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Furan Resins
7 Furan Resins Furan resins are condensation products of furfuryl alcohol (FA). The resins are derived from vegetable cellulose, a renewable resource [1]. Furans as constituents of polymers have been reviewed [2].
7.1 History In Latin, furfur means bran. Furfural was first isolated in 1832 (or 1821) by Döbereiner,1 as a by-product of the synthesis of formic acid. In 1840 the ability of furfural to form resins was discovered by Stenhous [3]. The industrial production of furfural started in 1922, and 1 year later the first furan-based resins emerged. Early patents on furan resins include that of Claessen [4] and one for synthetic resins (actually mixed phenol furan resins) suitable for use in molding gramophone records [5].
Table 7.1 Monomers for Furan Resins [6] Furan Compound
Remarks or Reference
Furan Furfural Furfuryl alcohol 5-Hydroxymethylfurfural (HMF) 5-Methylfurfural 2-Furfurylmethacrylate Bis-2,5-hydroxymethylfuran 2,5-Furandicarboxylic acid
OH
OH
O
OH OH
OH
[8] Glass fiber binder
OH
OH
OH
7.2 Monomers Monomers suitable for furan resins are listed in Table 7.1. One of the chief advantages of furan resins stems from the fact that they are derived from vegetable cellulose. Suitable sources of vegetable cellulose are corn cobs, sugarcane bagasse, oat hulls, paper mill by-products, biomass refinery eluents, cottonseed hulls, rice hulls, and foodstuffs such as saccharides and starch [6]. Pentoses hydrolyze to furfural and hexoses give 5-hydroxymethylfurfural on acid digestion [7].
7.2.1 Furfural Furfural is a by-product from sugarcane bagasse, which produces resins with an excellent chemical stability and low swelling. 2-Furan formaldehyde or furfural is made from agricultural materials by means of hydrolysis. The mechanism of formation of furfural is shown in Figure 7.1. It is a light yellow to amber colored 1 Johann Wolfgang Döbereiner, born in Hof an der Saale 1780, in
Germany, died in Jena 1849.
OH
HO
O
C
H
O
Figure 7.1 Mechanism of the formation of furfural.
transparent liquid. Its color gradually deepens to brown during storage. It tastes like apricot kernel. It is mainly used in lubricant refinement, FA production, and pharmaceutical production. Furfural is the chief reagent used to produce materials such as FA, HMF, bis(hydroxymethyl)furan, and 2,5-dicarboxyaldehyde-furan. The furan-containing monomers in turn can undergo reactions to produce various furan-containing monomers with a wide variety of substituents, as shown in Table 7.1.
7.2.2 Furfuryl Alcohol FA is made from furfural by reduction with hydrogen. It is a colorless transparent liquid and becomes brown, light yellow, or deep red when exposed in the air. It can be mixed with water and many organic solvents such as alcohol, ether, acetone, etc., but not in hydrocarbon products.
Fink: Reactive Polymers Fundamentals and Applications. http://dx.doi.org/10.1016/B978-1-4557-3149-7.00007-3 © 2013 Elsevier Inc. All rights reserved.
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7.2.3 Specialities 7.2.3.1 Furan-based Polyimides
APPLICATIONS
+
CH2
O
Polyimides based on poly (2-furanmethanol-formaldehyde) can be prepared by a Diels-Alder reaction (DA) of the respective furan resin with bismaleimides [9]. The Diels-Alder reaction proceeds in tetrahydrofuran or in bulk. The tetrahydrophthalimide intermediates aromatize in the presence of acetic anhydride. Polyimides based on the furan resin exhibit good thermal stability.
H
OH
+
O
CH2
O
CH2
O
CH2
O
7.2.4 Synthesis Furan-based monomers can polymerize through two well-known mechanisms. The first involves chain or polyaddition polymerization, which is initiated by free-radical, cationic, or anionic promoters. Polymerization produces macromolecules with furan rings pendant on the main chain. The second method is a polycondensation, also referred to as polymerization condensation. Polymers and copolymers resulting from acid-catalyzed condensation reactions result in macromolecules with furan rings in the main chain [6]. As a general rule, the furan resins formed by polycondensation reactions have stiffer chains and higher glass transition temperatures. These reactions may involve self-condensation of the furan monomers described above, as well as condensation reactions of such monomers with aminoplast resins, organic anhydrides, and aldehydes such as formaldehyde, ketones, urea, phenol, and other suitable reagents. Most common, furan resins are produced by acid-catalyzed condensation reactions. The condensation results in linear oligomers, the furan rings being linked with methylene and methylene-ether bridges, cf. Figure 7.2. The synthesis of furan resins proceeds in a pH range of 3–5, at a temperature range of 80–100 ◦ C. The condensation is stopped, when a desired viscosity value is reached, by neutralizing the liquid resin. FA can also be condensed with formaldehyde to obtain furan-formaldehyde resins. The content of free formaldehyde can be lowered by the addition of urea at the late stages of synthesis. The condensation of monomers from renewable resources, 2-furfural, vanillin, and 4-hydroxyacetophenone, at 80 ◦ C in the presence of potassium hydroxide gives an amorphous polymer resin with a yield of 85% [10]. The reaction is shown in Figure 7.3.
AND
CH2
CH2
O
+
OH
OH
Figure 7.2 Acid-catalyzed self-condensation of furfuryl alcohol. H
H3C
C
H3 C
O CHO
C
O
O O OH
OH
H
C
H3 C O
O H C 3
C
O
C OH
OH O
Figure 7.3 Terpolymer from 2-furfural, vanillin, and 4-hydroxyacetophenone [10].
A polydispersity index of the terpolymer of 1.52 was found. The terpolymer strongly inhibits the growth of a wide variety of microorganisms, including Gram-positive bacteria, Gram-negative bacteria, and fungi [10]. Upon pyrolysis, the major product is 4-hydroxyacetophenone.
7.3 Special Additives
7.3.1 Reinforcing Materials 7.3.1.1 Fibers Aramid fibers were used as reinforcing material for a phenol resin and a furan resin. A comparative study of
7: FURAN RESINS
the mechanical performance of the materials showed that the furan resin is more suitable as a matrix than the phenol resin [11]. Due to an emerging awareness of environmental issues a growing interest in biocomposites has been developed. Fully bio-based thermoset composites with aligned flax fiber textiles as reinforcement and a furan resin as the matrix resin were produced. After precuring, the prepregs were consolidated by compression molding [12]. Reinforcing fibers from a poly(phenylene sulfide) are suitable for epoxy and furan resins as binder materials [13]. The composite material is stable in 20% sodium hydroxide solutions at 60 ◦ C. Possible suggested applications are containers and pipes used in the transportation of alkaline fluids.
7.3.1.2 Nanomaterials In order to reduce the formaldehyde emission, furan was selected as an environmentally friendly and safe alternative to formaldehyde that is needed in phenolic resins. It is expected that the addition of organically modified and unmodified montmorillonite nanoparticles to the furan resin enhances its performance for use in metal coatings. Actually, viscosity measurements of the composites revealed that the addition of the nanoreinforcement led to a higher curing rate [14].
7.4 Curing Materials known to be suitable for curing furan resins include inorganic and organic acids. Examples of suitable organic and inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, tartaric acid, and maleic acid. Friedel-Crafts catalysts include aluminum trichloride, zinc chloride, aluminum bromide, and boron fluoride. Resins with improved fire resistance are cured with a mixture of trimethylborate, boric anhydride, and p-toluenesulfonic acid [15]. Salts of both inorganic and organic acids may also be used. Ammonium sulfate is preferred. Ammonium sulfate is a latent catalyst which may become active at approximately 110–150 ◦ C. Suitable organic salts are the urea salt of toluenesulfonic acid, the polyammonium salts of polycarboxylic acids such as the diammonium salts of citric acid, and the ammonium
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salts of maleic acid. Cyclic anhydrides such as maleic anhydride are also suitable for use as catalysts. It is believed that polyester copolymers are formed between the anhydride and the free hydroxylated species present in the resin. Maleic acid promotes the polymerization reaction. Furthermore, it is believed that maleic acid may preferentially reduce the emission of bis(hydroxymethyl)furan monomer during the curing process. A significant reduction of volatile organic compounds will use a catalyst system comprised of maleic acid and ammonium sulfate [6]. The curing mechanism of furfuryl alcohol and urea/formaldehyde furan resins was investigated with infrared spectroscopy (IR) [16]. The resins were modified with different agents, such as sorbitol, polyester polyol, phenol, and acetone. The influence of these compounds on the curing behavior was investigated. Except for the polyester polyol, the other modifiers have little effect on the thermal strength of the urea/formaldehyde furan resin. The curing kinetics of a modified furan resin therefrom was characterized by differential scanning calorimetry [17]. In particular, clay nanocomposites and phenol were used for modification. Free kinetic models were applied to get the activation energy of each process, the models according to Vyazovkin [18], Friedman [19], Ortega [20], and Ozawa [21]. The Vyazovkin numerical analysis was found to be the most accurate method. In the case of furan-based nanocomposites an additional peak in the end stage of curing was observed.
