The invention relates to a method as described m the preamble of claim l, and to an apparatus as described in the preamble of claim 2.

For protecting the surface of glass objects, it is known to apply to such surface a coating of a metal oxide, for instance tin oxide. To such a layer organic lubricants will adhere and the combined coating of metal oxide and organic lubricant has been found to be particularly effective in preventing scratches on the surface of the glass objects. Although this treatment method is generally applicable to any glass objects, it is employed in particular to treat glass containers such as bottles. For that reason, hereinafter a glass object will also be designated by the term "bottle", but it will be clear that the scope of the invention is not limited thereto.

When carrying out the treatment mentioned by means of chemical vapor deposition (CVD) , the bottles have a temperature of about 500 to 650 °C. The treatment is therefore normally carried out directly upon the molding process of the bottles, in which they have acquired their final form at such a temperature or even a higher temperature. The hot bottles are passed on a conveyor through a coating tunnel in which a suitable atmosphere is present. An example of such an atmosphere is a carrier gas which has been mixed with a suitable chemical substance, for instance a suitable metal chloride m vapor form, for instance tin tetrachloride, monoalkyl tin trichloride or titanium chloride. The carrier gas is normally air. On the hot surface of a bottle, the coating-forming chemical substance reacts to metal oxide and on the bottle surface a coating is formed having a thickness normally m the range of 5 to 80 nm. To the human eye, such a

coating having a thickness of about 5 nm is practically invisible; the presence of the thicker films can be perceived as a metallic light reflection or "gloss" .

US-A-4, 389,234 describes a method and apparatus as described in the preamble of claim 1 and claim 2, respectively. The bottles are conveyed through a series of flows or jets which consist of the gas mixture mentioned and are directed transversely across the conveyor track. The composition of the different jets may be different: the publication describes an example where the concentration of the coating-forming substance is greater at the central jets than at the outer jets. The system of jets s flanked by gate- shaped gas discharge stations for discharging carrier gas with unreacted coating-forming chemical substance.

The f lm deposition rate is a function of inter alia the concentration of the film-forming substance, the gas velocity and the surface temperature of the bottles. Because ot the necessity for rather high gas velocities within the tunnel, there is a large extent of exchange between ambient air and the gas in the tunnel owing to the outer gas jets tending to mix with the outside air. A first consequence thereof is that a part of the coating-forming substance is withdrawn from the process, which adversely affects the environment and entails financial loss. A second consequence is that outside air penetrates the outer jets and thus causes an undesirable dilution of the outer jets, which must be compensated by an increased supply of the coating-forming substance.

This problem is already recognized m US-A-4,389,23 . The publication tries to offer a solution to this problem by dimensioning the outer jets m such a manner that they form a gas curtain. For a satisfactory result, however, a laminar flow is needed and the narrow blow openings needed for that purpose tend to clog up with solid particles formed in the carrier gas from the coating chemicals.

The object of the invention s to remove the disadvantages mentioned. More particularly, the object of the invention s

to provide a coating apparatus in which the extent of exchange between ambient a r and the gas in the tunnel is reduced to a far-reaching extent.

To that end, a method of the above-mentioned type is characterized according to the invention in that at the inlet and/or the outlet, preferably at both, of the coating tunnel, a separation is effected between the atmosphere in that tunnel and the atmosphere outside that tunnel, which separation consists of at least one gas curtain which is fed with a chemical-free gas, preferably fresh ambient air.

In a preferred embodiment, the width of the gas curtain, measured in the direction of conveyance of the bottles, is greater than the diameter of the bottles to be treated.

These and ether aspects, features and advantages of the present invention will be clarified by the following description of a preferred embodiment of a coating apparatus according to the invention, with reference to the drawing, in which:

Fig. l schematically shows a top plan view, partly cutaway, of a conventional coating apparatus;

Fig. IA schematically shows on a larger scale a detail of the apparatus shown in Fig. l;

Fig. 2 schematically shows a side elevation of the apparatus shown in Fig. I; Fig. 3 schematically shows a cross section of the apparatus shown in Fig. i, along the line A-A of Fig. 1;

F g. 4 schematically shows on a larger scale an adaptation for the tunnel shown in Fig. l;

