WO2019160505A1

WO2019160505A1 - Abrasion-resistant silica coating on plastics

Info

Publication number: WO2019160505A1
Application number: PCT/SG2019/050084
Authority: WO
Inventors: Yee Fun Lim; Thi Bao Nguyen NGUYEN
Original assignee: Agency For Science, Technology And Research
Priority date: 2018-02-13
Filing date: 2019-02-13
Publication date: 2019-08-22

Abstract

According to the present disclosure, a method of forming a silica coated polymer substrate, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate, is provided herein. The method comprises providing a solution comprising a silica precursor and an organic solvent, mixing the solution with an aqueous acid catalyst to form a silica sol-gel solution, depositing the silica sol-gel solution on the polymer substrate, and curing the silica sol- gel solution on the polymer substrate to form the silica coated polymer substrate. A silica coated polymer substrate obtained according to the method described herein is also provided, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate.

Description

ABRASION-RESISTANT SILICA COATING ON PLASTICS

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10201801232T, filed 13 February 2018, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The present disclosure relates to an abrasion resistant silica coated polymer substrate and method of forming such an abrasion reistant silica coated polymer substrate.

Background

[0003] Light-weight and low-cost plastics, including thermoplastics such as acrylic and polycarbonate, have found diverse applications as optical, automotive and building materials (see FIG. 1A). In many of these applications, optical clarity is needed, in particular for windows and lenses. Due to their low hardness, such plastics are easily scratched and lose their optical clarity after a period of use. For such applications, a scratch resistant coating on the plastic is needed.

[0004] In general, plastics suffer from low glass transition temperatures and low melting points. As such, it tends to be challenging to use hard materials, like ceramics, as a coating material on plastics due to the high processing temperatures required for ceramics, which render ceramics incompatible with plastics for processing. Instead, hard coatings on plastics have traditionally been dominated by organic-inorganic hybrid coatings such as ORMOCER (organically modified ceramics), which may be composed of inorganic nanoparticles embedded within an organic matrix.

[0005] Such coatings, however, tend to be limited in terms of their hardness due to the high organic content, and may suffer from durability issue as the organic compounds are susceptible to degradation under ambient conditions over time.

[0006] There is thus a need to provide for a solution that addresses one or more of the limitations mentioned above. The solution should at least provide for a method of coating a scratch resistant material on plastics. Summary

[0007] In a first aspect, there is provided for a method of forming a silica coated polymer substrate, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate, the method comprising:

providing a solution comprising a silica precursor and an organic solvent;

mixing the solution with an aqueous acid catalyst to form a silica sol-gel solution;

depositing the silica sol-gel solution on the polymer substrate; and

curing the silica sol-gel solution on the polymer substrate to form the silica coated polymer substrate.

[0008] In another aspect, there is provided for a silica coated polymer substrate obtained according to the first aspect as described above, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate.

Brief Description of the Drawings

[0009] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0010] FIG. 1A shows various applications of conventional thermoplastics.

[0011] FIG. 1 B shows a silica coated acrylic sample prepared by doctor blading method.

[0012] FIG. 1C illustrates conversion of a tetraethyl orthosilicate (TEOS) sol-gel precursor to form a silica coating.

[0013] FIG. 2A shows 5H pencil scratch marks on an acrylic surface without silica coating as observed under an optical microscope. Scale bar denotes 200 pm.

[0014] FIG. 2B shows lesser 5H pencil scratch marks on an acrylic surface with silica coating observed under an optical microscope, as compared to FIG. 2A. Scale bar denotes 200 pm.

[0015] FIG. 2C shows the equipment of Lantek HT-6510P Pencil Hardness Tester with 7.5 N (750 g) weight and loaded with a 5H pencil at 45° angle. [0016] FIG. 3A shows a cross hatch cut pattern made with a 6 mm x 2 mm cutting blade on silica coating.

[0017] FIG. 3B demonstrates application of ISO 2409 adhesive tape onto a cut area.

[0018] FIG. 3C shows the cut pattern after tape removal, showing no visible removal of material thus qualifying for highest adhesion grade (ISO 2409 grade 0).

