WO2003061937A1

WO2003061937A1 - Acrylic cast sheet and process for its manufacture

Info

Publication number: WO2003061937A1
Application number: PCT/GB2003/000261
Authority: WO
Inventors: John Lynch
Original assignee: Lucite International Uk Limited
Priority date: 2002-01-18
Filing date: 2003-01-20
Publication date: 2003-07-31
Also published as: GB0201137D0

Abstract

A process for manufacturing an acrylic cast sheet having a thickness of greater than or equal to 5 mm, the process comprising: (a) providing a curable mixture comprising a polymerisable matrix having one or more mica particulates dispersed therein, wherein the polymerisable matrix is derived from 60 to 100 wt% methyl methacrylate monomer, 0 to 40 wt% of at least one other copolymerisable alkyl (alk)acrylate comonomer, 0 to 10 wt% of a copolymerisable cross-linking monomer; (b) casting the curable mixture; and (c) curing the curable mixture by heating the mixture at an initial cure temperature of at least 70 °C.

Description

ACRYLIC CAST SHEET AND PROCESS FOR ITS MANUFACTURE

The present invention relates to an acrylic cast sheet, a process for manufacturing an acrylic cast sheet, and an article and a method of producing an article from an acrylic cast sheet. In particular, although not exclusively, the present invention relates to an acrylic cast sheet obtained from a curable acrylic composition, wherein the cast sheet has a surface exhibiting a "honeycomb-like" appearance, especially an appearance comprising a number of lobes.

Polymer products, particularly acrylic products, and methods for their production are known. Such products have found application in the manufacture of flooring, countertops, spas, sanitary ware and other ornamental articles. Suitably, acrylic sheet products for use in spas, sanitary ware and bath ware may be prepared by a cell or continuous casting processes where a curable acrylic solution or syrup comprising a monomer or monomer/polymer mixture, and optionally one or more additives, is cured to form a thermoset sheet. The sheet may subsequently be shaped into the desired article. Although such thermoset sheets may exhibit desirable properties, such as hardness, chemical and abrasive resistance, which render them suitable for use in spas, bath ware and sanitary ware, it may be difficult to increase the rate of curing of such sheets without adversely affecting the aesthetic and mechanical properties of the sheet.

Typically, processes for producing thermoset polymeric sheets, particularly an acrylic cast sheet, require careful control of the reaction conditions and the type and amount of raw materials used. For example, the cure temperature, cure time, initiator levels and viscosity of the polymer syrup typically need to be carefully controlled.

Suitably, although the rate of cure of a cell or continuous casting process may be increased by increasing the cure temperature and/or initiator levels and/or by reducing cure time, increasing the rate of cure may produce a sheet having less desirable mechanical, physical and aesthetic properties. For example, if the cure rate is increased slightly above a threshold value, for example by increasing the cure temperature, eddy currents may be induced within the monomer mixture causing non-uniform polymerisation throughout the monomer mixture, thus producing a sheet having an uneven and unattractive mottled surface. If the cure rate is increased more significantly, the uneven and unattractive mottled surface of the resultant sheet becomes more pronounced. Moreover, the resultant sheet may exhibit undesirable mechanical and physical properties and may break and/or split during curing. The extent of these problems typically increase with increasing thickness of the resultant sheet, particularly if the sheet is greater than approximately 5 mm thick.

Suitably, in order to form an acrylic cast sheet having a thickness of approximately 2 mm having the desired mechanical, physical and aesthetic properties typically the curable acrylic mixture is cured at an initial cure temperature of about 70°C to polymerise substantially all of the curable mixture to form essentially the acrylic cast sheet end product. After which the product is typically subjected to a secondary (post) cure temperature of greater than 100°C to cure the remainder of the curable mixture. In contrast, in order to form an acrylic cast sheet of increased thickness, for example 4 mm thick, with the desired mechanical, physical and aesthetic properties, typically it is necessary to heat the curable mixture at a lower initial cure temperature (e.g.

60 to 65°C) than that used to form a comparable cast sheet having a lower thickness. Suitably, in order to form an acrylic cast sheet of increasing thickness it is typically necessary to decrease the initial cure temperature still further.

Moreover, as the resultant thermoset acrylic sheet is typically not thermoplastic, it is typically not capable of being repeatedly melt processed using techniques such as injection moulding and extrusion. Although it is possible to re-use and re-cycle waste and off-grade material by depolymerising the material, it is typically not economically viable to do so.

Suitably, a cell or continuous casting process typically has a narrow 5 processing window, because increasing the cure rate of the process may result in off-grade material that cannot be used to manufacture articles with satisfactory properties. Thus, it is typically difficult to increase production throughput and reduce cycle times of a cell or continuous casting process beyond established boundaries. These problems are typically particularly 0 pronounced with producing a thick cast sheet (i.e. sheets which have a thickness of greater than or equal to 5 mm) as the initial cure temperatures which need to be employed are lower and the cure times longer than those used to form a comparable thinner (i.e.2 mm thick) cast sheet.

s The present invention seeks to solve the aforementioned technical problems with producing an acrylic cast sheet, particularly by a cell or continuous casting processes.

