Brush filaments

The invention provides novel, improved wear indicating brush filaments and novel, improved coextrusion process for their manufacture. Essentially, the filaments of the present invention are coextruded filaments which include a longitudinal surface providing a boundary about the cross-sectional area of the filament and the longitudinal surface and/or the cross-sectional area presents a two colored region adapted to provide a visual signal indicative of wear in response to filament use. The filaments can be natural or synthetic materials. In filaments of the present invention, the colored region provides an initial color or color intensity viewable to the user. As wear is produced by continuing use of the filaments, the intensity of the colored region changes to a point which signals the user that the filament no longer provides the requisite performance characteristics for effectively performing its assigned function.

BACKGROUND OF THE INVENTION 
1. The Field of the Invention 
This invention relates to filaments (or fibers) for brushes. More 
precisely, this invention relates to novel, improved filaments for oral 
care brushes and to oral care brushes including the novel improved 
filaments. 
2. Description of the Prior Art 
Brushing the teeth is universally recommended as the most effective way to 
maintain oral hygiene. While there is disagreement as to the most suitable 
form of brushing, the toothbrush is acknowledged to be the most effective 
aid in cleaning the teeth. In turn, the cleaning effectiveness of a 
toothbrush is dependent on such factors as the brushing habits of the 
user, the frequency, intensity and duration of brushing and the quality of 
the brush filaments. There is considerable debate in the art relating to 
the features which provide a toothbrush having maximized cleaning 
effectiveness. These factors include the material, size, shape, strength 
and resiliency of the brush filament and the length, width and overall 
shape and area of the brushing surface. Other features affecting the 
cleaning effectiveness of a toothbrush include the number of tufts 
(bundles of individual filaments), the number of rows of tufts and the 
arrangement of the tufts on the brush head. However there is general 
agreement in the art that wear is a crucial factor which can dramatically 
diminish the effectiveness of a toothbrush in maintaining oral hygiene. 
For example, the art recognizes and acknowledges that diminished 
effectiveness of a toothbrush by wear can result in increased plaque 
accumulation and increased risks to periodontium tissue. 
The degree of wear of a toothbrush is primarily a function of the 
properties of the filament and the mechanical force applied to the brush 
during brushing. The degree of wear can also be accelerated to some extent 
by abrasive materials normally contained in dentifrices. Brush wear 
results in tearing, splaying, expansion and fraying of the filaments and a 
decrease in strength and resiliency of the filaments which is manifested 
by single filaments deviating from their original direction. Moreover, 
wear is manifested by a change in the overall shape and size of the 
brushing surface area and by changes in the texture of the filament. While 
toothbrush wear varies from use to user, studies indicate that the average 
toothbrush subject to average use has a useful effective life from about 
eight to twelve weeks. Thereafter, wear causes sufficient deterioration of 
the filaments to warrant replacement of the brush in order to assure 
continued maintenance of effective oral hygiene. 
Unfortunately, toothbrushes are not usually replaced regularly and often 
times are used far beyond their effective useful life. As mentioned, the 
dental profession has recommended replacement of toothbrushes after about 
three months of use. However, annual, toothbrush consumption figures 
indicate that toothbrush user replace their toothbrushes about once a 
year. The dental profession has made an earnest effort to educate the 
public about the need to assess the wear of a toothbrush being used to 
determine if it should be discarded and replaced. However, these efforts 
have had limited success since the user has the responsibility to remember 
the condition of a toothbrush which should be discarded and to remember to 
monitor and continually assess the condition of the toothbrush. 
Accordingly, a more effective approach is needed to provide reliable means 
to signal or warn a toothbrush user when a toothbrush has become 
sufficiently affected by the wear that it should be discarded and 
replaced. 
