HOMOGENEOUS POLYMERS HAVING ANTI-STATIC PROPERTIES

Abstract: A self-supporting body of homogeneous hydrophilic polymer, typically a cross-linked acrylic polymer based on 2-hydroxyethyl acrylate, contains water at a concentration of 1 to 20 weight % such that the the body has anti-static properties. The body is preferably clear and transparent, and may be used under conditions where a build-up of static electricity might otherwise have been expected. Also claimed is a reference standard for radioactive analyses which includes a sheet of the said body which has been radioactively labelled.

Description: Homogeneous Polymers having Anti-static Properties The build up of static electricty on synthetic polymers has been a major problem for many years, and various techniques have been used to overcome it. Anti-static agents may be applied either directly over the polymer surface (external type) or mixed with the polymer (internal type). Internal type anti-static agents are usually preferred but it is essential that such agents are compatible with properties of the polymer and are able to migrate to polymer surfaces in order to exert their anti-static properties. Tons of anti static are used annually in the plastics industry, but few successfully confer long-lasting non-static forming properties on the polymer because of their propensity to diffuse out of the polymer and/or be washed off the polymer surface. A carbon-loaded polyolefin (ethylvinylacetate) polymer which is volume conductive throughout is commerically available as Velostat (see D. Martin in Process Engineering September 1975, p.11). This material which is black, soft, flexible and easily formed has been described as having permanent antistatic properties. But by reason of the carbon filler, it is necessarily opaque and non-homogeneous. Radioactive analysers are instruments which scan flat samples, e.g. thin layer chromatography (TLC) plates and membranes, containing radioactive isotopes at spaced locations. Examples of such instruments are linear analysers, which perform a linear scan, e.g. along a run on a TLC plate or electrophoresis gels. Two-dimensional analysers, capable of scanning in two dimensions rather than one, are currently being developed. The build-up of static electricty on the sample being analysed is a known problem (Stephen R. Moss, "Electrostatic Effects seen on TLC analysers", poster presented at the International Symposium on TLC held at Brighton, March 1987). There is a need for reference standards for radioactive analysers, but these must avoid the problem of build up of static electricty. It is one object of this invention to provide a homogeneous polymer which is uniformly labelled with radioactivity and has anti-static properties making it suitable for use in such reference standards. But the invention extends to homogeneous polymers having antistatic properties, whether they are radioactively labelled or not. In one aspect this invention provides a selfsupporting body of homogeneous hydrophilic polymer, containing hydroxylic liquid at a concentration such that the body has anti-static properties. The body may be in the form of a sheet of uniform thickness. The body is preferably dimensionally stable in the presence of water. That is to say, when soaked in water at ambient temperature, it preferably does not swell so as to increase any linear dimension by more than 5%. Conversely, when held in a dry atmosphere at ambient temperature, the body preferably does not shrink so as to reduce any linear dimension by more than 5%. The homogeneous hydrophilic polymer is preferably an organic addition polymer such as an acrylic polymer. It may be derived from a monomer mix comprising a major proportion of a hydrophilic acrylic monomer and a minor proportion of a diacrylate or dimethacrylate. The hydrophilic acrylic monomer may be a hydroxyalkyl acrylate or methacrylate, such as hydroxymethyl, hydroxy -ethyl or hydroxypropyl acrylate or methacrylate, particularly hydroxyethyl methacrylate which is miscible with water in all proportions at ambient temperature. The diacrylate or dimethacrylate functions as a cross-linking agent, and acts to reduce water absorption. An example of a suitable material is ethyleneglycol dimethacrylate. The hydroxylic liquid is preferably water, although other liquids or liquid/water mixtures may occasionally be useful to increase compatibility with