BACKGROUND OF THE INVENTION The process of manufacturing paper consists of squeezing the water out of a solution, usually called"pulp,"composed of approximately 98% water and 2% fibers (from trees or recycled paper).

The principle of a paper machine lies in the headbox, whose function is to distribute the pulp uniformly over the Foudrinier wire. On this wire, between 30% and 35 % of the water is eliminated due to the effect of gravity.

Thereupon, the paper moves on to the press where, by means of the pressure exerted by a hard cylinder against another soft rubber cylinder, the water is transferred to an absorbent cloth. This hard cylinder is usually made of granite, ceramic or synthetic rubber. Subsequently, the cloth is dried by the action of suction boxes.

At the outlet of the press, the paper still has a water content of between 40% and 50%. Then, the paper moves through the drying zone where, thanks to several heated cylinders, the water evaporates the paper is left 98% dry paper.

Historically, the hard cylinder was made out of granite, since granite is a hard material and with good non-stickiness for the previously used technology.

However, with the changes in the paper machines toward higher production speeds, it has become necessary to seek materials to replace granite, to provide greater drying efficiency and, at the same time, a better detachment of the paper sheet. Currently, different alternative materials are used, such as, for example, synthetic ebonites, coatings of hard rubber, polyurethane or ceramics.

As has been mentioned previously, the function of the pressing cylinders is to squeeze the paper to eliminate the water present therein. Accordingly, the hard cylinder must be resistant to wear and nonstick to keep the sheet of paper from sticking to the cylinder.

There are primarily three variations associated with the mechanism by means of which the paper is detached from the cylinder: a) Thickness of the liquid film between the sheet and press. The thinner the film, the harder it will be to detach the sheet from the cylinder; b) Number and size of the pores on the surface of the cylinder. The surface pores can originate condensation (increased liquid in the pores and, accordingly, a greater thickness of the liquid film) and formation of air bubbles between the cylinder and the paper. These phenomena can facilitate the separation of the paper. If the surface of the cylinder is completely flat, the paper sticks more; c) Surface energy. The film interposed between the paper and the cylinder is affected directly by the surface energy of the coating. The higher the surface energy, the less the amount of accumulated liquid and, therefore, the film will be of thinner.

In summary, it could be stated that the hard cylinder has the following properties: -Resistance to high temperatures; -Resistance to wear; -Surface roughness and even distribution of the surface micropores; -Low surface energy; -Surface moistening; -Sufficient non-stickiness to work with all kinds of pulp, including recycled ones, which are rich in sticky substances.

To replace the granite cylinders heretofore used, in 1933 the Stowe Woodward Company introduced on the market the Stonite coating. This coating contains many of the mineral elements present in granite. However, certain temperature and wear problems gave rise to the appearance in 1950 of a new generation of rubber compounds, beginning with the Microrok coating. This product is nonstick and is characterized by minimum wear and a high coefficient of elasticity at ambient temperature. Subsequently, in 1985, Dynarok coatings began to be marketed for the purpose of creating a coating capable of resisting the increase of the operating temperature caused by the introduction of steam-distribution chambers and a progressive reduction of the water in the manufacturing process.

In 1989, there began to be marketed a new range of thermostable compounds, called Centerok, developed to withstand very high temperatures of around 110°C and provide, at the same time, a resistance to wear greater than that achieved with the previous rubber compounds or with granite cylinders. In 1991, Stowe Woodward launched on the market a new generation of Centerok with an even greater resistance to wear.

Despite the need to use an adequate coating for several years and the appearance of the aforementioned coatings, current presses still present different disadvantages, primarily because of the accumulation of fibers in the micropores, among which the following may be highlighted: -The quality of the paper diminishes as the press becomes less nonstick.

Since it is harder for the sheet to become detached from the cylinder, an abrupt tug occurs when the sheet is detached. Therefore, the side of the paper that is in contact with the press is less smooth (i. e., of poorer quality). This effect is known as peeling and begins to appear several months before it becomes necessary to send the press to be ground.

