A dental floss having improved abrasion resistance composed of dental fibers coated with a UV-curable resin, and a method for producing a dental floss having improved abrasion resistance.

BACKGROUND OF THE INVENTION
 Tooth decay and dental disease can be caused by bacterial action resulting
 from the formation of plaque about the teeth and/or the entrapment of food
 particles in interstices between the teeth. Removal of plaque and
 entrapped food particles reduces the incidence of caries, gingivitis, and
 mouth odors as well as generally improving oral hygiene. Conventional
 brushing has been found to be inadequate for removing all entrapped food
 particles and plaque. To supplement brushing, dental flosses and tapes
 have been recommended.
 Dental flosses including a thickened "brush" portion have been developed.
 These flosses may also include a thin "floss" portion and a threader. The
 brush portion, when drawn between tooth surfaces, provides good cleaning
 action which removes materials left by a standard thin floss used alone.
 To form a brush floss, it is necessary to provide bulked filaments in a
 strand of floss, i.e., filaments which are separated and have a somewhat
 sinuous, random orientation.
 Dental flosses, both in brush and thin floss form, often include additives
 such as flavors or colors. These flavors have been conventionally applied
 by coating the additive onto the surface of the floss.
 Conventionally, polymers coating a filament are applied as solutions
 dissolved in a solvent. The production of polymer-covered filaments
 entails delay in the time required for the solvent to evaporate,
 generation of polluting solvent vapors, and the danger of residual
 unevaporated solvents. There is a need for improved filament coatings and
 for an improved method of producing a polymer-coated filament.
 SUMMARY OF THE INVENTION
 The present invention features a dental floss having improved abrasivity
 consisting of a dental filament coated with a UV-curable resin. Improved
 abrasivity is achieved by coating the fibers with a resin binder curable
 by exposure to ultraviolet (UV) radiation. Preferably, the resin is a
 composition comprising a clear oligomer resin and a UV-sensitive
 component. Preferably the oligomer resin is comprised of monomers having
 one or more cross-linkable functional groups such as acrylate or
 methacrylate. In specific embodiments, the monomers are epoxy acrylates,
 polyurethane acrylates, polyester acrylates, and acrylic acrylates. The
 UV-sensitive component is a photoinitiating compound which absorbs
 ultraviolet light and initiates polymerization of the monomers. In
 specific embodiments, the photoinitiating compound is benzoin, a benzoin
 alkyl ether, a benzyl ketal, an acetophenone derivative, a benzophenone,
 Michler's ketone, an .alpha.-acyloxime ester, a thioxanthone or a
 thioxanthone derivative, a quinone, anthraquinone or derivative, an
 organic peroxide, an organic sulfur compound, a metal compound or metal
 ion, an alkali dichromate, an organic phosphorus compound, a chlorosilane
 or an azo compound. In a preferred embodiment, the photoinitiator is
 benzophenone.
 The UV-cured resin coated dental floss of the invention may further
 comprise additional components that provide desired floss properties
 and/or health treatment. In a preferred embodiment, the UV-cured resin
 includes a compound for control of the floss friction coefficient, e.g.,
 abrasivity. In a specific embodiment, polytetrafluoroethylene (PTFE or
 Teflon.RTM.) powder is added to the UV resin formulation. In another
 embodiment, a compound is added to the UV-cured resin formulation for
 anti-caries tooth treatment. In a specifically preferred embodiment, the
 anti-caries treatment compound is sodium fluoride, stannous fluoride, or
 monosodium fluoride phosphate.
 The UV-curable resin coated dental floss of the invention may have one or
 more distinct sections, including 1) a conventional floss section for
 cleaning the interproximal surfaces between the teeth, 2) a bulked brush
 section having improved abrasivity toward plaque and a larger dimension
 than the floss.
 The coated dental filaments of the invention may be composed of mono
 filaments, e.g., filaments composed of one fiber, and multifilaments,
 e.g., strands having multiple fibers. Specific embodiments of the method
 of the invention include coating bulked or non-bulked filaments having a
 high or low elasticity, respectively, to produce dental floss having
 improved abrasivity.
