Page 8 - Handbook of Surface Improvement and Modification

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Scratch and Mar Resistance

2.1 METHODS AND MECHANISMS OF PROTECTION

A single mechanism of anti-scratch protection does not exist because many influences
have an effect on properties of materials and our present knowledge permits their use. The
major influences include:
• the chemical composition of the matrix
• structure of polymer segments
• chain orientation
• chemical treatment of the surface
• properties of the substrate on which a coating is deposited
• surface segregation of additive and effect of loading
• thermal treatment
• polymer blending
• deposition of hard materials on surface
• surface texturing
• particle size and shape
• multilayer deposition
• fracture mechanism
• healing
The above major influences on anti-scratch protection are discussed in the above-
mentioned order.
The effect of the chemical composition of the matrix can be well illustrated by the
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example of SAN in which variable amounts of acrylonitrile were used. SAN contained
19, 27, and 35 wt% acrylonitrile and had different molecular weights (106-134 kg/mol). 1
The scratch deformation process leads to the formation of a scratch groove which is
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related to the secant modulus at a compressive yield point. Increase in the acrylonitrile
content and/or molecular weight lowered the secant modulus and slightly delayed scratch
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groove formation. Increase in the acrylonitrile content and/or molecular weight also con-
tributed to the increase of tensile strength of the copolymer which delayed the onset of
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periodic micro-cracking. Also, a higher acrylonitrile content strongly influenced the ori-
1
entation of illumination. The scratch visibility was significantly delayed with a high acry-
lonitrile content in SAN copolymer with illumination parallel to the scratch direction but
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no appreciable difference was noticeable at perpendicular illumination.
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Structure of polyurethane depends on the type of soft segments. The four model sys-
tems contain four different types of soft segment polyols: polytetramethylene ether glycol
(PT), polycaprolactone (PC), ethylene oxide, and propylene oxide based polyether polyol