The Magnetic Properties of Organic Coatings containing Nickel, Nickel-alloy and Stainless Steel Flakes

Transcription

1 Preface This paper was written for presentation at the Advances in Coatings Technology Conference held in Warsaw, Poland, November It was subsequently published in the proceedings of the Conference. It is still probably the only technical paper dealing with the magnetic properties of organic coating films containing magnetic pigments. The paper has been re-edited for the Hart Materials Limited website Copyright Hart Materials Limited, October 2012 The Magnetic Properties of Organic Coatings containing Nickel, Nickel-alloy and Stainless Steel Flakes Dr. Tony Hart - Hart Materials Limited, West Midlands, U.K. David Croan Novamet Specialty Products Corporation, New Jersey, U.S.A. Introduction The purpose of this study is to provide basic background data characterising the magnetic properties of organic coatings containing a selection of flake pigments manufactured from nickel and nickel-related alloys. Nickel metal and a very wide range of nickel-containing alloys have, of course, been readily available for very many years and the magnetic characteristics of relevant materials in bulk form are widely published. However, until this study was undertaken no data has been available on their behaviour when incorporated as pigments into organic media and used in the form of thin coatings. The flake products selected for this investigation are obviously ones that exhibit ferromagnetism and ones that, in addition, are readily available from established sources in commercial quantities. The range of materials chosen for the study includes flakes manufactured from pure nickel metal, from stainless steel and from two specialised magnetic nickel alloys.

2 Page 2) Metallic Flake Pigments Metallic flake pigments have been used in organic finishing systems such as paints, inks and powder coatings for very many years. Originally, the reason for incorporating them into these products was simply for their decorative effect and this still remains by far the most widespread reason for employing metallic pigments. Aluminium flakes are the most widely used metallic pigments in organic coatings since they exhibit excellent levels of brightness and reflectivity combined with a unique white colour that cannot be matched by any other metal or alloy. Since there has also been a great deal of development work carried out on these pigments an extensive range of products are readily available capable of giving a wide variety of decorative effects. Copper and copper alloy flakes are also used extensively in organic coatings, particularly for metallic printing inks, due again to the highly attractive aesthetic effects that they produce. Pure silver flakes can also be employed for but obviously their use is limited by cost. During the last 20 to 25 years, however, metallic flake pigments have been finding an increasing number of new applications that depend on their functional properties rather than merely on their aesthetic appeal. Almost certainly the most significant of these has been the production of electrically conductive coatings that are employed to shield electronic devices from radiofrequency and electromagnetic interference (RFI/EMI). The use of conductive coatings for RFI/EMI shielding has become increasingly important during the last decade because of the proliferation of the numbers of electronic devices in common use and also due to to legislation enacted by major industrial countries to control the effects of this type of interference. In this specialised application nickel pigments have assumed a position of key importance due to their unique combination of critical properties. They exhibit very good electrical conductivity, obviously essential for effective shielding, together with high resistance to corrosion, so that shielding effectiveness does not deteriorate with time. An additional benefit is that nickel pigments are commercially available in a number of different morphologies, including flake, spheres and filamentary. A further example where the functional properties of the pigments employed are of critical importance is the use of stainless steel flakes in powder coating and liquid paint systems. In these applications although an attractive aesthetic appearance is an important requirement other, more functional, properties such as high corrosion and/or abrasion resistance in aggressive environments are also essential

