New organic fiber-based core material for vacuum thermal insulation

Abstract

Melamine-formaldehyde fibers were synthesized from a meltable pre-polymer of etherified melamine-formaldehyde in a form of a low density fleece, subsequently thermally cured in a conveyor belt oven at temperatures of up to 200°C and post-heated at 260°C. High thermal stability and small fiber diameter below 5μm made it a serious candidate as a novel core material for vacuum thermal insulation panels. Two most crucial core properties, thermal conductivity and outgassing rate, were investigated in thin-walled stainless steel envelopes, enabling thermal processing combined with a pump-out procedure. A base thermal conductivity of ∼2.3mWm−1K−1 was achieved with randomly oriented fibers at a density of ∼250kgm−3. The long-term pressure-rise measurements revealed extremely low outgassing rates, q∼10−15mbarLs−1cm−2. Additional measurements of thermal conductivity in a wide pressure range from 10−3mbar to the atmosphere indicate that these melamine-formaldehyde fibers could be the first organic candidates applied as the core material in vacuum insulating panels with an adequate service lifetime. Their performance is comparable to selected inorganic core materials like glass fibers.