WO2004036114A1

WO2004036114A1 - Headlamp reflector made of a polymer composite and to be used in a vehicle

Info

Publication number: WO2004036114A1
Application number: PCT/SI2003/000037
Authority: WO
Inventors: Ales Kralj; Mihael Trcek; Ales Adamlje; Bostjan Smuc
Original assignee: Hella Saturnus Slovenija doo
Current assignee: Hella Saturnus Slovenija doo
Priority date: 2002-10-21
Filing date: 2003-10-17
Publication date: 2004-04-29
Anticipated expiration: 2005-04-21
Also published as: AU2003269793A1

Abstract

A reflector according to the invention is made of a thermoplastic polymer composite having a thermal conductivity transversely with regard to the reflector shell of above 0.5 W/mK and below 3 W/mK. Advantageously, the polymer composite reflector is slender in the sense of its geometry as well in the sense of the slenderness criterion at the cast injection molding. The polymer composite is made of any thermoplastic polymer of the group mentioned and as a thermally as well as electrically conductive filler a mixture of carbonaceous fibres and grains may be used. The reflector of the invention distinguishes itself by its low weight and price.

Description

Headlamp reflector made of a polymer composite and to be used in a vehicle

The invention concerns a headlamp reflector to be used in a vehicle, said reflector being made of a polymer composite. The headlamp is a small one having high luminous intensity and especially its reflector is thermally stressed. By the invention a slender reflector made of a thermally as well as electrically conductive polymer composite is proposed.

Modern large reflectors to be used in headlamps of road vehicles are produced mainly from a geometrically stable polymer composite (BMC) having a low thermal conductivity, which is made on the basis of a thermosetting unsaturated polyester resin. So a BMC reflector can withstand a temperature up to 200 °C, yet in a small BMC reflector the temperature exceeds this value. Therefore a small BMC reflector can not be used in said headlamp. The BMC reflector must be lacquered before the deposition of a reflecting layer. Since the reflector cast is brittle, the lacquering process is demanding and the scrap is high when a smooth cover glass is used. The BMC reflector is not recyclable, which aspect makes it less desirable as a road vehicle component.

Small reflectors made of cast metals like aluminium or magnesium or of a drawn steel sheet have been known. They are used when the temperature resistance of BMC reflectors is exceeded. A lacquer powder is electrostatically deposited on the metal and the lacquer layer is coated by a thin reflecting layer. Such small reflectors represent the backbone in the production of thermally stressed small reflectors. They have high heat conductivity; therefore below a light source no cold zone appears (this does not apply to drawn steel sheet reflectors because of their thin wall), where a high rate condensation of vapours of materials present within the headlamp would take place. Such reflectors are expensive and the aluminium ones are also massive. It is difficult to achieve the adhesion of the lacquer to metal. The life of the reflector is limited due to corrosion.

There have been known low weight, non-lacquered medium size and small reflectors with lower demands with regard to smoothness and appearance of the reflecting surface. They are made of amorphous thermoplastic polymers like polyetherimide (e. g. ULTEM) and polyethersulfone, both having a low thermal conductivity. Therefore, the reflector may even melt down in a hot zone, where a convective hot air column rising past the light source acts upon the reflector, whereas in a cold zone below the light source a condensation of vapours of materials present within the headlamp takes place. The reflector is recyclable. However, it has not been accepted as a large head lamp reflector because of the high price material. Also its appearance is rather ordinary.

In the prior patent application WO 03/021623 Al the applicant COOL OPTIONS, INC., USA, discloses a polymer composite reflector having a thermal conductivity above 3 W/mK and preferably above 22 W/mK. It is a small reflector that has to withstand a high temperature. It is recyclable and may be applied where a metal reflector is not appropriate because of corrosion and there are no high demands on the surface quality. The reflector shell can not be slender because the polymer composite thermal conductivity is high, and the polymer composite is expensive. The reflecting surface quality is low. Therefore the reflector is not appropriate to be used in a free-form headlamp. Further, low weight lacquered reflectors have been known, which are made of a partly crystalline thermoplastic, e.g. polyphtalamide, which has low thermal conductivity, but can withstand higher temperatures. Therefore the reflector does not melt in the hot zone, but in the cold zone a condensation of vapours of materials present within the headlamp takes place. The reflector has a moderate price. Nevertheless, it has not been accepted, since a special technology would have to be developed for its possible lacquering.