[0019] FIG. 4 shows the X-ray photoelectron spectroscopy (XPS) for Cls, Ols and Si2p scans of a silica film made according to embodiments disclosed herein.

[0020] FIG. 5 shows the binding energy of C, Si and O in a silica film, and its corresponding elemental composition. The silica film is made according to embodiments disclosed herein.

[0021] FIG. 6 shows the Fourier- transform infrared (FTIR) spectrum of the silica film surface on Si wafer.

[0022] FIG. 7 shows the morphology of the silica film deposited on Si wafer as observed using SEM at 50,000x magnification. Scale bar denotes 100 nm.

[0023] FIG. 8 shows an example of an applicator as used herein for coating the scratch resistant silica film onto plastics.

Detailed Description

[0024] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised.

[0025] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0026] The present disclosure relates to method of forming an abrasion (e.g. scratch) resistant coating based on a purely inorganic silica film, formed in situ from a solution sol-gel process without the use of nanoparticles. The present method and coating are superior over conventional abrasion resistant coatings that are composed of an organic- inorganic composite, which has inorganic nanoparticles incorporated in an organic material, or vacuum evaporated or sputtered oxide films.

[0027] The present method is advantageous as it provides for a purely inorganic film to be formed from sol-gel at low temperatures and the present method is compatible with various organic plastic substrates.

[0028] In various embodiments of the method disclosed herein, an abrasion resistant coating based on a purely inorganic silica film can be obtained. The silica film is formed in-situ from a solution sol-gel process without involving use of any nanoparticles. The silica coating may be deposited onto a plastic substrate at atmospheric pressure and low temperatures of up to 1 l5°C. No intermediate primer layer is required in the present method to coat a layer of non-porous silica on the plastic substrate.

[0029] Traditionally, inorganic ceramics formed by sol-gel are processed at high temperatures to drive the sol-gel reaction to completion and remove all organic residues, and this renders it difficult to form a purely inorganic silica film directly on a plastic substrate at low temperatures via a solution sol-gel process. However, the present method overcomes such a limitation.

[0030] Moreover, purely inorganic films tend to be incompatible with organic plastic substrates, as they tend to suffer from poor adhesion and cracking issues, such that an inorganic silica film is conventionally unable to adhere strongly to a plastic substrate without cracking. The present method and silica coated polymer substrate, however, do not suffer from such issues.

[0031] To obtain an abrasion resistant coating, conventional methods involve forming an organic-inorganic hybrid film, wherein the abrasion resistant coating comprises inorganic particles, e.g. silica, incorporated into the matrix of the organic plastic substrate. Conventionally, in certain instances, an intermediate primer layer may be used to improve adhesion and bonding properties between the inorganic abrasion resistant layer and the organic substrate.

[0032] Traditionally, the silica coating tends to be deposited by vacuum evaporation or sputtering process, both of which tend to require sophisticated equipment or non- atmospheric conditions that render the processing expensive. Moreover, silica coating deposited on non-plastic substrates may have to be thermally annealed at high temperatures (above 200°C). [0033] The present method circumvents at least one or more of the limitations highlighted above, by forming a silica coating through a facile sol-gel process to form the abrasion resistant coating. The present method also eliminates the use of further costly reagents or ultrasonic for the sol-gel process.

[0034] Details of various embodiments of the present method and silica coating are now described below.

[0035] In the present disclosure, there is provided for a method of forming a silica coated polymer substrate, wherein the silica coated polymer substrate consists of a non- porous silica layer disposed directly on a polymer substrate, the method comprising: providing a solution comprising a silica precursor and an organic solvent, mixing the solution with an aqueous acid catalyst to form a silica sol-gel solution, depositing the silica sol-gel solution on the polymer substrate, and curing the silica sol-gel solution on the polymer substrate to form the silica coated polymer substrate.

