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polymer faq

Q. What is a "Polymer"?

Ans. 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 "polymerization".

A homopolymer results from polymerizing 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.

Q. Abbreviations for Common Polymers?

Ans. Polymers are commonly refered to by both their names and abbreviations. Commercial polymers are also frequently refered to by the trade names of their manufacturer.

ABS - acrylonitrile-butadiene-styrene terpolymer
BMC - bulk molding compound
EVA - ethylene-vinyl acetate copolymer
LCP - liquid crystal polymer
PA - polyamide, commonly called nylon
PAN - polyacrylonitrile
PAS - polyarylsulfone
PBD - polybutadine
PBT - polybutadiene 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 - ultrahigh-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, subdivided as:
homopolymer, random, impact and block copolymers
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
PVC - polyvinylchloride, commonly refered to as vinyl RUBBER
EPR - ethylene propylene rubber
SBR - styrene butadiene rubber
EPDM - ethylene propylene diene monomer rubber
SAN - styrene acrylonitrile copolymer
SI - silicone
SMC - sheet molding compound
TPE - thermoplastic elastomer
UF - urea formaldehyde

Q. Different Properties of Polymers?

Ans. Polymers are characterized 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
Mold Shrinkage

Mechanical Properties:
Strength (Tensile and Flexural)
Modulus (Tensile and Flexural)
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
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

Composition (Neat, Blended, Filled)

Q. What is the difference between Polymers and Plastics?

Ans.Most Polymers need some additives before they can be used as plastics. The proportion of additives can vary from as low as 300 ppm (0.03%) (Oleamide in LDPE) to as high as 75% (Plasticisers in PVC).

Some other materials when compounded with polymers change their properties substantially. In fact, at times they have properties markedly different from those of either the polymer or additives e.g. Carbon fibre reinforced nylon/polyurethane fibre reinforced polyester resins. Talc in the thermoplastic polyester.

Q. How Plastics are formed into useful items?

Ans. The single most important property of plastics is the change that they undergo on heating.

Some plastics can be softened by heating and will then harden again on cooling. This cycle of softening and hardening can be repeated many times without any appreciable change in the properties. Such plastics are termed THERMOPLASTICS.

There is the other class of Plastics which soften on heating and if kept at that temperature for some time, harden irreversibly. That is, such products cannot be softened by further heating. This type is called THERMOSETTING. In thermosetting materials, chemical reaction takes place during heating and crosslinks are formed between adjoining chains.

Heat causes the polymer molecules to turn flexible and if the crosslinks were not formed, the process could be repeated any number of times.

On the above thermal property depends the processing of plastics i.e., formation of useful articles. The various processing techniques include:

Injection moulding
Rotational moulding
Compression moulding

Q. Are plastics biodegradable?

Ans. Yes, certain biodegradable plastics are available. Only trials have been made in India. No commercial quantities are available as yet.

To carry this topic further, it may be said that biodegradable plastics will not on their own solve plastics refuse problem. For example, 20 year old rubbish dumps when opened up showed half eaten hamburgers and pastries which could be recognised in that shape.

Q. Are plastics recycleable?

Ans. Yes, almost all thermoplastics can be recycled. Thermosets are not recycled in the conventional sense. These can be ground and used as fillers in other plastics.

Q. What are composites?

Ans. These materials consist of two descreet phases. A fibrous phase surrounded by polymeric resin. The resin matrix allows stress to be transferred from one fibre to another. Thus a consolidated structure results which has the properties of a uniaxially drawn fibres.

Typical reinforcing fibres are - glass, carbon or organic fibres.

The matrix may be thermoset or thermoplastics, e.g., unsaturated polyester resin, epoxies (Araldite), nylon, polycarbonate, etc.

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