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Influence of laser irradiation on the optical and structural properties of poly(ethylene terephthalate) fibres
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Optics & Laser Technology 39 (2007) 1301–1309 www.elsevier.com/locate/optlastec
Influence of laser irradiation on the optical and structural properties of poly(ethylene terephthalate) fibres V.N. Wijayathungaa, C.A. Lawrencea, R.S. Blackburna, M.P.U. Bandaraa, E.L.V. Lewisb, H.M. El-Dessoukya,, V. Cheungc a Centre for Technical Textiles, School of Design, University of Leeds, UK IRC in Polymer, School of Physics and Astronomy, University of Leeds, UK c Centre for Colour Design Technology, School of Design, University of Leeds, UK b
Received 27 October 2006; received in revised form 20 December 2006; accepted 22 December 2006 Available online 12 February 2007
Abstract Laser irradiation has been previously investigated for achieving uniform heating of polyethylene terephthalate (PET) fibres in the hotdrawing stage of the production process, so as to obtain better fibre mechanical properties. The optical properties and dye uptake of PET fibres also depend on the polymer chain orientation and crystallinity within the fibre structure. This paper reports an investigation of a concept whereby laser irradiation and interferometry could be used to modify and trace a small change in the optical properties of a PET monofilament fibre, but the corresponding change in the dye uptake would not be detected visually. A copper vapour laser (550–580 nm wavelengths) was used to expose consecutive 4 mm lengths along a running length of monofilament to 39.8 W cm2, at a pulse rate of 9.89 kHz in order to modify, in a controlled way, the polymer crystallinity and orientation. A 3D finite element simulation, based on uncoupled heat-transfer analysis, indicated that rapid heating and cooling could be obtained with the laser to give the small changes required. Irradiated and untreated samples were analysed by interferometry and a 0.16% change was detected in the birefringence profiles, corresponding to a small reduction in the degree of orientation and crystallinity of the irradiated samples. Density measurements and wide-angle X-ray scattering (WAXS) analysis confirmed the change in crystallinity. Tests conducted for dye adsorption and tensile strength showed a small increase in the former and only a very small decrease in the latter. It was concluded that these changes in property provide the opportunity for a laser-irradiated PET monofilament fibre to be used as a subtle tracer element in brand labels for textile garments as an anti-counterfeit measure. r 2007 Elsevier Ltd. All rights reserved. Keywords: Laser irradiation; Interferometry; Optical properties
1. Introduction Every year textile manufacturers and retailers are subject to significant financial losses because of counterfeit fashionable brand products, e.g. jeans, sportswear and premiership replica football shirts. Detecting such counterfeit goods has become increasingly difficult as identification is largely based on the manufactured quality. Today, the widespread availability of advanced textile machines with Corresponding author. Tel.: +44 0113 2172952; fax: +44 0113 3433704. E-mail addresses: [email protected], [email protected] (H.M. El-Dessouky).
0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.12.007
integrated computer process control enables the counterfeiter to produce copies of brand products virtually identical in appearance and quality to the legitimate goods. Although barcodes and miniature electronic devices may be used for anti-counterfeit they also can be copied or damaged. It is, therefore, of interest to develop security markings integral to the product and preferably within the identifying brand labels. Modifying the optical properties of a monofilament which is then incorporated into the weave of the brand label would offer the possibility of integral security markings, provided the differences from the neighbouring fibres are not visible to the human eye, say in the dyed state, but perceptible by optical measurements. Although
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the optical properties of a fibre may be modified by various means, the concept is to simply heat treat a fully drawn monofilament to effect just a traceable change in the crystallinity and refractive indices, nJ and n?, and thereby the birefringence. To achieve a very small change would require rapid heating and cooling of short lengths consecutively along a set length of the monofilament, and laser irradiation would seem an effective method. If a fibre is exposed to a narrow laser beam, respective proportions of the incident light will be transmitted, reflected, and give rise to radiative heat transfer. When considering methods of heat transfer into fibre, laserinduced heat flux offers important advantages. It provides almost instantaneous and more uniform heating than any other thermal treatment, and the temperature field cools faster when the incident flux is removed. Laser treatment of polyester fibres has been previously employed, primarily to overcome large temperature differences arising through the fibre cross-section, which occurs when traditional fibreheating methods are used at high fibre-running speeds. Conventional methods of heating based on surface contact are probably the most popular. However, polymer fibres have generally low thermal conductivity. Therefore, obtaining fast heat transfer is restricted. The first patent application for the laser heating methodology was put forward by the Asahi Chemical Industry Company of Japan in 1973 [1], which was followed by applications from the Mitsubishi Rayon Company in the same year [2] and the Toyobo Company in 1985 [3]. These patents are concerned with the localized drawing of a fibre under instantaneous and uniform heating across a narrow section of the fibre, so as to achieve a higher degree of polymer chain alignment (orientation) along the fibre axis. Okumura et al. [4] also utilized laser radiation in a continuous drawing process for PET fibres and compared experimental and theoretical analysis of the temperature profile along the heated fibre. The laser application carried out in the present study aims to slightly reduce the orientation and crystallinity of a pre-drawn monofilament, where the drawing was done by the widely used commercial process of heated godet rollers. A subtle alteration should make the fibre a tracer element among the untreated fibres of the same polymer type forming a brand label, detectable by a slight difference in the measurable optical properties but not visually apparent.
