Urethane Adhesion

Urethane Adhesion

Understanding climate factors related to choosing an adhesive

Adhesive properties can be affected by temperature and humidity in both the cured and uncured state, so it is important to establish proper storage and to manage the work environment.

Every country in the world undergoes seasonal changes, some more extreme in range than others. Global transportation methods encounter changes in environment as well. The result is variable and inconsistent levels of heat and humidity which, in most cases, can be largely controlled.

Heat and humidity differences exist in both time and space. Time: it is not unexpected to see 30% relative humidity at 70degF in winter jump in the summer to 80% humidity at 70DegF in the mid-Atlantic states of the US, for example. Space: A given day in June in the northern hemisphere will have warm weather, while in the southern hemisphere there will be cool weather.

The following advice will help you to make appropriate decisions.

STORAGE
Follow the manufacturer’s recommended storage guidelines for best results with any chemical product.
You can usually find this information on the label. For example, warehouse or refrigerator maintained between 45F and 75F, or refrigerate at 40F-45F.

WORK ENVIRONMENT
The work environment is just as important as proper storage, perhaps even more so. In this context, we are concerned with two measurable properties – heat and relative humidity – and their impact on adhesive application and cure.

To give an example, there was a multinational company with basically the exact same manufacturing plants set up in both Mexico and Eastern Europe. They wanted to simplify the supply chain by specifying a single product for use in all plants.

Shortly after both plants went into full production, QC tests reported a high number of component failures from the plant in Mexico. After comparing all the possible reasons for the difference in product performance, the native climate of the facility was found to be the overlooked factor in choosing the adhesive.

One location is a very cold, low-moisture country, while the other is very hot and humid. In this example, the adhesive spec’ed was a cyanoacrylate (instant) adhesive. These adhesives cure by reacting with trace amounts of moisture in the substrates. In the case of a high humidity environment, the adhesive begins to cure before the components can be fully assembled.

Many adhesive formulas are based on a “moisture-cure” reaction, not just the instant cyanoacrylates. These moisture-cure products include some of the poly-urethanes, silicones, and modified epoxies, to name a few. Adhesives with long open times (for precise assembly) can also be affected by heat or humidity.

Each application should be carefully considered in all stages, and tested across the full range of expected environmental conditions.

It is important to measure the environment with calibrated equipment and to monitor the work area regularly to maintain a constant temperature and relative humidity. It may be as simple as installing a fan or dehumidifier unit. This investment of time and money up front will ensure consistent cure speeds low reject rates.

If you’re supplying factories in a few different parts of the world, be sure to have new adhesive choices tested in each location. Don’t neglect to confirm that satisfactory storage arrangements are available.

Armed with the knowledge of both local climate and the application process, it is quite often possible to choose one adhesive that will meet all the requirements. For challenging conditions, contact the manufacturer who will have a technical support staff to provide recommendations.

About the Author

Permabond is a leading manufacturer of engineering adhesives, supplying high-quality industrial adhesives to customers worldwide. Permabond has offices in The Americas, Europe and Asia and a worldwide network of distributors to assist you with your specific bonding and sealing requirements. To learn more, please visit Epoxy.

How should the first coat of automotive urethane clear coat be sprayed?

I'm using PPG urethane clear coat. Most instructions and videos online say to mist a light first coat for adhesion ("tack coat"). The first coat will not appear glossy. Its purpose is to penetrate into the base coat for better adhesion and flexibility. Subsequent coats can be wet and achieve high gloss. But the guy from the paint supply store says the first coat should be a wet coat that looks the way you want the finished product to look. His explanation is, "People are used to spraying lacquer, in which each coat melts into the previous coat and levels out. Urethane is prone to orange peel if you use a light 1st coat because each coat doesn't melt into previous coats."

I've painted 3 times in the last week using wet 1st coats as the guy at the paint store suggested and they look great, but I'm afraid of the long term effects.

As a professional painter I can say that whatever method is used to apply clearcoat, the goal is to produce the best finish.What works for you is what is best.And the long term durability is not affected by starting with any particular first coat.Find a technique that suits you and be happy.

Polyurethane Adhesion Test, Part One, A BETTER SURFACE