The effect of autoclave processing on some properties of heat cured denture base material

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1 The effect of autoclave processing on some properties of heat cured denture base material Salwan S. Abdulwahhab, B.D.S. (1) Widad A.H. Alnakkash, B.D.S., D.D.H., M.Sc. (2) 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 s were conducted transverse strength (Instron universal ing machine), impact strength (charpy er), surface hardness (shore D), and porosity. The results were analyzed to ANOVA and LSD. Results: There were no significant differences between the results of the processing techniques regarding transverse, impact, and hardness s. While, there were a highly significant difference in porosity 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 ed 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). INTRODUCTION Poly (methyl methacrylate) (PMMA) is the most commonly used material in construction of denture base since (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) M.Sc. Student. Department of Prosthodontics. College of Dentistry, University of Baghdad. (2) Professor. Department of Prosthodontics, College of Dentistry, University of Baghdad, Baghdad, Iraq. 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. 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 and surface hardness ; then, a bar shaped specimen Restorative Dentistry 13

2 with dimensions of (80mm X 10mm X 4mm) length, width, thickness respectively (7) for impact strength ; finally, a disk shaped specimen with dimensions of (30mmX 3mm) diameter & thickness respectively (8,9) for Porosity. 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 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 : and twenty experimental specimens. The was achieved by using instron ing 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 : and twenty experimental specimens. Impact strength was conducted with charpy type impact ing instrument (IZOD CHARPY, ing 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 ing 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. Restorative Dentistry 14

3 b: is the width in millimeters of the specimens. d: is the thickness in millimeters of the specimens. (12) Surface hardness and twenty experimental specimens. Surface hardness was determined using (shore D) durometer hardness er (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 : 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 (8, 9) calculated for each specimen. The results of this study were analyzed by the following statistical methods: Descriptive statistic which includes (n, Standard deviation (S.D)). Inferential statistics: ANOVA (one-way analysis of variance ) was used for assessing the differences between more than two groups; LSD (Least Significant Difference ) 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<0.05) and P- value less than 0.01 accepted as a highly significant (P<0.01). RESULTS n and standard deviation (S.D.) values of examined s results were shown in (Table 1). A non-significant difference between groups of Vertex by using F- in transverse strength, impact strength, and surface hardness s; while, a highly significant difference between groups of Vertex by using F- in porosity showed in (Table 2). Verte x Table 1: n and standard deviation values of examined s results. Transver Impact se strength Surface Porosity strength hardnes (KJ/mm s (N/mm²) ²) S.D. S. n n D. Contr ol Fast Slow n S. D. n S. D Table 2: F- by ANOVA table of examined results. Between groups of Vertex Transvers e strength (N/mm²) Impact strength (KJ/mm² ) Surface hardnes s Porosit y F P-value Significan NS NS NS HS t P>0.05 non-significant (NS). P<0.01 highly significant (HS). Table 3: LSD between groups of Vertex in porosity Vertex groups P-value Significant Control & Fast 0.00 HS Control & Slow 0.00 HS Fast & Slow 0.09 NS P>0.05 non-significant (NS). P<0.01 highly significant (HS The LSD between groups of Vertex in porosity showed a highly significant difference between control and fast groups and control and slow groups; while, a non-significant difference between fast and slow groups showed. As shown in (Table 3). DISCUSSION An autoclave is a pressurized device designed to heat aqueous solutions above their boiling point to achieve sterilization. It was invented by Charles Chamberland in Under ordinary circumstances (at standard pressure), liquid water cannot be heated above 100 C in an open vessel. Restorative Dentistry 15

