Silica production process

Abstract

This disclosure relates to a gas phase production process of silica from a thermal treatment of a silicone feedstock, particularly a waste silicone feedstock. The application also encompasses the silica product resulting from the process.

Description

[0001] SILICA PRODUCTION PROCESS

[0002] This disclosure relates to a gas phase production process of silica from a thermal treatment of a silicone feedstock, particularly a waste silicone feedstock. The application also encompasses the silica product resulting from the process.

[0003] Silicon metal is a valuable and difficult-to-produce element. It is used in the preparation of silicon containing compounds such as silanes and siloxane (silicone) polymers, which siloxane (silicone) polymers comprise repeating units of the structure — [Si(R)2 - O]- where each R is a monovalent organic group.

[0004] Such silanes and siloxane (silicone) polymers or both together are used in curable compositions comprising several components which upon cure generate materials such as silicone elastomers. Silicone elastomers often comprise a silica (SiO2) reinforcing filler, in the form of, for example, precipitated silica or fumed silica.

[0005] The main commercial routes to making fumed silica (sometimes referred to as pyrogenic silica) are by the flame pyrolysis of chlorosilanes, such as tetrachlorosilane, trichlorosilane and alkyl trichlorosilanes, with tetrachlorosilane being the most regularly used. The flame pyrolysis of tetrachloro silane is undergone in a hydrogen / oxygen flame and fumed silica is generated in accordance with the following reaction:

[0006] SiCl4+ 2 H2+ O2→ SiO2+ 4 HCl

[0007] If carbon-containing feed materials such as alkyl trichlorosilane and dialkyldichlorosilanes are used, an oxidation process is also undertaken to convert the carbon present into carbon dioxide. Gaseous silicon tetrachloride is usually prepared by having a suitable inorganic silica source undergo carbothermal reduction in e.g., an electric arc furnace to make silicon metal and then treating the silicon metal with HC1 to produce SiCl4. Suitable inorganic silica source materials such as quartz and quartzite are usually used as the silica starting material for e.g., making fumed silica fillers for use in silicone elastomer compositions.

[0008] The silica produced from such processes is typically hydrophilic characterised by having surface hydroxyl groups which will readily interact with e.g., water and other hydroxylated compounds and consequently disperses easily in hydrophilic systems. However, it is difficult to mix into hydrophobic systems with non-polar or low-polarity compounds such as siloxane polymers and as such when fumed silica is to be utilised in a silicone composition as a reinforcing filler, it is typically rendered hydrophobic by means of suitable hydrophobing treating agents such as silanes, silazanes, short chain siloxanes and saturated fatty acids and their esters e.g., stearic acid and stearates. The hydrophobing process can be undertaken prior to use, if desired, or may be carried out in-situ by intermixing a siloxane, polymer, the silica filler and a suitable hydrophobing treating agent as a step in the compounding of silicone compositions for making silicone elastomers containing reinforcing silica fillers.

[0009] Silicone elastomers may be prepared via numerous cure processes including by condensation cure processes, sometimes referred to as room temperature vulcanisable processes, by hydrosilylation (addition) processes and by way of free-radical cure processes. However, given the resulting cured products are thermoset materials, silicone elastomers cannot be melted and reprocessed into polymers suitable for reuse, it is difficult to provide an effective recycling and / or reclaiming alternative to incineration or landfilling as an end-of-life option.

[0010] Manufacturers who use silicone elastomers and the like in their commercial activities are increasingly requesting sustainable solutions for dealing with silicone-based waste products. Furthermore, environmental legislation is requiring reductions in carbon dioxide (CO2) emissions and the use of recycled materials.

[0011] Several physical recycling processes and chemical recycling processes have been instigated for the end-of-life recycling of thermoset silicone elastomers. End of life, thermoset silicone elastomers may be physically recycled, i.e., they may undergo mechanical processes such as mechanical reclaiming, thermomechanical reclaiming and cryo-mechanical reclaiming and may alternatively undergo wet / solution grinding methods. Mechanical reclaiming / recycling requires significant separation and often generates solid residues that are typically of low value. It is known, however, that particulates made from elastomeric silicone materials may be prepared by recycling and / or reclaiming. That said, such recycled and / or reclaimed silicone elastomeric particulates are typically incorporated as fillers in conjunction with a binder material that serves as a matrix. The binder can be made from either a new silicone composition or an organic polymer material that can also be combined with an inorganic mixture such as an asphaltic or cementitious mixture and serves the function of entrapping and / or encapsulating the discrete particles. However, they have not been considered useful in valuable applications because of their inconsistency and variability in performance and therefore the value proposition for such recycled and / or reclaimed silicone elastomeric particulates is poor with the particulates only being deployed in lower value applications, lowering economic and technological incentives for reuse, as a result of:

[0012] (i) poor incorporation of the silicone elastomeric particulates within the binder matrix;

[0013] (ii) Interfacial adhesion and binding between the silicone elastomeric particulates and matrix being typically poor such that the preformed silicone elastomeric particulates can serve as defects in the matrix, creating voids, surface protrusions or other heterogeneities in the matrix, resulting in inferior properties to new silicone elastomers.

[0014] Chemical recycling of silicone elastomers is possible but not straightforward. Chemical recycling methods for recycling / reclaiming thermoset silicone elastomeric materials tend to be via “chemical processes” such as pyrolysis, chemical degradation and chemical reversion and are mainly designed to generate short chain (i.e., up to about 25 siloxane units), linear, branched and / or cyclic siloxane oligomers for use in new silicone compositions.

[0015] However, those techniques require waste stream sorting by composition, chemistry or additive e.g., filler. Such sorting becomes very complex when treating waste streams made of a mix of cured silicone rubber.

[0016] Alternatively, silicone elastomers have been chemically recycled to form silica-based materials. A selection of such processes is mentioned below:

[0017] In US4640901, a silica-base membrane was prepared by pyrolyzing a silicon-based material in an inert gas atmosphere at a temperature sufficiently high to drive off substantially all hydrocarbons in the silicon-based material. The resulting de -carbonized solid was then placed in an oxygenating atmosphere to crosslink the de-carbonized solid material, and optionally, submitting the resulting crosslinked material to re -pyrolysis to produce a membrane. Optionally the membrane could be coated with silicon material, or other thermo-setting materials and then re-pyrolyzed in an inert atmosphere.

[0018] CN103130230A describes a process for producing white carbon black (i.e., hydrated silica (SiO2•nH2O) or precipitated silica) from silicon rubber pyrolysis ash by introducing silicone rubber pyrolysis ash into a storage tank, removing inorganic salts with spent acid, putting into a dilute acid reaction tank to react with hydrochloric acid, and putting the ash subjected to pressure filtration into a high-temperature roasting furnace system to remove organic substances by oxidation. The resulting product was then ground into a powder.

[0019] CN111892061 describes a process to generate white carbon black by burning organic silicone solid residues and conducting a wet-chemistry process to obtain white carbon black fine powder.

[0020] In N.S.M. Stevens &, M.E. Rezac; Polymer 40 (1999) 4289–4298 a two-step pyrolysis process was carried out. Firstly, inert pyrolysis was conducted at 300-400°C in a nitrogen atmosphere followed by an oxidative pyrolysis process at 260-300°C in air. Hybrid materials with reduced carbon and hydrogen content and an increased oxygen / silicon ratio resulted.

[0021] In R. Hajj, et. al, Polymer Deg. and Stabil. 200 (2022) 109947 two types of fillers were prepared by controlled pyrolysis of a commercial silicone elastomer. In order to prepare high carbon content residues, the pristine crosslinked RTV silicone elastomer was cut into small parts of 2 x 2 x 1 cm3placed in two interlocking steel tubes in order to limit air exposure. These tubes were then placed in an oven and the temperature was ramped up to 850 °C, maintained around 15 min, then cooled down to room temperature. The black residue pieces obtained were then ground into powder. Furthermore, in order to prepare carbon-free residue, the same elastomer parts were pyrolyzed under air at 600 °C overnight. The obtained white residue was then ground into powder and pyrolyzed again overnight (still in air) to ensure the total removal of the remaining carbon and to obtain a white silica powder.

