With advancement of science and technology and the improvement of management level of ceramic enterprises, the quality and grade of Chinas architectural ceramic products have made a qualitative leap. 1. Advantages and disadvantages of old ball mill The old-fashioned intermittent ball mills that are currently in common use have not changed for decades. It is a closed structure, with low output, large floor space, high energy consumption, complicated operation, inconvenient automation, and the powders are extremely inconsistent, and it is not suitable to produce high-grade bricks. There is a serious over-grinding phenomenon. 2. Advantages and disadvantages of 2 combined continuous ball mill The combined continuous ball mill is a new type of ball mill with reasonable structure, optimized combination and high transmission efficiency. The combined continuous ball mill comprises a cylinder, a transmission, a feeding and discharging device for raw materials and grinding bodies, an automatic detection and an automatic control system. The driving system consisting of the motor, the reducer and the pulley is driven by the belt during operation, and the cylinder with the belt groove is rotated. The raw material and water enter the cylinder through the feed end hollow shaft driven by the feeding device. Driven by the cylinder, more than half of the raw materials, water and ball stones are used to impact each other and rotate. The raw materials is quickly ground under the strong impact of the ball. After more than one hour of strong grinding, the raw material flows out through the discharge port of the third cylinder to complete the ball milling process of the entire raw material. 3. Comparison between combined continuous ball mills and old-fashioned ball mills 1) Differences in principle The old mill is a closed grinding machine in which the raw materials, water and ball are filled with the entire ball. The grinding effect is poor; the combined continuous ball mill is open-type, only contains more than half, raw materials, water and ball stones, the drop difference on the ball stone is large, the potential energy impact on t he raw material is large, and the ball stone is brought to a high place and directly impacts the raw material. Products a good grinding effect, and the ground material can be immediately flowed away from the third cylinder without excessive grinding. 2) Structural differences There is not belt groove on the old mill cylinder. The belt is hung on the cylinder to be forced to rotate, and the belt often slips, which does not conform to the mechanical design principle. The continuous ball mill cylinder has 48 belt grooves, and the belt does not slip, which can achieve a good transmission effect. The old mill can only be filled with one ball, and then released after the ball is finished. In this way, the labor intensity of the workers is large, the operation is very difficult, and it is necessary to start and shut down several times, which also has a certain impact on the power grid. The continuous ball mill does not need to stop, can continuously fee and continuously discharge, has less impact on the grid, and can be operated and controlled automatically. Three-piece or multi-continuous ball mill structure, small footprint, under the same output, the area of the continuous ball mill is one-third of the area of the oil mill, greatly reducing the civil engineering and foundation investment. The continuous ball mill has a large output and has remarkable energy-saving effect. One set combined continuous ball mill. The raw materials are below 2mm and directly milled into a continuous ball mill. The output of powder per hour is above3 20T. Rough grinding by the old mill. The sieve residue is 8-12 and then into the continuous ball mill for fine grinding. It can produce 40T powder per hour and the sieve residue is about 0.8-1.2. Due to the large output, it saves about 15%-20% of electricity per ton of powder compared to the old ball mill processing technology. The continuous ball mill is equipped with three frequency converters. In the three-piece continuous ball mill, according to the first section rough grinding, its rotation speed can be appropriately slowed down, and the final section of the fine grinding speed can be adjusted appropriately. The quality of the raw materials ground by the continuous ball mill is good, and the fineness of the powder is uniform, which lays a good raw material foundation to produce high-grade bricks. Especially for the air-pressure molding machine, the brick has good exhaust performance.
Active lime is produced from limestone dolomite chalk and other minerals with high calcium carbonate content by the calcination process under the temperature of 1000-1100 C. There are various processes for the active lime production mainly
Would you be interested in a mill which produces nanosized particles, has sufficient energy input for long-term grinding without cooling breaks and is very easy to handle? Then join our free live webinar on the new Mixer Mill MM 500 - the first impact ball mill worldwide that efficiently grinds samples to nanosized particles with only minor warming effects.
The new Mixer Mill MM 500 features a maximum frequency of 35 Hz and is the first Mixer Mill in the market which comes with the crushing power to produce particles in the nanometer range. With its suitability for rapid pulverization and long-term processing, it is a real alternative to Planetary Ball Mills with all the benefits a Mixer Mills offers like comfortable handling and less warming effects.