monomers. An upper limit is placed on the permissible amount of hydroxylic liquid in the starting mix by the need for such liquid to be either evaporated or absorbed into the polymer formed during polymerisation. Based on 100 parts of polymer, the amount of hydroxylic liquid is likely to be from 0.5 to 50 parts, more usually from 1 to 20 parts, by weight. For brevity hereafter, the hydroxylic liquid is assumed to be water. The polymer may be derived from a starting mix containing one or more co-monomers. One reason for such inclusion may be to improve the physical or chemical properties of the polymer. For example, the homopolymer of hydroxyethyl methacrylate is rather brittle; and a minor proportion of an acrylic comonomer, e.g. hydroxyethlyacrylate or methyl - or butyl-methacrylate, may be incorporated to improve toughness and flexibility. Another reason may be to radioactively label the polymer. Thus, for example, methyl - or bu-tyl-methacrylate or butyl acrylate, labelled with carbon-14 or tritium, may be used as the co-monomer. The amount of co-monomer that can be incorporated in a starting mix containing water without destroying homogeneity is rather limited. Generally such comonomers will be present in proportions up to 10%, e.g. 0.5% to 5% by weight on the weight of the monomer mix. The water-containing monomer mix can be polymerized using standard free-radical initiators, such as for example azo bis-(di-isobutyronitrile) (azdn). More preferably especially if the water content is above 005X, a redox catalyst system is used, eOg. comprising: - an initiator selected from hydrogen peroxide, potassium persulphate, and ammonium persulphate, and - an activator selected from ascorbate, bisulphite, thiosulphate, ethylene diamine, ferrous ammonium sulphate, N, N, N', N' tetramethyl ethylene diamine, ferrous or ferric chloride and manganese chloride. An advantage of the use of a redox cataylst is that an ionic polymerisation catalyst species is present in the resulting polymer, generally in solution in the hydroxylic liquid, and may help conferring anti- static properties on the polymer. If desired, other ionic species not useful for catalytic purposes, such as sodium chloride, may be incorporated in the hydroxylic liquid. Preferred concentrations are 0.1 to 5%, particularly 0.2 to 1%, by weight of the weight of the polymer. Hydroxyethyl methacrylate is hygroscopic, and precautions are usually taken to keep it anhydrous. Polymerisation of this monomer gives rise to a homopolymer which has a substantial capacity for absorbing water, e.g. up to more than 40% by weight on soaking overnight Polymerisation of a mixture of hydroxyethyl methacrylate (hema) with a cross-linking agent such as ethyleneglycol dimethacrylate (egdma) results in a polymer whose water aborbing tendency is reduced in proportion to the egdma concentration in the monomer mix. A polymer derived from 58% hema and 42% egdma absorbs little or -no water; but the physical and chemical properties and more particularly the anti- static properties of such a polymer are adversely affected. It is preferred to use a minor proportion, generally from 1 to 40%, particularly from 2 to 30%, by weight of the monomer mix of a cross-linking agent, and to perform the polymerisation under conditions to trap so much water during polymerisation that the resulting polymer does not absorb more than 10% by weight of water on being soaked in water at ambient temperature overnight. The polymerisation conditions need to be chosen such that at least part of the water or other hydroxylic liquid present in the starting mix remains and becomes incorporated in the polymer. Polymerisation results in a self-supporting body of homogeneous hydrophilic polymer, that is to say any hydroxylic liquid is dispersed or absorbed in the polymer. This invention is not concerned with systems which generate a polymer in solution or dispersion in a liquid. As a result, the polymer can be formed in any desired shape, simply by introducing the liquid homogeneous polymerisation mix into a vessel of suitable shape. For the particular application to be described below, a preferred shape is a sheet of uniform thickness. Although not essential, the polymer is preferably clear, and colourless and contains no filler. Particularly when one of the co-monomers is radioactively labelled with a betaemitter such as carbon-14 or tritium, the presence of a