a Processor’s advice on how to get Moisture testing right

tips & techniques
a Processor’s advice on
how to get Moisture testing right
By Joel Lischefski and Alison Maicke, Teel Analytical Laboratories
gravimetric moisture analyzers are the most
commonly used instrument for determining
the percentage of moisture in resin. the
instrument is essentially a sensitive balance contained within an
oven. The sample is placed inside and the mass is compared before
and after the heating cycle to determine the percent moisture.
Performance specifications for these instruments are often
derived in tightly controlled laboratory environments rather than
in a production or plant setting. As a result the values displayed
on product brochures may not represent the instrument’s performance in a plant setting using your methods and materials. It is
important to verify your instrument’s performance where it’s
actually used and with your methods and materials.
eDitOR’S nOte: about a year ago (see Processor strategies,
Jan. 2012), we reported on how a 2010 decision to invest in a
state-of-the-art laboratory was paying dividends for custom profile
extruder teel Plastics in baraboo, Wis. here, two of teel’s key lab
personnel share their insights on gravimetric moisture analysis
and how to ensure your testing procedures are appropriate.
SenSitivity iS Key
It is helpful to get a sense of scale for what it is that you’re
measuring with a moisture balance. If you assume that your
target moisture for a resin is no greater than 0.02% and you
use a 40-g sample, than you need to be able to accurately
measure a difference in weight of 8 mg (0.008 g). That
amount of water is roughly the size of a pin head. If your
limits are even lower than 0.02%, the amount you must measure reliably is even tinier.
When dealing with measurements of such small quantities,
minor issues can cause large effects in results. A variety of interferences can affect the instrument reading, including these six :
•Air currents,
•Vibrations,
•Electromagnetic fields/static,
•Whether the unit is level,
•The materials used,
•The method used.
All of these factors indicate that you should read carefully
the manual that comes with your instrument and place it in the
best possible location.
In addition, take the following five steps to characterize the
performance of your moisture balance:
Step 1: Ensure that your moisture balance does not have a
bias or interferences. Run five replicate tests with a weight
placed in the center of an empty pan. The weight simulates the
mass of your sample, but will not give off any mass when heated. The results of all the tests should ideally be 0%, but there
are many reasons it may be otherwise.
Instrument sensitivity is crucial when measuring moisture,
as sometimes the amount of water is the size of a pin head.
Gravimetric moisture analyzers are commonly used to determine the percentage of moisture in resin. The instrument is
essentially a sensitive balance contained within an oven.
34 february 2013 Plastics technology
02 FEATURE ExtTips.indd 34
1/11/13 11:01 AM
moisture analysis
The six common interferences listed above may come into play
and cause a bias in the result. Also, something as simple as a single
beadFIve
of sweat
from the hand of the person running the test transS’ By RunnIng
SAmpLeS
ferring to the outside of the test pan would be enough to give you 8
mg Aof mass that
will evaporate.
Instrument
Instrument
B Other things may come into play
Weight Loss,
mg OilsWeight
Loss,
mghands to the pan, or residual oils on
as well.
transferred
from
the sample pan from its own manufacturing process may be lost
3.6
0.5
during the heating process. The moisture balance is indiscriminate
3.3 in measuring what evaporates
0.4
off your pan and sample. Any mass
being water, whether it is or not. By running
3.7 loss is interpreted as0.5
five replicates you will have a good idea of whether your moisture
3.3
0.5
balance has an issue with a biased result. Table 1 shows real data
4.1 from a pair of moisture
0.3 balances tested in this way.
Instrument A in Table 1 shows a definite bias that should be
3.6
0.44
reduced or eliminated in order to obtain true results. Instrument B
9.21shows a small amount
0.089
of bias that will provide a more accurate result.
