Field of the invention

The present invention relates to industrial printing methods of carriers through a temporary printing medium, in particular an ink, designed for post to be completely removed from its carrier

Background of the invention

Known methods for temporary imprinting aim generally at marking which is intended to provide a required process information to the operator of a later process step This is i a the case for glass panels, which are intended for a window where a marker is necessary to indicate for which window it is intended After the glass is incorporated in its frame, this flag should be eliminated in order not to obstruct the sight through the window on the one hand, and also for the sake of a neat presentation for esthetic purposes on the other hand Nevertheless this marking should remain perfectly visible until the window is incorporated in its final structure as finished product In other words, said temporary marking is required during the whole phase as semi-finished product, whereby the finished product may show no more trace of the meanwhile eliminated marking, in the final phase of the process

This problem further occurs with so called tracing of spectacle glasses Tracing is applying a temporary marker - with reference of i a optical features and type - on the primary processed product for mechanical treatment and/or after applying special coatings, for semi-process lenses The markings on the raw product determine in this case unequivocally the optical features which are required for the further processing of the lens to a spectacle glass These markings should be applied accurately with respect to the so-called prism reference point

After processing the raw product to a finished spectacle glass, the marking needs to be eliminated completely, i e without the slightest trace remaining It concerns therefore a temporary marking in this case.

An existing technology for temporary printing of spectacle glasses consists of tracing spectacle glasses with a so-called solvent ink, wherein the whole marking, which needs to be present on the spectacle glass, is applied in one imprinting cycle, thus without splitting up the image. The system may choose among various possible images to be considered as a whole.

It is further known to trace spectacle glasses with a so-called solvent ink, wherein the whole marking which needs to be present on the spectacle glass is applied in one or several imprinting cycles, by splitting up the image in parts that will be chosen on a separate ink module or on a part of an ink module.

It is also known to trace spectacle glasses with so-called solvent ink, wherein the whole marking which needs to be present on the spectacle glass is applied in one or several imprinting cycles, by splitting up the image in partial images that will be chosen on one single ink module, wherein the doctor blade system consists of a circular or oval ink well.

These technologies respectively consist of tracing spectacle glasses through one printing cycle. This classical tracing system provides a limited flexibility since one may choose only among a limited set of images.

One single pad is used here for printing all the images and may therefore show no trace of another imprinting. For each printing cycle, a pad purification is virtually necessary owing to problems occurring with ink getting dry on the pad.

Finally, there is the limit case consisting of tracing spectacle glasses through several imprinting cycles with splitting up the images and tracing the spectacle glasses through several imprinting cycles.

Wiping off the ink may cause a problem in these various methods of the known prior art, wherein it occurs that imprintings cannot be completely eliminated or possibly still leave traces after the operation of ink elimination. These traces may consist either of remaining ink or also through ink absorption of little images of the carrier itself. Drying thus causes a delicate problem. For systems that merely allow a selection among a limited set of images without splitting up these images, the flexibility is very limited. Given the plurality of combinations, it is necessary to often convert such a machine in case a plurality of products must be traced. In any case the products to be traced must be processed in groups with the same image.

A greater flexibility could yet be achieved by providing a greater amount of fixed images. In this case however, problems will come up more that are in connection with the distance between the ink system and the product to be imprinted.

Even greater flexibility may be achieved by constantly changeovers of the existing machines. However many ink systems always need to be converted. This is quite time consuming, including if all systems to be converted were available twice.

None of these both options thus provides a greater flexibility without generating additional problems.

As to the drying and adhesion properties, the marking systems used have drawbacks owing to the combination of an ink system, the recording and depositing of the image with pad displacement and ink. Indeed, the use of open or closed ink systems, the pad displacements which are inherent to the concepts of existing machines between the recording and the depositing of ink between several possible partial images and the product to be imprinted, all this for the validated solvent ink, is a combination causing quite some technical printing problems.

These problems are caused by the rather arbitrary nature of the drying of solvent inks. Indeed, solvent inks start to dry on the printing block, the pad and even in the image in the printing block, as soon as the solvents start evaporating. The beginning of the drying process corresponds to the time that the ink is exposed to the air, if not saturated with solvent.

