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Effect of binder to solid ratio on mechanical properties of granules processed using reactive and non-reactive binder
Powder Technology 229 (2012) 137–147
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Effect of binder to solid ratio on mechanical properties of granules processed using reactive and non-reactive binder Sujitkumar Hibare ⁎, Koushik Acharya Unilever R&D Bangalore, Hindustan Unilever Ltd., 64, Main Road, Whitefield, Bangalore-560066, India
a r t i c l e
i n f o
Article history: Received 7 December 2011 Received in revised form 12 April 2012 Accepted 8 June 2012 Available online 15 June 2012 Keywords: Binder to solid ratio Granule compression Granule strength Reactive binder Packing coefficient
a b s t r a c t In wet granulation of powders in high shear granulators, the properties of granules formed can be altered by controlling a number of parameters. One of the crucial parameters responsible for granule properties is the quantity of the binder used for granulation. This can be expressed as binder to solid ratio (B/S) or liquid to solid ratio (L/S). Researchers have reported in the past the effect of liquid to solid ratio on granulation mechanism and growth, granule structure and consequently on the deformation behavior of the granules. Some researchers have also reported the distribution of the binder and the size distribution of the granules formed as a function of the concentration of the binder used in the process. However, there is limited work reported in the literature on the effect of binder to solid ratio on the physical and mechanical properties of granules. The mechanical properties of the granules can determine the behavior of granules at various stages such as usage, handling, transportation and storage under compression. The work reported in this paper investigates the effect of binder to solid ratio (B/S) on the mechanical properties of soft detergent granules made via wet granulation in a high shear mixer. The effect of binder to solid ratio in wet granulation can vary depending upon the nature of the binder used. Hence two different types of binders were used viz. reactive (reacts with base powder) and non-reactive (acts only as a binder) to granulate sodium carbonate as primary particles. The mechanical properties were studied by conducting single and bulk compression measurements. Known models of Heckel and Kawakita and Ludde were also used to predict mechanical properties of the granules. The granulation time required to achieve similar physical properties (particle size distribution and circularity) of the granules decreased with increase in binder to solid ratio. Single granule compression measurements showed that apparent granule strength decreases with increase in binder to solid ratio for both reactive and non-reactive granules which may be due to more liquid binder content in the granules. The Heckel parameter was observed to be approximately 2.5 to 3 times the apparent granule strength obtained from single granule compression measurements for both reactive and non-reactive granules. Packing coefficient increased with increase in binder to solid ratio indicating higher compressibility behavior under similar load for both reactive and non-reactive granules. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The process of wet granulation talks about three mechanisms viz. nucleation, consolidation or coalescence, and attrition or breakage. Coalescence and breakage happen simultaneously and the equilibrium between these two mechanisms determines the outcome of the granulation process. The important components in a high shear granulator are a drum, an impeller and a vertically mounted chopper. The mechanical energy required for mixing the base powder homogeneously is provided by the impeller and the chopper is designed to help in breaking the granules formed. The chopper operates comparatively at a higher speed than the impeller. The effect of the impeller and chopper speed on granule
⁎ Corresponding author. Tel.: + 91 80 39831123; fax: + 91 80 28453086. E-mail address: [email protected] (S. Hibare). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.06.020
properties, therefore, depends on the response of the granules to the energy provided by both impeller and the chopper. A.M. Bouwman et al. [1] have investigated the role of deformability, which depends on the amount of binder liquid and the granule porosity, on the subsequent microcrystalline cellulose (MCC) granulation mechanisms and how this determines the final structure of granules produced in the high shear granulator. X-ray micro tomography was used to visualize and quantify the densification process of microcrystalline cellulose granules, comparing granules produced with different granulation times, and different amounts of liquid binder. The granules were also compared for differences in the porosity at different sites in the granule. Scanning electron microscopy (SEM) was used to visualize the external appearance of the granules. The deformability of the visualized granules was measured using micromanipulation. The authors recommended that the binder liquid should not only be regarded as a binder but also its effects on deformability of the granules should be taken into account, as the deformability determines the granulation mechanism occurring
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during the latter phases (consolidation or growth, and breakage or attrition) of the granulation process. Liquid phase is necessary for binding the powder particles and making the wet mass more deformable. With a low amount of binder liquid, densification of the granules occurs and the final phase of the granulation process results in equilibrium between attrition and growth. Since the granules are no longer broken during this phase, spheronization of the material may occur. However, when excess binder liquid is used, the deformability of MCC granules increases, which makes them susceptible to breakage upon continued granulation. Continued breakage will reduce the extent of densification of the granule core and the granules remain weak during the complete process. Even during the last (equilibrium) phase of the process, continuous breakage occurs. This breakage is now balanced by coalescence of the fragments with each other or with other granules. This finally results in weak, irregularly shaped granules. The function of the binding agent used in granulation is primarily to maintain the integrity of the granules prior to compression. However, it has been shown by Seager et al., 1979 [2], 1981 [3] and Rue et al., 1980 [4] that the distribution of the binding agent within the granule not only controls the intra-granular particulate adhesion, but also the intergranular bonding achieved during tabletting and hence the resultant tablet properties, such as tensile strength, disintegration and dissolution. Teresa Cutt et al. [5] have studied the friability of granules at different binder levels. An increase in binder concentration from 1.5 to 3% w/w decreased the friability of both the HPMC and Byco granules. The PVP granules were considerably less friable than the other granules at the lowest binder concentration, and increasing the concentration had little effect on their friability. An increase in binder concentration produced an increase in the resistance to crushing of granules prepared from all 3 binders. The aim of this paper is to look at the binder to solid ratio on mechanical properties of soft detergent granules formed in a high shear granulator. In addition, the role of nature of binder which also affects the granulation output was investigated. This is done by choosing reactive and non-reactive viscous binders for granulation experiments. The granules made at different binder to solid ratio were characterized for mechanical properties like shape, strength, Young's modulus etc.
Table 1 Properties of entire product (less than 2 mm). Binder type
Binder to solid ratio
Granulation time (min)
Moisture (% by wt.)
As-poured bulk density (g/cc)
Reactive
0.25 0.3 0.4 0.25 0.3 0.4
30 12 7 12 11 2
5.3 3.9 4.2 4.8 5.8 7.3
1.16 1.09 1.0 1.13 1.11 0.95
Non-reactive
2. Materials and methods 2.1. Processing of granules Light soda ash (d0.5 = 126 μm) was used as the base powder. Light soda ash is referred to as sodium carbonate in this paper. The particle size distribution of sodium carbonate was determined by laser diffraction using Malvern Mastersizer Scirocco 2000 (model: ADA 2000) and the results are shown in Fig. 1. The reactive binder used was linear alkyl benzene sulfonic acid (96% active) and the non-reactive binder was sodium lauryl ether sulfate paste (70% active). Granulation was carried out in a 40-L capacity plough share mixer (Special Purpose Machines, India). The jacket temperature during granulation was maintained at 85 °C by circulating hot water. The batch size was 6-kg for both the reactive and non-reactive granulation. The plough speed was kept at 285 rpm with the chopper on at 2800 rpm. At the end of the granulation process, the product was layered with Zeolite 4A (Henkel India Ltd.) to ensure free flowing nature of the granules. The granulation product obtained at different binder to solid ratio for both reactive and non-reactive binder was first sieved manually with standard sieve (ASTM Mesh No. 10) and the product going through this sieve (less than 2 mm size) is referred to as entire product. The bulk properties are measured on this entire product. Further sieving of the entire product was done using standard sieves (ASTM Mesh No. 10 and 12) to get the desired size range (1.7 mm to 2.0 mm) for
Fig. 1. Particle size distribution of base powder (sodium carbonate) determined for laser diffraction using Malvern Mastersizer (average of three repetitions).
