WO2007133378A1

WO2007133378A1 - Multi-wall plastic sheet having an internal plasma-enhanced chemical vapor deposition coating and process for manufacturing the same

Info

Publication number: WO2007133378A1
Application number: PCT/US2007/009447
Authority: WO
Inventors: Aaron M. Gabelnick; Paul D. M. Haemhouts
Original assignee: Dow Global Technologies Inc.
Priority date: 2006-05-11
Filing date: 2007-04-17
Publication date: 2007-11-22
Also published as: TW200801231A

Abstract

A process for producing a coating on at least one interior surface of a multi-watl sheet is described comprising (a) providing a multi-wall sheet (1) comprising multiple hollow flutes (1a) between the walls of the multi-wall sheet; (b) introducing at least one gaseous mixture capable of plasma deposition into at least one hollow flute (1a) of the multi-wall sheet; (c) generating a plasma in the at least one gaseous mixture in the at least one hollow flute of the multi-wall sheet, and (d) allowing the plasma of step (c) to form a solid deposit (9) derived from the at least one gaseous mixture on the interior surface of the at least one hollow flute. A multi-wall sheet (1) obtainable by the above process is also described, including a multi-wall sheet having a plasma-enhanced vapor deposition coating (9) on the interior surface of at least one flute (1a) of the multi-wall sheet, wherein the coating has an average thickness not greater than 5 μm and a variation in thickness relative to the average thickness not greater than 1 μm.

Description

MULTI-WALL PLASTIC SHEET HAVING AN INTERNAL PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION COATING AND PROCESS FOR MANUFACTURING

THE SAME

Cross Reference Statement

[0001] This application claims the benefit of U.S. Provisional Application No. 60/810,202, filed June 1, 2006 and of U.S. Provisional Application No. 60/799,740, filed May 11, 2006. Background of the Invention

[0002] The present invention relates to coating plastic sheet using plasma-enhanced chemical vapor deposition (PECVD).

[0003] Coating of materials using PECVD is known. Application of an organosilicon coating via PECVD is described in US-A-5,718,967, WO 03/066932, and US-A-6,815,014. Application of a silicon dioxide coating via PECVD is described in WO 2005/049228. Application of metal oxide coatings via PECVD is described in WO 2005/113856. In these references, the substrate may be a plastic material,

[0004] Multi-wall sheets are multi-layer hollow chamber plastic sheets in which plastic sheets are connected with one another by means of cross-pieces between the sheets, such that hollow chambers, cavities or flutes are formed between the sheets. Multi-wall sheets, and how to make them, are described in EP-A-O 110221, EP-A-O 110238, EP-A-O 054 856, EP-A-O 741 215, EP-A-O 774 551, and US-A-2004/0013882.

[0005] Multi-wall sheets are used for making glazing panels, protective panels, conservatories, verandas, carports, bus stops, advertising signage, windows, partitions, cash kiosks, viewing panels, displays, roofing, and films. The glazing panels may be used for greenhouses, swimming pool enclosures, patio enclosures, solar collectors, vehicles, petrol stations, laboratories and chemical plants. In most of these applications, multi-wall sheets are exposed to sunlight and moisture or rain. Exposure to sunlight often causes deterioration of the plastic material and substantial heat gain due to the "greenhouse effect" and exposure to moisture or rain can result in condensation within their hollow chambers. The latter problem results in growth of bacteria, fungi, and mildew and, when the multi-wall sheet is used as a transparent glazing or window, loss of optical transparency due to fogging.

[0006] The state of the art has addressed the problem of deterioration due to sunlight by co-extrusion of UV absorption layers with the multi-wall sheets or incorporating resistance to UV radiation into the plastic material used to make such sheets. External coatings are known which may be used to address the heat gain and external fogging on the sheet.

[0007] However, the above state of the art has not fully addressed the heat gain problem and does not address the problem of fogging between the sheets of the multi-wall sheets. The latter has been addressed by physically applying chemical compositions dissolved in a solvent to the hollow chambers to form an anti-fogging coating, such as an organosilicon coating, on the interior surfaces of the multi-wall sheets. The process of applying the coatings and removing the solvent is complicated and time-consuming and such a process has inherent limitations on the minimum thickness and evenness of the coating, which in turn results in the use of an excessive amount of the coating, which substantially increases the cost of the treated multi-wall sheets and may cause problems with optical transparency.

[0008] These and other problems relating to multi-wall sheets are solved by the present invention described below.

Summary of the Invention

[0009] The present invention provides a process for producing a coating on at least one interior surface of a multi-wall sheet comprising:

(a) providing a multi-wall sheet comprising multiple hollow flutes between the walls of the multi-wall sheet;

(b) introducing at least one gaseous mixture capable of plasma deposition into at least one hollow flute of the multi-wall sheet;

(c) generating a plasma in the at least one gaseous mixture in the at least one hollow flute of the multi-wall sheet, and

(d) allowing the plasma of step (c) to form a solid deposit derived from the at least one gaseous mixture on the interior surface of the at least one hollow flute.

