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Chemical Resistance of Silicone Rubber
Compared With That of Other Elastomers |
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  1
Materials, Chemicals, Time Needed |
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- Vials with snap-on lids
- Spring balance + press
- Tweezers
- Balance
- Various natural rubber specimens (shoe sole, rubber tube, natural
rubber)
- Various silicone rubber samples from WACKER's Experimental Kit,
e.g. HTV(b), HTV(s), HTV(w) and a self-made silicone elastomer RTV2(r)
- Acetone, F
- Gasoline (normal, unleaded), F
- Hydrochloric acid, c = 2 mol/l, hydrochloric acid, conc. C
- Sodium hydroxide, c = 2 mol/l, sodium hydroxide, conc.
w = 50 %, C
- Mineral oil (e.g. SAE 15 W 40), F
- Sulfuric acid, c = 1 mol/l, C
It is not possible to
state the precise duration for this experiment as the time needed
for sample preparation, analysis etc. will depend on the number
of pupils, samples and chemicals. |
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| 2
Procedure and Observations |
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| Protective gloves and safety glasses must be worn when working
with these chemicals. |
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| Sketch
of experiment: Chemical resistance of silicones |
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First,
slice the various rubber samples into rectangles. Investigate
the tensile strength of the samples manually and with a tensile
force of 10 N. Determine the weight of each sample.
Place the elastomer samples in the different vials (or test
tubes), cover each one with a few milliliters of a different
chemical, put the lids on and leave for about one week.
Then remove each sample from the chemical, dry it and weigh
it again. Examine the samples for changes in appearance and
tensile strength.
Tabulate the observations and rate the chemical resistance
on a scale of 1-3.
1 = High resistance (no changes), 2 = Average resistance (slight
changes, e.g. color of solution has changed) and 3 = Low resistance
(material is extensively attacked or destroyed).
The following
observations were made after one week in a reference experiment:
Elastomer
+ acetone: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
Slightly swollen,
but no other visible changes. No change in elasticity or tear strength |
2 |
| Shoe sole |
0.2 |
0.25 |
Yes |
Yes |
Liquid slightly
yellow. No visible changes to sole.
No change in elasticity or tear strength |
2 |
| Rubber tubing |
0.45 |
0.5 |
Yes |
Yes |
Liquid slightly
yellow. No visible changes to tubing.
No change in elasticity or tear strength |
2 |
| HTV(b) |
0.3 |
0.3 |
Yes |
Yes |
No visible changes.
No change in elasticity or tear strength |
1 |
| HTV(s) |
0.3 |
0.35 |
Yes |
Yes |
No visible changes.
No change in elasticity or tear strength |
1 |
| HTV(w) |
0.55 |
0.37 |
Yes |
Yes |
No visible changes.
No change in elasticity or tear strength |
1 |
| RTV2(r) |
0.55 |
0.65 |
Yes |
Yes |
No visible changes.
No change in elasticity or tear strength |
1 |
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| (Due to the low mass of the natural rubber samples,
they were not weighed.) |
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| Photo
1: Elastomer samples after one week in acetone |
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| Elastomer
+ gasoline: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
No |
No outer changes.
Sharp drop in elasticity and tear strength. |
3 |
| Shoe sole |
0.2 |
0.3 |
Yes |
No |
Sample slightly swollen.
Sharp drop in elasticity and tear strength. |
3 |
| Rubber tubing |
0.5 |
1.05 |
Yes |
No |
Sample very swollen.
Sharp drop in elasticity and tear strength. |
3 |
| HTV(b) |
0.4 |
0.7 |
Yes |
Yes |
Sample moderately swollen.
No change in elasticity or tear strength. |
2 |
| HTV(s) |
0.5 |
0.85 |
Yes |
Yes |
Sample moderately swollen.
No change in elasticity or tear strength. |
2 |
| HTV(w) |
0.2 |
2 |
Yes |
No |
Sample extensively swollen.
Sharp drop in elasticity and tear strength. |
3 |
| RTV2(r) |
0.2 |
0.77 |
Yes |
No |
Sample extensively
swollen.
Sharp drop in elasticity and tear strength.
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3 |
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| Photo
2: Elastomer samples after one week in gasoline |
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| Elastomer
+ hydrochloric acid, c = 2 mol/l: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
Slight yellow discoloration of sample.
