EP3398718B1

EP3398718B1 - Process for producing a flexible abrasive article

Info

Publication number: EP3398718B1
Application number: EP18166588.6A
Authority: EP
Inventors: Claudio Colombi; Carlo Ranieri
Original assignee: Biffignandi SpA
Current assignee: Biffignandi SpA
Priority date: 2017-04-10
Filing date: 2018-04-10
Publication date: 2020-06-10
Anticipated expiration: 2038-04-10
Also published as: EP3398718A2; IT201700039495A1; EP3398718A3

Description

The present invention relates to a process for producing a flexible abrasive article. More in particular the invention relates to a process for producing a flexible abrasive article in sheet or film form.
The abrasive article produced with the process according to the present invention is particularly, but not exclusively, intended for finishing operations of painted surfaces, such as, but not limited to, vehicle parts.
More in detail, the abrasive article is adapted to be used for one of the finishing operations of a painted surface in order to reduce possible imperfections before final polishing. In particular, the article produced with the process according to the present invention is suitable to smooth a painted surface, reducing the "orange peel" effect, and to reduce the roughness (micro finishing operation) of a painted surface during the "pre-polishing" step.
At the end of the abrasion process of the paint, the surface normally remains opaque with micro-scratches. The pre-polishing step aims to reduce and lessen the micro-scratches, providing a semi-gloss and homogeneous surface.
For these applications there are known flexible abrasive sheets in which the abrasive layer is formed on a flexible backing of elastomeric material.
An example of an abrasive sheet with these characteristics is known from the document EP 0771613 B1 . The abrasive sheet comprises a flexible film formed of a polyurethane resin, NBR or SBR, having the function of backing, and an abrasive layer applied to said film. The abrasive layer, called "make coat" in jargon, in turn comprises an adhesive layer formed of a phenolic, epoxy or acrylic resin, abrasive particles and a further flexible layer, called "top coat" in jargon, in a resin the same as or similar to the resin of the flexible film, applied over the adhesive layer and the abrasive particles.
The manufacturing process of the aforesaid abrasive sheet comprises:

coating the surface of the backing film with a layer comprising a solution of epoxy, phenolic or acrylic resin;
depositing the abrasive particles on said layer through an electrostatic deposition process;
drying the solution of epoxy or phenolic resin and the abrasive particles in a furnace;
applying a layer of polyurethane resin to the abrasive particles; and finally
drying the layer of polyurethane resin in a furnace.

The document EP 2179818 B1 describes a flexible abrasive sheet with a structure substantially identical to the structure of the abrasive sheet described in the document EP 0771613 B1 . In the abrasive sheet described in the document EP 2179818 , a further layer of an anti-clogging compound of stearate can be applied over the abrasive layer. Therefore, its manufacturing process comprises a further step of coating with said layer of anti-clogging compound and a step of drying in a furnace.
The manufacturing methods of the abrasive sheets described above use conventional processes and plants of known type.
Generally, these plants are particularly large and costly and therefore suitable for continuous production of large numbers of articles.
Generally, the various drying stages are carried out in furnaces of large sizes, even up to 40 meters in length, which, besides requiring large spaces for their installation, have very high energy consumptions and long heating and start-up times. Moreover, to manage transport of the product inside these plants, festoon dryers of large dimensions must be used.
Also for this reason, the aforesaid plants are somewhat inflexible for frequent production changes, for example when the size of the abrasive particles must be changed, but, on the contrary, are suitable for manufacturing large batches of articles with the same characteristics.
Moreover, the process for electrostatic deposition of the abrasive particles on the backing requires an isolated chamber to be provided, generally a hole excavated under ground level, in which the deposition device is positioned.
The device uses very high voltages, up to 20,000 volt or higher, to operate and therefore requires complex electrical systems provided with special safety systems.
Further, during its operation this device produces a large amount of fine particles or other volatile products that require the use of exhaust systems for the disposal of these particles. Notwithstanding this, part of these fine particles is still also dispersed outside the chamber into the surrounding environment.
Document WO2010/053729 A1 describes a manufacturing method of coating an abrasive slurry to produce a flexible abrasive article according to the preamble of claim 1.
In this context, the object of the present invention is to propose a process for producing a flexible abrasive article that solves the problems of the prior art described above.
It is therefore an object of the present invention to propose a process that allows the production of a flexible abrasive article with a less complex and less expensive plant with respect to those of the prior art.
In particular, an object of the present invention is to propose a process that comprises a smaller number of stages with respect to known methods, therefore requiring the use of a smaller number of machines.
Another object of the present invention is to provide a faster production process with respect to those of the prior art.
Another object of the present invention is to provide a process for producing an abrasive article with low environmental impact, in particular very low emission of volatile organic compounds (VOC).
Another object of the present invention is to provide a process for producing an abrasive article that guarantees permanent and durable fixing of the abrasive particles and that makes them less erodible during processing.
An advantage of the process according to the invention is the possibility to produce a plant that is easy to manage, for producing an abrasive article that allows variation of the characteristics of the article even in case of production of small batches.
According to an embodiment not part of the claimed invention, it is presented a flexible abrasive article that is less expensive with respect to those of the prior art.
For brevity and to facilitate reading, hereinafter in the description, in alternative to the term "article", the term "film" has been used alone to indicate more generically the article that can, as described previously, be in film or sheet form.
These objects are achieved with a process that comprises only a few stages during which a substrate formed of a base film is coated with a layer of polymeric material, similar to and compatible with the polymer forming said base film, in which the abrasive particles are dispersed.
The process according to the invention for preparing said article comprises the following stages:

a) laying a base film F_B of elastomeric or thermoplastic polymers so as to obtain an upper surface and a lower surface;
b) forming with an aqueous suspension SA, having viscosity from 1,000 to 15,000 mPa·s, a layer on the upper surface of the film F_B, said aqueous suspension SA comprising the following components:
1. (i) elastomeric or thermoplastic polymers, and
2. (ii) abrasive particles;
c) subjecting the product obtained from the preceding stage to a heat cycle that first comprises a step of heating and then a step of cooling to room temperature, in the heating step the temperature is increased until reaching a temperature at least close to the melting or softening temperature of the polymers of the film F_B and of the polymers P_SA.

In this process, the film F_B is formed with polymers P_FB that have the same chemical composition as the polymers P_SA present in the aqueous suspension SA.
The film F_B is produced with elastomeric or thermoplastic polymers. Preferably, these polymers are selected from: polymers of the families of polyester thermoplastic polyurethanes (TPU) and polyether TPUs, silicone-based elastomeric polymers, synthetic rubbers of NBR or SBR type. Preferably, said film is formed with polymers of the family of thermoplastic polyurethanes from polyester.
Said film F_B is typically a single layer film.
As stated previously, the aqueous suspension SA used in stage (b) is prepared with the same types of elastomeric or thermoplastic polymers P_FB with which the film F_B is formed. Therefore, the elastomeric or thermoplastic polymers P_SA used for preparing said aqueous suspension are the same mentioned previously, i.e., they are preferably selected within the families of polyester TPUs, of polyether TPUs, of silicone-based elastomers, and of synthetic rubbers of NBR or SBR type.
The most preferred polymers within the scope of the present invention are polymers of the family of polyester thermoplastic polyurethanes.
Said aqueous suspension (or slurry) is preferably prepared prior to its use in stage (b).
As mentioned previously, the manufacturing process of the abrasive article of the present invention comprises a first stage (stage a) in which the film of the polymer selected is laid, more in particular horizontally, to be subjected to the subsequent stages.
This stage is typically carried out at room temperature.
Room temperature, with reference to the process of the present invention, means a temperature of about 20-25°C.
The film thus arranged accesses the subsequent stage (stage (b)), in which a layer is formed on the upper surface of the film F_B with said aqueous suspension SA.
Preferably, at the end of stage (b) said layer S is of uniform thickness. Therefore, if necessary, said layer is subjected to leveling so as to make its thickness uniform.
In detail, the formation of said layer on the film F_B can be carried out, for example, by means of a deposition or spreading process with roll coating, doctor blade or slot die type systems, or by spraying onto the surface of the film with spray coating type systems.
In the process of the present invention, stage (b) is typically carried out at room temperature.
At the end of this stage (b), the layer that has formed has a thickness preferably comprised between 10 µm and 200 µm, more preferably between 50 µm and 150 µm.
In general, the thickness of this layer is also a function of the size of the abrasive particles present. Preferably, the thickness of this layer is always greater than the largest size of the aforesaid abrasive particles.
The subsequent stage (c) is the stage in which the film coming from stage (b) is subjected to heating and subsequent cooling.
Within the scope of the present invention, the term "melting temperature" and "temperature close to melting temperature" means a temperature comprised in a range of about ±20°C, preferably ±10°C, with respect to the melting or softening temperature of the polymer used.
Moreover, for practicality, hereinafter in the description the term "melting temperature" or "temperature close to melting temperature" will also refer to the softening temperature, where the polymer selected has the second in place of the first.
According to a first embodiment of this process, said heating step is implemented in a single step.
