdoor polyester resin, acrylic resin, polyurethane resin, fluorine resin and corresponding curing agent < br / > < br / > 2.1.1 pure polyester system < br / > < br / > at present, there are two kinds of powder coatings applied to outdoor weather resistance, one is polyester / TGIC system, the other is polyester / primed system The outstanding advantage of < br / > < br / > polyester / TGIC powder lies in its weatherability 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 < br / > The system has the advantages of safety, non toxicity, stable reaction activity, easy electrification, good storage stability, low consumption, etc < br / > < br / > 2.1.2 acrylic system < br / > < br / > the main advantages of acrylic powder coating: due to the light color of acrylic resin, transparent and bright light color can be prepared, and the coating film is bright, full, high hardness, not easy to scratch, good pollution resistance, excellent light and color retention, with 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 < br / > 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 < br / > < br / > its outdoor insolation and physical mechanical properties are comparable to pure polyester / TGIC powder coating At present, domestic powder coating manufacturers rarely choose this type of powder coating In addition to the expensive curing agent, this type of powder coating also has the following disadvantages: < br / > < br / > in the process of curing and film forming, it is necessary to release the sealer, resulting in air pollution in the baking environment, and the smell is unpleasant; the film should not be too thick, otherwise it is easy to cause pinholes, bubbles and other defects in the film; the curing temperature is high < br / > 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 The disadvantages of < br / > < br / > are poor dispersion of pigment, low gloss of coating surface and high temperature baking, which limits its application < br / > < br / > 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 < br / > < br / > 2.2 adding light stabilizers and antioxidants < br / > < br / > many light stabilizers 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 < br / > The commonly used ultraviolet absorbers are benzophenone compounds and Benzotriazole Compounds < br / > 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 < br / > < br / > They can reflect or absorb ultraviolet rays in powder coatings, so that light can not penetrate into the coating, thus reducing the harm of ultraviolet rays and playing a role of light shielding < br / > < br / > 2.2.5 mechanism and application characteristics of antioxidants < br / > < br / > in fact, antioxidants are substances that inhibit or delay the oxidative degradation of polymers 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 reaction chain interruption between such antioxidants and free radicals produced in polymers is also called as the main antioxidant The main antioxidants are hindered phenols and secondary aromatic amines; the other is preventive antioxidants, which can inhibit or slow down the formation of free radicals in the initiation process, also known as auxiliary antioxidants Preventive antioxidants include phosphite, metal dithiocarbamate, etc < br / > < br / > 2.2.6 selection of antioxidant < br / > < br/