Methods for polymer materials

Abstract

A polysilazane composition characterised by a resin system comprising a blend of a polysilazane polymer with a silazane oligomer, an organic monomer, such as 1,6-Hexanediol diacrylate, and a photoinitiator. Also disclosed is use of the polysilazane composition in 3D manufacturing of articles, for aerospace, automobiles, energy, oil and gas industries.

Description

The present invention relates to methods and compositions for processing polymers for use in ceramics technology, and products therefrom. In particular, the invention relates to Si-C-N ceramics, and improved products prepared therefrom. Existing methods for preparing Si-C-N ceramics include US 10,385,234, which discloses a method for the cross-linking of a polysilazane material with a fluoride containing catalyst in tetrahydofuran (THF) and O. Flores et al, J. Mat. Chern. A., 2013, 1,15406. Improved processes are required to allow polymer derived ceramics (PDC) materials to be easily fabricated and processed into new, large or complex shapes with improved properties - such as with improved strength and high temperature resistance. In particular improved manufacture methods are needed for the preparation of complex component geometries manufactured from high performance materials and for adaption for further methods, such as additive layer manufacture. Problems with existing processes and materials include that they do not allow the manufacture of certain composite materials, in particular machining materials from a block is expensive. Traditional methods for delivering complex geometries using complex manufacturing and post processing steps is also costly. There is also a need to enable the manufacture of finer features and to provide improved techniques to improve the strength of green bodies. According to the present invention there is provided a method according to the appended claims. In the process of the present invention the combination of silozane (preferably Durazane), crosslinked silozane (preferably cross-linked Durazane), organic monomer and a photo initiator allows the 3D printing of polysilozane structures with UV exposure times similar to conventional UV curable resins. In one embodiment the polymer composition provides scaffolding that allows parts to be produced with complex geometries relying on the mechanical properties of the polymer scaffolding to retain fine features. Benefits of the present invention include faster processes and finer product features. In a preferred embodiment the silozane can subsequently be driven to full crosslink density through additional heat treatment. In one embodiment, upon ceramification the polymer scaffolding is degraded leaving a SiCN ceramic with good mechanical properties. In one embodiment the combination and compositions optionally comprises fillers such as silicon carbide, boron carbide, milled carbon fibres, alumina-silicate fibres, glass fibres, silicon carbide fibres, chopped silicon carbide fibres, chopped glass fibres, chopped alumina silicate fibres, metal fillers (stainless steels, cobalt, nickel), Sialon particles, magnetic oxide fillers and stabilising fillers. In one embodiment the high temperature resistance is greater than 800, or greater than 1500, or up to about 2000 degrees C. In a further aspect the invention provides composites prepared by the invention method having a very uniform microstructure or reduced porosity approximately 75% of theoretical maximum density as measured using the Archimedes Principle. In a further aspect the invention provides the use of polymers or composites prepared by a method of any preceding claim in aerospace, automotive, oil and gas industries. Benefits of the invention methods include the ability to use any cross linked polysilazane to produce improved parts with relatively little deviation to existing manufacturing techniques, hence dramatically reducing the costs vs. standard ceramic matrix composites and in particular improved articles for use in automotive, energy and aerospace industries. Where catalysts are used, the catalyst may be a source of fluoride ions. The catalyst may be selected from tetraethylammonium fluoride or tetrabutylammonium fluoride. Preferably, the catalyst is tetrabutylammonium fluoride (TBAF). The solvent may be tetrahydrofuran. The solvent may additionally be toluene. The solvent may also be 2-methyltetrahydrofuran. The solvent may also be dibutylether. The solvent may be a mix of tetrahydrofuran or toluene in any particular ratio, with or without additional solvents selected from 2-methyltetrahydrofuran and dibutylether. The solvent may additionally be any other solvent known in the art, which dissolves both the oligosilazane, catalyst, and the quenching agent. The mass ratio of oligomer:solvent may be between 8:1 and 1:8. The mass ratio of oligomer:solvent may more specifically be within the range 8:1 and 1:1, yet more specifically be within the range 8:1 and 3:1, and further within the range 8:1 and 5:1. The mass ratio of oligomer:solvent may alternatively be within the range 1:1 and 1:8, more specifically within the range 1:3 and 1:8, yet more specifically within the range 1:5 and 1:8. The mass ratio of oligomer:solvent may be preferably within the range 1:3 and 3:1. More preferably the mass ratio of oligomer:solvent may be 1:2 and 2:1. Yet more preferably the mass ratio may be 1:1.5 to 1:1. Most preferably the mass ratio is 1:1. The molar ratio of catalyst to oligomer repeat units may be between 1 x 10-4 and 10 x 10-4. More preferably, the molar ratio may be between 2 x 10-4 and 10 x 10-4. Yet more preferably, the molar ratio may be between 2 x 104 and 7 x 10 4. Most preferably the molar ratio may be between 2x10' 4 and 6 x 10-4 