Amine crosslinked epoxy systems

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 ... 

Three homopolymer (diblock copolymer) phase boundary systems have been studied extensively the system of polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with diblock copolymers ofPS-PVP [22,25,28,31-33], the system of poly(methyl methacrylate) (PMMA) and PS reinforced with diblock copolymers of PMMA-PS [17,24,34,35] and the system of PMMA and poly(phe-nylene oxide) (PPO) reinforced by diblock copolymers of PMMA-PS [ 14,36,37]. Phase boundaries between PS and a crosslinked epoxy (XEp) were reinforced with carboxy-terminated PS chains whose -COOH ends reacted with either excess amines or epoxy to form a grafted brush at the interface [38,39]. In a similar manner, interfaces between rubber-modified PS (HIPS) and XEp reinforced with grafted PS-COOH chains have been investigated [40]. 

It is noted (Carswell et al., 1996) that the concentration of radicals almost reaches millimolar levels in the crosslinking system, i.e. 40 times that for the uncrosslinked polymer. Species other than those responsible for network formation may also be observed, if they are stable, and then used to monitor crosslinking. The oxidation of amine-containing epoxy resins may occur during cure or on UV irradiation, and ESR has been used to identify the cation radical species formed (Figure 3.10) (St John, 1993, Fulton et al., 1998). 

Water induces a similar phase separation at least in the bulk. It breaks the imino ether-like crosslinks specific for the DDA-cured epoxy systems in the bulk as well as in the layers on the two types of stainless steel. The proposed mechanism describes the aging behavior of DDA-cured epoxy systems well, since it explains all the observed effects of chemical modification, irreversible plasticization, and irreversible water uptake. Additionally, it is understood why there is no complete disintegration of the network the hydrolysis cleaves only the imino ether-like crosslinks but not the amine-like or the ether-like crosslinks that are also formed during curing. Hence, after hydro-thermal aging the macromolecu-lar mobility in EP2 is similar in the layers and in the bulk because the content of amine-like and ether-like crosslinks is similar. 

In this paper we address the area of the structure-property relations of epoxy matrices, with specific emphasis on amine-cured epoxides. In attempts to correlate the structure-property relations of amine-cured epoxides, there have been a number of studies on the mechanical properties of these glasses as a function of epoxide amine ratios and the chemical structure of the constituent epoxide and amine monomers.(1-15) Generally, there is no direct correlation between the chemistry of the epoxide and amine monomers and the mechanical properties of epoxies with the exception that as the distance between crosslinks becomes shorter, these glasses become more brittle.(1,10,15) Also, for a specific amine-cured epoxide system, the Tg is always highest for the fully reacted, highest crosslink density glass.(3,6,9,12,... 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

Other common binders include nitrocellulose (of lower than 12.6% nitrogen content, with acetone as the solvent), polyvinyl alcohol (used with water), and Laminae (an nnsaturated polyester crosslinked with styrene—the material is a liquid until cured by catalyst, heat, or both, and no solvent is required). Epoxy binders can also be used in liquid form during the mixing process, and then allowed to enre to leave a final, rigid product the caution here is the fact that many epoxy enring agents are amine compounds, which are basic. Be sure the epoxy system is compatible with all other components in the composition. 

Crosslinkers and Accelerators. Besides the polymers themselves, changes and improvements are taking place in curing agents, hardeners, catalysts, and polymer modifiers. For example, if epoxy adhesives with improved heat resistance are needed, the hardeners can be aromatic anhydrides and amines such as benzophenone tetracarboxylic dianhydride, py-romellitic dianhydride, diaminodiphenyl sul-fone. To enhance curing speed, epoxy systems utilize mercaptan hardeners, while acrylic monomer/polymer blends avail themselves of amine/aldehyde catalysts with benzosulfimide (saccharin) accelerators. Radiation-curing systems for today and tomorrow utilize both visible light and laser beams. 

The original development of non-isocyanate acrylics was targeted in the early 1980 s at the vehicle refinishing market. The products were developed to be direct alternatives to the isocyanate cured acrylics and polyesters used in this market, due to the inherent toxicity problems of the isocyanate crosslinker. However, the slower cure and lower resistance characteristics of the non-isocyanate products restricted their development in this field and wholesale replacement of two packs by non-isocyanates has not taken place. Nevertheless the anhydride/hydroxyl and amine/epoxy systems have developed well in the captive VR markets in North America and Western Europe, respectively. 

The effect of nano reinforcement (amino-POSS) was also dominant in the BAPPO cured systems as compared to the DDM cured systems (Table 3.4). The amine-terminated POSS reinforced tetrcdunctioncil epoxy nanocomposites exhibited the best results for the phosphorus (DOPO-based) skeletal modified tetra epoxy systems. This attention grabbing behavior may be attributed to the presence of additional crosslinking sites offered by POSS nano reinforcement paving the way... 

The products of the chemical degradation of PETP with triethylene tetramine and triethaneolamine can be used as epoxy resin hardeners, it is demonstrated. Products of PETP aminolysis with triethylene tetramine and aminoglycolysis with triethanolamine, were characterised using NMR and rheometric measurements. Characteristics of the crosslinking process for the system epoxy resin/ PETP/amine degradation product, and epoxy resin/TETA for comparison were investigated by DSC. Three classes of liquid epoxy resins based on bisphenol A, bisphenol F and epoxy novolak resins were used in the experiments. 16 refs. 

The plastic deformation in several amine and anhydride cured epoxy resins has been studied. The experimental results have been reasonably interpreted by the Argon theory. The molecular parameters determined from the data based on the theory reflect the different molecular structures of the resins studied. However, these parameters are in similar enough range to also show the structural similarity in these DGEBA based systems. In general, the mechanisms of plastic deformation in epoxy resins below T are essentially identical to those in amorphouE thermoplastics. The yield stress level being related to the modulus that controls the intermolecular energy due to molecular deformation will, however, be affected by the crosslinks in the thermosets. 

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