Pharmaceutical Manufacturing Handbook: Production and

1044 TABLET DESIGN
FIGURE 47
Temperature (°C)
120
100
80
60
40
20
0
0 2 4 6 8 10 12 14 16 18 20
Time (sec)
Temperature profi le inside compression chamber. (Courtesy of Tecnea Srl.)
Rodriguez et al. [87, 88] ) or when the materials used are not thermoplastic.
In these cases the machine acts as a multiple - impact mechanical press.
Thermal Effects Due to the poor conductivity for ultrasounds (low module of
elasticity and high quantity of air trapped inside) usually exhibited by the
materials included in pharmaceutical formulations, a fast decay of ultrasonic
energy to thermal energy is obtained. This process has been studied, monitoring
the temperature inside the compression chamber by means of a thermistor.
In the studied mixtures [87, 88] , a fast rise in temperature was obtained in
tenths of a second followed by a relatively fast decrease (see Figure 47 ).
The peak temperature obtained for low ultrasonic energy (25 J) is below 80 ° C,
whereas for high energies (125 – 150 J) it is above 140 ° C. In mixtures of ketoprofen
with acrylic polymers [90] , the increase in temperature was slightly
lower. In this respect it must be mentioned that a recent modifi cation of the
ultrasound - assisted tableting machine that involves the suppression of Tefl on
isolators in contact with the powder must result in a faster decrease in temperature
inside the compression chamber. Thermal effects can cause the total
or partial fusion of some components of the formulation. Nevertheless, in the
assayed controlled - release formulations, the components are usually below its
melting points.
Plastic Deformation Plastic deformation results from the combination of
thermal and mechanical effects. The thermoplastic excipient was subjected to
a temperature above its glass transition temperature ( T g ) and to a high - frequency
mechanical pressure that can avoid the elastic recovery of the
material.
Sintering The combination of temperature, pressure, and friction effects can
result in the sintering of particles, so that the limits between them are no
longer evident [46, 87] .
Recent studies [91 – 93] have shown that, for one component of the system undergoing
thermoplastic deformation, the continuum percolation model can be used to
predict the changes in the system with respect to a traditional pharmaceutical