There should be mentioned polymer composites, whose thermal conductivity is increased by means of included fillers. These polymer composites are used for casings of electronic components due to heat dissipation. They are electrically insulating. A normal thermal conductivity value up to 5 W/mK is achieved by a combination of fillers like ceramic powders and ceramic fibres or small ceramic sticks, e.g. alumina, aluminium nitride, boron oxide and boron nitride, but at an exceptional price since such fillers are very expensive and have a rather high density. Hence their application in headlamp production is unfavourably influenced by the extremely high price and great weight, which is comparable to the weight of metal reflectors. And also the surface of the cast is matt due to the fillers, which results in a poor reflectance of the reflecting layer deposited thereon.

There should also be mentioned polymer composites, whose thermal and electrical conductivities are increased by their fillers. They are used for radio-frequency interference suppressing casings as well as where it is important to prevent an accumulation of electrostatical charges. A thermal conductivity of 20 W/mK is achieved by filling up approximately 40 % of the final composite volume by a filler like a mixture of thermally and electrically conductive carbonaceous powders and thermally and electrically conductive carbonaceous or, exceptionally, metal fibres or small sticks. The said high level of the filling up with the filler represents the limit to an injection moulding processing, since above it the flowing is not satisfactory any more, and also, due to the fillers, the cast surface is matt, which results in a poor reflectance of the reflecting layer deposited thereon. The polymer composites with a thermal conductivity of 20 W/mK that are available on the market are too expensive and their processing is difficult due to their high thermal conductivity.

The use of said polymer composites only belongs to the general state of the art. Specialists in the field of the automotive illuminating engineering have started to become acquainted with these materials, but, for the said reasons, the experiments have mainly been unsuccessful.

The technical problem to be solved by the present invention is how to make, at a low price, a high quality reflector, which may be also a slender one, from a polymer composite so that the reflector operating temperature as well as its temperature gradient will be as low as possible and that, when required, at the same time it will be possible to lacquer the polymer composite cast also by electrostatical lacquer powder deposition, a technique that has so far been accepted at metal reflectors.

According to the invention the said technical problem is solved by a reflector made of a polymer composite and having the features cited in first claim.

The preferred embodiments of the reflector according to the invention, however, are characterized by the features from dependent claims.

The invention proposes an elegant solution of all major problems in the field of polymer composite reflectors for small, thermally heavy stressed headlamps to be used in vehicles. The reflector made of a recyclable thermoplastic polymer composite distinguishes itself over metal reflectors by its low weight, low price, both especially when it is slender, and non-corrosiveness, and over polymer reflectors by not being susceptible to melting in the hot zone, also the appearance of a hazy spot in the cold zone is weakened.

A thermoplastic polymer composite having a moderate thermal conductivity below 3 W/mK may be used, which on the one hand, because of a lower portion of expensive fillers, is cheaper than one having a higher thermal conductivity and, on the other hand, it makes possible the production of a reflector with a slender shell and, thus, with less material.

The reflector can be lacquered according to the accepted technique of electrostatical lacquer powder deposition.

At the production of the ellipsoid reflector the deposition of the reflecting layer is performed immediately onto the surface of the polymer composite cast at the expense of the reduction of the reflectivity to a certain degree.

By the invention also a fairly good quality immediate deposition of the reflecting layer onto the reflector double-layer-cast is proposed.

The invention will be now explained in more detail by way of the presentation of the technical solution of the invention and the description of an embodiment.

A reflector of the invention for a headlamp to be used in a vehicle is made of a thermoplastic polymer composite having a thermal conductivity, transversely with regard to the reflector wall, of at least 0.5 W/mK and below 3 W/mK. At the transverse thermal conductivity of about 3 W/mK, the filling up of a mould cavity, which is provided for the production of slender reflectors, gets difficult since the molten mass of the polymer composite solidifies too fast.

