US20050015907A1

US20050015907A1 - Toothbrush

Info

Publication number: US20050015907A1
Application number: US10/924,047
Authority: US
Inventors: Matthias Georgi; Hans Kraemer
Original assignee: GlaxoSmithKline Consumer Healthcare GmbH and Co KG
Current assignee: GlaxoSmithKline Consumer Healthcare GmbH and Co KG
Priority date: 1999-04-22
Filing date: 2004-08-23
Publication date: 2005-01-27
Also published as: US20060288510A1; US20070151058A1

Abstract

A toothbrush made partly of a hard plastic material and partly of an elastomeric material, characterized in that at least part of the elastomeric material comprises an inner core (23) of a first soft elastomeric material wholly or partly enclosed within an outer flexible skin (24) of a second polymer material having different physical characteristics to the inner core polymer. Processes for making such a toothbrush are also described.

Description

This invention relates to toothbrushes, in particular to toothbrushes made of two construction materials being a hard plastic and a softer elastomeric material.
Such toothbrushes are known. For example EP 0 336 641 A discloses a toothbrush having elastomeric material grip-enhancing pads on its handle. WO 92/17092 discloses a toothbrush having elastomeric material parts forming a flexible link between its head and its handle. WO 92/017093 discloses a toothbrush having elastomeric material parts in its head, thereby modifying the flexibility at its head. DE 3923495 A discloses a toothbrush having elastomeric material inserts in cut-outs between the handle and neck region, also to modify flexibility. EP 0310482 discloses a toothbrush having elastomeric material parts around its head as a soft buffer to absorb impact forces between the head and the mouth of the user. EP 0371293 A discloses a toothbrush having elastomeric material parts in its neck. WO 94/05183 discloses a toothbrush having elastomeric material parts in one, two or three places in its handle, neck and head. Other parts of the head, handle and neck region of the above-mentioned toothbrushes are made of hard plastic materials. The above-mentioned disclosures are only examples of many in the literature. Two component toothbrushes of this type have been made and sold commercially for many years, for example the applicant's Dr BEST™ and AQUAFRESH™ range of toothbrushes.
The above-mentioned two component toothbrushes are made by a process of injection moulding in which the hard plastic parts are first injection moulded to form a frame or skeleton having cavities which define the position, size and shape of the elastomeric material parts to be formed therein, and in which cavities the elastomeric material parts are subsequently formed by a second injection moulding of fluid elastomeric material. Such a process is described for example in WO 94/05183.
A problem with known two-component toothbrushes of this type is that the elastomeric materials which are used need to be relatively soft, for example to enable them to fulfill their function of forming soft grip handles, flexible links or soft buffers. Soft elastomeric materials are relatively vulnerable to physical damage during use, e.g. due to impact with and abrasion from tooth surfaces or other surfaces. Soft elastomeric materials tend to be absorbent, and consequently when such toothbrushes are used, especially when they are exposed to detergents, soaps or toothpastes, moisture and dirt etc, their elastomeric material parts can absorb these substances. As a consequence these elastomeric material parts can deteriorate, become discoloured or dirty, and such absorbed substances can be difficult to remove by cleaning.
It is an object of this invention to offer a solution to this problem.
According to this invention a toothbrush is provided comprising a handle, a head on which are mounted bristles, with optionally a neck region between the head and handle, the toothbrush being made partly of a hard plastic material and having at least one part made of an elastomeric material, characterised in that at least part of the at least one elastomeric material part comprises an inner core of a soft elastomeric material wholly or partly enclosed within an outer flexible skin of a polymer material having different physical characteristics to the inner core material.
The term “elastomeric” material as used herein includes natural and synthetic elastomeric materials which have a semi-rigid rubbery nature, i.e. being soft and resilient to the touch, deforming under hand pressure and springing, preferably rapidly, back to substantially or exactly its original shape on release of the hand pressure. The term also includes polymers which are not inherently of a rubbery nature but which are rendered rubbery in nature by for example foaming or other treatment.
By “soft” herein is meant deforming under the pressure of the hand, particularly under pressures applied during normal use of the toothbrush in brushing the users teeth.
The outer skin should be a flexible skin, so that the elastomeric core material can deform as pressure is applied to the outer skin and transmitted to the core elastomeric material via the skin. It is desirable that the outer polymer skin binds with, preferably has good bonding characteristics to, the hard plastic material of the toothbrush.
