The selection of SBC and rheology of
the compound are the most critical to control
surface smoothness of the finished product.
Figure 1 shows the TEM of two of SEBS/oil/PP
compound(1). Styrene domain is stained black.
The styrene domain of high molecular weight
SEBS is less than visible wavelength. However,
there is significant difference of appearance of
the two compounds. The one with low
molecular weight of SEBS is transparent and the
one with high molecular weight SEBS is
transclscent. It is difficult to make a judgement
of clarity from the difference of morphology
based on TEM pictures.
Service Temperature
DMA (dynamic mechanical analyzer)
can be used to evaluate the service temperature.
Test sample is clamped and strain-temperature
relation under constant load is measured. For
sterilization purpose, such as boiling water and
autoclave, low stress loading can be used. For
application at high stress and high temperature, a
stress loading similar to application environment
is preferred. Examples of typical DMA results
are shown in Figure 2.
Hardness and modulus
Soft touch feeling is a combination of
both hardness and modulus. Feeling of “softness” of two TPE of same hardness but different chemistry can be quite different, because of the difference of modulus. Examples are given later to show the difference of modulus of TPE of difference chemistry.
Mechanical properties such as tensile
and tear are specified design requirement. Often
there are more than one formula which satisfy
these requirements. Two examples in Figure 3
show are shown to have similar tensile and
elongation, while different in modulus. The
sample with a lower modulus is felt softer than
the higher modulus one.
SBC compound application
I. Silicone replacement for baby nipple
Majority of baby nipple/pacifier is made
from silicone and natural rubber. Transparent
SBC compound was developed, which has
similar appearance as silicone rubber and