DOI 10.1007/s11148-017-0008-0 Refractories and Industrial Ceramics

Vol. 57, No. 5, January, 2017

OXIDATION-RESISTANT NANO-REINFORCED PC-REFRACTORIES OF MODIFIED PHENOLFORMALDEHYDE RESIN. PART 1. MODIFICATION OF PHENOLFORMALDEHYDE RESINS WITH SILICON ALKOXIDES G. D. Semchenko,1,3 O. N. Borisenko,2 V. V. Povshuk,1 D. A. Brazhnik,1 L. A. Angolenko,1 E. E. Starolat,1 L. V. Rudenko,1 Yu. V. Permyakov,1 and O. A. Vasyuk1 Translated from Novye Ogneupory, No. 9, pp. 22 – 26, September, 2016.

Original article submitted April 8, 2016. Phenolformaldehyde resins (PFRs) were modified with silicon alkoxide and transformed during heating. The physicochemical processes occurring as the PFRs were heated were established. The oxidation-resistance of PFRs modified by silicon alkoxide increased upon heating. Low-temperature synthesis of SiC from them was confirmed. Keywords: phenolformaldehyde resin (PFR), silicon alkoxide, resit structure, nanoreactor, low-temperature synthesis of SiC, carbon binder.

Phenolformaldehyde resin (PFR) is an oligomeric polycondensation product of formaldehyde (CH2O) and phenol and its homologs, e.g., cresols. Synthetic phenol is usually used in industry. Phenol forms colorless transparent crystals with mp 181°C and is soluble to 8% in H2O. Its solubility in alkaline solutions is unlimited. Tricresol is a liquid with bp 185 – 205°C. Another component required to produce PFR, formaldehyde, is used as an aqueous solution, which is called formalin. Both thermoplastic and thermally active oligomers are obtained from the preparation of resins via polycondensation of trifunctional phenols (m-cresols) and formaldehyde. Polycondensation in acidic and alkaline solutions of the reaction products of phenol and formaldehyde forms the corresponding linear and branched polymers. The resin formed in acidic solution with an excess of phenol has molecular mass 600 – 1300 Da. This resin is called novolac. The products obtained from polycondensation of phenol and formaldehyde in alkaline solution are branched polymers or resols 1 2 3

with molecular masses from 400 to 1000 Da, i.e., resols have lower masses than novolac [1]. Polymerization results in branched polymers. Resol molecules contain reactive hydroxymethyl groups (–CH2OH). Therefore, resols can convert into three-dimensional polymers called resits upon heating. Linear novolac resins can be converted to resols via their reactions with formalin or addition to the system of solid formaldehyde derivatives, e.g., hexamethylenetetramine or paraformaldehyde. Liquid PFRs contain up to 25% H2O and up to 15% free phenol. Drying produces dry resol resins. Modification of PFR in the presence of silicon alkoxide and ash as polysilicic acid mixed with a formaldehyde polymer is assumed to form complexes as a result of a separate reaction with the latter. The reaction of tetraethoxysilane Si(OR)4 with PFR at room temperature is

(1)

National Technical University Kharkov Polytechnic Institute, Kharkov, Ukraine. Kharkov National Economic University, Kharkov, Ukraine. [email protected]

It was found that the performance of PFRs was degraded if >5% modifier was added. Therefore, less than 3% of the 479 1083-4877/17/05705-0479 © 2017 Springer Science+Business Media New York

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organosilicon compounds was used for the modification. Small amounts of silicon alkoxide additive caused the PFR to liquefy, become more durable, and mix better with refractory fillers. The organosilicon compounds were incorporated into the PFR structure during heat treatment [2]. Hydroxymethyl groups (–CH2OH) of the phenolformaldehyde polymer reacted readily with hydroxyls although they could also react with ethoxyls. Tetraethoxysilane (TEOS) will react with formaldehyde resin as follows: (5)

(2)

In a similar manner, hydroxyls of polysilicic acid obtained via hydrolysis of TEOS or ethylsilicate (ETS) by a large amount of H2O:

Various compounds such as novolac and resols, polymerization and polycondensation of which produces organic polymers of various structures, are formed because acidic and basic catalysts affect the course of the reaction between formaldehyde and a carboxylic acid. Addition of modifiers can probably affect not only the synthetic reaction itself but also the C–C cross-linking with polysiloxane bonds ºSi–O–Siº [3]. Modification of PFR by silicon alkoxide can incorporate ºSi–O–Siº bonds into the carbon structure:

Si(OC2H5)4 + 16H2O ® Si(OH)4 + 12H2O + 4C2H5OH, (3) (6) will react with formaldehyde hydroxymethyl groups:

(4) (7)

The resulting structures with incorporated polysiloxane bonds can react with each other to give a polycondensed bulky structure of rather high strength:

Considering that –OH groups remain inside during three-dimensional cross-linking and formation of the formaldehyde binder resit structure, they can react with an

Oxidation-Resistant Nano-Reinforced PC-Refractories of Modified Phenolformaldehyde Resin

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organosilicon compound, creating the conditions for incorporating polysiloxane bonds into the structure [3]. Incorporation of the organosilicon compound into the resit structure of resin condensed at 180 – 200°C is accompanied by clathrate formation in its cavity, which acts as a nanoreactor:

A clathrate is an inclusion compound, which is a prototype organic—inorganic complex (–CH3)–(SiO2)n–C, the components of which are converted into refractory SiC during operation. Remaining non-bonded –OR groups of the organosilicon compound can react not only with resol –OH groups but also with the organosilicon compound itself to form a chain of ºSi–O–Siº polysiloxane bonds inside the three-dimensional resit structure of the organic compound, i.e. resins. The possibility for cross-linking of ºSi–O–Siº bonds with resit bonds should result in modified resins with improved properties, primarily strength. It is important to know what amount of organosilicon modifier and what method of addition are most fruitful for creating a highstrength organosilicon composite structure, i.e., PFR, and which is better for PFR modification, a pure organosilicon compound or its derivatives. Addition of freshly prepared ash allows single ash molecules and not polymerized components to be incorporated into the structure because formaldehyde stabilizes ash, i.e., it prevents the formation of polysiloxane bonds. Then, the ash cross-linking mechanism with resin bonds will be more similar to the formation of inclusion compounds [3]. Also, the cavity in the resit structure will act as a nanoreactor for synthesizing b-SiC from the components of the created clathrate (–CH3)–SiO2. The organosilicon compound is incorporated into the resit structure that is created during PFR carbonization to form chemical Si–C bonds between tetraethoxysilane and the resin carbonization product, which is a prototype of the future SiC tetrahedron [4] of the additional b-SiC antioxidant that is synthesized in the nanoreactor. Differential thermal analysis (DTA) was used to study the reaction of PFR with modifiers, i.e., organosilicon compounds (TEOS or ETS-32), and ash based on them. X-ray phase analysis (XPA) was used to study the phase composition of the reaction product. The following samples were pre-

Fig. 1. DTA curves of pure and modified resins: CP 1001/2 resin (1 ), imported resin FL 9831 (2 ), resins modified with organosilicon compound (3 ), resins modified by ash from organosilicon compound (4 ).

pared for the analysis: domestic PFR, SP 1001/2 grade (1), imported PFR FL 8931 (2), PFR FL 9831 modified by an organosilicon compound (3), and PFR FL 9831 modified by ash based on the organosilicon compound (corundum was used as an inert sample filler because it undergoes no transformation on heating to 1000°C). Figure 1 shows DTA curves that exhibited exothermic peaks at 250°C for both resins (curves 1 and 2 ) and at 560 and 540°C for resins SP 1001/2 and FL 9831, respectively. The insignificant exothermic peaks at 250°C could be interpreted as resulting from a PFR structural rearrangement without destruction (the mass remains stable). The PFR underwent thermal destruction and subsequent oxidation in the range 400 – 1000°C that were characterized by exothermic peaks at 560°C for SP 1001/2 and 540°C for the imported resin. The DTA curves of both resins were identical. An analysis of the DTA curves for pure PFR found that resins FL 9831 and SP 1001/2 behaved the same on heating, i.e., it was confirmed that both resins had similar chemical compositions. Therefore, domestic resin SP 1001/2 could be used in place of the imported one. Domestic liquid PFR SP 1001/2 lost less mass than liquid imported FL 9831 during heat treatment to 1000°C (Fig. 2), i.e., SP 1001/2 released lower amounts of gaseous compounds during thermal destruction. This characterized it as more ecologically benign during use as a refractory binder for lining thermal systems.

