Patent Application
20100009202
The present invention relates to the storage of glass, and in particular, to the protection of the surface of glass sheets during storage and transportation.
Sheets of glass are vulnerable to staining due to corrosion of the glass surface during storage, and also to damage caused by transit rub (where two sheets of glass rub together and/or where glass fragments from the cutting process rub the surface of the glass) during transportation. Both staining and transit rub result in the glass having a poor surface quality, which then creates problems in subsequent uses, for example, coating, printing, silvering, laminating, etc. The damage to the surface of the glass is often also visible to the eye. Known solutions to both staining and transit rub involve using an interleavant between adjacent sheets of glass. The interleavant prevents adjacent sheets of glass from being in contact, reducing or eliminating transit rub. Typical interleavants include paper, PMMA (polymethyl methacrylate) beads and coconut husk flour.
Storing glass in humid conditions causes water to adsorb onto the surface of the glass. Staining of the glass occurs when water on the surface of the glass sheet reacts with the silicate network of the glass. Water diffuses into the glass and exchanges for alkali glass components, which are then leached to the surface of the glass. The leached alkali glass components, particularly sodium and potassium, dissolve in the surface water to form an alkaline solution, which can attack and dissolve the silicate matrix of the glass itself, creating a series of etch pits on the surface of the glass. Other glass components, such as calcium and magnesium, can then react with the silicate species dissolved by the alkali attack to form insoluble salts, causing a precipitate to be deposited on the surface of the glass. The main approach to reduce staining of the glass surface is to use a chemical stain inhibitor, which reacts on the surface of the glass to neutralise the leached alkali. Other approaches, such as the use of film coatings on the surface of the glass may also be used. Chemical stain inhibitors are typically used in conjunction with interleavants, for example, coconut husk flour and PMMA beads, in order to prevent transit rub. Interleavants, such as paper, may also reduce the amount of staining present on the surface of the glass by absorbing some of the water present on the surface of the glass. As the amount of surface water is reduced, the amount of alkali leached and consequential surface damage to the glass are reduced.
GB 1,477,204 discloses the use of a weakly acidic material as a stain inhibitor. A porous support material, such as coconut shell flour or hardwood flour is used to support a weak acid, such as maleic or adipic acid. The porous support material is then mixed with particles of a chemically inert plastics material, such as a thermoplastic homopolymer or copolymer, to form an interleavant. The interleavant is then applied to the glass as a powder.
GB 1,413,031 also discloses the use of weak acids as stain inhibitors, for example, adipic acid, citric acid, maleic acid and malic acid, suspended in a solvent and sprayed onto the surface of the glass to be stored. U.S. Pat. No. 3,723,312 discloses the use of salicyclic acid, or a mixture of dedusted agglomerated salicyclic acid and an inert separator material, such as polystyrene beads, as a stain inhibitor.
US 2005/0011779 A1 discloses the use of watery mixtures of adipic and malic, adipic and citric or citric and malic acids as stain inhibitors for glass storage in conjunction with a separating powder as an interleavant. Groups of glass sheets are then hermetically sealed to prevent further water ingress during storage.
All of the above examples are concerned with the direct application of acids to the surface of the glass. However, the application of acids directly to the surface of the glass can actually cause the alkali leaching that produces staining of the glass to become worse.
Under acidic conditions, for example when adipic acid is used, onium ions (H3O+ from the dissolution of the acid in the water present on the surface of the glass) diff-use into the glass and exchange for the alkali metal (sodium) present in the glass. This reaction releases sodium ions from the glass structure that then diffuse to the surface, and react with the acid stain inhibitor. As in the glass corrosion mechanism mentioned above, the alkaline solution of the sodium ions eventually neutralises all of the acid stain inhibitor and the pH on the surface of the glass then increases to initiate alkaline attack on the silicate network of the glass.
In the absence of the acid stain inhibitor, the diffusion of sodium ions to the surface of the glass would have occurred at a rate determined by the diffusion of any water present on the surface of the glass. This is because electrical neutrality must be preserved at the glass surface. Thus, any sodium ions diffusing to the surface must carry a counter-anion with them. In the absence of water, the only counter-anion available in the silicate network is the oxygen dianion, O2−, and this is immobile at temperatures below about 600° C. In the presence of acid stain inhibitors, however, the release of sodium from the network structure is simply an exchange of sodium ions for onium ions with no net change in charge and the counter-ion for the sodium and onium ions is the highly mobile hydroxyl anion, OH−. The direct application of an acid to the surface of the glass results in the mechanisms for sodium ion diffusion in the presence of surface water being catalysed, resulting in an alkaline attack on the silicate network of the glass. The direct application of an acid to the surface of the glass is therefore undesirable.
