Functional Finishes for Textiles || Ultraviolet protection finishes for textiles

Ultraviolet protection fi nishes for textiles Y. K. Kim University of Massachusetts – Dartmouth , N. Dartmouth, MA, USA

15

15.1 Introduction

Long term human exposure to solar UV radiation may result in acute and chronic health effects on the skin, eyes and immune system. Sunburn (erythema) is the most common acute effect of excessive UV radiation exposure. The World Health Organization (WHO) recommends that loose fi tting, full length clothing be used for outdoor activities to protect our skin from UV rays (WHO, 2013 ).

Textiles, coatings and fi nishes derived from synthetic and naturally occurring polymers absorb solar ultraviolet radiation, and then undergo photolytic, photo-oxidative and thermo-oxidative reactions that result in the degradation of the material in the forms of discoloration, chalking and reduced mechanical properties. The damaging effects of solar UV radiation on wood, paper, biopolymers, polymers, plastics and rubber are well known (Andrady et al ., 1998 ). Hence, the development of effective UV protective textile fi nishing is of great importance to human health, society and the environment.

Textiles have been used for solar radiation protection since ancient civilizations. Textile structures render unique characteristics required for sunscreening apparel such as pliability, good mechanical strength, softness, aesthetics and other engineered properties. In this chapter, the main focus is to explore UV fi nishing materials and techniques for preventing degradation of textiles from harmful effects of solar UV radiation and reducing health risks.

15.2 Mechanisms of UV protection

In this section, the solar radiation spectrum will be examined to identify the UV radiation to be attenuated with UV protective fi nishes. The UV absorbers and blockers interact with the harmful radiation to minimize transmission through the textile fabrics. Models for diffusive and direct transmission of UV light through the protective coating and textile fabrics are introduced to clarify the mechanisms of UV protection on a mathematical basis.

15.2.1 Solar radiation spectrum

The solar radiation spectrum at the top of Earth ’ s atmosphere is very close to that of a blackbody with a temperature of about 5500°C and consists of three parts: UV Functional Finishes for Textiles. http://dx.doi.org/10.1533/9780857098450.2.463Copyright © 2015 Elsevier Ltd. All rights reserved.

464 Functional Finishes for Textiles

190–400 nm, visible 380–780 nm and IR 780–2500 nm (NASA, 2013 ). UV radiation is part of the solar electromagnetic radiation spectrum, and is arbitrarily divided into three bands of different wavelengths for different disciplines. However, environmental and dermatological photobiologists commonly use slightly different divisions, more closely associated with the biological effect of the different wavelengths: UVA 400–320 nm, UVB 320–290 nm and UVC 290–200 nm. The characteristics of UV spectra are shown in Table 15.1 (EPA, 2013 ).

UVA passes through the atmosphere with little change, while 90% or more of UVB is absorbed by stratospheric ozone. UVC is totally absorbed by atmospheric ozone, water vapour, oxygen and carbon dioxide, which means that UVC has minimal penetration to the surface of the Earth and has little effect on human health. Thus, the solar ultraviolet radiation of importance to human health and UV protective textiles is of UVA and UVB (WHO, 2006 ).

15.2.2 Mechanisms of UV attenuation: Kubelka–Munk analysis

Textile materials are used for apparel and technical textiles. The purpose of UV protection for technical textile materials is to prevent photodegradation of the polymers, fi bres and fabrics used in technical textile products. The chemical pathways by which common polymers photodegrade are fairly well known, but various aspects of the mechanisms involved remain to be elucidated. However, it is important to understand the very signifi cant infl uence of additives and fi nishing agents such as dyes, pigments, extenders, photostabilizers and thermal stabilizers in modifying these pathways (Andrady et al ., 1998 ).

There are many approaches to protect polymeric materials from the harmful effects of UV radiation. One of the most widely used methods in industry is the dispersion of UV absorbing agents in the photodegradable fi bres and polymers. There are inorganic and organic UV absorbing additives that can be applied to protect against UV radiation. Inorganic materials are based mainly on mixed oxide fi lms or particles, able to absorb or scatter light, while there are classes of organic molecules which effi ciently absorb UV light and can also be used as additives for this purpose (Zayat et al ., 2007 ).

Spectra Wavelength (nm) Health effects

UVA 320–400 Cataracts, sunburn, indirect DNA damage, immune system suppression

UVB 290–320 Skin or eye burns, skin cancer, premature ageing, immune system suppression

UVC 190–290 Skin or eye burns

Table 15.1 Health effects of UV radiation

Ultraviolet protection fi nishes for textiles 465

Light entering a polymeric material containing dye molecules and pigment particles of various sizes (typically 0.1 to 1.0 μ m) is attenuated by absorption and scattering. When the pigment particles are smaller than 200 nm, they can be effectively considered as a solid solution and treated as dye solutions which absorb but do not scatter light (McDonald, 1987 ). This solid solution absorbs light according to the Lambert–Beer law: I = I 0 × 10 − ε c ℓ where I and I 0 are the intensity of the transmitted and incident light respectively, ε is the extinction coeffi cient of the absorber, c is the molar concentration of the absorbing species in the material, and ℓ is the path length through the material. When light travelling in a medium encounters pigment particles with a diameter larger than 10 times its wavelength, it is scattered by an amount in accordance with Fresnel ’ s law of refl ection: ρ = ( n 2 − n 1 ) 2 /( n 2 + n 1 ) 2 , where ρ is refl ectance, and n 1 and n 2 are the refractive indices of medium and pigment, respectively.

However, light interaction with UV protection fi nishes in textiles and polymers cannot be analysed with Lambert–Beer ’ s absorption and Fresnel ’ s refl ection theories. To derive a UV protection effi cacy of the additives, let us consider a thin polymeric fi lm with uniformly dispersed UV scattering particles and/or UV absorbing molecules as a model shown in Figure 15.1 .

