GB1602403A

GB1602403A - Coherent rigid solid material

Info

Publication number: GB1602403A
Application number: GB1252478A
Authority: GB
Original assignee: Westlund R C; Lebanon Steel Foundry
Current assignee: Westlund R C; Lebanon Steel Foundry
Priority date: 1977-03-30
Filing date: 1978-03-30
Publication date: 1981-11-11
Also published as: CA1104593A; DE2813745A1; FR2385654A1; JPS5439433A; BR7802008A

Description

(54) COHERENT RIGID SOLID MATERIAL (71) We, LEBANON STEEL FOUNDRY, a corporation organised and existing under the laws of the State of Pennsylvania, United States of America, of 1st Avenue & Lehman Streets. Lebanon. Pennsylvania 17042, United States of America and RODNEY CHARLES WESTLUND. a citizen of the United States of America, of 506 Braeburn Terrace, Lansdale. Pennsylvania 19446, United States of America, do hereby declare the invention. for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to coherent rigid solid materials particularly ones for use as insulating material.

Heat insulating materials prepared from alkaline ionic silicates and fillers having at least a 75% reactive expanded perlite content are known. Such materials are formed and cured so as to enable the perlite fraction of the filler to react with the silicate to produce a crystalline reaction product. U.S. patent 3.658.564 discloses such a material and a method of making such a material.

The material of U.S. patent 3,658,564 is made by using an extended curing period of at least three and preferably seven days to achieve relative water insensitivity. Moreover, curing is accomplished under carefully controlled conditions of humidity and temperature during this period. The required curing creates some difficulties in the large-scale production of the high temperature insulating material.

In addition. the material of U.S. patent 3.658,564 must be made with specific SiO2:K2O and SiO.:Na.O ratios in the alkaline ionic silicates. In particular, water resistive or insensitive products require SiO2:Na2O ratios of 3:1 to 4:1 and SiO2:K2O of 2:1 to 2.6:1 to achieve a water insensitive product. Furthermore, the material of U.S. patent 3,658,564 has a rough surface texture which is undesirable from a cosmetic standpoint. The roughness is also undesirable because of the inability to provide intricate shapes, e.g., mitre joints.

According to the present invention there is provided a coherent rigid solid material comprising the heat set product of a mixture comprising by weight from 20 to 50 parts expanded perlite, a total of from 9.5 to 19 parts sodium and/or potassium silicate solids, from 2 to 9 parts zinc oxide, and water to a total, including any that may be associated with the said silicate, of 21.5 to 67 parts.

The invention also provides a method of forming a coherent rigid solid material, the method comprising forming a mixture which comprises by weight from 20 to 50 parts expanded perlite, a total of from 9.5 to 19 parts sodium and/or potassium silicate solids, from 2 to 9 parts zinc oxide, and water to a total, including any that may be associated with the said silicate, of 21.5 to 67 parts. and setting the mixture by heating with reduction of its initial water content.

In preferred embodiments of the invention utilizing sodium silicate, the mixture comprises 29 to 45 parts expanded perlite, with 36 to 42 parts being particularly preferred.

11.5 parts to 18 of sodium silicate is preferred with 14.5 to 16.5 parts being particularly preferred. 3 to 8 parts of zinc oxide is preferred with 4.5 to 6.8 parts being particularly preferred. Water, including any which is associated with the sodium silicate, is preferably 26.5 to 57 parts of the mixture with 32.5 to 43.5 parts being particularly preferable.

Preferably, a solidifying agent is utilized. The solidifying agent may comprise 0.5 to 6 parts by weight of sodium fluosilicate or the mixture may be subjected to the action of carbon dioxide gas before being subjected to heat.

In order to provide green strength, the mixture may include organic or inorganic fiber material. Preferably, 0.2 to 5 parts by weight of fibre material are included. The fiber material may be from a class comprising 1 to 5 parts fiber-glass, 0.2 to 1.5 parts heat resistant nylon, 1 to 5 parts mineral wood and 1 to 5 parts of netting, all by weight as part of the mixture. Where netting is used, the fibers of the netting are preferably of a thickness in the range of 0.007 to 0.0125 inches with the openings of the mesh having areas preferably in the range of 0.06 to 1 square inch and the weight of the netting being less than 1 lb. per 1000 sq. ft. Where fiber material other than netting is utilized, an average length of 1/8 to 1 inch is preferred.

