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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
This book is a comprehensive reference manual that contains essential information on thermoforming processing and technology.
The field of thermoforming is experiencing rapid development driven by commercial factors; millions of tons of polymers are manufactured for use in various applications, both as commodity and specialty polymers. Building on the previous edition published about ten years ago, this edition includes new, as well as, fully revised chapters and updated information on materials and processes. The book is designed to provide practitioners with essential information on processing and technology in a concise manner. The book caters to both engineers and experts by providing introductory aspects, background information, and an overview of thermoforming processing and technology. The troubleshooting section includes flowcharts to assist in correcting thermoforming processes.
>em>Thermoforming: Processing and Technology offers a complete account of thermoplastics, covering properties and forming, with chapters providing perspective on the technologies involved. Readers will find it:
- serves as a handy knowledge source for professionals who occasionally work on thermoforming projects or need to refresh their knowledge;
- offers a troubleshooting guide that can help to identify and solve challenges that may arise in thermoforming processes;
- provides insights into process optimization, helping businesses improve efficiency, reduce waste, and enhance the quality of thermoformed products;
- acts as a course book to inform students about the thermoforming process.
Audience
The book will be of interest to mechanical, materials engineers, and process engineers who are involved in designing and optimizing thermoforming processes; professionals in the manufacturing and production industries who use thermoforming as a manufacturing method, such as in the production of plastic packaging, automotive components, and consumer goods; scientists, researchers, and students in plastics/polymer engineering and technology, materials science, polymer technology; professionals responsible for ensuring product quality and compliance with industry standards.
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Seitenzahl: 554
Veröffentlichungsjahr: 2024
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
1 Introduction
2 Polymers
2.1 Introduction
2.2 Physics and Chemistry of Polymers
2.3 Natural Polymers
2.4 Synthetic Polymers
2.5 Polymerization Methods
2.6 Polymer Molecules in Thermoforming
2.7 Classification
2.8 Primary Classification
2.9 Secondary Classification
2.10 Distinction Between Thermoplastics and Thermosetting
2.11 General Classification
3 Thermoplastics
3.1 Introduction
3.2 Polyolefins
3.3 Polyethylene
3.4 Polypropylene
3.5 Polystyrene (PS)
3.6 High-Impact Polystyrene (HIPS)
3.7 Polyvinylchloride
3.8 Acrylonitrile–Butadiene–Styrene (ABS)
3.9 Polyethylene Terephthalate (PET)
3.10 Acrylics
3.11 Nylon 6 (PA6)
3.12 Nylon 66 (PA66)
3.13 Polyoxymethylene (POM)
3.14 Polycarbonate (PC)
3.15 Poly(ether-ether-ketone) (PEEK)
3.16 Polyphenylene Oxide (PPO)
3.17 Polybutylene Terephthalate (PBT)
3.18 Liquid Crystalline Polymers
3.19 Cyclic Olefin Copolymer (COC)
3.20 Plastic Foams
3.21 Thermoplastic Elastomers
3.22 Thermoplastic Composites (TCs)
3.23 Bioplastics
4 Properties of Thermoplastic Sheet Materials
4.1 Introduction
4.2 Polymer Characteristics
4.3 Polymer Morphology
4.4 Molecular Structure
4.5 Molecular Weight
4.6 Molecular Weight Distribution
4.7 Melt Flow Index
4.8 Glass Transition Temperature
4.9 Melt Temperature
4.10 Heat Deflection Temperature (HDT)
4.11 Crystallization Temperature
4.12 Melt Strength
4.13 Rheological Properties
4.14 Viscoelastic Behavior
4.15 Coefficient of Friction
4.16 Thermal Conductivity
4.17 Thermal Diffusivity
4.18 Specific Heat
4.19 Stress
4.20 Strain Hardening
4.21 Plastic Strain
4.22 Tensile Strain
4.23 Tensile Yield Stress
4.24 Deformation
4.25 Stress Deformation
4.26 Modulus and Stiffness
4.27 Sag
4.28 Toughness
4.29 Effect of Additives
5 Thermoforming Technology
5.1 Introduction
5.2 Thermoplastic Sheet Materials
5.3 Mechanical Characteristics
5.4 Thermoformability
5.5 Thermoforming Cycle
5.6 Draw Ratio
5.7 Processing Window
5.8 Mold
5.9 Mold Design
5.10 Heating Elements
5.11 Plug Material
5.12 Plug Design
5.13 Product Design
5.14 Clamping
5.15 Process Control
5.16 Process Variables
5.17 Thermal History
5.18 Pre-Drying
5.19 Plug Movement
5.20 Plug Speed
5.21 Sheet Temperature
5.22 Mold Temperature
5.23 Forming Temperature
5.24 Wall Thickness Distribution
5.25 Sheet Deformation
5.26 Heat Transfer
5.27 Effects of Temperature Distribution
5.28 Effect of Drawing
5.29 Effect of Frictional Force
5.30 Effect of Plug-Assist/Vacuum
5.31 Effect of Applied Pressure
5.32 Effect of Heating
5.33 Effect of Cooling
5.34 Rate of Deforming
5.35 Rate of Sagging
5.36 Effect of Air Temperature
5.37 Effect of Air Pressure
5.38 Effect of Crystallinity and Morphology
5.39 Processing Technology
5.40 Thermoforming—Processing of Thermoplastic Sheet Material
5.41 Methods of Thermoforming
5.42 Low-Pressure Forming Technology
5.43 Plug-Assist Thermoforming
5.44 Pressure Forming
5.45 Snapback Thermoforming
5.46 Drape Forming
5.47 Matched Mold Forming
5.48 Foam Sheet Forming
5.49 In-Line Thermoforming
5.50 Industrial Versus Laboratory Thermoforming
6 Troubleshooting Thermoforming
6.1 Introduction
6.2 Product Quality Analysis
6.3 Product Quality
6.4 Product Defects
6.5 Fundamental and Inherent Defects
6.6 Troubleshooting
7 Thermoforming—Optimization
7.1 Introduction
7.2 Thermoforming
7.3 Optimization Process
7.4 Numerical Modeling
7.5 Constitutive Model
7.6 Key Findings—Modeling
7.7 Mold
7.8 Thermoforming Process
7.9 Viscoelastic Behavior
7.10 Method of Thermoforming
7.11 Heating
7.12 Cooling
7.13 Computer Simulation
7.14 Polyflow
7.15 PAM-FORM™ Software
7.16 Geometric Element Analysis (GEA)
8 Case Studies
8.1 Introduction
8.2 Case Studies—Brief Details
8.3 Case Study I
8.4 Case Study II
8.5 The Significance of Case Studies in Thermoforming
9 Applications
9.1 Introduction
9.2 World of Thermoformed Products
9.3 Market Trends
10 Conclusion
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Properties of low-density polyethylene (LDPE).
