Melt index values

The T of crystalline polymers may be determined by observing the first-order transition (change in heat capacity value) by DTA or by DSC (ASTM-D3418). Some comparative information on thermal properties of polyolefins may be obtained from the melt index. To determine the melt index, the weight of extrudate or strand under a specified load and at a specified temperature is measured. Melt index values are inversely related to the melt viscosity. 

Many technical problems that may be encountered, say, with a new thermoplastic, will already have been met and solved with polymers, like rubber, that have been in the marketplace for a comparatively long time. It is not often possible to recognize and use such parallels, however, if the parameters of the molecular weight distributions in the different cases are not measured in the same units. This results in much unnecessary rediscovery of old answers, and the engineer or scientist who can interpret both Mooney and melt index values in terms of statistical parameters of the molecular weight distributions of the respective rubber and thermoplastic may save considerable time and efl ort. 

Relative Melt Index Potential. Melt indices were obtained from polymer samples by the standard test (ASTM D 1238-73) at 190 C using a weight of 2160 grams. However, melt index values are not just affected by activation parameters, but also by reactor conditions, such as the temperature, monomer concentration, and residence time. Therefore for clarity in this report we have normalized melt index values against those of a reference catalyst run under the same reactor conditions. We call the normalized value the relative melt index potential (RMIP) because it is... 

FIGURE 209 The influence of cocatalyst type and amount on the response to shear stress (HLMI/MI) of polymers made with Cr/silica. The reaction temperature was adjusted (135-160 °C) to maintain a constant melt index value. 

Figure 236 shows an example of this same relationship taken from a commercial fluidized-bed process using a chromium catalyst. Three reaction temperatures were used to make polymers having a range of melt-index values. At each temperature, the addition of sub-ppm concentrations of O2 raised the polymer MI significantly, and broadened the MW distribution. 

The melt index values were obtained with ASTM D1238. 

Practically all resin suppliers provide a melt index value with every lot of material sold and most manufacturers use the melt index (either as reported by the supplier or as measured in-house) as both a check on incoming resin and a guide for processing conditions. Slight variations in the melt index (a few tenths) of incoming resin are usually... 

Flow Properties - Melt Index values for five of the polyesters were determined as a function of both temperature and inherent viscosity. While it is difficult to compare the flow behavior of polymers of differing molecular weights, it appears that all of the compositions for which values could be obtained would be attractive candidates for injection molding resins at the proper molecular weight range. 

Figure 3-89. An example of the effect of specimen thicknesses on Izod impact strength for various molecular weights or melt-index values for polycarbonates.

The ethylene polymerization of this catalyst was carried out in an autoclave reactor at 221°F in isopentane as the slurry solvent in the presence of triisobutylaluminum as cocatalyst and 50 psig of hydrogen and sufficient ethylene to achieve a total reactor pressure of 550 psig. The catalyst activity was 10,540 g of PE/g of catalyst/ hr, which corresponded to an activity of 146,000 g PE/g Ti/hr. The granular polyethylene product obtained was considered suitable for a particle-form slurry process such as the Phillips slurry process. The polyethylene sample displayed a Melt Index (I value of 0.70 and a High Load Melt Index ) value (HLMI) of 3 1 with a HLMI/MI ratio of 45, which indicates tfiat the polyethylene molecular weight distribution was of an intermediate value. 

Examination of Figure 3.10 clearly shows that both types of titanium-chromium catalysts provide polyethylene with a significant increase in Melt Index (lower molecular weight) if the air oxidation temperature step is carried out at about 425-550°C. However, the catalysts prepared by reacting the titanium tetraisopropoxide with CrO using carbon monoxide at 700°C, followed by an air oxidation step at about 500°C, provide polyethylene with a Melt Index value of about 45 under the polymerization conditions outlined. Consequently, this type of catalyst was able to produce significantly lower molecular weight polyethylene than the catalysts that were reduced with carbon monoxide at 400°C as the final catalyst preparation step. 

Until the mid-1990s, the polyethylene product space consisted of the manufacture of ethylene-based homopolymers and copolymers over a density range of about 0.910-0.965 g/cc with Melt Index values between about 0.05-100. 

The product mix of autoclave and tubular reactors are similar in terms of LDPE homopolymers (0.910-0.935 g/cc) and some specialty grades of polyethylene such as ethylene/vinyl acetate copolymers up to about 30 wt% vinyl acetate (VA). However, the autoclave process provides higher levels of vinyl acetate (40 wt%) in ethylene/VA copolymers and additional specialty grades of polyethylene such as ethylene/methyl acrylate, ethylene/acrylic acid and ethylene/n-butyl acrylate. Polyethylene molecular weight can be varied over a wide range with the high-pressure process, with Melt Index values (I ranging from 0.15 to 40. 

It was reported by F. Mirabella, Jr., and E. Ford that the amount of long-chain branching in LDPE samples over a density range of 0.917-0.922 and Melt index values of 0.25-2.5 was about 2.9-3.4 LCB/1000 carbon atoms [18]. 

The first plant operated in the solution mode for the first four years of operation at a production rate of approximately 9,000 lbs PE/hr. During the first two years of operation, the plant produced only ethylene homopolymers with a Melt Index value of less than one. As outlined by Hogan [22] in 1958, ethylene/1-butene HDPE copolymers were introduced in order to increase the characteristics of the product mix available. Soon after the... 

One of the most common instruments associated with the polyethylene industry is the Melt Index instrument that is found in research, product and manufacturing facilities around the globe. For commercial applications, polyethylene is sold according to Melt Index values summarized in material specification data sheets, usually by the Melt Index value determined with a 2.16 kg weight (MI ... 

Figure 6.1 shows the density and Melt Index values of various grades of polyethylene that are fabricated into finished polyethylene products. The... 

Examination of the data in Table 6.7 shows that the UNIPOL II resin processes with excellent bubble stability and similar output rate and head pressure as a LDPE resin with a lower Melt Index value. In addition, the UNIPOL II resin exhibited a higher modulus, dart-drop strength and higher puncture energy than the LDPE resin. 

The density (corrected to a 1.0 melt index value) vs comonomer content (mol%) relationship is dependent on variables such as branching distribution and branch length. For example, a single-site catalyst that provides a homogeneous branching distribution requires significantly less comonomer... 

The most common capillary instrument in the polymer industry is not a rheometer but an indexer. The quality of nearly every batch of thermoplastic made in the world is controlled by melt index. Because it is so widely used and has all the essential features of a capillary rheometer, and because rheologists ate often asked to compare their results to melt index values, we need to examine it here. 

The melt index apparatus (Figure 7-1) is preheated to a specified temperature. The material is loaded into the cylinder from the top and a specified weight is placed on a piston. The most commonly used test conditions are shown in Tables 7-3fl and l-3b. The material is allowed to flow through the die. The initial extrudate is discarded because it may contain some air bubbles and contaminants. Depending on the material or its flow rate, cuts for the test are taken at different time intervals. The extrudate is weighed and melt index values are calculated in grams per 10 min. 

An alternate method for making the measurement for materials with a high flow rate involves automatic timing of the piston travel by some electrical or mechanical device. The melt index value is calculated by using the following formula ... 

The melt index values obtained from the test can be interpreted in several different ways. First, a slight variation in the melt index value should not be interpreted as... 

Melt index is an inverse measure of molecular weight. Since flow characteristics are inversely proportional to the molecular weight, a low-molecular-weight polymer will have a high melt index value and vice versa. 

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