GB2150144A

GB2150144A - Polymeric substrates for optical information carriers

Info

Publication number: GB2150144A
Application number: GB08428359A
Authority: GB
Inventors: Jurgen Hennig; Ralf Liebler; Winfried Wunderlich
Original assignee: Rohm GmbH; Roehm GmbH Darmstadt
Current assignee: Rohm GmbH; Roehm GmbH Darmstadt
Priority date: 1983-11-11
Filing date: 1984-11-09
Publication date: 1985-06-26
Also published as: GB2150144B; GB8428359D0; IT8468127A1; FR2554951A1; NL8403187A; IT1179819B; JPS60118705A; DE3340875A1; FR2554951B1; IT8468127A0

Abstract

Polymeric substrates for optical information carriers comprise a copolymer of A) one or more aromatic vinyl compounds of formula I <IMAGE> wherein R1 represents a hydrogen atom or a methyl group and Ar represents a phenyl group optionally substituted by a C1-6 alkyl group, in amounts of from 90 to 60% by weight, based on the total weight of the polymeric substrate P B) one or more polymerisable aprotic monomers of formula II <IMAGE> wherein R2 represents a hydrogen atom or a methyl group and X represents a -CN group or a group <IMAGE> wherein A represents an optionally branched hydrocarbon group with 1 to 8 carbon atoms and Z represents a chlorine atom or a group selected from; <IMAGE> the monomers being present in amounts of from 10 to 40% by weight based on the total weight of the polymeric substrate P. Further monomers may be included.

Description

SPECIFICATION Polymeric substrates for optical information carriers The present invention relates to polymeric substrates for optical information carriers that are distinguishable by their low water uptake and favourable optical and mechanical properties.

Optically readable information carriers are often in the form of discs with a stratified structure, one of the inner layers containing the so-called information track.

DE-OS 28 53 262, discloses a disc-shaped information carrier which comprises on one or both sides a radiation-reflecting structure which contains the information and which can be optically, read, the reflecting structure having a radiation-permeable covering layer, this covering layer being formed by a radiationpermeable disc-shaped covering plate which is connected to the reflecting structure by means of a radiation-curable lacquer.

The radiation-curable lacquer is preferably a protic mixture of esters of acrylic acid, whilst the covering plate is a plastics plate having a minimum thickness of 200 Itm which may consist of transparent polyvinylchloride, polyvinylchloride-acetate copolymer, polycarbonate or polymethylmethacrylate. From the optical and mechanical point of view, polymethylmethacrylate itself (PMMA) and its derivatives appear to be particularly suitable for optical information carriers.

However, a number of publications have highlighted the problems which may arise as the result of moisture uptake, particularly the formation of a moisture gradient in the information carrier discs.

To solve these problems, DE-OS 30 28 498 proposes that the water uptake of the PMMA be minimised during the forming and subsequent finishing steps. The undesirable effects of a non-uniform water uptake (the socalled "umbrella effect") are also central to US-PS 4 310 919. There, the solution to the problem is seen as the construction of totally symmetrical information carrier plates.

DE-OS 30 19 189 recommends the use of a radiation-curable lacquer which comprises oligomeric acrylates or methacrylates which contain, on average between 1 and 4 acrylate or methacrylate groups per molecule, this lacquer having a viscosity of from 1000 to 1500 cP and which also comprises a photoinitiator in a concentration of from 0.1 to 0.5% by weight. The lacquer preferably contain at least 90% by weight of an opoxylated or alkoxylated bisphenol and/or novolak containing on average between 1 and 3 acrylate or methacrylate groups per molecule.

Our German Patent Application P 32 48 601.4, also proposes improved polymer materials as substrates for optically readable data carriers. Polymers with low water uptake and low birefringence characteristics are recommended, such polymers comprising 10-100% by weight of methylstyrene, preferably 20-80% by weight, optionally together with 0-20% by weight of other vinyl aromatic compounds, 0-50% by weight of aromatically substituted (meth)acrylic acid esters, 0-80% by weight of methylmethacrylate, 0.50% by weight of other (meth)acrylic acid esters, 0-30% by weight of other comonomers, the solubility of which does not exceed a level of 50 g of monomer per litre of water, and 0-10% by weight of cross-linking monomers, with the proviso that the proportion of monomers containing aromatic groups is at least 10% by weight, based on the total monomers.

