Effect of Stretching on UV

protection of Knitted Fabrics

Presenter: Dr. Jimmy K.C. Lam

Wai-yin Wong & Jimmy K.C. Lam*

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OVERVIEW

1. Introduction

2. Significance

3. Ultraviolet Radiation

4. Assessment of UV protection of textiles

5. Main Factors affecting UV protection of textiles

6. Stretch (Tension)

7. Methodology

8. Results

9. Conclusion

10. Future Works

11. Acknowledgement

12. References

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1. INTRODUCTION

Increasing number of people dying from skin cancer each year over

the world and over-exposure to ultraviolet radiation (UVR) deemed to

be one of the main reasons1

Clothing is recommended by physicians and medical experts as

one of the primary methods of protecting the skin form the harmful

UVR2-3

Clothing offers more durable protection against the deleterious

impacts of UVR than sunscreen

Limited protection against UVR is usually enhanced by chemical

approach with UV-absorbers (e.g. TiO2, ZnO)

Knitted fabrics are more porous and elastic than woven fabrics,

UV protection of knitwear enhanced by chemical is only sufficient when

the fabric structure is closed enough

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Skin is our biggest organ of the body

Clothing has the ability to absorb, reflect, or scatter radiant energy

thereby it can be considered as a kind of wearable sunscreen

which is essential and suitable to everyone

Fabric construction offers the simplest and healthiest

solution to achieve good UV protection without additional

finishing processes

Highly beneficial to those having extreme sensitivity to sunlight,

living in sunlight-intensive regions and for those with outdoor

occupations

Extensive exposure to UVR at the age between 10 and 24 has been

identified as a potent risk factor in developing skin cancer4

Worth to develop children’s UV-protective clothing

2. SIGNIFICANCE

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Electromagnetic radiation consisting visible light (50%), infrared

radiation (45%) and UVR (5%)

Essential for our well-being (synthesis of vitamin D, for growth and

maintenance of a healthy skeleton)

Depletion of stratospheric ozone layer is a serious environmental

problems

Decrease of 1% in ozone lead to increases in UVR at Earth’s surface

and may eventually lead to a 2.3% increase in the rate of skin

cancer5

3. ULTRAVIOLET RADIATION

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4. ASSESSMENT OF UV PROTECTION OF TEXTILES

(1) In vivo measures the minimum erythema dose (MED) (minimum quantity of

radiant energy required to produce first detectable reddening of skin, 22±2 hours after exposure) of UVR using human skin as a test indicator

tests the ability of a fabric to protect against sunburn with measurement of MEDs on the skin protected and unprotected by a fabric

SPF is the ratio of the time of solar radiation exposure required for the skin to show redness with and without protection by fabric6-7

Pro: gives the direct response of the human body to UVRCon: it is difficult to conduct it as a standard test method because it

involves human subjects (ethical issue) and relies on the optical measurement techniques to count the MED

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(2) In vitro Simpler, less time consuming, more reliable, widely adopted

UV protection ability of fabrics is expressed as Ultraviolet Protection Factor (UPF)

Measure UVR transmittance through a fabric over UV spectrum (290-400 nm)

A spectrophotometer equipped with an integrating sphere is used to measure

the UVR transmittance and UPF is calculated from the ratio of the average UVR

transmitted through air to the average UVR transmitted though the fabric8

Calculated UPF value is rounded into a multiple of 5 (from 5+ to 500+) and

UPF>50 are generally indicated as 50+

Although definition of UVR given in the international standards start at 280 nm,

UVR irradiance at wavelengths below 290 nm is not used in the calculation of UPF

because these wavelengths are unlikely to reach the earth surface9

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 Australian/New Zealand standard (AS/NZS 4399:1996)9

Using a solar spectrum measured in Melbourne

Measures UPF of fabric in dry and tensionless (un-stretched)

state

At least 4 specimens tested (2 from machine direction and 2 from

cross machine direction)

UPF 20 ~ allow 1/20th of UVR falling on its surface to pass through it;

which means that it would reduce UVR exposure by a factor of 20

For the purposes of labeling, sun protective clothing shall be

categorized according to its rated UPF

A textile product must have a minimum UPF of 15 to be rated as UV

protective

UPF RatingUVR Protection

CategoryEffective UVR

transmission (%)UPF Rating

15 – 24 Good Protection 6.7 – 4.2 15, 20

25 – 39 Very Good Protection 4.1 – 2.6 25, 30, 35

40 – 50, 50+ Excellent Protection ≦2.5 40, 45, 50, 50+

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5. MAIN FACTORS AFFECTING UV PROTECTION OF TEXTILES

Fiber Types

Fabric Construction and Yarn properties

Dye characteristics (Color)

Additives and Nanotechnology

Moisture content (Wetness)

Stretch (Tension)