[00010] The present invention also provides a multi-wall sheet obtainable by the above process.

[001 1] In one embodiment, the present invention is a multi-wall sheet having a PECVD coating on the interior surface of at least one flute of the multi-wall sheet, wherein the coating has an average thickness not greater than 5 μm and a variation in thickness relative to the average thickness not greater than 1 μm.

Brief Description of the Drawings

[0012] Fig. 1 is a side-view cross-section schematic diagram showing the section in the direction of the arrows along line C-D of Fig. 2 of an apparatus applying a plasma enhanced chemical deposition on the interior surfaces of a multi-wall sheet during extrusion of the multi-wall sheet according to the process of the present invention.

[0013] Fig. 2 is a top-view cross-section schematic diagram showing the section in the direction of the arrows along line A-B of Fig. 1 of the apparatus in operation shown in Figure 1.

Detailed Description of the Invention

[0014] For purposes of United States patent practice, the contents of any patent, patent application, or publication referenced herein are hereby incorporated by reference in their entirety (or the equivalent US version thereof is so incorporated by reference) especially with respect to the disclosure of synthetic techniques, raw materials, and general knowledge in the art. Unless stated to the contrary, implicit from the context, or customary in the art, all parts and percents are based on weight.

[0015] If appearing herein, the term "comprising" and derivatives thereof is not intended to exclude the presence of any additional component, step or procedure, whether or not the same is disclosed herein. In order to avoid any doubt, all compositions claimed herein through use of the term "comprising" may include any additional additive, adjuvant, or compound, unless stated to the contrary. In contrast, the term, "consisting essentially of if appearing herein, excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term "consisting of, if used, excludes any component, step or procedure not specifically delineated or listed. The term "or", unless stated otherwise, refers to the listed members individually as well as in any combination.

[0016] As used herein, the expression, "multi-wall sheet" refers to "multi-layer hollow chamber plastic sheet" comprising planar plastic layers connected with one another by means of cross-pieces between the layers, such that hollow flutes (i^that is. chambers) are formed between the layers.

[0017] As used herein, the term "flutes" refers to hollow elongated structures within a multi-wall sheet formed by the planar plastic layers and cross-pieces of the multi-wall sheet. Due to their hollow elongated shape, each flute is able to conduct a gas in the direction of elongation when at least one end of the flute is open.

[0018] Unless otherwise stated herein, the term "liter" refers to the volume of a specified gas, or mixture of gases, at ambient temperature and atmospheric pressure.

[0019] As used herein, ambient temperature refers to a temperature of about 21° C.

[0020] As used herein, atmospheric pressure refers to a pressure of about 1 atmosphere.

[0021] Unless otherwise stated herein, "cm²" refers to the cross-sectional area of the multi-wall sheet perpendicular to the direction of the flutes

[0022] As defined herein, each of the terms "electrode" and "counter-electrode" refer to a single conductive element or a plurality of conductive elements. The terms electrode and counter-electrode as used herein refer to a first electrode, or set of electrodes, on one side of the multi-wall sheet and a second electrode, or set of electrodes, on the other side of the multi-wall sheet.

[0023] As defined herein, "frequency" refers to the number of times per second that an applied voltage or electric current cycles from a minimum value to a maximum value and then back again to a minimum value, which may be expressed in Herz (Hz)

cycles per second).

[0024] As defined herein, "applied voltage" refers to a voltage potential applied to an electrode or counter-electrode.

Multi-Wall Sheet

[0025] The multi-wall sheet is a multi-layer hollow chamber plastic sheet comprising at least two parallel opposing plastic walls having multiple parallel cross- pieces connecting at least two of the plastic walls, each wall forming a plane of the multilayer hollow chamber sheet, and multiple parallel hollow flutes between opposing walls and adjacent cross-pieces, each hollow flute having at least one interior surface formed by opposing walls and/or adjacent cross-pieces.

[0026] The walls of the multi-wall sheets are preferably substantially planar, although they may also have various profiles, such as a corrugated profile. The number of walls is not limited other than by the inherent limitations in the thinness of the walls that can be produced while maintaining the integrity of each wall. Multi-wall sheets having two, three or four walls in two, three, and four parallel planes relative to each other, respectively, are known as a twin-wall, triple-wall, and quadruple-wall sheets, respectively.

[0027] The cross-pieces connect the walls with each other and preferably maintain a certain distance between the respective walls of the multi-wall sheet. The number of cross-pieces is not particularly limited and should be sufficient to prevent collapse of the hollow flutes between the walls. The cross-pieces may have various shapes and profiles, and may connect with the respective walls at various angles. The angles are preferably in the range from 45 to 90 degrees relative to the plane of the respective walls when measured in a direction perpendicular to the direction of the hollow flutes. The cross- pieces of an extruded multi-wall sheet are generally parallel to each other and parallel to the extrusion direction of the multi-wall sheet.

[0028] The outermost walls of the multi-wall sheet preferably has a thickness of at least 0.5 mm, more preferably at least 1 mm, and preferably up to 3 mm, more preferably up to 2 mm. The cross-pieces and any walls or planes between the outermost walls of the multi-wall sheet preferably have a thickness of at least 0.1 mm, more preferably at least 0.2 mm, and preferably up to 2 mm, more preferably up to 1 mm.

