Coloured Tracer Fibre Migration Studies in Nonwoven Materials

Transcription

1 Coloured Tracer Fibre Migration Studies in Nonwoven Materials M. Tausif and S.J. Russell Centre for Technical Textiles, School of Design, University of Leeds, UK STRCT: Multiple high velocity fluid jets are used during hydroentangling to mechanically bond and re-arrange fibres within a tenuous web structure to produce a coherent nonwoven fabric. In addition to bonding single layer nonwovens, the technology is also suitable for combining multiple layers of fibrous material without the need for lamination using adhesives or thermal treatment. The inter-layer bonding is therefore dependent on the frictional interaction and knotting together of fibres and the displacement of fibre segments particularly in the transverse direction. Fibre tracing studies performed using microscopy and image analysis can be a valuable tool in tracking the displacement of fibres so that it is possible to visualize migration of fibres along their length. In this study, bi-layer hydroentangled nonwoven fabrics, consisting of polyester (PET) and glass fibres were mechanically integrated during hydroentangling to produce structures with increased resistance to delamination without adhesives. s part of a wider study on the effect of hydroentangling parameters on the inter-layer bonding and fibre arrangement, a small fraction of pre-coloured PET fibres was incorporated in discrete layers of the fabric cross-section at different positions prior to hydroentangling to enable the migration of fibres within separate sections to be observed in optical microscopy. ased on the observed colour differences image processing was then performed to characterise aspects of the transverse fibrous structure. 1 INTRODUCTION: Conventionally, lamination of nonwovens relies on either wet application of an adhesive by dispersion in water or solvent or a dry method involving processes such as calendering, hot melt extrusion, flat bed, flame and hot melt spraying techniques. Mechanical lamination provides potential for an alternative approach. Hydroentanglement (HE) is a process of entangling webs by means of high velocity water jets. The water at high pressure impacts the web supported by a conveying surface to induce fibre entanglement, displacement and rearrangement. It is used commercially to mechanically bond two or more webs in multilayer fabric production [1, 2]. In the engineering of durable fabrics for bitumen impregnated roofing felts, there is interest in the development of hydroentangled glass-pet fabrics. These fabrics must be mechanically stable and resistant to delamination at elevated temperature, which depends in part on the durability of the bonding between each layer. To understand the integration of the layers it is important to understand how fibres originating in each layer are re-arranged through-thickness. Note that most of the fibres in webs and in nonwoven fabrics have in-plane orientations. Fibre tracing techniques have long been used to study fibre migration behaviour in fibrous materials such as webs and yarns for the purpose of probing their internal architecture. Various techniques have been developed based on optical, chemical and colour-based tracing methods. difference in the refractive indices of the material (fibre) and surrounding liquid medium is one approach that has been widely investigated in optical microscopy. This technique is normally employed by pre-blending a fraction (typically <1% by mass) of a different fibre to that of the bulk; the bulk fibre is then optically dissolved by using a mountant of similar refractive index to reveal the path of the tracer fibres. [3] Chemical dissolution also requires use of dissimilar fibres to trace the fibre paths as compositionally identical fibres will be chemically dissolved. More sophisticated techniques include digital volumetric imaging (DVI) in which a blockface fluorescence imaging technique is used. The fabric is embedded in a polymeric resin and is repeatedly sectioned and imaged. 3D microstructural image is then constructed from the 2D cross-sections. Unfortunately during embedding some disturbance of the fibre arrangement is likely. µ-ct tomography techniques have also be applied to probe the internal structure of nonwovens. In one example, employed in Leeds, X-ray computed microtomography, which is suitable for the non-destructive three-dimensional study of the internal structure involves the collection of shadow images by rotating a sample at discrete angular increments before reconstructing each individual cross-sections [4]. One of the challenges is that polymeric fibres with low specific densities of typically produce low X-ray attenuation coefficients, which limits image contrast. Therefore, for tracer fibre studies a small fraction of fibres with a greater x-ray attenuation coefficient are blended with the bulk material. This may be achieved by pre-doping or coating the fibres with an appropriate compound as well the use 400

