Epoxy powder coating

'powder coating is a kind of technical product used to decorate metal matrix and form protective film It has the advantages that the fluidity powder is suitable for dry coating and heating to form a continuous and economical film It also provides some value in terms of safety and health Its characteristics are: the technical film is more uniform, resulting in more consistent performance Good technology and chemical resistance No solvent consumption, no flash energy and space It is usually used for one-time coating, which can be highly automated and has a very high use efficiency The materials can be recycled in a rather simple way Safety and health: no released solvent pollutes the air, reduces the risk of fire or explosion, and the working environment is clean The first powder organic coating is based on thermoplastic polymer Later, the development of technology extends to thermosetting polymer powder coating, which is better known as epoxy resin The preparation of powder coating from epoxy resin has a considerable development scope and is suitable for application in various sectors of technology and economy, because the epoxy resin / curing agent system can obtain the following properties: There is no volatiles released during curing addition; primer is unnecessary, with excellent adhesion; there may be a wide range of film thickness; the surface gloss range can be adjusted from high gloss to no gloss; relatively high pigment fillers can make manufacturing more economical; high hardness, with good flexibility and impact resistance; very good resistance to a variety of chemicals; good heat resistance They also have some disadvantages: accurate color matching is more difficult than liquid pigments; it is more difficult to change the color in the production line; more systems need relatively high oven temperature Due to its many good properties, epoxy powder coating has a wide range of applications: office and school facilities, refrigerators and freezers, washing machines, radios, lampshades, bicycles and motor frames, car driving boards, components and bodies, agricultural machinery equipment, pipeline exterior coating The development of epoxy resin powder coating, in order to meet the needs of different development of industrial sectors, requires new coating preparation technology, new powder preparation method and new application technology, all of which are completely different from liquid coating The basic characteristics of the mixture of epoxy resin, curing agent, pigment and additive are required In the preparation of powder coating, the physical and chemical properties of the mixture will be affected First of all, the resin system must have the ability to wet pigments and fillers, so as to produce the minimum reaction between resin and curing agent under the conditions of temperature and time Products of uniform quality must be relatively easy to produce powder of required particle size; the powder must maintain fluidity, have no adhesion or caking during transportation and storage, and will not react under this condition The most commonly used is electrostatic, so the powder particles must have the ability to be charged When heated, the powder must melt and flow into a smooth continuous film Wet the substrate and cure it under baking The coating obtained in this production process has the required mechanical and chemical resistance In order to meet these performance requirements, the mixture system requires a suitable melting range and melting viscosity; in addition, there must be a suitable active range in the addition system of these resin curing agents, and appropriate temperature shall be used in each sequence of powder preparation, storage and output, curing and baking stage, etc In order to enable users to obtain powder coatings with different requirements, this can be composed of different epoxy resin / curing agent / additive systems The molecular weight of epoxy resin, the content of epoxy group and the range of softening point play an important role in the properties of powder coatings Generally speaking, the effect of the increase of molecular weight and softening point can be summarized as follows: performance, the effect of the increase of resin molecules, the difficulty of pigment dispersion, the requirement of high temperature storage stability, the improvement of membrane fluidity and the decrease of membrane fluidity Mechanical properties - flexibility / impact resistance improvement - viscosity reduced due to reduced substrate wetting - hardness not affected - chemical resistance reduced very little The best balance of properties should include physical storage stability (i.e resistance to adhesion or caking) When the epoxy content of the resin is 1000-1400 mmol / kg, the fluidity and mechanical properties of the membrane can be improved It can use the solid resin with low and high molecular weight, which is mainly used to adjust the special properties in the special composition Additive some powder coating components need to add flow control agent, which is leveling agent The dosage is generally 1-10% of the resin quantity Leveling agent can be added immediately after the preparation of epoxy resin to make epoxy resin containing leveling agent, such as apikote3003-4f-1, 