GB2082107A

GB2082107A - Plastics optical elements which comprise a moulded plastics material coated on one face with a photopolymerized resin

Info

Publication number: GB2082107A
Application number: GB8026706A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1980-08-15
Filing date: 1980-08-15
Publication date: 1982-03-03

Abstract

A method of duplicating a large number of plastics optical elements comprising moulding each block (1) of a plastics material in a mould tool at an elevated temperature and pressure to produce a required profile (2), allowing the block (1) to cool under pressure prior to removal and annealing the block (1) with other moulded blocks if desired, in the free state comprising further cooling followed by heating to just below the softening or glass transition point of the polymerized block, placing the block (1) in a second finishing glass mould (14) with a profile (13) filling a moulding space (16) remote to the required profile (2) with a liquid photopolymerizable resin, curing the liquid resin by irradiation with light at ambient temperatures through the mould (14) and removing the element so formed from the finishing mould (14).

Description

GB 2 082 107 A SPECIFICATION Plastics optical elements which comprise a moulded plastics material coated on one face with a photopolymerized resin 5 This invention relates to a method of duplicating a large number of optical elements in which each optical element has at least one accurately profiled optical surface of a required shape, and to a method of manufacturing such elements which comprises placing a block of a polymerizable monomer composition in a hot mould tool having at least one surface complementary to the required shape, moulding the block against the at least one complementary surface of the mould tool by the application of heat and pressure and 10 *. removing the moulded block therefrom. - It is known that composite optical elements may be produced by shaping a blank of a hard, stable, light " transmissive material such as glass, to a first approximately accurate shape and by moulding a light transmissive thermosetting resinous material thereon to reproduce a second, closer approximation of the required shape. 15 U.K. Patent No. 1,301,551 describes the preparation of an optical element in which the surface of a blank or substrate is processed to enable a thermosetting resin to adhere thereto by placing the blank in a mould having an aspheric mould surface and injecting the thermosetting resin material, in liquid form, into the space between the substrate and the mould surface, allowing the liquid material to harden and removing the optical element from the mould. The intrinsic properties of thermosetting resins, however, are such that an 20 application of sensible heat is required and that the cure time to harden the liquid resin material increases with a decrease in temperature. Thus, in order to keep the cure time of the liquid resin reasonably short elevated cure temperatures have to be used. This results in that the temperature of mould and also of the blank being moulded follows and/or even matches the elevated cure temperature of the heated liquid resin. Consequently, a thermal expansion of each of the components occurs and, as the thermal coefficients of 25 expansion of each of the mould materials used are different, a differential thermal expansion of the components results and the profile of the resultant optical element at ambient temperatures differs from the required profile in manner which can neither be accurately estimated for nor compensated against. Afurther disadvantage is that the shrinkage of the liquid resin, on curing or hardening, increases with an increase in the cure temperature for short cure times. The amount of shrinkage that the liquid resin 30 additionally shrinks on curing is related to the type of resin used, the degree of polymerization and whether or not special fillers can be incorporated therein in order to compensate for the shrinkage. Many resins however which have a desired optical clarity also have a high shrinkage on curing and an unexpected unreliable coefficient of thermal expansion which, priorto the present invention, were excluded from use for optical elements of high precision elements, particularly when produced by compression moulding 35 techniques and, it is undesirable to include more than very small amounts of fillers as additives to such resins to reduce shrinkage. For similar reasons it is also undesirable to add to the liquid resins heretofore used in more than very small quantities, other additives, such as, catalysts, plasticizers and/or accelerators in order to increase the rate of cure of the resin. The cure time for a large number of light transmissive thermosetting resins at slightly elevated ambient temperatures frequently is between 10 minutes to 30 40 minutes for each element. This time is still too long for quantity production purposes and the resins even after this time are still frequently in an easily