The variation in measurements shown by the percent relative stanreplicates will give
a good
ofalso
whether
dardyou
deviation
(%idea
RSD)
indicates that results from Instrument A
balance has an
issue
with
a
biased
result.
will have much more variation than results from instrument B.
al data from a pair of moisture balances testStep 2: Characterize the detection limits. One of the most
.
common methods of determining the method detection limit
comes from the U.S. EPA standard, 40 CFR 136, Appendix B. A
simplified version of this method is explained here for illustration.
Please refer to the CFR standard when characterizing your instruWeIght LoSS
CompARISon:
ment.
The process involves testing seven replicate samples the same
way
as in Step 1, AnALyzeR
except that to each sample a precise, known
BALAnCe vS.
moIStuRe
amount of water is added. The amount added should be around
three to five timesmoisture
the expected detection limit of the instrument.
moisture
milligrams
is a good
Balance,Twenty
%
Analyzer,
% place to start for many moisture
balances. The 20 mg can either be weighed out with purchased
0.2990
0.2119
solid standards certified to contain a certain amount of water or by
0.3030
0.1896
accurately measuring
out 20µl of water in a very accurate syringe.
of this test is to define the minimum amount of
0.3120 The purpose 0.2017
moisture that can be detected, and to identify the minimum
0.3050
0.1842
amount of water that can be quantified with a reasonable
0.2960
0.1640
amount of accuracy.
These two quantities are referred to as the
Method Detection
Limit (MDL) and the Limit of Quantitation
0.3010
0.1919
(LOQ). Ideally your LOQ should be below the moisture level
0.3070
0.2554
that you want to dry your materials to, ensuring that you can
0.2940
0.1262
accurately measure
a value below the specification limit. The
sidebar on p. 37 0.2169
shows the equations needed to calculate the
0.3020
MDL and LOQ from
the seven replicates.
0.3100
0.1992
Step 3: Verify that you’re actually testing water evaporation.
0.3029
This step involves0.1941
sending a split sample to an outside labora1.91tory to test the amount
17.4 of water that is given off by the sample.
The two instruments that can do this are a Karl Fischer-type
titration
instrument
a moisture analyzer with a relativeas conducted with
relative
humidity or
detection
humidity
detector.
Both instruments are selective in dee type of sample.
The % (RH)
relative
standard
tecting
justmoisture
the water
that is within the sample.
SD) is much larger
for the
analyzer
aller sample size, as
it is more
susceptible
Moisture
balances
onlytodetect changes in weight; because of
s in the moisture
quantities
of
the
sample.
By nothing to do with the moisture
this, other processes that have
0 replicates, variations in the samples can be
level
can obtained.
bias the result. Chemical reactions occurring at elevatand a true overall
value
ed temperatures in air can result in either gaining or losing mass.
Plastics technology february 2013 35
02 FEATURE ExtTips.indd 35
1/11/13 11:01 AM
moisture analysis
TAble 1
AvoId ‘BIAS’ By RunnIng FIve SAmpLeS
Sample
Replicate
Instrument A
Weight Loss, mg
Instrument B
Weight Loss, mg
1
3.6
0.5
2
3.3
0.4
3
3.7
0.5
4
3.3
0.5
5
4.1
0.3
Average
3.6
0.44
% RSD
9.21
0.089
Running five replicates will give you a good idea of whether
your moisture balance has an issue with a biased result.
This shows real data from a pair of moisture balances tested in this way.
TAble 2
mAteRIAL WeIght LoSS CompARISon:
moIStuRe BALAnCe vS. moIStuRe AnALyzeR
Sample
Replicate
moisture
Balance, %
moisture
Analyzer, %
1
0.2990
0.2119
2
0.3030
0.1896
3
0.3120
0.2017
4
0.3050
0.1842
5
0.2960
0.1640
6
0.3010
0.1919
7
0.3070
0.2554
8
0.2940
0.1262
9
0.3020
0.2169
10
0.3100
0.1992
Average
0.3029
0.1941
% RSD
1.91
17.4
This study was conducted with relative humidity detection
using the same type of sample. The % relative standard
deviation (RSD) is much larger for the moisture analyzer
due to the smaller sample size, as it is more susceptible to
slight changes in the moisture quantities of the sample. By
performing 10 replicates, variations in the samples can be
averaged out and a true overall value obtained.