Preferably, the expanded perlite of the mixture has a dry bulk density of 2 to 8 Ibs. per cubic foot and the AFS average screen size (Fineness No.) for the perlite is in the range of from 70 to 120 and most preferably 110.

Preferably the zinc oxide has an average fineness not in excess of 0.5 microns with a fineness of 0.1 to 0.2 microns being particularly preferred.

Preferably, the sodium silicate has a proportion of silicon dioxide to sodium oxide of 3.1 to 1 to 3.4 to 1. The sodium silicate is preferably introduced into the mixture in a solution having a solids content of 36 to 44% by weight. Where potassium silicate is used, the silicon dioxide to potassium oxide ratio is preferably from 2.0 to to 2.7 to 1 with a preferable solids content of 24 to 35% by weight.

Preferably, the mixture is formed by bringing together a dry powder material including the perlite with a slurry including the said silicate, zinc oxide and water and mixing the powder and the slurry at least to the time the mix appears damp and dust-free and short of the time the mix begins shrinking substantially in volume. After the mixing of the mixture is completed the mixture is preferably held in storage for a period not exceeding 2.5 hours so as to permit the material to be formed into shape under compression.

Compression to achieve the desired shape may be achieved prior to heating by vibration or a plurality of compression steps. The shaping may also be accomplished by blowing the material into shape.

Heating of the mixture may be accomplished by heating in an oven at a temperature of 93 through 99"C for four hours for each inch of minimum dimension of shape. In the alternative, the material may be subjected to heat by microwave energy.

A material according to the invention can have a bulk density of 13.0 to 14.0 Ibs. per cubic foot when subjected to ASTM test C-303 and preferably the bulk density is 13.0 to 13.5. The material can be characterized by less than 1% linear change and less than 4% weight loss when subjected to ASTM test C-356.

The flexural strength of the material can be in excess of 45 Ibs. per square inch for a bulk density of less than 13.5 Ibs. per cubic foot when tested in accordance with ASTM test C-303.

The thermal conductivities of the material can be less than 0.53, 0.65 and 0.78 Btu's per inch of thickness per square foot per degree F. per hour at 500"F and 900"F respectively when tested in accordance with ASTM test C-777.

The material can have a surface which is smoother than SIS-3 on the official alloy casting Institute Surface Indicator Scale and the surface is preferably as smooth as or smoother than SIS-2 on that scale.

In accordance with another important optional aspect of the invention, the material is formed into shaped bodies. Preferably, the material is formed into a hollow tubular configuration comprising a plurality of separable segments where the segments include mating tongues and grooves or mitre joints. The segments may be separable along surfaces extending substantially parallel to the axis of the tubular configuration with the tongues and grooves being formed in the surfaces.

A material of the present invention can start out as a mix comprising the following on a basis of 100 parts by weight of the mix: expanded perlite 20 to 50 parts, preferably 29 to 45 parts, with 36 to 42 being more preferable; sodium fluosilicate 0.5 to 4 parts, preferably 1 to 3.5 parts, with 2 to 3 parts more preferred; fiber material 0.2 to 5 parts, preferably 1 to 5 parts; sodium silicate or potassium silicate solids 9.5 to 19 parts, preferably 11.5 to 18 parts with 14.5 to 16.5 being more preferable; zinc oxide 2 to 9 parts, preferably 3 to 8 parts, with 4.5 to 6.8 being more preferable; water, to make a total of water, including any that may be associated with the sodium of potassium silicate, of 21.5 to 67 parts, preferably 26.5 to 57, with 32.5 to 43.5 parts being more preferable.

The mixture can be brought to this state by mixing the expanded perlite, the sodium fluosilicate and the fiber material together in powder form and separately mixing the sodium silicate in a solution containing preferably 36 to 44% solids, with 38% more preferred or the potassium silicate in a solution containing preferably 24 to 35% solids, zinc oxide and water together to form a slurry which is in turn mixed in with the powder. Such mixing is then continued up at least to the time the mix appears damp and dust-free, but not up to the time the mix starts to shrink substantially.

Various mixing devices, such as a planetary batch mixer, or a continuous screw feed mixer, are suitable for the mixing, a pug mill batch mixer also being suitable.

The material may be passed through a screen, which will preferably have half inch openings, after the mixing.

The material is then preferably held in storage under cover up to two and one half hours.

This step has been observed to have the effect of markedly increasing the strength of the final product.

The material can then be compressed, by a ram or vibration, into the shape in which it will ultimately be wanted. Another less preferred way of bringing it into shape is to put part of it into a mold or the like that may be used and compress that part and then put in part or all of the additional and compress it, and so on.