The rate of addition of catalyst may be between 10 and 100 (% total catalyst) hour1. Preferably the rate of addition may be between 15 and 35 (% total catalyst) hour1. In one aspect the invention uses a reaction that may be performed in a vessel with a height / width dimension ratio of > 1. The vessel may have an inlet aperture width / base dimension of 2 0.5. The resulting cross-linked polymer may yield a ceramic material without a significant and homogeneous oxygen content on pyrolysis at above 1200°C in an inert atmosphere. The catalyst may be added dropwise, and the inhibitor selectively added over the course of the reaction. In a further aspect the invention provides polymers and composites comprising polymer prepared according to a method of the invention, and use thereof in aerospace, automotive, energy including oil and gas industries. Preferably the polymers and composites have a low oxygen content such as lower than 10 wt% on average across a specimen. In particular embodiments of the invention, the solvent is THF, the catalyst is TBAF, the mass ratio of the oligomer:solvent is between 1:1.5 and 1:1, and the molar ratio of the catalyst:oligomer repeat unit is between 2 x 10 4 and 4 x 10-4 and the catalyst is added on average at around 17 (% total catalyst) hour1. In a particular embodiment, the solvent is THF, the catalyst is TBAF, the mass ratio of oligomer:solvent is 1:1, and the molar ratio of the catalyst:oligomer repeat unit is 3.6 x 10-4 and the catalyst is added on average at around 17 (% total catalyst) hour1. In other embodiments of the invention, the solvent is toluene, the catalyst is TBAF, the mass ratio of the oligomer:solvent is between 1:1.5 and 1:1, and the molar ratio of the catalystoligomer repeat unit is between 2 x 10-4 and 6 x 10-4 and the catalyst is added on average at around 33 (% total catalyst) hour1. In a particular embodiment, the solvent is toluene, the catalyst is TBAF, the mass ratio of the oligomer:solvent is 1:1, and the molar ratio of the catalyst:oligomer repeat unit is 2.6 x 10-4 and the catalyst is added on average at around 33 (% total catalyst) hour1. In a further embodiment, the solvent is toluene, the mass ratio of the oligomer:solvent is 1:1.1, and the molar ratio of the catalystoligomer repeat Unit is 5.2 x 10-4 and the catalyst is added on average at around 33 (% total catalyst) hour1. In one embodiment the present invention provides a method to chemically cross-link a particular polysilazane (Durazane(TM) 1800, Merck) using a catalyst (tetrabutylammonium fluoride, TBAF) comprising reactive F’ ions, which causes a number of reactions resulting in a higher molecular weight polymer material. In one embodiment, the method of the present invention is not carried out under dry inert gas and the arrangement is such that sufficient hydrogen gas is produced during the reaction so as to drive the atmosphere above the reaction liquid away, thereby providing in-situ ‘inert’ atmosphere (insofar as moisture is concerned). Also the actual extent of hydrolysis during the method of the present invention does not seem to produce a particularly high concentration of siloxane (and therefore oxygen) in the final ceramic material produced. A high concentration of oxygen may be defined as an appreciable quantity of the final ceramic comprising an oxide - such as 15 mol%. By not including a protection step, a significant bonus towards scalability is provided. In one embodiment cross-linked material produced from Durazane(TM) 1800 and TBAF in THF, followed by reaction quenching with Ca(BH4)2.2THF to produce CaF2 and tetrabutylammonium borohydride by-products, followed by filtration and drying under vacuum to produce a ceramic precursor. The preparation of cross-linked material prepared by the cross-linking of a polysilazane under ambient atmospheric conditions is particularly advantageous. In one embodiment the technique uses the inherent atmosphere generated by the synthesis for shielding and control of the synthesis time to ensure that sufficient H2 is being evolved to expel H2O from the reaction vessel atmosphere. A particular benefit arises from the lack of infrastructural requirement for complex gas supply to the reaction vessel, cleaning or drying procedures for gases and solvents. Modification or additions of any form can be added to the reaction without need for complicated air-sensitive techniques to be employed that are inherently small-scale solutions in synthesis. Hence, the synthesis is yet more flexible and scalable. Potential applications for an improved PDC precursor according to the invention include: • High temperature ceramics for aerospace applications: leading edges, exhaust ducts • High temperature resistance materials - energy materials, automotive, aerospace • Hard materials - Abrasives, bearings, cutting / machining tools • Chemical engineering - catalyst support, food- and biotechnology • Functional materials - electrical engineering, micro / nano-electronics As such, methods and products of the present invention have applications in a variety of industries including automotive, aerospace, oil and gas, and the like. The method of the present invention can also be utilised in combination with other methods as appropriate. The features of any aspect or embodiment of the invention may be used, alone or in any combination, with other aspects and embodiments as appropriate. Brief description of the Figures The invention will now be described in more detail and by way of example only, with reference to the following schematic Figures, in which: Figure 1 shows EDS (elemental analysis) data for a ceramic composite made from the precursor of the method of the invention. Figure 2 shows a round bottomed flask experimental