RESIN MOISTURE & DRYING

You may not realize that many plastic pellets are wet before they are processed. Some have only surface moisture, while other pellets can draw moisture into themselves at a molecular level and contain moisture deep inside the pellet. Even when plastic pellets appear dry on the outside, they can be loaded with moisture on the inside.

It is vital to the successful production of plastic products to understand the starting moisture of resin and how to thoroughly remove it by drying the material in preparation for processing.

Types of Resin Moisture

There are two major classifications of polymers/plastics resins, hygroscopic and non-hygroscopic.

Hygroscopic resins have a strong affinity to attract moisture and absorb it into their molecular structure when exposed to ambient air from the atmosphere. This absorption results in internal or intragranular moisture deep inside a pellet that must be properly dried before processing to achieve quality and consistent production. Intragranular moisture cannot be removed with hot air alone. In most cases, the water can be removed from the material by heating the pellets under dehumidified air in a vacuum or under a flow of dry gas such as nitrogen.

Hygroscopic polymers include nylon, ABS, acrylic, PET, PBT, polyurethane, polycarbonate, and others.

Non-hygroscopic (hydrophobic) resins used to make plastic products do not have an affinity for moisture. Any moisture collected is not absorbed inside the pellet but remains on the pellet as surface moisture which a hot air dryer can remove. Non-hygroscopic polymers include PVC, polypropylene, polystyrene, polyethylene, and more.

The chart lists common hygroscopic and non-hygroscopic polymers used to make plastic products. Photo Credit: Novatec

It is important to remember that hygroscopic materials absorb moisture, whereas non-hygroscopic (hydrophobic) materials do not absorb moisture from the environment. Knowing the classification of the resin to be processed provides important information about how it should be stored and dried.

How Moisture Gets into Resin

Many resins attract and absorb water molecules from moisture in the atmosphere and must be dried prior to processing to prevent surface/cosmetic and structural product defects. The amount of moisture absorbed by resins depends on many factors, including resin type, ambient temperature, seasonal changes, packaging and transportation issues, manufacturing inconsistencies, and more.

Moisture Issues with Specific Resins 

Nylon, more so than other plastic resins, is extremely susceptible to moisture and has a high capacity for moisture absorption and saturation. Nylon is a hygroscopic material with a strong affinity to absorb atmospheric water. It will also absorb moisture if it is directly exposed to water. It can absorb upward of 2-8% moisture by weight at saturation which is much higher moisture absorption than most other resins.

When water is absorbed into a Nylon pellet, it bonds with the polymer. These characteristics of Nylon can make it a challenge to dry and prevent moisture from regaining after drying. However, Nylon is widely used due to its high tensile strength, impact toughness, abrasion and temperature resistance, and chemical compatibility.

Protecting your Nylon material from moisture and knowing the starting moisture is critical to successfully drying Nylon. Suppose Nylon's moisture capacity exceeds 5,000 parts per million (PPM). In that case, it can be difficult to dry and maintain all the polymer properties. Finding the appropriate balance between too much and too little moisture is critical. If Nylon is molded too wet, it will drool, flash, or foam and can cause a pressure drop on the injection molding machine or extruder. If Nylon is processed when it is too dry, the final product could be brittle and break with stress.

To prevent moisture, regain in sensitive resins, Nylon, in particular, cover the Gaylord when processing and store it in a climate-controlled area if possible. Use closed-loop conveying (conditioned air), and keep a minimum amount of material over the machine's throat. Also, be aware of the material's immediate surroundings; the higher the relative humidity, the faster Nylon will absorb moisture.

Impact of Moisture on Plastics Products

Plastic resin processed when too much moisture is present will cause product defects. Common defects visible when the resin has been under-dried are sink marks or voids, cloudiness, splay, streaks, air bubbles on the product's surface, shrinkage, or irregular-shaped effects.

Importance of Initial Moisture Content

Even in sealed boxes and bags, resins can have wildly different internal moisture levels depending on ambient conditions, storage, packaging, and more.

Two Gaylords of material received from the same supplier on the same day can have dramatically different moisture content. Yet, the resin supplier's recommendation for drying time and temperature would be the same. It could lead to one Gaylord of material being overdried and one Gaylord being under-dried.

The graph depicts the moisture content of two Gaylords received on the same day from the same supplier. The initial moisture content of one Gaylord was 1750 PPM (Parts Per Million), and the other Gaylord had more than double the starting moisture content at 3950 PPM. For the Gaylord with an initial moisture content of 3950 PPM, the dryer manufacturer’s drying recommendation for processing moisture level was not met even though the material was dried per the recommendations. It remained too wet for processing. The other Gaylord of material with a starting moisture of 1750 PPM was overdried in the same time. Photo Credit: Novatec

By knowing and understanding the initial or starting moisture content of your resin before you begin processing the resin and then properly drying the resin to the resin manufacturer's recommended level of dryness, you will maintain the intended performance characteristics of the resin and yield a high-quality, cost-effective part in the least amount of time possible.