2.2. Laser irradiation A custom-designed copper vapour laser was used to provide the heating light with a measured irradiance at the fibre exposure point of 39.8 W cm2, a pulse rate of 9.89 kHz and a wavelength of 550–580 nm. The diameter of the laser was 4 mm at the fibre exposure point; the focal point was inside the outer aperture of the optical instrument used for focusing the laser beam and the fibre travel was 2 mm outside the outer aperture. The back plate was 15 mm away from the fibre path. The fibre exposure arrangement is shown in Fig. 1. The fibre was pulled across the laser beam with the help of a take-up spindle driven by a 50 W DC shunt motor, at a slow speed of 30 mm min1. The ratio of input and output roller speeds was 1.05, which gave the lowest possible tension of 5 mN. Two polished ceramic guides were used to maintain a straight fibre path across the laser beam. In order to visualize the heat transfer pattern due to laser irradiation, a 3D finite element simulation was attempted using ABAQUS software. This enabled an understanding of the temperature profile. The approach used eight-node continuum elements with heat transfer capability, and uncoupled transient heat transfer analysis was then performed with the application of irradiance. Fig. 2 depicts the simulated localized heating of a fibre as it passes through the laser beam. Within 0.2 s the central portion of the 4 mm heated length attains ca. 34 1C; after 4 s the heating has propagated further along the length and the maximum temperature is now at 144 1C; by 8 s (the duration for which the 4 mm length is exposed to the beam) a temperature of 204 1C is reached. On either side of the heated zone the temperature decreases rapidly along the fibre length, reaching 63 1C after 8 s of heating. As would be expected, the surface facing the projected beam reaches the highest temperature and has the most rapid temperature decrease through the fibre cross-section. However, the temperature difference between the front and back surfaces, relative to the beam contact, is only 3 1C. The laser application was therefore accepted as providing a reasonably uniform thermal treatment.
2. Theoretical and experimental considerations 2.1. Fibre material A commercial grade of fully drawn PET monofilament fibre of 70 decitex linear density and diameter 0.08 mm (commonly used in textile labels) was obtained from Teijin Monofilaments GmbH.
Fig. 1. Fibre exposure to laser arrangement.
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Fig. 2. Simulated transient temperature profiles during the heat transfer process; (d) shows the temperature contours across the fibre section.
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2.3. Optical and structural characteristics Investigation of the optical properties was based on the observations made with an Interphako interference microscope having a fitted Mach–Zehnder interferometer and CCD camera for image capture. For light (l ¼ 550 nm) vibrating parallel and perpendicular to the fibre axis, the samples were immersed in liquid having refractive indices nL ¼ 1.725 and 1.532, respectively. Subsequently, microinterferograms (see Figs. 3 and 4) were captured, recorded and analysed by the profile software [5,6].
2.3.1. Refractive indices (nJ and n?) profile The refractive index profile of a fibre reflects the variation of its refractive index across the fibre diameter. In the case of duplicated position of a two-beam interference microscope, the refractive index (n) profile of fibres (taking into account the refraction of the incident beam by the fibre layers) can be determined automatically using special software [7] based on Eq. (1) [5,6,8]: lZ Q ¼ b
Q¼1 X
2nj ½K 1 K 2 1=2 þ 2nQ K 3 n0 K 4 ,
(1)
j¼1
where K1, K2, K3 and K4 are given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d Q n0 2 2 K 1 ¼ ½R ðj 1Þa , nj
(2)
Fig. 4. Microinterferograms using Interphako microscope when the light is vibrating perpendicular to the axis of untreated (top) and irradiated (bottom) fibres.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi d n Q 0 , K 2 ¼ ½R ja2 nj sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi d n Q 0 K 3 ¼ ½R ðQ 1Þa2 , nQ
K4 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R2 d 2Q þ R2 X 2Q .
(3)
(4)
(5)
where R is the fibre radius, a is the layer thickness (a ¼ R/ N), N is the number of layers, Q and j are the limits of the layer numbers, n0 ¼ nL and is the liquid refractive index, nj and nQ are the refractive indices of the jth layer and Qth layer, respectively, dQ is the distance between the incident and the fibre centre, XQ is the distance between the emerging beam and the fibre centre, ZQ is the fringe shift corresponding to the point XQ in the two-beam interference pattern, l is the wavelength of light used and b is the interfringe spacing. 2.3.2. Birefringence Dn profile Birefringence profile of a fibre can be obtained from the measured refractive indices, nJ (parallel) and n? (perpendicular) as follows: Dn ¼ njj n? .
Fig. 3. Microinterferograms using Interphako microscope when the light is vibrating parallel to the axis of untreated (top) and irradiated (bottom) fibres.
(6)
2.3.3. Orientation function f(y) The orientation function, f(y), of polymer fibres can be calculated from the relation [9,10] f ðyÞ ¼ ð1 þ xÞf D xf 2D ,
(7)
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where the parameter x is given by x¼
2n2k n2? n3v ðnk n? Þ
1
(8)
and fD is the Herman’s factor [11] which is given by fD ¼
Dn . Dnmax
(9)
The strains developed in the crystallites due to the laser irradiation could manifest as changes in the lattice planes, causing line shifting. These changes in the lattice planes are sensitive to the X-ray diffraction methods, provided that variations in the scatter peaks are properly identifiable. As described by Mallick et al. [16] strain can be verified by differentiating the Bragg’s law: l , 2 sin y
Here, Dnmax is the maximum birefringence and its value for PET fibres is 0.24 [12,13].
d¼
2.3.4. Degree of crystallinity The volume crystallinity w is related to the density according to r ra w¼ , (10) rc ra
Dd 1 ¼ l cosec y cot y ¼ d cot y, Dy 2
where r is the density of the sample, ra is the density of the amorphous region (1.335 g cm3) and rc is the density of the crystalline region equal to (1.455 g cm3) [14]. The density r (g cm3) of PET fibres can be calculated from the relation [15] 2 n¯ 1 r ¼ 4:047 2 , (11) n¯ þ 2 where n¯ is the average refractive index of fibre, which is given by n¯ ¼
ðnjj þ 2n? Þ . 3
(12)
2.4. Density Fibre density was determined using a density gradient column according to BS2782-6: method 620D. Potassium iodide aqueous solution was used at 2370.1 1C. Five specimens of the laser treated and the untreated fibre were used to obtain an average reading. Volume crystallinity (w) was calculated using Eq. (10). 2.5. Wide-angle X-ray scattering (WAXS)
Dd ¼ cot y Dy. d Since diffraction angle is measured in 2y, Dd D2y ¼ cot y , d 2
(15)
(16)
(17)
Dd tan y. (18) d Eq. (18) allows the variation in strain, Dd/d to be calculated from the observed broadening of the scatter. If du indicates the untreated spacing and ds the spacing in the irradiated fibre, the micro-strain in the fibre elements in the direction normal to the diffracting plane (e) is Dd ds du ¼ ¼ . (19) d du If ds4du, then Dd/d is positive which indicates that the residual stress is tensile and if dsodu, then Dd/d is negative indicating compressive residual stress. This value of Dd/d, however, includes both tensile and compressive strains. Assuming both are equal for the micro-crystallites, Cullity [17] specified that the value of Dd/d must be divided by two to obtain the maximum tensile strain alone, or maximum compressive strain alone. Hence, maximum micro-stress present in the sample (s) can be defined as
2.6. Tensile properties
nl ¼ 2d sin y,
2.7. Dye sorption
where the value of integer n is equal to 1, d is the distance between the scatter planes, l is the wavelength of the incident ray and y is half the diffraction angle of the ray.
