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Recycling of Polymers ebook

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This timely reference on the topic is the only book you need for a complete overview of recyclable polymers. Following an introduction to various polymer structures and their resulting properties, the main part of the book deals with different methods of recycling. It discusses in detail the recycling of such common polymers as polyethylene, polypropylene and PET, as well as rubbers, fibers, engineering polymers, polymer blends and composites. The whole is rounded off with a look at future technologies and the toxicological impact of recycled polymers. An indispensable reference source for those working in the field, whether in academia or industry, and whether newcomers or advanced readers.

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Table of Contents

Cover

Title Page

Copyright

Preface

List of Contributors

Abbreviations

Chapter 1: Introduction

1.1 Introduction

1.2 Conclusion

References

Chapter 2: Common Additives used in Recycling of Polymers

2.1 Review on Different Additives Used in Polymer Recycling

References

2.2 Recent Trends and Future of Polymer Additives in Macromolecular Recycling Technology: A Brief Overview

References

Chapter 3: Methods of Recycling

3.1 Methods of Recycling of Polymers: Addition Polymers

References

3.2 Methods of Recycling of Polymers: Condensation Polymers

References

Chapter 4: Recycling of Plastics

4.1 Introduction

4.2 Plastic Waste Management Scenario

4.3 Ways of Recycling

4.4 Poly(Lactic Acid)

4.5 Poly(Vinyl Chloride)

4.6 Polyethylene

4.7 Polypropylene

4.8 Polystyrene

4.9 Poly(Ethylene Terephthalate) (PET)

4.10 Applications

References

Chapter 5: Recycling of Rubber

5.1 Introduction

5.2 Rubber

5.3 Recycling of Rubber Products

5.4 Applications of Recycled Rubber

5.5 Concluding Remarks

References

Chapter 6: Fibers

6.1 Introduction

6.2 Natural Fibers

6.3 Synthetic Fibers

6.4 Conclusion

References

Chapter 7: Recycling of Polymer Blends and Composites (Epoxy Blends)

7.1 Introduction

7.2 Polymer Blends and Composites

7.3 Characterization and Application of Recyclates

7.4 Conclusions

References

Chapter 8: Recycling of Other Layered Mixed Plastics or Resins: Polyurethanes

8.1 Introduction

8.2 Mechanical Recycling

8.3 Chemical Recycling

8.4 Thermochemical methods

8.5 Energy Recovery by Incineration

References

Chapter 9: Ecoprofiles of Recycled Polymers at a Glance

9.1 Advantages of Recycled Polymers on the Environment

References

9.2 Toxic or Environmental Effects of Recycled Polymers

References

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Begin Reading

List of Illustrations

Chapter 1: Introduction

Figure 1.1 Four methods of recycling.

Chapter 2: Common Additives used in Recycling of Polymers

Figure 2.1.1 Turnover of additives in 2004.

Figure 2.1.2 Consumption of plastics, light stabilizers, and antioxidants since 1950.

Figure 2.1.3 MFI vs the number of injection molding steps for stabilized and unstabilized PP.

Figure 2.1.4 DTST of recycled PVC as a function of the stabilizer content.

Figure 2.1.5 Effect of different compatibilizers on the tensile strength of HDPE/PP/PVC (8/1/1) blend.

Figure 2.1.6 Complex viscosity of HDPE/PP/PVC (8/1/1) blend with different compatibilizers.

Figure 2.1.7 Flow curves of stabilized and unstabilized polymers.

Figure 2.1.8 Effect of HDT modifier and impact modifier on PVC.

Figure 2.1.9 Stress distribution in polymer matrix surrounding a rubbery impact modifier particle.

Figure 2.2.1 Commonly used inorganic fillers in polymer formulations.

Figure 2.2.2 Chemical structure of organic brighteners.

Figure 2.2.3 Structure of antiblocking agents.

Figure 2.2.4 Structure of ethoxylated sorbitan ester (antifogging additive).

Chapter 3: Methods of Recycling

Figure 3.1.1 Recycling of polymers.

Scheme 3.1.1 Types of plastic waste recycling.

Figure 3.1.2 Schematic diagram of catalytic fluidized-bed reactor system [94].

Figure 3.1.3 Mechanism of PS cracking.

Figure 3.1.4 Schematic diagrams showing the NTK process diagram.

Figure 3.1.5 BASF processes [150].

Figure 3.2.1 Depolymerization of nylon 6,6 by hydrolysis.

Figure 3.2.2 PC glycolysis reaction pathway [34].

Chapter 4: Recycling of Plastics

Figure 4.1 Zero viscosity of PLA as a function of injection number.

Figure 4.2 Chemical recycling of PLA/PE and PLA/PBS.

Figure 4.3 (a) Complex viscosity and (b) MFI of LDPE after extrusion cycles.

Figure 4.4 Role of phenol in the thermal cracking of HDPE.

Figure 4.5 Effect of HDPE content on the viscosity of the recycled PET.

Figure 4.6 Reaction and analytical procedure for the glycolysis of PET wastes.

Figure 4.7 Scheme of PET super-clean recycling processes based on pellets.

Chapter 5: Recycling of Rubber

Figure 5.1 World production and consumption of natural rubber (NR) and synthetic rubber (SR).

Figure 5.2 Chemical structure of natural rubber (NR).

Figure 5.3 Recycling of tire using nitrous oxide.

Figure 5.4 Ultrasound reactor with a conical chamber [58].

Figure 5.5 Environmental scanning electron microscope image of

T. ferrooxidans

.

Figure 5.6 Schematic representation of desulfurization of the sulfur linkage by

T. ferrooxidans

.

Figure 5.7 Environmental scanning electron microscope image of

Sphingomonas

sp.

Chapter 6: Fibers

Scheme 6.1 Brief classification of fibers.

Figure 6.1 Morphological investigation. Field emission scanning micrographs of (a) a raw kenaf fiber, (b) alkali-treated fibers, and (c) bleached fibers. Transmission electron micrographs of cellulose nanocrystals produced after treatment for different hydrolysis times: (d) CNC20, (e) CNC30, (f) CNC40, (g) CNC60, (h) CNC90, and (i) CNC120.

Figure 6.2 The cotton recycling symbol.

