Polymers at Cryogenic Temperatures

Cryogenic processing is the one-time permanent treatment of the materials at very low temperatures to increase the physical and mechanical properties. Cryogenic processing is capable of treating a wide variety of materials such as metals, alloys, polymers, carbides, ceramics and composites. Cryogenic applications of polymers are not only limited to the fields of space, electrical and superconducting technology but also to other advanced technologies such as cryosurgery and cryobiology in the medical field.

The cryogenic properties of polymer materials have received great attention with new developments in space, superconducting, electronic and defense technologies as well as large cryogenic engineering projects such as International Thermonuclear Experimental Reactor (ITER), etc. Polymer materials developed for these applications are mainly employed as electrical insulators, thermal insulators, vacuum sealants, and matrix materials for composites used in cryogenic environments. The requirements are extremely severe and complicated for polymer materials in these unique applications. The polymer materials need to possess good mechanical and physical properties at cryogenic temperatures such as liquid helium (4.2 K), liquid hydrogen (20 K), liquid nitrogen (77 K), and liquid oxygen (90 K) temperatures, etc., to meet the high requirements by the cryogenic engineering applications. Herein the cryogenic mechanical and physical properties of polymer materials will be highlighted in this chapter. Cryogenic tensile properties/behaviors are first presented in some details for various neat polymers and filled polymers. Cryogenic shear strength, impact strength, and fracture toughness are then discussed. Afterwards, cryogenic thermal, creep, sliding, and dielectric properties of polymers are briefly summarized. Finally, discussions about effects of water absorption and cryogenic aging on cryogenic properties of some polymers are conducted.

Polymers are extensively used for sliding systems in cryogenic applications because of their favourable friction and wear behaviour in the absence of external lubrication. Since important new technologies are based on applications under extreme conditions, such as at low temperatures, new requirements on material properties, in particular regarding their operability and reliability, must be met. Up to now, most tribological investigations have been carried out in inert cryogens or cryogenic gas (He, N₂). Few experiments have been performed in vacuum environment at cryogenic temperatures. Rarely were testing in reactive media, such as LH₂ or LOX. Due to the wide range of operating conditions in cryogenic applications, it is difficult to state general rules. Therefore, this chapter tends to give an overview on theories and experimental studies on polymer tribology at cryogenic temperatures.

High specific strength, stiffness, excellent environmental fatigue resistance and low weight remain the winning alliance that impels fibrous composite materials into new arenas, but other properties are also equally important. Fibrous reinforced plastics (FRPs) offer good vibrational damping and a low coefficient of thermal expansion, characteristics that can be engineered for specialized applications. Commercial composites are used in large markets such as automotive components, boats, consumer goods, and corrosion-resistant industrial parts. Advanced composites, initially developed for military aerospace applications, offer performance superior to that of conventional structural metals and now find applications in satellites, aircraft, and sporting goods and in the energy sector in oil and gas exploration and wind turbine constructions. Cryogenic applications of polymeric fiber composites are mainly in superconductivity, space technology, and handling of liquefied gases. By contrast, because of the heterogeneous nature and anisotropic behavior of FRPs, a structural designer faces challenges in predicting the integrity and durability of FRP laminates during service periods. Polymer composites soften, creep, and distort when heated to high temperatures (>100 °C), accompanied by collapse of free volume as the molecular adjustments take place. This can result in buckling and failure of load-bearing composites structures. Severe environmental exposure affects the physical and mechanical properties of polymeric composite materials, resulting in an undesirable degradation and damage.

Cryogenic fuel tanks are the most common structural application of FRP at low temperatures. Expose to cryogenic temperatures can cause microcracks as well as delamination in the composites due to thermal residual stresses. These microcracks provide a pathway for the ingress of moisture or corrosive chemicals and are a possible pathway for loss of cryogenic fluids in the tanks. Matrix resins at low temperatures are brittle and do not allow relaxation of residual stresses or stress concentration to take place. At low temperatures, polymers are well below their glass transition temperature and show little viscoelastic behavior. Molecular motion of segments or side groups is still possible, but the degrees of freedom decrease with decreasing temperature. This motion influences the damping behavior of the polymers under cyclic mechanical load. If the temperature-dependent relaxation time of molecular motion is equal to the time of external deformation, maximum power dissipation occurs. Simultaneously, a change in the shear modulus is observed. The goal of this chapter is to extensively study the in-plane mechanical properties of FRP composites at cryogenic temperatures. The composites considered include carbon, glass, and Kevlar fiber-reinforced polymers with different resin matrices.

