Plexiglass, commonly used as a lightweight, shatter-resistant alternative to traditional glass, will melt when subjected to sufficient heat. Known chemically as poly(methyl methacrylate) or PMMA, this clear plastic is designed to be shaped by heat. Unlike glass, which softens over a vast temperature range, PMMA’s response to temperature is a predictable physical property that dictates how it must be handled.
Understanding Plexiglass as a Thermoplastic
Plexiglass is classified as a thermoplastic polymer, which explains its capacity for melting and reshaping. The long molecular chains are held together by relatively weak intermolecular forces, not strong chemical cross-links. Unlike thermoset plastics, which undergo an irreversible chemical change when heated, thermoplastics can be repeatedly softened and solidified.
When heat is applied, the energy causes the weak bonds between the PMMA polymer chains to loosen. This increased molecular movement allows the chains to slide past one another, transforming the rigid solid into a soft, moldable, or liquid state. This physical change is reversible, allowing the material to be cooled back down to a solid without significant degradation. This enables processes like thermoforming, where acrylic sheets are bent and molded into various shapes.
Specific Thermal Benchmarks
The transition from a rigid solid to a soft, pliable material occurs across a temperature range, typical for polymers. The first significant thermal benchmark is the Glass Transition Temperature (Tg), which for standard PMMA is around 105°C (221°F). At this temperature, the material changes from a hard, glassy state to a rubbery state, significantly reducing its rigidity.
The material’s heat deflection temperature, the point at which it deforms under a specific load, can be as low as 94°C (201°F). For practical purposes like thermoforming and welding, the material must be heated well beyond the Tg into the softening or melting range. The true melting point for PMMA typically falls between 160°C and 218°C (320°F to 424°F), depending on the specific grade.
This low melting range is a defining difference compared to traditional glass, which requires temperatures exceeding 1400°C (2600°F) to melt. The relatively low thermal benchmarks of Plexiglass make it easier to process and fabricate than plastics like polycarbonate, which have higher heat resistance. These figures serve as general guidelines, and manufacturers’ specifications should be consulted for precision applications.
Response to Excessive Heat and Fire
When temperatures continue to rise far beyond the melting point, the material’s response shifts from reversible softening to irreversible chemical decomposition and combustion. Plexiglass is considered a flammable material, with an ignition temperature of approximately 460°C (860°F). Once ignited, PMMA burns steadily and intensely.
Plexiglass is rated as B2, or “normally flammable,” under the DIN 4102 standard. A notable characteristic of burning PMMA is that it burns almost without smoke, and the resulting smoke gases are not considered acutely toxic or corrosive. The primary combustion byproducts include carbon dioxide, water, and some carbon monoxide.
In certain applications, such as roof glazing, the low melting point can serve a safety function by melting and opening up, allowing smoke and heat to escape the building. During processes like cutting or polishing, heat generation must be managed carefully to avoid localized melting, which can cause the plastic to gum up or crack. Using sharp tools and continuous cooling is the best practice to prevent thermal damage.