Background The group of fine silica materials consists of micro-silica, precipitated silica and fumed silica.

Micro-silica stems from the production of silicon metal in electric arc furnaces. It has a specific surface area of approx. 10-30 m2/g and is mainly applied in the concrete business.

The precipitated silica is produced in a hydrometallurgical process, has a specific surface area in the range of 90-600 m2/g and is used as reinforcement filler to silicon rubber and plastic as well as filler in paper and paints.

Fumed silica is produced in a pyrometallurgical process, has a specific surface area in the range of 100-800 m2/g and is used as a pseudoplastic and thixotropic property agent in resins, as a rheological property agent in plastisols, to reduce the water absorption of printing ink, viscosity control and thixotropic agent in paints and coatings, as thickener and gelling agent in rechargeable batteries, in chemical-mechanical polishing, as a reinforcing filler in silicon elastomers, as reinforcing and rheological properties agent in joint sealant, in electrophotographic toners and developers as well as in cosmetics, pharmaceuticals and foods. It is also used in paper to enhance opacity, reflection, printing, pitch adhesion on rollers, retention aid and to give a smooth surface and enhanced friction.

Silica soot stems from the production of high purity fused quartz glass for e g the optical fibre industry. The first step of the quartz glass produced can be to produce silica soot in a flame hydrolysis process, in which silicon tetrachloride and various volatile reactants are introduced to an oxygen/hydrogen flame. The product of this process takes a physical form of fine particles and these are usually designated as silica"soot". In the second step of the quartz glass production this silica soot is heated to or slightly above its melting point. The silica soot particles fuse with one another and create a quartz glass material without voids. However, some of the silica soot particles escape the sintering process trough the furnace off-gas system and must be collected in the off-gas cleansing filter. Up till now, this silica soot, collected in the off-gas filter, has mainly been considered as a waste material which has been pelletized and put on the municipal rubbish heap.

This solution of the waste slurry problem is unfortunate, both from an economical and environmental point of view. Especially when considering that the silicon soot are leftovers from one of the commercially available purest raw material, and thus constitute a valuable material which is well suited to be applied as raw material in a number of industrial applications.

Object of the Invention The objective of this invention is to eliminate the waste disposal by utilising the silica soot as a raw material in other industrial applications.

Another objective of this invention is to provide a method for using the silica soot as a substitute to fumed and precipitated silica.

Brief description of the invention The objects of this invention can be achieved by what is described in the appended claims and/or in the following description.

The objectives of this invention can be obtained by exploiting the fact that silica soot is a very pure X-ray amorphous silicon dioxide (Si02) which is physiologically inert. Thus silica soot may be utilised as a substitute for conventional fumed silica and/or precipitated silica as an active reinforcing filler and thixotropic agent in a variety of applications. This can for instance be seen from Table 1 which compares a set of properties of a conventional fumed silica sold under the name Wacker HDKOO H 15 and silica soot. Both materials have a bright white colour with light reflection above 94%.

Table 1 Comparison of silica soot and fumed silica of type Wacker HDK# H 15 Wacker Silica soot HDKO H 15 Surface area according to BET (m2/g) 120+20 50+10 Si02-content (%) 99, 8 99,9 Moisture (%) <0,6 0,5 Loss of ignition (%) <2,0 <0,01 pH-value 4,0-4, 8 2,8-3, 9 HCL-content (%) <0,02 0,02 Al203-content (%) <0,05 0,02 Fe203-content (%) <0, 005 <0,01 C-content (%) <2,0 0,03

From Table 1, one has that silica soot is purer than Wacker HDK H 15 while the surface area of silica soot is approx. half the size of the Wacker material. Thus for some applications where the surface area is the dominating property, one has to use roundabout twice the quantity of silica soot when substituting fumed silica.

Obviously, when the ratio of the surface area of silicon soot and the fumed or precipitated silica is different from the ratio of the example shown in Table 1, the quantity ratio of silicon soot/substituted silicon should be the adjusted accordingly.

However, even if one has to substitute fumed silica with larger amounts of silica soot, there is still considerable economical benefits involved since silica soot is a waste material which is very cheap compared to a highly purified raw material such as fumed silica or precipitated silica. And it is fairly obvious that the environment will also benefit when a waste materiel is given the opportunity of being utilised as a raw material in other industrial processes.

Detailed description of the invention The invention will now be described in more detail by way of examples of preferred embodiments. This should not be constructed as a limitation of the invention.

