Types of Non-Reactive Polyurethane (PU) Adhesives
Polyurethane adhesives, also known as PU glue, adhere well to many substrates and are divided into two major classes: non-reactive and reactive. The concept behind polyurethane adhesives is to apply a thin layer of high molecular weight polyurethane between two surfaces to be bonded. The adhesion strength is reinforced by hydrogen bonds. While reactive polyurethane adhesives are usually supplied as one- or two-component systems, non-reactive polyurethane adhesives are based on high molecular weight polymers and are applied as one-component solvent-borne, waterborne, or hotmelt adhesives.
Solvent-Borne PU Adhesives
Solvent-borne PU adhesives are created by dissolving high molecular weight polyurethane polymers in organic solvents, such as MEK, acetone, methyl acetate, and ethyl acetate. This process allows the solid polyurethane polymer to be applied to the substrate. The solvent promotes substrate wetting and then evaporates with heat, leaving a thin layer of solid polyurethane polymer on the surface. Once the substrates are joined with moderate heat and pressure, a strong bond rapidly develops due to the recrystallization of the polyurethane polymer backbone.
To enhance bonding strength, heat resistance, and solvent and water resistance, cross-linkers are often added to solvent-borne PU adhesives. However, this process transforms one-component non-reactive PU adhesives into two-part reactive adhesives with limited pot life. Therefore, adding cross-linkers is only necessary for certain applications, such as adhesives for footwear, automotive, and furniture. Additionally, highly dispersed silicas can be added as fillers to increase initial bond strength without chemical reaction, but higher adhesive activation temperature and adhesive formulation stability need to be considered.
During the preparation of solvent-borne PU adhesives, about 50% of the required solvent for the final adhesive solution is typically added to a closed mixer. When selecting the solvent, factors like compatibility, substrate wetting ability, adhesive viscosity, storage stability, activation behavior of the adhesive film, and process environment safety need to be carefully considered and controlled. In solvents, thermoplastic polyurethane granules need to be added gradually to avoid coagulation. The mixture is stirred, and the temperature, raised by shear forces, is controlled by adding the remaining solvent. The dissolution time varies based on process conditions, ranging from 24 hours using low-temperature equipment to two hours under reflux conditions.
Waterborne PU Adhesives
Waterborne PU adhesives consist of fully reacted, high molecular weight polyurethane polymers dispersed in water, containing minimal or even zero solvents, and thus emit very low levels of volatile organic compounds. The adhesive films form through water evaporation or water migration into the substrate. If the drying equipment is properly designed, the drying rates of waterborne PU adhesives can be comparable to those of solvent-borne PU adhesives. Temperature control is critical during the film-forming drying process of waterborne PU adhesives. If the temperature is too low, the adhesive film may not achieve the required tack level, and if the temperature is too high, the film will cool slower, causing low initial bonding strength. Moreover, the drying process must ensure that no residual water remains in the adhesive film, as trapped moisture can interfere with bond formation and lead to hydrolysis over time. After drying, substrates can be joined with moderate heat and pressure, and a strong bond quickly develops due to the recrystallization of the polyurethane polymer backbone.
Compared to solvent-borne PU adhesives, waterborne PU adhesives have lower viscosity and higher solid content. Like solvent-borne PU adhesives, cross-linkers can be added to improve bonding strength, heat resistance, and solvent and water resistance, transforming them into two-component reactive adhesives with limited pot life.
Waterborne PU dispersions (PUDs) are commonly used in the automotive, construction, packaging, and footwear industries, as well as in the manufacture of foils and textile laminates. Although they bond well with many materials, including PVC substrates, their adhesion to rubber or certain surface-treated materials is poor. Waterborne PU adhesives are typically coated by knife, roller, or sprayer, and heating is required to speed up the drying process.
Hotmelt PU Adhesives
Hotmelt PU adhesives are thermoplastic polyurethane polymers in granule form that can be extruded or melted by heat in holding tanks to form glue sticks for hotmelt guns or hotmelt adhesive films for industrial heat press machines. When the hotmelt PU adhesive granules are used to make hotmelt adhesive film, adhesive-coated nonwoven, or other substrates, they can be easily processed with conventional hotmelt bonding techniques, such as calendaring or membrane heat press. Hotmelt PU adhesives can be applied at temperatures ranging from 60 to 130°C, with bonding strength formed by physical cooling and recrystallization.
A common method of using hotmelt PU adhesives involves placing the adhesive in film or powder form between substrates, heating the laminate above the melting point, and then applying pressure to bond the layers together. Additionally, hotmelt PU powder adhesives can be spray applied or scatter coated, making them especially suitable for gentle lamination of heat-sensitive substrates, such as high-quality textiles, due to their low tacky temperature. The good flow properties of hotmelt PU powder adhesives in the heat-activated state allow them to form strong bonds even at low pressure and with short pressing times. Hotmelt PU adhesives in both film and powder form are suitable for bonding flat substrates, such as paper, metal film, plastic film, wood plate, leather, and mainly textiles.
Hotmelt PU adhesives can also be designed as reactive PU hotmelt adhesives. More information can be found here:
https://www.taiwanpu.com/en/about/What-is-PUR-Reactive-Hotmelt-Adhesive
Summary
Non-reactive polyurethane adhesives can be classified into solvent-borne, waterborne, and hotmelt PU adhesives. While solvent-borne PU adhesives are commonly used in the footwear and textile industries, they are under increasing scrutiny due to the environmental impact and health risks associated with solvent evaporation and volatile organic compound emissions. Consequently, waterborne and hotmelt PU adhesives are becoming more popular. For more information about TPUCO's solvent-borne, waterborne, and hotmelt PU adhesives, please visit
https://www.taiwanpu.com/en/product/PU-Adhesives-and-Hot-Melt?kindt=true
Link to TPUCO products:
Polyurethane Adhesive and Hot-melt:
https://www.taiwanpu.com/en/product/PU-Adhesives-and-Hot-Melt
Shoe adhesive:
https://www.taiwanpu.com/en/product/PU-Adhesives-and-Hot-Melt/Shoe-adhesive
Hot-Melt Thermoplastic Polyurethane:
https://www.taiwanpu.com/en/product/Hot-Melt-TPU
Water-borne Polyurethane Adhesive:
https://www.taiwanpu.com/en/product/Water-Based-PU-Adhesive
Waterborne Polyurethane Adhesive
https://www.taiwanpu.com/en/product/Waterborne-Polyurethane-Adhesive
Polyurethane Dispersion
https://www.taiwanpu.com/en/product/PU-Adhesives-and-Hot-Melt/Polyurethane-dispersion