On the one hand, there is the drying problem of ink on the pad. The pad not only needs to be moved over a relatively great distance, but there may be a great difference between the distances to be covered between recording and depositing the image for the various possible images or partial images. Determining and acheving the right surface tension for optimal depositing of the ink is therefore a process that needs to be done separately for each combination of ink system and product location.

That optimal setting needs to be performed for a constant production. Owing to the desired flexibility wherein a further combination of ink system and product may respectively be addressed, such an optimal situation will not come true.

Actually, problems arise with undesired drying of ink on the pad. In order to eliminate these residues, the pad needs to be purified regularly, resulting in a potential additional loss of capacity.

The draw back of the ink drying on the pad is greatly neutralised for those systems which gather many partial images on one ink system or printing block. However a new problem caused in such cases again consists in the drying of ink in the printing block on locations, notably partial images where ink was not absorbed.

On the other hand, there is the drying of ink in the printing block, which is less easily solvable. Owing to the desired flexibility, it may be possible in an extreme case that a certain part image never gets selected. This results in that the ink in the engraving section of the partial image will still get more viscous, even until it finally may dry up. The solving agent, as a necessary ingredient for pad printing, will evaporate more and more from the part image indeed. Indeed, the ink will not be replaced when scraping off, in case the part image is not recorded.

In case the ink of the part image dries into the engraving section, this may affect the scraping properties and the resulting printing quality, including for part images that are selected at times and thus recorded and deposited.

Prior art

Document GB-A-2 161 752 discloses essentially some features in connection with an imprinting device, having no connection however with the problem identified here as to the aimed method. The same applies for US-A1 -2005/154075 describing essentially features mainly relating to a device for imprinting, and not to the aforementioned problem about the aimed method.

Document WO-A-01 038101 however mentions lenses that show no trace of a print, yet before these lenses are printed.

Document GB-A-2 433 228 yet mentions a process for temporary printing of a carrier, comprising comprises the steps of printing an image on a carrier, and exposing the printed image to an external influence to the image to remove , i.e. by physically removing the image by ablation or pyrolysis, or the image to colorless. The external influence, however, caused a partial curing or hardening by cross connection of the printed image. Thus the current objective problem occurs that consists here of providing a means for temporary printing images on substrates, which images can be completely removed if desired without leaving traces of substrates.

Object of the invention

The aim of the invention is to provide a solution to these problems and shortcomings of current systems. This implies one or more of the following measures to be taken as an objective solution to the aforementioned problem.

Summary of the invention

Thus, the main measure proposed according to this invention is defined in the appended main claim, consisting of a method for temporary printing of a marker on a carrier with a printing medium, which is intended to be removed from the carrier after completing the process for which the marking was applied, which is remarkable in that it includes a first step in which said printing medium is deposited on said carrier, which is followed by a further step in which the printed carrier is submitted to an external influence which is such that the deposited printing medium undergoes a partial curing on said carrier.

It is thus clear that the aforementioned objective problem is solved in no way by means of the teachings of the latter document, either separately or in combination with the other documents cited, especially by exposing the image to such an irradiation that the print is only partially hardened. The main measure proposed above invention is based on the following findings indeed. The flexibility is optimized by the use of a limited number of ink systems, each providing a maximum number of possible prints. In practice, only one per ink system is used per color. Then it is also necessary that we may choose among multiple images, respectively section images in function of the selected marker. For printing the product in this case, an image, or share image should be recorded and deposited by the pad several times.

As to drying and adhesion properties, applying a temporary marker requires that the adhesion of the substance, in particular the ink on the product, in particular the glass spectacles, has a temporary nature. This problem has no easy solution, by selection of the ink, given the diversity of the contact surfaces of the product including glass, various plastics, coatings, etc.

One should not forget here that the ink does not only need a sufficient yet a no permanent adhesion, but it must also be possible to apply the ink via pad printing. Validation of a substance that meets these characteristics is not a trivial choice, esp. for the extended variety of potential surfaces such as glass, plastic, with special coatings or without same, etc.

Finally, the adhesion properties of the ink on the product need to remain unaffected by solutions achieved for to the drying problem of ink on the pad and/or cliche.

Based on the distinction between the possible embodiments of a spectacle glass, the invention can be further extended in to the temporary marking of other objects through printing of one or more images which are one or more ink systems.

