US 3087231 A
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United States Patent 01 Company, New York, N.Y., a corporation of New Jersey No Drawing. Filed Nov. 8, 1960, Ser. No. 67,929v
2 Claims. (Cl. 29-432) This invention relates to improved press rolls for use in paper making machines. More particularly, it relates to press rolls comprising a cylindrical core covered with a layer of polyurethane rubber, i.e. compositions of diisocyanate modified polyesters or polyethers, which rolls will not pick up fibers from the web of newly formed paper.
The purpose of the pressure rolls in the wet section of a paper forming machine is to remove water from the web of pulp fibers. This is done by passing the wet web of pulp fibers, carried on a felt blanket, through a plurality of squeeze rolls. These rolls are arranged in pairs to form pressure nips which squeeze the water from the web of pulp. The felt blanket protects one side of the web, but the other side, usually the top, comes in contact with the surface of the various rolls. If the composition used for manufacturing these rolls has the property of attracting pulp fibers from the web, some fibers will be pulled out causing a rough sheet as well as machine operating problems.
Various materials have been used for manufacturing these rolls. Metals such as brass were found unsatisfactory because the pulp fibers adhered to the surface. Wooden rolls have been used but are not uniform in composition and are difficult to maintain. Granite rolls have been standard for many years. The surface has little attraction for pulp fibers and will not wear excessively under the scraping action of a doctor blade. Granite rolls are very expensive, however, and due to their surfaces hardness it is difficult to maintain an even nip across the width of the web. This will result in uneven moisture removal.
Rolls covered with various compositions of matter have come into use over the years. These usually consist of a hard, elastomeric matrix loaded with crushed sand, quartz, granite, rock salt, etc., of various particle sizes. These rolls have served reasonably well but have the disadvantage that certain pulp fibers will adhere to the surface and Water removal across the web is not uniform.
Soft elastomeric coverings are also in use. Some of these are also loaded with a coarse agglomerate such as crushed sand, quartz, granite, rock salt, etc. They are successful for some types of paper manufacture but certain fibers adhere to the surface and render the rolls unsatisfactory. In addition they have rather poor wear resistance and become uneven across the face. These rolls are unsatisfactory when used in conjunction with doctor blades because the distortion of such rolls after wear enables the doctor blades to turn under or dig into the covering and break.
It is an object of our invention to provide improved press rolls for paper making machines which will not pick up fiber from the paper web.
It is a further object of our invention to provide a first top press roll for a Fourdrinier paper machine with exceptional fiber release properties and good wear resistance.
Other objects will be apparent to those skilled in the art of paper making from the description hereinafter set forth.
The press rolls of this invention combines the good 3,087,231 Patented Apr. so, 19 3 "ice 2,- qualities of the various known types of press rolls without any of their disadvantages. Surface properties are such that pulp fibers will not adhere as determined by tests conducted under actual paper making conditions.
The following table shows a comparison between the fiber pickup (given in grams of dry fiber) of a polyurethane covered roll and a granite roll.
Machine Operating Conditions Fiber Pickup Press Load, Polyure- Speed, ft. per min. lbs. per thane Granite linear inch In this test, the experimental paper machi e was run under four conditions of speed and press loading. The fiber pickup was measured as grams of fiber per one thousand square feet of paper surface. In each case the machine was run for the proper period of time, it was then stopped and the fiber accumulation scraped off h su f e The p r d of ime f r each run is n t cri ical and can be adjusted to suit conditions at the time. Usually, enough fibers will be picked up in five minutes to illustrate the difference between a satisfactory roll and an unsatisfactory roll. Thus, we used a five minute interval in these tests. In the case of the polyurethane roll, no fiber is accumulated on the surface.
The conditions used in the test are typical of a very small or experimental paper machine. The large commercial machines run very much faster, However, past experience indicates that a test made on the experimental machine can be used to predict performance in the larger paper machines.
The polyurethane covering used in these experiments was f the llowin mp s o Adiprene T V 100 oon 11 Adiprene L is a urethane Polyme mark t d by E- du Pont d e. Nemour 8: Co. t is d y reac tolylene diisocyana e with pplyiem mfilh km l q lecular weight 900 to 1,000 in the molecular ratio of 1.5 mols of tolylene diisocyanate to 1 mol of polytetramethylene glycol. MOCA is also marketed by E. I. du Pont de Nemours & Co., and chemically is 4,4'-mcthylene-bis- (2-chloroaniline) The covering was cast on a metal core and the assembly heated for four hours at 300 F. to produce a cured rubber-like composition. This temperature can be varied over a wide range as is well known to those familiar with curing polyurethanes. As a matter of fact these polyurethane compositions will eventually cure at room temperature without any heat or pressure. The conditions actually used are a matter of choice and convenience. We used an open mold to make the polyurethane coated rolls, however, closed molds may be used as desired.
