Polyurethane Types, Synthesis and its Applications

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Abstract

Polyurethanes are all around us. Polyurethanes (PUs) are a class of most versatile modern and safe plastic materials with great potential for use in different applications, especially based on their structure–property relationships. Their specific mechanical, physical, biological, and chemical properties are attracting significant research attention to tailoring PUs for use in different applications.  They are used in a wide variety of applications to create all manner of consumer and industrial products that play a vital role in making our lives more convenient, comfortable and environmentally friendly. Polyurethanes are formed by reacting a 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 and additives. Because a variety of diisocyanates and a wide range of polyols can be used to produce polyurethane, a broad spectrum of materials can be produced to meet the needs of specific applications. To Enhancement of the properties and performance of PU-based materials may be achieved through changes to the production process or the raw materials used in their fabrication or via the use of advanced characterization techniques. Clearly, modification of the raw materials and production process through proper methods can produce PUs that is suitable for varied specific applications. The present study aims to shed light on the types, and different kinds of PUs.

Introduction

Polyurethanes (PUs) are extraordinary and versatile polymeric materials having exceptionally balanced properties such as excellent flexibility, elasticity, a wide range of hardness, good tear strength, good abrasion resistance, good chemical resistance, heat saleability and wide processing window. By virtue of their tailor able properties, PUs have a variety of applications such as coatings, adhesives, sealants, foams (flexible or rigid), paints, varnishes, leathers, rubbers, fibres, films, bio mimetic materials and many more [1]. Despite their versatile properties, a major drawback of PUs is their inherent permeability to gases and vapours including oxygen (O2), nitrogen (N2), carbon dioxide (CO2), helium (He), water and organic vapours.

A good gas barrier property is one of the stringent requirements for numerous special applications where PU based membranes, films or coatings are extensively used such as coated or laminated envelop for lighter-than-air (LTA) applications (hot air balloon, aerostat, airship etc), food packaging, automotive applications (primer coating, tire etc), biomedical applications (dialysis membranes, wound dressings, encapsulating membranes, catheters, etc), flame retardant coatings, anti-corrosion coatings and so on. However, the high permeation of gases through PU based films and coatings seriously affect their service or performance in such applications. The incorporation of these nanomaterials into the PU matrix is also found to be effective in improving its mechanical, physical, thermal and many other functional properties. Here we are giving a briefing and discussing the types and application of polyurethanes [2].

Basic chemistry raw materials and Synthesis of PU

PUs are basically block copolymers which are formed by poly-addition reaction of three main components: a diol or polyol, a diisocyanate or polyisocyanate and a chain extender [3]. Sometimes catalysts are also used in the synthesis of PUs. In practice, only few isocyanates are commonly used to prepare PUs, which are mostly aromatic compounds such as 4,4′- methylenebis(phenyl isocyanate) (MDI), toluene diisocyanate (TDI) and some aliphatic compounds such as hexamethylene diisocyanate (HDI) or cycloaliphatic compounds such as 4,4′-diisocyanate dicyclohexylmethane, (H12MDI or hydrogenated MDI). More recently, because of the high reactivity of isocyanates, the blocked isocyanates are being preferred due to their better stability. Blocked isocyanates do not react until they are exposed to their deblocking temperature and after deblocking, they react with hydroxyl or amine functionalized co-reactants to form thermally stable urethane or urea bonds, respectively [4]. In comparison to diisocyanates, the versatility of polyols is much higher in term of chemical structure, functionality and molecular weight. However, polyether and polyester polyols are most commonly used in the synthesis of PUs. The effect of structure and characteristics of polyols on properties of PU has been thoroughly investigated in much literature [5, 6].

The properties of PU can be tailored as per requirements using right di or polyol, di or polyisocyanate and chain extender (diol or diamine) as raw materials. During polymerization, the aromatic diisocyanates generally show higher reactivity than aliphatic or cycloaliphatic diisocyanates. Different diisocyantes have different contributions towards properties of PU. For example, the UV and oxidative stability of aliphatic PUs are better than aromatic PUs, but it is less rigid than aromatic PUs [4].

