The present invention relates to a continuous process for producing polyester resins suitable as binders for compatible thermosetting paints, and the relative paints based on said polyester resins.

Regulations relating to the health of human beings and environmental protection, in force on a national and international scale, have imposed and impose increasing restrictions in the use of organic solvents in coating compositions, such as, for example paint or protective products. In particular, restrictions relating to pollution have reduced the degree of use of solvent-based coatings and have required the development of new types of paint products or paints, which do not pollute or in any case reduce atmospheric pollution to the minimum. At the same time, however, the resin forming the base (binder) of the paint must be formulated so as to give a coating- film having a good resistance to atmospheric agents, abrasion and solvents .

The growing demand for saturated polyesters for the production of thermosetting paints, in particular powder paints, has incentivized the study and research for a production process which allows polyesters to be obtained, with much more uniform characteristics. Each production cycle of polyester resins, also carried out in the same reactor, leads to the production of products with final parameters which are often different . The use in the subsequent production of paint compositions of these polyester resins which are not totally homogeneous leads to the production of paints, above all powder paints, which are often incompatible .

The necessity is therefore strongly felt for finding new processes which overcome the drawbacks described in the state of the art .

The compatibility between batches coming from different production cycles is in fact extremely important and requested, especially in the case of powder paints, and the process according to the present invention allows the technical problem indicated above to be solved.

Continuous processes for the production of thermoplastic polyesters (PET, PBT, PPT, etc.), known in the state of the art, are generally characterized by producing polyester, starting from a synthesis phase of the oligomer, followed by a polymerization phase by transesterification, by stripping glycol, so as to obtain a polyester with a high molecular weight, high viscosities and extremely low hydroxyl and/or carboxyl functionality values.

A particular process has now been surprisingly found, which solves the problem of incompatibility of powder paints as it allows the production of polyester resins coming from different production cycles, with a uniformity of physico-chemical characteristics, thanks to a homogenization of the final parameters of the polyester, before being discharged.

It has been very surprisingly observed, in fact, that paints, when cross-linked with the same quantity and type of hardening partner, starting from polyester resins obtained by applying the process according to the present invention, through different production cycles, have proved to be completely compatible with each other. Compatibility refers to the possibility of mixing powder paints having an identical formulation, but formulated starting from polyester resins coming from different production cycles, always obtaining the same characteristics with reference to gloss, film appearance , etc .

An object of the present invention relates to a continuous process for producing low-molecular-weight polyester resins, suitable as binders for thermosetting paints, characterized in that it comprises the following steps:

a) polycondensation of

an oligomer of one or more polyfunctional organic acids and one or more polyfunctional alcohols, both with functionalities ranging from 2 to 4, and/or one or more epoxides,

with one or more polyfunctional organic acids and/or one or more polyfunctional alcohols, both with functionalities ranging from 2 to , and/or one or more epoxides,

in the presence of an esterification catalyst and stabilizers, obtaining hydroxylated polyester resins with an OH number ranging from 15 to 300 mg KOH/g and with an acidity number ranging from 1 to 20 mg KOH/g and/or carboxylated polyester resins with an acidity number ranging from 15 to 100 mg

KOH/g with an OH number ranging from 1 to 20 mg KOH/g;

b) hot homogenizing at least two batches of polyester resin obtained at the end of step a) , filtration and continuous discharge.

A further object of the present invention also relates to polyester resins for compatible paints, obtained with the process according to the present invention, and paint compositions or paints containing the polyester resin produced according to the present invention, as binder.

Step b) can possibly be followed, again in continuous, by a static mixing and/or dynamic mixing step c) with addition to the homogenized polyester resin obtained at the end of step b) and continuous hot discharging.

The process according to the present invention can comprise the synthesis of the oligomer, effected before step a) starting from one or more polyfunctional organic acids or anhydrides (such as, for example, succinic, maleic, phthalic, tetrahydrophthalic , hexahydrophthalic, trimellitic, pyromellitic , methylnadic, methylhexahydrophthalic anhydrides) and one or more polyfunctional alcohols, having functionalities ranging from 2 to 4 and one or more epoxides, or the oligomer can be a commercial product.

The polycondensation step a) is carried out in the presence of esterification catalysts and stabilizers and further quantities of glycol (s) and di- and/or polyfunctional acid(s) and/or epoxides (s), are added to the base oligomer, transferred to a second reactor if synthesized in situ, and the mixture is esterified until the desired parameters relating to acidity, hydroxyl number and viscosity are reached.

