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poly(methyl methacrylate ) ( pmma ) is the most commonly used denture base resin , and its polymerization process may take place by different mechanisms8 .
several processing methods have been proposed to simplify the technique and to reduce denture manufacturing time , in spite of the long acceptance of compression molding with the water - bath polymerization method .
the convenience of using microwave energy in the food industry suggested similar advantages for denture processing .
however , microwaved system have gained only limited clinical acceptance despite extensive laboratory research1,2,5,7,24 . in previous studies ,
most of the mechanical properties of denture base resins polymerized by microwave energy and conventional water - bath method were comparable2,17,19,21 .
reitz , et al.17(1985 ) and shlosberg , et al.19 ( 1989 ) observed that the strength of the denture base resin polymerized by microwave was not significantly different by microwave - polymerized denture base resin .
alkhatib , et al.2 ( 1990 ) compared two microwave and one water bath - polymerized resins .
some authors11,12,14 have reported that high levels of residual monomer have a deleterious effect on the mechanical properties of denture base polymers . also , the level of residual monomer and the mechanical properties are closely related to the polymerization conditions10 .
harrison and huggett12 ( 1992 ) evaluated the effect of the polymerization cycle on residual monomer levels of denture base resins , and concluded that when terminal boils was used , the levels of residual monomer were markedly decreased .
these authors also cited that the rate of heat application and the thickness of the denture base polymers are influencing factors .
furthermore , these investigations have indicated that in addition to obtaining minimal levels of unpolymerized monomer , denture base resins must be porosity free and therefore an appropriate polymerization cycle is essential to prevent high exothermic reactions .
however , the important factor for consideration is the careful control of the temperature during polymerization processing .
de clerck9 ( 1987 ) reported that microwaves do not automatically heat the monomer to the correct temperature of 100.8c , which can be easily surpassed during polymerization using microwave energy .
therefore , this temperature must be monitored and controlled accurately , and the timing must be correct9 .
this author observed external porosity when the acrylic resin specimens have absorbed too much energy in the beginning of the polymerization , and internal porosity when too much energy was applied at the end of polymerization using microwave energy .
reitz , et al.17 ( 1985 ) and alkhatib , et al.2 ( 1990 ) also noted that the porosity in thick specimens polymerized by microwave energy could be reduced by a longer polymerization time at a lower wattage .
lai , et al.15 ( 2004 ) established the time and temperature required for the complete conversion of monomer to polymer , and compared physical and mechanical properties of microwave- and water bath - polymerized acrylic resin specimens .
the higher power applied to the system , the more the resin temperatures rises , the faster the benzoyl peroxide decomposes , and more rapid is the polymerization reaction .
the flexural strength of the specimens polymerized by microwave energy showed strength of 89 - 92% of water bath polymerized ones .
they concluded that microwave energy can effectively polymerize denture base polymer , and has a potential for saving time in processing dentures .
however , it has been stated that the choice of a suitable power and polymerization time is important in order to reduce porosity to a minimum level .
the aim of this study was to compare the flexural strength of a denture base resin polymerized by various microwave cycles to a conventional heat - polymerized and a autopolymerizing acrylic resin , based on the hypothesis that microwave - polymerized denture base resins exhibit flexural strength in the same range as other resins .
five protocols ( cycles ) were used to polymerize the acrylic resins ( table 1 ) .
clear acrylic resins were chosen in order to identify more easily whether specimens were free of porosity .
it has been shown that using a longer cycle at a lower - wattage may decrease porosity significantly 18,23 .
the denture resin specimens to flexural strength analysis were prepared by partially investing metal patterns in the conventional metal flasks for cycles t and q and in the fiber - glass - reinforced plastic flasks for microwave polymerization cycles ( a , b and c ) . to facilitate removal from the flask ,
the master patterns were individually invested in high - viscosity silicone ( zetalabor , zhermack s.p.a . , badia polesine , rovigo , italy ) and
after the invested material had set , the flasks were separated , and the master pattern was removed from the silicone mold . each acrylic resin was mixed according to the manufacturer 's recommendations .
