Analytical Pyrolysis Of Synthetic Organic Polymers

692 Pages·2005·65.97 MB·english

by  Moldoveanu S.

PREFACE
The widespread use of synthetic polymers has led to the development of a considerable
number of analytical tools for polymer characterization and analysis. Analytical
pyrolysis, consisting of pyrolysis coupled with an analytical technique, si one of these
tools. The technique can be invaluable ni solving many practical problems ni polymer
analysis. tI can be used alone or can provide complementary information to other
techniques such as thermal analysis, infrared spectroscopy, or even nuclear magnetic
resonance.
The applications of analytical pyrolysis to synthetic polymers range from polymer
detection and characterization to the microstructure elucidation of specific polymers and
the identification of additives present ni polymers (antioxidants, plasticizers, etc.).
These applications cover analysis of thermoplastics, fibers, paints, adhesives, and
elastomers for quality control characterization, competitor product evaluation,
identification of unknown materials, polymer identification for forensic purposes, etc. A
subject of major interest ni many practical applications regarding polymer properties si
the degradation of polymers during heating. Incomplete burning of common objects
made from plastics, fibers, or elastomers, with pyrolysis around the combustion area,
generates pyrolysates that can have complex compositions. Their analysis can be
important ni connection to health issues and environmental problems. Analytical
pyrolysis can be used for obtaining information ni all these areas. The technique si also
useful for better understanding of the processes occurring during the industrial pyrolysis
of polymers with the purpose of recycling.
The present book si a follow-up of a previous one with the title Analytical Pyrolysis of
Natural Organic Polymers published by Elsevier as vol. 20 ni the series "Techniques and
Instrumentation ni Analytical Chemistry." nI addition to the discussion on pyrolysis of
various natural polymers, the previous book contains information on chemically modified
celluloses, modified starches, etc. For this reason, the present book does not include
synthetically modified natural polymers. Information on the pyrolysis process and
pyrolytic techniques in general also can be found ni the book on natural polymers.
These subjects are only summarized here. For the internal consistency of the second
book, a few general sections are similar to those ni the previous one, although some
information is updated.
This book has two main parts, and the material si organized ni chapters and sections.
The first part of the book has five chapters including an introduction, a discussion on
physico-chemistry of thermal degradation of synthetic polymers, a short discussion on
instrumentation used ni analytical pyrolysis, a chapter discussing what type of
information can be obtained from analytical pyrolysis, and a chapter dedicated to the
applications of analytical pyrolysis for the analysis and characterization of synthetic
polymers. The chapter on applications includes only a few selected examples from the
multitude existent ni literature, and ti si not intended to be exhaustive. Excellent
monographs, such as .F .W Billmeyer Jr., Textbook of Polymer Science, .J Wiley, New
York, 1971; .H .H G. Jellinek, ed., Aspects of Degradation and Stabilization of ,sremyloP
Elsevier, Amsterdam, 1980; .S A. Liebman, .E .J Levy, ed. Pyrolysis and CG ni Polymer
Analysis, .M Dekker, New York, 1985; and T. .P Wampler, ed., Applied Pyrolysis
vi ecaferP
,koobdnaH .M Dekker, New York, 1995, can provide supplementary information
regarding these subjects.
The second part si the core of the book. nI this part are presented ni a systematic
manner the main results published ni literature regarding the analytical pyrolysis of
various classes of synthetic polymers. Some unpublished original results using
pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS) also are
included ni this part. Each polymer class si presented ni a different chapter and
includes polymers with a particular backbone structure. The polymers with the same
backbone but with different side chain groups are discussed ni different sections of the
same chapter. The main types of polymers discussed ni the book include those having
ni the backbone saturated carbon chains, unsaturated carbon chains, aromatic
hydrocarbon groups, ether groups, ester groups, carbonates, anhydrides, sulfides,
sulfones, imines, imides, amides, urethanes, ureas, various heterocycles, silicon,
phosphorus, etc.
Copolymers are very common synthetic materials, and a significant number of pyrolysis
studies reported ni literature are dedicated to copolymers. Pyrolysis results for different
copolymers are discussed ni connection to each homopolymer class. Also, a
considerable number of literature references si given ni the book for each subject.
