feedstock recycling of plastic wastes

Feedstock Recycling of Plastic Wastes
RSC Clean Technology Monographs
Series
Editor:
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Clark, University
of
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UK
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DC,
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UK),
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(Mayfield,
UK),
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(Frankfurt, Germany),
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The chemical process industries are under increasing pressure to develop
environmentally friendly products and processes, with the key being
a
reduction
in waste. This timely new series will introduce different clean technology
concepts to academics and industrialists, presenting current research and
addressing problem-solving issues.
Feedstock Recycling of Plastic Wastes
by
J.
Aguado,
Rey Juan Carlos University, Mdstoles, Spain;
D.P. Serrano,
Complutense University
of
Madrid, Spain
Applications

of
Hydrogen Peroxide and Derivatives
by C.W. Jones,
formerly
of
Solvay Interox
R
&
D,
Widnes,
UK
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CLEAN
TECHNOLOGY
MONOGRAPHS
Feedstock
Recycling
of
Plastic Wastes
Josk
Aguado
Department
of
Experimental Sciences and Engineering, Rey Juan
Carlos
University, Mdstoles, Spain
David
P.
Serrano
Chemical Engineering Department, Complutense University
of
Madrid, Spain
RSaC
ROYAL
SOCIETY
OF
CHEMISTRY
ISBN
0-85404-531-7
A
catalogue record for this book is available from the British Library

0
The Royal Society of Chemistry
1999
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Preface
The use of plastic materials in daily life has continuously increased over the last
30
years. The amount of plastic consumed per inhabitant in the industrialized
countries has increased by a factor of
60
over this period, while the generation of
plastic wastes has grown at a similar rate. Thus, over 17.5 million tonnes of
plastic wastes are generated per year in Western Europe, their environmental
impact being a matter of great public concern. The variation in properties and
chemical composition between different types of plastic materials hinders the
application of an integrated and general approach to handling these plastic
wastes. The light weight of plastic goods, and the fact that plastic wastes are
mainly found in MSW (municipal solid waste) mixed with other classes of
residues, are factors that greatly limit their recycling. As a consequence, the
primary destination of plastic wastes is landfill sites, where they remain for
decades due to their slow degradation. In
1996,
only around 10% of the plastic
wastes generated in Europe were recycled, whereas over 70% were disposed of
in landfills.
At present, there are three main alternatives for the management of plastic
wastes in addition to landfilling: (i) mechanical recycling by melting and
regranulation of the used plastics, (ii) feedstock recycling and (iii) energy
recovery. Mechanical recycling is limited both by the low purity of the
polymeric wastes and the limited market for the recycled products. Recycled
polymers only have commercial applications when the plastic wastes have been
subjected to a previous separation by resin; recycled mixed plastics can only be
used in undemanding applications. On the other hand, energy recovery by
incineration, although an efficient alternative for the removal of solid wastes, is

the subject of great public concern due to the contribution of combustion gases
to atmospheric pollution. There has also been some controversy in the past
about the possible relationship between dioxin formation and the presence of
C1-containing plastics in the waste stream.
Consequently, feedstock recycling appears as a potentially interesting
approach, based on the conversion of plastic wastes into valuable chemicals
useful as fuels or as raw materials for the chemical industry. The cleavage and
degradation of the polymer chains may be promoted by temperature, chemical
agents, catalysts,
etc.
The aim of this work is to describe and review the different alternatives
developed for the feedstock recycling of plastic wastes, with emphasis on both
the scientific and technical aspects. Due to the wide variety of plastic types, the
V
vi
Preface
work focuses on the major polymers present in household and industrial plastic
wastes: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene
terephthalate (PET), polyvinyl chloride (PVC), polyurethanes (PU) and poly-
amides (PA). These plastics account for more than
90%
of total plastic wastes.
Although elastomers are not usually considered as plastic materials, the book
also covers the feedstock recycling of rubber wastes, mainly used tyres. This is
supported by the fact that a number of degradation treatments have been
developed which can be used for both plastic and rubber wastes.
Five main types of feedstock recycling processes have been considered:
chemical depolymerization, gasification and partial oxidation, thermal degra-
dation, catalytic cracking and reforming, and hydrogenation. Each
of

