What is the effect of resin amount and film thickness on anti-bending sand grain coatings for fire doors?

Summary

After the sand pattern powder coating is sprayed and cured on the fire door, the performance is poor when it is bent with a bending machine, and the coating film is prone to cracking and peeling

The anti-bending sand grain powder coating for fire doors was prepared by comprehensive analysis of polyester resin curing conditions, total resin amount, coating film thickness, adjustment of additive amount and particle size of powder products

The exterior of fire doors is decorated and protected using coatings online coatingol.

The traditional powder coating coating process for fire doors is: sheet metal shearing, folding, punching and welding → pickling and phosphating → filling → gluing → bending → powder coating → baking and curing

In this process, the door panel is first formed and then painted, which can avoid damage to the coating film during processing, and has low requirements on the flexibility of the powder coating

However, since the fire door gluing process needs to add honeycomb paper, rock wool, glass wool, aluminum silicate and other materials in the interlayer, the quality of the workpiece increases, and it is difficult to hoist

The interlayer also absorbs a lot of heat, prolonging the heating time of the workpiece and reducing the production efficiency

There are various powder coating products applied to fire doors, including flat powder, wrinkle powder, sand grain powder and transfer powder

Sand grain powder coatings form dense stress concentration points during bending due to the shape of the texture, and cracks are formed after bending, and a large number of tiny cracks cause the cracking and peeling of the coating film
.

The ideal curing condition of powder coating is that the workpiece is heated to 190-200℃ and then kept for 10-15min
.
Due to the large size of the fire door workpiece, the curing temperature and time of the workpiece are limited during continuous production operations.
After the actual measurement of multiple manufacturers, the actual curing conditions are 185 ℃, 10 minutes, and the construction temperature and time in winter are lower, and the curing conditions are 180 ℃, 8 minutes.

.

In this paper, starting from the above two difficulties, the sand grain powder coating with high flexibility is produced.
One is that the 0T bending does not crack (the coating is on the outside of the curved surface, folded in half at 180°)
.

Second, it is suitable for harsh curing conditions.
The curing conditions are at 180 ° C and 8 minutes to meet most user applications
.
In this paper, polyester resin is used as the base material, and the anti-bending sand grain powder coating with stable quality is obtained through formula adjustment and production process control
.

1 Experimental part

1.
1 Main raw materials

Polyester resin (SJ4#E, SJ4700, SJ4800): industrial grade, Anhui Shenjian; curing agent (TGIC): industrial grade, Anshan Runde; leveling agent (KC758): industrial grade, Anhui Jingcheng Chemical; barium sulfate (precipitated barium sulfate): industrial grade, Jiaxin Chemical; wollastonite (800 mesh): industrial grade, Shanghai Huichao; sand texture agent (816): industrial grade, Hangzhou Zhongshun; organic bentonite: industrial grade, Anji; flexible Agent (F-105): industrial grade, Suzhou Paterna
.

1.
2 Experimental equipment and instruments

Electronic balance (TC3K): Changshu Shuangjie Testing Instrument Factory; Twin Screw Extruder (SFJ-32N): Yantai Yuanfeng Machinery Co.
, Ltd.
; Multifunctional Crusher (YB-700): Yongkang Sufeng Industry and Trade Co.
, Ltd.
; High Voltage Electrostatic Spray (CQ-SJ5): Shenzhen Chengqixin Technology Co.
, Ltd.
; Electric Blast Dryer (WGL-125B): Tianjin Test Instrument Co.
, Ltd.
; Laser Particle Size Distribution Analyzer (BT-9300H): Dandong Baite Instrument Co.
, Ltd.
; Coating Impactor (QCJ): Tianjin Hongjuli Testing Equipment Factory; Coating Crossing Instrument (QFH): Tianjin Jingke Material Testing Machine Factory; T-Bending Testing Machine (QZW) : Jingke; Cupping Tester: Germany BYK; Humidity and Heat Resistance Test Chamber: British Sheen; Salt Spray Test Chamber (F-120S): Dongguan Jingzhuo Instrument Co.
, Ltd.
; Film Thickness Tester (4500B): Nix
.

1.
3 Reference formula of anti-bending sand grain powder coating

According to the customer's construction environment and curing conditions, determine the reference formula of anti-bending sand grain powder coating, as shown in Table 1
.

1.
4 Preparation of powder coatings

Weigh the raw materials according to the formula in Table 1, melt and extrude them in the extruder after fully pre-mixing, the extruder speed is 50Hz, the temperature in the first zone is 95°C, and the temperature in the second zone is 105°C.
The pulverizer is crushed, and the powder coating is obtained by hand sieving
.

