Transcript The 11th Asian Textile Conference

Effect of Stretching on UV
protection of Knitted Fabrics
Presenter: Dr. Jimmy K.C. Lam
Wai-yin Wong & Jimmy K.C. Lam*
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
2
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
3
2. SIGNIFICANCE

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
4
3. ULTRAVIOLET RADIATION

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
5
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 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 measurement6
techniques to count the MED
(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
7
because these wavelengths are unlikely to reach the earth surface9
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

UPF Rating
UVR Protection
Category
Effective 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+
A textile product must have a minimum UPF of 15 to be rated as UV
protective
8
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)
9
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
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.
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. METHODOLOGY
Materials

Stoll CMS 822 14G computer flat knitting machine

100% cotton yarn with yarn count 3/40s

Bleached fabric specimen

10 knit structures
13
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
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
UPF =
𝐸𝑒𝑓𝑓
𝐸′
=
400 nm
290 nm
400 nm
290 nm
𝐸λ × 𝑆λ × ∆λ
𝐸λ × 𝑆λ × 𝑇λ × ∆λ

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
15
8. RESULTS
3.1 Effect of stretching on UPF of bleached single knitted cotton fabrics
Impact of stretch on UPF of bleached single
knitted cotton fabrics
Un-stretch
Stretch-Vertical
Stretch-Horizontal
25
19.7
14.3
13.4
UPF
20
15
13.8
10.8
10
8.1
13.9
12.4
7.6 7.7
9.4 9.8
5
0
All Knit

Knit + Tuck
Knit + Miss
(25%)
Single Knit Structures
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

Knit + Miss
(50%)
(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
16

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:
Single Knit
Reduction in UPF (%)
Structures
Stretched-Vertically Stretched-Horizontally
All Knit
21.6
41.1
Knit & Tuck
39.1
38.1
Knit & Miss (25%)
32.6
29.4
Knit & Miss (50%)
27.3
31.6
Average
31.4
38.6
31.0
29.5

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 unstretch state but also in the stretched state with higher reduction in UPF
(%) than the other three single knit structures
17
3.2 Effect of stretching on UPF of bleached double knitted cotton fabrics
Impact of stretch on UPF of bleached double knitted cotton
fabrics
Un-stretch
Stretch-Vertical
Stretch-Horizontal
60
50
45.0
UPF
40
30
22.3
17.7
21.2 17.9
20
11.2
10
12.8
7.8
10.6
14.2
30.3
29.4
28.9
29.2
21.6
20.1
7.8 9.5
0
1x1 Rib

Half Cardigan Full Cardigan
Half Milano
Double Knit Structures
Full Milano
All the double knit structures experienced significant reductions when the fabrics
were either stretched in vertical or horizontal directions

Interlock
(F2, 105 = 9.636, p ≤ 0.05)
No significant difference in UPF between fabrics stretched
in vertical and horizontal directions (p = 0.749)
18

Double knitted fabrics have better UV protection than the single
knitted fabrics in both un-stretch & stretched states
Double Knit
Structures
1x1 Rib
Half Cardigan
Full Cardigan
Half Milano
Full Milano
Interlock

Reduction in UPF (%)
Stretched-Vertically Stretched-Horizontally
15.8
47.0
38.9
17.0
45.1
33.4
22.6
38.8
26.7
31.6
32.6
35.1
Average
31.4
28.0
39.2
30.7
29.1
33.8
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
19
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
20
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
21
11. ACKNOWLEDGEMENT
The research is funded in part by the General Research Fund (ASA21) from the University Grants Committee, Hong Kong and The
Hong Kong Polytechnic University, Hong Kong.
22
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4.
5.
6.
7.
8.
9.
10.
11.
12.
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