deformable and damageable form with their full hardness not being reached for 24 hours or more. Furthermore, during the cure time whilst the resin is being hardened and cured in the mould tool the mould can not be used for the preparation of other optical elements. As such master mould tools are both expensive to prepare and in a limited supply this is most inconvenient and 45 effectively prevents the production of large numbers of substantially identical optical elements of high optical performance. One attempt to avoid the disadvantages of shrinkage of an all plastics element is described by U.K. Patent *No.858,218 wherein precision optical elements are made from moulded synthetic resins in which a main 'body of a polymerizable monomer composition is placed in a first mould having at least one substantially 50 Optically perfect mould surface, applying polymerizing conditions to set said body at least in an area ^adjacent said surface, shaping another surface of said body remote from the surface formed by said mould surface and affected by shrinkage on polymerization, applying to said other surface an additional body of monomer composition, placing the at least partly polymerized main body in a finishing mould with said other surface in engagement with a substantially optically perfect surface of said finishing mould and 55 applying conditions to complete the polymerization of said bodies and to fuse said bodies into a unitary whole. The Applicants' have discovered that certain internal stresses may be formed in composite plastics elements for example when formed by fusing one partly polymerized monomer to an additional body of a monomer. Such internal stresses can adversely affect the profile of the optical elements. The coefficient of 60 expansion of a polymer may reflect the thermal history that the polymer has been subjected to in the case of a composite body of plastics materials the thermal history of each component part should therefore either be identical or eliminated for at least the additional components. It is known from the Journal of Applied Physics, Vol 39, No.3, February 1968, pp. 1890-1899 that the coefficient of expansion of non-composite moulded plastics materials can change in accordance with the 65 GB 2 082 107 A thermal treatment applied below the glass transition temperature. In this article there is described the thermal treatment of plastics materials such as, poly-n-butyl methacrylate (PnBMA) when annealed at 0[deg.]C for 3 days and cooled to -190[deg.]C in one hour has a coefficient of expansion of 0.67 x 10<4>deg<-1>, when quenched from 100[deg.]C to -190[deg.]C has a coefficient of 0.83 x 10<4>deg<_1>; when quenched from -190[deg.]Cfor 1 hour from the glass transition temperature has a coefficient of 0.71 and when quenched from 55[deg.]C to - 78[deg.]C and held for 18 5 hours has a coefficient of expansion of 0.92 x 10<4>deg<_1>. These wide variations in the coefficient of expansion are thus produced as a function of the thermal treatment and are similarly applicable to other polymeric materials such as, poly n-alkyl methacryaltes, hydroxylated methacrylates, poly methoxy ethyl methacrylates and polymethyl acrylate over temperatures which range from -180[deg.]Cto the respective glass transition temperatures. The plastics materials were compression moulded in a mould at pressures of about 10 5000 Ib/sq. in. and at a temperature of between 100[deg.]C to 165[deg.]C and the following polymers were specifically * tested for various changes of linear coefficients of expansion belowtheir appropriate (major) glass transition temperatures (Tg). It is also of interest to note that certain of the polymers were also found inter alia to have minor or first (Tgg(1 )), second (Tgg(2)) and third (Tgg(3)) glass transition regions at different temperatures. 15 Polymethyl acrylate (PMA) Polymethyl methacrylate (PMMA) 20 Polyethyl methacrylate (PEMA) Poly /7-propyl methacrylate (Pn P Ma) Poly [pi]-butyl methacrylate (Pn BMA) 25 Poly [Lambda]-hexyl methacrylate (Pn HMA) Poly n-octyl methacrylate (Pn OMA) 30 Poly-/? decyl methacrylate (Pn DMA) Poly sec-butyl methacrylate (Ps BMA) Poly /so-butyl methacrylate (Pi BMA) 35 Poly tert butyl methacrylate (Pt BMA) Poly 2-hydroxy ethyl methacrylate (PHEMA) 40 Poly 2-hydroxy propyl methacrylate (PHP MA) and Poly 2-methoxy ethyl methacrylate (PMEMA) 45 The polymers were annealed at about 20[deg.]C below their glass transition temperatures prior to obtaining measurements with reproducible results. The Applicants' have found that in order to duplicate a large number of optical plastics elements of composite plastics materials each with at least one accurately profiled optical surface in which at least one of the composite polymeric material has a high coefficient of contraction ' on compression moulding and a uniform coefficient of thermal expansion after moulding that a main block 50 of the polymerizable material has to be subjected to an annealing treatment after moulding and prior to the application at ambient