Table 3.2 Properties of high-density polyethylene (HDPE).
Table 3.3 Properties of linear-low-density polyethylene (LLDPE).
Table 3.4 Properties of polypropylene (PP).
Table 3.5 Properties of polystyrene (PS).
Table 3.6 Properties of high impact polystyrene (HIPS).
Table 3.7 Properties of polyvinylchloride (PVC).
Table 3.8 Properties of acrylonitrile–butadiene–styrene (ABS).
Table 3.9 Properties of polyethylene terephthalate (PET).
Table 3.10 Properties of polymethylmethacrylate (PMMA).
Table 3.11 Properties of nylon 6 (PA6).
Table 3.12 Properties of nylon (PA66).
Table 3.13 Properties of poly(oxymethylene) (POM).
Table 3.14 Properties of polycarbonate (PC).
Table 3.15 Properties of polyetheretherketone (PEEK).
Table 3.16 Properties of polyphenylene oxide (PPO).
Table 3.17 Properties of polybutylene terephthalate (PBT).
Table 3.18 Properties of cyclic olefin copolymer (COC).
Table 3.19 Physical properties of Thermoplastic Urethane (TPU).
Table 3.20 Properties of poly(lactic acid) (PLA).
Table 3.21 Properties of poly(butylene succinate) (PBS).
Chapter 4
Table 4.1 Comparison of amorphous and semi-crystalline plastic materials.
Chapter 5
Table 5.1 Properties of mold material.
List of Illustrations
Chapter 2
Figure 2.1 Schematic representation of block copolymer.
Figure 2.2 Representation of alternating copolymer.
Figure 2.3 Representation of random copolymer.
Figure 2.4 Graft copolymer with backbone of polymer “o” to which a number of “...
Chapter 3
Figure 3.1 DSC traces for linear PE (a) cooled from the molten state at a cool...
Figure 3.2 Typical microstructures of HDPE. (a) with fine spherulites (spherul...
Figure 3.3 Scanning electron micrograph (SEM) of polypropylene. The DSC crysta...
Figure 3.4 Spherulite morphology in the extruded polypropylene
Figure 3.5 An SEM micrograph of pure PS sample.
Figure 3.6 An SEM micrograph of pure PMMA.
Figure 3.7 Optical micrograph of steady-state transfer films formed by PA6
Figure 3.8 Thermal conductivity of polycarbonate.
Figure 3.9 SEM micrographs of thermoformed PLA foam sheets (a) before thermofo...
Chapter 4
Figure 4.1 Schematic diagram of the molecular structure of polymers.
Figure 4.2 Increase in heat deflection temperature of some of the filled semi-...
Figure 4.3 Stretching polymer chains and their consequences on crystallization...
Figure 4.4 Schematic representation of the three types of non-isothermal cryst...
Figure 4.5 Melt strength of various polyethylene.
Figure 4.6 Extrusion temperature versus melt strength.
Figure 4.7 Measured thermal diffusivity of polyethylene.
Figure 4.8 Governance of strain uniformity.
Figure 4.9 Contour plots of the plastic shear strain rate of isotropic PMMA as...
Figure 4.10 Load–elongation curve.
Figure 4.11 Diagram of limiting deformations as a function of the deformation ...
Figure 4.12 Schematic representation of cup cut-out strips drawing and transve...
Figure 4.13 Stress–strain curves for PP cups with depths of (a) 58 mm and (b) ...
Chapter 5
Figure 5.1 A schematic representation of thermoforming cycle time.
Figure 5.2 Graphical representation of thermal history during thermoforming.
Figure 5.3 Representation of mold for thermoforming process.
Figure 5.4 (a) negative mold, (b) positive mold.
Figure 5.5 Schematic representation of draft angle and draw ratio vs. radius.
Figure 5.6 Factors influencing product quality.
Figure 5.7 Deformation acting on the network resistance
Figure 5.8 (a) Predicted profile of polymeric sheet thickness at different tem...
Figure 5.9 (a) Coefficient of variation and average thickness vs. bubble heigh...
Figure 5.10 (a) Variation in ultimate tensile strength with temperature. (b) V...
Figure 5.11 Numerical temperature distribution through the thickness.
Figure 5.12 Wall thickness measurement for the product.
Figure 5.13 Heat transfer and slip during plug assist.
Figure 5.14 Measured temperature distribution of the sheet (by infrared thermo...
Figure 5.15 (a) ABS sheet and (b) thermoformed product.
Figure 5.16 Effect of cooling temperature on the structure of PP sheets.
Figure 5.17 Illustration of a typical crystallization exotherm from the contro...
Figure 5.18 Schematic diagram of thermoforming technique.
Figure 5.19 (a) Basic operations in thermoforming process - male mould.(b) Plu...
Figure 5.20 Schematic representation of vacuum forming.
Figure 5.21 Plug-assist thermoforming.
Figure 5.22 Schematic representation of plug-assist vacuum forming. (a) clampi...
Figure 5.23 Schematic diagram of pressure forming.
Figure 5.24 Pressure forming.
Figure 5.25 Plug assist–air pressure forming.
Figure 5.26 Schematic illustration of the vacuum snapback thermoforming operat...
Figure 5.27 Hot drape forming equipment.
Figure 5.28 Schematic representation of drape forming.
Figure 5.29 Matched mold forming.
Figure 5.30 Schematic diagram of the thermoforming process.
Figure 5.31 (a) Variations in plug force and plug temperature with time during...
Chapter 6
Figure 6.1 Thermoforming problems.
Figure 6.2 Thermoforming defects related to material.
Figure 6.3 Thermoforming defects related to processing.
Figure 6.4 Thermoforming defects related to product quality.
Figure 6.5 (a) Fracture of polymeric sheet during plug-assisted vacuum thermof...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Begin Reading
References
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Thermoforming
Processing and Technology
Muralisrinivasan Natamai Subramanian
This edition first published 2024 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2024 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-55586-5
Cover image: Pixabay.ComCover design by Russell Richardson
Preface
The field of thermoforming is experiencing rapid development driven by commercial factors. Today, millions of tons of polymers are manufactured for use in various applications, both as commodity and specialty polymers.
The first edition of the book Update on Troubleshooting in Thermoforming was published nearly a decade ago by iSmithers Rapra, UK. The second edition, titled Thermoforming – Processing and Technology, aims to incorporate new and revised material on recent developments in thermoforming. This edition includes new, fully revised chapters and updated information on materials and processes.
