An overview of polyurethane

Abstract:

With increasing demand in highly sophisticated featured products, manufacturing of materials have gone complex with designs and implementation. Conventional materials have fallen below the par levels of the market requirement, so advance engineered materials possessing cumulative features (low weight, thermal insulation, cushioning effect, flexibility etc.) are preferred over the conventional material. One such material is polyurethane foam, they have properties that pared expectations with optimum market price but, their technical end application have been restricted to limited use like thermal barriers/insulators, cushioning, and packing materials these materials can be used at better levels also. This article will give a brief idea on the market value, synthesis of material, its properties and its application based on its varying properties.

Key word: Acoustic insulation, cushioning, foam, thermal insulating material.

1. Introduction:

Polyurethane (pur and puf) is an organic based polymer composed of chain units joined by urethane links. They are cellular foams or plastic foams which may be expandable sponge plastics or tough rigid structures. Generally it consists of single/two phases, the first phase may be a solid polymer matrix/gaseous phase matrix and the second phase may be present as foam in the form of fillers/fibers or other form. Polyurethane foams are prepared by reaction of polyol (an alcohol with more than two reactive hydroxyl groups per molecule) with a diisocyanate or a polymeric isocyanate in the presence of suitable catalysts, surfactants, blowing agents and additives. The polyurethane foams are chemically resistive to low range of pH levels and can withstand high temperatures. They can get back to normal stage without any adverse effect. Polyurethane foams are basically categorized into three types Flexible, Rigid and Elastomeric structure. Polyurethane foams are highly porous structures with lower density, highly flexible and can undergo compressive behavior under continues cyclic loading which are made use in mattresses, seats, insulation covers and many more technical applications[1].

In foams the properties of the material varies along with change in densities. Low density materials are categorized under flexible foam and rigid foams with higher density, the range varies from 1.6kg/m3 to 960 kg/m3. The mechanical properties (strength, compression etc.) are directly proportional to density.

2. Market value:

Polyurethane (PU) foams constitute the largest category of cellular polymeric materials. They are produced in either flexible or rigid form. Within these groups, the density and other properties vary depending on the end use. PU foams offer an attractive balance of performance characteristics (aging properties, mechanical strength, elastic properties, and chemical resistance, insulating properties) and cost.

Thermosetting elastomers and thermoplastic elastomers are main types of PU elastomers, of which thermosetting hold the largest market share of around 75%. As per Research and Markets, the global market for polyurethanes was estimated at 13,650  kilo tons in 2010 and is expected to reach 17,946  kilo tons by 2016, growing at 4.7% from 2011 to 2016. In terms of revenue, the market was worth US$33,033 million in 2010 and is expected to reach US$55479.68 million by 2016, growing at 6.8% from 2011 to 2016. North America, Asia-Pacific, and Europe dominate the polyurethane market and together they account for 95% of the global polyurethane demand in 2010. North America and Western Europe are mature markets and are expected to grow at a sluggish rate. However, Asia-Pacific, Eastern Europe and South America are expected to drive the demand for polyurethanes in the coming decade. The furniture and interior industry dominated the polyurethane market, accounting for 28.01% of the total demand in 2010. The second largest end-use of polyurethanes is in construction industry, which accounted for 24.98% of the overall market As per Global Industry Analysts, the global market for foamed plastics (polyurethane) is projected to reach 9.6 million tons by the year 2015 [2].

3. Synthesis:

Polyurethane foam system is prepared by reaction of short chain polyol (an alcohol with more than two reactive hydroxyl groups per molecule) with a long concentration urethane linkage of diisocyanate or a polymeric isocyanate in the presence of suitable catalysts, surfactants, blowing agents, cell opener and other required additives. The dispersing of gas (component A) takes place throughout the fluid polymer phase (component B) and stabilizing results in the formation of foam. Depending on the type of polyol and isocyanate used, the generated foam exhibits different properties. An alternative soft segment of polyol with weak segment of polyols with weak inter chain interaction is present in coiled form and a hard segment is formed by reaction of diol/diamine and the diisocyanate. By careful selection of the polyol and isocyanate, foam with varying properties can be generated. It could be soft and flexible with totally open cell structure

Fig 1: The base polymeric components of Polyurethane synthesis

3.1 Polyols

Polyol is a macro molecule of poly hydroxyl alcohols with molecular weights ranging from  200 – 7000 with functionalities ranging between 2 and 8. Monomeric polyols such as glycerin, pentaerythritol, ethylene glycol and sucrose often serve as the starting point for polymeric polyols. Functional polyols are used to obtain by combining with propylene oxide or ethylene oxide. Polyester polyols, Polyether polyols, Fire retardant based polyols are different varieties of polymeric functional alcohols.