In particular, the product is heated to a temperature close or corresponding to the temperature at which the polymers of the film F_B and in the suspension melt. More in particular, it is advantageous for this heating to allow melting at least of the polymers of the film F_B and of the layer S that are at the interface between the film and the layer S, so that the polymers at the interface can mix. In this way, at the end of stage (c), a film F_A is obtained in which the layer S and the layer below have formed a single layer in which the abrasive particles are incorporated. In this stage, the temperature is the temperature close to the melting temperature and therefore depends on the type of polymers involved. Typically, this temperature is comprised between 90 and 150 °C, preferably between 100 and 150 °C, more preferably between 110 and 140 °C.
During this heating, the aqueous suspension is reduced in thickness, typically in a proportion between 40 and 50%.
The total heating time is generally comprised between 2 minutes and 7 minutes.
During this stage, due to the temperature required for this stage, before melting of the polymers takes place, at least partial evaporation of the suspension water SA (or drying of the suspension SA) occurs.
According to a preferred embodiment, the heating stage is developed in two steps. The first step consists of a preheating of the product coming from stage (b). This preheating step is typically conducted at a temperature between 100°C and 150°C. Typically, the duration of preheating is comprised between 1 and 3 minutes.
After preheating has been carried out, the product is subjected to the actual heating step described above.
Heating can be conducted in different ways.
The heating step is preferably conducted also by applying a certain pressure to the product, as occurs, for example, in a hot air furnace in which one or more hot air flows are aimed at the side of the film coated with the aqueous suspension SA.
Besides heating, the hot air furnace also produces an advantageous effect due to the jet of air. This jet exerts a thrust on the abrasive particles, which are consequently better incorporated in the polymer layer obtained, i.e., thrust in greater depth toward the inside of said layer.
Alternatively, the heating step can be conducted locally, i.e., only at the level of the base film, for example by irradiating this latter from below with a heated plate or equivalent devices.
Typically, if preheating is required, it is conducted using an infrared (IR) furnace.
The advantage of carrying out preheating consists of the fact of being able to use a different apparatus with respect to the hot air furnace, such as the aforesaid infrared furnace. This latter has various advantages. Firstly, it reduces the duration of the subsequent heating step, i.e., the step in which evaporation, or drying, and melting takes place. Another advantage, resulting from the previous advantage, regards a lower energy consumption to complete the heating stage. Finally, the use of the infrared furnace, which has a structure of much smaller size with respect to the hot air furnace, allows the dimensions of the heating plant to be limited.
In the process of the present invention, the whole heating stage, in one or more steps, is conducted for a period of time that varies as a function of various parameters, such as the process used, the temperature to which the apparatus used is taken, the thickness of the layer to be melted, the amount of liquid to be evaporated, etc. Typically, this heating stage has a total duration of about 2 to 9 minutes.
In the case in which preheating is carried out, during which the majority of evaporation of the water, or drying, of the suspension SA occurs, this preheating is typically conducted for a period of time varying from 1 to 3 minutes. The subsequent heating stage, in which melting of the polymers typically occurs, generally requires a period of time varying from 2 to 4 minutes.
The duration of the aforesaid heating stage, in one or more steps, is compatible with continuous in line production and therefore does not require the use of festoon dryers or similar apparatus, limiting both the costs and the footprint of the plant.
In the process of the present invention, the base film F_B that is used typically has a thickness comprised between 50 µm and 300 µm and more preferably between 80 µm and 200 µm.
Moreover, the base film F_B preferably has the following mechanical properties:

