Preparation of Shape Memory NiTiNOL Filaments for Smart Textiles

A nickel-titanium alloy (NiTiNOL, nitinol) fi lament with a diameter of 200 μm was used for preparing a smart knitted textile fabric with a shape memory eff ect within the range of human body temperatures. The annealing of the fi lament at 500 °C for 30 minutes was followed by air cooling at 20 °C to achieve a suitable transition temperature from martensite to austenite phase within the range between room temperature and 75 °C. The tensile properties of fi laments before annealing and after it were analysed on Instron 6022 dynamometer. The measurements of exact transition temperatures from soft martensite state at room temperature to hard austenite state at heating were made on a dynamic mechanical analysis instrument. From the annealed fi lament a left-right knitted fabric was hand made. The fabric was trained into a selected 3D form by cyclic heating in a strained form at 75 °C for 10 minutes and then cooled at room temperature. For a stable two-way memory eff ect, the nitinol fabric needed to make fi fteen cycles of heating and cooling.


Introduction
NiTiNOL (nitinol hereaft er) is a shape memory nickel-titanium alloy, which can change its shape in a wide temperature range, from -50 °C to +166 °C [1].It is one of rare shape memory alloys which with certain chemical composition show a shape memory also within the range of human body temperatures.In comparison to other shape memory materials, like shape memory polymers, nitinol distinguishes itself by better stability of shape memory [2] and super-elastic properties with up to 8% elastic deformation [3].Nitinol alloys are the most often used shape memory alloys in the area of smart textile materials, especially in clothes.Th e ability to memorize a desired shape is related to nitinol special crystalline structure, to its austenite and martensite crystal phases, which under the infl uence of temperature changes, heating or cooling, or stress changes can reversibly convert from one phase to another [4].Temperatures at which the conversions happen depend on the chemical composition and the annealing process of nitinol.Transition temperatures are described: by a temperature at which the transformation from martensite to austenite phase starts (A s ), by a temperature at which this transformation is complete (A f ), by a temperature at which the transformation of austenite to martensite phase starts (M s ) and, fi nally, by a temperature at which this transformation is complete (M f ) (Figure 1a) [4].Although numerous studies and patents of nitinol alloy for diff erent uses were published in last decades, commercially successful products are today still relatively low, mainly due to the high price of nitinol and Preparation of Shape Memory NiTiNOL Filaments for Smart Textiles the complexity of nitinol alloys processing to achieve desired thermomechanical properties.Th e use of nitinol is meaningful primarily in the areas where traditional materials do not provide adequate solutions.Textiles are fl exible materials which can easily change shape and can also follow the forms of embedded shape memory fi brous materials.Beside the unique uses of nitinol for kinetic garments by designers [6,7], some interesting functional prototype solutions have been developed, like NiTi micro-hook in Velcro fasteners [8], elastic compression knitting [9], smart curtains [10,11], where nitinol fi laments were integrated into woven or knitted fabric structures and trained into desired temporary forms.Figure 1b showes an example of a shape changes diagram of a fabric with embedded trained nitinol fi ne fi laments.At room temperature (point a2) the fabric is soft and deformable while the nitinol fi laments are in a multiple variant of twinned martensite phase.Loading the fabric by stretching, shrinking, bending or folding etc. causes macroscopic wrinkling of the fabric which is accompanied with structural changes in the crystalline structure of nitinol fi laments into a single variant of detwinned martensite state (point a3 and a4).Aft er unloading, the fabric preserves the macroscopic form and soft touch.Th ere is no conversion to multiplevariants and only a small elastic strain is recovered, leaving the material with a large residual strain [5].Th e structure of nitinol fi laments stays in detwinned martensite state (point a5).Heating the fabric up to a certain temperature causes that the fabric changes its shape into a preprogramed (unwrinkled) form (point a1/a6), which is followed by the changes into the austenite crystal microstructure.Cooling the fabric to room temperature leads to transition from hard and rigid austenite phase to soft twinned martensite state (point a2).Th e area of using nitinol fi bres in textiles is nowadays still largely restricted to unique uses.Th e main problems are the complexity of annealing nitinol fi laments to set proper transition temperatures, the training process to set temporary desired shape and the problems connected with the integration of nitinol fi laments into fabrics when using existing weaving or knitting machines.Th e basic concept of the doctoral research, part of which is presented in this article, is the study of a weft knitted fabric made from 100% nitinol fi laments, which would be inserted as interlining into a garment to create an air gap for increasing thermal insulation properties of such garment.In the article, properties of cold drawn and annealed nitinol fi laments are compared beside the presentation of preparing a weft knitted fabric in its permanent and temporary shape memory forms.

