The Effect of Laundering on the Physical and Thermal Properties of Phase Change Textile Materials

Abstract

Phase change materials (PCMs) have been integrated into clothing and textiles to provide added value in terms of thermoregulation and thus added comfort to the wearer in intensely hot and cold weather conditions. Since clothing is laundered several times during the service life of the textile, the effects of simulated domestic laundering on the physical and thermal properties of a non-woven textile that contains PCMs were investigated. The thermal properties, such as the thermal degradation, melting, evaporation, and crystallisation were measured by the Thermal Gravimetric Analysis (TGA) and the Differential Scanning Calorimetry (DSC) analytical techniques. The physical properties, such as the microcapsule size distribution, and the microcapsule morphologies of the PCM microcapsules, were measured by the Scanning Electron Microscopy (SEM) and Raman analytical techniques. The primary objective of this study is to determine the effect of repeated laundering at different wash temperatures.

1. Introduction

Phase change materials (PCMs) are ‘latent’ heat storage materials. The thermal energy transfer happens when a material changes from solid to liquid or liquid to solid. PCM absorbs and releases heat at an almost constant temperature. Latent heat stores 5 to 14 times more heat per unit volume than sensible storage materials such as bricks or rocks. Latent heat storage is one of the highly effective techniques of storing thermal energy. Unlike the sensible heat storage method, the latent heat storage method requires a significantly higher storage density with a minor temperature difference between storing and releasing heat [1].

As described by Grynaeus et al. [2], PCMs are designed to utilise latent heat absorption associated with a reversible phase change transition, such as solid to liquid transition. The material can be used as an absorber of heat whereby several thermal energies will be absorbed by the PCM before its temperature can increase. The PCM can also be preheated and used as a barrier to cold, as a greater quantity of heat must be removed from the PCM before its temperature can begin to decline.

The development and application of PCM, as a class of thermal energy storage systems, are receiving increasing attention due to their contribution to a more efficient environmentally friendly energy utilisation. The PCM technology was first developed in the 1970s by the National Aeronautics and Space Administration (NASA) of the United States to protect delicate instruments in outer space from large temperature extremes. Today PCMs are one of the most widely used energy storage materials in the fields of solar energy utilisation, energy-conserving in buildings, thermal insulation, and thermal regulation [3].

According to Yazdi & Sheikhzadeth [1], textiles are one of the most used applications of PCMs for thermoregulating principles. During the phase change, the temperature remains unchanged; as a result, the PCM can stabilise the human body temperature. The cooling effect of the PCM depends on the ability to absorb heat during the periods when external heat load or body heat production surpasses heat loss.

In the highly competitive textile marketplace, customers are requiring not only comfort and fashion, but also functionality in both daily wear and sporting apparel. Phase change materials have been incorporated into clothing to provide added functionality and value in terms of thermoregulation and thus increased comfort to the wearer, in notably extreme weather conditions and sporting activities. Since clothing is laundered many times, it was deemed important to study the effect of repeated laundering on the thermoregulating properties of fabrics containing PCMs.

In this study, the effect of simulated domestic laundering on the physical and thermal characteristics of a non-woven textile that contains PCM is investigated since this is important in terms of the functional lifecycle and durability of PCMs and can contribute to improving the physical and thermal performance of PCMs and their design, as well as the laundering conditions [4, 5].

2. Experimental

2.1 Sample preparation (according to EN ISO 6330:2000 standard)

• Lay the fabric to relax for at least 4 hours in a standard atmosphere (temperature of 20°C ± 2°C and relative humidity of 65% ±2%).

• Cut out the samples using a template and mark accordingly.

• Overlock all edges to avoid fraying of the samples at the edges.

2.2 Washing procedure (according to EN ISO 6330:2000 standard)

• Wash load: Ensure no more than half of the load are consist of the test samples, the rest of the load being made up of makeweights.