[0022] In R. Chen et al.: Synthesis of high-purity mesoporous nanosilica microspheres from retired composite insulators based on orthogonal experiment. High Voltage. 7(6), (2022) 1111-1122 a silicone compound is exposed to a heat treatment in aerobic atmosphere. A sample of silicone rubber was placed in a crucible with a cylindrical stainless-steel container providing complete coverage to provide gas-pyrolysis sites. Circular openings were provided at the bottom of the container to ensure the gas circulation between the interior and exterior of the container, to provide enough oxygen for the oxidation reaction and to vent the carbon dioxide and gaseous water generated as by-products of the reaction.

[0023] The crucible was tightly covered with a 2000-mesh stainless steel filter screen to block large particles escaping and as a means of collecting the final nanosilica microspheres products. Then, the entire container was placed in a box-type, high-temperature muffle to achieve heating. After pyrolysis, the container was carefully taken out from the muffle furnace to cool naturally to room temperature. White powders (silica) were attached onto the filter screen and collected for analysis. Hence, many opportunities remain for tire provision of methods for the generation of usable materials from silicone elastomer waste. The present disclosure aims to provide a means of using a silicone feedstock and in particular reusing a waste silicone feedstock of mixed and / or unknown origins and the like as a means of generating silica.

[0024] There is provided herein a gas phase production process of silica from a thermal treatment of a silicone feedstock comprising:

[0025] (i) Providing a furnace having at least one gas stream inlet, a gas stream outlet and a means of introducing a silicone feedstock having a siloxane content comprising one or more silicone elastomers, one or more organopoly siloxane polymers or a mixture thereof and a means of removing a solid residue resulting from thermolysis of said silicone feedstock;

[0026] (ii) Heating the furnace to a predetermined temperature of from 100°C to 900°C;

[0027] (iii) Providing a stream of gas through the at least one gas stream inlet into the furnace and out of the gas stream outlet;

[0028] (iv) Introducing a silicone feedstock into the furnace and heating said silicone feedstock towards the predetermined temperature of step (ii) whilst depolymerizing and evaporating at least some of the siloxane content of the silicone feedstock into gaseous siloxane oligomers, prior to and / or during thermolysis of residual silicone feedstock;

[0029] (v) Mixing the gaseous siloxane oligomers, in the stream of gas;

[0030] (vi) Oxidizing said gaseous siloxane oligomers, in the stream of gas and producing silica particles therein;

[0031] (vii) Collecting the silica particles produced in step (vi) from the stream of gas; and

[0032] (viii) Thermolyzing any remaining solid and / or liquid silicone feedstock in the furnace simultaneously with or subsequent to steps (iv) to (vii) causing a solid residue to be formed therefrom and subsequently removing said solid residue from the furnace.

[0033] There is also provided silica obtained or obtainable from a gas phase production process of silica comprising the steps of

[0034] (i) Providing a furnace having at least one gas stream inlet, a gas stream outlet and a means of introducing a silicone feedstock having a siloxane content comprising one or more silicone elastomers, one or more organopoly siloxane polymers or a mixture thereof and a means of removing a solid residue resulting from oxidation of said silicone feedstock;

[0035] (ii) Heating the furnace to a predetermined temperature of from 100°C to 900°C;

[0036] (iii) Providing a stream of gas through the at least one gas stream inlet into the furnace and out of the gas stream outlet;

[0037] (iv) Introducing a silicone feedstock into the furnace and heating said silicone feedstock towards the predetermined temperature of step (ii) whilst depolymerizing and evaporating at least some of the siloxane content of the silicone feedstock into gaseous siloxane oligomers, prior to and / or during thermolysis of residual silicone feedstock;

[0038] (v) Mixing the gaseous siloxane oligomers, in the stream of gas; (vi) Oxidizing said gaseous siloxane oligomers, in the stream of gas and producing silica particles therein;

[0039] (vii) Collecting the silica particles produced in step (vi) from the stream of gas; and

[0040] (viii) thermolyzing any remaining solid and / or liquid silicone feedstock in the furnace simultaneously with or subsequent to steps (iv) to (vii) causing a solid residue to be formed therefrom and subsequently removing said solid residue from the furnace.

[0041] The silicone feedstock utilised in the gas phase production process of silica described above may comprise any silicone materials which may be solids and / or liquid and may comprise, for example silicone polymers, uncured silicone compositions, silicone emulsions and cured silicone products e.g., elastomers.

[0042] In one preferred embodiment the silicone feedstocks are waste silicone feedstocks comprising waste silicone material, i.e., post-industrial or post-consumer waste containing silicones, which would otherwise most likely be disposed of by e.g., incineration or landfilling as an end-of-life option. The postindustrial or post-consumer waste containing silicones comprise cured and uncured silicone elastomers, selected from condensation cure silicone elastomers, hydrosilylation curable silicone rubber compositions, peroxide cured silicone rubber compositions or UV cured silicone rubber compositions, emulsions containing silicones, and silicone fluids.

[0043] When the silicone feedstock is waste silicone feedstock, it may be, for example, derived from any suitable source such as (but not limited to) post-industrial or post-consumer waste mainly obtained from condensation cured (RTV) silicone elastomers formerly used as adhesives, refrigerant spacers, potting agents, coatings and sealants such as weatherproofing sealants and coatings and / or tire sealants or silicone rubber elastomers prepared from hydrosilylation curable compositions, peroxide free-radical cure compositions or UV cure compositions using photo-initiators or photo-catalysts, which elastomers may have been used in applications such as airbag coatings, gaskets and seals adhesives, coatings, foams, molded rubber articles, hoses and tubing like medical tubing, encapsulants and potting agents or the like. The waste silicone feedstock may alternatively be waste emulsions containing silicones.

[0044] Advantageously, however, when the silicone feedstock is in the form of an uncured composition or cured silicone products, such as elastomers, said feedstock does not need to be sorted or separated by the means of cure or the like prior to use in the process herein described.

[0045] The silicone feedstock may be supplied into the furnace in the form of lumps of waste elastomeric material as received after end use. This may contain silicone elastomers and other materials. Alternatively, or additionally, some of the silicone feedstock may be in a liquid form, for example, an uncured silicone polymer material which is outside customer requirements and thus not suitable for reuse.

[0046] However, the silicone feedstock, particularly waste silicone feedstock introduced into the furnace may be pre -treated or pre-prepared. For example, this may include one or more of the following:

[0047] (a) partially or wholly separating silicone materials to be used as waste silicone feedstock, from non-silicone materials such as metals, fabrics, plastics e.g., organic thermoplastics or the like, aluminum window frames, glass, air bag textiles; (b) reducing silicone materials to be used as silicone feedstock in size by one or more of the following shredding, cutting, grinding and granulation; prior to step (iv) of the above process, preferably to reduce the size of the silicone feedstock to particulates having a Dmax of less than (<) 50 mm diameter / cross-section, alternatively particulates having a Dmax of < 5 mm as determined with any appropriate particle size measurement instrument, such as sieves or optical instruments, one example being a HELOS H4084 particle size Analyzer from Sympatec GmbH of Pulverhaus Germany with their PAQXOS 4.1 software fas detailed further in the examples)

[0048] (c) when the silicone feedstock is a mixture of solids and liquids, adding said liquid and / or pastes onto the surface solid feed materials or mixing said liquid and / or pastes with solid feed materials in a suitable mixer such as, for the sake of example, a paddle mixer, a blender such as a ribbon blender, or extruder, prior to step (iv); and

[0049] (d) when the silicone feedstock is a liquid or a mixture of solids and liquids, adding said liquids and or pastes through a nozzle, atomiser, vaporiser or another preparation process to disperse the liquids and / or pastes into the gas phase, prior to step (iv).

