light - cured composite resins are being more used specially because they are 
 aesthetically advantageous and enable to be polymerized by physical and chemical 
 activators . however , the crucial point to be reached during 
 restorative dentistry procedures with composite resins is to obtain satisfactory 
 restorations with an adequate light activation technique . composites consist mainly of filler particles and a resin matrix based on different 
 monomers . currently , the most 
 widely marketed composite resins are based on the polymerization of bisphenol a glycol 
 dimethacrylate ( bis - gma ) and urethane dimethacrylate ( udma ) . the curing process occurs in the organic matrix where 
 there is a monomer - polymer conversion through an activation mechanism . however , an insufficient degree of 
 conversion directly affects the physical properties and chemical stability of 
 material . polymers used in the 
 composition of composite resins may absorb water and chemicals from the oral environment 
 ( sorption ) and may release some of their own components ( solubility ) . restorations are subject to tension due to chewing when an excessive 
 or continue non - functional load is applied , internal stresses and strain are generated 
 which can cause the material failure . in this context , the adequate polymerization plays an important role on the longevity of 
 composite resin restorations . this procedure requires sufficient light energy intensity 
 and an adequate wavelength in order to activate the photoinitiator within these 
 materials , which will react with the reducer agent to form free radicals and initiate 
 the polymerization process . variables such as adequate light intensity , correct wavelength and energy density ( power 
 density x exposure time ) are essential for achieving the proper depth of cure . different types of light curing units 
 have been proposed for the polymerization of light activated resinous materials 
 including conventional quartz tungsten halogen ( qth ) lamp , plasma arc , argon laser and 
 light - emitting diodes ( led ) . all these devices emit blue light in the spectrum of the 
 absorption of camphorquinone , which is the photoinitiator generally found in most 
 resinous material . the qth lights can 
 achieve satisfactory results , but they present certain limitations such as shorter 
 durability of the lamp and heat production . led have been recognized as a promising technology for 
 polymerization of resin - based materials because all the light emitted is within the 
 spectrum of maximum absorption of camphorquinone at 468 nm . different curing regimes can exert an influence on the resistance of the union , hardness 
 and crosslinking density . when high 
 power density is applied , more photoinitiator molecules are activated at the same time 
 and consequently acceleration occurs immediately upon light exposure , thereby generating 
 inner stresses that are confined into composite , dental structure and 
 interfaces . in the pulse - delay ( pd ) polymerization technique there is a decrease in shrinkage and 
 stress . in this technique , there is a short period ( about 1 - 3 min ) between initial exposition and final 
 cure . the interval between 
 both pulses , an initial pulse with low energy density and a final pulse with high light 
 irradiation , increases the composite resin pre - gel phase improving its flowability and 
 relieving the stress generate by composite shrinkage . it has been hypothesized that low power densities , 
 characterized by reduced power density in the initial seconds , may generate a small 
 number of free radicals . then , a more linear polymeric structure is obtained , with lower 
 crosslinking density , which negatively affects the material 's chemical and physical 
 properties . a previous 
 study concluded that marginal 
 integrity of restorations and the composite " plasticizing effect " are negatively 
 affected when a pd technique is used . the aim of this study was to investigate the effect of different light sources , pd 
 curing protocols and storage media on sorption , solubility and biaxial flexural strength 
 ( bfs ) of a composite resin . the null hypotheses tested were : ( 1 ) qth and led curing 
