Several Views on the Production and Use of Powder Coatings
Last Update: 2020-06-19
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< br / > severe tips of powder coatings manufacturing and use you need to know < br / > Bi Xuezhen (Hebei Lidong powder coating Co., Ltd.) < br / > Abstract: as a semi-finished powder coating, the product quality is not only related to the preparation technology but also closely related to the coating technologyThis paper discusses the problems that should be paid attention to in order to improve the coating quality of powder coating from the aspects of its electrified mechanism, internal quality and coating technology< br / > 1 preface < br / > materials, energy, information as the three pillars of modern human civilization, promote the rapid development of societyIn recent decades, new technologies and materials emerge in endlessly, which makes revolutionary changes in various industriesAs a new coating material developed in recent decades, powder coating, with its excellent performance and environmental protection, has triggered a revolution in the coating industrySince the birth of powder coating, it has been in a high-speed development stagePowder coating theory is a new subject based on polymer physics, polymer chemistry and electrostatic theoryIt is not mature in many aspectsThe existing theories can not fully explain some phenomena, some concepts are still in the stage of continuous renewal and development, and some problems are only limited to qualitative discussionAlthough people have accumulated a lot of experience and skills in the field of production and use, and the spraying process is becoming more and more mature, there are still many misunderstandings in many aspectsHow to choose powder reasonably and use powder correctly has become an important topic in the coating industryThis paper aims to put forward some controversial or fuzzy topics, and is willing to discuss them with people in the industry< br / > 2 the charge mechanism of powder coating < br / > electrostatic theory is the basic theory of powder coating spraying, which can be divided into corona gun and friction gun according to the way of electrostatic generationCorona charging is the working mechanism of corona gun spraying, that is, the electrode is used to ionize the air, and then the passing powder particles are chargedThe specific method is to set an electrode at the end of the gun barrel and apply high pressureThe electrode is used to ionize the air molecules near the gun head into a charged ion cloudThe powder coating is fluidized in the powder feeder to form a mixed gas powder with air, which is sent to the gun through a hose for uniform spraying, and the powder is negatively charged when passing through the ion cloud at the gun head; at the same time, The electric field and power line are formed between the spray gun and the workpiece Under the action of the Coulomb force, the powder particles with negative electricity are attracted by the positive electricity on the substrate (grounded), and fly to the workpiece under the action of the thrust and Coulomb force of the compressed air to make the powder particles adhere to the substrate surface < br / > some other scholars think that the powder is not electrified at the head of the spray gun, but is polarized into a dipole (collect the powder sprayed by the spray gun in an insulated container, check its electrified degree, and find that the powder is not charged) The dipoles move in the electric field and reach the surface of the workpiece They are tightly adsorbed on the surface of the workpiece to achieve the purpose of coating This theory explains the principle of electrostatic spraying from another angle, but it can't explain the work piece grounding well < br / > although the theory of electrostatic spraying is still controversial, it has been applied in powder coating industry for decades, and it is the most widely used spraying method at present With the continuous improvement of people's understanding and the continuous improvement of electrostatic theory, many phenomena can be perfectly explained < br / > 3 internal quality and appearance < br / > there are two main purposes of coating: decoration and protection Decoration is external quality and protection is internal quality Internal quality determines external quality If the protective function of coating is lost, there is no decoration Of course, different workpieces have different emphases on these two functions For example, pipeline anticorrosive powder coating mainly emphasizes anticorrosion, and the requirements for convective flatness, color and luster are not strict, while household appliances, furniture and other articles pay attention to decoration, so as to have a more beautiful appearance For most sprayed workpieces, decoration and protection are equally important, because there is a close relationship between various indexes of powder coating, such as hardness and toughness, adhesion and impact resistance, leveling and corner coverage, constructability and coating effect If one index is overemphasized and the overall performance index is ignored, one will lose the other and gain more than lose < br / > 3.1 leveling and corner coverage < br / > many domestic manufacturers of powder coating usually only pay attention to leveling and neglect corner coverage According to the author's observation, the general damage of the powder coating starts from the thin corner of the coating, such as household appliances such as refrigerators, washing machines, air conditioners and public facilities such as lamp posts and guardrails, and the risk of such damage and the damage to the decoration effect are the most serious Generally speaking, the edge and corner parts can be well powdered during spraying, but in the baking process, with the melting of the powder, under the effect of surface tension, the edge and corner parts produce shrinkage, resulting in the thickness of the sharp edge and corner parts is very thin Water vapor is easy to infiltrate from the weakest corner, and then drive straight into the substrate along the interface between the coating and the substrate Under the chemical and electrochemical action, the substrate is gradually rusted, forming iron oxide with crystal water If the volume expands, the coating is peeled off, and the corrosion continues to the depth of the substrate, the substrate will be broken If it is used in high-rise buildings, if it is rusted to a certain extent and breaks and falls, it will endanger personal safety < br / > the author has observed outdoor powder coating appliances, such as air conditioners, steel windows, traffic guardrails, etc., and found that due to the long distance of the observer, the orange peel and particles of the coating film are almost imperceptible, while the yellow rust caused by the poor coverage of the corners will form obvious rust flow marks on the surface, which can be seen far away, very conspicuous and unsightly In general, the lower the melting viscosity of the coating system, the longer the curing time, the better the leveling, and