Polyurethane Case: Manufacture, Applications, and Market

The study of the manufacturing process of polyurethane (PU) is complex and relies on the knowledge of chemistry. This paper will be looking at three crucial aspects of the production process of polyurethane. The first part is the discussion about the manufacturing process of the material. It divides this study into three sections including the production of isocyanates, the creation of polyols and ultimately the production of polyurethanes. The second significant discussion encompasses the application of PU materials. This section is broad identifying the primary uses of polyurethane such as thermal insulation, sole production, cushioning, in construction among others.

There is also an introduction for the production and application of the following components:

1. Coatings
2. Adhesives
3. Sealants, and
4. Elastomers.

The last part of the paper looks at the processing of thermoplastic polyurethane.

Polyurethane (PU) refers to a compound belonging to the polymers family. In simple terms, polymeric materials are some form of plastic but different in their composition because they have no urethane monomer (Lazonby). The polymer is a product resulting from the production process of different materials. Otto Bayer and other of his colleagues were responsible for the earliest discovery of PU back in 1937. They were working on other projects that focused the production of polyurea from aliphatic diisocyanate and diamine. It was while working at the laboratories of I.G Farben in Leverkusen Germany that the team discovered the formation of PU from aliphatic diisocyanate and glycol (Sharmin & Zafar 3). The production of PU continued but only became commercially available fifteen years. There was mass production of the compound after the Second World War. Initially, Bayer extracted PU from toluene diisocyanate (TDI) and polyester polyols for the mass production. However, in the year 1952 to 1954, he managed to obtain the compound from various polyester-polyisocyanate systems.

Over time, polyether polyols gained popularity due to their low cost, ease in handling as well as enhanced hydrolytic stability which led to the phasing out of polyester polyols. Polymerizing tetrahydrofuran was the process that DuPont used to produce the first polyether polyols in the form of poly (tetramethylene ether) glycol (PTMG). The compound was commercially available, and DuPont later produced Lycra by combining PTMG and other compounds such as ethylene diamine. In 1957, the chemistry industry witnessed the production of polyethylene glycols (Sharmin & Zafar 3). Chemist continued to transform PU from flexible to rigid foams that serve as blowing agents. However, there are other forms of blowing agents in the market including pentane and carbon dioxide among others. Today, PU has numerous applications and is referred to different types of plastic owing to its superior properties in the final product.

Polyurethane manufacturing process

Of importance also is understanding the production process of PU that chemists such as DuPont and Bayer and that is still in use to date. The first point is that the process involves an exothermic reaction between a mixture of alcohol and multiple hydroxyl (-OH) group molecules with isocyanates. The molecules groups can either be diols, triols or polyols. On the other hand, the isocyanates should be multiple isocyanate groups (-NCO) such as diisocyanates or polyisocyanates. The reaction between the two molecule groups forms the urethane linkage which is an essential component of a PU molecule (Lazonby). The original reactants dictate the resulting structure properties of a PU. Furthermore, the use of the final product (polymer) depends on features such as relative molecular mass. Nonetheless, the manufacturing process can be explained according to the reaction of the three compounds.

The first part is the production of isocyanates. Industries consider TDI (toluene diisocyanate or methylbenzene diisocyanate) and MDI (methylene diphenyl diisocyanate or diphenylmethane diisocyanate) as the most critical form of aromatic and polyisocyanates. The former consists of two isomers and toluene (methylbenzene) serve as the first material to the reaction. It produces nitromethylbenzene after mixing with an acid substance for example nitric acid. Nitration of nitromethylbenzene produces dinitromethylbenzenes. The next step is the reduction of the dinitrobenzenes to amines. Chemists then combine the amines with phosgene by heating to create diisocyanates. This stage takes place when the substances are in a liquid state with chlorobenzene as the solvent. However, it is also possible to carry out this step in the gas phase. In such a case, the chemist has to vaporize the diamines and then mix with phosgene at ca 600 K. At this point, the actual substance is a dinitrocompound isomeric mixture consisting of 80 percent 2,4- dinitrotoluene (DNT)and 20 percent 2,6-dinitrotoluene.

In other words, it is necessary to have the diisocyanates in equal proportions to avoid incurring extra expenses to purify the mixture through distillation. Furthermore, the best practice calls for the production of PU with different properties by using polyols that can react with the 80:20 mix. The process of producing MDI is more sophisticated compared to the production of TDI. There are more processes involved in the formation of MDI resulting in a product with higher versatility. In most cases, MDI processes are used in the production of rigid foams. Unlike in the TDI case, where the toluene is the starting material, phenylamine(aniline) and methanol(formaldehyde) constitute the starting materials for the MDI process. The amines produced in this case are known as methylenedianiline (MDA). However, the mixing of these amines with phosgene produces MDI just like in the manufacture of TDI. Separation of the three isomers in the resulting mixture is possible through distillation.

The second part in the part in the manufacture of PU involves the production of polyols. These polyols make up at least 90 percent of the total fabrication of PU. They can either be in the form of hydroxyl-terminated polyethers or hydroxyl-terminated polyesters. A specific feature of these compounds is that their reaction with the isocyanates aims at producing PU containing particular properties. Hence, the degree of molecular cross-linking depends on the molecular structure, size and flexibility of the chosen polyol. These aspects are also important because they influence the mechanical properties of the polymer. Some reactions with biopols such as soya bean oil and epoxypropane reveal that polymers can be derived from renewable sources. Production of polyurethanes is the last part of the manufacture of PU. This part involves the production of a linear polymer which results from the reaction of two hydroxyl groups with either TDI or MDI. A reaction between polyols containing multiple hydroxyl groups leads to the intertwining of long-chain molecules at their intermediate points. The cross-linking of the particles in the immediate course of a stiffer polymer structure with enhanced mechanical features creating a rigid PU. The PU’s also does need to undergo a variety of chemical reactions to control the formation process and produce the desired PU with specific properties. Different additives serve different uses in the manufacture of PU.

For instance, catalysts are the agents that speed up the reaction process between the polyols and polyisocyanate. Smoke suppressants help to reduce the rate of generating smoke when burning a PU. One of the most popular uses of PU materials is serving as blowing agents surfactants. The additive, in this case, creates a PU in the form of foam as a way to control the bubble formation. Consequently, the reaction produces cell structure foam. On the other hand, the cross-linking of molecular strands modifies the PU structure and creating greater reinforcement. Hence, the physical structures increase the functionality of PU. Pigments ensure that PU has enough colored polyurethanes for visibility and aesthetic purposes. Plasticisers reduce the density making the product flexible. Flame retardants and fillers minimize flammability and improve the stiffness of the end product respectively.

In essence, the manufacturing process requires the combination of two major components in the right proportions. These elements usually consist of polyisocyanate and polyols in a liquid state. The reaction produces a solid polymer either in an elastic or rigid form. In few stances, the product contains bubbles of gas in cellular foam. A foamed PU results from two possible ways including physical blowing. This condition involves a liquid with a low boiling point. The next step involves mixing the liquid into polyols. The liquid vaporizes with time because it involves an exothermic reaction. The air dispersed through the reaction produces a mixture nucleation seed. On the other hand, water is necessary for chemical blowing to create carbon dioxide from the polyol-polyisocyanate reaction. The transformation of liquid to a solid polymer leads to the expansion of the gas bubble to the highest point possible.

Application

It is of the essence to point out again that manufacture of PU differs from the production of other plastics. For example, chemical plants manufacture poly (ethane and propane) and sell them in the form of granules, powder or any other substance. The process entails subjecting the polymer to heat and cold temperatures to allow shaping. The product properties are significantly similar to the original polymer. However, PU is produced as the final product especially as large blocks of foam. The manufacturer then cuts the blocks to smaller pieces. In most cases, the foams serve as cushions or for thermal insulation. The end reaction is a solid substance or liquid reactant. The rigidity or flexibility of PU depends on the specific density levels of the PU. This aspect also influences the use of a particular PU material. For example, PU’s are suitable materials for cushioning because of the low-density levels, high flexibility as well as high resistance to fatigue. Another frequent use of PU is the insulation of electrical equipment.

PU’s are the best suit for these machines because of their resistance to oils. Also, it is possible to increase the density during manufacturing to make the cables tough. Patients suffering from heart-related problems usually undergo a procedure of artificial heart valve installation. The artificial valves are suitable because PU’s have high flexibility and biostability levels. The high flexibility ensures that the valves can expand and contract freely just like the regular heart valve. PU’s are also crucial in construction because they provide thermal insulation that is critical because of varying temperatures. The adhesive properties also ensure that the building panels offer enough strength to hold the building together and that it lasts longer. Besides, its durability and flexible physical properties, PU’s are also good materials for shoe soles because they are resistant to abrasion.

Introduction of PU Coating and Adhesive Industries

PU is of crucial importance to the coating and adhesive industries. PU coatings and adhesives have several similarities as well differences in the manufacturing process as well as in the technological trends. The similarities in product format and global utilization reflect the need for specific polymer design and requirements. For instance, both cases involve the use of an integral film, in a process that entails applying the film on a surface that will later be moisturized to adhere to the surface. Likewise, both coatings and adhesive technologies involve either reactive or unreactive systems. The number of adhesives sold in the market is twice the number of PU coatings sold. However, it is impossible to have the exact measure of PU consumption in the market as a result of the components high formulation and dilute state. Ebrary.net estimates that PU accounts for at least 7 percent of the over 800 pounds of adhesives and sealant binders used in the world. Furthermore, the chemical industry recorded 1 percent increase in consumption from 2015 to 2012.

Coatings

Coating is necessary for vehicles, cables, floors, walls, bridges, and roads among other areas. Ebrary.net records that the coating market realizes about £ 1.5 billion in sales. Its rate of consumption globally reflects the high demand for finished goods that require PU coating. It also indicates that high application in industrial and architectural industries. PU contains properties that allow for durability, corrosion and weather resistance that makes it a suitable material for coating. The main aim of a coating is to protect and shield these services from pollution or corrosion of any external substance. It also helps in maintaining items such as cables last longer and also gives surfaces a better look (polyurethanes.org).

In particular, the coating of vehicles ensures that the exterior part is highly glossy to protect the car from corrosion or scratching while also improving the color retention. Japan is among the countries that record the highest consumption for PU coating. This situation could be as a result of a large number of automotive assemblies and architectural companies in the region. The same principle applies to the external parts of an aircraft. However, planes also need to have the surfaces coated from extreme temperature differences. The airplanes fly at high altitudes where temperatures can either be scorching or at freezing levels. Bridges surfaces require a coating to prevent the support beams from rusting (American Chemistry Council).

Adhesives

On the other hand, PU serves as adhesives or binders. It resonates from the fact that the compound is highly versatile allowing for the production of glues. History records that modern development of polymeric resins and sealants dates back to the early 1900s. The polymer industry began around the same time. Adhesives (resins) refer to substances used to hold two surfaces together and preferably in a permanent state. The process through which the surfaces attach to each other is known as adhesion. The phenol-formaldehyde adhesives used in the plywood industry were among the earliest forms of modern adhesives to be developed.

However, the period between the 1940s and 1950s marked significant growth for adhesives and sealants due to the increased demand for military aircrafts. However, the durability of the aircraft joints was a significant challenge for the developers. Chemists resolved this situation in the late 1970s when they introduced advanced adhesive systems that are still in use to date. Adhesion happens in the final stage of PU production. This step can produce either structural adhesives or non-structural adhesives. The former denotes resins that have strength as the core elements necessary for assembling. On the contrary, non-structural adhesives have less tensile strength since they are mainly used for temporary fastening.

Adhesives play a major role in the development of green strength. This phrase refers to the process whereby the PU material creates an initial bond layer before the complete curing. This aspect gives elements a second kind of protection. It also ensures that industries do not need to incur extra costs for clamping and holding materials. The physical properties include high shear and tensile strength (Partie 3). Some of the industries that heavily rely on PU glues include construction, furniture and packaging industries (polyurethanes.org). PU glues have high resilience and strength that ensures that items remain bind together for a long time. Construction, packaging, transportation, furniture, and footwear constitute the major market for adhesives and sealants in order of demand (ebrary.net).

Industries have also discovered that PU adhesive qualities help in the recycling of end of use vehicle tires to produce surfaces such as sports tracks. The recycling developments are essential as a way of preserving the natural resources. Fiberboard is a product of PU binders combined with woodchips. At the same time, PU sealants are a vital component in the construction or manufacture of materials that require high-strength water resistant seals. The PU adhesive property also ensures that materials recovery with ease after being bent or pulled and that the item does not lose its shape. It is also important to note that not all bonding agents are used for painting in spite of the use of PU adhesives and sealants for painting purposes (American Chemistry Council). Furthermore, adhesive failure arises from the inability of surfaces to bond.

Introduction of PU Sealant

Sealants are substances used to attach two surfaces by filling the space between the surfaces. The process provides a protective coating. The use of sealants closely relates to that of adhesives. In fact, sealants form the perfect example of nonstructural sealants. This situation results from the fact that both components play an essential role in assembling and value addition of finished products (Partie 1). The chemical structure of the two elements is almost identical. Just like adhesives, sealants are resistant to their operating environments. Other standard features include the fact that at some point the components are in liquid form to facilitate bond formation. After the bond is complete, the substances harden. Adhesion is also possible by combining with other parts in an assembly. This process is essential to ensuring the end product is durable. However, sealants have better flexibility as opposed to adhesives.

The construction, consumer products, transportation, aerospace and electronics companies are some of the primary market segments for sealants. The construction industry precedes the transportation and industrial markets in terms of demand. Though each market demands a specific type of sealant, synthetic sealants account for over 70 percent of the market supply. Chemists, however, note that there is need to develop a multi-disciplined approach for the successful application of both components. Pertie adds that the successful use of either sealants or adhesives depends on the correct selection of materials. The user must also understand the necessary process for adhesion. The natural flow of the liquid substance onto a substrate surface followed by the solidification of the element is one indicator of a successful adhesion process. Besides, the adhesive material should not destroy the substrate surface. Examples of conventional sealants include silicones. On a different note, it is essential to understand that external influences can affect the effectiveness of adhesive and sealant materials. Thus, it is impossible to determine the lifespan of a bonded surface.

Introduction of PU Elastomers

Another type of PU is the cast elastomers. These are rubber-like polymers with the ability to stretch to great lengths (McKeen 5). In fact, these polymers stretch the most compared to other forms of PU. However, they return to their shape after the stretching force is withdrawn. The way elastomers operate can be equated to a spring. Also, elastomers can resist flow when distorted by external forces. just like other forms of PU, the versatility element allows cast elastomers to attain the optimum physical properties necessary in the application of specific task. At the same time, flexibility facilitates PU manufacturing industries to customize the elastomers for use in the different market segments.

PU’s can perform a variety of functions for metals and ceramics because of the dependability of cast elastomers. When they are in rubber form, cast elastomers have high resiliency and flexibility. PU elastomers have diverse applications and offer a wide range of hardness and processing features. This component exhibits high levels of resistance to substances with high viscosity levels such as oil and petrol as well as non-polar solvents. Another unique feature of this element is that only a few compounds can affect completely-cured cast elastomers (McKeen 7). Oxidizing agents and other strong basic and acidic elements constitute examples of factors that can affect wholly cured elastomers. Popular cast elastomers products include skateboard wheels, fork-lift, and press-on tires.

Processing of PU Elastomers

Thermoplastics PU (TPU) elastomers is a urethane material. Industries produce it in the form of granules, or pellets through different thermoplastic techniques. These methods include three different kinds of molding: extrusion, calendaring and injection molding. The casting of the TPUs to various shapes is a simple process that requires the appropriate molding equipment and the suitable injection molding tools. Drying is the first step to ensuring that the method is effective. An essential element in the process is removing moisture from the polymer. This act ensures that the TPU does not lose its molecular weight. Drying of TPU requires specific temperatures for specified time intervals (Foster). The best practice requires drying TPU to water content specification.

A hopper dryer can later be used to ensure that the component remains dry. If coloring is a priority, in this case, additives should be added before the drying process commences. Extrusion molding also requires selecting of the best parameters. The drying step is similar to the injection method. The emphasis is to dry the TPU to a water content level to allow for ease in molding, and a hopper dyer ensures that the TPU remains dry. On the other hand, melt pumps ensure that there is constant and consistent flow. The screw design and cooling are also important steps in molding. The crystallization levels determine the shape of the melting curve. Other determinants include the screw speed and configuration (Foster). Spray nozzle and chilled water are suitable for effective cooling. Experts also propose the use of lubrication film to enhance the strength of the extrudate between the surfaces.

Thermoplastic Polyurethane

TPU materials are elastic, melt-processable and can even be colored in a number of ways. Fabrication methodologies prefer these types of materials because of their solution-coated and vacuum forming processes. The high flexibility levels provide that industries can adapt to several applications (Drobny 217). In part, TPU composes of block copolymers that have both soft and hard segments. While the soft layer can either be polyether or polyester in type, the hard portion contains either aromatic or aliphatic properties. Like discussed in the previous text on the manufacturing process of PU, MDI is the chemical reaction between isocyanates and hydroxyl components. Short term diols and isocyanates create the hard segment. An aliphatic is useful when the process mainly considers color and clarity retention (American Chemistry Council).

Cast elastomers formation process involves the low-pressure combination of a degassed liquid substance into a mold (Drobny 218). The polyurethane substance is reactive. The transformation process can be undertaken using a casting machine. There are numerous uses for PU cast elastomers including the development of engineering parts. Nonetheless, the different environment call for different kinds of TPU. A polyether-based TPU is the best suit for moist environments while polyester-based TPU function best in oil or hydrocarbon resistance materials. TPU utility depends on the molecular weight of the compound including other factors such as the chemical type. The unique structure is a crucial ingredient for high resilience plus resistance impacts. Some people argue that TPU’s bridge the gap between rubber and plastics. Users carry out a number of activities on TPU materials such as welding, painting, coloring, and printing among others. In spite of the low temperatures, TPU materials contain flame retardant as well as anti-static properties.

Studies show that the use of TPU is most common in cases of higher structural integrity such as assembling of automotive in the process of side molding (Drobny 222). A good illustration would be the mixing of TPU with glass fiber or mineral fillers. The end product of this process increases the abrasion resistance, low-temperature flexibility levels including adding to the strength. Resistance to oil would also be evident. Nonetheless, TPU acts as an excellent agent of polymer blends. Therefore, this aspect reinforces specific compounds such as polycarbonates. TPU are key components of films and sheet, drive belts, cattle tags, architectural glass lamination, wire and cable coating among other uses. The versatility property adds to the toughness and processing of the component with ease. On the other hand, other machines that can manufacture parts of cast elastomers include the compact, spanning and universal and advanced machine series (Drobny 222). Standard features for these devices include powerful electrical heating, a mixing system, bubble-free molding compartment as well as process reliability.

It is evident that the PU manufacturing industry has witnessed tremendous growth over the past eighty years leading to the high demand for PU materials. The process of manufacturing of polyurethane(PU) requires an understanding of basic chemistry. The different parts of production are unique and require the manufacturer to follow the correct steps to have the desired outcome. However, the end product in one process may be necessary for the next process. In particular, all the procedures required the production of components in the same proportion. This condition ensures that the product does not require to undergo distillation to purify the content. It is, however, apparent that there are many uses for a different kind of PU. Coating, adhesives, and sealants are the most common types of PU in the market. However, adhesives appear to have greater demand compared to coatings and sealants. Some users utilize adhesives and sealants interchangeably because of their binding capability. The difference arises from the fact that adhesives will provide a lasting effect compared to sealants that may prove helpful temporarily. On the other hand, elastomers are the PU components that bridge the gap between rubber and plastic. The distinguishing feature of these materials is the high ability to stretch to great lengths and return to the initial shape after withdrawing the external force. These materials undergo molding in three phases namely; injection, extrusion, and calendaring. It is highly probable that technological advancements will result in more efficient products in the future.