(14)
D2y ¼ 2
WAXS analysis of the fibres was done on a Huber WAXS texture goniometer 4020 configuration, operating at 40 kV and 30 mA. The radiation was a crystal monochromated Cu-Ka (wavelength 1.5418 A˚). The detector slit size was 2 mm in width and 6 mm in height. Equatorial scanning was done for a fibre sample exposed to the laser and an untreated sample, for 2y angles starting from 101 up to 351 and 401, respectively. The correction for measured 2y angle based on a standard polyethylene sample was 0.401. The distance between scattering planes were calculated using Bragg’s equation as the following: (13)
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1 Dd E¼ E, 2 2 d where E is Young’s modulus of the material. s¼
(20)
The load-elongation characteristics of the fibre were measured before and after the laser irradiation in order to determine the significance of the changes to the mechanical properties.
To determine the visual influence of the laser treatment, 0.05 g fibre samples of before and after treatment, were
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1.7232 Refractive Index n1
dyed with C.I. Disperse Yellow 19 (supplied by DyStar) at 5.0% omf concentration at a liquor ratio of 1000:1, with the addition of 1 g dm3 of Levagal DLP (levelling agent) and 2 g dm3 of Ludigol AR (anti-reducing agent) and pH buffered to pH 4.5–5.0 (sodium acetate/acetic acid). The fibre samples were wound loosely around a stainless steel ‘H’ frame and placed in 300 cm3 stainless steel dyepots housed in a Roaches Pyrotec 2000 infra-red dyeing machine, providing agitation throughout the dyeing process. The temperature was increased at a rate of 2 1C min1 to 130 1C and held for 30 min and then cooled at a rate of 2 1C min1 to 60 1C. After dyeing, the samples were rinsed thoroughly in cold water and dried under ambient conditions. The exhausted dyebaths were subsequently measured using a Perkin-Elmer Lambda 9 UV/visible/NIR spectrophotometer in the visible region of the spectrum (400–700 nm), at 20 nm intervals. Concentrations were calculated from calibration graphs at the wavelength of maximum absorption (lmax). The residual dye solution was diluted using 50/50 v/v acetone/water and measured at lmax and percentage dyebath exhaustion (E%) calculated using Eq. (21), where A0 and A1 represent absorbance of dye solution before and after dyeing, respectively. No difference in the shape of the absorption spectrum before and after dyeing was noted: A1 E% ¼ 100 1 . (21) A0
Untreated
1.7224 1.7216 1.7208 Irradiated
1.7200 1.7192 1.7184 -40 -32 -24 -16
-8
0
8
16
24
32
40
Fibre Diameter (μm) Fig. 5. Refractive index profile nJ of untreated and irradiated fibres.
1.535 Refractive Index n2
1306
Untreated
1.534
1.533
1.532
1.531 -40
Irradiated
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm)
3. Results and discussion
Fig. 6. Refractive index profile n? of untreated and irradiated fibres.
3.1. Optical characteristics 0.190
Untreated Birefringence Δ Δn
The significant effect of the laser irradiation on the PET fibres can be seen from the difference between the microinterferograms in Figs. 3 and 4. For example, the relative value of the fringe displacement of the irradiated and untreated PET fibres was calculated to be Zirr/ Zuntr ¼ 1.61 for nJ (see Fig. 3). Figs. 5–9 show the profiles for the refractive indices nJ and n?, the birefringence Dn, the orientation function F(y) and the degree of crystallinity. An important consideration that has to be taken into account when interpreting these figures is that the profiling software quantifies the parameters for one half of the fibre and produces a mirror image about the vertical plane through the fibre centre axis; this is clearly seen from the symmetry that is observed in the figures. Figs. 5 and 6 show the measured refractive indices nJ and ? n , when the light is vibrating parallel and perpendicular to the fibre axis, respectively. As expected for PET fibres, the plots show positive birefringent properties (nJ4n?). The decrease in the refractive indices nJ and n? indicates that the shift factor which corresponds to the friction between
0.189 0.188 0.187 Irradiated
0.186 0.185 0.184 0.183 -40
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 7. Birefringence profile Dn of untreated and irradiated fibres.
the chain segments has reduced [18,19], which leads to an increase in the molecular mobility. From Fig. 7, it is evident that birefringence has decreased for laser-irradiated fibres, which indicates that PET fibre became more amorphous following laser
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Orientation Function F (θ))
0.802 0.798
Untreated
0.794 0.790 Irradiated 0.786 0.782
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be an inaccurate value for the constant in Eq. (11), which was obtained form published literature [15]. Nevertheless, both density and crystallinity can be said to have decreased after laser irradiation, which supports the view that the PET fibre became more amorphous following laser irradiation. Another observation from Fig. 9 is that the variation in the degree of crystallinity from the fibre outer surface to the fibre centre axis is less in the case of the irradiated fibre. The irradiation would appear to have resulted in a more uniform, although lower, level of crystallinity throughout the fibre cross-section.
0.778 0.774 -40
3.3. Wide-angle X-ray scattering -30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 8. Profile of orientation function F(y) of untreated and irradiated fibres.
Untreated 36.0 35.5 35.0
3.4. Tensile properties
34.5 34.0 Irradiated
33.5 33.0 -40
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 9. Profile of the degree of crystallinity (w) for untreated and irradiated fibres.
irradiation. The orientation function depends on both birefringence and the Herman’s factor, and it can be seen from Fig. 8 that the orientation function across the fibre diameter has reduced a small amount for the laserirradiated fibre. This result suggests that the average angle between the chain axis and the fibre axis across the fibre diameter has increased after laser irradiation. The results indicate a comparative disordering in the arrangement of the molecules within the fibre along the fibre axis [7], which may be attributed to chain relaxation [20].
