Knowledge Base

Humidity & Extreme Temperatures

Understanding How Environmental Conditions Affect Moisture-Curing Adhesives.

The StarFix range of adhesives and coatings are formulated to provide reliable performance across a wide range of environmental conditions. However, like all moisture-reactive systems, their curing behaviour is influenced by both temperature and relative humidity. Understanding these effects helps ensure consistent installation quality and avoids misconceptions when working in challenging weather conditions.

Temperature and Cure Rate

Under typical conditions, increasing temperature generally accelerates the chemical reactions responsible for curing. As the adhesive becomes warmer, its viscosity decreases, allowing it to flow and wet the substrate more effectively while reacting more quickly with atmospheric moisture.

Conversely, lower temperatures increase the viscosity of the adhesive, making application more difficult and reducing the rate at which the curing reaction takes place. At lower temperatures (<5°C), installers should therefore expect extended cure times before the adhesive reaches its full mechanical strength.

The Counter-Intuitive Effect of Extreme Heat and High Humidity

One phenomenon that is often misunderstood occurs during periods of extreme heat combined with high humidity. When temperatures exceed 30°C and relative humidity rises above 70%, the adhesive may appear to cure very rapidly. In reality, only the exposed surface is reacting quickly with the moisture in the air.

The result is a rapid surface skin or flash cure, which can give the impression that the adhesive has hardened. However, this surface layer acts as a barrier that slows the movement of moisture into the bulk of the adhesive beneath. As moisture penetration is restricted, the curing reaction within the body of the adhesive can slow significantly.

Although the surface may become tack-free within a short period, the adhesive beneath can require several additional days to achieve a complete cure. This behaviour is a recognised characteristic of moisture-curing polyurethane systems and is the opposite of what many people naturally expect.

Ideal Installation Conditions

For optimum performance, StarFix® moisture-curing adhesives should be applied under the following conditions:

Ideal Installation Conditions

For optimum performance, StarFix® moisture-curing adhesives should be applied under the following conditions:

Application temperature: 5°C to 25°C
Relative humidity: 40–70%
Adequate ventilation to allow normal moisture exchange

Under these conditions:

The adhesive will typically achieve a full cure within approximately 24 hours.
Bonded assemblies can generally be subjected to performance testing after 48 hours.

Where temperatures or humidity exceed the recommended ranges, curing times should be expected to increase, and testing or loading of bonded components should be delayed accordingly.

Climate Change and Increasing Weather Extremes

Historically, prolonged periods of very high temperatures and humidity were relatively uncommon across much of the UK. However, climate observations show that the UK is experiencing more frequent heatwaves, with higher average summer temperatures and an increasing number of days exceeding 30°C. Southern England, particularly London and the South East, is projected to experience the greatest increase in both heat intensity and warm, humid conditions during the coming decades.

These changing weather patterns mean that installers are increasingly likely to encounter environmental conditions capable of causing rapid surface skinning and delayed through-cure of moisture-curing adhesives. Planning installations with weather conditions in mind is therefore becoming increasingly important, particularly during the summer months.

Best Practice During Hot Weather

To achieve the best possible performance during periods of elevated temperature or humidity, Star Uretech recommends:

Store products in a cool, shaded environment prior to use.
Avoid applying adhesives to substrates that have been heated by direct sunlight.
Where practical, carry out installations during the cooler parts of the day.
Ensure adequate ventilation throughout the curing period.
Allow additional curing time whenever temperatures exceed 30°C or relative humidity exceeds 70%.
Confirm the adhesive has fully cured before applying significant loads or carrying out performance testing.

By understanding how temperature and humidity influence moisture-curing chemistry, installers can ensure the StarFix® range consistently delivers the high levels of performance, durability and reliability for which it has been developed.