The ball mill accepts the SAG or AG mill product. Ball mills give a controlled final grind and produce flotation feed of a uniform size. Ball mills tumble iron or steel balls with the ore. The balls are initially 510 cm diameter but gradually wear away as grinding of the ore proceeds. The feed to ball mills (dry basis) is typically 75 vol.-% ore and 25% steel.
The ball mill is operated in closed circuit with a particle-size measurement device and size-control cyclones. The cyclones send correct-size material on to flotation and direct oversize material back to the ball mill for further grinding.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.
By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.
The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.
where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.
Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.
When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.
In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).
Planetary ball mills. A planetary ball mill consists of at least one grinding jar, which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars according to a fixed ratio. The grinding balls in the grinding jars are subjected to superimposed rotational movements. The jars are moved around their own axis and, in the opposite direction, around the axis of the sun wheel at uniform speed and uniform rotation ratios. The result is that the superimposition of the centrifugal forces changes constantly (Coriolis motion). The grinding balls describe a semicircular movement, separate from the inside wall, and collide with the opposite surface at high impact energy. The difference in speeds produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill. Planetary ball mills are smaller than common ball mills, and are mainly used in laboratories for grinding sample material down to very small sizes.
Vibration mill. Twin- and three-tube vibrating mills are driven by an unbalanced drive. The entire filling of the grinding cylinders, which comprises the grinding media and the feed material, constantly receives impulses from the circular vibrations in the body of the mill. The grinding action itself is produced by the rotation of the grinding media in the opposite direction to the driving rotation and by continuous head-on collisions of the grinding media. The residence time of the material contained in the grinding cylinders is determined by the quantity of the flowing material. The residence time can also be influenced by using damming devices. The sample passes through the grinding cylinders in a helical curve and slides down from the inflow to the outflow. The high degree of fineness achieved is the result of this long grinding procedure. Continuous feeding is carried out by vibrating feeders, rotary valves, or conveyor screws. The product is subsequently conveyed either pneumatically or mechanically. They are basically used to homogenize food and feed.
CryoGrinder. As small samples (100 mg or <20 ml) are difficult to recover from a standard mortar and pestle, the CryoGrinder serves as an alternative. The CryoGrinder is a miniature mortar shaped as a small well and a tightly fitting pestle. The CryoGrinder is prechilled, then samples are added to the well and ground by a handheld cordless screwdriver. The homogenization and collection of the sample is highly efficient. In environmental analysis, this system is used when very small samples are available, such as small organisms or organs (brains, hepatopancreas, etc.).
The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).
The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere. Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.
A ball mill is a relatively simple apparatus in which the motion of the reactor, or of a part of it, induces a series of collisions of balls with each other and with the reactor walls (Suryanarayana, 2001). At each collision, a fraction of the powder inside the reactor is trapped between the colliding surfaces of the milling tools and submitted to a mechanical load at relatively high strain rates (Suryanarayana, 2001). This load generates a local nonhydrostatic mechanical stress at every point of contact between any pair of powder particles. The specific features of the deformation processes induced by these stresses depend on the intensity of the mechanical stresses themselves, on the details of the powder particle arrangement, that is on the topology of the contact network, and on the physical and chemical properties of powders (Martin et al., 2003; Delogu, 2008a). At the end of any given collision event, the powder that has been trapped is remixed with the powder that has not undergone this process. Correspondingly, at any instant in the mechanical processing, the whole powder charge includes fractions of powder that have undergone a different number of collisions.
The individual reactive processes at the perturbed interface between metallic elements are expected to occur on timescales that are, at most, comparable with the collision duration (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b). Therefore, unless the ball mill is characterized by unusually high rates of powder mixing and frequency of collisions, reactive events initiated by local deformation processes at a given collision are not affected by a successive collision. Indeed, the time interval between successive collisions is significantly longer than the time period required by local structural perturbations for full relaxation (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b).
