CA1198550A - Flame-retardant benzylic-ether phenolic modified foam and method of preparing same - Google Patents

Abstract

ABSTRACT
A method of preparing a flame-retardant benzylic-ether phenolic modified foam product and the resulting foam product produced thereby, which method comprises reacting and mixing benzylic-ether phenol-formaldehyde resin, having a hydroxyl number of from about 400 to 600, with from about 50 to 150 parts by weight of a boric-acid catalyst to each 100 parts of the resin, op-tionally in the presence of melamine, to form a resin prepolymer, and, there-after, reacting the prepolymer so formed, in the presence of a blowing agent and an organometal catalyst, with a methylene diisocyanate in an exothermic reaction, to provide a nonfriable, substantially closed-cell, flame-retardant benzylic-ether phenolic modified foam product characterized by substantially acid methylene linkages in the resulting thermosetting foam product.

Description

Applicant's copending Canadian applica~ion Serial No. 415~6~6, filed November 16, 1982, relates to the preparation of flame-retardant, rigid, thermosetting foam products, by reacting, in the presence of a blowing agent~
a catalyst, a furfural-alcohol polymer, a phenol-formalde]lyde or benzylic-ether formaIdehyde resin, a melamine resin and a methylene diphenyl isocyanate.
The reaction provides for a rigid foam characterized by low friability and good heat and flame-retardant properties.
Typically, thermosetting foam products are prepared by the use of a phenol-formaldehyde resin, a blowing agent and a strong acid catalyst. Such thermosetting foam products Qre often characterized by having a fairly brittle foam structure, with low compression strength and exhibiting high friability and brittleness properties. Such phenolic foams, usually have an open-cell foam structure and very little, if any, strength at low densities; for example, below about 2.5 pounds per cubic foot. These phenolic foams, are difficult to burn, melt or smoke, in that they are characterized substantially by methylene linkages. However, such phenolic foams are not generally commercially accept-able, because of low compression strength, poor insula-tion value and the corrosive characteristics of the foam.
Modified phenolic foams have been prepared employing polyisocyanates, to produce a phenolic urethane foam product. ~or example, United States Patent 3,948,324, issued April 6, 1976, describes a modified urethane benzylic-ether phenolic foam product. The cellular polymeric reaction product is pro-duced by the reaction of a benzylic-ether phenolic foam in the presence of a liquid methylene diisocyanate-type isocyanate, a blowing agent and amine catalysts, to form a ben~ylic-ether phenolic urethane foam product. The reac-tion mixture foams and cures spontaneously in an exotherm into a tough, non-friable foam. The resulting foam product is closed-cell and has good insula-~ion 55~

properties; however, the foam product, because it ~urns arld smokes in a manner very similar to a ure-thane foam product made from aromatic polyols, does not have commercially acceptable flame-retardallt properties, probably due in part to the presence o-f urethane linkages in the resulting foam mass.
Another difficulty associated with benzylic-ether phenolic urethane foams is that the resul-ting material is quite reactive, and, therefo-re, it is difficult to control the nature and extent of the reaction and is difficult to produce the thermosetting foam in a consistently high-quality manner in standard production foaming equipmen-t, due to the very high reactivity of the ingredients.
It would be desirable ~o prepare a phenolic foam material which pro-vides for a substantially closed-cell ultra low density foam s-tructure of low friability and which exhibits nonburning and nonsmoking characteris-tics.
~his inven-tion relates to an improved, modified, phenolic thermo-setting foam product and to the method of preparing and using the foam product.
In particular, the invention concerns a thermosetting foam product prepared by the reaction of a benzylic-ether phenol-formaldehyde resin and reacting the pre-polymer in the presence of a weak acid catalyst, to produce a prepolymer with a diisocyanate to provide a modified benzylic-e-ther pehnolic foam product.
More particularly, the invention relates to a method of preparing a nonEriable, closed-cell, benzylic-e-ther phenol-formaldehyde modified thermosetting foam product characterized by a low flame spread and smoke properties, which method comprises:
a~ reacting, under substantially anhydrous conditions, 100 parts of a benzylic-ether phenol-formaldehyde resill, having a hydroxyl number of greater than about 200, wi-th from about 50 to 150 parts by weight per 100 parts of the benzylic-ether phenol-formaldehyde resin of a weak acid catalys-t at a pH of from about ~.0 -to 6.5, -to form a benzylic-ether phenol resin prepolymer; and b) reacting the prepolymer, in the presence of a blowin$ amount o-f a blowing agent to form a Eoamable mass, and acatalytic amount of an organo-metallic catalyst, which i.s not affected by the pH o:E the prepolymer, with from about 50 to 200 parts by weight per 100 parts of the benzylic-e-ther phenol resin prepolymer of a methylene diphenyl polyisocyanate in an exothermic reac-tion, to provide for the further heat and acid condensation of -the phenol resin prepolymer and the reaction of the isocyana-te wi-th the prepolymer, to produce a low-friability, substantially closed-cell, phenolic modified thermosetting foam product composed substantially of acid methylene linkages.
It has been discovered that a modified benzylic-ether phenolic aldehyde foam product may be prepared containillg substantially methylene linkages and few, if any, urethane linkages in a controlled reaction, by the employment of a weak acid catalyst compound, to prepare a prepolymer, and the subsequent reaction of the weak acid-containing benzylic-ether phenol resin prepolymer with a methylene diphenyl isocyana-te compound in the presence of a blowing agent and a catalyst, to produce a phenol modified product. The thermosetting foam product of the invention is not friable or has low

