Thermosets

Structure, Properties, and Applications

Qipeng Guo, Qipeng Guo

• Editorial: Elsevier
• Año: 2017
• ISBN:
  9780081010280
• Edición: 2
• Categoría: Tecnologia e ingegneria
• Categoría: Scienza dei materiali

Thermosets: Structure, Properties, and Applications, Second Edition builds on and updates the existing review of mechanical and thermal properties, as well as rheology and curing processes of thermosets, and the role of nanostructures in thermoset toughening. All chapters have been updated or re-written, and new chapters have been added to reflect ongoing changes and developments in the field of thermosetting materials and the applications of these materials. Applications of thermosets are the focus of the second part of the book, including the use of thermosets in the building and construction industry, aerospace technology and as insulation materials. Thermoset adhesives and coatings, including epoxy resins, acrylates and polyurethanes are also discussed, followed by a review of thermosets for electrical applications. New chapters include coverage of thermoset nanocomposites, recycling issues, and applications such as consumer goods, transportation, energy and defence. With its distinguished editor and international team of expert contributors, the second edition of Thermosets: Structure, Properties, and Applications is an essential guide for engineers, chemists, physicists and polymer scientists involved in the development, production and application of thermosets, as well as providing a useful review for academic researchers in the field.

• Links structure, properties, and applications, making this book relevant to both academia and engineers in industry
• Includes entirely new chapters on the use of thermosets in aerospace, transport, defense, and a range of consumer applications
• Enables practitioners to stay current on the latest developments in recycling of thermosets and their composites

Part 1

Structure and Properties of Thermosets

Chapter 1

Overview of thermosets: Present and future

J.-P. Pascault^⁎; R.J.J. Williams^{† ⁎} University of Lyon and INSA-Lyon, Villeurbanne, France
^† University of Mar del Plata and National Research Council (CONICET), Mar del Plata, Argentina

Abstract

This chapter provides a background of thermosetting polymers analyzing network formation by step-growth, chain-growth, and controlled polymerization; the occurrence of gelation and vitrification; and the use of transformation diagrams. Innovations on chemistries and formulations are discussed and motivated by improvements in properties and processing, adaptation to new regulations, and the need of recycling. General rules for thermoset processing are analyzed considering pot life, processing window, and manufacturing methods involving low energy consumption and enabling rapid processing. A brief survey of vitrimers and advanced materials based on thermosets with specific properties such as recyclability, self-healing, or shape memory is provided.

Keywords

Chemistries of network formation; Gelation and vitrification; Transformation diagrams; Rules for thermoset processing; Bio-based thermosets; Vitrimers; Self-healing and shape memory

1.1 Thermosets

A conventional way to classify plastics is based on their behavior when they are heated. Thermoplastics flow when they are heated beyond a particular temperature while thermosets remain in the solid state until temperature becomes so high that degradation of the material takes place. Typical examples of thermoplastics are polyethylene, polypropylene, polystyrene, and poly(vinyl chloride). Their behavior when heated can be illustrated by the preparation of the fire for a barbecue using carbon or wood in a polyethylene package. When the whole package is used to make the fire, the formation of polyethylene droplets is observed before their combustion takes place. A different behavior is observed when the handle of a frying pan, typically made of a phenolic plastic, is exposed to fire. In this case, the handle keeps its shape and eventually produces smoke if the exposure to fire is prolonged. Phenolics, epoxies, and unsaturated polyesters (UPRs) are typical examples of thermosets.

The different behavior of thermosets and thermoplastics when heated arises from their chemical structures. Thermoplastics are linear polymers that in the solid state are either semicrystalline or amorphous glasses. When they are heated beyond the melting point of crystals (for semicrystalline thermoplastics like polyethylene) or beyond the glass transition temperature (for amorphous thermoplastics like atactic polystyrene), polymer chains are free to move, and flow takes place. On the other hand, thermosets are cross-linked polymers, and they remain in the solid state as far as the covalent chemical bonds are not destroyed.

Some linear polymers like poly(tetrafluoroethylene) do not flow when heated due to the presence of strong noncovalent bonds holding the polymer chains together (e.g., they are composed of linear chains but do not show the typical thermoplastic behavior). Other linear polymers like poly(phenylene oxide) degrade before they can flow. However, they are still classified as (intractable) thermoplastics because they are composed of linear chains. Polymers constituted by branched chains like low-density polyethylene synthesized by the high-pressure process are also thermoplastics because they can flow or dissolve in an adequate solvent indicating that branched chains can be separated one from the other. A thermoset cannot flow or be dissolved due to the interconnection of the cross-linked structure throughout the whole sample. Only a fraction of the material (the sol fraction) can eventually dissolve, while the interconnected structure (the gel fraction) will swell in an affine solvent. Therefore, a thermoset can be better defined as a polymer network cross-linked by covalent chemical bonds that percolates the whole mass.

However, the difference between thermoplastics and thermosets has been recently challenged by polymer networks designed as vitrimers by Leibler [1]. Vitrimers can interchange covalent bonds above a temperature T_v (located above their glass transition temperatures). Below T_v, vitrimers behave as conventional thermosets. Above T_v, they can undergo creeping and relax stresses due to the exchange of covalent bonds while being permanently cross-linked. Due to this behavior, vitrimers can be recycled, self-healed, or reshaped. Examples will be given in Section 1.6.

1.2 Network Formation

There a several ways in which a polymer network may be produced.

1.2.1 Step-Growth Polymerization

In the step-growth polymerization (also called polyaddition or polycondensation), the polymer network is generated by the reaction of functional groups of type A present in a monomer (A_f, with f functional groups in the molecule), with functional groups of type B present in a comonomer or hardener (B_g, with g functional groups in the molecule). When f = g = 2, a linear polymer is obtained. In order to produce a network, f and/or g must be higher than 2. The network is formed in consecutive steps starting from the mixture of monomers and generating branched structures of increasing size as reaction progresses. Any A-functional group can react with any B-functional group independently of the size of the species on which they are located. However, as the largest branched structures contain more unreacted A and B groups, they can enter reaction with a higher probability than the smallest species (the starting monomers). This finally leads to the formation of a polymer network. The step-growth network formation can also take place in the homopolymerization of a monomer with f functional groups (A_f); in the copolymerization of two monomers, with one of them containing functional groups of both types (A_f + A_gB_h); or in the polymerization of monomers with a distribution of molar masses and functional groups. A network can be also generated using difunctional monomers that can react in a concerted way generating cycles that involve three of them (a cyclotrimerization reaction). Every cycle involves the concerted reaction of three functional groups, leaving three other functional groups covalently attached to the cycle, giving the polyfunctionality required to produce a polymer network.

Examples of polymer networks generated by a stepwise mechanism are one-step phenolics (resoles) produced by reaction of phenol (A₃) with formaldehyde (B₂), epoxy-amine networks generated by reaction of diglycidyl ether of bisphenol A (DGEBA, A₂) with a diamine (B₄), polyurethanes formed by reaction of a polyisocyanate (A_f, f > 2) with a diol (B₂), and cyclotrimerization of a dicyanate ester...