This article will provide a detailed insight into polyurethane foams. You will learn:
Polyurethane foam is a synthetic material with a porous and cellular structure, created through the reaction between polyols and diisocyanates. The foam consists of two phases: a solid phase made of polyurethane elastomer and a gas phase consisting of air, introduced by blowing agents.
The significant gas phase within polyurethane foams provides excellent thermal and acoustic insulation, high impact absorption, low density, and flexibility. Polyurethane foams are sometimes categorized under the broader term "foam rubber," which also includes foams made from materials like latex, neoprene, and silicone.
Polyurethane foams are commonly utilized in the production of mattresses, furniture, automotive seating, thermal insulation, and packaging materials. They account for approximately 67% of the global polyurethane market, which was valued at around $37.8 billion in 2020 and is projected to reach $54.3 billion by 2025.
Polyurethanes were first discovered in 1937 by Otto Bayer and his colleagues at the I.G. Farbenindustrie A.G. laboratories in Germany. The initial polyurethane was produced through a reaction between a diamine that formed polyurea and an aliphatic diisocyanate. However, glycol eventually replaced polyurea, resulting in polyurethanes with improved properties.
The first patent for flexible polyurethane foam preparation was obtained by Zaunbrecher and Barth in 1942. The flexible polyurethane foam was created 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 of the reaction of diisocyanates with water. This one-step process was highly exothermic and resulted in scorching and fires, so it was then later transformed into a two-step process. This two-step process starts with the polymerization process (prepolymer preparation) and is followed by gas generation.
In 1956, DuPont introduced polyether polyols in the form of poly tetramethylene ether glycol (PTMEG). Following this, BASF and Dow Chemicals developed polyalkylene glycols, which have since become the predominant polyols in polyurethane production. Initially, polyurethane foam was produced through a two-step process, but advancements in catalysts and surfactants enabled a shift to a one-step process. The refinement of blowing agents, polyether polyols, and polymeric isocyanates further facilitated the evolution from flexible to 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.
Microcellular Polyurethane Foam features a fine cellular structure with a density ranging from 0.25 to 0.65 grams per cubic centimeter. It contains billions of gas bubbles, each less than 50 microns (µ) in size. These uniformly distributed bubbles provide the foam with exceptional physical properties, including excellent compression set resistance, making it durable, long-lasting, and resistant to chemicals and corrosion.
A well-known type of microcellular foam is Poron®, an open-cell foam commonly used for gaskets, protective gear, and footwear. Poron® is prized for its cushioning, padding, compression, and sealing abilities, available in thicknesses from 0.012 to 0.5 inches (0.3 to 12.7 mm). Its properties include impact and vibration absorption, a smooth finish, low outgassing, and ease of manufacturing, making it ideal for addressing issues related to improper sealing and cushioning.
Polyurethane foams consist of six primary components: polyols, diisocyanates, blowing agents, surfactants, catalysts, and curatives (which include cross-linkers and chain extenders). The polyols and diisocyanates react to create the main polymer chain of the foam. The blowing agent generates gas, forming the foam’s porous structure. Surfactants, catalysts, and curatives play a crucial role in stabilizing the polymer system and controlling the reaction rate. Additional additives are included to enhance specific properties based on the foam’s intended use.
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.
Diisocyanate: Together with the polyols, diisocyanate compounds form the prepolymer of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
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 an 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 compared to the TDI and MDI-dominated American market. NDIs offer superior performance and long service life for dynamic applications. One downside of NDIs is their high melting point making them difficult to process. Moreover, it is highly reactive, resulting in lower storage stability.
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 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 being replaced by water, pentane, methylene chloride hydrocarbons, halogen-free azeotropes, and other zero ozone depletion potential blends.
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 closed, which causes the foam to shrink as it 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.
The manufacturing of polyurethane foam is split into two stages: the preparation of the polymer system and the actual foam production process. The polymer system preparation involves blending and mixing components using a mixing head or a master batching system. The primary reactive elements are polyols, diisocyanates, and chemical blowing agents. Polyols and diisocyanates are essential for polymerization, while diisocyanates and blowing agents (like water) drive gas generation. The methods of combining these components vary depending on the specific type of polymer system preparation.
