Understanding Polymers

What is a polymer?

A polymer is a molecule, made from many joining smaller molecules called monomers. The word “polymer” can be broken down into “poly” (meaning “many” in Greek) and “mer” (meaning “unit”).

Conversational Nylon

Nylon is a member of the thermoplastic polyamide (PA) family, and is considered to be the first crystalline plastic. It was invented way back in the 1930’s but introduced for injection molding around 1943.

We all know nylon as a strong tough plastic. Why is nylon such a strong material? The basic strength lies in the electrical bonds between molecules. This is the same bond which occurs in nature in fibrous materials such as silk and animal tissue. Interestingly, the tensile strength of the muscle in your arm – at least those of us who have any muscle left – is very close to the tensile strength of nylon.

Many years ago when the chemists learned to analyze silk they discovered a very strong bond. From then on they were simply trying to duplicate this bond by using available chemicals. If you were to look through a microscope at a silk fiber and a nylon fiber, side by side, you would see a striking similarity. These early scientists produced, in reality, a synthetic silk. This was nylon.

Types of Nylon

Now, when you hear someone talk nylon, they talk numbers like nylon 6, nylon 66, nylon 610, nylon 612 and 11. You should be able to nod your head wisely and say, “Of course.” You should, that is, if you know what the numbers mean!

These numbers simply refer to the number of carbon atoms hooked together in the starting material.

For instance, nylon 66 is made from two starting materials, each of which has 6 carbon atoms hooked together in a row.

Similarly, nylon 610 is made from two starting materials, one has 6 carbon atoms in a row and the other has 10 carbon atoms in a row.

This is simple enough, but when we only say nylon 6, this means that there is only one starting material, caprolactam, which has 6 carbon atoms in a row. The single starting material link up with itself to produce a polymer.

The same for nylon 11. Here the single starting material has 11 carbon atoms in a row, and this material links up with itself to produce nylon 11.

What is Crystallinity?

Many trade journals have carried several articles about crystalline nylon. You may wonder what is meant by crystallinity?

First, a crystal is simply an orderly arrangement of molecules. This means that the molecules lie next to one another in a definite pattern.

Now you must take the first mental giant step! If all nylon were crystalline you might expect nylon to be stiff and brittle. Similarly, if all nylon were amorphous you might expect it to be soft and pliable like rubber.

Well you and I know that nylon is not completely one or the other. It is relatively stiff, yet can bend a nylon comb double and it doesn’t break.

Why is this?
Just as you might expect, the answer lies in between these two conditions. Nylon, in a molded or extruded piece both has crystalline and amorphous molecules. This is true of all nylons , whether you make a comb, a stocking thread, or a speedometer gear, the nylon is a mixture or crystalline and amorphous molecules – rigid sections and soft sections.

Are you convinced?
Do you understand that all nylon is a mixture of crystalline and amorphous molecules? Okay, now we come to a second mental giant step. (If you didn’t stumble on the first, this next one will be easy.) Why is there so much noise about a crystalline nylon? There is so much noise about crystalline nylon because of two critical factors which influence the behavior of molded nylon parts:

1) The amount or percentage of crystallinity.
2) The size and the distribution of the crystals.

Crystalline grade nylon is like the ice cream made on the hand freezer. The nylon crystals are small and well distributed. There is also a slightly higher amount of crystals present than in a non-crystalline grade nylon.

What does crystallinity mean to a molder?
First it means that the molded piece will solidify or set up faster in the mold. Molding cycles should be faster. This does not mean that the mold will fill easier. The ease of filling depends on other factors. Secondly, it means that in some molds the molded piece will release from the mold easier. Why? Reason being is that the highly crystalline piece is just slightly denser than a non-crystalline piece.

