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Molded Foams

Preparation of molded foams is a widely practiced technology, and the varieties of molded foam formulations are at least as varied as those for slabstock. The concept of molded foam production is simple: the components of the foam are mixed and injected or poured into a premade mold that the foam fills as it forms. The foam has a set residence time and is removed after a designed-for residence time in the mold. After removal from the mold, the foam is set aside to finish curing. As practiced industrially, molded foam technology is highly optimized from the point of view of reproducibility of processing and properties. In contrast to slab foam production, in which parts must be cut from a block, molded foams produce the final part as part of the foaming operation. While this aspect reduces scrap rates and removes the cutting operation of making a finished part, the mass throughput of a molded foam plant is lower and requires the presence of molds, which can be quite expensive. In typical molded foam operations, molds are placed on carousel or oval racetrack fixtures that rotate such that the timing of foam residence is matched to the time it takes for the carousel to make a cycle and the mold be refilled.

Besides the operational details, manufacturers choose to make molded foams because of the intricate shapes they are trying to fabricate and the ability to overmold specific objects within the shape such as functional parts, decorative skins, or reinforcements. The structure is engineered to provide the correct appearance, function, and shape as a composite part.

Figure 6.3 shows the components of a molded foam production in terms of handling of raw materials and then mold filling. The isocyanate-reactive parts (polyols, chain extenders, surfactants, catalysts, water, etc.) are put into their joint tank or individual tanks and then mixed with the isocyanate in a high-energy mixing head. The mixed components are injected into the mold, the mold closed, and the components allowed to cure.

Because of the need to optimize efficiency of molded foam manufacturing and at the same time produce foams of exceptional durability, comfort, and variety, the formulation of molded foams is complex and could be described to some extent as an art. The speed of the molding, pouring/injecting, and curing process is all controlled by the chosen foam formulation. Similarly, details of the manufacturing operation can have a significant influence on formulation design. For instance, if the foam molds are heated, then a less reactive system can be employed, and if the molds are unheated or heated to a lower temperature, then another formulation will be chosen. In some cases, the mold is partially filled with a polyurethane formulation to produce a specific characteristic and then filled subsequently with another formulation to produce desired characteristics for the rest of the part. This flexibility of molded foam operations makes it very desirable for specialty part production but also contributes greatly to the complexity of the formulation science.

As previously mentioned, examples of molded foam formulations are bound to be inadequate in reflecting the range of available options. However, the basic categories of components used in molded foams are similar to those used for HR slabstock foams. If the manufacturer is actively heating his/her mold, by passing through a

Molded foam operations, (a) Raw material handling and (b) foam fabrication. Reprinted with permission from Ref. [3]. © John Wiley & Sons, Inc.

FIGURE 6.3 Molded foam operations, (a) Raw material handling and (b) foam fabrication. Reprinted with permission from Ref. [3]. © John Wiley & Sons, Inc.

hot oven, for instance, this will be termed a "hot cure" molded foam. In this case, the formulation will employ polyols that may be 3000 molecular weight triols (1000 g/eq OH) that will be a triblock of PO capped by EO tips. The polyol will be approximately 20% by weight EO (more by molarity) to assure that the great majority of end groups will be faster-reacting primary hydroxyls. A manufacturer may use TDI, blends of TDI with polymeric MDI (pDMI), or pMDI. The choice is dependent on price, preference of the manufacturer, and in some cases geography where a particular isocyanate may be preferred based on convention. Table 6.5 offers examples of hot molded foam formulations to make foams of differing hardness. The formulation shows the role of copolymer polyol in hardening foam even in the case of lower overall hard segment. Of course, the reduced overall mass of hard segment is crowded into a smaller volume as well since the hard segment is excluded from the space occupied by the SAN particles.

HR molded foams are also sometimes referred to as "cold cure" HR foam in distinction to the "hot cure" foams described earlier. They are a more recent industrial innovation relative to hot cure foams and are differentiated by cooler mold and cure oven temperatures. These foams usually offer resiliency greater than 60%, and as high as 70% has been reported [8].

As with HR slabstock foams, HR molded foams utilize higher-molecular-weight polyols, typically 4000-6000 triols with relatively high amounts of EO capping (up to 85% primary hydroxyl functionality at 25% EO incorporation) to increase reactivity [9]. The higher reactivity of EO-capped polyols allows the formulator to use less gelation catalyst but can dictate the intensified use of blowing catalyst to make sure that the urea hard segment forms in kinetic equilibrium with the urethane reaction that builds molecular weight. Lower temperatures and retarded blow conditions often result in HR molded foams having a larger than desired population of foam cells with intact windows. It is often protocol in manufacturing to mechanically crush the foam after retrieval from the mold in order to break open cells and so increase airflow and foam breathability. While it is in principle possible to formulate so that the crushing operation is superfluous, insuring completion of the gelation

TABLE 6.5 Representative formulations for making "hot mold foam" of two hardnesses

Component

Soft foam

Harder foam

3000 molecular weight tnol 25% EO

80

1300 equivalent weight 26% solid SAN copolymer polyol

20

100

High-efficiency silicone surfactant

1

Medium-efficiency silicone surfactant

0.8

Bis(dimethylaminoethyl)ether (blowing catalyst)

0.1

0.05

Tnethylenediamine (strong gel catalyst)

0.1

N,-dimethylaminoethyl morphohne (less strong gel catalyst)

0.4

Tin dioctanoate

0.05

0.1

water

2.4

2.1

TDI (100 index)

30.9

27

reaction so the foam doesn't collapse supersedes the nuisance of the mechanical manipulation of the foam to maximize openness. These are the kinds of considerations of practical foam formulation. An example of a TDI HR molded foam formulation is provided in Table 6.6.

A common variation of the formulation of Table 6.6 is to substitute a blend of TDI and pMDI to provide foam properties intermediate between the foams produced by either isocyanate on its own. An example of such a formulation is provided in Table 6.7.

Some molded foam producers manufacture only with pMDI as the isocyanate. The choice is largely a matter of preference including the better health and safety profile of pMDI due to its lower volatility; however, there are several differences in foam properties between foams made with TDI and pMDI. One difference is that pMDI foams generally cure faster due to the uniform reactivity of MDI isocyanate functionality (the second isocyanate reaction on TDI is usually significantly slower than the first). While speed is a preferred attribute in molded foam operations, rapid viscosity increase can interfere with complete mold filling. However, when formulated effectively, pMDI foams can be made with shorter cycle times. This attribute can also allow the manufacturer to lower their mold and oven temperatures to further balance chemical and process kinetics. The larger molecular volume of pMDI relative to TDI also means that the volume of hard segment produced for an MDI foam is higher than that for a TDI foam. This different molecular volume results in MDI-

TABLE 6.6 Representative formulation for a high-resilience "cold mold foam"

Component

Parts by weight

40% SAN EO-capped PO tool of ~1800 g/eq OH polyol

70

40% solids EO-capped PO triol -2350 g/eq OH polyol

30

Diethanolamine (cross-linker to speed up gelation)

1.25

Triethylenediamine (gel catalyst)

0.25

Bis(dimethylaminoethyl)ether (blow catalyst)

0.1

Silicone surfactant for HR foam

1.0

Water

4.0

TDI (100 index)

43

TABLE 6.7 Representative formulation for a high-resilience "cold mold foam" using blended isocyanates

Component

Parts by weight

40% SAN EO-capped PO triol of ~ 1800 g/eq OH polyol

70

40% solids EO-capped PO triol -2350 g/eq OH polyol

30

Silicone surfactant for HR foam

0.7

Triethylenediamine

0.3

Bis(dimethylaminoethyl)ether (blow catalyst)

0.1

Water

3.5

80% TDI 20% pMDI with 136 g/eq isocyanate (100 index)

42.7

TABLE 6.8 Representative formulation for a high-resilience "cold mold foam" Using PMDI

Representative formulation for a high-resilience

based foams having potentially higher load bearing at a given water and isocyanate index than TDI. It has been noted however that higher-index molded foams may exhibit degradation in some dynamic properties while exhibiting higher hardness. A representative formulation is provided in Table 6.8.

A complicating aspect of foam formulation alluded to earlier is the conventional aspects of molded foam usage. This is greatly influenced by regional perceptions of comfort that is then related to the foam stiffness, resilience, and breathability (related to airflow). Another foam feature dictated by regional preference is the rate of change of resistance with increasing compression (sometimes called the "support" or "comfort" factor and defined quantitatively by the ratio of foam stiffness at two different compressions) [10]. Foam formulation factors that the foam designer may vary to meet these customer preferences are:

• Foam density (influenced by water level and percentage hard segment)

• Cell structure and foam openness (dictated in part by surfactancy and catalysis)

• Isocyanate type and index

• Polyol functionality and equivalent weight

• Copolymer polyol and percentage solids

• Additives, inserts, and seat design

 
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