filler could mask radiation in an unpredictable manner. The polymers of this invention have anti-static properties, which are maintained after storage for 16 hours at 60 0C or on storage at room temperature for many months and probably over their entire useful lifetime. The anti-static properties do not depend on the presence of water in the atmosphere or adjacent the polymer. The exact mechanism by which such properties are conferred is unknown, but the hydroxylic liquid (e.g. water) is an essential component without which the anti-static properties are lost. Anti-static properties can be demonstrated by simple qualitative tests. A polymer sheet is rubbed with a dry cloth fabric or paper to generate an electrostatic charge on the plastic surface. If so charged, the polymer sheet attracts small pieces of paper or expanded polystyrene. If so charged and placed beside a thin water jet from a tap, the polymer sheet will also cause deflection of the jet towards the sheet. On a somewhat more quantitative level, it is generally regarded as preferable that polymers with anti-static properties have a surface resistivity in the range of 108 to 101Q ohms. This is higher than most liquids, but lower than most polymers which do not have anti-static properties. More sophisticated equipment for measuring anti-static properties is commercially available, e.g. from Keithley Instruments Bud,, Reading, U.K. or from Eltex Electronic H. Grunenfelder Ing, Basel, Switzerland (model Q475A or Q475C). The anti-static polymers of this invention have utility in the electronics industry, in paper manufacture, where powders are handled, where there is frictional contact between insulators, indeed in any industry where static electricty is a problem. The polymers have the advantage over carbon-filled polymers of being able to be colour-coded. In another aspect, the invention provides a reference standard for a radioactive analyser, such as a linear analyser, comprising a plate with anti-static properties and having a substantially flat front surface, predetermined regions of said surface being formed of a uniform sheet of a body of radiolabelled polymer as described. The reference standard comprises a support plate, the front surface of which may be provided with one or more flat-bottomed depressions which are filled with pieces of the uniform sheet of radiolabelled polymer. The support plate should not be prone to static build up. It may be formed of aluminium metal, provided that any oxide layer on the front surface does not permit static build up. It may be formed of steel, provided that accurately dimensioned flat-bottomed slots or other depressions can be machined in its front surface; these slots or depressions may be as little as 0.5mm wide and deep, and separated by a gap as small as 0.5mm. To avoid possible problems of ionizing discharge (corona effects), sharp corners or edges with a radius of curvature below 50 microns are preferably absent. The support plate is preferably formed of an electrically conducting polymer, such as a carbonloaded polystyrene. Such plates can be manufactured to close tolerances and accurately shaped flat-bottomed slots milled in them. The depth of the slots needs to be controlled to ensure that the front surface (to face the radioactive analyser) of the uniform sheet of radiolabelled polymer is substantially flush with the front surface of the support plate. In order to test various aspects of the performance of the radioactive analyser, the support plate may have several slots or depressions, each carrying a body of radiolabelled polymer and which may differ as to: the radionuclide, e.g. carbon-14 or tritium; the surface area of the sheet of polymer; the radioactivity per unit area; the width of the slot; and the spacing of adjacent slots. Reference is directed to the accompanying drawings in which: figures la and Ib are respectively a diagramatic front elevation and a diagramatic side elevation of a conventional linear analyser; figures 2a and 2b are respectively a plan and a section of a reference standard for such an analyser; and figure 3 is a chart showing variations in radio activity along a uniform sheet of radiolabelled polymer according to the invention. Figure 1 shows a radioactive substrate 10, which may be a TLC plate or a reference standard. A linear analyser comprises a housing 12 a detector wire 14 and a delay line 16. When a