Resol-Type Foams

The P/F mol ratio is usually 1/1 to 1/3, but preferably 1/1.5 to 1/2.0. The basic catalysts to be used then should be hydroxides of barium, sodium, potassium and ammonium carbonates of sodium and potassium alkylamines, etc. The quantity of these catalysts to be used is 0.005 to 0.1 mol against 1 mol of P. The preferable quantity of these catalysts is 0.01 to 0.05 mol. After these materials, in aqueous solution, react at a temperature below lOO C they are neutralized with an appropriate acid and then dehydrated to provide a liquid-type resol. Representative properties of resol are shown in Table 57. 

The manufacturing conditions and the properties of resol are the most important factors in controlling the final properties f phenolic foams. For this reason all manufacturers have their own highly developed procedures regarding manufacturing conditions. 

Curing Catalysts. If resol is heated to a high temperature, it can be cured without catalyst but in order to make curing it at an ambient temperature (room temperature), an acidic catalyst must be used. Both organic acid and inorganic acid catalysts can be used. Inorganic acids used include hydrochloric acid, sulfuric acid, and phosphoric acid, whUe benzene sulfonic acid, toluene sulfonic acid, and phenol sulfonic acid, are organic acids used. 

In some cases, inorganic acid ( e.g., phosphoric acid) and organic acid (e.g., phenolsulfonic acid) are mixed with each other and used as a catalyst. Acid remaining in the final foam causes acidity of the foam. To minimize corrosion of the metal in contact with the foam the use of organic acid catal) t is preferable. 

Surfactants. Various types of surfactant can be used for forming phenolic foams. There are two types of surfactant, one is the silicone-oU type and the other is the non-sUicone-oil type. The latter is used most commonly. 

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Resol-Type Foam Chemistry. Resol is obtained as a result of the reaction between phenols (P) and formaldehyde (F) in the presence of basic catalyst. Generally, the reaction is made at a temperature below... 

In manufacturing phenolic foams, a blowing agent such as R-113 (trichlorotrifluoroethane) is evaporated by using an exothermic reaction (Equation 7), and the blowing agent is included in the polymer. The reaction involved in forming the benzylic ether-type foam is shown below. Similarly, this foam will finally form a network polymer as do resol-type foams. 

Materials for Resol-Type Foams. Resol (Resin). As described above, resol is formed as a result of the reaction of phenol (P) with aldehyde (F) in the presence of a basic catalyst. A representative preparation process is shown in Figure 56. Representative manufacturing conditions and the properties of resol are shown in Table 57 (5). 

It is easy to use these reinforcing materials for novolic-type foams, but they are difficult to use for resol-type foams because the viscosity of the latter foams is high and it is hard to distribute the reinforcing fibers evenly in the foam. However, if even distribution becomes technically possible, a bright futine is expected for these reinforcing materials in such applications. 

Foaming Process of Resol-Type Foam. The foaming processes for phenolic foam of both the resol type and benzylic-ether type are the same as those for rigid polyurethane foams. The block-foaming process (slab foaming), pouring process, continuous-laminate process, and spray process are used. See Figure 57. 

Table 60 Typical Formulation of Resol-Type Foam (5)...

General Properties. Resol-type foams are classified roughly into four types, as follows ... 

In the past, types C and D were common in resol-type foam. Type D is low in density and is an open-cell-type foam used widely. Type C is high in density and mechanical strength. Type B was introduced by the American manufacturers, Koppers, Monsanto and others. This type has a high closed-cell content and has attracted special interest recently. Since Type B is a high closed-cell it is excellent for thermal insulation. This type has been used in construction in the U.S. and Western European countries. 

The general properties of resol-type foams produced by the blockfoaming process are shown in Table 66, whUe those produced by the spraying process are shown in Table 67 (10, 12). 

Table 66 General Properties of Resol-Type Foam Prepared by the Block Foaming Process (12)...

Thermal Resistance. Examples of thermal resistance over a long term are shown in Figure 62. This figure shows the compressive strength of resol-type foams maintained in an air oven for aging. 

Examples of thermal gravimetric analysis (TGA) for phenolic foam are shown in Figure 63. The phenolic foam in this figure is the resol-type foam. In the case of novolac-type foams the values are almost the same (3). 

Drying. Resol-type foams contain 10 to 15% moisture when produced. The source of this moisture is water contained in both resol and catalyst and is the water yielded during the condensation reaction. This moisture is released within 5 to 10 days by leaving the foam in the room until it reaches its equilibrium state. The drying results for different laminates and slab board are shown in Figure 65 (5). These results indicate that it is necessary to leave newly-produced foams in the warehouse for annealing for about one week before shipment. 