The book is designed to provide practitioners and students with essential information on processing and technology in a concise and portable format. It is a valuable resource for polymer processors, engineers, technologists, and students, offering a comprehensive update on thermoforming. The book caters to both engineers and experts in physics and chemistry, providing introductory aspects, background information, and an overview of thermoforming processing and technology. The troubleshooting section includes flowcharts to assist in correcting thermoforming processes.
Thermoforming – Processing and Technology offers a complete account of thermoplastics, covering properties and forming, with chapters providing perspective on the technologies involved. The book is practical and mechanistic in its approach, making it useful for both industry professionals and academics. It serves as a self-contained reference manual and is engaging, accessible, and attractively presented. It would be a valuable addition to any research group with an interest in polymers, particularly in the field of thermoforming.
I am deeply thankful to the people who have made significant sacrifices of their personal time to prepare the book. This includes my parents who raised me, my wife Himachalaganga, and my children Venkatasubramanian, Amrutha and Sailesh. I am also grateful to those who have supported me in my education and career, including my professors and teachers. Above all Lord Natarajar, who brought me to this world, and my Guru, Thiruchendur Subramanya Swamy, who provided his knowledge to write this book. Furthermore, I would like to express my gratitude to Mr. Martin Scrivener and his team at Scrivener Publishing for their invaluable assistance in bringing this book to fruition.
Dr. MuralisrinivasanMadurai, India
1Introduction
Thermoforming was one of the earliest methods for fabricating thermoplastic materials. This involves shaping thermoplastic sheets into distinct parts. The United States has contributed to plastic processing methods. However, most of these techniques originated in Europe. It was not until the mid-twentieth century that a comprehensive understanding of its true nature was achieved [1].
Thermoforming has undergone significant growth and development with the introduction of cellulosic sheet materials, followed by acrylic and vinyl. In the 1890s, baby rattles and teething rings were thermoformed, but growth was slow until the 1930s, when roll-fed machines were developed in Europe. This process made significant progress in the 1950s using high-impact polystyrene for manufacturing containers and lids in the dairy industry. In addition, it has been used in the production of signage, exhibits, toys, and packaging materials. Initially, the thermoforming industry was limited by the lack of suitable sheet materials and forming equipment. However, in recent years, the process has rapidly advanced due to its advantages such as low machine and mold costs, low temperature and pressure requirements, ease of forming large parts, and fast mold cycles [2].
The emergence of plastics, synthetic materials that showcase human creativity and ingenuity, coincided with this understanding. Among the various methods utilized for processing plastics, thermoforming is a widely used technique for shaping extruded sheets into desired forms. This process is commonly used to produce large parts with low production volumes. It is a more cost-effective alternative to injection molding, which incurs higher expenses due to substantial fixed costs.
Thermoforming is a straightforward processing technology [2]. This involves modifying polymeric materials or systems to enhance their usefulness. Thermoforming is a cost-effective alternative to other plastic molding and forming methods. The process discussed here is often associated with the production of packaging materials such as blister packs and disposable coffee cup lids. However, it is important to note that the cost and time benefits of this process can be extended to a wide range of products in various industries.
Thermoplastics dominate the global polymer market and account for 85% of all polymers [3]. Most thermoplastic production focuses on large-volume, low-cost commodity resins, such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Engineering plastics, such as acrylics, acrylonitrile-butadiene-styrene (ABS), and high-impact polystyrene (HIPS), are chosen for their performance and cost-effectiveness. However, there is an increasing demand for high-performance materials, which has led to the utilization of engineering plastics such as polyacetal (POM), polyamides (PAs), and polycarbonate (PC), and polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polypropylene oxide, as well as their blends. Polymers such as liquid crystalline polymers, polyetheretherketone (PEEK), and fluoropolymers are commonly used in high-temperature applications [4].
Thermoforming presents numerous advantages over alternative thermoplastic fabrication techniques. These advantages encompass several aspects, such as the ability to fabricate components with a large surface area, reduced expenses for molds and equipment due to its low-pressure requirement, the capability to produce extremely thin-walled parts, increased production rates for high-volume thin-walled products, and the ability to manufacture low-volume heavy-gauge products at lower tooling costs [5].
This realization coincided with the advancement of plastics, which are synthetic materials that showcase human ingenuity and resourcefulness. The process involves shaping the thermoplastic sheets into precise components. Thermoforming has gained considerable importance as a method for processing plastic sheets, particularly for producing larger components [6].
Thermoplastics, which have gained significant popularity in the global polymer market, are exceptionally suitable for thermoforming processes because of their ability to be produced in high volumes at lower costs. This versatile procedure can be used for any thermoplastic sheet material, which demonstrates the necessary dimensional stability and impact resistance. Thermoforming presents numerous advantages over alternative thermoplastic fabrication techniques.
The market for thermoforming is experiencing growth due to its increasing use in the production of complex shapes and a wider range of materials. Thermoforming is widely regarded as a viable and economically advantageous alternative to various plastic molding and forming methods. The development of applications has largely been brought about by the introduction of new manufacturing methods. New methods have encouraged the development of new forming machines and techniques. This has brought the thermoforming industry to a new level in terms of processing technology.
2Polymers
2.1 Introduction
Polymers have experienced steady market growth and are widely acknowledged as a significant category of materials. These materials possess characteristics such as affordability, ease of manipulation, and versatility, rendering them suitable for a diverse array of applications. These applications encompass a wide range of household items, packaging materials, advanced fibers, medical devices, and wearable electronics. The application of polymers has greatly enhanced our standard of living and sparked revolutionary progress in various sectors [1, 2].
The prevalence of synthetic polymers can be attributed to the fact that 90% of these polymers are derived from finite fossil feedstock. The incorporation of these materials into diverse products is driven by their exceptional stability, processability, versatile mechanical properties, and durability. Although the terms plastics and polymers are often used interchangeably, it is crucial to acknowledge that there is a subtle distinction between them [3, 4].
Polymer engineering encompasses the technological processes used for the complete synthesis of meticulously regulated macromolecules. The objective of polymer engineering is to attain mastery over the physical characteristics of macromolecules, encompassing factors such as molecular weight, molecular weight distribution, end functionality, tacticity, stereochemistry, block sequence, and block topology. Polymers find applications in a wide range of consumer products such as carpets, furniture, glues, and clothing. In addition, they play a crucial role in advanced engineering, particularly in the development of materials used in the aerospace industry. Therefore, the utilization of polymers and their constituent monomers is important in our daily lives [5].
2.2 Physics and Chemistry of Polymers
The study of polymer materials is a complex and intriguing subject in physics and chemistry. The structure of a polymer is determined by the number and type of repeating units it contains. Polymer science investigates the characteristics, composition, and behavior of these substances at the atomic, molecular, and macroscopic levels. This multidisciplinary field combines concepts from biology, engineering, chemistry, and physics to understand and manipulate material properties for specific purposes. Many polymeric materials exhibit emergent properties, which are unique characteristics arising from the interaction of their constituent monomers.
