Introduction To Polyurethane Foam Engineering Essay

Polyurethanes are wide category of stuffs used widely in many applications. Polyurethanes are besides written as PUR. Polyurethanes besides called as urethanes are characterized by urethane linkage -NH- C ( =O ) – O – . This linkage is formed due to the reaction of isocyanate group with hydroxyl group as shown in the undermentioned equation. [ 2 ]

Equation 1.1 Equation demoing the reaction of urethane linkage in polyurethanes. [ 2 ]

Polyurethanes are discovered by Otto Von Bayer & A ; co workers in 1937. [ 3 ] The characteristic construction of urethanes is given as:

And they can be considered as esters or amide esters of carbonaceous acid. [ 3 ] The polyurethanes are besides sometimes called as urethanes merely and they are besides called as isocyanate polymers. They are synthesized by the reaction of polyfunctional hydroxyl compounds with polyfunctional isocyanates. [ 3 ] The construction of polyurethane formed from di hydroxyl compound and di isocyanate is given as:

Polyurethane is widely used in assorted Fieldss such as doing of plastics, shock absorbers, froths, gum elastic goods, man-made leathers and fibers. The entire one-year production of urethanes in the universe is estimated to be 8 million dozenss per twelvemonth and is increasing 4-5 % annually. [ 4 ] Polyurethane is a various polymer holding alone chemical science with first-class mechanical and optical belongingss and have good dissolver opposition. [ 5 ] Polyurethanes are stuffs with many applications but major applications are in furniture industry, foam industry, constructing building and shoe industry and besides polyurethanes can be used in medicative and agricultural applications. [ 6 ] Polyurethanes have big figure of applications and utilizations in different countries due to their versatility but most of import application of polyurethane which has covered about 29 % of the entire market of polyurethane is polyurethane froth. [ 7 ]

1.2 Polyurethane Foam

The major application of polyurethane is polyurethane froths which are extensively used in many applications. They are referred as PUR froths and they are prepared by the reaction of add-on, condensation or cyclotrimerization. [ 2 ] Following tabular array shows the type of reaction in the foaming system and the ensuing foam belongings nowadays in the froth produced by that several reaction.


Chemical reaction

Foam Property


Poly add-on

Flexible and stiff


Poly cyclo trimerization



Poly condensation

Flexible and stiff


Poly condensation

Semi stiff and stiff


Poly add-on

Flexible and stiff

Poly Carbodiimide

Poly condensation

Semi rigid

Poly Oxazolidone

Ringing opening Poly add-on

Rigid and semi rigid

Table 1.1 Table demoing different types of froth based on their synthesis reaction [ 2 ]

Chiefly PUR froths are classified into two types which is flexible and stiff froth and so flexible and stiff froth are classified into assorted sub types depending upon the different type of applications in which they are used.

Polyurethane construction consist of a polyol and polyisocyanate constituent we can state that polyols are the edifice blocks and isocyanates are jointing agents. So polyurethane froth chemical science is called as edifice block chemical science. All sorts of polyurethane froths are prepared with the proper pick of polyol and isocyanate constituent with regard to chemical construction, tantamount weight and functionality. [ 2 ] Following tabular array shows the categorization of PU froth with regard to polyol constituent and functionality.


Rigid froth

Semi stiff froth

Flexible froth

OH No.




OH Equivalent No.








Table 1.2 Categorization of polyurethane froth w.r.t polyol and functionality [ 2 ]

1.3 Foam Preparation Technology

Polyurethane froths are prepared by blending isocynate constituent in polyol holding accelerator, fillers, additives, wetting agents and other chemicals in it. No warming is required at room temperature. [ 2 ]

1.3.1 Foaming Systems

Foaming systems are classified into three types based on the type of chemicals used in the synthesis procedure. These types are:

One measure One shooting system

Quasi Pre polymer system

Full Pre Polymer system

One measure system and Quasi system are largely used in frothing industries now in which one measure procedure is used majorly while pre polymer system was used merely in the early times of urethane industry. [ 2 ] One measure one shooting System

