overcome the problem of cushioning which is found in a conventional ball mill as a result of aggregation of the fines and which contributes much of the exponential decrease of size in a tumbler ball mill used for soft materials. The effect of increased energy is illustrated in Fig. 1, where the slope of the grinding curve is modified at each adjustment of the mill to a higher energy level. A hypothetical curve is shown by the hatched line. One reason adduced for this effect in soft, .ag%lomer~ting~aterials is that the impacting grinding medIa In the VIbratIonal ball mill can penetrate the fluffy aggregates more effectively. A secondary effect is that the highe~ ball velocity· in this type of mill exerts a greater crushIng effect on the fluffy product lying between the balls~ From earlier studies with vibrational ball mills it was noted that, by pumping liquid through the chambers of such a mill, a sufficient velocity of flow between the balls would exert a hydrodynamic wedging effect, so preventing them from impacting. It is possible to quantify this effect, but the mechanIsm suggests that shape and size can be controlled for malleable materials by wet grinding and control of flow rates. It is known that the balls spin on their axes in the o~cill~tingchamber while circulating among each other, so YIeldInganother mechanism for the breakdown of very thin platelets. . It should be noted that, where a malleable material yields a. fibr?us texture on size-reduction, a polyester resin, a VIbratIonal ball or conventional tumbler mill are not considered to offer success, especially if cryogenic methods have failed. One must recognize such a limitation in this approach to size-reduction; materials of this class require devices which exert a tearing action or the use of hard, abrasive grinding aids which clearly must permit removal. after processing. There is a special problem in size-reducing or comminuting pow~e~s which have an intrinsic malleability. Silver, alumInIum, copper, and other soft materials tend to form flakes when ground in a ball mill, and this is often advantageous in the end-product. Where, however, particle sizes in th~ lower micrometre size range are required, special technIques must be employed or the grinding time required is long. The production of flake powders is generally done with the aid of grinding additions to the mill or the process can be carried out in the presence of a liquid. There ~re, nevertheless, cases where grinding aids may be undeSIrable as, for example, with silver for electrical applications. Similarly, there may be limitations with certain organic or inorganic solids in this respect. Where a grinding aid is used, chemical adsorption of the aid may occur, making its removal difficult after the size-reducing operation, and intercalation may also occur. In the course of development work on the use of vibrational ball mills in the size-reduction of silver, an interesting mechanical effect came to light. It was found that, with progressive size-reduction of the silver, microscopy showed that the aspect ratio of the particles increased with the time o.fgrinding; that is, the powder particles became progressIvelymore plate-like. At a median size of around 3--4 IJ-mit was found that size-reduction had slowed up significantly. When, however, the grinding energy of the vibrational mill was increased, the very small flakes were further size reduced, apparently by breakage across the platelets. It was deduced that the mechanical strength was insufficient for the particle to withstand snapping under the increased energy of the mill, and a particle size of around 1 IJ-mwas achieved. By repetition of this process of thinning and snapping across the basal plane of the particle it is possible to achieve very fine particles. .. Where an organic material shows relatively high elasticIty, such as a polymer, th~ problem of size-reduction presents greater difficulties. In such instances recourse must be m~de ~omore unusual methods, and it may be necessary to gnnd In the presence of, for example, water. Once again, the higher grinding energy offered by a vibrational ball mill may be needed. A grinding aid which increases friction between the grinding media and powder is sometimes effective, the limitation here generally coming from the removal of unwanted aid from the end product. Cryogenic grinding is clearly indicated, technically, with solids showing elasticity, but would have to be carefully considered in economic terms. Where a material is soft, with a relatively high modulus that cannot be classed as elasticity as it is generally understood, high milling energy of the type offered by vibrational milling is indicated. High energy of size-reduction tends to t « w n::: « w u ~ n::: :=) (/)
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Mar 1, 1980
Jiří Thýn + 1
