1. Analysis of Organic Functional groups in Monomers and Resins
George Porter III – Titration Product Manager – Metrohm USA.
Introduction
Synthetic polymers have infiltrated nearly every aspect
of modern life, finding use in packaging, construction,
consumer goods, medical devices, aerospace and nearly
every other manufacturing sector. Given their incredible
utility and variety, it is surprising to remember that
polymers are simply macromolecules made up of the same
repeating subgroups of molecules. The monomer building
blocks of which polymers are composed have no special
properties in and of themselves. A monomer is merely “a
molecule of low molecular weight capable of reacting with
identical or different molecules of low molecular weight
to form a polymer”i
. Yet the structural properties of a
polymer are entirely dependent on the chemical properties
of the substituent monomers, and on their purity.
In 2011, the production of polymers accounted for 18% of
the $610 billion chemical industry in the United States ii
.
The polymer industry is focused not only on the production
of finished polymers, but also on the production of the
many types of monomers used in the manufacturing of
common polymers. These range from simple polymers
such as poly(ethylene terephthalate) and polystyrene to
more sophisticated polymers such as Kevlar®
, nylon and
poly(urethane). To achieve proper synthesis of a polymer, it
Polymer Constituent Monomer(s) Example Functional Group Titrations
Poly(ethylene terephthalate) ethylene terephthalate • Acid end groups
• Aldehydes
Poly(styrene) styrene • Aldehyde content
Poly(ethyleneglycol) ethylene glycol • Molecular weight by acid number back titration
Nylon diamine dicarboxylic acid • Acid end groups
Poly(urethane) di or polyisoctanate polyol • NCO number
• Amine number
• Acid value
• Epoxide equivalent
Kevlar®
1,4-phenylenediamine
terephthaloyl chloride
• Acid value
isessentialtoensurethattheconstituentmonomermixtures
have the appropriate chemical characteristics. Those
characteristics are determined by the organic functional
groups of the monomers, and thus understanding their
composition is critical to high quality polymer synthesis.
Analysis of Monomer Organic
Functional Groups
Polymer manufacturing is performed on a bulk scale. The
methods used to monitor and predict its quality must
likewise involve rapid and robust analysis of bulk properties
and composition. Many different components of a
monomer or resin can contribute to the overall monomer
functional group properties at a bulk level. Rather than
attempting to analyze each component functional group
individually, it is easier to approach functional group analysis
in polymer manufacturing by evaluating the mixture’s bulk
chemical properties. Potentiometric titration is one of the
best known and best characterized methods to perform
this analysis. It is also one of the easiest and most accurate
techniques to scale up from single bench measurements
to fully automated in-process analysis, particularly when
powered by automation accessories that combine precise
fluid control with comprehensive software.
2. 2
Figure 1: Typical Modern Potentiometric Titrator
Potentiometric Titration
Together with gravimetric analysis, titration is one of the
oldest analytical methods, and both are based on chemical
reactions. Titration determines the volume of a standard
solution (titrant) that is necessary for complete chemical
reaction with the analyte. By identifying the endpoint or
equivalence point of the reaction and factoring in the
known amount of the reacting substance in the titrant, the
concentration of the unknown targeted within the analyte
can be determined.iii
The equivalence point can be measured several ways, the
most traditional being colorimetric or visual detection
with an indicator such as phenolphthalein. Potentiometric
indication of endpoint has become equally popular,
and works by measuring the difference in the electrical
potential between an indicator electrode and reference
electrode in the test solution as the reaction proceeds.
Potentiometric titration is particularly suitable for the
measurement of organic functional groups in monomers
because it is convenient, robust and well characterized.
Furthermore, modern potentiometric titrators (see figure
1) offer all of the features found in any other piece of
automated laboratory equipment, including computer
control, color touch screens, full traceability for auditing
and easy interfacing to LIMS or other data management
systems.
Overcoming Functional Group Titration
Challenges
Like any analytical technique, function group titration
of monomers poses some challenges, most of which
are related to the solvents, reagents, and viscosity of
the solutions involved. Metrohm titrators are designed
specifically with these challenges in mind.
Functional group titrations are typically carried out in
non-aqueous, and thus non-polar, solutions. Non-polar
solutions are poor conductors of electrical current, and
can quickly build up a static charge when a rapidly rotating
stirrer is used. This adds significant electronic noise, and
results in very low signal. Metrohm designs protect your
instrument and samples from static interference through
the use of galvanically-isolated input boards to prevent
static discharge and outside RF interference. Instruments
with optional grounding of the titration solution are also
available to prevent static charge build up and discharge
in solution.
The low conductivity of non-polar solutions can also result
in very little potential change at the titration equivalent
point. This leads to flat or erratic titration curves, and can
make equivalence points difficult or even impossible to
determine. Metrohm specialty sensors for non-aqueous
titration, including the Solvotrode and differential titration
input box, have extremely high sensitivity in non-polar
and non-aqueous solutions, allowing endpoints to be
determined accurately despite low conductivity.
Why use an “old-fashioned” titration?
• Titration is an absolute content determination providing
direct information about the sample.
• Titrations are very easy to execute. Simple and affordable
lab equipment with basic lab training is all that is needed.
• Titrations are versatile and scalable. A single automatic
titrator can be used for dozens of analyses and expanded
and automated as your needs grow.
• Titrations are highly reproducible. The typical %RSD of a
titration is 1%, with 0.01% being possible.
