Technical Guide to Water Absorption in Plastics: Molecular Mechanisms, Industrial Drying, Hydrolytic Degradation, Recycling, Dew Point Control, and Management of Residual Moisture in PA, PC, PET, ABS, PMMA, PBT, PE, and PP

Author: Marco Arezio. Expert in circular economy, polymer recycling, and industrial plastic processing. Founder of the rMIX platform, dedicated to enhancing the value of recycled materials and developing sustainable supply chains.

Original article: May 2020

Updated version: April 2, 2026

Reading time: 12 minutes

Moisture in polymers: why it is a decisive technical variable

Moisture in polymers is one of those variables that often seems secondary until the production line begins to generate apparently inexplicable defects. Bubbles, surface haze, abnormal brittleness, opacity, viscosity fluctuations, dimensionally unstable parts, or sudden deterioration in finish are almost never random phenomena. Very often, behind these problems lies inadequate management of the water present in the material.

Every polymeric material, during the stages of synthesis, packaging, transport, storage, and processing, comes into contact with environmental moisture. This interaction does not have the same meaning for all polymers. In some cases, water remains mainly on the surface; in others, it penetrates the pellet, diffuses into the material structure, and profoundly affects the behavior of the polymer during processing and in service.

In the current industrial context, where companies must ensure constant quality, reduced scrap, lower energy consumption, and increasing use of recycled materials, moisture management can no longer be treated as a simple auxiliary step. It has become an integral part of process control, on the same level as temperature, residence time, plasticizing speed, and raw material quality.

What hygrometric equilibrium means in plastics

All plastics, to varying degrees, tend to reach equilibrium with the surrounding environment. This equilibrium depends on various factors: relative air humidity, ambient temperature, exposure time, pellet size, specific surface area of the material, the possible presence of dust or regrind, and the chemical structure of the polymer.

Speaking of hygrometric equilibrium means describing the condition in which the material has absorbed or released water until it stabilizes with respect to the surrounding environment. This situation, however, is never absolute or final. A change in warehouse temperature, a seasonal shift, prolonged exposure to air, or a different storage method is enough to alter the moisture content of the material.

It is also important to clarify a point that in the past was oversimplified: in hygroscopic materials, water does not always “chemically bind” in the strict and irreversible sense. More accurately, water molecules diffuse into the material and interact with specific polar sites along the polymer chains through intermolecular interactions, often in the form of hydrogen bonds. This distinction is relevant because it explains why a pellet may appear dry on the outside while still containing a significant amount of moisture inside.

Hygroscopic and non-hygroscopic polymers: real differences in production

The distinction between hygroscopic and non-hygroscopic polymers remains fundamental for those working in the plastics industry, but it must be understood technically and not merely in a textbook sense.

In hygroscopic polymers, water penetrates into the pellet and distributes itself through the material by diffusion. This means that simply removing surface moisture is not sufficient. The polymer must undergo a drying treatment capable of extracting water also from the inner part of the pellet. Materials such as polyamide, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, and acrylonitrile-butadiene-styrene belong to this family in practical processing terms.

In non-hygroscopic polymers, by contrast, water tends to remain mainly on the surface, without diffusing significantly into the matrix. This behavior is typical of polyolefins such as polyethylene and polypropylene, as well as polystyrene. In these cases, the problem is often linked to condensation, surface water, or moisture associated with poor storage conditions, rather than a true phenomenon of internal absorption.

However, even this classification must be used intelligently. A material defined as non-hygroscopic is not automatically free from water-related problems. If the pellet has been exposed to moisture, if it comes from a washing cycle, if it has a high specific surface area due to grinding, or if it has been stored in cold environments and then exposed to warmer air, even a polyolefin can introduce into the machine an amount of water sufficient to generate defects.

For this reason, in modern production language, it is no longer enough to ask whether the polymer is hygroscopic or not. It is necessary to understand where the water is located, in what quantity, how it got into the material, and what effects it can produce under the specific processing conditions.

Why some polymers absorb water: polarity and hydrogen bonding

To truly understand the phenomenon of hygroscopicity, one must go down to the molecular level. The water molecule is polar: it has an asymmetric distribution of electric charges and is therefore capable of interacting with polar functional groups present in the macromolecular chains of polymers.

The structure of the water molecule, with the oxygen atom being more electronegative and the two hydrogen atoms bonded at an angle of about 104.5°, generates a permanent dipole. Oxygen assumes a partial negative charge, while the hydrogens assume a partial positive charge. This polarity makes water particularly prone to interacting with other polar regions.

When a polymer contains functional groups such as carbonyls, esters, amides, or other polar functions, the possibility increases that interactions with water molecules will occur. In polycarbonates, in polyesters such as PET and PBT, in PMMA, and in other technical families, the presence of the carbonyl group contributes to the polarity of the structure and facilitates the attraction of water molecules.

