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Fabrication and properties of thermal insulating glass fiber reinforced composites from low temperature curable polyphosphate inorganic polymers
Composites Science and Technology 63 (2003) 493–499 www.elsevier.com/locate/compscitech
Fabrication and properties of thermal insulating glass fiber reinforced composites from low temperature curable polyphosphate inorganic polymers Dong-Pyo Kima,*, Heung-Shik Myung, Jae-Seong Rho, Kyoo-Seung Han, Hee-Gweon Woob, Hunseung Hac, Feng Caod a
Department of Industrial Chemistry, Chungnam National University, Taejon 305-764, Republic of Korea b Department of Chemistry, Chonnam National University, Republic of Korea c Agency for Defense Development, Taejon 305-660, Republic of Korea d Key Lab of C.F.C, National University of Defense Technology, Changsha 410073, PR China Received 21 June 2001; accepted 10 April 2002
Abstract Glass fiber reinforced polymetalphosphate matrix composites prepared by a simple process displayed excellent thermal insulating and mechanical properties. Low-viscous Al3Cr(H2PO4)x=9,12 binders were prepared by dissolving Al(OH)3 and Cr(OH)3 or CrO3 in 85% phosphoric acid, and mixed with Al2O3 and Cr2O3 fillers. The glass fiber pre-pregs impregnated by the binder solution were laid-up and cured at 150–200 C for 12 h under pressure, which are similar conditions to those used for carbon fiber/phenolic resin matrix composites. The composites cured using the hot-press or autoclave showed outstanding hygroscopic resistance even after standing in air for 30 days, due to the chemical stability of the cured network. Hot-press cured composites with higher density exhibited maximum flexural strengths of 155 MPa and thermal conductivity in the range 1.12–3.45 W/mK, while the porous autoclave cured composites displayed 60–77 MPa and 0.4–0.6 W/mK, respectively. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); A. Preceramic polymer; B. Curing; B. Strength; B. Thermal properties
1. Introduction Aluminum phosphate compounds are used for various purposes including flame retardant specialties, such as adhesives, anti-freeze, and paints [1–7]. Phosphoric acid as a weak acid undergoes three ionization steps (pK1=2.15, pK2=7.1, and pK3=12.4 at 25 C) that are associated with the three phosphate ions, namely, 2 H2PO and PO3 4 , HPO4 4 , and they are known as ligands of monodentate, chelating or bridging forms. Generally, in the case of the thermal treatment of hydrated orthophosphate, it is known that dehydrolytic condensation occurs to form P–O–P bonds. As the reaction proceeds, polymeric phosphates convert mainly to linear polyphosphate or circular metaphosphate. It * Corresponding author. Tel.: +82-42-821-6695; fax: +82-42-8236665. E-mail address: [email protected] (D.-P. Kim).
contains more than one of three phosphate units such as end, middle, or branch, which depend upon the extent of condensation. The chemical equations for this reaction are given as follows [8–12]; Polyphosphate formation; Mx ðH2 PO4 Þy þ Mx0 ðHPO4 Þy0 ! Mz PnO3nþ1 þ m H2 O
ð1Þ Polyphosphate
Metaphosphate formation; nMx ðH2 PO4 Þy ! ðMz PO3 Þn þ m H2 O
ð2Þ
Metaphosphate Chemical stability is obtained by the gradual forming of a cured network of branching P that has these three functionalities. Naturally, circular phosphate could convert to a linear or cured network by ring-opening
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00229-4
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reaction. The degree of dehydrolytic condensation may depend on processing conditions such as the curing temperature and pressure, which also causes variations in the ratio of three type units and the curing density. Thus, we believe that these factors might significantly affect the overall physical properties, such as the thermal stability, hygroscopic resistance, and the strength of the prepared products. In the present study, ceramic fiber composites were prepared by a low temperature hardening process carried out at under 200 C on the pre-pregs impregnated with matrix solution, which is a homogeneous mixture of phosphate binder and Al2O3, Cr2O3 powders as fillers [5,13–15]. The prepared composites were evaluated for use as thermally insulating composites, which prevent unacceptable temperature increases when exposed to extreme heat as determined by measuring thermal conductivity, hygroscopic resistance and strength. Moreover, by establishing the layer-up forming method with pre-pregs and low-temperature curing process, the present study was conducted as a preliminary study for the production of inexpensive composites using the same production equipment as is used for carbon fiber/phenolic resin matrix composites.
2. Experimental
Table 1 Mixing ratios of reactants to prepare binder solution Sample name
Chemical formula
Mixing ratio (mol.%)
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
Al3Cr(H2PO4)9 Al3Cr(H2PO4)12 Al3Cr(H2PO4)9 Al3Cr(H2PO4)12
Al(OH)3:Cr(OH)3:H3PO4=3:1:9 Al(OH)3:Cr(OH)3:H3PO4=3:1:12 Al(OH)3:CrO3:H3PO4=3:1:9 Al(OH)3:CrO3:H3PO4=3:1:12
this was followed by dissolution with stirring for 24 h. The alternative binder was identified as ACP2 solution and was designed to replace the Cr(OH)3 by with CrO3, which was quickly dissolved within 1 h by adding several drops of methanol. The vigorous evolution of gas and foam was controlled by the drop-wise addition of methanol [20,21]. The matrix solution containing two kinds of fillers was prepared at a ratio of ACP binder solution: Al2O3:Cr2O3=48:41:11wt.% by stirring at 80 C for 30 min [16]. 2.3. Fabrication of composites Composites were fabricated as a particulate reinforced type to investigate the thermal conductivity and hygroscopic resistance of matrix. The glass fiber reinforced composite was cured by two processes using an autoclave (Nextinstrument Co, Korea) or a hot-press (Tetrahedron, O-100, USA).