Drying wastage of ink on the pad and/or printing block is minimized or even avoided by appropriate selection of the ink parameters, given the temporary nature of the printing, wherein enough yet temporary adhesion of the ink on the product is required, given the transfer properties of ink through pad. This requires that the ink does not dry randomly on the pad. Furthermore, the ink may neither dry out in the printing block or any of those places where the image or the ink is not irecorded. It is thus proposed a solution in which the ink will not and cannot dry arbitrarily, and this throughout the entire process of pad printing.

In contrast, the ink needs to adhere sufficiently, yet temporarily on the product. This means that the ink can not be removed from the product, e.g. by contact with other products, by inadvertently rubbing the printing, and others. A forced removal, in particular with an alcoholic solution, should be possible of course, without causing damage to the product or remaining residues of the printing on the product.

For this purpose, a controlled setting of the ink must be carried out in which the issues of sufficient hardening, sufficient adhesion on the one hand, and temporary marking on the other hand are of utmost importance. A random curing does not provide these desired properties.

In other words, at first such an ink or printing agent needs to be used that will come to cure only through an explicite impulse or action. The impulse or action is characterised in that it determines the degree of curing and of adhesion on the finished product to be achieved based upon product dependent parameters and that is to be processed from the semi-finished product concerned here.

It is clear that the issue here subtends conflicting conditions which are very difficult to reconcile. The main object of the present invention consists in proposing a printing ink indeed, wherein the ink is intended to be subsequently completely removed from its carrier and through which the adhesion of the printed figure on the carrier is improved, while the removability thereof in due time is equally promoted. In contrast stands however, that an ink either does not adhere well in which case it is easy to remove, or it adheres attach very well and is then very difficult to remove. In order to solve the problem according to the invention, it is necessary to propose a middle way between these two apparently contradictory conditions.

The connecting further measure proposed by the invention thus consists in that said influence consists of a radiation as explicite action that is set so that the hardening of the ink is not complete, wherein said hardening does not take place at the contact side thereof with the carrier. . According to an advantageous embodiment of the method of the invention, low-solvent inks are used.

According to a preferred embodiment of the method of the invention, UV radiation is a validated, preferably UV inks are used. After an ink type that meets the requirement of just drying under controlled impulse is formed by the so-called UV inks. To switch the ink from a liquid state, which is a necessary condition for transferring via pad ink from printing block on the product to a solid state on this, thereby curing an amount of light and thus energy is required, typically in the UV wavelength range. A polymerization through a photochemical reaction occurs thus occurs, resulting in that medium and pigments get in a solid state.

To obtain such a reaction a so-called photo-initiator is supplied to the medium that is contained in UV inks. The energy of the light makes the photo-initiator react, resulting in liberating chemical radicals. These radicals react in turn with chemical bonds in the monomers and/or oligomers of the medium. These liberate in turn radicals which causes a chain reaction. As the reaction progresses, compounds are formed in yhe medium, which results in the polymerization of the medium, and thus causing curing of the ink. The chain reaction stops when the molecules and radicals get immobilized.

If enough energy is supplied over the entire volume of the ink that is distributed over the ink layer, all molecules are compounds and polymerization and thus harden. If insufficient energy is supplied over the whole volume, certain molecules will yet be immobilized through polymerization and get cured, while neighboring molecules themselves do not form compounds. This effect depending on the energy supply will affect the adhesion and the mechanical and chemical resistance.

There is also a possible analogue chain reaction based on a known so-called cationic polymerization. Such UV inks are used less however, because they have some undesirable side effects such as toxicity and therefore risk for the operator.

In addition to the selection itself of UV ink as ink type, there is the determination of the ink composition that is not trivial, given the very specific target adhesion properties of the ink on the printable product, particularly true enough but not a permanent adhesion. UV inks contain a medium in which there are pigments. The printing medium contains said photo-initiator, monomers and/or oligomers, also chemical agents which are added according to the article, so that the required special adhesion properties are obtained.

In addition to optimization the composition of the ink medium for adhesion and hardening, the drying process should also be closely monitored and controlled. If an insufficient amount of energy suitable for the curing process is used, the ink will be removable randomly. Conversely, if an excessive amount of energy is used, the curing will be so strong that the ink simply cannot be removed or only if the product is also damaged.

The flag must be completely removed, typically after a period of some 15 days, preferably with an alcoholic solution or via tape. The results of these tests can be compared with the results obtained in the adhesion test standard ISO 2409 (NEN-EN-ISO 2409:2007). It is to be understood however that the marking in no case should be removable by rubbing. This would mean that the marker would be removable arbitrary too.