U.S. Pat. No. 4,802,255 and Pending PCT application Ser. No. 92/04589, both 
incorporated herein by reference, describe brush filaments, particularly 
toothbrush filaments which include a colorant which is adapted to provide 
a color intensity which can change in response to increased use of the 
filament to provide a signal indicative of filament wear. A line of 
toothbrushes covered by the claims of these patents has been successfully 
marketed by Oral-B Laboratories, Redwood City, Calif. under the 
Indicator.sup.R brand. In the manufacturing methods of these patents, an 
outer color region is provided by ring dying monofilaments. In a ring 
dying process, the filament is contacted with a dye for a time sufficient 
to at least color the outer surface and to also penetrate into a portion 
of the cross sectional area. 
Through a careful investigation of the Indicator.sup.R products we have 
discovered two previously unknown shortcomings or problems with the 
product. First, dye penetration is not uniform at different depths along 
the annular circumference of the bristle. As a result, the darker regions 
on the outside wear first, while lighter blue persists for a considerable 
period of time. It would be desirable to have a darker region which is 
uniform in color density for a fixed depth along the annular circumference 
of the bristle ring. This would result in a line of clear demarkation of 
dyed sheath material versus undyed core filament material. The second 
problem relates to overall dye intensity. During the ring dyeing operation 
if a high level of dye concentration is used, the resulting bristles are 
darker; however, the dye material penetrates into the core at a much 
greater degree. Thus, it is difficult to produce a dark colored, ring dyed 
material with a thin, dark ring coating. 
The following background patents also relate generally to the present 
invention: 
U.S. Pat. No. 2,328,998 to Radford, issued Sep. 7, 1943 discloses an 
attrition product, e.g., toothbrush filament including a co-mingled 
abrasive material. FIGS. 3 and 4 disclose a composite filament including a 
non-abrasive core with a co-mingled abrasive material. FIG. 8 shows a 
batch-wise apparatus for extruding the filament of FIGS. 3 and 4. 
U.S. Pat. No. 3,016,554 to Peterson, issued Jan. 16, 1962 discloses a wire 
brush filament on to which a plastic coating, e.g., nylon, is extruded as 
a vibration dampening coating. This bristle is then dip coated with a thin 
epoxy protective layer. The nylon layer is said to have a general 
thickness about one half the diameter of the wire core plus or minus 50% 
(see column 2, lines 67 to 69). 
U.S. Pat. No. 3,258,805 to Rossnan, issued Jul. 5, 1966 discloses a wear 
indicating toothbrush bristle comprised of a nylon filament which is 
entirely encased within a coating of colored rubber (see column 1, lines 
30 to 37). This bristle is said to produce an erasing, instead of 
grinding, action in cleaning the teeth. Also, it states that the wear on 
the tips of the bristles gives a visual indication of warning when the 
brush is ready to be discarded. 
U.S. Pat. No. 3,327,339, issued Jun. 27, 1967 and Re. No. 26,688, reissued 
Oct. 14, 1969 to Lemelson disclose composite plastic filaments formed by 
extruding different polymers one over the other and each imparting a 
different useful characteristic to the composite filament (see column 1, 
lines 11 to 15). The core member is said to be preferably a synthetic 
textile filament such as nylon. The outer jacket layer is described as a 
flexible cellular plastic material (see column 2, lines 37 to 41 and 
Column 4, lines 74 through column 5, line 7). FIGS. 3 and 4 show an 
element comprising a core made from a more rigid material than the 
surrounding jacket which is made by simultaneously extruding both the core 
and jacket as a unitary rod or filament (see column 3, line 74 to column 
4, line 20). The jacket is said to comprise the major portion of the 
filament and is in the order of 0.005 inches to 0.030 inches or greater in 
outside diameter while the core element or thread is about 0.010 inches in 
diameter or less (see column 4, lines 70 to 74). 
U.S. Pat. No. 3,403,070 to Lewis, issued Sep. 24, 1968, discloses a 
composite filament. These cellular foam filaments are said to be useful as 
bristles and mop fibers. The filament comprises an unoriented polyolefin 
shell and an inner core of polyurethane foam. The shell is described as a 
coating which has a radius to the annular wall thickness ratio of at least 
4 to 1. Furthermore, Applicants believe this coated filament is not an 
extruded bristle due to the soft, foamy nature of the core material. 
