Interfacial toughening

Yasaee M, Bond LP, Trask RS, Greenhalgh ES. Mode II interfacial toughening through discontinuous interleaves for damage suppression and control. Compos Part A Appl Sci Manuf 2012 43 121-8. http //dx.doi.Org/10.1016/j.compositesa.2011.09.026. 

Compressive interfacial stresses increase the interfacial shear resistance. Although usually detrimental to toughening, these stresses can enhance toughening if bridge pullout is the operative toughening process. 

The important factors that affect the rubber toughening are (1) interfacial adhesion, (2) nature of the matrix, (3) concentration of the rubber phase, and (4) shape and size of the rubber particles. In the PS-XNBR blend containing OPS, due to the reaction between oxazoline groups of OPS and carboxylic groups of XNBR, the interfacial adhesion increases and as a result, the minor rubber phase becomes more dispersed. The immiscible blend needs an optimum interfacial adhesion and particle size for maximum impact property. In PS-XNBR, a very small concentration of OPS provides this optimum interfacial adhesion and particle size. The interfacial adhesion beyond this point does not necessarily result in further toughening. 

Reactive impact modifiers are preferred for toughening of PET since these form a stable dispersed phase by grafting to the PET matrix. Non-reactive elastomers can be dispersed into PET by intensive compounding but may coalesce downstream in the compounder. Reactive impact modifiers have functionalized end groups. Functionalization serves two purposes - first, to bond the impact modifier to the polymer matrix, and secondly to modify the interfacial energy between the polymer matrix and the impact modifier for enhanced dispersion. Some examples of commercially available reactive impact modifiers for PET are shown in Table 14.3. An example of a non-reactive elastomer that can be used in combination with reactive impact modifiers is ethylene methyl acrylate (EMA), such as the Optema EMA range of ethylene methyl acrylates manufactured by the Exxon-Mobil Chemical Company (see Section 4.2). 

Elastomers with reactive end groups such as maleic anhydride (MA) or glycidyl methacrylate (GMA) are preferred for toughening PET. The reason that they are so effective is that they form a graft copolymer by reaction with the PET hydroxyl and carboxyl end groups (as shown below). The graft copolymer then acts as an emulsifier to decrease the interfacial tension and reduce the tendency... 

E-EA-GMA (see Table 14.3) and EEA are often used in combination as a toughening system. The optimum blend ratio of reactive elastomers non-reactive elastomers (e.g. Lotader Lotryl) is 30/70. Since the E-EA-GMA terpolymer and EEA copolymer are mutually miscible, when blended together with PET the mixture acts as a single elastomeric phase, which is interfacially grafted to the PET continuous phase. 

Sakai M., Takeuchi S., Fischbach D.B. and Bradt R.C. (1988). Delamination toughening from interfacial cracking in ceramics and ceramic composites. In Proc. Ceramic Microsiruclures 86 (J.A. Pask and A.G. Evans, ed.), Plenum Press, New York, pp. 869-876. 

In Secs. 13.2-13.3 the principles of toughening of thermosets by rubber particles, and the role of morphologies, interfacial adhesion, composition, and structural parameters on the toughening effect are analyzed. Section 13.4 is devoted to the use of initially miscible thermoplastics for toughening purposes. The effect of core-shell rubber particles is discussed in Sec. 13.5 and, in Sec. 13.6, miscellaneous ways of toughening thermosets (liquid crystals, hybrid composites, etc.), are analyzed. 

SBM) as a compatibilizer. As a result of the particular thermodynamic interaction between the relevant blocks and the blend components, a discontinuous and nanoscale distribution of the elastomer at the interface, the so-called raspberry morphology, is observed (Fig. 15). Similar morphologies have also been observed when using triblock terpolymers with hydrogenated middle blocks (polystyrene-W<9ck-poly(ethylene-C0-butylene)-Wock-poly(methyl methacrylate), SEBM). It is this discontinuous interfacial coverage by the elastomer as compared to a continuous layer which allows one to minimize the loss in modulus and to ensure toughening of the PPE/SAN blend [69],... 

Merchant, I.J., Macphee, D.E., Chandler, H.W. and Henderson, RJ. Toughening cement based materials through the control of interfacial bonding , Cement Concrete Research 31 (2001) 1873-1880. 

In both derivations of toughening behavior, increases in toughness for whisker-reinforced composites are dependent on the following parameters (1) whisker strength, (2) volume fraction of whiskers, (3) elastic modulus of the composite and whisker, (4) whisker diameter, and (5) interfacial fracture energies. 

M. H. Rawlins, T. A. Nolan, D. P. Stinton, and R. A. Lowden, Interfacial Characterizations of Fiber Reinforced SiC Composites Exhibiting Brittle and Toughened Fracture Behavior, in Advanced Structural Ceramics, Materials Research Society Symposia Proceedings, Vol. 78, MRS, Pittsburgh, PA, 1987, p. 223. 

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