Why Capsules Become Brittle, Soft or Sticky: Humidité, Stockage & Packaging Causes

A batch of capsules can look perfect when it enters the warehouse and still cause trouble a few weeks later. On the capsule filling line, shells may crack during separation. Another batch may feel soft, deform in the hopper, ou rester ensemble. Production may blame the remplisseuse de capsules, QA may suspect the shell supplier, and procurement may wonder whether the packaging material is inadequate.

Very often, the real issue is moisture balance.

Hard capsule shells exchange water with the surrounding air, le fill formulation, and the headspace inside their package. For gelatin shells in particular, losing too much moisture reduces flexibility and increases capsule brittleness; gaining too much moisture can cause softening, swelling, et sticky capsules. HPMC and pullulan behave differently, but they are not completely independent of humidity either. USP recognizes gelatin, hypromellose, and pullulan hard shells as water-containing polymer systems, while USP guidance separately treats water content, water activity, and packaging moisture transmission as important pharmaceutical quality notions.

This article focuses on hard two-piece capsules that become soft, not the separate dosage form commonly called a capsule molle.

How Moisture Balance Affects Capsule Quality

Quand capsules become brittle, start by looking for moisture loss. Hard gelatin capsules commonly contain about 13–16% water under suitable conditions. Research indicates that when their water content falls below the normal operating range—because of low relative humidity, very dry or hygroscopic fills, or moisture migration—the shell progressively loses flexibility and can fracture more easily. Inversement, excessive water uptake can plasticize gelatin, making shells softer and potentially sticky.

For production and QA teams, four points matter most:

1. Low RH generally increases brittleness risk; high RH generally increases softening and sticking risk. Gelatin is especially sensitive to this shift.
2. The fill can be the cause even when room humidity looks correct. A hygroscopic powder can draw moisture from a gelatin shell until the shell becomes brittle.
3. Packaging should manage moisture equilibrium, not simply make the package “as dry as possible.” USP evaluates container performance through moisture-vapor transmission, alors que emballage pharmaceutique research shows that barrier level strongly influences internal RH and product stability.
4. Do not immediately change capsule filling machine vacuum or closing pressure when breakage begins. First separate shell-condition problems from machine-setting, powder-flow, remplissage excessif, vide, and tooling problems. Jinlu’s production troubleshooting guidance identifies brittle shells as only one of several possible causes of cracking.

Below is a quick summary of common environmental factors and their effects on capsules:

The practical goal is therefore not “keep capsules dry.” It is to maintain the validated balance between capsule moisture content, relative humidity, water activity, formulation, conditionnement, and storage conditions.

Why Capsule Shells Are Moisture-Sensitive

A hard capsule shell is not an inert plastic container. Gélatine, Hpmc, and pullulan are hydrophilic polymer systems, and water interacts with their structure. USP monographs for all three hard-shell types explicitly include water as part of the shell composition.

For gelatin, water also contributes to flexibility. Under commonly cited conditions of roughly 15–25°C and 30–65% RH, hard gelatin shells typically contain about 13–16% water. Research reviewing capsule performance reports that falling below this normal range causes gradual loss of flexibility, while excess water makes the shell softer and stickier.

That is why capsule humidity cannot be judged from room RH alone. The system is continuously moving toward an equilibrium involving the surrounding air, enveloppe de capsule, formulation, and package headspace.

USP also distinguishes teneur en eau depuis water activity (aw). Total moisture tells you how much water is present, while water activity helps describe how much of that water is available to participate in physical, chimique, or microbiological processes. For a hygroscopic powder inside a capsule, this distinction is useful because two materials with similar total moisture may still have different tendencies to exchange water.

A simplified moisture-migration model looks like this:

The key point is that water can move in both directions. A package may slow moisture from entering from outside, while water is still redistributing between the shell and the fill inside the sealed package. Studies of capsules de gélatine dure have demonstrated exactly this shell-to-fill moisture transfer.

Why Capsules Become Brittle, Soft or Sticky

Why do capsules become brittle?

The most common mechanism is excessive moisture loss from the shell.

Faible relative humidity creates a moisture gradient between the shell and its surroundings. As gelatin loses water, it becomes less flexible and less able to absorb mechanical stress. Classic research found a strong relationship between gelatin capsule brittleness and RH, while later studies have repeatedly confirmed that mechanical behavior changes as shell moisture decreases.

En pratique, brittle shells may show up as:

• hairline cracks around the body or cap;
• splitting during cap/body separation;
• broken edges after feeding or orientation;
• fractures during closing;
• powder leakage from damaged shells.

The important diagnostic question is where the capsule breaks. If empty capsules crack before powder enters the body, investigate shell condition, stockage, capsule age, environmental RH, tooling contact, and vacuum first. If cracking occurs mainly during closing, overfill, closing depth, alignement, and excessive mechanical force become more likely contributors. Jinlu’s capsule filling defect analysis makes the same distinction between shell weakness and equipment/process causes.