Measurement of the mechanical properties showed only a 3% decrease in modulus and tensile strength, and 14% increase in breaking elongation. This suggests that Table 1 Measured density and % volume crystallinity calculated from density Sample
Measured density (g cm3) average
Standard deviation
% Volume crystallinity
Untreated Irradiated
1.3806 1.3734
0.00070 0.00182
38.070.58 32.071.52
12 23.9
10 16.9
~21.8
8 Intensity
Degree of Crystallinity χ (%)
36.5
Scatter plots obtained from the Huber WAXS goniometer for the untreated and irradiated PET fibres are shown in Figs. 10 and 11, respectively. The spatial values are given in Table 2. The main observation from the two scatter plots is the depressed presentation of the peaks for the laser-irradiated fibre, which is an indication of reduction in crystallinity. The variation in micro-strains due to laser irradiation, with respect to the 1 1¯ 0 plane, was determined as compressive strain (see Table 3).
6 4
3.2. Density and crystallinity Table 1 gives the density column values and the corresponding volume crystallinity. The data imply that the laser irradiation decreased the fibre crystallinity. The degree of crystallinity derived from the optical properties (Fig. 9), shows slight discrepancies with the values obtained with the density column measurements. The reason could
2 0 10
15
20
25
30
35
2Theta / deg.
Fig. 10. Wide-angle X-ray scatter plot for untreated PET fibre.
40
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1308 10
Table 4 % dye exhaustion for untreated and irradiated samples
9 17.1
8
Intensity
7
~22.0
Dyeing temperature (1C)
24.5
6
Untreated
Irradiated
% Exhaustion
Std. deviation
% Exhaustion
Std. deviation
37.3 25.5
0.3618 0.0012
46.7 28.8
1.8555 1.2875
5
130 110
4 3 2 1 0 10
15
20
25
30
35
2Theta / deg.
Fig. 11. Wide-angle X-ray scatter plot for laser irradiated PET fibre.
Table 2 Spacing between the diffracting planes for untreated and laser irradiated PET fibre Sample
Peak
2y (deg)
Corrected 2y (deg)
Spacing (A˚)
hkl
Untreated
1 2 3
16.9 21.8 23.9
17.3 22.2 24.3
5.126 4.004 3.663
010 1 1¯ 0 100
Laser Irradiated
1
17.1
17.5
5.068
010
2 3
22.0 24.5
22.4 24.9
3.969 3.576
1 1¯ 0 100
Table 3 Micro-strain for 1 1¯ 0 plane Spacing (A˚) for the plane 1 1¯ 0 Untreated
Laser irradiated
4.004
3.969
Micro-strain (no units)
0.00882
although the laser treatment was effective, the fibre retained good crystallinity.
3.5. Dye adsorption The results of the dyeings are given in Table 4; it is observed that there was a significant increase in dye adsorption (% exhaustion) for irradiated PET fibre, with respect to the untreated (non-irradiated) fibre, at the standard dyeing temperature, 130 1C. It is also evident that laser irradiation enables a small, but statistically significant, increase in dye adsorption at lower temperatures (110 1C). The liquor ratio used in this study was 1000:1 (due to the limitation of a small mass of fibre for the standard laboratory dyeing apparatus), typically, PET
would be dyed at a liquor ratio of 20:1 or lower, where % exhaustion could be higher [21]. As determined by the analytical methods, laser irradiation had increased the proportion of amorphous areas in the fibre. Hence, dye adsorption should have increased since dyes are only adsorbed and diffused into the amorphous regions of a polymer sorbent. Colour tests were made on two different regions along the dyed untreated and laser-irradiated fibres. A colour camera was used to image the two samples and the RGB values of the image regions were measured. The values of a subregion (4 106 pixels) of each region were averaged to generate the mean RGB values. Converting the mean RGB values into device-independent colour, then enabled the colours of the two regions to be compared—in this case, RGB-sRGB-XYZ (under 1964 CIE observer and D65 illuminant)—and using the CIELAB colour difference formula. The CIELAB colour difference was found to be 1.99. For spatially uniform stimuli, industrial tolerances are usually close to 1.0 CIELAB unit [22]. Further, it is known that for images, as opposed to spatially uniform stimuli, the magnitude of DE that is visually acceptable is between three and five CIELAB units [23]. Given that the magnitude of DE corresponding to the difference between the dyed untreated and laser-treated fibres is significantly less than these values, it can be assumed that the colour difference is too small to be detected visually. Therefore, while the increase in dye take-up was statistically significant, it is a very small change, which would be imperceptible to the eye. 4. Conclusions The results obtained would appear to confirm the notion that laser irradiation could be used as a method for inducing a small reduction in the polymer chain orientation and crystallinity of a fully drawn PET monofilament fibre, thereby subtly altering the measurable optical properties and density. Although small changes also occurred in the mechanical properties and dye take-up of the fibre, these were of little practical significance. The concept that very small changes in the optical properties of a monofilament could be made to be detectable by measurement but not visually, would seem to be feasible. A laser-irradiated monofilament could therefore be used as a tracer fibre within a brand label. Placed at a given location within the
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weave, the fibre could be extracted and its birefringence compared with neighbouring fibres. It would also seem possible to use density measurements to support the optical analysis of the label. Acknowledgements The authors acknowledge the financial support of the Engineering and Physical Sciences Research Council (EPSRC) for this research through Grant GR/S64929/01. References [1] [2] [3] [4]
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[8] Hamza AA, Sokkar TZN, El-Farahaty KA, El-Dessouky HM. J Opt A: Pure Appl Opt 1999;1:41. [9] Hermans PH, Platzek P. Kolloid Z 1939;88:67. [10] Kratky O. Kolloid Z 1933;64:213. [11] Hermans PH. Contributions to the physics of cellulose fibres. NorthHolland: Amsterdam; 1946. [12] Purvis J, Bower DI, Ward IM. Polymer 1974;14:398. [13] Nobbs JH, Bower DI, Ward IM, Patterson DH. Polymer 1974;15:287. [14] Kequiang C, Zhongxun C, Jranwei X. Text Res J 2002;72(4): 367. [15] De Vries H, Bonnebat C, Beautemps J. J Polym Sci Polym Symp 1977;58:109. [16] Mallick B, Behera RC, Patel T. Bull Mater Sci 2005;28(6):593. [17] Cullity BD. Elements of X-ray diffraction. London: Addison-Wesley Publishing Company Inc; 1978. p. 286. [18] Tager A. Physical chemistry of polymers. MIR: Moscow; 1978. [19] van Meerveld J, Peters GWM, Hu¨tter M. Rheol Acta 2004;44: 119. [20] Le Bourvellec G, Beautemps J. J Appl Polym Sci 1990;39:329. [21] Burkinshaw SM. Chemical principles of synthetic fibre dyeing. London: Blackie Academic and Professional; 1995. [22] Westland S, Ripamonti C. Computational colour science using MATLAB. New York: Wiley; 2004. [23] Stokes M, Fairchild MD, Berns RS. Colorimetrically quantified visual tolerances for pictorial images. Proceed TAGA 1992;757.