Kubelka and Munk (K–M) analysed light transmittance and refl ectance of this paint fi lm model. They assumed incident light is monochromatic radiation and particles are randomly oriented. The model considers two diffuse light fl uxes: one downward ( i ) and the other upward ( j ). The i fl ux in its passage through d x is decreased by an amount Ki d x by absorption and is also decreased by an amount Si d x by scattering, which reverses the direction of some of the light rays. These rays belong then to j . The K and S values are called the absorption coeffi cient and the scattering coeffi cient of diffusive light fl uxes , respectively, of the colourant layer, which are not identical with the normally defi ned coeffi cients of parallel beam traversing the material medium. They are assumed to be constant within the entire volume of the fi lm (Wyszecki and Stiles, 1982 ). The changes in the upward and downward fl uxes traversing the thin slab of thicknes dx at distance x from the top surface can be written:

d d dj S K j x Si x= − +( ) + (15.1)

Figure 15.1 Kubelka–Munk analysis.

dx

x

Lj(x)

i(x)

r(x)

Rg

R

I J


− = − +( ) +d d di S K i x Sj x (15.2)

We defi ne r ( x ) = j ( x )/ i ( x ) to derive a differential equation with respect to r ( x ), the local refl ectance at depth d x . Kubelka obtained the most general hyperbolic solution with boundary conditions R = J/I for diffusive refl ectance at x = 0 (the top of fi lm surface) and diffusive refl ectance R g at x = L :

RR a b bSL

a R b bSLa

S KS

b a=− − ( )[ ]

− + ( )= + = −

112g

g

wherecothcoth

, , (15.3)

From this solution, two important cases useful for UV protection additive effi cacy analysis can be derived. The most signifi cant case is when L is so thick that further increase in thickness fails to change the refl ectance of the fi lm layer. Let R ∞ = R x → ∞ and R g = 0 in this case. From Eqn 15.3 ,

R a b K S K S K S∞ = − = +( ) − +( ) +⎡⎣ ⎤⎦1 1 22 1 2 (15.4)

By inverting Eqn 15.4 ,

K S R R= −( )∞ ∞1 22 (15.5)

Since diffusive refl ectance R can be measured by a spectrophotometer used for colour measurement, K / S can be determined in the UV-Vis spectrum. From Eqn 15.5 one can see that the K / S value is very small when, for example, the layer refl ectivity is 90%. This case arises when the particle has low absorbing power and high scattering power, such as TiO 2 and other UV scattering additives. Another limiting case of interest is a very high K / S value for a particle with low scattering power and high absorption power, which is a typical characteristic of UV absorbing molecules and dye molecules. In this case refl ectance is very low in the UVA and UVB spectral range.

One more valuable application of K–M analysis (Eqn 15.5 ) is that the absorption and scattering coeffi cients of a coloured material can be built up from the individual absorption and scattering coeffi cients of the individual colourants as shown in the equation below:

K S c K c K c K K c S c S c S S= + + + +( ) + + + +( )1 1 2 2 3 3 1 1 2 2 3 3… …s s (15.6)

where subscripts 1, 2, 3, etc. are for individual colourants and subscript s identifi es substrate. In the K–M analysis, it is assumed that the colourants have both absorbing and scattering power, as is the case for inorganic UV protecting pigments such as TiO 2 , ZnO and CeO 2 .

When dyed textile fabrics with UV protection pigments are assessed with K–M analysis, textile dyes are considered to be dissolved in fi bre and thus have no scattering


power. Any scattering power can be from the UV blocking particles in the fi bre. Accordingly, Eqn 15.6 reduces to:

K S c K c K c K K S

c K S c K S c K S K

= + + + +( )= ( ) + ( ) + ( ) + +

1 1 2 2 3 3

1 1 2 2 3 3

……

s s

s s s s SSs

(15.7)

The ratio of K / S s makes it possible to characterize a dye with only one substrate specifi c parameter. For example, we can assess effects of dye or UV absorbers ( K i ) and concentrations ( c i ) on different substrates (i.e., fi bre composition or coating medium). The utility of Eqn 15.7 can be demonstrated using data from of plain weave cotton fabric dyed with natural colourants (Zimniewska and Batog, 2012 ).

Assuming that the K / S value is measured at the maximum absorption wavelength, and the natural colourants are dispersed in the fi bre in a solid solution state with no scattering, Eqn 15.7 indicates that K / S = c i (K i / S s ). From Figure 15.2 , slope is proportional to the absorption power (K i ) of the given colourant. For example, Indigo shows 3.4 times the absorbing power of Cochineal, while Cochineal has twice the absorbing power of Madder.

From Eqn 15.3 , the internal transmission of the layer ( T i ) is given as:

T b a bSL b bSLi = ( ) + ( )[ ]sinh cosh (15.8)

When the dyed sample has very small S compared to K , then it is a semi-transparent medium that absorbs light but does little scatter. For this case Eqn 15.8 is reduced to:

T KL KLi = ( ) + ( )[ ]1 sinh cosh (15.9)

For KL > > 1, Eqn 15.9 is reduced to T i = exp ( − KL) , which is a Lambert–Beer law. By defi nition UPF is inversely proportional to T i , so it is evident that the higher the

Figure 15.2 K / S value depends on colourant concentration.

3.5

3

2.5

2

1.5

1

0.5

00 2 4 6 8

K/S

Colourant concentration (%)

Madder

Cochineal

Indigo

Linear (madder)

Linear (cochineal)

Linear (indigo)


K / S value is, the better the UPF performance of the UV absorbing chemical will be. This can be seen from Figure 15.3 derived from Table 15.2 . It can be observed that dark shaded Indigo at 4% and 6% concentration shows UPF > 50, plotted here as UPF = 50, and that UPF increases exponentially as predicted by Eqn 15.9 .

Although K–M analysis has limitations in certain circumstances, it can be utilized successfully to quantitatively assess the effi cacy of UV protective additive particles and chemicals by measuring refl ectance and transmittance with a UV-Visible spectrophotometer. Moreover, the diffusive refl ectance and transmittance measurement can be used as a screening and UV protective fi nish product development.

