57 (1989) 83 - 87
Creep Compliance Behaviour of Direct Compression J. N. STANIFORTH
and C. I. PATEL
Particle Engineering Research Group, School of Pharmacy Claverton Down, Bath BA2 7AY (U.K.) (Received February l&1987;
in revised form August 4, 1988)
Creep compliance was studied during tablet compaction by treating powder deformation or flow as analogous to the rheologtcal behaviour of semi-solids. The creep curves could be differentiated into three distinct regions, corresponding to different types of rheological behaviour: elastic, visco-elastic and plastic deformation. Such curves were found useful in describing the rheological characteristics of direct compression excipients and in interpreting the different mechanical behaviour of powders.
The formation of a tablet with satisfactory mechanical properties is dependent on the degree of inter-particle bonding and bond strength, both of which are largely governed by the magnitude of true intimate contact areas between particles. The areas of true contact are both formed and destroyed sequentially during the compression and decompression stages of tablet compaction. One major factor that governs bond formation and destruction is the rheological behaviour of the powders compressed. Knowledge of their rheological properties, especially for direct compression excipients, may give information regarding the compactibility of powders. A test which has been used to examine the rheological behaviour of semi-solids is the creep test. The process of creep may be defined as the slow progressive deformation of a material with time, under constant stress. Creep compliance measurements therefore differ from stress relaxation studies where the strain is maintained constant and the 0032-5910/89/$3.50
force decay is plotted versus time. Creep compliance analysis can be achieved by graphical methods as used by Barry and Warburton [l] and Sherman [2,3]. According to convention, compliance is plotted with respect to time. The numerical value of compliance J is calculated according to eqn. (1) and has the units of l/Pa. J=
The rheological behaviour of semi-solids can be represented in the form of a canonical model  where a Maxwell unit is placed in series with several Voigt or Voigt-Kelvin units. The spring and the dashpot of the Maxwell unit are separated using three Voigt units connected in series (Fig. l), although the number may vary according to the viscoelasticity of the material being modelled.
Fig. 1. Canonical model used to describe creep behaviour . @ Elsevier Sequoia/Printed
in The Netherlands
The total strain in the individual elements, and thus compliance, in series are additive.
where J(t) is the compliance at time t, Ji is the compliance at the ith element and ri is the retardation time. The Voigt model therefore describes a visco-elastic solid. Equation 2 was further extended [ 1] to v&o-elastic liquids by including the compliance of the viscous nature of liquids by including the compliance of the viscous dashpot, which is calculated according to eqn. (3).
Jn(O = ;
where J,(t) represents compliance of the viscous dashpot and q. is the viscosity of liquid in the dashpot. Thus, for a real v&co-elastic liquid, eqn. (4) may be used [ 41.
Fig. 2. (a) Creep compliance curve [ 31; (b), example of creep curve obtained using Emcocel, microcrystalline cellulose N.F.
where Jo represents compliance of the elastic spring and q. is the residual shear viscosity of liquid in the dashpot of the Maxwell model. A creep compliance-time curve can be divided into three regions [ 31, (Fig. 2), where A-B is the region of instantaneous compliance (Jo). A simple method for determining the value of Jo is to measure it directly using an experimental creep plot. The A-B region represents the elastic deformation of the powder during compaction and is modelled by spring stretching in the Maxwell unit (Fig. 1). B-C is the timedependent retarded elastic or v&o-elastic region. This is modelled by the retarded stretching of the spring caused by the dashpot connected in parallel (Fig. 1). C-D is the linear region of Newtonian compliance and it represents residual viscous flow which is modelled by the viscosity of the liquid in the dashpot of the Maxwell unit. The residual shear viscosity q. can be determined by regression of the linear portion C-D.
The direct compression excipients studied were known to differ in their mechanical compactibility and were selected to give a range of rheological behaviour to allow evaluation of the sensitivity of creep compliance determinations in distinguishing this different behaviour quantitatively. The excipients studied were Avicel, type PH102, (FMC Corp., Philadelphia, U.S.A.); Emcocel (Edward Mendell Co. Inc., Carmel, U.S.A.); Microtal (Tate and Lyle plc, Plaistow Wharf, U.K.); Starch 1500 (Colorcon Ltd., Orpington, U.K.). All powders were stored at 25 “c, 55% relative humidity for 48 h prior to compression and the mass of powder required to produce a tablet of 2.5 mm thickness at zero theoretical porosity was manually transferred into a pre-lubricated die. The punches and die were fitted onto a mechanical cage (Fig. 3) which was then mounted onto a tensile tester (type T22K, J. J. Lloyd
Fig. 3. Experimental pliance of powders.
set-up to monitor creep com-
Instr., Southampton, U.K.) which was capable of maintaining constant stress levels. Two linear variable displacement transducers were attached to the cage in such a way that the movement of a 12.7-mm flat-faced punch tip could be determined. The rate of load application used was 14 kN/min and the constant loads applied were 5, 10 and 15 kN, thus giving constant stress values of 39.5, 79.0 and 118.5 MPa respectively. The complete set-up and data acquisition and manipulation have been described in detail elsewhere [ 51.
