Hygroscopicity and Osmosis in Plastics – Are They the Same?
In my recent quiz, I asked: Which of these plastics absorb moisture from the air? The options were ABS, PVC, and PE.
The discussion in the comments was very interesting, and one of the responses suggested that PVC can absorb moisture and reduce osmosis in CIPP (Cured-In-Place Pipe) applications. This is a great example of how many fascinating and complex topics exist in the world of plastics!
Each of us has had moments when we didn’t know something – I certainly did when I first entered this industry. Even though my father, Ryszard, had been teaching me about plastics since I was 10 years old (because my mother wasn’t particularly interested in listening, so he explained everything to me instead), I’m still discovering new things every day. That’s why I share knowledge – so we can grow together and better understand the materials we work with.
I also received questions via email about the hygroscopicity of plastics, so I decided to answer them on the blog and clear up any doubts. If this topic interests you, keep reading!
1️⃣ What Does Hygroscopicity Mean?
Some plastics have the ability to absorb moisture from the surrounding air, a property known as hygroscopicity. This isn’t just surface moisture; it’s the ability of the plastic granules to pull water molecules deep into their structure.
This happens because certain polymers have polar molecular structures, which attract and retain water molecules within the material. The presence of polar groups in the polymer chain creates an affinity for water, allowing moisture to diffuse into the granules.
Examples of Hygroscopic Plastics:
✔️ ABS (Acrylonitrile-Butadiene-Styrene)
✔️ PA (Polyamides, e.g., Nylon)
✔️ PC (Polycarbonate)
✔️ PET (Polyethylene Terephthalate)
These materials can absorb moisture not only on their surface but also internally, which can significantly affect processing and final product quality.
2️⃣ Why Does Hygroscopicity Matter in Plastics Processing?
When hygroscopic plastics absorb moisture, the water remains trapped inside the granules, and if not properly removed before processing, it can cause serious issues such as:
✔️ ABS – Must be dried before processing; otherwise, excess moisture can cause defects like bubbles, streaks, and reduced mechanical strength in the final product.
✔️ PA (Nylon) – Highly hygroscopic; absorbs moisture that changes its mechanical properties. A dry nylon part will be rigid, but after absorbing water, it becomes more flexible.
✔️ PET – Must be thoroughly dried before injection molding. If processed with excess moisture, the material can degrade, leading to weak, brittle parts.
If a hygroscopic material is not properly dried, it can lead to processing defects, reduced mechanical strength, and poor surface quality in the final product.
3️⃣ What About PVC and PE?
Now, let’s go back to the quiz. One of the answers suggested that PVC can absorb moisture and reduce osmosis in CIPP applications. Does this mean PVC is hygroscopic? No!
✔️ PVC (Polyvinyl Chloride) is not hygroscopic – it does not absorb moisture into its structure.
✔️ PE (Polyethylene) is also non-hygroscopic, meaning it does not absorb water from the air.
Some plastics, like PVC and PE, are hydrophobic, meaning they repel water. This happens because their molecular structure lacks polar groups, so water molecules cannot penetrate and become trapped inside the material.
However, PVC can come into contact with water in applications such as CIPP (Cured-In-Place Pipe) sewer repair, where it is used as an inner liner. In this case, PVC does not absorb water but acts as a barrier that reduces osmosis, preventing water from penetrating other layers of the system.
4️⃣ Osmosis vs. Hygroscopicity – The Key Difference
💧 Osmosis is the movement of water molecules through a semi-permeable material, often due to a concentration gradient. This is a physical process that occurs when water moves through a barrier from an area of lower solute concentration to an area of higher solute concentration.
🌡 Hygroscopicity, on the other hand, is the ability of a material to absorb and retain moisture within its structure.
✔️ PVC can act as an osmotic barrier, but it is not hygroscopic.
5️⃣ How to Manage Moisture in Plastic Processing?
✔️ Dry hygroscopic materials before processing – ABS, PA, and PET must be dried in industrial dryers under controlled conditions to remove moisture before molding or extrusion.
✔️ Monitor humidity in the processing environment – Excess moisture can lead to injection molding defects, reduced mechanical strength, and poor surface quality.
✔️ Choose the right materials for applications exposed to water – Hydrophobic plastics such as PE and PP are ideal for applications requiring resistance to moisture and water exposure.
Summary – Who Was Right?
✔️ Is PVC hygroscopic? No, it does not absorb moisture into its structure.
✔️ Can PVC reduce osmosis? Yes, it can act as an osmotic barrier in certain applications.
✔️ Which material from the quiz is hygroscopic? ABS!
Osmosis and hygroscopicity are two very different phenomena – osmosis involves water movement through materials, while hygroscopicity refers to absorbing moisture into the material itself.
Thank you for participating in the quiz and engaging in the discussion! I hope this explanation helps clarify the differences.
If you want to explore more key topics in polymer science, check out our page where we discuss 100 essential plastic-related topics, including ABS, PVC, PE, hygroscopicity, and hydrophobicity of plastics.
If you're interested in practical aspects of plastics processing, check out our specialized courses for deeper insights!