These few considerations suffice to point out the two fundamental features of powder processing by ball milling, which in turn govern the MA processes in ball mills. First, mechanical processing by ball milling is a discrete processing method. Second, it has statistical character. All of this has important consequences for the study of the kinetics of MA processes. The fact that local deformation events are connected to individual collisions suggests that absolute time is not an appropriate reference quantity to describe mechanically induced phase transformations. Such a description should rather be made as a function of the number of collisions (Delogu et al., 2004). A satisfactory description of the MA kinetics must also account for the intrinsic statistical character of powder processing by ball milling. The amount of powder trapped in any given collision, at the end of collision is indeed substantially remixed with the other powder in the reactor. It follows that the same amount, or a fraction of it, could at least in principle be trapped again in the successive collision.
This is undoubtedly a difficult aspect to take into account in a mathematical description of MA kinetics. There are at least two extreme cases to consider. On the one hand, it could be assumed that the powder trapped in a given collision cannot be trapped in the successive one. On the other, it could be assumed that powder mixing is ideal and that the amount of powder trapped at a given collision has the same probability of being processed in the successive collision. Both these cases allow the development of a mathematical model able to describe the relationship between apparent kinetics and individual collision events. However, the latter assumption seems to be more reliable than the former one, at least for commercial mills characterized by relatively complex displacement in the reactor (Manai et al., 2001, 2004).
A further obvious condition for the successful development of a mathematical description of MA processes is the one related to the uniformity of collision regimes. More specifically, it is highly desirable that the powders trapped at impact always experience the same conditions. This requires the control of the ball dynamics inside the reactor, which can be approximately obtained by using a single milling ball and an amount of powder large enough to assure inelastic impact conditions (Manai et al., 2001, 2004; Delogu et al., 2004). In fact, the use of a single milling ball avoids impacts between balls, which have a remarkable disordering effect on the ball dynamics, whereas inelastic impact conditions permit the establishment of regular and periodic ball dynamics (Manai et al., 2001, 2004; Delogu et al., 2004).
All of the above assumptions and observations represent the basis and guidelines for the development of the mathematical model briefly outlined in the following. It has been successfully applied to the case of a Spex Mixer/ Mill mod. 8000, but the same approach can, in principle, be used for other ball mills.
The Planetary ball mills are the most popular mills used in MM, MA, and MD scientific researches for synthesizing almost all of the materials presented in Figure 1.1. In this type of mill, the milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial (milling bowl or vial) and the effective centrifugal force reaches up to 20 times gravitational acceleration.
The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed, as schematically presented in Figure 2.17.
However, there are some companies in the world who manufacture and sell number of planetary-type ball mills; Fritsch GmbH (www.fritsch-milling.com) and Retsch (http://www.retsch.com) are considered to be the oldest and principal companies in this area.
Fritsch produces different types of planetary ball mills with different capacities and rotation speeds. Perhaps, Fritsch Pulverisette P5 (Figure 2.18(a)) and Fritsch Pulverisette P6 (Figure 2.18(b)) are the most popular models of Fritsch planetary ball mills. A variety of vials and balls made of different materials with different capacities, starting from 80ml up to 500ml, are available for the Fritsch Pulverisette planetary ball mills; these include tempered steel, stainless steel, tungsten carbide, agate, sintered corundum, silicon nitride, and zirconium oxide. Figure 2.19 presents 80ml-tempered steel vial (a) and 500ml-agate vials (b) together with their milling media that are made of the same materials.
Figure 2.18. Photographs of Fritsch planetary-type high-energy ball mill of (a) Pulverisette P5 and (b) Pulverisette P6. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).
Figure 2.19. Photographs of the vials used for Fritsch planetary ball mills with capacity of (a) 80ml and (b) 500ml. The vials and the balls shown in (a) and (b) are made of tempered steel agate materials, respectively (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).
More recently and in year 2011, Fritsch GmbH (http://www.fritsch-milling.com) introduced a new high-speed and versatile planetary ball mill called Planetary Micro Mill PULVERISETTE 7 (Figure 2.20). The company claims this new ball mill will be helpful to enable extreme high-energy ball milling at rotational speed reaching to 1,100rpm. This allows the new mill to achieve sensational centrifugal accelerations up to 95 times Earth gravity. They also mentioned that the energy application resulted from this new machine is about 150% greater than the classic planetary mills. Accordingly, it is expected that this new milling machine will enable the researchers to get their milled powders in short ball-milling time with fine powder particle sizes that can reach to be less than 1m in diameter. The vials available for this new type of mill have sizes of 20, 45, and 80ml. Both the vials and balls can be made of the same materials, which are used in the manufacture of large vials used for the classic Fritsch planetary ball mills, as shown in the previous text.
