A Study Into the Hot Tack and Cooled Seal Performance of Emerging Coated Papers for Primary Flexible Food Packaging

Abstract

In response to the need to employ recyclable materials for food packaging, the resurgence of paper as a primary flexible packaging material is driven by consumer trust in paper and its renewable wood fibre composition, strengthened by a well-established recycling infrastructure. A diverse range of coated papers and coatings has become accessible in the market, specifically tailored for applications in horizontal and vertical form-fill sealing. Within the framework of the CORNET-TETRA project HBC.2021.0288 REPAC2, a careful selection of 16 food-grade coated papers and/or coatings, slated for introduction to the market either presently or within the next 2 years, has been undertaken. This study focused on evaluating the processing window for superior hot tack and cooled seal performance of coated papers, in relation with the composition of paper coatings as identified using Fourier transform infrared (FTIR) measurements, and their thermal softening or melting characteristics as identified through differential scanning calorimetry (DSC). Thermal behaviour in DSC thermograms is used to categorize the commercial coated papers into distinct classes. Class I papers, mainly with acrylic-based coatings, exhibit high hot tack strengths (0.2-0.7 N/mm) and a glass transition temperature (Tg) close to the seal initiation temperature but do not considerably gain strength with further cooling. Class II papers are thermally inert in DSC with minimal thermoplastics, leading to weak seals. In contrast, classes III and IV, including wax-based and polyolefin-based or polyvinylalcohol-based (PVOH) coatings, respectively, show low initial hot tack strengths that considerably increase upon adequate cooling. Particularly, the Class IV papers with polyolefin-based coatings have well-performing seals. Despite having relatively low hot tack strengths after cool times of 0.1 s, below 0.3 N/mm, high strengths can be obtained after adequate cooling with outliers reaching 0.74 and 1.14 N/mm. Additionally, the influences of seal parameters on seal performance were evaluated. The study reveals that cool time, seal pressure and, to a lesser extent, seal time significantly impact hot tack strength, consistent with prior research. The critical role of jaw temperature in heat conductive sealing is affirmed, as it dictates the efficacy of other seal parameters. Seal initiation occurs at 75 degrees C for four papers, and nine others necessitate temperatures equal to or exceeding 100 degrees C, with one paper only displaying seal initiation at 195 degrees C. This variation highlights the requisite for tailored temperature windows for effective sealing of these papers. As such, deeper insights into the intricate interplay between coating composition, thermal properties and seal performance are obtained in order to support advances in sustainable packaging technology. This study evaluated the influence of seal parameters on the heat seal performance of 16 food-grade coated papers, focusing on their coating compositions and thermal characteristics. Seal pressure, cool time and jaw temperature were most influential. Additionally, seal performance varied significantly with coating type: Acrylic-based coatings (Class I) showed high hot tack strengths but minimal strength gain upon cooling, whereas polyolefin-based coatings (Class IV) demonstrated low initial hot tack strengths that increased substantially with adequate cooling.image