In one measure system Component A and Component B are taken individually. Component A contains merely polyisocyanate constituent while constituent B contains polyol, wetting agent, blowing agent and accelerator. Both constituents are assorted which led to the formation of froth. Quasi Pre polymer System

In this system in constituent A polyisocyanate constituent is taken with polyol and in constituent B remainder of the ingredients are taken with polyol which includes accelerator, blowing agent etc. Blending signifiers a bubbling merchandise. Full Pre polymer System

In full prepolymer system in constituent A poly isocyantae constituent is taken and polyol is besides added in it while in constituent B polyol is non added while the remainder of chemicals like blowing agents, wetting agents and accelerator are taken. Components A & A ; B are assorted and froth is formed.

These frothing systems show merely two constituent system nevertheless in industrial procedures there are alterations. For illustration in slabstock procedure three or four watercourses are employed transporting different reactants. [ 2 ]

1.3.2 Foaming Procedures

Chiefly three types of foaming procedures are used, which are cup foaming, box foaming and machine foaming. Machine foaming is farther classified into assorted frothing procedures. [ 2 ] Cup Foaming

This foaming procedure is besides known as manus commixture. This method is largely used for lab scale synthesis of foaming merchandises. In this procedure commixture is done in paper or plastic cups that ‘s why called as cup foaming. In cup foaming poly isocyanate constituent is taken in a cup and remainder of the reactants are besides taken in a separate cup which includes polyols, accelerators etc. Then both the constituents are assorted utilizing electric balance or by manus commixture and a bubbling stuff is formed in the form of a cup. This is the simple and inexpensive foaming procedure that ‘s why used widely on lab graduated table. Box Foaming

Box foaming is better than cup frothing to obtain sufficient sum of samples for measuring froth belongingss for machine foaming. The sizes of the box scope from 15x15x15 centimeters and 30x30x30 centimeter. The interior of the box is lined with craft paper. The process is about similar to that of cup foaming and commixture is done through electric drill equipped with sociable. Immediately after blending system is poured into the box and allowed to lift to obtain froth. Foam is cured at room temperature after some clip. [ 2 ] This method is besides used mostly on lab graduated table to bring forth froth. Machine Foaming

In machine frothing assortment of machine procedures are available which are one by one described as under. [ 2 ] Large Box Foaming

It is a discontinuous procedure used to bring forth little froth blocks with smaller volumes. The size can be 1x1x2 metres. Due to exothermic conditions little alteration is required in the procedure as compared to the box frothing procedure. Slabstock Foaming

Slabstock froth is made by the uninterrupted pouring of foamable liquids on a traveling conveyor. A cut-off section of the continuously produced foam loaf is called a block or roll [ 2 ] . In the instance of polyether-based flexible urethane froth, a slabstock froth size can be about 1 metre high and 2 metres broad, with a foam denseness of 1.6 to 2.0 pound/ft3. The cross subdivision is rectangular with a crown-shaped top surface. The higher the Crown, the less the output of available froth. Two techniques canto used to cut down the Crown. One method is to draw perpendicular sides of lifting froth. Another technique is to compact the top of crown surface. Equipment for making this is commercially available. [ 2 ] Pour-in-Place Foaming

This procedure includes pouring of frothing ingredients into a null infinite to organize an built-in portion of the froth and substrates. Merchandises which use this procedure are household iceboxs and deep deep-freezes. Sandwich Foaming and Molding

Sandwich foaming is used to bring forth sandwich panels from froth nucleuss. It can be produced continuously utilizing conveyors or discontinuously utilizing gigues. Surfaces stuff can be craft paper in it. Molding is used to bring forth molded froth merchandises such as auto seats and furniture. Reactants are placed in mold pit and cured in the cast. After bring arounding molded froth is removed. [ 2 ] Spraying and foaming

Spraying is a alone procedure used for urethane and iso cyanurate froths.It creates insularity beds non merely on level surfaces like roofs but besides on non level surfaces such as spherical armored combat vehicles and pipes and constructing constructions such as frames. Foaming procedure in urethane systems is similar to that of shaving pick. There are different types of frothing procedures which are conventional foaming procedures, chemical frothing and thermic frothing etc. Foaming process use depends upon the reactants and conditions and which type of frothing merchandise is required. [ 2 ]

1.4 Types of Polyurethane froths

Major categories of polyurethane froths are flexible and stiff froth further these two types are more classified harmonizing to their extended applications.