• Detergent: Use 5 grams of European Colourfastness Establishment (ECE) detergent and 2 grams Sodium Perborate. ECE detergents do not contain any optical bright agents (OBA).

• Wash Program:

1. Select the correct program for the type of wash required. Set temperature and water levels. Place makeweights and then the test samples in the washing machine. Dissolve the detergent with a small quantity of warm water in a beaker.

2. Start the machine. When the water is above the base level in the sight tube, add the dissolved detergent into the machine by lifting the lid on top of the machine.

3. On completion of the program remove the wash load.

2.3 Drying method (according to EN ISO 6330:2000 standard)

Flat dry – place the samples on a drying rack without stretching the samples. Wait at least 24 hours until completely dry before assessing samples.

3. Testing equipment and testing procedures

3.1 TGA analytical instrument (according to ISO 11358 standard)

TGA test procedure was followed where 10 to 15 mg of samples were heated from 30 to 500°C at a heating rate of 20°C/min under nitrogen atmosphere.

3.2 DSC analytical instrument (according to ISO 11357-1:2016 standard)

The DSC instrument was used to evaluate the latent heat of the PCM samples. The samples were cut into small pieces with a sharp blade after which they were spread evenly on the surface of the pans. Some 10 to 15 mg of samples were added to the pan and weighed on the balance which measures up to the accuracy of 0.0001 mg.

3.3 DSC testing procedure

• Turn on the machine and allow it to warm up for about an hour.

• Ensure the compressed nitrogen tank and liquid nitrogen tank are both full and the valve connection is open. Set the compressed nitrogen pressure flow.

• Prepare empty pans and place the pans on the two circular sensors within the furnace.

• Activate the machine software and input all necessary information in the program.

• Input a temperature 10°C higher than the highest temperature set in the temperature program. This is a protective setting, which stops the machine from surpassing a set temperature in case of machine malfunction.

• To start the program, the current furnace temperature must be 5°C of the initial temperature.

• Start and the measurement will begin, after baseline scan has run, remove the empty baseline pan, and replace it with the pan containing the samples.

• To ensure even heat flow and accurate DSC readings, a thin layer of sample pieces is placed so the entire bottom of the pan is covered.

• After the measurement has finished, close the program, turn off the compressed nitrogen tank and turn off the machine.

3.4 Scanning electron microscopy (SEM)

3.4.1 SEM sample preparation (according to ISO 22493:2014 standard)

• A small amount of the fabric was mounted on a 12 mm aluminium SEM stub using carbon-coated glue.

3.4.2 SEM analysis procedure

SEM settings: Backscatter detector, 20 kV accelerating voltage, working distance of 10 mm. Magnification between 1000 and 4000x.

• Insert the sample into the chamber of the SEM and observe in low vacuum mode.

• SEM is equipped with lenses to compress the spot and direct the focused electrons on the sample.

• The image of the sample is generated point by point dependent on the movement of the scan coils, which causes the electron beam to move to positions in a form of straight lines until a rectangular construction is formed on the surface of the sample.

• If necessary, change to a higher magnified image, and the scan coils will then make the beam deflect a smaller area.

• The electron detector detects the emitted electrons (signals) from the scanned sample.

• The signals are then displayed on the viewing screen, the brightness and the intensity can be controlled until a clear image is obtained.

3.5 Raman microscopy and spectroscopy

3.5.1 Raman sample preparation

Raman analysis needs no sample preparation. Images of the fibres were taken with the SEM after which the sample was moved from under the SEM pole piece to the Raman objective in the SEM chamber and a Raman map was acquired of the same area.

3.5.2 Raman analysis procedure

Raman Settings: Laser wavelength 532 nm, the laser power was 2 mW, map areas were 50μmx50μm with data points every 0.5 μm, integration time (time spent on each point) 0.2 seconds.

• Insert the sample, and make sure it is flat enough to avoid scratching the lenses.

• Repeat the position and focus procedure until the sample is focused on the desired magnification.