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Fig. 4. Diffraction patterns of PFR modified with silicon alkoxide and heat treated at 180 – 200 (a) and 1000°C (b ). Fig. 2. Mass losses of pure PFRs (corundum filler) as functions of temperature: imported resin (1 ) and CP 1001/2 resin (2 ).

Fig. 3. Mass losses of pure and modified imported PFR (corundum filler) as functions of temperature: pure resin (1 ), resin modified by ash from organosilicon compound (2 ), resin modified by organosilicon compound (3 ).

Mass losses during heat treatment to 1000°C decreased even more significantly if modifiers were added. The mass losses were ~1.8% for pure imported PFR, 0.6% for resin modified with organosilicon compound, and only 0.2% for resin modified with ash based on the organosilicon compound (Fig. 3). This indicated that the carbon content in the coked modified resins was elevated and was explained by the formation of inclusion compounds of the organosilicon compound in the resit cavities and involvement of the TEOS—PFR composite components in b-SiC synthesis if the modified resins were heated above 700°C. It was found that the domestic modified resin lost even less mass upon heating, e.g., 0.5 and 0.15%, although the DTA curves of the modified resins were in full agreement. Coking of the modified PFR caused the precursors necessary to synthesize SiC to accumulate, i.e., carbon, via recombination of (–CH3) radicals into carbon and hydrogen; mass losses during resin thermal destruction to decrease, and oxidation of the carbon resin and SiO formed within the

nanoreactor as a result of reducing amorphous SiO2 from TEOS or gel to slow. Thermal destruction of modified PFRs stopped at 500 – 600°C and was accompanied by stabilization of the mass losses whereas thermal destruction continued above 700°C for the pure resins. The bonds formed between the modifier and resin carbonization product during carbonization of the modified PFRs hindered access of oxygen into the structure and helped to decrease the environmental releases of CO, CO2, and phenol during heat treatment and use of uncalcined refractories (e.g., periclase-carbon). This improved the ecological situation during use of such materials. Therefore, modification of PFRs with organosilicon compounds and ash based on them increased the yield of coke residue, slowed its oxidation during heat treatment to 1000°C, and reduced releases of hazardous compounds [5]. SiC was synthesized during heat treatment of PFRs modified by TEOS or ETS organosilicon compounds. Figure 4 shows XPA results for samples of FPR SP 1001/2-1 modified with silicon alkoxide (1%) before and after heat treatment. SiC was synthesized from the components of the modified FPR (Fig. 4b ) during heat treatment at 1000°C, i.e., low-temperature synthesis of SiC from the components of the organic—inorganic complex (–CH3)–(SiO2)n–C that was formed during modification of the resins was observed [6]. The carbon bonds formed by coking during use of PFRs modified by an organosilicon compound as binders of PC-refractories will be self-reinforced by the synthesized SiC nanoparticles. This improves the physicomechanical properties of the material and also increases its oxidation resistance because the SiC will act as an antioxidant. CONCLUSION Modification of PFRs by silicon alkoxide increases its service life, increases its flowability, improves wetting of the charge fillers, decreases moss losses during heating, and improves the ecological situation for production using these resins. SiC nanoparticles that reinforce the carbon bonding and improve the physicomechanical properties of pressed

Oxidation-Resistant Nano-Reinforced PC-Refractories of Modified Phenolformaldehyde Resin items is synthesized during service from the modified PFR components. REFERENCES 1. A. Knop, in: Phenol Resins and Materials Based on Them [in Russian], A. Knop and V. Sheib (eds.), Khimiya, Moscow, 1983, 280 pp. 2. A. A. Maiboroda, O. N. Slepchenko, and G. D. Semchenko, “New techniques for increasing the strength of uncalcined magnesia-carbon refractories,” in: VIth All-Russian Student Scientific-Practical Conference “Chemistry and Chemical Engineeering in the XXIst Century” [in Russian], May 11 – 12, 2005, Tomsk, Internet Session, 2005; http://chemstud.chtd.tpu.ru.

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