Performance of Natural Fibre Nonwoven for Oil Sorption from Sea Water Zmogljivost vlaknovin iz naravnih vlaken za sorpcijo olj iz morske vode

This work deals with the study of the oil sorption behaviour of needlepunched nonwoven fabrics produced from natural fi bres such as cotton, cotton fl at waste, cotton/kapok blend, and nettle fi bres. Polypropylene nonwoven fabric, which is used as a commercial oil sorbent, was also prepared using the same needling parameters for comparison purposes. The eff ect of the type of fi bre, oil, and fabric parameters on oil sorption and retention capacities was investigated. All of the fabrics displayed higher oil sorption capacities for engine oil (high viscosity) than diesel oil (low viscosity). Among natural fi bre nonwovens, cotton and cotton/kapok nonwovens displayed higher oil sorption capacities than that of polypropylene nonwovens, while nettle fi bre nonwoven fabric displayed poor oil sorption capacity. An increase in kapok content in cotton/ kapok nonwovens led to an increase in oil sorption behaviour. More than 95% of the diesel oils adsorbed by the nonwoven fabrics could be recovered by simple compression. Oil sorption capacity of the nonwovens were reduced signifi cantly during repetitive cycles of use due to higher thickness loss. This study indicated that cotton and cotton/kapok nonwovens displayed better oil sorption behaviour than polypropylene, and may be used as an alternative natural oil sorbent material.


Introduction
Oil spills generally occur on the ocean's surface and also in nearby land areas due to tanker disasters, wars, operational failure, equipment failure, accidents and natural disasters during the production, transportation, storage and use of oil [1−3]. It is a serious problem that causes environmental and ecological imbalance, as well as fi nancial loss. Th e immediate and eff ective decontamination and cleanup of spilled oils are necessary in order to protect the environment and human health [4]. Various methods are available for oil spill clean-up, such as mechanical recovery, dispersants, burning, etc. However, not a single system has been found to be completely eff ective. Oil spill clean-up through oil sorption using sorbents is one of the most effi cient and economical methods [5−6]. Commercially, polypropylene is most widely used as oil sorbents due to its oleophilic and hydrophobic characteristics, but it is non-biodegradable [7−8]. Th is presents a great challenge in the disposal of the sorbent aft er usage [9]. Th e use of natural fi bres such as milkweed, kapok, cotton, wool, fl ax, ramie, etc., as oil sorbents has been reported [10−16]. Cotton fi bre was studied for oil sorption behaviour and reported to have higher oil sorption capacity than polypropylene fi bre [17]. Th e crude oil sorption capacity of low-micronaire cotton is also signifi cantly higher than that of high-micronaire cotton because it contains a higher number immature fi bres [4,18]. Milkweed and kapok fi bres were reported to exhibit better oil sorption behaviour than the rest of the above-mentioned fi bres. Higher surface waxes and non-collapsing lumens are believed to be the reason [9,17]. Kapok/polypropylene blend needlepunched nonwovens were investigated as oil sorbents. It was reported that a 50/50 blend ratio of kapok and polypropylene demonstrated higher oil sorption [19]. Choi, Kwon, and Moreau investigated cotton/polypropylene blend needlepunched nonwovens as oil sorbents, and reported that an increase in cotton content increases oil sorption capacity [17]. Nettle fi bre was also tested as another alternative material due to its hollow structure and the presence of surface waxes [20]. Loose fi bres demonstrated a higher oil sorption capacity than structured fi brous assemblies due to a less eff ective or accessible fi bre surface area [21]. Th e collection of fi bres in loose form from a spill area aft er use has been found to be a challenge. Hence, the nonwoven form is the best choice where the accessible fi bre surface area is closer to loose fibres due to structural openness and the easy collection of nonwovens aft er use [9]. Th e oil sorbent characteristics of stitch-bonded, needlepunched and spunlaced nonwovens based on polypropylene were investigated. It was determined that porosity, pore size, and fi bre fi neness are important parameters for oil sorption [6]. However, the oil sorption behaviour of nonwovens does not depend on web forming technologies such as carding and air-laid techniques [9]. Based on literature review it is understood that as natural resources, immature cotton, kapok and nettle fi bres may have great potential of oil sorption from seawater. Cotton fl at waste has immature fibres in majority and may be a potential candidate for oil sorption. But it is not explored for this application. Th ere is lack of information in literature on oil sorption capacity of needlepunched nonwovens made of cotton fl at waste, cotton/kapok blend and nettle fi bres. Th erefore, in this work, needlepunched nonwovens were produced from cotton, cotton fl at waste, cotton/kapok blend of three diff erent proportions, nettle and polypropylene fi bres using the same needling parameters. Oil sorption behaviour, mechanical properties, and the re-usability of all these nonwoven specimens were tested and compared with polypropylene nonwoven to fi nd a sustainable alternative of the same.

