Transient Thermal Conductivity Minimum in Phenolic Foam Insulation: Closed-Cell Structural Dependence and Long-Term...

Area of Science:

• Materials Science
• Polymer Science
• Thermal Engineering

Background:

• Closed-cell phenolic foam (PF) insulation is widely used for its thermal properties.
• It is generally assumed that thermal conductivity of PF insulation monotonically increases after cutting due to blowing agent release.

Purpose of the Study:

• To investigate the thermal conductivity behavior of closed-cell phenolic foam (PF) insulation after cutting.
• To characterize a previously unobserved transient thermal conductivity minimum (TTCM) and its underlying mechanism.

Main Methods:

• Tracking thermal conductivity and mass of n-pentane-blown PF foam under various aging conditions (23 °C, 70 °C, 110 °C).
• Utilizing analytical techniques including gas chromatography-flame ionization detection (GC-FID), ATR-FT-IR, TGA/DSC, and SEM.
• Comparing closed-cell PF foam with a reference foam exhibiting cell-wall micro-perforations.

Main Results:

• A consistent decrease-minimum-rebound pattern, termed TTCM, was observed in closed-cell PF foam.
• TTCM reductions ranged from 3.8% to 16.7% across tested specimens.
• A reference PF foam with micro-perforations did not exhibit TTCM, confirming the necessity of an intact closed-cell structure.

Conclusions:

• The TTCM phenomenon in PF insulation is attributed to the redistribution of n-pentane blowing agent within the foam structure.
• This redistribution is thermodynamically favored by the proximity of n-pentane's boiling point to the heat flow meter's hot plate temperature.
• Post-TTCM thermal conductivity increase is governed by n-pentane diffusion and displacement by air, indicating distinct aging stages.