The preferred embodiment of the invention relates to the polymer composite reflector which is slender in the sense of its geometry as well in the sense of the slendemess criterion at the cast injection molding.

The reflecting surface of the reflector cast is lacquered and provided with a reflecting layer.

In order to obtain a satisfactory lacquer adhesion to the polymer composite of the cast reflector of the invention and also in order to remove the cast from the mould cavity, the polymer composite must have a surface tension of about 50.10^"3 N/m and below 60.10^"3 N/m.

The polymer composite is made of any polymer of the thermoplastic polymer group comprising polyphenylene sulfide, polyphtalamide, polyethylene terephthalate, polyamide 6, polyamide 66, polyamide 6T and polybutylene terephthalate, and of the thermally as well as electrically conductive fillers, which are carbonaceous or metal materials. The appropriate carbonaceous filler is a mixture of carbonaceous fibres and carbonaceous grains without an emphasized elongation.

The required mechanical properties and the coefficient of thermal expansion are achieved by adding glass fibres.

If the polymer composite has an electrical conductivity of at least 10^"5

the reflector of the invention in its preferred embodiment is lacquered by the electrostatical technique. The reflecting surface of the reflector cast is lacquered by electrostatical lacquer powder deposition.

The reflector of the invention conducts the heat so well that no melting occurs in the hot zone nor is any such low temperature achieved in the cold zone that the reflecting surface would get clouded. The reflector of the invention has small mass and a low price. The reflecting surface is somewhat rippled like the skin of an orange, yet not disturbingly so.

Experiments directed towards improving the surface of the naked cast have not been successful. Namely, the fillers in the polymer make it impossible that a smooth surface would develop already by mere injection moulding. In individual cases, with approximate losses of 10 % of luminous flux, such reflector might be applied for less demanding headlamps with an ellipsoid reflector where the unattractive reflector appearance is hidden by a convex lens.

The lacquer layer is gastight. Thus, additionally to the already mentioned functioning as a substrate for the depositing of the reflecting layer, the lacquer layer also blocks the outgassing of the incompletely polymerized components of the polymer composite into the headlamp.

The teaching of the invention relating to the proposed efficacious technical solution is founded on the understanding of the physical background at the making of the reflector according to the invention as well as at the functioning of said reflector.

Within the electric field set up e.g. during the electrostatical lacquer powder deposition, the surface of the electrically conducting body of the polymer composite reflector functions like an electrode surface attracting the said lacquer powder. However, the polymer composite must have some lowest electrical specific conductivity defined above so that the electrical potential established on the surface of the reflector body can sufficiently quickly follow the electrical field changes at the electrostatical deposition.

In a similar way the higher thermal conductivity of the polymer composite parallelly to the reflector wall makes it possible that, in the stationary state, the temperature differences between the points on the reflector body are decreased and at the same time the reflector holder better conducts heat from the reflector body out of the headlamp.

In a known, thermally poorly conducting polymer reflector - the thermal conductivity of the polymer is about 0.25 W/mK - within a headlamp with a diameter of 50 mm to 100 mm, a rising convective hot air flow flowing around a filament light source heats up the reflector in the hot zone even above 250 °C, whereas the cold zone situated in the lower portion of the reflector may have a temperature even of only 50 °C to 70 °C. The harmful effects of the occurrence of a hot zone as well as of a cold zone have already been mentioned.

An anysotropy of the thermal conductivity of the polymer composite in a slender reflector shell appears as a result of the thermally conductive carbonaceous fillers in the form of fibres and the production of a slender reflector shell by injection moulding: its thermal conductivity parallelly to the reflector wall considerably exceeds the one transversely to said wall.

The hot zone temperature on a reflector of a polymer composite having a thermal conductivity transversely with regard to the reflector wall equal to or above 0.5 W/mK, is considerably lower, the typical temperature lowering being 50 K, than on a thermally nonconductive reflector, whereas the cold zone temperature rises only insignificantly, typical is an increase for 1 K. At such a reflector no harmful hot zone effect occurs, whereas there still develops a hazy spot, in the cold zone on the visible reflecting surface of the reflector, which is inconvenient for aesthetical reasons.

