Structural characteristics of temperature-sensitive polyurethane and its application prospects

Zhou Hu Chen Dongchu Zeng Jianxian Seeking Gold Ingot Yue Kunliang (School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China)

Abstract: Temperature-sensitive polyurethanes usually have a typical block and micro-phase separation structure. The vapor permeability, moisture permeability, internal pore size and mechanical properties of the membranes will change sharply before and after the soft phase transition temperature, showing uniqueness. Temperature sensitive properties. The unique structural characteristics and temperature response characteristics of temperature-sensitive polyurethanes have shown broad application prospects in textile materials, food packaging materials, agricultural greenhouse films, separation membranes, industrial control reactors and other fields.
Key words: polyurethane; temperature-sensitive property; microstructure; application CLC number: TQ323.8 Document code: AArticle ID: 1009-1815(2011)02-0094-03
Temperature-sensitive polyurethane (TSPU) is a smart material that can respond to external temperature stimuli. The soft segment will undergo a significant phase change before and after the phase transition temperature, resulting in a free-volume hole size and free volume fraction inside the membrane. The microscopic structure changes correspondingly, so that the macroscopic properties such as vapor permeability, moisture permeability, water absorption and mechanical properties of the membrane also undergo a corresponding sudden change, showing significant temperature-sensitive properties [1~3]. This paper mainly combines the intelligent response characteristics of temperature-sensitive polyurethane to introduce its application in many fields of national economy, and provides guidance and reference for the in-depth research and development of temperature-sensitive polyurethane materials.
1. Structural characteristics of temperature-sensitive polyurethanes TSPU usually has a typical block structure. The main chain is generally composed of a flexible segment (soft segment) and a rigid segment (hard segment), and the soft segment is a regular linear structure. The hard segment is a urethane group formed by a small molecular chain extender and a diisocyanate, which is easy to form a microphase crystal region and is microphase separated from the soft segment, and the melting point of the crystallite can reach 100 to 250 ° C [4, 5]. According to the glass transition (T)g or crystalline melt transition (Tm) of the soft segment in the main chain, TSPU can be divided into Tg type and Tm type [6].
The soft segment of the TSPU has an apparent phase transition temperature (defined as the switching temperature, T) s over the operating temperature range (0 to 60 ° C), by adjusting the soft segment structure, chain length, ratio of soft segment to hard segment, etc. Temperature sensitive polyurethane materials with different switching temperatures can be produced [7].
2. Application of temperature-sensitive polyurethane Polyurethane materials generally have excellent mechanical properties, wear resistance, oil resistance, tear resistance, chemical resistance, radiation resistance, and good adhesion. As one of the polyurethane materials, the temperature-sensitive polyurethane material has its own unique temperature-sensitive properties in addition to the above-mentioned excellent properties. This unique temperature-sensitive intelligence makes it a potential application in many areas of the national economy.
2.1 Textile Materials The development of temperature-sensitive polyurethanes and their application to textile materials dates back to the 1990s. In 1993, Hayashi et al. claimed to invent a polyurethane polymer with a glass transition temperature of 0 to 60 ° C to form a film for textiles, which can control the body sweat by adjusting the moisture permeability in the glass transition temperature range. The amount of evaporation from the outside [8]. In 1998, MHI established Diaplex in New York, USA, and announced the development of smart fabrics for outdoor wear. The fabric is compounded with a very thin non-porous polymer film. At low temperatures, the polymer has a tight molecular chain arrangement, which prevents the outward transfer of water vapor and heat in the human body, thereby achieving warmth; when the temperature rises When the temperature is up to the predetermined range, the molecular chain movement suddenly increases, resulting in a sharp increase in the free volume, which provides more ways for the discharge of water vapor and heat in the human body, thus helping to prevent the occurrence of micro-environment in the garment. Comfortable and sticky feeling [9].
With the improvement of people's living standards, people's requirements for the comfort and functionality of fabrics are getting higher and higher. Future textiles should be versatile, that is, have "smart" characteristics. Smart textiles made with temperature-sensitive polyurethane can be both rain and windproof, wicking, breathable, and comfortable to wear, and can be made under certain pressure during wearing. The water droplets do not soak the fabric, but the sweat emitted by the human body can pass through the fabric or conduct to the outside in the form of water vapor, and does not condense and accumulate between the surface of the human body and the fabric, keeping the wearer dry and warm. In short, temperature-sensitive polyurethanes have shown broad application prospects in the textile field, especially in the field of smart textiles [10, 11].
