Year : 2008 | Volume
: 19 | Issue : 1 | Page : 2--5
Effect of chemical surface treatments and repair material on transverse strength of repaired acrylic denture resin
Mahroo Vojdani, Sakineh Rezaei, Lila Zareeian
Department of Prosthodontic, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
Department of Prosthodontic, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz
Purpose: This study was performed to evaluate the transverse strength of a denture base resin (H), repaired with an autopolymerizing acrylic resin (A) or a visible light-curing (VLC) resin (T) following the use of three chemical solvents: methyl methacrylate monomer, aceton or chloroform.
Materials and Methods: Eighty specimens (65.0 × 10.0 × 3.3 mm) of H were fabricated and stored in distilled water at 37°C for seven days. Specimens were divided into eight equal groups of 10. In each group, specimens were sectioned in the middle to create a 10 mm gap. Two groups served as controls and had no surface treatment. They were repaired with A or T materials. In the remaining six experimental groups, specimen surfaces were treated with ac for 30 sec or mma for 180 sec or ch for 5 sec. Then A or T material was placed on the treated surfaces, using the same preparation molds. After seven days«SQ» storage at 37°C, the transverse bond strength (MPa) of the specimens was measured using a three-point bending test. A two-way ANOVA and a Tukey HSD were performed to identify significant differences ( P < 0.05). The nature of the failures was noted as adhesive, cohesive or mixed.
Results: Significant differences were found between the controls and experimental groups ( P < 0.05). In the control groups, repair with A showed significantly higher strength (60.3 MPa) than those repaired with T (51.3 MPa). Mean transverse strength of experimental specimens repaired with A was (75.06 MPa) which was significantly greater than those repaired with T (67.9 MPa). Although surface treatment increased repair strength, no significant differences were detected between the effects of the chemical etchants.
Conclusions: The autopolymerizing resin exhibited significantly higher repair strength than VLC resin. The transverse strength of the repaired specimens was increased significantly after chemical treatments.
|How to cite this article:|
Vojdani M, Rezaei S, Zareeian L. Effect of chemical surface treatments and repair material on transverse strength of repaired acrylic denture resin.Indian J Dent Res 2008;19:2-5
|How to cite this URL:|
Vojdani M, Rezaei S, Zareeian L. Effect of chemical surface treatments and repair material on transverse strength of repaired acrylic denture resin. Indian J Dent Res [serial online] 2008 [cited 2013 Dec 9 ];19:2-5
Available from: http://www.ijdr.in/text.asp?2008/19/1/2/38923
Acrylic resin complete dentures are susceptible to fracture after long periods of clinical use.  The repair of fractured prosthesis can be accomplished using acrylic resins that are light polymerized, autopolymerized, heat polymerized or microwave polymerized. 
Among these acrylic resins, Triad, which is a visible light-curing (VLC) material and also autopolymerized acrylic resin can be used for chairside repair of dentures.  Although the conventional and microwave polymerized materials demonstrate superior strength, these materials require a significant amount of working time due to necessary packing and flasking procedures and also present the added risk of denture distortion by heat. ,
However, recent developments in processing methods and materials may result in superior strength of autopolymerizing acrylic resins to those previously reported. For example, the use of air pressure and low heat were reported to reduce the porosity of chemically accelerated materials, thus increasing the repair strength. 
The suitability of Triad for denture repair was tested by Khan et al . who reported that it was satisfactory in terms of strength.  Craig et al . also supported these results since they observed no problems in repairing resins with this material. 
Although the physical and mechanical properties of VLC resins were reported well with conventional heat-activated resins, some studies showed lower elastic modules and lower transverse strength for VLC resins, which could increase deformation of dentures during function. 
The purpose of this in vitro study was to evaluate the transverse strength of a heat-polymerized acrylic resin, when repaired with an autopolymerizing acrylic resin or a VLC resin following the use of three chemical solvents- methyl methacrylate, aceton or chloroform.
Materials and Methods
The materials used in this study are recorded in [Table 1]. A heat-curing denture base resin was used to prepare 65.0 × 10.0 × 3.3 mm specimens. Eighty heat-polymerized (H) specimens were processed by placing denture resin under compression in 74°C water for eight hours in metal flasks. Once processed, all the flasks were bench cooled for 30 min. The specimens were ground with 320-grit silicon carbid paper to remove surface irregularities and excess materials. Afterwards, specimens were stored in water at 37°C for seven days before the repair procedures. After storage, the specimens were sectioned in the middle using a double-sided diamond disc. The cut ends of each specimen were ground to 45° bevel joints with 600-grit silicon carbide papers until 10 mm of the total length of the specimens was removed. The surfaces were then ultrasonically cleaned with distilled water and dried with a blast of air. The specimens were divided into eight equal groups, each with 10 specimens [Table 2].
In two groups (H and G) as controls, without any surface treatment, repair was done with Triad (T) or autopolymerized (A) acrylic resin respectively. In the remaining 60 specimens, the exposed surfaces were treated with chemical etchants, either by immersion in acetone (Ac) for 30 sec,  in methyl methacrylate (MMA) for 180 sec  or chloroform (Ch) for 5 sec.  Once polished and cleaned, the specimens were returned and positioned into the same preparation mold in such a way that a 10-mm gap existed between the two sections of the specimens. For 30 specimens the material A was mixed according to the manufacturer's recommendations and added to the gap in a free-flowing stage. These specimens were processed under two bars in 45°C for 15 min.