[0050] Any suitable furnace may be utilised such as, but not limited to, a tube furnace, a fluid bed reactor, a rotary kiln, a linear stationary furnace, a rotary hearth furnace, a retort furnace, a muffle furnace or a moving grate furnace.

[0051] In one embodiment, the furnace has walls and a roof and at least one gas stream inlet and at least one gas stream outlet. The gas stream inlet and the gas stream outlet are not the same. The gas stream inlet may be in any suitable position within the furnace, which is suitable for directing a gas stream towards and passing over the silicone feedstock.

[0052] Any suitably positioned opening may be used as an entrance into the furnace enabling a means of inserting silicone feedstock into the furnace to introduce said silicone feedstock therein.

[0053] When desired, the silicone feedstock may be placed on a support upon entry into the furnace. The support may be stationary or may be moveable. A moveable support may be rotatable or in the form of a conveyor belt or the like. Any suitably positioned furnace outlet may be used as an exit out of the furnace enabling a means of removing waste residue from the furnace as and when required. The furnace inlet and outlet may be the same opening or there may be one inlet and one outlet for introducing feedstock and removing waste residue respectively. In the latter instance the inlet and outlet may be positioned opposite each other to enable, for example, a conveyor belt to function as the means of introducing the silicone feedstock into the furnace and the means of removing any solid residue from the furnace. When required the opening, or each opening, may have a door to seal the furnace shut.

[0054] When the process may be deemed a batch process in which case, for example, there may be an opening defined by the furnace wall sized to allow the silicone feedstock to be introduced in a suitable receptacle in the furnace, after which said opening can be closed by means of a suitable door. The suitable receptacle may, for example be a tray, a crucible or any other type of container used for holding materials in a furnace but must be open to the gas stream such that the gas stream may pass over the receptacle and mix with or entrain gaseous siloxane oligomers generated from the silicone feedstock. Hence, in a batch process the silicone feedstock may be introduced into the furnace and placed in a static position on an immovable base or support, alternatively the base or support may be moveable, i.e., it may be designed to vibrate or rotate or the like during heating to ensure even heating throughout the evaporation period.

[0055] However, in the case of a continuous process the silicone feedstock may be transported through the furnace for a predetermined residence time with the speed of the transportation method retaining the silicone feedstock in the furnace for said predetermined residence time. The means of transportation can involve a continuous gradual, or stepwise movement through the furnace.

[0056] Dependent on the furnace temperature, irrespective of whether a batch or continuous process is being used, any suitable residence time may be selected as desired for example the residence time of the silicone feedstock in the furnace at 600 °C may be from 0.01 to 120 minutes, alternatively from 1 to 120 minutes, alternatively from 5 to 90 minutes, alternatively from 5 to 60 minutes.

[0057] A continuous process is preferred because a process comprising continuous gas mixing step (iv) and a continuous collection of silica particles from the gas stream after oxidation of tire oligomers allows for a much larger volume of waste silicone to be converted to silica in a shorter timescale. The feed rate of the silicone feedstock into the furnace may be for example 1 kilogram per hour (Ikg / h) or greater, for example from Ikg / h to lOkg / h but may be more or less if required. A continuous process was found to reduce or minimise deposition of the oligomers on the furnace walls causing silica to be subsequently formed thereon.

[0058] The velocity of the gas in the gas stream may be from about 0.1 to about 100 ms'1but is really dependent on the geometry of the gas stream inlet e.g., a gas inlet nozzle, the furnace and pathway of the gas stream to ensure it is able to mix with the gaseous siloxane oligomers evaporating from the silicone feedstock and transport them to the section, region or chamber of the furnace where oxidation thereof takes place and subsequently suspending the silica particles produced to transport them out of the furnace for collection. When fully oxidised and after having been resident in the furnace for a longer required period, the silica produced in the present process will be hydrophilic. However, by reducing the ability for the gaseous siloxane oligomers to participate in oxidation in the furnace hydrophobic silica is produced. The hydrophobic nature of the silica in such embodiments is as a result of the carbonaceous groups in the gaseous siloxane oligomers not being fully oxidised and therefore when the gaseous siloxane oligomers break down to form silica they have hydrophobic surfaces. Hence, it has been found that the silica particles produced in the gas stream may be tuned as to their hydrophilic or hydrophobic nature. Tuning may be achieved by:

[0059] (i’) Limiting the time period the gaseous siloxane oligomers having exposure to oxygen in the furnace;

[0060] (ii’) Having a low furnace temperature or;

[0061] (iii’) controlling the oxygen content of the gases to which the gaseous siloxane oligomers are exposed; or

[0062] (tv’) any combination thereof. The silica particles in the gas stream resulting from oxidation of the gaseous siloxane oligomers may be carried out of the furnace in the gas stream through the gas stream exit and the resulting gas stream containing said particles may be cooled and the particles collected by being separated from each other and the gas stream itself, using a filter or the like to capture the silica particles.

[0063] The furnace may have one or more than one designated sections, regions and / or chambers and the gas introduced through the gas stream inlet(s) is selected dependent on the structure of the furnace.

[0064] When the furnace has a single section, region or chamber the stream of gas introduced into the furnace through the gas stream inlet in step (iii) comprises a gaseous oxidizing agent, typically oxygen. In such an instance the gas contained in the stream of gas may comprise or consist of air or a mixture of oxygen in a chemically inert gas. The chemically inert gas may be any suitably inert gas but is typically nitrogen.

[0065] When the furnace has two sections, regions or chambers, in one embodiment, the gas stream in the first section, region or chamber into which the silicone feedstock is introduced i.e. in step (v) of the process may comprise any suitable one or more inert or non-oxidative gases such as nitrogen or combustion gases from a natural gas flame which is / are introduced into said first section, region or chamber through a first gas stream inlet in step (iii) whilst an oxidizing gas e.g., oxygen or air is introduced into the gas stream through a second gas stream inlet at the entrance into the second furnace section, region or chamber the gaseous siloxane oligomers are mixed into the gas stream in step (v).

[0066] Alternatively, when the furnace has two sections, regions or chambers tire gas stream in the first section, region or chamber into which the silicone feedstock is i nt re uced in step (v) may comprise one or more suitable inert or non-oxidative gases and an oxidizing gas introduced into said first section, region or chamber through a gas stream inlet during step (iii) and no additional oxidizing gas is introduced in the second section, region or chamber.

[0067] In one embodiment the furnace is provided with two sections, regions or chambers wherein the first operates in an anaerobic atmosphere e.g., a nitrogen atmosphere or in an oxidative atmosphere below the temperature threshold of oxidation or in an inert or non-oxidative atmosphere. When present this section, region or chamber is used for the depolymerisation resulting in the formation of volatile siloxane oligomers and their subsequent evaporation to gaseous siloxane oligomers (step (iv)) and the consequential mixing (entrainment) of the gaseous siloxane oligomers into the gas stream (step (v)). Consequently, the second section, region or chamber is designed for oxidation of the gaseous siloxane oligomers including oligomer decarbonization at an oxidative temperature e.g., in the range of from 400° C to 900° C, alternatively of from 450° C to 850° C, alternatively from 500° C to 800° C, alternatively 500° C and 750° C prior to collection of the silica particles.

[0068] In one alternative where the furnace has multiple sections, regions or chambers, e.g., three section, region or chambers and the silicone feedstock is conveyed through the furnace on a conveyor belt or the like, there may be a first evaporating section, region or chamber i.e., for the introduction of the stream of gas through the gas stream inlet in step (iii) initially heating the silicone feedstock to generate silicone oligomers and consequently evaporating the siloxane oligomers in step (iv) and entraining the gaseous siloxane oligomers into the gas stream (step (v)); a section, region or chamber into which the evaporated siloxane oligomers mixed (entrained) in the gas stream are transported prior for oxidation (step (vi)) and consequential collection of the fumed silica resulting from the oxidation process (step (vii)) and a third section, region or chamber into which the waste left after the evaporation process may be transported to complete thermolysis of the waste material into the solid residue (step ( viii)).

