Epoxy network impact properties

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. 

Several theories have been proposed to explain the toughening effect of rubber particles on the brittle thermoset matrix. These are based on the fractographic features and fracture properties of rubber-toughened thermoset networks. Garg and Mai [113] proposed 13 mechanisms to explain the impact behaviour of rubber-toughened epoxy... 

For this reason, HMTA cured novolac networks are generally brittle in nature and have low fracture energies and impact strengths, probably due to the voids combined with the high crossliiik densities. Curing novolac resins with epoxy resins can produce networks without volatile by products. These can be reinforced with glass or carbon fibers to yield tough, void-fiee composites, which also retain much of the flame-retardant properties of phenolics [39],... 

Direct-write assembly has been used to embed fully interconnected 3D microchannel network in an epoxy matrix. 3D scaffolds were fabricated with a fugitive organic ink using a robotic deposition apparatus in a layerwise scheme. As the presence of channels impacts the structural properties of the polymer, networks with maximum channel spacing and minimum channel diameter are desirable. Autonomic healing efficiency is evaluated on the basis of the ability of the healed polymer to recover fracture toughness. 

A complete epoxy group conversion is obtained already in the presence of 20 wt% of AZ129. The Tg data are reported in Table 8 together with the impact resistance and E values. It is particular interesting to observe a peculiar results due to the multicrosslinker behaviour of HBP the increase on toughness properties is accompanied with with an increase on E values notwithstanding the flexibilization of the network due to the presence of the HBP flexible structure. 

A few other strategies for the synthesis of epoxy nanocomposites can be foimd in the literature. Brown et al and Salahuddin et al used acetone as a low-boiling solvent to enhance the processability of the network precursors. In both studies, the solvent is used as a processing aid only, i.e the acetone is removed under vacuum before thermal cure is initiated. Whilst the work by Brown et al. did not show any impact on the properties of the material after cure, the processing aid allowed the production of highly filled epoxy nanocomposite systems, as reported by Salahuddin et al ... 

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