According to a first aspect, the present invention provides a process for o manufacturing an acrylic cast sheet having a thickness of greater than or equal to 5 mm, the process comprising

(a) providing a curable mixture comprising a polymerisable matrix having one or more mica particulates dispersed therein, wherein the 5 polymerisable matrix is derived from 60 to 100 wt% methyl methacrylate monomer, 0 to 40 wt% of at least one other copolymerisable alkyl (alk)acrylate comonomer, 0 to 10 wt% of a copolymerisable cross-linking monomer;

o (b) casting the curable mixture; and (c) curing the curable mixture by heating the mixture at an initial cure temperature of at least 70°C.

Such a process is referred to hereinafter as the process of the present invention.

Unexpectedly, the inclusion of the one or more mica particulates in the polymerisable matrix permits the polymerisable matrix to be processed at higher rates, for example higher cure rates and higher initial cure temperatures, compared to a comparable polymerisable matrix not including mica particulates, thereby allowing formation of a thick (i.e. greater than or equal to 5 mm thick) acrylic cast sheet having an attractive, substantially smooth surface and desirable mechanical and/or physical properties. Consequently, the process of the present invention typically widens the processing window associated with forming thick acrylic cast sheets. Suitably, the throughput and efficiency of a process for forming an acrylic cast sheet having a thickness of greater than or equal to 5 mm is typically increased using the process of the present invention.

By the term "initial cure temperature", it is meant the temperature at which the curable mixture is initially heated to effect substantial curing of the curable mixture. In other words, the temperature at which the curable mixture is heated so that the curable mixture essentially resembles the ultimate cast sheet end product.

Preferably, the curable mixture is cured by heating at an initial cure temperature of greater than or equal to 80°C, more preferably greater than or equal to 85°C, even more preferably greater than or equal to 90°C, most preferably greater than or equal to 95°C. Preferably the curable mixture is cured by heating at the above defined initial cure temperatures for at least 20 minutes, preferably at least 30 minutes, more preferably at least 60 minutes, even more preferably at least 70 minutes, even more preferably at least 80 minutes, especially at least 120 minutes.

Preferably, after the curable mixture has been heated at the initial cure temperature, the resultant mixture is heated at a secondary (post) cure temperature to effect curing of any residual monomers. Preferably, the resultant mixture is heated at a secondary cure temperature of greater than or equal to 105°C, more preferably greater than or equal to 110°C, even more preferably greater than or equal to 115°C, most preferably approximately 118 to 130°C. Preferably, the resultant mixture is heated at the above secondary cure temperatures for greater than or equal to 20 minutes, preferably greater than or equal to 30 minutes, even more preferably greater than or equal to 40 minutes, even more preferably 50 minutes, especially approximately 60 minutes.

Alternatively, or additionally, the composition of the present invention may be subjected to a different cure cycle by heating at a temperature of at least 105°C, preferably at least 110°C, more preferably 115°C, especially 118°C. Suitably, the composition of the present invention is cured at the above defined temperatures for at least 20 minutes, preferably at least 30 minutes, especially 40 minutes.

The residual monomer contents of the acrylic cast sheet is typically less than 1 wt% (based on the total weight of the monomers and monomer units present) preferably less than 0.5 wt%.

Suitably, the curable mixture may be cast by continuous or cell casting. For example, the curable mixture may be cast in conventional moulds i.e. between glass sheets, or the curable mixture may be cast on a continuous conveyor belt by methods well known to those skilled in the art. Suitably, the curable mixture is cured using methods well known to those skilled in the art, for example using a heated water bath, a polymerisation oven or pressurised polymerisation oven.

Suitably, the polymerisable matrix after curing may comprise homopolymers and copolymers of methyl methacrylate. Suitably, the homopolymers and copolymers of methyl methacrylate in the polymerised matrix are linear (co)polymers which are optionally cross-linked. In other words, the polymerised matrix is preferably not a core-shell type polymer.

As used herein, the term "alkyl (alk)acrylate" refers to the corresponding acrylate ester or alkacrylate ester, which are usually formed from the corresponding acrylic or alkacrylic acids, respectively. In other words, the term "alkyl (alk)acrylate" refers to either an alkyl alkacrylate or an alkyl acrylate.

Preferably, the one or more alkyl (alk)acrylates is a (Cι-C₂₂)alkyl ((Cr Cιo)alk)acrylate. Examples of C1-C₂₂ alkyl groups of the alkyl (alk)acrylates include methyl, ethyl, n-propyl, n-butyl, iso-butyl, tert-butyl, iso-propyl, pentyl, hexyl, cyclohexyl, 2-ethyl hexyl, heptyl, octyl, nonyl, decyl, isodecyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, behenyl, and isomers thereof. The alkyl group may be straight or branched chain. Preferably, the (Cι-C₂₂)alkyl group represents a (Cι-Cβ)alkyl group as defined above, more preferably a (d-CβJalkyl group as defined above, even more preferably a (Cι-C4)alkyl group as defined above. Examples of C-_MO alk groups of the alkyl (alk)acrylate include methyl, ethyl, n- propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, pentyl, hexyl, cyclohexyl, 2-ethyl hexyl, heptyl, octyl, nonyl, decyl and isomers thereof. The alk groups may be straight or branched chain. Preferably, the (C-ι-Cιo)alk group represents a (C-i- C₆)alk group as defined above, more preferably a (C₁-C4) alk group as defined above. Preferably, the one or more alkyl (alk)acrylates is a (Cι-Cβ)alkyl ((d- C )alk)acrylate, even more preferably a (Cι-Cβ)alkyl (meth)acrylate, most preferably a (Cι-C4)alkyl methacrylate . It will be appreciated that the term (d- Cβ)alkyl (meth)acrylate refers to either (CrC₈)alkyl acrylate or (CrC₈)alkyl methacrylate. Examples of (Cι-Cβ)alkyl (meth)acrylate include the alkyl methacrylates: methyl methacrylate (MMA), ethyl methacrylate (EMA), n-propyl methacrylate (PMA), isopropyl methacrylate (IPMA), n-butyl methacrylate (BMA), isobutyl methacrylate (IBMA), tert-butyl methacrylate (TBMA), octyl methacrylate (OMA) and the alkyl acrylates: methyl acrylate (MA), ethyl acrylate (EA), n-propyl acrylate (PA), n-butyl acrylate (BA), isopropyl acrylate (IPA), isobutyl acrylate (IBA), tert-butyl acrylate (TBA), octyl acrylate (OA) and combinations thereof.