 light units using different energy densities produce no influence on sorption , 
 solubility and bfs ; ( 2 ) qth and led curing light units using different curing techniques 
 produce no influence on sorption , solubility and bfs ; ( 3 ) storage procedure in water and 
 ethanol produces no influence on sorption , solubility and bfs . a nanohybrid composite resin ( esthet - x , dentsply , rio de janeiro , rj , brazil ) ( batch 
 # 071116 ) , a2 shade , was used to perform the sorption and solubility test and biaxial 
 piston - on - ring flexural strength test . specimens were made using a stainless steel mold with 2 mm in thick 
 and 8 mm in diameter . two hundred and forty specimens were divided into 24 groups 
 ( n=10 ) established according to the 4 study factors : 1- light - curing units : qth lamp ( variable intensity polymerizer , bisco inc . , 
 shaumburg , il , usa ) and led ( ultra blue is , dmc , so carlos , sp , brazil ) , 2- energy 
 densities : 16 j / cm and 20 j / cm , 3- curing modes : conventional 
 mode ( cm ) , pulse - delay using 3 s ( pd3s ) and pulse - delay using 10 s ( pd10s ) ; 4- 
 storage media ( for 28 days ) : deionized water and 75% ethanol ( figure 1 ) the light irradiance for each curing mode was checked with the in - built radiometer 
 prior to use to ensure consistency of light output . a single increment of composite 
 material was placed into a stainless steel mold and confined between two opposing 
 polyester strips ( k - dent , quimidrol , joinville , sc , brazil ) . the specimens were then removed from the mold and 
 excesses were eliminated with a scalpel blade ( free - bac , wuxi xinda medical device 
 co. ltd . , the sorption and solubility tests were performed in compliance with iso 
 4049:2000 standard 
 specifications , except for the specimen dimensions , types of permeant and storage 
 time . the specimen was placed individually into an open glass bottle of 20 mm 
 ( verallia ; saint - gobain vidros s.a . , so paulo , sp , brazil ) inserted into the 
 desiccators ( vidrolabor ; vidrolabor ind . com . , so paulo , sp , brazil ) containing 
 freshly dried white silica , batch # 0506198 , ( vetec , vetec qumica fina ltda , rio de 
 janeiro , rj , brazil ) , and maintained for 22 h at 371c in a vacuum oven ( model 440 
 d , lf equipamentos , so paulo , sp , brazil ) . thereafter , desiccators containing the 
 specimens were removed from the oven and placed on a bench for 2 h , at a temperature 
 of 231c , completing a cycle of 24 h. specimens were weighed daily on an analytical 
 scale accurate to 0.001 mg ( marte ay220 , marte balanas e aparelhos de preciso ltda , 
 santa rita do sapuca , mg , brazil ) . the complete cycle was repeated until a constant 
 mass ( m1 ) was obtained , that is , until the mass loss of each specimen was not more 
 than 0.1 mg per 24 h cycle . afterwards , the specimens were returned to their 
 respective labeled bottles , and 15 ml of either deionized water or 75% ethanol were 
 added with manual pipettes . the bottles were capped , replaced in the oven and kept at 
 371c for 28 days . after this period , all bottles were removed from the oven and kept at room 
 temperature , 231c for 2 h. the specimens were removed from the bottles , dried with 
 absorbent paper for 15 s and left in a sterile bucket ( duflex , sswhite , rio de 
 janeiro , rj , brazil ) for 1 min . then the 
 specimens were returned to the desiccators until they reached a constant weight ( m3 ) 
 using the same procedure described to obtain m1 . the values for water sorption and solubility were calculated in g / mm 
 using the following equations : wsp=(m2-m3)/v ; wsl=(m1-m3)/v , where wsp was the 
 sorption , wsl was the solubility , m1 was a initial mass of the sample in mg , m2 was a 
 mass of the specimen after immersion into storage media in mg , m3 was a final mass of 
 the specimen recorded after to evaporation of the water or ethanol in mg , and v was a 
 volume of the specimen in mm . after sorption and solubility tests , all specimens were submitted to a biaxial 
 piston - on - ring flexural strength test . the specimen was positioned on a circular 
 metallic device with 3 mm in radius that contain 3 symmetrically spaced steel 
