Phenolic foam is a kind of foamed plastic formed by foaming phenolic resin. Compared with polystyrene foam, PVC foam, polyurethane foam and other materials that dominated the market in the early days, it has particularly excellent performance in flame retardant. It has the advantages of light weight, high rigidity, good dimensional stability, chemical corrosion resistance, good heat resistance, flame retardancy, self-extinguishing, low smoke, flame penetration resistance, no overflow in case of fire, and low price. It is an ideal insulating material for electrical appliances, instruments, construction, petrochemical and other industries, so it has been widely valued by people. At present, phenolic foam has become one of the fastest growing foam varieties. The dosage is increasing and the application scope is expanding, and the research and development at home and abroad is quite active. However, the biggest weakness of phenolic foam is its brittleness and high open porosity, so improving its toughness is the key technology to improve the performance of phenolic foam. In this paper, the foaming additives used in the preparation of phenolic foam, the foaming mechanism and the new progress of foam toughening are introduced.
2 foaming AIDS
2. 1 catalyst/curing agent
Phenolic foam is usually prepared at room temperature or low heat, so acid is needed as catalyst. When acid is used as catalyst, acid can accelerate the polycondensation reaction between resin molecules, and the heat released from the reaction promotes the rapid gasification of foaming agent, which makes the emulsified resin swell and solidify. The catalyst of the reaction is also the curing agent of the resin. At ambient temperature, the type and quantity of curing agent are extremely important for obtaining high-quality foam. The choice of curing agent should make the curing speed of polymer match the foaming speed. Therefore, it is required that the curing agent used can make the curing speed change in a wide range, and the curing reaction itself can be carried out at a lower temperature.
Curing agents are divided into inorganic and organic acids, such as sulfuric acid, hydrochloric acid and phosphoric acid. Organic acids include oxalic acid, adipic acid, phenylnitric acid, phenylnitric acid, toluene sulfonic acid, benzene sulfonic acid and petroleum sulfonic acid. Inorganic acids are cheap, but they cure too fast and have a strong corrosive effect on metals. Therefore, anticorrosion has become a major problem in the use of phenolic foam. The research shows that inorganic acids can be diluted with methanol, ethanol and propanol to achieve corrosion inhibition, and corrosion inhibitors such as calcium oxide, iron oxide, calcium carbonate, anhydrous borax, alkali metal and alkaline earth metal carbonates, zinc and aluminum can also be added. Some people have considered treating foam with alkali neutralizer, but the effectiveness of this method has not been proved. The research in this field is still in progress. It is reported in the literature that the use of acidic naphthalene sulfonic acid phenolic aldehyde not only plays a catalytic role, but also participates in the condensation reaction of phenolic aldehyde, which reduces the permeability of acid and has little corrosion to metals. Other methods to reduce the corrosiveness of foam materials have also been mentioned in the literature, such as using hydrochloric acid as curing agent, removing volatile compounds in molded products by vacuum method, removing residual acid by NH3, or heat-treating at 80- 130℃, or adding neutralizer into resin formula. These methods complicate the production process and increase the cost.
At present, curing agents based on aromatic sulfonic acid are very common. This is because it is less corrosive and has plasticizing effect. There are also mixtures of organic acids and inorganic acids. In order to ensure uniform dispersion, solid organic sulfonic acid should be prepared into high-concentration aqueous solution, and the concentration of the general solution is 40-65%.
2.2 foaming agent
Foaming agent is the source of foaming force in plastic foaming molding. Plastic foaming methods are generally divided into mechanical foaming, physical foaming and chemical foaming. Mechanical foaming is to mix gas evenly into resin with strong mechanical stirring to form bubbles. Physical foaming is to change the physical state of foaming agent dissolved in resin and form a large number of bubbles. The above two kinds of foaming are completely physical processes without any chemical changes. Chemical foaming is the chemical change of chemical foaming agent in the foaming process, and the decomposition produces a large amount of gas to make the foaming process proceed. The type and dosage of foaming agent have an important influence on foaming effect. It directly affects the foam density, and then affects the physical and mechanical properties of the product. In addition, the use of foaming agent makes the foam have a large number of spherical micropores, which improves the flame retardancy and toughness of the foam.
