One-part solvent-borne polyurethane adhesive The one part adhesive is essentially a thermoplastic PU (TPU) dissolved in a solvent. The solvated TPU is applied to the prepared surface as a contact cement, or is applied, allowed to dry, and then adhesion generated by contacting the adherends with heat. polyether soft segments dominate the adhesive application, many onecomponent solventborne PU adhesives will employ a polyester soft segment such as polybutylene adipate for the simple reason that its ability to crystallize can increase the initial adhesion between adherends.
In one part solvent borne PU adhesives, pretreatment of the surface ie. extensive roughening of the surface and the use of a primer coating that has preferential adhesion to the substrate and the TPU.
Typical primer polymers are substances such as chlorinated polyethylene at high dilution in an aromatic solvent such as toluene. For these systems, the solvent can also be a major participant in the growth of adhesion by swelling both the substrate and the adhesive polymers to promote polymer chain entanglement or physical interlocking of larger polymer segments that form as the adhesive laminate swells and then contracts.
Two part solvent borne polyurethane adhesive A two part solvent borne version of PU adhesives is typically an isocyanate tipped prepolymer with relatively low isocyanate content (5–12%) on the one side, and chain extender (diol or diamine), catalyst, and often a crosslinker for the second component. A two part solvent borne system has the benefit of allowing the opportunity to tune adhesive modulus and improve solvent resistance via the crosslinking capability. Blocked isocyanates are standard polyisocyanates prereacted with weakly bonded active hydrogen groups. When heated, the weakly reacted components decouple releasing the original polyisocyanate and the blocking agent. In addition, PU side reactions, particularly with ambient water, are capable of producing CO2 gas. In the limit of high humidity, this can have the very undesirable effect of foaming the adhesive layer. Hotmelt Adhesives Hot melt adhesive (HMA) is a form of thermoplastic adhesive that is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. hotmelt adhesives are thermoplastic materials that (i) are heated and applied to a substrate in the molten state, (ii) the bonding substrates are assembled, and
(iii) the thermoplastic resolidifies back to its thermoplastic state. non-reactive hot melt adhesive This is ascribed to the higher viscosity of TPU at standard application gun temperatures that limit substrate wetting. It is possible to reduce viscosity and thereby improve hotmelt adhesive properties by addition of a monol (e.g., replacing some amount of butanediol with 1butanol), substitution of even 5% of the butanediol chain extender with 1butanol can reduce molecular weight of the chain limited polymer by a factor of 2–3 and reduce the viscosity of the polymer melt by over an order of magnitude based on the α Mn rule formulation of a nonreactive polyurethane hotmelt adhesive
Reactive Hotmelt Adhesive Reactive PU hotmelt adhesives are essentially a low percentage isocyanate prepolymer that is applied to a substrate and allowed to cure by reaction via adventitious moisture, or by direct reaction with substrates possessing active hydrogen on their surface. Reactive hotmelts possess a highly desirable grouping of properties including low viscosity at application temperatures, rapid generation of green strength when using crystalizable soft segments, gradual development of very strong adhesive bonding, no VOCs, and the excellent material properties of PU elastomers. Formulation of a reactive polyurethane hotmelt adhesive
Polycaprolactone based MDI prepolymer with 2.3% isocyanate useable as a reactive PU hotmelt adhesive. Polyether polyols can also be used to make very good reactive hotmelt adhesives, especially when the prepolymers are made of blended molecular weights to provide the mix of low viscosity and rapid green strength build.
Waterborne PU Adhesives (PUd) PUds enable delivery of a low viscosity urethane polymer to a substrate, while reducing or eliminating the use of fugitive organic solvents The polyol and isocyanate structures in PUd prepolymers can vary widely depending on final application and requirements. While PUd prepolymers are typically isocyanate terminated, they can also employ hydroxylterminated prepolymers for two component waterborne urethanes.
Prior to dispersing in water, the carboxylic acid groups in the prepolymer are neutralized with a strong base such as triethylamine to convert the acid to the carboxylate. It is this hydrophilic neutralized carboxylate that serves to disperse and stabilize the particles in the aqueous phase. Immediately following this aqueous dispersion step, a chain extender is added to the dispersion to complete conversion of the remaining isocyanate groups to the final polymer within the particles. Due to the competing reactivity of water, the selection of chain extender is limited to chain extenders with higher reactivity toward isocyanates than water. Most commonly, these chain extenders are primary or secondary polyamines that chain extend the polymer via formation of urea bonds
Batch Process for PUd The PUd ionomer process is often done with low levels of a solvent, such as acetone or nmethyl pyrrolidinone (nMP) to lower prepolymer viscosity in the process. In the case of acetone, the solvent is removed in a final step, while low levels of high boiling solvents such as nMP may be left in the final product.
PUd ionomer process