Example 1 Production of silicone sealant Silicon sealants with Silica Soot has been produced and tested according to ISO 116600 Building construction-Sealants-Classification and requirements in order to find out if the in silicone sealants widely used pyrogenic silica can be replaced by Silica Soot.

As described, the Silica Soot is a pyrogenic silica with a specific area (BET) of ca 50 m2/g. When incorporated in transparent formulations of silicone sealants the sealant appears opalescent.

The pyrogenic Silica Soot with a specific surface area (BET) of 47.9 m2/g was used instead of a pyrogenic silica with a BET surface area of 150 m2/g in a transparent (tr) and a pigment containing formulation (pig).

Exchanging pyrogenic silica in a silicone sealant formulation with oxin-cure by Silica Soot the following compositions have been obtained: Table 2 Transparentlopalescent silicone-sealant (parts by weight) Formulation Si-tr-1 Si-tr-2 Silicone-polymer MG 50 000 58,00 58,00 Silicon-oil 1000 27,00 27,00 MOS Methyloximino-Silane 4,00 4,00 VOS Vinyloximino-Silane 1,30 1,30 HDK V 15 Wacker 9,23 Silica Soot 35,00 Silane 3024 AMMO 0,40 0,40 Catalyst DBTA 0,07 0,07 TOTAL 100, 00 125,77

For the exchange of the pyrogenic silica the sealants have been tested according to: ISO 11600 Building construction-Sealants, Classification and requirements (1993) construction sealants (F) Table 3 Pigment-containing silicone-sealant (parts by weight)

Formulation Si-pig-1 Si-pig-2 Silicone-polymer MG 50 000 38, 00 38, 00 Silicon-oil 1000 20,50 20,50 MOS Methyloximino-Silane 3,50 3,50 VOS Vinyloximino-Silane 0,90 0,90 HDK V 15 Wacker 5,00 Silica Soot 15,00 calsium carbonate Omyacarb 5 GU 30,00 30,00 Silane 3024 AMMO 0,50 0,50 Titandioxide-paste 1,50 1,50 Catalyst DBTA 0,10 0,10 TOTAL 100, 00 110, 00

Sealants with a total movement capacity of 25% are classified according to their modulus into two types: Type LM (low modulus) tensile modulus E 100 at 23 °C < 0,4 N/mm2 at-20 °C < 0,6 N/mm2 Type HM (high modulus) tensile modulus E 100 at 23 °C > 0,4 N/mm2 at-20 °C > 0,6 N/mm2 (E 100: Tensile modulus at 100% elongation) If either at 23 °C or at-20 °C the tensile modulus is higher than the type LM given values, the sealant is designed as Type HM. The results of the tests are given in the tables 4 and 5: Table 4 Test results of transparentlopalescent silicone sealants Nr. Test Test Results Results Requirements class 25 HM according Si-tr-2 Si-tr-1 ISO 11600 to Adhesion-cohesion Type LM 1 properties under at ISO 8340 23 °C < 0,4 maintained extension 23 °C 0,48 23 °C 0,45-20 °C # 0,6 tensile modulus E 100 - 20 °C 0,49 - 20 °C 0, 47 Type HM Nlmm2 23 °C > 0,4 - 20 °C > 0, 6 Tensile strength 2 N/mm2 ISO 8339 0,78 0,68 cohesion failure max. Elongation % 225 269 Adhesion-cohesion properties at variable no cohesion failure 3 temperature ISO 9047 no failure no failure Amplitude 25 % no adhesion failure Adhesion-cohesion properties under at no cohesion failure 4 maintained extension ISO 10590 no failure no failure after water immersion no adhesion failure elatic recovery % ISO 7389 vertical : vertical: vertical: 5 °C 0 5 °C 0 5 °C zu 2 70 °C 0 70 °C 0 70 °C # 2 6 Resistance flow m m ISO 7390 horizontal: horizontal: horizontal: 5 °C 0 5 °C 0 5 °C < 2 70 °C 0 70 °C 0 70 OC 2 7 Chyange of volume % ISO 10563 0,3 0,3 # 10 Table 5 Test results pigmented silicone sealants Nr. Test Test Results Results Requirements class 25 HM according Si-pig-2 Si-pig-1 ISO 11600 to Type LM Adhesion-cohesion 23 °C < 0,4 properties under at ISO 8340 23 °C 0,57 23 °C 0,58-20 °C < 0,6 1 maintained extension-20 °C 0, 57-20 °C 0,60 Type HM tensile modulus E 100 23 °C > 0,4 N/mm2-20 °C > 0,6 Tensile strength 2 N/mm2 ISO 8339 0,81 0,59 cohesion failure max. Elongation % 232 244 Adhesion-cohesion properties at variable no cohesion 3 temperature ISO 9047 no failure no failure failure Amplitude 25 % no adhesion failure Adhesion-cohesion properties under at no cohesion 4 maintained extension ISO 10590 no failure no failure failure after water immersion no adhesion failure 5 elastic recovery % ISO 7389 89 90 #07 vertical: vertical: vertical: 5 °C 0 5 °C 0 5 °C < 2 70 °C 0 70 °C 0 70 °C < 2 6 Resistance flow m m ISO 7390 horizontal: horizontal: horizontal: 5 °C 0 5 °C 0 5 °C < 2 70 °C 0 70 °C 0 70 °C < 2 . Change of volume % ISO 10563 0,3 0,3 #10

