A MULTI-LAYER ABRASIVE PRODUCT WITH ANTI-SCRATCH FUNCTIONALITY

Abstract

 The invention relates to a multi-layer abrasive product configured to comprise a plurality of elements capable cooling of an elastic layer based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof by means of air flowing through the elements, when the abrasive product is used in machine abrasion. The multi-layer structure distributes the sanding pressure over a wider area of a workpiece surface being abraded while the elastic layer enables a low scratch pattern, which improves quality of the abrasion result in demanding surface finishing applications, such as mattering or lacquer repairments.

Description

Field of invention

[0001] The invention relates to a multi-layer abrasive product suitable for use in machine abrasion with dust extraction, wherein the backing has been arranged to comprise a structure which provides anti-scratch functionality.

Background

[0002] Abrasive products used for surface finishing are a specific type of abrasive products. Surface finishing and smoothness is of particular interest in industries where coated surfaces are used, such as in the automotive and aviation industries. Scratches on an exposed metal surface, for instance, are subject to differing electrical potential due to surface stresses and predispose the surface to formation of local corrosive cells.

[0003] Abrasive products conventionally used for surface finishing purposes have a backing and an abrasive layer adjoined to one side of the backing, onto which abrasive particles have been firmly embedded into a suitable matrix, which is denoted as a make coat. The other side of the backing is free of the make coat and typically configured to comprise releasable means for attaching the abrasive product onto a power tool comprising a rotating or oscillating backing pad. The backing is meant to withstand forces generated during machine abrasion and is therefore typically prepared of a durable material, such as polyethylene terephthalate. The abrasive particles, also denoted as abrasive grains, are grains prepared of durable material, such as aluminium oxide or silicon carbide, which have been produced and shaped specifically for surface finishing purposes. The shape and size of the grains may be controlled to a certain extent by employing specific materials and different manufacturing methods, such as specific crushing techniques. The abrasive grains are also categorized based on a grit designation, which categorizes the abrasive particles by their grain size distribution, which can be determined by sedimentation. While the shape of grains in each grit designation is relatively similar, variation in shape and size between grains belonging to the same grit designation cannot be completely eradicated.

[0004] The size variation of the abrasive grains is particularly challenging when the grains are fixed parts of abrasive products, which are intended to be used in demanding surface finishing applications together with power tools, such as in mattening and paint rectification applications. Such applications typically use abrasive grains having a grit designation in the range of P240 to P5000, when determined according to FEPA standard 43-2:2017(en). The workpiece surface to be abraded in such applications may comprise non-planar regions, such as convex or concave shapes. Further, the surface roughness of the workpiece to be abraded can be very small. For instance, clear coat thicknesses used in the automotive industry, referring to the optically clear layer of lacquer designed to protect the base colour from UV degradation and oxidation, are typically less than 100 micrometers, such as in the range of 30 to 50 micrometers. Such sanding applications necessitate that the abrasive product provides a low final scratch pattern and a good speed of cut. When abrading non-planar regions with power tools, even sanding pressure is required, as well. These requirements are challenging to meet with conventional abrasive products, without compromises.

[0005] When the abrasive grains are embedded into a make coat, grain size variation becomes an issue, since all grains will not be elevated to the same distance from a planar surface of the make coat. This uneven height profile of the abrasive grains which protrude from an abrading product surface is of particular relevance during machine sanding, where a power tool part comprising the abrasive product is used. The abrasive product is typically attached to a backing pad of the power tool, which is often operated at high speed. The abrasive grains on the abrasive product may obtain high rotational velocity and produce unintentional scratches on the workpiece, when grains that extend furthest away from the make coat surface contact the workpiece surface prior to other grains. An impact with high velocity can lead to a more aggressive cut performance and deeper cuts during the abrasion. Further to this, high speed generated by the power tool and a more aggressive cut performance may warm up the abrasive product considerably. Also the amount of loose, abraded material produced between the abrasive product and the surface of the workpiece being abraded, denoted as debris, may increase. The debris, unless efficiently removed, may clog the abrasive product surface or lead to further scratching of the workpiece, due to abrasive grains becoming loose within the debris. In demanding surface finishing applications the removal of coatings, such as paints or lacquers, may also become problematic due to coatings which contain substances that are hazardous to environment or health, unless properly handled and collected.

Summary

[0006] The invention solves the problems discussed above by providing a multi-layer abrasive product and a method for manufacturing the same.

[0007] An abrasive product capable for dust extraction may be configured to comprise a supportive core layer and an elastic layer, wherein the composition and thickness of the elastic layer is selected to enable reduction of local compression exerted by individual abrasive grains on local points of a workpiece surface. The reduction of local compression exerted by individual abrasive grains on local points of a workpiece surface is referred to as anti-scratch functionality. The core layer, on the other hand, may be configured to improve distribution of sanding pressure, which a backing pad attached to the abrasive product exerts on a workpiece surface area upon machine abrasion, when the abrasive product surface is in contact with the workpiece surface area. The abrasive product may thus comprise a structure, which is configured to provide sufficient overall hardness for machine abrasion, while allowing conformability to follow the shape of a workpiece surface, and ability to reduce pressure of individual grains on local points of a workpiece surface.

[0008] An abrasive product, when used in machine abrasion, produces two kinds of compressive forces which affect a workpiece surface being abraded, which may be described as a surface compression and a local compression. The surface compression, in this context, relates to a sanding pressure which the backing pad of the power tool exerts on the workpiece surface area, which is in contact with the abrasive layer of the abrasive product. The surface compression is therefore transmitted via the backing and the abrasive layer and spread over the abrasive product surface. The local compression, in this context, relates to a point compression exerted on local points on a workpiece surface, for instance by individual abrasive grains. As compressive force is dependent of the surface area over which the force is distributed, the local compression caused by a tip of an abrasive grain is a significant factor for surface scratching. The local compression may also become significant, when machine abrasion is used over a workpiece surface area comprising high curvature, such as edges or corners. An example of a workpiece area comprising high curvature is a body of an automotive or an airplane, which comprises multiple edges. Upon passing over a thin area of workpiece surface, such as an edge, the workpiece surface area beneath the abrasive product rapidly diminishes, whereby the compressive forces exerted on the suddenly reduced surface area increase rapidly. In such case, the reduction of local compression exerted by the abrasive product towards such workpiece surface area comprising high curvature may be used to improve the softness of the abrasion.

[0009] An abrasive product suitable for use in machine abrasion with dust extraction in demanding surface finishing applications preferably uses high quality abrasive grains. Preferably, abrasive grains belonging to the microgrit range are used, when determined according to FEPA standard 43-2:2017(en). Microgrits are defined by a grit designation in the range P240 to P5000, wherein P240 corresponds to grains which have a grain size distribution of less than 59 µm, when defined by median grain size d_s50-value, by means of sedimentation. The microgrit range may further be divided into two sub-ranges, wherein the first range comprises grit numbers P240 to P1200, while the second range, denoted as superfine abrasive grains, comprises grit numbers P1500 to P5000. A larger grit designation value indicates grains having a smaller average size. In very demanding surface finishing applications, abrasive grains may have a grit designation equal to or higher than P600. In highly demanding surface finishing applications, abrasive grains may have a grit designation equal to or higher than P800. Advantageously, the grit designation value is selected based on the intended application. Superfine abrasive grains have a grain size distribution of less than 10 µm, when defined by median grain size d_s50-value. For instance, a superfine abrasive grit designation value may contain grains which are less in size than the size of the defects on a workpiece surface. This is particularly advantageous for reducing the local compression exerted by the abrasive grains to local points on a workpiece surface.

[0010] In order to obtain a good relation between the abrasion cut rate and efficient debris removal, the abrasive layer surface typically comprises both abrasive areas and non-abrasive areas. Abrasive area, in this context, refers to a surface portion of the abrasive layer which contain abrasive grains fixed into a make coat. A non-abrasive area, in this context, refers to a surface portion of the abrasive layer which is free of abrasive grains and make coat. The abrasive layer may comprise abrasive areas, for example, in the range of 40 to 90% of the total area of the abrasive layer surface. Advantageously at least 20% of the total area of the abrasive layer surface is free of abrasive areas. When over 50% of the total area of the abrasive layer surface is free of abrasive areas, the abrasion cut rate may diminish to levels which are not sufficient. Further, the abrasive product may wear down faster than desired. Advantageously the abrasive layer comprises abrasive areas separated by the non-abrasive areas, which form a non-random pattern, such as a tessellation, a fractal or a pattern of geometric or self-similar shapes. In addition to controlling the grit designation of the abrasive grains and the ratio of abrasive area to non-abrasive area on the abrasive layer surface, the spreading of the sanding pressure evenly on a larger area may be further adjusted by controlling the conformability of the backing and the abrasive layer of the abrasive product with the workpiece surface. However, by controlling the elasticity of the layers beneath the abrasive grains, the local compression exerted by individual abrasive grains may be further adjusted.