Synthesis of PU

Two main methods for preparation of PU are:

i) One-shot process: In this process, PU synthesis takes place in one step where the raw materials (polyol, diisocyanate, chain extender and catalyst) are mixed simultaneously. It is a very exothermic reaction and generally requires a similar reactivity for different hydroxy and isocyanate compounds.

ii) Prepolymer process: It is a two-step process. In the first step, the polyol and diisocyanate are reacted to form a prepolymer of intermediate molecular weight of about 20,000. The prepolymer can be OH-terminated or NCO-terminated depending on the stoichiometry of the raw materials. In the second step, the prepolymer is reacted with a chain extender (diamine or diol) to obtain a high molecular weight PU [3].

Types of Polyurethane

Flexible Polyurethane Foam

Flexible polyurethane foam accounts for about 48.55 percent of the entire world polyurethane market. Flexible polyurethane foam is used as cushioning for a variety of consumer and industrial products, including bedding, furniture, automotive interiors design, carpet underlay and packaging industry [7, 8]. Flexible foam can be created in almost any variety of shapes and immovability. It is very light, durable, supportive and very much comfortable.

Rigid Polyurethane Foam

Rigid polyurethane and polyisocyanurate (polyiso) foams create one of the world’s most famous, energy-efficient and versatile insulations. To maintain uniform temperature and lower noise levels in houses and commercial buildings, builders turn to rigid polyurethane and polyisocyanurate foam. These foams are effective insulation materials that can be used in rooftop and wall insulation, insulated windows, doors and air boundary sealants [9].

Coatings, Adhesives, Sealants and Elastomers (CASE)

The uses of polyurethanes in the coatings, adhesives, sealants and elastomers market offer a wide and developing range of utilizations and advantages. Polyurethane coatings can improve a product’s appearance and lengthen its lifespan. Polyurethane adhesives can gives the strong bonding advantages, while polyurethane sealants provide tighter seals [10]. Polyurethane elastomers can be formed into practically in any shape, are lighter than metal, offer superior stress recovery and can be resistant to many environmental factors such as weather, Air and water.

Thermoplastic polyurethane (TPU)

TPU is an elastomer that is completely thermoplastic. Thermoplastic polyurethane (TPU) offers a myriad of physical property combinations and processing applications. It is highly elastic, flexible and resistant to abrasion, impact and climate. TPUs can be shaded or manufactured in a wide assortment of techniques and their use can increase a product’s overall the durability [11].

Reaction Injection Molding (RIM)

Vehicle guards, electrical housing panels, computer and telecommunication equipment are some of the parts are manufactured with polyurethanes using reaction injection moulding method (RIM). In addition to high strength and low weight of material, polyurethane RIM parts can exhibit heat resistance, thermal insulation, dimensional stability and an abnormal state of dynamic properties.

Binders

Polyurethane binders are used to adhere numerous types of particles and fibres to each other. Their primary areas of use are in the manufacturing of wood panels, rubber or elastomeric flooring surfaces and sand casting for the foundry industry [12]. These wood panels are used in structural sheathing and flooring, manufactured housing, joists and beams and shop panels.

Waterborne Polyurethane Dispersions (PUDs)

Waterborne polyurethane dispersions (PUDs) are coatings and adhesives that use water as the primary solvent to apply on any material. With expanding government guideline on the measure of unpredictable natural mixes (VOCs) and risky air toxins (HAPs) that can be discharged into the environment, PUDs are being used in more industrial and commercial applications. The list of applications is long and getting longer all the time, as new uses is found for this versatile material [13].

Applications and uses of polyurethanes

Apparels applications

When scientists discovered that polyurethanes could be made into fine threads, they were combined with nylon to make more lightweight, stretchable garments. Over the years, polyurethanes have been improved and developed into spandex fibers, polyurethane coatings, and thermoplastic elastomers. With the advancement in techniques for producing PUs, it has opened up the possibilities for producers to manufacture a wide variety of PU-based leathers, bra cups and man-made skins, which may be also used for several sport attires and a wide range of accessories.