The polycondensation of the oligomer with one or more polyfunctional organic acids and/or one or more polyfunctional alcohols, both having functionalities ranging from 2 to 4 , and/or one or more epoxides, in the presence of an esterification catalyst and stabilizers, leads to the production of hydroxylated polyester resins with an OH number ranging from 15 to 300 mg KOH/g and with an acidity number ranging from 1 to 2 mg KOH/g and/or carboxylated polyester resins with an acidity number ranging from 15 to 100 mg KOH/g with an OH number ranging from 1 to 20 mg KOH/g.

In particular, the acids, alcohols or epoxides can be the same or different from those used for the synthesis of the oligomer or from those forming the commercially available oligomer.

Acids which can be used in the process according to the present invention are: terephthalic acid, isophthalic acid, phthalic acid, methylterephthalic acid, trimellitic acid, pyromellitic acid, adipic acid, sebacic acid, succinic acid, maleic acid, fumaric acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, hydroxybenzoic acid, ricinus fatty acid and anhydrides, such as succinic anhydride, maleic anhydride, phtalic anhydride, tetrahydrophtalic anhydride, hexahydrophtalic anhydride, trimellitic anhydride, pyromellitic anhydride, methylnadic anhydride, methylhexahydrophthalic anhydride. Preferred acids are terephthalic acid, isophthalic acid and/or adipic acid.

Polyfunctional alcohols or glycols which can be used in the process according to the present invention are: ethylene glycol, propylene glycol, 1, 3-butanediol, 1, 4-butanediol, 1, 6-hexanediol, neopentyl glycol, isopentyl glycol, bishydroxyethyl terephthalate , bisphenol A hydrogenate, bisphenol A ethoxylate (chemical name (2- [4- [2- [4- (2- hydroxyethoxy) phenyl] propan-2-yl] phenoxy] ethanol) , bisphenol A propoxylate, trimethylolethane , trimethylolpropane, glycerol, pentaerythrite and 2,2,4- trimethylpentane-1, 3-diol, and relative mixtures.

Preferred polyfunctional alcohols or glycols in the process according to the present invention are neopentyl alcohol and/or 1 , 3 -propanediol alcohol.

Epoxides which can be used in the process according to the present invention are monoepoxide compounds such as glycidyl ether, glycidyl methacrylate, glycidyl versatate (also known with the commercial name of Cardura E10P of Hexion) , glycidyl methacrylate. The oligomer is preferably selected from neopentyl terephthalate and/or neopentyl isophthalate and/or neopentyl adipate with a hydroxyl functionality and/or from 1-3 propanediol terephthalate and/or 1-3 propanediol isophthalate and/or 1-3 propanediol adipate (oligomer) with a hydroxyl functionality (terminal hydroxyl groups) .

If the oligomer has been synthesized before effecting the polycondensation, the synthesis is carried out starting from neopentyl glycol with terephthalic acid and/or isophthalic acid and/or adipic acid and leads to the production of neopentyl terephthalate and/or neopentyl isophthalate and/or neopentyl adipate (oligomer) with a hydroxyl functionality or starting from 1-3 propanediol with terephthalic acid and/or isophthalic acid and/or adipic acid and leads to the production of 1-3 propanediol terephthalate and/or 1-3 propanediol isophthalate and/or 1-3 propanediol adipate (oligomer) with a hydroxyl functionality (terminal hydroxyl groups) .

The polycondensation step (a) is preferably carried out starting from a quantity of oligomer which ranges from 10 to 70% by weight with respect to the total weight of the reaction mixture, a quantity of polyfunctional organic acids that ranges from 5 to 50% by weight with respect to the total weight of the reaction mixture and/or a quantity of polyfunctional alcohols that ranges from 5 to 50% by weight with respect to the total weight of the reaction mixture and/or a quantity of epoxides that ranges from 0.5 to 10% by weight with respect to the total weight of the reaction mixture .

The quantity of polyfunctional acids and/or polyols added to the oligomer is calculated and controlled so that the Tg parameters, hydroxyl number and/or acid number desired are reached in the polyester resin obtained at the end of this step.

In the case of a polyester resin obtained using, in step a) , a quantity of acids which ranges from 5 to 25% by weight with respect to the total weight of the reaction mixture, selected from 5-sulfoisophthalic acid, lithium salt of 5-sulfoisophthalic acid (Li- SIPA) , sodium salt (Na-SIPA) of 5-sulfoisophthalic acid, the polyester resin obtained with the process according to the present invention, regardless of the starting oligomer, is hydro- dilutable and suitable for water paints .

