Sodium silicate binder as a binder for sand casting has the advantages of green, non-polluting, low cost, but the problem of strong hygroscopicity of sodium silicate bonded ceramic sand (SSBCS) has been pending solution. In this paper, the effect of methyl silicone oil and polyvinyl alcohol on the moisture resistance of ester-hardened SSBCS was studied, aiming to solve the problem of high moisture absorption of SSBCS. The results showed that the optimum ratio was 15% methyl silicone oil and 3% polyvinyl alcohol. Compared with unmodified SSBCS, the moisture absorption rate of SSBCS modified by methylsilicone oil and polyvinyl alcohol at 2, 4, 6 and 8 h decreased by 0.2%, 0.18%, 0.32% and 0.51%, respectively, and the strength loss rate at 2, 4, 6 and 8 h decreased to 0. Characterization analysis concluded that methyl silicone oil and polyvinyl alcohol improved the strength of SSBCS by absorbing free sodium ions (Na+) from the sand and by covering the surface of SSBCS with a film to reduce water absorption.
Research Article
Effect of methyl silicone oil and polyvinyl alcohol on the moisture resistance of ester-hardened sodium silicate bonded ceramic sand
https://doi.org/10.21203/rs.3.rs-4869162/v1
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Sodium silicate bonded ceramic sand
Polyvinyl alcohol
Methyl silicone oil
Moisture resistance
As a kind of binder in sand casting, sodium silicate binder has the advantages of easy production, low cost, and environmental friendliness [1–4]. Based on the many advantages of sodium silicate binder and the increasingly improved modification processing of sodium silicate binder, sodium silicate binder is more and more widely used in sand casting [5–8]. However, sodium silicate bonded sand (SSBS) is hygroscopic and difficult to maintain strength in high humidity environments, so it is of great practical importance to solve this problem [9].
Several researchers have now investigated the moisture resistance of microwave-hardened SSBS [10–13]. For example, Xin et al. [10] studied the effect of microsilica powder, PVB ethanol solution and cement on the moisture resistance of SSBS. It was found that the composite additives strengthened the bonded bridge of the sand core and blocked the diffusion of water molecules, which significantly improved the storage performance of the sand core. Liu et al. [11] used the surface coating method to significantly improve the moisture resistance of microwave-hardened SSBS. Tang [12] found that preventing the hydration of Na+ and stopping water molecules is the key to improve the moisture resistance of SSBS. Hua et al. [13] found that ester-microwave composite hardening can significantly improve the moisture resistance of SSBS. However, microwave hardening has high requirements for the material of the mold [14], so it is difficult to popularize.
Ester-hardened SSBS has the advantages of green environmental protection and good collapsibility, but it also has the disadvantages of poor moisture resistance [15–17]. For example, Hai [18] adopted ester-hardened modified SSBS process, which can reduce the amount of sodium silicate binder, and significantly improve the SSBS collapsibility, but also can effectively improve the recycling efficiency of old sand. Wang et al. [19] found that ester-hardened SSBS absorbed more crystal water than microwave-hardened SSBS.
Some additives have been shown to improve the moisture resistance of materials [20–22]. For example, Ba et al. [20] studied methyl silicone oil on silicone antifouling coatings and showed that methyl silicone oil significantly improved the hydrophobicity of the coatings. Xia et al. [21] used starch and polyvinyl alcohol to prepare an adhesive with high bond strength and moisture resistance. Zhou [22] found that the moisture resistance of polyvinyl alcohol films can be improved by processes such as esterification cross-linking. Nevertheless, less research had been done on the effects of the amount of methicone and polyvinyl alcohol on the moisture resistance of ester-hardened SSBCS. Therefore, this paper aimed to investigate the influence of the amount of methicone and polyvinyl alcohol on the moisture resistance of ester-hardened SSBCS. The corresponding mechanism for the improvement of humidity resistance of SSBCS was also discussed, providing the theoretical basis for the development of SSBCS application.
2.1 Materials
The test was carried out with ester-hardened modified sodium silicate. Ester-hardened modified sodium silicate binder was sodium silicate (modulus 2.7, density 1.43–1.45 g/cm3), 7.5% ethylene glycol diacetate, 5% triethylene glycol diacetate, 10% butylbenzyl titanate, 5% branched-chain starch, 7.5% sodium metaaluminate and 2.5% γ-aminopropylmethyldiethyloxysilane (wt.% of sodium silicate binder). The raw sand used was spherical ceramic sand with an average grain size between 0.020 mm and 3.350 mm and its physical and chemical properties are shown in Table 1. The moisture resistant modifiers were methyl silicone oil and polyvinyl alcohol. In the experiment, the quantity of sodium silicate was 2.5% of 1 kg ceramic sand weight. The addition amount of methyl silicone oil was 9%, 12%, 15%, 18%, and the addition amount of polyvinyl alcohol was 2.5%, 2.75%, 3%, 3.25% (wt. % of sodium silicate binder).
