Hydrophobicity rate testing is a fundamental laboratory procedure for evaluating how effectively thermal insulation materials, such as mineral wool, foam glass, and aerogel blankets, repel liquid water and maintain their thermal performance in humid environments. This article provides a comprehensive technical reference covering testing principles, sample requirements, standard methods such as GB/T 10299 and EN 1609, calculation procedures, and practical applications, serving engineers, quality-control professionals, and procurement specialists in the insulation industry.
Technical Overview
Hydrophobicity rate, also referred to as the water-repellency rate, quantifies the capacity of a thermal insulation material to resist the penetration and absorption of liquid water when its surface is exposed to a controlled water spray. The result is expressed as the percentage of applied water that is repelled rather than retained by the material. For hydrophobic-grade mineral wool products, typical acceptance thresholds are 98% or higher, with many specifications requiring 99% or more.
The significance of this property is substantial. Water has a thermal conductivity of approximately 0.6 W/(m·K), roughly twenty-five times greater than that of still air. Even limited moisture uptake can therefore dramatically degrade the insulating capability of a fibrous or cellular material, while also adding weight, promoting corrosion of adjacent metal substrates, accelerating freeze-thaw deterioration, and encouraging microbial growth. Hydrophobicity is typically engineered into mineral fiber products through the application of silicone-based water-repellent agents during manufacture, and laboratory verification of treatment uniformity and effectiveness is essential.
The primary reference method for hydrophobicity rate determination in China is GB/T 10299, the standard test method for the hydrophobicity of thermal insulation materials. It is complemented by related European and international methods for water absorption behavior, including EN 1609, EN 12087, EN 12088, and ISO 29767, as well as surface wettability evaluation methods such as ASTM D5946 and ISO 15989. Accredited third-party laboratories routinely perform these tests for product certification, factory quality control, and project compliance verification.
Testing Samples
Mineral Wool Products
Rock (stone) wool and glass wool materials represent the largest category of samples submitted for hydrophobicity testing. This group includes boards, blankets, felts, wire-mesh blankets, and preformed pipe sections, all of which commonly receive silicone-based hydrophobic treatment. Their open fibrous structure makes them particularly dependent on effective water-repellent additives, so hydrophobicity rate is a key delivery specification.
Organic Foam Materials
Closed-cell organic foams, including phenolic foam, polyurethane (PU/PIR) foam, and flexible elastomeric (NBR/PVC) foam, are also tested. Although their closed-cell structure provides inherent resistance to water uptake, surface wettability and long-term immersion behavior still require verification, particularly for outdoor and below-ambient service conditions.
Inorganic Rigid Materials
Cellular (foam) glass, calcium silicate, expanded perlite boards, and microporous insulation panels form a third sample category. These materials are used in high-temperature industrial applications where condensation and wash-down exposure can occur, and their surface water-repellency behavior is evaluated alongside mechanical and thermal properties.
Advanced and Composite Materials
Aerogel blankets, composite insulation panels, and products with facings such as aluminum foil or glass tissue may be tested with the facing intact or removed, as agreed between the parties. The influence of the facing on measured water behavior should always be documented in the test report.
Specimen Preparation and Conditioning
- Cut representative rectangular specimens to the dimensions prescribed by the applicable standard, preserving the product's original thickness and surface condition.
- Use at least three specimens per test condition; five specimens are recommended when statistical evaluation is required.
- Reject specimens exhibiting visible damage, delamination, or non-uniform binder distribution.
- Dry specimens to constant mass, typically at 105°C ± 5°C for inorganic materials or 60°C ± 5°C for organic and faced products to avoid thermal degradation.
- Cool dried specimens in a desiccator and weigh them using a balance with a readability of 0.01 g before testing.
Testing Items
Depending on the project specification and product standard, a hydrophobicity evaluation program typically includes the following testing items:
- Hydrophobicity rate by spray method, expressed as a percentage of repelled water.
- Short-term water absorption by partial immersion (WS class), determined per EN 1609 or ISO 29767 for boards, or EN 13472 for preformed pipe insulation.
- Long-term water absorption by full or partial immersion, determined per EN 12087, and by diffusion per EN 12088.
- Static and dynamic contact angle measurements for surface wettability characterization per ASTM D5946 or ISO 15989.
- Thermal conductivity before and after moisture exposure to quantify the performance penalty of water uptake.
- Water vapor transmission properties, tested per ASTM E96 or GB/T 17146, as supporting data for condensation risk assessment.
Testing Methods
Spray Method (Primary Procedure)
The spray method specified in GB/T 10299 is the principal procedure for hydrophobicity rate determination. The apparatus consists of a calibrated spray head, a constant-flow water supply, an inclined specimen holder, and collection vessels for runoff. The test sequence is as follows:
- Condition and weigh the dried specimen to obtain its initial mass, recorded as m1.