In particular, polyester resins obtained with the process according to the present invention starting from unsaturated polyfunctional acids or polyfunctional alcohols, can be applied to UV cross- linkable paints.

The esterification catalysts are preferably selected from dibutyl tin oxide, sodium acetate, stannous octoate, hydroxylbutyl tin chloride, zinc acetate, titanium glycolate, etc. Preferred catalysts are catalysts containing tin such as di-butyl-tin-oxide DBTO and mono-butyl-tin-oxide MBTO.

The catalyst is present in a quantity which ranges from 5 to 5000 ppm, preferably from 1000 and 2000 ppm, with respect to the total weight of the reaction mixture .

The stabilizers are preferably phosphorous acids and/or derivatives such as polyphosphoric diacids, phosphoric acid, organophosphoric compounds, organophosphates, organophosphites , and organophosphonates ; orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, tri- polyphosphoric acid, phosphorous acid, hypophosphorous acid, bismuth phosphate, monoammonic phosphate, phosphate, monoammonic phosphorite; triphenylmethylphosphonium iodide , triphenylmethylphosphonium bromide , triphenylbenzylphosphonium chloride, salts of phosphoric acid esters, phosphites, etc.

The stabilizer is present in a quantity ranging from 5 to 2000 ppm with respect to the total weight of the reaction mixture .

The hot homogenization step b) of at least two batches of polyester resin obtained at the end of step a) , and subsequent filtration and continuous discharging, is essential for making the paint obtained compatible for the subsequent formulation of the polyester resin thus produced.

Once at least two batches of polyester resin have been produced at the end of step a) , in fact, the homogenization step b) is initiated.

Step b) is carried out at a constant temperature within the range of 180 - 210°C, with continuous stirring for the time necessary for obtaining the homogenization.

The homogenized product is then continuously discharged, effecting a fine filtration with filters having a pore dimension lower than 40 microns. The discharging of the homogenized product is dosed so that once a quantity of homogenized product corresponding to a batch of polyester resin coming from step a) has been discharged, a new batch of product coming from said step a) is ready and will be fed to step b) to be homogenized with the remaining mix of the previous batches. In particular, when the third production batch of polyester resin is ready at the end of step a) , it will be fed to step b) and will be homogenized with the same quantity of product, corresponding to the mix of the first two batches, not discharged at the end of step b) . Step b) then proceeds in continuous according to this procedure : the discharging of a batch from step b) , possibly to be sent to the subsequent step c) , will be effected when a new batch of polyester resin is ready at the end of step a) to be fed to step b) , thus effecting a continuous discharge of a quantity of homogeneous polyester resin, calculated in relation to the step times.

As previously indicated, step b) can be followed in continuous by a possible static and/or dynamic mixing step c) which provides for the addition of the homogenized polyester resin obtained at the end of step b) and subsequent hot discharging in continuous.

A first important advantage of the process according to the present invention is that it allows polyester resins to be obtained, which are suitable as binders for compatible thermosetting paints, in particular for powder and/or liquid and/or hydro- dilutable paints, which are substantially free of byproducts such as acetaldehyde and colouring.

The process according to the present invention, moreover, allows low-molecular-weight polyester resins to be produced with hydroxyl and/or carboxyl terminations, suitable for the production of thermosetting paints by formulation with suitable hardening partners .

In the present description, low-molecular-weight resins refer to resins with a weight average molecular weight ranging from 500 to 10,000, preferably from 2, 000 to 4, 000.

The polyester resins produced with the process in continuous according to the present invention create resins which are particularly suitable for producing powder paints. Paints are in fact obtained, having the same formula, but deriving from resins obtained from different production cycles, characterized by a high compatibility. The compatibility between paints having the same chemical formula, but obtained from different production cycles, is greatly desired by appliers who wish to and must avoid cleaning the painting plant with each change of batch of paint . When present, in the preliminary synthesis phase of the oligomer, the neopentyl glycol reacts, in a first reactor, with terephthalic acid and/or isophthalic acid to obtain neopentyl terephthalate and/or isophthalate (oligomer) with a hydroxyl functionality.

Substantially, this preliminary phase is carried out in a reactor in an inert atmosphere, under nitrogen, and the reaction water is removed.