7.4.1 Acidic Curing The resin can be crosslinked using an acidic catalyst. The reaction is not sensitive to air. The main route of curing is an additional condensation reaction at the free α-hydrogen of furan rings. These positions are connected by methylene bridges.
7.4.2 Oxidative Curing The oxidative crosslinking of FA polycondensates proceeds at temperatures of 100–200 ◦ C. Structures with tertiary carbon atoms, as shown in Figure 7.4, could be identified.
7.4.3 Ultrasonic Curing Ultrasonic treatment, i.e., sonication during the curing process of a furan resin, showed changes of the curing performance. p-Toluenesulfonic acid was added
REACTIVE POLYMERS FUNDAMENTALS
206
O O
CH CH2 CH
O O
CH2 CH2
Figure 7.4 Crosslinked methylene bridges.
as curing catalyst in the proportion of 0.3%. Fine carbons were also incorporated. Using an ultrasonic homogenizer in the presence of carbonaceous fine particles showed an increased curing rate of the furan resin. This, in turn, increased the polymerization degree with an increase in ultrasound intensity. An increase of curing rate was also observed with small additions of carbonaceous fine particles. In this case, the curing accelerated with an increase in the specific surface area of the additives [22]. The increase of curing rate is believed to result from cavitation. The curing reaction proceeds slowly in the absence of cavitation and simple stirring fails to produce such a marked increase in the rate of reaction. The curing is accelerated by heat, oxygen, and the addition of phenol and urea.
7.5 Properties
7.5.1 Recycling Research has been conducted to introduce pendant furan groups into polymers such as poly(styrene) via copolymerization with a suitable comonomer. The pendant furan moieties can be crosslinked with a bismaleimide to produce polymers with better performance. In order to recycle these crosslinked materials, heating experiments with an excess of 2-methylfuran were performed in order to induce the retro DielsAlder reaction and break up the network. The reaction proceeded in this manner and the original copolymers could be recovered from the treatment. Therefore, the introduction of furan units is a potential path of recycling crosslinked polymers by thermal treatment with a diene in excess [23]. The DA between styrene-furfuryl methacrylate copolymer samples and bismaleimide can be monitored by the ultraviolet absorbance of the maleimide group at 320 nm or by 13 C NMR spectroscopy [8].
7.6 Applications and Uses Furan resins are used mainly in the foundry industry, as sand binders for casting molds and cores. They are
AND
APPLICATIONS
often used in combination with other resins. They are highly corrosion resistant. Therefore, they have found use in mortars and in cements. Improved mechanical properties are implemented by reinforcing with glass fibers.
7.6.1 Carbons 7.6.1.1 Carbon and Graphite Formation Furan resins form a porous carbon by pyrolysis at 450 ◦ C. A characterization of furanic foams by MALDI-ToF before and after carbonization at 900 ◦ C has been done. The results revealed that some of the original structures remained intact and thus resist the process of carbonization. This means that some furanic oligomers are particularly stable and are not prone to carbonization [24]. The carbonization of rigid foams based on polyflavonoid, tannin, formaldehyde, and furfuryl alcohol results in a three-dimensional network in which polynuclear aromatic hydrocarbon chains with a high molecular weight are observed. Some furan moieties that survive pyrolysis are also covalently linked to these chains [25]. A monolithic honeycomb sorbent bed for removing mercury and other toxic metals from flue gas of a coal combustion system has been described [26]. The sorbent bed contains an activated carbon catalyst and a toxic metal adsorption co-catalyst bonded to the activated carbon catalyst. The sorbent bed is produced from a synthetic carbon precursor, e.g., a furan resin. Graphitization is the transformation of disordered carbon materials into three-dimensional graphite, which typically occurs at temperatures of 2500– 3000 ◦ C. However, it has been found that some carbon materials, such as those derived from the thermosetting resin, are non-graphitizable even after heating to temperatures above 3000 ◦ C. Increased pressure or catalysts can be used to accelerate the graphitization of such materials [27]. From furan resin-derived glass-like regions with carbon spherical graphite in the carbon matrix can be obtained by hot isotropic pressing at 200 MPa and 2500 ◦ C [28]. The spherical regions have a diameter of 20–150 µm. A micrograph of the carbon spherical graphite is shown in Figure 7.5. Several metal-based catalysts are efficient for the catalytic graphitization of carbon [29,30] obtained from organic polymers. However, the residual
7: FURAN RESINS
Figure 7.5 Micrograph of the carbon spherical graphite. Reprinted from [28] with permission from Elsevier.
catalysts can be harmful for further applications of the thus obtained graphite materials. Graphite oxide can be used to accelerate the graphitization of a non-graphitizing furan resin carbon. The use of this catalyst does not introduce metal-based catalysts and should not cause environmental harm [31]. Graphite oxide can be synthesized from natural graphite powder by treatment of graphite with an anhydrous mixture of sulfuric acid, sodium nitrate, and potassium permanganate [32]. Yttrium and praseodymium can be used as catalysts for the graphitization of a carbon obtained in a first step from a furan resin [27,33]. The extent of graphitization of the furan resin carbon can be followed by X-ray diffraction and Raman spectroscopy. The content of praseodymium and the temperature of pyrolysis are important factors in the catalytic graphitization. A significant catalytic graphitization was achieved at 2400 ◦ C and a content of praseodymium of 15%. The addition of multiwalled carbon nanotubes (MWCNTs) into a furan resin can induce an ordered arrangement of planar carbon microlites in the course of a high-temperature treatment [34]. Also, the degree of graphitization is enhanced. Graphite and carbonaceous materials are promising materials for lithium-ion batteries. Natural graphite exhibits a high specific capacity, but a violent irreversible capacity may occur after a few charge/discharge cycles. A carbon coating from the pyrolysis of a furan resin on natural graphite surface can enhance graphite surface structure and promote the electrochemical properties [35]. In addition a surface fluorination can improve the lithium-ion intercalation and deintercalation during charging and discharging cycles.
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Both methods of carbon coating and surface fluorination can increase the charge capacity. MWCNTs were used as filler in a furan resin to fabricate an electrically conducting polymer composite for electrode applications [36]. The orientation is preferably unidirectional, as this may result in a higher electric conductivity in a specific direction at a lower loading of the MWCNTs. The preparation by means of the doctor blade technique can induce a preferential alignment of the MWCNTs in the composite. The orientation is believed to create more junctions between the MWCNTs, which results in the formation of more conducting channels in the polymer matrix parallel to the blading direction [36]. Cross-sections through the composites are shown in Figure 7.6. Nanoshell carbon is a type of nanocarbon with a hollow, round, shell-like structure. The diameters are
Figure 7.6 Aligned multiwalled carbon nanotubes. Reprinted from [36] with permission from Elsevier.
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REACTIVE POLYMERS FUNDAMENTALS
20–50 nm. It can be used in elctrochemical applications, e.g., in polymer electrolyte fuel cells. Nanoshell carbons can be prepared by the carbonization of a furan resin in the presence of acetylacetonates and of phthalocyanines with Fe, Co, and Ni as catalysts [38,39].
7.6.1.2 Glass-like Carbon Glass-like carbon is identified as an excellent carbon artifact due to characteristics such as hardness and shape stability. The microstructure of glass-like carbon consists of a non-graphitic alignment of hexagonal sheets. It has unique properties such as great hardness compared with other carbon materials and impermeability for gases [40]. Glass-like carbon is of interest in the battery and semiconductor industries. Glass-like carbon is prepared by heat treatment on thermosetting resins in inert atmosphere. During the heat treatment of a furan resin, weight loss is very rapid up to 500 ◦ C, then continues gradually up to 1000 ◦ C, and then the weight stays almost constant above 1000 ◦ C. Scanning electron microscopy photographs of heat-treated glass-like carbon reveal a large increase of micro-grain size in the range of 60–105 nm when treated at 2000 ◦ C, cf. Figure 7.7. Up to 2500 ◦ C, the grain size decreases to 27–40 nm due to graphitization [37]. There is a structural correlation between the micro-texture of the furan resin and the glass-like carbon formed from the particular resin. The pore structure in glass-like carbon can be characterized using the small-angle X-ray scattering technique. The scattering intensities grow gradually with increasing heat treatment temperature
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APPLICATIONS
up to 1600–1800 ◦ C, and then the intensities increase abruptly at a temperature higher than 1800 ◦ C. The dependence of the structural change of a glasslike carbon from a furan resin is almost the same as that of a phenolic resin. However, it was found that the carbon prepared at 1200 ◦ C from furan resin shows the largest interlayer spacing in the carbon matrix and at the same time the smallest value of the gyration radius for the pores [40]. The surface oxidation and the corrosion behavior of carbon materials obtained from a furan resin in nitric acid, sulfuric acid, and hydrofluoric acid were investigated [41]. The carbons from the furan resins were both neat and Ta-alloyed carbons. The heat treatments occurred at 1200 ◦ C. The weights of the neat specimens treated at 1200 ◦ C decreased after the treatment with the acids. Spectroscopic analysis proved that the oxygen in the internal area as well as on the external surface was increased. The Ta-alloyed specimens treated at 1200 ◦ C have smaller amounts of oxygen after treatment with nitric acid and sulfuric acid. It was concluded that the use of Ta prevents the oxygen from penetrating into the depths of the specimens [41].