1.4.1 Flexible Polyurethane froth

Flexible polyurethane froths are synthesized by slabstock procedure or by modeling procedure and they are besides classified further on the footing of technique used for their synthesis. They are besides classified on the footing of polyol in polyether and polyester froths. Slabstock froths include conventional polyether froth, high-resilience ( HR ) froth, visco-elastic froth, super-soft froth, endoergic ( EA ) froth, semi flexible froth, and flexible polyester froth. Molded froths are classified in two categories: hot molded froth and cold molded froth. [ 2 ] All categories of flexible froths are explained one by one as follows. Slab stock froth

The contionous piece of froth by the uninterrupted pouring of frothing ingredients on a continuously traveling conveyor is called slab stock froth. A cut off section of slab stock froth is called as roll froth. Slab stock procedure is widely used in industry in production of flexible polyurethane froth. [ 2 ]

In slab stock froth procedure inclined conveyor is used from the beginning of urethane industry. Following figure shows the production line of contionuous slab stock machine.

Figure1.1 Continuous slabstock machine [ 2 ]

The cross subdivision of slabstock froths produced by the inclined conveyor procedure is non precisely rectangular and has crown form. A crown-shaped cross subdivision of froth roll consequences in a lower output of fancied froths. Accordingly, some improved procedures to obtain rectangular cross subdivisions have been proposed. One measure procedure is widely used in these frothing systems. All ingredients are pumped into the commixture caput through several watercourses and are poured from the blending caput onto the traveling conveyor. From the point of view of viscousness and metering truth, some ingredients are preblended. These blends include a blend of H2O and aminoalkane accelerator, a blend of Sn accelerator and polyol, and a blend of physical blowing agent and polyol. The reaction temperatures in nucleus froth normally reach about 160 to 170A°C. The exothermal heat is chiefly a map of the water-isocyanate reaction. The reaction temperatures are affected by the isocyanate index, H2O degree, and accelerator concentration. [ 2 ]

Poly ether slab-stock froth is of import in slab-stock flexible PU froth and it is produced by this slab-stock procedure widely in industries. Molded Flexible froths

Molded flexible froths are used for doing molded merchandises such as automotive seats, kid seats, caput restraints, quiver muffling for cars, arm remainders, furniture shock absorbers and mattresses. The denseness distribution of shaped froth is composed of high denseness tegument froth and low denseness nucleus froth. In the casting procedure assorted foaming ingredients are poured through a mixing caput into a preheated cast made of aluminium, steel, or epoxy rosin. The assorted ingredients flow and expand in the cast, and so the molded froths are kept at the needed hardening temperature for a limited clip, which is followed by de-molding. [ 2 ]

Molded flexible froths are farther classified into hot molded and cold molded flexible polyurethane froths. Hot Molded flexible froths

Hot molded froth is produced utilizing conventional polyether polyol with TDI 80/20. In 1961 hot-molded urethane froth mattresses was commercialized that provided better kiping comfort than slabstock froth mattresses and greater lastingness. Because of the comparatively higher cost, nevertheless, the shaped mattress was non competitory with slabstock froth mattresses, and production was discontinued. [ 2 ] Cold Molded flexible froths

Cold molded flexible froth is prepared at really low cast temperatures of 60 to 70 A°C with mold keeping clip of 10 proceedingss and so cured in a separate oven or at room temperature. In this type largely isocyante constituents used are blends of TDI & A ; MDI and polyol used is polymer or graft polyol. Double hardness automotive seats are made by this procedure utilizing transplant polyols. [ 2 ] High Resilience HR froths