Preferably, where the polymerisable matrix includes at least one copolymerisable alkyl (alk)acrylate comonomer, the copolymerisable alkyl (alk)acrylate comonomer comprises at least one alkyl acrylate, preferably at least one (Ci-Cβ) alkyl acrylate, as defined herein, especially ethyl and/or butyl acrylate and/or octyl acrylate and isomers thereof.

Preferably, the polymerisable matrix is derived from greater than or equal to 60 wt%, more preferably greater than or equal to 75 wt%, even more preferably greater than or equal to 80 wt%, even more preferably greater than or equal to 85 wt%, more preferably greater than or equal to 90 wt%, even more preferably greater than or equal to 95 wt%, especially greater than or equal to 98 wt% methyl methacrylate monomers based on the total weight of the polymerisable matrix. In a particularly preferred embodiment the polymerisable matrix is derived from essentially methyl methacrylate monomers, so that after curing a methyl methacrylate homopolymer (optionally cross-linked) is formed.

Preferably, where the polymerisable matrix is derived from at least one other copolymerisable alkyl (alk)acrylate comonomer as defined herein, the polymerisable matrix is derived from less than 98 wt%, suitably less than 95 wt%, suitably less than 92 wt%, more preferably less than 88 wt% methyl methacrylate monomer based on the total weight of the polymerisable matrix. Preferably, where the polymerisable matrix is derived from at least one other copolymerisable alkyl (alk)acrylate comonomer as defined herein, the polymerisable matrix is derived from at least 80 wt%, preferably at least 82 wt%, more preferably at least 84 wt%, especially at least 85 wt% methyl methacrylate monomer based on the total weight of the polymer.

Preferably, where the polymerisable matrix is derived from at least one copolymerisable alkyl (alk)acrylate comonomer as defined herein, the polymerisable matrix is derived from at least 2 wt%, preferably at least 5 wt%, preferably at least 8 wt%, more preferably at least 12 wt% of at least one alkyl

(alk)acrylate comonomer, as defined herein, based on the total weight of the polymerisable matrix. Preferably, where the polymerisable matrix is derived from at least one copolymerisable alkyl (alk)acrylate comonomer as defined herein, the polymerisable matrix is derived from less than 20 wt%, preferably less than 18 wt%, more preferably less than 16 wt%, especially less than 15 wt% of at least one alkyl (alk)acrylate comonomer, as defined herein, based on the total weight of the polymerisable matrix. Preferably the at least one alkyl (alk)acrylates comonomer is an alkyl acrylate, especially a (CrC₈)alkyl acrylate as defined hereinbefore.

Preferably, where the polymerisable matrix is derived from at least one copolymerisable alkyl (alk)acrylate comonomer, the polymerisable matrix is derived from a single alkyl acrylate as defined herein, preferably a (Cι-Cβ)alkyl acrylate, especially ethyl acrylate, butyl acrylate or octyl acrylate and isomers thereof.

Thus according to a further preferred embodiment the polymerisable matrix is derived from methyl methacrylate monomer and at least one other copolymerisable alkyl (alk)acrylate as defined herein, so that after curing a methyl methacrylate/alkyl (alk)acrylate copolymer (optionally cross-linked) is formed.

Preferably, the polymerisable matrix includes a copolymerisable cross-linking monomer. Preferably, the polymerisable matrix includes at least 0.01 wt%, preferably at least 0.03 wt%, preferably at least 0.05 wt%, preferably at least 0.08 wt%, preferably at least 0.1 wt%, preferably at least 0.2 wt%, preferably at least 0.3 wt% of a copolymerisable cross-linking monomer based on the total weight of the polymerisable matrix. Preferably, the polymerisable matrix includes less than 10 wt%, suitably less than 5 wt%, preferably less than 4 wt%, more preferably less than 2 wt%, especially less than 1 wt% of a copolymerisable cross-linking monomer based on the total weight of the polymerisable matrix.

Preferably, the copolymerisable cross-linking monomer comprises a difunctional (alk)acrylate. Suitable copolymerisable cross-linking monomers include, for example, ethylene glycol dimethacrylate, polyethylene glycol diacrylate and dimethacrylate, propylene glycol dimethacrylate and diacrylate, glycidyl methacrylate and divinyl benzene. A preferred copolymerisable cross- linking monomer is ethylene glycol dimethacrylate.