Figure 15.3 Natural dyes with high UV absorption ( K ) with very low scattering ( S ) improve UPF. Note that dark shading of Indigo at 4% and 6% concentration show UPF > 50, which are plotted here as UPF = 50.

60

50

40

30

20

10

00 1 2 3 4

UP

F

K/S

Madder

Cochineal

Indigo

Poly. (madder)

Poly. (cochineal)

Poly. (indigo)

Colourant Concentration (%) UPF K / S

Madder 2 11.1 0.24 15.8 0.286 16.6 0.38

Cochineal 2 28.5 0.634 34 0.796 36.6 0.99

Indigo 2 43.1 1.784 50 2.56

Table 15.2 UPF and K / S values of cotton fabric dyed with natural colourants

Source: after Zimniewska and Batog ( 2012 ).


Fibre type/knit structure Approximate UPF

Cotton tricot 4

Wool tricot 45

Silk twill 7

Nylon/Spandex tricot 12

Polyester tricot 26

Table 15.3 UV protection factors of undyed knitted fabrics of similar construction

Source: Zimniewska and Batog ( 2012 ).

15.2.3 UV protection mechanisms by textile structures

Textile fabrics are used for UV protective apparel products. The UPF of apparel textiles are infl uenced by fi bre contents, fabric construction and cover factor, fabric colouration and UV protective fi nishes. During the service life, deformation, moisture content and washing of the fabrics affect the UPF performance (Hoffmann et al., 2001 ). The effect of fi bre type on the UPF is shown in Table 15.3 . Cotton and silk show low protection from UV radiation due to the fact that their fi bre structure and chemical composition have no signifi cant UV absorbing capacity. However, wool and polyester have very high UV protection, since these fi bres have signifi cant UV absorption capability.

In this section, UPF effi cacy assessment of fabrics made of yarns, fi bre and polymers treated with UV protective fi nishing additives and/or chemicals will be discussed. It is assumed that a fabric structure with appropriate UV protective fi nishing is exposed to solar radiation. This situation is schematically presented in Figure 15.4 .

Figure 15.4 Plain weave structure: (a) top view; (b) view from weft side; (c) light–fabric interaction: 1 incident light; 2 specular and diffuse refl ection; 3 diffusive refl ection by scattering ( S ); 4 absorption ( K ); 5 direct transmission; 6 diffusive transmission by scattering (Ti).

1

26

5

6

3

3

2

4

(a) (b) (c)


It is evident from the model that total refl ectance R = ρ 2 + ρ 3 ; S = s 3 ; K = κ 4 ; and T = τ 5 + τ 6 , where ρ is refl ectance, s is scattering coeffi cient, κ is absorption coeffi cient and τ is transmittance, and the subscripts 1, 2, 3, … are associated with the light beams indicated in Figure 15.4 (c). The most important factors controlling the UPF of fabric are total UV transmittance by the direct ( τ 5 ) and diffusive ( τ 6 ) transmission mechanisms. The former is determined by the fabric structure and its fractional coverage, while the latter transmission depends on the absorbtion ( K ) and scattering ( S ) power of the fi nishes. Diffusive transmission through a UV protection additives/polymeric composite fi lm has been discussed in the previous section with the K–M model. Diffusive transmission is minimized by UV protective fi nishes with UV absorption molecules and/or scattering particles.

The direct transmission is minimized by lowering the fractional cover of fabric structures. There is a class of fabric called jammed structures with maximum weavability that shows the highest fractional coverage, up to 100%. However, UV protection fi nishing is needed for light summer apparel fabrics which have relatively low fractional coverage.

The proposed fabric UV radiation transmission model is derived from a modifi ed K–M model, where the fabric slab has interstitial holes that allow the light to be transmitted directly ( τ 5 ). This, fabric–light interaction model is approximated in Figure 15.1 as a thin polymeric slab with colourant, with rectangular holes. The UV protection of a fabric depends on direct transmission through the fabric interstices ( τ 5 ) of the uncovered area by yarns when the fi bre content and level of UV fi nishes are kept constant, which maintains diffusive transmission ( τ 6 ) at a constant level.

The fraction of the area covered by yarns per unit area (accordingly with no direct transmission) for textile fabrics is defi ned as the ‘fractional coverage’. For plain weave fabric the fractional coverage, FK , is given by Eqn 15.10 :

FK = + −wd fd wfd d1 2 1 2 (15.10)

where w and f are warp and fi lling yarns per cm, and d 1 and d 2 are warp and fi lling yarn diameters, respectively. Assuming a yarn packing factor of 0.65 (typical for ring spun yarn), the yarn diameter is given by d (cm) = 4.44 × (tex/ ρ f ) 1/2 × 10 − 3 , where ρ f is fi bre density in g/cm 3 . For cotton yarn, ρ f = 1.52; yarn diameter cm( ) = √ tex 280 . For example, for a plain cotton woven fabric: warp 20 tex, 28 ends/cm; weft 35 tex, 25 picks/cm; FK = 0.451 + 0.533 − (0.451 × 0.533) = 0.744. Theoretically, 74.4% of the cotton fabric area is covered by the yarns.

For other weaves such as twill and satin and derivatives (Weiner, 1966 ), w and f can be modifi ed with weave factor, M , average yarn spacing and yarn diameter according to the yarn clustering pattern (no compression and compression, race track, etc.). As an example, for 2 × 1 twill with uncompressed yarns under the fl oat,

1 1 01w Mp M d= − −( )a1 (15.11)

where M is the weave factor = (number of yarns per weave repeat)/(number of interlacings per weave repeat), p a1 is the average warp yarn spacing, and d 01 is the


uncompressed warp yarn diameter. It can be easily found that the M value for 2 × 1 twill is 3/2 and p a1 and d 01 are measured to determine w . Similarly f can be worked out. Once w and f are determined using a modifi ed Eqn 15.10 , fractional coverage for other weaves can be calculated. Fractional coverage should not be confused with cover factor K , which is relative coverage in either the warp or the weft direction. In SI units, K n= √( )tex 10 , where n is the number of threads per cm. Hence, the cover factor is defi ned as K = constant × FK by considering the fact that yarn diameter is proportional to √tex and removing yarn density factor.

