Development and application of long-lasting, easy-to-wipe and anti-graffiti powder coatings

1 Introduction

The word graffiti is of Italian origin and means "to draw a line on a hard surface" and has been around for thousands of years as an art form of expression

The so-called anti-graffiti and easy-to-wipe powder coatings are electrostatically sprayed powder coatings on the workpiece.

2 Application areas and standards, test methods

Easy to wipe, anti-graffiti powder coatings are widely used in municipal public facilities, furniture, hospitals, electronics, instrumentation, hardware decoration, automobiles and other fields, such as: car wheels, range hoods, outdoor guardrails, household aluminum furniture, schools, hospitals Public tables and chairs,

In 2000, the American Society for Materials issued a standard for the test method of graffiti resistance, ASTM D6578-00 Standard for determination of Graffiti Resistance.

In 2011, the Ministry of Housing and Urban-Rural Development issued a product standard for the construction engineering industry, JG/T 304-2011 "Anti-Graffiti and Anti-Stick Coatings for Construction".

Table 1 JG/T 304-2011 Graffiti Resistance Rating

3 Easy to erase, anti-graffiti reaction mechanism

The main difference between the anti-graffiti powder coating and other products is that the coated surface can form a dense low surface energy structure, which makes it difficult for pollutants to adhere to the surface of the paint film and penetrate into it, so as to obtain a better anti-graffiti effect.

Easy-to-clean, anti-graffiti powder coatings are classified according to protective properties as sacrificial anti-graffiti coatings, semi-permanent anti-graffiti coatings and permanent anti-graffiti coatings
.
Sacrificial, semi-permanent anti-graffiti coatings typically consist of wax, which is removed along with the contaminants, and the biggest problem with this method is that the cleaning effect deteriorates with the number of removals
.
Permanent, long-lasting powder coating with a long coating life, the use of a safe neutral cleaning agent can remove graffiti, stickers or other contaminants on the surface of the coating many times without losing the performance of the coating itself
.

Permanent, easy-to-erasure, anti-graffiti powder coatings have the following common characteristics:

(1) The surface energy of the coating is low, which is hydrophobic and oleophobic, and the graffiti is difficult to adhere to;

(2) It is easy to remove the pollutants adhering to the surface;

(3) The coating has certain anti-corrosion and anti-aging properties and can be used outdoors;

4 Experimental part

Experimental raw materials and tests

Experimental extruder: 35 machines, screw arrangement: medium arrangement, extrusion temperature, set according to different systems; testing equipment: ESPEC brand low temperature oven (0-250 ℃), Shanghai Institute of Chemical Industry, resin A: hydroxyl value 35-45mgKOH/g, resin B: hydroxyl value 75-85mgKOH/g, resin C: hydroxyl value 120-150mgKOH/g, resin D: hydroxyl value 250-300mgKOH/g, curing agent is Degussa's B1530, NCO content 14.
8-15.
7%, leveling agent, benzoin, filler, titanium dioxide, etc.
are commonly used mature materials in the market.
The test results are as follows:

Table 2 Comparison of film-forming properties of resins with different hydroxyl values

One of the outstanding advantages of polyurethane powder coatings is that they are super-leveling.
The highly smooth and flat surface prevents the penetration of contaminants, and the residual spray paint or similar paste on the surface of the coating film can be easily removed with cleaning agents
.
Obviously to have such properties, the coating requires high smoothness and high crosslink density
.

It can be seen from the above results that the low hydroxyl value resin has better mechanical properties, but poor anti-graffiti performance.
With the increase of hydroxyl value, the anti-graffiti performance gradually improves.
It can be easily removed for ink and oily markers, but the mechanical properties are deteriorated, and the impact can only reach 30cm positive punch, which is caused by the high crosslinking density of the high hydroxyl value resin film-forming material and the high hardness of the coating film
.

The formula is designed by mixing and matching resin A with a hydroxyl value of 35 mgKOH/g and resin D with a hydroxyl value of 300 mgKOH/g.
The film-forming properties are compared as follows:

Table 3 Comparison of film-forming properties of high/low hydroxyl value resins

It can be seen from the above results that with the increase of the amount of high hydroxyl value resin, the gloss of the coating film shows a downward trend of high gloss-semi-gloss-matt.
When the dosage is higher than 3:1, the mechanical properties decrease, and the anti-graffiti performance does not As the crosslinking density increases, this is because there are many microscopic irregularities on the surface of the low-light coating film, pollutants can penetrate into the coating film, and the anti-graffiti performance is reduced.
From the test results, the best ratio A:D =4:1
.

Application of anti-graffiti additives

When the polyurethane system is matched with high and low hydroxyl values, and different contents of anti-graffiti additives are added, the test results of the obtained coating film are as follows:

Table 4 Performance comparison of different anti-graffiti additives

From the experimental results, after the addition of anti-graffiti additives reaches 1%, the repetition times of the oil-resistant marker are all more than 50 times, and the coating film has excellent long-term and easy-to-clean effect
.
However, if the addition amount exceeds 1%, the mechanical properties of the coating will be significantly reduced, and the impact performance will be deteriorated.
The best recommended addition amount of anti-graffiti additives is 1%
.

The application of anti-graffiti additives comes from the functions of natural organisms, such as the lotus leaf effect, which is mainly due to the special morphology and hydrophobicity of the lotus leaf surface.
The hydrophobicity makes the lotus leaf hydrophobic, so water droplets roll off its surface and carry dirt down
.

Figure 1 Young's formula

Figure 2 Contact angle and hydrophobicity and hydrophilicity

It can be seen from Young's formula that when the contact angle is greater than 90°, the coating film exhibits good hydrophobicity.
The application of anti-graffiti additives in powder coatings can reduce the surface tension of the coating, and its contact angle with water droplets is about 160° °, the surface of the coating film is smooth, and can endow the coating with anti-graffiti properties
.
When oil-based pen or spray paint graffiti on the surface of the coating film, the writing shrinks and can be easily erased by a lint-free cotton cloth
.

Figure 4 The graffiti resistance test repeated 20 times by the oil-based pen

Figure 5 Wipe with oily pen

Figure 6 The iron red on the coating is rinsed with water

Figure 7 Coating oil stain comparison

Note: The picture on the right is an anti-graffiti sample.
Compared with the ordinary sample, the oil shrinks and is not easy to flow, indicating that the surface tension of the coating film is small, while the oil spread of the ordinary sample is larger
.

Figure 8 The test effect of spray paint on the anti-graffiti sample

Note: For the anti-graffiti sample, the spray paint can be easily wiped off with a lint-free cotton cloth compared with the ordinary sample.

5 Conclusion

Graffiti is a global problem and presents a huge challenge to the coatings industry
.
While not a complete stop to graffiti, the use of anti-graffiti powders not only protects the substrate, but also makes graffiti cleaning much easier
.

From the experimental results, using the combination of high and low hydroxyl value of the polyurethane system and adding 1% of anti-graffiti additives, the obtained coating film has excellent long-term, easy-to-wipe and anti-graffiti properties
.