the resin was then packed and polymerized according to the cycles listed in table 1 .
t and q specimens were processed , respectively , in an automatic polymerization tank ( termotron p-100 , termotron equipamentos , piracicaba , sp , brazil ) and in a press chamber ( termo press , promeco ind eletro mecnica ltd , so paulo , sp , brazil ) . for the microwave polymerization method , a domestic microwave oven with a rotating table was used ( continental aw-30 , bosh eletrodomsticos , manaus , am , brazil ) .
ten specimens from each cycle were prepared by investing metal patterns with dimensions of 67  12  5.3 mm .
after processing , the resin specimens were polished in a polishing machine ( metaserv 2000 , buehler , uk ltd . ,
coventry , england ) to a final dimension of 65  10  3.3 mm , according to the international organization for standardization ( iso / fdis 1567)13 .
all surfaces of the specimens were finished with 280 and 320-grit wet - dry sandpaper ( norton , saint - gobain abrasivos ltda , vinhedo , sp , brazil ) followed by 400 and 600-grit sandpaper .
all specimens were stored in distilled water at 37c for 50  2 hours before testing13 .
the flexural strength of the five cycle groups were measured using a three - point bending test in a material test system machine ( model 810 , mts system corp , edden praire , minnesota , usa ) at a crosshead speed of 5 mm / min . the flexural strength ( ts )
was calculated using the formula : ts = 3wl / 2 bd2 , where w is the maximum load before fracture , l is the distance between supports ( 50 mm ) , b is the specimen width and d is the specimen thickness .
statistical analyses were completed using a one - way analysis of variance ( anova ) and tukey 's test for post - hoc comparisons .
data were analyzed with spss for windows software ( version 12.0.0 , chicago , il , usa ) .
there was a significant difference between the three polymerization methods ( microwave , conventional water bath and autopolymerizing method ) , with cycle t exhibiting the lowest value ( table 2 ) .
tukey 's test showed a significantly higher flexural strength for microwave - polymerized denture resin , regardless of the polymerization cycle , and there were no differences among any of these groups .
flexural failure of denture base resins is considered the primary mode of clinical failure6 . the flexural three - point bending test is useful in comparing denture base materials as it simulates the type of stress that is applied to the denture during mastication25 .
this study compared the flexural strength of a microwave - polymerized denture base resin ( onda - cryl ) with a conventional heat - polymerized ( clssico ) and an autopolymerizing acrylic resin ( jet ) .
based on the results , the hypothesis that microwave - polymerized denture resin specimens exhibit flexural strength in the same range as that of heat- and autopolymerizing resins was not accepted .
the results showed higher flexural strength for onda - cryl , regardless of the polymerization cycle .
clssico denture base resin demonstrated the lowest value of flexural strength ( 84.4 mpa ) , indicating its inferior strength and low resistance to fracture .
these findings are consistent with previous studies2,3,21 that found a statistically higher flexural strength for microwave - processed denture resins . in spite of these results ,
the flexural strength values for the heat - polymerized resin were in accordance with those found by other authors2 .
for both autopolymerizing and heat - polymerized acrylic resins , the level of residual monomer in the resin and the mechanical properties are closely related to the polymerization conditions11,12,14 . according to harrison and huggett12 ( 1992 ) the long water bath cycles without terminal boil show levels of residual monomer approximately 3 times greater than when a terminal boil is used
. the residual monomer could affect the flexural strength because of a plasticizing effect , which effectively reduces interchange forces14 .
the heat - polymerized specimens also showed the lowest strength values , even though autopolymerizing resin .
the resin specimens of that have been polymerized for a long period in water may absorb water continuously20 , and it could act as a plasticizer , reducing the glass transition temperature ( tg ) and the evaluated mechanical property . it might have happened with clssico denture base resin .
vallittu , et al.22 ( 1998 ) demonstrated that the increase of the polymerization temperature for autopolymerizing resins from 30c to 60c , decreased the residual monomer content of the polymer from an average of 4.6 wt% to 3.3 wt% .