End-use conditions (Wash and wear)

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6. Stretch (Tension)

Stretching is a common end-use conditions

Knitwear fabric is easily deformed or stretched

Stretching a fabric will normally cause ↓UPF:

Open up spaces in fabrics & alter fabric structure

↑Porosity accompanied by a decrease in fabric thickness

Knitted fabrics construction changed in openness/closeness more

than woven fabrics

Yarns in knitted fabrics have a greater freedom of movement

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Moon and Pailthorpe (1995) found there is 15.5% of elongation in

average of elastane garments worn and caused a remarkable

reduction of UPF.10 Measurements of elongations were measured on body

part expecting to have a high incidence of skin cancer

Kimlin et al. (1999) also studied effect of stretch of stocking against UVR

transmission11

Clark et al. (2000) investigated the effects of areal and linear stretches12

Osterwalder et al. (2000) also found increase of UVR penetration is

almost linear with stretch.13 The relationship between stretch and

UVR penetration (%) is illustrated.

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UPF label of garment may not reflect actual UV protection in

a wearing situationis as fabric is measured in a relaxed state

Limited research on UV protection of knitted fabrics with

different structures

Aim: Investigate impact of stretching on UV protection of

bleached single and double knitted cotton fabrics constructed with

different knit structures

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7. METHODOLOGY

Materials Stoll CMS 822 14G computer flat knitting machine 100% cotton yarn with yarn count 3/40s Bleached fabric specimen 10 knit structures

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Weft knitted fabrics (stitches)

Knit stitch: formed by needle receives new

loop and knock–over the old loop that held from

the previous knitting cycle

Tuck stitch: formed when needle reaches to a

height during rising that the old loop is not

cleared but the needle hook catch a new yarn

during downloading movement

Miss stitch: formed at the height that neither

the old loop is cleared nor needle hook can

catch new yarn during downloading movement

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UPF Measurement

Ultraviolet Protection Factor (UPF) of fabric specimens was measured by

Cary 300 Conc UV-Vis spectrophotometer (In-vitro method)

Australian/New Zealand Standard AS/NZS 4399:1996

Dry, flat and tensionless state (un-stretched)

Both machine and cross-machine directions (Wale and course directions)

Linear stretch (stretch in one direction only) by 15% elongation from its

original un-stretch state

UPF= 𝐸𝑒𝑓𝑓𝐸′= σ 𝐸λ × 𝑆λ × ∆λ400nm290nmσ 𝐸λ

400nm290nm × 𝑆λ × 𝑇λ × ∆λ

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8. RESULTS

3.1 Effect of stretching on UPF of bleached single knitted cotton fabrics

All Knit Knit + Tuck Knit + Miss (25%)

Knit + Miss (50%)

0

5

10

15

20

25

13.8 12.4

13.9

19.7

10.8

7.6 9.4

14.3

8.1 7.7 9.8

13.4

Impact of stretch on UPF of bleached single knit-ted cotton fabrics

Un-stretch Stretch-Vertical Stretch-Horizontal

Single Knit Structures

UP

F

Significant difference in UPF between fabrics in un-stretch state and fabrics stretched

in vertical and horizontal directions (F2, 69 = 24.28, p ≤ 0.05)

No significant difference in UPF between fabrics stretched in vertical and

horizontal directions (p = 0.616)

All the single knitted cotton fabrics here cannot be rated as UV protection (minimum

UPF 15) and the situation is even worse after stretching

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Resulted UPF in both stretching directions are averaged for

further calculation of reduction in UPF (%) of fabrics

Reductions in UPF (%) by stretching for the four single knit structures

are listed in table:

The reduction in UPF of Knit & Tuck fabrics are the highest (38.6%)

Other three single knit structures have similar reductions in UPF by

stretching (range of reduction in UPF: 29.5 - 31.4%)

Fabrics with tuck stitches have worse UV protection not only in un-

stretch state but also in the stretched state with higher reduction in

UPF (%) than the other three single knit structures

Single Knit Structures

Reduction in UPF (%)

Stretched-Vertically Stretched-Horizontally AverageAll Knit 21.6 41.1 31.4Knit & Tuck 39.1 38.1 38.6Knit & Miss (25%) 32.6 29.4 31.0Knit & Miss (50%) 27.3 31.6 29.5

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3.2 Effect of stretching on UPF of bleached double knitted cotton

fabrics

1x1 Rib Half Cardigan Full Cardigan Half Milano Full Milano Interlock05

101520253035404550

21.2

12.8 14.2

28.9 29.4

45.0

17.9

7.8 7.8

22.3 21.6

30.3

11.2 10.6 9.5

17.7 20.1

29.2

Impact of stretch on UPF of bleached double knitted cotton fabrics

Un-stretch Stretch-Vertical Stretch-Horizontal

Double Knit Structures

UP

F

All the double knit structures experienced significant reductions when the fabrics

were either stretched in vertical or horizontal directions (F2, 105 = 9.636, p ≤ 0.05)

