High temperature powder coating formulation design and problem
Powder coating due to its solvent-free, construction simple and utilization high and rapid growth in the global market, high temperature powder coating of silicone in the United States in the eighties in the oven first to be applied quickly in the mid- European and American markets are growing rapidly. With China becoming a global stove, oven and other major production base. The market is also growing need for high temperature powder coating. The high temperature powder coating formulation design, problem solving, production process was introduced.
2, the principle and performance introduced
2.1 Principles of The reaction mechanism of the silicone resin is very similar to its own cross-linking. At a high temperature curing reaction is as follows: ~ Si – OH + HO – Si ~ —> ~ Si – O – Si ~ + H2O ~ Si – OR + HO – Si ~ —> ~ Si – O – Si ~ + ROH
In addition, the thermal stability of the silicone resin side groups of different organic groups: phenyl> methyl> ethyl> propyl> butyl> hexyl
Typically, the curing temperature of the silicone resin can not be less than 200 degrees. Temperature range between 270 and 350 ° C for silicone high temperature powder coating is more sensitive to the range of points. Because in the silicone composition has not been fully sintered, while the organic components has already begun burning and decomposition.
In addition, the silicone resin due to the low Tg of the silicone resin agglomeration problems encountered in the process of storage and production of transport to the development of high Tg (glass transition temperature) is inevitable. , Germany, Wacker Chemical Corporation has introduced the Tg> 65 silicone resin used in high temperature powder coating, successfully resolving the problems of the hot weather, transportation, storage.
2.2 Silicone powder coating should have a performance?
With the organic resin, silicone resin used in conjunction with the appropriate Yan, packing should have excellent long-term heat resistance (200 – 650 ° C).
In addition, the occasions for food contact, the silicone resin should also meet FDA 175.300 and BGVV – XV. Good hot and cold denaturation. Hot plate directly into cold water, while the film is not damaged.
3. Formulation Design
3.1 the choice of base material:
The silicone resin is essential for the high temperature powder coating base material, silicone resin can be used alone as the base material, or fight with polyester, epoxy resin to improve the heat resistance of the coating. Formulations should be used in high temperature inorganic pigments and fillers and additives.
Silicone resin for high temperature powder coating is divided into the following two:
Methyl polysiloxane, which is characterized by: good hardness; Hydrophobicity; low surface energy, with non-stick effect; – high temperature resistance to yellowing; but poor miscibility with other organic resin. Phenyl polysiloxane, a good miscibility; good flexibility; security light, good color retention.
Silicone resin in the high temperature grade requirements: Heat to 250 ° C for 15% -45% Heat 250 – 400 30% -45%
3.2 the selection of pigments
The selection of pigments and fillers is also essential for the heat-resistant silicone powder coating, heat resistant coatings due to the selection of inappropriate Yan, fillers eventually leading to film discoloration and poor adhesion.
High temperature powder coating in black or silver: Black: manganese dioxide, the ferrochrome black (iron oxide, chromium oxide, manganese dioxide mixed burn is made). But the black somewhat less. Silver: aluminum White: titanium dioxide (350-400), zinc (about 370) hiding poor, antimony oxide, light, heat, chalking small, can be used for fire retardant. Yellow: strontium yellow (SrCrO4,): heat resistance, hiding power, suitable for primer. Chrome yellow (CdS CIPY.37; CINo.77199): beautiful color, good hiding power. However, toxic, and is mainly used for non-food contact coating system. Red: red chrome (Pigment Red 108 CIPR108 the CINo77202) bright color, hiding power, weatherability, dispersion, and heat resistance are good. Blue: Cobalt Blue (CoAl2O4, Pigment Blue, CIPB28 CINo77346), the main component of cobalt oxide and zinc oxide. A small amount of alumina and chromium oxide. Chemical stability, low tinting strength. Non-toxic. Green: Yanghuagelv.
3.3 fillers choice: Talc (3MgO.4SiO2.H2O): anti-cracking, easy to lose a light, easy powder. Mica (K2O.3Al2O3.6SiO2.2H2O): resistance to chalking. Precipitation of barium sulfate (BaSO4): inert material, the largest proportion of the pigment, fillers, acid, alkali, no negative impact. Silica Fume (SiO2): precipitated silica. Improve mobility. Wollastonite: calcium silicate, high hardness, low oil absorption.
4 Production Process:
Pre-mixed: Recommended mixing time :6-15 minutes. Too short: poor efficiency, increase the the extruder burden. Pigments unite into a group. Too long: generate heat, the resin melt agglomeration. Screw extruder temperature: Can not be too high to reach the curing temperature, recommended extrusion temperature :95-105 Particle size distribution: Size too small and poor: mobile, easy to plug, gather into a group. Live good, easy to spray, available flat film. Recommended particle size of 10-80 microns
5 deal with the problem:
5.1 How to improve the rate of the powder
The influence of powder particle size Too large: the appearance of lower quality, orange peel, gravity caused the powder on the rate. Too small: with the reduction in power consumption, Recommended particle size 10 – 90 microns Base material: enough base material: not less than 45% Coating moisture content: the particle size is finer about Yi absorbent; Storage ventilation: dehumidification of compressed air piping; spraying air humidity is not too much. Spray voltage :60-90 KV Spraying distance :250-350 mm
5.2 How to eliminate the pinhole?
Pinhole generation from water, while water is mainly from three aspects: 1, the water contained in the raw materials. 2, the resin curing process to produce the water. 3, production, transportation, storage absorb the moisture in the air.
In addition to the aqueous isocyanate (IPDI), and water the following reaction. 2R-N = C = O + H2O à R-NH-CO-NH-R + CO2
A two-step curing. Can not only enhance the film’s effective leveling time. At the same time allows the moisture in the powder coating process volatilization. 130 C / 5 min 230 C/30 min An appropriate increase in the extrusion temperature, moisture can remove part of the raw materials.
5.3 How to improve the adhesion? Substrates require preprocessing Sandblasting or sanding. The dry film thickness should be controlled between 20-60 microns Substrate should be maintained at the proper thickness
5.4 How to improve the anti-caking?
The particle size is not too small. Too thin filler is easy to absorb moisture, pigments and fillers absorb moisture very easily assembled into a group in the process of production, transportation, storage.
Add a hydrophobic fumed silica. The hydrophobic fumed silica in the surface of the pigments and fillers to form an effective package, hydrophobic groups of the silica surface water, according to the door, and water is the main cause of caking. The appropriate pigment to binder ratio (usually resin should be 45-55%)
6. Reference formula 1) SILRES 604 260 silicone resin Wacker 2) Vestagon B 1530 6.5 isocyanate Degussa 3) FERRO PK 3095 35 black pigment Ferro 4) Glimmer SC 80 mica Microfine Minerals 5) Talkum EL-10 99 talc Naintsch Minerals 6) Heucophos SAPP 75 phosphoric acid strontium Heubach Minerals 7) Benzoin 1.5 benzoin 8) Resiflow PV 88 2.5 leveling agent Worlee