METHOD OF COATING HEAT SENSITIVE MATERIALS WITH POWDER PAINT

Abstract: A method of coating heat sensitive materials with powder paint is described, which is based on repetitive heating cycles of paint coating by radiation for example infrared, interspersed with cooling cycles, for which there is a temperature difference between the paint coating and substrate such as to allow the filmation and subsequent curing of the paint without damaging the substrate.

Description: METHOD OF COATING HEAT SENSITIVE MATERIALS WITH POWDER PAINT. The present invention refers to a method by which it is possible to coat heat sensitive materials with powder paints. In the context of tn s report the term "heat sensi t e materials" means materials which undergo physical and/or cnemicai modifications when treated at temperatures and times currently used in filming processes and possibly c ring of paints in powder form, while "heat resistant materials" are tho e which oo not undergo t ese modifications. Powder paints a r e assuming an always greater importance fo ■ ■ coating metall ic or heat resistant objects of any type, given that these have el iminated numerous oroblems of en ironmenta pollution and danger to the healtn of workers employed in painting, other than forming coatings i th excellent qual ities. Powder paints, after having been appl ied to the objects by means of various systems, and today principally electrostati al ly, they must however be subject to melting, filmation and possibly curing to adhere permanently to the objects, and this generally takes place in kilns of various structure according to the objects to be coated, and always at rather high temperatures, made necessary by the melting temperatures of the paint components and maintaining these temperatures for continuous set times. However until today it has been considered impossible to coat heat sensitive materials with powder paints, such as plastic materials, wood, paper and cardboard, leathers, textiles and so on, because these materials absolutely cannot resist the temperatures necessary for powder paint treatment for a continuous time. On the other hand it would be highly desirable to be able to coat these materials with powder paints too, to eliminate the abovementi oned dangers and risks also in these industrial sectors due to the use made, currently necessary, of solvent-containing paints to coat the above mentioned heat sensitive materials. The structures and methods carried out until now for t ie treatment of objects coateo with powder paints do not however allow the treatment to be extended to heat sensitive materials which would be irremediably damaged. All the existing structures and methods are only suitable for metallic or heat resistent objects. In particular treatments by means of infrared radiation exist, but the temperatures and times of the treatment are prohibitive for heat sensitive materials. From German patent DE-34 06 789 Cl (Berkmann) a process is known for filming and curing objects coated with powder paints by infrared radiation, in which an intermediate neutral area is provided between two heating areas which allows a certain cooling of the objects during treatment, but also here the treatment temperatures and times in the two areas a r e such as to exclude any util ization for heat sensitive materials. In particular according to the details of the description of the above mentioned patent the temperature of the ooject is lowered only sl igntly, so that one can compensate for the temperature differences _ between thicknesses of different entity in the object. As a whole howe er, also in this case the temperature of the object remains excessive. Furthermore, the whole object is heated b ' means c f air circulation, so ething to be voided in the presence of heat sensitive materials. In W. Bruegel , Physik und Technik ό e r Ul trarotstranlung, Chapter F. Lackhaertung (pages 383-395) , Curt Vincsnt∑ Verlag, Hannover 1961, i t is proposed (page 394) to subject paints on wood to a "shock hardening", applying a strong irradiation for a brief period, without first heating the (wooden) base. However it is not explained how such a treatment is to be carried out. With solvent based paints, as opposed to that which occurs with powder paints, one is unl ikely to have spreading problems. In "Pulver + Lack", No. 2, 1979, page 110 and following, in particular page 112, the subdivision is proposed of the hardening of the powder paints in a "spreading phase", and a true and proper "curing phase". First, however, a low temperature is maintained and then the hardening temperature is immediately increased, which in practice does not produce a good spreading and exposes the material to high temperatures for a rather long period. The present invention for the first time resolves this inf ential problem and finally allows the coating of heat sensitive materials with powder paint, overcoming the prevention threshold imposed by the powder paint treatment temperature, which until now seemed absolutely impassable. The procedure according to the present invention consists essentially of treating the substrate (t.hat is t e object) and the paint applied to it at short alternate intervals, with short operation times at high temperature by means of radiation, such as for example medium or short waves in the infrared range and ith cooling times or passive zones of treatment interruption interspersed between subsequent operation times, the various parameters of the procedure being variable and adjustable in an optimum way as a function of the materials in play, in order that a perfect layer of paint