1. Experiment is contained in the WACKER's Experimental Kit.

No

 2. Experimental procedure has been modified

/

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

No

 5. Flash animation available

No

 6. Other materials: Worksheet 4

Chemical Resistance of Silicone Rubber Compared With That of Other Elastomers

TopDown 1 Materials, Chemicals, Time Needed
  • 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.

TopDown 2 Procedure and Observations
Protective gloves and safety glasses must be worn when working with these chemicals.
Sketch of experiment: Chemical resistance of silicones

TopDown  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:

TopDown Elastomer + acetone:

 
Weight in g
Tear strength at 10 N
   
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
(Due to the low mass of the natural rubber samples, they were not weighed.)
TopDown 
Photo 1: Elastomer samples after one week in acetone
TopDown Elastomer + gasoline:
 
Weight in g
Tear strength at 10 N
   
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.
3
TopDown 
Photo 2: Elastomer samples after one week in gasoline
TopDown Elastomer + hydrochloric acid, c = 2 mol/l:
 
Weight in g
Tear strength at 10 N
   
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
TopDown Elastomer + hydrochloric acid, c = 10 mol/l (HCl conc.):
 
Weight in g
Tear strength at 10 N
   
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
TopDown 
Photo 3: Elastomer samples after one week in concentrated hydrochloric acid
TopDown Elastomer + sodium hydroxide, c = 2 mol/l:
 
Weight in g
Tear strength at 10 N
   
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
TopDown Elastomer + sodium hydroxide, w = 50 %:
 
Weight in g
Tear strength at 10 N
   
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
TopDown Elastomer + mineral oil SAE 15 W 40:
 
Weight in g
Tear strength at 10 N
   
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
TopDown 
Photo 4: Elastomer sample after one week in mineral oil SAE 15 W 40
TopDown Elastomer + sulfuric acid, c = 1 mol/l:
 
Weight in g
Tear strength at 10 N
   
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

TopDown 3 Discussion of Results

The observations for the chemical resistance of the elastomers investigated at room temperature may be tabulated as follows:
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
TopDown  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):
TopDown 
Depolymerization of silicones by hydroxide ions
TopDown 
Depolymerization of silicones by inorganic acids HX
TopDown 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.
Fig.: cis-1,4-Polyisoprene (natural rubber) Source: Thieme Computerlexikon

TopDown 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.

TopDown 4 Tips and Comments

  • 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.

TopBottom  5 Supplementary Information

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.

  1. 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.
  2. 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.
TopBottom 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
Top