A Look at Caprolactone Technology in Reactive Hot Melt Adhesives
As market drivers accelerate the adoption of polyurethane reactive hot melt adhesives away from solvent-borne solutions, the use of polycaprolactone technology in reactive hot melt adhesive applications should be examined. According to a recent study conducted by supplier Ingevity, reactive hot melt adhesives provide advantages in a variety of end-application areas that include textiles, woodworking, construction, automotive, bookbinding, and electronics. A combination of the initial recrystallisation after application with the subsequent polyurethane polymerization gives this type of adhesive platform superior adhesion properties with enhanced chemical and heat resistance compared to traditional non-reactive hot melt adhesives.
Polycaprolactone is polymerized via a ring opening synthesis route, where caprolactone monomer is ring-opened via an initiator moiety. The initiator moiety is a hydroxyl functionalized molecule, which can be a diol, triol, or tetrol. In reactive hot melt adhesives, polycaprolactone polyols are commonly diols of molecular weight between 2,000 and 4,000 and are used in the soft segments of the polyurethane structure. In addition to polycaprolactone polyols, polycaprolactone thermoplastics are used to enhance the performance of the adhesive. The polycaprolactone thermoplastics provide increased green strength to the adhesive and are used in combination with polyurethane pre-polymers.
The study conducted at Ingevity’s new Innovation Centre in Warrington, UK, has fully characterized the performance of a 4,000 MWt caprolactone diol versus that of a commercially available polyester diol. Table 1 shows the details of the polyols used in the study. Formulators have a variety of polyol chemistries to select from when considering the desired properties of their polyurethane soft segment. Polyether polyols from 1000-2000 molecular weight are typically used, with polyester chemistry being incorporated to increase the levels of adhesion or to tune crystallinity for control of adhesive open times.
In the Ingevity study, reactive polyurethane hot melt adhesives were synthesized with a 3% excess isocyanate content. The formulations were kept simple to allow for a direct comparison between the caprolactone and the polyester diols respectively. The formulations characterized in this study are based on a reaction between a diol and monomeric methylene diphenyl diisocyanate (MDI), with a 3% excess of isocyanate content, to allow for the subsequent reaction between the isocyanate and ambient moisture, converting the final adhesive into a thermoset polymer.
Source: Ingevity / Adhesives Magazine