A still higher thermal conductivity of the polymer composite transversely with regard to the reflector wall, however, influences the temperature level of the hot and the cold zones of the reflector in the following way. The temperature lowering of the hot zone proved to be about 100 K at a thermal conductivity of 3 W/mK and about 110 K at a thermal conductivity of 6 W/mK, the temperature rise of the cold zone, however, proved to be about 10 K at a thermal conductivity of 3 W/mK and about 15 K at a thermal conductivity of 6 W/mK. If the minimal practical needs are considered, at a thermal conductivity of 3 W/mK the harmful effect of the cold zone is removed.

The correctness of the aforementioned empirical limit value 3 W/mK for a useful thermal conduction transversely to the slender reflector shell - said practical experience has been derived from too fast cooling down of the polymer composite through the mould wall, i.e. transversely to the reflector shell - can be demonstrated by the following consideration. The difference of the hot zone temperatures at the inner and the outer sides of the known polymer composite reflector shell having the thermal conductivity of 0.22 W/mK - see the first reference embodiment below - under the conditions mentioned below is equal to 80 K. The said temperature difference at the reflector of the invention made of the polymer composite having the transverse thermal conductivity of 0.5 W/mK is about 35 K, at the transverse thermal conductivity of 3 W/mK, however, it is only about 6 K and no further increase of the transverse thermal conductivity of the polymer composite is necessary as regards the hot zone temperature.

The targeted choice of the narrow interval between the transverse thermal conductivity values 0.5 W/mK and 3 W/mK rendered the invention of a slender, thermally conductive reflector made of a polymer composite possible. It is, of course, not difficult to make also a reflector having a stronger wall from such polymer composite. The said interval is situated far away from the preferred interval, which according to the technical teaching of the mentioned prior patent application WO 03/021623 Al comprises values exceeding 22 W/mK.

Actually, the polymer composite must also have the following features, which can be realized in the solution according to the invention. The thermal expansion coefficient in the temperature range from the vitreous transition temperature T_g to 180 °C should not exceed 40.10^"6 K¹ so that the optical properties of the headlamp are retained. Since rigidity is needed, Young's modulus must be above 10 GPa and the tensile strength should exceed 40 MPa. Although at the operation a reflector made of a polymer composite having thermal conductivity above 3 W/mK seldom attains a temperature above 160 °C, the heat deflection temperature (HDT) should exceed 220 °C since at a normal lacquering the electrostatically coated casts are baked at the temperature of 220 °C.

The injection moulding of the reflector casts is performed on standard injection moulds for the injection moulding of thermoplastic materials. However, the moulds must be cooled by a liquid with a possibility provided to control the tempering medium temperature to at least 160 °C, whereat it is recommended that the polished face of the steel mould is additionally heated, controlled by an PID controller and with a possibility of the control up to 190 °C.

The injection mould retains ordinary injection velocities, but an unusually short hold pressure time - up to 2 seconds - is set. Due to said thermal conductivity of the polymer composite, the time the cast with a thickness of the walls even exceeding 3 mm takes to cool off in the mould cave is only from 7 s to 15 s. Thus, the mould productivity is high with such a material. All embodiments described below represent a slender reflector for a fog lamp, with a diameter of 70 mm and a thickness of 2.1 mm. The headlamp is provided with a halogen bulb having a testing power of 65 W. The reflectors are round free-form ones, except in one case where it is an ellipsoid one. When tested the reflector is inserted into a 35 mm long cylindrical casing, which is made of black polyetherimide thermoplastic and covered by a smooth headlamp cover glass. The rear of the reflector is exposed to surroundings. A tight contact between the reflector and the casing is provided by a temperature resistant rubber seal. Tests were performed at room temperature.

All polymer composite reflectors of the invention are made of polyethylene terephthalate carbonaceous composite produced by COOL POLYMERS, INC. They are cast by injection moulding.

The reflecting layer in all embodiments is a vacuum deposited aluminium thin layer, which is covered by transparent thin hexamethyldisiloxane and SiOx layers.