S. Hibare, K. Acharya / Powder Technology 229 (2012) 137–147 Table 2 Properties of the granules in desired size rage of 1.7 mm to 2 mm. Binder type
Binder to solid ratio
Moisture (% by wt.)
As-poured bulk density (g/cc)
Reactive
0.25 0.3 0.4 0.25 0.3 0.4
3.2 3.3 3.0 4.0 3.5 6.5
1.04 0.94 0.90 1.06 1.03 0.89
Non-reactive
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2.2. Particle size distribution for the product The entire product obtained from all the batches was analyzed for particle size distribution using a Malvern Mastersizer Scirocco 2000 (model: ADA 2000). The volumetric mean diameter, D4,3, was calculated according to 4
D4;3 ¼
∑d ∑d3
ð1Þ
where d is the equivalent diameter of sphere. mechanical strength measurements. This product is referred to as desired cut. Moisture content (% by weight) and as-poured bulk density were measured for both desired cut and the entire product. Moisture content was measured using an IR moisture balance (Sartorius MA 150). The measurement was carried out by placing ~1 g of the product on the aluminum plate and the temperature maintained was 130 °C.
2.3. Granule shape and surface structure analysis The granule circularity was used as a shape parameter for the desired cut (1.7 mm to 2 mm) from all the batches of granulation. The granule circularity was determined from images recorded using a stereo
Fig. 2. Particle size distribution of entire product determined for laser diffraction using Malvern Mastersizer (average of three repetitions) for reactive binder.
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microscope using the same procedure described in Mangwandi et al. [6]. The circularity, Ф, of the granules was determined from [7] 4πA ϕ¼ 2 : S
ð2Þ
The circularity data were fitted to a Gaussian distribution: f ¼ aeð
−0:5ððϕ−ϕ0:5 Þ=bÞ2 Þ
ð3Þ
where f is number density, φ0.5 is the median circularity and b is the standard deviation of the distribution. For surface structure analysis, images of both reactive and nonreactive type of granules were taken using field emission scanning electron microscope (model: Hitachi S4700). A qualitative comparison of the surface structure was made from these images.
2.4. Measurement of the granule strength Single and confined bulk compression techniques have been traditionally used to characterize granular material in terms of yield pressure and failure strength. To estimate the apparent strength, yield strength, and Young's modulus of individual granules, the standard technique of diametric compression between two parallel plates was used by Samimi et al. [8] and Hibare et al. [9] for soft detergent granules. The load–displacement behavior is used to characterize individual granules in terms of their mechanical properties. Yap et al. [10] have used micromanipulation technique for characterizing mechanical properties of small pharmaceutical agglomerates (b200 micrometer). 2.4.1. Single granule diametric compression Single granule compression tests were carried out using texture analyzer (model: Texture Analyzer TA-HDi, Stable Micro Systems Ltd., UK) with 5-kg load cell. About 40 granules of each type and each binder to solid ratio were compressed individually between
Fig. 3. Particle size distribution of entire product determined for laser diffraction using Malvern Mastersizer (average of three repetitions) for non-reactive binder.
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two flat plates. The top plate had a diameter of 25 mm. The punch crosshead speed of 0.01 mm/s was set for the measurement with maximum compression load of 5 kg. On-line load–displacement data were recorded at the rate of 20 points per second. The data were then used to draw load displacement curves for individual granules. Despite the wide variation in the compression behavior in a given type of granules, one can estimate mechanical properties of granules such as yield stress, failure strength and Young's modulus from the load–displacement curves obtained by single granule compression tests. Apparent single granule strength, σos, can be calculated by substituting the value of the peak failure force, Ff, obtained from load–displacement curve, in the following equation. σ 0s ¼
4F f πd2
ð4Þ
where d is the granule size, taken as the average size from the load displacement measurement when the first contact with the granule was made during compression. To minimize the variation observed in load–displacement behavior of a given type of granules, single granule compression experiments were conducted for 40 granules. The apparent granule strength was calculated for all 40 granules of a given type and the average value was estimated and reported to be the apparent strength of that type of granules. 2.4.2. Confined uni-axial bulk compression Models of Heckel [11], Kawakita and Ludde [12] have been widely used in recent literature to characterize bulk compression data. These models have been developed based on the first order rate process relating the bed pressure to the bed porosity. Shapiro [13] had developed a model for bulk compression of metal powders which was later modified by Heckel [11] and the constant in the equation was characterized. The Heckel equation is given by: −
dφ ¼ Kφ dP
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Kawakita and Ludde [12] proposed the following empirical model: P 1 P þ ¼ εe ab a
ð8Þ
where, εe is the extent of volume reduction, which is equivalent to engineering strain. The constant a is related to initial bed porosity and the constant b is related to resistance force. The term 1/b is termed as Kawakita parameter and represents the failure stress. Recent research on physical interpretation of Kawakita and Ludde parameters done by Frenning et al. [16] and Nordström et al. [17] suggests that parameter a approximated the maximal engineering strain of the granular solids and both the Kawakita parameters, a and 1/b, reflected the plasticity of the granules. The linear relationship between P/εe allows the constants to be evaluated. By simple modification of Eq. (8), Denny [18] proposed new form of Kawakita equation as
ln
1 1 ¼ ln þ bð1−φ0 ÞP: φ φ0
ð9Þ
In a special case, when the term b(1 − φo)P is lower than 0.1, i.e., at very low pressures Eq. (9) reduces to same form of original equation of Heckel given by Eq. (6).
ln
1 1 ¼ ln þ ln½1 þ bð1−φ0 ÞP φ φ0
ð10Þ
ð5Þ
where φ is the bed porosity, P is the applied pressure and K is a constant. The reciprocal of the constant K is called the Heckel parameter. Integration of the above equation with the boundary conditions of φo at zero pressure gives the following equation. ln
1 1 ¼ ln þ KP φ φ0
ð6Þ
The bed porosity, φ, is related to relative bed density, D. The usual form of the Heckel equation can be obtained by re-arranging Eq. (6), as follows. ln
1 ¼ A þ KP 1−D
ð7Þ
Heckel [11] showed that, for metal powders, the reciprocal of K is 3 times the yield stress (σy) of individual particles. The Heckel model initially developed for metal powders was later widely used in pharmaceutical applications by Robert and Rowe, Rowe and Roberts [14,15]. Robert and Rowe further investigated the relation between 1/K and mechanical properties of the granules viz. hardness and yield stress. They found that the value of 1/K was in agreement with the yield stress obtained according to indentation tests for a wide range of materials (metals, inorganic halides and polymers). In addition, they suggested that K is inversely related to the ability of the material to deform plastically. Therefore Heckel model can be used for materials that can be compressed by plastic deformation.
Fig. 4. Volumetric mean diameter, D4,3 for (a) reactive binder and (b) non-reactive binder.
