Molecular weight melt index, relation

The weight average molecular weight of acetal copolymers may be estimated from their melt index (MI, expressed in g/10 min) according to the relation... 

Molecular Weight. The range of molecular weights of commercial LLDPE resias is relatively narrow, usually from 50,000 to 200,000. One accepted parameter that relates to the resin molecular weight is the melt index, a rheological parameter which, broadly defined, is inversely proportional to molecular weight. A typical melt index range for LLDPE resias is from 0.1 to 5.0, but can reach over 30 for some appHcations. 

Some properties, such as heat capacity, refractive index, and density, are not particularly sensitive to molecular weight but many important properties are related to chain length. Figure 3.2 lists three of these. The melt viscosity is typically proportional to the 3.4 power of the average chain length so 17 is proportional to Thus, the melt viscosity increases rapidly... 

To a plastic producer (i.e. processor), melt index is one property that is needed in order to evaluate whether the same process can be used irrespective of whether it uses virgin or recycled polymers. This will tell if it is possible to process the recycled polymeric materials in the same set-up as usual. Several other properties are needed in order to quality mark the materials. The melt index is related to what final tensile properties a product obtains, this in turn has an impact on the expected life-time. The purity of a recyclate stream with respect to the amount of foreign polymer in the stream has an impact on melt-index, but will also be an important factor for the final mechanical properties. Another very important property is the amount of low molecular weight compounds, which may be of vastly different types. Typically such an analysis will show the presences of additives and their degradation products, degradation products of the polymeric matrices, traces of solvents, initiators, or catalysts, compounds related to the use of the plastics and others. 

Molecular weights are not often measured directly for control of production of polymers because other product properties are more convenient experimentally or are thought to be more directly related to various end uses. Solution and melt viscosities are examples of the latter properties. Poly(vinyl chloride) (PVC) production is controlled aceording to the viscosity of a solution of arbitrary concentration relative to that of the pure solvent. Polyolefin polymers are made to specific values of a melt flow parameter called melt index, whereas rubber is characterized by its Mooney viscosity, which is a different measure related more or less to melt viscosity. These parameters are obviously of some practical utility, or they would not be used so extensively. They are unfortunately specific to particular polymers and are of little or no use in bringing experience with one polymer to bear on problems associated with another. 

Molecular weight is most often judged by the viscosity of the polymer melt, to which it is inversely related. One convenient measure of this parameter is melt index. Manufacturers and customers of polyethylene are usually more concerned with melt index than with the actual molecular weight because melt index gives a direct indication of the flow of the polymer during processing. 

The fats and oils obtained from various sources differ from one another in the proportion of the several esters present in each. This difference in composition results in a difference in physical properties, such as specific gravity, viscosity, index of refraction, and melting point. All the fats and vegetable oils are soluble in ether, however,—a fact made use of in the analysis of foods to separate fats from the other constituents of food-products. As the physical properties of a fat or an oil obtained from a definite source are more or less constant, the determination of these properties is of value in the analysis of such substances. The chemical analysis of fats and oils is based upon determinations of the proportion of unsaturated compounds present, of the relation between the acids of low and high molecular weight obtained on hydrolysis, and of the proportion of substances which do not undergo hydrolysis. This statement will be made clearer by a brief consideration of a few of the more important methods employed in the analysis of fats and oils. 

Various processing conditions can require the tie resin to fall into a particular melt index (MI) classification. MI is inversely related to molecular weight (MW) (see Chapter 1) a high-MW adhesive will have a low MI. Most adhesive are available in a range of Mis to meet different requirements. 

The traditional melt index test (230°C/2.16kg for PP) is widely used in the plastics industry to estimate the polymer melt processability by a melt flow index (MFl) which is inversely related to the viscosity and the average molecular weight of the material. The MFI has to be considered very cautiously because polymers are usually processed under very different shear rates and this fairly simple index is far from giving a complete picture of the material s flow behavior. 

Some type of melt viscosity is included in the specification for almost every polymeric or plastic product. This is because viscosity is related to the molecular weight and to the performance of a polymer. Equipment used for rheological measurements range from the simple and ubiquitous melt flow indexer to the precise and quantitative capillary and cone-and-plate rheometers. 

Each of the two quantitative spectra of poly(ethylene glycol) (PEG) depicted in Fig. 41 exhibits an end-group (OH) resonance at 3.2 ppm and a main -O-CH2 CH2O- peak at 3.9 ppm. The molecular weights of these two products can easily be calculated by NMR, based on the relative intensity ratio of the end to main peaks. Rare spins ( C and Si) can also be used to determine molecular weight, as demonstrated for several polysilox-anes [117]. A commercially available on-line system tracks both density and melt index (which is related to molecular weight) based on wide-line H experiments [118]. GPC/NMR, a variant of LC/NMR, has been used to track the molecular-weight distribution of poly(methyl methacrylate) [119]. 

This phenomenon has been observed in both Newtonian and power law fluids (non-Newtonian) and is experimentally found to be closely related to polymer properties (molecular stmcrnre, molecular weight distribution, shear viscosity, relaxation time, power law index) as well as to the spinning variables, such as draw ratio, cooling rate, melt temperature, and die geometry, and so on. 

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