Engineering Insights: Thermoplastic Drying Part 1
Understanding Moisture Behavior in Thermoplastics
Material behavior can make or break an injection molding process, and few characteristics are more important than how a thermoplastic resin interacts with moisture. The first part in this series covering thermoplastic drying takes a closer look at the difference between hygroscopic and hydrophobic materials. Explore the basics of hygroscopic and hydrophobic materials below and why moisture control is critical to ensure product quality and performance. Then read part two for more information on thermoplastic drying technologies and key process considerations.
What Are Hygroscopic and Hydrophobic Materials?
A hygroscopic material is one that naturally absorbs moisture from the surrounding environment. In hygroscopic thermoplastics, water molecules permeate deep beyond the surface level of the material into the resin’s molecular structure. Any embedded moisture can compromise the material during high-heat processing like injection molding and extrusion processes.
In contrast, hydrophobic materials resist moisture absorption. While hydrophobic materials can still be affected by direct contact with water, they don’t take on moisture in the air. This characteristic makes hydrophobic materials less sensitive to ambient conditions and reduces their drying requirements.
Whether a material is hygroscopic or hydrophobic is a key consideration during the material selection process, as it impacts drying requirements, handling and storage needs, production timelines, and the risk of moisture-related defects.
Hygroscopic Resins: What Material Buyers Should Know
Hygroscopic materials such as nylon (polyamide), polycarbonate (PC), polyester (PET, PBT) and ABS are commonly used resins in plastics processing, but their absorbent nature requires additional consideration and care. If not adequately dried before processing, hygroscopic materials can undergo hydrolysis, in which absorbed moisture chemically breaks down polymer chains, resulting in diminished mechanical properties. Other issues include cosmetic defects (e.g. bubbles, spay marks, poor surface finish) and inconsistent processing, where variability in moisture content leads to fluctuations in resin viscosity.
When selecting a hygroscopic resin, consider more than just its performance specifications. Evaluate whether the appropriate drying equipment, handling protocols and storage conditions are in place to protect the material from ambient moisture and avoid inconsistent quality and performance.
“For materials like nylon or PET, drying isn’t optional, it’s an essential part of material processing,” said Myles Wegner, Technical Development Engineer at M. Holland. “Even small amounts of moisture can compromise performance. Ensure every partner from your supplier to your processor to your in-house team is sufficiently equipped with the knowledge and tools to meet the resin’s drying requirements.”
Following the manufacturer’s recommended drying temperature and time is essential for hygroscopic materials. If a material is improperly dried, it may oxidize or degrade, causing discoloration or brittleness.
Hydrophobic Resins: Simpler Handling, Fewer Risks
Hydrophobic materials such as polypropylene (PP) and polyethylene (PE), do not require pre-drying under normal conditions, so their performance is more consistent across changing humidity levels. This makes hydrophobic materials a strong choice for applications where processing time and equipment capacity are limited, moisture-sensitive defects would be especially costly, or resin will be stored for extended periods of time before processing.
“Polyethylene and polypropylene are popular not just for cost and versatility, but because they are forgiving when it comes to moisture,” Myles explained. “Applications using hydrophobic materials do not have to factor in the same level of drying infrastructure, which simplifies purchasing and production planning.”
While hydrophobic materials are resistant to water absorption, they can still become contaminated by direct exposure, so proper storage and transportation practices are key to keeping processes streamlined and quality consistent.
Start Material Selection with Moisture Behavior
Understanding how a resin interacts with moisture is essential to making the right material choice. Consider process readiness and end product performance requirements prior to the final material selection.
“From a material sourcing perspective, moisture behavior should always be part of the conversation,” said Myles. “Buyers need to confirm whether a resin is hygroscopic or hydrophobic, understand the drying requirements for proper processing, and ensure that suppliers or third-party processors can meet those requirements.”
He continued, “It always helps to think ahead about how moisture sensitivity could impact product delivery lead times, storage conditions, or even logistics.”
Any resin’s moisture profile directly influences how it must be handled from purchase order to production floor. In Engineering Insights: Thermoplastic Drying Part 2, we explore ideal drying technologies and why following drying guidelines is key to maintaining material performance.
Engineering Insights: Practical Plastics Solutions
Technical plastics pros like Myles are sharing their expertise in M. Holland’s Engineering Insights series. To ensure you don’t miss future technical insights, subscribe to receive the latest news and insights from M. Holland.
Disclaimer: The information provided in this post is intended solely for general guidance. Outcomes vary based on individual circumstances, and M. Holland Company, LLC does not ensure a specific result. Clients shall use their own independent skills and expertise when testing any application of technical support. M. Holland is not responsible and will not be liable for any discrepancies between expected and actual outcomes. M. HOLLAND DISCLAIMS ALL WARRANTIES, WHETHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF MERCHANTABILITY OR FITNESS OF A PRODUCT FOR A PARTICULAR PURPOSE.