7.6.1.3 Wood Ceramics Wood ceramics are carbon-carbon composites from wood charcoal and glassy carbon. A wood ceramic has been developed from wood powder with a furan resin as binder [42]. The content of the furan resin has a significant effect on the microstructure of the wood ceramic. With a higher content of resin, generated glassy carbon increases and the connection between wood charcoal and the glassy carbon is strengthened.
7.6.2 Chromatography Support
Figure 7.7 SEM photographs of glass-like carbon derived from furan resin. (a) 300 and (b) 600 ◦ C. Reprinted from [37] with permission from Elsevier.
Conventionally used packing materials for liquid chromatography are a chemically bonded type of packing materialbasedonsilicagelandapackingmaterialbased on synthetic resin. The silica gel-based packing material is relatively strong in mechanical properties and in its swelling or shrinking characteristics against various organicsolvents.Therefore,ithasahighresolvingpower and is superior in exchangeability of eluent for analysis. However, the silica gel-based packing material has problems in that the silica gel dissolves under acidic or alkaline conditions and the solubility of the silica gel in an aqueous solution increases when warmed, resulting in durability problems.
7: FURAN RESINS
The packing material of synthetic resin, on the other hand, is known to be high in acid and alkali resistivity and has good chemical durability as a packing material. However, since the mechanical strength of the particles is small, it has been difficult to convert them into finer particles. Raw materials which are highly chemically stable and exhibit high mechanical strength are graphitized carbon black. A packing material for liquid chromatography is produced by mixing carbon black, a synthetic resin which can be graphitized, and pitches. Suitable synthetic resins are phenolic resins, furan resins, furfural resins, divinylbenzene resins, or urea resins [43]. The pitches can be petroleum pitches, coal-tar pitches, and liquefied coal oil. The mixture is granulated and heated up to 3000 ◦ C in an inert atmosphere.
7.6.3 Composite Carbon Fiber Materials Impregnation of carbon fibers and subsequent pyrolysis at 1000 ◦ C improves strength of carbon fibers [44]. A yarn is passed through a bath containing a carbonizable resin precursor, such as a partially polymerized FA. It is advantageous to add a latent catalyst along with the precursor. Suitable catalysts are a complex of boron trifluoride and ethylamine or maleic anhydride. The use of a latent catalyst allows the application of a low-viscosity solution to the fiber with subsequent polymerization at the elevated temperatures. If the precursor were to polymerize significantly prior to application, the treating bath would be so viscous that it would allow only a coating to be formed. For high-performance composite carbon fiber-reinforced carbonaceous material which is compositely reinforced with carbon fibers, prepregs of woven fabrics of carbon fibers are impregnated with a resin such as phenol resin, furan resin, epoxy resin, urea resin, etc. They then are laminated as a matrix and molded under heat and pressure, and after carbonization they are further graphitized by heating to a temperature of 3000 ◦ C [45].
7.6.4 Foundry Binders Furans are somewhat more expensive than other binders, but the possibility of sand reclamation is advantageous. One of the most commercially successful no-bake binders is the phenolic urethane no-bake binder. This binder provides molds and cores with
209
excellent strengths that are produced in a highly productive manner. Furan-based binders have less VOC, free phenol level, low formaldehyde, and produce less odor and smoke during core making and castings. However, the curing performance of furan binders is much slower than the curing performance of phenolic urethane nobake binders. Furan binders can be modified to increase their reactivity, for instance by formulating with urea/ formaldehyde resins, phenol/formaldehyde resins, novolak resins, phenolic resol resins, and resorcinol. Nevertheless, these modified furan binders do not provide the cure speed needed in foundries that require high productivity. Therefore, an activator, which promotes the polymerization of FA, is added. Resorcinol pitch is used for this purpose [46]. Further components in such a formulation are polyester polyols or polyether polyols, and a silane, such as (3-aminopropyl)triethoxysilane. The curing process of urea-modified furan resins in sands has been investigated by IR [47]. A composition for binder resins has been described that contains both a furan resin, furfuryl alcohol, and oligomers from bis-hydroxymethyl furan [48]. These binders are particularly useful in warm-box applications. The advantages of using these binders over conventional heat-cured furan binders are [48]: 1. the curing rate of the binders is much faster than that of conventional heat curable furan binders, and 2. the hot and cold tensile strengths of cores prepared from these compositions are higher earlier on than the cores prepared with conventional heat curable furan binders. Spent foundry furan sands have a turbidity that is almost 15 times higher than that of other sands such as a CO2 sand or green sand. The turbidity of a furan sand can be reduced by stabilization of the furan resin by a thermal treatment [49]. The performance and the mechanism of no-bake sands with different binder resins have been compared [50]. The binders were specifically a furan resin, ester-sodium silicate sand, CO2 -sodium silicate, and phosphate. It turned out that the furan resin sand and the estersodium silicate sand have a higher dry strength and a
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REACTIVE POLYMERS FUNDAMENTALS
better humidity resistance. This is explained by their fracture modes which are cohesive fracture and plastic fracture. The fracture modes of the phosphate sand and the CO2 -sodium silicate sand are mainly cohesive with a few adhesive and brittle fracture modes, but there is more cracking on their bond membrane. Phosphate sand has higher dry strength and better humidity resistance than CO2 -sodium silicate sand, lower cost and less pollution than a furan resin sand. As an alternative adhesive for sand cores in foundry applications instead of a furan resin, a solid watersoluble modified starch, carboxymethyl starch, was tested. The modified starch was used as a binder for sand in shell-core applications. Using these compositions, high-quality iron castings with smooth inner cavties can be produced. Thus, the carboxymethyl starch-bonded shell-cores can be used as a replacement for the conventional furan resin-bonded sand cores [51].
7.6.5 Glass Fiber Binders An alternative to phenol/formaldehyde-based fiberglass binders is furan-based binders. Furan binders provide many of the advantages of phenolic binders while resulting in substantially reduced VOC emissions. Water as a significant component can be used. Formaldehyde is not a significant curing or decomposition by-product, and the furan resins form very rigid thermosets. Emulsified furan resins can be used. Emulsified furan-based glass fiber binding compositions are advantageous since they allow the use of furan resins that have high molecular weights or the addition of other materials which give rise to the formation of two-phase systems [6]. A suitable surfactant to be added to the furan binder compositions is sodium dodecylbenzene sulfonate. It may be added in an amount from 0.05% to 1.0%.
7.6.6 Aluminum Electrolysis Most aluminum reduction cells in commercial use employ prefabricated carbon blocks as the cell lining and as the cathodic working surface. These blocks are formed into a liquid-tight container surfaced by filling the joints between the blocks with a ramming paste. The efficiency of sealing of the ramming paste is an important factor in determining the life and energy efficiency of a reduction cell, which depends to a great
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APPLICATIONS
degree on the extent and rate of electrolytic penetration into the cell bottom [52]. An ecofriendly cold ramming paste for an aluminum electrolysis was synthesized from electrocalcined anthracite, with artificial graphite as an aggregate and a furan resin as the binder material [53]. The synthesized paste is an ecofriendly material and it has also some superior properties such as low electrical resistivity, high compressive strength, appropriate sodium penetration, and thermal expansion.
7.6.7 Panels and Fiberboards The properties of fiberboards made from wood fiber with ammonium lignosulfonate and urea as filler resins have been tested according to standardized procedures [54]. The specifications of EN 622-2 [55] can be met with these composite types. This is of interest because the composites are free from formaldehyde. Further, the properties are better than those obtained from conventional filling resins. Panels made from natural fibers, e.g., recycling paper and thermosetting biopolymers, have been described [56]. The base materials for the binder resins are 2,5-bis(hydroxymethyl)-furan, 2,3,5-tris (hydroxymethyl)furan, or 2,2 -hydroxymethyldifurylmethan. Curing of the composites can be done with acid catalysis at temperatures of 120–160 ◦ C.