Suitably, the polymerisable matrix of the composition of the present invention includes at least one free-radical initiator. Suitable free radical initiators include peroxy, hydroperoxy and azo initiators, for example, azo-bis(isobutyronitrile) (AIBN), azo-bis(α-methylbutyronitrile), acetyl peroxide, benzoyl peroxide. Preferably, the polymerisable matrix includes at least 0.01 wt%, more preferably at least 0.02 wt%, most preferably at least 0.04 wt% free-radical initiator based on the total weight of the polymerisable matrix. Preferably, the polymerisable matrix includes less than 5 wt%, more preferably less than 2 wt%, most preferably less than 1 wt%, especially less than 0.5 wt% free- radical initiator based on the total weight of the polymerisable matrix. Suitably, the balance of the polymerisable matrix may contain one or more adjuvants, such as dyes, antioxidants, ultraviolet stabilisers, processing aids (such as lubricants and mould release agents), flame retardants and viscosity controlling materials. Preferably, the polymerisable matrix includes less than 5 wt%, more preferably less than 3 wt%, most preferably less than 2 wt%, of one or more of such adjuvants.

Preferably, the polymerisable matrix is derived from a major amount of the total amount of methyl methacrylate and at least one other copolymerisable alkyl (alk)acrylate comonomer as defined herein when present.

In the context of this specification a major amount refers to at least 80 wt%, more preferably at least 85 wt%, more preferably at least 90 wt%, preferably at least 95 wt%, preferably at least 97 wt%, especially at least 98 wt%.

Preferably, the polymerisable matrix, as defined herein, is in the form of a solution or syrup having the one or more mica particulates dispersed therein. Suitably, the polymerisable matrix comprises partially polymerised methyl methacrylate and/or alkyl (alk)acrylate. Suitably, the polymerisable matrix may comprise poly(methyl methacrylate) in methyl methacrylate monomer and/or at least one copolymerisable alkyl (alk)acrylate monomer. Alternatively, the polymerisable matrix may comprise at least one poly(alkyl (alk)acrylate) in methyl methacrylate monomer and, optionally, at least one copolymerisable alkyl (alk)acrylate monomer. Still further, the polymerisable matrix may comprise poly (methyl methacrylate/alkyl (alk)acrylate) copolymer in methyl methacrylate monomer and/or at least one copolymerisable alkyl (alk)acrylate comonomer. Suitably, the syrup may be prepared by partial polymerisation as defined herein. Alternatively, the syrup may be formed by dissolving a commercially available polymer (e.g. PMMA or a PMMA/alkyl (alk)acrylate copolymer) in a methyl methacrylate monomer and/or at least one other copolymerisation alkyl (alk)acrylate comonomer. Suitably, the polymerisable matrix comprises polymethyl methacrylate (PMMA) solids and an alkyl (alk)acrylate monomer as defined hereinbefore, such as methyl methacrylate and alkyl acrylate comonomers, and optionally copolymerisable cross-linking monomers, initiators and/or one or more adjuvants as described hereinbefore. Suitably, the syrup is prepared by subjecting methyl methacrylate, and/or the alkyl (alk)acrylate comonomer, by conventional partial polymerisation processes as exemplified in US Patent 4152501. After which, further methyl methacrylate monomer and optionally alkyl (alk)acrylate comonomer as defined herein is typically added to the partially polymerised syrup in a ratio of about 1 :1 to about 1:5, preferably 1:3 (syrup to monomer). Optionally copolymerisable cross-linking monomers, initiators and/or one or more adjuvants as described hereinbefore are added to form a polymerisable matrix as defined herein.

Preferably, the viscosity (pour viscosity) of the polymerisable matrix is less than or equal to 1200 centipoises at 25°C, preferably less than or equal to 1000 centipoises at 25°C, more preferably less than or equal to 800 centipoises at 25°C.

Preferably, the viscosity (pour viscosity) of the polymerisable matrix is greater than or equal to 250 centipoises at 25°C, preferably greater than or equal to 300 centipoises at 25°C, preferably greater than or equal to 350 centipoises at 25°C, more preferably greater than or equal to 400 centipoises at 25°C.

Most preferably, the viscosity (pour viscosity) of the polymerisable matrix is greater than or equal to 500 and less than or equal to 700 centipoises at 25°C. Suitably, the viscosity (pour viscosity) of the polymerisable matrix may be controlled within the above limits by altering the polymer to monomer ratio in the polymerisable matrix. Suitably, the viscosity (pour viscosity) of the polymerisable matrix may be measured by techniques well known to those skilled in the art, for example using a Brookfield viscometer fitted with the appropriate spindle. Preferably, the ultimate end product, the acrylic cast sheet, comprises greater than or equal to 80 wt%, preferably greater than or equal to 85 wt%, preferably greater than or equal to 98 wt% of the polymerised matrix as defined hereinbefore.

Preferably, the ultimate end product, the acrylic cast sheet, comprises less than or equal to 99.7 wt%, preferably less than or equal to 99.6 wt%, preferably less than or equal to 99.4 wt%, preferably less than or equal to 99.3 wt% of the polymerised matrix as defined hereinbefore.

Preferably, the curable mixture and the ultimate end product, the acrylic cast sheet, comprises less than or equal to 10 wt%, preferably less than or equal to 5 wt%, preferably less than or equal to 2 wt% of the one or more mica particulates.

Preferably, the curable mixture and the ultimate end product, the acrylic cast sheet, comprises greater than or equal to 0.3 wt%, preferably greater than or equal to 0.4 wt%, preferably greater than or equal to 0.6 wt%, more preferably greater than or equal to 0.7 wt% of the one or more mica particulates.

Suitable mica particulates include muscovite, paragonite, margarite, and celadonite. An especially preferred mica is muscovite.