The intention of the author was to provide information on pyrolysis for a wide range of
readers, including chemists working ni the field of synthetic polymers as well as for
those applying pyrolysis coupled with specific analytical instrumentation as an analytical
tool. Some theoretical background for the understanding of polymer structure using
analytical pyrolysis si also discussed. The book si mainly intended to be useful for
practical applications of analytical pyrolysis ni polymer identification and
characterization.
The author expresses his thanks to Mrs. Nancy Qian for assistance with performing the
pyrolysis experiments on many synthetic polymers.
Serban .C Moldoveanu
CHAPTER 1
Overview of Organic Synthetic Macromolecules
1.1 INTRODUCTION TO POLYMER CHEMISTRY
- General aspects
Polymer chemistry is an important branch of science, and polymer analysis and
characterization is a common subject in scientific literature. Analytical pyrolysis is one of
many tools used particularly for polymer identification and for the evaluation of polymer
thermal properties. Before a more in-depth discussion on analytical pyrolysis and its
application to polymer science, some basic concepts regarding the chemistry of
synthetic polymers will be briefly discussed.
A term more general than polymer is that of macromolecule. Macromolecules are
chemical compounds formed from at least one thousand atoms linked by covalent
bonds. They are common as natural substances like cellulose, proteins, lignin, etc., and
also as synthetic compounds including plastics, fibers, elastomers, coatings, and
adhesives. Many synthetic and some natural macromolecules have repetitive structures
and are known as polymers. For example, cellulose si made from 13-D-glucose residues
interconnected by 13-glucoside (1-->4)links, polystyrene is made from 1-phenylethylidene
units, etc.
1-Phenylethylidene
Glucose residue unit unit
...... CH--CHi--CH--CHi--CH--CH 2 .....
HO2HC O ! \-ON I I \13!OrU-?k" /OH l
The polymers formed from identical repeating units are sometimes called
homopolymers, and the name "polymer" is frequently extended to nonrepetitive
macromolecules such as various organic geopolymers, lignin, or proteins. The number
of repeating units is defined as the degree of polymerization (DP). The molecules
formed by the union of a few repeating units are known as oligomers, and they have a
low DP. The molecule that generates the repetitive unit is called a monomer. Certain
polymers can be formed from the repetition of more than one kind of unit, and they are
known as copolymers. The repeating units in copolymers may have an ordered
repetition or a random one.
Polymers can be viewed as consisting of a backbone on which are attached atoms or
groups of atoms. The polymer backbone may have a linear, branched, or network
structure. More unusual polymer structures may have peculiar characteristics such as
star, comb-like, ladder, or other structures. For linear polymers the backbone extends
mainly in one dimension, for sheets in two dimensions, and for reticulate polymers in
4 Overview of organic synthetic macromolecules
three dimensions. However, the coiling of linear polymers or the intermixing of
branched structures leads to tridimensional macromolecules.
nI their molecules, the polymers also contain end groups. An end group is the last
group in a chain, these not being identical to the repeating units. Depending on the
frequency of occurrence of the end groups (related, for example, to the length of
polymer chains), the role of the end groups ni the property of the polymer can be more
or less important. The end groups are counted ni the value of the DP.
- Formation of polymers
The formation of polymers from the monomers is known as polymerization reaction.
When more than one basic unit forms the polymer, the process is also named
copolymerization. The polymerization reactions can be classified into two main groups,
addition polymerizations and condensation polymerizations (or polycondensations). For
the addition polymerizations, the resulting polymer has the repeating unit with the same
molecular formula as the monomer, and the molecular mass of the polymer is the sum
of the molecular masses of all the monomer molecules. For the condensation
polymerizations, the resulting polymer has the repeating unit with fewer atoms than that
of the monomer or monomers, and the molecular mass of the polymer is less than the
sum of molecular masses of the original monomer unit or units because small molecules
are eliminated following this reaction. This classification is not adequate for the
characterization of the polymer itself, because the same polymer can be formed by
more than one type of reaction. For example, a polyamide can be formed by addition
from a lactam or by condensation from an co-aminocarboxylic acid as shown below:
OH //c~__~N O-~ 1017
R ..... HNyC--R--HNTC--R .....
