Monday, 6 November 2017

How can a dense (hard) clear coat be so easily scratched?


The Scleroscope Hardness Test [: the Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a glass tube under the force of its own weight from a fixed height, onto the test specimen. The height of the rebound travel of the hammer is measured on a graduated scale].

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. (Newton's third law of motion)

Hardness is dependent on ductility, elasticity, plasticity, strain, strength, toughness and viscosity; suffice to say it is extremely important that paint hardness is properly taken into account when polishing, because if it is not then inappropriate polishing products and pads may be selected that either remove too much paint or turn out to be completely ineffective

Hard and soft are relative terms, and 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]

If you press down on your paint finish with your palm it feels really hard and tough, but that’s because the surface area of your palm is relatively large and what you’re actually feeling is the resistance of the steel underneath the paint. Try pressing your thumbnail into the paint with the same amount of force you used with your palm if you dare.

Scratch resistance can be related to higher cross-link density and elasticity of the polymer network The theory is that a dense cross-linked (hard) paint provides better protection from scratches and stone chips, whereas a less dense (soft) paint absorbs the impact is not based on scientific fact.
In solid mechanics, solids generally have three responses to force, depending on the amount of force and the type of material:

Elasticity—the ability to temporarily change shape, but return to the original shape when the pressure is removed.

Plasticity—the ability to permanently change shape in response to the force, but remain in one piece
Fracture—split into two or more pieces.

Paints need to be hard enough to protect a surface, yet not so soft that they could dent and be tacky. Hardness is a measure of the cure of the film or how the coating has softened when in contact with various liquids. When an organic coating “cures,” it does so in either of two processes. It can allow the solvent used in the formulation to evaporate, thereby leaving a hard film.

Lacquer is an example of a coating that cures by this process. The second process by which paints cure is when the coating crosslinks. Crosslinking is a chemical process where the polymer portion of the paint formulation reacts to form a dry, hard and, typically, solvent-resistant film. Test Method D 3363 is an easy and inexpensive technique to determine how hard or soft an organic coating is.

Most factory paint does not contain ceramic particles for hardness; there is more advanced technology today utilized to ensure hardness of the clear coat and the composition varies by application and curing temperature, but typically consists of acrylate-type resins and isocyanate-type cross-linkers.

Modern clear coat urethane even though they are harder than lacquer paints, they still scratch more easily. And because they tend to be harder it's more difficult and time-consuming to remove swirls and scratches. This is the main reason to take vendor claims of ‘provides 9H hardness ‘for what it is – totally irrelevant.

Copyright © 2002 - 2015 TOGWT® (Established 1980) all rights reserved

No comments:

Post a Comment