With the rapid development of industrial technology, environmental protection issues have been paid more and more attention all over the world. In China in particular, the government agency announced that it would impose a consumption tax of up to 4% on solvent paint manufacturers in early 2015.
Since the VOC of powder coatings is almost zero, this is good news for powder coating manufacturers, and will provide more opportunities for development and strong support from the government.
Powder coatings have been widely used in many fields, such as construction, automobiles, plumbing, mobile communications and household appliances. In the powder coating industry, the work and research on coating anticorrosion have never stopped.
Reports of corrosion damage to bridges, buildings, aircraft, automobiles and gas pipelines are commonplace. Although an effective form of cathodic protection has been developed in the oil and gas pipeline industry, instances of coating failure still exist.
There are many factors affecting coating damage, which may be affected by the coating system including film-forming substances, pigments and fillers, additives, substrate type, pre-treatment process, curing process, coating thickness and coating adhesion , and may also include external environment such as temperature, humidity, bacteria and ultraviolet rays.
For powder coating manufacturers, the key to improving anti-corrosion performance lies in the design of the chemical composition of the coating itself, which requires the selection of suitable film-forming substances, pigments and fillers, and anti-corrosion additives. Next, these influencing factors will be introduced one by one.
The resins used in powder coatings are mainly polyester, acrylic, polyurethane and epoxy. There are ester groups in the polyester structure, which are prone to hydrolysis in humid environments; acrylic structures also contain ester groups, which have been proven to be easily damaged in acid rain environments.
Polyurethane can be used for anti-corrosion coatings, but the price is high, and epoxy resin with reasonable price and excellent anti-corrosion performance is a good choice.
The determinants of the excellent anticorrosion performance of bisphenol A epoxy are as follows:
(1) The side hydroxyl endows good adhesion with the metal;
(2) The ether bond in the skeleton endows excellent chemical resistance and alkali resistance;
(3) There is no ester group in the structure, and it has good water resistance;
(4) High cross-linking density endows better shielding effect on the substrate.
In addition to bisphenol A epoxy, novolac modified epoxy and bisphenol F can also be used in anti-corrosion powder coatings.
From the analysis of chemical structure, compared with bisphenol A type epoxy and F type epoxy, novolac modified epoxy contains more epoxy groups and aromatic rings per unit mole, and the crosslinked structure after curing is denser and has better anti-corrosion performance, but it also has the defect of brittleness, and is generally rarely used alone in powder coatings .
In heavy-duty anti-corrosion powder coatings, novolac-modified epoxy is often blended with bisphenol-A epoxy to achieve excellent anti-corrosion and mechanical properties. The bisphenol F type epoxy used in powder coatings is mainly to improve the leveling property of the coating, and it can also be blended with bisphenol A type epoxy.
In addition to chemical structure, molecular weight is also an important factor affecting various properties of epoxy, and its influence is shown in Figure 5. Compared with high-molecular-weight epoxy, since low-molecular-weight epoxy contains more epoxy groups, the crosslinking density of the cured coating increases, and the hardness of the coating increases, while the flexibility and impact strength decrease.
Low molecular weight epoxies contain fewer hydroxyl groups than high molecular weight epoxies, which also reduces the wettability and adhesion between the coating and the substrate.
In consideration of various performances of epoxy, the molecular weight of epoxy must be within a certain range, whether large or small will affect the anti-corrosion performance of the coating. In the field of heavy anti-corrosion, the molecular equivalent weight of epoxy is generally 700-900g/eq, and the molecular weight distribution is narrow.
In the heat-curable powder coating formula, the film-forming substance is mainly composed of resin and curing agent. Common curing agents for epoxy powder coatings include imidazole, amines (dicyandiamide and aromatic amines), anhydrides and phenolic curing agents .
Among them, imidazole is rarely used alone due to its low activation energy and violent curing reaction, large internal stress of the cured coating and poor mechanical properties, but is often used as a catalyst for epoxy powder coatings. Dicyandiamide has good comprehensive performance and cost performance, and is a common curing agent for powder coatings.