[0036] To form a non-porous silica layer directly on the plastic substrate, a solution comprising the silica precursor for forming the non-porous silica layer may be first prepared. In various embodiments, the silica precursor may comprise or may consist of a tetra-alkyl orthosilicate. The term“alkyl” used herein as a group or part of a group refers to a straight or branched saturated aliphatic group having from 1 to 10 carbon atoms, preferably having 1 to 6 carbon atoms unless otherwise noted. For example, an “alkyl” group can have 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms. The term“alkyl” includes, but is not limited to, methyl, ethyl, l-propyl, isopropyl, l-butyl, 2-butyl, isobutyl, tert- butyl, amyl, l,2-dimethylpropyl, l,l-dimethylpropyl, pentyl, isopentyl, hexyl, 4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 2,2- dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2- trimethylpropyl, 1,1,2-trimethylpropyl, 2-ethylpentyl, 3-ethylpentyl, heptyl, 1- methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2- dimethylpentyl, 1,3 -dime thylpentyl, 1,4-dimethylpentyl, 1,2,3-trimethylbutyl, 1,1,2- trimethylbutyl, l,l,3-trimethylbutyl, 5-methylheptyl, 1-methylheptyl, octyl, nonyl, decyl, and the like. In various embodiments, the silica precursor may comprise or consist of tetramethyl orthosilicate, tetraethyl orthosilicate, or tetrapropyl orthosilicate. In certain embodiments, the silica precursor may comprise or consist of tetraethyl orthosilicate. [0037] The silica precursor may be dissolved in an organic solvent such as an alcohol, and according to various embodiments, the organic solvent may comprise or consist of ethanol, isopropanol, butanol, or 2-methoxyethanol. The use of such alcohols, e.g. ethanol, has several advantages. The alcohols are good solvents for tetra-alkyl orthosilicate, e.g. tetraethyl orthosilicate, as tetra-alkyl orthosilicate does not dissolve or does not easily dissolve in other solvents. The alcohols have low boiling points and low surface tensions, and thus are ideal for use as solvents for coating formulations due to the need for good surface wetting and film forming properties. The low boiling point allows the solvent to evaporate quickly, and not aggregate into droplets on a surface. The alcohols are less toxic compared to other organic solvents with similar properties, such as acetone or xylene. The alcohols are miscible with water and allow aqueous reactions to take place within the alcohol medium.

[0038] An acid catalyst may then be added to the solution. The acid catalyst may be a suitable aqueous mineral acid or an organic acid. In various embodiments, the aqueous acid catalyst may comprise or consist of hydrochloric acid, sulfuric acid, phosphoric acid, or acetic acid. Advantageously, the acid catalyzes the cross-linking of the silica precursor to form a silica sol-gel in the organic solvent. This may constitute a gelation step that enables low temperature processing of silica materials. This gelation step taking place before coating of a silica material onto plastics is also advantageous, as silica can be formed on a plastic substrate without the traditional use of high temperatures required for coating inorganic materials. Said differently, as a silica sol- gel solution is already formed prior to depositing it on a polymer substrate, the silica sol-gel solution that is deposited on the polymer substrate can be easily converted to a silica coating the polymer substrate, simply through low temperature curing. The term “sol” used herein refers to a colloidal solution converted from precursors (i.e. the silica precursor) and the colloidal solution can gradually evolve into an integrated network of cross-linked particles or a network polymer, which is referred to as the“gel” . In the sol state, colloidal particles tend to be dispersed in the liquid. In the gel state, both a liquid and a solid may be dispersed in each other, presenting the solid network containing liquid component(s). A“sol-gel” solution used herein, accordingly, refers to a solution containing both the sol (liquid phase) and the gel (solid phase). [0039] After adding the acid catalyst to the solution, the present method may comprise mixing the solution with the aqueous acid catalyst, and mixing the solution with the aqueous acid catalyst may comprise aging the solution for at least 24 hours. The term “aging” used herein means that the solution with the added acid catalyst is left to stand for a period of time. During aging, cross-linking of the silica precursor takes place to form a silica sol-gel, and this continues through the aging process.

[0040] After aging, the silica sol-gel solution may be deposited directly on the polymer substrate. This means that no other intervening steps are required. The silica sol-gel solution is contacted directly with the polymer substrate, and no other materials exists between the deposited silica sol-gel solution and the surface of the polymer substrate. In other words, the silica sol-gel solution is disposed adjacent to the polymer substrate or its surface thereof. No primer (i.e. adhesives), as used in conventional methods, are required.