• Titrations are very economical. ROI on automated
systems vs manual titrations is typically reached in 6
months to 2 years.
3. 3
Sensor fouling is another common issue when analyzing
monomers. Functional group titrations are often carried
out with resins and solvents that are viscous, and can
easily clog the pores in a traditional reference electrode
diaphragm. Metrohm’s solution lies in electrodes which
utilize a ground glass joint in lieu of the traditional porous
ceramic pin. The ground glass joint provides a larger
surface area for greater sensitivity and smaller pore size,
reducing probability of fouling.
Finally, functional group titrations tend to use reagents
that are toxic, noxious, volatile or pose health risks in
some other fashion. Metrohm titration systems provide
full laboratory automation to allow completely hands
free analysis of one to 100 samples, improving workplace
safety.
Important Organic Functional Group
Titrations
Literally hundreds of titrations are used in the monomer
and polymer industry. These include acid value, alkalinity,
chloride number, epoxy number, amine number, and
carboxyl number. Of these, several stand out as the most
common and readily automated analyses, including acid
value, hydroxyl number, isocyanate concentration, and
epoxide equivalent number. Other analyses can easily
be accommodated by working in contact with a sales
specialist to identify the correct off-the-shelf or custom
system.
Acid Value
Acid value (or acid number) is the most common non-
aqueous titration used by the monomer manufacturing
industry. Acid value corresponds to the amount of
carboxylic acid functional groups in various types of resins
such as polyester acrylates and alkyl monomers, but it is
also an indicator of solvent purity. Acid value is usually
reported as the amount of base needed to bring the
sample to the potentiometric equivalence point, typically
expressed in terms of weight of potassium hydroxide per
unit weight of sample.
When performed using an automatic titrator in dynamic
dosing mode, an automatic burette, stirrer, and a non-
aqueous electrode such as the Metrohm Sovlotrode
easyClean, preparation of the sample is as simple as
dissolving a known quantity of the sample in the solvent
mixture.
Titration to the first equivalence point using alcoholic
potassium hydroxide can be performed after just 30 s,
though if the sample is not particularly soluble, a solvent
mixture of one volume ethanol and three volumes tert-
butyl methyl ether or toluene is suggested.
To ensure accurate results, all solvents should have either a
blank determination or be neutralized to a phenolphthalein
endpoint prior to analysis. This is important to prevent
atmospheric carbon dioxide from providing an erroneous
bias to the acid value. A self-rinsing automatic burette
such as the Metrohm Dosino will help prevent excess wear
on the instrument from the rapid buildup of non-soluble
carbonates in the non-aqueous titrant, extending lifetime
and reducing need for service.
Hydroxyl Number according to ASTM E1899-08iv
Hydroxyl Number (or OH number) provides insight to
the degree of esterification of a sample. Degree of
esterification is a critical number in the production of
polyols used in the production of polyurethane. Hydroxyl
number is represented as weight of potassium hydroxide
equivalent to the hydroxyl content in one gram of polyol or
other hydroxyl compound.
Figure 2: Titration curve from acid value titration of alkyl resin
4. 4
While hydroxyl number has traditionally been a laborious
analysis requiring extended refluxing in acetic anhydride
as sample preparation, the more modern and rapid ASTM
E1899-08 method requires neither the noxious acetic
anhydride nor the extended sample preparation, resulting
in analysis times as short as 12 minutes. The Metrohm
Dis-Cover system for fully automated OH number analysis
shown in figure 5 offers the unique capability of allowing
the titration vessels to be uncovered and covered as
needed to minimize contamination of the sample with
atmospheric moisture.
Full automation of an extended sample preparation shows
the ease of use of this system. It includes weighing of
the sample into a titration vessel, where it is dissolved
with acetonitrile and stirred before toluene-4-sulfonyl-
isocyanate solution is added for further stirring, while
covered. Distilled water is added after 5 minutes, the lid
is again closed, and the solution stirred further before
acetonitrile is added and the solution is titrated past the
second end point with tetrabutylammonium hydroxide.
Not only is the sample preparation and titration automated,
but cleaning between titrations is achieved by rinsing the
burette and vessel with ethanol and distilled water, after
which the electrode is also conditioned in distilled water.
Determination of Isocyanates (NCO Number)v
Isocyanate number is a critical organic functional group
that needs to be determined during the manufacture of
polyurethane foam. The isocyanate number is required to
formulate polyurethane foams with the proper flexibility
and thermal stability for their desired application.vi
An NCO
Number titration is a classic example of a back titration, in
which excess reactant is titrated to give the answer instead
of a direct analyte equivalence point reading.
This titration can also be performed using the Dis-Cover
system shown in figures 5 and 6. Special precautions
in sampling must be taken when performing this
determination, as organic isocyanates react with
atmospheric moisture. Typical sampling methods can
cause contamination of the sample with insoluble ureas,
so it is advisable to cover the sample with a dry, inert gas
such as nitrogen, argon or even dried air at all times.
To perform this type of back titration, the sample is weighed
into a titration vessel and dissolved with toluene. Once a
reaction solution of dibutylamine is added, the vessel is
covered and allow to react while stirring. Methanol is then
added and the excess of dibutylamine is back titrated with
a known concentration of hydrogen chloride. Knowing the
total amount of dibutylamine added and the amount of
hydrogen chloride needed to reach endpoint, the initial
concentration of isocyanates can be calculated.
Figure 3: Titration curve for ASTM Hydroxly Number Figure 4: Example NCO Number Titration Curve