The physical characteristics of the inner core and the skin polymer may differ in various ways. The skin may be a polymer material which is harder than the soft core elastomeric material. Additionally or alternatively the skin material may be a polymer material which is more resistant to physical damage or penetration by absorption of substances than the core elastomeric material as mentioned above. Additionally or alternatively the skin polymer material may have better surface properties than the core elastomeric material, e.g. having better tactile properties or being more easy to print upon than the core elastomeric material. Additionally or alternatively the core and skin may differ in quality, e.g. the inner core elastomeric material being a relatively cheap, low quality recycled elastomeric material and the outer skin material may be a higher quality, e.g. non-recycled polymer. For example there may be an inner core of a less dense, or soft or easily damaged or absorbent elastomeric material and an outer skin of a material which is denser or relatively less soft or more resistant to damage or less absorbent. The inner core elastomeric material may be a cheap elastomeric material, e.g. a recycled low quality e.g. easily damaged, elastomeric material, and the outer skin may be a thin skin of an expensive polymer of high quality or having desirable surface properties high quality. In this way the superior surface properties of the more expensive polymer may be enjoyed whilst minimising the quantity used.
By means of the invention the elastomeric material parts of the toothbrush may benefit from the softness of the core elastomeric material, e.g. in providing a comfortable grip, in forming a flexible region in the toothbrush, or in providing a soft buffer around the head, whilst the outer skin can provide a coating having advantageous properties, e.g. of physical resistance, resistance to absorption of substances, hand-feel, or of printing onto its surface. The outer skin can enhance the bonding of the elastomeric material and hard plastic parts of the toothbrush as this bonding can be made between the outer skin and the hard plastic parts, and a polymer material may be selected for the outer skin which has better bonding characteristics with the hard plastic material than the core elastomeric material.
In the toothbrush of this invention the elastomeric material part(s) may be in any or all of the parts where elastomeric material is at present used in two-component toothbrushes of the state of the art. For example the elastomeric material part(s) may comprise a pad on the grip handle, a flexibility modifying region, for example in the handle, neck or head or between the handle and neck or the neck and the head, or a soft buffer around the head. The elastomeric material part is preferably a pad on the grip handle. The toothbrush of the invention may also incorporate parts which are made of conventional elastomeric material, i.e. of soft elastomeric material throughout without the less soft coating skin.
The outer skin and inner core elastomeric material polymers should preferably be compatible to encourage bonding between them. The outer skin and inner core materials may be inherently compatible, or alternatively they may be of materials which are modified to be compatible with each other.
The outer skin polymer may be an elastomeric or non elastomeric material.
For example the inner core material and the skin material may comprise the same polymer, i.e. including the same monomeric units, but including different additives or quantities of additives to give the core and skin materials such different physical characteristics.
For example the core and outer skin may comprise the same polymer, i.e. both being an elastomeric material and comprising the same monomer units, but being different grades of the same polymer. For example such different grades may differ in their relative density, softness, resistance to damage or ability to absorb environmental substances or other characteristics.
The core and outer skin may be different polymers, i.e. comprising different monomer units or different combinations of monomer units.
In a preferred embodiment the core comprises a foamed elastomeric material. With such a core the outer skin may be an elastomeric material or a non-elastomeric material. If the skin in this embodiment is an elastomeric material it may be the same elastomeric material as the core but in a different grade, e.g. harder, more resistant or higher quality than the core. Such an elastomeric skin, whether or not it is the same elastomeric material as the core, may be a non-foamed elastomeric material, or it may be a foamed elastomeric material having different characteristics, e.g. different void characteristics, to the inner core material so as to give the outer skin different properties to the inner core, e.g. so as to make the outer skin more dense, less soft and/or more resistant to penetration or absorbency than the inner polymer. For example a higher number of gas bubbles per unit volume in such a foamed polymer can result in a softer polymer than a lower number of gas bubbles per unit volume. For example the difference in void characteristics may comprise the skin having a lower number of gas bubbles per unit volume than the core.
