A method for forming a composite slab

Abstract

A method for forming a composite slab from a matrix of cured resin and aggregate particles is disclosed. The method includes filling a mould with resin, broadcasting aggregate particles over the resin surface so they settle into and are covered by the resin, and curing without need for mechanical pressing. Curing may occur at ambient or elevated temperatures below 85°C. An apparatus is also provided, including a mould and a broadcaster tower with a height-adjustable hopper for uniform particle discharge. The resulting composite slab has less than 1 wt% crystalline silica.

Description

A METHOD FOR FORMING A COMPOSITE SLABField of the Invention

[0001] The present invention relates to a method for forming a composite slab which is suitable for replacement of engineered stone containing crystalline silica.Background of the Invention

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Engineered stone is a useful material for use in a wide range of applications, typically in the building industry and including benchtops, sinks, tabletops, panels or slabs for other building applications such as walkways and decorative features, jewellery, garden ornaments and sculptures. This is especially so because it is easier to work than natural stone such as marble or granite. Typically, such engineered stone contains silica in the form of crystalline silica. When the engineered stone is worked, particulate crystalline silica is respirable by people nearby and this respirable crystalline silica has been associated with lung disease, in particular through a substantial risk of contracting silicosis or other crystalline silica associated dust related diseases. This risk is noticeably higher than associated with natural stone.

[0004] With silicosis risk in view, a ban on the use, supply and manufacture of engineered stone containing more than 1 % (w / w) crystalline stone has come into effect in Australia from 1 July 2024. Specifically, in Model WHS Regulations supporting the ban engineered stone is defined as an artificial product that: a) contains at least 1 % crystalline silica as a weight / weight concentration; b) is created by combining natural stone materials with other chemical constituents with other chemical constituents (such as water, resins, or pigments); and c) becomes hardened. Excluded from the proposed definition of engineered stone under the Model WHS Regulations are concrete and cement products; bricks, pavers and othersimilar blocks; ceramic wall and floor tiles; sintered stone; porcelain products; roof tiles; grout, mortar, and render; and plasterboard.

[0005] Composite materials which could substitute for engineered stone are currently available. Such composite materials are typically formed by curing a mixture of unsaturated polyester resin and filler particles (such as milled recycled glass particles) under heat and pressure to form a slab in which the milled recycled glass particles are bound together by the polyester resin. One such composite slab is described in Australian Patent Application No. 2020217435 which observes that resins other than polyester resins used to manufacture composite products were found to be problematic and provided inferior properties.

[0006] It is against the above background that the above invention has been developed.Summary of the Disclosure

[0007] In one embodiment, the present invention provides a method for forming a composite slab formed from a matrix of a cured resin and aggregate particles comprising the steps of: (a) filling a mould with a resin; (b) spreading a feed comprising aggregate particles from a distance over a surface of the resin, the distance from the surface and the viscosity of the resin being selected such that aggregate particles settle into, and are covered by, the resin; and (c) curing the resin and aggregate particles to form the composite slab.

[0008] Advantageously, curing can be conducted at ambient temperature without need for a curing oven. Alternatively, curing may be accelerated by curing at higher than ambient temperature, for example in a curing oven. In either case, no mechanical pressing is required.

[0009] The method is preferably conveniently conducted continuously.

[0010] Conveniently, the mould may be filled with a mixture of a resin and a hardener for the resin. In other words, the mould is preferably filled with a two component resin system wherein a first component of the resin system is constituted by the resin and the second component of the resin system is constituted by the hardener. A hardenermay also be termed a catalyst or curing agent. While a range of potential hardeners are available, methyl ethyl ketone peroxide is desirably avoided.

