1 Introduction. Keywords:Air permeability, Fibre cross-sectional shape, Filter fabric, Filtration efficiency, Nonwoven,Polyester,Scrim

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1 Indian Journal of Fibre & Textile Research Yol. 21, December 1996, pp Performance characteristics of filter fabrics in cement dust control: Part II-Influence of fibre cross-sectional shape and scrim on the performance of nonwoven filter fabrics Department K N Chatterjee, A Mukhopadhyay & S C Jhalani The Technological Institute of Textile and Sciences, Bhiwani , India and B PMani of Chemical Engineering, Indian Institute of Technology, New Delhi , India Received 4 October 1995; revised received & accepted 19 February 1996 The influence of fabric weight, reinforcing material and polyester fibre of three different crosssectional shapes on the characteristics of needle-punched nonwoven polyester filter fabrics has been studied. Hollow fibre fabrics exhibit higher air permeability followed by trilobal and normal round fibre fabrics. Trilobal fibre fabrics show higher filtration efficiency. However, for a given pressure drop level, higher filtration efficiency is achieved by hollow fibre fabrics. Improved performance in terms of mechanical properties is seen with the hollow fibre filter fabric, followed by trilobal and round fibre fabrics. Increased fabric weight causes reduction in air permeability, increase in filtration efficiency at the cost of higher pressure drop, and better mechanical properties. Use of reinforcing material leads to reduction in air permeability, increased filtration efficiency and pressure drop, and improved mechanical properties. Keywords:Air permeability, Fibre cross-sectional shape, Filter fabric, Filtration efficiency, Nonwoven,Polyester,Scrim 1 Introduction To bring out some desirable characteristics in synthetic fibres, manufactures have been paying much attention in recent years to the chemical and physical modifications of the structure of the existing popular fibres. As a result, many more variants, such as conjugate fibres, low- and highshrink fibres, hollow fibres, split fibres, fibres with trilobal, multilobal and other profiled cross-sections and micro fibres, are available for use in textiles as well as for other applications including filtration. It has been observed that the trilobal fibre filter fabrics give higher filtration efficiency than the normal round filter fabrics owing to the greater projected area available for surface filtration I. During filtration, if the number of particles captured by depth filtration is high, cleaning becomes more difficult, and repeated use of filter fabric is restricted due to blinding of the filter fabric as a result of particle residue in the filter. Hollow fibres have tubular cross-section which results in increased bulkiness. Greater surface area is also responsible for lower effective density, thus providing a higher cover power. Hollow fibres made from cellulose acetate, acrylic, nomex, etc. are generally used as membranes in reverse osmosis for desalination, separation of gases and ultrafiltration. Industrial applications of hollow polyester fibres such as insulated tents, geotextiles, air filters, carpets, etc. are also claimed2 Having assessed the speciality in fibre properties, it was anticipated that useful filter fabrics can be made using hollow polyester fibres. The important features expected to be derived in the filter fabric made from hollow polyester fibres over the normal nonwoven filter fabric constructed from regular fibres are as foilows: (i) Since the hollow polyester fibre has greater bulk, fabrics will also be of greater surface area, resulting in better fabric cover; (ii) Due to its tubular structure, the hollow poly~ster fibre is more stiff and thus ensures beur quality characteristics such as durability,etc., (iii) Because of its increased surface "" area, hollow polyester fibre can capture fine particles as compared to regular round fibres; and (iv)