4 Further heating results in boiling, but does not raise the temperature of the liquid water. However, when water is heated in a sealed vessel such as an autoclave, it is possible to heat liquid water to a higher temperature. As the container is heated, the pressure rises due to the constant volume of the container. The boiling point of water is then raised because the amount of energy needed to form steam against the higher pressure is increased. (13).Tuttnauer 2540EA autoclave is an example of a gravity displacement. The autoclave is a pressure cooker. A pressure cooker is a container with an airtight lid that traps steam from boiling water. The steam increases the pressure inside the cooker, which raises the water's boiling point. The higher temperature kills bacteria much faster than at lower temperatures. The autoclave is no different than the simple pressure cooker used in home cooking which less costs as compared to the autoclave (14). Transverse strength : The transverse (flexural) strength is especially useful in comparing denture base materials in which a stress of this type is applied to the denture during mastication. The transverse (flexural) strength is a combination of compressive, tensile, and shear strengths, all of which directly reflect the stiffness and resistance of a material to fracture. (2) The results in Vertex showed that there was a non-significant difference in transverse strength between autoclave and water-bath methods, but there were a slight increase in slow curing and fast curing values than the control values as a result of high heat treating could be attributed to increase in cross linking. Cross-linkage provides a sufficient number of bridges between linear macromolecules to form a three-dimensional network that decreases water sorption, decreases solubility, and increases the strength and rigidity of the resin. (12) Durkan et al in 2008 studied the effect of autoclave polymerization on the transverse strength of denture base polymers, Under 3atm (303KPa) pressure, the specimens were subjected to one of the processing cycles: autoclave-cured for 60ºC/ 30 min. followed by130ºc/10 min., or autoclave-cured for 60ºC/ 30 min. followed by130ºc/20 min., the results revealed that polymerization in an autoclave led to a statistically significant increase in transverse strength for the two materials evaluated when compared to the control. However, there were no statistically significant differences in transverse strength between the two time durations of autoclave polymerization system, and there were no statically significant differences between the two acrylic resin control groups. Curing cycles that used by Durkan et al in 2008 were different from this study. Impact strength : Impact strength defined as the energy required fracturing a material under an impact force (12), impact strength is an important property for acrylic denture base materials which have tendency to fracture if accidentally dropped on to a hard surface. (15), the results in Vertex showed that there was a non-significant difference in impact strength between autoclave and water-bath methods, but there were a slight increase in slow curing and fast curing values than the control values due to the pressure. The pressure played an important role in speeding up the initial polymerization and elevating the boiling temperature of the monomer and thus might reduce the residual monomer content (5) the most important determinant of resin strength is the degree of polymerization exhibited by the material. As the degree of the polymerization increase, the strength of the resin also increases; In this regard, the polymerization cycle employed with a heat activated resin is extremely important. (12) Surface hardness Hardness may be broadly defined as the resistance to permanent surface indentation or penetration. (16) In this study, shore (D) hardness er was used which is suitable for measuring the hardness of acrylic resin, Shore durometer type (D) hardness er eliminate problem with elastic recovery owing to its use of a method that measures the depth of the loaded indentation under loading condition directly by screen which show the number of it (17). In Vertex, The results showed that there was a non-significant difference in surface hardness between autoclave and water-bath methods. The results of Vertex in this study coincide with the results obtained by Ming et al in1996 when the resin was polymerized in automatic pressure cooker with different pressure and time duration than this study, the pressure cooker was filled with water and inflated with 6KgF/cm² (580KPa) air pressure and heated to 120ºC (248 F) for 10 min., the results revealed no statistically significant differences between the pressure cooker and conventional polymerization methods. Porosity : Porosity in acrylic resin is a complex phenomenon of multifactorial origin. It appears to depend partly on the material/ polymerization method combination and the flasking technique used. (18). A number of methods have been used to study porosity, including microscopic observation Restorative Dentistry 16