 spheres . each specimen was placed concentrically on the supporting spheres to ensure 
 that the load was applied at the center of the specimen . a steel piston of 2 mm in 
 diameter was attached to a universal testing machine ( emic , dl2000 , so jos dos 
 pinhais , pr , brazil ) and the test was performed at a crosshead speed of 0.5 mm / min 
 using a 50 kgf load cell until specimen failure . the bfs was calculated according to 
 the astm f394 - 78 specifications as follows : s=-0.2387 p(x - y)/d ; where s is the flexural strength in mpa , p is the 
 load at failure in n , and d is the specimen thickness in mm of the failure area . x 
 and y were determined as follows : x=(1+v ) ln ( b / c)+[(1-v)/2 ] 
 ( b / c ) and y=(1+v ) [ 1+ln ( a / c)]+(1-v ) ( a / c ) , 
 where v is poisson 's ratio of the composite resin ( 0.24 ) , a is the radius of the support circle , b is the 
 radius of the tip of the piston , and c is the radius of the specimen . data obtained from the sorption , solubility and bfs tests were subjected to three - way 
 analysis of variance and tukey 's tests for parametric values , and kruskal - wallis and 
 mann - whitney tests for non - parametric values ( =0.05 ) . pearson 's correlation was used 
 to verify the correlation among the sorption , solubility and bfs . all tests were 
 performed using the spss 17.0 for windows statistics software ( spss inc . , chicago , 
 il , usa ) . a nanohybrid composite resin ( esthet - x , dentsply , rio de janeiro , rj , brazil ) ( batch 
 # 071116 ) , a2 shade , was used to perform the sorption and solubility test and biaxial 
 piston - on - ring flexural strength test . specimens were made using a stainless steel mold with 2 mm in thick 
 and 8 mm in diameter . two hundred and forty specimens were divided into 24 groups 
 ( n=10 ) established according to the 4 study factors : 1- light - curing units : qth lamp ( variable intensity polymerizer , bisco inc . , 
 shaumburg , il , usa ) and led ( ultra blue is , dmc , so carlos , sp , brazil ) , 2- energy 
 densities : 16 j / cm and 20 j / cm , 3- curing modes : conventional 
 mode ( cm ) , pulse - delay using 3 s ( pd3s ) and pulse - delay using 10 s ( pd10s ) ; 4- 
 storage media ( for 28 days ) : deionized water and 75% ethanol ( figure 1 ) the light irradiance for each curing mode was checked with the in - built radiometer 
 prior to use to ensure consistency of light output . a single increment of composite 
 material was placed into a stainless steel mold and confined between two opposing 
 polyester strips ( k - dent , quimidrol , joinville , sc , brazil ) . the specimens were then removed from the mold and 
 excesses were eliminated with a scalpel blade ( free - bac , wuxi xinda medical device 
 co. ltd . , wuxi city , jiangsu , china ) . the sorption and solubility tests were performed in compliance with iso 
 4049:2000 standard 
 specifications , except for the specimen dimensions , types of permeant and storage 
 time . the specimen was placed individually into an open glass bottle of 20 mm 
 ( verallia ; saint - gobain vidros s.a . , so paulo , sp , brazil ) inserted into the 
 desiccators ( vidrolabor ; vidrolabor ind . com . , so paulo , sp , brazil ) containing 
 freshly dried white silica , batch # 0506198 , ( vetec , vetec qumica fina ltda , rio de 
 janeiro , rj , brazil ) , and maintained for 22 h at 371c in a vacuum oven ( model 440 
 d , lf equipamentos , so paulo , sp , brazil ) . thereafter , desiccators containing the 
 specimens were removed from the oven and placed on a bench for 2 h , at a temperature 
 of 231c , completing a cycle of 24 h. specimens were weighed daily on an analytical 
 scale accurate to 0.001 mg ( marte ay220 , marte balanas e aparelhos de preciso ltda , 
 santa rita do sapuca , mg , brazil ) . the complete cycle was repeated until a constant 
 mass ( m1 ) was obtained , that is , until the mass loss of each specimen was not more 
 than 0.1 mg per 24 h cycle . afterwards , the specimens were returned to their 
 respective labeled bottles , and 15 ml of either deionized water or 75% ethanol were 
 added with manual pipettes . the bottles were capped , replaced in the oven and kept at 
 371c for 28 days . after this period , all bottles were removed from the oven and kept at room 
