POLYURETHANE (PU) AND THERMOPLASTIC POLYURETHANE (TPU)

Polyurethane is the name given to materials produced by the reaction of isocyanates with polyols. I say "materials", in the plural, because polyurethanes are extremely versatile and allow as the manufacturing of hard products as elastomeric/rubbery products and foams/sponges. In addition, polyurethanes can be thermoplastic or thermosetting (or thermoset).

The properties of polyurethanes can be changed by varying the type or quantity of three basic substances of their production: diisocyanate, short-chain diol and long-chain diol. The classification is determined by the type of polyol used, for example, polyester polyurethane (PUR) or polyether polyurethane.

Polyether PU is more resistant to hydrolysis and generally has lower glass transition temperature, while polyester PU has better resistance to oils and fuels.

Thermoplastic Polyurethane (TPU)

TPU's contain linear flexible coiled and rigid packaged segments, able to merge without degradation of urethane links. They have essentially linear chains, with molecular weight between 15000 and 40000 with a glass transition temperature below 32°F (0°C). TPU's are usually produced with polyols of molecular weight between 600 and 4000, chain extenders with a molecular weight between 61 and 400 and diisocyanates. Because of the diversity of combinations of flexible and rigid segments, TPU's can be formulated as different materials, from soft and flexible elastomers (elastic materials that bridge the gap between plastics and rubbers), to rigid plastics with high modulus.

A TPU can be processed using common techniques, such as injection molding, extrusion, blow molding etc. Drying is recommended prior to use until it get below 0.05% moisture content, which may take from 3 to 6 hours between 194°F (90°C) and 230°F (110°C) depending on the hardness of the material.

Processing of TPU requires attention to the speed of the injection molding machine (or extruder) screw and the barrel temperature because of its high viscosity, since a too low temperature and/or high screw speed may cause shear degradation, in addition, any cold point (mainly in injection nozzle) can be a major problem resulting in clogging. The table below is a guide for injection molding:

Hardness Feed zone Transition zone Metering zone Nozzle
  °F °C °F °C °F °C °F °C
70A 338 - 356 170 - 180 365 - 374 185 - 190 374 - 383 190 - 195 374 - 392 190 - 200
80A 365 - 374 185 - 190 383 - 392 195 - 200 392 - 401 200 - 205 392 - 401 200 - 205
85A 383 - 392 195 - 200 392 - 401 200 - 205 401 - 410 205 - 210 401 - 410 205 - 210
90A 392 - 401 200 - 205 401 - 410 205 - 210 410 - 419 210 - 215 410 - 419 210 - 215
95A 392 - 401 200 - 205 410 - 419 210 - 215 419 - 428 215 - 220 419 - 428 215 - 220
98A 392 - 401 200 - 205 410 - 419 210 - 215 419 - 428 215 - 220 419 - 428 215 - 220
64D 401 - 410 205 - 210 410 - 419 210 - 215 419 - 428 215 - 220 428 - 437 220 - 225
72D 401 - 410 205 - 210 419 - 428 215 - 220 428 - 437 220 - 225 428 - 437 220 - 225

Another recommendation is to post-cure the part with the goal of improving product performance, this treatment is to keep the part in an oven dryer for a period as shown below:

Hardness Curing conditions
Shore A 70-85 176°F x 16h (80°C x 16h)
Shore A 90-98 194°F x 16h (90°C x 16h)
Shore D 64-72 212°F x 16h (100°C x 16h)

Features (Polyurethanes in general):
- Increased hardness by adding fillers such as glass fiber, talc etc
- Good resistance to oils, solvents, oxidation and ozone
- Resistant to the action of microorganisms
- Good resistance to hydrolysis
- Loss of properties at low temperatures and high temperatures
- It can be flexible or rigid, thermoset or thermoplastic

Applications:
Smartphone case (TPU), hidraulic seals (TPU), footwear sole (TPU), hoses (TPU), tire chains (TPU), caster, ball (TPU), foam (expanded PU), expanding PU foam spray etc.
Smartphone case (TPU)
Hydraulic seals (TPU)
Footwear sole (TPU)
Hoses (TPU)
Tire chains (TPU)
Caster (TPU)
Ball (TPU)
Foam (Expanded PU)
Expanding PU foam spray

Bibliography:
HARPER, Charles A.; PETRIE, Edward M. Plastics Materials and Process: A Concise Encyclopedia. Hoboken: John Wiley & Sons, Inc., 2003.
WIEBECK, Hélio; HARADA, Júlio. Plásticos de Engenharia: Tecnologia e Aplicações. São Paulo: Artliber Editora, 2005.
Article posted in Oct 2, 2014
About the author: Daniel Tietz Roda is Plastics Technologist graduated from the FATEC/ZL and Mechanical Design Technician from ETEC Aprígio Gonzaga, in São Paulo, Brazil. Roda worked 5 years with technical assistance and development of plastics in industries and nowadays is the publisher of this website.
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