Polymer properties epoxy

Cycloahphatic diamines react with dicarboxyUc acids or their chlorides, dianhydrides, diisocyanates and di- (or poly-)epoxides as comonomers to form high molecular weight polyamides, polyimides, polyureas, and epoxies. Polymer property dependence on diamine stmcture is greater in the linear amorphous thermoplastic polyamides and elastomeric polyureas than in the highly crosslinked thermo set epoxies (2—4). 

As it has been noted above, at present it is generally acknowledged [2], that macromolecular formations and polymer systems are always natural nanostructural systems in virtue of their structure features. In this connection the question of using this feature for polymeric materials properties and operating characteristics improvement arises. It is obvious enough that for structure-properties relationships receiving the quantitative nanostructural model of the indicated materials is necessary. It is also obvious that if the dependence of specific property on material structure state is unequivocal, then there will be quite sufficient modes to achieve this state. The cluster model of such state [3-5] is the most suitable for polymers amorphous state structure description. It has been shown, that this model basic structural element (cluster) is nanoparticles (nanocluster) (see Section 15.1). The cluster model was used successfully for cross-linked polymers structure and properties description [61]. Therefore, the authors of Ref [62] fulfilled nanostmetures regulation modes and of the latter influence on rarely cross-linked epoxy polymer properties study within the frameworks of the indicated model. 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Although the main use of impact modifiers is with thermoplastics, thermosets also benefit. The agent is added at the monomer stage. Thus epoxy polymers can be made less brittle by the addition of rubbers. Care has to be taken that the high temperature properties of the thermoset are not compromised. 

It is evident that relaxation studies in the solid state can look at the motions which are responsible for the mechanical properties of the cured epoxy systems 43). Therefore, Garroway, Moniz and Resing continued to do relaxation studies 61). Garroway, et al. looked at four epoxy polymers based on the DGEBA resin. Two of the epoxy resins were cured with amines and the other two were cured with anhydrides. Proton enhanced spectra of the epoxy systems were generated. The solid state spectra were compared to the solution spectra of the unreacted epoxy. The< epoxy resin of interest was again DGEBA which was reacted with ... 

The dynamic mechanical properties of the siloxane-modified epoxy networks were also investigated. The DMTA curves for the control epoxy network exhibit the two major relaxations observed in most epoxy polymers 39 40,41>. A high temperature or a transition at 150 °C corresponds to the major glass transition temperature of the network above which large chain motion takes place. The low temperature or (5 transition is a broad peak extending from —90° to 0 °C with a center near —40 °C. It has been attributed predominantly to the motion of the CH2—CH(OH)—CH2—O (hydroxyether) group of the epoxy 39-40 2 ... 

The most common advanced composites are made of thermosetting resins, such as epoxy polymers (the most popular singlematrix material), polyesters, vinyl esters, polyurethanes, polyimids, cianamids, bismaleimides, silicones, and melamine. Some of the most widely used thermoplastic polymers are polyvinyl chloride (PVC), PPE (poly[phenylene ether]), polypropylene, PEEK (poly [etheretherketone]), and ABS (acrylonitrile-butadiene-styrene). The precise matrix selected for any given product depends primarily on the physical properties desired for that product. Each type of resin has its own characteristic thermal properties (such as melting point... 

Oligophenylethoxysiloxanes are used as modifiers for various polymers to improve their weather resistance and other technical characteristics, as well as to increase the heat resistance of coatings. E.g., PES-50 is used to modify polyethers, aciylic and epoxy polymers PES-80 is used to modify alkyd and urea-formaldehyde resins. Besides, PES-80 is used as an additive in paints and enamels (to improve their flow properties, gloss and colour), as well as in concrete mixes (to improve the water resistance and durability of concrete works). 

Other interesting examples of the behaviour of the considered samples are given in Fig. 32. Samples with high Tg which are chemically less defective have lower E25 values. These anomalies in the mechanical properties of epoxy polymers prepared at low Tcure are very difficult to understand in terms of conventional concepts of the relationship between the structure of polymer glasses with their mechanical properties. 