In the case of polyamides, the phenomenon is even more evident. The presence of the amide group makes these macromolecules particularly sensitive to water. Water molecules can interact both with the carbonyl group and with the hydrogen bound to nitrogen, forming hydrogen bonds that promote the absorption and retention of moisture in the polymer matrix.

Hydrogen bonds are interactions weaker than the covalent bonds of the polymer chain, but sufficiently stable to allow the adsorption and absorption of water up to a certain equilibrium value. This value changes from one polymer to another and is also influenced by environmental conditions. By contrast, non-polar polymers, such as many polyolefins, do not have a structure favorable to this type of interaction and therefore do not absorb moisture in the same way.

What happens during extrusion and molding when the material is wet

When plastic material enters the plasticizing barrel or the extruder, the presence of water becomes a critical factor.

If, on the other hand, the polymer is hygroscopic and water is present within the pellet volume, the problem is deeper. During melting, moisture can promote hydrolysis reactions or, more generally, degradation processes that reduce the molecular weight of the material. This leads to a decrease in viscosity, a change in melt flow index, a loss of mechanical strength, and, in some cases, increased brittleness of the finished product.

From a production standpoint, this means that the material no longer behaves as expected. The process technician may observe filling instability, pressure fluctuations, less stable cycle times, poorer dimensional stability, and defects that seem attributable to the machine or the mold, but which in reality derive from insufficient preparation of the raw material.

The most common mistake is to consider moisture only as a defect of the material. In reality, it is a variable that alters the process. A wet polymer changes its rheological, thermal, and mechanical behavior, and therefore alters the entire balance of the transformation process.

Hydrolytic degradation, surface defects, and loss of performance

One of the most serious effects of moisture in engineering polymers is hydrolytic degradation. In the presence of water and high temperatures, some macromolecular chains may undergo scission. This phenomenon is particularly relevant in polyesters, polycarbonate, and other sensitive families, in which contact between residual moisture and processing temperatures can lead to a reduction in molecular mass.

When molecular mass decreases, the material loses part of the properties for which it was originally selected. Toughness, impact resistance, the ability to withstand prolonged stress, and surface quality may all decrease. In many cases, the part may even appear visually acceptable but prove inferior under load, during laboratory testing, or in actual use.

In polyamides, the relationship with water is even more complex. On the one hand, moisture in service can act as a plasticizer, increasing chain mobility and modifying stiffness, elongation, and dimensional stability. On the other hand, during processing, the uncontrolled presence of water can contribute to qualitative deterioration, which becomes especially pronounced when the material has already undergone other thermal histories, as happens in recycling or reprocessing cycles.

This explains why the simple elimination of visible defects cannot be considered sufficient. A part free of bubbles is not necessarily a correctly processed part. True quality is measured by the preservation of the molecular structure and by the ability of the finished product to maintain the required performance over time.

Industrial drying: hot air, dry air, and dew point

From a plant engineering perspective, moisture management is based on a distinction that remains valid today. In non-hygroscopic materials, where the problem is predominantly superficial, water removal can be carried out through hot-air dryers. In these cases, the goal is mainly to eliminate moisture adhering to the pellet surface or to prevent the effects of condensation.

For hygroscopic materials, however, hot air alone is not sufficient. If the air introduced into the system still contains a significant amount of vapor, it will not be able to effectively extract water from inside the pellet. It therefore becomes necessary to use dehumidified air systems, in which the moisture content of the air is lowered before it comes into contact with the material.

In this context, the concept of dew point becomes highly important. The lower the dew point of the process air, the greater its ability to absorb moisture from the polymer. For this reason, modern drying is not evaluated solely as a function of the set temperature, but rather by the relationship between temperature, residence time, air flow rate, dew point, and the integrity of the entire system.

Another often overlooked aspect concerns reabsorption. A properly dried material can quickly take up moisture again if it is left exposed to the ambient air of the production department, if the hopper is not adequately protected, or if too much time passes between drying and processing. From this point of view, good drying depends not only on the quality of the dryer, but also on the discipline with which the material is handled and fed to the machine.

In 2026, moreover, the subject of drying is closely linked to energy efficiency. Drying a polymer correctly is essential, but doing so inefficiently can significantly increase the industrial cost of the process. For this reason, the most advanced plants now seek a balance between drying quality, energy savings, and adaptation of parameters to the actual condition of the material.

Moisture and recycled polymers: an even more important critical factor

If moisture management is already fundamental in virgin material, it is even more so in recycled material. Recycled polymers may show greater variability, a previous thermal history, higher sensitivity to degradation, and, in many cases, a surface more exposed to interaction with the environment.