Moisture Content vs. Time-Based Drying 

For many years the standard practice has been time-average drying of plastics resins. Resin suppliers recommend the drying time and temperature for drying a material based on the resin's assumed initial moisture content level in the supplied technical data. So, the amount of time it would take to dry resin is determined without consideration given to the actual wetness of the resin.

With time-average drying methods, sometimes the material is under-dried, sometimes over-dried, often leading to a whole host of product aesthetic and quality issues. If the material being processed is less wet than the material supplier's assumption when providing time average drying recommendations, you would waste valuable injection molding machine or extruder time and energy by drying when you could have started production. You would also risk over-drying material, leading to aesthetic problems like yellowing and product strength issues like brittleness. Suppose the material you are processing is wetter than the material supplier assumed when recommending a blanketed time and temperature. In that case, you could be under-drying material leading to product issues like air bubbles, splay, and distorted sizing, which leads to wasting expensive resin, time, and energy required to fix the problem and restart the process.

Drying based on knowing the starting moisture content before you begin drying is critical to successful drying. When you know the moisture of the pellet before you start the drying process and adjustments are made to the dryer to account for that information, you can reliably depend on the appropriate dryness level of the final product, which eliminates many concerns of over-drying and under-drying material and ultimately saves a processor time and money.

There are many pellet moisture analyzing products available for purchase by plastics processors. Offline lab and in-line units that detect moisture at the end of the process have been available for many years. More recently, products have become available that measure the moisture in a pellet before the drying process begins.

Types of Resin Moisture Measurement Devices

Moisture analysis is critical to knowing if resin is properly dried. Resin manufacturers recommend that their customers dry material to a maximum level of moisture for a stable process and to produce aesthetically pristine parts with the ideal mechanical properties. Monitoring the resin’s moisture content before you begin processing and during processing is the key to making high-quality plastic parts with reduced waste and increased efficiencies.

When selecting a moisture measurement device, it is essential to understand how they function and their differences. All moisture analyzers measure the percent of moisture in the resin.

Laboratory/Offline Devices

Laboratory devices can measure moisture content or even provide complete chemical analysis, which can be time-consuming, labor-intensive, and costly.

Loss-in-weight analyzers measure the total change in weight of the material when the resin sample is heated at a specific temperature. As the polymer heats up, it burns off moisture and volatiles, and when the weight stabilizes, it records the weight difference. However, volatile burn-off can lead to inaccurate moisture results. In cases where volatiles are considerable, the moisture readout will be higher than the actual moisture, which can lead to over-drying. The manufacturers of these units typically supply recipes that at least partially account for the discrepancy. Karl Fischer Titration is a well-known loss-in-weight laboratory device in the plastics industry and was the gold standard of moisture analyzers for many years before other types of devices became available.

Moisture specific analyzers use a chemical process that only detects moisture levels, not volatiles. These analyzers are expensive and need to be used offline in a lab setting; however, they are reliable and repeatable. It is recommended that plastic processors have one of these devices to confirm their resin's moisture level.

Inline Devices

Inline devices are an alternative to laboratory/offline devices. Inline devices are often less expensive and easier to use then lab devices.

Capacitive sensor moisture analyzers work by measuring the changes in capacitance caused by the changes in the dielectric. Water moisture presents a very high dielectric constant compared to the low dielectric constant of plastics, allowing the sensors to detect the presence of moisture reliably. These units are relatively inexpensive and simple to use on the shop floor. When installed after the hopper, tunneling in hopper can give an inaccurate overall moisture level.

Microwave moisture analyzers work on the principle that water has a significantly higher dielectric constant than most other materials. Due to a water molecule's dipolar effect, a microwave resonator's resonant frequency changes with variations in moisture content. These variations are detected by the sensor electronics, which are scaled by the calibration process to provide a precise readout of the moisture present.

Near-Infrared (NIR) moisture analyzers excite water molecules by bouncing light beams off the sampled resin and measuring how much light is absorbed in wavelengths. This measurement provides the moisture content of the resin and may also be used to measure residual oils or solvents, which can cause contamination issues if not detected and removed. This equipment can be handheld, desktop, online, or inline. These units are costly and rely on a small sample for an overall moisture reading, which will not represent the product's total moisture.

Pre-Drying Devices

Pre-Drying moisture analyzers are a new entry to the market. They include a patent-pending moisture analyzer that consists of a capacitance sensor, an accelerometer, and a gyroscope near the bottom of a material pickup lance to provide reliable inline sampling. The moisture is measured from the lance's cavity on a part per million (PPM) basis before it is drawn into the dryer for further processing.

Capacitance sensors measure the dielectric constant of plastics. Since moisture presents a sizeable dielectric constant compared with the low dielectric constant of plastics, the presence of moisture can be precisely and reliably detected. The plastic pellets' initial and ongoing moisture content is continually communicated to the dryer control, where drying parameters like blower speeds, temperature, and drying times are automatically fine-tuned to dry the resin to the manufacturer's prescribed dryness level.