Will HPMC empty capsules deform or disintegrate in high temperature and high humidity environments?

1. Moisture absorption and physical deformation of HPMC capsules

hygroscopic mechanism
HPMC molecular chains contain hydrophilic hydroxyl groups, which absorb moisture when humidity exceeds 60%, resulting in an increase in the moisture content of the capsule shell. Experiments have shown that storing HPMC capsules in a 25 ℃/75% RH environment for 3 months can increase the moisture content from 5% to 8% -12%, which may cause the following problems:
Softening adhesion: The surface of the capsule becomes sticky, especially when stacked and stored, and is prone to sticking together into blocks.
Slight expansion: The diameter expansion rate is about 1.5% -2.0%, which may affect the accuracy of automatic filling equipment.

Deformation risk under high temperature
The glass transition temperature (Tg) of HPMC is approximately 170-180 ℃, but in practical applications, temperatures above 40 ℃ can accelerate water migration. For example, under the conditions of 40 ℃/75% RH, the elastic modulus of the capsule shell decreases by 30%, and the compressive strength decreases, which may lead to mechanical damage during transportation.

2. Disintegration performance: the combined effect of humidity and temperature

High humidity shortens disintegration time
The disintegration time of HPMC capsules in a dry environment (25 ℃/40% RH) is about 15-30 minutes (in accordance with USP<701>standards), but the disintegration time can be shortened to 8-12 minutes in a 75% RH environment. If the content is hygroscopic drugs (such as traditional Chinese medicine extracts), the capsule shell may absorb water too quickly, leading to drug clumping or degradation.

High temperature accelerated swelling
At high temperatures (40 ℃), the movement of HPMC molecular chains intensifies, and the disintegration time is further shortened to 5-8 minutes. For sustained-release formulations, this may disrupt the drug release curve and stability needs to be improved by adjusting the capsule formulation (such as adding hydrophobic agents).

3. Process improvement: Enhance weather resistance

Formula optimization
Plasticizer: Adding glycerol (≤ 5%) or polyethylene glycol (PEG 400) can reduce moisture absorption, but excessive amounts can soften the capsule.
Crosslinking agent: Some manufacturers use citrate crosslinking technology to stabilize the moisture content of the capsule shell at 6% -7% at 80% RH.

Coating Technology
Spraying hydroxypropyl starch or ethyl cellulose film on the outside of HPMC capsules can block more than 80% of water vapor penetration and reduce the deformation rate under high temperature and high humidity to<0.5%.