Preferably, the one or more mica particulates includes an inorganic pigment.

Suitably, the inorganic pigment comprises a metallic oxide pigment. Preferred metallic oxide pigments are commercially available and comprise oxides of cobalt, chromium, iron, aluminium, zinc, titanium, nickel, manganese and antimony. Various combinations of the aforementioned metallic oxide pigments may be blended together to produce a wide range of colours. Preferably, the inorganic pigment comprises an oxide of titanium, especially titanium dioxide, and/or an iron oxide. Preferably, the one or more mica particulates is coated with the inorganic pigment as described above. Preferably, the one or more mica particulates is totally coated with the inorganic pigment.

Suitable inorganic particulates for use in the present invention comprising mica coated with metal oxide pigments are available from Merck and Co. and sold under the trade names Iriodin. Preferred Iriodins include Iriodin 103, Iriodin 111, Iriodin 600, Iriodin 225, Iriodin 219 and Iriodin 225.

Preferably, the density at 25°C of the one or more mica particulates is greater than or equal to 1.9 gem^"3, preferably greater than or equal to 2.0 gem^"3, preferably greater than or equal to 2.2 gem^"3, more preferably greater than or equal to 2.4 gem^"3, most preferably greater than or equal to 2.5 gem^"3. Preferably, the density of the one or more mica particulates at 25°C is less than or equal to 4.0 gem^"3, preferably less than or equal to 3.8 gem^"3, preferably less than or equal to 3.6 gem^"3, most preferably less than or equal to 3.4 gem^"3.

Preferably, the bulk density at 25°C of the one or more mica particulates is 200 to 450 kg/m³ more preferably 250 to 350 kg/m³, most preferably 280 to 340 kg/m³.

Preferably, a major amount, more preferably essentially all, of the one or more mica particulates have a maximum dimension of 500 μm, more preferably 350 μm, more preferably 300 μm, most preferably 250 μm. Suitably, a major amount, more preferably essentially all, of the one of more mica particulates have a maximum dimension of less than or equal to 500 μm, more preferably less than or equal to 350 μm, even more preferably less than or equal to 300 μm, even more preferably less than or equal to 250 μm, most preferably less than or equal to 200 μm. Suitably, the one or more mica particulates are in the form of flakes or platelets. Suitably, the term "maximum dimension" means the largest dimension measurable across the major surface from one side of the mica particulate to an opposite side of the mica particulate. Such a dimension may, for example, represent the length of a diagonal across the major surface of the particulate from one corner of the mica particulate to an opposite corner of the mica particulate. Suitably, the "maximum dimension" may be measured by techniques well known to those skilled in the art, for example electron microscopy.

Preferably, a major amount, more preferably essentially all, of the one or more mica particulates have a minimum dimension of 5 μm, more preferably 10 μm. Suitably, a major amount, more preferably essentially all, of the one or more mica particulates have a minimum dimension of greater than or equal to 5 μm, more preferably greater than or equal to 10 μm. Suitably, the term "minimum dimension" means the smallest dimension measurable across a surface of the particulate from one side of the particulate to an opposite side of the particulate. Typically, the minimum dimension represents the thickness of the particulate. Suitably, the minimum dimension of the one or more mica particulates may be measured by electron microscopy.

The scope of particle size distribution of the one or more mica particulates which may be used in the present invention is preferably between 5 to 350 μm.

Preferably, the aspect ratio (i.e. maximum dimension to minimum dimension) of a major amount, more preferably essentially all, of the mica particulates is greater than or equal to 50:1, suitably greater than or equal to 70:1, suitably greater than or equal to 80:1.

Suitably, the weight average particle size of the one or more mica particulates is in the range of 2 to 300 μm, preferably 5 to 250 μm, more preferably 5 to 200 μm. Particularly preferred weight average particle size ranges of the one or more mica particulates include 10 to 50 μm, 10 to 60 μm, and 40 to 200 μm.

Conveniently, the cast acrylic sheet formed by the process of the present invention typically has at least one smooth surface of high gloss, particularly a major surface, that does not break or split during the curing process. By the term "smooth" we mean the surface essentially does not include pits or fissures which are visible to the naked eye and it is essentially smooth to human touch. Moreover, at least one surface of the sheet, particularly a major surface, typically exhibits an attractive three-dimensional appearance comprising a number of metallic lobes separated by crevices, rather than an unattractive mottled appearance. Suitably, the acrylic cast sheet may be formed into other articles, such as bath ware, spas and sanitary ware by methods well known to those skilled in the art, such as thermoforming.

Although only theory, it is believed that eddy currents develop within the monomer mixture of the polymerisable matrix during curing due to the increased cure rate e.g. increased initial cure temperature. It is believed that one or more factors relating to the type of mica particulate, size of mica particulate, aspect ratio of the mica particulate, density of the mica particulate, amount of mica particulate in the polymerisable matrix and/or the viscosity of the polymerisable matrix permit the mica particulate to rise to and align with the upper surface of the polymerisable matrix. In particular, it is believed that the combination of one or more factors of the size and density of the mica particulate and viscosity of the polymerisable matrix may permit the mica particulate to rise within the polymerisable matrix, and the aspect ratio of the mica particulate may permit horizontal alignment of the particulate with the upper face of the polymerisable matrix. Consequently, it is believed the eddy currents generated in the monomer mixture may be frozen into the cured material. Suitably, an acrylic cast sheet may be formed having the desired mechanical and physical properties for further processing, such as thermoforming. Moreover, at least one surface, typically the upper show face, of the sheet may be smooth, have a high gloss, and exhibit an attractive three dimensional appearance comprising a number of metallic lobes.

