Thermosets epoxies

Although the discussion will mainly be concerned with the movement of water molecules in epoxy thermosets, the character of this investigation is rather general and valid for different polymers and penetrants. 

This study reveals the need for separate investigative tools for quantitatively characterizing the influence of manufacturing defects and chemical characteristics on the hygrothermal fatigue response and morphology of epoxy thermosets. 

The growth in production of epoxies is up from 183 million pounds in 1972 [17], to an estimated sales of epoxy products of 464 million pounds in 1990 [18], Epoxy thermosetting resin sales in 1990 are also reported to be 499 million pounds [19]. 

Fig, 3. Transmission electron micrograph of osmium-tetroxide stained section of a typical rubber-modified epoxy thermosetting polymer... 

The complex sorption behavior of the water in amine-epoxy thermosets is discussed and related to depression of the mechanical properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental vapor and liquid sorption tests, differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis. In particular, two different types of epoxy materials have been chosen low-performance systems of diglycidyl ether of bisphenol-A (DGEBA) cured with linear amines, and high-performance formulations based on aromatic amine-cured tetraglycidyldiamino diphenylmethane (TGDDM) which are commonly used as matrices for carbon fiber composites. 

Studies in larger temperature intervals showed that expansion obeys a parabolic law, both for linear polymers (Bongkee, 1985), and epoxy thermosets (Skourlis and McCullough, 1996) ... 

A good agreement was found for thermosets, as demonstrated by Yamini and Young (1980) and Cook et al. (1998). The exponent n for all amorphous thermoplastics was about 1.6 and about 1.9 for epoxy thermosets (Fig. 12.7). As n is significantly higher than 1, Brown s rule cannot be applied. 

The Paris power law is generally well verified with neat epoxy thermosets (Fischer et al., 1995 Sautereau et al., 1995 Rey et al., 1999). The exponent varies between 9 and 17. The crack resistance increases continuously when Me increases AK at da/dN = 1.5 x 10 4 mm/cycle is linearly related to KIc for numerous networks, as already described for thermoplastics (Hwang et al., 1989 Rey et al., 1999). 

Other substances are also used as polymerization processing aids, like solvents (e.g. benzyl alcohol) and accelerators (e.g. nonylphenol). These processing aids are not significantly or not to a measurable degree chemically incorporated into the crosslinked polymer. Under the conditions of the epoxy thermoset reaction the epichlorohydrin, for example, is completely decomposed. Under the current, state of the art hardening technology, practically no epichlorohydrin can be detected in the finished product. 

Curing of epoxy thermosets requires a knowledge of the chemical kinetics and the crosslinking reactions. This information is necessary to optimize the cure cycle. The parameters that define the cure cycle ultimately determine the crosslink density and the final physical properties of the polymer. In addition to temperature, these parameters include the rate of temperature increase, the number of stages in the cure, the hold temperature at each stage, the pressure at which cure takes place, and the time allotted for the cure cycle. These parameters are usually determined empirically. Once the kinetics are understood and the actual chemistry behind the cure is established, these cure cycle parameters can be chosen based on the desired end properties. Usually the cure cycle seeks to establish a certain degree of cure that is in line with the expected final properties. 

Ophir, Z. Structure-Property Relationships in Solid Polymers I — Segmented Polyurethanes and II — Epoxy Thermosets. Ph. D. Thesis, Princeton University (1979)... 

Equations (9) and (10) assume that the reactions are not diffusion oontroUed and only one temperature-independent reaction mechanism is operable. Epoxy thermosetting reactions are actually complex, and complicated kinetic expressions and competing reaction mechanisms have been proposed... 

The microstructure of epoxy thermosets can be complex, and both molecular and physical microstructures are presumed. Unfortunately, the intractable nature of these materials makes direct structural characterization extremely difficult. The most accessible technique for direct structural characterization is evaluation of epoxy rubber-like properties above Tg. Sometimes, indirect characterization of epoxy structure is possible due to the fact that the chemistry of several epoxy systems is well behaved (e.g., epoxy-amine chemistry). This permits epoxy network structure to be modeled accurately as a function of the extent of the crosslinking reaction(s). This approach has been developed extensively by Du ek and coworkers for amine-linked epoxies ... 

The most popular method by which epoxy thermoset structure is altered for structure-property investigations is the intentional variation of the curative/epoxy resin functional group ratio (A/E). Unfortunately, it is impossible to alter independently only one structural feature at a time using this technique. For example, the... 

Above the glass transition temperature, thermosets are weak elastomers (because of their densely crosslinked structure) and are of no known practical use. Apparently, only King and Andrews Swetlin and LeMay have investigated the cohesive fracture or tear of thermosets above Tg, all using amine-linked epoxies. These studies have demonstrated that the rubbery fracture of epoxy thermosets is quite similar to that of more conventional crosslinked elastomers. 

The work of King and Andrews and Swetlin has shown that the rubbery fracture energies of epoxy thermosets are time-temperature superposable and sensitive to network structure. These studies incorporated different amine/DGEBA... 

The fracture behavior of epoxy thermosets has been of growing interest since the mid-1960 s when investigations by Broutman and McGarry and Mostovoy and Ripling were published. Literature references seem to have peaked in the late 1970 s and early 1980 s when studies on crack blunting mechanisms speculations... 

Like the testing variables just described, material variables can influence the fracture behavior of epoxy thermosets. Material variables discussed herein include the types of epoxy resins and amine curatives. 

In recent years new thermoplastic matrices have been developed to improve the stiffness/toughness balance and the service temperature, in comparison to the epoxy thermoset matrices used in high performance composites. These materials, usually referred to as advanced thermoplastic matrices (8), include polymers that have great structural similarities, with aromatic moieties in the main chain spaced by groups of the type diagrammed below. 

Barclay, C.G. McNamee, S.G. Ober, C.K. Papathomas, K.I. Wang, D.W. Liquid crystalline epoxy thermosets mechanical and magnetic alignment. J. Polym. Sci. Polym. Chem. 1992, 30, 1845-1853. 

Phase Separation of Two-Phase Epoxy Thermosets That Contain Epoxidized Triglyceride Oils... 

The preparation and characterization of the VR and ESR liquid rubbers and of the two-phase epoxy thermosets were described in detail previously (3-6). DGEBA is a solid epoxy resin (Epon 825) from Shell Chemical Company. ESO and ELO are commercial products from Atochem. Samples of VO and crambe oil were obtained from the U.S. Department of Agriculture. All other reagents and solvents were purchased from Aldrich Chemical Co. and Fisher Scientific Co. and used without additional purification. 

Preparation of Two-Phase Epoxy Thermosets. Two-phase epoxy thermosets were prepared from homogeneous stoichiometric mixtures of DGEBA and diamine (DDM or DDS), which contained varying amounts of liquid rubber (ESR or VR). The rubber was dissolved first in DGEBA at 70 °C. Then the diamine (DDM or DDS) was added and the mixture was stirred at 70 °C until the diamine dissolved (about 15 min). The homogeneous (one-phase) transparent mixture was then degassed before it reached its cloud point (phase separation). The formulations cross-linked with DDS were cured at 150 °C for 2 h, whereas the formulations cross-linked with DDM were cured first at 75 °C for 4 h and then at 150 °C for 2 h. 

Morphology. The morphology of the fracture surface of the two-phase epoxy thermosets was examined by scanning electron microscopy (SEM, Amray model 1000B). SEM specimens were sputter-coated with a thin film of gold. 

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