JPH0192242A - Phenolic resin foam - Google Patents

Abstract

PURPOSE:To obtain the title foam suitable for building components with excellent corrosion resistance for the metal components, by foaming and curing a mixture containing a basic phenolic resin containing methylol groups, an organic ester as a gelling agent, a foaming agent and a foam stabilizer. CONSTITUTION:100pts.wt. of a methylol group-containing basic phenolic resin (preferably of 400-1,500 weight-average molecular weight, containing a metallic ion such as alkali or alkaline earth metal ion) are combined with, preferably 5-70pts.wt. of an organic ester (preferably hydroxycarboxylic acid cyclic ester and/or aliphatic ester), 1-50pts.wt. of a foaming agent such as methylene chloride, and 0.1-10pts.wt. of a foaming stabilizer such as tetraalkyl ammonium salt. The composition is foamed and cured to give the subject foamed product of high adhesion to basic coating substance, high resistance to surface brittleness, no formation of sulfur dioxide gas, even it is exposed to fire.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is a phenolic resin-based material useful for construction parts, heat insulating materials for refrigerator-freezers Plan 1, soundproofing/vibration proofing materials, structural panels, etc. Regarding foam.

More specifically, it has an extremely excellent function of preventing or improving corrosion of metal parts such as iron and aluminum, adhesion with alkaline adherents, and surface brittleness, and the generation of toxic sulfur dioxide gas. The present invention relates to a novel phenolic resin thread foam that does not involve

[Conventional technology]

In recent years, phenolic resin foams have been given superior performance compared to other plastic foams in terms of heat resistance, fire resistance, low smoke emission, dimensional stability, etc., and as a result, plastic foams have been subject to a review, especially in line with revisions to the Building Standards Act. It has attracted much attention in the field of architecture, where it is in demand, and is even put into practical use in some areas.

Conventionally, phenolic resin foams are produced by treating resol-type phenolic resin with a strong acidic catalyst such as organic sulfonic acid in the presence of a blowing agent, foam stabilizer, or various additives blended as necessary at low temperatures. It has been produced by a method of foaming and curing, or a method of blending hexamethylenetetramine as a curing agent with a novolak type phenol resin and heating the mixture at high temperature to foam and harden it.

However, the recent phenolic resin foams used in the above fields have significantly improved product quality compared to conventional ones, and have improved adaptability to various foaming methods such as continuous foaming, in-situ foaming, and injection foaming. Acid-curable phenolic resin foams manufactured by the former method, which have come to be widely adopted because of their richness and excellent productivity, are common.

[Problem that the invention seeks to solve]

However, acid-curing phenolic resin foam (
1) Because the strong acidic catalyst is present as a residual acid, there is a risk of corrosion of metal parts such as iron and aluminum during long-term use. (2) Alkaline facing materials,
Poor adhesion to alkaline fillers, alkaline reinforcing materials, concrete or mortar base materials, etc. (hereinafter referred to as alkaline adhering materials) (3) Prone to surface brittleness when foamed and cured at room temperature. (4) When exposed to high temperatures or fire, the curing catalyst thermally decomposes and generates sulfur dioxide gas, which is toxic to the human body.

The present invention has been made in view of this point of view, and includes:
The objectives are, firstly, to provide a phenolic resin foam that prevents or reduces corrosive effects on metal components, and secondly, to provide a phenolic resin foam that has extremely excellent adhesion to alkaline adherends. to donate the body,
A third object is to provide a phenolic resin foam that has excellent surface brittleness resistance and does not generate toxic sulfur dioxide gas.

[Means for solving problems]

As a result of extensive research to achieve the above object, the present inventors have created a new phenolic resin foam based on a completely different technical idea from conventional foams using strong acidic catalysts, and The present invention was achieved by discovering that foam is extremely effective in resolving the above-mentioned problems.

That is, the present invention is made by foaming and curing a mixture containing an alkaline phenolic resin having a methylol group, a gelling agent, a foaming agent, and a foam stabilizer as essential components, and the gelling agent is an organic ester (as described in the first summary). ) or cyclic carbonate esters (second aspect).

Hereinafter, the phenolic resin foam of the present invention will be specifically explained.

The phenolic resin according to the present invention must have a methylol group and be alkaline, and if the phenol resin has these essential requirements, it may be resol resin, ammonia resol resin, benzyl ether resin, or novolac resin. Any type of phenolic resin, exemplified by novolac resol resins having methylol groups added to the resin, can be used alone or as a mixture of two or more.

The phenolic resin usually has a weight average molecular weight adjusted to 200 to 2000, preferably 400 to 1500, but if the molecular weight is less than 200, foaming and curing will be slow, making it impractical. becomes high and difficult to handle. Although it varies depending on the purpose of use and is not absolutely limited, - For boat fishing, the solid content is about 30%.
~80% by weight of aqueous resin.

Such phenolic resins can be prepared by, for example, mixing phenols and aldehydes in the presence of a catalyst to give a temperature of 0.5 to 0.5 to reflux at 0°C to reflux temperature.
An initial condensate having a predetermined weight average molecular weight is prepared by reacting for 24 hours, and then the initial condensate is concentrated using an appropriate condensation means (e.g., reduced pressure, decantation, etc.) according to the purpose of use. Prepared with adjusted solids content or without concentration. In addition, the prepared resin may contain an appropriate amount of an alkali metal compound or an alkaline earth metal compound, especially an alkali metal compound, in order to adjust the curing characteristics, convert to an alkaline system, prevent separation (improve storage stability), etc. It is adjusted by further adding a compound.

In addition, when it is necessary to reduce the viscosity of the phenol resin according to the present invention depending on its intended use, for example, ethylene glycol, diethylene glycol, N-methyl-
It is also possible to incorporate a diluent such as 2-pyrrolidone within a range that does not inhibit the foaming properties. In addition, in order to improve adhesion with inorganic fillers, inorganic or mineral fiber reinforcing materials, etc. that are blended as desired, γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxy Epoxy thread silanes such as cyclohexyl)ethyltrimethoxysilane; amino-based silanes such as N-β(aminoethyl)γ-aminopropyltrimethoxysilane and γ-aminopropyltriethoxysilane; mercaptosilanes such as γ-mercaptopropyltrimethoxysilane A silane coupling agent such as thread silane can be included in the mix.

Examples of phenols include monohydric phenols such as phenol, cresol, xylenol, paratertiary butylphenol, paracumylphenol, and phenylphenol, or resorcinol, catechol, hydroquinone, pyrogallol, and bisphenols (e.g., bisphenol A, bisphenol F, etc.). polyhydric phenols,
Alternatively, phenolic residues produced as by-products during the production of phenols (for example, resorcinol residues, catechol residues, cresol residues, xylenol residues, etc.) are exemplified, and among these, phenol, cresol, bisphenols, and phenol thread residues are particularly preferably used. be done. On the other hand, as the aldehyde, for example, formaldehyde supplying substances such as formaldehyde, paraformaldehyde, trioxane, acetals, hexamethylenetetramine, glyoxal, furfural, benzaldehyde, etc. are used.

The amount of aldehydes to be added to phenols is as follows:
A suitable amount is 0.6 to 5.0 mol per 1 mol of phenol, and a blending amount that can generate methylol groups depending on the type of catalyst used for resin production, for example, an alkaline catalyst or a divalent metal salt catalyst is used. In cases where 0.6 mol or more of aldehydes are blended per 1 mol of phenonyls, and when using strong acidic catalysts such as inorganic acids or organic acids, approximately 1 mol or more of aldehydes are blended, etc., as appropriate. It is necessary to.

The reaction catalyst used in the production of the resin can be arbitrarily selected from alkaline catalysts or acidic catalysts depending on the desired resin type, and can be used alone, in the same combination, or in different combinations (
For example, conversion from an acidic reaction system to an alkaline reaction system)
It can be used in other ways.

Examples of alkaline catalysts include alkali metal compounds such as alkali metal hydroxides, oxides, phosphates, carbonates or hydrogen carbonates; alkaline earth metal compounds such as alkaline earth metal hydroxides or oxides; Amine compounds and the like are used.

Typical examples of this alkaline catalyst include, for example, K
OH, NaOH, LiOH, 20, Na2O, K3P
O4, Na3PO4, K2CO3, KHCO3B a(
OH)2, Ca(OH)2, CaO, MgO, ammonia, hexamethylenetetramine, triethylamine,
Examples include triethanolamine and ethylenediamine.

