Thermoset blend systems

Many commercial thermosets are quite often used as complex mixtures of several co-reacting monomers and prepolymers, specifically formulated to suit a given end-use application. For example, in coating and adhesive applications, often mixtures of different epoxies, differing in chemical structure and/or molecular weights, are used as required. Even two different thermoset- 

Although it is sometimes hard to distinguish some thermoset blends from co-reacting thermosets, thermoset blends can be ideally classified as follows  

B) Thermoset-thermoplastic blends and semi-interpenetrating networks (Semi-IPN) 

As previously mentioned, several commercial hybrid thermosets are known to be co-reacting thermosets, i.e. when the mixture of two different thermosetting monomers or prepolymers is cured, there is a simultaneous graft or co-reaction between the components along with the crosslinking reactions. These systems may therefore be considered as co-polymerizing thermosets and not as true blends. Examples of such systems are phenolic novolak/epoxy resin (or epoxy novolaks)  

Some examples of what may be considered as actual thermosetithermoset blends are the epoxy and bismaleimide thermosets modified with aromatic dicyanate esters such as bisphenol A dicyanate. Aromatic cyanates crosslink by cyclotrimerization to form a network of triazine ethers. 

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Although the dynamically vulcanized blends such as EPDM/PP (Santoprene ) and NBR/PP (Geolast ) have sometimes been referred in the literature as semi-IPNs, we considered them as blends of crosslinked elastomer dispersions in a thermoplastic matrix and as such treated them under the elastomer blends. There is yet another class of thermoplastic/thermoset blend system in which a minor amount of the crosslinkable monomer(s) is allowed to polymerize in the thermoplastic matrix forming a loose network. 

For polymer melts and polymer solutions, numerous excellent reviews [1-5] have provided detailed descriptions of the concepts, test methods and rheological transitions in these systems. The rheological behaviors of thermoplastic blends, including both homopolymers and block copolymers, have also been extensively reviewed [6]. In the case of thermosetting blends, the majority of the information related to rheological studies has been reported in journals. Consequently, after a brief introduction of some of the concepts related to these studies, recent developments in thermosetting blend systems will be reviewed in the following sections. 

Thermosetting Blend Systems with Rubbers and Thermoplastics... 

Figure 2.9. A time-temperature-transformation (TTT) diagram for a phase-separating thermoset/ thermoplastic blend system. Reprinted from Figure 12 from (Kim et al., 1993). Copyright (1993), with permission from Elsevier.

Pascault JP, Williams RJJ. Phase morphology of nanostruc-tured thermosetting multiphase blends. In Harrats C, Thomas S, Groeninckx G, editors. Micro- and Nano structured Multiphase Polymer Blend Systems Phase Morphology and Interfaces. Boca Raton, FL CRC-Taylor and Francis 2006. p 359 ff. 

Daron 40 Unsaturated polyester/polyurethane thermosetting blends for fiber-filled systems DSM... 

Glass Transition Temperature Tg (DSC, TMA, and DMA). The resin systems used in today s copper-clad laminates are almost all three-dimensionally cross-linked thermoset polymers. This means that the materials are manufactured (in a curing reaction) into a hard final product and cannot be remelted many times the way thermoplastic polymers can. In many cases, different resin systems are blended to create a material with specific properties. The functionality of the individual resins and their ratio in the blend define the properties of the thermoset.The degree of cure of the hardened thermoset resin system can be described by characteristics such as percent of uncured monomer and heat of final cure or by mechanical properties such as hardness, modulus, and yield stress. But one of the most commonly used characteristics to describe the degree of cure and cross-hnk density of a thermoset polymer is the glass transition temperature (Tg). 

In a system with a phase inversion structure, the complex viscosity of a blend system can be described by the Einstein equation (Eq. (4.11)). During viscoelastic phase separation, in which the interfadal tension plays a minor effect, the change in curing conversion is quite low and the viscosity difference between thermoplas-tics-rich matrix and dispersed thermoset-rich (low conversion and molecular weight) is very large and increases with the phase-separation process. When the viscosity of the dispersed thermoset-rich phase was neglected, the viscosity of the blend can be simplified as that of the thermoplastic-rich phase. 

Zhu et al. and Huang et al. employed a block copolymer/homopolymer system (poly(ethylene oxide)-h-polystyrene/polystyrene) in order to study the crystallization of PEO under hard and soft confinement [180, 244]. In a related work, Xu et al. prepared blends of poly(oxyethylene)-h-poly(oxybutylene) and polystyrene/poly(oxybutylene) and compared confined versus breakout crystallization [249, 308]. Guo et al. studied a block copolymer/thermoset blend constituted by poly(ethylene)-h-poly(ethylene oxide)/bisphenol A type epoxy resin. The authors reported the nanoconfinement effect on the crystallization kinetics of the PE block [309]. 

Usage of phosphoms-based flame retardants for 1994 in the United States has been projected to be 150 million (168). The largest volume use maybe in plasticized vinyl. Other use areas for phosphoms flame retardants are flexible urethane foams, polyester resins and other thermoset resins, adhesives, textiles, polycarbonate—ABS blends, and some other thermoplastics. Development efforts are well advanced to find appHcations for phosphoms flame retardants, especially ammonium polyphosphate combinations, in polyolefins, and red phosphoms in nylons. Interest is strong in finding phosphoms-based alternatives to those halogen-containing systems which have encountered environmental opposition, especially in Europe. 

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