Appearances can be deceiving: What's in your thermal barrier?
by Helen Sanders, Ph.D
Polyamide (PA) thermal barrier systems (thermal breaks) have been used successfully in aluminum fenestration systems for 50 years. With higher thermal performance increasingly required by code, wider and more complex PA thermal barriers are being utilized. PA thermal barriers are an integral part of the structural integrity, as well as the thermal performance, of fenestration. They also provide additional advantages, such as facilitating easy, dual color finishes and straightforward, safe processing.
As manufacturers seek to optimize supply chain costs, there are several important criteria to ensure choices related to thermal barriers do not negatively impact structural performance of fenestration.
While PA thermal barriers sourced from different suppliers may look alike, their structural, temperature resistance and sealant adhesion performance can be substantially different. Since fenestration systems are designed around pre-determined thermal barrier material properties, it is extremely important to evaluate sourced material against the performance criteria relied upon in the design.
Typically, PA thermal barriers contain glass fibers for strength, which must be oriented in three dimensions to provide uniform strength in all directions. Substitutions of calcium carbonate for glass fibers, using mono-directional glass fibers, or non-homogeneity, weakens the material. Altering the glass fiber content or orientation reduces the ability of the thermal barriers to transfer loads, and thus, reduces the structural integrity of the system. The presence of voids and impurities in the material can also reduce PA strength substantially. Even extrusion processing has an impact, with differences in tensile strength of up to 65% observed. Dimensional precision is also critical.
While none of these material properties are visually distinguishable, they are easily tested.
If a fenestration system has been engineered using a thermal barrier made of PA 6.6, the most prevalent thermoplastic type used for this application, substituting the barrier system for other less expensive thermoplastic blends also can significantly impact its ability to meet the original design criteria. An engineering evaluation should be done before making such a change.
Substitutions of PA 6.6 and glass fiber materials by other, less expensive materials or the use of reduced quality materials may reduce the thermal barrier cost, but doing so introduces substantial risk. In addition to changing the strength of the barrier, changes in surface adhesion properties can occur, which impacts a fenestration system’s air and water integrity. Material substitutions or impurities can also lower the heat deflection temperature, which is a critical performance parameter in profiles used in spandrel or other high-temperature applications.
It is, therefore, crucial to ensure that the PA barriers’ properties being sourced for production meet those assumed in the as-engineered fenestration. Quality checks should be done when evaluating a new material supply and for regular incoming inspection. Fabricators should measure material density and tensile strength as good first indicators of product quality. More sophisticated “fingerprint” tests, such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), identify polymers present and glass fiber content. Using manufacturers with ISO 9001 certified processes and requiring transparency to their manufacturing control plan and material testing is highly recommended. Thoroughly evaluate any new material against the design performance as looks can be deceiving.
The image to the right is a cutaway of an aluminum window system that shows a complex polyamide thermal break.