GB2329389A

GB2329389A - Fire-resistant coatings

Info

Publication number: GB2329389A
Application number: GB9719680A
Authority: GB
Inventors: Brian Huckstepp
Original assignee: Nullifire Ltd
Current assignee: Nullifire Ltd
Priority date: 1997-09-17
Filing date: 1997-09-17
Publication date: 1999-03-24
Also published as: GB9719680D0

Abstract

The invention relates to intumescent fire-resistant coating compositions comprising one or more hydrated alkali metal silicates and one or more rheology modifiers. The rheology modifiers include elemental carbon, silicon, silicon carbide, polysaccharides or modified polysaccharides.

Description

FIRE-RESISTANT COATINGS The current invention relates to intumescent fireresistant coatings, in particular those coatings comprising hydrated alkali metal silicates ("water glass").

Intumescent fire-resistant coating compositions are well known in the art. Such compositions expand on exposure to heat to form a fire-resistant barrier between the source of the fire producing the heat and the surface on which the composition has been placed. Typical intumescent fireresistant compositions comprise one or more blowing agents, such as ammonium phosphates or melamine, which decompose upon heating to produce one or more gases such as water vapour, carbon dioxide or ammonia, together with a source of carbon such as pentaerythritol, sugar or starches to produce an expanded char. Such compositions are technically complex and expensive to produce.

Hydrated alkali metal silicates are known to produce an expanded fire-resistant layer upon heating. The hydrating water within the compound is released upon heating to form water vapour which expands the alkali metal silicate to form a solid foam. Such compounds are relatively inexpensive and are readily available. However, solutions of hydrated metal silicates are newtonian in character and flow readily when applied to a surface. Thus, in order to produce a thick coating of an alkali metal silicate on a vertical surface it is necessary to apply a large number of thin layers of the compound onto the substrate to be protected.

The inventors have investigated the flow of hydrated alkali metal silicates and have unexpectedly found that it is possible to modify the rheology of a hydrated alkali metal silicate by the addition of one or more compounds. It is therefore an object of the current invention to produce an intumescent fire-resistant coating composition comprising hydrated alkali metal silicate which has improved rheological characteristics to allow it to be applied in relatively thick layers onto a substrate.

The inventors have also unexpectedly found that the rheological modifiers also improve the behaviour of the expanded alkali metal silicate layer upon exposure to heat.

The compositions of the invention have been noted to improve the thermal stability of the alkali metal silicate layer.

Accordingly, the invention provides an intumescent fireresistant coating composition comprising hydrated alkali metal silicate and one or more rheology modifiers, selected from elemental carbon, silicon, silicon carbide, a polysaccharide or a modified polysaccharide.

The alkali metal silicate used is preferably sodium silicate, potassium silicate or lithium silicate. The alkali metal silicates may be used individually or in combination. Preferably the combination of alkali metal silicates used is sodium silicate in combination with potassium silicate. Typically the ratio of sodium silicate to potassium silicate is between 1:1 and 10:1, preferably 3:1 to 6:1, especially approximately 4:1.

The elemental carbon used may be in the form of graphite, amorphous carbon, carbon black (furnace black, channel black or lamp black), diamond or buckminsterfullerene.

The polysaccharide is preferably a gelling agent or gum.

Preferred polysaccharides include xanthan gum, guar gum, locust bean gum, alginate derivatives, scleroglucan gum, welan gum, starch and derivatives thereof. Chemically modified polysaccharides such as hydroxyethylcellulose and carboxymethylcellulose may also be used.

An especially preferred polysaccharide is xanthan gum. xanthan gum is a naturally occurring polysaccharide from Xanthomonas campestris. It consists of a polymer backbone of ss 1,4-D glucose units. At the 3 position of alternate glucose monomer units branches a trisaccharide side chain containing 1 glucuronic acid and 2 mannose residues.

The elemental carbon may be used separately as a rheological modifier. Alternatively, it may be used in combination with one or more polysaccharides.

Each polysaccharide may be used independently or be used in combination with another polysaccharide and/or with elemental carbon. A preferred ccmbination of polysaccharides is xanthan gum with guar gum.

