Evaluation of thermal aging life of hot melt adhesives using high-temperature oven method

Principle

In the process of long-term use, the performance of hot melt adhesive will undergo irreversible degradation due to the action of heat, oxygen and other factors, which is called thermal aging. Evaluating its thermal aging life is crucial for predicting the material's reliability under real-world operating conditions. The high-temperature oven method is a widely used accelerated aging testing method, and its core principle is the Arrhenius equation. This method uses high temperature conditions to accelerate the internal chemical and physical changes of the material and simulate the long-term thermal aging effect, so as to evaluate the life index in a short time.

The Arrhenius equation describes the relationship between the reaction rate constant and temperature and is the theoretical cornerstone of accelerated aging testing. Its expression is as follows:

k = A exp(-Ea/(RT))

Among them, k represents the reaction rate constant, A refers to the pre-factor, Ea is the reaction activation energy, R is the molar gas constant, and T is the thermodynamic temperature. By determining the time it takes for the performance of a hot melt adhesive to degrade to a certain critical value at different temperatures, its service life at the expected service temperature can be extrapolated.

Experimental methods

The evaluation begins with the preparation of the specimen. The hot melt adhesive to be tested needs to be made into a standard-sized spline or film to ensure that the specimen is uniform and defect-free. Subsequently, the specimens are grouped into different positions in the high-temperature oven to ensure uniform heating. The oven temperature should be set higher than the normal service temperature of the hot melt adhesive, and multiple temperature points are usually selected for testing, such as 20°C, 30°C, 40°C, etc. gradient higher than the expected use temperature.

During the experiment, the specimen should be removed from the oven at regular intervals, cooled to room temperature in a standard laboratory environment, and then tested for performance. Key performance indicators include, but are not limited to: bond strength, melt viscosity, open time, softening point, and cosmetic changes (e.g., color, morphology). Records data on performance metrics decaying over time at each temperature point until performance drops to a preset failure threshold.

Performance metrics

During the thermal aging process, a number of properties need to be systematically monitored. Bond strength is at the heart of measuring the effectiveness of its application; Melt viscosity directly affects the construction workability; Opening hours are related to the operation window; The softening point reflects its heat resistance. All data should be clearly documented, and below are examples of key items that are recommended to be recorded.

Life prediction

Once performance-time data at different temperatures are obtained, data analysis is required. Typically, the aging curve at each temperature is plotted using the logarithm of performance retention (e.g., bond strength retention) as the ordinate and the logarithm of the aging time as the abscissa. When the performance degrades to a certain critical value (such as 50% of the initial value), the corresponding time is the failure time at that temperature.

Subsequently, the failure time at each temperature is substituted into the Arrhenius equation for linear fitting. The logarithm of the failure time (lnt) is used as the ordinate and the thermodynamic temperature reciprocal (1/T) is used as the abscissa, and the slope of the obtained line is correlated with the activation energy Ea. By extrapolating the straight line to the 1/T coordinate corresponding to the expected service temperature, the logarithm value of the predicted service life can be read from the ordinate coordinate, and then the thermal aging life estimate at the conventional service temperature can be calculated.

Notes:

The high-temperature oven method is an effective evaluation tool, but its premises and boundaries need to be paid attention to when applying. First, accelerated aging testing is based on a key assumption: the aging mechanism of materials at high temperatures is the same as at service temperatures. If the temperature is too high to trigger a new degradation mechanism (such as thermal decomposition of a hot melt matrix resin), the extrapolation results will be biased. Secondly, the air circulation, oxygen concentration and humidity in the oven may be different from the actual environment, and the experimental conditions need to be specified in the report. Finally, this method evaluates the aging life of thermal oxygen, and if there are other factors such as ultraviolet rays and chemical media in the actual environment, it needs to be combined with other test methods for comprehensive judgment.

Conclusion

The high-temperature oven method combined with the Arrhenius equation provides a relatively efficient and theoretically well-founded experimental scheme for evaluating the thermal aging life of hot melt adhesives. Through standardized sample preparation, accelerated aging at multiple temperature points, system performance testing and scientific data extrapolation, the durability of materials under expected service temperatures can be reasonably predicted in a short period of time. This provides important data support for formulation development, quality control and application selection of hot melt adhesive products.

References

ASTM D3045-92, Standard Practice for Heat Aging of Plastics Without Load.

ISO 11346, Rubber, vulcanized or thermoplastic — Estimation of life-time and maximum temperature of use.

Research on aging test methods for hot melt adhesives, Polymer Materials Science and Engineering.