set up. Figure 3 shows a wide-brimmed container experimental set up Figure 4 shows a schematic showing physical arrangement for the fabrication of composites via traditional composite vacuum bagging with the Polysilazane prepreg. Figure 5 shows the oxidation resistance of a number of polymer derived ceramic matrix composite (PDCMC) samples. The present invention will now be described more fully with reference to the accompanying Examples and Figures in which embodiments of the invention are shown. This invention should not be construed as limited to the embodiments set forth herein. Example 1 400 g of an oligosilazane, Durazane(TM) 1800 (Merck Life Sciences, India), was added to tetrahydofuran THF (Sigma-Aldrich, Gillingham, UK) in a 1:1 mass ratio under magnetic stirring in a 3 L round bottomed flask, at room temperature. The vessel was 24.3 cm tall, 18.8 cm wide, with high curvature and a 2.5 cm neck opening diameter. No reflux condenser or gas line was added. After homogenisation, tetrabutylammonium fluoride (TBAF) 1 M in THF (Sigma Aldrich, Gillingham, UK) was added dropwise. The rate of addition was controlled to prevent excessive evolution of H2 gas, defined as when the entire surface was a covered by a foam, without significant disruption of the surface due to large bubbles. The total quantity of TBAF solution added was 2.3 cm3 over the course of approximately 6 hours, including time during which there was no addition due to the appearance of a full surface foam. Addition of catalyst on top of such a foam would result in overpolymerisation of the thin bubble films, resulting in insoluble scum formation. This corresponded to a final TBAF concentration of 0.00282 M, with an addition rate of 0.00038 mol It1. The final molar ratio of TBAF:oligosilazane repeat units was 0.000356. The mixture was left to stir until gas evolution had ended. 0.1 cm3 TBAF solution was added at the end to ensure no further evolution of gas was forthcoming. Ca(BH4)2.2THF (Sigma Aldrich, Gillingham, UK) was added in a 1.5:1 molar ratio with respect to F-as a suspension in THF (approximately 5 cm3). The suspension was stirred for 10 minutes, and was then filtered to remove aggregates and the filtrate collected and used as synthesised in solution in compositing applications. Example 2 300 g of an oligosilazane, Durazane(TM) 1800 was treated with TBAF 1M in THF (1.7 cm3) and Ca(BH4)2.2THF as described in Example 1. This corresponded to a final TBAF concentration of 0.00278 M, with an addition rate of 0.00028 mol h1. The final molar ratio of TBAF:oligosilazane repeat units was 0.000351. Subsequently, the solution was concentrated in a rotary evaporator at 85°C at pressures between 800 and 300 mBar until a pale yellow oil was obtained. This solidified on cooling to room temperature as a pale glassy solid. Example 3 400 g of an oligosilazane, Durazane(TM) 1800 was treated with TBAF and Ca(BH4)2.2THF as described in Example 1. This corresponded to a final TBAF concentration of 0.00282 M, with an addition rate of 0.00038 mol h1. The final molar ratio of TBAF:oligosilazane repeat units was 0.000351. The suspension was then filtered to remove aggregates and the filtrate collected. Subsequently, the solution was concentrated in a rotary evaporator at 85°C at pressures between 850 and 700 mBar for a total of 15 minutes until a pale yellow liquid was obtained. This contained 22 wt% retained THF solvent. This liquid was used as a viscous oil in compositing applications. Example 4 30 g of an oligosilazane, Durazane(TM) 1800 (Merck Life Sciences, India), was added to toluene (Sigma-Aldrich, Gillingham, UK) in a 1:1 mass ratio under magnetic stirring in a 0.4 L round bottomed flask with a height of 13.5 cm, a width of 10 cm and a neck opening diameter of 2.5 cm, at room temperature. No reflux condenser or gas line was added. After homogenisation, tetrabutylammonium fluoride (TBAF) 1 M in THF (Sigma Aldrich, Gillingham, UK) was added dropwise after 2-fold dilution in toluene. The rate of addition was controlled to prevent excessive evolution of H2 gas. The total quantity of TBAF solution added was 0.25 cm3. The mixture was left to stir until gas evolution had ended. 0.05 cm3 TBAF solution was added at the end to ensure no further evolution of gas was forthcoming. This corresponded to a final TBAF concentration of 0.00202 M, with an addition rate of 2.08 x 10 5 mol h1. The final molar ratio of TBAF:oligosilazane repeat units was 0.000258. Ca(BH4)2.2THF (Sigma Aldrich, Gillingham, UK) was added in a 1.5:1 molar ratio with respect to F-as a suspension in THF (approximately 5 cm3). The suspension was stirred for 10 minutes, and was then filtered to remove aggregates and the filtrate collected and reduced in a rotary evaporator at 85°C and 80 mBar. A pale yellow gel was obtained at room temperature. Example 5 30 g of an oligosilazane, Durazane(TM) 1800 (Merck Life Sciences, India), was added to toluene (Sigma-Aldrich, Gillingham, UK) in a 1:1 mass ratio under magnetic stirring in a 1 L round bottomed flask, at room temperature. No reflux condenser or gas line was added. After homogenisation, tetrabutylammonium fluoride (TBAF) 1 M in THF (Sigma Aldrich, Gillingham, UK) was added dropwise. The rate of addition was controlled to prevent excessive evolution of H2 gas. The total quantity of TBAF solution added was 0.15 cm3. The mixture was left to stir until gas evolution had ended. 