Fig. 5 schematically shows a top plan view of a coating apparatus according to the invention;

Fig. 6 schematically shows a top plan view of a gas curtain screening apparatus according to the invention;

Fig. 6A schematically shows on a larger scale a detail of the apparatus shown m Fig. 6. Fig. 7 shows a longitudinal section of a blow tube;

Fig. 8 shows a front v ew of a blow tube; and

Fig. 9 shows constructional details of a variant embodiment of a gas curtain screening apparatus according to the invention. In the drawings, equal or comparable parts are designated by the same reference numerals. For a detailed description of some examples of a coating apparatus, reference may be had to the above-mentioned publication US-A-4,389,23 . For an explanation of the general construction and operation of a coating apparatus, reference may be had to Figs. 1-4, where a conventional coating apparatus is generally designated by reference numeral l. The apparatus l comprises a coating tunnel 2 which has sidewalls 3, 4 and a, preferably vertically adjustable, ceiling 5, as best identifiable in Fig. 3. The bottom of the tunnel 2 is defined by a conveyor 6 which is supported by a frame, not shown for the sake of simplicity. The conveyor 6 carries bottles 10 through the tunnel 2 in a direction as designated by the arrow Pi, in Fig. 1 from right to left. The bottles 10 are represented only diagrammatically in Fig. 1 by circles. Arranged along the sidewalls 3, 4 of the tunnel 2 are blow nozzles 31, 41. Each blow nozzle 31, 41 has at least one blow opening 32, 42, which terminates in the sidewall 3, 4, as well as an inlet opening 33, 43. Further arranged along the sidewalls 3, 4 of the tunnel are exhaust nozzles 36, 46. Each exhaust nozzle 36, 46 has at least one suction opening 37, 47, which terminates in the sidewall 3, 4, as well as a discharge opening 38, 48. The inlet openings 33 of the right-hand (as viewed in the direction of travel Pi) blow nozzles 31 are connected via a common blow duct 52 with a blow outlet of a first blower 51. The discharge openings 38 of the right-hand exhaust nozzles 36 are connected via a common exhaust duct 53 with a suction inlet 55 of the first blower 51, which blows out the drawn-in gas mixture via its blow outlet 54. In comparable manner, the left-hand blow nozzles 41 are connected via a common blow duct 57 with a blow outlet of a second blower 56, and the left-hand exhaust nozzles 46 are connected via a common exhaust duct 58 with a suction inlet of the

second blower 56. The blow openings 32 of the right-hand blow nozzles 31 are arranged opposite the suction openings 47 of the left-hand exhaust nozzles 46, and the blow openings 42 of the left-hand blow nozzles 41 are placed opposite the suction openings 37 of the right-hand exhaust nozzles 31. In the embodiment shown, in each sidewall four blow nozzles and four exhaust nozzles are provided, the blow nozzles being all four arranged next to each other and the exhaust nozzles being all four arranged next to each other, always at some distance from each other.

The gas flow thus induced is designated with arrows P2. It is clear to see in this figure that this gas flow at the location of the tunnel 2 is directed transversely across the conveyor 6, substantially at right angles to the direction of conveyance PI, as indicated more clearly by the arrows P3 and P4 in Fig. IA. Further, it can be seen in the figure that a bottle 10 entering the tunnel 2 meets in the first half of the tunnel 2 a gas stream (P3) coming from the left and in the second half of the tunnel 2 a gas stream (P4) coming from the right. It is also possible, however, that the blow nozzles and the exhaust nozzles are arranged in alternation, so that the bottle 10 meets gas streams alternately from the left and from the right, as illustrated in US-A-4,389,234.

It is noted that Fig. IA is only diagrammatic and not drawn to scale. More particularly, in practice the length of the nozzles (measured in the direction of the flow P3 and P4) will be greater than the width of the tunnel 2. It is further observed that the walls between the successive nozzles are hollow and that air can be blown therethrough for cooling. The gas flow referred to is formed by a carrier gas such as air, having included therein a substance (precursor) capable of generating a desired coating. An example of such a substance is a suitable metal chloride in vapor form, for instance tin tetrachloride, monoalkyl tin trichloride, or titanium chloride. The bottles 10 entering the tunnel 2 have just left a forming machine, not shown for the sake of simplicity, and have a fairly high temperature, typically in

the range of 500 to 650°C. The gas circulating in the apparatus l is warmed up by these hot bottles to a temperature of about 70 to 250°C.