3 Page 3) Many of these functionally dependant applications have been established for many years. However, more recently, there has been an increasing interest in metallic pigments mostly in flake form that exhibit useful magnetic properties. Of the three elements that naturally exhibit ferromagnetism iron, cobalt and nickel it is only nickel and nickel-based materials can be readily used to manufacture flakes in a suitable morphological form for incorporation into coatings. These are therefore the only ones that are readily available in commercial quantities for such applications. Pure nickel flakes, for example, are produced in a number of different forms for use in decorative as well as electrically conductive applications. Both categories, however, exhibit ferromagnetic properties. In addition, commercial available grades of stainless steel flakes, used because of their combination of excellent durability and aesthetic attractiveness, also exhibit ferromagnetic properties. This is somewhat unusual since it is normally ferritic and martensitic grades of stainless steel that are recognised as magnetic, not austenitic ones. Furthermore there are two magnetic nickel-based alloys commercially available in flake form and therefore suitable for coating manufacture an 81% Ni: 3% Mo:16% Fe Permalloy type material and a Nickel: 3% Al alloy with a low Curie temperature. The magnetic properties of nickel and nickel alloys Magnetic materials can generally be divided into two categories. Hard magnetic materials retain their magnetism once the magnetising field has been removed. Soft magnetic however becomet to a greater or lesser extent de-magnetised in the absence of the magnetising field. The nickel-related products examined in this study all fall into the former category of soft magnetic materials. Materials selected for evaluation All of the materials selected for evaluation in this work are produced in commercial quantities under industrial manufacturing conditions by NOVAMET Specialty Products Corporation at their plant in Wyckoff, New Jersey, United States of America. They have become well established in many very varied applications either as a result of their decorative appearance combined with durability as a result of their electrical properties.

4 Page 4) These flake products are all produced by ball milling suitable metallic powders using steel balls in a mineral spirit medium containg surfactant. Specific details of the processing are naturally commercially confidential. The characteristics of the materials selected for this study are given below together with the abbreviation codes used in the results and discussion sections. NOVAMET Nickel Flakes These products are all made from high purity carbonyl nickel powders manufactured by the nickel carbonyl vapour deposition process Conductive Nickel Flake Grade HCA-1 (Code Ni-HCA-1) This product was developed almost 35 years ago in response to a need for a metallic pigment that could be used to make electrically conductive paints with the high level of performance required for RFI/EMI shielding applications. HCA-1 Nickel Flake is treated, following the flaking process, to remove all traces of lubricant and surfactant and additionally undergoes a proprietary heat treatment process designed to maximise conductivity. HCA-1 Nickel Flakes are approximately 1 µm thick and the material has a random shape (with a typical aspect ration of around 30:1. This ensures that 98% of the material is below 37 µm (minus 400 US Sieve mesh size). Figure 1 NOVAMET Conductive Nickel Flake Grade HCA-1 Nickel Flake Fine Leafing Grade (Code NiFL) This product is milled under conditions that maximise brightness rather than electrical conductivity and, since the surfactant is not removed after processing, the material possesses leafing properties within the organic carrier film. The flake is also random in shape with a thickness within the range of µm and an aspect ratio somewhat larger than that of the HCA-1 grade. This results in a screen analysis of 95% below 44 µm ( minus 325 US Sieve mesh size).

5 Page 5) Nickel Flake Water Grade (Code NiFW) This grade is manufactured under similar conditions to Nickel Fine Leafing Grade but using a surfactant specifically selected for its compatibility with aqueous-based resin systems. Consequently it exhibits non- leafing behaviour. It has a thickness around 0.6 µm with an aspect ratio similar to that of Nickel Fine Leafing Grade. This results in a screen analysis of 95% below 44 µm. NOVAMET Stainless Steel Flakes These are produced, by technology similar to that used to make decorative nickel flakes using Stainless Steel Type UN powder. This is a high performance 18% Cr, 10% Ni, 3% Mo alloy formerly known as AISI Type 316 frequently used in chemical plant construction. 5 grades of NOVAMET Stainless Steel flake were examined in this study. Stainless Steel Fine Leafing Grade (Code StSFL) Like all of the NOVAMET flake materials this is a random shaped product (Figure II) with a flake thickness of about 0.8 µm and a screen analysis 95% below 44 µm ( minus 325 US Sieve mesh size). Figure 2 NOVAMET Stainless Steel Flake Fine Leafing Grade Stainless Steel Standard Leafing Grade (Code StSStL) This is in most of its properties identical to Stainless Steel Fine Leafing Grade apart from its larger flake size which yields a screen analysis of 80% below 44 microns and a somewhat greater flake thickness of 1.0 µm. This gives a brighter more sparkling decorative effect than the Fine Leafing Grade