2 of higher density fibre materials e.g. glass, metals and ceramics. Unfortunately, such materials are mechanically dissimilar to the bulk polymer fibre and therefore do not behave in an equivalent manner during hydroentangling. Furthermore, coatings can be removed during hydroentangling. ccordingly, although the technique has been long established, amongst the various methods, coloured fibre tracing techniques remain as an important complementary tool for fibre migration studies. This paper outlines a simple approach that relies on the differentiation of coloured fibres within the cross-section and provides a basis for quantitative assessment of fibre segment migration in nonwoven materials. 2. MTERILS ND METHODS: PET (dvansa) and glass (ptec) fibres were obtained for the preparation of experimental fabrics (Table 1). The opening of the PET fibres was performed manually. Carded webs were produced from PET and glass fibre using a volumetric feed laboratory-scale, 1m wide two-part card operating with a single doffer. Carded and cross-lapped PET webs were prepared. For the glass fibre webs, 20% by weight of red PET was added to increase the web strength. Webs of 0.5m x 0.5m were prepared from both the PET and glass fibres. Samples were prepared for hydroentangling by layering carded webs of 20% Red PET & 80% glass (50 g/m 2 ) in the cross direction and cross-lapped 100% PET web (White PET: 130 g/m 2 & lack PET: 20 g/m 2 ), giving a total web weight of 200 g/m². ll samples were pre-wetted prior to the first injector. Hydroentangling was conducted using Leeds Hydrolace system such that the PET side was hydroentangled first and then the glass fibre web side was treated. summary of the layered assemblies and the pressure profiles are given in Fig. 1. Through-air drying of samples was conducted at 100 C for 5 min. Sample 4 was prepared with the same conditions as sample 3 except that an inverse arrangement of injectors was used during fabric formation. Sections were directly obtained without resin embedding and were analysed by optical microscopy (Olympus H-2) at 40X magnification followed by image processing (Image Pro 6.2). Images were segmented on the basis of histogram colour masking of the target regions; in this case the black tracer fibres. Red tracer fibres were used to identify the effect of pressure profile by surface imaging of layer in samples 3 and 4. Red tracer fibres were not segmented in the present study but could be selected if required, on the basis of colour-cube segmentation. In this preliminary approach, the segmented images were quantified by obtaining the mean intensity values (MIV) from subsequent zones in the image. The MIV data were calculated for 0.1mm wide sequential zones through thickness. n exemplar segmented image for Sample 1 is shown in Fig. 7. These zones were selected on the basis of the largest fibre migration depth (LMD). The LMD was determined as the maximum distance between the starting and end point of the tracer fibre segments (Intensity 0) in the thickness direction of the sample as indicated in Fig. 7. Thereafter, the LMD was equally divided to obtain 0.1mm zones for the MIV, an example of which is shown in Fig. 8. The effective fibre migration (EFM) was calculated as the product of the MIVs in the 0.1mm zones through-thickness and the LMD. The same procedure was extended to samples 2-4 and the results are summarized in Table RESULTS ND DISCUSSION: Morton and Summers pioneered the fibre tracing technique [5] for studying the effect of process variables on the fibre arrangement in assemblies such as yarns. The technique requires the blending of a small fraction of dyed fibres of the same chemical composition as the bulk fibre material and then randomly distributing them throughout the assembly. The assembly is placed in a liquid mountant with a similar refractive index to that of the bulk material and observed by optical microscopy. Previously, the approach of the Minimum-width Rectangle has been used to study fibre configurations in needle punched nonwovens revealing how a planar fibre configuration is converted to a 3D curve by the transference of fibre segments through thickness. The number of hooked, U-bent, straight, looped and entangled fibres have been identified using this approach.[6] In another study concerned with needlepunching, layers of dyed tracer fibres were located at different positions in the web thickness to study the effect of fibre transport based on the original position of tracer fibres in each layer and the influence of the depth of needle penetration and needle design. Characterisation of fibre transport based on the number of tracer fibres transported to the reverse side of the fabric has also been reported using the optical fibre diameter analyser (OFD) by exploiting differences in fibre opacity. [7] Fibre tracing techniques have been widely employed for fibre migration studies in yarn but this technique has not been employed in the detailed study of fibre configuration in hydroentangled nonwovens. Fig.(s) 2-5 are examples of cross sections obtained for each of the four hydroentangled samples produced in the present work. It is evident that most of the fibre segment migration associated with the black PET (tracer fibre layer) takes place as a result of fibres migrating from the top layer, with less migration associated with the middle and bottom layers. Use of image analysis to study and quantify fibre segment migration in the cross-section by differentiating fibres on the basis of colour is a new approach. The EFM data 401

