Interphase internal stresses

The new phases are characterized by considerable volume changes in relation to the basic material. The stresses occur on the interphase boundaries, and microcracks are formed. The example of such damage is metal embrittlement when forming hydride phases. The internal stresses also have an effect on kinetics of a new phase growth. Let us consider the residual stresses in a hollow cylinder. Maximal concentration of the impurity atoms occurs on the area boundary, where the new phase formation takes place. Its further growth is realized at the expense of impurity atoms diffusion. The task of defining kinetics of the new phase growth in the hollow cylinder is mathematically formulated as follows... 

From a general point of view, the challenge in the case of reaction-induced phase separation is to create new morphologies (particle sizes at the nanometer scale, transparent two-phases materials, interconnected phases, etc.) and to control the properties of the interface (adhesion, and also the internal stress concentration). For size adjustment, it is possible to superimpose a thermal quench to the reaction-induced phase separation [144], In the case of thermoplastic blends the interfaces or interphases can be modified and modelled by the use of block copolymers, especially triblock terpolymers ABC [145]. A similar systematic approach can be developed in thermoset blends and the results are expected to be different to those obtained by the use of functionalized modifiers [71,138, 146-148]. 

Let us consider a particular example of the effect of RS substances on the water resistance of adhesive-bonded joints. Adhesives based on imsaturated polyester resins, such as PN-1, are distinguished by low water resistance. The influence of water on a steel joint cemented by such an adhesive actually results in some initial increase of the specific electrical resistance along the adhesive-steel interface and then in an abrupt drop (Fig. 5.5). The increase is explained by more complete consumption of the monomer in the system. When ATG is added to the adhesive (which decreases the interphase tension) the specific electrical resistance stabilizes after a drop. The decrease seems to be related to the processes of relaxation of the internal stresses in the adhesive interlayer. The stresses facilitate the diffusion of liquids in polymeric materials, in particular the stress concentration at the polymer-metal interface. 

Thus, minimizing the adhesive-substrate interphase tension is a necessary condition for obtaining both strong and liquid-resistant adhesive-bonded joints. But it is not necessarily sufficient, because the strength and the water resistance can be determined by various factors, such as internal stresses, low hydrolytic stability, etc. 

Two other specific problems complicate the process of sealing pipelines with adhesives. One is that gas in gas mains is transported under 70-75 atmospheres pressure, and noticeable increase of gas main diameter is observed under this pressure. Thus, on the adhesive-metal interphase boimdary heavy internal stresses occur, which can lead to strength reduction or to failure of the adhesive joint. Joint failiu-e will also be promoted by vibration, which occurs with passage of fluid or gas through a deformed section of the pipe. 

Keywords particulate filled composites, filler, aggregate, homogenization, mixing, internal mixer, single-screw extruder, twin-screw extruder, mechanical properties, tensile yield stress, tensile strength, stiffness, impact resistance, structure-property relationships, interface, interphase, reactive treatment, nonreactive treatment, surfactant, encapsulation, functionalized PP, coupling, specific surface area, application. 

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