For knitted fabrics, direct transmission of UV radiation is proportional to the fraction of the area uncovered by intermeshed loops. More specifi cally, the fractional coverage of single jersey fabric (plain knit) is given by

FK cpc wpc cpc wpc= ( ) × × × − × ×[ ]1 100 4 2l d d (15.12)

where wpc and cpc are wales per cm and courses per cm, l is loop length in mm, and d is yarn diameter in mm. The fractional coverage for other knitted structures can be deduced in a similar manner from geometrical models of these structures.

It cannot be overemphasized that the fractional coverage (FK) should not be confused with cover factor ( K ), which is a relative coverage indicator. For plain weave, K = 1/( pN 1/2 ), where p is yarn spacing and N is yarn size in cotton count, while for knit fabrics K = 1/( lN ½ ), where l is loop length and N is yarn size in cotton count.

For dark shades with effective UV absorption, the only source of transmitted light is from the interstices between yarns and is proportional to (1 − FK). From the defi nition of UPF, the theoretical maximum UPF is expressed as:

UPF FKmax = −( )1 1 (15.13)

In summary, apparel fabrics for the summer season and UV protective industrial fabrics can be engineered to give maximum protection from harmful radiation by fi nishing with additives and colourant chemicals of most refl ection, absorption and scattering power. The substrate fabrics should be selected based on fi bre content, surface texture and fractional coverage.

15.3 Chemistry and mechanisms of UV protection fi nishes

It is well known that UV light causes damage to organic materials by photodegradation. Organic materials used for UV protection textiles and sunscreening structures, such as most fi bres, polymers and plastics from natural or manufactured origin, absorb ultraviolet radiation and undergo a rapid photolytic and photo-oxidative reaction that results in their photodegradation. The energy of the photons in the ultraviolet region (290–400 nm) is suffi cient energy (315–400 kJ/mol) to break chemical bonds in these


organic materials, resulting in the formation of free radicals (Schindler and Hauser, 2004 ). In air, the UV damage of polymers and other organic materials proceeds with a photo-oxidative degradation. This photo-oxidation reaction occurs as a result of activation of a molecule or polymer chain in the material from its ground state (S 0 ) to its excited singlet (S*) and/or triplet (T*) states by the photon energy of UV light to form free radicals. The free radicals produced by the UV radiation react with other molecules in the polymer to form oxy and peroxy radicals, which result in chain scissions. The chain scission reaction continues until two free radicals are combined with each other or form stable non-radical compounds. The loss of strength, impact resistance and mechanical integrity of polymers exposed to UV light is due to polymer chain scission as a result of this photo-oxidation process (Zayat et al., 2007 ).

UV light can cause signifi cant damage to human tissue as shown in Table 15.1 . UVC can damage DNA and other biomolecules and is often used as a germicidal agent. Fortunately UVC from solar radiation is absorbed completely by atmospheric ozone and other molecules, thus health risks can be minimized by limiting exposures to artifi cial UV sources such as tanning and sterilization UV lamps. Exposure to both UVB and UVA leads to tanning and sunburn (erythema) and can affect the immune system. UVA penetrates deeper into the skin due to its longer wavelength, and plays a role in skin photo-ageing.

Thus the required properties of UV protective fi nishing chemicals should include but not be limited to effi cient absorption of UVB radiation at 290–320 nm quick transformation of the high UV energy into vibrational energy in UV absorber molecules and then into heat energy in the surroundings without photodegradation (Schindler and Hauser, 2004 ). There are two types of UV absorbing/scattering molecules and particles used commercially: inorganic UV blocking particles and organic UV absorbing molecules. In this section, the chemistry and mechanisms of the UV protection additives and molecules will be discussed.

15.3.1 Inorganic UV blocking chemicals

Many inorganic UV blocking systems based on particles or thin fi lm coatings have been developed. Inorganic oxides such as TiO 2 , CeO 2 and ZnO are the most widely used for protection against UV radiation (Zayat et al., 2007 ). These metal oxides share the same UV protection mechanism; the attenuation of the UV radiation in these materials is accomplished by both band gap absorption and scattering of light (Kasap, 2006 ).

In absorption the loss of intensity of light is due to the conversion of light energy to other forms of energy such as lattice vibration (heat) during the polarization of molecules in the medium, local vibration of impurities or defects, and excitation of electrons from the valence band to the conduction band. However, the UV light is strongly absorbed by excitation of electrons from the valence band to the conduction band. This absorption wavelength can be estimated from the band gap energy E g in eV units:

λ nm g( ) = 1204 E (15.14)


Table 15.4 shows TiO 2 , CeO 2 and ZnO have band gap energies corresponding to absorption spectra and refractive index. Light below these wavelengths has suffi cient energy to excite electrons and hence is absorbed by the metal oxides. Light having a wavelength longer than the band gap wavelength will not be absorbed.

The intensity of scattered light is a function of particle size as well as the refractive indices of the particles and the media. Recently, many researchers have reported a relationship between particle size and optical properties of these metal oxides (Ghrairi and Bouaicha, 2012 ; Lin et al ., 2006 ; Abbott and Holmes, 2013 ). Through careful control of particle size a maximum scattering of UV light can be obtained, while the scattering of visible light must be eliminated to obtain high transparency or ‘colour pollution’. It is evident that to minimize colour pollution, the matching of the refractive index ( n ) of the metal oxide particles to that of target fi bre and polymer matrices should be as close as possible. The refractive index of typical polymers used for coating or fi bres is about 1.46. ZnO (n = 2.0) will be better served, other conditions being equal, since the refractive index of ZnO is the nearest to 1.46 among the four UV absorbing metal oxides as listed in Table 15.4 . For example, mixed CeO 2 /TiO 2 fi lms were shown to be an effi cient UV absorber; however, they exhibit a slight yellow coloration due to the tail of the absorption peak that enters the visible range of the spectrum shown in Table 15.4 .

