Introduction To Polyurethane Foam Engineering Essay

Polyurethanes are broad class of materials used widely in many applications. Polyurethanes are also written as PUR. Polyurethanes also 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 following equation. [2]

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

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

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

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Polyurethane is widely used in various fields such as making of plastics, cushions, foams, rubber goods, synthetic leathers and fibres. The total annual production of urethanes in the world is estimated to be 8 million tons per year and is increasing 4-5 % yearly. [4] Polyurethane is a versatile polymer having unique chemistry with excellent mechanical and optical properties and have good solvent resistance. [5] Polyurethanes are materials with many applications but major applications are in furniture industry, foam industry, building construction and shoe industry and also polyurethanes can be used in medicinal and agricultural applications. [6] Polyurethanes have large number of applications and uses in different areas due to their versatility but most important application of polyurethane which has covered almost 29% of the total market of polyurethane is polyurethane foam. [7]

1.2 Polyurethane Foam

The major application of polyurethane is polyurethane foams which are extensively used in many applications. They are referred as PUR foams and they are prepared by the reaction of addition, condensation or cyclotrimerization. [2] Following table shows the type of reaction in the foaming system and the resulting foam property present in the foam produced by that respective reaction.

Foam

Reaction

Foam Property

Polyurethane

Poly addition

Flexible and rigid

Polyisocyanurate

Poly cyclo trimerization

Rigid

Polyamide

Poly condensation

Flexible and rigid

Polyimide

Poly condensation

Semi rigid and rigid

Polyurea

Poly addition

Flexible and rigid

Poly Carbodiimide

Poly condensation

Semi rigid

Poly Oxazolidone

Ring opening Poly addition

Rigid and semi rigid

Table 1.1 Table showing different types of foam based on their synthesis reaction [2]

Mainly PUR foams are classified into two types which is flexible and rigid foam and then flexible and rigid foam are classified into various sub types depending upon the different type of applications in which they are used.

Polyurethane structure consist of a polyol and polyisocyanate component we can say that polyols are the building blocks and isocyanates are jointing agents. So polyurethane foam chemistry is called as building block chemistry. All kinds of polyurethane foams are prepared with the proper choice of polyol and isocyanate component with respect to chemical structure, equivalent weight and functionality. [2] Following table shows the classification of PU foam with respect to polyol component and functionality.

Polyol

Rigid foam

Semi rigid foam

Flexible foam

OH No.

350-560

100-200

5.6-7.0

OH Equivalent No.

160-100

560-280

10,000-80

Functionality

3.0-8.0

3.0-3.5

2.0-3.1

Table 1.2 Classification of polyurethane foam w.r.t polyol and functionality [2]

1.3 Foam Preparation Technology

Polyurethane foams are prepared by mixing isocynate component in polyol having catalyst, fillers, additives, surfactants and other chemicals in it. No heating 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 process. These types are:

One step One shot system

Quasi Pre polymer system

Full Pre Polymer system

One step system and Quasi system are mostly used in foaming industries now in which one step process is used majorly while pre polymer system was used only in the early times of urethane industry. [2]

1.3.1.1 One step one shot System

In one step system Component A and Component B are taken separately. Component A contains only polyisocyanate component while component B contains polyol, surfactant, blowing agent and catalyst. Both components are mixed which led to the formation of foam.

1.3.1.2 Quasi Pre polymer System

In this system in component A polyisocyanate component is taken with polyol and in component B rest of the ingredients are taken with polyol which includes catalyst, blowing agent etc. Mixing forms a foaming product.

1.3.1.3 Full Pre polymer System

In full prepolymer system in component A poly isocyantae component is taken and polyol is also added in it while in component B polyol is not added while the rest of chemicals like blowing agents, surfactants and catalyst are taken. Components A & B are mixed and foam is formed.

These foaming systems show only two component system however in industrial processes there are modifications. For example in slabstock process three or four streams are employed carrying different reactants. [2]

1.3.2 Foaming Processes

Mainly three types of foaming processes are used, which are cup foaming, box foaming and machine foaming. Machine foaming is further classified into various foaming processes. [2]

1.3.2.1 Cup Foaming

This foaming process is also known as hand mixing. This method is mostly used for lab scale synthesis of foaming products. In this process mixing is done in paper or plastic cups that's why called as cup foaming. In cup foaming poly isocyanate component is taken in a cup and rest of the reactants are also taken in a separate cup which includes polyols, catalysts etc. Then both the components are mixed using electric balance or by hand mixing and a foaming material is formed in the shape of a cup. This is the simple and cheap foaming process that's why used widely on lab scale.