[0011] The resin is desirably an epoxy resin having lower viscosity than a polyester resin. The epoxy resin may include bisphenol resins or a mixture of bisphenol resins such as bisphenol A and bisphenol F epoxy resins. Preferably, the resin viscosity is selected with respect to particle size for the aggregate particles. Resin viscosity is desirably in the range 200 to 400 centipoise which allows, without limitation, settling of aggregate particles having particle size less than 4mm, optionally 0.7 to 3.5 mm, optionally 0.8 to 3.3 mm. A solvent, such as benzyl alcohol, may be mixed with the epoxy resin to adjust viscosity to the desired range.

[0012] The aggregate particles are desirably spread from the discharge of a hopper (or other storage) at a rate controlled to achieve substantially uniform spreading of aggregate particles across a surface of the resin in the mould. The hopper discharge, which may include a tube dispersion system, is located at the selected distance above the surface of the resin, said distance being for example less than 500 mm, preferably less than 300 mm, optionally in the range 200 mm to 300 mm. The hopper and a mounting structure for the hopper may be termed a “broadcaster” or “broadcaster tower”. The spreading of aggregate particles may be termed “broadcasting” or “casting” which differs from the premixing method conventionally used in the art of composite slab manufacture. Discharge of aggregate particles from the hopper may be facilitated by vibration of the hopper at a controlled rate.

[0013] Preferably, the mould is vibrated for a selected duration, preferably in the order of minutes, to promote settling or infiltration of aggregate particles into the resin.

[0014] During vibration, a deaeration step may be conducted to displace entrained air from the resin. Deaeration may involve agitation, desirably of an upper layer of the resin. The resin may also be levelled within the mould as deaeration proceeds. This may be done manually in normal atmospheric conditions.

[0015] Following vibration of the mould for the selected duration, the mould may be directed to a curing station where curing takes place at ambient temperature. Alternatively, curing may be accelerated by directing the mould to a curing oven forhigher than ambient temperature curing. Preferred curing temperature range in either case is less than 85°C, more preferably less than 70°C, optionally between 20°C and 60°C and desirably at ambient temperature, for example 15 to 40°C. Curing time is preferably less than 6 hours, optionally less than 5 hours. Curing under other than ambient or atmospheric pressure is not required. Use of a platen press or other mechanical pressing equipment is not required.

[0016] A range of aggregate particle materials may be suitable for inclusion in the composite slab alone or in mixtures provided that the resin and aggregate particle mixture contains less than 1 wt% crystalline silica. Recycled glass, for example a fine grade crushed glass, may be included as a component of the aggregate particles. Different colours (for example blue, green, yellow and clear) of crushed glass may be employed as aggregate particles. A mixture of different aggregate particles may be used.

[0017] The feed comprising aggregate particles may include a filler material if required to fill any voids left between the aggregate particles and support strength and rigidity of the composite slab. Preferably, aluminium trihydrate (ATH) is included as a filler material for this purpose.

[0018] The feed comprising aggregate particles may comprise at least one of the following additives: wetting agent, dye and pigment whether alone or in admixture. Such additive(s) may also be added directly to the mould.

[0019] In another embodiment, the present invention provides an apparatus for forming a composite slab formed by the method described above.

[0020] The present invention further provides a composite slab or an article formed from a composite slab produced by the method or apparatus as described above.

[0021] Composite slabs as described above desirably include less than 1 wt% crystalline silica and have desirable mechanical and aesthetic qualities. At the same time, significant advantage in production cost and ease of production is achieved by curing at ambient temperature without need for a curing oven; and the absence of vacuum systems and mechanical pressing equipment.

[0022] Further features of the composite slab of the present invention are more fully described in the following description of a non-limiting embodiment thereof which is included solely for the purposes of exemplifying the present invention , but should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above.Description of Embodiments

[0023] In preferred embodiments of the invention, a composite slab is formed from a matrix of a cured resin and aggregate particles by a broadcasting method which contrasts from a mixing method. A mould is filled with a resin and a feed comprising aggregate particles is spread from a selected distance over the surface of the resin. The distance from the surface of the resin and the viscosity of the resin are selected such that aggregate particles settle into, and are covered by, the resin.