2() friability characteristics, ;~
- 2a -is composed of closed cells or subs-tantially closed cells, and has comparable strength to unmodified urethane foam, without the burning and smoking character-istics and properties of such urethane foam. The resulting foam product, for example, is substantially nonburning and nonsmoking foam with a low density of generally less than 3.0 pounds per cubic foot. The foam product has a flame spread under AST~I E-84 of lO or less and a smoke value of ~0 or less.
The benzylic-ether phenol-formaldehyde modified foam product of *he invention is prepared by reacting 100 parts of a substantially anhydrous benzylic-ether formaldehyde resin, typically having a hydroxyl number of greater than 200; for example, ~00 to 600, with about 50 to 150 parts by weight per 100 parts of the benzylic-ether resin of a ~eak acid catalyst, to form a benzylic-ether resin prepolymer. Typically, the reaction is carried out without the use of external heating at a tempera-ture of less than 100F; that is, at room temperature 60-80F. The resulting prepolymer then is reacted in the presence of a blowing agent and in the presence of a catalyst, to eE:Eect the reaction with the isocyanate, typically an organometal catalyst not affected by the acid pH of the prepolymer, with Erom about 50 to 200 parts by weight of a methy-lene diphenyl isocyanate (MDI) per 100 parts of the benzylic-ether phenolic resin. The prepolymer MDI reaction produces an exothermic reaction, with the exothermic reaction providing and generating heat to complete further the heat and acid condensation of the benzylic-ether phenol-formaldehyde resin, to pro-duce a thermosetting :Eoam product which comprises substantially methylene linkages and, therefore, having desirable flame spread and smoke properties, but having desirable friability and compression strength properties of a typical phenolic foam.
In one preferred embodiment, it has been found that the reaction to form the weak acid-catalyst benzylic-ether formaldeh~de prepolymer should be ;t~

carried out with mixing of the components of the reaction for at least 6 hours and preferably 8 to 12 hours, e.g., 12 to 24 hours, to provide :Eor a substantial-ly condensed prepolymer with methy]ene linkages.
Further, the formaldehyde odor of the prepolymer is often quite powerful so that prior to the reaction of the prepolymer with the polyisocyanate to form the modified foam of the invention a sufficient amount oE melamine or other amine reactive with formaldehyde should be added to and mixed with the phenolic resin or the prepolymer reaction components to react with all or substantially all of the free formaldehyde of the prepolymer reaction mixture.
One mole of melamine is added to about every six moles of free formaldehyde to react the free formaldehyde stoichiometrically and provide a substantially formaldehyde-odor-free foam product. Commercially available benzylic-ether phenol-formaldehyde has about two percent free formaldehyde so that about 2.5 parts of melamine is added to the resin prior to or with the addition of the weak acid catalyst. The amount of melamine to be used often ranges from about 1 to 5 parts per 100 parts of the resin.
In operation, the melamine is added and mixed with the free formal-dehyde containing benzylic-ether phenol-formaldehyde resin to react the free formaldehyde, then the weak acid catalyst is added with the blowing agent and other fillers and additives 9 and the mixture reacted with continuous mixing and blending for 6 to 24 hours to provide a condensed prepolymer. The prepolymer is then reacted with the ~1DI typically in a foam mixing head wherein the ~IDI
containing the tin catalysts in one stream is intimately admixed with the pre-polymer in another separate stream to provide an exothermic foam mixture dis-charged from the mixing head on a production line.
The method of the invention overcomes the difficulties associated with the preparation of the cellular polymeric masses, as disclosed in the \~