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Differentiating above equation, it is clear that Kawakita and Ludde model also follows a first order lumped-parameter model as given by: −
dφ ¼ bð1−φ0 Þφ dP
ð11Þ
Comparing Eq. (5) and Eq. (11), parameters of Heckel and Kawakita and Ludde can be related as follows: K ¼ bð1−φ0 Þ ¼ bD0
of the granules was determined from the data obtained by the confined compression [21]:
Ct ¼
h0 −h0:5 100% h0
ð13Þ
where h0 and h0.5 are the initial bed height and that at a pressure of 0.5 MPa.
ð12Þ 3. Results and discussions
where, D0 is the relative bed density at zero pressure. 2.4.3. Packing coefficient The packing coefficient expresses the ability of particles or granules to rearrange at low compression pressures [19]. Values of b25% indicate a tendency to close pack under load while those in the ranges 25–30% and >30% indicate an intermediate and high resistance to pack. The packing coefficient is related to the flow characteristics with low values having a large flowability index according to Jenike cell measurements [20]. Thus the packing coefficient can be used as a convenient procedure for assessing flow behavior. In the current work the packing coefficient
Moisture content (% by weight) and as-poured bulk density for the entire granulation product and for the product in the desired size range of 1.7 mm to 2 mm are given in Tables 1 and 2 respectively. Aspoured bulk density for the entire granulation product and also the desired size range decreases with increase in binder to solid ratio. This is observed for both reactive and non-reactive granulation products. This may not be only attributed to increase in binder to solid ratio but also to the granulation time. As-poured bulk density as mentioned in Tables 1 and 2 increases with increase in granulation time. It is generally known in wet granulation that increase in granulation time makes the
Fig. 5. Granule circularity distribution for the desired size range of 1.7 mm to 2 mm processed by (a) reactive granulation and (b) non-reactive granulation.
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granules more compact (less porous) and this naturally reflects in increased as-poured bulk density of the granulation product. 3.1. Particle size distribution The particle size distribution for the entire product obtained from all the three batches (at different binder to solid ratio) for reactive and non-reactive binder measured using a Malvern Mastersizer is shown in Fig. 2 and Fig. 3 respectively. The volumetric mean diameter, D4,3, was calculated according to Eq. (1) and the results are shown in Fig. 4. For both reactive and non-reactive granules, the volumetric mean diameter of the product remains similar as a function of binder to solid ratio. This may be due to the granulation time provided during processing as detailed in Table 1. The higher the binder to solid ratio, the lower is the time required to achieve a similar product for both reactive and non-reactive granules. In other words, it is possible to get similar particle size distribution at different binder to solid ratios by manipulating the time for granulation in the operating range studied. The effect of reactive and non-reactive nature of the binder used for this study on the mechanism of wet granulation viz. wetting and nucleation, consolidation and growth, and attrition and breakage may be different. With reactive binder, the spreading on the powder bed should happen uniformly due to less viscous nature of the binder but the reaction of this binder with sodium carbonate may take more time. This will increase the nucleation and wetting stage of granulation mechanism and thereby the total time required for granulation to achieve the end product as shown in Table 1. Also the particle size distribution for reactive binder should be more skewed towards finer side due to better spreading compared to non-reactive binder. This is reflected in the data on particle size distribution as shown in Figs. 2 and 3 and in the volumetric mean diameter as shown in Fig. 4. 3.2. Circularity Fig. 5 shows the circularity distribution profiles for the desired size range (1.7 mm to 2 mm) as a function of binder to solid ratio for both reactive and non-reactive binder granulation. They are well described by a Gaussian function. The data were fitted using a non-linear regression analysis with R 2 values >0.99. The mean granule circularity for reactive and non-reactive granules was calculated from circularity distribution profile and is shown in Fig. 6. The mean circularity remains the same at different binder to solid ratios and this is true for both reactive and non-reactive granules. This again confirms that it is possible to get similar granule circularity at different binder to solid ratios by manipulating the time for granulation in the operating range studied.
Fig. 6. Mean circularity, Ф, from granule circularity distribution for (a) reactive and (b) non-reactive granules.
binder in the granules make them soft and that reflects in apparent strength of the granules. For a given binder to solid ratio, it can be seen that non-reactive granules have higher apparent strength compared to reactive granules. This contradicts with the results reported by Hibare et al. [9] where reactive granules were observed to be stronger than non-reactive granules. However, the difference was in
3.3. Unconfined single granule compression Unconfined single granule compression measurements were carried out on all the types of granules. A representative load displacement data to compare reactive and non-reactive granules are shown in Fig. 7 for the granules processed at a binder to solid ratio of 0.4. A clear difference was observed between the nature of the curves for the two types of granules. Non-reactive granules showed multiple fractures which indicates that the granules break several number of times under compression. This shows the brittle nature of the nonreactive granules. On the other hand, reactive granules showed a gradual increase in load as a function of displacement after the first peak indicating significant plastic flow. Similar observations have been reported in the past by Hibare et al. [9]. The apparent granule strength is calculated using Eq. (4) for both reactive and non-reactive granules processed at different binder to solid ratio. These results are given in Table 3. The apparent strength decreases with increase in the binder to solid ratio for both reactive and non-reactive granules. This suggests that higher levels of liquid
Fig. 7. Representative force–displacement curve for uni-axial compression of single granules made by reactive and non-reactive granulation.
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Table 3 Apparent granule strength calculated from uni-axial single granule compression studies at different binder to solid ratio. Granule type
Binder to solid ratio
Avg. single granule apparent strength, kPa
Standard deviation
Reactive
0.25 0.30 0.40 0.25 0.30 0.40
543 391 257 563 623 390
140 135 77 111 117 83
Non-reactive
the moisture content of the granules studied in current and the previous work. The granules studied as part of this work were at a moisture content of ~ 4 to 6% by weight where as the previous work was done with a moisture content of ~ 12% by weight. With decrease in moisture content from 12% to 4%, the strength of reactive granules does not increase significantly compared to non-reactive granules. 3.4. Confined bulk granule compression The force–displacement data obtained from confined bulk granule compression measurements on desired size range granules are fitted to Heckel model (Eq. (6)) and Kawakita and Ludde model (Eq. (8)). The models have shown a good linear fit for both reactive and non-
reactive granules over the entire data range except the initial part i.e. for pressures above 40 kPa. Fig. 8 shows the fitment for reactive granules at binder to solid ratio of 0.4 and Fig. 9 shows fitment for nonreactive granules at binder to solid ratio of 0.4, analyzed for confined bulk compression behavior. The slopes and intercepts of the best linear regression fits were determined (initial non-linear part of the curve was excluded from the linear fit). The model parameters were estimated and are listed in Tables 4 and 5 for both reactive and non-reactive granules respectively. The Heckel parameter (1/K), which is related to apparent strength of individual granules, was found to be decreasing with increasing binder to solid ratio for both reactive and non-reactive granules. This also confirms the data obtained from un-confined single granule compression. The Heckel parameter (1/K) was observed to be approximately 2.5 to 3 times of the apparent strength of single granules for both reactive and non-reactive system. For a given binder to solid ratio, Heckel parameter (1/K) was found to be higher for non-reactive granules compared to reactive granules. This again confirms the higher strength of non-reactive granules over reactive granules. The Kawakita and Ludde parameter (1/b), which represents failure stress of individual granules, showed a decreasing trend with increase in binder to solid ratio for both reactive and non-reactive granules. For a given binder to solid ratio, parameter (1/b) was estimated to be higher for non-reactive granules over reactive granules again confirming higher strength for non-reactive granules over reactive granules.
Fig. 8. Compression curves for granules made by reactive granulation process at binder to solid ratio of 0.4. (a) Heckel model Eq. (2), (b) Kawakita and Ludde model Eq. (4).
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Fig. 9. Compression curves for granules made by non-reactive granulation process at binder to solid ratio of 0.4. (a) Heckel model Eq. (2), (b) Kawakita and Ludde model Eq. (4).
3.5. Variation of final bed height
than non-reactive under similar load at all the binder to solid ratios studied. This may be due to the lower single granule strength for reactive granules compared to non-reactive granules.
The final bed heights of the granules post confined bulk compression measurements were noted for both reactive and non-reactive granules. The initial aspect ratio for these measurements was 0.9 and the load applied for these measurements was 0.5 MPa. The results are reported in Table 6. It is quite evident from this table that for both reactive and non-reactive granules, the final bed height decreases with increase in the binder to solid ratio used for granulation. In other words, increase in binder to solid ratio makes the granules more compressible. This indicates that increase in binder content decreases granule strength (as shown in Table 3) which results in increased compression of the bed during bulk compression. It can also be seen from Table 6 that reactive granules are more compressible
The effect of binder to solid ratio on packing properties of reactive and non-reactive granules in the size range of 1.7 to 2.0 mm was estimated using Eq. (13) and is plotted in Fig. 10. The packing coefficient Ct increases from 28% to 42% for reactive granules and from 25% to 36% for non-reactive granules with increase in binder to solid ratio. This suggests that the packing ability of the granules deteriorates with increasing binder to solid ratio for both reactive and non-reactive granules. It can also be seen from Fig. 10 that for any
Table 4 Parameters of Heckel, Kawakita and Ludde for reactive granules at an aspect ratio of 0.9.
Table 5 Parameters of Heckel, Kawakita and Ludde for non-reactive granules at an aspect ratio of 0.9.
Model
Heckel
Kawakita and Ludde
Parameters
1/K, kPa A = ln(1/φ0) D0 a 1/b, kPa
Reactive
3.6. Packing coefficient
Model
B/S = 0.25
B/S = 0.30
B/S = 0.40
1429 0.62 0.53 0.49 382
833 0.67 0.51 0.50 130
769 0.68 0.50 0.53 133
Heckel
Kawakita and Ludde
Parameters
1/K, kPa A = ln(1/φ0) D0 a 1/b, kPa
Non-reactive B/S = 0.25
B/S = 0.30
B/S = 0.40
1667 0.64 0.52 0.59 664
1429 0.62 0.54 0.61 606
1000 0.57 0.56 0.57 287
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Table 6 Final bed height post confined bulk compression of reactive and non-reactive granules at an initial aspect ratio 0.9. Binder to solid ratio
Final bed height, mm Reactive granules
Non-reactive granules
0.25 0.30 0.40
16.6 13.5 12.9
17.0 16.4 14.4
given solid to binder ratio, packing coefficient for non-reactive granules is lower than reactive granules suggesting better packing ability of non-reactive granules compared to reactive granules. This may be due to the more plastic nature of reactive granules compared to non-reactive granules which show brittleness under compression as shown in Fig. 7. This may be also explained from the images of surface structure characterization of reactive and non-reactive granules captured using field emission scanning electron microscope at a magnification of 35×. The images are shown in Fig. 11. The surface of nonreactive granules looks smoother compared to reactive granules. This may lead to higher packing ability of non-reactive granules compared to reactive granules.
4. Conclusions The effect of binder to solid ratio on physical and mechanical properties of soft detergent granules processed in a high shear mixer using reactive and non-reactive binder was investigated. The granulation time required to achieve a similar particle size distribution as well as similar circularity of the granules decreases with increase in binder to solid ratio. The mechanical properties like apparent granule strength, yield strength, and packing coefficient were evaluated by conducting uni-axial single granule compression and confined bulk compression measurements. Single granule compression measurements showed that apparent granule strength decreases with increase in binder to solid ratio for both reactive and non-reactive granules and this may be due to more liquid binder content in the granules. The confined bulk compression data were analyzed using known models of Heckel and Kawakita and Ludde. Good linear fitment was obtained for both these models. The Heckel parameter (1/K) was observed to be approximately 2.5 to 3 times the apparent granule strength obtained from single granule compression measurements for both reactive and non-reactive granules. Kawakita and Ludde parameter (1/b) which represents failure strength decreases with increase in binder to solid ratio for both the types of granules. Packing coefficient increases with increasing binder to solid ratio. In addition, packing coefficient for non-reactive granules was lower compared to reactive granules suggesting better packing ability of non-reactive granules. Acknowledgments Authors would like to thank Mr. Manu George for his support in carrying out scanning electron microscopy experiments. References
Fig. 10. Effect of binder to solid ratio on the packing coefficient for reactive and nonreactive granules in the size range of 1.7 to 2.0 mm.
[1] A.M. Bouwman, M.J. Henstra, D. Westerman, J.T. Chung, Z. Zhang, A. Ingram, J.P.K. Seville, H.W. Frijlink, The effect of the amount of binder liquid on the granulation mechanisms and structure of microcrystalline cellulose granules prepared by high shear granulation, International Journal of Pharmaceutics 290 (2005) 129–136. [2] H. Seager, I. Burt, J. Ryder, P.J. Rue, S. Murray, N. Beal, J.K. Warrack, The relationship between granule structure, process of manufacture and the tabletting properties of the granulated product. Part I, International Journal of Pharmaceutical Technology (1979) 36–42. [3] H. Seager, P.J. Rue, I. Burt, J. Ryder, J.K. Warrack, The relationship between granule structure, process of manufacture and the tabletting properties of the granulated product. Part III, International Journal of Pharmaceutical Technology 2 (1981) 41–50. [4] P.J. Rue, H. Seager, J. Ryder, I. Burt, The relationship between granule structure, process of manufacture and the tabletting properties of the granulated product. Part II, International Journal of Pharmaceutical Technology and Product (1980) 2. [5] T. Cutt, J.T. Fell, P.J. Rue, M.S. Spring, Granulation and compaction of a model system. I. Granule properties, International Journal of Pharmaceutics 33 (1986) 81–87.
Fig. 11. Surface characterization of reactive and non-reactive granules processed at 0.4 binder to solid ratio. Images were taken using field emission scanning electron microscope (model: Hitachi S4700).
S. Hibare, K. Acharya / Powder Technology 229 (2012) 137–147 [6] C. Mangwandi, M.J. Adams, M.J. Hounslow, A.D. Salman, Effect of impeller speed on mechanical and dissolution properties of high-shear granules, Chemical Engineering Journal 164 (2010) 305–315. [7] J. Vertommen, P. Rombaut, R. Kinget, Shape and surface smoothness of pellets made in a rotary processor, International Journal of Pharmaceutics 146 (1997) 21–29. [8] A. Samimi, A. Hassanpour, M. Ghadiri, Single and bulk compressions of soft granules: experimental study and DEM evaluation, Chemical Engineering Science 60 (2005) 3993–4004. [9] S. Hibare, R. Sivanathan, S. Nadakatti, Behaviour of soft granules under compression: effect of reactive and non-reactive nature of the binder on granule properties, Powder Technology 210 (2011) 241–247. [10] S.F. Yap, M.J. Adams, J.P.K. Seville, Z. Zhang, Single and bulk compression of pharmaceutical excipients: evaluation of mechanical properties, Powder Technology 185 (2008) 1–10. [11] R.W. Heckel, Density–pressure relationship in powder compaction, Transaction of the Metallurgical Society of AIME 221 (1961) 671–675. [12] K. Kawakita, K.H. Lüdde, Some considerations on powder compression equations, Powder Technology 4 (1970) 61–68. [13] I. Shapiro, Ph.D. Thesis, University of Minnesota, USA (1944). [14] R.J. Roberts, C. Rowe, Mechanical properties of powders, in: G. Ganderton, T.M. Jones, J.W. McGinity (Eds.), Advance in Pharmaceutical Sciences, 7, 1995, pp. 1–62.
