FLOOR COVERINGS

Abstract: Vinyl floor coverings having a wear layer of a flexible PVC composition are characterised in that the flexible PVC composition contains core-shell polymer particles in addition to conventional ingredients.

Description: FLOOR COVERINGS Field of the Invention This invention relates to vinyl floor coverings with improved anti-slip characteristics, particularly to those in sheet or tile form and especially to those which have a clear wear layer. Background of the Invention Vinyl sheet and vinyl tile floor coverings are well-known and have enjoyed considerable commercial success. They are based on homopolymers or copolymers of vinyl chloride, for example vinyl chloride/vinyl acetate copolymers. The homopolymers and the copolymers are both commonly referred to as PVC resins or PVC polymers. Compositions which comprise PVC resin are commonly referred to as PVC compositions or simply as PVC. PVC compositions which contain little or no plasticiser (for example up to about 2% by weight) are relatively rigid and brittle, and they are commonly referred to as rigid PVC, unplasticised PVC, or UPVC. PVC compositions which contain relatively large amounts of plasticiser (for example around 10-40% by weight) are more flexible and have a lower modulus, and they are commonly referred to as flexible PVC, plasticised PVC, or PPVC. Vinyl sheet and vinyl tile floor coverings are based on flexible PVC and are resilient floor coverings. "PVC Technology", ed. W.V. Titow, Elsevier Applied Science Publishers, 4th Edition (1984), describes the manufacture, properties and uses of PVC. It uses the same terminology as that defined above. An article entitled "Vinyl Chloride Polymers" in Encyclopaedia of Polymer Science and Engineering, iley-Interscience, 3rd Edition, Supplement Volume (1989), pages 822-889, also describes the manufacture, properties and uses of PVC. An article entitled "Flooring Materials" in Encyclopaedia of Polymer Science and Engineering, Wiley-Interscience, 3rd edition, Volume 7 (1987), pages 233-241 , describes the manufacture and uses of vinyl sheet and vinyl tile floor coverings. Vinyl sheet and vinyl tile floor coverings are usually composite materials constructed from a number of layers, in which each layer is specially formulated for a particular duty. Thus, the uppermost layer, which is commonly called the wear layer, is formulated for good resistance to abrasion and wear. In one common form of construction, the wear layer is transparent, so that the colouring or patterning of lower layers can be seen through it, and it may then be referred to as a clear wear layer. Flexible PVC compositions for use in a clear wear layer preferably have high clarity and transparency. It is also important that the wear layer should have good anti-slip characteristics. PVC floor coverings generally have good anti-slip characteristics when dry, but they may be less satisfactory when wet. Both the abrasion resistance and the anti-slip characteristics of vinyl floor coverings can be improved by providing a wear layer which incorporates particles of a harder material than the PVC resin matrix, for example mineral particles or fibres. EP-A-100595 discloses anti-slip vinyl floor coverings which comprise a wear layer composed of synthetic resin particles 100 micron to 1 mm in size dispersed in a flexible PVC matrix. The resin particles are chosen to have a higher softening temperature and a better abrasion resistance than the PVC matrix. Some of the particles are exposed at the surface of the matrix. Floor coverings which contain relatively large particles such as these develop a rough appearance in service which may be aesthetically undesirable. PVC compositions which incorporate particles of a harder material will be optically clear only if the particles have the same refractive index as the PVC resin matrix. This greatly restricts the choice of particles which may be used in a clear wear layer. Furthermore, it is most desirable that a clear wear layer should retain a smooth appearance in service. Core-shell polymer particles are known and are described for example in GB-A-1583671, US-A-4141935 and US- A-4351875. These particles consist basically of (1) a polymeric core formed by emulsion polymerisation of one or more unsaturated monomers and (2) a polymeric shell formed on the core by emulsion polymerisation of one or more unsaturated monomers in the presence of the core. Polymer particles are also known which have one or more further polymeric shells formed on the particles by a further step or steps of emulsion polymerisation. Core-shell polymer particles are typically about 50 nm to 5 micron in size, often 100 to 200 nm. The core polymer and the shell polymer are both acrylic polymers. One type of