HR froths have higher resilience than conventional flexible froth. These froths are prepared by cold molded procedure. The preparation rule is to cut down intermolecular interactions such as H bonding of urethane linkages, irregular constructions of polymeric molecule, and so on. High Molecular weight polyether polyols are used in the preparation of these froths. Polyisocyante constituents used are blends of TDI & A ; MDI with the weight ratio of 80/20. Besides polymeric MDI entirely can be used. HR foms are widely used in furniture and automotive applications. [ 2 ] Visco-elastic froth

Visco-elastic VE froths are characterized by slow recovery after compaction. The froth is besides called low-resiliency froth, slow-recovery froth, ergonomic froth, better-riding comfort froth, and temperature-sensitive froth. VE froths are used by NASA in early 1960 but they are highlighted commercially merely in the recent old ages. VE froths are synthesized utilizing polyol blend of high and low molecule weight polyols with TDI or MDI, and they can besides be produced utilizing blend of MDI & A ; TDI with polyether polyol. TDI-based VE froths are soft to the touch and have really low resiliency and slow recovery. MDI-based froths are easier to treat and can be produced in a wider scope of isocyanate indexes. However, physical strengths of MDI-based froths are lower, resiliency is higher, and compaction set is better than TDI-based froths.

The VE froth market includes seat shock absorbers for people who spend a long clip sitting such as truck drivers, office workers, and air hose pilots, every bit good as for NASAs infinite bird seats, wheelchair seats, and so on ; featuring goods that require energy soaking up such as gym mats, helmet liners, leg guards for ice hockey, ski boots, and ice skates. [ 2 ]

Our chief merchandise is VE froth and it is farther explained in the ulterior chapters. Super soft slab-stock froth

This froth can be prepared by utilizing a high molecular weight polyether triol with TDI 80/20. Blowing agent can be H2O entirely or some physical blowing agent can be used with H2O. It is a low denseness froth. Major market of ace soft froth is high quality back shock absorbers for couch. This froth besides have low cross associating denseness. [ 2 ] Semi flexible slab-stock froth

These froths have high unfastened cells, high burden bearing and high energy absorbing features. These froths are used in big figure of automotive applications such as bumper nucleuss, interior articulatio genus long pillows, side impact absorbing door panels, and stars under the automotive roof. Due to energy absorbing features this froth may be called as Energy absorbing EA froth. [ 2 ] Reticulated froth

It is 100 % unfastened cell construction froth with no cell membranes. Removal of cell membranes for polyether froth is possible when membranes are dissolved with an alkalic solution. Reticulated foam happen its applications in assorted mats, filters and tablets. [ 2 ] Mostly the froth used in floor mats is this type of froth and it is largely used in doing mats and filters because it is wholly unfastened celled froth. Built-in tegument flexible froth

Built-in tegument froth is besides called ego tegument froth. It consists of high denseness tegument froth and low denseness nucleus froth that is sandwich construction. The mold temperature for built-in tegument froth is carefully controlled at 40 to 70 A°C. Water can be used as a blowing agent but it produces thin teguments. Built-in tegument froth is prepared by unfastened cast procedure or by reaction injection casting. [ 2 ] Micro-cellular Elastomers

These are besides called foamed urethane elastomers. The denseness is in the scope of approximately 320 to 960 kg/m3. Micro-cellular froths are prepared by utilizing aliphatic polyester glycols with poly tetra methylene quintessence ethanediol ( PTMEG ) . The poly isocyanate constituent used is TDI pre-polymer and liquid MDI. Besides concatenation extenders and cross linkers are used. Water is used as a blowing agent. These elastomeric froths are used for assorted daze absorbing elements in vehicle suspensions, preciseness machines, shoe colloidal suspensions and athleticss places etc. [ 2 ] Shoe Sole froth

The advantages of polyurethane froth in shoe colloidal suspensions include high scratch opposition, high flexibleness, low denseness, high padding, and solvent opposition. In add-on, the direct casting of the sole with the upper portion of shoe makes it possible to increase production efficiency and lower production costs. Casual places are produced utilizing polyether-based froth systems. Some athleticss places such as tennis places are produced by polyester froth systems, because polyester-based froth has higher scratch opposition, tensile strength, and elongation at interruption in comparing with polyether-based froths. [ 2 ]