 The invention further features a method for producing a dental floss having
 improved abrasivity. An oligomer resin containing a photoinitiating system
 is coated onto a dental fiber and exposed to UV radiation such that the
 resin coat is cured to the desirable degree of hardness.
 The method of the invention allows the degree of abrasivity of a floss to
 be controlled as desired.
 One object of the invention is to provide a dental floss having improved
 abrasivity.
 Another object of the invention is to provide an improved method of
 producing a coated dental floss having improved abrasivity.
 One advantage of the invention is the production of a resin-coated dental
 floss without the use of solvents.
 These and other objects, advantages and features of the present invention
 will become apparent to those persons skilled in the art upon reading the
 details of the compositions, composition components, methods and method
 steps of the invention as set forth below.

DETAILED DESCRIPTION
 Before the present UV-cured resin coated dental floss and methods of
 production are described, it is to be understood that this invention is
 not limited to particular materials and methods described, as such
 materials and methods may, of course, vary. It is also to be understood
 that the terminology used herein is for the purpose of describing
 particular embodiments only, and is not intended to be limiting, since the
 scope of the present invention will be limited only by the appended
 claims.
 Unless defined otherwise, all technical and scientific terms used herein
 have the same meaning as commonly understood by one of ordinary skill in
 the art to which this invention belongs. Although any methods and
 materials similar or equivalent to those described herein can be used in
 the practice or testing of the present invention, the preferred methods
 and materials are now described. All publications mentioned herein are
 incorporated herein by reference to disclose and describe the methods
 and/or materials in connection with which the publications are cited. The
 publications discussed above are provided solely for their disclosure
 prior to the filing date of the present application. Nothing herein is to
 be construed as an admission that the invention is not entitled to
 antedate such disclosure by virtue of prior invention.
 Definitions
 By the term "dental floss" is meant a filament or yarn suitable for use for
 removal of food particles in the interstices between teeth. A dental floss
 may be composed of a single filament (monofilament) yarn of a
 multifilament yarn, and may contain discrete sections of different
 thickness, e.g., a threader portion and a brush portion (FIG. 1).
 By the term "UV-curable resin", "ultraviolet-curable resin", or "UV-cured
 resin", and the like, is meant a coating which achieves the desired degree
 of hardness upon exposure to ultraviolet radiation. This occurs due to the
 presence of a photoinitiating compound which absorbs UV light, generates a
 free radical, and causes cross-linking of functional groups on resin
 monomers.
 By the term "cross-linkable functional group" is meant a chemical group,
 e.g., an acrylate or methacrylate, which is induced to react with another
 functional group upon activation by a free radical, resulting in
 cross-linking of the monomers containing the functional groups.
 By the term "photoinitiator system" is meant a molecule which absorbs
 visible or ultraviolet light, forming an excited state which can then
 either fragment into free radical functional groups ("type I") capable of
 initiating polymerization of the monomers, or alter a second molecule by
 abstracting a hydrogen from the second molecule and converting the second
 molecule into a free radical capable of initiating polymerization ("type
 II" photoinitiator).
 By the term "bulked" yarn is meant a multifilament strand composed of
 multiple individual filaments, where the individual filaments are in a
 random nonlinear conformation and remain entangled together. This results
 in an overall shortening of the overall length of a section of yarn
 relative to the length of the yarn when the filaments are placed under
 tension.
 UV-Cured Resin Coated Dental Floss
 The coating of solid material onto a dental material, such as dental floss,
 is generally achieved by dissolving the coating material in a solvent such
 as ethanol, acetone, ethyl acetate, triethylene glycol. Commonly used
 natural and synthetic resins are generally dissolved in a solvent, and the
 resin becomes hard after evaporation of the solvent. Examples of commonly
 used resins are shellac, benzoin resin, polyvinyl pyrrolidone, and
 colophony. U.S. Pat. No. 5,393,516 describes the coating of dental
 material with an antibacterial chlorhexidine adduct. The chlorhexidine
 adduct is formulated into a light-curing sealant applied with a
 paint-brush onto a molar fissure.