is obtained, without damaging and/or worsening the properties of the painted material . In this way the heat necessary for the melting, filmation and curing if any, of the paint layer does not have time to attack the heat sensitive material of the substrate in a damaging way, in as much as the heat is dissipated during the cool ing intervals. It is however important that the heating speed is high, which is obtained with a high irradiation power. Preferably the paint layer is heated rapidly to a temperature at which thermal damaging does not yet take place, t en it is cooled to 3 suitable temperature for the curing of the paint and this temperature is maintained until the curing is completed. The first heating cycle is preferably interrupted as soon as the spreading of the paint has finished. The cool ing is preferably carried out each time at temperatures lower than 100 C. 2 IR irradiation powers of at least 40 kW/m are used. The parameters of the process, such as the heating time, the maximum temperature allowed, cool ing and so on, each time must be adapted to the substrate and the powder paint and determined by means of prel iminary tests. This physical action of alternation of short treatment and cool ing periods can be assisted by combining it with the incorporat on of additives in the powder paint composition, which in a certain way could function as a buffer zone with respect to the heat's action. One could also foresee the appl ication of an intermediate protective layer of materials suitable for protecting the substrate from the heat and which is at the same time compatible both with the substrate and the paint, in order to guarantee and preserve the perfect adhesion of the paint layer to the substrate. Moreover by means of the use of suitable additives, one could also think, for example in the case of synthetic thermoplastic or thεrmoset materials, of producing composite materials for the substrate, which could increase the instantaneous heat resistance. These additive substances could be both organic or inorganic and could for example include known thermo- insulating compounds such as iron oxides, silicon oxides, titanium fibres and so on. To ca r ry out the method according to the present invention one could construct apparatus in various manners and forms, provided that they fulfil the objective of obtaining alternation of i radiation and cooling times of the object to be treated. It is expecially important that the heating is rapid, which is obtained with high irradiation power. Thus for example an apparatus could provide an alternation of the treatment ana irradiation areas obtained by positioning the objects on a mobile support such as a conveyor belt, which passes under a series of narrow slits at intervals, from which the infrared radiation is directed on the objects, which thus pass in rapid succession through active treatment areas and passive cool ing areas. In this way continuous cycle treatment of the objects would be obtained. It is important only that the irradiation power is suffi iently high; the heating time will then be determined simply by the length of the action range of the lamp and by the velocity of the .conveyor. The cool ing occurs in an area free from the lamp and i f necessary can be assisted for exa pl e by cold air. Another form of apparatus could however provide a chamber in which the sources of infrared radiation are placed be ind mobile screens whic al ternatively cover and uncover the sources, in such a way that the objects are treated when the sources ar uncovered, and cooled when the sources are covered by the screens which could for example assume the form of rotating and oscillating slats in the manner of bl inds or 1 ouve rs . Other systems could provide the switching on or switching off of the irradiation sources at brief intervals, for example by means of a phase delay command, while the power could be regulated for example by a thyristor. The objects, characteristics and advantages of the method are still c1 ear r and more evident from the following detailed description of some specific exa plary embodiments, in which some practical values of some parameters which must be taken into consideration in the carrying out of the method a r e s u p p l i e d . Experiments of application of the method have been carried out on various substrates such as synthetic materials, wood and leather, after having chosen powder paints most suitable for such a treatment. As far as infrared raoiation is concerned, it has been found that the best results are obtained with short length infrared radiation in the range between about 0.76 and 2.0 jum, which permit more rapid heating and subsequent cooling of treated materials, as well as maximum speed of regulation of the heat sources, for which one can easily perform temperature and time cycles according to the requirements dictated by arious types of substrates and powoer paints. In the experiments an apparatus comprising various infrared radiators was used, each independently adjustable by an electronic circuit of control and phase delay regulation of the sinusoidal input tension and by measuring the temperature produced with a radiation pyrometer as regards the surface temperature of the paint and with thermo- elements positioned at various depths within the substrate to measure the changes of the temperature as a function of its thickness . By means of preliminary tests on the slabs of sheet- steel used as a reference it was determined that the optimum spreading of a powder paint is obtained if it is heated very rapidly to the maximum temperature at which there is no