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Swelling characteristics of fluorocarbon rubber in gasoline blended with ethanol: Relation to vulcanizate thickness.

Rubber based automotive components play an important role. O-rings, seals, gaskets, etc., are vital components for sealing and related functions. Performance demands for such under-the-hood components, as well as rubber products exposed to weather, have increased greatly over the last few decades. Preferred elastomers are HNBR, FKM and FPM, as well as EPDM for these applications.

India is the largest manufacturer of two-wheelers, including motorcycles, scooters and three-wheelers used for transport of both passengers and goods. These two-wheelers use a number of rubber items exposed to solvent/fuel (present day gasoline contains as high as 35% ethanol) which require resistance to fuel swelling and weather resistance, including exposure to ozone.

Two-wheeler manufacturers in India observed that the FKM component is changing in volume and other properties with each successive exposure to atmospheric heat and gasoline fumes. According to them, with the first exposure, certain volumetric expansion takes place (X), and then when the part is kept out (dried in the atmosphere), it does not return to its original state. In the second exposure, it starts swelling again from an already swollen state. If, in stage 2, the swelling is Y2, the total swelling shall be X + Y2 (figure 1).

Figure 1 shows a schematic presentation of these states, swelling and drying of an FKM seal, as explained above, where X = original state; [X.sub.1] = swelling in a particular fuel in time [t.sub.1] (swollen state); [Y.sub.2] = after drying in time [t.sub.2], does not reach X stage; and Z = ([X.sub.1] - [Y.sub.2]) C, with C a constant.

Swelling and permeability phenomena

In terms of the effect on elastomers, the two most important fuel composition variables are aromatic hydrocarbons (toluene, benzene, xylenes) and oxygenated additives (alcohol and ethers). Aromatic hydrocarbons have a greater swelling effect (and a more adverse effect on physical properties) than aliphatic or olefinic hydrocarbons (the other primary constituents of petroleum).

Volume change occurs as a result of exposure to fuel (usually swelling); and the amount of swell is a measure of the resistance of the particular elastomers to the fuel. A certain amount of volume change is sometimes tolerable; but in many cases, it will compromise the functionality of the component. O-rings, for example, must function within a predefined groove space, and excessive swelling will cause groove overfill, usually followed by seal failure and leakage. In fuel metering systems, large changes in volume will often compromise the accuracy of the metering system and reduce engine efficiency.

Physical property change usually goes hand in hand with volume swell. The greater the volume swell, the greater the loss in other properties. As would be expected from its low volume swell, FKM provides excellent retention of physical properties after fuel immersion. This can be translated into excellent long term compression set resistance and retention of sealing characteristics, giving functionality for the life duration of the vehicle.

The process by which elastomeric seals function is rather complex, and while it is generally true that the loss of physical properties caused by volume swell will be accompanied by a reduction in sealing integrity, this may not always be the case.

All elastomers are permeable to fuels, as well as other gases and liquids; however, the resistance of individual classes of elastomers to fuels varies widely. Fluroelastomers have the lowest permeability to fuel than any other class of elastomer used commercially in automotive fuel systems. High fluorine types are particularly resistant to permeation by oxygenated fuel blends.

Effect of liquids

The degree of swelling can be related to the state of cure of the rubber.

The standardized test procedures are concerned with the resistance of the rubber to the liquid, not the estimation of degree of cure, and generally recommend the measurement of change in dimensions, tensile properties and hardness, as well as volume change.

The time to reach equilibrium or 'maximum' swelling will increase with increased test piece thickness, in a manner roughly proportional to the square of the thickness.

By assuming that swelling is isotropic, i.e., swelling in the thickness direction is equal to that in other directions, volume change can be calculated from:

V = [[(AB/ab).sup.3/2] - 1] x 100%

Where V = volume change; A and B = lengths of diagonals after swelling; and a and b = lengths of diagonals before swelling (ref. 1).