-The speed difference between both sets of continuous presses is difficult to control, which may cause the paper to rip. The paper sticks at the outlet of the press of the first position and the next cylinder yanks the paper, creating a tug that can tear the paper sheet. If the sheet tears, the machine stops manufacturing paper until the sheet again travels through the entire machine. The process of the sheet traveling through the machine per se is not that long (from 15 to 30 minutes), but if the sheet breaks several times a day, productivity will be affected significantly.

-It is necessary to stop periodically to clean the cylinder, which means a reduction in production time, which results in a drop in productivity.

-The cleaning process may be long and difficult, since to clean off the sticky material a pressurized water jet must be used or else the surface must be scraped with solvents. This makes it necessary to remove the cylinder from the machine, or else assume that the machine will be stopped for a longer period of time if the cleaning is done without removing it. Even so, not all of the fibers are completely eliminated.

-The doctor blades, which are always in contact with the hard cylinder to remove part of the fibers, must be replaced frequently, since they suffer the wear of abrasion against the press.

-The press must be ground, which increases the maintenance costs.

-The coating of the press is subjected to premature wear as a result of the different grindings.

Furthermore, there is another type of cylinder, called embossing cylinders, which are housed in embossing machines to manipulate paper, whose function is to form a relief design on the paper. In this case, the embossing cylinder, which is the one that has the design pattern engraved, is currently metal, while the other press cylinder is made of soft rubber. By means of the pressure applied by one cylinder against the other, the design is stamped on the paper.

As the paper runs through the cylinders, the paper fibers become detached because of the effect of the friction with the cylinders. The pressure exerted by the cylinders causes the paper fibers to accumulate in the hollows of the cylinder engraving. Little by little, the relief on the cylinders disappears, so that the embossing is of poor quality. Accordingly, it is necessary to stop the embossing machine frequently to take the fibers out of the cylinders, which results in a loss of productivity.

DESCRIPTION OF THE INVENTION With the pressing device of the invention, resolving the aforementioned disadvantages is achieved, along with other advantages, which will be described.

The device for pressing laminated products of the invention, which comprises a pair of cylinders between which the laminated product passes, with said cylinders applying pressure on said product, with at least one of said cylinders comprising a nonstick coating, is characterized by the fact that said nonstick coating contains a polymerized fluoropolymer.

Advantageously, the device of the invention, which comprises a blade, called a doctor blade, in contact with one of the cylinders, which is used to remove the material that becomes incrusted in this cylinder, is also characterized by the fact that this blade also comprises a nonstick coating which contains a polymerized fluoropolymer.

When the pressing cylinders and/or doctor blades are coated with a fluoropolymer-based coating, the coated surface adopts the characteristics typical of fluoropolymers: high surface energy (maximum non-stickiness), extremely low friction coefficient and resistance to corrosion and chemical attack.

Therefore, the operating principle of the fluoropolymer coatings is completely different and opposite the traditional theories about the separation of the paper. Currently, the cylinders are manufactured so that they have the roughness and the pores evenly distributed using materials with a low surface energy to obtain the film effect of liquid. The coatings currently used tend to imitate the properties of granite, but gaining in non-stickiness and resistance to abrasion and thermal stability.

With the fluoropolymer coatings, the contrary effect is sought: the surface has a very high surface energy to the point that it repels water and keeps the cylinder completely dry. The paper does not become detached by the effect of the liquid film, but by the high surface energy. This high surface energy causes the angle of contact to be much greater for the fluoropolymers, which prevents any type of material from sticking, whether water, additives or fibers.

What is more, fluoropolymers can withstand temperatures between 150°C and 290°C, depending on the product used. As a result of this, fluoropolymers are thermally more stable than the coatings used today and, at the paper machine's working temperatures, neither do they suffer an appreciable alteration in hardness or variation in the coefficient of elasticity.

In the light of all of this, the advantages obtained are the following: -The coated surface is self-cleaning, i. e., it is so nonstick that the fibers and additives practically do not stick. As a result of this, the frequency of shutdowns for cleaning the press is significantly drawn out.