[0029] The average distance between planes of the multi-wall sheet is preferably at least 1 mm, more preferably at least 3 mm and preferably up to 8 mm and more preferably up to 5 mm. The average thickness of the overall multi-wall sheet is preferably at least 1 mm, more preferably at least 5 mm, and even more preferably at least 8 mm and preferably up to 40 mm and more preferably up to 20 mm.

[0030] The material of the multi-wall sheets is not particularly limited. Preferably, the material comprises one or more polymers, optionally containing various adjuvants known in the industry. The polymers may be selected from the group consisting of polycarbonates, polyurethanes, poly(meth)acrylates, polypropylenes, polyethylenes, ethylene/α-olefin copolymers, styrene-acrylonitrile copolymers, polyethylene terephthalates, and polybutylene terephthalates, and combinations thereof. Combinations of polymers may be present in admixture or concentrated in layers via coextrusion. The plastic material may be homogeneous or heterogeneous. When a combination of optical clarity and mechanical strength is desired, polycarbonates are preferred. [0031] Multi-wall sheets, and how to make them, are well-known in the industry. Processes for manufacturing multi-wall sheets are described, for example, in EP-A-O 110 221 , EP-A-O 110238, EP-A-O 054 856, EP-A-O 741 215, EP-A-O 774 551 , and US-A- 2004/0013882. In general, multi-wall sheet is produced by extruding a molten plastic through a special die designed for making multi-wall sheet, such as the dies available from Breyer GmbH of Singen, Germany. Ports are provided in the die for injecting air into the flutes of the multi-wall sheet to provide pressure from within the flutes to help maintain their shape after the multi-wall sheet exits the die. Upon exiting the die and while still in a warm, semi-soft state, the multi-wall sheet is passed through one or more vacuum calibrators. Each vacuum calibrator pulls the outermost layers of the multi-wall sheet against a pair of platens located above and below the multi-wall sheet via a vacuum applied through multiple openings in the platens to ensure that a set thickness of the multi- wall sheet is maintained while the multi-wall sheet cools and hardens. Further equipment for conveying and cutting the multi-wall sheet may be located downstream from the calibrator.

[0032] Preferably, steps (b) and (c) are carried out downstream from extrusion of the multi-wall sheet, such as downstream from one or more vacuum calibrators or simultaneously with vacuum calibration by applying a vacum through or between one or more flat electrodes.

[0033] In particular, steps (b) and (c) are preferably applied during extrusion of the multi-wall sheet as further described below in reference to Figs. 1 and 2.

Gaseous Mixture

[0034] The gaseous mixture of step (b) may be any gaseous mixture capable of forming a deposit on a substrate when subjected to plasma enhanced chemical vapor deposition (PECVD). The deposit is preferably at least one macromolecule, and more preferably at least one polymer. The gaseous mixture preferably comprises at least one inert gas, at least one working gas, and optionally at least one oxidative gas.

[0035] As used herein, "inert gas" refers to a gas which does not react with the working gas or oxidative gas, if present, and which is capable of providing a plasma environment for activating the working gas during step (c) of the process of the present invention. Preferred inert gases include helium, argon, nitrogen, and carbon dioxide, or a combination thereof. Nitrogen and carbon dioxide may be introduced as such or in the form of air. [0036] The inert gas is preferably introduced into the multi-wall sheet at a rate in the range from 0.1 to 100 liter/minute/cm² (wherein "cm²" refers to the cross-sectional area of the multi-wall sheet perpendicular to the direction of the flutes). Preferred sub-ranges include minimum rates of 1 liter and 10 liter per minute/cm² and maximum rates of 10 liters and 1 liter per minute/cm² . The preferred ranges and sub-ranges include all possible combinations of the aforementioned lower and upper endpoints.

[0037] As used herein, "oxidative gas" refers to at least one gas capable of oxidizing at least one working gas. Examples of suitable oxidizing gases include oxygen, nitrous oxide (N₂O), ozone (O₃), nitric oxide (NO), and nitrogen tetraoxide (N₂O₄), as well as combinations thereof. Oxygen may be introduced as such or in the form of air.

[0038] The oxidative gas is preferably introduced into the multi-wall sheet at a rate in the range from 0 to 5 liter/minute/cm². Preferred sub-ranges include minimum rates of 50 ml and 1 liter per minute/cm² and maximum rates of 1 liter and 50 ml per minute/cm² . The preferred ranges and sub-ranges include all possible combinations of the aforementioned lower and upper endpoints.

[0039] The rate ratio of oxidative gas to inert gas is preferably in the range from 0 to 3: 10. Preferred rate ratio sub-ranges include a minimum of 1:50 and 1: 10 and maximum ratios of 1:10 and 1 :50. The preferred ranges and sub-ranges include all possible combinations of the aforementioned lower and upper endpoints.

