Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2003-04-28
2004-08-10
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S162000, C427S299000, C427S331000, C427S385500, C427S421100, C427S422000, C427S425000, C427S457000, C427S508000, C427S521000, C427S532000, C427S561000
Reexamination Certificate
active
06773761
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of applying a varnish, including a solvent or free of any solvent, on a component which may in particular be a vehicle headlight reflector.
BACKGROUND OF THE INVENTION
The majority of basic compositions of varnishes which are applied on components of the type that include vehicle headlight reflectors made from solid moulding mixtures such as BMC, CIC and so on, contain large quantities of solvents. The presence of these solvents is judged necessary in order to reduce the viscosity of the products applied and to improve the application performance, for example by spraying. During the spraying step and then during the period of removal of the solvents, the solvents evaporate wholly or partly. The time during which trapped air bubbles are able to escape and the varnish film is able to spread, depends on the viscosity of the products used and the nature and residual concentration of the solvents. The longer is the solvent removal time, the greater are the risks of increased flowing out of the varnish on the surface of the components, so giving rise to numerous rejects.
DISCUSSION OF THE INVENTION
The Applicant has accordingly set the particular objective of obtaining excellent performance of the application of a varnish on components of the vehicle headlight reflector type, while limiting its tendencies to “flow out” on the surface after application and without reducing the qualities of appearance in the base coating film.
This object is achieved by the present invention, which provides a method of applying a varnish, whether or not it includes a solvent, on a component, in particular a vehicle headlight reflector, characterised in that it comprises the following steps, namely: (i) selecting a varnish having a viscosity at ambient temperature in the range from about 500 mPa.s to 2000 mpa.s, and a viscosity at the application temperature which is lower than about 200 mPa.s, with a viscosity which is substantially independent of variation in the temperature within the range of application; (ii) heating the surface of the component before the step of moistening the varnish at a selected temperature &thgr;p
1
; (iii) heating the varnish at the moment of application to a selected temperature &thgr;v; and (iv) during the spreading step, maintaining the component at a selected temperature &thgr;p
2
for a selected time tp
2
.
The varnishes, whether or not they contain a solvent, generally consist of resins the viscosity of which is directly linked to temperature.
The method according to the invention, which may be performed equally well, whether or not the varnish contains a solvent, through the choice of appropriate varnishing temperatures, enables excellent application performance to be guaranteed, in particular by spray application. The selection of temperatures for the component to be varnished, before the application step and during spreading, also enables, in particular, the wettability of the varnish and its mobility to be controlled and optimised. In practice, the viscosity of the varnish achieves its optimum value at these selected temperatures. When applied on a hot surface, excellent wettability is obtained on the one hand, and removal of bubbles from the varnish is accelerated on the other hand. On cooling, viscosity increases rapidly so that the risks of flowing out are reduced.
Preferably, in the method according to the invention, step (ii) consists in heating by radiation, convection or conduction, for example using a forming tool, and step (iv) consists in heating by radiation or convection. This type of heating enables correct spreading to be obtained for the smoothest possible optical surface.
In a first embodiment in which the varnish includes a solvent, &thgr;v is about 20 to 40° C., &thgr;p
1
is about 20 to 50° C., and &thgr;p
2
and tp
2
about 20 to 50° C. and about 0.5 to 3 min, respectively.
By comparison with methods with traditional solvents, in which the solvent conventionally represents about 20% to 70% by weight of the overall varnish composition, in the method of the invention the solvent does not represent more than 10% by weight of the overall composition of the varnish used. Solvent removal times in the method according to the invention are therefore diminished overall, which greatly reduces the cost.
In a second embodiment of the method of the invention, the varnish being solvent free, the temperature &thgr;v is about 40 to 60° C., the temperature &thgr;p
1
is about 70 to 110° C., the temperature &thgr;p
2
is about 50 to 90° C., and the time tp
2
is about 0.5 to 1.5 min.
One of the major advantages of this embodiment is first that the varnish formulations are simplified and it is performed without solvents. Apart from their volatile nature, solvents have a toxicity level which may be more or less acceptable, and are a significant fire risk. These constraints usually make it necessary to have in place installations for treatment of the volatiles, together with fire fighting equipment in all the zones concerned. This embodiment therefore enables economies of investment and exploitation to be achieved by simplification of the plant, since the application zone is equipped with fire prevention means because of the need for cleaning the material. It also has the advantage of putting the plant into conformity with even stricter regulations in regard to the environment.
The complementary or alternative features of the method according to the invention are as follows:
The varnish is applied by spraying under pressure (“airless”), with a compressed air or electrostatic spraygun; preferably, the spraygun is equipped with a thermo-regulated flow loop, which enables the viscosity of the varnish to be reduced and enables the temperature &thgr;v and the varnish flow to be constant at the moment of spraying by the spraygun. Without the said loop, temperature control problems would arise because the spraygun works intermittently; the thickness of the varnish is obtained in a single pass of the spraygun: the varnish is deposited perfectly on the workpiece and no retouching is necessary; when the varnish applied by the spraygun is a solvent-free varnish, this enables major variations to be obtained in the thickness which are compatible with the complex geometry of the surface being covered, and this increases the robustness of the method; with identical thickness, two passes of the spraygun are in general necessary in the case where the varnish does have a solvent.
The varnish is a UV varnish: compared to so-called thermal varnishes, UV varnishes save a certain amount of time, because once the required appearance has been obtained the latter is “set” practically instantaneously, compared with times of the order of 5 to 10 minutes which are necessary with thermal varnishes. During these setting times, the atmosphere of the plant must remain very clean and “dust free”.
When applied on a component such as a BMC component, the method includes a step of pre-treatment with UV radiation at about 1 to 4 J/cm
2
with a maximum power of about 130 to 250 mW, measured in the UVA band and in a plane substantially at right angles to the mean radiation of the UV illumination zone. Effectiveness of treatment of the surface of the workpiece, and adherence of the varnish on the BMC are thus guaranteed.
The method includes a step of polymerisation step with UV radiation at between about 4 and 8 J/cm
2
, and preferably about 2 to 4 J/cm
2
with a maximum power of about 80 to 200 mW measured in the UVA band and in a plane substantially at right angles to the mean radiation of the UV illumination zone. In this way a brilliant spread-out surface, capable of being metallised, is obtained.
The orientation of the light radiation from each emitter, or the putting of the workpiece in motion, are optimised, which enables homogeneous illumination to be guaranteed regardless of the complexity of the exposed surface; in practice, the emitters are inclined so as to take the orientation of the faces of the component into account;
In the
Cote Frédéric
Montaudouin Michel
Morgan & Finnegan , LLP
Pianalto Bernard
Valeo Vision
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