EP0803381B1 - Buffer reservoir for writing implement with fluid and writing implement comprising such reservoir - Google Patents

Description

The present invention relates to the field of ink writing articles liquid, i.e. in which the ink is in the free state, not being trapped in a fibrous reservoir. Such a writing article has a reservoir for ink and a point ensuring by capillary action the transfer of ink from this reservoir up to the actual writing head which may consist of the end of said point. When the ink tank is not isolated from air, a variation in the conditions of use and in particular the increase in air pressure in the tank due to a heating of the air results in an abnormal inflow of ink in the tip transfer and may cause stains or drips when the item is uncapped. To avoid this phenomenon, we plan to equip this type of article a buffer tank whose function is to absorb excess ink from of the reservoir before it reaches the write head, i.e. the front end of the point used to write, then, once normal conditions have been restored, restore the ink it contains to the point.

The applicant has already proposed in document EP 0516538 an article of this type including the buffer tank, which is made of a hydrophobic material open pore, consists of at least one compact, adjusted block without clamping around the transfer point, coaxial with it. Preferably the porous hydrophobic material is based on microbeads or microspheres, by example in polypropylene.

The buffer tank provided in document EP 0516538 gives functional satisfaction, i.e. it technically fulfills the absorption and restitution function of the excess ink arriving in the tip if the pressure in the ink tank changes. However the applicant has found that the efficiency of the buffer tank decreases radially as one moves away from the area of contact with the transfer tip. In other words the ink diffuses without difficulty from the point transfer to the buffer tank in an area relatively close to the tip transfer, an area which, depending on the case, may be of the order of 2 to 4 mm. The more we moves away from the transfer tip and the more difficult the ink diffusion is for become zero at a distance which can be of the order of 6 to 7 mm.

Since the role of the buffer tank is to be able to absorb a excess ink due to a pressure variation prevailing in the ink tank, understands that the useful volume of the buffer tank is a function of the volume maximum excess ink likely to be caused by such variation. This excess ink depends in particular on the volume of air contained in the tank liquid ink, since it's usually an increase in temperature of air contained in this tank which will cause the pressure variation causing this excess ink in the tip. The less ink remains in the tank, the more air it contains, the more excess ink, for a same temperature increase.

The difficulty which was noted by the applicant currently leads either to constitute a very high buffer tank, that is to say having a large contact surface with the transfer tip, i.e. to limit the capacity of the tank liquid ink.

The aim which is sought by the applicant is to propose a buffer tank which overcomes the aforementioned drawback.

This object is perfectly achieved by the buffer tank of the invention which in a manner known from document EP 0516538 is a buffer tank for a writing article containing liquid ink, said writing article comprising a reservoir, a transfer point opening into the reservoir, said reservoir pad capable of absorbing and restoring excess ink in the event of variation in the pressure prevailing in the tank and being produced in the form of a compact block made of an open pore material based on microbeads.

Typically, according to the invention, the buffer tank is consisting of microbeads of a hydrophobic nature with respect to said ink and, in reduced proportion of microbeads of hydrophilic nature with respect to said ink.

In this text, the concept of hydrophilia and hydrophobia is a relative concept, which is related to the nature of the ink, in particular its viscosity and its surface tension. Is considered hydrophilic compared to an ink determined - and even if this ink would not have water as a solvent - a porous material in which said ink naturally diffuses into normal conditions of use; is considered hydrophobic compared to the same ink, a porous material in which said ink does not diffuse in normal conditions of use. A given material can be hydrophobic for a given ink and hydrophilic for another ink. For example, the polypropylene is hydrophobic for an aqueous ink without additive likely to modify its surface and hydrophilic tension for a solvent type ink to alcohol. However, and this is the principle of the buffer tank, excess ink can diffuse in the hydrophobic material when the conditions of use are changed, especially by abnormal increase in pressure.

The presence of hydrophilic microbeads, in reduced proportion, among the hydrophobic microbeads unexpectedly promotes the diffusion of ink throughout the volume of the buffer tank, well beyond the peripheral zone which is in direct contact with the ink, especially on the surface of the transfer tip.

