SAMPE Journal - November/December 2016 - 40
Feature Article
Figure 4. Shrinkage test values.
Figure 5. Measured values of shrinkage vs. hardness at 0
cures of tested material.
Most of technical literature deals not with rubber
shrinkage after thermal cycling but shrinkage after the
rubber part is made, that is shrinkage after molding.
In this latter case, shrinkage is a result of ingredients,
such as process aids, plasticizers and other constituents
being leached out13. Shrinkage after molding is also
affected by cure time, temperature, pressure, inserts,
post-cure, etc. Soft elastomers generally shrink more
than harder elastomers14. Figure 5 is the plot of shrinkage
vs. hardness of tested material at zero cures, with the
dotted line being the best fit, and showing that in three
out of four materials higher hardness is accompanied by
lower shrinkage, as stated in14.
The standard deviation values in Table 5 are equivalent
to less than 3% of average values, indicating contained
scattering, and hence a quite defined trend.
The hardness increase for KE 1310ST was consistent
with the 12% hardness increase after 24 hr at 180°C
reported by Shin-Etsu for KE 1300, precursor of KE
1310ST. It also has to be added that general purpose
silicone rubbers made by Shin-Etsu feature hardness
increase or decrease (depending on the material) upon
continuous not intermittent heating exposure.
The hardness decrease in V-340 was confirmed
by mechanical indentation data (force/penetration/
thickness) collected at room temperature on V-340
using thermo-mechanical analyzer (TMA), and plotted in
Figure 7. The TMA test was conducted applying a max
load of 1N at 2 N/min for three times, and calculating
the maximum force/strain value, with strain equal to
maximum penetration depth over coupon's thickness.
Each coupon's result was the average of the three
loading cycles.
Hardness Results
Hardness was measured on two 1.2 cm thick strips.
On each strip 15 reading were taken every 7.5 cm, for a
total of 30 readings for every value in Table 5. Hardness
variation spanned from -2.5% to +13%. Although
the final values across materials differed in sign and
magnitude, they shared a tapering off trend, pictured in
Figure 6, and common to shrinkage readings in Figure 4.
Table 5. Hardness values.
Shore A = average ± standard deviation. CC = cure cycle
40
SAMPE Journal, Volume 52, No. 6, November/December 2016
Table of Contents for the Digital Edition of SAMPE Journal - November/December 2016
Contents
SAMPE Journal - November/December 2016 - Cover1
SAMPE Journal - November/December 2016 - Cover2
SAMPE Journal - November/December 2016 - Contents
SAMPE Journal - November/December 2016 - 2
SAMPE Journal - November/December 2016 - 3
SAMPE Journal - November/December 2016 - 4
SAMPE Journal - November/December 2016 - 5
SAMPE Journal - November/December 2016 - 6
SAMPE Journal - November/December 2016 - 7
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