ORGANIC ALKALI METAL SALT ESTIMATION BY ICP-OES IN THERMOPLASTIC
Rosa AD, SABIC Technology Center, Bangalore, India; Xu Jenny, Wen Liang, STC-Shanghai, China;
Akshay Gouda, Benoi, Singapore; Lin Chen, Nansha, China.
Abstract
Thermoplastic based films are widely used for electrical
and electronic (EE) insulation applications. For EE
applications, high flame retardant properties are required
with increasingly stricter regulations against chlorinated
or brominated formulations. The ongoing research is
focused on developing new grades with alternate flame
retardant (FR) additives giving comparable VTM0 and V0
ratings. In order to deliver this excellent FR performance,
one of the key factor is to control the loading of the FR
additive (organic alkali metal salt) as per formulation
during the production stage. Hence, there was a need to
develop XRF based fast screening method that could be
implemented in manufacturing sites for regular
monitoring of additive loading in new
grade of
thermoplastic containing complex inorganic fillers.
Establishment of XRF method requires generation of
absolute standard values for the organic alkali metal salt
with this new formulation. Analytical efforts were tried to
extract the FR additive by two extraction techniques
followed by analysis using instrumental techniques such
as ion chromatography and LC-MS. However the results
were not consistent due to insufficient extraction of the
salt from the thermoplastic containing complex organic
and inorganic matrix and other inorganic fillers due to
adsorption issues. To overcome these challenges, absolute
method using ICP-OES was developed to quantitatively
estimate the potassium content in the organic alkali metal
salt in this grade and back calculate the % FR additive.
Specific formulations with known concentration of
organic alkali metal salt were compounded and analyzed
by ICP-OES to generate standard values which were used
for XRF calibration.
This presentation covers the
development of successful XRF method based on ICPOES results. Details of method development approach,
comparison of results obtained by two different
techniques
(ICP
and
XRF),
translation
and
implementation of methods to manufacturing sites and
detailed round robin studies at multiple manufacturing
sites to validate the new method are discussed.
Introduction
The use of additive in thermoplastics always has an
influence on the characteristics of the polymer, but the
improvement of some of its properties is often
accompanied by unfavorable side effects. In the case of
some thermoplastics often the addition of small quantity
of an additive may affect the optical characteristics and
the transparency of the final product. Also the
characteristics of the additive itself can limit its
utilization. In the case of brominated flame retardants,
increasing concern for environmental and health issues, is
encouraging users to look for bromine and chlorine free
flame retardants. This increases the interest of
thermoplastic producers and compounders to find
alternatives to these additives. The main objective of this
study was to establish a standard curve in XRF for the FR
additive in the new grade of thermoplastic. In order to
deliver excellent UL performance, one of the key factor
was to control the loading of the FR additive as per
specification during the production stage. The FR additive
is quantitatively characterized using a XRF method. In
order to establish the XRF method for routine analysis of
FR additive in the new grade, absolute quantification of
FR additive was essential to build a calibration curve in
XRF. In our efforts to develop quantification method, we
first chose chromatographic techniques such as ion
chromatography and LC-MS (1) by two different extraction
techniques. The extraction methods used failed to extract
the complete FR additive into the solvents used for
extraction. The samples were then estimated for
potassium by ICP-OES and back calculated for FR
additive as it is a salt containing potassium. The ICP-OES
data was accurate however due to inhomogeneity in the
samples, consistent data was not obtained. Therefore an
in-house formulation and compounding study by
controlled conditions was carried out to obtain accurate
results by ICP-OES. This report covers the result for the
round robin study conducted with the absolute numbers
generated by ICP-OES to validate the new ICP-OES
method.
Methodology
For ion chromatography studies, two different extraction
procedures were used to extract the FR additive into the
solvents followed by ion chromatography.
Extraction 1: Around 0.50 g sample was accurately
dissolved in 10 ml chloroform for 1 hour or till the sample
dissolved completely. Then 25 ml of water was added
and shaken for 5 minutes and allowed for settling till
chloroform and water layer separated. The aqueous layer
was filtered and injected into IC. The instrument
conditions used is as given below.
Instrument used: Dionex ICS 3000, Column: AS 19 Ion
Pac Analytical Column: 4 × 250 mm, Ion Pac AG19
Guard Column: and 4 × 50 mm, Eluent: 20mM KOH,
Flow rate: 1.0 ml, Detector: Conductivity.
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Extraction 2: Around 0.5 g sample was accurately
dissolved in 5 ml chloroform and precipitated with 25 ml
methanol. From this solution, 10ml of the methanol was
filtered and evaporated and residue re-dissolved in 10 ml
of water and injected into IC. The instrument conditions
were same as followed for extraction 1.
ICP for total potassium content: About 0.2 g of the
sample was digested with 7 ml conc. nitric acid using
Anton paar microwave digester. The solutions were
analyzed for potassium using ICP-OES. The instrumental
conditions were as follows.
Instrument make: Spectro Cirros, Nebulizer- cross flow,
Plasma power- 1400 W, Coolant flow14 l/min,
Nebulizer flow- 0.8 l/min, auxiliary flow-1 l/min,
calibration standards- 0.5 ppm to 5 ppm range, internal
standard -1 ppm of scandium.
ingredients. Microwave digestion was carried out for
sample preparation and estimated total potassium using
ICP–OES. The values obtained by the two extraction
processes and also by ICP-OES Vs the formulation values
are as shown in Figure 1.
