This paper discusses the different mechanisms to improve scratch and wear resistance of solvent and waterborne coatings by incorporation of nano oxide particles. Depending on the concentration of nano particles and their orientation in the coating film, different results were obtained. Not only the nano particle core, but also surface modifications (boundary phase) have great impact on the performance of the Al2O3 or SiO2.

The demand for enhanced scratch and wear resistance in today’s clear and pigmented coatings is increasing. The idea of an everlasting surface that retains its initial properties is the driving force for the ongoing research in this field. Today, not only automobiles or high-end furniture, but also substrates such as ceramic tile, marble, wood, wood flooring, plastic, paper and vinyl floorings have clear topcoats to protect their surfaces from abrasion and scratching. There are various ways to enhance scratch resistance in these coatings.

These new nano silica and alumina particles can also be incorporated into pigmented coatings. The nano silica and alumina particles not only offer scratch resistance but can assist in wear resistance, better adhesion, staining and corrosion resistance because the nano particles create a denser coating structure. The nano particles have a synergistic effect with silicone, acrylates, silica and waxes.

Recently introduced additives containing alumina nanoparticles can provide improved scratch resistance for solvent, waterborne and UV coatings. These new nano alumina particles of 20-80 nm diameters are dispersed in different media and easily incorporated with low shear into aqueous, solventborne or solvent-free UV-curing systems. Typically, low dosages of 0.5-2.0% provide significant and long-term scratch, mar and/or abrasion resistance without adversely affecting gloss, color, clarity or other physical properties of the coatings.

The inclusion of surface-active-modified poly(dimethyl-siloxanes), acrylates or waxes due to their orientation and crosslinking at the coatings surface, as well as throughout the coating, enhances the performance of the nanoparticles. Where high gloss is not desirable, combinations of the nano alumina particles with flatting waxes and matting agents allow the formulation of semi-gloss to flat coatings.

Among inorganic UV absorbers, zinc oxide and cerium oxide stand out with their almost complete adsorption of UV-A, B and C. Reduction of particle size from micron to nano enables the formulator to formulate clear coatings that can both enhance the appearance of wood substrates as well as provide long-term protection for the wood, metal and plastic substrates against UV degradation. Comparisons between the typical organic UV absorbers will be made with available inorganic UV absorbers.

Testing Procedures

The following additive incorporation steps into coating formulations were used. The additives were (post added) admixed using a Dispermat CV with a 40 mm Cowles blade at 600 rpm for 2 minutes. For combinations of wax and NANO, the wax was admixed first at 600 rpm for 2 minutes and then NANO admixed at 600 rpm for 2 min. All additive dosages (nano Al2O3 or SiO2, wax, acrylates, silicones) mentioned are based on total formula weight.

Application
Substrate: Masonite board (for abrasion test) and sealed Byko chart (for scratch test).

Application tool: 3 mil bird blade.

Coats: 3 on Masonite board and 1 on Byko chart.

Thickness: 4 mil wet of each coat. Thickness of dry film depends on percent solids of the coating.

Dry time: 2 hours between each coat and at least 48 hours after final coat.

Sanding: after 2 hours dry, (1st and 2nd coat) each was sanded with 200 grit paper to ensure proper adhesion of additional coats.

Definitions
Scratch – To make a thin shallow cut or mark on the surface with something pointed or sharp.

Mar – To degrade the soundness, perfection or integrity of a coating; to spoil or blemish the surface.

Scratch Resistant Coating (SRC) – A coating with thin layers applied to a substrate intended to prevent marking, breaking or cutting into the surface with something pointed or sharp.

Mar Resistance – The resistance of a material to abrasion. It is measured by abrading a specimen, then measuring the gloss of the abraded areas with a glossmeter and comparing the results with an untested area of the specimen.

Wear or Abrasion – Occurs when there is removal of coating material during contact with hard particles. The particles either may be located at the surface of a second material (two-body wear) or may exist as loose particles between two surfaces (three-body wear). Abrasive wear can be measured by loss of mass as by Taber Abrasion.

Hardness – The resistance of a solid material to permanent deformation.

Scratch Test
Each sample drawdown was evaluated for scratch resistance by giving 20 double rubs with (9 micron) polishing paper pads mounted on an Abrasion Tester (Dry) from BYK-Gardner. Gloss (20o, 60o) was measured (statistical average of 5 readings) by micro-TRI-gloss from BYK-Gardner before and after scratching the panels. Gloss retention (%) was calculated as follows:



% Gloss Retention =  100   x                Gloss of scratched area

               Original gloss

Testing for scratch covers both areas of mar and scratch.

Abrasion Test
Test methods: ISO 9352 or ASTM D 1044. Method uses a Taber abrader (from TABER Industries) CS-10 abrading wheel, 1000 gm load, 100, 250, 500, 750 and 1000 cycles.


Pendulum Hardness Test – König
After allowing a coating to dry for 8 h and 48 h, a König pendulum (König pendulum counts the number of oscillations) is used to measured hardness.