core-shell polymer particle comprises an elastomeric polymeric core of lower glass transition temperature (Tg) and a hard polymeric shell of higher Tg. Such particles have been blended into rigid PVC compositions as impact modifiers in order to reduce the typical brittleness of rigid PVC and so to allow the production of articles with improved impact resistance. This procedure is described ^ for example in US-A-4448932, EP-A-2813, EP-A- 42091, EP-A-50848, EP-A-82717 and EP-A-338486. Flexible PVC exhibits adequate impact resistance for its end-uses, such as vinyl floor coverings, without the addition of impact modifiers. JP-A-61-215612 describes plastisol compositions which comprise (1) a PVC resin, (2) core-shell polymer particles 10-150 micron in size, in which the core contains at least 30% by weight vinyl halide and the shell contains at least 60% by weight methyl methacrylate, and (3) a plasticiser. JP-A-61-215611 describes similar compositions except that the core-shell polymer particles are 0.1-10 micron in size. A plastisol is a paste which consists of a suspension of PVC resin in a liquid plasticiser. These compositions are said to have excellent low-temperature workability and viscosity stability. They can be stored for 7 days or more without setting solid. They are said to be useful for example in the coating of textiles. Disclosure of the Invention A vinyl floor covering having a wear layer of a flexible PVC composition according to the invention is characterised in that the flexible PVC composition comprises core-shell polymer particles in addition to the conventional ingredients. The floor coverings of the " invention have good anti- slip properties. They have the advantage that they can be manufactured using conventional equipment for the manufacture of vinyl floor coverings. Core-shell polymer particles which have a refractive index close to that of flexible PVC can readily be made, and these are particularly suitable for the manufacture of those floor coverings according to the invention which have a clear wear layer. Because the core-shell polymer particles are relatively small, for example a few microns or less in size, the floor coverings of the invention retain a visually smooth appearance in service. The core-shell polymer particles can be made in known manner, for example as described in the prior art hereinbefore mentioned. They are preferably made by emulsion polymerisation of one or more unsaturated monomers to form core polymer particles, followed by emulsion polymerisation of one or more unsaturated monomers in the presence of the core polymer particles to form core-shell polymer particles. They may have one or more further polymeric shells as hereinbefore mentioned, although this is not generally necessary. The ratio of shell to core polymer in the particles is determined by the quantities of monomers consumed in the two stages of polymerisation. This ratio is typically 0.5:1 to 5:1 by weight, preferably 1:1 to 2:1. The core-shell polymer particles are preferably substantially in the range 50 nm to 1 micron in size, more preferably 100 to 200 nm on average. Such particles are markedly smaller than those particles previously added to PVC compositions to give improved anti-slip properties in floor coverings. In one embodiment of the invention, the core polymer, has a lower Tg than the shell polymer. The Tg of the core polymer in this embodiment is preferably in the range -50°C to 60°C, but it is more preferably around or below normal room temperatures, and it is most preferably in the range -20°C to 20°C. The Tg of the shell polymer in this embodiment is preferably above normal room temperatures, and it is more preferably in the range 50°C to 110°C. Such particles have a relatively hard shell and a relatively soft core at normal room temperatures. T.G. Fox, Bulletin of the American Chemical Society, Vol. 1 (1956), page 123, provides an equation which can be used to predict the Tg of a copolymer, and this equation can be used to advantage when formulating core-shell polymer particles for use in the invention. This equation is: 1/T AB = X A /T A + d-X A )/T B where A _ is the Tg of a binary copolymer of monomers A and B; T A and T„ are the Tg's of the homopolymers of A and B; and X A is the weight fraction of A in the copolymer. The equation can be adapted appropriately for copolymers of more than two monomers. Glass transition temperatures of a wide range of homopolymers are tabulated in Polymer Handbook, edd. J. Brandrup & E.H. Immergut, 3rd edition, iley- Interscience (1989), at pages VI/213-258. The core polymer preferably comprises major amounts of monomers such as styrene and alkyl acrylates in which the alkyl group contains 1 to 8, more preferably 4 to 8, carbon atoms. The