Once the polymer system is ready, the foaming process is controlled through various techniques, such as slabstock, molded, laminated, or sprayed methods. Some proprietary processes allow for more precise control of foaming at higher rates. After foaming is complete, the material undergoes secondary processes, including additional curing and cutting.
The polyurethane formation process starts with a polyaddition reaction, where a polyol component (a carbon-based molecule with alcohol functional groups) reacts with a diisocyanate component (a molecule containing two isocyanate groups). This reaction forms a polymer chain with a reactive alcohol group at one end and a reactive isocyanate group at the other. The alcohol end connects with another isocyanate group or terminal, while the isocyanate end continues to react with additional polyols or cross-linking agents and chain extenders, resulting in the creation of long polyurethane chains.
Polyurethane formulation can be achieved using various methods, including the single-shot (one-step) process, quasi-prepolymer, and full prepolymer systems.
Outlined below are the various foaming techniques applicable to 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 prevents 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 heat release 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 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 sheets. 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. 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 59° F (15°C.) When performed at lower temperatures, foaming efficiency and adhesion strength becomes poor.
A wide array of machines is available for producing polyurethane foam, which plays a crucial role in modern society. Polyurethane foam is a highly adaptable material extensively used across industries like construction, automotive, furniture, and packaging due to its superior insulation, cushioning, and structural support properties. These machines provide precise control over the foam's formulation, density, and shape, enabling efficient and customizable production to meet various industrial and consumer requirements. Below, we explore some prominent brands of polyurethane foam production machines in the United States and Canada.
Linden Industries provides the LPU™ Series of machines, recognized for their cutting-edge metering technology and precise control over foam density and composition. These machines offer customizable production options, guaranteeing consistent and superior-quality polyurethane foam output.
Cannon USA's EPU Pro is a polyurethane foam production machine equipped with state-of-the-art mixing and metering systems, efficient temperature regulation, and programmable controls for adjusting foam density and composition. This machine offers flexibility and dependability in foam production.
Hennecke's STREAMLINE HP machines are engineered for polyurethane foam manufacturing, boasting high-pressure mixing and metering systems, sophisticated process controls, and the capability to produce various foam types, densities, and configurations.
Saip's UNIFLOW™ HP machines are renowned for their high-pressure polyurethane foam production features. They offer exact control over mixing ratios, temperature, and foam density, along with customizable options for foam formulation and product dimensions.
PMC's AP-3 Spray Foam Machine is tailored for polyurethane foam production using the spray foam application technique. It boasts accurate metering and mixing, adjustable pressure and temperature settings, and compatibility with diverse foam formulations, ensuring efficient and high-quality foam production.
This chapter explores the key properties of polyurethane foam. Due to its highly porous nature, polyurethane foam is easily compressible. The extent of this compressibility is influenced by its cell structure, which may be either open or closed cell. The main characteristics of polyurethane foam, such as its density, load-bearing capacity, and durability, are closely related to its compressibility.
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 testing methods for the load-bearing capacity are indentation force deflection (IFD) and compression load deflection (CLD).
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.
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.
Polyurethane foam's broad application stems from its flexibility and versatility. It is commonly utilized in products designed for comfort, protection, and relaxation. Additionally, as an insulation material, polyurethane foam contributes to environmental sustainability by reducing emissions. Its durability and resilience make it an ideal choice for a wide range of products and applications.
Car Interiors: In the auto industry, polyurethane foam is used for foam seating due to its resilience and rigidity. It is also used for panels, B pillars, headliners, suspension insulation, and bumpers. Due to customer complaints and technological necessity, polyurethane foam is used in cars as a noise and vibration suppressant for safety and comfort reasons.
Additionally, one of the main goals of manufacturers is to increase the miles per gallon of cars by making cars lighter. Polyurethane foam is integral to planning and design to meet automaker weight goals.
The few products listed above are only a small sampling of the many products made from polyurethane foam. It has found use in every aspect of society and has become a dependable material for residential and industrial use.
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