Plastics technology february 2013 35
1/11/13 11:01 AM
moisture analysis
Calculating MDL & LOQ
The six common interferences listed above may come into play
TAble 1
and cause a bias in the result. Also, something as simple as a single
Calculations:
bead of sweat from the hand of the person running
the test transAvoId ‘BIAS’ By RunnIng FIve SAmpLeS
ferring to
the outside of theResult,
test pan would be enough
to give
you 8
StanDarD
DEviatiOn:
LiMit OF QuantitatiOn (LOQ):
Sample
mg of mass
that will evaporate.
Replicate
mgOther things may come into play
Sample
Instrument
Instrument B
10A
Replicate
Weight Loss, mg
Weight Loss, mg
as well. Oils transferred from hands to the pan, or residual oils on
Using these calculations with the
1 pan from its own22.9
the sample
manufacturing process may be lost
1
3.6
0.5
example data set:
sample standard deviation
during the heating process. The moisture balance=isThe
indiscriminate
0.4
Mean 3.3
value = 21.94 mg
= The number
of values in2the data set
in measuring
off your pan andNsample.
Any mass
2 what evaporates
23.4
Standard
deviation
=
1.02
mg
AnBy
individual
loss is interpreted as being water, whether it is xori =not.
running value of 3the data set
3.7
0.5
MDL = 3.2 mg
x-baryour
= The
mean of the values in the
five replicates you will have a good idea of whether
moisture
3
20.6
4
0.5
LOQ =3.3
10.2 mg
set real data
balance has an issue with a biased result. Tabledata
1 shows
5
4.1the instrument detects
0.3
Since
mass, the
from a pair of moisture balances tested in this way.
DEtECtiOn LiMit (MDL):
4
21.2a definite biasMEthOD
detection limits are in units of mass. This
Instrument
A in Table 1 shows
that should be
Average
3.6
0.44
means that the percent moisture that can
reduced or eliminated in order to obtain true results. Instrument B
% RSD
9.21 is a function of the0.089
5 amount of bias21.8
= “Student’s
t value” for the
number
be detected
sample
shows a small
that will provide a tmore
accurate result.
of replicates
the 99% confidence level.
size. In other words the MDL for a 10The variation in measurements shown by the percent
relative at
stanRunning
will give
you
a good
of whether
(Can
beInstrument
found in a A
reference
table in afive replicates
gram sample
size
would
be idea
3.2 mg/10
g=
dard deviation
indicates that results
from
6 (% RSD) also22.5
your
moisture
balance
has
an
issue
with
a
biased
textbook,
or on the internet. The
0.032%. For a 50-g sample size, result.
the MDL
will have much more variation than results fromstatistics
instrument
B.
This shows real data from a pair of moisture balances testvalue
changes
level and would be 3.2 mg/50 g = 0.0064%. The
Step 2: Characterize the detection limits. One
of the
mostwith the confidence
ed in this way.
7
21.2
the number
of replicates used. For this
larger the sample size, the lower the percommon methods of determining the method detection
limit
example
the student’s
t value equals 3.143). cent moisture you will be able to detect.
comes from the U.S. EPA standard, 40 CFR 136,
Appendix
B. A
simplified version of this method is explained here for illustration.
TAble 2
Explanation
Figure
PleaseMDL
refer to
the CFR standard
when characterizing your instrumAteRIAL
WeIght LoSS CompARISon:
ment. The process involves testing seven
replicate samples the same
MDl
loQ
way as in Step 1, except that to each sample a precise, known
moIStuRe BALAnCe vS. moIStuRe AnALyzeR
amount of water is added. The amount added should be around
three to five times region
the expected
1 detection limit of the instrument.
region 2
region
3
Sample
moisture
moisture
Twenty milligrams is a good place to start for many moisture
Replicate
Balance, %
Analyzer, %
increasing
Mass lost
balances. The 20 mg can either be weighed
out with purchased
1
0.2990
0.2119
solid standards certified to contain a certain amount of water or by
2
region
1: Starting
fromofthe
leftinata0very
mg,accurate
up to thesyringe.