2 252 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1996 nal frequency extension Tenacity Fineness 6.6 dtex Breaking Crimp cn/tex % mmlength 51.~. Table % 1-Specifications of polyester fibres used FI FlO F12 F2 F3 ref. F4 F8 F7 F5 F6 Fll F9 no" Due to its greater bulk, it will have F15 F18 F17 F13 F14 F16 better air flow. Fabric Filtration and mechanical characteristics of a filter fabric for dusty air and gas filtration are greatly influenced by the type of fibres, fibre cross-sectional shape, lobe number, lobe depth, fibre crimp, fibre fineness, etc. In the present work, nonwoven needle-punched filter fabrics produced from polyester fibres of three different cross-sectional shapes have been studied for filtration and mechanical properties in terms of air permeability, filtration efficiency, pressure drop, tenacity, breaking elongation, abrasion resistance, bursting strength. The effects of fabric weight and presence of scrim on the filtration and mechanical properties have also been studied. " Table 2-Specifications cross-sectional Fabric glm2 Fibre " Trilobal Hollow Round shape weight of nonwoven fabric samples [Needle density, 400 tration, punches/cm2; 12 mm] ).~ and Needle pene- 2 Materials and Methods 2.1 Materials Polyester fibres of three different cross-sectional shapes, viz. normal round, circular hollow and trilobal, were used. The specifications of these fibres are given in Table 1. A light weight cotton fabric having the following specifications was used as reinforcing material: "Fabrics F16-F18 contained scrim. Warp count: 14.8 tex Weft count: 17.4 tex Ends/cm: 12.6 Picks/ cm: 9.4 Fabric weight:31.5 glm2 Breaking extension: 17.25% Tenacity at break: 6.35 gltex 2.2 Methods Preparation of Fabric Samples In all, eighteen nonwoven needle-punched fabric samples were prepared from 100% polyester fibres having three different cross-sectional shapes, each variety having 51 mm length and 6.6 dtex fineness (Table' 2). From each cross-sectional variety, five fabric samples of 200, 300, 400, 500 and 600 glm2 weights were prepared keeping the f: needling density and depth of penetration constant at 400 punches!cm2 and 12 mm respectively. For studying the effect of reinforcing materi- al a cotton scrim was placed centrally between the two webs in such a way that the resultant fabric weight was 400 glm2 This was done with all the three cross-sectional shapes of fibre keeping the needling density and depth of penetration constant at 400 punches/cm2 and 12 mm respectively. The fabrics were made from parallel-laid web which was obtained by feeding opened fibres in the TAIRO Laboratory Model Stationary Flat Card. The fine web emerging out of the card was built up into several layers to get the desired fabric weight. After preparing the web, the bonding was carried out in a James Hunter Laboratory Fibre Locker, Model 26 (315 mm), having stroke frequency of 170 strokes/min with needle dimension as 15 x 18 x 36 x R/SPX 31 in. x 1t in. x 9. The needling was done alternatively on each "'I ", ii' 'I III ~I I r I It i' I'

3 CHATTERJEE et al.: PERFORMANCE CHARACTERISTICS OF FILTER FABRICS 253 side of the fabric. The machine speed and needle density on the board were chosen in such a way that in a single passage, the needling density that can be obtained on the fabric is 50 punches!cm2 Each of the fabric sample was tested for thickness, air permeability, filtration efficiency, pressure drop and mechanical properties in terms of tenacity, breaking elongation, abrasion resistance and bursting strength. Before testing, all the samples were conditioned at 27 ± 2 C and 65 ± 2% RHfor24h Measurement of Physical Properties Fabric thickness was measured by 'R & B' cloth thickness tester. Thickness values were measured at 10 different places at 20 glcm2 pressure and the average value was taken. After measuring the average thickness, the density was calculated from thickness and known values of fabric weight. Density(glcc) Fabric weight (glm2) x 10-3 Fabric thickness (mm) Porosity of the nonwoven fabrics was calculated from