2.1. Materials and reagents Binder solution was prepared from H3PO4 (DAEJUNG, 85%), Al(OH)3 (Samchun Pure Chemicals), Cr(OH)3 (Kojundo, Japan), CrO3 (Fluka, 99%), and methanol (Aldrich, 99.99%) as starting compounds. Al2O3 (Aldrich, < 10 um, 99.8%) and Cr2O3 (Aldrich, 1 um, 98+%) powders were used as fillers, and woven glass fabric (Karam Co, Ceramic wool) was used for reinforcement. Teflon mold (diameter 26 mm depth 5 mm) was used for pellet type of composites and a stainless steel mold (25 mm 45 mm) for plate type. Graphite foil (Korean Aerospace Co, 0.2 mm thick) was used for mold release after curing. 2.2. Preparation of binder solution and matrix solution Highly viscous Al3Cr (H2PO4)x=9,12 solutions were used as a binder solution, and were prepared by dissolving Al hydroxide, Al(OH)3, and either CrO3 or Cr(OH)3 in 85% phosphoric acid (15% water) using two mixing ratios, as shown in Table 1; one at Al:Cr:H3PO4=3:1:9 (mol%), and the other at Al:Cr:H3PO4=3:1:12 (mol%) based on the stoichiometric ratio of the two metals and the phosphate [16– 19]. Binder described as ACP1 solution was synthesized by dissolving Al(OH)3 in 85% phosphoric acid at 80 C for 20 min and the subsequent addition of Cr(OH)3, and
2.3.1. Particulate reinforced composite The matrix solution was poured into a Teflon mold (26 mm dia., mm deep) and dried in a muffle furnace at 100 C for 24 h in an air atmosphere. The pelletized specimen was cured in an autoclave under a pressure of 10 atm of air to 150–200 C using a heating rate of 10 C/h and held at the desired temperature for 12 h [13,16]. During the cooling stage, the pressurized air containing water as a byproduct was released to prevent rehydration. 2.3.2. Glass fiber reinforced composite Woven glass fabric was cut into 25 45 mm2, brushcoated with the viscous matrix solution, and dried at room temperature for 24 h. The ratio was designed to yield a pre-preg of 56% matrix and 44% glass fiber by weight. Laying-up of 10 pre-pregs was then undertaken in a stainless steel mold (25 45 mm2), which was then cold-pressed by applying a pressure of 68 atm for 5 min at room temperature. The specimen released from the mold was cured in an autoclave under the same conditions as the particulate reinforced composite. Alternatively, the laid-up 10 pre-pregs in the stainless steel mold were hot-pressed uniaxially at 80–90 C at a rate of 10 C/h under a pressure of 34 atm, and this was followed by heating to 150 C under a pressure of 68 atm, which was held for 12 h [13].
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was added instead of methanol, the similar reducing effect was reported to occur [20].
2.4. Properties The viscosity of binders was measured using an AFNOR #4 viscosity cup. Hygroscopic resistance of the cured composites was evaluated by measuring the difference between the weight loss in the as-cured samples and that of the cured sample standing in air for 3–30 days, as determined by TGA (TGA2050, TA Instrument) at temperatures up to 1000 C at a rate of 5 C/ min in dry air [13,22]. The morphology of composite specimens was observed by SEM (Scanning Electron Microscope, JSM 5410LV, JEOL). The apparent density was calculated with the volume obtained by water displacement. The cured composites were cut with a diamond saw into 4 mm 45 mm 1.5mm pieces to measure the three-point flexural strength using an Instron (Model 4505) according to the ASTM (C116194) method [23]. The crosshead speed used was 0.5 mm/ min, the support span was 40 mm, and nominal bearing diameter was 4.5 mm. Four specimens of the composite were tested to determine the average strength for each sample type. For thermal conductivity measured using the hot disk AB(TPA-501, Sweden) method, specimens were cut into a pellet shape (26 mm dia 5 mm thick) and polished.