[0069] The section, region or chamber, or each section, region or chamber, is preheated to a predetermined temperature before introduction of the silicone feedstock in step (iv). The stream of gas may pass through the furnace during the heating of the furnace or the introduction of the stream of gas (step (iii)) may commence once the desired temperature(s) has or have been reached in the section, region or chamber of the furnace, or each section, region or chamber of the furnace. The gas in the gas stream when entering the furnace through the gas stream inlet in step (iii) may be at any suitable temperature, for example it may be at room temperature which may be, for example, but not restricted to a temperature of from 20 °C to 25 °C or thereabouts or may have been pre-heated up to about 250 °C, alternatively up to about 150 °C alternatively from around room temperature up to about 100 °C, alternatively from around room temperature up to about 75 °C, alternatively from around room temperature i.e., from 20 °C to 30 °C, up to about 50 °C. In one embodiment the gas in the gas stream, the silicone feedstock, or both are at the same or a different temperature from room temperature to 250 °C prior to introduction into the furnace during steps (iii) and (iv) of the process respectively.

[0070] Depending on the number of sections, regions or chambers present in the furnace, the temperature of the section, region or chamber, or each section, region or chamber, may be controlled by any suitable means e.g., appropriate temperature sensors, e.g., a series of thermocouples which temperature sensors can initiate further heating when needed using local burners or the like when the temperature strays from the required range.

[0071] However, external supply of energy to heat the furnace e.g., from either fossil fuel or electricity is preferably only required during step (ii) of the process whilst the furnace is brought up to the required initial pre-determined temperature. A control means may be used in the furnace to ensure that any variation in temperature is kept within a set range above or below the predetermined temperature of the furnace and to cause the furnace to be heated or cooled back to being within the range. However, heat generated by the exothermic oxidation of the gaseous siloxane oligomers may be used as a means of heating the furnace under autothermal conditions to maintain the furnace temperature or at least the section, region or chamber temperature in which oxidation takes place at the pre-determined temperature.

[0072] Preferably, as much of the silicone feedstock as possible is depolymerized during step (iv) of the process and forms volatile siloxane oligomers which subsequently evaporate. Residual feedstock which has not so evaporated consequently undergoes thermolysis i.e., an oxidative cross-linking process effectively ceramifying the residual silicone feedstock e.g., solid silicone rubber pieces into a solid waste residue of silica and other components remaining from the silicone feedstock. In this case the resulting silica containing material is not considered part of the product of the process herein, but it may be collected as a by-product subsequent to exiting the furnace as the waste residue herein. The solid thermolyzed waste residue may be salvaged and crushed or may be disposed of.

[0073] The silicone feedstock may be introduced into the furnace either as a liquid or as a gas in step (iv). The temperature of the silicone feedstock at the time of entry into the furnace (step (iv)) may be any suitable temperature up to about 150°C, alternatively up to about 125 °C. Whilst the silicone feedstock may be introduced into the furnace at temperatures below 0 °C if desired or required, typically it is more often introduced into the furnace at a temperature of around room temperature i.e., from 20 °C to 30 °C, typically at a temperature of from 23°C to 25 °C up to about 100 ° C, alternatively from around room temperature i.e., from 20 °C to 30 °C, typically at a temperature of from 23 °C to 25 °C up to about 75 °C, alternatively from around room temperature i.e., from 20 °C to 30 °C, typically at a temperature of from 23 °C to 25 °C up to about 50 °C.

[0074] Hence in one embodiment, the furnace is pre-heated during step (ii) to the pre-required temperature in tire section, region or chamber of the furnace, or each section, region or chamber of the furnace, before the introduction of the silicone feedstock and the silicone feedstock is at e.g., room temperature before it is introduced into the furnace and is heated at an approximately constant rate until it reaches a temperature approaching the pre-defined furnace temperature. As the feedstock temperature increases there is a gradual depolymerization of the silicone materials in the feedstock and the resulting volatile siloxane oligomers generated evaporate and are mixed / entrained into the gas stream. The residual silicone feedstock such as e.g., solid silicone rubber which does not undergo the depolymerization and evaporation (i.e., the first reaction) is subsequently thermolyzed.

[0075] Some of the evaporated siloxane oligomers may deposit on the walls of the furnace etc. and can be oxidized to form silica thereon. The silica on the wall, like the silica from the oxidized waste thermolyzed residue does not form part of the product obtained by the process described herein. Again, any such silica can be collected and used separately and is believed to be a less pure product than the silica particles mixed / entrained in the gas of the gas stream collected separately. Such silica can be collected and used as a filler and / or can be disposed of.

[0076] Hence, the process described herein is designed to depolymerize the siloxane content of the silicone feedstock into volatile siloxane oligomers which subsequently evaporate into gaseous siloxane oligomers as the silicone feedstock is heated in the furnace from the initial temperature at the time of entry of the silicone feedstock into the furnace (step (iv)) towards the predetermined temperature of the furnace i.e., between 400 °C and 900 °C inclusive which is reached by the time the solid residue resulting from the thermolysis of the remaining solid silicone feedstock is removed from the furnace (step (viii)).

[0077] The furnace dimensions, the silicone feedstock residence time or both are designed to provide time for as much of the siloxane content as possible to depolymerize from the silicone feedstock to volatile siloxane oligomers and then evaporate the siloxane oligomers from the silicone feedstock as it is heated in the furnace (step (iv)) prior to when the residual silicone materials in the silicone feedstock residue are thermolyzed and / or oxidized dependent on the gas(es) in the stream of gas to form a final solid residue (step (viii)). The furnace dimensions and the velocity of the gas stream transporting the evaporated siloxane oligomers after mixing in step (v) are designed to give sufficient time for the evaporated siloxane oligomers to at least partially oxidize within the gas stream in step (vi).

[0078] In one embodiment the gas in the gas stream may be continuously recirculated so that the furnace is heating the gas from the moment it enters the furnace in step (iii) as it transports the mixed gaseous siloxane oligomers through the furnace from the moment it enters the furnace in step (iii) until it has been separated from the silica particles (step (vii)) and then the gas can be prepared for re-entry into the furnace by taking appropriate steps such as heating and / or cooling to the desired temperature of entry to the furnace, removing unwanted gaseous species and replenishing the levels of oxidizing gases such as oxygen.

[0079] Forming an inorganic material in the gas phase starting from volatile gaseous siloxane oligomers creates a separation process in that the organic functionalities which form part of the gaseous siloxane oligomers decompose as the temperature of the oligomers increase. Hence, the removal / decomposition of these organic groups leads to the breakdown of the oligomers and formation of the silica and as such the silica produced, particularly when the oligomers have undergone total oxidation, is not dependent on the functionality of the starting organic groups in the feedstock and consequently the oligomers in the gas stream. However, when the silica product is undergoing tuning by (f), (if), (iii’) or (iv’) above it may be possible to at least partially design the surface of the tuned silica by selecting the organic groups present in the feedstock and consequently the oligomers produced therefrom after depolymerisation. Hence, not only can the silica be made hydrophobic but the organic groups creating this hydrophobic nature can be at least to an extent selected.

[0080] It will be appreciated that inorganic filler present in the materials contained in the silicone feedstock stays trapped in the solid residue and is removed as part of step (viii) for disposal or alternative use.

[0081] The gaseous siloxane oligomers resulting from the depolymerization and evaporation during step (iv) from the siloxane content of the silicone feedstock are believed to be a mixture of linear, branched and cyclic siloxane oligomers. The vast majority of the gaseous siloxane oligomers will have from about 2 to 25 repeating siloxane units of the structure –[Si(R)2-O]- where each R is a monovalent organic group, alternatively from about 2 to 20 siloxane units, alternatively from about 2 to 15 siloxane units. The cyclic siloxane oligomers may, for example, mainly be cyclic siloxane oligomers comprising from 3 to 10 siloxane units. The depolymerized and evaporated siloxane oligomers will typically have two or three organic groups e.g., alkyl, aryl, alkoxy and / or alkenyl groups linked to each silicon atom. Subject to the tuning by (i’), (ii’), (iii’) or (iv’) above, such organic groups are also oxidized into combustion products, such as carbon dioxide and / or water.