No significant difference in UPF between fabrics stretched

in vertical and horizontal directions (p = 0.749)

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Double knitted fabrics have better UV protection than the single

knitted fabrics in both un-stretch & stretched states

The reduction in UPF of Full Cardigan is the highest (39.2%) while

the other five double knit structures have similar reductions in UPF

by stretching (range of reduction in UPF: 28-33.8%)

Full Cardigan has the highest proportion of tuck loops among the six

double knit structures

A tuck loop tends to extend wider than a knit loop which increases the

fabric width and thus more UV radiation can be transmitted

Double Knit Structures

Reduction in UPF (%)

Stretched-Vertically Stretched-Horizontally Average1x1 Rib 15.8 47.0 31.4 Half Cardigan 38.9 17.0 28.0 Full Cardigan 45.1 33.4 39.2 Half Milano 22.6 38.8 30.7 Full Milano 26.7 31.6 29.1 Interlock 32.6 35.1 33.8

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9. CONCLUSION

Both the bleached single knitted and double knitted cotton exhibit a

significant reduction of 30-40% in UPF when stretched by 15% linear

directions

Vertical and horizontal stretching give similar reductions in UPF

Bleached single knitted cotton fabrics cannot be rated as UV protective

(minimum UPF 15) in the un-stretch state and the situation get worse when

the fabrics were stretched

Bleached double knitted fabrics retain to be UV protective even if they are

stretch by 15% in linear direction

Fabrics with miss loops and the Interlock structure retain UV protective ability

when the fabric is stretched

Knit & Tuck structure (single knit) and Full Cardigan (double knit with tuck

loops) exhibited the highest reduction in UPF

Fabrics with tuck loops are not recommended for UV protective knitwear

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10. FUTURE WORKS

Stretching in different percentages (e.g. 5%, 10%, 15%, 20%) of

elongation

Stretching and wetting simultaneously

Stretching colored fabrics, different materials (e.g. elastane added

fabrics)

Improvement on the stretching apparatus

Standardized method for measurement of UPF of a stretched fabric

in the future

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The research is funded in part by the General Research Fund (A-SA21) from the University Grants Committee, Hong Kong and The Hong Kong Polytechnic University, Hong Kong.

11. ACKNOWLEDGEMENT

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http://www.who.int/mediacentre/factsheets/fs305/en/index.html2. Hurwitz, S. (1988). The Sun and sun protection: recommendations for children. Journal of

Dermatologic Surgery and Oncology, 14(6), 687-680.3. Hacker, S.M., Browder, J.F. and Ramoscaro, F.A. (1993). Basal-cell carinoma – choosing the best

method of treatment for a particular lesion.2. Postgraduate Medicine, 93(8), 101-111.4. Capjack, L., Kerr, N., Davis, S., Fedosejevs, R., Hatch, K.L. and Markee, N.L. (1994). Protection of

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5. Roy, C.R., Gies, H.P., and Toomey, S. (1995). The Solar UV Radiation Environment: Measurement Techniques and Results. Journal of Photochemisty and Photobiology. B, Biology, 31(1-2), 21-27.

6. Menter, J.M. and Hatch, K.L. (2003). Clothing as solar radiation protection. Current Problem in Dermatology, 31, 50-63.

7. The American Society for Testing and Materials, ASTM D6544-2011, Standard Practice for Preparation of Textiles Prior to Ultraviolet (UV) Transmission Testing.

8. Gies, H.P., Roy, CR., Elliot, G. and Wang, Z. (1994). Ultraviolet radiation factors for clothing. Health Physics, 67(2), 131-139.

9. Australian/New Zealand Standard, AS/NZS 4399:1996, Sun protection clothing – Evaluation and classification.

10. Moon, R. and Pailthorpe, M. (1995). Effect of Stretch and Wetting on the UPF of Elastane Fabrics. Australasian Textiles, 15(5), 39-42.

11. Kimlin, M.G., Parisi, A.V. and Meldrum, L.R. (1999). Effect of stretch on the ultraviolet spectral transmission of one type of commonly used clothing. Photodermatology Photoimmunology Photomedicine, 15(5), 171-174.

12. Clark, I.E.S., Grainger, K.J.L., Agnew, J.L. and Driscoll, C.M.H. (2000). Clothing Protection Measurements. Radiation Protection Dosimetry, 91(1-3), 279-281.

13. Osterwalder, U., Schlenker, W., Rohwer, H., Martin, E. and Schuh, S. (2000). Facts and fiction on ultraviolet protection by clothing. Radiation Protection Dosimetry, 91(1-3), 255-260.

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