[0040] As used herein, the term "working gas" refers to a least one reactive substance in a gaseous state. The reactive substance may or may not be gaseous at ambient temperature and atmospheric pressure. When the reactive substance is not gaseous at ambient temperature and atmospheric pressure, the reactive substance may be evaporated by the application of heat, agitation, electron discharge, microwave energy and/or vacuum in order to combine the working gas with the inert gas(es) and any oxidizing gas(es). A well-known device which may be used to evaporate the reactive substance to form the working gas and combine it with the other gases for introduction to the flutes according to step (b) is a controlled evaporator mixer.

[0041 ] The reactive substance preferably comprises a precursor capable of forming at least one macromolecule and/or at least one polymer when subjected to PECVD. The reactive substance also preferably comprises at least one precursor for forming an organosilicon, silicon oxide, or metal oxide deposition.

[0042] Examples of suitable precursors for making an organosilicon or silicon oxide deposit include silicon-containing compounds such as silanes, siloxanes, and silazanes. Preferred silicon-containing compounds may be represented by the chemical formula (I):

R₃Si[X-Si(RZ)₂] _rR" (I) wherein R, R', and R", independently at each occurrence, represent -H, -OH, a C₁. J_Q hydrocarbyl group, or a C_{1 - I 0} hydrocarbyloxy group, X represents -O- or -N(R"")-, wherein R"", independently at each occurrence, represents -H or a CJ.JO hydrocarbyl group, and r represents zero or a number in the range from 1 to 10. The hydrocarbyl and hydrocarbyloxy groups may be saturated or unsaturated, substituted or unsubstituted, and branched or unbranched. The silicon-containing compounds are preferably organosilicon compounds and preferably comprise at least one, more preferably at least three, C₁. _{Q hydrocarbyl group(s) and/or at least one, preferably at least three, C_| .JO hydrocarbyloxy group(s). Each of the C HQ hydrocarbyl groups and CJ.JQ hydrocarbyloxy groups preferably comprise up to three carbon atoms.

[0043] Examples of silanes include dimethoxydimethylsilane, methyltrimethoxysilane, tetramethoxysilane, methyltriethoxysilane, diethoxydimethylsilane, methyltriethoxysilane, triethoxyvinylsilane, tetraethoxysilane (also known as tetraethylorthosilicate or TEOS), dimethoxymethylphenylsilane, phenyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3- methacrylpropyltrimethoxysilane, diethoxymethylphenylsilane, tris(2- methoxyethoxy)vinylsilane, phenyltriethoxysilane, and dimethoxydiphenylilane.

[0044] Examples of siloxanes include tetramethyldisiloxane (TMDSO), hexamethyldisiloxane (HMDSO), and octamethyltrisiloxane.

[0045] Examples of silazanes include hexamethylsilazanes and tetramethylsilazanes.

[0046] These precursors may be used to make deposits or coatings comprising polymeric organosilicon, polymeric siloxane, and/or silicon oxide depending on the starting material and the amount of oxidizing gas present. The presence of a low or zero concentration of oxidizing gas may be used to produce polymeric organosilicon deposits or coatings, particularly when the precursor comprises silanes or silazanes. Increasing the concentration of the oxidizing gas may be used to promote the production of a polysiloxane or silicon oxide deposit or coating.

[0047] When the working gas comprises an organosilicon or silicon oxide precursor, helium or helium combined with argon are preferred inert gases. Gaseous mixtures useful for application of an organosilicon coating via PECVD are described in US-A-5,718,967, WO 03/066932, and US-A-6,815,014. Gaseous mixtures useful for application of a silicon dioxide coating via PECVD are described in WO 2005/049228.

[0048] As used herein, the term "metal-oxide precursor" refers to a material capable of forming a metal oxide when subjected to PECVD. Examples of suitable metal-oxide precursors useful as the working gas include diethyl zinc, dimethyl zinc, zinc acetate, titanium tetrachloride, dimethyltin diacetate, zinc acetylacetonate, zirconium hexafluoroacetylacetonate, zinc carbamate, trimethyl indium, triethyl indium, cerium (IV) (2,2,6,6-tetramethyl-3,5-heptanedionate), and mixtures thereof. Examples of metal oxides include oxides of zinc, tin, titanium, indium, cerium, and zirconium, and mixtures thereof.

[0049] When the working gas comprises a metal oxide precursor, nitrogen is a preferred inert gas. Gaseous mixtures useful for application of metal oxide coatings via PECVD are described in WO 2005/113856.

[0050] The working gas is preferably introduced into the multi-layer sheet according to step (b) at a rate from 0.1 mg up to 0.5 g per minute/cm² (wherein "cm²" refers to the cross-sectional area of the multi-wall sheet perpendicular to the direction of the flutes). Preferred sub-ranges include minimum rates of 5 mg, 50 mg, and 0.25 g per minute/cm² and maximum rates of 0.25 g, 50 mg, and 5 mg per minute/cm² . The preferred ranges and sub-ranges include all possible combinations of the aforementioned lower and upper endpoints.

