Visible Light Shielding Performance of Fabrics with Non-Circular Cross Section Fiber Xiaosong Liu, Fumei Wang College of Textiles, Donghua University, Shanghai CHINA Correspondence to: Fumei Wang email: jl_textile@mail.dhu.edu.cn ABSTRACT The models of polyethylene terephthalate (PET) non-circular cross-section (NCCS) fibers were established, and the characteristic parameters of non-circular cross-section fiber used in this numerical simulation were extracted. Furthermore, the differences of refracted light intensity ratio Irz and the direction of refracted light βt among circular, trilobal and quadri-lobal cross-section fibers were analyzed theoretically. The results show that the refracted light intensity ratios Irz of these fibers were in the range of 0.94~0.95. Both the trilobal and quadri-lobal cross-section fibers' βt, which was the angle of refracted light to Y-axis, changed non-monotonically and much more intricately than that of circular cross-section ones. Moreover, according to the theory of geometrical optics, the effects of trilobal and quadri-lobal cross-section shapes on the changes of internal refracted light and transmitted light in fibers were also conducted. The results suggested that the refracted and transmitted light were changed more effectively in the quadri-lobal cross-section fiber. The results of the experiment show that the shielding properties of quadri-lobal cross-section filament fabrics were better than that of the counterparts with circular fibers, but the difference was limited or insignificant. Consequently, people began to study the effects that the special technologies [2, 3] made on the NCCS fibers with respect to the yarn’s and fabric’s aesthetics, appearance and ‘feel’ in the early 1970s. NCCS fibers usually have excellent capillary and wicking properties for removing moisture, therefore, it was suggested for use in many applications, such as nappies, sanitary towels, filter media, shoe liners and wound coverings [4, 5, 6, 7]. There is much research on the cross section of the NCCS fiber, and the drafting behavior of the NCCS polyester fiber was investigated to determine the optimal conditions for spinning the P/C blend yarns [8]. A new method was developed to calculate the refractive index [9]. The Japanese [10-15] have carried out studies on the properties of NCCS fibers, to some extent, concentrating on performances of the fiber assemblies, for instance the handle, moisture permeability, or imitating the style of the natural silk fiber fabric. Recently, various types of NCCS fibers were used to produce fiber reinforced composites (FRCs) [16-19]. Due to its high surface adhesion on their larger specific areas, not only the characteristics of the structure and surface were improved, but also the properties of mechanical and thermal conductivity. Thorough studies on the optical performance of single fibers with conventional circular cross sections or cylindrical cross sections were conducted [20-26]. The geometric and photometry theory were used to analyse the optical property of the light reflection and transmission [20-23], dispersion [24, 25], the Descartes-ray and Rainbow phenomenon [25, 26] of circular fibers. The shielding fabrics were developed by [27-31] adjusting the weaving process, developing multi-layer or double-layer fabrics, coating the surface of fabrics, or improving pigment printing technology. Several studies were focused on the reflection goniophotometric curves of woven INTRODUCTION The synthetic fibers, polyethylene terephthalate (PET) and polyamide (nylon in market), were normally round in their cross-sections, and natural fibers don’t have a perfect circular cross-section. Cotton has a ‘kidney-type’ or ‘dog-bone’ shape, and silk has a roughly triangular shape. DuPont developed the first non-circular cross-section (NCCS) fiber made by the expanded adhesion spun technology [1]. Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 50 http://www.jeffjournal.org fabrics [32-35]. Tsuneyo, et al [32], studied the goniophotometric curves of the monochromatic specular and diffuse light to evaluate the optical properties and color change of fabrics, which was suitable for fabrics made of conventional circular fibers. Nobuo, et al [33,34], developed two models on entire cross section of a yarn and simulated the goniophotometric curves in a refractive index range [35] from 1.3 to 1.8. The effects on the cross sectional shape of the NCCS fiber on the radiative properties, such as extinction and scattering efficiencies [36], was carried out numerically to predict the intensity distribution of the scattered radiation using the finite element