Patent Publication Number: US-2016236326-A1

Title: Abrasive medium comprising a phosphate-based filler

Description:
The present invention relates to abrasive media bonded with a thermosetting material consisting of a mixture of abrasive particles, thermosetting binder, and optionally further fillers, as well as a method for producing corresponding grinding wheels bonded with a thermosetting material. 
     The abrasive media bonded with a thermosetting material of the present application can be grinding wheels bonded with a thermosetting material or also other abrasive media, such as for example coated abrasives. 
     A grinding wheel essentially consists of abrasive particles which are pressed with a binder to form a grinding wheel. Thermosetting materials, in particular phenolic resins, modified phenolic resins, e.g. epoxy- and/or rubber-modified phenolic resins, are frequently used as binder. In addition, a grinding wheel can also contain further fillers. These can be active in the grinding process (e.g. cooling) and thus support the grinding process or be used for stabilization (bursting strength, flexibility) of the grinding wheel. 
     Coated abrasives likewise essentially consist of abrasive particles and binder. This abrasive mixture here is, however, not pressed to form a self-supporting body, but rather is applied as a thin layer onto a carrier material such as paper, fabric or plastic. For this, a binder layer used as an adhesive layer is first applied to the carrier. The abrasive particles are then applied onto the adhesive layer, usually directed under the action of an electric field. For stabilization, the abrasive particles are then also covered with a second binder layer. 
     EP 0 855 948 relates to abrasive media such as grinding wheels which are covered with a phosphate-containing coating. The grinding wheels can be, for example, grinding wheels bonded with synthetic resin which essentially consist of abrasive particles, which are shaped, together with binder, to form a grinding wheel. The coating likewise consists of a binder, in which an inorganic metal phosphate salt devoid of hydrogen is dissolved. The phosphate additive is thus exclusively contained in the external surface of the grinding wheel. The grinding wheels provide a useful grinding power on metal surfaces, such as titanium, with reduced energy consumption. 
     DE 2 324 222 relates to a ceramically bonded grinding wheel, wherein the ceramic binder is provided with a phosphate additive. In the case of ceramics, phosphates are frequently added in order to reduce the curing temperature of the ceramics. Moreover, by adding from 10 to 20 wt.-% alkali or alkaline-earth metal phosphates to the ceramic binder, the adhesion of the binder to the abrasive particles, as well as the mechanical strength of the grinding tool, is increased. The grinding wheels are cured at temperatures of up to 1280° C. 
     WO 2010/040472 A2 relates to ceramically bonded abrasive particle agglomerates. The binder essentially contains aluminosilicate, water glass and water. The binder mixture can in addition also contain inorganic phosphates and further grinding aids or fillers. To produce the abrasive particle agglomerates, the abrasive particles are mixed with the binder, wherein primary particles cluster together to form abrasive particle agglomerates. The abrasive particle agglomerates are then dried and cured at temperatures of up to 400° C. 
     It is generally known in the abrasives industry that abrasive media, in particular grinding wheels, are subject to certain ageing effects. A cause of these ageing effects appears to be the adsorption of moisture by the material of the grinding wheel. It is currently believed that the boundary surface between the particles and the surrounding bond matrix is weakened by moisture diffusion, which leads to the abrasive particles breaking off more easily. These phenomena are intensified by alkaline-reacting bond components, which contribute to a decomposition of the phenolic resin matrix. The service life of conventional grinding wheels is considerably impaired by these ageing effects. 
     Earlier investigations (C. Stang; “Rezeptur and Feuchteeinflüsse auf die Standzeit von phenolharzgebundenen Trennschleifscheiben”; Lehrstuhl für Kunststofftechnik 2009, Erlangen and investigations at Rhodius Schleifwerkzeuge GmbH) have shown that grinding wheels usually reach a saturated state of equilibrium with their environment with respect to the moisture bound in the matrix after a storage time of 8 to 12 months. This corresponds to a conditioning time of 14 to 21 days in a damp alternating atmosphere according to DIN 50016. Based on a standardized testing method, the determined power of the grinding wheels decreases purely through these ageing effects to approximately 40% to 50% of the initial power, wherein the loss in power takes place considerably more sharply at the start and the curve levels out towards the saturation point. 
     The ageing effects appear to be relatively independent of the resin systems and particle coatings used. In extensive series of tests, no positive effects with respect to extending the service life or preserving power of the grinding wheels could be achieved by conventional modifications of the resin systems, the particle coatings and different fillers active in the grinding process. 
     Until now, it has been attempted to prevent ageing phenomena in grinding wheels using a suitable packaging, such as for example to shrink-wrap grinding wheels in plastic film. However, ageing effects can, of course, only be prevented in this way while the packaging is closed. If the packaging is opened or if the diffusion barriers of the packaging are exceeded for a sufficient length of time, ageing effects cannot be prevented. Resealable, diffusion-resistant packaging, on the other hand, is comparatively laborious and expensive. 
     An object of the present invention is to provide an abrasive medium with longer service life and higher grinding power, without significantly increasing the production costs and without over-complicating the handling of the abrasive media. 
     This object is achieved by adding a phosphate-based additive to the abrasive medium of the type named at the beginning. The phosphate-based additive is contained in the starting mixture of the abrasive medium in a concentration of from 1 to 10 wt.-% and is distributed homogeneously in the abrasive medium. 
