INFLUENCE OF STORAGE IN THE DEGRADATION OF SODA...

Christina-Amalia Drosou, Nikolaos Papadopoulos, Angeliki Moutsatsou

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Journal of Chemical Technology and Metallurgy, 50, 4, 2015, 415-422

INFLUENCE OF STORAGE IN THE DEGRADATION OF SODA LIME GLASS CONTAINERS

Christina-Amalia Drosou1, Nikolaos Papadopoulos2, Angeliki Moutsatsou1

1 School of Chemical Engineering, National Technical University of Athens, Athens, Greece2 Yioula Group, 5 Orizomylon Street, 122 44 Aegaleo (Athens), Greece, E-mail: [email protected]

ABSTRACT

The aim of this study is to examine the weathering influence during storage and the rate of degradation of soda lime glass. Glass is thought of as a corrosion proof material but under certain exposure conditions its chemistry, structure and properties are affected. Key variable in this phenomenon is the cations’ behavior. In order to examine the effect of vari-ous foods, glass samples (flint and green) from production lines, industrial storage and recycling sites are subjected to an investigation. The latter refers to an exposure to acidic, alkaline and neutral environments for a month. The solutions obtained are studied in respect to the concentration of glass formers and glass modifiers (silicon and calcium, respectively) for both colors and that of chromium for the green colored samples. In addition, the thin film formed on the surface of the glass is studied via XRD and FT-IR. The rate of the degradation and the extent of corrosion in each sample is determined by measuring the concentrations of silicon, calcium and chromium. The XRD and FT-IR patterns reveal new crystalline phases formed on the glass surface and help to clarify the behavior of the cations leached from the glass surface. Opti-cal observation of the samples and SEM examinations are integrated in the study. The results indicate that the rate of degradation in the course of the alkaline attack increases exponentially whereas that of the acidic and neutral one stays almost constant with a slight tendency of decrease. Differences are also observed between the two colored glasses with the flint containers having a higher rate than that of the green colored samples and between the samples coming from the production line, the storage and the recycling sites. The examination of the film formed on the surface of the samples studied reveals the formation of several crystalline phases - mainly compounds of silicon, calcium and magnesium. The microstructural evaluation of the weathering tests confirms the previous results.

Keywords: weathering, rate, soda lime glass, corrosion products.

Received 30 January 2015Accepted 15 May 2015

INTRODUCTION

As far as packaged food products are concerned, people, more often than not, tend to concentrate only on the quality of the food product itself and seem to over-look relevant quality issues in respect to the packaging. The latter is far from a trivial issue. Indeed, inappropriate packaging can lead to grievous health risks, not to men-tion the adverse effects it can have on both the quality of the food product and its characteristics, for example

its flavor and aroma. As such it should not come as a surprise that glass bottle packaging has attracted, espe-cially in recent years, the attention of the food industry. Apart from the fact that glass is a sustainable material, something extremely important from both environmen-tal and economic perspectives, this safe and inert type of packaging seems to be by far the most appropriate material for the food industry ensuring the quality of the food products [1 - 7].

The particular chemical composition of soda lime

Journal of Chemical Technology and Metallurgy, 50, 4, 2015

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glasses, which are low in silica and rich in alkali and alkaline earth components, in combination with acidify-ing gases, water or chemical agents in food, can result in severe degradation of glass. Therefore, the high demand on glass packaging requires detailed knowledge about the quality of glasses used especially regarding chemical resistance and strength.

Following the production the glass is stored in pallets wrapped with a plastic film of LDPE in open air storage sites prior to their release to the consumers. Even then they are subjected to weathering conditions - a phenom-enon of a greenhouse effect type occurs and starts the containers’ degradation. The corrosion rate depends on numerous parameters such as the temperature, the aging time, the chemical composition of the glass as well as the chemical composition of the aging solution [8 - 15].

The extent to which the glass weathers depends on the behavior of the alkali ions released from the glass surface. If they are released from non-bridging oxygen sites in the glass and occupy sites previously filled by hydrogen ions (either on the surface or in the bulk of the glass), then once the available sites are saturated, the re-action must stop and the glasses will appear unweathered. If the alkali ions react with gases such as CO2, CI2, or SO2

to form crystalline phases, then the reaction can continue and the glass will appear weathered. The precipitates act as an alkali sink and continued weathering can lead to hydrolysis of the glass and subsequent cracking and spalling. The hydrolysis of SiO2 - rich materials as a func-tion of temperature, pH, and time has been the subject of considerable research. In most cases the overall reaction is presumed to be surface controlled [16 - 17].