[0051] Desirably, the concentration of precursor compound present in the gaseous mixture is maintained in the range from at least 50 ppm, preferably at least 200 ppm, and more preferably at least 500 ppm; and not greater than 10000 ppm, preferably not greater than 8000 ppm, and more preferably not greater than 7000 ppm.

[0052] The flow rate and concentration is preferably adjusted to obtain an average thickness of the coating on the inner surface of the multi-wall sheet not greater than 5 μm, more preferably not greater than 2 μm, even more preferably not greater than 1 μm, and even more preferably not greater than 0.1 μm and preferably at least 1 nm, more preferably at least 10 nm, and even more preferably at least 30 nm, and a variation in thickness relative to the average thickness not greater than 1 μm, more preferably not greater than 0.5 μm, and even more preferably not greater than 0.1 μm. The variation in thickness relative to the average thickness is preferably not greater than 50 percent, more preferably not greater than 20 percent, and even more preferably not greater than 10 percent. [0053] Preferably at least 0.2 liters, more preferably at least 1 liter, and even more preferably at least 2 liters, and preferably up to 20 liters, more preferably up to 10 liters, and even more preferably up to 4 liters, of the gaseous mixture are introduced into the multi-wall sheet per cubic centimeter of multi-wall sheet during step (b). For one square meter of a multi-wall sheet having an average thickness of 2 cm for example, the preferred gaseous mixture amount ranges would be at least 10 liters, more preferably at least 50 liters, and even more preferably at least 100 liters, and preferably up to 1000 liters, more preferably up to 500 liters, and even more preferably up to 200 liters.

[0054] Preferably the velocity of the gas mixture is such that a stable plasma is formed allowing for uniform deposition of polymerized product. Desirably, the velocity of the gas passing through the flutes is at least about 0.05 m/s, more preferably at least about 0.1 m/s, and most preferably at least about 0.2 m/s; and preferably not greater than about 1000 m/s, more preferably not greater than about 500 m/s, and most preferably not greater than about 200 m/s. Preferably, the volumetric flow of the gas mixture is from 10 to 1 ,500 cc/minute per cm² of the surface area exposed to a plasma-generating energy.

[0055] The gaseous mixture is preferably introduced in step (b) at a pressure in the range from 1 kPa to 800 kPa above atmospheric pressure. The gaseous mixture is more preferably introduced at a pressure sufficiently above atmospheric pressure to obtain the desired gas flow past the electrode(s), such as at least 20 kPa, more preferably at least 100 kPa, and even more preferably at least 200 kPa above atmospheric pressure.

[0056] The gaseous mixture is introduced into at least one flute of the multi-wall sheet. In a preferred embodiment, the gaseous mixture is introduced into multiple flutes simultaneously. In an even more preferred embodiment, the gaseous mixture is introduced into all the flutes of the multi-wall sheet simultaneously. The gaseous mixture may be introduced into the flutes after extrusion of the multi-wall sheet has been completed or preferably during extrusion of the multi-wall sheet.

[0057] In a preferred embodiment of the preferred process, the gaseous mixture is introduced into the at least one hollow flute, preferably all hollow flutes, via ports in the extrusion die. In that case, the gaseous mixture may simultaneously serve as a means for maintaining an appropriate thermal and pressure equilibrium between the interior and exterior of the multi-wall sheet while cooling and solidifying after exiting the extrusion die. This embodiment is illustrated in Figs. 1 and 2 describe below. Plasma Generation

[0058] Any suitable apparatus may be used to generate a plasma in step (c). Examples include apparatus for generating a plasma via plasma discharge such as a dielectric barrier discharge apparatus. Such devices comprise one or more electrodes and one or more counter-electrodes and the ability to apply a voltage potential between the electrode(s) and the counter-electrodes. The electrodes and counter-electrodes are positioned so that the multi-wall sheet may be passed between the electrodes and the counter-electrodes, so that the electrode(s) are located on one side of the multi-wall sheet and the counter-electrode(s) are located on the other side of the multi-wall sheet. A plasma is produced by electrical discharge between the electrode(s) and the counter- electrode(s).

[0059] Dielectric barrier discharge is also known as "silent" and "atmospheric- pressure-glow" discharge. The application of dielectric barrier discharge for making an organosilicon coating via PECVD is described in US-A-5 ,718,967, WO 03/066932, and US-A-6,815,014. The application dielectric barrier discharge for making a silicon dioxide coating via PECVD is described in WO 2005/049228. The application of dielectric barrier discharge for making metal oxide coatings via PECVD is described in WO 2005/1 13856.

[0060] In the process of the present invention, sufficient electric power density and frequency are applied to an electrode and/or counter-electrode to create and maintain a glow discharge in the gaseous mixture located in a gap between the electrode and counter- electrode. The power density (based on electrode surface area exposed to the plasma) is preferably at least 1 W/cm², more preferably at least 5 W/cm², and most preferably at least 10 W/cm²; and preferably not greater than 200 W/cm², more preferably not greater than 100 W/cm², and most preferably not greater than 50 W/cm². The current applied to the electrodes may vary from 10 to 10,000 watts, preferably from 100 to 1000 watts, at potentials of 10 to 50,000 volts, preferably 100 to 20,000 volts. The frequency is preferably at least 2 kHz, more preferably at least 5 kHz, and most preferably at least 10 kHz; and preferably not greater than 100 kHz, more preferably not greater than 60 kHz, and most preferably not greater than 40 kHz.