- Mount the specimen on the holder inclined at 45° to the horizontal, positioned at the prescribed distance below the spray head, commonly 150 mm.
- Spray deionized or distilled water at a constant, calibrated flow rate onto the specimen surface for the specified duration, while collecting the runoff water.
- After spraying, gently blot the specimen surface with absorbent lint-free paper to remove adhering droplets without pressing water into the material.
- Reweigh the specimen immediately to obtain the final mass, recorded as m2.
The absorbed water mass is calculated as the difference between the final and initial specimen masses, and the hydrophobicity rate H is determined from the ratio of repelled water to total applied water:
H = (m applied − m absorbed) / m applied × 100%, where m absorbed = m2 − m1. The mass of applied water is verified gravimetrically from the metered supply or by mass balance between the applied water and the collected runoff plus absorbed fraction.
Partial Immersion Method
For short-term water absorption classification, specimens are immersed to a defined shallow depth in water for 24 hours per EN 1609 or ISO 29767. The water absorbed per unit of exposed area is reported in kg/m², and products achieving ≤ 1.0 kg/m² are classified in the highest WS class. This method evaluates capillary uptake through the surface rather than spray repellency and is often performed in parallel with the hydrophobicity rate test.
Contact Angle Measurement
Surface wettability is characterized using the sessile drop technique, in which a droplet of deionized water is deposited on the material surface and the static contact angle is measured optically. Surfaces exhibiting contact angles greater than 90° are classified as hydrophobic, while angles approaching or exceeding 150° indicate superhydrophobic behavior. This method is particularly useful for research and development of water-repellent treatments.
Result Evaluation and Precision
The hydrophobicity rate is reported as the arithmetic mean of all valid specimens, rounded to the nearest whole percentage point. Product standards for mineral wool, such as those aligned with GB/T 11835 and GB/T 13350, commonly require hydrophobicity rates of no less than 98%. Key factors influencing repeatability include the uniformity of the water-repellent agent distribution, specimen density and thickness, water temperature, blotting technique, and the operator's adherence to the specified spray parameters. Interlaboratory comparisons and periodic apparatus calibration are recommended quality assurance practices.
Applications
Hydrophobicity rate testing supports a wide range of engineering and commercial applications across the insulation industry:
- Building envelope quality control, including external thermal insulation systems, curtain wall insulation, roof insulation, and ventilated facade layers exposed to wind-driven rain.
- Industrial thermal insulation for power plants, petrochemical facilities, and LNG installations, where pipe sections and equipment lagging must resist condensation, wash-down water, and weather exposure during construction and operation.
- Cold storage and refrigeration engineering, where sustained water repellency prevents moisture accumulation that would otherwise destroy insulation efficiency at below-ambient temperatures.
- Marine, offshore, and outdoor-exposed systems subject to salt spray, precipitation, and high humidity.
- Product certification, factory production control, incoming material inspection, and procurement compliance verification performed by accredited third-party laboratories.
- Research and development of silicone-based water-repellent agents and optimization of their application rates on mineral fiber production lines.
FAQ
What hydrophobicity rate is considered acceptable?
Most national product standards and project specifications for hydrophobic-grade mineral wool require a hydrophobicity rate of at least 98%. For premium products and demanding applications such as offshore or below-ambient service, requirements of 99% or higher are common.
How does hydrophobicity rate differ from water absorption?
Hydrophobicity rate measures the percentage of sprayed water that a material repels under a short, controlled surface exposure, whereas water absorption tests measure the mass of water taken up by a material under partial or full immersion over an extended period, reported per unit area or volume. The two properties are complementary and are frequently specified together.
How many specimens are required and how long does testing take?
A minimum of three specimens per condition is standard practice, with five recommended for statistical purposes. The spray method itself is rapid, and a complete test including drying, conditioning, and reporting typically requires one to three working days. Immersion-based methods require 24 hours or longer, and long-term diffusion tests may extend to several weeks.
Can faced or laminated insulation products be tested?
Yes. Testing can be performed with the facing intact to evaluate the system as installed, or with the facing removed to evaluate the core material. The tested configuration must be clearly stated in the report, since facings significantly influence water behavior.
Does a high hydrophobicity rate guarantee long-term moisture resistance?
No. A high spray-method result confirms surface water repellency at the time of testing but does not alone guarantee durability. For long-term performance assessment, hydrophobicity testing should be combined with long-term immersion tests, diffusion tests per EN 12088, aging exposure, and freeze-thaw cycling as appropriate to the intended service environment.