Finally, crystalline nylon has a significantly different appearance than conventional, amorphous grade nylon. Crystalline nylon is more opaque. If you were to look at the ends of the teeth of a comb molded from crystalline grade nylon you would see they would be opaque while the end of the teeth of a comb molded from conventional grade nylon will almost be completely clear. The difference can be important in a gear, or a bearing, or any wearing surface. The crystalline nylon surface is harder, will wear better, and absorbs moisture at a slightly slower rate.

Let’s go onto other characteristics such as viscosity, extractables, and moisture content with one word of caution. There is no single factor in nylon. The molding performance and the properties of the molded piece are the result of several factors. In many cases it is impossible to predict which material or formulation will be best for a particular job, without conducting actual molding trials. However if you understand the factors and how they work, together you will be in a much better position to analyze the customers requirements and discuss the job with confidence.

Viscosity

The viscosity of the polymer is probably the property which is the easiest for all of us to understand. Viscosity means – if you melt it in a pot – how hard is it to stir. The higher the viscosity the harder it is to stir the mess in the pot. The viscosity is dependent on how big the molecules are.

Extractables

In nylon 66, only about 1% of the starting molecules don’t get hooked up. However, in nylon 6 about 10% of the starting molecule don’t get hooked up. This amount of un-reacted molecules is called the extractable content, because most of this material can be removed by extracting the polymer in water. Whether this starting material it taken out or not can produce some interesting results.

Nylon with a lot of monomer (starting material) left in it is soft and pliable, and has a high impact strength. Nylon with most of the monomer removed is stiff and has lower impact strength.

Now, if you are really on your toes you may begin to wonder. We said earlier that high viscosity nylon is not necessarily a stiff nylon. Stiffness depends more on crystallinity. Then we come along and throw in this fact about monomer. What gives? It is really not as bad as it seems.

High monomer – low stiffness, high impact
Low monomer – high stiffness, lower impact
Low viscosity – stiffness depends on monomer content – easy flow
High viscosity – stiffness depends on monomer content – stiff flow Highly crystalline – highest stiffness, usually lower impact

When you try to mix all these up you get just what you might expect – a compromise. All this means there are several factors controlling the behavior of nylon and you should understand how they work together to really understand the material.
Now we arrive at the real bugaboo – simple, everyday water or moisture content.

Moisture Content

It is common knowledge that nylon is moisture sensitive. It picks up and gives off moisture depending on the surrounding humidity and temperature. Mother Nature is consistent though. If you know the temperature and humidity, then you can depend on her to see that nylon always has a moisture content to match the conditions. How long it takes nylon to get there is another story.

Of course nylon is not really different than any other things in nature. There is an equilibrium moisture content for all things, even lead pipes, concrete blocks and 2″x4″s and you and me. IT’s just that nylon is tempermental about getting there.

Basically, moisture works as a plasticizer. When the nylon soaks up all the water it can hold the nylon becomes soft and flexible. When you dry nylon it becomes stiff, and if you dry it low enough, nylon becomes brittle. Happily though, under most conditions encountered in the normal use nylon picks up and retains enough moisture to make it just about the toughest plastic material know.

Moisture content is not something to worry about if you understand it. Its effects can be used. Years ago when nylon was first being molded, some few molders understood the effect of moisture on nylon and installed dehumidifying dryers. By drying all material before molding they were able to run nylon like other fellows run styrene, and some of them made a mint.

For example, on a job molding hammer heads, one of the operators may get the ideas of checking the quality by beating on the molded piece. This is fine, and a nylon hammer head will take a lot of beating – but not when it first comes out of the mold. At this time the foreman had better wear his safety glasses because beating freshly molded nylon hammer hear will produce mainly a lot of pieces of nylon all over the room.

Have the operator moisture the condition pieces first. Then point is where impact strength is critical, nylon needs all the moisture content it can get. Most moldings, however just left in a normally humid room will be sufficiently conditioned in three or four days.