[0024] The combination of resin and aggregate particles is then cured to form the composite slab.

[0025] The method is a so-called broadcasting method in which a broadcaster tower or ‘broadcaster’ is provided with a supply of aggregate particles which do not include crystalline silica. Aggregate particles may be provided to the broadcaster in a variety of ways including conveying. In one embodiment, sacks of aggregate particles are hoisted up to the broadcaster to fill a hopper with the discharge of the hopper being disposed a selected distance or height above the mould. In practice, a number of moulds would be used to produce a desired composite slab production rate. The selected distance or height of hopper discharge above resin surface is selected to achieve sufficient potential energy for aggregate particles to sink into and be covered by the resin. Where different size aggregate particles are used - the broadcasting method allowing easy adjustment of aggregate particle mixtures - the selected distance and location of the hopper discharge may be adjusted accordingly.

[0026] The aggregate particles may be mixed in the hopper with one or more filler materials - preferably aluminium trihydrate (ATH) which the Applicant has found to improve slab strength and rigidity. Other additives as known in the art of composite slab manufacture, such as wetting agents and dyes, may be included with theaggregate particles. The aggregate particle mix is tailored to achieve desired aesthetics of the product composite slab.

[0027] In some embodiments, the aggregate particle mixture may be adjusted during a broadcaster run to allow formation of layers having different aggregate particle and filler material composition.

[0028] The mould may be filled with resin by pouring or pumping the resin from a resin dispenser. When the mould is filled to the required level, the broadcaster is activated to operate a drive to cause the hopper to travel back and forth across a length of the mould to spread the aggregate particles evenly across the mould. Such hopper travel may be accompanied by vibration of the hopper to facilitate discharge of aggregate particles. During spreading of aggregate particles, the hopper discharge may be maintained at selected distance, for example 200mm, above the surface of the resin in the mould. This approach contrasts with the premixing of resin and aggregate particles of well known methods of composite slab manufacture.

[0029] The mould may be vibrated to assist settling of aggregate particles into the resin. Vibration may be achieved by operating pneumatic vibro motors, or other suitable vibration means, fixed below the mould. Such vibration may be conducted at a vibration station comprising a vibration table which is disposed away from the broadcaster.

[0030] Following a selected vibration duration, for example 7 minutes, the mould may be transferred - by a suitable conveyor - to a curing station where curing of the resin - as described below - is preferably conducted at ambient temperature, for example in the range 20 to 40°C. However, in alternative embodiments, the curing process may be accelerated by curing in an oven where curing may occur in a 50-70°C temperature range for 4 to 6 hours. In embodiments, curing may have duration of 6 hours at 70°C or potentially 60°C for 4 hours. Mechanical pressure need not be applied with the emphasis being on achieving chemical strength through bonding of resin and aggregate particles and filler material without use of a mechanical press.

[0031] The composite slab may be removed from the mould and subjected to further processing, for example by evening, for example by sanding in a calibration machine,the surface and / or polishing in a polisher as known in the art of composite slab manufacture. Plural polishing steps may be conducted if necessary with 140 mil diamond thicket polishing heads being used in one embodiment. Following such further processing, the composite slabs may be packaged and distributed for sale.

[0032] The method may involve inspection of composite slabs for presence of air holes following exit from the polisher. Air holes may be skim coated with a resin conveniently cured by use of an infrared oven. A second polishing stage is desirable in this embodiment. Inspection can also assess whether binding between resin and aggregate particles is acceptable. Gaps of higher than 0.5mm between resin and aggregate particles - as identified by visual inspection using a magnifying glass - is likely to indicate unacceptable binding and a substantial risk of composite slab failure.

[0033] Water used by the polisher for cooling and lubrication during the polishing process may be cleaned in water treatment equipment and recycled.

