WO2010097263A1

WO2010097263A1 - Method and sizing rule for dimensioning and producing fresnel lenses for focusing light

Info

Publication number: WO2010097263A1
Application number: PCT/EP2010/050958
Authority: WO
Inventors: Stefan Delp; Heiko Rochholz; Peter Battenhausen
Original assignee: Evonik Röhm Gmbh
Priority date: 2009-02-24
Filing date: 2010-01-28
Publication date: 2010-09-02
Also published as: DE112010000878A5; DE102009001102A1; TW201043452A

Abstract

The invention relates to a method for producing linear Fresnel lenses by forming a melt strand of a thermoplastic plastic, such as polymethyl (meth)acrylate, on the surface of a rotating drum that can be heated and cooled in certain areas, wherein the melt strand is pressed onto the rotating drum by means of a device, characterized in that the profile of the rotating drum is engraved according to the stipulations of the optimization program.

Description

Method and design rule for dimensioning and fabrication of Fresnel lenses for light focusing

Field of the invention

Compared to solid lenses, Fresnel lenses are also suitable for large-area applications due to their low material requirements. In the field of solar thermal energy production there is a useful potential for large collection lenses. You can focus sunlight on heat transfer media and heat them up to high temperatures. Cheap Fresnel lenses can be molded from plastics and used in overhead projectors, passive infrared sensors, simple hand loupes, and wide angle lenses in automobile rear windows and counters to control the shopping carts

The invention relates to the use and the application of an improved manufacturing method of a linear Fresnel lens made of transparent plastics, whose geometry has been optically and manufacturing optimized with a new calculation program. A linear Fresnel lens is understood to mean a Fresnel lens which acts like a cylindrical lens and has a linear focal zone. State of the art

US Patent No. 5,656,209 (Röhm GmbH) describes a process for the extrusion of Fresnel lenses from transparent plastics, such as polymethyl (nneth) acrylate (PMMA) or polycarbonate (PC). The melt stream emerging from a slot die wraps around the embossing roll, which imprints the profile of the Fresnel lens on the melt stream. The thickness of the lens is about 6 mm, the lenses are made of an unspecified polymethylmethacrylate.

WO 01/196000 (Schröder, Nawrath) describes another process for the production of endless, optically imageable films, webs or plates and an apparatus for carrying out the process. The object of the invention is to improve the previously unsatisfactory imaging accuracy of the structures on the film o. Ä.

The solution is to press the emerging from a slot die PMMA melt belt by means of a rotating steel belt on the engraved roller. The temperature of the gravure roll at the point of impact of the plastic melt (in the case of PMMA) is close to the maximum permissible temperature for extrusion processes, along the circumference of the gravure roll is then cooled, so that a demolding of the resulting Fresnel lens is easily possible.

EP 1 559 528 (Kark AG) describes an apparatus for molding a plastic molding on a rotating drum, wherein the drum can be heated and cooled in individual areas. task

It was now the task of optimizing the process for the production of linear Fresnel lenses made of transparent plastics with the known devices of the prior art so that one is able to give an embossing roller such an optimized structure that a so shaped linear Fresnel lens perpendicular to them incident parallel (solar) light with optimal light collection intensity and energy efficiency in a narrowest possible focus line bundles.

The construction of this Fresnel step structure has the following problems that have been solved or considered:

1. Aberration: The occurrence of the spherical aberration, ie the variation of the focal position of axis-far and near-axis light rays was avoided in the calculation rule by an optimized design of the flank angle. (For this purpose, the refractive index of the polymer used was determined for the middle wavelength range with which the lens is to be irradiated and used in the calculation.) The color dispersion, ie the variation of the focus position as a function of the wavelength, can be due to the dispersion of the refractive index of the polymer used not influenced by a single Fresnel lens, for example, the average wavelength was assumed to be 630 nm.)

2. Minimizing the material to be reshaped by the embossing roll to the width of a flank structure along the entire width of the Fresnel lens. 3. By minimizing the rounding of the step edges in the Fresnel lens arise interference, which also direct perpendicular to the plane of the plate incident light in unwanted directions. The surfaces of the intermediate steep flanks cause additional losses due to their refraction according to a diverging cylindrical lens. The design of this angle was minimized to the value required for demolding. The angle is for example about 1 ° - 4 °.