Recycled pellets or regrind may also come from washing, shredding, prolonged storage, or handling in environments that are not perfectly controlled. This means that their water content may be more variable and less predictable than that of first-supply virgin material.

In polymers sensitive to hydrolysis, this condition is particularly critical. If the material has already undergone an initial thermal and mechanical cycle, its tolerance to further degradation may be lower. The presence of residual moisture, combined with processing temperatures, can therefore accelerate the loss of molecular weight and further worsen the performance profile of the recycled material.

For those working in the circular economy, this is an essential point. Recycling does not depend only on the ability to recover material, but on the ability to preserve its technical value. If moisture is not rigorously controlled, a significant share of the recycled material’s potential can be lost already during the processing phase.

How residual moisture is measured in plastic pellets

A modern production department cannot rely only on visual or tactile experience to assess whether a material is dry enough. Residual moisture must be measured or, at the very least, controlled through standardized procedures.

At the technical level, there are reference standards for evaluating water absorption and moisture content in polymer materials. Absorption can be studied using specific standardized methods, while the quantification of residual moisture in pellets is often carried out with dedicated laboratory techniques. Among these, Karl Fischer titration remains one of the most reliable approaches when it is necessary to measure very low amounts of water with precision.

In addition to laboratory controls, more and more plants use in-line monitoring instruments or indirect verification procedures based on dew point, residence time, feeding conditions, and process behavior. True quality, however, arises from the combination of measurement, experience, and organization.

Knowing how much water is present in the pellet is important, but it is equally important to know when the material was dried, how long it remained exposed to the environment, and whether the transport system to the machine maintained adequate conditions. Without this overall view, the numerical value alone may not be enough.

Correct strategies for managing PE, PP, PA, PET, PC, ABS, PMMA, and PBT

From an operational point of view, polyolefins such as PE and PP mainly require attention to storage, condensation prevention, and the elimination of surface moisture. If stored correctly, these materials present fewer critical issues related to internal water absorption, but they can still generate problems when they come from washing cycles or humid environments.

Polyamides, on the other hand, require much more rigorous management. Their strong affinity for water demands accurate drying, control of exposure time to air, and careful evaluation of the final dimensional and mechanical conditions of the part. PET and PBT, as engineering polyesters, must be processed with very low levels of residual moisture in order to avoid hydrolysis and loss of performance. Polycarbonate and PMMA also require careful preparation to preserve transparency, surface quality, and stability of the molecular structure. ABS, while not always reaching the same critical level as polyamide, must not be underestimated and still requires proper pre-drying.

The best strategy does not consist in applying one general rule to all materials, but in building a protocol consistent with the polymer family, the type of plant, the pellet format, the possible presence of recycled material, the season, the humidity of the production area, and the quality objectives of the final product.

Moisture in polymers is a much more complex issue than it may appear at first reading. It is not simply a matter of wet or dry material, but a phenomenon involving chemistry, diffusion, environmental equilibrium, drying technology, rheology, aesthetic quality, and preservation of mechanical properties.

Hygroscopic polymers absorb water into their structure and require dehumidification systems and rigorous procedures. Non-hygroscopic polymers, although less sensitive to internal absorption, are by no means free from problems and must still be protected from surface moisture, condensation, and poor storage practices.

In the contemporary industrial landscape, marked by growing attention to recycling, energy efficiency, and quality stability, moisture control has become a fundamental competence. Only by understanding the specific behavior of each polymer and by building a coherent process of drying, measurement, and handling is it possible to process plastic materials in a reliable, technical, and sustainable way.

FAQ

What does it mean for a polymer to be hygroscopic?

It means that the material is capable of absorbing water even داخل the pellet and not only on the surface. This requires a true dehumidification stage before processing.

Do PE and PP need to be dried?

In many cases it is sufficient to remove surface moisture, but if the material has been washed, poorly stored, or subjected to condensation, even these resins may require careful drying.

Why is moisture dangerous for PET, PBT, and polycarbonate?

Because during processing it can promote hydrolytic degradation, reduction of molecular weight, and deterioration of mechanical and optical properties.

Do polyamides change behavior when they absorb water?

Yes. Water can act as a plasticizer, modifying stiffness, elongation, dimensions, and the mechanical behavior of the material.

Is recycled material more sensitive to moisture?

Often yes, because it has a previous thermal history, greater variability, and may come from washing or less controlled storage stages.

How is residual moisture controlled in polymers?

Through standardized procedures, laboratory instruments, dew point control, verification of residence times, and monitoring of material handling along the entire line.

Sources

Scientific literature on water diffusion in polymer materials

Technical standards for measuring water absorption in polymers

Technical standards for determining residual moisture in plastic materials

Scientific studies on the hydrolytic degradation of engineering polymers

Technical publications on the processing of virgin and recycled polymers

Regulatory documentation on methods for measuring moisture and water absorption

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