The aim of this study is to examine the weathering influence during storage and the rate of degradation of soda lime glass. To attain this goal, samples of flint

and green colored glasses are obtained from industrial production lines and storage sites as well as from recy-cling sites and exposed to alkaline, acidic and aquatic conditions for a month.

EXPERIMENTAL

Materials and methodsCharacterization of the raw materialsFor the purpose of the present study three types of

soda lime glass samples were used. These samples were collected from: i) a production line (post-industry), ii) exterior storage sites of glass industries (pre-consumer) and iii) recycling units (post-consumer). The samples were exposed to acidic, alkaline and neutral environ-ments. Flint and green colored glass samples were selected since these were the colors most frequently produced by the industry. The glass samples were chemically analysed using X-ray Fluorescence spectros-copy technique (THERMO A.R.L. ADVANT’XP). The chemical composition identified is presented in Table 1.

The samples were collected in the form of bottles. Prior to the chemical attack the samples were washed, cleaned and any metal, plastic or paper parts were removed.

Weathering tests and Characterization TechniquesIn order to study and evaluate the extent of degrada-

tion of the glass surfaces when in contact with various products, the glass samples were exposed to acidic (HCl), alkaline (Na2CO3, NaOH) and neutral conditions (ultrapure water) The effect of temperature was followed as well. Therefore, 1M solutions were prepared and the specimens were exposed to each solution for a month at 60oC. The samples were taken out of the solution at predefined intervals to measure the silicon and calcium

Glass type

(Si-Na-Ca)

Component, %

SiO2 Al2O3 K2O MgO CaO Na2O SO3 Fe2O3 Cr2O3

flint 70.6 1.75 0.55 2.45 10.70 13.25 0.45 0.45 -

green 70.5 1.80 0.45 2.75 10.15 12.95 0.25 0.45 0.25

Table 1. Chemical composition of the samples.


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Crystalline phase

Origin of the gel sample (alkaline attack NaOH) Flint Green

Production line

Storage site

Recycling site

Production line

Storage site

Recycling site

Quartz (SiO2) + + + + + + Calcite + + + Brucite + + + + + Diopside (CaMg (Si2O6)) + + + Calcium silicate + + + Sodium silicate + + + Sodium carbonate (Na2CO3)

+ + +

Stishovite + + +

Christobalite (SiO2) + + +

Table 2. Crystalline phases of the surface layers after the alkaline attack with NaOH.

Crystalline phase

Origin of the gel sample (Na2CO3) Flint Green

Production line

Storage site

Recycling site

Production line

Storage site

Recycling site

Quartz ( SiO2) + + + + + + Calcite + + + + + Aragonite (CaCO3) + + + + + + Brucite + + + + + Magnesium silicate + + Hydrogen silicate + + Sodium carbonate + Stishovite + + Potassium hydroxide (KOH)

+ + +

Table 3. Crystalline phases of the surface layers after the alkaline attack with Na2CO3.

ions concentration in the corresponding solutions aiming to determine the rate of dissolution [3, 9, 12]. The con-centration of chromium was determined in the solutions containing green colored glass samples. Flame Atomic Absorption Spectrometry (AAS, VARIAN AA240FS) was applied.

The thin layers formed on the glass surfaces after the alkaline attack were analysed via X-ray Diffraction (Sie-mens D5000, Cu Ka, λ = 1,5406 Å) and FTIR (JASCO FTIR-4200). XRD measurements were performed to as-sess the degree of crystallinity of the corrosion products, while FT-IR was applied to detect chemical alterations

of the specimens’ surface during the chemical attack. The latter effect on the glass surfaces was also evaluated through microstructural observation of the specimens via Scanning Electron Microscopy (FEI Quanta 200 equipped with an EDAX detector).

RESULTS AND DISCUSSION

Chemical attackDuring an acidic, an alkaline or an aquatic attack

different constituents of the glass (formers and modi-fiers) are leached from its surface at different rates. As


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shown in Fig. 1, the rate of the dissolution of silicon ions in the samples of flint glass increases exponentially. The samples obtained from the glass industry and the storage sites appear to have a higher rate than those from the recycling sites. The rates differ by approximately 30 %. However, the rates during the aqueous and acidic attack are less by 60 % than that of the alkaline one. The rates of silicon dissolution in the green glass samples show the same tendency. Likewise, the samples from the recycling

sites have a rate of dissolution less by approximately 35 %. The fast dissolution rate during the alkaline attack is consistent with the mechanism of glass corrosion ac-cording which a rapid breakdown of the glass surface occurs at pH > 9 [8, 12 - 15, 17].

