Nylon 6,6 Nonwoven Fabric Separates Oil Contaminates from Oil-in-Water Emulsions

The spunbond nylon samples used in this study sorb oils following a power model ( Fig 4 ). At the lowest fabric densities measured, the fabric sorbed approximately 16x its weight in crude oil and 26x its weight of the more viscous gear lube oil. Sorption of oil for both viscosities is a function of the power of the fabric’s area mass density (basis weight, osy) raised to the power of -0.638 for crude oil and -0.62 for the gear lube oil. These powers are almost identical, indicating similar sorption behavior for the nylon fabric, with differences in oil viscosity affecting the coefficient of the power function (12.93 for gear lube oil and 7.09 for crude oil). This indicates that the material properties of the nylon fabric (constant for this experiment) control the value of the exponent of the power function, and therefore the shape of the curve, while the viscosity of the oil affects the coefficient of the power function and the separation between the two curves in Fig 4 . Higher basis weight fabrics have a larger mass of nylon packed into approximately the same area as lower basis weight fabrics. Though the lower basis weight fabrics have less total material, they have more void space for containing oil droplets and more available sorbent surface area due to reduced contact between the less dense nylon strands.

Nylon 6,6 is known to be amphiphilic.[ 15 ] The amphiphilic nature of nylon is confirmed by an investigation of the contact angle of water and oil with nylon fabric. Fig 3A shows the contact angle formed by water on 4 osy nylon fabric. The contact angle measured for water on nylon is 73°, indicating that the fabric is hydrophilic.[ 19 ] Similarly, the contact angle formed by the same nylon fabric and high viscosity gear lube oil is 56°, indicating that the material is also oleophilic ( Fig 3B ).

Oil-in-water emulsion separation with nylon bags

Both the two denier per filament and four denier per filament nylon bags filtered more than 97% of the oil contaminates from the oil-in-water emulsion (Table 1). Nylon spunbond fabric contains oleophilic aliphatic chains which are connected by hydrophilic polyamide linkages.[20] Upon contact with water molecules, the polyamide linkages are solvated while the aliphatic chains in the nylon are available to participate in Van der Waals attractions with the non-polar oil.[16] Because of the ability of the shell hydration around each polyamide linkage to grow large enough to merge with each other in the aqueous environment, the emulsified oil initially attracted to the aliphatic chain separates from the nylon as it is repelled by the polarity of the relatively large hydration shell. The enlarging oil aggregation is held together by the surface tension of the oil which is in an aqueous environment. If the force applied by the flowing water upstream of the coalesced oily aggregate is lower than the repulsive force applied by the oleophobic, solvated polyamide linkages, the oily aggregate will be kept from flowing through the hydrated nylon barrier. The oil aggregation increases in size by coalescing oil particles out of the oily water being filtered. In an aqueous system, the water on the opposite side of the fabric represents a significant thermodynamic barrier to oil release from the nylon, resulting in oil retention in the fabric as opposed to oil release into the water on the other side.

When a filtration media is placed in the flow path of an oil and water emulsion to separate the oil from the water, there will be a maximum amount of oil that the fabric will retain. The maximum oil retained is referred to as the critical oil exposure volume (COEV), or saturation volume. It is known that COEV decreases with increasing fabric thickness because fluids will take a longer time to diffuse though thicker fabrics.[21] Although the body of water on the far side of the fabric creates a significant barrier that resists oil transport through the fabric, oil droplets occasionally pass through the fabric filter. In these cases, droplets passing through thicker fabric have had more time to coalesce, resulting in the release of larger oil droplets from the fabric. Fig 5 shows that the thickness of the studied fabric increases linearly with respect to basis weight, providing evidence that the mechanism driving the oil separation efficiency is the mechanism described above, first hypothesized by Briscoe, et al.[21]

The nylon spunbond fabric also affects the stability of the interfacial distance between the surface of the fabric and the oil. Since the oil is emulsified in water, it is conjectured that the hydrophilicity of nylon spunbond fabric provides a motive force to attract the aqueous emulsion to the fabric increasing the initial rate at which the oil adheres to the fabric. The surface energy of nylon in an n-alkane/water system, 52.9 mJ/m2, is higher than that of polyester, 40 mJ/m2, or polyethylene, 23.1 mJ/m2, commonly used materials in sorbent booms used for oil reclamation.[22,23] This suggests that nylon is more hydrophilic than polyester or polyethylene in a crude oil-in-water system. Polyethylene behaves similarly to polypropylene and has surface tension that is near identical to polypropylene.[24] The moisture regain of nylon 6,6 at 65% relative humidity and 23°C is 4.5% which is higher than the moisture regain of polyethylene terephthalate (PET) and polypropylene at about 0.4% and about 0.05%, respectively.[25] Thus, nylon 6,6 is both hydrophilic and oleophilic. In most cases large bodies of water, lakes, rivers, and streams will not flow fast enough to provide the force needed to overcome the attractive forces within the oil and the repulsive forces of the shell of hydration surrounding the polyamide linkages. Therefore, the oil will remain upstream of the nylon spunbond fabric. This will allow execution of methods to remove the oil from the environment.