No change in elasticity or tear strength |
2 |
| Shoe sole |
0.3 |
0.3 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| Rubber tubing |
0.35 |
0.4 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(b) |
0.45 |
0.5 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(s) |
0.3 |
0.3 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(w) |
0.3 |
0.3 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| RTV2(r) |
0.4 |
0.4 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
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| Elastomer
+ hydrochloric acid, c = 10 mol/l (HCl conc.): |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
Sample turned
white. No change in elasticity or tear strength |
2 |
| Shoe sole |
0.25 |
0.27 |
Yes |
Yes |
Solution turned
deep yellow.
Sample feels somewhat harder.
No change in elasticity or tear strength
|
2 |
| Rubber tubing |
0.25 |
0.3 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(b) |
0.45 |
Not measurable |
Yes |
No |
Solution turned
slightly yellow.
Sample itself turned deep blue and crumbles on removal and when handled.
No longer elastic or tear resistant |
3 |
| HTV(s) |
0.3 |
0.25 |
Yes |
Yes |
No visible changes.
No change in elasticity or tear strength
|
1 |
| HTV(w) |
0.3 |
0.3 |
Yes |
Yes |
Sample slightly tacky; no visible changes otherwise.
Sample tears somewhat faster, but passes the test. |
2 |
| RTV2(r) |
0.6 |
0.7 |
Yes |
No |
Sample turned yellow and is very tacky and greasy.
Solution also turned yellowish.
Sample no longer elastic and snaps when pulled. |
3 |
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| Photo
3: Elastomer samples after one week in concentrated hydrochloric
acid |
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| Elastomer
+ sodium hydroxide, c = 2 mol/l: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
Sample no longer as transparent; no other changes discernible.
No change in elasticity or tear strength |
1 |
| Shoe sole |
0.2 |
0.2 |
Yes |
Yes |
Sample somewhat softer and solution has turned yellow.
Slight increase in elasticity. No change in tear strength. |
2 |
| Rubber tubing |
0.35 |
0.48 |
Yes |
Yes |
Sample feels somewhat softer.
Liquid has turned deep yellow-orange.
Elastic, but not as tear resistant |
2 |
| HTV(b) |
0.35 |
0.35 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(s) |
0.25 |
0.25 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(w) |
0.3 |
0.3 |
Yes |
Yes |
Sample slightly tacky; no visible changes otherwise.
No change in elasticity or tear strength |
1 |
| RTV2(r) |
0.27 |
0.28 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
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| Elastomer
+ sodium hydroxide, w = 50 %: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
Sample no longer as transparent; no other changes discernible.
No change in elasticity or tear strength |
1 |
| Shoe sole |
0.2 |
0.25 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| Rubber tubing |
0.922 |
0.908 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(b) |
0.223 |
0.192 |
Yes |
Yes |
Surface of sample is dark blue, crumbly and soapy. No change beneath surface
Elastic and tear resistant |
3 |
| HTV(s) |
0.682 |
0.695 |
Yes |
Too soapy to measure |
Surface of sample is tacky and soapy.
No change beneath surface.
Elastic and tear resistant |
3 |
| HTV(w) |
0.677 |
0.697 |
Yes |
Too soapy to measure |
Surface of sample is tacky and soapy.
No change beneath surface.
Elastic and tear resistant |
3 |
| RTV2(r) |
0.645 |
0.639 |
Yes |
Yes |
Somewhat soapy; no changes otherwise. No change in elasticity or tear strength |
2 |
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| Elastomer
+ mineral oil SAE 15 W 40: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
No |
Sample slightly swollen; no other changes discernible.
