EP1115672A1

EP1115672A1 - Refractory mastics

Info

Publication number: EP1115672A1
Application number: EP99946345A
Authority: EP
Inventors: Paul Nigel-Morgan Materials Technology Lim EATON
Original assignee: Morgan Crucible Co PLC
Current assignee: Morgan Crucible Co PLC
Priority date: 1998-09-15
Filing date: 1999-09-14
Publication date: 2001-07-18
Also published as: CZ2001948A3; GB9820123D0; BR9912238A; PL346610A1; GB2337993A; CA2336974A1; AU5875999A; GB2337993B; JP4319355B2; ID27836A; WO2000015573A1; CN1316981A; MXPA01002622A; JP2002524384A; HK1024226A1; KR20010079828A

Abstract

Mastics are provided in which separately or in combination the mastic comprises: 1) a) alkaline earth metal silicate refractory fibres; and, b) colloidal silica having a pH of below 8. 2) a) alkaline earth metal silicate fibres; and b) a non-ionic polymer viscosity modifier. 3) a) alkaline earth metal silicate fibres; and b) a multi-dentate calcium-complexing ligand.

Description

REFRACTORY MASTICS

This invention relates to refractory mastics and is particularly applicable to mastics comprising saline soluble fibres bonded with a binder comprising colloidal silica.

Refractory ceramic fibres (RCF) are well known materials and typically comprise an alumino- silicate inorganic fibre formed from an oxide melt which is spun, blown, drawn, or otherwise formed into fibres. Such RCF fibres are used in the manufacture of various industrial and domestic articles. Typical uses of RCF are for applications in which resistance to temperatures in excess of 800°C is required.

Much RCF fibre is used in the form of needled blankets of fibre in which structural integrity is provided by the fibres that are tangled together in the needling process. (Such products are known as "blanket"). Sometimes a binder is used to lock the fibres together subsequent to exposure to high temperature. Blanket can be processed further to form cut shapes or folded to form insulating modules.

RCF fibre is also used in the production of so-called "Converted Products". Converted products comprise materials in which the RCF is processed further to provide materials in which the RCF is present as either a minor or major constituent. Typical converted products include the following:- "Board" - substantially rigid flat sheets containing inorganic and/or organic binders produced by a wet process (for example made by dehydrating a suspension of RCF and binders); "Paper" - a flexible fibrous insulating material with a thickness of less than or equal to

6mm, formed on paper making machinery (for example RCF in sheet form with a binder); "Shapes" - substantially rigid shapes made of ceramic fibre with the addition of inorganic and/or organic binder, fired or unfired (for example,RCF formed by vacuum foπriing into a variety of shapes);

'Tire shapes"- RCF formed by a vacuum forrning route and used for domestic and industrial fires either as radiant bodies or for decorative appearance; "Castables"- ceramic fibre with inorganic and/or organic binder which may be cast (for example, RCF in the form of cements, concretes and mortars); "Mastics" - A mouldable material containing RCF with binders and which may be trowelled, hand moulded, or dispensed from a pressure gun and which sets upon drying/heating;

'Εxtrusion" - A mastic-like material that may be used in the manufacture of extruded sections and tubes;

^•Textiles" - ceramic fibre which has been woven with or without the addition of other filaments, wires, or yarns (for example, RCF formed into rope, yam, mats and the like by textile technology).

In many of the above mentioned applications binders are used. There are two broad classes of binders:- "Organic binders" - which serve to improve the handling characteristics of the product concerned at low temperatures but which bum off at higher temperatures. Organic binders include, for example, such materials as starch. "Inorganic binders" - which may be effective to improve the handling characteristics of the product concerned at low temperatures, but which also give integrity to the product after exposure to high temperatures. Inorganic binders include, for example, such materials as colloidal silicas, aluminas, and clays.

All of the above materials and concepts are well known in the refractory industry.

Although extremely useful, RCF is an inorganic fibrous material. Inorganic fibrous materials can be either glassy or crystalline. Asbestos is an inorganic fibrous material one form of which has been strongly implicated in respiratory disease.

It is still not clear what the causative mechanism is that relates some asbestos with disease but some researchers believe that the mechanism is mechanical and size related. Asbestos of a critical size can pierce cells in the body and so, through long and repeated cell injury, have a bad effect on health. Whether this mechanism is true or not regulatory agencies have indicated a desire to categorise any inorganic fibre product that has a respiratory fraction as hazardous, regardless of whether there is any evidence to support such categorisation. Unfortunately, for many of the applications for which inorganic fibres are used, there are no realistic substitutes. Accordingly there is an industry and regulatory demand for inorganic fibres that will pose as little risk as possible (if any) and for which there are objective grounds to believe them safe.