In the above described laboratory tests it could be confirmed that the pyrogenic Silica Soot can be used in silicone sealants by replacing the widely used pyrogenic silica with a specific surface area of 150 m2/g.

The tested transparent/opalescent silicone sealant fulfil the requirements of ISO 11600 for high modulus sealants with a total movement capacity of 25 % and is design as: Sealant ISO 11600 F-25 HM

Because of the lower specific area the amount of pyrogenic Silica Soot is about three times higher to have a non sag type sealant. Due to this effect it is possible to incorporate more silica into the sealant.

The whitish colour of the pyrogenic Silica Soot is suitable for the production of transparent/translucent silicone sealants.

Example 2: Production of water based acrylic sealant Acrylic sealants with Silica Soot has been produced and tested according to ISO F/DIS 116600: 2001 Building construction-Jointing products-Sealants- Classification and requirements in order to find out if the in acrylic sealants widely used pyrogenic silica Aerosil R 972 can be replaced by Silica Soot.

As described, the Silica Soot is a pyrogenic silica with a specific area (BET) of ca 50 m2/g The pyrogenic Silica Soot with a specific surface area (BET) of 47.9 m2/g was used instead of a pyrogenic hydrophobic silica Aerosil R 972 with a BET surface area of 110 m2/g in a acrylic water-based sealant.

Exchanging pyrogenic silica in an acrylic sealant formulation by Silica Soot the following compositions have been obtained: For the exchange of the pyrogenic silicas the sealants have been tested according to: ISO F/DIS 11600: 2001, Building construction-Jointing products-Sealants, Classification and requirements construction sealants (F) Table 6 Formulation water based acrylic sealants (parts by weight) Formulation AC-1 AC-2 AC-3 Acronal V 271 with NaOH (20%) >pH 8 315, 00 315, 00 315, 00 Plastilit 70,00 70,00 70,00 TiO2 ILronos 2056 50,00 50,00 50,00 Lumiten NOG 2,00 2,00 2,00 Pigment distributer N 1,00 1,00 1,00 Omya BLP-3 555,00 555,00 555,00 Silica Soot 7,00 0,00 10,00 Aerosil R 972 0,00 7, 00 0, 00 TOTAL 1000, 00 1000, 00 1003, 00

Acrylic sealants with a total movement capacity of 12,5 % are classified according to their modulus into two types: - Class 12,5 E elastomeric type Elastic recovery > 40% - Class 12,5 P plastic type Elastic recovery < 40% The tested acrylic sealants correspond to a sealant with mainly elastomeric properties.

The following tests given in table 7 have been carried out: Table 7 Test program No. Test Test according to 1 Elastic recovery ISO 7389 2 Tensile properties at maintained extension ISO 8340 3 Adhesion-cohesion properties at varible temperature ISO 9047 Adhesion-cohesion properties 4 under at maintained extension ISO 10590 after water immersion 5 Change of volume ISO 10563 6 Resistance to flow ISO 7390 7 Secant tensile modulus at 60 % and 100 % elongation ISO 8339 8 Dynamic viscosity ISO 3219 The tests of the adhesion and cohesion properties have been carried out with test specimens of mortar coated with acrylic based primer DISBON 486 Acryl Tiefgrund. The sealant was applied after 30 min drying time of the primer.