The applicant considers that the difficulty for the ink to diffuse through the network formed by the hydrophobic microbeads of the buffer tank is due to pressure losses increasing as one moves away radially from the periphery of the transfer point. Having locally hydrophilic microbeads on the path of the ink in the microbead network hydrophobic allows ink to reach hydrophobic sites that were inaccessible in the case of a buffer tank consisting exclusively of hydrophobic microbeads.

The presence of hydrophobic and hydrophilic microbeads can result from a homogeneous mixture throughout the volume of the buffer tank.

It can also consist of a differentiated mixture, the proportion of hydrophilic microbeads being weaker in the area immediately in contact with excess ink and being more prominent in the most distant area.

On average, the proportion of hydrophilic microbeads is between 2 and 10% relative to the total weight of the buffer tank.

In a preferred embodiment, it was a mixture homogeneous which was composed of 95% hydrophobic microbeads and 5% of hydrophilic microbeads.

A second object of the invention is to propose a writing article comprising such a buffer tank, said buffer tank being adjusted without clamping around the transfer point coaxially with the latter.

La figure 1 est une vue schématique en coupe longitudinale de l'article d'écriture et,

La figure 2 est une vue schématique en coupe transversale, selon la ligne II.II de la figure 1.

The present invention will be better understood on reading the description of an exemplary embodiment of a liquid ink writing article comprising a buffer tank composed of hydrophobic microbeads and, in reduced proportion, hydrophilic microbeads, illustrated by the appended drawing in which :

FIG. 1 is a schematic view in longitudinal section of the writing article and,

Figure 2 is a schematic cross-sectional view along line II.II of Figure 1.

The writing article 1 which is represented in FIG. 1 is in accordance with the teaching of the document EP 0516538 of the applicant. It is composed of a body 2, a transfer tip 3 and a buffer tank 4. The body 2 has its rear part which constitutes a tank 5 for the liquid ink, tank which is closed by an internal partition 6. The transfer point 3 has its rear end 3 a which opens into the tank 5 through the interior partition 6; its front end 3b constitutes the writing tip and opens out of the body 2 through a central orifice 7. The buffer tank 4 which is able to absorb and restore the excess ink coming from the tip 3 by capillarity in the event of of variation of the pressure prevailing in the ink tank 5, is made of an open-pore material based on microbeads and is in the form of a compact block adjusted loosely around the tip 3 coaxial thereto .

In such a writing article 1, the ink 8 coming from the reservoir 5 is consumed by the writing tip 3 b as it is used and is replaced by air which can circulate freely between the ambient space and said reservoir 5 thanks to the natural porosity of the transfer point 3 but also to a communication orifice 9 made at the level of the central orifice 7 through which the writing point 3 b passes and to a free space 10 surrounding the buffer tank 4. The buffer tank 4 is a compact porous block based on microbeads whose grade, particle size as well as distribution, molecular weight and morphology are known. According to the teaching of document EP 0516538, this compact block can be obtained by thermofusion of a mixture of microbeads of at least two thermoplastic materials of different grade, that is to say two materials having different melting temperatures. After a homogeneous mixture of the microbeads in a suitable mold, they are brought to a given temperature which is higher than the melting temperature of the material of the first type and lower than the melting temperature of the material of the second type. In this way the material constituting the microbeads of the first type bottom, which on the one hand creates the microcavities corresponding to the open pores and on the other hand ensures the cohesion of all the microbeads of the materials of the second type.

This embodiment is not limiting. In particular the compact block can be obtained from microbeads of a single thermoplastic material by simple sintering, the capillary network being formed by the interstices between the microbeads, after localized fusion and bonding thereof.

In normal operation, the ink 8 which is contained in the reservoir 5 and which is in contact with the rear end 3 a of the transfer point 3 is absorbed and migrates by capillary action in the point 3 to its front end 3 b . The ink consumed by this front end 3 b serving as a writing head is gradually replaced by the ink 8 coming from the reservoir 5. In this reservoir 5, the ink which diffuses in the transfer point 3 is itself replaced by air coming from inside the body 2. A pressure balance is thus established between the air contained in the tank 5 and the air prevailing in the rest of the body 2.

