PET heat-seal film What are some common issues?

　　1. PET heat-seal films that have turned yellow, gray, or developed yellow streaks—especially when rolled into a spool and visibly yellow or gray—are problematic. If yellow streaks are present, unroll the film and examine it against the light, or lay it over a white backing, to better assess the issue.

　　Analysis: Gel spots, yellow spots, and black spots all stem from the same underlying cause. Yellowing occurs when the resin has poor thermal oxidative stability or when excessive air enters the extruder. As the resin undergoes thermal oxidation, it generates substantial acetaldehyde, causing the film (or resin) to turn gray because the acetaldehyde reduces antimony trioxide to metallic antimony. Yellow streaks frequently appear after changing the filter or the resin; this is due either to air ingress during the changeover or to prolonged residence time of certain resins in the machine. In some cases, resins themselves become yellowed; once molded, these yellowed resins cannot achieve uniform mixing with the normally melted and plasticized material, resulting in yellow streaks.

　　II. The adhesion strength of PET heat-seal films—whether in printing, lamination, or aluminum coating—is insufficient. For assessing the sealing strength of printed, laminated, and vacuum‑aluminized materials, a self‑adhesive tape test is typically employed. There are three possible failure modes: complete delamination, partial delamination, and no delamination. The first two indicate inadequate peel strength.

　　Analysis: The adhesion strength between the PET heat-seal film and the ink–mink mixture is limited by the surface tension of the PET heat-seal film. The untreated PET heat-seal film exhibits a surface tension of 42–48 mN/m, which fails to meet the requirements for bonding with the ink adhesive. Therefore, during production, measures such as corona treatment should be employed to increase the surface tension to above 55 mN/m.

　　After being printed with gravure patterns, PET heat-seal films serve as an effective promotional tool for products, offering advantages that other printing materials cannot match. However, conventional PET heat-seal films are non-polar polymers with very low surface free energy—only 2.9 to 3.0 × 10⁻⁶ J/cm². Theoretically, if a material’s surface free energy falls below 3.3 × 10⁻⁶ J/cm², it will be virtually unable to adhere to any known adhesive. Therefore, to ensure adequate ink adhesion and print durability on polyolefin surfaces, it is necessary to increase the surface free energy; according to current processing standards, this should be raised to 3.8 to 4.0 × 10⁻⁶ J/cm². To date, corona treatment remains the most widely adopted and effective solution for enhancing the surface properties of plastic films.

　　Corona treatment of PET heat-seal films is typically carried out in air. Under the impact of high‑voltage, high‑frequency spark discharge, weak electrolytes are generated in the gas phase, giving rise to various polar functional groups; meanwhile, the double bonds within polyolefin molecules—particularly those on carbon–carbon linkages—are readily cleaved. As a result, during the brief period of dissolution, these polar functional groups undergo grafting reactions with the polymer surface, transforming the originally nonpolar polyolefin surface into a polar one. In essence, after arc discharge, the surfaces of many polyolefins with thicknesses ranging from 10 to 100 μm are converted into polar chemical species that differ substantially from the original polyolefin structure, leading to a further increase in surface free energy, which has been measured to reach 3.8–4.0 × 10⁻⁶ J/cm². Moreover, the high‑voltage, high‑frequency spark discharge roughens the surface of the plastic film (as observed by transmission electron microscopy at high magnification, revealing fine tubular grooves and irregularities), thereby enhancing ink wettability and increasing the total contact area. Consequently, the adhesion strength of the ink on the surface is improved through both organic chemical and physical effects.