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[15] C. Rowe, R.J. Roberts, Mechanical properties, in: G. Ganderton, C. Nystrom (Eds.), Drugs and Pharmaceutical Sciences, Marcel Dekker, New York, 1996, pp. 283–321. [16] G. Frenning, J. Nordström, G. Alderborn, Effective Kawakita parameters for binary mixtures, Powder Technology 189 (2009) 270–275. [17] J. Nordström, K. Welch, G. Frenning, G. Alderborn, On the physical interpretation of the Kawakita and Adams parameters derived from confined compression of granular solids, Powder Technology 182 (2008) 424–435. [18] P.J. Denny, Compaction equations: a comparison of the Heckel and Kawakita equations, Powder Technology 127 (2002) 162–172. [19] F. Chantraine, M. Viana, S. Cazalbou, N. Brielles, O. Mondain-Monval, C. Pouget, P. Branlard, G. Rubinstenn, D. Chulia, From compressibility to structural investigation of sodium dodecyl sulphate—Part 2: a singular behavior under pressure, Powder Technology 177 (2007) 41–50. [20] M. Viana, C.M.D. Gabaude-Renou, C. Pontier, D. Chulia, The packing coefficient: a suitable parameter to assess the flow properties of powders, Kona 19 (2001) 89–93. [21] C.M.D. Gabaude, J.C. Gautier, P. Saudemon, D. Chulia, Validation of a new pertinent packing coefficient to estimate flow properties of pharmaceutical powders at a very early development stage, by comparison with mercury intrusion and classical flowability methods, Journal of Materials Science 36 (2001) 1763–1773.
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Effect of binder to solid ratio on mechanical properties of granules processed using reactive and non-reactive binder Sujitkumar Hibare ⁎, Koushik Acharya Unilever R&D Bangalore, Hindustan Unilever Ltd., 64, Main Road, Whitefield, Bangalore-560066, India
a r t i c l e
i n f o
Article history: Received 7 December 2011 Received in revised form 12 April 2012 Accepted 8 June 2012 Available online 15 June 2012 Keywords: Binder to solid ratio Granule compression Granule strength Reactive binder Packing coefficient
a b s t r a c t In wet granulation of powders in high shear granulators, the properties of granules formed can be altered by controlling a number of parameters. One of the crucial parameters responsible for granule properties is the quantity of the binder used for granulation. This can be expressed as binder to solid ratio (B/S) or liquid to solid ratio (L/S). Researchers have reported in the past the effect of liquid to solid ratio on granulation mechanism and growth, granule structure and consequently on the deformation behavior of the granules. Some researchers have also reported the distribution of the binder and the size distribution of the granules formed as a function of the concentration of the binder used in the process. However, there is limited work reported in the literature on the effect of binder to solid ratio on the physical and mechanical properties of granules. The mechanical properties of the granules can determine the behavior of granules at various stages such as usage, handling, transportation and storage under compression. The work reported in this paper investigates the effect of binder to solid ratio (B/S) on the mechanical properties of soft detergent granules made via wet granulation in a high shear mixer. The effect of binder to solid ratio in wet granulation can vary depending upon the nature of the binder used. Hence two different types of binders were used viz. reactive (reacts with base powder) and non-reactive (acts only as a binder) to granulate sodium carbonate as primary particles. The mechanical properties were studied by conducting single and bulk compression measurements. Known models of Heckel and Kawakita and Ludde were also used to predict mechanical properties of the granules. The granulation time required to achieve similar physical properties (particle size distribution and circularity) of the granules decreased with increase in binder to solid ratio. Single granule compression measurements showed that apparent granule strength decreases with increase in binder to solid ratio for both reactive and non-reactive granules which may be due to more liquid binder content in the granules. The Heckel parameter was observed to be approximately 2.5 to 3 times the apparent granule strength obtained from single granule compression measurements for both reactive and non-reactive granules. Packing coefficient increased with increase in binder to solid ratio indicating higher compressibility behavior under similar load for both reactive and non-reactive granules. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The process of wet granulation talks about three mechanisms viz. nucleation, consolidation or coalescence, and attrition or breakage. Coalescence and breakage happen simultaneously and the equilibrium between these two mechanisms determines the outcome of the granulation process. The important components in a high shear granulator are a drum, an impeller and a vertically mounted chopper. The mechanical energy required for mixing the base powder homogeneously is provided by the impeller and the chopper is designed to help in breaking the granules formed. The chopper operates comparatively at a higher speed than the impeller. The effect of the impeller and chopper speed on granule
⁎ Corresponding author. Tel.: + 91 80 39831123; fax: + 91 80 28453086. E-mail address: [email protected] (S. Hibare). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.06.020
properties, therefore, depends on the response of the granules to the energy provided by both impeller and the chopper. A.M. Bouwman et al. [1] have investigated the role of deformability, which depends on the amount of binder liquid and the granule porosity, on the subsequent microcrystalline cellulose (MCC) granulation mechanisms and how this determines the final structure of granules produced in the high shear granulator. X-ray micro tomography was used to visualize and quantify the densification process of microcrystalline cellulose granules, comparing granules produced with different granulation times, and different amounts of liquid binder. The granules were also compared for differences in the porosity at different sites in the granule. Scanning electron microscopy (SEM) was used to visualize the external appearance of the granules. The deformability of the visualized granules was measured using micromanipulation. The authors recommended that the binder liquid should not only be regarded as a binder but also its effects on deformability of the granules should be taken into account, as the deformability determines the granulation mechanism occurring
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during the latter phases (consolidation or growth, and breakage or attrition) of the granulation process. Liquid phase is necessary for binding the powder particles and making the wet mass more deformable. With a low amount of binder liquid, densification of the granules occurs and the final phase of the granulation process results in equilibrium between attrition and growth. Since the granules are no longer broken during this phase, spheronization of the material may occur. However, when excess binder liquid is used, the deformability of MCC granules increases, which makes them susceptible to breakage upon continued granulation. Continued breakage will reduce the extent of densification of the granule core and the granules remain weak during the complete process. Even during the last (equilibrium) phase of the process, continuous breakage occurs. This breakage is now balanced by coalescence of the fragments with each other or with other granules. This finally results in weak, irregularly shaped granules. The function of the binding agent used in granulation is primarily to maintain the integrity of the granules prior to compression. However, it has been shown by Seager et al., 1979 [2], 1981 [3] and Rue et al., 1980 [4] that the distribution of the binding agent within the granule not only controls the intra-granular particulate adhesion, but also the intergranular bonding achieved during tabletting and hence the resultant tablet properties, such as tensile strength, disintegration and dissolution. Teresa Cutt et al. [5] have studied the friability of granules at different binder levels. An increase in binder concentration from 1.5 to 3% w/w decreased the friability of both the HPMC and Byco granules. The PVP granules were considerably less friable than the other granules at the lowest binder concentration, and increasing the concentration had little effect on their friability. An increase in binder concentration produced an increase in the resistance to crushing of granules prepared from all 3 binders. The aim of this paper is to look at the binder to solid ratio on mechanical properties of soft detergent granules formed in a high shear granulator. In addition, the role of nature of binder which also affects the granulation output was investigated. This is done by choosing reactive and non-reactive viscous binders for granulation experiments. The granules made at different binder to solid ratio were characterized for mechanical properties like shape, strength, Young's modulus etc.