[0041] In various embodiments, depositing the silica sol-gel solution on the polymer substrate may comprise spreading the silica sol-gel solution on the polymer substrate. The spreading may be carried out by any suitable deposition method that includes, but is not limited to, spray coating, spin coating, doctor blading, etc.

[0042] After depositing the silica sol-gel solution on the polymer substrate, the silica sol-gel solution may be cured directly thereon. The term“curing”, and its grammatical variants thereof, used herein refers to hardening of the silica sol-gel that involves cross- linking of the silica precursor.

[0043] In various embodiments, curing the silica sol-gel solution may comprise drying the silica sol-gel solution on the polymer substrate. The drying may be carried out by any suitable means that removes liquid from the silica sol-gel solution. In certain embodiments, the drying may comprise heating at 40°C to 80°C, 50°C to 80°C, 60°C to 80°C, 70°C to 80°C, etc. for at least 5 mins, such as 10 mins. In certain embodiments, the drying may comprise heating at 80°C for 10 mins. The heating temperature may depend on what is required to cure the silica sol-gel solution. If temperature is too high, the polymer substrate may be damaged while temperatures that are too low may adversely increase curing duration.

[0044] In various embodiments, curing the silica sol-gel solution may comprise heating the silica sol-gel solution on the polymer substrate to a temperature of H5°C or less after drying the silica sol-gel solution. The heating is carried out with the silica sol-gel solution directly on the polymer substrate. Advantageously, temperatures within this range can help to accelerate the cross-linking without polymer deformation. Hence, the highest possible temperature that is below the glass transition temperature of the polymer substrate may be used as long as the temperature does not cause deformation of the polymer substrate.

[0045] Alternatively, the curing temperature may be l00°C or less, 90°C or less, 80°C or less, 70°C or less, etc. The curing may be carried out for a duration of 60 mins, 50 mins, 40 mins, 30 mins, 20 mins, etc. The curing may be carried out in a heating oven.

[0046] In various embodiments, curing the silica sol-gel solution may comprise subjecting the silica sol-gel solution on the polymer substrate to a temperature ranging from 20°C to 40°C, or 20°C to 30°C, etc., for at least 3 days after heating the silica sol- gel on the polymer substrate. In other words, the silica sol-gel solution that has been deposited on the polymer substrate may be left to stand at room temperature and atmospheric pressure after drying. This step advantageously provides for sufficient time to allow the cross-linking reaction to complete, so as to have a complete silica coating formed directly on the polymer substrate.

[0047] In the present disclosure, there is also provided for a silica coated polymer substrate obtained, or obtainable, according to various embodiments described for the first aspect, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate. Various embodiments of the present method, and advantages associated with various embodiments of the present method, as described above are applicable to the present silica coated polymer substrate, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity.

[0048] The non-porous silica layer may be transparent. By the term“transparent”, this means any form of electromagnetic radiation, e.g. visible light, can pass through the silica layer, that is coated on the polymer substrate, even when the silica layer is non- porous.

[0049] In certain embodiments, the polymer substrate may consist or may be composed of acrylic or polycarbonate. Nevertheless, other plastic substrates may be used. [0050] Advantageously, in embodiments where the polymer substrate is composed of acrylic as an example, the silica coated polymer substrate may have a pencil hardness of 5H. The silica coated polymer substrate is rendered abrasion (e.g. scratch) resistant due to such pencil hardness of the silica coating formed thereon.

[0051] In embodiments where the polymer substrate is composed of acrylic as an example, even when the silica coating has been scratched, the silica coating remains adhered to the polymer substrate, and in various embodiments, the non-porous silica layer may be adhered to the polymer substrate after being subjected to abrasion, and wherein the polymer substrate is composed of acrylic.

[0052] In various embodiments, the non-porous silica layer may remain adhered to the polymer substrate even when an adhesive tape stuck on the non-porous silica layer is pulled away from the non-porous silica layer. This may be demonstrated through an adhesion test using an ISO 2409 standard. The polymer substrate used in such a test may be composed of acrylic. In such a standard, the silica coated polymer substrate may be subjected to certain damage, such as being scratched as shown in FIG. 3A, an adhesive tape is then stuck onto the non-porous silica layer and pulled away in an attempt to remove the non-porous silica layer from the polymer substrate to evaluate the adhesion. The present non-porous silica coating remains adhered to the polymer substrate even after such a harsh test.