abovementioned United States Patent NQ. 3,9~8,824, in that the reaction is a controlled reaction) by the employment of a weak acid catalyst and the avoidance of the typically strong acid catalyst employed phenolic foams, such as the sulfonic acid catalyst, such as methane sulfonic acid and paratoluene sulfonic acid and similar strong acids which are far too reactive in curing the benzylic-ether phenol-formaldehyde resin. Purther, the benzylic-e~her phenol-formaldehyde resin comprises a novola~c resin which is substantially anhydrous; that is, having less than about 1% by weight water, in order to avoid the intentional creation of urethane linkages, which leads to unacceptable flame and smoke properties in the foam.
The benzylic-ether phenolic resins employed in the present invention are characterized by containing one or more units having the formula:

Oll R' R' ~ CH - 0 -lH
A ~ B

_, wherein A, B and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals or halogen and R' is individually a hydrogen or a hydrocarbon radical of 1 to 8 carbon atoms. The resins have average degrees of polymerization, as measured by the number of repeating aromatic rings, of 3 to 100, and preferably about to 10. Although higher molecular weight resins are operable in the curing reactions above described, such resins are difficult to handle from the stand-point of viscosity.
The described benzylic-ether resins are condensation polymers of a phenol having the general formula:
OH

A ~ B

S5 L) wherein A, B and C are hydrogen, hydrocarbon radicals, oxyhydrocarbon radicals or halogen, with an aldehyde having the general formula R'CHO, wherein R' is a hydrogen or a hydrocarbon radi.cal of 1 to 8 carbon atoms, prepared in the liquid phase in the substantial absence of water a~ temperatures below about 130C in the presence of catalytic concentrations of a metal ion dissolved in the reac-tion medium. The molar ratio of aldehyde to phenol generally can be varied from

3:1 to 1:1, although some resin is also formed outside these ratios. ~he pre-paration and characterization of these resins is disclosed in greater detail in United States Patent 3,485,797. In the preferred form, these resins have the general formula:
nH n~l m X ~ CH2-0-CH2 ~ CH2 [~

wherein R is a hydrogen or a phenolic substituent meta to the phenolic hydroxyl group, the sum of m and n is at least 2 and the ratio of m to n is at least l, and X is an end group selected from the group consisting of hydrogen and methyl-ol, the molar ratio of said methylol-to-hydrogen end groups being at least 1.
The most preferred benzylic-ether resins employed in the resin com-positions of the present invention are -those in which R is hydrogen. The phenols employed in the formation of the benzylic-ether resins are generally all phenols which have heretofore been employed in the formation of phenolic resins general-ly, and which are not substituted at either of the carbon atoms ortho to the hydroxyl group. Any one, all or none of the remaini.ng carbon atoms of the phenol ring can be substituted. The nature of the substituent can vary widely, and it isonly necessary that the substituent not interfere in the polymerization of the aldehyde with the phenol at the ortho position. Substituted phenols 3 ~ r~ r ~
,, ~;9 ~