• Adjust the instrument parameters (exposure time, laser filter, spectrometer offset, etc.) until the desired spectrum is shown on the display and capture Raman Image and Spectrum.

• Press stop and turn off the lamp.

4. Analysis

Comparative analysis was applied to the DSC, TGA, SEM and Raman analytical results.

5. Results and discussions

The main objective of this study is to determine the effect of repeated laundering at different wash temperatures, on the thermal and physical related properties of the PCM as measured by TGA, DSC, SEM and Raman.

5.1 Thermal gravimetric analysis (TGA)

5.1.1 TGA results of multiple washed samples at different temperatures

Thermal gravimetric analysis (TGA) is generally used to investigate chemical reactions in which weight changes occur. Most TGA curves illustrate mass losses, for example in Figures 1–3 the TGA thermograms show 3 different degradation regions that indicate a combination of paraffin wax that forms the core of the microcapsule, an acrylic resin as the polymeric shell of the microcapsule and polymeric fibres which can be identified as Polyester (PES), Polyamide (PA) and Polyethylene Terephthalate (PET) present in the PCM sample.

Figure 1.

TGA thermograms showing the effect of repeated laundering on mass loss (number of washes at 30°C).

Figure 2.

TGA thermograms showing the effect of repeated laundering on mass loss (number of washes at 40°C).

Figure 3.

TGA thermograms showing the effect of repeated laundering on mass loss (number of washes at 60°C).

5.1.2 Mass loss percentage (%) comparison of different wash temperatures after 10 washes

In Figure 4 the thermogram of the first segment was identified as the paraffin wax, where it is observed for the sample washed at 30°C there is a mass loss of 15.8% at a heating temperature of around 208°C, for the sample washed at 40°C there is a mass loss of 18.8% at a heating temperature of approximately 205°C and for the sample washed at 60°C a mass loss of 19.1% was observed at a heating temperature of nearly 200°C. The thermogram indicate that the mass loss at wash temperature 60°C is slightly higher with a lower heating temperature compared to the wash temperature of 30°C and 40°C where the mass loss is lower at a higher heating temperature.

Figure 4.

TGA thermograms illustrating the effect of wash temperatures on mass loss after 10 washes.

The thermogram of the second degradation segment, was identified as the acrylic resin of the microcapsule, which exhibit a mass loss of 24.5% of the sample at wash temperature 30°C, a mass loss of 31.8% of the sample at wash temperature 40°C and a mass loss of 35.1% of the sample at wash temperature 60°C with heating temperatures ranging from 300 to 400°C. This proves that a higher mass loss occurred for the sample at wash temperature 60°C compared to the samples of the lower wash temperatures, this is due to the PCM microcapsules breaking during the laundering process.

The last segment which indicates the thermal degradation of the polymeric fibres (PES, PA, and PET) show a mass loss of around 87.5% ±2% for the samples washed at temperatures 30°C, 40°C and 60°C at a heating temperature up to 500°C.

The difference in onset degradation is caused by the different wash temperatures where loss of mPCM content occurred during the laundering process. The thermogram clearly indicate that there is a higher mass loss at wash temperature 60°C, than at wash temperature 30°C and 40°C, respectively. The difference in average mass loss percentage between the sample at wash temperature 60°C compared to the sample at wash temperature 30°C is about 4.6%.

5.1.3 Mass loss percentage (%) of unwashed sample vs. multiple washed sample at 60°C

Figure 5 shows a transition at around heating temperature of 150°C for the sample at wash temperature 60°C, which is not present for the unwashed sample. The onset degradation for the sample washed at 60°C, differs with a mass loss percentage of 26.12% compared to the unwashed sample. The difference in mass loss is substantial and is due to the PCM capsules being broken at the wash temperature of 60°C and the associated loss of thermoregulating content, whereas with the unwashed sample, the thermoregulating content was unchanged.

Figure 5.

TGA thermogram demonstrating mass loss for the unwashed sample and a multiple washed sample at 60°C.