1.3.2.2 Box Foaming

Box foaming is better than cup foaming to obtain sufficient amount of samples for evaluating foam properties for machine foaming. The sizes of the box range from 15x15x15 cm and 30x30x30 cm. The inside of the box is lined with craft paper. The procedure is almost similar to that of cup foaming and mixing is done through electric drill equipped with mixer. Immediately after mixing system is poured into the box and allowed to rise to obtain foam. Foam is cured at room temperature after some time. [2] This method is also used largely on lab scale to produce foam.

1.3.2.3 Machine Foaming

In machine foaming variety of machine processes are available which are one by one described as under. [2]

1.3.2.3.1 Large Box Foaming

It is a discontinuous process used to produce small foam blocks with smaller volumes. The size can be 1x1x2 meters. Due to exothermic conditions small modification is required in the process as compared to the box foaming process.

1.3.2.3.2 Slabstock Foaming

Slabstock foam is made by the continuous pouring of foamable liquids on a moving conveyer. A cut-off segment of the continuously produced foam loaf is called a block or bun [2]. In the case of polyether-based flexible urethane foam, a slabstock foam size can be about 1 meter high and 2 meters wide, with a foam density of 1.6 to 2.0 pound/ft3. The cross section is rectangular with a crown-shaped top surface. The higher the crown, the less the yield of available foam. Two techniques canto used to reduce the crown. One method is to pull vertical sides of rising foam. Another technique is to compress the top of crown surface. Equipment for doing this is commercially available. [2]

1.3.2.3.3 Pour-in-Place Foaming

This process includes pouring of foaming ingredients into a void space to form an integral part of the foam and substrates. Products which use this process are household refrigerators and deep freezers.

1.3.2.3.4 Sandwich Foaming and Molding

Sandwich foaming is used to produce sandwich panels from foam cores. It can be produced continuously using conveyers or discontinuously using jigs. Surfaces material can be craft paper in it. Molding is used to produce shaped foam products such as car seats and furniture. Reactants are placed in mold cavity and cured in the mold. After curing molded foam is removed. [2]

1.3.2.3.5 Spraying and frothing

Spraying is a unique process used for urethane and iso cyanurate foams .It creates insulation layers not only on flat surfaces like roofs but also on non flat surfaces such as spherical tanks and pipes and building structures such as frames. Frothing process in urethane systems is similar to that of shaving cream. There are different types of frothing processes which are conventional frothing processes, chemical frothing and thermal frothing etc. Frothing process usage depends upon the reactants and conditions and which type of foaming product is required. [2]

1.4 Types of Polyurethane foams

Major classes of polyurethane foams are flexible and rigid foam further these two types are more classified according to their extensive applications.

1.4.1 Flexible Polyurethane foam

Flexible polyurethane foams are synthesized by slabstock process or by molding process and they are also classified further on the basis of technique used for their synthesis. They are also classified on the basis of polyol in polyether and polyester foams. Slabstock foams include conventional polyether foam, high-resilience (HR) foam, visco-elastic foam, super-soft foam, energy-absorbing (EA) foam, semi flexible foam, and flexible polyester foam. Molded foams are classified in two classes: hot molded foam and cold molded foam. [2] All classes of flexible foams are explained one by one as follows .

1.4.1.1 Slab stock foam

The contionous piece of foam by the continuous pouring of foaming ingredients on a continuously moving conveyer is called slab stock foam. A cut off segment of slab stock foam is called as bun foam. Slab stock process is widely used in industry in production of flexible polyurethane foam. [2]

In slab stock foam process inclined conveyer 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 section of slabstock foams produced by the inclined conveyer process is not exactly rectangular and has crown shape. A crown-shaped cross section of foam bun results in a lower yield of fabricated foams. Accordingly, some improved processes to obtain rectangular cross sections have been proposed. One step process is widely used in these foaming systems. All ingredients are pumped into the mixing head through several streams and are poured from the mixing head onto the moving conveyer. From the standpoint of viscosity and metering accuracy, some ingredients are preblended. These blends include a blend of water and amine catalyst, a blend of tin catalyst and polyol, and a blend of physical blowing agent and polyol. The reaction temperatures in core foam usually reach about 160 to 170°C. The exothermic heat is mainly a function of the water-isocyanate reaction. The reaction temperatures are affected by the isocyanate index, water level, and catalyst concentration.[2]

Poly ether slab-stock foam is important in slab-stock flexible PU foams and it is produced by this slab-stock process widely in industries.

1.4.1.2 Molded Flexible foams

Molded flexible foams are used for making shaped products such as automotive seats, child seats, head restraints, vibration damping for automobiles, arm rests, furniture cushions and mattresses. The density distribution of molded foams is composed of high density skin foam and low density core foam. In the molding process mixed foaming ingredients are poured through a mixing head into a preheated mold made of aluminum, steel, or epoxy resin. The mixed ingredients flow and expand in the mold, and then the molded foams are kept at the required curing temperature for a limited time, which is followed by de-molding.[2]

Molded flexible foams are further classified into hot molded and cold molded flexible polyurethane foams.