[0082] Other species will evaporate during the heating of the silicone feedstock. These may include or be derived from oils or plastics that are present in the silicone feedstock and which also breakdown into volatile compounds during the heating process. Typically, it is believed that the vast majority, even if oxidised during the process herein will not form solids. The other species which evaporate in the furnace and which may be mixed into the gas stream may include volatile organic compounds (VOCs) and / or hazardous air pollutants (sometimes referred to as HAPs). If desired any such VOCs and / or HAPs may be suitably heated before release into the atmosphere. For example, they may be heated in a thermal oxidizer. A thermal oxidizer can be designed to heat these species to a pre-defined temperature, i.e., until any present VOCs and / or HAPs have been broken down into e.g., carbon dioxide and water.

[0083] In a further embodiment the gas stream containing the mixed gaseous siloxane oligomers and other gaseous species may exit the furnace after or during step (v) of the process and be oxidized to form particulate silica in a thermal oxidizer during step (vi) of the process simultaneous with the oxidation of the VOCs and HAPs, if any, also mixed into the gas stream during step (v). In such an embodiment the particulate silica resulting from the oxidation of the gaseous siloxane oligomers transferred into a thermal oxidizer can be collected during step (vii) of the process after leaving the thermal oxidizer.

[0084] Unlike several prior art processes, the present silica preparation process is designed to have a gradual silicone feedstock temperature increase in the furnace after its introduction. By doing this, time is provided for the silicone materials in the silicone feedstock to be intentionally depolymerized and / or evaporated and consequently mixed in the gas stream during steps (iv) and (v) of the process prior to during thermolysis of the remaining solids left in the furnace. It is particularly pertinent to appreciate that the thermolyzed solid waste residue remaining at the end of step (viii) is removed from the furnace. It may be removed from tire furnace and then ground into a potentially contaminated silica product containing other species, e.g., fillers, metals, thermoplastic residues etc, in the solid residue resulting from the thermolysis of the silicone feedstock and as such is not considered a product of the process described herein but a potentially contaminated by-product which may be disposed of as waste.

[0085] The silica particles in the gas stream resulting from oxidation of the gaseous siloxane oligomers may be carried out of the furnace through the gas stream exit and the resulting gas stream containing said particles may be cooled and the particles collected by being separated from each other using a filter or the like to capture the silica particles. These silica particles produced in the present process may be collected via any suitable method for example they may be collected when prevented from passing through a screen in the pathway of the gas stream and / or using for example a bag filter, cyclone or another gas-solid separation technique.

[0086] The gaseous siloxane oligomers mixed in the gas stream are believed to condense in the gas stream and consequently oxidize to produce “fluffy powder” of silica having an apparent median particle size of from approximately 1 to 10μm measured using a HELOS H4084 particle size Analyzer from Sympatec GmbH of Pulverhaus Germany with their PAQXOS 4.1 software (as detailed further in the examples).

[0087] The term “fluffy powder” as referred to above is intended to mean that the particles are composed of, containing, or resembling fluff or loose flocculent matter. In the situation described herein the silica particles produced by the process and collected in step (vii) of the process, exhibit above-described behavior, being made of a light open structure, fractal-like, micron size aggregates made of nanosized primary particles.

[0088] In the present disclosure, the depolymerization and evaporation in steps (iv) and (v) of the process led to silica being produced inside the gas stream and collected therefrom in step (vii) enabling the silicone materials in the silicone feedstock to be converted to a “fluffy” silica product produced herein and collected during step (vii).

[0089] The silica particles produced by the process herein were determined to have a branched-like, non-uniform particle / aggregate morphology rather than generally comprising smaller uniform spherical particles which are more akin to precipitated silica. Hence, advantageously, but in line with the process described herein, the product produced in the process herein is more similar to fumed silica or pyrolytic silica, with the silica generated “dendritic (branched) growth” being due to promoting particle growth in the gas stream (and not having the gaseous siloxane oligomers condense and oxidise on the furnace walls.

[0090] The samples produced using this process preferably have a robust hydrophilic silica characterized by visually showing a good wettability with water. The product resulting from the process herein is generated with a BET (Brunauer-Emmet-Teller) surface area comprised from 10 to 450 m2 / g, alternatively from 10 to 400 m2 / g, alternatively from 10 to 300 m2 / g, alternatively from 10 to 200 m2 / g, alternatively from 15 to 100 m2 / g, alternatively from 15 to 95m2 / g alternatively from 20 to 95 m2 / g, determined with N2physisorption using Micromeritics Tristar II 3020 Surface Area and Porosity Analyzer with version 1.04 of Tristar II 3020 software package. Before the measurement, samples were degassed for 2 hours at 200 °C under nitrogen flow in a Micromeritics SmartPrep 065 Programmable Degas System. This BET range is partially outside the range of commercial fumed silica (50 to 400 m2 / g).

[0091] The structure of the silica produced with the process described above is made of aggregates of primary particles and shows a dendritic open structure, like fumed silica being produced with a more complex, higher energy consuming process starting from a virgin stream of silane. Preferably it possesses a lower density of surface silanol, providing unique properties compared to hydrophilic silica. The silica generated by this process herein will provide mechanical reinforcement, allowing larger filler loading than standard fumed silica while keeping flowability or dispensing capabilities.

[0092] Advantageously, the fact that the gaseous siloxane oligomers are mixed in a gas stream in the present process and subsequently oxidized in the gas stream significantly reduces the opportunity for said silica particles to condense on the internal wall of a part of the furnace or the like.

[0093] One advantage of this process compared to other processes for re-using waste silicone materials is that the source of the silicone feedstock does not need to be known or identified, particularly when fully oxidized, resulting in hydrophilic silica. Not needing such information greatly simplifies the process described herein as no analysis of the silicone feedstock is necessary prior to its introduction into the furnace and as such silicone materials cured via multiple cure routes can be present in a batch of silicone feedstock which is introduced into the furnace. However, if the process is being used to provide a tuned form of silica, then knowledge of the feedstock content, is advantageous. It will be appreciated that the oligomer conversion to silica in the gas stream provides a robust separation of valuable Si-containing molecules from the silicone feedstock. In other words, this process can enable an efficient separation and vaporization of the silicone contained in a silicone feedstock (irrespective of its origin and / or method of cure) which would potentially have otherwise been sent to landfill.

[0094] BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Fig. 1 is a depiction of an overlaid attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy (ATR-FITR) spectra of the silica powders from examples 1-5, normalized to the SiO band at 1100 cm-1using the method and equipment described in the Examples below.

[0096] Figs. 2A and 2B show the X-ray photoelectron spectroscopy (XPS) comparing the silicon 2p spectra acquired from the surfaces of silica powder produced in accordance with Ex. 5 (Fig. 2B) and commercial silica powder sold under the trade name CAB-O-SIL™ LM-150D by Cabot Corporation of Alpharetta Georgia, USA C. 1 (Fig. 2A).

[0097] Figs. 3A-3F depicts SEM images in secondary electron contrast at 50000x magnification of the silica powders comparative C. 1 (top, Figs. 3A and 3B) commercial fumed silica (CAB-O-SIL™ LM-150D), the silica product of Ex. 1 (middle row, Figs. 3C and 3D) and the silica product of Ex. 2 (bottom row, Figs. 3E and 3F) with the scale bars being 500 nm.

[0098] Figs. 4A-4F depicts SEM images in secondary electron contrast at lOOOOOx magnification of the silica powders comparative C. 1 (top, Figs. 4A and 4B) commercial fumed silica (CAB-O-SIL™ LM-150D), the silica product of Ex. 1 (middle row, Figs. 4C and 4D) and the silica product of Ex. 2 (bottom row, Figs.4E and 4F) with the scale bars being 300 nm.

