Phase change materials: overview

Abstract

Phase change materials (PCMs) are environmentally benignsalts or organic compounds with variable environmental credentials, which storeand release latent heat by changing chemical bonds through a phasetransformation, unlike sensible heat storage materials such as water ormasonry, which change structure mechanically.

Initially, solid-liquid PCMs perform like conventionalstorage materials; their temperature rises as they absorb heat. However, whenPCMs reach melting point (phase change temperature) they absorb large amountsof heat without getting hotter. When the ambient temperature drops, the PCMsolidifies, releasing its stored latent heat. PCMs absorb and emit heat whilemaintaining a nearly constant temperature.

Within the human comfort range of 20-30C, PCMs store 5 to14 times more heat per unit volume than sensible storage materials. PCMs can beused for any heating and cooling requirement in buildings, vehicles or fabrics,including insulation and engine cooling, refrigeration, process cooling orcontributing to process heat, and combined heat and power systems.

The challenge to greater use of PCMs is their packaging,cost and knowledge, both technical and among potential customer and usercommunities.

Introduction:

Protection from extreme environment has always been acritical requirement of textile industries. Clothing that protects from water,extreme winter intensive heat, open fire, high voltage, propelled bullets,toxic chemicals, nuclear radiations and biological toxins, etc. are some of theexamples.

These clothing finds application as a sportswear, firefighting wear, defence wear, bulletproof jackets and other professional wear.Textile products can be made more comfortable when the properties of textilematerial adapt to the environment.

One of such important intelligent material at the present isPhase Change Material (PCM), which absorb, stores or release heataccording to the change of temperature during Phase Change Process are mostfrequently used in the manufacturing of smart textiles.

What are PCMS?

Phase change is the process of going from one physical stateto another i.e. from solid to liquid. Substances that undergo the process ofphase change are better known as Phase Change Materials (PCM). These material stores,release or absorb heat as they oscillate between solid and liquid form. Theygive off heat as they change to a solid state and absorb as they return toliquid state. The three fundamentals Phases of a matter solid, liquid and gasare known, but other are consider to exist, including the crystalline, colloid,glassy, amorphous and plasma phase.

The fundamental phenomena of a science was initiallycultivated and employed for building space suits for astronauts for spaceprograms. These suits kept astronaut warm in a black void of space and cool ina solar glare. Phase Change Materials are compounds, which melts and solidifyat certain temperature and in doing so are capable of storing or releasinglarge amount of energy. The storage of thermal energy by changing the phase ofmaterial at a constant temperature is called latent heat, that is, changingfrom a liquid state to solid state. When a PCM undergoes a phase change a largeamount of energy is required. The most important characteristic of latent heatis that it involves the transfer of much larger packets of energy than sensibleheat transfer.

Some of these PCMs change phase within a temperature rangejust above and below human skin temperature. This property of certain substanceis used for making protective all season outfits, and for abruptly changingclimatic conditions. Fibre, fabric and foam with built-in PCMs store thewarmth of body and then release it back to the body, as it needs it. Since theprocess of phase change is dynamic, the material are constantly changing from asolid to liquid and back depending upon level of physical activity of the bodyand outside temperature. Moreover, phase change material are used, but neverget used up.

Phase change materials are waxes that have the unique ability to absorb heat energy and emit heat energy without changing temperature themselves. These waxes include Eicosane, Octadecane, Nonadecane, Heptadecane and Hexadecane. They all have different freezing and melting points and when combined in a microcapsule will store heat energy and emit heat energy and maintain their temperature range of 30-34C, which is comfortable for the body.

The amount of heat absorbed by a PCM during the actual phase change with amount of heat absorbed in an ordinary heating process can be compared by considering water as PCM. The melting of ice into water leads to the absorption of latent heat of about 335j/g. If water is further heated, a sensible heat of only 4j/g is absorbed while the temperature rises by one degree. Therefore, the latent heat absorption during the phase change from ice into water is nearly 100 times higher than the sensible heat absorption.

How to incorporated PCMs into Fabric?

The micro encapsulated PCM can be combined with woven, non-woven or knitted fabrics. The capsules can be added to the fabric in many ways such as:

Microcapsules of different shapes-round, square and triangular within fibres at the polymer stage. The PCM microcapsules are permanently locked within the fibre structure during the wet spinning process of fibre manufacture. Micro encapsulation provides a softer hand, increased stretch, more breath ability and air permeability to the fabrics.

Matrix coating onto fabrics during finishing: the PCM microcapsules are embedded in a coating compound like acrylic, polyurethane, etc. and applied to the fabric. There are various coating processes available such as knife-over-roll, knife-over-air, pad-dry-cure, gravure, dip coating, and transfer coating.

