NL8602018A - Prodn. of polycarbonate strip for optical recording plate - by extruding, propelling extrudate between polished cooling rollers and biaxially stretching - Google Patents

Abstract

A polycarbonate strip is made by extruding the polymer through a nip, propelling the extrudate to a nip between 2 polished cooling rollers, rotating counter-currently, and held at a temp. such that the surface temp. of the extrudate falls to below the Tg of the polycarbonate, stretching the polycarbonate biaxially, either at the same time or in sequence, at stretching temp., in such a way that the surface roughness in the transverse direction is less than 0.01 microns, and the ratio between the transverse and longitudinal stretching results in molecular orientation in these directions of equal value and gives a stretched extrudate with a thickness at which the optical wavelength difference is less than 50 nM, and the thickness of the extrudate is 0.100-1.300 mm; the height of the nip is such that the stretching operation(s) can be carried out; the stretched extrudate is cooled to room temp.

Description

V 'u JJS / WP / mjh STAMICARBON B.V.

Inventors: Armand A.M.A. Duijsens in Bunde Rudolf J.H. Burlet at Ge Leen -1- PN 5573

METHOD FOR MANUFACTURING A COURSE POLYCARBONATE AND CARRIERS MADE THEREOF FOR OPTICALLY READABLE INFORMATION AND THE OPTICALLY READABLE INFORMATION CARRIER

A method of manufacturing a polycarbonate web suitable for producing carriers of optically readable information such as audio and video plates therefrom, said method comprising planar slit extrusion of polycarbonate and cooling the extrudate using polished cooling rolls.

Such a method is known from JP-A-0214-923. This film-making method is unsuitable for making polycarbonate sheet of about 1 mm thick that meets the specifications for producing carriers 10 therefrom for optically readable information. Namely, due to the flow in the extrusion gap, a longitudinal molecular orientation in the extrudate arises and, due to wear and contamination of the slit walls, even when polished, extrusion streaks appear on the surface of the extrudate after some time. Extrusion stripes are 15 longitudinal bumps. Even when using the polished cooling rollers as in JP-A-0214.923, these irregularities cannot be sufficiently eliminated. Both phenomena are likely to adversely affect the desired birefringence properties of the sheet material.

Namely, the plate suitable for the purpose should preferably be free from birefringence caused by unbalanced molecular orientation and must have a uniform thickness in all directions.

An optically readable information carrier may consist of a polycarbonate plastic disc provided on one of its parallel surfaces with optically readable information, which is formed by a surface structure track of pits or ridges. The dimensions of these pits or cams as well as the distances between them are usually in the size of 1 to 2 μιη. This optically readable information is covered by an inwardly reflecting, mirroring layer, for example an aluminum or silver layer, such that a laser beam, which enters the information carrier through the other surface, after a certain distance, depending on the thickness of the carrier, reaches the structure track, scans it and then is reflected by the specular layer. The reflected beam which, as it were, has been converted into an intermittent light beam by the presence of the pits or cams, forms a signal for further processing into image and / or sound.

The more the carrier material has the property of causing birefringence of a light beam, and the more irregular its surface structure its surface becomes, the less the usefulness of the reflected beam as a processable signal becomes. Namely, birefringence, the phenomenon in which a light beam with a certain wavelength is split into two components with different refractive index when entering a medium, leads to a greatly reduced usability of the reflected beam for conversion to image and / or sound. The degree of this unusability of the reflected beam further depends on the thickness of the support material or the path length that the beam must travel after refraction through the support material. Irregularities in the thickness as a result of extrusion defects of the sheet material thus also have an adverse effect on the quality of the image and / or the sound. A practical requirement that can be imposed on carriers for optically readable information is that the optical path length difference, represented by Δ nxd, is less than 50 nM, preferably less than 25 nM, where Δ n is the difference in refractive index of the light components and d is the thickness of the support expressed in nanometers (nM). Further requirements are that the surface roughness of the carrier is less than <0.01 μιη and that a polycarbonate carrier has a thickness between 1.0 and 1.3 mm for stiffness reasons. For a compact disc, the thickness is preferably between 1.05 and 1.15 mm and has §602018

yC

-3- - for normalization reasons a thickness of approx. 1.1 mm. Carbonate plate suitable for image plates can have a thickness of 1.25 mm.

The object of the invention is to provide a method for manufacturing a polycarbonate sheet suitable for manufacturing carriers therefrom for optically readable information which lacks the disadvantages of the known method and which can be flexibly adapted to changed requirements.