Materials
Th e raw materials used for this work were cotton, cotton fl at waste (collected from carding machine during spinning of the cotton fi bre), kapok, nettle (Girardinia diversifolia) and polypropylene fi bres. Th e African variety of cotton, its fl at waste and virgin polypropylene fi bres (3.33 dtex, 50 mm cut length) were collected from local industrial producers in Punjab, India. Properties of cotton, cotton fl at waste and kapok were measured using a high volume instrument (HVI) and advanced fi bre information system (AFIS) ( Table 1). Th e nettle fi bre (fi neness 1.4 dtex and 50 mm cut length) was purchased from the Uttaranchal Bamboo and Fibre Development Board, India. Th e kapok was collected from industrial producers in Coimbatore, India. Engine oil (high viscosity) and diesel (low viscosity) were used to conduct oil sorption testing. Th e specifi cations of the oils are given in Table 2.

Sample preparation
All needlepunched nonwoven samples were prepared using a DILO (Germany) needlepunching machine using a punch density of 50 punches/cm 2 , a needle depth penetration of 8 mm, and a mass per unit area of 200 g/m 2 . Parallel-and cross-laid nonwovens were prepared for 100% cotton fi bre only. A cross lapper was used for the preparation of crosslaid nonwovens. Th e compositions of all prepared nonwoven samples are shown in Table 3.

Measurement of nonwoven properties
Th e mass per unit area of the nonwoven samples was determined according to the ASTM D6242-98 standard. Th e nonwoven fabric thickness was determined according to the ASTM D5729-97 standard at a pressure of 4.14 kPa. Th e bulk density (kg/m 3 ) of nonwoven samples was calculated using equation 1.
where W is the mass per unit area of the sample (g/m 2 ) and t is the thickness of the sample (m).
Th e porosity and pore size distribution of the nonwoven fabrics were measured using a capillary fl ow porometer (CFP-1100-AEHXL, PMI Inc.). Th e measurements were carried out in a dry-up/wet-up test mode using a Galwick solution (surface tension 15.9 mN/m) to saturate the samples aft er the dry test. Th e minimum, maximum, average pore diameters and pore size distribution of all samples were measured.   [14][15][16][17][18][19][20][21][22][23][24][25][26] Th e tensile strength and breaking elongation in machine direction and in a cross direction of nonwoven fabrics was measured using a universal testing machine (Zwick) according to the ASTM D 5035-09 standard and the CRE principle, with a sample size of 20 cm × 10 cm, gauge length of 75 mm and testing speed of 300 mm/min.

Measurement of oil sorption capacity
Th e ASTM F716-82 (sorbent performance of absorbents) and ASTM 726-81 (sorbent performance of adsorbents) standards were followed for measurement of oil sorption capacity of the prepared nonwoven samples. Th e testing procedure of oil sorption capacity was classifi ed in two ways: (a) oil sorption from oil in an artifi cial seawater bath; and (b) oil sorption from an oil bath. Th e artifi cial seawater bath was prepared according to the AATCC 106-8 standard.

a) Measurement of oil sorption from oil bath
To study the oil sorption capacity of oil sorbents without a water medium, a simple procedure was used. 60 g of sample oil was placed in a 1000 ml size glass beaker, and the dry nonwoven specimen was immersed in the oil for 10 minutes. As a result, the nonwoven specimen was soaked with the oil and the excess oil was allowed to drain by free hanging the soaked specimen vertically for 5 minutes. Th e specimen was then weighed. Th e oil sorption capacity of sorbents nonwoven was determined using equation 2.