There are also described three reference embodiments.

1st embodiment of the invention

The reflector according to the invention was made of polyethylene terephthalate with 20 % of glass fibres (Rynite 520, producer DuPont) and to 1 kg of the material the following fillers, were added:

540 g of Al₂O₃ powder with 2μm - 3μm granules (any producer), i.e. 15 vol. %,

120 g of aluminium fibres A1F, 0.1 mm thick and 2 mm long (producer Green Steel

Espana), i.e. 5 vol. %, and 79 g of graphitized soot (Carbon Black Printex L, producer Degussa), i.e. 7 vol. %.

Graphitized soot was added in order to achieve an appropriate electrical conductivity. The mixture was mixed in an extrusion mixer. The extruded material was cut to granules, which were inserted into an injection mould. The moulding faces had to be made of steel.

Test casts of the reflector according to the invention, having a diameter of 70 mm, were made.

The reflector casts were electrostatically covered by the powder lacquer Polydrox 11 Naturel 22P11 EP 9072527 (producer Akzo Nobel Coatings GmbH), thereafter in a furnace the lacquer was melted for 10 minutes at a temperature of 150 °C and thereafter it was cross-linked at a temperature of 220 °C.

Afterwards the cooled-off lacquered reflector casts were coated by a thin reflecting layer in a vacuum deposition device.

The thermal conductivity of the reflector polymer composite was 0.6 W/mK. The electrical conductivity was sufficiently high for electrostatical powder lacquering. The surface tension of the polymer composite was 55 mN/m and it made possible an excellent lacquer adhesion.

On the reflector no harmful effects appeared in the hot zone, but they were not eliminated in the cold zone. The tensile strength was too low for a practical application, which was a consequence of the presented recipe for the polymer composite production.

2nd embodiment of the invention

An ellipsoid reflector of the invention of a headlamp to be used in a vehicle was made of a polymer composite having a thermal conductivity of at least 0.5 W/mK. It was provided with a reflecting layer that was deposited immediately on the reflector cast made of the polymer composite having a well-defined thermal conductivity. It should be mentioned that the ellipsoid reflectors provided with the reflecting layer deposited on a lacquer layer are already incorporated in the first embodiment of the invention.

The reflector conducted the heat so well that no deformation or even material damage took place in the hot zone nor was any such low temperature achieved in the cold zone that the reflecting surface would get clouded.

Since the fillers in the polymer made it impossible that a smooth surface would develop already by mere injection moulding, at the ellipsoid reflector of the invention approximately 10 % of the luminous flux were lost. The unattractive appearance of the reflector was hidden by a convex lens.

No harmful effect of the hot zone appeared even at a thermal conductivity just above 0.5 W/mK, whereas in this case a hazy spot appeared in the cold zone of the reflector, but was hidden by the convex lens.

3rd embodiment of the invention

A polymer composite having a thermal conductivity of between 1.5 W/mK and 2 W/mK was used. The cast was immediately coated by a reflecting layer. The maximum reflector temperature nowhere exceeded 140 °C. The specular reflectivity was about 80 %. The luminous flux values were not satisfactory , especially the border between light and dark areas was blurred. Also the reflector surface was rather matt. The reflector is appropriate as an ellipsoid reflector provided with a collector lens where the reflecting surface is hidden and the requirement of a precise light beam adjustment is low. 4th embodiment of the invention

A polymer composite having a thermal conductivity of between 1.5 W/mK and 2 W/mK transversely to the shell and an electrical conductivity above 10^"5 Q-'m^"1 was used. The cast was lacquered by electrostatical powder deposition. The maximum reflector temperature nowhere exceeded 135 °C. The specular reflectivity was about 87 % to 88 %. The luminous flux values were satisfactory . The appearance of the reflecting surface was fine.

5th embodiment of the invention

A polymer composite having a thermal conductivity of between 1.5 W/mK and 2 W/mK transversely to the shell was used. The cast was lacquered by the lacquer which was hardened by ultraviolet irradiation. The maximum reflector temperature nowhere exceeded 135 °C. The specular reflectivity was about 88 % to 89 %. The luminous flux values were satisfactory . The appearance of the reflecting surface was excellent.