Judging from the foaming reaction mechanism of phenolic resin, most of them are carried out by physical foaming. Physical foaming agents can be divided into inert gas and low boiling point liquid. Commonly used foaming agents for phenolic foam are various volatile liquids with boiling point between 30-60℃, such as freon, chlorinated hydrocarbon, n-pentane, etc. At present, the vast majority of foaming agents used in scientific research and factory production are still chlorofluorocarbons, among which freon-1 1 and freon -2 1 1: 4 (molar) mixtures are widely used. The effect of HCFC foaming agent is very good, but HCFC will destroy the ozone layer in the atmosphere, so the use has been restricted and alternatives have been selected. In recent patents, in order to reduce the harm to the atmospheric ozone layer, less harmful chlorofluorocarbons, such as CF32CF2CHC 12 and HCF2CF2CEt, are selected, which are called ozone-free foaming agents. Others use reducing the dosage of fluorine-containing foaming agent and adding some substitutes, such as F- 1 1 and pentane. Among the new substitutes, the most promising are inert gas foaming agents such as carbon dioxide and nitrogen. They are non-toxic and pollution-free, with zero ozone depletion coefficient (ODP) and small greenhouse effect coefficient (GwP). They are non-combustible and cheap, which is the focus of the research on freon substitutes, but it is difficult. Fortunately, it is reported that researchers of Asahi Chemical Company in Japan used CO2 instead of HCFCs as foaming agent to produce phenolic foam, and the effect was good. They are phenolic foams made of a mixture of phenolic copolymer resin (containing hydroxymethyl urea), foaming agent CO2 and catalyst. The closed-cell content is 96.0%, the pore size is 190μm, the thermal conductivity (JISA 14 12) is 0.023 1 kcal/m.h.c., the CO2 content is 5.2%, and it is brittle (JIA9512). Among chlorinated hydrocarbons, dichloromethane is the most commonly used, its chemical properties are relatively stable, and its gas production is also higher than that of chlorofluorocarbons. Therefore, in recent years, many manufacturers have used it to replace chlorofluorocarbons or both. In the plastic foaming industry, the mixture of low-boiling aliphatic alkanes G4-G7 such as n-pentane is used as foaming agent, but its effect is not ideal and it is flammable. Sometimes, the combination of several foaming agents can solve the problem of matching the vaporization temperature of foaming agent with the curing reaction speed of resin, so that when the foaming agent is vaporized, the resin has appropriate viscosity, which is beneficial to the formation and stability of cell structure.
Chemical foaming agents, such as foaming agent H(N, N- dinitrpentamethylene tetramine), are also used, which will decompose strongly in acid and release nitrogen, thus foaming the resin.
2.3 surfactant
The molecules of surfactant contain hydrophilic structure and hydrophobic structure, which have the functions of interface orientation and reducing the surface tension of liquid resin, so that the raw materials with great differences in hydrophilicity and hydrophobicity in foam plastics are emulsified into a uniform system, and all components are fully contacted, so that various reactions can be carried out more balanced. Although the dosage of surfactant is small, accounting for only 2-6% of resin, it has great influence on foaming process and product performance. It can ensure that all components are fully and evenly mixed in the foaming process, form a uniform microporous structure and a stable closed-cell rate, accelerate the reaction process, shorten the curing time, and have a great influence on the compressive strength and cell size of foam products.