The apparatus 1 comprises a first gas discharge station 61, arranged before the inlet of the tunnel 2, which may for instance have the construction as described in US-A- 4,389,234 and serves to prevent the chemical substance from leaving the tunnel 2 in uncontrolled manner to contaminate the ambient air. Similarly, the apparatus 1 comprises a second gas discharge station 62 arranged beyond the outlet of the tunnel 2. The gas extracted by the gas discharge stations 61 and 62 can be stripped of the unused coating-forming chemical substance entrained therein, which can subsequently be removed in controlled manner. Such an apparatus typically consumes a few kilograms of the coating-forming chemical substance a day. This consumption is compensated by the supply of the coating-forming chemical substance to the gas stream in a manner known per se, such as for instance by the introduction into the suction inlet of one or both of the blowers 51, 52. The coating-forming chemical substance evaporates in the carrier gas because of the relatively high temperature thereof.

In a practical embodiment, the jets blown into the tunnel 2 have a width of, for instance, 5 cm and a flow velocity of 5 m/sec. For the outer jets such parameters mean that they attract much ambient air into the tunnel 2. This has the disadvantages already mentioned, such as a dilution of the gases in the tunnel 2 and the lowering of the concentration of the coating-forming chemical. Fig. 4 shows a detail of the apparatus 1 in the situation where the measure proposed in US-A-4,389,234 to solve this problem has been used therein. Fig. 4 shows on an enlarged scale the left-hand sidewall 4 of the tunnel 2 adjacent the inlet thereof. More particularly, Fig. 4 shows the first three blow nozzles 41i, 1 ₂ , and 41 ₃ . The second and third blow nozzles 41^ and 41 ₃ have inlet openings 43 ₂ and 43 ₃ , respectively, whose dimensions have been chosen such that at a

particular pressure in the common blow duct 57, the above- mentioned parameters of the jets set themselves, which jets are designated by the arrows P3. The first blow nozzle 4lχ has an inlet opening 43ι of smaller dimensions, in such a manner that the outflow velocity of the jets is smaller, for instance 3 m/s, as illustrated with the smaller arrows P3' .

In a practical example, the tunnel 2 has a width of about 15 cm, and the speed of travel of the conveyor 6 is about 0.3 to 1 m/s. Then a jet speed of 2 to 3 m/s is required to ensure that the jet reaches the other side of the tunnel 2. In this connection it is desirable for a good screening action that the flow of the outermost jet be laminar since then the mixture with the environment is minimal. For that purpose, a jet with a Reynolds number of less than 2300 is desired. Herein the term "outer jet" is understood to mean the jets located closest to the inlet and outlet of the tunnel 2, as for instance the jet P3' furnished by the first blow nozzle 4lι.

In this example, in good approximation the following parameters apply to the gas in the jets: composition air; temperature 100°C; specific mass 0.95 kg/m* ³ ; viscosity 2.2-10" ⁵ Ns/m ² . For a round blow opening and a gas velocity of 3 m/s, it can then be calculated that the diameter of the jet may be 18 mm at a maximum. If the blow openings have an elongate slotted form, it holds that the equivalent diameter is approximately equal to twice the width of the slot; in that case the width of the slot may therefore be 9 mm at a maximum.

Because of the relatively high rate at which the surfaces of coating tunnels are soiled, such openings would have to be cleaned too often, which is undesirable.

According to the invention all of the disadvantages mentioned are overcome by the use of a gas curtain which is fed with pure gas. In the context of the present invention,

the expression "pure gas" is understood to mean: a gas to which the coating-causing chemical substances mentioned in the foregoing have substantially not been added. An example is nitrogen, or fresh ambient air, which may or may not be dried to a greater or less degree. Since the use of ambient air is preferred, the gas curtain referred to will hereinafter be designated as an air curtain.

By the use of a pure gas, preferably pure air, the advantage is achieved that the outflow openings will not clog by gathering particles entrained in the air stream.