The hardness of the cured polyurethane roll covering was 20 on the Pusey and Jones hardness scale. Pusey and Jones hardness values are standard in the rubber industry, particularly for measuring the hardness of rubber covered rolls. The use of the Pusey and Jones hardness tester is described in ASTM D-53 149. The hardness value for polyurethane press rolls varies over a rather Wide range depending on type of paper being produced and personal preference of the individuals operating the paper mill. The range suitable for our press roll covering goes from 0 to about 100 P and I; however, values in the 5 to 30 range are generally preferred.
It is a known fact that the abrasion resistance of polyurethane compositions is much superior to other types of elastomeric compounds used for roll coverings. Rolls covered with polyurethane compositions will wear much slower and therefore maintain a proper crown much longer than other types of coverings.
The absence of fiber pickup on the polyurethane surface should make doctoring unnecessary but, if for op erating reasons on the paper machine doctoring should be desired, the blade will cause little wear on the surface.
Polyurethane compositions are formed by reacting a base polymer with a curing agent. The base polymer may be in a liquid or a rubber-like solid form. Liquid base polymers may be processed by casting, dipping, spreading or spraying. Norm-ally, rolls of this type are made by casting in a mold. However, if only a thin coating is desired, it could be applied by the usual spray techniques. Also, if desired, the metal core could be covered by dipping it in a polyurethane composition and then curing it in air. By putting a doctor blade against the roll and rotating the roll, it could be covered by a spreading technique. The solid base polymers may be processed on conventional rubber machinery.
Commercial base polymers of both the liquid and solid type are available from several manufacturers. Typical are the Adiprene series marketed by the Du Pont Company, Vibrathane marketed by the Naugat-uck Chemical Division of the United States Rubber Company and Genthane marketed by the General Tire and Rubber Company.
These base polymers can be cured to rubber-like compositions by reaction of the isocyanate group with polyamine or polyol compounds. Certain other compounds such as titanate esters also cure the base polymers. The type of curing agent used determines the curing time and has a major efiect on cured properties of the polyurethane.
Typical polyurethane compositions which might be used for top press roll coverings are:
(1) Liquid polyether system Parts by weight Polyether-and-diisocyanate reaction product 100 4,4 -methylene-'bis- (2-chloroaniline 1 l (2) Solid (rubber-like) polyether system- Unsaturated polyether-and-diisocyanate reacfion product 100 High abrasion furnace type carbon black.. 50
Coumarone-indene resin Benzothiazyl disulfide 4 2 mercaptobenzothiazole 1 (2) Solid, etc.Con. Parts by weight Sulfur 0.75 Zinc chloride-benzothiazyl disulfide activator" 0.35
Coumarone-indene resin 5 Stearic acid 0.2 High abrasion furnace type carbon black Adiprene L.
4 Vibrathane A-509 or Genthane-3.
The preparation of the various diisocyanate reaction products or prepolymers referred to in the above examples will not be described here in detail because they have been described elsewhere in the patent and technical literature, particularly in U.S. Patent 2,927,905 to Eckert, issued March 8, 1960, and copending application of R. N. Kienle, Serial No. 13,136, filed March 7, 1960'.
Having thus described our invention, what we claim and desire to protect by Letters Patent is:
1. In a paper making machine having a top press roll which squeezes water out of a wet web of pulp fibers, the improvement comprising as said top press roll, a cylindrical core covered with a layer of polyurethane rubber selected from the group consisting of polyesterdiisocyanate reaction product and polyether-diisocyanate reaction product having a Pusey and Jones hardness value of less than 10 0, said layer being free from undesirable tendency to pick up said wet pulp fibers.
2. A paper making machine having a top press roll which squeezes water out of a wet web of pulp fibers, said top press roll comprising a cylindrical metallic core covered with a layer of polyurethane rubber which is a polyether-diisocyanate reaction product having a Pusey and Jones hardness of from 5 to 30, said layer being non-adherent to said wet pulp fibers.
References Cited in the file of this patent FOREIGN PATENTS 749,541 Great Britain May 30, 1956