Among the alkaline catalysts, alkali metal compounds, alkaline earth metal compounds, and mixtures thereof (hereinafter referred to as alkali metal compounds, etc.) are suitable because they promote the curing (crosslinking) reaction during foam production. In particular, alkali metal compounds are more preferably used because they have a large function of promoting the curing (crosslinking) reaction and contribute to providing a resin with excellent storage stability.

The amount of such alkali metal compound used is 0.01 to 3.0, preferably 0.1 to 2.0, per mole of phenol.
5 mol, and the metal compound and the like are blended during or after the production of the resin as described above to obtain the metal ion-containing phenol resin suitably used in the present invention.

The appropriate amount of the amine compound catalyst to be used is 0.005 to 0.5 mol per 1 and a half mol of the phenol. However, since resins prepared using such catalysts generally tend to cure slowly, it is desirable to use the alkali metal compound or the like in combination during or after the production of the resin.

Examples of acidic catalysts include inorganic acids such as hydrochloric acid and sulfuric acid, organic acids such as oxalic acid and organic sulfonic acids, divalent metal salts (such as carboxylic acids, Mn-Zn-Pb salts such as naphthenic acid or boric acid), and Lewis acid catalysts. Acids (for example, zinc chloride, etc.) are used. The appropriate amount to be used is 0.001 to 0.05 mol per mol of phenol. It should be noted that a resin prepared using only an acidic catalyst needs to be converted to alkalinity using the above-mentioned alkali metal compound or the like after resin production from the viewpoint of curing properties.

Note that the alkali metal compound and the like and the silane coupling agent described above do not necessarily have to be included in the phenol resin, and may be added at the time of preparing the foamable mixture.

Next, the gelling agent, foaming agent, and foam stabilizer used together with the alkaline phenol resin in producing the phenolic resin foam of the present invention will be explained.

Examples of organic esters used as gelling agents for phenolic resins in the present invention include oxyacid cyclic esters such as β-propiolactone, γ-butyrolactone, δ-valerolactone, and ε-caprolactone; Ethyl, methyl acetate, ethylene glycol monoacetate, ethylene glycol diacetate,
Suitable examples include aliphatic esters such as diacetin and triacetin. These organic esters can be used alone or in combination of two or more.

On the other hand, examples of cyclic carbonate esters include dioxolone-2 (commonly known as ethylene carbonate), 4-methyldioxolone (commonly known as propylene carbonate), 4-ethyldioxolone, 4-butyldioxolone, 4,4' −
Examples include dimethyldioxolone, 4.5-dimethyldioxolone, 4-pentyldioxolone, and the like. Among these, ethylene carbonate, propylene carbonate, and mixtures thereof are preferably used because they are commercially easily available, inexpensive, and have low toxicity and odor.

The amount of the gelling agent to be blended is usually 2 parts by weight or more, preferably 5 to 70 parts by weight, per 100 parts by weight of the phenolic resin. If the amount is less than 2 parts by weight, foaming and curing will be slow, which is a practical problem.

Note that the above-mentioned organic esters and cyclic carbonate esters can be used in combination, and the purpose of the present invention is not hindered in any way.

Examples of the blowing agent used in the present invention include methylene chloride,
Carbon tetrachloride, trichloroethane, trichloromonofluoromethane, dichloromonofluoromethane, 1.1.2
- Illustrative examples include, but are not limited to, halogenated hydrocarbons such as trichlorotrifluoroethane and 1,2-dichlorotetrafluoroethane, and aliphatic hydrocarbons such as butane, pentane, and hexane. It's not a thing.

The blowing agents can be used alone or in combination of two or more, and the amount blended is usually 1 to 50 parts by weight per 100 parts by weight of the phenol resin.

Examples of the foam stabilizer used in the present invention include cationic surfactants such as tetraalkylammonium salts, anionic surfactants such as alkylphenol sulfonates, polyoxyethylene sorbitan fatty acid esters, and castor oil ethylene oxide adducts. Examples thereof include nonionic surfactants such as siloxane/oxyalkylene copolymers, among which foam stabilizers mainly composed of nonionic surfactants are particularly preferred. The amount of foam stabilizer is usually 10% of phenol resin.
It is 0.1 to 10 parts by weight per 0 parts by weight.