Preferably, the air curtain is operated in such a manner that the Reynolds number of the flow is m the laminar range or m the transitional range. With regard to the transitional range, reference is made to the definition as used m the publication "VDI Warmeatlas", 6. Auflage, 1991, Chapter Lbl, viz. a flow having a Reynolds number between 2300 and 8000. At Reynolds numbers greater than 8000, the flow is turbulent; at Reynolds numbers smaller than 2300 the flow is laminar. When the flow is operated as a "transitional flow", advantages over the prior art are already achieved.

Preferably, the flow is operated as a substantially laminar flow, because of the particularly small extent of gas exchange with the environment.

A coating apparatus improved in accordance with the invention is diagrammatically illustrated in Fig. 5 and designated by the reference numeral 100. This apparatus is also referred to by the term "hood" . Designated centrally in this figure by the reference numeral 110 is a reaction gas circulation section of the hood 100. The construction of this so-called "active" section of the hood 100 does not form a subject of the present invention and knowledge thereof is not necessary for a skilled person to properly understand the present invention. Therefore, this construction w ll not be described m detail. Suffice t to note that this construction may be the same as constructions which per se are already known, for instance the construction as described m US-A-

4,389,234 or as discussed in the foregoing with reference to Figs. 1-4, but excluding the gas discharge stations 61 and 62; the active section 110 therefore comprises only that section of a coating apparatus 100 as defined by the blowers 51, 52, the blow ducts 52, 57, the exhaust ducts 53, 58, the blow nozzles 31, 41, and the exhaust nozzles 36, 46, as is designated by means of a dotted line in Fig. l for the sake of clarity.

In accordance with the invention, the improved coating apparatus 100 comprises a gas curtain screening apparatus 111 arranged in front of the inlet of the tunnel 2. In Fig. 5 it is indicated that there may be some distance between the active section 110 of the hood 100 and the gas curtain screening apparatus 111, in which case a gas-tight connecting tube 112 connects the active section 110 of the hood 100 and the gas curtain screening apparatus ill with each other. However, the gas curtain screening apparatus ill may also be arranged directly against the active section 110 of the hood 100 with a gas-tight connection, permitting omission of the connecting tube 112.

In fact, the gas curtain screening apparatus 111 can be considered as a replacement for the first gas discharge station 61 in the apparatus of Figs. 1-4.

The embodiment of the improved coating apparatus 100 according to the invention illustrated in Fig. 5 further comprises a second gas curtain screening apparatus 113 arranged beyond the outlet of the tunnel 2. The construction thereof may be the same as that of the first-mentioned gas curtain screening apparatus 111, whose construction will be described in more detail hereinafter. This second gas curtain screening apparatus 113 can in fact be considered as a replacement for the second gas discharge station 62 in the apparatus of Figs. 1-4. According to the invention, it is preferred that the coating apparatus 100 comprises two gas curtain screening apparatuses 111 and 113, as shown, but the presence of either of the gas curtain screening

apparatuses 111 or 113 already provides an improvement over the prior art.

Now the construction of the first-mentioned gas curtain screening apparatus 111 will be described in more detail with reference to Figs. 6-10. The gas curtain screening apparatus ill comprises a vertically arranged blow tube 120 and an exhaust tube 160, also arranged vertically. The blow tube 120 has at one end - the top end 121 in the case shown - an inlet connection 122 for receiving ambient air. The inlet connection 122 can be suitable for a connection to a blow outlet of a blower drawing in ambient air, not shown for the sake of simplicity. Preferably, means are provided for preheating the ambient air supplied to the inlet connection 122. In an embodiment, such air preheating means are provided in that the ambient air is passed through a double wall (not shown for the sake of simplicity) in the ceiling 5 of the tunnel 2 before that air is passed to the inlet connection 122. An additional advantage of such an embodiment is that the ceiling 5 of the tunnel 2 is cooled by the air passed through. Provided in an outlet sidewall 123 of the blow tube 120 are openings 124 which connect the blow tube 120 with curtain blow nozzles 130 mounted against that outlet sidewall 123. Each curtain blow nozzle 130 has a substantially box-shaped configuration with two substantially vertically arranged sidewalls 131. The sidewalls 131 are substantially perpendicular to the direction of travel Pi of the conveyor 6.

