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At Pounds of Plastic Inc., we have sold many pounds of nylon. I have been nicknamed “the nylon guy”. My theory is you must love what you sell. However, I was blessed with representing large firms producing nylon and I believe I’ve learned much. Nylon although a fantastic polymer that I am very passionate about has many idiosyncrasies and over many years the challenge has been to educate Engineers, Designers and Processors to facilitate good parts. Some of our recent articles related to nylon may be perceived as negative toward nylon.
Today again we are writing about nylon 6 and nylon 6,6 the two most prevalent nylons procured. In my plastic early days, I almost lost my finger to nylon 6,6. I was a “Production Engineer”. I was called to a molding machine. The Technician was no where to be found. The sprue was stuck in the mold. I pulled the sprue on a mold to get the mold/machine running. There was very hot material waiting for the next shot and it blew through the nozzle through the sprue and coated my hand after I unplugged the orifice. It was steam that pushed out the molten nylon.
I was rewarded with a 3rd degree burn. It was predominantly on my index finger on my right hand. Molten nylon sticks very well to skin. Just call me Lefty. As I reflect on this incident, it was caused by lack of knowledge. In hindsight I should have backed off the nozzle from the mold. I was in too much of a hurry.
The reason the sprue stuck was because the nylon was not dry, and the nozzle became too cold. The water in the nylon was not a liquid any longer but a pressurized gas. My theory on why poorly dried nylon exhibits “freeze off” and spue “stick” has to do with the “refrigeration effect”. When gas goes from a pressurized to unpressured it expands rapidly and thus cools. This is how a refrigerator works, expansion of a gas. The water was at greater than 509o F so obviously it was a gas called steam. Water boils at 100o C or 212o F. Above this temperature under atmospheric pressure the water is a gas. The pressurized gas pushed the nylon against the wall of the sprue with excessive pressure. The force was too large to overcome when the mold opened. The sprue stuck. Nylon 6,6 is a solid at 509o F and below. The nozzle was pushed up against a relative cold mold. Nozzles are cooled being pushed against molds that are relatively cool.
Nylon has played significantly in the world of thermoplastics. Many items are manufactured from nylon. Nylon was invented during the time frame 1928-1935 by a gentlemen named Wallace Carothers who worked for E.I. du Pont de Nemours. Nylon 6,6 was commercialized in 1935. I presume there were others on his team that contributed greatly to bring this synthetic fiber to market. Was nylon the first commercialized thermoplastic? There were products such as nitrocellulose and cellulose acetate that were conceived and marketed prior to nylon. Bakelite (Phenol formaldehyde) was also being sold. Bakelite is a thermoset. World War Two initiated shortages of goods. As a result, products such as nylon in cord or rope became very useful and popular. Think of parachutes. World War two also brought on mind set related to costs. Attitudes like “cost is no object”. This was a way to save lives. Once a product is used and has a successful track record it continues to be used. We use the term “Grandfathered”
If one could procure a thermoplastic that had most of the characteristics/physical properties of nylon and maybe physical properties that were even better than that of nylon, you (rhetorical) as a Designer would likely be interested in procuring this material into thermoplastic applications.
What is the easiest material to process? What is the easiest material to design parts from? The answer is “the one you know the most about.”
The negatives of nylon. Nylon is hygroscopic. This means that it absorbs moisture. I am not sure if this term (word) was developed along with nylon. Since nylon parts expand when they absorb moisture, “hygroscopic”. Nylon’s commercialization proceeded polycarbonate and other water absorbing polymers. Nylon therefore requires drying prior to processing it, as does polycarbonate and polyesters. We use the term hygroscopic to describe materials that absorb moisture. Polycarbonate molded parts do not expand post molding after absorption of moisture.
My largest “pet peeve” regarding nylon is this fact that it, Nylon changes as it absorbs and loses moisture. It is not that nylon absorbs moisture but the issues that come with that absorption. I dislike how moisture absorption impacts the processing of the nylon and the physical properties of the finished parts. This phenomena must be paid very much attention to when designing and processing nylon.
Processing data says to dry unreinforced nylon to yield between 0.08 to 0.20 percent water by weight for the nylon portion of the compound. Impact modified nylon can have as much as 20% other polymer that doesn’t absorb moisture, hence the nylon portion of the compound.
