Coating processes – Spray coating utilizing flame or plasma heat – Metal oxide containing coating
Reexamination Certificate
2001-03-12
2002-10-08
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Metal oxide containing coating
C427S453000, C427S331000, C427S355000
Reexamination Certificate
active
06461687
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a roll for use in the manufacture of paper and board, in particular a press roll, a center roll in a press, a backup roll for an extended-nip press roll, a hot press roll, or an equivalent press roll which is in direct contact with a wet paper web, or a calender roll.
The present invention also relates to a method for manufacturing a roll for a paper or board machine or calender.
The present invention further relates to a coating composition for a roll for a paper or board machine or calender.
BACKGROUND OF THE INVENTION
Press rolls and calender rolls are critical components in a paper machine both from the point of view of the runnability of the machine and from the point of view of the quality of the product, because in a press and in a calender, the roll face is often in direct contact with the paper web. Direct contact with the face of a roll has a significant effect on the surface properties of paper, which imposes high requirements on the quality of the face of the roll. The surface properties of the roll are also critical from the point of view of the paper-making process. The wet or coated paper web must adhere to the roll in a suitable way, however, on the other hand, it must also be readily separable from the roll. It must be readily possible to doctor the roll, easy to keep it clean, and the roll must remain in good operating condition for a long period of time. Further, from the point of view of runnability, it is essential that the surface properties of the roll do not change during the process so that, for example, the separability of the web from the roll face cannot be suitably controlled. Owing to the direct contact of the roll face with the web, fibers and contaminations adhere to the roll face and block the surface layer and, consequently, the surface properties of the roll are changed. In order to amend the effect of contamination, the roll is doctored, which again imposes its requirements on the mechanical surface properties of the roll, i.e., the roll face must adequately withstand the effects of doctoring.
In a press, particularly demanding surface properties are required from center rolls in presses. At center rolls, the web is pressed against the roll face in two or more nips. Also, demanding surface properties are required from backup rolls of extended-nip presses in which the contact face between the web and the roll is longer and the nip load higher. In these cases, the tendency of sticking of the web and contamination of the roll in web contact are particularly intensive. In such positions, in particular in wide paper machines, variable-crown rolls are used. The roll face is further subjected to particularly demanding conditions when the pressing takes place at an elevated temperature either by heating the web before the center roll (e.g., by means of a steam box) or by means of the roll when the web is on the roll face (heatable center roll or so-called impulse drying).
The granite roll, which has-been traditionally used in the press section of a paper machine, has been abandoned in recent years in spite of the excellent surface properties of granite. The reason has been mainly the requirements of strength and durability increased along with higher running speeds as well as the need to profile and to heat the roll. Synthetic rock rolls, which have been coated with a mixture of ceramic powder added to a hard rubber and polyurethane or some other polymer, have performed poorly because of their low mechanical strength and excessive adhesion of the paper web to the roll face.
By means of calender rolls, the surface of paper or board can be given the desired gloss, smoothness, and finished face. Traditionally, calendering has been used mainly for compacting the surface of paper, which has taken place by means of rolls which are as smooth as possible. For this purpose, chilled rolls and hard-chromium plated rolls have been commonly used. Electrolytic hard-chromium plating has been highly laborious as a process for rolls of large size, and with respect to the quality of the face to be chromium-plated, it has been highly demanding. From the point of view of the plating process, the high susceptibility of wear of the hard-chromium plated face has constituted a problem, which wear has been increased further by the micro-particles contained in the coating material, such as clay. Owing to the wear of the plated face, the capability of holding/adhesion of the face is deteriorated, and doctoring becomes more difficult and causes streaks in the web. The production of dull-finish grades by means of the prior art rolls has not been suitable because of the rapid smoothing of the face.
Owing to the circumstances mentioned above, alternative solutions for coatings of rolls have been developed both for press rolls and for calender rolls. Most commonly, at present, rolls with metal frames are used which have been coated with a metal, a ceramic, ceramic-metal, polymers or elastomers and various mixtures of the same.
Ceramic and ceramic-metal coatings and coating processes have been described, for example, in the following publications.
In Finnish Patent Application No. 853544, a roll coating is described which consists of a metal or a mixture of a metal and a ceramic material.
In Finnish Patent No. 70,273, mixtures of metal powder and inorganic material are suggested as a coating for a press roll.
On the other hand, in Finnish Patent Application No. 861803, a construction of a press roll is described, in which a metallic adhesion layer has been applied onto the face of a metal frame, which adhesion layer has a thermal expansion coefficient lower than that of the metal frame, and onto the adhesion layer, a ceramic surface layer has been applied.
In Finnish Patent No. 84,506, a press roll is described whose metallic frame cylinder is coated with an intermediate layer consisting of a composite compound made of a ceramic material and a metal, and then with a ceramic surface layer. The mixing ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll.
In Finnish Patent No. 86,566, a center roll for a press is described, which has been manufactured so that the mantle of the roll frame has been coated with a corrosion-protection layer which consists of chromium-containing stainless steel as well as of a surface layer which has been prepared by thermal spraying of a powder in which the metal and ceramic phases are contained in the same powder particle.
In International Patent Application WO 93/01326, a composition of a coating for a press roll is described, which comprises a mixture of aluminum silicate and alkaline-earth metal oxide that has been plasma-sprayed onto the face of the roll.
In European Patent Application 0 657 237, thermal spraying of cobalt-containing tungsten carbide granules and nickel-containing chromium carbide granules onto the roll face is described.
In Finnish Patent No. 89,950, a press roll for a paper machine is described, in which the mantle of the metal core of the roll has been coated with a metal layer that contains molybdenum-based and nickel-based metal alloys. Onto the metal layer, a ceramic coating is prepared by means of thermal spraying. Suitable ceramic compounds are grey aluminum oxide (95% A1203-2.5% TiO
2
), white aluminum oxide (99% Al
2
O
3
), titanium dioxide (TiO
2
), etc. and mixtures of same.
In European Patent Application 0 481 321, a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by plasma spraying, and onto this layer a ceramic layer has been applied by plasma spraying, which layer consists of metal oxides or of mixtures of same. Finally, the roll is coated with an organic polymer to fill the pores in the ceramic coating.
International Patent Application WO 96/41918 is an example of a hot press roll having a coating prepared by means of thermal spraying of a metal-ceramic and a
Kuosa Harri
Lehtonen Pentti
Majava Jorma
Niemi Kari
Tuomela Jussi
Bareford Katherine A.
Metso Paper Inc.
Steinberg & Raskin, P.C.
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