these
alternatives is reviewed in an independent chapter, highlighting the most recent
progress with extensive literature references. Besides conventional treatments
(pyrolysis, gasification,
etc.),
the book includes new technological approaches
for the degradation of plastics such as conversion under supercritical conditions
and coprocessing with coal.
The first chapter gives a general introduction to the types and applications of
polymeric materials, as well as to the various plastic waste management and
recycling alternatives. Data are provided about the volume of plastic wastes
generated, their origin and their composition. Previous separation and classifi-
cation of the plastics is required in many feedstock recycling processes, and
so
the different methods available for plastic sorting are described: manual, density
differences, selective dissolution, automated methods based on spectroscopic
techniques,
etc.
Chapter
2
discusses depolymerization processes based on the chemical
cleavage
of
polymer molecules to convert them back into the raw monomers.
The latter can be reused in the manufacture
of
new polymers, with properties
similar to those
of
the virgin resins. However, this alternative is mainly used for

condensation polymers, and is not successful for the degradation of most
addition polymers. Glycolysis, methanolysis, hydrolysis and ammonolysis are
the main treatments considered. Chemical depolymerization of polyesters,
polyurethanes and pol yamides is reviewed.
Chapter
3
deals mainly with gasification processes leading to synthesis gas,
which is a mixture useful for the preparation of a variety of chemical products
(ammonia, methanol, hydrocarbons,
etc.).
Gasification processes based on
treatment with oxygen, air and steam are described. In many cases, gasification
of plastic wastes takes place simultaneously with that of other organic residues,
coal and petroleum fractions. In addition to gasification, other degradation
alternatives based on partial oxidation methods are described in this chapter.
The degradation of plastic wastes by thermal treatments in the absence of
oxygen is reviewed in Chapter
4.
Depending on the raw polymer and the
degradation conditions, a variety of thermal processes have been considered:
thermal depolymerization into the raw monomers, thermal cracking, pyrolysis,
steam cracking and thermal treatment in the presence of solvents. For each
treatment both the products derived and the different types of reactors used are
described.
Preface
vii
Chapter
5
is devoted to catalytic processes for plastic waste recycling.
Through selection

of
the right catalysts, the plastic degradation can be used to
obtain
a
number of valuable products. The properties
of
the main types of
catalysts are reviewed. Both direct catalytic cracking processes, and the
combination of a previous thermal cracking
of
the plastic wastes with a catalytic
reforming of the gases generated in the former are considered.
Chapter
6
deals with hydrogenation processes, usually based on the use of
bifunctional catalysts. Plastic and rubber degradation in a hydrogen atmo-
sphere is an effective treatment yielding highly saturated oils. Coliquefaction of
plastics or rubber with coal is also considered.
The last chapter highlights the main conclusions and establishes a compara-
tive study of the various alternatives for the feedstock recycling of plastic
wastes. The final conclusion is that feedstock recycling of both plastic and
rubber wastes has a high potential for growth in the next few years, although to
be commercially successful a number of technical and economic aspects still
have to be addressed.
Acknowledgements
We are indebted to those who took the time to help
us
in drawing many of the
figures and schemes in this book: Raul Sanz, Josk
M.

Escola, Rafael Garcia,
Luis
M.
Garcia and Araceli Rodriguez. We also greatly appreciate the efforts
and work of Prof. Rafael van Grieken, who patiently reviewed all the chapters.
Finally, we would like to thank all those colleagues that gave permission for the
reproduction of figures from their work.
Contents
Preface
Acknowledgements
V
Vlll

Chapter
1
Introduction
1
2
Significance
of
Plastic Materials in Today’s Society
Classes of Organic Polymers and their Main Applications
Classification
of
Polymers
Thermoplastics
Thermosets
Plastic Additives
Rubber
The Economic and Environmental Impact of

Management
of
Plastic Wastes
Mechanical Recycling
Feedstock Recycling
Sorting and Separation
of
Mixed Plastics
Future Trends in Plastic Waste Management
3
Plastic Wastes
Plastic Wastes
4
References
Chapter
2
Chemical
Depolymerization
1
Introduction
2
Polyesters
Glycolysis
Methanolysis
Hydrolysis
Ammonolysis and Aminolysis
Combined Chemolysis Methods
Comparison of the Various PET Chemolysis Methods
Glycolysis
Hydrolysis

Ammonolysis and Aminolysis
Combined Chemolysis Methods
3
Polyurethanes
1
1
3
4
8
11
11
12
13
15
16
19
20
22
27
28
31
31
32
33
37
38
41
42
44
45

46
47
49
50
ix
X
4
5
6
Chapter
3
1
2
3
4
5
6
7
Chapter
4
1
2
3
4
5
6
7
8
Chapter
5