Foam forming is usually divided into three stages. The first stage is to form a large number of uniform and fine bubble nuclei in the melt or liquid of the foaming matrix, and then expand into bubbles with the required bubble structure. Finally, the foamed plastic product is obtained by heating, curing and molding. The first stage of foaming is to prepare emulsion with foaming agent as dispersed phase and resin as continuous phase, and form a large number of foaming agent droplets (bubble cores) with uniform distribution and tiny particle size in the resin. If the foaming agent is dispersed into the resin only by high-speed stirring, the dispersion system is extremely unstable and easy to be destroyed. Surfactants can reduce the interfacial tension and make the dispersion system thermodynamically stable. At this time, the surfactant acts as an emulsifier or a foam homogenizer. When the curing agent is added to the emulsion of phenolic resin and foaming agent under high-speed stirring, phenolic foam molding enters the second stage. Under the action of curing agent, A-grade resin undergoes condensation reaction, which is transformed into B-grade resin and finally solidified into C-grade resin. At the same time, a large amount of reaction heat released by resin condensation vaporizes the foaming agent droplets, while the foaming material thickens and the volume increases rapidly, and the original emulsion has been transformed into foam. This kind of foam is unstable, and the formed bubbles can continue to expand, or may merge, collapse or rupture. Surfactants play a role in stabilizing phenolic foam before curing.
The compatibility of phenolic foam components is poor, so emulsifying properties should be considered more when selecting surfactants. Good emulsifying performance can improve the mixing uniformity of each component, help to form uniform and fine cell structure, speed up the reaction process and shorten the curing time. In addition, the surfactant must be stable to the strong acidity of the curing agent. Although there are many kinds of surfactants that can be used in phenolic foam plastics, nonionic surfactants have the best effect, such as ① fatty alcohol polyoxyethylene ether and polyoxypropylene ether; (2) alkylphenol polyoxyethylene ethers, such as adducts of nonylphenol and ethylene oxide; (3) Block copolymer of polysiloxane, polyoxyethylene and polyoxypropylene. These surfactants not only have good foam stability, but also have strong emulsifying effect.
In recent years, researchers have also used various surfactant mixtures to obtain foams with specific properties. For example, Ikeda Hiroshi et al. mixed surfactants with silicone and sodium dodecyl benzene sulfonate to make super absorbent foam.
Study on toughening of 3 foam
The structural weakness of phenolic resin is that phenolic hydroxyl and methylene are easily oxidized. Its foam has low elongation, brittleness, high hardness and bending resistance. This greatly limits the application of phenolic foam, so it is necessary to toughen the foam. The toughening of phenolic foam can be achieved by the following ways: ① adding external toughening agent into the system to achieve the purpose of blending and toughening; (2) The purpose of toughening is achieved through the chemical reaction between phenolic resole and toughening agent; ③ Using partially modified phenol with tough chain instead of phenol to synthesize resin.
3. 1 Add external toughening agent
This modification method requires that the resin and toughening system must have certain miscibility to improve their brittleness, toughness and compression resistance. The miscibility between organic compounds can be predicted according to the solubility parameter δ. The implementation of this modification method is generally carried out according to the following steps. Firstly, ordinary phenolic resole resin was synthesized, then modifier was added and dehydrated and foamed. There are three commonly used modifiers.
The first category is rubber elastomer modifier. Rubber toughened phenolic resin is modified by physical blending, but the elastomer usually has active end groups (such as carboxyl and hydroxyl) and double bonds, which can be grafted or block copolymerized with hydroxymethyl in phenolic resin to varying degrees. In the process of resin curing and foaming, these rubber elastomer segments can generally be separated from the matrix, physically forming an island-in-sea two-phase structure. The fracture toughness of rubber toughened thermosetting resin and foam is much higher than that of non-toughened resin and foam. Commonly used rubbers include nitrile rubber, styrene-butadiene rubber, natural rubber, carboxyl-terminated nitrile rubber and other rubbers containing active groups. The toughening effect is also related to the blending ratio. Too little rubber can't achieve the effect, but if the rubber content is high, it will affect the compatibility between heat resistance and phenolic rubber. Generally, the addition of rubber should be controlled between 5% and 20%.
The second category is thermoplastic resin. Polyvinyl alcohol and polyethylene glycol were used to modify phenolic foam. It is possible that hydroxyl groups in polyvinyl alcohol molecules react with hydroxymethyl groups in phenolic polycondensates to form graft copolymers. PVA modified phenolic resin can improve the compressive strength of foam. According to the literature, the compressive strength of foam is related to the amount of polyvinyl alcohol added. The addition of polyvinyl alcohol is too small, and the improvement of compressive strength is not obvious; Adding too much polyvinyl alcohol will cause the pan to stick and it is difficult to continue the reaction. The addition of polyvinyl alcohol is 1.5-3% of the weight of phenol.
