Melt-index

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 

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. 

Schematic of slit rheometer with flush-mounted pressure transducers /, . 

After the sample comes to temperature, the weight is applied and flow rate is determined by cutting and weighing the extrudate obtained in 10 minutes. This weight in grams is the melt flow index (MFI) of the polymer. It resembles an inverse viscosity at moderately low stress. High MFI means low r)o and low molecular weight, hence its use to control polymerization reactors (Bremner and Rudin, 1990). 

Because MFI is used so widely, there is interest in quantitatively relating it to rheological material functions. Michaeli (1984) gives a nomogram to relate MFI to melt viscosity. Such calculations generally ignore reservoir and entrance pressure losses. 

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Testing. Melt index or melt flow rate at 190°C, according to ASTM D1238, is the test most frequently appHed to the characterization of commercial acetal resins. The materials are typically grouped or differentiated according to their melt flow rate. Several other ASTM tests are commonly used for the characterization and specification of acetal resins. 

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... 

This is perhaps the most commonly encountered melt index. 

Techniques for evaluating processing stabiHty and mechanochemical effects include using a Brabender torque rheometer (29,30), injection mol ding (26,28), capillary rheometry (26,28), and measuring melt index as a function of residence time (26). 

Fig. 12. Tensile strength vs draw ratio (6) 0.42 melt index spun at 50 m /min, B, and 500 m /min, A, A 12.0 melt index spun at 100 m /min, , O and 500 m /min, , <). Open symbols = cold drawn and annealed at 140°C filled symbols = drawn at 140°C. To convert GN/m to dyne/cm, multiply by...

Material Percent neutralized Melt index at 190°C, dg/min Secant modulus, % extension, MPa Ultimate tensile strength, MPa Elongation at break, %... 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

LDPE homopolymers in the 0.2—150 melt index or 100, 000 20,000 mPa s(= cP) viscosity range. Specialty polymers such as greases and waxes or highly cross-linked polymers are not included. 

Melt Index or Melt Viscosity. Melt index describes the flow behavior of a polymer at a specific temperature under specific pressure. If the melt index is low, its melt viscosity or melt flow resistance is high the latter is a term that denotes the resistance of molten polymer to flow when making film, pipe, or containers. ASTM D1238 is the designated method for this test. 

A wide variety of chromium oxide and Ziegler catalysts was developed for this process (61,62). Chromium-based catalysts produce HDPE with a relatively broad MWD other catalysts provide HDPE resins with low molecular weights (high melt indexes) and resins with a narrower MWD (63,64). 

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. 

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

The molecular weight distribution of LLDPE resins is usually characterized in industry by the ratios of melt indexes measured in the same apparatus using different loads (2.16, 10.16, and 21.6 kg). The commonly used ratios are melt flow ratio, MFR) and I q/I2. Both of these ratios... 

Melt Index Tester MTS Capillary Viscometer Automatic Capillary Rheometer ProcessibiLity Tester... 

NB The data compares a syndiotactic material with a melt index of 30 and an isotaclic material with an index of 0.3. This should however not invalidate the large differences seen in such properties as modulus, impact strength and opacity.)... 

Melt Index ASTM D.1238 (300 C) g/10 min Notched impact strength ASTM D.256 hod ftlbfin jin Recommended processing method... 

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