The material can then be heated to cure it, this being preferably done in a microwave oven. If a conventional oven utilizing hot air is used then the heating should preferably be done at temperatures in the range from 93"C to 990C and preferably for approximately two to four hours per inch of the smallest dimension of its shape, and most preferably at 960C for four hours for each inch of the smallest dimension of the shape.

The fiber material may comprise a class of organic or inorganic materials including 1 to 5 parts fiberglass or a heat resistant nylon-type fibrous material, such as poly (1, 3-phenylene isophthalamide), sold commercially under the name Nomex (Trade Mark), can be used less preferably instead of the fiberglass, in which case the amount will be 0.2 to 1.5 parts by weight and most preferably 0.5 parts by weight. Another less preferable possibility for this is mineral wool, such as rockwool in the amount of 1 to 5 parts or most preferably 1 part.

Cotton or wood fibers may also be used.

The fiberglass or other fibrous material can be in the form of a floc - that is, a set of fibers in short lengths, averaging preferably 1/8 to 1 inch long and most preferably averaging 1/4 inch long.

The fiber material may also be 1 to 5 parts of netting which may be organic or inorganic materials including polypropylene, polyester, nylon or Dacron (Trade Mark). The netting comprises fibers of a thickness in the range of 0.007 to 0.125 inches, openings having areas in the range of 0.06 to 1 square inch and weight of less than 2 Ibs. per thousand square feet.

The netting strength should preferably exceed 4 grams per denier when subjected to an Instron tester at 65% relative humidity. The openings in the netting may take on a variety of shapes including squares. rectangles, circles or ovals.

All fiber materials must be stable at temperatures in excess of 250"F, i.e., there is no substantial softening below these temperatures.

The expanded perlite preferably has a dry bulk density of 2 to 8 Ibs. per cubic foot, and more preferably 2 to 3.5 Ibs. per cubic foot. Perlite is a complex sodium potassium aluminum silicate volcanic granular glass. Its screen sizing preferably is AFS (American Foundry Society) average screen size designation of 70 through 120 and most preferably 110. A perlite with 25lie maximum contaminants including no more than 0.5% each of iron and calcium and no more than 0.16T of each of arsenic, barium, beryllium, boron, chlorine, chromium, copper, gallium, lead, manganese, molybdenum, nickel, sulphur, titanium, yttrium and zirconium is suitable. Expanded perlite includes all perlite made from naturally occurring perlite sand which is expanded by heat. The fusion temperature is in excess of 2300"F and has a solubility of less than 1% in water, less than 10% in 1N NaOH and less than 3%' in mineral acids.

The sodium silicate preferably becomes part of the mix in the form of a water solution capable of being handled in a practical manner. Any commercially available solution of appropriate concentration will suffice but it is preferred that its ratio of silicon dioxide to sodium oxide should be in the range of 3.1 to 1 to 3.4 to 1, and most preferably 3.22 to 1, and it should have a solids content preferably in the range of 36 to 44% by weight, and most preferably 38%. An example of a suitable sodium silicate grade to use is the N grade of Philadelphia Quartz. Where potassium silicate is used, the ratio of silicon dioxide to potassium oxide should preferably be in the range of 2.0 to 1 to 2.7 to 1 with a preferable solids content of 24 to 35%.

For good results, it is important that the zinc oxide should be finely divided and fairly clean, e.g., the type that is produced by the so-called French process. Use of the zinc oxide in finely divided form has been found to quite substantially increase the strength and the water resistivity. A zinc oxide having an average fineness of no more than 0.50 microns is suitable, 0.10 to 0.20 is preferred, and 1.7 most preferred; 98 to 99% purity or, more particularly, reagent grade zinc oxide is suitable from the standpoint of purity.

Instead of including the sodium fluosilicate as a solidifying agent to add green stength, a less preferred alternative which nevertheless has special advantages is to pass carbon dioxide gas under pressure into or through the material after it has been compressed into shape.

The curing or drying step can well take place in an oven, for example in an electric or other conventional oven with air circulating within it, and in such a case the piece of material being subjected to the heating should preferably be so supported that the maximum area of the piece will have direct access to the air in the oven. For example, a piece with relatively longer and shorter dimensions should be supported with its longest dimension in the vertical direction, which will achieve this result and also tend to prevent warping. Dehydrating and otherwise curing the material can also be accomplished by the use of microwave energy in heating devices such as ovens, which have a capability of securing the result in a much shorter time, for example for some time such as 5 minutes to the inch of minimum dimension. Use of this has been found on the average to enhance the strength more than 20% as compared to using a conventional oven.

