THERMOFORMABLE CAST POLY(METHYL METHACRYLATE)

Abstract: A curable composition capable of being cured to form a thermoformable cast poly(methacrylate) comprising at least from 95 % by weight of a syrup which is a solution of a poly(methacrylate) in a curable monomeric methacrylate, from 0.05 to 0.1 % by weight of at least one initiator capable of initiating the curing of the curable monomeric methacrylate, from 0.0001 to 0.0005 % by weight of a catalytic chain transfer agent which is or which is derived from a cobalt chelate, from 0.05 to 0.1 % by weight of a cross-linking agent; and optionally one or more of conventional additives selected from splitting aids, pigments, peak suppressors and stabilisers. A thermoformable sheet formed from such a composition and a thermoformed article produced from such a sheet. The thermoformable sheet does not need to be thermoformed at relatively high temperatures, has an acceptable Vicat Softening Point, does not emit unpleasant odours on heating and meets stringent industry standards.

Description: Thermoformable Cast Poly(methyl methacrylate) The present invention relates to thermoformable cast poly(methyl methacrylate). Conventionally, as described in US 4113803, thermoformable cast poly(methyl methacrylate) is prepared by polymerising suitable curable compositions between sheets of glass (cell casting) or between two opposing endless metal bands (band casting) so as to provide the thermoformable cast poly(methyl methacrylate) in the form of sheet. T e sheet can be subsequently thermoformed to manufacture articles such as sanitary ware, e.g. baths, washbasins and shower trays. In the thermoforming process, the sheet is preheated to a predetermined temperature, for example as calculated according to the formula stated in GB 1577517, and is then drawn into a mould under the influence of pressure or vacuum. Problems associated with the thermoforming of such sheet are discussed in US 4113803, in particular the difficulty of forming complex shaped articles and the conflict between increasing the rate at which the articles are formed by raising the thermoforming temperature and the risk of thermal degradation at such higher temperatures. US 4113803 states that the prior art solutions to the above problems of including a plasticiser into the curable composition or to ensure that the poly(methyl methacrylate) is of a sufficiently low molecular weight result in a deterioration of mechanical and haze resistance properties of a thermoformed article. US 4113803 proposes a further solution in which a small amount of cross-linking agent is added to the curable composition which has a reduced viscosity as therein defined of between 1.5 and 4.0. Unfortunately, the sheet produced according to US 4113803 still has to thermoformed at relatively high temperatures thereby increasing the time it takes to produce an article. Alternatively, current commercially available sheet contains a small percentage of lower alkyl acrylate, e.g. butyl acrylate, in order to allow the sheet to be thermoformed at lower temperatures. Unfortunately, such sheet has, according to certain industry standards, an unacceptably low Vicat Softening Point and thereby is unsuitable for use in sanitary ware. Further alternatives wherein the molecular weight and polydispersivity of the poly(methyl methacrylate) is controlled by the use of mercaptan based chain transfer agents results in sheets which have good thermoformability but poor performance in the standard heat tests and also emit unpleasant odours on heating. The present invention is directed towards an improved thermoformable cast poly(methacrylate), such as poly(methyl methacrylate), particularly in the form of sheet, which does not need to be thermoformed at relatively high temperatures, which has acceptable Vicat Softening Point, which does not emit unpleasant odours on heating and which meets stringent industry standards. Accordingly in a first aspect the present invention provides a curable composition capable of being cured to form a thermoformable cast poly(methacrylate) which composition comprises (a) at least from 95 % by weight of a syrup which is a solution of a 5 poly(methacrylate) in a curable monomeric methacrylate, (b) from 0.05 to 0.1 % by weight of at least one initiator capable of initiating the curing of the curable monomeric methacrylate; (c) from 0.0001 to 0 001 % by weight of a catalytic chain transfer agent or residue thereof which is or which is derived from a cobalt chelate, and optionally 10 (d) from 0.05 to 0 1 % by weight of a crosslinking agent; (e) one or more of conventional additives selected from splitting aids, pigments, peak suppressors and stabilisers. In a second aspect the present invention provides a thermoformable cast poly(methacrylate) sheet capable of being formed from a curable composition as hereinbefore 15 defined and having a Vicat Softening Point of at least 105°C and exhibiting a weight loss when subjected to thermogravimetπc analysis (TGA) between 260 and 300°C. The poly(methacrylate) in the syrup is preferably formed from the same monomeric methacrylate which is also present in the syrup. Typically, the poly(methacrylate) represents from 5 to 35 %, more typically from 5 to 15 %, by weight of the syrup and preferably from 7 to 2025 %, more preferably from 7 to 9 % by weight, of the syrup Although