Preliminary investigations showed that creep curves for the direct compression excipients were similar in profile to the theoretical curve. The intercept on the ordinate was used to indicate the degree of elastic behaviour of the powder. In terms of the canonical model, this represented the instantaneous stretching of the spring of one Maxwell unit when a load is applied (Fig. 1). The largest intercept value was obtained with Starch 1500, suggesting that this material undergoes the greatest degree of elastic deformation and this was true for all stress levels (Tables 1 - 3). This is in agreement with the results obtained using stress relaxation experiments
. It is therefore likely that during the decompression phase of tablet production, destruction of inter-particle bonding by elastic recovery was responsible for the low mechanical strength of Starch 1500 tablets reported. Avicel PH102 had lower elastic deformations than Starch 1500, Microtal and 5Owt.B Avicel in Emcompress had approximately equal amounts of elastic deformation. The smallest elastic deformation was exhibited by Emcompress. The C-D portion is a linear region of Newtonian compliance in semi-solids and can be considered as the irreversible deformation of a material. In terms of a canonical model, when the load has been applied for a sufficient time, ensuring that all the Voigt units are fully extended, the deformation is analogous to viscous flow and is irreversible. A rank order of decreasing value of the C-D slope was obtained at all stress levels (Tables 1 -3): Starch 1500 > Avicel PH102 > Microtal = 5Owt.s Avicel PH102 + 50wt.s Emcompress > Emcompress. When a powder undergoes a considerable amount of strain through plastic deformation, the slope CD will have a large numerical value. High strains are only possible when mechanisms or conditions exist within a compact which can reduce the applied load and such reduction can be much more efficiently accomplished through plastic deformation than brittle fracture. The results indicate that Starch 1500 had the greatest plasticity. The slope for Emcompress was very much lower than those of the other direct compression excipients, suggesting that Emcompress had minimal plastic properties. The use of creep curves in order to study the rheological properties of a powder has been reported by Morri et al. [ 71. However, these workers used compressive strain (creep strain) instead of creep compliance uersus time and since strain determinations were made during dynamic compression (i.e., not constant stress), their plots are not true creep curves. Unlike this present study, Morri et al.  were unable to show a distinction betwen plastic and elastic or visco-elastic properties. The v&o-elastic characteristics were derived from the B-C region of the creep curve (Tables 1 - 3). The value of viscoelastic compliance for Starch 1500 and
86 TABLE 1 Constant stress 39.5 MPa* Materials $N~X lo-lo Emcompress Emcompress (A) Microtal Avicel PH102 Starch 1500
0.49 4.07 4.30 6.10 9.20
(0.04) (0.30) (0.38) (0.69) (1.02)
B-C (JR x lo+ 2.43 2.20 3.10 2.13
(0.16) (0.11) (0.28) (0.61)
Intercept (JO X lo-’ 0.38 3.10 2.80 6.20 8.51
(0.09) (0.46) (0.47) (0.25) (1.00)
*Tables 1 - 2 show the numerical value of slopes of two regions of the creep curve and the intercept when the powders were subjected to a constant stress of 39.5, 79 and 118.5 MPa at load rate of 14 kN/min. The slope and intercepts are the mean of five determinations and figures in parentheses are corresponding standard deviations. Emcompress (A) refers to a binary mix of 50 wt.% Avicel PH102 in Emcompress. TABLE 2 Constant stress 79.0 MPa a Materials
C-D (JN X lo-lo
Emcompress Emcompress (A) Microtal Avicel PH102 Starch 1500
0.28 1.67 2.31 2.78 3.30
(0.04) (0.20) (0.07) (0.11) (0.17)
B-C (JR x 10V8 spa-‘)
Intercept (Jo X lo-’
2.70 11.70 8.00 15.30 19.40
6.90 8.50 10.30 19.00
(0.72) (0.60) (0.10) (2.60)
(0.60) (0.50) (0.19) (0.30) (0.76)
TABLE 3 Constant stress 118.5 MPaa Materials
Emcompress Emcompress (A) Microtai Avicel PH102 Starch 1500
2.45 12.10 12.75 14.84 16.50
(0.50) (1.30) (0.55) (0.40) (1.54)