Alternatively xanthan gum may be used with locust bean gum.

The rheology modifier is used at an amount of 0.5 to 10% of the content of aqueous silicate medium, preferably at 1 to 5% thereof, and especially 1.5 to 3.5%.

Preferably a solvent is used with the composition of the invention. Preferably the solvent is water, which is environmentally friendly. Alternatively, it may be desired to use an organic solvent such as glycerol, ethylene glycol, or C 1 - C 4 lower alcohols, optionally as mixtures of themselves, or with water. Preferably the lower alcohol is methanol, ethanol or isopropanol.

It may be desired to incorporate pigments, such as titanium dioxide, into the coating composition to produce an improved finish.

The invention also provides a support coated with a composition according to the invention. The support may be brick, steel, plastics, wood or glass. Typically, the intumescent fire-resistant coating composition is applied onto the support to produce a dry film thickness of between 0.5 mm and 4 mm. Preferably the thickness is between 0.5 mm and 1.5 mm.

Where glass is used as the support the preferred coating composition of the invention does not contain carbon or a pigment. This allows a transparent intumescent fireresistant coating to be applied to the glass, thus allowing the glass to remain transparent whilst having the coating on its surface in its non-intumesced state.

The coating compositions of the invention are easy to produce and may be relatively inexpensive. Expensive materials such as char-forming agents or further binding agents, such as latex are not required, though they may be used if special specific properties are required.

A further aspect of the invention provides a process for producing an intumescent fire-resistant coating composition comprising the steps of mixing polysaccharide with a small amount of water to produce a thick paste, and mixing with a solution of alkali-metal silicate. The thick paste may be added to the solution of the silicate or alternatively the silicate solution may be added to the polysaccharide paste.

The alkali metal silicate and polysaccharide are as defined above.

This method improves the dispersion of the polysaccharide within the final intumescent fire-resistant coating composition.

The invention will now be described by way of example only with reference to the following figures: Figure 1 shows a vertical plate fire test assembly used to test the efficiency of intumescent fire-resistant compositions, by attaching to the door aperture of a Carbolite EAF 11/14 electric furnace which had been preheated to 8009C.

Figure 2 shows the effect of dry film thickness on the fail time of a variety of compositions using the Carbolite furnace vertical plate fire test: (i) no rheological modifier (0), (ii) "Rhodopol" xanthan gum (A), (iii) carbon black (t2), and (iv) special black (0).

Figure 3 is a schematic diagram showing the effects of additives on intumescent material production.

The effect of xanthan gum ("Rhodopol" 50MD Rhône-Poulenc), carbon black (S 160 Degussa) and special black (5, Degussa) on a sodium/potassium silicate solution as an intumescent coating was studied.

The sodium/potassium silicate solution was 77.4% sodium silicate (40% aqueous): 19.4% potassium silicate (40% aqueous): 3.2% water (weight/weight). Sodium silicate (9.4% Na2 0: 29.9% SiO2) was obtained from Brunner Mond (P84). Potassium silicate (13% Ka 2 0: 27.2% SiO 2 ) was obtained from Crosfield (K78). Demineralised water was used throughout the experiment.

The Rhodopol (xanthan gum) was combined with some additional water to produce a thick paste. This was then mixed with the silicate solution. This produced improved dispersion of the Rhodopol within the final composition.

Carbon black and special black were mixed within the sIlicate solution to form a dispersion.

The formulations used are indicated in Table 1.

Table 1 Silicate project formulations.

i Components (percent by weight) Formulation Na/K sincate R@o@@@@@ Car@on blac@ Spe@@@@ blac@ Additional soiunon number S160 5 water 100% I !:.r:. 1- I 2 94.1% 1.3% - - 4.6% 3 98.4% - 1.6% - 4 98.35% - - 1.65% - The Na/K silicate solution was 77.4% Na silicate (40% aqueous); 19.4% K silicate (40% acqueous); 3.2% water (w/w).

The formulations were cast onto steel plates using a mould and then left to dry in the laboratory. After drying, the moulds were removed and the coated plates were fire tested.

The Carbolite furnace as described was used for the test.