7.6.8 Oil Field Applications Wells in sandy, oil-bearing formations are frequently difficult to operate because the sand in the formation is poorly consolidated and tends to flow into the well with the oil. Sand production is a serious problem because the sand causes erosion and premature wearing out of the pumping equipment. It is a nuisance to remove from the oil at a later point in the operation. Furan resin formulations can be used for in situ chemical sand consolidation [57]. Besides furan resins other thermosetting resins can also be used; however, the choice of the resin type depends on the bottom hole static temperature of the well. In the range of 150–315 ◦ C a furan resin is most suitable for controlling the particle migration [58]. In hydraulic fracturing treatments, a viscous fracturing fluid is pumped into a production zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Particulate solids, commonly referred to as the proppant, are suspended in a portion of the fracturing fluid so that the proppant is deposited in the fractures.
7: FURAN RESINS
211
O
Table 7.2 Efficiency of Cleaning [60] Components
Coated Metal Plate for 1 h at 100 ◦ C (% by volume)
Ammonium diacetate 5 5 0 0 Sodium hydroxide 0 0 5 5 Water 70 70 70 70 Ethylene glycol 25 25 25 25 monobutyl ether Cleanup (%) Epoxy Residue Furan Residue Percent cleanup 98 97 94 90
H 3C
O
O
7.6.9 Photosensitive Polymer Electrolytes Both conjugated furan chromophores and polyethers can be grafted onto chitosan to result in a photosensitive polymer electrolyte. The furan
H
Figure 7.8 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde.
OH
OH O
O
O
O NH
NH
The proppant deposited in the fractures functions to prevent the fractures from fully closing so that conductive channels are formed through which produced hydrocarbons may flow. Some portion of the proppant particulates may crush under the pressure of the formation and create unconsolidated particulates. For these reasons, the proppant is often coated with resins to facilitate the consolidation. In the same way as already described above, furan resin coating may be used to enhance the consolidation process [59]. One disadvantage associated with the use of resins is the removal and cleanup of the resin from the equipment. For example, a residue of resin may remain in fracturing equipment used during fracturing operations, e.g., connecting hoses, valves, sand hoppers, sand screws, or blender tubs. Without proper cleaning or removal, there is potential that a layer of resin may build up each time the equipment surface is exposed to the resin. This build-up of resin coat could result in plugging of equipment or cause the equipment not to function properly. Moreover, particulates, such as sand or proppant, may become entrapped within the resin coat and lead to further equipment damage. Residues from furan resin coating can be removed from a surface, e.g., valves, blender tubs, or hoses, by applying to the surface a cleaning solution, which is an acid or a base for adjusting the pH, followed by a solvent, e.g., dipropylene glycol monomethyl ether [60]. The efficiency of cleaning is shown in Table 7.2. As can be seen from Table 7.2, the cleanup is somewhat better for an epoxy resin in comparison to a furan resin.
C
CH 2
CH 3
O
O
CH 2 O
CH 3
O
O
CH 3
CH 2
O
O
CH 3
O CH 2
NH
NH
O
O OH
O
O OH
Figure 7.9 Photo crosslinking of the furylene vinylene units grafted on chitosan.
chromophore consists of conjugated furan chromophores of 5-[2-(5-methyl furylene vinylene)]furancarboxyaldehyde [61], cf. Figure 7.8. The graft polymer can be photo crosslinked. The photochemical reaction consists of a π 2 + π 2 cycloaddition reaction of the vinylene double bonds of the furan moiety so that two pendant vinylene groups form a four-membered ring. The crosslinking reaction is shown in Figure 7.9.
7.6.10 Plant Growth Substrates Conventional mineral wool plant growth substrates are based on a coherent matrix of mineral wool of which the fibers are mutually connected by a cured binder. There is a need to reduce the phytotoxicity of the chemicals used. The phytotoxicity may result from the phenolic binder materials. If a phenolic resin is used as binder, a wetting agent must be added in order to impart the hydrophobic mineral wool matrix with hydrophilic properties. However, the use of a furan resin allows the abandonment of the use of a wetting agent.
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A disadvantage of the use of a furan resin is its comparatively high price. Therefore, a traditional phenol/ formaldehyde resin substituted only partly by a furan resin is sufficient to maintain or to achieve the desired properties [62,63].
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[22] K. Hoshi, T. Akatsu, Y. Tanabe, E. Yasuda, Curing properties of furfuryl alcohol condensate with carbonaceous fine particles under ultrasonication, Ultrason. Sonochem. 8 (2) (2001) 89–92. [23] C. Gousse, A. Gandini, P. Hodge, Application of the Diels-Alder reaction to polymers bearing furan moieties. 2. Diels-Alder and retroDiels-Alder reactions involving furan rings in some styrene copolymers, Macromolecules 31 (2) (1998) 314–321. [24] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, K. Rode, MALDI-TOF investigation of furanic polymer foams before and after carbonization: Aromatic rearrangement and surviving furanic structures, Eur. Polym. J. 44 (9) (2008) 2938–2943. [25] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, Structure degradation, conservation and rearrangement in the carbonisation of polyflavonoid tannin/furanic rigid foams—a MALDI-TOF investigation, Polym. Degrad. Stabil. 93 (5) (2008) 968–975. [26] K.P. Gadkaree, L. He, Y. Shi, Activated carbon honeycomb catalyst beds and methods for the use thereof, US Patent 7 722 705, Assigned to Corning Incorporated, Corning, NY, May 25, 2010. [27] S. Yi, Z. Fan, C. Wu, J. Chen, Catalytic graphitization of furan resin carbon by yttrium, Carbon 46 (2) (2008) 378–380. [28] Y. Teranishi, E. Yasuda, T. Maeda, T. Nishizawa, T. Kobayashi, M. Fukushima K. Nakamura, Y. Tanabe, Interior graphitization of furan resinderived carbon by hot isostatic pressing, Mater. Sci. Eng. B – Solid-State Mater. Adv. Technol. 148 (1–3) (2008) 270–272. [29] S.-S. Tzeng, Catalytic graphitization of electroless Ni-P coated PAN-based carbon fibers, Carbon 44 (10) (2006) 1986–1993. [30] S. Yi, C. Wu, Z. Fan, Y. Kuang, J. Chen, Catalytic graphitization of PAN-based carbon fibers by spontaneously deposited manganese oxides, Transit. Metal Chem. 34 (5) (2009) 559–563. [31] S. Yi, J. Chen, H. Li, L. Liu, X. Xiao, X. Zhang, Effect of graphite oxide on graphitization of furan resin carbon, Carbon 48 (3) (2010) 926–928.
