Epoxy crosslinked polymers

Broutman and McGarry [98] examined the effects of crosslinking on toughness as early as 1965. Bell [99] observed a threefold increase in notched impact strength as the molecular mass between crosslinks was increased. Schmid et al. [100] and Lohse et al. [101] pointed out the dominating effect of molecular strand length on the ultimate properties and the toughness of crosslinked polymers. Later, Batzer et al. [46], Schmid [44], and Fischer et al. [45] compared the behavior of various networks composed of epoxy resins. 

As is usually characteristic of crosslinked polymers of commercial importance, epoxy resins are prepared in two stages, with the initial reaction leading to a linear prepolymer and the subsequent reaction introducing the crosslinks between the molecules. The prepolymers from which epoxy resins are prepared are diglycidyl ethers with the structure shown in Figure 4.2. 

Bisphenol F (BPF) is a mixture of three isomers 2,2 -, 2,4 -, and 4,4 -dihydroxydiphenylmethane, in the ratio 15%, 50% and 35%, respectively. It has also found application in the manufacture of epoxy resins, but as a fully crosslinked polymer it is rarely used in food-contact materials. Residues of... 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

The next breakthrough came with the discovery of how to make almost linear polymers with just a few crosslinked connections between molecules. These materials will not flow after crosslinking, and they become elastomers. A final type is the heavily crosslinked polymers such as epoxy and urea-formaldehyde, which are processed as monomers and then crosslinked at a suitable time and rate to make paints, adhesives, and very hard and durable plastic materials. 

The chemical structure of the epoxy matrix constituent as well as processing are reported to strongly influence 11 -I3> the thermoset network and hence the properties and durability of the crosslinked polymer 11 ,4-16). The cure of a reactive prepolymer involves the transformation of low-molecular-weight reactive substances from liquid to rubber and solid states as a result of the formation of a polymeric network by chemical reaction of some groups in the system. Gelation and vitrification are the two macroscopic phenomena encountered during this process which strongly alter the viscoelastic behavior of the material. 

Figure 11.2 Shape of relaxation maps (coordinates of transitions a, p, and y in a graph In (frequency) - reciprocal temperature). Left polymers having their a and p transitions well separated (example polycarbonate, amine-crosslinked epoxy). Right polymers with close a and p transitions (example polystyrene, unsaturated polyester).

We can prepare functionalized chromophore and crosslinking reagents in a thermosetting composition that is, the chromophores and crosslinking reagents can react to form a three-dimensional crosslinked polymer matrix. Thermosetting polyurethane, sol-gel, or epoxy chemistries are frequently used. Many variations on the above themes can be and have been executed for example,... 

An investigation of the thermal and thermooxidative stability of crosslinked polymers obtained from BPA/DC and BPA/DC compositions with BPA/ECH epoxide resin or epoxynovolak resin has demonstrated that the epoxy resin decreased the thermal stability of BPA/DC based non-modified polycyanurate [71]. The systems with epoxynovolak resin behave somehow better. 

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. 

The middle infrared region of the cured crosslinked polymers shows bands in the 3480 cm"i, the 1200-1300 cm" and the 1000-1100 cm l region, the latter two increase with phthalic anhydride concentration. A band at 1420 cm"l along with the 3480 cm i 0-H stretch band appears in the spectra of the epoxy resin cured with curezol only. This band is a weak band in the uncured epoxy resin and is interpreted to be associated with a secondary hydroxyl group arising from epoxy ring opening (1 ). Bands at 1080 cm"i and 1290 cm"i also are present and are probably the C-0 stretch... 

We also will have little to discuss about the behavior of non-crazable polymers which range from the very tough inflexible chain polyimides to the quite brittle epoxies, and other extensively crosslinked polymers. For a summary of the behavior of the latter the reader is referred to the work of Kinloch... 

Yarovsky, L Evans, E. Computer simulation of structure and properties of crosslinked polymers application to epoxy resins Polymer 2001, 43 (3), 963-969. 

Hartmann and Lee [20] extended the types of calculations possible by using Ur to crosslinked polymers. They then developed [21] an alternative additive quantity (not favored by us, and presented here mainly for historical reasons) similar to UR, to calculate the shear modulus. Although the additive quantity for the shear modulus was initially developed [21] by using crosslinked epoxies as test cases, it is also useful for the much simpler uncrosslinked polymers. In the latest edition of his book, van Krevelen [7] provided tentative values for the group contributions to this new additive quantity, which he named the molar Hartmann function UH. Ujj has the same units as UR. Its value can be used in Equation 11.18 to estimate the shear... 

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