Objectives:
It is well known that, even in the absence of water, highly acidic monomers in dental adhesives and cements lower the curing capability of the chemical cured reaction much more than that of the light cured, although both use aromatic tertiary amines as their co-initiators. This work investigated the factors that were likely responsible for the difference.
Methods:
Model neat resins were made from a 62:38 bis-GMA/TEGDMA blend. The light cured resins (LCR) contained 0.3% CQ, and for one series, 1.0% of one of the three amines: ethyl dimethyl aminobenzoate (EDMAB), p-tolyl imino diethanol (PTID), or N,N' dimethyl p-toluidine (DMPT), and for another series, varying concentrations of DMPT. Chemical cured resins (CCR) contained 2.2% benzoyl peroxide (BPO) in Part A and also varying concentrations of DMPT in Part B's. The acidic monomer was bis-[2-(methacryloyloxy)ethyl] phosphate (2MP). The degree of conversion (DC) was measured with a differential scanning calorimeter (modified from Shimadzu DSC-50).
Results:
DC (%) as function of the type of amines
Amine | EDMAB | DMPT | PTID |
Relative basicity * | Low | Medium | High |
CCR, with no 2MP | 0 | 50 | 62 |
CCR, with 4% 2MP | - | - | 0 |
LCR, with no 2MP | 62 | 65 | 56 |
LCR, with 10% 2MP | 62 | 33 | 8 |
* Based on structural analysis.
Normalized DC (%) as a function of DMPT concentration (ppm), with no 2MP added.
10,000 | 1,000 | 300 | 100 | 10 | |
LCR | 100 | 99 | - | 82 | 40 |
CCR | 100 | 88 | 38 | 20 | - |
Conclusions:
The higher susceptibility of chemical cured resins to the presence of acidic monomers is probably due to the requirements for higher basicity of their amine co-initiator (usually PTID) and its higher concentration. An amine with higher basicity gets protonated more thoroughly by an acid, leaving a lower concentration of non-protonated type, which is the active form for co-initiation.