beta particle leaves a source under the linear analyser, it is accelerated towards the detector wire (anode) where it ionizes the gas to form a charge cloud 18. This is not only detected as a pulse but its energy is measured and is proportional to the energy of the original beta particle. The weaker the energy accepted the more likely it will have originated normal to the detection point. The delay line also operates to determine the position of detection. When the gas ionizes, positive and negative ions are produced. The negative Xons are mopped up by the detector wire (anode) leaving the positively charged ions to fall onto the surface of the substrate. If the substrate is an electrical conductor or has anti-static properties, this charge is dissipated, with the result that total radioactive counts and analyser height are directly proportional to analysis time. If on the other hand the substrate is an insulator, then a charge builds up which interferes with subsequent beta particles leaving the substrate. The increase in total counts and peak height is no longer directly proportional to analysis time. The peak becomes lower and wider and the total counts are less than those expected. A reference standard that included radioactive sources not having anti-static properties would give results which varied depending not only on the analysis time but also on the distribution, localisation and size of the electrostatic fields, and hence would be of little value or accuracy. Figure 2 shows a reference standard for a linear analyser comprising a plate of carbon-loaded polystyrene 20, 200mm by 200mm by 2.5mm thick, adhered to an aluminium backing plate 22, 2mm thick. Slots or depressions with flat bottoms and vertical sides have been milled into the upper surface of the polystyrene plate to a depth of imam, In each slot or depression has been stuck an appropriately sized piece of 24 of a uniform sheet lmm thick of a self-supporting body of radiolabelled anti-static polymer according to this invention. At each end of the standard, a rib 28 serves to locate the head of the linear analyser. In figure 2a the radio-labelled polymer in different slots or depressions has been indicated by a number from 1 to 6, and these numbers mean: 1. 500 microCuries of tritium per gram of polymer 2. 1 microCurie of carbon-14 per gram of polymer 3. 10 microCuries of carbon-14 per gram of polymer 4. 0.1 microCuries of carbon-14 per gram of polymer 5. 6.01 microCuries of carbon-14 per gram of polymer 6. 0.001 microCuries of carbon-14 per gram of polymer The slots or depressions have been arranged in five columns, marked A to E in figure 2a, whose function is: A. Alignment of plate with reading head (for tritium). B. Linearity of response over the whole wire of the linear analyser. C. Response to wide dynamic range; detection of 1% impurity; resolution. The column includes resolution gaps of 5, 4, 3, 2, 1 and 0.5mm. D. Spatial recording. E. Alignment of plate with reading head (for carbon-14) The following examples illustrate the invention. EXAMPLE 1 There was formed a homogeneous polymerisation mix containing (in parts by weight): - hydroxyethylmethacrylate - 98 - methylmethacrylate - 1 - ethyleneglycol dimethacrylate - 4 - 106 aqueous solution of ascorbic acid - 2 - 10X aqueous solution of ammonium persulphate - 2 The resulting mix was poured onto a flat surface and left to polymerise at ambient temperature for three hours. There was obtained a clear colourless homogeneous polymer sheet, which was removed from the flat surface and tested for anti-static properties in various ways. The polymer sheet was rubbed with dry cloth fabric or paper and subjected to the qualitative tests mentioned above for static electricity. While a sheet of polymethylmethacrylate charged in this manner gave positive results in both tests, the polymer sheets prepared from the monomer composition described in this Example gave no response in both tests indicating that the surface of such polymers carried no detectable static charge. A Keithley meter (Model Number 610C - Keithley Instruments Ltd., Reading UK) used to make some comparative static and resistance measurements gave the following results after rubbing: Static (voltage) Measurements (at 1cm from probe head) Sample: a) 15cm square of polymethymethacrylate - 20 volts b) 15cm square of INVENTION POLYMER - none recorded Static (Coulombs) Measurement