Textile finishes generate value through perceptive and protective effects, points out Dr Ashok Athalye.
The way a textile is finished can change its personality. Conventional textile finishes help modify basic handling of the fabric and provide surface softness or stiffness. Functional effect finishes impart comfort and performance properties like managing moisture management, retarding flame, repelling oil and water, protecting from sun and improving hygiene. Functional finishes also correct deficiencies or introduce specific properties. Textile substrates which are deficient in some properties or where some improved properties are desired are treated with specialty effect finishes. So, conferring desired effects, meeting end-use expectations, ensuring sustainable ECO norms and enhancing value for money are major considerations for such perceptive and protective finishes.
Functional finishes are categorised as:
1. Sensory perceptive - As the name suggests, these impart effects which impact senses such as touch, smell, visual appeal and offer physical comfort.
2. Protective and repelling - effects that can shield the wearer from external factors
Skin care - rejuvenating
Health care - antimicrobial
Aroma - fragrance
Fire protective - flame retardant
Sun protection - UV absorbers
Soil free - anti-static
Stain free- oil and water repellent
Quick dry - moisture management
Wrinkle free - resins
Selection criteria - depends on desired functional effect and various factors such as
i) Intended end-use
ii) Desired effect - either singular or multiple properties
iii) Regulatory norms - Ecological, toxicological, sustainable
iv) Requirements of international brand
v) Fastness testing and evaluation methods
vi) Washing method and durability expectations
Further, there are regulations for environment protection, liquid effluent discharge, landfills and safeguarding human health which the textile material is expected to maintain.
Each treatment has its own application. This could be padding through low liquor ratio or spraying with suitable equipment or coating to achieve different effects on two sides of a fabric with two different finishes (moisture management on inner side and water repellant on outer side). Some are put through micro-encapsulation or active ingredient application while others go through multi layer or layer on layer application or even electro-chemical deposition.
1. Skin care
Finishes can generate a feel-good factor by providing skin nourishing chemicals through textile. These chemicals provide various effects such as moisturising during dry or low humidity conditions, antioxidants like Vitamin E to overcome wrinkle formation or delayed ageing.
Likewise, various chemicals are applied as textile finishing agents for effects such as anti-inflammation, improved blood circulation, soothing and rejuvenating. The transfer and percolations of such chemicals into the blood through the pores of the skin benefits the wearer because of their anti-oxidant effect.
Vitamin E and F - protects skin against premature ageing
Aloe vera gel - moisturises skin
Jojoba oil - improves softness of skin
Avocado oil - improves comfort
Their performance is evaluated by commercial testing methods such as qualitative assessment and ratings by a panel and quantitative evaluation by mass spectroscopy and scanning electron microscopy.
2. Health care
Anti-microbial finishes are applied to improve health, hygiene and freshness of textiles. These are used especially on undergarments in direct contact with the body and which have high probability of microbial growth due to warmth and sweat generated by the body. To have control over growth of the microbes, the material is treated with specialty chemicals. Antimicrobials are defined as bacterio-cidal - that kills bacteria -- or bacterio-static - that only inhibits further growth.Antimicrobial effect finishes exhibit:
Effective control of undesirable microorganisms
Non-toxicity to human
Durability to washing, laundering and dry cleaning
Compatibility with other finishing agents
Simple application and evaluation
Products belonging to various chemical compositions such as triclosan, chitosan, quaternary ammonium compounds, silver halides which have antimicrobial mechanisms are available in the global market. The most popular method of evaluation of effect of antimicrobial finish are:
(i) Quantitative assessment - AATCC 100 test method - counting bacterial growth percentage
(ii) Qualitative assessment - AATCC 147 - parallel strip for estimating zone of inhibition
Some other test methods are also followed based on specific end-use requirement.
Table 1 - List of anti-bacterial test methods
3. Aroma Sensual
The wearer and those in close proximity enjoy unique benefits from aromatic chemicals and fragrances that impart a pleasant smell. These stimulate different moods like relaxation and encouraging sleep.