U.S. Pat. No. 4,263,691 to Pakarnseree, issued Apr. 28, 1981, discloses a 
toothbrush bristle comprised of a stiff core made of a hard thermoplastic 
material such as nylon 6 or high density polyethylene and an outer 
elastomeric sheath. The sheath is said to be made of a soft thermoplastic 
such as ethylene vinyl acetate co-polymer, polyethylene, poly-vinyl 
chloride, or natural or synthetic rubbers. Regarding thicknesses, it is 
disclosed that if the diameter of the core is d and the outer diameter of 
the sheath is D, then the ratio d:D can be in the range of 1:1.01 to 1:2.5 
(column 2, lines 10 to 13). FIG. 4 illustrates a filament of material 
produced by continuous extrusion molding of a substance suitable for the 
core and a substance suitable for the sheath of the bristle. The filament 
may be made into bristles with the stiff core exposed at the ends or with 
the ends of the stiff core covered by the sheath (see column 2, lines 14 
to 28 and FIG. 7). 
U K. patent application, Ser. No. 2,137,080, to Weihrauch, assigned to 
Coronet-Werk Heinrich Schlerf GmbH discloses plastic bristles or filaments 
for brushes which also change color in response to wear. The filaments 
disclosed in the U.K. Application are composite filaments and include a 
colored core completely surrounded by an outer cover material having a 
color different from the core color. The cover is injection molded 
directly onto the core. In the disclosed filaments, the core is a 
reinforcing element and is relatively hard and stiff to control the 
rigidity of the filament while the outer cover material is softer than the 
core material and is more susceptible to wear. In use, the cover material 
becomes worn in the area of the rounded end of the filament and peels or 
breaks off to expose the core color to signal that the brush should be 
discarded. 
It is an object of the present invention to produce a composite toothbrush 
filament containing a colored outer sheath and a different colored inner 
core material which has a clear line of demarkation at a magnification of 
about 250X. 
It is yet another object of the present invention to provide a wear 
indicating toothbrush filament with a greater degree of color intensity in 
the core and/or sheath. 
It is another object of the present invention to produce striped wear 
indicating bristles as an alternative to ring dyed bristles. 
It is yet another object of the present invention to provide a more 
efficient method of producing wear indicating bristles. 
Still yet another object of the present invention to provide color-change 
type, wear indicating bristles with a more desirable color change. 
These and other objects will be evident by the following: 
BRIEF SUMMARY OF THE INVENTION 
The invention provides novel, improved wear indicating brush filaments and 
novel, improved coextrusion process for their manufacture. Essentially, 
the filaments of the present invention are coextruded filaments which 
include a longitudinal surface providing a boundary about the 
cross-sectional area of the filament and the longitudinal surface and/or 
the cross-sectional area presents a two colored region adapted to provide 
a visual signal indicative of wear in response to filament use. The 
filaments can be natural or synthetic materials. In filaments of the 
present invention, the colored region provides an initial color or color 
intensity viewable to the user. As wear is produced by continuing use of 
the filaments, the intensity of the colored region changes to a point 
which signals the user that the filament no longer provides the requisite 
performance characteristics for effectively performing its assigned 
function.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
As used herein, the term "core" refers to the central portion of a bristle 
material as examined at the cross-section. The FIGS. designate the core as 
28. 
As used herein, the term "sheath" refers to an outer coating layer(s) over 
the core material 28 on a bristle. The Figures designate sheaths as 26 and 
32. Preferably the sheath provides a uniform coating around the entire 
perimeter of the core. However, also encompassed in the present invention, 
are bristles wherein the sheath material only partially covers the 
perimeter, forming a stripe(s) or spiral(s) down the length of the bristle 
material. Preferably, the sheath region extends inwardly into a portion of 
the overall cross-sectional area of the filament for a distance equivalent 
to about 10 per cent or less of the filament diameter. 