In the subsequent polycondensation step a) , said oligomer is transferred to a second reactor where the esterification reaction - polymerization is effected. The oligomer is transferred through a filter, again in an inert atmosphere, under nitrogen and heat. If the oligomer is a commercial product, the process according to the present invention comprises feeding said oligomer directly to the second reactor. In this step a) , there is the addition of a further quantity of di- and/or polyfunctional acids and/or polyfunctional alcohols, with functionalities ranging from 2 to 4, and/or epoxides and esterification is then effected until the desired acidity, hydroxyl number and viscosity parameters are reached. In particular, hydroxylated polyester resins are obtained, with an OH number which ranges from 15 to 300 mg KOH/g and with an acidity number ranging from 1 to 20 mg KOH/g and/or carboxylated polyester resins with an acidity number ranging from 15 to 100 mg KOH/g with an OH number which ranges from 1 to 20 mg KOH/g.

When the desired parameters have been reached, the polyester is transferred, again under nitrogen and heat, through a filter, into the homogenization reactor-boiler where the homogenization, filtration and discharge step b) , is effected.

The hot homogenization, in continuous, forms the start of the process for then making the paint preparations deriving from the different production batches of polyester resins, compatible.

The homogenization reactor-boiler has a volume which is at least double with respect to the volume of the reactor of step a) . In this step b) , a first batch of polyester resin coming from step a) is fed to the homogenization reactor-boiler, where the discharge of a subsequent second batch of polyester resin coming from step a) is waiting. Once the two batches of polyester resin are present, the homogenization step b) of two productions coming from step a) , is initiated.

The homogenization reactor-boiler is maintained at a constant temperature in the order of 180-210°C and with a stirrer continuously functioning, for the time necessary for reaching homogenization.

The homogenized product is then continuously discharged, effecting a fine filtration with filters having a pore dimension lower than 40 microns. The discharging of the homogenized product is dosed so that once a quantity of product corresponding to a batch of product coming from step a) has been discharged, a new batch of product coming from said step a) is ready and will be fed to the reactor/boiler of step b) to be homogenized with the remaining mix of the previous batches. In particular, when the third production batch of polyester resin is ready in step a) , it will be fed to step b) and will be homogenized with the same quantity of product, corresponding to the mix of the first two batches, remaining in the reactor/boiler of step a) . Step b) then proceeds in continuous according to this procedure: the discharging of a batch from step b) and contemporaneous feeding of a new batch of polyester resin of step a) to step b) , thus effecting a continuous discharge of a quantity of homogeneous polyester, calculated in relation to the step times. The batch of polyester resin which is discharged from step b) can be sent to the subsequent step c) for a further processing or it can be packaged as an end- product (homogeneous polyester resin not containing additives) .

The homogenization step b) is essential for making the paint obtained by the subsequent formulation of the polyester resin thus produced, compatible.

The process according to the present invention can comprise a further static and/or dynamic mixing step c) with the possible addition and discharge.

The homogenization step b) of the polyester resins is in fact followed by the possible addition of useful and/or necessary additives in relation to the characteristics of the thermosetting paints deriving from said polyester resins.

The polyester resin, as such at the end of step b) or possibly containing additives at the end of step c) , is discharged in continuous using static and/or dynamic mixers. The addition, with respect to both liquid additives and solid additives, is effected in continuous by means of suitable dosage pumps . The polyester resin, to which additives have possibly been added, is then discharged effecting a fine filtration with filters having a pore dimension lower than 40 microns. At the end of step b) a homogeneous polyester resin not containing additives is obtained.

Step c) of the process for the production in continuous of polyester resins, uses Sulzer mixers and/or analogous mixers and/or systems with extrusion and dosage pumps, for the continuous addition of the additives.

In particular, the polyester resin coming from step b) is fed to step c) , at a temperature of about 180- 210°C, through a static mixer of the Sulzer type or a dynamic mixer of the extruder type, where the possible doses of additives in the molten state are pumped in continuous .

The additives generally used for the addition in step c) of the process according to the present invention are selected from antioxidant agents and/or thermal stabilizing agents and/or UV absorbers and/or tribo additives, and/or stress-relieving promoters and/or anti-crater agents and/or HALS agents, pigments, stress-relieving agents, degassing agents, photoinitiator agents, etc. Hardening-reticulation agents can also be added, such as epoxy resins, β- hydroxy-alkyl-amide, TGIC (triglycidyl isocyanurate) , isophorone uretdione isocyanate and melamine resins.

A polyester resin produced with the process according to the present invention can be applied to thermosetting powder paints, thermosetting paints for coil coating, liquid and hydro-dilutable paints, UV cross-linkable paints.