Retsch has also produced a number of capable high-energy planetary ball mills with different capacities (http://www.retsch.com/products/milling/planetary-ball-mills/); namely Planetary Ball Mill PM 100 (Figure 2.21(a)), Planetary Ball Mill PM 100 CM, Planetary Ball Mill PM 200, and Planetary Ball Mill PM 400 (Figure 2.21(b)). Like Fritsch, Retsch offers high-quality ball-milling vials with different capacities (12, 25, 50, 50, 125, 250, and 500ml) and balls of different diameters (540mm), as exemplified in Figure 2.22. These milling tools can be made of hardened steel as well as other different materials such as carbides, nitrides, and oxides.
Figure 2.21. Photographs of Retsch planetary-type high-energy ball mill of (a) PM 100 and (b) PM 400. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).
Figure 2.22. Photographs of the vials used for Retsch planetary ball mills with capacity of (a) 80ml, (b) 250ml, and (c) 500ml. The vials and the balls shown are made of tempered steel (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).
Both Fritsch and Retsch companies have offered special types of vials that allow monitoring and measure the gas pressure and temperature inside the vial during the high-energy planetary ball-milling process. Moreover, these vials allow milling the powders under inert (e.g., argon or helium) or reactive gas (e.g., hydrogen or nitrogen) with a maximum gas pressure of 500kPa (5bar). It is worth mentioning here that such a development made on the vials design allows the users and researchers to monitor the progress tackled during the MA and MD processes by following up the phase transformations and heat realizing upon RBM, where the interaction of the gas used with the freshly created surfaces of the powders during milling (adsorption, absorption, desorption, and decomposition) can be monitored. Furthermore, the data of the temperature and pressure driven upon using this system is very helpful when the ball mills are used for the formation of stable (e.g., intermetallic compounds) and metastable (e.g., amorphous and nanocrystalline materials) phases. In addition, measuring the vial temperature during blank (without samples) high-energy ball mill can be used as an indication to realize the effects of friction, impact, and conversion processes.
More recently, Evico-magnetics (www.evico-magnetics.de) has manufactured an extraordinary high-pressure milling vial with gas-temperature-monitoring (GTM) system. Likewise both system produced by Fritsch and Retsch, the developed system produced by Evico-magnetics, allowing RBM but at very high gas pressure that can reach to 15,000kPa (150bar). In addition, it allows in situ monitoring of temperature and of pressure by incorporating GTM. The vials, which can be used with any planetary mills, are made of hardened steel with capacity up to 220ml. The manufacturer offers also two-channel system for simultaneous use of two milling vials.
Using different ball mills as examples, it has been shown that, on the basis of the theory of glancing collision of rigid bodies, the theoretical calculation of tPT conditions and the kinetics of mechanochemical processes are possible for the reactors that are intended to perform different physicochemical processes during mechanical treatment of solids. According to the calculations, the physicochemical effect of mechanochemical reactors is due to short-time impulses of pressure (P = ~ 10101011 dyn cm2) with shift, and temperature T(x, t). The highest temperature impulse T ~ 103 K are caused by the dry friction phenomenon.
Typical spatial and time parameters of the impactfriction interaction of the particles with a size R ~ 104 cm are as follows: localization region, x ~ 106 cm; time, t ~ 108 s. On the basis of the obtained theoretical results, the effect of short-time contact fusion of particles treated in various comminuting devices can play a key role in the mechanism of activation and chemical reactions for wide range of mechanochemical processes. This role involves several aspects, that is, the very fact of contact fusion transforms the solid phase process onto another qualitative level, judging from the mass transfer coefficients. The spatial and time characteristics of the fused zone are such that quenching of non-equilibrium defects and intermediate products of chemical reactions occurs; solidification of the fused zone near the contact point results in the formation of a nanocrystal or nanoamor- phous state. The calculation models considered above and the kinetic equations obtained using them allow quantitative ab initio estimates of rate constants to be performed for any specific processes of mechanical activation and chemical transformation of the substances in ball mills.