A test assembly rig is shown in Figure 1. A steel plate 10, together with its coating 12 were attached to a Monolux 500 board 14 by means of Duraboard clamps 16 and bolts 18.

The assembly was then exposed to the chamber of heated oven 20. The temperature of the steel plate was measured by means of thermocouples 22.

The overall steel plate dimensions were 180 x 128 x 3 mm.

The actual surface area exposed to heat was 0.138m2 .

The oven was heated to 800"C with the door closed. The test assembly rig was then placed against the open door (within 5 seconds). The thermocouples 22 (two copper surface contact thermocouples) gave the temperature on the back of the steel plate. This was recorded on a Kane and May temperature logger. The test was run until the back of the plate reached 5300C + ambient temperature. The time taken to reach this temperature (the fail time) was recorded. The fail times for steel coated with the various formulations are shown in Table 2. For comparison the fail time of unprotected steel is also shown.

Table 2 Fire test restilts for forniulations 1-4

Tesr specimen I Coating | Dry film thickness | Fail time (mins) formulation (mm) Steel plate (Sp) - - 6.25 Sp 1 1.023 24.13 Sp 2 0.765 29.75 Sp 2 1.180 41.25 Sp 2 1.550 50.75 Sp 3 0.565 26.20 Sp 3 1.247 48.10 Sp 4 0.805 43.00 The results are also shown in Figure 2.

Figure 3a shows steel plate 10 covered with a coating 12 before it has been subjected to heating.

Figure 3b shows the effect of heating on sodium/potassium silicate solution, without any rheology modifiers being present.

Figure 3b(i) shows the appearance of the coating at approximately 6500C after it has been subjected to heat for approximately 15 minutes. Large vesicles of gas 32 were observed. If the plate continued to be heated the silicate was observed to begin to melt.

This resulted in indentations 34 in the surface of the intumescent coating and resulted in a decrease in the ability of the coating to prevent the passage of heat through to the steel plate 10. The effects are shown schematically in Figure 3b(ii) after the surface coating has been subjected to heat for approximately a hour, with the external temperature being approximately 8000C.

Figure 3c shows the effect of heating to 8000C a carbon black containing sodium/potassium silicate composition.

The carbon black produces a layer 36 which has smaller vesicles 38 than those observed in silicate compositions without rheology modifiers at 6500C, but the intumescent layer formed 36 is considerably more heat stable than sodium/potassium silicate without modifiers.

This shows that the rheology modifiers not only improve the coating characteristics of the sodium/potassium silicate intumescent material, but also improve the thermal stability of the intumescent product.

Claims

CLAIMS 1. An intumescent fire-resistant coating composition comprising at least one hydrated alkali metal silicate and one or more rheology modifiers.
2. A coating composition according to claim 1, wherein the rheology modifier is selected from elemental carbon, silicon, silicon carbide, a polysaccharide or a modified polysaccharide.
3. A coating composition according to claims I or 2 wherein the hydrated alkali metal silicate is selected from one or more of sodium silicate, potassium silicate and lithium silicate.
4. A coating composition according to claims 2 or 3 wherein the elemental carbon is selected from graphite, amorphous carbon, carbon black, diamond and buckminsterfullerene.
5. A coating composition according to claims 2 or 3 wherein the polysaccharide is selected from xanthan gum, guar gum, locust bean gum, alginate, scleroglucan gum, welan gum, starch, or derivative thereof.
6. A coating composition according to claims 2 or 3 wherein the modified polysaccharide is selected from hydroxyethylcellulose and carboxymethylcellulose.
7. A coating composition according to any preceding claim wherein the rheology modifiers comprise one or more elemental carbon rheology modifiers in combination with one or more polysaccharide or modified polysaccharide rheology modifiers.
8. A support coated with a coating composition according to any preceding claim.
9. A process for producing an intumescent fire-resistant coating composition according to claims 1-8, comprising the steps of: mixing polysaccharide with an amount of water sufficient to produce a thick paste, followed by the step of mixing the paste with a solution of alkali metal silicate.
10. An intumescent fire-resistant coating composition comprising one or more hydrated alkali metal silicates and one or more rheology modifiers, substantially as hereinbefore described with reference to the description and accompanying figures.