thermal damage of the paint coating, and naturally also of the substrate, then it is cooled immediately and subsequently the paint coating hardens or cures. Maintenance of the maximum temperature is not recommended, in that it does not br ng further ev dent improvements to the spreading, while damage to the painted surface beg i ns . However the following a r essential for the spreading of the paint; the granul o e ri c composition, the viscosity in the mel ted stage as well as the heating speed which as mentioned is optimum when the infra-red radiation is in the range of 0.76 to 2.0 Ai and especially around 1.2 uπι . Thus there is the experimental confirmation t at i f one attempts to make a fil of the powder paint on substrates of heat sensitive materials with only one heating cycl e, damage to the substrate and defects in the paint coating layer inevitably occur, while the temperature difference between the superficial paint coating and the substrate is too small for the said substrate to be protected from the effects of the temperature necessary for the powder paint treatment. In contrast to this, one also has confirmation of the feasibil ity and excellent results which can be obtained with the method of the present invention. In fact subjecting the materials to two or more shorter heating cycles with intermediate cool ing, one obtains strong temperature differences between the surface of the paint coating and the substrate, which can thus stand the thermal treatment much better . As regards some types of wood, some problems exist due to the emission of water or resin from the pores of the wood especially in the case of young timber and soft pulp, but the procedure is however feasible particularly by subjecting the material to various brief treatment cycles. For the moment experiments on the SMC (Sheet-Moulding Compound) have been concentrated on, carried out by means of moulding of unsaturated polyester resins and layers of fibreglass and utilised particularly to produce component parts of vehicle bodies, using a thermosetti g paint with a base of epoxy-pol yester resin. The experiments were carried out with SMC plates of 4 mm in which holes of various depth were made, starting from the lower side of the sheet, that is to say the opposite side to that on which the powder paint coating is applied. In , these holes Ni-CrNi thermoelements are affixed. By means of this adhesion one can determine exactly the measuring point of the material and furthermore there is a good heat transfer between the sheet material and the point of temperature measurement . γ e figures of the appended drawings show the most important characteristics of the experiment, and specifically Figure 1 sho s schematically the position of the IR radiator or the SP radiation pyrometer with respect to t e SMC sheet; Figure 2 shows schematical ly the position of the five ther oelements TE1, TE2, TE3, T and TE5 on the SMC sheet; Figure 3 is a graph showing the profile of the temperature in the SMC sheet during powder paint treatment with ^nrrared radiation according to the method of the present invention. The preparation thus carried out on the S-MC sheets therefore al lows the recordal of the upper surface temperature witn t e SP radiation pyrometer, in the SMC sheet at a de th of C.l mm (under the upper surface) with the ιerπιoε!ement TEI, at 1 mm with TE2, at 2 mm with TE3, at 3mm with TE a d at the lower surface of the sheet (4 mm) with TE5. The SP pyrometer and the thermoelements TE are cal ibrated and can be recorded directly. The measuring error is +. 1.5 e C. Al l the experiments are carried out with the paint coatings just appl ied, thus the absorption relationships are equal and the influences of the endothermic or exothermic cycles are constant. The repetitive treatment cycle is carried out as fol 1 ows : heating up to 240°C (of the paint coating layer) within 3 minutes, maintaining the temperature at 240°C for one minute, cooling for 10 minutes at 65 ° C, heating to 240 C, ma ntai nence at 24θ " C for 1 minute, cool ing. 2 The irradiation power is of 50 kW/m and the distance between the radiator and the sheet is 120 mm. As the graph of Figure 3 shows clearly, when a temperature 240° C is reached, the temperatures of the thermoelements register the following values: TE1 = 200° C, TE2 = 192 ° C; TE3 = 185°C; TE4 = 180 ° C; TE5 = 176 P C. It can therefore be seen that the temperatu e at the inside of the sheet is always lower than that of the paint layer, that the maximum temperature difference (40 C C) occurs right in the delimiting layer bet een the paint coating and the sheet, while the difference between the upper and lower- edges of the sheet is aoout 20 C, and furthermore that in the second heating cycle, even if the cooling does not reach room temperature, the temperature does not exceed that recorded during the first cycle. One can thus verify that by knowing the heat resistance of the substrate material , one can choose the maximum treatment temperature and the number of heating cycles necessary or possible. The period of maintaining the maximum temperature is the deciding factor for the feasiblity of the process and generally must not exceed 1 minute. It is in fact establ isneα that witn 2 minutes of irradiation no heat sensitive material manages to pass the test intact " and detachment of the paint coating, emission of gas and other defects take place. Moreover one must obviously choose a type of powder paint compatible with the substrate in order to avoid buooi e formation, dullness and other defects. By subdividing the treatment into more than two cycles, each of