There are two classes of ball mills: planetary and mixer (also called swing) mill. The terms high-speed vibration milling (HSVM), high-speed ball milling (HSBM), and planetary ball mill (PBM) are often used. The commercial apparatus are PBMs Fritsch P-5 and Fritsch Pulverisettes 6 and 7 classic line, the Retsch shaker (or mixer) mills ZM1, MM200, MM400, AS200, the Spex 8000, 6750 freezer/mill SPEX CertiPrep, and the SWH-0.4 vibrational ball mill. In some instances temperature controlled apparatus were used (58MI1); freezer/mills were used in some rare cases (13MOP1824).
The balls are made of stainless steel, agate (SiO2), zirconium oxide (ZrO2), or silicon nitride (Si3N). The use of stainless steel will contaminate the samples with steel particles and this is a problem both for solid-state NMR and for drug purity.
However, there are many types of ball mills (see Chapter 2 for more details), such as drum ball mills, jet ball mills, bead-mills, roller ball mills, vibration ball mills, and planetary ball mills, they can be grouped or classified into two types according to their rotation speed, as follows: (i) high-energy ball mills and (ii) low-energy ball mills. Table 3.1 presents characteristics and comparison between three types of ball mills (attritors, vibratory mills, planetary ball mills and roller mills) that are intensively used on MA, MD, and MM techniques.
In fact, choosing the right ball mill depends on the objectives of the process and the sort of materials (hard, brittle, ductile, etc.) that will be subjecting to the ball-milling process. For example, the characteristics and properties of those ball mills used for reduction in the particle size of the starting materials via top-down approach, or so-called mechanical milling (MM process), or for mechanically induced solid-state mixing for fabrications of composite and nanocomposite powders may differ widely from those mills used for achieving mechanically induced solid-state reaction (MISSR) between the starting reactant materials of elemental powders (MA process), or for tackling dramatic phase transformation changes on the structure of the starting materials (MD). Most of the ball mills in the market can be employed for different purposes and for preparing of wide range of new materials.
Martinez-Sanchez et al.  have pointed out that employing of high-energy ball mills not only contaminates the milled amorphous powders with significant volume fractions of impurities that come from milling media that move at high velocity, but it also affects the stability and crystallization properties of the formed amorphous phase. They have proved that the properties of the formed amorphous phase (Mo53Ni47) powder depends on the type of the ball-mill equipment (SPEX 8000D Mixer/Mill and Zoz Simoloter mill) used in their important investigations. This was indicated by the high contamination content of oxygen on the amorphous powders prepared by SPEX 8000D Mixer/Mill, when compared with the corresponding amorphous powders prepared by Zoz Simoloter mill. Accordingly, they have attributed the poor stabilities, indexed by the crystallization temperature of the amorphous phase formed by SPEX 8000D Mixer/Mill to the presence of foreign matter (impurities).
Ball mill and high-intensity ultrasonication were used to obtain nanocellulose (NC).Understand the ball mill challenges and optimize the methodology.Mechanical grinding resulted in a random and irregular break of the fibers.Ultrasound waves generate more defibrillation, decreasing the NCs diameter.
A combination of mechanical methods was developed to obtain a suspension of nanocellulose (NC) from Eucalyptus sawdust. In this two-step process, the ball milling (14h) was followed by a high-intensity ultrasound (1030min) irradiation. The prepared samples were characterized using Fourier-transform infrared spectroscopy, Raman, dynamic and static light scattering, Zeta potential, X-ray diffraction, X-ray photoelectron spectroscopy, Thermogravimetric analysis, Field-emission scanning electron microscopy, and Transmission electron microscopy. The crystallinity was calculated using different methods and compared. The mechanical grinding results in a random and irregular break of the fibers. In a combined method, the ultrasound waves generate more defibrillation and separation of the nanofibers and, consequently, the reduction of fiber diameters. The high-intensity ultrasound promoted an increase in the homogeneity, crystallinity, and electrostatic and thermal stability of the NC suspension. The samples milled for 2h and 20min ultrasound irradiation presented the high-efficiency in obtaining NC.