Our German Patent Application P 32 48 602.2 proposes acrylic resins with low water uptake characteristics as suitable carrier materials for optically readable information. The composition is made up of at least three comonomer groups, 20-85% by weight consisting of methylmethacrylate whilst the remainder consists of aromatically substituted (meth)acrylic acid esters and/or aliphatic or alicyclic hydrocarbon groups other than methylmethacrylate, and/or styrene or ex-me- thylstyrene, and up to 9.9% by weight of 4methylstyrene and optionally other monomers with limited water solubility and/or crosslinking monomers.

Both the latter two publications stress the problem of the birefringence as an important parameter in addition to that of water uptake.

Our practical experience with optical information carriers based on plastics has resulted in our recognition of a number of characteristics which would appear to be desirable if the product is to be technologically useful: These include -transparency -low water uptake -low birefringence -low corrosion in the presence of solvent and other chemical substances used in processing -adequate mechanical strength and -adequate impact resistance.

The publications discussed above reveal, on the one hand, the tendency to use typically hydrophobic monomers for polymeric optical information carriers and on the other hand the efforts to counteract the disadvantageous consequences of water uptake. There is therefore a need to provide materials which satisfy the above technical requirements asfaras possible.

We have now been able to develop a special range of compositions based on vinyl compounds which largely satisfy the above criteria for an optically readable data carrier.

According to the invention we provide a polymeric substrate for optical information carriers, wherein the polymeric substrate P is a copolymer of inter alia, A) one or more aromatic vinyl compounds of formula I

wherein R1 represents a hydrogen atom or a methyl group and Ar represents a phenyl group optionally substituted by a C1 6 alkyl group, in amounts of from 90 to 60% by weight, based on the total weight of the polymeric substrate P B) one or more polymerisable aprotic monomers of formula 11

wherein R2 represents a hydrogen atom or a methyl group and X represents a-CN group or a group

wherein A represents an optionally branched hydrocarbon group with 1 to 8 carbon atoms and Z represents a chlorine atom or a group selected from;;

the monomers being present in amounts of from 10 to 40% by weight based on the total weight of the polymeric substrate P.

An aprotic monomer in the sense of this invention is a monomer which does not contain any hetero atoms such as oxygen or nitrogen directly associated with hydrogen atoms.

The polymeric substrate may also include comonomers (C) other than those of (A) and (B) above the solubility of which does not exceed a level of 50 g of monomer per litre of water at 25 C, in amounts of from 0 to 30% by weight based on the total weight of the polymeric substrate P.

Aromatic vinyl compounds of formula I had been proposed as monomers for use in the production of polymeric optically readable data carriers, but appeared to be ruled out by their tendency to form birefringent units.

It was not to be expected that, by including comonomers of formula 11 which have a cohesion-promoting group in the terminal position, it would be possible to satisfy the technological requirements to a higher degree than with the measures and materials proposed in the prior art, and indeed the presence of the (polar) groups X in the copolymers would be expected to increase rather than reduce the tendency to water uptake.

Examples of aromatic vinyl compounds of formula I include styrene, (E-methylstyrene and substituted styrenes. Styrene is preferred.

Examples of monomers of formula li include acrylonitrile and methacrylonitrile, and ss-cya- noethyl, chlorotethyl. 2,3-epoxypropyl. N-morpholinoethyl, methoxyethyl and ethoxyethyl esters of acrylic and methacrylic acid. Acrylonitrile is preferred.

The monomers of formula II are known per se (cf H. Rauch-Puntigam and T. Völker in "Acrylic and Methacrylic Compounds", Springer-Verlag, 1967 and Houben-Weyl, vol.

XIV/1, 4th Ed., Georg-Thiem-Verlag, Stuttgart 1961, pages 1036-1037).

The monomers of type C) which may be present are monomers different from A) and B). Preferred monomers of type C) include olefins such as butadiene, isoprene and isobutylene, esters such as itaconic, maleic and fumaric acid esters, particularly the methyl and ethyl esters thereof, vinylidene chloride and vinyl esters of carboxylic acids with more than two carbon atoms. The monomers of type C) are preferably present at 0-10% by weight, based on the total weight of polymer.

Cross-linking monomers, e.g. butanediol-dimethacrylate, may be also be present in small amounts (generally less than 10% by weight).

The choice of monomers from B) and C) is preferably such that the glass temperature Tg determined by differential scanning calorimetry (DSC) or by dilatometry of the resulting polymer P is not less than 70"C, more preferably not less than 85 C. Polymerisation may be carried out by conventional techniques.

