Polymerisation kinetics in a fibre reinforced resin-based composite
2 x 16 mm. Irradiation occurred with the above
described LCU on top and bottom of the specimens,
with three light exposures of 20 seconds per side,
overlapping one irradiated section no more than
1 mm of the diameter of the light guide, in order
to prevent multiple polymerization. After removal
from the mould the specimens were grinded with
silicon carbide sand paper (grit size P 1200/4000
(Leco)) in order to remove excess material. All
specimens were then stored in distilled water at
37°C for 24 h prior to testing. The specimens were
loaded until failure using a universal testing machine
(Z 2.5, Zwick/ Roell, Ulm, Germany) in a three-point
bending test device, constructed according to the
guidelines of NIST No. 4877 with 12 mm distance
between the supports. Specimens were immersed
in distilled water at room temperature during
testing. The crosshead speed was 0.5 mm/min.
The universal testing machine measured the force
during bending as function of deflection of the
beam. The bending modulus was calculated from
the slope of the linear part of the force-deflection
diagram.
2.3. Micro-mechanical properties
Fragments (n = 10) of the three-point-bending
test specimens were used to determine the
micro-mechanical properties – Vickers hardness
(HV), indentation modulus (Y HU ) and Creep (Cr) -
according to DIN 50359-1:1997-10 by means of a
universal hardness device (Fischerscope H100C,
Fischer, Sindelfingen, Germany). Prior to testing,
the specimens were polished with a diamond
suspension (mean grain size: 1 µm). Measurements
were done on the first polymerised side of the
specimens (6 measurements per specimens, 60
measurements in total). The test procedure was
carried out force controlled; the test load increased
and decreased with constant speed between 0.4
mN and 500 mN. The load and the penetration
depth of the indenter were continuously measured
during the load-unload-hysteresis. The Universal
hardness is defined as the test force divided by the
apparent area of the indentation under the applied
test force. From a multiplicity of measurements,
a conversion factor between Universal hardness
(Martens Hardness) and Vickers hardness was
calculated and implemented in the software, so that
the measurement results were indicated in the more
familiar Vickers hardness (HV) units. The indentation
modulus (Y HU ) was calculated from the slope of the
tangent of indentation depth-curve at maximum
force. By measuring the change in indentation
depth with constant test force, a relative change in
the indentation depth can be calculated. This is a
value for the creep of the materials.
2.4. Curing unit characteristics
The analysis of the variation in irradiance delivered
at distances up to 10 mm away from the light tip was
performed in 1 mm steps (n = 3) on a laboratory-
grade NIST-referenced USB4000 Spectrometer
(MARC (Managing Accurate Resin Curing) System,
Bluelight Analytics Inc., Halifax, Canada). The
miniature fiber optic USB4000 Spectrometer uses
a 3648-element Toshiba linear CCD array detector
and high-speed electronics. The spectrometer
has been spectro-radiometrically calibrated using
Ocean Optics’ NIST-traceable light source (300-
1050 nm). The system uses a CC3-UV Cosine
Corrector to collect radiation over 180º field of view
thus mitigating the effects of optical interference
associated with light collection sampling geometry.
2.5. Statistical analysis
Statistical analysis were performed using statistical
software (SPSS Inc. 23.0, Chicago, IL, USA). A
Shapiro-Wilk test verified the normal distribution of
the data. Descriptive statistics and 95% confidence
interval are indicated. DC and irradiance results
were statistically compared using one-way ANOVA
followed up with Tukey’s post hoc test. In all
statistical tests, p-values < 0.05 were considered
statistically significant.
3. Results
3.1. Degree of conversion (DC) and polymerisation
kinetic
The degree of conversion as function of increment
thickness measured 300 s after initiating the
polymerisation as well as the kinetic parameters
are summarised in Table 1.
One way ANOVA revealed no significant difference
in DC measured 300 s post irradiation in a depth
of 100-µm and 2-mm (p = 0.281). Similarly, no
significant difference was identified between DC
measured at 2-mm and 4-mm (p = 0.724), while
the DC measured at 6-mm depth was significantly
lowest (p < 0.001).
The polymerisation kinetic (Fig 1, Table 1) was
well described (R² > 0.95) by an exponential sum
function. Parameters “a” and “b” are describing
the gel phase of the polymerisation process.
Table 1. DC (%) measured 300 s post-irradiation (mean and standard deviation in parenthesis; superscript letters indicate
statistically homogeneous subgroups as function of specimen’s thickness, Tukey’s HSD test, α = 0.05), maximal rate of
polymerisation, Rate max , and parameters of the exponential sum function (95% c