the syrup may be prepared by the mixing of the poly(methacrylate) with the monomeric methacrylate, it is preferred that the syrup is prepared by partially polymerising the monomeric methacrylate so as to form the poly(methacrylate) in situ The conditions are controlled such that the poly(methacrylate) formed in situ is of an appropriate molecular weight for use in the 25 subsequent casting stage. Particularly preferred is when -he poly(methyl methacrylate) is formed in situ by the use of a catalytic chain transfer agent thereby providing improved control over both the molcular weight and polydispersivity The poly(methyacrylate) and monomeric methacrylate are preferably derived from lower alkyl methacrylates, e.g. C, to C 4 methacrylates, and in particular methyl methacrylate 30 The poly(methacrylate) may be a copolymer Additionally, one or more monomeric methacrylates may be used. Preferably, the poly(methacrylate) is a homopolymer, particularly poly(methyl methacrylate) and that the monomeric methacrylate is methyl methacrylate. Nevertheless, the use of the catalytic chain transfer agent allows for the incorporation of a minor proportion of one or more lower alkyl acrylates, C, to C 4 acrylates, into the curable 35 composition without the deleterious effect of lowering the Vicat Softening Point being exhibited Where such an acrylate is used it is prefered that the acrylate is butyl acrylate The at least one initiator may be one or more initiators commonly used in the preparation of conventional thermoformable cast poly(methacrylate) Suitable initiators include azodnsobutyronitrile, peroxydicarbonates such as di-isopropyl, di-n-butyl, di-isobutyl, 5 dicyclohexyl, or bιs-(4-t-butyl cyclohexyl) peroxydicarbonate, and t-butyl perpivalate Adventitiously, two or more initiators are chosen such that the lifetime and activity of the initiators are optimised during curing. The catalytic chain transfer agent, or residue thereof, is or is derived from a cobalt chelate Suitable cobalt chelates are as described in US 4694054, US 5028677 and US 10 4680352 Particularly preferred is a cobalt chelate having the following general formula I 0 wherein R can be selected from of a variety of groups such as methyl and phenyl, particularly phenyl, and which is known generally as CoBF. Alternatively or additionally, the catalytic chain transfer agent is or is derived from cobaloxime. The catalytic chain transfer agent is added at very low concentrations, i e. from 0.0001 to 0.001 , for example from 0.001 to 0.0005, % by weight on the curable composition 5 Preferably, the catalytic chain transfer agent is present from 0 00025 to 0.0003 % by weight, i e 2 5 to 3 ppm, in the curable composition At these levels of addition the average molecular weight ( of the cured curable composition (absent cross-linking agent) as determined by GPC (chloroform solvent; poly(methyl methacrylate) standard) is in the range 350000 to 500000 and in particular from 380000 to 480000. Increased amounts of the catalytic chain transfer agent reduce the molecular weight. Of further note is that it is believed that due to the manner in which the chain transfer agent effects the growing polymer chain, a surprisingly high proportion (compared with the proportion expected from using conventional free radical polymerisation) of the terminal groups of the poly(methacrylate) are not saturated. Consequently, when undergoing TGA the thermoformable cast poly(methacrylate) of the present invention shows some decomposition between 260 and 300°C typical of non-saturated terminal groups. Where the poly(methacryiate) is formed in situ by the use of a catalytic chain transfer agent as hereinbefore described the syrup will contain residues of such a chain transfer agent. Such residues will be present in the syrup at the levels hereinbefore stated. Subsequent casting and curing of the syrup may then use further amounts of the same or of a similar chain transfer agent or may employ more conventional catalysts. The crosslinking agent may be chosen from any conventionally used. Suitably, the crosslinking agent is chosen from at least one of ethylene glycol dimethacrytate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, triallyl cyanurate, allyl methacrylate, allyl acrylate, triallyl phosphate, diallyl maleate, methallyl acrylate, vinyl methacrylate and divinyl benzene. Particularly preferred is ethylene glycol dimethacrylate. Other conventional additives include splitting aids such as Texol (obtained from Imperial Chemical Industries pic) and CR3009 (Wilco Corporation), pigment, peak suppressors such as Terpinolene (White Sea and Baltic Co Ltd) and stabiliser such as Irganox (Ciba Geigy). The following examples illustrate the present invention. Example 1 A reactive mixture was prepared containing 2000 kg of monomeric methyl methacrylate, 100 g of UV stabiliser (Aduvex - The Great Lakes Fine Chemicals Ine), 1200 g of splitting aid (Texol) and 38 g of initiator (azodiisobutyronitrile - AZDN). The mixture was stirred and heated to about 82°C until a syrup containing about 8% by weight of poly(methyl methacrylate) was formed. Cast Poly(methyl methacrylate) A curable composition was formed having the following composition: Component % by weight Splitting aid 0.2 Pigment 2.5 AZDN 0.018 Peak suppressor 0.01 Sheet stabiliser 0.05 Crosslinker 0.08 CoBF 0.00025 Syrup