This article talks about adhesives, looks at various polymer chemistries and curing options available, and provides information.
Adhesives for decal and label applications need to adhere readily to the label or decal and the substrate to which it is attached, and the label usually needs to be applied to the substrate without the use of heat or excessive mechanical force. For this reason, pressure-sensitive adhesives (PSAs) are the product of choice. PSAs provide a durable solution that is easy to apply, replacing other materials, such as thermally activated adhesives, liquid glues, or varnishes.
PSAs are often supplied in liquid form, which has several advantages over transfer tape. Since adhesive is only printed on the label where needed, this can eliminate the waste inherent in designs where excess tape material would need to be disposed of, potentially reducing cost. It can be designed not to cover the die-cutting line for labels that will be die cut, avoiding adhesive buildup on the cutting tool. Liquids can be printed in various thicknesses for different applications and can incorporate fine lines or intricate patterns, providing extensive design flexibility.
Liquid adhesives can be applied to label stock using many different methods including screen printing, flexographic printing, gravure printing, pad printing or Mayer rod coating. Adhesive suppliers often specify a preferred application process to achieve optimum results. For example, some adhesives are not suited to screen printing, whereas others are specifically formulated for that method. The elasticity and viscosity of the adhesive affect its ability to effectively transfer to the label material under a given set of coating conditions.
The combination of polymers, tackifiers, thickeners, and other additives in a given formulation determine the properties of the adhesive. PSAs are available in three different polymer families: rubber, silicone, and acrylic. Rubber PSAs have the advantage of excellent adhesion and are resistant to moisture and humidity. One drawback is that rubber degrades with ultraviolet (UV) exposure and is, therefore, not recommended for outdoor applications where it is not possible to protect the rubber from such exposure. Rubber adhesives are best suited to end products that will not be exposed to high temperatures or sunlight and require a strong bond.
Many hot melts are rubber-based. They are applied at elevated temperatures and cure as they cool, forming a high tack material for instantaneous, strong bonds. Some new products with especially aggressive tack are designed for freezer-grade applications.
Silicone can be a good option for labels exposed to extreme temperatures, as it remains flexible and maintains adhesion at temperatures ranging from -50 to +300°C. Silicone PSAs can be customized for various degrees of adhesion, depending on whether the application requires a temporary or permanent bond. They can wet to many different substrates, including many hard-to-bond plastics such as polyethylene, polypropylene, nylons, or even Teflon.
Acrylic PSAs are often made from copolymers that may include vinyl or styrene. They are the most commonly used all-purpose class of materials, as the raw materials can be easily polymerized in atmospheric pressure and the resulting polymers are well-suited to a variety of label applications. Acrylics tend to be priced lower than other options. They are stable over a fairly broad temperature range, from around -40 to +175°C, and are UV resistant. Some formulations have excellent moisture resistance.
Acrylics come in solvent-based or water-based formulations. The polymers in solvent-based products have relatively low molecular weight but are highly crosslinked, producing an easily applied adhesive with very high strength. Water-based formulations are an emulsion with discrete particles of longer polymer chains that coalesce as the water evaporates.
The goal with water-based PSAs is to achieve the performance of solvent-based products without using solvents, and manufacturers are making progress in this direction. One difficulty is in providing good adhesion to a wide variety of substrates. While water-based PSAs bond well to paper and polypropylene substrates, their performance on other plastic surfaces is not as good. Also, the cohesive strength of solvent-based acrylics is much higher.
PSAs are available from many different manufacturers (Table 1). Some boast a wide array of products and pride themselves on being able to supply the right adhesive for any application. Others focus on expertise in a particular technology. The resource list below includes suppliers contacted for this article and specifies what categories of adhesives each manufacturer produces. Within each category there may be many specific products available, and selection depends on the specific application.
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