Structure–property relationships in carnauba wax-based coatings for water-resistant Kraft paper

The growing demand for sustainable alternatives to plastic packaging has positioned paper substrates as a key material in the transition toward the use of bio-based solutions for such applications. This study investigates carnauba wax-based coatings emulsified with Tween 80, focusing on how the emulsifier concentration affects the structure, morphology, air permeance, water barrier, and mechanical properties of coated Kraft paper. Scanning electron microscopy (SEM) analysis revealed that the 1:4 (Tween 80:wax, in weight) formulation forms a continuous, defect-free film that fully covers the fibrous paper structure, reducing air permeance from 6.80 to 0.11 µm·Pa −1 ·s −1 . Higher wax-to-emulsifier ratios (1:6 and 1:7) resulted in severe cracking, incomplete coverage, and an increase in air permeance up to 0.60 µm·Pa −1 ·s −1 . The 1:4 coating reduced the Cobb 60 from 104.3 to 21.7 g·m −2 and the water-vapor transmission rate (WVTR) from 1151 to 223 g·m −2 ·day −1 at 23 °C and 50% relative humidity. Mechanically, the machine direction tensile strength (~110 N/15 mm) was preserved, while the transverse direction (TD) tensile strength improved from ~ 32.6 to 40.4 N/15 mm. Simultaneously, elongation at break in TD increased from 3.5% to ~ 5%. Overall, the results demonstrate the potential of carnauba wax as a bio-based coating for improving the barrier performance of paper substrates, contributing to the development of more sustainable alternatives to conventional fossil-based paper coatings.