 temperature , 231c for 2 h. the specimens were removed from the bottles , dried with 
 absorbent paper for 15 s and left in a sterile bucket ( duflex , sswhite , rio de 
 janeiro , rj , brazil ) for 1 min . specimens were weighed to obtain m2 . then the 
 specimens were returned to the desiccators until they reached a constant weight ( m3 ) 
 using the same procedure described to obtain m1 . the values for water sorption and solubility were calculated in g / mm 
 using the following equations : wsp=(m2-m3)/v ; wsl=(m1-m3)/v , where wsp was the 
 sorption , wsl was the solubility , m1 was a initial mass of the sample in mg , m2 was a 
 mass of the specimen after immersion into storage media in mg , m3 was a final mass of 
 the specimen recorded after to evaporation of the water or ethanol in mg , and v was a 
 volume of the specimen in mm . after sorption and solubility tests , all specimens were submitted to a biaxial 
 piston - on - ring flexural strength test . the specimen was positioned on a circular 
 metallic device with 3 mm in radius that contain 3 symmetrically spaced steel 
 spheres . each specimen was placed concentrically on the supporting spheres to ensure 
 that the load was applied at the center of the specimen . a steel piston of 2 mm in 
 diameter was attached to a universal testing machine ( emic , dl2000 , so jos dos 
 pinhais , pr , brazil ) and the test was performed at a crosshead speed of 0.5 mm / min 
 using a 50 kgf load cell until specimen failure . the bfs was calculated according to 
 the astm f394 - 78 specifications as follows : s=-0.2387 p(x - y)/d ; where s is the flexural strength in mpa , p is the 
 load at failure in n , and d is the specimen thickness in mm of the failure area . x 
 and y were determined as follows : x=(1+v ) ln ( b / c)+[(1-v)/2 ] 
 ( b / c ) and y=(1+v ) [ 1+ln ( a / c)]+(1-v ) ( a / c ) , 
 where v is poisson 's ratio of the composite resin ( 0.24 ) , a is the radius of the support circle , b is the 
 radius of the tip of the piston , and c is the radius of the specimen . data obtained from the sorption , solubility and bfs tests were subjected to three - way 
 analysis of variance and tukey 's tests for parametric values , and kruskal - wallis and 
 mann - whitney tests for non - parametric values ( =0.05 ) . pearson 's correlation was used 
 to verify the correlation among the sorption , solubility and bfs . all tests were 
 performed using the spss 17.0 for windows statistics software ( spss inc . , chicago , 
 il , usa ) . however , to 
 facilitate understanding , the data presented in the tables are the means of the groups 
 with their respective standard deviations ( s.d . ) . the results for sorption and 
 solubility at energy densities of 16 j / cm and 20 j / cm are shown 
 in tables 1 and 2 , respectively . in general , no significant difference was found between the 
 light sources and between the curing modes tested in this study ( p>0.05 ) . however , in 
 the interaction between permeant and light source with a density of 16 j / cm , 
 it was seen that the specimens light - cured with the led unit device in pd10s and stored 
 in ethanol led to higher values of sorption and solubility than those light - cured with 
 qth ( p<0.05 ) . ethanol showed higher sorption and solubility than water irrespective 
 of curing unit or curing method ( p<0.05 ) . means , standard deviations ( sd ) for comparison between light source and permeant 
 for the measurement of sorption and solubility at 16 j / cm values in the column with same superscript lower caser letter denote no 
 statistical differences ( p>0.05 ) means , standard deviations ( sd ) for comparison between light source and permeant 
 for the measurement of sorption and solubility at 20 j / cm values in the column with same superscript lower caser letter denote no 
 statistical differences ( p>0.05 ) in the bfs there were interactions between light sources , curing modes and permeants at 
 an energy density of 16 j / cm ( p<0.05 ) ( table 3 ) , which did not occur at a density of 20 j / cm , where 
 there were significant differences only between the permeants ( water and ethanol ) ( table 4 ) . means , standard deviations ( sd ) for biaxial flexural strength at 16 