[0011] Referring to above, a multi-layer structure, wherein the components have been selected for surface finishing purposes, may be arranged to comprise an abrasive layer adhered to a backing, which can be configured to provide both conformability towards the workpiece being abraded as well as a low scratch pattern for the individual abrasive grains, which have been embedded into the make coat. In particular, such dual effect of conformability and low scratch pattern may be arranged by providing a backing, which comprises a combination of a core layer and an elastic layer applied between the core layer and a make coat, wherein the core layer is arranged to distribute the sanding pressure exerted by the power tool towards the workpiece, whereas the elastic layer is configured to act as a spring that is responsive to local compression variation caused by grains that extend furthest from the make coat surface. When the sanding pressure is evenly distributed, the largest local compression is experienced at surface points wherein the grains extend furthest from the make coat surface.

[0012] Advantageously, the abrasive product is configured to comprise an elastic layer comprising polyurethanes. Polyurethanes are copolymers that have been formed of two types of monomers. Polyurethanes are generally synthesized from mixtures of polyols and polyisocyanates in the presence of a catalyst, or upon exposure to ultraviolet light. First a prepolymer containing isocyanate end groups is formed, which is further reacted with a chain extender, such as a short-chain diol, which reacts with the isocyanate end groups of the prepolymer, such that urethane linkages are formed. The increase in the molecular weight and the reaction kinetics may be controlled by the stoichiometry of the starting materials.

[0013] Thermoplastic polyurethane, hereafter abbreviated as TPU, denotes a special type of polyurethane, which is a block copolymer. The polymer structure of TPU comprises segments formed by the reaction of diisocyanates with short-chain diols, which alternate with segments formed by the reaction of diisocyanates with long-chain diols. This leads to a polymer structure containing both lower and higher polarity segments and to the formation of both crystalline regions and flexible chains in the polymer structure, respectively. The crystalline regions provide high elasticity, whereas the flexible chains provide elongation characteristics to the polymer. The formed TPU material may be further moulded or shaped by heating, which enables to control, inter alia, thickness of a layer formed of TPU, while preserving the mechanical strength and elastic properties of the layer. Of notice is, that due to the polymer structure, TPU may be configured to comprise both adjustable hardness and tensile strength, which enable the material to be both reversibly stretched in a first direction S_x parallel to a surface plane and reversibly compressed in a second direction S_z perpendicular to said surface plane, at the same time. The material may thus be temporarily stretched and compressed, without permanent deformation. TPU may be formed of a polyester polyurethane resin or a polyether polyurethane resin, depending on the starting materials used for forming the segments of the copolymer. TPU based on a polyester polyurethane resin may be derived from adipic acid esters. TPU based on a polyether polyurethane resin may be derived from tetrahydrofuran ethers.

[0014] TPU formed of polyester polyurethane or polyether polyurethane resin may be provided as a film having a defined thickness, which enables an elastic layer having a defined thickness to be applied on a multi-layer abrasive product. Advantageously, a TPU film used as an elastic layer is a blown film, having a thickness which is equal to or less than 250 micrometers, such as in the range of 25 to 250 micrometers. A blown TPU film may be produced using conventional thermoplastic processing machinery. A blown TPU film may be orientated uniaxially into machine direction or biaxially into both machine direction and cross-direction, which is perpendicular to the machine direction, to adjust the mechanical properties of the film, such as the tensile stress at break. Preferably, TPU film used as an elastic layer comprises a tensile stress at break which is equal to or higher than 50 MPa, when determined in accordance with ISO 527. Preferably, the tensile stress at break is substantially symmetrical in both machine direction and cross-direction.

[0015] Alternatively, the elastic layer of a multi-layer abrasive product as disclosed above may be a TPU formed of polyester polyurethane or polyether polyurethane resin by extrusion coating, wherein the thermoplastic polyurethane has been coated in an extrusion process on top of a core layer. This is particularly advantageous when the core layer is a lattice formed of thermoplastic polymer threads or a knitted textile layer of thermoplastic material, since the TPU material may be applied in a melt form and thereby firmly adhered directly onto the core layer surface, thereby forming the elastic layer. The surface of a thus formed elastic layer may subsequently be flattened and smoothened into a defined thickness as disclosed above, for instance by means of a release liner or a calender roll. This is highly beneficial compared to adhering layers together with phenolic resins, which have been predominantly used as binders on coated abrasive products comprising a core layer formed of thermoplastic polymer threads, since the phenolic resins are typically partially absorbed into the polymer threads, and, once hardened, cause substantial stiffening of the backing, whereby conformability is reduced.

[0016] Reactive hot-melt polyurethane adhesive, hereafter abbreviated as HMPUR, is an adhesive which is based on the same chemistry as disclosed above for polyurethanes. HMPUR is generally prepared of amorphous prepolymers which are melted and mixed together in a melt state, whereafter the prepared mixture in a melt state is degassed under reduced pressure and sealed into an air tight container, such as an aluminum cartridge for later use. Upon exposure to moisture in the air, the chemical composition of the prepared HMPUR starts to react rapidly with the moisture, whereby very strong bonds begin to form into the adhesive. This reaction is denotes a curing. The curing typically continues 24 to 48 hours from the initial exposure to the moisture, which strengthens the structure. Due to the curing chemistry, HMPUR is typically solvent-free and does not emit volatile organic compounds during application or curing. HMPUR adhesives have a fast curing time and a strong bonding strength to different materials. The elastic properties of the HMPUR, in particular, may be controlled by selecting prepolymers having a suitably low glass transition temperature. A prepolymer, wherein glass transition takes place below room temperature (23°C), are preferred, as such prepolymer tends to have a longer setting time, wherein the adhesive remains tacky, upon application. Thus, the abrasive product may be configured to comprise an elastic layer which is based on reactive hot-melt polyurethane adhesive.

[0017] HMPUR may be used as a coating having a defined thickness for providing the elastic layer of a multi-layer abrasive product. Advantageously, an elastic layer may comprise a cross-linked elastomer formed of HMPUR having a coat weight equal to or higher than 20 g/m², such as in the range of 20 to 100 g/m², the coat weight determinable according to ASTM F2217-02. Advantageously, HMPUR is used in combination with TPU for providing the elastic layer on a multi-layer abrasive product, such that the HMPUR is used as an adhesive interface to firmly adhere a TPU film having a defined thickness with a core layer.

[0018] Advantageously, the elastic layer comprises polyester polyurethane or polyether polyurethane resin that has a hardness equal to or less than 95 ShA, preferably equal to or less than 90 ShA, such as in the range of 75 ShA to 95 ShA, preferably in the range of 80 ShA to 90 ShA, when determined from the resin prior to forming the elastic layer, at a temperature of 22°C, in accordance with ASTM D2240. When the elastic layer has been formed of a reactive hot-melt polyurethane, the cross-linked elastomer advantageously has a hardness equal to or less than 65 ShA, preferably equal to or less than 60 ShA, such as in the range of 40 ShA to 65 ShA, preferably in the range of 45 ShA to 60 ShA, when determined after 7 days from cross-linking, at a temperature of 22°C, in accordance with ASTM D2240.

[0019] A restriction in the use of a thin, elastic layer which is based on thermoplastic polyurethane has been the thermal properties of the material. TPU, in particular, may begin to soften already in temperatures significantly below 200°C, such as in the range of 160 to 200°C, whereby it no longer provides sufficient hardness for the make coat and the abrasive grains. The softening temperature of a thermoplastic material may be determined as Vicat softening temperature, in accordance with ISO 306, Method A120 using a force of 10 Newton and a heating rate of 120°C / hour. Softening of the elastic layer can become problematic upon machine sanding, wherein the abrasive product is prone to warm up relatively fast. However, when the abrasive product has been configured to comprise a plurality of elements capable of cooling of the elastic layer by means of air flowing through the elements, when the abrasive product is used in machine abrasion, the warming of the abrasive product may be reduced. This enables an elastic layer which is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof. The elements capable of cooling of the elastic layer may be, for instance holes, such as cut holes provided with laser light or mechanically punched holes. When a knitted textile layer comprising individual polymer threads is used as a core layer, the elements capable of cooling of the elastic layer may comprise openings defined by the boundaries of crossing polymer threads. Advantageously, the elements capable of cooling may simultaneously be configured to be suitable for dust extraction, whereby debris formed during the machine sanding may be effectively removed by suction. This enables also the removal of coatings, such as paints or lacquers, which contain substances that else would become hazardous to environment or health.

[0020] The multi-layer structure comprising an elastic layer and disclosed above therefore enables a distribution of sanding pressure over a wider area of a workpiece surface being abraded while simultaneously enabling a low scratch pattern, which improves quality of the abrasion result in demanding surface finishing applications, such as mattering or lacquer repairments.