4. The calculated contour on the impression roll is unscrewed with a wedge-shaped tool. The lens is optically optimized under the constraint of the possibilities of the tool.

solution

The object is achieved by the provision of the profile coordinates, which allow the machine for engraving the embossing roll to be controlled in such a way that an embossing roll can be produced with which linear fresnel lenses are shaped in accordance with the task using the prior art can be. The invention further provides an optimized linear Fresnel lens made of transparent plastics.

Implementation of the Invention Transparent plastic

As transparent plastics, poly (meth) acrylate (PMMA), polycarbonate (PC), cyclic olefin copolymers (OCP) or polystyrene (PS) can be used.

Poly (meth) acrylate (PMMA)

Suitable for carrying out the invention is z. B. a Polymethylnnethacrylat - molding composition consisting of 80 to 100 wt .-% methyl methacrylate units and optionally from 0 to 20 wt .-% of other copolymerizable monomers. To name a few are z. As hydroxyethyl methacrylate, butyl acrylate, ethyl acrylate or preferably methyl acrylate. The molecular weight M w (weight average, determined, for example, by DSC or by gel chromatography) can be determined, for example, by B. in the range of 5 x 10 ⁴ to 2 x 10 ⁵ lie. The appropriate PMMA types are available under the trademark Plexiglas ^® from Evonik Röhm GmbH, such as Plexiglas ^® POQ 62 or Plexiglas ^® POQ64 or Plexiglas ^® 6N.

Readily flowing types of PMMA molding compositions with a melt volume rate of 25 are preferred - 5 cm ³ / 10min used, particularly preferably PMMA molding compositions with a melt volume rate of 20 - used tenth The melt volume rate is determined according to ISO 1133 (230 / 3.8). Impact-resistant polymethylmethacrylate (sz-PMMA)

Impact-modified polymethyl methacrylate and its preparation is z. B. from EP-A 0 733 754 known.

The impact polymethyl methacrylate may, for. From p1) from 4 to 30% by weight of an elastomer phase and from p2) from 70 to 96% by weight of a thermoplastic matrix phase of polymethyl methacrylate containing up to 20 parts by weight, based on 100 parts by weight of P, may contain at suitable comonomer proportions, wherein the refractive indices of the elastomer phase E and the matrix phase M differ by a maximum of n ≤ 0.02 and wherein the sum of p1) + p2) makes up 100 wt .-%.

The elastomer phase of crosslinked polymer phase is from 60 to 99.9 parts by weight of alkyl acrylate and / or aryl acrylate, from 0.1 to 10 parts by weight of suitable crosslinking agents and optionally from 0 to 30 parts by weight of suitable monofunctional ethylenically unsaturated monomers built up.

C ₂ -C ₁₀ -alkyl acrylates are preferably used as alkyl acrylates, for example ethyl acrylate, propyl acrylate, isopropyl acrylate, amyl acrylate, hexyl acrylate, octyl acrylate, decyl acrylate, and particularly preferably butyl acrylate and 2-ethylhexyl acrylate. Preferred acrylates are phenyl acrylate, 2-phenylethyl acrylate, 3-phenyl-1-propyl acrylate, 2-phenoxyethyl acrylate, 2-phenoxyethoxyethyl acrylate, and particularly preferably benzyl acrylate.

The crosslinking agents are generally compounds having at least two ethylenically unsaturated, radically polymerizable radicals. As representatives of compounds having two ethylenically unsaturated, radically polymerizable radicals may be mentioned by way of example: (meth) acrylic esters of diols, such as ethylene glycol di (meth) acrylate or 1, 4-Butandioldi (meth) acrylate, aromatic compounds such as divinylbenzene, and compounds with at least an allyl group such as allyl (meth) acrylate. As crosslinking agents with three or more ethylenically unsaturated, radically polymehsierbaren residues are exemplified by triallyl cyanurate, trimethylolpropane tri (meth) acrylate and pentaerythritol (meth) acrylate mentioned. Further examples of this are given, for example, in US Pat. No. 4,513,118.

The comonomers optionally contained in 0 to 30 parts by weight in the elastomer phase, serve primarily to approximate the generally lower refractive index of the elastomer phase to those of the matrix phase M. Preferably, therefore, comonomers are selected with comparatively high refractive indices, such as free-radical polymerizable aromatic compounds. Examples include: vinyl toluene, styrene or α-methyl styrene, which are used in amounts such that they do not affect the weather resistance of the impact polymethylmethacrylate.

