Preparation and Weathering Resistance of Fluorinated Polyester for Powder Coatings

Abstract: Fluorinated polyesters were prepared by polymerizing conventional polyesters with commercialized FEVE fluorocarbon resins

introduction

Polyester powder coating for outdoor use is a widely used thermosetting coating, which currently accounts for about 70% of the total output of powder coatings.

With the expansion of the powder coating market, people's weather resistance is getting higher and higher, such as aluminum profiles for high-rise buildings and anti-corruption of sea-crossing bridges.

FEVE fluorocarbon resin (FC) is mainly obtained by free radical block polymerization of chlorotrifluoroethylene and alkyl vinyl ether

Because the CF bond in fluorocarbon resin is extremely short, the bond energy is large, and the polarizability is low, the fluorine atoms are closely arranged around the polymer carbon chain, which has a good protective effect on the resin matrix, making FEVE fluorocarbon resin excellent.

However, the high cost of pure FEVE fluorocarbon resin and poor mechanical properties and heat resistance limit its application in some fields

In this study, a polyester resin containing fluorine in the molecular chain was prepared by vacuum polycondensation of polyester resin and FEVE fluorocarbon resin.

1.

1.
1 Main raw materials

Neopentyl Glycol (NPG), 2-Methyl-1,3 Propanediol (MPDI), Trimethylolpropane (T MP), Butyl Propylene Glycol (BEPD), 1,4 Cyclohexanedimethanol (CHMD), Ethylene glycol (EG), terephthalic acid (PTA), isophthalic acid (IPA), adipic acid (AOA), 1,4 cyclohexanedicarboxylic acid (CHDA), trimellitic anhydride (TMA), fumaric acid (FCC), monobutyltin oxide (FC4100), white powder, barium sulfate, leveling agent, benzoin, etc.
are all industrial products
.

Fluorocarbon resin: a certain type of FEVE fluorocarbon resin, the theoretical fluorine content is 23±2%, the hydroxyl value is 50±5, and the AV is 1.
0±0.
5
.

1.
2 Main test equipment

A set of 5L glass reactor, 30 twin-screw extruder and other small preparation equipment, electrostatic spraying equipment, impact tester, gloss tester, film thickness tester, artificial accelerated aging tester and other secondary performance testing equipment
.

1.
3 Fluorine-containing polyester resin synthesis process

Add polyols, polyacids, and FC4100 into a 5L spherical glass reactor according to the ratio, slowly heat up to 240-255°C and maintain, sampling and testing the acid value (AV) to reach 5-15 (unit mgKOH/g resin, the same below) When adding secondary acid IPA, ADA, CHDA, TMA and FCC, etc.
for acid deblocking
.

When the acid value of the polyester reaches 40-60, the temperature is lowered to 230-240°C and vacuum polycondensation is carried out for a period of time, so that the acid value of the polyester is maintained at about 25-35
.

Then the temperature was lowered to 200-230°C and the fluorocarbon resin was added and maintained for 0.
5-3h, then the temperature was lowered to 180-230°C and the auxiliary agent was added and maintained for 5-30min before discharging
.

Pure resins and fluororesins with effective fluorine contents of 1% and 2% were prepared by this method, designated PE PE+1%F and PE+1%F, respectively
.
 

1.
4 Preparation of powder coatings and their coatings

The powder coating was prepared according to the basic formula in Table 1.
The technological process was as follows: batching→premixing→extrusion→tabletting→crushing→sieving→product
.

Subsequently, the prepared powder coating was placed on the sample plate by electrostatic spraying, and then placed in an oven at 200° C.
for 10 minutes to cure and obtain a coating
.
The formulations of pure fluorocarbon resin and fluoropolyester in the process of preparing powder coatings are shown in Table 1
.

1.
5 Secondary performance testing of powder

Gel time (gt): Using a self-made gel time tester, take an appropriate amount of powder and place it on the surface of the copper plate at 200°C and continuously stir and stretch.
Second (s) to represent
.

The gloss of the sample was obtained by BAK gloss meter; the thickness was obtained by the Positionst DFT film thickness meter; the impact property was obtained by the impact meter of the state-owned Tianjin Instrument Testing Machine Factory; the 200 ℃ baking experiment was carried out in a binder FP oven
.