The mass loss for each degradation phase, indicate that the mass loss is higher for the sample washed at 60°C. The difference in average mass loss between the sample washed at 60°C compared to the unwashed sample is about 9.4%. It is evident that both samples burn out around the same heating temperature of around 500°C, where the ash content can be calculated as a repeatability average of approximately 12.5%.

5.2 Differential scanning calorimeter (DSC)

The DSC analysis is used to assess the melting point, crystallisation, and heat of fusion to determine the change in thermal properties of the PCM due to the different wash temperatures. The DSC analysis results are presented in the form of thermograms as illustrated in Figures 6–8. The primary focus is to evaluate the effect of the different wash temperatures in terms of the heating and cooling peaks of wash temperature (30°C, 40°C and 60°C), also of the samples at 10 washes to observe the worst-case scenario.

Figure 6.

DSC analysis of washed samples (wash temperature 30°C).

Figure 7.

DSC analysis of washed samples (wash temperature 40°C).

Figure 8.

DSC analysis of washed samples (wash temperature 60°C).

Figure 9 shows 3 distinct melting peaks for each of the samples at wash temperatures (30°C, 40°C and 60°C), these melting peaks are probably related to the molecular structure of the hydrocarbons in the paraffin wax PCM (tetradecane, hexadecane, and octadecane). The main interest is the enthalpy values of the third melting peaks where the enthalpy of the octadecane content changed, indicating phase transition of the PCM from solid to liquid state at a heat temperature of 28°C ± 2°C for all three of the samples at different wash temperatures.

Figure 9.

Endothermic results showing the effect of different wash temperatures on the heat flow for samples washed 10 times.

It is apparent that the heat capacity of 1.8 W/g for the sample washed at 60°C is lower than the sample washed at 30°C, with a heat flow of 2.62 W/g, and for the sample washed at 40°C, with a heat flow of 2.16 W/g. It can be established that the heat loss is greater at wash temperature 60°C, this is due to the loss of thermoregulating content, broken microcapsules, and loss of fibres.

5.2.1 Exothermic results of multiple washed samples at different temperatures

The crystallisation temperature is defined as the lowest point of the cooling peak. The enthalpy of crystallisation is being determined by the area under the curve. Figure 10 shows details of crystallisation occurring where the cooling peaks of samples washed at different wash temperatures coincide at heating temperatures of 9°C ± 2°C at wash temperature 30°C, 14°C ± 2°C at wash temperature 40°C and 23°C ± 2°C at wash temperature 60°C.

Figure 10.

Exothermic results showing the effect of different wash temperatures on the heat flow for samples washed 10 times.

The primary focus is on the third cooling peaks where the heat flow was at its peak. It can be observed that the sample at wash temperature 60°C has significantly less degree of crystallinity with a heat flow of −0.99 W/g compared to the samples at wash temperature 30°C, where the heat flow is −1.48 W/g, and at wash temperature 40°C, where the heat flow is −1.26 W/g. These results could be explained by leakage or evaporation of paraffin after the rupture of the PCM microcapsules during the laundering process at wash temperature 60°C.

5.2.2 Effect of 10 washes on thermal properties

The effect of multiple laundering (10 washes at 60°C) on the thermal properties are shown in Figure 11. The multiple washing exhibits lower treatment heat capacity and cooling capacity than the unwashed sample. There is a difference of approximately 50% latent heat capacity between the unwashed sample and the sample washed at 60°C, this is due to the loss of PCM content after 10 wash cycles at a higher wash temperature.

Figure 11.

Effect of 10 washes at wash temperature 60°C compared to the unwashed sample.

5.3 SEM analysis

The SEM analysis will discuss the effect of multiple washes at different temperatures on the morphology (shape and size of PCM microcapsule) that was examined by the means of SEM images at different magnifications.

