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An initiator/activator system must be added to the cement formulation in order to polymerise the monomer component. The effect of varying the initiator and activator concentrations on the setting reaction of the cement was determined using surface hardness measurement.

4.6.1 Methods

The powder component of the cement consisted of the glass and the dried polyacid mixed in the ratio 80:20. The liquid contained the 70/30 THFMA/bis-GMA mbrture and distilled water in the ratio 1:1 (i.e. the monomer concentration was 50 wt%). This was used to represent the maximum concentration of the monomer that produced a workable cement. Different combinations of the initiators and the activator (2.5-10.0 % of the liquid weight) were added to either the powder or the liquid portion. The powder and liquid were mixed in the ratio 3:1. The pastes were inserted into stainless-steel split rings resting on a glass slide with a piece of polyethylene sheet on top, covered with another piece of polyethylene sheet and another glass slide. The light cured specimens were cured through the top glass slide for 60s using a light-curing unit. The specimens were stored in the mould at 25°C and 50% relative humidity in the dark to prevent any polymerisation by ambient light. The surface hardness of the specimens was determined using the Wallace indentation tester at 1 0 ,2 0 ,3 0 , 45, 60,120 min and 24h after mixing. The method has been described in Section 3-3. Three specimens were determined for each condition. The indentation depths at each condition were averaged from the nine indentations made on the same surface of the three specimens.

Statistical analysis of the indentation depths among initiator/activator concentration and time were carried out using two-way AN OVA and Tukey's HSD multiple range tests at a 0.05 significance level.

4.6.2 Results

The effects of varying the initiator/activator concentrations on the surface hardness of the light-cured and chemically-cured specimens are shown in Figures 4.6-1 and 4.6-2, respectively. 3 0 min 4 5 min 6 0 min 1 2 0 min 2 4 h Indentation depth (|j,m) 5 0 0 , LIGHT CURE 4 0 0 - 3 0 0 - 200 - 100 - 0 /0 2 .5 /2 .5 2 .5 /5 5 /5 7 .5 /5 1 0 /5 5 /1 0 1 0 /1 0 % w t CQ / % w t DMPT

Figure 4 .6 -1 Effect of CQ and DMPT concentration and tim e on hardness of specim ens

Indentation depth (^m) 5 0 0 4 0 0 3 0 0 20 0 ^ 100 0 # 2 0 min ^ 3 0 min g 4 5 min ^ 6 0 min ^ 1 2 0 min A 24 h CHEMICAL CURE

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All the specimens polymerised using the initiator/activator system were significantly harder (p<0.05) at the corresponding time than the cement set by acid-base reaction alone, i.e. 0 wt% initiator/activator concentration. The hardness of all the specimens also increased with time.

At the early stages of setting (30-45 min after mbcing), the surface hardness of the light- cured specimens was similar when the concentrations of CQ were 5 and 7.5 wt% while the DMPT concentration was constant at 5 wt% (Figure 4.6-1). The specimens were slightly harder when CQ concentration increased to 10 wt% and softer when the concentration decreased to 2.5 wt%. Increasing the DMPT concentration from 5 to 10 wt%, while the concentration of CQ was constant at 10 wt%, resulted in a significant decrease in the hardness.

The specimens formulated using 2.5/5 wt% BP/DMPT were slightly softer than those formulated from 2.5/2.5 and 5/5 wt% BP/DMPT only at 20 min after mixing. At other times, there were no differences in the hardness values among the cements using the three formulations (p>0.05).

4.6.3 Discussion

Venz and Antonucci^^^^^ reported that increasing the CQ percentage resulted in a 15% increase in degree of conversion of composite resins whereas increasing amine percentage had less effect. Using a large amount of CQ concentration, besides causing the yellowness of the restorations, has a detrimental effect on the shelf life of the resins due to its slow initiation of the polymerisation(^°). From Figure 4.6-1, it was reasonable to use 5 wt% (=1.2 % of the total cement weight) CQ and DMPT since this is the lowest concentration that gave high initial surface hardness. This is also in the range suggested for use in the commercial RMGICs(^^^'^^'^^2,4i6)

The initiator systems used in commercial RMGICs are summarised below.