In the event of abnormal operation, due in particular to an increase in pressure in the reservoir 5, for example during heating of the article 1, an excess of ink 8 flows from the reservoir 5 into the transfer tip 3. Thanks to the presence of the buffer tank 4, this excess ink does not reach the writing tip 3b ; by capillarity, the excess ink diffuses into the buffer tank 4 in contact with the capillaries of the tip 3 until the pressure balance is restored. Then, when the reuse of the article 1, the ink that is consumed by the writing head 3 b preferably comes from the buffer tank 4.

It is important that the buffer tank 4 can store all the excess ink that may leak from transfer tip 3 if abnormal increase in pressure inside the tank 5. This capacity is a function of the capacity of the reservoir 5 of liquid ink 8, this excess being maximum when the ink tank contains a large proportion of air which tends to expand strongly when the temperature of the article increases 1.

It is possible by knowing the proportion of air in the compact block porous constituting the buffer tank 4 to perform theoretical calculations used to define the volume required for the buffer tank of an article of writing given.

However, the applicant has found that, depending on the configuration of the buffer tank 4, this theoretical volume was not always sufficient to absorb all of the excess ink, as long as there was no complete saturation of the buffer tank, the ink not being able to diffuse radially beyond a certain distance from the face which is in contact with the transfer point 3. This distance is a function of the nature, in particular of the surface tension and viscosity of the ink 8 and also of the constitution from buffer tank 4. In some cases, the effective distance is say where the buffer tank is saturated, can be limited to 2 to 3 mm.

Certainly to resolve this difficulty it would be possible to increase the surface of the buffer tank 4 which is in immediate contact with the ink. However, this would require modifying the structure of this reservoir 4, for example example by increasing the height of it for a theoretical volume determined. This structural modification would require modifying correlatively the configuration of the article.

According to the invention, the aforementioned difficulty has been resolved by using in the constitution of the buffer tank 4, from microbeads, microbeads to hydrophobic character, and in a lesser proportion, microbeads with character hydrophilic, according to the particular definition which has been given above to the notions hydrophilicity and hydrophobia.

The presence of this small proportion of hydrophilic microbeads has the effect of facilitating the diffusion of the ink throughout the volume of the buffer tank. These are polyethylene microbeads with an average diameter between 25 and 250 µm, the adequate proportion to obtain the technical effect above was between 2 and 10% hydrophilic polyethylene relative to all the microbeads constituting the buffer tank.

During the production of the buffer tank 4, use is made of microbeads which are hydrophobic for the most part and a hydrophilic character for a reduced proportion. Hydrophobic character or hydrophilic microbeads is obtained depending on the type of ink used or the made of the choice of the constituent material of the microbeads or because of a treatment suitable prerequisite, for example by fluorinated plasma to give the character hydrophobic or by an oxidation treatment to give the character hydrophilic or by the deposition of a surfactant to bring the energy of surface of the material at a determined value, depending on the desired character.

According to one embodiment, a mixture of hydrophobic microbeads and hydrophilic was produced homogeneously throughout the tank volume buffer. In a specific embodiment, being an aqueous ink and polyethylene microbeads, having an average diameter of around 140 µm, the mixture contained 95% hydrophobic microbeads and 5% hydrophilic microbeads. The buffer tank had a shape cylindrical in the order of 12.5 mm in diameter, the transfer point arranged axially in the buffer tank had a diameter of the order of 5 mm. In bringing the temperature of the article, cyclically, during use, to temperatures up to 50-55 ° C, complete saturation of everything was obtained the volume of the buffer tank, at the end of the cycle when the ink tank 5 was almost empty.

It should be noted that if we increase the proportion of microbeads hydrophilic, the buffer tank behaves as if it were a material completely hydrophilic. In other words, the ink diffuses into the material in question even if the conditions of use are normal conditions. This is of course not the technical function that is sought for a tank buffer. The buffer tank must absorb and return the excess ink from abnormal use, and not consist of a storage medium where the ink would be stored in addition to the liquid ink tank.