the ratio of volume of void present in the fabric to the total volume of the fabric- expressed as percentage: POroSIty. (010) = Volume of voids x 100 Volume of fabric Volume of fabric - Volume of fibres = x 100 Volume of fabric Fabric density x 100 = 1 Fibre density Measurement of Filtration Properties In the experimental set-up (Fig. 1), the pressure drop across the orifice meter and across the fabric was measured at different flow rates by controlling the valves. From the orifice meter calibration chart, flow rates in litres/ s can be determined for subsequent pressure drop across the orifice meter. From these flow rate results, air permeability (cc/cm2/s) was calculated by dividing the area of fabric. Sectional permeability, which is defined as the product between permeability and thickness of the fabric, was also calculated. A very thick cloth with rather open structure may have the same permeability as a thinner clotp with dense struc- FAIIRIC HOLDER r N1\JRl ROTA:ING~ ~ OZZLE J/D1scr Fig. i-experimental IoWIOlolETER ORIFICE loleter VAlVE ~ set-up fof dust characterization OMPRESS ture. If the two cloths were of equal thickness, the dense structure would have a lower permeability. It is, therefore, desirable to have a figure independent of the thickness to represent the 'openness' of a fabric. For the measurement of filtration efficiency, cement dust having a particle size range of f.l was used. Dust feeding was done at the rate of 0.5 glmin. The dust laden air was sucked through the fabric by the suction pump. The fabric was weighed after every 10 s up to one minute and thereafter after every 60 s up to 360 s. It was observed that as the dusty air was fed to the fabric the reading of both the manometers Mi and M2 changed. The orifice meter reading was adjusted constantly to the initial value (2 cm water gauge) to keep the face velocity constant at 30 cm/s and the corresponding manometer (Mi) reading was noted at 60 s intervals. The fabric- sample was then taken out and weighed. The dust concentration was kept constant at 5.25 glm3 From these readings, the filtration efficiency and pressure drop were calculated Measurement of Mechanical Properties Tenacity and Breaking Elongation Tenacity and breaking elongation were measured in both machine and cross directions using an Instron Tensile Tester, Model The sample size and rate of straining were chosen according to ASTM standard D (Sample size, 10 cm x 2.5 cm; crosshead speed, 10 cm/min; chart sp~ed, 5 cm/min; full-scale load, 100 kg for machine direction and 20 kg for cross direction) Abrasion Resistance The abrasion resistance was measured using a C.S.I. Abrasion Tester. The sample size and other

4 254 INDIAN J. FIBRE TEXT. RES., DECEMBER 1996 specifications were: Sample size, 4.4 in. diam; Abradent ately in aii the cases. Sectional air permeability, used, C-320, P 027; Pressure on abrasion which is a product of air permeability and fabric head, 2.0 Ib; and Air pressure, 3.0 psi. thickness, was also measured to have permeability values independent of the thickness and are Bursting Strength shown in Table 3. Bursting strength is more important for filter cloths where.fabrics are stressed in all directions Effect of Fibre Shape It was measured by Mullen Diaphragm Burst Tester Tables 3 and 4 show that in all the cases at any according to BS standard level of fabric weight the hollow fibre fabric 3 Results and Discussion The physical properties of the fabrics in terms shows higher air permeability, followed by trilobal and round fibre fabrics. This may be attributed to two factors. Firstly, in case of hollow fibre fabric of fabric thickness, density and porosity are given the total surface area exposed to air is more, thus in Table 3 and the results of the experiments on more resistance to air flow. This should cause a permeability and sectional permeability are given decrease in air permeability. However, the density in Table 4. of hollow fibre fabric is less. Within the range of air pressure studied, density factor is