GB/T 7762-2014Rubber, vulcanized or thermoplastic -- Resistance to ozone cracking -- Static strain test (English PDF)

5.1 Test chamber

This shall be a closed, non-illuminated chamber, thermostatically controlled to within ±

2°C of the test temperature, lined with, or constructed of, a material (for example aluminium) that does not readily decompose ozone. The chamber can be provided with a window through which the surface of the test pieces can be observed. A light to examine test pieces can be installed, but this shall remain switched off at all other times.

5.2 Source of ozonized air

The ozonized air shall be largely free of nitrogen oxides in order to avoid errors in the ozone concentration.

One of the following items of apparatus shall therefore be used.

5.3 Means of adjusting the ozone concentration

When an ultra-violet lamp is used, the ozone concentration can be controlled by adjusting either the voltage applied to the tube or the input-gas flow rate, or by shielding part of the tube from the UV light.

5.4 Means of determining the ozone concentration

A means of sampling the ozonized air from the vicinity of the test pieces in the chamber and a means of estimating the ozone content are specified in ISO 1431-3.If required, the methods given in Annex B can also be a reference.

5.5 Means of adjusting the gas flow

A mechanism shall be provided that is capable of adjusting the average velocity of the flow of ozonized air in the test chamber to a value of not less than 8 mm/s and preferably to a value between 12 mm/s and 16 mm/s, calculated from the measured gas flow rate in the chamber divided by the effective cross-sectional area of the chamber normal to the gas flow.

5.6 Mounting test pieces for static strain testing

Clamps shall be provided to hold the test pieces at the required elongation and with both sides in contact with the ozonized air in such a manner that the longitudinal axis of each test piece is substantially parallel to the direction of gas flow. The clamps shall be made of a material (for example aluminium) which does not readily decompose ozone.

6.2 Wide test piece

This test piece shall consist of a strip of not less than 10 mm in width, of thickness 2.0 mm ± 0.2 mm and of length not less than 40 mm between the grips before stretching.

The ends of the test piece held in the grips can be protected with an ozone-resistant lacquer. Care shall be taken when selecting the lacquer to ensure the solvent used does not appreciably swell the rubber.

6.3 Narrow test piece

This test piece shall consist of a strip of width 2.0 ± 0.2 mm, thickness 2.0 ± 0.2 mm and length 50 mm, between enlarged tab ends 6.5 mm square (see Figure 3). This test piece shall not be used for procedure A.

7.1 Conditioning in the unstrained state

For all tests, the minimum time between vulcanization and straining of the test pieces shall be 16 h.

7.2 Conditioning in the strained state

After stretching, the test pieces shall be conditioned for a period of between 48 h and

96 h in an essentially ozone-free atmosphere in the dark; the temperature for this conditioning shall normally be a standard laboratory temperature (see GB/T 2941-

2006), but other temperatures can be used if appropriate for particular applications.

The test pieces shall not be touched or otherwise disturbed in any way during the conditioning period.

8.1 Ozone concentration

The test shall be carried out at one of the following ozone concentrations, expressed in volume fraction.

8.2 Temperature

The preferred test temperature is 40°C ± 2°C. Other temperatures, such as 30°C ± 2°C or 23°C ± 2°C, can be used if they are more representative of the anticipated service environment, but the results obtained will differ from those obtained at 40°C ± 2°C.

8.3 Relative humidity

The relative humidity of the ozonized air shall normally be not more than 65% at the test temperature.

8.4 Maximum elongation

Tests shall normally be carried out using one or more of the following strain levels.

(5 ± 1) %, (10 ± 1) %, (15 ± 2) %, (20 ± 2) %, (25 ± 2) %, (30 ± 2) %, (40 ± 2) %, (50 ±

2) %, (60 ± 2) %, (80 ± 2) %

9.1 General

Adjust the rate of flow and temperature of the ozonized gas and its ozone concentration to that required and place the strained test pieces suitably conditioned, in the test chamber. Maintain the test conditions at the required levels.

9.2 Procedure A

Unless otherwise specified, strain the test pieces at 20% elongation, condition them in accordance with 7.2, and examine them after 72 h in the test chamber for the development of cracking (an alternative elongation and an alternative exposure period may be given in the appropriate material specification).

9.3 Procedure B

Strain the test pieces at one or more of the elongations given in 8.4 and condition them in accordance with 7.2.If only one elongation is used, this shall be 20%, unless otherwise specified. Examine the test pieces after 2 h, 4 h, 8 h, 24 h, 48 h, 72 h and

96 h and, if necessary, at suitable intervals thereafter in the test chamber and note the time until the first appearance of cracks at each elongation.

9.4 Procedure C

Strain the test pieces at no fewer than four of the elongations given in 8.4 and condition them in accordance with 7.2.Examine the test pieces after 2 h, 4 h, 8 h, 24 h, 48 h, 72 h and 96 h and, if necessary, at suitable intervals thereafter in the test chamber and note the time until the first appearance of cracks at each elongation so that the threshold strain can be estimated.

......