The at least 5 wt .-% covalently bonded to the elastomer phase matrix phase M consists of a polymethyl methacrylate P, which is composed of 80 to 100 parts by weight of methyl methacrylate units, and has a glass transition temperature of at least 70 ⁰ C. Furthermore, in the polymethyl methacrylate 0 to 20 parts by weight of further ethylenically unsaturated, free-radically polymerizable comonomer units may be present, preferably alkyl (meth) acrylates having 1 to 4 carbon atoms in the alkyl radical. The average molecular weight M _w of

Polymethylmethacrylate is favorably between 10 and 10, preferably between 3 x 10 and 5 x 10 Dalton (for the determination of M _w see for example HF Mark et al., Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 10, p ff, J. Wiley, New York, 1989).

Preferably, the elastomer phase is part of two- or multi-stage emulsion polymers which consist in the outer shell of the polymethyl methacrylates forming the matrix phase. Particular preference is given to emulsion polymers with an at least three-stage structure, formed from a core of polymethylmethacrylate, a first shell S1 of the elastomer phase and a second shell S2 of polymethylmethacrylate, wherein further shells can connect in accordance with the shells S1 and S2 alternately. The proportion of emulsion polymers in the impact-resistant polymethyl methacrylate is between 5 and 70 wt .-%, preferably between 10 and 50 wt .-%, the remaining parts by weight of the non-contained in the latex particles polymethyl methacrylate plastic are made.

Preferably, the impact-resistant polymethyl methacrylate is prepared by mixing the emulsion polymer with the polymethyl methacrylate, for example, the components are mixed and then the water phase and the emulsifiers are separated or wherein first the emulsion is isolated from the aqueous phase and then with, for example by continuous bulk polymerization Polymethylmethacrylate is melt-mixed. Overall, the latex particles which form the emulsion polymer should have a diameter between 0.1 and 3 μm, preferably between 0.15 and 1 μm. In principle, the structure of such latex particles and the isolation of the emulsion polymer for two-stage emulsion polymers is described, for example, in EP patent 0 033 999 (= US Pat. No. 4,543,383) and for three-stage emulsion polymers, for example in EP patent 0 113 925 (= US Pat. No. 4,513,118). In the aqueous emulsion polymerization is advantageously carried out in the neutral or slightly acidic pH range, the use of long-chain alkyl sulfates or alkyl sulfonates is advantageous as emulsifiers. The polymerization initiators used suitably the relevant known azo compounds or organic or inorganic peroxides, such as persulfates, which are generally used in amounts between 10 ^"and 1 wt .-%, based on the monomers: For adjusting the molecular weight of the above-described M _w of the present in the emulsion polymer

Polymethylmethacrylate serve the pertinent known molecular weight regulators, such as for example, mercapto compounds, such as 2-ethylhexyl thioglycolate or tert-dodecyl mercaptan.

Particularly preferred are those emulsion polymers which are coagulated and dehydrated in an extruder. The melt is divided in the dewatering zone of the extruder into several sections, which are each conveyed in separate screw flights. The melt phase is thereby stowed in at least one of these flights in the intake nip of the twin screw to form a locally narrow pressure gradient to a coherent melt cake. In this case, the water is allowed to flow down before the border of the melt cake under the action of gravity down through at least one exhaust port that the melt cake is not in contact with a continuous water phase. As a result, the additives and impurities contained in the water are effectively removed, so that a particularly weather-stable material not prone to yellowing is obtained (see also EP-A 0 683 028 and as a two-stage process DE 197 18 597 C1). The appropriate im PMMA types are available under the trademark Plexiglas ^® from Evonik Röhm GmbH, such as Plexiglas ^® zk6BR, zk6HF or zk6HT.

polycarbonates

The present invention also relates to thermoplastic moldable polycarbonates. Polycarbonates are plastics known to the skilled person. They refer to thermoplastic polymers having the general structural formula

which can be considered formally as carbonic acid polyesters and aliphatic or aromatic dihydroxy compounds. In this case, the radical R denotes divalent aliphatic, cycloaliphatic or aromatic groups which are derived from the corresponding dihydroxy compounds.