[0053] The combination of the pencil hardness and good adhesion imparts the abrasion resistant property of the silica coating onto the polymer substrate.

[0054] In summary, the method disclosed herein produces an abrasion resistant film on plastic substrates, based on purely inorganic silica film coated using a solution sol-gel process at low temperatures no higher than 1 l5°C. A 600 nm thick silica film deposited on acrylic sheet improved the pencil hardness of the acrylic from 2B to 5H, and exhibited film adhesion of the highest grade (ISO 2409 grade 0).

[0055] The word“substantially” does not exclude“completely” e.g. a composition which is“substantially free” from Y may be completely free from Y. Where necessary, the word“substantially” may be omitted from the definition of the invention.

[0056] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements. [0057] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0058] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0059] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0060] While the methods described above are illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases.

Examples

[0061] The present disclosure relates to a method that involves applications of a purely inorganic silica coating onto plastics. The coating method is based on sol-gel chemistry, with the whole process performed at low temperatures no higher than H5°C. Low temperature processing is enabled by a gelation step which is a controlled acid- catalyzed cross-linking of sol-gel precursors in an alcohol medium. This step takes place before the actual coating process onto plastics, so that subsequent curing can take place at low temperature. Scalability of the coating is demonstrated via large-area coating techniques such as doctor blading.

[0062] The silica coating on acrylic, based on the present method, improves the acrylic’s pencil hardness by 8 pencil grades from 2B to 5H, as determined by ASTM standard D3363 testing. The hardness is superior to that of a commercially available scratch resistant acrylic sheet, which was found to have a hardness rating of 4H. The purely inorganic coating is also more durable than an organic-inorganic hybrid coating.

[0063] An adhesion test of the coating according to ISO 2409 standard was also performed. The coating qualified for the best adhesion grade (ISO 2409 grade 0), indicating excellent adhesion between the present coated inorganic film and the acrylic substrate.

[0064] The present method and the present coating are described in further details, by way of non-limiting examples, as set forth below.

[0065] Example 1: Preparation of Silica Sol-Gel Solution

[0066] A sol-gel solution of tetraethyl orthosilicate (TEOS) (0.2 mol, 44.6 mL) in ethanol solvent (0.4 mol, 23.36 mL) was stirred rigorously at room temperature for 10 minutes followed by drop-wise addition of aqueous hydrochloric acid (0.1 M, 7.2 mL). The TEOS/ethanol/HCl(aq) solution was subsequently stirred for 2 hours at room temperature and allowed to age for at least 24 hours prior to film deposition. The room temperature used may range from 20°C to 40°C, or 20°C to 30°C, etc.

[0067] Example 2: Coating of Silica Thin Film

[0068] Cast acrylic substrates were chosen for demonstration of the anti-scratch property of the silica film. The acrylic samples were 3 mm thick, with protective cover films. The cover films were removed just prior to coating, and the silica sol was subsequently deposited on the acrylic without any further acrylic surface treatment. Silica films were also deposited onto silicon (i.e. Si(l00)) substrates for Fourier- transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis. The silicon substrates were cleaned by sonication with acetone and water followed by oxygen plasma for five minutes.

[0069] The sol-gel solution was filtered using Whatman PTFE membrane filters with pore size of 0.2 pm prior to deposition. Small substrates (3 x 3 cm in size) for film characterization were coated by spin coating with spinning speed of 1500 rpm for 30 seconds. The films were subsequently dried at 80°C for 10 mins, and further heated to 1 l5°C for 30 mins in an oven, following which they were kept at room temperature for three days prior to testing. The sol-gel reaction continues to proceed at room temperature after the heating step, and the reaction takes about three days to reach completion. The doctor blading method was used for demonstrating coating of the sol- gel solution onto large substrates over 10 cm (see FIG. 1B for an example). The blade of the applicator was maintained at a gap of 0.36 mm above the sample surface, and the blade coating speed as it moves across the sample was 2 mm/s. Both spin coating and doctor blading methods as described above produce films that are 600 nm thick, as measured using a surface profilometer.