1
2
3
4
Polyamides, Polycarbonates and Polyacetals
Summary
References
Gasification and Partial Oxidation
Introduction
Gasification of Carbonaceous Materials and Uses
Gasification of Plastic and Rubber Wastes
Gasification of Mixed Solid Wastes
Other Plastic and Rubber Partial Oxidation Processes
Summary
References
of the Syngas
Thermal Processes
Introduction
Mechanism of the Thermal Degradation of Addition
Thermal Conversion of Individual Plastics
Polymers
Polyethylene
Polypropylene
Polystyrene
Polyvinyl Chloride
Other Plastics
Interactions Between Components During Thermal
Thermal Conversion of Plastic Mixtures
Degradation
Contents

52
55
56
59
59
59
62
67
69
71
71
73
73
74
77
78
85
86
91
98
100
101
Processes for the Thermal Degradation of Plastic Wastes
Thermal Coprocessing of Plastic Wastes with Coal and
Thermal Conversion of Rubber Wastes and Used Tyres
Summary
References
Lignocellulosic Materials
Catalytic Cracking and Reforming
Introduction

Types and Properties of Polymer Cracking Catalysts
Catalytic Conversion of Individual Plastics
Polyethylene
Polypropylene
Polystyrene
Catalytic Conversion
of
Plastic Mixtures and
105
115
117
122
124
129
129
130
133
133
145
148
Rubber Wastes
150
Con tents
5
6
7
Chapter
6
1
2

3
4
5
6
7
Chapter
7
Subject Index
Conversion of Plastics by a Combination of Thermal and
Summary
References
Catalytic Treatments
Hydrogenation
Introduction
Hydrocracking
of
Plastics
Hydrocracking
of
Rubber and Used Tyres
Coliquefaction of Coal and Plastics
Coliquefaction of Coal with Rubber and Used Tyres
Summary
References
Concluding
Remarks
xi
151
157
158

161
161
161
168
171
173
176
177
179
185
To
Maribel and
to
Juany
CHAPTER
1
Introduction
1
Significance
of
Plastic Materials
in
Today’s Society
Plastics are not, as many people believe, new materials. Their origin can be
traced to 1847 when Shonbein produced the first thermoplastic resin, celluloid,
by reaction of cellulose with nitric acid. However, the general acceptance and
commercialization of plastics began during the Second World War when
natural polymers, such as natural rubber, were in short supply. Thus, poly-

styrene was developed in 1937, low density polyethylene in 1941, whereas other
commodity plastics such as high density polyethylene and polypropylene were
introduced in 1957.
Today, plastics are very important materials having widespread use in the
manufacture of a variety of products including packaging, textiles, floor cover-
ings, pipes, foams, and car and furniture components. Plastics are synthesized
mainly from petroleum-derived chemicals, although only about 4% of total
petroleum production is used in the manufacture of plastics.
The main reasons for the continuous increase in the demand for commodity
plastics are as follows:
0
Plastics are low density solids, which makes it possible to produce
lightweight objects.
0
Plastics have low thermal and electric conductivities, hence they are
widely
used
for
insulation
purposes.
0
Plastics are easily moulded into desired shapes.
0
Plastics usually exhibit high corrosion resistance and low degradation
rates and are highly durable materials.
0
Plastics are low-cost materials.
Engineering plastics, particularly thermosets, are also used in composite
materials. Their excellent technological properties make them suitable for
applications in cars, ships, aircraft, telecommunications equipment,

etc.
In
recent years, important new areas of application for plastics have emerged in
medicine (fabrication of artificial organs, orthopaedic implants, and devices for
the controlled release of drugs), electronics (development of conductive poly-
1
2
Chapter
I
mers for semiconductor circuits, conductive paints, and electronic shielding),
and computer technology (use of polymers with non-linear optical properties
for optical data storage).
The above paragraphs show that today plastic materials are used in almost all
areas of daily life. Accordingly, the production and transformation of plastics
are major worldwide industries. Consumption of plastics in Western Europe is
forecast to grow from 24.9 million tonnes in 1995 up to about 37 million tonnes
in 2006,' an annual growth rate of 4%. This prediction places plastics among
the most important materials in the next century
also.
Table
I. I
summarizes the changes in total plastic consumption in Western
Europe from 1992 to 1996.* These data refer to the final market for plastic
products consumed by end-users but they do not include sectors such as textile
fibres, elastomers, coatings, or products in which plastics are present in small
quantities, because these are not considered as plastic products. If non-plastic
applications are also taken into account, the total plastic consumption in
Western Europe in 1996 increases up to 33.4 million tonnes. By comparison,
the consumption of plastics in the
USA