MDL,
confidence
level, but the 0.3030
actual numerical value0.1896
has a large
accurately
measuring
out 20µl
water
the instrument
reliably
the amount
of of
degree
The
purpose of can’t
this test
is to differentiate
define the minimum
amount
3 of uncertainty. 0.3120
0.2017
water that
lost from
thedetected,
measurement
Anythe
result
in this
region 3: This region is where the numerical values assigned
moisture
can be
and tonoise.
identify
minimum
4
0.3050
0.1842
region
less than
canwith
detect.
to the mass loss have a reasonable degree of confidence. You
amount
of iswater
that the
caninstrument
be quantified
a reasonable
5 be able to determine
0.2960
0.1640
2: In this
region,
between the
and to
theas the
should
whether your materials
are dry or
amountregion
of accuracy.
These
two quantities
areMDL
referred
LOQ,Detection
the amount
of mass
loss
is greater
than
0 with a 99%
not 6by producing values from
your instrument in Region
Method
Limit
(MDL)
and
the Limit
of Quantitation
0.3010
0.1919 3.
(LOQ). Ideally your LOQ should be below the moisture level
7
0.3070
0.2554
that you want to dry your materials to, ensuring that you can
8 shown in Table 2, the
0.2940
0.1262
Some additives
may aevaporate
out the
of the
plastic at higher
As
% RSD is much larger
for the moisaccurately
measure
value below
specification
limit. temThe
peratures
that
weight
will be needed
interpreted
by the instruture9analyzer due to the smaller
it more
sidebar
onand
p. 37
shows
theloss
equations
to calculate
the
0.3020sample size, making
0.2169
ment as
losing
susceptible to slight changes in the moisture content of the samMDL
and
LOQmoisture.
from the seven replicates.
10
0.3100
0.1992
By sending
outthat
a split
sample
you’lltesting
be ablewater
to measure
and
ple. By performing 10 replicates, variations in the samples can be
Step
3: Verify
you’re
actually
evaporation.
Average
0.3029
0.1941
compare
resultssending
from your
moisture
to the laboraamount
averaged
out and a true value
for the overall sample
can be
This
step the
involves
a split
samplebalance
to an outside
% RSDThe moisture balance
1.91 required a much17.4
of actual
water
to makeof
sure
the that
material
isn’toff
undergoing
some
measured.
larger sample
tory
to test
the amount
water
is given
by the sample.
othertwo
process
that is producing
a biased
size for testing.
The
instruments
that can do
this areresult.
a Karl Fischer-type
The moisture
balance
is only capable
of with
detecting
weight
Step 4: Evaluation. Once all the data is collected it needs to be
titration
instrument
or a moisture
analyzer
a relativeThis study was conducted with relative humidity detection
loss, regardless
of the kind
of instruments
mass loss that
occurring.
A
evaluated.
your of
blank
results
or nearstandard
0 mg so that
humidity
(RH) detector.
Both
areisselective
in deusing theIdeally
same type
sample.
Theare
%atrelative
true moisture
is capable
detecting
there
are no biases
within
the instrument
or the
tecting
just theanalyzer
water that
is withinofthe
sample.only water loss
deviation
(RSD)orisinterferences
much larger for
the moisture
analyzer
due toIfthe
smaller
samplethey
size,need
as itto
is be
more
susceptible
to
by Moisture
utilizing the
RH detector,
which
can discern
the because
difference
method.
there
are biases,
identified
and elimibalances
only detect
changes
in weight;
of
slight
changes
in
the
moisture
quantities
of
the
sample.
By
between
and other
volatile
vapors
being
released
from
nated to the best of your ability. This may mean moving the
this,
otherwater
processes
that have
nothing
to do
with
the moisture
performing 10 replicates, variations in the samples can be
the sample.