In particular, in relation to the presence or absence of the addition step c) and type of addition effected in step c) , the end-product of the process according to the present invention consists of a homogeneous polyester resin not containing additives, a homogeneous polyester resin containing additives or paint compositions or paints .

The viscosity of the polyester resins was measured according to the method ASTM D4287 or ISO 2884, with an I.C.I. Cone & Plate, viscometer, with an impeller D (with a 400/viscosity factor ranging from 900 to 3,500 mPa. s) .

The process according to the present invention allows uniform product-process conditions to be effected in the distribution, concentration and temperature parameters .

Some examples are provided for a better understanding of the invention, which should be considered as being purely illustrative and non- limiting of the present invention.

Example 1 Preliminary phase

6 moles of neopentyl glycol 100% and 3 moles of terephthalic acid were charged under stirring, in an inert atmosphere, under nitrogen, into a 3 litre four- necked glass reactor. The reaction mixture was heated and, once a temperature of 120-150°C had been reached, 0.10% by weight of DBTO (di-butyl tin oxide) was added as catalyst.

The temperature was then increased to 220°C to proceed with the distillation of the reaction water by means of a rectifying column. The temperature at the head of the column was maintained within a range of 101 to 103 °C, whereas the temperature in the reactor was maintained at 220°C approximately until the end of the distillation of the reaction water, i.e. until the formation of the hydroxylated oligomer.

This temperature of the product of 220°C was maintained until the acid value was lower than 15 mg KOH/g and was kept under control so as not to exceed 240°C. Once the reaction time necessary for obtaining a limpid solution and with an acidity number lower than 15 mg KOH/g had been completed, the reaction mixture was transferred to the reactor of step b) through a filter with a pore size equal to 100 microns.

Step a)

In step a) , 3 moles of isophthalic acid, 1 mole of adipic acid, 0.333 moles of trimethylolpropane , 0.10% by weight of H ₃ P0 ₃ , as stabilizer, 0.10% by weight of DBTO (di butyl tin oxide) , as catalyst, were added to the reaction mixture containing the oligomer coming from the preliminary phase, maintaining the temperature between 160 and 180°C.

The reaction temperature was then increased to 235- 240°C, constantly keeping the temperature at the head of the column under control so as not to exceed 103 °C.

Once the reaction time necessary for obtaining a limpid solution and with an acidity number lower than 45 mg KOH/g had been completed, an increasing vacuum was applied, up to a pressure value equal to 40 mm Hg. This pressure was maintained until a viscosity of the product equal to about 2,200 mPa.s had been reached. The measurement of the viscosity was effected with an ICI Cone-Plate viscometer (of the type Hb-Epprecht) at 200 °C and impeller D. The polymerization was continued until the final reaction point had been reached, corresponding to an acidity number lower than 40 mg KOH/g, a hydroxyl number lower than 10 mg KOH/g and a Tg > 35 °C (method ASTM D4287) .

The pressure inside the reactor was then brought to atmospheric pressure by the addition of nitrogen. An aliquot equal to 0.5 kg was discharged from the polyester resin thus obtained (batch Bl) having a total weight of 1.5 kg, to be used for effecting the characterization test of the batch. The remaining part of batch Bl (1 kg) , on the other hand, was transferred under heat (at a temperature of about 220°C) through a filter to the reactor/boiler for step b) .

Step b) The hot homogenization, in continuous, represents the start of the process for making the formulation of paints deriving from different production batches of polyester resins, compatible.

The homogenization reactor/boiler is a glass reactor having a total volume of 6 litres . In said step b) , the polyester resin coming from step a) which was fed to the homogenization reactor/boiler (batch Bl) is maintained at a temperature of 200°C, in a nitrogen atmosphere. A second batch of polyester resin (batch B2) coming from step a) is then fed to the same reactor. Batch B2 is equivalent by composition to batch Bl, but was obtained separately from this. The overall quantity of batch B2 which was prepared in step a) is equal to 1.5 kg, of which 1.0 kg were fed to the reactor/boiler for step b) , whereas an aliquot equal to 0.5 kg was discharged to be used for effecting the characterization test of the batch.

Once the two batches Bl and B2 of polyester resin had been introduced into the reactor/boiler, the homogenization step b) was carried out, maintaining the reactor/boiler at a constant temperature in the order of 200 °C and with the stirrer constantly functioning.