With regard to each curtain blow nozzle 130, the openings 124 can consist of a series of round openings provided vertically above each other, such as bores in the outlet sidewall 123 of the blow tube 120. The openings 124 can also consist of one or a few vertically oriented slots in the outlet sidewall 123 of the blow tube 120. It is also possible that, with regard to each curtain blow nozzle 130, a few openings are provided next to each other. With a view to lowering the gas flow velocity m such a manner that a laminar gas flow can be maintained, the transverse dimensions of the openings 124, in accordance with the concept of the invention,

can be chosen sufficiently small with no risk or only a minor risk that those openings 124 will be soiled and/or clog up, since the gas passed therethrough consists of ambient air, which is substantially free of additive material. The ends of the sidewalls 131 of each curtain blow nozzle 130 remote from the blow tube 120 define an outflow opening 132 of the curtain blow nozzle 130. The outflow opening 132 terminates in a vertical wall 134 which is directed parallel to the direction of travel PI of the conveyor 6, and which is positioned at only a slight distance from that conveyor 6. This vertical wall 134 can be an extension or a part of the sidewall 4 of the tunnel 2.

Each curtain blow nozzle 130 is constructed such that an air flow leaving the curtain blow nozzle 130 is at least substantially laminar. In an illustrative exemplary embodiment, the walls 131 can be parallel to each other, and may have a mutual distance of about 10 mm. A preferred embodiment of a curtain blow nozzle 130 to ensure the effect mentioned will be discussed later on with reference to Fig. 9. For providing an adequate air curtain 141, it is preferred that several curtain blow nozzles 130 are present next to each other, in the embodiment shown, to that end, three curtain blow nozzles 130 are grouped to form a blow nozzle group 143. During the use of the coating apparatus 100, bottles 10 are conveyed through the tunnel 2 by the conveyor 6. This implies that with some frequency a bottle 10 is passed through the air curtain 141. The laminar flow m the air curtain 141 is thereby disturbed, so that the screening action of the air curtain 141 is reduced or even cancelled. To reduce this problem, the number of curtain blow nozzles 130 m a blow nozzle group 143 can be chosen to be so large that the width of the curtain 141 (measured m the direction Pi) is greater than the diameter of the bottles 10, preferably even greater than about 1.5 times the diameter of the bottles 10.

Fig. 6, however, illustrates another solution, in the preferred embodiment illustrated m Fig. 6, a second blow

nozzle group 144 is present, which may be identical to the first-mentioned blow nozzle group 143, for generating a second air curtain 142. In this connection, the mutual distance of the two blow nozzle groups 143 and 144 is preferably chosen in relation to the mutual distance between the bottles 10 on the conveyor 6. In Fig. 6 the mutual distance between the centerlines of the air curtains 141 and 142 is approximately equal to half the mutual distance between the centerlines of the bottles 10 on the conveyor 6, so that a situation is realized where always at least one of the two air curtains 141 and 142 blows between the successive bottles 10 substantially without being disturbed.

In a preferred embodiment, at least one of the blow nozzle groups is detachably mounted on the blow tube 120, and the position thereof in the direction of conveyance PI can be varied in order to enable adjustment of the mutual distance between the centerlines of the air curtains 141 and 142 to a changing mutual distance between the centerlines of the bottles 10 on the conveyor 6.

On the other side of the conveyor 6, opposite the tube 120, a tube 160 is arranged, in a sidewall facing the conveyor 6, the tube 160 has inlet openings 161, 162 for receiving the air flow of the air curtains 141 and 142. At its top end the tube 160 comprises an outlet connection for a blower which discharges the air received in the tube.

The inlet openings 161, 162 of the tube 160 are connected via funnel-shaped guides 163, 164 with a vertical wall 165 which is directed parallel to the direction of travel Pi of the conveyor 6 and is positioned at only a slight distance from that conveyor 6, opposite the first-mentioned vertical wall 134. This vertical wall 165 can be an extension or a part of the sidewall 3 of the tunnel 2.

