City Plastics Plastic FAQ's

WHAT IS A POLYMER 
ABBREVIATIONS
PROPERTIES
DEFINITIONS
CLASSIFICATIONS
PROCESSING
RECYCLING

What is a "Polymer"
The word Polymer comes from the Greek "poly" meaning many, and "meros", parts or units. A polymer is a group of many units. You combine many monomers (one unit) to create a polymer.
Polymer is often used as a synonym for "plastic", but many biological and inorganic molecules are also polymeric. All plastics are polymers, but not all polymers are plastics. Plastic actually refers to the way a material melts and flows.
Commercial polymers are formed through chemical reactions in large vessels under heat and pressure. Other ingredients are added to control how the polymer is formed and to produce the proper molecular length and desired properties. This chemical process is called "polymerisation".
A homopolymer results from polymerising only one kind of monomer. A copolymer results from using different monomers.
Homopolymers have the same repeating unit while copolymers (which can be random, block, or graft) can vary have different numbers of repeating units. A terpolymer results from using three different monomers.

Abbreviations for Common Polymers:
Polymers are commonly referred to by both their names and abbreviations. Commercial polymers are also frequently referred to by the trade names of their manufacturer.

ABS - acrylonitrile-butadiene-styrene terpolymer
BMC - thermoset polyester bulk Moulding compound
EVA - ethylene-vinyl acetate copolymer
LCP - liquid crystal polymer
PA - polyamide, commonly called nylon
PAN - polyacrylonitrile
PAS - polyarylsulfone
PBD - polybutadine
PBT - polybuylene terephthalate
PC - polycarbonate
PE - polyethylene see also:
HDPE - high density PE
LDPE - low density PE
LLDPE - linear low density PE
VLDPE - very low density PE
HMW-HDPE - high molecular weight HDPE
UHMWPE – ultra high-molecular-weight polyethylene
PEEK - polyetheretherketone
PEK - polyetherketone
PEI - polyetherimide
PES - polyethersulfone
PET - polyethylene terephthalate
PET-G - glycol modified PET
PI - polyisoprene
PS-b-PI - polystyrene/polyisoprene block copolymer
PI - polyimide
PK - polyketone
PMMA - polymethyl methacrylate, commonly called acrylic
PMP - polymethylpentene
POM - polyoxymethylene, commonly called acetal
PP - polypropylene
PPA - polyphthalamide
PPO/PPE - polyphenylene oxide, polyphenylene ether
PPS - polyphenylene sulfide
PS - polystyrene
EPS - expanded polystyrene
HIPS - high impact polystyrene
PSO,PSU - polysulfone
PTFE - polytetrefluoroethylene
PU,PUR - polyurethane
PVA - polyvinyl alcohol (sometimes used for polyvinyl acetate)
PVAc - polyvinyl acetate
PVC - polyvinylchloride, commonly refered to as vinyl
PVOH - alternate abbreviation for polyvinyl alcohol

RUBBER
EPR - ethylene propylene rubber
SBR - styrene butadiene rubber
EPDM - ethylene propylene diene monomer rubber
SAN - styrene acrylonitrile copolymer
SI - silicone
SMC - thermoset polyester sheet Moulding compound
TPE - thermoplastic elastomer
TPO - thermoplastic olefin
TPU - thermoplastic urethane
UF - urea formaldehyde

A more comprehensive list of TRADE and GENERIC NAMES is available at this page.

Polymer Properties
Polymers are characterised in many ways - by chemical or physical structure, by strength or thermal performance, by optical or electrical properties, etc.
Most textbooks will give qualitative and some quantitative data on polymer properties. Properties can vary widely however, between manufacturers, for different performance grades, due to additives and reinforcements, or other reasons. For more precise data, contact a representative from a polymer producer, compounder, or distributor for a spec sheet on a particular material and grade. Often grades are offered to suit the needs of specific types of applications.