H2N--R__C~ O ~ ..... NH --R--HNq--C--R ..... + 2H ~
~OH
The reactions with formation of polymers also are classified based on another difference
in their mechanism. This classification distinguishes step reactions and chain reactions.
nI step reactions the polymers are built from the monomer by random individual
reactions to form dimers, trimers, tetramers, etc., each resulting molecule being able to
participate in a subsequent reaction with a monomer or with an oligomer molecule. This
type of reaction may start with molecules having two reactive functional groups in one
molecule such as an e)-aminocarboxylic acid. Another possibility consists of reactions
between two different types of bifunctional molecules such as a diamine and a
dicarboxylic acid as shown below:
HOOC(CH2)xCOOH + H2N(CH2)yNH2 -~ -OOC(CH2)xCOO-+H3N(CH2)yNH3 § ~--
-+ -OOC(CH2)xCOHN(CH2)yNH3 § + H20 -~ ....
~-- H2N(CH2)yNH-{OC(CH2)xCONH(CH2)yNH}n-OC(CH2)xCOOH + n H20
Overview of organic synthetic macromolecules 5
nI chain reactions the polymer is formed by successive linking of monomer molecules to
the end of a growing polymer chain. These reactions typically have an initiation, a
propagation, and a termination stage. The initiation can be caused using a specific
compound that generates free radicals (R'), these reacting with the monomer as shown
below for vinyl chloride:
Initiator ~ *R
R" + CH2=CHCl--> RCH2-CHCI*
Further, the polymerization takes place by a radicalic mechanism, this stage being
known as propagation:
RCH2-CHCI* + CH2=CHCI -+ RCH2-CHCI-CH2-CHCI* +-- ..... R-{CH2-CHCI}n-CH2-CHCr
The reaction is terminated either by radical coupling or by disproportionation:
R-{CH2-CHCI}n-CH2-CHCr + R" (or any other free radical) ~-- R-{CH2-CHCI}n-CH2-CHCIR
or R-{CH2-CHCI}n-CH2-CHCI ~ + R'-+ R-{CH2-CHCI}n-CH=CHCI + RH
Among the most common free radical initiators are benzoyl peroxide, tert-butyl
hydroxyperoxide, tert-butyl peroxide, dicumyl peroxide, 2,2'-azobisisobutyronitrile
(AIBN), potassium persulfate, etc.
A similar role to that of the free radical can be played in certain polymerizations by an
ion. For example, cations can be generated from mineral acids or Lewis acids in the
reaction with compounds such as water or alkyl chlorides, as follows:
X + H20 >-- XOH'+ H § ro AICI3 + RCI-> AICI4 + R §
The resulting cation reacts with the monomer in a chain reaction:
R § + CH2=CRa2 -~ RCH2-CRa2 § ~ §2aRC-2HC-2aRC-2HCR ~-- 2aRC-2HC-n}2aRC-2HC{-R §
For obtaining a cationic polymerization, the new carbocation generated between R § and
the monomer should have enough stability to be relatively easily formed and to continue
the polymerization (for example, CH2=CH2 is not polymerized using a cationic initiator,
while (CH3)2C=CH2 can be polymerized because the species RCH2-C(CH3)2 § is stable
enough to be formed). The stability of the carbocation increases as the chain length
increases. Chain transfer reactions are common in carbocation polymerization. The
termination reactions typically occur because of the combination of the cationic
component with a counterion.
Anions can be used to promote a chain reaction similarly to cations, except that they act
as nucleophiles in their reaction with the organic monomer. The anions are frequently
formed in a reaction with alkali metals such as Na, ,K or .iL As an example, potassium
in liquid ammonia forms KNH2, which generates NH2-anions. These anions further
react with the organic monomers in a reaction as shown below:
NH2-+ CH2=CHX >--- H2NCH2CHX-
6 Overview of organic synthetic macromolecules
The reaction is continued with a new molecule of monomer. The anion formation ni the
reaction of an alkali metal and an organic monomer also may take place with a radical
anion intermediate that is followed by the formation of a dianion, as ni the following
case:
Na ~ + CH2=CHR ~-- CH2-CHR ~ Na § -> Na § -CHR-CH2-CH2-CHR- Na §
The dianion further reacts with the monomer ni a propagation reaction.
