CA1341069C - Safety razor and process for manufacture of a surface of low frictional resistance on a safety razor - Google Patents

Abstract

The invention concerns a safety razor with a platform for at least one razor blade, where a surface in contact with the skin of the user during the shaving process is provided with a lubricant and also a process for making such a surface of low frictional resistance on a safety razor. To further develop the safety razor of the aforementioned type in such a way that the surface work to be done when shaving is reduced without the disadvantages occurring produced by a lubricant which is dissolved out of the carrier material on contact with water when staying and forms a film of lubricant on the skin of the user improving slidability, it is proposed that the lubricant be A xerogel which on the absorption of water as a dispersion medium becomes a lyogel with a coefficient of friction µ < 0.25 which is highly slidable on the skin of the user.

Description

1 341 Ofi 9 Safety razor and process for manufacture of a surface of low fricaional resistance on a safety razor The invention concerns a safety razor with a platform for at least one razor blade, where a surface in contact With the skin of the user during t:he shaving process is provided with a lubricant and a process: for manufacturing such a surface with low frictional resi::tance on a safety razor.
Various aids are known for wet shaving which should perform the task of reducing the frictional resistance between skin and safety razor during t:he shaving process, namely shaving foam, shaving soap, stubble softening agents as well as medical or cosmetic substances or combinations of all these.
Such shaving aids reduce either the cutting work which has to be done to separate the stubble by softening the stubble or act as a lubricant which reduces the frictional force between the plastic parts of the safety razor in contact with the skin and the skin of the user.
A new proposal consists in using polyethylene oxide as an intergral, water-soluble shaving aid on a safety razor or fitting an infection moulded strip of a hydrophobic polymer, such as polystyrene, and a hydrophilic polymer, such as polyethylene oxide, in a recess on the cap of the safety razor. In both cases they endeavour is to dissolve out the ~ 341 06 9 water-soluble polymer connponents continuously in the course of the wet shave, d.ue to the presence of water on the skin surface to be shaved and to produce a sliding film on the skin. This reduces the i:rictional forces occurring during shaving and makes the shaving process softer and more comfortable.
When using such shaving aids there are more or less serious disadvantages due to the fact that these essentially consist of a water-soluble, polynneric material which dissolves out during the shaving process and forms a film on the skin of the user which dries on i:he already shaved skin surfaces. It is thus necessary to wash off the dried-on lubricant from the skin after shaving, which may be difficult depending on the degree of hardness of they water available and can possibly leave a sticky feeling with the user. It cannot thus be excluded that resid':ues o1: the dissolved out lubricant remain on the skin and with repeated use and sensitive skin may cause the danger of skin irritations. In addition the process of dissolving oui: the lubricant during the shaving process takes places uncontrolled which is a disadvantage.
For example, as a function of the temperature of the water used, the time the safety razor remains immersed in the water, the shaving time annd the number of shaving strokes.
It has therefore proved :Ln practice that despite the known sliding strips on safety razors with water-soluble polymers as a lubricant, other shaving aids such as shaving foam or shaving cream cannot be dispensed with, as was originally intended with theses developments. On the contrary it must be said in summing up that previous attempts to simplify wet shaving by using a shaving aid so that the frictional forces occurring when shaving are reduced were in fact partially successful, but dial involve some serious disadvantages. In particular, the large quantity of water-soluble polymer dissolved out of tree lubricant carrier in the course of shaving produces ar.~ uneven action which at the start of use produces an excessive lubricating effect in such a way that a drying film that i~; diff:icult to wash off again forms on the skin of the user causing a sticky feeling and can involve the danger of skin irritations, while at the end of its life only extremely little lubricant is given off so that just at that time when the cutting action of the razor blade has deteriorated throu~fh wear and with it shaving comfort, the additional lubricar.~t effect of the substance is no longer adequate. In addition, ithe method of making razor blade units and/or safet~~ razors with such sliding strips is very costly due to the complicated production either by injection moulding of the cap and :sliding strip separately, joining them together afterwards,. e.g. by bonding, or due to an expensive 2-component injection moulding process and also due to the high costs of the water-soluble polymers used as lubricant.
In view of this prior arit, the invention is based on the problem of further developing the known razor of the type mentioned at the berginning in such a way that surface work to be done in shaving is reduced without the disadvantages occurring which a i.ubric<~nt produces, which in the shaving process is dissolved out of the carrier material on contact with water and which forms a lubricating film on the skin of the user improving slidability. The objective of the invention is rat: her t:o~ develop the safety razor with a highly slidable surface when wet, in particular with a coefficient of friction which i.s le;~s than that between skin and metal, whereby it is to be x:~orne in mind that it has been established as a result of mc~asur~ements made that the frictional forces between the partners polystyrene (usual material of safety razors)/skin when wet:'under the same test conditions are considerably greater than those when dry. In contrast with these results thEe aim a:F the inmention is precisely the opposite effect, namel~,r a reduction of the frictional forces when wet compared with those when dry, with the same partners.
The invention provides a razor blade unit comprising a support for at least one razor' blade, a surface for slidably engaging the skin of a user, and a xerogel on the surface wherein the xeroc~el comprises a homogeneous mixture of homopolymers and copolymers of polyvinyl pyrrolidone with polyurethane in which tree ratio of the amount of polyvinyl pyrrolidone to polyurethane is about (~-5):1.
Preferably :L.n a safety razor in accordance with the invention the lubricant is a xerogel which on the absorption of water as a dispersion medium becomes a lyogel with a coefficient of fracti.c>n a < p.25 and is highly slidable on the user's skin. With the invention a proposal is made for reducing the frictional farce wh:Lch is in principle different from the prior art, as the use of a water-soluble, polymeric film forming agent. is a~aaided ad instead the surface of the _5 _ ~ 341 06 9 safety razor in contacts with the skin of the user when shaving is highly slidah~le because of the properties of the xerogel when wet which are typical of the substance. Xerogels are gels which have lost their fluidity in some way, e.g. by evaporation. The bas~.<: gel structure is, however, retained which consists in one or more colloidally distributed substances with long ar heavily branched particles and a liquid (usually water) as dispersian a..

