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Microfibers in nonwoven production.

an examination of the various steps that comprise microdenier fiber technology; many areas must be considered

The use of microdenier fiber (either as sub 1 denier or bicomponent "sea-island" with sub denier islands) in dry laid needlepunched nonwovens, although quite established in Japan, has just recently begun to assert itself in the North American nonwovens industry. The application of microdenier fiber to various high technical end uses, as well as the types of microdenier fiber, is growing.

Opening/Blending, Carding

The opening of microfibers can be made through the traditional methods of either a bale opener feeding a pneumatic transport system or through bale opening devices feeding weighing hopper feed systems. If any blending is required with lots of multiple components, the easiest and best method of insuring blend ratio consistency would be through use of weigh pan hopper feeders, feeding to a bin in which simultaneous feed and discharge is possible, to thoroughly homogenize the mix. This is due to the significantly higher number of individual fibers within the blend as compared to conventional 1.5 denier or higher.

Thus, the opening and blending of the fiber is critical to obtain consistency of the carded web. Due to the relatively low rates of production (as compared to the normal rates seen with 1.5 denier and greater), the opening machine will not be required to generate high levels of throughput but rather would be required to maximize fiber opening with the minimum of fiber damage.

Carding Rates of Production. Rates are for 2.5 meter cards assuming, for example, a doffer speed of 60 m/min:

Single Doffer 5-10 g sq. meter=45 kg/hr.

Double Doffer 9-18 g sq. meter=80 kg/hr.

Wire Considerations. The most important wire choice is on the doffer(s) and main cylinder. The wire for the main cylinder must take into account the population, points per square inch (PPSI), height and profile of the teeth and the dimension and sharpness of the point. The PPSI typically would be 600 with a front angle of 75-80 [degrees]. The doffer would normally have a PPSI of 400 with a front angle of 55-65 [degrees].

The surface finish is fundamental, especially on the doffer. The largest quality problem encountered is the formation of neps and the largest handling problem is loading the fiber onto the wire.

At present, carding of >0.5 denier is successful with 100% microfiber, with <0.5 denier it is necessary to blend in >0.5 denier as a carrier fiber. It is not yet generally possible to card 100% <0.5 denier due to wire limitations.

Recommended Carding Configurations. As rates of production are quite low, a double doffer format would be advantageous in regard to both production capacity as well as generation of webs having greater weight and thus, better strength and coverage.

The double doffer format would present two variations in regard to number of cylinders (i.e. embellishment of carding cylinders). A card having two main cylinders--a breast cylinder followed by a main cylinder--will be well suited to bicomponent type microdenier as cylinder speeds are progressive and do not apply the same stress to the fibers as a compact card.

The card giving the highest web quality would be a tandem card with two main cylinders and intermediate doffing. A card of this type would be well suited for applications such as synthetic leather.

Difficulties Or Special Considerations. The largest problem with carding of microdenier (both monofilament and bicomponent) is the formation of neps.

The problem with neps is derived from the fiber type (bicomponent PA fibers are traditionally more difficult due to the fiber softness, flexibility, crimp levels and low resilience) as well as:

* speed ratios within the carding points

* the amount of fiber recycling

* gap settings

* wire specification

* edge suction

* prevention of turbulence.

The objective of the volumetric hopper feeder should be to make a thin batt (small settings on chute feed zone) to enable higher feed roll speed. The reason would be to reduce both torque at the feed-in as well as the density of the batt being fed into the feed roll/lickerin zone, keeping the number of individual fibers per single tooth of lickerin wire at a low density.

The use of a feed plate may also be successfully used in lieu of a dual feed roll with the effect of being able to minimize the area of uncontrolled fiber due to the short length of staple (i.e., 38 mm).

The use of feed plate versus feed roll will be determined in large part by the fiber, length of fiber and most importantly, its crimp levels, specifically the amplitude and number of crimps per inch.

A fiber with low amplitude and high number of crimps per inch will be better suited to feed plate, while a fiber having high amplitude/low crimp population will be better suited to feed roll.

Crosslapping

The web weight and staple length of the microfibers play an important part in the difficulties of crosslapping. The specific areas of concern are the tension (draft) between the doffer of the card and the feed-in apron of the crosslapper (typically an integrated front apron of the machine).

The angle of the front incline apron is important; the smaller this angle the better, so it may be advantageous to have the card on a small wall that would lower the feed-in angle. The control of the carded web through the crosslapper would dictate a crosslapper being able to sandwich the web throughout the process of crosslapping, preventing distortions by both air turbulence and draft, which occurs on open roller, curved type machines.