15.3.2 Organic UV protection fi nishes

Organic UV absorbers have been used to reduce the damaging effects on textile materials and health risks of UV radiation. The UV absorber molecules must be colourless or nearly colourless compounds having high absorption coeffi cients in the UV range of 290–400 nm spectra. In order to offer effective protection against UV radiation, the UV protection fi nish molecules must quickly transform the absorbed energy into less harmful vibrational (phonon) energy before reaching the surrounding substrate, and must exhibit good photostability.

One of the most important families of UV absorber molecules includes a phenolic group that forms intramolecular O–H–O bridges, such as salicylates, 2-hydroxybenzophenones, 2,2 ′ -dihydroxybenzophenones, 3-hydroxyfl avones or

Oxide Band gap, E g (eV) Wavelength, λ (nm) Refractive index, n

TiO 2 (anatase) 3.2 376 2.5

TiO 2 (rutile) 3.02 398 2.5

ZnO 3.3–3.4 354–368 2.0

CeO 2 3.0–3.2 376–401 2.2

Table 15.4 Selected optical properties of UV absorbing metal oxides


xantones, and compounds forming O–H–N bridges, such as 2-(2-hydroxyphenyl)benzotriazoles and 2-(2-hydroxyphenyl)-l,3,5-triazines (Schindler and Hauser, 2004 ). The structures of these compounds are shown in Figure 15.5 . These UV absorbers with an intramolecular hydrogen bond effi ciently dissipate the UV energy without radiation via an Excited State Intramolecular Proton Transfer (ESIPT) mechanism (Zayat et al ., 2007 ; Seo et al ., 2005 ).

The ESIPT energy dissipation mechanism can be explained with help of Figure 15.6 . The UV absorber 2-(2-hydroxy-5-methylphenyl)benzotriazole molecule contains acid and basic groups within reasonably close proximity. The acid group in the UV absorbing molecule is the phenolic hydroxyl, while the basic acceptor is the

Figure 15.5 Chemical structures of UV absorbers (after Schindler and Hauser, 2004 ).

X

(R1)n

OH

ROO

N

OHO

O R

Phenyl salicylates Benzophenones

Benzotriazoles

RO OR

OH OHO

NHO

R2

R2 R3

R1

N

N

Benzotriazole derivatives

NHO

X2X1

R

N

N

NN

N

Cyanoacrylates Triazine derivatives

R2R3

(R1O)nArSO3M

NH NH

O O

Oxalic acid dianilide derivatives


nitrogen atom in the heterocyclic benzotriazole or the oxygen atom in the carbonyl group in benzophenones. When the UV–absorber molecule in ‘enol form’ is excited to the fi rst singlet state (S 1 ) by UV radiation, the increased acidity promotes a proton transfer to create a tautomeric species (keto form) in its fi rst excited singlet state (S ′ 1 ). In most cases, the keto species loses its energy by a non-radiative thermal energy decay process, which is indicated as intersystem crossing (IC) in Figure 15.6 . It should be noted that the contribution of this energy to the thermal degradation of the material is negligible compared to the much stronger thermal energy absorbed by the sample from the solar radiation. In some cases, for instance 3-hydroxyfl avones, salicylates and xantones, the dissipation of the energy from the tautomeric species is accomplished by a fl uorescent emission with an unusually large Stokes shift. A reverse proton transfer regenerates the ground state of the enol form of the UV absorber (S 0 ), which completes the UV protection chemical processes, quickly transforming the absorbed UV energy into less harmful vibrational (phonon) energy before reaching the surrounding substrate material to be protected (Zayat et al ., 2007 ; Seo et al ., 2005 ).

The ESIPT mechanism is responsible for the exceptionally high photostability of these UV absorber molecules, due to the fact that the process occurs on an ultrafast picosecond timescale, which decreases the probability of chemical reactions in the excited state. Phenolic UV fi nishes show very small quantum yields of photodecomposition in the 10 − 7 to 10 − 6 range. However, the nature of the matrix in which the organic UV absorber is embedded may affect the photostability of the molecule. Basic and polar groups in matrices may form intermolecular H-bonds with the acid phenolic hydroxyl groups which disrupt the intramolecular H-bond formation. Therefore the intramolecular H-bond formation will be limited, preventing the ESlPT mechanism from taking place. This is usually accompanied by the appearance of fl uorescence and increased photoreactivity (Zayat et al ., 2007 ).

The ESIPT mechanism proceeds effectively only in planar fi ve- or six-membered rings having an intramolecular H-bond between the phenolic hydrogen and the heteroatom (i.e., nitrogen atom in benzotriazoles and triazines, and oxygen atom in salicylates, benzophenones, fl avones or xantones). In benzotriazoles, the formation

Figure 15.6 UV absorption by ESPIT mechanism.

N

NCH3

N

H O

N

NCH3

N

HO

Light

Keto form S1

Enol form S0

Excitation by light

ESIPT

Back transfer

S1´

S0´

IC


of an intermolecular hydrogen bond with the matrix results in the loss of planarity due to the rotation of the phenolic ring around the N–C bond resulting in the loss of photostability of the molecule.

The most important non-phenolic UV absorbers are oxanilides and cyanoacrylates. The mechanism of energy dissipation in oxanilides is believed to be similar to the ESIPT mechanism occurring in phenolic UV fi nish molecules. Cyanoacrylates are remarkably photostable molecules, considering the absence of an ESIPT mechanism for energy dissipation. It seems likely that a charge separated species could be formed from the excited state, allowing the dissipation of energy through rotation or increased vibration about the central bond (Figure 15.5 ) (Schindler and Hauser, 2004 ).

15.4 UV protection fi nishing for various textile structures

The UV protective fi nishing molecules can be incorporated in protective coatings to be applied on the textile structures and fabrics. Alternatively, the molecules can be dispersed or impregnated in the fi bres and yarns that will be converted into UV protective textiles. Most of the coatings developed are based on dispersions of the UV fi nishes in polymeric matrices. In this section, fi nishing techniques for improving UV protection effi ciency of textile fabrics and structures are discussed.

