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

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 5. Flash animation available

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UV Resistance of Silicone Rubber Compared With That of Other Elastomers
TopDown 1 Materials, Chemicals, Time Needed
  • High-pressure mercury lamp (e.g. Hanau TQ 150 with quartz
  • cooling shaft; see table opposite for spectrum)
  • Cardboard
  • Spring balance + press
  • Scotch tape
  • Laboratory platform
  • Glass rod
  • Petri dish
  • Microscope
  • Carpet knife
  • Stop-watch
  • Various rubber specimens, e.g. bicycle inner tube, rubber tubing (laboratory tubing), natural rubber
    Various silicone rubber specimens from WACKER's Experimental Kit, e.g. HTV(b), HTV(s) and HTV(w).

Preparation and examination of each specimen will take around 15 minutes. Irradiation alone will take a further 30 minutes. Overall, ¾ of an hour should be allowed for each specimen.
  TQ 150 naked emitter
l(nm) Radiation Flux Mole quanta per hour
238/40 1.0 8
248 0.7 5
254 4.0 30
265 1.4 11
270 0.6 5
275 0.3 2
280 0.7

6
289 0.5 4
297 1.0 9
302 1.8 17
313 4.3 41
334 0.5 5
365 6.4 71
390 0.1 1
405/8 3.2 39
436 4.2 55
492 0.1 1
546 5.1 84
577/49 4.7 82
Source: Schule Chemie System SCS UV-Tauch-
lampenreaktor für Schulversuche, M. Tausch
TopDown 2 Procedure and Observations
Naked UV light must be avoided at all costs because it can quickly damage eyes and skin. The irradiation step may lead to the formation of ozone. A fume cupboard and UV protection (cover with cardboard or aluminum foil) must be used when working with the quartz lamp!
TopDown Set up the apparatus in the fume cupboard as shown in the diagram above. Using the laboratory stand, place the petri dish as close as possible to the radiation source. To ensure that comparable results are obtained, do not alter the distance between the petri dish and the radiation source during the series of experiments.

Keep the apparatus covered with an appropriately shaped piece of cardboard during irradiation.
Before irradiation, cut each elastomer specimen into a rectangle and test its tensile strength both manually and under a tensile force of 10N.
Then tape the specimen to the glass rod and stretch it under a tensile force of 10N. Place the specimen face up on the petri dish (right photograph + sketch). Irradiate the specimen for 30 minutes and then examine it for mechanical, macroscopic and microscopic changes.
Repeat this procedure for all the specimens.
The following reproducible observations were made in reference experiments:
TopDown 1) Natural rubber: (force = 1 N, width = approx. 1 cm)
The natural rubber specimen was only stretched under a tensile force of 1 N, as otherwise it would have become too long.
The properties of the natural rubber were severely affected by the radiation.
Macroscopically, the specimen tears under load and turns whitish. The surface of the specimen is tacky.
Examination under the microscope shows that irradiation has led to the formation of a net-like structure consisting of black lines (see photo 2).
Prior to irradiation, the specimen had been transparent, except for a few dirt particles (see photo 1).
TopDown 2) Bicycle inner tube: (force = 10 N, width = approx. 0.9 cm)
The 30 minutes of irradiation failed to produce any macroscopic, microscopic (see photos 3 and 4) or mechanical changes in the bicycle inner tube.

The differences in the brightness of the two photos stem from differences in light intensity when the photos were taken.

3) Silicone rubber HTV(b): (force = 10 N, width = approx. 0.5 cm)
The 30 minutes of irradiation failed to produce any macroscopic, microscopic (see photos 3 and 4) or mechanical changes in the HTV(b) specimens.
Unfortunately, no photomicrographs could be taken as the nature of the specimens made it impossible to focus the lens.

TopDown 4) Silicone rubber HTV(s): (force = 10 N, width = approx. 0.5 cm)
The 30 minutes of irradiation failed to produce any macroscopic, microscopic (see photos 5 and 6) or mechanical changes in the HTV(s) specimens.

