Constant Temperature and Humidity Chamber Testing for Environmental Adaptability of Hot Melt Adhesive

Background and purpose

As a thermoplastic adhesive, the properties of hot melt adhesive are significantly affected by ambient temperature and humidity. During storage, transportation, and practical applications, it may face complex climatic conditions such as high temperature, high humidity, and low temperature drying, which may lead to changes in key characteristics such as bonding strength, opening time, and curing speed. Therefore, evaluating the environmental adaptability of hot melt adhesives is crucial to ensure their reliability under different operating conditions. Constant temperature and humidity chambers provide a standardized testing method for simulating these conditions by precisely controlling the temperature and humidity of the internal environment. This test aims to systematically evaluate the physical and chemical stability of a specific hot melt adhesive sample under set temperature and humidity conditions, and provide data support for the definition of its application scope and quality control.

Test Principle:

The test is based on the law that the performance of a material changes over time under constant temperature and humidity conditions. The constant temperature and humidity chamber works together with the refrigeration system, heating system, humidification system and dehumidification system to maintain the target temperature and humidity in the confined space. When the hot melt adhesive sample is exposed, the polymer segment movement, crystallization behavior, additive migration, and other processes may be accelerated, thus reflecting the possible effects of long-term environmental exposure in a short period of time. The test mainly refers to the general standards for environmental testing of adhesives at home and abroad, such as some standard methods related to environmental testing of electrical and electronic products, as well as relevant guidelines for wet and hot aging testing of non-metallic materials. The core is to look at the performance response of the sample to specific environmental stresses, rather than simulating absolute natural aging.

Test equipment

The test uses a programmable thermostatic humidity chamber, and its key performance indicators need to meet the test requirements. The temperature range typically covers -40°C to +150°C, and the humidity range is between 20% RH and 98% RH, with fluctuations and uniformity in accordance with relevant measurement specifications. The sample is the hot melt adhesive to be tested, usually prepared as a standard bonding specimen (e.g., a single-lap shear specimen) or a specific shape of the glue block. The preparation process needs to ensure that the colloid is fully melted, uniformly coated, and cured under consistent conditions to eliminate the dispersion of performance caused by the process. Samples should be conditioned in a standard laboratory environment (e.g., 23±2°C, 50±10% RH) for no less than 24 hours before testing.

Test conditions

The design of test conditions should be combined with the expected application scenarios of hot melt adhesives. Common test combinations include high temperature and high humidity, high temperature and low humidity, low temperature drying, and temperature and humidity cycling. For example, a typical test procedure might include: 500 hours at 85°C and 85% RH, followed by 20 hours at -20°C and 30% RH. The test procedure should specify the time points for the initial measurement, intermediate inspection (if any), and final measurement. When the sample is placed in the box, ensure that the surrounding air circulation is unobstructed and that there is no direct contact between different samples. The temperature and humidity in the box reach the set value and stabilize and start the timer.

Evaluation methodology

Before and during the test, the key performance parameters of the hot melt adhesive need to be quantitatively evaluated. The main evaluation methods include:

1. Mechanical properties test: Use a universal material testing machine to measure the shear, peel or tensile strength of the bonded specimen. Strength retention rate is the core evaluation index, and the calculation formula can be expressed as:R = (σ₁ / σ₀) × 100%, among themRis the strength retention rate,σ₀is the initial strength,σ₁It is the strength after the test.

2. Thermal Performance Analysis: The differential scanning calorimetry was used to measure the changes of glass transition temperature, melting temperature and crystallinity.

3. Appearance and morphology inspection: Visually or with the help of a light microscope to observe whether the colloid has cracking, powdering, discoloration, exudation or phase separation.

4. Rheological performance test: Measure the change of melt viscosity with temperature and humidity exposure, and evaluate the stability of its construction performance.

Typical test data

The following table shows a summary of the key performance data of a hypothetical hot melt adhesive under two different test conditions. The data has been simplified.

Data interpretation: Under high temperature and humidity conditions, a slight decrease in strength is accompanied by color changes, which may indicate a slight hydrolysis or thermal oxidation reaction. Under low-temperature drying conditions, the strength increases slightly, which may be related to stress release or further crystallization in the colloid. On the whole, the glue sample showed good stability under the test conditions, but the long-term high temperature and high humidity environment may need attention.

Notes:

The accuracy and reproducibility of test results are influenced by multiple factors. The uniformity and control accuracy of temperature and humidity in the chamber are fundamental and need to be calibrated regularly. The placement and loading of the sample affect the airflow circulation in the chamber, which affects the actual microenvironment in which the sample is actually located. Test cycles and measurement intervals need to be set up scientifically and rationally to capture key change nodes. In addition, tested samples often need to be reconditioned in a standard laboratory environment before being removed for performance testing to eliminate interference with measurement results by surface condensation or transient thermal stress. The entire testing process should document the equipment parameters, environmental conditions, and any observed anomalies in detail.

Conclusion

The use of constant temperature and humidity chambers for environmental adaptability testing is an effective method to evaluate the performance stability of hot melt adhesive products under different climatic conditions. By systematically designing test conditions, standardizing sample preparation, precisely controlling equipment, and comprehensively evaluating performance changes, quantitative data reflecting a material's environmental resistance can be obtained. These data are valuable for product development, specification development, quality control, and application guidance. Testing has shown that different formulations of hot melt adhesives are sensitive to environmental factors, so targeted testing for specific products is necessary.

References

1. Environmental Test of Electrical and Electronic Products Part 2: Test Method Test Cab: Constant Humidity and Heat Test.

2. Guidelines for adhesive environmental resistance test methods.

3. Plastic damp heat aging test method.

4. Calibration specification of constant temperature and humidity chamber.