Polyurethane adhesives are one of the most important types of structural adhesives. Their properties can be tailored over a wide range for a large number of applications. They can be rigid and hard or flexible and soft. They are formed from the reaction of an organic (poly)diisocyanates with (oligomeric) diol compounds, which leads to urethane linkages in the backbone (-NH-C(=O)-O-). The high reactivity of the isocyantes allows for fast cure but slow cure can also be achieved by adjusting the catalyst level.
Reactive urethane adhesives are available as 100% solids or as solvent based systems and as one or two part adhesives. Unlike epoxies, polyurethanes can be formulated to be extremely flexible. They form tough bonds with high peel strength and medium to high shear strength. Since they are very flexible, they are often an excellent choice to bond films, foils, and elastomers. They have excellent adhesion properties to sheet molding composites (SMC), and therefore, are used extensively in the automotive industry. They also bond exceptionally well to wooden substrates and find many uses in the wood industry. Unlike many other structural adhesives, urethane adhesives have excellent low temperature properties similar to silane based adhesives. However, urethanes do not have high temperature resistance and decompose at much lower temperatures than epoxy adhesives. Another major drawback of many urethane adhesives is that they do not bond well to metals unless a primer is applied to the substrate prior bonding.
The most important commercial aromatic isocyanates are toluenediisocyanate (TDI), diphenylmethane diisocyanate (MDI), and naphthalene diisocyanate (NDI) and their polymeric forms.
Toluene diisocyanate is usually supplied as a mixture of two isomers: 2,4-TDI and 2,6-TDI. Both are are more toxic than MDI. For this reason,
MDI based polyisocyanate are often prefered for coatings, sealants and adhesives.
One way to eliminate or minimize the release of isocyanate vapor is the preparation of reactive urethane oligomers. This can be achieved by reacting them with polyols to oligomers (prepolymers) with a slight excess of isocyanate.
Besides (polymeric) MDI and TDI some aliphatic isocyanate have gained industrial importance. The most
common aliphatic isocyanate are hexamethylene diisocyanate (HDI),
isophorone diisocyanate (IPDI), and hydrogenated MDI (HMDI). In contrast to aromatic isocyantes, urethane asdhesives made with these isocyantes are UV-stable, i.e. do not discolor and
are less susceptible to oxidation and degradation. However, they are more expensive. For this reason, aromatic isocyantes are used when oxidative discoloration on exposure to UV
radiation is not an issue, whereas aliphatic isocyantes are preferred for more demanding applications.
The most important triisocyanate is triphenylmethane triisocyanate. It is sometimes blended with other isocyanate and acts as a cross-linker.
A great advantage of isocyanates is that they react with any compound that contains active hydrogen. For example, they react with amines to polyurea.
The addition of other co-reactants leads to many hybrid adhesives
with many interesting properties.
Most polyurethane adhesives are either polyester or polyether based. They are present in the isocyanate prepolymers and in the active hydrogen containing hardener component ( polyol). They form the soft segments of the urethane, wheeras the isocyanate groups form the hard segments. The soft segments usually comprise the larger portion of the elastomeric urethane adhesive and, therefore, determine its physical properties. For example, polyester-based urethane adhesives have better oxidative and high temperature stability than polyether-based urethane adhesives, but they have lower hydrolytic stability and low-temperature flexibility. However, polyethers are usually more expensive than polyesters.
Many urethane adhesives are sold as two-component urethane adhesives. The first component contains the diisocyanates and/or the isocyanate prepolymers, and the second consists of polyols (and amine / hydroxyl chain extenders). A catalyst is often added, usually a tin salt or a tertiary amine, to speed up cure. The reactive ingredients are often blended with additives, and plasticizers to achieve the desired processing and/or final properties, and to reduce cost.
One-component urethane adhesives are formulated to cure when exposed to moisture at room temperature, or by heating. The majority of one component polyurethanes are based on isocyanate terminated prepolymers. To accelerate cure, a catalyst, usually a tin salt or a tertiary amine, is added. In the case of moisture curing urethanes, the moisture in the air is usually sufficient to cure the prepolymers. However, the substrate must have some permeability to allow the moisture to reach the adhesive. The other type of one-component polyurethane adhesives is based on ‘‘blocked isocyanates’’. The diisocyanates or isocyanate prepolymers are reacted with certain active hydrogen containing compounds that block the isocyanate, i.e. preventing it from reacting with the polyols present in the mixture prior cure. Common blocking agents are caprolactams, 3,5-dimethylpyrazole, acetoxime, and methylethyl ketoxime which vary in the deblocking agents. The blocked isocyanates are usually stable for several month and can be formulated with polyols and certain chain extenders without additional reactions. To cure these adhesives, the blocked isocyanate has to be unblocked, which is achieved by heating the adhesive to the temperature where the blocked isocyante reverts back to the free isocyanate.