1.
5 Performance Measurement

The prepared powder coatings were uniformly coated on a tinplate plate with a size of 50mm × 120mm × 0.
3mm using an electrostatic spray gun, and the film thickness was controlled at (60±10) μm.
, Take out the sample plate after holding for 8min
.

The sample is placed in a constant temperature and humidity environment of (23±2) ℃ and relative humidity of 50%±5%, and various performances are tested according to the standard
.
Referring to GB/T1732-93 "Determination of Impact Resistance of Paint Films", the test was carried out with a film impactor (QCJ)
.

Refer to GB/T13448-2006 standard part 7 bending test test method, use T-type bending testing machine to test
.
With reference to ISO8130-8: 1994 "Powder Coatings Part 8: Evaluation of the Storage Stability of Thermosetting Powder Coatings", the test was carried out using an electric heating blast drying oven (WGL-125B)
.

2 Results and discussion

2.
1 Selection of polyester resin

Polyesters used in the powder coating industry are mostly saturated types, which are synthesized from polyols and polyacids through polyesterification reaction.
There are many systems and different uses
.
From the structure of the end group, it is divided into carboxyl-terminated polyester resin and hydroxyl-terminated polyester resin
.

The acid value of the carboxyl-terminated polyester varies from 20 to 100 mgKOH/g and has different properties, and is used in different powder coatings
.

The polyester powder coating with excellent weather resistance was prepared by using triglycidyl isocyanurate (TGIC) as a curing agent for polyester with low and medium acid value (20-45 mgKOH/g) in this paper
.
However, TGIC is a restricted product and should be paid attention to in future use
.

Three polyester resins with different reaction rates were selected and cured at 180°C for 8 minutes according to the reference formula
.
The impact resistance, gelation time, 0T bending, and storage stability were tested under the same conditions, and the suitable polyester resin was selected.
The experimental results are shown in Table 2
.

It can be seen from Table 2 that the curing speed of SJ4800 is slow, the coating film does not fully react under the conditions of 180 ° C and 8 min, and the impact resistance and 0T bending are very poor
.

The curing speed of SJ4#E is relatively slow, the coating film does not fully react under the conditions of 180 ° C and 8 minutes, and the impact resistance and 0T bending are relatively poor
.

SJ4700 has a fast reaction speed and can be fully cured under the conditions of 180 ℃ and 8min, with impact resistance and 0T bending passing, but SJ4700 has high activity and poor storage stability
.

The two polyesters SJ4#E and SJ4700 were mixed and matched according to the mass ratio of 1:1, and the obtained coating film had no change in the impact resistance and 0T bending under the conditions of 180℃ and 8min
.

To sum up, it is appropriate to use polyester SJ4#E and SJ4700 (1:1) to make sand grain powder
.

2.
2 Resin amount selection

The amount of resin can be controlled by the ratio of pigment to base
.
When the amount of resin added is small, the melt viscosity of the powder coating is increased, the molecular motion speed and curing rate are reduced, and the curing time is prolonged.
The coating film is insufficiently cured under the specified conditions and has poor mechanical properties
.

Three resins with different contents were selected, and the curing reaction was carried out at 180°C and 8min according to the reference formula
.
Under the same conditions, the gloss, sand grain effect, and 0T bending tests were carried out, and the suitable amount of resin was selected.
The experimental results are shown in Table 3
.

It can be seen from Table 3 that when the resin content is 50%, the sand pattern of the sprayed board surface is rough, the gloss is low, and the 0T bends and cracks
.
When the resin content is 60%, the sprayed board surface has fine sand pattern, low gloss, and no cracking in 0T bending
.

When the resin content is 70%, the sand pattern of the sprayed board surface is smooth, the gloss is bright, and the 0T bending has no cracking
.
Under the condition that other conditions remain unchanged, with the increase of resin amount, the powder loading rate of the powder gradually increases, the lower the viscosity of the system, the worse the sand grain effect, the higher the gloss, and the loss of the frosting effect
.

The amount of resin is too low, the powder coating effect is poor, the roughness of the sand grain is stronger, and the 0T bending is easy to crack
.
A suitable amount of resin can control the material cost
.

To sum up, 60% of the resin content has the best comprehensive effect on the anti-bending sand grain powder for fire doors
.

2.
3 Coating thickness

The thickness of the coating film is an important measurement method to detect the actual application and quality of the protected workpiece, and the thickness has an important impact on the properties of the coated workpiece
.

Three samples with different spray thicknesses were selected and cured at 180°C for 8 minutes according to the reference formula
.
The hardness, sand grain effect, impact resistance, and 0T bending were tested under the same conditions to determine the appropriate film thickness.
The experimental results are shown in Table 4
.