predominating 3.1 Air Permeability and, therefore, resulted in higher air permea Experiments were conducted with dust-free air bility for hollow fibre fabrics as compared to that to estimate the effect of variables on air permeability for other two varieties. Trilobal fibre fabric has and sectional permeability at 10 mm water less density than normal round fibre fabric and gauge pressure drop. Experiments were also carried out for clean air permeabilities at different round fibre fabrics. thus shows more permeability as compared to pressure drops ranging from 2 mm to 10 mm water In all cases, the air permeability decreases with gauge. The variation of permeability with the increase in fabric weight. The decrease in change in pressure drop for individual fabric is permeability with the increase in fabric weight is shown in Table 4. From Tables 3 and 4, it may be due to the fact that increase in fabri~ weight observed that with the increase in pressure drop causes more number of fibres per unit area. the air permeability increases almost proportion- Therefore, the total surface area exposed is more, Table permeability Sectional Actual weight 3- Thickness glm' glcc Porosity ll permeability cc/cm'/s ] ] % Thickness, mm Density Airair, density, porosity, air permeability andcc/cm'/s sectional air permeability of fabrics Fabric 'I I I' '"' 'I I ; I I' I! I ~11 t1 1III ~I' "111111

5 , CHATTERJEE et al. : PERFORMANCE Pressure CHARACTERISTICS drop, mm-water gaugeqf FiLTER FABRICS permeability (cc/cm2/s) no. with change in pressure drop ic 255 resulting in less air permeability. Secondly, increase in fabric weight causes the fabric density to increase, which, in turn, increases the resistance to air flow, which is in agreement with the findings of Kothari and Newton Effect of Reinforcing Material The effect of reinforcing material (scrim) on the perfueability of fabrics (400 glm2) made from fibres of three different cross-sectional shapes is shown in Tables 3 and 4. It is observed that the presence of scrim causes reduction in air permeability of nonwoven fabrics which is due to the better consolidation of fibres with the presence of scrim. The woven fabric scrim itself restricts the air flow, leading to less air peimeability. Further, it is also clear that this decrease in air permeability is more for circular hollow fibre fabric. Normal round and trilobal fibre fabrics show almost the same amount of decrease. This IS due to thejgfeater increase in fabric density in case of hollow fibre fabric. Although, with the presence of scrim, the permeability decreases, but from the durability point of view, scrim should be used as a reinforcing material to stabilize the loose nonwoven structure. 3.2 Filtration Properties Experiments were conducted with dusty air using cement dust, keeping face v~locity constant at 30 cm/s. Tb.e' filtration properties in terms of filtration efficiency and pressure drop were measured. The experimental values of filtration efficiency and pressure drop are given in Tables 5 and Filtration Efficiency It may be seen from Table 5 that with the passage of time, the filtration efficiency increases and after some time, it almost reaches a steady state. The increasing efficiency with time can be explained.on the basis of the fact that as the dust particles reach the fabric surface ~ong with air, deposition of dust particles occurs and as the dust deposition continues, cake formation takes place which also acts as a filter medium. It is important that filter cake should be formed immediately so that the filteration takes place due to resistance offered by the cake than by fabric. This demands a fabric to have a high filtration rate and maximum j'iltration efficiency should be achieved in the shortest possible time. The slow filtration rate of 200 glm2 fabric indicates that the dust is getting lo~t with time. It may also be seen that the filtration efficiency of 100% is reached in a much shorter time for trilobal fibre fabric compared to round and hollow fibre fabrics.

