UV Protection Finishing

UVR:

UV rays are a type of electromagnetic radiation, with wavelengths longer than those of soft X-
rays and shorter than those of visible rays. Of all the light projected onto the earth by the sun,
only 47% reaches the earth’s surface. Of the remainder, 34% is reflected by the atmosphere and
19% is absorbed. There are three categories of UV radiation:

 UVA: 320 - 400 nm

 UVB: 280 - 320 nm
 UVC: 100 - 280 nm

UVA (320400 nm) UVB (280-320 nm) UVC (100-280 nm)

98.8% of the UV that reaches 1.1% of the UV that reaches Almost none reaches the
the ground. the ground. earths surface.
Penetrates ozone layer. Mostly absorbed by ozone in Absorbed by ozone at
Penetrates buildings through the stratosphere; damaged altitudes.
windows, sunlamps and UV ozone layer has resulted in
lamps have UVA. increased levels.
Can cause obvious harm to the
human body.

Effect of UVR:

Benefits for Human:

1. A certain level of UV radiation is needed for human existence and life on earth. UV rays
synthesize vitamin D, which prevents rickets.
2. Humans also use UV radiation for sterilization and disinfection.
3. An appropriate amount of sunbathing promotes blood circulation, invigorates metabolism and
improves resistance to various pathogenic bacteria.

Risk for Human Skin:

1. Both UVA and UVB rays can cause permanent changes to the skin. Packed with energy,
the UVB rays are responsible for the tanning effect on skin.
2. However, these rays are also responsible for burns and the degradation of fibres in the
dermis can also cause skin cancers by destroying chromosomes.
3. Radiation at shorter wavelengths of 290–320 nm, designated as UVB, causes damage at
the molecular level to the fundamental building block of life, DNA.

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Countries risks of high UVR:

Australia
New Zealand
USA
Switzerland
Norway
Scotland
Britain
Scandinavian countries etc.

The Global Solar UV Index (UVI)

The UVI is a simple measure of the UV radiation level at the Earth's surface. It has been
designed to indicate the potential for adverse health effects and to encourage people to protect
themselves. The values of the Index range from zero upward and the higher the Index value, the
greater the potential for damage to the skin and eye, and the less time it takes for harm to occur.
While the levels of UV radiation vary during the day, they reach a maximum around mid-day.
The UVI is usually presented as a forecast of the maximum amount of UV radiation expected to
reach the Earth's surface at solar noon. In countries close to the equator, the UVI can reach up to
20. Summer-time values in Northern latitudes rarely exceed 8.

The following harmonized exposure categories and colours are associated with various values of
the UVI:

UV Index Exposure risk Sun bask limit Protection method

0-2 Light Cap or umbrella
3-4 Low Cap or umbrella
5-6 Moderate 30 min Cap or umbrella
Sunblock
Sunglasses
Stay in cool places
7-9 Excessive 20 min Cap or umbrella
Sunblock
Sunglasses
Keep staing in cool places
Long sleeve clothing
Stay indoors from 10:00 am to
2:00 am
10-16 Dangerous 15 min Cap or umbrella
Sunblock
Sunglasses
Keep staying in cool places
Long sleeve clothing
Stay indoors from 10:00 am to
2:00 am

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Solar Protection Factor:

To quantify the protective effect of textiles against UVR, the solar protection factor (SPF) is
determined. The SPF is the ratio of the potential erythemal effect to the actual erythemal effect
transmitted through the fabric by the radiation and can be calculated from spectroscopic
measurements. The larger the SPF, the more protective the fabric is to UV radiation. In Europe
and Australia, the SPF is referred to as the ultraviolet protection factor (UPF). The SPF is also
used with so-called ‘sun blocking’ skin creams, giving a relative measure of how much longer a
person can be exposed to sunlight before skin damage occurs. Typically, a fabric with an SPF of
> 40 is considered to provide excellent protection against UV radiation (according to AS/NZS
4399: Sun protective clothing – Evaluation and classification, Standards Australia, Sydney)

Table: Grades and classification of UPF

UPF Transmission (%) Classification Grade

>40 <2.5 Excellent Protection III
30-40 3.3-2.5 Very Good Protection II
20-29 5.0-2.4 Good Protection I

A UPF of 20 means that 1/20th, i.e. 5%, of the biologically effective UV radiation striking the
surface of the fabric actually passes through it

Mechanism of UV protection

When radiation strikes a fibre surface, it can be reflected, absorbed, transmitted through the fibre
or pass between fibres (Fig.). The relative amounts of radiation reflected, absorbed or transmitted
depend on many factors, including the fibre type, the fibre surface smoothness, the fabric cover
factor (the fraction of the surface area of the fabric covered by yarns) and the presence or
absence of fibre delustrants, dyes and UV absorbers.

Factor Affecting UV protection of textile materials:

1. Nature of Fibre:

UPF is strongly dependent on chemical structure of fibres. Natural fibres like cotton, silk, wool,
have lower degree of UVR absorption than synthetic fibres Grey cotton provide high UPF due to
natural pigment while bleached cotton provide less.

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 2. Moisture & Swelling:

In case of moisture, the influence largely depend on the type and hygroscopicity of fibres and the
conditioning time. Relative Humidity & moisture content affect UPF in two ways-
i) The swelling of fibres due to moisture absorption which reduces interstices and
consequently the UV transmittance.
ii) On the other hand, the presence of water reduces scattering effect as the refractive
index of water is closer to that of textile polymer and hence a greater UV
transmission & a lower UPF.
A typical cotton fabric transmit 15-20% UVR rising to moisture that 50.5 if it is wet.