2 shows
that thereactions
sample isoccurring
losing roughly
level
can biasTable
the result.
Chemical
at elevataveraged out and a true overall value obtained.
0.1%
of its massinthat
not
water.
ed
temperatures
air is
can
result
in either gaining or losing mass.
Continued on p. 47...
Plastics technology
technology february
february 2013
2013 35
37
Plastics
02 FEATURE
FEATURE ExtTips.indd
ExtTips.indd 35
37
02
1/11/13 11:01
2:03 AM
PM
1/11/13
tips & techniques
moisture analysis
a
Processor’s
advice on
…Continued from p. 37
how to get Moisture testing right
By Joel Lischefski
and Alison
Maicke,
Teel Analytical
Laboratories
moisture
balance
to a new
location
or altering
the
eDitOR’S nOte: about a year ago (see Processor strategies,
gravimetric
moisture
analyzers
are
the
most
method,
as using rubber
gloves to avoid
transferJan. 2012), we reported on how a 2010 decision to invest in a
voId ‘BIAS’
By such
RunnIng
FIve SAmpLeS
state-of-the-art laboratory was paying dividends for custom profile
commonly used instrument for determining
ring
sweat or grease onto the sample or sample
pan.
extruder teel
Plastics in baraboo, Wis. here, two of teel’s key lab
the percentage of moisture in resin. the
personnel share their insights on gravimetric moisture analysis
andenough
how to ensure your testing procedures are appropriate.
The
detection
limits
should
be
low
ample
Instrument
A contained within
Instrument
B
instrument is
essentially a sensitive balance
an
oven. The
sample
is placed
inside and
themeasure
mass is compared
eplicate that
Weight
Loss,
mg
Weight
mg
you
can
accurately
tobefore
theLoss,
specification
ble 1
Dedicated to quality laboratory testing
and after the heating cycle to determine the percent moisture.
Performance
specifications
for these instruments
often
levels
of your
materials.
If the are
amount
you’re trying
3.6
0.5
derived in tightly controlled laboratory environments rather than
in
a
production
or
plant
setting.
As
a
result
the
values
displayed
to measure is below your LOQ that means that the
on product brochures 3.3
may not represent the instrument’s perfor- 0.4
mance inof
a plant
setting
using your methods
and materials.
is
value
the
instrument
reading
is Ithighly
variable
important to verify your instrument’s performance where it’s
0.5ultimately
actually
used andunreliable
with3.7
your methods
materials.
and
more
atandthat
level. That
SenSitivity iS to
Keymore
translates
mois3.3 variability in the measured
0.5
It is helpful to get a sense of scale for what it is that you’re
measuring
with a material
moisture balance.
If you
thatprocessed.
your
ture
of the
that
isassume
being
target moisture for a4.1
resin is no greater than 0.02% and you 0.3
sample,
than you need
to be ablebe
to accurately
use a 40-g
The
samples
should
confirmed to be giving
measure a difference in weight of 8 mg (0.008 g). That
verage amount of water is roughly
3.6 the size of a pin head. If your 0.44Instrument sensitivity is crucial when measuring moisture,
off only water in the
moisture balance. If asyou
can’tthe amount of water is the size of a pin head.
sometimes
limits are even lower than 0.02%, the amount you must mea% RSD adjust
9.21
0.089
sure reliably
is
even
tinier.
the method to make sure that only water is
When dealing with measurements of such small quantities,
minor issues
can cause large
effectsainmoisture
results. A variety
of interbeing
released,
then
balance
is not an
ferences can affect the instrument reading, including these six :
unning fiveappropriate
replicates
will give you
good ideathe
ofpercent
whether
•Air currents, instrument
to adetermine
ur moisture •Vibrations,
balance has an issue with a biased result.
•Electromagnetic
fields/static,
moisture
of your
material.
his shows real
datathefrom
a pair of moisture balances test•Whether
unit is level,
materials used,
•The When
these four steps are completed you can
in this way.