Suitable elastomeric materials for use as the core polymer include thermoplastic elastomer (“TPE”) materials, for example compounds based on styrenic block copolymers such as styrene-ethylene-butadiene-styrene (SEBS) copolymers, e.g. the material Thermolast-K™ (available from Gummiwerk Kraiburg GmbH & Co, Germany), polyblends based on EPDM/PP such as the material Sarlink™ (available from DSM Thermoplastic Elastomers NL), or polyacrylate/PP blends such as the material Pacrel™ (available from Optatech Corporation, Suomi), thermoplastic polyurethanes such as Desmopan™ (available from Bayer AG, Germany), thermoplastic copolyesters such as Hytrel™ (available from Du Pont De Nemours, USA) natural or synthetic latex type elastomers such as Baystal S™ or Baypren Latex™ (available from Polymer Latex, DE), polychloroprenes such as Baypren™ (available from Bayer AG, DE), natural rubber and foamed polymers such as polyurethane foams. Preferably such core polymers are used in a foamed form, e.g. incorporating a dispersion of gas bubbles in their structure.
A core of a foamed material such as a foamed elastomeric material may be formed in situ in an injection mould cavity by incorporating into the elastomeric material, such as a known TPE material, a foaming agent, such as the known foaming agents Exocerol AB40E™ or Hydrocerol™, typically at 1-2.5 weight %, typically 2% of the former or 1.5% of the latter. These foaming agents generate a gas on heating to elevated temperatures, typically those used for injection moulding, and if such a foaming agent is included in the elastomeric material, then when it is injected in a heated state into the mould, the foaming agent will form bubbles within the elastomeric material, resulting in a foam.
Suitable polymers for the outer polymer skin include the above-mentioned elastomeric materials in non-foamed or other grades of foaming e.g. having a lower number of gas bubbles per unit volume than the core. This may be achieved by using an elastomeric material without a foaming agent, or with less foaming agent than the core material, or a different foaming agent than the core which generates less gas bubbles. Other suitable polymers for the skin include the above-mentioned elastomeric materials with different hardness, resistance etc. to the core, and inherently harder polymers different to such elastomers, e.g. polypropylenes, polyamides (such as Nylon™), polyurethanes etc. Preferred polymers for the outer skin are those which bind with the hard plastic material of the toothbrush. For example the polymer for the outer skin may be chemically similar to the hard plastic material of the toothbrush. A suitable combination of inner and outer polymers is an inner core of a thermoplastic elastomer and an outer skin of the same thermoplastic elastomer in a less soft grade, preferably being a thermoplastic elastomer which binds to the hard plastic material. Thermolast-K™ is such an elastomer, and is known for use in two-component toothbrushes where it is bonded to the hard plastic material of the toothbrush.
Suitably if the inner elastomeric polymer is a relatively soft elastomer and the outer skin is a relatively less soft polymer, e.g. a foamed elastomer core and a non foamed or less foamed elastomer skin, the inner core polymer may have a hardness of ca. Shore A 5 to 30 preferably ca. 20-20, and the outer skin may have a hardness of ca. Shore A 65±5. The thickness of the outer skin may vary from application to application but typically a thickness of 100 microns to 1.5 mm may be used. It will be appreciated by those skilled in the art that the shape and size of a toothbrush will inevitably impose dimensional constraints, for example only a relatively thin outer skin may be possible in some parts of the toothbrush where only a relatively thin mass of the elastomeric parts may be used.
Suitably the elastomeric part(s) of the toothbrush may comprise part(s) of a grip handle in which the elastomer is of sufficient thickness, i.e. from their outer surface to the hard plastic internal skeleton, that the elastomer parts resiliently deform under the pressure of a gripping hand, so that the grip handle has a soft hand feel. For example the elastomer parts of the toothbrush may have a thickness of 3 mm-1.5 cm, e.g. 5 mm-1.0 cm. A suitable grip handle having thick elastomer regions of this type is known for example from WO 97/29663 the contents of which are included herein by reference.
The hard plastics material of the other parts of the toothbrush may be made of any plastics material which is used at present in the manufacture of toothbrushes, particularly in the manufacture of two component toothbrushes which incorporate parts of elastomeric material, and in which the elastomeric parts and plastics parts are bonded together. Suitable hard plastics include polypropylenes such as
