Influence of moisture content on Mooney viscosity of epichlorohydrin-ethylene oxide.

Influence of moisture content on Mooney viscosity of epichlorohydrin-ethylene oxide

It has been known for many years that epichlorohydrin elastomer (ECO) will absorb up to approximately 2% moisture if stored in moderate to high humidity conditions. Unfortunately the presence of moisture in ECO, as well as other elastomers, causes a reduction in raw polymer Mooney viscosity of over 10 points when volatile levels approach 2%.

Because Mooney viscosity is a standard control criterion for ECO, variations in viscosity due to changing moisture content during normal storage creates serious problems. Not only is it difficult to utilize Mooney viscosity as a control tool during production, without relating the values to volatile matter, but also it is impossible to correlate data between laboratories due to these variations.

When ECO is mixed with carbon black or other fillers to form a compound, the moisture is volatilized, and has no significant influence on vulcanizate properties.

A study was undertaken, therefore, to show:

* The effect of moisture content on Mooney viscosity,

* to demonstrate the influence, or lack thereof, of the moisture content on vulcanizate properties of an ethylene thiourea (ETU) cured compound, and

* to establish a conversion between any observed Mooney viscosity at a given moisture content, and the Mooney viscosity at a specific low moisture content, which will allow for the use of this test as a reliable quality control tool.

Experimental procedures

ASTM standard procedures were used for all testing. The various ECO elastomer samples were conditioned in the laboratory in several ways, in addition to utilizing samples stored in a warehouse in uncontrolled conditions. One series of samples were aged in a circulating air oven at 40 [degrees] C and unspecified low humidity. The second set were aged in a room controlled at 40 [degrees] C and 85% relative humidity (RH). A third group of samples, taken from second set exposed to high humidity, were vacuum dried (conditions) to a constant volatile level.

In addition to testing the ECO samples for Mooney viscosity and moisture content, some of them were mixed into carbon black filled compounds using the formulation in table 1 both on an open mill and in an internal mixer.

A statistical study was also made using many samples of six grades of ECO and the results of Mooney viscosity versus volatile matter were plotted.

Results and discussion

A comparison was made between samples of ECO exposed for up to two weeks at 40 [degrees] C and low humidity in a circulating air oven. The results of their moisture contents are given in figure 1 and the corresponding Mooney viscosity is shown in figure 2. There was a slight drop in heat loss after one day, and the values remained constant thereafter. There was no significant change in Mooney viscosity over the test period.

Figures 3 and 4 show the moisture content and Mooney viscosity results respectively of samples exposed to 85% RH at 40 [degrees] C for up to two weeks. There were significant increases in heat loss during the first three days of exposure, with an equilibrium established at 2% for the balance of the period. There was a corresponding dramatic drop in viscosity the first day and a further decrease after three days, at which point the viscosity remained relatively constant for the balance of the test period. At each test period, duplicate samples were vacuum dried and tested for moisture and viscosity. The results, plotted on the same graphs for comparison, showed the moisture and viscosities were constant after vacuum drying.

The heat losses and Mooney viscosities of samples stored under different environments are given in figures 5 and 6. The vacuum dried samples exhibited no significant level of moisture and the Mooney viscosity may be considered the base line. The samples stored in a warehouse with uncontrolled humidity showed a significant increase in heat loss up to approximately 1%, and a corresponding loss in viscosity of six to seven points. When the samples were exposed to 85% RH at 40 [degrees] C, the moisture contents increased to about 2% and the viscosities dropped by 14 to 16 points.

The same samples were then mixed into compounds using the formulation in table 1, on both a mill and in a mixer. The volatile matter and compound viscosities of the mill mixed compounds shown in figures 7 and 8 indicate that the moisture content of the base elastomer was driven off during mixing, resulting in constant values for both the heat loss and compound Mooney viscosity. The results of the internal mixer mixed compounds showed some minor differences in moisture loss and corresponding compound viscosities. These differences were attributed to closed chamber of the mixer which may have somewhat reduced the escape of moisture during mixing.