Step c)

The homogenized polyester resin coming from step b)

(batch B3) was discharged under heat at a temperature of about 180-190°C, into a TSA 21/25 twin-screw extruder. 1% by weight with respect to the polyester, of the additive TRIBO in the molten state (Ciba ^® TINUVIN ^® 144 HALS corresponding to Bis (1,2,2,6,6- pentamethyl-4-piperidinyl) - [ [3 , 5 -bis (1,1- dimethylethyl) -4 -hydroxyphenyl] methyl] -but- ylmalonate) ) , was also pumped contemporaneously in continuous to the extruder (by means of a dosage pump) . The polyester resin containing the additive (homogeneous polyester resin with additive - batch B3- add) was then discharged at 140°C onto a calender (cooled with water to 15°C) and scaled.

Example 2

The procedure described in Example 1 was repeated a second time, producing at the end of step a) two further batches of polyester resin B4 and B5, equivalent to batches Bl and B2. At the end of the hot homogenization step b) of batches B4 and B5, a homogeneous polyester resin was obtained (not containing additives) labelled as batch B6.

Example 3

The polyester resins of each batch B1-B6 obtained according to examples 1 and 2 were formulated as powder paints (paints V1-V6) according to the following formulation:

465 parts by weight of polyester resin;

250 parts by weight of Ti0 ₂ Kronos 2160, as pigment;

8.0 parts by weight of Modaflow Powder (Cytec) polyacrylate, anticrater stress-relieving agent;

2.0 parts by weight of benzoin, as degassing agent and 25.0 parts by weight of Primid XL 552 (E SPrimid) as cross-linking agent.

This formulation was mixed and cold homogenized with a Plasmec mixer and then subsequently extruded with a twin-screw extruder of the TSA 21/25 type under the following operative conditions:

Extruder feeding: 0.5 kg/minute

Temperature 5 zones: 1 zone (cold feeding) about 15°C, 2 zone 40°C, 3 zone 80°C, 4 zone 120°C, 5 zone 120°C

Screw rev number: 500 revs/minute

Outgoing temperature of extruded product: 130°C.

Temperature of water on calender by cooling of extruded product: 10°C.

The extruded product thus obtained was then cooled, scaled, micronized, sieved at 105 microns and packaged. Part of the powder paint was then applied with an electrostatic gun of the type Wagner EPG Prima (with air at 3.4 m3/hour, with 60 KV) and with a triboelectric gun of the type Tribomatic2 of Nordson on degreased aluminium panels (of the Qpanel type) .

Table 1 indicates the acidity number and viscosity values measured for batches B1-B6. Table 1

Table 2 indicates the gloss data for each paint measured with an instrument of the GLOSSMETRO BYK- GARDNER type at a light incidence angle of 20° and 60°.

Table 2

In order to verify the compatibility of the paints obtained starting from different batches of polyester resins, the gloss of a mixture of powder paints V3 and V6 (mix "V3/V6"), i.e. a mixture of paints obtained starting from polyester resins prepared according to the present invention, was compared with the gloss of mixtures of powder paints obtained starting from batches B1-B2 (mix V1/V2) and B4-B5 (mix V4/V5) , i.e. mixtures of paints obtained starting from polyester resins prepared according to the state of the art .

The compatibility level was evaluated according to the following scale:

"excellent" for gloss reductions lower than 4% with respect to the average gloss value of the paints at 60°;

"sufficient" for gloss reductions lower than 8% with respect to the average gloss value of the paints at 60°;

"poor" for gloss reductions higher than 8% with respect to the average gloss value of the paints at 60°;

In determining the compatibility of the powder paints, the instrumental gloss value is purely indicative, whereas the visible appearance of the film is more determinant, as incompatibility causes a microstructure in the film, which the value measured by the glossmeter is often not capable of revealing. In addition to the instrumental measurement of the gloss, the compatibility of the paints was then evaluated by visibly determining the quality of a film obtained starting from a mixture of paints with respect to a reference sample. The reference sample is normally a plate painted by the client, which acts as visual reference of the minimum acceptable compatibility. Table 3 indicates the gloss data, the compatibility- degree of the different paint mixtures determined starting from the gloss values and a visual evaluation of the appearance of the film applied.

Table 3

The results of Table 3 show that the compatibility of the powder paints based on polyester resins obtained with the process of the present invention is higher than those of the powder paints based on polyester resins obtained according to the state of the art.

The powder paints V3 , V3-add and V6 proved to be absolutely suitable for electrostatic and/or triboelectric applications.