[0099] Applications

[0100] The silica powders prepared in accordance with the process herein may be used in any application in which conventional silica powders such as fumed silica and / or precipitated silica are used for example as a filler in silicone elastomers as well as in organic rubbers and plastics. The silica particles collected in step (vii) of the process herein can then be used as hydrophilic or hydrophobic fillers dependent on the tuning when being prepared.

[0101] The silica produced herein by following the enclosed process and extracting same from the gas stream may be utilised as a filler in silicone elastomers as well as in organic rubbers and plastics; as a carrier material or support and as an additive in both silicone and organic sealants, adhesives, paints, and coatings.

[0102] Typically, the silica produced using the method described herein, particularly the hydrophilic silica produced, may be surface treated with any low molecular weight organosilicon compounds disclosed in the art applicable. For example, they may be treated with organosilanes, polydiorganosiloxanes, organosilazanes, short chain siloxane diols, fatty acids or fatty acid esters such as stearates. Once treated the silica produced using the method described herein is rendered hydrophobic and consequently easier to handle and to obtain a homogeneous mixture with the other ingredients, such as silicone polymers. Specific examples of treating agents include but are not restricted to silanol terminated trifluoropropylmetliyl siloxane, silanol terminated vinylmethylsiloxane, tetramethyldi(tritluoropropyl)disilazane, tetramethyldivinyl disilazane, hexamethyl disilazane (HMDZ), silanol terminated MePh siloxane, liquid hydroxyl-terminated polydiorganosiloxane containing an average from 2 to 20 repeating units of diorganosiloxane in each molecule, hexaorganodisiloxane, hexaorganodisilazane. A small amount of water can be added together with the silica treating agent(s) as a processing aid.

[0103] The silica produced using the method described herein may be pre -treated prior to introduction into a suitable curable composition such as a hydrosilylation curable silicone composition, a peroxide curable silicone composition or a condensation curable silicone composition. Alternatively, the silica may be treated in situ, i.e., in the presence of at least a portion of the other ingredients of the curable silicone composition by blending these ingredients together at room temperature or above until the filler is completely treated. Typically, when present untreated silica is treated in situ with a treating agent in the presence of organopolysiloxane polymer which results in the preparation of a silicone base material which can subsequently be mixed with other ingredients.

[0104] Examples

[0105] The following examples further illustrate the invention, but, of course, should not be construed as in any way limiting its scope. They were conducted using a high-temperature tunnel furnace having a rectangular cross-section equipped with two primary heating burners and five secondary process burners. The primary heating burners were used to initially heat the furnace to a desired process temperature (step (ii)). The temperatures within the furnace were tracked using five evenly spaced thermocouples. Secondary process burners were used as and when required to maintain temperatures within the furnace within tire desired ranges in conjunction with the heat released from tire waste combustion.

[0106] Two axial air injectors were used to introduce the gas stream into the furnace (step (iii)). Each air injector (13 mm diameter) delivered 4 m3 / h of air at room temperature, resulting in a mean gas residence time of 90 seconds (s) in the furnace.

[0107] The base of the furnace was used as a sample support in conjunction with a pneumatic cylinder feeding rod system having a t-shaped cross-section and which moved between a forward and retracted position. A row of rectangular alumina crucibles (150 mm x 100 mm) containing samples of approximately the same weight were placed on a “start line’’ at the enhance to the furnace with the pneumatic cylinder feeding rod in the retracted position. Once the row of crucibles were in place the pneumatic cylinder feeding rod was moved to the forward position pushing said first row of crucibles into the furnace. The pneumatic cyclinder feeding rod was then returned to the retracted position and a second row of the rectangular alumina crucibles containing samples of approximately the same weight were placed on the “start line” and after a set period of time the pneumatic cyclinder feeding rod was moved to the forward position pushing said second row of crucibles into the furnace and as a consequence pushing the first row of crucibles further into the furnace. This is repeated after the same time intervals until the first row of crucibles reached the exit of the furnace at which point they were removed and allowed to cool. Once the allotted amount of silicone feedstock had been introduced into the furnace sequential rows of empty crucibles were introduced until all samples in crucibles had been through the furnace for the allotted residence time period.

[0108] The furnace had a vertical exhaust tube out of the roof thereof which functioned as the gas stream exit designed to transport the gas stream containing mixed siloxane oligomers, silica particles and other species from the furnace into and through a post combustion chamber. The post combustion chamber was a thermal oxidizer having an entrance and an exit which was designed to oxidize species such as carbon monoxide to carbon dioxide and residual silicone oligomers not previously oxidized into silica. A pipe means was fixed to the exit of the thermal oxidizer to enable the resulting gas stream to pass through a water-cooling system and subsequently through a bag filter designed to capture silica particles in the gas stream and allow the gas to pass through the bag filter for disposal or recycling.

[0109] Four different types of silicone feedstock was introduced into the furnace on rectangular alumina crucibles (150 mm x 100 mm) in the examples. These are identified in Table 1 below.

[0110] Table 1: Silicone feedstocks used in the Examples

[0111] Description Product Type

[0112] A Cured silicone sealant DOWSIL™ SST- 2 -component RTV 2650 sealant

[0113] B Expired and partially cured silicone sealant DOWSIL™ 799 1 -component RTV sealant

[0114] C 1:1 (w / w) mix of waste type A and B with 20 DOWSIL™ SST- Potential window

[0115] wt.% organic contamination of a 1:1 (weight 2650 + DOWSIL™ facade waste mix

[0116] : weight) mix of Norton Tape & PE cartridge 799

[0117] material

[0118] D Industrial waste of self-lubricating liquid Ph-silicone

[0119] silicone rubber containing HTV

[0120] rubber

[0121]

[0122] Silicone feedstock type A was applied on the crucibles via extrusion through a 6 mm die.

[0123] Silicone feedstock type B was applied directly from caulk cartridges with a hand-held pneumatic caulking gun.

[0124] For silicone feedstock type C, A and B were applied as specified above together with 5-8 mm pieces of Norton tape and polyethylene.

[0125] Silicone feedstock type D were rectangular gaskets of roughly 25 mm x 80 mm and a thread diameter of 2 mm.

[0126] Five examples were carried out and the process and criteria for each one is described below. Example 1

[0127] The furnace was heated overnight to a pre-determined value of about 800 °C using both primary heating burners and once the target temperature was reached, it was maintained between 800 °C and 850 °C mainly reliant of the secondary heating burners and the heat released from the waste combustion.

[0128] Each air injection unit was set at 4 m3 / h which resulted in a mean gas residence time of 90 seconds (s).

[0129] Silicone feedstock A was introduced into the furnace on the alumina crucibles in the manner described above with approximately 50 g of silicone feedstock per crucible at a feed rate of 2 kg / h resulting in each crucible containing 50 g of silicone feedstock having a residence time of 33 min in the furnace. The samples of silicone feedstock gradually warmed up in the furnace and the silicone materials in the waste feedstock gradually depolymerized to form the volatile siloxane oligomers which were mixed in the gas stream together with other gaseous species. The gas stream exited the furnace after having mixed the gaseous silicone oligomers and any other species resulting from the heating of the silicone feedstock in the furnace, passing through the post-combustion chamber at 860 °C designed to guarantee the elimination of combustible off gases such as CO to CO2 for safe process operations and oxidizing any residual oligomers in the gas stream before cooling. Once cooled the resulting gas stream was directed to the filler bag system where the silica particles were collected.

[0130] In example 1, a total of 6 kg of silicone feedstock A was transported through tire furnace. At the end of the process, 0.83 kg of grey silica powder were collected from the bag filter. Furthermore, 1.7 kg residue was collected from the crucibles and 1.2 kg silica as a deposit on the furnace walls. The residue from the crucibles contained inorganic and non-degradable substances present in the original waste stream as well as silica that was formed inside the crucible. The silica deposited in the furnace would be very compacted with much lower surface area than the fluffy silica collected was from the bag filter, this can be mostly attributed to the much longer residence time in the oven, leading to particle coalescence.