tensile strength comprised between 15 and 45 MPa according to the standard DIN 53504,
elongation at break comprised between 500% and 800%, according to the standard DIN 53504.

The abrasive particles used in the present process are the particles commonly used for producing known abrasive articles commonly used for these applications. Typically, these particles are minerals, for example, aluminum or pure silicon carbide (NGC) oxides. In the present process it is also possible to use abrasive particles with other compositions.
Moreover, the abrasive particles used typically have a size comprised between about 3 µm and 150 µm.
According to a preferred embodiment of this process, the aforesaid aqueous suspension SA comprises the following components (parts by weight):

from 30 to 80, preferably from 35 to 70, parts of water,
from 2 to 20, preferably from 2 to 18, more preferably from 2 to 16, parts of elastomeric or thermoplastic polymers;
from 10 to 30, preferably from 20 to 30, more preferably from 20 to 28, parts of abrasive particles,
from 4 to 15, preferably from 4 to 12, more preferably from 6 to 12, parts of both rheological additives and suspending agents, and optionally
from 3 to 20, preferably from 4 to 15, parts of a functionalizing additive.

Preferably, the rheological additive is from 2 to 8, more preferably from 3 to 8, even more preferably from 3 to 6, parts by weight in the aqueous suspension SA.
Preferably, the suspending additive is from 2 to 7, more preferably from 2 to 6, parts by weight in the aqueous suspension SA.
The rheological additives are selected, for example, from anti-settling additives, such as those based on urea. Examples of these suitable rheological additives are additives known with the trade names BYK®-E420, marketed by BYK Additives & Instruments, and AD500, marketed by IMA.
Suitable suspending additives are, for example, anti-terra, such as salts of unsaturated polyamine amides and low molecular weight acidic polyesters, such as the additive known with the trade name BYK®-U100, marketed by BYK Additives & Instruments.
The functionalizing additives are typically selected from saturated fatty acids and their derivatives such as paraffin hydrocarbons (so-called mineral fats), typically saturated, in particular their mixtures; more in particular, they are selected from esters of fatty acids with long chain alcohols and their mixtures, liquid mineral waxes and their mixtures, for example in the form of emulsion or dispersion, and mineral fatty acids (in particular fatty acids with C18 and C20 chains) and their salts. Mineral fatty acids mean paraffin hydrocarbons mixed with one or more medium or long chain saturated fatty acids (for example stearic acid), microcrystalline waxes and liquid paraffins. Said functionalizing additives are typically mixed with water as carrier and suitable surfactants. Examples of the aforesaid additives are compounds marketed by Lubrizol with the trade names Aquaslip™ 677 and LANCO™ GLIDD 4370, compounds marketed by Michelman with the trade marks Michem® Emulsion 66035 and Michem® LUBE 270R and by Byk (Altana Group) with the trade mark AQUACER 537.
Naturally, the aqueous suspension SA can comprise any other additive considered suitable.
The aqueous suspension SA also comprising the aforesaid functionalizing additive is particularly preferred. Therefore, the aforesaid process according to the present invention is also preferred in the variant in which in stage (b) the layer S is formed with the aqueous suspension comprising the functionalizing additive (hereinafter identified as aqueous suspension SA_AF).
Typically, the aqueous suspension SA has viscosity from 1,000 to 15,000 mPa·s, preferably from 2,000 to 10,000 mPa·s, measured at 25° C according to the Brookfield method.
Therefore, a further aqueous suspension, not part of the claimed invention and comprising the functionalizing agents (aqueous suspension SA_AF) for preparing the aforesaid flexible abrasive article in sheet or film form is disclosed.
This suspension (whether SA or SA_AF) is typically prepared by mixing the various components.
More precisely, in a preferred embodiment, the process is implemented by starting to add the additives to the water and after mixing the polymers and the abrasive particles are added to the same mixture thus obtained.
This mixing process is preferably conducted in a mixer, for example a turbo mixer.
Typically, the whole mixing process takes place at room temperature and atmospheric pressure. However, if the functionalizing additive is added (on its own or at the same time as the other additives), mixing of this additive to the water, or to the solution with the components already added, is conducted at a temperature of at least 40°C, preferably between 40° and 70°C, more preferably between 45° and 65°C. Mixing of the subsequent components can take place at room temperature.
Typically, mixing continues for a time between 20 and 90 minutes.
The mixing speed is typically comprised between 200 and 800 rpm.
The aforesaid objects are also achieved with a flexible abrasive article in sheet or film form, comprising a single layer of an elastomeric or thermoplastic polymeric material comprising the abrasive particles, at least partially incorporated in said single layer. Said polymeric material of said single layer is selected from the aforesaid polymers.
Said flexible abrasive article in polymeric material has a thickness preferably comprised between 55 µm and 400 µm and more preferably between 100 µm and 220 µm.
The abrasive article produced with the method described above typically has the following mechanical properties:

tensile strength: 80-250 N, according to the standard UNI EN ISO 1421:2000,
elongation at break: 350% - 800%, according to the standard UNI EN ISO 1421:2000,
longitudinal modulus of elasticity at 300% of elongation: 3000-9000 gf, according to the standard UNI EN 14704-1:2005.

Without wishing to be held to any particular theory, it is deemed that during stage (c), following melting of the polymer film, and evaporation of the water in the aqueous suspension, the abrasive particles are incorporated in depth in the film offering greater resistance to detachment. The abrasive article therefore guarantees a greater duration.
According to a preferred variant, this result is preferably reached also through the thrust exerted by the hot air flows on the upper surface of the film, and therefore also on the abrasive particles, thrusting them even deeper into the polymer film.
At the end of the heating step the film is cooled.
The flexible abrasive article obtained with the process of the present invention as described previously can be used as is in abrasive processes such as those dedicated, for example, to the finishing of painted surfaces, as mentioned previously. However, the abrasive article obtained with the process of the present invention, described previously and hereinafter defined "primary process", can preferably be finished, on the one hand to facilitate its application to tools that require its presence for their operation and on the other to increase the usage cycles of this article. Therefore, preferably, in the same way as similar articles of the prior art, this article will be subjected to further processing. This further processing, defined hereinafter as finishing operation, includes stages known per se, which for example include the use of an anti-clogging compound, and therefore these stages will not be described in detail.
In particular, the flexible abrasive article obtained with the process of the present invention is, preferably and typically, subjected to a subsequent finishing operation to thus obtain a finished flexible abrasive article. Hereinafter, the whole process with which said finished flexible abrasive article is produced is defined as "complete process". This subsequent processing comprises one or both the following treatments:

i. coating the abrasive face of the article with a layer of anti-clogging compound, and
ii. coupling the smooth face of the unfinished article with a coupling layer.

