What is the meaning of polymer?
This word stems from Greek: Poly = many, mers = particles. So this
term describes a molecule composed of many identical parts, called mers.
The large molecule is therefore termed: macromolecule. What is left for us,
is the practical definition of the term “many”. The minimum considers hundreds
of mers. However, there are no sigdicant mechanical properties below
about 30 mers, while the useful average reaches 200-2,000 mers. If one wants
to speak about molecular weights (which is usually done in chemistry) a
broad range between 5,000 up to 2 x IO6 may be representative, while in
some cases it may reach 10’.
The sheer existence of such a broad range of molecular weights indicates
that we deal with long molecules that have no fixed or standard length (or
weight). This presents a most comprehensive difference between macromolecules
and the smaller molecules that are characterized by a smgle and fixed
molecular weight (e.g. water = 18). Prior to delving deep into the world of
polymers, it is essential also to explain the common term “plastics”. The
name itself actually, describes the stage of processing the polymer while it is
plastic or soft-enabling smooth flow and shaping. On the other hand, plastics
(or the scientific term, plastomers) refer to the major group of polymers
that in combination with various additives leads to materials of construction.
In addition to plastomers, polymers are also used as elastomers (rubber like), textiles, coatings and adhesives. Many polymers may appear in various
utility groups, determined by the final desired composition. Because plastics
use is dominated by polymers, many people do not care about the difference
between those two terms, which are similar but by no means identical.
While the synthetic polymers are, for good reasops, of major interest in
this book one should acknowledge the historical role of natural polymers
since the beginning of mankind: in food [protein, starch and others); in clothing
(wool, cotton, silk); and in various other uses (cellulose for writing and
natural resins for ornaments). Later (in the 18th century) the resin exuded
from rubber trees (Hevea Braziliensis) turned into a very useful product in
transportation (wheels and tires), in industry (conveyor belts) and in general
uses, including toys. Even today, many natural polymers are still in use, reaching highly developed processing methods. However, modem industry is
mady based on raw material that is suitable for mass production. Hence the
vitality of the synthetic polymers.
In intermediate stages (the 19th century) polymers made by the modification
of natural resins have appeared, the most prominent ones being the
cellulose derivatives. Celluloid (obtained by nitration of cellulose ) represents
the first semi-synthetic polymer. It became useful after compounding with a
plasticizer (mainly camphor) to reduce its brittleness. Many cellulose derivatives
are still currently in use (as plastomers, textiles or coatings) but the
major development in the 20th century is definitely attributed to many families
of synthetic polymers-the era of polymers.
Table 1-1 presents by historical year, the beginning of commercial production
of the most useful polymers. While many useful polymers appeared in
the 1930s, the process of introducing new polymers continued during the
1950s, 1960s and so on. It apparently takes about 10 years for a newborn
polymer to reach maturity. During this period it has to undergo infant development
and compete with other polymers or nonpolymeric materials. The
major issue has become the tremendous cost of commercialization of a new
polymer, because the period of research and development (including marketing)
is long and costly. In spite of all this, novel polymers appear every year,
as long as they demonstrate uniqueness. However, there appears an increasing
trend towards polyblends (mixtures of existing polymers). There is a distinction
between homogeneous polymers (homopolymers), consisting of
identical mers, and heterogeneous ones made of a random combination of
two (or more) mers, namely, the copolymers. The latter may involve various
compositions differing in the type, concentration and order of the distinct
mers in the macromolecule.
In conclusion, a wide array of polymers is already in use, so that a systematic
presentation of their chemical structure, as well as the relationship between
structure and behavior at the final stage, is essential.
[ 3 ]
PROBLEMS
1. Describe 3 products made of plastics. What are the advantages over
other materials? State other options.
2. Describe 3 natural polymers and the scope of applications. What synthetic
polymers may replace them and what are their advantages?
3. Describe 5 thermoplastic polymers and 5 thermosetting polymers.
What are the principal differences between the two groups? Give details on
the structure of the mer in each case.