[0061] To obtain efficient conversion of electric power into plasma, the frequency is preferably selected to coincide with the resonance frequency of the electrode(s)/counter- electrode(s) configuration. The resonance frequency is equal to the inverse of 2π V(L»C) in which L is the inductance, provided almost entirely by the high-voltage side of the transformers, and C is the capacitance provided almost entirely by the electrode-to-ground arrangement at the coating station. The inductance is preferably provided primarily by the high-voltage side of an electrical transformer and the capacitance is preferably provided primarily by a counter-electrode-to-ground arrangement.

[0062] Tn a preferred embodiment, electric power is applied to the electrode(s) on one side of the multi-wall sheet and the counter-electrode(s) on the other side of the multi- wall sheet are grounded (herthatjs, connected to the earth via an electric conductor). In this embodiment, the voltage applied to the electrode(s) preferably cycles between a minimum voltage value and a maximum voltage value at a substantially constant frequency. The maximum voltage value is preferably either a positive value or a negative value in each cycle in contrast to cycles in which the maximum voltage alternates between a positive value and a negative value.

[0063] The electrode(s) and the counter-electrode(s) are preferably located within 4 mm, more preferably within 1 mm of the outer surface of the respective outermost sheets of the multi-wall sheet. In one embodiment, the electrode(s) and/or counter-electrode(s) is/are in contact with the outer surface of the respective outermost sheets of the multi-wall sheet. The electrode, the counter-electrode or both the electrode and the counter-electrode may be fitted with an elevated temperature resistant dielectric sleeve such as a ceramic, if desired. The electrode, the counter-electrode or both the electrode and the counter- electrode may be integrated with a vacuum calibrator used to calibrate the multi-wall sheet.

[0064] Plasma generation is preferably applied to the multi-wall sheet as the multi- wall sheet and the plasma-generating apparatus are displaced relative to one another over time in a direction parallel to the orientation of the flutes. In one embodiment, the multi- wall sheet is is displaced relative to a stationary plasma-generating apparatus over time during step (c). In a preferred embodiment, the multi-wall sheet is displaced relative to a stationary plasma-generating apparatus over time during step (b) and step (c). The displacement of the multi-wall sheet is preferably due to simultaneous extrusion of the multi-wall sheet.

[0065] The movement of the multi-wall sheet relative to the plasma-generating apparatus is preferably at least 20 cm, more preferably at least 50 cm, per minute and preferable up to 5 m, more preferably up to 2 m, per minute.

[0066] Fig. 1 provides a side cross-sectional schematic diagram of apparatus in operation for extrusion of a twin-wall sheet (1) from extrusion die (2) according to the process of the present invention. The multi-wall sheet has multiple hollow flutes (Ia) between the upper and lower sheets (Ib, Ib'). The extrusion die (2) has gas ports (3) located between the upper and lower sheets (Ib, Ib') for introducing a gaseous mixture via gas channels (4) into the hollow flutes (Ia). The gaseous mixture contains a gaseous component capable of forming a deposit on the inner surface of the flutes (Ia) when the gaseous mixture is ignited into a plasma state.

[0067] The gaseous mixture also preferably is introduced at a temperature, pressure and composition suitable for maintaining an appropriate thermal and pressure equilibrium between the interior and exterior of the multi-wall sheet while cooling and solidifying the hot multi-wall sheet exiting the extrusion die to ensure the integrity of the hollow flutes (Ia) and the flatness of the multi-wall sheet (1).

[0068] Downstream from the extrusion die (2) is a dielectric barrier discharge apparatus (5) comprising one or more electrodes (6) and one or more counter-electrodes (7). The space between electrode(s) (6) and counter-electrode(s) (7) defines a plasma generating zone (8). When the multi-wall sheet (1) exiting the extrusion die (2) passes through the plasma generating zone (8) while applying an appropriate electric potential and frequency to the electrode(s) and counter-electrode(s), a plasma is created in the gaseous mixture within the flutes (Ia). The plasma causes formation of a deposit (9) on the interior surface of the flutes (Ia) within the multi-wall sheet (1).

[0069] Fig. 2 is a top-view cross-section schematic diagram of the apparatus shown in operation in Fig. 1 showing the presence of multiple flutes (Ia) and multiple gas ports (3). In addition, Fig. 2 shows the presence of multiple crosspieces (Ic) connecting the twin sheets (Ib, Ib') shown in Fig. 1 which were coated with a deposit formed on the surface of the cross-pieces while passing through the plasma generating zone (8).

[0070] Although electrode (6) and counter-electrode (7) are shown as a unitary blocks , they may comprise multiple electrodes arranged in a planar array adjacent the multi-wall sheet.

