Hot melt glues are 100% solid thermoplastics that are applied in a molten state, forming a bond on cooling. Because they only have to lose heat to set, curing is very rapid when compared to the loss of carrier liquids used in solvent, water and starch-based adhesives. Furthermore, they do not require drying areas, which saves space, or need special extractor and recovery systems to remove harmful volatile organic compounds (VOCs) during the application process.
Hot melt glue composition
Hot glues typically consist of five main ingredients: polymers, plasticisers, resins, waxes/oils, and antioxidants - all of which have a role to play but must be fully compatible with one another for the glue to work as intended.
Polymers
Seen as the backbone of the formulation, polymers provide the strength and flexibility that is essential to the performance of glue. Different types of polymers are often used, typically these include ethylene vinyl acetate (EVA), thermoplastic rubber, metallocene (mPO) and amorphous poly alpha olefin (APAO). EVAs, known for their versatility and compatibility to many types of waxes and oils, and metallocenes, ideal for advanced bonding where improved precision, mileage and temperature performance is required, are the most commonly used in modern hot melts.
Increasing the polymer strength in any formula will raise its viscosity and flexibility, as well as impact its heat and chemical resistance.
Resins
Imparting tack is the main function of resins, allowing the adhesive to form the initial bond between substrates, due to the resin having a softening point above room temperature. Tackifying resins also influence many important characteristics of glue: its stability, colour, odour, and heat resistance are all affected by the amount, type and compatibility of resin used in the formula.
Plasticisers
Acting as a second base polymer, plasticisers can be added to formulations to compensate for base resins that are too hard, providing the adhesive with greater flexibility and workability. Ultimately, this improves wetting and reduces the melt viscosity of the glue.
Waxes/oils
Paraffin, micro & synthetics are common types of waxes and oils that are used as diluents within formulations. They help to control the open time (amount of time it takes to form a bond) and setting speed (how quickly a bond of acceptable strength forms) of adhesives; characteristics which should be tailored to the exact requirements of production. Waxes and oils also influence heat resistance and decrease viscosity.
Antioxidants
Small amounts of antioxidants are added to formulations to support thermal stability, to protect hot melts from degradation during their shelf life and application, which can occur if glues are kept at elevated temperatures for too long. Antioxidants do not influence the physical or adhesive properties of hot melts.
Other additives
Other additives, such as fillers, UV, pigments, dyes, and stabilisers, can be added to formulations to fine-tune certain properties, providing stability against exposure to light, in the case of UVs, or to alter viscosity, open time, and set time, as in the case with fillers.
If you would like further information, you can visit our hot-melt adhesive technology page, alternatively, send an email to: marketing@beardowadams.com