6 Page 6) Stainless Steel Fine Water Grade (Code StSFW) This grade is milled with the same aqueous-system compatible surfactant used for the Nickel Fine Water Grade and is therefore also non-leafing. It has a screen analysis of 95% below 44 microns and a flake thickness of 0.6 µm. Stainless Steel Standard Water Grade (Code StSStW) This has a larger flake size than Stainless Steel Fine Water Grade, with a screen analysis of 80% below 44 microns, together with a greater flake thickness of 0.8 µm. Stainless Steel Grade SSC (Code StSSSC) This is a specialised grade milled to give a flake thickness in the region of 0.4 µm which is used make coatings with an appearance similar to that of pewter metal. NOVAMET Nickel Alloy Flakes Permalloy Flake This product is manufactured from a commonly available Permalloy 81% Ni:3% Mo:16% Fe alloy powder which has a particularly high inherent magnetic permeability. Nickel Aluminium Alloy Flakes Nickel alloys containing relatively small additions of aluminium (around 3%) are useful in that they exhibit Curie temperatures much lower than that of pure nickel (358 o C). Since these materials are available in a flake form two examples, one containing 3.0% Al and the other 3.85% Al, were included in this work.

7 Page 7) Particle size analysis Figures 1 and 2 above, the shape of flake particles is random; therefore it is not easy to express the size of the materials accurately. The overall size of this type of material is determined by particle size separation techniques used in production which consists of passing the product through mesh screens with precisely controlled apertures. To provide a better appreciation of the typical particle size distribution that can be achieved Table 1 shows the particle size analysis for two nickel flake materials compared to one for the Permalloy flake. These were determined by the Microtrac method that employs laser scanning of suspended particles of the material. Table 1 Particle size comparison of three grades of flake Channel (µm) Size Percentage Passing Ni-HCA-1 Ni FL Permalloy d d d

8 Page 8) Experimental techniques Sample preparation The flake pigments were manually dispersed into an acrylic resin base using a low shear stirring technique. A number of series of samples were prepared with pigment to binder ratios varying from 0.67:1.00 to 3.00:1.00 by weight. The resulting mixture was then used to produce a simple draw-down onto a vellum drafting paper and air dried to provide a dried film approximately 125 µm thick. Measurement of magnetic properties The measurements of the magnetic properties of the samples ware carried on a Vibrating Sample Magnetometer (VSM) under a maximum applied field of 10k Oe at an average sweep rate of 3.3k Oe per minute. Measurements were all taken at room temperature. The magnetic measurements were carried out as two separate series of determinations designed to investigate the effect on the magnetic properties firstly of flake type and secondly of increasing the amount of metallic pigment present in the organic film. The second series of determinations carried out some time after the first and was essentially an extension of the original work intended to evaluate more fully the effects observed in this first series. In the first series the area of sample measured was approximately 50 mm 2 whereas in the second series it was 10 mm 2. The typical hysteresis loop as determined for Nickel Fine Water Grade Flake - is shown below. Figure 3 Typical Hysteresis Loop for the Metallic Flake Products

9 Page 9) The magnetic values recorded were:- Magnetic Saturation MS a measure of the total magnetic content of the coating in a saturating applied magnetic field. Remanent MR a measure of the level of magnetisation which remains after the application of a saturating magnetic field Coercivity HC a measure of the opposing magnetic field required to reduce the magnetisation to zero from the remanant state. The values for MS and MR are reported in electromagnetic units (emu) which is the c.g.s. unit of magnetic moment. No calculation is given for volume or mass dependence of the magnetisation properties. Results and Discussion The results obtained in the two series of measurements are differentiated in all of the tables by showing those for the first series in bold type and those for the second series in italics. The main difficulty in interpreting these results lay in the fact that the results obtained from the two series of measurements were not always consistent. The effect of nickel flake content in the film Table 2 demonstrates the effect obtained of increasing the metal content of the film, using Nickel Fine Leafing Grade Flake Pigment/ Binder Ratio 0.67:1 2.00:1 2.50:1 3.00:1 Saturation MS 10-3 emu Remanent Magnetsation MR 10-3 emu Coercivity HC Oe The most consistent value is quite obviously that for Coercivity which appears to be effectively constant over the range of metal flake contents examined.