[0021] Referring to above, there is provided an abrasive product suitable for use in machine abrasion with dust extraction, the abrasive product comprising

• an abrasive layer adhered onto a backing,
• the abrasive layer comprising an adhesive size coat adhered on top of a make coat and a plurality of abrasive grains fixed into the abrasive layer,
• the backing comprising a core layer and an elastic layer adhered on top of the core layer,

wherein

• the elastic layer has a thickness in the range of 25 to 250 micrometers, and the elastic layer is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, and

wherein

• the abrasive product has been configured to comprise a plurality of elements capable of dust extraction and cooling of the elastic layer by means of air flowing through the elements, when the abrasive product is used in machine abrasion.

[0022] There is further provided a method for manufacturing an abrasive product suitable for use in machine abrasion with dust extraction, the method comprising

• applying a make coat on top of an elastic layer,
• fixing a plurality of abrasive grains on the make coat,
• applying an adhesive size coat on top of the make coat and the plurality of abrasive grains, thereby forming an abrasive layer,
• adhering the elastic layer to a core layer, thereby forming an abrasive product, and
• providing a plurality of elements into the abrasive product,

wherein

• the elastic layer has a thickness in the range of 25 to 250 micrometers, and the elastic layer is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, and

wherein

• the elements are capable of dust extraction and cooling of the elastic layer by means of air flowing through the elements, when the abrasive product is used in machine abrasion.

[0023] The invention is described in more detail in the independent and dependent claims.

Short description of the figures

[0024] In the figures, S_x, S_y and S_x denote orthogonal directions.

Fig. 1

is a schematic, cross-sectional illustration of a backing pad of a power tool comprising air conducts for dust extraction, a multi-layer abrasive product suitable for use in machine sanding with dust extraction and a workpiece surface, which comprises non-planar regions.

Fig.2

is a schematic, cross-sectional illustration of a multi-layer abrasive product comprising an abrasive layer and a backing. The abrasive layer comprises abrasive grains fixed into a make coat, wherein the abrasive grains have been elevated into different heights from the make coat surface. The backing comprises a core layer capable of spreading sanding pressure and an elastic layer capable of reducing local compression exerted by the abrasive grains which extend furthest away from the make coat surface. The abrasive product comprises elements capable of dust extraction and cooling of the elastic layer.

Fig.3

is a schematic, cross-sectional illustration of a multi-layer abrasive product in use during machine abrasion, wherein sanding pressure having a force F is applied to a core layer of the backing, which causes local reversible compression of the elastic layer on areas where the abrasive grains extend furthest away from the make coat surface. At the same time, a suction through the elements creates an air flow from the abrasive product surface toward the backing, which enables a cooling effect for the elastic layer.

Fig.4

is a schematic, top-view illustration of a multi-layer abrasive product, showing a surface of the abrasive product comprising elements capable of dust extraction and cooling of the elastic layer.

Fig.5a

is a schematic, top-view illustration of a multi-layer abrasive product, showing a surface of the abrasive product wherein the core layer is formed of thermoplastic polymer threads in the form of a lattice, wherein the elements capable of dust extraction and cooling of the elastic layer are positioned between the thermoplastic polymer threads.

Fig.5b

is a schematic, 3D magnification of Fig. 5a, illustrating a multi-layer abrasive product surface comprising individual thermoplastic polymer threads in the form of a lattice, wherein the polymer threads define the boundaries of an element capable of dust extraction and cooling of the elastic layer.

Fig 6.

is a 150 times magnified image produced with scanning electron microscope of a multi-layer abrasive product surface, which comprises an elastic layer made of TPU film beneath the abrasive layer, showing abrasive grains extending to varying heights from the surface.

Fig.7

is a 30 times magnified image produced with scanning electron microscope of a multi-layer abrasive product surface, which comprises an abrasive layer on top of a textile core layer, which comprises a flattened HMPUR layer as an elastic layer between the abrasive layer and the textile core layer, and elements having a generally rectangular shape, which are capable of dust extraction and cooling of the elastic layer.

Fig. 8

shows comparative experimental results of air flow measurements and air pressure drop determined from disc shaped multi-layer samples with or without an elastic layer.

Fig. 9

shows comparative experimental results of surface and point compression measurements determined from disc shaped multi-layer samples with or without an elastic layer.

Detailed description

A multilayer abrasive product

[0025] Reference is made to Figs.1 to 3. Figure 1 illustrates an abrasive product PROD1 suitable for use in machine abrasion with dust extraction and anti-scratch functionality, which is attached into a backing pad PAD1 of a power tool. The abrasive product PROD1 has a multi-layer structure and comprises a backing BCK1 and an abrasive layer ABR1 adjoined to one side of the backing BCK1.

[0026] The abrasive layer ABR1 comprises a make coat 11, a size coat 12 and a plurality of abrasive grains 14, 14a, 14b. The make coat 11 is typically a water-based dispersion of polyurethane acrylate, which is cured by means of ultraviolet light. The make coat may be coated on a suitable substate, such as on an elastic layer 10. The purpose of the make coat is to firmly adhere the abrasive grain 14, 14a, 14b from one side into the abrasive layer ABR1. Once the plurality of abrasive grains 14, 14a, 14b has been fixed into the abrasive layer ABR1, an adhesive size coat 12 is applied on top of the make coat 11. The size coat typically comprises a mixture of acrylate oligomers and monomers, and is also cured by means of ultraviolet light. The size coat is meant to tightly envelop the abrasive grains 14, 14a, 14b from other sides, such that once the adhesive size coat is cured, the abrasive grains 14, 14a, 14b are firmly supported also horizontally in direction S_x by the adhesive size coat 12. When desired, the abrasive layer ABR1 may further comprise a super-size coat 13, which can be applied on top of the adhesive size coat 12. The super-size coat 13 is typically a mixture of zinc stearate and acrylate dispersion, which is cured by means of ultraviolet light. The super-size coat 13 acts as a protective layer and also lubricates the debris forming into the space between the workpiece OBJ1 surface and the abrasive product PROD1, which reduces clogging. This also prolongs the lifetime of the abrasive product PROD1.

[0027] The make coat 11 is typically applied on the elastic layer 10 by means of screen coating, whereby the make coat forms a substantially planar layer in a direction S_x. The thickness of the make coat 11 and the size coat 12 is relative to the height h1, h2 of the abrasive grains 14, 14a, 14b such that height h1, h2 of the abrasive grains 14, 14a, 14b is more than the thickness of the make coat 10 and the size coat 12. Typically thickness of the make coat 10 and the size coat 12 is a few micrometers, such as in the range of 2 to 20 micrometers. The thickness of the make coat 110 and the size coat 12 may be determined, for example, by using image analysis software on cross-sectional images produced with scanning electron microscope of the abrasive product PROD1.

[0028] The abrasive grains 14, 14a, 14b typically have a body comprising a broad end and a sharp, pointed end. Advantageously, the abrasive grains 14, 14a, 14b have a grit designation equal to or higher than P240, preferably equal to or higher than P600, most preferably equal to or higher than P800, such as in the range of P240 to P5000, preferably in the range of P600 to P4000, most preferably in the range of P800 to P3000, when determined according to FEPA standard 43-2:2017(en), as disclosed above. The abrasive grains 14, 14a, 14b are preferably attached to the make coat 11 by means of electrostatic coating. Electrostatic coating refers to a process, wherein the abrasive grains are pulled up towards a backing comprising a make coat by a sufficiently high electric voltage. This generates an electric field, wherein the grains orient themselves while travelling through air. This results into the dull end of the abrasive grains to become embedded into the make coat while the pointed, sharp end of the abrasive grain protrudes away from the make coat. Examples of materials used in abrasive grains 14, 14a, 14b are aluminum oxide, silicon carbide, zirconia, alumina zirconia, diamond, cubic boron nitride or any combinations thereof. The abrasive product PROD1 may contain a plurality of abrasive grains 14, 14a, 14b which are of the same material or which may consists of a mixture of grains, which are of different grain types.

[0029] The backing BCK1 comprises a core layer 15 and an elastic layer 10. Typically, the backing BCK1 comprises a further attachment layer 16 with means for adhering the adhesive product PROD1 onto a receiving layer 17 of a backing pad PAD1 of a power tool in a releasable manner. The attachment layer 16 and the receiving layer 17 may be, for instance, a hook and loop -type of fastening system, wherein the attachment layer 16 comprises soft loops while the receiving layer 17 comprises harder hooks capable of intertwining into the soft loops. When the two layers are pressed together, the hooks mingle into the loops and fastens the abrasive product PROD1 temporarily, but strongly, against the substantially planar surface of backing pad PAD1.

[0030] Reference is made to Figs. 2 and 3. The core layer 15 is meant to act as a supportive element of the abrasive product PROD1, which is capable of withstanding shear forces experienced by the abrasive product when the backing pad PAD1 is in motion. The core layer 15 further serves as an intermediate layer which is meant to spread a compressive force F, such as sanding pressure caused by the surface of backing pad which is in contact with the abrasive product, when the abrasive product PROD1 is pressed against a workpiece OBJ1 surface. Upon machine sanding, the core layer 15 distributes the sanding pressure F which the backing pad PAD1 of the power tool exerts on the surface of the abrasive product PROD1, which is in contact with the workpiece surface OBJ1. The compressive force F is therefore transmitted and distributed via the backing BCK1 and the abrasive layer ABR1.