Elasticity almost zero
Sharp drop in tear strength |
3 |
| Shoe sole |
0.25 |
0.28 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| Rubber tubing |
0.3 |
0.43 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(b) |
0.3 |
0.303 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(s) |
0.27 |
0.276 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(w) |
0.25 |
0.246 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| RTV2(r) |
0.6 |
0.613 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
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| Photo
4: Elastomer sample after one week in mineral oil SAE 15 W 40 |
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| Elastomer
+ sulfuric acid, c = 1 mol/l: |
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Weight in g |
Tear strength at 10 N |
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| Sample |
Before |
After |
Before |
After |
Observation |
Chemical
resistance |
| Natural rubber |
/ |
/ |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| Shoe sole |
0.2 |
0.246 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| Rubber tubing |
0.2 |
0.249 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(b) |
0.5 |
0.42 |
Yes |
Yes |
Sample has turned somewhat darker; no other changes discernible.. No change in elasticity or tear strength |
1 |
| HTV(s) |
0.2 |
0.235 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| HTV(w) |
0.25 |
0.255 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
| RTV2(r) |
0.6 |
0.637 |
Yes |
Yes |
No visible changes. No change in elasticity or tear strength |
1 |
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3
Discussion of Results |
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| The observations for the chemical resistance of
the elastomers investigated at room temperature may be tabulated
as follows: |
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Natural rubber |
Shoe sole |
Rubber tubing |
HTV(b) |
HTV(s) |
HTV(w) |
RTV2(r) |
| Acetone |
1 |
2 |
2 |
1 |
1 |
1 |
1 |
| Gasoline |
3 |
3 |
3 |
2 |
2 |
3 |
3 |
| Hydrochloric
acid 2 mol/l |
2 |
1 |
1 |
1 |
1 |
1 |
1 |
| Hydrochloric
acid 10 mol/l |
2 |
2 |
1 |
3 |
1 |
2 |
3 |
| Sodium
hydroxide 2 mol/l |
1 |
2 |
2 |
1 |
1 |
1 |
1 |
| Sodium
hydroxide 50 % |
1 |
1 |
1 |
3 |
3 |
3 |
2 |
| Mineral
oil |
3 |
1 |
1 |
1 |
1 |
1 |
1 |
| Sulfuric
acid 1 mol/l |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
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|
Comparison
of the chemical resistances shows that silicones are resistant
to mineral oil, acetone, and fairly dilute solutions (c < 2
mol/l) of inorganic acids and bases. They are attacked by concentrated
solutions of strong acids and bases, and by gasoline.
The observed discrepancies among the silicones investigated are
due to different additives and degrees of crosslinking.
The observed behavior of the silicones toward acids and bases
can be explained in terms of the chemical structure of their
molecules. Due to the polarity of the siloxane linkage Si-O,
the negatively charged oxygen atom could undergo electrophilic
attack, while the positively charged silicon atom could undergo
nucleophilic attack.
This means that the following reactions are theoretically possible
for the degradation of silicones by inorganic acids and bases
(the curved arrows in the equations indicate the movement of
electron pairs):
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| Depolymerization
of silicones by hydroxide ions |
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| Depolymerization
of silicones by inorganic acids HX |
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However,
high concentrations of hydronium or hydroxide ions, i.e. concentrated
solutions of strong acids and bases, are needed before these
reactions will proceed. This explains why silicones are stable
to dilute solutions of inorganic acids and bases.
The action of gasoline on silicones is not a chemical reaction.
Instead, physical processes are at work. The swelling of the
silicones in gasoline is due to the fact that nonpolar molecules
penetrate from the gasoline into the macromolecular silicone
material and accumulate in the molecular network.
The organic elastomers studied show moderate to very good resistance
to acids and alkalies. But, they are attacked by organic substances,
such as engine oil, gasoline and acetone (which cause swelling
or dissolution).
The resistance of the organic elastomers to acids and bases can
be explained in terms of their particle structure.
Organic elastomers generally are built up from unsaturated polymers.
Natural rubber consists of cis-1,4-polyisoprene macromolecules
(see diagram) that become linked to each other via sulfur atoms
during vulcanization.
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| Fig.: cis-1,4-Polyisoprene (natural
rubber) Source: Thieme Computerlexikon |
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The
primary points of attack by reagents in polyisoprene molecules
are the carbon-carbon double bonds. These, however, are stable
to direct nucleophilic attack by, e.g. hydroxyl ions. On the
other hand, hydronium ions from acidic solutions can attack
the carbon-carbon double bonds by electrophilic addition. This
explains why the examined organic elastomers were found to
be more resistant in concentrated sodium hydroxide than in
concentrated hydrochloric acid.
The fact that the organic elastomers, which essentially consist
of nonpolar molecules, gradually swell up and dissolve in organic
solvents such as gasoline, acetone and engine oil is due to
interactions between the molecules of the solvents and the
molecules of the organic elastomers (Like dissolves Like).
These interactions are primarily Van der Waals forces. |
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4
Tips and Comments |
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- As an alternative to this test, the chemical
resistance of the elastomer samples was studied by exposing
them to gasoline,
concentrated hydrochloric acid, c = 10 mol/l, sodium hydroxide,
w = 50% and acetone at 45°C. The elastomer samples were
placed along with the respective liquid in a round-bottom flask,
which
was then sealed and heated for roughly one hour at approx.