ogawa , et al.16 ( 2000 ) found that the transverse strength of autopolymerizing resin increased with an increase in water temperature , and concluded the polymerization of the autopolymerizing resin in hot water greatly improves its mechanical properties .
heat may have activated the chemical reaction between the monomer and polymer components of the resin and produces more complete polymerization .
this mechanism may explain why hot water conditions improved the mechanical properties of the autopolymerizing acrylic resin .
the results of flexural strength indicated that the evaluated microwave - polymerized resin can be polymerized by cycles a , b and c without strength impairment . even though there were no significant differences among microwave cycles , cycle a produced the lowest flexural strength values .
the specimens polymerized on that cycle absorbed more energy in the beginning of the reaction , and then there was a rapid rise in the resin temperature9,15
. it could lead to the formation of porosities , and decreased the flexural strength values .
lai , et al.15 ( 2004 ) demonstrated the effectiveness of denture base resin polymerization by microwave energy .
they suggested 7 , 10 , and 15 min to polymerize acrylic resins at 240 , 160 and 80 w , respectively .
then , the flask should be turned over , and polymerized for additional 2 minutes at 560 w to complete polymerization . in the same way of the present study
, they did not find any significant difference among specimens polymerized by any of the microwave cycles .
although it had been observed , they found that the acrylic resin blocks polymerized at 240 w seemed to be superior to other microwave - polymerized blocks because of their little porosity and sufficiently polymerized state .
this study involved a limited analysis of a mechanical property for only one denture base resin polymerized by microwave energy .
further research should be conducted to compare different microwave ovens , other denture base resins , and how they could affect other physical and mechanical properties of these materials . to overcome the limitations of in vitro tests ,
the clinical implication of this study is that the polymerization of denture base resin using microwave energy may have a positive effect on the strength and longevity of complete dentures .
also , microwave energy has a potential for saving a great amount of time in processing dentures .
within the limitations of this in vitro study , the results suggested that the microwave - processed specimens showed a significantly higher flexural strength than the other ones .
the flexural strength means for the specimens polymerized by conventional water bath ( t ) were comparable to those of the autopolymerizing resin specimens ( q ) . the microwave - cycles ( a , b and c ) can be used without compromising damages to the acrylic resin strength .
however , it has been stated that the choice for a suitable power and polymerization time is important in order to reduce porosity to a minimum level .
despite the large number of studies addressing the effect of microwave polymerization on the properties of acrylic resin , this method has received limited clinical acceptance . 
 this study evaluated the influence of microwave polymerization on the flexural strength of a denture base resin . a conventional heat - polymerized ( clssico ) , 
 a microwave - polymerized ( onda - cryl ) and a autopolymerizing acrylic ( jet ) resins were used . 
 five groups were established , according to polymerization cycles : a , b and c ( onda - cryl , short cycle  500w/3 min , long  90w/13 min + 500w/90 sec , and manufacturing microwave cycle - 320w/ 3 
 min + 0w/3 min + 720w/3 min ) ; t ( clssico , water bath cycle  74c/9h ) and q ( jet , press chamber cycle  50c/15 min at 2 bar ) . 
 ten specimens ( 65 x 10 x 3.3 mm ) were prepared for each cycle . the flexural strength of the five groups was measured using a three - point bending test at a cross - head speed of 5 mm / min . 
 flexural strength values were analyzed by one - way anova and the tukey 's test was performed to identify the groups that were significantly different at 5% level . 
 the microwave - polymerized groups showed the highest means ( p<0.05 ) for flexural strength ( mpa ) ( a = 106.97  5.31 ; b = 107.57  3.99 ; c = 109.63  5.19 ) , and there were no significant differences among them . 
 the heat - polymerized group ( t ) showed the lowest flexural strength means ( 84.40  1.68 ) , and differ significantly from all groups . 
 the specimens of a microwavable denture base resin could be polymerized by different microwave cycles without risk of decreasing the flexural strength .