3. Results and discussion 3.1. Preparation of binder solution Clear viscous Al(H2PO4)3 solutions were readily prepared by adding Al(OH)3 to phosphoric acid. However, when homogenous Al3Cr(H2PO4)x=9,12 binder solutions were prepared, the dissolution time varied significantly, and depended upon Cr sources. Cr(OH)3 dissolved in 24 h whereas CrO3 completely dissolved in only 1 h when methanol was added slowly. This was attributed to the fact that the strong oxidation CrO3 as brown Cr(VI) was reduced to green Cr(III) presumably as it formed Cr(H2PO4)3. Simultaneously, the methanol was oxidized to volatile aldehyde [21]. As is shown in the following chemical equation, as HCHO vaporized vigorously as H2 gas to produce foam, therefore, the methanol needs to be added very slowly to the solution at a reaction temperature of 80 C. When H2O2 or sugar
CrO3 þ CH3 OH ! HCHO " ðbp: 19:5 CÞ þ Cr3þ þ H2 " ð3Þ The complete dissolution of Cr(OH)3 in ACP1 binder solution requires not only a long time but is also relatively expensive, so that the preparation method using ACP2 binder solution with CrO3 is believed to be a superior method. Furthermore, it is most likely that ACP-3112 sample is simply a mixture with the stoichiometric ratio of Al(H2PO4)3 and Cr(H2PO4)3, while ACP-319 solution is believed to be chemically more homogenous due to multi-dentate phosphate, Al and Cr moieties. The density of the binder solutions were as follows: ACP1-319 was 1.89 g/cm3, ACP1-3112 as 1.76 g/cm3, ACP2-319 as 1.90 g/cm3, and ACP2-3112 as 1.84 g/cm3, Viscosities were readily controlled by the addition of water. 3.2. Hygroscopic resistance of composites In order to determine the chemical stability of particulate reinforced composites cured in an autoclave, the extent of moisture take-up was measured by TGA. As shown in Table 2, the composites immediately after curing showed a weight loss of ca. 3 wt.%, whereas most composites left in air for 3 days show a weight loss of ca. 4 wt.%. The additional weight loss is caused mainly by rehydrolytic reaction of the unstable cured structure with the moisture in air. Therefore, it can be used as a measure of hygroscopic resistance. Thus, since most of the particulate reinforced composites show a relatively low hygroscopicity about 1 wt.%, a relatively stable cured matrix for glass reinforced composites is also believed to have formed under low-temperature curing conditions. However, because they show more than 2 wt.% hygroscopicity, ACP2-3112 composites are believed to have relatively low stability compared with ACP-319. Generally, a severe weight gain more than 10 wt.% on exposing to air was observed for specimens cured under 150 C or with reaction times of less than 5 h due to the rehydrolytic reaction. We are currently investigating the curing reaction mechanism and the chemical effect of various additives.
Table 2 Hygroscopic resistance of particulate reinforced composites Conditions
ACP1 319
Autoclave at 200 C for 12 h under 10 atm Unit: wt.%.
Initial 3 Days
2.62 2.45
ACP2 3112 2.60 3.47
319 3.00 3.43
3112 3.90 6.48
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Table 3 Hygroscopic resistance of glass fiber reinforced composites Conditions
ACP1 319
ACP2 3112
319
3112
Autoclave at 200 C for 12 h under 10 atm
7 Days 30 Days
1.01 1.43
– 5.26
2.45 2.38
– 3.61
Hot-press at 150 C for 12 h under 68 atm
7 Days 30 Days
3.98 6.60
– 7.10
4.48 7.57
– 6.96
Unit:wt.%
Table 3 shows the hygroscopic resistance of glass fiber reinforced composites cured under various conditions. At first, the composites shown in Fig. 1 generally showed less than a 5 wt.% weight loss when exposed to air for 7 days. Autoclave cured composites exhibited better hygroscopic resistance than hot-press cured composites, indicating that a higher curing temperature may be proceeded to induce dehydrolytic condensation. In Table 3 the autoclave cured specimens standing in the air for 30 days showed almost no weight change compared with those left in the air for 7 days. This confirmed that the specimens were composed of a stable cured network of matrix with hygroscopic resistance. By comparison, hot-press cured composites displayed lower stability to rehydration, which is partially attributed to the fact that the water as a dehydrolytic byproduct
captured in the mold suppressed the curing reaction of the matrix. This is consistent with the fact that, in case when cooled in pressurized air containing abundant water as a by-product, the autoclave cured specimens showed more than 10 wt.% weight loss. Furthermore, a slight difference in hygroscopic resistance could be attached to the mixing ratio of the reactants, but no difference to the used Cr sources. 3.3. Properties of composites Table 4 shows thermal conductivity of various composite specimens measured at 50 C. The thermal conductivity of particulate reinforced composites was 1.35–2.16 W/mK, whereas these of glass reinforced composites showed significant differences with respect
Fig. 1. Typical TGA thermogram of glass fiber reinforced composites exposed to air for 7 days (A: Autoclave 200 C, P: Hot-press 150 C).
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D.-P. Kim et al. / Composites Science and Technology 63 (2003) 493–499 Table 4 Thermal conductivity and density of various composites Composite type
Curing condition
Sample name
Thermal conductivity (W/mK)
Density (g/cm3)
Particulate reinforced composite
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
1.44 1.76 1.35 2.16
2.67 3.33 2.84 3.63
Fiber reinforced composite
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
0.45 0.57 0.64 0.52
1.91 1.53 1.83 1.84
Hot-press
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
1.36 1.13 1.12 3.45
2.2 2.45 1.83 1.92
