Closed Cell Foam
Closed cell foam is a type of foam where the “cells” are tightly pressed together and enclosed, contrasting with the open, traditional polyurethane foam variation or interconnected cells of the open cell...
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This article will provide a detailed insight into polyurethane foams. You will learn:
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 has good thermal and acoustic insulation, high force absorption, low density, and flexibility. Polyurethane foams are sometimes referred to as foam rubber, a broader type of material that include foams made from latex, neoprene, and silicone.
Polyurethane foams are used in the manufacturing 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.
The discovery of polyurethanes is attributed to Otto Bayer, dating back to the year 1937 together, and 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 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.
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 have become 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 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.
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 has a fine cellular structure with a density of 0.25 grams to 0.65 grams per cubic centimeter. It contains billions of gas bubbles less than 50 microns (µ) in size. The bubbles form in uniform designs to give the foam exceptional physical properties. It has excellent compression set resistance that makes it highly durable and long-lasting with low outgassing, resistance to chemicals, and is non-corrosive.
A type of microcellular foam is Poron®, an open-cell foam used to make gaskets, protective gear, and footwear. It is widely used for its cushioning, padding, compression, and sealing properties. It comes in thicknesses of 0.012 inches and 0.5 inches (0.3 and 12.7mm) and is valued for its compression set resistance to resolve mproper sealing and cushioning issues. Its many properties include impact and vibration absorption, smooth finish, low outgassing, and ease of manufacturing.
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 in stabilizing and maintaining the desired rate of reaction of the polymer system. Additives 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.
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 production process of polyurethane foam can be divided into 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 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. Different types of polymer system preparation differ in 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 can also 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.
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 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 by making long-chained polyurethane.
The formulation can be done through different systems such as single shot (one step), quasi-prepolymer, and full prepolymer systems.
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 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.
There are many machines available to produce polyurethane foam, and these machines are important in today's society because polyurethane foam is a versatile material widely used in various industries such as construction, automotive, furniture, and packaging due to its excellent insulation properties, cushioning abilities, and structural support. These machines enable precise control over foam formulation, density, and shape, allowing for efficient and customizable production of polyurethane foam products to meet diverse industrial and consumer needs. Below, we discuss some notable brands of machines used for producing polyurethane foam in the United States and Canada.
Linden Industries offers the LPU™ Series of machines, known for their advanced metering technology, precise control over foam density and formulation, and customizable production options, ensuring consistent and high-quality polyurethane foam production.
Cannon USA's EPU Pro is a polyurethane foam production machine that features advanced mixing and metering capabilities, efficient temperature control, and programmable controls for foam density and formulation, providing versatility and reliability in foam manufacturing.
Hennecke's STREAMLINE HP machines are designed for polyurethane foam production, featuring high-pressure mixing and metering systems, advanced process control, and the ability to produce a wide range of foam types, densities, and shapes.
Saip's UNIFLOW™ HP machines are known for their high-pressure polyurethane foam production capabilities, including precise control of mixing ratios, temperature, and foam density, as well as customizable options for foam formulation and product dimensions.
PMC offers the AP-3 Spray Foam Machine, specifically designed for polyurethane foam production through the spray foam application method. It features precise metering and mixing, adjustable pressure and temperature controls, and compatibility with various foam formulations, enabling efficient and high-quality polyurethane foam production.
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 polyurethane foam properties revolve around its compressibility, such as its density, load-bearing properties, and durability.
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.
The wide use of polyurethane foam is due to its flexibility and versatility. It is often used for products for human comfort, protection, and relaxation. In addition, polyurethane foam is a "green" product that helps reduce emissions as an insulation material. Its durability and resilience are why it has been chosen to fit the needs of so many 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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