and Japan in 1995 were 33.9 and 11.3
million tonnes, respective~y.~
The main sectors of plastic consumption in Western Europe are shown in
Figure 1.1. The major field of plastic consumption is packaging, accounting for
more than 40% of the total volume, followed by the building and automotive
sectors. The most important uses of plastics in packaging are the production of
films and sheets, sacks, bags, bottles and foams. In the building sector, plastics
are used in a variety of applications: insulation, floor and wall coverings,
window and door profiles, pipes,
etc.
The automotive sector is a good example
of the continuous increase in the use of plastic materials.
A
car's weight can be
reduced by 100-200 kg through the replacement of conventional metallic
materials by plastics. Fuel tanks, bumpers, bonnets, insulation, seats, dash-
boards, textiles, batteries,
etc.
are examples of car components commonly
manufactured with plastic materials. Plastics are used for a variety
of
applica-
tions in the agricultural sector such as greenhouses, tunnel and silage films,
pipes
for
both
drainage and
irrigation,
drums
and

tanks,
etc.
Figure 1.2 illustrates plastic consumption in Western Europe by product for
1995,4 confirming that plastics are versatile materials which can be found in a
wide range of products. The production and consumption of plastics have
continuously increased over recent decades. The plastic consumption per capita
in Western Europe has increased from
-
1
kg per inhabitant in 1960 to about
65
kg per inhabitant in 1995.
Table
1.1
Total plastic consumption in Western Europe (based on reference
2)
Year
1992 1993 1994 1995 1996
Plastic consumption (million tonnes)
24
730
24 360 26 260 24 909 25 905
In t
r
oduc t ion
Others
30%
3
Automoth
7%

Building
1
9%
Agriculture
2
%
Figure
1.1
Plastic consumption
by
sector in Western Europe (1996)
.2
TOTAL:
24.9
Mtonnes
25.00%
I1
Figure
1.2
Plastic consumption
by
product in Western Europe (1995)
.4
2
Classes
of
Organic Polymers and their Main
Applications
Polymers are long-chain molecules composed
of

a large number
of
identical
units called repeating units.
A
polymer can be expressed as
follows:
where
RU
is the repeating unit and
n
the number
of
units present in the polymer
molecule. The number
of
repeating units must be large enough that no
variations in the polymer macroscopic properties occur by small changes in the
4
Chapter
I
number
of
repeating units. This concept enables
a
distinction to be made
between polymers and oligomers. Oligomers are molecules with a small
number of repeating units, hence their properties vary significantly by just
adding or removing a repeating unit.
Most of the polymers with commercial applications are synthetic materials.

They are prepared by polymerization reactions involving the chemical linkage
of small individual molecules (monomers) to give long-chain polymeric
molecules. In some cases, polymers are synthesized by reaction between several
monomers. The product
so
obtained is called a copolymer while the starting
molecules are known
as
comonomers. The structure
of
copolymers depends on
both the relative proportion and the sequence of the different comonomers
along the macromolecular chain. Depending on the polymerization conditions,
it is possible to obtain random, alternating, block or graft copolymers, as
illustrated in Figure
1.3.
It is not easy to define the term ‘plastic’, which is usually considered as
equivalent to the term polymer. Plastics are polymeric materials, but not all
polymers are plastics. In general, the term ‘plastic’ is used to refer to any
commercial polymeric material other than fibres and elastomers. Moreover,
commercial plastics include other components such as additives, fillers, and a
variety of compounds incorporated into the polymers to improve their proper-
ties. The term ‘resin’ is usually used to describe the virgin polymeric material
without any of these components.
Classification
of
Polymers
Polymers are commonly classified according to two main criteria: thermal
behaviour and polymerization mechanism.
As

explained further below, these
classifications are important from the point
of
view
of
polymer recycling,
because the most suitable method for the degradation of a given polymer is
closely related to both its thermal properties and its polymerization mechanism.
8)
-A-B-B-A-A-A-B-A-B-B-8-A-B-A-A-B-A-A-B-
b)
-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-
C)
-A-A-A-A-A-0-0-0-0-B-0-B-A-A-A-A-A-
I
?
?
d)
-A-A-A-A-A-A-A-A-A-A-A-A-A-A-
Figure
1.3
Possible structures
of
copolymers containing
A
and
B
repeating units:
(a)
random,