5.3.1 Unwashed PCM: different areas at 500X magnification (100 μm)

SEM images at magnification 500X was captured at 3 different areas on the sample to illustrate distribution of the PCM microcapsules within the sample. In Figure 12 image A, B and C shows visible bulges on the fibres and in an uneven webbing that links the intercedes of micron sized PCMs. The bulges indicates that the PCM content is wrapped around the fibre and the micron sized PCM observed in the uneven webbing indicates the PCM content was incorporated through a pad-dry-cure process method.

Figure 12.

SEM images of unwashed sample at different areas.

5.3.2 Unwashed PCM: different magnifications (100X, 500X, and 3000X)

In Figure 13, image A at magnification 100X shows the web linking oriented fibres of the non-woven PCM sample. Image B at magnification 500X there are 2 different form of PCMs observed, the PCM microcapsules in the web and the mPCM wrapped around the fibre. Image C at magnification 3000X clearly illustrates the PCM capsules within web.

Figure 13.

SEM images of unwashed PCM at different magnifications.

5.3.3 SEM images: Multiple laundered (10 washes) samples at different wash temperatures

5.3.3.1 Sample at wash temperature 30°C

Figure 14 shows that image A at 500X magnification there is fibre interlacing and holes within the web, whereas image B at magnification 3000X clearly shows the broken PCM capsules, which indicate PCM content has evaporated during laundering as indicated by the TGA and DSC results.

Figure 14.

SEM images: mPCM of sample at wash temperature 30°C.

5.3.3.2 Sample at wash temperature 40°C

In Figure 15 the sample at wash temperature 40°C where image A at 500X magnification illustrates brittle fibres in the webbing, and Image B at magnification 3000X evidently illustrates collapsed PCM capsules in the web, which is due to multiple laundering at higher wash temperatures as quantified in TGA and DSC analysis.

Figure 15.

SEM images: mPCM of sample at wash temperature 40°C.

5.3.3.3 Sample at wash temperature 60°C

In Figure 16 it can be observed that image A at magnification 1620X the mPCM around the fibre are brittle and fragmented and image B at magnification 3280X it is evident that the PCM capsule around the fibre is damaged, therefore the conclusion can be made that multiple laundering at higher wash temperatures are damaging to the PCM microcapsule, which causes the loss of thermoregulating content as identified in the TGA and DSC test results.

Figure 16.

Damaged mPCM around fibre of sample at wash temperature 60°C.

5.4 Raman analysis

The Raman analysis will investigate the effect of multiple laundering at different temperatures on the polymer changes in the crystalline phase through the sample, the crystallinity distribution, and the orientation of the polymers, that was examined by the means of Raman imaging and spectrum.

5.4.1 Raman imaging and Spectrum of unwashed sample

In Figure 17 the scanning position of image A illustrates the PCM in the web and on the fibre, and image B shows the dept. profiling of the Raman image which displays 4 different wavelengths in the spectrum represented by colours: red, dark blue, green, and light blue.

Figure 17.

Raman scan position of unwashed sample (left: Image a); Raman scan depth of unwashed sample (right: Image B).

Figure 18 illustrates the peaks of the Raman spectrum, these shifting peaks can be identified as the chemical bond lengths of the n-alkanes paraffins, namely tetradecane, hexadecane and octadecane, which form the core of the microencapsulated PCM, and the fourth shifting peak observed could be the polymeric shell microcapsule which is an acrylic resin.

Figure 18.

Raman spectrum of unwashed sample.

The width of the shifting peaks on the graph illustrates a high degree of crystallinity, which is important, because most physical and mechanical properties of PCMs are affected by the degree of crystallinity. The crystallinity distribution in the sample can be found from the band shift area and the orientation of the polymers in the spectrum can be seen.

