You Should Know About Thermal Conductivity and Aging of Low-Temperature Mechanical Insulation

June 12, 2014 - PRLog -- PURPOSE
This Technical Bulletin is another in Dyplast’s series to provide objective information to decision-makers and end-users on important issues related to mechanical insulation (i.e. insulation for equipment). Although technically intense in some paragraphs, this Technical Bulletin objectively encapsulates more than can be gleaned from other documents generally available.
DEFINITION
Thermal aging is a term that refers to the tendency of some insulants to lose some thermal resistance over time - - predominantly due to the very slow diffusion of low-thermal-conductivity cellular gases out of the cells within the insulation, only to be replaced by "air" with higher thermal conductivity. Since thermal aging is so often used as an argument for or against a particular insulation, it is important to understand this somewhat complex phenomenon.
VARIABLES IN THERMAL AGING
Thermal aging occurs very gradually over many years, yet not linearly. At low temperatures, the vast majority of thermal aging occurs in the early years, though is likely not complete even after several decades. At very low temperatures (e.g. cryogenic), the rate of thermal aging is dramatically less.
The rate of thermal aging can depend on the following, sometimes interdependent, factors:
The temperature: the lower the temperature, the slower the rate of thermal aging Cell size: The smaller the better
Closed Cell Content (the blowing agent must be retained in the cell to be effective)
Closed cell density and space between the cells (interstitial space): Can "short circuit" heat flow (such as with EPS)
Thickness of the insulation (outer layers impede diffusion of blowing agents from inner layers)
Voids or holes in the foam: Like the interstitial space between the cells, these can provide a pathway for the escape of the blowing agents contained within the cells
The solubility of the blowing agent in the polymer (blowing agent must be retained in the cell to be effective)
The diffusion rate of the blowing agent out of the foam, and rate of air diffusion into the foam
Can depend on the structure and thickness of the cell walls
Physical enclosures over the insulation, such as vapor barriers, jackets, laminates, or "skins" dramatically retard the rate of gas diffusion.
"PREDICTING" THERMAL AGING
The practical problem has been how to accurately assess the thermal performance of the foam at a point in time. Two classical approaches have been developed to accelerate thermal aging so as to accurately predict thermal conductivity over the lifetime of the insulation, either: 1) aging at higher temperature or 2) aging a thin slice.
ASTM C591 requires that 12 x 12 x 1-inch samples be cut from the bun/block. After aging the material for 180 days in a controlled environment at 75°F, the foam’s R-value is measured at mean temperatures ranging from -200°F to +200°F
It is important to note that the mechanical insulation service temperatures referenced in this document are normally well below 75F, and that thermal aging is reduced considerably at lower operating temperatures. Thicker insulation, vapor barriers, and metal constraints also limit gas diffusion.
PERSPECTIVE
To be effective, pipe and equipment below-ambient temperature conditions require insulation systems quite different from those in above-ambient conditions - - meaning that water condensation and freezing on pipe surfaces or the outer surfaces of the insulation system can have dramatic impact on the ultimate performance of the process. Admittedly, the insulant itself is the last line of defense against performance degradation, yet improper design or installation of vapor barriers, mastics, or jackets can doom the best of insulants. There have been dozens of scholarly papers and hundreds of marketing documents that provide sometimes credible and sometimes misleading or at least confusing information.
Since there is so much complexity and so much information, it is easy to lose sight of the forest when trees block the view. We do not trivialize the complexities of the related issues, yet we also understand that specifiers and end-users need informed and objective information upon which to base their decisions - - without the need to convene a panel of experts for each application.
As the basic perspective, there are many insulants with "poor" thermal efficiencies that exhibit no thermal aging, and there are some "superior" insulants that do so. It can be generalized that the Aged Thermal Conductivities of the "superior" insulants are still materially better than the "poor" insulants, and thus over a lifetime the energy savings can greatly outweigh any small premium in cost.