The level of humidity, which would be high to improve the carding aspect, can be dangerous should the fiber become too damp or the crosslapper apron becomes laden with moisture, as the fibers will then have a tendency to stick to the aprons. The delivery floor apron of the crosslapper should be with a smooth surface, but covered with a substance such as silicone that would be able to maintain the web sufficiently without allowing deformation to the batt through web tension at the transition of the bottom carriage or through premature draft by way of the following apron.

It is important to feather the card web edge while preventing the edge to fold onto itself (due to air turbulence and web tension). Due to this, the closer the gap between the doffing mechanism (preferably roller doffer type) and the feed-in apron, the less chance web deformation will occur. Another important factor in maximizing crosslapping quality is the deceleration/acceleration time at the transition of the traversing carriage. The weight uniformity of the batt, its profile, is in large part determined by the slope of acceleration/deceleration, which can be controlled through use of correct engineering of the crosslapping masses and its drive. Any side to side weight variation (defect) of the batt is further aggravated by drafting, which occurs due to the textile effect of machine to machine feed (line tension) as well as to uncontrolled fiber migration.

The advent of the "Profile" crosslapper, through use of a computer program, will allow for the crosslapper to develop batts that would mirror the image of line draft. This profiled batt will allow for much higher levels of finished side-center-side uniformity of the end product, as the batt formed in crosslapping is generated specifically to the line draft condition of the particular end product. Each nonwoven product has a different draft requirement for which a profiled batt can be created to enhance the uniformity of that particular product. This level of uniformity is extremely important as the coefficients of variation for high technical felts--for example, filtration--will have tighter tolerances.

Needlepunching

The product end use will dictate the needle loom configuration, such as the level of needle densities required (per machine, total line) and direction of stroke. The preneedle loom and type of needle used are the most important considerations in the needlepunch line, as the batts will be very susceptible to deformation prior to stabilization through preneedling.

The control of the batt entering the needle zone is very important as it is here that the most damage to batt uniformity can occur due to uncontrolled draft created by tension of the delivery rolls of the loom. Systems like the DCIN batt compressive feed, which progressively compress the batt and deliver the product to the needle zone, have been developed. Delivery of the batt into the needle zone reduces the uncontrolled length-distance between release of the fibers and the first row of needles. The batt driven into the needle zone through use of aprons is important, as tension within the batt is no longer derived from the delivery rolls of the loom but is stabilized and can be adjusted through adjustment of feed-in delivery speed ratios.

The selection of needles for use with microfibers is very important. The needles typically will be 40-42 gauge, with preneedling utilizing a 40 gauge RB needle and the finishing loom(s) using 40-42 gauge with less aggressive barb specifications. This is specially important in the production of synthetic leather where needlepunching is sequential. The barb treatment on the finishing needle, normally 42 gauge, will have depth as little as 0.4 mm as the needlepunching objective is surface enhancement. The needle may also have only one barb on a single apex or the depth of penetration used will engage only one barb.

Production Example

The production of synthetic leather typically uses bicomponent microdenier, which after felting is subjected to a caustic soda bath that dissolves the sheath, leaving the multiple nylon 6 fiber. An example of a bicomponent fiber used would have the center (island) of nylon 6 with 36 individual fibers with a ring (sea) of polyester, with the whole fiber being 3.5 denier x 51 mm staple length. The weights of felts for synthetic leather or associated products may be as high as 1400 g. sq. meter, which will be needled to obtain exact thickness and density requirements. With microfibers, the speeds of production are typically slow--less than 60 m/min doffer speeds--with the emphasis on quality of web and felt preparation.

The needlepunching will use multiple zones, having both single direction looms and double punch looms incorporated into the line. The double punch looms are typically used for finishing, positioned at the end of the production line. Double punch looms are used at the conclusion of the line to ensure surface appearance and overall uniformity in regard to fabric structure. The needle density and penetrations per square inch (PPSI) for the line is high, with progressive needlepunching machine to machine with penetration becoming less in each loom as the product flows towards the windup apparatus.

For example, in a line with three looms after the preneedle machine, the PPSI value could be 1250 for the first loom, 1100 for the intermediate loom and 850 on the finishing double punch loom. Clearly the work stabilization and maintenance of fabric strength occurs in the first looms, with overall densities and surface appearance occurring in the final loom.

The width of the production line is typically 2.5-2.6 meters, with fabrics after trimming being 1.8-2.0 meters in width. Other end uses for microdenier (either a bicomponent or monofilament type) include synthetic leather; PVC polyurethane coated products for shoes, handbags, luggage; medical, such as blood filtration; silicone polishing chips in the computer industry; filtration--air and other; glass polishing cloths and wiping cloths.
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Nonwovens Technology
Author:Feroe, Daniel
Publication:Nonwovens Industry
Date:Mar 1, 1992
Words:1921
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