Properties of interest typically include:

Physical Properties:
Specific Gravity, Heat Capacity, Mould Shrinkage

Mechanical Properties:
Strength (Tensile and Flexural) Modulus (Tensile and Flexural) Elongation, Hardness, Impact Resistance

Thermal Properties:
Heat Deflection Temperature, VICAT Softening Temperature, Glass Transition Temp, Thermal Conductivity, Thermal Expansion

Processing Characteristics:
Melt Flow Index, Melting Point, No-flow Temp, Shear Rate/Viscosity Relation, Compressibility (Pressure/Volume/Temperature Relation)

Optical Properties:
Light Transmission, Haze, Refractive Index

Electrical Properties:
Surface and Volume Resistivity, Dielectric Constant, Dielectric Strength, Dissipation Factor, Breakdown Voltage

Environmental Properties:
Chemical Resistance, UV Resistance, Flame Resistance (UL Rating) Oxygen Index, Water Absorption

Morphology:
Crystallinity, Orientation, Composition (Neat, Blended, Filled)

Polymer definitions
All definitions are given only within the context of the field of polymers.

ADDITIVE - A compound or substance added to a polymer during the final synthesis stages or in subsequent processing to improve or alter some characteristic of the polymer. Additives as a class of materials are not intended to increase strength properties. Examples: Pigments, lubricants, anti-stats, flame-retardants, and plasticisers. (See REINFORCEMENT and FILLER)

ALLOY (also BLEND) - A mixture of two chemically different polymers to form a homogenous substance having properties different but often a combination of the two original polymers. An alloy differs from a copolymer in that any polymer molecule in the mixture is representative of only one monomer. In a copolymer, all molecules have units of both monomers. Alloys are often referred to as blends or hybrids.

AROMATIC - A Description used for chemicals that have at least one ring structure derived from benzene in their chemical structure. Six carbon atoms forming a hexagonal structure with alternating single and double bonds (three of each) makes benzene rings. The description is very general and covers a wide range of chemicals. The word "aromatic" is used because of the strong offensive smell of benzene it its raw state. Many of the chemicals classified as aromatic have a very different smell or no smell at all. A benzene ring structure with one bonding site is a "phenyl" ring or group. (See ENZENE RING)

ATOM - The most basic composition unit of the elements composed of protons, electrons, and neutrons. Elements are any substance composed solely of chemically identical atoms. Carbon, oxygen, sulfur, nitrogen, iron, aluminium, silver and gold are but a few of over 100 known elements. New elements are still being discovered.

BENZENE RING - A chemical structure composed of six carbon atoms arranged in a stable cyclic structure. Each carbon atom is single bonded to the next carbon atom on one side, and double bonded to the carbon atom on the other side. Each also has a hydrogen atom bonded to it. Phenyl groups are benzene rings where one of the carbon atoms is bonded to another molecule, making the entire cyclic structure a substituent or side group of that molecule. Phenyl groups are relatively large and bulky, and generally have a significant influence on the properties of the polymer they are present in.

BONDS or BONDING - Forces between atoms, which hold them in relative proximity to each other resulting in larger structures, called molecules (organic materials only). Primary bonds are between atoms of the same molecule and are the strongest. They result from the sharing of two electrons by both atoms. Secondary bonds are between atoms of different molecules or remote sections of the same molecule. They are the result of attractions due to polarity, induced polarity due to displaced electrons, and temporary polarity due to vibration and spinning. These bond forces are weak in comparison to primary bonds.

COPOLYMER - A polymer composed of two different monomers where the repeating structural units of both are present within each molecule. The frequency and order of the units can vary as in the three most common types: random, block, and alternating. The end product generally has properties intermittent between those of polymers of the two composing monomers. The two materials must have compatibility to be copolymerised.

COVALENT BOND - A bond where one or pairs of electrons are equally shared between two atoms producing a stable electron configuration and a very stable molecule. Covalent is the strongest of the molecular bonds.

CRAZING - A series of, or the forming of very fine cracks in the surface of a material, usually a polymeric substance. Chemical attack or other degrading agents such as ultraviolet radiation generally causes crazing. (See "STRESS CRACKING")

DEGREE OF POLYMERIZATION (DP) - The number of repeat units in the chain of a molecule. In a condensation polymer a repeating unit is composed of a monomer group from each reactive species.

DIELECTRIC CONSTANT – Is the comparison of the capacitance of an insulating material to that of air. Capacitance is the ability of a material to store electrical charge when exposed to electrical current. A low dielectric constant is desired for plastic components used to insulate and isolate electrical components from each other. High dielectric constant materials are desirable for use as the insulator portion of capacitors, so that the electrical energy can be stored in as small a volume of material as possible.