The comparison of the flint and green samples shows that the green glasses have a less rate than that of the flint samples in the course of the alkaline attack as well as in that of the acidic and aqueous one varying

Fig. 1. Rate of dissolution of silicon.


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Bond Wavenumber [cm-1] Type of group Band width

CO3-2 680-720 in-plane bending medium

CO3-2 850-880 out of plane bending narrow

CO3-2 1400-1450 asymmetric stretching narrow

Si-O- 460 bending stress broad

Si-O- 950-1050 anti symmetrical stretching medium

OH- ~3450 vibration broad

Table 4. Infrared absorption bands of silicates and carbonates

Fig. 2. Rate of dissolution of calcium.


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from 13 % to 19 % depending on the sample. This is a fact that can be attributed to the chromium content of green glass and to the higher content of aluminum and magnesium oxide which affect the glass structure by making it more durable. The addition of these two oxides reinforces the main glass structure and prevents the leaching of glass formers and modifiers. This results in glass durability increase.

The behaviour of calcium (Fig. 2) during the chemi-cal attack differs from that of silicon. The leaching of calcium ions does not appear to have a constant rate which can be attributed to the surface precipitation of secondary products of the dissolution process. As far as the alkaline attack is concerned the initial contact of the glass surface with the attacking medium (of duration of up to 3 days) causes calcium leaching but then the latter stops. Secondary products start to develop on the glass surface and create a protective barrier towards further leaching. Calcium leaching during the acidic and aque-

ous attack shows fluctuations which can be attributed to the precipitation of less soluble calcium compounds created on the surface of the glass (CaCl2). Calcium dis-solution in the green glass is of a similar pattern. The rate is also dependent on that of the secondary products precipitation. The alkaline attack rate on calcium brings about equilibrium in all samples after 15 days of expo-sure [8 - 10, 15].

SEM Characterization Images (a) through (d) present scanning electron

microscopy images of the surface of reference and glass samples subjected to an alkaline attack. Images (a) and (c) refer to the white and green reference samples, re-spectively, collected from the production line (NF and NG). Images (b) and (c) refer to the corroded samples. The surface of the reference samples appears to be free of corrosion since there are no signs of craters, cracks or pits. On the contrary, the surfaces of the samples

Fig. 3. SEM images of a) NF reference, b) NF alkaline attack, c) NG reference and d) NG alkaline attack.

a) b)

c) d)


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which have been attacked by alkaline agents are rougher. Specifically, the surface of the flint glass (Image b) has many cracks as well as white spots which are attributed to secondary corrosion products. It is clear that the extent of corrosion in the green samples (Image d) is greater since the surface of the samples appears very coarse with a sponge like pattern containing many pits and craters. The white spots and flakes in this case are also attributed to the presence of secondary corrosion products [18, 19].

Corrosion products analysisThe thin layer formed on the glass surface (corro-

sion products) after the alkaline attack was probed with x-ray diffraction and FTIR analytical technique. The XRD patterns of the gel layer formed on the cullet dur-ing the weathering experiments reveals the presence of several crystalline phases, mostly compounds of silicon, calcium and magnesium. These rich in silicon layers, whose thickness increases with time, have a different composition when compared to that of the bulk glass. Tables 2 and 3 summarize the main crystalline phases observed in the x-ray diffraction patterns [20 - 23].

The main frequencies observed in the FTIR spectra of all samples are summarized in Table 4, [21, 22, 24].

CONCLUSIONS

The glass is a very appealing material and has many advantages when it comes to food and beverage packaging as it can protect the product and at the same

time renders the containers attractive for the consum-ers. The weathering phenomena when combined with the effect of the food stored can lead to degradations of the container quality. That is why samples of the most commonly used glasses (flint and green) are selected from industrial line and storage sites as well as from recycling sites and subjected to artificial weathering tests. The main conclusions drawn from this study in relation to the parameters studied are:l The rate of silicon dissolution in the course of an

alkaline attack increases exponentially whereas the rate is significantly less in case of an acidic and a neutral attack.l The rate during an aqueous and an acidic attack

is less by 60 % than that in an alkaline one. l Flint containers have a corrosion rate higher by

13% to 19 % than that of the green glass depending on the origin of the sample. Post-consumer samples have a less rate of corrosion than those coming from industrial lines and storage sites. The difference goes up to ca 30 % and 35 % in case of flint and green colored samples, correspondingly.l The rate of calcium dissolution is dependent on

that of secondary products precipitation.l The examination of the film formed on the surface

of the samples studied reveals the formation of several crystalline phases, mainly compounds of Si, Ca and Mg.

AcknowledgementsThe authors would like to thank Yioula Group S.A.

for their support and valuable insight on this study.

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