Engineering SAN-core architecture in waterborne acrylic latexes for paper coating applications

Abstract

Waterborne dispersion coatings are a potential alternative to extrusion coating and lamination for paper-based food packaging, but require latexes that combine barrier performance, film formation, and industrial processability. Here, poly(styrene- co -acrylonitrile) (SAN) was explored as the hard core of film-forming core–shell latexes. Latexes with SAN cores and a soft styrene–acrylic shell were synthesized by seeded semi-batch emulsion polymerization, an industrially established process, varying the acrylonitrile (AN) content of the core (0–75 wt%, at 35 wt% core) and the core fraction (35–65 wt%, at 50 wt% AN). Free-standing films and coated papers (10 μm) were characterized by TEM, SEM, AFM, DSC and TGA, tensile testing, water vapor transmission rate (WVTR), contact angle, and water and oil Cobb 1800 test. The cores remained phase-separated, well dispersed across the AN series but aggregated at high core fractions. All films in the AN series exhibited high flexibility, with elongations at break above 800%, whereas the increase of the core fraction promoted core aggregation and, at 65 wt% core, resulted in brittle films with an elongation at break of only 2%. All coatings reduced the WVTR (434 g m −2 d −1 ) and water uptake (51.3 g m −2 ) of the paper. The shell alone gave the lowest values (30 g m −2 d −1 and 3.0 g m −2 ), closely followed by the 25 wt% AN core (32 g m −2 d −1 and 4.2 g m −2 ), whereas AN content ≥50 wt% (54–71 g m −2 d −1 ) and core aggregation (up to 181 g m −2 d −1 ) compromised the water vapor barrier. For most formulations, the cores reduced oil uptake from 9.2 g m −2 (shell alone) to 3.4–4.7 g m −2 . Cores containing up to 25 wt% AN therefore largely preserve the water barrier properties of the shell while improving its oil resistance.