to the curing conditions used. In other words, glass reinforced composites cured with hot-press had thermal conductivities in the range 1.12–3.45 W/mK, which is similar to the value of matrix, whereas autoclave cured specimens had conductivities of 0.45–0.64 W/mK. As reported, low-density insulators are generally known to have lower conductivity than high-density insulators [24]. The apparent density of particulate reinforced composites lay in the range 2.6–3.6 g/cm3, and similarly glass reinforced composites cured with a hot-press were from 1.8 to 2.5 g/cm3, which was significantly higher than 1.5–1.9 g/cm3 of autoclave cured composites. These figures indicate that hot-press cured composites facilitated binding of the layers between pre-pregs, and resulted in high density. Thus, hot-press cured composites with compact structure lost the beneficial lowering effect to thermal conductivity as given by high porosity. Table 5 shows the three-point flexural strength of glass reinforced composites. The strength of the specimens cured with hot-press was between 100 and 155 MPa whereas that of the specimens cured in the autoclave was 60–77 MPa, showing about half the value of the hotpress cured samples. Moreover, moduli were compared,
the hot-press cured specimens were 2.5–3.0 GPa, which was approximately 50% higher than the 1.27–2.4 GPa achieved by the autoclave cured samples. When compared with the displacement at yield, the specimens cured with a hot-press showed 0.86–1.01 mm, whereas the specimens cured in the autoclave showed 0.37–0.67 mm. When considered the measured stress-strain curve, the hot-press cured specimens showed higher toughness. These results agreed well with the measured densities, and correlate with the general rule that higher density expresses higher strength [24]. Fig. 2 shows SEM micrographs of the surface and cross-section of composites. The particulate reinforced specimens shows homogeneous distribution of Al2O3 (< 10 mm) and 1 mm Cr2O3 fillers in the binding matrix, indicating the high wettability of fillers suitable for mixing with binders. Moreover, the matrix solution was found to easily wet the fiber permitting excellent infiltration into the interstices between fibers and into the pre-preg lay-ups. It was observed that the porous structure was rather more developed in the autoclave cured specimens than the hot-press cured specimens, and this was consistent with the density measurements. On the
Table 5 Three-point flexural strength of glass fiber reinforced composites Curing method
Sample
Stress at yield, MPa (standard deviation)
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
59.48 76.13 60.17 77.60
Hot-press
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
99.87 128.83 155.80 138.90
Displacement at yield (mm)
Modulus (GPa)
(10.03) (14.84) (5.83) (10.66)
0.368 0.666 0.498 0.610
2.41 1.97 1.93 1.27
(12.61) (10.83) (1.95) (8.90)
0.865 0.891 0.924 1.017
2.51 3.00 3.01 2.68
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Fig. 2. SEM microphotographs of the surface and the cross-section of typical composite specimens: (a) surface 200 (autoclave), (b) cross-section 500 (autoclave), (c) surface 200 (hot-press) and (d) cross-section 500 (hot-press).
other hand, glass fiber was damaged severely when uniaxial pressure of over 136 atm was applied during curing. The indication of acidic degradation, such as the discoloration of glass fiber, when curing temperatures exceeded 200 C, demands a coating barrier to prevent the direct contact between glass fiber and matrix. Studies are also being conducted to overcome these problems.
temperatures provide a stable matrix with outstanding moisture resistance. Compared with the composite cured in an autoclave, specimens cured in the hot-press were of higher density and had about twice the flexural strength, achieving 155 MPa, with a maximum modulus of 3.0 GPa, and with significant toughness. However, the density of autoclave cured specimens was low and they had a well defined pore structure and the lowest thermal conductivity of about 0.4–0.6 W/mK.
4. Conclusions Acknowledgements Highly viscous Al3Cr(H2PO4)x=9,12 binders were prepared by adding Al(OH)3 and Cr(OH)3 or CrO3 in 85% phosphoric acid, using CrO3 as a preferred Cr source. The binder containing Al2O3, Cr2O3 fillers cured under a pressure of 10 atm for 12 h at 200 C in an autoclave showed superior hygroscopic resistance, with a weight decrease of less than 1 wt.% after 3 days of air exposure versus weight immediately after curing. The thermal conductivity of the specimens cured under the same conditions was 1.35–2.16 W/mK. Evenness of distribution, and superior wettability of the filler and the binding solution, was confirmed by SEM. The glass fiber reinforced composites cured in autoclave at 200 C and those cured for 12 h at 150 C in a hot-press were stable in terms of hygroscopic resistance even when exposed to air for 30 days. It is evident that curing processes at low
This work was supported by Korea Research Foundation Grant (KRF-2001-005-E00033).
References [1] Griffith EJ. US Patent, 5,223,336, 1993. [2] Griffith EJ. US Patent, 5,486,232, 1996. [3] Beious NK, Samuskevich VV, Stukalenko AA. Curing behavior of phosphate binders mixed with pyrophosphoric acid. Inorg Mater 1998;34:1132–5. [4] Svatovskaya LB, Latutova MN, Makarova OY. Reactive ornamental materials capable of normal setting. Russian J Appl Chem 1998;71:1565–7. [5] Mukhutdinov RK, Samoilov NA, Perlov RA. Mechanical strength and catalytic properties of coatings based on metal oxide
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powers and aluminum-chromium phosphate adhesive. Russian J Appl Chem 1998;71:447–51. Chiou JM, Chung DDL. Improvement of the temperature resistance of aluminum-matrix composites using an acid phosphate binder. J Mater Sci 1993;28:1435–46. Gumaste JL, Swain BC, Mohanty BC, Murty JS. Chemical reaction bonding of building blocks using red mud and orthophosphate acid binder. J Mater Sci Lett 1996;15:1667–8. Cotton FA, Wilkinson G. Advanced inorganic chemistry, vol. 5. Willy Interscience; 1988. p. 424–7. Geoff RC,. Descriptive inorganic chemistry. W. H. Freeman and Company; 1995. p. 294–305. Morris JH, Perkins PG, Rose AE, Smith WE. Interaction between aluminum dihydrogen phosphate and quartz. J Appl Chem Biotechnol 1976;26:385–90. Nriagu JO, Moore PB. Phosphate minerals. Springer-Verlag; 1984. Yamada K, Murae K. Synthesis and application of inorganic polymer. CMC; 1982. p. 241–5. Gonzalez FJ, Halloran W. Reaction of orthophosphoric aicd with several forms of aluminum oxide. Bull Am Ceram Soc 1980; 59:727–32. Miyake M, Uehara H, Suzuki H, Yao Z, Matsuda M, Sato M. Syntheses of chromium(III)-doped aluminophosphate-5 molecular sieves from various gels. Micro Meso Mater 1999;32:45–53. Lapko KN, Tikavy VF, Markova LN, Vydumchik GN, Krylova ZF. The thermorisistant phosphate composition hardening at low temperature. Phosph Res Bull 1996;6:245–8. Arclanova NI, Bushgev UH, Kamalov AD, Kirillov VN, Pronin