Experimental
In the research, a cold worked nitinol fi lament with a diameter of 0.2 mm (Fort Wayne Metals, Ireland) with the characteristics listed in Table 1 was used.

Results and discussion
Th e purchased cold worked nitinol fi lament was characterised by its mechanical properties and a range of austenite temperatures (Table 1).For being used in smart textiles as shape memory material, the worked cold nitinol fi laments were fi rstly annealed to set proper transition temperatures.For NiTi alloy application the transition temperature should be within the range slightly above the temperature of a human body.DSC thermograms showed the transition temperatures (M s , M f , A s and A f ) of annealed nitinol fi laments.Th e most proper annealing temperature for the intended use of nitinol fi laments was at 500 °C, where nitinol fi laments existed in a soft pure martensite phase at room temperature and in a stiff pure austenite state at 75 °C (Figure 3).Th e tensile properties of the cold worked and annealed nitinol fi laments at 500 °C were determined at room temperature (20 °C) and at 100 °C (Figure 4): the average measured breaking force of the cold worked nitinol fi laments at 20 °C was 55.2 N at average breaking elongation 5.1% (Figure 4a), which is consistent with the declared data given in Table 1; the average measured breaking force of the an-nealed nitinol fi laments at 20 °C was (Figure 4b) 34.0 N at average breaking elongation 7.6%; the breaking force of the cold worked nitinol meas-ured at 100 °C (Figure 4c) showed a little lower average breaking force of 50.9 N at average breaking elongation of 4.5% than at room temperature; the average average breaking force of the an-nealed nitinol fi laments at 100 °C (Figure 4d) was 33.3 N at average breaking elongation of 12.2%.From the stress/elongation diagrams of cold worked and annealed nitinol fi laments (Figure 5) the comparison of their tensile properties in the whole deformation range can be made: a cold worked fi lament shows a similar tensile be-havior at 20 °C and 100 °C up to 2% elongation and a little higher tensile stress and elongation within the range between 2% and breaking elongation at 20 °C in comparison with 100 °C.Both curves are very steep with distinct elastic behavior.Th e curves demonstrate a similar microstructure of the cold worked nitinol fi lament at 20 °C and at 100 °C; the shape of the stress/elongation curves of an-nealed nitinol fi lament diff ers substantially from the shape of the curves of cold worked fi laments and is typical for shape memory nitinol alloys.At 20 °C, the annealed material is in martensite state.
Th e curve has a short elastic deformation, and shows no pseudoelasticity.Th e annealed nitinol fi lament at 100 °C is in austenite phase.On the stress/ elongation curve it shows a more pronounced elastic area, and extremely higher modulus of elasticity of 60.4 GPa as the annealed fi lament at 20 °C that is only 8.95 GPa.Th e breaking stress of the annealed nitinol fi lament at 100 °C is 1.06 GPa which is for about 38% higher than for annealed fi lament at 20 °C, with the breaking stress of 0.656 GPa.Th e annealed nitinol fi lament is at 100 °C in a pseudoplastic state in the elongation range of 1−8%, where the stress-induced austenite to martensite transformation occurs.It is followed by a plastic deformation that leads to the breaking of the fi lament.From the results of the DMA measurements the transition temperatures, storage modulus (E'), loss modulus (E'') and tangent delta (tgδ) depending on the heating temperature and the frequency (ν) have been detected (Figure 6): storage modulus (E') of the cold worked nitinol fi l-ament (Figure 6a) decreases with heating.Th e value of 28.44 GPa at temperature of 11 °C monotonously decreases to a value of 26.29 GPa at 98 °C.
Th ere is no heat loss in the absence of relaxation passages, that is why the storage modulus (E') remains within the range of 0.2 GPa (0.213− 0.205 GPa).Even the tangent delta (tgδ) curve, which illustrates the internal movements and damping, shows an extremely small value, about 0.007 (0.007−0.009), as there is no relaxation oscillations and displacements.On the curve of length changes in dependence of temperature spontaneous fl at shrinking of the sample is seen: -0.029 mm, or 0.38% to a temperature of 101 °C; according to the data of the manufacturer (Table -1), the annealed nitinol fi lament has transformation temperatures from martensite to austenite phase between 40 °C and 80 °C.Th is transition is seen on Figure 6b   From the cold worked nitinol fi lament, a hand weft knitted fabric was prepared (Figure 7).For the purpose of annealing and training of the knitted fabric, a special metal prefabricated mould was prepared from a stainless steel and aluminium.Before annealing, the fabric was clamped in a mould framework in a fl at state without a pre-stress (Figure 8a).Th e annealing in a furnace at 500 °C for 30 minutes was followed by cooling in the air at temperature below 22 °C for 20 minutes.Th e annealed fabric was trained to achieve a two-way shape memory.Th e training process covered heating of annealed fabric at 75 °C in an oven for 10 minutes with a subsequent cooling in the air below 22 °C for 20 minutes.Aft er repeating this procedure several times (more than 10-times), the material got a two-way shape memory: it took a temporary form at heating and returned to a permanent form at cooling.Th e knitted fabric was trained to a temporary three-dimensional half-sphere form (Figure 8b) and into a fl at permanent form (8c). Th e most important properties of nitinol alloy for integration in textiles and suits are high enough fi neness and related fl exibility, abrasion and fatigue resistance and rate of shape changing.Due to the high price of shape memory alloys, only unique products and prototypes have been developed until now.
In the study, we successfully trained the nitinol fi laments and prepared a functional knitted fabric to be potentially useful as interlining in a personal protecting suit that could dynamically regulate a thickness of air layer to protect a body from high environment heating and feel comfortable in a suit at normal environmental temperatures.