After curing through said explicit action, the marker needs to be removable again, preferably with an alcoholic solution. This means that the marker may not be completely chemically stable. As such, the mark can be released and it is therefore removable.

This was validated in a number of tests, with a previous selection of a type of UV ink, i.a. with a photo-initiator, medium, additives for adhesion properties, defoaming, and a selection of printing block properties such as depth and grid.

When irradiating with too little energy, the flag does not adhere to the product so that a random removal of the marker occurs in case ofcontact with other objects. When illuminating with too much energy however, the marker cannot be removed anymore. In contrast, illuminating with the appropriate amount of energy makes the marker insensitive to random contact. Once put in contact with an alcoholic solution, the image is completely removable without damaging the product.

For a sake of clarity, the advantages of the invention compared to the current systems are summarized below. The use of inks which do not arbitrarily dry on the pad or printing block and which do not arbitrarily dry on the product under conditions of being sufficiently dry and of a temporary marking has the overriding advantage that previous problems have been solved, especially no more unwanted drying wastage on the pad, no unwanted drying on the printing block, sufficient adhesion for further manipulation of the product, forced removable marking without damage to the product and marking completely forced removable.

Besides this application, there are still other uses of temporary markers provided by this invention, according to what is defined in the relevant subclaims and/or described below, which further features and characteristics of the invention are defined in further subclaims.

Thus the aforementioned print can take place advantageously on a medium that is essentially transparent, or preferably even translucent, mainly composed of glass, especially glass spectacles, in particular an optical lens or even plastic materials.

This invention also concerns a device for printing via pad by applying the above method according to this invention.

Although there are known devices based on pad printing implementing classic pad printing processes, as described in GB-A-2 433 228 which yet discloses a pressure device, however, that is not a priori specifically designed for to implement a pad printing method according to this invention.

There is thus proposed according to the invention an adapted printing device as defined in the concerned device claim for applying the method which is remarkable in that it includes a radiation unit which is adjustable so that the ink used is properly transformed through irradiation thereof by means of aforementioned radiation unit.

According to a preferred embodiment of the device according to the invention, it comprises at least one lamp to illuminate said inked carrier with a predetermined wavelength, particularly in the UV region, which also has a predetermined amount of energy. Thus this device is best adapted for applying the method according to the invention as defined in the subclaims. Classic machines do not allow these products to be printed with the guarantee of the temporary printing quality required here. According to a further preferred embodiment of the device according to the invention, it consists of a pad printing device. In the above classicly known machines for printing on products using pad printing, there are undue restrictions related to the merely temporary nature of the applied marking aimed at here. The machine parameters such as image size, position of the image relative to the product, technical print parameters of the media including ink, pigment, drying time, bonding, etc., shape and size of the pad, hardness thereof, and of the printable product, yet each require a specific setting which is own to the invention. The remedy to the set constraints described above regarding the peculiar method according to the invention involves a technological solution that is not reached in traditional devices that would implement this method.

According to an additional embodiment of the printing device, the classical solid pad may be replaced by a hollow pad. The shape of the pad and of the cavities, specifically for a particular product and images to be printed, result in a reduction of the pressure force on the printable product. This addition may be particularly more important with larger images and as a function of the characteristics of the printable product related to deformation and/or fragility, such as glass for example.

In addition, this invention also relates to a temporarily printed product in itself as a special application of the printing method applied here for the temporary printing lenses, esp spectacle glass, which pad printing is applied advantageously. Document WO-A-01 038101 yet mentions lenses that don't show any trace of a print, but only before lenses are printed, and without offering the proposed solution according to the present invention.

According to a product-oriented embodiment of the invention, an optical lens is proposed indeed, in particular glass spectacles, which is produced by a process as defined in one of the claims 1 to 23, respectively a device as defined in one of the claims 24 to 26, which is remarkable in that the printing of this is only temporary, especially during the production phase of that, after finishing with virtually no trace of ink residue on the lens, respectively. glass glasses.

According to a preferred embodiment of the invention shows the optical lens as semiproduct, only a temporary marking during the production phase thereof, which is designed after finishing half of the processed lens to be completely removed them with almost no trace of the ink achterblijfsel the lens, respectively glass glasses. It is always made sure explicitly that the final product quality remains guaranteed. Quality in broad sense is then determined by the preserved quality of the final product even after removal of the only temporary printing.

