Polyurethane Foam (Foam Fabricating)

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Introduction

This article will provide a detailed insight into polyurethane foams. You will learn:

• What is Polyurethane Foam
• Raw Materials in Polyurethane Foam
• Production of Polyurethane Foam
• Properties of Polyurethane Foam
• And much more

Chapter 1: What is an overview of polyurethane foam?

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 and is projected to reach $54.3 billion by .

History and Development of Polyurethane Foam

Polyurethanes were first discovered in 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 . 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 , 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.

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.

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.

Chapter 2: What are the raw materials used in polyurethane foam production?

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 foams 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 foams 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.

  
  
  • 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). Of 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 produces 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 to manufacture 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 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 foams 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 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.

  
  
  • Chemical Blowing Agents: Chemical blowing agents generate gas by adding compounds that react with the isocyanate groups to form carbon dioxide. 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, preventing cell shrinkage after curing, 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 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.

  
  
• 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.

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Chapter 3: How is polyurethane foam produced?

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.

Polymer System Preparation

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.

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Polyurethane formulation can be achieved using various methods, including the single-shot (one-step) process, quasi-prepolymer, and full prepolymer systems.

• Single-shot system prepare a premix of polyols, blowing agents, surfactants, catalysts, and additives combined with isocyanates. The polymerization and gas generation processes occur simultaneously.
• Quasi-prepolymer systems have the polyol 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.
• Full prepolymer system involve mixing the polyols and isocyanates to create the prepolymer, which is mixed with the blowing agent, catalysts, and other additives. This system is less often used compared to the single-shot and quasi-prepolymer systems.

Polyurethane Foam Production Processes

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.

  
  

Chapter 4: Who are the leading manufacturers of polyurethane foam producing machines?

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 - Model: LPU Series

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 - Model: EPU Pro

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 - Model: STREAMLINE HP

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 - Model: UNIFLOW HP

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.

Polyurethane Machinery Corporation (PMC) - Model: AP-3 Spray Foam Machine

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.

Chapter 5: What are the properties of polyurethane foam?

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.

• 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 testing methods 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. Depending on the test method used, the degree of compression can be 50%, 75%, or 90%. The foam is compressed for a certain number of hours before 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 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 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 foams 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 and kept for a specified duration. Afterward, final measurements are performed.

Chapter 6: What are the different products made from polyurethane foam?

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.

• Mattresses: Since polyurethane foam comes in blocks, it can be easily trimmed and cut to produce various sizes of mattresses. The density of the foam determines the type of mattress it is used to manufacture, with softer polyurethane foam used for baby mattresses while tougher foams are used for long-lasting adult beds.
• Thermal Insulation: Rigid polyurethane foam is inserted into the walls of buildings and homes during construction to reduce heating and cooling costs as well as carbon emissions. Additionally, it lasts for over a half-century without needing replacement or repair.
• Elastomers: Elastomers are one of the more predominant uses for polyurethane foam due to their resilience, elasticity, flexibility, and capability to withstand heavy weight loads. A common use for polyurethane is as gasket material due to its ability to be stressed or torqued and return to its original shape.
• Sponges: Polyurethane foam is used to produce sponges due to its flexibility and tensile strength, which makes it less likely for sponges to tear or flake. They can be used to scrub any surface regardless of the surfaces texture and do not deteriorate. The manufacturing process for polyurethane sponges does not produce biocides, substances that destroy living things that would cause the sponges to interfere with other 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.

• Packaging: The versatility of polyurethane foam makes it an ideal material for protecting sensitive and delicate equipment during shipping. Cut by CNC machines into the shape and configuration of products, it provides cushioning, protection, and impact resistance for shipped goods.

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.

Conclusion

• 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 have 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 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 polyurethane foam properties revolve around its compressibility, such as its density, load-bearing properties, and durability. Other important properties are its tensile strength, tear strength, elongation at break, and air permeability.

Leading Manufacturers and Suppliers

VALCO GROUP to discover the process of Polyurethane manufacturing

Polyurethanes, also known as polycarbamates, are some of the most versatile polymers. First developed in late s, they are used in building insulation, surface coatings, adhesives, and solid plastics. Polyurethanes can be produced in four different forms including elastomers, coatings, flexible foams, and cross-linked foams. Each one has specific uses based on theirs properties.

Polyurethanes are produced by reacting an isocyanate and apolyol of various types (most common isocyanates: TDI (toluenediisocyanate) and MDI (methylenebisdiphenyldiisocyanate); mostcommon polyols: PTMEG (polyetetramethylene ether glycol), PPG (polypropylene ether glycol)).

During the process of Polyurethane manufacturing, the raw materials are pumped from their own storage tank to a common mixing vessel kept under controlled pressure and temperature. Here, additives are incorporated in accordance with a specific recipe. The temperature control inside the reacting vessels is usually made by a recirculating system such as the one illustrated in figure 1.

Figure 1. Schematic of a high-pressure vessel with re-circulation system (a: feed tanks, b: metering pumps, c: mixhead, d: safety valve, e: recirculation throttle, f: low-pressure recirculation valve, g: filter, h: hydraulics for mixhead, M: drive motor)

Polyurethane foam is the most widely used flexible foam plastic. It is produced by adding a blowing agent such as carbon dioxide and methylene chloride. Hard polyurethane plastic can be formed if the polymerization reaction is carried out without blowing agent. In some applications, continuous systems are a good alternative to the batch polyurethane production process, as it can reduce the risk of failed batches.

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