The Self Accelerating Decomposition Temperature (SADT) is the lowest temperature at which an organic peroxide in a typical vessel or shipping package will undergo a self-accelerating decomposition within one week. The SADT is the point at which the heat evolution from the decomposition reaction and the heat removal rate from the package of interest become unbalanced. When the heat removal is too low, the temperature in the package increases and the rate of decomposition increases in an uncontrollable manner. The result is therefore dependent on the formulation and the package characteristics.
A self-accelerating decomposition occurs when the rate of peroxide decomposition is sufficient to generate heat at a faster rate than it can be dissipated to the environment. Temperature is the main factor in determining the decomposition rate, although the size of the package is also important since its dimensions will determine the ability to dissipate heat to the environment.
All peroxides contain an oxygen-oxygen bond that, on heating, can break apart homolytically to generate two radicals. As mentioned previously, this decomposition also generates heat. But the stability of the oxygen-oxygen bond is dependent on what else is present in the molecule. Some peroxides, due to their chemical make-up, are very unstable and need to be refrigerated to avoid a self-accelerating decomposition. Others, particularly those used for crosslinking purposes, are much more stable and can be stored at normal ambient temperatures without risk of self-acceleration. Due to the large variations in the stabilities of peroxides, each is tested to determine the safe maximum temperature for which the peroxide may be stored, shipped, and handled. The result of this test is the self-accelerating decomposition temperature (SADT).
Although a number of organic peroxides can safely be stored at room temperature, most require some form of temperature control. For long storage periods, the organic peroxide is usually kept at a lower temperature than the maximum safe storage temperature as determined by the SADT.
The SADT for an organic peroxide formulation is usually lower for more concentrated formulations. Dilution with a compatible, high boiling point diluent will usually increase the SADT since the peroxide is dilute and the diluent can absorb much of the heat minimizing the increase in temperature. Also, for an organic peroxide formulation, larger packages generally have a lower SADT because of the poorer heat transfer of the larger package due to lower surface area to volume ratio. Most organic peroxides react to some extent with their decomposition products during thermal decomposition. This often increases the rate since the decomposition proceeds more rapidly as the decomposition products are generated.
The SADT measurement is made as follows:
- The package containing the peroxide is placed in oven set for test temperature
- The timer starts when product reaches 2°C below intended test temperature
- The oven is held at constant temperature for up to one week or, until a runaway event occurs.
- Test “Passes” if product does not exceed test (oven) temperature by 6°C within one week
- Test “Fails” if product exceeds test temperature by 6°C within one week
- The test is repeated in 5°C increments until a failure is reached
- Fail temperature is reported as SADT for that package and formulation
- Secondary information about the violence of the decomposition can also be recorded
As an alternative to the oven test the SADT for larger packages can be determined by substituting a Dewar flask for the package. The heat transfer of the Dewar flask can be matched to the heat transfer of a larger package size. This test is called the Heat Accumulation Storage Test (HAST).
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