where Ai is the average effect of factor A at level i, yj
i
represents the j-th observation (result) of factor A at
level i, and n stands for total number of observations
for factor A. By plotting the average factor effect
against the corresponding factor level, an average
plot is obtained from which the trend of influence of
a factor on the result is extracted. The relative influence
of factors (contribution) on the result can also
be obtained by somewhat more rigorous statistical
calculations known as analysis of variance
(ANOVA), see the details in Ref. 25. In this study,
calculations of both ANOVA and average factorlevel
effect are performed by using QT4 software.25
Preparation of compounds
The compounds are prepared by a laboratory scale
two-roll mill. The rubbers are first masticated at temperature
around 508C for 5 min. Then the masticated
rubbers are mixed further with PH and other ingredients
for around additional 25 min. The obtained
compounds are cured into a sheet using compression
molding at 1508C for 1 h under a pressure of 3.5
MPa in accordance to ASTM D-3182. The samples
required for mechanical and physical tests are cut
from the cured sheets.