Termination reactions of this type of polymerization usually involve an ion transfer.
Since the ion transfer requires the presence of a molecular species to which the transfer
can occur, it is possible by conducting the reaction with very pure compounds to
eliminate the termination step. By unterminated polymerization the final material can
remain active indefinitely and is known as a living polymer.
Another important method for the formation of synthetic polymers si that using complex
coordination catalysts involving transition metal compounds, also known as Ziegler or
Ziegler-Natta catalysts. These catalysts are used mainly ni the polymerization of
alkenes. They are combinations of a transition metal compound such as a halide of
titanium, vanadium, chromium, molybdenum or zirconium and an organometallic
compound of a metal from groups I to III of the periodic table such as an alkyl, aryl or
even a hydride of aluminum, lithium, zinc, magnesium, etc. The most common
coordination catalysts are probably the combinations of a titanium chloride and a
trialkylaluminum compound. nI the TiCI4 + AIR3 system, organotitanium compounds are
formed that also suffer reduction leading to Ti )111( combinations. This type of
coordination catalyst is insoluble ni the monomer or ni monomer solutions, and the
catalysis takes place ni heterogeneous phase. The dicyclopentadienyl metallocenes ni
the presence of alkyl aluminum can be used ni homogeneous catalysis. The precise
mechanism of coordination catalysis is still not known, two main models being accepted,
the monometallic and the bimetallic ones. nI the monometallic mechanism, the
organotitanium halide with incomplete coordination si assumed to induce polymerization
following a scheme as shown below:
bR
aR Rb~ /H .%bR H~ Rb~,.(,/H aR _....~,C.,~aR H
I lC.... C aR ............ C I
CI----~Ti-- + II = .,Cl ~- t ~'cl I 2HC
Cl~Ti .......... ,~C Cl----;Ti" " 2HC I .Cl
,c,-,c :iTc,C
c'cl' . H H H
ab
R a J....~,C..~ H
Rb /H bR
bR C R a""C'~f H Rb\ /H bR
C H C2 ~..C,~..~H
R 1.~,C.,~,a H lt I ab lr
/C\ H C2 ~...~,C..,~ H /C\ I bR
I H H N H C2H ~.c,~H
CH2 I
I .Cl = CH 2
C I----;Ti" I IC. 2HC
C Idc~ I C l__Ti" I IC.
c/I C I---;Ti
Cl
C IdOl
Overview of organic synthetic macromolecules 7
Similarly to other chain reactions, the termination takes place by transfer reactions,
internal hydride transfer, etc.
A special type of polymerization si that of cyclic compounds such as lactones, lactams,
cyclic ethers, cyclic anhydrides, or cyclic N-carboxyanhydrides that can be polymerized
by ionic mechanisms. These compounds can undergo an addition reaction with
characters of both chain and step polymerization.
Although a large number of step-reactions are condensations and a large number of
radical chain reactions are additions, there are also numerous exceptions. Including
both classifications, a more detailed one distinguishes four groups. These groups are
(a) polycondensation (step-polymerization with small molecules eliminated following this
reaction), (b) polyaddition (step-polyaddition), (c) chain polymerization (chain-
polyaddition), and (d) condensative chain polymerization (chain-polymerization with
small molecules eliminated following the reaction).
Different reaction mechanisms, although leading to a polymer with identical repeating
units, may have a significant impact on the physical properties of the polymer. The
polymers may differ ni (average) molecular weight, end groups, stereochemistry, chain
branching, etc. 1.
- Formation of copolymers
When a mixture of two (or more) types of monomers si used as starting material ni a
polymerization reaction, the result can be the formation of a copolymer. However,
different monomers differ significantly ni their tendency to enter into copolymers. Even
some monomers that are very difficult to polymerize alone or do not form polymers at all
may participate very easily ni the formation of some copolymers. One such example is
maleic anhydride that gives easily copolymers with styrene or with vinyl chloride and
forms very difficultly a homopolymer.
nI their structure the copolymers may contain the monomeric units randomly, and their
overall composition si determined by the composition of the initial feed mixture of
monomers (see Section 2.3). Alternating copolymers (alt-copolymers) also are known,
where the monomers alternate regularly along the chain. Other types include block
polymers where a linear arrangement of groups of one type of monomers si present,
graft polymers that have side chain blocks connected to a polymer main chain, per-
copolymers where ordered sequences of more than two units are present, etc.