Oil sorption capacity
where W S is the mass (g) of a dry and fresh nonwoven, W SO is the mass (g) of the nonwoven saturated with oil and W O is the mass (g) of oil soaked by the nonwoven.
b) Measurement of oil sorption from artifi cial sea water bath in static and dynamic conditions One litre of artifi cial seawater was prepared by dissolving 30 g of sodium chloride and 5 g of magnesium chloride anhydride in 1000 ml of distilled water. 500 ml of this artifi cial seawater was poured in a 1000 ml size beaker and 50 g of sample oil was added to it and stirred with a digital magnetic stirrer (Cole Parmer) at 200 rpm, for 5 minutes to prepare an oil in a water emulsion. A dry nonwoven specimen of known weight (m o ) was immersed in the emulsion beaker for 10 min for soaking in static condition. In case of dynamic condition of test, aft er immersing the dry sample into the emulsion beaker stirring was conducted using the same stirrer for 10 minutes at a frequency of 50 cycles/minute to simulate the actual ocean waves. Aft er soaking, the specimen was taken out of the beaker and hanged vertically for 5 min so that excess solution can be drained out of the specimen. Aft er that weight of the soaked specimen (m f ) was taken for analysis. Th e amount of sorbed solution was extracted from the soaked specimen by squeezing with a roller squeezer at a roller pressure of 1.5 kg/cm2 and collected. Th e solution contained both oil and water, from which the water (mw) was separated using a separation funnel. Hence, the oil sorption capacity of sorbent material was determined using equation 3.

Oil sorption capacity
where m f is the mass (g) of the soaked specimen after draining, m o is the initial dry mass (g) of the specimen and m w is the water content (g) extracted from the specimen.

Theoretically defi ned oil sorption capacity
Th e oil sorption capacity of all nonwoven samples was also calculated theoretically and compared with experimental value. Th e theoretical oil sorption capacity of nonwovens indicates the maximum oil that a nonwoven fabric can adsorb. It is assumed that when all the pores in the nonwovens are fi lled with oils, the theoretical oil sorption capacity can be calculated using equation 4 [22].

Th eoretical oil sorption capacity
where V p and V f indicate the volume of pores (equation 6) and fi bres (equation 7) in the nonwovens, and ρ i and ρ f represent the density of oil and fi bre respectively. Th e volume of pores (V p ) in a given fabric volume (V F ) can be calculated from the porosity of the fabric (equation 5).
where ρ F and ρ f represent the bulk density of fabric and density of fi bre respectively.

Calculation of normalised oil sorption capacity
All nonwovens produced for this study had diff erent levels of porosity with fi bres of varying density. It was thus necessary to normalise the oil sorption capacity for comparison purposes. Normalised oil sorption capacities provide information about the eff ect of fi bre characteristics (oleophilicity, contact angle and surface tension). Th e normalised oil sorption capacity can be expressed using equation 8 [9].

Normalised oil sorption capacity
where ϕ denotes the porosity, ρ i and ρ f represent the density of oil and fi bre respectively.

Measurement of rate of oil release from sorbed nonwovens
Drainage/release of excess oil aft er soaking from oil bath by nonwoven specimens was measured by hanging the specimens freely in vertical manner so that loose oil can be drained automatically with time. Th e amount of oil releases from the specimens was calculated by measuring gradual weight loss of the soaked samples aft er various interval viz. 0, 1, 3, 5, 7, 10, 20 and 30 minutes. Th e amount of oil retained was determined by taking the diff erence between the initial weight of the soaked nonwoven specimen and the weight of the specimen aft er drainage for predetermined time.

Measurement of oil sorption rate
Th e sorption rate is defi ned as the amount of oil adsorbed by the nonwovens from oil bath over a period of time. An experimental setup was fabricated to measure the sorption rate of the nonwoven specimens. Th e experimental setup is shown in Figure 1. A reservoir with oil was placed over an electronic scale that was connected to a computer. Th e known weight of the specimen was placed over a mesh. Th e mesh was connected with a vertical rod that hung vertically from the wicking apparatus. Th e bottom surface of the specimen was then placed in contact with oil in the reservoir, as shown in Figure 1. Th e oil from the reservoir penetrated into the specimen due to wicking/capillary pressure. Th e change in weight of the reservoir was recorded over time and thus the sorption rate over time was calculated.