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BF, Furhina XK, Chuirul NP, Chistiakov AM. RU Patent 2015948, 1978. Bautista FM, Campelo JM. Conversional of 2-propanol over chromium aluminum orthophosphate. Catal Lett 1995;35:143154. Bautista FM, Campelo JM. Chromium-aluminum orthophosphates, III. Acidity and catalytic performance in cyclohexene and cumene conversions on CrPO4–AlPO4 (20–50 wt.% AlPO4) catalysts obtained in aqueous ammonia. React Kinet Catal Lett 1994;53:55–63. Bautista FM, Campelo JM. Chromium-aluminum orthophosphates, II. Effect of AlPO4 loading on structure and texture of CrPO4–AlPO4(20–50wt.% AlPO4) catalysts obtained in aqueous ammonia. React Kinet Catal Lett 1994;53:45–53. Kopeikin BA, Petrova AP, Pashkovan IL. Metalphosphate materials, Chemistry; 1976. Morrison RT, Boyd RN. Organic Chemistry. Prentice Hall International; 1992. p. 235–6. Beppu MM, Lima ECO, Galembeck F. Aluminum phosphate particles containing closed pores; preparation, characterization, and use as a white pigment. J Colloid Interface Sci 1996;178:93– 103. American Society for Testing and Materials. Standard test method for flexural strength of advanced ceramics at ambient temperature. Annual Book of ASTM Standard. C 1161-94. ASTM; 1994. p. 306–12. Hiroshi M, Tadahiko T. High-performance fiber. In: Elvers B, Hawkins S, Ravenscroft M, Schulz G, editors. Ullmann’s encyclopedia of industrial chemistry, vol. 5. VCH; 1989. p. 75–6.
Fabrication and properties of thermal insulating glass fiber reinforced composites from low temperature curable polyphosphate inorganic polymers Dong-Pyo Kima,*, Heung-Shik Myung, Jae-Seong Rho, Kyoo-Seung Han, Hee-Gweon Woob, Hunseung Hac, Feng Caod a
Department of Industrial Chemistry, Chungnam National University, Taejon 305-764, Republic of Korea b Department of Chemistry, Chonnam National University, Republic of Korea c Agency for Defense Development, Taejon 305-660, Republic of Korea d Key Lab of C.F.C, National University of Defense Technology, Changsha 410073, PR China Received 21 June 2001; accepted 10 April 2002
Abstract Glass fiber reinforced polymetalphosphate matrix composites prepared by a simple process displayed excellent thermal insulating and mechanical properties. Low-viscous Al3Cr(H2PO4)x=9,12 binders were prepared by dissolving Al(OH)3 and Cr(OH)3 or CrO3 in 85% phosphoric acid, and mixed with Al2O3 and Cr2O3 fillers. The glass fiber pre-pregs impregnated by the binder solution were laid-up and cured at 150–200 C for 12 h under pressure, which are similar conditions to those used for carbon fiber/phenolic resin matrix composites. The composites cured using the hot-press or autoclave showed outstanding hygroscopic resistance even after standing in air for 30 days, due to the chemical stability of the cured network. Hot-press cured composites with higher density exhibited maximum flexural strengths of 155 MPa and thermal conductivity in the range 1.12–3.45 W/mK, while the porous autoclave cured composites displayed 60–77 MPa and 0.4–0.6 W/mK, respectively. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); A. Preceramic polymer; B. Curing; B. Strength; B. Thermal properties
1. Introduction Aluminum phosphate compounds are used for various purposes including flame retardant specialties, such as adhesives, anti-freeze, and paints [1–7]. Phosphoric acid as a weak acid undergoes three ionization steps (pK1=2.15, pK2=7.1, and pK3=12.4 at 25 C) that are associated with the three phosphate ions, namely, 2 H2PO and PO3 4 , HPO4 4 , and they are known as ligands of monodentate, chelating or bridging forms. Generally, in the case of the thermal treatment of hydrated orthophosphate, it is known that dehydrolytic condensation occurs to form P–O–P bonds. As the reaction proceeds, polymeric phosphates convert mainly to linear polyphosphate or circular metaphosphate. It * Corresponding author. Tel.: +82-42-821-6695; fax: +82-42-8236665. E-mail address: [email protected] (D.-P. Kim).
contains more than one of three phosphate units such as end, middle, or branch, which depend upon the extent of condensation. The chemical equations for this reaction are given as follows [8–12]; Polyphosphate formation; Mx ðH2 PO4 Þy þ Mx0 ðHPO4 Þy0 ! Mz PnO3nþ1 þ m H2 O
ð1Þ Polyphosphate
Metaphosphate formation; nMx ðH2 PO4 Þy ! ðMz PO3 Þn þ m H2 O
ð2Þ
Metaphosphate Chemical stability is obtained by the gradual forming of a cured network of branching P that has these three functionalities. Naturally, circular phosphate could convert to a linear or cured network by ring-opening
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00229-4
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D.-P. Kim et al. / Composites Science and Technology 63 (2003) 493–499
reaction. The degree of dehydrolytic condensation may depend on processing conditions such as the curing temperature and pressure, which also causes variations in the ratio of three type units and the curing density. Thus, we believe that these factors might significantly affect the overall physical properties, such as the thermal stability, hygroscopic resistance, and the strength of the prepared products. In the present study, ceramic fiber composites were prepared by a low temperature hardening process carried out at under 200 C on the pre-pregs impregnated with matrix solution, which is a homogeneous mixture of phosphate binder and Al2O3, Cr2O3 powders as fillers [5,13–15]. The prepared composites were evaluated for use as thermally insulating composites, which prevent unacceptable temperature increases when exposed to extreme heat as determined by measuring thermal conductivity, hygroscopic resistance and strength. Moreover, by establishing the layer-up forming method with pre-pregs and low-temperature curing process, the present study was conducted as a preliminary study for the production of inexpensive composites using the same production equipment as is used for carbon fiber/phenolic resin matrix composites.