FILLER (also EXTENDER) - A compound or substance added to a polymer during the initial synthesis process or in subsequent processing to decrease the volume of resin needed to produce a given product. Fillers are generally much lower in cost than the resins they are used in, thus reducing resin cost per part. Fillers or extenders are generally not used with engineering resins. (See ADDITIVE and REINFORCEMENT)

GRADES - Refers to polymers, which belong to the same chemical family, and are produced by the same manufacturer. They may vary in processing or performance due to differences is molecular weight, additives, or other structural features. For example, a supplier of polycarbonate may have flame resistant grades, glass fibre reinforced grades, a conductive grade, and easy flowing grades.

MOLECULAR WEIGHT DISTRIBUTION - Statistically describes the sizes and frequency of occurrence of different molecular chain lengths within a given sample or lot of polymer. Wide and skewed distributions result in significant variance in properties. Narrow distributions are more consistent.

MOLECULE - A group of atoms bonded together which forms the fundamental structural unit of most organic substances. The number of atoms can range from two to millions. A molecule is the smallest unit of a substance that still retains the properties of that substance.

MONOMER – Are small molecules of an organic substance which are the most basic structural unit of polymers. Monomers are generally gases or liquids. When bonded together in long chains they form solid materials or polymers.

ORGANIC - Refers to a general class of substances whose composition is based on the element carbon. Organic infers some relationship to materials that at some point in time were alive.

PLASTIC - A synthetic or naturally occurring organic substance generally characterised by being formable or pliable at some stage during its formation or subsequent manufacturing process. Many materials, such as glass, become plastic under the right conditions.

POLYMER - A substance formed by a chemical reaction in which two or more small organic units join to form large units composed of repeating small units. This term is often used interchangeably with "plastic". (See - PLASTIC)

POLYMERISATION - The process or chemical reaction by which low molecular weight monomers are converted to high molecular weight polymers. Most polymerisation processes are classified as condensation (step) reactions or addition (chain) reactions.

REINFORCEMENT - A substance or material added to a polymer during the final synthesis stages or in subsequent processing to improve the strength properties of the polymer. Examples: Glass fibres, carbon fibres, glass beads, mica, clay, and organic fibres. (See ADDITIVE and FILLER)

RESIN - Any of a large class of synthetic substances that have some of the properties of natural resin (or rosin) but differ chemically. "Resin" is often used as a general term for polymers or plastics, and denotes a class of material.

STRESS CRACKING - A series of or the process of cracking under induced mechanical stress. Stress cracking generally initiates with microscopic surface cracks causes by chemical attack or other degrading agent such as ultraviolet radiation. Under mechanical stress, the microcracks propagate eventually producing a localised failure.

TERPOLYMER - A polymer composed of three different monomers where the repeating structural units of all three are present within each molecule. The influences of all three types of monomer are evident in the property profile of the polymer. Common terpolymers include ABS and ASA (Acrylonitrile/Styrene/Acrylic)

VISCOSITY - A measure of a material's resistance to flow when a mechanical stress is applied. Viscosity is quantitatively defined in terms of shear stress and shear rate.

Classification of Polymers
There are many ways in which polymer properties or behaviour are classified to make general descriptions and understanding easier. Some common classifications are:

Thermoplastic vs. Thermoset:

Thermoplastics can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.

Thermosets undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network. Once they are heated and formed they can not be reprocessed - the three- dimensional molecules can not be made to flow under pressure when heated.

Amorphous vs Crystalline:

Polymers with nearly linear structure, which have simple backbones, tend to be flexible and fold up to form very tightly packed and ordered areas called crystals. Levels of crystallinity can vary from zero to near 100%. Time and temperature during processing influence the degree of crystallinity. Crystalline polymers include polyethylene, polypropylene, acetals, nylons, most thermoplastic polyesters, and in some cases polyvinyl chloride. Crystalline polymers have higher shrinkage, are generally opaque or translucent, good to excellent chemical resistance, low friction, good to excellent wear resistance.