[0031] The workpiece OBJ1 surface to be abraded has a macroscopic shape, which may contain both planar and non-planar regions. Non-planar regions may comprise an inclination angle α, which defines a curvature for the non-planar region. The workpiece OBJ1 surface may contain regions having high curvature, such as edges or corners, wherein the inclination angle α is close to 90°. Regions having high curvature are particularly demanding for surface finishing applications. Advantageously, the core layer 15 is manufactured of material having sufficient conformability to follow the macroscopic shape of the workpiece OBJ1 surface. Examples of suitable core layer materials are for instance, paper and film, such as a cellulose fiber based paper substrate or a polymeric film made of polyester, polyamide, or polypropylene. Advantageously, the core layer 15 is composed thermoplastic polymer material, such as polyamide, polyester or polypropylene. Thermoplastic polymer material may be used as threads which can be a knitted together to form a textile layer. Such textile layer construction enables a dense network threads onto which an elastic layer may be adhered to. However, when the core layer 15 is a weaved lattice of thermoplastic polymer threads, such as a knitted textile layer, the structure remains relatively open, which enables very effective air flow to be arranged between substantially across the entire abrasive layer ABR1 surface. A knitted textile layer comprising individual polymer threads may further be arranged to provide superior conformability for demanding surface finishing applications, since the arrangement of the polymer threads enables a better dimensional stability of the core layer upon machine abrasion, when the abrasive product PROD1 is in contact with a non-planar region comprising an inclination angle α. By dimensional stability it is meant that upon applying a compressive force F, the structural integrity and thickness of the core layer is substantially maintained, even though the core layer shape locally conforms to surface deviations of a workpiece, which it is pressed against. A core layer 15 which is a knitted textile layer of thermoplastic material, may further be configured to comprise an arrangement of interlaced loops protruding from the core layer in a manner which enables the loops to act as releasable fastening means for a receiving layer 17, such as a backing pad PAD1 of a power tool, whereby a backing BCK1 may be produced without a separate attachment layer 16. Reference is made to Figs 5a and 5b, which illustrate a multi-layer abrasive product PROD1, showing a top surface of the abrasive product wherein the core layer 15 is formed of thermoplastic polymer threads in the form of a lattice, and wherein the crossing polymer threads THR1 define the boundaries of an element VOID1 capable of dust extraction and cooling of the elastic layer 10.

[0032] The core layer 15 may further consist of or comprise a foam layer based on polyurethane. Referring to the synthesis of polyurethanes disclosed above, polyurethane may be foamed in the presence of a blowing agent, such as carbon dioxide gas, during the polymerization step. When a small amount of water is added to the polymerization reaction, the moisture reacts with isocyanates to form carbon dioxide gas and an amine, which may further react with isocyanates to form urea groups. Depending on whether the formed gas bubbles have been broken or remained intact in the foam layer, the layer can be denoted either as closed-cell or open-cell layer. An open-cell foam layer based on polyurethane is flexible and allows air to flow through, whereas a closed-cell foam layer does not. The type of foam layer produced may be controlled by the amount of blowing agent and by the addition of surfactants, which can be used to adjust the rheology of the polymerising mixture. In addition to being a core layer 15, a foam layer may further be used as a spacer layer positioned between the core layer and a power tool, wherein the spacer layer may be used to even out the sanding pressure exerted by the power tool upon use. The spacer layer may have a thickness which is equal to or less than 7 millimeters, preferably equal to or less than 5 millimeters, most preferably equal to or less than 3 millimeters, such as in the range of 2 to 7 millimeters, preferably in the range of 3 to 6 millimeters, most preferably in the range of 3 to 5 millimeters.

Functionality of the abrasive product upon machine abrasion

[0033] Reference is made to Figure 2. When the abrasive grains 14 are embedded into the make coat 11, all grains will not be elevated to the same distance from a reference plane REF1. Hence, a tip of an abrasive grain 14a may be elevated to a height h2, whereas a tip of another abrasive grain 14b may be elevated to a height h1, which is less that the height h2, when measured substantially perpendicular in direction S_z from the reference place REF1. The height difference Δh denotes the portion of the abrasive grain length h2, which extends furthest away from the reference place REF1. A reference plane REF1 in this context denotes a substantially planar surface which can be used to determine the height profile of the abrasive grains. A reference plane REF1 can be the surface of a substantially planar make coat 11. Alternatively, the surface of a super-size coat 13, when present, may be denoted as a reference plane REF1 for the determination of the height profile of the abrasive grains. The height difference Δh is relative to the grain size, and therefore to the grit designation, which defines the shape and average size of the abrasive grains 14. In a microgrit range the height h1, h2 of the abrasive grains 14, 14a, 14b in general is less than 59 micrometers, such as in the range of 2 to 59 micrometers.

[0034] To provide an abrasive product which upon machine abrasion produces a low scratch pattern, the backing BCK1, may be arranged to comprise a combination of a core layer 15 and an elastic layer 10, wherein the elastic layer 10 has been applied between the core layer 15 and a make coat 11. While the core layer 15 is primarily configured to distribute a general compressive force F, such as a sanding pressure caused by a power tool towards the workpiece, the elastic layer 10 is configured to act as a spring that is responsive to local compression variation caused by the height difference Δh due to abrasive grains 14b that extend furthest from the reference place REF1. A preferred elastic layer 10 is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, such as a film that has been formed of polyester polyurethane resin or polyether polyurethane resin and/or a cross-linked elastomer formed of a reactive hot-melt polyurethane adhesive. When the elastic layer 10 is a film that has been formed of polyester polyurethane resin or polyether polyurethane resin, it advantageously has a hardness in the range of 75 ShA to 95 ShA, preferably in the range of 80 ShA to 90 ShA, when determined from the resin prior to forming the elastic layer, at a temperature of 22°C, in accordance with ASTM D2240. When the elastic layer 10 is a cross-linked elastomer formed of a reactive hot-melt polyurethane adhesive, it advantageously has a hardness in the range of 40 ShA to 65 ShA, preferably in the range of 45 ShA to 60 ShA, when determined after 7 days from cross-linking, at a temperature of 22°C, in accordance with ASTM D2240.

[0035] Reference is made to Figure 3. The elastic layer 10 may be provided into a defined thickness d1. The thickness d1 may be selected based on the height h1, h2 of the abrasive grains, such that the thickness d1 is at least equal to the height h1, h2 of the abrasive grains. This enables the elastic layer 10 to be reversibly deformed in response to local compressive force caused by an abrasive grain 14b which is pushed against a workpiece OBJ1 surface. The thickness may be equal to or less than 250 micrometers, preferably equal to or less than 200 micrometers, most preferably equal to or less than 150 micrometers, such as in the range of 25 to 250 micrometers, preferably in the range of 30 to 200 micrometers, most preferably in the range of 40 to 150 micrometers. When the elastic layer 10 is based on a reactive hot-melt polyurethane adhesive, the thickness d1 may be controlled by the coat weight of the adhesive. Advantageously, the elastic layer 10 may comprise reactive hot-melt polyurethane adhesive having a coat weight equal to or higher than 20 g/m², preferably equal to or higher than 30 g/m², most preferably equal to or higher than 40 g/m², such as in the range of 20 to 100 g/m², preferably in the range of 30 to 90 g/m², most preferably in the range of 40 to 80 g/m², the coat weight determinable according to ASTM F2217-02. Preferably, the elastic layer 10 is a combination, wherein a TPU film having a defined thickness is adhered to the core layer 15 by means of reactive hot-melt polyurethane adhesive. Thereby a backing BCK1 may be obtained, wherein the elastic layer 10 and the core layer 15 are strongly adjoined to one another.

[0036] Upon machine abrasion, when a compressive force F is applied to the abrasive product PROD1 from the direction of the backing pad PAD1, the surface of the abrasive layer ABR1 becomes into contact with the workpiece OBJ1 surface. Due to the compressive force F, the tip of the abrasive grain 14b, which is the furthest point from the reference plane REF1, is first to contact the object OBJ1. When the elastic layer 10 spot beneath the abrasive layer ABR1 is sufficiently elastic, it will enable the broad end of the abrasive grain 14b to reversibly penetrate into the elastic layer 10 by a distance Δd, denoting a difference between the original thickness d1 at rest and a thickness d2 under compressive force. This enables to reduce the local compression caused by height difference Δh of the abrasive grains on a workpiece surface.

