Tuesday, 22 August 2017

Surface Hardness and Scratch Resistance

Surface Hardness

Definition [: the ability of a material to resist local deformation (or penetration) from externally applied pressure, and is directly related to its tensile strength; stronger materials are generally harder]

The enamel paint finishes on vehicles from the 50’s and 60’s era were as tough as porcelain. But rightly due to environmental concerns, those high percentage petroleum based paints have been generally superseded, resulting in the softer water-based paint finishes of today and the unavoidable orange-peel seen on many new and re-painted vehicles.

Today’s paints, unfortunately, rank somewhere near the bottom of the scale of hardness, especially single coat black/red paint the exception being a white single stage and CeramiClear, when compared to all the materials your paint can possibly come in contact with (always bear that in mind).
Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material

An adaptation of that hardness scale (1 - 10)
• Talc = 1
• Carbon Black [black paint pigmentation] = 2
• Glass = 6
• Titanium dioxide [white paint pigmentation] = 7
• Corundum 9
• Diamond =10

Pencil Hardness Test
This type of test uses special graphite pencils with different degrees of hardness to scratch the coating, which then determines its hardness - Testing Your Coating's Hardness
Pencil Hardness is one of many tests that are done to evaluate a coating's performance. Other tests are abrasion, reverse impact resistance, direct impact resistance, cross-hatch adhesion, oxidation, gloss retention, UV resistance, yellowing, blistering, drying times, chemical/solvent resistance (using both the rubbing and spot/time tests), salt spray resistance, humidity resistance, acid and caustic resistance
Most coatings are formulated for specific types of finishes, various conditions or different substrates. So use the pencil hardness test as one criterion for selection.

But do not judge any coating by coating thickness or pencil hardness alone, as there are many other significant characteristics to consider. Be cognizant that whatever the hardness rating it only offers scratch resistance, which only helps to minimize the appearance of light scratches

Pencil Hardness for Common Coatings
• Catalyzed polyester: 9H
• Catalyzed polyurethane: 9H
• Catalyzed modified acrylic polyurethane: 4H
• Catalyzed acrylic polyurethane: 2H
• Water-based polyurethane: 3H
• Water-based urethane/Isocyanate catalyst: 2H
• Urethane/nitrocellulose lacquer: F (24 hours)
• Water reducible lacquer: 2H
• Water-based polyurethane wipe-on finish: HB-F
• Clear shellac aerosol: 3B
• Polyurethane/nitrocellulose aerosol: HB
• Nitrocellulose aerosol: 3B

Hard and soft are both relative terms; you can scratch the hard surface of vehicles paint with a soft towel by the application of enough pressure.

Both pressure and mechanical stress are defined as force per unit area. These two forces are the subject of Newton's third law of motion; the law of reciprocal actions [: to every action, there is an equal and opposite reaction] 

How can a hard clear coat be so easily scratched?
Force acts through a body that has a surface area; if the surface area is really small while maintaining an equal force, the pressure becomes astronomical and the object under pressure capable of penetrating the surface of an otherwise tough material.
That’s why a micro fine thread that is twice as fine as silk and 100 times finer than a human hair, in an otherwise soft towel will scratch your paint. And the same reason a mosquito can penetrate a rhino hide with its proboscis (stinger)

Scratch Resistance
Preserving a scratch-free, high gloss finish over a longer vehicle life has challenged the auto industry for decades. One problem is acid etch, degradation of the surface by environmental pollution (Acid rain, Industrial fallout, etc) a major factor in the phenomenon more generally known as weathering; another is scratch resistance to abrasion from many sources, not least of which is the car wash. 

Unfortunately, any solution comes with tradeoffs in paint chemistry. A scratch resistant coating was not as environmentally resistant, and vice-versa. And chemistries which offered the best combination were not VOC compliant (water- based) so applying them released solvent volatiles from the paint
Thickness of a paint or coating has little relevance; a coating will provide 2-4 µ (microns) a clear coat is approx 50-76µ and yet a micro fibre can cause scratches

Do not judge a coating solely by the thickness it provides or pencil hardness alone, as there are many other significant characteristics to consider. Be cognizant that whatever the hardness rating it only offers scratch resistance, which only helps to minimize the appearance of light scratches

Relevant Articles
1. American Society for Testing and Materials (ASTM) publications - http://www.astm.org/Standard/standards-and-publications.html

2. ASTM D1014 - 09 Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates
 3. Nanotechnology coatings “

No comments:

Post a Comment