Although bristles have been produced by coextrusion in the past, these 
processes suffered from at least two shortcomings. First, the bristle 
materials were made of radically different core and sheath materials 
and/or processing conditions which resulted in severe failure of the 
sheath upon wear. Secondly, the coating materials were too thick. 
Applicants have alleviated these problems by coextruding the bristles 
through the use of a coextruding die in combination with shorter contact 
times, preferably through the additional use of a rotating coextruder die 
head. 
In a preferred toothbrush embodiment of the present invention, the novel 
filaments are included in toothbrushes of the type shown in FIG. 1. The 
toothbrush shall have at least one tuft securely affixed in or attached to 
the head, said tuft including a plurality of filaments according to the 
present invention. As shown there, the toothbrush 10 includes a handle 12 
and a head 14 having a plurality of tufts 16. Tufts 16 comprise a 
plurality of individual filaments and, tufts 16 are securely affixed in or 
attached to head 14 in manners known to the art. The configuration of head 
14 and tufts 16 can vary and may be oval, convex curved, concave curved, 
flat trim, serrated "V" or any other desired configuration. Additionally, 
the configuration, shape and size of handle 12 or tufts 16 can vary and 
the axes of handle 12 and head 14 may be on the same or a different plane. 
The longitudinal and cross-sectional dimensions of the filaments of the 
invention and the profile of the filament ends can vary and the stiffness, 
resiliency and shape of the filament end can vary. Preferred filaments of 
the present invention have substantially uniform longitudinal lengths 
between about 3 to about 6 cm., substantially uniform cross-sectional 
dimensions 24 between about 100 to about 350 microns and have smooth or 
rounded tips or ends. 
FIGS. 2 and 4 diagrammatically represent a most preferred filament of the 
present invention. As shown in the Figures filament 20 includes 
longitudinal surface 22 which terminates at a tip or end 18 and defines 
the boundary of the cross-sectional area 24 of the filament. 
Cross-sectional area includes two colored regions 26 and 28 which have 
different color or different intensities. As used herein the term "colored 
region" can mean a core or sheath material which is colored by a colorant 
prior to being extruded. It can also mean a core or sheath which is made 
of a plastic with a unique color. Furthermore, transparent or translucent 
regions are also considered by be "colored" as they are at least of 
different optical appearance than a truly pigmented or dyed region, as is 
also the case for a sheath/core varying degrees of color intensity. 
However, it is important that the core 28 and sheath 26 materials have 
visually different color, e.g., white core and blue sheath, transparent 
core and red sheath, light red core and dark red sheath, etc. Preferenced 
bristles according to the present invention comprise a white or 
transparent core and a dyed or pigmented sheath. 
Typically, a colored region 26 extends at least about surface 22 or 
preferably extends from surface 22 inwardly into a portion of 
cross-sectional area 24 to a distance 30 (FIG. 4) of region 26 into 
cross-sectional area . Preferably, colored region 26 provides an annular 
ring having a substantially uniform depth 30. Most preferably, this depth 
should not vary more than 20%, preferably not more than 10%, from the mean 
depth around the annular ring. In either event, colored core region 28 
occupies the remaining portion of the overall cross-sectional area defined 
by maximum diameter 24. Accordingly, sheath color region 26 provides an 
initial color intensity or color which is predominant and more conspicuous 
to the toothbrush user while the color intensity of core region 28 is less 
conspicuous. However, in response to wear produced by progressive 
brushing, the region 26 wears, and after sufficient wear the perceived 
change in color of the bristle to that of core region 28 signals the user 
that the filament is no longer effective. 
As was mentioned previously, colorants can be added to the core and/or 
sheath of the present invention. These colorants can be dyes or pigments. 
Preferred dyes providing region 26 are food dyes or certified food 
colorants. Suitable food dyes or colorants are F D & C red No. 40, 
erythrosine (F D & C red No. 3), brilliant blue F C F (F D & C blue No. 