employed in the formation o:E the phenolic resins include: alkyl-substituted phenols; aryl-substituted phenols; cycloalkyl-substituted phenols; alkenyl-substituted phenols; alkoxy-substituted phenols; aryloxy-substituted phenols;
and halogen-substituted phenols, the foregoing substituents containing from 1 to 26 and preferably from 1 to 6 carbon atoms. Specific examples of suitable phenols, aside from tlle preferred unsubstituted phenol, include: m-cresol; p-cresol; 3,5-xylenol; 3,4-xylenol; 3,4,5-trimethyl phenol; 3-ethyl phenol; 3,5-diethyl phenol; p-butyl phenol; 3,5-dibutyl phenol; p-amyl phenol; p-cyclohexyl phenol; p-octyl phenol; 3,5-dicyclohexyl phenol; p-phenyl phenol; p-crotyl phenol; 3,5-dimethoxy phenol; 3,4,5-trimethoxy phenol; p-ethoxy phenol; p-butoxy phenol; 3-methyl-4-methoxy phenol; and p-phenoxy phenol.
A prepolymer of the ben~ylic-ether phenol-formaldehyde resin is pre-pared employing at least 50 parts by weight to 100 parts by weight of the resin of a weak acid catalyst, and often from 50 to 150 parts may be employed. The weak acid catalyst may, for example, be an organic or inorganic weak acid cata-lyst and should be anhydrous or substantially anhydrous, and typically in pow-dered form. Representative weak acid catalysts, which may be useful in the practice of the invention, would include, but are not limi~ed to: oxalic acid;
citric acid, tartaric acid; and boric acid, and combinations thereof. The preferred weak acid catalyst comprises an anhydrous or substantially anhydrous powdered boric acid catalyst.
The boric acid catalyst, in a l/~Oth normal solution, provides for a pil of about 5.2 and, in addition to initiating effectively polymeri~ation of the phenolic resin as a prepolymer, also provides and incorporates excellent flame-retardant properties and increases the dimensional s-tability o the resulting foam product, and raises the upper temperature range of the Eoam from about 250F to 350F. In the preferred embodiment, the boric acid is employed in an amount ranging from about 50 to 120 per lO0 parts of the benzylic-ether resin.
It has been found that sodium tetra borate, substituted for a boric acid in the reaction of the inven-tion, is not effective, indicating clearly that the improved flammability and flame-spread properties of the resulting foam are not the result of the borate acting as a flame-retardant agent, but rather in-dicates the controlled reaction provided by the boric acid and the preparation of foam with substantially methylene, rather than urethane, linkages. The use of the borate resulted in a phenolic foam which burned and smoked and had high friability and an unacceptable K factor, due to the open-cell nature of the resulting foam.
The preparation of the thermosetting foam product is carried out in thepresence of a catalyst, which catalyst should n~t be affected by the acid environment of a low pH, that is, one which is not hydrolyzed in an acid environ-ment. The catalyst employed is in an amount suf-ficient to effect the complete cure, cross-linking or condensation of the reactant, to provide for the rigid urethane modified foam product. The catalyst should provide for the condensa-tion of the resulting phenolic resin, as well as reaction of the diisocyanate with the hydroxyl groups of the prepolymer. It has been found that organo-metallic catalysts, such as cadmium, tin and the like; for example, those catalysts, or accelerators, used as urethane foam-type catalysts, particularly catalysts used as trimerization catalysts for polyisocyanurate foams~ may be employed in the practice of the invention. For example, a suitable organo-metallic catalyst may comprise alkyl fatty acid, metal catalyst, such as a tin catalyst, such as stannous octoate, dibutyl ti.n dilaureate and similar type catalysts. The catalysts may be employed in an amoun~ sufficient to effect the condensation reaction and typically ranges from about 0.05 to 10 parts per 100 parts of the benzylic-ether resin; for example, 0.5 to 5 parts.