Measurement of recovery of absorbed oil
Oil recovery is defi ned as the ratio of the amount of oil recovered from a soaked specimen by mechanical squeezing to the total amount of oil soaked by the specimen. Squeezing of soaked specimens was conducted with the help of a squeezing roller keeping a roller pressure of 1.5 kg/cm 2 . Th is extracted amount of oil is recovered oil (W r ). Th e amount of oil soaked by the specimen (W O ) was measured by the method discussed in section 2.3.2 a). Th en percentage recovery of oil was calculated by equation 9.
3 Results and discussion

Nonwovens properties
An engineered fi bre structure needlepunched nonwovens are fl exible thick sheets which are porous, thick, bulky, and strong. Th ey are developed in such a way that they resemble a spongy low density fabrics, wherein textile fi bres are loosely interlocked via fi bre entanglement without disturbing the active surface area of fi bres and capillary network between the fi bres much. No external adhesive was employed for fi bre bonding so that surface characteristics of fi bres, pore structure and capillary network are not aff ected and the capillary network is responsible for absorbing oil from seawater. Performance of nonwoven structure for any application depend on its mass per unit area, thickness, tensile properties etc. Average values of mass per unit area, thickness, tensile strength and breaking elongation in machine and cross directions of prepared nonwoven samples are reported in Table 4.  , 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] Th e results depict that all nonwoven specimens are suffi cient thick and strong for the application of oil spill clean-up from seawater.

Oil sorption capacity of nonwovens from oil bath
Th e oil sorption capacities of all nonwovens were determined for engine oil and diesel from oil baths. Th e results are graphically represented in Figure 2. All the nonwovens displayed higher oil sorption capacity for high viscosity oil (engine oil) than that of low viscosity oil (diesel). It can be observed from Figure 2 that, among all types of nonwovens, the cotton/kapok blended nonwovens exhibited the highest oil sorption capacity. Th e oil sorption capacity increased with an increase in kapok content (samples S6−S4). Th is is due to the lower bulk density of kapok enriched nonwovens and oleophilic nature of the kapok fibres. Porosity and bulk density of all nonwovens are shown in Figure 3 and Figure 4 respectively. Good correlation has been observed between oil sorption capacity of the nonwovens and their porosity. Coeffi cients of correlation (r) between sorption capacity and porosity are found to be 0.92 and 0.97 for engine oil and diesel oil respectively. Kapok fi bres are oleophilic in nature and have good affi nity to oil. Th e oleophilicity is related to the surface waxes of fi bres and kapok has higher surface waxes than cotton, that makes the kapok more oleophilic [25].  Tekstilec, 2020, 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] All of the cotton nonwovens displayed an oil sorption capacity just below kapok blended nonwovens. Th e cotton fl at waste nonwoven (S3FW) displayed signifi cantly lower oil sorption than other cotton nonwovens for both engine and diesel oil because of the lower porosity and higher bulk density of the nonwovens, as shown in Figure 4. Good negative correlation has been observed between oil sorption capacity of the nonwovens and their bulk density. Coeffi cients of correlation (r) between sorption capacity and bulk density are found to be -0.87 and -0.79 for engine oil and diesel oil respectively. Th e higher bulk density is attributed to the higher number of short fi bres in cotton fl at waste. Th e cotton cross-laid nonwovens (S2CC) displayed signifi cantly higher oil sorption capacity than cotton parallel-laid (S1CP) nonwoven for high viscosity oil (engine oil). Th is might be due to diff erence in fi bre orientation. For lower viscosity oil (diesel), both nonwovens displayed similar oil sorption capacity. Th e polypropylene nonwoven (S7PP) displayed signifi cantly lower oil sorption capacity than cotton and cotton/kapok blend nonwovens. Th is can be explained as follows. Th e oil sorption capacity of a nonwoven is generally infl uenced by the oleophilic nature of the fi bre, fi bre fi neness and the structure of the nonwoven fabric prepared thereof. Th e oleophilic nature of the fi bre was one of the important factors that favourably infl uenced oil sorption behaviour. A structure that facilitates capillary fl ow should be able to adsorb more liquids. Th e capillary fl ow through a structure depends on the number of pores and their size in the structure. A structure made of fi ner fi bre should yield more pores but with a smaller size. Th us, a structure made of fi ner fi bre is expected to have more oil retention capacity due to higher capillary pressure. If the pore size is higher, then capillary pressure will be lower. In the present experiment, the fi neness of polypropylene was 3.33 dtex, which was coarser than all other fibres. Th erefore, the nonwoven prepared by the polypropylene fi bres had larger pores, which was experimentally verifi ed by the measurement of mean pore diameters, as shown in Figure 5. Hence, oils drained more easily due to a higher gravitational force than capillary pressure on account of a larger pore size. As a result, polypropylene nonwoven fabric displayed a lower oil sorption capacity than that of the cotton and cotton/kapok nonwovens.

