Effect of moisture exposure on scratch resistance of PMMA

• Model PMMA systems with three different levels of polarity were studied to determine their water adsorption mechanisms.
• Scratch performance of each PMMA was examined after moisture exposure.
• Scratch behavior of polar PMMA systems are greatly influenced by moisture exposure.
• Mechanical properties and surface characteristics of PMMA plaques throughout moisture exposure process were determined.
• Polar site bonding and free volume concepts were proposed to explain the resulting scratch resistances.

Effect of environmental conditioning on scratch performance of polymethylmathacrylate (PMMA) is investigated. Three different grades of PMMA with varying levels of polarity were chosen and their scratch resistance compared in both dry and moist conditions. Linear increasing normal load scratch tests were performed according to ASTM D7027/ISO 19252 standards. Results indicate a drop in scratch resistance with initial exposure to moisture in all three systems. In the two highly polar PMMA systems, the scratch resistance recovers to that of the dry condition after long exposure to moisture. It is proposed that the moisture absorbed initially acts as a plasticizer causing weakening of the surface mechanical integrity. In the case of more polar systems this moisture absorption continues until saturation where water molecules cluster and impart a degree of lubrication and consequently improves scratch resistance.

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