There is therefore a need for a stain inhibitor, which reduces the staining on the surface of the glass, and which does not act to promote leaching of the alkali content that leads to the dissolution of the silicate network of the glass.
The present invention aims to address the above problems by providing a stain inhibitor, which is not acidic but acts to neutralise the alkali leached to the surface of a sheet of glass in the presence of water, comprising a compound, wherein the compound reacts with the water to produce an acid, and the acid acts to neutralise the alkali leached to the surface of the glass.
The advantage of using a compound that reacts with the water present on the surface of the glass to form an acid, rather than directly applying an acid to the surface of the glass, to neutralise the leached alkali is that the leaching of the alkali that results in dissolution of the silica network of the glass is not catalysed. This results in the reduction of haze and improvement in the surface quality of the glass.
Preferably, the compound hydrolyses to produce the acid. The hydrolysis reaction may be base catalysed.
The compound may be soluble in water, or may be dispersed in water using a surfactant.
Preferably, the compound is an ester.
The compound may comprise one of epsilon caprolactone, polycaprolactone triol, diacetin, triacetin, diethyl malate, dihydroacetic acid, diethyl tartrate, triethyl citrate, diethylene glycol diacetate or Borester 240.
A solution comprising the compound and a solvent may be applied to the surface of the glass. In particular, the solution may be sprayed onto the surface of the glass. The solution may further comprise an alkali to neutralise any acid formed from the compound in storage. The alkali may be caustic soda. The solvent may be one of isopropanol, acetone and DI water. The solution may also comprise a surfactant.
The solution may be used in conjunction with an interleavant. The interleavant may be one of PMMA beads, UHMWPE beads, coconut husk flour, hard wood flour or paper.
Preferably, the compound has a pH between 6 and 9.4 when dissolved in DI water. More preferably, the compound has a pH between 7 and 9 when dissolved in DI water.
The invention also provides the use of a compound that reacts with water to form an acid, to neutralise alkali leached to the surface of a sheet of glass in the presence of water.
The invention further provides the use of an ester that hydrolyses in water to form an acid, to neutralised alkali leached to the surface of a sheet of glass in the presence of water.
The invention yet further provides a method of reducing the haze of the surface of a sheet of glass in storage, comprising applying a stain inhibitor to the surface of the glass, the stain inhibitor comprising a compound that reacts with water to produce an acid to neutralise alkali leached to the surface of the sheet of glass in the presence of water. Preferably, the compound hydrolyses. Preferably, the compound is an ester. Glass treated with the stain inhibitor is also provided, as is the use of an ester as a stain inhibitor to prevent the corrosion of glass in storage.
The invention will now be described by way of example only, and with reference to the accompanying drawings in which:
The corrosion of silicate glass occurs when water from an adsorbed surface film diffuses into the silica network of the glass, and establishes an equilibrium:
The reaction is catalysed by the hydroxyl anion, and so is strongly pH dependent:
Thus, the silicate network is stable under acid conditions, but is attacked rapidly at pH>9. However, under acid conditions, the oxonium ion, H3O+ exchanges rapidly with the alkali in the glass:
If the released alkali is not washed away, it will increase the pH of the water in contact with the glass surface, and as discussed above, if the pH exceeds 9, dissolution of the silicate network will commence. In addition, CO2 dissolves in the adsorbed water film, creating carbonic acid, which also diff-uses into the surface of the glass. At the same time as Na diffuses to the surface of the glass, the protons in the water are also exchanged for other elements, such as K, Ca, Mg, Ca and Mg precipitate at the surface of the glass when they react with dissolved carbonate and silicate anions to form insoluble salts (carbonates and silicates). Such insoluble salts are then re-deposited on the glass surface. The combination of precipitated salts and etched regions (from the dissolution of the silicate network) causes an increase in haze (decrease in direct light transmission of the glass). In addition, when alkali is leached to the surface of the glass, a region of the glass just below the surface becomes depleted of sodium. This can by verified by use of XPS (X-ray photon spectroscopy).
The corrosion process therefore starts with the diffusion of water and onium ions into the glass, resulting in leaching first of the alkali metals and then the alkaline earth metals. If the pH increases sufficiently, the actual silicate network will break down.