What is the tie-in between crystallinity and moisture?
It’s no secret. A nice stiff crystalline nylon comb becomes just like an old inner tube if you soak it in water for a few days. Then if you leave it on the shelf to dry for a few days, it will be it’s old self again. The answer is that most items are never subjected to such extreme environments.

Finally when nylon is packaged in drums it is very dry. IT can be molded directly out of the drum. When it is molded it is still very dry. In this condition most nylon appears brittle. Give it a couple of days and you will have a high strength, resilient, long wearing part.

Summary

We have tried to present a simple picture of four factors which influence the behavior and properties of nylon resins:
Crystallinity
Viscosity
Extractables
Moisture Content

We have tried to show that these four factors are interrelated so that anyone in the plastics industry can understand how nylon works. OF the four factors discussed, moisture content is, perhaps the most important. Any nylon which is good and dry usually molds well. Any molded piece of nylon which is permitted to pick up a little moisture is usually tough and resilient.

Crystallinity is the primary factor in stiffness and molding cycle.
Viscosity is the primary factor in mold filling and ultimate toughness.
Extractables is only a factor in nylon 6, and in nylon 6 it is a factor in stiffness and impact strength.

Even with the introduction and subsequent dramatic growth of other engineering resins, such as polypropylene, polycarbonate, nylon usage continues to grow because of nylons unique combination of properties.

Base Resins Overview

Amorphous vs. crystalline
Most all of today’s thermoplastics can be lumped into these two categories. There are, however, very distinct differences between the two as follows:

CRYSTALLINE polymers have a very dense “ordered” structure, in which the molecules in certain regions get tightly aligned. As heat is added, they remain solid until they reach their melting point, then all crystalline structure is destroyed and they become very easy flowing liquid like substance. Crystalline polymers include: Nylon, PBT, PET, Polypropylene and Polyethylene.

AMORPHOUS polymers don’t really melt well. Instead, they have a broad softening range. The molecular structure is more like random coils or “spaghetti like”. Very stiff flowing at low temperatures, but as heat is increased: space is added between the molecules making it more easily flowing. Amorphous polymers include: ABS, Acrylics, Styrene and Polycarbonates.

Fillers, Reinforcements and Additives

Processing and property characteristics can be altered greatly by incorporating different types of fillers, reinforcements, and additives.

Fillers: Nonmetallic minerals, metallic powders, glass spheres and organic material are added at fairly high percentages to nylons. They usually act as “extenders” to reduce the resin cost, but many also reinforce the resin to some extent or provide thermal property improvements.

Reinforcements: Usually fibrous in nature. The principal ones in use today are glass, carbon and aramid fibers. Less common are ceramic, alumina and boron fibers.

Additives: Flame retardants, pigments, plasticizers, lubricants, heat and UV stabilizers and impact modifiers are typical examples of other additives that are commonly used with nylon and other polymers.

