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Compaction pressure (MPa)

Fig. 9 The effect of compaction pressure on the compressive (squares) and tensile (triangles) Young's modulus for paracetamol DC (filled symbols) and Dipac sugar (unfilled symbols) (taken from Al-Hassani eta/., 1989).

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Compaction pressure (MPa)

Fig. 9 The effect of compaction pressure on the compressive (squares) and tensile (triangles) Young's modulus for paracetamol DC (filled symbols) and Dipac sugar (unfilled symbols) (taken from Al-Hassani eta/., 1989).

reflected and transmitted strain, c is the longitudinal stress wave velocity and ÍÓ(er)dt is the area under the reflected strain curve.

The technique has been used to evaluate the compressive and tensile Young's modulus of two pharmaceutical materials: paracetamol DC (spray granulated paracetamol with 4% hydroysed gelatin) and Dipac sugar (Al-Hassani etal., 1989; Sarumi and Al-Hassani, 1991). Data from these workers are reproduced in Fig. 9. Unfortunately the workers did not report porosity values for the specimens measured and hence the data cannot be evaluated in terms of calculating Young's modulus at zero porosity. However, it can be seen that for all specimens Young's modulus in tension is always lower than in compression.

The tensile modulus (taken as the maximum values reported by Al-Hassani etal., 1989) for Dipac sugar and paracetamol DC are 10.8 and 1.8 GPa, respectively. The value for paracetamol DC is a factor of 10 lower compared with the value measured using three-point beam bending (Roberts, 1991) but the value for Dipac sugar compares favourably with that measured using four-point beam bending (Bassam etal., 1990).

As stated by Al-Hassani etal. (1989) the technique has many advantages in that it is simple and precise. For pharmaceutical materials it has the added advantage that the specimens used are 10 mm diameter flat-faced tablets that are easily prepared using standard punches and dies in a tablet machine.

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