This is achieved by feeding the extrudate under some backwater to a gap between two oppositely rotating polished cooling rollers which are maintained at a temperature such that the surface temperature of the extrudate drops to a temperature below the glass point temperature of the polycarbonate, in that the extrudate is then subjected to a simultaneous or sequential biaxial stretching at a stretching temperature of the polycarbonate such that the surface roughness in the transverse direction is less than 0.01 µm, and the ratio between the transverse and the longitudinal thereby has that the molar orientation in the stretched extrudate in those directions reaches substantially the same value and the stretched extrudate assumes a thickness where the optical path length difference is less than 50 nM, and the thickness of the stretched extrudate is between 0.100 and 1.300 mm, in that adjust the height of the roll gap nig sets that the stretching operations can be performed and that the stretched extrudate is further cooled to ambient temperature.

In the method according to the invention, extrusion heads normally used for plate extrusion can be used without placing undue demands on the degree of smoothness of the walls of the extrusion gap, because extrusion defects in the form of longitudinal irregularities are largely eliminated by the subsequent operations. Irregularities in the extrusion gap walls caused by wear or contamination, no longer play an annoying role during operation. Feeding the extrudate to the gap between the cooling walls under some back-up is one of the mentioned operations for eliminating extrusion defects. The 860? C i 8

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The defects still present and the Longitudinal - i.e. Longitudinal - in the longitudinal direction of the extrudate - orientation of the polymer molecules are eliminated by choosing the correct transverse and longitudinal stretching ratio. The stretching further aims to achieve the desired thickness of the Ultimate sheet. The advantage of supplying the extrudate to the roll gap under some thrust is that the gap remains well filled and the extrudate remains pressed against both roll surfaces. The advantage of cooling an extrudate with a thickness greater than that of the final sheet is likely that the core of the extrudate remains at a high enough temperature to reheat the cooled surface layers after leaving the chill rollers, causing a more uniform voltage-relaxing cooling process can be obtained. The entire execution of the production process requires no more knowledge than the normally trained skilled person possesses. The adjustment is made on the basis of measurement data on the formed plate, which measurement data relate to the birefringence, the surface roughness and the thickness of the plate. If the surface roughness in the transverse direction is too great, more transversal stretching has to be done, whereby if the thickness of the plate thereby falls below the lower limit, the gap width between the cooling rollers must be increased. If Δn is too high, the stretching must be reduced in the transverse or longitudinal direction, respectively, or increased in the longitudinal or transverse direction, respectively, depending on too great a molecular orientation in the transverse or longitudinal direction. Changes in the stretching ratios must always be made taking into account the desired surface roughness. These changes, in turn, should not result in sheet thicknesses beyond specifications and should be accompanied by roll gap width changes if necessary.

The drawing is preferably carried out above the glass point temperature of the polycarbonate used.

It has been found that the draw ratios do not usually have to exceed three.

Suitable polycarbonates are 35 bisChydrophenylalkane polycarbonates. BisChydrophenyDalkane 8602018 0 / -5- polycarbonates means aromatic hydrocarbon polymers of the formula 0

II

A - 0 - 0 Jn 5 wherein A is a divalent phenol derived from a two OH groups containing phenol derived aromatic group, which polycarbonate can be obtained in a known manner by a condensation reaction of phosgene and a hydroxy (one or multi-ring aryl ) compound such as, for example, bis (4-hydroxyphenyt) propane of the formula: 10 CH3

noO-c -O OH

C 3 which yields the known bisphenol A (see Kunststoffhandbuch Band VIII, Polyester Seite 12, Carl Hanser Verlag Munich 1973).

The polycarbonates have a molecular weight such that they can be processed with an extruder and have sufficient strength to prevent sagging of the sheet during stretching.

The process can be carried out with some loss of process flexibility in two steps by ending the first step by cooling the extrudate leaving the cooling rollers and heating and stretching the cooled plate in a subsequent step.

The advantage of the two-step process is that the intermediate product can be stored and, if desired, further processed elsewhere into a polycarbonate sheet suitable for information carriers. The invention will be explained in more detail below with reference to a few examples. In all examples, a polycarbonate (LEXAN RL 3198-112 from General Electric Plastics B.V.) is extruded using a Kleinewefers single screw extruder. The diameter D of the screw is 60 mm and its length is 25 D. The extrusion temperature 30 of the polycarbonate is always 250 B. The extrudate with a width of approximately 0.6 m is fed to the gap between two polished rollers with a equal diameter of 0.21 m, which rollers are kept at a temperature of 146 hc so that at least both surfaces of the plate drop below the glass point Tg of the polycarbon-8602018 yv-6-carbonate. This glass point temperature is + 150 hC. The plate is then passed over a third roll kept at a temperature of 152 ** C.