Preferably, the acrylic cast sheet formed by the process of the present invention exhibits an essentially uniform thickness extending across the entire sheet. However, it will be appreciated that a sheet of non-uniform thickness (for example a wedge-shaped sheet) may be formed by the process of the present invention. Suitably, when a sheet of non-uniform thickness is produced, the thickness of the sheet as defined herein refers to the thickness when measured at the thickest part of the sheet.

Preferably, the acrylic cast sheet is at least 5.5 mm thick, more preferably at least 7 mm thick, especially at least 8 mm thick. Preferably, the acrylic cast sheet is less than or equal to 12 mm thick, more preferably less than or equal to 10 mm thick. Most preferably, the acrylic cast sheet is between 8 to 10 mm thick.

Thus, in accordance with a second aspect of the present invention, there is provided an acrylic cast sheet obtainable by the process of the present invention as defined herein.

Thus according to a third aspect the present invention provides an acrylic cast sheet having a thickness of greater than or equal to 5 mm comprising a polymer matrix having one or more mica particulates as defined herein dispersed therein, wherein the polymer matrix comprises a homopolymer or copolymer derived from a monomer mixture comprising 60 to 100 wt% of methyl methacrylate, 0 to 40 wt% of at least one other copolymerisable alkyl (alk)acrylate comonomer as defined herein and 0 to 10 wt% of a copolymerisable cross-linking monomer as defined herein.

For the avoidance of doubt, the preferred features of the polymerisable matrix as defined hereinbefore are to be regarded as preferred features of the polymer matrix of the acrylic cast sheet (i.e. after polymerisation of the polymerisable matrix).

Preferably, as mentioned hereinbefore, at least one surface of the cast acrylic sheet, particularly at least one or more major surfaces of the sheet, is a smooth surface, particularly a smooth surface of high gloss. Preferably, the cast acrylic sheet is essentially opaque.

Suitably, when a surface, particularly the major surface, of the cast acrylic sheet is tested for impact resistance in accordance with ISO 179 (1993), the unnotched Charpy resistance is approximately 12 kJm^"2.

Suitably, at least one surface, particularly at least one or more major surfaces, of the acrylic cast sheet exhibits an attractive three-dimensional appearance comprising a number of lobes separated by crevices. Suitably, the lobes exhibit a metallic appearance. Preferably, only one of the major surfaces, the "show face", of the acrylic cast sheet exhibits such an attractive appearance.

Unexpectedly, it has been found that an acrylic cast sheet which is at least 5 mm thick having desired aesthetic and mechanical properties may not only be formed by the process of the present invention without the disadvantageous effects of mottling, splitting and cracking but also the cast sheet may be formed at increased cure rates.

Suitably, the acrylic cast sheet may be shaped by techniques well known to those skilled in the art. For example, the sheet, may be thermoformed or vacuum formed into products for use in bath ware, sanitary ware and spas. Suitably, the acrylic cast sheet may be formed into a laminate comprising a substrate and the acrylic cast sheet. Preferably, the substrate is a thermoplastic polymer well known to those skilled in the art, for example an acrylic polymer or PVC. Preferably, the substrate is transparent. Suitably, the substrate is extruded or bonded onto the acrylic cast sheet, preferably the show face of the sheet, by techniques well known to those skilled in the art.

Thus according to a fourth aspect, the present invention provides a component for use in bath ware, sanitary ware or spas comprising an acrylic cast sheet as defined herein.

Thus according to a fifth aspect, the present invention provides a laminate as defined herein comprising a substrate as defined herein and an acrylic cast sheet as defined herein.

According to a sixth aspect, the present invention provides an acrylic cast sheet as defined herein having at least one smooth surface exhibiting an appearance comprising a number of lobes.

According to a seventh aspect, the present invention provides the use of one or more mica particulates as defined herein for forming an acrylic cast sheet as defined herein having a smooth surface exhibiting an appearance comprising a number of lobes.

For the avoidance of doubt the features of the first, second, third, fourth, fifth, sixth and seventh aspect of the present invention, respectively, are regarded as preferred features of the other aspects of the present invention.

The present invention will now be illustrated by way of example with reference to the following drawings wherein:

Figure 1 is a photograph of an acrylic sheet of the present invention having a smooth surface exhibiting the appearance of lobes.

The following materials are obtainable from the following suppliers: Azo-bis-(isobutryronitrile) - AIBN - supplied by Atofina.

Octadecyl 2-(4-hydroxy-3,5-di-t-butyl phenyl)ethanoate a heat stabiliser supplied by Ciba Geigy under the trade name Irganox 1076.

Di-octyl sodium sulphosuccinate a release agent supplied by Citec under the trade name Aerosol OT.

Ethylene glycol dimethacrylate (EGMA) a cross-linker supplied by Aldrich

Chemical Company.

Titanium dioxide coated muscovite mica supplied by Merck and Co. under the trade name Iriodin 103. Iriodin 299 coated mica supplied by Merck and Co.

Iriodin 255 coated mica supplied by Merck and Co.

Iriodin 600 coated mica supplied by Merck and Co.

Iriodin 303 coated mica supplied by Merck and Co.

Titanium dioxide coated mica supplied by Merck and Co. under the trade name Iriodin 111.

