The effect of autoclave processing on some properties of heat cured

J Bagh College Dentistry

Vol. 24(3), 2012

The effect of autoclave

The effect of autoclave processing on some properties of heat cured denture base material Salwan S. Abdulwahhab, B.D.S. (1) (2) Widad A.H. Alnakkash, B.D.S., D.D.H., M.Sc.

ABSTRACT Background: Although most of the physical and mechanical properties of denture base resin polymerized by the conventional heat polymerization have been studied, the effect of autoclave processing in these properties has not been fully determined. The aim of the present study is to investigate the effect of two different cycles of autoclave processing on the transverse strength, impact strength, surface hardness and the porosity of acrylic denture base material. Materials and methods: Vertex was the heat- cured acrylic denture base material included in the study. A total of 120 specimens were prepared, the specimens were grouped into: Control groups (Group A) in which acrylic resins processed by conventional water- bath processing technique (74°C for 1.5 hours then boil for 30 minutes) and experimental groups in which acrylic resins processed by autoclave at 121°C,210KPa.The experimental groups were divided into Group B(Fast) for15min. , and Group C (Slow) for 30min... To study the effect of the autoclave processing (Tuttnauer 2540EA), four tests were conducted transverse strength (Instron universal testing machine), impact strength (charpy tester), surface hardness (shore D), and porosity test. The results were analyzed to ANOVA and LSD test. Results: There were no significant differences between the results of the processing techniques regarding transverse, impact, and hardness tests. While, there were a highly significant difference in porosity test results. Conclusions: The autoclave processing technique might also be a good alternative to the conventional water bath processing technique. Regarding to autoclave processing technique, the slow (long) curing cycle provide better denture bases material including the tested physical and mechanical properties as compared with the fast (short) curing cycle. Keywords: Autoclave, Transverse, Impact, Hardness, Porosity. (J Bagh Coll Dentistry 2012;24(3):13-17).

Previous studies of pressure cooker polymerization have shown comparable physical and mechanical properties to the water bath technique. (5) There is no previous Iraqi study deal with the investigation of the effect of varied autoclave processing conditions on the final properties of acrylic resins. Therefore, the aim of this study was to investigate the effect of different time durations of autoclave processing on some physical and mechanical properties of acrylic denture base material.

INTRODUCTION Poly (methyl methacrylate) (PMMA) is the most commonly used material in construction of denture base since 1936. (1) Attempts to improve the mechanical properties of poly (methyl methacrylate) have taken the researcher through many avenues. (2) Over the years, curing procedures have been modified with a view to improve the physical and mechanical properties of resin materials. Different polymerization methods have been used: heat, light, chemical and microwave energy. (3) The water bath processing technique has been the most conventionally used polymerization technique. In spite of the advantages provided by this technique like the ease, simplicity and costeffectiveness, a major disadvantage has been the long processing time required. (4) Indian researchers extensively investigated the pressure cooker polymerization technique, Conventional acrylic resin material can be used for this technique and requires less than 1h for polymerization and utilizes conventional equipment. (1) (2)

MATERIALS AND METHODS A total of 120 specimens were prepared, the specimens were grouped into: Control groups (Group A) in which acrylic resins processed by conventional water- bath processing technique and experimental groups in which acrylic resins processed by autoclave. The experimental groups were divided into Group B (Fast), and Group C (Slow). 40 specimens were divided for each group. Three different metal patterns were prepared, a bar shaped specimen with dimensions of (65mm X 10mm X 2.5mm) length, width, thickness respectively (6) for transverse strength test and surface hardness test; then, a bar shaped specimen

M.Sc. Student. Department of Prosthodontics. College of Dentistry, University of Baghdad. Professor. Department of Prosthodontics, College of Dentistry, University of Baghdad, Baghdad, Iraq.

