Foams are cellular structures created by gas bubbles during the polymerization of polyurethane. Flexible polyurethane (PU) foams have limited resistance to applied loads, are permeable to air, and are deformable. There are two main types of flexible foams, hot cure foam and cold cure foam, which differ in composition and processing temperature.

Hot polyurethane foam is widely used and represents the main composition of the actual foam, while cold polyurethane foam has several advantages in processing and properties, for example, faster molding times, better moisture resistance, and greater variety.

Cold foam processing is also notable due to its low energy consumption (mold temperature from 30 degrees to 65 degrees Celsius) compared to hot foam (mold temperature from 30 degrees to 250 degrees Celsius). Another advantage is the high variety of soft materials for processing molds at low temperatures. Cold foam is made based on diphenylmethane diisocyanate (MDI) while hot foam is based on toluene diisocyanate (TDI).

Methods of Producing Cold Polyurethane Foam

Producing molded foams is a common technology, and the types of molded foam formulations vary at least as much as the types of slabs. The concept of producing molded foam is simple: foam components are mixed and injected or poured into a pre-made mold that the foam fills during its formation. The foam has a specific residence time and is removed from the mold after an appropriate time for residence.

After being removed from the mold, the foam is set aside to cure completely. As is done in the industry, the technology of molded polyurethane foam has been highly optimized in terms of processing repeatability and properties. Unlike the production of block foam, where pieces must be cut from a block, molded foams undergo the final part as part of the foaming operation.

While this method reduces waste rates and eliminates the cutting operation of the finished part, the throughput of a molded foam factory is lower and requires the presence of molds, which can be very expensive. In typical molded foam operations, molds are placed on rotating or elliptical equipment that rotates in such a way that the residence time of the foam matches the time required for rotating and refilling the mold.

In addition to operational details, manufacturers choose molded foams due to the complex shapes they attempt to create and the ability to mold specific objects in shape such as functional parts, decorative skins, or reinforcements. This structure is designed to provide the correct appearance, performance, and shape as a composite part.

Reactive isocyanate components (polyols, chain extenders, surfactants, catalysts, water, etc.) are placed in a common tank or their separate tanks and then mixed with isocyanate at the head of a high-energy mixer. The mixed components are injected into the mold, the mold is closed, and the components are allowed to cure.

Due to the need to optimize the yield of molded foam production while producing durable, comfortable, and diverse foams, the formulation of molded foams is complex and can be partially described as an art. The speed of molding, pouring/injecting, and curing process are all controlled by the selected foam formulation. Similarly, the details of the production operation can have a significant impact on the design of the formulation. For example, if the molds are heated, a less reactive system may be used, and if the molds are not heated or are heated at lower temperatures, a different formulation is chosen.

In some cases, the mold is partially filled with a polyurethane formulation to create a specific characteristic and then subsequently filled with another formulation to produce the desired properties for the rest of the part. This flexibility of molded foam operations makes it very desirable for producing specialized parts, but it also contributes significantly to the complexity of formulation science.

As previously mentioned, examples of molded foam formulations in reflecting a wide range of available options are insufficient. However, the main categories of components used in molded foams are similar to those used for block cold foams. If the manufacturer actively heats their mold by passing through a hot oven, in this case, it is referred to as hot molded foam.

In the formulation of hot foam, polyols that may have a high molecular weight are used, and in them, a tri-functional polyol is surrounded by isocyanate. A manufacturer may use TDI, a mixture of TDI with polymeric MDI (pDMI). The choice depends on price, manufacturer preferences, and in some cases, geography, where a specific isocyanate is preferred based on contract.

Cold molded polyurethane foams, in contrast to the hot foams previously described, have a new industrial innovation and are produced by cooler molds and distinguished by lower oven temperatures. These foams typically exhibit more than 60 to 70 percent flexibility.

Similar to cold block foams, higher molecular weight polyols are used in cold molded foams. The lower temperature often results in cold molded foams having a greater number of untouched foam cells. Often, in the production protocol for mechanically crushing the foam after recovery from the mold to break the cells and increase airflow and breathability of the foam, protocols are in the agenda. While it can originally be formulated in such a way that additional crushing operations are not needed to ensure the completion of the gelling stage. Some molded foam manufacturers produce foam using only pMDI as the isocyanate.

In fact, the choice of the type of isocyanate is mainly a matter of preference, with various factors playing a role. However, there are several differences in the properties of the produced foams between foams made with TDI and pMDI. One difference is that pMDI foams generally cure faster due to the uniform reaction of the MDI isocyanate (the second reaction of isocyanate in TDI is usually significantly slower than the first). While speed is a preferred characteristic in molded foam operations, rapid viscosity increase can interfere with the complete filling of the mold.

However, when formulated effectively, pMDI foam can be made with a shorter cycle. This feature can also allow the manufacturer to lower the temperature of their mold and oven to further balance the chemical kinetics and process. The higher molecular volume of pMDI compared to TDI also means that the volume of the hard part produced for MDI foam is greater than that of TDI foam. This different molecular volume results in MDI-based foams having a higher potential load in water index and isocyanate index compared to TDI.

However, it has been noted that foams with a higher index may be destructive in some dynamic properties while showing greater hardness.

The complex aspect of polyurethane foam formulation previously mentioned is the conventional aspects of using molded foam. This is largely influenced by a rough understanding of comfort, which then relates to the firmness of the foam, flexibility, and breathability (related to airflow). Another feature of foam dictated by regional preference is the degree of change in resistance with increasing compression, which is sometimes referred to as the “support” or “comfort” factor and is quantitatively defined by the foam stiffness ratio at two different pressures.

Factors that foam designers may consider to match customer preferences include:

Foam density (affected by water level and percentage of hard component)

Cell structure and openness of the foam (partly dictated by surfactant and catalyst)

Type and index of isocyanate

Polyol performance and equivalent weight

Polyol copolymer and percentage of solids

Additives for design

Methods of producing cold foam

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