• Ozone apparatus with UV lamp, available, for example, from Hedinger, Stuttgart
• Aluminum foil
• Glass tube
• Glass rod
• 2 stoppers
• Wash bottle
• Spring balance
• Press
• Microscope
• Scotch tape
• Carpet knife
  

• Various rubber samples (bicycle inner tube, rubber tubing, natural rubber)
• Various silicone rubber specimens from WACKER's Experimental Kit, e.g. HTV
• Oxygen, O
• Sulfuric acid, conc., C

Allow about 10 minutes to set up the apparatus. It takes about 20 minutes to conduct the experiment, including examining the specimens under the microscope.

 As ozone will be used, conduct the experiment in a fume cupboard.
Cover the ozone reactor with aluminum foil (protects against UV radiation; see photo below).
Cut various rubber specimens into long rectangles of uniform thickness and about 0.5 cm in width.
Clamp each rubber specimen lengthwise, ensuring it is held adequately in the retort (see photo), and measure the elongation under a force of 10 N.
Use Scotch tape to attach the specimen under the same elongation, i.e. under a force of 10 N, to the glass rod and then place this assembly face up in the glass tube (see diagram above).
Generate the ozone by first flushing the entire apparatus with dry oxygen (see diagram above) for about 2 minutes. Then switch on the the UV lamp and slowly (roughly 1 bubble per second) pass the mixture of oxygen and ozone generated in the UV reactor over the rubber specimen inside the glass tube. Continue the ozone treatment for 15 minutes. Examine the specimen for macroscopic, microscopic and mechanical changes.
It is important to ozonate all the specimens under the same conditions in order that their resistance to ozone may be compared.

 Bicycle inner tube (force = 10 N, width = approx. 0.5 cm)
Even as the ozone was being fed in, small cracks formed initially in the surface of the specimen perpendicular to the acting force. The cracks gradually grew until the rubber tore completely after about 10 minutes.
A check of the mechanical properties revealed an extensive decline in tear strength and elasticity.
Examination under the microscope after ozonation revealed small black veins (see photos 1 and 2).

 Silicone rubber HTV(s): (force = 10 N, width = approx. 0.5 cm)
Again, no macroscopic, microscopic (see photos 9 and 10) or mechanical changes were observed.

 Silicone rubber HTV(w): (force = 10 N, width = 0.5 cm)
No macroscopic or mechanical changes were observed either before or after ozonation.
As with silicone HTV(s), the slight differences in the photomicrographs (see photos 11 and 12) may be explained in terms of differences in light intensity during photography and the different structures created by cutting. They are not due to ozonation.

From the observations above, it may be said that “silicone rubber” (more accurately: the silicone rubbers from WACKER's Experimental Kit) is not attacked by ozone under the conditions of the experiment.
In contrast, “normal rubber” of the kind used in rubber tubing (laboratory tubing), bicycle inner tubes and natural rubber, is extensively attacked by ozone and is destroyed.
The order of ozone resistance is therefore as follows:

The “normal types of rubber” are attacked much more extensively than the silicones because they are made from polymers that contain carbon=carbon double bonds (C=C). These react with ozone (see below).
Silicone polymer molecules generally do not contain any carbon=carbon double bonds and thus cannot be attacked by ozone. For example, a 2 % content of ozone in the atmosphere will not bring about any changes in the properties of silicone rubber, not even for 70 hours exposure at 40 °C. Given natural aging processes, silicone rubber is virtually totally resistant to oxygen and ozone.

  The specimens in the experimental apparatus were exposed to ozone concentrations that are much higher than occur naturally (compare the ozone content of air in 5 Supplementary information).
Despite its low concentration at the earth’s surface, ozone does still play an important role in the atmospheric aging of plastics and elastomers. Ozone causes aging in two different ways.

In saturated polymers, it initiates oxidation. The free radicals formed at the start of the reaction then participate in further reactions.

 Mechanical stress (e.g. stretching or elongation) leads to the formation of typical ozone cracks, with the cracks always occurring perpendicular to the acting force.
One possible explanation for the accelerating effect of mechanical stress on the degradation of elastomers by ozone is that the deformation mechanically destroys the ozonated surface layer. This exposes fresh surfaces and facilitates further reactions between the ozone and the elastomer.
Crack formation is a serious type of damage because, if the cracks grow as degradation is happening, the product concerned may be totally destroyed. Ozone cracking depends both on the composition of the rubber and on the mechanical stress, ozone concentration and the temperature.
Ozone-induced changes in the polymer chain of the elastomer manifest themselves as a reduction in tensile strength, extensibility and structural strength. Furthermore, the oxidation generates functional groups containing oxygen that reduce the mobility of the polymer chains and thereby impair the material’s dynamic properties.

• Since the tensile stress acting on the specimens varies with the width and thickness of the specimens, quantitative comparison is not possible. Simple tools (e.g. carpet knife) are unable to cut the specimens uniformly.
• The length of time needed for the specimens to tear fluctuates in accordance with the tensile stress, the ozone concentration, the temperature and the degree of crosslinking.
• Except for the temperature and possible tensile stress, these measuring parameters cannot be kept constant. However, as the observations show, this has no effect on the qualitative results. If the tensile stress is too low, the specimens show no reaction.
• Since no changes were observed in the silicone specimens at 10 N, the experiment was repeated on these specimens by stretching them as far as possible by hand. However, there were no observable changes. Additionally, the silicones were placed in an ozone atmosphere (tube was sealed with Scotch tape) for longer than a day. Again, there were no changes.
• The experiment has a fairly high bearing on everyday life, because resistance to ozone affects the scope for using the elastomers concerned and is thus regularly checked in industry.
• The experiment can serve to familiarize the pupils with an important property, namely the great difference between the resistance of silicone rubber and other natural and synthetic elastomers to ozone.
• This difference in materials can be graphically illustrated at the particle level on the basis of the structural feature of the “C=C double bond”, even if the mechanism of ozonolysis itself is not discussed. It is enough to restrict the classroom lesson to the overall reaction involved in ozonolysis.
  

Ozone O₃ is formed in the ozonosphere at a height of 20 to 60 km above the earth’s surface by the following photochemical reaction of oxygen at wavelengths less than 242 nm.

M represents a collision partner, e.g. a nitrogen or oxygen molecule, which is needed to absorb the energy released.
Ozone formation is counteracted mainly by photolysis at longer wavelengths to maintain a photostationary equilibrium between oxygen and ozone in the ozonosphere.

The constant turbulence present causes some ozone to be transported by airstreams down as far as the earth’s surface. Some of the ozone in the lower layers of the atmosphere is also formed by electrical discharges (lightning) and some ozone close to the ground is formed as photochemical smog when intense sunlight acts on airborne impurities (vehicle exhaust fumes).
Nevertheless, the average concentration of ozone at the earth’s surface is very low. It varies from 0.02 ppm to 0.05 ppm as a function of the season, the location of the measuring point and the meterological conditions.
Further information (in German) on the subject of ozone may be found under http://www.chemiedidaktik.uni-wuppertal.de/ > Unterrichtsmaterial > Ozon.