The Materials Analyst, Part 51: Where does the moisture go? (Part 1)

In the last case study we recounted the difficulty detecting a contaminant that has been changed from its original form by the molding process. There is a common substance that has a similar ability to contaminate plastic raw material. If the raw material is molded with moisture present, the moisture may alter its chemical form, making it difficult to trace. Plastics processors spend more than a billion dollars a year in equipment, maintenance, and energy to remove moisture, yet evidence suggests that on occasion they are unsuccessful, sometimes with disastrous consequences. 

The resin drying process is such a common part of molding that it is often taken for granted. But the interaction between moisture and plastic materials is a complicated one. When considering interactions with moisture, there are three main categories of plastic materials. First there are the hydrophobic (water-hating) materials. These are materials that have no chemical affinity for water. In chemistry these materials are classified as nonpolar while water is polar. In chemistry like attracts like and therefore nonpolar materials have no use for polar materials. If you don't believe it, try mixing a hydrocarbon like motor oil or cooking oil with water. Oils are nonpolar and will separate as soon as the motion stops. The same thing occurs between water and nonpolar plastic materials such as polyethylene. 

The second category of materials is called hydrophilic (water-loving) materials. These are polar compounds that attract water to varying degrees depending on the exact chemical makeup of the polymer. These materials require drying before processing—otherwise the result is molded parts with splay, streaks, and bubbles that are created by trapped water that boils during processing. 

But a subset of this hydrophilic family does more than just absorb water and release it during melt processing. The materials in this special class actually enter into chemical reactions with the water that they absorb. These chemical reactions consume some of the water and in the process the long polymer chains break into shorter polymer chains, a process known as hydrolysis or hydrolytic degradation. Table 1 categorizes common polymers that represent each of these categories. 

Blood From a Turnip 
Many processors and end users are aware of the fact that the properties of some polymers are compromised when they are molded with a high moisture content. So when parts fail, one of the things that the customer wants to determine is whether the material was dry or wet when the part was molded. Often the customer believes it can tell how wet the pellets were at the time of molding by measuring the moisture content of the molded part. 

There are two problems with this approach. First, the moisture content of a molded part at any point in time is largely a function of the atmosphere to which the part is exposed after molding. A part molded with a very dry resin may produce parts with good properties. But regardless of how dry the material was at the time of molding, the parts will absorb moisture as a function of their environment. 

Second, even if the polymer is degraded by excess moisture because of poor drying, some of the moisture that was in the pellets will have reacted with the polymer during melt processing. Since the water has been chemically changed, it will not be present at the same level as it was in the pellets. The moisture content at the time of processing must therefore be inferred from the state of the final product. Viscosity tests that compare molded parts to pellets will confirm the degradation of the material. 

In a subsequent segment of this topic, we will quantitatively track the water in a polymer as it is converted from pellets to parts and illustrate what really happens to it. But before we can address the question of where the moisture goes during the molding process, we must first address the process of moisture measurement. 

Loss-in-weight Myth 
There are three methods in the ASTM test protocol for plastics that deal with moisture content determinations. These are D 789, D 4019, and D 5336. All of these methods refer to measurement techniques that involve a chemical reaction between water and another substance. Two of these three techniques refer to a method known as Karl Fischer titration. These techniques are designed to be moisture specific. 

Unfortunately, moisture measurement within the molding community is rarely done using these methods. Instead, the last decade has seen the introduction of a series of instruments that are sold as moisture monitors but operate strictly on the principle of loss in weight. (Some loss-in-weight systems use a downstream treatment in order to make the measurement moisture specific. We will come to that discussion in our next installment.) 

Loss-in-weight systems heat a material to the point where volatile material is driven off. The weight of this material is compared to the original mass of the sample and the moisture content is calculated as a simple ratio of lost weight to original sample weight. This would be fine if water was the only substance that evolved when a plastic compound was heated. Unfortunately, it is not that easy. Along with water come residual monomers, plasticizers, and other additives. If the test temperature is high enough, it is even possible to start driving off degradation byproducts. This is particularly likely to occur with instruments that use air as the test atmosphere. 

