Drying and gas or vapor contact with solids – Process – By centrifugal force
Reexamination Certificate
2002-03-01
2004-07-06
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
By centrifugal force
C034S315000, C034S386000
Reexamination Certificate
active
06757987
ABSTRACT:
DESCRIPTION
1. Field of Application
In its more general aspect the present invention refers to the coating industry sector.
In particular, the invention regards a method for producing powder paints.
2. Prior Art
The use of powder paints began at the start of the 60′s in the United States, applied to the coating of metal objects and then underwent a substantial impulse from the moment when in 1966 the first electrostatic plant for applying powders was made operative in Germany.
From then on coating with electrostatically applied powders has won an ever greater market share in the sector of surface protection, in virtue of the substantial advantages, which it gives with respect to traditional application methods which use solvents.
Amongst the most significant advantages one can recall:
the elimination of risks of fire, of air pollution and of toxicity connected with the presence of solvents;
the lower porosity of the obtained set coatings;
the obtainment of high thickness in a single coating operation;
the negligible material losses;
the excellent chemical strength of the obtained coatings;
the absence of powder inside the obtained films;
the ease with which the application equipment is cleaned;
the saving in manpower;
The formulations for powder paints generally include the following components: a resin, a curing agent, a flow control agent, pigments and fillers.
Amongst resins, the most widely used are epoxy resins but polyester, polyurethane and polyacrylic resins are also used, as well as mixed polyester-epoxy systems.
The aforementioned formulations are produced fundamentally according to two procedures.
A first procedure foresees the mixing of all of the formulations inside a mixer (for example a zed mixer). Firstly the resin is melted (at about 130-150° C.) and then the flow control agent is added; finally the pigments and the fillers are added and the temperature is allowed to drop down to 90-110° C., before adding the curing agent. After about 15 minutes, the mass is discharged onto a flat surface and cooled, to then be subjected to milling, for example in a hammer mill. The powder thus obtained is sieved and the particles which are too big are put back through the mill.
When this procedure is used, the choice of curing agent is limited to those agents which are inactive at the temperatures which are reached inside the mixer, like for example dicyandiamide and boron trifluoride complexes.
The second procedure for preparing powder paints, on the other hand, foresees an initial mixing of the ingredients, which must all be powdered. The uniform mixture thus obtained is fed inside an extruder, the chamber of which has been preheated to 70-90° C. The melted mass leaving the extruder is gathered under the form of pieces about 12 mm thick and then is pulverized and sifted.
In this case more reactive curing agents can also be used, since the permanence of the resin/curing agent mixture in melted state in the extrusion chamber is very short. Nevertheless, since the extrusion must be preceded by a mixing, the curing agent must be a powder and not a liquid.
As has been seen, both of the procedures referred to above have limitations in the choice of curing agents and, moreover, are characterized by a somewhat low hourly productivity, as a consequence of the low yields of the extrusion and milling apparatuses.
SUMMARY OF THE INVENTION
The problem underlying the present invention is that of providing a method for producing powder paints which allow the range of curing agents which can be used to be widened and above all to increase productivity.
Such a problem has been solved, according to the invention, by a method for producing powder paints, which comprises the steps of:
a) preparing a paste comprising at least one thermosetting plastic resin, at least one curing agent, at least one pigment and water and making said paste flow in the form of a thin layer and under conditions of high turbulence in contact with a wall at a predetermined temperature;
b) drying said paste by making it flow into a thin and turbulent layer in contact with a heated wall and optionally with a flow of heated gas.
The temperature of the heated wall is generally between 50° C. and 100° C. and the temperature of the heated gas is generally between 80° C. and 160° C. Preferably the gas consists of air.
To carry out the method according to the invention an apparatus known as a turbomixer is advantageously used. In such a case the method according to the present invention comprises the steps of:
providing a turbomixer comprising a cylindrical tubular body, equipped with an optional jacket for heating/cooling its inner wall, with at least one inlet opening for a flow of a mixture comprising at least one thermosetting plastic resin, at least one curing agent and at least one pigment, with at least one inlet opening for a flow of water, at least one discharge opening and a bladed rotor, rotatably supported in the cylindrical body and having the blades arranged helically;
continuously feeding a flow of said mixture and a flow of water in cocurrent into the turbomixer;
centrifuging said flows by means of said bladed rotor against the inner wall of said turbomixer, with the formation of a paste arranged in a turbulent and dynamic thin tubular layer which is made to advance by the blades of said rotor towards said at least one discharge opening, said blades subjecting the paste of said turbulent and dynamic thin tubular layer to intense mechanical work;
continuously discharging a flow of said paste from said at least one discharge opening; and
drying said paste obtaining said powder paint.
In the procedure according to the invention, the bladed rotor of said turbomixer is rotated at a speed of at least 600 rpm, advantageously 800-1200 rpm.
In the turbomixer, the aforementioned at least one inlet opening for the flow of water can consist of a feeding mouth or of distributor nozzles or of both.
Preferably, the paste leaving the turbomixer contains an amount of water equal to about 10-40%, advantageously about 20%, of its total weight.
Conveniently, the drying step can be carried out in a turbodryer structurally analogous to the previous turbomixer from which it differs for a few characteristics, described hereafter, to be able to carry out the drier function.
The turbodryer comprises a cylindrical tubular body equipped with a jacket for heating its inner wall, with at least one inlet opening for said paste, with at least one discharge opening for the powder paint, at least one outlet opening for the steam released from the drying and with a bladed rotor, rotatably supported in the cylindrical body, with blades oriented so as to make said paste advance from said at least one inlet opening to said at least one discharge opening.
A fan is connected to the turbodryer to suck out the steam which is released during drying.
The turbodryer can also comprise at least one inlet opening for a gas, preferably air.
In accordance with such an embodiment, the drying of the procedure of the invention comprises the steps of:
continuously feeding a flow of the paste coming out of the aforementioned turbomixer into the aforementioned turbodryer, in which the inner wall is kept at a temperature of 50-100° C. and in which the bladed rotor is rotated at a speed of at least 600 rpm;
centrifuging the aforementioned paste in the turbodryer with the formation right from its entry into the turbodryer of a turbulent and dynamic tubular thin layer which is made to advance by the blades of the rotor of the turbodryer towards said at least one opening for discharging it, in a heat exchange relationship with the inner wall of the turbodryer, said blades subjecting the turbulent and dynamic thin tubular layer of paste to intense mechanical work;
simultaneously sucking out the steam which is released from said paste, and
continuously discharging a flow of a powder paint.
In accordance with a feature of the drying step of the procedure of the invention, a flow of gas, preferably air at a temperature of 80-160° C., is fed into the turbodryer at the
Lazarus Ira S.
O'Malley Kathryn S.
Sughrue & Mion, PLLC
Vomm Chemipharma S.r.l.
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