4. Can you convert thermoplastic polymers to thermosetting polymers?
Thermosets to thermoplastics? Describe hybrids.
5. Find the output of polymer production in the recent two years in the
US. Give details of production of the major polymer families and state the
changes for each of them.
6. Which polymers are products from the coal industry and which are
produced by the petrochemical industry?
2
IN THIS CHAPTER we deal with a very basic concept-from
where does everything start? We begin with the raw materials for the polymer
industry, the so-called monomers, and explain how they are produced. The
process for polymer synthesis - polymerization - should be well understood
regarding both the mechanism as well as the industrial technology. We avoid
going into too many details but still point out the basic principles.
Polymerization reactions involve careful control of purity of monomers,
ratio of reactants, special additives, temperature and pressure, separation and
recovery. Each family of polymers represents a wide range of variables. They
appear in a large number of grades, differing in molecular weght, distribution,
degree of crystallinity, size of particles and types of special additives.
The polymer leaves the reactor as a powder or as pellets (often, after passing
through extrusion and granulation). Some stabilizers are directly added in the
polymerization or granulation stage. The polymer is then stored in big silos
for homogenation. This process is mostly carried out at special compounding
units, which are responsible for exact mixing and composition.
2.2 THE PETROCHEMICAL INDUSTRY
The petrochemical industry, that branch of the chemical industry which
is based on the exploitation of the crude-oil distillation products has turned out to be the leading industry in chemistry, wherein most monomers and
polymers are produced. It was preceded by the coal industry, developed
mainly in Germany and Britain during the 19th century. By decomposition of
coal at high temperature (an anaerobic process called cracking) products llke
acetylene, methanol, or phenol are derived. These chemicals serve as the
primary source for an extended array of polymers.
Acetylene is derived from carbide, the latter obtained by a reaction between
lime and coke (the major solid fraction obtained from coal pyrolysis).
The basic chemical reactions are as follows:
CaO + 3C --* CaCz + CO (2-1)
CaC, + 2Hz0 + Ca(OH)z + HC=CH
(acetylene)
Just another step leads to the manufacture of one of the oldest monomers
-vinylchloride - along with other vinyl derivatives (including acrylonitrile).
CHsCH + HCl + CHz=CHC1
(vinylchloride)
(2-3)
Ethylene, which serves as a source of many other monomers, can also be
synthesized by the hydrogenation of acetylene.
CHzCH + Hz + CHz=CHz (2-4)
Methanol is obtained in another process, by oxidizing coke with steam to
form a mixture of carbon monoxide and hydrogen, as follows:
C + HzO -, CO + Hz (2-5)
Next steps will lead to the formation of methanol via a catalytic reaction:
CO + 2Hz + CH30H (2-6)
Methanol may be further oxidized to formaldehyde:
CHjOH + %Oz --* HCOH
(formaldehyde)
(2-7)
A whole family of polymers is derived from formaldehyde: polyacetal
(polymethylene oxide), phenol-formaldehyde, urea-formaldehyde and melamine-
formaldehyde. It is interesting to note that the other components (phenol,
urea and melamine) are also products of coal pyrolysis. Like many other aromatics, phenol is produced from another fraction of
the cracking of coal, coal-tar. These aromatics serve as the basis for a large
list of other polymers, like nylons, epoxides and polycarbonates, in addition
to polystyrene and other styrenic derivatives (ABS, SB rubber) and polyesters.
The petrochemical industry (based on crude oil and partly on natural gas)
essentially replaced the coal industry as the major source of monomers. This
industry was developed in Europe during the 1950s, but started in the United
States as early as 1920.
The lowest homolog in the paraffin family, methane (CH,), (derived from
crude oil but frequently found in natural gas), serves as the basis for the
manufacture of methanol:
[ 7 ]
CH, + H20 -+ CO + 3Hz (2-8)
CO + 2H2 -+ CHBOH (2-9)
Alternatively, methane can be converted to acetylene, and through it to all
kinds of monomers.