temperatures of a thin film of another polymerizable resin on a surface remote to the * moulded surface. In this manner internal stresses can be removed from the main polymeric block of material and a uniform coefficient of thermal expansion can be obtained which can be estimated to produce the required profile of main polymeric material and the profile of the film of the another polymeric material on 55 the remote surface. Also the cure time for moulding the main block of polymeric material can be greatly reduced by selecting resins which are easily mouldable and by providing a polymeric material in the form of a thin film on one surface which has a short cure time. The annealing of the main blocks of polymeric material after moulding can be effected in batch quantities and therefore the additional stage in the process does not provide any significant time delay. 60 One object of the present invention is to provide a method of duplicating large numbers of optical elements quickly by automatic means which method mitigates the above mentioned disadvantages. Another object is to provide a method whereby a substantially stress free plastics optical element is produced with a main body part from an optically good refractive plastics material having the one required profile and a minor part with a resin which has a low shrinkage on curing at ambient temperatures and a 65 GB 2 082 107 A highly accurate performance optical profile and surface finish on a surface remote from the required profile. A further object is to provide a light stable, composite plastics element which is substantially free from physical stresses which effect the optical performance, the coefficient of thermal expansion and the life of the optical element over various working temperature ranges. 5 According to the present invention there is provided a method of duplicating a large number of optical 5 elements, each optical element having at least one accurately profiled optical surface of a required shape, comprising placing a block of a polymerizable monomer composition in a hot mould tool which has at least one mould surface complementary to the required shape, moulding the block against the at least one complementary mould surface of the mould tool by the application of heat and pressure and removing the 10 moulded block therefrom, characterised in the an approximate profile and surface finish of the required 10 shape is formed on at least one surface of the block by moulding each of the large number of blocks of an - optically transparent, polymerizable monomer composition at a temperature above the softening or glass transition temperature of the composition with the application of pressure in a first hot mould tool, allowing - the or each moulded block to cool to ambient temperature under the applied pressure before removal from 5 the mould tool, annealing by further cooling and subsequently reheating the or each moulded block in the 15 free state to a temperature which is just below the softening point or glass transition temperature of the polymerized monomer composition so that on re-cooling to the ambient temperature, the moulded block or blocks, in the free state, are physically stress relieved and optically stabilized, sequentially placing each cold, stress relieved, moulded block in a second finishing mould tool comprising an optically polished, 0 transparent glass mould having at least one surface, complementary to the required surface at ambient 20 temperature, locating the moulded block in the second, optically polished finishing glass mould tool without accurately centering the mould so that there is a space between the surface of the moulded block remote to the at least one moulded surface and the complementary surface of the finishing glass mould tool, centering in the mould by filling the space with a low viscosity liquid, photopolymerizable resin, adhesively bonding 5 the resin, at ambienttemperatures, to the surface of the moulded block by passing a polymerizing radiation 25 through the transparent glass finishing mould tool to polymerize and harden the liquid photopolymerizable resin and removing the optical element so formed from the finishing glass mould tool. The expression, "a large number" is understood to mean any number of optical elements which may be produced relatively quickly from one or more master moulds and which, if desired may be produced at the 0 reate of up to several thousand per year per mould. 30 Preferably, the photopolymerizable liquid resin forms a thin liquid layer on the moulded block which is hardened by irradiating the resin with ultra-violet light which passes through at least one part of the transparent finishing, glass mould tool, and wherein said finishing glass mould tool comprises a glass which is substantially transparent to ultra-violet light radiation. 