     In a preferred embodiment, the phosphate-based additive is contained in the starting mixture of the abrasive medium in a concentration of from 1 to 5 wt.-% and preferably in a concentration of approximately 4 wt.-%. 
     In a further preferred embodiment, the proportion of the phosphate-based additive is 2 to 10 wt.-%, preferably 5 to 7 wt.-%, of the proportion by weight of the abrasive particles contained in the starting mixture. 
     The phosphate-based additive preferably contains phosphates of alkaline-earth metals, such as magnesium, calcium, strontium and barium phosphates, iron(II) phosphates, iron(III) phosphates, manganese, aluminium and zinc phosphates, synthetic or mineral phosphates, as well as mixtures thereof. The phosphate-based additive is particularly preferably a calcium phosphate such as dicalcium phosphate or the anhydride thereof or monetite or brushite, tricalcium phosphate or hydroxylapatite or pentacalcium hydroxy-triphosphate (Ca 5 (PO 4 ) 3 OH), fluorapatite, dimagnesium phosphate, trimagnesium phosphate, magnesium diphosphate or a mixture thereof. The phosphate-based additive is preferably insoluble in water and not subjected to any chemical or physical change during the production of the abrasive medium. 
     Any thermosetting material is used as binder in the abrasive medium of the present invention. Synthetic resins such as phenolic resins or modified phenolic resins, e.g. epoxy- and/or rubber-modified phenolic resins, are preferably used as binder. 
     The proportion of the binder is preferably 1 to 40 wt.-%, preferably 5 to 30 wt.-% and particularly preferably 10 to 20 wt.-% in the starting mixture of the abrasive medium. 
     Sodium cryolite, potassium cryolite, potassium aluminium fluorides, iron sulfides, zinc sulfides, manganese sulfides, manganese chlorides, copper sulfides, chalcopyrites, pyrites, iron(II) oxides, iron(III) oxides, iron(II/III) oxides, lime, chalk, carbon modifications such as graphites, carbon blacks, coal, as well as mixtures thereof can be used as fillers. 
     Tricalcium phosphate (TCP) has previously been used e.g. in foodstuff technology as an acidity regulator, firming agent or release agent. It is also used as a so-called “anticaking agent” in order to counteract the formation of clumps of moisture-sensitive bulk material. The concentrations used are, as a rule, in the single-figure percentage range. Furthermore, it is known to a person skilled in the art that TCP, because of the large specific surface area (up to 70 m 2 /g) of the powder particles, can reversibly adsorb up to more than 10% moisture, relative to the particles&#39; own weight, on the particle surface. 
     Comprehensive experiments have shown that both the service life and the power of the abrasive medium according to the invention are increased by the addition of a phosphate-based additive. The phosphate additive appears to prevent the bonds between the abrasive particles and the binder from being attacked and impaired by the penetration of moisture. 
     It is known that e.g. TCP has good adsorption capability for moisture. However, as the moisture can only be adsorbed latently and reversibly by TCP, it was not to be expected by a person skilled in the art that the service life of a synthetic resin-bonded abrasive medium could be increased by the addition of such a phosphate-based additive. In particular, the increase in the grinding power of the abrasive media according to the invention was not to be expected, and as yet it also cannot be explained precisely how it comes about. 
     A further advantage of the abrasive media according to the invention is that the phosphate addition is not subject to any physical or chemical change during the production of the abrasive media. Crystalline apatite phases can therefore be clearly demonstrated in incineration residues of abrasive media by means of X-ray diffractometry. In addition, the phosphate proportion in the incineration residue can be precisely quantified by means of X-ray fluorescence spectrometry and thus the phosphate proportion of the underlying abrasive medium composition can be calculated. 
     The abrasive media of the present invention can be grinding wheels, coated abrasives or any other abrasive media. In the case of coated abrasives, the percentage weight indications used here relate to the weight of the starting mixture (abrasive particles, binder, additives) of the abrasive medium. The weight of the support layer is not included in the weight of the starting mixture. 
     A subject of the present application is additionally a method for producing a grinding wheel with a phosphate-based additive, wherein the phosphate-based additive is contained in the starting mixture of the grinding wheel in a concentration of from 1 to 10 wt.-% and is distributed homogeneously in the abrasive medium. The method comprises the steps of preparing a pulverulent phosphate-based additive, mixing the pulverulent phosphate-based additive with abrasive particles, binder and optionally further fillers, pouring the starting mixture into a mould provided for this, adding glass fabric, pressing the mixture to form a green compact and curing the green compact to obtain a grinding wheel. 
     In the method for producing a grinding wheel according to the invention, the pulverulent phosphate-based additive is preferably first mixed with the abrasive particles. This mixture is then combined with the binder and optionally further fillers. Through the intensive mixing, the phosphate additive is distributed homogeneously both in the starting mixture and in the finished grinding wheel. 
     The production of a coated abrasive according to the invention essentially corresponds to the production of conventional coated abrasives. The only thing to bear in mind is that a homogeneous distribution of the additive in the abrasive medium is to be achieved. In particular, if the abrasive particles are applied to the support layer electrostatically, it could be advantageous to combine the additive with the binder of the adhesive layer and the final coating in order to ensure a homogeneous distribution of the additive in the abrasive medium. 
    
