Preparation of foamed glasses from CRT TV glass by means of hydrothermal hot-pressing technique

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Abstract

The feasibility to prepare foamed glass specimens by using waste TV monitor (panel) glass as starting material under hydrothermal hot-pressing (HHP) conditions, followed by a conventional heating of the compact, was investigated. Hydrothermally hot pressed glass compacts were preliminarily prepared at a constant temperature of 200 °C, with a loading pressure of 20 MPa and two different water contents (5 and 10 wt%), for reaction intervals between 0.5 and 2 h, employing a glass particle size <38 μm. After the powder densification stage, firing of the compacted disk glass was conducted over a temperature range of 650–850 °C for 1 h. Microstructural observations conducted by SEM showed the formation of a new glass phase which incorporates water, this phase was formed during the glass densification stage at 200 °C for 2 h, with a water content of 10 wt%. The lowest apparent density achieved on these glass compacts heated at 700 °C for 1 h was 0.36 g/cm3. This value is similar to that of other foamed glasses that have close cell network and low thermal conductivity (0.0021 W/cm/°C).

Introduction

In general, the Cathode Ray Tube (CRT) represents nearly two thirds of the total television or computer monitor weight, where 85 wt% of the CRT weight corresponds to glass. The development of novel materials by recycling this type of glasses has recently been the research subject of some investigations.1, 2 Some of the most recent studies have been directed towards for preparing novel glass-ceramic materials with a very high level of controlled morphology of its microstructural constituents.1 One of the emerging routes for future processing of CRT glass is being orientated towards the production of foamed glasses. At present, the route proposed for foaming monitor glasses involves the addition of SiC and TiN which act as reducing agents at high temperature. These compounds lead to the formation of gas (CO2 and N2) at high treatment temperatures (800–900 °C) and simultaneously the foaming of a softened glass phase is also promoted.3

The present authors have recently reported an alternative route for preparing porous glasses, which involves the previous preparation of a precursor glass compact by treating glass particles in the presence of aqueous solvents, by means of the hydrothermal hot-pressing technique (hereafter referred to as HHP). Subsequently, the exposure of the glass compacts at temperatures in the range of 650–750 °C in air, resulted in a remarkable foaming of the glass. This process was applied to common glass used for the preparation of glass containers (bottle glass). In this particular case, it was found that the formation of pores proceeds by decomposition of a new glass phase formed during the hydrothermal hot pressing compaction, which incorporates water molecules in its structure. The decomposition of this phase at high temperature results in gradual water evaporation. Simultaneously, the formation of a pyroplastic mass of softened glass promoted a bulk expansion of the glass compact. The maximum compressive strength for the close cell porous glass was 14 MPa and its apparent density was 0.40 g/cm3; the thermal conductivity of this porous specimen was 0.21 W/(m K).4 These porous materials are likely to have a good performance as thermal insulators because their thermal conductivities lie within the values corresponding to thermal insulating materials such as wood or fiber glass wool.5 Furthermore, based on the increasing interest for developing glass porous materials with stable and controlled pore size, particularly for thermal and acoustic insulation proposes, from engineering, economic and environmental points of view, the production of glass foams is an interesting route for the use of waste glasses.6, 7, 8 Therefore, in the present work we intended to study the effect of some parameters such temperature, time and water content on the densification of CRT powder glass during HHP treatments. Additionally, the feasibility for preparing foamed glass was investigated by heat treatments of HHP’ed CRT TV glass compacts in air.

Section snippets

Hydrothermal hot pressing of TV panel glass

The as received CRT TV panel glass was employed for preparing HHP glass compacts. The panel glass was milled for 12 h and then sieved to obtain a small glass particle size, less than 38 μm. The chemical composition determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES) showed that the glass consisted basically of SiO2 (50.05 wt%), BaO (24.14 wt%), Na2O (9.5 wt%), K2O (8.90 wt%) and Al2O3 (2.21 wt%), and other oxides in minor amount. Initial experiments were conducted to

Densification and microstructural evaluation of hydrothermal hot pressed CRT waste glass compacts

In Fig. 1 is shown the typical behavior of the bulk density of hydrothermally densified CRT glass specimens as a function of the water content, the CRT glass compacts were prepared with a glass particle size smaller than 38 μm, at a temperature of 200 °C for 2 h, with a loading pressure of 20 MPa. A slight decrease on the bulk density was obtained by increasing the water content, with a maximum value of 2.74 g/cm3 achieved for the compact prepared with only 5 wt% of water. The bulk density values

Conclusions

In accordance with the current results, it was found that the addition of a small amount of water (10 wt%) influences the expansion process of glass compacts prepared under HHP conditions and subsequently heated in air. An increment in the water amount allows increasing the reactivity of glass particles with water during the hydrothermal hot pressing process. We assumed that the Na+ Ba+2 and K+ ions had an active participation in the softening of the glass particles in order to produce a new

Acknowledgments

One of the authors, ZMV, wants to thank for the financial support given to conduct the present research through the research grants ITS-DEPI/1106/002 and DGEST-524-07. The authors want to acknowledge Eng. Felipe de Jesus Marquez Torres for his help on the SEM sample observations.

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