The rheological and physical properties of these compounds were also determined, and are given in tables 2 and 3. It is evident that the cure rate, as determined with the Monsanto Rheometer, was unaffected by the moisture content of the various original ECO samples. The tensile strength, elongation at break, modulus and hardness of the compounds were all unaffected by variations in the volatiles of the base ECO polymers.

Figure 9 shows the differences in moisture contents of ECO samples after storing for four months under different storage conditions. The corresponding Mooney viscosity results are given in Figure 10, with the viscosities increasing or decreasing depending upon the moisture contents. Six different grades of Hydrin C ECO were selected and a statistical correlation made of the observed data for heat loss and Mooney viscosity. The results of the Mooney viscosity versus heat loss were plotted in figure 11 and lines drawn through the observed points to give the best statistical correlation, the coefficient being better than 0.97. The slopes of these lines were:

S = - (Mooney units/heat loss %)

MV calc. = MV obs. - S (HL% - 0.5) where MV calc is the Mooney viscosity calculated for a sample at 0.5% heat loss and MV obs is the Mooney viscosity measured on a specimen from the same sample with a given heat loss HL%.

Conclusions

It was evident that the effect of moisture on ECO raw polymer was a physical absorption and desorption, which were reversible. There was no chemical effect, as shown by the lack of influence on compound and vulcanizate properties, hence no need for concern about the moisture content of the base ECO polymer. The influence of moisture content on the Mooney viscosity of ECO elastomers has presented problems in using this test as a key quality control criterion. For example, if the observed Mooney viscosity was outside of the specification limits, usually below the lower limit, it does not necessarily mean that molecular weight or molecular structure, e.g. gel-content of the polymer was not within design limits.

It was very evident that with ECO polymers, it is necessary to measure heat loss as well as Mooney viscosity and convert these readings to a standard moisture level of 0.5%. Specification limits for Mooney viscosity of ECO must also be established at this moisture content.

Figure 12 is a conversion chart for translating the Mooney viscosity of six different grades of Hydrin C ECO from the observed reading at a measured heat loss to the standard Mooney viscosity at 0.5% moisture level. This will allow both the producer of ECO and the user to accurately and reproducibly measure its Mooney viscosity, adjusting for variations in moisture content to a standard established at 0.5%.

Table 1 - Test formulation

ECO polymer                                           100
Stearic acid                                            1
N550 FEF carbon black                                  40
Nickel dibutyldithio carbamate                          1
Red lead oxide dispersion (71% active)               6.25
Ethylene thiourea dispersion (71% active)            1.43
                                             Total 149.68

[Tabular Data 2 and 3 Omitted]

PHOTO : Figure 1 - heat loss of ECO stored at 40 [degrees] C and low humidity

PHOTO : Figure 2 - Mooney viscosity change of ECO stored at 40 [degrees] C and low humidity

PHOTO : Figure 3 - heat loss of ECO stored at 40 [degrees] C and 85% relative humidity vs. vacuum drying

PHOTO : Figure 4 - Mooney viscosity change of ECO stored at 40 [degrees] C and 85% relative humidity vs. vacuum drying

PHOTO : Figure 5 - heat loss of ECO stored under various conditions

PHOTO : Figure 6 - Mooney viscosity of ECO stored under various conditions

PHOTO : Figure 7 - heat loss of ECO compounds after mill mixing

PHOTO : Figure 8 - Mooney viscosity of ECO compounds after mill mixing

PHOTO : Figure 9 - heat loss of ECO stored for four months under various conditions

PHOTO : Figure 10 - Mooney viscosity of ECO stored for four months under various conditions

PHOTO : Figure 11 - Mooney viscosity vs. heat loss of various grades of ECO

PHOTO : Figure 12 - conversion chart of Mooney viscosities adjusted for heat loss

References

[1]D.L. Hertz, Jr., "Epichlorohydrin elastomers," in "The Vanderbilt Rubber Handbook, 13th ed., R.F. Ohm, ed., R.T. Vanderbilt Co., Norwalk, CT, 1990, p. 234.