A line of study has proposed that if inorganic fibres were made that were sufficiently soluble in physiological fluids that their residence time in the human body was short; then damage would not occur or at least be minimised. As the risk of asbestos linked disease appears to depend very much on the length of exposure this idea appears reasonable. Asbestos is extremely insoluble. As intercellular fluid is saline in nature the importance of fibre solubihty in saline solution has long been recognised. If fibres are soluble in physiological saline solution then, provided the dissolved components are not toxic, the fibres should be safer than fibres that are not so soluble. Accordingly, in recent years, a number of different types of fibre have been proposed which are refractory and yet soluble in body fluids. Such fibres comprise alkaline earth silicates (e.g. WO87/05007, WO89/12032, WO93/15028, WO94/15883, WO96/02478, and WO97/49643) which are soluble to varying extent in body fluids.

A problem with saline soluble fibres is that by their nature they are more reactive than RCF and therefore cannot always be used as a dire t replacement for RCF. Mastics are required to have a reasonable shelf life, which for RCF containing mastics is generally about 6 months. Mastics made using alkaline earth metal silicates have had such a short shelf life as to be unusable. The applicants have realised that this is due to the reactivity of the fibres with the binders. Calcium ions released from the alkaline earth metal silicate fibres set the organic and inorganic constituents of the mastic.

The present invention provides a mastic comprising:- a) inorganic refractory fibres; and, b) colloidal silica characterised in that the inorganic refractory fibres are alkaline earth metal silicates and the colloidal silica has a pH of below 8. Preferably the pH is below 7 and may usefully lie in the range 4 to 7. Further features of the invention are made apparent in the attached claims and the following description with reference to the drawings in which:-

Fig. 1 is a schematic view of a penetrometer used in measuring the characteristics of mastics; Figs. 2 to 5 are graphs showing penetrometer readings for various mastic compositions.

The invention is exemplified in the following with reference to the alkaline earth silicate fibres SUPERWOOL 607™ and SUPERWOOL 612™ (both available from Thermal Ceramics Limited of Bromborough, England).

SUPERWOOL 607™ has a nominal composition (by weight) of SiO₂ 65%, CaO 29.5%, MgO 5.5%, and Al₂O <1% and is usable at temperatures up to 1050°C.

SUPERWOOL 612™ has a nominal composition (by weight) of SiO₂ 64%, CaO 17%, MgO 13.5%, ZrO₂ 5%, impurities 0.5% and is usable at temperatures up to 1260°C.

Fibre mastics or mouldables are used to repair fibre linings of kilns. SUPERWOOL 612™ has been used to replace refractory ceramic fibre in many applications but when used in current mastic formulations as a replacement for RCF the shelf life is not satisfactory and certainly will not be useable 6 months after manufacture. The applicants have shown that by using certain types of colloidal silica and/or a different viscosity modifier and/or calcium scavenging multi- dentate ligands then the shelf life can be much extended over the standard formulation.

The fibres used in experiments of mastic compositions were SUPERWOOL 612™ and SUPERWOOL 607™. The latter fibre was used in an attempt to predict the long-term effects on the SUPERWOOL 612™ fibre. Typically a standard mastic mix with SUPERWOOL 612™ as a one for one replacement of RCF will be useable up to about three weeks compared to only 1-2 days with the SUPERWOOL 607™ fibre which is much more reactive. A standard RCF fibre (HY20™ (46%Al₂O₃/54%SiO₂) ob^tainable from Thermal Ceramics Limited, Bromborough, England) was used as a comparison. The standard procedure for producing a mastic in these experiments was to first combine the ingredients of colloidal silica, water, biocide and dye (if appropriate) with a paddle-type stirrer. Then the viscosity modifier was slowly added to this liquid which was allowed to thicken on standing (3 - 5 minutes). A Hobart-type mixer was used to break down the bulk fibre for 10-15 seconds on low speed before the thickened liquor was added. After about 1 minute the speed was then increased to its maximum for 4 minutes to obtain a homogenous mixture that had a consistency that was stiff/creamy. The mastic would then be ready for use or for storage in airtight containers.

Measurement of the stability of the formulations tested was made with the use of a penetrometer (Fig. 1). The penetrometer had two rams 1 (one aluminium and the other steel of respective masses 109 g and 336g) which gave the ability to measure quite different consistencies due to the different weights involved. The lever 2 on top of the penetrometer was used to release the ram to drop down and penetrate the mastic. Indicator mark 3 was viewed through window 4 and compared with scale 5 (in mm). Higher penetration readings represented mastic of lower viscosity.