method (FEM). Although the effects of light on the NCCS fiber materials in nature was similar to sound [37] and acoustics [38], there was little reported in the literature on the effects on the light shielding performance of fiber itself and its assembly. Theoretical Calculation The Cartesian coordinates were set as shown in Figure 1, and it was clear that r1<r<R. The refractive index of the outside medium and the fiber were n0 and n1 (n0<n1), respectively. According to the Fresnel formula [39], the reflection coefficient R(θR) was obtained from Eq. (1). (1) And the refraction angle was derived from the refractive formula as expressed in Eq. (2) (2) When the light on the surface of the fiber was a vertical incidence, the reflection coefficient was fixed value as shown in Eq. (3) In this paper, a theoretical model was developed based on the principles of geometrical optics; the optical properties on the surface of a single trilobal and quadrilobe cross-section fiber were conducted. Then experimental studies were carried out on the effect of a cross-section shape on the visible light shielding properties of the fabrics with NCCS fibers, and light shielding performance between the fabrics with NCCS fiber were tested. (3) Now, the trilobal cross section of the fiber was set to calculate the distribution, and the results of the theoretical formula of each component are shown as follows. On the surface of the protruded arc CA, supposing θtCA was the angle of normal on one point of arc CA in top ground circle O1 to Y axis forward, the range was 0 to θtA, therefore THEORETICAL CALCULATION OF NCCS FIBERS Model of Fiber Cross-Section Characteristics The transverse cross-section of a trilobal and a quadri-lobal fiber were painted using Pro/Engineer Wildfire 4.0 under conditions where the surface was smooth, and then the fibers’ cross-section shape outlines were divided into several independent parts, including circular arcs and linear portions linked in sequences. Their design sketches are shown as insets in the upper-right hand corner of Figure 1, in which different colors stand for different modalities of the NCCS fiber, and the same colors represent the same properties of light reflectivity on each segment surface. Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 (4) And θRCA the incident angle on the surface of the arc CA was calculated from Eq. (5). (5) 51 http://www.jeffjournal.org FIGURE 1. Sketches of a single trilobal (a) and a quadrilobe (b) fiber cross-section and the incident light coordinate of the trilobal and the quadrilobe cross-section, respectively. With θ0 and θt changed, the distribution of reflective light intensity Ir(θRCA) on the surface of the arc CA was derived. Taking no consideration of light absorption in the fiber, the total incident light intensity I0 was equal to the reflected light intensity Ir (described as Eq. (6) plus the refraction light intensity I0-Ir) so the relative intensity ratio Irz could be expressed as Eq. (7). TABLE I. Values of angles Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 t ~ (6) (7) and 't ~ 52 of single trilobal and quadrilobe. http://www.jeffjournal.org light intensity declined and formed a trough as shown in Figure 2(c); or, under particular conditions of θ0, the incident light would not fall on the boundary point αt=2π/3rad. But, the simulation was still working for these condition values at tangent points, so there would be a sharp rise and a peak was formed in βt shown in Figure 3(c), and could not be calculated correctly due to the very complex process at boundary point αt=2π/3rad. TABLE II. Computation parameters. In Figure 3, the platform regions represented the angle value of βt, so that the refracted light to the forward Y-axis, on the surface of AB and B1A1 linear segments of trilobal and quadri-lobal cross section fibers, and βt at different locations on the surface of the circular cross section, increased gradually with αt. * Refractive index is 1.65, a selected value of polyester fiber (PET) [20]. Numerical Simulation To investigate the reflective ability and the distribution morphology of the reflected light intensity on each segment’s surface, and predict the effects of the optical properties on the assemblies of the NCCS fibers, especially that of trilobal and quadri-lobal fibers, the numerical simulation results were obtained by writing MATLAB programs. The values of the parameters, marked in Figure 1, used in the programs were listed in Table I and Table II, and θt of different circular arcs were divided into m segments. Simulation Results (1) The relative intensity ratio Irz and the direction of refracted βt: The simulation results of the relative intensity ratio Irz and the direction of refracted βt are shown in Figure 2 and Figure 3, respectively. And αt in each figure was the angle of any point on the surface of the fiber with respect to the forward Y-axis, the range of which was 0-π/2 both in circular and quadri-lobal, but 0-2π/3 in trilobal. From Figure 2, we can see that the overall light intensity refractive ratio of the fiber-translucent material was roughly in the range of 0.94-0.95. The overall differences among the light intensity refractive rations in different parts of the circular, trilobal and quadri-lobal cross sections in different conditions were not remarkable, so the differences in cross-section morphology had little effect on the light intensity. As the fiber morphology itself could make the incident light tangented at the border point αt=2π/3rad on the surface of the trilobal cross-section and, thus, the relative refraction of Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 FIGURE 2. Irz vs. αt of three cross section fibers (a: θ0=π/3, b: θ0=π/4, c: θ0=π/6). 53 http://www.jeffjournal.org (2) Geometrical Optics Analysis: The bump point in the βt-αt line was for the value of βt when the incident angle was 0 clearly in the curves of the circular cross section, which was exactly equal to the value of θ0. Under the three light incident angles θ0, the values of βt for the circular, trilobal and quadri-lobal cross sections were 0.7, 0.7, 1.0rad, respectively. However, compared with that of the circular cross section, the number of changed frequencies on the optical path of trilobal and quadri-lobal cross section were increased significantly. The light paths transmitted through the trilobal and quadri-lobal cross section fibers could be drawn as shown in Figure 4(a) and Figure 4(b) according to the boundary conditions of the numerical simulation results shown in Figure 2 and Figure 3. When θ0=π/3, the emitted light in AB and B1A1 of trilobal produced a total reflection phenomenon (Figure 4(a-1)). The total reflection phenomenon happened partly due to the transmitted light in AB of the quadri-lobal and less in B1A1 (Figure 4(a-2)), which led to more luster in the trilobal than that of the circular and quadri-lobal cross section. When θ0=π/6rad partly transmitted light in AB, of the trilobal and the quadri-lobal cross sections, it generated a total reflection. θ0=π/6rad generated transmitted light in B1A1, of the trilobal cross section (Figure 4(b)). The recurrence of the reflection phenomenon and refraction of partially transmissive light falling on the bottom surface of the quadri-lobal cross section was clearly shown, but the intensity was reduced and messy in that direction. Figure 4 suggested that the role of the circular cross section was the focused light, and the same thing also occurred on the arc AC and the arc A1C1 in the trilobal and quadri-lobal cross sections. The changes on these cross sections varied more dramatically than that on the circular cross section. However, the refracted light angle βt of the arc BB1, expressed as a scattering of light, was gradually decreased with the change of αt. FIGURE 3. βt vs. αt of three cross section fibers (a: θ0=π/3, b: θ0=π/4, c: θ0=π/6) Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 54 http://www.jeffjournal.org MATERIALS AND EXPERIMENT Fiber Materials Two comparable chemical circular and quadrilobe cross-section fibers made of the semi-dull polyethylene terephthalate (PET) slices interwoven with cotton plied yarns and knitted ones were used to compare the light shielding properties. The specifications of the different PET drawings of textured yarns (DTY) are shown in Table III. In the theoretical simulation, the most significant difference in the light reflective properties between the trilobal and quadri-lobal cross sections were realized by the light reflection effects of the concave arc. But a better dispersion of light appeared in the non-circular cross-section fiber compared to the surface light reflection of the circular fiber. Cross section morphological examinations of the circular and quadri-lobal cross section fibers were performed by using scanning electron microscope (SEM) images as shown in Figure 5. As can be seen from Figure 5(a) and Figure 5(c), the samples Y1# and Y3# named the same as the quadri-lobal cross section, but differed in their actual cross-section because the Y1# cross-section was very close to the theoretical mode described above as in Figure 1. Under the same magnification, the lobes of sample Y3# were significantly thicker and shorter, and the degree of the uneven surface was lower than that of Y1#, and this may have a pronounced effect on its optical performance results. TABLE III. Specifications of different cross-section polyester filament yarns. FIGURE 4. Geometrical optics analysis of transmitted light (a: θ0=60°and b: θ0=30°). Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 55 http://www.jeffjournal.org used as the warp. The fabric parameters were calculated based on the covering factor [40] ET with the value of 37.61% and the specific weaving parameters are shown as follows in Table IV. The knitted weft plain stitch samples and their specification parameters of F6#, F7# in Figure 6(f) and Figure 6(g) are shown in Table V. The alkali boil-off and soaping processing on these fabrics were carried out to remove the natural impurities of the cotton fiber and the spinning oil on the polyester filaments, thereby reducing the impact on the fabrics’ shielding performance. All the samples were placed for 24 hours at 20±2Ԩ and 65±5% relative humidity before being tested. Morphologies of Fabrics Figure 6 shows the surface morphologies of the treated fabrics viewed using a Koshida-1000 3D microscope (United States) under 200 times magnification. The Shielding Properties Testing A Hitachi U-4100 UV-Vis spectrophotometer tested the light shielding properties of the non-circular cross-section polyester/cotton union and the knitted fabrics in the visible range. The average transmission coefficient Tj was chosen as an objective indicator for evaluating the shielding performance of the fabrics by analyzing the transmission coefficient spectrogram. FIGURE 5. SEM images of different cross-sectional polyester fibers. RESULTS AND DISCUSSION Figure 6 shows the surface images of the designed fabric specimens described above. The number of holes and its size distribution in the unit length of 0.5 mm indicates that the structure of the fabrics impacted the light shielding properties as they were different, but the light shielding performance was influenced by the cross section of fiber/monofilament under the same fabrics’ structure. Fabric Samples Since there are many grooves on the surface, the non-circular cross-section filament yarns cannot be used as the yarns warp through sizing. Meanwhile, the sizing agents have unpredictable effects on the optical performance. To avoid this impact, the non-circular cross section polyester filament yarns were used as weft yarns to produce a suitable fabric structure that could maximize the non-circular cross-section polyester filament yarns appearing on the surface of the fabric. Cotton plied yarns were Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 56 http://www.jeffjournal.org TABLE IV. Structure parameters of woven fabric samples. TABLE V. Structural parameters of knitted fabric samples. *measured date FIGURE 6. Surface images of the treated NCCS polyester fabrics (200×). Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 57 http://www.jeffjournal.org The average transmission coefficient Tj using the UV-Vis spectrophotometer analysis and the typical measured curves of the transmission coefficient spectrogram, for evaluating the shielding performance of the fabrics, are shown in Table VI and Figure 7, respectively. This is because the refraction light intensity ratio Irz on the surface of both the circular and quadrilobe cross section PET fiber was roughly in the range of 0.94-0.95, according to the theoretical calculations of optical properties on the surface of the single non-circular cross section fibers. There was not as much visible light absorption of the non-circular cross section fibers. The quadrilobe cross section fibers increased the number of surface reflection and refraction and the frequencies of the light intensity and the direction changed in the refraction lowered the transmission coefficient of light intensity in the range of 0°-90°(0-π/2), compared to that of circular ones. This would reduce the amount of light transmission, but the multiple reflection and refraction have limited impacts on the attenuation of light. CONCLUSIONS The differences of the refracted light intensity ratio Irz and the refracted light direction among the single circular, trilobal and quadri-lobal cross-section fiber, and the effect of the different fiber cross sections on the changes of refraction and the transmission of light from the view of geometrical optics in fiber were analyzed theoretically. Experiments of different cross-section fibers and their fabrics were carried out on the light shielding performance and the following conclusions were reached: FIGURE 7. UV-Vis spectrophotometer transmissions coefficient spectrums of