How to improve the scratch resistance of water-based wood paint?

Author｜Michelle Bauer, Tanya Hunter, and Yvette Gomez, ICL Phosphate Specialty—Halox, USA

At present, the demand for high-performance water-based coatings in many market segments is still the main trend.

Introduction

Generally speaking, durability needs to be paid attention to in all coating applications, but the meaning of durability may vary greatly depending on the end use of the coating and its required protective properties.

At this stage, most common anti-scratch additives rely on wax changes on the coating surface.

Other anti-scratch additives rely on high-hardness minerals to improve scratches.

In recent years, the latest nanotechnology has provided an opportunity to improve the formulation of high-gloss transparent materials.

ICL invented an anti-scratch additive that combines the advantages of hard silicate materials and nanoparticles.

experiment

Six commercially available anti-scratch additives were selected, representing the commonly used chemicals used by paint manufacturers to improve the scratch resistance of industrial wood coatings.

Table 1: Evaluation of anti-scratch additives and their added amount according to the quality of the formulation

The performance of the three additives was tested in a water-based UV-curable polyurethane acrylate system.
This type of formula can provide protection and beautification for wooden cabinets and furniture.
The coating has undergone key performance tests determined by the Kitchen Cabinet Manufacturers Association (KCMA) and office furniture standards to prove its durability, cold crack resistance, chemical resistance, pencil hardness, water immersion, Taber abrasion, and adhesion , Gloss, haze, focusing, scratch resistance and other properties.

The formula composition of the UV curable wood paint used for evaluation is shown in Table 2.
This is a formulation with a solids content of 25% and a resin with a Tg of 51°C.
Each layer of coating adopts a three-step curing method: first air-dry at ambient temperature for 15 minutes, then oven dry at 66°C for 15 minutes, and finally pass through the U.
S.
UV curing system for three times.
The system uses a standard mercury halogen lamp.
When the radiance is about 300 mj/cm2.

Table 2: Aqueous UV formulations used to evaluate anti-scratch additives

Hardness and durability are the same terms that have some ambiguities.
There are many test methods in the entire coating industry, but it is impossible to define its hardness with the test results of a single method.
Hardness can be explained in many ways, whether it is abrasion resistance, penetration by an object, or hardness scratches.
Therefore, a variety of test methods are usually used to characterize the hardness properties of the coating.
In this study, the scratch resistance was measured by comparing the changes in the 20° gloss value of No.
1 steel wool after 10, 25, and 50 back and forth rubbings under a load of two pounds.
Taber's abrasion resistance is 1000 times abrasion test using CS-17 grinding wheel under a weight of 1000 grams.
Before the test, apply three coats of paint on the birch veneer for Taber test.
The pencil hardness test is to test a single-layer paint film on a glass substrate.

The Leneta card was coated with a 3 mil wet film and cured under the conditions described above to evaluate gloss and haze.
Use a gloss meter to test the value of 20°/60°/85°.
By coating the paint film on the glass plate, the haze of the paint film can be evaluated intuitively.

The adhesion of the paint film is tested on birch and glass substrates according to the American Standard ASTM D3359.
The test planks were sprayed with four clear coats using HPLV sprayer, each with a thickness of 1-1.
25 mils.

The test method for crack resistance is to cyclically test the birch panel coating under low and high temperature conditions.
The coated board was placed in the refrigerator at -20°C for one hour.
Then immediately transfer the panel to an oven at 80°C for one hour.
After each cycle, observe whether the panel is discolored or cracked.

The anti-deformation test is to apply a small amount of common household foods and chemicals on the coating film, then cover with a watch glass and leave it for 24 hours.
The tested substances are water, 50% 409 solution, red wine, vinegar, lemon juice, orange juice, grape juice, mustard, ketchup, coffee, olive oil and 100% ethanol.
The stain is removed for about 24 hours, and the recovery of the coating is observed.

In addition to the water stain test, a layer of coating was applied to the glass plate, cured, and then immersed in water for observation at a distance of 4 hours and 24 hours, and the coating test was carried out after 24 hours of recovery.
Observe the redness and wrinkles of the coating.

Experimental result

In certain coating applications, the resin system is the main component that provides the required scratch resistance.
UV curable coatings can reach a high level of hardness immediately after the UV curing stage is completed.
Traditional water-based coatings require a longer drying time under ambient conditions or forced air drying to achieve similar results.
The application formula can further enhance the anti-scratch performance by adding the additives identified in this study.
Figure 1 shows the scratch resistance of the UV curable coating system when each additive is at the optimal addition level.
The key measurement value of the curve in the figure is the percentage of 20° gloss loss measured after a fixed number of times of wiping with steel wool.
The results show that the nano-stable silicate dispersion (NSSD) has a more significant (scratch resistance) improvement effect than all other products in this series.
The key component of NSSD has both hardness and rheological properties suspended on the surface of the coating, making it exhibit excellent scratch resistance in this high-gloss transparent formulation.
Second, the better-performing additive is a wax dispersion containing polytetrafluoroethylene, and its gloss loss is three times that of NSSD.
In addition, the nanocomposite exhibits scratch resistance similar to that of polytetrafluoroethylene.
In general, compared with the blank control, other additives have a certain improvement in anti-scratch performance.

Figure 1: The scratch resistance is measured according to the percentage of gloss loss at a 20° angle

ASTM D3363, commonly referred to as pencil hardness, describes the use of lead with a known hardness to measure the scratch resistance of coatings.
Although the results may vary depending on the operator and the lead itself, this method is a valuable tool when the data set of the test technique is strictly controlled.
In this data set, the hardness shown in the scratch test is also reflected in the pencil hardness results.
The hardness of NSSD is twice that of the second-ranked competitor in performance.
Figure 2 shows the effect of each additive on the hardness of the coating.

