METHOD ACCURACY DETERMINATION
The silicon content of organosilicon materials was determined by AAS after dilution of
the siloxane of interest in an organic solvent. However, the silicon absorption signal, or
signal intensity, can vary, depending on the chemical structure of the siloxane. Calibra-
tion error is minimized if a siloxane of known purity and having the same chemical
structure and vapor pressure as the material to be assayed is found from which to pre-
pare calibration standards. In actual practice, it was more convenient to compare the
signal intensity of the compounds of interest to calibrating solutions prepared from
known purity standard materials and then to decide if the signal response differences
between the materials introduced an acceptable level of error. If the error was only a
percent or so relative to the actual silicon content of the material, that error is negli-
gible when measuring silicon levels of a few parts per million. Normal instrumental
signal-to-noise ratios will impart far greater measurement error, especially with
readings close to the detection limit. Standard deviation of the measurement is given in
all of the data tables.
To test the accuracy of the method for detecting the proper ratio of Si polymer emul-
sion, several samples were assayed. Table I contains the results from samples of three
different aminofunctional siloxane emulsions: amodimethicone and two trimethylsilyl-
amodimethicone (TSA) polymers [see Figure 1] with different polymer chain lengths
and percent amine functionality. The assayed Si values are within about 10% of the
calculated values.
The calibrating standard used in the previous example was a well-characterized di-
methyl fluid with a viscosity of 1000 centistokes. To check the correlation of various
neat organofunctional siloxane fluids with this standard, several samples of different
functionality siloxane fluids were analyzed. Those data are reported in Table II. The Si
assay of these siloxane polymers matched very closely to the calculated % Si values also,
except for one polymer (siloxane E) which contained a very long dimethyl siloxane
chain, and which because of its high molecular weight is less soluble in the organic
solvent. As a result of these tests, it was decided to use the 1000-centistoke dimethyl
fluid as the primary standard for all of the future silicon determinations.
PRECISION OF METHOD
The method was tested for reproducibility (precision) by analyzing duplicate samples