| composition | refractoriness (°C) | Angular coefficient | Packing density(kg/m3) | Coefficient of expansion (20 ~ 1000℃)/ ℃ | |||
|---|---|---|---|---|---|---|---|
| Al2O3 | SiO2 | Fe2O3 | others | ||||
| ≥ 75% | > 16 | ≥ 2.5 | < 5% | ≥ 1800 | < 1.1 | 1.95 ~ 2.05 | 6\(\:\times\:\)10−6 |
2.2 Experimental scheme
First, methylsilicone oil, polyvinyl alcohol, ethylene glycol diacetate, triethylene glycol diacetate, butylbenzyl titanate, amylopectin, sodium metaaluminate and gamma-aminopropyl methyl diethoxysilane were added to the sodium silicate binder and stirred for 60 s. Secondly, the modified sodium silicate binder was mixed with ceramic sand for 120 s. Finally, the standard “8”- shaped sand core was made manually by the three-hammering method, as shown in Fig. 1.
2.3 Performance test
Firstly, the specimens were weighed. Then the specimens were put into a salt spray test chamber with a humidity of 98%RH. Finally, the tensile strength of the specimens was tested by XQY-Ⅱ Intelligent Sand Strength Tester for different time periods. The tensile strength of 6 specimens were measured in each group. The final value is the average of the maximum and minimum values.
2.4 The characterization of the sample
A sand sample with a size of 8 mm×8 mm×3 mm was prepared from the section where the sample was prepared. The samples were glued to the platform with conductive adhesive and sprayed with gold. The morphology of the bridge was observed by QUANTA-200 scanning electron microscopy (SEM).
Part of the sand particles were ground from the section where the sample was prepared, and ground to more than 150 mesh, and put into a drying oven to dry fully. Finally, the micromorphological phases of the dried samples were characterized by XRD-6100 X-ray diffractometer (XRD).
Some sand grains were ground on the cross-section of the specimen and placed in a ceramic crucible with a lid, then heated to 1000°C at 40 K/min and finally analyzed by NETZSCH 404F3 Synchronous Thermal Analyzer (DSC).
3.1 Experimental results of unmodified SSBCS
Table 2 shows the room temperature strength of unmodified SSBCS. The moisture absorption and strength loss rate of unmodified SSBCS at 98% RH humidity for different times are shown in Fig. 2. With the increase of time, the moisture absorption rate and strength loss rate of SSBCS showed an increasing trend. The hygroscopicity and strength loss rate can be calculated according to Eqs. 1 and 2.
In Eq. 1, η is the hygroscopicity; mt (g) is the mass of the sand sample in the salt spray tester under the condition of 98% RH high humidity; m0 (g) is the mass of the sand sample under the condition of room temperature (63% RH). In Eq. 2, δ is the rate of strength loss; εB is the strength value of the sand sample placed at room temperature (63% RH); εA is the strength value of the sand sample placed at 98% RH humidity.
|
Time(h) |
2 |
4 |
6 |
8 |
|---|---|---|---|---|
|
Strength(MPa) |
0.172 |
0.245 |
0.286 |
0.371 |
3.2 Effect of methyl silicone oil on SSBCS
The tensile strength results of modified SSBCS with different amounts of methsilicone oil added at different times under 98% RH humidity are shown in Fig. 3 (The tensile strength of the sample at different placement times under 98% RH humidity environment was collectively referred to as 2 h strength, 4 h strength, 6 h strength and 8 h strength). With the increase of the amount of methyl silicone oil additive, 2 h strength, 6 h strength and 8 h strength increased first and then decreased, and 4 h strength increased first, then decreased and then increased.
Figure 4 shows the hygroscopicity and strength loss rate of samples containing different additive amounts of methyl silicone oil at 98% RH humidity for different times.
As shown in Fig. 4a, with the increase of the amount of methyl silicone oil additive, 4 h hygroscopicity, 6 h hygroscopicity and 8 h hygroscopicity decreased first and then increased, and 2 h hygroscopicity increased first, then decreased and then increased. As shown in Fig. 4b, with the increase of the amount of methyl silicone oil additive, 2 h strength loss rate, 6 h strength loss rate and 8 h strength loss rate decreased first and then increased, and 4 h strength loss rate decreased first and then increased.
According to the results of tensile strength, moisture absorption rate and strength loss rate, the optimal addition amount of methyl silicone oil was 15%.
3.3 Effect of polyvinyl alcohol on SSBCS
The tensile strength results of SSBCS modified with different additive amounts of polyvinyl alcohol at 98% RH humidity for different times are shown in Fig. 5. With the increase of the amount of polyvinyl alcohol additive, 2 h strength,4 h strength, 6 h strength and 8 h strength decreased first, then increased and then decreased.
Figure 6 shows the moisture absorption rate and strength loss rate of samples containing different additive amounts of polyvinyl alcohol at 98% RH humidity for different times.
As shown in Fig. 6a, with the increase of the amount of polyvinyl alcohol additive, 4 h hygroscopicity, 6 h hygroscopicity and 8 h hygroscopicity decreased first and then increased, and 2 h hygroscopicity increased slowly. As shown in Fig. 6b, with the increase of the amount of polyvinyl alcohol additive, 4 h strength loss rate, 6 h strength loss rate and 8 h strength loss rate decreased first and then increased, and 2 h strength loss rate decreased first and then increased.
