This article will give you a detailed insight into polyurethane foams. Read further to learn more about:
- Overview of polyurethane foam
- Raw materials
- Production of polyurethane foam
- Properties of polyurethane foam
- And much more…
Chapter One – Overview of Polyurethane Foam
Polyurethane foam is a porous, cellular-structured, synthetic material made from the reaction of polyols and diisocyanates. Its structure is a composite of a solid phase and a gas phase. The solid phase is made from polyurethane elastomer while the gas phase is air brought about by blowing agents. The huge gas phase of polyurethane foams ascribes desirable properties such as good thermal and acoustic insulation, high force absorption, low density, and flexibility. Polyurethane foams are sometimes referred to as foam rubber. Foam rubber is a broader type of material that include foams made from latex, neoprene, and silicone.
Polyurethane foams are used in the manufacturing of mattresses, furniture, car seats, thermal insulations, and packaging materials. They comprise around 67% of global polyurethane production with an estimated market of around $37.8 billion in 2020. This is expected to grow to $54.3 billion by 2025.
History and Development of Polyurethane Foam
The discovery of polyurethanes is attributed to Otto Bayer dated back in the year 1937 together with his coworkers at the laboratories of I.G. Farbenindustrie A.G. in Germany. The first polyurethane was formed by the reaction between a diamine forming polyurea and an aliphatic diisocyanate. The polyurea was later replaced by glycol due to the enhanced properties of the polyurethane created.
The first patent for flexible polyurethane foam preparation was obtained by Zaunbrecher and Barth in 1942. The flexible polyurethane foam was done through simultaneous polyurethane synthesis and gas generation by combining organic toluene diisocyanate (TDI), aliphatic polyester, water, and catalyst. In this one-step process, polyurethane was formed by the reaction of the isocyanates with the hydroxyl-functional groups from the polyester while its gas phase was formed by the generation of carbon dioxide. Carbon is a product from the reaction of diisocyanates with water. This one-step process was highly exothermic and resulted in scorching and fires. This one-step process was then later transformed into a two-step process. This two-step process starts with the polymerization process (prepolymer preparation) followed by gas generation.
Polyether polyols in the form of poly tetramethylene ether glycol (PTMEG) were introduced in 1956 by DuPont. Later, BASF and Dow Chemicals developed polyalkylene glycols. Since then, polyether polyols became the major polyols in the polyurethane manufacturing industry. Initially, polyurethane foam production was done in a two-step process. But with the development of catalysts and surfactants, this was then replaced by a one-step process. Further refining of blowing agents, polyether polyols, and polymeric isocyanates led to the advancement from flexible polyurethane foams to rigid polyurethane foams.
Flexible and Rigid Polyurethane Foams
The two main types of polyurethane foams are flexible and rigid polyurethane foams. Flexible polyurethane foams can be made slabstock or molding processes from either polyether or polyester polyols. They have lower bulk densities, higher sag factors, and permeable structures. Flexible polyurethane foams are mostly used in furniture, seat cushions, mattresses, and acoustic dampers.
Rigid polyurethane foams are denser and have a high percentage of closed cells. Foams with closed cells do not easily allow air to escape from the foam. This gives the foam higher load-bearing capacities, good water resistance, and lower thermal conductivity. This makes rigid polyurethane foams suitable as construction and insulation material. Rigid polyurethane foams can also be manufactured through slabstock and molding processes, with the addition of lamination and spraying.
Chapter Two – Raw Materials
Polyurethane foams are made of six main components: polyols, diisocyanates, blowing agents, surfactants, catalysts, and curatives (cross-linkers and chain extenders). The polyols and diisocyanates react to form the main polymer chain of the polyurethane foam. The blowing agent is responsible for generating gas creating the material’s porous structure. The surfactants, catalysts, and curatives aid the polymer system to stabilize and maintain the desired rate of reaction of the polymer system. Additives are used to impart additional properties that depend on the foam’s intended application.
Polyol: A polyol is an organic molecule containing one or more hydroxyl (OH) groups. Polyols used in polyurethane foam production are mainly categorized into either polyether or polyester types.
- Polyether Polyols (PETP): These are made by the reaction of organic oxides and glycol. Polyurethane developed from polyether polyols have high moisture permeability, good hydrolysis resistance, better functionality, and a cheaper method of production. However, in contrast with polyurethanes from polyesters, polyether polyurethane foams are less resistant to oxidation. Common types of polyether used in the polyurethane industry are PTMEG and polypropylene glycol (PPG). Between the two, PTMEG offers superior quality but is more expensive.