One particularly fascinating aspect of chemistry and polymer science is the ability to transition materials between different phases, particularly in relation to the monomers. These transitions allow the manipulation of materials for real-world applications. This process is not only scientifically intriguing, but also essential for the development of new and innovative materials with a wide range of uses in modern technology and everyday life. This is particularly true for monomers that form polymers [6, 7].
Polymer molecules consist of a sequence of monomers that are joined by chemical bonds. The orientations of these bonds between successive monomers are correlated with those of adjacent monomers and crossing energy barriers is necessary for a bond to change its orientation. The arrangement of monomers within a polymer significantly affects its characteristics. Describing the behavior of materials at different length scales, from atomic interactions to macroscopic properties, is challenging. The development of polymer materials with specific properties is crucial for numerous technological advancements.
Even small variations in the arrangement of monomers can lead to significant differences in the material behavior. Polymers are highly versatile due to their flexible design, structure, and chemical composition, allowing them to exhibit a wide range of properties that can be tailored to meet specific requirements [8].
2.3 Natural Polymers
Natural polymers, which are associated with biopolymers, are formed through metabolic processes in living organisms [9]. These polymers consist of monomeric units that are linked by covalent bonds. Examples of natural polymers include proteins such as collagen and silk fibroin as well as polysaccharides such as chitosan, alginate, hyaluronic acid, and cellulose [10]. These polymers play important roles in nature, including the preservation and transmission of genetic information and the storage of cellular energy. One of their key advantages is their ability to biodegrade, with the released CO2 rapidly absorbed by agricultural crops and soil. Among polysaccharide biopolymers, cellulose is particularly abundant and is present in approximately 33% of all plant components [11, 12]. Other notable natural polymers include chitin/chitosan, starch, and lignin. Chitosan, alginate, cellulose, lignocellulose, starch, and PVA are among the most promising and frequently studied natural polymers, either as standalone materials or in combination with other advanced materials [13].
2.4 Synthetic Polymers
Synthetic polymers belong to a distinct class of polymers derived from crude oil, petrochemicals, natural gas, or biomass. These polymers possess a wide range of desirable characteristics such as low density, high durability, and resistance to deterioration. In addition to their advantages in terms of weight and cost, polymer products offer enhanced durability that can help prevent damage during transportation. Furthermore, many polymers exhibit corrosion resistance, making them well suited for use in harsh environments, such as chemical manufacturing facilities [14].
Polymerization, which involves chemically bonding monomers to form long chains, is utilized to produce synthetic polymers, which are subsequently used in various manufacturing processes. Synthetic polymeric networks consist of repeatable inert units and are generally superior to natural polymers in terms of mechanical properties and immunogenic responses [15].
Synthetic polymers offer tailored structures and properties through the appropriate design of their functional groups. These advantages ensure predictable, reproducible, and adjustable properties, which can vary according to specific applications. For example, the degradation rate of synthetic polymers can be altered by manipulating their chemical compositions, crystallinities, and molecular weights.
The ability to process and shape polymers enables the efficient mass production of a diverse range of items. Techniques, such as injection molding, extrusion, blow molding, and thermoforming, facilitate the creation of complex designs and ensure reliable quality. The lightweight nature of plastic products contributes to reducing shipping expenses and energy consumption. Moreover, manufacturing products from plastics is often more cost-effective than manufacturing conventional materials [16].
2.5 Polymerization Methods
During the polymerization process, monomers undergo chemical bonding to form extensive chains or networks, resulting in the development of unique polymer properties. This chemical reaction, known as a polymerization reaction, leads to the formation of high-molecular-weight molecules from the monomers [17]. The structural features of polymers, such as linear, branched, or network configurations, are determined by the arrangement of monomers and the types of chemical bonds that connect them. The ability to manipulate the structure and composition of polymers is crucial in various industries and applications. Two primary approaches are utilized in the production of polymers: addition and condensation polymerization. Both mechanisms can be used in the polymerization of the same monomer or different monomers can be used to create the same polymer through both approaches, provided that suitable functional groups are available for each individual polymerization. Addition and condensation polymerizations are both essential in the creation of diverse polymers, fibers, rubbers, and other materials that have extensive applications in everyday life and industry. The selection of specific monomers and the desired characteristics of the final polymer product determine the appropriate polymerization method [18, 19].
2.5.1 Addition Polymerization
In this process, the reaction between monomers containing double or triple bonds leads to the formation of polymer chains. This involves the breaking of the double or triple bonds and the subsequent connection of the monomers without the production of any byproducts. The double bonds of the monomers react without releasing any molecules. These reactions occur via the addition of monomer molecules via unsaturated (double) bonds [20].
The initiation of the reaction can be achieved using chemical molecules, such as azo compounds or peroxides, or through physical sources, such as heat or electromagnetic radiation. These initiators create radicals, anions, or cations in the monomer. Depending on the type of initiation, addition polymerization can be classified as radical, anionic, cationic, or coordination polymerization. All addition polymerizations involve three stages: initiation, propagation, and termination [18, 21].
In the case of addition polymerization, a reactive center such as a radical, an anion, or a cation must first be created on a molecule that contains a double bond. New monomer molecules are then successively added to this active molecule, creating a new active center for further addition. This process continues until the reaction is terminated by other reactions or events. Termination results in the formation of “dead” polymer, which is no longer capable of further reaction. During these reactions, addition must be initiated and typically involves a termination reaction. No small molecules, such as water or alcohol, are released during this process [22].
The most common type of addition polymerization is free-radical polymerization. Most radical polymerizations require an initiator to generate the initial active radical and initiate a chain of addition reactions. The most common initiation reaction is the thermal decomposition of molecules containing weak bonds such as peroxides (‒OO‒) or azo compounds (‒N=N‒). Addition polymerization is generally rapid and can be moderately or highly exothermic. Additionally, polymerization of an individual chain is completed quickly, resulting in the formation of high-molecular-weight products even in the early stages. The concentration of monomers in the medium slowly decreased over time [23]. The most significant group of addition polymerizations involves the combination of monomers, such as ethylene, propylene, styrene, and vinyl chloride, which give rise to polyethylene, polypropylene, polystyrene, and polyvinylchloride, respectively.
