Development of Fluorocarbon-Polyester Composite Super Weather Resistant Powder Coatings

Abstract: In view of the defects of pure fluorocarbon powder coatings in adhesion and poor wetting of pigments, from the perspective of cost reduction and popularization and application, a composite method of fluorocarbon resin system and polyester resin system is adopted, and electrodeless ceramic pigments are preferred.
A cost-effective fluorocarbon-polyester composite super-weather-resistant powder coating was developed
.

introduction 

At the end of the 20th century, Japan's Daikin developed a thermosetting powder fluorine coating based on fluoroolefin-vinyl ether, and built an industrialized device

Compared with other common powder coatings (such as epoxy, polyester, etc.
), thermosetting fluorocarbon powder coatings have similar melting temperature, baking temperature and coating process, but their properties (such as chemical resistance and weather resistance) are similar to those of common epoxy coatings.
, polyester resin powder coating several times
.

According to the information, the accelerated weather resistance test for 300h, the gloss retention rate is 82%~90%, and almost no aging occurs; the salt spray test remains unchanged for 4000h, and the durability is expected to reach more than 30a, far exceeding the existing weather resistance powder coatings

However, the inherent deficiencies of pure fluorocarbon system powder coatings in terms of adhesion, pigment and filler wettability, and its high material cost make the market application and promotion of pure fluorocarbon system powder coatings slow
.

Aiming at the disadvantages of pure fluorocarbon system and from the perspective of cost reduction, this paper adopts a composite method of fluorocarbon system and polyester system to study a more cost-effective super weather-resistant powder coating

The basic formula of fluorocarbon-polyester composite super weather-resistant powder coating is shown in Table 1
.

1.

Test equipment: electronic balance, extruder, pulverizer, electrostatic spray gun, oven, salt spray test chamber, thickness gauge, artificial processing aging test chamber, impact tester,

The formula weighs the raw materials, mixes them evenly, extrudes them with a twin-screw extruder, crushes them, crushes them with a high-speed pulverizer, and passes them through a 180-mesh standard sieve.
various tests
.

2.
Results and discussion

There are two main reasons for the super weather resistance of fluorocarbon coatings: for the organic polymer used outdoors, when it absorbs a photon with energy greater than its chemical bond energy, it can cause bond breakage, so that the compound is damaged.
to destruction
.

The bond length of the CF bond is 0.

From a principle point of view, the composite ratio of fluorocarbon resin and polyester resin is the key to affecting its weather resistance.
Selecting the appropriate composite ratio makes the fluorocarbon system and polyester system complement each other and improve the lubrication of the fluorocarbon system.
Wetting properties, improve adhesion, improve its mechanical properties, and make up for the weather resistance of pure polyester systems
.

2.
1 Selection of fluorocarbon resin

The weather resistance of the fluorocarbon-polyester composite system is mainly determined by the fluorocarbon resin, the isocyanate curing system, and the types and proportions of pigments and fillers
.

However, considering that the research and development cycle is short, and the test cycle for the aging resistance and salt spray performance of fluorocarbons is very long, a complete salt spray test takes 5-6 months
.

Therefore, it is unrealistic to test the long-term performance of the coating film and then feed it back to the formulation design based on the test results
.

Compared with other coatings, in addition to excellent weather resistance, fluorocarbon coatings have chemical resistance that is unmatched by conventional coatings; and the reason why fluorocarbon resins have excellent performance in the above two aspects is due to the high strength of fluorocarbon bonds.
energy and shielding effect, so we believe that fluorocarbon weatherability and chemical resistance correspond to each other
.

Therefore, by comparing the alkali resistance of different types of fluorocarbon resins and isocyanate cured coating films, the film-forming system with better expected weather resistance was screened out
.

The 2# and 5# experiments compared the performance of the two fluorocarbon resins.
The technical parameters of the two resins are very close to each other, but the results obtained are somewhat different
.

The reason may be that in the molecular structure of the fluorocarbon resin, the F atom and other group atoms on the main chain are not arranged in ABAB orderly, or the F atom is not connected to the CC main chain, and the F atom cannot play a "shielding effect on the CC chain in the molecule".
”, so that the alkali resistance of the configured coating is also reduced
.

Through the above comparison, fluorocarbon resin 1 with better alkali resistance was selected
.

Experiments 1#-4# mainly compared three types of isocyanate curing agents, and formula 1# selected hexamethylene isocyanate (HDI)
.

The test results show that its impact performance is excellent, but its alkali resistance is the worst among the four groups of formulas
.
Formulation 2# uses caprolactam-terminated dimer isophorone diisocyanate (IPDI), which is the best in the comparative test in terms of impact performance and alkali resistance
.

Both formulations 3# and 4# use trimeric IPDI.
Considering its self-sealing structure, formulation 4# is added with an organotin catalyst (DBTL), and the two formulations have obtained similar performance, and the overall performance is average
.

Compared with the outer closed IPDI, the inner closed IPDI has a higher softening point because it is a trimer, and has a large gap with the fluorocarbon resin
.

