Multi-layer structure suitable for use as a reflector

Abstract

The invention relates to a filler-containing multi-layer structure on the basis of a polycarbonate composition, having a metal layer as a reflective layer and having a balanced property profile of CLTE, CLTE ratio, heat conductivity and lustre. This multi-layer structure can be used, inter alia, for reflectors or as a mirror element in head-up displays. Expanded graphite and burned silicon dioxide are contained as fillers. In the composition provided according to the invention, there is no need for an additional heat sink when implementing the component.

Claims

1. A layer structure, comprising i) a substrate layer made of a thermoplastic polycarbonate-based composition and ii) a metal layer applied to the substrate layer i, wherein the thermoplastic polycarbonate-based composition of substrate layer i contains the following components: a) 44% to 63% by weight of aromatic polycarbonate, b) 3% to 8% by weight of expanded graphite, c) 34% to 38% by weight of fused silica, d) 0% to 10% by weight of one or more further additives, wherein the total amount of expanded graphite and fused silica is at least 40% by weight.

2. The layer structure as claimed in claim 1, wherein the thermoplastic polycarbonate-based composition of substrate layer i contains a) 54% to 60% by weight of aromatic polycarbonate, b) 5% to 7.5% by weight of expanded graphite, c) 35% to 37.5% by weight of fused silica, d) 0% to 5% by weight of one or more further additives.

3. The layer structure as claimed in claim 1, wherein the substrate layer has been produced by injection molding with dynamic temperature control of a mold.

4. The layer structure as claimed in claim 1, wherein the layer structure comprises no further layers other than optionally one or more protective layers.

5. The layer structure as claimed in claim 1, wherein the thermoplastic polycarbonate-based composition of substrate layer i consists of components a to c and optionally d.

6. The layer structure as claimed in claim 1, wherein the group of the further additives consists of the group of flame retardants, heat stabilizers, antistats, UV absorbers, IR absorbers, anti-dripping agents, impact modifiers, antioxidants, inorganic pigments, carbon black, organic colorants, inorganic fillers and/or mold-release agents.

7. The layer structure as claimed in claim 1, wherein a D(0.5) of the expanded graphite is 700 m to 1200 m, determined by sieve analysis in accordance with DIN 51938:2015-09.

8. The layer structure as claimed in claim 1, wherein a D(0.5) of the fused silica is 2.5 m to 8.0 m, determined in accordance with ISO 13320:2009-10.

9. The layer structure as claimed in claim 1, wherein D(0.5) of the fused silica is 3 m to 5 m, determined in accordance with ISO 13320:2009-10.

10. The layer structure as claimed in claim 1, wherein for the substrate layer a ratio of a longitudinal and transversal CLTE value (longitudinal: transversal), determined in accordance with DIN 53752-Method A: 1980, is 0.84 and an in-plane thermal conductivity, determined in accordance with ASTM E 1461:2013, is >0.7 W/(m.Math.K) and a Vicat temperature, determined in accordance with DIN ISO 306:2014-3, is 141 C. and a gloss, determined in accordance with ASTM D 523-14, is >90.

11. The layer structure as claimed in claim 1, wherein the metal layer ii has a thickness of 60 nm to 300 nm, determined by atomic force microscopy.

12. A component comprising a layer structure as claimed in claim 1.

13. The component as claimed in claim 12, wherein the component is a reflector or a mirror element of a head-up display.

14. The component as claimed in claim 13, wherein the component is a headlamp reflector.

15. A method for producing a layer structure as claimed in claim 1, wherein a) the substrate layer is formed from the thermoplastic polycarbonate-based composition by one-component injection molding with dynamic temperature control of a mold and then b) a metal layer is applied to this substrate layer.