medium. The colloidally distributed substance forms a spatial network in the dispersion medium, the particles adhering to each other at various points (adhesion points) due to secondary or main valences. If the spaces between the particles are filled witlh liquid, it is called a lyogel. The xerogels produced by evalporation represent a boundary state with the solid. 85~ adding the dispersion medium, e.g. water, xerogels swell aga~'.n into lyogels. The surface becomes slippery and has a low coefficient of friction. In this process the colloidal sultistance forming the network does not pass into solution. Consequently when shaving it does not form a film of a d9'.ssolved out lubricant on the skin as is the case with the ~~rior art .
Preferably the xerogel consists of a natural unary system, such as polysaccharides of high molecular weight, or a homogeneous mixtures of homopolymers and copolymers of vinyl pyrrolidone, preferably polyvinyl pyrrolidone, as well as an aromatic polyurethane as a further component. Absorption of the dispersion med~.um water during the shaving process produces the high E:lidab.ility, there being no need to fear the dissolving out of substances because of the intermolecular forces typical in gels.
It is appropriate v~ith a xerogel consisting of polyvinyl pyrrolidone and the' polyurethane to set the proportion of polyvinyl pyrrolidone to a PVP:PUR ratio between (3-5):1, ie.
there is a 3-5fold surplus of PVP, namely 3.6-4:1. This ratio is optimal a~> rega:rds setting a low coefficient of 1'41 069 friction, the value of which is retained over the life of the razor blade (norma7.ly put at seven days), also as regards resistance to wear as well as the drying time to be spent in the course of the manufacturing process. There are the fol lowing dependencies 1. The coefficient: of friction is lower, the more polyvinyl pyrrolidone there is in the xerogel.

2. The greater they proportion of polyvinyl pyrrolidone in the xerogel, the greater the danger that the intermolecular linkage forces are not sufficient to hold the polyvinyl F~yrrol:idone in the gel structure so that there is the danger o.f polyvinyl pyrrolidone dissolving out.

3. Abrasion resistance increases with the proportion of polyurethane.
Great advantages are achieved, if the polyurethane is a dispersion With wager and N-methyl pyrrolidone as solvent, where preferably the pol~,turethane dispersion has a solids content between 30 and 3;3%, in particular 31% w/w and the proportion of N-methyl p!~rro7.idone (NMP) is between 10 and 12% w/w, in particular 1:1% w/w, with the rest water. NMP is a high boiling solvent with a boiling point of about 204'C, which involves a cerrtain lengthening of the evaporation time, which is accepted though on account of its high environmental compatibility. They usual organic solvents can basically be dispensed with.

