by Bob Beranek

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.

Comments (4)

  1. FRANK KRENSLAK said on 30-05-2013


  2. daniel said on 30-05-2013

    As a matter of fact, I faced strange situation this tuesday. The job was to replace a windshield of a 2008 Volvo S-40. Huge was the surprise when I removed the windshield and realized it was not the one from the original vehicle birth and more of this, the bondage was performed with butyl worm and not with urethane. Note that the original primary bead of adhesive was urethane ! I decided to use the factory solution, so I layed urethane on the pinchweld after peeling of all the butyl remains and pthoroughly primering the frame. Operation seems to be a success and customer was completely satisfied.

  3. Glasseye said on 31-05-2013

    Your article and the comment made by Daniel raises several questions. How can a technician identify HMU sealant compared to other PU sealants?. How many glazing operations would recognise and store varying sealant types?. The fact Daniel found a windshield fitted with butyl tape, demonstrates there is still ignorance/unawareness in the automotive glazing industry. I no longer do automotive glazing except for the occasional doorglass or indirectly glazed windshield but I am concerned that work done in the field is keeping up to date with new sealants and their correct application.

  4. Mitch Becker said on 03-06-2013

    To add to Bobs comment. Stiffening the structure is and can be related to structural integrity. It also relates to longevity of repair. If a urethane is used that allows to much flex in the vehicle structure, where does that flex go? What may happen is the body twists and rolls repeatedly beyond desired limits while driving. This movement creates stress on welds and metals in the weld joints. This may lead to metal fatigue or weakening of joints compromising the structure. The seam sealers also move beyond recommended tolerance. If the seam sealers do become compromised, then the corrosion protection is defeated. This opens the area up for corrosion in critical structural joints. A technician could do everything right by not producing cause any scratches in paint or any surface damage and still cause corrosion by using wrong material. Steels today are stronger than ever but also much thinner. Corrosion spreads and can compromise steel much faster today than ever before. Add to the mix chemicals on roadways such as salt and magnesium chloride for ice conditions and you have a mixture that can eat through the toughest of metals. His comment as to “don’t out think the engineers” are good words to live by.

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