GB2048288A

GB2048288A - Process for Using Residual Sludges from Industrial Painting with Oven-curable Paint in Spray Booths

Info

Publication number: GB2048288A
Application number: GB8011375A
Authority: GB
Original assignee: LIVIO ROSSI Srl; ROSSI LIVIO Srl
Current assignee: LIVIO ROSSI Srl; ROSSI LIVIO Srl
Priority date: 1979-04-09
Filing date: 1980-04-03
Publication date: 1980-12-10
Also published as: ES8102503A1; JPS55142636A; FR2453688B1; IT7967749A0; DE3013511A1; GB2048288B; ES490342A0; FR2453688A1; BR8002150A; IT1119064B

Abstract

The water content of paint sludge obtained from painting with oven- curing paint is reduced to a residual value not exceeding 35% by weight by mechanical working such as compressing, extruding or rolling. The obtained moist plastic sludge and a fibrous reinforcing material are combined by for example kneading or sandwiching to form a heat-curable panel without the addition of extra heat-curable resin. The sludge and fibrous material are hot-pressed at an elevated temperature and for a time sufficient to at least partially heat-set the panel.

Description

SPECIFICATION Process for Using Residual Sludges from Industrial Painting with Oven-curable Paint in Spray Booths In industrial painting in spray booths, only part of the atomised paint is deposited on the object to be painted. To avoid the remaining part forming deposits on the walls of the booth, the walls are protected by cascades of water which capture the excess paint. The resulting liquid mixture is collected in a decantation pool; after decantation and, if desired, filtration, the aqueous phase is recycled to the cascades and a more or less dense mud, called the paint sludge, is obtained.

This invention is specifically concerned with paint sludges originating from oven-curable paints such as those used in coating motor car bodies, cabinets for domestic or industrial refrigerators, housings for washing machines, and the like. An essential component (film forming component) of such paints is a heat-curable synthetic resin, which is typically selected from the group consisting of polyester, epoxy, alkydic, acrylic, ureic and melaminic thermosetting resins.

Formulations of said paints are widely variable and, generally, are not specified in detail by the manufacturers. Typical average compositions are given hereinbelow by way of example, wherein the parts are by weight.

Composition I (Finishing Enamel Paint) Heat-curable resin 33 Inorganic pigments 18 Mineral fillers 10 Solvents: Alcohols 3 Glycols 4 Aromatics 29 Additives 3 100 Composition II (Primer): Heai-curable resin 32 Inorganic pigments 20 Mineral fillers 15 Solvents: Alcohols 10 Glycols 3 Esters 5 Aromatics 1 5 100 Composition Ill (Metaiized Finishing Paint): Heat-curable resin 40 Metal pigments 3 Solvents: Ketones 12 Alcohols 10 Esters 5 Glycols 5 Aromatics 25 100 It may be noted that the paints are of "nonaqueous" type and that, accordingly, they may be (and actually are) negatively affected to a more or less detrimental degree by the presence of water, so that recovery of the paint from a sludge for reuse is practically impossible.

Various industries give rise to enormous quantities of paint sludge with consequent problems. In particular, sludges resulting from painting with oven-curable paints do not offer any prospect of recovery of the resins and other valuable components contained therein because of the degradation, agglomeration and other changes produced by the water, time and other factors. The uncontrolled disposal of these sludges is not possible for obvious ecological reasons. Incineration or controlled disposal are not economically satisfactory because of the high costs involved in investment in, and operation of, the necessary plants.

An attempt at using paint sludges containing oven-drying paints is known to have been made, in which the sludge is first freed from the greater part of its water content by mechanical means (in particular by extrusion through apertures of a few millimetres diameter) and then drying in an oven for several hours at a temperature of the order of 200"C until a product is obtained which can be converted into particles of a suitable size by grinding and screening; the particles thus obtained are then used (as an inert filler) mixed with wood particles, with the addition of a suitable thermosetting binder (for example urea/formaldehyde resin) in the manufacture of panels by hot pressing, by the conventional method for the production of chipboard.

The idea has also been considered, in very general terms, of making use of the binding properties of the resins in sludges derived from oven-curing paints, again for the purpose of manufacturing chipboard and the like, since the resins under consideration essentially are thermosetting resins; this idea, however, has been discarded since the very beginning in view of the difficulties connected with the treatment of a material (sludge) which is of a gum-consistency and is sticky, damp, impregnated with solvents and contains impurities of various types (in particular mineral or metallic pigments). Even more serious problems were foreseen in the drying of the material described above without prejudicing its thermobinding properties.