Example 1

A syrup of partially polymerised methyl methacrylate is prepared by charging methyl methacrylate and AIBN (0.0019 wt%) into a stirred vessel, and heating the resultant mixture at 50°C (preferably 80°C) for 1 hour. The syrup is cooled to room temperature. After which Irganox 1076 (0.05 wt%), Aerosol OT (0.05 wt%), EGMA (0.1 wt%), Iriodin 103 (0.6 wt%) and AIBN (0.03 wt%) is stirred into the syrup at room temperature.

The resultant mixture is poured into a glass plate mould, comprising glass plates, having a depth of 8 mm, length of 900 mm and width of 400 mm. The mould is placed in an oven and heated at 90°C for 80 minutes, followed by heating at 120°C for 50 minutes. The mixture is then cooled at 1°C per minute to room temperature. The resultant product, comprising a cured sheet of length

880 mm, width 380 mm and depth 6.5 mm is removed from the mould. Both of the major surfaces of the sheet are smooth and free from cracks, splits and distortions. As shown in Figure 1, the upper surface of the sheet e.g. the show face, exhibits a high gloss and an appearance comprising a number of three-dimensional metallic lobes separated by crevices.

Example 2

A syrup of partially polymerised methyl methacrylate is prepared by charging methyl methacrylate and AIBN (0.0019 wt%) into a stirred vessel, and heating the resultant mixture at 80°C for 1 hour. The syrup is cooled to room temperature to produce a polymerisable matrix having a pour viscosity of approximately 600 centipoises at 25°C. After which Aersol OT (0.05 wt%), EGMA (0.1 wt%), AIBN (0.03 wt%) and Iriodin 299 (0.225 wt%) is stirred into the syrup.

The resultant curable mixture is poured into a glass plate mould having a depth of 10 mm, length of 900 mm and width of 400 mm. The mould is placed in an oven and heated at 90°C for 150 minutes, followed by heating at 120°C for 60 minutes. The mixture is then cooled at 1°C per minute to room temperature. The resultant acrylic cast sheet of length 880 mm, width 380 mm and depth 8.5 mm is removed from the mould.

Both of the major surfaces of the sheet are smooth and free from cracks, splits and distortions. The upper surface of the sheet e.g. the show face, exhibits a high gloss and an appearance comprising a number of three-dimensional lobes separated by crevices. The sheet is essentially opaque.

Examples 3 to 5

Example 2 was repeated employing different Iriodins and in the case of Examples 3 and 5 an additional pigment was also included. The formulation details are set out below:

* CU 88 masterbatch dispersion available from Lucite International UK Limited.

** CT 67 masterbatch dispersion available from Lucite International UK Limited.

Both of the major surfaces of the acrylic cast sheets produced from Examples 3 to 5 were smooth and free from cracks, splits and distortions. All of the sheets were essentially opaque.

The show face of the acrylic cast sheet of Example 3 exhibited a high gloss and an appearance comprising three-dimensional dark blue lobes on a light blue background.

The show face of the acrylic cast sheet of Example 4 exhibits a high gloss and an appearance comprising three-dimensional gold lobes on a white background. The show face of the acrylic cast sheet of Example 5 was similar to that of Example 1 and exhibited a high gloss and an appearance comprising a number of three-dimensional metallic lobes separated by crevices.

Examples 6 to 9

Examples 2 to 5 were repeated with a polymerisable syrup comprising 15% by weight poly(methyl methacrylate), 12% by weight n-butyl acrylate, and 73% by weight methyl methacrylate monomer.

The polymerisable mixtures were subjected to the same cure cycle to produce an acrylic cast sheet comprising a poly(methyl methacrylate/n-butyl acrylate) (88:12) copolymer matrix. The acrylic cast sheet of Examples 6 to 9 exhibited similar mechanical and aesthetic appearance to the acrylic cast sheets produced by Examples 2 to 5, respectively.

Example 10

A syrup of partially polymerised methyl methacrylate is prepared by charging methyl methacrylate and AIBN (0.0019 wt%) into a stirred vessel, and heating the resultant mixture at 80°C for 1 hour. The syrup is cooled to room temperature to produce a polymerisable matrix having a pour viscosity of approximately 650 centipoises at 25°C. After which Aersol OT (0.05 wt%), AIBN (0.02 wt%) and Iriodin 111 (0.6 wt%) is stirred into the syrup.

The resultant curable mixture is poured into a glass plate mould having a depth of 12 mm, length of 900 mm and width of 400 mm. The mould is placed in an oven and heated at 90°C for 150 minutes, followed by heating at 120°C for 60 minutes. The mixture is then cooled at 1°C per minute to room temperature. The resultant acrylic cast sheet of length 880 mm, width 380 mm and depth 10.5 mm is removed from the mould. Both of the major surfaces of the sheet are smooth and free from cracks, splits and distortions. The upper surface of the sheet e.g. the show face, exhibits a high gloss and an appearance comprising a number of three-dimensional metallic lobes separated by crevices, as shown in Figure 1.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A process for manufacturing an acrylic cast sheet having a thickness of greater than or equal to 5 mm, the process comprising:

(a) providing a curable mixture comprising a polymerisable matrix having one or more mica particulates dispersed therein, wherein the polymerisable matrix is derived from 60 to 100 wt% methyl methacrylate monomer, 0 to 40 wt% of at least one other copolymerisable alkyl (alk)acrylate comonomer, 0 to 10 wt% of a copolymerisable cross-linking monomer;

(b) casting the curable mixture; and

(c) curing the curable mixture by heating the mixture at an initial cure temperature of at least 70°C.

