Application of Thermochromic Substances in Textile and Apparel Field

Application of Thermochromic Substances in Textile and Apparel Field

Sayak Nandi* & Piyali Mandal**

*Department of Textile Technology, Government College of Engineering and Textile Technology, Serampore, West Bengal, India

**Department of Apparel Production Management, Government College of Engineering and Textile Technology, Serampore, West Bengal, India

Introduction

When we think of coloring textile and apparel goods majorly dyes and pigments pop into our minds. Now the application of dyes and pigments are vast and depends on the substrate. However the dyes that we generally think of are monochromatic and are meant to produce sustained colouration. In this article we have tried to give an insight on a special kind of colour changing substances called “Thermochromic Substances”. This substance has limited use in Textiles; however its market is predicted to be worth $2.1 Billion by 2024 [1], so this has a potential to be an important aspect in Textile and apparel fields.


What are Thermochromic substances?

The word “Thermochromic” can be divided as “Thermal” is heat and “Chrome” is colouration. So, from the very word it can be said, these substances are related to heat coloration. Thermochromic substances developed in the 1970s [2] are substances which show reversible transition from colour to colorless due to change in temperature [3]. The change in temperature may occur due to various stimuli like sunlight, ultraviolet irradiation [4]. There are mainly two types of materials used to produce thermochromic effects, namely liquid crystals and organic dyes called leuco dyes.

Whenever we talk about Thermochromic substances, Photochromic substances are also referred. However, Photochromic substances show coloration due to change in intensity of light [4] or frequency of incident light [5] or electromagnetic radiation [6]. If the wavelength of light absorbed is suitable then, reversible color change occurs from colorless or light shade to a different color [4]. This is the main difference between photochromic and thermochromic substances.


Liquid Crystals

The general mechanism of Liquid crystals, showing thermochromic effect is shown in fig. 1.


Fig. 1: Mechanism of Thermochromic Liquid Crystals [7, 8]


Leuco Dyes

A leuco dye composite consists of 3 parts the dye, colour developer and an organic solvent. The dyes are electron donors, and the developers are proton donors which is a main component for the coloration. For instance Crystal violet lactone (CVL) has a lactone ring which is colorless in normal conditions. When it comes in contact with a developer (proton donors), it is protonated giving it blue/violet colour at a high temperature [9]. Even certain dyes undergo keto-enol tautomerism, leading to increase in conjugation, forming chromophore [10]. In a research the mechanism for 6′-(Diethylamino)-3′-methyl-2′-(phenylamino)spiro[isobenzofuran-1(3H),9′-[9H]-xanthene]-3-one (ODB) leuco dye and N-(4-hydroxyphenyl)docosanamide (PA21) developer is shown, it is given in Fig. 2 [11].



Fig. 2: Mechanism of reversible coloring/decoloring reaction of PA21/ODB mixture. [11]

Application of Thermochromics in Textile

Camouflage Application for military:


Fig. 3: Adaptive Camouflage (Warm, left; Cold, right) [12]


  • In the figure Camouflage used is a dessert Camouflage. Here the thermochromic inks are microencapsulated into the fabrics. And at transition temperature of 36.6°C (Acceptable) the colour of the inks changes to warm thus forming a color quite similar to dessert. [12]
  • Camouflage suit for radiant flash protection. In this when the flashlight arrives, the dyes absorbs energy and gets heated, turning colorless [10]
  • Camouflage nets are meant to conceal military equipments and objects. Generally two terrains are considered one for green vegetation and second one for the dessert region [10]. Depending on surrounding temperature the color of camouflage nets differ, providing camouflage according to terrain [10].
  • It is also used for manufacturing camouflage defense equipments like body armors, helmets etc. [13]

Smart and fashionable clothing and accessories

Thermochromics can be applied to textiles fabrics using dyes directly by exhaust method or continuous method, screen printing, microencapsulation are some techniques for application in fabrics, even it can be applied in core-spun yarns where core fibre contains microencapsulated thermochromic materials[4] . However for repetitive color changing microencapsulation is preferred Fig 4. For end use purpose it is used in T-shirts, pants, children’s clothing, curtains, wall cloth, toys [14], cosmetics [15], umbrella [10], bag packs, sling bags [16] and caps ,etc. for producing aesthetic functional design [4]. Due to the dynamic color changing effect these materials are in line with the demands of modern customers.

Researchers like Kooroshnia have extensively worked on different complex and dynamic designs using thermochromic inks [17], it has also been used in color changing patterns [18].

Fig. 4: Appearance of thermochromic mixture and microencapsule in revisable color changed process [14]

Fig 5: Application of Thermochromic substances (a) Thermochromic Designs [18], (b) Thermochromic dresses [16], (c) Thermochromic Yarns [16], (d) Thermochromic sport dresses [16]

(a)                                                        (b)


(c)                                                          (d)

Sport-Tech application

Thermochromics are also used in sport-tech applications; it is used in swimming suit, leisure sport clothing [14] shown in fig 5(d). In a study thermochromic pigments were applied through surface treatment; it was used for visually signaling physical exhaustion due to high sport application [19].


Conclusion:

The application of thermochromic substances is not known wide spread in textile and apparel sectors. One of the main reasons is for major thermochromic substances the activation temperature lies within 10°-20°C, which is considered room temperature [4]. Moreover the temperature sensation of different materials varies (sensation of leuco dye is much less than liquid crystals). Also the material is expected to undergo multiple cycles of coloration and decolouration producing same bright shade. Due to all these major drawbacks the application is limited, however with companies like Kolorjet (India), Olikrom (France), LCR Hallcrests (USA), New Prismetic Enterprise (Taiwan) etc. producing thermochromic materials and with recent technologies it might be possible this might gain more application in textile and apparel sectors in future.

References


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5 Comments

  1. It's very helpful.. Good information

    ReplyDelete
  2. Very interesting and informative article.

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  3. Well informative and interesting.

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  4. This is a document with lot of technical insights. Interesting and eye opener for the limitless possibilities in the field of apparel.

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  5. Rohit kumar DebnathJuly 27, 2020 at 3:35 AM

    Great work .
    Waiting to get such article in future...

    ReplyDelete