[0034] In the preferred embodiments, composite slab manufacture can be conducted without requirement for cool rooms and humidity control. No vacuum is required. The selected epoxy resin system - exemplified below - is less harmful and toxic than polyester resin systems conventionally selected in the art of composite slab manufacture. The composite slab also presents no health issues due to crystalline

[0035] silica which is absent from the aggregate particle mixture used to form the composite slab.Example 1 : Preparation of a composite slab

[0036] A composite slab for use as a benchtop was formed using the method as described above according to the following example wherein the aggregate particles comprised powderised recycled glass with nominal particle size between 0.85 and 3mm. The recycled glass may be clear or coloured dependent on market preference. Aggregate materials other than recycled glass may be employed as known in the art of composite slab manufacture and, in preferred embodiments, would have the same nominal particle size between 0.85 and 3mm. Blends of aggregate materials may be used.

[0037] The aggregate particles were spread over a surface of the resin accommodated within a rectangular steel mould, having depth 20-30mm, from a selected distance or height of 200mm from the broadcaster hopper discharge which is moved over the surface at a controlled speed to ensure even spreading of the aggregate particle mixture. The hopper was vibrated at a controlled vibration rate to facilitate discharge of the aggregate particle mixture.

[0038] While an epoxy resin was selected, a standard laminating epoxy resin is not suitable due to the slab depth (laminating epoxy resins are designed to cure at a shallow depth of 1 to 4mm, not at a depth of 20-30mm) and the requirement for a resin with controlled exothermic curing properties to minimise risk of warping of the composite slab due to high internal temperatures within a 20-30mm deep composite slab. Desirably, the exothermic peak internal temperature would not exceed 50 to 70°C. To this end, a two component solid surface epoxy resin system - previously applied to windscreen repair - was obtained from Trojan Fibreglass, Cardiff, NSW, Australia. The solid surface epoxy resin liquid contains, according to the SDS data sheet, 30-60 wt% 4,4’-(propane-2,2-diyl)diphenol, 10-30 wt% of reactive / non-reactive diluents (in this embodiment a non-reactive diluent is benzyl alcohol) and <50 wt% of a proprietary blend of filler(s) / additive(s). The epoxy resin system includes a mixtu re of bisphenol-A and bisphenol-F epoxy resins.

[0039] Curing of the above epoxy resin at ambient temperature, in the range 20 to 40°C, is initiated by a solid surface epoxy hardener liquid, also available from Trojan Fibreglass. The solid surface epoxy hardener contains, according to the SDS data sheet, 30-60 wt% 5-Amino-1 ,3,3-Trimethyl, Cyclohexanemethanamine and the balance a trade secret formulation.

[0040] The composite slab is suitable as a benchtop comprising smaller aggregate particle size, light texture and neutral colour (where colourless glass aggregate particles are used).

[0041] The composite slab, as formed above, was tested for compliance with the ASTM C97M-18 Standard Test Methods for Absorption and Bulk Specific Gravity of Dimension Stone. Water absorption was between 0.12 and 0.16 vol% (0.06 to 0.08wt%) with an average 0.14±0.09 vol% (0.07±0.0 4wt%) indicating low porosity, good binding and good stain resistance and chemical resistance. Measured specific gravity was 2.03 to 2.04 g / cc.

[0042] The composite slab was found to be stain resistant after 50 hours and UV stabilised with strong aggregate particle-resin binding as assessed visually using a magnifying glass as described above.

[0043] Such composite slab could also be used for other applications. Such a composite slab differs from a terrazzo style surface utilising larger nominal aggregate particle sizes of 6mm to 12mm.Accelerated weathering testing

[0044] Accelerated weathering testing was performed on composite slab samples produced according to the method described above, in order to assess the stability and performance of the slabs under extended ultraviolet (UV) exposure conditions. The testing was carried out in accordance with ASTM-G154 (2012a), using a Q-Lab QUV / SPRAY chamber equipped with UVA-340 lamps to simulate the effects of longterm outdoor weathering. A test duration of 1043 hours was selected, which is commonly used in the industry as an empirical equivalent to approximately 1 to 3 years of outdoor exposure, depending on environmental conditions.

