Shape Memory Polymers for Smart Textile Applications

INTRODUCTION TO SMART GARMENTS AND SHAPE MEMORY MATERIALS 

The Textile industry is facing new challenges: intelligent textiles, smart garments and smart clothes are only a few of the keywords which revolutionize whole industry within the next 10 years. The integration of high-technologhy into garments, e.g. modern communication or monitoring systems or development of new materials with new functions, hast just started with timidly, but obviously we will see these technologies in every part of our daily life. 

Smart garments are a set of materials or structures which sense and react to stimuli, such as those from mechanical, chemical, thermal, electrical, magnetic or other sources. There are two types of smart materials, passive and active smart materials. Passive smart materials are which can only sense the enviromental conditions or stimuli. They cannot react any kind of senses. Active smart materials are which can sense and react to the conditions or stimuli. For examle, a curtain which is painted with colormatic colorant can react to amount of sun light in the way of changing colours automaticly for keep the temperature at same level. 

Shape memory means the ability of a material to remember its original shape, either after physical deformation which is called one-way effect, or by heating and cooling which is called two-way effect. To date, a large variety of alloys, gels and polymers have been found to exhibit shape memory behaviour. Engineering aspects of shape memory materials(SMM) have been investigated extensively and a rather wide variety of different SMMs are presently commercial materials. The most common shape memory products have been based on metallic shape memory alloys. Also shape memory polymers (SMP) is reproducing itselves at a quick rate. Shape memory materials (SMMs) belong to the group of smart or intelligent materials which due to external situmulus can change their form from temporary shape to previously programmed shape. The most common thing to activate shape change is changing ambient temperature. However, some certain materials also stres, mechanical force, electric field, magnetic field, pH-value can be the triggering stimulus. 

Shape Memory Material 

Shape Memory Alloys 

History 

The first steps towards the discovery of the shape memory effect were taken in the 1930’s. In 1932, the pseudoelastic behavior of the Au-Cd alloy was discovered by A. Ölander. In 1938, Greninger and Mooradian observed the formation and extinction of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy. In 1949 Kurdjumov and Khandros reported more comprehensively about the phenomenon of the memory effect and the thermoelastic behavior of martensite phase. Also Chang and read did it too in 1951. However, in 1960’s important developments of shape memory materials really begun. The nickel-titanium(NiTi) alloys were first developed in 1962 by the “United States Naval Ordnance Laboratory” and commercialized under the trade name “Nitinol”. Also remarkable properties of Nitinol were discovered by accident. Since the birth of Nitinol, extremely intensive research has been made world wide to clarify the characteristics of the basic behaviour of shape memory effect.Nowadays NiTi alloys are the most important and best characterized of all the alloys ever found shape memory behaviour. 

General Principles of Shape Memory Alloys 

The Shape memory alloys are quite fascinating materials characterized by a shape memory effect and superelasticity, which ordinary metals and alloys do not have and which can memorize a predetermined shape. This shape memory effect is due to a phenomenon known as a “thermoelastic martensitic transformation” which is reversible, transformation between two different crystal microstructures that occurs when a shape memory alloys heated or cooled beyond specific transition temperatures. These temperatures depented phases are called “martensite” and “austenite”. The shape memory alloys change from austenite to martensite upon cooling; Mf is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating As and Af are the temperatures at which the transformation from martensite to austenite starts and finishes. 

                   

Shape Memory Effect 

The Shape memory alloys have different effects. There are two common effects; one way and two way shape memory. When a shape memory alloy is in its cold state which is below As, the metal can be bent and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. With the one way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low temperature shape. On heating, transformation starts at As and is completed at Af. However, The two way shape memory effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature shape. A material that shows a shape-memory effect during both heating and cooling is called two-way shape memory. Special alternative training processes can be used to build up two-way SME, first a specific heat treatment, and then several complicated and time consuming thermomechanical cycles. 