2.5.2 Condensation Polymerization
The homogeneous two-component polycondensation reaction holds significant historical importance in the field of polymer science. Achieving a consistently high degree of polymerization (DP) is a crucial consideration in polymer synthesis, as the desired physical properties often arise from high molecular weights. Condensation polymerization involves the reaction between two distinct functional groups present in monomers. This process entails the elimination of small molecules, such as water, alcohol, or ammonia, while simultaneously forming a polymer chain from monomers that possess two or more functional groups, such as ‒OH and ‒COOH. The reaction occurs in multiple phases, with each phase releasing a small molecule, hence earning the designation of a “condensation” reaction [24] and presented in Equation (2.1).
It is applicable to monomers that possess functional groups, such as ‒COOH, ‒COOR, ‒COOOC‒, ‒COCl, ‒OH, ‒NH2, ‒CHO, ‒NCO, and epoxy. In condensation polymerization, the reactive groups located at the ends of each monomer react with one another. Consequently, a growing chain is formed with reactive groups at both ends, and when two chains combine, the length of the chain immediately increases.
However, the increase in the molecular weight of the product in condensation polymerization is slow due to the addition of growing chains to each other, leading to a depletion of monomers at the initial stages. Condensation polymerization is typically characterized by its slow rate, limited by equilibrium, and slightly exothermic nature [25].
In condensation polymerization, two functional groups form a bond by releasing a small molecule such as H2O. While there are some condensation reactions where no molecule release occurs, such as polyurethane polymerization, nylon 6 is an example of a polymer with repeating units ‒NH(CH2)5CO‒. It can be synthesized through either condensation of 6-aminocaproic acid or addition polymerization of caprolactam [26].
Furthermore, there have been recent advancements in polymerization techniques, including click polymerization, atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer polymerization (RAFT). These techniques employ specialized catalysts and agents to produce polymers with controlled molecular weights, offering new possibilities for polymer design [27–29].
2.6 Polymer Molecules in Thermoforming
The behavior of polymeric materials is influenced by the chemical composition and physical structure of both the penetrant molecule and polymer itself. These factors play a crucial role in determining the mobility of the chain segments, presence of defects, and interactions that govern the extent of sorption and molecular mobility of the penetrant within the polymer.
Anisotropy, or directional dependence, occurs in polymers when the links of the polymeric chain align in the direction of stretching. In thermoforming, anisotropy is associated with the temperature at which a transition between two or more phases occurs as well as the temperature at which a change of state, such as melting, occurs. The molecular structure of the polymer contributes to these transitions, which depend on the monomer structure, degree of orientation, and extent of stretching at specific temperatures. It is important to note that a given transition in polymers can arise from either the amorphous or the crystalline phase.
Generally, polymers exhibit significant changes in their physical properties as a function of temperature. The theory of rubber elasticity states that strained elastomers with a low degree of crosslinking exhibit a combination of orientation and segmental motion. However, in stretched amorphous polymers, segmental motion is absent because it can only exist below the glass transition temperature. This resulted in a frozen-in orientation. By subjecting amorphous linear polymers to strain at temperatures above the glass transition temperature, they can be brought into an anisotropic state. Once cooled below the glass transition temperature, the polymer remains in the stretched state even when unloaded.
In mechanical applications, the amorphous regions between crystallites largely determine the response to stress, giving polymers desirable qualities such as ductility and toughness. In electronic applications, it is believed that tie molecules spanning adjacent crystalline lamellae can improve field-effect charge carrier mobility [30].
2.7 Classification
Polymers are classified on the basis of their unique properties, characteristics, or attributes. The objective of this categorization is to methodically arrange these elements and their connections, promoting comprehension and enabling a deeper investigation within each specific category.
2.8 Primary Classification
In the field of polymer science, plastics can be classified into two fundamental groups: homopolymers and copolymers, which are distinguished by their distinct molecular structures.
2.8.1 Homopolymers
Homopolymers possess a consistent and uniform structure throughout their composition because they are composed of identical repeating units of a single monomer. Despite their uniformity, the properties of homopolymers can still be modified by adjusting various factors, such as the processing parameters, molecular weight, or monomer structure [31].
The chemical structure of homopolymers is relatively straightforward, allowing relatively easy synthesis. Depending on the specific monomer and polymerization conditions, homopolymers can exhibit varying degrees of crystallinities. Higher levels of crystallinity can enhance mechanical properties but may reduce transparency. The homogeneous structure of homopolymers generally makes them more suitable for processing, making them applicable to a range of production techniques, including injection molding, extrusion, blow molding, and thermoforming. Additionally, homopolymers typically possess high thermal stability, enabling them to withstand high temperatures without significant degradation. This thermal resistance makes them valuable for applications that require heat resistance [32].
Homopolymers are known for their inherent rigidity, which is attributed to their high elastic modulus. This characteristic renders them particularly advantageous in scenarios where maintaining structural integrity is of utmost importance. Additionally, many homopolymers exhibit exceptional optical clarity, making them highly suitable for applications that necessitate transparency and visual acuity, such as display screens, optical lenses, and transparent packaging materials. Moreover, homopolymers generally offer a more cost-effective solution compared to copolymers or more intricate polymers due to their straightforward chemical composition and ease of production. However, it is crucial to acknowledge that homopolymers possess a limited range of properties, which can be perceived as a drawback [33].
2.8.2 Copolymers
The blending of polymeric substances is a cost-effective and straightforward method to create new materials with enhanced mechanical properties. It is anticipated that copolymers will exhibit a wider melting temperature range and a sigmoidal relationship between their crystallinity and temperature. When two polymer surfaces that are not soluble in each other come into contact, it is expected that their surface functionality will arrange or structure themselves in a way that minimizes free energy. Understanding the structure of polymer/polymer interfaces is crucial in various areas such as adhesion, polymer blends, and nanocomposites [34].
These new multiphase materials can be obtained as blends, block copolymers, and graft copolymers, all of which typically consist of two or more polymeric phases in the solid state. It is important to differentiate these materials from composite materials. Most homopolymer phases are insoluble in each other, resulting in materials with low strength due to the lack of interfacial adhesion [35].
Copolymers have a narrower range of compositions than blends, but they offer several advantages. The different segments in the copolymers are covalently bonded, eliminating the interface problem [36]. The molecular architecture of copolymers can be precisely controlled to produce novel materials. Copolymers can also strengthen blends of immiscible polymers by acting as emulsifiers and facilitating physical connections between the phases. This improves the interfacial adhesion and ability to transfer loads between components [37].
In an ordered copolymer, where structural units of a specific type are arranged in long sequences, crystallinity should disappear at a temperature slightly below the melting temperature of the pure polymer. The phase rule cannot be applied to the crystal-amorphous transformation in polymers because the free energy per unit amount of the amorphous “phase” is not a unique function of the composition [38].