This chapter describes the results of our studies on the interlaminar delamination fracture and fatigue of woven glass fiber reinforced polymer composite laminates under Mode I, Mode II, and Mode III loadings at cryogenic temperatures. Delamination fracture tests were carried out at cryogenic temperatures, and the critical energy release rate at the onset of delamination propagation, i.e., fracture toughness, was evaluated based on a finite element analysis coupled with damage. In addition, cryogenic fatigue delamination tests were performed, in order to obtain the delamination growth rate as a function of the range of the energy release rate. After the tests, fractographic observations were made to assess the delamination mechanisms at cryogenic temperatures.

Dielectrics is ubiquitous in all electrical systems. In this chapter we introduce many aspects in a systematic manner in order for the reader to be able to follow it in a sequence. Firstly, the various media and the salient features of each are discussed. The media range from vacuum to highly compressed gases, liquids, and solids. The basic mechanism of dielectric behavior is discussed in each different case when subject to electric field. The imposed electric field can be of different forms such as steady-state AC and DC voltages. It can also be due to transients in the system brought about by switching operations or they could be due to naturally occurring phenomena namely lightning. The configuration of electrode geometries and the polarity of the electrodes bring about phenomena within the media that need to be understood in order to design electrical systems for a variety of applications. We also focus our attention particularly when the media are at cryogenic temperatures. The electronic and ionic reaction mechanics change drastically at low temperatures. In the early part of this chapter the discussion is centered around the basics such as partial discharge, electrical breakdown, dielectric losses, and permittivity. This is followed by applications of power cable design and operation at or near the boiling point of liquid helium. The reader is directed to some real life experiences of such cable systems. Once again the emphasis here has been on the dielectric aspects and the role that materials play in enabling such technology. With the advent of high temperature superconductors (HTS) the outlook was more promising as superconductivity could be achieved at around the boiling point of liquid nitrogen (77 K).

Most dielectric systems were designed with the cryogen playing a dual role of being the cooling medium and also being an integral part of the dielectric. As HTS became more widespread cold compressed gaseous helium is now considered viable for some special applications. Advances in the development of polymeric materials for cryogenic applications have largely kept up with HTS technology. However, there are problems that have to be overcome, in particular mechanical strength at low temperatures. Another problem inherent to devices such as cables and coils is winding gaps and inclusions. This presents opportunities for partial discharges to start and if not avoided leads to aging of the device and finally failure. Important dielectric properties such as permittivity and loss tangent have been discussed at some length. The measurement of these parameters not only gives their numerical values but also provides insight into the behavior of the material properties in a general sense. Composites is another area that is being actively pursued and this topic is discussed in particular the differences between micro- and nano-fillers in polymer resins. The large increase in surface area with the reduction in particle size to the nano-scale has opened up great opportunities for advancement. The interfacial region is one that holds the key to future advancement in this technology. Cryogenic nanocomposites are a fascinating technology that has opened up new horizons, and many laboratories are making great progress in understanding the behavior of these materials both theoretically and with experiments.

Cryo-treatment of polymeric hydrogels, such as poly(vinyl alcohol) cryogels (PVA-C), have long been developed for a variety of medical applications. In the case of PVA-C the simplicity of its fabrication in its most basic form, obtained through a series of freezing and thawing cycles from low to room temperatures, along with its highly tailored microstructure, soft-tissue-like mechanical properties, and excellent biocompatibility makes it one of the most promising medical hydrogels researched today. In this chapter, we discuss the properties of soft tissues, with an emphasis on human vasculature, as a preamble for our investigation of current techniques in PVA-C manufacturing. The mechanical properties of PVA-C are then presented, outlining its behavior with varying cryo-treatments and PVA concentrations. We then highlight an example of PVA-C used for a coronary imaging phantom, and lastly provide brief examples of PVA-C and its derivatives in a variety of biomedical applications.