Figure 2: Test pencil hardness according to ASTM D3363

The third method of measuring the hardness of the coating is the Taber Wear Resistance of ASTM D4060.
As shown in Table 3, the mass loss of the coating was measured after 500 and 1000 cycles.
This test measures the ability of the coating to resist gradual wear and the deformation caused by scratches.
This method can effectively determine the balance point of scratch and wear performance.
The results show that all additives provide more or less certain benefits to the coating system.
The chemicals HDPE, NAOD and NCS can all improve wear, while NSSD has a mediocre effect on improving wear resistance.
The abrasion resistance is mainly controlled by the hardness of the resin system, but the test results show that additives can also play a certain role.

Table 3: Taber abrasion resistance test under mass loss

The remaining properties in this series of tests are very important for the study of high-performance, well-balanced wood coatings, but the additives in this study do not intend to further improve these properties.
In the preparation of coatings, an increase in one performance often leads to a decrease in the other performance.
Each formulator can give different evaluations on the balance of performance.
As a result, any particular coating has certain advantages and disadvantages.
Therefore, all key properties must be tested to obtain true measurements of coating durability.

One of the key properties that anti-scratch additives can affect is gloss change (Figure 3).
The initial gloss value shows that many additives have the least gloss loss at the optimal addition level, including HDPE, NAOD, NSD and NCS.
Three of these four products are based on nanoparticle technology, which may explain why they have less impact on gloss.
The gloss of NSSD has decreased slightly, but still belongs to the high gloss level.
Particle size may be a key attribute that determines the degree of gloss reduction.
Among the tested additives, PEW has the largest particle size, so the gloss is significantly reduced.
The haze or opacity of the clear coating has a similar conclusion.
It is important that the transparent coating makes the substrate as undistorted as possible.
The transparency is controlled by the refractive index of the material used and its main particle size.
The haze can be explained by the change of the gloss value at the 20° angle compared to the blank sample.
The visual performance of haze is by coating a layer of film on the glass, placing a pattern on the film, and observing the distortion of the pattern, as shown in Figure 4.
In this article, PEW is the only additive with visual distortion.

Figure 3 The gloss of the coating after adding each anti-scratch additive

Fig.
4 The haze test is to apply the coating film on the glass plate and place a pattern on the back to observe the distortion degree of the pattern

Regardless of the type of coating, adhesion is a key performance that must be maintained when improving other properties.
According to the ASTM D3359 cross tape peel test adhesion standard, this method can be used to characterize the results of large differences in adhesion between samples, represented by 0 to 5.
Generally speaking, due to its porosity, it is easy to bond with the wood surface.
The adhesion of all additives on wood can reach level 5B.
However, PTFE and HDPE make the adhesion between different coatings of wood invalid, which means that the additives have a significant effect on the surface energy of the coating.
Therefore, when there are multiple coatings when applied, their performance tends to be very poor.
In order to amplify the differences between samples, adhesion is usually tested on glass substrates.
Table 4 lists the test results of adhesion between the glass substrate, wood and coating.

Table 4 Cross-cut method adhesion test results

The chemical stain resistance test is to expose the stain on the paint film under the transparent glass that is easy to observe, and leave it for 24 hours for evaluation.
This simulated test method is relatively harsh.
Table 5 summarizes the test results.
The results are classified using the following levels:

5 No effect    

4 Leave a round mark   

3 Color/gloss change   

2 The paint film becomes soft   

1 bubbling   

0 Paint film peeling off

Table 5—Chemical stain resistance test grades after 24 hours of exposure

The key difference between the blank control and the formulation with additives is the slight decrease in water resistance.
Only the PTFE sample showed the same performance level as the blank control sample.
From the comprehensive results, the NSD performance score is higher than that of the blank control sample.
The chemical resistance of PEW is slightly worse, and there is no significant difference among other products.

The results of the recovery level test are shown in the following table (Table 6).
After 24 hours, NSSD had the same cumulative score as the blank control group.
In fact, all products will improve their performance ratings after the recovery period.
Among them, PTFE, PEW and NSD three products are better than the control group.

Table 6: Recovery test of chemical stain resistance after 24h

In the chemical stain test, water resistance is also measured.
Moreover, the water resistance will be further tested in the water immersion test of the glass substrate.
For most coatings, testing adhesion on glass substrates is a harsh detection method.
After immersing in water at room temperature for 4 hours and 24 hours, the difference between the hydrophobicity of the coating can be characterized.
It is difficult to take the test photos, but the performance results of the coating after immersion in water are shown in Table 7.
The test further shows that all products have varying degrees of influence on the water resistance of this waterborne UV curable coating.

Table 7: Observation results of water immersion test

The additives in this article have no effect on the low-temperature crack resistance.
All samples can pass 8 cycles of extreme cold and extreme heat temperature change tests.
This feature is very important to ensure that the coating material can withstand a variety of climates and still maintain stable performance.

in conclusion

For wooden cabinets and furniture, scratch resistance is a very critical performance, because it can extend the use time of these wooden products.
As an important formulation additive, additives can improve coating performance.
Based on the test results, it can be seen that the additives can affect the performance of the high-gloss transparent coating.
During the performance test of PTFE and HDPE, it was found that wax-containing products affected the surface energy of the coating, resulting in irreparable defects-reduced adhesion.
And in most tests, anti-scratch additives have a certain risk of performance degradation in terms of chemical resistance and water resistance.
The expected effect of these additives is to improve the durability of the coating by reducing the possibility of scratches.
Only NSSD has better performance in anti-scratch performance, while performance in other performance aspects has declined.
These results show that (the performance of the test additives in this article) exceeds the performance of the currently commercially available additives in the coatings market.