Patent Application
20070235890
The invention provides molds that have coated mold surfaces, as well as methods and components associated with coating. The mold surface coatings may be formed of a polymeric material, such as polytetrafluoroethylene (PTFE), and may be very thin (e.g., 50 microns or less). The coatings may facilitate the release of articles formed in the mold and may also reduce, or eliminate, the build up of contaminants on mold surfaces during processing which, thus, increases mold lifetime. The coatings may be formed in an initiated chemical vapor deposition (i-CVD) process with process conditions selected to enable formation of uniform, conformal coatings, even on mold portions including mold features having small widths and/or high aspect ratios, as described further below. The coatings are particularly well suited to be used in connection with rubber tires molds, though also can be used in other types of molds and other types of articles.
The tire mold has dimensions that provide the desired tire product. For example, the tire mold may be sized to produce tires having a width between about 150 mm and 500 mm; and, in some cases, between 180 mm and 300 mm. The tire mold may have a radius of between 200 mm and 500 mm. The tire mold may be formed of any suitable material used in the art including metals such as aluminum or stainless steel.
In general, the coating may be formed of any suitable polymeric material. Examples of coating materials include polymeric materials such as fluorocarbon polymeric materials (e.g., PTFE), polyoxymethylene, crosslinked siloxanes, vinyl polymers (e.g., methacrylates, acrylates, styrenic polymers) and co-polymers of these materials. Suitable coating material compositions have been described, for example, in U.S. Pat. No. 5,888,591 which is incorporated herein by reference.
In some embodiments, it may be particularly preferred for the coating material to comprise PTFE. PTFE coating materials may be particularly well-suited in providing the above-described advantages including increasing lubricity (e.g., by reducing coefficient of friction of mold surface), enhancing release from the mold surface, reducing the formation of contamination on mold surfaces, enhancing chemical resistance, and lowering surface energy. For example, in these embodiments, the compositional CF2 fraction (i.e., atomic fraction) of the coating material may be at least about 50%; in some cases, at least about 75%; in some cases, at least about 90%; and, in some cases, at least about 95%. In some of these embodiments, the fluorine to carbon ratio (F/C ratio) is between about 1.1/1 to 2.2:1. In some cases, the coating material consists essentially of PTFE, or consists of PTFE. In some embodiments, the coating material compositions (e.g., PTFE compositions) are formed during deposition and do not need to undergo additional steps (e.g., curing) to form the final composition. Thus, these coating materials may be un-cured.
The coating may have any suitable thickness, though in certain embodiments, it is preferable that the coating is very thin. For example, the coating may have a thickness of less than 50 microns. In some embodiments, it is preferable that the coating be even thinner. For example, the coating may have a thickness of less than 25 micron; a thickness of less than 10 micron; a thickness of less than 5 micron; a thickness of less than 2 micron; and, even, a thickness of less than 1 micron. In some cases, it may be preferable for the coating to have a thickness of greater than 10 nm (or greater than 100 nm), for example, to ensure sufficient coverage. It should be understood, however, that other thicknesses may be suitable.
Advantageously, coating methods of the invention, as described further below, can provide coatings that strongly adhere to mold surfaces. Such adherence enhances the ability to coat mold features having small cross-sectional widths and/or high aspect ratios.
In some embodiments, it is preferable that the coating be formed on substantially the entire mold surface area. That is, substantially all of the area of the mold surfaces that defines the mold cavity is coated. However, in certain embodiments, only a portion of the mold surface is coated.
As described further below, methods of the invention enable forming the coating on mold features having small cross-sectional widths. As used herein, a cross-sectional width of a feature is a lateral cross-sectional distance across the portion of the feature between mold surface portions adjacent the feature (i.e., width at h=0). The width is indicated as (w) in
In some embodiments, the mold features may have high aspect ratios which may be in combination with, or separate from, the small cross-sectional widths noted above. As used herein, aspect ratio is defined as the feature height (h) divided by the cross-sectional width (w). Feature height is defined as the vertical distance a feature extends away from a plane defined between mold surface portions adjacent the feature. Feature height is shown as (h) in
A mold feature may include features that deviate from adjacent mold surface portions. For example, the mold features may be an indentation (e.g., hole, groove) that extends into the mold surface, or a protrusion (e.g., post, ridge) that extends upward from the mold surface. In some embodiments, the mold may include both indentations and protrusions.
The mold features having the above-described dimensions may form corresponding features (e.g., portions of a tire tread) on the resulting molded tires. The mold features may also be vent holes which are open to the ambient to release vapor during vulcanization. One aspect of the invention is that the coating may be provided on mold surfaces without clogging the vent holes.
Another aspect of the invention is that the coating may be formed with a relatively constant thickness, even on portions of the mold surface that includes mold features (e.g., indentations or protrusions) having small cross-sectional widths and/or high aspect ratios, and mold surface regions adjacent such features. That is, as shown in
It should be understood that though the embodiment shown in
It should be understood that the present invention is not limited to the coating apparatus shown in
Vacuum chamber 22 may be formed of any suitable material such as stainless steel or aluminum. The dimensions of the vacuum chamber are selected to enable placement of a tire mold having desired dimensions in to the chamber. For example, the vacuum chamber may have a diameter of between about 30 and about 50 inches, though it should be understood that other dimensions are also possible. In the illustrative embodiment, the vacuum chamber has an annular shape. However, in other embodiments, the vacuum chamber may be rectangular, or have other shapes. In addition to the gas inlet and exhaust ports, the vacuum chamber may be equipped with a number of ports for other purposes including providing access for instruments (e.g., thermocouples) and cooling water.
Gas distributor 30 is designed to ensure sufficient distribution of the feed gas around the mold surfaces to be coated. In the illustrative embodiment, the gas distributor has an annular shape. However, other shapes are also be possible. The distributor has a series of small holes in its outer surface through which gas passes. The number and position of the holes is preferably selected so that the flow rate of gas is relatively uniform over the entire area of the gas distributor. The selection of the number and position of holes may depend on process parameters (e.g., temperature and pressure, amongst others), as known to those of ordinary skill in the art. In certain embodiments, the apparatus may include a flow rate controller to provide additional control over the gas flow rate.
Any suitable type of vacuum pump system may be connected to exhaust port 30. For example, the pumping system may include a roots blower and a rotary vane pump. The pumps operate to provide sufficiently low pressure in the vacuum chamber which may be between about 0.01 Torr and 10 Torr (e.g., 1 Torr).
In the embodiment of
The first and second frame portions may be non-planar and lead to the filament array being non-planar. As shown, the first and second frame portions may be curved to conform to the shape of the mold being coated enabling the distance between the filament array (which is also curved) and mold surface to be relatively constant which promotes uniform heating. Thus, in the apparatus of
The pins may be arranged within the first and second frame portions to move back and forth (in the direction of the arrow in
The first and second frame portions may be connected by one or more spring assemblies 47 which include a spring 48 disposed around a rod 49. The first and/or second portions can slide along the rod to increase or decrease the distance between the first and second frame portions over which the filament segments extend to compensate for the expansion/contraction of the filament segments during heating and cooling. Thus, in some embodiments, the spring assembly enables self-adjustment of this distance.
During use, the flow rate of gas may be substantially perpendicular to the filament array. The filament array provides substantially uniform heating conditions to which the reactive gas is exposed.
Though the filament assembly of
It should also be understood that other types of heating sources may be used in connection with the tire mold coating apparatus of
The mold coating apparatus includes supporting device 26 to support the tire mold. In general, any suitable supporting device may be used including clamping devices. In some cases, it may be preferable for the supporting device to be adjustable to accommodate tire molds having different sizes. That is, the supporting device may be designed to support molds having a range of sizes. It may also be preferable for the supporting device to be capable of cooling the mold surface. As described further below, certain methods of the invention involve maintaining the mold surface to be coated at a relatively cool temperature in relation to the temperature of the feed gases. When cooling is desired, the supporting device may be formed of a thermally conductive material (e.g., metals such as aluminum) which can be cooled using conventional techniques. For example, the supporting device may include channels through which cooling fluid flows.
Though the supporting device of
Methods of the invention may be used to form the mold surface coatings described above. The methods involve introducing a gas, or mixture of gases, into the vacuum chamber. The gas(es) is heated to a desired temperature and decomposes (i.e., pyrolyzes) into reactive species (e.g., radicals) that serve as monomer units. The units migrate to the mold surface, which is at a lower temperature, where the units combine and grow into a coating having desired dimensions. It should be understood that the process parameters (e.g., feed gas composition, gas flow rate, vacuum chamber pressure, filament temperature, mold surface temperature,) may be controlled to deposit a coating having the desired characteristics. The following description of the process parameters relates to certain embodiments of the invention, though it should be understood that the invention is not limited in this regard.
Processes of the invention include certain initiated chemical vapor deposition (iCVD) processes. iCVD processes have been described in U.S. Pat. No. 5,888,591 which is incorporated herein by reference. In general, iCVD processes have different process steps than other conventional CVD processes which may involve heating the substrate which is coated and/or a plasma to generate reactive species, amongst other differences. One advantage of the iCVD process is that the process does not involve “line-of-sight” deposition and that, instead, reactive species are free to penetrate and conform to small features. iCVD processes of the invention are also very well suited to form polymeric material coatings, and in particular the PTFE material coatings described above.
The feed gas composition depends on the composition of the coating being deposited. When forming a fluorocarbon polymeric material, such as PTFE, suitable feed gases include those that decompose (or pyrolyze) to form fluorocarbon (e.g., CF2) monomer units. Examples of feed gases that decompose to form CF2 monomer units include C3F6O(HFPO or hexafluoropropylene oxide)C2F4, C3F8, CF3H, CF2H2, CF2N2 (difluordiaxirine), CF3COCF3, CF2ClCOCF2Cl, CF2ClCOCFCl2, CF3COOH, difluorohalomethanes such as CF2Br, CF2HBr, CF2HCl, CF2Cl2 and CF2FCl, difluorocyclopropanes such as C3F6, C3F4H2, C3F2Cl4, C2F3Cl3 and C3F4Cl2, trifluoromethylfluorophosphanes such as (CF3)3 PF3, (CF3)3PF3, and (CF3)PF4; or trifluoromethylphosphino compounds such as (CF3)3P, (CF3)2P—P(CF3)2, (CF3)2PX and CF3PX2, wherein X is F, Cl or H. In some embodiments, it may be preferable to use HFPO feed gas. It is also be possible to use mixtures of the feed gases described above. In some cases, an inert gas (nitrogen, argon) may be added to the feed gas; though, it may be preferable not to add an inert gas to certain feed gases (e.g., HFPO). It should be understood that other feed gases may also be suitable.
The feed gas is introduced into the chamber at a desired flow rate. Suitable flow rates may be between about 0 sccm and 5000 sccm; and, more typically, between about 200 sccm and 5000 sccm. The specific flow rate may depend on a variety of factors including other processing parameters (e.g., chamber pressure), the geometry of the coating apparatus, as well as the desired properties of the coating. During the deposition process, the partial pressure of the feed gas is preferably kept to a sufficiently low level that prevents homogeneous gas-phase reactions, which could form particles in the gaseous environment rather than a coating on the mold surface.
The pressure in the vacuum chamber is generally maintained between about 1 milliTorr and 50 Torr; and, more typically, between about 1 milliTorr and 10 Torr (e.g., about 1 Torr).
In general, the feed gas is heated to a temperature sufficient to decompose (or pyrolyze) to form the desired monomer units. As noted above, a heat source (e.g., a filament) may be used to heat the feed gas. Typical heat source temperatures are between about 200° C. and about 800° C. In some cases, the heat source temperature is greater than about 350° C. It is generally preferable that the temperature of a filament heat source is set to be less than that which causes thermionic emission from the filament.
In general, the mold surface is maintained at a lower temperature than the heat source and the feed gas. Such temperature conditions promote formation of a coating having the desired characteristics. For example, the mold surface may be maintained at a temperature of less than about 200° C., in some cases, less than about 100° C. In some methods, it may be preferable for the temperature of the mold to be between about 10° C. and about 30° C. (e.g., 20° C.) As noted above, the mold surface may be cooled to achieve such temperature conditions.
The reaction conditions are maintained to provide a coating having the desired thickness and other characteristics. The thickness of the coating may be monitored by a sensor placed within the chamber.
In certain methods, it may be preferable to include a post-deposition annealing step. The annealing step may relieve stress in the coating, passivate dangling bonds in the coating, enhance thermal stability of the coating, amongst other advantages. The annealing step may be performed by heating the coating to a temperature between about 50° C. and 400° C. In some methods, the annealing step may be performed in the presence of air or an inert gas (e.g., nitrogen). The annealing step may be conducted in-situ in the coating apparatus.
In some embodiments, it may be preferable to treat the mold surface prior to coating deposition to promote adhesion of the monomer units. Pre-treatment may include exposing the mold surface to a plasma and/or spraying an adhesion promoting layer on the mold surface. In some methods, an adhesion promoting layer can be vapor-deposited in situ in the deposition chamber prior to deposition of the coating (e.g., fluorocarbon). Examples of suitable adhesion promoters include 1H,1H,2H,2H-Perfluorodecyltriethoxysilane; 1H,1H,2H,2H-Perfluorooctyltriethoxysilane; 1H,1H, 2H,2H-Perfluoroalkyltriethoxysilane; perfluorooctyltriclorosilane; and all classes of vinyl silanes. It should be understood that other adhesion promoters known to those skilled in the art may also be suitable. In some embodiments, it may be desirable not to have an adhesion promoting layer and that the coating is deposited directly on the mold surfaces.
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.