10 page 10) The trends in relation to both MS and MR values are less easy to interpret due to the differences in values obtained between the two series of determinations. The first series shows an increase in both MS and MR values as the metal content of the film is increased. However, the values obtained in the second series are considerably lower than those obtained in the first series and do not follow the pattern of increase with increasing metal content. Table 3 demonstrates that the coercivity values obtained with the other grade of decorative flake, Nickel Fine Water, are similarly consistent over the range of metal contents examined and also consistent between the two series of determinations. The absolute values obtained are about 8% greater than those obtained for the Fine Leafing Grade. Table 3 Nickel Fine Water Grade Pigment/ Binder Ratio 0.67:1 2.00:1 2.50:1 Saturation MS 10-3 emu Remanent MR 10-3 emu Coercivity HC Oe Very much higher values of MS and MR values were recorded in the single determination carried out on Nickel Fine Water Grade flake in the first series of experiments than those obtained in second series. This is, however, consistent with the result obtained with Nickel Fine Leafing Grade, where lower absolute values were obtained in the second series of measurements. There is, however, a clear trend established with the three samples tested in the second series in that both MS and MR values increase markedly with increasing metal content of the film. The effect is not linear but much more pronounced with both parameters as the pigment to binder ratio increases from 2.00:1 to 2.50:1 than it was as the metal content increased from 0.67:1 to 2.00:1. The absolute values obtained at the highest metal loading were much greater than those obtained in the single determination from the first serried of tests even though in general the first series of tests yielded much higher MR and MS values than the second

11 Page 11) All three grades of nickel flake were evaluated in the first series of tests to determine if there were any significant differences in behaviour. Table 4 shows that the differences in the values obtained of either MR, MS or HC were relatively small. Effect of Nickel Flake Grade Nickel Flake Type HCA-1 NiFW NiFL Pigment Binder Ratio Saturation MS 10-3 emu Remanent MR 10-3 emu Coercivity HC Oe Nickel HCA-1 conductive l flake did exhibit a lower value of MR and the slightly higher value of HC than the two decorative grades. This might possibly be attributed to the specific properties of this grade, i.e. greater flake thickness and smaller overall particle size, but the evidence cannot be considered conclusive. Effect of Stainless Steel Flake Grade Flake Type StSFL StSStL StSFW StSStW StSSSC Pigment/ Binder Ratio Saturation M S 10-3 emu Remanent M R 10-3 emu Coercivity H C Oe With the measurements carried out on the stainless steel flakes it is quite significant that all of the five grades examined showed levels of ferromagnetism of a similar order of magnitude to those exhibited by the nickel flakes. This is a significant finding, given that the UN material from which the flakes are manufactured would normally, since they are basically austenitic materials, be considered to be nonmagnetic.

12 Page 12) This is probably attributable to the changes in metallurgical structure that takes place in the stainless steel, either during the atomisation process used to produce the powder from the bulk alloy or as a result of the mechanical effect of the milling process used to convert the powder product into flake. It is indicative of the presence of ferritic structures in the flaked material The inconsistencies between the two series of tests was apparent with the stainless steel flakes as it was with nickel flakes. However, it is notable that MS values (18 x 10-3 emu) obtained for stainless steel flakes in the first series are significantly lower than those obtained (29-35 x 10-3 emu see Table 4) in the same series for nickel flakes at similar pigment/binder ratios. Similarly the range of MR values (6-8 x 10-3 emu) obtained for the stainless steel flakes in the first series of tests are notably lower than those obtained (9-16 x 10-3 emu see Table 4) for nickel flakes in the same series. The grade of stainless steel flake that stands out from the other four is the Standard Water Grade. It exhibits significantly lower values of both MS and MR and a Coercivity (HC) value which is much greater than any other observed in either series of tests for any of the other materials. It is not clear if this is due to the greater thickness of the Standard Grade flakes since an effect of a similar magnitude is not observed with the Standard Leafing Grade. Unfortunately, useful comparisons between both of the Fine Grades and the standard Grades cannot be made since the former were tested in the first series of tests and the latter in the second. Properties of Nickel Alloy Flakes Nickel Flake Type Pigme/ Binder Ratio Saturation M S 10-3 emu Remanent Magnetisatio n M R 10-3 emu Coercivity H C Oe Ni 3%Al Ni3.85% Al Permalloy The three nickel-alloy flakes which were all evaluated in the first series of experiments exhibited magnetic properties that did not appear to be vastly different from the nickel or stainless steel flakes.