0.05 cm3 TBAF solution was added at the end to ensure no further evolution of gas was forthcoming. This corresponded to a final TBAF concentration of 0.00403 M, with an addition rate of 8.33 x 10 5 mol h1. The final molar ratio of TBAF:oligosilazane repeat units was 0.000516. Ca(BH4)2.2THF (Sigma Aldrich, Gillingham, UK) was added in a 1.5:1 molar ratio with respect to F_ as a suspension in THF (approximately 5 cm3). The suspension was stirred for 10 minutes, and was then filtered to remove aggregates and the filtrate collected and reduced in a rotary evaporator at 85°C and 80 mBarfor 3 minutes. The product was collected as a viscous pale liquid at room temperature. Example 6 A layer of the oil of Example 3 was extruded and spread evenly on a layer of a plastic release film. A layer of Torayca COR8112 fibre was placed on this layer of oil. A coating was applied using an extruder and rolling device. Further layers of fibre and oil were sequentially applied until a total of 25 fibre plies had been laid up. A further layer of release film was placed on the top of the stack, and the stack placed between two steel plates which were spaced at 4.6 mm with spacers. The lay-up width as produced was 5 mm. This stack was dried in an oven in ambient conditions at 160°C, and subsequently pyrolysed in Ar(g) at 1280°C for 1 h. The sample was then tested in flexure according to ASTM C1341, and energy dispersive spectroscopy (Oxford Instruments, Oxford, UK) was performed on the matrix of the subsequently fractured ceramic composite in a scanning electron microscope (SEM). Figure 1: The atomic percentages from a number of test sites of each of the primary constituents -Si, C, N and O. O was introduced as a contaminant variously during synthesis or pyrolysis. Overlaying the experimental data points are box-and-whisker plots displaying the median and interquartile ranges of the atomic percentage distributions for data collected across the test sites. From Figure 1, it is clear that there are, location dependant, large quantities of all the elements are observed. However, statistically speaking, there is a negligible quantity of oxygen across the composite at large. Relative to the abundance of silicon, the matrix has an average stoichiometry of Sii±o.48Ci.58±o.6iNo.68±o.5zOo.ii±o.ii, indicating that the ceramic is predominantly a Si-C glass with some N, with statistically negligible oxygen content. While a few locations are high in oxygen, this is likely due to localised regions of oxygen enrichment, formed during strength testing or pyrolysis due to insufficiently dry inert gas supply. A wholesale hydrolysis of the precursor would manifest itself as a significant increase in the average oxygen content compared to that observed here. Thus, it is seen that the method of the invention can produce Si-C-N ceramics with minimal oxygen content, without many of the procedural inefficiencies and large-scale inhibiting complications associated with air-sensitive chemical handling techniques. Example 7 6 further composite samples were produced following the method described in Example 6. The masses of these composites and the components thereof are shown in Table 1. Table 1: the mass of the composites and the components thereof before and after the pyrolysis, with the calculated ceramic yield of the precursor. Sample Green composite component mass / g Pyrolysed composite mass / g Ceramic yield / wt% Fibre Precursor 1 13.3 23.7 21.3 0.77 2 13.7 24.3 22.1 0.79 3 13.9 24.7 21.7 0.72 4 13.7 22.1 19.8 0.72 5 14.0 22.6 20.3 0.73 6 14.7 23.8 20.8 0.67 The average ceramic yield for the precursor was calculated to be 74±4%. The variation is likely due to lack of consistency in the gas flow and temperature in all parts of the heat treatment furnace. EXAMPLE 8 - Resin systems and Initiators Resin systems can be prepared from a blend of polysilazane polymer with silazane oligomer wherein said oligomer acts as a reactive diluent to tune the resin system to the desired viscosity to use as a prepreg. Said systems allow for the processing of polysilazane into ceramic matrix composite parts using suitable manufacturing techniques to process standard polymer composites and to allow the manufacture of composite parts with high temperature resistance. The reactive diluent comprises any oligomer of lower viscosity than the polysilazane and capable of reacting with said polysilazane to form a cross linked polymer network. Resin systems of crosslinked durazane: durazane oligomer: organic monomer (methyl methacrylate): UV initiator have been prepared. The following Figures illustrate the benefits of preferred embodiments. Figure 4 schematic shows how the polysilazane prepreg could be used with a vacuum bagging process to produce a polymer derived ceramic matrix composite (PDCMC) part. As the properties of the polysilazane based resin are similar to the properties of existing polymer resins used in prepregs for the manufacture of PMC components, the process needs little to no alteration vs. existing techniques. Figure 5 shows through heat treatment of a number of test coupons at 1000 °C oxidation resilience has been demonstrated, as is shown Figure 4. After heat treatment the coupons were testing using a 4 point bend to measure the residual strength of the coupons vs samples that were not exposed to elevated temperatures after pyrolysis. Polymer chemistries based on acrylate based resin systems (1,6-Hexanediol diacrylate), have been trialled with a number of common photo-initiators (4-Dimethylaminobenzophenone & Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide)) and combined with both the polysilozane oligomer and the crosslinked BB resin. 5 A wide range of common photo-resin systems can be used. The use of the photo resins provides polysilozane greenbodies with increased mechanical strength and increased interlayer adhesion. This increased mechanical strengths allows parts with fine features to be successfully printed without fractures caused by de-bonding from the FEP film of the 3D printer both during and after 10 the print has completed.