According to a preferred farm of execution of the safety razor in accordance with the invention it is proposed developing xerogel as a coating with a thickness between 5 and 150 um on the c;ap and/or guide surface of the safety razor. Preferably a coating thickness > 30 um is chosen, as this has sufficient: stability even in the swollen state of the lyogel. With t:oo great coating thicknesses > 150 um losses of material can occur due to abrasive forces, as a correspondingly high absorption of water (swelling capacity) reduces the resistance to abrasive forces.
The coating can be formed either directly or indirectly on a surface of the safe~.ty raaor in contact with the skin of the user during the shaving process. For indirect formation it is proposed in a further development of the invention to arrange the xeroge7, on a self-adhesive carrier film and arrange it on the appropriate surface of the safety razor.
' The carrier film can conaist of polyvinyl chloride or polyester. To improve adhesion it is advisable to affix the coated carrier film either on a flat part of the safety device or on a curved part with a radius R > 20 mm. This ensures adequate adhesion on the safety razor and the frictional forces accurr.ing are evenly distributed over the whole sliding surface. When using a carrier film it can be an advantage to arrange at in a recess, in particular of the cap, in order to achieve an even contact surface.
H-With the xerogel coating in accordance with the invention on a safety razor, a short activation time is necessary by the water present when shaving. The structure of the coating material is constituted so that small molecules, such as water, can penetrate into the gel network and bring about swelling of the xerogel within a short time. This can be controlled by the choice of matrix material and the width of the gel network. Unlike the previously known deterioration of the sliding properties. over the useful life, when using a xerogel as sliding coating there is an improvement in the sliding effect with increasing time of use due to the orientation of the molecular chains at the surface.
Durability against leaching is considerably improved with the safety razor in accordance with the invention. Even 24 hours' immersion in water does not change the properties of the xerogel sliding coating. Even storage at high humidity (> 90%), which causes previously known lubricants to become ' unusable, has no effect on the quality of the safety razor in accordance with the invention. Finally the temperature stability of the xerogel is very high, as it is basically determined by the temperature stability of the carrier material. This results in problem-free handling for the consumer with assured sliding characteristics over the whole time of use of the safety razor or the razor blade unit.
A new method for making a sliding surface on a safety razor is proposed by the invention, which is characterized in accordance with the invention in that a solution of a linear, unlinked polyurethane and a water-soluble polymer, in particular polyvinyl pyrrolidone, is made, that this solution is applied directly or indirectly in one operation as a r coating to a surface, in particular the cap of the safety razor, in contact with the skin of the user during the shaving process and. the :solvent is evaporated at raised temperature to form, a xerogei. In this way the desired xerogel is obtained, in one step immediately after evaporation of the solvent without additional curing, so that the cost in time and technique for making the sliding coating is reduced to a minimum, thereby enabling use for a mass-produced safety razor.
For indirect application it is proposed in a suitable development of the inveni:ion first to coat a self-adhesive carrier film of polyvinyl chloride or polyester, preferably polyethylene phthalate, possibly pretreated with a caustic or polymer bonding aid., and then to loin the carrier film to the desired surface of the s<~fety razor, in particular the cap, by bonding. It may also be appropriate with direct coating of the desired surface oi: the safety razor to carry out pretreatment with a. primer or to pretreat the surface to improve adhesion for 15 nnin at a raised temperature, preferably 40'C, in. a solution of chromatisulphuric acid.
The plastic then used is an ABS suitable for electroplating.
It is important in the manufacturing process to produce a homogeneous mixturer of polyurethane and polyvinyl pyrrolidone, whereby the ratio by weight of polyvinyl pyrrolidone to polyurethane should lie between 3:1 to 5:1 and the homogeneity is to be ensured by stirring the solution right up until application. The evaporation process to be effected after application is ideally done at a temperature of about TO'C over a period of less than 15 min.

1 341 d6 9 With thE~ method in accordance with the invention an application techn_Lque h;as been developed which can be used for mass products and with which excellent anchoring of the xerogel on the carrier matserial is ensured.
According to one practical example the procedure is as follows:
A polyvinyl pyrrolidone/polyurethane blend is first made to the following formulation:
Polyuret;bane dispersion 81 g 1C Water 250 g Diacetone alcohol 50 g Polyvin~rl pyrrolidone K90 100 g Isopropanol 519 g Fluorad FC430* 1 g 15 The cap of a safety razor is immersed in the above solution and then subje<:ted to drying for 10 min at 70°C. It may be advantageous to i=urther dilute the dip solution. A
xerogel is formed which during shaving absorbs water as a dispersion medium and obtains highly slidable surface 20 characteristics.
In a further practical example the following mixture of polyvinyl pyrrolidone~ and polyurethane is used:
Polyurethane dispersion 81 g Water 300 g 25 Diacetone alcohol 60 g Polyvin~~l pyrrolidone K90 90 g Isopropa.nol 469 g Fluorad FC430 1 g *Trade-mark ~.