[0070] Example 3: Pencil Hardness Testing

[0071] The abrasion resistance of the silica film was measured according to ASTM standard D3363 testing, using a Lantek HT-6510P Pencil Hardness Tester with a constant load of 7.5 N. Five lines with an approximate length of 6 cm were drawn across the film using a pencil loaded at an angle of 45° to the surface. The film hardness reading was recorded as the highest pencil grade which produced less than three visible scratch lines on the film, after the pencil mark was removed with an eraser. Using this method, the 600 nm silica film was determined to have a pencil hardness of 5H. Compared to untreated acrylic sample which has a pencil hardness of 2B, the silica film has significantly improved the surface hardness by 8 pencil grades. 5H pencil scratch marks on acrylic surface with and without silica coating are observed under optical microscope and shown in FIG. 2A and FIG. 2B, respectively.

[0072] Example 4: Adhesion Testing

[0073] The film adhesion was measured by an Elcometer 107 Cross Hatch Adhesion Tester according to ISO 2409 standard for a thin film coated onto a soft surface. A 6 mm x 2 mm cutter blade is loaded onto the cross hatch cutter, and a cross hatch pattern is formed by making 2 sets of deep cuts on the coating at a 90° angle. ISO 2409 adhesive tape was then applied firmly onto the cut pattern, and then removed parallel to the substrate after 90 seconds. Adhesion result is judged based on the amount of coating material that was removed by the tape. Our silica coating showed no visible loss of material (FIG. 3A to 3C), thus qualifying for the highest adhesion grade (ISO 2409 grade 0). This indicates strong adhesion between the silica film and the acrylic substrate.

[0074] Example 5: X-ray Photoelectron Spectroscopy (XPS) Measurement

[0075] The XPS measurement was carried out using a Thermo Scientific Theta Probe XPS, with monochromatic Al Ka X-ray of energy of 1486.7 eV. Survey scan was measured with step of 1 eV and pass energy of 200 eV while narrow high-resolution scan was performed with step of 0.1 eV and pass energy of 40 eV.

[0076] The high-resolution XPS spectra of Cls, Ols and Si2p peaks are shown in FIG. 4. In addition, the measured binding energy, full width at half maximum (FWHM), and peak area percentage of the elements are presented in FIG. 5. The Si spectrum shows a single Si2p peak at 103.7 eV, corresponding to the binding energy of Si-O. No unexpected Si-Si peak (indicative of bulk silicon) at approximately 99 eV is observed.

[0077] Ols signal shows a sharp symmetrical peak at 533.14 eV, indicating the presence of bulk O² . The absence of Ols peak shoulder at a higher binding energy suggests a hydroxyl-free (OH) silica surface, indicating elimination of Si(OH)₄ by product under the low temperature treatment process. The ratio of elemental percentage of Si to O is very close to 1:2, consistent with empirical formula of Si0₂.

[0078] Only a very minute amount of C content of 2.9 % was detected, suggesting an almost complete reaction with an insignificant amount of carbon-containing residue remaining.

[0079] The observed peak position of Ol s and Si2p at 532.9 eV and 103.5 eV, respectively, represents for Si0₂, further confirming the formation of the desired Si0₂ product.

[0080] Example 6: Fourier-Transform Infrared (FTIR) Measurement

[0081] The FTIR reflection spectrum of silica film on Si wafer was scanned in a range of 1400 cm ¹ to 600 cm ¹, using a Spectrum 2000 FTIR instrument with Perkin Elmer Autoimage Microscope. The FTIR spectrum of silica film is shown in FIG. 6. A broad prominent peak at the wavenumber of 1087 cm ¹ and a weak peak at 800 cm ¹ represent Si-O-Si bond stretching and bending, respectively, and confirm the formation of Si0₂. A major signal at the wavenumber of about 620 cm ¹ is attributed to Si-Si bond stretching of the background silicon wafer. A small peak at 950 cm ¹ corresponds to Si- OH bond bending, which suggests a minute amount of hydroxyl group present on silica film surface. FTIR, being more sensitive than XPS, was able to detect the hydroxyl group.