The presence of copolymers at the interface between immiscible polymers has been widely recognized for their significant influence on interfacial properties, provided that an appropriate copolymer structure is utilized. The degree of toughening varies depending on the copolymer architecture and the interactions between the copolymer and polymer [39, 40].
2.8.2.1 Block Copolymers
Block copolymers are polymers that consist of two or more distinct sequences joined together by covalent bonds and have different chemical compositions. Blending different homopolymers together to form a block copolymer structure offers unique properties that cannot be achieved by simply blending individual homopolymer components. This is due to the ability of the block copolymer to segregate its chains, resulting in distinct characteristics. There are two types of block copolymers: coil–coil and rod–coil, with the latter having a rigid and inflexible rod-like segment [41].
Unlike homopolymers, block copolymers consist of distinct blocks of different monomers, rather than repeating units of the same monomer. Block copolymers are a fascinating group of polymeric materials that are of great interest in both the scientific and technological fields. These consist of sequences of different homopolymers within the same molecule. These block copolymers had distinct hard and soft blocks with substantially different compositions. In the strong segregation limit, they exhibit highly ordered long-range organization, which is a result of the interplay between the thermodynamics of polymer interactions, configuration of domain-forming blocks, and minimization of free energy with the morphology of microphase-separated domains [42].
These blocks can vary in size, and their arrangement can result in different morphologies and properties, as depicted in Figure 2.1. Within a block copolymer, blocks can form distinct and specialized regions. This phase separation led to the formation of microdomains with unique characteristics. This property makes block copolymers important in industries, such as nanotechnology and the production of self-assembling materials.
These copolymers tend to separate into different phases on the nanoscale due to the repulsive interaction between their repeating units. This results in the formation of well-organized morphologies, such as alternating lamellae, gyroids, hexagonally packed cylinders, and body-centered cubic spheres. The equilibrium morphology and domain size of block copolymers depend on the balance between the enthalpic contribution associated with short-range segmental interactions and the entropic contribution associated with chain packing and distribution within the microdomains [43].
At equilibrium, a dense collection of monodisperse diblock copolymer chains is arranged in minimum free energy configurations (ordered). Lowering the temperature, i.e., increasing the energy parameter, favors a reduction in the A–B monomer contacts (disordered) [44]. To manipulate the phase behavior of block copolymers, factors such as the volume fraction of components, degree of polymerization, and segmental interaction parameters are considered. Another approach to modify the phase behavior of block copolymers is to blend them with other polymers such as homopolymers or other block copolymers. This blending introduces an additional degree of freedom into the system, resulting in a more complex and diverse phase behavior than that of a neat block copolymer. This enhanced behavior includes microphase separation, macrophase separation, order–order transition, and order–disorder transition. For instance, studies have been conducted on binary block copolymer blends where two block copolymers have the same repeating units but different molecular weights [45].
Figure 2.1 Schematic representation of block copolymer.
Commercially available block copolymers are more cost-effective than the other sequestering phases. These materials are designed to possess thermomechanical properties that meet engineering requirements, such as those required for automobile parts. However, there is limited knowledge about the structure-property relationships that govern their interactions with small molecules, which ultimately determine their performance in various applications. Notably, exceptional mechanical properties can be achieved by carefully selecting the type and molecular weight of the block copolymer.
Block copolymers have frequently been used as compatibilizers in polymer blends that undergo phase separation. The ability of block copolymers to act at interfaces is attributed to their preferential location at the interface and the extension of each block into the homopolymer phase, where their energetic interactions are favorable [46]. However, block copolymers have certain drawbacks when used as interfacial agents: they are relatively challenging to synthesize and tend to form micelles, which significantly reduces their ability to segregate at the interfaces [47, 48].
In the context of film crystallization, the aggregation of block copolymers significantly affects the optical and electronic properties of materials. This is because interchain contacts alter the absorption characteristics, quench emission, and affect conduction behavior. Block copolymer architectures have been successfully used to control the orientation and morphology of optically and electronically active materials that can be processed. The flexible segments in these block copolymers primarily serve to increase the overall solubility, reducing intermolecular contact and aggregation of the functional segments.
Block copolymers can enhance the bonding between substances with different surface characteristics. Block copolymers are utilized in the fabrication of nanoscale patterns and templates, which play crucial roles in nanolithography and nanodevice production. These materials have applications in various fields. Some block copolymers are suitable drug delivery systems because they can self-assemble into vesicles (polymersomes) or spherical micelles in solution. The mechanical characteristics and toughness of regular polymers can be significantly improved by incorporating small amounts of block copolymers. Due to their unique permeability characteristics, block copolymers are used in membrane technologies for water purification and gas separation [49].
2.8.2.2 Diblock Copolymers
The most basic forms of copolymers are diblocks, which consist of a sequence of A monomers connected to a sequence of B monomers. From a thermodynamic perspective, the impact of diblock copolymers on interfaces can be described as follows: copolymers exhibit amphiphilic behavior in a mixture of homopolymers [33]. As the concentration of the copolymers increases, the interfacial tension decreases continuously until a third phase, rich in copolymers, is formed, terminating this process. Block copolymers, which are partially compatible with both the A-rich and B-rich phases, tend to aggregate at interfaces, reducing the number of direct contacts between the A and B homopolymers, thus reducing the interfacial tension [50]. Simple AB-block copolymer melts undergo microphase separation on a single characteristic length scale. However, when more than two types of monomers are involved, microphase separation often occurs at different scales [7, 51, 52].
These blocks are composed of two separate units, A and B, which are alternately connected to each other, such as A-B-A-B-A-B. The blocks tend to separate into various domains and do not mix easily, resulting in a variety of microstructures, including spherical, cylindrical, and lamellar structures [53, 54].
2.8.2.3 Multiblock Copolymers
In the field of polymer chemistry, multiblock copolymers are commonly composed of consecutive blocks of the same type of monomers. Among these copolymers, the simplest and most interesting are diblock copolymers, which consist of two homopolymers connected end-to-end, with the first polymer composed of type A monomers and the second polymer composed of type B monomers [55]. At low temperatures, the A and B monomers undergo local demixing due to the irreversible chemical links between the blocks, but this demixing does not occur globally. Depending on the average composition of the chain and the temperature, various microdomain textures spontaneously appear at thermal equilibrium. By manipulating the displacement of different volumes by the monomers or by making one block significantly stiffer than the other, the boundaries between the different phases can be shifted [56].
The development of block copolymers has recently attracted significant attention due to their intriguing characteristics and potential applications in cutting-edge technologies. Phase segregation and microphase segregation in block copolymers provide them with distinct qualities. The two primary forms of block copolymers are multiblock and diblock copolymers. These materials display more complicated phase-separated microstructures and feature more than two types of monomer blocks in their structures, such as A-B-C or A-B-A-C-B-A [57].