Polyethylene glycol is also an effective toughening agent for phenolic resin. OH in polyethylene glycol may combine with OH in resin, but it is difficult to react under alkaline conditions. One OH in the polyethylene glycol and one OH in the resin may also form partial hydrogen bonds, so that long flexible ether chains are introduced into the resin, thus playing a toughening role. Ge Dongbiao et al. toughened the foam with polyethylene glycol series with different molecular weights, and found that the modification effect increased with the increase of polyethylene glycol molecular weight, reaching the peak when the molecular weight was 1000, and then decreased with the increase of polyethylene glycol molecular weight. The conclusions are as follows: firstly, with the increase of molecular weight, the flexible chain of polyether introduced with phenolic resin becomes longer, which is beneficial to the improvement of tensile strength and elongation at break; However, when the molecular weight of polyethylene glycol is greater than 1000, because the added polyethylene glycol has a certain mass, the proportion of hydroxyl groups at both ends of the molecular chain is relatively reduced, which reduces the probability of the reaction between hydroxyl groups and hydroxymethyl groups of phenolic resin and affects the modification effect of polyethylene glycol. The foams modified by medium molecular weight polyethylene glycol 1000 and 800 have the best toughness.
Compared with pure phenolic foam, the phenolic foam toughened by polyethylene glycol not only has good dimensional stability, high compressive strength and moderate apparent density, but also has high closed cell rate, uniform and compact size, easy processing and cutting, and no or less debris in cross section. In addition, chlorinated polyethylene (CPE) and polyvinyl chloride (PVC) toughened resins and foams are also reported.
The third category is small molecular substances such as ethylene glycol. The preparation method of ethylene glycol toughened foam is to synthesize phenolic resin, add ethylene glycol according to a certain proportion, mix evenly, add stabilizer, foaming agent and foam homogenizer in turn, stir evenly, then add curing agent, stir vigorously, quickly pour into the prepared mold for block foaming, and demould after complete curing.
According to the difference of infrared spectra between pure phenolic foam and ethylene glycol modified phenolic foam (ethylene glycol content is 65438 05% of phenol), it is speculated that ethylene glycol may partially or completely generate glycerol alcohol derivatives under acid catalysis and participate in the main reaction. The addition of ethylene glycol can improve the properties of phenolic foam, improve its compressive strength and brittleness to a certain extent, without losing its flame retardancy too much. The optimum dosage is 10- 15 parts/100 parts of resin. At this time, the oxygen index is 37-38, the compressive strength is 0.40MPa and the density is 0.059 g/cm3, as shown in Table 1.
Table 1 foam characteristics and ethylene glycol addition
British and French troops
Ethylene glycol/weight percentage 25 20 15 10 5 0
Density/g cm-3 0.064 0.060 0.059 0.058 0.056 0.062
Compressive strength/MPa +0
Oxygen index 35 37 37 38 40
Adding chopped glass fiber is also an external toughening method. Chopped glass fiber is an inorganic material, colorless, odorless and nontoxic at room temperature, and easily mixed with phenolic resin. After being treated by coupling agent, chopped glass fiber was blended with phenolic resin and foamed into phenolic foam. The influence of chopped glass fiber content on the main properties of modified phenolic foam is shown in Table 2.
Table 2 chopped fiber content and phenolic foam properties
Chopped glass fiber/weight% 0 3 4 5 6 8 10
Bulk density/kg cm -3 60 60 60 60 62 68 80
Brittle mass loss/%40.0 28.0 25.0 22.021.017.715.0
Oxygen index 45 45 46 48 48 50 50
Compressive strength/MPa 0.20 0.25 0.26 0.28 0.310.39 0.43
It can be seen from Table 2 that with the increase of chopped glass fiber content, the compressive strength, bulk density, brittleness and oxygen index of phenolic foam plastics are obviously improved, but the viscosity of the blend increases with the increase of chopped glass fiber content, which makes the foaming process difficult to control, so the chopped glass fiber content is generally controlled below 65438 00%. The literature also reported that organic substances such as dioctyl phthalate and triphenyl phosphate were used to toughen foams.
