Process for combating the appearance of mist during the...

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Reexamination Certificate

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C427S515000, C427S180000, C427S387000

Reexamination Certificate

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06511714

ABSTRACT:

The invention relates to the general field of the silicone coating on high speed rolls of various flexible supports, such as sheets of paper or synthetic polymer (polyolefin, polyester and the like), or alternatively of textile.
More specifically, the invention relates to the coating of flexible materials with liquid compositions comprising one or more polyorganosiloxanes (POSs) which can be crosslinked by polyaddition, by polycondensation (in emulsion), by the cationic route or by the radical route, so as to form a protective film or coating having in particular adhesion-resistant and/or hydrophobic properties.
The flexible supports can be paper, board, plastic film or metal film. The applications of these silicone-coated supports are, for example: paper for foodstuffs (pastry molds, packaging), adhesive label/tape, seal, and the like.
The coating of these flexible supports with crosslinkable liquid silicones is carried out on coating devices which operate continuously and at very high speed. These devices comprise coating heads composed of several rolls, including in particular a pressure roll and a coating roll, which roll is fed continuously with crosslinkable liquid silicone composition, by means of a series of rolls placed next to one another. The strip of flexible supports moves at high speed between the pressure roll and the coating roll to be coated on at least one of its faces with a silicone film intended to crosslink via crosslinking means positioned downstream of the coating head. These crosslinking means can be emitters of heat, of radiation (e.g. UV radiation) or of electron beams, for example.
In the drive to be ultraproductive, manufacturers of flexible supports coated with adhesion-resistant silicone are demanding liquid silicone coating formulations suitable for increasingly high linear speeds of forward progression of the strip of flexible supports. The economic factor is obviously not insignificant in this search for novel silicone formulations for high speed coating.
In point of fact, it is known that high speeds on continuous coating machines are synonymous with problems of transfer of the silicone liquid film from the coating roll onto the strip of forwardly progressing flexible support. These problems of transfer (splitting) are reflected in particular by the appearance of a mist or aerosol (misting, fogging) in the surroundings of the coating head and more particularly at the points of contact between the rotating rolls and/or between the coating roll and the flexible support to be coated. The density of this mist or of this aerosol increases when the linear speed of forward progression and thus the rotational speed of the rolls increases.
The consequence of this phenomenon is first of all a loss in consumable and in particular a deposition of droplets of coating liquid on the support downstream (for example in the oven) which seriously harms the quality of the coating: orange peel appearance, lack of coverage, mechanical qualities of the crosslinked coating (ruboff), adhesion resistance.
In addition, this undesirable formation of mist has harmful consequences with regard to industrial hygiene and the safety of the operators, who are exposed in the vicinity of the roll coating device to a high aerosol content. This can prove to be harmful.
Furthermore, the misting results in the rapid fouling of the roll coating device, leading to maintenance restrictions and premature wear.
In order to guard against the consequences of this mist, a suction system, which makes it possible to capture said mist, is generally positioned around the coating head.
Furthermore, a number of adjustments of the coating head to thwart this phenomenon are known to a person skilled in the art. A few examples thereof will be mentioned below:
lowering the speed at the expense of the productivity;
decreasing the level of deposition of silicone at the expense of the properties of the silicone-coated flexible support which it is desired to obtain (appearance, coverage, adhesion resistance, mechanical properties);
increase in the difference between the tangential speed of the coating roll and the linear speed of the paper. However, beyond a certain differential, the homogeneity of the coated layer is seriously disrupted. Furthermore, this makes it possible to reduce the density of the mist without for all that eliminating it sufficiently to allow a significant increase in the coating speed;
increase in the pressure between the coating roll and the pressure roll; here again, within a certain limit and without advantageous suppression of the phenomenon of formation of mist.
Another approach for combating the formation of mist in roll coating machines consists in exerting an effect on the formulation of the liquid silicone coating composition.
According to this approach, it is known to reduce the number-average degree of polymerization of the POSs constituting the silicone coating liquid and consequently to reduce the viscosity of the silicone coating bath in order to limit the density of the mist.
These known methods suffer from a serious disadvantage, which is that of substantially modifying the properties and in particular the adhesion resistance of the silicone-coated flexible support which it is desired to obtain.
Mention may be made, by way of illustration of this approach through the silicone formulation, of European Patent Application EP-0 716 115, which discloses a process for manufacture of a silicone composition for high speed coating with rolls, this composition being presented as making possible the reduction in the mist density. According to this process, use is made of a polydimethylmethylhydrosiloxane with trimethylsilyl ends with a degree of polymerization of 12 and 0.01% of a polydimethylsiloxane substituted by perfluoethylbutyl and methylvinyl functional groups, the ends of which are of the dimethylvinylsiloxyl type, and with a degree of polymerization of 300, and polypropylene glycol and optionally a stearyl or oleyl alcohol. This results in polydimethysiloxanes functionalized by polyoxypropylene groups. These functionalized PDMSs are used in combination with other functionalized PDMSs, e.g. functionalized by hexenyl units, and with a platinum-based hydrosilylation catalyst, to form silicone coating compositions which make it possible to reduce the formation of mist. The functionalization units can be hydrophobic residues, such as stearic or oleic acid residues.
It is clear that such compositions are relatively complex and thus expensive to obtain. Furthermore, they still remain capable of improvement as regards combating the formation of mist in silicone coating on rolls at high speed.
U.S. Pat. No. 4,806,391 relates to silicone-based inks and varnishes and more specifically to a process for the application of these inks/varnishes to a substrate using a roll coating machine operating at high speed. This patent discloses in particular compositions comprising polydimethylsiloxanes PDMSs with vinyl ends with a viscosity at 25° C. of between 15 000 and 50 000 mPa.s. (Examples XII and XIII: 30 000 and 18 000 mPa.s at 25° C. respectively). These liquid coating compositions also comprise a platinum-based catalyst and a rheological additive composed of silica with a high specific surface area, in particular fumed silica, in a proportion of less than 1% and less than 5% by weight on a dry basis in Examples XII and XIII respectively.
This composition also comprises a polymethylhydrosiloxane capable of reacting with the Si—Vi POS in the presence of a platinum catalyst.
According to this patent, it is the choice of a specific viscosity interval between 15 000 and 50 000 mPa.s for the majority Si—Vi POS which makes it possible to introduce a solution to the problem of formation of mist. In reality, in this patent, only the liquid coating compositions devoid of rheological additive of the silica type were tested in a roll coating device operating at high speed. From the results obtained, the acceptable reduction in the mist/rheological behavior compromise lies in the viscosity inte

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