The remaining 30 specimens were also placed in their mold and the gaps were repaired with T material. The material T was adapted into the gap by finger pressure. They were placed in the light-curing unit for 5 min initially, then removed from the mold and cured for an additional 8 min on the other side. After polymerization, all the specimens were finished to a final dimension of 65.0 × 10.0 × 3.3 mm (ISO/FDIS 1567).  After the repair procedures, the specimens were stored in distilled water at 37°C for seven days. The transverse strength (s) of the controls and repaired specimens was measured using a three-point bending test in a universal testing machine with a 100 kg load cell and a crosshead speed of 5 mm/min.
The transverse strength was determined using the formula: S = 3W1/2bd 2
Where W is the fracture load, l is the distance between supports (25 mm) b is the width and d is the thickness of the specimens. In addition, the nature of failure was noted as adhesive (AD) cohesive (Co) or mixed (MI). Mean values and standard deviations were calculated. Data were analyzed by two-way ANOVA and mean values were compared by using the Tukey (HSD) test (significance level equal 0.05).
The mean transverse strength values and SDs of the groups are presented in [Table 3]. The 2-way ANOVA in [Table 4] shows the differences among the groups. The type and frequency of failures for specimens are presented in [Table 5].
The transverse strength of repaired experimental specimens was significantly greater than the controls ( P -value = 0.000). In the control groups, G showed a significantly higher (60.3 MPa) ( P -value = 0.022) transverse strength than H (51.3 MPa). After surface treatment, all experimental specimens showed higher transverse strength. Significant differences ( P -value = 0.000) were detected between the specimens repaired with A (mean = 75.13) and those repaired with T (mean = 65.96). The specimens repaired with A showed a significantly higher transverse strength than the specimens repaired with T. The mean transverse strength of the examined groups which were repaired with A or T was 75.06 MPa and 67.9 MPa respectively.
The effect of surface treatment of three kinds of chemical etchants on transverse strength showed no significant differences between the experimental groups. However, after chemical etching of cutting surfaces, significant differences were observed between the controls and the experimental specimens ( P -value = 0.000).
The specimens showed three types of failures including adhesive (interface), cohesive (only at repair material) and mixed (interface and repair material ones). Mixed and cohesive fractures were the most common types of failure for the examined groups. However, the controls showed adhesive and mixed failures.
The results of this study supported the hypothesis that the chemical and physical properties of acrylic resins, as well as surface treatment affect the bond strength of repair material. The control groups showed significantly lower transverse strength than the experimental groups ( P -value = 0.000). In control groups, repair with A material showed a higher (60.3 MPa) and with T material showed a lower (51.3 MPa) transverse strength value.
Razavi et al. detected that the bond strength of Triad to denture base resin was sufficiently high to suggest its clinical applicability as reline material.  Some studies demonstrated that both visible light-cured and heat-cured resin have comparable values for transverse strength and also similar results with regard to their mechanical properties. , Lewinstein et al. reported that transverse bond strength of Triad to heat-cured resin did not differ significantly from that of autopolymerized acrylic resin. 
In the present study transverse bond strength of T to H was significantly lower than A to H. This finding was supported by the work of Goto et al.,  Andreopoulos et al .  and Dixon et al .  too, because of Triad's high viscosity and poor adhesion. However, surface treatment led to stronger repairs.
Andreopoulos et al. indicated that chemical etching with mma was useful to increase the bond strength of Triad. 
Lewinstein et al . reported that pretreatment of a denture with either monomer or bonding agent is essential for a stronger bond of Triad VLC reline resin. In this study, the use of monomer rather than bonding agent resulted in a stronger bond.  In the present study surface treatment increased the transverse bond strength of both A and T materials but the effect of ac, ch and mma was approximately similar and no significant differences were detected between the experimental groups. Successful denture repair relies on the phenomenon of adhesion. Strong bonding of the surfaces improves the strength of the repaired unit and reduces stress concentration. Adhesion can be improved by first applying appropriate chemicals to the acrylic resin surfaces. , These chemicals etch the surface by changing the morphology and chemical properties of the materials. Surface treatments cause superficial crack propagation, as well as the formation of numerous pits approximately 2 mm in diameter.  Chemical surface dissolution of acrylic resin could be affected by the degree of cross-linking of the polymer chain.  Normally this change is obtained by wetting the surfaces with mma. Organic solvents such as ch, ac and methylen chloride have also been used for this process. ,, Vallitte et al . stated that 180 sec of wetting of polymethacrylate with mma enhanced adhesion, compared with shortened duration of wetting.  Shen et al . noticed a duration of 5 sec wetting with chloroform, because longer contact with chloroform undermined the structure of PMMA denture at repair surface. 
On acetone, no guidelines were available in the literature concerning the most effective application time of acetone. However, the application time for other chemical treatments was generally stated as 30 sec. , The application of aceton to repair surfaces for different time periods should be investigated in the future.
In the present study, the repair joint design was 45° bevel. According to the results of Ward et al . the strength of repairs made with round and 45° bevel joint contour were similar and significantly greater than those with a butt joint design. 
We can conclude within the limitation of this study that (1) The specimens repaired with Triad-VLC resin showed significantly lower transverse bond strength than specimens repaired with autopolymerizing acrylic resin, (2) For the denture base resin tested, aceton, MMA and chloroform provided statistically identical bond strength values, (3) The transverse bond strength of repair materials to denture base resin increased significantly with chemical treatments.
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