Optics & Laser Technology 39 (2007) 1301–1309 www.elsevier.com/locate/optlastec
Influence of laser irradiation on the optical and structural properties of poly(ethylene terephthalate) fibres V.N. Wijayathungaa, C.A. Lawrencea, R.S. Blackburna, M.P.U. Bandaraa, E.L.V. Lewisb, H.M. El-Dessoukya,, V. Cheungc a Centre for Technical Textiles, School of Design, University of Leeds, UK IRC in Polymer, School of Physics and Astronomy, University of Leeds, UK c Centre for Colour Design Technology, School of Design, University of Leeds, UK b
Received 27 October 2006; received in revised form 20 December 2006; accepted 22 December 2006 Available online 12 February 2007
Abstract Laser irradiation has been previously investigated for achieving uniform heating of polyethylene terephthalate (PET) fibres in the hotdrawing stage of the production process, so as to obtain better fibre mechanical properties. The optical properties and dye uptake of PET fibres also depend on the polymer chain orientation and crystallinity within the fibre structure. This paper reports an investigation of a concept whereby laser irradiation and interferometry could be used to modify and trace a small change in the optical properties of a PET monofilament fibre, but the corresponding change in the dye uptake would not be detected visually. A copper vapour laser (550–580 nm wavelengths) was used to expose consecutive 4 mm lengths along a running length of monofilament to 39.8 W cm2, at a pulse rate of 9.89 kHz in order to modify, in a controlled way, the polymer crystallinity and orientation. A 3D finite element simulation, based on uncoupled heat-transfer analysis, indicated that rapid heating and cooling could be obtained with the laser to give the small changes required. Irradiated and untreated samples were analysed by interferometry and a 0.16% change was detected in the birefringence profiles, corresponding to a small reduction in the degree of orientation and crystallinity of the irradiated samples. Density measurements and wide-angle X-ray scattering (WAXS) analysis confirmed the change in crystallinity. Tests conducted for dye adsorption and tensile strength showed a small increase in the former and only a very small decrease in the latter. It was concluded that these changes in property provide the opportunity for a laser-irradiated PET monofilament fibre to be used as a subtle tracer element in brand labels for textile garments as an anti-counterfeit measure. r 2007 Elsevier Ltd. All rights reserved. Keywords: Laser irradiation; Interferometry; Optical properties
1. Introduction Every year textile manufacturers and retailers are subject to significant financial losses because of counterfeit fashionable brand products, e.g. jeans, sportswear and premiership replica football shirts. Detecting such counterfeit goods has become increasingly difficult as identification is largely based on the manufactured quality. Today, the widespread availability of advanced textile machines with Corresponding author. Tel.: +44 0113 2172952; fax: +44 0113 3433704. E-mail addresses: [email protected], [email protected] (H.M. El-Dessouky).
0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.12.007
integrated computer process control enables the counterfeiter to produce copies of brand products virtually identical in appearance and quality to the legitimate goods. Although barcodes and miniature electronic devices may be used for anti-counterfeit they also can be copied or damaged. It is, therefore, of interest to develop security markings integral to the product and preferably within the identifying brand labels. Modifying the optical properties of a monofilament which is then incorporated into the weave of the brand label would offer the possibility of integral security markings, provided the differences from the neighbouring fibres are not visible to the human eye, say in the dyed state, but perceptible by optical measurements. Although
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the optical properties of a fibre may be modified by various means, the concept is to simply heat treat a fully drawn monofilament to effect just a traceable change in the crystallinity and refractive indices, nJ and n?, and thereby the birefringence. To achieve a very small change would require rapid heating and cooling of short lengths consecutively along a set length of the monofilament, and laser irradiation would seem an effective method. If a fibre is exposed to a narrow laser beam, respective proportions of the incident light will be transmitted, reflected, and give rise to radiative heat transfer. When considering methods of heat transfer into fibre, laserinduced heat flux offers important advantages. It provides almost instantaneous and more uniform heating than any other thermal treatment, and the temperature field cools faster when the incident flux is removed. Laser treatment of polyester fibres has been previously employed, primarily to overcome large temperature differences arising through the fibre cross-section, which occurs when traditional fibreheating methods are used at high fibre-running speeds. Conventional methods of heating based on surface contact are probably the most popular. However, polymer fibres have generally low thermal conductivity. Therefore, obtaining fast heat transfer is restricted. The first patent application for the laser heating methodology was put forward by the Asahi Chemical Industry Company of Japan in 1973 [1], which was followed by applications from the Mitsubishi Rayon Company in the same year [2] and the Toyobo Company in 1985 [3]. These patents are concerned with the localized drawing of a fibre under instantaneous and uniform heating across a narrow section of the fibre, so as to achieve a higher degree of polymer chain alignment (orientation) along the fibre axis. Okumura et al. [4] also utilized laser radiation in a continuous drawing process for PET fibres and compared experimental and theoretical analysis of the temperature profile along the heated fibre. The laser application carried out in the present study aims to slightly reduce the orientation and crystallinity of a pre-drawn monofilament, where the drawing was done by the widely used commercial process of heated godet rollers. A subtle alteration should make the fibre a tracer element among the untreated fibres of the same polymer type forming a brand label, detectable by a slight difference in the measurable optical properties but not visually apparent.