Description

EXAMPLES

(1) 1. Description of Raw Materials and Test Methods

(2) The polycarbonate compositions described in the examples which follow were produced by compounding on an MX58 co-kneader from BUSS at a throughput of 80 kg/h. The melt temperature was between 250-310 C., and the kneader housing, kneader shaft and discharge housing and shaft of the discharge screw had a defined temperature of 260 C. The temperature of the die plate was 300 C. Component b was added together with a powder premix (polycarbonate powder+additives) via a side extruder and component c was added directly via the main intake. Component a: Linear polycarbonate based on bisphenol A having a melt volume-flow rate MVR of 19 cm.sup.3/(10 min) (according to ISO 1133:2012-03, at a test temperature of 300 C. and under a load of 1.2 kg). Component b-1: Expanded graphite: Ecophit GFG 900 from SGL Carbon GmbH with a D(0.5) of approx. 900 m according to DIN 51938:2015-09. Component b-2: Expanded graphite: SC 4000 O/SM with a D(0.5) of approx. 1000 m according to DIN 51938:2015-09 from Graphit Kropfmuhl GmbH. Component c: Fused silica: Amosil FW 600 from Quarzwerke GmbH in Frechen, unsized, with a median particle size D(0.5) of approx. 4 m, D(0.98) of approx. 13 m, a D(0.1)/D(0.9) ratio of approx. 1.5/10 and a specific surface area of approx. 6 m.sup.2/g, determined in accordance with DIN-ISO 9277:2014-01. Component c*: Compacted talc having a talc content of 98% by weight, an iron oxide content of 1.9% by weight, an aluminum oxide content of 0.2% by weight, ignition loss (DIN 51081/1000 C.) of 5.4% by weight, pH (according to EN ISO 787-9:1995) of 9.15, D(0.5) (sedimentation analysis) of 2.2 m; BET surface area according to ISO 4652:2012-06 of 10 m.sup.2/g, brand: Finntalc M05SLC, manufacturer: Mondo Minerals B. V. Component d-1: Wax. A maleic anhydride-modified polypropylene copolymer from Honeywell (AC907P) having an average molecular weight (gel permeation chromatography in ortho-dichlorobenzene at 150 C. with polystyrene calibration) M.sub.W=20 700 g/mol, M.sub.n=1460 g/mol and with an acid number of 78 mg KOH/g (ASTM D-1386:2015).

(3) As a measure of the heat distortion resistance, the Vicat softening temperature VST/B50 or B120 was determined in accordance with DIN ISO 306:2014-3 on test specimens measuring 8 mm10 mm4 mm with a 50 N ram load and a heating rate of 50 C./h or 120 C./h using a Coesfeld Eco 2920 instrument from Coesfeld Materialtest.

(4) The coefficients of thermal expansion (CLTE) were measured in accordance with DIN 53752:1980-12 (coefficient of linear thermal expansion, parallel/perpendicular, at 23-60 C. (with a heating rate of 3 K/min).

(5) The thermal conductivity TC in injection molding direction (in-plane) at 23 C. was determined in accordance with ASTM E 1461:2013 on specimens having dimensions of 60 mm60 mm2 mm.

(6) The thermal conductivity TC in injection molding direction (through-plane) at 23 C. was determined in accordance with ASTM E 1461:2013 on specimens having dimensions of 60 mm60 mm2 mm.

(7) The melt viscosities were determined in accordance with ISO 11443:2014-04 with a Gttfert Visco-Robo 45.00 instrument (cone/plate arrangement).

(8) The density was determined in accordance with ISO 1183-1:2012.

(9) The gloss was determined in accordance with ASTM D 523-14.

(10) The test specimens used for determining the surface quality were produced by the injection molding process using dynamic temperature control of the mold. The materials were processed with a melt temperature of 330 C. and a mold temperature of 166 C. (heating) and 85 C. (cooling). The injection speed was 40 mm/s and the hold pressure was 900 bar.

(11) Following the tests, a metal layer was applied to the substrate layer by the following method:

(12) The coating system consisted of a vacuum chamber in which the specimens were positioned on a rotating specimen holder. The specimen holder rotated at about 20 rpm. Before they were introduced into the vacuum chamber, the test specimens were blown with ionized air in order to free them of dust. For the metallization, Ar gas was admitted at a pressure of 5.Math.1 10.sup.3 mbar. Using a DC magnetron, an aluminum layer of approx. 200 nm thickness was applied to the specimens with a power density of 6.4 W/cm.sup.2.