Examples of the invention will now be described and reference made to the accompanying drawings, in which: Figure 1 is a graph of thermal conductivity against temperature for material according to the invention, Figure 2 is an exploded perspective view of a tube, and Figures 3 to 5 are sectional views of further tubes.

In each of the Examples the amount of total water, including any which may be associated with the silicate, and the amount of silicate are both within the ranges given in claim 1.

Example I This example involves the making of a block 12 inches by 8 inches by 2 inches, for the purpose of which 200 cubic inches of material is made in order to have enough to make the block together with a small excess to accommodate handling losses.

The following materials are obtained: 1. PFF 10 perlite (powder) 352 g (29%) 2. Sodium fluosilicate (powder) 24 g ( 2%) 3. Fiberglass (1/8" fibers) 12 g ( 1%) 4. Water 290 g To) 5. Sodium silicate (3.22:1) 473 g (39%) (liquid) 6. Zinc oxide (powder, rubber pigment type) 61 g ( 5%) 1212 g (100%) Items 1, 2 and 3 are added to a ten gallon bowl capacity Hobart (Trade Mark) mixer.

Items 4, 5 and 6 are pre-mixed in a slurry. The mixer is turned on and the slurry is poured into bowl in a way that the stream will meet the path of the impeller. It is important in this particular case that the mixing does not exceed 35 seconds nor be less than 20 seconds.

The material may be passed through a 1/2 inch opening screen. The material is then held in storage 90 minutes under cover. After this time, the material is added to a mold box.

About one third is added and spread evenly across the bottom of the box. This is gently and carefully rammed, after which the remaining portion is added. The box which is designed to receive about 2.3 pounds of the damp mix is then positioned under a hydraulic press head.

A close-fitting, smooth, wooden block is then inserted into the loaded mold box.

This block being high enough will act as a piston and will drive the mixture downward, compressing it into the proper sized block.

The mold box is so constructed that it is more than deep enough to receive the batch as given and rammed according to the instructions given above.

The box surfaces have three coats of pattern lacquer, rubbed smooth, and paste wax applied giving it a final shiny, good-releasing surface.

The box is constructed in a way that the corners can be loosened after block forming, thus, allowing easy, scuff-free removal of the molded piece. At this point, the driving piston block can be used to eject the formed piece, for its subsequent transfer to a dryer plate.

Oven drying then takes place as follows: The thickness of the block is 2 inches, and therefore eight hours at 960C eliminates sufficient free water and apparently properly accomplishes the formation of willemite which is believed to be important for securing the best results with such a block. This involves four hours of such that heat for each inch of minimum dimension, in an ordinary air heat oven.

At this time, if the particular material in question was being produced for commercial use, the block could be stored, packed or shipped.

The sodium fluosilicate used is of pharmaceutical grade finely powdered with an analysis purity of in excess of 23% Na2SiF6 and a fineness of 95% through a 200 mesh screen. If carbon dioxide gas were used, it would be at more than 99% purity, and applied under 15 lbs. gauge pressure for four to five seconds per inch of thickness of matrix, i.e., inside dimension to outside dimension.

In the forming step, a compressive action by the moving head of 30 to 100 lbs. per square inch is used and of course the mold has a strength to withstand this. The compacting step with vibration should take no longer than ten seconds per cubic foot of material. Vibration can be at a rate of 300 or 1000 pulses per minute, for example.

The material can instead be formed by blowing it into a suitable cavity in a mold to make a pre-shaped piece, in the manner of core blowing in foundry practice. Suitably screened vents permit the escape of the air which is carrying the material, while keeping the material in the cavity and in a shape formed under pressure of the blowing.

In the mold used in the forming step, a coating of Teflon (Trade Mark) or epoxy with high gloss can take the place of the pattern lacquer.

The box construction with provision for loosening the corners may be made unnecessary by securing a good straight travel in ejection.

Example 2 The following mixture is prepared: 1. PFF 10 perlite (powder) 164 g (39.00%) 2. Sodium fluosilicate (powder) 10 g ( 2.40%) 3. Water 45 g (10.70%) 4. Zinc oxide 27 g ( 6.50%) 5. Sodium silicate solution 174 g 41.40%) Items l and 2 are dry mixed in a Hobart mixer for 10 seconds at which time the slurry made by separately mixing 3, 4 and 5 is added to the running mixer and such mixing is continued until the mixture appears dust free but short of a time when it starts to shrink in volume.