45 °C.
The results for conc. hydrochloric acid and gasoline were
the same as those observed in the long-term test. No changes
were
observed, however, in the case of either conc. sodium hydroxide
or acetone. Quite likely, the reason for this is that the
reaction rate and the diffusion rate were too low. The
exposure period
was therefore not long enough for a visible change to occur.
- A
further addition to the test was to examine all samples
under a microscope (magnification: x100) before and after exposure
to the liquids. No microscopic changes were observed,
however,
in any of the examined samples. For time reasons, microscopic
examination of the samples should not be performed in
class.
This experiment teaches the pupils how to determine the
chemical resistance of elastomers and other polymers
to different
liquids. It enables them to choose the best polymer for
every application.
For example, there would be little point in using seals
made from natural rubber in a gasoline pipe.
- As for silicones, the
pupils learn how silicones react to different chemicals,
especially inorganic acids and bases. Here, again,
they learn the didactically important relationship between
substance property and particle structure.
- Since the experiment is relatively
safe if the safety advice is followed (wear gloves, safety
glasses), it makes a good classroom
experiment. It is advisable to let the pupils work in
groups to save time and to encourage communication. Each group
should
examine the behavior of the samples in one liquid. The
samples should be prepared at the beginning or the end of a
class
to prevent valuable teaching time from being wasted. The
results could then be discussed the following week.
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5
Supplementary Information |
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The chemical resistance of
polymer materials, such as natural rubber and silicone rubber,
can be
divided into two groups according to the form of exposure.
- Physically active media do not react with macromolecular
material, but they
will contribute to swelling to the point of dissolution,
thereby causing reversible changes to the material’s
properties.
The underlying mechanism of physically active media is based
on the destruction of the molecular bonds between the macromolecules.
Chemically inert hydrocarbons and some of their derivatives
are examples of such liquids.
Physically active media may cause leaching of additives (e.g.
plasticizers) from plastics and elastomers, as a result of
which the properties of the material are irreversibly damaged.
The reaction of a macromolecular material to physically active
media can be estimated from the polarity of each reagent.
Generally, materials made from nonpolar molecules will swell
up and dissolve
in nonpolar solvents whereas they will be physically resistant
to polar compounds. Macromolecules containing polar groups
will either be soluble in polar solvents or will swell up,
whereas
they will be resistant to nonpolar compounds.
The resistance of macromolecular materials to physically
active liquids is dependent on other factors, especially
the degree
of crosslinking between the macromolecules. Swelling in solvents
decreases with increasing crosslinkage. Thus, ebonite swells
much less than soft rubber in hydrocarbons.
Fillers can also influence the resistance of polymers to
physically active media.
- Chemically active media react with
the macromolecular material and change its properties
irreversibly. Chemical degradation
of macromolecules is typified by the fact that minor
chemical changes are enough to cause very distinctive changes
in
physical properties.
Macromolecular substances obey the same laws as low-molecular
substances during chemical reactions. The course of
the reaction, however, is different for macromolecular
and
low-molecular
substances.
As a rule, the reaction between the aggressive chemical
and the macromolecular material takes place in a heterogeneous
system
(solid/liquid or solid/gas), which is why the course
of the
reaction, just like swelling and dissolution, is heavily
influenced by
diffusion. The consequence in some chemical reactions
is that a film of reaction products forms between the
macromolecular
material and the aggressive medium to impede diffusion
of the
medium and therefore reduce the degradation rate. Such
protective surface coatings explain the high chemical
resistance of
natural rubber to hydrochloric acid and sulfuric acid.
Furthermore, the resistance of macromolecular materials
to chemically active substances is influenced over
wide limits
both positively
and negatively by fillers and other additives.
The crosslinking density also has an influence on the
chemical resistance.
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| 6
References |
|
- M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT,
C.C. Buchner, Bamberg (1993), (1998), S. 337 - 352
- M. Tausch, M. von Wachtendonk (editors), STOFF-CHEMIE S
I, FORMEL-UMWELT, C.C. Buchner, Bamberg (1996), (1997), S.
228 - 233
- M. Tausch, M. von Wachtendonk (editors), CHEMIE 2000+,
C.C. Buchner, Bamberg (2001), S. 60 - 67
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