The solution of these substances was applied to a polyvinyl chloride film 150 um thick and the solvent evaporated within min at T5'C. The PVC film thus coated was cut into strips. A strip was joined with the cap of a safety razor by bonding. A waterproof polyacrylate from 10 to 15 Nm thick was used as the adhesive.
Further details, features. and advantages of the subject matter of the patented irwention can be seen from the following description of the accompanying drawing. The drawing shows rr~ig. 1 a razor blade unit, partially in section Fig. 2 an individual s7.iding strip for use on a razor blade unit as in Fig. 1 in section and Fig. 3 top view of the sliding strip.
The razor blade unit 1 in Fig. 1 is joined in a known manner to a safety razor handle fitted with suitable fastenings and in this way forms the safety razor to be used by the user.
It has a platform 2 for t:wo razor blades 3, 4 and a cap 5. A
flat surface 6 is shaped on the front of cap 5 which is suitable for taking a sliding strip Z. The flat surface 6 ends in the front in a rounded pratective lip 10, which is used firstly to facilitate positioning of the sliding strip Z
on assembly and secondly protects its front edge during the shaving process. Its construction can be seen from Figs. 2 and 3 of the drawing. The sliding strip T has on the front ~~4~ 06g a sliding surface E~ made from a xerogel, Which is so arranged in relation to the front end of cap 5 that a smooth transition is created from an imaginary connecting line forming a tangent Hrith the cutting edges of the two razor blades via the front end of cap 5 to the sliding surface 8 in the direction of tree shaving movement to be made. The sliding surface 8 is set angled slightly forward to the geometric imaginary line 9 connecting the cutting edges of the two razor blades to ensure increased skin contact when used.
According to Fig. ~: of the drawing the sliding strip T is built up of several. layers. The middle layer is a polyvinyl chloride carrier film 150 Nm t 10% thick, the bottom of which is coated with a polyacrylate adhesive. On the top is applied a coating of a polyvinyl pyrrolidone/polyurethane blend as per the above formulations as a xerogel. The ' coating has a thiclc:ness of 25 arm t 5%.
Such a sliding strip Z with sliding surface 8 is stuck on a razor blade unit 1 with 'the waterproof polyacrylate adhesive layer, which is 10 to 15 um thick, rounded corners as in Fig. 3 being provided to improve this process. When the safety razor is used a lyogel with excellent sliding properties forms very spontaneously from the xerogel through the absorption of water, which significantly reduces the surface work to be done when shaving. A comfortable and efficient shave is achieved because of the reduced coefficient of friction. No appreciable wear of the sliding surface 8 due to abrasion can be found over the usual life of the razor blades 3, 4.
-I4~

~ 341 06 9 Reference list:
1 Razor blade unit 2 Platform 3 Razor blade 4 Razor blade Cap 6 Flat surface 7 Sliding strips 8 Sliding surface 9 Connecting line Protective lip '~t ~J

Claims (44)

1. A razor blade unit comprising a support for at least one razor blade, a surface for slidably engaging the skin of a user, and a xerogel on the surface, wherein the xerogel comprises a homogeneous mixture of homopolymers and copolymers of polyvinyl pyrrolidone with polyurethane in which the ratio of the amount of polyvinyl pyrrolidone to polyurethane is about (3-5):1.

2. The razor blade unit of claim 1, wherein the xerogel has a coefficient of friction less than 0.25 when contacted with a dispersion medium.

3. The razor blade unit of claim 1, wherein the xerogel has a coefficient of friction less than 0.25 when contacted with a hydrous dispersion medium.

4. The razor blade unit of claim 1, wherein the polyurethane is an aromatic polyurethane.

5. The razor blade unit of claim 1, wherein the ratio of polyvinyl pyrrolidone to polyurethane is about (3.6-4):1.

6. The razor blade unit of claim 1, wherein the polyurethane is a dispersion with water and N-methyl pyrrolidone as a solvent, the polyurethane dispersion has a solids content between 30 and 33% by weight, the N-methyl pyrrolidone is from about 10 to 12% by weight and the balance is water.

