Polyurethanes

Thermoplastic polyurethanes are unique products. They are of high interest for a variety of applications due to their properties as following:

TPU is therefore successfully applied in many fields such as textile-, sports-, leisure and automotive-industry.

 

Schematic representation of the diisocyanate-addition-method

Today thermoplastic polyurethanes are usually placed on the market ready for processing. For the use of hot melt adhesives preferably linear PU are offered.





As polyols typically in use are polyester, polyether or polycarbonate diols with molecular weights of 1.000 to 4.000.
Short-chain diols are usually butanediol, hexanediol, or aromatic diols. The diisocyanate is mostly as diphenylmethane- or hexamethylene diisocyanate. The diisocyanate represents the chemical bond between the diols. The chemist calls the resulting binding "urethane". From the accumulation of these bonds in the long-chain polymer molecule the naming "polyurethane" is derived.

Chemical structure of thermoplastic polyurethanes

What do the following items have in common: A shoe, a hose, a wood glaze and a sponge, a textile melt glue?
They all contain polyurethane (PUR).

These plastics are characterized by the urethane group:


 
                        Figure 1: Structure of a urethane bond

Due to the isocyanates the user has the choice between aliphatic and aromatic components. The aromatic components tends to yellow under UV radiation.

Processing

To prepare polyurethanes for use in the domain of thermoplastics, it is not enough to bring forward the "curing reaction" to have it available as granules.
In fact it is important to tailor the material to the well established processing procedures. There are several well known widely used processing techniques in the plastics industry:

Injection molding, extrusion, blow molding, powders, pastes, calendering and sintering

For each of these techniques the demand on the material is widely differing with the processing characteristic.

For the calendering it needs materials which are easily meltable, highly homogeneous (free from flocks) and solidified slowly.
Additionaly the Powder-Slush- or  Sintering-Method require a particularly good flow property.

The soft flexible disordered segments are responsible for the chemical properties such as resistance to hydrolysis, chemicals and microbes.
Also the important cold temperature flexibility can be adjusted by the soft segment.
The ordered "crystalline" hard segments determine the processing behaviors such as the melting, solidification and thermal properties (heat resistance).

Resistance

Products based on polyethers or polyesters behave fundamentally different depending on their hardness. Furthermore environmental conditions such as temperature, concentration or duration of exposure have a major role.

Solvents

Aliphatic alcohols such as ethanol and isopropanol cause a slight swelling of TPU.
In ketones e.g. acetone, methyl ethyl ketone (MEK) cyclohexanone, as well as aliphatic esters such as ethyl acetate and butyl acetate, TPU is swelling strongly.
Polar organic solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), N-Methylpyrrholidon and tetrahydrofuran (THF) dissolve TPU.

Hydrolysis resistance


Soft TPU based on polyesters show at high temperatures and high humidity, a certain susceptibility to hydrolysis. This is due to the water molecules splitting the ester groups and hence decreasing the molecular weight significantly which leads to a reduction in mechanical properties.
In practice a failing of soft polyester TPU is nearly not happening.


UV-Radiation

TPU based on aliphatic diisocyanates show no yellowing caused by UV rays.
In weathering tests it was found out that for longer periods of UV radiation and hydrolysis the degradation can cummulate.

Shore hardness A amd D

Shore A is indicated for soft elastomers, as measured by a needle with a blunted tip.  If values up to 90A are measured it shall be measured by the Shore D device.
Shore D is specified for  Stiff elastomers and is measured by a not blunted tip needle penetrating with a 30 ° angle.  If values below 20D is measured  the device Shore A shall be used.

Glass transition temperature

The glass transition temperature Tg (glass or glass transition temperature) is characterized by the conversion of a more or less hard, amorphous glassy or semicrystalline polymer sample in a rubber-like to viscous phase.
The reason for the phenomenon of Tg freezing or thawing, the movements of relatively long chain segments (20 to 50 chain atoms) of the macromolecules.
The macro-conformation does not change by reaching the glass transition temperature.

Tensile strength

This test is used for evaluation of plastics under uniaxial stress.
The advantage of the tensile test is that even ductile materials can be tested until complete breakage.
The modulus of elasticity (Young's modulus) is used as a comparative characteristic of various materials and is a measure of the stiffness.
The test is usually carried out at 23 ° C.


Standards: DIN 53504, measured at S1-beam at 200mm/min feed speed.

 

Dimensional stability under heat

For hard thermoplastics the determination of heat resistance is often performed with the Vicat - softening temperature (Vicat - Softening - Temperature, VST ISO 75) or the heat resistance (Heat-Deflection-Temperature, HDT  ISO 306).
In the foil production and the production of coated fabrics in addition to good meltability, a high melt elasticity is required.

Applications TPU melt adhesive

AUTOMOTIVE:
Decorative fabrics for headliner
Door panels, seat backs, rear shelves
Abrasion-resistant respiration active fabric combinations for car seats
Leisure and sports
Respiration active and waterproof materials

MILITARY TECHNOLOGY:

Protective clothing for firefighters' suits
Police and Military clothing


MEDICINE:
mouthguards
Surgical clothing
Mattress protection cover
incontinence products
Anti-allergy system


Of course there are, besides the above listed applications still a variety of applications. ROWAK AG is capable of both granules and powders to deliver for the right purpose. Ask us!

Competence

We are happy to advise you further. Contact our experienced technicians.

ROWAK AG