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[32] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339–1339. [33] S. Yi, J. Chen, X. Xiao, L. Liu, Z. Fan, Effect of praseodymium on catalytic graphitization of furan resin carbon, J. Rare Earth. 28 (1) (2010) 69–71. [34] J. Chen, X. Xiong, P. Xiao, The effect of MWNTs on the microstructure of resin carbon and thermal conductivity of C/C composites, Solid State Sci. 11 (11) (2009) 1890–1893. [35] Y.-S. Wu, Y.-H. Lee, Z.-W. Yang, Z.-Z. Guo, H.-C. Wu, Influences of surface fluorination and carbon coating with furan resin in natural graphite as anode in lithium-ion batteries, J. Phys. Chem. Solids 69 (2–3) (2008) 376–382. [36] L.J. Lanticse, Y. Tanabe, K. Matsui, Y. Kaburagi, K. Suda, M. Hoteida, M. Endo, E. Yasuda, Shear-induced preferential alignment of carbon nanotubes resulted in anisotropic electrical conductivity of polymer composites, Carbon 44 (14) (2006) 3078–3086. [37] Y. Korai, K. Sakamoto, I. Mochida, O. Hirai, Structural correlation between micro-texture of furan resin and its derived glass-like carbon, Carbon 42 (1) (2004) 221–223. [38] J.-I. Ozaki, S.-I. Tanifuji, N. Kimura, A. Furuichi, A. Oya, Enhancement of oxygen reduction activity by carbonization of furan resin in the presence of phthalocyanines, Carbon 44 (7) (2006) 1324–1326. [39] J.-I. Ozaki, S.-I. Tanifuji, A. Furuichi, K. Yabutsuka, Enhancement of oxygen reduction activity of nanoshell carbons by introducing nitrogen atoms from metal phthalocyanines, Electrochim. Acta 55 (6) (2010) 1864–1871. [40] K. Fukuyama, T. Nishizawa, K. Nishikawa, Investigation of the pore structure in glass-like carbon prepared from furan resin, Carbon 39 (13) (2001) 2017–2021. [41] K. Nakamura, Y. Tanabe, E. Yasuda, Analysis of the oxidation behavior of neat and Ta-alloyed glass-like carbons heat-treated at 1200 and 3000 ◦ C by nitric, sulfuric and hydrofluoric acid, J. Alloys Compd. 414 (1–2) (2006) 186–189. [42] W. Zhou, Y. Yu, X. Xiong, Basic properties of wood-ceramics made from furan resin/wood
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powder composite, Adv. Mater. Res. (Zurich, Switzerland) 168–170 (3) (2011) 2602–2605. H. Ichikawa, A. Yokoyama, T. Kawai, H. Moriyama, K. Komiya, Y. Kato, Packing material for liquid chromatography and method of manufacturing thereof, US Patent 5 270 280, Assigned to Nippon Carbon Co. Ltd., Tokyo, JP, Tosoh Corporation, Yamaguchi, JP, December 14, 1993. M. Katz, Improvement of carbon fiber strength, EP Patent 0 251 596, Assigned to Du Pont, January 7, 1988. T. Kawakubo, E. Oota, Process for preparation of carbon fiber composite reinforced carbonaceous material, US Patent 5 096 519, Assigned to Mitsubishi Pencil Co. Ltd., JP, March 17, 1992. K.K. Chang, Furan no-bake foundry binders and their use, US Patent 6 593 397, Assigned to Ashland Inc., Dublin, OH, July 15, 2003. M. Bilska, M. Holtzer, Application of Fourier transform infrared spectroscopy (FTIR) to investigation of moulding sands with furan resins hardening process, Arch. Metall. 48 (2) (2003) 233–242. K.K. Chang, Heat-cured furan binder system, US Patent 7 125 914, Assigned to Ashland Licensing and Intellectual Property LLC, Dublin, OH, October 24, 2006. S.-W. Rhee, A study on thermal stabilization of spent foundry sand, Materials Science Forum 544–545, Eco-Materials Processing and Design VIII, 2007, pp. 507–510. L. Xia, J. Huang, Y. Zhang, Performance comparison and mechanism analysis of several noback sand, Adv. Mater. Res. (Zurich, Switzerland) 189–193 (Pt. 5, Manufacturing Process Technology) (2011) 3828–3831. X. Zhou, J. Yang, F. Qian, G. Qu, Synthesis and application of modified starch as a shell-core main adhesive in a foundry, J. Appl. Polym. Sci. 116 (5) (2010) 2893–2900. A.A. Mirchi, W. Chen, L. Lavigne, E. Bergeron, J. Bergeron, High swelling ramming paste for
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aluminum electrolysis cell, US Patent 7 785 497, Assigned to Alcan International Limited, Montreal, Quebec, CA, August 31, 2010. L. Tian, Y. m. Zhou, G. Xie, R. x. Li, X. h. Yu, Ecofriendly cold ramming paste for aluminum electrolysis cell with furan resin as binder, Ind. Eng. Chem. Res. 51 (17) (2012) 6018–6024. M. Guo, Y. Wang, F. Liu, Performance analysis of ammonium lignosulfonate/urea formaldehyde-free fiberboards, Adv. Mater. Res. (Zurich, Switzerland) 113–116 (Pt. 3, Environment Materials and Environment Management) (2010) 1774–1778. Fibreboard—Specifications Part 2: Requirements for Hardboard, EN Standard EN 622-2: 2004, European Committee for Standardization, Brussels, Belgium, 2004. D. Schaefer, L. Reitzel, Biocomposite panel, WO Patent 2 012 041 521, Assigned to Roemmler H Resopal Werk GmbH and Reitzel Lutz, April 5, 2012. P. Shu, Water compatible chemical in situ and sand consolidation with furan resin, US Patent 5 522 460, Assigned to Mobil Oil Corporation, Fairfax, VA, June 4, 1996. J.D. Weaver, P.D. Nguyen, Methods for controlling particulate flowback and migration in a subterranean formation, US Patent 8 136 595, Assigned to Halliburton Energy Services, Inc., Duncan, OK, March 20, 2012. J.D. Weaver, P.D. Nguyen, Methods for maintaining conductivity of proppant pack, US Patent 8 136 593, Assigned to Halliburton Energy Services, Inc., Duncan, OK, March 20, 2012. P.D. Nguyen, J.D. Weaver, J.A. Barton, Methods and compositions for removing resin coatings, US Patent 7 198 681, Assigned to Halliburton Energy Services, Inc., Duncan, OK, April 3, 2007. A. Gandini, S. Hariri, J.-F. Le Nest, Furanpolyether-modified chitosans as photosensitive polymer electrolytes, Polymer 44 (25) (2003) 7565–7572.
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7.1 History In Latin, furfur means bran. Furfural was first isolated in 1832 (or 1821) by Döbereiner,1 as a by-product of the synthesis of formic acid. In 1840 the ability of furfural to form resins was discovered by Stenhous [3]. The industrial production of furfural started in 1922, and 1 year later the first furan-based resins emerged. Early patents on furan resins include that of Claessen [4] and one for synthetic resins (actually mixed phenol furan resins) suitable for use in molding gramophone records [5].
Table 7.1 Monomers for Furan Resins [6] Furan Compound
Remarks or Reference
Furan Furfural Furfuryl alcohol 5-Hydroxymethylfurfural (HMF) 5-Methylfurfural 2-Furfurylmethacrylate Bis-2,5-hydroxymethylfuran 2,5-Furandicarboxylic acid
OH
OH
O
OH OH
OH
[8] Glass fiber binder
OH
OH
OH
7.2 Monomers Monomers suitable for furan resins are listed in Table 7.1. One of the chief advantages of furan resins stems from the fact that they are derived from vegetable cellulose. Suitable sources of vegetable cellulose are corn cobs, sugarcane bagasse, oat hulls, paper mill by-products, biomass refinery eluents, cottonseed hulls, rice hulls, and foodstuffs such as saccharides and starch [6]. Pentoses hydrolyze to furfural and hexoses give 5-hydroxymethylfurfural on acid digestion [7].
7.2.1 Furfural Furfural is a by-product from sugarcane bagasse, which produces resins with an excellent chemical stability and low swelling. 2-Furan formaldehyde or furfural is made from agricultural materials by means of hydrolysis. The mechanism of formation of furfural is shown in Figure 7.1. It is a light yellow to amber colored 1 Johann Wolfgang Döbereiner, born in Hof an der Saale 1780, in
Germany, died in Jena 1849.
OH
HO
O
C
H
O
Figure 7.1 Mechanism of the formation of furfural.
transparent liquid. Its color gradually deepens to brown during storage. It tastes like apricot kernel. It is mainly used in lubricant refinement, FA production, and pharmaceutical production. Furfural is the chief reagent used to produce materials such as FA, HMF, bis(hydroxymethyl)furan, and 2,5-dicarboxyaldehyde-furan. The furan-containing monomers in turn can undergo reactions to produce various furan-containing monomers with a wide variety of substituents, as shown in Table 7.1.
7.2.2 Furfuryl Alcohol FA is made from furfural by reduction with hydrogen. It is a colorless transparent liquid and becomes brown, light yellow, or deep red when exposed in the air. It can be mixed with water and many organic solvents such as alcohol, ether, acetone, etc., but not in hydrocarbon products.
Fink: Reactive Polymers Fundamentals and Applications. http://dx.doi.org/10.1016/B978-1-4557-3149-7.00007-3 © 2013 Elsevier Inc. All rights reserved.
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7.2.3 Specialities 7.2.3.1 Furan-based Polyimides
APPLICATIONS
+
CH2
O
Polyimides based on poly (2-furanmethanol-formaldehyde) can be prepared by a Diels-Alder reaction (DA) of the respective furan resin with bismaleimides [9]. The Diels-Alder reaction proceeds in tetrahydrofuran or in bulk. The tetrahydrophthalimide intermediates aromatize in the presence of acetic anhydride. Polyimides based on the furan resin exhibit good thermal stability.
