Epoxy monomers

General Electric Co.) were added to the acrylic and epoxy monomers, respectively, to initiate photopolymerization. The acrylic monomers that polymerized by a free-radical mechanism were irradiated under nitrogen. The output of the lamp measured at the substrate was about 4 mW/cm2 at 260 nm and about 11 mW/cm2 at 310 nm. 

Cationic photoinitiators are used in coatings, printing inks, adhesives, sealants, and photoresist applications. Most of the applications involve vinyl ether polymerizations or ringopening polymerizations of epoxy monomers (Sec. 7-2b). 

Mixtures of PEI/epoxy monomer and PEI/BPACY monomer were prepared by dissolving PEI and thermoset monomers in methylene chloride at room temperature and evaporating the solvent. The epoxy monomer/PEI solution was heated on a hot plate to 140°C, and stoichiometric amounts of DDS were added while stirring for about 7 min. For the catalyzed system, 2-methyl imidazol catalyst was also added. 

The mixtures of PEI/(epoxy monomer+DDS) and PEI/BPACY monomer cured isothermally in the aluminum mold according to the schedule shown in Table 3.1. 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

Alteration of this epoxy structure is the result of the fact that the epoxy molecules are both reacting and diffusing at the same time. This process forms a concentration gradient with a high epoxy monomer concentration at the surface which gradually reduces to the bulk concentration away from the surface. The properties of an epoxy with an excess of resin can be quite different from the stoichiometric amount. Figure 2, for example, illustrated the alteration of cured epoxy mechanical properties with epoxy/amine ratio. Excess epoxy or less than the stoichiometric amount of amine produces a brittle material if the mixture is cured in the same manner as the stoichiometric amount (Fig. 2). The stoichiometric sample has the lowest modulus while excess amine produces increased brittleness. The potential for variation in local properties within the epoxy due to the presence of a 200 nm or less layer must be considered. 

Another type of thermoset polymers is based on epoxy monomers. These thermosets are cured by use of a two-component system or by photo initiators. Disadvantages of epoxies are high water uptake in service and polymerization shrinkage (22). 

Figure 8. Schematics of the epoxy monomer N,N-(diglycidyl)-4-nitroaniline (a.) and of the amine monomer N-(2-aminophenyl)-4-nitroaniline (b.) Also shown is the SHG signal vs time at 80C for the already annealed sample.

It is well established that formaldehyde reacts with amino groups on protein side chains (Fig. 7.2), introducing mostly reversible protein crosslinks. Epoxy monomer reacts with the... 

The epoxy or oxirane group is characterized by its reactivity toward both nucleophilic and electrophilic species and it is thus receptive to a wide range of reagents. Epoxy monomers polymerize through step-growth and chain-growth processes. The ionic polymerization initiated by both Lewis bases or acids will be discussed later (Sec. 2.3.4). The case of polyaddition polymerizations is mainly represented by epoxy-amine reactions. 

A major type of epoxy monomers is that derived from the reaction of bis(4-hydroxy phenylene)-2,2 propane (called bisphenol A) and 1-chloropropene 2-oxide (called epichlorohydrin) in the presence of sodium hydroxide (condensation reaction). The structure of the major product, bisphenol A digly-cidyl ether (DGEBA) and its condensed forms (Table 2.7a), is dependent upon the stoichiometry of the reactants. 

Another major group of epoxy monomers derived from epichlorohydrin is that comprising monomers synthesized with an aromatic amine, such as aniline (DGA), para-aminophenol (TGpAP) and methylene dianiline (TGMDA) - (Table 2.1 e, f, and g). The reaction of epichlorohydrin with an alcohol is more difficult. Liquid monomers based on butanediol, neo-pentylglycol, and polypropylene oxide (molar mass in the range of 500 g mol-1), are the most common. 

Table 2.7 Main commercial epoxy monomers (resins)... 

The mechanisms of these reactions have been the subject of considerable controversy for many years those presented in Tables 2.21 and 2.22 are still questionable. As in the case of classical chain polymerization, many (unknown) transfer reactions with impurities, or with the polymer formed, stop the chain propagation, which explains why degrees of polymerization less than 10 are obtained when a monofunctional epoxy monomer is poly-... 

Table 2.21 Different steps of the anionic polymerization of an epoxy monomer... 

The reactivity of cycloaliphatic epoxy monomers with cations is higher than that of glycidyl ether monomers. For this reason, cycloaliphatic epoxies are used for UV-cure coatings. The most efficient initiators are complex aromatic salts of Lewis acids such as diaryl iodonium, triarylsul-fonium, or arene diazonium (Table 2.23) (Crivello, 1999). 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

The ring-opening polymerization of cyclic monomers can be performed by ionic chain polymerization, as is the case of epoxy monomers. Anionic polymerization of ethylene oxide propylene oxide, and caprolactone can be initiated by alkoxides ... 

Figure 3.13 shows the variation of the gel conversion of the limiting reactant as a function of the stoichiometric ratio. For r > 3, no gel is formed and the polymer remains in the liquid state after complete reaction of epoxy groups. If the amount of epoxy monomer necessary to obtain a stoichiometric system is added in a second step, polymerization restarts, leading to gelation and the formation of a network. The two-step polymerization is the basis of several commercial thermosetting polymers. 

Figure 4.16 TTT diagram for a trifunctional epoxy monomer, triglycidyl ether of tris(hydroxyphenyl)methane, cured with 4,4 -diaminodiphenylsulfone (DDS), under He. (Chan et ai, 1984 - Copyright 2001 - Reprinted by permission of John Wiley Sons, Inc.)...

Figure 8.2 Schematic phase diagrams for thermoplastic-epoxy monomer (diglycidyl ether of bisphenol A) blends, (CPC = cloud point curve, and VC = vitrification curve), (a) and (b) UCST (upper critical solution temperature) behaviour for PPE and PEI (respectively) - DGEBA n = 0.15 (c) LCST (lower critical solution temperature) behaviour for PES-DGEBA y n = 0.15. (Pascault and Williams, 2000 - Copyright 2001. Reprinted by permission of John Wiley Sons Inc.)...

Experimental cloud-point curves are fitted by Eq. (8.1), selecting an adequate function for /. Depending on their structures, thermoplastics are more or less soluble in epoxy monomers poly(methyl methacrylate), PMMA and poly(styrene-co-acrylonitrile), SAN are quite soluble in liquid DGEBA, but the other thermoplastics shown in Table 8.1 are only partially miscible (Pascault and Williams, 2000). 

At their melting temperature these powders can be dissolved in the epoxy monomers and they are able to react and participate in the cross-linking reaction through the amide groups (Lennon et al., 2000). For this reason, the cure cycle must be selected in order to keep the polyamide particles below their melting point (in the range 170°C or 220°C, depending on the type of polyamide used), and thus keep their initial shape and size. But in some cases a partial dissolution of the powder surface can improve the particle-polymer network interactions. 

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