The preconditioning was carried out according to method B of DIN EN 28 340.

Test according to ISO F/DIS 11600: 2001: The test of the adhesion and cohesion properties was carried out according to the test conditions described in the test procedures.

Test of dynamic viscosity: The dynamic viscosity of the sealants have been tested with the Haake rotavisco with cone and plate according to ISO 3219 at 23 °C.

Using the system PK5-1, 0 wiyh a share speed from 0 to 50 s-1 the hysteresis was determined.

The results of the tests are given in table 8: Table 8 Test result water-based acrylic sealant No. Test Test Results Results Results Requirements class 12,5 E accor-AC-1 AC-2 AC-3 ISO 11600 ding to Elastic recovery 1 elongation 60% ISO 7389 58,3 51,2 59,2 40% Adhesion/cohesion no adhesion failure 2 properties under ISO 8340 no failure no failure no failure no cohesion failure maintained extension 60% Adhesion-cohesion no cohesion failure properties at no adhesion failure 3 variable ISO 9047 no failure no failure no failure temperature Amplitude 12, 5 % Adhesion-no cohesion failure cohesion no adhesion failure 4 properties ISO 10590 no failure no failure no failure under maintained extension after water immersion extension 60% 5 Change of ISO 10563 14, 9 15, 1 15 25 volume % vertical: vertical: vertical: 5°C 0 5 °C 0 5 °C 0 70 °C 1 70 °C 1 70 °C 0 6 Resistance ISO 7390 horizontal : horizontal : horizontal : < 3 flow mm 5°C 0 5 °C 0 5 °C 0 70 °C 0 70 °C 1 70 °C 0 Adhesion/ cohesion 7 properties ISO 8339 0,28 0,28 0, 28 no E 601 N/mm2 0, 34 0,34 0,35 E 1002 N/mm2 8 Dynamic ISO 3219 9300 9500 11800 no viscosity mPas 1) E 60: secant tensile modulus at 60% elongation 2) E 100: secant tensile modulus at 100% elongation

In the above described laboratory tests it could be confirmed that the pyrogenic Silica Soot can be used in water-based acrylic dispersion sealants by replacing the widely used pyrogenic silica with a specific surface area of 110 m2/g.

The tested acrylic sealant fulfill the requirements of ISO F/DIS 11600: 2001 for elastomeric sealants with a total movement capacity of 12,5 % and is design as: Sealant ISO 11600 F-12,5 E Production of silicone rubber sealant Silicone rubber is often produced from dimethyl-dichlorine-silane, in a process where the chlorine ions are exchanged with oxygen through hydrolysis until the whole material is polymerised. A number of additives are added to the liquid material to provide it with the ideal rheological properties and also the necessary mechanical strength as well as a smooth surface. Such additives are among others; filler, plasticizer, adhesion promoter, crosslinker, and cure catalyst. Fumed silica, e g the above mentioned Wacker HDKW H 15, is the most commonly used filler in silicone rubber. Normally approximately 8-10% fumed silica is added as filler material.

Trials have shown that by adding 20% silica soot in stead of 8-10% fumed silica, a silicone rubber sealant with almost the same properties is achieved. The rheological properties, the mechanical strength and the rubber surface arc excellent. Yet two properties are different: The silicone rubber is somewhat heavier.

The transparency has decreased; it has become"milky".

The additional weight might reduce the price in the market. The loss of transparency will make silica soot useful only in coloured silicone sealer applications.

Polysulphide rubber sealant Another class of sealants is polysulphide rubber sealants, a two component sealant comprising a liquid polysulphide elastomer and a curing agent. Such sealants often contain calcium carbonate and have encountered problems with segregation. These problems can be solved by replacing the calcium carbonate filler with silica soot.

This is a surprising effect since it is known that conventional fumed silica gives a too large viscosity of the sealant.

Thixotropv agent Silica soot has been mixed with water to a slicker in the soot/water weight ratio 1: 1. The mixture got stiff after a few hours. After nine months it is still set, showing no signs of segregation. A touch with e g a brush brings it back to the

fluid state. Left to rest the mixture get stiff again. This indicates that silica soot is an excellent viscosity control and thixotropic agent.

It is also possible to employ silica soot from the production quartz glass as a replacement of filler materials in other sealants, such as for instance acrylic sealants, butyl rubber sealants etc. , and in other types of products such as for instance urethane prepolymers etc.