1.4.1.2.1 Hot Molded flexible foams

Hot molded foam is produced using conventional polyether polyol with TDI 80/20. In 1961 hot-molded urethane foam mattresses was commercialized that provided better sleeping comfort than slabstock foam mattresses and greater durability. Because of the relatively higher cost, however, the molded mattress was not competitive with slabstock foam mattresses, and production was discontinued. [2]

1.4.1.2.2 Cold Molded flexible foams

Cold molded flexible foam is prepared at very low mold temperatures of 60 to 70 °C with mold retention time of 10 minutes and then cured in a separate oven or at room temperature. In this type mostly isocyante components used are blends of TDI & MDI and polyol used is polymer or graft polyol. Dual hardness automotive seats are made by this process using graft polyols. [2]

1.4.1.2.3 High Resilience HR foams

HR foams have higher resiliency than conventional flexible foams. These foams are prepared by cold molded process. The formulation principle is to reduce intermolecular interactions such as hydrogen bonding of urethane linkages, irregular structures of polymeric molecule, and so on. High Molecular weight polyether polyols are used in the formulation of these foams. Polyisocyante components used are blends of TDI & MDI with the weight ratio of 80/20. Also polymeric MDI alone can be used. HR foms are widely used in furniture and automotive applications. [2]

1.4.1.2.4 Visco-elastic foam

Visco-elastic VE foams are characterized by slow recovery after compression. The foam is also called low-resiliency foam, slow-recovery foam, ergonomic foam, better-riding comfort foam, and temperature-sensitive foam. VE foams are used by NASA in early 1960 but they are highlighted commercially only in the recent years. VE foams are synthesized using polyol blend of high and low molecule weight polyols with TDI or MDI, and they can also be produced using blend of MDI & TDI with polyether polyol. TDI-based VE foams are soft to the touch and have very low resilience and slow recovery. MDI-based foams are easier to process and can be produced in a wider range of isocyanate indexes. However, physical strengths of MDI-based foams are lower, resilience is higher, and compression set is better than TDI-based foams.

The VE foam market includes seat cushions for people who spend a long time sitting such as truck drivers, office workers, and airline pilots, as well as for NASAs space shuttle seats, wheelchair seats, and so on; sporting goods that require energy absorption such as gym mats, helmet linings, leg guards for ice hockey, ski boots, and ice skates. [2]

Our main product is VE foam and it is further explained in the later chapters.

1.4.1.2.5 Super soft slab-stock foam

This foam can be prepared by using a high molecular weight polyether triol with TDI 80/20. Blowing agent can be water alone or some physical blowing agent can be used with water. It is a low density foam. Major market of super soft foam is high quality back cushions for sofas. This foam also have low cross linking density. [2]

1.4.1.2.6 Semi flexible slab-stock foam

These foams have high open cells, high load bearing and high energy absorbing characteristics. These foams are used in large number of automotive applications such as bumper cores, interior knee bolsters, side impact absorbing door panels, and headliners under the automotive roof. Due to energy absorbing characteristics this foam may be called as Energy absorbing EA foam. [2]

1.4.1.2.7 Reticulated foam

It is 100% open cell structure foam with no cell membranes. Removal of cell membranes for polyether foam is possible when membranes are dissolved with an alkaline solution. Reticulated foam find its applications in various mats, filters and pads.[2] Mostly the foam used in floor mats is this type of foam and it is mostly used in making mats and filters because it is completely open celled foam.

1.4.1.2.8 Integral skin flexible foam

Integral skin foam is also called self skin foam. It consists of high density skin foam and low density core foam that is sandwich structure. The mold temperature for integral skin foam is carefully controlled at 40 to 70 °C. Water can be used as a blowing agent but it produces thin skins. Integral skin foam is prepared by open mold process or by reaction injection molding. [2]

1.4.1.2.9 Micro-cellular Elastomers

These are also called foamed urethane elastomers. The density is in the range of about 320 to 960 kg/m3. Micro-cellular foams are prepared by using aliphatic polyester diols with poly tetra methylene ether glycol (PTMEG). The poly isocyanate component used is TDI pre-polymer and liquid MDI. Also chain extenders and cross linkers are used. Water is used as a blowing agent. These elastomeric foams are used for various shock absorbing elements in vehicle suspensions, precision machines, shoe soles and sports shoes etc. [2]

1.4.1.2.10 Shoe Sole foam

The advantages of polyurethane foam in shoe soles include high abrasion resistance, high flexibility, low density, high cushioning, and solvent resistance. In addition, the direct molding of the sole with the upper part of shoe makes it possible to increase production efficiency and lower production costs. Casual shoes are produced using polyether-based foam systems. Some sports shoes such as tennis shoes are produced by polyester foam systems, because polyester-based foam has higher abrasion resistance, tensile strength, and elongation at break in comparison with polyether-based foams. [2]