fabrics. The transmission coefficients of monochromatic light and the average transmission coefficient of fabrics (in Figures 6(a)F1#, (c)F3#, (f)F6#) with the quadri-lobal cross section fibers were lower than those of the circular cross-section ones (in Figures 6(b)F2#, (d)F4#, (g)F7#) looking at both Table VI and Figure 7. This data suggested that the transmission coefficient of fabrics would be lowered by fibers’ cross-section shapes, which meant higher light shielding performance, But, the overall differences between them were small, which indicated that the improvement in the fabrics’ shielding performance due to the fibers’ cross-section was limited. (1) Under the same characteristic parameters, the difference of the theoretical simulation on the refracted light intensity ratio Irz of the circular, trilobal and quadri-lobal cross section fiber did not differ much in general. Irz was within the range of 0.94-0.95. Considering the different positions on the surface of the trilobal and quadri-lobal cross section fiber, the changes of the light refraction angle βt were more complex than that in the circular one, showing non-monotonic changes and reflected the differences in the optical properties between the fibers. TABLE VI. Objective testing results of light shielding of fabrics with non-circular cross section fiber. Type Weaved Knitted No. Rj Tj F1# 60.11 29.78 F2# 59.59 30.11 F3# 71.85 19.90 F4# 69.10 21.48 F5# 71.86 20.64 F6# 67.25 26.56 F7# 65.78 27.49 Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 (2) As can be seen from the geometrical analysis on the changes in the path of the refraction light in the fiber before ejecting, the effects of the quadri-lobal cross section in the refraction light path was more obvious compared to that of the round and trilobal cross-sections. (3) Under the same parameters of the yarn count, density and other structural conditions in the 58 http://www.jeffjournal.org fabrics, the light shielding properties of the fabrics with the quadri-lobal cross-section fibers were superior to that of the ordinary circular ones. The effect of fibers’ cross-section morphology on differences in light shielding of the fabrics were not very large. [2] [3] [4] ACKNOWLEDGMENT This research was funded in partly by the National Science Foundation of China (NSFC5097315). The authors wish to thank Dr. Jin Luo in College of Textiles and Materials Testing Center of Donghua University for their help and efforts completing this study. [5] [6] [7] Nomenclature r1 =radius of top ground circular O1 [mm] r = radius of center ground circular O [mm] R = radius of circumscribed circular [mm] I0 = incident light intensity [lx] Ir = reflected light intensity [lx] Irz = relative light intensity ratio [-] θ0 = angle of incident light to forward of Y-axis [rad] θt = angle of the normal of the incident light to forward of Y-axis [rad] θR = incident angle of light on the surface of fiber [rad] = refraction angle on the surface of medium [rad] φ αt= angle of any point on the surface of fiber to forward of Y-axis [rad] βt= angle of refracted light on the surface of fiber to forward of Y-axis [rad] θt~ , θ’t~-the angles of incident light normal to Y axis forward of each segments on surface of trilobal and quadri-lobal fibers θR , θ’R-the incident angle of each segment on the surface of trilobal and quadri-lobal fibers R(θR~),R(θ’R~)-the reflection coefficient of each segments on surface of trilobal and quadri-lobal fibers Ir(θR~), I’r(θ’R~)-the distribution of reflected light intensity of each segment on the surface of trilobal and quadri-lobal fibers Where, ~ could be arc CA, AB, arc BB1 ,B1A1, arc A1C1 which were listed in Table I. 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PhD Thesis, 2007, pp 20-25. [27] Ren Q.N. Weaving Process of Shade Cloth [J]. Technical Textiles, Vol.17, No.4, 1999, pp 14-17. Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 60 http://www.jeffjournal.org [38] Mevlut T., Edward A. V. Effects of Fibres Denier, Fibre Cross-Sectional shape and Fabric Density on Acoustical Behaviour of Vertically Lapped Nonwoven Fabrics[J]. Journal of Engineered Fibres and Fabrics, Vol.3, No.2, 2008, pp 32-38. [39] Wu J., Yan G.S. Basic Course on Principles of Optics [M]. Beijing: National Defence Industry Press, 2007, pp26-52. [40] Yu X.F. Experimental Technology of Textile Materials [M]. Beijing: China Textile Press, 2004, pp 232-240. AUTHORS’ ADDRESSES Xiaosong Liu Fumei Wang College of Textiles, Donghua University 2999 North Renmin Road, Songjiang District Shanghai 201620 CHINA Journal of Engineered Fibers and Fabrics Volume 7, Issue 3 – 2012 61 http://www.jeffjournal.org