Accelerated Aging Test Methods for Predicting the Long Term Thermal Resistance of Closed-Cell Foam Insulation

摘要

The accurate estimation of the thermal performance of insulation products used in buildings over their expected lifetimehas been a recognized challenge for over 25 years. This is because the lifetime of such products is long, thermal aging iscaused by the diffusion of a multitude of gases, and the insulation product is not homogeneous. The task of developing astandard test method for predicting long term thermal performance which applies to a variety of closed-cell foam products iseven more complicated as diffusion processes occur at rates that depend on the type of polymer, the cellular structure, thetemperature, the gas type, and its pressure. Both classical approaches to accelerating diffusion controlled phenomena,namely aging at higher temperature and aging a thin slice, present challenges especially if a single method is desired for avariety of cellular foam products such as polyisocyanurate (polyiso) and extruded polystyrene (XPS).Though Europe has favored standard test methods based on aging at elevated temperature, slicing and scaling techniqueshave been the leading approach in North America. Lately, two prescriptive test methods, ASTM C 1303 in the USA andCAN/ULC-S770 in Canada have emerged in North America. Both are based on accelerating the foam aging process byslicing the foam into thin specimens. Both methods use the projected thermal conductivity at five years of age to representthe insulation’s long term thermal resistance value (LTTR). The two methods have many other similarities, such as use ofFickian law for one-dimensional diffusion to calculate aging period, and use of a thin slice from the core and surface areasof the foam. However, they do vary in precisely how the long term thermal resistance is calculated. The C 1303 testmethod prescribes that the thermal resistance value of a stack of thin slices after aging for a calculated time is the long termthermal resistance value. The S770 test method calls for multiplication of an aging factor to an initial thermal resistancevalue to determine the five-year value. Though the basic frameworks of the two methods are in place, the preciseparameters are still being debated and balloted.This paper compares the two methods, ASTM C 1303-07 and CAN/ULC-S770-03 for their suitability for use as standardtest methods by the polyiso and XPS insulation industry for their boardstock products. Mathematical modeling andcalculation algorithms developed by Huntsman and described in earlier papers are used along with published thermal agingdata to evaluate how effective the two methods will be to meet the various criteria for being an industrially useful method.It looks at the impact on the bias for each of the method for the various parameters still being debated, such as slicethickness and stack composition. This study demonstrates that with the appropriate choice of test conditions, each of thetwo methods have potential to give low bias with polyiso boards. For XPS boards, ASTM C 1303-07 appears to be the onlyreal choice.