Hydroentanglement

Hydroentanglement is a bonding process for wet or dry fibrous webs made by either carding, airlaying or wet-laying, the resulting bonded fabric being a nonwoven. It uses fine, high pressure jets of water which penetrate the web, hit the conveyor belt (or "wire" as in papermaking conveyor) and bounce back causing the fibres to entangle.

Hydroentanglement is sometimes known as spunlacing, this term arising because the early nonwovens were entangled on conveyors with a patterned weave which gave the nonwovens a lacy appearance. It can also be regarded as a two-dimensional equivalent of spinning fibres into yarns prior to weaving. The water pressure has a direct bearing on the strength of the web, and very high pressures not only entangle but can also split fibres into micro- and nano-fibres which give the resulting hydroentangled nonwoven a leatherlike or even silky texture. This type of nonwoven can be as strong and tough as woven fabrics made from the same fibres.

Hydroentanglement

The process of hydroentanglement was invented as a means of producing an entanglement^[1] similar to that made by a needleloom, but using a lighter weight batt. A successful process was developed during the 1960s by Du Pont and was patented. However, Du Pont decided in the mid-1970s to dedicate the patents to the public domain, which resulted in a rush of new development work in the major industrial countries, Japan, USA, France, Germany and Britain. As the name implies the process depends on jets of water working at very high pressures through jet orifices with very small diameters.A very fine jet of this sort is liable to break up into droplets, particularly if there is any turbulence in the water passing through the orifice. If droplets are formed the energy in the jet stream will still be roughly the same, but it will spread over a much larger area of batt so that the energy per unit area will be much less. Consequently, the design of the jet to avoid turbulence and to produce a needle-like stream of water is critical. The jets are arranged in banks and the batt is passed continuously under the jets held up by a perforated screen which removes most of the water. Exactly what happens

to the batt underneath the jets is not known, but it is clear that fibre ends become twisted together or entangled by the turbulence in the water after it has hit the batt. It is also known that the supporting screen is vital to the process; changing the screen with all other variables remaining constant will profoundly alter the fabric produced. Although the machines have higher throughputs compared with most bonding systems, and particularly compared with a needleloom, they are still very expensive and require a lot of power, which is also expensive.The other considerable problem lies in supplying clean water to the jets at the correct pH and temperature. Large quantities of water are needed, so recycling is necessary, but the water picks up air bubbles, bits of fibre and fibre lubricant/fibre finish in passing through the process and it is necessary to remove everything before recycling. It is said that this filtration process is more difficult than running the rest of the machine. Fabric uses include wipes, surgeons’ gowns, disposable protective clothing and backing fabrics for coating. The wipes produced by hydroentanglement are guaranteed lint free, because it is argued that if a fibre is loose it will be washed away by the jetting process. It is interesting to note that the hydroentanglement process came into being as a process for entangling batts too light for a needleloom, but that the most recent developments are to use higher water pressures (400 bar) and to process heavier fabrics at the lower end of the needleloom range.

Methodology of hydro-entanglement Spun-lacing is a process of entangling a web of loose fibers on a porous belt or moving perforated or patterned screen to form a sheet structure by subjecting the fibers to multiple rows of fine high-pressure jets of water. Various steps are of importance in the hydro-entangling process [5, 6]. While some of them are typical in a nonwoven process, some of them are unique to the process of spun-lacing. The steps characteristic for producing hydro-entangled nonwoven fabric include:

• Precursor web formation
• Web entanglement
• Water circulation
• 
  Web drying

The formed web (usually air-laid or wet-laid, but sometimes spun bond or melt-blown, etc.) is first compacted and pre-wetted to eliminate air pockets and then water-needled. The water pressure generally increases from the first to the last injectors which are used to direct the water jets onto the web.

This pressure is sufficient for most nonwoven fibers, although higher pressures are used in specialized applications. It has been argued that 10 rows of injectors (five from each side of the fabric) should achieve complete fabric bonding [5].

Injector hole diameters range from 100-120 µm and the holes are arranged in rows with 3–5 mm spacing, with one row containing 30-80 holes per 25 mm [7].

The impinging of the water jets on the web causes the entanglement of fibers. The jets exhaust most of the kinetic energy primarily in rearranging fibers within the web and, secondly, in rebounding against the substrates, dissipating energy to the fibers.

A vacuum within the roll removes used water from the product, preventing flooding of the product and reduction in the effectiveness of the jets to move the fibers and cause entanglement.

Usually, hydro-entanglement is applied on both sides in a step-wise manner. As described in the literature[8] , the first entanglement roll acts on the first side a number of times in order to impart to the web the desired amount of bonding and strength.

The web then passes over a second entanglement roll in a reverse direction in order to treat and, thereby, consolidate the other side of the fabric. The hydro-entangled product is then passed through a dewatering device where excess water is removed and the fabric is dried.
Hydro-entanglement carried out at standard conditions (six manifolds of needles, 1500 psi, and web weighing 68 g /m2) requires 800 pounds of water per pound of product [9].

For that reason it is necessary to develop a new filtration system able to effectively supply clean water with this high throughput; otherwise, water jet holes become clogged. This system consists of three stages: chemical mixing and flocculation, dissolved air flotation and sand filtration [9]. Spun-laced fabrics have led to a lot of speculation regarding their manufacture because most of the manufacturing process details are considered as proprietary [10].

References

• M. G. Kamath, Atul Dahiya, Raghavendra R. Hegde (April 2004). "Spunlace (Hydroentanglement)". University of Tennessee's College of Engineering. Retrieved 2013-11-25.