2.8.3 Alternating Copolymers
The distribution of monomers within the chain of an alternating copolymer follows a predictable sequence. This type of copolymer is characterized by the composition of two or more distinct monomer units. In alternating copolymers, monomer units are arranged alternately along the polymer chain. Creating, a regular pattern in the polymer backbone, with each monomer unit followed by a different monomer unit [58].
The repetitive arrangement of monomers in alternating copolymers leads to distinct characteristics. Typically, these copolymers exhibit higher melting temperatures and increased stiffness compare to random copolymers [59].
For example, a basic alternating copolymer of type AB can be represented as A-B-A-B-A-B-A-B-A-B-… as shown in Figure 2.2.
In this structure, “A” and “B” denote different monomer units that are repeated in an alternating manner. The arrangement of monomers in alternating copolymers is often determined by the reactivity ratio of the monomers during polymerization. If the reactivity ratios are balanced, the monomers tend to undergo alternate integration. These copolymers often possess unique characteristics compared to homopolymers or other types of copolymers [60]. The regularity in their structure can lead to changes in their mechanical, thermal, and optical properties, resulting in more orderly packing in the solid state. Furthermore, the chemical reactivity exhibited by alternating copolymers makes them highly valuable for various applications, including optics, electronics, and specialty materials. Different polymerization processes, such as step-growth polymerization and chain-growth polymerization, can be used to produce alternating copolymers. Well-known examples of alternating copolymers include poly(styrene-alt-maleic) and poly(ethylene-alt-propylene) [61].
Figure 2.2 Representation of alternating copolymer.
Alternating copolymers constitute a distinct category of copolymers wherein the two comonomers polymerize in a regular alternating sequence along the polymer chain. The intriguing physical and chemical properties of these copolymers, as well as the underlying mechanisms governing their formation, have garnered significant attention from both the academic and industrial sectors. In an alternating copolymer, the two comonomers arrange themselves in a consistent alternating sequence [50, 62]
2.8.4 Random Copolymers
Random copolymers are considered viable alternatives to block copolymers because of their cost-effectiveness and ability to offer a high degree of design flexibility. A simple representation of the random copolymer is shown in Figure 2.3. When added to an immiscible polymer blend, random copolymers can effectively lower the cloud-point temperature. Additionally, the interfacial activity of random copolymers allows for the desired reduction in interfacial tension when incorporated into homopolymer blends. For instance, the inclusion of styrene-methyl methacrylate random copolymers has been found to effectively reinforce the interface between polystyrene and polymethyl methacrylate, although the presence of the random copolymer may affect fracture energy [63].
The effectiveness of a long random copolymer in strengthening the interface between immiscible homopolymers surpasses that of a short, symmetric block copolymer. While random copolymers have a relatively weak impact on interfacial strengthening, both block and alternating copolymers show promise as interfacial modifiers [61, 64].
2.8.5 Graft Copolymers
Graft copolymers possess characteristics of a combination of physical blends. The presence of chemical linkages within these copolymers significantly affects their physical properties. The number of initiating groups along the backbone can vary, resulting in copolymers with different grafting densities and distinct phase-separation behaviors. The hydrodynamic volume of a graft copolymer changes as its molecular weight increases, which is responsible for its unique attributes. Grafting is widely recognized as an important technique for modifying the physical and chemical properties of polymers [65, 66].
Figure 2.3 Representation of random copolymer.
Figure 2.4 Graft copolymer with backbone of polymer “o” to which a number of “•” sequences.
All graft copolymers comprise two general structural features: a backbone of polymer “o” to which a number of “•” sequences are grafted, as shown in Figure 2.4.
Graft copolymers, known as A-g-B copolymers, are large molecular structures that resemble combs consisting of a linear backbone A with attached side B blocks. Typically, these two blocks are incompatible with each other. These copolymers have significant technological importance, particularly in their ability to make polymer blends compatible and create thermoplastic elastomers. In fact, they are the main agents used for compatibilization in commercial applications. The specific aggregation structures of these copolymers depend on various factors, including the level and distribution of grafting along the backbone, molecular weights of the backbone and grafts, and nature of the pendant grafts. In commercial applications, polydisperse graft copolymers with randomly distributed grafting points are commonly used because of their ease of production and cost-effectiveness [61].
2.8.6 Impact Copolymers
Impact copolymers, also known as elastomeric copolymers or impact modifiers, are a type of polymer blend that combine two distinct monomers to provide a unique set of properties. These copolymers are specifically designed to maintain a balance between stiffness and impact resistance, particularly at low temperatures where ordinary polymers may become brittle and prone to fracturing [67].
One of the key characteristics of impact copolymers is their ability to enhance the resilience of materials to impact and mechanical stress. This is achieved through the inclusion of elastomeric segments, which can efficiently absorb and distribute energy, reducing the likelihood of fracture or shattering upon impact. Additionally, the formulation of impact copolymers allows for the adjustment of stiffness to suit the intended purpose, resulting in improved performance under different conditions [68].
Unlike regular polymers, which may shatter under cold conditions, impact copolymers exhibit ductility and resilience. This exceptional impact resistance makes them commonly used to enhance the durability and toughness of materials, such as plastics and rubbers. Furthermore, impact copolymers are often more processable than conventional elastomers, making them easier to produce and shape into desired forms [69].
The versatility of impact copolymers is evident in their utilization across various sectors, including consumer products, electronics, automotive, construction, and buildings. They are frequently used to enhance the properties of materials, such as polystyrene, polypropylene, and polyvinyl chloride (PVC). By adjusting the ratio and content of the two monomers, the characteristics of the impact copolymers can be tailored to meet the specific requirements for different applications. The flexibility of impact copolymers, even at low temperatures, is a significant advantage that makes them essential materials for many technical and industrial applications [70].
2.9 Secondary Classification
In materials science, plastics can be classified into two distinct categories: thermoplastics and thermosets. These groups are distinguished by their unique responses to changes in temperature and processing properties.
2.9.1 Thermoplastics
Thermoplastics are polymeric materials that can be reprocessed and fully recycled. In contrast to thermosets, thermoplastics consist of linear polymer chains that are not chemically linked. When heated, the intermolecular forces between the polymer chains weaken, allowing the material to reshape without any chemical bonding. This unique property enables the melting and reshaping of thermoplastics multiple times without significant degradation of their properties.
The classification of thermoplastics as either amorphous or semicrystalline based on their transition temperatures is a valuable categorization scheme in the field of materials science and engineering. The transition temperature provides insights into the molecular arrangement and behavior of polymer chains at different temperatures. It also aids in predicting the mechanical properties, heat resistance, and other relevant material characteristics for various applications.
