What will influence tablet film coating II
Writer： admin Time：2020-04-02 17:29 Browse：℃
3）Atomizing method and principle in film coating
Mist are made from liquid, which breaks into little drops. Ideal mist can be uniformly distributed on tablet surfaces and form even, smooth and continuous film. In fact, coating machine always applies air pressure and hydraulic pressure. The following are four types of atomizing method and principle.
Ultrasonic atomization
Through strong high frequency, liquid is broken into little drops and mist forms. This process is called ultrasonic atomization. Its atomizing efficiency is high and most energy consumed is used to enlarge mist area, while another two atomizing methods use only less than 1% to do so. However, it is difficult to control liquid flowing rate so that mist produced will not obtain enough momentum or will block spraying nozzle easily.
Hydraulic pressure airless spraying
In hydraulic spraying device, coating liquid passes through a small hole in high pressure. Then static pressure changes to momentum and liquid to mist. When liquid pressure,liquid flowing direction through small hole or spraying nozzle is changed, mist shape will change and form belt or cone mist screen. If organic solvent is used to dissolve polymer, corresponding method should be hydraulic pressure spraying. The reason is that mist will not be dried too early in an airless environment. However, some other factor should be noted, namely, liquid of certain viscosity, pretty high pressure and very small hole. Only all of these together can guarantee liquid flowing rate.
Air pressure spraying
When it collide and friction against high speed air flow, liquid will form mist. Relative shearing force of high speed and pressure air flow atomizes solution. Since air is of small density, large volume of high speed air is needed to generate enough energy to atomize solution of certain viscosity and tension.
Centrifugal spraying
In centrifugal spraying, coating liquid rotates in high speed in centrifugal plate. And generated centrifugal force will throw it out to form film, filament or droplet. Meanwhile, they friction against and are teared by air flow so that mist is formed.
Through research, it can be found that mist forming and moving can be influenced by those factors:
A.Atomizing air pressure
B.Mist shape
C.Distance between spraying gun and tablet container
D.Liquid spraying nozzle diameter
E.Radial distance of mist center
F.Design of atomizing device, features of coating liquid and confitions in atomization
G.The larger liquid viscosity, tension and tensity are, the larger mist droplets are.
H.Coating liquid flowing rate and relative speed between atomized gas and liquid. This is because different relative speed can lead to different shearing speed.
I.As the ratio of gas weight to liquid weight increases, the size of mist droplets decreases until the ratio is 4:1.
J.Preheating liquid can reduce its viscosity and decrease mist size as a result. Different atomizing device and evaporate and aggregation degree can lead to differences of mist droplets.
1)Coating liquid approaching and colliding against tablets
This process is shown in figure 2
Figure 2
a)Adhesion and wetting (sketch 1)
b)Adhesion and wetting (sketch 2)
c)Immersion and wetting sketch (tablet capillary)
d)Distribution and wetting sketch
The picture a) and b) are two different conditions of adhesion and wetting. When a particle is stick with a droplet which is not totally dried and another particle is close to and collide against the droplet, another adhesion and wetting process occurs. The second adhesion is not advantageous to coating, because after the droplet is absolutely dried, these two particles will bind to each other. However, as tablet cores rotate in coating device, the particles will separate and film will be teared. Therefore, an area on tablet will not be coated and film on the other tablet is not even. Such an defect is named adhesion. Fundamental solution is to better droplet wetting ability or tablet ability of being wet (with surfactant). Another solution is not so effective, i.e., controlling spraying time (or volume), drying time and roller speed. the picture c) indicates immersion and wetting, which is advantageous. It can enhance adhesion between dried film and tablet. There are two reasons. One is the increased contacting area of coating liquid and tablets. The other is that when dried, the film in capillary will be rooted in the tablet surface. And the picture d) show the process of mist distribution on tablets, which is also a future of mist and tablets in figure a). Being advantageous to coating, distribution and wetting can increase mist cover area and smooth dried film.