•The method used.
of these factors
indicate that
you are
shouldgetting
read carefully
be All
certain
whether
you
accurate results
the manual that comes with your instrument and place it in the
best possible
location.
from
your
moisture balances. You will be able to
In addition, take the following five steps to characterize the
confirm
whether
performance of
your moistureyour
balance:current procedures for testing
ble 2
Step 1: Ensure that your moisture balance does not have a
moisture
appropriate
the materials. You
bias or
interferences. are
Run five
replicate tests withto
a weight
mAteRIALfor
WeIght
LoSS
CompARISon:
placed
in the center of
an empty pan.
The weight simulates the
Gravimetric moisture analyzers are commonly used to deteralso
be able
tonotidentify
potential
mass ofwill
your sample,
but will
give off any mass
when heat- problems
mine the percentage of moisture in resin. The instrument is
moIStuReed.BALAnCe
vS.should
moIStuRe
AnALyzeR
The results of all the tests
ideally be 0%, but there
essentially a sensitive balance contained within an oven.
with
the
testing
if there are any, and if no
are many
reasons
it may be process,
otherwise.
problems exist, your production personnel will have
ample
moisture
moisture
that they
values on %
which
eplicate confidence
Balance,
% have reliableAnalyzer,
to base their decisions. All of this translates into less
1
0.2990
0.2119
variation in the results, which ultimately means less
2
0.1896
variation in0.3030
processing, which reduces
processing
3
0.3120
0.2017
problems and
saves time and money.
4
5
6
7
8
9
10
We are continually focused on improving the
services offered to our polymer customers.
Teel Analytical Laboratories is expanding its
capabilities with a variety of instrumentation
and test methods. Current and future
instrumentation includes:
• A Differential Scanning Calorimeter (DSC)
• A shore hardness Durometer
• A thermogravimetric analyzer coupled with
a Fourier transform infrared spectrometer
(TGA-FTIR)
• A high performance liquid chromatograph (HPLC)
• A Gas Chromatograph coupled with a Mass Spectrometer (GC-MS)
• A Gel Permeation Chromatograph (GPC)
• A chromameter (for determining the color of samples)
34 february 2013 Plastics technology
02 FEATURE ExtTips.indd 34
1/11/13 11:01 AM
• Accelerated weathering chambers
• Microscopy
• A moisture analyzer
• Various physical test methods such as crush, peel, impact, and tensile testing
0.3050
0.1842
About the authors
0.2960
0.1640
Joel Lischefski
is
laboratory
manager
0.3010
0.1919 at Teel
Analytical Laboratories, div. of Teel Plastics,
0.3070
Inc., Baraboo,
Wis. His specialty is0.2554
analytical
chemistry and
he has spent time0.1262
developing
0.2940
methods, performing research, and analyzing
samples in0.3020
the fields of polymer0.2169
materials and
environmental
science.
0.3100
0.1992
For more information please contact:
Average Alison Maicke
0.3029
0.1941
is a chemist at Teel
Analytical
specializing in chromatographic
% RSD Laboratories,
1.91
17.4
separations, method validation, and quality
control. She has worked in thermal analysis and
his study was
conductedofwith
relative
humidity
identification
active
ingredients
in detection
polymers
sing the same
of sample. The % relative standard
andtype
pharmaceuticals.
eviation (RSD) is much larger for the moisture analyzer
ue to the smaller sample size, as it is more susceptible to
Contact: (608)355-4520; email: [email protected];
ight changes in the moisture quantities of the sample. By
website: teelplastics.com
erforming 10 replicates, variations in the samples can be
veraged out and a true overall value obtained.
I
I
Plastics
technology
PLASTICS TECHNOLOGY
FEBRUARY
2013 47february 2013 35
1/11/13 11:01 AM
Teel Plastics, Inc.
1060 Teel Ct.
Baraboo, WI 53913
(608) 355-3080
www.teel.com
Joel Lischefski
Laboratory Manager
608-355-4520
[email protected]