Figure 6.3 SEM images of the fracture point of composite films of pellet LDPE with a pCot filler content of (a) 2.5%, (b) 5%, and (c) 10% and of films fabricated with powdered LDPE with an MFC–pCot filler content of (d) 2.5%, (e) 5%, and (f) 10%.

Figure 6.4 Sisal fiber SEM micrographs: fibrillar structure in fracture image (a), unmodified (b), and mercerized (c) fiber surface images.

Figure 6.5 Asbestos fibers.

Figure 6.6 Carpet construction.

Figure 6.7 Extrusion process developed by Monsanto for recycling carpet waste.

Figure 6.8 Classification of the chemical recycling methods with the products of each reaction.

Scheme 6.2 Chain-extending reaction of PET by a multifunctional epoxidic oligomeric additive (Joncryl).

Figure 6.9 Cradle-to-factory gate system boundary of recycling PET fibers from waste PET bottles, splitting the first life and the second life based on the “cut-off” approach.

Figure 6.10 Flow sheet of the production of recycled PET flakes.

Scheme 6.3 Products of PET pyrolysis.

Figure 6.11 The fluidized-bed process [3].

Figure 6.12 Photographs (a) GRP waste ground sample, (b) separated glass fiber and polyethylene, and (c) GRP waste powder.

Figure 6.13 Mechanical recycling.

Figure 6.14 Pyrolysis process [36].

Figure 6.15 The fluidized-bed process.

Figure 6.16 A schematic of the supercritical propanol flow system.

Figure 6.17 SEM micrograph of (a) virgin carbon fibers and (b) carbon fibers obtained after scPrOH treatment.

Chapter 7: Recycling of Polymer Blends and Composites (Epoxy Blends)

Figure 7.1 Classification of composite materials (a) based on matrix materials and (b) based on reinforcement materials.

Figure 7.2 (a) Structural composites made of fiber-reinforced thermosets as excellent replacements for metal [2]. (b) Safety helmets [3]. (c) Wind turbine blades [4]. (d) Carbon fiber reinforced polymer composites are used in aircraft wings [5].

Figure 7.5 Schematic model of SMC manufacturing process.

Figure 7.4 Methods of recycling taken from Pickering.

Figure 7.3 Epoxy resin formation.

Figure 7.6 DMC recyclate fiber–particulate bundle.

Figure 7.7 Schematic model for methods of chemical recycling.

Figure 7.8 Fluidized-bed recycling process.

Chapter 8: Recycling of Other Layered Mixed Plastics or Resins: Polyurethanes

Scheme 8.1 Schematic diagram of urethane formation.

Figure 8.1 Toluene diisocyante (TDI) structure.

Figure 8.2 Methylene diphenyl diisocyanate (MDI).

Figure 8.3 Examples of flexible polyurethane foam applications.

Figure 8.4 Rigid polyurethane foam applications.

Figure 8.5 Summary sheet of polyurethane recycling.

Figure 8.6 Structure reaction injection molding (SRIM) recycling of PU scrap [9].

Scheme 8.2 First step in glycolysis of polyurethane foam with glycerol to form a mixture hydroxyl oligomers [19].

Scheme 8.3 Second step in the glycolysis process: the glycolyzed products are used as initiators in a reaction with alkylene oxide to produce oxyalkylated polyols [19].

Scheme 8.4 Hydrolysis of polyurethane [27].

Scheme 8.5 Aminolysis of polyurethane [32].

Figure 8.7 Gasification process flowchart [42].

Chapter 9: Ecoprofiles of Recycled Polymers at a Glance

Figure 9.1.1 Plastic pollution.

Figure 9.1.2 Recycled polyethylene terephthalate used for making bottles.

Figure 9.1.3 Various applications of recycled HDPE.

Figure 9.1.4 Recycled PVC used for making plastic floor tiles.

Figure 9.1.5 Recyclable polypropylene.

Figure 9.1.6 Recyclable EPS forms.

List of Tables

Chapter 1: Introduction

Table 1.1 Various polymers with their characteristic recycling codes for particular applications

Chapter 2: Common Additives used in Recycling of Polymers

Table 2.1.1 Elongation at break of stabilized and unstabilized PP

Table 2.1.2 Mechanical properties of glass fiber (GF)-filled PET/HDPE blends

Table 2.1.3 Mechanical properties of PET/PE/PVC blend and effect of the addition of 10% of an elastomer

Table 2.1.4 Commonly used antistatic agents

Chapter 4: Recycling of Plastics

Table 4.1 Summary of the main products of LDPE/HDPE blend degradation at reaction temperature of 360 °C over various catalysts

Chapter 7: Recycling of Polymer Blends and Composites (Epoxy Blends)

Table 7.1 Various grinding equipment

Table 7.2 Aramid fiber recovered from aramid–epoxy composites [66]

Table 7.3 Mechanical properties of bulk molding compounds [50]

Chapter 8: Recycling of Other Layered Mixed Plastics or Resins: Polyurethanes

Table 8.1 Properties of long-chain polyether polyols

Table 8.2 Properties of short-chain polyether polyols

Table 8.3 Polyurethane particle size distribution after granulation [12]

Table 8.4 Recycling of polyurethane [26]

Table 8.5 Thermal dissociation temperature for various linkages in polyurethanes [49]

Edited by Raju Francis

Recycling of Polymers

Methods, Characterization and Applications

 

Editor

 

Dr. Raju Francis

Mahatma Gandhi University

School of Chemical Sciences

Priyadarsini Hills

Kottayam 686560

India

 

Cover

Blue bottles: [email protected] (No 475893605)

Background: PhotoAlto/James [email protected] (No 107906856)

 

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Preface

Polymer products are indispensible to humans because of their several advantages, such as easy processability into various shapes, low cost, lightweight, and durability, over conventional products. But the irony is that some of these advantages make polymeric materials a threat to life on Earth through widespread and irreparable damage to environment. This comes mainly because some of us still believe that polymer products are of the “use and throw” type. Because of this, our soil, water, and air are catastrophically affected. Therefore, it is high time to think and work on alleviating the serious ill effects of polymers and attempt to regenerate our environment for future generations. One of the possible remedies that is being considered and debated by the general public, scientists, and academicians is polymer recycling. This is because all other alternatives are either extremely dangerous or economically unviable. One can see that the two common substitutes for polymer recycling are (i) the simple burning of used polymers in open air, which is more dangerous to the environment, and (ii) the use of biodegradable polymers, which is uneconomical. Therefore, “recycle and reuse” is considered the best option for a sustainable environment.