Claims

1. A process for the preparation of a polymer composition comprising the combination of silozane, cross-linked silozane, organic monomer and a photo initiator to allow the 3D printing preferably sterolithography of a polysilozane structure with UV exposure times similar to conventional UV curable resins, wherein the polymer composition allows article parts to be produced at speed with complex geometries relying on the mechanical properties of the polymer composition to retain fine features.

2. A process according to claim 1 wherein the photo initiator is selected from 4-Dimethylaminobenzophenone and Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

3. A process according to any preceding claim wherein the organic monomer is 1,6-Hexanediol diacrylate.

4. A process according to any preceding claim wherein the polysilazane composition includes a resin system comprising a blend of polysilazane polymer with silazane oligomer, said oligomer acting as a reactive diluent to tune the resin system to the desired viscosity to use as a prepreg, optionally comprising fillers, said system allowing the processing of polysilazane into ceramic matrix composite parts using suitable manufacturing techniques to process standard polymer composites and to allow the manufacture of composite parts with high temperature resistance wherein the reactive diluent comprises any oligomer of lower viscosity than the polysilazane and capable of reacting with said polysilazane to form a cross linked polymer network.

5. A process according to any preceding claim wherein the silozane is subsequently driven to full crosslink density through additional heat treatment.

6. A process according to any preceding claim wherein upon ceramification the polymer composition is degraded leaving a SiCN ceramic with good mechanical properties.

7. A method according to claim 1 wherein the cross-linked polymer has been prepared by cross linking under ambient conditions.

8. A method according to any preceding claim wherein fillers are included such as silicon carbide, boron carbide, milled carbon fibres, alumina-silicate fibres, glass fibres, silicon carbide fibres, chopped silicon carbide fibres, chopped glass fibres, chopped alumina silicate fibres, metal fillers (stainless steels, cobalt, nickel), Sialon particles, magnetic oxide fillers and stabilising fillers.

9. A method according to any preceding claim wherein the high temperature resistance is greater than 800, or greater than 1500, or up to about 2000 degrees C.

5.

10. Composites prepared according to the method of any preceding method having a uniformmicrostructure or reduced porosity.

11. Articles or complex structures prepared using the method of any preceding claim.

012. Use of polymers or composites prepared by a method of any preceding claim in aerospace, automotive, energy, oil and gas industries.15

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