Product Layer Structure Requirements for High-Temperature Retort Materials in Solvent-Free Lamination

Although in theory solvent-free lamination can produce any product structure that dry lamination can, in practical applications many structures that are dry laminated smoothly show abnormalities when they are solvent-free laminated, especially high-temperature (above 121℃) retort products.

For example, structures laminated with aluminum foil and high-temperature retort products with three or more layers often encounter various problems. Among the high-temperature retort product structures commonly found on the market today, such as BOPET//RCPP, BOPA//RCPP, BOPET//BOPA//RCPP, BOPP//PVDC//RCPP, and BOPET//A1//BOPA//RCPP, in terms of layer count, two-layer high-temperature retort materials are easier to make than three-layer or higher-layer structures. Once a two-layer structure has been laminated, it can be placed in the curing room for curing.

Because solvent-free lamination has very low initial tack, the laminated film will wrinkle at the bottom of the roll and on the outer few wraps of the film surface, and the wrinkling becomes more obvious after curing, especially at both ends of the film. Especially when laminating three or more layers, due to the low initial tack of solvent-free lamination, the film must be fully cured after the first lamination before the second lamination can be carried out, and the wrinkled portions will be carried into the product of the second lamination, causing certain quality risks and material loss. In addition, the unwind and rewind units of solvent-free laminating machines are currently all single-unwind and single-rewind, with no automatic roll transfer function; the more layers that are laminated, the greater the loss caused by wrinkling at the beginning and end of the roll.

Therefore, when solvent-free laminating high-temperature retort products, a lamination structure of three layers or fewer should be selected.

From the perspective of structural materials, solvent-free lamination is more stable for plastic-plastic lamination than for aluminum-plastic lamination, mainly because the dry coating weight of solvent-free lamination is basically below 3g/m². For aluminum-plastic high-temperature retort products, the coating weight baseline is low, making it difficult to ensure the peel strength after retorting.

In addition, for patterns printed with large areas of light-colored ink, when laminated with aluminum foil or metallized film, white spots are difficult to control because of the low adhesive coating weight. Some manufacturers now produce the lamination of the printed layer with aluminum foil on dry laminators and the other layers on solvent-free equipment, combining dry lamination and solvent-free lamination to integrate the advantages of both. For plastic-plastic structures, a dry coating weight of about 2.5/m² can meet the requirements of high-temperature retorting, but the adhesive must have good retort resistance.

In addition, the same high-temperature retort adhesive is not necessarily suitable for different material structures; to achieve stable use, experience needs to be continuously accumulated. (This article is excerpted from 'Breakthroughs in Solvent-Free Lamination of High-Temperature Retort Products', author: Wang Dongjing. For more technical content, please see Packaging Frontier or the Packaging Frontier Information Network at www.pack168.com. Different opinions are welcome; please continue the discussion at www.pack168.com. Friends who provide more solutions can receive reading credits for technical articles on the website. To claim credits, please contact Xiao Yang: 15813331361, Xiao Lai: 18475695660.)