[0037] The abrasive product PROD1 is configured to comprise a plurality of elements VOID1, which are capable of dust extraction and cooling of the elastic layer 10 by means of air AIR1 flowing through the elements, when the abrasive product is used in machine abrasion. The cooling of the elastic layer 10 by means of air AIR1 flowing through the elements VOID1 facilitates the use thermoplastic material having a relatively low softening temperature in the range of 160 to 200°C as an elastic layer 10. Upon abrasion of the workpiece OBJ1 surface, debris formed during the machine sanding may further be effectively removed and collected through the same elements VOID1 by suction. This enables also a safe removal of coatings, such as paints or lacquers, which may contain substances that else would become hazardous to environment or health. When a knitted textile layer comprising individual polymer threads is used as a core layer 15, the elements VOID1 may comprise openings defined by the boundaries of crossing polymer threads, which are provided into the core layer 15 upon manufacturing said layer. Hence, the shape of the void may comprise angularity, such as a generally rectangular shape, such as a square or a rhomboid shape.

[0038] Reference is made to Figs. 4, 5a and 5b. Fig.4 is a top view illustration of a multi-layer abrasive product PROD1 in the form of a disc, showing a surface of the abrasive product comprising a plurality of elements VOID1 capable of dust extraction and cooling of the elastic layer. The elements VOID1 may be provided on the abrasive product PROD1 as holes, which are cut into the abrasive product. The elements VOID1 may be, for instance, substantially circumferential holes provided with laser light or punched mechanically. The elements VOID1 may be cavities extending through the abrasive layer ABR1 and the backing layer BCK1. Advantageously, the elements VOID1 are produced radially in rings RAD1 - RAD5 which are not radially equidistant. By having a large continuous area without elements VOID1 between the rings RAD3 and RAD4, the air flow from the edge of the disc inwards towards the centre may be improved, due to a longer distance between the elements VOID1. The positioning both also reduces wear and tear of the outer perimeter of the disc upon machine abrasion.

[0039] Fig.5a and 5g are illustrations of another version of a multi-layer abrasive product PROD1, wherein the core layer 15 is a knitted textile layer formed of crossing polymer threads THR1 in the form of a lattice, wherein the polymer may be of thermoplastic material, such as polyamide, polyester or polypropylene. The core layer 15 may further comprise an arrangement of interlaced loops protruding in a direction opposite to S_z away from the abrasive layer. When the backing BCK1 comprises an arrangement of interlaced loops in a manner which enables the loops to act as releasable fastening means for a receiving layer 17, a separate attachment layer 16 is not necessary. The elements VOID1 capable of dust extraction and cooling of the elastic layer may thus comprise angularity and be positioned between the thermoplastic polymer threads.

[0040] The shaping of the abrasive product PROD1 into a disc form and the cutting of the elements VOID 1 into the abrasive product PROD1 is preferably done by laser light, when using thermoplastic materials. This enables to melt fibers around the cut edges, which reduces the risk of premature disintegration of material from the abrasive product PROD1 surface during machine sanding. Further, the focus point and power of a laser light can be adjusted to provide an element VOID1 having a depth which will reach through the abrasive layer ABR1 but not to the backing BCK1. This is advantageous in products where the core layer 15 is a knitted textile layer of thermoplastic material and the elastic layer 10 is provided as a film that has been formed of polyester polyurethane resin or polyether polyurethane resin.

[0041] The number of elements VOID1 and their diameter d2 on the abrasive product PROD1 may be configured to enable air flow rate and air pressure, which are capable of cooling of the elastic layer and remove debris from the abrasive layer surface by means of suction AIR1. Advantageously, the diameter d2 of an element is at least 1 millimeters, preferably at least 2 millimeters, such as in the range of 1 to 20 millimeters. The diameter and shape of the elements in the abrasive product PROD1 may differ. The elements VOID1 may comprise, for example a centre hole provided on the middle of the disc with a larger diameter. Advantageously, the abrasive product PROD1 comprising the elements VOID1 has a capability for air flow rate which is equal to or higher than 25 litre per second, preferably equal to or higher than 27 litre per second, such as in the range of 27 to 30 litre per second. Advantageously, the abrasive product PROD1 comprising the elements VOID1 has a capability for providing a vacuum having a measured air pressure difference which is equal to or less than 2,2 kPa, preferably equal to or less than 2,0 kPa, and a ratio of the air flow rate to the air pressure difference equal to or higher than 12, when determining the air flow rate and the air pressure difference through the abrasive product PROD1 with a differential pressure meter.

Method for manufacturing an abrasive product comprising an elastic layer

[0042] A coated abrasive product comprising an elastic layer 10 based on TPU film is typically manufactured from roll to roll, by first providing a radiation curable make coat 11 on one side of an unwinded TPU film adjoined to a polyethylene carrier film and then attaching abrasive grains 14, 14a to the make coat 11, for example by means of electrostatic coating, as disclosed above, prior to curing the make coat, thereby binding the abrasive grain. The make coat 11 may be provided, for instance, by means of a rotary screen unit. Subsequently, a radiation curable size coat is applied onto the make coat and the abrasive grains, which is also cured by radiation. Preferably, ultraviolet (UV) light curable compounds are used, whereby the radiation may be performed by means of infrared heaters to remove excess water from the make coat and followed by UV light for curing the compounds. Alternatively, the compounds may be curable with electron beams. A supersize coat can be applied and cured separately, in a similar manner, to reduce clogging of the abrasive layer surface, during machine abrasion.

[0043] In a subsequent step, the polyethylene carrier film is separated from the TPU film comprising the abrasive layer ABR1. This step is typically performed from roll to roll, as well. A reactive hotmelt polyurethane adhesive may be applied on a core layer 15 and the TPU film may be firmly adhered to a core layer 15 by means of the HMPUR. Alternatively, a reactive hotmelt polyurethane adhesive may be applied to the other side of the TPU film, which is free of the make coat, whereby the TPU film may be firmly adhered to a core layer 15 by means of the HMPUR. When HMPUR is applied on the TPU film, however, a higher coat weight is typically used. A higher coat weight will stiffen the formed abrasive product structure, and thereby reduce the conformability of the abrasive product to follow the shape of a workpiece surface The core layer 15 may be, for instance, a foam layer based on polyurethane, such as an open-cell foam layer based on polyurethane. Alternatively, the core layer 15 may be formed of crossing thermoplastic polymer threads THR1, such as polyamide, polyester or polypropylene threads, such that the core layer 15 is a knitted textile layer. When the core layer 15 is a foam layer based on polyurethane, an attachment layer 16 comprising soft loops suitable for a hook and loop - type of fastening system may be adjoined to the core layer 15, for example by means of the HMPUR. When a knitted textile layer is used as a core layer 15, an arrangement of interlaced loops protruding from the core layer may be provided, wherein the interlaced loops act as releasable fastening means for a receiving layer 17, such as a backing pad PAD1 of a power tool.

[0044] Subsequently, the abrasive product PROD1 may be shaped from a finished roll by cutting with laser light or by mechanical punching. Simultaneously, the abrasive product PROD1 may be arranged to contain a plurality of elements VOID1 capable of dust extraction and cooling of the elastic layer. Fig. 6 is a 150 times magnified image produced with scanning electron microscope of a multi-layer abrasive product surface, which comprises an elastic layer made of TPU film beneath the abrasive layer, showing abrasive grains extending to varying heights from the surface.

[0045] An alternative method for manufacturing an abrasive product involves the use of reactive hot-melt polyurethane adhesive as an elastic layer 10 on a knitted textile layer which functions as a core layer 15. The use of HMPUR on a knitted textile layer reduces both the number of materials and process steps needed. Further, core layer 15 comprising a textile is more open for air to flow through the abrasive product.

[0046] In the alternative method, HMPUR in a melt state is coated on the core layer 15 by means of a roll coating unit, using a kiss coating technique, such that the HMPUR obtains a self-similar in shape with the core layer 15. Subsequently, a release liner or a structured film is pressed against the HMPUR until it solidifies, whereby an elastic layer 10 comprising a flat or structured surface is obtained, which may be coated with an abrasive layer ABR1 comprising a make coat 11, abrasive grains 14, 14a, 14b, size coat 12 and an optional supersize coat 13, if needed. Hence, an elastic layer 10 self-similar in shape with a core layer 15, which is a knitted textile layer, is obtainable.

[0047] The abrasive layer ABR1 is preferably manufactured in separate steps. First, a make coat 11 is applied on the flat or structured surface of the elastic layer 10 with a gravure roller unit using a kiss coating technique. Subsequently, the abrasive grains 14, 14a, 14b, are applied using electrostatic coating, prior to curing the make coat 11, to orientate the grains. After curing the make coat, a size coat 12 is applied on top of the make coat and grains, only. A gravure roller unit using a kiss coating technique or inkjet printing may be used to avoiding filling the cavities VOID1 between the flattened surface. An air knife may be used to remove excess material, prior to curing the size coat 12. A supersize coat 13 may optionally be applied which can be, for instance, a stearate coating on top of the cured size coat 12 or incorporated totally or partly into the size coat 12. Reference is made to Fig. 7, which is a 30 times magnified image produced with scanning electron microscope of a multi-layer abrasive product surface, comprising an abrasive layer ABR1 on top of a textile core layer 15, which comprises a flattened HMPUR layer as an elastic layer between the abrasive layer ABR1 and the textile core layer 15, and elements VOID1 having a generally rectangular shape, which are capable of dust extraction and cooling of the elastic layer.

