feedstock recycling of 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 the

Feedstock Recycling of Plastic Wastes

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Feedstock Recycling of Plastic Wastes

by J Aguado, Rey Juan Carlos University, Mdstoles, Spain; D.P Serrano, Complutense University of Madrid, Spain

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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

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

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

Hydrolysis Ammonolysis and Aminolysis Combined Chemolysis Methods

Gasification of Carbonaceous Materials and Uses

Gasification of Plastic and Rubber Wastes

Gasification of Mixed Solid Wastes

Other Plastic and Rubber Partial Oxidation Processes

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

Thermal Conversion of Rubber Wastes and Used Tyres

Types and Properties of Polymer Cracking Catalysts

Catalytic Conversion of Individual Plastics

Polyethylene Polypropylene Polystyrene Catalytic Conversion of Plastic Mixtures and

Hydrocracking of Rubber and Used Tyres

Coliquefaction of Coal and Plastics

Coliquefaction of Coal with Rubber and Used Tyres

To Maribel and to Juany

CHAPTER 1

Introduction

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-

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

In t r oduc t ion

Others 30%

3

Automoth 7%

Figure 1.2 Plastic consumption by product in Western Europe (1995) .4

2 Classes of Organic Polymers and their Main

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

(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

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)

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 ( P E )

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

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 ( P P )

Polypropylene is synthesized by polymerization of propylene, which may result

in two main types of PP with commercial applications:

0 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 ( 8 0 4 5 % ) 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

0 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

10 Chapter I

Polystyrene ( P S )

Polystyrene is produced by styrene monomer polymerization, which leads to an amorphous, non-flexible polymer having good electrical insulation properties and a density of about 1.04 g/cm3 However, its high brittleness and low softening temperature ( < 100 "C) are important limitations on its industrial application PS is used in the manufacture of radio and TV parts, toys,

electronic components, etc

Expanded polystyrene (EPS) is prepared by impregnation of commercial PS

beads with a blowing agent, such as isopentane Steam heating of the

impregnated beads leads to a cellular structure with a very low density EPS is

commercialized as beads or foams having widespread use in the packaging and building insulation sectors

High impact polystyrene (HIPS) is synthesized by emulsion polymerization of styrene in styrene-butadiene latex The higher impact characteristics of HIPS

make it suitable for use in the manufacture of sheets, food containers, window

frames, household goods, etc

Polyvinyl Chloride ( P V C )

PVC is a plastic of low crystallinity, prepared by polymerization of

vinyl chloride at temperatures of about 50°C There are two main grades of

PVC, rigid and flexible Rigid PVC is the product directly obtained from

the polymerization and, as its name indicates, it is a stiff, hard and often

brittle polymer Flexible PVC is obtained by blending with a variety of plasticizers, which leads to a soft and pliable material Rigid PVC is used in

the manufacture of sheets, pipes, window profiles, etc., whereas the applications

of flexible PVC include wire coating, toys, floor coverings, films and

tubing

In addition to plasticizers, PVC usually incorporates other components such

as impact modifiers, fillers and extenders

Polyethylene Terephthalate ( P E T )

Several routes are available for synthesis of PET, starting from different monomers: terephthalic acid (TPA), dimethylterephthalate (DMT) and bis-

hydroxyethylterephthalate (BHET) The most common method of PET syn-

thesis is based on the copolymerization of TPA and ethylene glycol PET is a

thermoplastic which can exist in amorphous, partially crystalline and highly

crystalline states For most PET applications, crystallinity is desired because it

leads to enhanced strength and increases the maximum working temperature

PET is widely used in the manufacture of fibres, bottles and films In

recent years, rapid growth in the use of the moulding grades of PET has

occurred

Introduction 11

Thermosets

Thermosets are used in a similar proportion for both plastic and non-plastic applications Plastic uses of thermosets include vehicle seats, sports equipment, electrical and electronic components, etc., while typical non-plastic applications include coatings and adhesives