a shorter duration, one can reach a temperature di fference between the paint coating (SP) and the del imiting la er of the substrate (TE1) also notably greater with a noteworth increase of the possibil ities of the process's use for other materials also. Tnus furt a 1 - experiments were carried out to find a treatment cycle as advantageous as possi le for obtaining an opti u result. The powder paint layer is first heated it a high irradiation power as rapidl y as possible and at 3 hig temperature, maintaining this temperature for a very short time to reduce the thermal load. In this first so-called "physical" stage the paint layer must not cure, but only melt and thus in a brief time reach a very low dynamic velocity which causes a better spreading of the paint film with respect to the conventional slow heating process up to the curing temperature. Onl in the subsequent so-cal led "chemical" stage, carried out at an appreciably lower temperature, does the complete curing of the powder paint film occur, wherein this temperature corresponds at least to that of curing of the paint. The tests are carried out with a powder paint with very 5 rapid hardening, on degreased sheet steel , with a paint layer t ickness of 60 ± 10 μ π\ of har ened film and a heating to the temperature TA as rapid as possible. These parameters a re deduced from the table hεreinbelow. The measurements carried out on the samples are as follows: - h ickness of t e layer Cmagnetoinducti e method - Standard DIN 50981) 5 - ball fall ing test (Ericksen 304 type - height of fall 50 cm no damage ) - resistencs to acetone (no dissolving of the nardenεo paint 1 aye ) - brill iance ι " 60 - ref 1 εc ome t ε r value according to 10 Stanαarαs Dirt 67530, ASTM D 523) - sorsaαing accordin to arbit ary scale (points f om 1 = .oecular to 4 - lacking) - c-romatic deviation E (according to Standard DIN 6174) N ana the res l ts art reported in trie following tabl e, Radiator - sample distance 120 mm; p ' ower absorption of IR. radiator 3 x 1 W Test T^ IX- - 7g l\- profile Brill¬ Chromatid Spreadin iance ( • C/MC « °C/miπ) (sα β ) Shift ! Jg (Δ≤) 220/1 * 1 good a 1 • 80/10 £3-75 yes 87 0.5S 2,5 a 2 240/1 180/10 5S-70 good yes 86 2.0 2,5 a 3 25-3/1 - 180/ 0 52-61 good yes 83 2,34 3 H 240/30 - 180/10 £5-76 good y.es 85 2.24 2,3 -5 260/30 - 180/10 50-55 good yes 84 4,34 1,9 240/60 ■ 180/10 45-50 good yes S3 2.14 2 -7 250/60 • 180/10 53 • 7; good yes 81 <i,Q5 1,5 180/15 50-73 good yes 83 0,05 reference 2HSO, From the results of the table it is clear that by means of the subdivision of the process into a prel iminary "physical" and a subsequent "chemical " stage one obtains a noteably • better spreading of the paint film than with conventional methods . Thanks to the brief duration of the "physical" stage the thermal load of the paint layer and therefore the danger of chromatic alteration are notably reduced. One a r. provide a further improvement to the spreading °f the paint and a further decrease in the chromatic al teration again by optimising the parameters of the method, and in particular the heating velocity, the value of the f . , maximum temperature T , the duration period at T ,the A AX velocity T and the value of temperature T . Here also one S S can subdivide the "chemical" phase in several intervals of limited duration to obtain better results and avoid any thermal damage due to a too lonq exposure to temperature T . S

Claims: C L A I M S 1. Method of coating heat sensitive and non-sensitive materials, ith powder paint, cons sting of heating the paint coating ap p l ied to the substrate by means of radiation at a suitable temperature to cause the filmation and possible curing of the paint, if necessary maintaining this temperature for a brief p re-dete rm i ned period, carrying out cool ing to a lower temperature and if necessary repeating the above mentioned cycle one or more times. 2. Method according to claim 1, characterized in that the heating temperature of cycles subsequent to the first one is equal or less than that of the first cycle, but of a value nich allows the continuation of thε filmation and curing, if any, of the paint. 3. Method according to cl aims 1 or 2 characterized in that the layer of paint is heated rapidly to a temperature at which thermal damage stil l does not occur; in that then it is cooled to a temperature suitable for the curing of the paint and the fact that this temperature is maintained until the curing is f i ni shed . 4. Method according to one or more of the claims 1, 2 and 3, characterized by the fact tnat in the first cycle the heating is interrupted as soon as the spreading of the paint is completed. 5. Method according to one or more of the preceding claims, characterized in that each time cooling to c temperatures lower than 100 C is carried out. 6. Method according to one or more of the preceding claims, characterized in that IR irradiation powers of at 2 least 40 kW/ a r e used. 7. Method according to claim 1, characterized in that the infrared radiation is included in the range from 0.76 to 2.0 Aim and preferably is around 1.2 /urn. 8. Method of coating heat sensitive materials with powder paints, substantially as described hereinbefore, with particular reference to the illustrated exemplary embodiments and for the above mentioned objects.

Inventors: GAZZEA SERGIO, KROEGER ROLF, ORTLIEB KONRAD, GAZZEA, SERGIO, KROEGER, ROLF, ORTLIEB, KONRAD
Applicants: FRAUNHOFER GES FORSCHUNG, HOECHST ITALIA, FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWAND, HOECHST ITALIA S.P.A.
Application year: 1991
Main classification: B05D7
IPC codes: B05D3, B05D7