Paper making and fiber liberation – Processes and products – Multi-layer waterlaid webs or sheets
Reexamination Certificate
2002-04-10
2003-03-25
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Multi-layer waterlaid webs or sheets
C162S130000, C162S132000, C162S146000, C162S157200, C162S157300
Reexamination Certificate
active
06537424
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
Provided is a high temperature paper structure comprised of two outside layers and at least one inside layer. The two outside layers are generally comprised of substantially cellulosic pulp fiber, and the inside layer is comprised of cellulosic pulp fiber, a high temperature fiber and a polymeric binder. The paper structure can be used as a high temperature E-board for application in transformers, and offers enhanced thermal resistance.
2. Description of Related Art
High temperature E-board is used in transformers and performs two functions. First, the E-board provides electrical insulation. This keeps the coils in the transformer from short circuiting. Secondly, the board provides mechanical strength. When there is a large passage of current through the transformer, there is force on the layers of the coil to move the board, which is glued to the coils. The glueing of the board to the coil keeps the various coils from telescoping. Each coil acts like a solenoid and tries to move. It is the E-board which prevents this telescoping.
Improving the mechanical strength of the E-board would aid in avoiding problems with telescoping coils. Having a reinforced E-board to strengthen the paper would help to provide the necessary mechanical strength. The paper, however, would have to be made in an efficient and effective manner.
There is also interest in increasing the temperature resistance of E-board for use in transformers so that a less expensive transformer could be designed. By reducing the diameters of the wires in a transformer, the coils would become smaller. Smaller coils require smaller cores and smaller metal containers. Smaller containers hold less oil, and this means that less copper for the wire, steel for the cores and oil for the insulation are needed. Because of the thinner wire, however, the transformer would have more electrical resistance and would run hotter. Thus, the E-board would have to exhibit enhanced thermal resistance before such a transformer would be practical.
A paper which exhibits such enhanced thermal resistance, as well as enhanced mechanical strength would allow the industry to design transformers which can recognize the economic benefits and performance benefits discussed above.
Accordingly, it is an object of the present invention to provide a paper structure which exhibits enhanced thermal resistance.
Yet another object of the present invention is to provide a paper structure which exhibits enhanced mechanical strength.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, and the claims appended hereto.
SUMMARY OF THE INVENTION
In accordance with the foregoing objectives, provided is a paper structure comprised of two outside layers and at least one inside layer. The two outside layers are preferably comprised of substantially cellulosic (wood) pulp fiber. The inside layer is comprised of cellulosic pulp fiber, a high temperature fiber and a polymeric binder. In a preferred embodiment, the structure comprises at least three inside layers, all comprised of cellulosic pulp fiber, high temperature fiber and a polymeric binder. The most preferred polymeric binder is polyvinyl alcohol.
The present invention also provides a process for making the paper structure of the present invention. A cylinder machine, as is known in the art, is employed with at least three different cylinders. A stock composition comprised substantially of cellulosic pulp fiber is fed to the cylinders corresponding to the outer layers, and a stock solution comprised of the cellulosic pulp fiber, high temperature fiber and a polymeric binder is fed to the other cylinder which corresponds to the inner layer of the paper structure.
The resulting paper structure provides a paper quite useful as E-board in transformers due to its enhanced thermal resistance. Moreover, the high temperature fiber also helps to reinforce the paper to avoid the problems in telescoping coils. The process used to prepare the paper also permits one to efficiently and effectively prepare the paper structure while avoiding problems with sticking due to the presence of the polyvinyl alcohol binder.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The paper structure of the present invention is quite useful as a high temperature E-board. The paper exhibits enhanced thermal resistance as well as good mechanical strength. The good thermal resistance would allow the use of the paper in a transformer with coils of smaller size as it would allow the coils to run hotter. Furthermore, the paper is also reinforced so that when glued to the coils, it would keep the various coils from telescoping. In actual use in a transformer, the paper is coated with an adhesive, such as an epoxy adhesive, and heat bonded to the wire coil surface. It is this bonding to the coils that permits the board to keep the various coils from telescoping. The mechanical strength of the paper structure of the present invention, including its high temperature reinforced fiber in combination with the polymeric binder, permits the E-board to act efficiently and effectively with regard to preventing the coils from telescoping.
The paper structure of the present invention generally comprises two outside layers, and at least one inside layer. The two outside layers are comprised of substantially wood pulp fiber, but can contain a minor amount of synthetic fiber. Such synthetic fiber can be, for example, polyester or nylon fiber. By minor amount, is meant less than 50 wt % fiber, and preferably less than 10 wt percent fiber. It is most preferred that the cellulosic (wood) pulp fiber used is a Kraft fiber.
The inside layer generally comprises cellulosic (wood) pulp fiber, high temperature fiber and a polymeric binder. The high temperature fiber is generally a fiber that has a T
g
of at least 400° C., and most preferably higher than 550° C. Preferred high temperature fibers include the polyaramide fibers available commercially, such as NOMEX®. Generally, the fiber is about ¼ inch in length, and has about a 2 denier.
The preferred polymeric binder is polyvinyl alcohol. It can be added in the form of a synthetic fiber or as a dry powder. If the binder is added as a fiber, it is important that the fiber has the proper chemical characteristics. Polyvinyl alcohol fiber is available with a wide range of water solublization temperatures. The temperature at which the polymer becomes soluble depends on the properties of the polymer like the degree of polymerization, degree of hydrolysis, and crystallinity. This solublization temperature can range from about 60° C. to over 100° C. It is important to match this solublization temperature to the paper making process. To be most effective the polyvinyl alcohol fiber should behave as a binder while it is in the fiber form. It should not be allowed to fully dissolve. The strongest binding occurs when the surface of the fiber just starts to dissolve. Then upon drying, the polyvinyl alcohol fiber will bond to all of the other fibers, both synthetic and natural, that it contacts.
This means that a polyvinyl alcohol fiber with a low solublization temperature should be used with a low to medium basis weight paper (roughly 25 to 120 pounds per 3000 square feet) that is typically run at high machine speeds. Because of the higher machine speed and low sheet mass, evaporation will cool the paper. It will dry before it gets very hot. The maximum temperature that the paper will reach is likely to be less than 70° C.
With high basis weight papers (200 pounds per 3000 square feet and above) a polyvinyl alcohol fiber with a higher solublization temperature can be used. These papers are typically run at slower machine speeds so that the sheet temperature is much higher.
When the powder form of the polyvinyl alcohol binder is used, the polymer should be fully hydrolyzed (99% or higher) and the polymer should be ground to a particle size of 100 mesh or smaller. The powder can be added to the wood f
Chin Peter
FiberMark, Inc.
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