Experimental

Typical FKM compounds were used for this trial (table l). FKM was sourced from Shin Etsu Chemical Co. Ltd. Mixing was carried out on an open mixing mill at ambient temperature, and curing of test slabs and buttons was done for 12 minutes at 150[degrees]C using a standard electrically heated laboratory hydraulic press followed by post-cure for 30 hours at 230[degrees]C in an oven.

Testing was carried out after 24 hours per ASTM specification (ref. 2).

Results and discussion

Swelling is linked to "mass" (in turn thickness, in this case) of the item which may be explained by the mathematical relationship proposed below:

SW [alpha] P * PC * SOL * [mu] * T * SH * CD * 1/TH, where

SW = swelling; P = polymer type; PC = polymer content; SOL = solvent type; T = temperature; SH--hardness (SH-A); CD = crosslink density; TH = thickness; and [mu] = interaction constant characteristic of the rubber and swelling liquid.

For a particular case: P/PC/SOLAT/SH/CD/p are all constant; so swelling is inversely proportional to the thickness:

SW [alpha] 1/TH

Figure 2 presents the swelling in gasoline 70 + ethanol 30 of the samples of different thickness for a fixed time. It shows, irrespective of test temperature, lower thickness exhibits much higher volume swell compared to higher thickness samples. The swelling gets magnified at higher test temperature (55[degrees]C/47 hours compared to RT/47 hours).

Figure 3 shows the effect with respect to dimension of FKM vulcanizate samples in gasoline (70) + ethanol (30). Dimensional change (%) of the sample is higher with higher thickness, irrespective of temperature (RT and 55[degrees]C for 47 hours).

The effect of swelling on mass change (%) of FKM vulcanizate in gasoline (70) + ethanol (30) of different thickness is presented in figure 4. As expected, mass change (%) is reduced with increase in thickness at both temperatures (RT and 55[degrees]C for 47 hours).

Swelling of FKM vulcanizate is related to the inverse of sample thickness, as shown in figure 5. Swelling decreases linearly with the inverse of thickness at RT, but at higher temperature (55[degrees]C), it is not a straight line (figure 5).

Conclusion

After analyzing the swelling results obtained, the following are concluded:

* The volume swell of thinner samples after a fixed time, irrespective of test temperature, shows much higher volume swell for lower thickness compared to higher thickness samples.

* This gets magnified in the case of swelling at higher temperature (55[degrees]C/47 hours compared to RT/47 hours).

* Dimensional change effect is not that pronounced with lower thickness compared to higher thickness samples.

* Higher mass change takes place with lower sample thickness compared to a higher thickness sample.

By S.N. Chakravarty, Polym Consultants, India

References

(1.) Testing of Rubber by Roger Brown,, p. 263.

(2.) ASTMD 471.

Caption: Figure 1--schematic diagram of swelling phenomenon

Caption: Figure 2--swelling of FKM vulcanizate of different thickness in gasoline (70) + ethanol (30)

Caption: Figure 3--dimensional change (%) of FKM vulcanizate of different thickness in gasoline (70) + ethanol (30)

Caption: Figure 4--mass change % of FKM vulcanizate of different thickness in gasoline (70) + ethanol (30)
Table 1--FKM compound formulation

Ingredients                                    Phr

FKM                                            100
Extra light MgO                                3.0
Ca(OH)2                                        5.0
Carnauba wax                                   1.0
Red oxide iron + N 990    As required for coloring

Figure 5--swelling versus inverse of
thickness of FKM vulcanizate in gasoline
(70) + ethanol (30)

Thickness (TH)    1/TH           At FIT/    At 55[degrees]C/
(mm)                            47 hours            47 hours
                          volume swell %      volume swell %

0.5                  2              9.47               18.91
0.8               1.25              7.05                21.9
2.0                0.5              2.77               20.47
4.0               0.25              0.78               13.61
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Author:Chakravarty, S.N.
Publication:Rubber World
Date:Apr 1, 2018
Words:1336
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