15.4.1 Fibre materials and fabric structures

The UV protection factor (UPF) of fl at textile products is measured with an integrating sphere spectrophotometer and defi ned by international standards, including ASTM 6603, AATCC Test Method 183 and EN 13758-1. In the UPF system, ‘Good’ ranking needs a minimum UPF rating of 15, and ‘Excellent’ ranking requires 40 to 50 + . A UPF rating greater than 50 is rated as 50 + with just ‘Excellent’ ranking. As explained in Section 15.2.3, UPF ranking can be enhanced by selecting fi bre materials with high UV absorption capacity and fabric structures with high fractional coverage. This strategy is based on the fact that UPF values are inversely proportional to the total transmittance through the fabric layer. Hence, an optimal combination of raw fi bre material and fabric structure will reduce the amount of harmful UV radiation reaching the skin.

Fibres are classifi ed in two categories: natural fi bres and manufactured fi bres. Among the natural fi bres from plant origin, untreated hemp, fl ax, jute, kenaf and raw cotton show good UV protection, due to the fact that chemical components of these fi bres (e.g., natural pigments, lignin, waxes and pectin) are effi cient UV absorbers. Once these fi bres are chemically scoured and bleached, the UV absorbing capacity of these fi bres will be diminished. This is because the treatments will remove a substantial proportion of the UV absorber chemicals in these raw fi bres. For natural fi bres from animal origin, wool provides excellent UV protection, while silk shows poor UV resistance (Zimniewska and Batog, 2012 ). Thus, raw natural fi bres with


built-in UV absorbers are excellent fi bre materials for UV protection, but fi nal fi nished products from these fi bres need to have UV absorbing chemicals applied. This is because the downstream manufacturing processes, including dyeing and printing, will remove or reduce the UV absorbing pigments and chemicals in the natural fi bres.

Manufactured fi bres can be extruded with inorganic UV blocking pigments such as TiO 2 , CeO 2 and ZnO to impart UV protection properties. Traditionally, synthetic fi bre manufacturers utilized about 1–3 wt% of TiO 2 for delustering fi bres. The majority of commercial UV protective fi bres for UV barrier apparel fabrics are extruded with spinning dope containing 5–10% TiO 2 , for example Sunpaque ® (sheath/core PET), Elacool ® (viscose sunpaque), Lecture ® (bicomponent PET) and Oboe ® (PAN staple fi bres) to name a few (Rupp et al ., 2001 ). It is advisable for a UV barrier apparel product design team to begin with evaluating the UV protection properties of fi bres to be used. Then they proceed to cost effectiveness and downstream UV fi nishing requirements for selected fi bre materials.

Han and Yu ( 2006 ) reported that fabric made from fi lament yarns containing 1–2% of nano TiO 2 particles show 50 + UPF rating. This excellent UV protection fabric was obtained from fi bres with good UV absorber additives.

Fabric quality parameters such as structures, fractional cover and fabric weight strongly infl uence the UV radiation transmission of the fabric and in turn its UPF rating. Zimniewska and Batog ( 2012 ) reported that linen fabrics with high fractional coverage (96.6%) show a UPF rating of close to 50. The fabric weave pattern determines the maximum cover factors at jammed structures as shown in Figure 15.7 (Behera et al ., 2012 ).

It is interesting to note that the maximum weavability (maximum coverage) behaviour is similar for different weaves of hypothetical fabrics made of circular yarn cross-section. With the increase in fl oat, however, the curve shifts towards higher

Figure 15.7 Relation between warp and weft cover factor for jammed fabric (circular cross-section) (after Behera et al ., 2012 ).

30

25

20

15

10

5

00 5 10 15 20 25 30 35 40

Weft c

ove

r fa

cto

r

Warp cover factor

SatinBasketTwill

Plain


values of weft cover factor. This shows that the higher the number of yarns fl oating, the higher the fractional coverage of the fabric. Thus, non-plain weave fabric affords advantages for increasing fabric mass and fabric cover, and in turn UPF ratings.

It is evident that careful selection of raw fi bre materials and fabric quality parameters such as weave, cover factors, areal density and thread counts is required to design a superior UV protective fabric with or without UV attenuating additives and chemicals.

15.4.2 UV protective textile fi nishing

Conventional textile chemical fi nishing processes can be employed to apply UV absorbing molecules and UV blocking pigments to fabrics. However, it is not uncommon that the UV fi nishing process can be applied to yarns or garments. This wet process typically takes place after dyeing and printing of fabrics, when the base fabric has a poor inherent UV barrier property with a suffi cient fractional coverage. There are both organic and inorganic UV attenuators. Inorganic UV blockers are usually certain semiconducting metal oxides such as TiO 2 , ZnO, CeO 2 , and Al 2 O 3 , reviewed in Section 15.3.1. The organic attenuators are also called UV absorbers because they mainly absorb UV rays as discussed in section 15.3.2. Compared with the existing organic UV absorbers, the inorganic UV blockers are more robust because of their unique features including, among others, non-toxicity and chemical stability under both high temperature and UV radiation exposure.

The organic UV absorbers listed in Section 15.3.2 and some other UV absorbing dyes, including natural dyes, can be applied with traditional textile fi nishing techniques: pad application of chemicals on dry coloured fabric, or pad application of chemicals to wet fabric after dyeing (wet-on-wet process). Some organic UV absorbers (Figure 15.5 ) and UV absorbing dyes can be applied with dyes in the same bath (Schindler and Hauser, 2004 ).

The UPF of fabrics can be improved by application of polymeric coatings. The polymeric coatings are formulated by dispersing UV blockers and photostabilizers. Examples of commercially available UV absorbers are shown in Table 15.5 .

Trade name Manufacturer Type

Tinuvin Ciba Benzotriazole

LA-34 Asahi Denka Benzotriazole

Cyasor UV-531 Cytec Benzophenone

UV-CHEK-AM-340 Ferro Hydroxybenzoate

Table 15.5 Commercially available UV absorbers

Source: after Becker et al . (2012).