[0045] Ten benchtop composite slab samples, each of differing colour formulations including off white, white, green, mid grey, speckled white, black, dark grey green, and dark grey, were subjected to the weathering test. The samples were placed in the chamber and exposed to repeated cycles comprising 8 hours of UV light at 60°C, followed by 15 minutes of water spray, and 3 hours and 45 minutes of condensation at 50°C. Inspections were conducted at intervals throughout the test, including at 102, 299, 559, 736, and 1043 hours.

[0046] The results demonstrated that the majority of samples exhibited only minor or no visible colour change. Specifically, the green, mid grey, speckled white, dark grey green, and dark grey samples consistently passed the visual inspections, with little to no colour shift or gloss loss noted over the full test period. In contrast, several of the lighter-coloured samples — specifically the off white and white formulations —showed moderate colour change, categorised as anomalous but not reaching failure criteria. These included samples designated Off White 1 , Off White 2, White 1 , White 2, and Black, all of which were classified as showing moderate visual changes evident to the naked eye, though no structural degradation or material breakdown was observed.

[0047] The testing confirmed that the composite slabs formed using the broadcasting method and epoxy resin system described herein maintain acceptable visual and structural integrity under extended UV exposure. The minor to moderate colour shifts in some lighter pigments are consistent with typical behaviour of such formulations under accelerated weathering conditions and do not indicate performance failure. Importantly, the slabs did not exhibit any signs of warping, material delamination, or mechanical degradation, indicating strong durability of the resin-aggregate bond and general UV stability of the composite matrix. This performance further supports the suitability of the described composite slab as a viable alternative to traditional engineered stone for both indoor and, potentially, limited outdoor applications where UV exposure may occur.Comparative Example

[0048] In a comparative embodiment, a composite slab was produced using a polyester resin preferred generally in the art of composite slab manufacture for reasons of cost effectiveness. However, slab performance was inconsistent during the curing process resulting in slab warping / bowing, failed glass transition and failed binding. Performance issues manifested through post curing bowing, where the slab would slowly bow towards the ground, days and weeks after the post curing process had been completed. Stain and chemical resistance testing failed on almost all test composite slabs. This performance was hypothesised to be the result of the lack of glass transition binding between the glass aggregate surface and the polyester resin causing excessive porosity and low strength and rigidity. Additional issues with residual styrene smell and general feel of the slabs rendered the use of polyester resin unsuitable.

[0049] Those skilled in the art of composite slab manufacture will appreciate that the method for forming a composite slab described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0050] Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application or patent cited in this text is not repeated in this text is merely for reasons of conciseness.

[0051] Any manufacturer’s instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention.

[0052] The invention described herein may include one or more range of values (eg. Particle size, distance, temperature and time etc). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range.

[0053] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers

[0054] The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practise the invention. Thus, the foregoing descriptions of specific embodiments ofthe invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed as obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.

Claims

Claims1 . A method for forming a composite slab formed from a matrix of a cured resin and aggregate particles comprising the steps of:(a) filling a mould with a resin;(b) spreading a feed comprising aggregate particles from a distance over a surface of the resin, the distance from the surface and the viscosity of the resin being selected such that aggregate particles settle into, and are covered by, the resin; and(c) curing the resin and aggregate particles to form the composite slab.

2. The method of claim 1 , wherein the curing is conducted at ambient temperature without application of mechanical pressure.

3. The method of claim 1 , wherein the curing is conducted at a temperature above ambient temperature, without application of mechanical pressure.

4. The method of claim 1 , wherein the curing is conducted at a temperature less than 85°C.

5. The method of claim 4, wherein the curing time is less than 6 hours.

6. The method of claim 1 , wherein the mould is filled with a two-component resin system comprising a resin and a hardener.