shell polymer preferably comprises major amounts of monomers such as styrene and methyl methacrylate. The core polymer may comprise a polyfunctional olefinic compound, for example ethylene glycol dimethacrylate, as crosslinking agent. This has been found to offer improved results, possibly as a consequence of better separation between the core and the shell polymers in the particles. Such polyfunctional olefinic compounds preferably comprise 0.1 to 5% by weight, more preferably 0.5 to 2% by weight, of the monomers used in the manufacture of the core polymer. In another embodiment of the invention, the core polymer has a higher Tg than the shell polymer. The Tg of the core polymer in this embodiment is preferably above normal room temperatures and is more preferably in the range 50°C to 110°C. The Tg of the shell polymer in this embodiment is preferably in the range -50°C to 60°C, but it is more preferably around or below normal room temperatures, and it is most preferably in the range -20°C to 20°C. Such particles have a relatively soft shell and a relatively hard core at normal room temperatures. The core and the shell polymers may suitably be formulated by use of the Fox equation as previously described. Plasticised PVC normally has a refractive index in the range 1.52 to 1.55, typically 1.54. The flexible PVC composition in the wear layer of a floor covering according to the invention can be considered to consist of plasticised PVC and the core and shell polymers. If it is desired to make floor coverings according to the invention which have a clear wear layer, then the refractive indexes of the plasticised PVC, the core polymer and the shell polymer in the flexible PVC composition should be substantially the same. They preferably differ by no more than about 0.02, more preferably by no more than 0.01, from each other. R. Albert and .M. Malone, Journal of Macromolecular Science - Chemistry, Vol. A6 (1972), pages 347-374, present at pages 371-373 an equation which can be used to predict the refractive index of a copolymer. This equation is: n AB = n X + n B - X'X A- where n is the refractive index of a binary copolymer of monomers A and B; n A. and n„B are the refractive indexes of the homopolymers of A and B; and X. is the weight fraction of A in the copolymer. The equation can be adapted appropriately for copolymers of more than two monomers. Refractive indexes of a wide range of homopolymers are tabulated in the aforementioned Polymer Handbook at pages VI/453-461. Examples of homopolymers having a higher refractive index than that of plasticised PVC include poly(styrene) and poly(vinylidene chloride). Examples of homopolymers having a lower refractive index than that of plasticised PVC include poly(butyl acrylate) and poly(methyl methacrylate) . This equation can suitably be used as a guide to choosing formulations for the core and the shell polymers. It is preferably used in combination with the aforementioned Fox equation for the prediction of Tg. It is a particular advantage of the invention that it is able to provide vinyl floor coverings which have a clear wear layer and improved anti-slip characteristics. Core-shell polymer particles prepared by emulsion polymerisation are normally obtained in the form of a latex, containing for example 30 to 50% by weight particles suspended in an aqueous medium. It is generally desirable to isolate the polymer particles from the aqueous medium before use in a PVC composition. This may be done by methods known in the art, for example by spray drying or by coagulation followed by filtration. Coagulation may be effected for example by -freezing and thawing the latex, or by adding a solution of a salt as a coagulant. Preferred coagulants are weak aqueous solutions of aluminium salts, for example aluminium sulphate or alum. Known compositions useful in the manufacture of vinyl floor coverings typically comprise: Secondary stabiliser 2-8 all parts being by weight. Flexible PVC compositions which are suitable for the manufacture of floor coverings according to the invention, and which comprise PVC, a plasticiser and core-shell polymer particles, can be prepared using conventional blending equipment known in PVC technology. Such compositions may also comprise other components such as pigments, processing aids and stabilisers known in the art. They can conveniently be prepared by powder blending. The flexible PVC compositions used in the manufacture of the floor coverings of the invention preferably comprise 5 to 60% by weight core-shell polymer particles, more preferably 10 to 40%, most preferably about 20% to about 40% or about 25% to 40%. It has been found that the anti-slip characteristics of the floor coverings improve as the content of core-shell polymer particles