The polymeric substrates P according to the present invention generally have, as copolymers, an average molecular weight Mw in the range from > 106 to about 5.106. Their VICAT softening temperature (according to DIN 53460) is generally > 90 C. The copolymers generally have an elasticity modulus (according to DIN 53457) of > 2700 Nm-2 and an elongation at break (according to DIN 53455) of > 2%.

The water uptake of the copolymers (according to DIN 53495) usually does not exceed 1 % by weight, and often does not exceed 0.5% by weight.

Polymerisation may be carried out using radical intiators, e.g. organic peroxides and peresters, which are soluble in the monomers, such as dibenzoyl peroxide or t-butyl-per-2ethylhexanoate or using azo compounds such as azo-iso-butyronitrile.

The quantities of initiator used may be within the conventional range, usually from about 0.01 to 0.5% by weight, based on the total weight of monomers.

The molecular weight may be adjusted using polymerisation regulators, e.g. sulphur regulators such as n- and tert-dodecylmercaptan and 2-ethylhexylthioglycolate. As is known, the molecular weight of the polymers may be controlled by means of the initiator and, more particularly, by the addition of regulator. Preferably the quantities of regulator used are from 0 to 0.5% by weight, based on the total weight of monomers.

Polymerisation processes already known for styrene and the copolymers thereof with other monomers may be used. (Cf. Houben-Weyl, Methoden der organischen Chemie, 4. Edition, Volume 14/1. (Bulk polymerisation has proved particularly suitable for the purpose herein described. (Cf. Rauch-Puntigam and T.

Völker loc. cit., pages 203-207, 274-275.) According to a further aspect of the invention we provide a process for the production of a polymeric substrate useful as a carrier for optical information which comprises copolymerising from 90 to 60% by weight one or more aromatic vinyl compounds of formula I as defined above and from 10 to 40% by weight of polymerisable aprotic monomers of formula II as defined above.

In accordance with the known methods of bulk polymerisation, the polymerisation is preferably carried out between two panes of glass held at a suitable spacing by means of spacers. The copolymers P may be introduced between the panes of glass by pouring either in the form of the comonomer mixture or as a prepolymeric syrup with suitable polymerisation adjuvants and then polymerised whilst the steps appropriate and convenient to produce good surface quality and low thickness tolerance are taken (Cf. Winnacker-Küchler, Chemische Technologie, Volume 6, pages 412-413, Hanser-Verlag, Vienna 1982.) The actual polymerisation generally takes from 1 to 1 + days, the substances being heated to a temperature above ambient temperature and below 60 C, e.g. 45 C. During the last stage of polymerisation (about 30% of the time) the substances should be heated to a high temperature, for example 105 C.

The polymerisation produces polymeric substrates which are characterised, inter alia, by optical clarity and perfect surface quality. The substrates usually show no signs of any undesirable birefringence and have a water saturation content which does not exceed 0.7% by weight according to present observations. The good transparency (TD = 90%) and the favourable mechanical properties such as tensile strength, elongation at break, tension on cracking, and elasticity modulus (see above) should be emphasized.

For producing the information storage discs according to the present invention, circular glass chambers are suitably used for polymerization. The discs so obtained have a diameter from 30-400 mm and are suitably 0.5-2 mm thick.

The following non-limiting Examples illustrate the present invention.

The physical data were determined by the following methods: VICAT softening temperature ( C) according to DIN 53460 tensile strength according to DIN 53455 Elasticity modulus (E modulus) according to DIN 53457 Elongation at break according to DIN 53455 Birefringence (An): compensation method using a polarisation microscope. The results given as 0 are within the accuracy of measurement of the method.

Water uptake according to DIN 53495 Example 1 A mixture of 75% by weight of styrene and 25% by weight of methyl methacrylate is polymerised for 5 minutes at 120 C with the addition of 0.3% of tert-butylperpivalate. The syrup is cooled and, after the addition of 0.7% of tert-butylperpivalate, 0.3% of glycoldimethacrylate and 0.005% of dinonylphosphate, poured into glass chambers. After a polymerisation period of 48 hours at 50 C and 16 hours tempering at 100 C, a clear, colourless polymer plate is obtained with a VICAT softening temperature of 90 C.

The following measurements were also obtained: Water uptake: 0.3% Birefringence An: O E modulus: 2900 N.mm-2 Tensile strength: 53.4 N.mm-2 Elongation at break: 2%.