7. The razor blade unit of claim 6, wherein the solids content of the polyurethane is 31% by weight and the N-methyl pyrrolidone is 11% by weight.

8. The razor blade unit of claim 1, including a cap and wherein the xerogel is disposed on the cap.

9. The razor blade unit of claim 8, wherein the xerogel is on a carrier film banded to the cap.

10. The razor blade unit of claim 9, wherein the surface includes a recess where the carrier film is bonded.

11. The razor of claim 10, wherein the recess is a flat surface, and the front-end of the recess forms a rounded protective lip being about equivalent in height to the thickness of the carrier film and the xerogel.

12. The razor as in claim 10, wherein the xerogel and carrier film is angled forward from the plane of shaving to give increased skin contact.

13. The razor blade unit of claim 1, including a guardbar, and wherein the xerogel is disposed on the guardbar.

14. The razor blade unit of claim 1, including a cap and a guardbar, and wherein the xerogel is disposed on both the guardbar and cap.

15. The razor blade unit of claim 1, wherein the thickness of the xerogel is in the range of about 5 to 160 µm.

16. The razor blade unit of claim 1, wherein the thickness of the xerogel is less than 30 µm.

17. The razor blade unit of claim 1, wherein the xerogel is on a carrier film bonded to the surface.

18. The razor blade unit of claim 17, wherein the surface includes a recess where the carrier film is bonded.

19. The razor blade unit of claim 17, wherein the carrier film is polyvinyl chloride.

20. The razor blade unit of claim 17, wherein the carrier film is polyester.

21. The razor blade unit of claim 17, wherein the carrier film is polyethylene phthalate.

22. The razor blade unit of claim 17, wherein the carrier film is arranged on a flat portion of the cap.

23. The razor blade unit of claim 17, wherein the carrier film is arranged on a curved portion of the cap having a radius greater than 20 µm.

24. A process for making a sliding surface on a razor blade unit comprising forming a solution of polyurethane, polyvinyl pyrrolidone and a solvent, wherein the weight of polyvinyl pyrrolidone to polyurethane is in the range of about 3:1 to 5:1, evaporating the solvent to form a xerogel, and bonding the xerogel to a surface of the razor blade unit which engages the skin of a user during shaving.

25. The process of claim 24, wherein the polyurethane is linear unlinked polyurethane.

26. The process of claim 24, wherein the forming step includes applying the solution to a carrier film.

27. The process of claim 26, wherein the carrier film is polyvinyl chloride.

28. The process of claim 26, wherein the carrier film is polyester.

29. The process of claim 26, wherein the carrier film is polyethylene phthalate.

30. The process of claim 26, including pretreating the carrier film with a bonding aid prior to the bonding step.

31. The process of claim 30, wherein the bonding aid is caustic.

32. The process of claim 31, wherein the bonding aid is a polymer.

33. The process of claim 24, including pretreating the surface of the razor blade unit in a solution of chromati-sulphuric acid.

34. The process of claim 33, wherein the pretreating occurs for about 15 minutes at a raised temperature.

35. The process of claim 34, wherein the raised temperature is about 40°C.

36. The process of claim 24, wherein the solution is a homogeneous mixture of polyurethane and polyvinyl pyrrolidone.

37. The process of claim 36, including the step of constantly stirring the solution to ensure homogeneity.

38. The process of claim 24, wherein the evaporating step is carried out at a temperature of greater than 70°C for a time of less than 1.5 minutes.

39. A process for making a sliding surface on a razor blade unit comprising applying a solution of polyurethane and polyvinyl pyrrolidone and a solvent to a surface of the razor blade unit which engages the skin of a user during shaving wherein the weight of a polyvinyl pyrrolidone to polyurethane is in the range of about 3:1 to 5:1, and evaporating the solvent to form a xerogel.

40. The process of claim 39, wherein the polyurethane is linear unlinked polyurethane.

41. The process of claim 39, including pretreating the surface of the razor blade unit with a primer prior to the applying step.

42. The process of claim 39, wherein the solution is a homogeneous mixture of polyurethane and polyvinyl pyrrolidone.

43. The process of claim 42, including the step of constantly stirring the solution to ensure homogeneity.

44. The process of claim 39, wherein the evaporating step is carried out at a temperature of greater than 70°C for a time of less than 15 minutes.