The unfinished article, obtained at the end of the heat cycle of stage (c), can, for example, be wound on reels and only later subjected to one, or both, of said further treatments or can be immediately subjected to at least one of the aforesaid treatments.
In particular, said coating treatment of the abrasive face comprises a first step consisting of coating the article as obtained with the aforesaid primary process with a layer of anti-clogging compound and a subsequent step of drying in a furnace.
In particular, a generally aqueous solution or suspension comprising the anti-clogging compound is poured onto the abrasive face. Said layer of anti-clogging compound is preferably applied by means of spreading or roll coating by means of a roller. The subsequent drying step allows removal of the liquid medium to produce said coating.
The coupling treatment of the article with a coupling layer consists of applying a coupling layer to the smooth surface of the article. This treatment (ii) can be carried out in various known ways, hereinafter mentioned briefly.
According to a first method, following this treatment, said smooth face of the article is coated, as coupling film, with a layer of fibrous material (hereinafter defined as layer MF), for example a warp knit nylon fabric, also called "velveteen" in jargon. This layer MF is typically present in an amount from 60 to 100 g/m².
The smooth face and the layer MF are typically joined by means of an adhesive material, which is applied previously to one of the two faces to be coupled. Generally, this adhesive material is a pressure sensitive adhesive (PSA) or a heat sensitive adhesive.
According to the desired thickness of the finished article, a heat sensitive adhesive film (generally between 50 and 100 µm thick) or a heat sensitive adhesive foam (generally between 1 and 10 mm thick) is interposed between said smooth face and said layer MF.
Typically, the aforesaid coupling treatment is carried out by calendering.
Typically, calendering is conducted by pressing the film and the layer to be coupled between rollers, heated or unheated, according to the type of adhesive material interposed between the film and the layer to be coupled. In the case of the use of PSA material, the rollers are not heated. Instead, in the case of the use heat sensitive adhesive material the rollers are heated, typically to a temperature equal to or greater than 90°C. These temperatures facilitate softening of the polymeric material. This softening makes the pressing action exerted by the rollers more effective. On one hand the pressure compacts the polymeric material and, on the other, it thrusts the abrasive particles in greater depth, incorporating them even more firmly with the film.
If the coating treatment (i) is carried out, the coupling treatment (ii) can take place before or preferably after the coating treatment.
It has also been surprisingly found that the implementation of said preferred variant, consisting of the use of the aforesaid aqueous suspension SA_AF, of the primary process according to the present invention allows the finishing operation as described above to be carried out according to a variant thereof, being said preferred variant.
In fact, due to the use of the aqueous suspension SA_AF during the aforesaid stage (b) of the primary process it is possible at the end of the stage (c) to carry out a shortened finishing operation comprising only the second of the two aforesaid treatments (i) and (ii), or only the treatment (ii). Therefore, to produce said finished flexible abrasive article it is possible to implement a complete process that is operationally simplified due to the fact that the treatment (i), i.e., the step of coating the abrasive face of the film with a layer of anti-clogging compound, is no longer necessary.
Therefore, a further object of the present invention is a complete process for producing a finished flexible abrasive article according to this embodiment of the process, in which during the finishing operation only the treatment (ii), i.e., the coupling or lamination treatment, is carried out.
This complete process, comprising said shortened finishing operation, has numerous advantages with respect to the process in which the finishing operation that of the prior art. In fact the process as a whole is faster, as just one step is sufficient to finish the unfinished article, and also less expensive, as the processing line requires the use of fewer machines and operations with respect to a process that also includes the aforesaid treatment (i). Moreover, as the shortened process does not include a step of washing and of cleaning from process aggregates, or a drying step (for evaporation of solvents, diluents or carriers), the consumption of water and power is also reduced and consequently it is more environmentally friendly.
The finished abrasive articles produced with both the aforesaid processes according to the present invention are, preferably, subjected to further procedures known per se to give characteristics also already found in prior art articles similar in make-up and use manufactured with known processes. In particular, with these further procedures, it is possible to give the articles a suitable shape, typically the shape of a disc, and/or to make, on the working side of the article, a through hole, central with respect to the shape of the article, and further "suction" through holes, which are distributed over this side.
Typically, the suction holes have a smaller diameter with respect to the central hole. The number of the suction holes depends on the size of the abrasive article.
The holes are typically made on the finished article by punching and/or laser cutting.
More in particular, as mentioned at the start of the description, the finished abrasive article obtained with said process is used to rub a painted surface with a dry process to improve its appearance, in particular to reduce or eliminate the orange peel effect present on this surface, and increase its glossiness. For this use, the article obtained according to the complete unsimplified process according to the present invention also has said further through holes. For this application the discs have a diameter, for example, of 150 mm, and, if they are this size, have 14 suction holes smaller with respect to the central hole.
If a finished article is obtained with the simplified complete process, i.e., comprising the shortened finishing operation, the article is suitable to carry out micro finish operations on a painted surface. Preferably, for use in this application the article does not have the central through hole and, optionally, nor does it have the aforesaid suction holes. The advantage of such article without suction holes is that of providing an abrasion surface larger, indicatively by 10-15%, with respect to an article with holes such as the one typically used to reduce or eliminate the orange peel effect. The abrasive capacity of an article with no holes or with a surface with fewer holes is greater than that of an article with holes, with the result that the time required for the processing step is considerably reduced with this article.
The finished articles obtained with the aforesaid processes according to the present invention provide performance and yield substantially comparable to those of similar finished articles of prior art, as shown by the data of the examples.
Further characteristics and advantages of the present invention are described with reference to some preferred embodiments, as illustrated in the accompanying figures, wherein:

Figs. 1a to 1c are schematic views representing the product obtained at the end of each of the three stages of the process according to the invention for producing an abrasive article;
Figs. 1d and 1e are schematic views representing the product obtained at the end of each of the optional further subsequent stages of the process according to the invention;
Fig. If is a schematic view that represents the product obtained at the end of an alternative embodiment of the process according to the present invention;
Figs. 2a and 2b are schematic sectional views showing an example of an abrasive article obtained according to two different embodiments of the process according to the invention;
Fig. 3a is a schematic view of a plant for producing the abrasive article not part of the claimed invention;
Fig. 3b is a schematic view of a plant for producing the abrasive article not part of the claimed invention;
Fig. 4 shows graphs that show the reduction of thickness of the paint by removal during sanding with abrasive articles not part of the claimed invention compared with a known abrasive article;
Fig. 5 shows graphs relating to the reduction of roughness in during pre-polishing of a surface with an abrasive article not part of the claimed invention compared with a known abrasive article.

The process for producing the abrasive article in sheet or film form according to the present invention is described with reference to the accompanying Figs. 1a to 1c.
The first stage of the process comprises preparing a base film F_B 10 (Figs. 1a and 3a). As the process is typically carried out in a continuous production plant 100, this stage comprises feeding a web with the base film F _B 10 and guiding it along a process direction Dp (Fig. 3a).
The subsequent stage of the process comprises forming on the upper surface 11 of the polymer film F_B a layer, as uniform as possible in thickness, of aqueous suspension SA 20 in which particles 30 of abrasive material are dispersed (Fig. 1b).
Said stage (b) is carried out by spreading with a doctor blade system using a smoothing blade B associate with a roller C (Fig. 3a).
In the subsequent stage (c), the product is subjected to preheating with an infrared furnace IO and heating in a hot air furnace AO (Fig. 3a). During this heating, drying of the aqueous suspension SA 20 takes place, reducing its thickness.
After drying is complete, heating causes the polymers present in the film F_B and in the aqueous suspension SA 20 to melt to form a single layer and consequently the article 40 (Figs. 1c and 3a). More in particular, this heating allows the polymers to melt so that the polymers of the film F_B and of the aqueous suspension SA 20 at the interface, i.e., at the upper surface 11, blend with one another to form the article 40.
Subsequently, as illustrated in Figs. Id and 3a, an anti-clogging layer 50 is laid on the article 40 by means of spreading or roll coating using a roller S. After the layer of anti-clogging stearate compound 50 has been spread (Figs. Id and 3a), the article 40 enters a furnace SO to be dried. The subsequent stage consists in application of the coupling layer 60 (Figs. 1e and 3a); application takes place by coupling the article 40 and the coupling layer 60 in a pair of rollers D. Finally, the article is calendered between heated calender rollers L.
According to an alternative embodiment according to the preferred variant described above and illustrated in Figs. If and 3b, the article as obtained at the end of stage (c) is treated directly to apply the coupling layer 60 thereto, according to the methods described previously.
In the examples indicated the following products were used:

water based dispersion (A) of aliphatic polyester thermoplastic polyurethane with the trade name IDROCAP® 501 PF, marketed by ICAP-SIRA POLYMERS SPA, having a viscosity of 250 mPa·s, measured at 25° C according to the Brookfield method; the dispersion comprises (% by weight) 45% of polymer and 55% of water; and

polyester based polyurethane film F_B, trade name Irogran® A85 P4722 marketed by Huntsman International LLC, and having the following physical and mechanical properties:
- softening point: 105 °C, according to the Vicat method;
- elongation at break: 620%, according to the standard DIN 53504;
- tensile strength 35 MPa, according to the standard DIN 53504.

The following tests were conducted on the articles obtained from the following examples to verify the efficacy in the following applications.

- Dry sanding test

The test is carried out on a test piece with a flat surface having a size of 550 mm x 700 mm and painted with an acrylic paint with the trade name PPG, marketed by MaxMayer AcquaMax. The parameter measured during the test is the reduction of the thickness of the layer of paint. The value of this parameter was obtained from the difference between the thickness of the layer before the test and the remaining thickness measured at different times during and at the end of the test. A portable feeler gauge produced by Helmut-Fisher S.r.l. was use for this measurement. The total test time was 3 minutes. Sanding was carried out continuously for the whole of the test. The thickness was measured 6 times at regular intervals of 30 seconds.