The different appearances of the two photomicrographs stem from differences in light intensity and the different structure (parallel stripes) caused by cutting to size. The stripes visible under the microscope were already present before irradiation and were caused by cutting the specimen.

TopDown 5) Silicone rubber HTV(w): (force = 10 N, width = approx. 0.5 cm)
The 30 minutes of irradiation failed to produce any macroscopic, microscopic (see photos 7 and 8) or mechanical changes in the HTV(w) specimens.

The different appearances of the two photomicrographs stem from differences in the light intensity and the different structure (parallel stripes) caused by cutting to size. The stripes visible under the microscope were already present before irradiation and were caused by cutting the specimen.

TopDown 6) Rubber tubing (laboratory tubing): (force = 10 N, width = approx. 1 cm)
Irradiation of the rubber tubing specimens produced different results. In most of the experiments, the microscopic structure of the specimens was altered by the irradiation to the extent that, following irradiation, longitudinal lines and signs of a lattice structure are visible (see photos 9 and 10).
In several cases, however, there were no changes.
None of the rubber tubing specimens showed either macroscopic or mechanical changes.
A possible reason for the different observations may be the different tensile stresses placed on the various specimens. Since the specimens are not exactly the same size and since the Scotch tape yields slightly, the tensile force varies somewhat between the different specimens and only approximates to 10 N.
Closer examination revealed that the microscopically observed changes depended heavily on the distance between the specimen and the radiation source.

TopDown The experimental results are summarized in the following table:
Specimen Observation after 30 minutes’ irradiation
Natural rubber Natural rubber undergoes extensive changes in macroscopic, microscopic and mechanical properties when exposed to UV radiation.
Bicycle inner tube No identifiable changes
Rubber tubing (laboratory tubing) Microscopic changes were found in the rubber tubing. However, there were no changes in the macroscopic and mechanical properties.
HTV(b) No identifiable changes
HTV(s) No identifiable changes
HTV(w) No identifiable changes

TopDown 3 Discussion of Results

The experimental results are in good agreement with literature values concerning the response of different macromolecular compounds to irradiation with UV light.
Whereas UV radiation does not affect silicones, it does affect rubber, which contains cis-1,4-polyisoprene. Its elastic properties, in particular, are damaged. These changes are chemical by nature and are induced by the high-energy UV radiation (see also 5 Supplementary Information). Even visible light, and UV light much more so, causes photochemical oxidation (photooxidation) of natural rubber in the presence of oxygen. This starts off with a reduction in molar mass (i.e., fragmentation of the macromolecules) followed by crosslinking of the fragments. The material turns soft and its surface becomes tacky. A brittle layer forms later. The light causes homolytic scission of several C-H bonds in the macromolecules that make up the natural rubber (scission occurs mainly at allyl C-H bonds, i.e. those in α-position to the carbon=carbon double bonds). This homolysis generates free-radicals that initiate chain reactions in which oxygen is also involved. This is called autoxidation. The primary products of autoxidation are fragments of the original macromolecules, which have now been converted into hydroperoxides (R-O-OH) or peroxides (R-O-O-R) (see reaction mechanisms). These primary products are generally unstable and disintegrate into other products.
TopDown In addition to autoxidation, natural rubber can also undergo photochemical [2+2] cycloaddition because the polymer chains often contain carbon=carbon double bonds and because these chains are close together.
This reaction proceeds as shown below for the example of dimerization of two cis-1,4-polyisoprene molecules (natural rubber):
TopDown Autoxidation and [2+2] cycloaddition are behind the changes observed both in the natural rubber and in the rubber tubing (laboratory tubing), which, as the manufacturer says, consists of a mixture of natural rubber and butadiene-styrene rubber (BSR). The changes are not as pronounced in this specimen as they are in the natural rubber because the rubber tubing specimen is thicker and has been cured more fully. If this specimen were to be exposed to the radiation for longer, there would be greater changes in the properties. However, since the experiment is designed to be performed during one lesson, the effects of longer exposure to the radiation were not examined.
There are two reasons why the bicycle inner tube is resistant to UV radiation: First, the inner tube is made from butyl rubber, which is a copolymer consisting of just 1 to 3 % isoprene (2-methyl butadiene) and 97 to 99 % isobutene. Butyl rubber therefore contains far fewer C=C double bonds than does natural rubber. This means that there are also far fewer groups of atoms that can undergo autoxidation and [2+2]-cycloaddition. Second, the inner tube contains added carbon black, which improves its mechanical properties and wear resistance. Carbon black absorbs light of any wavelength (hence its black color) and thus reduces the number of photons that cause degradation. Even rubber that contains a higher proportion of natural rubber (cis-1,4-polyisoprene) can be made more resistant to UV radiation by adding carbon black.