 j / cm values with same superscript capital letter ( a , b or c ) denote no statistical 
 differences ( p>0.05 ) means , standard deviations ( sd ) for biaxial flexural strength at 20 
 j / cm values in the same column with same superscript minute letter ( a or b ) denote 
 no statistical differences ( p>0.05 ) values in the same line with same superscript capital letter ( a or b ) denote no 
 statistical differences ( p>0.05 ) the correlation among sorption , solubility and bfs was performed by pearson 's 
 correlation coefficient test . otherwise , bfs were negatively correlated with sorption and 
 solubility ( table 5 ) . successful composite resin restoration depends on the association of low rate shrinkage , 
 good flowability , appropriate cure and satisfactory mechanical properties . led sources seem to be a promising 
 technology for polymerization of dental resin materials , but its application in 
 composite curing remains controversial . in this study , qth lamp was adopted as a control in order to 
 compare its performance with led . led units produce a narrow band of wavelengths 
 ( 450 - 490 nm ) , conveniently situated within the absorption spectrum of camphorquinone , 
 which is the photoinitiator present in the most of light - activated dental 
 materials . in general , the led 
 unit showed similar behavior to the qth unit , suggesting that similar polymerization 
 quality was reached by the equipment , except when led using 16 j / cm in pd10 
 s intensity produced higher values of sorption and solubility than qth ( p<0.05 ) . led 
 curing unit using cm at 16 j / cm produced lower values of bfs than qth used 
 with the same protocol ( p<0.05 ) . this fact could be associated with the greater heat 
 generated by the qth unit , which may speed up the polymer chain induction process in 
 composite , increasing the mobility of molecules during the reaction and allowing more 
 monomers to react before the curing process ends . the polymerization does not stop right after the photoactivation 
 period , and heat contributes for this post - activation polymerization . although camphorquinone presents its maximum absorbance at 
 468 nm , it has an absorption band ( 380 - 510 nm ) that is coincident with the light band 
 emitted by qth lamps resulting in more camphorquinone molecules being 
 activated . consequently , the 
 crosslinked polymer was probably lower in these led subgroups than in the corresponding 
 qth subgroups and more susceptible to softening in solvents . the size of the specimens used in this study differed from the iso 4049:2000 , as the standard dictates that they must 
 be 15 mm in diameter . this contrasts with the diameter of the light guide tip of the 
 curing units , which are 8 to 10 mm in diameter , and which would limit the uniform 
 irradiation throughout the circumference of the specimen . traditionally , the manufacturers have recommended high light intensity or power density 
 to provide a higher degree of monomer conversion into polymer , thereby improving the 
 mechanical properties of composite resin . high power density 
 used for short light exposures led to a lower degree of cure and lower flexural strength 
 and modulus than when the composite was cured with intermediary power densities for 
 longer exposures . in some 
 studies that used 24 j / cm of energy density , the 
 curing modes tested presented similar behavior . it has been speculated that this 
 occurred as a result of the formation of a densely crosslinked polymer network because 
 an adequate energy density had been used . the same finding was seen in this study in the groups that used 20 
 j / cm , where there were no significant differences between the subgroups 
 studied . the subgroups that used 16 j / cm presented different values of 
 sorption , solubility and bfs storage in ethanol for the qth and led units ( p<0.05 ) . the use of low initial intensity followed by high intensity light with an interval 
 between them seems to create a uniform polymerization of the composite resin , providing 
 its best adaptation to cavity walls and possibly the least polymerization contraction 
 stress . in the delay 
