This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at [email protected].
Conditioning nylon isnâ€™t always the best way to solve brittleness if it masks a bigger problem.
Most nylon polymers, including those used in the largest volumes, nylon 6 and nylon 6/6, have an unusual affinity for water. Nonpolar polymers such as polyethylene and polypropylene absorb only about .01% of their weight in water because the polar water molecule has no affinity for these polymers. These materials have a chemistry that resembles a hydrocarbon oil or gasoline and it is well known that oil and gasoline do not mix with water. For the same reason, polymers like polyethylene do not absorb water.
Most polar polymers, like polycarbonate and acrylic, absorb moisture but typically reach equilibrium at .10-.20% by weight. However, nylons contain a chemical feature known as an amide group, illustrated in Figure 1 as part of a short segment of a nylon 6/6 polymer chain. The (N-H) portion of this linkage creates a very polar group that gives rise to a relatively strong attraction between the nylon chains. It is this attraction, known as a hydrogen bond, that is responsible for the relatively high melting point of most nylon materials. This hydrogen bond is also the constituent that attracts water molecules to each other so strongly and is responsible for waterâ€™s unusually high boiling point.
This similarity in bonding types produces an unusual affinity between water and nylon, and at equilibrium, materials like nylon 6, nylon 6/6, and nylon 4/6 can hold approximately 1.5-2% of their weight in water. This value can be substantially higher if the polymer is immersed in water and is allowed to reach a saturation point.
Glass transition temp
When water molecules become absorbed into the nylon matrix, they attach themselves to the amide groups in the nylon chains. This forces the nylon chains farther apart, reducing the attractive forces between the chains and lowering a property known as the glass transition temperature. Below the glass transition temperature, polymers like nylon 6 and nylon 6/6 are relatively strong, stiff, and somewhat brittle, particularly in areas where stress concentrations due to features like sharp corners are present. Above the glass transition temperature, an unfilled nylon 6 or nylon 6/6 loses approximately 80% of its room-temperature stiffness and strength, but in the process becomes substantially tougher.
In a so-called dry-as-molded nylon, the glass transition temperature is approximately 75Â°C. With sufficient moisture absorption, this transition temperature can decline to room temperature or even below. The effect of this can be observed in Figure 2. This shows the modulus vs. temperature behavior of a nylon 6/6 with .2% absorbed water and 1.2% absorbed water.
The effect on the glass transition is easily observed. Because in our world we are stuck at room temperature, what we observe is a material that becomes increasingly flexible as this process of moisture absorption progresses. This is the reason that most nylon materials have two data sheetsâ€”one for dry material and one for conditioned polymer. The strength and stiffness of the conditioned material is substantially lower, but the impact properties are significantly improved. If we take the time to measure some dimensions on a molded nylon part before and after moisture conditioning, we will also notice that the conditioned part is larger. This is a direct result of the water forcing its way between the nylon chains and pushing them farther apart than they would be in a dry product.
Processors and end users who use nylon have become very familiar with the effects that water absorption has on that material. In applications where high loads are generated, such as in snapfit assemblies, nylon that is still close to its dry-as-molded state may exhibit brittle failure, and we have learned that this failure mode can be mitigated by conditioning the parts to bring them up to their equilibrium moisture content. This frequently solves problems with the assembly process.
The moisture conditioning process takes many forms. Some simply pour a prescribed amount of water into molded parts contained in a moisture-proof package such as a polybag. Others prefer placing saturated paper towels into the package with the nylon parts and allowing the water to migrate out of the paper and into the nylon. Some go as far as boiling the parts. This not only increases the moisture uptake rate, but also ensures that the moisture is absorbed more uniformly throughout the wall of the part.
While rapid moisture conditioning is a legitimate method for improving the impact resistance of nylon products, there should be concerns with using it indiscriminately. A nylon product may be temporarily brittle while it comes to equilibrium with the atmosphere. But it may also be brittle because the material has been degraded during the molding process. In such situations, the brittle condition is not simply a temporary symptom of low moisture content, but rather is a permanent condition brought about by reduced molecular weight.
The problem is that this shortcoming can be covered up by pumping large amounts of moisture into the polymer. Under such conditions, the polymer becomes sufficiently flexible so that it no longer appears to be brittle. But a moisturizing process that is performed rapidly often introduces more moisture into the polymer than it can retain in the long term. If this happens, then when the excess moisture comes back out of the polymer, the brittle condition can return, usually after the part has gone into the application.
