Decomposition Temperature

Detailed Description

Decomposition temperature is the temperature at which a substance chemically decomposes, breaking down into simpler substances or elements. This process typically involves the breaking of chemical bonds and often results in the release of gases, heat, or both. In the context of a Safety Data Sheet (SDS), decomposition temperature is a critical safety parameter that indicates the thermal stability of a substance and the potential hazards associated with its decomposition.

Key concepts related to decomposition temperature include:

Onset temperature: The temperature at which decomposition begins to be detected by analytical instruments.
Peak temperature: The temperature at which the rate of decomposition is at its maximum.
Endset temperature: The temperature at which the decomposition process is essentially complete.
Exothermic decomposition: Decomposition that releases heat, potentially leading to self-accelerating reactions.
Endothermic decomposition: Decomposition that absorbs heat, typically less hazardous than exothermic decomposition.
Self-accelerating decomposition temperature (SADT): The lowest temperature at which self-accelerating decomposition may occur in a substance in the packaging as used in transport.
Thermal stability: The resistance of a substance to chemical change or breakdown when subjected to heat.

Importance in Safety Data Sheets

Decomposition temperature information in an SDS is important for several reasons:

Process safety: Defines safe operating temperature ranges for processing, handling, and storage.
Storage requirements: Informs appropriate storage temperature conditions and thermal protection needs.
Hazard assessment: Helps evaluate the risk of thermal decomposition in various scenarios.
Decomposition products: Often linked to information about hazardous decomposition products that may be generated.
Equipment design: Guides selection of appropriate equipment and materials for handling and processing.
Emergency response: Provides critical information for firefighting and emergency planning.
Transportation: Informs appropriate transportation conditions and hazard classifications.
Shelf life determination: Helps establish appropriate shelf life and storage conditions.

Measurement Methods

Several analytical techniques are used to determine decomposition temperature:

Method	Description	Advantages	Limitations
Differential Scanning Calorimetry (DSC)	Measures heat flow associated with thermal transitions as a function of temperature	Small sample size, quantitative heat measurement, widely available	May miss subtle decomposition, pressure effects not captured
Thermogravimetric Analysis (TGA)	Measures weight changes as a function of temperature	Directly measures mass loss, can identify multi-stage decomposition	No heat flow information, not sensitive to decomposition without mass loss
Differential Thermal Analysis (DTA)	Measures temperature difference between sample and reference as a function of temperature	Can handle larger samples, simpler instrumentation	Less quantitative than DSC, lower sensitivity
Accelerating Rate Calorimetry (ARC)	Adiabatic calorimetry that measures self-heating rates	Simulates real-world scenarios, measures pressure effects	Requires larger samples, more complex and expensive
Isothermal Stability Testing	Holds sample at constant temperature and monitors for changes	Provides time-dependent stability information	Time-consuming, may require multiple tests
Evolved Gas Analysis (EGA)	Analyzes gases released during heating (often coupled with TGA)	Identifies decomposition products, provides mechanistic insights	Requires specialized equipment, complex data interpretation
UN Test Series H	Standardized tests for self-reactive substances and organic peroxides	Internationally recognized, directly applicable to transportation regulations	Designed for specific substance classes, resource-intensive

Types of Thermal Decomposition

Type	Description	Examples	Safety Implications
Exothermic Decomposition	Releases heat during decomposition	Organic peroxides, nitro compounds, some metal nitrates	Can lead to thermal runaway, fires, or explosions if heat cannot dissipate
Endothermic Decomposition	Absorbs heat during decomposition	Metal carbonates, metal hydroxides, some hydrates	Generally self-limiting, less hazardous than exothermic decomposition
Oxidative Decomposition	Involves reaction with oxygen	Polymers, oils, fats	Can be accelerated by oxygen concentration, may lead to fires
Hydrolytic Decomposition	Involves reaction with water	Esters, amides, acid chlorides	Can be accelerated by humidity, may generate acidic or corrosive products
Photolytic Decomposition	Triggered by light exposure	Some pharmaceuticals, dyes, hydrogen peroxide	Requires light protection in packaging and storage
Catalytic Decomposition	Accelerated by catalysts	Hydrogen peroxide (metals), organic peroxides (metal ions)	Trace contaminants can significantly reduce stability
Self-Accelerating Decomposition	Rate increases as decomposition progresses	Organic peroxides, self-reactive substances	Can lead to rapid temperature and pressure increase, potential explosion

Thermal Stability Categories

Category	Typical Decomposition Temperature Range	Stability Classification	Examples
Thermally Unstable	<100°C	Very Unstable	Some organic peroxides, diazo compounds, azides
Low Thermal Stability	100-150°C	Unstable	Many organic peroxides, some nitro compounds
Moderate Thermal Stability	150-250°C	Moderately Stable	Many polymers, some pharmaceuticals
High Thermal Stability	250-400°C	Stable	Many organic compounds, some inorganic salts
Very High Thermal Stability	>400°C	Very Stable	Many inorganic compounds, ceramics, metals

Note: These categories are approximate and may vary depending on the specific context and regulatory framework.