[0071] Although not shown in Figs. 1 and 2, one or more apparatus known for carrying out various multi-wall sheet manufacturing functions such as vacuum calibrating, transporting, laminating, cooling and/or cutting the multi-wall sheet exiting the extrusion die may be present at various locations, such as between the extrusion die (2) and the plasma-generating zone (8) or downstream from the plasma-generating zone (8). If calibrating is carried out at a location separate from the location of the plasma-generating zone (8), the plasma-generating zone (8) is preferably located downstream from the calibrating apparatus.

[0072] Organosilicon PECVD, such as polysiloxane, and silicon oxide on the inner surface of the flutes reduce condensation of moisture in the flutes, which improves optical clarity and reduces growth of algae, fungus, mildew, and bacteria.

[0073] Metal oxide PECVD such as zinc oxide may be used to reduce UV transmission, help reduce heat build-up caused by the greenhouse effect when exposed to sunlight via light spectrum management, reduce the refractive index for improving visible light transmission, and increase ignition resistance for improving fire safety.

[0074] The process described herein produces a multi-wall sheet having a PECVD coating on the interior surface of at least one flute of the multi-wall sheet, wherein the coating has an average thickness not greater than 5 μm and a variation in thickness relative to the average thickness not greater than 1 μm. The average thickness of the coating is more preferably not greater than 2 μm, even more preferably not greater than 1 μm, and even more preferably not greater than 0.1 μm and preferably at least 1 nm, more preferably at least 10 nm, and even more preferably at least 30 nm. The variation in thickness of the coating relative to the average thickness of the coating is preferably not greater than 1 μm, more preferably not greater than 0.5 μm, and even more preferably not greater than 0.1 μm. The variation in thickness relative to the average thickness is preferably not greater than 50 percent, more preferably not greater than 20 percent, and even more preferably not greater than 10 percent.

[0075] The coating preferably comprises a polysiloxane, silicone oxide and/or metal oxide and is preferably present on the interior surface of all the flutes of the multi-wall sheet.

[0076] Plasma polymerization as carried out by the process of the present invention preferably results in an optically clear coating deposited on the interior surface of the flutes. The term "optically clear" is used herein to describe a coating having an optical clarity of at least 70 percent, more preferably at least 90 percent, and most preferably at least 98 percent and a haze value of preferably not greater than 10 percent, more preferably not greater than 2 percent, and most preferably not greater than 1 percent. Optical clarity is the ratio of transmitted-unscattered light to the sum of transmitted- unscattered and transmitted-scattered light (<2.5°). Haze is the ratio of transmitted- scattered light (>2.5°) to total transmitted light. These values are determined according to ASTM D 1003-97.

Examples

[0077] The invention is further illustrated by the following examples that should not be regarded as limiting of the present invention. Unless stated to the contrary or conventional in the art, all parts and percents are based on weight.

Test Protocol

[0078] A 10mm thick twin wall sheet is cut 400mm wide x 1350mm long. The twin wall sheet has about 30-32 flutes. The flutes are closed at one end with a heavy metallic tape, so that no gas can escape. A slit is cut in the top surface of the sheet at a location 75mm from the closed end of the sheet, and in such a way that 4 flutes are cut open, followed by 1 closed flute. Six groups of four flutes are cut open with a slit in this manner. By dividing the 400mm wide sheet in 6 sections of 4 flutes each (each section separated by a flute without a slit), the sheet can be used for different trials, each time with one section, while the others have the slit sealed with a tape.

[0079] Each open slit is fitted with a cap for feeding precursor gas into the respective flute through the slit in the top surface. This cap is clamped on the sheet and is sealed gas tight by way of a rubber seal over the sheet surface. The cap is used as a reservoir for feeding the gas to each flute through the slit in the sheet surface. It can be placed on top of a second slit after doing the coating trial on the first one. The helium plus working gas supply line and the oxygen supply line meet on top of the cap in a T-junction with a venturi system. The helium + working gas line is heated to prevent any condensation. For this example, the working gas is TEOS.

[0080] The multi-wall sheet is placed between the electrode and counter-electrodes of a dielectric barrier discharge apparatus in a configuration similar to that shown in Figs. 1 and 2 and the following gases are introduced into the cap located over the slit of the test flute:

5.0 liters/minute helium

1.0 liter/minute mixture of helium and TEOS

1.2 liter/minute oxygen.

[0081] The amount of TEOS in the mixture of helium and TEOS may be varied to increase or decrease the concentration of TEOS in the overall gas mixture. When the gas composition has been confirmed by testing the gas exiting the open end of the multi-wall sheet, sufficient electric power is applied to the electrode at a frequency of 58 kHz to generate a plasma field in the gas within the flutes.

[0082] The test is conducted for 30 seconds. The test is concluded by switching off the power, blowing out the reactive gases with helium, removing the cap from the multi- wall sheet, and blowing the slit clean with air.

[0083] Test samples are cut from the sample sheet and tested by visual observation of fogging when the sheet is positioned over hot water.

Test Data and Results

[0084] The data obtained using the above test protocol is shown in Table 1 below.