The importance of solving the compatibility of fluorocarbon systems has been discussed above.
The large difference in softening point between the two will inevitably lead to a weakening of the compatibility, thereby affecting the performance of the coating
.
It is also understandable that the coating performance does not improve after adding the catalyst
.

In summary, the externally closed IPDI with good compatibility and chemical resistance was selected as the curing agent for the fluorocarbon system
.

2.
2 Selection of different ratios of fluorocarbon resin and polyester

The wetting performance of fluorocarbon resin is extremely poor, and the film forming effect of the sample is not good.
Generally speaking, it is believed that the viscosity of the resin is too large
.

However, the viscosity value of fluorocarbon resin measured by rotational viscometer is small, only half of the viscosity of commonly used polyester resin
.
The reason is that the poor wetting performance of fluorocarbon resin is determined by its own structure
.

Due to the weak polarity of fluorocarbon resins, the wettability to other components is poor
.
In this way, on the one hand, the resin cannot be well combined with the curing agent, and on the other hand, the film-forming substance cannot wrap the pigment and filler well, resulting in a significant decrease in the mechanical and weather resistance of the coating film
.

Although carboxyl groups are generally introduced during resin synthesis to improve the wettability of resins to pigments, excessive carboxyl groups will reduce the degree of crosslinking of the coating film
.

Therefore, it is a preferred idea to improve the poor wettability of the pure fluorocarbon system through the compounding of the fluorocarbon resin system and the polyester resin system
.
The ratio of fluorocarbon resin system and polyester resin system is the key to the success of compounding
.

The following discusses the various mechanical properties of the coating under different compound ratios of fluorocarbon resin and polyester resin, as well as the effect of different ratios on the artificial accelerated aging of the coating, so as to screen out suitable fluorocarbon resin and The composite ratio of polyester resin is used to prepare fluorocarbon-polyester composite super weather-resistant powder coating
.

It is not difficult to see from the experimental data in Table 3 that the addition of polyester resin significantly improves the adhesion defects and mechanical properties of the pure fluorocarbon system
.
At the same time, the addition of fluorocarbon resin significantly improves the weather resistance and alkali resistance of the polyester system
.

In the fluorocarbon-polyester composite system, with the change of the ratio of fluorocarbon/polyester, its weather resistance is proportional to the proportion of fluorocarbon within a certain range.
When the proportion of fluorocarbon increases to a certain limit , its weather resistance is no longer improved
.
In the composite system, when the composite ratio of fluorocarbon/polyester is 1:1, its weather resistance is better
.

2.
3 Influence of pigments

Due to the poor wetting performance of the fluorocarbon system, the selection of pigments is very harsh.
The first condition is that the oil absorption cannot be too large, and secondly, there are certain requirements for the particle size of the pigment
.

At the same time, in the field of powder coatings, the aging resistance of the dark series of coatings is generally different from that of the light-colored series, which has a great relationship with the choice of pigments
.
Therefore, in the study of ultra-weather-resistant powder coatings, the influence of pigments must be taken into account
.

The experimental results in Table 4 show that under the same conditions, the gloss retention and color retention of inorganic ceramic pigments are significantly better than that of common inorganic pigments, organic pigments and aluminum pigments
.
Inorganic ceramic pigments are not affected by sunlight, chemicals and thermal attack
.

They are also compatible with fluorocarbon-polyester composite systems, and their great inertness prevents migration from the coating
.
Therefore, in the fluorocarbon-polyester composite system, inorganic ceramic pigments are preferred for the selection of pigments
.

2.
4 Influence of coating conditions

Fluorocarbon-polyester composite powder coatings have higher requirements on the pretreatment of sheets, and are generally used for aluminum spraying
.
Before construction, it must be ensured that the aluminum plate is cleaned and passivated
.

The spraying process of fluorocarbon powder coating is similar to that of general powder coating.
Electrostatic spraying can be used.
The spraying thickness is controlled at 50-70um, and it can be cured by baking at 200℃/15min
.

3.
Conclusion

Compared with other coatings, fluorocarbon coatings are not only excellent in weather resistance, but also in chemical resistance unmatched by conventional coatings; and the reason why fluorocarbon resins have excellent performance in the above two aspects is due to the high strength of fluorocarbon bonds.
energy and shielding effect, so we believe that fluorocarbon weatherability and chemical resistance correspond to each other
.

At the same time, the introduction of the polyester resin system solves the wettability defect existing in the pure fluorocarbon system and improves the mechanical properties of the coating
.

Through the above research, it is found that the combination of the fluorocarbon resin system and the polyester resin system can not only overcome the inherent deficiencies of the pure fluorocarbon system in adhesion and pigment wetting, but also improve the weather resistance of the pure polyester system.
A cost-effective ultra-weather-resistant powder coating
.

At the same time, through the screening and research of pigments, the formula was optimized, and the inorganic ceramic pigments were selected as the pigments of the fluorocarbon-polyester composite system to improve its weather resistance
.