(2-10)
Currently, by cracking the light fraction naphtha (with a boiling point
between gasoline and kerosene), the unsaturated gases that serve the primary
monomers -ethylene, propylene and butylene - as well as aromatics (including
phenol) are obtained. From these, many monomers are derived. By the
1960s, 90% of all organic chemicals were derived from oil, and this trend
continued growing. Only 3% to 5% of crude oil is used as chemicals, while
the major portion is utilized as fuel. The world forecast for production of
petrochemicals during 1996 was:
Ethylene 65 million tons
Propylene 40 million tons
Butadiene 7 million tons
Benzene 38 million tons
Xylenes 28 million tons
Let us describe the routes to some selected monomers, produced by the petrochemical
industry.
Monomers Derived from Ethylene
(VCM) CH,=CHCI
(vinyl chloride) CH2=CH2 + 2HC1 + Y202 + CH2Cl-CH2Cl -+
CH,=CHCl + HC1
(2-1 1)
VCM can also be manufactured by an alternative reaction:
Styrene can be synthesized by reacting ethylene with benzene (the latter
present in the aromatic fraction of the oil cracking process- benzene, toluene
and xylene).
2CH2=CH2 + 2C6H6 -+ CbHS-C2H5 + CH,=CH I (2-13)
C6H5
(styrene)
Styrene serves as the monomer for the well-known polymer-polystyrene. It
also serves as the source of many copolymers, that is polymers made from
two monomers at varying compositions, such as SAN = styrene-acrylonitrile;
SBR = styrene-butadiene rubber (the major synthetic rubber); SBS =
styrene-butadiene-styrene (a modern family of thermoplastic rubbers which
are not cross-linked); and the well-known terpolymer ABS which is based on
3 monomers - acrylonitrile-butadiene-styrene.
Vinyl acetate (a monomer frequently used in adhesives and coatings) may
also be synthesized by reacting ethylene with acetic acid:
H2C=CH2 + CH3COOH + CH2=CH-O-C-CH3 8
Monomers Derived from Propylene
Acrylonitrile is obtained by reacting propylene with ammonia:
(2-14)
(2-15)
Another important monomer, methylmethacrylate (acrylic), is obtained
by reacting propylene with carbon monoxide, oxygen and methanol:
where does everything start? We begin with the raw materials for the polymer
industry, the so-called monomers, and explain how they are produced. The
process for polymer synthesis - polymerization - should be well understood
regarding both the mechanism as well as the industrial technology. We avoid
going into too many details but still point out the basic principles.
Polymerization reactions involve careful control of purity of monomers,
ratio of reactants, special additives, temperature and pressure, separation and
recovery. Each family of polymers represents a wide range of variables. They
appear in a large number of grades, differing in molecular weght, distribution,
degree of crystallinity, size of particles and types of special additives.
The polymer leaves the reactor as a powder or as pellets (often, after passing
through extrusion and granulation). Some stabilizers are directly added in the
polymerization or granulation stage. The polymer is then stored in big silos
for homogenation. This process is mostly carried out at special compounding
units, which are responsible for exact mixing and composition.
2.2 THE PETROCHEMICAL INDUSTRY
The petrochemical industry, that branch of the chemical industry which
is based on the exploitation of the crude-oil distillation products has turned out to be the leading industry in chemistry, wherein most monomers and
polymers are produced. It was preceded by the coal industry, developed
mainly in Germany and Britain during the 19th century. By decomposition of
coal at high temperature (an anaerobic process called cracking) products llke
acetylene, methanol, or phenol are derived. These chemicals serve as the
primary source for an extended array of polymers.
Acetylene is derived from carbide, the latter obtained by a reaction between
lime and coke (the major solid fraction obtained from coal pyrolysis).
The basic chemical reactions are as follows:
CaO + 3C --* CaCz + CO (2-1)
CaC, + 2Hz0 + Ca(OH)z + HC=CH
(acetylene)
Just another step leads to the manufacture of one of the oldest monomers
-vinylchloride - along with other vinyl derivatives (including acrylonitrile).