Table 1 IlNTERNAL COATING TEST RESULTS

[0085] As can be seen from the data in Table 1, fogging was prevented by the coating applied by the process described herein.

[0086] The multi-wall sheets according to this invention are useful for making glazing panels, protective panels, conservatories, verandas, carports, bus stops, advertising signage, windows, partitions, cash kiosks, viewing panels, displays, roofing, and films. The glazing panels may be used for greenhouses, swimming pool enclosures, patio enclosures, solar collectors, vehicles, petrol stations, laboratories and chemical plants. When treated with a silicon-containing compound, the multi-wall sheets resist condensation between the walls, making them particularly useful for applications exposed to rain or moisture. When treated with a metal oxide such as zinc oxide, the multi-wall sheets provide greater resistance to UV light, reduced heat gain related to the green-house effect due to improved light spectrum management, and greater optical transparency due to reduction of the refractive index of the coated surfaces.

Claims

CLAIMS:

1. A process for producing a coating on at least one interior surface of a multi-wall sheet comprising:

(a) providing a multi-wall sheet comprising multiple hollow flutes within the multi- wall sheet;

(b) introducing at least one gaseous mixture capable of plasma deposition into at least one of the hollow flutes of the multi-wall sheet;

(c) generating a plasma in the at least one gaseous mixture in the at least one hollow flute of the multi-wall sheet into a plasma, and

2. The process according to claim 1, wherein the step of providing a multi-wall sheet (a) comprises extruding the multi-wall sheet from at least one molten plastic material.

3. The process according to claim 2, wherein the extrusion is carried out simultaneously with steps (b) and (c).

4. The process according to any one of the preceding claims, wherein the at least one gaseous mixture of step (b) is introduced into the hollow flutes at a pressure in the range from 1 kPa to 800 kPa above atmospheric pressure.

5. The process according to claim 4, wherein at least 0.2 liters of the gaseous mixture are introduced into the hollow flutes of the multi-wall sheet per cubic centimeter of multi- wall sheet during step (b).

6. The process according to any one of the preceding claims, wherein the gaseous mixture of step (b) comprises an inert gas, a working gas, and optionally an oxidizing gas for the working gas.

7. The process according to claim 6, wherein the working gas comprises a precursor for forming an organosilicon, silicon oxide, or metal oxide plasma deposition.

8. The process according to claim 6 or 7, wherein the inert gas comprises helium, argon, nitrogen, or carbon dioxide, or a mixture thereof.

. The process according to any one of claims 6 to 8, wherein the oxidizing gas is present and comprises oxygen, nitrous oxide, ozone, nitric oxide, or nitrogen tetraoxide, or a combination thereof.

10. The process according to any one of claims 6 to 9, wherein the working gas is introduced continuously into the multi-wall sheet at a rate from 0.1 mg up to 0.5 g per minute/cm², wherein cm² refers to the cross-sectional area of the multi-wall sheet perpendicular to the direction of the flutes.

1 1. The process according to any one of the preceding claims, wherein the plasma- generating step (c) is carried out by passing the multi-wall sheet through at least one dielectric barrier discharge apparatus comprising one or more electrodes on one side of the multi-wall sheet and one or more counter-electrodes on the other side of the multi- wall sheet.

12. The process according to claim 11, wherein the dielectric barrier discharge apparatus generates a plasma discharge a) at a frequency of at least 10 kHz and not greater than 60 kHz, and b) at a power density of at least 5 W/cm² and not greater than 50 W/cm².

13. The process according to any one of the preceding claims, wherein the multi-wall sheet is passed through the plasma-generating device at a rate of at least 50 cm per minute.

14. The process according to any one of the preceding claims, wherein the plastic comprises at least one polymer selected from the group consisting of polycarbonates, polyurethanes, poly(meth)acrylates, polypropylenes, polyethylenes, ethylene/α-olefin copolymers, styrene-acrylonitrile copolymers, polyethylene terephthalates, and polybutylene terephthalates.

15. A multi-wall sheet obtainable by the process according to any one of the preceding claims.

16. A multi-wall sheet having a plasma-enhanced chemical vapor deposition coating on at least one interior surface of the multi-wall sheet, wherein the coating has an average thickness not greater than 5 μm and a variation in thickness relative to the average thickness not greater than 1 μm.

17. The multi-wall sheet according to claim 16, wherein the coating has an average thickness not greater than 1 μm and a variation in thickness relative to the average thickness not greater than 0.5 μm.

18. The multi-wall sheet according to claim 16 or 17, wherein the multi-wall sheet has a thickness of at least 1 cm.

19. The multi-wall sheet according to any one of claims 16 to 18, wherein the plastic comprises at least one polymer selected from the group consisting of polycarbonates, polyurethanes, poly(meth)acrylates, polypropylenes, polyethylenes, ethylene/α-olefϊn copolymers, styrene-acrylonitrile copolymers, polyethylene terephthalates, and polybutylene terephthalates.

20. The multi-wall sheet according to any one of claims 16 to 19, wherein the coating comprises an organosilicon, silicone oxide or metal oxide, or combination thereof.