Brief introduction of the test method on the heat spreading and conduction behavior of graphene textile
Functional textile is the unified term for the textile with the function of coolness, heat preservation, water/wind proof, anti-bacterial/odor, flame retardant or anti-abrasion. Different from fashion textile always with colorful, design and modeling, the main goal of functional textile is focusing on how to improve its functionality and value added with the innovative materials, multi-steps processing, or complex function. In general, the functional of coolness and heat preservation is the most common requirement for the functional textile, but the touch feeling to the human from the temperature change always depends on different people, that is why an effective and consistency standard for definition on specification of functional textile will always be an important objective for customer.
# The internation certification for the coolness and heat preservation of textile
In the previous article “An overview of measuring methods and international standards in the field of thermal environment, thermal characteristics of the clothing ensembles and the human subjects assessment of the thermal comfort”, Ivana Špelić mentioned that a proper and credible test standard is a crucial issue to the end customers, brand companies, fabric makers and material suppliers. Therefore, the two major certificate authority, ASTM and ISO, have proposed different certification method for variable demand of thermal comfort on functional textile. In 1985, ASTM had first proposed a certification standard for thermal comfort of functional textile, named ASTM D1518:” Standard Test Method for Thermal Transmittance of Textile Materials”. It is not only to define a heat resistance under a condition of one-dimensional steady heat flow for the determination to heat conduction of textile, but it also could compare the difference of heat conduction under water absorption by using the dry thermal resistance (Rct) and wet thermal resistance (Ret). After the appearance of ASTM D1518, ASTM organization continuous to provide several different certification methods for different environment, such as sports activities, high temperature environment, or heat conduction behavior on multi-structural textile. For example, ASTM D7024-2004: “Standard Test Method for Steady State and Dynamic Thermal Performance of Textile Materials” shows a definition of heat conductive coefficient by the heat conductance of textile at dry state, but also provide a temperature regulating factor (TRF) to determine the difference to thermal comfort of the functional textile. Other related ASTM standards are shown as the figure 1 in below.
Figure 1 Certification methods from ASTM for the evaluation of thermal comfort
ISO organization also present the relative certification standard ISO 11092-1993: ” Textile - Physiological effects–Measurement of thermal and water–vapour resistance under steady–state conditions (Sweating guarded–hotplate test)” for the variation of thermal comfort to textile at 1993. Although a different testing module are be used in this method, it could define the heat conduction behavior to the textile using the dry heat resistance under the one-dimensional heat flow state. After ISO-11092-1993, ISO organization also determine several different certification method for the thermal comfort in particular environment, such as ISO7730-2005:” Ergonomics of the thermal environment-Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria” try to quantify the average coolness or warmth to the most of people by a index named of Predicted mean Vote (PMV) and also want to predict the variation for the thermal comfort while we are wearing. Other related ISO standards are shown as the figure 2 in below.
Figure 2 Certification methods from ISO for the evaluation of thermal comfort
# The common certification methods used for coolness or heat preservation of textile in Taiwan
In addition to the regular certification method determined by the international standard organizations, most of brands or fabric manufactures will use some specific methods to define their product specifications. The common methods include the JIS L1927 or FTTS-FA-019-2010 for coolness, the FTTS-FA-010 for the FIR test, or the JIS 1096:2010 for the heat insulation. The relative certification methods are shown in the figure 3.
Figure 3 The common certification methods used for coolness or heat preservation of textile in Taiwan
Recently, ASTM organization had defined a whole new method of ASTM D7984 for the determination of heat conduction to functional textile. Base on this testing results, ASTM D7984 could further determine a thermal effusivity to the fabric. But since the determination of “thermal effusivity” is easy to confuse with the one of “thermal Diffusivity”, we will show the physical determination first in below:
- Thermal Diffusivity: the thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy, that is, it means that the thermal diffusivity is related to the speed at which thermal equilibrium can be reached. The formulation is:
α = K/ ρ× C p
where
k is thermal conductivity (W/(m·K))
Cp is specific heat capacity (J/(kg·K))
ρ is density (kg/m3)
- Thermal Effusivity: the thermal effusivity is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity. The thermal effusivity is the rate at which a material can absorb heat. That is, we can feel warm while is the thermal effusivity is lower, and we will feel much more coolness while thermal effusivity is much higher. The formulation is:
E =( K × ρ × c p)½
where
k is thermal conductivity (W/(m·K))
Cp is specific heat capacity (J/(kg·K))
ρ is density (kg/m3)
Figure 4 Brief introduction for the ASTM D7984
From here we can see that the value of thermal effusivity is highly related to the ability of heat absorption to the material, especially the heat flow from the hot face and cold face. That is, it only represents the interface behavior for the fabric but not to represent the ability for heat transfer inside the fabric texture or from the fabric to the environment. From this point of view, we still cannot have a clearer description to the heat conduction or heat dispassion of the fabric.
On the other hand, since the meaning to the thermal diffusivity is speed at which thermal equilibrium of the material, the major benefit of the thermal diffusivity is to present a comparable value for the heat transfer of the fabric, no matter what the material we used or the texture or the structure we choose. It will help us to have a unify determination for how to compare the heat conduction of the fabric.