According to the results of tensile strength, moisture absorption rate and strength loss rate, the optimal addition amount of polyvinyl alcohol was 3%.
3.4 Results of the final program
The optimized sample was composed of 15% methylsilicone oil and 3% polyvinyl alcohol on the basis of raw material. Figure 7 shows the moisture absorption rate and strength loss rate of the optimized sample at 98% RH humidity for different times. Compared with Fig. 2, the moisture absorption rate and strength loss rate of the modified sample were significantly reduced.
3.5 Micromorphology results
Figure 8 shows the bonded bridge morphology of the unmodified SSBCS and the modified SSBCS by methyl silicone oil and polyvinyl alcohol placed at 98% RH humidity for 4 h. It can be observed that the bonded bridge of unmodified SSBCS were very loose and full of cracks (Fig. 8a) and the bonded bridge of modified SSBCS were smoother and tighter (Fig. 8b). In addition, compared with the unmodified SSBCS, the surface of the modified SSBCS by methyl silicone oil and polyvinyl alcohol was covered by a white film.
3.6 Phase results
The XRD patterns of unmodified SSBCS and modified SSBCS by methyl silicone oil and polyvinyl alcohol placed at 98% RH humidity for 24 h is shown in Fig. 9.The bonded layer of the unmodified SSBCS placed at 98% RH humidity for 24 h emerged six kinds of phases, including Al2(Al2.8Si1.2)O9.6、Al2O3、SiO2、NaO2、Na2Si2O5、Na(Si2O4(OH))H2O、Na0.91Al1.88(Si3.45Al0.55)O10(OH)2, as shown in Fig. 9a. Compared with the unmodified SSBCS placed at 98% humidity for 24 h, the bonded layer of the modified SSBCS by methyl silicone oil and polyvinyl alcohol placed at 98% RH humidity for 24 h appeared to have four new phases, including NaO2、Na2Si2O5、Na(Si2O4(OH))H2O、Na0.91Al1.88(Si3.45Al0.55)O10(OH)2, as shown in Fig. 9b.
Ester-hardened SSBCS contains Na+ and OH-, which have affinity for water and can absorb water molecules, thus breaking the silica-oxygen bond and reducing the strength of SSBCS. But the hydrophilicity of Na+ is much stronger than hydroxide, so the hygroscopicity of SSBCS is mainly determined by the amount of Na+ [23].According to the phase composition of XRD (Fig. 9), it is inferred that methicone and polyvinyl alcohol react with SSBCS to form NaO2、Na2Si2O5、Na(Si2O4(OH))H2O and Na0.91Al1.88(Si3.45Al0.55)O10(OH)2 phases, which decreased free sodium ions, thus reducing the absorption of water molecules by the sodium silicate binder, and improving the moisture-proofing properties of the SSBCS (Fig. 7).
Since the surface tension of water is very small, and sand particles are surface high-energy substances, water molecules can easily replace the physical adsorption between water glass binder and sand particles through desorption, and quickly penetrate into the bonding interface between water glass binder and sand particles, making water glass binder ineffective [19]. The modified SSBCS produced a white adhesive film (Fig. 9b), and it can be inferred that the white adhesive film is composed of these new phases (Fig. 8b), which changes the surface potential and solubility of the sodium silicate binder, so that fine gel particles are obtained after curing and covered on the surface of SSBCS. Therefore, the white bond film can prevent the intrusion of water molecules, thus reducing the cracking of the bond bridge and improving the tensile strength and moisture resistance of the modified SSBCS (Fig. 7).
1、The optimal solution was 15% methylsilicone oil and 3% polyvinyl alcohol. Compared with unmodified SSBCS, the moisture absorption rate of modified SSBCS at 2, 4, 6 and 8 h decreased by 0.2%, 0.18%, 0.32% and 0.51%, respectively, and the strength loss rate at 2, 4, 6 and 8 h decreased to 0.
2、The modified SSBCS by methyl silicone oil and polyvinyl alcohol have four new phases, including NaO2、Na2Si2O5、Na(Si2O4(OH))H2O、Na0.91Al1.88(Si3.45Al0.55)O10(OH)2.
3、Methyl silicone oil and polyvinyl alcohol improved the strength of SSBCS by absorbing free sodium ions (Na+) from the sand and by covering the surface of SSBCS with a film to reduce water absorption.
Funding
This work is funded by the Henan Provincial Key Research and Development Special Project "Processing and Industrialization of Key Structural Components of Ultra Large Mining Height Support Equipment" (Approval No. 241111221500)
Author Contribution
H.P. and X.D. wrote the major manuscripts.M.D. ,Y.S.,F.M.,X.M.,J.Z. and Y.L. made grammatical changes to the paper.W.C. made changes to the iconography of the paper.All authors reviewed the manuscript.
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No competing interests reported.