The weather resistance data of the sample is obtained by Xenon artificial accelerated aging tester.
The main technical parameters used are: light temperature 65℃, light + spray temperature: 65℃, irradiation intensity 0.
51w/m2 (340nm), relative humidity 50%, the box temperature is 38°C; the circulation mode is: continuous operation, 18min wetting, 102min drying
.

2.
Experimental results

2.
1 Acid value and viscosity data and resin color

It can be found from Table 2 that with the addition of fluorocarbon resin, the overall viscosity (η, in mPa.
s/200°C, the same below) shows an increasing trend.
This trend is in the polyester with 2% fluorine content.
It is more obvious in the system
.

The analysis thinks that it is because the hydroxyl groups on the side chain of the fluorocarbon resin participate in the reaction with the hydroxyl groups in the polyester system, which increases the chain entanglement density of the polyester system, so its viscosity increases significantly
.

Since fluorocarbon resin is copolymerized by chlorotrifluoroethylene and alkyl vinyl ether, the presence of alkyl vinyl ether will cause a small amount of oxidative degradation of polyester resin at high temperature, and the color during induction period will change from colorless and transparent to light.
Yellow to dark yellow transition
.

2.
2 Molecular weight data of fluorocarbon resin and polyester resin

Figure 1 shows the time distribution curves of different polyesters during the GPC test
.
Among them, through the data of the outflow time of different samples, it can be found that the main chain of the fluorocarbon resin has been successfully connected to the molecular chain of the polyester resin, indicating that a chemical polycondensation reaction has occurred between the two
.

Table 3 is a summary of the number-average molecular weight (Mn), weight-average molecular weight (Mw) and distribution (PDI) data of fluorocarbon resins and fluorine-containing polyesters with different fluorine contents
.

Quantitative molecular weight distribution data show that with the increase of fluorine content, its molecular weight distribution broadens, and the analysis believes that part of the vinyl segment reacts with the polyester molecular segment, resulting in a broadening of its distribution, which is consistent with the viscosity data mentioned above.
match
.
The above data show that the fluorocarbon resin was successfully grafted onto the polyester backbone
.

2.
3 Secondary properties of powder samples

Table 4 summarizes the secondary performance data of different types of polyester resins
.
Compared with pure fluorocarbon resin, the surface gloss of fluoropolyester is higher.
For example, when the effective fluorine content is 2%, the gloss of the sample can be maintained at about 70-85%;

The samples prepared from fluorine-containing polyester have excellent impact.
For example, when the effective fluorine content is 2%, the impact of the sample can reach 100++; the baking experiment shows that the gloss retention and color difference of the samples prepared from fluorine-containing polyester are comparable to those of pure polymer.
Ester resins are relatively close, showing that they have excellent heat resistance
.

The gel time of fluoropolyester is close to that of pure polyester, which makes it have excellent leveling performance compared with fluorocarbon resin
.
The above secondary properties show that the leveling, impact and baking resistance properties of fluoropolyester are close to those of conventional polyesters
.

2.
4 Artificial aging

Table 5 summarizes the gloss retention data of pure polyester and fluorine-containing resin at different stages in the artificial aging process
.
It can be observed from the table that the gloss retention of pure polyester is 55-60% at 886h, the gloss retention of polyester with 1% fluorine content is 60-65% and the gloss retention of polyester with 2% fluorine content is 65% -70%
.

It was also observed that the gloss retention of the fluorine-containing polyester resin increased significantly at 1119h, and the magnitude of the increase increased significantly with the increase of the fluorine content
.

Therefore, it can be judged that the introduction of fluorocarbon resin significantly enhances the weather resistance of pure polyester, and with the higher fluorine content, the greater the increase in weather resistance
.

3.
Experimental conclusion

The polyester resin containing fluorine in the molecular chain was successfully prepared by adding FEVE fluorocarbon resin into the resin in the synthetic stage of polyester resin
.
Experimental tests show that its leveling, impact properties and heat resistance are close to those of pure polyester resins
.

The artificial aging shows that the weather resistance of the fluoropolyester resin is significantly increased compared with the pure polyester, and the magnitude of the increase increases with the increase of the fluorine content
.
It was shown that ultra-weatherable fluoropolyester resins can be successfully prepared by this method
.