Development and Application of Thermoplastic Powder Coatings

Since the oil crisis in the 1970s, powder coatings have developed rapidly with the characteristics of resource conservation, environmental protection, pollution-free and easy automated production.

In the 1940s, due to the development of the petrochemical industry and other industries, the output of polyethylene, polyvinyl chloride, polyamide resin and other resins increased rapidly, thus starting the research on thermoplastic powder coatings.

Thermoplastic powder coatings in China started relatively late.

01 Polyethylene (PE), Polypropylene (PP) powder coating

As one of the first materials used in thermoplastic powder coatings, polyethylene and polypropylene are the two most important thermoplastic polymers in the last century

Because the molecular chains of polyethylene and polypropylene are carbon-carbon bonds, and both have the non-polar characteristics of olefins, both polyethylene and polypropylene powders have good chemical resistance, so they are widely used in the field of anti-corrosion.

Polyethylene resin, as the most widely used thermoplastic powder coating with the largest output, has the following advantages:

a.

b.

c.
Excellent tensile strength, flexibility and impact resistance;

d.
It is resistant to low temperature and can be kept at minus 40 ℃ for 400 hours without cracking;

e.
The raw materials are relatively cheap, non-toxic and environmentally friendly
.

Similarly, due to the nature of the substrate polyethylene, polyethylene powder coatings also have some unavoidable disadvantages:

a.
The hardness, wear resistance and mechanical strength of the coating film are relatively poor;

b.
The adhesion of the coating film is poor, and the substrate generally needs to be strictly treated;

c.
Poor weather resistance, easy to produce stress cracking after being irradiated by ultraviolet rays;

d.
Not resistant to high temperature and poor heat and humidity resistance
.

Table 1.
Modified resins and modified characteristics of polyethylene

02 Polyvinyl chloride (PVC) powder coating

Polyvinyl chloride is an amorphous polymer containing a small amount of incomplete crystals.
Most PVC resin products have molecular weights between 50,000 and 120,000.
Although high molecular weight PVC resins have better physical properties, because low molecular weight PVC resins have Lower melt viscosity and softening temperature, so it is more suitable as a thermoplastic powder coating material than high molecular weight PVC resin
.

PVC itself is a rigid material and cannot be used as a raw material for powder coatings alone.
When making coatings, a certain amount of plasticizer should be added to adjust the flexibility of PVC.
At the same time, adding plasticizers also reduces the tensile strength, modulus and Hardness, by selecting the appropriate type and amount of plasticizer, the desired balance between flexibility and hardness of the material can be achieved
.
For a complete PVC powder coating formulation, stabilizer is also an essential part.
In order to solve the thermal stability of PVC, mixed salts of calcium and zinc with good thermal stability, barium and cadmium soaps have been developed.
Tin mercaptide, dibutyltin derivatives, epoxy compounds, etc.
Although lead stabilizers have excellent thermal stability, they are withdrawn from the market due to environmental protection reasons
.

At present, the products that use the most PVC powder coatings are various living utensils and dishwasher racks.
PVC products have good washing resistance and food contamination resistance.
The dish rack makes no noise when placing cutlery
.
PVC powder coatings can be constructed by fluidized bed or electrostatic spraying, but the required powder particle size is different
.
At the same time, it should be pointed out that the PVC powder coating will emit a pungent odor during dip coating, which is harmful to the human body.
It has been banned abroad
.

Polyvinyl chloride powder coating has the following advantages:

a.
Low price of raw materials;

b.
Good pollution resistance, washing resistance and corrosion resistance;

c.
High mechanical strength and good electrical insulation performance;

PVC powder coatings also have the following disadvantages:

a.
The temperature difference between the melting temperature and the decomposition temperature of PVC resin is small, and the temperature needs to be strictly controlled during the coating process to prevent the coating from decomposing during the coating process
.

b.
The coating film is not resistant to aromatic hydrocarbons, esters, ketones and chlorinated solvents
.

03 Polyamide (Nylon) Powder Coating

Polyamide resin is a widely used thermoplastic resin, commonly known as nylon
.
Nylon has good comprehensive properties, high hardness, especially outstanding wear resistance.
Nylon coating has small dynamic and static friction coefficients and has lubricating ability.
Therefore, it is used in textile machinery bearings, gears and valves.
Nylon powder coating has Good lubricity, low operating noise, good flexibility, excellent adhesion and chemical resistance, solvent resistance make it an ideal wear-resistant lubricating coating to replace copper, aluminum, cadmium, steel, etc.
, nylon The density of the coating film is only 1/7 of that of copper, and the wear resistance is 8 times that of copper
.

Nylon powder coatings are non-toxic, odorless and tasteless, combined with the fact that they are not attacked by fungi and do not promote bacterial growth, making them successful in coating machine parts and piping systems in the food processing industry, or for coating and food products.
surfaces that can be directly contacted
.
Because of its good water resistance and salt water resistance, it is also often used in the coating of washing machine parts
.

An important area of ​​application for nylon powder coatings is the coating of various types of handles, not only because of their important characteristics such as abrasion and scratch resistance, but also because their low thermal conductivity makes the handle feel soft, making these The material is ideal for painting tool handles, door handles and steering wheels
.

Compared with other coatings, nylon coating has poor chemical resistance and is not suitable for use in chemical environments such as acid and alkali
.
Therefore, some epoxy resins are generally added as modifiers, which can not only improve the corrosion resistance of the nylon coating, but also improve the bonding strength between the coating and the metal substrate
.
Nylon powder has high water absorption and is easy to get damp during construction and storage, so it needs to be stored under sealed conditions, and it is not suitable for long-term use under humid and hot conditions
.
Another point is that the plasticization time of nylon powder is relatively short, and there is even a coating film that can achieve the desired effect without plasticization.
This is a special point of nylon powder
.

04 Polyvinylidene fluoride (PVDF) powder coating

The most representative coating of weather resistance in thermoplastic powder coatings is polyvinylidene fluoride powder coatings
.

As the most representative weather-resistant vinyl polymer, polyvinylidene fluoride (PVDF) is characterized by good mechanical and impact properties, excellent abrasion resistance and excellent flexibility and hardness, and can resist acid, alkali , most corrosive chemicals such as strong oxidants, and it is insoluble in chemical solvents commonly used in the coating industry, which are due to the FC bonds contained in PVDF
.
At the same time, PVDF also meets the requirements of the FDA, can be used in food processing, and can be in contact with food
.

Because of its high melt viscosity, PVDF is easy to cause pinholes and poor metal adhesion during thin coating and film formation.
The material price is too high, so in most cases, it will not be used as the base material of powder coatings alone.
Generally, 30 % acrylic resin to improve these properties, too high acrylic resin content will affect the weather resistance of the coating film
.

The gloss of PVDF coating is relatively low, generally around 30±5%, which limits its application in surface decoration
.
It is currently mainly used as an architectural coating for large buildings, applied to roof panels, walls and extruded aluminum window frames, with excellent weather resistance
.

references

[1] Zhang Junzhi, Powder Coating and Coating Technology, Chemical Industry Press, 2008.
2

[2] Nan Renzhi, Powder Coating and Coating Technology, Chemical Industry Press, 2000.
5

[3] Zhang Huadong, Thermoplastic Powder Coatings, Chemical Industry Press, 2000.
5

Source: 2018 Powder Coatings and Coating Industry Annual Conference   

Author: Zhu Jinxiao, Lu Bin Huangshan Huajia Surface Technology Co.
, Ltd.