As discussed above, the use of adipic acid catalyses the first stage of the corrosion mechanism by increasing the onium ion concentration. The pKa value for the first ionisation of adipic acid is 4.4, and a 1% solution of adipic acid in water has a pH of 2.8, giving an increased concentration of onium ions compared with a glass surface where there is no acid present. This also explains why use of a weaker acid, such as boric acid, results in a more effective stain inhibitor.
However, even the use of a weaker acid, such as boric acid, also creates problems. Although the pKa value is higher (9.47), giving a pH of 5.0 and therefore the concentration of onium ions for a solution of boric acid in water is ≈150 times less than for a solution of adipic acid in water, the boric acid is rapidly neutralised by the leached sodium hydroxide. This increases the pH of the water on the surface of the glass to approximately >9, thereby triggering the dissolution of the silicate network in the glass. Therefore, although boric acid does not catalyse the leaching of sodium to the glass surface as much as adipic acid, the pH of the boric acid that is partially neutralised by leached alkali will approach the pKa of 9.47 and this is high enough to initiate alkaline attack of the silicate network, with the result that the glass will begin to corrode.
On this basis, an ideal stain inhibitor should have an initial pH of 5 to 8, a pKa of 6 to 8.5 and will neutralise a large amount of caustic soda. In addition, the stain inhibitor should be non-toxic, water soluble and non-volatile.
Based on the above theories and experimental results, it is possible to list criteria that a compound should ideally satisfy, in order to be stain inhibitor:
The approach taken based on this analysis in the present invention is to consider chemical compounds that are initially pH neutral, yet will act to buffer any alkali leached from the glass. Such chemical compounds hydrolyse slowly in neutral water but rapidly at high pH by base catalysis to produce acids that can then neutralise the leached alkali. As the compound is initially pH neutral, the concentration of onium ions is low, so the diffusion of sodium and resultant silicate network dissolution is not catalysed by the presence of the compound. At higher pH, acid is produced rapidly by base catalysis, neutralising the leached alkali before the silicate network dissolves, and effectively buffering the alkali produced. Suitable compounds include anhydrides, imides, amides, esters, cyclic esters(lactones) and cyclic amides(lactams).
One particularly suitable class of chemical compounds is esters, for example, caprolactone. Esters hydrolyse to produce an acid and an alcohol, the hydrolysis rate being pH dependent. If an ester hydrolyses very slowly in pure water, it will not cause a sudden decrease in pH when placed on a glass surface, but if pH increases due to leached alkali, the hydrolysis rate will increase producing acid more quickly to counter the pH increase. The hydrolysis of the ester should be base catalysed, such that the rate of hydrolysis increases as the pH increases, to achieve maximum resistance to glass corrosion.
In order to determine the effectiveness of esters as stain inhibitors, a series of trials were carried out, using the esters listed in Table 1 below:
The caprolactone monomer is a cyclic monomer and a mobile liquid at room temperature, and is designated epsilon caprolactone or delta caprolactone, depending on the chemical structure. Only epsilon caprolactone was used in the ester trials.
Table 2 below lists the experimental conditions used in the ester trials, including the solvent used to dissolve the ester in before spraying onto the surface of the glass, the amount of ester dissolved in terms of grams of inhibitor applied per m2 of glass, whether the solution was pre-neutralised, and the type of interleavant used.
Any free acid in the ester, produced by hydrolysis during storage, was first neutralized by adding a small amount of dilute sodium hydroxide until the pH was >=6. The hydrolysis of all of the esters listed in Table 1 is base catalysed.
Samples were prepared from 4 mm thick float glass, cut into 30 cm by 30 cm plates, and washed using a flatbed washer with hot, de-ionised water (at 60° C.), but with no detergent, to remove any glass fragments present on the glass surface from the cutting process. Once washed, the plates of glass were dried using an airknife to avoid drying marks on the surface of the glass. The stain inhibitor was then applied by spraying in the form of a solution in Iso-propyl alcohol or acetone, as shown in Table 2 above. The solvent evaporated leaving the ester stain inhibitor on the glass and either PMMA (poly(methyl methacrylate)) or UHMWPE (ultra high molecular weight polyethylene) interleavant beads were applied at 100 mg/m2. Each stain inhibitor was tested with an interleavant to mimic real life situations where the interleavant is necessary to reduce transit rub and to separate the plates of glass.