The main commercial thermosets are urea-formaldehyde resins (UF), melamine-formaldehyde resins (MF), phenol-formaldehyde resins (PF), epoxy resins, unsaturated polyesters, alkyd resins and polyurethanes Changes in thermoset consumption in Western Europe during the period 1994-1996 are shown in Table 1.2 UF/MF resins and polyurethanes are produced in the greatest quantities, making up about 70% of the total thermosets market

Plastic Additives

Commercial plastics are typically prepared by mixing one or more polymers with a variety of additives in order to adjust and improve the properties and performance of the polymer The main types of plastic additives are classified as follows:6

Plasticizers The major role of plasticizers is to reduce the polymer

modulus by lowering its glass transition temperature ( Tg) Plasticizers are usually low molecular weight organic compounds having a Tg of about

- 50 "C Common plasticizers used for PVC include dialkyl phthalate, aliphatic diesters and trialkyl phosphate

Fillers and reinforcements Fillers are inert materials used primarily to

reduce the resin cost but also to improve the polymer processability Typical fillers include clay, talc, silica, fly ash, mica, sand, glass beads, graphite and carbon black

Thermal stabilizers and antioxidants These additives are used to protect

the polymer against the effects of temperature and oxygen during processing Free-radical scavengers such as hindered phenols and aromatic amines are typically added for this purpose

Light stabilizers These are usually compounds which absorb ultraviolet

light, in order to avoid degradation of the polymer by radiation Products derived from benzophenone are typically used as light stabilizers

Flame retardants These components are incorporated to inhibit or

modify the polymer combustion when heated in an oxidative atmosphere Typical flame retardants are Cl-, Br- and P-containing organic compounds, as well as antimony oxide and hydrated alumina

Colorants Addition of soluble dyes and the dispersion of pigments are the

methods used to provide plastics with desired colours Dyes include azo compounds, anthraquinones, xanthenes and azines whereas a variety of inorganic compounds are used as pigments such as iron oxides, cadmium and titanium dioxide

Chapter I

Antistatic agents Static electrical charges may build up on the surface of polymers due to their low electrical conductivity, which may cause dust accumulation and sparking problems These charges can be dissipated through the addition of external or internal antistatic agents (phosphate and fatty acid esters, sulfated waxes, quaternary ammonium compounds,

amines, etc.)

Blowing agents These are used in the preparation of foamed plastics, mainly polystyrene and polyurethanes Both physical and chemical blowing agents are used, that volatilize or decompose into gases after being mixed with the polymer Examples of blowing agents include short- chain hydrocarbons (pentanes and hexanes), fluorocarbons, gases

(nitrogen, air, carbon dioxide), hydrazine derivatives, etc

Both natural and synthetic rubber are commercially used in the manufacture of

a variety of goods As mentioned earlier, rubbers are elastomeric polymers, characterized by the presence of a network structure that may be temporarily deformed when subjected to external forces

Natural rubber accounts for about 25% of total rubber consumption It is

produced from the Hevea brasiliensis tree, being formed by isoprene units

with cis-1,4 links Natural rubber is used in tyres and for retreading, latex,

mechanical goods, etc

A variety of synthetic rubbers are commercially used: styrene-butadiene rubber (SBR), polybutadiene, ethylene-propylene rubber, butyl and halobutyl

rubber, etc The most important is SBR, which is mainly used as a major component of all passenger tyres and in significant amounts in most tyre products

Rubbers are usually subjected to a vulcanization or curing process to improve their properties Vulcanization is carried out commonly by reaction with sulfur, which leads to the formation of a three-dimensional structure through the formation of sulfur bridges between the polymer chains Other vulcanizing

agents include peroxides, metal oxides, amines, etc As in the case of plastics,

rubber goods also incorporate a number of additives7

0 Accelerators of the curing process that allow control of the time and rate

of vulcanization, as well as the number and type of sulfur crosslinks which are formed Typical accelerators include guanidines, mercaptobenzo-

thiazoles and sulfenamides, etc

Introduction 13

Activators which increase the vulcanization rate by reacting with the

accelerators yielding rubber-soluble complexes Zinc oxide and stearic acid are widely used as activators

