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| Issue date:01/04/2010 |
| ATA Journal for Asia on Textile & Apparel - Apr 2010 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Polly Chiu and Dr Jimmy K.C. Lam |
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| Textile researchers in Hong Kong found that tightness factor* and knit structures play an important role in preventing UV radiation from reaching our skin |
People have been using textile materials and clothing since the ancient times for the purposes of protection, comfort and adornment. More recently, advanced material technology makes it possible to turn clothing into a guard against skin cancer and in particular melanoma (a kind of skin cancer) caused by excessive exposure to ultraviolet radiation (UVR) from the Sun.
Before the 1990s, consumers were largely unaware of the role of clothing in UV protection and showed little interest in this feature. Now, there is a growing demand for casual and active apparel giving added value to wearers without hindering the level of comfort. Fabrics with greater levels of sun protection can have a competitive edge in an increasingly health-conscious age.
Recent statistics of the World Health Organization (WHO) show that the number of people died of skin cancer is increasing each year.
Australia has one of the highest incidences of skin cancer in the world. Over 1,000 people died of skin cancer in Australia annually in addition to approximately 270,000 new cases of skin cancer being diagnosed. Scientists believe that the dominant reason for the occurrence of skin cancers is related to cumulative UVR exposure. Reasons for the high levels of UVR include geographical location, close proximity to the equator, relatively clear atmospheric conditions, and the influence of ozone depletion.
The general public often perceives that applying sun creams and wearing lightweight summer clothing, hats and sunglasses, are ways to prevent harmful solar ultraviolet radiation from reaching our skin. This is a misconception. These protective methods against UVR are effective when they are properly used, which is often neglected by users. For example, sunscreen of at least SPF 15 and preferably SPF 30+ should be applied to all areas of our body not covered by clothing. More importantly, it should be re-applied at regular intervals, i.e. every two hours, even on cloudy days.
Re-applying is essential because sunscreen can wear off after perspiring or swimming. People often apply it too thinly and unevenly, or do not apply it 20 minutes before going out under the sun. If these happen, full protection is not received and people are at risk of harmful effects due to UVR exposure.
Further, people are exposed to substantial UVR even when they are shaded from the direct sun because UVR is present in the open sky and reflected from the environment. For instance, a person wearing a hat in a boat can still be exposed to UVR reflected from the water to his/her face.
Clothing as a basic necessity for people can provide good solar UVR protection, if textile scientists correctly engineer the related parameters of UV protective clothing.
In this respect, there are few systematic studies on the interaction between the structure and physical properties of the textile materials to UV protection especially on summer lightweight knitted fabrics. It is believed that fabric cover factor has a direct influence on the ultraviolet protection factor of fabrics. Important fabric properties in this context include tightness factor, type of fibre, yarn and fabric construction, finishing processes, colour, UV absorbers, wash and wear, fabric stretch and wetting.
Against this background, a research project was initiated. It was found that tighter fabrics with a closer structure allow a lower UVR transmission. In other words, they were more effective to protect wearers from UVR.
Tightness factor alone was insufficient in explaining the effectiveness of UV protection, the research found. Fabric thickness, i.e. more materials per unit area, can also absorb or block more UVR and a higher UPF was recorded as a consequence.
The knitted fabric porosity to UV protection in terms of loop length, knit structures and fabric weight were studied systematically in this research. Seven knit structures were investigated, namely single-jersey plain knit, single “knit and tuck”, single knit, single “tuck and miss”, as well as double-jersey fabrics of 1x1 rib, interlock, full cardigan and full milano.
Experiments
Two experiments were conducted to investigate the effect of UV protection of knitted fabrics in association with:
- tightness / loop lengths of plain knitted fabrics to UV blocking, and
- knit structures, tightness, washing and dyeing to UV blocking.
In the first experiment, two sets of commercial knitted samples were used, using two machine gauges, 5G and 12G. Three loop lengths (tightness) of loose, normal and tight stitch were examined.