Table 1 Properties of entire product (less than 2 mm). Binder type
Binder to solid ratio
Granulation time (min)
Moisture (% by wt.)
As-poured bulk density (g/cc)
Reactive
0.25 0.3 0.4 0.25 0.3 0.4
30 12 7 12 11 2
5.3 3.9 4.2 4.8 5.8 7.3
1.16 1.09 1.0 1.13 1.11 0.95
Non-reactive
2. Materials and methods 2.1. Processing of granules Light soda ash (d0.5 = 126 μm) was used as the base powder. Light soda ash is referred to as sodium carbonate in this paper. The particle size distribution of sodium carbonate was determined by laser diffraction using Malvern Mastersizer Scirocco 2000 (model: ADA 2000) and the results are shown in Fig. 1. The reactive binder used was linear alkyl benzene sulfonic acid (96% active) and the non-reactive binder was sodium lauryl ether sulfate paste (70% active). Granulation was carried out in a 40-L capacity plough share mixer (Special Purpose Machines, India). The jacket temperature during granulation was maintained at 85 °C by circulating hot water. The batch size was 6-kg for both the reactive and non-reactive granulation. The plough speed was kept at 285 rpm with the chopper on at 2800 rpm. At the end of the granulation process, the product was layered with Zeolite 4A (Henkel India Ltd.) to ensure free flowing nature of the granules. The granulation product obtained at different binder to solid ratio for both reactive and non-reactive binder was first sieved manually with standard sieve (ASTM Mesh No. 10) and the product going through this sieve (less than 2 mm size) is referred to as entire product. The bulk properties are measured on this entire product. Further sieving of the entire product was done using standard sieves (ASTM Mesh No. 10 and 12) to get the desired size range (1.7 mm to 2.0 mm) for
Fig. 1. Particle size distribution of base powder (sodium carbonate) determined for laser diffraction using Malvern Mastersizer (average of three repetitions).
S. Hibare, K. Acharya / Powder Technology 229 (2012) 137–147 Table 2 Properties of the granules in desired size rage of 1.7 mm to 2 mm. Binder type
Binder to solid ratio
Moisture (% by wt.)
As-poured bulk density (g/cc)
Reactive
0.25 0.3 0.4 0.25 0.3 0.4
3.2 3.3 3.0 4.0 3.5 6.5
1.04 0.94 0.90 1.06 1.03 0.89
Non-reactive
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2.2. Particle size distribution for the product The entire product obtained from all the batches was analyzed for particle size distribution using a Malvern Mastersizer Scirocco 2000 (model: ADA 2000). The volumetric mean diameter, D4,3, was calculated according to 4
D4;3 ¼
∑d ∑d3
ð1Þ
where d is the equivalent diameter of sphere. mechanical strength measurements. This product is referred to as desired cut. Moisture content (% by weight) and as-poured bulk density were measured for both desired cut and the entire product. Moisture content was measured using an IR moisture balance (Sartorius MA 150). The measurement was carried out by placing ~1 g of the product on the aluminum plate and the temperature maintained was 130 °C.
2.3. Granule shape and surface structure analysis The granule circularity was used as a shape parameter for the desired cut (1.7 mm to 2 mm) from all the batches of granulation. The granule circularity was determined from images recorded using a stereo
Fig. 2. Particle size distribution of entire product determined for laser diffraction using Malvern Mastersizer (average of three repetitions) for reactive binder.
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microscope using the same procedure described in Mangwandi et al. [6]. The circularity, Ф, of the granules was determined from [7] 4πA ϕ¼ 2 : S
ð2Þ
The circularity data were fitted to a Gaussian distribution: f ¼ aeð
−0:5ððϕ−ϕ0:5 Þ=bÞ2 Þ
ð3Þ
where f is number density, φ0.5 is the median circularity and b is the standard deviation of the distribution. For surface structure analysis, images of both reactive and nonreactive type of granules were taken using field emission scanning electron microscope (model: Hitachi S4700). A qualitative comparison of the surface structure was made from these images.
2.4. Measurement of the granule strength Single and confined bulk compression techniques have been traditionally used to characterize granular material in terms of yield pressure and failure strength. To estimate the apparent strength, yield strength, and Young's modulus of individual granules, the standard technique of diametric compression between two parallel plates was used by Samimi et al. [8] and Hibare et al. [9] for soft detergent granules. The load–displacement behavior is used to characterize individual granules in terms of their mechanical properties. Yap et al. [10] have used micromanipulation technique for characterizing mechanical properties of small pharmaceutical agglomerates (b200 micrometer). 2.4.1. Single granule diametric compression Single granule compression tests were carried out using texture analyzer (model: Texture Analyzer TA-HDi, Stable Micro Systems Ltd., UK) with 5-kg load cell. About 40 granules of each type and each binder to solid ratio were compressed individually between
Fig. 3. Particle size distribution of entire product determined for laser diffraction using Malvern Mastersizer (average of three repetitions) for non-reactive binder.
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two flat plates. The top plate had a diameter of 25 mm. The punch crosshead speed of 0.01 mm/s was set for the measurement with maximum compression load of 5 kg. On-line load–displacement data were recorded at the rate of 20 points per second. The data were then used to draw load displacement curves for individual granules. Despite the wide variation in the compression behavior in a given type of granules, one can estimate mechanical properties of granules such as yield stress, failure strength and Young's modulus from the load–displacement curves obtained by single granule compression tests. Apparent single granule strength, σos, can be calculated by substituting the value of the peak failure force, Ff, obtained from load–displacement curve, in the following equation. σ 0s ¼
4F f πd2
ð4Þ
where d is the granule size, taken as the average size from the load displacement measurement when the first contact with the granule was made during compression. To minimize the variation observed in load–displacement behavior of a given type of granules, single granule compression experiments were conducted for 40 granules. The apparent granule strength was calculated for all 40 granules of a given type and the average value was estimated and reported to be the apparent strength of that type of granules. 2.4.2. Confined uni-axial bulk compression Models of Heckel [11], Kawakita and Ludde [12] have been widely used in recent literature to characterize bulk compression data. These models have been developed based on the first order rate process relating the bed pressure to the bed porosity. Shapiro [13] had developed a model for bulk compression of metal powders which was later modified by Heckel [11] and the constant in the equation was characterized. The Heckel equation is given by: −
dφ ¼ Kφ dP
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Kawakita and Ludde [12] proposed the following empirical model: P 1 P þ ¼ εe ab a
ð8Þ
where, εe is the extent of volume reduction, which is equivalent to engineering strain. The constant a is related to initial bed porosity and the constant b is related to resistance force. The term 1/b is termed as Kawakita parameter and represents the failure stress. Recent research on physical interpretation of Kawakita and Ludde parameters done by Frenning et al. [16] and Nordström et al. [17] suggests that parameter a approximated the maximal engineering strain of the granular solids and both the Kawakita parameters, a and 1/b, reflected the plasticity of the granules. The linear relationship between P/εe allows the constants to be evaluated. By simple modification of Eq. (8), Denny [18] proposed new form of Kawakita equation as
ln
1 1 ¼ ln þ bð1−φ0 ÞP: φ φ0
ð9Þ
In a special case, when the term b(1 − φo)P is lower than 0.1, i.e., at very low pressures Eq. (9) reduces to same form of original equation of Heckel given by Eq. (6).
