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Characterization of the PCM fibers

  1. Abstract:

The PCM technology was developed by Triangle Research and Development Corporation (TRDC) for NASA in 1988 to protect astronauts from extreme temperature fluctuations in space. In 1991, Gateway Technologies Inc. acquired the exclusive patent rights for incorporating phase change technology in fibers, fabrics and foam from TRDC. However it was not until 1997, the same year that Gateway Technologies changed its name to outlast technologies Inc., the first commercial products gloves and footwear containing phase change materials were introduced to consumers. Outlast technology is the leading developer of the PCM technology. The possibility of preparing clothes and fabrics that heat or cool the body depending on the temperature of the skin and the surrounding environment believed to have great commercial value for numerous business applications.

Keywords: PCM Microcapsules, Latent heat.

  1. Introduction:

Comfortable skin temperatures are within the 28-33°c. Outside this temperature range body feels discomfort. Clothes with built in thermo-regulating properties would provided maintained comfort without putting on or taking off clothes frequently. Such smart clothes will reduce the discomfort caused by accumulation of sweat in the clothing and shivering. Integration of PCM in clothing is an important method to obtain the thermo regulating properties. When body temperature increase, the PCM melts and absorbs heat from the body.

The process of melting or solidifying takes place at a constant temperature until all material is melted or solidified. Currently the most common method of incorporating PCMs into textiles is by coating fabric with a polymeric binder containing the PCM in microcapsules. But applying microencapsulated PCM as part of a coating has several demerits besides the high cost of the microencapsules. Properties like air permeability and moisture permeability are impaired and will affect the thermal comfort in a negative way. Further, increasing add on the coating results in a stiffer and less elastic fabric, and thus, less comfortable to wear. Another problem is that durability of the micro capsules in the laundering.

The principle of PCM materials working
Figure 1: The principle of PCM materials working

These drawbacks can be avoided if the PCM microcapsules are incorporated inside the fibers. An advantage is that those microcapsules are more durably bound to the fiber and can protect themselves laundering in a decent way. However, incorporation of microcapsules is so far only possible in solution spun fibers. Addition of microcapsules in acrylic fibers has so far produced latent heat up to some 10-15 J/g only and the amount of PCM that can be incorporated is limited by spinability and a negative effect on the strength and thickness. The low amount of microcapsule in the fibers leads to a very low thermoregulation effect.

The dominating synthetic fibers used (PET, PA, PP) are produced by means of melt spinning. Now a day no commercial melt spun fibers containing PCM are available on the market. Additions of microcapsules in melt spun fibers containing PCM are impossible for some reasons. For instance, the capsules do not withstand the high temperatures and shear forces created in the melt spinning process and are being crushed.

To be used in melt spun fibers, the PCM should be immobilized within the fiber. It can be done by using bi-component fiber with a core/sheath structure or an island-in-the-sea structure so that the PCM is attached inside the core of the fibers.

Producing bi-component fibers with PCM
Figure 2: Producing bi-component fibers with PCM

For process ability: ηcore = ηsheath , (η= melt viscosity)

The melt spinning process of the bi-component fibers put strict demands on the melt viscosity of the materials in the sheath and core. The viscosity of the core material should preferably be similar to the melt viscosity of the sheath material. The most reliable PCM materials in terms of latent heat and cost are low molecular weight compound, e.g. hydrocarbon waxes (paraffin’s). Such compounds have fewer viscosities at the processing temperatures (180-300°c). For overcome this, HDPE is used with Paraffin waxes as viscosity modifier that help to ensure the safe spinning of the bi-component fiber. Where, HDPE means high density polyethylene.

  1. PCM materials:
  • Hydrated inorganic salts;
  • Polyhydric alcohols;
  • Polymers;
  • Polyethylene glycol (PEG);
  • Block polymers;
  • Other polymers;
  • Aliphatic polyesters,
  • Polytetramethylene glyc
  • Linear chain hydrocarbon

Here we will discuss about the linear chain hydrocarbons only.

3.1. Linear chain hydrocarbon:

The most important type of PCMs for thermo regulating and heat storage applications in clothing and textiles are the linear hydrocarbons of the general formula:

CnH2n+2

For n> 13-14, these compounds are waxy by products of oil refining and are therefore inexpensive. They are represented in table 1.

