The automotive industry uses a wide range of polymer products, including adhesives and sealants. As the industry increasingly strives for light-weight materials, adhesives are replacing traditional connection methods. Each application requires the adhesive to have specific characteristics, including curing conditions (temperature, moisture and speed of cure) and long term material properties, such as flexibility, UV resistance and bond strength. Fourier Transform Infrared (FTIR) Spectroscopy is a fundamental analytical tool to help ensure quality control of adhesive products.
The curing and working properties of adhesives generally result from polymerization reactions, which form a lattice of chemical bonds. Basic chemical kinetics identifies four steps in these polymerizations – initiation, propagation, termination and branching. The relative rates of these determine the properties of the final polymer. For instance, the termination step can control overall polymer chain length, branching impacts the cross-linking, and propagation rate determines curing times.
The initiation step is critical. Early initiation may result in ruined product, while sluggish initiation can lead to poor or slow curing. The initiation can be stimulated chemically, as in most two-part epoxies (the hardener stimulates a reaction in the resin), via UV-irradiation (many modern dental sealants) or using temperature. Storage needs require that the initiation reaction be halted until the proper moment. Urethanes provide an excellent example, where the initiation step must be blocked until the proper moment. Failing to do this can result in railroad cars filled with solid, useless, product.
The polyurethane reaction starts with a diisocyanate (or a poly-isocyanate) reacting with a co-monomer like an alcohol (frequently a diol):
R-O-H + O=C=N-R’ → R-O-C(=O)-NH-R’
Thiols and amines can also be used (instead of the alcohol) – it is the reactivity of the acidic hydrogen which drives the reaction. This reaction can be very rapid, even at room temperature, so the liquid mixture rapidly becomes a solid. This rapidity can be used to produce unique products. For instance, during manufacturing, a little water can be added to the reaction mixture. The water reacts with the diisocyanate to produce a diamine and CO2. The CO2 forms bubbles in the reaction mixture which are trapped within the rapidly forming polymer matrix, yielding polyurethane foam.
Shipping and storage of the liquid urethanes requires preventing initiation. This can be done by reacting the isocyanate with a “blocker”:
R”-NHC(=O)-B → R”-N=C=O + BH
where BH is the blocking agent. The removal of the blocking group – by chemical or thermal processes – yields the reactive isocyanate, initiating the reaction. Different blocking agents will eliminate at different temperatures, so research into the best adapted blockers (least toxic, optimal deblocking temperature, etc.) is underway. A key part of investigating blocking agents requires studying the temperature dependence of the initiation and the time-evolution of the reaction mixture. Infrared is ideally suited to this, as the spectrum gives specific information regarding the progressing reaction. The multi-component mixture can be analyzed as a function of time, yielding an exhaustive view of the reaction kinetics.
Read Time-Based FT-IR Analysis of Curing of Polyurethanes to learn about a study where FTIR was able to elucidate both the progression and the mechanism for a crosslinking reaction.