title: Post-irradiation hardness development, chemical softening, and thermal stability of bulk-fill and conventional resin-composites.
abstract: OBJECTIVE
  To measure bottom/top hardness ratio of bulk-fill and conventional resin-composite materials, and to assess hardness changes after dry and ethanol storage. Filler content and kinetics of thermal decomposition were also tested using thermogravimetric analysis (TGA).
  
  
  METHODS
  Six bulk-fill (SureFil SDR, Venus bulk fill, X-tra base, Filtek bulk fill flowable, Sonic fill, and Tetric EvoCeram bulk-fill) and eight conventional resin-composite materials (Grandioso flow, Venus Diamond flow, X-flow, Filtek Supreme Ultra Flowable, Grandioso, Venus Diamond, TPH Spectrum, and Filtek Z250) were tested (n=5). Initial and 24h (post-cure dry storage) top and bottom microhardness values were measured. Microhardness was re-measured after the samples were stored in 75% ethanol/water solution. Thermal decomposition and filler content were assessed by TGA. Results were analysed using one-way ANOVA and paired sample t-test (α=0.05).
  
  
  RESULTS
  All materials showed significant increase of microhardness after 24h of dry storage which ranged from 100.1% to 9.1%. Bottom/top microhardness ratio >0.9 was exhibited by all materials. All materials showed significant decrease of microhardness after 24h of storage in 75% ethanol/water which ranged from 14.5% to 74.2%. The extent of post-irradiation hardness development was positively correlated to the extent of ethanol softening (R(2)=0.89, p<0.001). Initial thermal decomposition temperature assessed by TGA was variable and was correlated to ethanol softening.
  
  
  CONCLUSIONS
  Bulk-fill resin-composites exhibit comparable bottom/top hardness ratio to conventional materials at recommended manufacturer thickness. Hardness was affected to a variable extent by storage with variable inorganic filler content and initial thermal decomposition shown by TGA.
  
  
  CONCLUSIONS
  The manufacturer recommended depth of cure of bulk-fill resin-composites can be reached based on the microhardness method. Characterization of the primary polymer network of a resin-composite material should be considered when evaluating its stability in the aqueous oral environment.
authors.affiliation: School of Dentistry, The University of Manchester, Higher Cambridge Street, Manchester, UK; Department of Oral and Maxillofacial Rehabilitation, King Abdulaziz University, Jeddah, Saudi Arabia.
authors.forename: Ruwaida Z
authors.initials: RZ
authors.lastname: Alshali
authors.affiliation: Prosthodontic Department, University of Jordan, Amman, Jordan.
authors.forename: Nesreen A
authors.initials: NA
authors.lastname: Salim
authors.affiliation: School of Dentistry, The University of Manchester, Higher Cambridge Street, Manchester, UK.
authors.forename: Julian D
authors.initials: JD
authors.lastname: Satterthwaite
authors.affiliation: School of Dentistry, The University of Manchester, Higher Cambridge Street, Manchester, UK. Electronic address: nick.silikas@manchester.ac.uk.
authors.forename: Nick
authors.initials: N
authors.lastname: Silikas
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry*; D006244:Hardness; D008422:Materials Testing; D012117:Resins, Synthetic / Q000737:chemistry*; D013696:Temperature; D013818:Thermogravimetry
chemical_list: D003188:Composite Resins; D003764:Dental Materials; D012117:Resins, Synthetic
keywords: Bulk-fill; Cross-link density; Microhardness; Post-irradiation; Resin-composite; Thermogravimetric analysis