 period , little amount of free radicals and double bond conversion are produced and the 
 composite resin has more time to molecular rearrangement and stress relief . in the pd cure , there was reduced gap 
 formation without any mechanical properties being compromised , which can be considered 
 as an indicative that composite has the same quantity of remaining double 
 bonds . the use of slow - cure 
 methods ( pulse curing modes ) in combination with the interval between two irradiations 
 seem not to interfere with solvent sorption and could be useful for adhesive composite 
 restorations . in this study , 
 there were no significant differences among the curing modes in terms of sorption , 
 solubility and bfs ( p>0.05 ) except for qth comparing the cm and pd3s at 16 
 j / cm , where pd3s produced lower values of bfs than the cm ( p<0.05 ) . however , this 
 result was different from those of other studies , where the pd mode 
 resulted in a linear polymer structure that was less crosslinked , thereby increasing the 
 susceptibility of polymers to softening in ethanol . some studies 
 have shown a significant reduction in mechanical properties , such as flexural modulus 
 and hardness , when composites were photo - cured by pd mode and submitted to solvent 
 action . however , based on the methodology used in this study and the obtained results , 
 the curing modes presented similar behavior . according to lopes , et al . ( 2009 ) , this could have occurred because 
 another study used unfilled resins 
 for their tests instead of filled composite resin . another explanation for the 
 difference between the results of this study and that where the photo - cured composites 
 using the pd mode were more susceptible to softening in solvent could be attributed to 
 the type of tests applied to assess the amount of crosslinking , such as hardness 
 deterioration , degree of conversion and remaining double bonds , which only make an 
 indirect assessment of the quality of the polymer . the effect of the different solvents was clearly significant in all groups ; ethanol 
 resulted in more solubility , sorption and lower bfs than water media . 4049:2000 specifications , the storage time is 7 days , but in the 
 present study the storage time of 28 days in the same way as established for other 
 studies in which most of the composites studied reached 
 saturation within 7 - 60 days . the amount of solvent uptake by the polymer is determined 
 by differences in solubility parameters between the polymer and the permeant . water or solvent uptake into the resin 
 phase of cr causes two opposing processes : the solvent will extract unreacted 
 components , mainly monomer , thereby resulting in shrinkage , loss of weight and reduction 
 in mechanical properties ; conversely , solvent uptake leads to a swelling of the 
 composite resin and an increase in weight . these phenomena of sorption and solubility may serve as precursors 
 to a variety of chemical and physical processes which create biological concerns and 
 produce deleterious effects on the structure and function of the polymer 
 material . it is desirable that composites have all monomers converted into polymers during the 
 polymerization reaction and remain stable for long time . it may be assumed that , whereas the increase in the 
 degree of conversion reduced the solubility because the amount of unreacted monomers 
 available for leaching out was lower due to the high percentage of reacted aliphatic c = c 
 bonds from the dimethacrylate monomers . most dental polymers present ionic functional groups with water 
 affinity , which in turn optimizes their hydrophilicity . the susceptibility for more linear or less crosslinked 
 polymer to softening in solvents may be explained by the solvent - polymer interaction 
 and , consequently , by hoy 's solubility parameter for polar forces . solvents that can form strong secondary 
 bonds with the polymer chains can penetrate and replace the interchange secondary bonds , 
 and thereby pull apart and dissolve linear and branched polymers . distilled water , a solvent indicated in 
 iso specifications for resin - based 
 filling materials , simulates the wet intraoral environment provided by saliva and water . 
