ROLL COVERING

Description

Field of the invention

The present invention is directed to a roll covering which is used in the field of paper or board manufacturing and processing, and especially to a calender roll comprising said roll covering.

Background of the invention

Cylindrical rolls are utilized in a number of industrial applications, especially those relating to papermaking. Such rolls are typically employed in demanding environments, in which they are exposed to high dynamic loads and temperatures. Moreover, the surface of the roll may be required to conduct webs which may be damaged by the roll if the surface is not sufficiently free from abrasiveness or other damaging characteristics.

As the replacement of an entire metallic roll can be quite expensive, many papermaking rolls include a polymer cover that surrounds the circumferential surface of a metallic roll body. By varying the polymer employed as a cover, the designer can provide the roll with different performance characteristics as the papermaking application demands. The replacement of a cover of a metallic roll is much less expensive than the replacement of an entire metallic roll. The polymers used in the cover can be thermosets or thermoplastics .

In order to further improve hardness, wear resistance, surface quality, impact toughness, sufficient thermal

resistance and low heat generation tendency of the polymer roll cover, roll coverings derived from a non-woven fiber material have been proposed. Cover manufacturing on the basis of non-woven technique is workable and cost- effective. Today, a majority of polymer covers are manufactured by winding techniques, in which a non-woven fiber strip of carbon or glass is used as a carrier substrate for a matrix material. These fibers, although exhibiting several good properties, tend to make the surface heterogenous and anisotropic, and reduce both micro and macro homogeneity of the surface. This unhomogeneity may adversely affect the calendering results, but above all it reduces the life time of the cover due to an increased risk of disintegration or damage of the cover.

U.S. 3,707,752 describes a roll covering having an outer cover comprising a composite material formed from a needled nonwoven mat selected from acrylic fibers, polyester fibers and mixtures thereof. The mat is bonded with a thermosetting resin of the epoxy type. The described roll covering serves to achieve high operating speeds and high loading conditions without adverse effects on the covering from the environmental conditions. It is disclosed that the modulus of elasticity of the fibers to that of the resin is between 10:1 to 1:10.

However, there is an ongoing demand for a further improvement of the roll coverings known in the art, especially with respect to the surface quality and durability of the covering.

Summary of the invention

In view of the foregoing circumstances, it is an object of the present invention to provide a roll covering having an

improved life time and an improved surface quality which would not adversely affect the object with which it is in contact .

Another object of the invention is to provide a roll covering which is suitable for use in the papermaking process .

The present invention provides a roll covering comprising a nonwoven fabric and a binder resin, wherein the ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of said binder resin is within a range of 2:1 to 1:2. Preferably, the ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of said binder resin is within a range of 3:2 to 2:3. Particularly preferred is a ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of said binder resin within a range of 5:4 to 4:5.

With respect to the nonwoven-fabric of the roll covering, it is preferable that it is a resin having a glass transition temperature of at least 100 ⁰ C, preferably at least 115°C.

In a preferred embodiment, the binder resin of the roll covering is an epoxy resin.

More specifically, it is preferred that the fiber material used for the nonwoven fabric is a synthetic polymer fiber which is selected from the group consisting of epoxy resin fibers, polyether sulfone resin fibers, polycarbonate resin fibers, polyphenylene sulfone resin fibers, polyetheretherketone resin fibers, and mixtures thereof.

The present invention further provides a calender roll comprising the roll covering as defined above.

Detailed description of the invention

The roll covering of the present invention comprises a nonwoven fabric and a binder resin, wherein the ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of said binder resin is within a range of 2:1 to 1:2. By setting the ratio of the modulus of elasticity of the fiber material used for the nonwoven fabric to that of the binder resin within such a range, the surface quality of the roll covering is rendered uniform and homogenous. When a calender roll comprising the roll covering having the afore-mentioned features is used in a papermaking process, the calender results and the durability of the cover are improved. Especially, marks imparted to a treated material, for example paper or board web, can be prevented and smoothness and gloss of the paper are improved.

Further preferred features, embodiments and advantages of the present invention are discussed below.

The roll covering according to the present invention comprises a nonwoven fabric and a binder resin, wherein the ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of said binder resin is within a range of 2:1 to 1:2.

Preferably, the ratio of the modulus of elasticity of the fiber material used for said nonwoven fabric to the modulus of elasticity of the binder resin is set within a range of 3:2 to 2:3. Thereby, the surface homogeneity of the roll

covering is further enhanced, so that paper having excellent smoothness and gloss can be obtained and the life time of the roll is increased. These effects become even more prominent, if the ratio of the modulus of elasticity of the fiber material to the modulus of elasticity of the binder resin is within 5:4 to 4:5.

In the present invention, it is preferred that the nonwoven fabric fiber and the matrix material, respectively, have a modulus of elasticity within a range of 1000 to 10000 MPa, preferably within a range of 2000 to 6000 MPa.

The nonwoven fabric is selected from fiber material from the group of high temperature resistant polymers having an elastic modulus of the same level as the binder resin surrounding it and being compatible with the binder resin.