~ ~ r ~

The reaction is carrled out in the presence of a blowing agent, and typical blowing agents which may be employed include physical and chemical blow-ing agents, as well as mechanical blowing techniques. Ilowever, the preferred blowing agent and technique comprise the employment of liquid physical blowing agents which are volatile liquids introduced into the reaction mixture, and which produce a blowing gas, through vapori.~.ation of the blowing agent or thro-ugh decomposition of the blowing agent during the exotherm of the reaction.
Suitable blowing agents are short-chain aliphatic hydrocarbons; for example, in the C3-C7 range, and their chlorinated and fluorinated analogs~ such as the fluoro and chloro alkanes known as Freon (a registered trademark of E.I. du Pont de Nemours ~ Co.~, methylene chloride and similar blowing agents. The blowing agents may be employed in amounts ranging from 0.5 to 50 parts; for example, 1 to 10 parts, per 100 parts by weight of the phenol resin employed in the reaction mixture.
It is usually also desirable to provide a small, but ef-fective, amount of surfactant to act as a cell-control or nucleating agent, to produce a lmiform, fine cellular structure to the resulting rigid foam. Typical sur-factants employed would include, but not be limited to, silicone surfactants in an amount generally of Erom about 0.15 to 5 parts by weight; for example, 0.1 to 2 parts by weight, of the suractant to 100 parts by weight of the phenol resin in the reaction mixture. Typical silicone surfactants which may be employ-ed include those nonhydrolyzable silicone surfactants, such as those described in United States Patents 3,772,224 and 33849~156J and those polyalkylene glycol silicones and dimethyl silicone surfactant and block copolymers commonly employ-ed in foam preparation.
Optionally, if desi.red, a wide variety of other chemicals, additives, fillers, property enhancers and reinforcers may be incorporated ;n the resinous 9 _ r-- F~ ~

reaction mixture, such as, for example, antioxidants, stabilizers, antistatic agents, biocides, dyes, fibers, ~illers, particles, clays, flame re-tardants, fungicides, heat stabili.zers, lubricants, plasticizers, viscosity-control agents, ultraviolet absorbers and other addltives.
The polyisocyanate useful in the invention comprises polyisocyanates having 2 to 5 isocyanate groups and typically a methylene diphenyl isocyanate, such as toluene diisocyanate, or mixtures of isocyanates, such as those of the abovementioned ~nited States Patent ~o. 3,9~,824. The isocyanates may be employed alone or in combination or as prepolymers with excess isocyanate, such as in the abovementioned IJnited States Patent. Typically, the average func-tionality of the isocyanate would range from 2.~ to 3.2. The isocyanates are employed typically in a range of from about 50 to 200 parts by weight of the isocyanate per 100 parts of benzylic-ether phenolic resin; for example, 70 to 120 parts per 100 parts of the benzylic-ether resin.
The modified thermosetting foam of the invention is prepared by, firstly, reacting the substantially anhydrous benzylic-ether phenol-formaldehyde resin with melamine and with the weak acid, such as the anhydrous boric acid powder, to institute prepolymerization at a pH range of from ~.5 to about 6.5 and typically from about 5 to 5.6. The reaction mixture is then mixed with a silicone surfactant as a cell-control agent, the liquid blowing agent, such as ~reon, and the tin catalyst which is not affected by the low pH employed in the reaction mixture. The reaction mixture is then reacted with mixing to provide the prepolymer which is reacted with the methylene diisocyanate, to cause an exotherm and to produce a closed-cell, low-friability, low-density foam with good smoke and burning characteristics.
It has been ~ound that if the same procedure is repeated with a resole resin; that is, one containing water, no reacti.on takes place, since the MWI in the water and the resole resin react. The employment of the weak cata-lyst, such as boric acid, provides for a controlled reaction, and strong acids, which would lead to a lower pil and a very vigorous, uncontrolled reactive product, such as sulfonic acid, should be avoided. ~ecause of the lower pH of the resin mixed, after the addition of the boric acid, the reactivity of the benæylic-ether resin prepolymer is slowed down, so that it is much easier to handle on a continuous foam machine, unlike the modified urethane foam resin system of the abovementioned United States Patent No. 3,948,824, wherein the presence of low pHs and formic acid leads to a very reactive system, which is difficult to handle with conventional foam equipment.
The invention will be illustrated with reference to certain specific examples and embodiments; however, it is recognized that various changes and modifications may be made in the formulations and practice of my invention, as described hereinafter, all within the spirit and scope of the invention.

i3S

Example 1.
A thermosetting phenolic foam product was prepared by mixing to-gether the following ingredients as shown:
Ingredients Parts by Weight 1. Benzylic-ether phenol-formaldehyde resin #220-000 (Ashland Chemical Co~) - hydroxyl number 525 ratio phenol/formaldehyde 1/1.5 water less than 0.5% 100 2. Blowing agent - trichlorofluoro methane (Freon-ll (du Pont)) 25 3. Catalyst - triethanol amine (TEA) 0.3

4. Silicone surfactant - DC-193 (Dow Corning Co.) 2

5. Flame retardant - chlori.nated (Thermolin 101 (Olin Corp.)) 10

6. Methylene diphenyl diisocyanate (MDI) (Rubicon XI-171) 125 Cream Time 6 seconds Tack free time 30 seconds Cure time 90 seconds Flame spread (E-84) 25 Smoke 250 Density 2.0 psi Compressive strength 30 psi K factor .125 The resulti.ng foam prepared in accordance with the disclosure of United States Patent 3,948,824 is surface-friable.

