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Abstract: By compounding acrylic resins with different epoxy equivalents, a transparent powder coating that can be cured at low temperature at 150~160℃ was synthesized
By adding a bisphenol A epoxy resin as a curing accelerator to increase the cross-linking density of the system, thereby increasing the density of the coating, and alleviating the coating's corrosion resistance due to insufficient cross-linking density under low-temperature curing The problem of falling
By adding hydrophobic silica, the copper-accelerated acetic acid salt spray resistance of the coating is increased; the effect of the type and amount of ultraviolet absorbers and hindered amines on the weather resistance of the coating is studied
The product was characterized by means of DSC differential calorimetry and infrared spectroscopy
In recent years, the government and society have paid more and more attention to environmental, safety, and energy issues.
Generally, the curing temperature of conventional overcoat powder coatings is 175-200°C, the baking temperature is higher, and the energy consumption is higher in practical applications
Compared with curing at 175-200℃, powder coatings cured at 150~160℃ have obvious energy-saving advantages.
This article introduces a powder coating product that can be used for auto parts, high-end hardware materials, and sanitary products, and a low-temperature curing transparent powder coating made of epoxy-based acrylic resin, which meets the special requirements for decoration and protection of automobiles and buildings.
Experimental part
01 raw materials
High epoxy equivalent acrylic resin: industrial grade, domestically made;
Low epoxy equivalent GMA resin: industrial grade, self-made;
Dodecanedioic acid: industrial grade, imported;
Fumed silica R972: industrial grade, Degussa;
Ultraviolet absorber Tinuvin 327, light stabilizer Tinuvin783: industrial grade, former Ciba Fine Chemical Company;
Bisphenol A epoxy polyester: industrial grade, domestically made;
General additives for powder coatings (leveling agent, benzoin): industrial grade, imported
.
02 Main equipment
Ф30 type twin-screw extruder, universal grinder medicine, ACM milling system, a small electrostatic spraying equipment, impact tester, a xenon lamp artificial accelerated aging test, salt spray chamber, a laser particle size analyzer and the like
.
03 Powder coating and coating preparation process
Weigh the components according to the designed formula, and after the pulverizer is fully premixed, the twin-screw extruder is used for melt extrusion at the set temperature.
The extruded sheet is cooled, pulverized, sieved, electrostatic sprayed, and thermally cured.
Film, and finally the performance test of the coating film
.
04 performance test
The powder coating performance test is shown in Table 1
.
Results and discussion
Generally, the powder coating from being applied to the workpiece becomes a coating, and finally cured to form a coating film.
It needs to go through three stages: the first stage is to coalesce individual powder particles into a continuous uneven film;
In the second stage, flow from a continuous uneven surface to a relatively smooth and even surface, that is, the leveling process;
In the third stage, the viscosity of the molten coating increases continuously through the cross-linking reaction, and finally solidifies to form a coating film
.
In the leveling stage, the resistance that affects the flow is the viscosity of the molten coating.
At this stage, as the temperature increases, the viscosity of the coating becomes smaller and smaller, and when the time reaches the solidification of the coating liquid, the viscosity of the molten coating suddenly increases.
.
The heat dissipation of the aluminum wheel workpiece is fast, and the surface temperature is often difficult to maintain.
During the solidification process, after the powder is melted, because the temperature is low, it will gel and solidify without sufficient leveling.
This will result in poor surface flatness of the coating film and easy to form orange peel.
, Poor appearance and other shortcomings
.
01 Determination of the synthesis scheme
By selecting high and low epoxy equivalent acrylic resins to cure powder coatings at a low temperature of 155°C, conventional powder coatings are cured at a film temperature of about 175~200°C, with high epoxy equivalent acrylic resin as the main resin, and low epoxy equivalent.
Glycidyl acrylate (GMA) resin improves the crosslinking density of the system;
In addition, a bisphenol A epoxy resin is selected as a curing accelerator to further increase the density of the coating, and solve the lack of corrosion resistance and weather resistance caused by insufficient cross-linking density due to low-temperature curing of the coating
.