It can be seen from Table 4 that when the thickness of the coating film is 50-70 μm, the sand pattern on the board surface is fine, the hardness and impact resistance meet the standards, and there is no cracking at 0T bending
.
When the thickness of the coating film is 70-90μm and 90-110μm, the sand pattern on the board surface is rough, the hardness and impact resistance meet the standard, and the 0T bends and cracks
.
The thicker the sand grain powder is sprayed, the rougher the surface and the worse the 0T bending effect, so the sand grain powder is not suitable for thick coating
.

In summary, the thickness of the anti-bending sand grain powder coating film should be controlled at 50-70 μm
.

2.
4 Determination of the dosage of additives

Flexibility additives have good impact resistance and tensile properties in powder products, and play a good role in regulating the adhesion and bending of products
.

Three comparisons with different dosages were selected, and the curing reaction was carried out at 180°C and 8min according to the reference formula
.
The hardness, impact resistance, sand grain effect, and 0T bending were tested under the same conditions to determine the suitable addition amount.
The experimental results are shown in Table 5
.

It can be seen from Table 5 that the impact resistance and hardness of the powder without the addition of flexibility additives can meet the requirements, and the 0T bending is slightly worse
.
The impact resistance, hardness and 0T bending of 0.
5% flexibility additive powder can meet the requirements
.

The impact resistance, hardness and 0T bending of 1.
0% flexibility additive powder can also meet the requirements
.
The tensile properties of the sand grain powder itself are very poor.
With the addition of flexibility additives, the 0T bending tensile effect begins to improve, and there is no obvious change after the addition of the amount gradually increases
.

To sum up, it is appropriate to add flexibility aids in an amount of 0.
5%
.

2.
5 Effect of particle size control

The particle size distribution of powder coatings will affect the leveling, powdering rate and recovery rate of coatings
.
Generally speaking, the smaller the particle size of the coating, the more favorable the leveling property during curing, and the smoother and smoother the appearance of the coating film
.

At the same time, the small particle size of the paint particles can also reduce the thickness of the coating, and the paint of the same quality can coat a larger area, but the particle size of the powder coating is not as small as possible
.

If the particle size of the powder is small, the chargeability will be poor, resulting in a decrease in the powder loading rate.
Therefore, it is necessary to strictly control the particle size distribution of the particles
.
Sand grain powder has more stringent requirements on particle size, which affects the sand grain effect and powder coating rate on the surface
.

Three products with different median particle sizes were selected and cured at 180°C and 8min according to the reference formula
.
The tests of impact resistance, sand grain effect, 0T bending and storage stability were carried out under the same conditions to determine the appropriate particle size.
The experimental results are shown in Table 6
.

It can be seen from Table 6 that when the particle size is 28 μm, the sand pattern is fine, the 0T bending and impact resistance meet the standard, but the storage stability is poor
.
When the particle size is 35μm, the sand grain is fine, the 0T bending and impact resistance meet the standard, and the storage stability does not change
.

When the particle size is 45 μm, the sand grain is rough, the impact resistance meets the standard, the 0T bending crack, and the storage stability does not change
.
The same product has different particle sizes, and has little change in 0T bending and impact resistance performance.
The larger the particle size, the rougher the sand grain effect, and the smaller the particle size, the finer the sand grain effect, but the storage stability of the powder gradually deteriorates
.

In summary, the particle size D50 of the powder product should be controlled at 35 μm
.

3 Product performance test results

Through the above experiments, the formula of the suitable product was determined, and the coating performance test was carried out with reference to HG/T2006-2006.
The specific performance test results are shown in Table 7
.

4 Conclusion

(1) The reaction speed of polyester resin has a great influence on the anti-bending sand grain powder coating.
It is determined by experiments that polyester m(SJ4#E):m(SJ4700)=1:1 is used in combination
.

(2) The amount of resin and the thickness of the coating film have a certain influence on the anti-bending sand grain powder coating.
It is determined by experiments that the amount of resin is 60%, and the thickness of the coating film is controlled at 50-70 μm
.

(3) The flexibility additives have a great influence on the anti-bending sand grain powder coating, and there is no obvious change after the addition of the amount is gradually increased
.
Make sure the dosage is 0.
5%
.

(4) The particle size has a certain influence on the 0T bending of the bending sand grain powder coating.
The median particle size is 35 μm, and the obtained coating film is fine and has good storage stability
.

(5) Through the selection of polyester resin, the adjustment of resin dosage, the comparison of coating film thickness, the adjustment of additive amount and the control of particle size, a fire door with complete curing at 180 ° C, 8 minutes, good 0T bending and fine sand pattern was obtained.
Coated with flex resistant sand grain powder
.