The procedure involved stirring the mastic thoroughly in the container before attempting to take any reading. Then the penetrometer (with the aluminium ram) was placed on top of the mixture before pressing the lever 2. This reading was then recorded before repeating in a different area until 5 readings were obtained. The process was then repeated using the steel ram. Usually the mastic (~2.5kg) was split into three containers and each of these measured in the same way. Readings were then averaged and the result plotted against age of the mastic. Tables 1 and 2 below indicate the mastic formulations used (amounts in weight %) and Table 3 indicates the nature of the colloidal silicas and viscosity modifiers used.

n c

CD CO

H C H m n x m

- π

Table 3. Colloidal Silicas and Viscosity Mod fiers

The penetrometer tests show that the stability of the SUPERWOOL 612™ mastic, as measured using the steel ram, can be extended from approximately 20 days to over 120 days. This was achieved by substituting the standard colloidal silica used in such mastics (Nyacol™ 1430 - a colloidal silica with a pH of 10.2) by an acidic colloidal silica (Ludox™ TMA - a colloidal silica with a pH of 4-7), a different viscosity modifier (Magnafloc 351), or a combination of both (Ludox SK™ - a colloidal silica with a pH of 4-7 & Magnafloc 351). The results of the tests are shown in Figures 4 & 5.

Although the values obtained for the alkaline earth metal silicate fibres were not as consistent as those for the RCF fibre the overall trend could easily be seen. Variations may have been due to temperature fluctuations and indeed if curves are plotted on a date basis the variations are in conjunction with each other. The sequence in which the Magnafloc 351 is added also affected the mastic properties. The SUPERWOOL 612™ Magnafloc II formulation had the viscosity modifier dissolved in water before adding the other ingredients as opposed to the usual method of adding it to the colloidal silica. The Magnafloc II formulation decreased in viscosity within 1-2 days whereas the Magnafloc mixture took over 20 days before its viscosity also dropped. A drawback to simply using the non-ionic viscosity modifier is that separation tends to result, so requiring stirring of the mastic before use but for many applications this is acceptable.

An alternative alkaline silica (Ludox LS) produced very similar results to the standard formulation confirming that alkaline solutions are detrimental to the performance of the mastic. Use of an alternative low sodium content acidic silica (Nyacol 2034DI) which has a pH of around 3 confirmed that the acidic nature and alkali content of the silica is of importance as this too gave good results.

Formulations using SUPERWOOL 607™ fibre (Figures 2 & 3) were used as accelerated tests of the SUPERWOOL 612™ fibre. Values for the standard formulation (using Nyacol colloidal silica) had bottomed out after only 1-2 days. The Ludox TMA gave about 14 days before it plummeted within a day to its lowest level. This mastic gave very high readings to start with and could not be measured with the steel ram until day 10. The Ludox SK™ with Magnafloc 351 produced fairly linear values at least up to 50 days; if this difference were to be reproduced with the SUPERWOOL 612™ fibre then a shelf life of at least 6 months could be expected. Magnafloc 351 substituted in the standard SUPERWOOL 607™ mix alone did not make such a large difference, reaching the same value in 2 days as the standard and then continued to drop below this.

An alternative standard formulation for a pumpable mastic containing ethylene glycol (Detrick) was made with SUPERWOOL 607™ fibre and this set in less than 1 day. When this formulation was repeated but using LUDOX SK™ colloidal silica the mastic was almost unchanged after 18 days.

As an alternative to the acidic colloidal silicas or the Magnafloc 351, EDTA has also proven to be reasonably effective extending the normal life of the standard mix by adding only 0.4% EDTA. The mix thickened very quickly over the first few hours but then remained stable. Extra initial water may achieve the required viscosity without altering other properties.

A production formulation for a mastic comprises approximately :- Ludox TMA silica 42.2wt%

Superwool 612 blown fibre 30.2wt%

Magnafloc 139 viscosity modifier 2.2wt%

Water 25.4wt%

The applicants surmise that provision of an acidic colloidal silica reduces liberation of calcium ions from the fibre; use of the non-ionic viscosity modifier reduces the risk of setting reactions with the calcium ions released; and provision of multi-dentate ligands such as ethylene glycol and EDTA scavenges and locks up the calcium produced by the fibres. The present invention is not limited to the particular viscosity modifiers and multi-dentate ligands disclosed however.

Claims

1. A mastic comprising:- a) inorganic refractory fibres-, and, b) colloidal silica characterised in that the inorganic refractory fibres are alkaline earth metal silicates and the colloidal silica has a pH of below 8.

2. A mastic as claimed in claim 1 in which the colloidal silica has a pH of below 7.

3. A mastic as claimed in claim 2 in which the colloidal silica has a pH in the range 4 to 7.

4. A mastic as claimed in any preceding claim in which the colloidal silica is a negatively charged de-ionised sol.

5. A mastic comprising alkaline earth metal silicate fibres and a non-ionic polymer viscosity modifier.

6. A mastic as claimed in claim 5 also comprising colloidal silica as specified in claims 1-4.

7. A mastic comprising alkaline earth metal silicate fibres and a multi-dentate calcium- complexing ligand.

8. A mastic as claimed in claim 7 in which the multi-dentate calcium-complexing ligand is, or is derived from, ethylene glycol.

9. A mastic as claimed in claim 7 in which the multi-dentate calcium-complexing ligand is or is derived from ethylenediaminetetraacetic acid (EDTA).

10. A mastic as claimed in any of claims 1 to 6 and also comprising a multi-dentate calcium-complexing ligand as claimed in any of claims 7 to 9.