02 Selection of high epoxy equivalent acrylic resin
Acrylic transparent powder coating is suitable for spraying of high-end automobile wheels, high-end hardware, high-end fitness equipment, etc.
Because of its high requirements for decoration, weather resistance, and secondary processing performance, the requirements for acrylic resin are very high.
The selected acrylic resin should have the characteristics of high leveling, high transparency, and high weather resistance
.
At present, the resins that can be used for this kind of powder coatings are mainly imported resins, and only a few domestic manufacturers have the production capacity of acrylic resins for the preparation of transparent powder coatings
.
In this study, several domestic and foreign acrylic resins with epoxy equivalent weight of 500~700g/Eq, softening point of 80~100℃, and glass transition temperature of 40~70℃ were selected for comparison.
Dodecanedioic acid (DDDA) was used as the curing agent.
Add an appropriate amount of leveling agent and cure at 150°C for 20 minutes.
The comparison results are shown in Table 2
.
It can be seen from Table 2 that the resin A imported from Japan has better copper-accelerated acetate spray resistance and weather resistance, but the leveling and impact resistance are slightly worse, while the resin B imported from the United States has leveling and impact resistance.
Better
.
However, the weather resistance and copper-accelerated acetic acid salt spray performance are slightly worse.
This is because the imported resin A contains relatively more GMA monomer content and relatively less styrene (St) monomer content;
The performance of domestic resin D is roughly equivalent to that of imported resin B, which can meet product requirements and has a lower price.
This study uses domestic resin D for research
.
03 Selection and synthesis of acrylic resin with low epoxy equivalent
In this study, a low epoxy equivalent GMA resin with an epoxy equivalent of 280~350g/Eq was used as the second copolymer.
The epoxy equivalent is small, so the crosslinking density is large, which can increase the density of the coating and relieve the coating due to low temperature curing.
Insufficient and reduced crosslink density
.
The polymerization process of self-made resin a is as follows: 100 parts of raw material monomers (raw material monomers are shown in Table 3) and 2 parts of dibenzoyl peroxide (BPO) initiator are mixed uniformly according to the ratio, and then added to the drop funnel
.
Add 300 parts of toluene solvent into a three-necked flask equipped with a stirrer, a reflux condenser and a dropping funnel, stir, and raise the temperature to 120°C.
After the temperature is constant, drop the raw monomers dissolved in the initiator at a constant rate
.
After 3 hours, the polymerization reaction is finished.
After the reaction solution is cooled to room temperature, the synthesis device is replaced with a vacuum distillation device, and vacuum distillation is performed until the toluene solvent is completely removed, and the GMA resin is obtained
.
In the experiment, domestic acrylic resin D with high epoxy equivalent was used as the main resin, and low epoxy equivalent GMA resin with epoxy equivalent of 280~350g/Eq was selected: homemade resin a, domestic resin b, domestic resin c were compared with high and low The mass ratio of epoxy equivalent resin is 80:20
.
The total addition amount of the two is 78% (based on the total mass of the formula, the same below), and 20% DDDA curing agent, 1.
5% leveling agent, and 0.
5% benzoin are added to prepare low-temperature curing transparent powder coatings.
Compare different low epoxy resins Equivalent GMA resin coating performance
.
The results are shown in Table 4
.
Table 4 shows that the self-made low epoxy equivalent GMA resin in this study has relatively high content of styrene and GMA monomer, which improves the copper-accelerated acetate spray resistance of the coating while retaining the impact resistance.
The overall performance is better than the other two resins
.
04 Study on the ratio of high and low epoxy equivalent acrylic resin
When two acrylic resins with different epoxy equivalents are used in the same formulation, the difference in the ratio of the two acrylic resins will have a significant impact on the weather resistance and recoating performance of the acrylic transparent powder coating
.
In the experiment, high epoxy equivalent resin D and self-made low epoxy equivalent GMA resin a were used for compounding, and the best combination curing ratio was studied.
The experimental results are shown in Table 5
.
It can be seen from Table 5 that the higher the content of the low epoxy equivalent resin a, the better the resistance to copper-accelerated acetate spray, but the poorer recoatability and leveling properties
.