13 Page 13) Conclusions The results of this investigation are less conclusive than they might have been due to the differences observed between the two series of magnetic determinations. There are a number of factors that contribute to this situation and it seems probable that most of these are related to experimental techniques involved in the preparation of the samples. Firstly, since this was the first time an investigation of this type had been carried out there was no guidance available regarding the techniques that were necessary in order to obtain consistent results. It was logical, therefore, to see if the simple sample preparation techniques used to determine other properties of this type of film were adequate to the task. The one employed for the preparation of the draw down samples was the obvious one to use since it has been employed for many years as a laboratory control technique. In this it has provided consistent and reproducible results in relation to properties such as electrical conductivity. The possibility of using other sample preparation methods that might have yielded more consistent film thicknesses and metal flake distribution within the film was considered. However, to move to a more sophisticated technique would have involved the production of an extensive range of samples of printing inks containing the appropriate amounts of each flake, followed by controlled printing trials. This would have been prohibitively expensive, particularly when it was not clear at the initiation of the investigation whether anything useful would come out of the work. There are, however, a number of useful conclusions can be drawn from this investigation even taking into account the reservations and experimental shortcomings of this investigation bearing in mind that this is an area of increasing industrial interest where currently there is no quantitative information on the behaviour of this type of system. 1) Nickel flakes when incorporated into organic matrices and laid out in the form of thin films do exhibit quantifiable magnetic properties although the values are somewhat different to massive nickel. This is reported as being a fully soft material with a Coercivity value of around 10 Oe and very low values of MR. This difference could be attributable to structural changes in the metal during the flaking process. Obviously from the observation that the stainless steel flakes - which would be expected to be non-magnetic exhibit ferromagnetism it can be concluded that the shear used in the flaking process can influence structure.

14 Page 14) 2) There is an indication that the Nickel Fine Water Grade Flake provides higher levels of Saturation and Remanent than the other two grades of nickel flake examined. The results also suggest that increasing the quantity of this particular grade of flake in the film produces significant increases in both MS and MR values. This effect did not seem to be exhibited by the Nickel Fine Leafing Grade for reasons that are not clear. It is not possible to be absolutely certain of these findings from the present evaluation in view of the factors discussed above. 3) All of the stainless steel flakes examined showed magnetic properties when incorporated into organic films, the values of the parameters measured being reasonably consistent with those obtained for nickel flake materials. The fact that these stainless steel flakes exhibit magnetic properties, despite being nominally austenitic in structure, was not entirely surprising since it had been noted by qualitative observation. This work, however, provides some quantitative measure of the magnetic properties that has not been available previously. These magnetic properties are almost certainly due to changes in the metallurgical structure of the stainless steel that are bought about either by the powder production process, or by the flaking procedure or by a combination of both. 4) One of the grades of stainless steel flake the Standard Water was observed to behave in a significantly different manner to the others. It gave by far the highest value for Coercivity (HC) of any of the materials, either nickel or stainless steel, examined in this work together with much lower values of MS and MR. Similar anomalous behaviour was noted with the Nickel Fine Water Grade Flake. 5) The magnetic properties provided by the three nickel alloy flakes included in this work did not indicate that they might provide any specific advantage, in respect of these particular properties measured, compared to the nickel and stainless steel products. 6) Since these results relate to measurements on thin films it might be expected that they will differ form those obtained in bulk measurements.