Next, a method for producing a phenolic resin foam according to the present invention will be explained.

First, various additives (such as flame retardants, fillers, reinforcing materials, etc. , pH adjuster, plasticizer, coloring agent, etc.) and premixing, then a predetermined amount of blowing agent and gelling agent are added sequentially or simultaneously, and further stirred and mixed to prepare a uniform foamable mixture. .

Next, the prepared foamable mixture is immediately injected into a molding model adjusted to room temperature or a desired temperature, and is foamed and cured under normal pressure or pressure to produce the phenolic resin foam of the present invention. can.

In addition, the foamable mixture prepared using foaming methods commonly employed in the technical field, such as batch or continuous high-speed mixing, spray mixing, and froth mixing, is continuously conveyed onto an endless conveyor. Method of foaming and curing by flowing out and promoting self-adhesion with the single body or various surface materials (continuous foaming), method of filling and foaming and curing while press-fitting into the space of a molded body or assembly frame (injection foaming), or spraying. The present invention can be applied without restriction to one-way in-situ foaming or even impregnated foaming, and the phenolic resin foam of the present invention can be produced by ordinary foaming operations.

In addition, by subjecting the phenolic resin foam of the present invention to after-curing, it is possible to completely cure the remaining methylol groups and further improve the strength of the foam, and after-curing is recommended as a useful method. be done.

The phenolic resin foam of the present invention may exhibit weak acidity based on the acidity of the phenolic hydroxyl group depending on the selection and adjustment of the type or amount of the alkaline component, the type of gelling agent, etc.; is alkaline.

The phenolic resin foam of the present invention obtained in this way has corrosion resistance against metal members such as iron plates, aluminum plates, iron frames, aluminum frames, metal fasteners such as nails; asbestos paper, rock wool fibers, Foaming that requires performance such as adhesion to alkaline adherents such as mortar cement boards, concrete, or mortar base materials; resistance to surface brittleness; or safety without generating toxic sulfur dioxide gas even under fire/high temperatures. Fields of application such as ceiling materials, interior wall materials, exterior wall materials, flooring materials, building materials such as rain shutters, ceiling materials for automobiles, insulation materials for freezers and refrigerators, heat insulation materials for plants, soundproofing and
Suitable for use as vibration isolators, structural panels, etc.

[Effect]

Although some aspects of the production mechanism or the mechanism of effect expression of the phenolic resin foam in the present invention have not yet been logically elucidated, it is presumed as follows. Ring-opening decomposition or hydrolysis products (alcoholic components and acidic components) of gelling agents that occur in the presence of alkaline components that act catalytically to promote the decomposition and curing (crosslinking) of gelling agents.
and a phenol resin having a methylol group, or curing (cross!
) The polymerization mechanism of the resin is based on a reaction, and the neutralization reaction between the alkaline component bonded to the resin and the acidic component (e.g. carbon dioxide, oxyacid, carboxylic acid, etc.) occurs due to the elimination of the alkaline component. It is presumed that the hydrophobization mechanism of the resin works together to form a polymerized and hydrophobized resin foam. Furthermore, since the foam contains neutral or alkaline salts or excess alkaline components generated by the neutralization reaction, it is presumed to exhibit weak acidity based on the acidity of the alkaline or phenolic hydroxyl groups.

Since the phenolic resin foam of the present invention exhibits alkalinity or weak acidity as described above, it is thought to have the effect of preventing or reducing corrosive effects on metal members such as iron and aluminum. In addition, in the present invention, which does not use a strong acidic catalyst such as an organic sulfonic acid, there is no possibility of adhesion inhibition due to uncuring caused by a decrease in catalytic function due to an alkaline adhering substance, as observed in the prior art. Rather, not only the curing (crosslinking) is promoted by the alkali component, but also the homogeneous surfaces form a strong bond, resulting in an excellent bond between the foam of the present invention and the alkaline adherend. A resin that exhibits strength and does not generate toxic sulfur gas even when exposed to fire or high temperatures, ensuring safety for the human body, and that has a relatively high molecular weight as the foaming resin. It is presumed that a phenolic resin foam having characteristics such as good resistance to surface brittleness can be provided due to differences in the formation mechanism of the foam.

〔Example〕

Hereinafter, the present invention will be specifically explained based on Examples, but the Examples are not intended to limit the technical idea. In addition,
All "%" in the examples are based on weight unless otherwise specified.

