Method for preventing bubbles or small bubbles when connecting substrate parts of optical data carriers by means of an adhesive

A method of preventing bubbles or small bubbles when connecting substrate parts of optical data carriers by means of an adhesive. At least one of the substrate parts and/or the adhesive is provided with electric charges.

BACKGROUND OF THE INVENTION
 The invention relates to a method of connecting substrate parts, in
 particular the two halves of a DVD (digital versatile disc) by means of an
 adhesive.
 In DVDs, information which is stored in the form of very small elevations
 and pits in the surface of one DVD half is read out through the other DVD
 half as well as the adhesive layer between the two halves. Since the pits
 are very small, slight flaws in the adhesive layer in the form of small
 business may impair the reproduction of the information in the pits. Even
 if there is only a single small bubble in the adhesive layer, the DVD has
 to be considered defective. There may occur small bubbles even if the
 substrate part (DVD half) is uniformly coated with the adhesive. It has
 not been entirely clarified why these small bubbles occur. The small
 bubbles may be gas inclusions from the ambient atmosphere which are formed
 when pressing the two substrate parts against each other. There is also
 the possibility that they are formed in the adhesive by themselves. One
 reason therefor could be the gas evolution of solvents of the adhesive.
 Preferably, an adhesive having a specific viscosity is applied to the
 inner region of a substrate part in the form of a thick circumferential
 path and is flattened during the connection with the other substrate part,
 wherein excess adhesive is removed preferably by rapidly rotating the
 substrate part, i.e., the DVD. Other methods of applying the adhesive are,
 for instance, uniformly spraying it onto a rotating substrate part or
 injecting it into two convex substrate parts. In all known methods, the
 aforementioned small bubbles, which are detrimental to the reproduction of
 the data, may occur.
 BRIEF SUMMARY OF THE INVENTION
 An object of the invention is to overcome the aforementioned drawbacks and
 reliably prevent bubbles, and particularly small bubbles, from being
 present between the substrate parts after their connection by means of an
 adhesive.
 The above and other objects are achieved by applying electric charges to
 one or the other part of an optical data carrier, or to an adhesive
 connecting the parts together. The optical data carrier may be, for
 example, a DVD.
 It has surprisingly been found that the occurrence of small bubbles between
 two substrate parts after their connection by means of an adhesive can be
 prevented by the application of charges onto at least one substrate part
 and/or the adhesive before or after applying the adhesive to a substrate
 part.
 Therefore, according to the present invention, the solution to this problem
 is based on the concept of applying electric charges to at least one of
 the substrate parts and/or the adhesive. This may be done before and/or
 after applying the adhesive and before and/or after connecting the two
 substrate parts. It is also possible to bond more than two substrate parts
 (halves) to each other to form a composite of optical data carriers. In
 this case, the method according to the invention is applied in each of the
 successive steps of bonding substrate parts to each other.
 The advantages of the method according to the invention reside in ensuring
 a complete and faultless reproduction of the information stored in the
 pits of the DVD.
 It is known per se from DE-A1-37 21 703 to provide work pieces, e.g., car
 bodies, having an electrically insulating surface with a uniform
 electrostatic charge prior to lacquering or enameling. Thus, it is
 possible to uniformly coat the surface with a lacquer or enamel layer.
 However, the problems occurring in the production of optical storage
 media, such as DVD, namely the formation of small bubbles, which impair
 the storage or the reproduction of the data, do not play a role in this
 state of the art.
 In the following, the invention will be explained in more detail by means
 of the drawings.