Example I

A plate obtained with the aid of the described extrusion and rolling equipment in which the material is fed under some back-up to a roller gap with a height of 1.1 mm is then cooled to ambient temperature. The optical path length difference Δ n x d is 215 nM at a Δ n of 19.5 x 10 5 and the surface 10 roughness Ra in the transverse direction is <0.04 µm. Thickness of the plate is 1.1 mm.

Example II

A plate obtained with the aid of the described extrusion and rolling equipment in which the material, now without accumulation, is fed to the roller gap with a height of 1.1 mm is then cooled to ambient temperature. The optical path length difference Δ nxd is 125 nM at a Δ n of 11.3 x 10 "^. The cooled plate with a thickness of 1.1 mm has visible extrusion stripes, has a thickness of 1.1 mm and has a surface roughness in transverse 20 direction which is very much larger than 0.04 µm.

Example III

A plate obtained using the described extrusion and rolling equipment, the material being fed under some back-up to a 4 mm high roll gap, is then heated with hot air to a drawing temperature of 205 nC and then to a biaxial stretch where the longitudinal draw ratio is 1.82 and the transverse draw ratio is 2.0. The stretched plate cooled to ambient temperature has a thickness of 1.1 mm, an optical path-30 difference Δ n x d of 27 at a Δ n of 2.5 x 10 “5. pe surface roughness Ra in the transverse direction is less than 0.01 µm.

8602018> + 4 -7-

Example IV

The plate obtained in Example III is cooled to ambient temperature before stretching. Thickness is 4 mm. Optical path length difference Δ n x d is 380 at a Δ n of 9.5 x 10 * 5, pe 5 surface roughness Ra in transverse direction is between 0.02 and 0.04 µm.

The tests and results are summarized in Table 1.

The measurements of the surface roughness Ra are carried out in accordance with the ANSI B 46.1 standard.

From the examples given and from the table it appears that the plate according to the invention obtained in example III gives a result in which the plate meets the specifications for the production of carriers for optically readable information from this plate. The plate obtained according to example IV can be stretched after heating to a drawing temperature of 205 ° C into a plate suitable for manufacturing carriers for optically readable information therefrom.

The importance of pushing up the material in front of the roll gap is clear from Examples I and II.

The accumulation in front of the slit can be brought about by giving the extrudate a greater thickness than the height of the slit or by adjusting the velocity of the extrudate supplied and the removal rate by the cooling rollers such that a certain distance is the slit creates a bench that is thicker. then the height of the roll gap.

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Claims (5)

1. A method of manufacturing a web of polycarbonate suitable for producing therefrom carriers for optically readable information such as audio and video plates, which method comprises the flat gap extrusion of polycarbonate and the cooling of the extrudate using polished cooling rolls, characterized in that the extrudate is fed under some backwater to a gap between two oppositely rotating polished cooling rollers which are kept at a temperature such that the surface temperature of the extrudate drops to a temperature below the glass point of the polycarbonate, which is then, at a stretch temperature of the polycarbonate, subject the extrudate to a simultaneous or sequential biaxial stretch such that the surface roughness in the transverse direction is less than 0.01 µm and the ratio between the transverse and the longitudinal stretch thereby results in the molecular orientation orientation in those directions substantially equal and the stretched extrudate assumes a thickness where the optical path length difference is less than 50 nH and the thickness of the stretched extrudate is between 0.100 and 1.300 mm, such that the height of the roll gap is adjusted such that the stretching operations can be performed and the stretched extrudate is further cooled to ambient temperature. 2. A method of manufacturing optically readable information carriers from the sheet obtained according to claim 1, characterized in that the optically readable information carriers are obtained from the sheet by punching, cutting or sawing. Method according to claim 2, characterized in that the optically readable information is applied by pressing and that the punching and pressing are simultaneously found. 9602018 -V ψ -10- 4. Carriers for applying optically readable information obtained from the plate manufactured using the method according to any one of claims 1-3. 5. Carriers of optically readable information obtained using the method according to any one of claims 1-4. 860 2 0 IS