5 The liquid photopolymerizable resin preferably may be bonded and hardened to form a transparent, 35 light-stable layer by irradiation with ultra-violet light of intensity of approximately 7.4. milliwatts per. sq. cm. at an exposure time from a few seconds to less than 5 minutes. Other light radiation sources may be used to effect the curing of the resin such as laser light pulses, infra-red light or visible light radiations. 0 In one preferred method according to the invention the polymerizable monomer composition is moulded 40 against a heated, nickel plated steel surface of the mould tool having the required surface which surface is rapidly cooled to ambienttemperatures by cooling means comprising a cooling and or heating coil through which a heat exchange medium is passed prior to removal of the applied pressure and prior to the removal of the mould block from the mould tool, said moulded block or blocks in the free state, then being further 5 cooled followed by being slowly raised to a temperature just below the softening temperature or the resin of 45 the moulded block or blocks, re-cooled to ambient temperature after which each block is sequentially placed, without centering, in close proximity to the complementary surface of the finishing glass mould tool, the or each moulded block being centrally positioned by surface tension of the liquid photopolymerizable resin * which is admitted into said space via an inlet aperture, and wherein residual air in the spaced is expelled via f an output aperture above the inlet apertures. 50 The physical and optical properties of various moulded plastics materials may differ greatly and it is<">desirable to use only those light transmissive plastics materials for the blocks oc polymerizable monomer which can be readily moulded with heat and applied pressure in a compression moulding tool and which when moulded have a refractive index substantially the same as optical glasses and have good light stability 5 in use. 55 A high coefficient of thermal contraction or expansion is thus no longer a disadvantage in being variable and in causing distortions, when bonded to other polymeric materials, over the range of temperatures that the optical element is subject to in normal use. Suitable plastics materials for the main block of the optical element according to the invention may 0 therefore be polymerizable thermoplastic or thermosetting resins. Suitable resins are known perse and are 60 described in the Applicants' copending U.K. Patent Application No. 42482/77 in which there is described mono, di, tri, or tetra acrylic acid resins, polyesters, polycarbonate resins, epoxy resin monomers, and polyurethane resin monomers or compositions which contain combinations of these resins. The polymerizable thermoplastics or thermosetting resins may be moulded against the complementary 5 surface of the mould at an elevated temperature which is sufficient to cause either the full or partial 65 GB 2 082 107 A polymerization of the resin. No difficulty is experienced in obtaining the release of the moulded resins from the mould because, after heating to effect curing, the moulded monomer resin block is cooled whilst being pressed against the complementary profile of the required surface and, when cold, on releasing the pressure applied to the moulded block the block easily separates from the mould. When a metal mould is used the coefficient of 5 contraction on cooling the mould is greaterthan that of the moulded resin and release from the mould is facilitated. The moulded block then in the free state or in a loose manner, is further cooled followed by being placed in an oven, if desired with other moulded blocks, and brought to a temperature just below the softening point or glass transition point of the polymerized resin. Heating of the mould block or blocks is continued until all 10 the physical and optical stresses normally present in the compression moulded blocks are released. The moulded blocks in the mould were held at a pressure of approximately 5000 Ib/sq. in. until the I moulded profile was effected and then the temperature was lowered from between 100[deg.]C to 165[deg.]C to ambient temperature. On removal from the mould each block was cooled to a temperature of at least -10[deg.]C and preferably to - 40[deg.]C after which the block(s) were gradually heated in the free state to just below the 15 glass transition point of the polymer. A poly n-butyl methacrylate block was cooled after moulding to -40<C>C and a linear coefficient of thermal expansion was obtained after annealing at 20[deg.]C. Similarly, poly n-propyl methacrylate block was annealed at just below 40[deg.]C. polyethyl methacrylate block at just below 60<C>C and polymethyl methacrylate block was annealed at just below 100[deg.]C. Each of the above series of moulded blocks were removed from the oven and sequentially placed in a 20 finishing mould with the surface remote to the moulded surface facing a required surface profile. The profile of the moulded block is normally the same shape as the profile of the remote face after coating with the liquid photopolymerizable resin. No preshaping or machining of the remote surface is required prior