Retarders to delay the initial onset of vulcanization, providing the time

necessary for processing the uncured rubber

Antioxidants and antiozonants

Process oils and plasticizers

Fillers Carbon black, clays, calcium carbonate, silica, etc are typical

rubber fillers Carbon black is used in relatively high proportions to improve the strength of the rubber The performance of carbon black incorporated into rubber largely depends on its particle size distribution Tyres contain over 30 wt% of carbon black

The disposal of scrap tyres is currently an important environmental problem, as most used tyres are dumped in landfills In some cases, accumulations of used tyres have accidentally caught fire, causing the release of toxic substances into the atmosphere The generation of scrap tyres has been estimated to be of the order of 1.5 million tonnes per year in the European Union, 2.5 million tonnes per year in North America and 0.5 million tonnes per year in Japan.8

The increase in the use of plastic materials in all sectors of industry and in everyday life, as well as the reduction in the lifetime of most plastic products, have led to a continuous increase in the generation of plastic wastes Figure 1.7 illustrates the distribution of solid wastes by sector in Western Europe for 1996 The total volume of waste was 2.6 x lo9 tonnes, although the total post-user plastics waste accounted for only 1.7 x lo7 tonnes, i.e., only 0.6 wt% of the solid waste was plastic materials

Due to the extensive use of plastics in packaging, most of the plastics waste is found in domestic refuse Table 1.3 summarizes the plastics waste generated by

TRlBUTlON AND

MUNICIPAL SOLID WASTE

14 Chapter I

Europe for 1996 (based on reference 2 )

The average composition of household waste in Western Europe is shown in Figure 1.8, the plastic content in MSW being just 8 wt% A similar proportion

is found in the USA, where in 1993 plastics made up 9.3 wt% of the total

household waste The percentages by weight of the different resins found in plastics household waste are shown in Figure 1.9 Polyolefins (L/LDPE, HDPE and PP) account for approximately 60% of the total plastics waste in MSW,

which is in close correspondence with the production of these polymers Likewise, over 90% of the total plastics waste is made up of just six resins (LDPE, HDPE, PP, PS, PVC and PET)

The Economic and Environmental Impact of Plastic Wastes

From an economic point of view, used plastic can be considered as both an important source of valuable chemicals, mainly hydrocarbons, and an energy source The calorific value of most plastics is similar to that of fuel oils and higher than that of coals Plastic wastes can therefore be viewed as potential fuels, when other alternatives of valorization are not possible.'

Plastic wastes represent a significant environmental impact due to the following facts:

Because of their resistance to degradation, plastic materials exist for a long time when disposed of in landfills Although some biodegradable plastics have been successfully synthesized, most of the commercial resins are not found within this category Moreover, the slow degradation of plastics is responsible for the progressive reduction of landfill capacity The risk of accidental fires with highly polluting emissions is increased in landfills containing large amounts of plastics Plastic wastes account for

about 25% of all solid wastes accumulated in landfills

Plastics usually contain a variety of additives such as fillers, stabilizers, plasticizers, reinforcing agents, colorants, etc Both organic and inorganic compounds are added to plastics to improve and modify their properties, containing in many cases heavy metals Thus, according to the United States Environmental Protection Agency, plastics contribute 28 Yo of all cadmium present in MSW and about 2% of all lead."

As a consequence of their low density, plastics cause a greater visual

impact on disposal than many other materials Similarly, the light weight

of plastics is the origin of important limitations on the recycling of plastic wastes, due to increased collection and transportation costs Thus, to

Despite all these problems, substitution of plastics by other materials is not environmentally sound According to Gebauer and Hofmann, l 1 replacement of plastics in packaging by glass, paper, cardboard or metals would lead to drastic increases in the weight ( > 400%), cost ( > 200%) and volume (> 200%) of the packaging products, as well as in the energy consumed (>200%) in their manufacture The advantages of using plastic materials are demonstrated by the following examples:

0 Energy requirements for polyethylene grocery sacks are 2 0 4 0 % lower than paper, while generating about 75% less solid waste, 65% less atmospheric emissions and 90% less waterborne waste '*