Polypropylenes P 1600™ (Shell) and Apryl 3400 MA1 from Elf Atochem, Novolene 1100 HX™ (BASF), and polyamides such as Ultramil B3™ (BASF). Polypropylenes for example bond readily with Thermolast-K™. Such hard plastics materials typically have a hardness Shire R 80-100.
The hard plastic parts of the toothbrush may be made in a first stage by a conventional process of injection moulding as is common in the art and has long been used for making toothbrushes. In this first stage the hard plastic part(s) of the toothbrush are made, typically in the form of a “skeleton” or “frame” of the plastics material is made, having one or more cavities therein corresponding to the intended position(s) and dimensions of the elastomeric material part(s).
The elastomer parts may then be made in a second stage by containing the so plastic material part(s) in a second mould having one or more cavities therein corresponding to the intended position of the elastomeric material part(s), then forming the elastomer part(s) by injection of the fluid elastomeric material into the one or more cavities. Such a process is for example described in WO 94/05183. Suitable conditions of temperature and pressure etc. for the injection moulding process to achieve a bond between the plastic material skeleton and the elastomer material injected into the mould are known in the art, for example as used currently to make two-component toothbrushes.
The elastomer parts of the present invention, having an inner core and an outer skin, may be formed by the known injection moulding process of “sandwich moulding” also called the “skin/core” method, in which the elastomeric material part(s) is/are formed by an injection moulding process in which at least two materials being the materials of the skin and core are injected into a mould cavity simultaneously or successively in such a way that the injected polymer is in the form of an inner core and an outer skin. This process is known from the technical literature. Suitable processes and moulding equipment for such a sandwich moulding process are for example disclosed in U.S. Pat. No. 5,789,033 and U.S. Pat. No. 5,798,069, the contents of which are incorporated herein by way of reference. In sandwich moulding, a core is introduced into an outer skin component. This process takes place in two or three stages. First the material of the outer skin is injected into the mould cavity to partially fill the cavity with a mass of the material of the outer skin, and then the core component is injected into the mass of introduced skin material to fill and “inflate” the skin material. To finish off, a closure can be produced with the first, i.e. the skin component near the sprue (the point in the cavity at which the materials are injected). This prevents the core material remaining on the surface and at the same time cleans the system for the next shot of polymer to be injected into the mould. U.S. Pat. No. 5,789,033 and U.S. Pat. No. 5,798,069 disclose sandwich moulding processes in which opposed gating injection is used, i.e. the core and skin materials are injected into the mould cavity via respective separate injection nozzles. Sandwich moulding processes are also known in which a single nozzle is used to inject both skin and core materials. Hot runner injection moulding equipment is generally preferred for the sandwich moulding process.
Suitable injection moulding machinery for sandwich injection moulding is available from inter alia Mannesman Demag (Germany). The operating conditions, e.g. temperature, pressure, flow rate etc. conditions of the sandwich moulding process are preferably selected such that the elastomer part outer skin material bonds to the plastic material of the toothbrush, “skeleton” or “frame” referred to above. Such conditions can be determined by practice in the art.
For example in a sandwich moulding process suitable for making the toothbrush of the present invention a skeleton or frame may be made as described above and this skeleton or frame may be enclosed in a second mould having cavities therein corresponding to the intended position of the elastomer parts, then the skin material, e.g. a material such as those described above, for example an elastomeric material, may be injected into the cavity, typically in an amount comprising ca 20-30% of the total cavity volume. Then the core material, e.g. a material such as those described above, for example an elastomeric material, e.g. the same elastomeric material as the skin material and containing a foaming agent may be injected into the cavity to form the core, and the core material fills and inflates the skin. Suitably if the core material includes a foaming agent expansion of the core material within the mould may occur as a result of the temperature of the mould. If the core material is a foaming elastomer, e.g. includes a foaming agent, then a quantity of such a core material may be injected into the cavity such that the combined volume of skin and core materials fills ca. 50-70%, e.g 60% of the cavity, and the expansion of the core material as foaming occurs may then cause sufficient expansion that the core and skin materials fill the cavity.