Avicel PH102 were approximately similar, as were those for Microtal and 50% Avicel PH102 in Emcompress. The B-C region was found to be absent in data obtained for Emcompress, indicating that it had no apparent visco-elastic behaviour. Visco-elasticity can be represented in the canonical model by a series of Voigt or Voigt Kelvin units (Fig. 1) which produce retarded elastic deformation of the powder. The occurrence of capping and lamination during tablet compression may depend on the rheological characteristics of the powders compressed. Pilpel et al. [ 81 have determined the plasto-elasticity of mixtures of paracet-
B-C (JR X 10VQ spa-‘)
Intercept (JO X 10dq Pa-‘)
5.70 5.65 6.79 8.50
1.73 3.00 2.64 5.10 7.23
(1.20) (0.18) (0.30) (1.40)
(0.41) (0.50) (0.43) (0.60) (0.72)
am01 and Avicel powders and the tensile strength of their tablets. They state that the logarithms of tensile strength were inversely proportional to the ratio of the sample elastic recovery to its plastic compression. Table 4 shows the ratio of the elastic to plastic deformation for the direct compression excipients in the present study. Avicel had the largest value at a constant stress ratio of 39.5 MPa, suggesting according to Pilpel et al.  that it should possess the lowest tensile strength. This appears to suggest that the degree of inter-particle bonding was less for Avicel PH102 than for Emcompress, unlike results reported elsewhere
87 TABLE 4 Ratio of elastic:plastic deformation, as determined from creep curves for various powders. Emcompress (A) refers to binary mix of 5Owt.% Avicel in Emcompress. Material
Ratio of elastic:plastic deformation at three stress levels 39.5 MPa 79.0 MPa 118.5 MPa
Emcompress Emcompress (A) Microtal Avicel PH102 Starch 1500
775.6 762.0 651.0 1016.0 952.0
964.0 659.0 346.0 550.0 589.0
70.6 24.8 18.3 34.4 43.8
[ 51. However, at a constant stress of 79 MPa, Emcompress possessed a much higher ratio than Avicel PH102, according to the relationship stated by Pilpel et al. . Microtal possessed the lowest ratio at all three stress levels (Table 4) suggesting that Microtal tablets would have the highest degree of inter-particle bonding and highest tensile strengths, which is probably incorrect since it has been previously reported  that Avicel had a higher degree of inter-particle bonding than either Emcompress or Microtal. At a constant stress of 118.5 MPa, according to the scheme proposed by Pilpel et al. , 5Owt.S Avicel in Emcompress would have a higher degree of inter-particle bonding than tablets compressed using pure Avicel. This is again considered unlikely, since the degree of plastic deformation will be greater in Avicel than in Emcompress, because the latter consolidates purely by brittle fragmentation processes. A probable reason for the lack of agreement between the data reported by Pilpel et al.  and those in Table 4 is the difference in the methods used to determine the plastic parameters. Pilpel et al.  have calculated “plastic compression (PC)” and “elastic recovery (ER)” according to eqns. (5) and (6).
at the end of loading and HR is the tablet thickness after tablet ejection. Hence, PC calculated by these workers is a combination of elastic, visco-elastic and plastic deformation of the powders and not purely plastic deformation.
It can be concluded that the rheological behaviour of direct compression excipients can be characterized and differentiated using analysis of creep compliance uersus time relationships. Creep curves can be divided into three distinct regions corresponding to elastic, visco-elastic and plastic behaviour. The quantitative values and relative magnitudes of elastic, visco-elastic and plastic parameters differed for different types of excipients. The elastic:plastic ratio is considered to be a poor method of predicting the compaction behaviour of powders, whereas examination of creep compliance curves was found to predict the compactibility of powders within acceptable limits, for these materials, although subsequent work has revealed certain limitations of the technique
PILIST OF SYMBOLS
Ji Jll PC t r)O Ti
elastic recovery tablet thickness at end of loading tablet thickness at point of formation tablet thickness after ejection creep compliance compliance at ith element viscous compliance plastic compliance time viscosity of plastic element retardation time at ith element
where Ho is the tablet thickness when tablet is first formed, HL is the tablet thickness
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