    
     
       With the help of the following figures, the abrasive media of the present invention and the method for the production thereof are discussed in more detail. There are shown in: 
         FIG. 1  a diagram of the normalized power data of the grinding wheels according to Example 1 and 2 and the reference wheel  1  in the automated test; 
         FIG. 2  a diagram of the normalized power data of the grinding wheels from  FIG. 1  in the manual test; 
         FIG. 3  a diagram of the normalized power data of the grinding wheels according to Example 3 and the reference wheel  3  in the manual test; 
         FIG. 4  a diagram of the normalized power data of the grinding wheels according to Example 4 and the reference wheel  1  in the manual test. 
     
    
    
     Production of the Grinding Wheels: 
     To illustrate the advantageous properties of the abrasive media according to the invention, the grinding power of grinding wheels with phosphate additives was compared with the grinding power of conventionally produced phenolic resin grinding wheels. Table 1 shows an overview of the grinding wheels investigated. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Grinding wheels investigated 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Example 1/2 
                 Reference 1 
                 Example 3 
                 Reference 3 
                 Example 4 
               
               
                   
                 (Test no.: 
                 (Test no.: 
                 (Test no.: 
                 (Test no.: 
                 (Test no.: 
               
               
                   
                 13290.1/.2) 
                 13290) 
                 13290.4) 
                 13290.4) 
                 13366) 
               