Shape Memory Polymers 

Shape memory polymers are polymers, which have a special chemical makeup that gives them their “shape memory” properties. The shape memory effect requires two components at the molecular level: cross-links, which determine the permanent shape and so called switching segments, which are used to maintain the temporary shape. Above its glass transition temperature a shape memory polymer can seem just like any other polymer. At this temperature it is in its memory shape, if no loads are applied. Because of its flexibility it can be deformed into a different shape quite easily. This is shown in step. Cooling “freezes” the deformed shape. After it is completely cooled the force no longer needs to be applied and it stays in the deformed shape. This is a temporary shape and at that state the polymer is a fairly rigid and hard polymer. The temporary state can be removed by heating the polymer above the transition temperature. During the transition the material goes from its temporary shape to its memory shape. This is a very elastic and flexible state and it can be repeated over and over again. Shape memory polymers can be useful in many ways,since they are considered smart materials. Smart materials respond to an external stimulus, for SMP this is typically temperature. Based on the concept of SMP some applications some applications that would be ideal high performance textiles such as wrinkle-free clothing. 

Smart garments and textiles 

Smart materials and structures can be defined as the materials and structures that sense and react to environmental conditions or stimuli, such as those from mechanical, thermal, chemical or other sources. Three components may be present in smart materials: sensors, actuators and controlling units. The sensors provide a nevre system to detect signals, thus in a passive smart materials. The actuators act upon the detected signal either directly or from a central control unit; together with the sensors, they are essential element for active smart materials. At even higher levels, like very smart materials, another kind of unit is essential which works like a brain. Some of the research areas can be grouped as follows: 

For sensors / actuators: 

- Photo sensitive materials 

- Fibre optics 

- Conductive polymers 

- Thermal sensitive materials 

- Shape memory materials 

- Đntelligent coating / membrane 

- Chemical responsive polymers 

- Mechanical responsive materials 

- Micro and nanomaterials. 

Smart garments are made possible due to advances in many Technologies coupled with the advances in textile materials and structures. Such textile materials and structures are becoming possible as the result of a succesful marriage of traditional textiles technology with material science, structural mechanics, sensor and actuator technology, advanced processing technology, communication, etc. 

Smart garment applications with shape memory materials 

Shape memory materials usually use for heat protective products which when it is activated by environmental stimuli, they make some space between tight layers. Thus, it creates barrier layer between outside and body to protect body temperature. A few important trade name about smart garments with shape memory materials are Diaplex® and Oricalco®. All of these researches are based on fabrics which knitted by shape memory materials. 

Diaplex® 

Winter adventurers usually wear nylon fabrics to keep warm. These nylon fabrics are laminated with a thin coating which contains thousands of tiny pores, or openings. The pores allow moisture, such as sweat from your body, and excess thermal energy to escape. However, the pores would not let moisture and cold air into the fabric. Because the nylon fabrics which used for these clothes are windproof and waterproof. Diaplex is also made from laminated nylon, but the coating is different. Diaplex does not have pores; it is solid film that makes it even more waterproof and breathable than other laminated nylon fabrics. Diaplex keeps you warm by taking advantage of how participle move. When the air outside is cold, the particles of diaplex arrange themselves into a solid sheet, forming an insulator and preventing the transfer of thermal energy from your body to colder surroundings. As your body gets warm, such as physical movement or exercising, the fabrics particles respond to your bodys increased thermal energy. Their kinetic energy increases, and they rearrenge to create millions of tiny openings that allow excess thermal energy and moisture escape. 

Elasticity : Diaplex offers 200% elasticity to allow freedom of movement. 

Durability : The diaplex membrane is extremely hard wearing, and because it is net porous. 

Wind Resistance : The non-porous form of Diaplex has excellent wind resistance. 

Oricalco® 

In this product, a company which called Grade Zero Escape has used shape memory materials for the manufacturing of a shirt on with long sleeves. The sleeves fabric could be programmed to shorten immediately as the room temperature became a few degree hotter. The fabric can be screwed up into a hard ball, pleated and creased then just by a flux of hot air (even a hairdyer) pop back automatically to its former shape. 

Elasticity : Like diaplex, it offers 200% elasticity to allow freedom of movement. 

Texture : Can be used in light, normal and heavy clothes. 

Resistance : With the non-porous form, it is good resistant by clogging by salt, air and oil. 

Wind resistance : It has got excellent wind resistant with non-porous form. 

Thermal insulation : It can rearrange itself to outside temperature. 

Water Proof : It has got excellent protective to water. 

Thermo-Formable Yarn 

Luxion Industries NV and Massebeuf have developed a new material which called thermo-formable yarn. It is a new polymer that becomes bi-stable after being spun into a monofilament. After activation it is flexible and rigid. It can be used as a yarn for textile knitting or weaving. Also it becomes thermofomable when heated for around a minute at 65 °C, and stiffens as it cools down again. It can be reshaped by the reintroduction of the heat source.