Other thermoplastics such as polyesters like polycarbonate and PBT (polybutylene terephthalate) the recommended moisture content is zero prior to the material entering the throat of the molding machine. Old literature recommends 0.03% by weight for polycarbonate and polyesters. This is due to the drying technology from decades past. Drying technology has improved. It is highly recommended that polyesters be “bone” dry prior to processing.
Polycarbonate is reported to have been invented by Dr. Dan Fox of General Electric in 1953. However, Bayer’s Herman Schnell was quicker in patenting polycarbonate. There is a bit of humor in the previous sentence. GE paid Bayer royalties for many years, the life of the patent. It is reported that Wallace Carothers invented polycarbonate in the 1930s, the same time frame as his nylon invention. More than twenty years prior to the Bayer patent. He was also working on polyesters such as PET (polyethylene terephthalate) at the time.
Why does the literature say that it is ok to have moisture present in the nylon entering the throat of the molding machine? Why have any moisture present? Was this literature written prior to reciprocating screw injection molding machines? Having moisture present in a plunger machine may have been beneficial to filling the part?
I was told that the purpose for the moisture in the nylon is that it acts like a plasticizer. During injection, the moisture enhances flow of the nylon into the mold. It is therefore my belief that nylon does not suffer hydrolytic attack by moisture at process temperature other wise the literature would say to dry to as close to zero percent moisture as possible. Too much moisture in nylon 6,6 can cause in the case of injection molding for the nozzle to freeze off and spit. (See the above reference to burnt finger). In the case on nylon 6 we don’t experience the freeze off or spitting that we do with nylon 6,6.
Unreacted monomer or dimers and oligomers (these are short nylon molecules also known as volatiles) in nylon 6 will come to the molded part surface as the steam in the molten nylon expresses force and pushes these relatively low length molecules to the surface. Caprolactam and volatiles will be blown to the surface of the parts. We call this phenomena “caprolactam bloom”. White streaks will be present on the surface of the molded part. Since the Manufactures recommend some moisture, we can deduce that nylon doesn’t suffer hydrolytic attack.
Caprolactam is a crystalline cyclic amide with a melting point of 70 °C. It is soluble in water and most oxygenated and chlorinated solvents, and some hydrocarbons. Caprolactam; Melting point, 69.2 °C (156.6 °F); Boiling point, 270.8 °C (519.4 °F; Solubility in water. 866.89 g/L (22 °C).
Note that Molecular weight (length of molecules) has a relationship to flow. The shorter the molecules the easier the nylon flows. This can be demonstrated in a “Melt Index” machine with polyethylene. It is very difficult to create consistent flow data on nylon in a Melt Index machine. Melt Index machines are generally not used for nylon. I believe it is due to the moisture absorption of the nylon and the moisture affect on flow at process temperature. This moisture absorption inconsistency makes for flow changes that reflect on the accuracy of the grams per 10 minutes test in a melt Index machine.
The next question relates to drying temperature. Both nylon 6 and nylon 6,6 the recommended drying temperature is 85o C or 185o F. This means the dry air entering the hopper of the dryer is at 185o F (85oC). It is reported that nylon 6 has a melting temperature of 420o F or 228o C. Nylon 6,6 it is reported melts at 509o F or 268.8o C. Both are semi crystalline thermoplastics, and both melt over a temperature range. The crystallinity holds the molecules together and this “bonding” requires a certain amount of energy to allow the molecules to move. Think it like the molecules are tiny magnets. Neither nylon 6 nor nylon 6,6 have a definitive melting point. Both are dissimilar to water in this respect. Water freezes and melts @ 32o F or 0o Celsius. This is a melting point and the freezing point of water. Nylon 6 and nylon 6,6 have melting ranges, not melting points.
Nylon should be dried in a desiccant bed dyer. The moisture content of the dried unreinforced nylon needs to be at 0.08 to 0.2 % by weight entering the throat of the molding machine. I reiterate that the literature suggests a wee bit of moisture present in the nylon assists filling the part. Processors of nylon need a moisture analyzer. Processors need to use them. The addition of glass or mineral or impact modifier to a nylon compound will change the allowable moisture content.
Note that it is my opinion, Molders cannot control the moisture content of nylon in drying process. The moisture content of the nylon going down the throat of the machine. Most Processors dry the nylon to 0.03% or lower, AKA bone dry.
* If nylon 6 melts at 420o F and nylon 6,6 @ 509o F, why is the recommended drying temperature for both nylons 185o F (85o C)? Nylon will certainly dry more readily at a higher dryer temperature than 185o F. Nylon is solid at 185o F (85o C).
