This is the third part of a three-part series on castable urethane adhesives. The additional parts of the series include Understanding the New Castable Urethane Adhesives – Part 1 and Fundamentals of the New Castable Urethane Adhesives – Part 2.
The new 1K Chemlok castable urethane adhesive covers a duel role for bonding. In addition to bonding castable urethanes, it is also an excellent adhesive for Thermoplastic Urethanes (TPUs). TPUs fall into a sub-category of TPEs (thermoplastic elastomers). The type of crosslinking that occurs in TPEs (dipole or hydrogen bonding) is much weaker than the covalent crosslinking that occurs within thermosetting elastomers. From an adhesion standpoint, TPUs are difficult to bond due to a lack of covalent crosslinking functionality.
TPUs are block-type polymers that consist of three main components: a polyol, a chain extender, and a diisocyanate. These components impart the physical properties inherent to the polymer. For instance, long chain polyols can impart flexibility, and hard blocks made from the extender and isocyanate can impart toughness and affect other performance parameters. TPUs are known for their abrasion resistance, elasticity, low temperature performance, resistance to oils and solvent, flexibility, and good weather resistance. These attributes contribute to the versatile nature of TPUs and opens a door to many different applications.
Applications like load bearing wheels, caster wheels, gaskets, seals, etc. encompass a variety of markets for TPU, including industrial, automotive, and oil and gas to name a few.
Most articles are made through a melt injection process that utilizes mechanical fasteners that are expensive, time-consuming, and require complicated mold geometries. Using an adhesive to bond injected TPU to metal or plastic is a cost-effective and easy-to-use system. Yet, there aren’t many adhesives on the market that provide this type of performance, and certainly not many that can achieve a rubber tearing bond on TPUs.
New adhesive for TPU
The new 1K adhesive developed for TPU and castable urethane is easy to use - It can be sprayed or brushed directly out of the container without dilution and can achieve rubber tearing bonds with a variety of TPUs.
This new adhesive bonds polyester, polyether, aliphatic/aromatic isocyanate based TPUs in a wide variety of hardness levels. The manufacturers guidelines for injection molding were followed to bond TPU to grit blasted Steel (2-3 mil anchor profile).
The following parameters were used for testing, the adhesive was spray applied at 1.0 mil (25 microns) and then dried in a forced air oven for 30 minutes at 93 degrees C (200 degrees F) prior to molding. The parts were injection molded with TPU onto the coated GBS sample.
A variety of prebake and postbake conditions were evaluated for primary bond testing on a tensile tester. The best combination was no prebake with a 30-minute 121 degrees C (250 degrees F) post bake. The new adhesive gave nearly 100% rubber tearing bonds on all elastomers for this test.
The new 1Kand 2K adhesives work with a variety of castable urethane grades to provide superior bonding in tough environments. It should not be forgotten that the 1K adhesives were developed for use with both TPU and castable urethanes.
The advantages of these adhesives are:
- Easy to use
- Work in demanding high temperatures
- Work in hot/wet (humid) environments
- Do not need a prebake, but can tolerate one
- No free isocyanate in 1K formulas