[0082] Example 7: Scanning Electron Microscopy (SEM) Imaging

[0083] Surface morphology of silica film was observed under JSM-6700 Field Emission Scanning Electron Microscope (FESEM), under an accelerating voltage of 5 kV. Even at a magnification of 50,000x (FIG. 7), dense and compact silica films are observed without any visible pores.

[0084] Example 8: Scalability of Present Method

[0085] The coating technologies as described in the present disclosure include spin coating and doctor blading. Spin coating may not be suitable for scaling up, but doctor blading is appropriate for large substrates. The size of the substrate is limited only by the size of the applicator and length of the coating machine, both of which can be made very large. FIG. 8 shows the applicator used in the present method, which has a width of 18 cm but can be made much larger.

[0086] The present method, nevertheless, is compatible with other large-area coating method, e.g. spray coating. The present films have also been produced by spray coating. While the films are coated over a large area (15 cm x 15 cm) and quite uniform, spray coating may be more suitable for applications that do not require low film roughness, as high film roughness may arise due to droplets produced in spray coating.

[0087] The present coating method is also compatible with dip coating, although substrate size may be limited by the size of coating tanks.

[0088] Example 9: Commercial and Potential Applications

[0089] The present coating derived from the present method is highly durable due to zero organic content. Through the present method, there is no need to optimize ratio of organic to inorganic components. The present coating also does not suffer from light scattering (haze) from nanoparticles. The present coating is a one-step coating method where a primer layer is not needed. Advantageously, the present method and coating present a low cost, scalable and fast solution coating process, wherein the coating of the scratch resistant layer onto the plastic can be achieved by, for example, doctor blading and spray coating. A resultant abrasion resistant polycarbonate, derived from the present method, can serve as a replacement for conventional vehicle glass, used as a protective coating for optical lenses, and/or mobile devices.

[0090] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A method of forming a silica coated polymer substrate, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate, the method comprising:

providing a solution comprising a silica precursor and an organic solvent; mixing the solution with an aqueous acid catalyst to form a silica sol-gel solution;

depositing the silica sol-gel solution on the polymer substrate; and

2. The method of claim 1, wherein the silica precursor comprises a tetra-alkyl orthosilicate.

3. The method of claim 1 or 2, wherein the organic solvent comprises an alcohol.

4. The method of any one of claims 1 to 3, wherein the aqueous acid catalyst comprises hydrochloric acid, sulfuric acid, phosphoric acid, or acetic acid.

5. The method of any one of claims 1 to 4, wherein mixing the solution with the aqueous acid catalyst comprises aging the solution for at least 24 hours.

6. The method of any one of claims 1 to 5, wherein depositing the silica sol-gel solution on the polymer substrate comprises spreading the silica sol-gel solution on the polymer substrate.

7. The method of any one of claims 1 to 6, wherein curing the silica sol-gel solution comprises drying the silica sol-gel solution on the polymer substrate.

8. The method of any one of claims 1 to 7, wherein curing the silica sol-gel solution comprises heating the silica sol-gel solution on the polymer substrate to a temperature of 1 l5°C or less after drying the silica sol-gel solution.

9. The method of any one of claims 1 to 8, wherein curing the silica sol-gel solution comprises subjecting the silica sol-gel solution on the polymer substrate to a temperature ranging from 20°C to 30°C for at least 3 days after heating the silica sol- gel on the polymer substrate.

10. A silica coated polymer substrate obtained according to any one of claims 1 to 9, wherein the silica coated polymer substrate consists of a non-porous silica layer disposed directly on a polymer substrate.

11. The silica coated polymer substrate of claim 10, wherein the polymer substrate is composed of acrylic or polycarbonate.

12. The silica coated polymer substrate of claim 10 or 11, wherein the non-porous silica layer is adhered to the polymer substrate after being subjected to abrasion, and wherein the polymer substrate is composed of acrylic.

13. The silica coated polymer substrate of claim 12, wherein the non-porous silica layer remains adhered to the polymer substrate even when an adhesive tape stuck on the non-porous silica layer is pulled away from the non-porous silica layer.

14. The silica coated polymer substrate of claim 12 or 13, wherein the silica coated polymer substrate has a pencil hardness of 5H.

15. The silica coated polymer substrate of any one of claims 10 to 14, wherein the non-porous silica layer is transparent.