Table 2 Mechanical and physical properties of the unmodified and modified DGEBA/DDM epoxy polymers. Data taken from [85,86], AM Acetamide derivative (see Table 1)...

Moisture-Temperature Effects on the Dynamic Mechanical Properties of Epoxy Polymers... 

H. S. Chu, Processing-Structure-Property Relations for High Performance Amine-Cured Epoxy Polymers, M. S. Thesis, Department of Chemical Engineering, University of Washington, Seattle, Washington, (1980). 

These examples barely touch the wide variety of epoxy polymer structures and curing reactions. They illustrate the point that the latent functionality of the prepolymers and the chemical and mechanical properties of the linal polymeric structures will vary with the choice of ingredients and reaction conditions. 

Epoxy polymers (including epoxy novolacs) have been designed to meet most of these requirements and are almost universally used in such encap-sulant applications. Epoxy polymers exhibit superior adhesion that in many cases eliminates the need for a barrier or junction coating. They have a low coefScient of thermal expansion low shrinkage and low injection velocity, which means that low transfer or injection pressures can be used. These polymers also possess excellent mechanical properties coupled with low moisture and gas permeability. Above all, they are cheap and readily available. Other transfer-molding materials used to a limited extent include silicones, phenolic materials, and even polyesters. Most molding formulations are highly filled (70-75%) with materials such as quartz, fused silica, short... 

Composite piezoelectric transducers made from poled Pb-Ti-Zr (PZT) ceramics and epoxy polymers form an interesting family of materials which highlight the advantages of composite structures in improving coupled properties in soilds for transduction applications A number of different connection patterns have been fabricated with the piezoelectric ceramic in the form of spheres, fibers, layered, or three-dimensional skeletons Adding a polymer phase lowers the density, the dielectric constant, and the mechanical stiffness of the composite, thereby altering electric field and concentrating mechanical stresses on the piezoelectric ceramic phase. 

Polymer properties are highly sensitive to temperature with transitions between physical states typically occurring over many tens of degrees Celsius (A). Additionally, the properties are sensitive to the rate at which the temperature changes. For example, the apparent glass transition temperature of a given polymer sample increases with the rate of temperature scan. For thermosets (such as epoxies and polyimides) the thermal history is especially important because of its coupled effect on the physical state of the polymer and the reaction kinetics (11). 

Predictive modeling of rubber-toughened epoxy polymers is a powerful tool for the investigation of failure mechanisms. The new material model described in this chapter is important for modeling both the mechanical properties and the fracture processes. We have shown that the exact nature of the properties of the rubber may influence the sequence of failure events in the material. 

Novel hydroxyurethane modifiers (HUM) for cold-cured epoxy composite materials were synthesized. It is established that the compositions with HUM demonstrate a significant increase in the speed of the curing process, a nontrivial increase in abrasion resistance, and a marked improvement in strength properties. The HUM, which possesses a wide range of hydrogen bonds, is embedded in an epoxy polymer network without a direct chemical interaction. 

Primary and secondary aliphatic polyamines, their derivatives, and modified aliphatic polyamines and aromatic amines react with and cure epoxy resins as indicated earlier. The aliphatic systems usually give adequate cures at room temperature (7 days above 60 F) however, under most conditions aromatic amines are less reactive and require curing temperatures of about 300 F to give optimum cured polymer properties. 

Reiser reports that in order not to terminate the reaction and hence inhibit propagation, the counter anion must have very low nucleophilicity, since strong nucleophiles or bases will terminate the reaction immediately. Nevertheless, the polymerization can tolerate a small amount of water (1-2%), which is important for the practical usefulness of the system. Oxygen, which acts as a biradical, shows no effect on cationic polymerization—quite an important practical advantage. Characteristically, the epoxy polymers that are the result of the curing process tend to have excellent mechanical properties, including thermal and dimensional stability, nontoxicity, and chemical inertness. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

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. 

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