After 30 minute storage under a plastic sheet cover, a 50 gram portion is lightly tamped into a special Dietert transverse metal mold box with split corners. A drop weight rammer is actuated 8 to 12 times making an exact 1 x 1 x 8 inch bar of compressed material whose density is 13.5 to 14.0 Ibs. per cubic foot after drying the bar by microwave energy for 4 to 6 minutes.

Example 3 The sodium fluosilicate of Example 2 is omitted and the procedure to make and test bars as set forth in Example 2 is otherwise followed. After the compressed bars are formed, they are treated by passing carbon dioxide gas through them at a pressure of 15 libs. per square inch for 5 seconds.

Example 4 The following mixture is prepared using the method of Example 2: 1. PFF 10 perlite (powder) 168 g (40%) 2. Sodium fluosilicate (powder) 12.5 g ( 3%) 3. Water 12.5 g ( 3%) 4. Zinc oxide 25 g ( 6%) 5. Sodium silicate solution 202 g (48%) Example 5 The following mixture is prepared using the method of Example 2: 1. PFF 10 perlite (powder) 151 g (36%) 2. Sodium fluosilicate (powder) 14.5 g 3.5%) 3. Water 89.5 g 21.5%) 4. Zinc oxide 33 g ( 8%) 5. Sodium silicate 130 g 31% Example 6 The following mixture is prepared using the method of Example 2: 1. PFF 10 perlite (powder) 168 g (40%) 2. Sodium fluosilicate (powder) 12.5 g ( 3%) 3. Water 12.5 g ( 3%) 4. Zinc oxide 25 g ( 6% 5. Potassium silicate 202 g 48% Example 7 The following mixture is prepared using the method of Example 2: 1. PFF 10 perlite (powder) 151 g (36%) 2. Sodium fluosilicate (powder) 14.5 g (3.5%) 3. Water 89.5 g (21.5%) 4. Zinc oxide 33 g ( 8% 5. Potassium silicate 130 g (31%) Example 8 The procedure of Example 2 is utilized with the mixture of Example 6 except the sodium fluosilicate is omitted.

The material of each example is particularly resistant to boiling water. For example, the material of Example 2 retained its exact shape when subjected to boiling water for 5 hours and only suffered a weight loss of 10.0%. Similarly, the material of Example 3, when subjected to 5 hours of boiling water retained its exact shape and only suffered a weight loss of 11.5%. In contrast, the samples similar to Example 2 but without the sodium fluosilicate or the carbon dioxide of Examples 3 and 8 completely lost their shape and disintegrated into fine particles within 5 minutes of the time they were introduced into boiling water. In the absence of zinc oxide, the material of Example 2 completely disintegrated within 30 minutes after being introduced into the boiling water.

The properties of the material made in accordance with this invention make the material particularly well suited for use as a high temperature insulation.

When a material according to the invention was tested for bulk density in accordance with ASTM standard test C-303, it was found that the material had a bulk density ranging from 13.0 to 14.0 Ibs. per cubic foot, more than satisfying the ASTM requirement of 13.5 through 14.0 Ibs. per cubic foot.

The material of Example 2, when tested in accordance with ASTM standard test C-356 demonstrated a percent linear change of less than 1% at 12000F and a percent weight loss of less than 4% at 1200"F as contrasted with the standard of less than 2% linear change and less than 5% weight loss at 12000F.

The flexural strength of a material made in accordance with this invention and tested pursuant to ASTM standard test C-610 produced a flexural strength in excess of 45 Ibs. per sq. inch for bulk densities of less than 13.5 Ibs. per cubic foot. The flexural strength of bars made in accordance with Examples 4 and 5 is 66 and 55 Ibs. per square inch respectively.

Material according to the invention also passed the ASTM standard test C-421 for mechanical stability, test C-165 for compressive strength, test C-411 for hot surface performance and test E-84 for surface burn characteristics. The ASTM standard test C-692 for chlorine corrosion and the DANA stress corrosion test on stainless steel per military specification I-24-244 were also passed.