(b)
alternating,
(c)
block,
(d)
graft.
Introduction
5
Class@ntion According to Thermal Behaviour
Plastics are divided into two major groups depending on their behaviour when
they are heated:
0
Thermoplastics
are plastics which undergo a softening when heated to a
particular temperature. This thermoplastic behaviour is a consequence of
the absence of covalent bonds between the polymeric chains, which
remain as practically independent units linked only by weak electrostatic
forces (Figure 1.4(a)). Therefore, waste thermoplastics can be easily
reprocessed by heating and forming into a new shape. From a commercial
point
of
view, the most important thermoplastics are high density
polyethylene
(HDPE),
low density polyethylene
(LDPE),
polypropylene
(PP), polystyrene (PS), polyvinyl chloride
(PVC),
polyethylene tereph-

thalate
(PET),
polyamide (PA), polymethyl methacrylate (PMMA),
acrylonitrile-butadiene-styrene
copolymer (ABS), and styrene-acrylo-
nitrile copolymer
(SAN).
0
Thermosets
are plastics whose polymeric chains are chemically linked by
strong covalent bonds, which lead to three-dimensional network
structures (see Figure 1.4(b)). Once formed into a given shape, thermosets
a)
Thermoplastic
b)
Thermoset
Figure
1.4
Schematic structures
of
thermoplastic and thermoset polymers.
6
Chapter
1
cannot be reprocessed or remoulded by heating. Examples
of
thermosets
with significant commercial applications are polyurethanes
(PU),
epoxy

resins, unsaturated polyesters and phenol-formaldehyde resins. Thermo-
sets are produced in smaller amounts than thermoplastics, as can be seen
in Table
1.2.
Table
1.2
summarizes the production of different plastics in Western Europe
over the period 1994-1996. Thermosets account for just
16%
of total plastic
production.
A
similar ratio
of
thermoset to thermoplastic production is found in
the
USA.’
Elastomers constitute a third class
of
polymers. Similarly to thermosets,
elastomers have a network structure formed by crosslinking between the
polymer chains. However, the number
of
links is less than in the case of
thermosets which gives these materials elastic properties. Elastomers can be
deformed by the application of external forces. When these forces are
suppressed, the polymer recovers its original form. From
a
commercial point
Table1.2

Consumption
of
plastics in Western Europe
by
resin (based on reference
2)
Consumption
(k
tonnes per year)
Resin
1994 1995 I996
LDPE/LLDPE
HDPE
PP
PVC
PS/EPS
PET
ABS/SAN
PMMA
Acetals
Polycar bonates
PA
Acrylics
Others
Total thermoplastics
UF/MF resins
PU
Phenolic resins
Unsaturated polyesters
Alkyd resins

Epoxy resins
Total thermosets
Total plastics
5825
3718
4982
540 1
2352
1971
550
234
1 04
202
993
433
29 1
27 056
1174
1720
600
436
340
257
4527
31 583
5723
3613
5020
5300
2525

2101
567
270
110
225
1028
34 1
232
27
055
1915
1755
610
430
343
270
5323
32 378
5969
3846
5397
5406
255 1
2093
540
257
111
233
1027
349

232
28011
1934
1773
610
420
343
275
5355
33
366
Introduction
Thermodrstics
PE
PP
PS
PVC
PET
PMMA
PA
*
CH2-CH
rn
I
w NH-R-NH-CO-R'-CO AW
Polycarbonates
0
-CO
-0
-

R
rn
Pol
yacetal
s
wCH2-0-CH2-0
7
Thermosets
Polyurethanes
w#O-R-O-CO-NH-R'-NH-COM
Pol yureas
ww
NH -CO -NH-R
>,wb
Epoxy resins
Phenolic resins
Unsatured Polyesters
wCO-CH=CH-CO-O-R
-0
*VA
Elastomers
Natural Rubber
Pol ybutadi ene
'MN
CH*-CH=CH-CHz
'*
Figure
1.5
Repeating
units

of
diferent
polymers.
of
view, rubbers are the main class
of
elastomers, being mainly used in the
manufacture of tyres.
The repeating units corresponding to a variety
of
organic polymers are shown
in Figure
1.5.
ClasslJica t ion
A
cco
rding to Polymerization Mechanism
Depending on the mechanism
of
polymerization, two groups
of
plastic ma-
terials can be identified:
Addition
polymers.
The polymerization proceeds by a sequential
incorporation
of
monomeric molecules into the growing polymer chain,
without the release

