Semi-solid state, rheology

The attention paid to the polymer solid state is minimized in favour of the melt and in this chapter the static properties of the polymer are considered, i.e. properties in the absence of an external stress as is required for a consideration of the rheological properties. This is addressed in detail in Chapter 3. The treatment of the melt as the basic system for processing introduces a simplification both in the physics and in the chemistry of the system. In the treatment of melts, the polymer chain experiences a mean field of other nearby chains. This is not the situation in dilute or semi-dilute solutions, where density fluctuations in expanded chains must be addressed. In a similar way the chemical reactions which occur on processing in the melt may be treated through a set of homogeneous reactions, unlike the highly heterogeneous and diffusion-controlled chemical reactions in the solid state. 

Perhaps the best way to define semi-solids is based on their rheological characteristics. They show flow behaviour intermediate between liquids and solids, i.e. they are viscoelastic systems. To understand the flow behaviour of semi-solids, let us first consider the characteristics of elastic solids and viscous liquids. Elastic solids follow Hooke s law, which states that the relative strain (y, dimensionless) increases linearly with the applied stress (r/Pa),... 

Collisions between solid particles and semi-solid droplets in the state of solidification are frequent in spray processing of composite particles. The semi-solid or mushy state of the droplet is characterized by a variable viscosity related to the solid fraction in the droplet. For evaluating the effect of the solid fraction f on the penetration behaviour, the apparent viscosity is used to calculate the Reynolds number Re). The rheological model developed in Chen and Fan [53] is employed to evaluate the apparent viscosity in the present analysis ... 

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