Restorative Dentistry

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J Bagh College Dentistry

Vol. 24(3), 2012

with dimensions of (80mm X 10mm X 4mm) length, width, thickness respectively (7) for impact strength test; finally, a disk shaped specimen with dimensions of (30mmX 3mm) diameter & thickness respectively (8,9) for Porosity test. By the use of dental stone (Bluejey, Extra hard type IV, Italy) as investment material, these metal patterns were invested in their corresponding flasks (Broden, Sweden) After metal patterns removal, a fine brush was used to spread the separating medium (Isodent, Spofa Dental, Czechoslovakian, Europe) on to the exposed surfaces of a warm, clean stone moulds. Powder and liquid of heat cured acrylic resin (Vertex, Vertex-Dental by J.v.Oldenbamevetin Zeist, The Netherlands) were proportioned and mixed according to the manufacturer’s instructions. The specimens were packed at the dough stage according to the conventional dough molding method. The flasks were trial closed and then final closed under pressure using hydraulic press (Germany); then, Flasks were clamped with clamps (HANUA, Engineering corp. U.S.A) and be ready for processing. Curing cycle for conventional water-bath processing technique: For control specimens, curing was carried out by placing the clamped flask in a thermostatically controlled water-bath (memmert, Germany) and processed by short curing cycle (74°C for 1.5 hours then boil for 30 minutes.).(6) Curing cycle for autoclave processing technique: For experimental specimens, Curing was carried out by placing the clamped flask in a fully automatic autoclave (Tuttnauer 2540EA, Tuttnauer USA Co., NY, USA) and processed by the preprogrammed cycles (Fast 121ºC/210KPa, 15 min. & Slow 121ºC/210KPa, 30 min.). Before placing the clamped flask inside the autoclave, the autoclave must be leveled and filled. Then, the clamped flask placed in the tray and pushed inside the chamber, then closed and secured the door. Fast curing cycle In this cycle, held and selected the program (Fast 121ºC).The stages of operation of autoclaves include air removal, steam admission and sterilization cycle (includes heating up, holding/exposure, and cooling stages) (10,11). The autoclave operated and started heating the water, then the temperature and pressure were raised till its reached (121ºC &210 KPa) respectively. When the temperature reached (121 ºC), temperature and pressure held automatically at (121ºC &210KPa) respectively for 15 min., then Restorative Dentistry

The effect of autoclave

automatically exhausted the steam and the programmed cycle was finished. Unsecured and opened the door, then removed the clamped flask. Slow curing cycle In this cycle, held and selected the program (Fast 121ºC). The autoclave operated and started heating the water, then the temperature and pressure were raised till its reached (121ºC &210KPa) respectively. When the temperature reached (121 ºC), temperature and pressure held automatically at (121ºC &210KPa) respectively for 30 min., then automatically exhausted the steam and the programmed cycle was finished. Unsecured and opened the door, then removed the clamped flask. All the specimens were carefully finished and polished according to (6). After that, all the specimens were stored in distilled water at 37ºC by the use of an incubator (Gallen bamp, England) for 48 hours. (6) Transverse strength test: The total number of specimens for Transverse strength was 30: Ten acrylic control specimens and twenty experimental specimens. The test was achieved by using instron testing machine (instron corporation, 1122, canton mass), each specimen was positioned on bending fixture, consisting of 2parallel supports (50) mm apart, the full scale load was 50kg, and the load was applied with cross head speed of 1mm/min by rod placed centrally between the supports making deflection until fracture occurred. The transverse bend strength was calculated in N/mm² using the following formula: Transverse strength= 3Pl/2bd² P: is the peak load in Newton. l: is the span length in millimeter. b: is the sample width in millimeter. d: is the sample thickness in millimeter.(12) Impact strength test: The total number of specimens for Transverse strength was 30: Ten acrylic control specimens and twenty experimental specimens. Impact strength test was conducted with charpy type impact testing instrument (IZOD CHARPY, testing machines, Inc.) The specimen was supported horizontally at its ends and struck by a free swinging pendulum which released from a fixed height in the middle. A pendulum of 2 joules testing capacity was used. The scale reading gave the impact energy absorbed to fracture the specimen in joules when struck by a sudden blow. The charpy impact strength of unnotched specimen was calculated in KJ/mm² as given by the following equation: Impact strength= E/b.d ×10³ E: is the impact absorbed energy in joules. 14

J Bagh College Dentistry

Vol. 24(3), 2012

The effect of autoclave

b: is the width in millimeters of the test specimens. d: is the thickness in millimeters of the test specimens.(12) Surface hardness test The total number of specimens for Transverse strength was 30: Ten acrylic control specimens and twenty experimental specimens. Surface hardness was determined using (shore D) durometer hardness tester (TH210, TIME group Inc. company, Italy.) according to (6) which is suitable for acrylic resin material. The instrument consists of blunt-pointed indenter 0.8mm in diameter that tapers to a cylinder 1.6mm. The indenter is attached to a digital scale that is graduated from 0 to 100 units. The usual method is to press down firmly and quickly on the indenter and record the maximum reading as the shore “D” hardness measurements were taken directly from the digital scale reading. Five readings with 1 cm apart between each two indentation along the specimen (the same selected area of each specimen), and an average of five readings was calculated. Porosity test: The total number of specimens for Transverse strength was 30: Ten acrylic control specimens and twenty experimental specimens. The specimens were immersed in a solution of permanent black ink for 30 min., then washed for 10 seconds and dried with absorbent paper. A surface area of 1 cm² was limited in the center of each specimen and observed under 40X in light microscope (OLYMPUS, Japan). The number of pores per area were determined and calculated for each specimen. (8, 9) The results of this study were analyzed by the following statistical methods: Descriptive statistic which includes (Mean, Standard deviation (S.D)). Inferential statistics: ANOVA (one-way analysis of variance test) was used for assessing the differences between more than two groups; LSD (Least Significant Difference test) was used for examining the differences between each group. P-value more than 0.05 statistically considered as a non-significant (P>0.05) while P-value less than 0.05 accepted as a significant (P