2. Experimental
Table 1 Mixing ratios of reactants to prepare binder solution Sample name
Chemical formula
Mixing ratio (mol.%)
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
Al3Cr(H2PO4)9 Al3Cr(H2PO4)12 Al3Cr(H2PO4)9 Al3Cr(H2PO4)12
Al(OH)3:Cr(OH)3:H3PO4=3:1:9 Al(OH)3:Cr(OH)3:H3PO4=3:1:12 Al(OH)3:CrO3:H3PO4=3:1:9 Al(OH)3:CrO3:H3PO4=3:1:12
this was followed by dissolution with stirring for 24 h. The alternative binder was identified as ACP2 solution and was designed to replace the Cr(OH)3 by with CrO3, which was quickly dissolved within 1 h by adding several drops of methanol. The vigorous evolution of gas and foam was controlled by the drop-wise addition of methanol [20,21]. The matrix solution containing two kinds of fillers was prepared at a ratio of ACP binder solution: Al2O3:Cr2O3=48:41:11wt.% by stirring at 80 C for 30 min [16]. 2.3. Fabrication of composites Composites were fabricated as a particulate reinforced type to investigate the thermal conductivity and hygroscopic resistance of matrix. The glass fiber reinforced composite was cured by two processes using an autoclave (Nextinstrument Co, Korea) or a hot-press (Tetrahedron, O-100, USA).
2.1. Materials and reagents Binder solution was prepared from H3PO4 (DAEJUNG, 85%), Al(OH)3 (Samchun Pure Chemicals), Cr(OH)3 (Kojundo, Japan), CrO3 (Fluka, 99%), and methanol (Aldrich, 99.99%) as starting compounds. Al2O3 (Aldrich, < 10 um, 99.8%) and Cr2O3 (Aldrich, 1 um, 98+%) powders were used as fillers, and woven glass fabric (Karam Co, Ceramic wool) was used for reinforcement. Teflon mold (diameter 26 mm depth 5 mm) was used for pellet type of composites and a stainless steel mold (25 mm 45 mm) for plate type. Graphite foil (Korean Aerospace Co, 0.2 mm thick) was used for mold release after curing. 2.2. Preparation of binder solution and matrix solution Highly viscous Al3Cr (H2PO4)x=9,12 solutions were used as a binder solution, and were prepared by dissolving Al hydroxide, Al(OH)3, and either CrO3 or Cr(OH)3 in 85% phosphoric acid (15% water) using two mixing ratios, as shown in Table 1; one at Al:Cr:H3PO4=3:1:9 (mol%), and the other at Al:Cr:H3PO4=3:1:12 (mol%) based on the stoichiometric ratio of the two metals and the phosphate [16– 19]. Binder described as ACP1 solution was synthesized by dissolving Al(OH)3 in 85% phosphoric acid at 80 C for 20 min and the subsequent addition of Cr(OH)3, and
2.3.1. Particulate reinforced composite The matrix solution was poured into a Teflon mold (26 mm dia., mm deep) and dried in a muffle furnace at 100 C for 24 h in an air atmosphere. The pelletized specimen was cured in an autoclave under a pressure of 10 atm of air to 150–200 C using a heating rate of 10 C/h and held at the desired temperature for 12 h [13,16]. During the cooling stage, the pressurized air containing water as a byproduct was released to prevent rehydration. 2.3.2. Glass fiber reinforced composite Woven glass fabric was cut into 25 45 mm2, brushcoated with the viscous matrix solution, and dried at room temperature for 24 h. The ratio was designed to yield a pre-preg of 56% matrix and 44% glass fiber by weight. Laying-up of 10 pre-pregs was then undertaken in a stainless steel mold (25 45 mm2), which was then cold-pressed by applying a pressure of 68 atm for 5 min at room temperature. The specimen released from the mold was cured in an autoclave under the same conditions as the particulate reinforced composite. Alternatively, the laid-up 10 pre-pregs in the stainless steel mold were hot-pressed uniaxially at 80–90 C at a rate of 10 C/h under a pressure of 34 atm, and this was followed by heating to 150 C under a pressure of 68 atm, which was held for 12 h [13].
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was added instead of methanol, the similar reducing effect was reported to occur [20].