is increased to around 20 or 25% by weight, and they then remain more or less constant or improve less significantly thereafter. It has also been found that the compositions become softer, for example as measured by Shore D hardness, as the content of core-shell polymer particles is increased. This can be compensated by adjusting the formulation of the plasticised PVC composition, preferably by reducing the level of plasticiser. Such compensation is generally desirable in order to provide good resistance to scuffing and abrasion. Floor coverings according to the invention can be prepared using conventional equipment for the manufacture of vinyl tile floor coverings. A typical procedure for the manufacture of vinyl film suitable for the manufacture of vinyl floor coverings according to the invention is as follows. PVC powder, plasticiser and core-shell polymer particles are blended in a Z-blade mixer. Other components, for example pigments, processing aids and stabilisers, may be blended into the mixture at the same time. In general, no pigment is added if it is desired to produce a film suitable for use as a clear wear layer. The powdery blend is then charged into an intensive mixer, for example a Banbury mixer, and worked to produce a hot melt, for example at a temperature of 140-180°C. The melt is further worked by being fluxed on a two-roll mill heated to a similar temperature. It is then extruded between rollers to form a vinyl film having a controlled thickness, for example in the range 0.2-1.0 mm, and this film is cooled and collected on a reel. The floor coverings of the invention are preferably film laminates. A typical procedure for the manufacture of film laminate vinyl floor coverings is as follows. Two or more vinyl films are fed between heated nip rollers in order to fuse the films together. One or more sets of nip rollers may be used. The films may all be fed between the first set of nip rollers or one or more of the films may be introduced between later sets of nip rollers. The resulting laminated film may then be bonded to a backing fabric by passage of the film and the fabric between a further set of heated nip rollers. The fabric may be coated with a plastisol containing a blowing agent to provide a foam layer. Alternatively, the lower face of the laminated film may be embossed to provide a roughened surface, for example by contact with a fabric carrier or a drum. The wear layer may be embossed with a pattern by passage over a heated drum whose surface is the negative of the desired pattern. The resulting vinyl floor covering is then cooled and is typically cut into strips or squares ready for use. A film laminate floor covering may consist of one or two clear wear layers according to the invention, a patterned printed layer, a white or other pigmented face-ply layer to provide visual depth to the pattern and a black backing layer to ensure opacity. Brief Description of the Drawing Figure 1 illustrates a preferred floor covering according to the invention. Referring to Figure 1, a floor covering according to the invention is a laminate of films in the following sequential order: (1) Clear wear layer, 500-750 micron thick, consisting of a flexible PVC composition which comprises core-shell polymer particles in addition to the conventional ingredients; (2) Clear wear layer as (1); (3) Patterned printed layer, 50-125 micron thick; (4) White or other pigmented face ply-layer, 250-500 micron thick; (5) Black backing layer 750-1000 micron thick, embossed on the underside with the pattern of a fabric carrier belt. In an alternative construction, the clear wear layer (2) consists of plasticised PVC and contains no core-shell polymer particles. The invention is illustrated by the following Examples, in which parts and proportions are by weight unless otherwise specified: Example 1 ( a ) Preparation of Core-Shell Polymers The following general procedure was used. Water (700 ml), sodium dodecyl sulphate emulsifier (6 g) and monomers for the core polymer (75 g) were charged into a 1.5 L polymerisation reactor. The mixture was stirred under nitrogen and heated to 60°C in a thermostatted bath. A 1.5% aqueous solution of potassium persulphate (100 ml) was added as initiator. Monomers for the core polymer (225 g) were then added at a rate of 2 ml/min, after which the reaction was allowed to continue for a further 1-2 hours. The product was a latex consisting of a suspension of core polymer particles in water. Monomers for the shell polymer (300 g) were then added at a rate of 2 ml/min, after which the reaction was allowed to continue for a further 1-2 hours. The product was a latex consisting of a suspension of core-shell polymer particles in water, weighing 1500 g and with a 40% solids content. The