2.2. Laser irradiation A custom-designed copper vapour laser was used to provide the heating light with a measured irradiance at the fibre exposure point of 39.8 W cm2, a pulse rate of 9.89 kHz and a wavelength of 550–580 nm. The diameter of the laser was 4 mm at the fibre exposure point; the focal point was inside the outer aperture of the optical instrument used for focusing the laser beam and the fibre travel was 2 mm outside the outer aperture. The back plate was 15 mm away from the fibre path. The fibre exposure arrangement is shown in Fig. 1. The fibre was pulled across the laser beam with the help of a take-up spindle driven by a 50 W DC shunt motor, at a slow speed of 30 mm min1. The ratio of input and output roller speeds was 1.05, which gave the lowest possible tension of 5 mN. Two polished ceramic guides were used to maintain a straight fibre path across the laser beam. In order to visualize the heat transfer pattern due to laser irradiation, a 3D finite element simulation was attempted using ABAQUS software. This enabled an understanding of the temperature profile. The approach used eight-node continuum elements with heat transfer capability, and uncoupled transient heat transfer analysis was then performed with the application of irradiance. Fig. 2 depicts the simulated localized heating of a fibre as it passes through the laser beam. Within 0.2 s the central portion of the 4 mm heated length attains ca. 34 1C; after 4 s the heating has propagated further along the length and the maximum temperature is now at 144 1C; by 8 s (the duration for which the 4 mm length is exposed to the beam) a temperature of 204 1C is reached. On either side of the heated zone the temperature decreases rapidly along the fibre length, reaching 63 1C after 8 s of heating. As would be expected, the surface facing the projected beam reaches the highest temperature and has the most rapid temperature decrease through the fibre cross-section. However, the temperature difference between the front and back surfaces, relative to the beam contact, is only 3 1C. The laser application was therefore accepted as providing a reasonably uniform thermal treatment.
2. Theoretical and experimental considerations 2.1. Fibre material A commercial grade of fully drawn PET monofilament fibre of 70 decitex linear density and diameter 0.08 mm (commonly used in textile labels) was obtained from Teijin Monofilaments GmbH.
Fig. 1. Fibre exposure to laser arrangement.
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Fig. 2. Simulated transient temperature profiles during the heat transfer process; (d) shows the temperature contours across the fibre section.
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2.3. Optical and structural characteristics Investigation of the optical properties was based on the observations made with an Interphako interference microscope having a fitted Mach–Zehnder interferometer and CCD camera for image capture. For light (l ¼ 550 nm) vibrating parallel and perpendicular to the fibre axis, the samples were immersed in liquid having refractive indices nL ¼ 1.725 and 1.532, respectively. Subsequently, microinterferograms (see Figs. 3 and 4) were captured, recorded and analysed by the profile software [5,6].
2.3.1. Refractive indices (nJ and n?) profile The refractive index profile of a fibre reflects the variation of its refractive index across the fibre diameter. In the case of duplicated position of a two-beam interference microscope, the refractive index (n) profile of fibres (taking into account the refraction of the incident beam by the fibre layers) can be determined automatically using special software [7] based on Eq. (1) [5,6,8]: lZ Q ¼ b
Q¼1 X
2nj ½K 1 K 2 1=2 þ 2nQ K 3 n0 K 4 ,
(1)
j¼1
where K1, K2, K3 and K4 are given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d Q n0 2 2 K 1 ¼ ½R ðj 1Þa , nj
(2)
Fig. 4. Microinterferograms using Interphako microscope when the light is vibrating perpendicular to the axis of untreated (top) and irradiated (bottom) fibres.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi d n Q 0 , K 2 ¼ ½R ja2 nj sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi d n Q 0 K 3 ¼ ½R ðQ 1Þa2 , nQ
K4 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R2 d 2Q þ R2 X 2Q .
(3)
(4)
(5)
where R is the fibre radius, a is the layer thickness (a ¼ R/ N), N is the number of layers, Q and j are the limits of the layer numbers, n0 ¼ nL and is the liquid refractive index, nj and nQ are the refractive indices of the jth layer and Qth layer, respectively, dQ is the distance between the incident and the fibre centre, XQ is the distance between the emerging beam and the fibre centre, ZQ is the fringe shift corresponding to the point XQ in the two-beam interference pattern, l is the wavelength of light used and b is the interfringe spacing. 2.3.2. Birefringence Dn profile Birefringence profile of a fibre can be obtained from the measured refractive indices, nJ (parallel) and n? (perpendicular) as follows: Dn ¼ njj n? .
Fig. 3. Microinterferograms using Interphako microscope when the light is vibrating parallel to the axis of untreated (top) and irradiated (bottom) fibres.
(6)
2.3.3. Orientation function f(y) The orientation function, f(y), of polymer fibres can be calculated from the relation [9,10] f ðyÞ ¼ ð1 þ xÞf D xf 2D ,
(7)
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where the parameter x is given by x¼
2n2k n2? n3v ðnk n? Þ
1
(8)
and fD is the Herman’s factor [11] which is given by fD ¼
Dn . Dnmax
(9)
The strains developed in the crystallites due to the laser irradiation could manifest as changes in the lattice planes, causing line shifting. These changes in the lattice planes are sensitive to the X-ray diffraction methods, provided that variations in the scatter peaks are properly identifiable. As described by Mallick et al. [16] strain can be verified by differentiating the Bragg’s law: l , 2 sin y
Here, Dnmax is the maximum birefringence and its value for PET fibres is 0.24 [12,13].