Ingredients (%) Resol Type Foam Novolac-Type Foam... 

The cost of raw materials for use in phenolic foams is low, but since this foam has the weak points listed above, it cannot exceed polyurethane foam in cost performance. Resol-type foams and benzylic ether-type foams use chlorofluorocarbon (CFC) as a foamimg agent. It is necessary to develop a new foaming agent which does not destroy the ozone layer in the atmosphere. When these problems are solved phenolic foam applications will make further rapid progress. 

A comparison between theoretical predictions and experimental results is illustrated for the foaming of a phenolic resin of the resol type (Vazquez and Williams, 1986). 

Classification. Phenolic foam is divided broadly into two categories. One is the novolac type and the other is the resol type (see Figure 55). In the novolac type, phenol and formaldehyde react in the presence of an acid catalyst and link the linear condensation product by a methylene bond. The basic curable crosslinking agent, such as hexamethylene tetramine and blowing agent are added to the linear condensation product and the resultant product is molded at elevated temperatures and high pressure. 

In the resol type, phenol and formaldehyde react in the presence of a basic catalyst and provide a liquid type resol to which an acid catalyst and blowing agent are added and the foam is then molded. In forming the resol a method is available for obtaining a benzylic ether-type liquid oligomer by reacting phenol and formaldehyde in nonaqueous phase. 

Foaming Mechanism. The process for producing resol-type phenolic foams is simUar to that of polyurethane foam. The foaming process in phenolic foam may be divided into five steps. This process does not progress by step-by-step, but the several phenomena progress spontaneously. 

Figure 57. Foaming processes for resol-type and benzylic ether-type foam.

Spraying Process. This process has been theoretically possible, but practical application has not been realized. In Japan, however, Bridgestone Ltd. has developed (10) a method which makes spraying-inplace possible. By this method three components of resol-type composition can be foamed at a temperature 10°C and higher (preferably at 20°C) with a spray-gun. DetaUed technical data are unavailable, but it is believed that the material as well as foaming machine should have been improved very much. 

Foaming Characteristics. When producing resol-type and benzylic ether-type foams, the index expresses apparent reactivity. Following are the foaming times ... 

Examples of foaming characteristics of resol-type phenolic foams (free-rise) are shown in Table 64 in comparison with rigid urethane foam (3). Rise-profile curves, foaming-pressure curves and temperature curves for phenolic and polyurethane foams are shown in Figure 61. 

Thermal Coefficients. The thermal coefficients of phenolic foams are almost the same as those of resol- and novolac-type foams. Examples are as follows specific heat, 0.38 to 0.42 cal/g°Q flash point, 520 to 540 Q ignition point, 570° to 580°C softening point, do not soften. 

For making sandwich panels both the resol-type and benzylic ether-type foams can be used. These sandwich panels are used as building materials (partition panels, ceiling panels, wall panels, panels for clean rooms, door panels, panels for prefabricated refrigeration storehouses, etc.). 

Floral Foam. In the U.S. and Western European countries the most popular use of phenol foam for floral applications is a water-soaked foam using a resol-type composition. It is important to maximize the open-cell content in order to inq)rove its water penetration. Floral foam is produced as laige block or slab by a batch process. The foam is easily cut and packaged with a minimum of loss due to waste. Floral foam is commonly used for living flowers, but is not used for dried and artificial flowers. 

The early technique for PF manufacturing was based on the expansion of novolac-hexamethylenetetramine (FiMTA) mixtures. Now, the manufacturing is based on acid catalysed process of a resole type PF with added foaming agent and surfactant [43]. 

Phenol-formaldehyde (PF) resins were synthesized to manufacture non-flammable insulating foam. When alkali catalyst, for example, barium hydroxide (Ba(OH)2), was present, lesol resins are produced[l]. In the analj s of molecular species of resol, capillary GC-MS had been used to separate hemiformal-type compoimds(acetylated hydroxybenzylhaniformals)... 

Several other types of multicomponent ol otiKric foams have been proposed Foams based on mixtures of resol phenolic oligomers with novolac oligomers with urea oligomers 2—44) polycarboxamides with polyacrylonitrile with polystyrene ... 

Madders. Phenolic polymers are known to have high rigidity, and this property extends to phenolic foams, which are highly friable. In order to reduce friabUity and permit some flexibUity and toughness, various kinds of modifiers are sometimes used. Reactive modifiers are used in the course of resol-resin preparation, and they become integral parts of the polymer structure. Examples include PVA (polyvinyl alcohol), PVA-PVC (polyvinyl alcohol-polyvinyl chloride-copolymer), resorcinol, o-cresol, furfuryl alcohol, and other various types of polyols. 

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