[Example 1] 1. Preparation of alkaline phenol resin After charging 1-000 g of phenol and 1358 g of 47% formalin into a reaction flask equipped with a reflux condenser, thermometer, stirrer, and dropping funnel, 50% While dropping 715 g of an aqueous potassium hydroxide solution from a separating funnel, the temperature was gradually raised and the reaction was carried out at 75° C. for 8 hours. Subsequently, the reaction mixture obtained was concentrated under reduced pressure to a solids content of 70% and the viscosity at 30'C was 5000°P (B
(type viscometer) An aqueous alkaline phenol resin having a weight average molecular weight of 800 was prepared. This is called resin A.

2. Foaming operation To 100 g of the above resin A, add 2 g of 5H-193 (product name: Siloxane/oxyalkylene copolymer nonionic surfactant) manufactured by Resilicone Co., Ltd. as a foam stabilizer and premix. As a blowing agent, 20g of Freon 11 (trade name: Trichloromo, /Fluoromethane) manufactured by Asahi Glass Co., Ltd. was blended, and a homogenizer (rotation speed: 5000 r, p, m) was added.
) for 10 seconds, followed by adding 15 g of an 80% aqueous ethylene carbonate solution (1-2% pure content based on the resin) as a gelling agent and mixing for an additional 10 seconds to prepare a homogeneous foamable mixture.

The prepared foamable mixture was preheated to about 80°C on a press heating plate.
Quickly inject into four 200 x 100 x 25 mm + molds (laying materials: 0.5 mm iron plate, 0.5 mm aluminum plate, 9 mm mortar cement board, and release paper) that have been preheated to 200 x 100 x 25 mm, and foam and harden while holding under pressure for 10 minutes. In this way, a foam of the present invention was obtained.

Furthermore, this foam was placed in an oven at 100°C for 1 hour.
After-cure was performed for 0 minutes.

Evaluation of the created foam (density, pH, concentration of So2 gas generated, adhesion of two corrosion-resistant materials to metal parts, etc.) was carried out using the methods described below, and the results are shown in Table 1. . Note that a foamed product without a face material made using release paper was subjected to a nail corrosion test.

[Comparative Example 1] 20 g of phenol sulfonic acid curing agent CA-045 was added to 100 g of phenol resin PF-0009 (contains foam stabilizer) manufactured by Asahi Yokuzai Kogyo Co., Ltd., which is a resin system for acid-curable phenol foam. A foam was prepared in the same manner as in Example 1, except that 20 g of Freon 113 (1,1,2-trichlorotrifluoroethane), a trade name manufactured by Glass Co., Ltd., was used. The results of the evaluation of the foam according to Example 1 were as shown in Table 1.

[Examples 2 to 3] The same procedure was carried out except that instead of ethylene carbonate, a gelling agent with a purity of 20% γ-butyrolactone (Example 2) or 10% methyl formate (Example 3) was used with respect to resin A. A foam was prepared using the same procedure as in Example 1. The results of evaluation of each foam according to Example 1 are as shown in Table 1.

[Example 4 and Comparative Example 2] 2 g of 5H-193 as a foam stabilizer for 100 g of resin A
, 30 g of Freon 11 as a blowing agent and 40 g of propylene carbonate as a gelling agent (40 g in pure proportion to the resin)
A homogeneous foamable mixture was prepared in the same manner as in Example 1 using (0%). Next, this was quickly poured into a stainless steel container with an open top and allowed to foam freely to create a room temperature hardening foam (Example 4). For comparison purposes, 100g of phenolic resin PF-0009 and acid curing agent CA
A foam (Comparative Example 2) was prepared using 30 g of -045 and 30 g of Freon 113 and performing the same operations as in Example 4.

Next, in order to examine the surface brittleness resistance of the created foam, we rubbed the surface layer of the foam with our fingers and observed the degree of brittleness. As a result, the foam of Example 4 had excellent surface brittleness resistance without any powdering phenomenon. On the other hand, the foam of Comparative Example 2 had a powdered surface and was inferior in surface brittleness resistance.