The present invention provides a process for using residual sludges from painting with ovencuring paints in spray booths, in which the sludge is used, together with other ingredients, for the manufacture of thermoset panels reinforced with fibrous materials, which process is characterised essentially in that the fibrous material is combined (associated) with the sludge while the latter is still in its moist, plastic state, having a water content not greater than 35% by weight (preferably from 10 to 20%) without addition of a commercial thermosetting resin, and in that the panel is formed under mechanical pressure at a temperature sufficient to produce at least partial thermosetting of the resin in the sludge.

It follows that, for the purposes of the present invention, it is not necessary for the sludge to be completely dried; the residual reduced water content, indicated above, may be achieved by mechanical means such as pressing, rolling or extrusion, by which excess water is squeezed out of the sludge. Moreover, the consumption (obviously costly) of suitable thermosetting, commercial binders, of the urea/formaldehyde or other types, is eliminated, because, under the conditions of the process of the present invention, use is made of the binding capacity of the thermosetting resin present in the sludge, which is sufficient for the purposes of this invention.

The resins most frequently present in ovencuring paints such as those used, for example, in the painting of motorvehicle bodies, are of the alkyd-melamine or acrylic-melamine type.

Sludges containing other thermosetting resins, particularly of the types listed hereinbefore, may also be used in the present invention however.

For example a sludge resulting from an enamel paint, decanted into a decantation pool, may have the following composition (IV) by weight: Binder (resin) 35% Titanium dioxide 1 6.4 % Iron 0.25% Silica 0.74% Calcium 0.3% Magnesium 0.03% Chromic Oxide 0.7% Lead 0.3% Water and solvent 46.28% A sludge resulting from painting with a primer may have the following composition (v) by weight:: Binder (resin) 26% Titanium Dioxide 35.1% Silica 0.2% Iron 0.18% Calcium 0.5% Magnesium 0.2% Phosphate 0.2% Water and solvent 37.62% For the purposes of the present invention it does not matter if the enamel paint sludges are mixed with those resulting from painting with primer, as frequently happens in practice.

As usable fibrous materials there may be exemplified: - vegetable fibres, such as coconut, jute, cotton, wood fibres, in loose or matted form; - synthetic fibres, such as polyamide, polyester, acrylic, vinylic, polyolefin fibres, especially in matted or felted form; - reclaim materials, such as matted rag scraps of various types, shredded or milled; pressed sheets of various types of materials such as shredded rag scraps, paper cuttings, wood sawdust or shavings, or asbestos fibre residues; woven fibres of animal, vegetable or synthetic origin, woven asbestos, glass wool or mineral wool; - mineral fibres, such as asbestos, glass wool, mineral wool, especially in sheet or matted form.

According to one embodiment, the combining of the fibrous material with the sludge may be carried out by forming a sandwich of a sheet of sludge with at least one sheet or mat of fibrous material. In this case it is convenient to effect the squeezing of the starting sludge by rolling between cylinders to obtain a sheet which may be placed between two sheets or mats of fibrous material. The thickness of the sheet of sludge may vary, generally from 0.2 to 10 millimetres, depending upon the perviousness of the two sheets or mats of fibrous material.

According to another embodiment, the combining of the fibrous material with the sludge may be carried out by means of direct blending or kneading, preferably by means of a roll kneader, of the squeezed sludge and the loose fibrous material. Preferably this is carried out at a temperature higher than ambient temperature so as to reduce the viscosity of the sludge without the need to add solvents (which are difficult to recover without considerable expense). The preferred temperatures are from about 400C to about 800C, but, in any case should not be so high as to produce undesirable cross-linking of the resin at this stage.Temperatures of the order of 100 to 1 00C may even be reached but only for a few minutes (for example 10 minutes at 1 00C). The product which leaves to roll kneader is in the form of a sheet, with the fibrous filler incorporated in it, which may be not pressed as it is or with covering sheets of paper, woven textile fabric or other material.

In each case, the sludges: fibre weight ratio may vary between wide limits, of about 1:4 up to about 9:1. With ratios less than 1:4 the sludgefibre binding is hardly homogeneous, while with ratios greater than 9:1 the product may be excessively fragile when pressing of the panel is carried out at temperatures greater than 1 5O0C, or excessively plastic when pressing is carried out at temperatures less than 1 00 or.

As has been mentioned above, the panel is formed with the aid of mechanical pressure and a sufficiently high temperature.

The mechanical pressure, which favours elimination of both the residual water and solvents, may be achieved by a hot-platen press.

For continuous working, calendering through hot cylinders, or on continuous hot-plate conveyors, preferably with several calendering passes, are to be recommended. In each case, the minimum pressure necessary is 0.5 kg/cm2. The temperature may vary from 800C to about 2200C. At temperatures lower than 1 00 C, very flexible, relatively plastic panels are obtained and the elimination of the residual water is slow and incomplete. Thus temperatures higher than 1 000C, and preferably higher than 1 50 C are preferred. For rigid panels to be obtained, temperatures of from 1 750C to 2200C are recommended. The treatment times are generally in inverse relationship to the temperature and to the thickness. With small thicknesses and with temperatures of the order of 200 to 2200C two minutes are sufficient.For rigid panels, as stated above, 5 to 1 5 minutes are generally necessary.

