Die-wall lubricants

There are three types of lubricants employed in solid dosage form manufacture. The first class of lubricant is the glidant. The flow properties of a powder can be enhanced by the inclusion of a glidant. These are added to overcome powder cohesiveness. The two other classes of lubricant are antiadherent excipients, which reduce the friction between the tablet punch faces and tablet punches, and die wall lubricant excipients, which reduce the friction between the tablet surface and the die wall during and after compaction to enable easy ejection of the tablet. The level of a lubricant required in a tablet is formulation dependent and can be optimized using an instrumented tableting machine. 

The third class of lubricant activity is the antiadherent. Some materials have adhesive properties and can adhere to the punch surfaces during compression. This will induce tablet disorders sticking, with a film forming on the surface of the tablets, or picking, where solid particles from the tablet stick to the punch surface. Most die wall lubricants also have antiadherent actions, and in many formulations, the addition of a specific antiadherent will not be required separately. The antiadherent includes talc, maize starch, and microcrystalline cellulose. 

Stearic acid Lubricant 1 More preferred as a die wall lubricant... 

Most die-wall lubricants have antiadherent actions, and in many formulations, the addition of a specific antiadherent will not be required. Materials that can be added include talc, maize starch and microcrystalline cellulose. 

This technology was also adapted for other applications. Particularly the pressing of metal powders is much improved by lubricating the die cavity. Parts become more uniform, die wear is reduced, and the lubricant, which is often a contaminant, is eliminated from the metal powder mixture. Fig. 8.98 shows schematically and as a photograph a die wall lubrication system which uses an electrostatically charged powder for... 

Fig. 8.98 Schematic and photograph of a die wall lubrication system using an electrostatically charged powder (courtesy Casbarre, DuBois, PA, USA).

Process additives are most often employed in the ram extrusion process to minimize the back pressure exerted on the ram by the surface friction of the compacted powder on the die walls. Lubricants can be added which will migrate to the die surface during sintering to create a slip layer between the die... 

The friction coefficient between the powder and a rough wall is also dependent on the direction of the grooves in the wall. The friction is lower if the grooves run in the direction of relative motion between the powder and the die wall. The effect of die-wall lubricants (e.g., stearic acid) can be fairly complex. For fine particles (dp/R < 1), the coefficient of friction decreases gradually as the thick-... 

Dry lubricants are usually added to the powder in order to decrease the friction effects. The more common lubricants include zinc stearate [557-05-17, lithium stearate [4485-12-5] calcium stearate [1592-23-0] stearic acid [57-11-4] paraffin, graphite, and molybdenum disulfide [1317-33-5]. Lubricants are generally added to the powder in a dry state in amounts of 0.25—1.0 wt % of the metal powder. Some lubricants are added by drying and screening a slurry of powder and lubricant. In some instances, lubricants are appHed in Hquid form to the die wall. 

Insoluble Lubricants. Lubricants act by interposing an intermediate layer between the tablet constituents and the die wall. The smaller the amount of stress needed to shear the material, the better its lubricant properties will be. Since they are primarily required to act at the tooling/material interface, lubricants should be incorporated in the final mixing step after all granulation and... 

Another example of the use of DOE during pilot studies is the study of factors affecting the ejection and take-off force. Measurement of ejection force and take-off force will determine if the formula is sufficiently lubricated. Ejection force is measured as an indication of the release of the tablet from the die wall forces, and the take-off force related to the adhesion forces to the punch face. Take-off force is an appropriate measurement to determine if a formula has a tendency to stick. Based on these designs, an optimal formula, including the lubricant level, and process can be predicted for scale-up (Figs. 12 and 13). 

One very common beneficial interaction involving an excipient is the interaction between magnesium stearate and the metal of tablet punches and dies, or the equivalent parts on a powder encapsulation machine. Magnesium stearate is an example of a boundary lubricant. As such it has a polar head and a fatty acid tail. It is believed that the polar head of the magnesium stearate is oriented toward the die wall or tablet punch face. In these ways it is able to reduce the ejection force (the force required to eject the tablet from the die after compaction) and prevent sticking to the punch faces. The other boundary lubricants, e.g., calcium stearate and sodium stearyl fumarate, will also function in a similar manner. However, the so-called liquid film lubricants function in a very different manner (19). 

Lubricated (to prevents the powder/granulation and the tablet from sticking to punches and die, and to enable the formed tablet to be ejected from the die wall and released cleanly from the punch faces)... 

Lubrication is an important unit operation in manufacturing solid oral dosage forms, particularly when using a direct compression platform. Pharmaceutical lubricants can have a significant impact on product performance (e.g., disintegration and dissolution) as well as manufacturability. Lubrication is one of the most critical aspects of a tablet formulation. A lubricant is intended to reduce the friction between the tablet surface and die wall during and after compaction to enable easy ejection of the tablet. In low-dose dmg product development, three issues are associated with lubricating a direct compression formulation ... 

Third, after the tablet is compressed, the upper punch is withdrawn from the die and lower punch moves upwards to eject the tablet. Successful ejection of tablets without chipping or sticking requires sufficient lubrication of the powder blend so there is minimum adhesion between the tablet and the die wall. Lower ejection forces are preferred during tablet production to avoid unnecessary mechanical wear on the tablet press. 

Alternatively, a stiff paste of ceramic and binder solution can be forced through a wire mesh and granules of the required size extracted from the dried product by sieving. The flow of the granules can be enhanced and die-wall friction reduced by tumbling with a small quantity of a powdered lubricant such as calcium stearate. 

Lubricants act by interposing an intermediate layer between the tablet constituents and the die wall, to prevent adherence of the granules to the punch faces and dies. Thus, they ensure smooth ejection of the tablet from the die. In addition, many lubricants also enhance the flow properties of the granules. Stearic acid and its magnesium and calcium salts are widely used. The most effective lubricants, such as magnesium stearate, are very hydrophobic and can also prevent wetting of powders and hence retard dissolution (Figure 6.9). 

Wall friction depends on the roug ess of the die wall and its lubrication. For walls with a scale of roughness larger than the diameter of the powder, the wall friction is essentially the same as the internal friction of the powder. This is because the rough wall traps small amounts of the powder in its reguosity and the sliding of the powder mass against this trapped powder produces the effective wall friction. If the wall has a scale of rou mess smaller than the diameter of the powder, the wall friction is controlled particle—wall friction. In both... 

Lubricants To reduce the friction between the granules and the die wall during compression and ejection of the tableting process Water-insoluble metal stearates, stearic acid, talc Water-soluble boric acid, sodium chloride, benzoate and acetate, sodium or magnesium lauryl sulfate Carbowax 4000 or 6000... 

The primary function of tablet lubricants is to reduce the friction arising at the interface of tablet and die walls during compression and ejection. Lubricants also possess antiadherent (prevention of sticking to the punch and, to a lesser extent, to the die wall) and gli-dant (improvement of flow characteristics of powders or granulates) characteristics and are useful in the processing of hard gelatin capsules. 

The force required to push the tablet up the die wall, which is typically lower than the ejection peak force. However, inadequate lubrication or damaged dies may result in slip-stick behavior where the tablet continues to adhere and break adhesions to the die wall surface. These conditions typically result in tablet failure. 

When the tablet formulation is compressed, the sides of the tablet are brought into intimate contact with the die wall. The tablet must then be ejected from the die, involving the movement of the side of the tablet relative to the die wall. Therefore, friction between the tablet and the die wall must be overcome. With materials such as lactose, friction resistance can be considerable, and it may be impossible to remove the tablet from the die without damage to the tablet or to the tablet press. Therefore, a lubricant is almost invariably included in a tablet formulation. A lubricant is a substance that deforms easily when sheared between two surfaces, and hence when interposed between the tablet and the die wall, provides a readily deformable film. Details of some tablet lubricants are shown in Table 4. 

The reason why the lower punch maximum force is less than that of the upper punch is because a fraction of the force applied by the upper punch is transmitted to the die wall, where it results in die wall friction. This is reduced by the presence of a lubricant. Hence, the ratio between lower punch maximum force and upper punch maximum force, often called the R value, is used as the basis of comparison between lubricants. " R has a maximum value of unity and lubricants based on stearates usually exhibit R values greater than 0.95. 

Radial and axial die-wall force measurements also provide an insight into the compaction mechanism of the material and may indicate a die-wall binding (sticking) that is, in effect, a negative pull on lower punch. The radial die-wall pressure due to friction is material-specific and is more evenly distributed inside the die with an addition of a lubricant. Instrumentation of the die presents a technological challenge because pressure is distributed non-linearly with respect to tablet position inside the die and depends on tablet thickness. " ... 

A certain quantity of lubricant must be present in the granulation to reduce the friction that occurs at the die wall as the tablet is being ejected, as well as to prevent sticking of the tablet to the face of the punches. Without instrumented ejection cam and take-off bar. 

Schrank-Junghani, H. Bier, H.P. Sucker, H. The measurement of die wall forces to determine the minimum concentration of lubricant needed for tablet formulations. Acta Pharm. Technol. 1984, 30 (3), 224—234. 

Hydrogenated castor oil is additionally used to lubricate the die walls of tablet presses and is similarly used as a lubricant in food processing. 

---