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Why do plastics need hardcoats?


Frequently Asked Questions

Whilst mechanical damage to the surface of transparent plastics can be defined in a number of ways, the most useful general descriptions of damage are mar, abrasion and scratching.

Mar is the general term relating to contact damage from touching the surface with soft items e.g. fingers, cloth etc.

Abrasion refers to abrasive or erosive wear of the surface and the subsequent loss of surface integrity. This loss of integrity can be measured by the loss of transparency of the plastic specimen (commonly referred to as an increase in haze); or by loss of weight, as material is removed by the erosive action.

Scratching is potentially a significantly more aggressive act and involves scoring the surface with a point load. Since transparent plastics are very soft, the pressure applied does not have to be significant to induce a scratch into the surface. Even with a protective coating in place, plastic articles are readily scratched, since even the hardest coating is only a few microns thick and is relatively easily compromised by the underlying plastic deforming under the point load, this is the so-called 'ice on mud' scenario. However, this phenomenon is well understood and can be designed for. If resistance to point loads is the significant operational requirement then alternative substrate materials should be considered; if, however, the operational requirements demand a plastic article, then the most appropriate protection should be used.

To be considered for automotive glazing, plastics have to show glass-like abrasion resistance; FMVSS 205 sets this standard as an increase in haze of less than 2% after 1000 abrasion cycles. This is the target that a number of organisations, including TWI are aiming towards.

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Further information

FAQ: What protective hardcoats are available for plastics?

FAQ: Why replace glass with plastics?

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