5.4.2 Raman analysis of multiple washed samples at different temperatures

5.4.2.1 Raman imaging and Spectrum of multiple washed sample at 30°C

5.4.2.2 Raman imaging and Spectrum of multiple washed sample at 40°C

In Figures 19 and 20 the scanning position of images A illustrates the PCM microcapsules in the web and therefore the depth profiling of the Raman in images B only show 2 different wavelengths which are recognised as the n-octadecane paraffin and the acrylic resin in the spectrum represented by colours: blue and red.

Figure 19.

Raman scan position of washed sample at 30°C (left: Image a); Raman scan depth of washed sample at 30°C (right: Image B).

Figure 20.

Raman scan position of washed sample at 40°C (left: Image a); Raman scan depth of washed sample at 40°C (right: Image B).

Figure 21 represents the Raman spectrum of the washed sample at 30°C and Figure 22 represents the Raman spectrum of the washed sample at 40°C. In both these spectrums it can be observed that there is a change in band positioning, the width and shape of the peaks have transformed, which can indicate polymer changes in the crystalline phase, as indicated in the DSC analysis. The change in band shift is also owing to the temperature and stress applied from to the laundering effect, the more orientated the washed sample, the more shift is seen.

Figure 21.

Raman spectrum of washed sample at 30°C.

Figure 22.

Raman spectrum of washed sample at 40°C.

5.4.2.3 Raman imaging and Spectrum of multiple washed sample at 60°C

In Figure 23 the scanning position, image A shows the PCM in the web and on the fibre, however the dept. profiling of the Raman image B only exhibits 3 wavelengths in the spectrum identified as hexadecane, octadecane and the acrylic resin, represented by the colours: red, blue, and green. It can be understood that due to multiple laundering, and rupture of the PCM microcapsules, tetradecane content has evaporated at wash temperature 60°C.

Figure 23.

Raman scan position of washed sample at 60°C (left: Image a); Raman scan depth of washed sample at 60°C (right: Image B).

Figure 24 illustrates the Raman spectrum of the washed sample at 60°C, the Raman shift, intensity, peak width, and shape is different to the unwashed sample and the washed samples at 30°C and 40°C, the chemical composition, crystal size, molecular chain length and morphology vary for different areas of the samples.

Figure 24.

Raman spectrum of washed sample at 60°C.

It is evident that the washed sample at 60°C show a Raman peak with a wider width and thus indicates a lower degree of crystallinity, which possibly means that the latent heat effectiveness of the PCM has reduced. It is also apparent that the intensity and polarisation of the Raman peaks have changed compared to the unwashed sample, which means the crystallinity and orientation of the PCM has changed due to loss of thermoregulating content, and therefore validate that crystallinity is a structural arrangement in the material and can be calculated from the DSC analysis.

6. Conclusions

The main objective of this study was to investigate the effect of multiple laundering at different wash temperatures, on the thermal and physical related properties, as measured by TGA, DSC, SEM and Raman, of a non-woven textile fabric containing PCM.

Thermal degradation as determined by TGA thermograms was found to be higher at a wash temperature of 60°C relative to that of a wash temperature at 30°C, the mass loss being 4.6% more at 60°C than at 30°C. This was asserted to physical changes such as size and shape of the PCM microcapsules as evident in SEM images. The SEM images showed broken and collapsed PCM microcapsules in the web and around the fibre, indicating evaporation of the thermoregulating content of the PCM.

It was found that the heat capacity of the sample washed at 60°C was much lower than that of the unwashed sample, the difference being approximately 50% in terms of the latent heat capacity as determined by DSC thermograms. This implies that the latent heat effectiveness of the PCM has decreased significantly. The physical properties of the PCM such as the intermolecular interaction, the intensity and width of peaks, polarisation, degree of crystallinity and the orientation changed as illustrated in the Raman analysis.

It can be concluded that the multiple laundering of textiles which contain PCM causes damage to the PCM capsules, causing the thermoregulating effectiveness to be reduced as confirmed by the results from the analytical techniques. In addition, the results also indicate that reducing the wash temperature and improving the design and formation method of the PCM microcapsules can extend the usable life and value of a textile containing PCMs.