Reference will now be made to Figure 1 for an illustration of the thermal insulating properties of a material according to the invention. The curve as shown in Figure 1 represents the results of ASTM standard test C-177 which measures the thermal conductivity K in Btu's per inch thickness per square foot per degree F per hour. In Figure 1, the K factor is on the ordinant and the temperature is on the abscissa. It will be observed that a K factor of the material of this invention is substantially lower (i.e. better) than the K factor of the ASTM standard. For example, the K factor of the material of this invention is 0.42 at 300 as compared with the ASTM test standard of 0.50. The K factor at 5000F for the material is less than 0.53 and can typically be 0.44 whereas the ASTM standard is 0.60. At 700"F, the ASTM standard is 0.71 whereas the material has a K factor of less than 0.65 and typically the K factor can be 0.55. At 900"F, the K factor of a material according to this invention can be less than 0.78 and typically is less than 0.72.

The ASTM tests referred to in the foregoing are described in detail in the ASTM 1975 Annual Standards Part 18 which is incorporated herein by reference.

A material according to the invention has a cosmetically pleasing appearance due in large measure to the smoothness of the material. In this regard, the surface which is smoother than SIS-3 on official alloy Casting Institute Surface Indicator Scale and preferably as smooth as or smoother than SIS-2 on that scale.

A material according to the invention will provide solid rigid shapes suitable for enclosing and insulating hot pipes, furnace outer walls, oven walls, cold pipe and walls, and fittings and valves.

The ability of material according to the invention to hold solid rigid shape and the smoothness of the material allows the material to be formed for making a tongue and groove or mitred joints such as that shown in Figures 2-5. As shown in these Figures, the tongues 10(a-d) and the grooves 12(a-d) in surfaces 14 may take on various shapes, some of which are fairly intricate. However, due to the ability of the material to hold its shape and the relative smoothness of the surfaces, efficient mating of the tongues and grooves is assured.

As shown in Figure 2, the material has a hollow tubular configuration having two segments 16 which include the tongues and grooves 10(a-d) and 12(a-d) in the surface 14 which extend parallel to the axis of the tube. However, it will be appreciated that the tongues and grooves may be utilized in planar sheets and other configurations such as might be required to accommodate the configurations of furnaces, fittings, valves and oven walls, etc. It will be noted that the dove-tail tongues and grooves of Figure 5 permit the two segments to be joined without use of straps.

As already largely indicated, the material of the invention forms heat insulation of special strength and good heat resistance and water resistivity, made in a very practical, economical way.

It is thought that the particular components of the present invention work together in a very special way to give a high quality product. For example, it is believed that the sodium silicate, water and zinc oxide when dealt with in the way indicated herein as part of the present overall material react together to form willemite in large part, a complex material which is mainly a form of zinc silicate (Zn2SiO4), plus some basic zinc silicate plus some zinc sodium silicate (Zn2Na2SiO4H2O) and is believed to contribute greatly to the strength and water resistivity of the material. It is also believed that potassium silicate, water and zinc oxide will react to produce an analogous complex material.

The sodium fluosilicate as applied in this particular setting is believed to help secure dimensional stability while the material is undergoing dehydration, and also help prevent disinteg