(13) 2. Formulations and Results

(14) TABLE-US-00001 TABLE 1 Compositions I1 to I4 according to the invention and comparative examples C1 to C7 C1 C2 C3 I1 I2 I3 C4 I4 C5 C6 C7 Formulation a % by wt 59.37 59.37 54.37 59.37 56.87 54.37 60.00 60.00 59.60 74.75 74.00 b-1 % by wt 1 1 5 7.5 7.5 10 5 b-2 % by wt 5 10 5 c % by wt 40 39 44 35 35 37.5 30 35 30 c* % by wt 20 20 d-1 % by wt 1 further additives d-2, d-3, d-4: % by wt 0.075/ 0.075/ 0.075/ 0.075/ 0.075/ 0.075/ 0.1/ 0.25/ / (heat stabilizer/mold-release 0.4/0.16 0.4/0.16 0.4/0.16 0.4/0.16 0.4/0.16 0.4/0.16 0.3/ agent/carbon black) Tests melt viscosity Pa .Math. s 286 320 330 358 406 438 395 296 357 145 223 at 300 C. TC (in-plane) W/(m .Math. K) 0.32 0.49 0.47 0.96 1.58 1.71 1.88 0.89 1.87 1.7 1.57 TC (through-plane) W/(m .Math. K) 0.26 0.36 0.38 0.47 0.59 0.61 n.m. n.m. n.m. 0.4 n.m. VST/B50 [ C.] 142 142 142 142 142 142 148 147 148 139 147 CLTE parallel (RT-60 C.) ppm/K 44 43 43 40 36 35 36 42 36 37 40 CLTE perpendicular (RT-60 ppm/K 45 46 44 46 43 41 45 46 43 49 55 C.) ratio of CLTEparallel/CLTE 0.98 0.93 0.96 0.87 0.84 0.84 0.81 0.90 0.84 0.76 0.72 perpendicular density g/cm.sup.3 1.46 1.46 1.50 1.46 1.48 1.50 1.44 1.43 1.46 1.38 1.37 gloss (at 20) 102 n.m. 100 100 97 94 73 103 90 n.m. 103 n.m.: not measured

(15) The thermal conductivity of a composition is determined via the fillers introduced and the proportion by weight/volume thereof. The filler-specific thermal conductivity decisively determines the degree of thermal conductivity of the overall composition. The use of a poorly thermally conductive filler such as quartz results, at a concentration of 40% by weight (C1), in an overall thermal conductivity (in-plane) of 0.32 W/mK, which is too low for the intended purpose. A major advantage of quartz as spherical filler is the maintenance of high dimensional stability of the composition (expressed by the ratio of parallel to perpendicular CLTE, here 0.98). The addition of or partial substitution of the filler with graphite makes it possible to increase the thermal conductivity; however, the dimensional stability of the overall composition is reduced as a result of the platelet-form particle geometry thereof. At small concentrations of graphite (C2 and C3), although the CLTE ratio (and hence the degree of isotropy) is still very close to the ideal value (CLTE.sub.parallel=CLTE.sub.perpendicular=1.0), the thermal conductivity is increased only minimally, even with a total filler content of 45%. The addition of 5%-7.5% by weight of graphite (I1-I3) offers an optimal balance between in-plane thermal conductivity (>0.85 W/mK) and isotropy (0.84).

(16) The choice of the graphite also has an influence on these properties on account of the particle size distributions. For instance, with graphite b2, at least in a low concentration, a relatively low thermal conductivity is achieved (cf. I4 and I1). Although the compositions I3 and C5 have the same CLTE quotients, the higher filler content in I3 (45%) is preferred since it results in a lower CLTE and hence reduced shrinkage characteristics.

(17) A further disadvantage of high graphite amounts is the surface quality (measured using the degree of gloss). This becomes clear from C4 and C5, the degree of gloss of which is far below that of the other compositions comprising less graphite. While the combination of graphite and talc as filler (C6 and C7) leads to high thermal conductivities even at relatively low total filler contents, it has (as a result of the platelet-form particle geometry as in the case of graphite) the major disadvantage of reduced dimensional stability (see the CLTE quotient). Moreover, talc can be incorporated into polycarbonate only with the aid of a waxy stabilizer (d). However, this stabilizer results during processing in the formation of streaks on the surface, which has an adverse effect on the metal adhesion and the thermal stability of a reflector structure.

(18) It can be seen on the basis of the examples shown that only a balanced ratio of quartz and graphite (I1-I3, I4) results in a good property combination profile of CLTE, CLTE quotient, thermal conductivity and surface quality (gloss).