 Dental filaments and flosses. Dental filaments for use in the flosses of
 the invention are preferably mono filaments or multifilaments. In one
 embodiment, the dental floss is manufactured as multicomponent coextruded
 filaments. By "multicomponent" is meant a filament having two or more
 components; by "coextruded" is meant that at least two of the components
 are present in the form of substantially separate phases having a distinct
 interface between them, rather than being intermixed. The filaments may be
 formed by processes referred to in the art as "coextruded", but may also
 encompass filaments having the structure described above which are
 manufactured by other processes known to the art.
 The dental flosses having improved abrasivity include bulked and non-bulked
 fibers. A bulked nylon yarn alone is not adequate in stiffness to form an
 abrasive brush. If stretched or immersed in water its brush geometry will
 collapse. In order to provide brush abrasivity, a bulked yarn needs to
 have its geometry stabilized with a polymer over-coating. This coating
 acts to bind individual filaments of the bulked yarn together at contact
 points. The result of binding of filament to filament is a network
 structure similar to a fishing net but with three dimensions, which is
 flexible yet maintains its overall geometry when stretched or immersed.
 The abrasivity of such a bulked brush section is dependent on the
 properties of the polymer over-coating. If a brush section has a soft
 stretchy over-coating, the brush will be soft and stretchy; a hard brittle
 over-coating results in a brush with a hard brittle feel.
 The properties of a dental floss when wet are important. If the bulked
 nylon brush polymer over-coating is water soluble, the brush structure is
 likely to collapse when wet. A highly hydrophilic coating will be overly
 softened when wet, resulting in decreased abrasivity of the floss and less
 effectiveness for plaque removal.
 Ultraviolet radiation-curable resins. Any natural or synthetic resin can be
 used for coating the dental floss. The resin is composed of a mixture of
 crosslinkable resin or resins and a UV-sensitive component which initiates
 polymerization upon exposure to UV radiation. Preferably, the resin is a
 clear copolymer resin of a relatively high molecular weight, such as a
 polyurethane with molecular weight 1000.
 A variety of resinous compositions are known in the art to be susceptible
 to crosslinking by means of exposure to free radicals derived from
 excitation of added photoinitiators by (UV) radiation. Certain acrylate,
 methacrylate, and bismaleate vinyl ether blends are susceptible; see, for
 example, U.S. Pat. Nos. 3,066,112, 3,179,623, 3,256,266, and 3,301,743.
 Resinous composition such as urethane acrylates can be combined with
 radiation sensitive initiators such as benzophenone, and are cured upon
 exposure to UV radiation. In thin films, the cure can be accomplished in
 less than 1 second. Methods of photocuring a UV-sensitive resin are known
 in the art. See, for example, U.S. Pat. No. 4,380,435.
 Traditional UV-curable resins have molecular weights from 150 to 550.
 Generally, larger molecules pose a decreased toxicity risk than smaller
 molecules. For example, UV-curable monomer trimethylolpropane triacrylate
 (TMPTA) has a molecular weight of 296 and a toxicity (Drais) value of 5.
 When the molecular weight of TMPTA is increased to 912 by ethoxylation,
 the toxicity value drops to 0. Accordingly, the preferred oligomer monomer
 used in the present invention, such as polyurethane, has a high molecular
 weight, e.g., 1000.
 The UV-curable resin of the present invention is composed of a
 monofunctional or multifunctional monomer molecule and a photoinitiating
 compound. Monomers typically used for UV curing have an acrylate or
 methacrylate functional groups. The number of acrylate, methacrylate,
 bismaleates and vinyl ether groups per monomer molecule is typically two,
 e.g., diacrylates or dimethacrylates, but monomers may be monofunctional
 or multifunctional.
 Photoinitiators and photosensitizers absorb ultraviolet light to form free
 radical groups which can initiate polymerization. A critical element in
 the selection of the appropriate photoinitiating compound is that it
 absorb light at a wavelength at which the remaining resin components do
 not absorb. Further, the appropriate photoinitiator must be soluble in the
 selected resin monomer. Additionally, the photoinitiator must have a high
 activity so that it initiates polymerization at very low concentrations,
 and must be nontoxic to humans in the final polymer. Suitable
 photoinitiators include benzoins, benzoin alkyl ethers, benzyl ketals,
 acetophenone derivatives, benzophenones, Michler's ketone,
 .alpha.-acyloxime esters, thioxanthones or thioxanthone derivatives,
 quinones, anthraquinones or derivatives, organic peroxides, organic sulfur
 compounds, metal compounds or metal ions, alkali dichromates, organic
 phosphorus compounds, chlorosilanes and azo compounds.