Why do capsules become soft or sticky?

Usually the moisture gradient is moving in the opposite direction.

At high humidity, gelatin absorbs water. Water plasticizes the polymer, reducing hardness and stiffness. UN 2025 controlled study exposing gelatin and HPMC shells to 25°C/75% RH found substantial moisture-related softening in gelatin and a marked decline in its mechanical strength; HPMC retained mechanical integrity better under the same test.

On a production floor, excessive moisture can appear as soft bodies, dimensional changes, poor cap/body movement, capsules sticking to one another, feeding instability, or deformation. Earlier mechanical studies also found capsule softening became especially noticeable as RH moved above roughly 60%, although the exact response depends on shell formulation.

Temperature matters because it changes both material behavior and moisture-transfer dynamics. Sudden movement between cool and warm environments can also create local condensation risk. Pour cette raison, avoid treating RH and temperature as unrelated controls; the product experiences both simultaneously.

Can the Fill Material Cause Capsule Problems?

Yes—and this is one of the most frequently overlooked causes of gelatin capsule brittleness.

Suppose your warehouse is controlled correctly, your unopened capsules passed incoming inspection, and the empty shells run well on the machine. After filling and several weeks of storage, cependant, the finished capsules crack easily.

The fill may be pulling water from the shell.

A well-known Journal of Pharmaceutical Sciences study investigated a highly hygroscopic drug substance filled into hard gelatin capsules. Moisture transferred from the shell into the powder, lowering shell water content enough to create a brittleness problem. The researchers used moisture sorption/desorption relationships to predict equilibrium and redesign the moisture balance.

This is why formulation teams should evaluate more than the powder’s initial loss on drying.

UN hygroscopic powder has an affinity for atmospheric water. If its water activity and equilibrium moisture characteristics differ strongly from those of the capsule shell, moisture migration can continue even after the capsule is sealed. USP’s water-activity framework is useful here because aw addresses the thermodynamic availability of water rather than simply measuring total water percentage.

En termes pratiques, QA and formulation teams should ask:

Is the fill very dry? Is it strongly hygroscopic? What is its sorption isotherm? What is its water activity? What happens after the shell and formulation reach equilibrium?

Do not assume that specifying shell moisture and powder moisture independently is enough. Finished-product stability depends on the combined system.

Recommended Storage Temperature, RH, and Capsule Material Choice

For empty hard gelatin capsules, pharmaceutical literature commonly cites approximately 15–25°C and 30–65% RH as a suitable working/storage window associated with the normal 13–16% shell-water range. Some commercial and industry guidance uses a slightly narrower humidity band such as 35–65% RH. These values should be treated as general technical guidance—not as a universal specification. Your capsule supplier’s approved storage instructions and certificate/specification should take priority.

Pour gelatin capsule storage, avoid prolonged exposure to very dry air, humidité élevée, excessive heat, direct sunlight, and repeated package opening. Keep empty capsules in their original protective packaging until they are needed, and allow sealed containers to equilibrate to the processing room before opening when there has been a significant temperature difference. These practices follow the broader USP principle that pharmaceutical storage should protect materials from moisture and excessive environmental exposure and maintain labeled storage conditions.

Pour le filled drug product, do not simply copy the empty-shell storage range onto your product label. Shelf-life conditions must be supported by stability data generated with the formulation in its intended container-closure system. Je Q1A(R2), as adopted by FDA, establishes stability-testing frameworks that include controlled temperature/RH conditions; those are stability-study conditions, not automatically the operating specification for every empty-capsule room.

Gélatine vs. HPMC et. Pullulane

The comparison below summarizes peer-reviewed moisture-sorption research, USP shell definitions, ACG/Qualicaps technical specifications, and recent humidity studies. HPMC generally has lower initial moisture and greater low-RH mechanical resilience; pullulan absorbs less moisture than gelatin but still changes mechanically with moisture variation.

For a deeper material comparison, see Jinlu’s Gelatin Capsule vs HPMC Capsule guide, Pullulan Capsule guide, et Guide des capsules vides. Jinlu’s existing content also helps avoid overloading this article with capsule-material topics that deserve their own search intent.

How Packaging Prevents Moisture Damage and Why Filling Machines Notice First

Bien emballage en capsules does not “freeze” the capsule’s moisture content. Its job is to control the rate at which the outside environment changes the internal system.

USP <671> evaluates container performance using moisture-vapor transmission, and pharmaceutical packaging research shows why that matters: different package structures can produce very different rates of moisture ingress and therefore different product stability.

In one published accelerated study of a moisture-sensitive oral product, plain PVC blister packaging performed substantially worse than higher-barrier blister structures, while cold-form aluminum and an induction-sealed HDPE bottle provided markedly stronger protection. The exact numbers are product- and geometry-specific, so the lesson is not to copy another product’s package—it is to validate the barrier against your own formulation.