Further details and particulars of the invention are explained on the basis of the following description of an exemplary embodiment of the device according to the invention with the appended drawings.

Brief description of the drawings

Figure 1 is a diagrammatic view of an imprinting device according to the invention that is applied on a first embodiment of the method according to the invention. Figure 2 is a diagrammatic representation of said imprinting device applied on a further embodiment of the method according to the invention.

Figure 3 is a diagrammatic representation of a cross section of a carrier that is coated with an ink layer by implementing the method according to the invention.

Figure 4 is a diagrammatic representation of the process that is generated by a main embodiment of the method according to the invention on a carrier as represented in

Figure 3.

Description

In general, this invention relates to temporary imprinting of a carrier, which is transparent or translucent, more particularly made of glass or plastics material. In a particular application thereof, the imprinting method is used for the temporary imprinting of lenses, especially spectacle glass, using pad printing.

Figure 1 shows the basic functioning of an imprinting device with a pad, without thereby entering into details of the known elements which intervene in this process and for which reference is made to the relevant known prior art. The functioning thereof is as follows: a pad 1 , which is fastened to a linear shaft 7, is used to absorb a specific amount of ink in a fixed position of an inked printing block 2. The printing block is selected as a function of a lens 3 to be imprinted.

For the take up of an image 4 by the pad 1 , the printing block 2 is inked, wherein the movement 6 of a doctor blade (not shown) with respect to the imprinting device of the printing block can take place both parallel to the aforementioned shaft 7 and perpendicularly thereto.

After recording the image 4 by the pad 1 , the pad is displaced to a position B which is fixed with respect to the product 3 to be imprinted, in particular the lens. The selected image 4 is deposited by means of the pad 1 on the object to be imprinted, in this case the lens 3. The direction of movement of the pad 1 during the recording and the deposition of the image is perpendicular to the aforementioned shaft 7.

After the image 4 has been deposited on the object 3 to be imprinted, the pad 1 is preferably cleaned by means of a purification system 5 of the pad, which is therefore also provided with a purifying pad 15, the pad 1 being brought into contact with the purifying pad 15 which wipes away the remaining ink from the pad 4.

The aforementioned image 4 can if necessary be replaced by various partial images 14. This is shown in figure 2. The functioning of the system takes place essentially in a first step consisting in the applying of a marking via the pad, followed by a second step for the setting of the marking with adhesion which is adequate yet merely temporary.

Figure 3 shows a carrier 9 to which ink has been applied as a layer denoted by Y. For the sake of clarity, the ink layer Y has been symbolically split up into a number of virtual successive layers Y1 to Yn in order to be able to clarify the functioning of the process in greater detail. For this purpose, the physical ink layer is split up in any desired manner in the depth direction into parallel partial layers having a substantially constant thickness ε which is defined in an arbitrary manner for the present, purely illustrative purposes. In this case, the virtual layer Y1 is the fictitious external top layer forming the free surface of the inked carrier 9. The virtual layer therefore also forms the layer which is directly exposed to external influences such as radiation I, for example. The directly adjoining ink layer is formed by the virtual partial layer Y2, and so on until the last virtual partial layer Yn which is formed by the bottom layer which is in direct contact with the carrier 9 itself at its top surface.

Fig. 4 shows schematically the functioning of the process that is brought about as a result of the use of the present method. The aforementioned external influence or explicit pulse is in this case formed by radiation, preferably UV radiation I which acts on the printing medium used, in this case UV ink which is deposited on the carrier 9. The radiation has initially an energy I ₀ which is represented schematically by a pentagon having a specific initial surface area which may be seen in the figure, with the top directed toward the surface of the ink layer in question. It is this energy I ₀ that strikes the first top layer Y1 of the ink.

Once the radiation has penetrated this top layer Y1 over the entire depth ε thereof, a part A ₀ of the initial energy I ₀ of the radiation is absorbed as a result so that the energy level thereof is limited to a value presented by I ₁ which is lower than the initial value I ₀ according to the formula I ₁ = I ₀ - A ₀ . This difference in energy is represented symbolically in the figure by the smaller surface area of the directed pentagon which I ₁ presents. The energy dissipation corresponding thereto is converted into a setting of the layer Y1 penetrated thereby, for which A ₀ forms a measure.