Also, crock fastness, fastness of washing and the soap fastness of reactive dyes, acid dyes and direct dyes on dyed fabrics may be greatly improved by using WPUs as dye finishing agents [14]. In a different study, the low molecular weight of chitosan was used to extend the PU prepolymer chain for the preparation of chitosan–PU dispersion. This dispersion was applied to different quality plain weave poly-cotton dyed and printed fabric pieces to obtain improved stiffness, pilling resistance, and better mechanical properties. It was suggested that the quality of pure cotton and woolen fabrics can also be improved by applying this technique.

Automotive applications

The uses of PU material in the automotive industry are vast. Aside from its common use as foam to make vehicle seats more comfortable, it may also be used in car bodies, bumpers, doors, windows and ceiling sections. PUs also help to provide better automobile mileage through reduced weight, increased fuel efficiency, good insulation with proper sound absorption, great comfort for passengers and high corrosion resistance properties [15].  Due to the low density of PU foams, they are suitable for the manufacture of stiff and very light components, which may then be used as interior panels in aircraft, ships, sporting cars & racing cars, structural shapes, such as bulkhead cores, stringers and transform cores in reinforced plastic boats, etc. Coatings are another prime need for automobiles industry and can also be prepared by using PU. The development of modern technology related to nanofillers or nanoparticles can add some important features in PU-based advanced coating materials for automobiles.

Building and construction applications

Now day’s buildings need to meet certain requirements in terms of the use of construction materials, including high-performance with strong materials, lightweight, easy to install, durable and versatile. These properties may be achieved through the incorporation of PUs into building and construction materials. In fact, the use of PUs could offer great conservation of natural resources and help the environment through reduced energy consumption. The use of PUs for construction and building applications is on the increase due to their specific properties, such as excellent heat insulation capacity, highly desirable strength-to-weight ratio, versatility, and durability [16].

PUs can be used in almost any part of the building, such as for floors, e.g. in the form of pads of a flexible cushion for carpets, or for roofing, e.g. in form of heat and light-reflecting materials. In the roofing application, the plastic coverings on the PU surface can help to keep the building cool on the one hand and help to reduce energy consumption on the other hand. Generally, PU materials help to add flexibility to new buildings, such as the entry door and garage doors, which contains panels with foam cores.

Coating applications

Over the years, there has been continuous research is going on suitable materials for coating applications. PUs has been reported to possess great potential as paint and surface-coating materials. Research in this area saw the development of certain non-linear hyperbranched polymers, which have metamorphosed into other hyperbranched PUs with gloss, high solubility, and flexible coating properties. However, reports in the literature revealed that most of the synthesized hyperbranched polymers cannot withstand fire outbreak as they are non-flame retardants. To modify these hyperbranched materials for certain flame-retardant coating applications, compounds containing nitrogen, halogen or phosphorus may be incorporated into them [17].

Recently, triol, tris(bisphenol-A)monophosphate, which contained phosphorus, was reacted with polyethylene glycol and castor oil using different diisocyanates, such as toluene diisocyanate (TDI), hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI). A highly flame-retardant hyperbranched PU was produced, which was suitable for application in nanocomposites and nano-coatings.

In another research, a two-step, the one-pot pre-polymerization approach was used to manufacture hyperbranched castor-oil-based PUs, which were observed to have highly desirable potential to be used as advanced surface-coating materials. Another type of coating material suitable as a marine antifouling material was produced from the polyester-based polyol.  The synthesized antifouling coating material was also found to be highly degradable. Other sources from which PU have been recently synthesized for coating applications includes fatty acids, soybean and lignin isocyanate trimmers, and polyester polyols [17].

Electronic appliances, Flooring and packaging applications

Most of the electronics appliances that consumers use these days are based on PUs material. Rigid PU foams lead the way in the number of applications as they can be used as thermal insulators for refrigerators and freezers. These materials have become so essential due to their cost effectiveness, which make them suitable for use to meet the required energy ratings in most freezers and refrigerators. The advantages that rigid PU foams provide to these appliances are due to the combination of cell gases and fine foams with a closed-cell structure, which helps to prevent heat transfer from one to other places.

