Two types of adhesives or glues are in common use to glue aqueous coated substrates. These are cold white glues and hot melt adhesives.
The most widely used of these are hot melt adhesives which are a form of solvent-free thermoplastics. These are solid at room temperature and are melted to be applied as a molten film or in a pattern of dots or beads, with clean cut-off, to a material to be glued. Hot melt adhesives rely on cooling to set, offering a quick cure and set time. They are known for great machinability, and acceptance in box gluing and high-speed carton fabrication.
Hot melt adhesives are most often used in the paper and packaging industries because they provide strong bonds and set fast. Hot melts work effectively on a variety of porous and non-porous packaging materials including coated and uncoated paper, paper board, films, foils, and plastics. They are the most effective glues for use with aqueous, UV, and varnish coatings, producing good bonds on stocks that are flood coated.
When difficult coatings are an issue, hot melt adhesives are preferred. They also perform well in applications that see a fairly wide range of temperature from over +100° F. in unrefrigerated shipping trailers to below zero when shipping and storing frozen foods. Temperature exposure higher than glue application temperature will soften bonds.
The most popular types of hot melt adhesives are based on ethylene vinyl acetate (EVA). Others are polyolefins based on catalyzed metallocenes. Hot melt adhesives are applied using slot head dies, jetting applicators, and sprayers. Jetting applicators are mainly used in packaging applications, gluing where the applicator applies a continuous, or an interrupted bead of hot melt.
Cold white glue, is a type of water-based adhesive, most often opaque white in color, hence its name. Cold white glues are effective in bonding a wide variety of materials including, paper, paper board, some plastics, wood, metals, etc.
Cold white glues are less popular than hot melt adhesives, but nevertheless, they still find use in wide-ranging applications. The decision to use one or the other generally is decided by bond requirements, type of equipment, and production speed demands. Cold white glues offer some advantages over hot melts in that they offer heat, chemical, and water resistance, and are generally more economical. Cold white glues set and dry as water is evaporated and/or absorbed into a stock with the glue film said to break and form. After setting, adhesive bonding is permanent and irreversible. Cold white glues may not perform satisfactorily on some coated stocks due to an inability to adequately penetrate them. However, they do offer the ability to bind to a wide range of substrates producing extremely strong and lasting bonds. Additionally, they remain stable across a wide temperature range and high humidity and are not susceptible to softening, as hot melts are at elevated temperatures.
Common among cold white, water-based glues are Dextrin (vegetable) types, based on starch, which deliver a flexible solid bond. They are applied at room temperature with a slow set speed. Also, there are economical and versatile water-based resin adhesives based on EVA/PVA polymers, PVOH, acrylics, homopolymers, and copolymers formulated as water-based emulsions. These resin-based adhesives are known as the workhorse of the adhesive industry. Applied at room temperature, they range in set speed from fast to slow, adhering well to clay-coated paper, SBS, UV coated papers, films, and foils. Typical applications are found in the folding carton and packaging industries.
Water-based cold white glues can be dispensed-applied by several means, including spray and roll techniques. Cold white glues are thinner than hot melts in viscosity terms, and therefore support faster line speeds. They are favored for long-run folding carton and box forming, and gluing. Cold glue/adhesive application equipment is commonly installed on packaging fabrication box gluers, paper bag, folding carton-folder gluers, and 3-D box machines.
Flood coating is common in the application of aqueous top coatings, which can sometimes lead to an adhesion problem in gluing. This problem can be avoided by blocking out the area to be glued so that gluing to an applied top coating is avoided.
Coatings that contain high percentages of waxes, especially PTFE (“Teflon”) wax, and or silicones; additives that may be used to provide very good slip, low slide angles, and low coefficient of friction (COF) may present bonding issues. The excessive use of spray powder can also be detrimental to establishing effective glue bonds, so minimum amounts should be used. Heat resistance is needed in topcoats that will be glued with a hot-melt adhesive.
Considering a lengthy commercial aqueous coating history and experience, it can be said that the majority of aqueous coatings are capable of being glued successfully with cold white glues and hot melts. Also, it’s been shown that most problems will occur with cold glue applications.
Whenever there is doubt, request a co-polymer cold white glue that is formulated to adhere to foil or PVC. Testing is always important to determine what works best. Contact Cork Technical experts if you require an aqueous coating with enhanced performance properties for a demanding application.
Corks’ business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives.