Patent Publication Number: US-2006012086-A1

Title: Method for surface treatment of clay, ceramic or cementitious articles

Description:
This invention relates to a method for the surface treatment of clay, ceramic or cementitious articles, particularly roofing, floor and wall tiles, roofing panels and wall cladding panels. The method increases the smoothness, and can increase the density and hardness, of surfaces of such articles, thereby producing a glaze effect and increasing resistance to water penetration and to mould, moss or algae growth.  
     BACKGROUND TO THE INVENTION  
      Tiles for flooring, roofing or wall cladding are commonly made from clay or concrete, but can also be made from cement paste with a high loading of fibres, especially glass fibres. The latter are often formed as panels, larger in area than normal roofing tiles of clay or concrete.  
      Concrete tiles are produced in an extrusion process, wherein an extrudable, concrete mass is extruded as a ribbon and is passed through elements of the manufacturing apparatus which press, mould and cut the sheet into individual roofing tile format. Clay tiles are usually produced in a pressing process, the clay mass being pressed into moulds to form and shape the tiles. After extrusion or pressing, the tiles are then hardened, usually by accelerated curing methods involving heat.  
      The surfaces of conventionally produced tiles tend to be somewhat rough and porous, and susceptible to scratching, especially in the case of concrete tiles. Porosity is undesirable because it affects the surface smoothness and results in water penetration, which carries the risk of degradation of the tile in freeze-thaw conditions, and makes the surface susceptible to moss, mould and algae growth, which is unsightly, and in the case of moss can lead to degradation of the tile. The surfaces of the tile are vulnerable to these adverse effects of water deposited on the weather exposed surface through rainfall, humidity, fog and the like, and on the interior facing surfaces through condensation. To reduce the roughness and porosity, and to improve the appearance of the tiles they are often glazed by either applying silicate frits to the surface and firing at high temperature or by painting with a hardenable lacquer prior to curing.  
      It would therefore be desirable to improve tile processing to reduce surface roughness and porosity, and to impart an aethetically pleasing smooth appearance to the tiles without the necessity and additional expense of a separate lacquering step. Other articles of clay, and concrete, for example pipes, guttering, ornamental panelling and paving and the like would also benefit from such processing improvement. Ceramic articles too, such as wall and floor tiles could also benefit.  
     BRIEF DESCRIPTION OF THE INVENTION  
      The present invention is based on the finding that the surface smoothness, and in many cases the surface density and hardness, of clay, ceramic and cementitious articles may be increased by applying a smooth flexible membrane to an exposed surface of the preformed unhardened article, and vibrating the surface of the article covered by the membrane. Surface vibration has the effect of modifying the packing characteristics of the particles at the surface, increasing the density, and homogeneity of particles in the surface layer of the mass, to a depth which varies according to the composition of the mass, and the frequency, amplitude and duration of the vibration. After vibration, the membrane may be separated, e.g. peeled, from the article prior to or after partial or substantially complete hardening. Preferably, the article is hardened or partially hardened with the membrane in place and the membrane is subsequently peeled from the surface of the article, to expose the smoothed surface.  
      Application of vibration to the surface of uncured concrete mixes has been used and proposed as a means of compacting the mass to varying depths, the compacted layer having a higher density and greater hardness than the uncompacted layer. Such proposals have usually been made in the context of surface compaction and finishing of floors. However, known methods have involved direct contact between the means of vibration and the concrete surface, or casting the uncured mass into a mould which is lined with a smooth membrane and vibrating the mould (GB patent application 2078603A, Japanese patent applications Nos. 2001/019568 and 1125202, Russian patent application no. 2065815 and SU1065209). Vibration of a clay or cementitious surface through a flexible membrane which is in intimate contact with an existing exposed surface of a preformed article does not appear to have been used or proposed.  
      Furthermore, the value of vibrational surface treatment specifically in tile production does not seem to have been recognised despite the long standing availability of vibration techniques in the concrete art generally. For example, the normal way to decrease surface porosity and roughness of tiles has been to apply a glaze, which is expensive and adds an additional step to the manufacturing process.  
     DETAILED DESCRIPTION OF THE INVENTION  
      According to the invention, there is provided a method for the surface treatment of a clay, ceramic or cementitious article comprising (i) providing a hardenable, water-containing clay, ceramic or cementitious mass shaped in the form of the article, then (ii) covering an exposed surface area of the article with a flexible membrane having an upper-surface and a smooth under-surface, such that the latter is in intimate contact with and conforms to the contours of that surface area of the article, thereby providing a membrane-covered area of the article, (iii) vibrating the membrane-covered area of the article, such that vibration is transmitted through the membrane, to the surface of the article, and (iv) either removing the membrane then hardening the article or at least partially hardening the article with the membrane in place.  
      The Mass to be Surface Treated  
      The method of the invention is applied to a hardenable, water-containing clay, ceramic or cementitious mass shaped in the form of the desired article. The article may be pressed or otherwise moulded from clay or ceramics material, or formed from a cementitious mass such as concrete or fibre-loaded cement paste by extrusion, rolling, pressing or a combination of such techniques. The water content of the mass to be treated by the method of the invention is not critical, but is preferably as low as possible, consistent with the shaping and handling requirements of the particular article. Clay and cementitious articles are hardenable at ambient or elevated temperatures, or by microwave irradiation. Ceramic articles are hardenable by firing at high temperatures. Articles to which the invention is particularly applicable include, floor, wall and roofing tiles, as well as roofing and wall cladding panels, and drainage pipes.  
      In the case of concrete tiles, especially roofing tiles, the shaped mass to which the method of the invention is applied will normally be provided by the pre-curing production stages of a conventional tile production process. In such processes, a mouldable, eventually hardenable mass comprising at least water and reactive binder particles, the latter including at least cement particles, is extruded from an extrusion orifice onto conveyor means adapted to carry the extruded mass as a ribbon away from the extrusion orifice. The ribbon has a lower surface in contact with the conveyor means and an upper surface, and is passed under a compacting and smoothing plate (known as a “slipper” or “glitter”), the lower surface of which contacts the upper surface of the ribbon across its width as it is conveyed under the plate by the conveyor means. The plate is positioned such that the extruded ribbon is pinched between the lower plate surface and the conveyor means as it passes under the plate, thereby compacting the ribbon and smoothing its upper surface as it slides in contact with the lower plate surface. The pressed, smoothed ribbon is then cut across its width into individual tile format. Usually, the conveyor means is a conveyor belt provided with a plurality of longitudinally closely adjacent pallets or moulds of individual tile dimensions onto which the ribbon is extruded, and the ribbon is cut into individual tiles across its width between adjacent pallets of moulds.  
      The method of the invention can be applied to conventional concrete tile mixes, based on cement particles, sand and water. However, good results are often obtained when the composition also includes microsilica powder, for example fly ash or silica fume, whose incorporation into the mix may be aided by a surfactant. Fibres of steel, glass or plastics material such as polyethylene may also be included. Best results will generally be obtained when the particle sizes of the cement, sand and microsilica are selected for dense packing, for example where the sand has a volume average particle size in the range 0.1 mm to 10 mm (or where two or more grades of sand are used, each grade has a volume average particle size in that range) and the microsilica powder has a volume average particle size in the range 0.001 μm to 100 μm, (or where two or more grades of microsilica are used, each grade has a volume average particle size in that range). Fibres of length 3 mm to 100 mm are useful for increasing toughness.  
      The Membrane  
      The membrane is applied to an exposed surface of the article, ie a pre-formed surface of the article which is openly accessible to the covering membrane. Surfaces of the article which are in contact with supporting substrates are not exposed surfaces in this sense, nor are surfaces which are formed by casting the article in direct contact with the membrane.  
      The membrane which covers the exposed surface of the shaped mass should be flexible, so that it may be laid in intimate contact with and conforming to the contours of the area of the surface of the article which it is to cover. Air bubbles between the membrane and the article surface are preferably avoided, as are wrinkles in the membrane itself. Steps may be taken, if desired, to reduce the air content of the article before applying the membrane, for example by vibrating the article or by vacuum de-gassing. Generally the membrane should be laid as a skin on the area of the mass to be covered. The under-surface of the membrane in contact with the surface of the clay or cementitious mass should be smooth, since the surface smoothness of the article after vibration in accordance with the invention is in part a function of the smoothness of the membrane undersurface. This follows because vibration causes the particles in the surface layer of the mass to be agitated into increasingly intimate contact with the membrane under-surface, so that the surface characteristics of the article mirror those of the membrane undersurface to a large extent.  
      Preferably, the membrane has low adhesion affinity for the clay or cementitious mass of the article, so that it may eventually be peeled from the article, which has preferably been hardened or partially hardened, without significant damage to the article surface. Flexible, smooth membranes for use in the invention include plastics films, for example of polyethylene or polypropylene, but is some cases metal foils may be suitable. The hardening process for some articles may involve heating in an oven, and in such cases it will of course be desirable to choose a membrane material which is compatible with the hardening temperature and duration, or to separate the membrane from the article prior to exposure to the hardening temperature.  
      For tile production, the surface to be treated in accordance with the invention will normally be the upper surface, i.e. the surface which is visible when the tile is in use, although the invention can also be applied on both surfaces of the tile if required. For roofing tiles, the nose, ie the bottom edge of the tile, is also visible, and the surface of that edge may benefit from treatment. Hence, the membrane may be cut to a size which at least covers the upper tile surface and extends over the edge of the tile to contact the bottom edge surface. In a production process, the membrane covers may be dispensed onto the tiles from a pre-cut stockpile, and applied to the upper surface of the tile forms cut from the extruded ribbon. Alternatively, the membrane may be applied from a continuous supply roll onto the uppersurface of the uncut extruded ribbon, and the indibidual tile forms may then be cut fron the covered ribbon, with the membrane already in place.  
      Surface Vibration Through the Membrane  
      Conveniently, the surface of the article is vibrated through the membrane by pressing into intimate contact an area of the membrane-covered area of the article and a membrane-contact surface of a vibratable plate element contoured to match that of the membrane-covered area of the article which it contacts, and causing the vibratable plate element to vibrate while maintaining pressure contact between it and the membrane-covered area of the article, such that vibration is transmitted from the vibratable plate element, through the membrane, to the surface of the article. Thereafter contact between the vibratable plate element and the membrane-covered surface of the article is broken and the membrane is removed from the article or, preferably, the article is at least partially hardened with the membrane in place.  
      The vibratable plate element is conveniently of sheet metal or relatively rigid plastics such as acrylic plastics, contoured to match the contours of the membrane-covered area of the article which it contacts. Such a sheet metal or plastics plate may be vibrated by contacting a vibrating head element with the side of the plate not in contact with the membrane-covered area of the article, and if necessary causing relative movement between the head element and the contacted plate and membrane-covered area of the article, such that the vibrating head element traverses a desired area of that side. Since most tiles are rectangular in configuration, the vibratable plate element may also be rectangular with uniform transverse cross sectional profile, matching the contours of the upper tile surface In such cases, the vibrating head element may be contoured to match that profile, and the head may be caused to move longitudinally relative to the plate.  
      The axis or main axis of vibration of the vibratable plate may be perpendicular to the plane of the plate, but the vibration may also have components in other directions. Surface improvements are often obtained when vibration of a frequency of at least 150 Hz is transmitted from the vibratable plate element, through the membrane, to the surface of the article. However, the frequency, amplitude and duration of the vibration may vary within wide ranges, as may the pressure applied to the membrane-covered surface of the article by the vibratable plate element during vibration. Optimum parameters will be selected according to such factors as the composition of the mass being treated; the depth to which it is desired to influence the surface of the mass; the degree of surface glaze required on the finished article; and whether the production process for the article is a batch process or a continuous process. Good surface effects are often obtained when the vibratable plate is vibrated at ultrasonic frequencies, for example in the range 15 kHz to 50 kHz, or 20 kHz to 35 kHz, or using a combination of first mechanical vibration for example in the range of 100 Hz to 800 and then vibration at ultrasonic frequency. The amplitude of vibration of the vibratable plate may be in the range 1 mm to 3μ. In one embodiment of the invention, the vibratable plate is alternately vibrated at two or more different frequencies and/or amplitudes.  
      As foreshadowed above, the frequency and amplitude of the vibration of the vibratable plate and the duration of the vibration may be selected to increase the surface density of the article, relative to its density prior to vibration, to a depth of at least 0.5 mm, or at least 1 mm, or at least 2 mm.  
      In the case of the continuous production of concrete tiles referred to above, i.e. by extrusion as a ribbon onto moulds carried on a conveyor belt, followed by cutting between moulds into individual tile format, the speed of production is conventionally relatively high, for example of the order of 100-150 tiles per minute. The speed of a single cycle of membrane application and vibrational surface treatment may be too slow to be performed on each tile sequentially on a single conveyor belt. Hence, in one embodiment of the invention, he conveyor means divides into a plurality of tracks after the ribbon is cut into individual tiles. Tiles queued on the conveyer are successively transported onto separate tracks for the application of the membrane and the vibrational treatment on each tile at individual stations associated with each track. The tracks recombine thereafter to reconstitute the queue of now membrane-covered tiles for transport to hardening.  
      Hardening  
      After vibrational treatment in accordance with the invention, the membrane is removed or, preferably, the article is at least partially hardened with the membrane still in place. The latter is preferable for two main reasons. Firstly, attempting to peel the membrane from the surface of the article immediately after the vibrational treatment may disturb the smoothness of the still unhardened surface to some extent (though this may be minimised by careful removal of the membrane and by choice of membrane and article surface characteristics which minimise adhesion of the membrane to the surface of the article). Surface smoothness damage is increasingly less likely as the article hardens. Secondly, the membrane protects the treated surface from handling damage during or after handling. In fact, it may be desirable in the case of tile manufacture to keep the membrane in place until the point of end use, for this very reason.  
      In the case of tiles formed from Portland cement compositions without curing accelerators, the membrane may stay in place on the tile surface for several hours, eg at least 6 hours, during partial curing of the tiles. At that point the membrane may be peeled from the tiles, or left in place during complete curing and storage of the tiles, only to be removed at the point of end use.  
      Special Effects  
      In accordance with another aspect of the invention, a dry, particle-containing composition may be applied to the surface of the article prior to its being covered by the membrane. The vibrational treatment then causes the particles of that composition to become embedded in the vibrated surface of the article. Particles such as colour pigment, metal, or polymer particles may be incorporated in this way.  
      In another embodiment of the invention, the vibrational treatment may be utilised for the secondary purpose of impressing a pattern on the vibrated surface of the article. For example, a relief-pattern may be formed on the contact surface of the vibratable plate or interposed between the contact surface of the vibratable plate and the membrane-covered area of the article, such that when the vibratable plate is pressed into contact with the membrane-covered area of the article and/or vibrated the relief pattern impresses the surface of the article. Alternatively, or in addition, a relief-pattern may be formed on the under-surface of the membrane, such that when the vibratable plate is pressed into contact with the membrane-covered area of the article, and/or vibrated, the relief pattern impresses the surface of the article. 
    
    
      The principles of the invention will now be further discussed by reference to the following Drawings, wherein  
       FIG. 1  is a perspective view of an assembly of a membrane-covered tile, with a vibratable plate in contact with the membrane, and a vibrator head in pressure contact with the plate.  
       FIG. 2  is a longitudinal cross-sectional view of the assembly of  FIG. 1 .  
       FIG. 3  is a perspective view of a membrane-covered tile mass having an S-profile, in contact with a contoured vibratable plate, and vibrator head.  
       FIG. 4  shows in schematic cross-section how a membrane may be laid on the surface of an extruded ribbon or cut tile form. 
    
    
      Referring to  FIGS. 1 and 2 , an unhardened water-containing clay or cementitious mass  1  is moulded in mould  2  (shown in  FIG. 2 , but omitted for clarity in  FIG. 1 ) into the form of a plain, generally flat roofing tile. Subsequent to the formation of the moulded mass, a membrane in the form of a polyethylene film  3  having a thickness of about 0.1-0.3 mm covers the exposed upper surface of the tile and lies in intimate contact with that surface. To avoid trapped air bubbles or wrinkles in the membrane, the membrane has been smoothed onto the tile surface with the aid of a soft brush, but the precise mode of applying the membrane is not critical provided trapped air and wrinkling is minimised. In many cases, air entrained in the article may be reduced by vibration or degassing prior to membrane application. The membrane has a smooth undersurface in contact with the tile mass  1 , and is sized slightly larger in area than the area of the tile plus perimeter mould wall, with marginal overhangs  4 . A plate of relatively rigid plastic (such as acrylic) or metal (such as steel) sheet  5  about 1 mm thick, of the same area as the upper surface of the tile mass lies on the membrane-covered upper surface of the tile mass, and is pressed into contact therewith by a resiliently mounted vibrator head  6  of the same width as the plate  5 , vibrating at about 20 kHz mainly in the plane perpendicular to the plane of the plate- and membrane-covered tile mass. The vibrator head is movable, while still in pressure contact with the plate- and membrane-covered tile mass, in the direction indicated by arrow A, to traverse the entire length of the plate. As an alternative, the vibrator head could remain stationary while the tile mass passes under it on the production line.  
      After the vibrating head has traversed the length (and thus the area) of the plate  5 , that process being optionally repeated as many times as desired, the head is lifted out of contact with the plate, which in turn is lifted out of contact with the membrane-covered tile mass. The membrane-covered tile mass, still in its mould  2 , is then transported to be at least partially hardened at ambient temperature, or in an oven at a temperature below the melting temperature of the polyethylene membrane  3 . The tile may be demoulded when sufficiently hardened or after full hardening. The membrane may be peeled from the upper surface of the tile after partial hardening and before demoulding, or after hardening and demoulding, or later, at the point of use of the tile.  
      The principles of the invention, illustrated in relation to a flat roofing tile in  FIG. 1 , are equally applicable in the case of a profiled roofing tile as in  FIG. 3 . In  FIG. 3 , an unhardened water-containing cementitious mass  7  is cut from an extruded ribbon and shaped on a mould (not shown) into the form of an S-profiled roofing tile. (In practice the tile would have longitudinal grooves on the underside of edge  8  and corresponding longitudinal mating grooves on the upperside at edge  9 , so that when two adjacent tiles are laid side by side upperside grooves of one interlock with the underside grooves of the other. Likewise, there would be grooves on the underside at the bottom edge of the tile and corresponding interlocking grooves on the upperside at the top edge, to interlock tiles laid one above the other in adjacent courses on a roof. These grooves have been omitted from  FIG. 3  for clarity).  
      As in  FIGS. 1 and 2 , a polyethylene membrane  10  has been applied to the exposed upper surface of the pre-formed uncured tile and lies in intimate contact with that surface. Again the membrane has a smooth undersurface in contact with the tile mass  7 , and is sized slightly larger in area than the area of the tile plus perimeter mould wall. A stiff plate of sheet plastics or steel  11  (shown partially cut away) about 1 mm thick, of the same area as the upper surface of the tile mass, and contoured to match the S-profile of the tile, lies on the membrane-covered upper surface of the tile mass. The plate  11  is pressed into contact with the membrane-covered surface of the tile mass by a resiliently mounted vibrator head  12  of the same width as the plate  11 , which vibrates principally in the direction indicated by arrows  13 . The vibrator head is also contoured to match the S-profile of the plate and, like that of  FIGS. 1 and 2 , is movable longitudinally over the plate while still in pressure contact therewith.  
      As an alternative to the embodiments of  FIGS. 1-3 , the vibrator head could be resiliently mounded for pressure contact with the plate  5  or  11 , and arranged to traverse the plate across its width, rather than along its length as in  FIGS. 1-3 . In that case, the head need not be S-profiled as in  FIG. 3 .  
      In  FIG. 4 , an unhardened tile ribbon or cut tile form  21 , which may be generally flat as in  FIGS. 1 and 2  or profiled as in  FIG. 3 , is being conveyed in the direction of arrow A. A roll of membrane feedstock  22  having a smooth under-surface is positioned to dispense a continuous sheet of membrane material  24  onto and into intimate contact with the surface of the ribbon or cut tile form, via a membrane application roller  23 , contoured as appropriate to match the tile profile. The roller  23  is in slight pressure contact with surface of the ribbon or tile form (although for clarity, the roller and membrane are drawn out of contact with the surface). When the passage of the ribbon or tile form past the application roller has covered the ribbon or tile form surface with membrane, a knife tool (not show) cuts the membrane and tile form from the ribbon or cuts the membrane across the pre-cut tile form to allow the membrane covered tile to pass downstream for the hardening stage. Brushes may be positioned upstream or downstream of the knife tool, to brush the membrane-covered tile surface to encourage intimate bubble- and wrinkle-free contact between membrane and tile surface.