TopDown 4 Tips and Comments
  • This experiment shows the pupils that irradiation can cause photochemical reactions in elastomers that will adversely affect their properties. The conditions employed in the experiment are much more drastic (short-wave, intense UV radiation) than the rubber would experience in reality. It thus is typical of the experiments used to test the properties of materials.
  • There are simpler experiments than this one for introducing pupils to the interaction between light and materials, as well as photochemical reactions. However, the results presented here could be used in project work and/or young scientist experiments and be supplemented with further experiments.

TopBottom  5 Supplementary Information

The starting point for the chemical reactions of polymers on exposure to visible and UV light is the electronic transition induced by absorption of the electromagnetic radiation (see also http://www.chemiedidaktik.uni-wuppertal.de/ > Licht und Farbe, Licht und Energie).
Under normal conditions, i.e. at room temperature and away from light, molecules are in their base electron configuration (see Fig. 1). The permitted energy levels in the molecule are each fully occupied from the bottom up to the top with pairs of electrons. In molecular orbital theory, the highest occupied energy level is termed the HOMO (highest occupied molecular orbital). Figure 1 illustrates occupancy of the HOMO in the form of two opposing arrows to signify that the electrons have opposite spin. The next energy level above the HOMO permitted for the molecule but not occupied with electrons when the molecule is in the ground state is called the LUMO (lowest unoccupied molecular orbital).
By absorbing a photon of energy , i.e. precisely the energy difference between the LUMO and the HOMO, the molecule shifts from the ground state to its excited state. When the electron jumps from the HOMO to the LUMO, a process that takes approx. 10-15 s, it retains its spin.
TopDown The excited state is said to be a singlet state (see Fig. 2; the term singlet will not be explained here). Having reached the excited singlet state, the system (the excited molecule) can return to its ground state within approx. 10-9 s again by emitting a light quantum (observed as fluorescence) or emitting heat. A third possibility is for it to enter into a chemical reaction, e.g. homolytic scission of a bond. Homolysis of a C-H bond, the first step in autoxidation, is an example of this type of photochemically induced reaction in the excited singlet state.
TopDown Under certain conditions, which will not be explained here, the excited electron can reverse its spin. The resultant excited triplet state (see Fig. 3) has a much longer life time of 10-3 to 101 s than the singlet state. The reason is that a return from the excited triplet state to the ground state is forbidden by quantum mechanics and thus is much more unlikely than a return from the excited singlet state to the ground state.
When excited molecules in the triplet state emit light quanta and so become deactivated, the observer sees this as phosphorescence, which lasts longer than fluorescence. Because of its relatively long life time, an excited molecule in the triplet state can also participate in bimolecular reactions, i.e., in reactions that require a collision between it and another molecule. In the discussion above of the photochemical reactions occurring in natural-rubber elastomers (cis-1,4-polyisoprene), triplet states are likely also involved, especially in the bimolecular reaction steps.
TopBottom  6 References
  • M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), S. 297 - 302
  • D. Wöhrle, M. W. Tausch, W.-D. Stohrer, PHOTOCHEMIE - Konzepte, Methoden, Experimente, Wiley-VCH, Weinheim (1998)
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