Figure 1 :
Figure 1: Shape memory eff ect: (a) diagram of crystal phases transformations of nitinol alloy by temperature and stress [5], (b) an example of shape changing of smart textile fabric with Nitinol fi laments as function of stress (σ), temperature (T) and deformation (ε)

Figure 3 :
Figure 3: Transition temperatures of nitinol fi laments versus annealing temperatures

Figure 5 :
Figure 5: Stress/elongation curves of cold worked and annealed nitinol fi laments at 20 °C and 100 °C between 60 °C and 80 °C.Th e value of storage modulus (E') of 30.38 GPa is reducing by applying heat up to the 50 °C and then continuously increasing due to the transformation of a less regulated monoclinic cell crystal structure of martensite phase in a highly regulated cubic crystal structure of austenite phase.Due to increasing regulation of the crystal structure of austenite, the stress and the storage modulus (E') for the same deformation are higher: up to 100 °C the storage modulus (E') rises up to 59.87 GPa.Th e maximum transition temperature is at 66.95 °C.Within the temperature range between 60 °C and 80 °C the storage modulus increases from 30 GPa to 56 GPa; loss modulus (E'') also detects the transformation of martensite to austenite phase at temperature of 58.96 °C: this is the largest heat energy dissipation at the crystal lattice transformation; a) cold worked b) annealed

Figure 6 :
Figure 6: Dynamic mechanical behaviour of cold worked (a) and annealed (b) nitinol fi laments

Figure 7 :Figure 8 :
Figure 7: A knitted fabric hand-made from a nitinol fi lament of a diameter 0.2 mm