Paint and coating thermal conductivity testing with ASTM D5930

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.

A sample with flat substrate and thick paint or coating
  • 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.

Arrangement of measurement probe and two test samples

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.

SRI testing: Can membrane products be tested without substrate?

We are often requested to test the solar reflectance index (SRI) of membrane products, particularly liquid applied membrane products.

Unlike paints, membrane products can be free-standing. A membrane product sample can be prepared either as a standalone membrane layer or with a substrate. The question from many customers is whether the SRI of membrane products can be tested without substrate.

Two examples

To answer this question, two examples are illustrated below:

A translucent membrane sample
An opaque membrane sample

The sample on the left is translucent (with large transmission). For such samples, a substrate is required.

The sample on the right is opaque (with zero transmission). For such samples, a substrate is not required.

The theory

For a material, we have the following relationship:

Solar transmittance + solar reflectance + solar absorptance = 1

In SRI calculations, the solar transmittance is assumed 0 (solar transmittance = 0). The solar reflectance is directly measured by the instrument. Therefore, we use the following relationship to calculate the solar absorptance:

Solar absorptance = 1 – solar reflectance

However, if the material is not opaque (solar transmittance ≠ 0, for example, the left example), the equation above is not valid.

For such translucent samples, the calculated solar absorptance is higher than the actual solar absorptance, as the solar transmittance is counted as part of the solar absorptance, and the resultant SRI is therefore lower.

In order to eliminate this error, translucent samples shall not be used for SRI testing. If a membrane product is translucent, it shall be applied onto a suitable substrate for testing.

The practice

To determine if a membrane sample is translucent, a simple method is to check if the flashlight from a phone can transmit through the sample, as shown in the examples above.

Theoretical range of solar reflectance index (SRI)

In our website, we explain that the solar reflectance index (SRI) is the surface temperature in a 0 – 100 scale. In the same section, we also mention that It is possible for SRI to be negative or larger than 100. So, what is the theoretical range of SRI? What is the possible minimum SRI and maximum SRI?

We can easily find the minimum and maximum SRIs with our online SRI calculator.

  • Minimum SRI: the minimum SRI is the SRI of a surface with solar reflectance = 0 (perfect black color in the solar radiation spectrum) and emittance = 0 (perfect white body in the infrared radiation spectrum)
    • Mininum SRI = -244.6 (low-wind), -99.7 (medium-wind), or -44.9 (high-wind)
  • Maximum SRI: the maximum SRI is the SRI of a surface with solar reflectance = 1 (100%, perfect white color in the solar radiation spectrum) and emittance = 1 (perfect black body in the infrared radiation spectrum)
    • Maximum SRI = 133.0 (low-wind), 129.6 (medium-wind), or 128.0 (high-wind)

Therefore, the theoretical range of SRI is:

  • Low-wind: -244.6 ~ 133.0
  • Medium-wind: -99.7 ~ 129.6
  • High-wind: -44.9 ~ 128.0

Shown below are the calculation screenshots.

Minimum SRI calculation results
maximum SRI calculation results

With the theoretical SRI range presented above, is it still valid to say that the SRI is the surface temperature in a 0 – 100 scale?

Yes, it is still valid. As most natural surfaces are with high emissivity (emittance > 0.8), the SRIs of most natural materials are in the range of 0 – 100.

Surfaces with low emissivity (low-e, e.g. emissivity < 0.2) are typically bare metal surfaces (e.g. aluminum or stainless steel). In practice, they are rarely directly used as the top layer of roof or pavement materials. When such low-e surfaces are painted, the emissivity is high (as the paint layer becomes the top layer).

Why the SRI of a low-e surface is low?

There are 3 heat transfer modes: conduction, convection, and radiation. For a low-e surface, the radiative heat exchange between the surface and the ambient environment is weak (i.e. less heat transfer via radiation). More heat is kept on the low-e surface and it results in higher surface temperature and, therefore, lower SRI.

In the low-wind condition, the convection is weak and the radiation is more dominant. This is the reason that the dependence of SRI on emittance is stronger at the low-wind condition.

Thermal conductivity of thin materials (paint, coating, metal sheet)

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.

Thermal resistance of thin material

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.

Online glass U-value, SHGC & shading coefficient calculator: V2.0.0

We are pleased to introduce our upgraded online glass U-value, SHGC & shading coefficient calculator. The current version is V2.0.0. Click the screenshot below to access this online calculator.

Online glass U-value, SHGC & shading coefficient calculator

Refer to this early post for the features of the V1.1.0 version.

New features added in V2.0.0

  • SHGC and shading coeffcient calculations
    • Note: limited to single glazing systems; this is due to the restriction of the mathematical model in ISO 9050/EN 410
  • More compact user interface
  • Tooltips on glass configuration schematics

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.

OTM Insights Newsletter: Issue 5

Our newsletter OTM Insights Issue 5 was sent to our customer on 27 Jul 2020. We are sharing some technical insights of accuracy of glass optical & thermal property test results in the newsletter. Click the image to read the full newsletter.

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5 certainties amid uncertainties: our COVID-19 strategy

COVID-19 has brought in many uncertainties to the business environment and will bring in more uncertainties in the future. As OTM’s COVID-19 strategy, we remain committed to the following 5 certainties:

  • Certainty 1: our long-termism principle
  • Certainty 2: consistent efforts on innovation
  • Certainty 3: consistent efforts on branding
  • Certainty 4: consistent efforts on global opportunities
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Certainty 1: our long-termism principle

We will stay focused on optical & thermal testing.

Certainty 2: consistent efforts on innovation

We will consistently develop our own instruments, tools and methods for optical & thermal testing.

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One of the online optical & thermal calculators developed by OTM

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We will consistently update our website, newsletter and lab profile, with rich information on optical & thermal testing.

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Online ETTV U-value calculator: V1.1.0

We are pleased to introduce our online ETTV U-value calculator. The current version is V1.1.0. Click the screenshot below to access this online calculator.

Online ETTV U-value calculator

Features:

  • No download or installation required.
    • The calculator works in all mainstream browsers with javascript enabled.
  • User friendly and responsive
    • In fact, you don’t need to click the “Calculate U-value” button and the results are updated instantly after your changes.
  • Full compliance to BCA ETTV code
    • With built-in ETTV material library and standard conditions for air gaps and surface film resistances
    • Accuracy has been validated with other independent codes.
  • Flexible and powerful
    • User materials can be added and edited conveniently
    • The thermal resistance of each layer is listed clearly
    • Informative tooltips are displayed when the curve is moved over the schematics

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.

Online glass U-value calculator: V1.1.0

We are pleased to introduce our online glass U-value calculator. The current version is V1.1.0. Click the screenshot below to access this online calculator.

Features:

  • No download or installation required.
    • The calculator works in all mainstream browsers with javascript enabled.
  • User friendly and responsive
    • In fact, you don’t need to click the “Calculate Glass U-value” button and the results are updated instantly after your changes.
  • Full compliance to ISO 10292 or EN 673
    • Accuracy has been validated with other independent codes.
  • Flexible and powerful
    • Major factors influencing glass U-value are supported: insulating glazing units, low-e coating, gas fills, laminated glasses.

Future developments

It is planned to include US NFRC U-value calculation in the future. SHGC and shading coefficient calculations may be included too.

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.