3. Fabric Construction:

UPF increases with fabric density & thickness for a similar weave. The relative order of UV
protection is given by Cover% > Fibre Type > Fabric Thickness. According to 3rd Asian
Conference 1995 “Sun protection of apparel textile” Woven fabric has more UPF than knit fabric
due to type of construction. But knitted fabric made from synthetic blend fibres with stretch yarn
(e.g. Lycra) offer better protection.

Cover% UPF
99. 5 200
97. 5 40
95 20
90 10

4. Colour / Shade

 The colour or shade of a fabric is a very significant factor in preventing UV radiation

transmission through a textile.

 For fabrics of identical weight and construction, darker coloured fabrics show higher
UPF ratings than lighter shades.
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  In a subsequent report, the spectral transmission of fabrics dyed in different colours was
illustrated (Fig.) to show that darker colours had a UPF rating of 50+.
 Dyes with red hues (CI Direct Red 28 and CI Direct Red 24) provided greater UV
protection than a dye with a yellow hue (CI Direct Yellow 28). CI Direct Black 38
provided greater protection than all other dyes.
 It must be remembered that while colour can dramatically increase a fabric's protective
ability, the dyestuffs responsible for the protection must be colourfast to washing,
perspiration, sunlight, crocking, and bleaching for the life of the fabric.

5. Presence of OBA
 OBA can improve the UPF of cotton and cotton blends, but not of fabrics that are 100%
polyester or nylon.
 Another limitation of many OBAs is that they mostly absorb in the UVA part of the day
light spectrum but have a weak absorption in UV absorption around 308 nm, which plays
an important role in skin disease.

6. Additives and UV protection

 Undyed fibres, be they natural fibres such as cotton, linen and silk or manufactured
fibers such as viscose rayon, nylon and acrylic, offer little protection against UV
radiation. It is necessary for these fibers to be treated with additives in order to improve
their UV protection abilities.
 The simplest type of additive, albeit a very effective one, is the addition of the delustrant
pigment TiO2 which acts as an UV absorber.
 Since TiO2 is incorporated during fiber manufacturing the effect is permanent.
Treatment with ultraviolet absorbers (UVAs) is another efficient method of increasing
UPF.
 UVAs are colorless compounds with chromophore systems that are effective absorbers in
the UV band (290±400 nm).
 UVAs can be applied to fabrics by either exhaust or pad batch or continuous processes.

7. Stretch and UV protection

 When a textile material is stretched, the porosity increases accompanied by a decrease in
fabric thickness thereby allowing increased levels of UVR transmission.
 Under significant stretch the decrease for Lycra was an order of magnitude from UPF 200
to UPF 20.

Effects of laundering on UV protection

 The process of laundering led to compaction due to shrinkage presumably decreasing
porosity and hence resulting in an improvement in UV protection.
 Stanford et al. subjected five jersey-knit cotton T-shirts to a total of 36 washes. Fabric
UPF was measured after one wash and after 36 washes. Mean fabric UPF approximately
doubled for all T-shirts after the first wash and further additional change after another 35
washes was not significant.

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Chemical Finishing Process to improve UPF of Textiles

1. Organic and inorganic substances with UV absorbing characteristics

2. Application of fluorescent whitening agents (FWAs)
3. Application of polymeric coatings
4. Application of Dyes that act as UV absorbers
5. Application of nanolignin on UV blocking

1. Organic and inorganic substances with UV absorbing characteristics

UV absorbers include all organic and inorganic compounds that preferentially absorb UV
radiation. These compounds have negligible absorption in the visible region and consequently a
high light fastness. UV absorbers have to be distributed monomolecularly in the substrate for
maximum effect. In addition, they should meet other criteria such as:

i) An UV protection finish include efficient absorption of UV radiation at 300–320 nm,

quick transformation of the high UV energy into the vibration energy in the absorber
molecules and then into heat energy in the surroundings without photo degradation.
ii) Convenient application to textile fibres.
iii) Should be heat stable and compatible with other additives in the finish formulation
iv) Should be non toxic and non- skin irritant.

Structures of UV absorbers for synthetic fibres

2. Application of fluorescent whitening agents (FWAs)

 FWAs are applied to fabrics during laundering to enhance the whiteness of textiles by
inducing fluorescence by UV excitation and visible blue emission.
 Most FWAs have excitation maxima in the range of 340–400 nm, and have been known
to improve the UPF of textiles.
 However, the efficiency of FWAs can be affected adversely by use of some UV blocking
agents, depending on the respective absorption patterns of the two compounds.

3. Application of polymeric coatings

 In recent years, several Japanese companies have introduced specialized coatings which
provide UV protection.
 Ipposha Oil Industries of Japan have developed a homopolymer/copolymer-based high
molecular-weight coating for imparting prolonged light resistance and UV screening to
fabrics.

4. Dyes that act as UV absorbers

 Many dyes absorb UV radiation. Studies show that for a given colour, the darker the
shade, the higher the protection.
 In general, navy, black and olive shades would provide better protection than light pastel
and consequently are characterized by a higher UPF rating.

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5. Influence of nanolignin on UV blocking

 A new way of improving the UV barrier properties of textiles is application of technical

lignin in nanostructure during the finishing process, in order to increase the lignin content
in textiles.
 Nanolignin was obtained from kraft lignin by ultrasonic treatment.
 Treatment of linen fabrics with a solution of nanolignin significantly improves the
fabric’s UV barrier properties.
 The nanolignin treatment does not worsen the biophysical properties of fabrics and
guarantees comfort and a positive effect on the human body.

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