Photostabilizers (UV stabilizers) could be classifi ed as primary and secondary stabilizers. The former act as additives that interfere with the oxidation cycle of substrate material photodegradation, thus they are called ‘radical scavengers’. The latter decompose hydroperoxides and prevent new oxidation cycles from beginning, and are described as ‘peroxide decomposers’. However, some of the secondary stabilizers may have primary characteristics.

Primary antioxidants used for polymeric materials are hindered phenols (HP) and hindered amines (HA). Commercially available HA are listed in Table 15.6 . The sterically hindered amines, also known as HALS (hindered amine light stabilizers), are typically used as photostabilizers in polymeric coating binders. HALS operate as UV protectors by combining with oxygen when exposed to light to form stable nitroxide radicals, which trap the radicals that have developed from the photodegradation of polymer coating binders through exposure to UV rays. The most important feature of the nitroxide radicals is their regeneration capacity. Thus, a cyclical reaction is possible, which can repeat hundreds of times until the HALS itself has been degraded (Becker et al ., ( 2005 ).

The secondary photostabilizers are used synergistically with a suitable primary antioxidant, and include phosphites, phosphonites and thiocompounds. One common synergistic example is a phosphite (or phosphonite) with a hindered phenol.

Once the coating formulation and substrates are prepared, a laboratory scale trial coating should be conducted to produce test samples for UV protection performance and to identify potential problems before full scale production. There are coating methods available to fi t the product characteristics and production schedule. UV protective coated fabrics usually show very low transmittance of harmful UV radiation, thus apparel made of the coated fabric will have very high UPF ( > 50). Moreover, the coating formulation can include other additives to impart multifunctional performance to the fabric, for example fl ame retardance, antimicrobial and water vapour transmission (WVT). However, coated fabrics have some drawbacks such as poor drape, reduced tensile and tear strength, and poor transmission properties (if not

Trade name Manufacturer Type MW (dalton)

Tinuvin ® 770 CIBA Monomeric 481

LA-57 Asahi Denka Monomeric 326

Chimabsorb ® 944 CIBA Polymeric 2500 or higher

Cyabsorb ® 3346 Cytec Polymeric 1600

Cyabsorb ® UV-500 Cytec Monomeric 522

Uvasorb ® HA-88 3-V Chemical Corp Polymeric About 3000

Table 15.6 Commercially available hindered amines (HA)

Source: after Becker et al . (2012).


intended). To overcome these disadvantages, many researchers have been applying newly emerging nanotechnologies.

15.5 Nanotechnology based UV protection fi nishing

Polymer nanocomposites (PNCs) are made by introducing inorganic nanoparticles into polymer systems. PNCs exhibit multifunctional, high performance polymer characteristics beyond those possessed by traditional fi lled polymeric materials. This remarkable improvement of PNC properties stems from the ultra large interfacial area per volume between nanoparticles and the host polymer (Kim, 2010 ). Multifunctional PNCs may improve their thermal resistance, moisture resistance, reduced permeability, chemical resistance and UV stability and absorption property. For example, Katangur et al . ( 2003 ) reported that Kevlar fabrics coated with 5 wt% commercial ZnO and TiO 2 nanoparticles of 15–70 nm nanoparticle embedded acrylic coatings showed adequate protection from sun induced degradation.

Tsuzuki and Wang ( 2010 ) reviewed nanoinorganic UV blocker fi nishing on cotton, polyester and blended fabrics. They pointed out that the inorganic nanoparticles have better UV blocking properties than that of microsized ordinary UV blockers, but commercial TiO 2 and ZnO nanoparticles pose challenges of durability and uniform dispersion in fi nishing media to realize this advantage due to agglomeration of these nanoparticles. Moreover, these nanoparticles are photocatalysts for promoting photodegradation of fi bre materials, although the inorganic UV blockers may possess other functionalities such as antibacterial and self cleaning properties.

One of nanocoating techniques to apply nanoinorganic UV absorbers to fabric substrates is based on sol–gel methods. A transparent TiO 2 nanofi lm formed by this technique imparted an excellent UV protection (UPF 50 + ) after 50 washings, which is classifi ed as a durable fi nish (Xin et al, 2004 ). Xu et al . ( 2005 ) also reported similar results with the sol–gel nanocoating technique. Zhang et al . ( 2012 ) formed nano TiO 2 fi lm on a PET polyester fabric via hydrothermal reaction. The treated fabric demonstrated very good UV protection and wash fastness. Paul et al . ( 2010 ) found that cotton yarns treated with ZnO nanoparticles withstand the abrasion and other mechanical interaction with knitting elements during conversion of the treated yarn to a knit fabric, and the knitted fabric shows moderate to high UPF. They also reported that bleached and reactive dyed cotton fabrics treated with TiO 2 nanoparticles along with succinic acid as linking agent deliver 50 + UPF together with excellent launderability.

Wool is one of those natural fi bres which has a very low photostability. It is advisable for imparting photostabilty and UV protection of wool fabrics to apply nano TiO 2 coating together with organic UV absorbers and photostabilizer such as HALS. It can be envisaged that the maximum UV protection of inorganic nanoparticles with limited photocatalytic activities is achievable when these nano UV blockers are combined with UV stabilizers such as HALS, HP and phosphites (Section 15.4.2).

A new approach for UV protection of cotton fabrics based on the electrostatic self assembly (ESA) technique was investigated by Wang and Hauser ( 2010 ). Three


fl uorescent brightening agents (FBAs) and poly(diallyl dimethyl ammonium chloride) (PDDA) were stepwisely fabricated on cationized cotton fabrics through direct layer by layer (LbL) deposition technique. The self assembled cotton fabrics could obtain excellent UV protection ratings of UPF > 40 after depositing several bilayers of FBA/PDDA and excellent washing fastness, which indicates good adhesion between the multilayer nanocoatings and the cotton surfaces.