Comparison of Filler or Additive

Filler or Additive Description Advantages Disadvantages
Glass Fiber 1/8″ short strand 10-60% loadings are common. Increases: strength, stiffness, HDT and dimensional stability. Reduces shrinkage and cycle time. Decreases true toughness and flexibility. Increases density and can cause warpage.
Glass Beads 40-50 micron solid spheres.Usually at 25 to 40% loadings. Increases compressive strength, stiffness and dimensional stability. Reduces shrink, cycle time and warpage. Decreases flexibility and toughness. Increases notch sensitivity and brittleness.
Calcined Kaolin Mineral (clay), very small particle size (2-3 microns) 25-40% loadings are typical. Increases stiffness and HDT. Reduces cost, sink marks and warpage. Improves paintability. Increases notch sensitivity, decreases flexibility and toughness and can increase warpage
Wollastonite Mineral. “needle like” particles of various size with aspect ratio larger than clay. 25-40% loading are typical. Increases HDT and stiffness when compared to clay, while decreasing shrinkage and mineral splay. Improved flow. Slightly rougher surface finish when compared to clay. Decreases toughness and can increase warpage.
Talc neral in the form of small “plates”. More common with Nylon 6. Mi Increases stiffness, decreases tool wear when compared to other minerals. Reduces cost. Decreases flexibility and toughness. Increases notch sensitivity.
Glass Fiber & Mineral Various minerals used together with short strand glass fibers. Has similar stiffness to a GF product with less warpage, lower cost and improved surface finish. Slightly lower strength and impact resistance, and increased notch sensitivity when compared to a GF product.
Metallics Stainless steel, copper, bronze and aluminum etc. in various fibers, flakes or powders. Improves thermal and electrical conductivity. Can improve static dissipation. Reduces toughness, increases density and cost.
Flame Retardants Various halogenated compounds. Can improve fire retardancy to a UL94 V0 rating. Decreased flexibility. High heat sensitivity can make processing more difficult.
Impact Modifiers Various thermoplastic elastomers used in various loadings. Improves impact resistance, flexibility and toughness. Decreases stiffness and tensile strength. Increases melt viscosity.
Heat Stabilizers Various copper salts, iodides and bromides used at low percentages. Reduces degradation from oxidation in high heat applications. Can discolor natural materials and make color matching more difficult.
Lubricants Aluminum, sodium, zinc, magnesium and various other metallic stearates. Can improve mold release and machine feed characteristics. Can reduce paint adhesion and increase mold deposit.
Pigments Carbon black, Titanium dioxide etc. Carbon black improves UV resistance and weathurability. Ti02 can improve appearance and cycle time. Decrease physical properties slightly.

Physical Properties

In this section we will discuss some of the physical properties most often reported on Material Data Sheets. It is important to realize that property data can be influenced by varying test speeds, specimen preparation, specimen thickness, etc. Therefore, the importance of end-use testing must be kept in mind.

Tensile Strength: Standard test: ASTM D638 / ISO 527 Tensile strength is a measure of materials ability to resist being pulled apart. Testing is carried out in a universal testing machine using dry as molded test bars or “dog bones”. The dog bone is gripped between a fixed and moveable crosshead. The moveable crosshead is made to travel at a constant rate until breakage occurs. The testing machine is equipped with sensors to measure the stress being exerted on the specimen.

With nylons, tensile strength can range dramatically depending on the specific grade being tested. With more flexible nylon (impact modified) results can be as low as 7,000 Psi.( 48 Mpa). Where higher strength formulations such as glass filled, can well exceed 30,000 Psi.(207 Mpa).

Elongation at Break: Standard test: ASTM D638 / ISO 527 Elongation is the total amount of stretching that occurs during the tensile test until the final breakage point is reached. As extensometer (strain gauge) is attached to the dog bone to record the amount of elongation or strain.

The more flexible nylons will typically register greater then 50% elongation, where the higher strength formulations, on the other hand, may break with less than 5% elongation.

Tensile Modulus Standard test: ASTM D638 / ISO 527 During tensile testing the amount of stress exerted on the dog bone, and the amount of strain as measured by the extensometer, is captured on a computer and a graph of the stress / strain curve can then be obtained.

Tensile Modulus is the ratio of the tensile stress to the corresponding strain before the plastic begins to deform.

Flexural Strength Standard test: ASTM D790 / ISO 178 Flexural strength is an indication of “stiffness”, and is a measure of how well a material resists bending. During this test, a dry molded test specimen ( 80 mm long x 10 mm wide x 4mm thick ) is supported at each end ( please visualize ) and a load is applied to the middle. The load is forced downward at a constant rate until a break occurs on the outer surface. The maximum stress applied is recorded as the Flexural Strength, and is expressed in megapascals. The data is captured and then plotted in the form of a stress / strain curve.

Flexural Modulus Standard test: ASTM D790 / ISO 178 Flexural modulus is an approximation of “Young’s Modulus of Elasticity” and is expressed as the ratio of stress to corresponding strain below the materials yield point.