Foam dispersion: Microcapsules are mixed into a water-blown polyurethane foam mix and these foams are applied fabric in a lamination process water is taken out of the system by drying process.

Body and clothing Systems

The required thermal insulation of clothing systems primarily depends on the physical activity and on the surrounding conditions, such as temperature and relative humidity. The quantity of heat produced by humans depends very much on the physical activity and can vary from 100W while resting to over 1000W during maximum physical performance. Particularly during the cooler seasons (approx OC). The recommended thermal insulation is defined in order to ensure that the body is sufficiently warm when resting. At a more intensive activity, this is often the case with winter sports, the body temperature increases with enhanced heat production. To keep this increase within a certain limit, the body perspires in order to withdraw energy from the body by evaporative cooling. If thermal insulation of clothing is reduced during a physical activity, part of the produced heat can be removed by convection, thus the body is not required to perspire so much.

The quality of insulation in a garment against heat and cold will be extensively governed by the thickness and density of its component fabrics. High thickness and low density improve insulation. In many practical examples, thermal insulation is provided by air gaps between the garment layers.

However the external temperature also affects the effectiveness of the insulation. The more extreme the temperature, be it very high or very low the les effective the insulation becomes. This, a garment designed for its ability to protect against heat or cold is selected by its wearer on the expectation of the climate in which the garment is to be worn.

However, a garment made from a thick fabric will have greater weight, and the freedom of movement of the wearer will be impaired. Clearly then a garment made from an intel1igent fabric whose nature can vary depending on the external temperature can provide superior protection. At the same time, such a garment must still be comfortable to wear.

Temperature change effect of PCM's

PCM microcapsules can produce small, temporary heating and cooling effects in garment layers when the temperature of the layers reaches the PCM transition temperature. The effect of phase change material on the thermal comfort of protective clothing systems will be maximized when the wearer is repeatedly going through temperature transients (i.e., going back and forth between a warm and cold environment) or intermittently 'touching or handling cold objects. The temperature of the PCM garment layers must change over and over again for the buffering effect to continue.

The most obvious example is [when] water changes to ice at f ' zero degrees and to steam at 100- degrees. There are other products that change phase at around body temperature and are being incorporated now in fibres and laminates or coating substrates, that will change phase at or near body temperature and so help to balance the body temperature and keep it more constant, it is for athletes in extreme conditions. People who are engaged in extreme sports such as mountaineering, trekking. It is going to be used in industrial applications where people are moving, for example, in and out of cool rooms.

How does it work In Fabrics?

When the encapsulated PCM is heated to the melting point, it absorbs heat energy as it moves from a solid state to a liquid state. This phase change produces a temporary cooling effect in the clothing layers. The heat energy may come from the body (e.g. when the wearer first dons the garment) or from a warm environment. Once the PCM has completely melted the storage of heat stops.

If the PCM garment is worn in a cold environment where the temperature is below the PCM's freezing point and the fabric temperature drops below the transition temperature, the micro encapsulated liquid PCM will change back to a solid state. Generating heat energy and a temporary warming effect. The developers claim that this heat exchange produces a buffering effect in clothing, minimizing changes in skin temperature and prolonging the thermal comfort of the wearer. The clothing layer(s) containing PCM must go through the transition temperature range before the PCMs will change phase and either generate or absorb heat. Consequently, the wearer has to do something to cause the temper- 1 3ture of the PCM fabric to change. PCMs are a transient phenomenon. They have no effect under steady state thermal conditions.

Active microclimate cooling systems require batteries pumps, circulating fluids. And sophisticated control systems to provide satisfactory body cooling, but their performance can be closely adjusted and made to last for extended periods of time. They are, however, expensive and complex. Current passive microclimate systems utilize latent phase change; either by liquid to gas evaporation of water (Hydro-weave), a solid to liquid phase change by a cornstarch/water gel, or with a paraffin that is contained in plastic bladders.

The Liquid evaporation is inexpensive. But will only provide a limited or short-term cooling in the high humidity environment found in protective clothing. They must also be re-wetted to recharge the garments for re-use. The water\starch gel-type cooling garment is currently favoured by the military and can provide both satisfactory and extended cooling near 32F (OC), but it can also feel very cold to the skin and requires a very cold freezer (5F) to fully recharge or rejuvenate the garment. When fully charged its gel-PCMs are relatively rigid block, and the garment has limited breathability.