- Polyester Polyols (PEP): These are made by the polycondensation reaction of dicarboxylic acids/anhydrides and glycols. The properties of the resulting polyurethane depend on the degree of cross-linking and the initial molecular weight of the prepolymer. High branching results in a more rigid structure with good heat and chemical resistance while low branching produces the opposite. Regarding molecular weights, low molecular weight means more rigidity while high molecular weight yields more flexibility. Polyester polyols are generally prone to degradation through hydrolysis due to ester functional groups present in the polymer chain.
Diisocyanate: Together with the polyols, diisocyanate compounds form the prepolymer of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
- Aliphatic Diisocyanates: Its most popular characteristics are its non-yellowing appearance and lower reactivity. Aliphatic diisocyanates are mostly used in applications where color stability is required. The most common ADIs are hexamethylene (HDI), hexamethylene (HMDI), and isophorone (IPDI). These are less often used in foam production since they are better suited in the manufacturing of elastomeric coatings and seals.
Aromatic Diisocyanates: Aromatic diisocyanates represent more than 90% of total diisocyanate consumption. This type is further divided into NDI, TDI, and MDI. Polyurethane foams made from aromatic diisocyanates can be formulated into different levels of rigidity. However, they have lower oxidation resistance and ultraviolet radiation stability.
Toluene Diisocyanate (TDI): TDI is obtained from the phosgenation of diamino toluene taken from the reduction of nitrotoluene. Typical forms of TDIs used on an industrial scale are the 2,4 and 2,6 isomers at 80/20 blend. Producing different proportions other than the 80/20 requires an additional process. TDI is used in the preparation of flexible polyurethane foams.
Methylene Diphenyl Diisocyanate (MDI): MDIs are manufactured by the phosgenation of the condensation product of aniline with formaldehyde. The most common isomer used in polyurethane production is purified 4,4 isomers. MDIs are generally used in producing rigid and semi-rigid polyurethane foams.
Naphthalenic Diisocyanates (NDI): This type is extensively used in Europe as compared to the TDI and MDI dominated American market. NDIs are known to offer superior performance and long service life for dynamic applications. One downside of using NDIs is their high melting point making them difficult to process. Moreover, it is highly reactive resulting in lower storage stability.
- Toluene Diisocyanate (TDI): TDI is obtained from the phosgenation of diamino toluene taken from the reduction of nitrotoluene. Typical forms of TDIs used on an industrial scale are the 2,4 and 2,6 isomers at 80/20 blend. Producing different proportions other than the 80/20 requires an additional process. TDI is used in the preparation of flexible polyurethane foams.
Blowing Agents: Blowing agents are used to generate gas to produce the foam’s cellular structure. Gas can be introduced into the polymer system through chemical and physical means. The first blowing agent used was CFC-11 or trichlorofluoromethane. This was considered as an ideal blowing agent due to its non-combustibility, appropriate boiling point, good compatibility with polyurethane, and non-toxicity. However, the chemical, along with other hydrochlorofluorocarbons, is now banned through the Montreal Protocol in 1987 because of its tendency to cause ozone layer depletion. Today, CFCs are now being replaced by water, pentane, methylene chloride hydrocarbons, halogen-free azeotropes, and other zero ozone depletion potential blends.
- Chemical Blowing Agents: Chemical blowing agents generate gas by adding compounds that react with the isocyanate groups to form carbon dioxide gas. A common chemical blowing agent is water. Using water alone poses several problems such as higher temperatures from the exothermic reaction, high polymer system viscosity, high consumption of isocyanates, and inefficient mixing. Thus, it is usually paired with a physical blowing agent. Other chemical blowing agents include enolizable organic compounds, polycarboxylic acid, and boric acid.
- Physical Blowing Agents: Physical blowing agents, on the other hand, operate by vaporizing a volatile compound from the heat generated by the exothermic polymerization reaction. Since the ban on CFCs and HCFCs, new physical blowing agents have been developed such as cyclopentane, n-pentane, liquified CO2, methyl chloride hydrocarbons, and halogen-free azeotropes.
- Surfactants: Surfactants are additives that help form, stabilize, and set polyurethane foams. The most widely used are silicone-based surfactants. Silicone surfactants perform important functions such as reducing surface tension, preventing foam collapse until cross-linking, controlling cell size, prevention of cell shrinkage after cure and counteracting any deformities induced by adding solids into the system.
Catalysts: Catalysts are used to control the rate of reaction of the isocyanate and hydroxyl groups and the rate of gas generation. These polymerization and gas generation processes usually need to occur simultaneously. If the polymerization process proceeds faster than the gas generation, the cells tend to remain close which causes the foam to shrink as the cures and cools. Consequently, if the gas generation is faster, the cells expand before the polymer can cure and provide support. The rates of these two reactions must be balanced to produce uniform open cells.