 Polymerization of the resin on the floss/filament material is accomplished
 by exposure of the UV-sensitive photoinitiator and free radical curable
 resin to UV radiation. Preferably, the source of UV radiation is a medium
 pressure mercury arc lamp. For example, resin-covered floss can be exposed
 to UV radiation from a commercial 85 watt Mercury vapor lamp for a time
 sufficient to cure the resin. Other sources of UV radiation known in the
 art may be used in the method of the invention such that the resin
 exhibits a desired hardness value, as determined by methods known in the
 art, including Barcoll or Vickers hardness scores (Mark's Standard
 Handbook for Mechanical Engineers, 9th Ed., pp. 5-13).
 Degree of cure is determined by measuring the solubility of the cured UV
 polymer in an appropriate organic solvent. Uncured monomers are completely
 soluble in organic solvents and will extract out of a partially cured UV
 resin into an appropriate solvent. Extraction amount decreases as degree
 of cure increases. A fully cured, cross-linked polymer will be insoluble
 in solvents. Additives which do not crosslink with the UV resins such as
 type II photoinitiators or their reaction products will be extractable
 after the UV resin polymerizes.
 The degree of cure of the UV-cured floss of the invention is determined by
 refluxing the floss in a suitable solvent. For example, in one method of
 determining degree of cure, refluxing floss that has been coated with a
 UV-curable coating in placed in 50:50 water/ethanol for 24 hours; the
 solvent is decanted and evaporated, and the residue weighed. The
 determination of the extractable portion of the coating provides a measure
 of the degree of cure.
 This degree of cure extraction test can be used to optimize the UV exposure
 required to cure a UV coating for specific coating formulations. A UV
 curable mixture can be coated onto the floss, passed through a 300 watt
 per inch medium pressure mercury arc UV lamp at a variety of speeds, and
 the percent extractable content after refluxing for 24 hours in a 50:50
 water/ethanol solution determined.
 Production of a UV-resin coated dental floss for use in the oral cavity of
 a human requires the careful selection of materials which will produce a
 floss with the desired mechanical and safety characteristics. The present
 invention provides a series of test protocols and test results that must
 be achieved in order to achieve the UV-resin coated dental floss of the
 invention. Specifically, final UV-cured resins and photoinitiating
 compounds must be nontoxic to humans and nonsensitizing in the oral
 cavity.
 In the method of the invention, one or more specific compounds can be added
 to the UV-curable resin coating for controlling the floss friction
 coefficient. In one embodiment, the compound added for controlling the
 floss friction coefficient is polytetrafluoroethylene (PTFE). A preferred
 floss friction coefficient is between 0.08 and 0.25. PTFE has a
 coefficient of friction value of 0.04. The addition of PTFE to the UV
 resin composition results in a lowering of the coefficient of friction
 value of the floss.
 The Examples below describe the test protocols used to evaluate the
 acceptability each component of the UV-curable resin composition in terms
 of toxicity (Example 1) and mechanical properties such as abrasiveness,
 brittleness, and flexibility (Example 2). Example 3 describes the test
 protocols used to determine important processibility characteristics, for
 example, ease of handling (viscosity and sensitivity to UV light
 (.lambda.)).
 Example 4 evaluates three UV-cured resin formulations for the threader
 portion of a dental floss in terms of their mechanical and toxic
 properties. Example 5 provides detailed toxicity results for a UV-cured
 resin formulated for application to the brush portion of a dental floss.
 Example 6 evaluates a UV-cured coating formulation in terms of percent
 extractables and toxicity.