2.4. Properties The viscosity of binders was measured using an AFNOR #4 viscosity cup. Hygroscopic resistance of the cured composites was evaluated by measuring the difference between the weight loss in the as-cured samples and that of the cured sample standing in air for 3–30 days, as determined by TGA (TGA2050, TA Instrument) at temperatures up to 1000 C at a rate of 5 C/ min in dry air [13,22]. The morphology of composite specimens was observed by SEM (Scanning Electron Microscope, JSM 5410LV, JEOL). The apparent density was calculated with the volume obtained by water displacement. The cured composites were cut with a diamond saw into 4 mm 45 mm 1.5mm pieces to measure the three-point flexural strength using an Instron (Model 4505) according to the ASTM (C116194) method [23]. The crosshead speed used was 0.5 mm/ min, the support span was 40 mm, and nominal bearing diameter was 4.5 mm. Four specimens of the composite were tested to determine the average strength for each sample type. For thermal conductivity measured using the hot disk AB(TPA-501, Sweden) method, specimens were cut into a pellet shape (26 mm dia 5 mm thick) and polished.
3. Results and discussion 3.1. Preparation of binder solution Clear viscous Al(H2PO4)3 solutions were readily prepared by adding Al(OH)3 to phosphoric acid. However, when homogenous Al3Cr(H2PO4)x=9,12 binder solutions were prepared, the dissolution time varied significantly, and depended upon Cr sources. Cr(OH)3 dissolved in 24 h whereas CrO3 completely dissolved in only 1 h when methanol was added slowly. This was attributed to the fact that the strong oxidation CrO3 as brown Cr(VI) was reduced to green Cr(III) presumably as it formed Cr(H2PO4)3. Simultaneously, the methanol was oxidized to volatile aldehyde [21]. As is shown in the following chemical equation, as HCHO vaporized vigorously as H2 gas to produce foam, therefore, the methanol needs to be added very slowly to the solution at a reaction temperature of 80 C. When H2O2 or sugar
CrO3 þ CH3 OH ! HCHO " ðbp: 19:5 CÞ þ Cr3þ þ H2 " ð3Þ The complete dissolution of Cr(OH)3 in ACP1 binder solution requires not only a long time but is also relatively expensive, so that the preparation method using ACP2 binder solution with CrO3 is believed to be a superior method. Furthermore, it is most likely that ACP-3112 sample is simply a mixture with the stoichiometric ratio of Al(H2PO4)3 and Cr(H2PO4)3, while ACP-319 solution is believed to be chemically more homogenous due to multi-dentate phosphate, Al and Cr moieties. The density of the binder solutions were as follows: ACP1-319 was 1.89 g/cm3, ACP1-3112 as 1.76 g/cm3, ACP2-319 as 1.90 g/cm3, and ACP2-3112 as 1.84 g/cm3, Viscosities were readily controlled by the addition of water. 3.2. Hygroscopic resistance of composites In order to determine the chemical stability of particulate reinforced composites cured in an autoclave, the extent of moisture take-up was measured by TGA. As shown in Table 2, the composites immediately after curing showed a weight loss of ca. 3 wt.%, whereas most composites left in air for 3 days show a weight loss of ca. 4 wt.%. The additional weight loss is caused mainly by rehydrolytic reaction of the unstable cured structure with the moisture in air. Therefore, it can be used as a measure of hygroscopic resistance. Thus, since most of the particulate reinforced composites show a relatively low hygroscopicity about 1 wt.%, a relatively stable cured matrix for glass reinforced composites is also believed to have formed under low-temperature curing conditions. However, because they show more than 2 wt.% hygroscopicity, ACP2-3112 composites are believed to have relatively low stability compared with ACP-319. Generally, a severe weight gain more than 10 wt.% on exposing to air was observed for specimens cured under 150 C or with reaction times of less than 5 h due to the rehydrolytic reaction. We are currently investigating the curing reaction mechanism and the chemical effect of various additives.
Table 2 Hygroscopic resistance of particulate reinforced composites Conditions
ACP1 319
Autoclave at 200 C for 12 h under 10 atm Unit: wt.%.
Initial 3 Days
2.62 2.45
ACP2 3112 2.60 3.47
319 3.00 3.43
3112 3.90 6.48
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Table 3 Hygroscopic resistance of glass fiber reinforced composites Conditions
ACP1 319
ACP2 3112
319
3112
Autoclave at 200 C for 12 h under 10 atm
7 Days 30 Days
1.01 1.43
– 5.26
2.45 2.38
– 3.61
Hot-press at 150 C for 12 h under 68 atm
7 Days 30 Days
3.98 6.60
– 7.10
4.48 7.57
– 6.96
Unit:wt.%
Table 3 shows the hygroscopic resistance of glass fiber reinforced composites cured under various conditions. At first, the composites shown in Fig. 1 generally showed less than a 5 wt.% weight loss when exposed to air for 7 days. Autoclave cured composites exhibited better hygroscopic resistance than hot-press cured composites, indicating that a higher curing temperature may be proceeded to induce dehydrolytic condensation. In Table 3 the autoclave cured specimens standing in the air for 30 days showed almost no weight change compared with those left in the air for 7 days. This confirmed that the specimens were composed of a stable cured network of matrix with hygroscopic resistance. By comparison, hot-press cured composites displayed lower stability to rehydration, which is partially attributed to the fact that the water as a dehydrolytic byproduct
captured in the mold suppressed the curing reaction of the matrix. This is consistent with the fact that, in case when cooled in pressurized air containing abundant water as a by-product, the autoclave cured specimens showed more than 10 wt.% weight loss. Furthermore, a slight difference in hygroscopic resistance could be attached to the mixing ratio of the reactants, but no difference to the used Cr sources. 3.3. Properties of composites Table 4 shows thermal conductivity of various composite specimens measured at 50 C. The thermal conductivity of particulate reinforced composites was 1.35–2.16 W/mK, whereas these of glass reinforced composites showed significant differences with respect
Fig. 1. Typical TGA thermogram of glass fiber reinforced composites exposed to air for 7 days (A: Autoclave 200 C, P: Hot-press 150 C).