[0131] Example 2

[0132] The furnace was heated overnight to a pre-determined value of about 600 °C using both primary heating burners and once the target temperature was reached, it was maintained between 600 “C and 650 °C mainly reliant of the secondary heating burners and the heat released from the waste combustion. Each air injection unit was set at 4 m3 / h which resulted in a mean gas residence time of 90 seconds.

[0133] Silicone feedstock B was introduced into the furnace on the alumina crucibles in the manner described above with approximately 50 g of silicone feedstock per crucible at a feed rate of 3 kg / h resulting in each crucible containing 50 g of silicone feedstock having a residence time of 22 min in the furnace. In total, 9 kg of silicone feedstock B were passed through the furnace and converted. At the end of the process, 1.7 kg of grey silica powder were collected from the bag filter. 2.1 kg residue was collected from the crucibles and 1.4 kg silica as a deposit on the furnace walls.

[0134] Example 3

[0135] The furnace was heated overnight to a pre-determined value of about 750 °C using both primary heating burners and once the target temperature was reached, it was maintained between 680 °C and 730 °C mainly reliant of the secondary heating burners and the heat released from the waste combustion. Each air injection unit was set at 4 m3 / h which resulted in a mean gas residence time of 90 seconds.

[0136] Silicone feedstock A was introduced into the furnace on the alumina crucibles in the manner described above with approximately 50 g of silicone feedstock per crucible at a feed rate of 2 kg / h resulting in each crucible containing 50 g of silicone feedstock having a residence time of 33 min in the furnace. In total, 8.4 kg of silicone feedstock A were passed through the furnace and converted. At the end of the process, 1.1 kg of grey silica powder was collected from the bag filter. 2.0 kg residue was collected from the crucibles and 2.4 kg silica as a deposit on the furnace walls.

[0137] Example 4

[0138] The furnace was heated overnight to a pre-determined value of about 750 °C using both primary heating burners and once the target temperature was reached the waste feedstock was introduced and an average temperature between 580 °C and 630 °C mainly reliant of the heat released from the waste combustion and intermittent operation of the secondary burners. Each air injection unit was set at 4 m³ / h which resulted in a mean gas residence time of 90 seconds (s).

[0139] Silicone feedstock A was introduced into the furnace on the alumina crucibles in the manner described above with approximately 50 g of silicone feedstock per crucible at a feed rate of 2 kg / h resulting in each crucible containing 50 g of silicone feedstock having a residence time of 33 min in the furnace. In total, 8.7 kg of silicone feedstock A was passed through the furnace and converted. At the end of the process, 1.6 kg of grey silica powder was collected from the bag filter. 1.9 kg residue was collected from the crucibles and 2.5 kg silica as a deposit on the furnace walls.

[0140] Example 5

[0141] The furnace was heated overnight to a pre-determined value of about 750 °C using both primary heating burners and once the target temperature was reached the waste feedstock was introduced and an average temperature between 580 and 630 °C was maintained mainly reliant on the heat released from the waste combustion and intermittent operation of the secondary burners. Each air injection unit was set at 4 m3 / h which resulted in a mean gas residence time of 90 seconds.

[0142] Silicone feedstock C was introduced into the furnace on the alumina crucibles in the manner described above with approximately 55 g of silicone feedstock per crucible at a feed rate of 1.7 kg / h resulting in a crucible residence time of 44 min in the furnace. After 5.3 kg of waste feedstock C, the feed was switched to waste feedstock D at 1.6 kg / h with 40 g of waste material per crucible, resulting in a crucible residence time of 40 min in the furnace. All other conditions were kept constant from waste feedstock C. The trial was terminated after 1.2 kg silicone feedstock D had been through the furnace. In total, 6.5 kg of waste feedstock C and D were converted. At the end of the process, 1.0 kg of grey silica powder was collected from the bag filter, 1.5 kg residue was collected from the crucibles, and 1.5 kg silica as a deposit on the furnace walls.

[0143] Upon completion of each of Examples 1 to 5, the respective silica powders collected from the bag filter in each example were characterized by Brunauer, Emmett and Teller (BET) surface area measurements, particle size measurements, and thermogravimetric analysis (TGA) under air. All Brunauer-Emmet-Teller (BET) surface area results were determined with N2physisorption using a TriStar™ II 3020 Surface Area and Porosity Analyzer from Micromeritics Instrument Corporation of Norcross, Georgia, USA with version 1.04 of TriStar™ II 3020 software package. Before the measurement, samples were degassed for 2 hours at 200 °C under a sufficient nitrogen flow in a SmartPrep™ 065 Programmable Degas System from Micromeritics Instrument Corporation of Norcross, Georgia, USA.

[0144] All particle sizes were measured on a HELOS H4084 particle size Analyzer from Sympatec GmbH of Pulverhaus Germany with their PAQXOS 4.1 software. The material was dispersed using a RODOS / L dry dispersion unit from Sympatec GmbH of Pulverhaus Germany with compressed air at 1.2 bar (0.12MPa) and a vacuum of 57 mbar (5,700Pa). Dosing was accomplished with a VIBRI vibratory feeder from Sympatec GmbH of Pulverhaus Germany (VIBRI) at a 60% feed rate and 4 mm gap.

[0145] All thermogravimetric analysis (TGA) experiments were performed using a TGA / DSC 3+ Thermal Analysis system from Mettler-Toledo™ Instrument Co. Ltd, with Al₂O₃ pans (150 μL) and a pinhole lid. In a typical TGA measurement, 5-7 mg of the silica powder were placed in the crucibles and subjected to a dynamic heating ramp from 30 to 900 °C at 10 °C / min under air flow (50 mL / min).

[0146] The results are provided in Table 2 below and are compared with a commercially obtained silica sold under the trade name CAB-O-SIL™ LM-150D by Cabot Corporation of Alpharetta Georgia, USA (identified in Table 2 as comparative example C. 1).

[0147] Table 2.: BET (Brunauer, Emmett and Teller) surface area, particle size, and thermogravimetric mass loss above 420 °C in air for silica powder of Ex. 1 - 5 and C. 1

[0148] BET (m² / g) d₁₀ (μm) d₅₀ (μm) d90(μm) Mass loss >420 °C (wt. %)

[0149] Ex. 1 68 0.85 2.9 7.6 2.9

[0150] Ex. 2 93 0.94 3.4 8.9 1.3

[0151] Ex. 3 58 1.9 9.0 28 3.0

[0152] Ex. 4 44 1.6 7.1 21 3.5

[0153] Ex. 5 87 1.6 9.4 37 3.6

[0154] C. 1 176 8.5 28.0 103 0.23

[0155]

[0156] The TGA mass loss under air from 420 to 900 °C was considered as residual carbonaceous material from coking processes. This was further supported by the color change of the silica from grey to white upon exposure to the temperature treatment in the TGA up to 900 °C in air. The mass loss steps at lower temperatures can be attributed to the loss of physically adsorbed water and chemically bonded water.

[0157] Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy was performed on samples of each form of silica using a System 2000 FTIR spectrometer from PerkinElmer U. S. LLC with a Golden Gate™ Single Reflection Diamond Attenuated Total Reflectance Accessory (ATR-GG, single reflection diamond crystal with KRS-5 windows). Each sample was pressed on the crystal and measured over 15 scans from 450 to 4000 cm-1with a resolution of 4 cm-1. The background was recorded with no sample on the diamond crystal.

[0158] The Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy scans are provided in Fig. 1 herein and confirmed the expected broad SiO absorption band at 1100 cm"1in each instance, analogous to the commercial product C. 1. The results also showed an absence of CH or SiCH3signals which were considered to indicate full oxidation of the samples into silica.