H
OH
+
O
CH2
O
CH2
O
CH2
O
7.2.4 Synthesis Furan-based monomers can polymerize through two well-known mechanisms. The first involves chain or polyaddition polymerization, which is initiated by free-radical, cationic, or anionic promoters. Polymerization produces macromolecules with furan rings pendant on the main chain. The second method is a polycondensation, also referred to as polymerization condensation. Polymers and copolymers resulting from acid-catalyzed condensation reactions result in macromolecules with furan rings in the main chain [6]. As a general rule, the furan resins formed by polycondensation reactions have stiffer chains and higher glass transition temperatures. These reactions may involve self-condensation of the furan monomers described above, as well as condensation reactions of such monomers with aminoplast resins, organic anhydrides, and aldehydes such as formaldehyde, ketones, urea, phenol, and other suitable reagents. Most common, furan resins are produced by acid-catalyzed condensation reactions. The condensation results in linear oligomers, the furan rings being linked with methylene and methylene-ether bridges, cf. Figure 7.2. The synthesis of furan resins proceeds in a pH range of 3–5, at a temperature range of 80–100 ◦ C. The condensation is stopped, when a desired viscosity value is reached, by neutralizing the liquid resin. FA can also be condensed with formaldehyde to obtain furan-formaldehyde resins. The content of free formaldehyde can be lowered by the addition of urea at the late stages of synthesis. The condensation of monomers from renewable resources, 2-furfural, vanillin, and 4-hydroxyacetophenone, at 80 ◦ C in the presence of potassium hydroxide gives an amorphous polymer resin with a yield of 85% [10]. The reaction is shown in Figure 7.3.
AND
CH2
CH2
O
+
OH
OH
Figure 7.2 Acid-catalyzed self-condensation of furfuryl alcohol. H
H3C
C
H3 C
O CHO
C
O
O O OH
OH
H
C
H3 C O
O H C 3
C
O
C OH
OH O
Figure 7.3 Terpolymer from 2-furfural, vanillin, and 4-hydroxyacetophenone [10].
A polydispersity index of the terpolymer of 1.52 was found. The terpolymer strongly inhibits the growth of a wide variety of microorganisms, including Gram-positive bacteria, Gram-negative bacteria, and fungi [10]. Upon pyrolysis, the major product is 4-hydroxyacetophenone.
7.3 Special Additives
7.3.1 Reinforcing Materials 7.3.1.1 Fibers Aramid fibers were used as reinforcing material for a phenol resin and a furan resin. A comparative study of
7: FURAN RESINS
the mechanical performance of the materials showed that the furan resin is more suitable as a matrix than the phenol resin [11]. Due to an emerging awareness of environmental issues a growing interest in biocomposites has been developed. Fully bio-based thermoset composites with aligned flax fiber textiles as reinforcement and a furan resin as the matrix resin were produced. After precuring, the prepregs were consolidated by compression molding [12]. Reinforcing fibers from a poly(phenylene sulfide) are suitable for epoxy and furan resins as binder materials [13]. The composite material is stable in 20% sodium hydroxide solutions at 60 ◦ C. Possible suggested applications are containers and pipes used in the transportation of alkaline fluids.
7.3.1.2 Nanomaterials In order to reduce the formaldehyde emission, furan was selected as an environmentally friendly and safe alternative to formaldehyde that is needed in phenolic resins. It is expected that the addition of organically modified and unmodified montmorillonite nanoparticles to the furan resin enhances its performance for use in metal coatings. Actually, viscosity measurements of the composites revealed that the addition of the nanoreinforcement led to a higher curing rate [14].
7.4 Curing Materials known to be suitable for curing furan resins include inorganic and organic acids. Examples of suitable organic and inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, tartaric acid, and maleic acid. Friedel-Crafts catalysts include aluminum trichloride, zinc chloride, aluminum bromide, and boron fluoride. Resins with improved fire resistance are cured with a mixture of trimethylborate, boric anhydride, and p-toluenesulfonic acid [15]. Salts of both inorganic and organic acids may also be used. Ammonium sulfate is preferred. Ammonium sulfate is a latent catalyst which may become active at approximately 110–150 ◦ C. Suitable organic salts are the urea salt of toluenesulfonic acid, the polyammonium salts of polycarboxylic acids such as the diammonium salts of citric acid, and the ammonium
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salts of maleic acid. Cyclic anhydrides such as maleic anhydride are also suitable for use as catalysts. It is believed that polyester copolymers are formed between the anhydride and the free hydroxylated species present in the resin. Maleic acid promotes the polymerization reaction. Furthermore, it is believed that maleic acid may preferentially reduce the emission of bis(hydroxymethyl)furan monomer during the curing process. A significant reduction of volatile organic compounds will use a catalyst system comprised of maleic acid and ammonium sulfate [6]. The curing mechanism of furfuryl alcohol and urea/formaldehyde furan resins was investigated with infrared spectroscopy (IR) [16]. The resins were modified with different agents, such as sorbitol, polyester polyol, phenol, and acetone. The influence of these compounds on the curing behavior was investigated. Except for the polyester polyol, the other modifiers have little effect on the thermal strength of the urea/formaldehyde furan resin. The curing kinetics of a modified furan resin therefrom was characterized by differential scanning calorimetry [17]. In particular, clay nanocomposites and phenol were used for modification. Free kinetic models were applied to get the activation energy of each process, the models according to Vyazovkin [18], Friedman [19], Ortega [20], and Ozawa [21]. The Vyazovkin numerical analysis was found to be the most accurate method. In the case of furan-based nanocomposites an additional peak in the end stage of curing was observed.
7.4.1 Acidic Curing The resin can be crosslinked using an acidic catalyst. The reaction is not sensitive to air. The main route of curing is an additional condensation reaction at the free α-hydrogen of furan rings. These positions are connected by methylene bridges.
7.4.2 Oxidative Curing The oxidative crosslinking of FA polycondensates proceeds at temperatures of 100–200 ◦ C. Structures with tertiary carbon atoms, as shown in Figure 7.4, could be identified.
7.4.3 Ultrasonic Curing Ultrasonic treatment, i.e., sonication during the curing process of a furan resin, showed changes of the curing performance. p-Toluenesulfonic acid was added
REACTIVE POLYMERS FUNDAMENTALS
206
O O
CH CH2 CH
O O
CH2 CH2
Figure 7.4 Crosslinked methylene bridges.
as curing catalyst in the proportion of 0.3%. Fine carbons were also incorporated. Using an ultrasonic homogenizer in the presence of carbonaceous fine particles showed an increased curing rate of the furan resin. This, in turn, increased the polymerization degree with an increase in ultrasound intensity. An increase of curing rate was also observed with small additions of carbonaceous fine particles. In this case, the curing accelerated with an increase in the specific surface area of the additives [22]. The increase of curing rate is believed to result from cavitation. The curing reaction proceeds slowly in the absence of cavitation and simple stirring fails to produce such a marked increase in the rate of reaction. The curing is accelerated by heat, oxygen, and the addition of phenol and urea.
7.5 Properties
7.5.1 Recycling Research has been conducted to introduce pendant furan groups into polymers such as poly(styrene) via copolymerization with a suitable comonomer. The pendant furan moieties can be crosslinked with a bismaleimide to produce polymers with better performance. In order to recycle these crosslinked materials, heating experiments with an excess of 2-methylfuran were performed in order to induce the retro DielsAlder reaction and break up the network. The reaction proceeded in this manner and the original copolymers could be recovered from the treatment. Therefore, the introduction of furan units is a potential path of recycling crosslinked polymers by thermal treatment with a diene in excess [23]. The DA between styrene-furfuryl methacrylate copolymer samples and bismaleimide can be monitored by the ultraviolet absorbance of the maleimide group at 320 nm or by 13 C NMR spectroscopy [8].
7.6 Applications and Uses Furan resins are used mainly in the foundry industry, as sand binders for casting molds and cores. They are
AND
APPLICATIONS
often used in combination with other resins. They are highly corrosion resistant. Therefore, they have found use in mortars and in cements. Improved mechanical properties are implemented by reinforcing with glass fibers.