Oil sorption of nonwovens from artifi cial sea water bath
Oil spills generally occur on the ocean's surface and in nearby land areas [1]. It was thus necessary to test the oil sorption capacities from oil in a water bath. Th e dynamic test conditions simulated the actual condition of ocean waves. Th e oil sorption capacities of all nonwovens from the artifi cial seawater bath for dynamic condition are shown in Figure 6 and on the Figure 7 the diff erence between oil sorption capacity from artifi cial seawater bath in static and dynamic condition is given.   , 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] under dynamic conditions except polypropylene. In dynamic condition the agitation hampered the oil sorption mechanism. Polypropylene showed exceptional behaviour may be due to its hydrophobicity. For low viscosity oil (diesel), the cotton/kapok nonwovens (S4−S6) displayed higher oil sorption under static condition than that of dynamic condition due to the same reason as mentioned above. Exceptional behaviour observed in case of cotton nonwovens because agitation helps better penetration of oil inside these nonwoven structures which are relatively compact due to bulk density in higher side.

Diff erence between theoretical and measured oil sorption capacity of nonwovens
Th e oil sorption capacity of all nonwoven samples was calculated theoretically from equation 4 and then experimentally measured. Th e results of both theoretical and experimental sorption capacities for both high and low viscosity oil are represented in Figures 8 and 9. It is evident from these fi gures that for engine oil, actual oil sorption capacity was higher than theoretical oil sorption capacity for all kinds of nonwovens whereas in case of diesel oil actual oil sorption was lower than that of theoretical. When oil is sorbed by a nonwoven structure the oil molecules are entered and occupied all pores in fi bre interstices as well as attached over the surface of the nonwoven structure. As a result actual oil sorption should be higher than the theoretical value that actually happened in case of high viscosity engine oil. In case of low viscosity diesel oil due to poor surface tension there was weak bonding between diesel oil molecules and fi bre surface and therefore diesel oil drain out easily from the nonwoven structure during vertical hanging and as a result actual sorption capacity become lower than that of theoretical value.

Normalised oil sorption capacity
All nonwovens produced for this study had diff erent levels of porosity with fi bres of varying density. Sorption capacity also depends on density of oil. For comparison purposes, it was thus necessary to normalise the oil sorption capacity of nonwovens to nullify the eff ect of density of sorbent fi bres, sorbing oil and porosity of the nonwoven structure. Th e normalised Engine oil  , 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] sorption capacities of all nonwovens are calculated as per equation (8) and shown in Figure 10.

Figure 10: Normalised oil sorption capacity of nonwovens
Among cotton-based nonwovens (S1−S3), cotton fl at waste nonwoven fabric (S3FW) showed the highest normalised oil sorption capacity. Th ough the nonwoven fabric made of cotton fl at waste had more immature and shorter fi bres resulting in lower porosity, a higher normalised oil sorption capacity was observed due to improved oleophilicity. An immature fi bre generally contains higher surface waxes that improve its oleophilicity [23]. Th e sorption capacity of fi bres is infl uenced by their oleophilic nature. In the case of cotton/kapok nonwovens (S4−S6), the normalised oil sorption capacities were found to be close to that of cottonbased nonwovens (S1−S3). Th is was due to both the oleophilic nature of kapok fi bres and higher fabric porosity. Cotton/kapok nonwovens had a higher porosity because of poor compaction during needling on account of poor cohesiveness between kapok fi bres [9,18].