Deflection Temperature Under-load Standard test: ASTM 648/ ISO 75 DTUL, sometimes referred to as Heat Deflection Temperature (HDT), is used as an indication of high temperature performance, by measuring how elevated temperatures affect stiffness.

This test is very similar to the flexural strength test, accept the applied load is held constant at the required force (in newtons). The test specimen is held on edge and is placed in an oil bath. The temperature of the oil is then increased by 120 K/h, until a bar deflection of 0.32 mm is detected (based on a test specimen height of 10 mm). The temperature (C) is then recorded at DTUL.

Impact Strength Standard test: ASTM D256 / ISO 180 Impact strength is an indication of material “toughness”. Impact data can be obtained by a number of different testing methods, but common tests used in the U.S. are the IZOD IMPACT and CHARPY IMPACT.

For measuring IZOD IMPACT (ISO 1A method) a dry molded test specimen measuring (please visualize) 80 x 10 x 4 (mm) is used. This bar can be tested un-notched or notched with a 8 mm, “V” cut into the bar with a 0.25 mm radius at the base of the groove. Both tests utilize a swinging pendulum type machine which delivers an impact on the notched or un-notched specimen. The machine records the loss of energy, and results are reported in kilojoules per square meter (kJ /m2 ) of specimen width.

Toughened nylons can exceed 80 kJ/m2, where the more notch sensitive formulations, such as mineral filled, can break at less than 3 kJ/m2.

Coefficient of Linear Thermal Expansion Standard test: ASTM D696 Like all other materials, including metal, plastics will contract when cooled and expand when heated. The CLTE is the ratio of the change in dimension from the original dimension, per degree change of temperature.

Specific Gravity Standard test: ASTM D792 / ISO 1183 Specific Gravity and Density are often times used interchangeably, there is, however a difference between the two.

Density is the measure of mass per unit volume, and is typically expressed as; grams / cm3, or kg/m3 (per ISO 1183).

Specific gravity is a dimensionless quantity, and is defined as the ration of the density of a given material, to the density of water.

Specific gravity = Density of the material / Density of Water
Density (g/cm3) = Specific gravity (23 C) x 0.998

Water Absorption 24Hr Standard test: ASTM D570 Water absorption (24 Hr) is the percent increase in weight of a material due to the absorption of H20.

Melt Flow Rate Standard test: ASTM D1238 / ISO 1133 The melt flow rate test, also referred to as the melt index test, measures the amount of polymer flow through an extrusion plastometer.

The test is carried out by feeding the material into a cylinder where it is heated to a specific temperature and then forced down by a weighted piston through a small orifice, where it is weighed. The results are then calculated to reflect what amount (measured in grams) would have been extrude in 10 minutes time.

This test is best suited for quality assurance reasons, such as checking lot consistency, rather then for comparing different materials flow or processing characteristics.

Flammability Standard test: UL 94 Flammability testing attempts to measure how material reacts upon exposure to an actual flame.

UL-94 Flammability class (V-0, V-1, V-2, 5V, HB) tests are carried out using separate specimens per class. In each test a specimen is subjected to a specified flame exposure. Whether or not burning continues after the flame is removed is the basis for classification.

The series of tests are performed by first exposing material to a very hot flame. If it does not ignite or drip it is given a 94-5V rating, which is the best rating. If the material ignites, it must then undergo the VERTICAL BURN test, where it is given a 94-VO rating if it extinguishes itself in a short amount of time. A 94-V1 rating can be given if it takes longer to self extinguish. If the material drips and takes longer to extinguish itself it can be given a 94-V2 rating. If the material does not extinguish itself, then it must undergo the HORIZONTAL BURN test, where the burning rate is measured and calculated in inches per minutes. If the material burns slowly and does not exceed 1.5″ per minute for the specimens having a thickness of 0.12″ to 0.500″, or exceeds 3″ per minute for thinner specimens less than 0.120″, a UL-94 HB rating can be given.