Acrylic-based polymer waterproofing material - Methods of the test, Part 4: Determination of Minimum Film Forming Temperature and White Point
1992 Edition

Minimum Film Forming Temperature (MFT) is defined as the lowest temperature at which a polymer dispersion forms a continuous, uniform film free from cracks. This temperature is critical because coatings applied below the MFT may result in films that are discontinuous or cracked, compromising protective properties. Ensuring application above the MFT guarantees film integrity, enhances durability, prevents moisture penetration, and supports quality control by verifying suitability for environmental conditions.

The White Point (WP) temperature is identified by applying one or more films of an acrylic polymer dispersion onto a metal plate and establishing a temperature gradient along its surface. The film is dried using moisture-free air, and the temperature at which the film changes from transparent to opaque (whitens) is noted as the WP. This visual transition indicates the temperature below which the film fails to coalesce properly, thus marking the limit for suitable application temperature.

The apparatus consists of a rectangular metallic plate—made from aluminum, stainless steel, or copper—with precisely spaced small holes for thermometer insertion. One end is heated via an electric resistance heater, while the opposite end is cooled through a refrigerant reservoir, creating a stable temperature gradient spanning 20°C to 40°C. Optional features include shallow surface channels to contain the polymer and dry air inlets for humidity control. Insulation surrounds the setup to maintain thermal consistency during testing.

A calibrated film spreader is used to apply the polymer dispersion uniformly on a level or channeled metal plate, beginning from the hottest region of the plate. The dry film thickness should be maintained between 0.10 and 0.15 mm with a width of approximately 20 mm. The coated plate is covered with a glass sheet and dried either by passing a slow, constant flow of dry air from the cooler to warmer side or by placing silica gel with a moisture indicator above the film. Temperature is controlled to ensure a gradient of 20°C to 40°C, with the MFT positioned near the midpoint for accurate observation.

Acceptable temperature measuring instruments include mercury thermometers, resistance temperature detectors (RTDs), thermocouples, and semiconductor probes. These devices must operate accurately within a range of -10°C to +15°C, with a measurement precision of ±0.1°C. Proper installation into designated holes on the test plate is essential, and readings are taken once thermal equilibrium is reached to validate the linearity of the temperature gradient.

A combination of an electric heating element and a refrigerant-based cooling system establishes a consistent temperature gradient across the test plate. The gradient must be linear, meaning equal spatial intervals correspond to equal temperature drops. Thermometers are placed in uniformly spaced holes along the plate, starting at the cold end, to monitor temperatures. Data collected are plotted to confirm linearity, ensuring the Minimum Film Forming Temperature is positioned near the center of the gradient for reliable evaluation.

The test report should clearly include the identification information of the polymer dispersion (such as batch number and type), the type of apparatus used during testing, the measured White Point temperature in degrees Celsius, and the Minimum Film Forming Temperature also in degrees Celsius rounded to the nearest whole number. All numerical data should conform to rounding rules specified in IS 2:1960. This documentation ensures traceability, clarity, and compliance with the standard's requirements.