               
                   
                 Proportion by 
                 Proportion by 
                 Proportion by 
                 Proportion by 
                 Proportion by 
               
               
                 Component 
                 mass [%] 
                 mass [%] 
                 mass [%] 
                 mass [%] 
                 mass [%] 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Abrasive 
                 65 
                 65 
                 69 
                 73 
                 65 
               
               
                 particles 
               
               
                 (corundum) 
               
               
                 Phenolic resin 
                 18 
                 18 
                 16 
                 14.5 
                 18 
               
               
                 Active fillers 
                 13 
                 17 
                 11 
                 12.5 
                 13 
               
               
                 Tricalcium 
                 4 
                 — 
                 4 
                 — 
                 — 
               
               
                 phosphate 
               
               
                 (TCP) 
               
               
                 Dicalcium 
                 — 
                 — 
                 — 
                 — 
                 4 
               
               
                 phosphate 
               
               
                 (DCP) 
               
               
                   
               
            
           
         
       
     
     The grinding wheels according to Example 1 and 2 (Test no. 13290.1 and 13290.2) contain 65 wt.-% corundum abrasive particles with a grit size F46 and F60. The abrasive particles were first combined with 4 wt.-% pulverulent TCP with a grain size D50 (corresponds to 3 to 5 μm). 7.5 wt.-% liquid phenolic resin was then added to the mixture. Finally, this mixture was added to a receiver of 10.5 wt.-% pulverulent phenolic resin and active fillers, namely 10 wt.-% cryolite and 3 wt.-% potassium aluminium fluorides and mixed intensively for 30 minutes using an Eirich intensive mixer. The thus-produced granular starting mixture was sieved with a sieve with a mesh of 2 mm in order to remove larger chunks. The granular material was then distributed weighed out into moulds, glass fabric, labels and flange rings were added to it and it was pressed to form green compacts. The green compacts were lastly cured at an increased temperature of 185° C. over 18 hours in total. The grinding wheels according to Example 1, 2, 4 and according to Reference  1  have a radius of 125 mm and a thickness of 1 mm. 
     The reference grinding wheels  1  and  3  were produced according to the same method, wherein these reference wheels were, of course, produced without addition of TCP. Instead, these wheels contained correspondingly more fillers. 
     In the case of the grinding wheel according to Example 2, the same components were used as in the case of the grinding wheel according to Example 1. However, in the case of the grinding wheel according to Example 2 the production method was somewhat modified. The TCP was not admixed into the abrasive particles, but rather into the mixture of pulverulent phenolic resins and active fillers. 
     The grinding wheel according to Example 4 was essentially produced like the grinding wheel from Example 1. However, in the case of the grinding wheel according to Example 4 a different phosphate additive, namely dicalcium phosphate anhydride (CaHPO 4 ), was used. DCP anhydride is also called monocalcium hydrogen phosphate and is found in natural form as monetite or brushite. 
     The grinding wheels according to Example 3 and Reference  3  have dimensions of 230×1.9 mm. 
     Investigation of the Grinding Power of the Grinding Wheels: 
     To analyse the grinding power of the grinding wheels, the grinding wheels were subjected to standardized power tests, namely an automated test and a manual test. The grinding wheels all underwent the same program of grinding testing. 
     The automatic test equipment used in the automated test consists of a device in which an angle grinder (from Flex, 1400 W) is clamped on a moveable arm and the arm is moved up and down by a motor with fixed feed. In the downward movement, an approx. 2 to 3 mm wide piece of a steel tube (V2A steel 15×15×1.5 mm) is cut off. After the upward movement, the tube is pushed along using an automatic device, with the result that in the next downward movement again a piece of tube is cut off. The machine continuously determines the remaining diameter of the wheel and counts the cutting processes. When the remaining diameter of the grinding wheel is 95 mm, the machine stops the test. 