In the second experiment, seven sets of knitted fabrics in varied structures were examined: a) single knit plain; b) single "knit and tuck"; c) single "knit, tuck and miss"; d) double knit 1x1 rib; e) double knit interlock; f) double knit full cardigan; and g) double knit full milano.
All the samples were placed in a conditioned room at 21+/-1°C with relative humidity of 65+/- 2 % for 24 hours. Fabric thickness was measured by a fabric thickness tester, Hans Baer AG CH-Zurich Telex 57767, under a pressure of 10g/cm2 onto the fabric. Fabric weight was measured according to the ASTM D3776-96 and tightness factor (TF) calculated in accordance to Munden's Theory:
where Tex is yarn tex and l is loop length (mm) |
Tightness factor is very useful in setting up knitting machine and it is the ratio between yarn count and loop length. A higher value refers to tighter fabrics.
Tightness factor of single jersey fabrics normally fall between 1.29 and 1.64. A tighter fabric provides less space for UVR to pass through it, thus improving the UV protective ability.
Test method
In this research, protection factor (PF) was used to quantify the UVR protection of fabrics. The PF indicates how much UVR is blocked by a textile material. For instance, a material with a UPF rating of 20 would allow 1/20th of UVR falling on its surface to pass through it. In other words, it would block 95% of the UVR and transmit the rest. The level of PF is calculated by:
Where
E l is the relative erythemal spectral effectiveness (unit-less),
S l is the solar UVR spectral irradiance in W m-2 nm-1,
T l is the spectral transmission of the item,
l is the bandwidth in nm, and
l is the wavelength in nm |
Effect of fabric cover on UPF
Knitted fabrics are formed by a yarn travels across the fabric inter-looping with a loop in the previously formed row.
Different cover factors on UPF values |
Knitted fabrics generally have greater openness than woven fabrics due to the formation of loops. Therefore, fabric tightness factor was predicted to be an important factor affecting UPF.
In this experiment, commercially knitted fabrics of two gauges (5G and 12G) were made with differing knitting tension (i.e. tight, normal and loose).
The results indicated that a higher tightness factor provided greater UV protection regardless machine gauges.
In the 5G samples, tightness factor of loosely and tightly knitted fabrics increased from 1.373 to 1.57. The mean UPF increased from 15.653 to 32.294 respectively. A similar trend was found with 12G fabric samples, but a smaller percentage of difference in mean UPF was obtained.
The results indicated that denser fabrics (i.e. higher level of tightness) give greater UV protection. These fabrics have smaller micropores providing less space for UV radiation to pass through them, resulting in greater UV protection.
Other fabric properties, like fabric thickness and weight, also contributed to UV blocking.
From the study, fabric samples of 5G have an average thickness of 2.3mm while samples of 12G have an average thickness of 1.0mm. Additionally, 5G samples were about twice heavier than 12G samples. A positive relationship between fabric thickness and weight to UPF was determined. For instance, doubling the fabric thickness and weight resulted in doubled mean UPF.
The research found that higher fabric weight per unit area of the same fabric construction and fiber composition led to a larger amount of fiber per unit area, achieving greater UV protection.
Effect of fabric constructions on UPF
Single knit structures
Results of single knit structures on UPF | 
Effect of tightness of single-knit 5G samples on UPF | 
Effect of tightness of single-knit 12G samples on UPF |
In this study, three single knit structures of plain knit, “knit and miss”, and “knit, tuck and miss” were prepared on a Stoll machine using 2x20Ne 100% cotton yarn.
The three single knit structures showed a similar but relatively low average mean UPF value of 4.61. A "knit and tuck" structure provides the weakest UV protection, whereas the "knit, tuck and miss" structure provided the greatest UV protection in the family of single knit structures.
Tuck stitch
A tuck stitch is formed by feeding a new yarn to the needle, which the old loop remains around the latch and stays open. The tuck needle did not clear the old loop due to its intermediate rising position. A tuck loop has a shape like an inverted "V" forming a wider, shorter and thicker fabric than plain knit alone. Hence, fabrics with tuck loops would have a more open work effect, leading to increased space for UV radiation to pass through it and weaker UV protection.