For the cases examined by the claimant, the maximum percentage of microbeads, allowing to obtain the desired effect, improved migration of the ink was around 10%. This proportion may change depending on the nature of the ink, and the nature and size of the microbeads. So it comes down to those skilled in the art to determine precisely, as a function of these parameters, what is the optimal proportion to keep for hydrophilic microbeads by compared to all of the microbeads used to build the reservoir buffer.

As previously pointed out, the ink 8 is slowed down in its diffusion from the face in contact with the transfer point 3 due to the existence pressure losses in the capillary network of the buffer tank 4. However this diffusion is very good in the area immediately in contact with the ink. Consequently, in this zone, it would be possible to use microbeads having only the hydrophobic character, or possibly a proportion still more reduced of hydrophilic microbeads. In other words, it would be possible to vary continuously or discontinuously the proportion of hydrophilic microbeads from the area immediately in contact with the ink to the area furthest from the buffer tank. Being a variation discontinuous, the buffer tank can then be in the form of elements annular, fitting into each other, the most annular element exterior with the highest proportion of hydrophilic microbeads.

In Figure 2, there is shown a buffer tank 4 which is constituted of three successive elements 4 ', 4' ', 4' '', fitted one inside the other, in which the proportion in hydrophilic microbeads was respectively 2% for the innermost 4 'element, 6% for the intermediate 4' 'element and 10% for the outermost 4 '' element.

The present invention is not limited to the embodiment which comes to be described by way of non-exhaustive example. In particular the choice of microbeads entering the composition of the buffer tank is determined by depending on the characteristics sought. It can be polyethylene as in the above example or other materials such as in particular the polypropylene. Hydrophilic microbeads can be in a material different from hydrophobic microbeads.

In addition, it may be advantageous to choose microbeads having different dimensions or even a different grade for the realization of microbeads with hydrophobic characteristics and / or microbeads with hydrophilic character. These differences in dimensions and also the differences in temperature of fusion in case of different grade will affect the capillary properties of the buffer tank thus obtained. This diversity of parameters that can enter into the realization of the buffer tank widens the possibilities offered to the man of the job.

Claims (8)

1. A buffer reservoir (4) for a writing instrument (1) containing a liquid ink (8), said writing instrument (1) including a reservoir (5) for the liquid ink (8), and a feed tip (3) opening out in the reservoir (5), said buffer reservoir (4) being suitable for absorbing the surplus ink (8) in the event that the pressure in the reservoir (5) varies and for subsequently returning it, and being implemented in the form of a compact block made of an open-pore material based on microbeads, characterised in that said buffer reservoir is made up of microbeads that are hydrophobic relative to said ink and a small proportion of microbeads that are hydrophilic relative to said ink.
2. A buffer reservoir according to claim 1, characterised in that the hydrophobic microbeads and the hydrophilic microbeads are distributed in a mixture that is uniform throughout the entire volume of the buffer reservoir (4).
3. A buffer reservoir according to claim 1, characterised in that the hydrophobic microbeads and the hydrophilic microbeads are distributed in a mixture that is not uniform, the proportion of hydrophilic microbeads being smaller in the zone immediately in contact with the surplus ink, and being larger in the zone that is furthest away therefrom.
4. A buffer reservoir according to claim 3, characterised in that it is made up of annular elements (4', 4'', 4''') nesting in one another, the proportion of hydrophilic microbeads being smallest in the element (4') that is the most immediately in contact with the surplus ink, being largest in the element (4''') that is furthest away therefrom.
5. A buffer reservoir according to one of claims 1 to 4, characterised in that the proportion of hydrophilic microbeads lies in the range 2% to 10% relative to the total weight of the buffer reservoir (4).
6. A buffer reservoir according to claim 2, characterised in that 95% of the uniform mixture is made up of hydrophobic microbeads, and 5% of said uniform mixture is made up of hydrophilic microbeads.
7. A buffer reservoir according to one of claims 1 to 6, characterised in that the hydrophilic microbeads are made of a different material and/or have different dimensions, and/or have a different grade than the hydrophobic microbeads.
8. A writing instrument (1) including a reservoir (5) for liquid ink (6), a feed tip (3) opening out into the reservoir (5), and a buffer reservoir (4) according to one of claims 1 to 7, said buffer reservoir fitting snugly around the feed tip (3) and being coaxial therewith.

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