Alternatively the elastomer parts of the present invention, having an inner core and an outer skin, may be formed by an injection moulding process that employs internal foaming of an injected elastomer material to generate both skin and core. For example an elastomer material such as those described above may have incorporated into it a foaming agent, such as those described above. These foaming agents generate a gas on heating to elevated temperatures, typically those used for injection moulding, and if such a foaming agent is included in the elastomeric material, then when it is injected in a heated state into the mould, the foaming agent will form bubbles within the elastomeric material, resulting in a foam. However an injection mould for a toothbrush is normally made of a heat conducting metal and will normal include cooling fluid channels in its metal mould blocks to rapidly cool the toothbrush formed therein. Consequently the outer parts of the moulded elastomer being in contact with the metal mould cavity surfaces can be arranged to cool more rapidly so as to foam less, resulting in an outer skin in a non foamed state or in a less foamed state than the inner core parts of the material further away from the metal walls of the mould cavity.
Accordingly this invention further provides an injection moulding process for making a toothbrush comprising hard plastic and elastomeric material parts, in which a hard plastic material “skeleton” or “frame” is provided having one or more cavities therein corresponding to the intended position of the elastomeric part(s), this skeleton or frame is located within a mould having one or more cavities corresponding to the intended position of the elastomeric parts, and the elastomeric part(s) are formed within the said mould cavities by an injection process which forms an inner core of a soft elastomeric material wholly or partly enclosed within an outer skin of a second polymer material. The injection process is preferably a sandwich moulding or foaming process.
The invention further provides a mould suitable for use in such an injection moulding process, having cavities therein corresponding to the intended position of the elastomeric parts, and provided with means to carry out a sandwich moulding process therein. The mould is suitably provided with injection ports to facilitate a sandwich moulding process.
The invention further provides a mould suitable for use in such an injection moulding process, having cavities therein corresponding to the intended position of the elastomeric parts, suitable for injecting an elastomeric material which includes an internal foaming agent therein.
The invention will now be described by way of example only, referring to:
FIG. 1 which shows a toothbrush according to this invention.
FIG. 2 which shows an enlarged cross section through part of the toothbrush of FIG. 1.
FIG. 3 which shows an enlarged cross section through an alternative construction of part of the toothbrush of FIG. 1.
FIG. 4 which shows the sandwich moulding process used for making a toothbrush of FIG. 1.
Referring to FIG. 1 a toothbrush is shown in longitudinal section, comprising a handle 1, a head 2 on which are mounted bristles 3, and a neck region 4 between the handle 1 and head 2. The toothbrush of FIG. 1 is made partly of a hard polypropylene material, which is in the form of a frame 5 having cavities 6 therein, and partly of parts 7, 8, 9, 10, 11, 12 which comprise an elastomeric material. The cavities 6 are in positions which correspond to the positions of the elastomeric material parts 7-12.
Part 7 is a grip pad on the handle 1. Part 8 is a flexible link between the handle 1 and neck 4 of the toothbrush comprising an aperture in the hard plastic material which is occupied by the elastomeric material. Part 9 is a flexible link in the toothbrush neck 4, part 10 is a flexible link between the head 2 and neck 4 of the toothbrush 2 comprising apertures in the hard plastic material of the neck which are occupied by the elastomeric material. Parts 11 are flexible links in the head 2 of the toothbrush, each comprising a widthways extending groove being occupied by the elastomeric material. Part 12 is a soft buffer around the head 2, comprising a projecting rim around the head 2.
Each of parts 7-12 comprises an elastomeric core made of a soft thermoplastic elastomeric material of hardness Shore A ca 5; and an outer enclosing skin of a less soft grade of the same thermoplastic elastomeric material. Two alternative structures of the elastomeric material parts 7-12 are shown in FIGS. 2 and 3.
Referring to FIG. 2, a cross section through the part 7 of the toothbrush of FIG. 1 which is circled in FIG. 1 is shown, although sections through other elastomeric parts shown in FIG. 1 would be identical. FIG. 2 shows part 21 of the plastic material of the toothbrush. The elastomeric material part 22 comprises a core 23 of a first soft elastomeric material polymer material and an outer skin 24 of a second polymer. The second polymer of the outer skin 24 is a flexible polymer which is harder and less permeable than the elastomeric material of the core. At the interface 25 between the elastomeric material part 22 and the plastic material 21 the outer polymer skin 24 is bonded to the plastic material 21. The elastomeric material parts 7-12 are formed by a process of sandwich moulding.