- Evaluation test of the reduction of the roughness of a painted surface (micro finish)

The test is conducted on a painted test piece having the same characteristics as the one used for the dry sanding test. During the test the average roughness parameter Rz was measured, evaluated on the measurement length, according to the JISB0601-2001 & ISO method. Measurement is carried out with a Mitutoyo SJ-210 contact profilometer. The total test time was 3 minutes. The roughness was measured 6 times at regular intervals of 30 seconds.

Example 1 - Primary process

A) Preparation of the aqueous suspension

Initially an aqueous mixture (M1) of rheological additives and suspending agents is prepared, placing the following components in a turbo mixer (parts by weight):

46 parts of water
4 parts of rheological additive known with the trade name BYK®-E420, and
3 parts of suspending additive known with the trade name BYK®-U100.

Mixing is conducted at the following conditions: temperature of 20-25 °C, mixing time of 45 minutes and mixing speed of 200-400 rpm. At the end of mixing, the following components are placed in said mixer containing all the aqueous mixture M1 prepared previously (parts by weight):

15 parts of said dispersion (A), and
23 parts of aluminum oxide based abrasive particles having a size equivalent to P3000 (about 5 µm).

The aforesaid mixing takes place by mixing at the following operating conditions:

room temperature;
mixing time: 60 min.
mixing speed 760 rpm

The aqueous suspension 20 obtained has a viscosity of 2200 mPa·s, measured at 25° C according to the Brookfield method.

B) Preparation of the flexible abrasive film

In a continuous production plant 100 a flexible abrasive single layer film is prepared according to the process of the present invention carrying out the stages indicated below.
Stage (a): the plant 100 is fed with a film F_B with a thickness of 150 µm.
The film is guided along a process direction Dp, as illustrated in Fig. 3.
In this stage (a) and in the subsequent stage (b), the process temperature corresponds to the room temperature.
Stage (b): the aqueous suspension 20 prepared previously is poured onto the upper surface of the film F_B coming from the previous stage and spread with an apparatus B of doctor blade type, associated with a roller C, so as to form a layer with a thickness of 60 µm.
Stage (c): the product coming from the preceding stage is subjected to heating which is implemented in two consecutive steps, precisely:

preheating: the product is placed in an infrared furnace IO set at a temperature of 120 °C, the dwell time in this furnace is 1 minute, and
heating: the product removed from the infrared furnace IO is immediately placed in a hot air convection furnace AO set at a temperature of 110° C, the dwell time in this furnace is 3 minutes.

The total heating time is 4 minutes.
After being removed from the hot air furnace, the film is cooled to room temperature.
At the end of this stage a flexible abrasive single layer film 40 having the following physical and mechanical properties is obtained:

total thickness 180 µm;
tensile strength: 135 N, determined according to the standard UNI EN ISO 1421:2000;
elongation at break: 503%, determined according to the standard UNI EN ISO 1421:2000;
longitudinal modulus of elasticity at 300% of elongation: 5980 grf, determined according to the method UNI EN 14704-1:2005

Example 2 - Finishing operation with anti-clogging agent

The flexible abrasive film 40 obtained with the process of example 1 is subjected, according to the following known process, to a finishing operation in which the following is carried out:

a coating treatment (Fig. 3a) comprising:
- laying of an anti-clogging stearate compound 50 (Fig. 1d) on the film 40 and subsequent spreading or roll coating thereof by means of a roller S; this layer has a thickness of about 4 µm, and
- drying to a temperature of about 105° C in a hot air convection furnace SO; and
a coupling treatment in which the flexible abrasive single layer film obtained with the previous coating treatment is subjected to a further treatment to apply, to the lower surface 12 of the film F_B, a coupling layer 60 (Fig. Id). Said layer 60 comprises a layer of a warp knit nylon fabric of 80 g/m².

As illustrated in Fig. 3a, application of the coupling layer 60 is carried out by coupling the article 40 and the coupling layer 60 in a pair of rollers D.
The face of the coupling layer 60 that bonds to the lower surface 12 of the polymer film F_B, has an adhesive layer formed of an adhesive material sensitive to the pressure exerted by the rollers D, unheated, to allow said coupling.
Finally, the film is subjected to the calendering stage in which the rollers L exert a pressure on the film to compact the polymeric material. The rollers are heated to a temperature of 120 °C.
Finally, the finished film thus obtained is given the shape of a disc with a diameter of 150 mm and is subjected to punching to form on the working side according to the prior art a central through hole and 14 suction holes with a diameter of 10 mm.
This article is suitable to be used for dry sanding a painted surface, typically to eliminate the "orange peel" effect of said painted surface.

