BASIC SCIENCE RESEARCH The Effect of Water Absorption on Acrylic Surface Properties Hugh Devlin, PhD, MSc, BSc, BDS; 1,2 and Prashant Kaushik, DMD, MBA, BSc 1 Purpose : The aim of the present study was to determine whether an increased water content during thermal cycling of hot water-treated acrylic was associated with a reduction in surface hardness and an increased opacity or whitening of the surface. Materials and Methods : Ten acrylic samples were treated with 30 soak cycles (cycle duration, 24 hours), using warm water (40 ◦ C) and an alkaline peroxide tablet (Efferdent  control group); a further ten samples were treated with boiling water (100 ◦ C) and one Efferdent  tablet (experimental group). Indentation hardness of the acrylic specimens was measured prior to and immediately following the completion of the warm and hot water treatments, using an automated micro-indentation system. The hydrated acrylic specimens were then allowed to air dry at room temperature (20 ◦ C) and were weighed weekly until they had obtained a constant dry weight. The loss in weight of the acrylic specimens represented the maximum water absorption. Results : The hot water-treated specimens were much whiter than the warm water-treated specimens. The mean reduction in hardness (H IT ) of the acrylic specimens following the treatment with hot water and alkaline peroxide tablet was 12.9%. Treatment with warm water and alkaline peroxide resulted in a slight increase in mean hardness (2.63%). There was a significant correlation between the water content of the acrylic specimens after treatment and the percentage of change in indentation hardness (r = 0.495, p = 0.026). Conclusions : The hot water treatment of the acrylic was associated with a significant reduction in hardness. We attribute the whitening and reduction in the hardness of the hot water-treated specimens to absorption of water and a disruption of the acrylic surface structure. J Prosthodont 2005;14:233-238. Copyright C  2005 by The American College of Prosthodontists. INDEX WORDS: whitening of acrylic, plasticizing effect, indentation hardness I T HAS been shown that when patients use hot water to clean their acrylic dentures(usually by immersing them in an alkaline peroxide solution), the dentures take on a bleached appearance. The denture opacity may result from an extensive sur- face degradation of the acrylic, 1 with measured reductions in the physical properties of the acrylic resulting from the plasticizing effect of absorbed water. 1 Department of Restorative Dentistry, Temple University School of Dentistry, Philadelphia. 2 School of Dentistry, University of Manchester, Manchester. Accepted November 2, 2004. Financial Support: Dr. Kaushik was a recipient of the Myerson Student Research Grant Award (Summer 2003). Correspondence to: Dr. H. Devlin, School of Dentistry, University of Manchester, Higher Cambridge St., Manchester M15 6FH, U.K. E-mail: Hugh.Devlin@man.ac.uk Copyright C  2005 by The American College of Prosthodontists 1059-941X/05 doi:10.1111/j.1532-849X.2005.00050.x Methyl methacrylate is also known to have a plasticizing effect on the acrylic, but there is no known mechanism in which hot water immer- sion could increase the concentration of residual methyl methacrylate in the surface of the denture. The tendency of denture cleansers to produce a color change in the acrylic soft lining materials and a severe deterioration in the physical properties of tissue conditioners is a well-known shortcoming; 2 however, when peroxide cleansers are used in a warm water solution as recommended by the manufacturer, no deleterious effects on correctly processed denture acrylic have been found. These cleansers contain alkaline detergents and oxygen- releasing compounds (such as sodium perborate or percarbonate). 3 Acrylics cannot withstand the operating tem- peratures observed in high-temperature applica- tions, such as solar water heating system appli- cations. 4 Von Fraunhofer and Suchatlampong 5 showed that water at room temperature could enter denture acrylic and cause a small change Journal of Prosthodontics, Vol 14, No 4 ( December), 2005: pp 233-238 233