Nylon brittle-ductile transition

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

Impact Modifiers. Notched impact strength and ductility can be improved with the incorporation of impact modifiers, which can also lower the brittle-ductile transition temperature and give much improved low temperature toughness. Impact modifiers are rubbers (often olefin copolymers) that are either modified or contain functional groups to make them more compatible with the nylon matrix. Dispersion of the rubber into small (micrometer size) particles is important in order to obtain effective toughening (19). Impact modifiers can be combined with other additives, such as glass fiber and minerals, in order to obtain a particular balance of stiffness and toughness. Modified acrylics, silicones, and polyurethanes have also been proposed as impact modifiers. 

With polymers there is the added complication of there being a strong dependence of the mechanical properties upon strain rate as well as upon temperature. For example, nylons can be cold-drawn at room temperature when relatively low strain-rates are used, but become brittle as the strain-rate is increased. It is thought that this is because the curves of yield stress and brittle stress against temperature are moved to the right by increasing the strain-rate and so the temperature of the brittle-ductile transition is increased (Fig. 5.51(b)). 

Residual strains in injection moulded Nylon-6 have been measured but the stress distribution is parabolic, with a compressive stress as much as 6 Mn m on the surface and a tensile one in the middle of 4 Mn m dimension in water reverses this distribution making it tensile on the surface. This has been attributed to recrystallization in presence of water and not simple to adsorption and volume changes. The yield and fracture toughness of the Nylon-6 is affected by water content, moulding conditions due to changes in yield stress. There is a brittle to ductile transition with temperature associated with cc-transition. The yield stress increases linearly with crystallinity, whereas the fracture toughness falls consistent with a move from plane strain to plane stress conditions. 

Figure 4 Impact behavior of nylon 66 toughened with a functionalized ethylene-propylene rubber at different loadings (BD= brittle to ductile transition).

Above I wt% of fibres, for both coatings K c increases steeply with the amount of reinforcement, the higher values being obtained with the type A fibres which are strongly bonded to the nylon. As the fibres concentration is increased, higlier stress levels are needed to develop the frontal plastic zone. This trend was analysed in terms of the mean distance between the fibre ends, d, [5, 37] and the results confirm a transition from brittle to ductile behaviour when d is lower than about 60. im which is six times the fibre diameter (Fig. I Ib). 

Commercial impact modified PPE/PA blends exhibit notched Izod impact strengths ranging from 175 to 500 J/m at room temperature. They also differ in their ductile brittle transition temperature and low temperature impact behavior. The type of nylon used (PA-6 or PA-66 or copolymer type), its end group concentrations and molecular weight, and more importantly, the nature of the rubber modifier used (compatible... 

Flexman, E. A. Jr. (1979) Impact behavior of Nylon-66 compositions ductile-brittle transitions, Polymer Eng. Sci., 19, 564-571. 

Nylon 6/EPR-g-MA EPR-g-MA 0-25 wt% Cloisite 30B Nylon 6 phase Impact strength increased with the addition of elastomer at a fixed MMT content. Ductile-brittle transition temperature decreased with the addition of elastomer and with the decrease in the MMT ctmtent Ahn and Paul 2006... 

Another valued result in determining performance is obtained by studying impact behavior as a function of temperature. Materials that behave in a ductile fashion at room temperature become brittle at a low temperature. This transition in mechanical behavior is known as the Brittle to Ductile Temperature. Figure 5 gives an example of the temperature dependence of the toughness of nylon 6 modified with core/shell particles. 

Oshiiiski A J, Keskkula H and Paul D R (1996) The role of matrix molecular weight in rubber toughened nylon 6 blends III - Ductile-brittle transition temperature. Polymer 37 4919-4928. 

Polycarbonate is perhaps the most notoriously notch-sensitive of all thermoplastics, although nylons are also susceptible to ductile/brittle transitions in failure behaviour caused by notch sharpening. Other plastics such as acrylic, polystyrene and thermosets are always brittle - whatever the crack condition. 

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