5 of a cut specimen (19, 20), a photographic method, and mercury porosimetry, which is generally regarded as the best method available for the routine determination of pore size. (18).There are two major causes of porosity: polymerization shrinkage-associated contraction porosity, and volatilization of the monomer, termed gaseous porosity. (21) The results in Vertex showed that there was a highly significant difference in porosity between autoclave and water-bath methods. The lower value of porosity in slow curing cycle and fast cycle than the highest value in control curing cycle could be attributed to the role of the pressure accelerating the polymerization, the higher pressure is instantly transmitted to the resin dough and prevents the monomer from boiling (22) The results of porosity in Vertex were coincide with the results of Undurwade and Sidhye in 1989 demonstrated a reduction in porosity in a denture base resin polymerized in the domestic pressure cooker with steam pressure rise to 1520mm Hg (197KPa) and held for 30min.While, Ming et al in1996 cured 10 acrylic resin in an automatically controlled pressure cooker at 0.58MPa (580KPa) of 120ºC for 10min. Ming et al in1996 showed no porosity in any specimens. REFERENCES 1. Sideridou ID. Polymeric Materials in Dentistry. New York, Nova Science Publishers, Inc; p Jagger DC, Jagger RG, Allen SM, Harrison A. An investigation into the transverse and impact strength of high strength denture base acrylic resins. J of Oral Rehabilitation, Blackwell Science Ltd 2002; 29; Azzari MJ, Cortizo MS, Alessandrini JL. Effect of the curing conditions on the properties of an acrylic denture base resin microwave polymerized. J Dent; : Banerjee R, Banerjee S, Prabhudesai PS, Bhide SV. Influence of the processing technique on the flexural fatigue strength of denture base resins: An in vitro investigation. Indian Dent Assoc; : Undurwade JH, Sidhaye AB. Curing acrylic resin in a domestic pressure cooker: A study of residual monomer content. Quintessence Int 1989; 20(2): American Dental Association Specification No.12 Guide to dental materials and devices.10 th Ed., Chicago; p ISO International organization for standardization. Plastics. Determination of charpy impact properties-part 1: Non-instrumented impact ; Rodrigues Garcia, RCM. & Del Bel Cury AA. Accuracy and porosity of denture bases submitted to two polymerization cycles. Indian J Dent Res 1996; Vol.7, , ISSN Al-Fahdawi IH. The effect of the poly vinyl pyrrolidone (PVP) addition on some properties of heat- cured acrylic resin denture base material. PhD thesis, College of dentistry, University of Baghdad, Judelson HS. Operation of the autoclaves. An excellent overview of autoclave operation posted by Dr. Howard Judelson at University of California at Riverside; Oyawale FA and Olaoye AE. Design and Construction of an Autoclave. Pacific J Science and Technology 2007; 8(2): Anusavice KJ. Phillips s sciences of dental materials. 11 th Ed. Saunders Co. Philadelphia; p Durkan R, Ozel MB, Bagis B, and Usanmaz A. In vitro, Comparison of autoclave polymerization on the transverse strength of denture base resins. Dental Materials J 2008; 27(4): Monson MH. The pressure cooker as a steam sterilizer. Trop Doct 1988; 18(4): McCabe JF, Walls AWG. Applied dental material. 9 th Ed. Blackwell Publishing Ltd; p Powers J M, Sakaguchi R L. Craig s Restorative dental materials. 13 th Ed. Philadelphia, PA Mosby Co; p Unalan F, Dikbas I. Effects of mica and glass on surface hardness of acrylic tooth material. Dent. Material J 2007; 26(4): Yannikakis S, Zissis A, Polyzois G, Andreopoulos Evaluation of porosity in microwave-processed acrylic resin using a photographic method. J Prosthet Dent 2002; 87: Reitz PV, Sanders JL, Levin B. The curing of denture acrylic resins by microwave energy. Physical properties. Quintessence Int. 1985; 16: Wolfaardt JF, Cleaton-Jones P, Fatti P. The occurrence of porosity in a heat-cured poly (methyl methacrylate) denture base resin. J Prosthet Dent 1986; 55: Noort RV. Introduction to dental materials. 3 rd Ed. Elsevier limited; p Ming XC, Changxi S, Weizhou H. Rapid processing procedure for heat polymerization of poly (methyl methacrylate) in a pressure cooker with automatic controls. J Prosthet Dent 1996; 76: Restorative Dentistry 17