 EXAMPLES
 The following examples are put forth so as to provide those of ordinary
 skill in the art with a complete disclosure and description of how to make
 and use various UV-curable resin compositions and perform the various
 methods of the present invention and are not intended to limit the scope
 of what the inventors regard as their invention. Unless indicated
 otherwise, parts are parts by weight, temperature is in degrees
 centigrade, and pressure is at or near atmospheric pressure. Efforts have
 been made to ensure accuracy with respect to numbers used, (e.g.,
 molecular weights, amounts, particular components, etc.) but some
 deviations should be accounted for.
 Example 1
 Acceptability Test Protocols
 Toxicity Test. Cytotoxicity is evaluated by using the USP Elution Test
 Method (MG057), which measures the biological reactivity of polymeric
 materials. Briefly, monolayers of L-929 mouse fibroblast cells were grown
 to confluency in duplicate flasks and exposed to an extract of the test
 article. The extract was prepared according to USP guidelines by placing
 the test article in 13 ml of MEM and extracting for 24 hours at 37.degree.
 C. The current NAmSA positive control was used as a positive control and a
 USP negative bioreaction control was used as a negative control. The cells
 were examined for evidence of cytotoxic effect after exposure to the
 extracts for 48 hours at 37.degree. C. Reactivity and grade were recorded
 according to Table 1. The NAmSA positive control was scored at 24 hours.
 The test article meets the requirements of USP if the biological response
 is less than or equal to grade 2 (Mild).
 TABLE 1
 GRADE REACTIVITY OBSERVATIONS
 0 None Discrete intracytogranules; no cell lysis
 1 Slight Not more than 20% of cells are round,
 loosely attached and without
 intracytoplasmic granules; some lysed
 cells are present
 2 Mild Not more than 50% of the cells are round
 and devoid of intracytoplasmic granules;
 extensive cell lysis and empty areas
 between cells
 3 Moderate Not more than 70% of the cell monolayer
 contain rounded and/or lysed cells
 4 Severe Nearly complete destruction of the cell
 monolayer
 Sensitization Test. Sensitization was measured with the Repeated Insulin
 Patch Test (RIPT) (Belsity (1989) J. Am. Acad. Dermatol. 21:822-829). 40
 grams of floss or threader were incubated at 37.degree. C. in 200 ml of
 0.9% saline for 24 h. The saline was then sterilized by filtration through
 a 0.2 micron filter. In the "induction phase" of the test, approximately
 0.2 ml of the test solution was applied as a patch to the back of the hand
 adjacent to the spinal mid-line of 50 male or female adult human volunteer
 subjects. The subjects were instructed to remove the patch 24 h after
 application. The procedure was repeated 3 times per week (Monday,
 Wednesday, Friday) for 3 weeks. 24 h rest periods followed the Tuesday and
 Thursday patch removals; a 48 h rest period followed the Saturday patch
 removal. The application site was scored just prior to each test
 application according to the following 6 point scale: 0, no evidence of
 any effect; +, barely perceptible (minimal, faint, uniform, or spotty
 erythema); 1, mild (pink, uniform erythema covering most of the contact
 site); 2, moderate (pink-red erythema uniform in the entire contact site);
 3, marked (bright red erythema with/without petechiae or papules); and 4,
 severe (deep red erythema with/without vesiculation or weeping).
 In the challenge phase of the sensitization test, a challenge patch with
 the same amount of test extract previously applied was applied to a
 previously unpatched test site approximately 2 weeks after the induction
 phase was complete. The site was scored 24 and 72 h after application.
 Flexibility and brittleness. Flexibility and brittleness are measured using
 an Instron stress strain tester on cast films of polymer (ASTM D638).
 Viscosity. Viscosity is determined with a rheometer having cone and plate
 geometry, constant stress mode (TA Associates).
 Sensitivity to UV light (.lambda.). The UV-visible light absorption spectra
 of resin and resin+photoinitiator is compared. A basic requirement for
 suitable resin and photoinitiator systems is that the photoinitiator
 absorbs UV radiation at a .lambda. different from that of the resin.