Failure in the field
This occurred in an application involving a critical part in a consumer product. Parts produced in an unfilled nylon 6/6 were received from the molder that appeared to be more brittle than usual. The explanation was that the parts had been molded just a few days earlier. It was winter in a northern state where the indoor humidity was very low and therefore moisture uptake was slow.
The corrective action was to moisture condition the parts. However, this was done very aggressively, and the final moisture content of the conditioned parts was 3.2%. The parts worked initially, going through the assembly and testing process without any obvious problems. However, once in the field the parts began to fail. When the product was brought back in for evaluation, the moisture content of the product had declined to 1.5-1.6%.
Field experience has shown that this is a consistent value that is obtained for parts that have been allowed to come to a true equilibrium with ambient surroundings. It will be higher in extremely hot, humid environments or in situations where the part is immersed in water or used in close proximity to water, but in most cases a part molded in unfilled nylon 6/6 can only hold about 1.5% water by weight.
This experience contradicts a lot of the data published by material suppliers showing the conditioned moisture content at 2.5%. But much of this early work was performed using accelerated techniques that had a tendency to introduce more moisture into the polymer than it could hold in the long term. Field experience shows that values of 1.5% for an unfilled material are much closer to the norm.
It is also important to emphasize that this value is by weight of polymer. If a material contains 33% glass fiber, then one-third of the polymer has been replaced by an inorganic material that is not hygroscopic, and therefore the amount of water that this compound can hold will be proportionally lower.
Testing for MFR
So how would the end user of the parts have been able to detect the root cause of the brittle condition? It was accomplished quite simply by performing melt-flow-rate (MFR) tests on virgin resin, good molded parts, and brittle molded parts. The MFR of the virgin material was 9 g/10 min while the good product had an MFR of 11 g/10 min and the brittle parts had an MFR of 26 g/10 min. Remembering that for unfilled polymers the upper threshold for an acceptable increase in MFR is 40%, the good parts are clearly below this benchmark at 22% while the bad parts are far above it at nearly 200%.
Analysts who use MFR tests to evaluate nylons run into some significant problems. While MFR is used as an indicator of relative average molecular weight in many materials like polycarbonate and polypropylene, few nylon suppliers list MFR values for their materials. The reason again has to do with the unusual relationship between nylon and water. Unfilled nylon is considered to be dry at moisture contents between zero and .20%. This is unusual. Most polymers that are dried need to have moisture contents below .10% and for many of the more critical polymers the upper limit is .02%. Therefore, for most materials, the range of allowable moisture content values is quite small, but for nylon it is relatively large.
In addition, because water acts as a plasticizer in nylon, the moisture content has a significant effect on the measured MFR of the material. The table above shows this effect on an unfilled nylon 6. At .20% the MFR of the material is nearly 40% higher than it is at .04%. This makes it difficult to detect polymer degradation because a change in moisture content can either mask or accentuate a difference in molecular weight.
There are ways around this. One is to be sure that whenever products are being compared to each other, they are all dried to approximately the same moisture content. This can be done, but it requires that the moisture content of each sample be verified at the time the MFR tests are performed. And since more than 95% of the devices called moisture analyzers used in the industry today do not actually measure moisture, this is a difficult proposition for most manufacturers.
Another solution is to establish a calibration curve between moisture content and MFR. Figure 3 shows the graphical relationship contained in the table. MFR changes with moisture content in an approximately linear manner; therefore, the math behind a correction factor is fairly simple. A convenient benchmark might be to normalize all the raw data to a moisture content of 1000 ppm (.10%). This does not relieve us of the responsibility of making an accurate moisture content measurement, but it does allow us to directly compare results from samples with widely varying moisture levels as long as they are all dry.
Donâ€™t mask it, fix it
All of this is just too much work for most people. Therefore, in many cases the average molecular weight of nylon is measured using a property known as relative viscosity. This removes the need to control and measure moisture content because the sample is tested at room temperature where no breakdown of the polymer can occur due to the presence of water.
However, it also involves dissolving the sample in formic acid, an activity not for the faint of heart and therefore seldom attempted by processors. And while this is often the preferred technique for evaluating the molecular weight of nylon polymers, you seldom find the value for relative viscosity on the data sheet.
But whatever choice is made for evaluating the molecular weight of the polymer, a check for polymer degradation must be a consideration before water is added to a nylon product to correct for brittle behavior. Otherwise, there is the risk that the added moisture will simply mask a bigger problem that will be part of the partâ€™s property profile for the life of the product. In cases where the polymer is degraded, this lifetime may be shorter than expected.