The other paraffin PCM garments are relatively inexpensive, but their plastic bladders can rupture, thus spilling their content or posing a serious fire hazard. Furthermore, their paraffin PCM melts near 65F (18C) and must be recharged at temperatures below 50F (10C) in a refrigerator or ice-chest. Their rate of cooling also decreases with time because paraffin blocks are thermal insulators and limit the heat and can be transmitted into or through them. The plastic bladders used to contain the PCM also severely restrict airflow and breathability of the garment, thus decreasing their comfort.

Application on PCM

Automotive textiles: The scientific principle of temperature regulation through PCMs has been deployed in different ways for the manufacturing of textiles. During the summer, the temperature inside the passenger compartment of an automobile can rise substantially -- for instance, when the car is parked outside. In order to stabilise the interior temperature while driving the car, many models are equipped with air conditioning systems; however, providing sufficient cooling capacity required a1 lot of energy. Thus, the use of Phase Change Material technology in different applications for automotive interior could supply energy savings as well as increasing the thermal comfort of car interior.

Apparel active wears: Active wear needs to provide a thermal balance between the heat generated by the body while engaging in a sport and the heat released into the environment. Normal active wear garments do not always fulfill this requirement.

The heat generated by the body .during strenuous activity is often not released into the environment in the necessary amount, thus resulting in a thermal stress situation. On the other hand, during periods of rest between activities, less heat is generated by the human body. Considering the same heat release, hypothermia is likely to occur. Use of PCM in clothing helps in regulating the thermal shocks, and thus, thermal stress to the wearer, and helps in enhancing of work under extreme situations.

Lifestyle apparel: smart fleece vest, mens and womens hats, gloves and rainwater.

Outdoor sports apparel: jackets and jacket linings, boots, golf shoes, running shoes, socks, and ski and snowboard gloves.

From original applications in space suits and gloves, Phase Change Materials are finding their way into consumer product.

Aerospace textiles: Phase Change Materials found in today's consumer products originally were developed for use in space suits and gloves to protect astronauts from extreme temperature fluctuations while performing extra-vehicular activities in space.

The effectiveness of the insulation stems from micro encapsulated Phase Change Material (micro-PCMs) originally made to keep warm the gloved hands of space-strolling astronauts. The materials were considered ideal as a glove liner, to help during temperature extremes of the space environment.

Medical textiles: Textiles containing Phase Change Materials (PCMs) could soon find uses in the medical sector

• To increase the thermo-physical comfort of surgical clothing such as gowns, caps and gloves.
• In bedding materials such as mattress covers, sheers and blankets.
• A product, which supports the effort to keep the patient warm enough during an operation by providing insulation tailored to the body's temperature.

Other application of PCM

Phase Change Materials are at the moment being used in textiles, which cover the extremities: gloves, boots, hats, etc.

Different PCMs can be chosen for different applications. For instance, the temperature of the skin around the torso is about 33C (91F). However, the skin temperature of the feet is around 30C-3 IC. These PCM materials can be effective down to 16C, sufficient to ensure the comfort of someone wearing a ski boot in the snow. They are increasingly used in body-core protection and it will move into the areas of blankets, sleeping bags, mattresses and mattress pads.

Types of PCM

Standard phase change materials are typically a polymer/carrier filled with thermally conductive filler, which changes from a solid to a high-viscosity liquid (or semi-solid) state at a certain transition temperature. These materials conform well to irregular surfaces and have wetting properties similar to thermal greases, which significantly reduce the contact resistance at the different interfaces. Due to this composite structure, phase change materials are able to withstand mechanical forces during shock and vibration, protecting the die or component from mechanical damage. Additionally, the semi-solid state of these at elevated temperature resolves issues related to "pump-out" under thermo-mechanical flexure.

When heated to a pre-determined transition temperature, the material significantly softens to a near liquid-look physical state in which the thermally conductive material slightly expands in volume. This volumetric expansion forces the more thermally conductive material to flow into and replace the microscopic air gaps present in between the heat sink and electronic component. With air gaps filled between the thermal surfaces, a high degree of wetting of two surfaces minimizes the contact resistance.

There are two categories of phase changes materials in general:

• Thermally conductive and electrically insulating.
• Electrically conductive

The primary difference between the thermally and electrically conductive materials is the film or carrier that phase change polymer is coated unto. With the electrically insulating material, minimum voltage isolation properties can be obtained.

Measurements for the thermal Barrier function of Phase Change Materials in Textiles

Manufacturers can now exploit PCMs to provide thermal comfort in a wide variety of garments. But to understand how much and what kind of PCM to use, as well as refining the structure of the textile, in order to make a garment fit for its purpose, it is necessary to quantify the effect of the active thermal barrier provided by these materials.

