Effect of crystallinity of bio-based polymers on compatibility and the tortuous path model of cellulose-based industrial products

Polylactic acid (PLA) is chosen as a coating material on industrial Kraft paper substrates to impart a barrier against air, grease, and water. This is an excellent initiative to replace the existing non-biodegradable and petroleum-based coating materials used in many industrial products such as packaging and coated abrasives. In this study, the effect of the crystallinity of PLA on its barrier properties is evaluated. Initially, amorphous and semi-crystalline PLA (sc-PLA) are compared for properties such as coating weight, gloss, heat seal strength, air resistance, grease resistance, contact angle, water resistance, and water vapor transmission rate (WVTR). Sc-PLA is observed to show better barrier properties than its amorphous counterpart. To understand if the barrier properties imparted by a biodegradable coating material on Kraft paper will improve with an increase in crystallinity, sc-PLA is compared to a more crystalline biodegradable polymer, polycaprolactone (PCL). The results indicate that PCL shows compromised results with respect to sc-PLA as a barrier coating material. The theory behind this effect is discussed, and the two bio-based polymers, namely, PLA and PCL, are also comparatively evaluated against the conventionally used synthetic barrier coating materials, acrylic, and varnish, on identical paper substrates under identical process conditions. SEM and DSC analyses on amorphous PLA, sc-PLA, and PCL coatings are also conducted to decipher the microstructural traits of these macromolecules. Sc-PLA serves as an ideal barrier coating material on Kraft paper, and it possesses better hydrophobic properties than untreated, PCL-treated, acrylic-treated, or varnish-treated Kraft paper substrates.