Oil sorption rate and rate of release of engine oil from the nonwovens
Th e oil sorption rate of the nonwovens was measured for engine oil, and the results are shown in Figure 11. It is evident that all nonwovens adsorb engine oil more rapidly until 1 minute, followed by a slowdown in next two minutes till the nonwovens finally becomes saturated within 5 minutes. Th e initial steep rise in oil sorption was due to the porous structure of nonwovens that had small pores that exerted high capillary pressure. Th e next gradual rise in oil sorption might be attributed to larger pores. Th is can be explained in light of the Young-Laplace equation of the relationship between capillary pressure and pore radius, as shown in equation 10 [1−2].
where p indicates capillary pressure, r c represents pore radius, θ denotes the oil contact angle and γ indicates surface tension of oil.
Hence, the oil sorption rate depends on the capillary pressure, surface tension and contact angle of liquid, while capillary pressure depends on the size of the capillary. Th erefore, the diff erences in the oil sorption rate among the nonwovens were due to the fibre-oil contact angle and mean pore diameter of the nonwovens. In the case of high viscosity oil, all cotton nonwovens displayed a similar oil sorption rate that was signifi cantly lower than polypropylene nonwovens (S7PP). Th e cotton parallel-laid (S1CP) and cross-laid (S2CC) fabrics followed an almost similar pattern of oil sorption. It is clear that the diff erence in fi bre orientation did not cause any signifi cant difference in the oil sorption rate in the fabrics. Th e release or draining-out of adsorbed oils from the nonwovens due to free vertical hanging is approximately an inverse phenomena of oil sorption. Th e oil release rate of all nonwovens for high viscosity engine oil is shown in Figures 12. Each oil release curve consists of three distinct phases. First phase is the initial stage of release that occurs within 1 minute. Th e rate of release is highest during this period. Th e second or transition phase occurs from 1 to 10 minutes. During this period, the rate of release decreased substantially. Th e third phase represents the steady-state period. In this period, the  Tekstilec, 2020, 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] nonwoven sorbent tended to begin a descent towards a steady state. High viscosity engine oil drained very slowly from nonwovens, and thus reached a steady-state aft er 10 minutes.

Oil sorption rate and retention capacity for diesel oil
Th e oil sorption rate of the nonwovens was measured for engine oil, and the results are shown in Figure 13. It is evident that nonwovens adsorb diesel oil very fast and reach saturation within 10 seconds. Low viscosity oil (diesel) would enter pores more quickly than high viscosity oil (engine oil), which leads to the quicker absorption of diesel oil. All nonwovens displayed a slightly higher oil sorption rate for lower viscosity oil (diesel) than high viscosity oil (engine oil). High viscosity oils were not able to adsorb upward through larger pores due to insuffi cient capillary pressure. Th e heavier oil (engine oil) would require a higher capillary pressure than lighter oil (diesel) to raise the oil to a particular height. Th e polypropylene nonwoven fabric (S7PP) displayed similar oil sorption rates for both engine and diesel oil, but the time taken to reach the saturation point is higher for high viscosity oil (engine oil). It was thus determined that the fi bre type is a critical factor in determining the oil sorption rate. Th e rate of release of diesel oil for all nonwovens is shown in a Figure 14. Each of these curves consists of two distinct phases. Th e fi rst phase is the initial stage of release, which occurs within 1 minute. Th e rate of release is much high during this period. Th e second or transition zone lasts from 1 to 10 minutes. During this period, the rate of release was achieved a steady-state. Low viscosity oil (diesel) drained from nonwovens more rapidly and reached a steady-state quickly, i.e. within 1 minute.
Th e sorption of low viscosity oil ( Figure 11) by all the nonwovens from the oil bath was quicker than that of the high viscosity oil ( Figure 13). Also, low viscosity oil was found to drain away more rapidly during the draining period (1 minute) (Figure 12), while the draining of high viscosity oil was found to be slow ( Figure 14). Th is is the reason for the ultimately higher oil sorption capacity for high viscosity oil exhibited by all kinds of nonwoven specimens.