Secondly, recycling minimizes the need for raw materials so that the rainforests can be preserved. Great amounts of energy are used when making products from raw materials. Recycling requires much less energy and therefore helps us preserve natural resources.

This book Recycling of Polymers is a collection of recent research and academic studies on the methods of recycling, followed by applications and, finally, the merits and demerits of recycled polymer products. It is noteworthy that this book encompasses almost all categories of polymers, namely plastics, elastomers, and fibers, and, in addition, also blends, composites, and resins.

This book consists of nine chapters. The first chapter mainly presents the overall idea that recycling is one of the best options for making a positive impact on the world in which we live. It gives a general idea about its importance, why we should do recycling, what are the sources of recycling, various stages of recycling, and so on.

Chapter 2 (Parts 2.1 and 2.2) provides the different types of additives that are commonly used for recycling. Additives play a leading role in the success of commercial plastics, elastomers, rubbers, coatings, and adhesives. It also describes the common additives used in the recycling of polymers. This includes a study of the different classes of additives that are employed alone or in combination with other additives in the polymer recycling or manufacturing process. After describing the different additives that are not included in the first part of this chapter, a quick look into the recent trends, advancements, and the future of additives is included in the second part.

The third chapter includes the method of recycling of polymers. Part 3.1 of the chapter comprises a significant review of the chemical recycling of the generally used addition polymers such as polypropylene (PP), polystyrene (PS), low-density polyethylene (LDPE), high-density polyethylene (HDPE), poly(vinyl chloride) (PVC), and poly(methyl methacrylate) (PMMA), and Part 3.2 includes chemical recycling of condensation polymers such as poly(ethylene terephthalate) (PET), polycarbonate (PC), nylon, and so on.

Fourth chapter reviews the recycling of thermoplastic waste from some traditional polymers such as polyethylene (PE), PP, PS, PVC, PS, PET, and so on.

Chapter 5 includes the production and world consumption of rubber products and applications of recycled rubber. The recycling of rubber products is not a trivial process because their crosslinked structure restricts reprocessing. Efficient methods of devulcanization include chemical, mechanical, biological, thermal, microwave, and ultrasonic techniques.

Chapter 6 mainly focuses on the recycling of fibers. The most commonly recycled natural and synthetic fibers are included in this chapter. Natural polymers are biodegradable. They can be blended with plastics to produce materials that are more biodegradable while retaining the more desirable features of conventional plastics. Synthetic polymers are non-biodegradable. So this chapter mainly gives an idea about the recycling and use of recycled products of synthetic fibers.

Chapter 7 deals with the recycling of polymer blends and composites. Epoxies are thermoset polymers and are very difficult to degrade. Therefore, the different types of recycling techniques used for the epoxy thermosets are presented in this chapter. Examples of recycled epoxy thermosets that are converted into useful products are highlighted.

Chapter 8 deals with the recycling of polyurethanes. Mainly polyurethanes are used to obtain rigid and flexible foams. Nowadays, recycling of polyurethanes is drawing more and more attention worldwide because of the variety of products developed with them for various applications.

Chapter 9 gives an idea on the benefits of recycling and the impact of some significant recycled polymers on the environment. First part of this chapter discusses the advantages of recycling with the help of six major recycled polymers. Recycled polymers leads to the following positive impacts: (i) they save the Earth, (ii) they conserve energy, (iii) they help in mitigating global warming and in reducing pollution, (iv) they minimize waste products placed in landfills, (v) they help save money, (vi) they reduce the need for allied activities such as transportation and mining, and (vii) they spread awareness for the environment. The second part of this chapter evaluates the effects of recycled polymers from three angles – environmental, human health, and economic.

Raju Francis

India

August 11, 2016

List of Contributors

S. Anil Kumar

Mahatma Gandhi University

NSS Hindu College

Department of Chemistry

Changanacherry

Kottayam

Kerala

 

Preetha Balakrishnan

Mahatma Gandhi University

International and Inter University Centre for Nanoscience and Nanotechnology

Kottayam

Kerala

India

 

Raju Francis

Mahatma Gandhi University

School of Chemical Sciences

Priyadarsini Hills

Kottayam

Kerala 686560

India

 

Geethy P. Gopalan

Mahatma Gandhi University

School of Chemical Sciences

Priyadarsini Hills

Kottayam

Kerala 686560

India

 

Nidhin Joy

Mahatma Gandhi University

School of Chemical Sciences

Priyadarsini Hills

Kottayam

Kerala 686560

India

 

Beena Sethi

Department of Chemistry

K. L. Mehta D. N. College for Women

New Industrial Township

K. L. Mehta Marg, N.H-3

Faridabad

Haryana 121001

India

 

Anjaly Sivadas

Mahatma Gandhi University

School of Chemical Sciences

Priyadarsini Hills

Kottayam

Kerala 686560

India

 

M. S. Sreekala

Department of Chemistry

Sree Sankara College – Kalady

Sankar Nagar, Mattoor

Ernakulam

Kerala 683574

India

 

Ranimol Stephen

Department of Chemistry

St. Joseph's College (Autonomous)

Devagiri, Calicut

Kerala 673008

India

 

Jyothi V. Sunny

BASF Corporation

889 Valley Park Drive

Shakopee, MN 55379

USA

 

V.P. Swapna

Department of Chemistry

St. Joseph's College (Autonomous)

Devagiri, Calicut

Kerala 673008

India

 