The polycarbonates which can be used according to the invention include homopolycarbonates, copolycarbonates, unbranched polycarbonates, branched polycarbonates and mixtures of the polycarbonates mentioned.

In the context of the present invention, aromatic radicals R are preferred. These include, but are not limited to, hydroquinone, resorcinol, 4,4'-dihydroxydiphenol, 2,2-bis- (4-hydroxyphenyl) -propane, 2,4-bis (4-hydroxyphenyl) -2-methylbutane, 2,2-bis (4-hydroxy-3,5-dimethylphenyl) -propane, 2,2-bis (4-hydroxy-3,5-dichlorophenyl) -propane, 2,2-bis (4-hydroxy -3,5-dibromophenyl) -propane, 1,1-bis (4-hydroxyphenyl) -cyclohexane or derived from 1,1-bis (4-hydroxyphenyl) -3,3,5-trimethylcyclohexane. Particularly preferred radicals R are derived from 2,2-bis (4-hydroxyphenyl) propane or from 1,1-bis (4-hydroxyphenyl) cyclohexane.

The radicals R may optionally carry further substituents, preferably methyl or halogen groups. Particularly preferred substituents are bromine and chlorine atoms. The polycarbonates of the invention preferably have a weight average molecular weight in the range between 10,000 g / mol and 200,000 g / mol. Particularly preferred is a weight average molecular weight in the range between 10,000 g / mol and 100,000 g / mol, in particular between 15,000 g / mol and 45,000 g / mol.

The polycarbonates of the invention may contain other polycarbonate-miscible polymers. These include, inter alia, poly (meth) acrylates, polyesters, polyamides, polyimides, polyurethanes, polyethers, ABS, ASA and PBT.

Miscibility of the various substances in the context of the present invention means that the components form a homogeneous mixture.

Furthermore, the polycarbonates may contain additives well-known in the art. These include antistatics, antioxidants, dyes, fillers, light stabilizers, pigments, UV absorbers, weathering agents and plasticizers.

polystyrene

Polystyrene may for example be composed of the following monomers: styrene, substituted styrenes having an alkyl substituent in the side chain, such as α-methylstyrene and α-ethylstyrene, substituted styrenes having an alkyl substituent on the ring, such as vinyltoluene and p-methylstyrene, halogenated styrenes, such as monochlorostyrenes, dichlorostyrenes, tribromostyrenes and tetrabromostyrenes. Implementation of the invention

1 Determination of the refractive index

2 Optimization of the lens structure

3 flowchart

ad 1

The refractive index value to be used in the calculation follows from a measurement on the transparent polymer to be used at a wavelength which is in the central region of the spectrum with which the lens is to be irradiated. It was determined at 23 ° C. As an example, one can assume 1.50.

ad 2

The coordinates for the control of a CNC machine for the production of the Fresnel lens are determined in the following steps.

figure description

Fig. 1 shows the geometry that is assumed. A bundle of parallel light rays is focused by the Fresnel lens to the point F. It should be noted that the linear Fresnel lens spatially gives a line focus. It is generally assumed that the bundle of parallel rays hits the lens at the angle α. In addition, the focus is removed by the distance f from the optical axis. In the following, only α = 0 ° and f '= 0 are considered.

The width of the lens is b. It is a Fresnel lens to construct with these optical properties.

Fig. 2 shows that a Fresnel lens is made up of many elementary flank combinations arranged side by side. A flank combination (3) consists of an optical flank (1), which directs the light beam in the direction of focus. In addition, two interfering flanks (2) lead up and down, leading to the steps of the lens. These are to be aligned in such a way that as few rays of light as possible should impinge on them. These rays of light mean a reduction in the energy yield, since these light rays do not reach the focus.

To calculate the optimized geometry, a process is described, which is described by the flow chart (Fig. 3).

1. Set Start Coordinates: In the case of a = 0 ° and f '= 0, the lens is symmetrical to the optical axis and the lens is constructed upward from the axis. Then the upper part is mirrored and the lens is completed downwards. 2. Align lower interfering flank: The flank is aligned parallel to the light rays in the lens. At a = 0 ° the demolding is exactly perpendicular to this surface. Therefore, a demoulding ability of the edge of about 1 ° - 4 ° is used.