Recovery of oil and reusability
Th e sorbed oil from nonwovens was recovered by compressing the nonwovens using a roller squeezer with a roller pressure of 1.5 kg/cm 2 . Th e percentage of recovered diesel oil from diff erent nonwovens for consecutive four sorption cycles is shown in Figure  15. Th e recovery of diesel for PP nonwoven in the fi rst cycle was observed to be around 94% which was found to be lowest among all nonwovens. Th e oil recovery showed a higher value in second cycle is attributed to the presence of residual oil inside nonwoven structure even aft er the squeezing in fi rst cycle. Th is is the same reason due to which 100% recovery Tekstilec, 2020, 63(1), [14][15][16][17][18][19][20][21][22][23][24][25][26] cannot be achieved. It can be seen from Figure 15 that oil recovery did not deteriorate much aft er 4 th cycle of test.

Figure 15: Percentage of diesel oil recovered from nonwovens aft er diff erent cycles
An oil sorbent can be considered reusable if it can be easily compressed or squeezed to retain its original size and shape [12,24]. Figure 16 shows the reusability of nonwovens for diesel oil. All nonwovens displayed a signifi cant reduction in oil sorption capacities of around 50% (10 to 20 g/g) during the second cycle. Th e oil sorption depends on the porosity of the fabric, while the fabric porosity is in direct correlation with fabric thickness. Fabric thickness reduced aft er every cycle of padding, leading to a change in porosity and pore size. Th e fl attening of pores was expected, which might result in the inability to hold much liquid. Th e thickness of the fabric was reduced due to padding aft er every cycle, which led to a reduction in oil sorption capacity. Th e percentage of thickness retained by nonwovens aft er every cycle is given in Figure 17. Th e reduction in oil sorption capacities was much higher during the second cycle, while the reduction was not very signifi cant during further successive cycles. During reuse, the reduction in oil sorption for polypropylene nonwoven fabric (S7PP) was found to be lower than in other nonwovens. Th e oil sorption capacity of polypropylene nonwoven fabric (S7PP), even after four cycles, was found to be lower than the nonwovens from natural fi bres. All nonwovens prepared from natural fi bres displayed poor oil sorption capacity during reuse. Sorption is dependent on the availability of pores. Bulkier fabrics with similar mass per unit area should off er more oil retention sites. Nonwovens from natural fi bres suff ered more loss in thickness. Th is led Oil sorption capacity [g/g]

Conclusion
All the studied nonwovens displayed signifi cantly higher oil sorption capacity for high viscosity oil (engine oil) than that of the low viscosity oil (diesel). Nettle fi bre nonwoven exhibited lowest oil sorption capacity and poor compressional recovery and therefore considered poor material for this application. Except nettle fi bre nonwoven fabric (S8), all other natural fibre nonwovens (S1−S6) displayed higher oil sorption capacity than polypropylene nonwoven fabric (S7). Cotton/kapok blended nonwovens (S4−S6) were the best performer in terms of higher oil sorption and retention capacity, and oil sorption rate. An increase in kapok content in the cotton/kapok nonwoven led to a better oil sorption capacity. Even nonwovens prepared from cotton fl at waste fi bres exhibited very good normalised oil sorption capacity, which could open up a new door for sustainable usage of cotton waste. All these natural fi bre nonwovens achieved a steady-state of sorption quickly, within 1 minute for low viscosity oil (diesel) and within 10 minutes for high viscosity oil (engine oil). In addition, more than 95% of the oils adsorbed by the nonwoven fabrics can be recovered through simple compression. During reuse, the oil sorption capacity of nonwovens gradually fell down due to thickness loss during compression. Th us, cotton/kapok fi bres and cotton fl at waste may be a sound choice as alternative materials to polypropylene as sea-water oil sorber in terms of low-cost, biodegradable and sustainable material.