# The application for graphene in textile: highly related to the wearing times and effective area
We know graphene is a nanomaterial with excellent thermal properties, not only for the highest thermal conductivity of 5300 W/m‧k among all materials but also for more than 0.93 of thermal radiation emissivity, that makes it a wonderful material for heat dissipation with the synergy of conduction and radiation effects. In textile application, graphene could be applied as additive into the fiber spinning or be added on the surface of the fabric by the means of the printing or the membrane lamination. Graphene will enhance the heat conduction through a conductive path inside the fabric and further increase the effective heat dissipation area and the heat exchange to the outside environment. The most important thing is that, with a proper design on the fabric texture for changing the way of the heat storage or heat loss, graphene could both create a long-time coolness or the rapid warming-up, and further regulate the surface temperature for the human being to a comfort status. Therefore, how to build up a suitable test method for the variation of the heat conducting behavior to the “graphene related textile” by measuring the change of the heat transfer or the heat resistance as function of graphene content, processing method, or texture design is a crucial problem for practical application. In addition, since most of effects for the graphene are related to the heat transfer, it should also need to take a consideration on the time as a key factor. That means we also need to check how heat transfer affect to the microclimate to the apparel system in different time while we are wearing.
# What is the proper certification method for the graphene added textile ?
UC built up a heat flow measuring module in accordance with the ASTM D1518 standard, as shown in the figure 3. It has been used an aluminum block with a dimension of 7x7x3 cm3 as the heating source, and to control the heating temperature at 36.5 degree for the simulation the human body temperature. At the same time, we record the surface temperature at the top of the sample and the interface temperature at the point between the sample and heating block representing the fabric temperature and the skin temperature. In addition, the measurement will test in a close system for eliminating the wind effect. Before the test, the room temperature will be kept at 25±2 degree and 60±5%RH to make sure the thermal balance in environment. After putting sample at the surface of the heating source, we close the upper cover of the test module and start to record the temperature as function as the time. Usually, the recording time is 30mins for a period. The typical temperature plot is shown in the figure 5 in below.
Figure 5 The measuring module for heat flow analysis designed by UC
Through the calculation to the recorded temperature, we can analysis the heat transfer and determine the heat conductive behavior. In the beginning, we found the temperature for the heating source will keep at 36.5±0.2℃ under a constant input power (Q) as shown as blue line in figure, in the meantime the input power could be consider as the unit body heat. When the sample has been put on the surface of the heating aluminum block, the interface temperature will have a significant drop in the first few seconds and then raise back as shown as selected circle area in figure. A heat absorption could be calculated from this temperature drop as the familiar rapid coolness (Qmax). The interface temperature will raise up to a stable state as the test time goes on, and it could be considered as the skin temperature under a thermal equilibrium state as show as red line in figure. Typically, the skin temperature will affect by the material selection, texture design, functional additive, and processing method. In addition, the surface temperature of the fabric also will increase as the test time goes on, it could be represented as the fabric temperature as shown as black line in figure.
Figure 6 Heat flow analysis of textile and the determination on the performance indexes
A heat resistance could be calculated according by the equation of ASTM D1518. It is worth to mentioned that the calculated heat resistance will be determined as “dry heat resistance” because there is no any humidified air input during the test. The formulation is:
Rct=A(TS-TE)/Q-Rct0
Where,
Rct is the dry heat resistance.
Q is the input power.
A is the sample area.
TE is the environment temperature.
TS is the skin temperature.
Rct0 is the blank value for the dry heat resistance.
From the dry heat resistance, we can then calculate the clo value of sample, as shown in below:
clo value = Rct/0.155
In the article “The Effect of Thermal Insulation of Clothing on Human Thermal Comfort”, Tuğrul Oğulata further describe how to apply the clo value to calculate the convection of heat of the fabric, and the concept is based on the Newton's law of cooling with the adjustment for the fabric texture. The formulation is:
Q(convectionHeatTransfer)=hc×fcl×(Tf-Te)
Where
hc is the heat transfer coefficient.
fcl is the clothing area factor.
Tf is the surface temperature on the fabric.
Te is the environment temperature.
The heat transfer coefficient is highly related to the environment conditions, and mainly affected by humidity and wind speed. The formulation is:
hc=12.1×Va½
Although there is no wind speed in the practical test in accordance with ASTM D1518 standards, the heat convection will be generated from the surface of the heating source due the temperature difference between the heat source to environment, and further cause the surface wind speed inside the system. Generally, the wind speed will set as 0.2-0.4m/s.
In the other hand, the clothing area factor is highly related to the variation on the structure of the fabric, material selection to the yarns, or the surface texture due to the fiber fineness. The formulation is:
fcl=1.05+0.1×clo
Based on the results calculated from the formulation, the higher value for convection of heat transfer means to have more obvious coolness. On the contrary, lower one means more warming-up behavior.
# The key performance indexes for the evaluation of the heat conductive performance of textile introduced by UC
UC’s introduce those calculated results above to transform several key performance indexes for the evaluation the thermal comfort for the fabric, also try to make the difference to the current certification method on the thermal comfort of the textile. The reason we want to do this is because that the current certification method only shows the simple result on coolness or heat preservation but does not help us to realize the effect on the wearing time to the heat conduction of fabric. Four key performance indexes introduced by UC , the rapid coolness (Qmax) for the heat absorption, the time for thermal balance for heat conduction of the fabric, the clo value for the heat insulation of the fabric, and the convection of heat transfer for the heat dissipation of the fabric, will help us not only to define the heat conduction behavior of the fabric, but also to predict the effect of heat conduction to the microclimate of the apparel system while long-time wearing. It will provide us a better way to evaluate the specification of the heat conductive fabric with graphene additive for the designer, fabric manufactures, brands and end users.
Figure 7 Key performance indexes for the evaluation of the heat conduction of textile
# Summary
In summary, it has been provided a comprehensive study of how to verify the coolness and heat preservation of textiles from the current certification standards. But more importantly, we also understand how to use a proper method to verify the characteristics of graphene in textile, and bring the advantages of the graphene to the end users more clearly.