S~

Example 2.
A thermosetting phenolic foam product was prepared by mi.xing to-gether the following ingredients as shown:
Ingredient Parts by Weight 1. Ben~ylic-ether phenol-formaldehyde resin #220-000 100 2. Boric acid (anhydrous powder) 75 3. Blowing agent - Freon 11 25 . Silicone surfactant DC-193 2 5. Organo metallic catalyst -stannous octoate 2 6. DMI (Rubicon XI-171) 100 Cream time 20 seconds Tack free time 60 seconds Cure time 90 seconds Density 1.6 pcf Compressive strength 20 psi K factor .125 E-84 flame spread 5 Smoke 15 The thermosetting modified foam of the invention provided a closed-cell foam with good compressive strength and low smoke and flame-spread properties (as measured by ASTM E-84 tunnel test). The same formulation used with a water-containing phenol-formaldehyde resole resin provides a friable foam of very low or no strength and with a density of 4.0 pcf. The use of an alkaline catalyst, such as an amine catalyst, will not be effective, since the acid of the resin mixture will render the catalyst ineffective. An exotherm occurs on -the addition of the isocyanate, which increases the acid reacti.vi.ty of the phenolic resin, so that both condensation and polymeri~ation occur in ?t~
~ 3~

the reaction mixture, providing a Eoam with substantially methylene bridges connected with other methylene bridges, which provides excellent flame and smoke properties to the foam.
It has also been found that the above formulation, when mixed for a short period of time, provides mixture having a white, creamy texture; how-ever~ on continuous mixing for long periods of time, such as 12 hours, -the mixture has a bluish coat or color with condensed water on the wall of the mixing vessel, providing a substantially reacted prepolymer with methylene link-ages. The use of melamine, either to the phenolic ether or to the reacted prepolymer, removes the free formaldehyde odor. The long-mixed prepolymer reacted with MDr on the formulation produces a low-density foam with low smoke and flame spread values, yet with good compressive strength and good friability.
The foam is suitable for use as insulation and other uses for closed-cell low-density foam products.

Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing a nonfriable, closed-cell, benzylic-ether phenol-formaldehyde modified thermosetting foam product characterized by a low flame spread and smoke properties, which method comprises:
a) reacting, under substantially anhydrous conditions, 100 parts of a benzylic-ether phenol-formaldehyde resin, having a hydroxyl number of greater than about 200, with from about 50 to 150 parts by weight per 100 parts of the benzylic-ether phenol-formaldehyde resin of a weak acid catalyst at a pH of from about 4.0 to 6.5, to form a benzylic-ether phenol resin prepolymer;
and b) reacting the prepolymer, in the presence of a blowing amount of a blowing agent to form a foamable mass, and a catalytic amount of an organo-metallic catalyst, which is not affected by the pH of the prepolymer, with from about 50 to 200 parts by weight per 100 parts of the benzylic-ether phenol resin prepolymer of a methylene diphenyl polyisocyanate in an exothermic reaction, to provide for the further heat and acid condensation of the phenol resin prepolymer and the reaction of the isocyanate with the prepolymer, to produce a low-friability, substantially closed-cell, phenolic modified thermosetting foam product composed substantially of acid methylene linkages.

2. The method of claim 1 wherein the weak acid catalyst is selected from the group consisting of oxalic acid, citric acid, tartaric acid and boric acid, and combinations thereof.

3. The method of claim 1 wherein the weak acid catalyst comprises boric acid.

4. The method of claim 1 wherein the weak acid catalyst comprises from about 50 to 120 parts by weight of a powdered boric acid catalyst.