The total thermal capacity of the PCM in a certain product depends on its specific thermal capacity and its quantity. The necessary quantity can be estimated by considering the application conditions, the desired thermal effect and its duration, and the thermal capacity of the specific PCM. The construction of the carrier system and the end-use product also influences the thermal efficiency of the PCM, which has to be considered in regard to the material selection and the product design.

Future of PCM

The main challenge in developing textile PCM structures is the method of their application. Encapsulation of PCMs in a polymeric shell is an obvious choice, but it adds dead weight to the active material. Efficient encapsulation, core-to-wall ratio, yield of encapsulation, stability during use and integration of capsules onto fabric structure are some of the technological issues being investigated by our research group.

Although PCMs are being promoted in a wide range of apparel and related products, the applications in which they can truly perform are limited. As better test methods are developed for PCMs, producers of PCM materials and garments will have to more carefully target the markets in which their products do perform.

Technology Status

Phase change materials are either organic compounds or inorganic (salts). A range of materials and their properties have been mapped in detail. Examples of the former are paraffin wax and carboxylic acid, and of the salts, Glauber's salt (sodium sulfate decahydrate) and calcium chloride hexahydrate. The challenge is in overcoming the disadvantages listed above and in effective and economic heat transfer mechanisms in the wider system in which PCMs do their work.

PCMs are most developed for industrial refrigeration and ice production. The scope for technical improvement lies in the materials in which the PCMs are contained and in the development of new PCMs.

Containers - Requirements for containers include:



• Small scale in the range of about 25 mm width /diameter produces best performance (However, larger containers can also be effective.)

• Good heat conductance

• Strength to contain changes in PCM volume with phase change (organics).
• Impermeable to fluids and corrosion-resistant

Market Status

As mentioned above, cold storage for space cooling and refrigeration is the biggest market for PCMs. PCM sales are not monitored separately so it is not possible to give a value for this market. The potential value of markets for PCMs is large. The following are areas where PCMs could offer significant economic, energy and carbon reduction advantages:

• Storage of thermal energy when electricity supply and demand are out of phase. This particularly applies to intermittent renewable, and they are particularly appropriate for combining with solar domestic hot water heating or passive solar space heating systems.
• Providing a thermal mass effect for lighter weight buildings.
• Thermal storage when cheaper systems are too large.

PCMs themselves are not expensive, but the packaging and processing to achieve reliable, consistent performance puts a cost premium onto the products. An additional barrier to a wider market is lack of awareness of the technology, how it can be used, and its benefits.

Environmental and economic benefits

The environmental benefits of PCMs are:

Energy efficiency Per unit volume, PCMs store and release between 2 and 14 times the amount of thermal energy those latent heat-storing materials do. For example, calcium chloride hexahydrate, at its melting point of 29C, can store/release 190 kJ of energy. To store the same amount of energy water would have to be heated to 45C and concrete to 190C.

In addition, because PCMs can absorb free energy, PCM thermal storage can shift most of the electrical load for buildings with mechanical air conditioning from peak to off-peak periods. If applied to enough buildings, this would reduce the requirement for peak power generation, and therefore greenhouse gas emissions, and any upgrading of transmission networks which would otherwise be necessary. There is research to be done to measure the u-values of PCMs in structures and more efficient ways to transfer stored heat from them into spaces. This would enable us to calculate potential carbon savings.

Materials efficiency The corollary of the energy efficiency is that much less material is needed to store, upgrade and release thermal energy if PCMs are used. PCMs therefore have the potential significantly to reduce the tonnage of material mined or imported for construction, by up to 14 times. Figure 1 below illustrates the potential.

Benign chemistry PCM salts are not pollutants or toxins, and are expected to be readily biodegradable, according to PCM safety datasheets. Their maximum harm factor is as eye irritants if directly in contact with the eye.

Figure 1: Energy storage capacities of building materials, with and without PCM (carboxylic acid)

Improving PCMs Environmental Credentials

Whilst PCMs themselves have advanced environmental credentials, the materials in which they are encapsulated as finished thermal storage products could be improved. Organic compounds have a more heterogeneous environmental profile, including their origin and embodied energy, so each should be considered before use.

Conclusion

Day is not far when all-season outfits are mass-produced, since lot of money is being spent on research and development in these areas in the developed countries. For example, in Britain, scientists have developed an acrylic fibre by incorporating microcapsules containing Phase Change Materials. These fibres have been used for making lightweight all-season blankets.

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