6 ",,,, 'I' I 'II III' "'I I' II '''' I 'r IIt I' I ' 256 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1996 Fabric No Table 5-Filtration Time efficiency interval, ofs fabrics ~ } Effect of Fibre Shape It may be observed from Table 5 that initially the filtration efficiency is less for hollow fibres at each level of fabric weight. Trilobal fibre fabric gives higher filtration efficiency followed by round fibre fabric at each level of fabric weight. However, after some time, the filtration efficiency approaches to 100% for each type of fibres. It is also found from Table 5 that filter fabrics with trilobal fibres take less time to reach 100% efficiency, followed by filter fabrics with hollow and round fibres. The initial decrease in filtration efficiency with the hollow fibres may be attributed to the fact that hollow fibre filter fabri s show lower fabric density and hence chances of surface filtration is less as compared to that in normal round fibre filter fabrics. Trilobal fibre, because of its peculiar structure with projected area, can capture particles more effectively as compared to round and hollow fibres. Another reason may be the lodging of particles in the concave region of the trilobal fibre. This is in agreement with the findings of Lamb eta!.l The plot of filtration efficiency versus time interval would be useful in deciding the type of filter fabrics which can reach 100% efficiency at minimum possible time. It may be concluded from the Table 5 that trilobal fibre fabric shows better filtration efficiency as compared to hollow and round fibre fabrics. However, one should see the acceptable pressure drop level before selecting the fibre shapes. However, with increase in fabric weight, the filtration efficiency improves for all types of fabric, the effect of which will be discussed in more detail in Part IV Effect of Reinforcing Material It is seen from Table 5 that the use of reinforcing material gives increased filtration efficiency at all intervals of time. Trilobal fibre fabric with scrim shows higher efficiency followed by round and hollow fibre fabrics. The increased filtration efficiency may be due to the increase in fabric compactness and density with the use of reinforcing material. It may be concluded from above that fabrics with reinforcing material not only reach 100% efficiency faster but also have increased bal fibre fabric reinforced with scrim life. Trilo takes 1-2 min to reach 100% efficiency, whereas the round and hollow fibre fabrics take 3-4 min to reach 100% efficiency Pressure Drop Table 6 shows the pressure drop variation with the filtration time. It is observed that with the increase in filtration time, the pressure drop increases in all the cases. The increase in pressure drop is due to the increase in resistance offered

7 CHATIERJEE et al. : PERFORMANCE CHARACTERISTICS OF FILTER FABRICS 257 No Initial Table 6- Time Pressure interval, drop s(nun-water gauge) variation with Fabricfiltration time mcrease m fabric weight the pressure drop m creases Effect of Reinforcing Material It is observed from Table 6 that the filter fabrics with reinforcing material show higher pressure drop as compared to filter fabrics without scrim. the increase in pressure drop with the use of scrim is due to increased fabric density with the consolidation of fibres. Woven fabric in the centre layer may also increase the compactness of the nonwoven fabric, which, in turn, increases the pressure drop. Thus, the use of reinforcing material causes increased drag, but it helps in increasing the life of the filter fabric by consolidating the nonwoven structure. -.- by the cake of dust particles deposited on the fabric surface. The pressure drop study is useful for determining the time for cleaning cycle. The filter fabrics can be operated as long as the system can take maximum pressure drop. The time in which the maximum pressure drop is reached is known from Table 6. This time can be adjusted for reverse cleaning Effect of Fibre Shape It is observed from Table 6 that the pressure drop increases with the filtration time for all the fabrics. Pressure drop does not show any significant increase at the initial stages followed by abrupt increase at the later stages for most of the fabrics. The slow increase in pressure drop with time for the hollow fibre filter fabric may be due to the lower fabric density as compared to that of trilobal and round fibre filter fabrics. Thus, hollow fibre filter fabrics perform better than other types of fabric. The plot of filtration time versus pressure drop is very much useful in determining the time of cleaning the filter fabrics. It is usually observed in practice that the adjustment of the timer is left to the discretion of the plant operator who with the intention of running the plant and to avoid the choaking of th~ fabric, tries to increase the pressure of the reverse jet air and increases the frequency