Referring to FIG. 3, a cross section through the part 7 of the toothbrush of FIG. 1 which is circled in FIG. 1 is shown, although sections through other elastomeric parts shown in FIG. 1 would be identical. FIG. 3 shows part 31 of the plastic material of the toothbrush. The elastomeric material part 32 is a soft foamed thermoplastic elastomeric material, i.e. within which there are numerous internal bubbles 33. As is shown in FIG. 3 the extent of foaming decreases towards the outer surface 34 of the elastomeric material part 32, and the part 35 of the elastomeric material part 32 immediately adjacent to the outer surface 34 is essentially non-foamed. Consequently the outer parts 35 of the polymer layer are less soft than the inner core part closer to the plastic parts 31. At the interface 36 between the elastomeric material part 32 and the hard plastic material part 31 these parts are bonded together.
The elastomeric material part 32 has been formed by positioning the hard plastic parts 31 in the cavity of a injection mould (not shown), and injecting an elastomeric material which includes a foaming agent into the cavity under known conditions such that the elastomeric material and plastic parts bond. At the temperature of the mould the foaming agent generates gas bubbles 33, but the metal walls of the mould (not shown) are a good heat conductor and the outer layers 35 of the injected elastomeric material rapidly cool down immediately after the elastomeric material is injected in so that less foaming occurs in these layers. The plastic material part 31 is not as good a heat conductor as the metal injection mould (not shown) in which the elastomeric material part 32 has been formed, consequently bubbles 33 are formed in the parts of the elastomeric material 32 adjacent to the plastic parts 31.
The thickness of the elastomeric material part, e.g. the distance from the outer surface 34 to the plastic part 31 may be typically 3 mm-1.5 cm.
Referring to FIG. 4A, part of an injection mould 40 is shown overall, being a two part mould having two halves 41, 42 with a split line A-A, and defining a mould cavity 43 between the two halves 41, 42. There is an injection gate 44, of a known type suitable for sandwich moulding. The mould 40 encloses a hard plastic part 45 of a toothbrush handle which has previously been made in a separate first stage injection moulding process. The part 45 shown is part of the grip handle of the toothbrush, with a neck 46 extending therefrom, but the head is not shown. Part of the cavity 43 is also defined by a cavity in the handle part 45.
Referring to FIG. 4B a first material 47 has been injected into the cavity 43 via gate 44. The material 47 is a thermoplastic elastomer such as Thermolast-K™, and the quantity injected is sufficient to fill ca. 20-30% of the cavity 43. The moulding conditions are such that the the material 47 is injected in a hot fluid state, but cools slightly in the cavity 43 to increase its viscosity.
Referring to FIG. 4C, a second material 48 has been injected into the cavity 43, in a way such that the material 48 flows into the centre of the mass of first material 47, to inflate the mass of material 47 so that the material 47 surrounds the inner core of material 47. The second material 48 is typically a thermoplastic elastomer such as Thermolast-K™ but including a foaming agent such as Exocerol AB40E™ or Hydrocerol™, typically at 1-2.5 weight %, typically 2% of the former or 1.5% of the latter. The quantity of second material 48 injected is such that the combined first and second material 47, 48 occupy ca. 60% of the cavity 43. The second material 48 is of a lower viscosity than the first material 47 in the cavity 43.
Referring to FIG. 4D, the second material 48 has foamed under the hot conditions inside the cavity 43, and the second material 48 has consequently expanded within the skin of first material 47 so that the mass of combined first and second material fills cavity 43. The small residual hole (not shown) in the skin 47 where the second material has penetrated into the core of first material 47 has been closed by a small injection of first material 47 through gate 44.
The conditions inside the cavity 43 are such that the first material 47 bonds to the plastic material part 45. Suitable conditions to achieve bonding are known in the art, e.g in conventional toothbrush manufacturing processes.
In FIG. 4 the sandwich moulding process shown uses a single gate 44, but it will be understood that further gates (not shown) may be provided to inject first and second materials into other parts of the toothbrush. It will also be appreciated that the process shown in FIG. 4 may be adapted for use with opposed gate techniques, although id materials 47, 48 are injected from sides of the cavity 43 on opposite sides of the handle 45, it will be necessary to provide some communication channels either through or around the part 45 to enable the core to be injected. In the manner described.