[0048] Comparative experimental studies are provided hereafter, to better understand the aspects of the invention, as disclosed above.

Comparative example 1 - Air flow rate and air pressure testing

[0049] A comparative experimental study was performed to measure the air flow rate through multi-layer abrasive products, when using dust extraction to create suction. The air flow rate through the abrasive product is indicative of the cooling efficiency of the elastic layer, when the abrasive product is used in machine abrasion with suction. A higher air flow rate correlated with a higher cooling efficiency.

[0050] In the experimental study, also the vacuum generated by the dust extraction system was measured. A vacuum, in this context, refers to measured air pressure difference (in units of kPa), in relation to the surrounding atmospheric pressure (1 atm = 101,3 kPa). The measured air pressure difference, as disclosed herein, is therefore indicative of the air pressure decrease, which has occurred due to suction. A dust extractor, when used, generates a pressure decrease by suction, towards which the air flows from the surroundings through the abrasive product. The air pressure difference serves as a further indication of the ability of the abrasive product to convey air and particles through the product layers. A higher air pressure difference indicates lower air pressure at the measurement point. A low air flow and a high air pressure difference may indicate the presence of one or more layers in the abrasive product, which restrict the passage of air through the abrasive product. A high air flow and a low air pressure difference, on the other hand, indicates an abrasive product structure, which enables the air flow through the layers to cool the structure while enabling an efficient removal of debris from the surface.

[0051] The following equipment and experimental setup was used for the air flow rate and air pressure measurements:

• Mirka DEROS circular backing pad having a 150 millimeter diameter and a grip attachment surface, the backing pad connected to an extractor system comprising an adapter jig for the backing pad (106 millimeter diameter), a sleeve and a hose (4 meter length, diameter 32 millimeters),
• a hollow steel tube for measuring air velocity (2 meter length, diameter 32 millimeters) between the hose and an dust extractor
• Mirka Dust Extractor 1025 L with an unused dust bag and filter installed and an adapter hose (0.5 meter length, diameter 32 millimeters) connecting the dust extractor with the hollow steel tube,
• TSI Anemometer TA465 with a straight air velocity probe 964, and
• Testo 512 pressure meter having a 4mm diameter pressure hose.

[0052] A pressure system was set up, wherein an abrasive product having the same diameter of 150 mm as the backing pad was firmly attached to the DEROS backing pad over the whole area of the disc surface (hook and loop type of attachment). An adapter jig having a smaller outer diameter of 106 mm and a center hole (diameter 21 millimeters) was, in turn, firmly attached to the backing pad. The adapter jig contained a rim at the outer perimeter for sealing the outer perimeter between the adapter jig and the backing pad, such that when the parts were connected, the sealing prevented air from leaking in or out via the outer perimeter. This forced all the incoming air to flow through the pressure system comprising an abrasive product, a backing pad and an adapter jig. A pressure hose for measuring air pressure difference caused by the dust extractor, in relation to surrounding atmospheric pressure, was inserted into a 4mm diameter through-put hole of the adapter jig, such that the free end of the pressure hose was level with the surface of the adapter jig that was facing the backing pad. The center of the through-put hole was located at a radial distance of 30 mm from the center of the adapter jig. Upon connecting the backing pad with the adapter jig, the two parts were oriented such that the free end of the pressure hose in the through-put hole was visible, when viewed through a hole on the DEROS backing pad. The opposite end of the adapter jig,which was not connected to the backing pad, was firmly attached to the sleeve and hose leading to the hollow steel tube, which was connected to the dust extractor.. The air velocity probe 964 was placed inside the hollow steel tube, such that the sensor in the probe was located in the middle of the tube, both lengthwise and cross-sectionally. The pressure system was operated for 30 minutes before the tests, to warm up the system and thereby providing constant operating conditions for the measurements.

[0053] The TSI Anemometer TA465 and Testo 512 pressure meter were used according to manufacturer's instructions. The air pressure was measured as a pressure difference in relation to surrounding atmospheric pressure, in units of kilopascals (kPa). Air pressure difference of 0 kPa therefore indicates a measured pressure, which is equal to the surrounding atmospheric pressure. Air pressure difference of 1 kPa, in turn, indicates a measured pressure, which is 1 kPa less than the surrounding atmospheric pressure. The air flow rate was measured in units of litres per second. The test was performed in a room having normal temperature and pressure (20 ± 1°C, 1 atm) during the measurements.

[0054] The dust bag and the filter of the Dust Extractor was changed between each measurement, such that an unused dust bag and filter was used for each abrasive product in the experiment. Deros backing pad, abbreviated as "pad only", without any abrasive product attached on the surface, was used as a reference, to improve the comparability of the air flow rate and pressure difference measurements.

[0055] The abrasive product samples tested in the experiment are explained hereafter. All abrasive product samples tested in the experiment were shaped into circular discs having a 150 mm diameter, to be compatible with the DEROS backing pad.

[0056] The air flow capability and pressure difference created by core layer which is a knitted textile layer was tested with a layer having a thickness of 5 millimeters and which had been formed of crossing polyamide threads. This sample is abbreviated as "CL 5mm net only". The layer comprised an arrangement of interlaced loops protruding from the layer in a manner which enabled the loops to act as releasable fastening means. The layer was therefore as such suitable for direct attachment to the backing pad, without a further attachment layer.

[0057] "P1200 MH (Film + CL 5mm net)", "P1200 MF (Film + CL 5mm net)" and "P1200 MH Film + CL 5mm net" and "P1200 MF Film + CL 5mm net" denote a further set of tested abrasive product samples, comprising the above-disclosed core layer (denoted as CL) that was a net formed of crossing polyester threads, having a thickness of 5 millimeters, and an elastic layer, which was a blown TPU film that has been formed of polyether polyurethane resin. On top of the elastic layer was an abrasive layer comprising an adhesive size coat, a make coat and abrasive grains having a grit designation P1200.

[0058] The blown TPU film had a thickness of 60 micrometers. The blown TPU film had hardness in the range of 80 to 90 ShA (22°C, ASTM D2240, determined from the polyether polyurethane resin) and a tensile stress at break in the range of 50 to 60 MPa (ISO 527), in both machine and cross direction. The softening temperature of the TPU film was in the range of 160 to 200°C (Vicat method, ISO 306, Method A120, 10 N, heating rate of 120°C / hour). The TPU film had been produced on a polyethylene carrier, which was removed prior to use.

[0059] "P1200 MH (Film + CL 5mm + velcro)" and "P1200 MF (Film + CL 5mm + velcro)" denote a still further set of tested abrasive product samples, comprising a core layer which was an open-cell foam layer based on polyurethane having a thickness of 5 millimeters, and an elastic layer, which was a blown TPU film that has been formed of polyether polyurethane resin on one side of the core layer and an attachment layer (Velcro) for a backing pad on the other side of the core layer. The elastic layer and the abrasive layer was the same as for the samples disclosed above.

[0060] The designations MH and MF refer to circular holes, which were cut with laser light either as through-holes extending through the abrasive product or as holes only extending through the abrasive layer and the elastic layer, but not extending into the core layer. A computer aided laser cutting machine was used to cut holes into the products. A central hole having a 15 mm diameter was cut into each of the samples. In addition, a plurality of smaller holes having a diameter of 4.5 mm was cut into the sample, either into a pattern wherein the holes were spread in a radially equidistant manner from the center, denoted as a "multihole" arrangement and abbreviated as MH, or in a manner wherein the pattern of the holes was radially symmetrical, but not in equidistant manner, denoted as a "multifit" arrangement and abbreviated as MF. Reference is made to Figure 4, which illustrates an abrasive product surface comprising a "multifit" arrangement. Sample names wherein parentheses have been used, indicate a sample comprising a through-hole arrangement, whereas sample names without parentheses indicate an arrangement wherein the core layer has not been provided with circular holes cut with laser light.

[0061] Reference is made to Table 1 (below) and Figure 8, which show results of the experimental study. In Figure 8, the axis on the left indicates the measured air flow rate, in units of litre per second for the samples represented by grey bars. The axis on the right indicates the measured air pressure difference, in units of kiloPascal (kPa), for the same samples represented by the continuous black line.

Table 1. Experimental study results of air flow rates and air pressure differences determined from a backing pad, 5 mm core layer and abrasive products.