3.2 Isocyanate

This is the second basic component in manufacturing foam material. Isocyanate is the functional group with the formula R–N=C=O. Organic compounds that contains an isocyanate group. Isocyanate is electrophiles, and are reactive toward nucleophiles including alcohols, amines, and even water. Upon treatment with an alcohol, an isocyanate forms urethane linkage, Isocyanates are extremely reactive and they react with materials containing active hydrogen.

3.3 Catalyst

Catalyst are substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change, the teriary amine catalyst is commonly found in rigid polyurethane foam formulation and are strongly basic and often have higher vapor pressure, it    causes skin irritation and precautionary measures are must while handling.

3.4 Blowing agent

A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, CFC are the most preferred though they are low toxic by standards if wrongly dealt it might cause heart related problem [3].

During thermal decomposition of chemical blowing agents nitrogen/carbon dioxide generation takes place these gas phase molecules helps in forming bubbles in the foam material creating the resilience effect.

4. Principles of foam formation

• Bubble formation: bubble formation may be due to self nucleation due to truly homogenous liquid. The bubbles will be easily be formed at the liquid solid interface this is called nucleation process and the solid phase is called nucleating agent.
• Bubble growth: the bubble once formed may grow by diffusion of gas from solution in the liquid phase into bubble, the system will be stable with even fewer larger cells when compared to smaller cells
• Bubble stability: stability is an important aspect because the liquid phase are thermodynamically unstable so an adsorber must be added to act at the surface giving good surface tension which can control the adsorbed solute [1].

5.Properties of polyurethane foams

5.1 Thermal behavior of polyurethane foam
Thermal Insulation has always been and still remains one of the fundamental tools for achieving energy conservation. Many organic and inorganic materials like PVC, polystyrene, polyurethane, asbestos, ceramic, phenols and many more fiber are used for insulation purpose. Among them polyurethane foams are much preferred because of their flexible nature. About 65-80% of the insulation capacity of foam is due to the cell gas mixture while cell size and density are secondary parameters. High temperature thermal behavior of the foam is a complex process driven by:
– Bubble size and distribution
– Cell anisotropy
– Pore pressure
– Polymer degradation

Thermal conductivity of specific material is represented by (λ). It represents the heat flow in watts (W) through a 1 m² surface and 1 m thick flat layer of a material when the temperature difference between the two surfaces in the direction of heat flow amounts to 1 Kelvin (K). The unit of measurement for thermal conductivity (λ) is W/(m·K).

The thermal resistance denoted by (R) describes the thermal insulation effect of a constructional layer. Which is obtained by dividing the thickness (d) by the design thermal conductivity value of a building component: R = d/λ (in accordance with EN ISO 6946). The unit of thermal resistance (R) is (m²·K)/W. In building components comprising several layers, the thermal resistances of the individual layers are added together [4].

The effect of chemical structure of the APP containing fragments of glycerol, adipic acid, poly(propylene glycol) or hexanediol on thermal stability and flame resistance of the PU-PIR foams was elucidated. PU-PIR foams prepared from APP containing fragments of glycerol and/or adipic acid had higher thermal stability and lower weight loss even at 330 °C [5].

The thermal conductivity of the PU foam at 300K was reduced by as much as 70% by evacuating the gases in the foam cells. Radioactive heat transfer is found to accounts for about 10 to 20 % at room temperature. To enhance its thermal performance at room temperature, PU foams should be blown with as a high percentage of CO2 as possible [6].

At a reference temperature of 25°C, the thermal conductivity of water is λ = 0.58 W/(m·K). As the thermal conductivity of most common insulation materials ranges between 0.020 W/(m·K) and 0.050 W/(m·K), water absorption due to immersion in water leads to an increase in thermal conductivity. However, water absorption has only a small impact on the thermal conductivity of rigid polyurethane foam [7].

Major drawback in predicting the thermal behavior with age, the thermal conductivity/insulation changes with age and environmental of the material as usage prolongs, the retentivity of thermal behavior may improve in some foams may not in some foam .