Claims

1-47. (Cancelled)

48. A toothbrush comprising a handle, a head on which are mounted bristles, with optionally a neck region between the head and handle, the toothbrush being made partly of a hard plastic material and having at least one part made of an elastomeric material, wherein;

at least part of the at least one elastomeric material part comprises an inner core of an elastomeric material wholly or partly enclosed within an outer flexible skin of the same elastomeric material comprising the same monomer units as the core but having different physical characteristics to the inner core material.

49. A toothbrush according to claim 48 wherein the core comprises a foamed elastomeric material and the skin comprises the same elastomeric material in a non-foamed state.

50. A toothbrush according to claim 48 wherein the core comprises a foamed elastomeric material and the skin comprises the same elastomeric material in a foamed state having different void characteristics to the inner core material.

51. A toothbrush according to claim 50 wherein the skin comprises the same elastomeric material in a foamed state having a lower number of gas bubbles per unit volume than the core.

52. A toothbrush according to claim 48 wherein the elastomeric material of the core comprises a foamed elastomeric material having a hardness of Shore A 5 to 30, and the outer skin comprises a non foamed elastomeric material or an elastomeric material less foamed than the material of the core having a hardness of Shore A 65±5, the thickness of the outer skin being in the range 100 microns-1.5 mm.

53. A toothbrush according to claim 52 wherein the elastomer part of the toothbrush has a thickness of 3 mm-1.5 cm.

54. A toothbrush according to claim 48 wherein the elastomeric material of the core and skin is selected from the group of compounds based on styrenic block copolymers such as styrene-ethylene-butadiene-styrene (SEBS) copolymers, polyblends based on EPDM/PP, polyacrylate/PP blends, thermoplastic polyurethanes, thermoplastic copolyesters, natural or synthetic latex type elastomers, polychloroprenes, natural rubber and polyurethane foams.

55. A toothbrush according to claim 48 when made by a process in which:

a hard plastic part of the toothbrush is made in a first stage by a process of injection moulding in the form of a skeleton of the hard plastic material having one or more cavity therein corresponding to the intended position(s) and dimensions of the elastomeric material part(s), then;

this skeleton is enclosed in a mould having at least one cavity therein corresponding to the intended position of an elastomer part, then

the skin material is injected into the cavity, then

the core material is injected into the cavity to form the core and to fill and inflate the skin.

56. A toothbrush according to claim 55 wherein the skin material is injected into the mould cavity in an amount comprising 20-30% of the total cavity volume then

57. A toothbrush according to claim 55 wherein the core comprises an elastomer material which includes a foaming agent and the quantity of such core material injected into the cavity is such that the combined volume of skin and core materials fills ca. 50-70% of the cavity and the expansion of the core material as foaming occurs then causes sufficient expansion that the core and skin materials fill the cavity.

58. A toothbrush according to claim 48 when made in a process in which:

this skeleton is enclosed in a mould having at least one cavity therein, bounded by a wall, corresponding to the intended position of an elastomer part, then

an elastomer material having incorporated therein a foaming agent is injected into the mould and outer parts of the elastomer material injected into the mould are arranged to cool more rapidly so as to foam less to result in an outer skin in a non foamed state or in a less foamed state than the inner core parts of the material further away from the walls of the mould cavity.