d¼
2.3.4. Degree of crystallinity The volume crystallinity w is related to the density according to r ra w¼ , (10) rc ra
Dd 1 ¼ l cosec y cot y ¼ d cot y, Dy 2
where r is the density of the sample, ra is the density of the amorphous region (1.335 g cm3) and rc is the density of the crystalline region equal to (1.455 g cm3) [14]. The density r (g cm3) of PET fibres can be calculated from the relation [15] 2 n¯ 1 r ¼ 4:047 2 , (11) n¯ þ 2 where n¯ is the average refractive index of fibre, which is given by n¯ ¼
ðnjj þ 2n? Þ . 3
(12)
2.4. Density Fibre density was determined using a density gradient column according to BS2782-6: method 620D. Potassium iodide aqueous solution was used at 2370.1 1C. Five specimens of the laser treated and the untreated fibre were used to obtain an average reading. Volume crystallinity (w) was calculated using Eq. (10). 2.5. Wide-angle X-ray scattering (WAXS)
Dd ¼ cot y Dy. d Since diffraction angle is measured in 2y, Dd D2y ¼ cot y , d 2
(15)
(16)
(17)
Dd tan y. (18) d Eq. (18) allows the variation in strain, Dd/d to be calculated from the observed broadening of the scatter. If du indicates the untreated spacing and ds the spacing in the irradiated fibre, the micro-strain in the fibre elements in the direction normal to the diffracting plane (e) is Dd ds du ¼ ¼ . (19) d du If ds4du, then Dd/d is positive which indicates that the residual stress is tensile and if dsodu, then Dd/d is negative indicating compressive residual stress. This value of Dd/d, however, includes both tensile and compressive strains. Assuming both are equal for the micro-crystallites, Cullity [17] specified that the value of Dd/d must be divided by two to obtain the maximum tensile strain alone, or maximum compressive strain alone. Hence, maximum micro-stress present in the sample (s) can be defined as
2.6. Tensile properties
nl ¼ 2d sin y,
2.7. Dye sorption
where the value of integer n is equal to 1, d is the distance between the scatter planes, l is the wavelength of the incident ray and y is half the diffraction angle of the ray.
(14)
D2y ¼ 2
WAXS analysis of the fibres was done on a Huber WAXS texture goniometer 4020 configuration, operating at 40 kV and 30 mA. The radiation was a crystal monochromated Cu-Ka (wavelength 1.5418 A˚). The detector slit size was 2 mm in width and 6 mm in height. Equatorial scanning was done for a fibre sample exposed to the laser and an untreated sample, for 2y angles starting from 101 up to 351 and 401, respectively. The correction for measured 2y angle based on a standard polyethylene sample was 0.401. The distance between scattering planes were calculated using Bragg’s equation as the following: (13)
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1 Dd E¼ E, 2 2 d where E is Young’s modulus of the material. s¼
(20)
The load-elongation characteristics of the fibre were measured before and after the laser irradiation in order to determine the significance of the changes to the mechanical properties.
To determine the visual influence of the laser treatment, 0.05 g fibre samples of before and after treatment, were
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1.7232 Refractive Index n1
dyed with C.I. Disperse Yellow 19 (supplied by DyStar) at 5.0% omf concentration at a liquor ratio of 1000:1, with the addition of 1 g dm3 of Levagal DLP (levelling agent) and 2 g dm3 of Ludigol AR (anti-reducing agent) and pH buffered to pH 4.5–5.0 (sodium acetate/acetic acid). The fibre samples were wound loosely around a stainless steel ‘H’ frame and placed in 300 cm3 stainless steel dyepots housed in a Roaches Pyrotec 2000 infra-red dyeing machine, providing agitation throughout the dyeing process. The temperature was increased at a rate of 2 1C min1 to 130 1C and held for 30 min and then cooled at a rate of 2 1C min1 to 60 1C. After dyeing, the samples were rinsed thoroughly in cold water and dried under ambient conditions. The exhausted dyebaths were subsequently measured using a Perkin-Elmer Lambda 9 UV/visible/NIR spectrophotometer in the visible region of the spectrum (400–700 nm), at 20 nm intervals. Concentrations were calculated from calibration graphs at the wavelength of maximum absorption (lmax). The residual dye solution was diluted using 50/50 v/v acetone/water and measured at lmax and percentage dyebath exhaustion (E%) calculated using Eq. (21), where A0 and A1 represent absorbance of dye solution before and after dyeing, respectively. No difference in the shape of the absorption spectrum before and after dyeing was noted: A1 E% ¼ 100 1 . (21) A0
Untreated
1.7224 1.7216 1.7208 Irradiated
1.7200 1.7192 1.7184 -40 -32 -24 -16
-8
0
8
16
24
32
40
Fibre Diameter (μm) Fig. 5. Refractive index profile nJ of untreated and irradiated fibres.
1.535 Refractive Index n2
1306
Untreated
1.534
1.533
1.532
1.531 -40
Irradiated
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm)
3. Results and discussion
Fig. 6. Refractive index profile n? of untreated and irradiated fibres.
3.1. Optical characteristics 0.190
Untreated Birefringence Δ Δn
The significant effect of the laser irradiation on the PET fibres can be seen from the difference between the microinterferograms in Figs. 3 and 4. For example, the relative value of the fringe displacement of the irradiated and untreated PET fibres was calculated to be Zirr/ Zuntr ¼ 1.61 for nJ (see Fig. 3). Figs. 5–9 show the profiles for the refractive indices nJ and n?, the birefringence Dn, the orientation function F(y) and the degree of crystallinity. An important consideration that has to be taken into account when interpreting these figures is that the profiling software quantifies the parameters for one half of the fibre and produces a mirror image about the vertical plane through the fibre centre axis; this is clearly seen from the symmetry that is observed in the figures. Figs. 5 and 6 show the measured refractive indices nJ and ? n , when the light is vibrating parallel and perpendicular to the fibre axis, respectively. As expected for PET fibres, the plots show positive birefringent properties (nJ4n?). The decrease in the refractive indices nJ and n? indicates that the shift factor which corresponds to the friction between
0.189 0.188 0.187 Irradiated
0.186 0.185 0.184 0.183 -40
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 7. Birefringence profile Dn of untreated and irradiated fibres.
the chain segments has reduced [18,19], which leads to an increase in the molecular mobility. From Fig. 7, it is evident that birefringence has decreased for laser-irradiated fibres, which indicates that PET fibre became more amorphous following laser
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Orientation Function F (θ))
0.802 0.798
Untreated
0.794 0.790 Irradiated 0.786 0.782
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be an inaccurate value for the constant in Eq. (11), which was obtained form published literature [15]. Nevertheless, both density and crystallinity can be said to have decreased after laser irradiation, which supports the view that the PET fibre became more amorphous following laser irradiation. Another observation from Fig. 9 is that the variation in the degree of crystallinity from the fibre outer surface to the fibre centre axis is less in the case of the irradiated fibre. The irradiation would appear to have resulted in a more uniform, although lower, level of crystallinity throughout the fibre cross-section.