Composites are the modern trend materials replacing monolithic metals and alloys in many industrial applications. It has also been widely used in many of engineering applications where materials are subjected to very low temperatures. Hence it finds essential to investigate its mechanical properties after cryogenic treatment of composite materials and compare the results with behavior of materials at ambient conditions. This chapter describes the preparation of glass/epoxy composites for different volume fraction. The developed specimens were subjected to cryogenic treatment with the aid of liquid nitrogen. Mechanical properties such as tensile, impact, and flexural behavior were presented separately of cryogenic treated specimens and untreated specimens. The obtained results exhibited enhanced mechanical properties.

External tanks of the spacecrafts need not only efficient, but also safe cryogenic insulation materials and the issues of their development are still urgent. At present, polyurethane (PUR) or polyisocyanurate (PIR) foams’ cryogenic insulation is widely applied in the partially reusable launch systems. Factors influencing the cryogenic resistance of external thermal insulation performed of spray-on PUR or PIR foams are characterised based on a wide literature search. They include chemical structure and macromolecules’ architecture of polymeric matrix, physical and mechanical properties of foams, thermal stability and combustibility, cellular structure of foams etc. Experimental data on physical and mechanical properties at room and cryogenic temperatures are presented for IWC developed PUR foams further named as IWC-Cryo. Technological processes of spray-on PUR and PIR foams cryogenic insulation, cryo-pumping and the main defects of external tank insulation are analysed as well as applications of foams’ insulation in space technologies (Space Shuttle, Ariane and Buran).

Cryogenic treatment (CT), a supplementary process to conventional heat treatment process, is the process of deep-freezing materials at cryogenic temperatures to enhance the mechanical and physical properties of materials being treated. CT is one of the field in which the materials to be treated play a very imperative role in its technological development. Some of its successful applications in nonferrous materials defy the conventional reasoning that the only affect it has is to convert retained austenite to martensite. For example tungsten carbide cutting tools, electronics materials, some plastics, composites, and polymers show significant improvements that cannot be supported by the conventional theories as to why the process works. CT technologies have applications in wide range of areas including cutting tools, polymers, plastics, power industry, medicine, rocket propulsion and space simulation, food processing, to name but a few of them. The execution of CT on cutting tool materials increases wear resistance, hardness, and dimensional stability and reduces tool consumption and down time for the machine tool set up, thus leading to cost reductions. Similarly, improvements in wear resistance, hardness, dimension stability, crystallinity, tensile strength and elongation have been reported for polymers subjected to CT. The effects of CT on tool materials (steels and carbides) and polymers along with their applications are reviewed for manufacturing industry in this chapter. Although it has been confirmed that CT can improve the service life of tools and polymers, the degree of improvement experienced and the underlying mechanism remains ambiguous. The steps involved in CT are critical enough to account for the significant incongruity in post-treated performance of treated materials. If we look toward the next century, the topic of the-state-of the art and future developments in CT areas has several aspects to it. Firstly it is essential to look at the history of the development of CT. It is equally important to evaluate the current status of science of CT and to identify research and development trends in this area which can act as starting point for future developments. In this chapter, an attempt has been made to present the past, present, and future of CT technology for tool materials and polymers.

As the use of polymers in structural components and power transmission systems has grown in the last decades, the demand for improved mechanical performances of these materials is increasing. Besides fibre reinforcement, the research about thermal treatments and surface coatings of these materials is focused on obtaining more strength and resistance to fatigue and wear. In this direction, a contribution can be given by unconventional treatments such as cryogenics. The use of deep cryogenic treatment (DCT) on polymers is already claimed as miraculous by some companies. The analysis of peer-reviewed literature shows that it is not all marketing: many pure and reinforced polymers have shown DCT improvements in hardness and in wear resistance under controlled experimental conditions. Starting from these results, a general overview of potential DCT applications on polymeric products is given in order to suggest further developments and research areas.