The synthesis of different types of copolymers is done following particular procedures
depending on the nature of the copolymer. For free radical polymerization, the outcome
of the copolymerization depends very much on the nature of the monomers, while in
step polymerization it si more common to generate random polymers since the reactivity
of the functional groups is less influenced by the length of the molecule to which the
groups are attached. Special types of copolymers such as block copolymers can be
obtained, for example, by using unterminated anionic polymerization for one monomer,
followed by the addition to the living polymer of a second monomer that will continue the
polymerization process. The structure of copolymer can be further complicated. For
example, block copolymers may have a diblock structure, where the copolymer si made
8 Overview of organic synthetic macromolecules
from two chemically distinct polymer blocks, or a structure where only one of the
monomers forms uninterrupted blocks. The blocks can be in a predominantly linear
structure, or they can be part of a radial (star) type structure.
Copolymers have a very large range of applications in practice since they may possess
properties difficult to attain in homopolymers. For this reason, many polymer samples
from common sources are copolymers.
- Polymerization conditions
A large number of experimental conditions to obtain polymers have been developed.
The polymerization can be done in homogeneous systems including bulk polymerization
and solution polymerization, or heterogeneous system polymerization including
suspension and emulsion polymerization. Bulk polymerization of pure monomers may
appear as the simplest procedure for polymer formation, but this type of procedure is
limited to reactions that are only mildly exothermic. Solution polymerization can avoid
overheating, but the final removal of a solvent from the polymer is often difficult.
Polymerization in suspension is usually done in water with the monomer as a dispersed
phase and the polymer resulting as a suspension. The procedure usually applies
stabilizers and agitation of the solution to maintain the dispersion of the monomer. The
initiator is typically dissolved ni the monomer phase. nI emulsion polymerization, the
monomer forms a true emulsion in water where also the initiator is present. Detergents
are used as emulsion stabilizers and may play an active role in the progress of the
polymerization reaction (see e.g. 2). Other polymerization procedures are known such
as solid phase polymerization, inverse phase emulsion polymerization, etc. The subject
of polymerization conditions is covered by an enormous body of literature.
- The degree of polymerization and molecular mass distribution
Polymeric materials are typically obtained with a range of DP values. For this reason
the characterization of polymers is done using an average degree of polymerization.
For a polymerization reaction starting with oN molecules of monomer, at a certain point
during the reaction, the number of molecules is reduced to .N The average degree of
polymerization is defined as follows:
DP- ~N (1.1.1)
N
An average molecular mass (weight) of the polymer also needs to be defined, since
components of various molecular masses (weights) iM are present in the polymer. The
number of moles of species "i" in the polymer can be obtained from the typical formula ~n
= w~/M~ where ~w is the weight fraction of the component "i", and jM is the molecular
mass of species "i". The masses of molecules or groups can be calculated using two
different conventions. One convention considers the natural isotopic abundance of
elements and takes their sum based on the compound chemical formula. For the
masses of polymers, the first convention is typically used. The other convention
considers only the masses of the most abundant isotope, which is useful for MS
Overview of organic synthetic macromolecules 9
interpretations (as in the case of pyrolysis GC/MS of polymers). For this latter case, the
resulting mass is rounded to the unit 3.
The number-average molecular mass nM expressed in g/mol is given by the formula:
~'-~ niMi ~'~ w i W
(1.1.2)
Mn = '~-~'xiMi = ~--~n~ : -~-~'M/iw~~' =--n --
where W is the total weight and the summation is done over all "i" values, and the mole
fraction is given by ix = n~/n where n is the total number of moles of polymer.