     In the manual test, the angle grinder (from Flex, 1400 W) was operated by a test person. In each case, a predetermined number of cuts were carried out in the V2A steel solid material. The solid material (square bar V2A steel, 25×25 mm) was firmly clamped in a vice. The grinding power was determined here by a comparison of the grinding wheel diameter before and after the test. The determined wheel wear represents a measure of the grinding power. 
     In  FIG. 1 , the results of an automated test on tubular V2A steel 15×15×1.5 mm for the grinding wheels according to Example 1 and 2 and Reference  1  are shown. 
     For some of the grinding wheels, the grinding test was carried out directly after the production. The grinding power of the grinding wheels is highest at this point in time. In order to investigate the effect of moisture on the grinding power, the other grinding wheels were stored suspended individually in a climate chamber and exposed to a damp alternating atmosphere according to DIN 50016. The storage in such a climate chamber over 14 to 21 days approximately corresponds to storage of the grinding wheels under normal conditions of a central European climate over 8 to 12 months. As already discussed above, after this storage time, phenolic resin-bonded grinding wheels have usually reached a state of equilibrium with respect to the moisture bound in the material of the grinding wheels and the grinding power has reached a saturation level. 
     After different storage times of 3, 7, 14 and 21 days, in each case test batches of the grinding wheels were removed and likewise subjected to the program of grinding testing. In  FIG. 1 , the normalized power values are plotted against the storage time. The grinding wheels investigated directly after the production (storage time: 0 days) showed the highest grinding power. The number of cuts which could be carried out with these grinding wheels was normalized to 100%. As follows from  FIG. 1 , the grinding power, i.e. the number of cuts carried out, of the reference grinding wheel  1  continuously decreases with the storage time and, after 21 days&#39; storage, approaches a grinding power level of approximately 50% of the original power. The grinding power of the grinding wheels according to Example 1 and 2 likewise decreases as the storage time increases. However, the tests clearly show that the decrease in the grinding power is smaller and that the saturation grinding power achieved after 21 days&#39; storage is higher, at more than 60%, than the saturation grinding power of the conventionally produced reference grinding wheel  1 . 
     Surprisingly, the grinding wheel according to Example 1 showed a further advantageous effect. The grinding wheel according to Example 1 had a higher initial grinding power compared with the reference grinding wheel  1 . The grinding wheel according to Example 1 investigated directly after its production achieved 237 cuts until a remaining diameter of 95 mm was reached. With the reference grinding wheel  1 , in contrast, only 208 cuts were possible. This corresponds to an increase in power of the grinding wheel according to the invention compared with the reference grinding wheel  1  of approximately 14%. 
     A grinding wheel from a retest of the above-described Example 1 (test no. 13290.3) showed an initial power in the automated test of 241 cuts on tubular V2A steel 15×15×1.5 mm. To investigate the saturation grinding power, this wheel was left in the climate chamber for 69 days (2 months and 1 week). The normalized remaining power was still 59% of the initial power (141 cuts in the automated test on tubular V2A steel 15×15×1.5 mm) even after this long exposure to the alternating atmosphere. This shows that the grinding power which is reached after 21 days in the alternating atmosphere actually does represent a saturation grinding power and that even after longer dwell times no further significant change in the grinding power occurs due to the storage conditions. 
     Interestingly, with a grinding wheel according to Example 2, only 189 cuts could be carried out. The initial power of the grinding wheel according to Example 2 was thus 10% lower than the initial power of the conventional wheel. The cause of this appears to lie in the manner in which the TCP additive is incorporated in the production of the grinding wheel. While the grinding wheels according to Example 1 and 2 in each case have clearly improved properties regarding the preservation of power compared with conventional grinding wheels, only the grinding wheel according to Example 1 provides the additional advantage of an increased initial grinding power. 