3. Align the optically effective flank: The flank is aligned so that the middle light beam is directed into the focus. The refraction of the light rays is calculated according to the Snellius law of refraction, taking into account the refractive index to be applied. The flank is no longer aligned if, due to the total reflection condition, a light beam can no longer be directed to the focus.

4. Optimize upper disturbing flank: The flank is aligned parallel to the light beam in the lens. Also, the Entformungsneignung of about 1 ° - 4 ° is considered again.

5. Scale flank combination: Steps 2 to 4 form a flank combination. A limitation in the production is the maximum height h of the flank combination. In addition, only optical edges with a maximum length z can be manufactured with optical precision. Under these two boundary conditions, the largest possible edge combination is selected by scaling and the coordinates are entered. In addition, an equal Matehalfluss is to be achieved in the production. Therefore, the edge combination is aligned in the horizontal so that averaged over an edge combination, the material consumption lies on the dashed line.

6. Has the nominal width been reached? The juxtaposition of edge combinations is continued until the desired width is reached or no longer makes sense due to incoming total reflection. 7. Prepare coordinates for the next flank combination: If the set width has not yet been reached, the next combination is appended.

8. Mirror coordinates: In the case a = 0 ° and f '= 0, the lens is symmetrical and the constructed half must be additionally mirrored. Otherwise, the lower half will be calculated separately.

A circular Fresnel lens is understood to mean a lens with concentric steps. A linear curved Fresnel lens is understood to mean a plate, whereby the steps run straight. This plate can then -. B. by clamping - be installed like a vault.

Engraving the embossing roll

The embossing roll is in suitable mech. Machining centers provided by milling and or turning with the invention optimized Fresnel structure.

Production of Fresnel Lenses According to the Invention

Conventional single-screw kneaders (manufacturers, for example, Kühne, Breier, Krauss Maffei) can be used to plasticize the selected thermoplastics for the production of the Fresnel lenses according to the invention. Depending on the plastic and plastic type, melt temperatures of approx. 200 ^° C to 300 ^° C must be used. The pre-distribution of the plastic melt takes place through a slot die. The melt film thus applied to the gravure roll is pressed into the gravure roll by a circulating mirror strip of highly polished stainless steel, which leads to the impression of the predetermined inventive Fresnel structure in the solidifying melt strand. Instead of the rotating mirror band of highly polished Stainless steel can also be arranged one or more rollers to generate the contact pressure of the melt web on the gravure roll. The contact pressure of the rollers may be the same or different. The gravure roll can be equipped to be heatable and / or coolable, wherein individual areas can be heated and / or cooled.

The dimension of the lens depends essentially on the performance of the screw kneader as well as the slot die used and the width of the embossing drum.

The extrusion speed can vary from about 0.1 to 10 meters per inch, and is mainly affected by the emboss roll diameter and the lens thickness.

For example, a lens was manufactured with the following parameters:

Width 1000 mm thickness 2.0 mm

Refractive index n = 1, 49

Focal length f = 1500 mm

Impression angle φ = 4 ° (slope to the vertical line)

All values given are to be understood as examples which are not restrictive.

Considering the Fresnel reflections, 89.9% of the light is transmitted.

Claims

claims

I. Method for producing linear Fresnel lenses by shaping a melt strand of a thermoplastic on the surface of a rotating drum which can be heated and cooled in certain areas, the melt strand being pressed against the rotating drum by means of a device,

characterized in that

the profile of the rotating drum is engraved according to the specifications of the optimization program.

2. A process for the production of circular Fresnel lenses by molding a melt strand of a thermoplastic material on the surface of a rotating drum, which can be heated and cooled in certain areas, wherein the melt strand is pressed by means of a device to the rotating drum,

characterized in that

3.A method for producing curved linear Fresnel lenses by molding a melt strand of a thermoplastic material on the surface of a rotating drum, which can be heated and cooled in certain areas, wherein the melt strand is pressed by means of a device to the rotating drum,

characterized in that

4.A method according to any one of claims 1 - 3,

characterized,

that the device with which the melt strand is pressed against the rotating drum, consists of one or more rollers.

δ.Method according to one of the claims 1 - 3,

characterized,

that the device with which the melt strand is pressed against the rotating drum consists of a band. Θ.Method according to claim 5, characterized in that the band consists of steel.