Polymers, thermosetting

Thermosetting polymers (also called thermosets) are a family of plastics characterized by the fact that they are formed starting from a liquid solution that irreversibly leads to a solid material during a heating step. In this sense, they exhibit an opposite behavior than the one of thermoplastic polymers that, with some exceptions, show a reversible solid-to-liquid transition when heated to a convenient temperature. Although the irreversible liquid-to-solid transition can be also produced by other means such as UV or electron beam irradiation, the resulting products are also called thermosetting polymers. The process by which the initial liquid solution is transformed into a solid is usually known as the cure of the material. 

The only possible action is to mill the solid material and use it as a filler. Knowledge of the chemical reactions taking place and the reaction heat is essential to design a cure process. Thermoplastic polymers are easier to process in the sense that their transformation from a solid to a liquid (heating) and back to a solid (cooling) is reversible. If there is a mistake, the final material (e.g., without the required thickness, size, or shape) can be recycled to the process. 

Many other families of thermosetting polymers have also found their niches in the global market. Some common examples are urea-formaldehyde and melamine-formaldehyde polymers, epoxies, unsaturated polyesters (UPs), and polyurethanes (PUs). Some typical 

Handbook of Polymer Synthesis, Characterization, and Processing, First Edition. Edited by Enrique Saldivar-Guerra and Eduardo Vivaldo-Lima. 2013 John Wiley Sons, Inc. PubUshed 2013 by John Wiley Sons, Inc. 

Although epoxies are mainly classified as thermosets, it is also possible to produce linear epoxy polymers using comonomers with two reactive sites per molecule. These linear polymers behave as thermoplastics and can be amorphous or semicrystalline. They exhibit some outstanding optical and barrier properties. Similarly, PUs can be either thermoplastics or thermosets depending on the number of reactive sites per molecule of monomers and comonomers. 

Manufacture of components from thermosetting resins or from rubbers involves not only shaping the material, but also carrying out an exothermic chemical reaction. Proper control of temperature is required in order to obtain an acceptable rate of reaction without overheating and causing unwanted reactions which lead to thermal runaway. It may therefore be necessary to 

The maximum stress occurs at the midspan, in the outer fibres, and is given by 

The maximum shear stress occurs in the neutral plane at the centre of the bar 

These equations hold for small deflections, when the polymer is linearly viscoelastic. 

Values of modulus determined in tension or flexure at one or more temperatures are provided in tables of data supplied by manufacturers. Whilst these single-point data are useful for materials selection, they are obviously inadequate for detailed design of load-bearing components. Here the engineer must look for information about time-dependence, which is usually obtained 

The thermoset plastics generally have superior abrasion and dimensional stability characteristics compared with the thermoplastics, which have better flexural and impact properties. In contrast to the thermoplastics, thermosetting polymers, as the name implies, are changed irreversibly from fusible, soluble products into highly intractable cross-linked resins that cannot be molded by flow and so must be fabricated during the cross-linking process. Typical examples are  

Phenolic resins, prepared by reacting phenols with aldehydes. They are used for electrical fitments, radio and television cabinets, heat resistant knobs for cooking utensils, game parts, buckles, handles, and a wide variety of similar items. 

Amino resins are related polymers formed from formaldehyde and either urea or melamine. In addition to many of the uses listed earlier, they can be used to manufacture lightweight tableware, and counter and table surfaces. Being transparent they can be filled and colored using light pastel shades, whereas the phenohcs are already rather dark and, consequently, have a more restricted color range. 

Thermosetting polyester resins are used in paints and surface coatings. In these, oxidation during drying forms a cross-linked film, which provides a tough, resistant finish. 

Epoxy resins are polyethers prepared from glycols and dihalides and are extensively used as surface coatings, adhesives, and flexible enamel-like finishes because of their combined properties of toughness, chemical resistance, and flexibility. 

Phenol Formaldehyde (PF). Phenol formaldehyde is known for its high strength, stiffness, hardness and its low tendency to creep. It is also known for its high toughness, and depending on its reinforcement, it will also exhibit high toughness at low temperatures. PF also has a low coefficient of thermal expansion. Phenol formaldehyde can be compression molded, transfer molded and injection-compression molded. Typical applications for phenol formaldehyde include distributor caps, pulleys, pump components, handles for irons, etc. It should not be used in direct contact with food. 

The condensation of a diphenol, (bisphenol-A), with dicarboxylic acid 

The reaction of epichlorohydrin with bisphenol-A forms the diepoxy or diglycidyl ether. 

The reaction of the diglycidyl bis-phenyl ether with a polydiol or polydiamine will result in a hardened thermosetting resin. Other thermosetting polymers are discussed in the next section. 

Polymers which form 3D network solids are thermosetting and decompose when heated and thus cannot be reshaped once they have set. A selection of the common polymers is listed in Table 11.3. 

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In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

Polymers are characteri2ed as thermosetting and thermoplastic with respect to the methods by which they are joined. Thermosetting polymers are permanently hard and do not soften upon the apphcation of heat they are joined by mechanical fasteners and adhesives. Several methods have been devised to join thermoplastic polymers, as weU as thermoplastic composite materials, which soften upon heating. 