Decomposition Temperatures of Common Substances

Substance	Decomposition Temperature (°C)	Decomposition Type	Major Decomposition Products
Sodium Bicarbonate (Baking Soda)	50 (begins), 270 (complete)	Endothermic	Sodium carbonate, water, carbon dioxide
Calcium Carbonate	825	Endothermic	Calcium oxide, carbon dioxide
Ammonium Nitrate	210	Exothermic	Nitrogen oxides, water, oxygen
Hydrogen Peroxide (90%)	80-100	Exothermic	Water, oxygen
Benzoyl Peroxide	105	Exothermic	Benzoic acid, carbon dioxide, phenyl radicals
Polyethylene	335-450	Exothermic	Various hydrocarbons, carbon monoxide, carbon dioxide
Polyvinyl Chloride (PVC)	200-300	Exothermic	Hydrogen chloride, benzene, toluene
Nylon-6,6	350-400	Exothermic	Various nitrogen compounds, hydrocarbons
Aspirin (Acetylsalicylic Acid)	140	Endothermic	Salicylic acid, acetic acid
Potassium Chlorate	400	Exothermic	Potassium chloride, oxygen
Sodium Azide	300	Exothermic	Sodium, nitrogen gas
Sucrose (Table Sugar)	186	Endothermic	Carbon, water, carbon dioxide
Urea	133	Endothermic	Ammonia, isocyanic acid

Factors Affecting Decomposition Temperature

Chemical Structure

The molecular structure significantly affects decomposition temperature:

Bond strength: Stronger bonds generally result in higher decomposition temperatures.
Molecular weight: Higher molecular weight compounds often have lower thermal stability.
Functional groups: Certain functional groups (e.g., peroxides, nitro groups, azides) can significantly lower thermal stability.
Crystallinity: Crystalline materials often have higher decomposition temperatures than amorphous forms.
Isomerism: Different isomers of the same compound may have different decomposition temperatures.

Physical and Environmental Factors

Several physical and environmental factors can influence decomposition temperature:

Pressure: Higher pressure can affect decomposition temperature, either increasing or decreasing it depending on the decomposition mechanism.
Particle size: Smaller particles may decompose at lower temperatures due to increased surface area.
Sample mass: Larger samples may show apparent lower decomposition temperatures due to self-heating effects.
Heating rate: Faster heating rates typically shift observed decomposition temperatures higher.
Atmosphere: The presence of oxygen, moisture, or other reactive gases can significantly affect decomposition.
Container material: Catalytic effects of container materials can lower decomposition temperature.
Impurities: Even trace impurities can catalyze decomposition, lowering the observed temperature.
Aging: Some materials show decreased thermal stability with age due to oxidation or other degradation processes.

Safety Considerations

Understanding decomposition temperature is critical for safety for several reasons:

Process temperature limits: Process temperatures should include an adequate safety margin below the decomposition temperature.
Scale-up considerations: Decomposition behavior can change dramatically with scale due to heat transfer limitations.
Hazardous decomposition products: Thermal decomposition often generates toxic, corrosive, or flammable products.
Pressure generation: Decomposition that generates gases can cause pressure build-up in closed containers.
Self-accelerating reactions: Exothermic decomposition can lead to thermal runaway if heat cannot dissipate.
Incompatible materials: Some materials can catalyze decomposition, lowering the effective decomposition temperature.
Storage conditions: Proper temperature control during storage is essential for thermally unstable materials.
Transportation restrictions: Materials with low decomposition temperatures may be subject to special transportation requirements.

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), decomposition temperature should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is particularly important for thermally unstable substances.

For self-reactive substances and organic peroxides, the Self-Accelerating Decomposition Temperature (SADT) is a critical parameter for classification and transportation requirements under regulations such as UN Recommendations on the Transport of Dangerous Goods, ADR, IMDG, and IATA.

Best Practices

When reporting decomposition temperature in an SDS:

Specify the value in degrees Celsius (°C)
Indicate the analytical method used (e.g., DSC, TGA)
Specify whether the value represents onset, peak, or endset temperature
For complex decomposition processes, consider providing a temperature range
For exothermic decomposition, indicate the energy released if available
For self-reactive substances and organic peroxides, include the SADT
Provide information on hazardous decomposition products in Section 10 of the SDS
For substances where decomposition temperature is not applicable, clearly state this with a brief explanation
For substances with low decomposition temperatures (<100°C), highlight this as a special hazard in Section 2 of the SDS
Include information on factors that may lower the decomposition temperature (e.g., contaminants, confinement)
Ensure consistency between decomposition temperature information and stability/reactivity information in Section 10 of the SDS