Vinh Q. Tran, Hai T. Doan, Nhiem T. Nguyen, Cuong V. Do, "Preparation of Graphene Nanoplatelets by Thermal Shock Combined with Ball Milling Methods for Fabricating Flame-Retardant Polymers", Journal of Chemistry, vol. 2019, Article ID 5284160, 6 pages, 2019. https://doi.org/10.1155/2019/5284160
Graphene nanoplatelets were successfully prepared from graphite powder by simple and scalable thermal shock combined with ball milling methods. The formation of the graphene nanoplatelets were observed by field-emission scanning electron microscopes and BrunauerEmmettTeller methods with the much smaller number of layers and the considerable increase of specific surface area in comparison to the initial expanded graphite material. The other characterizations such as Fourier transform infrared spectroscopy and X-ray powder diffraction methods of graphene nanoplatelets showed unchanged structure. These graphene nanoplatelets were combined with aluminum trihydroxide and zinc borate to prepare flame-retardant polycarbonate plastic and chlorine-sulfonated polyethylene rubber. The prepared composites showed the improvement of flame resistance properties with V0 level according to the UL-94 test method, and the limiting oxygen index value was higher than 27.
Graphene nanoplatelets (Gnps) and their derivatives are potential flame-retardant additives applied for preparation of flame-retardant polymers . It can be combined with polymer matrix as a nanofiller for the case of plastic , rubber [5, 6], or coating agent for the case of fabric . The flame resistance effect of Gnps depends mainly on their dispersity in polymer matrix. Beside the factor of compatibility between Gnps and polymers, the size and layer number of Gnps also play an important role. Gnps can be prepared by two main methods, the bottom-up and top-down methods [10, 11]. The bottom-up methods, such as chemical vapor deposition and epitaxial growth, are expensive and cannot meet the requirement of big quantitative production of Gnps. On the contrary, the top-down methods are expected to meet large-scale production of graphene with low cost. Graphite material can be cracked and exfoliated using a very useful mechanical method reported by Babak Alinejad et al. . This method simply used NaCl as an exfoliation agent because this salt is harder than graphite. The mixture of graphite powder and NaCl salt particles were grinded by using a planetary ball mill for 25hours to crack graphite flakes to nanosized pieces. In this paper, we apply another simple method to produce Gnps material by using thermal shock combined with ball milling methods. An expanded graphite (EG) is firstly prepared from graphite powder. Next, a planetary ball milling machine is used for EG exfoliation to form Gnps. The ball milling of the EG instead of graphite powder is expected to improve the exfoliation effectiveness. Properties of the obtained Gnps are characterized by fourier transform infrared the spectroscopy (FTIR), X-ray powder diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and BrunauerEmmettTeller (BET) methods. The prepared Gnps are used to fabricate the plastic and rubber flame-retardant materials. The obtained flame-retardant materials are measured the fire resistance property by UL-94 and limiting oxygen index (LOI) methods.
Graphite powder (<20m, synthetic, Sigma-Aldrich), hydrogen peroxide (H2O2, AR, 3032%, Xilong Scientific), sulfuric acid (H2SO4, 98%, AR, Xilong Scientific), sodium chloride (NaCl, AR, 99.5%, Xilong Scientific), aluminum trihydroxide (AR, Xilong Scientific), zinc borate (99%, Kayseri, Turkey), polycarbonate plastic (Lotte Chemicals, Korea), and chlorine-sulfonated polyethylene rubber (CSM-3304, KunLun Co. Ltd, China) were used.
Gnps were prepared from graphite powder via two steps. In the first step, graphite powder was undergone a thermal treatment. Typically, graphite powder was pretreated by being added to the mixture of H2O2 (30%):H2SO4 (98%)=1:1.5 (v/v) at 20oC for 2h. The solid was filtered and washed by DI water to have the pH=7. The obtained solid was dried at 80C for 24h. This solid was thermal shocked by using a microwave at 800W for 60s. The solid was subjected to the pretreatment and the thermal shock once again by the same procedure described above to obtain EG sample. In the second step, the EG was grinded by using NaCl salt as disucssed in . 5g EG sample was mixed with NaCl salt (EG:NaCl=3:1 in molar ratio). A planetary ball milling machine (Pulverisette 5) with ball to powder weight ratio of 20:1 was used to grind the obtained mixture with a rotational speed of 350rpm for 2h in argon inert gas medium. NaCl salt was removed from the obtained powder by DI water using an ultrasonic bath. The final sample was centrifuged and dried at 80C under vacuum to get the final sample.