For the flooring purposes, PUs has several specific applications, such as top coatings or as carpet underlay foams. They can help to make floors more durable, aesthetically pleasing and easy to maintain. The lifespan of carpets and their appearance can be increased though the use of PU foam underlay’s, which can also help to provide better comfort with reduced ambient noise. PU Based protective finishes can also be used as floor coatings, where they can provide solvent and abrasion resistance on the one hand and ease of cleaning and maintenance on the other hand [18].

For the packaging applications, PU can also be used as a printing ink or as packaging foams. A PU plasticizer was prepared from palm olein and castor oil for packaging applications. This PU plasticizing resin showed high flexibility with good mechanical and freeze resistivity properties. On the other hand, PU packaging foams (PPFs) offer a wide range of packaging options, which should help to overcome most onsite packaging challenges. Custom-fit packaging materials have also been made available to almost all shipments using PUs.

Medical applications

PUs are used in several medicine-related applications, including, but not limited to, general-purpose tubing, surgical drapes, catheters, hospital bedding, wound dressing and several other injection-molded types of equipment. They are used for these applications due to their availability, good mechanical and physical properties and biocompatibility. However, the most frequent use is in short-period implants. The incorporation of PUs in the medicine-related application helps to offer cost-effective and provides adequate room for toughness and longevity of materials. This feature has allowed polymeric materials to replace conventional materials, such as metals, ceramics, and metal alloys [19].

Furthermore, other medical-application-based studies were performed, including studies on a chitosan-based PU for antibacterial properties and biodegradable electro-active PUs for cardiac tissue engineering. From these research studies on the medicinal applications of PUs, it was observed that some of the produced materials often perform only at a moderate level, especially in terms of their resistance towards bacterial adhesion.

This is because most of them are susceptible to bacterial attack, thereby leading to the risk of infection. New strategies for producing antibacterial PUs have therefore become necessary. These could be achieved via the incorporation of certain surfaces that have the capability to resist or repel the attachment of bacteria to the material surface. These bacterial resisting surfaces could be produced either through the incorporation of some antibacterial coatings or via some other surface modifications that could enhance the antibacterial or anti-biofouling properties of the materials.

Marine applications

PU materials have contributed a large innovation to the recent development in boat and ship technology. PU-Based epoxy resins help to protect boat hulls from weather, corrosion and water as well as other substances that may increase drag. In addition, PU-based rigid foam helps to insulate boats from extreme temperatures and noise. It helps to provide increased tear and abrasion resistance, and offers good load-bearing capacity with the properties even at minimum weight. PUs provides the specific advantages including elasticity, durability and ease of processing ability with good suitability for cable and wire coatings, drive belts, hydraulic seals and hoses and engine tubing as well as ship construction [20].

Wood composite applications

PUs are very important inclusions in many present day materials, including wood composites. Recently PU-based flat composites were prepared by using activated carbon for electromagnetic interference (EMI) shielding. Different amounts of activated carbon were loaded into PUs for microwave absorption and complex permittivity. The results showed the suitability of the composites in place of materials based on polyethylene and polyester filled with metal additives [21].

Conclusion

Polyurethanes (PUs) are some of the most common, versatile and researched oriented materials in the world. They combine the durability and toughness of metals with the elasticity of rubber, making them suitable replacements for metals, plastics and rubber in several engineered products. They have been incorporated into many types of industrial equipment and for making different items, such as paints, liquid coatings, elastomers, rigid insulations, elastic fibres, soft flexible foams and even as integral skins. PUs may be produced from a wide range of diisocyanates, a variety of polyols and other chain extenders and cross-linking agents. PU could be considered to be environmentally non-hazardous and more economical compared to other conventional polymers available, due to its good recycling and recoverable properties.

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Brijesh Kumar, Supriyo Chakraborty, Shubhankar Maity

Uttar Pradesh Textile Technology Institute Kanpur, India