After several consecutive days of rain, warehouses are still busy shipping and receiving goods, making moisture protection one of the biggest challenges for manufacturers and processors. Plastic raw materials stored outdoors can easily become soaked by rain if proper precautions are not taken. In mild cases, this may affect the material’s appearance; in more serious cases, it can lead to bubbles, silver streaks, and a high rate of defective products during processing.

Today, we’ll take an in-depth look at what really happens when plastic pellets get wet. By the end of this article, you’ll know which materials can simply be air-dried, which must be thoroughly dried before processing, and which may be beyond recovery altogether.

What Happens When Plastic Pellets Get Wet?

When plastic pellets are exposed to rain or moisture, their performance and processability may be affected. Whether they can still be used depends largely on the type of plastic and the extent of moisture absorption.

In simple terms, some plastics, such as PE and PP, do not absorb water and can often be used after surface drying. However, most engineering plastics, including PA, ABS, and PC, absorb moisture internally and require proper drying before processing. Simply air-drying or sun-drying is usually not sufficient.

Will Moisture Affect Material Performance?

Plastics can generally be divided into two categories, each reacting very differently to moisture exposure.

Non-Hygroscopic (Hydrophobic) Plastics

Examples include PE (Polyethylene), PP (Polypropylene), PS (Polystyrene), and PVC (Polyvinyl Chloride).

Water molecules remain only on the surface of the pellets. In most cases, the material can be restored by air drying or using a hot-air dryer. After proper surface drying, material performance is typically unaffected.

Hygroscopic Plastics

Examples include PA (Nylon), ABS, PC (Polycarbonate), PET, and PBT.

These materials absorb moisture into the interior of the pellets, which can lead to significant processing and performance issues.

Appearance Defects

During molding, absorbed moisture vaporizes under high temperatures, causing defects such as:

Silver streaks
Bubbles
Whitening
Flow marks
Splay marks
Reduced Mechanical Performance
PA (Nylon): Tensile strength and stiffness decrease, while toughness and impact resistance may increase.
ABS: Moisture primarily affects appearance and flowability, and severe moisture contamination can reduce overall performance.
PC (Polycarbonate): Mechanical and optical properties can deteriorate significantly.
Material Degradation

This is especially critical for PA, PET, and PBT.

At processing temperatures, moisture can trigger hydrolysis, breaking down polymer molecular chains and causing irreversible damage. As a result, molded parts may become brittle and suffer permanent strength loss.

How Should Rain-Soaked Plastic Pellets Be Treated?

The following procedures are recommended, particularly for hygroscopic materials.

1. Assess the Moisture Exposure and Material Type
Check whether the pellets have clumped together or become sticky.
Conduct a small trial production run, especially for PA and PC materials, to inspect for silver streaks or bubbles.
Review the material datasheet to determine whether the resin is hygroscopic and to identify the recommended drying conditions.
2. Dry the Material Properly (Critical Step)
Non-Hygroscopic Plastics (PP, PE, PS, PVC)

A hot-air dryer is usually sufficient.

Typical drying conditions:

Temperature: 60–80°C
Drying time: 2–4 hours
Hygroscopic Plastics (ABS, PC, PA, PET, PBT)

A professional desiccant dryer is strongly recommended.

Conventional hot-air drying is often unable to remove moisture trapped inside the pellets.

3. Key Drying Principles
Best Practice: Use a Desiccant Dryer

How it works:

The system circulates high-temperature, low-dew-point air (typically ≤ -20°C) through the material, effectively drawing moisture out from within the pellets.

Important:

After drying, the material should be protected from ambient humidity and processed as soon as possible to prevent reabsorption of moisture.

Emergency Solution: Drying Oven

If a desiccant dryer is unavailable, a drying oven can be used as a temporary solution.

Typical drying temperatures are above 80°C.

For example:

PA6: 100–120°C for 4–6 hours

Actual drying conditions should always follow the resin supplier's recommendations.

4. Test for Irreversible Damage

After drying, moisture-sensitive materials such as PA and PBT should undergo a small-scale production trial.

Recommended evaluations include:

Tensile strength testing
Impact strength testing
Visual inspection of molded parts

While physical moisture absorption can generally be reversed through proper drying, hydrolytic degradation is permanent. If mechanical properties remain significantly reduced after drying, the material may need to be downgraded for less demanding applications or blended with virgin resin.

Common Moisture Exposure Treatment Guide for Plastic Pellets

Note: The drying conditions listed above are general reference values. Actual drying temperatures and times should always follow the resin supplier's technical datasheet and processing recommendations. For moisture-sensitive engineering plastics such as PA, PET, and PBT, using a desiccant dryer with a low dew point is strongly recommended to ensure optimal processing performance.