By operating, as is preferable, at temperatures higher than 1 500C the panel dries practically completely if care is taken to choose a sufficiently long period of time. According to a preferred embodiment of the invention, the pressing heat treatment is advantageously carried out at increasing temperatures, starting from temperatures less than 100 C, so as to favour, initially, the dewatering and, only subsequently, the cross-linking; with this type of treatment and with the use of fibrous materials in sheets or matted form, it is recommended, according to the invention, that a pressure suitable for the impregnation of the material by the sludge should be chosen from the beginning and should be maintained throughout the subsequent stages.

Example 1 A sludge resulting from an indefinite number of painting cycles with primer and with enamel paint was extruded through a 2x50 millimetre slot at 250C and then cold rolled down to a thickness of 1 millimetre, with a residual water content of about 20%. The sheet thus obtained was interposed between two mats of vegetable fibre (principally jute) of a density of 430 g/m2.

The pressing of the panel was carried out on a platen press, under a pressure of 5 kg/cm2 at 190"C for 20 minutes.

A panel was obtained with a thickness of about 2 mm, having a modulus of elasticity of 20,000 and a bending strength of about 220 kg/cm2.

Example 2 Rags of polyamide material were ground until they had been reduced to a mass of fibrils of 2 to 3 millimetres in length. The mass was subsequently compacted by means of hot pressing to obtain a felt with a thickness of about 2 millimetres. The felt was placed between two sheets of sludge, 0.6 millimetres thick, obtained in the manner described in Example 1 and having a residual water content of about 15%.

The pressing was carried out on a platen press, under a pressure of 2 kg/cm2 at 1 750C for 10 minutes. The obtained panel had a tensile strength of about 50 kg/cm2 and a modulus of elasticity of 4,500.

Example 3 Example 1 was repeated, the time for the heat treatment being increased to 30 minutes. The obtained panel had a modulus of elasticity of 20,000, a bending strength of about 230 kg/cm2 and a tensile strength of 200 kg/cm2.

Example 4 Scrap rag (cuttings) from a shirt-making factory, composed by 70% 6f cotton fibres and by 30% of poly(ethylene-terephthalate fibres) were defibrated until fibrils of 2-6 mm length were obtained. 30 parts by weight fibrils were compounded with 70 parts by weight paint sludge, the original composition was that denoted by (V) hereinbefore and the water content of which was reduced to about 8% by weight by repetitive extrusion in the form of spaghettis.

Compounding was effected at about 50"C and the mass was calendered at ordinary temperature to obtain a sheet of 2.5 mm thickness. A sheet of Kraft paper was superposed on each face of the sludge sheet and the obtained sandwich structure was pressed on a platen press at 1 800C for 30 minutes at a pressure of 0.5 kg/cm2. A panel was obtained having a flexural strength of 1 82 kg/cm2, a tensile strength of 134 kg/cm2 and a flexural modulus of elasticity of 14,600 kg/cm2.

Example 5 The sludge used was identical to that of Example 4, having residual water content of 8% by weight. The sludge was rolled at ordinary temperature to give a sheet of 1.5 mm thickness.

A mat of iso-oriented glass fibres was superposed on each face of the sheet. The overall proportion of fibres (by weight) to the sludge sheet was 40%.

The sandwich was pressed on a platen press for 10 minutes at 1 900C and 0.5 kg/cm2. A panel of a thickness of about 2.3 mm was obtained, having a flexural strength of 295 kg/cm2 and a flexural modulus of elasticity of 27,200. The tensile strength was 468 kg/cm2 in the fibre direction and 323 kg/cm2 in transverse direction.

For test purposes, the panel was sub-merged in water until np increase in weight was observed (that is, to saturation conditions); the water absorption determined in this manner was as low as 6.5% by weight, only.

Example 6 In a spray-coating booth an enamel paint was used composed (by weight) of about 25% alkyd resin, about 10% melamine resin, about 18% inorganic pigments, about 10% mineral filler, about 36% organic solvent and about 1% stabilizers. A sludge was collected comprising the above paint and about 40% water. The sludge was extruded through a 2x50 mm slot at 250C and subsequently rolled at ordinary temperature to give a sheet of 1.5 mm thickness containing 15% water. A mat of juta fibres needled onto a Kraft paper sheet was prepared apart. Two such mats were superposed on the opposite faces of the sludge sheet, with the paper layer turned to the outside, and coupled to the sheet by compression at 10 kg/cm2 at ambient temperature.Hot pressing of the panel was effected on a platen press at 1800 C, at 25 kg/cm2 during first 2 minutes, and 0.5 kg/cm2 during subsequent 4 minutes. The resulting panel had a flexural strength of 273 kg/cm2, flexural modulus of elasticity of 26,200 and tensile strength of 201 kg/cm2.