True Moisture Measurement 
The essential task in moisture measurement is to drive off all of the water in the sample quickly without producing degradation byproducts. This means the temperature must be elevated but not to the point where the polymer begins to break down. One of the best ways to extend the safe temperature range for a polymer is to heat it in an atmosphere that is free of oxygen. But the use of inert or relatively inert gas streams such as argon or nitrogen is treated as an optional feature in many moisture measurement systems. As the ASTM methods are quick to point out, even chemical techniques can be fooled by degradation byproducts that come from certain polymers, so it is essential that a temperature be established that gets all the water, ignores everything else that might come from the compound, and preserves the polymer. 

Even though the title of ASTM D 789 suggests that the method is designed strictly for nylon materials, this method provides a protocol for establishing the correct temperature at which a moisture measurement should be made. Figure 1, below, shows the result of such a method development for a PET polyester, but the method is essentially the same for any material. First the material is tested at a baseline temperature. The test is then repeated at successively higher temperatures. A convenient interval is 10 deg C. If the chosen test temperature is too low, then each increase in temperature results in an increase in measured moisture. 

Once the temperature is sufficiently high to completely dry out the sample, the moisture content stops increasing. Ideally the plateau is 20 to 30 deg C wide and the method can use a midpoint in this plateau region for repeatable results. In the case of the PET the plateau occurs at 230C. Any test temperature between 230C and 260C should produce the same result. For some materials, continued increases in test temperature may produce a new ascent. This new increase does not represent real moisture in the sample. Instead it is due to the formation of water from solid-state polymerization or byproducts of polymer degradation. A graph in ASTM D 789 shows an example of this type of behavior. 

The Air Out There 
If the tests are performed in air, some polymers will never establish a reliable plateau. In other words, for these materials the new weight loss due to degradation starts before the legitimate moisture removal process is complete. An inert atmosphere is essential for these materials and is an insurance policy against bad results even in materials that are not as sensitive. 

Figure 2, below, shows the result of attempting to establish a reliable test temperature for PET in air. At low test temperatures, the test values appear to be independent of the type of atmosphere. But note that as the values in nitrogen reach a plateau, the values in air skyrocket. The reason for the sudden increase in apparent moisture content is that PET, when exposed to elevated temperatures in air, generates a chemical called acetaldehyde (AA). Processors who produce PET bottles are well aware of AA because it contributes undesirable taste and odor properties and its presence in bottles must be kept to a minimum by carefully controlling processing temperatures. AA reacts with the methanol in the Karl Fischer reagent to form water. 

Method development is important in any moisture measurement technique. But when moisture measurement is turned over to a loss-in-weight system, none of the safeguards built into the chemical methods is in force. The weight loss can be almost anything, and the higher the measurement temperature goes, the more mass will be lost. The same AA that skewed the results of the Karl Fischer instrument will also be recorded as moisture in the simpler loss-in-weight systems. 

In these more basic systems the tendency is to cope with the runaway moisture measurements by reducing the test temperature, in this case to a temperature near 200C. But at these conditions it can be shown that not all of the water is being removed from the material. If we refer back to Figure 1 it is easier to see that the measured moisture at 200C is 212 ppm, which is probably close enough to the recommended limit for good-quality molding. But the actual moisture content of this material is actually 297 ppm, or almost .03 percent. 

This small difference can be crucial to part quality. The problem is that the vast majority of all molding operations that are measuring moisture are using a loss-in-weight system. These systems are attractive because they are less expensive and do not require the use of any chemicals. But the fact that they are simple to use does not make them accurate. And the fact that they are called moisture monitors or moisture analyzers gives their users a false sense of security and the incorrect belief that they are really monitoring moisture content. 

In our next installment, we will explain the details of loss-in-weight moisture measurements and chemical methods.