Stabilization of the Cellular State. The increase in surface area corresponding to the formation of many ceUs in the plastic phase is accompanied by an increase in the free energy of the system hence the foamed state is inherently unstable. Methods of stabilizing this foamed state can be classified as chemical, eg, the polymerization of a fluid resin into a three-dimensional thermoset polymer, or physical, eg, the cooling of an expanded thermoplastic polymer to a temperature below its second-order transition temperature or its crystalline melting point to prevent polymer flow. 

Amino resins are thermosetting polymers made by combining an aldehyde with a compound containing an amino (—NH2) group. Urea—formaldehyde (U/F) accounts for over 80% of amino resins melamine—formaldehyde accounts for most of the rest. Other aldehydes and other amino compounds are used to a very minor extent. The first commercially important amino resin appeared about 1930, or some 20 years after the introduction of phenol—formaldehyde resins and plastics (see Phenolic resins). 

Many different thermosetting polymers are used in pultmsion, eg, polyester, vinyl ester, epoxy, and urethane. Reinforcements must be in a continuous form such as rovings, tows, mats, fabrics, and tapes. Glass fibers are the low cost, dominant composition, but aramid and carbon fibers are also used. 

Thermosetting Reactive Polymers. Materials used as thermosetting polymers include reactive monomers such as urea—formaldehyde, phenoHcs, polyesters, epoxides, and vinyls, which form a polymerized material when mixed with a catalyst. The treated waste forms a sponge-like material which traps the soHd particles, but not the Hquid fraction the waste must usually be dried and placed in containers for disposal. Because the urea—formaldehyde catalysts are strongly acidic, urea-based materials are generally not suitable for metals that can leach in the untrapped Hquid fractions. Thermosetting processes have greater utiHty for radioactive materials and acid wastes. 

Synthetic Resins. Various polymers and resins are utilized to produce some specialty carbon products such as glassy carbon or carbon foam and as treatments for carbon products. Typical resins include phenoHcs, furan-based polymers, and polyurethanes. These materials give good yields of carbon on pyrolysis and generally carbonize directly from the thermoset polymer state. Because they form Httle or no mesophase, the ultimate carbon end product is nongraphitizing. 

It is found that a force F will inject a given weight of a thermosetting polymer into an intricate mould in 30 s at 177°C and in 81.5 s at 157°C. If the viscosity of the polymer follows an Arrhenius Law, with a rate of process proportional to calculate how long the process will take at 227°C. 

Thermoplastics are the largest class of engineering polymer. They have linear molecules they are not cross-linked, and for that reason they soften when heated, allowing them to be formed (ways of doing this are described in Chapter 24). Monomers which form linear chains have two active bonds (they are bifunctional). A molecule with only one active bond can act as a chain terminator, but it cannot form a link in a chain. Monomers with three or more active sites (polyfunctional monomers) form networks they are the basis of thermosetting polymers, or resins. 

If the bismaleimide-amine reaction is carried out with a deficiency of amine the polymer will have terminal double bonds which allows a cure site to give a thermosetting polymer via a double bond polymerisation mechanism. This approach was developed by Ciba-Geigy with their product P13N (Figure 18.42). 

Methacrylic acid and its esters are useful vinyl monomers for producing polymethacrylate resins, which are thermosetting polymers. The extruded polymers are characterized by the transparency required for producing glass-like plastics commercially known as Plexiglas ... 

In these reactions, the monomers have two functional groups (whether one or two monomers are used), and a linear polymer results. With more than two functional groups present, crosslinking occurs and a thermosetting polymer results. Example of this type are polyurethanes and urea formaldehyde resins (Chapter 12). 

Acid catalysts, such as metal oxides and sulfonic acids, generally catalyze condensation polymerizations. However, some condensation polymers form under alkaline conditions. For example, the reaction of formaldehyde with phenol under alkaline conditions produces methy-lolphenols, which further condense to a thermosetting polymer. 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

Amino resins are condensation thermosetting polymers of formaldehyde with either urea or melamine. Melamine is a condensation product of three urea molecules. It is also prepared from cyanamide at high pressures and temperatures ... 

The product is a linear random copolymer that can he cured to a thermosetting polymer. This is made possible through the presence of some unsaturation from isoprene. 

These are thermoset polymers made from phenol or, less commonly, phenolic-type compounds such as the cresols, xylenols, and resorcinol, together with formaldehyde. They had been known for some time - G.T (later Sir Gilbert) Morgan discovered them in the early 1890s when attempting (unsuccessfully) to make artificial dyestuffs by reaction of phenol with formaldehyde. But this knowledge had not been exploited before 1907, the year in which Leo... 

Amino resins are those polymers prepared by reaction of either urea or melamine with formaldehyde. In both cases the product that results from the reaction has a well crosslinked network structure, and hence is a thermoset polymer. The structures of the two parent amino compounds are shown in Figure 1.1. 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and poly ethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different diisocyanates are used commercially typical examples are shown in Table 1.2. 

Composites consist of two (or more) distinct constituents or phases, which when combined result in a material with entirely different properties from those of the individual components. Typically, a manmade composite would consist of a reinforcement phase of stiff, strong material, embedded in a continuous matrix phase. This reinforcing phase is generally termed as filler. The matrix holds the fillers together, transfers applied loads to those fillers and protects them from mechanical damage and other environmental factors. The matrix in most common traditional composites comprises either of a thermoplastic or thermoset polymer [1]. 

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