7.6.1 Carbons 7.6.1.1 Carbon and Graphite Formation Furan resins form a porous carbon by pyrolysis at 450 ◦ C. A characterization of furanic foams by MALDI-ToF before and after carbonization at 900 ◦ C has been done. The results revealed that some of the original structures remained intact and thus resist the process of carbonization. This means that some furanic oligomers are particularly stable and are not prone to carbonization [24]. The carbonization of rigid foams based on polyflavonoid, tannin, formaldehyde, and furfuryl alcohol results in a three-dimensional network in which polynuclear aromatic hydrocarbon chains with a high molecular weight are observed. Some furan moieties that survive pyrolysis are also covalently linked to these chains [25]. A monolithic honeycomb sorbent bed for removing mercury and other toxic metals from flue gas of a coal combustion system has been described [26]. The sorbent bed contains an activated carbon catalyst and a toxic metal adsorption co-catalyst bonded to the activated carbon catalyst. The sorbent bed is produced from a synthetic carbon precursor, e.g., a furan resin. Graphitization is the transformation of disordered carbon materials into three-dimensional graphite, which typically occurs at temperatures of 2500– 3000 ◦ C. However, it has been found that some carbon materials, such as those derived from the thermosetting resin, are non-graphitizable even after heating to temperatures above 3000 ◦ C. Increased pressure or catalysts can be used to accelerate the graphitization of such materials [27]. From furan resin-derived glass-like regions with carbon spherical graphite in the carbon matrix can be obtained by hot isotropic pressing at 200 MPa and 2500 ◦ C [28]. The spherical regions have a diameter of 20–150 µm. A micrograph of the carbon spherical graphite is shown in Figure 7.5. Several metal-based catalysts are efficient for the catalytic graphitization of carbon [29,30] obtained from organic polymers. However, the residual
7: FURAN RESINS
Figure 7.5 Micrograph of the carbon spherical graphite. Reprinted from [28] with permission from Elsevier.
catalysts can be harmful for further applications of the thus obtained graphite materials. Graphite oxide can be used to accelerate the graphitization of a non-graphitizing furan resin carbon. The use of this catalyst does not introduce metal-based catalysts and should not cause environmental harm [31]. Graphite oxide can be synthesized from natural graphite powder by treatment of graphite with an anhydrous mixture of sulfuric acid, sodium nitrate, and potassium permanganate [32]. Yttrium and praseodymium can be used as catalysts for the graphitization of a carbon obtained in a first step from a furan resin [27,33]. The extent of graphitization of the furan resin carbon can be followed by X-ray diffraction and Raman spectroscopy. The content of praseodymium and the temperature of pyrolysis are important factors in the catalytic graphitization. A significant catalytic graphitization was achieved at 2400 ◦ C and a content of praseodymium of 15%. The addition of multiwalled carbon nanotubes (MWCNTs) into a furan resin can induce an ordered arrangement of planar carbon microlites in the course of a high-temperature treatment [34]. Also, the degree of graphitization is enhanced. Graphite and carbonaceous materials are promising materials for lithium-ion batteries. Natural graphite exhibits a high specific capacity, but a violent irreversible capacity may occur after a few charge/discharge cycles. A carbon coating from the pyrolysis of a furan resin on natural graphite surface can enhance graphite surface structure and promote the electrochemical properties [35]. In addition a surface fluorination can improve the lithium-ion intercalation and deintercalation during charging and discharging cycles.
207
Both methods of carbon coating and surface fluorination can increase the charge capacity. MWCNTs were used as filler in a furan resin to fabricate an electrically conducting polymer composite for electrode applications [36]. The orientation is preferably unidirectional, as this may result in a higher electric conductivity in a specific direction at a lower loading of the MWCNTs. The preparation by means of the doctor blade technique can induce a preferential alignment of the MWCNTs in the composite. The orientation is believed to create more junctions between the MWCNTs, which results in the formation of more conducting channels in the polymer matrix parallel to the blading direction [36]. Cross-sections through the composites are shown in Figure 7.6. Nanoshell carbon is a type of nanocarbon with a hollow, round, shell-like structure. The diameters are
Figure 7.6 Aligned multiwalled carbon nanotubes. Reprinted from [36] with permission from Elsevier.
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20–50 nm. It can be used in elctrochemical applications, e.g., in polymer electrolyte fuel cells. Nanoshell carbons can be prepared by the carbonization of a furan resin in the presence of acetylacetonates and of phthalocyanines with Fe, Co, and Ni as catalysts [38,39].
7.6.1.2 Glass-like Carbon Glass-like carbon is identified as an excellent carbon artifact due to characteristics such as hardness and shape stability. The microstructure of glass-like carbon consists of a non-graphitic alignment of hexagonal sheets. It has unique properties such as great hardness compared with other carbon materials and impermeability for gases [40]. Glass-like carbon is of interest in the battery and semiconductor industries. Glass-like carbon is prepared by heat treatment on thermosetting resins in inert atmosphere. During the heat treatment of a furan resin, weight loss is very rapid up to 500 ◦ C, then continues gradually up to 1000 ◦ C, and then the weight stays almost constant above 1000 ◦ C. Scanning electron microscopy photographs of heat-treated glass-like carbon reveal a large increase of micro-grain size in the range of 60–105 nm when treated at 2000 ◦ C, cf. Figure 7.7. Up to 2500 ◦ C, the grain size decreases to 27–40 nm due to graphitization [37]. There is a structural correlation between the micro-texture of the furan resin and the glass-like carbon formed from the particular resin. The pore structure in glass-like carbon can be characterized using the small-angle X-ray scattering technique. The scattering intensities grow gradually with increasing heat treatment temperature
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up to 1600–1800 ◦ C, and then the intensities increase abruptly at a temperature higher than 1800 ◦ C. The dependence of the structural change of a glasslike carbon from a furan resin is almost the same as that of a phenolic resin. However, it was found that the carbon prepared at 1200 ◦ C from furan resin shows the largest interlayer spacing in the carbon matrix and at the same time the smallest value of the gyration radius for the pores [40]. The surface oxidation and the corrosion behavior of carbon materials obtained from a furan resin in nitric acid, sulfuric acid, and hydrofluoric acid were investigated [41]. The carbons from the furan resins were both neat and Ta-alloyed carbons. The heat treatments occurred at 1200 ◦ C. The weights of the neat specimens treated at 1200 ◦ C decreased after the treatment with the acids. Spectroscopic analysis proved that the oxygen in the internal area as well as on the external surface was increased. The Ta-alloyed specimens treated at 1200 ◦ C have smaller amounts of oxygen after treatment with nitric acid and sulfuric acid. It was concluded that the use of Ta prevents the oxygen from penetrating into the depths of the specimens [41].
7.6.1.3 Wood Ceramics Wood ceramics are carbon-carbon composites from wood charcoal and glassy carbon. A wood ceramic has been developed from wood powder with a furan resin as binder [42]. The content of the furan resin has a significant effect on the microstructure of the wood ceramic. With a higher content of resin, generated glassy carbon increases and the connection between wood charcoal and the glassy carbon is strengthened.
7.6.2 Chromatography Support
Figure 7.7 SEM photographs of glass-like carbon derived from furan resin. (a) 300 and (b) 600 ◦ C. Reprinted from [37] with permission from Elsevier.
Conventionally used packing materials for liquid chromatography are a chemically bonded type of packing materialbasedonsilicagelandapackingmaterialbased on synthetic resin. The silica gel-based packing material is relatively strong in mechanical properties and in its swelling or shrinking characteristics against various organicsolvents.Therefore,ithasahighresolvingpower and is superior in exchangeability of eluent for analysis. However, the silica gel-based packing material has problems in that the silica gel dissolves under acidic or alkaline conditions and the solubility of the silica gel in an aqueous solution increases when warmed, resulting in durability problems.
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The packing material of synthetic resin, on the other hand, is known to be high in acid and alkali resistivity and has good chemical durability as a packing material. However, since the mechanical strength of the particles is small, it has been difficult to convert them into finer particles. Raw materials which are highly chemically stable and exhibit high mechanical strength are graphitized carbon black. A packing material for liquid chromatography is produced by mixing carbon black, a synthetic resin which can be graphitized, and pitches. Suitable synthetic resins are phenolic resins, furan resins, furfural resins, divinylbenzene resins, or urea resins [43]. The pitches can be petroleum pitches, coal-tar pitches, and liquefied coal oil. The mixture is granulated and heated up to 3000 ◦ C in an inert atmosphere.
7.6.3 Composite Carbon Fiber Materials Impregnation of carbon fibers and subsequent pyrolysis at 1000 ◦ C improves strength of carbon fibers [44]. A yarn is passed through a bath containing a carbonizable resin precursor, such as a partially polymerized FA. It is advantageous to add a latent catalyst along with the precursor. Suitable catalysts are a complex of boron trifluoride and ethylamine or maleic anhydride. The use of a latent catalyst allows the application of a low-viscosity solution to the fiber with subsequent polymerization at the elevated temperatures. If the precursor were to polymerize significantly prior to application, the treating bath would be so viscous that it would allow only a coating to be formed. For high-performance composite carbon fiber-reinforced carbonaceous material which is compositely reinforced with carbon fibers, prepregs of woven fabrics of carbon fibers are impregnated with a resin such as phenol resin, furan resin, epoxy resin, urea resin, etc. They then are laminated as a matrix and molded under heat and pressure, and after carbonization they are further graphitized by heating to a temperature of 3000 ◦ C [45].
