Photothermal Storage Performance Testing: Principles, Methods, and Applications

Photothermal storage performance testing measures how effectively a material absorbs radiant light, converts it into heat, and retains that thermal energy, with the irradiation temperature rise serving as the key quantitative indicator. This article explains the evaluation principles, applicable standards such as GB/T 35263-2017, sample requirements, test procedures, and typical applications across functional textiles and related industries.

Technical Overview

Photothermal storage performance refers to the ability of a material, most commonly a functional textile, to absorb radiant energy across the ultraviolet, visible, and near-infrared regions of the solar spectrum, convert that energy into heat, and then store or slowly release the heat to provide a sustained warming effect. The performance is quantified primarily through the temperature rise of the specimen surface when it is exposed to a calibrated, simulated sunlight source under controlled laboratory conditions.

Photothermal functionality is typically engineered into materials through additives and structural design, including carbon-based fillers, ceramic micro- and nanoparticles, metal or metal-oxide nanoparticles, and phase change material (PCM) microcapsules. These components increase light absorption and photothermal conversion efficiency, and some also emit far-infrared radiation to enhance the perceived warmth of the finished product.

In China, the reference method is GB/T 35263-2017, which specifies the testing and evaluation of the light-to-heat storage performance of textiles. Internationally, no single ISO method addresses this exact property, so accredited third-party laboratories commonly combine GB/T 35263-2017 with complementary thermal and spectroscopic methods to provide a complete performance profile.

Testing Samples

Functional Fibers and Yarns

Staple fibers, filaments, and spun or textured yarns incorporating photothermal or heat-storage modifiers. These are usually knitted or wound into a standardized fabric form before testing so that a uniform, reproducible test surface is obtained.

Woven, Knitted, and Nonwoven Fabrics

Finished fabrics at any stage of processing, including greige, dyed, and functionally finished materials. Fabrics are the most common test objects because they represent the form in which photothermal performance is delivered to the consumer.

Coated, Laminated, and Composite Materials

Fabrics with photothermal coatings, PCM microcapsule finishes, metallized layers, adhesive-bonded laminates, and multilayer thermal-insulation composites used in outerwear, protective equipment, and outdoor products.

End Products

Garments such as warming underwear, outdoor jackets, skiwear, gloves, and hats, as well as home textiles including blankets, quilts, and bedding, can be evaluated either as whole items or as representative cut specimens taken from designated locations.

Sample preparation generally requires conditioning to moisture equilibrium, for example at 20 ± 2 °C and 65 ± 4 % relative humidity in accordance with GB/T 6529, before specimens of the specified dimensions are mounted for irradiation testing. A sufficient number of specimens is tested so that the mean temperature rise can be reported with statistical confidence.

Testing Items

The core and supplementary test items for a photothermal storage performance evaluation typically include:

• Light-to-heat temperature rise (ΔT) of the specimen surface under simulated solar irradiation
• Maximum surface temperature reached and the time required to reach the peak temperature
• Thermal retention behavior after irradiation stops, expressed as a cooling or heat-release curve
• Photothermal conversion efficiency derived from absorbed radiant power versus generated heat
• Latent heat storage capacity of PCM-containing materials, measured as melting and crystallization enthalpy by differential scanning calorimetry
• Complementary far-infrared indicators, such as far-infrared emissivity and far-infrared temperature rise, evaluated with reference to GB/T 18319
• Thermal resistance and heat retention ratio of the fabric, with reference to GB/T 11048
• Durability of photothermal performance after repeated standard laundering cycles

Testing Methods

Simulated Solar Irradiation Method

The principal method follows the concept established in GB/T 35263-2017. A calibrated xenon arc or tungsten-halogen light source simulating solar radiation illuminates the test specimen and a blank or reference specimen inside a controlled enclosure. Thin-film thermocouples attached to the specimen surfaces, or a calibrated infrared thermal camera, record the surface temperature continuously. After the system stabilizes in the dark, irradiation is applied for a defined period, commonly around 15 minutes, and the temperature rise is calculated as the difference between the illuminated specimen temperature and the reference temperature. Under the evaluation rules of the standard, a mean temperature rise reaching the specified threshold, commonly cited as 3 °C or higher, is required for the product to be classified as possessing light-to-heat storage performance, with higher ΔT values indicating stronger photothermal capability.

Differential Scanning Calorimetry

For materials incorporating phase change materials, DSC is used to quantify the latent heat storage capacity. Heating and cooling scans identify the phase transition temperatures and the associated enthalpies, which describe how much heat the material can absorb, store, and re-release during use.

Infrared Thermography

Full-field infrared imaging complements point measurements by visualizing the uniformity of the photothermal response across the specimen, revealing hot spots, coating defects, or uneven distribution of functional additives that single-point thermocouples may miss.

Wash Durability Assessment

To verify that photothermal performance survives real-world use, specimens are subjected to repeated laundering using a standard domestic washing procedure, after which the irradiation temperature rise test is repeated. The retention rate of ΔT after a defined number of wash cycles is reported as the durability index.

Instrumentation and Quality Control

Reliable results depend on strict control of test conditions, including radiometric calibration of the light source, verification of irradiance at the specimen plane, stable ambient temperature and humidity, the use of dark references, and traceable temperature sensors. Accredited third-party laboratories maintain these controls through regular instrument calibration and inter-laboratory verification.

Applications

Photothermal storage performance testing supports product development, quality control, and market claims substantiation in multiple sectors:

• Cold-weather and outdoor apparel, including skiwear, mountaineering clothing, and lightweight insulated jackets that use solar gain to enhance warmth
• Thermal underwear and heat-generating knitwear promoted with warmth-enhancement claims
• Home textiles such as blankets, quilts, and bedding with sunlight-activated warming properties
• Military, occupational, and protective clothing for cold environments
• Health-care and thermal-comfort textiles where gentle, sustained warmth is desired
• Tents, awnings, and flexible shading or insulation composites where light absorption and heat management interact

Manufacturers use the test data to benchmark formulations, optimize additive loading and fabric construction, verify supplier declarations, and provide objective evidence for warmth-related marketing claims, while brands and retailers rely on reports from accredited third-party laboratories to manage compliance and procurement risk.

FAQ

Q: Which standard governs the determination of light-to-heat storage performance? A: GB/T 35263-2017 is the primary standard specifying the test method and evaluation criteria for the photothermal storage performance of textiles, based on the temperature rise measured under simulated solar irradiation.

Q: What does the temperature rise value indicate? A: The temperature rise ΔT quantifies how much warmer the specimen becomes relative to a reference under identical irradiation. A higher ΔT indicates stronger light absorption, faster photothermal conversion, and greater heat generation; under the evaluation rules, products achieving the specified threshold, commonly a mean rise of at least 3 °C, are considered to have demonstrable light-to-heat storage performance.

Q: How is photothermal storage testing different from far-infrared testing? A: Photothermal storage testing measures heat generation caused by exposure to light, whereas far-infrared testing, such as that based on GB/T 18319, evaluates the emissivity of the material and the temperature rise produced by its emitted far-infrared radiation. The two properties are complementary and are frequently tested together for warming textiles.

Q: Can the performance be tested after washing? A: Yes. Specimens are laundered for a defined number of standard cycles and then retested, and the retention of the temperature rise is reported to demonstrate the durability of the functional finish.

Q: What sample information should be provided before testing? A: Typical requirements include the sample name, fabric structure and composition, the type of functional additive or finish, the number of specimens available, the intended test items, and any relevant laundering conditions, which allows the laboratory to select the appropriate procedure and reporting basis.