Blends Involving Thermosetting Polymers

This section will cover polymer blends containing thermosetting polymers as at least one of the constituents. Crosslinked elastomers will not be discussed in this section, as they are covered in Section 4.2. Specific thermosetting polymers include epoxies, phenolics, unsaturated polyesters, bismaleimides, vinyl esters, cyanate esters and polyurethanes, which are the major commercial thermoset polymers. Preparation of thermoset polymer blends comprised of thermoset/thermoset or thermoset/thermoplastic combinations will require in-situ polymerization as melt processing of thermosetting polymers is not possible. The in-situ polymerization procedure can include  

One of the more common thermoset systems is termed epoxy and the most common structure for the epoxy resin component of an epoxy thermosetting systems is based on Bisphenol A, as noted below  

The diglycidylether of Bisphenol A (DGEBA) is where n = 0. With increasing n, the viscosity of the epoxy resin increased imtil it becomes a solid epoxy. At high molecular weight, the structure is basically the polyhydroxyether of Bisphenol A termed Phenoxy (PHE). Epoxidized novolacs (phenol-formaldehyde resins) are also employed in epoxy thermosetting systems. Epoxies are typically crosslinked with di- (or higher) amines (either aliphatic or aromatic). 

The thermosetting polymers noted above are generally high modulus, amorphous materials with a high crosslink density. As such, they are brittle and thus many blends have been inves- 

Cyanate esters are generally prepared from bisphenols and cyanogen bromide to yield the structure depicted below  

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Scanning electron microscopy (SEM) involves scanning an electron beam (5-lOnm) across a surface and then detecting the scattered electrons. Literature abounds, with work focussing on the use of SEM in the fracture and failure of epoxy resins and other thermoset polymers. Also work on multiphase thermosets (thermoset-thermoplastic blends, thermoset nanocomposites, interpenetrating network (IPN) polymers) is abundant. 

The rheological properties of polymer blends - and especially of thermosetting polymer blends - share a close relationship to the morphology and reactions involved. Moreover, these properties can cause changes in shear or strain conditions that may lead to dramatic variations in the phase structure and other properties of polymer blends. The time-temperature superposition principle (WLF-type equations) and power laws have been widely applied to the linear viscoelastic behavior of both neat polymers and blends, despite the fact that they may reflect different types of structure transitions for either thermoplastic or thermosetting resins. 

Chemical engineers have worked with polymers since the first Bakelite articles were produced early in this century. Since then, the class of polymeric materials has grown to encompass a whole range of thermoset and thermoplastic resins, as well as copolymers and polymer blends, and chemical engineers have played major roles in the rise of these materials to commercial success. From production of the resins (which involves heat and mass transfer, kinetics, fluid dynamics, process design, and control) to the fabrication of final articles (involving many of the same processes, as well as some unit operations not part of traditional chemical engineering, such as extrusion... 

While most of the subjects discussed up to this point involve injection molding or extrusion applications, there are a considerable number of other applications where polymer blend technology is and will be employed including adhesives, sealants, coatings, and thermosetting systems, polymeric membranes, and water-based polymers. 

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