Discussion
Figure 1: % FR additive recovery by IC, ICP-OES from
actual loading from formulation
FR additive estimation was carried out based on the report
which was used to estimate the FR additive in
thermoplastic without inorganic fillers. However, the
formulation of the samples in our study contained
complex inorganic fillers. Based on the solubility property
of FR additive, it has solubility in water and also good
solubility in methanol. In the first extraction method,
water was used as solvent to extract and in the second
extraction method, methanol was used for extraction. In
all these samples there were inorganic fillers which
remained undissolved in the solution after the sample
dissolution in chloroform. The extracts were analyzed by
ion chromatography. However the extraction process was
not efficient enough to extract the complete FR additive
into water or even methanol by both the extraction
techniques due to adsorption properties of the additives.
The values obtained from extraction 1 was only one third
of the actual FR loading in the formulation. This indicated
that the extraction was incomplete in water and hence
extraction 2 process was followed. In this method,
polymer was dissolved first in chloroform and
reprecipitated with methanol in which the FR additive is
known to have good solubility. This solution was
evaporated to remove methanol and the residue was
redissolved in water and injected into IC. By extraction
method 2, values obtained were higher compared to
extraction method 1 however the actual FR additive
content as per formulation could not be extracted. All the
extractions were carried out at room temperatures.
ICP-OES, an elemental technique, was considered as both
the extraction techniques failed to extract the FR additive
completely into the extraction solutions. ICP-OES is a
technique which can determine the total metals present in
a sample at sub ppm levels. Since this FR additive is
made of an organic alkali metal salt containing potassium,
total potassium content estimation in the sample present
can be back calculated to % FR additive, provided there is
no contamination of potassium from other formulation
The values based on total potassium by ICP-OES was
higher than the two extraction techniques by IC by 2 to 3
times. However the values were still lower than the
expected formulation values. Although ICP-OES is an
absolute technique by which the absolute number could be
estimated, however there was inhomogeneity in the
sample due to which inconsistent readings were obtained.
Therefore an in-house formulation and compounding with
small batch (100 g) was carried out to understand how the
distribution happens during mixing and compounding. 5
batches were formulated and compounded and FR
additive was analyzed by ICP-OES. Batches 1, 2, 3 had
FR additive in formulation whereas the batch 4 and batch
5 did not contain FR additive. The results of first three
batches were close to the formulation values which was
above 93% of actual loading. Batches 4 & 5 and also plain
thermoplastic did not show any presence of potassium by
ICP-OES, indicating that the other formulation ingredients
do not contain potassium as contaminant. If the mixing
and compounding is done under controlled conditions, the
FR additive distribution in the sample can be controlled
and accurate results can be obtained as shown in Figure 2.
Figure 2: % FR additive recovered based on ICP-OES
from in-house formulation with 100g batch
SPE ANTEC™ Indianapolis 2016 / 1409
This study generated the values of FR additives to be used
as standards for generating standard calibration by
XRF (2). The values were repeatable with standard
deviations between 0.006 to 0.01. Based on these data,
10 more samples were re-compounded and absolute
values for FR additive were generated by ICP-OES. The
values lied between 80 to 88 % of the actual loading but
with good standard deviation as given in Table 1.
The Z-score test indicated the data lies within allowable
limit (±2) thus validating the round robin as shown in
Figure 4.
Table 1: % of FR measured by ICP-OES with standard
deviation
% Recovery of FR from formulation
std dev
Sample 1
80
0.001
sample 2
79
0.005
Sample 3
88
0.015
Sample 4
87
0.012
Sample 5
83
0.013
Sample 6
83
0.036
Sample 7
84
0.021
Sample 8
79
0.006
Sample 9
88
0.011
Sample 10
83
0.005
These results were used as calibration values in XRF and
it was translated to 5 manufacturing sites. To validate the
calibrations set up in XRF at the different sites based on
the values generated by ICP-OES, an unknown sample
was analyzed using the new calibration at all sites and the
same was analyzed by using ICP-OES (3). The values of
all the sites were very close to the actual loading of FR
both by XRF and ICP-OES as shown in Table 2.
Table 2: % of FR data match with the unknown sample by
XRF and ICP-OES
Site 1 (XRF) Site 2 (XRF) Site 3 (XRF) Site 4 (XRF) Site 5 (ICP-OES)
% match
93
95
96
93
96
std dev
0.004
0.005
0.006
0.005
0.170
Statistical tools were used to evaluate the data obtained
from across the sites. Homogeneity of variance test was
used to see variances are equal across groups or samples.
The Levene test was used to verify that assumption.
The homogeneity of variance test showed the P value
greater than 0.05 (0.096) indicating there is no significant
variation between the values as shown in Figure 3.
Figure 4: Z-score statistical analysis
Z scores (4) are measures of standard deviation. This
distribution relates standard deviations with probabilities
and allows significance and confidence to be attached to Z
scores and p-values. The statistical tools further validated
the method adopted for the estimation of FR additive.
Conclusions
In summary, a method was developed to accurately
estimate FR additive concentration in thermoplastic
formulations containing inorganic fillers. It was observed
that while the FR additives estimation in neat
thermoplastic was straight forward, the presence of
inorganic fillers in formulations inhibited the complete
extraction and estimation of the FR additives due to
adsorption. This issue is compounded if the samples are
not homogeneous in nature (well mixed and
compounded). To address and understand this issue, inhouse formulation and compounding studies were carried
out. ICP-OES was used to estimate total potassium
content as this FR additive is an organic salt containing
potassium. The potassium values were used to back
calculate the % FR additive. The results generated by
ICP-OES was used as calibration standards for XRF. This
method would enable the preparation of similar
formulations for improved FR performance.
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Figure 3: Homogeneity of variance test
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