2. A process as claimed in claim 1 wherein the curable mixture is cured by heating the mixture at an initial cure temperature of at least 80°C.

3. A process as claimed in claim 1 or 2 wherein the curable mixture is heated at the initial cure temperature for at least 20 minutes.

4. A process as claimed in any one of the preceding claims further including the step of heating the curable mixture at a secondary cure temperature of at least 105°C following curing at the initial cure temperature.

5. A process as claimed in claim 4 wherein the curable mixture is heated at the secondary cure temperature for at least 20 minutes.

6. A process as claimed in any one of the preceding claims wherein the sheet has an essentially uniform thickness extending across the entire sheet.

7. A process as claimed in any one of the preceding claims wherein the acrylic cast sheet has a thickness of greater than or equal to 7 mm thick.

8. A process as claimed in any one of the preceding claims wherein the acrylic cast sheet has a thickness of less than or equal to 12 mm thick.

9. A process as claimed in any one of the preceding claims wherein the curable mixture comprises less than or equal to 10 wt% of one or more mica particulates.

10. A process as claimed in any one of the preceding claims wherein the one or more mica particulates include an inorganic pigment.

11. A process as claimed in claim 10 wherein the one or more mica particulates is coated with the inorganic pigment.

12. A process as claimed in claim 10 or 11 wherein the inorganic pigment comprises a metal oxide, preferably titanium dioxide and/or an iron oxide.

13. A process as claimed in any one of the preceding claims wherein a major amount of the one or more mica particulates has a density of less than or equal to 3.8 gem^"3 at 25°C.

14. A process as claimed in any one of the preceding claims wherein a major amount of the one or more mica particulates has a maximum dimension of δOO μm.

15. A process as claimed in any one of the preceding claims wherein a major amount of the one or more mica particulates has an aspect ratio of greater than or equal to 50:1.

5 16. A process as claimed in any one of the preceding claims wherein the polymerisable matrix is partially polymerised.

17. A process as claimed in claim 16 wherein the polymerisable matrix includes polymethyl methacrylate (PMMA), methyl methacrylate o monomer, 0 to 40 wt% of at least one of other copolymerisable alkyl

(alk)acrylate comonomer, 0 to 10 wt% of a copolymerisable cross-linking monomer.

18. A process as claimed in any one of the preceding claims wherein the s polymerisable matrix is derived from a monomer mixture comprising at least 75 wt% methyl methacrylate.

19. A process as claimed in any one of the preceding claims wherein the polymerisable matrix is derived from a monomer mixture comprising at o least 1 wt% of a copolymerisable alkyl (alk)acrylate comonomer.

20. A process as claimed in any one of the preceding claims wherein the copolymerisable alkyl (alk)acrylate comonomer is an alkyl acrylate.

5 21. A process as claimed in any one of the preceding claims wherein the polymerisable matrix includes at least 0.01 wt% of a copolymerisable cross-linking monomer.

22. A process as claimed in any one of the preceding claims wherein the viscosity of the polymerisable matrix is less than or equal to 1300 centipoises at 25°C.

23. An acrylic cast sheet having a thickness of greater than or equal to 5 mm obtainable by the process as defined in any one of claims 1 to 22.

24. An acrylic cast sheet having a thickness of greater than or equal to 5 mm comprising a polymer matrix having one or more mica particulates dispersed therein, wherein the polymer matrix comprises a homopolymer or copolymer derived from a monomer mixture comprising 60 to 100 wt% of methyl methacrylate, 0 to 40 wt% of at least one other copolymerisable alkyl (alk)acrylate comonomer and 0 to 10 wt% of a copolymerisable cross-linking monomer.

25. An acrylic cast sheet as claimed in claim 24 wherein the acrylic cast sheet has a thickness of greater than or equal to 7 mm thick.

26. An acrylic cast sheet as claimed in claim 24 or 25 wherein the one or more mica particulates includes an inorganic pigment, preferably the one or more mica particulates is coated with the inorganic pigment.

27. An acrylic cast sheet as claimed in claim 26 wherein the inorganic pigment comprises a metal oxide, preferably titanium dioxide and/or an iron oxide.

28. An acrylic cast sheet as claimed in any one of claims 24 to 27 wherein the sheet comprises less than or equal to 10 wt% of one or more mica particulates.

29. An acrylic cast sheet as claimed in any one of claims 24 to 28 wherein the sheet has an essentially uniform thickness extending across the entire sheet.

30. An acrylic cast sheet as claimed in any one of claims 24 to 29 wherein at least one surface of the sheet exhibits an appearance comprising a number of lobes.

31. An acrylic cast sheet as claimed in claim 30 wherein said at least one surface of the sheet exhibiting an appearance comprising a number of lobes is a smooth surface.

32. A method of forming an article comprising shaping an acrylic cast sheet as defined in any one of claims 24 to 31 or shaping an acrylic cast sheet as prepared according to any one of claims 1 to 22.

33. A component for use in bath ware, sanitary ware or spas comprising an acrylic cast sheet as defined in any one of claims 24 to 31 or an acrylic cast sheet as prepared according to any one of claims 1 to 22.

34. A laminated product comprising a substrate and an acrylic cast sheet as defined in any one of claims 24 to 31 or an acrylic cast sheet as prepared according to any one of claims 1 to 22.