High modulus urethane (HMU) can be confusing if you do not know all the specifics concerning its use. I remember when HMU first came out. I wanted to get the facts, so I called urethane manufacturers and vehicle manufacturers and asked for a definition of “high modulus.” Most of the answers I received were the same: “it is higher in modulus than normal urethane.”

I’m glad we got that cleared up, but what does a vehicle with an original equipment windshield installed with HMU mean to the technician in the field? Do you have to replace the windshield using HMU?

HMU is stiffer and has less elongation than normally cured urethane. While the shore harness of normal urethane is 50 to 60, HMU has a level of 60 to 70. As far as elongation, a normal urethane may have as much as 400-percent elongation while a HMU may only have 250 percent. According to vehicle manufacturers, HMU is used to stiffen the body for performance reasons. HMU, in conjunction with other stiffly bonded or welded panels, makes the vehicle less flexible than a normal unibody constructed vehicle, thus making a vehicle’s handling more precise and firm.

So is high modulus urethane a safety device? Does the customer have the right to waive its use? Some vehicle manufacturers contend that HMU is a feature built into the automobile for performance only, not protection of the occupants. Engineers and crash re-constructionists might not agree completely with that statement.

Does HMU boost performance? Yes. However, the argument on the safety side is that when a vehicle body is modified from original design, such as when a windshield installed with HMU at the factory is replaced with normal urethane, the crash dynamics change. In this case other panels and components may not act as predicted. If any of the other panels are structurally designed for safety crush zones, and counts on the rigidity of the HMU bonded windshield, then the safety would be compromised.

I have the tendency to avoid playing with what works. For the safety of my customers I will replace the glass in the same way the engineers designed it. If an installation calls for HMU or non-conductive urethane, I will replace it with HMU or non-conductive. As a trainer, I always try to find the reasons why a certain process or product is used. However, sometimes it is better to replicate the OE method rather than try to outthink the engineers.

I invite any and all urethane chemists out there to comment on this post. I would love to hear your layman’s explanation for the use of HMU and whether it makes a difference in the vehicles’ safety cage.