ln
1 1 ¼ ln þ ln½1 þ bð1−φ0 ÞP φ φ0
ð10Þ
ð5Þ
where φ is the bed porosity, P is the applied pressure and K is a constant. The reciprocal of the constant K is called the Heckel parameter. Integration of the above equation with the boundary conditions of φo at zero pressure gives the following equation. ln
1 1 ¼ ln þ KP φ φ0
ð6Þ
The bed porosity, φ, is related to relative bed density, D. The usual form of the Heckel equation can be obtained by re-arranging Eq. (6), as follows. ln
1 ¼ A þ KP 1−D
ð7Þ
Heckel [11] showed that, for metal powders, the reciprocal of K is 3 times the yield stress (σy) of individual particles. The Heckel model initially developed for metal powders was later widely used in pharmaceutical applications by Robert and Rowe, Rowe and Roberts [14,15]. Robert and Rowe further investigated the relation between 1/K and mechanical properties of the granules viz. hardness and yield stress. They found that the value of 1/K was in agreement with the yield stress obtained according to indentation tests for a wide range of materials (metals, inorganic halides and polymers). In addition, they suggested that K is inversely related to the ability of the material to deform plastically. Therefore Heckel model can be used for materials that can be compressed by plastic deformation.
Fig. 4. Volumetric mean diameter, D4,3 for (a) reactive binder and (b) non-reactive binder.
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Differentiating above equation, it is clear that Kawakita and Ludde model also follows a first order lumped-parameter model as given by: −
dφ ¼ bð1−φ0 Þφ dP
ð11Þ
Comparing Eq. (5) and Eq. (11), parameters of Heckel and Kawakita and Ludde can be related as follows: K ¼ bð1−φ0 Þ ¼ bD0
of the granules was determined from the data obtained by the confined compression [21]:
Ct ¼
h0 −h0:5 100% h0
ð13Þ
where h0 and h0.5 are the initial bed height and that at a pressure of 0.5 MPa.
ð12Þ 3. Results and discussions
where, D0 is the relative bed density at zero pressure. 2.4.3. Packing coefficient The packing coefficient expresses the ability of particles or granules to rearrange at low compression pressures [19]. Values of b25% indicate a tendency to close pack under load while those in the ranges 25–30% and >30% indicate an intermediate and high resistance to pack. The packing coefficient is related to the flow characteristics with low values having a large flowability index according to Jenike cell measurements [20]. Thus the packing coefficient can be used as a convenient procedure for assessing flow behavior. In the current work the packing coefficient
Moisture content (% by weight) and as-poured bulk density for the entire granulation product and for the product in the desired size range of 1.7 mm to 2 mm are given in Tables 1 and 2 respectively. Aspoured bulk density for the entire granulation product and also the desired size range decreases with increase in binder to solid ratio. This is observed for both reactive and non-reactive granulation products. This may not be only attributed to increase in binder to solid ratio but also to the granulation time. As-poured bulk density as mentioned in Tables 1 and 2 increases with increase in granulation time. It is generally known in wet granulation that increase in granulation time makes the
Fig. 5. Granule circularity distribution for the desired size range of 1.7 mm to 2 mm processed by (a) reactive granulation and (b) non-reactive granulation.
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granules more compact (less porous) and this naturally reflects in increased as-poured bulk density of the granulation product. 3.1. Particle size distribution The particle size distribution for the entire product obtained from all the three batches (at different binder to solid ratio) for reactive and non-reactive binder measured using a Malvern Mastersizer is shown in Fig. 2 and Fig. 3 respectively. The volumetric mean diameter, D4,3, was calculated according to Eq. (1) and the results are shown in Fig. 4. For both reactive and non-reactive granules, the volumetric mean diameter of the product remains similar as a function of binder to solid ratio. This may be due to the granulation time provided during processing as detailed in Table 1. The higher the binder to solid ratio, the lower is the time required to achieve a similar product for both reactive and non-reactive granules. In other words, it is possible to get similar particle size distribution at different binder to solid ratios by manipulating the time for granulation in the operating range studied. The effect of reactive and non-reactive nature of the binder used for this study on the mechanism of wet granulation viz. wetting and nucleation, consolidation and growth, and attrition and breakage may be different. With reactive binder, the spreading on the powder bed should happen uniformly due to less viscous nature of the binder but the reaction of this binder with sodium carbonate may take more time. This will increase the nucleation and wetting stage of granulation mechanism and thereby the total time required for granulation to achieve the end product as shown in Table 1. Also the particle size distribution for reactive binder should be more skewed towards finer side due to better spreading compared to non-reactive binder. This is reflected in the data on particle size distribution as shown in Figs. 2 and 3 and in the volumetric mean diameter as shown in Fig. 4. 3.2. Circularity Fig. 5 shows the circularity distribution profiles for the desired size range (1.7 mm to 2 mm) as a function of binder to solid ratio for both reactive and non-reactive binder granulation. They are well described by a Gaussian function. The data were fitted using a non-linear regression analysis with R 2 values >0.99. The mean granule circularity for reactive and non-reactive granules was calculated from circularity distribution profile and is shown in Fig. 6. The mean circularity remains the same at different binder to solid ratios and this is true for both reactive and non-reactive granules. This again confirms that it is possible to get similar granule circularity at different binder to solid ratios by manipulating the time for granulation in the operating range studied.
Fig. 6. Mean circularity, Ф, from granule circularity distribution for (a) reactive and (b) non-reactive granules.
binder in the granules make them soft and that reflects in apparent strength of the granules. For a given binder to solid ratio, it can be seen that non-reactive granules have higher apparent strength compared to reactive granules. This contradicts with the results reported by Hibare et al. [9] where reactive granules were observed to be stronger than non-reactive granules. However, the difference was in
3.3. Unconfined single granule compression Unconfined single granule compression measurements were carried out on all the types of granules. A representative load displacement data to compare reactive and non-reactive granules are shown in Fig. 7 for the granules processed at a binder to solid ratio of 0.4. A clear difference was observed between the nature of the curves for the two types of granules. Non-reactive granules showed multiple fractures which indicates that the granules break several number of times under compression. This shows the brittle nature of the nonreactive granules. On the other hand, reactive granules showed a gradual increase in load as a function of displacement after the first peak indicating significant plastic flow. Similar observations have been reported in the past by Hibare et al. [9]. The apparent granule strength is calculated using Eq. (4) for both reactive and non-reactive granules processed at different binder to solid ratio. These results are given in Table 3. The apparent strength decreases with increase in the binder to solid ratio for both reactive and non-reactive granules. This suggests that higher levels of liquid
Fig. 7. Representative force–displacement curve for uni-axial compression of single granules made by reactive and non-reactive granulation.
