sound band morphology and thickness, nominal air bubble
formation, low leak rates and no band /capsule separation.
The high percentage of ethanol in banding solution 1 allowed the solution to dry quickly, form smooth band edges
and eliminate air bubble formation. The lack of air bubbles
in HPMC capsules banded with solution 1 likely resulted
from increases in viscosity (to several thousand cps) caused
by rapid evaporation of ethanol from the banding solution.
Gelatin-based banding solution 2 (22% gelatin/ 78% H2O)
exhibited a low air bubble formation rate but bound poorly
with HPMC hard shell capsules. Not surprisingly, capsules
banded with solution 2 exhibited 100% band/capsule/
separation rates and had the highest leak rates among all
banding solutions tested. Interestingly, the relatively high
gel strength of solution 2 (afforded by the gelatin) likely
prevented air bubble formation.
Banding solution 3 (22% HPMC/ 78% H2O) exhibited low
band/capsule separation rates and moderate leak rates but
a 70% air bubble formation rate (Table 1, Fig. 1D). The high
bubble formation rate observed with capsules banded with
solution 3 likely resulted from an inability of the solution to
prevent air venting during drying. Bubbles are known to increase capsule leak rates and also cause cosmetic problems.
The addition of gelling agents and gelling aids such as
~1% carrageenan, locust bean gum, and KCl to solution
4 (11% HPMC/87% H2O) completely eliminated air bubble
formation (Fig 1 D), band/capsule separation, and drastically reduced capsule leak rates. These improvements
were likely caused by the gelling agents that allowed
uniform band thickness at cap/body junctions, which, in
turn, helped to minimize band shrinkage and lower leak
rates. Generally speaking, it is difficult to avoid band
shrinkage with HPMC capsules. This is because, after
banding, water rapidly migrates from the banding solution into the capsule shell; softening it and sometimes
causes band shrinkage during drying. The addition of
gelling agents to solution 4 likely retarded the migration of water from the banding solution to the capsule
shell and was responsible for lower band shrinkage rates
observed with solution 4 as compared with solution 3
The results of this study suggest that HPMC-containing
water based solutions with gelling agents can be used as
viable alternatives to ethanol-containing solutions to band
Fig.1.Microscopic examination of banded HPMC capsules A through D. (A) 16% HPMC/ 31% H2O/ 53% ethanol; (B) 22% gelatin/78% H2O ; (C)
22% HPMC/78% H2O; (D) 11% HPMC/ 87% H2O/ ~1% carrageenan, locust bean gum, and KCl.