Polyvinyl chloride (PVC) is a widely used polymer in construction and manufacturing. When exposed to heat, the material exhibits two distinct responses: predictable expansion and a high-heat shrinkage effect known as reversion. Understanding these opposing thermal behaviors is necessary for the proper design and installation of PVC products. Linear expansion dominates under moderate temperature changes, but shrinkage under extreme heat is a consequence of how PVC is manufactured.
The Primary Response: Thermal Expansion
The most common reaction of PVC to a temperature increase is expansion, a behavior governed by its Coefficient of Linear Expansion (CLE). The CLE describes how much a material’s length changes per degree of temperature change. For PVC, this coefficient is notably high compared to traditional materials like metal or concrete, meaning it expands and contracts at a much greater rate. For instance, a long run of PVC pipe can expand approximately one inch over 50 feet for every \(50^\circ\text{F}\) rise in temperature. This dimensional change occurs even with moderate exposure, such as hot water flowing through a plumbing system or direct sunlight hitting exterior siding.
Understanding Thermal Memory and Reversion (The Shrinkage Effect)
While general heating causes expansion, PVC can shrink when exposed to specific, high-temperature conditions due to a phenomenon called “thermal memory” or reversion. This shrinkage is not a simple thermal contraction but rather the material reverting to a previous, less-stressed state.
During the manufacturing process, such as extrusion for pipes or profiles, the PVC is heated, shaped, and then rapidly cooled while under significant mechanical stress. This stress stretches the polymer chains in the direction of the extrusion. These stretched polymer chains are essentially frozen in an oriented, high-energy state as the material cools, retaining a kind of physical “memory” of the stretching.
The release of this internal stress occurs when the material is heated above its Glass Transition Temperature (\(T_g\)), which for rigid PVC is around \(80^\circ\text{C}\) (\(176^\circ\text{F}\)). Once the temperature exceeds \(T_g\), the polymer chains regain enough mobility to relax and pull the material back toward its original, unstretched dimensions.
Managing Thermal Movement in PVC Applications
Because PVC exhibits both expansion at moderate temperatures and shrinkage at high temperatures, managing thermal movement is a necessity in construction applications. In plumbing and drainage systems, this is primarily achieved through the use of expansion joints or offsets in long, straight runs of pipe. These components act as flexible couplings that absorb the length changes without transferring stress to fixed joints or fittings. Without these allowances, linear expansion could generate enough force to crack fittings, resulting in leaks or system failure.
For exterior applications like siding and window frames, installers must avoid anchoring the material too rigidly, instead using slotted holes and fasteners that allow for slight sliding movement. The color of the material plays a role in heat absorption; dark-colored PVC absorbs more solar energy, leading to higher surface temperatures and greater movement. Using lighter colors or shading exposed areas helps keep the temperature lower, minimizing the effects of both thermal expansion and the risk of high-heat reversion.