The results show that when the mass ratio of resin D to resin a is 60:40~80:20, the overall performance is better.
Experiments have confirmed that resin D: resin a=70:30, resin D+resin a=77%, DDDA curing agent Is 22%
.
05 Curing promotion effect of bisphenol A epoxy polyester
In order to further increase the crosslinking density of the coating system and increase the density of the coating, this study adds a bisphenol A epoxy polyester as a curing accelerator to promote the curing of the system.
The epoxy polyester has an epoxy equivalent of 700 ~850g/Eq, the effect of its addition on the performance of the coating was investigated in experiments, and the results are shown in Table 6
.
It can be seen from Table 6 that when the addition amount of bisphenol A epoxy polyester is 0.
5% to 5%, the copper-accelerated acetic acid salt spray resistance of the coating is improved, and the weather resistance does not change significantly; When it reaches 10%, the weather resistance of the coating is affected
.
Therefore, the addition amount of 1% to 5% is more appropriate
.
06 Research on leveling agent
By examining the effects and effects of different leveling agents on the transparency and surface leveling of the system, it provides a coating system with excellent leveling properties and fullness and clarity of the coating without affecting the recoating performance
.
The choice of leveling agent has an impact on the performance of all aspects of the product.
There are 3 types of leveling agents that can be used:
(1) 100% high viscous liquid leveling agent.
This leveling agent addition method generally uses acrylic resin as the base material for dispersion, the production process is relatively cumbersome, and it has a certain impact on the transparency of the coating;
(2)60%~70%, , ;
(3)100%,,
。,7
。
7,CE,C,,5%10%.
,,E,
。
07
150~160℃,,, , ,8
。
,2:1,1%~1.
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。
08 SiO2
, 150~160℃,(180~200℃)
。
,,9
。
9,,,1%,2%,1%
。
09
(DSC),
。1
。
1DSC,、、AKZO 165℃、182℃、184℃
。
20℃, ,150~160℃
。
21730cm-1,、AKZO, —COO—
。
,,,
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,GMA
。3,Wb=1.
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5~2mm
。
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。
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。
In this study, GMA resin with low epoxy equivalent was added, and its glass transition temperature was relatively low compared with high epoxy equivalent acrylic resin, so the storage stability of powder coatings was relatively poor
.
Hydrophobic silica has hydrophobic properties and the ability to increase powder flow.
In this study, the addition of hydrophobic silica has improved the storage stability of powder coatings to a certain extent
.
Color and transparency
Transparency directly affects the appearance of the workpiece.
If the transparency is high, the color of the paint and the surface of the finished car will be full and bright.
Therefore, transparency is an important performance of the gloss powder coating for high-end automotive aluminum wheels.
This research has also done a lot of this performance.
The results of the experiment are shown in Figure 5
.
When the leveling agent used in the formula is different, the transparency is obviously different.
When the resin is selected differently, it also has a significant impact on the transparency, so the formula with good transparency should be selected
.
#Conclusion
Generally, the curing temperature of conventional varnish powder coatings is 175~200℃, the baking temperature is higher, and the energy consumption is higher in practical applications.
In addition, automotive aluminum wheels require sufficient toughness to ensure impact resistance, and the curing temperature is too high It will also reduce the toughness of cast aluminum wheels
.
Reducing the curing temperature of powder coatings can increase the toughness of aluminum wheels, reduce the production energy consumption of enterprises, and achieve the purpose of energy saving and environmental protection
.
At present, the low-temperature curing aluminum wheel base powder has been widely used in the market, but there is no related product for the matching low-temperature curing over-glazing transparent powder coating
.
In this paper, epoxy-based acrylic resin is used to prepare low-temperature curing transparent powder coating products, which can be used in conjunction with low-temperature curing aluminum wheel base powder products to reduce the production energy consumption of enterprises and achieve the purpose of energy saving and environmental protection
.
Low-temperature curing powder coatings are not only the requirements of environmental protection and human health, but also energy-saving and consumption reduction and carbon emission reduction are inevitable requirements, which is an inevitable trend in the development of powder coatings
.