T a b le 4 .6 - 1 Initiator s y s t e m u s e d in co m m ercial RMGIC fo rm ulations.

A uthor Initiator/activator A m o u n t

Mitra' Cam phorquinone and a diaryliodonium salt

Reducing ag en t such as ascorbic acid or thiourea

Oxidising ag en t such as potassium or am m onium persulfate 0 ,1 -5 % of total w eight 0 .0 2 -5 % of total w eight 0 .0 2 -5 % of total w eight

Engelbrecht'”*'®’ A mixture of an a-diketone and a tertiary amine and/or a tertiary phosphine

A mixture of an organic peroxide and a com pound th a t contains a tertiary amine and sulfur salt, for exam ple BP and sodium para-toluene sulfinate

5 % of liquid w eight

A ntonucci e t W ater com patible peroxide such as hydrogen

peroxide

Amine com pounds su ch as ascorbic acid or salts

Redox com pounds such as copper salts and iron salts 0 .3 -3 .0 % of liquid w eight 0 .1 -2 % of pow der w eight 0 .0 0 0 1 -0 .1 % of pow der w eight

From Section 4 A we found that CQ and DMPT concentrations as low as 0.5 wt% could be used to polymerise 70/30 THFMA/bis-GMA mixture. The polymerisation of the monomer occurred after 5 min as determined using the spatula. However, in the cement formulation, the presence of glass, acid and water has an effect on the polymerisation of the monomer. The glass and acid both attenuate the intensity of the light passing through the cement paste. This reduces the quantity of free radicals essential for initiation of the polymerisation reaction. The free radicals formed can be quenched in the presence of water. Therefore, in this study the concentrations of CQ and DMPT started from 2.5 wt%. The same explanation may apply for the reason why the ‘command set’ was not obtained in this system. The cement pastes were soft at 5 min. The particle size of the initiator also accounts for its efficiency in initiating the

polymerisation reaction; finer particles are more effective. In this study, CQ was ground before incorporation into the system.

The BP and DMPT concentrations as low as 2.5 wt% could be used without an effect on the surface hardness of the specimens. Comparing the two initiator systems at the same concentrations (2.5/2.S, 2.5/5, 5/5 wt%) (Figures 4.6-1 and 4.6-2), the cements formulated using BP/DMPT were harder initially; the cements had the indentation depth of 200 jim at 20 min compared to 45 min for those formulated with CQ/DMPT. It was considered that this was not due to changes in the light curing device since the light output was checked regularly with a radiometer. The difference is attributable to the attenuation of light in the cement which reduced the efficiency of the system. The system tested was probably not sufficiently optimised. It is highly likely that the glass- resin mbc had a mismatch of optical properties.

4.6.4 Conclusion

Increasing the CQ concentrations from 2.5-10 wt% resulted in an increase in the surface hardness of the cements while increasing the DMPT concentrations from 5-10 wt% decreased the hardness. The 5 wt% CQ/DMPT was the lowest concentration that gave the cements with high initial hardness. There were no differences in the hardness when the three concentrations of BP/DMPT were used.

In the following experiments, the efficiency of the 70/30 THFMA/bis-GMA monomer mixture was compared with that of other monomers such as HEMA and 70/30 HEMA/bis-GMA mixture. HEMA was used because it is the monomer found in commercial RMGICs. These model systems were used instead of the commercial materials since the differences found would reflect the efficiency of the monomers themselves. In the commercial materials, the system has already been optimised. The

setting reaction and the temperature rise during setting of cements prepared from these monomers were compared initially. Other properties such as, surface hardness on storage in distilled water, water sorption, and flexural strength were also investigated.