5. The method of claim 1 wherein the blowing agent comprises a liquid fluorocarbon.

6. The method of claim 1 wherein the organometal catalyst comprises a fatty acid tin catalyst.

7. The method of claim 1 wherein the reaction is carried out at a pH
of from about 5.0 to 5.7.

8. The method of claim 1 which includes reacting the resin with the weak acid catalyst at a temperature of less than about 100°F.

9. The method of claim 1 wherein the methylene diphenyl polyisocyanate has an average functionality of from 2.4 to 3.2.

10. The method of claim 1 wherein the methylene diphenyl polyisocyanate comprises toluene diisocyanate.

11. The method of claim 1 wherein the benzylic-ether phenol-formaldehyde resin has a hydroxyl number ranging from about 400 to 600.

12. The method of claim 1 which includes mixing the benzylic-ether phenol-formaldehyde resin and weak acid catalyst to form the prepolymer for a period of at least 6 hours.

13. The method of claim 1 which includes adding an amine reactive with formaldehyde to the prepolymer to react with free formaldehyde in the prepolymer.

14. The method of claim 1 which includes adding sufficient melamine to react with substantially all of the free formaldehyde of the phenolic-formaldehyde resin.

15. The method of claim 1 which includes adding from about 1 to 5 parts by weight per 100 parts of the resin of melamine to the prepolymer.

16. The method of claim 1 wherein the benzylic-ether phenol-formalde-hyde resin has the structural formula:

wherein R is a hydrogen or a phenolic substituent meta to the phenolic hydroxyl group, the sum of m and n is at least 2 and the ratio of m to n is at least 1, and X is an end group from the group consisting of hydrogen and meth-ylol, the molar ratio of said methylol-to-hydrogen end groups being at least 1.

17. A method of preparing a nonfriable, closed-cell, benzylic-ether phenol-formaldehyde modified thermosetting foam product characterized by a low flame spread and smoke properties, which method comprises:
a) reacting, under substantially anhydrous conditions, 100 parts of a benzylic-ether phenol-formaldehyde resin, having a hydroxyl number of from about 400 to 600, with from about 50 to 150 parts by weight per 100 parts of the benzylic-ether phenol-formaldehyde resin of a boric acid catalyst at a temperature of less than about 100°F and a pH of from about 4.0 to 6.5, to form a benzylic-ether phenol resin prepolymer; and b) reacting, in the presence of a blowing amount of a blowing agent to form a foamable mass, and a catalytic amount of an organometallic catalyst which is not affected by the pH of the prepolymer, the prepolymer with from about 50 to 200 parts by weight per 100 parts of the benzylic-ether phenol resin prepolymer of a methylene diphenyl diisocyanate in an exothermic reaction, to provide the further condensation of the phenol resin prepolymer and the reaction of the isocyanate with the prepolymer, to provide a low-friability, substantially closed-cell, phenolic modified thermosetting foam product com-posed substantially of acid methylene linkages.

18. A method of preparing a nonfriable, closed-cell, benzylic-ether phenol-formaldehyde modified thermosetting foam product characterized by a low flame spread and smoke properties, which method comprises:
a) reacting, under substantially anhydrous conditions, 100 parts of a benzylic-ether phenol-formaldehyde resin, having a hydroxyl number of from about 400 to 600, with from about 50 to 150 parts by weight per 100 parts of the benzylic-ether phenol-formaldehyde resin of a boric acid catalyst at a temperature of less than about 100°F and a pH of from about 4.0 to 6.5, and with a sufficient amount of melamine to react with substantially all of the free formaldehyde of the phenolic-formaldehyde resin and mixing the resin, melamine and weak acid catalyst for a period of about 6 to 24 hours, to form a benzylic-ether phenol resin prepolymer; and b) reacting, in the presence of a blowing amount of a blowing agent to form a foamable mass, and a catalytic amount of an organometallic catalyst which is not affected by the pH of the prepolymer, the prepolymer with from about 50 to 200 parts by weight per 100 parts of the benzylic-ether phenol resin prepolymer of a methylene diphenyl diisocyanate in an exothermic reaction, to provide the further condensation of the phenol resin prepolymer and the reaction of the isocyanate with the prepolymer, to produce a low-friability, substantially closed-cell, phenolic modified thermosetting foam product composed substantially of acid methylene linkages.