0.778 0.774 -40
3.3. Wide-angle X-ray scattering -30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 8. Profile of orientation function F(y) of untreated and irradiated fibres.
Untreated 36.0 35.5 35.0
3.4. Tensile properties
34.5 34.0 Irradiated
33.5 33.0 -40
-30
-20
-10
0
10
20
30
40
Fibre Diameter (μm) Fig. 9. Profile of the degree of crystallinity (w) for untreated and irradiated fibres.
irradiation. The orientation function depends on both birefringence and the Herman’s factor, and it can be seen from Fig. 8 that the orientation function across the fibre diameter has reduced a small amount for the laserirradiated fibre. This result suggests that the average angle between the chain axis and the fibre axis across the fibre diameter has increased after laser irradiation. The results indicate a comparative disordering in the arrangement of the molecules within the fibre along the fibre axis [7], which may be attributed to chain relaxation [20].
Measurement of the mechanical properties showed only a 3% decrease in modulus and tensile strength, and 14% increase in breaking elongation. This suggests that Table 1 Measured density and % volume crystallinity calculated from density Sample
Measured density (g cm3) average
Standard deviation
% Volume crystallinity
Untreated Irradiated
1.3806 1.3734
0.00070 0.00182
38.070.58 32.071.52
12 23.9
10 16.9
~21.8
8 Intensity
Degree of Crystallinity χ (%)
36.5
Scatter plots obtained from the Huber WAXS goniometer for the untreated and irradiated PET fibres are shown in Figs. 10 and 11, respectively. The spatial values are given in Table 2. The main observation from the two scatter plots is the depressed presentation of the peaks for the laser-irradiated fibre, which is an indication of reduction in crystallinity. The variation in micro-strains due to laser irradiation, with respect to the 1 1¯ 0 plane, was determined as compressive strain (see Table 3).
6 4
3.2. Density and crystallinity Table 1 gives the density column values and the corresponding volume crystallinity. The data imply that the laser irradiation decreased the fibre crystallinity. The degree of crystallinity derived from the optical properties (Fig. 9), shows slight discrepancies with the values obtained with the density column measurements. The reason could
2 0 10
15
20
25
30
35
2Theta / deg.
Fig. 10. Wide-angle X-ray scatter plot for untreated PET fibre.
40
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1308 10
Table 4 % dye exhaustion for untreated and irradiated samples
9 17.1
8
Intensity
7
~22.0
Dyeing temperature (1C)
24.5
6
Untreated
Irradiated
% Exhaustion
Std. deviation
% Exhaustion
Std. deviation
37.3 25.5
0.3618 0.0012
46.7 28.8
1.8555 1.2875
5
130 110
4 3 2 1 0 10
15
20
25
30
35
2Theta / deg.
Fig. 11. Wide-angle X-ray scatter plot for laser irradiated PET fibre.
Table 2 Spacing between the diffracting planes for untreated and laser irradiated PET fibre Sample
Peak
2y (deg)
Corrected 2y (deg)
Spacing (A˚)
hkl
Untreated
1 2 3
16.9 21.8 23.9
17.3 22.2 24.3
5.126 4.004 3.663
010 1 1¯ 0 100
Laser Irradiated
1
17.1
17.5
5.068
010
2 3
22.0 24.5
22.4 24.9
3.969 3.576
1 1¯ 0 100
Table 3 Micro-strain for 1 1¯ 0 plane Spacing (A˚) for the plane 1 1¯ 0 Untreated
Laser irradiated
4.004
3.969
Micro-strain (no units)
0.00882
although the laser treatment was effective, the fibre retained good crystallinity.
3.5. Dye adsorption The results of the dyeings are given in Table 4; it is observed that there was a significant increase in dye adsorption (% exhaustion) for irradiated PET fibre, with respect to the untreated (non-irradiated) fibre, at the standard dyeing temperature, 130 1C. It is also evident that laser irradiation enables a small, but statistically significant, increase in dye adsorption at lower temperatures (110 1C). The liquor ratio used in this study was 1000:1 (due to the limitation of a small mass of fibre for the standard laboratory dyeing apparatus), typically, PET
would be dyed at a liquor ratio of 20:1 or lower, where % exhaustion could be higher [21]. As determined by the analytical methods, laser irradiation had increased the proportion of amorphous areas in the fibre. Hence, dye adsorption should have increased since dyes are only adsorbed and diffused into the amorphous regions of a polymer sorbent. Colour tests were made on two different regions along the dyed untreated and laser-irradiated fibres. A colour camera was used to image the two samples and the RGB values of the image regions were measured. The values of a subregion (4 106 pixels) of each region were averaged to generate the mean RGB values. Converting the mean RGB values into device-independent colour, then enabled the colours of the two regions to be compared—in this case, RGB-sRGB-XYZ (under 1964 CIE observer and D65 illuminant)—and using the CIELAB colour difference formula. The CIELAB colour difference was found to be 1.99. For spatially uniform stimuli, industrial tolerances are usually close to 1.0 CIELAB unit [22]. Further, it is known that for images, as opposed to spatially uniform stimuli, the magnitude of DE that is visually acceptable is between three and five CIELAB units [23]. Given that the magnitude of DE corresponding to the difference between the dyed untreated and laser-treated fibres is significantly less than these values, it can be assumed that the colour difference is too small to be detected visually. Therefore, while the increase in dye take-up was statistically significant, it is a very small change, which would be imperceptible to the eye. 4. Conclusions The results obtained would appear to confirm the notion that laser irradiation could be used as a method for inducing a small reduction in the polymer chain orientation and crystallinity of a fully drawn PET monofilament fibre, thereby subtly altering the measurable optical properties and density. Although small changes also occurred in the mechanical properties and dye take-up of the fibre, these were of little practical significance. The concept that very small changes in the optical properties of a monofilament could be made to be detectable by measurement but not visually, would seem to be feasible. A laser-irradiated monofilament could therefore be used as a tracer fibre within a brand label. Placed at a given location within the
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weave, the fibre could be extracted and its birefringence compared with neighbouring fibres. It would also seem possible to use density measurements to support the optical analysis of the label. Acknowledgements The authors acknowledge the financial support of the Engineering and Physical Sciences Research Council (EPSRC) for this research through Grant GR/S64929/01. References [1] [2] [3] [4]
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