0 Replacement of about 200-300 kg of conventional materials in a modern car by plastics leads to a reduction in the fuel consumption of 750 litres over a lifespan of 150 000 km If the whole automotive sector of Western Europe is considered, this reduction would cause a decrease in the oil consumption of 12 million tonnes and in CO2 emissions of 30 million tonnes every year.2

0 In the case of glass containers, 43% by volume of a lorry load would be the packaging, whereas if using plastic containers this value is reduced to

7 Y 0 l ~

Management of Plastic Wastes

The general concerns about environmental protection and resource conserva- tion have led to the development of a variety of solid waste management techniques to reduce both the environmental impact of the different types of waste and the depletion of natural resources Management of plastic wastes cannot be treated as an individual problem; it must be considered as an integral part of the global waste management system

Current waste management is based on a four-level hierarchical approach:

0 Reduction Minimizing the consumption of raw materials through improvements in the design of products may allow a significant reduction

in the amount of wastes generated when they reach the end of their life

cycle As examples of the progress in this area, Table 1.4 shows the

decrease in the weight of different food containers over the period 1970-

1990 However, it is clear that there is a limit to the advances which can be made by weight reduction, since the mechanical properties and performance of the products are also affected by this decrease

Reuse This is mainly applied to packaging goods, being defined as any

Introduction 17

Table 1.4 Reduction in weight of food containers (1970-1990)

Container Weight in 1970 (8) Weight in 1990 (g) Reduction (%)

Wine bottle (glass) 450

Beer bottle (glass) 210

Supermarket bag (PE) 23

Yoghurt container (PS) 6.5

3 50

130 6.5 3.5

option can be applied especially for containers such as bottles, bags, etc

Recycling This allows the wastes to be reintroduced into the consumption

cycle, generally in secondary applications because in many cases the recycled products are of lower quality than the virgin ones Recycling must be applied only when the amount of energy consumed in the recycling process is lower than the energy required for the production of new materials Plastics can be recycled using two different approaches: mechanical and feedstock recycling In the first case, the plastics are recycled as polymers, whereas in the second, plastic wastes are transformed into chemicals or fuels

Energy recovery When the recycling of wastes is not feasible or there is no

market for the recycled product, incineration can be used to generate energy from the waste combustion heat Plastics are materials of high calorific value, hence plastic wastes greatly contribute to the energy produced in incineration plants Alternatively, they can be used as fuels in

a number of applications: power plants, industrial furnaces, cement kilns,

etc Incineration of Cl-containing plastics has been the subject of great

controversy due to the possible formation and release into the atmosphere

of dioxins However, the relationship between PVC content in the waste stream and dioxin concentration has not been clearly demonstrated In fact, it seems that the formation of dioxins depends mainly on the incineration conditions rather than on the waste composition

the basis of this hierarchical approach, only non-recyclable and non- energy valuable wastes should be disposed of in landfills However, the current picture of plastic wastes management is far from this situation Figure 1.10 shows the destiny of plastic wastes in Western Europe during

1996 Most of the plastics are still disposed of by landfilling, followed by energy recovery and with small proportions of mechanical and chemical recycling This distribution has been forecast to change in the next few years, with more recycling and energy recovery and a large decrease in the amount

of plastic wastes sent to landfills In fact a rapid growth in feedstock recycling has been observed in recent years in Europe, although this is concentrated mainly in one country, Germany

Targets for the minimization of wastes have been established by the EU

Directive on Packaging and Packaging Waste, which came into force in December 1994, fixing minimum and maximum recovery rates of 50 and 60%, as well as minimum and maximum recycling targets of 25 and 45%

Moreover, a minimum recycling rate of 15% was established for each individual material

There are several reasons for the lack of recycling of plastic wastes, compared

to other solid materials such as paper/cardboard and glass A number of problems arise from the large variety of chemical compositions and properties

of the different types of plastics, which makes it difficult to establish a general recycling procedure In addition, plastic wastes are mainly contained in MSW, mixed with other solids Therefore, costly and complex separation treatments must be applied in many cases to obtain a plastic waste stream having a more or less homogeneous composition Likewise, the low density of most plastics makes it necessary to deal with large volumes of wastes in order to produce a given mass of recycled material

