Understanding the New Castable Urethane Adhesives – Part 1
This is the first of a three-part series on castable urethane adhesives. The additional parts of the series include Fundamentals of the New Castable Urethane Adhesives – Part 2 and Thermoplastic Urethanes and the New Castable Urethane Adhesives – Part 3.
Castable urethanes are expanding into many industries. In particular, castable urethanes are used for wheels, rollers, bumpers, and mining and oilfield equipment. It’s important to understand the use and properties of castable urethanes before discussing the adhesives.
Castable urethanes are popular in part because they are easy to use, process, mix, pour and cure. They also provide a low barrier to entry into a market. They offer superior properties including abrasion resistance, oil resistance, tear and cut resistance, and capability for high load bearing.
A castable urethane is a two-part urethane, with an A-side that consists of liquid urethane prepolymer with a known percentage of isocyanate. The B-side is hydroxyl or an amine-type curative. The A-side may contain plasticizers, catalysts, fillers, degassing agents, UV stabilizers, antioxidants, as well as colorants.
Certain variables exist that can affect a castable urethane’s performance. The composition of the urethane, such as whether it is aromatic versus an aliphatic isocyanate or polyester versus polyether polyol, can result in difference performance properties. The index number, or ratio of the number reactive groups remaining in the prepolymer to the number of groups present in the curative, can fluctuate from 85 – 130 (and is typically in the range of 95 – 105). The polyol chain length, multifunctional crosslinkers and additives are also variables that can contribute to performance.
Castable urethanes do have a few disadvantages, such as high cost and the associated use of ketone solvents. Castable urethanes are not suggested for continuous high temperature service above 121°C (250°F), or for wet /hot environments.
The current industry trend is toward higher heat applications and use in hot/wet environments as new urethane grades are in development to withstand demanding environmental conditions.
In accordance with this trend, wheels and similar parts are being engineered for high load-bearing applications. Vibration, flexing or impact can generate an internal temperature increase that can lead to failures within the urethane and the adhesive. As the development of urethanes leads to higher heater resistance within compounds, the demand for heat-resistant adhesive offerings will continue to increase.
Problems with existing adhesives
Many of the current adhesives in the market were not designed to perform at elevated temperatures or in hot, wet conditions because those were considered disadvantages of castable urethanes. However, the industry has moved forward with better urethane formulas that allow manufacturers to push the boundaries of performance. We have followed these trends and developed adhesives that work in these demanding conditions.
As explained in Part 2, new adhesives are available in both 1K and 2K systems. These new, stable adhesives can bond a variety of cast urethanes.
Learn more on castable urethane adhesives, the testing, grades and failure modes or contact Parker LORD at +1 877 ASK LORD.