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 0.42 at 300 as compared with the ASTM test standard of 0.50. The K factor at 5000F for the material is less than 0.53 and can typically be 0.44 whereas the ASTM standard is 0.60. At 700"F, the ASTM standard is 0.71 whereas the material has a K factor of less than 0.65 and typically the K factor can be 0.55. At 900"F, the K factor of a material according to this invention can be less than 0.78 and typically is less than 0.72. The ASTM tests referred to in the foregoing are described in detail in the ASTM 1975 Annual Standards Part 18 which is incorporated herein by reference. A material according to the invention has a cosmetically pleasing appearance due in large measure to the smoothness of the material. In this regard, the surface which is smoother than SIS-3 on official alloy Casting Institute Surface Indicator Scale and preferably as smooth as or smoother than SIS-2 on that scale. A material according to the invention will provide solid rigid shapes suitable for enclosing and insulating hot pipes, furnace outer walls, oven walls, cold pipe and walls, and fittings and valves. The ability of material according to the invention to hold solid rigid shape and the smoothness of the material allows the material to be formed for making a tongue and groove or mitred joints such as that shown in Figures 2-5. As shown in these Figures, the tongues 10(a-d) and the grooves 12(a-d) in surfaces 14 may take on various shapes, some of which are fairly intricate. However, due to the ability of the material to hold its shape and the relative smoothness of the surfaces, efficient mating of the tongues and grooves is assured. As shown in Figure 2, the material has a hollow tubular configuration having two segments 16 which include the tongues and grooves 10(a-d) and 12(a-d) in the surface 14 which extend parallel to the axis of the tube. However, it will be appreciated that the tongues and grooves may be utilized in planar sheets and other configurations such as might be required to accommodate the configurations of furnaces, fittings, valves and oven walls, etc. It will be noted that the dove-tail tongues and grooves of Figure 5 permit the two segments to be joined without use of straps. As already largely indicated, the material of the invention forms heat insulation of special strength and good heat resistance and water resistivity, made in a very practical, economical way. It is thought that the particular components of the present invention work together in a very special way to give a high quality product. For example, it is believed that the sodium silicate, water and zinc oxide when dealt with in the way indicated herein as part of the present overall material react together to form willemite in large part, a complex material which is mainly a form of zinc silicate (Zn2SiO4), plus some basic zinc silicate plus some zinc sodium silicate (Zn2Na2SiO4H2O) and is believed to contribute greatly to the strength and water resistivity of the material. It is also believed that potassium silicate, water and zinc oxide will react to produce an analogous complex material. The sodium fluosilicate as applied in this particular setting is believed to help secure dimensional stability while the material is undergoing dehydration, and also help prevent disintegration of the other components as a result of the presence of water. The fiber material is believed to help increase the final strength and also reduce any tendency towards fracture of the structure by impact. A material useful for insulation is preferably made according to the invention by making up a mix having the following proportions in parts by weight: 20-50 parts expanded perlite; 0.5-4 parts sodium fluosilicate; 0.2-5 parts fiber material; a water solution having 9.5-19 parts total of solid content of sodium or potassium silicate; 2-9 parts zinc oxide; and water which, along with the water in the sodium silicate solution totals 21.5-67 parts; thereafter storing the mix under cover for less than 2 1/2 hours, compressing the mix to a desired form, and curing and drying it by heating. WHAT WE CLAIM IS:

1. A coherent rigid solid material comprising the heat set product of a mixture comprising by weight from 20 to 50 parts expanded perlite, a total of from 9.5 to 19 parts sodium and/or potassium silicate solids, from 2 to 9 parts zinc oxide, and water to a total, including any that may be associated with the said silicate, of 21.5 to 67 parts.

2. A material according to claim 1 wherein the mixture includes a solidifying agent.

3. A material according to claim 2 wherein the solidifying agent comprises sodium fluosilicate.

4. A material according to claim 3 wherein the mixture contains from 0.5 to 6 parts by weight sodium fluosilicate.

5. A material according to claim 1 wherein the mixture is subjected to the action of carbon dioxide gas before being subjected to heat.

6. A material according to any of claims 1 to 5 wherein the mixture includes organic or inorganic fiber material.

7. A material according to claim 6 wherein the mixture includes 0.2 to 5 parts by weight

of the fiber material.

8. A material according to claim 6 or 7 wherein the fiber material is selected from fiberglass, heat resistant nylon, mineral wool and netting.

9. A material according to claim 8 wherein the mixture contains, by weight of the mixture, 1 to 5 parts fiberglass or 0.2 to 1.5 parts heat resistant nylon or 1 to 5 parts mineral wool or 1 to 5 parts of netting.

10. A material according to claim 8 or 9 wherein the mixture contains netting which comprises fibers of a thickness of 0.007 to 0.125 inches, has openings of areas of 0.06 to 1 square inch, and weighs less than 1 pound per 1000 square feet.

11. A material according to any of claims 6 to 10 wherein the fibers, other than any in the form of said netting, have an average fiber length of 1/8 to 1 inch.

12. A material according to any of claims 1 to 11 wherein the mixture contains 29 to 45 parts by weight expanded perlite.

13. A material according to claim 12 wherein the mixture contains 36 to 42 parts by weight expanded perlite.

14. A material according to any of claims 1 to 13 wherein the mixture contains 11.5 to 18 parts by weight sodium silicate.

15. A material according to claim 14 wherein the mixture contains 14.5 to 16.5 parts by weight sodium silicate.

16. A material according to any of claims 1 to 15 wherein the mixture contains 3 to 8 parts by weight zinc oxide.

17. A material according to claim 16 wherein the mixture contains 4.5 to 6.8 parts by weight zinc oxide.

18. A material according to any of claims 1 to 17 wherein the mixture contains a total of 26.5 to 57 parts by weight water.

19. A material according to claim 18 wherein the mixture contains a total of 32.5 to 43.5 parts of weight water.

20. A material according to any of claims 1 to 19 in which the expanded perlite of the mixture has a dry bulk density of 2 to 8 pounds per cubic foot.