15.6 Test methods for effi cacy of UV protection fi nishes

Many standards organizations around the world have developed in vitro testing methods to rate UV protection performance of textile fabrics. Before the development of instrumental methods, UV protection factor (UPF) values were determined by in vivo testing, where human subjects were exposed to UV radiation for determining the dosage necessary to cause sunburn. In recent years, investigators and manufacturers of UV protective textiles have paid keen attention to understanding the solar UV protection properties of textiles and to hangtag labelling of the UV performance rating. Several related standards have been published by Australia and New Zealand (AS/NZS 4399: 1996 ), Europe (EN 13758-1: 2001 , UV Standard 801) and the United States ( AATCC TM 183 , ASTM D6603 and ASTM D6654). The AATCC 183 test method defi nes the UPF rating for a fabric as the ratio of UV measured without the protection of the fabric to that measured with the protection of the fabric: UPF = 100/(1 − %UV transmission). A fabric rated UPF 30 will transmit 1 out of 30 units of UV, therefore it transmits 3.3% of UV radiation through the fabric. UPF tests are normally conducted in a laboratory with a double beam spectrophotometer equipped with a proper fabric sample holder at the transmission measurement port, which should be suffi cient to collect all the scattered and transmitted lights through an integrating sphere, to include all the erythemal active wavelength (UVA and UVB) spectral measurements (Figure 15.8 ).

The AATCC 183 test method should be used in conjunction with other related standards including the American Society for Testing and Materials (ASTM) D6544 and ASTM D6603 testing standards. ASTM D6544 specifi es simulating the lifecycle of a fabric, which includes the exposure of a specimen to laundering, simulated sunlight and chlorinated pool water, and to present in a state that simulates the conditions at the end of two years of normal seasonal use, so that the UV protection level fi nally stated on the label estimates the maximum transmittance of the garment fabric during a two-year lifecycle. ASTM D6603 defi nes the specifi cation for visible hangtag and care labelling of sun protective clothing and textiles. A manufacturer may publish a test result to a maximum of UPF 50 + . Table 15.7 shows the various grades and the related protection factors for the textile materials. UV labelling is required in addition to other garment labelling, including Permanent Care labels, Fibre Content labels and other required labels.

The UV Standard 801 has been developed recently by the International Test Association for Applied UV Protection, for determining the UV protection of a consumer product under everyday loads applied to the material. (UV Standard 801,


2013 ). The UV protection factor is determined for fabrics when new, abraded, laundered and/or dry cleaned, both in a stretched and in a stretched and wet condition, which simulate the fabric being exposed to an end user ’ s everyday usage. So UV Standard 801 provides a more realistic UPF rating than the other standards. It utilizes Australian/New Zealand Standard AS/NZS 4399: 1996 to determine the UPF ratings with consideration of the erythemal effectiveness and the spectral irradiation of the sun.

15.7 Future trends and challenges

The current state of UV protective textile fi nishing research focuses mainly on inorganic nanoparticle coating onto fabrics. Various nanocoating techniques such as

UPF rating Protective category UV radiation blocked (%)

40–50 + Excellent 97.5–99 +

25–39 Very good 96.0–97.4

15–25 Good 93.3–95.9

Table 15.7 UPF Rating and Protective Category

Source: after ASTM D6603-12 .

Figure 15.8 Double beam integrating sphere UV-Vis spectrophotometer. Courtesy of PerkinElmer ( www.perkinelmer.com ), accessed 5 May 2013 .

150 mm sphere

M2

M2

M1

M1

M3

Transmittancesampleholder

Light trapor port plug

Referenceholder

Reference beamSample beamSampleDetector

Reflectancesampleholder


the sol–gel method and self assembly layer-by-layerhave been studied to synthesize and apply nanolayers and nanoparticle coatings onto cotton, polyester and Kevlar fabrics. In the past, UV blocking fi nishes of textiles by polymeric coatings have been investigated mostly using micro-TiO 2 and ZnO particles, which signifi cantly increase the UV blocking effi ciency. Although nanosized particles offer additional advantages, such as an enhanced UV shielding effect and higher affi nity towards fi bre surfaces, their photocatalytic activities within the host materials cause a deterioration in the integrity of the polymeric materials used in the fabrics and coatings.

Thus, future challenges lie in how to reduce or eliminate the photocatalytic capability from the inorganic nano UV blockers, and improvement of wash fastness of organic UV absorbers and inorganic UV blockers in achieving practical UV blocking textiles. Potentially challenging issues are the health and safety risks of manufactured nanoparticles. There are many studies on this topic that appear to be contradictory and the methodology employed is nevertheless not uniform. Thus researchers and manufacturers involved in nanoparticle handling and processing should exercise caution and pay attention to current developments and information on nano safety from regulatory authorities (NIOSH, EPA, and FDA in the US) (Abbott and Holmes, 2013 ). The European Union has formed a group to study the implications of nanotechnology, called the Scientifi c Committee on Emerging and Newly Identifi ed Health Risks, which has published a list of risks associated with nanoparticles. Thus, manufacturers and importers of nanoparticles will have to submit full health and safety data within a year or so in order to comply with REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) (ECHA, 2013 ).

As the development of nanotechnology innovation progresses, more choices in commercial nanoparticles and application technologies will be available on the market. By combining different types of nanoparticles and nano textile fi nishing techniques, UV protective textiles with multifunctionality can be realized in the near future.

15.8 Conclusion

Textiles have been used for solar radiation protection since the time of ancient civilizations. Textile structures render unique characteristics required for sunscreening apparel such as pliability, good mechanical strength, softness, aesthetics and other engineered properties. In this chapter, UV textile fi nishing materials and techniques were explored. The physical and chemical principles of UV protection mechanisms were discussed. Innovative nanotechnology based textile fi nishing techniques were reviewed to enhance understanding and develop more effi cient and cost effective UV protective textiles, which prevent the degradation of textiles from harmful effects of solar UV radiation and reduce health risks from UV radiation. Once the new fi nishing technologies mature to be translated into cost effective manufacturing practices, UV protective fabrics with multifunctional fi nishes will be widely available for apparel and technical textile products.


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Functional Finishes for Textiles || Ultraviolet protection finishes for textiles