to placing the moulded block in the finishing mould. This is true even if the remote surface of the moulded block is substantially flat when a larger space will be formed between the two surfaces in the finishing block 25 and a thicker coating of photo-polymerizable resin is formed. Thus the need for conventional shaping procedures is avoided and time consuming grinding and or centering the moulded block in the finishing glass mould is eliminated. The moulded block when assembled in the finishing glass mould tool and the space between the remote moulded surface and the complementary surface of the glass mould tool is filled with liquid resin which self centres the moulded block by surface tension of fthe liquid resin. The liquid resin 30 before curing is preferably a low viscosity resin for this reason. If high viscosity liquid resins are used the glass mould tool may be placed with the complementary moulding face uppermost so that gravity assists the self centering process. It may be necessary in this case afterthe space is filled with liquid resin to remove the inlet pipe through which the liquid resin is passed to the inlet aperture of the space and then to seal both the inlet and outlet apertures of the space before re-orientating the glass mould. 35 Suitable liquid photopolymerizable resins are substantially solvent free resin liquid compositions comprising a light curable resin which is preferably capable of co-polymerizing with the transparent polymerized polymer of the stress relieved moulded block. Such liquid light curable resins or liquid ultra-violet light curable resins are preferably aryl azido compounds which contain a substantial proportion of side chains having azide (-N3) groups and wherein not more than two -N3groups are attached to any 40 one side chain. Aryl azido compounds are photopolymerizable film forming polymers which on exposure to actinic radiation are hard, transparent, light-stable and have good adhesive properties. Their shrinkage on curing is minimal and after exposure a resulution geometry of the layer or film so formed can be obtained to an accuracy of 1 micron. THe aryl azido compounds are represented by the formula 45 > A - B - N3 in which formula A represents the recurring atoms of a polymer chain and B represents the linkage joining 50 the azide (-N3) group. The linkage group B may be a heterocyclic group joined to a dihydric phenol. The condensation product of cyanuric azido dihalide and an alkali metal salt of bis (p. hydroxyphenol) propane-2 is paricularly suitable. P. azidobenzoic acid condensed with an epoxy resin monomer also gives a suitable light sensitive reaction product. In this case two moles of p. azidobenzoic acid (16.3 grams) were reacted in 150 ml. toluene and 2 ml. 55 benzyl tri methyl ammonium hydroxide with one mole of an epoxy resin monomer {17 grams of resin of molecular weight 340). After refluxing for 7 hours the product was separated and 2 grams of Michler's ketone was added and the composition so formed was polymerized with an optimum exposure time of 1 minute. Novalac epoxy resin monomer and p. azidobenzoic acid give a suitable condensation product with a polymerization exposure time of 15 seconds and a profile resolution geometry accuracy of the layer so 60 formed of 0.6 microns. A higher molecular weight epoxy resin (MW 1400) with p. azidobenzoyl chloride give a condensation product with an optimum exposure time of 40 seconds and with a profile geometry resolution of the polymerized resin layer of 1 micron. An optimum exposure time of 20 seconds was obtained with the reaction product of p. azidobenzoyl 65 GB 2 082 107 A chloride and phthalic anhydride glycerol. P. azidobenzoyl chloride condensed with sucrose gave a photo-sensitive product of exposure time of 1 minute and a resolution geometry ofthe exposed layer of 0.6 microns and p. azidaobenzoyl chloride condensed with phthalic anhydride sorbitol had an exposure time of 45 seconds for polymerization and the 5 polymerized layer resulted in a resolution geometry of the exposed profile layer with a 1 micron accuracy. 5 Physical mixtures ofthe light sensitive polymerizable resins may be used with other photopolymerizable resins or with fillers, sensitizing agents, catalysts and accelerators if desired. Strong light radiations, in general, are not required to initiate the curing and polymerization ofthe resins. An 80 watt mercury vapour lamp, for example, is normally sufficient particularly if located so as to be 0 directed to focus through the lens like glass finishing mould. 