Example 7 A paint sludge was collected containing about 33% by weight acrylic- and melamine resin, about 16% pigments and fillers, and about 51% aqueous liquid containing traces only of organic solvent(s). The mass was of a cellular appearance with cells filled by the said aqueous liquid. Water was squeezed from the sludge by repeated rolling at 200C until the residual water content was about 15%. The obtained sheet (about 1.5 mm thickness) was interposed between two webs of a loosely woven fabric of glass fibres, the joint weight of which was about 30% of the weight of the sludge sheet. The fabrics were preliminarily coupled with the sheet by compression at 30 kg/cm2 at 200C.Hot pressing was subsequently effected on a platen press during 1 5 minutes at 0.5 kg/cm2 and 1 900 C. The resulting panel had a modulus of elasticity of 29,800, a flexural strength of 376 kg/cm2 and a tensile strength of 320 kg/cm2. Its water absoprtion (on saturation) was as low as 9% by weight.

It will be evident to those skilled in the art that hot pressing may be effected between embossed or, generally, non-planar dies, so as to obtain embossed or three-dimensional panels.

Owing to the favourable properties eivdenced by the Examples hereinbefore, the panels are successfully usable in many fields, in lieu of panels obtained heretofore from costly starting materials (as are commercial synthetic resin formulations for moulding purposes, for example).

A practical advantage of the invention also resides in that the proportion of the sludge material in the final panel is relatively high; this means that the enormous amounts of paint sludges accumulating daily in spray-booth plants of a factory can be disposed of without difficulty, particularly without necessitating a considerable supply of materials from sources outside of the factory. Also, owing to low water absorption of panels reinforced with glass fibres, such as those obtained in Examples 5 and 7, such panels (and similar panels obtained with the use of rock wool or slag wool, for example) may successfully be used for roof coverings, sheds and similar artifacts, notoriously representing a consumption of thousand of square metres of covering sheets or plates. Undulated panels obtained according to this invention may thus successfully be used in lieu of conventional asbestos-cement plates.

Panels obtained according to this invention may also advantageously be used as internal upholstery in motor cars and railways cars.

The color of the panels obtained according to this invention is, in most cases, indefinite or randomly variable, because the collected sludge ordinarily results from painting with differently colored paints. However, a color of the panel may be enhanced or, generally, colored panels may be obtained by additionally incorporating into the sludge convenient pigments. Also fluorescent pigments may be incorporated, if desired.

Claims

1. Process for using a residual sludge from industrial painting with an oven-curing paint in a spray booth, in which the sludge in used together with other ingredients for the manufacture of a thermoset panel reinforced with fibrous material, characterised in that the fibrous material is combined with the sludge while the latter is in plastic, moist state, having a water content not greater than 35% by weight, without addition of a commercial thermosetting resin so that the only thermosetting binder is that comprised by the sludge, and in that the panel is formed under mechanical pressure at a temperature sufficient to induce at least partial thermosetting of the resin contained in the sludge.

2. Process according to Claim 1, in which the combining of the fibrous material with the sludge is carried out by forming a sandwich of a sheet of sludge with at least one sheet or mat of the said fibrous material.

3. Process according to Claim 1, in which the combining of the fibrous material with the sludge is carried out by kneading.

4. Process according to Claim 3, in which the kneading is carried out under hot conditions, at a temperature and in a time insufficient to produce thermosetting.

5. Process according to claim 1, in which the mechanical pressure is at least 0.5 kg/cm2.

6. Process according to claim 1, in which the said temperature is from 80 to 2200C and is applied for a time of at least 2 minutes.

7. Process according to Claim 6, in which the said temperature is higher than 1000C.

8. Process according to claim 6, in which said temperature is from 175"C to 200"C.

9. Process according to claim 1, wherein the plastic sludge with the water content not exceeding 35% by weight is obtained by expelling excess water from the original sludge by mechanically processing the latter.

1 0. Process according to claim 9, wherein the original sludge is mechanically processed by extrusion or rolling.

11. Process according to claim 9 or 10, wherein the water content not exceeding 35% by weight is at least 10%.

12. Process according to claim 1, wherein the sludge: fibre weight ratio is from 1:4 to 9:1.

1 3. Process for using a residual sludge from industrial painting with an oven-curing paint in a spray booth, substantially as hereinbefore described with reference to any one of the Examples 1 to 7.

14. A panel when made by the process according to any one of the preceding claims.