Miss stitch
Different stitches of knitted fabrics |
Miss stitch, or float or welt, is formed as the needle does not rise up during yarn feeding, and it appears as a short length of horizontal yarn at the back of the fabric. It will form a narrower and higher density fabric than plain knit.
The fabric will be thicker and heavier if larger numbers of miss-stitch yarns are used. A fabric made of miss stitches has higher density reducing space for UV radiation to pass through it, increasing UV protection. It should be noted that long floats in a "knit and miss" structure are not stable and undesirable.
Tuck stitches are thus used to tie-in long floating yarns and increase the anchorage of the long float. This is known as accordion stitch, and was used in making our "knit, tuck and miss" samples. It was found that UV protection increased in relation to knit structures – from tuck, plain knit to miss stitches.
Tightly knitted fabrics in any single knit structure offered greater UV protection than loosely knitted fabrics. The differences between tight knits and loose knits increased from 33.15% to 38.31% for "knit, tuck and miss" and single jersey plain knit; and up to 51.39% for "knit and tuck". This suggested that higher fabric tightness led to greater UV protection.
Double knit structures
Effeects of double knit structures on UPF |
Four double knit structures of 1x1 rib, interlock, full cardigan and full Milano were examined.
All of the four structures of double knits examined provided a stronger UV protection than single knits. Double knit structures have an average mean UPF values of 24.81, and that of full cardigan was 5.85.
The UV protection ability increased in an ascending order as: full cardigan, 1x1 rib, full milano and interlock. Samples with an interlock structure offered the greatest UV protection among them. The results match with our understanding in the knitting science in terms of the properties of knitted loops, and show that knit structure is an important factor affecting UPF.
Double knit can be classified as a fabric knitted by two sets of needles of the machine or by at least two needle beds. It is different from a single knitted fabric, which is produced by using only one set of needles. With more fiber materials (i.e. one more set of yarns) to block or absorb solar UV radiation, double knitted fabrics can block or absorb more UV than single knits.
Full cardigan
Full cardigan contains no rib courses but two courses of knit and tuck with alternatively in direction, and has identical face and back sides.
Tuck stitches with more open work effect have a weaker ability in terms of UV protection. Unwashed full cardigan samples had a relatively low average mean UPF value of 7.225, 5.362 and 4.968 with tight, normal and loose knit respectively.
Full milano
In contrast, full milano composes of knit and miss stitches only, without tuck stitch. Its fabric structure is three courses per repeat. The first course is a rib course, and the second and third courses are one knit in the front and one knit at the back plain courses. As a result, it provides a narrower and denser fabric with miss stitches, offering better UV protection than full cardigan. The average mean UPF values with tight, normal and loose knits were 47.495, 19.51 and 11.687 respectively.
Rib structure
Rib structure is the simplest double knit structure, involving one course per repeat. In this case, all courses are the same and the fabric is in a zig-zag shape providing high elasticity in width direction.
In this study, 1x1 rib samples composed of alternate wale of plain and reversed plain stitches were put into examination. 1x1 rib fabric has the appearance of alternate vertical column of technical face and back loops. The structure offered a medium UV protection among the four double knit structures, and the average mean UPF values were 17.058, 14.673 and 10.433 with tight, normal and loose knits respectively.
Interlock
Interlock is another structure in the family of double knit. Interlock gating involves two sets of needles facing each other on the same line. An interlock fabric is formed by two courses of 1x1 rib meshed with each other, and can be described as a double 1x1 rib. Interlock has a close surface structure with both sides identical with face loops, and hence provides the greatest UV protection among the double knits. As shown in the study, the average mean UPF values were 29.857, 23.203 and 21.046 with tight, normal and loose knits respectively.
To sum up, textile fabric can be engineered to reduce harmful UV radiation. Experiments show that the tighter the knitted fabric, the smaller is the opening of the fabric, and the lower of UV transmission through the fabric.
Knit structure also plays a role. Double knit structures, with more materials per unit area and closer structure, show a greater UV protection than single knit structures. Interlock structure shows the greatest UV protection. |
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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