Melt index Testing

The melt Index test measures the flow property at a fixed wall shear stress In the capillary. The shear stress depends on the load specified for the condition and it is provided in Table 3.8. The apparent shear rate at the capillary wall that the resin experiences depends on the Ml value measured, and it can be calculated using a modification of Eq. 3.33 as follows ... 

Table 3.8 Test Conditions and Shear Stress at the Capillary Wall for Melt Index Testing as Prescribed by ASTM D1238-04c [15]...

The melt index test, widely used for technological purposes, measures the rate of extrusion of a polymer melt through a given capillary (57). This is a... 

FIGURE 50 Pore volume distribution of Cr/silica-titania catalysts dried by various methods and then activated at 800 °C. The mesoporosity of the catalyst influences its activity and the polymer melt index (tested at 105 °C with 1.5 mol C2H4 L ). 

The melt index test measures the rate of extrusion of a thermoplastic material through an orifice of specific length and diameter under prescribed conditions of temperature and pressure. This test is primarily used as a means of measuring the uniformity of the flow rate of the material. In this study, the melt flow index (MFI) value increased from 1.35 to 1.69 g/10 min with increase in EMA content from 0 to 10% in the PMMA/EMA blends (Table 5.3). This may be due to the reduction in cohesive strength as well as the plasticizing effect of EMA in PMMA/EMA blends. However, further addition of EMA content above 10% by weight shows no more increase in MFI. 

Rheological data provides information about how a melted material will perform (flow) under processing conditions. Perhaps the most important rheological test used by plastics processors is the melt index test. Though rheologists scoff at this notion, it is nevertheless vitally important because virtually every manufacturer that employs an extruder, injection molder, or blow molder uses the test method or the data it yields. 

Incidentally, a true rheologist has a right to scoff The melt index test provides only the smallest amount of useful data from a vast array of information that the field of rheology can provide about a material. Further, the small amount of data yielded by a... 

One of the reasons for the popularity of melt index data is that rheometer data is significantly more expensive. Because of the higher cost of rheometry equipment and the advanced training needed by operators, most plastics manufacturers do not have a rheometer on site. Nonetheless, the data obtained from a rheometer is quite valuable to processors, so a technique has been developed to use an extruder to obtain more complete rheological data than is available from the melt index test. 

Table 6.1 Standardized Conditions for Melt Index Testing...

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. 

The melt index is a measure of the viscosity of the plastic material at very low-shear rate. The melt index is the amount of mass of aplastic that flows through a small orifice in a die over a 10-minute time period. A melt index test machine is shown in Figure B.5. The temperature for plastics can range from 190°C to 300°C and the plunger mass can vary... 

Quite often, the melt index test results simply do not provide sufficient information because the melt index is a single-point test. The prespecified test temperature is not necessarily the actual processing temperature. The test is conducted at a low shear rate, which is significantly lower than the actual processing shear rate. The flow rates are measured at a single shear stress and shear rate performed at one set of temperatures and geometric conditions. Furthermore, since the melt index measurement takes account of the behavior of the polymer at only one point, it is quite... 

Receiving Inspection. Many types of tests have been devised for testing raw materials. Depending upon the severity of the need for inspection, the types of tests selected may vary from being basic and simple to sophisticated and complex. Some of the most common basic tests are the melt index test, specific gravity, bulk density, spiral flow test, and viscosity tests. Gel permeation chromatography, infrared analysis, thermal analysis, and rheometry are some of the more elaborate raw material quality control tests. These tests are discussed in detail in Chapter 7. Some processors also choose to mold test bars from a small sample of raw material and conduct physical tests such as tensile, impact, and flexural tests and then evaluate the results to see if they meet the preestablished specifications. 

Melt index test Melt index test measures the rate of extrusion of a thermoplastic material through an orifice of specific length and diameter under prescribed conditions of temperature and pressure. Melt index value is reported in grams per 10 minutes for specific condition. 

Another approach to finding a good way to use the capillary rheometer viscosity to estimate the Melt Index is to determine the shear rates for all samples that produce the same shear stress in the capillary rheometer as in the Melt Index test. The estimated value for the shear stress in the Melt Index with a 2.16 kg weight is 19348 Pa. Polyethylene values with Melt Indexes of less than 3.3 required an extrapolation to determine this shear rate. Samples greater than 3.3 grams/10 minutes required an interpolation. In either case, the data was put into the following equation to calculate the Shear Rate at 19348 Pa of Shear Stress ... 

Since polymers shear thin, K is equal to the viscosity at a shear rate of 1. Therefore, to measure K, the melt index test should be done at a shear rate of 1 or at a few different piston weights and extrapolated to a shear rate of 1. Equation 8 (5) below shows the relationship between shear rate (y) and melt index (MI) ... 

A check of shear rate during the melt index test using equation 8 yields 1.27 s". This value is close enough to 1 that a value of 0.015 MPa.s" can be used for K. 

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