title: Water absorption characteristics of two types of acrylic resin obturators.
abstract: BACKGROUND
  The mechanisms of water accumulation in the inner space of a closed hollow obturator are not understood, and it is not known whether the porosity of acrylic resin is involved in these mechanisms.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate water absorption by solid and closed hollow obturators and to determine the mechanisms of water accumulation in the inner space of the closed hollow obturator.
  
  
  METHODS
  Twenty solid and 20 closed hollow spherical specimens (30 mm in diameter) were fabricated from heat-polymerized acrylic resin (Acron Clear). Closed hollow specimens consisted of 2 hemispheres (30 mm in diameter, 1.5 mm in thickness) joined with autopolymerizing resin (Unifast II). Ten solid and 10 closed hollow specimens were immersed in distilled water at 37 degrees C, while the other specimens were stored in an atmosphere of 100% relative humidity at 37 degrees C. Each specimen was weighed every 24 hours for 180 days, and weight changes between each group were compared by 2-way ANOVA and the Tukey HSD test. Split-plot design ANOVA was used to compare the course of the weight change over 180 days (alpha=.05).
  
  
  RESULTS
  The weight of the closed hollow specimens increased immediately and became saturated by day 30, with an increase of 1.44% from the initial weight. Water accumulation was not observed inside the closed hollow specimen during the 180-day storage period. The weight of solid specimens increased more slowly than that of closed hollow specimens; by day 180 the weight of the latter had increased by 1.16%. However, no significant difference in changes in mean mass of the same type of specimen was observed between the 2 storage conditions.
  
  
  CONCLUSIONS
  These results suggest that water accumulation inside a closed hollow obturator is unrelated to the water absorption properties of the acrylic resin.
authors.affiliation: Division of Aging and Geriatric Dentistry, Department of Oral Function and Morphology, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan. tsuboi@mail.tains.tohoku.ac.jp
authors.forename: Akito
authors.initials: A
authors.lastname: Tsuboi
authors.forename: Kazuhito
authors.initials: K
authors.lastname: Ozawa
authors.forename: Makoto
authors.initials: M
authors.lastname: Watanabe
mesh_terms: D000042:Absorption; D000180:Acrylic Resins / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry; D006801:Humans; D006813:Humidity; D008422:Materials Testing; D010158:Palatal Obturators; D019904:Polymethyl Methacrylate / Q000737:chemistry; D016062:Porosity; D011474:Prosthesis Design; D013499:Surface Properties; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D000180:Acrylic Resins; D003764:Dental Materials; C112073:unifast II; D014867:Water; D019904:Polymethyl Methacrylate