Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1996-08-16
2001-04-03
Cole, Elizabeth M. (Department: 1771)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S296000, C156S308200, C156S309900, C428S198000, C442S409000
Reexamination Certificate
active
06210511
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for rapid formation of a highly uniform nonwoven web of staple fibers and is particularly suitable for the formation of the low basis weight webs of thermoplastic fibers at a high rate of speed.
Nonwoven fabrics are produced by a variety of methods, and in general, such methods involve the continuous laydown of fibers or filaments in the form of an unconsolidated flat web on a conveyor, followed by consolidation of the web, such as by bonding or locking the fibers together to form the web into a cohesive fabric.
The carding of staple fibers into an unconsolidated web followed by point bonding with a hot calender is one well known method of producing a nonwoven fabric. In such a process the fibers, which are received in bales, are first opened with standard textile opening equipment. The opened fibers are then fed to single or multiple cards which are installed in line, each forming a thin web. The webs are then layered together, then usually spread to increase web width, and fed to a hot calender for thermal bonding. The customary calender consists of two heated rolls, one being a smooth steel anvil roll, the other being a roll with an embossed pattern. The high points of the pattern are the area where the fibers are bonded together through partial melting. Such systems can produce webs which are reasonably uniform at a given speed and basis weight. Typically, a reduction in unit weight or an increase in speed results in a noticeable degradation in the uniformity of the fiber distribution. More precisely, at lower basis weights the web develops a more blotchy appearance due to areas of higher and lower concentrations of fibers. In the worst case, holes will form where the concentration of fiber is low. The degradation in web uniformity for the traditional system is also linked to the need of additional draw on the unbonded web to eliminate the bulging of the web which would otherwise occur at various points in the process. The amount of draw used to control the web during transport to the calender is inversely proportional to the cohesion of the unbonded web. A low cohesion web will require a higher draw. The spreading section and the calender nip point are prime areas where the bulging occurs. This bulging, if not eliminated, causes extremely poor web uniformity. A lighter web, when submitted to such increase in draw, develops even greater defects because the extremely light areas are now deformed into holes in the web.
The prior art has tried to minimize the requirement for draw by using equipment transfer geometry and higher cohesion fiber to produce nonwoven material at higher production speeds. Both modifications have produced only moderate improvements in speed or uniformity.
Other prior art has been the development of a machine which reorganizes the carded unbonded web (with minimal or no increase in output speed) by reforming it on a vacuum collector such as described in U.S. Pat. No. 4,475,271. This process can produce a web with a more uniform balance in tensile strength between the MD and CD direction but, it does not deliver the desired level of uniformity in fiber distribution as judged by visual appearance.
SUMMARY OF THE INVENTION
In accordance with the present invention, a slow moving thick or high basis weight web of fibers having a high degree of cohesion, is formed using conventional cards, or other mechanisms. This web may be first spread in the cross machine direction.
The thick web is fed into a relatively fast moving toothed reforming roll, which carries a layer of excess recirculating fibers needed to form the final web. A uniform portion of the layer of fibers is continuously removed from the reforming roll by a toothed web forming roll, and this web layer is transferred as a web to a conveyor by a transfer roll. The web is subsequently bonded.
In the preferred embodiment, the reformed web is fed from the conveyor around an air control transfer roll, which allows the web to change direction without lifting or disruption, with the exit of the air control roll being located closely adjacent the upper heated roll of the rotating calender rolls.
The web is not fed directly into the nip between the calender rolls. Rather, the web is transferred to the upper hot calender roll into a secondary nip between the transfer roll and hot calender roll, in an area upstream of the nip. The unconsolidated web is then heated and compressed in the secondary nip and is supported on the hot roll prior to entry into the calender nip to become thermally bonded.
As the web passes through the secondary nip, the web is compressed, causing fibers to move relative to each other in a more uniform arrangement. This effect is aided by contact of the web with the heated roll in which individual heated fibers may shrink, curl or relax as they are being physically rearranged by compression. The rearranged web is partially wrapped and supported on the heated roll, which tends to eliminate any bulging of the web due to passage through the calender.
Downstream of the reformer roll, all rolls and conveyor operate at substantially the same surface speed, and no substantial machine direction draw is imparted to the reformed web due to transport or thermal bonding. Thus, very light weight or low cohesive webs may be processed at high speeds without any loss in uniformity, and, in fact, uniformity is increased in the final stages of processing.
In summary, the invention can be considered as having several general aspects. First, a web of staple fibers having a first basis weight and moving at a first speed is converted into a second, more uniform web having a second, lower basis weight and moving at a second, higher, surface speed. This is accomplished by continuously metering a layer of fibers from the first web onto a rapidly rotating toothed cylinder and removing a uniform portion of said layer to form the second web moving at the second speed. The second web is subsequently bonded.
In a broad second aspect, a web of individual fibers, including at least some thermally bondable fibers, is subjected to preconditioning immediately prior to passage through a nip of a bonding calender. The preconditioning involves subjecting the web to heat and compression which is sufficient to at least partially rearrange the fibers in a more uniform array, but insufficient to thermally bond the fibers.
A third broad aspect comprises supporting a web of unbonded thermoplastic fibers on a heated surface immediately prior to entry into the nip of a calender. The second and third aspects are preferably accomplished using a heated roll of the calender to heat, compress and support the web upstream of the bonding nip.
A fourth broad aspect is to support the web of individual fibers to be thermally bonded at a substantially constant surface speed between the zone of formation and into and through the bonding zone in order to minimize any draw on the web after final web formation and to prevent loss of uniformity due to draw.
REFERENCES:
patent: 4003783 (1977-01-01), Groome et al.
patent: 4574433 (1986-03-01), Brunnschweiler
patent: 5361451 (1994-11-01), Fehrer
patent: 5494736 (1996-02-01), Willey et al.
patent: 03269154 (1991-11-01), None
Merker Joseph F.
Nolan Benjamin Michael
Cole Elizabeth M.
Fibertech Group
Pyle & Piontek
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