Polymers with bulkier molecular chains or large branches or functional groups tend to be stiffer and will not fold up tight enough to form crystals. These polymers are referred to as "amorphous" and include polystyrene, polycarbonate, acrylic, ABS, SAN, and polysulfone. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated (no melting point), average to poor chemical resistance, high friction, and average to low wear resistance.

Addition vs. Condensation:

Polymers such as nylons, acetals, and polyesters are made by condensation or step-reaction polymerisation where small molecules (monomers) of two different chemicals combine to form chains of alternating chemical groups. The length of molecules is determined by the number of active chain ends available to react with more monomer or the active ends of other molecules.

Polymers such as polyethylene, polystyrene, acrylic, and polyvinyl chloride are made by addition or chain-reaction polymerisation where only one monomer species is used. The reaction is begun by an initiator which activates monomer molecules by the breaking a double bond between atoms and creating two bonding sites. These sites quickly react with sites on two other monomer molecules and so on. This continues until the initiator is used up and the reaction stops. The length of molecules is determined by the number of monomer molecules, which can attach to a chain before the initiator is consumed and all molecules with initiated bonding sites have reacted.

Commodity, Engineering, High Performance:

Another common classification of plastics is "commodity" versus "engineering". Commodity polymers have relatively low physical properties. They are used for consumer products, which require low cost, disposability, packaging or container related, low stress and low temperature resistance, limited product life, and high volume production. (e.g. PE, PS, PP)

Engineering polymers have properties towards the high end of the spectrum. Strength and thermal resistances are the most significant. Their price may range from two to ten times as much as a commodity polymer. They are used in: housings, brackets, load bearing members, machine enclosures, and applications requiring wear resistance, long life expectancy, flame resistance, and the ability to endure cyclic stress loading. (e.g. PC, POM, PBT)

The properties of high performance polymers are at the highest end of the spectrum, generally with very high strength and thermal resistance. They tend to be very expensive, priced above most engineering polymers. They are used in high temperature, high stress applications, in harsh environments, and low to medium volume production. (e.g. PEEK, PEI, LCP)

Processing Methods for Polymers
There are many processing methods for polymers. Commercial processing equipment can range from a few thousand dollars to many millions of dollars.
In addition to the equipment itself, tooling is generally required to make a particular shape.

Moulding:
Compression Moulding, Transfer Moulding, Injection Moulding, Gas Assisted Injection Moulding (GAIN) Reaction Injection Moulding (RIM/SRIM) Injection/Compression Moulding, Blow Moulding, Extrusion Blow Moulding, Injection Blow Moulding, Injection Stretch Blow Moulding, Rotational Moulding

Extrusion:
Rod, Pipe, Sheet, Profile Extrusion, Coextrusion, Extruded/Blown Film, Extruded Foam, Pultrusion,

Casting:
Cast Film, Cast Shape, Vacuum Casting

Forming:
Vacuum forming, Thermo forming, Pressure Forming

Coating:
Powder Coating, Dispersion Coating, Extrusion Coating and Laminating, Spray Coating, Dip Coating

Recycling
Most thermoplastic polymers can be recycled - that is converted from their initial use as a consumer, business, or industrial product, back into a raw material from which some other product can be manufactured. Recycled materials are often classified as Post-Industrial and Post-Consumer. Post-Industrial includes such things as manufacturing scrap, containers and packaging.
Post-Consumer is basically any product, container, packaging, etc. that has passed through the hands of a consumer, e.g. plastics bags, beverage containers, carpeting, home appliances, toys, etc.

Thermoset polymers can only be recycled for use as an inert filler (something to take up space) in another material.

The keys to effective recycling are:
1.an efficient infrastructure for collecting used materials
2.ease of separation and low levels of contamination
3.an established market for reprocessing/reusing the materials

The contamination issue is very important for plastics. While oil, grease, paper labels, glue, etc. will burn off when glass or metals are recycled, they become contaminants and degrade thermoplastics during reprocessing.

There are three versions of the recycling logo. The original one was three arrows chasing each other in the shape of a triangle, the second was just a triangle, and the current one is a pair of angle brackets. < 1 >

The number inside the triangle or brackets indicates the material used in the part.

There are six specific categories and a generic seventh for "other". In the case of "other" it is good practice to put the material name under the recycling logo.

See more hints at the pages below