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Table 3 Apparent granule strength calculated from uni-axial single granule compression studies at different binder to solid ratio. Granule type
Binder to solid ratio
Avg. single granule apparent strength, kPa
Standard deviation
Reactive
0.25 0.30 0.40 0.25 0.30 0.40
543 391 257 563 623 390
140 135 77 111 117 83
Non-reactive
the moisture content of the granules studied in current and the previous work. The granules studied as part of this work were at a moisture content of ~ 4 to 6% by weight where as the previous work was done with a moisture content of ~ 12% by weight. With decrease in moisture content from 12% to 4%, the strength of reactive granules does not increase significantly compared to non-reactive granules. 3.4. Confined bulk granule compression The force–displacement data obtained from confined bulk granule compression measurements on desired size range granules are fitted to Heckel model (Eq. (6)) and Kawakita and Ludde model (Eq. (8)). The models have shown a good linear fit for both reactive and non-
reactive granules over the entire data range except the initial part i.e. for pressures above 40 kPa. Fig. 8 shows the fitment for reactive granules at binder to solid ratio of 0.4 and Fig. 9 shows fitment for nonreactive granules at binder to solid ratio of 0.4, analyzed for confined bulk compression behavior. The slopes and intercepts of the best linear regression fits were determined (initial non-linear part of the curve was excluded from the linear fit). The model parameters were estimated and are listed in Tables 4 and 5 for both reactive and non-reactive granules respectively. The Heckel parameter (1/K), which is related to apparent strength of individual granules, was found to be decreasing with increasing binder to solid ratio for both reactive and non-reactive granules. This also confirms the data obtained from un-confined single granule compression. The Heckel parameter (1/K) was observed to be approximately 2.5 to 3 times of the apparent strength of single granules for both reactive and non-reactive system. For a given binder to solid ratio, Heckel parameter (1/K) was found to be higher for non-reactive granules compared to reactive granules. This again confirms the higher strength of non-reactive granules over reactive granules. The Kawakita and Ludde parameter (1/b), which represents failure stress of individual granules, showed a decreasing trend with increase in binder to solid ratio for both reactive and non-reactive granules. For a given binder to solid ratio, parameter (1/b) was estimated to be higher for non-reactive granules over reactive granules again confirming higher strength for non-reactive granules over reactive granules.
Fig. 8. Compression curves for granules made by reactive granulation process at binder to solid ratio of 0.4. (a) Heckel model Eq. (2), (b) Kawakita and Ludde model Eq. (4).
S. Hibare, K. Acharya / Powder Technology 229 (2012) 137–147
145
Fig. 9. Compression curves for granules made by non-reactive granulation process at binder to solid ratio of 0.4. (a) Heckel model Eq. (2), (b) Kawakita and Ludde model Eq. (4).
3.5. Variation of final bed height
than non-reactive under similar load at all the binder to solid ratios studied. This may be due to the lower single granule strength for reactive granules compared to non-reactive granules.
The final bed heights of the granules post confined bulk compression measurements were noted for both reactive and non-reactive granules. The initial aspect ratio for these measurements was 0.9 and the load applied for these measurements was 0.5 MPa. The results are reported in Table 6. It is quite evident from this table that for both reactive and non-reactive granules, the final bed height decreases with increase in the binder to solid ratio used for granulation. In other words, increase in binder to solid ratio makes the granules more compressible. This indicates that increase in binder content decreases granule strength (as shown in Table 3) which results in increased compression of the bed during bulk compression. It can also be seen from Table 6 that reactive granules are more compressible
The effect of binder to solid ratio on packing properties of reactive and non-reactive granules in the size range of 1.7 to 2.0 mm was estimated using Eq. (13) and is plotted in Fig. 10. The packing coefficient Ct increases from 28% to 42% for reactive granules and from 25% to 36% for non-reactive granules with increase in binder to solid ratio. This suggests that the packing ability of the granules deteriorates with increasing binder to solid ratio for both reactive and non-reactive granules. It can also be seen from Fig. 10 that for any
Table 4 Parameters of Heckel, Kawakita and Ludde for reactive granules at an aspect ratio of 0.9.
Table 5 Parameters of Heckel, Kawakita and Ludde for non-reactive granules at an aspect ratio of 0.9.
Model
Heckel
Kawakita and Ludde
Parameters
1/K, kPa A = ln(1/φ0) D0 a 1/b, kPa
Reactive
3.6. Packing coefficient
Model
B/S = 0.25
B/S = 0.30
B/S = 0.40
1429 0.62 0.53 0.49 382
833 0.67 0.51 0.50 130
769 0.68 0.50 0.53 133
Heckel
Kawakita and Ludde
Parameters
1/K, kPa A = ln(1/φ0) D0 a 1/b, kPa
Non-reactive B/S = 0.25
B/S = 0.30
B/S = 0.40
1667 0.64 0.52 0.59 664
1429 0.62 0.54 0.61 606
1000 0.57 0.56 0.57 287
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Table 6 Final bed height post confined bulk compression of reactive and non-reactive granules at an initial aspect ratio 0.9. Binder to solid ratio
Final bed height, mm Reactive granules
Non-reactive granules
0.25 0.30 0.40
16.6 13.5 12.9
17.0 16.4 14.4
given solid to binder ratio, packing coefficient for non-reactive granules is lower than reactive granules suggesting better packing ability of non-reactive granules compared to reactive granules. This may be due to the more plastic nature of reactive granules compared to non-reactive granules which show brittleness under compression as shown in Fig. 7. This may be also explained from the images of surface structure characterization of reactive and non-reactive granules captured using field emission scanning electron microscope at a magnification of 35×. The images are shown in Fig. 11. The surface of nonreactive granules looks smoother compared to reactive granules. This may lead to higher packing ability of non-reactive granules compared to reactive granules.
4. Conclusions The effect of binder to solid ratio on physical and mechanical properties of soft detergent granules processed in a high shear mixer using reactive and non-reactive binder was investigated. The granulation time required to achieve a similar particle size distribution as well as similar circularity of the granules decreases with increase in binder to solid ratio. The mechanical properties like apparent granule strength, yield strength, and packing coefficient were evaluated by conducting uni-axial single granule compression and confined bulk compression measurements. Single granule compression measurements showed that apparent granule strength decreases with increase in binder to solid ratio for both reactive and non-reactive granules and this may be due to more liquid binder content in the granules. The confined bulk compression data were analyzed using known models of Heckel and Kawakita and Ludde. Good linear fitment was obtained for both these models. The Heckel parameter (1/K) was observed to be approximately 2.5 to 3 times the apparent granule strength obtained from single granule compression measurements for both reactive and non-reactive granules. Kawakita and Ludde parameter (1/b) which represents failure strength decreases with increase in binder to solid ratio for both the types of granules. Packing coefficient increases with increasing binder to solid ratio. In addition, packing coefficient for non-reactive granules was lower compared to reactive granules suggesting better packing ability of non-reactive granules. Acknowledgments Authors would like to thank Mr. Manu George for his support in carrying out scanning electron microscopy experiments. References
Fig. 10. Effect of binder to solid ratio on the packing coefficient for reactive and nonreactive granules in the size range of 1.7 to 2.0 mm.
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