03Linear hydrocarbons are commercially available under the brand name Rubitherm as phase change materials for different applications. If the latent heat is high, price is low.

  1. Measurements:

4.1. Bholin Rheometer:

Bohlin Instruments Inc, Sweden is to count the viscosity of the material.

4.2. DSC

The Perkin-Elmer DSC 7, USA and DSC Q1000 V9.8 Build 296 is Differential Scanning Calorimeter is used to measure the melting temperatures, melting enthalpies, glass transitions, solid state transition or crystallization etc.

4.3. Vibrodyn and Vibroscope:

The Vibrodyn and Vibroscope is Lenzing Technique GmbH & Co KG, Austria is to measure the tenacity (cN/tex), Elongation%, force (cN), Young-M (cN/tex), work (cN/cm).

5. PCM capsules:

i. Thermasorb 83 (Outlast technologies, 28°c)

ii. Thermasorb 95 (outlast technologies, 35°c)

iii. Lurapret TX PMC 18 (BASF)

iv. Lurapret TX PMC 28* (BASF)

 v. Encapsulance PC 140 (Ciba)

*Means 180 J/g

PCM Capsules ( Micronal of BASF)
Figure 3: PCM Capsules ( Micronal of BASF)
  1. Microencapsulate PCMs applied as coating:

The most common method of incorporating PCMs into clothing by coating individual fibers or fabrics with a polymeric binder containing the PCMs in microcapsules. Linear chain hydrocarbon and polyhydric alcohols are PCMs within the microcapsules. The inventors told that the combination of microcapsules and a polymeric binder retained the PCMS within the fabric also during washing. The technique also allowed incorporation of different kinds of PCMs to the same fabric, which is a splendid advantage since it is possible to prepare fabrics with thermo-regulating activity over specified and relatively broad temperature duration.

On the other hand, research has showed that the commercial coating equipment often fails to introduce the microcapsules evenly throughout the coating. Thermoplastic extrusions are also problematic because of high temperature and physical damage to the microcapsules caused by the extrusion screw. Applying a binder with microcapsules directly to a fabric is also problematic since a significant amount if binder is necessary to get a high content of the PCMs. Furthermore, the addition of a coating can stiffen extensible fabrics, and the non-extensible microcapsules can also lower the elongation at break of the composition.

 Microscope image of microcapsules containing phase change materials applied as coating into fibers
Figure 4: Microscope image of microcapsules containing phase change materials applied as coating into fibers
  1. Introducing PCMs during fibre spinning:

There is different way of doing this work. But in this report we will concentrate on the two processes-

  • Microencapsulated PCMs incorporated during fiber spinning
  • Bi-component fiber of PCMs in the cor

We will discuss briefly on the first method and describe the second method more.

7.1. Microencapsulated PCMs incorporated during fiber spinning:

When the PCM microcapsules are inside the fibers by spinning, they are more durably bound to the fibers and can intact after laundering. An important advantage of having the microcapsules inside the fibers that it allows the preparation of knitted, woven and non-woven fabrics having thermo-regulating properties. Another advantage is that the PCMs are protected both by the wall of the microcapsules and the fiber wall and that no coating layer stiffens the fabric or alters its breathing ability.

But microencapsulated PCMs had only been incorporated in acrylic fibers.

Microscopic image of microcapsules containing PCMs embedded within the body of the fibers.
Figure 5: Microscopic image of microcapsules containing PCMs embedded within the body of the fibers.

7.1.1. Problems with PCM microcapsules in fibers:

1. Can not be used in the melt spun fibers (PET, Nylon, and PP)

2. Low titers (fiber diameter) are not possib

3. Low thermal effect (5-15) J/g) since loading has to be low for process ability and strength

4. High cost (20 €/kg)

7.2. Bi-component fiber of PCMs in the core:

The total process of manufacturing a bi-component PCM fiber is described below:

7.2.1. Materials:

A high molecular weight, high density polyethylene was used as viscosity modifier. The PCM was n-octadecane. To improve the thermal stability during processing of the PCM/HDPE mixture, 0.2% of a phenolic based anti-oxidant was used. PA6 was used as sheath material in fibers.