S. Vishnu Sankar

Mahatma Gandhi University

NSS Hindu College

Department of Chemistry

Changanacherry

Kottayam

Kerala

Abbreviations

ABS

Acrylonitrile/butadiene/styrene

AC

Acidification

AIBN

Azobisisobutyronitrile

ADN

Adiponitrile

BLL

Blood lead level

BPA

Bisphenol A

CD

Circular disk

CPE

Chlorinated polyethylene

CSBR

Conical spouted bed reactor

DCP

Dicumylperoxide

DFE

Design for environment

DMC

Dimethyl carbonate

DMF

Dimethylformamide

DSC

Differential scanning calorimetry

DTST

Dynamic thermal stability time

EPDM

Ethylene–propylene–diene monomer rubber

EPR

Ethylene–propylene rubber

EPS

Expanded polystyrene

FCC

Fluid catalytic cracking

FRP

Fiber-reinforced plastic

FTIR

Fourier transform infrared

GC

Gas chromatography

GF

Glass fiber

GHG

Greenhouse gas

GOP

Vaccum gas oil product

GW

Global warming

HALS

Hindered amine light stabilizers

HCl

Hydrogen chloride

HDPE

High-density polyethylene

HDT

Heat distortion temperature

HIPS

High-impact polystyrene

HMDA

Hexamethylenediamine

HT

Human toxicity

IPP

Isopropenyl phenol

IR

Infrared

LCA

Life-cycle analysis

LDPE

Low-density polyethylene

LHV

Lower heating value

LLDPE

Linear low-density polyethylene

LPG

Liquefied petroleum gas

MBS

Methacrylate/butadiene/styrene

MFI

Melt flow index

MMA

Methyl methacrylate

MSW

Municipal solid waste

MW

Microwave

MWD

Molecular weight distribution

NE

Nutrient enrichment

OPS

Oriented polystyrene

PAH

Polyaromatic hydrocarbons

PAHs

Polycyclic aromatic hydrocarbons

PBCDDs

Polybrominated–chlorinated dibenzo-

p

-dioxins

PC

Polycarbonate

PCDFs

Polychlorinated dibenzofurans

PE

Polyethylene

PEHD

Polyethylene high density

PET

Poly(ethylene terephthalate)

PHAs

Polyhydroxyalkanoates

PLA

Poly(lactic acid)

PMMA

Poly(methyl methacrylate)

POF

Photochemical ozone formation

POP

Persistent organic pollutant

PP

Polypropylene

PS

Polystyrene

PSBD

Poly(styrene–butadiene)

PSW

Plastic solid waste

PT

Persistent toxicity

PVC

Poly(vinyl chloride)

R-PVC

Rigid poly(vinyl chloride)

SAPO

Silicoaluminophosphate

SEP

Styrene–ethylene–propylene block copolymer

TBE

Tetrabromoethane

TDF

Tire-derived fuel

TG

Thermogravimetry

TGA

Thermogravimetric analysis

TPH

1,3,5-Triphenylhexane

VCC

Veba Combi cracking

VOC

Volatile organic compound

WEEE

Waste electrical and electronic equipment

Chapter 1Introduction

Raju Francis, Geethy P. Gopalan and Anjaly Sivadas

Mahatma Gandhi University, School of Chemical Sciences, Priyadarsini Hills, Kottayam, Kerala, 686560, India

“Recycling saves energy, preserves natural resources, reduces greenhouse-gas emissions, and keeps toxins from leaking out of landfills.”

–Marc Gunther

1.1 Introduction

1.1.1 Why Recycling?

During the past decades, the enormous population increase worldwide, together with the need for people to adopt improved conditions of living, has led to a dramatic increase of the consumption of polymers (mainly plastics). Materials appear interwoven with our consumer society, where it would be hard to imagine living without plastics, which have found a myriad of uses in fields as diverse as household appliances, packaging, construction, medicine, electronics, and automotive and aerospace components. The unabated increase in the use of plastics has led to an increase in the quantity of plastics ending up in the waste stream, which has stimulated intense interest in the recycling and reuse of plastics [1]. Worldwide, the production of plastics was 168 million tons in the year 1999 and approximately 210 million tons in 2010 .

Since the treatment of plastic wastes has become a serious problem, the development of effective recycling processes is urgently needed [2].

1.1.2 Sources of Waste

Plastics play an important role in almost every aspect of our lives. Plastics are used to manufacture products of daily use such as beverage containers, toys, and furniture. The widespread use of plastics demands proper end-life management [3]. A large number of items can be easily recycled in most curbside programs, including all kinds of paper and cardboard, glass of all colors and types, plastic bottles, aluminum cans, and yard trimmings. In addition, a number of localities offer drop-off programs for recycling other items, such as household hazardous wastes (paints, cleaners, oils, batteries, and pesticides), automobile items (tires, used engine oil, car batteries, antifreeze), wood construction materials, certain metals, appliances, and consumer electronics [4].

The largest amount of plastics is found in containers and packaging (e.g., soft drink bottles, lids, shampoo bottles), but they also are found in durables (e.g., appliances, furniture) and nondurables (e.g., diapers, trash bags, cups, utensils, and medical devices). Commercial waste is often produced by workshops, craftsmen, shops, supermarkets, and wholesalers. Agricultural waste can be obtained from farm and nursery gardens outside the urban areas. This is usually in the form of packaging (plastic containers or sheets) or construction materials (irrigation or hosepipes). Municipal waste can be collected from residential areas (domestic or household waste), streets, parks, collection depots, and waste dumps [5].

Around 50% of plastics are used for single-use disposable applications, such as packaging, agricultural films, and disposable consumer items; between 20% and 25% for long-term infrastructure such as pipes, cable coatings, and structural materials; and the remainder for durable consumer applications with intermediate lifespan, such as in electronic goods, furniture, and vehicles [6].

1.1.3 Plastics

Plastics are made up of polymers and other materials that are added to give the polymer increased functionality. The polymer content in a plastic can vary widely from less than 20% to nearly 100%. Those plastics consisting virtually entirely of polymers are termed prime grades. The level and type of the other additives used depend on the application for which the plastic is intended. Plastics are inexpensive, lightweight, and durable materials, which can readily be molded into a variety of products that find use in a wide range of applications. As a consequence, the production of plastics has increased markedly over the last 60 years [6]. Thermosets and thermoplastics are the two major classifications of plastics. This distinction is based on both the molecular structure and the processing routes that can be applied. It also relates to recycling routes, as each category needs a different approach to utilize its recovery potential. Thermoplastics and thermosets will now be discussed.

Thermoplastics

These materials melt and flow when heated and solidify when cooled. On subsequent reheating, they melt and regain the ability to flow. This means that they can be used again and hence recycled by remelting them. Thermoplastics are used to make consumer items such as drinks containers, carrier bags, and buckets.