Sample: : a) 15cm square of polymethylmethacrylate - 4 x 10 18 b) 15cm square of INVENTION POLYMER - no detectable response c) White acrylic plate 20 x 20 x 0.3cm - ca.10 x 10-10 d) White acrylic laminates with 2mm aluminium sheet - ca.2 x 10-10 Resistance (Ohms) Measurements (using small "bull-dog" clips 3-4mm apart) Sample: a) Polymethylmethacrylate - off-scale > 1010 b) INVENTION POLYMER - 7.5 x 108 For additional information the resistance of some monomers and solvents were measured by placing a large drop of the liquid sample on a clean dry standard microscope slide and touching the liquid with small "bull-dog" clips held 3-4mm apart. Sample: a) tap water - 2 x 105 ohms b) ethanol (water free) - 2.5 x 109 ohms c) methanol (water free) - 8 x 105 ohms d) glycerol - > 1012 ohms e) HEMA monomer - 2.5 x 107 ohms f) methylmethacrylate monomer - > 1012 ohms g) butylmethacrylate monomer - > 1012 ohms h) hydroxypropylmethacrylate monomer - > 10 ohms i) ethylene glycol dimethacrylate monomer - > 1012 ohms A preferred surface resistivity for a clear transparent polymer with anti-static forming properties is 108 to 1010 ohms. Polymers prepared according to this Example had surface resistivity in this range. The polymerisation experiment above was repeated using methylmethacrylate labelled with carbon-14 to an extent that the resulting polymer contained 1 micro- Curie of carbon-14 per gram of polymer. A lmm thick sheet of the body of radiolabelled polymer was placed in column b of a reference standard as shown in figure 2, and used to test the performance of a linear analyser. Figure 3 is a chart showing counts against distance along the wire. Except in the final half centimetre, the linearity of response is excellent. If the response had not been linear, it would have been possible to turn the reference standard through 1800 This provides a simple check that the radiolabelling in the polymer is indeed uniform. EXAMPLE 2 Different formulations were made up, as shown in table 1 below, and polymerised generally as described in example 1 above. When azdn was used as catalyst, polymerisation was carried out overnight at 7OOC. When hydrogen peroxide/ascorbic acid was used as catalyst, polymerisation was at ambient temperature for two hours followed by holding overnight at 70 C. When subjected to the qualitative tests indicated above, all the polymers were found to have anti-static properties. TABLE 1 Ex. HEMA GLYMA EGD H20 H202 AA AZDN NaCl 2 90 10 2 2 2 3 90 10 2 2 4 90 10 5 1 0.1 5 80 20 5 1 0.1 6 70 30 5 1 0.1 7 90 10 10 0.1 8 90 10 15 0.1 KEY Hema-hydroxyethylmethacrylate GLYMA-glycerylmethacrylate EGD-ethyleneglycol dimethacrylate AA-Ascorbic acid AZDN-azobis- (di-isobutyronitri le) EXAMPLE 3 Further formulations were made up as indicated in Table 2 below, and polymerised generally as described in Example 1 Polymerisation conditions using azdn were at 70 C overnight. Polymerisation conditions using aa/aps were at abient temperature, followed optionally by heating at 400 C overnight. When subjected to qualitative testing all the polymer bodies were found to have anti-static properties. Each was soaked in water at ambient temperature overnight, and the uptake (expressed in percent by weight) recorded in the column headed "uptake". These figures are all 10% or less, indicating that the polymer bodies were diamensionally stable in the presence of water. TABLE 2 Ex HEMA EGD H20 MeMA AZDN AA/APS Uptake 9 70 30 5 0.1 10 10 70 30 7.5 0.1 4.5 11 70 30 10 0.1 7.7 12 70 30 3.5 2/2 2.3 13 70 30 6 2/2 5.5 14 70 30 3.5 1 2/2 7.2 15 70 30 3.5 0.5 2/2 0.9 16 7Q 30 6 1 2/2 5,5 17 70 30 6 0.5 2/2 5.1 KEY MeMA-methylmethacryl ate AA/BPS - 10g w/v solutions of ascorbic acid and ammonium persulphate. EXAMPLE 4 To make radiolabelled polymer bodies for use in reference standards, methylmethacrylate radiolabelled to the appropriate specific activity may be used. In a typical preparation: hydroxyethyl methacrylate 70 parts ethyleneglycol dimethacrylate 30 parts [14C]methyl methacrylate 1 part and water 6 parts were polymerised in the presence of 10% w/v ascorbic acid solution 2 parts and 10% W/v ammonium persulphate solution 2 parts, at room temperature (200C) for 2 hours. Autoradiography of the polymer sheet showed that the radioactivity was uniformly distributed through the volume of the polymer body. The bodies of anti-static polymer of this invention can be used in the following application: 1. Radioactive versions to yield uniform radioactive emission rates in the case of charged particle emissions. 