5.2 Compressive nature:

Foams are materials with high porous nature having air sacks/bubbles between the structures. The pore density and the pore size distribution are directly proportional to the volumetric pressure applied to the foam system during preparation of the material. When external applied forces on the solid foam, the air escapes from the system to give the cushioning/compressive resilience effect and gets back to the original form when the external forces are nullified.

The rigid foams have excellent load-bearing weight capability compared to many other plastics of natural products such as wood. This property is a highly important factor in many designs, e.g. use in boats, building panels or other composite parts. The flexible foams have an excellent load-bearing weight capability but the emphasis is different; comfort is the key feature for a wide assortment of applications such as seating or bedding.

On compression test, the engineering stress found by Quasi-static compression tests reveals that by dividing the load recorded at each data point by the original cross-sectional area of the PU foam cylinder the calculated engineering strain analyzed by  the displacement of the machine’s  actuator head (at each data point) was found to have 3% energy absorbed to yield (deformation) [8].

Under cyclic compression the structures absorbs energy when compressive loads were applied and released energy when the loads were removed with a very small deformation. The very large foam cells get fully damaged in the initial cycles of compression due to high air compression ratio and in the subsequent cycles, there would not be any effect of these small cells. The smaller cells act as dampers because of low air compression ratio. In compression the circular holes of PU foam become elliptical perpendicular to the axis of load. The compressive strain signature is non-linear due to the stiffening of the sandwich with increase in compressive load. The deformation mechanism of foam is localized under-static compression conditions and become even more localized through the material [9].

The glass transition temperature is one main point to be taken into concern. Creep strain increases as temperatures increases. The proximity of the glass transition temperature was the dominant factor influencing creep strain. The creep compression curves at the higher temperatures had a significant acceleration in creep strain over the period of time. Increased temperatures were known to cause disruption in hydrogen bond which increased the amount of chain slippage. It was assumed that this chemical behavior was the primary reason for the accelerated creep rate at these higher temperatures [10].

5.3 Abrasion resistance

In applications where severe wear problem are faced polyurethane foams offer outstanding durability when compared with other rubbery plastics. In many applications the unusual combination of properties has made it possible to design and fabricate Shoe soles, seat cushioning etc requires outstanding abrasion resistance When the material under goes cyclic compression there are better chances of the material to fail in composite structures that Crack propagation occurred in a discontinuous manner which was attributed to the brittle nature of both phases and the relatively coarse composite structure Under cyclic loading, crack propagation appeared to follow the Paris law, (da/dN) = A(ΔK)m, with some scatter [11].

5.4 Moisture Absorption Behavior

Affinity towards water is because of the existence of hydroxyl. The level of absorption depends on the pore size and the void space that helps in reshaping the air with liquid particles. Moisture flexure tests carried on samples over time different periods of two days, one week and two weeks showed almost 17% increase in modulus and 12% increase in flexure strength were found when compared to the dry foam samples. The amount of moisture absorbed at saturation was about 8% and the saturation time was about 400 hours, all samples had same contact area but differed in moisture absorption behavior, it could have attributed to the anisotropy in the cell morphology (size and shape) along the thickness of the foam (flow direction) [12].

During elevated temperatures the absorption levels of water varies. The effect of moisture absorption at glass transition temperature (Tg) exhibited a maximum water uptake of 8.0% (by mass) after exposure to 100% relative humidity for 96 h [13].

6. Application methods of Coating on substrate

 6.1 Foam coating

The coating or finishing material is converted into foam from solution or emulsion and applied on the fabric knife as roll; the penetration of foam is low but gives much better handle and drape. This is used for open type cell construction. The foam coated substrate is passed through a stenter for finishing process, care should be taken such that high heat is not supplied and neither lowers than the finishing temperature. With 5-10% of moisture content the fabric is passed through steel rollers crushing the foam and curing at 160-170ÌŠ C. The foam collapses on drying leaving a thin film on the surface.

6.2 Spray coating

Spray foam is long lasting, durability and light-weight. Instead of replacing your entire system because one area of the surface is an issue, spray foam coating is a more cost effective solution. It is sprayed onto the surface to fill small cracks and crevices; it then expands approximately 30 times its original liquid volume to form a hard, closed cell roof surface. The foam dries almost instantly and self adhere to the surface. Not only is the foam coating is an expensive solution to protect your roof, but it insulate the building exterior which will lower the utility cost. This method involves use of single coating material or multiple materials to spray. The main drawback of this method is the non-homogeneity and expensive cost for manufacturing but this can be used for specialized narrow web bonding for design complicated structures.