       FIG. 2  shows the normalized power data of the grinding wheels from  FIG. 1  in the manual test on solid material V2A steel 25×25 mm. In this test, in each case 10 cuts were carried out and then the remaining diameter of the grinding wheels was determined. In qualitative terms, the results agree with the findings of the automated test described with the help of  FIG. 1 . In the manual test it was also shown that the grinding power of the grinding wheels according to Example 1 and 2 is fundamentally higher than the grinding power of the reference grinding wheel  1 . In particular, the results after 21 days&#39; storage show that the saturation grinding power of the reference grinding wheel  1  is somewhat less than 50%, whereas the saturation grinding power of the grinding wheels according to Example 1 and 2 is still approximately 60% of the initial power. 
     The somewhat fluctuating outcomes in the results of the manual test can presumably be attributed to the removal time points of the grinding wheels from the climate chamber. As (in particular) at storage times of less than 21 days, possibly no saturation of the moisture absorption has yet occurred, here the grinding power is presumably influenced by the moisture prevailing at the time point of the removal of the grinding wheels. In addition, the day-to-day condition of the test person must also be taken into account. 
     In the diagram of  FIG. 3 , the results of the power test for grinding wheels according to Example 3 and reference grinding wheels  3  are shown. In this test, the power and the ageing of the grinding wheels was determined using a manual test on a solid material V2A steel 25×25 mm. Here, 20 cuts were carried out with each grinding wheel and the wheel wear was determined by measuring the wheel diameter before and after the test. The results, in turn, show the positive influence of the TCP additive on the power preservation of the grinding wheels. In addition, with the reference wheel  3 , after 21 days&#39; storage only 19 cuts could be carried out; the remaining diameter did not allow for any further cut in the solid material V2A steel 25×25 mm. As represented in  FIG. 3 , with the conventional grinding wheel after 21 days&#39; storage only a grinding power of less than 15%, taking into account the omitted cut, could be achieved compared with the initial grinding power. The grinding power of the grinding wheel according to Example 3, in contrast, after 21 days achieved a saturation grinding power of approximately 50%. 
     In addition, in this test it is again shown that the initial grinding power, i.e. the grinding power directly after the production (storage: 0 days) of the grinding wheel according to the invention is higher than the initial grinding power of the reference grinding wheel  3 . In the grinding wheel according to Example 3, the wear after 20 cuts was only 15.7 mm, while the wear of the reference grinding wheel  3  was 16.2 mm. This is all the more surprising as the reference grinding wheel has 4 wt.-% more abrasive particles than the grinding wheel according to Example 3 and thus a higher initial power of the reference grinding wheel was to be expected. As the initial grinding power is not yet influenced by any possible moisture absorption of the grinding wheel material, the improved grinding power cannot be explained by the adsorptive properties of the TCP additive alone. Rather, further effects must be present in consequence of which the TCP additive increases the grinding power of synthetic resin grinding wheels. 
     The same power tests were carried out for grinding wheels according to Example 4. As stated above, the formulation of these grinding wheels corresponds to the formulation of the grinding wheels according to Example 1 with the difference that DCP anhydride was used instead of TCP. The DCP was likewise added to the abrasive particles. The initial grinding power of the grinding wheels according to Example 4 was 258 cuts in the automated test, thus again higher than the initial power of the grinding wheels according to the preceding examples. However, in the automated test on tubular V2A steel 15×15×1.5 mm no significant positive effect was shown after 14 days&#39; ageing. The determined remaining power in this test was 52% of the initial power. 
     In the diagram of  FIG. 4 , the results of the power test for grinding wheels according to Example 4 are shown. The reference wheel corresponds to the reference wheel from  FIG. 2 . In the manual test (10 cuts on solid material V2A steel 25×25 mm), however, the normalized power after 7 days was, at 78% of the initial power, significantly above the reference, with 56% remaining power, at this point in time. After 14 days&#39; ageing, the remaining power was, at 66%, still significantly above the remaining power of the reference, 32%. At the time of the application, this series of tests had not yet been completed, with the result that no information can be provided about the grinding power of the grinding wheels according to Example 4 after 21 days. 
     The preceding examples relate exclusively to synthetic resin-bonded grinding wheels. However, it is to be assumed that the advantageous effects of the phosphate additive likewise occur in all other abrasive media bonded with a thermosetting material, such as for example in coated abrasives.