Thermosets

These materials are processed by melting, often in a similar manner to thermoplastics. However, once formed and cooled, they cannot be reprocessed; they decompose before they can melt. This is because they are chemically crosslinked by a process termed

curing

. The material becomes stiff and brittle with a highly dense molecular network [7].

1.1.4 Recycling of Plastics

Recycling of plastics is one method for reducing environmental impact and resource depletion. Recycling can therefore decrease energy and material usage per unit of output, leading to improved eco-efficiency. The only way to decrease the environmental problems caused by polymeric waste accumulation produced from day-to-day applications of polymer materials such those used in packaging and construction is by recycling. This helps to conserve natural resources because most polymer materials are made from oil and gas [8].

Recycling is the final result of the intermediate stages of collection, sorting by type, and processing of polymers. It reduces the quantity of residues in landfills and those indiscriminately discarded in the environment. Thus, it also leads to a reduction of problems such as the spread of diseases as well as contamination of soil, air, and water bodies [9]. It is one of the most important options currently available to reduce these impacts and represents one of the most dynamic areas in the plastics industry today. It provides opportunities to reduce oil usage, carbon dioxide emission, and the quantities of waste requiring disposal.

Recycling plastics encompasses four phases of activity, namely collection, separation, processing, and manufacturing and marketing. Because only the use of clean, homogeneous resins can produce the highest quality recycled plastic products in the existing secondary process (material recycling) and high-value chemical products in the existing tertiary process (feedstock recycling) [10], an effective separation of mixed plastics waste is necessary.

1.1.5 Municipal Solid Waste

The growth of plastics waste has a great impact on the management of municipal solid waste (MSW) by landfilling and incineration, because the available capacity for landfill of MSW is declining and plastics incineration may cause emission and toxic fly and bottom ash containing lead and cadmium [10]. Plastics waste recycling is a method of reducing the quantity of net discards of MSW. Although the benefits have not been quantified, plastics recycling also offer the potential to generate demonstrable savings in fossil fuel consumption, both because the recycled plastics can supplement and even compete with “virgin” resins produced from refined fossil fuel and because the energy required to yield recycled plastics may be less than that consumed in the production of the same resins from virgin feedstock. Therefore, plastics waste recycling conserves both material and energy and provides a comparatively simple way to make a substantial reduction in the overall volume of MSW [11].

The major plastics recycled are polyolefins (high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP)) and poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC), polystyrene (PS), and polycarbonate (PC). The recyclable polymers and the recycling codes are shown in Table 1.1.

Table 1.1 Various polymers with their characteristic recycling codes for particular applications

There are several options for how this can be done: primary recycling, mechanical or secondary recycling, tertiary or chemical recycling, and energy recovery or quaternary recycling (Figure 1.1).

Primary recycling involves the use of the same product without essential changes in a new use cycle (e.g., refillable packaging after cleaning).

Mechanical recycling implies the application of the material used, without changing the chemical structure, for a new application.

Chemical recycling implies the chemical structure of the material is changed, which means that the resulting chemicals can be used to produce the original material again [12].

Energy recovery refers to the recovery of plastic's energy content. Incineration aiming at the recovery of energy is currently the most effective way to reduce the volume of organic materials. Although polymers are actually high-yielding energy sources, this method has been widely accused as ecologically unacceptable owing to the health risk from airborne toxic substances, for example, dioxins (in the case of chlorine containing polymers).

Figure 1.1 Four methods of recycling.

1.1.6 Various Stages of Recycling Plastic Wastes

There are various stages of recycling:

Collection

: Plastic waste is collected from different locations. This can be achieved by keeping special containers at home, public places, farms, and so on. These wastes are then collected by professional waste collectors and transported to the recycling sites.

Cleaning

: The cleaning stage consists of washing and drying the plastic items. Cleaning is important since clean waste materials fetch better prices and they improve the quality of end products. Plastics can be washed at various stages of recycling process: before, after, or even during sorting.

Sorting

: This involves not only the separation of the polymers from recoverable foreign bodies but also the separation of these polymers themselves.

Size reduction

: It aims to reduce the size of the waste, which in turn facilitates not only in the separation of different polymers but also recovery of the micronized powder which is used to feed processing machines. The end products of shredding can be irregularly shaped pieces of plastics, which can be sold to reprocessing industries and workshops.

After processing, these materials are further subjected to various techniques such as extrusion, injection molding, blow molding, and film molding. Finally, the processed materials are converted into various products such as pipes, tubes, bags, sheets, and miscellaneous items.

1.1.7 Additives

Polymer industry cannot survive without additives. Additives in plastics provide the means whereby processing problems, property performance limitations, and restricted environmental stability are overcome. In order to get a technical effect additives used to incorporate into the plastics. So additives are expected to be the key part of the finished particle. A few examples of additives are antistatic agents, antioxidants, emulsifiers, antifogging agents, impact modifiers, fillers, plasticizers, lubricants, solvents stabilizers, UV absorbers, release agents and thickeners. It might be either inorganic (e.g., oxides, salts, fillers), organic (e.g., alkyl phenols, hydroxybenzophenones), or organometallic (e.g., Ni complexes, Zn accelerators, metallocarboxylates) [13].

Benefits of adding additives in plastics significantly shows varying properties with one or more directions such as stiffness, and strength, general durability, thermal resistance, impact resistance, resistance to flexure and wear, acoustic isolation and so on. In the broadest sense, these are essential ingredients of a manufactured polymeric material. An additive can be a primary ingredient that forms an integral part of the end product's basic characteristics or a secondary ingredient that functions to improve performance and/or durability.

The other recyclable materials are fibers, rubbers, mixed plastics, blends and composites, and so on. The recycling techniques, use of additives, and reusing applications are discussed in the following chapters.

Rubber recycling is growing in importance worldwide because of increasing raw material costs, diminishing resources, and the growing awareness of environmental issues and sustainability [14]. The rubber industry faces a major challenge in finding a satisfactory way to deal with the increasing quantities of rubber goods that reach the end of their useful life and are rejected from factories as scrap. The main source of waste rubber is discarded rubber products, such as tires, rubber hoses, belts, shoes, flash, and so on [15].