A weight-average molecular mass wM also is used for polymer characterization. This
parameter is given multiplying the weight fraction w,/W by the molecular mass ~M and is
defined by the formula:
Mw = ~ t__Miw = ~ i__Miw ._ ~ niM~ (1.1.3)
W ~w~ '-~' niM ~
For an ideal polymer with all molecules having the same molecular mass, nM = ,wM and
the polymer is known as a monodisperse system. nI most synthetic polymers, wM > ,nM
and the ratio Q = wM / nM is called polydispersity index. The molecular mass (weight) for
small molecules is noted MW and does not represent an average.
References 1.1
.1 .M .P Stevens, Polymer ,yrtsimehC nA ,noitcudortnI Oxford Univ. Press, weN York, .9991
.2 .F .W Billmeyer Jr., Textbook of Polymer ,ecneicS .J Wiley, weN York, .1791
.3 .F .W McLafferty, Interpretation of Mass ,artcepS University Science Books, Mill Valley, .0891
1.2 NOMENCLATURE OF POLYMERS
- General aspects
Polymers are typically complex molecules and in some cases not sufficiently
characterized regarding their structure. Even the polymers with a simple model
structure are always mixtures of macromolecules of different molecular masses
(weights). For these reasons a name of the polymer describing the structure with the
same exactness as for small molecules is not possible. Besides the commercial names
of the polymers, two different scientific systems for naming are in use. The first is a
source-based nomenclature and the other si a structure-based one. The source-based
nomenclature uses the name of the monomer or of the starting reactant for naming the
polymer. For homopolymers that are derived from only one species of monomer, the
name of the polymer can be formed by attaching the prefix "poly" to the name of the
monomer or of the starting reactant (parentheses are used when the name of the
monomer has two or more words). Examples are polyacrylonitrile, polyethylene,
polystyrene, poly(vinyl chloride), etc.
01 Overview of organic synthetic macromolecules
The name based on starting reactants is also applied for some condensation polymers
such as poly(ethylene terephthalate) or phenol-formaldehyde resin.
The structure-based nomenclature is rather straightforward for linear single strand
polymers, which are named "poly(constitutional repeating unit)" 1,2. The name of the
repeating unit is based on the IUPAC nomenclature of bivalent organic groups. For
repeating units containing a sequence of subunits, a seniority order from left to right
must be followed. This seniority decreases in the following order: heterocycles >
heteroatomic groups > carbocycles > acyclic carbon groups. Within each structural
type, further seniority is established. For example, for heteroatomic groups the seniority
decreases in the order O > S > N > P >Si, etc. For carbocycles a larger number of
cycles is senior to a lower number, and a larger cycle is senior to a smaller one. For
acyclic carbon groups the seniority is based on the number of ,C and for equal number
by the number of substituents. nI all cases, the seniority is applied only for the polymer
backbone.
As an example, the polymer with the structure:
....
is named poly(oxy-1,4-phenylene-sulfonyl-1,4-phenylene-oxy-1,4-phenylene-
isopropylidene- 1,4-phenyle ne).
Specific rules also are applied for polymers with other regular structures such as ladder
type polymers or spiro polymers 3. For single-strand linear copolymers with irregular
structures, the name is given as "poly(first constitutional repeating unit/second
constitutional repeating unit)", etc. Detailed explanations for polymer nomenclature can
be found in various reference materials (see e.g. 4).
Although there are clear IUPAC rules for naming the polymers, in practice various
names for the same polymer are frequently encountered. As an example, a simple
polymer such as nylon 6 (commercial name) can be named poly(~-caprolactam),
polyimino(1-oxo-l,6-hexandiyl), poly(6-aminohexanoic acid), polycaproamide or
poly(pentamethylenecarbonamide).
Besides scientific names, many polymers are indicated by their common name, trade
name, brand name, or abbreviation (commercial names) 4. Typical examples of
common names are nylon or silicone. Several nylons are known and the name nylon 6
is used for polyimino(1-oxo-1,6-hexandiyl), the name nylon 66 is used for
poly(iminohexa-methyleneiminoadipolyl), etc. Trade names such as Teflon | for
|
poly(tetrafluoro-ethylene) made by DuPont, Nomex for poly(iminoisophthaloylimino-l,3-
phenylene), or Kevlar | for poly(iminoisophthaloylimino-l,4-phenylene) are also common.
Many abbreviations are in use for both homopolymers and copolymers. Table 1.2.1
gives some of the common abbreviations for polymers.