POLYURETHANE

Polyurethane

•Polyurethane, any of a class of synthetic resinous, fibrous, or elastomeric are compounds belonging to the family of organic polymers made by the reaction of diisocyanates (organic compounds containing two functional groups of structure −NCO) with other difunctional compounds such as hydroxyl.

• •Otto Baer and his colleagues in Germany first manufactured polyurethane in 1937.
• The one of polyurethane disadvantages it is a flammable solid material and when we burn it by fire it will produce high quantities of ( CO, hydrocyanidein ,in addition to nitrogen's oxides, isocyanates , and other toxic products).
• It Consists of linked organ units with urethane radical . This polymer is has a very wide using especially in industries everything surrounding us which has an insulation, corrosion &( cooled , heat) resistance properties is almost consists of polyurethane.

PREPARATION:

• Polyurethanes are made by the exothermic reactions between alcohols with two or more reactive hydroxyl (-OH) groups per molecule (diols, triols, polyols) and isocyanates that have more than one reactive isocyanate group (-NCO) per molecule (diisocyanates, polyisocyanates). For example a diisocyanate reacts with a diol.

Materials used :

•Polyol: it is a viscosus liquid susbstance of light yellow color.

•MDI : the texture of the texture is dark yellow.

•Water.

•Triethylamine.

•Rigid foams are made with PMDI and polyether glycols, along with low-molecular-weight dialcohols to increase the rigidity. Use of PMDI, which contains a larger number of reactive functional groups, results in a network polyurethane. A blowing agent such as pentane is normally added to augment the foaming. (Chlorofluorocarbons such as Freon [trademark] used to be employed as blowing agents before they were declared unacceptable for depleting ozone in the stratosphere.) 

THE PROPRIETIES :

•Wide Range of Hardness :The classification of hardness for polyurethane relies on the prepolymer's molecular structure.

•High Load Bearing Capacity :Polyurethane has a high load capacity in both tension and compression. Polyurethane may undergo a change in shape under a heavy load, but will return to its original shape once the load is removed with little compression set in the material when designed properly for a given application.

•Abrasion & Impact Resistance:For applications where severe wear prove challenging, polyurethanes are an ideal solution even at low temperatures.

•Flexibility : Polyurethanes perform very well when used in high flex fatigue applications. Flexural properties can be isolated allowing for very good elongation and recovery properties.

•Strong Bonding Properties: Polyurethane bonds to a wide range of materials during the manufacturing process. These materials include other plastics, metals, and wood. This property makes polyurethane an ideal material for wheels, rollers, and inserts.

•Performance in Harsh Environments: Polyurethane is very resistant to temperature extremes, meaning harsh environmental conditions and many chemicals will not cause material degradation.

•Color Ranges: Varying color pigments can be added to polyurethane in the manufacturing process. Ultraviolet shielding can be incorporated into the pigment to provide better color stability in outdoor applications.

•Electrical Properties :Polyurethanes exhibit good electrical insulating properties.

THE TYPES OF PU:

•Rigid polyurethane foams:   Rigid polyurethane foams represent one of the most commonly known versatile and energy saving insulation materials , these foams can significantly reduce energy costs on the one hand and can make commercial and residential appliances more comfortable and efficient on the other hand.

•Flexible polyurethane foams : comprise some block copolymers whose flexibility is based on phase separation between the soft and hard segments, thus pu. Foams may be modified through deliberate control of the individual compositional ratios of these segments.

•Polyurethane ionomers: the presence of ionic groups in the polyurethane backbone chain has many advantages , such as better dispersion in polar solvents due to their enhanced hydrophobicity and improved thermal and chemical properties .

•Coatings , adhesives ,sealants and elastomers: there is growing range of applications and advantageous markets that maybe derived from the use of pus  as coatings ,adhesives , sealants or elastomers ,this is because pus of ten reveals excellent and versatile mechanical , chemical and physical properties

•Binders: polyurethane binders are often used to bond different types of fiber and other materials to each other , binders made from polyurethane help to provide a permanent gluing effect between organic materials and long strand lumbers , medium density fiber board , particle board and straw board .

•Waterborne polyurethane dispersion: coating and adhesive that  make use of water primarily as the solvent are often referred to as waterborne polyurethane, there are several pieces of legislation that place restrictions on the amount of allowed volatile organic solvents and other hazardous air pollutions that maybe release into the environment.

•Thermoplastic polyurethane: reveal vast combination of both physical properties and processing applications    , Usually they’re flexible and elastic with good resistance to impact, abrasion and water, there is the possibility for coloring as well fabrication using a wide range of techniques. 

THE ADDITIVES WHICH COULD ADD FOR PU :

Colornats:

•Many flexible foam products are color coded during manufacture to

           -  To identify product grade

           -  To conceal yellowing,

           -  To make an appealing consumer product

• The historical method of coloring foam was to blend in traditional pigments or dyes.

•Typical inorganic coloring agents included titanium dioxide, iron oxides and chromium oxide.

• Organic pigments originated from the azo/diazo dyes, phthalocyanines and dioxazines, as well as carbon black .

•Typical problems encountered with these colorants included high viscosity, foam instability,  migrating color and a limited range of available colors .

Flame retardants :

•Low-density, open-celled flexible polyurethane foams have a large surface area and high permeability to air and thus will burn given the application of sufficient ignition source and oxygen .

•Flame retardants are often added to reduce this flammability.

•The choice of flame retardant for any specific foam often depends upon the intended service application of that foam and the attendant flammability testing scenario governing that application

•The most widely used flame retardants are the chlorinated phosphate esters, chlorinated paraffins and melamine powders have also been used.

Antistatic Agents:

•Some flexible foams are used in packaging, clothing and other applications where it is desired to minimize the electrical resistance of the foam so that buildup of static electrical charges is minimized. This has traditionally been accomplished through the addition of ionizable metal salts, carboxylic acid salts, phosphate esters and mixtures thereof.

•These agents function either by being inherently conductive or by absorbing moisture from the air.

• The desired net result is orders of magnitude reduction in foam surface resistance.

•Bacteriostats:

•Under certain conditions of warmth and high humidity, polyurethane foams are susceptible to attack by microorganisms.When that is a concern, additives against bacteria, yeast or fungi are added to the foam during manufacture.

THE APPLICATIONS OF PU:

•Polyurethane is one of most widely polymers which it is used in industries and many applications.

•It is achieve this publicity because it ease to dealing with it & manipulating its properties.

•We use it in coating,insulation,different types of foams ,textiles , foot wears , and many other applications and they used it in different percentage as in following table(according to USA usage):

in renewable energy sector (PU) is ideal for manufacturing wind turbine rotor blades. It offers advantages over other plastic systems such as epoxy and unsaturated polyester resins in terms of process, cost efficiency and mechanical properties

•one of most unique application is oleo sponge: it is  a technology for recovering oil and other petroleum products from bodies of water .

•MDF WOOD : IS A type of wood which use specially in furniture due to the unique properties of its such as :

•Excellent mechanical strength properties

     Emission-free gluing

     Outstanding long-term stability under permanent load.

•In the past they were blend sawdust  & other additives with urea resins but with the time they noted that it’s released  toxic formaldehyde so they replace it with polyurethane which has greater stability than urea and gives MDF more properties .

•It has this widely usage because of its physical & chemicals  properties and it’s easy to paint with many different colors than normal wood and also it has less pores than wood as shown in fig.