2. Electronic/Optoelectronics industry: protection in manufacture, assembly, servicing, use, assembly, packaging and transport of electronic devices or items incorporating electronic components, e.g. use in manufacturing of clean rooms. 3. Office copiers, facsimile machines and other office equipment. 4. Screens for TV computer terminals. 5. Electric field screening. 6. Medical a) Operating theatres - flooring, surface eoatings, clothing, equipment. b) Emergency services equipment, eg in ambulances. 7. Optical devices - lenses, screens, coatings, waveguides, light-guides. 8. Gramophone records. 9. Industries using powders or moving and transporting material where static charges tend to arise, e.g. flours, pharmaceuticals, explosives, coal mining, other mining operations. 10. Manufacture and use of photographic films and emulsions 11. Printing and paper manufacture. 12. Industries handling other sheet material. 13. In filters and filtration equipment to avoid/ minimise static generation and retention, e.g. filtration of fuels and other liquids. 14. In pipework handling material likely to generate static, e.g. oils, solvents. 15. Anti-static or non-static versions of adhesive tapes. 16. Paint spraying and other spraying applications. 17. In belting and belting machinery. 18. Clothing and fabrics. 19. Paints and other coatings. 20. Glazing and double glazing. 21. Information storage tapes and discs and housing, eg. computer discs, audio and video tapes electronic and optical types. 22. Component housings, mounting boards/supports and casings of static sensitive devices, eg silicon chip housings, printed circuit boards, computer casings, cable and wire insulation. 23. Encapsulation material/shielding for static sensitive and electric field sensitive devices. 24. Static earthing devices/leads for vehicles of all kinds. 25. Replacement of rubber mats and carbon fibre antistatic mats in record decks. 26. Replacement of device components which attract dust to the detriment of the optimum operation of the device. 27. Bomb disposal equipment.

Claims: CLAIMS 1 A self-supporting body of homogeneous hydrophilic polymer, containing hydroxylic liquid at a concentration such that the body has anti-static properties. 2. A body as claimed in claim 1 which is dimensionally stable in the presence of water. 3. A body as claimed in claim 1 or claim 2, wherein the polymer is an acrylic polymer. 4. A body as claimed in any one of claim 1 to 3, wherein the polymer is derived from a monomer mix comprising a major proportion of a hydroxyalkyl acrylate or methacrylate and a minor proportion of a diacrylate or dimethacrylate. 5 A body as claimed in any one of claim 1 to 4, wherein the hydroxylic liquid is water present at a concentration of 0.5 to 50 parts by weight per 100 parts of polymer. 6. A body as claimed in any one of claim 1 to 5, wherein at least one ionic polymerisation catalyst species is present. 7. A body as claimed in any one of claim 1 to 6, wherein the polymer is labelled uniformly with a radioactive isotope. 8. A body as claimed in any one of claim 1 to 7, wherein the body is clear and transparent. 9. A body as claimed in any one of claim 1 to 8, wherein the body is in the form of a sheet of uniform thickness. 10. A reference standard for a radioactive analyser, comprising a plate with anti-static properties and having a substantially flat front surface, predetermined regions of said surface being formed of a uniform sheet of a body of radiolabelled polymer as claimed in claim 9. 11. A reference standard as claimed in claim 10, wherein a front surface of the plate is provided with one or more flat-bottomed depressions which are filled with pieces of the uniform sheet. 12. A method which comprises subjecting a body as claimed in any one of claims 1 to 9 to conditions which tend to lead to the build-up of static electricity. 13. A method as claimed in claim 12, wherein the conditions involve frictional contact between the body and an insulator.

Inventors: BELL COLIN DAVID, CARTWRIGHT PETER, MANGION ANDREW JOHN, BELL, COLIN, DAVID, CARTWRIGHT, PETER, MANGION, ANDREW, JOHN
Applicants: AMERSHAM INT PLC, AMERSHAM INTERNATIONAL PLC
Application year: 1988
Main classification: C08K3
IPC codes: B29D11, B29D17, C08F20, C08F220, C08K3, C08L33, G01T1