of
any molecules or fragments to the reaction medium.
As
a consequence, the repeating units
of
addition polymers have the same
8
0
Chapter
I
chemical composition as the monomers. Examples of addition polymers
include PE, PS, PVC, PMMA,
etc.
Condensation polymers.
In this case the polymerization reactions take
place with the liberation of small molecules, such as water, hydrochloric
acid,
etc.
Nylon-6,6, obtained by polycondensation of adipic acid and
hexamethylenediamine is a classic example of a condensation polymer. As
shown in Figure 1.6, this polymerization reaction proceeds with the
release of two water molecules by each repeating unit.
Thermoplastics
Thermoplastics account for the majority of plastics consumption. They are used
in a wide variety of products and applications. It can be seen from Table
1.2
that
about
90%
of the total thermoplastics consumption in Western Europe

corresponds to just five thermoplastics: PE, PP, PVC,
PS
and PET. The main
properties of these resins are briefly described below.
Polyethylene (PE)
Polyethylene
is
synthesized by polyaddition of ethylene molecules, which leads
to different types of PE depending on the reaction conditions:
0
High density polyethylene
(HDPE) is produced at relatively low temperature
(60-200
"C)
and pressure (1-100 atm) and is a highly linear polymer having a
specific gravity in the range
0.94-0.97
and a high degree of crystallinity
(80-
95%).
The main applications of HDPE are for the manufacture of films,
food and domestic containers, crates, toys, gas tanks, pipes,
etc.
by blow
moulding and injection moulding. The production
of
blown films for bags
accounts for about
7%
of the HDPE market.

8 8
n HO-CfCH21f4C-OH
+
n H2NfCH2fNH2
6
Adipic
acid
Hexame thylenediam ine
1
+
2nH20
Nylon-6,6
(
repeating unit
)
Figure
1.6
Synthesis
of nylon-6,6
by condensation polymerization.
Introduction
9
Ultrahigh molecular weight polyethylene
(UHMWPE) is really a variety
of
HDPE with a molecular weight greater than
3
x
lo6.
UHMWPE is a

strong and lightweight plastic used in the fibre industry and for
specialized applications such as its use in medicine for the manufacture
of artificial hips.
Low
density polyethylene
(LDPE).
Unlike HDPE, this type of poly-
ethylene is synthesized at very high pressures (1200-1500 atm) and at
temperatures
of
about 250°C. LDPE is a highly branched polymer
characterized by its lower crystallinity and specific gravity than HDPE
but with greater flexibility. Both the flexibility and crystallinity of LDPE
can be controlled by adding low concentrations
of
acryl or vinyl
monomers during the polymerization. LDPE has widespread use in films
for bags and food packaging, greenhouses, bottles, cable insulation and
injection moulded products.
Linear low density polyethylene
(LLDPE) is synthesized by copolymeriza-
tion of ethylene and a-olefins, mainly 1-butene and 1-hexene. The role of
the a-olefinic comonomers is to control both the number and the length of
the side branches.
As
a consequence, LLDPE is a polymer with
intermediate properties with respect to LDPE and HDPE. Main
applications for LLDPE are films, injection moulded parts and wire
insulation.
Polypropylene (PP)

Polypropylene is synthesized by polymerization of propylene, which may result
in two main types of PP with commercial applications:
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Isotactic polypropylene
(1-PP) is the most widely produced type. In this
polymer, all the pendant methyl groups are located on the same side of
the backbone, which results in a high crystallinity
(8045%).
Isotactic
polypropylene is synthesized at temperatures in the range
50-80
"C and at
pressures of 5-25 atm. The main commercial applications of 1-PP are the
manufacture of injection moulded containers, pipes, sheets and textile
fibres for carpets. 1-PP is more rigid and crack resistant than HDPE,
having good electrical insulation properties. Moreover, i-PP has a higher
crystalline melting temperature
(Tm)
which enables its use in products that
must be steam sterilized. These facts explain the continuous increase in
the use of i-PP in various sectors.
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Syndiotactic polypropylene
(s-PP)
is
produced at lower temperatures than
1-PP in the presence of Ziegler-Natta catalysts. The side methyl groups in
this case are
in
alternating positions along the chain, which results in a

non-crystalline polymer with lower density, mechanical strength and
Tm
than 1-PP. Accordingly, s-PP
is
consumed in significantly lower amounts,
being used as a coating material and in hot melt adhesives.