Effect of Stretching on UV protection of Knitted Fabrics Presenter: Dr. Jimmy K.C. Lam Wai-yin Wong & Jimmy K.C. Lam*

O VERVIEW 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 2

1. I NTRODUCTION 3 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 reasons 1 Clothing is recommended by physicians and medical experts as one of the primary methods of protecting the skin form the harmful UVR 2-3 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. TiO 2, 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

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 cancer 4 Worth to develop children’s UV-protective clothing 4 2. S IGNIFICANCE

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 cancer U LTRAVIOLET R ADIATION

4. A SSESSMENT OF UV PROTECTION OF T EXTILES 6 (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 fabric 6-7 Pro: gives the direct response of the human body to UVR Con: 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

7 (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 ( 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 fabric 8 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 surface 9

8 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 Rating UVR Protection Category Effective UVR transmission (%) UPF Rating 15 – 24Good Protection6.7 – 4.215, – 39Very Good Protection4.1 – 2.625, 30, – 50, 50+Excellent Protection ≦ , 45, 50, 50+

5. M AIN FACTORS AFFECTING UV PROTECTION OF T EXTILES 9  Fiber Types  Fabric Construction and Yarn properties  Dye characteristics (Color)  Additives and Nanotechnology  Moisture content (Wetness)  Stretch (Tension)  End-use conditions (Wash and wear)

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 10

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 transmission 11 Clark et al. (2000) investigated the effects of areal and linear stretches 12 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. 11

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 12

7. M ETHODOLOGY 13 Materials Stoll CMS G computer flat knitting machine 100% cotton yarn with yarn count 3/40s Bleached fabric specimen 10 knit structures

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 14

15 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

8. R ESULTS 3.1 Effect of stretching on UPF of bleached single knitted cotton fabrics 16 Significant difference in UPF between fabrics in un-stretch state and fabrics stretched in vertical and horizontal directions (F 2, 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

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: %) 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 17 Single Knit Structures Reduction in UPF (%) Stretched-VerticallyStretched-HorizontallyAverage All Knit Knit & Tuck Knit & Miss (25%) Knit & Miss (50%)

3.2 Effect of stretching on UPF of bleached double knitted cotton fabrics 18 All the double knit structures experienced significant reductions when the fabrics were either stretched in vertical or horizontal directions (F 2, 105 = 9.636, p ≤ 0.05) No significant difference in UPF between fabrics stretched in vertical and horizontal directions (p = 0.749)

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: %) 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 19 Double Knit Structures Reduction in UPF (%) Stretched-VerticallyStretched-HorizontallyAverage 1x1 Rib Half Cardigan Full Cardigan Half Milano Full Milano Interlock

9. C ONCLUSION 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 20

10. F UTURE W ORKS 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 21

22 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. A CKNOWLEDGEMENT

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