- Cross-linkers and Chain Extenders: These are low molecular weight polyols that are used to cure the polyurethane system. The elastomeric properties of the polyurethane solid phase such as tensile, flexural, and tear strength are derived from the degree of cross-linking and chain extension. Common cross-linkers and chain extenders are glycerol, ethylene glycol, and diamines.
- Other Additives: These include antioxidants, UV stabilizers, anti-static agents, plasticizers, flame retardants, pigments, mold release agents, and fillers. A specific set of additives is blended into the polymer system to impart properties desired for the intended application.
Chapter Three – Production of Polyurethane Foam
The production process of polyurethane foam can be divided into two: the polymer system preparation and the foam production process. The polymer system preparation involves blending and mixing the components through a mixing head or in a master batching system. The main reactive components are the polyols, diisocyanates, and chemical blowing agents. Polyols and diisocyanates are the components for the polymerization process while diisocyanates and chemical blowing agents (water) are for the gas generation process. There are different types of polymer system preparation that differ on the method of combining these components.
After the polymer system is prepared, its foaming or rise is controlled through different foaming technologies. This can be slabstock, molded, laminated, or sprayed. Proprietary processes also exist which can have more controlled foaming capabilities at higher rates. Once the foaming process is complete, it passes through secondary processes such as additional curing and cutting.
Polymer System Preparation
The polyurethane reaction involves the formation of a simple polymer chain from the reaction of a polyol component (a carbon-chained molecule with alcohol functional groups) to a diisocyanate component (a molecule with two isocyanate functional groups) through polyaddition polymerization. This results in a molecule with reactive alcohol on one end and a reactive isocyanate on the other. The alcohol end further links to another isocyanate end or terminal, while the isocyanate end of the same chain further reacts with other polyols or cross-linkers and chain extender compounds. This process continues making long-chained polyurethane.
Preparing the formulation can be done through different systems: single shot (one step), quasi-prepolymer, and full prepolymer systems. The single-shot system involves preparing a premix of polyols, blowing agents, surfactants, catalysts, and additives which are then combined with isocyanates. Thus, the polymerization and gas generation process occur simultaneously. In a quasi-prepolymer system, a part of the polyol is premixed with the isocyanates while the rest is blended with the blowing agents, catalysts, and other components. The polyol-additives premix and the prepolymer are then mixed to form the polyurethane foam. Lastly, the full prepolymer system involves mixing the polyols and isocyanates to create the prepolymer which is then mixed with the blowing agent, catalysts, and other additives. This system is less often used in comparison with the single shot and quasi-prepolymer systems.
Polyurethane Foam Production Processes
Enumerated below are the different foaming methods for both flexible and rigid polyurethane foams.
Slabstock Foam: A slabstock foam is a continuous loaf of foam made by pouring the foaming polymer system onto a moving conveyor. The slabstock process typically uses a single shot polymer system with water as the blowing agent. The polymer system foams or rises as it spreads across the conveyor. Waxed paper is used to prevent the polymer system from adhering to the conveyor and forming plates. As the polyurethane continues its polymerization and gas generation, heat is released from the reaction. Preventing excessive release of heat is controlled by the isocyanate index, water level, use of physical blowing agents, and catalyst concentration. Ventilation systems are used to aid in removing this heat to prevent spontaneous combustion or scorching.
Molded Foam: Unlike slabstock foam, molded foams are usually produced in a discontinuous process. Foam molding is used to create products with intricate shapes such as seat cushions, paddings, head restraints, dampers, and construction materials. This process involves pouring or injecting the components through a mixing head and into a preheated mold. The components react inside the mold causing the polymer system to foam and rise. The molded foam process can be further divided into two types: the hot-molded foam process and the cold-molded foam process. As their name suggests, they are classified according to the mold temperature. The hot-molded process involves conventional polyethers mixed with TDI. The cold-mold process, on the other hand, uses polymer systems prepared from polyethers and a blend of TDI and MDI, or 100% MDI. The faster reaction of MDI results in lower mold temperatures.
Lamination: This is similar to slabstock production but is mostly used in producing rigid polyurethane foams. Rigid polyurethane foam laminates consist of a rigid foam core with either flexible or rigid facings. Examples of flexible facings are craft paper, aluminum foil, and polyethylene-coated paper. Rigid facings include gypsum board and steel sheet. In this process, a continuous slab is produced by pouring the polymer system on a moving conveyor. Another belt system is present to form the top-side of the foam. Both the conveyor and the top-side belt system feed the facings onto the polyurethane foam.
