ELABORATION AND MECHANICAL BEHAVIOR OF HIGH STRENGTH FOAM GLASS

GUO Hong-wei, GONG Yu-xuan, LIU Xin-nian, LI Ying-hui

(School of Materials Science & Engineering, Shaanxi University of Science & Technology, Xi′an 710021,China)

0 Introduction

Foam glass-ceramics, which feature high strength, thermal and sound insulation properties, and exhibit good moisture-proof and fire-proof abilities, are new constructional materials used in the internal and external wall of buildings. Fernandez, Bernardo, and etc[1-8].prepared the foam glasses using waste glasses as raw materials with other additives. Fly ash was used as raw materials in the foam glasses′synthesis of Boccaccini, Ducman, and etc[9-12]. Vladimir and Méar synthesized the foam glasses with non-metallic minerals[13-16].

As a country with an extensive manufacturing and consuming amount of domestic appliances, China has stepped into the peak of replacing TVs and computers. The study of Fernandez[1]suggested that the increasing speed of the amount of waste CRTs in China is reaching more than 6 million sets per year. Also Méar and Shuya[17，18]revealed that the waste CRTs, which contain Pb and consist 55～65 wt% of a set, should be recycled properly. Producing foam glass-ceramics from waste CRTs is an excellent method of electronic glass waste recycling: environment protection and achievement of substantial economic benefits.

In this paper, the influence of the process parameters on mechanical properties of foam glass-ceramics is discussed. The preparation of foam glass-ceramic which used waste CRTs as raw material and SiC as foaming agent is described. The pore distribution and the microstructure were determined by SEM micrograph. And the influence of processing techniques on mechanical properties of the foam glass-ceramic sample was observed.

1 Experimental Procedures

1.1 Processing

Fig.1 Flow chart of experiment (each step ofprocessing is displayed summarily)

Fig.1 is the flow chart of experimental process. In the preparation of foam glass-ceramic, the waste CRT, which is the main material for synthesis, comes from Caihong Corp of Xianyang, Shaanxi. Analytical reagent SiC (through 100 sieve mesh) was used as foaming agent, and Suzhou soil and quartz sand were used as release agent ( weight ratio=1/3). First, peel off the glasses from CRT. Then, cleaned glass was ball milled through 200 sieve mesh. Next, the CRT glass powder and SiC powder were mixed in ball mill. Last, the evenly mixed mixture (3～7 wt% of SiC) was put in the mold on which has brushed release agent for sintering in the furnace.

1.2 Tests

The foaming temperature of the mixture was measured by the Ladder-temperature furnace[19].The crystallization and softening temperature of the mixture was determined using a differential scanning calorimeter (NETZSCH STA-409) with a heating rate of 10 ℃/min. During all runs the sample chamber was purged with dry nitrogen. The deviation in glass transition temperature gained from the DSC test is ±1 ℃. Later with the assistance of JXD-Ⅱ reading microscope, the bubble diameter of the sample was carried out accurately. In the mechanical property test, in order to qualify the testing size, the samples of foam glass-ceramics were cut and polished. The specimens used in bending strength test were machined into 45 mm×4 mm×3 mm test bars. The three-point bending strength was measured for specimen bars with a span of 20 mm at a cross-head speed of 2 mm/min, using a PT-1036PC testing machine. Each final result is an average of five measurements. Square foam glass-ceramics samples of 12.5 mm length and 5 mm×5 mm section area were subjected to uniaxial compressive loading. These tests were conducted at a cross-head speed of 2 mm/min on the same testing machined used as the measurements of compressive strength. In the microstructural characterization of foam glass-ceramics, the crystalline phase precipitated in the prepared sample was examined by the Rigaku D/max2200PC X-ray (CuKα, 40 kV, 40 mA, scanning rate 5°/min, 0.01°) and the morphology was investigated by Quanta 200 SEM produced by Philip-FEI Corp.

2 Results and Discussion

In the heating process, reactions occurred between the compositions, and weight of the mixture changed apparently. Fig.2 is the DSC and TG curves of sample with foaming agent SiC in amounts of 5 wt%. The occurrence of an apparent endothermic peak at 80～100 ℃ is attributed to the evaporation of small amount of mixed water which absorbed heat. Another apparent exothermic peak is attributed to the heat release of reaction between foaming agent SiC and the oxygen at 600 ℃. The TG curve in Fig.2 shows that the weight of the sample was starting to increase. The mixture absorbed heat and became softened quickly at 780 ℃ with a intensified reaction between foaming agent SiC and PbO in the waste glass, and small amount of heat were released at the same time; when heated to 830 ℃, the reaction between SiC and PbO were intensified while the viscosity of the softened mixture lowered (shown as Eq.(1)). Apparently, foaming process is relatively ideal at the temperature of 830 ℃.

Fig.2 DSC and TG curves of sample (foam agent SiC dosage of 5 wt%)

Fig.3 is the foaming temperature determined by Ladder-temperature furnace at the bubble diameter of 2 mm. When the dosage of SiC was 5 wt% the foaming temperature came to the lowest. This is because that less gas has been produced when the SiC dosage was relative low. The foaming temperature roses, when the SiC dosage was in the amount of 7 wt%.