6th embodiment of the invention

A double-layer cast made of a polymer composite was produced according to any known process. A polymer composite having a thermal conductivity of between 1.5 W/mK and 2 W/mK transversely to the shell was injected to give a reflector rear part having a thickness of 2.1 mm. Hereon a layer of amorphous polycarbonate copolymer having a thickness of 0.7 mm was injected. Thereby a smooth substrate surface for a reflecting layer deposition was achieved. It withstood a temperature of up to 170 °C. The maximum reflector temperature reached the value of 160 °C. The specular reflectivity was about 86,5 % to 87 %. The luminous flux values were satisfactory . The appearance of the reflecting surface was satisfactory .

1 st reference embodiment

A reflector was made of a transparent polyetherimide polymer having the thermal conductivity of 0.22 W/mK. A reflecting layer is deposited directly to the cast. The maximum reflector temperature exceeded 220 °C and was too high for the polymer used. The specular reflectivity was about 86.6 % to 87.6 %. The luminous flux values were satisfactory . The appearance of the reflecting surface was satisfactory .

2nd reference embodiment

A reflector was made of aluminium by die casting and it was lacquered by electrostatical lacquer powder deposition. The maximum reflector temperature did not exceed 90 °C. The specular reflectivity was about 87 % to 88 %. The luminous flux values were satisfactory. The appearance of the reflecting surface was excellent.

3rd reference embodiment

A BMC polymer composite having the thermal conductivity of slightly above 0.22 W/mK was used. The cast was lacquered by a lacquer which was hardened by ultraviolet irradiation. The reflector form differed from that of the first reference embodiment and therefore a temperature comparison was meaningless. The specular reflectivity was about 88 % to 89 %. The appearance of the reflecting surface was excellent.

Claims

1. Headlamp reflector made of a polymer composite comprising a thermally conductive filler and to be used in a vehicle, characterized in that the polymer composite has a thermal conductivity, transversely with regard to the reflector shell, of above 0.5 W/mK and below 3 W/mK.

2. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 1 , characterized in that the polymer composite reflector is slender in the sense of its geometry as well in the sense of the slendemess criterion at the cast injection molding.

3. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 1 or 2, characterized in that a reflecting surface of the reflector cast is lacquered.

4. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 1 or 2, characterized in that the polymer composite has at least such an electrical conductivity that an electrostatical lacquer powder deposition on the reflector cast is made possible.

5. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 4, characterized in that the polymer composite has an electrical conductivity of at least 10^"5 Ω^'W.

6. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 4 or 5, characterized in that the reflecting surface of the reflector cast is lacquered by the electrostatical lacquer powder deposition.

7. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in any of claims 3 to 6 , characterized in that the laquer is hardened by UN irradiation.

8. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claims 3, 6 or 7, characterized in that on the lacquered reflecting surface a reflecting layer is deposited.

9. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 1 or 2, characterized in that a double-layer reflector shell consists of a polymer composite rear layer and of a thin polymer front layer having a smooth surface.

10. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 9, characterized in that on the polymer surface of the reflector cast a reflecting layer is deposited.

11. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in claim 1 or 2, characterized in that on the ellipsoid reflecting surface of the reflector cast a reflecting layer is deposited.

12. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in any of preceding claims, characterized in that the polymer composite has a surface tension from 50.10^" Ν/m to 60.10^" Ν/m.

13. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in any of preceding claims, characterized in that the polymer composite is made of any thermoplastic polymer in the polymer group comprising polyphenylene sulfϊde, polyphtalamide, polyethylene terephthalate, polyamide 6, polyamide 66, polyamide 6T and polybutylene terephthalate and that as the thermally as well as electrically conductive filler a mixture of carbonaceous fibres and carbonaceous grains is used.

14. Headlamp reflector made of a polymer composite and to be used in a vehicle as recited in any of preceding claims, characterized in that the required mechanical properties and the coefficient of thermal expansion of the polymer composite are achieved by adding glass fibres.