3 given in Table 2 confirms that fibre segment migration decreases as the starting layer position gradually shifts from the surface to the inner regions of the web. The EFM from the top layer of the web is 41% and 131% greater than the middle and lower layers respectively. Fig. 9 illustrates the fact that the largest fibre segment migration is associated with fibres whose origin is in the top layer, followed by the middle and lower layers. This technique also reveals large differences in the fibre segment migration as a function of pressure profile. Samples 3 and 4 were prepared with the same construction of layers but a different pressure profile. Sample 4 shows 87% higher EFM than sample 3 confirming that the pressure profile greatly effects the fabric s microstructure. Furthermore, surface imaging of samples 3 and 4 confirm differences in fibre segment migration as a result of a different pressure profile: Fig. 6 clearly shows migration of black and red PET from layer to of sample 4 whereas sample 3 shows no such migration. Fig. 9 also indicates the effect of pressure profile: sample 4 has a higher LMD than sample 3. The first injector gives rise to a large increase in fabric density and reduces the mobility of fibre segments when they are acted upon by subsequent jets. Fibre entanglement, loops and knots produced by the first two injectors, (samples 1-3), also limits the extent to which fibre segments in layer can physically penetrate into layer as a result of injectors 3 and 4. The pressure profile is therefore an important parameter in controlling fabric structure, especially in the formation of multilayer fabrics. The microstructure therefore depends not just on the total specific energy consumed by the web but also on the pressure profile that is used to deliver the energy. 4. CONCLUSIONS: Coloured tracer fibre analysis is a useful technique for the study of fibre configurations and through-thickness fibre segment migration in nonwoven fabrics, which complements more sophisticated techniques based on methods such as µct. Simple image processing facilitates the calculation of the effective fibre migration within the fabric structure based on differences in fibre colour. The method has revealed differences in the extent to which fibre segments migrate through thickness based on their original planar position within the web and the pressure profile that is adopted during hydroentangling. The technique needs to be further developed to improve methods of sectioning and quantitative analysis of resulting image data. CKNOWLEDGEMENT: The authors are highly indebted for the support of the University of Engineering and Technology Lahore (Faisalabad Campus), Pakistan. REFERENCES: 1. Russell, S.J., ed. Handbook of nonwovens. 2007, CRC Press ; Woodhead: oca Raton, Fla. : Cambridge :. 2. Munstermann, U., W. Moschler, and. Watzl, eds. Nonwoven fabrics. ed. W. lbrecht, H. Fuchs, and W. Kittelmann. 2003, Wiley-VCH: Weinheim ; [Cambridge]. 3. Garner, W., Textile laboratory manual / [by] W. Garner. 3rd ed. ed. 1966, London : New York :: Heywood ooks ; merican Elsevier. 4. Eberhardt, C.N. and.r. Clarke, utomated reconstruction of curvilinear fibres from 3D datasets acquired by X-ray microtomography. Journal of Microscopy-Oxford, : p Morton, W.E. and R.J. Summers, Fibre arrangement in Card Slivers. Journal of the Textile Institute Proceedings, (2): p Miao, M. and H.E. Glassey, n Experimental Study of the Needled Nonwoven Process: Part I: Fiber Geometry efore Needle Punching. Textile Research Journal, (4): p Miao, M., n Experimental Study of the Needled Nonwoven Process Part II: Fiber Transport by arbed Needles. Textile Research Journal, (5): p Table 1. Fibre Specifications Fibre Type Colour Mean fibre length (mm) Fibre linear density (dtex) PET White as received PET lack as received PET Red* Glass * Disperse dyed with Dianix Red (4G 150%) at 130ºC for 45 minutes, followed by reduction clearing at 60ºC for 15 minutes Table 2. Mean intensity and migration depth ID MIV of zones (0-255) LMD (mm) EFM (MIV x LMD)

4 % White PET (65 g. m -2 ) 100 % lack PET (30 g. m -2 ) 100 % White PET (65 g. m -2 ) 3 4 Fig.1. Experimental Design with sample ID Fig. 2. Sample 1 Fig. 3. Sample 2 403

5 i Sample 3 Fig. 4. Sample 3 Zone 1 Width (2.79 mm) Fig. 5. Sample 4 Sample 4 Zone 2 Zone 3 Thickness 1.48 mm Fig. 6. Surface image of Sample 3 and 4, Layer Fig. 7. Segmented Image showing zones and LMD (Sample 1) Line Profile 200 Intensity I n t e n s t y Distance (mm) Distance (millimeters) Fig. 8. Intensity (y-axis) vs. Distance for Zone 6 in Fig. 7 Fig. 9. MIV in all zones vs. LMD 404