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Mercury free Polyurethane for underwater application

 

The situation:

The customer was using a polyurethane compound, incorporating a mercury based catalyst, to pot connectors used under water. Stock of the existing product was running low and the customer wanted to switch to a more environmentally friendly, safer system.

 

The basic requirement

In addition to being suitable for continuous operation under water, the handling parameters for the new Polyurethane compound had to be very close to the old product in order to accommodate the manufacturing process presently in place. The most critical handling parameters to be considered were the time to develop “green strength” and the overall cure time.

 

The formulating problem

Shifting to Alternative Catalysis in a Polyurethane Formulation

Taking a traditional organometallicly catalyzed polyurethane and replacing the catalyst with a more environmentally friendly alternative has its’ challenges. The traditional catalyst imparts delayed reactivity which is followed by a relatively fast cure which allows for optimum handling while developing the required "green strength" quickly thereafter. Furthermore, these catalysts also have the added benefit of selectively promoting the polyol reaction with the isocyanate. Based on our experience, just substituting another type of catalyst would change many of the handling and physical properties such as; pot life, gel time, green strength development and could even affect the storage stability.

 

One of the major issues to be considered was the customer’s need for a minimum 4 minute pot life, while developing the required green strength within 20 minutes. It was our experience that the alternative catalysts available simply could not provide the kind of gel/cure profile required without modification. If the pot life was long it would take much longer for the green strength to develop. If the pot life was short then the urethane system would gel too quickly making the handling difficult. The solution to this problem was to select a catalyst that would allow for the green strength to develop relatively quickly while at the same time being compatible with a poly-urea to control the pot life and gel time. Such a formulation might result in a moderately slow pot life of 4 minutes and good green strength after 20 minutes.

 

Unfortunately this solution caused another problem in the formulation. It appeared that the longer pot life in combination with the modified catalyst allowed bubbles to form in the cured product. It was later theorized that bubbles formed as a result of a competing reaction with moisture in the Isocyanates. As a next step, air release agents and surfactants were incorporated to help eliminate the bubbles in addition to making certain that both the resin and hardener components of the formulation were stripped of water. Unfortunately the bubbles were still present in the final product. Further analysis revealed that some of the additives which were added to help release the bubbles contained volatile components and in fact these volatiles were the source of the problem. No bubbles were visible in the cured product once these additives were removed.

 

As one might conclude from the above, there are no simple, ready made solutions to everyday application problems, but solutions are possible with a bit of innovation, clever formulating and troubleshooting applied to each issue at hand. The benefits of formulating in this manner provided not only a system that is tailor made for the customer, but it also allowed us to add a new, more environmentally friendly product to our portfolio.

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