Is FAS stable at high temperature?

Direct Answer

Direct Answer The solid is stable at ambient temperature when dry, but strictly speaking it is not a heat-resistant material.

Thermal behaviour

• Ambient and dry: stable, and can be stored for at least 6 months in a sealed container
• Above 100 C: according to our technical data sheet, slow decomposition begins, releasing sulphur dioxide
• Above 123 C: according to the safety data sheet, this is the region of pronounced decomposition
• 124 to 127 C: melting range, accompanied by decomposition
• 126 C: exothermic decomposition
• Damp: yellows and releases heat, which must be avoided

Decomposition products

The safety data sheet states that thermal oxidative decomposition of formamidinesulfinic acid can produce toxic fumes including carbon monoxide, carbon dioxide, hydrogen cyanide, sulphur oxides and nitrogen oxides. That has to be taken into account in high-temperature operations and fire emergency planning, and protection cannot be planned on the assumption that only sulphur dioxide is released.

Comparison with sodium hydrosulfite

Hydrosulfite decomposes noticeably above about 85 C, can decompose explosively at 190 C, and reacts on contact with water. formamidinesulfinic acid (FAS) has a higher decomposition threshold and a more controllable exotherm, which is the basis of its better storage and transport profile.

Two concepts that need separating

The thermal stability of the solid product and its decomposition as a reducing agent in a bleaching liquor are two different things. In bleaching, thermal decomposition is part of the mechanism: it is that decomposition which generates the sulfoxylate that does the reducing. Being unstable at high temperature is therefore not a drawback for bleaching, it is how the product works.

Storage recommendations

Bottom line

The solid is stable under normal storage and starts to decompose above 100 C, which is simply the product doing its job. The point that needs particular attention is that thermal oxidative decomposition can produce toxic fumes including hydrogen cyanide.

Sources

1. 我司产品 SDS 与 TDS（危害分类、运输分类、热分解温度、溶解度等物性数据）
2. US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en
3. 我司产品安全数据表《Material Safety Data Sheet — THIOUREA DIOXIDE》（MSDS，签发日期 2023-03-03）
4. 我司产品技术数据表《THIOUREA DIOXIDE Technical Data Sheet》（TDO-TDS，签发日期 2024-05-08）