Reclaimed rubber is the product resulting when waste vulcanized scrap rubber is treated to produce a plastic material that can be easily processed, compounded, and vulcanized with or without the addition of either natural or synthetic rubbers. Regeneration can occur either by breaking the existing crosslinks in the vulcanized polymer, or by promoting scission of the main chain of the polymer, or a combination of both processes. Reclaiming of scrap rubber is, therefore, the most desirable approach to solve the disposal problem. Reclamation is done from vulcanized rubber granules by breaking down the vulcanized structure using heat, chemicals, and mechanical techniques. Reclaimed rubber has the plasticity of new unvulcanized rubber compound, but the molecular weight is reduced so reclaimed compounds have poorer physical properties when compared to new rubber [16].

Natural fibers are obtained from plants and animals whereas synthetic fibers are obtained by chemical processing of petrochemicals. Natural fibers have recently attracted the attention of scientists and technologists because of the advantages that these fibers provide over conventional reinforcement materials; the development of natural fiber composites has been a subject of interest during the past few years [17–19]. These natural fibers are of low cost, low density, and high specific properties. These are biodegradable and nonabrasive unlike other reinforcing fibers. Also, they are readily available and their specific properties are comparable to those of other fibers used.

Fiber-reinforced plastics (FRPs) are inherently difficult to separate into the base materials, that is, fiber and matrix, and the FRP matrix into separate usable plastic, polymers, and monomers. These are all concerns for environmentally informed design today, but plastics often offer savings in energy and cost in comparison to other materials. Also, with the advent of new and more environmentally friendly matrices such as bioplastics and UV-degradable plastics, FRP will similarly gain environmental sensitivity [20].

One of the biggest challenges posed by FRPs is their recycling. Many different recycling techniques have been studied during the last two decades, such as mechanical processes (mainly grinding) [21–24], pyrolysis and other thermal processes [25, 26], and solvolysis [27–29].

1.1.8 Mixed Plastics

Another type of plastics used for recycling is mixed plastics. Mixed plastics contain different types of plastics with different processing behavior and stability. Usually, these plastics are not compatible (or thermodynamically miscible) with each other, and the resulting properties are very often inferior to those of the parent polymers. In its broadest sense, mixed plastics constitute a a mixture of plastic resins or a mixture of package/product types which may or may not be the same plastic type or color category, and may not have been fabricated using the same manufacturing techniques.

1.1.9 Composites

Composites are generally considered high-value, high-performance materials that are employed in producing end products of high net worth. The term composite can be used to describe a large number of multiphase materials, consisting of a wide variety of matrix materials along with a correspondingly large array of different fillers and reinforcements. Composites can be easily recycled. Additionally, composites have been demonstrated to often have a better ecological track than traditional materials such as steel, aluminum, and concrete [30].

1.2 Conclusion

Recycling or reuse is one approach for end-of-life waste management of plastic products. It makes increasing sense economically as well as environmentally, and recent trends demonstrate a substantial increase in the rate of recovery and recycling of plastic wastes. This process has advantages and disadvantages. The foremost advantage of recycling is that it helps in protecting the environment in the most balanced manner. It helps in conserving important raw materials and protecting natural habitats for the future. Protecting natural resources such as wood, water, and minerals ensure their optimum use. Governments and various environmental organizations regularly emphasize the many benefits of recycling. First and foremost, recycling reduces the amount of waste that must be placed into landfills or incinerated. The recycling of metals, glass, and other materials reduces the pollution that would be caused by the manufacturing of products from virgin materials. Using recycled materials also saves energy because it takes less energy to use recyclables than to make a product from raw materials.

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Chapter 2Common Additives used in Recycling of Polymers

2.1 Review on Different Additives Used in Polymer Recycling

Sivasankarapillai Vishnu Sankar and Sivasankarapillai Anil Kumar

2.1.1 Introduction

Polymer recycling is a process involving both environmental and economic issues. The choice of the recycling technique is influenced by factors such as the purity of the polymer stream, chemical composition, and the nature of additives present in the polymeric material. During the past two decades, polymer recycling methods have attracted considerable interest because they open a way to reduce environmental issues caused by the accumulation of polymer waste generated from day-to-day use of polymer materials. The various methods include reuse, mechanical recycling, and chemical recycling. Even though various strategies are available for the recycling of polymer materials, still there remain certain problems during the post-consumer recycling stage. These problems are due to the degradation of polymers during the processing steps and to the incompatibility of different polymer types. Additives can be used for reducing these problems in the recycling stage and to increase the properties of the recycled products. The primary goal of this chapter is to give an overview of the different types of additives used in polymer (plastic) recycling.

2.1.1.1 Challenges in Recycling – Need for Additives

An increase in the use of plastics has led to a corresponding increase in the amount of plastic waste, which plays a major role in environmental pollution. This has resulted in great interest in the recycling of polymer waste generated from our daily lives. With advances in polymer technology, the processing of plastic mixtures for recycling has been attempted with some success, but poor mechanical properties of the secondary materials and uncertain economic value limit their versatile reuse [1–14].

For homogeneous polymeric materials, two main methods can be adopted to reduce the negative effects during the recycling process [15]:

1.

Restabilization

avoids or retards the degradation process undergone by polymers.

2.

Addition of fillers and modifiers

improves the performance of thermoplastic polymers without increasing the final cost of secondary material.

The major challenge in the recycling of homogeneous polymers is only related to the degradation phenomena that take place during recycling [16]. These phenomena are generally much more dramatic than in virgin polymers since the oxygenated groups, formed during the processing or during the lifecycle, remarkably accelerate the degradation of plastic materials. This causes serious deterioration of the end properties of the secondary materials [15].

For heterogeneous polymers, the major problem associated with the recycling process is its heterogeneous composition due to the presence of polymers having different chemical structures; so compatibilization is a necessary step in the recycling process. Incompatibility gives rise to blends with poor properties, in some cases worse than those of the virgin polymers [16]. Thus, additives such as compatibilizers can be used to obtain secondary materials with acceptable properties. Therefore, it is clear that additives play a crucial role in the recycling and processing of polymer materials.