The chemicals are volatile. Fragrance is transported to and detected by the olfactory system in the upper part of the nose which transmits the stimuli to the brain where it is perceived as a pleasant odour.
The pleasant smell tends to elicit a variety of feelings and emotions such as relaxation, happiness and well-being. Various essential oils like lavender, rosemary, and jasmine were used in this finish. These are used in home textiles such as bed linen, pillow covers, bed sheets to keep them fresh even with everyday use. Also, aroma compounds infuse a feeling of well-being and freshness. Aroma fabrics have several uses in medicine and alternative healing.
The following table gives a list of those aroma chemicals.
Table 2 List of aroma-imparting chemicals
Fire protective finishes make textiles flame-resistant. They minimise risk of a fire starting in case of contact with a small heat source. The fabric does not ignite or the process is delayed. When a fire starts, retardants reduce spread of flames and rate of fire spreading. Fire retardants interfere with combustion at different stages of the process like during heating, decomposition, ignition or spreading of the flame.
Increase decomposition temperature
Decrease the amount of combustible gases
Promote char formation
Prevent access of oxygen to the flame
Increase combustion temperature
Commonly used chemicals include boric acid or borax, tetrakis (hydroxymethyl) phosphonium chloride (THPC) and phosphoric acid derivatives. Flame-retardant fibres confer high comfort and safety to textiles and are, thus, preferred for use in garments and in decorative fabrics and home textiles.
Testing is crucial for compliance to fire-safety laws and standards. Some common test methods are:
Limiting Oxygen index
Vertical and horizontal flame test
Char length measurement
Specified ASTM, JISL tests
Limiting oxygen index (LOI) is the minimum concentration of oxygen expressed as volume per cent in a mixture of oxygen and nitrogen that supports flaming combustion of a material initially at room temperature.
UL-94 vertical burning test - provides a preliminary assessment of relative flammability and dripping. It addresses such end-use characteristics as ease of ignition, burn rate, flame spread, fuel contribution, intensity of burning and products of combustion. In the test, a bar measuring 5 x 0.5 inches is mounted vertically in a draft-free enclosure. A burner is placed beneath the sample for 10 seconds and the duration of flaming is timed. The test is repeated for five specimens. Any dripping that ignites surgical cotton placed 12 inches below the bar is noted.
5. Oil and water repellant
The effect finishes that prevent liquids from wetting the surface and repel water, oil and release dry dirt are important. Release of soil and preventing its re-deposition improves aesthetic appeal of textiles and also makes it more functional when the garments are worn in rain. Such water-repellent fabrics permit passage of water and are resistant to penetration of water under much higher hydrostatic pressure.
Water repellant finishes resist penetration of water into the fabric but permit passage of moisture or air. The fabric is coated with the repellant chemical which permits the passage of air and vapour between the interlacing in fabric. Water and the other liquid remain on the surface in small beads rather than spreading out and getting absorbed. This finish
Prevents oily fluids from wetting treated textiles
Protects textiles from both dry and wet soil
Reduces free energy at fibre surfaces
Minimises internal cohesive interaction of the liquid
Exhibits low interactions with liquids and fibre interface
Specialty chemicals including paraffin wax emulsion, pyridinium compound, steramides, poly siloxanes, acrylates, fluro-chemicals and dendrimers are widely used as water repellants.
Table 3 - Test methods and evaluation of water and oil repellants
6. Antistatic finish
Static electricity is produced or created when two non-conducting surfaces such as synthetic textiles rub together. The two surfaces become oppositely charged and an electrical charge builds up. The wearer can experience electric shocks and the fabric clings to the body. Anti-static finishes are chemical substances applied to reduce and eliminate static charge. They absorb moisture from the atmosphere and reduce dryness of the fabric that causes static charge build up. These finishes
Avoid charge creation upon rubbing
Provide wicking effect
Impart soft handle
Improve fat and soil release
Avoid impairment of whiteness
Table 4 - Common evaluation methods
7. Moisture management
The ability of a garment to transport moisture away from the skin is one of the key performance enhancement criteria in modern sports and active outdoor wear. This phenomenon is termed moisture management. Various factors including the physio-chemical characteristic and construction of the textile coupled with application of different type of finishing chemicals help impart this effect.