2.2 Food packaging materials In general, the amount of moisture in food has a direct impact on the color, aroma, taste and stability of food [12]. The temperature-sensitive polyurethane film can be used as a food packaging material to adjust the moisture content of the packaged food by adjusting the temperature and humidity of the outside world. For example, when the food having a small moisture content is packaged, the humidity is greater than the temperature of the switch. In the environment, the internal free volume pore size of the temperature-sensitive polyurethane film becomes larger, and the water vapor in the external environment passes through the temperature-sensitive polyurethane film to enter the inside of the food, thereby increasing the moisture content of the food, and once the moisture content of the food meets the demand, Immediately remove the food from the above-mentioned hot and humid environment for temperature treatment, so that the temperature-sensitive polyurethane film will return to the glass state (crystalline state), the internal free volume hole size will be immediately reduced, and the internal water vapor will no longer be exchanged with the external environment, thereby improving The moisture content of the food. On the contrary, when the packaged food with a large moisture content is placed in an environment where the humidity is small and the temperature is higher than the temperature of the switch, the internal free volume pore size of the temperature-sensitive polyurethane film becomes large, and the moisture inside the food passes through the temperature-sensitive polyurethane. The film enters the external environment, so that the moisture content of the food is reduced. Once the moisture content of the food meets the demand, the food is immediately taken out and cooled, so that the temperature-sensitive polyurethane film immediately returns to the glass state (crystalline state), and the interior is free. The volume of the pores is immediately reduced, and the internal water vapor is no longer exchanged with the external environment, thereby reducing the moisture content of the food [13]. Therefore, it can be seen that the free-volume pore size of the temperature-sensitive polyurethane film determines the water vapor transmission like the "switch", so that the moisture content of the packaged food can be arbitrarily changed without opening the food packaging bag, thereby satisfying Its processing, transportation and storage needs.
2.3 Agricultural greenhouse film temperature-sensitive polyurethane film has low temperature warmth, high temperature vapor permeability, moisture permeability and other characteristics [3], if the temperature sensitive polyurethane film is applied to agricultural greenhouse film, this will certainly adjust the temperature and moisture content inside the greenhouse. Thereby controlling the growth rate of crops to suit the best selling season. In addition, the temperature-sensitive polyurethane film also has the function of intelligently regulating the transmission intensity of light, and can also control the growth rate of crops to a certain extent. Before the switching temperature, the light-sensitive polyurethane film has a low light transmittance, so that the photosynthesis of the crops in the greenhouse is weak, and the crop growth is slow; after the switching temperature, the light transmittance of the temperature-sensitive polyurethane film is significantly increased, and the photosynthesis of the crops is increased. The effect is strong and the crops grow faster. Therefore, as long as the temperature-sensitive polyurethane film and the electric resistance wire are used in combination, an intelligent greenhouse film which can adjust the temperature, moisture content and light intensity in the greenhouse can be prepared.
2.4 Separation membranes Temperature-sensitive polyurethane membranes with different initial pore structures can be prepared by techniques such as solvent evaporation or wet phase inversion. When Zhou et al. applied a temperature-sensitive polyurethane film to the separation of different kinds of substances, it was found that sodium ions and chloride ions (small size) were able to pass through at a lower temperature, while glycine (slightly larger molecular size) was transmitted. It shows temperature dependence, that is, the flux through the barrier effect is small at low temperatures, and the flux is high at high temperatures (especially after the temperature exceeds the switching temperature). Because of its large molecular size, collagen is not permeable even at temperatures above the switching temperature and remains in the mother liquor. Therefore, selective separation of molecules of different sizes can be achieved by temperature control [14].
2.5 Industrial Controlled Reactors Temperature sensitive polyurethane membranes can be used to make industrial controlled reactors. It is well known that membranes are often used as an ideal separator for gas or liquid mixtures [15]. When the separated materials need to be mixed together for reaction, the membrane can act as a control reactor in this process.
For a continuous or semi-continuous reaction system, the feed rate of the raw materials, the reaction temperature, the type of the catalyst, etc. have a great influence on the formation speed of the product. We mainly consider the use of temperature-sensitive polyurethane membranes to control the rate of addition of materials (liquid, gas materials) to control the entire reaction process. The permeation flux of the temperature-sensitive polyurethane film changes significantly with temperature. When the temperature is lower than the switching temperature, the permeation flux of the film to the material (liquid or gas) is small, the material is added slowly, and the concentration of the raw material is low. Lower, the reaction process is slower; when the temperature is higher than the switch temperature, the material (liquid or gas) has a large flux, the material is added faster, and the reaction speed is faster, thus controlling the entire reaction process [16] . In this way, according to the actual needs, by adjusting the temperature of the temperature-sensitive polyurethane film body to change the speed of the addition of a certain key material, the control of the entire reaction process can be realized.
3. Conclusions and Prospects As can be seen from the above analysis, the temperature-sensitive polyurethane has a typical block and microphase separation structure, and the phase transition temperature of the soft segment is between 0 and 60 °C. Temperature sensitive polyurethane has unique temperature response characteristics and has broad application prospects in textile materials, food packaging, agricultural greenhouse film, separation membrane, industrial controllable reactor and other fields. As a new type of intelligent polymer material, temperature-sensitive polyurethane has attracted extensive attention from scholars at home and abroad, and its research has been deepened. It is believed that in the near future, people will have a further understanding of the microstructure and temperature response mechanism of temperature-sensitive polyurethane materials. Various products made with temperature-sensitive polyurethane materials will enter people's lives in large numbers.
References: slightly

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