Example 2 80% by weight of p-methylstyrene were mixed with 20% acrylonitrile, 0.2% i-propylpercarbonate, 0.2% tert.-butylperpivalate and 0.2% peroctoate and polymerised in glass chambers for 40 hours at 45 C. The product was then tempered at 105 C for 6 hours. The clear colourless polymer had the following properties: VICAT softening temperature: 106 C Water uptake: 0.5% Birefringence An: O E modulus: 3200 N.mm-2 Tensile strength: 70 N.mm-2 Elongation at break: 2.8%.

Example 3 60% by weight of a-methylstyrene and 40% by weight of acrylonitrile were polymerised with the combination of initiators used in Example 2 in glass chambers at 45 C for one week and then tempered for 6 hours at 105 C. Clear, virtually colourless plates were obtained with the following properties: VICAT softening temperature: 104 C Water uptake: 0.7% Birefringence An: O.

Example 4 75% by weight of styrene and 25% by weight of acrylonitrile were mixed with the initiator combination used in Example 2 and polymerised in glass chambers at 45 C for 40 hours. A clear colourless polymer was obtained with the following properties: VICAT softening temperature 108 C Water uptake: 0.6% Birefringence An: O E modulus: 3500 N.mm Tensile strength: 75 Elongation at break: 2.6%.

The acrylonitrile as used in Examples 2-4 may be substituted by at least one of the following monomers yielding clear, colourless copolymers with similar properties.

cyanoethylmethacrylate glycidylmethacrylate morpholinoethylmethacrylate methacrylonitrile 2- [ 2-(2-ethoxy)ethoxyjethylmethacrylate chloroethylacrylate.

chloroethylmethacrylate

Claims

1. A polymeric substrate for optical information carriers, wherein the polymeric substrate P is a copolymer of inter alia, A) one or more aromatic vinyl compounds of formula I

wherein R, represents a hydrogen atom or a methyl group and Ar represents a phenyl group optionally substituted by a C1.6 alkyl group, in amounts of from 90 to 60% by weight, based on the total weight of the polymeric substrate P B) one or more polymerisable aprotic (as hereinbefore defined) monomers of formula II

wherein R2 represents a hydrogen atom or a methyl group and X represents a -CN group or a group

2. A polymeric substrate as claimed in claim 1 which also includes conomoners (C) other than these as defined in claim 1, the solubility of which does not exceed a level of 50 g of monomer per litre of water at 25 C, in amounts of from 0 to 30% by weight based on the total weight of the polymeric substrate P.

3. A polymeric substrate as claimed in claim 1 or claim 2 which also includes a cross-linking monomer.

4. A polymeric substrate as claimed in any one of claims 1, 2 or 3 wherein the aromatic vinyl compound of formula I is styrene.

5. A polymeric substrate as claimed in any one of claims 1 to 4 wherein the polymerisable aprotic monomer of formula II is acrylonitrile.

6. A polymeric substrate as claimed in any one of claims 1 to 5, wherein the copolymer has an average molecular weight Mw in the range from > 106 to 5. 106.

7. A polymeric substrate as claimed in any one of claims 1 to 6, wherein the copolymer has a VICAT softening temperature (according to DIN 53460) of > 90 C.

8. A polymeric substrate as claimed in any one of claims 1 to 7, wherein the copolymer has a elasticity modulus (according to DIN 53457) of > 2700 Nmm-2 and an elongation at break (according to DIN 53455) of > 2%.

9. A polymeric substrate as claimed in any one of claims 1 to 8, wherein the water uptake of the copolymer (according to DIN 53495) does not exceed 1% by weight.

10. A polymeric substrate as claimed in claim 9 wherein the water uptake of the copolymer (according to DIN 53495) does not exceed 0.5% by weight.

11. A polymeric substrate as claimed in claim 1 substantially as hereinbefore described.

12. A process for the production of a polymeric substrate useful as a carrier for optical information which comprises copolymerising from 90 to 60% by weight one or more aromatic vinyl compounds of formula I as defined in claim 1 and from 10 to 40% by weight of polymerisable aprotic monomers of formula II as defined in claim 1.

13. A process for the production of a moulding useful as carrier for optical information, wherein a polymeric substrate as claimed in any of claims 1 to 11 is used.

14. A process for the production of a moulding useful as a carrier of optical information from a polymeric substrate P as claimed in claim 1 wherein comonomers (A) and (B) as defined in claim 1 and optionally a comonomer (C) as defined in claim 2 are polymerised between glass plates in the form of a mixture of comonomers or a prepolymeric syrup.

15. A process as claimed in any one of claims 12 to 14 substantially as hereinbefore described.

16. Optical information carriers based on a polymeric material which is comprised of a polymeric substrate as claimed in any one of claims 1 to 11.