Balance The curable composition was stirred and then poured into casting cells formed from sheets of glass so as to eventually provide sheets having thicknesses of 3.2 and 5 mm respectively. Curing followed a two stage heating process. The filled cells were first slowly heated to 90 D C at which they were held for 1 hour. Thereafter the temperature was slowly raised to about 120 C C at which they were held for a further 30 minutes. After curing, the cells were slowly cooled to room temperature. The 5 mm thick sheets had a Vicat Softening Point (VSP) of 107.4°C and the 3 mm thick sheets had a VSP of 108.0°C. TGA of samples of the sheet showed decomposition occurring between 260 and 300°C which was absent from conventional thermoformable cast poly(methacryiate) prepared in the absence of cobalt chelate. The ease with which sheets of the present invention could be thermoformed was compared against the ease at which commercially available sheets could be thermoformed. Sheets of each were preheated to different thermoforming temperatures and thereafter thermoformed into a standard shape. The accuracy to which the thermoformed sheet fitted the standard shape was then determined. Those commercial sheets formed from poly(methyl methacrylate) had to be preheated by about an additional 20°C above the preheat temperature used for the sheets of the present invention in order to achieve the same accuracy of fit. Conversely, the commercial sheets containing copolymer need not be heated by the same amount as the sheets of the present invention but had significantly inferior VSPs of about 100°C Example 2 Example 1 was repeated except that the CoBF was replaced by cobaloxime. The 5mm sheet had a VSP of 109.2°C and showed a decomposition between 260 and 300 C C when subjected to TGA. Example 3 Example 2 was repeated except that the cobaloxime was used at a level of 3ppm. The 5mm sheet had a VSP of 109.6°C and again showed decomposition between 260 and 5 300°C. Example 4 Example 1 was repeated except that the syrup contained 3.4 % by weight of butyl acrylate. The 3.2 mm sheet had a VSP of 105.4°C and the 5 mm sheet had a VSP of 105.2°C

Claims: Claims 1. A curable composition capable of being cured to form a thermoformable cast poly(methacrylate) which composition comprises (a) at least from 95 % by weight of a syrup which is a solution of a poly(methacrylate) in a curable monomeric methacrylate; (b) from 0.05 to 0.1 % by weight of at least one initiator capable of initiating the curing of the curable monomeric methacrylate; (c) from 0.0001 to 0.001 % by weight of a catalytic chain transfer agent or residue thereof which is or which is derived from a cobalt chelate; and optionally (d) from 0.05 to 0.1 % by weight of a crosslinking agent; (e) one or more of conventional additives selected from splitting aids, pigments, peak suppressors and stabilisers. 2. A curable composition as claimed as claim 1 wherein the poly(methacrylate) in the syrup represents from 5 to 15 % by weight of the syrup. 3. A curable composition as claimed in either claim 1 or claim 2 wherein the poly(methacrylate) and monomeric methacrylate are derived from C→ to C, methacrylates. 4. A curable composition as claimed in any one of claims 1 to 3 wherein the at least one initiator is one or more initiators selected from azodiisobutyronitrile, peroxoydicarbonates such as di-isopropyl, di-n-butyl, di-isobutyl, dicyciohexyl, or bis-(4-t-butyl cyclohexyl) peroxoydicarbonate, and t-butyl perpivalate. 5. A curable composition as claimed in any one of claims 1 to 4 wherein the catalytic chain transfer agent , or residue thereof, is or is derived from cobalt oxime. 6. A curable composition as claimed in any one of claims 1 to 4 wherein the catalytic chain transfer agent , or residue thereof, is or is derived from a cobalt chelate having the following general formula I wherein R is methyl or phenyl. 7. A curable composition as claimed in any one of claims 1 to 6 wherein the crosslinking agent is selected from at least one of ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, tπethylene glycol dimethacrylate, triallyl cyanurate, allyl methacrylate, allyl acrylate, triallyl phosphate, diallyl maleate, methallyl acrylate, vinyl methacrylate and divinyl benzene. 8. A thermoformable cast poly(methacrylate) sheet formed from a curable composition as hereinbefore defined in any one of claims 1 to 7 having a Vicat Softenng Point of at least 105°C and exhibiting weight loss when subjected to thermogravimetric analysis (TGA) between 260 and 300°C. 9- thermoformable cast poly(methacrylate) sheet as claimed in claim 8 wherein the average molecular weight (M of the cured curable composition, absent cross-linking agent, as determined by GPC, using chloroform solvent and poly(methyl methacrylate) standard, is in the range 350000 to 500000 10. A thermoformed article produced from a thermoformable cast poly(methacrylate) sheet as defined in either claim 8 or claim 9.

Inventors: BEVERLY GORDON MAXWELL, IRVINE DEREK JOHN, LYNCH JOHN PATRICK, BEVERLY, GORDON, MAXWELL, IRVINE, DEREK, JOHN, LYNCH, JOHN, PATRICK
Applicants: BEVERLY GORDON MAXWELL, ICI PLC, IRVINE DEREK JOHN, LYNCH JOHN PATRICK, BEVERLY, GORDON, MAXWELL, IMPERIAL CHEMICAL INDUSTRIES PLC, IRVINE, DEREK, JOHN, LYNCH, JOHN, PATRICK
Application year: 1997
Main classification: C08F265
IPC codes: C08F2, C08F265