Fig.4 Variation of mechanical strengthalong foam agent dosage (Held in thefurnace for 30 min)

Fig.4 is the curve between SiC and the mechanical properties after holding at the foaming temperature for 30 min. As can be seen from Fig.4, the compressive strength and the bending strength decreases as the SiC dosage increases when the dosage of foam agent SiC is relatively low. While the compressive strength and the bending strength increases as the SiC dosage increases when the dosage of SiC is relatively high. This is due to the low gas rate when the sample contains little SiC. And that leads to a close integration between glasses. With the increasing dosage of foam agent, a considerable amount of gas was produced in the foaming process, contributing the separation of small bubbles in the sample. And the separation caused a great reduction of the mechanical properties of the sample. The reaction Eq.(1) occurred between foam agent and PbO in the mixture, a great number of micro-crystal PbO were precipitated during the reaction when the foam agent dosage is 7 wt% (Fig.5 shows the existence of SiO2in the sample as the state of glass phase). And the mechanical properties of the sample were meliorated by the precipitated micro-crystals[19].

4PbO+SiC→Pb+SiO2+CO2↑

(1)

By using the sintering schedule offered by the reference[1], the experimental results revealed that the strength of the samples is obviously affected by the heating rate between 600 ℃ and the foaming temperature. Fig.6 is the curves of heating rate and mechanical properties. With the increasing of heating rate, the compressive strength and the bending strength decreased gradually. And at the heating rate of 20 ℃/min, the compressive strength and the bending strength decreased sharply. In addition, in the experiment, we found that to reach the same bubble diameter at a low heating rate the foaming temperature must be roused. In Fig.6 the foaming temperature are 860 ℃，850 ℃，830 ℃ and processed at the heating rate of 8 ℃/min，10 ℃/min，15 ℃/min correspondingly. This is because when the heating rate is low, the CO2released by foam agent have enough time to escape from the surface of our sample. After this process the bending strength and the compressive strength increased, because the gas rate decreased and the sample became relatively dense. When the heating rate reaches 20 ℃/min, the strength of the sample decreased. Seen from the cross-section of the sintered sample, larger pores are generated in the bottom of the sample. This is caused by the thermal difference between the top and the bottom of the sample. Obviously the bottom part of the sample touched the mold, nearer to the heat source and being foamed more rapidly than other parts of the sample. The lower thermal conductivity of foamed sample also contributed to these lager pores.

Fig.6 Variation of mechanical strength alongheating rate (foam agent SiC dosage is 5 wt%)

Fig.8 Variation of mechanical propertiesalong holding time

Fig.7 presents the relation between cooling rate and the bending strength (foam agent SiC dosage of 5 wt%). With a lower cooling rate, the sample behaves better of bending strength. This can be explained by the following truth: as an excellent thermal insulation material, foam glass-ceramics is tend to have great difference of temperature between internal and external areas when being heated or cooled at a high rate. Then stresses which may generate cracks were generated in internal areas. As a result of the generation of the stresses, the bending strength decreased.

As can be seen from Fig.8, the compressive strength and the bending strength of the sample increased simultaneously with increased holding time, caused by the increasing of micro-crystal Pb, and the precipitation of other crystals such as Pb3O4，Al6Si2O13(shown as Fig.9). And these large number of micro-crystals are produced via the reaction of Eq.(2) and (3) during the process of thermal insulation of the foaming process.

2O2+3Pb→Pb3O4

(2)

2O2+SiC→SiO2+CO2↑

(3)

Larger pores were precipitated at the bottom of the cross-sectional area of our samples. The bottom of the samples is nearer to the heat source and the pores grown faster than the top. The larger pores precipitated at the bottom of the sample were due to the worse thermal conductivity caused by the asymmetrical heating of the bottom and the top theoretically. The SEM visual examination enabled the description of the samples in terms of their microstructural parameters such as pore distributions. Fig.10 show that with the addition of 5 wt% SiC and held for 30 min at the foaming temperature of 840 ℃, the pores range from 0.5 mm to 2 mm and are distributed evenly (closed pore structure). The white dots on the bubble wall are the crystals precipitated at high temperature.

Fig.10 SEM micrographs of foam glass-ceramic in different magnificationfactors (840 ℃，30 min, and the foam agent SiC dosage is 5 wt%)

3 Conclusions

In this study, the specific properties of foam glass-ceramics acquired form waste cathode ray tube were investigated. The data of crystalline temperature was obtained from the DSC test, and the morphology of the precipitated micro-crystals was observed by the using of SEM.

Our results of regarding the mechanical properties of foam glass-ceramics indicated that the bending strength and the compressive strength are strongly affected by the dosage of foam agent dosage, heating rate, cooling rate and holding time. The experimental results show that foam glass-ceramics with a strength of 2～20 MPa can be prepared from adding 1～7 wt% foaming agent of SiC through the heating rate of 8～25 ℃/min and the cooling rate of 1～3 ℃/min. With the extension of the thermal insulation, crystals like Pb, Pb3O4and Al6Si2O13were precipitated from the sample, which generally contribute to the improvement of the foam glass-ceramics′mechanical property.

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