Each funnel-shaped guide 163, 164 is substantially aligned with the respective blow nozzle groups 143 and 144 to receive the respective air curtains 141 and 142 and to guide them to the inlet openings 161, 162 of the tube 160. The number of

funnel-shaped guides preferably corresponds to the number of blow nozzle groups. If the blow nozzle groups 143, 144 are adjustably mounted on the tube 120, preferably the funnel- shaped guides 163, 164 are adjustably mounted on the tube 160 m a comparable manner.

The capacity of the exhaust system 160 is preferably chosen to be so large that per unit t me more air is drawn in than is blown out by the blow nozzle groups 143, 144. The funnel-shaped guides 163, 164 are preferably oriented n such a manner that the excess of gas being drawn m preferentially originates from the external world, viewed with respect to the apparatus 100, m other words, the excess of gas sucked in mainly consists of ambient air, as indicated by the arrow P5. What is thus achieved is that the amount of gas able to escape from the tunnel 2 to the environment is minimized.

In the exemplary embodiment shown, the air curtains 141, 142, as stated, are directed substantially at right angles to the direction of conveyance Pi. However, depending on ambient conditions, m particular the flow of the ambient air along the hood, it may be desirable, rather, that the air curtains 141, 142 make a certain angle with respect to the right-angled direction. To be able to optimally adjust the direction of the air curtains 141, 142 to the conditions such as they prevail at the location where the gas curtain screening apparatus ill is to be installed, preferably the blow nozzles 130 and/or the funnel-shaped guides 163, 164 are adjustable as to direction.

In order to ensure that each air curtain 141, 142 extends over the full height of the tunnel 2, which height m a practical example may be about 25 cm, each tube 120, 160 has a height which is at least as great, and the vertical walls 131 of the blow nozzles 130 and those ot the funnel-shaped guides 163, 164 reach at least to the height mentioned.

The inlet openings 124 for the blow nozzles 130 are preferably so dimensioned that the pressure drop across these openings is much higher than the dynamic pressure of the a r

flow n the tube 120 as measured at a position a few centimeters before those openings 124. Thus a uniform outflow over the full height is achieved. If the inlet openings 124 are dimensioned as circular holes of a diameter of 5 mm, there is a substantial chance that the flow in these holes will become turbulent at the highest permissible laminar outflow velocity at the outlet 132 of the blow nozzles 130. This means that the length of the walls 131 of the blow nozzles 130 (measured m the direction of the flow through the blow nozzles 130) would have to be chosen to be rather large to convert the air flow from turbulent flow at the inlet openings 124 to laminar flow at the outlet openings 132. It s therefore preferred to dimension the inlet openings 124 as elongate slots of a height of about 40 mm and a width of about 1.5 mm, because then the flow in the inlet slot 124 remains laminar as long as the flow at the outlet 132 of the blow nozzles 130 is laminar. This means that the length of the walls 131 of the blow nozzles 130 may be chosen to be fairly small. Fig. 9 illustrates a preferred embodiment of a curtain blow nozzle 130 which is constructed m such a manner that a laminar flow behavior of an air curtain is ensured to a far- reaching extent. In this preferred embodiment the sidewalls 131 of each nozzle 130 are not directed parallel to each other but are arranged so as to diverge, m such a manner that the width of each nozzle 130 increases from the inlet 124 towards the outlet 132. The sidewalls 131 can include an angle with each other m the range of about 5° to 10° and may have a length of about 50 to 100 mm.

It will be clear to a person of ordinary skill in the art that it is possible to change or modify the embodiment of the apparatus according to the invention shown, without departing from the concept of the invention or the scope of protection. Thus it is for instance possible that the air in the screening curtain is preheated to much higher temperatures m order to keep the surface temperature of the bottles high. It is also

possible that the blow and exhaust nozzles of the screening jets are adjustable as to position along the conveyor and/or as to direction, to enable adjustment of the apparatus to local conditions, such as air flows in the environment of the tunnel, for minimizing any mixture of tunnel air with curtain air and/or to enable adjustment of the apparatus to other bottle diameters.

Also, the number of nozzles per group, and/or the number of groups of nozzles can be chosen differently. It is then possible that different tubes are present for different groups of nozzles. It is also possible that the mutual distances between the nozzle groups are mutually different.

Further, it is foreseen that the amounts of curtain air and discharged air are measured and adjusted, for instance by hand or with automatic means.