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D.-P. Kim et al. / Composites Science and Technology 63 (2003) 493–499 Table 4 Thermal conductivity and density of various composites Composite type
Curing condition
Sample name
Thermal conductivity (W/mK)
Density (g/cm3)
Particulate reinforced composite
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
1.44 1.76 1.35 2.16
2.67 3.33 2.84 3.63
Fiber reinforced composite
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
0.45 0.57 0.64 0.52
1.91 1.53 1.83 1.84
Hot-press
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
1.36 1.13 1.12 3.45
2.2 2.45 1.83 1.92
to the curing conditions used. In other words, glass reinforced composites cured with hot-press had thermal conductivities in the range 1.12–3.45 W/mK, which is similar to the value of matrix, whereas autoclave cured specimens had conductivities of 0.45–0.64 W/mK. As reported, low-density insulators are generally known to have lower conductivity than high-density insulators [24]. The apparent density of particulate reinforced composites lay in the range 2.6–3.6 g/cm3, and similarly glass reinforced composites cured with a hot-press were from 1.8 to 2.5 g/cm3, which was significantly higher than 1.5–1.9 g/cm3 of autoclave cured composites. These figures indicate that hot-press cured composites facilitated binding of the layers between pre-pregs, and resulted in high density. Thus, hot-press cured composites with compact structure lost the beneficial lowering effect to thermal conductivity as given by high porosity. Table 5 shows the three-point flexural strength of glass reinforced composites. The strength of the specimens cured with hot-press was between 100 and 155 MPa whereas that of the specimens cured in the autoclave was 60–77 MPa, showing about half the value of the hotpress cured samples. Moreover, moduli were compared,
the hot-press cured specimens were 2.5–3.0 GPa, which was approximately 50% higher than the 1.27–2.4 GPa achieved by the autoclave cured samples. When compared with the displacement at yield, the specimens cured with a hot-press showed 0.86–1.01 mm, whereas the specimens cured in the autoclave showed 0.37–0.67 mm. When considered the measured stress-strain curve, the hot-press cured specimens showed higher toughness. These results agreed well with the measured densities, and correlate with the general rule that higher density expresses higher strength [24]. Fig. 2 shows SEM micrographs of the surface and cross-section of composites. The particulate reinforced specimens shows homogeneous distribution of Al2O3 (< 10 mm) and 1 mm Cr2O3 fillers in the binding matrix, indicating the high wettability of fillers suitable for mixing with binders. Moreover, the matrix solution was found to easily wet the fiber permitting excellent infiltration into the interstices between fibers and into the pre-preg lay-ups. It was observed that the porous structure was rather more developed in the autoclave cured specimens than the hot-press cured specimens, and this was consistent with the density measurements. On the
Table 5 Three-point flexural strength of glass fiber reinforced composites Curing method
Sample
Stress at yield, MPa (standard deviation)
Autoclave
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
59.48 76.13 60.17 77.60
Hot-press
ACP1-319 ACP1-3112 ACP2-319 ACP2-3112
99.87 128.83 155.80 138.90
Displacement at yield (mm)
Modulus (GPa)
(10.03) (14.84) (5.83) (10.66)
0.368 0.666 0.498 0.610
2.41 1.97 1.93 1.27
(12.61) (10.83) (1.95) (8.90)
0.865 0.891 0.924 1.017
2.51 3.00 3.01 2.68
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Fig. 2. SEM microphotographs of the surface and the cross-section of typical composite specimens: (a) surface 200 (autoclave), (b) cross-section 500 (autoclave), (c) surface 200 (hot-press) and (d) cross-section 500 (hot-press).
other hand, glass fiber was damaged severely when uniaxial pressure of over 136 atm was applied during curing. The indication of acidic degradation, such as the discoloration of glass fiber, when curing temperatures exceeded 200 C, demands a coating barrier to prevent the direct contact between glass fiber and matrix. Studies are also being conducted to overcome these problems.
temperatures provide a stable matrix with outstanding moisture resistance. Compared with the composite cured in an autoclave, specimens cured in the hot-press were of higher density and had about twice the flexural strength, achieving 155 MPa, with a maximum modulus of 3.0 GPa, and with significant toughness. However, the density of autoclave cured specimens was low and they had a well defined pore structure and the lowest thermal conductivity of about 0.4–0.6 W/mK.
4. Conclusions Acknowledgements Highly viscous Al3Cr(H2PO4)x=9,12 binders were prepared by adding Al(OH)3 and Cr(OH)3 or CrO3 in 85% phosphoric acid, using CrO3 as a preferred Cr source. The binder containing Al2O3, Cr2O3 fillers cured under a pressure of 10 atm for 12 h at 200 C in an autoclave showed superior hygroscopic resistance, with a weight decrease of less than 1 wt.% after 3 days of air exposure versus weight immediately after curing. The thermal conductivity of the specimens cured under the same conditions was 1.35–2.16 W/mK. Evenness of distribution, and superior wettability of the filler and the binding solution, was confirmed by SEM. The glass fiber reinforced composites cured in autoclave at 200 C and those cured for 12 h at 150 C in a hot-press were stable in terms of hygroscopic resistance even when exposed to air for 30 days. It is evident that curing processes at low
This work was supported by Korea Research Foundation Grant (KRF-2001-005-E00033).
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