10 The optimum degree of exposure time for polymerization may be obtained by incorporating into the resin - a catalytic quantity of a sensitizing agent or a photo initiator for example, an aromatic carbonyl compound<>such as Michler's ketone and analogues thereof, or quinolizine, pyrazoline or benzanthraquinone. Such r sensitizer's or photoinitiators form free radicals on absorbing light rays and the free radicals on absorbing 5 light rays and the free radicals are capable of enhancing the polymerization reactions, preferably in the 15 absence of oxygen, to effect the curing and hardening ofthe liquid resin film. Figure 1 represents an optical element when prepared according to the method ofthe invention. Figure 2 represents a moulded block of a transparent polymerizable polymer in a compression mould tool, Figure 3 illustrates a moulded block of a transparent polymerizable polymer in a finishing glass mould for 0 application of a coating of a photopolymerizable liquid resin to the surface ofthe moulded block remote from 20 the moulded surface ofthe block. The optical element shown by Figure 1 comprises a stress relievedmoulded block {1 ) of a transparent polymerizable polymer with one surface (2) ofthe block (1) having an optical surface finish and shape ofthe required profile and a surface (3) remote from the required surface (2) with a coating (4) of a 5 photopolymerized resin adhesively bonded thereto. The coating (4) has an optical surface finish and a profile 25 (5) which is formed by being moulded in a finishing transparent glass mould through which light radiations are passed to harden and cure the photopolymerizable resin from liquid state to the solid state and to form a profile complementary to the profile of the finishing glass mould. Figure 2 shows the moulded block (1) in a compression mould tool prior to removal ofthe block (1) after 0 the block (1 ) is mouled by the application of heat and pressure and also after cooling. The mould tool has a 30 mould surface (6) and a platen (7) which is complementary to the required surface (2) ofthe block (1 ) and a movable compression platen (8). Heating and cooling coils (9, 10) are shown in the respective platens (7) and (8) for the passage of a heat exchange medium to heat, or to rapidly cool, the moulded block (1 ). In Figure 3 the moulded block (1) is held in a support (11 ) with a gasket sealing member (12) so that the face (3) remote from the moulded surface (2) is in close proximity to an optically polished and profiled face 35 (13) ofthe finishing glass mould (14). An inlet aperture (15) leading to a space (16) between the face (3) and face (13) allows a quantity of liquid photopolymerizable resin from a reservoir (17) to fill the space (16) and an outlet aperture (18) permits residual air to leave the space (16). When the (16) is completely filled with liquid resin the reservoir (17) the inlet pipe (19) is removed and light radiations are passed through the finishing glass mould (14). The liquid resin is hardened by the light radiations and adhesively bonds to the 40 surface (3) ofthe moulded block (1 ). The finishing glass mould (14) may be treated if desired with a surface release agent to give a monomolecular layer of release agent, such as, silicone oil. The glass block (14) is removed and the optical element so formed was found to require no further treatment before use, except for the removal of residual resin from the periphery ofthe element. In carrying outthe method according to the invention the surface ofthe finishing glass mould is ground 45 and optically polished to a surface finish and shape which is complementary to the required surface finish and shape and the surface ofthe mould is then preferably treated or annealed, so as to render it both physically 'and chemically inert to the liquid photopolymerizable resin which adhesively bonds to the<>moulded block. * The finishing glass mould is prepared from a transparent glass material which preferably has a low linear 50<">coefficient of thermal expansion, for example, of 0.65 x 10<-4>or less and preferably less than 0.05 x 10<~4>. - The glass mould should be substantially transparent to the photopolymerizing radiation. A number of impurities may be present on the surface ofthe glass finishing mould after grinding and polishing operation is complete and these impurities may contaminate the liquid resin or change the radiation transmissivity ofthe glass mould. These impurities can adversely effect both the rate of 55 polymerization and the degree of polymerization produced in the liquid photopolymerizable resin particularly for liquid photopolymerizable resins when in the form of a very thin film. The surface ofthe glass mould may, for example, contain a number of undesirable reactive sites and also have an undesirable surface porosity after grinding and polishing which sites can cause the liquid photopolymerizable resin to firmly adhere to the mould. The Applicants' have found that the glass finishing 60 mould can be treated and sealed or annealed on order to obtain an easy and a good release from the mould whe[alpha].the hardened liquid resin is bonded to the moulded plastics block. The glass finishing mould after grinding and polishing by conventional means with an aqueous based grinding and/or polishing agent (such as, an aqueous based mixture containing cerium oxide or aluminium oxide), has a silica surface which is slightly polar and acidic. This surface exhibits, in the untreated state, a 65