Spraying: The polyurethane spraying process involves projecting and impinging the blended polymer system on a surface or inside a cavity. This provides a seamless insulation layer that is particularly useful for roofing, wall, and tank insulation. This is usually done at temperatures above 15°C. When performed at lower temperatures, foaming efficiency and adhesion strength becomes poor.
Chapter Four – Properties of Polyurethane Foam
This chapter tackles the main properties of polyurethane foam. Because of its highly porous structure, polyurethane foams can easily be compressed. The degree of compression largely depends on the cell structure which can be open or closed cells. Most of the polyurethane foam properties revolve around its compressibility such as density, load-bearing properties, and durability.
- Density: The polyurethane foam density is characterized by the bulk density, not the true polymer density. Density indirectly reflects the cost of the foam and its load-bearing capacity. This is measured by getting the average densities of several layers from a sample block. The sample block is cut into several layers perpendicular to the growth axis.
Load-bearing Capacity: The load-bearing capacity is the measure of how much compressive force the foam can support. This determines the firmness or stiffness of the polyurethane foam. The two common methods of testing for the load-bearing capacity are indentation force deflection (IFD) and compression load deflection (CLD).
- Indentation Force Deflection (IFD): This test is performed by measuring the load applied to an indenter pressed against the foam specimen at 25%, 50%, and 65% of its thickness. The higher the force recorded, the higher the load-bearing capacity of the foam.
- Compression Load Deflection (CLD): The CFD test involves pressing a plate to apply uniform force over an entire sample block. The principle is the same as IFD. The force required to compress the specimen is recorded at 25%, 50%, and 65%.
- Compression Set: This is the loss of thickness after compressing the foam under particular conditions. The test is done by compressing a specimen between two plates. The degree of compression can be 50%, 75%, or 90% depending on the test method used. The foam is kept compressed for a certain number of hours before doing the final measurement.
Tensile Strength: Tensile strength is the amount of force required to break a specimen with a given cross-sectional area. The specimen is die-cut into a “dog-bone” or dumbbell-shaped profile. The test is done by clamping the specimen at both ends and pulling it at a constant rate until it breaks.
- Elongation at Break: This is the amount of stretching the foam can hold before breaking. This is measured by getting the percentage change between the initial length and length at the break. Elongation measurement is performed simultaneously with the tensile strength test.
- Tear Resistance: This is the force required to break the foam with a puncture or slit at one side. The concept of measuring the force is similar to that of the tensile strength test. But in this test, the specimen is slit with a specified length and location. The tear is intended to propagate from this slit.
- Air Flow: Air flow or air permeability is defined as the volume of air that can pass through the foam at a specific rate. This is a measure of the porosity of the foam. Airflow determines the foam’s sealing properties and its ability to insulate heat.
Resilience: Resilience is the ability of the foam to elastically rebound an applied force. This is determined by measuring the bounce height of a calibrated steel ball dropped at a specific height. Resilience is expressed in terms of the ratio or percentage of the rebound height with the starting height.
- Dynamic Fatigue (Dynamic Durability): This is the measurement of the loss in load-bearing capacity and thickness after the foam has been compressed repeatedly for several cycles. This is done by comparing the initial and final compression load deflections and thicknesses. An indenter or plate is used to apply compressions to the foam sample. Final measurements are done after several thousand cycles at a specified rate.
- Static Fatigue (Static Durability): This is a measurement of the loss of load-bearing capacity and thickness after subjecting the foam to static loading. Similar to dynamic fatigue, static fatigue measurement is obtained by comparing the initial and final compression load deflections and thicknesses. The specimen is compressed at a specific thickness percentage kept for a specified duration. Afterward, final measurements are performed.
- Polyurethane foam is a porous, cellular-structured, synthetic material made from the reaction of polyols and diisocyanates.
- The two main types of polyurethane foams are flexible and rigid polyurethane foams. Flexible polyurethane foams are mainly used for cushioning and acoustic and force damping, while rigid polyurethane foams are used for construction and thermal insulations.
- Polyurethane foams are made of six main components: polyols, diisocyanates, blowing agents, surfactants, catalysts, and curatives (cross-linkers and chain extenders).
- The production process of polyurethane foam can be divided into two: the polymer system preparation and the foam production process. The polymer system preparation involves blending and mixing the components while the foam production process involves controlling the rate and profile of foam formation.
- Most of the polyurethane foam properties revolve around its compressibility such as density, load-bearing properties, and durability. Other important properties are tensile strength, tear strength, elongation at break, and air permeability.