The drying process of nylon can be confusing. At above 185o F (85o C) nylon in the presents of oxygen will oxidize. Another term for oxidization is “burning”: Both materials nylon 6 and nylon 6,6 will turn yellow after exposure to temperatures above 185o F (85o C) in the presents of oxygen.
How does a molder mold parts when the moisture content of the material going down the throat of the machine varies? It is my belief that this is not considered when drying nylon. The drying equipment in my opinion could be improved. For example, vacuum dryers could be employed. I believe most processors go for “bone dry”. Targeting moisture content percentage of 0.08 to 0.2% of the nylon portion of the compound going down the throat of the molding machine is darn near impossible with current familiar technology. This could be accomplished with a considerable outlay of capital. Here we are speaking of moisture & temperature sensors placed strategically in the flow of the nylon pellets. Does nylon once the moisture level of zero achieved start to polymerize? How does a molding company control this as the extending of molecule lengths will affect the flow of the molten nylon? The nylon will become more difficult to push.
Over the last several years experimenting with vacuum drying nylon, has in my opinion improve the processing issues associated with nylon when vacuum dryers are used. These experiments have concluded that a small amount of moisture present in the nylon going down the throat of the machine is not necessary.
Another issue with processing and designing with nylon is shrinkage. Nylon exhibits differential shrinkage. In the case of injection molding the nylon shrinks more in the crossflow direction than in the direction of flow. This phenomena is much more apparent with glass reinforced nylons. This type of shrinkage is named anisotropic. Amorphous thermoplastics such as ABS exhibit isotropic shrinkage.
How does one design a precision part that will undergo anisotropic shrinkage and dimensional expansion upon moisture absorption post molding. How does one mold when the material entering the throat of the machine has a moisture content that is too high, the part may over pack? Nylon material that has a moisture content that is too low, the part will underfill?
Suppose, Pounds of Plastic Inc. could offer a product with all the attributes of nylon, but the material did not have to dried. This would save grief, ambiguity (variable concern) and considerable amount of money. Would you be interested in this material? Of course, you would.
We believe we have a great alternative to nylon in a polymer called #3. #3 doesn’t require drying prior to molding. Therefore, the molded part doesn’t absorb moisture. The molded #3 part doesn’t grow post molding. #3 doesn’t absorb sufficient moisture post molding to affect dimensions.
Large cost savings and energy savings are realized when a polymer doesn’t have to be dried. The cost savings relative to drying are huge. The liability of transferring material from one place to another is large. From Gaylord to dryer, from dryer to molding machine for example. The potential for contamination among other things.
Nylon regrind is a science in itself. Once regrind nylon absorbs moisture it is very difficult to evolve due to particle size relative to surface area. # 3 regrind is not dissimilar to handling polyethylene regrind
#3 doesn’t exhibit an isotropic shrink. #3 shrinks like amorphous polymers isotropically.
#3 has wear resistance that exceeds acetal.
#3 has a coefficient of friction that is less than acetal and nylon 6,6.
Wear and lubricity are different although often lumped together. Wear and coefficient of friction are different.
#3 has fantastic chemical resistance. Nylon has very poor chemical resistance against acids and bases.
#3 with glass reinforcement doesn’t undergo a reduction in physical properties with moisture absorption. A designer doesn’t have to worry about a drop in tensile strength, flexural modulus, flexural strength nor an impact property change with the absorption of moisture using #3. A Designer could use a 18-23% glass reinforced #3 in current application where 33% glass reinforced nylon is used and obtain the same or better part. Radiator end caps comes to mind to as a potential to use Number Three with glass reinforcement. Nylon seat belt components could be replaced by #3. This would result in large cost savings. Cost in electricity (drying) as well as processing costs (inconsistencies in the melt causing poor parts).
An Isotropic shrinkage is the type of shrinkage that amorphous polymers such as ABS exhibit.
Currently nylon 6,6 is very expensive and there are several factors that have contributed to this high price. It is time to change to #3.
The caveat to using #3 versus nylon is the difference in specific gravity. #3 has a higher specific gravity than does nylon: 1.24 versus 1.13. Part wall thickness reduction may be an option to compensate for the higher specific gravity.
If you require addition information pertaining to #3 and nylon, please don’t hesitate to contact us.
Have you ever wondered why our garbage bins (3) supplied by our town are made of virgin material?