Light Microscopy
A photomicrograph of each sample was taken after scratch testing. Images were taken with MOTIC lens at 4/0.10 magnification.<

Nano Dispersion

Just like any other solid particle that we try to incorporate into a resin solution, if we just try to mix the particle into solution it will sink to the bottom because the solid particle has no surface area compatible with the solution. Figure 1 represents the dispersion in the resin of these solid particles or nano particles with a proper wetting and dispersing additive that is imperative to the functionality of the solid particles within today’s coatings systems. Much more energy must be used to disperse the nano particles because of their small size. Once these solid nano particles are stable via stearic hindrance or electrical charge repulsion the solid particles are easily mixed (1,000 rpm for 2 minutes) into the resin or coating.

How Nanoparticles Function

Nano particles have the capability to set up a uniform distribution throughout the length and depth of a resin or coating thus allowing the resin or coating to have a solid networking protective layer of Al2O3 or SiO2 within the resin or coating. This is the unique nano advantage: as the coating is worn away there continues to be another layer of nano available to resist the scratching (Figure 2).

Evaluation of Nanoparticles in Waterborne Coatings

Coating Fracturing

Figure 4 shows another problem called coating fracturing. This is where the coating becomes brittle and, when shear force is applied to the coating, the coating breaks apart or fractures. Even though this coating shows excellent adhesion to the substrate, the coating breaks away from the surface when shear force (saw blade) is applied. This can be seen by the whiteness marks in a clear coating or, in an opaque coating, the breaking away from the substrate when force is applied. Here nano particles can assist with their uniform distribution to form a network of resistance and an almost elastomeric characteristic to fracturing. This again can enhance the water-based coating.

Wear Resistance

Another situation with water-based coatings is wear resistance. Today, to solve that problem, waxes (PTFE and paraffin combinations) are used to increase wear or abrasion resistance. The drawback with just waxes alone is that the waxes are soft and do not provide any resistance to scratching, fracturing or mar. This is where nano and waxes can have an excellent synergetic effect. The combination of waxes and nano can increase the scratch, fracture, mar and wear resistance of clear and pigmented coatings. A similar principal applies with waxes but here the combination creates a more durable surface capable of protecting the film from scratches and wear. Figure 5 shows the acrylic/PUD formulation where the CF-10 wheel of a tabor abrader has been used. The result shows that the wax and nano combination gives the best over all results.

Marring Resistance

When nano is used in conjunction with silicones a synergetic effect is also created. Silicones and nano increase the mar resistance of most types of coating. Together they work on the principal of lower surface tension; they can migrate together based on the eddy currents of the coating within the Bénard cell migration (Figure 6).

Burnishing or Polishing of a Satin or Flat Coating

Polishing or burnishing a satin or flat clear coating occurs when silica is used in the formula to flatten the finish look. The problem with silica is that the silica orientates to the surface of the coating and when the coating is rubbed, washed, or marred, the top layer of silica is quickly worn away. With the use of nano particles once again a synergetic effect is created between the silica and nano giving a uniform distribution to the silica. This gives the coating scratch resistance and no more polishing (glossing up), Figure 7.

UV Protection

Among inorganic UV absorbers, zinc oxide and cerium oxide stand out with their almost complete adsorption of UV-A, B and C. Reduction of particle size from micron to nano enables the formulation of clear coatings that can both enhance the appearance of wood substrates as well as provide long-term protection for the wood, metal and plastic substrates against UV degradation. Comparisons between the typical organic UV absorbers to available inorganic UV absorbers are shown in Figure 8.

Conclusion

Nano Al2O3 and SiO2, when dispersed properly, can enhance water-based, solvent-based and UV coatings. This paper focused on water-based coatings because they are the most difficult to improve. Nano can improve scratch resistance by developing a network of particles to create resistance to the elements. This network also helps in resistance to coating fracturing and marring.

In clear high-gloss coatings nano particles are small enough to enhance scratch resistance without creating haze. In satin and flat coatings, nanoparticles can not only provide scratch and mar resistance but help in the burnishing or non-polishing of the coatings.

When used in conjunction with waxes, nanoparticles can enhance the wear and abrasion resistance for pigmented or clear coatings. UV absorbers, zinc oxide and cerium oxide stand out with their almost complete adsorption of UV-A, B, and C. Also the nano oxides provide permanent protection because they do not degrade from the film. This is what nano technology can bring to today’s coating technology.

Acknowledgements
Mandar Mulay, Nanotechnology Scientist; Dr. Thomas Sawitowski, Global Manager Nanotechnology; Dr. Michael Berkei, Nano Laboratory Manager; Robert Schroeder, Senior Chemist – Wallingford Lab.

This paper was presented at The Waterborne Symposium sponsored by The University of Southern Mississippi School of Polymers and High Performance Materials and The Southern Society for Coatings Technology, 2009, New Orleans, LA.