Thermoplastics possess several key characteristics that make them widely used in various industries. They have a defined melting point, which is the temperature at which they transition from a solid to molten state. This property allows them to be melted, molded into various shapes, and then cooled and solidified again, thus enabling convenient processing and recycling. Thermoplastics are also more ductile than thermosetting plastics, meaning that they can be stretched and elongated without breaking.
Furthermore, thermoplastics exhibit a wide range of properties including flexibility, rigidity, transparency, impact resistance, chemical resistance, and electrical insulation. This versatility makes them suitable for various applications. Their mechanical properties can vary widely, with some thermoplastics being rigid and strong, whereas others are more flexible. Various processing techniques such as injection molding, extrusion, blow molding, thermoforming, and 3D printing can be used to manufacture thermoplastic products with intricate shapes and sizes.
Thermoplastics have applications in numerous industries including packaging, automotive components, household products, medical devices, toys, pipes, and cables. Common examples include high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), and polycarbonate (PC). Their lightweight nature makes them useful in industries where weight is a critical factor. Additionally, they often exhibit good chemical resistance and weatherability and can be easily colored and molded into complex shapes. The recyclability of thermoplastics helps reduce waste and minimize environmental impacts [71].
Thermoplastics generally exhibit greater ductility than thermosets, although they can become brittle under certain conditions such as high strain rates, triaxial stress states, and low temperatures. The tensile moduli of most thermoplastics range from 1 to 4 GPa at room temperature, whereas thermosets have moduli ranging from 2 to 10 GPa. Due to their relatively low elastic moduli, pure resins are not commonly used in structural applications.
Overall, thermoplastics play a crucial role in modern manufacturing processes and have widespread applications in automotive, aerospace, packaging, electronics, construction, and consumer goods. Their ease of processing, recyclability, and wide range of properties continue to make them a preferred choice for various applications.
2.9.2 Thermosetting Plastics
Thermosets, which are referred to as highly crosslinked covalent network polymers, possess exceptional mechanical properties, chemical and heat resistance, and dimensional stability. Nevertheless, the irreversible nature of their chemical bonds renders thermosets incapable of reprocessing or recycling once they fail. Furthermore, the occurrence of shape alteration due to reversible bond-exchange reactions, commonly known as “creep,” is regarded as a drawback of these polymers. Consequently, the irreversibility and absence of recyclability in thermoset networks are deliberate design characteristics [72, 73].
Thermosets are a class of polymers that undergo a chemical reaction known as curing or crosslinking during their curing process. This reaction causes them to transition from the liquid or soft solid state to the hardened or solid state. Unlike thermoplastics, which can be melted and reshaped multiple times, thermosetting plastics cannot be reprocessed once they have undergone curing. This characteristic distinguishes them from other thermoplastics.
The curing process of thermosetting polymers involves the formation of strong covalent bonds between polymer chains, resulting in the creation of a three-dimensional network structure. These crosslinked networks provide thermosets with superior mechanical properties, including high strength, stiffness, and resistance to heat and chemicals [74].
Thermosetting plastics are used in various industries, including aerospace, automotive, electrical insulation, adhesives, and composite materials for structural components. Their main advantage lies in their ability to maintain their shape and structural integrity at high temperatures, which makes them suitable for applications that require high heat resistance.
Common examples of thermosetting plastics include epoxy resins, phenolic resins, melamine-formaldehyde, and urea-formaldehyde. Each type of thermosetting plastic has specific properties and applications.
2.10 Distinction Between Thermoplastics and Thermosetting
The primary difference between thermoplastics and thermosetting plastics is their response to heat. Thermoplastics can be melted and reformed multiple times, whereas thermosetting plastics undergo irreversible chemical changes and become permanently hardened after their initial formation. The disparity in recyclability between these two types of plastics is a crucial factor to consider when evaluating their environmental impact and potential for integration into a circular economy. In a circular economy, materials are continuously reused and recycled to minimize waste and resource consumption. It is important to note that the distinction between thermoplastics and thermosetting plastics is vital when selecting materials for specific applications because their properties and behavior under different conditions can vary significantly.
2.11 General Classification
Plastics are a category of man-made materials derived from polymers, which consist of elongated chains of molecules comprising recurring units known as monomers. The primary distinction between commodity plastics and engineering plastics is their respective performance attributes and costs. Commodity plastics are more affordable and are commonly used for everyday objects, whereas engineering plastics offer superior properties, albeit at a higher price. The selection between these two options depends on the specific requirements of the application and desired level of performance [75]. Engineering plastics are favored when elevated strength, temperature resistance, chemical resistance, or other advanced characteristics are imperative, whereas commodity plastics are suitable for less-demanding applications. These are classified as follows:
2.11.1 Commodity Plastics
Plastic materials have undoubtedly brought about a significant transformation in numerous industries and everyday life, due to their exceptional properties and versatile applications. These materials offer several advantages that have resulted in their widespread adoption over traditional materials, such as metals, glass, and wood. Plastics often exhibit superior performance in specific applications compared with conventional materials. They can be engineered to possess a wide range of physical, chemical, and biological properties that render them suitable for diverse applications. They can be flexible or rigid, transparent or opaque, lightweight, or heavyduty, and can even be electrically conductive or insulating [76].
Commodity plastics, also referred to as bulk or general-purpose plastics such as polyethylene, polypropylene, polystyrene, polyvinylchloride, and polyethylene terephthalate, constitute a group of widely utilized and economically viable polymers. They are typically produced in large quantities and can be applied in various industries due to their low cost and versatility [1].
2.11.2 Engineering Plastics
Recently, there has been a growing preference for the use of plastics in industrial structural components. This preference can be attributed to the advantageous characteristics of plastics such as their capacity to decrease weight, facilitate the creation of intricate shapes, and reduce expenses. Consequently, the engineering sector has witnessed a notable surge in the adoption of plastics in various applications. Presently, there is a prevailing inclination towards the production of substantial load-bearing components through molding techniques [77].
Engineering plastics, also referred to as high-performance plastics or advanced plastics, encompass a category of polymers, such as polyamide, polyacetal, polycarbonate, polyphenylene sulfide, and polyetheretherketone, which exhibit exceptional mechanical, thermal, and chemical characteristics. These materials are purposefully engineered to fulfill stringent performance criteria and are widely used in industries where conventional commodity plastics may prove inadequate [78].
In conclusion, despite the challenges encountered, ongoing research on polymers continues to make significant advancements, facilitating progress in diverse industries, including electronics, healthcare, energy, and transportation. The study of materials will remain indispensable in addressing global issues and constructing a more sustainable future, even as technology continues to evolve [76].