latex was cooled to room temperature and filtered through glass wool to remove any coagulum. The latex was frozen solid by immersion in a bath containing solid carbon dioxide and was subsequently allowed to thaw, following which the core-shell polymer was recovered by filtration. The freeze-thaw process was found necessary to permit the collection by filtration of the coalesced polymer particles. The collected polymer was dried in a fan oven at 40°C to yield a powder. Photon Correlation Spectroscopy (Malvern Autosizer IIC - Trademark) of the latex showed that core polymer particles had an average size around 70 nm and that core-shell polymer particles had an average size around 80-90 nm. Scanning - 12 - Electron Microscopy (SEM) of the powder showed it to contain loose aggregates of particles, up to 100 micron in size. The following core-shell polymers were prepared (BA = n-butyl acrylate; EGDM = ethylene glycol dimethacrylate; MMA = methyl methacrylate; St = styrene; VDC = vinylidene chloride). The shell polymer was 50 MMA/50 St in each case. Reference Core Polymer Mean particle Core Shell Core Shell size in latex Tg°C Tg°C R.I. R.I. nm CEC1 60 BA/40 St 85.2 17 104 1.51 1.54 CEC2 41 BA/59 St 80.3 56 99 1.54 1.54 CEC3 40.5 BA/58.5 St 80.7 51 100 1.54 1.54 /1.0 EGDM Core polymer and shell polymer Tg were measured by Differential Mechanical Analysis (DMA) on the core-shell polymer particles, it being assumed in each case that the core would have a lower Tg than the shell in accordance with the Fox equation. Core and shell polymer refractive indexes (R.I.) were calculated using the equation of Albert and Malone. (b) Preparation of Core-Shell Polymers A polymer latex was prepared according to recipe CEC3 of Example 1(a), but containing 50% solids. This latex was allowed to cool, filtered through glass wool and added to an equal volume of 0.5% aqueous aluminium sulphate solution at 60°C. This gave coagulated polymer particles which could be collected by filtration and dried to yield a fine powder. (c) Preparation of Sample Tiles (i) 100 g PVC composition consisting of 100 parts PVC, 26 parts dioctyl phthalate as plasticiser, 5 parts epoxidised soya bean oil and 2 parts stabiliser was fluxed on a 2-roll mill at 160°C to form a melt. Core- shell polymer powder prepared as in 1(a) or 1(b) was added directly into the mill, and the mixture was fluxed for a further 5 minutes before extrusion to form a sheet uniformly 1-2 mm thick. (ii) 100 g PVC composition of the same formulation as 1(c) (i) in powder form was blended with core-shell polymer powder prepared as in 1(a) or 1(b). The powder blend was fluxed on a 2-roll mill at 160°C for about 5 minutes. The resulting melt was extruded and pressed between bead-blasted sheet aluminium or siliconised paper at 158°C and 5500 kPa (800 psi) to form a sheet uniformly 0.1-2 mm, typically 1 mm, thick. (d) Testing of Sample Tiles Slip resistance in this and the following Examples was measured by the technique published by RRL (Road Research Laboratories) or by the technique published by SATRA (Shoe and Allied Trades Research Association) . Higher values indicate higher coefficients of friction and therefore better slip resistance. The RRL technique is described in a booklet entitled "Portable Skid Resistance Tester - Instructions for Use" by W.S. Stanley & Co. Ltd, who supply apparatus designed for this test. The SATRA technique is described and assessed by M.P. Wilson in "Development of SATRA Slip Test and Tread Pattern Design Guidelines", Slips, Stumbles and Falls: Pedestrian Footwear and Surfaces, ASTM STP 1103, B.E. Gray. Ed., American Society for Testing and Materials, Philadelphia, 1990, pp. 113-123. The tests were performed using either a TPU (thermoplastic polyurethane) heel or a 4S (rubber to RAPRA specification) heel. (RAPRA is the Rubber and Plastics Research Association) . A desirable minimum value for RRL slip resistance may be taken to be 30. A desirable minimum value for SATRA slip resistance may be taken to be 0.30. Conventional PVC tiles generally have a SATRA wet slip resistance, measured with a TPU heel, below that value. The following test results were obtained: The advantage in appearance of using core and shell polymers with refractive indexes matched to that of plasticised PVC can be seen. A " significant improvement in slip resistance was observed in all cases. Example 2 Vinyl floor tiles having a clear wear layer, which was a flexible PVC composition comprising 10% core-shell polymer CEC3, were made on conventional factory equipment. They showed a SATRA wet slip resistance of 0.35 using a TPU heel. Control samples omitting the core-shell polymer typically showed a value of 0.26 in this test. Example 3 Vinyl floor tiles were made according to Example 