-If fibers accumulate on the surface of the cylinder by the effect of its pressure, they can be easily eliminated with a simple jet of water so that the cleaning time drops very significantly.

-The output of the sheet from the presses is more constant and the angle of contact is also less; therefore, the paper is manufactured with greater quality.

-Grinding the cylinder with so much frequency is not necessary.

-The coating remains nonstick for a longer period of time.

-The coating is much easier to repair.

In summary, it could be stated that the fluoropolymer coating offers important advantages due to the reduction in manufacturing and maintenance costs of the machine, the considerably increased production of paper also and, finally, the greater paper quality.

Preferably, this fluoropolymer is chosen from among perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethyelene (ETFE), ethylene chlorotetrafluoro-ethylene (ETCFE), modified fluoroalkoxy (MFA) and fluorinated ethylene propylene (FEP).

Also, preferably, the nonstick coating, which contains a fluoropolymer, is applied to a cylinder with a greater hardness.

According to one embodiment of the invention, the cylinder provided with the nonstick coating, which contains a fluoropolymer, presents several relief designs.

Preferably, the thickness of this nonstick coating is between 10 um and 2 Also, preferably, this laminated product is paper.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of everything that has been said, several drawings are attached in which, schematically, and only by way of a non-limiting example, a practical embodiment is represented.

The only figure is a schematic elevated view of a machine for the manufacture of paper, which includes the pressing device of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT AND THE TESTS PERFORMED Just as can be appreciated in the sole figure, a conventional machine for the manufacture of paper comprises a headbox 1, a Foudrinier wire 2, several presses 3--three in the case of the embodiment represented--and a drying zone 4.

Each one of the presses 3 is made up of a hard cylinder 5 and a soft cylinder 6, between which the paper travels. The soft cylinder 6 presents in connection a cloth 7 to absorb the water and a water-suction box 8. For its part, the hard cylinder 5 presents in connection a doctor blade 9, which removes the fiber present in the cylinder 5.

According to the invention, the hard cylinder 5 comprises a coating that contains a fluoropolymer with this fluoropolymer being chosen from among perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethyelene (ETFE), ethylene chlorotetrafluoroethylene (ETCFE), modified fluoroalkoxy (MFA) and fluorinated ethylene propylene (FEP).

Preferably, the doctor blades 9 are also coated with a coating that contains one of the aforementioned fluoropolymers.

The operation of the paper machine has already been described and is also known by a technician in the field.

In the machines for embossing toilet paper (not represented), a press composed of an embossing cylinder equipped with a relief design and a soft-rubber cylinder is also used. In this case, the embossing cylinder is also equipped with a nonstick coating that contains one of the fluoropolymers referred to above.

To show that the coatings with fluoropolymers are less sticky that the coatings currently used, the angle of contact of a drop of water has been measured with a surface of the different materials of the coatings currently used and the coatings of the invention, according to the attached table.

It should be stated that, when the angle of contact is less than 90°, the surface is considered to be moistened, while, when the angle of contact is greater than 90°, the surface is considered not to be moistened. This angle of contact depends on the surface energy, the temperature, the relative humidity and the surface roughness of the coating. The lower the surface energy and the temperature and the greater the relative humidity and the roughness of the surface, the thicker the film of liquid between the paper and the cylinder. Coating Angle of Contact Stonite 82°-86° Microrok 87°-90° Dynarok 85°-89° Centerok'89 87°-90° Centerok'91 77°-82° Granite 77°-79° PTFE104°-111° FEP 95°-105° PFA104°-111° ETFE 90°-100° It must be stressed that the angle of contact of the fluoropolymers always exceeds 90°, which means that the surface adopts an unmoistened configuration.

Despite the fact that reference has been made to a specific embodiment of the invention, it is evident to an expert that the pressing device is capable of numerous variations and modifications and that all the details mentioned can be replaced by other technically equivalent ones without deviating from the scope of protection defined by the attached claims.