The individual plates of glass were then stacked in groups of 7, comprising 5 test plates and 2 cover plates, placed on a mini-stillage (i.e. stacked almost vertically on an L-shaped holder) and put into a humidity cabinet for accelerated ageing. The accelerated aging cycle chosen was 40° C./80% relative humidity for 10 days and then 60° C./80% humidity for 40 days. The weathering of the glass is affected by changes in temperature and humidity. For example, weathering for 30 days at 60° C. may result in glass having similar corrosion to that weathered for 6 to 20 years at 20° C., depending on the precise activation energy for sodium diffusion. A two-stage test is used to balance light weathering under slightly accelerated conditions, for example 3-6 months at 20° C., and the minimum amount of weathering measurable using haze detection.
Once ageing was complete, the mini-stillage was removed from the humidity cabinet and each glass plate washed individually to remove the stain inhibitor and the interleavant, and inspected visually for any sign of staining. The haze of each plate was then measured using a BYK-Gardner Haze-gard Plus machine, in accordance with ASTM D 1003.
In general, the performance of all the esters was good, as each gave less haze than the LBK paper reference. Of the esters, epsilon caprolactone gave the best performance, showing zero % haze after 50 days in the weathering cabinet. Diethyl malate, dehydroacectic acid, diethyl tartrate, diethylene glycol diacetate and polycaprolatone triol also gave good results, with a haze of 0.06% or less after 50 days. Borester 240 and triethyl citrate gave mixed results, with diacetin and triacetin showing a slight increase in haze after 40 days. As a comparison, LBK paper gave a steady increase in haze over the measurement period, with samples showing 0.21% haze after 50 days in the weathering cabinet.
Other suitable esters include delta caprolactone, glycerol acetate, sucrose acetate (in particular, sucrose tetraacetate and sucrose octaacetate), glucose acetate, diethyl(bis hydroxymethyl)malonate, diethyl(ethoxymethylene)malonate and poly(vinyl pyrrolidone-co-vinyl acetate). In addition to water soluble, non-toxic, non-volatile compounds, esters that are not soluble in water, for example, whiskey lactone and decanolactone, can be mixed with a surfactant, for example, benzalkonium chloride (available under the trade name Quadrilan BC), before application to the surface of the glass. It may also be necessary to dissolve the ester in acetone, rather than Iso-propyl alcohol. For example, although glycerol diacetate and poly(vinyl pyrrolidone-co-vinyl acetate), may be dissolved in Iso-propyl alcohol, sucrose octaacetate needs to be dissolved in acetone with the addition of 1% beiixalkonium chloride surfactant, based on the weight of the sucrose octaacetate used, to be applied to the surface of the glass. The hydrolysis of such esters needs to be base catalysed to be suitable as stain inhibitors. However, during storage, some esters may react with water in the atmosphere, and hydrolyse to produce a small amount of acid. For example, caprolactone may produce a small amount of hydroxypentanoic acid during storage. When a solution of caprolactone and a solvent, such as Iso-propyl alcohol is formed to be applied to the surface of the glass, an alkali may be added to neutralise any acid produced during the storage of the ester. Caustic soda is a suitable alkali for this purpose.
Although the measurement of the haze of the glass gives a good indication of the ability of the ester to act as a stain inhibitor, haze is generally perceived subjectively by the human eye. Results from techniques such as AFM (Atomic force microscopy) are time consuming to obtain, and inconsistent. The early stages of glass corrosion are typified by extremely small etch pits and precipitated deposits, each of the order of tens of nanometres in size. As the major issue with haze is the detrimental effect that the haze has on coatings deposited on stored glass, a more objective test is to coat the stored glass, after weathering, once such pits and deposits have appeared on the surface of the glass, and to examine the quality of the coating.
Samples were coated with a multilayer stack as shown in
As can be seen from
Light transmission was measured for epsilon caprolactone, polycaprolactone and LBK paper using a Perkin Elmer Lambda 900 spectrophotometer. Nine samples were measured and an average light transmission obtained, as shown in
From these results it is clear that esters, such as epsilon caprolactone, give better protection to glass when used in conjunction with a standard interleavant from weathering than a traditional interleavant. Unlike other acid-based stain inhibitors, the esters tested do not appear to accelerate the corrosion of glass, but neutralise any alkali leached from the glass in the presence of water with the minimum of damage to the glass surface.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0522640.2 | Nov 2005 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2006/004089 | 11/2/2006 | WO | 00 | 8/19/2009 |
