The thermal conductivity of agar gel (with 5-gram agar powder per liter of water) is tested, as part of the quality control measures when we test soil or similar gel-like materials.
We’ve helped a few customers in determining the thermal conductivity of thin materials, such as paint and coating, according to ASTM D5930.
In general, the thermal resistance of thin materials is negligible. In case it is necessary to determine the thermal conductivity of paint and coating. A pair of special samples, with thick paint or coating, need to be prepared, as illustrated below.
- Substrate (the blue color part): the substrate needs to be flat and rigid. The substrate type does not affect the measurement result. Typical substrate types include glass, metal plate, and wood plate.
- The preferred substrate size is 50 mm x 50 mm; The minimum size is 30 mm x 30 mm and the maximum size is 100 mm x 100 mm.
- Thick paint or coating (the yellow color part): the paint or coating needs to be applied onto one side of the substrate, with large enough thickness
- The preferred thickness is 3 mm or larger; the minimum thickness is 1 mm.
- Sample quantity: two samples in a pair are needed.
Shown below is the arrangement during testing.
The measurement probe is a thin film (less than 0.1 mm in thickness, the red color part) with a tiny wire inside (refer to our thermal conductivity test page for details). The probe is sandwiched between the two test samples, next to the paint or coating material.
Because the probe is in contact with the paint or coating material and the measurement duration is very short, it is equivalent to insert a tiny wire into a bulk material block made of the paint or coating. Only the thermal conductivity of the paint or coating is measured and the result is not affected by the substrate material.
We are sometimes requested to test the thermal conductivity of thin materials, such as paint, coating and metal sheet.
Negligible thermal resistance of thin materials
For a wall (or a roof) system, the influence of such thin materials to the overall wall system U-value is negligible.
Shown below is an example calculated with our online ETTV U-value calculator. it is obvious that the thermal resistance of a 0.2 mm thick paint layer with 0.2 W/(m⋅K) thermal conductivity is only 0.001 (m2K)/W, which is negligible comparing to the thermal resistances of other layers (e.g. concrete, plaster, or insulation wool).
For thin metal sheets, e.g. 0.7 mm thick aluminium plates, the thermal resistance is further smaller, as the thermal conductivity of metal is much larger.
The reason is that thermal resistance is dependent on both thermal conductivity and thickness, with the following relationship:
Thermal resistance = Thickness / Thermal conductivity
In practice, due to the small thickness of thin materials (typically less than 1 mm), it is not practical to reduce the thermal conductivity of thin materials to achieve better insulation.
In wall/roof U-value calculations, the thin materials can be simply ignored. It is not meaningful to get the thermal conductivity of thin materials.
Thin material thermal conductivity testing
It may be still necessary to determine the thermal conductivity of thin materials. For example, the thin material is not used in a wall/roof system, but in a system with low thermal resistance.
For such scenarios, we can test the thermal conductivity of thin materials according to ASTM D5930, with the following practices:
- For low thermal conductivity paint and coating, the paint/coating can be applied onto flat substrates for testing.
- There is no specific requirement on the substrate type, as long as the substrate surface is flat. We recommend flat metal plates or glass plates.
- The preferred substrate size is 50 mm x 50 mm (minimum size: 30 mm x 30 mm; maximum size: 100 mm x 100 mm). There is no requirement on the substrate thickness, as long as it is strong enough.
- The thickness of paint or coating should be thick enough (3 mm or thicker preferred, 1 mm minimum)
- 2 pieces of painted/coated samples are requried
- We cannot test materials with large thermal conductivity [i.e. > 10 W/(m⋅K)], for example, aluminium or stainless steel.
Please refer to our thermal conductivity page for more details.
We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.
What are in the laboratory profile?
- A comprehensive overview of the laboratory
- Detailed introduction of 10 popular tests
- 14 informative technical insights articles
You may download a soft copy of our lab profile.