1), indigotine (F D & C blue No. 2), tartrozine (F D & C yellow No. 5), 
sunset yellow F C F (F D & C yellow No. 6) and fast green F C F (F D & C 
green No. 3). The thermal stability of these materials is less of a 
concern due to the relatively short contact times of the present process 
Suitable pigments for use as colorants include any food grade pigments, 
such as titanium dioxide, metal flake pigments and nacreous pigments which 
impart a pearl luster. For a further discussion of colorants see Juran, 
Modern Plastics Encyclopedia. Vol. 67 (11), pp 167-175 (October 1990). 
The core and sheath of the present bristles can be extruded from a variety 
of polymeric materials. Preferably these materials are polyamides, acetal 
resins, such as Delrin 900 (mfg. DuPont) and polyesters, such as Rynite 
530, Rynite 545, Rynite 555 (mfg. by Dupont). Most preferably, the core 
and sheath are extrusions of Nylon 616, preferably Zytel 158L, Zytel 330 
or Zytel ST901, all manufactured by Dupont. Preferably the core and sheath 
are of the same material. Optionally, other additives known to those 
skilled in the art may be added to the bristle material such as 
polyethylane glycol, antioxidants, plasticizers, etc. 
The thickness of the sheath material 30 is coordinated with the wear 
characteristics of the filament so that the change in color provides a 
reliable indication of filament deterioration due to 12 weeks of typical 
wear. In general, with nylon core/nylon sheath filament, suitable 
coordination between the sheath thickness and colorant fastness (if any) 
and filament wear characteristics can be achieved if region 26 (FIG. 2) 
has an average depth equal to about 10% or less of the filament diameter. 
Preferably the average depth 30 is equal to about 5% or less of the 
diameter, especially when dealing with nylon/nylon filaments with a dye 
such as indigotine, also known as FD C No. 2. 
Filaments of FIGS. 2 and 4 may also be prepared with combinations of 
colored regions, each colored region providing a unique color intensity 
having substantially the same resistance to change in response to wear and 
use wherein the alternative each dye may provide a color intensity having 
a different resistance to change in response to wear and use. For example, 
as in FIGS. 3 and 5, a filament may be prepared with two colored sheath 
regions in which one colored layer 26 is more resistant to change in 
response to wear and use than the other 32. In this case the color of the 
outer region 26a will abrade in response to wear and use to provide a 
color which will be predominantly provided by the more resistant colored 
region. 
Another embodiment is a bristle wherein the sheath material only partially 
covers the perimeter. When a stationary die head is used, this would 
result in a bristle which has stripe(s) down the length of the bristle. 
When a rotating die head is used, this would result in a bristle which has 
sprial(s) down the length of the bristles. Optionally, a plurality of 
different colored bristle stripe(s) or spiral(s) may be utilized for 
greater visual impact. 
The bristle filaments of the present invention are produced by a 
coextrusion process. For a general discussion of coextrusion technology 
see Levy, Plastics Extrusion Technology Handbook, Industrial Press Inc., 
pages 168-188 (1981), incorporated herein by reference. FIG. 8 shows a 
schematic cross-sectional view of a coextrusion filament die 41. The die 
head unit comprises the core orifice 42, the sheath orifice 35. The sheath 
material inlet manifolds 48 and 48', and the core inlet manifold 47. 
Typically the entire die is heated. The best condition for making 
coextruded bristles is to have the melt viscosity of both resins, core 43 
and sheath 44, as close together as possible at the point of stream 
combination. This results in the minimum disturbance at the interface 
between the two materials and results in a clear line of demarkation along 
the cross-sectional area at a magnification of about 250.times.. A sharp 
interface between the core and the sheath can also be produced by 
adjusting contact time, material grades or by using different resins. This 
can clearly be seen in photomicrograph FIG. 6. 