3.2 Chemical Toughening of Phenolic Resin
The chemical toughening modification method is to add a modifier when synthesizing the first-grade resin, and graft the flexible chain through the chemical reaction of phenolic hydroxyl and hydroxymethyl to obtain the modified first-grade resin with internal toughening. This modification method has better effect than blending method.
Polyurethane modified phenolic foam is a good chemical toughening method, which has been studied in Japan and the United States and achieved good results. According to the method adopted, there are two ways: ① Furfuryl alcohol resin, aromatic amine polyol, etc. As polyhydroxy compounds in polyurethane components, phenolic resin, polyisocyanate (MDI, PAPI) and the above polyols are mixed, and foaming agents and other additives are added for compound foaming. (2) Polyether, polyester polyol and isocyanate synthesize a prepolymer with an NCO group at the end, and then mix it with phenolic resin and foaming additives for composite foaming.
In the preparation process of polyurethane modified phenolic foam, the reaction mechanism is the same no matter which modification method is adopted. There are two main reactions: ① the isocyanate group in the component and the hydroxyl group of the polyhydroxy compound cross-link or extend the chain; (2) The isocyanate group and hydroxymethyl group in phenolic resole resin are cross-linked. As a result of two reactions, flexible segments were introduced into the rigid molecular structure of phenolic resin, which fundamentally changed the rigid molecular structure of phenolic resin, thus improving the toughness and reducing the brittleness of foam products. At the same time, the characteristics of polyurethane are introduced, such as improving closed cell ratio, reducing water absorption, accelerating curing reaction speed and improving product strength.
Phenolic foam modified by prepolymer with NCO group was synthesized by the reaction of TDI with polyethylene glycol with molecular weight of 1000. Its properties are shown in Table 3.
Table 3 Properties of TDI Modified Polyethylene Glycol Toughened Phenolic Foam
Compressive strength /MPa density /kg cm-3 water absorption/%oxygen index
0.288 0. 177 1 14.39 38.3
3.3 Substitute partially modified phenol with tough chain for phenol synthetic resin.
The third modification method is to use a tough substance containing functional groups similar to phenol, and partially replace phenol with formaldehyde to achieve the purpose of toughening. According to the literature, it was modified by resorcinol, o-cresol, p-cresol and hydroquinone. The addition of 0.2- 10% can reduce the brittleness of foam and improve the strength and toughness of products. Modification of alkylphenol and cashew nut shell oil has also been reported. The main structure of cashew nut shell oil is a long chain of mono-or diolefins with meta-phenol 15 carbon, so cashew nut shell oil has both the characteristics of phenolic compounds and the flexibility of aliphatic compounds. After modification with it, the toughness of phenolic foam is obviously improved.
There are also attempts to modify phenol with tung oil and linseed oil. Conjugated trienes in tung oil react with phenol under the catalysis of acid, and there is little chance that the remaining double bonds will participate in the reaction due to steric hindrance. The reaction product further reacts with formaldehyde under the catalysis of alkali to produce tung oil modified phenolic resole resin. Linseed oil is glyceride of octadecatrienoic acid, and its molecular structure has three double bonds. Under the action of catalyst, ortho-and para-carbon atoms of phenol are alkylated on the double bond of linseed oil to synthesize modified phenol, and then the modified phenol is copolymerized with formaldehyde, and the flexible alkyl chain connects the fragile phenol molecular chain, which effectively improves the brittleness of phenolic foam. Tung oil modified phenol is shown in the figure.
4 conclusion
In recent years, a lot of research work has been done on the raw materials, foaming technology and technological process of phenolic foam at home and abroad. The preparation technology of foam is becoming more and more perfect, and it has entered the stage of industrial production. With the improvement of people's requirements for fire-retardant materials, the deepening of foam modification research and the continuous improvement of foam toughness, the application of phenolic foam plastics will be more extensive.