2.1.1.2 Equipment for Additive Processing

There are a few vital pieces of equipment and machinery that are essential for additive processing. The most effective of these for the incorporation and handling of additives into polymer networks are volumetric or gravimetric feeders and blenders. Volumetric devices measure the volume that passes through a metering disk, while gravimetric devices measure and control the weight of additives dispensed over a given time interval. Gravimetric type equipments are better when two or more additives are being dispensed at either the machine throat or in a blending system. The blending system can be a dosing or mixing unit, which functions by dividing the main component under consideration into different streams of material such that the streams can combine with the additives at the feed inlet, which helps produce a homogeneous mixture. Another important class of equipment is “Level sensors,” which can measure insufficient additive material in order to prevent unnecessary downtime.

2.1.2 Different Types of Additives

Polymer additives are materials that are added to polymers in the processing stage in order to improve or change the visual, process, environmental resistance, or degradation properties, thereby enhancing their overall performance and application potential. The discovery and research in the field of natural polymer materials has created a need for additives to modify their intrinsic properties so that the polymers have improved characteristics to allow for a wider variety of uses. Today, both natural and synthetic polymers depend on additives during processing. To create new blends, additives and polymer resins are mixed to produce improved materials. Standard mixtures are produced that can be further modified by adding various additives, which can develop a variety of chosen materials with different application potentials to meet individual processing needs.

The term additive means all substances, inorganic and organic, that can alter one or more properties of polymers [15]. Additives are essential components of plastic materials, providing maintenance and modification of the polymer's properties, performance, and long-term use. The main purpose of adding these compounds in the recycling stage is to prevent unwanted degradation phenomena during the recycling process and to improve the properties of the secondary materials generated in the recycling stage. The additives in plastics recycling play the same role as in their manufacturing process [15].

Additives can be categorized based on their primary objectives, which are to

Add bulk or volume while controlling properties and costs of production

Modify the intrinsic chemical or physical properties of the polymer material

Reinforce the mechanical properties of the polymer, thereby increasing its impact strength.

Additive materials differ from fillers and reinforcements in following aspects:

Fillers are added mainly to reduce cost and may or may not serve any other objectives

Reinforcements are added to increase the structural and mechanical properties of polymers (e.g., fibers and flakes).

The role of additives on the recycling process can be summarized as follows:

They prevent the thermomechanical degradation of materials occurring during recycling operations.

They improve the properties of the secondary materials.

They make heterogeneous polymers compatible.

The most important classes of polymer additives and their function can be listed as follows [15]:

Antioxidants and stabilizers

delay the degradative processes of polymers.

Mineral fillers

can decrease the cost and enhance the properties of the polymer.

Impact modifiers

increase, in particular, the impact strength.

Compatibilizers

improve the compatibility between incompatible polymeric materials.

In addition to these, there are various other additives such as lubricants, coloring agents, and so on.

Increase in the use of plastics leads to an increase in the growth of additives and their consumption (Figures 2.1.1 and 2.1.2) [17].

Figure 2.1.1 Turnover of additives in 2004.

(Pfaendner [17] Reproduced with permission of Elsevier.)

Figure 2.1.2 Consumption of plastics, light stabilizers, and antioxidants since 1950.

(Pfaendner [17] Reproduced with permission of Elsevier.)

The most commonly used additives in polymer recycling and processing methods can be classified as follows:

2.1.2.1 Stabilizing Agents

The role of stabilizing agents during polymer processing or recycling is to prevent radiation-induced and thermal degradation affecting the polymer product quality over time. The efficacy of stabilizers appears to decay with lifetime. Restabilization of the material is one of the key steps in polymer recycling by compounding recycled plastics with a fresh reagent. In addition to extending the polymer product lifetime, stabilizing agents also help in protecting virgin plastics and recycled materials from the inevitable degradation that occurs during the relatively short-lived and severe conditions that are associated with polymer processing operations [18].

For example, thermal stabilizers are employed to maintain the stability of the polymer up to the processing temperature, and ultraviolet (UV) absorbers and radical traps are incorporated to withstand photodegradation of the polymer. The nature of the stabilizing agent will depend on the nature of the polymer system into which it has to be incorporated. For example, polymeric phenolic phosphites represent a new family of polypropylene (PP) stabilizers acting through their phenolic group as free-radical scavengers and through their phosphite group as hydroperoxide decomposers.

2.1.2.1.1 Thermal Stabilization

A combination of heat and oxygen will cause oxidation of the polymer, resulting in degradation. The mechanism is the formation of free radicals, which are highly reactive chemical species. This reaction can be visually observed, as the products tend to show discoloration to yellow or brown. Additives called antioxidants can be used to arrest this mechanism. The chemicals most commonly employed for this purpose are hindered phenols, which act as peroxide radical decomposers.

Other additives are also employed apart from hindered phenols for these purposes. Additives called phosphites combined with hindered phenols have a synergetic effect. This combination is especially effective for polyolefins. The well-known example of the use of additives to prevent thermal degradation is the thermal stabilization of polyvinylchloride (PVC). The free radicals produced in this case are chlorine, leading to the formation of hydrochloric acid. The stabilizers must be able to stop these reactions, which can lead to acid corrosion of the processing equipment.

2.1.2.1.2 Photostabilization

Light, especially in the UV range, can induce photoxidation, which results in the degradation and cleavage of the polymer chains. To prevent this effect, three classes of additives are employed. They are usually called UV absorbers, quenchers (scavengers), and radical traps.

UV Absorbers

These are among the oldest light stabilizers and work by absorbing the harmful UV radiation and converting it to heat energy. By absorbing these rays, they protect the vulnerable polymer chains. Examples of additives of this class are benzophenones and benzotriazoles.

For example, hydroxybenzophenone and hydroxyphenylbenzotriazole are effective UV stabilizers that can be conveniently used for applications requiring neutrality or transparency. Hydroxyphenylbenzotriazole is not very useful when employed in thin parts, below about 100 µm. Other UV absorbers include oxanilides for polyamides, benzophenones for PVC, and benzotriazoles and hydroxyphenyltriazines for polycarbonates.

Quenchers

Quenchers contain chromophore groups (light-absorbing species) that can absorb energy and convert it into less harmful forms. Nickel compounds are the most commercially available quenchers and these are used in applications such as agricultural film production.