DETAILED DESCRIPTION OF THE INVENTION
 According to the method of the invention, electric charges are applied to
 at least one substrate part. This may be done by forming charges by means
 of an electric field or by means of spraying electrons onto the substrate
 part (e.g., corona discharge).
 However, it is also possible to achieve a separation of the charges on the
 substrate part by means of friction and thus to generate frictional
 electricity. Depending on the substrate material, the adhesive material,
 the temperature and/or the atmospheric humidity, the surface of the
 substrate part can be charged so as to have a predetermined charge
 distribution and a predetermined polarity. Substrate parts having a
 non-conductive surface are preferably positively charged. Metalized
 substrate parts--i.e., conductive substrate parts--may be charged both
 positively and negatively.
 Preferably, the surface is charged so as to achieve a homogeneous charge
 distribution. However, the charge distribution may also be inhomogeneous,
 e.g., radially increasing or decreasing on the substrate part. Above all,
 the charge condition in that area of the substrate where the first contact
 with the adhesive occurs during the bonding step is important for the
 method of the invention to function properly. It is an intended effect of
 the method of the invention to prevent an electrostatic repulsion of the
 adhesive. It has been observed that without the method according to the
 invention an adhesive path applied to a substrate part was changed in its
 shape during the bonding step by the approaching other substrate part,
 possibly by electrostatic repulsion, i.e., the convex form of the
 bulge-like adhesive path obtained a concave indentation. Thus, cavities
 were formed when joining the substrate parts, which led to the detrimental
 formation of bubbles in the adhesive.
 According to an advantageous further development of the method, the charges
 are applied during the production process such that, on the one hand, they
 yield sufficient effect and, on the other hand, the application of the
 charges can be incorporated in the process as easily as possible. It is,
 for instance, also possible to charge both substrate halves. Furthermore,
 the adhesive may be grounded or charged positively or negatively at its
 outlet nozzle. In this way, the adhesive can be applied to at least one
 substrate part in a charge-neutral or charged state.
 FIG. 1 shows a device for applying the method according to the invention in
 the DVD production. Two DVD halves 1 are placed on a rotary table 3
 comprising a drive 6, which halves are fixed on the rotary table in an
 electrically insulated manner in the center 4 of the rotary table 3 and by
 means of mountings 5 at the edge of the disks. However, it is also
 possible that only one DVD half or more than two DVD halves are arranged
 on the rotary table 3. A charging means 2 is arranged above the DVD halves
 1 at a specific distance of e.g., 10 to 50 mm, preferably 30 mm. An
 electric field is applied, i.e., charges are transported between the
 charging means 2 and the DVD halves 1. The DVD halves are thus
 electrostatically charged or, if there are already charges on the halves,
 they are neutralized. The rotation of the rotary table effects a uniform
 distribution of the charges.
 It can be recognized in the top view according to FIG. 2 that the DVD
 halves 1, which are mounted on the rotary table 3 and can be supplied and
 removed in a transport system via a gripper 7, rotate below the charging
 means 2. However, it is also possible in principal that the charging means
 2 is moved over the substrates and the substrates are stationary.
 Preferably, one of the DVD halves is electrostatically charged and lowered
 onto the other DVD half which has already been coated with an adhesive in
 order to bond said halves to each other. However, it is also possible that
 charges are applied to both DVD halves before they are coated with an
 adhesive.
 FIG. 3 shows a schematic representation of a production line which shows
 the method according to the invention in connection with the step of
 bonding two substrate parts. From the left side to the right side,
 substrate parts 1a are supplied and their surfaces are provided with
 charges by means of a charging device 2. The surface may be electrically
 charged, but also neutralized if there have already been electric charges
 on the surface of the substrate part. By means of a pivot arm 8 comprising
 a gripping means 8a, the substrate part 1a is taken at its lower side and
 rotated about an axis A so that the upper side faces downwards.
 Subsequently, the substrate part 1a is placed onto a substrate part 1b
 which is supplied from the right side and is covered with an adhesive 9.
 When said substrate part 1a is placed onto said substrate part 1b, the
 bulge-like adhesive path 9 is flattened and said parts are joined by means
 of the adhesive. FIG. 3 shows an embodiment of the method according to the
 invention, wherein only substrate part 1a, which is placed onto substrate
 part 1b being covered with the adhesive is charged. However, it is also
 possible that only substrate part 1b being covered with the adhesive is
 provided with charges, or that both substrate parts 1a and 1b are provided
 with charges. This can be done before or after the application of the
 adhesive. As an alternative, substrate part 1b may be charged on the rear
 side after the application of the adhesive 9. When charging the substrate
 part covered with adhesive, charges which are contained in the adhesive
 itself or are formed during the application step may be neutralized.
 The thickness of substrate parts in the DVD production (0.6 mm) is small in
 comparison to the range of electrostatic forces. Therefore, the charges
 can also be applied to the rear side of the substrate parts, i.e., to that
 surface which subsequently does not come into contact with the adhesive.
 It is furthermore possible that only the adhesive is provided with electric
 charges or grounded or that additionally charges are applied to at least
 one substrate part.
 The application of charges to the substrate part or the substrate parts can
 be performed, as explained above, during the rotation of the substrate
 parts relatively to the charge electrode (charging means), during any kind
 of relative motion, e.g., linear motion or arc-shaped motion, but also
 during static positioning of the substrate parts relative to the charge
 electrode, wherein it must only be ensured that the entire desired surface
 of the substrate part is provided with the desired charge. If necessary,
 the position and the size of the charge electrode must be adapted to the
 size of the substrate.
 The advantage of the method according to the invention consists in that
 bubbles or small bubbles, i.e., air/gas inclusions or cavities in the
 adhesive can be reliably prevented.
 Apart from being used in the DVD production, the method according to the
 invention can also be used in other processes, in which plastics
 substrates, i.e., substrates which are easily electrostatically charged,
 are to be coated without bubbles. The method according to the invention
 can be used, for instance, in lacquering or enameling a substrate.
 This application relates to subject matter disclosed in German Application
 number 198 50 832.8, filed on Nov. 4, 1998, the disclosure of which is
 incorporated herein by reference.
 While the description above refers to particular embodiments of the present
 invention, it will be understood that many modifications may be made
 without departing from the spirit thereof. The accompanying claims are
 intended to cover such modifications as would fall within the true scope
 and spirit of the present invention.
 The presently disclosed embodiments are therefore to be considered in all
 respects as illustrative and not restrictive, the scope of the invention
 being indicated by the appended claims, rather than the foregoing
 description, and all changes which come within the meaning and range of
 equivalency of the claims are therefore intended to be embraced therein.