Generally, the hydrophobic textiles repel moisture and the insufficient transport of moisture leads to damper feeling, restricting evaporative cooling. Whereas, the hydrophilic textiles have more moisture absorption, but at the same time retains moisture due to slower drying and leads to clinging and sagging. However, a combination of inner hydrophobic and outer hydrophilic textile can provide desired rapid transport of moisture from skin and then evaporated from the outer surface.
Such finishes are used in combination to impart additional desirable properties like antibacterial, antistatic and soil release effects. They control moisture balance between the garment and skin and keep body temperature constant by evaporating moisture both from the skin and from outside of the textile material. The schematic mechanism of moisture management is given below.
The common methods of determination and evaluation are:
Wicking- vertical wicking test (BS 3424 or AATCC 197:2011)
Water Absorbency (AATCC 79: 2000)
8. Thermal protection
Textile material capable of absorbing or releasing heat energy can provide desired thermal protection under adverse climatic conditions. The chemicals that provide such effect are called phase-change material. Such chemicals have the ability to change their state with a certain temperature range. These materials absorb energy during the heating process as phase change takes place and releases energy to the environment during a reverse cooling process.
These chemical finishes have the capability of melting during heat absorption and crystallising during heat release, maintaining temperature of the wearer constant under varying surrounding conditions. Certain specialty paraffin microcapsules are applied on the textile substrate for such thermo-regulating and temperature controlling effects.
Sunlight consists of harmful ultra violet radiation in the wavelength region of 150 to 400 nm, which can result in acceleration of skin ageing, photodermatosis (acne), erythema (skin reddening), sunburn, melanoma (skin cancer) on prolonged exposure. Though the intensity of UV radiation is much less than that of visible or infrared radiation, the energy per photon is significantly higher.
When radiation strikes the surface of a fibre, it can be reflected, absorbed, transmitted through the fibre or pass between fibres. The relative amounts of radiation reflected, absorbed or transmitted depend on many factors, including fibre type, fibre surface smoothness, fabric cover factor and the presence or absence of fibre delustrants and dyes.
Therefore, to protect human skin and other body parts from harmful UV component of natural sunlight, the material needs to be finished with an effective UV absorber which absorbs UV radiation and then dissipates the absorbed energy to avoid degradation of fabric and detrimental effect on the wearer.
Sun protection factor or UV protection factor is used as a measure of how well a UV absorber will protect skin from harmful rays. While skin normally burns after 10 minutes in the sun, finish chemicals with SPF 15 allow you to stay in the sun without burning for approximately 150 minutes (a factor of 15 times longer). For best protection, a minimum SPF of 30 is desired. Another method of estimation of sun protection is Erythema Damage Factor (EDF) where the chemical is applied on the skin of test animals and the extent of skin reddening is noted after specified time and the effect is compared.
Storage and disposal of functional finishes
Storage conditions for such functional finishes are generally specified by the manufacturer or supplier of these chemicals. They vary from product to product and according to chemical composition or concentration. In general, most chemicals are recommended to be stored in a cool and dry place maintaining below conditions for a shelf life of about one year.
- Relative humidity - 65 %
- Temperature - 20 /- 20C
The disposal mechanism and conditions vary and depend on specific chemical composition and is specified by the manufacturer and provided in the material safety data sheet (MSDS). This contains information on precautions to be ascertained during handling, safety and storage of these products.
With growing awareness, user expectations are evolving. To meet this challenge, specialty chemicals are being developed to impart specific performance characteristics. Major areas for clothing and home textiles are comfort, easy care, health and hygiene while such textile material also needs protection against thermal, mechanical, chemical or biological attacks. Thus, such perceptional and protective finishes are a combination of many functions.
About the author:
Dr Ashok Athalye is from Technical Service, Atul Ltd-Colors Business, Valsad, Gujarat.