of cleaning. It results in decrease in the life of fabric. Table 6 also shows that with the Effect of Pressure Drop on Filtration Efficiency It is observed from Tables 5 and 6 that with the increase in pressure drop, the filtration efficiency increases for all the three fibre shapes. The hollow fibre fabric shows higher filtration efficiency for a given pressure drop level followed by trilobal and round fibre fabrics. It is also seen that there is a steep increase in efficiency with increase in pressure drop for hollow fibre fabrics and gradual increase for round fibre fabrics. It is always desirable to have steep increase in efficiency so that 100% efficiency is achieved at minimum pressure drop. Therefore, from the study of filtration efficiency and pressure drop, it may be concluded that hollow fibre fabric will be suitable for long-term filtration purposes. 3.3 Mechanical Properties Tenacity Effect of Fibre Shape It is observed from Table 7 that in both machine and cross directions, hollow fibre fabric shows higher tenacity at each level of fabric weight followed by trilobal and normal round fibre fabrics respectively. This may be attributed to the following reasons. Firstly, the higher bulk of hollow and trilobal fibres provides higher surface area, which, in turn, increases the fibre cohesiveness. Secondly, the higher tenacity of hollow and trilobal fibres compared to that of round fibres gives less breakage of fibres during needling, resuiting in corresponding increase-in strength. Apart from that, the stronger fibre is expected to produce stronger fabric.,i, I I' 1 1 III 't, I 'I I' 'I' '~I' ii' I I" r T' I" 'I

8 Effect of Fabric Weight Table 7 shows that in almost all cases the tenacity at break increases with the fabric weight in both machine and cross directions. However, the rate of increase is more pronounced at the initial stages and after a certain level it approaches a steady state. The increased tenacity with the increase in fabric weight may be attributed to the following reasons. The initial increase may be due to the increase in the number of fibres in the web with fabric weight, which, in turri, increases the number of vertical loops and increases the density and entanglement, causing less freedom of fibre movement and greater frictional restraints. The reduction in rate of increase of tenacity bevond a fabric weight level may be due to fibre breakage during the needling of a consolidated fabric. It may be concluded from above that higher fabric weight (in this case glm2) gives better performance as far as the tenacity is concerned. Therefore, from the filter fabric life point of view, one should choose a fabric of glm Effect of Reinforcing Material The us~ of reinforcing material causes an increase.in fabric tenacity at break in all the cases in both machine and cross directions (Table 7). This may be due to the fact that the base woven fabric (scrim) do contribute effectively when the final breakage takes place. The results are in agreement with the findings of Hearle and Sultan4 Therefore, base woven fabric in the form of reinforcing material (scrim) may be helpful in increasing the strength of nonwoven fabric Breaking Elongation Effect of Fibre Shape It is observed from Table 7 that the hollow fibre filter fabrics show highest breaking elongation for both the directions, followed by trilobal and round fibre fabrics respectively. The reduced breaking elongation for round fibre fabrics is due to the increased fabric consolidation, which reduces the mobility of fibres in the fabrics. It may be concluded from above that the hollow fibre fabric shows better performance as compared to the other two types because the more is the breaking elongation, the more will be the durability Effect of Fabric Weilht In almost all the cases, the breaking elongation decreases gradually in both machine and cross dire<;tions with the increase in fabric weight (Table 7). The initial decrease may be due to the better

9 CHATIERJEE et al. : PERFORMANCE CHARACTERISTICS OF FILTER FABRICS 259 compactness of fibres, causing reduced slippage, and further decrease may be attributed to the fibre breakage which reduces the fibre length and hence fibre-to-fibre cohesion. These results are in agreement with the finding of Hearle and Sultan6 It may be concluded from the above study that the lower fabric weight (200 glm2) gives better performance as far as the breaking elongation is concerned. However, higher fabric weight ( glm2) is necessary to achieve optimum strength for desirable performance in the long run Effect of Reinforcing Material It is observed from Table 7 that the breaking elongation of reinforced fabric is marginally less than that of the fabric without reinforcing material. This may be attributed to the fact that on the one hand, increased fabric density should cause reduced breaking elongation for the nonwoven fabrics with scrim but on the other