 Example 2
 UV-Sensitive Resins on Threader Portion of Floss
 The following formulas were evaluated by coating onto 640 denier yarn at 15
 feet per minute and UV curing with a 300 watt per inch medium pressure
 mercury vapor light:
 1. Cationic System

Union Carbide Cyracure 6110 84%
 Union Carbide PEG 300 11%
 Union Carbide 6990 5%
 FD&C Lake Pigment 0.15%
 Test results: The resin provided adequate stiffness, but had an irregular
 surface. The resin failed the cytotoxicity test (4).
 2. Bisphenol Epoxy Diacrylate System

Radcure Ebecryl 8402 93%
 Radcure Ebecryl BPO 7%
 Toxicity Results: The Radcure 8402 formulation had a cytotoxicity score of
 "1" (Table 2).
 TABLE 2
 Negative Negative
 A B Control A Control B
 Confluent Monolayer -- -- + +
 % Cells without 10 10 0 0
 Intracellular
 Granulation
 % Rounding 10 10 0 0
 % Lysis 10 10 0 0
 Grade 1 1 0 0
 Reactivity Slight Slight None None
 Example 4
 Production of UV-cured Resin Coated Dental Floss Threader
 Coating of floss with UV resins was accomplished on a coating line
 consisting of a yarn unwind station, a tensioning device, a coating die
 (FIG. 2), a medium pressure mercury arc lamp, and a speed controlled
 winding station. The UV curable resin was supplied to the extrusion die
 through a side injection port where it is delivered to the yarn at a
 controlled rate. The exit orifice of the extrusion die was sufficiently
 narrow to spread the UV resin uniformly on the yarn.
 The coating composition Radcure 8402:220:BPO (46.5:46.5:5:7) was coated and
 cured as described above at a variety of speeds, and percent extractables
 determined. The following results were obtained with the use of a Fusion
 Systems Model F300 medium pressure mercury arc UV lamp.
 The stiffness of this composition was compared with that of the
 conventional solvent-based polyurethane threader. As measured by Gurley
 stiffness, the conventional threader had a stiffness of 2.8 with a 38%
 coating (% of yarn substrate), while the UV formulation had a stiffness of
 3.3 with a lower coating weight of 33%.
 TABLE 3
 Speed UV Exposure
 (ft/min) Time (sec) % Extractable Cytotoxicity
 22.5 0.89 1.1 0
 26 0.77 1.0 0
 30 0.67 1.7 4
 Example 5
 Fluoride-Containing UV-Sensitive Resins
 Sodium fluoride (2.2%, 4.4%, and 6.6%) was-added to a 95:5 ratio of Radcure
 8402 and BPO. After coating and UV curing onto nylon yarn as described in
 Example 3 above, the release of fluoride was measured. Fluoride release
 was determined by stirring a 10 meter length of floss in TISAB II water
 and measuring the fluoride concentration with an Orion 96-09BN fluoride
 specific electrode. The results are shown in Table 4:
 TABLE 4
 Radcure 8402:BPO Sodium Fluoride Release
 (% sodium fluoride) (mg/0.5 m floss)
 2.2 0.05
 4.4 0.15
 6.6 0.20
 Commercial Floss 0.16
 The method used to add sodium fluoride to commercially available dental
 floss was adding sodium fluoride to a wax, and coating the wax onto nylon
 yarn. The addition of sodium fluoride to a commercial floss was found to
 weaken the wax coating, resulting in a dental floss more prone to fray
 between teeth. In contrast, the UV-cured floss containing sodium fluoride
 was not prone to fraying.
 Example 6
 UV-Cured Dental Floss with Altered Geometry
 A UV-cured dental floss with flattened geometry was produced by first
 coating nylon yarn as with the composition of Example 3 above, and
 UV-curing the floss while it was in contact with the surface of a drum.
 This allowed the coated uncured floss to flatten against the drum surface
 and to be fixed in that geometry by exposure to UV light. A flattened
 floss was found to be easier to insert between teeth relative to floss
 having a round geometry.
 The instant invention is shown and described herein in what is considered
 to be the most practical, and preferred embodiments. It is recognized,
 however, that departures may be made therefrom which are within the scope
 of the invention, and that modifications will occur to one skilled in the
 art upon reading this disclosure.