the worse the corner coverage We can observe the powder coating of foreign countries Generally, leveling is not very good, but the corner coverage is often very good Some people think that foreigners like the surface of big orange peel In fact, this is a way of sacrificing the external quality and improving the internal quality that they have summed up based on years of experience < br / > in some other fields, the workpiece has no large plane and many edges and corners For example, the road protection net, bicycle basket, chicken cage and other products need to be coated with thermoplastic PE powder coating This material has very high molecular weight and poor melting fluidity, so it will not produce the phenomenon of edge and corner shrinkage Although the adhesion of this material is relatively poor, due to the coating, The coating is wrapped on the filiform workpiece without peeling, and the substrate can be well protected < br / > 3.2 workability and coating effect < br / > the effect of powder coating after coating is very important If the construction performance is not good, the effect after coating is difficult to guarantee Workability refers to whether the powder coating is easy to use, including powder loading rate, fluidization performance, electrified uniformity, storage stability, sensitivity to baking temperature and time, etc., which is also a reflection of the inherent quality of powder coating, In today's highly competitive powder coating industry, improving the construction performance is also one aspect of quality competition It is not advisable to emphasize the coating effect and ignore the construction performance < br / > 3.3 toxicity of powder coating and MSDS table < br / > relevant experts pointed out that China has joined WTO, domestic chemical enterprises must be familiar with and master the legal and standardized MSDS (material safety date sheet) and warninglabel (warning label) in line with international standards in order to integrate into the global economy and participate in international competition MSDS and warninglabel are related to the provisions of laws, regulations and standards (including writing content, format, etc.), which is a standard, systematic, rigorous and important specification procedure integrating regulations and technical knowledge (Chemistry, biology, environment, toxicology, hazardous chemicals assessment, audit, occupational health, etc.) < br / > the domestic powder coating industry seldom prepares safety instructions Enterprises with export needs may provide wrong data when preparing MSDS or warninglabel because they do not know the relevant laws and technical specifications of foreign countries or lack the toxicity information of chemicals Moreover, they do not meet the requirements of laws and specifications of their countries, Often delay business opportunities Even may be subject to legal proceedings, resulting in irreparable losses MSDS and warninglabel will help domestic products to enter the foreign market < br / > not only MSDS is required for export, but also for domestic sales It is understood that a large number of foreign-funded enterprises in China also purchase powder coatings at home However, due to the lack of MSDs for domestic products, they do not want to purchase them, so they have to import them from abroad < br / > toxicity is an intrinsic index of powder coating The material selection of powder coating follows the principle of low toxicity However, some materials used for many years, such as TGIC and lead chrome yellow, are irritant or contain heavy metals, which may cause harm or potential danger to human body Developed countries use laws to limit the use of toxic and harmful substances, for example, for the content of heavy metal lead in coatings, North America, Europe, Australia, New Zealand, Japan, South Korea have legislative restrictions, China, Russia and other countries are tending to legislative restrictions, but the implementation and implementation still need some time The preparation and application of high standard MSDS and warninglabel can undoubtedly show the advanced business philosophy and high level of production and management level of the enterprise, which will help to improve the brand image of the enterprise in the international market and lead the domestic powder coating industry The use of < br / > 4 nano materials < br / > nano and nano materials are very fashionable terms in the field of materials nowadays In fact, nano is only a unit of length, equivalent to one billionth of a meter Nano technology is to make materials into 1-100nm particle materials It has been found that when the size of matter becomes very small, such as tens of nanometers, tens of nanometers, or even several nanometers, due to the small size and large specific surface of nanoparticles, they have quantum effect and surface effect, It will take place and show a series of peculiar physical and chemical properties, such as mechanics, optics, electricity, magnetism, catalysis, sterilization and bacteriostasis In fact, only about three or five atoms can be arranged in a nano particle The essence of nano technology is to construct a man-made structure with brand-new molecules and brand-new arrangement from the precise manipulation of atoms and molecules < br / > nano materials have magical effects, such as improving strength, enhancing heat resistance, antibacterial, weather resistance, etc the effective use of nano materials can completely change people's traditional habits of clothing, food, housing and transportation But its application must be scientific and rigorous In recent years, there have been many manufacturers of nano materials in China, which have launched so-called "nano" materials, such as fillers, pigments, additives and so on Through analysis and test, the author found that most of the so-called "nano" materials sold in the market have particle diameters of several microns or dozens of microns, that is to say, thousands or tens of thousands of nanometers, which are not nano materials at all, and the magic function publicized by the manufacturer is not available at all, and the mechanical strength, weather resistance, heat resistance and other indicators of the coating have not changed significantly So when we use the so-called "nano" materials, we must know whether they are nano materials, whether they are reasonable applications, and whether they can stand the test of time < br / > 5 powder loading rate and spraying area < br / > some people think that if the powder loading rate is high, the spraying area must be large, which may not be comprehensive Because the powder loading rate is related to the powder electrification and powder particle size, it refers to the amount of powder adsorbed on the workpiece under the same electrostatic voltage, air pressure and spraying equipment Spray area refers to the area of powder coating per unit weight, which is related to coating thickness, powder specific gravity and utilization rate Sometimes due to the experience of spraying personnel
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