3003-4f-2 and 3003-4f-10, which contain 1, 2 and 10% acronal 4f respectively A certain amount of leveling agent can also be added to the powder coating according to the fluidity requirements of the product Curing agent curing agent is an essential part of resin mixture system, which plays a decisive role in the performance of powder coating One of the obvious requirements is the activity of their resin / curing agent system, which should be very low in normal transportation and storage temperature, but high in industrial baking temperature During the preparation of the powder, it should be stable for mixing, and the curing agent should be solid at room temperature The quantity and type of curing agent are related to the chemical reactivity of the basic resin and the different requirements of the crosslinking density of the final film Therefore, different levels of hardener can be used for products with different proportion of hardener In different humidity and the balance between various properties, such as price, chemical resistance, mechanical properties, over baking performance and color retention, industrial users have different quite wide range of requirements Many kinds of commonly used curing agents are generally established through tests according to the user's use conditions, product quality requirements, equipment environmental conditions, etc Pigments and fillers are usually used in powder coatings First of all, they must be dry, because humidity will reduce its fluidity and the pinholes that cause the appearance of the film Second, they must have enough heat resistance to withstand the expected baking system Third, they must be resistant to the chemical properties of the curing agent and do not hinder the curing reaction Increasing the amount of pigment in the powder coating will decrease the flexibility of the film It is proved by the impact strength data that the cone axis bending and slow penetration test decrease At this time, the level of pigment precipitation value becomes very important, which is related to the particle size of filler and packaging characteristics There are two methods of industrialization: Z-blade mixing and extruder mixing Their advantages and disadvantages are compared as follows: square method, advantages and disadvantages, point preparation, injection, Z-blade mixing method, excellent dispersion in the melting humidity, long-term retention, use in the powder with slow solidification in one processing, easy color adjustment, batch unloading, need to improve the temperature, simple mixing operation is not suitable for the batch process of rapid solidification system, and now most of them are replaced by extrusion method Can mix liquid mixture, can remove any humidity in the ingredients, long-term cleaning material loss is relatively high, extruder mixing is very good, mixing investment is high, applicability is good, continuous production requires skilled operation, material loss is low, components need to be mixed preliminarily, retention time at melting temperature is short, color adjustment is difficult, Z-blade mixing Large Z-blade mixing requires the ability to heat up to 180 ° C and an ammeter on the power line to check the shear load The resin melts and adds pigments and fillers at 130-140 ℃; this temperature allows dispersion to occur at maximum shear Add leveling agent and complete dispersion; finally add curing agent and thixotropic agent Heat to obtain fully flowing uniform melt, put it in the plate, cool it to room temperature, and then crush, grind and screen the product It is possible for the shear and dispersion time of different components to change, and the best dispersion of each component is allowed, but the mixing yield is low, only limited curing agent can be used, and the method is generally not used for the long heating time of materials The main requirement of extruder mixing extruder is to make the thermosetting mixture continuously and completely displace, and the substance can be fully exchanged through extruder under the condition of heating up in a relatively short time; therefore, twin-screw or reciprocating screw extruder is often used In addition, it is required to have good temperature control for different areas, simple feeding area and screw cooling, easy disassembly for cleaning cylinder body and excellent dispersion The process is as follows: resin → crushing → pigment + filler mixing and finally adding curing agent mixing → extrusion → strip cooler cooling → grinder rough grinding → fine grinder grinding → screening → packaging powder particle size is controlled according to different application methods: application method: particle size micrometre; electrostatic spraying < 100; fluidized bed or flocculation spraying < 300; electrostatic current bed < 100 The electrostatic spraying voltage is 30-80kv, the coating object is not preheated, and the solidified layer is about 30-60 μ m fluidized bed or flocculent spraying coating object needs preheating There are many formulations of powder coating, including general type, art type, anticorrosion type, insulation type, flame retardant type and so on It is a special formula developed by each factory according to the requirements of users, with its own technical characteristics and application skills