Polymethyl methacrylate (PMMA), commonly known as acrylic or acrylic glass, is a widely used transparent thermoplastic. This synthetic material is found in a vast array of products, from windows and protective barriers to paints and fabrics. Like most synthetic polymers derived from petroleum, acrylic possesses inherent combustibility, distinguishing it from non-flammable materials such as mineral glass. Understanding why acrylic burns requires examining its chemical structure and how it breaks down when exposed to heat.
Understanding Acrylic Chemical Composition
Acrylic is an organic polymer whose susceptibility to fire is rooted in its chemical makeup. The polymer chains are built primarily from carbon and hydrogen atoms, making PMMA a hydrocarbon-based material. This structure means acrylic is essentially a solid form of chemical fuel, similar to wood or oil. Unlike ceramics or metals, the covalent bonds holding the carbon and hydrogen together are susceptible to high temperatures. Sufficient thermal energy can break these bonds, releasing stored chemical energy and classifying acrylic as a combustible material.
The Specific Process of Acrylic Burning
The process of acrylic burning begins with thermal decomposition, also known as pyrolysis. When exposed to heat, acrylic starts to degrade above approximately 220°C, though sustained burning requires an ignition temperature around 460°C (860°F). The distinctive feature of this breakdown is depolymerization, often described as the “unzipping” of the polymer chain. Under heat, the long polymer chains rapidly revert back to the methyl methacrylate (MMA) monomer, which accounts for over 90% of the volatile matter released. This volatile MMA gas acts as the primary fuel source, and the visible flame results from these gaseous molecules undergoing rapid oxidation in the presence of oxygen.
Fire Behavior and Combustion Hazards
The combustion of acrylic presents specific physical and toxicological hazards. Acrylic burns with a high heat release rate, generating a significant amount of thermal energy quickly, comparable to burning hardwood. This rapid heat generation accelerates the involvement of nearby materials. As a thermoplastic, acrylic softens around 100°C and melts as the temperature rises. Extruded PMMA tends to melt and drip flaming liquid, which can spread the fire, while cast acrylic resists dripping due to its higher molecular weight.
Toxicological Hazards
A major hazard arises from the specific chemical byproducts released during the fire. While the combustion is relatively clean, generating carbon dioxide and water vapor, it also produces hazardous compounds. These include carbon monoxide (CO), an acutely toxic gas, and formaldehyde. The fire also releases the uncombusted methyl methacrylate (MMA) monomer as a volatile vapor, which is itself a flammable and toxic substance.