Hygroexpansion, Surface Roughness and Porosity Affect the Electrical Resistance of EVOH-Aluminum- Coated Paper

Abstract

When aluminum is applied to paper by physical vapor deposition, substrate roughness contributes to the defect density and hygroexpansion can cause defects that impair the aluminum coating. Both effects can manifest as an increase in electrical resistance. We quantified the effect of substrate paper hygroexpansion (0–95% relative humidity) and paper surface roughness on the effective resistivity (ρEFF) of aluminum coatings. To create different degrees of roughness, five different papers were used. Each of them had one pigment coated side and one side without pigment coating. These different rough paper surfaces were pre-coated with ethylene vinyl alcohol co-polymer (EVOH). Hygroexpansion was promoted by pre-coating and increased more when the coating was applied on rough and porous surfaces. Simultaneously, the pre-coating reduced surface roughness; especially porosity. The reduction of porosity decreased effective resistivity (ρEFF). Based on these results, an aluminum thickness of ≥35 nm is recommended to ensure maximum mechanical stability during hygroexpansion in combination with minimum material usage. Moreover, the resistivity did not regain its initial value when the paper substrate shrank during re-drying.