Gnps were used as nanofiller additive for flame-retardant polycarbonate (PC) plastic and chlorine-sulfonated polyethylene (CSPE) rubber preparation. Typically, a mixture of Gnps, conventional flame-retardant aluminum trihydroxide (ATH), and/or zinc borate (ZB) was melt-compounded with PC plastic or CSPE rubber in a Brabender closed mixing machine. PC plastic or CSPE rubber was melted in the first 4min with a rotor speed of 50rpm at 200C (PC) and 85C (CSPE). The above mixture with different percentage of Gnps, ATH, and/or ZB was then added to the compound with PC plastic or CSPE rubber for 10min. The compounded sample was hot-pressed in a Brabender molding machine at 210C (PC) and 95C (CSPE) for 7min to obtain the sample.
The FTIR spectra were recorded using an IMPACT-410 (Nicolet, Germany) infrared spectrophotometer at room temperature in the range 4000400cm1 on thin wafer of KBr in which 1% (w/w) of sample was dispersed. XRD diffraction patterns were recorded on a HUT-PCM-Bruker D8 Advance instrument diffractometer system equipped with Ni-filtered Cu Ka radiation (operating at 40kV; 40mA; wavelength k=0.154nm). FE-SEM was carried out using a Jeol JSM-7500F instrument. Sample was dried, mounted on a thin plate, and coated by a thin gold layer before recording. The BET method was measured by ASAP2010 equipment (Micrometrics-USA). The sample was treated in vacuum of 106mmHg, at 120C for 4h and at 350C for 9h.
The fire resistance property of flame-retardant PC plastic and CSPE rubber was tested by the UL-94 method on Atlas Electric HVUL-94 flame test station and LOI analysis on a JF-3 instrument according to GB/T 10707-2008 standard.
The FTIR spectra of the graphite and GNPs samples are shown in Figures 1(a) and 1(b), respectively. Both FTIR spectra show the peaks at about 1635cm1 and 1450cm1 assigned to C=C skeletal band of aromatic domains  and the peak at 3430cm1 assigned to OH groups . These OH groups may be corresponded to OH stretching vibrations of adsorbed water molecules and structural OH groups which do not allow a distinction between COH and H2O peaks. In the FTIR spectrum of Gnps sample, there are more peaks at 2975.09cm1 assigned to CH2 (the symmetric and antisymmetric vibrations of methylene)  and peaks at 1089.48 and 1048.86cm1 assigned to OH stretching vibrations . This result shows that by the pretreatment process using H2O2 and H2SO4, more OH groups and CH2 were formed. The intercalation of OH groups may help improve the ball milling effectiveness.
In addition, there is no considerable change between the XRD patterns of graphite and Gnps samples by using XRD method (Figure 2). There is a peak with high intensity at 2-theta of 26.4 assigned to the layer-by-layer structure of both graphite and Gnps samples [10, 12]. This result shows that by using thermal shock and ball milling, the Gnps sample was formed without structural transformation in comparison to the graphite sample.
The FE-SEM images (Figure 3) show clearly that the structure of Gnps is layer-by-layer assembly. This structure is similar to the initial graphite structure. However, in comparison to the FE-SEM image of the EG sample (Figure 3(a)), the FE-SEM of the Gnps sample (Figures 3(b)3(d)) shows clearly the exfoliation of EG to obtain Gnps with a monolayer or a few layers in its morphology.
The exfoliation of EG to Gnps is further demonstrated with the increase of specific surface area by BET result that is shown in Figure 4 and Table 1. The similarity of nitrogen adsorption/desorption curves shows the unchanged structure between EG and Gnps samples (Figure 4). The data in Table 1 show that the EG sample obtained by thermal shock has a BET specific surface area of 5.33m2/g. After the ball milling process of EG mixed with NaCl salt, BET specific surface area of the obtained GNPs is increased to 638.11m2/g (Table 1). It is 130 times higher in comparison to BET specific surface area of the EG sample and also higher than the reported result (524.4m2/g) that used virgin graphite powder for the ball milling . In addition, the pore volume of Gnps sample is decreased to 0.02cm3/g from 0.04cm3/g (the pore volume of EG sample before the ball milling). This result is in agreement with the FE-SEM result that showed the exfoliation of EG to obtain Gnps material with a few layers.
Gnps have been well known as an effective nanofiller flame-retardant additive with only small amount compounded with polymers (<5wt.%) . However, to improve the self-extinguishing property of polymers, it is better to use Gnps combined with conventional flame retardants. In this study, the ATH and ZB conventional flame retardants were applied. The compositions of the composite samples are shown in Table 2.