7. The method of claim 6, wherein the hardener is selected to exclude methyl ethyl ketone peroxide.

8. The method of claim 1 , wherein the resin is an epoxy resin having a viscosity in the range of 200 to 400 centipoise.

9. The method of claim 8, wherein the epoxy resin comprises a bisphenol resin or a mixture of bisphenol A and bisphenol F epoxy resins.

10. The method of claim 8, wherein a solvent is mixed with the epoxy resin to achieve the viscosity in the range.1 1 . The method of claim 1 , wherein the aggregate particles have a particle size between 0.7 mm and 4 mm.

12. The method of claim 1 , wherein the aggregate particles are spread from a discharge point located between 200 mm and 500 mm above the resin surface.

13. The method of claim 1 , wherein the aggregate particles are discharged from a hopper mounted on a broadcaster tower.

14. The method of claim 13, wherein the hopper is vibrated at a controlled rate to facilitate uniform discharge of aggregate particles.

15. The method of claim 1 , further comprising vibrating the mould for a selected duration to promote settling of the aggregate particles into the resin.

16. The method of claim 15, wherein a deaeration step is performed during vibration by agitating an upper layer of the resin to displace entrained air.

17. The method of claim 1 , wherein the resin is levelled within the mould during or after deaeration.

18. The method of claim 1 , wherein the feed comprising aggregate particles includes recycled glass.

19. The method of claim 1 , wherein the feed comprising aggregate particles further comprises a filler material to fill voids and enhance strength and rigidity of the composite slab.

20. The method of claim 19, wherein the filler material comprises aluminium trihydrate.21 . The method of claim 1 , wherein the feed comprising aggregate particles includes at least one of a wetting agent, dye, or pigment.

22. An apparatus for forming a composite slab comprising: a mould for receiving a resin; a broadcaster tower positioned above the mould, the broadcaster tower comprising a hopper configured to store and discharge aggregate particles over a surface of the resin in the mould; wherein the hopper is configured to discharge aggregate particles such that the particles settle into and are covered by the resin.

23. The apparatus of claim 22, wherein the hopper is mounted at a fixed or adjustable height between 200 mm and 500 mm above the mould.

24. The apparatus of claim 22, wherein the hopper is mounted on a motorised track or guide rail configured to move the hopper across a length of the mould to achieve uniform distribution of the aggregate particles.

25. The apparatus of claim 22, wherein the hopper includes a vibration mechanism configured to facilitate controlled discharge of aggregate particles.

26. The apparatus of claim 22, further comprising a vibration table configured to vibrate the mould for a selected duration to promote settling of aggregate particles into the resin.

27. The apparatus of claim 26, wherein the vibration table is positioned downstream of the broadcaster tower in a continuous production line configuration.

28. The apparatus of claim 26, further comprising a curing station positioned downstream of the vibration table, the curing station configured to permit curing of the resin at ambient temperature or at a temperature less than 85°C.

29. A composite slab formed from a matrix of cured resin and aggregate particles, wherein the cured resin comprises an epoxy resin, and the aggregate particles are distributed across a surface of the resin by broadcasting prior to curing such that aggregate particles settle into, and are covered by, the resin .

30. The composite slab of claim 29, wherein the aggregate particles have a particle size between 0.7 mm and 4 mm.31 . The composite slab of claim 29, wherein the resin is cured at a temperature less than 85°C and without application of mechanical pressure.

32. The composite slab of claim 29, wherein the aggregate particles include recycled glass.

33. The composite slab of claim 32, wherein the composite slab contains less than 1 wt% crystalline silica.

34. The composite slab of claim 29, further comprising a filler material located between aggregate particles, the filler material comprising aluminium trihydrate.

35. The composite slab of claim 29, wherein the composite slab has a water absorption value of less than 0.2 vol% and a specific gravity in the range of 2.0 to 2.1 g / cc.

36. The composite slab of claim 29, wherein the composite slab exhibits UV stability after 1043 hours of exposure in accordance with ASTM-G154, with no material degradation and only minor colour change.