Moisture in Recycled Plastic: Why It Matters and How to Dry It
PlasticSetu sourcing desk · 15 Oct 2026 · 4 min read · moisture · drying · hygroscopic · processing defects
Moisture is one of the most common and least visible causes of processing problems with recycled plastic. It can cause splay marks and bubbles in moulded parts, voids in extruded profiles, holes in film and, in some polymers, permanent loss of strength. Recycled material is especially prone to it because much of it has been washed, stored outdoors or packed while still damp.
This guide explains why moisture matters, which polymers need drying, how to check moisture and the basics of drying.
Why moisture matters
Water in plastic causes two kinds of problems:
- Physical defects: when material is melted, water turns to steam. This produces bubbles, silver streaks (splay), voids, rough surfaces and unstable film bubbles.
- Chemical degradation: in some polymers, water reacts with the polymer chains at melt temperature in a process called hydrolysis. This shortens the chains, lowers viscosity and weakens the product. The damage cannot be reversed by drying afterwards.
Moisture also affects commercial value. If you buy by weight, water is weight you pay for but cannot use.
Hygroscopic vs non-hygroscopic polymers
Polymers differ in how they take up water:
| Type | Polymers | What it means |
|---|---|---|
| Hygroscopic (absorb moisture into the material) | PET, PC, nylon (PA), ABS, PMMA | Must be dried before melt processing |
| Non-hygroscopic (moisture stays on the surface) | PP, HDPE, LDPE, LLDPE | Do not generally absorb water, but surface moisture from washing or storage must be removed |
PET and nylon are particularly sensitive because they degrade through hydrolysis when processed wet. For PET, this shows up as a drop in intrinsic viscosity. See our pages on nylon and polycarbonate for more on these materials.
For polyolefins such as PP and PE, the issue is usually water trapped between flakes, inside film agglomerate or in poorly dried granules after washing and pelletising.
How to check moisture
Several methods are available, depending on how precise you need to be:
- Visual checks: look for condensation inside bags, damp patches or clumping. A quick melt check on a hot plate that shows bubbling is a sign of moisture, though not a measurement.
- Loss-on-drying (moisture analyser): weighs a sample, heats it and measures the weight lost. Common in factories and useful for routine checks.
- Karl Fischer titration: measures water specifically and suits low moisture levels in hygroscopic polymers such as PET and nylon. Usually done in a lab.
- Processing signs: splay, bubbles and unstable melt flow can indicate moisture, although other causes are possible.
Ask suppliers to state moisture on the test report and check incoming lots yourself. More on moisture content.
Drying basics
The right drying method depends on the polymer:
- Hot air dryers blow heated air through the material and are often adequate for removing surface moisture from polyolefins.
- Desiccant (dehumidifying) dryers use dried air and are the usual choice for hygroscopic polymers such as PET, nylon, PC, ABS and PMMA, because ordinary hot air may still carry too much humidity.
- Vacuum dryers and crystallisers are used in some setups. Amorphous PET, for example, is typically crystallised before drying so that it does not stick together.
Drying temperature, time and air dew point vary by polymer and grade. Follow the guidance from your resin supplier and dryer manufacturer rather than a generic figure. Drying at too high a temperature or for too long can itself cause yellowing or degradation.
Storage and handling
- Store material indoors, off the floor and away from rain, especially during the monsoon.
- Keep bags closed until use, and process dried material promptly.
- Do not leave dried hygroscopic material standing in open hoppers for long periods.
- Make sure washed flakes and film are thoroughly dried, mechanically and thermally, before pelletising.
- Label lots with their arrival date so older stock is used first.
Source dry, documented material through PlasticSetu
When you post a requirement on PlasticSetu, include your acceptable moisture level and whether you need a test report with each lot. The sourcing desk passes this to matching suppliers and arranges samples for you to check before bulk dispatch. Get quotes to begin.