CHsCH + HCl + CHz=CHC1
(vinylchloride)
(2-3)
Ethylene, which serves as a source of many other monomers, can also be
synthesized by the hydrogenation of acetylene.
CHzCH + Hz + CHz=CHz (2-4)
Methanol is obtained in another process, by oxidizing coke with steam to
form a mixture of carbon monoxide and hydrogen, as follows:
C + HzO -, CO + Hz (2-5)
Next steps will lead to the formation of methanol via a catalytic reaction:
CO + 2Hz + CH30H (2-6)
Methanol may be further oxidized to formaldehyde:
CHjOH + %Oz --* HCOH
(formaldehyde)
(2-7)
A whole family of polymers is derived from formaldehyde: polyacetal
(polymethylene oxide), phenol-formaldehyde, urea-formaldehyde and melamine-
formaldehyde. It is interesting to note that the other components (phenol,
urea and melamine) are also products of coal pyrolysis. Like many other aromatics, phenol is produced from another fraction of
the cracking of coal, coal-tar. These aromatics serve as the basis for a large
list of other polymers, like nylons, epoxides and polycarbonates, in addition
to polystyrene and other styrenic derivatives (ABS, SB rubber) and polyesters.
The petrochemical industry (based on crude oil and partly on natural gas)
essentially replaced the coal industry as the major source of monomers. This
industry was developed in Europe during the 1950s, but started in the United
States as early as 1920.
The lowest homolog in the paraffin family, methane (CH,), (derived from
crude oil but frequently found in natural gas), serves as the basis for the
manufacture of methanol:
[ 7 ]
CH, + H20 -+ CO + 3Hz (2-8)
CO + 2H2 -+ CHBOH (2-9)
Alternatively, methane can be converted to acetylene, and through it to all
kinds of monomers.
(2-10)
Currently, by cracking the light fraction naphtha (with a boiling point
between gasoline and kerosene), the unsaturated gases that serve the primary
monomers -ethylene, propylene and butylene - as well as aromatics (including
phenol) are obtained. From these, many monomers are derived. By the
1960s, 90% of all organic chemicals were derived from oil, and this trend
continued growing. Only 3% to 5% of crude oil is used as chemicals, while
the major portion is utilized as fuel. The world forecast for production of
petrochemicals during 1996 was:
Ethylene 65 million tons
Propylene 40 million tons
Butadiene 7 million tons
Benzene 38 million tons
Xylenes 28 million tons
Let us describe the routes to some selected monomers, produced by the petrochemical
industry.
Monomers Derived from Ethylene
(VCM) CH,=CHCI
(vinyl chloride) CH2=CH2 + 2HC1 + Y202 + CH2Cl-CH2Cl -+
CH,=CHCl + HC1
(2-1 1)
VCM can also be manufactured by an alternative reaction:
Styrene can be synthesized by reacting ethylene with benzene (the latter
present in the aromatic fraction of the oil cracking process- benzene, toluene
and xylene).
2CH2=CH2 + 2C6H6 -+ CbHS-C2H5 + CH,=CH I (2-13)
C6H5
(styrene)
Styrene serves as the monomer for the well-known polymer-polystyrene. It
also serves as the source of many copolymers, that is polymers made from
two monomers at varying compositions, such as SAN = styrene-acrylonitrile;
SBR = styrene-butadiene rubber (the major synthetic rubber); SBS =
styrene-butadiene-styrene (a modern family of thermoplastic rubbers which
are not cross-linked); and the well-known terpolymer ABS which is based on
3 monomers - acrylonitrile-butadiene-styrene.
Vinyl acetate (a monomer frequently used in adhesives and coatings) may
also be synthesized by reacting ethylene with acetic acid:
H2C=CH2 + CH3COOH + CH2=CH-O-C-CH3 8
Monomers Derived from Propylene
Acrylonitrile is obtained by reacting propylene with ammonia:
(2-14)
(2-15)
Another important monomer, methylmethacrylate (acrylic), is obtained
by reacting propylene with carbon monoxide, oxygen and methanol:
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