Introduction 19

Mechanical Recycling

As shown in Figure 1.1 1, mechanical or material recycling of plastics involves a

number of treatments and operations: separation of plastics by resin, washing to remove dirt and contaminants, grinding and crushing to reduce the plastic particle size, extrusion by heat and reprocessing into new plastic

Because thermosets cannot be remoulded by the effect of heat, this type of recycling is mainly restricted to thermoplastics

Mechanical recycling is limited by the compatibility between the different types of polymers when mixed, as well as by the fact that the presence of small amounts of a given polymer dispersed in a matrix of a second polymer may dramatically change the properties of the latter, hindering its possible use in conventional applications Thus, the presence of low amounts of PVC in recycled PET strongly reduces the commercial value of the latter, due to the possible release of HC1 during the PET reprocessing This problem is enhanced

by the fact that PVC and PET are difficult to separate from other plastic wastes Another difficulty with mechanical recycling is the presence in plastic wastes of products made of the same resin but with different colours, which usually impart an undesirable grey colour to the recycled plastic

20 Chapter I

In addition, most polymers suffer a certain degradation during their use due

to the effect of a number of factors such as temperature, ultraviolet radiation, oxygen and ozone This degradation leads to a progressive reduction in length and to a partial oxidation of the polymer chains

Therefore, recycled polymers usually exhibit lower properties and perfor- mance than the virgin material, and are useful only for undemanding applica- tions Recycling plastics without prior separation by resin produces a material with mechanical properties similar to timber, hence it is often used for the replacement of timber in certain applications A higher quality of recycled plastics is achieved when separation by resin is carried out prior to the remoulding step However, even in this case, recycled plastics cannot be used

in food containers, unless direct content with the food can be avoided

An alternative developed in recent years for promoting the use of recycled plastics has been the preparation of containers with a three-layer wall The middle layer is the thickest and is made of recycled polymer, whereas the thinner external and internal layers are made of virgin material With this approach direct contact between the recycled polymer and both the consumer and the product in the container is avoided

A wide variety of procedures and treatments have been investigated for the

feedstock recycling of plastic and rubber wastes For the purposes of this book, these methods have been classified into the following categories (Figure 1.12):

0 Chemical depolymerization by reaction with certain agents to yield the starting monomers

0 Gasification with oxygen and/or steam to produce synthesis gas

0 Thermal decomposition of the polymers by heating in an inert atmosphere

0 Catalytic cracking and reforming The polymer chains are broken down by the effect of a catalyst, which promotes cleavage reactions

0 Hydrogenation The polymer is degraded by the combined actions of heat, hydrogen and in many cases catalysts

The progress and the current status of these alternative methods of feedstock recycling are described in the following chapters At present, feedstock recycling

is limited by the process economy rather than by technical reasons Three main factors determine the profitability of these alternatives: the degree of separation

Introduction

PLASTICS AND RUBBER WASTES

Figure 1.12 Alternatives for the feedstock recycling of plastic and rubber wastes

required in the raw wastes, the value of the products obtained and the capital

investment in the processing facilities In most of the above methods, some

pretreatments and separation operations must be carried out on the plastic

wastes prior to feedstock recycling, which results in an increase in recycling

costs According to the separation steps required, the different methods of

feedstock recycling can be ordered as follows: gasification < thermal treat-

ments % hydrogenation < catalytic cracking < chemical depolymerization

Many of the projects on chemical recycling of waste plastics have failed in the

past due to the relatively low price of the derived products In recent years, there

has been a trend towards the production of added value compounds such as

olefinic gases, paraffins, activated carbons, etc In general terms, the commercial

value of the products obtained in the different treatments can be ordered as:

thermal oils z synthesis gas < hydrogenation oils z catalytic oils < mono-

mers

It is interesting to note that the required pretreatments and product value

follow almost reverse orders However, many other factors should be included

for an adequate comparison of these alternatives: the possibility of carrying out

the treatment in existing or new facilities, minimum size of the industrial plants

needed to be profitable, required investment, plant location, etc Hofmann and

Gebauerlg have identified the main problems involved in several of the feed-

stock recycling methods:

High capital expenditure, especially in hydrogenation plants

Lack of a regional plastic waste volume to support the continuous

operation of large-scale plants For gasification, a minimum capacity

about 400 000-500 000 tonnes per year is necessary

Pyrolysis and hydrogenation lead to a wide range of end-products, which

then have to be further upgraded and processed, mainly in refinery units

Several petrochemical companies have considered the possible feedstock recy-

cling of plastic wastes in existing refinery facilities, which would avoid the need

to invest and build new processing This alternative is based on the

similarity of elementary composition between plastics and petroleum fractions

Moreover, taking into account the differences in production between plastics

and the total of petroleum-derived products, plastic wastes could be incorpo-

22 Chapter I

rated into refinery streams in relatively low amounts The main problem associated with this approach is the possible presence of undesired elements and compounds (Cl, N, metals, etc.) in the plastic wastes that would be

introduced into the refinery In such cases, even a small drop in yield or efficiency, when multiplied by hundreds of tonnes, would have dramatic effects

on the refinery economy Accordingly, plastic wastes should be intensively pretreated and conditioned prior to being added to petroleum fractions Figure 1.13 shows the main refinery units that may be used in the processing of plastic wastes according to Hofmann and Gebauer ’’ (visbreaker, hydrogenation and gasification) Meszaros2’ has also proposed the processing of plastic wastes in the cokers and catalytic cracking units of refineries The economic viability of this alternative greatly depends on the proximity of the plastic waste generation points and the refineries, due to the high costs of the plastic transportation

Sorting and Separation of Mixed Plastics

The previous sections point out the relationship between the ‘purity’ of the raw plastic wastes and the quality of the products obtained by both mechanical and feedstock recycling Therefore, removal of contaminants and separation of plastics by resin can be considered as treatments required before the plastic recycling is carried out A variety of methods have been developed for the separation of plastics: manual, flotation, dissolution, spectroscopic identifica- tion, etc

Manual separation of plastics is carried out by personnel placed on either side

of a conveyor transporting the plastic goods Figure 1.14 illustrates a manual sorting facility with a number of operators specialized in the separation of different plastic containers.22 Three sorters initially remove HDPE goods, and two sorters are responsible for separating green and clear PET products Another operator separates mixed color HDPE and the final three sorters are

in charge of the recovery of PP and PVC bottles and any remaining PET and HDPE containers As the incoming material stream progresses towards the end

of the conveyor, plastic goods are progressively removed until nothing but trash and undesirable materials remain

Plastic separation by flotation is based on density differences between the polymers (see Table 1.5) Mixed plastics are dispersed in a liquid having a density between those of the different plastics Polymers with densities lower than that of the medium float, while those of higher density sink, and so can be easily separated In successive steps the density of the liquid medium can be changed in order to achieve separation of the resulting fractions The density of aqueous solutions can be adjusted by adding salts or alcohols In some cases the separation of a given polymer is promoted by adding a wetting agent and bubbling a gas through the solution, which selectively adheres to the polymer surface, causing it to float.23 Likewise, addition of solvents that are absorbed by PVC, so reducing its density, has been proposed to favour the separation of PVC and PET by flotation.24 In a recent patent, the use of a solvent in supercritical conditions has been reported as an effective method for the

Synthesis gas ( CO, H, )

Met h a no1 synthesis

I

v

Methanol

Figure 1.13 Processing of plastic wastes in rejnery units.” VVA: pre-treated mixed

plastic scrap; AR: residue from the atmospheric distillation process; VR: vacuum distillation residue; VisR: visbreaker residue; VVR: vacuum vis- breaker residue

separation of plastics.25 The extremely high compressibility of a fluid in the vicinity of its critical point allows its density to be varied by minimal changes in the temperature or pressure Plastics with similar densities can thus be separated

by adjusting the fluid density, usually using C 0 2 , to a value between those of the

two polymers Plastic separation by density differences can be accelerated by centrifugation instead of flotation techniques

Differences in solubility have also been exploited for the separation of