Fig 2: Multi jet nozzle spray gun for synthesis of polyurethane

6.3 Rod Mayer coating technique

The Mayer rod is a stainless steel rod that is wound tightly with stainless steel wire of varying diameter. The rod is used to doctor of the excess coating solution and control the coating weigh. The wet thickness after doctoring is controlled by the diameter of the wire used to wind the roll and is approximately 0.1 times the wire diameter.

Fig 3: Rod Mayer method

Lower setup cost and precise thickness control (control the web coating thickness in 0.1-0.2 mil increments) are features that make the process favorable over the other methods, but low viscosity liquids (viscosity parameter) and coating capacity of only 1000 ft/min are found to be the limitations.

6.4 Immersion/dip coating

Simplest and most commonly used coating technique used. The substrate is dipped into the solution bath containing coating substrate and taken out, the direction of gravity determines the geometry of application the excess coating material on the substrate is removed using doctor blade.

6.5 Knife coating

The substrate is pulled by rollers and the excess amount of coating material is fed and the application is controlled by the doctor knife. The coating thickness is calculated based on the gap between the knife and the substrate excess coating material is removed by the knife.

  Fig 4 : Knife coating

6.6 Roll coating

This method is also called as kiss roll coating and has procedure similar to that of immersion coating, N numbers of rollers are used with textured patterns to achieve desired results this also uses heat and pressure to fix the coating material, the thickness of the coating is determined by the texture pattern and the pressured involved.

6.7 Transfer coating

The coating material is applied to an external material with required value then it is transferred to the substrate by application of heat and pressure using hot rollers.

6.8 Screen coating

The surface of the screens has mesh structure with minute pores allowing the coating material to flow through them. The amount of coating is determined by the screen mesh number; squeeze pressure, angle between the squeeze blade and the screen, the viscosity of the coating substrate. This method helps in producing very fine and delicate film coating on to the surface having no external pressure or heat to seal the coating material binders (fixing agents) are used.

6.9 Powder coating

Powder coating is one of the methods that involve use of solid phase coating material. In this method pre-prepared coating powder in solid foam is deposited directly onto the substrate, since no water is used drying of material is not required and wastage of energy is also reduced. The scattering of solid powder is comparatively uniform with rotating scatter roller.

7. Testing standards

International standards by ISO and ASTM are widely used for manufacturing and testing the products without causing any physical harm to consumers and the environment.

8. Applications:

Polyurethane foams have started gaining more importance in technical application products rather than being used as auxiliary items, some of the most used products along with the industries are cited below.

9. Conclusion:

In the advanced world with requirements of special featured high end products from customer side the manufacturers are competing to bring in their best possible results and polyurethane foams are one such material that has given hand to do it in an easier manner. The market has started growing rapidly in the last few years for polyurethane foams because of their special properties, researchers are finding new functional polyurethane foams to satisfy all the end users and the market value also tends to do so.

10.  Bibliography

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• Purvi SD Patel, Duncan ET Shepherd, and David WL Hukins, Behavior of compressive nature of polyurethane foam using Quasi-static compression tests  (on BMC Musculoskelet Disord. 2008; 9: 137. Published online 2008 October 9. doi: 10.1186/1471-2474-9-137  )
• Sripathy M, Sharma K V, Krishna M, Effect of Cyclic Compression Loading On Crushing Response of Polymer Based Composites Sandwich Panels, International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-2, Issue-6, January 2013.
• Conor Briody, Barry Duignan, Stephen Jerrams, Stephen Ronan, Prediction of Compressive Creep Bhaviour in Flexible Polyurethane Foam Over Long Time Scales and at Elevated Temperatures, Centre for Elastomer Research 2012-12-01.
• Tilbrook M T, Rutgers L, Moon R J & Hoffman M, Effective Crack-Propagation Resistance Under Monotonic And Cyclic Loading, Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836, SIF 2004.
• Saha M. C, Mohan S, Hickman D, and Balakrishnan.A, Moisture Absorption Behavior and Its Effect on Flexure Properties of Polyurethane Foams, Society for Experimental Mechanics, June 2-5, 2008.
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(Data retrieved from online sources on and before 7/9/2013).