Density, melt index, and molecular

Table 2.3 Density, melt index, and molecular weight influence PEs performances...

Three basic characteristics of PEs determine their processing and end-use properties their density, melt index, and molecular weight distribution (see Chapter 2). Their range in density, from 0.890 to above 0.96 g/cm, is a result of their crystalline structure. This difference accounts for their property variations seen in Chapter 2. As one example, reducing PE s crystallinity increases its impact resistance, cold flow, tackiness, tear... 

Table 6-8. The Effects on PEs of Increases in Density, Melt Index, and Molecular Weight... 

Direct comparisons between melt index and molecular weight of polyethylene should be made with caution. Such comparisons are only appropriate when the polymers have similar histories (made using the same catalyst, by the same process, at near identical densities, etc.). An example of the relationship between melt index and molecular weight for a series of LLDPEs with similar histories is... 

FIGURE 39 High-load melt index and molecular weight (Mw) values of polymers made at three densities by adjusting the amounts of 1-hexene added to the reactor. The addition of comonomer tended to decrease the polymer MW (raise HLMI). 

First, wc purify the compound and determine its physical properties melting point, boiling point, density, refractive index, and solubility in various solvents. In the laboratory today, we would measure various spectra of the compound (Chap. 13), in particular the infrared spectrum and the nmr spectrum indeed, because of the wealth of information to be gotten in this way, spectroscopic examination might well be the first order of business after purification. From the mass spectrum we would get a very accurate molecular weight. 

Melting involves a change from the crystalline solid state into the liquid form. For low-molecular-weight (simple) materials, melting represents a true first-order thermodynamic transition characterized by discontinuities in the primary thermodynamic variables of the system such as heat capacity, specific volume (density), refractive index, and transparency. Melting occurs when the change in free energy of... 

Abstract Polyethylene, PE, is characterized by molecular weight (MW), molecular weight distribution (MWD) and density. Melt index (MI2) is measured in the QC lab and is used as an indication of resin MW. Melt flow ratio (MFR or MI20/MI2) is a calculated QC lab number, which is used as an indication of MWD. Density is a measure of crystallinity. In polyethylene production, having fast reliable feedback on MW and MWD is critical for producing high quality resins. This article describes a novel/unique idea how a capillary rheometer can be used to predict MI2 and MI20. 

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystalhsation does not alter it, the compound may be regarded as pure. For a hquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boUing point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

Polyethylene and polypropylene are semitransparent plastics made by polymerization. They are produced from ethylene and propylene in a variety of grades. Their mechanical properties are determined mainly by density (degree of crystallinity) and molecular weight, characterized by the Melt Index (MI). 

Important physical properties include the density, melt flow index, crystallinity, and average molecular weight. Mechanical properties of a polymer, such as modulus (the ratio of stress to strain), elasticity, and breaking strength, essentially follow from the physical properties. 

The two reactor feeds were controlled to give copolymers with the desired densities, and a physical blend and a diblock OBC were produced. DEZ was added to the first reactor to achieve the desired melt index (/2 = 20 dg min1, equivalent to a Mn of -15-20 kg mol1). This material was fed to the second reactor, and production was continued under different conditions. The material collected after the second reactor had a lower melt index (/2 = 3.9 dg min1), indicating a higher molecular weight consistent with the chain extension reaction from the CCTP process. 

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... 

Many publishers make available their databases on floppy disks or CD-ROMs, which are often found inside the back cover of the book. An example is the Properties of Organic Compounds (Tide and Milne 1999), which is a CD-ROM list of 27,500 known organic compounds and their formulas, molecular weights, melting points, boiling points, densities, refractive indexes, colors, solubility scales, vapor pressures, and threshold limit values (TLVs) that workers can be exposed to in a normal 8 h workday and 40 h workweek. This reference gives only a limited number of properties, and does not go into flammability or toxicity. Solubility in water is given only as a... 

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