Comparison example 1c

A film of the type Kovax Tolex K 1500 was subjected to the same dry sanding test for removal of paint to which the film of example 2 was subjected.
The graph of Fig. 4 indicates the removal data of the 6 measurements relating to each of the two dry sanding tests carried out, in which the article of example 2 was used in the first and the article of the comparison example 1c was used in the other.
As can be noted by observing the graphs, 3 minutes after starting the test the paint removal capacity of the abrasive article according to the invention is on the whole greater with a smaller variation in efficiency of the performance of the article over the whole of the test period with respect to that of the article of the comparison example 1c. In particular, the finished article of example 2 gives better removal and machining speed results with respect to the article of the prior art in the first 90 seconds, while subsequently they have similar efficacy.

Example 3 - Primary process

Example 1 is repeated with the following differences in the preparation of the aqueous mixture (M2), which also contains functionalizing additives (process of Fig. 3b). The following components (parts by weight) are placed in a turbo mixture with impeller:

40 parts of water,
5 parts of rheological additive known with the trade name BYK®-E420,
5 parts of suspending additive known with the trade name BYK®-U100,
5.5 parts of functionalizing additive known with the trade name Aquaslip™ 677,
3 parts of functionalizing additive known with the trade name MICHEM 66035, and
3 parts of functionalizing additive formed of an aqueous mixture of stearin, composed of 70 parts of water and 30 parts of stearin.

Said mixing is conducted at a temperature of 50°-60° C, for 40 minutes at a speed of 200-300 rpm.
The aqueous suspension 21 obtained has a viscosity of 4000 mPa·s, measured at 25° C according to the Brookfield method.

Example 4 - Finishing operation

The finishing operation as described in the previous example 2 is repeated on the flexible abrasive single layer film and then a coupling layer 60 is applied to the lower surface 12 of the film F_B (Figs. If and 3b).
The finished film thus obtained (Fig. If) is subjected to the same operation to give it the shape of a disc, but without the operations to create the through holes.
This article is suitable to be used, during the pre-polishing step of a painted surface, for the micro finish of this surface.

Comparison example 2c

A film of the type Kovax Buflex K 3000 was subjected to the same test to reduce the roughness of a painted surface to which the film of example 4 was subjected.
The graph of Fig. 5 indicates the roughness data of the 6 measurements relating to each of the two micro finish tests to reduce the surface roughness carried out, in which the article of example 4 was used in the first and in the article of the comparison example 2c was used in the other.
As can be noted by observing the graphs, 3 minutes after starting the test the efficacy in reducing the roughness profile of the painted surface of the test piece by the abrasive article not part of the claimed invention is initially better and, as from the second minute, exactly the same in absolute terms with respect to the efficacy of the article of the comparison example 2c.

Claims

A process for producing a flexible abrasive article (40), in sheet or film form, comprising the following stages:
a) laying a film F_B of elastomeric or thermoplastic polymers PFB so as to obtain an upper surface (11) and a lower surface (12);

b) forming with an aqueous suspension SA, having viscosity from 1,000 a 15,000 mPa·s, a layer on the upper surface (11) of the film F_B, said aqueous suspension SA comprising the following components:
(i) elastomeric or thermoplastic polymers P_SA, and

(ii) abrasive particles (30);

c) subjecting the product obtained from the preceding stage to a heat cycle that first comprises a step of heating and then a step of cooling to room temperature, in the heating step the temperature is increased until reaching a temperature at least close to the melting or softening temperature of the polymers of the film F_B and of the polymers P_SA;
characterized in that in this process the film F_B is formed with polymers P_FB that have the same chemical composition as the polymers P_SA present in the aqueous suspension SA.
Process according to claim 1, wherein the polymers are selected from: polymers of the families of polyester thermoplastic polyurethanes (TPU) and polyether TPUs, silicone-based elastomeric polymers, synthetic rubbers of NBR or SBR type.
Process according to claim 1 or 2, wherein said aqueous suspension SA comprises the following components (parts by weight):
- from 30 to 80 parts of water,

- from 2 to 20 parts of polymers,

- from 10 to 30 parts of abrasive particles,

- from 4 to 15 parts of both rheological additives and suspending agents.
Process according to any one of the preceding claims, wherein after stage (c) the article is subjected to a finishing treatment comprising at least a coupling treatment (ii) of the smooth face of the article with a coupling layer.
Process according to any one of the preceding claims, wherein after stage (c) the article obtained is subjected to a finishing treatment comprising a coating treatment (i) of the abrasive face of the article with a layer of anti-clogging compound.
Process according to any one of claims 1 to 4, wherein the aqueous suspension SA further comprises from 3 to 20 parts by weight of a functionalized additive selected from paraffin hydrocarbons and saturated fatty acids and their derivatives.
Process according to the preceding claim, wherein the functionalized additive is from 4 to 15 parts by weight with respect to the whole aqueous suspension SA_AF.
Process according to any one of the preceding claims, wherein in stage (c) the step of heating the product obtained from stage (b) is conducted at a temperature comprised between 90 and 150 °C.
Process according to any one of the preceding claims, wherein in stage (c) heating is conducted with hot air flows.
Process according to any one of the preceding claims, wherein the heating stage (c) comprises an initial preheating step, in which the product is irradiated with infrared waves.