7.6.4 Foundry Binders Furans are somewhat more expensive than other binders, but the possibility of sand reclamation is advantageous. One of the most commercially successful no-bake binders is the phenolic urethane no-bake binder. This binder provides molds and cores with
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excellent strengths that are produced in a highly productive manner. Furan-based binders have less VOC, free phenol level, low formaldehyde, and produce less odor and smoke during core making and castings. However, the curing performance of furan binders is much slower than the curing performance of phenolic urethane nobake binders. Furan binders can be modified to increase their reactivity, for instance by formulating with urea/ formaldehyde resins, phenol/formaldehyde resins, novolak resins, phenolic resol resins, and resorcinol. Nevertheless, these modified furan binders do not provide the cure speed needed in foundries that require high productivity. Therefore, an activator, which promotes the polymerization of FA, is added. Resorcinol pitch is used for this purpose [46]. Further components in such a formulation are polyester polyols or polyether polyols, and a silane, such as (3-aminopropyl)triethoxysilane. The curing process of urea-modified furan resins in sands has been investigated by IR [47]. A composition for binder resins has been described that contains both a furan resin, furfuryl alcohol, and oligomers from bis-hydroxymethyl furan [48]. These binders are particularly useful in warm-box applications. The advantages of using these binders over conventional heat-cured furan binders are [48]: 1. the curing rate of the binders is much faster than that of conventional heat curable furan binders, and 2. the hot and cold tensile strengths of cores prepared from these compositions are higher earlier on than the cores prepared with conventional heat curable furan binders. Spent foundry furan sands have a turbidity that is almost 15 times higher than that of other sands such as a CO2 sand or green sand. The turbidity of a furan sand can be reduced by stabilization of the furan resin by a thermal treatment [49]. The performance and the mechanism of no-bake sands with different binder resins have been compared [50]. The binders were specifically a furan resin, ester-sodium silicate sand, CO2 -sodium silicate, and phosphate. It turned out that the furan resin sand and the estersodium silicate sand have a higher dry strength and a
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better humidity resistance. This is explained by their fracture modes which are cohesive fracture and plastic fracture. The fracture modes of the phosphate sand and the CO2 -sodium silicate sand are mainly cohesive with a few adhesive and brittle fracture modes, but there is more cracking on their bond membrane. Phosphate sand has higher dry strength and better humidity resistance than CO2 -sodium silicate sand, lower cost and less pollution than a furan resin sand. As an alternative adhesive for sand cores in foundry applications instead of a furan resin, a solid watersoluble modified starch, carboxymethyl starch, was tested. The modified starch was used as a binder for sand in shell-core applications. Using these compositions, high-quality iron castings with smooth inner cavties can be produced. Thus, the carboxymethyl starch-bonded shell-cores can be used as a replacement for the conventional furan resin-bonded sand cores [51].
7.6.5 Glass Fiber Binders An alternative to phenol/formaldehyde-based fiberglass binders is furan-based binders. Furan binders provide many of the advantages of phenolic binders while resulting in substantially reduced VOC emissions. Water as a significant component can be used. Formaldehyde is not a significant curing or decomposition by-product, and the furan resins form very rigid thermosets. Emulsified furan resins can be used. Emulsified furan-based glass fiber binding compositions are advantageous since they allow the use of furan resins that have high molecular weights or the addition of other materials which give rise to the formation of two-phase systems [6]. A suitable surfactant to be added to the furan binder compositions is sodium dodecylbenzene sulfonate. It may be added in an amount from 0.05% to 1.0%.
7.6.6 Aluminum Electrolysis Most aluminum reduction cells in commercial use employ prefabricated carbon blocks as the cell lining and as the cathodic working surface. These blocks are formed into a liquid-tight container surfaced by filling the joints between the blocks with a ramming paste. The efficiency of sealing of the ramming paste is an important factor in determining the life and energy efficiency of a reduction cell, which depends to a great
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degree on the extent and rate of electrolytic penetration into the cell bottom [52]. An ecofriendly cold ramming paste for an aluminum electrolysis was synthesized from electrocalcined anthracite, with artificial graphite as an aggregate and a furan resin as the binder material [53]. The synthesized paste is an ecofriendly material and it has also some superior properties such as low electrical resistivity, high compressive strength, appropriate sodium penetration, and thermal expansion.
7.6.7 Panels and Fiberboards The properties of fiberboards made from wood fiber with ammonium lignosulfonate and urea as filler resins have been tested according to standardized procedures [54]. The specifications of EN 622-2 [55] can be met with these composite types. This is of interest because the composites are free from formaldehyde. Further, the properties are better than those obtained from conventional filling resins. Panels made from natural fibers, e.g., recycling paper and thermosetting biopolymers, have been described [56]. The base materials for the binder resins are 2,5-bis(hydroxymethyl)-furan, 2,3,5-tris (hydroxymethyl)furan, or 2,2 -hydroxymethyldifurylmethan. Curing of the composites can be done with acid catalysis at temperatures of 120–160 ◦ C.
7.6.8 Oil Field Applications Wells in sandy, oil-bearing formations are frequently difficult to operate because the sand in the formation is poorly consolidated and tends to flow into the well with the oil. Sand production is a serious problem because the sand causes erosion and premature wearing out of the pumping equipment. It is a nuisance to remove from the oil at a later point in the operation. Furan resin formulations can be used for in situ chemical sand consolidation [57]. Besides furan resins other thermosetting resins can also be used; however, the choice of the resin type depends on the bottom hole static temperature of the well. In the range of 150–315 ◦ C a furan resin is most suitable for controlling the particle migration [58]. In hydraulic fracturing treatments, a viscous fracturing fluid is pumped into a production zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Particulate solids, commonly referred to as the proppant, are suspended in a portion of the fracturing fluid so that the proppant is deposited in the fractures.
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211
O
Table 7.2 Efficiency of Cleaning [60] Components
Coated Metal Plate for 1 h at 100 ◦ C (% by volume)
Ammonium diacetate 5 5 0 0 Sodium hydroxide 0 0 5 5 Water 70 70 70 70 Ethylene glycol 25 25 25 25 monobutyl ether Cleanup (%) Epoxy Residue Furan Residue Percent cleanup 98 97 94 90
H 3C
O
O
7.6.9 Photosensitive Polymer Electrolytes Both conjugated furan chromophores and polyethers can be grafted onto chitosan to result in a photosensitive polymer electrolyte. The furan
H
Figure 7.8 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde.
OH
OH O
O
O
O NH
NH
The proppant deposited in the fractures functions to prevent the fractures from fully closing so that conductive channels are formed through which produced hydrocarbons may flow. Some portion of the proppant particulates may crush under the pressure of the formation and create unconsolidated particulates. For these reasons, the proppant is often coated with resins to facilitate the consolidation. In the same way as already described above, furan resin coating may be used to enhance the consolidation process [59]. One disadvantage associated with the use of resins is the removal and cleanup of the resin from the equipment. For example, a residue of resin may remain in fracturing equipment used during fracturing operations, e.g., connecting hoses, valves, sand hoppers, sand screws, or blender tubs. Without proper cleaning or removal, there is potential that a layer of resin may build up each time the equipment surface is exposed to the resin. This build-up of resin coat could result in plugging of equipment or cause the equipment not to function properly. Moreover, particulates, such as sand or proppant, may become entrapped within the resin coat and lead to further equipment damage. Residues from furan resin coating can be removed from a surface, e.g., valves, blender tubs, or hoses, by applying to the surface a cleaning solution, which is an acid or a base for adjusting the pH, followed by a solvent, e.g., dipropylene glycol monomethyl ether [60]. The efficiency of cleaning is shown in Table 7.2. As can be seen from Table 7.2, the cleanup is somewhat better for an epoxy resin in comparison to a furan resin.
C
CH 2
CH 3
O
O
CH 2 O
CH 3
O
O
CH 3
CH 2
O
O
CH 3
O CH 2
NH
NH
O
O OH
O
O OH
Figure 7.9 Photo crosslinking of the furylene vinylene units grafted on chitosan.
chromophore consists of conjugated furan chromophores of 5-[2-(5-methyl furylene vinylene)]furancarboxyaldehyde [61], cf. Figure 7.8. The graft polymer can be photo crosslinked. The photochemical reaction consists of a π 2 + π 2 cycloaddition reaction of the vinylene double bonds of the furan moiety so that two pendant vinylene groups form a four-membered ring. The crosslinking reaction is shown in Figure 7.9.
7.6.10 Plant Growth Substrates Conventional mineral wool plant growth substrates are based on a coherent matrix of mineral wool of which the fibers are mutually connected by a cured binder. There is a need to reduce the phytotoxicity of the chemicals used. The phytotoxicity may result from the phenolic binder materials. If a phenolic resin is used as binder, a wetting agent must be added in order to impart the hydrophobic mineral wool matrix with hydrophilic properties. However, the use of a furan resin allows the abandonment of the use of a wetting agent.
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A disadvantage of the use of a furan resin is its comparatively high price. Therefore, a traditional phenol/ formaldehyde resin substituted only partly by a furan resin is sufficient to maintain or to achieve the desired properties [62,63].
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