In a similar manner, the partial layer Y2 is subsequently penetrated by an energy I ₂ which is lower than I ₁ according to the formula I ₂ = I ₁ - A ₁ .

For the same reason, the partial layer Y3 is penetrated in a similar manner by an energy I ₃ which is again less than I ₂ according to the formula I ₃ = I ₂ - A ₂ .

This process continues at the level of a further fictitious partial layer Y, which is penetrated in a similar manner by an energy I, which is still lower than the earlier energy level I _1-1 according to the formula I ₁ = ^ _-1 - A _1-1 .

This process is repeated on and on until the last fictitious partial layer Yn which is penetrated by an energy I _n which is again lower than the earlier energy level I _n ^, still according to the formula I _n = l _n -i - A _n ^ . This is represented by the respective surface areas of the corresponding pentagons, the surface area of which decreases in each case for each partial layer Yi, during progressive advancement toward the carrier 9 per se and for which the corresponding absorption factor A ₁ forms a respective measure which responds to a law. The result of this is that the deepest partial layers Yn, Yn-1 , ..., which are positioned closest to the carrier 9, are penetrated by much less energy than the surface of the top layers Y1 , Y2, ... , as may be seen from the relevant system of equations.

I ₁ J ₀ - A ₀ appropriate substitutions which then provided the following overall equation can be deduced:

I _n = I ₀ - ∑ ⁿ ,=i A with A, > 0.

This equation establishes a connection between the originally incoming energy and that at the level of the surface of the object to be imprinted; the possibility cannot be ruled out that the latter will be at 0, depending on the absorption factors in question. This noteworthy method accordingly allows the realization of an adjustment of the impinging radiation I that allows the partial layers Y1 , Y2,... , which are more remote with respect to the carrier 9, to set completely, whereas the partial layers Yn-1 , Yn, which are positioned closer to the carrier, set only incompletely, and may even remain liquid, as may be derived from the iteration formula assumed hereinbefore.

Indeed, because the energy level decreases as the radiation progresses toward the surface of the carrier 9, the setting is also weaker as a result of the fact that the energy level I _n of the contact layer Yn is much lower than that of the top layer Y1. This has the result that the available energy that is necessary for the setting of the layer in question is much lower for the contact layer Yn than for the top layer Y1 , so that the setting of the former will therefore logically also be able to take place using less energy than for the latter Y1.

Ink may not in this case enter into a chemical reaction with the product to be imprinted, since no visible traces may remain after the removal, such as for example a solvent does do with a plastic. Photoinitiated polymerization of the ink as a medium occurs preferably over a part of the thickness of the marking, particularly in the top part. At most, polymerization may occur over the entire thickness of the marking, up to the product 9. Care must be taken to ensure that the marking can still be removed afterwards, i.e. the setting, and in particular the chemical resistance, should not be complete. The situation is such that complete out- and-out polymerization, with the associated good adhesion, mechanical and chemical resistance, may not occur in any case.

In practice, the pad 1 is subjected to a rising and falling linear movement 6. Precise positioning of the image 4 to be imprinted is carried out in the method, the printing block cartridge and pad 1 being aligned with each other during the absorption of the ink from the printing block plate 2 and that carrier 9 and pad 1 being aligned with each other during depositing of the ink on the carrier 9.

The method consists for example of the following steps: absorbing the ink from a printing block cartridge with the image 4 to be printed by means of the pad 1 of the printing block plate 2 via a linear movement 6 in the direction in which deformation of the pad 1 takes place. The image 4 recorded by the pad 1 is then deposited on the top surface of the carrier 9 via a linear movement 6 in the direction in which deformation of the pad 1 takes place. During this cycle, the printing block plate 2 is displaced in such a way that, in the first phase, the printing block cartridge is aligned with the pad 1 and that, in the deposition phase, the printing block plate 2 is displaced to a waiting position in such a way that the path between the pad 1 and the carrier 9 to be imprinted is completely free.

As used UV inks, they consist for example of epoxy resin, N-butyl acetate, propylene carbonate and calcium carbonate. Example: 25 to 30% epoxy resin,

20 to 25% N-butyl acetate, up to 5% propylene carbonate and 15 to 20% calcium carbonate.