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Specifying the temperature for the melt elasticity test.

Specifying the Temperature for the Melt Elasticity Test

With the recognition that the elasticity of a polymer melt is just as important in polymer processing as the viscosity of the melt, the need for a standard test for elasticity for quality control has become apparent. The melt elasticity index, or MEI (see PE, September 1987, p. 41), is a test that consists of applying a specific strain history to the melt and then releasing the specimen so that its elastic strain may be measured. As with any standard test, significant variables must be specified, one of which, of course, is the test temperature.

This article reports on an investigation into the effect of temperature on the melt elasticities of a series of ten polypropylenes. PP melts are much less elastic than many other polymer melts, and therefore, it is more difficult to differentiate between various grades for quality control purposes.

The materials tested were all intended for the same application, but some had been found to process better than others. Because differences in melt elasticity were suspected, the MEI test was used to determine if the better processing materials could be differentiated.

Experimental

The melt elasticity index test was described in detail by the author at ANTEC '89. Figure 1 is a schematic drawing of a recently introduced commercial model of the Melt Elasticity Indexer, which is interfaced with a computer (Custom Scientific Instruments Inc., Cedar Knolls, N.J.). A central cylindrical rotor is surrounded by the polymer specimen contained in the cup. A controlled heater system melts the specimen and holds it at the test temperature. A disk attached to the top of the rotor is turned by the drive, thereby shearing the specimen. After the prescribed amount of shearing has taken place, the drive pin moves up, releasing the rotor. The rotor is then free to rotate about its axis in response to the elastic strain recovery of the specimen.

As the strain recovery takes place, the slots in the disk pass between the LED and the detector, chopping the radiation from the LED. The essential square wave is sensed by the detector and converted to a voltage that is sent to the data processor for analog-to-digital conversion and processing. This information is sent to the computer for data storage and display - either on the monitor in the form of a table of strain recovery values and corresponding times, or as a printout together with a test description.

Results

Typical data, plotted as amount of recoverable strain vs. time after release, is shown in Fig. 2 for material F, which exhibits a high melt elasticity, and for material D, which exhibits a low melt elasticity. After a large initial elastic recovery, the recovery continues at a decreasing rate. The recovery for F is about 19% greater at 190 [Degrees] C than at 230 [Degrees] C. Although for D the difference in elasticity between 230 [Degrees] C and 190 [Degrees] C is less on a strain unit basis than for F, the difference for D on a percentage basis is much larger, 44%.

The best way to compare the melt elasticities of different materials is through their strain recovery curves, but they are difficult to use in quality control and in setting specifications. It would be much better to have a single number to characterize the melt elasticity, as is done to characterize the melt flow. Two possibilities have been proposed.

One is to use the time required to reach a specific amount of strain recovery as an index of melt elasticity. This would be easy to measure but it presents a problem in that the more elastic the melt, the shorter the time. As a result, the index would be the reverse of the property, as is the case with melt flow. In addition, some materials may never reach the specified strain recovery, and the index would indicate infinite melt elasticity.

The alternative is to define the index as the amount of recovery that takes place after a specific period of time, for example, 20 sec: MEI (20), or 300 sec: MEI (300). The shorter recovery time is desirable because the test can be done quickly. The longer time has some interest since the recovery is then progressing very slowly and the index is indicative of "total" recovery.

Data on the amount of strain recovery in 20 sec, MEI (20), at 190 [degrees] C for the ten PPs is presented in Table 1. Two different instruments with three different operators were used. Many tests were replicated, and all materials were evaluated by at least two operators using different instruments. The reproducibility of the measurements is seen to be quite good.

Table : TABLE 1. PP Melt Elasticity Indices at 190 [degrees] C and 20 sec Strain Recovery Time.
PP MEI (20) PP MEI (20)
A 1.73 E 1.60
A 1.84 E 1.52
A 1.85 E 1.58
A 1.76 E 1.59
A 1.97 F 1.70
B 0.75 F 1.72
B 0.70 F 1.68
B 0.76 G 0.55
B 0.70 G 0.59
B 0.69 G 0.63
C 0.45 H 1.44
C 0.45 H 1.38
C 0.46 H 1.44
C 0.46 I 0.78
C 0.48 I 0.74
D 0.34 I 0.76
D 0.29 J 0.60
D 0.33 J 0.57
D 0.32 J 0.58


In Table 2, the melts are ranked in order of decreasing melt elasticity according to their MEI (20) and MEI (300) results at 190 [Degrees] C and 230 [Degrees] C. Regardless of test temperature or recovery time, materials A, F, E, H, and I ranked 1, 2, 3, 4, and 5, respectively, and materials C and D ranked least elastic. The materials in between also ranked essentially in the same sequence with only small exceptions.

Table : TABLE 2. Ranking of PPs in Order of Decreasing Melt Elasticity.
190 [Degrees] C 230 [Degrees] C
MEI (20) MEI (300) MEI (20) MEI (300)
1 A A A A
2 F F F F
3 E E E E
4 H H H H
5 I I I I
6 B B G B
7 J G B G
8 G J J J
9 C C C C
10 D D D D


Conclusions

Neither the test temperature nor the time used to define the index appeared to significantly change the relative rating of the melt elasticity index. This may indicate that an arbitrary test temperature may be selected for PP, and, since all of the strain recovery for these materials takes place in the first few second, a time associated with the "total" recovery may be used.

These conclusions should be taken with caution. Other polymers, including some modified PPs, are known to exhibit large, slow strain recoveries where test temperature and strain recovery time have a strong effect on the relative ratings of melt elasticity.

PHOTO : FIGURE 1. The Melt Elasticity Indexer; computer, data processor, and printer not shown.

PHOTO : FIGURE 2. Strain recovery curves for two polypropylenes.
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Author:Maxwell, Bryce
Publication:Plastics Engineering
Date:Nov 1, 1990
Words:1197
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