21. A material according to claim 20 in which the density is from 2 to 3.5 pounds per cubic foot.

22. A material according to any of claims 1 to 21 wherein the expanded perlite of the mixture has an AFS Fineness No. of 70 to 120.

23. A material according to any of claims 1 to 22 in which the zinc oxide of the mixture has an average fineness of not over 0.5 microns.

24. A material according to claim 23 in which the zinc oxide of the mixture has an average fineness of from 0.1 to 0.2 microns.

25. A material according to any of claims 1 to 24 in which the mixture contains sodium silicate having a proportion of silicon dioxide to sodium oxide of 3.1 : 1 to 3.4 : 1.

26. A material according to any of claims 1 to 25 in which the mixture contains potassium silicate having a proportion of silicon dioxide to potassium oxide of 2.0 1 to 2.7 :1.

27. ' A material according to any of claims 1 to 26 having a bulk density of 13.0 to 14.0 lb.

per cubic foot as measured by ASTM test C-303.

28. A material according to any of claims 1 to 27 exhibiting less than 1% linear change when subjected to ASTM test C-356.

29. A material according to any of claims 1 to 28 exhibiting less than 4% weight loss when subjected to ASTM test C-356.

30. A material according to any of claims 1 to 29 having a flexural strength in excess of 45 lb. per square inch for a bulk density of less than 13.5 lb. per cubic foot when tested in accordance with ASTM test C-303.

31. A material according to any of claims 1 to 30 having thermal conductivities of less than 0.53, 0.65 and 0.75 Btu's per inch of thickness per square foot per degree F at 500"F, 700"F, and 900"F respectively as measured by ASTM test C-777.

32. A material according to any of claims 1 to 31 having a surface smoother than SIS-3 on the official Alloy Casting Institute Surface Indicator Scale.

33. A material according to claim 32 having a surface as smooth as/or smoother than SIS-2 on the official Alloy Casting Institute Surface Indicator Scale.

34. A method of forming a coherent rigid solid material, the method comprising forming a mixture which comprises by weight from 20 to 50 parts expanded perlite, a total of from 9.5 to 19 parts sodium and/or potassium silicate solids, from 2 to 9 parts zinc oxide, and water to a total, including any that may be associated with the said silicate, of 21.5 to 67 parts, and setting the mixture by heating with reduction of its initial water content.

35. A method according to claim 34 in which sodium silicate is introduced to the mixture as a solution having a solids content of 36 to 44 weight percent.

36. A method according to claim 34 or 35 in which potassium silicate is introduced to the mixture as a solution having a solids content of 24 to 35 weight percent.

37. A method according to any of claims 34 to 36 in which the mixture is formed by bringing together a dry powder including the perlite with a slurry including the said silicate, zinc oxide and water, and mixing the powder and the slurry at least until the mix appears damp and dust free but short of the time it begins to shrink substantially.

38. A material according to any of claims 34 to 37 in which the completed mixture is subjected to the action of carbon dioxide gas.

39. A method according to any of claims 34 to 38 in which the completed mixture is held in storage for a period not exceeding 2 1/2 hours before being heat set.

40. A method according to any of claims 34 to 39 in which the mixture is compressed into shape before being heat set.

41. A method according to claim 40 wherein the mixture is compressed under vibration.

42. A method according to claim 40 or 41 in which the mixture is compressed into shape in a plurality of separate compression steps.

43. A method according to any of claims 34 to 37 in which the mixture is blown into shape before being heat set.

44. A method according to any of claims 34 to 43 in which the mixture is heat set at 93" to 990C.

45. A method according to claim 44 in which the mixture is heat set in a hot air oven at 93"C to 99"C for two to four hours for each inch of minimum dimension of the shape of the mixture.

46. A method according to any of claims 34 to 44 in which the mixture is heat set by microwave energy.

47. A material obtained by a method according to any of claims 34 to 46.

48. A material according to any of claims 1 to 33 and 47 in the form of separate or separable segments having mating tongues and grooves.

49. Segments according to claim 48 shaped to form a hollow tube with their tongues and grooves in surfaces extending longitudinally of the tube.

50. A coherent rigid solid material substantially as hereinbefore described in any one of Examples.

51. A method of making a coherent rigid solid material, the method being substantially as hereinbefore described in any one of the Examples.

52. Segments according to claim 48 substantially as hereinbefore described with reference to any of Figures 2 to 4 of the accompanying drawings.