Claims

GB 2 082 107 A marked preference for the absorption and adherence thereto of basic and polar resinous compounds. An untreated surface of such polished glass moulds may also have small, but not insignificant, amount of water bonded thereto. In one instance, this may be due to capillary action (as capillary-held-water molecules) and in another instance, this may be due to surface hydroxyl groups (as silanol groups, [Xi] (SiOH). Forthe purpose ofthe present invention the capillary-held-water molecules are removed by heat 5 treatment ofthe mould to a temperature of between 150[deg.]C to 200[deg.]C and the hydroxyl bound (silanol) groups are removed preferably by the continued heat treatment of the mould to a temperature of between 200[deg.]C to 400<C>C. A particularly suitable temperature is 200[deg.]C. It can be shown that the surface silanol groups are then substantially converted to siloxane groups in accordance with the equation, 10 2( [identical to] SiOH) -≥ Si-O-Si [identical to] + H20 The treated surface ofthe glass finishing mould when treated in the above manner has no preferential 15 absorption for liquid photopolymerizable resins which contain basic groups, polar groups and/or unsaturated organo groups and such resins exhibit good mould release properties. Good mould release may be further enhanced by sealing the aforesaid surface porosity ofthe glass finishing mould, which may comprise a micro-porosity of up to 5 to 15 micron diameter, by the application of a mould release agent. Surface porosity ofthe mould can also be sealed by reacting residual surface silanol groups with a 20 silanizing agent, such as, dimethyldichlorosilane to form silyl ether surface groups, such as -Si-0-Si(CH3)3groups. In orderto avoid any undesired dimensional changes ofthe surface ofthe finishing mould, mould release agents which comprise the formation of a silyl ether group, should be restricted to form a very thin surface layer, which is for example, positioned substantially within the microporous structure. A monomolecular 25 thickness of silyl ether was found to be satisfactory for sealing such micro porosity. Other more conventional mould release agents may be used in orderto assist mould release, in which case the surface ofthe mould may be treated with an inert release agent, such as, a monomolecular layer of a silicone oil or a inert wax, which can be freshly applied after each mould use. Preparation ofthe finishing glass mould after grinding and polishing may be summarised as being 30 treated: (1 ) to remove surface mineral impurities as the result of grinding and polishing with an acid and/or a base wash, (2) to remove physically bound surface water and/or chemically bound hydroxyl groups by the application of heat, 35 (3) optionally to react residual surface chemcially bound hydroxy (silanol) groups with a silanizing agent to produce surface silyl ether groups and, (4) to treat the surface of the mould before each use with an inert mould release agent. The required surface ofthe glass finishing mould after treatment, should have an optionally polished profile with a profile accuracy of at least 10 microns, and if desired, an acuracy of 0.5 microns which profile is 40 hard,_durable and is accompanied with good resin release properties. A similar profile accuracy may be obtained for the mould for producing the moulded plastics blocks but for reasons given above regarding thermal expansion during the moulding process such accuracy is not normally necessary. The moulded plastics blocks may be moulded from a steel mould having the at least one surface ofthe required complementary shape and the steel mould for convenience of producing the 45 required profile may be electroplated with a layer of nickel. The layer of nickel when electrodeposited is sufficiently thick so that it can be suitably finely machined, for example, with a single point diamond machine cutting tool and the finely machined surface so produced can be protected with a thin layer of an inert scratch resistant material, such as, vapour deposited layer of vitreous carbon by thermal degradation of an<s>organic polymer. 50 CLAIMS

