Azodicarbonamide decomposition temperature

A most widely used decomposable chemical blowing agent is azodicarbonamide. Its decomposition temperature and rate of evolution of gaseous components are greatly influenced by the stabilizers containing zinc. Lead and cadmium are considered moderate activators for, -oxybis benzenesulfonyl hydrazide (OBSH). OBSH can also be used as a blowing agent for PVC foams. 

An 850 kg batch of a slightly doped form of azodicarbonamide exploded violently, with a TNT equivalence of 3.3 kg, 5 minutes after sampling at the end of drying. The probable initial temperature was 65°C, the lowest self accelerating decomposition temperature 90°C, and such decomposition is not explosive. Full explosibility tests, including detonability, had shown no hazard. Further study demonstrated that slightly contained azodicarboxamide, thermally initiated at the bottom of a column or conical vessel could explode even at the 5 kg scale. The above TNT equivalence corresponds to decomposition of 4% of the available charge. The cause of the presumptive hot spot is unknown. 

Examples of chemical blowing agents are azo compounds, N-nitroso-compounds, and sulphonyl hydrazides, which yield 10()-3()() cm of nitrogen per gram of compound at temperatures of 90-2 7 5°C. Azodicarbonamide is widely used, having a decomposition temperature of 230-235°C, which can be reduced to 155-200°C by means of metal compounds such as lead and zinc stabilizers. It can thus match the temperatures at which the melt viscosity of many polymers is suitable for foaming, and is used (typically) in calendered PVC and PVC plastisols and in structural foam forms of polyethylene, polypropylene, PVC, polystyrene, and ABS. 

Type Azodicarbonamide Appearance Orange/Yellow fine powder Decomposition Temperature Range 200>205C Gas/Volume (ml/g) 9 STP 215-225 Average Particle Size (Fisher Subsiever) ... 

The polymer used was high density polyethylene (HOPE) PADMEX-60120 from PEMEX (Mexico). Its melt flow index is 12 g/10 min with a density of 961 kg/m and a melting temperature of 124°C. The chemical blowing agent was azodicarbonamide (ACA) from Electroquimica Mexicana S.A. de C.V. The decomposition temperature of ACA was around 167°C [8]. 

Figure 110 The effect of reactants on temperature of decomposition and yield of gas from azodicarbonamide reaction medium 400 g dioctylphthalate, 3 g azodicarbonamide reactants (1) 1 g zinc oxide (2) 1 g zinc oxide, 0.25 g trimellitic anhydride (3) no reactant (4) 1.5 g thiourea...

Expansion by means of azodicarbonamide and associated metal salt activators was mentioned earlier many substances have been suggested for use as inhibitors for such systems, their mode of action typically being to raise the temperature of decomposition of the blowing system and hence reduce the volume and speed of evolution of gas. Some inhibitors act by neutralizing metal salt activators so that they no longer have the effect of reducing the temperature of decomposition. Examples of this type include fumaric acid, trimellitic anhydride, and benztriazole. Others react with the azodicarbonamide and transform it into a different compound substantially resistant to the activation thiourea is an example of this type. Inhibition in these different forms is exemplified in Figure 110. 

Secondly, the mixture produced is thermoformed in a hot-plates press at 80 °C for 5 min using a pressure of 1 ton with the aim of obtaining circular samples. The thermoformed sample of activated NR was then introduced into a circular mould. The mould was also placed in the hot-plates press where the vulcanization and foaming process were performed simultaneously. The reason for choosing a hot-plates press instead of a conventional oven was that the heat transfer is faster in a press than in an oven, which consequently allows a reduction in the production time. Nevertheless, no pressure was applied to the material. The plates of the press were only in contact with the upper and lower surfaces of the mould. Temperature and time were set up respectively to 160 °C and 30 min. During this time, NR was crossUnked and expanded due to the decomposition of azodicarbonamide in gases, mainly N2 and CO, allowing NR to fill the mould completely. Finally, the mould was water cooled and the foam was removed from it. 

Prasad et al. [106] reported a linear relationship between A7/d (0-425 J/g) and azodicarbonamide content (0-36%). DSC thus allows detection of the level of undecomposed CBAs present in processed foam products and establishes the onset temperature for the decomposition. Advantages of DSC over EGA techniques are ease of operation, shorter analysis time, and detection of azodicarbonamide concentrations as low as 1%. Dixon etal. [107] have correlated thermal analysis data (DSC, TGA) of a variety of CBAs with cell morphology of extruded, expanded PP rod samples. CBAs with a higher temperature and rate of gas evolution lead to foams displaying a finer cell size structure and higher cell density. 

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