The same principle applies to déshydratant. Desiccants lower package-headspace humidity and can be extremely useful for moisture-sensitive fills. But because gelatin shell flexibility itself depends on water, aggressively drying the headspace can theoretically shift equilibrium water away from the shell. This is an engineering inference from established shell/fill moisture-transfer and desiccant-equilibrium models: the correct objective is a validated moisture balance, not the lowest achievable RH.

For more packaging detail, Jinlu already has a dedicated Guide complet des options d'emballage des capsules, so this article should remain focused on moisture failure rather than repeat a full blister-material guide.

How humidity problems affect capsule filling machines

Le filling machine of capsule is often where an upstream moisture problem becomes visible.

A brittle shell may survive storage and feeding but fracture when the capsule body is pulled from the cap, when the body enters the segment, or when closing force is applied. A softened shell can create the opposite problem: dimensional changes and higher surface tack may interfere with smooth feeding, orientation, cap/body separation, and locking. Dry conditions can also increase static-related handling problems, while moist or cohesive powder can create its own feeding and dosing difficulties.

This is why production teams should resist the instinct to compensate immediately with more vacuum, more closing pressure, or slower speed. First inspect unused capsules from the same lot. Compare their physical condition with capsules that have spent time in the filling room. Review warehouse and room RH records, open-bag time, formulation moisture behavior, and shell moisture if your QC program measures it. Then inspect vacuum, alignement des outils, volume de remplissage, paramètres de dosage, and closing depth.

Par exemple, if empty capsules already crack during gentle manual handling, increasing machine vacuum will not correct the underlying problem. If capsules are sound before filling but split only after an overfilled body reaches the closing station, the machine or dosing process deserves more attention.

Jinlu Défauts de remplissage des capsules: Problèmes courants, Causes & Guide de dépannage covers the mechanical side—including separation failure, rupture, remplissage excessif, vide, alignement, and closing settings—in greater detail.

Dépannage et Prévention

Troubleshooting checklist

When capsules suddenly become brittle, doux, ou collant, use the same sequence every time:

1. Record actual warehouse and filling-room temperature and RH rather than relying on the HVAC set point.
2. Compare unopened capsules with capsules already exposed to the room.
3. Review how long each capsule bag or container remained open.
4. Check shell moisture condition and the supplier’s specification.
5. Review formulation hygroscopicity, water activity, and moisture-sorption data.
6. Inspect the primary package, intégrité du joint, and desiccant configuration.
7. Only then adjust filling-machine vacuum, alignement, closing depth, dosage, or speed.

Capsule humidity troubleshooting table

These symptom-to-cause relationships are supported by gelatin moisture-transfer studies, recent capsule humidity research, USP packaging principles, and filling-machine troubleshooting evidence.

Prevention checklist

For routine prevention, keep empty capsules sealed until use; monitor the actual production environment; minimize unnecessary open exposure; characterize hygroscopic formulations rather than judging them by appearance; select the shell material around the formulation; choose packaging from moisture-barrier and stability data; size desiccant scientifically; and confirm the finished product in the final market package through an appropriate stability program.

Conclusion

En résumé, capsule brittleness or stickiness is almost always a moisture issue. By controlling humidity, choosing the right capsule material, and using appropriate packaging, manufacturers can maintain capsule quality and avoid production headaches. Par exemple, Jinlu Packing’s solutions include precise environmental controls and high-barrier packaging machinery to keep capsules in spec.

Maintaining 40–60% RH, storing at 15–25 °C (gélatine), using foil blisters or desiccated bottles, and selecting moisture-stable capsule types are key to preventing brittleness or stickiness. When issues arise, refer to the troubleshooting checklist and flowchart above: identify if the problem is due to low or high moisture, then correct the environment or equipment accordingly. With best practices in place, capsules will fill and ship successfully, maintaining product efficacy and customer trust.

Prêt à optimiser votre production de capsules? Jinlu Packing offers advanced encapsulation and packaging equipment engineered for moisture-sensitive pharmaceuticals. Contact our specialists for guidance on humidity control, solutions d'emballage, and high-speed filling lines that keep capsules perfectly stable from warehouse to patient.

FAQs About Why Capsules Become Brittle, Soft or Sticky

Références：
1.A study on gelatin capsule brittleness: moisture tranfer between the capsule shell and its content —— PubMed
2.Gelatin capsule brittleness as a function of relative humidity at room temperature —— ScienceDirect
3.Propriétés de sorption et de désorption d'humidité de la gélatine, Gélules HPMC et pullulane —— PubMed
4.Water Activity —— USP <922>
5.Évaluation comparative des coques de capsules d'inhalation de gélatine et de HPMC exposées à des conditions d'humidité simulées —— PubMed
6.Package selection for moisture protection for solid, oral drug products —— Bibliothèque en ligne WILEY