As described above, the phenolic resin foam of the present invention has superior prevention or improvement in (1) corrosion resistance to metal members and adhesion to alkaline adherends than conventional acid-curing phenol resin foams; It was found to be effective. It was also confirmed that (2) it exhibited very good resistance to surface brittleness, and (3) it did not generate toxic sulfur dioxide gas even when exposed to high temperatures, making it useful as a foam.

Measurement of the weight average molecular weight of the phenolic resin and evaluation of the foam in the present invention were carried out by the following method.

Note 11 Method for measuring weight average molecular weight The sample is an alkaline phenol resin prepared with formic acid at pH 5 to 6.
The resin content obtained by adjusting the content to tetrahydrofuran (TH
F) was prepared by dissolving it in F).

Next, high performance liquid chromatography H manufactured by Toyo Soda Kogyo Co., Ltd.
Using a molecular weight distribution measuring device consisting of LC802 (THF eluent flow rate: 1, 2 ml/min, 2 separation power, 0OOH8XG2000H8, precolumn: GH8P) and data processor chromato processor CP800o (with built-in polystyrene standard calibration curve). The weight average molecular weight of the sample was determined.

Note 2゜The density of the foam (Kg/m3) is JIS A 95
Measured according to 14.

Note 3.9H measurement method After weighing 0.5 g of powdered foam into a beaker, add distilled water (),
pH of the supernatant obtained by adding 0Oce) and stirring for 30 minutes.
was measured with a pH meter.

Notes・1. Method for measuring sulfur dioxide gas concentration: 0°5 g of powdered foam is placed in a high-temperature tubular furnace (temperature: 700°C).
After inserting the gas, quickly attach a gas collection bag to collect the generated gas for 10 minutes, and measure the sulfur dioxide concentration (
%) was measured.

Note 5. Corrosion resistance test method (1> A test piece with a nail inserted into the center of a foam with a 20 mm trx depth was placed in a thermostatic chamber at a temperature of 60°C/humidity of 90% and left for 30 days, and the corrosion state of the nail was observed. did.

(2) A test specimen made by self-adhering a specified metal surface material during foam molding was placed in a constant temperature M shaker at a temperature of 60℃/90% humidity and left for 30 days to check the corrosion state of the metal surface material. Observed.

Note (3) How to measure adhesion: After fixing a spring scale to the end of the face material of a test specimen made by self-adhering the specified face material during foam molding, the face material was peeled off and its adhesive strength (K g /100mm width) was measured.

Table 1-2 - [Effects of the Invention] As detailed above, the phenolic resin foam of the present invention has (1) extremely superior corrosion resistance to metal members than acid-curing conventional foams; It is possible to prevent accidents caused by corrosion and reduce repair work, and (2) it has very good adhesion to alkaline adherends, so surface treatment/pre-coating for these adherends, which has been conventionally carried out, is It is possible to easily provide a composite foam with excellent durability without the need for complicated pretreatments such as (
3) It exhibits excellent surface brittleness resistance when foamed at room temperature, making it particularly effective for on-site foaming.In addition, it does not generate toxic sulfur dioxide gas even when exposed to fire or high temperatures, ensuring safety for the human body. It has useful effects such as vines, and has a wide range of applications and high utility value as an industrial material.

Patent applicant: Asahi Yokuzai Kogyo Co., Ltd.

Claims (1)

[Claims] 1. Alkaline phenol resin having a methylol group,
A phenolic resin foam obtained by foaming and curing a mixture containing a gelling agent, a foaming agent, and a foam stabilizer as essential components, and characterized in that the gelling agent is an organic ester. 2. Organic esters are oxyacid cyclic esters and/or
or an aliphatic ester, the phenolic resin foam according to claim 1. 3. Claim 1 or 3, wherein the alkaline phenol resin is an aqueous resin containing at least one metal ion selected from alkali metals or alkaline earth metals and having a weight average molecular weight of 200 to 2000. The phenolic resin foam according to item 2. 4, alkaline phenolic resin having a methylol group,
A phenolic resin foam obtained by foaming and curing a mixture containing a gelling agent, a foaming agent, and a foam stabilizer as essential components, and characterized in that the gelling agent is a cyclic carbonate ester. 5. Claim 4, wherein the alkaline phenol resin is an aqueous resin containing at least one metal ion selected from alkali metals or alkaline earth metals and having a weight average molecular weight of 200 to 2,000. Phenolic resin foam.