Sample	Air pressure difference (kPa)	Air flow (litre/second)
pad only	0,86	28,4
CL 5mm net only	0,99	28,1
P1200 TPU MH (Film + CL 5mm net)	1,33	27,7
P1200 TPU MF (Film + CL 5mm net)	1,39	27,5
P1200 TPU MH (Film + CL 5mm + velcro)	1,30	27,5
P1200 TPU MH Film + CL 5mm net	1,29	27,4
P1200 TPU MF (Film + CL 5mm + velcro)	1,41	27,2
P1200 TPU MF Film + CL 5mm net	1,38	27,0

[0062] The experimental study results show that the abrasive product samples comprising a blown TPU film, which comprised a MH or MF arrangement, comprised an air flow which was in the range of 27 to 28 litres per second and was in the range of 95 to 98% of the air flow, which was obtainable with the pad only (28,4 litres per second). The air pressure differences determined from abrasive product samples comprising a blown TPU film demonstrated an air pressure difference, which was in the range of 1,3 to 1,4 kPa. The air pressure difference was 50 to 60 % higher than the pressure difference measured from the pad only (0,85 kPa). The ratio of air flow to air pressure difference determined from abrasive product samples comprising a blown TPU film was in the range of 19 to 22. These parameters are indicative of the dust extraction and cooling capability of the abrasive product, upon machine sanding.

[0063] The experimental study results demonstrate that an abrasive product comprising an elastic layer may be configured to comprise a plurality of elements capable of dust extraction and cooling of the elastic layer by means of air flowing through the elements, when the abrasive product is used in machine abrasion. In particular, an abrasive product suitable for use in machine abrasion with dust extraction may be provided having a capability for air flow rate which is equal to or higher than 25 litres per second, such as in the range of 25 to 28 litters per second, when determining the air flow rate through the multi-layer structure with an anemometer, as disclosed above. The abrasive product may further provide a vacuum having a measured air pressure difference which is equal to or less than 2,2 kPa, preferably equal to or less than 2 kPa, most preferably equal to or less than 1,8 kPa, such as in the range of 2,2 kPa to 1,3 kPa, when determining the air pressure difference through the multi-layer structure with a differential pressure meter, as disclosed above. The abrasive product may further provide a ratio of the air flow rate to the air pressure difference is equal to or higher than 12, preferably equal to or higher than 16, such as in the range of 12 to 22, when determining the air flow rate and the air pressure difference through the multi-layer structure, as disclosed above.

Comparative example 2 - Surface and point compressibility testing

[0064] A further comparative experimental study was performed to measure surface compression and point compression from multi-layer abrasive products.

[0065] The purpose of the surface compression test was to measure how much force in Newtons is required to compress the entire surface of an abrasive product to a target thickness, which is 60% of the original thickness.

[0066] The surface compression test was performed with a Shimadzu EZ-LX tester, using 150mm diameter compression plates and the Trapezium program, according to the manufacturer's instructions. The abrasive product sample is placed on the lower compression plate such that the abrasive layer is upwards and facing the upper compression plate. After measuring the original thickness at rest, without compression, the target thickness corresponding to 60% of the original thickness is set. The sample is first compressed at a speed of 50 mm/min until a 5 N preload is reached. The determination of force required to compress the entire surface of an abrasive product to the target thickness is done subsequently, using a speed of 10 mm/min.

[0067] The surface compression test is indicative of the ability of the abrasive product to resist surface compression, which correlates with a sanding pressure which the backing pad of a power tool would exert, when the abrasive product is used in machine abrasion with a power tool. When a larger force is required to compress the entire surface of an abrasive product to the target thickness, the abrasive product is more resistant to surface compression.

[0068] The purpose of the point compression test was to measure how much force in Newtons is required to compress a small area of an abrasive product surface to a target thickness, which is 60% of the original thickness.

[0069] The point compression test was performed with a Shimadzu EZ-LX tester, as explained above for the surface compression test, but using a 150mm diameter compression plate below the sample and a rod comprising a spherical end with a diameter of 15mm, according to the manufacturer's instructions. The abrasive product sample is placed on the lower compression plate such that the abrasive layer is upwards and facing the rod comprising the spherical end. After measuring the original thickness at rest, without compression, the target thickness corresponding to 60% of the original thickness is set. The rod comprising the spherical end is pushed against the abrasive layer at a speed of 1 mm/min until a 0,1 N preload is reached. The determination of force required to compress the entire surface of an abrasive product to the target thickness is done subsequently, using a speed of 30 mm/min.

[0070] The point compression test is indicative of the ability of the abrasive product to resist point pressure, which simulates a local compression point which the abrasive grains would exert towards the elastic layer, when the abrasive product is used in machine abrasion with a power tool. When a larger force is required to compress the area of an abrasive product to the target thickness, the elastic layer of the abrasive product is more resistant.

[0071] The abrasive product samples tested in the experiment were to a large extent the same as in the experimental study explained above. All abrasive product samples tested in the further experiment were also shaped into circular discs having a 150 mm diameter. The abbreviations used are also the same. The abbreviation MF denotes a "multifit" arrangement, as explained above. Sample names wherein parentheses have been used indicate a sample comprising a through-hole arrangement, as explained above.

[0072] "P800 PET MF (Film + CL 5mm net)" and "P1200 PET MF (Film + CL 5mm net)" denote abrasive product samples comprising the above-disclosed core layer that is a net formed of crossing polyester threads, having a thickness of 5 millimeters, and a commercially available biaxially oriented high-strength polyethylene terephthalate film layer, abbreviated as PET, adjoined next to the core layer. The PET used for P800 grit samples had a thickness of 100 micrometers (Hostaphan^® RN CT02O). The PET used for P1200 grit samples had a thickness of 75 micrometers (Hostaphan^® RNK CT02O). On top of the PET was an abrasive layer comprising an adhesive size coat, a make coat and abrasive grains having a grit designation P800 or P1200.

[0073] "P800 TPU MF (Film + CL 5mm net)" and "P1200 TPU MF (Film + CL 5mm net)" denote abrasive product samples comprising the above-disclosed core layer that is a net formed of crossing polyester threads, having a thickness of 5 millimeters, and an elastic layer, which was a blown TPU film that has been formed of polyether polyurethane resin, as disclosed in the experimental study above. On top of the elastic layer was an abrasive layer comprising an adhesive size coat, a make coat and abrasive grains having a grit designation P800 or P1200, respectively.

[0074] "P1200 PET MF (Film + CL 5mm + velcro)" and "P1200 TPU MF (Film + CL 5mm + velcro)" denote abrasive product samples comprising an above-disclosed core layer which was an open-cell foam layer based on polyurethane having a thickness of 5 millimeters and an attachment layer (Velcro) for a backing pad on one side of the core layer. The other side comprised either a high-strength PET film layer or an elastic layer, which was a blown TPU film, as disclosed above. On top of the PET and TPU layer was an abrasive layer comprising an adhesive size coat, a make coat and abrasive grains having a grit designation P1200.

[0075] "P800 HMPUR (coat + CL 3mm net)" denotes an abrasive product sample comprising an above-disclosed core layer that is a net formed of crossing polyester threads, having a thickness of 3 millimeters, on top of which had been coated a reactive hot-melt polyurethane adhesive as an elastic layer, having a coat weight of 55 grams per square meter (ASTM F2217-02). On top of the elastic layer was an abrasive layer comprising an adhesive size coat, a make coat and abrasive grains having a grit designation P800.

[0076] The sample set above enabled a comparison of products comprising a TPU film and a conventional polyester film (PET), as well as comparison of the effect of core layers which were of homogeneous material (an open-cell foam layer based on polyurethane) or manufactured into a textile form (crossing polyester threads). The sample set above further enabled a comparison of products having different grit designation (P800 vs P1200) and wherein the elastic layer was either a blown TPU film or a cross-linked elastomer formed by reactive hot-melt polyurethane adhesive coating.

[0077] Reference is made to Table 2 (below) and Figure 9, which show results of the experimental study. In Figure 9, the axis on the left indicates the surface compression, in units of Newton, for the samples represented by grey bars. The axis on the right indicates the point compression, in units of Newton, for the same samples represented by the continuous black line.

Table 2. Experimental study results of surface compression and point compression measurements from tested abrasive products.

Sample	Surface Compression [N]	Point Compression [N]
P800 PET MF (Film + CL 5mm net)	87,5	6,3
P1200 PET MF (Film + CL 5mm net)	91,8	5,9
P800 TPU MF (Film + CL 5mm net)	81,4	2,2
P1200 TPU MF (Film + CL 5mm net)	84,1	1,9
P1200 PET MF (Film + CL 5mm + velcro)	337,6	8,2
P1200 TPU MF (Film + CL 5mm + velcro)	355,8	4,7
P800 HMPUR (coat + CL 3mm net)	299,3	2,2

[0078] The experimental study results show that the measured surface compression of abrasive product samples comprising a core layer that was a net formed of crossing polyester threads was less than 100 N and thus much lower than in the abrasive product samples comprising a core layer which was open-cell foam layer based on polyurethane, where the measured surface compression was over 300 N. The measured surface compression values in abrasive product samples comprising a TPU film and a core layer that was a net formed of crossing polyester threads were in the range of 7 to 9 % smaller in the corresponding samples wherein a PET film was used.