 however , intraoral conditions are clearly more complex than those achieved with 
 distilled water in the laboratory . according to the us food and drug administration guidelines , a 75% ethanol - water 
 solution is a clinically relevant food / oral simulating liquid . the 75% 
 ethanol solvent is the most frequently used to simulate accelerated ageing of 
 restorations as it has a solubility parameter matching that of bisgma when ethanol penetrates the polymer network , it causes the structure 
 swell , thereby allowing for the release of uncured monomers and causing the dissolution 
 of linear polymer chains . this 
 expansion is facilitated when crosslink density is low , based on the fact that more 
 space and pathways are available for solvent molecules to diffuse within the 
 structure . the properties of composite resins are affected by the degree of conversion , but an 
 analysis of the amount of crosslinking in the resins may provide a closer correlation to 
 mechanical properties . water 
 sorption may deteriorate polymer mechanical properties and because of this the resin 
 specimens were submitted to a mechanical test after the sorption and solubility tests . tensile strength is generally considered to be the most meaningful property of these 
 brittle materials in assessing the potential for failure of dental 
 restorations . flexural strength 
 can be measured using a three - point bending flexural test , a four - point bending flexural 
 test or a biaxial flexure test ; the drawback of the three - point and four - point flexural 
 tests is the inherent sensitivity to flaws and defect near specimen edges . as ban and anusavice ( 1990 ) related , the biaxial flexure test 
 is recognized as a reliable technique since the maximum tensile stress occurs within the 
 central loading area and edge failures are eliminated . in the evaluation of the correlation among the variables ( sorption , solubility and bfs ) , 
 it was possible to see how much higher the sorption and solubility were and how much 
 lower the bfs ( p<0.05 ) was . the degree of conversion and crosslink density can 
 influence the chemical degradation that is usually caused by oxidation and hydrolysis 
 processes , and consequently 
 reduce the physical and mechanical properties . thus , polymers with lower crosslink 
 densities are expected to undergo more softening in solvents , which results in greater sorption and solubility values 
 but lower bfs values . within the limitations of this in vitro study methodology , it was 
 concluded that , in general , the light sources ( qth and led ) and curing modes ( cm and pd ) 
 did not influence the sorption , solubility or bfs of the tested resin . however , the 
 different solvents ( water and ethanol ) did influence its sorption , solubility and bfs 
 behavior . the authors would like to thank nbia miranda ferreira ( dentsply ) for supplying 
 materials and paulo csar freitas santos filho for his assistance at the federal 
 university of uberlndia 's laboratory .
adequate polymerization plays an important role on the longevity of the composite 
 resin restorations.objectives the aim of this study was to evaluate the effect of light - curing units , curing 
 mode techniques and storage media on sorption , solubility and biaxial flexural 
 strength ( bfs ) of a composite resin . 
 material and methods two hundred and forty specimens were made of one composite resin ( esthet - x ) in a 
 stainless steel mold ( 2 mm x 8 mm  ) , and divided into 24 groups ( n=10 ) 
 established according to the 4 study factors : light - curing units : quartz tungsten 
 halogen ( qth ) lamp and light - emitting diodes ( led ) ; energy densities : 16 
 j / cm2 and 20 j / cm2 ; curing modes : conventional ( cm ) and 
 pulse - delay ( pd ) ; and permeants : deionized water and 75% ethanol for 28 days . 
 sorption and solubility tests were performed according to iso 4049:2000 
 specifications . 
 all specimens were then tested for bfs according to astm f394 - 78 
 specification . 
 data were analyzed by three - way anova followed by tukey , 
 kruskal - wallis and mann - whitney tests ( =0.05 ) . 
 results in general , no significant differences were found regarding sorption , solubility 
 or bfs means for the light - curing units and curing modes ( p>0.05 ) . only led 
 unit using 16 j / cm2 and pd using 10 s produced higher sorption and 
 solubility values than qth . 
 otherwise , using cm ( 16 j / cm2 ) , led 
 produced lower values of bfs than qth ( p<0.05 ) . 
 75% ethanol permeant produced 
 higher values of sorption and solubility and lower values of bfs than water 
 ( p<0.05 ) . 
 conclusion ethanol storage media produced more damage on composite resin than water . in 
 general the led and qth curing units using 16 and 20 j / cm2 by cm and pd 
 curing modes produced no influence on the sorption , solubility or bfs of the 
 tested resin .