Due to elevated temperatures and high load requirements in calenders, the nonwoven fabric is advantageously made of fibers of a resin having a glass transition temperature (Tg) of at least 100 ⁰ C, preferably at least 115°C. More preferably, the glass transition temperature is within a range of 115 to 200 ⁰ C. In case of using epoxy binders, the glass transition temperature of the resin fiber used for the nonwoven fabric preferably is of the same level as the glass transition temperature of the binder material.

The glass transition temperature of the fiber resin advantageously is not less than that of the binder resin.

The nonwoven fabric may be a mat of nonwoven fibers. The method of manufacturing fibers into a needled nonwoven mat is well known in the art and many needled nonwoven mats are commercially available.

Most often, the fabric is used in a form of a continuous nonwoven fibre strip which is impregnated by the resin and winded around the roll core. Alternatively, the impregnation can be made after winding.

The amount of nonwoven-fabric in the finally cured roll covering ranges from 10 to 40 percent by weight. More preferably, the amount of fabric is within a range of from 15 to 30 percent by weight.

In the present invention, the binder resin advantageously is a resin having a glass transition temperature within a range of 115 to 170 ⁰ C. Preferably, the binder resin is an epoxy type resin. Epoxy type resins are commercially available. Conventionally, epoxy type resins are prepared from an epoxide which contains epoxy groups that are cured with either diamines or dibasic anhydrides. Epoxides are prepared in a number of ways, such as by the reaction of phenol and acetone to produce Bisphenol-A, which in turn is reacted with epichlorohydrin to yield the epoxy intermediate. Polyamines or other conventional curing agents are then reacted with the epoxy intermediates to produce the final resin. It is preferred that the epoxy type resin contains an epoxide equivalent of at least 100 and it is more preferred that the epoxide equivalent weight ranges from 175 to 200.

It is preferred that the amount of binder resin in the finally cured roll covering ranges from 60 to 90 percent by weight. More preferably, the amount of binder resin is within a range of from 70 to 85 percent by weight.

Methods for manufacturing the roll covering of the present invention are well known in the art. Basically, roll coverings containing a nonwoven fabric and a binder resin

are prepared in the following manner. A strip of a nonwoven mat is impregnated with a binder resin and is wound around a roll support. In many instances, an excess of resin is applied as much as needed, which excess is later drained from the roll prior to curing of the resin. The tension under which the roll is being wound acts to squeeze excess resin from the saturated mat, so as to leave a mat containing from 10 to 40 percent by weight of fibers and 90 to 60 percent by weight resin. Preferred composite materials contain from 15 percent to 30 percent by weight fibers and from 85 percent to 70 percent by weight of said resin .

Once the resin has been cured, machining, grooving, grinding and other modifications of the roll may be performed to prepare the roll for use in a wide variety of applications. Many uses in the paper industry have been found for rolls having the roll covering of the present invention .

Suitable roll supports for preparing the calender rolls of the present invention may be made of metals such as cast iron and chilled iron, steel or fiber reinforced plastic. In consideration of a small inertia mass of the roll as well as low material costs, it is preferable that the roll support is designed in form of a hollow body. Additionally, one or more intermediate layers may be present between the roll support and the roll covering. In case of a metal support, the preferred embodiment of such an intermediate layer is a steel layer, preferably made of stainless steel having a chromium content, specifically a chromium content in the range of 5 to 45 % and more preferably 10 to 30%. Such an intermediate layer protects the roll support from corrosion and can enhance the bonding strength of the outermost surface layer to the roll. In case of a fiber

reinforced plastic support, the intermediate layer could be an adhesive, preferably of the epoxy-type.

Roll covers of the invention have a good combination of fibre reinforcement and binder material which are compatible in terms of elasticity and thermal properties. Due to this optimal compatibility they show better performance properties, as shown in the following examples.

Examples

Test rollers simulating calender rolls were manufactured by wrapping the respective resin-impregnated nonwovens shown in the following Table 1 over a steel roll body (diameter: 200 mm; length: 200 mm) until the functional layer thickness reached the desired level and cured at about 160°C. Typical thicknesses were between 10 and 15 mm, but also thicknesses up to 25 mm have been used. The covered rolls were tested in a nip simulator comprising two rollers in nip contact, maximum line load of the nip simulator being 400-600 kN/m. One of the rollers was a hard faced metal roller and the other was covered by a cover according to the invention. The temperature of the both rollers could be controlled. The rollers were rotated with a rotation speed of 800 - 1400 rpm. The temperature of the cover was monitored during the test. Durability of the covers was determined by running the simulator with different line loads until visible markings of disintegration or inhomogeneity on surface of the cover as seen by naked eye occurred or, more often, presence of a hot spot (local overheating) was measured which is known to indicate a forthcoming cover damage in unpredictably short period of time, even very rapidly if the load is increased. The results are summarized in Table 1. In Table 1, character "A" indicates an optimal durability and homogene surface

appearance of the cover. Characters "B", "C" indicate some lowering in durability or integration of the cover in this order. Character "D" denotes that the calendering result was not at an acceptable level.

Table 1

1) Modulus of elasticity was determined by dynamic mechanical testing according to ASTM D4065 using a DMA test equipment with 10 Hz frequency, and as a testing mode 3-point bending, single cantilever.

By the above-described embodiments of the present invention, the above-mentioned object is properly solved and the present invention provides a roll covering having an improved life time and an improved surface quality which

would not adversely affect the object with which it is in contact .