1. A method of duplicating a large number of optical elements, each optical element having at least one accurately profiled optical surface of a required shape, comprising placing a block of a polymerizable 55 monomer composition in ahot mould tool which has at least one mould surface complementary to the required shape, moulding the block against the at least one complementary mould surface ofthe mould tool by the application of heat and pressure and removing the moulded block therefrom, characterised in that an approximate profile and surface finish ofthe required shape is formed on at least one surface ofthe block by moulding each ofthe large number of blocks of an optically transparent, polymerizable monomer 60 composition at a temperature above the softening or glass transition temperature of the composition with the application of pressure in a first hot mould tool, allowing the or each moulded block to cool under the applied pressure before removal from the mould tool, annealing by further cooling and reheating the or each moulded block in the free state to a temperature which is just below the softening point or glass transition temperature ofthe polymerized monomer composition so that on re-cooling to the ambient temperature, the 65 GB 2 082 107 A moulded block of blocks, in the free state, are physically stress relieved and optically stabilized, sequentially placing each cold, stress relieved, moulded block in a second finishing mould tool comprising an optically polished, transparent glass mould having at least one surface, complementary to the required surface at ambient temperature, locating the moulded block in the second, optically polished finishing glass mould tool 5 without accurately centering the mould so that there is a space between the surface of the moulded block 5 remote to the at least one moulded surface and the complementary surface of the finishing glass mould tool, centering in the mould by filling the space with a low viscosity liquid, photopolymerizable resin, adhesively bonding the resin, at ambienttemperatures, to the surface ofthe moulded block by passing a polymerizing radiation through the transparent glass finishing mould tool to polymerize and harden the liquid 10 photopolymerizable resin and removing the optical element so formed from the finishing glass mould tool. 10

2. A method according to Claim 1, characterized in that the photopolymerizable liquid resin forms a thin . liquid layer on the moulded block which is hardened by irradiating the resin with ultra-violet light which passes through at least one part or the transparent finishing glass mould tool, and wherein said finishing . glass mould tool comprises a glass which is substantially transparent to ultra-violet light radiation. 15

3. A method according to Claim 1 or Claim 2, characterised in that the liquid photopolymerizable resin is 15 bonded and hardened to form a transparent, light-stable layer irradiation with ultra-violet light of intensity of approximately 7.4 milliwatts per sq. cm. at an exposure time from a few second to less than 5 minutes.

4. A method according to any one of Claims 1 to 3, characterised in that the polymerizable monomer composition is moulded against a heated, nickel plated steel surface ofthe mould tool having the required 20 surface which surface is rapidly cooled to ambienttemperatures by cooling means comprising a cooling and 20 or heating coil through which a heat exchange medium is passed prior to removal ofthe applied pressure and priorto the removal ofthe moulded block from the mould tool, said moulded block or blocks in the free state, then being further cooled followed by being slowly raised to a temperature just below the softening temperature ofthe resin ofthe moulded block or blocks, re-cooled to ambient temperature after which each 25 block is sequentially placed, without centering, in close proximity to the complementary surface of the 25 finishing glass mould tool, the or each moulded block being centrally positioned by surface tension ofthe liquid photopolymerizable resin which is admitted into said space via an inlet aperture, and wherein residual air in the space is expelled via an output aperture above the inlet apertures.

5. An optical element when manufactured by any ofthe methods as claimed in Claims 1 to 4. 30

6. A method of duplicating optical elements substantially as hereinbefore described. 30

7. An optical element according to Claim 5 substantially as hereinbefore described. Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.