2, except that the level of plasticiser (dioctyl phthalate) was altered to provide tiles having a hardness as close as possible to 58 Shore D. The following results were obtained, slip resistance being measured by the SATRA test: Core-shell polymer particles identified by the reference CEC52 were made by the procedure of Example 1(a) to the following composition (MA = methyl acrylate) : Core 40.5 BA/58.5 St/1.0 EGDM Shell 44.8 MA/55.2 St The Tg values of the core and shell polymers estimated by the Fox equation were 16°C and 53°C respectively. The R.I. of both polymers estimated by the equation of Albert and Malone was 1.54. The mean size in the latex of the core polymer particles was 93 nm and of the core-shell polymer particles was 111 nm. Vinyl floor tiles incorporating CEC52 were made by the method of Example 2, and the following results were obtained: CEC52 Shore D SATRA Slip Resistance content % Hardness TPU 4S Wet Dry Wet Dry 19 51.5 0.48 0.57 0.65 0.85

Claims: CLAIMS 1. A vinyl floor covering having a wear layer of a flexible PVC composition containing conventional ingredients, characterised in that the flexible PVC composition additionally contains core-shell polymer particles. 2. A vinyl floor covering according to claim 1, wherein the size of the core-shell polymer particles is substantially in the range 50 nanometre to 1 micron. 3. A vinyl floor covering according to claim 2, wherein the average size of the core-shell polymer particles is in the range 100 to 200 nanometre. 4. A vinyl floor covering according to any preceding claim, wherein the Tg of the core polymer is lower than the Tg of the shell polymer. 5. A vinyl floor covering according to claim 4, wherein the Tg of the core polymer is in the range -50°C to 60°C. 6. A vinyl floor covering according to claim 5, wherein the Tg of the core polymer is in the range -20°C to 20°C. 7. A vinyl floor covering according to any of claims 4 to 6, wherein the Tg of the shell polymer is in the range 50 to 110°C. 8. A vinyl floor covering according to any of claims 4 to 7, wherein the core polymer comprises a polyfunctional olefinic compound. 9. A vinyl floor covering according to claim 8, wherein the core polymer comprises 0.1 to 5 percent by weight of units of the polyfunctional olefinic compound. 10. A vinyl floor covering according to claim 9, wherein the core polymer comprises 0.5 to 2 percent by weight of units of the polyfunctional olefinic compound. 5 11. A vinyl floor covering according to any of claims 1 to 3, wherein the Tg of the core polymer is higher than the Tg of the shell polymer. 12. A vinyl floor covering according to claim 11, wherein the Tg of the core polymer is in the range 50 to 10 110°C. 13. A vinyl floor covering according to either of claims 11 and 12, wherein the Tg of the shell polymer is in the range -50°C to 60°C. 14. A vinyl floor covering according to claim 13, 15 wherein the Tg of the shell polymer is in the range -20°C to 20°C. 15. A vinyl floor covering according to any preceding claim, wherein the wear layer is a clear wear layer. 16. A vinyl floor covering according to claim 15, 0 wherein the refractive indices of the core polymer, the shell polymer and the plasticised PVC, which together form the flexible PVC composition, differ by no more than about 0.02 from each other. 17. A vinyl floor covering according to claim 16, 5 wherein the refractive indices of the core polymer, the shell polymer and the plasticised PVC, which together form the flexible PVC composition, differ by no more than about 0.01 from each other. 18. A vinyl floor covering according to any preceding claim, wherein the flexible PVC composition comprises 10 to 40 percent by weight core-shell polymer particles. 19. A vinyl floor covering according to claim 18, wherein the flexible PVC composition comprises about 20 to 40 percent by weight core-shell polymer particles. 20. A vinyl floor covering which is a laminate of films in the following sequential order: (1) clear wear layer, 500 to 750 micron thick, consisting of a flexible PVC composition which comprises core-shell polymer particles in addition to the conventional ingredients; (2) clear wear layer as (1); (3) patterned printed layer, 50 to 125 micron thick; (4) pigmented face ply layer, 250 to 500 micron thick; and (5) black backing layer, 750 to 1000 micron thick, embossed on the underside with the pattern of a fabric carrier belt.

Inventors: CLIFFORD CATHERINE EMMA, JAMES MARTIN IAN, WILSON GARY JOHN, CLIFFORD, CATHERINE, EMMA, JAMES, MARTIN, IAN, WILSON, GARY, JOHN
Applicants: AMTICO CO, CLIFFORD CATHERINE EMMA, COURTAULDS PLC, JAMES MARTIN IAN, WILSON GARY JOHN, THE AMTICO COMPANY LIMITED
Application year: 1993
Main classification: D06N7
IPC codes: B32B27, C08L27, C08L51, D06N3, D06N7