[0159] X-ray photoelectron spectroscopy was used to analyze the surfaces of the samples provided using an Axis Supra4" XPS apparatus from Kratos Analytical of Manchester, U. K. using the conditions listed in Table 3 below

[0160] Table 3: XPS apparatus conditions

[0161] X-ray source Monochromatic Al Ka 375 Watts (15kV 25mA)

[0162] Analyzer Pass Energy 160 eV (survey spectra) & 20eV (High resolution spectra)

[0163] Take-off Angle 90°

[0164] Lens Mode Hybrid

[0165] Aperture Slot

[0166] Flood Gun Conditions Filament current 0.44 A, Bias 1.0V and Charge Balance 4.8V Charge Correction Method O Is to 532.6 eV

[0167]

[0168] Samples were placed into aluminum dishes and mounted onto a sample bar using conducting double sided adhesive tape. Two results for Ex. 5 and C. 1 are provided in Fig. 2A and Fig. 2B.

[0169] It will be seen that main elements detected on the surface were silicon and oxygen, with trace amounts of carbon. The relative amount of the said elements detected on the surfaces of the samples are shown in Table 4 below. Because the samples were mounted in clean aluminum dishes, the carbon detected is associated with the surface composition of the silica.

[0170] Table 4.: Elements and their relative atomic percentages detected on the surfaces of Ex. 5 and C. 1

[0171] Example C O Si commercial silica mean 1.2 68.7 30.1

[0172] St. deviation 0.2 0.8 0.8

[0173] Ex. 5 mean 3.7 66.8 29.5

[0174] St. deviation 0.6 0.7 0.3

[0175]

[0176] The amounts of carbon detected are values expected for adventitious carbon contamination on the surface. Nevertheless, the example 5 sample had significantly more surface carbon than the commercial fumed silica sample, which is in line with the higher TGA mass loss in air. After charge referencing the O Is peaks to 532.6 eV, the Si 2p spectra from all of the samples have a binding energy consistent with Q units (SiO4 / 2). Scanning electron microscopy (SEM) was used to study the morphology of selected silica powders. SEM imaging was conducted by placing a small amount of a sample on carbon adhesive, the excess was removed with compressed air. No coating was applied. The SEM images were obtained using with NOVA nanoSEM 600 (FEI, Eindhoven, The Netherlands) operated at low vacuum mode at 40Pa with 2kV and spot 4.5. The results can be seen in Figs. 3A-3F and 4A-4F for Ex. 1 (Figs. 3C, 3D, 4C, and 4D) and Ex. 2 (Figs. 3E, 3F, 4E, and 4F) which are compared with C. 1 silica (Figs. 3A, 3B, 4A, and 4B).

[0177] It was found that the silica powders from example 1 and 2 both showed fractal-like aggregates and agglomerates of primary particles. Larger particles show rough surface and appear to have been formed by coalescence of small primary particles. Generally, the morphological features of the example 1 and 2 powders are larger, and less uniform compared to commercial fumed silica. The larger particles appeared to have formed by coalescence of multiple smaller particles and showed rough “raspberry-like” surfaces formed from smaller particles that nucleated on the surface. Such unique particle morphology can facilitate additional pathways for interfacial interactions that can be beneficial in applications requiring specific particle-mediated functions.

Claims

Claims1. A gas phase production process of silica from tire combustion and / or thermal treatment of a silicone feedstock comprising(i) Providing a furnace having at least one gas stream inlet and a gas stream outlet and a means of introducing a silicone feedstock and a means of removing a solid residue resulting from thermolyzed of said silicone feedstock having a siloxane content comprising one or more silicone elastomers, one or more organopolysiloxane polymers or a mixture thereof; (ii) Heating the furnace to a predetermined temperature of from 400°C to 900°C(iii) Providing a stream of gas through the gas stream inlet into the furnace and out of the furnace through the gas stream outlet;(iv) Introducing the silicone feedstock into tire furnace and heating said silicone feedstock towards the predetermined temperature of step (ii) and depolymerizing and evaporating the siloxane content of the silicone feedstock into gaseous siloxane oligomers, whilst said silicone feedstock is being heated prior to or during thermolysis thereof;(v) Mixing the gaseous siloxane oligomers, in the stream of gas;(vi) Oxidizing said gaseous siloxane oligomers, in tire stream of gas to produce silica particles in the stream of gas;(vii) Collecting tire silica particles produced in step (vi) from the stream of gas and(viii) thermolyzing any remaining solid and / or liquid silicone feedstock in the furnace simultaneously with or subsequent to steps (iv) to (vii) causing a solid residue to be formed therefrom and subsequently removing said solid residue from the furnace.

2. The gas phase production process of silica in accordance with claim 1, wherein the silicone feedstock is post-industrial or post-consumer waste containing silicone.3 The gas phase production process of silica in accordance with claim 2, wherein the post-industrial or post-consumer waste containing silicones comprise cured and uncured silicone elastomers, selected from condensation cure silicone elastomers, hydrosilylation curable silicone rubber compositions, peroxide cured silicone rubber compositions or UV cured silicone rubber compositions, emulsions containing silicones, and silicone fluids.

4. The gas phase production process of silica in accordance with claim 2 or 3, wherein the silicone elastomeric materials utilised in the waste silicone feedstock is not sorted or separated prior to introduction into the furnace in respect of their prior use or by their method of being cured.

5. The gas phase production process of silica in accordance with claim 1, 2, 3 or 4, wherein the silicone feedstock is pre -prepared before it is introduced into the furnace by one or more of the following:(a) partially or wholly separating silicone materials to be used as silicone feedstock, from nonsilicone materials;(b) reducing silicone materials to be used as silicone feedstock in size by one or more of the following shredding, cutting, grinding and granulation; prior to step (iv) of the above process; (c) when tire silicone feedstock is a mixture of solids and liquids, adding said liquid and / or pastes onto tire surface solid feed materials or mixing said liquid and / or pastes with solid feed materials in a suitable mixer, prior to step (iv); and(d) when tire silicone feedstock is a liquid or a mixture of solids and liquids, adding said liquids and or pastes through a nozzle, atomiser, vaporiser or another preparation process to disperse the liquids and / or pastes into the gas phase, prior to step (iv).

6. The gas phase production process of silica in accordance with claim 1, 2 or 3, wherein the silicone feedstock is introduced into tire furnace during step (iv) on to a stationary or moveable support.

7. The gas phase production process of silica in accordance with claim 5, wherein the process is a continuous process.

8. The gas phase production process of silica in accordance with any preceding claim, wherein a control means is present in the furnace to ensure that any variation in temperature is maintained within a set range above or below the predetermined temperature of the furnace in step (ii) and to cause the furnace to be heated or cooled back to being within said temperature range when required.

9. The gas phase production process of silica in accordance with any preceding claim, wherein heat generated by exothermic oxidation of the gaseous siloxane oligomers in step (vi) is used as a means of heating the furnace.

10. The gas phase production process of silica in accordance with any preceding claim, wherein the gas in the gas stream, the silicone feedstock, or both are at the same or a different temperature from room temperature to 250°C prior to introduction into the furnace during steps (iii) and (iv) of the process respectively.

11. The gas phase production process of silica in accordance with any preceding claim, wherein the gas stream containing the mixed gaseous siloxane oligomers and other gaseous species may exit the furnace after or during step (v) of the process and be oxidized to form particulate silica in a thermal oxidizer during step (vi) of the process simultaneous with the oxidation of the VOCs and HAPs if any also mixed into the gas stream during step (v).

12. The gas phase production process of silica in accordance with claim 11, wherein particulate silica resulting from the oxidation of the gaseous siloxane oligomers transferred into a thermal oxidizer can be collected during step (vii) of the process after leaving the thermal oxidizer.

13. The gas phase production process of silica in accordance with any preceding claim, wherein tire silica particles produced in the gas stream may be tuned as to their hydrophilic or hydrophobic nature by: (i') Limiting the time period the gaseous siloxane oligomers having exposure to oxygen in the furnace: (if) Having a low furnace temperature; (iif) controlling the oxygen content of the gases to which the gaseous siloxane oligomers are exposed; or (iv’) any combination thereof.

14. The gas phase production process of silica in accordance with any one of preceding claims, wherein tire silicone feedstock is introduced into the furnace as a liquid, a gas, a mixture thereof in step (iv).

15. Silica obtained or obtainable from the gas phase production process in accordance with any one of claims 1 to 14.

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