Rheological properties characterisation

Much less is known about the settling of particles in fluids exhibiting a yield stress. Barnes (39) suggests that this is partly due to the fact that considerable confusion exists in the literature as to whether or not the fluids used in the experiments do have a true yield stress 39. Irrespective of this uncertainty, which usually arises from the inappropriateness of the rheological techniques used for their characterisation, many industrially important materials, notably particulate suspensions, have rheological properties closely approximating to viscoelastic behaviour. 

Blends of poly (ethylene terephthalate) (PETP) and polypropylene (PP) with different rheological properties were dry blended or compounded, and extrusion foamed using both physical blowing and chemical agents, and the foam properties compared with those of foam produced from the individual components in the absence of compatibilisers and rheology modifiers. The foams were characterised by measurement of density, cell size and thermal properties. Low density foam with a fine cell size was obtained by addition of a compatibiliser and a co-agent, and foamed using carbon dioxide. The presence of PP or a polyolefin-based compatibiliser did not effect... 

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

In order to use these powders for chocolate seeding, a characterisation of their mechanical and rheological properties is needed and this will be done in future work. 

The behaviour of a foam at deformation can be described also by the Newton s law but then the rheological properties would be characterised by a variable viscosity [20]... 

The Role of Gases and Specific Ingredients in Characterising Interfacial and Rheological Properties... 

It is difficult to study the rheological properties of a foam since, on deformation, its properties are changed. The most convenient geometry to measure foam rheology is to use a parallel plate. The rheological properties could be characterised by a variable viscosity [4],... 

Unresolved problems include overall theory and related experiments on the contribution of chemical reactions at interfaces to the relaxation spectrum for a better theoretical description of surface rheological parameters. As discussed above each chemical reaction is characterised by at least one relaxation time. On the other hand surface rheological properties, for example dilational elasticity, are connected to relaxations taking place at interfaces. 

Adhesion results from the combination of an adequate surface energy couple between the PSA and skin and rheological properties. Adhesion is commonly measured by a peeling test, which involves the measruement of the force required to peel an adhesive, spread on to a flexible backing, from a substrate whose surface properties are well characterised. 

Additionally, other features of polymer characterisation are discussed such as the determination of molecular weight, polymer fractionation techniques, chemical and thermal stability, resin cure, oxidative stability, photopolymers, glass and other transitions, crystallinity, viscoelasticity, rheological properties, thermal properties, flammability testing, particle size analysis and the measurement of the mechanical, electrical and optical properties of polymers. 

Basically this technique involves the measurement of the mechanical response of a polymer as it is deformed under periodic stress and is used to characterise the viscoelastic and rheological properties of polymers. 

S. Malik, and M. Ahuja, Gum kondagogu-g-poly (acrylamide) Microwave-assisted synthesis, characterisation and release behaviour, Carbohydr. Polym., 86 (1), 177-184, 2011. H. Gong, M. Liu, J. Chen, F. Han, C. Gao, and B. Zhang, Synthesis and characterization of carboxymethyl guar gum and rheological properties of its solutions, Carbohydr. Polym., 88 (3), 1015-1022, 2012. 

Like any disperse system, foams produce non-Newtonian systems and to characterise its rheological properties one need to obtain information on the elasticity modulus (modulus of compressibility and expansion), the shear modulus, yield stress and effective viscosity, elastic recovery, etc. 

Raman spectroscopy has also been applied as a rapid characterisation tool of ex-reactor aliphatic polyketones. Chalmers et al. [104] have described off-line compositional analysis by means of Raman and FT-Raman of EO-PO copolymers (non-ionic surfactants) for QA/QC purposes PLS modelling can importantly decouple the spectral influences of crystallinity and orientation on Raman spectra. Simultaneous monitoring of composition and rheological properties of EVA copolymers by means of inline fibre-optic Raman spectroscopy was reported [188,189],... 

The addition of a nanofiller to a polymer melt can significantly affect the rheological properties in certain cases, both processing and final composite properties can be enhanced with the same filler. To date, rheological studies of nanotube/nanofibre-filled systems have focused mainly on the shear behaviour however, shear tests alone cannot characterise the... 

A number of peculiar properties are displayed, including rheology characterised by viscoelasticity. Viscosities are far higher than that of either bulk phase this is a result of the large amount of energy required to deform the network of thin films of the continuous phase. A yield stress is observed, below which HIPEs behave as elastic solids and will not flow. Resistance to flow occurs from the inability of compressed droplets to easily slip past each other. Above the... 

Rheology is an important technique for looking at polymer properties and association characteristics. Gels are characterised by their solid-like rheology even though they comprise ca. 99 % immobilised liquid material. 

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