In a preferred coextrusion unit according to the present invention, the 
system includes a coextrusion die as shown in FIG. 8 which includes a 
cross head sheath die which rotates about the axis of extrusion 49. The 
set up also includes two 3/4" Haake extruders, a cooling trough, a puller 
and a winder. Each extruder is equipped with a screw with a L/D ratio of 
25:1 and a compression ratio of 3:1 and a 5 HP motor capable of operating 
at screw speeds and processing temperatures of up to 250 rpm and 
500.degree. C., respectively. Each extruder incorporates six temperature 
controllers to control processing temperatures. The extrusion die has a 
core orifice 42 without exit diameter of 0.080 inches and a sheath orifice 
35 with an exit diameter of 0.085 inches. The core melt 43 is uncolored 
nylon (Zytel 158L) and the sheath melt 44 is a 1% indigotines/nylon blend. 
Both melts and the die 31 are maintained at a temperature of 
190.degree.-230.degree. C. The core extruder operates at 20 rpm, 608 psi; 
and 5263 m.gm torque. The sheath extruder operated at 2 rpm, 1827 psi and 
1416 m.gm torque. The screw speeds are optimized to minimize interfacial 
shear stresses. The particular connections between these physical 
properties would be apparent to one skilled in the art. Furthermore, a 
full production line in this area will also include additional processing 
hardware for orienting (draw process), annealing and finishing. 
Finally, to produce a 0.008" filament from the above extrusion dye (orifice 
equals 0.085") the draw down ratio is set at 10.625:1. By employing this 
technique the thickness of the outer sheath layer 26 ranges from 0.0001" 
to 0.0004", and can be produced at a diameter of 0.0002"plus or minus 20%, 
typically plus or minus 10%. This highly uniform coating layer thickness 
is achieved by optimizing the ratio of the two extruder speeds and 
cross-head design. For example, to extrude the above-mentioned 0.008" 
nylon bristles with a layer thickness of 0.0002", the ratio of the screw 
speed (sheath/core) is set at 10:1. Increasing the ratio results in a 
thinner outer layer up to a point when the outer layer becomes 
discontinuous, while increasing both screw speeds increases dye pressure 
and ends up degrading polymeric material. On the other hand reducing both 
screw speeds lowers the die pressure but reduces input. A discontinuous 
outer layer would of course appear as a stripe down the side of the 
bristle. Optionally a gear pump can be added to meter the materials more 
precisely. 
As mentioned previously, the die may incorporate a rotating sheath orifice 
45 to produce a more uniform coating on the filament. The technique 
involves rotating the outer frame (sheath frame) of a coextrusion die of 
from about 0.5 to about 50 RPM's depending on the rheological properties 
of the polymer used for forming the outer layer. When coating nylon 
bristles like the ones described above, a rotational speed of from about 
0.5 to about 10.0 is utilized, most preferably from about 0.5 to about 
5.0. A chain sprocket is added to the dye for the frame rotation. During 
the filament coextrusion the sprocket is rotated at a set speed controlled 
by a motor with a chain drive. This is depicted as the rotation arrow 39 
in FIG. 10. This frame rotation helps disperse the melt stream in the 
outer layer, thereby producing a uniform ultra thin layer. When the sheath 
screw speeds are metered back, discontinuous sheath coatings are produced. 
On a rotating die, this results in a swirling stripe around the filament 
similar to a barber's pole. Either of these concepts could also be used as 
a wear indicating bristle. 
Applicants consider equivalent embodiments to be part of the present 
invention. For example, noncircular bristles, such as square, hexagonal, 
or other geometric cross-sections, are also contemplated by the present 
invention. The invention and manner of making and using the invention will 
be more fully appreciated from the following non-limiting, illustrative 
examples: 
______________________________________ 
SELECTED EXAMPLES 
Overall Sheath 
Diameter Thickness Core Sheath 
(inches) (inches) Material Material 
______________________________________ 
1) .008 .003 Zytel 158 
2% blue, Zytel 
158, 1% Ti02 
2) .008 .003 Zytel 158 
3% blue, Zytel 
158, 1% Tio02 
3) .008 .003 Zytel 158L 
Zytel 158L (blue) 
4) .012 .001 Zytel 330 
Zytel ST901 (Black) 
5) .008 .003 nylon EVA (black) 
6) .008 .003 Zytel 158L 
EVA (blue) 
______________________________________