[0079] The measured point compression values in abrasive product samples comprising a TPU film and a core layer that was a net formed of crossing polyester threads were in the range of 32 to 57 % of the measured point compression values in the corresponding samples wherein a PET film was used. In the samples comprising an attachment layer (Velcro) for a backing pad on one side of the core layer, the measured point compression values in the sample comprising a TPU film was 57% of the corresponding samples wherein a PET film was used. Interestingly, the abrasive product sample comprising a net formed of crossing polyester threads, on top of which had been coated a reactive hot-melt polyurethane adhesive as an elastic layer, demonstrated a relatively high surface compression value but a very low point compression value. An abrasive product demonstrating a low resistance to point compression value is well suited for surface finishing and capable of conforming to surface areas comprising high curvature, such as edges or corners. A sufficiently high resistance to surface compression, on the other hand, indicates a structure, which is capable to provide sufficient overall hardness for machine abrasion.

[0080] The experimental study results demonstrate that abrasive products having a grit designation equal to or higher than P800 and comprising a core layer and an elastic layer adhered on top of the core layer, wherein the elastic layer is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, are showing much lower point compression resistance values than a conventional abrasive product comprising a polyester layer beneath the abrasive layer, such as a PET film layer.

[0081] The observed difference in compression resistance values was less, when comparing the surface compression values, in particular within samples wherein the core layer was a net formed of crossing polyester threads. This is due to the core layer being a knitted textile layer of thermoplastic material, such as polyamide, polyester or polypropylene, wherein the lattice formed by the individual threads forms a structure that better resists the lateral movement of the layer, when compressive force is applied to the surface. In a core layer formed of homogeneous material, such as open-cell foam layer based on polyurethane, the material was compressible.

[0082] Of notice, in samples comprising a core layer that was a net formed of crossing polyester threads was present and an elastic layer based on thermoplastic polyurethane or reactive hot-melt polyurethane adhesive beneath the abrasive surface, almost three times lower point compression resistance was observable, in comparison to samples comprising the same core layer but a polyester layer beneath the abrasive layer, such as a PET film layer. An abrasive product comprising a structure wherein an elastic layer based on thermoplastic polyurethane or reactive hot-melt polyurethane adhesive is positioned between the core layer and the abrasive layer is thus better equipped to distribute sanding pressure exerted by the power tool towards the workpiece, while also being responsive to local compression variation, such as pressure caused by abrasive grains that extend further from the make coat surface.

[0083] Of notice is also the sample "P800 HMPUR (coat + CL 3mm net)", which demonstrated very low point compression and a relatively high surface compression. This combination was considered to be very promising for distributing sanding pressure exerted by the power tool towards the workpiece, while also being responsive to local compression variation. Due to an open textile net structure, the abrasive product comprises a plurality of elements capable of both dust extraction and cooling of the elastic layer.

[0084] The results also demonstrated, that by using a larger mircrogrit designation, the point compression required to compress the abrasive product could be adjusted. A decrease in point compression was observable both in the comparative PET film samples P800 and P1200 comprising a net backing, as well as in the TPU film samples P800 and P1200 comprising a net backing.

[0085] It is to be understood that many variations to the experimental studies above may be conceived. The disclosure above, including the appended figures, is provided to aid understanding of the invention, which is defined by the appended claims.

Claims

1. An abrasive product (PROD1) suitable for use in machine abrasion with dust extraction, the abrasive product comprising

- an abrasive layer (ABR1) adhered onto a backing (BCK1),

- the abrasive layer comprising an adhesive size coat (12) adhered on top of a make coat (11) and a plurality of abrasive grains (14, 14a, 14b) fixed into the abrasive layer,

- the backing (BCK1) comprising a core layer (15) and an elastic layer (10) adhered on top of the core layer (15),

wherein

- the elastic layer (10) has a thickness (d1) in the range of 25 to 250 micrometers, and the elastic layer (10) is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, and

wherein

- the abrasive product (PROD1) has been configured to comprise a plurality of elements (VOID1) capable of dust extraction and cooling of the elastic layer (10) by means of air flowing through the elements, when the abrasive product is used in machine abrasion.

2. The abrasive product (PROD1) according to claim 1, wherein the elastic layer (10) comprises

- a film that has been formed of polyester polyurethane resin or polyether polyurethane resin that has a hardness equal to or less than 95 ShA, preferably equal to or less than 90 ShA, such as in the range of 75 ShA to 95 ShA, preferably in the range of 80 ShA to 90 ShA, when determined from the resin prior to forming the elastic layer, at a temperature of 22°C, in accordance with ASTM D2240, and/or

- a cross-linked elastomer formed of a reactive hot-melt polyurethane adhesive, the cross-linked elastomer having a hardness equal to or less than 65 ShA, preferably equal to or less than 60 ShA, such as in the range of 40 ShA to 65 ShA, preferably in the range of 45 ShA to 60 ShA, when determined after 7 days from cross-linking, at a temperature of 22°C, in accordance with ASTM D2240.

3. The abrasive product (PROD1) according to any of the previous claims, wherein the elastic layer (10) comprises a tensile stress at break which is equal to or higher than 50 MPa, when determined in accordance with ISO 527.

4. The abrasive product according (PROD1) to any of the previous claims, wherein the backing (BCK1) further comprises a spacer layer, for instance a foam layer based on polyurethane, preferably an open-cell foam layer based on polyurethane.

5. The abrasive product (PROD1) according to any of the previous claims, wherein the core layer (15) is a lattice of thermoplastic polymer threads (THR1), such as polyamide, polyester or polypropylene threads, wherein the thermoplastic polymer threads (THR1) define the boundaries of the plurality of elements (VOID1) capable of dust extraction and cooling of the elastic layer (10).

6. The abrasive product (PROD1) according to any of the previous claims, wherein the core layer is a knitted textile layer of thermoplastic material, such as polyamide, polyester or polypropylene,

7. The abrasive product (PROD1) according to any of the previous claims, wherein the backing (BCK1) comprises an arrangement of interlaced loops protruding from the core layer in a manner which enables the loops to act as releasable fastening means for a receiving layer (17), such as a backing pad (PAD1) of a power tool.

8. The abrasive product according (PROD1) to any of the previous claims, wherein the elements (VOID1) capable of cooling of the elastic layer (10) are cavities extending through the abrasive layer (ABR1) and the backing layer (BCK1).

9. The abrasive product according (PROD1) to any of the previous claims, wherein the elastic layer (10) is self-similar in shape with the core layer (15), and wherein the elements (VOID1) capable of cooling of the elastic layer (10) have a substantially similar shape on the abrasive layer (ABR1) and on the backing layer (BCK1).

10. The abrasive product according to any of the previous claims, wherein the plurality of abrasive grains (14, 14a, 14b) consists of a mixture of grains, which are of different grain types, such as aluminum oxide, silicon carbide, zirconia, alumina zirconia, diamond, cubic boron nitride or any combinations thereof.

11. The abrasive product according (PROD1) to any of the previous claims, wherein the plurality of abrasive grains (14, 14a, 14b) have a grit designation equal to or higher than P240, preferably equal to or higher than P600, most preferably equal to or higher than P800, such as in the range of P240 to P5000, preferably in the range of P600 to P4000, most preferably in the range of P800 to P3000, when determined according to FEPA standard 43-2:2017(en).

12. The abrasive product (PROD1) according to any of the previous claims, wherein the abrasive layer (ABR1) comprises abrasive areas separated by the non-abrasive areas, which form a non-random pattern, such as a tessellation, a fractal or a pattern of geometric or self-similar shapes.

13. The abrasive product (PROD1) according to any of the previous claims, having a capability for

- air flow rate which is equal to or higher than 25 liters per second,

- vacuum having an air pressure difference which is equal to or less than 2,2 kPa, and

- a ratio of the air flow rate to the air pressure difference equal to or higher than 12

, when determining the air flow rate and the air pressure difference through the multi-layer structure with a differential pressure meter.

14. An apparatus (PAD1) comprising an abrasive product (PROD1) according to any of the previous claims.

15. A method for manufacturing an abrasive product (PROD1) suitable for use in machine abrasion with dust extraction, the method comprising

- applying a make coat (11) on top of an elastic layer (10),

- fixing a plurality of abrasive grains (14) on the make coat (11),

- applying an adhesive size coat (12) on top of the make coat (11) and the plurality of abrasive grains (14), thereby forming an abrasive layer (ABR1),

- adhering the elastic layer (10) to a core layer (15), thereby forming an abrasive product (PROD1), and

- providing a plurality of elements (VOID1) into the abrasive product (PROD1),

wherein

- the elastic layer (10) has a thickness (d1) in the range of 25 to 250 micrometers, and the elastic layer (10) is based on thermoplastic polyurethane, reactive hot-melt polyurethane adhesive or a combination thereof, and

wherein

- the elements (VOID1) are capable of dust extraction and cooling of the elastic layer (10) by means of air flowing through the elements, when the abrasive product is used in machine abrasion.

Drawing