Fig. 31 Internals of colloid mill. (From Ref. 29.) colloid mills, typically equipped with rotor diameters of 10-30 cm, provide flow rates in the area of 4000-6000 L/hr, depending upon the viscosity. The key operating requirements of colloid mills are to feed the mill with a well-blended premix and to set the gap at the correct and reproducible setting. There is of-ten some difficulty with setting the gap at exactly the required distance, since the cali-bration of the gap can only be done at the manufacturer. This is less of a problem if the mill is well made and the product is not abrasive. If abrasive wear attacks the ro-tor or stator, the gap may become larger than the setting on the machine indicates. Colloid mills are generally used as "polishing" machines for emulsions or sus-pensions. That is, after the product has been totally and uniformly blended, the batch is passed through the colloid mill one or two times to further reduce the droplet or particle size. Whether or not multiple recycling passes are required depends on prod-uct requirements. Generally speaking, the colloid mill produces emulsions and suspen-sions with particle-size distributions smaller than the particle sizes obtainable using fixed gap rotor/stator mixers. They do represent an extra step in the process, and their use is suggested only when it is found that this added ability to disperse is necessary to produce a fine enough particle- or droplet-size product to enhance a product's stabil-ity. 3. Piston Homogenizers The most powerful device for producing emulsions and suspensions is the piston ho-mogenizer or high-pressure homogenizer. This device uses a high-power positive dis-placement piston-type pump to produce pressures of 3000-10,000 psig and then force

doi:10.1201/9781420000955-52

Chapter

Fig. 31 Internals of colloid mill. (From Ref. 29.) colloid mills, typically equipped with rotor diameters of 10-30 cm, provide flow rates in the area of 4000-6000 L/hr, depending upon the viscosity. The key operating requirements of colloid mills are to feed the mill with a well-blended premix and to set the gap at the correct and reproducible setting. There is of-ten some difficulty with setting the gap at exactly the required distance, since the cali-bration of the gap can only be done at the manufacturer. This is less of a problem if the mill is well made and the product is not abrasive. If abrasive wear attacks the ro-tor or stator, the gap may become larger than the setting on the machine indicates. Colloid mills are generally used as "polishing" machines for emulsions or sus-pensions. That is, after the product has been totally and uniformly blended, the batch is passed through the colloid mill one or two times to further reduce the droplet or particle size. Whether or not multiple recycling passes are required depends on prod-uct requirements. Generally speaking, the colloid mill produces emulsions and suspen-sions with particle-size distributions smaller than the particle sizes obtainable using fixed gap rotor/stator mixers. They do represent an extra step in the process, and their use is suggested only when it is found that this added ability to disperse is necessary to produce a fine enough particle- or droplet-size product to enhance a product's stabil-ity. 3. Piston Homogenizers The most powerful device for producing emulsions and suspensions is the piston ho-mogenizer or high-pressure homogenizer. This device uses a high-power positive dis-placement piston-type pump to produce pressures of 3000-10,000 psig and then force

Chapter

Fig. 31 Internals of colloid mill. (From Ref. 29.) colloid mills, typically equipped with rotor diameters of 10-30 cm, provide flow rates in the area of 4000-6000 L/hr, depending upon the viscosity. The key operating requirements of colloid mills are to feed the mill with a well-blended premix and to set the gap at the correct and reproducible setting. There is of-ten some difficulty with setting the gap at exactly the required distance, since the cali-bration of the gap can only be done at the manufacturer. This is less of a problem if the mill is well made and the product is not abrasive. If abrasive wear attacks the ro-tor or stator, the gap may become larger than the setting on the machine indicates. Colloid mills are generally used as "polishing" machines for emulsions or sus-pensions. That is, after the product has been totally and uniformly blended, the batch is passed through the colloid mill one or two times to further reduce the droplet or particle size. Whether or not multiple recycling passes are required depends on prod-uct requirements. Generally speaking, the colloid mill produces emulsions and suspen-sions with particle-size distributions smaller than the particle sizes obtainable using fixed gap rotor/stator mixers. They do represent an extra step in the process, and their use is suggested only when it is found that this added ability to disperse is necessary to produce a fine enough particle- or droplet-size product to enhance a product's stabil-ity. 3. Piston Homogenizers The most powerful device for producing emulsions and suspensions is the piston ho-mogenizer or high-pressure homogenizer. This device uses a high-power positive dis-placement piston-type pump to produce pressures of 3000-10,000 psig and then force

ABSTRACT

There are several limitations of high-pressure piston-type homogenizers. First, the high-pressure piston pumps are not designed to handle product feeds above 200 centipoise. This does not, however, mean that high viscosities cannot be prepared with piston homogenizers, but that the initial feed of the premix cannot be above this limit. Second, despite the fact that the homogenizing valve parts are made of very hard materials, such as stellite, appreciable wear can arise on these precision parts when abrasive solids are present in the formula. This can result in high maintenance costs and downtime. Third, the valve setting may not conform to specifications due to regular wear and spring fatigue. This may cause lack of product homogeneity and batch-tobatch variability [1]. Finally, there are two reasons to try to prevent the inclusion of air into the premix. First, the air causes the machine to run with vibration, and second, the air most certainly is very well dispersed into the product, which is almost always undesirable.