Radical Traps

One of the major effects of light on a polymer material is the formation of radicals in the polymer system. Radicals are highly unstable reactive species and can cause rapid degradation of the polymer material. To prevent further damage to the plastic, these can be “scavenged” using radical traps. The most important radical traps are known as hindered amine light stabilizers (HALSs). It should be noted, however, that once the additive has been consumed, the degradation process will commence. For this reason, it is important that the correct levels of additives are employed. Interestingly, the mechanism of the radical trap can also be applied to flame-retardant technology in plastics. Thus, instead of the chemicals trapping damaging UV radiations, the additives also help in limiting the combustion process. Different chemicals are employed for this purpose, but the mechanisms of action are very similar.

The category of additives that act as radical traps usually contain the 2,2,6,6-tetramethylpiperidine ring system in their structure as seen in commercially available HALS products. Even though there are wide structural differences in these products, all of them act by trapping free radicals during the photoxidation of the system, thus preventing further degradation of the polymer system.

Polymers that have poor thermal stability, such as PP and PVC, have to be stabilized against thermomechanical degradation [15]. Klemchuk and Thompson [19] have reported that stabilization is necessary during reprocessing to prevent, or at least to retard, the degradation of these polymers and the subsequent deterioration of their rheological and mechanical properties. Ma and La Mantia [20] have investigated the variation in the molecular structure of PP during repeated reprocessing. They reported the change in the melt flow index (MFI) of a molding-grade PP as function of the number of injection molding steps and in presence of two stabilizers, namely B900 (CIBA), which is a mixture of Irganox 1076 and Irgafox 168, and P-EPQ (Sandoz), which is a phosphite stabilizer (Figure 2.1.3).

Figure 2.1.3 MFI vs the number of injection molding steps for stabilized and unstabilized PP.

(La Mantia [15] Reproduced with permission of John Wiley and Sons.)

The dramatic increase in the MFI of the unstabilized sample suggests a drastic degradation of PP due to the thermomechanical stress during the processing. The MFI value is about twice that of the virgin sample, after the first recycling. The degradation is considerably reduced by adding the stabilizer before each injection molding step. In this case, the MFI slightly increases, indicating a small change in the molecular weight. It can be seen that the two stabilizers probably act in a similar way. The study also revealed that the stabilizer also helps in preventing the dramatic deterioration of some mechanical properties, as shown in Table 2.1.1 [15]. They found that without any stabilization, the PP sample shows a brittle fracture after five extrusion steps, while the elongation at break is very close to that of the virgin polymer if the stabilizer is added before each extrusion.

Table 2.1.1 Elongation at break of stabilized and unstabilized PP

Sample

Elongation at break (%)

PP virgin

680

PP after five extrusions

20

PP + 0.3% B900 after five extrusions

520

PP + 0.3% P-EPQ after five extrusions

560

Source: La Mantia [15]. Reproduced with permission of John Wiley and sons.

Thermal stability studies of PVC have revealed that the poor thermal stability of PVC requires the addition of heat stabilizers to prevent degradation to a large extent. The stabilizer in the PVC products is consumed both during processing and, sometimes, during their lifetime. The thermal stability is remarkably reduced if PVC is subjected to the reprocessing steps [15].

Thermal stability of the polymer can be best evaluated using the dynamic thermal stability time (DTST) index, which is the time at which the torque in a mixing test at constant temperature starts to rise [20]. Thus to enhance processability, it is essential to increase the DTST value by adding suitable stabilizing agents that have been consumed during both processing and the lifetime of the PVC products. The effect of a lead stabilizer (which is employed as a thermal stabilizing agent for PVC) on the processability of recycled PVC is shown in Figure 2.1.4, where the DTST values under some processing conditions are reported as a function of the stabilizer content [15].

Figure 2.1.4 DTST of recycled PVC as a function of the stabilizer content.

(La Mantia [15] Reproduced with permission of John Wiley and Sons.)

It can be seen that the thermal stability of PVC is greatly enhanced by adding 1 phr of the stabilizer. Then the curve tends to flatten, and the processability does not remarkably improve by increasing the concentration of the stabilizer. The stabilizer is particularly effective at high temperature and rotational speeds. Indeed, under these processing conditions, DTST increases nearly 3 times by adding 1 phr of the lead compound. On the contrary, the improvement is limited when the processing is carried out under softer conditions. From Figure 2.1.4, it is evident that the improvement in DTST at 180 °C and 20 rpm is only about 50% [15].

2.1.2.1.3 Testing the Effects of Stabilizers

In order to measure the stabilization of a plastic by a stabilizing additive, whether it is virgin or a recyclate, information is required on the effects of processing, heat, and light.

2.1.2.1.4 Processing Stability

A common technique to investigate processing stability is through repeated cycling of a sample, such as by extrusion or injection molding. Mechanical tests that rely on parameters such as tensile and impact strength as well as rheological tests such as MFI can then be used to monitor the changes occurring in the properties of the plastic.

2.1.2.1.5 Heat Stability

When discussing heat stability, there are a number of different monitoring methods that can be employed.

1.

The temperature at which the plastic decomposes, which can be measured by techniques such as differential scanning calorimetry (DSC). DSC measures the heat output from the polymer as it is heated up or cooled down. It can be used across a wide range of temperatures, from −180 °C to above 600 °C. This technique also permits the measurement of changes occurring in the plastic, which gives information on the polymer properties, such as the melting point and the temperature at which thermal degradation occurs.

2.

The maximum temperature at which the polymer material can be effectively processed without affecting its color or other problems of the polymer formulation. These problems include decomposition of the polymer or other additives employed (e.g., colorants).

3.

In many applications in daily life, plastics are employed where they may be subjected to prolonged use at high temperatures. For these types of applications, data is needed on how the material will behave in service. This is usually tested by aging a sample in an oven for an appropriate time interval to reflect the behaviour that is to be expected when the plastic is used in a component.

2.1.2.1.6 Light Stability

There are two techniques commonly employed to assess the light stability of a polymer. One is to utilize artificial weathering equipment as described in ISO 4892. These can simulate weathering cycles by controlling the light (including UV) intensity, heat, and humidity. In this method, polymers are exposed to detailed and programmed cycles, which are similar to those that the components would be exposed to in real applications.

The second method is to simply leave the samples exposed to natural environmental conditions. The effects occurring in the polymeric system can be measured in a number of ways. For example, the surface of the sample can be evaluated in terms of chalking, gloss, and surface texture as a function of the weathering time.

2.1.2.1.7 Stabilizer Combinations for Specific Applications of Polymers