Reduced die lip buildup extrusion of polymer compositions

Details Reduced die lip buildup extrusion of polymer compositions Reduced die lip buildup extrusion of polymer compositions

1999-11-22

2001-06-12

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

With severing, removing material from preform mechanically,...

Making hole or aperture in article

C264S173120, C264S210600, C264S211000, C264S288800

Reexamination Certificate

active

06245271

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to processes for the die extrusion of polymers and resulting extruded products. The process of the invention permits extended run times without shutting down the extrusion line for clean-up due to die lip build-up and improves resulting products.
BACKGROUND OF THE INVENTION
Many products today require highly engineered components and yet, at the same time, must be produced at a cost consistent with limited use or disposability. By limited use or disposable, it is meant that the product and/or component is used only a small number of times or possibly only once before being discarded. Examples of such products include, but are not limited to, surgical and health care related products such as surgical drapes and gowns, disposable workwear such as coveralls and lab coats and personal care absorbent products such as diapers, training pants, incontinence garments, sanitary napkins, bandages, wipes and the like. All of these products can and do utilize as components, films and fibrous nonwoven webs. While both materials are often used interchangeably, films tend to have greater barrier properties, especially to liquids, while fibrous nonwovens webs have, among other things, better tactile, comfort and aesthetic properties. When these materials are used in limited use and/or disposable products, the impetus for maximizing engineered properties while reducing cost is extremely high. To this end, it is often desirable to use either a film or a nonwoven to achieve the desired results because the combination often becomes more expensive. In the area of films, there have been previous attempts to make multilayer films with reduced thicknesses. One advantage in forming multilayer films is that specific properties can be designed into the film, and, by making the films multilayer, the more costly ingredients can be relegated to the outer layers where they are most likely to be needed.
In addition, in the production of a breathable filled film it is common to employ a significant percent (by weight) of filler such as, for example, calcium carbonate. As is known in the art, stretching of the filled film creates a fine pore network which allows the film to continue to act as a barrier to liquids and particulate matter yet allows air and water vapor to pass therethrough. In order to obtain more uniform barrier and vapor transmission properties throughout the film it is desirable to have the filler equally distributed throughout the film. Thus, although such breathable barriers may act as a barrier to liquids and particulate matter they may themselves be a source of unwanted particles (i.e. the filler) which can be a source of die lip contamination and buildup. This filler accumulation and/or detachment may also be an undesirable cause of defects in various applications or articles employing the barrier fabric. A filled film which retains good breathability and low defect levels produced without die lip buildup is therefore desirable. In this regard, there exists a continuing need for a multilayer film having outer layers with little or no filler, yet which does not significantly reduce the breathability of the multilayer film. Moreover, many filled films fail to provide good adhesion to additional layers, such as, for example, nonwoven fabrics. Multilayer films which are capable of providing good adhesion to a support fabric without loss of breathability are likewise needed.
As mentioned, production of such films and nonwovens has, however, been accompanied by persistent problems of buildup of the composition being extruded on the die tip causing machine downtime for cleanup, frequently after only a few hours of operation. Various mechanisms are known to facilitate the cleaning and maintenance of dies used for the extrusion of polymer materials, while minimizing downtime. Molten polymers are extruded through dies to form films, strands, nonwoven webs, and other finished polymer forms. Particularly with polymer compositions containing fillers, as polymer exits the die, some of the polymer composition clings to the die openings or “lips,” accumulating on the exterior surface of the die. This die lip build-up gradually increases until it accumulates to a point where it breaks off, possibly causing a defect in the product, which can be, for example, in the form of thin spots or tears or otherwise deleterious effects on the texture or other esthetic properties of the product. Considerable engineering goes into the design of dies and selection of extrusion compositions to minimize this build-up. Diverging, converging, radiused, and angled die lip geometries all are examples of methods developed to minimize this build-up. However, no die design completely eliminates it. It is common practice to temporarily halt the extrusion operation to perform maintenance on the die to remove this build-up.
U.S. Pat. No. 5,435,708, issued to Kaun, discloses a melt blowing die head with opposing die lips mounted on lip guides pivotally connected to respective setback bars. To facilitate cleaning and maintenance of the die, the lips can be swung away from the spinnerette on the respective lip guides. This allows for reduced down time during maintenance of the die, but does not permit maintenance during operation of the line.
U.S. Pat. No. 5,720,986, issued to Gohlisch et al., discloses an extrusion head having a stationary part fixed on an extrusion installation. The installation includes a plurality of extrusion cylinders. The stationary part has flow channels, each in communication with a respective extrusion cylinder and a common extrusion die. Two pivotal outer parts hinged on the stationary part are selectively swingable individually between open and closed positions. The outer parts define an extrusion orifice for the die in their closed position. When the outer parts are opened, maintenance can be performed on the die.
U.S. Pat. No. 4,413,973, issued to Peters, discloses a die which has a removable extrusion plate covering the die head. The plate can be removed and replaced to minimize downtime required for maintenance.
Co-assigned U.S. patent application Ser. No. 09/075,510 filed May 8, 1998 to Cook entitled “Extrusion Die System with Removable Insert” describes an insert for cleaning die tips “on the fly”.
Also, the selection of polymer compositions and extruded structures has been varied in an effort to minimize die lip buildup. For example, co-assigned U.S. patent application Ser. No. 08/882,715 to McCormack et al. entitled “Low Gauge Films and Film/Nonwoven Laminates” filed Jun. 25, 1997 and incorporated herein in its entirety by reference describes film structures having thin outer skins that reduce die lip buildup.
A common feature of the prior art devices and processes is that routine cleaning and maintenance requires at least some downtime. While efforts have been made to reduce maintenance and downtime, it remains a goal to further increase operating times between required cleanings.
SUMMARY OF THE INVENTION
The present invention is directed to die extrusion of products such as films and nonwovens. The films may be made by conventional film forming techniques such as cast and blown co-extrusion film forming processes and the nonwovens by conventional processes such as spunbonding and meltblowing. The films in a particular embodiment are created with a core layer made from an extrudable thermoplastic polymer with the core layer defining a first exterior surface and a second exterior surface. In basic embodiments of the present invention, the core layer may have a first skin layer attached to the first exterior surface of the core layer and a second skin layer attached to the second exterior surface of the core layer. In such situations the first skin and the second skin layer may have a combined thickness which does not exceed about 15 percent of the overall thickness and more desirably for applications requiring breathability, neither the first skin thickness nor the second skin thickness exceeds more than about 7.5 percent of the overall thick

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