hand, presence of woven fabric should bear some load before breaking takes place. Therefore, a marginal decrease in breaking elongation is observed with the reinforced nonwoven fabrics. From the above study, it may be concluded that the presence of reinforcing material (scrim) will be useful for producing nonwoven filter fabric as it gives better strength and elongation properties Abrasion Resistance Effect of Fibre Shape ~ It is observed from Table 7 that at any level of fabric weight the trilobal fibre fabric shows higher abrasion resistance (number of cycles) followed by normal round and hollow fibre fabrics Effect of Fabric Weight Table 7 show that increase m fabric weight increases the total number of abrasion cycles required for destruction of sample. The same trend is observed for all the three varieties of fibre. The reason for this may be the increase in compactness and density of fabric with the increase in fabric weight. Therefore, higher fabric weight ( glm2) will be useful for better abrasion resistance Etrect of Reinforcing Material The use of reinforcing material causes an increase in abrasion resistance of hollow, trilobal and round fibre fabrics. The reason for the increase in abrasion resistance when reinforcing material is used may be as exp'ained earlier, i.e. better compactness and higher fabric density. Also, the woven base cloth gives higher abrasion resistance. It is clear from above that the scrim should be used to reinforc~ the nonwoven structure so that longer life of filter fabrics in terms of abrasion resistance would result Bursting Strength Effect of Fibre Shape It is observed from Table 7 that -the hollow fibre fabric shows highest bursting strength, followed by trilobal and round fibre fabrics respectively. Hollow fibre fabrics, because of its higher breaking elongation, can resist bursting pressure. Alsq,fibre tenacity of hollow fibre fabric is higher as compared to that of trilobal and round fibre fabrics, thus resulting in more bursting strength. It may be concluded from above that the hollow fibre fabric gives better performance as far as bursting strength is concerned as compared to trilobal and round fibre fabrics Effect of Fabric Weight Table 7 shows that as the fabric weight increases, the bursting strength increases in all the cases, as expected. This is because with the increase in fabric weight, the number of fibres is increased which plays an important role in resisting the bursting pressure. Therefore, higher fabric weight ( glm2) will be useful in producing filter fabrics to achieve better bursting strength Effect of Reinforcing Material Table 7 shows that reinforced filter fabrics show higher bursting strength as compared to the filter fabrics without reinforcing material. The increased brusting strength of reinforced nonwoven fabric is because of the woven fabric which also resists some bursting pressure and hence causes more bursting strength. Hollow fibre fabric shows higher bursting strength, followed by trilobal and round fibre fabrics respectively. It may be concluded that the use of reinforcing material (scrim) is essential to achieve better bursting strength and hence, durable filter fabric. 4 Conclusions 4.1 Hollow fibre fabrics exhibit higher air permeability followed by trilobal and normal round fibre fabrics. Trilobal fibre fabrics show higher filtration efficiency. However, for a given pressure drop level, higher filtration efficiency is achieved by hollow fibre fabncs. Improved performance in terms of mechanical properties is seen with the

10 260 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1996 hollow fibre filter fabric, followed by trilobal and round fibre fabrics respectively. 4.2 Increased fabric weight causes reduction in air permeability, increase in filtration efficiency at the cost of pressure drop, and better mechanical properties. 4.3 Use of reinforcing material leads' to reduction in air permeability, increased filtration efficiency and pressure drop, and improved mechanical properties. Acknowledgement The authors wish to thank Shri ReD Kaushik, Director, TIT&S, Bhiwani, for providing the facilities to carry out this work. They are grateful to the management and staff members of JTRL, Calcutta, for preparing the samples and to IEL and JK Synthetics for supplying raw material free of cost. References 1 Lamb G E R, Costanza P & Miller B, Text Res J, 45 (1975) Chattopadhyay S K, Srinathan B, Parthasarathy, Indiiln TextJ, Jan (1991) Kothari V K & Newton A, J Textlns~ 65 (1974) HearleJ W S & Sultan MAl, J Text Inst, 59 (1968) Subramanian V, Madhusoothanan M, Debnath C R, A study on the properties of needle-punched non-woven fabrics using factorial design technique, Resume of papers, 31st joint technological conference of ATIRA, BTRA, SITRAandNITRA(1990) 124. I) Hearle J W S & Sultan MAl, J Text Inst, 58 (1967) 251. I I I,ll!II I", l'i'lnl'i"1 t" 1'" 11'1 'I 'I" 'I " 'II!II r T' r