7.2.2. Compounding:

Small amounts of PCM/HDPE mixture for rheological and thermal measurements were prepared in a Brabender Kneader (type 350 s, Brabender OHG, Duisburg, Germany) machines. In the process it was observed that PCM/HDPE mixture of 70/30 is better.

7.2.3. Fiber spinning:

Fibers were produced by a bi-component melt spinning line manufactured by Extrusion Systems Limited (ESL), Leeds, England. The process is shown below.

Bi-component melt spinning apparatus with in line hot drawing
Figure 6: Bi-component melt spinning apparatus with in line hot drawing

Drawing:

Two polymer melts are combined to form a co-axial co-extrusion flow in the two lower spin pack discs
Figure 7: Two polymer melts are combined to form a co-axial co-extrusion flow in the two lower spin pack discs.

The materials are melted in two 18mm single screw extruders and metered to the spin pack via gear pumps. The spin pack was arranged for bi-component fibers with core/sheath structure.

Photograph oh the two lower spin pack discs
Figure 8: Photograph oh the two lower spin pack discs

The lower picture describes the two lower dismantled spin pack disks.

Thermal properties of fibers with varying amount of PCM
Table 2: Thermal properties of fibers with varying amount of PCM

7.2.4. Result:

The thermal properties of the fibers are summarized in the table 2.

 Heat of fusion of fibers vs. wt-% octadecane in the fiber
Figure 9: Heat of fusion of fibers vs. wt-% octadecane in the fiber

The heat of fusion is increasing with increasing the amount of PCM in the fiber.

The dashed line in the figure-9 indicates the theoretical heat of fusion of the fiber based on the nominal weight fraction of octadecane in the fiber and heat of fusion of pure octadecane. It is seen that experimental values are close to that line. This means that the PCM is utilized in a very effective way.

The highest heat of fusion was 86 J/g, which is very competitive to existing commercial fibers based on microencapsulated PCM.

Mechanical properties of the fibers are shown in the table 3.

Linear density (titer), tenacity and elongation at break of produced bi- component fibers
Table 3: Linear density (titer), tenacity and elongation at break of produced bi- component fibers

The fiber strength (tenacity) is decreasing with increasing core content and the elongation decreases. This is natural since the cohesive strength of the octadecane/HDPE blend is low. For most clothing application tenacity of 20 CN/Tex is sufficient.

The figure shows the effect of washing on the different percentage of PCM content in the fiber
Figure 10: The figure shows the effect of washing on the different percentage of PCM content in the fiber
  1. Effect of washing in the PCM fibers:

From the figure-10, we can say that the PCM fiber loses its thermo- regulating properties continuously as PCM from the fiber is washed away after repeated exposure to wash.

  1. Heat flow into bi-component PCM fiber:
The duration of warming
Figure 11: The duration of warming

From the figure-11, we can say that the PCM loses its strength of the thermo- regulating properties with the duration of the exposure/time.

  1. Conclusion:

To get better thermo-regulating properties the amount of PCM content will be high. Except that, the heat absorbance or heat release will last for a very short time. But it was shown PCM microcapsules content in the fiber is not more than 10%. If we increase the PCM percentage than the fiber strength will fall. If we perform coating process then it is possible to increase PCM content some extent. The resulting garment will have lower breathability and high stiffness. Bi-component fibers that have a core mainly PCM material there we can increase the percentage of PCM content but this process also lower the fiber strength. Today only a few companies around the world are trading PCM fibers. To improve the performance fibers with a large amount of PCM and good strength have to be developed. Then the manufacturers will able to get a worldwide success than present condition.

12. References:   

01. Hagstrom Bengt, Temperature regulating Melt spun bi-component textile fibers containing a Phase Change Material in the core, Proceedings of the Polymer Processing Society Europe/Africa Regional meeting-PPS-October 18-21, 2009 Larnaca (Cyprus)

02. Engstrom Jonas, Phase Change Materials in clothing fabrics, A literature review, IFP Research, P.O Box-104, SE-431 22 Molndal, Sweden

03. www.outlast.com

 

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