Several aromatic amines can also be used as curing agents for powder coatings. For example, Z.Wang et al. studied three aromatic amine curing agents (diaminophenyl sulfone, m-phenylenediamine and phenalkamine) cured coatings in a hot sulfuric acid solution with a concentration of 10%. Anticorrosion performance, and found that diaminophenyl sulfone cured coatings have better anti-corrosion properties.
Because acid anhydride is easy to produce irritating gas and has poor storage stability, its application in powder coatings is limited to a certain extent. Phenolic curing agent has become the first choice in the field of heavy corrosion environment such as oil and gas pipelines due to its advantages of low baking temperature, fast curing and excellent anti-corrosion performance.
Pigments and fillers for anti-corrosion coatings
As an indispensable component in powder coatings, anti-corrosion pigments and fillers achieve the purpose of anti-corrosion and anti-corrosion through physical barrier, electrochemical reaction and corrosion inhibition reaction in terms of anti-corrosion mechanism.
Physical barrier protection is achieved by means of layered fillers with coating thickening and anti-seepage functions. Layered fillers widely used in anti-corrosion powder coatings are mainly micaceous iron oxide, sericite and glass flakes .
The corrosive medium tends to migrate and diffuse linearly in the coating containing spherical fillers, which greatly delays the corrosion progress of the coating. In addition, it should be pointed out that it may be difficult for flaky fillers to maintain their original flaky shape during the extrusion process of powder coatings, which limits their application in powder coatings.
In principle, metals that are more active than the electrochemical properties of the substrate can be used as pigments and fillers for anti-corrosion coatings. But at present, metal zinc particles are the most widely used, and it is reported that the average diameter of annular zinc particles with the best anti-corrosion performance is 2 μm.
The anti-corrosion mechanism of metal zinc is that zinc participates in the corrosion reaction to produce insoluble substances such as ZnFe 2 O 4 and basic zinc carbonate. When the flake metal zinc developed by Eckave-Werke replaces the traditional spherical zinc, the anti-corrosion performance of the coating is further improved due to the unique parallel lapping and shielding functions of the flake metal zinc.
Slow-release pigments and fillers can be divided into cathode type and anodic type according to the type of reaction involved. Cathodic corrosion inhibitors, such as inorganic salts of magnesium and aluminum, inhibit the corrosion of coatings by reacting with hydroxide ions in a neutral environment to form insoluble substances.
Anodic corrosion inhibitors, such as phosphates, silicates or hydroxides, form a protective oxidation layer on metal surfaces. Insufficient RER pigments and fillers will result in poor electrode zones which will accelerate corrosion progress.
At present, the most widely used slow-release pigments and fillers are phosphate-containing pigments and fillers, such as zinc phosphate and magnesium phosphate . In addition, there is a class of slow-release pigments and fillers that are less toxic, which are spinel-type pigments based on metal oxide mixtures .
In theory, all additives that help to improve the adhesion between the coating and the metal substrate can be used to improve the anti-corrosion performance of the coating. Silanes or modified silanes are a common one, which improve the interlayer adhesion of the coating to the metal substrate.
In addition to silanes, there are other types of adhesion promoters. For example, Chartaway International has developed a series of adhesion promoters for powder coatings.
Such as Chartsil B 515.2H/1H and ChartsilC containing high concentrations of amino groups, hydroxyl groups or hydroxyl groups , these can be introduced into the coating before extrusion processing. In addition, it has been reported that adding polyphenylamine to epoxy powder coatings can improve its anti-corrosion properties.
With more and more attention paid to environmental protection, powder coatings will have greater application prospects in the near future. But so far, due to the limitations of various conditions, powder coatings have not been applied to C5 and C6 corrosion levels. For powder coating technicians, equipment suppliers and substrate pretreatment professionals, there is still a lot of work and research to do.
Among the many anti-corrosion additives, low-toxic green corrosion inhibitors are the development trend of anti-corrosion coatings . Some green corrosion inhibitors such as biodegradable polymers and plant extracts have been mentioned in the liquid coating industry, which show outstanding anti-corrosion properties in many corrosive environments.
Although these green corrosion inhibitors have the advantages of low toxicity and biodegradability, there are still various technical bottlenecks. How to widely use these green corrosion inhibitors at the industrial level, especially in powder coatings, needs further research and exploration.