The results in Table 2 show that by using the same content (5wt.%) of graphite and Gnps as nanofillers to PC material, their flame resistance abilities are UL-94 V2 and V1 levels, respectively. This result shows that the Gnps material is better flame retardant than the graphite material. However, at high content of Gnps, it may result in the agglomeration of Gnps and the decrease of the flame resistance ability. The experiments of decreasing Gnps content have been performed. The observed results show that by decreasing Gnps content to 1.5 and 1.0wt.%, the UL-94 V0 level is attained. The lower content (0.5wt.%) of Gnps did not show the good result (UL-94 V2 level).
The self-extinguishing property was improved by combining the Gnps material with the ATH and ZB conventional flame retardants. The ATH and ZB conventional flame retardants were measured to get the UL-94 V1 level of the PC-ATH and PC-ZB composites. The idea is that by adding more Gnps combined with the PC-ATH and/or PC-ZB composites, the Gnps can help improving PC flame resistance ability to the UL-94 V0 level. The obtained results reach the expectation. By adding Gnps 1.5wt.% combined with ATH (25wt.%) or ZB (15wt.%) to PC, the obtained composites attain the UL-94 V0 level. However, the LOIs of these composite samples are lower than 27% (PC-Gnps-ATH: 22.9%; PC-Gnps-ZB: 24.0%). The LOI of 27% is needed for a flame-retardant composite to become a self-extinguishing material . Thus, the LOI of the composite sample was improved by adding Gnps, ATH, and ZB with the measured optimal contents (1.5wt.% Gnps, 25wt.% ATH, and 15wt.% ZB) to the PC plastic. The result shows that the obtained composite sample (PC-Gnps-ATH-ZB) reaches the UL-94 V0 level and LOI of 29.7%. This shows the role of Gnps to increase the LOI of this sample. Besides the fire resistance measurement, the mechanical property of the samples was also evaluated. The result indicates that the flame-retardant PC (PC-Gnps-ATH-ZB sample) has higher tensile strength (39.5 MPa) in comparison to the virgin PC plastic sample (34 MPa). This shows that adding Gnps can enhance the mechanical property of PC plastic .
Another CSPE rubber polymer was used to illustrate the flame resistance effectiveness of Gnps. ZB has been well known as flame-retardant filler for rubber [17, 18]. The contents of Gnps (1.0wt.%) and ZB (15wt.%) were measured to add to CSPE rubber. The result in Table 2 shows that both the CSPE-Gnps and CSPE-ZB composites attain the UL-94 V1 level. In addition, by combining Gnps and ZB with the contents mentioned above, the CSPE-Gnps-ZB composite (84.0-1.0-15.0wt.%) sample attains the UL-94 V0 level. However, the LOI of this composite sample attains only 23.1%. This LOI has been improved by increasing Gnps content up to 1.5 wt.%, and the LOI of the CSPE-Gnps-ZB composite (83.5-1.5-15.0wt.%) reaches the value of 28.6%. This result once again shows that Gnps can be used very effectively as flame-retardant additive, especially for increasing the self-extinguishing property of polymer composite by combining with conventional flame retardants.
The Gnps material was successfully prepared with the simple and scalable method of thermal shock combined with ball milling. The characterization methods (FTIR, XRD, FE-SEM, and BET) showed that the initial graphite material was exfoliated to obtain the Gnps material with few layer numbers and unchanged structure. The obtained Gnps material was applied as flame retardant for PC plastic and CSPE rubber. The results obtained by UL-94 and LOI measurement methods showed that by combining the Gnps material with ATH and ZB with suitable content, the PC-Gnps-ATH-ZB and CSPE-Gnps-ZB composite materials can be prepared with highest fire resistance properties (UL-94 V0 level and LOI is higher than 27%). The results also showed that Gnps have important role in improving the LOI of composite material with small added amount (1.5 wt.%).
This work was funded by a research grant from Vietnam Academy of Science and Technology under the grant number TPCCC.03/1820. The authors would like to thank Prof. Dr. Nguyen Van Tuyen, Director of Chemistry institute, Vietnam Academy of Science and Technology, for the scientific guidance during the research.
Copyright 2019 Vinh Q. Tran et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.