Patent Application
20110318576
The subject of the invention is a process for treating an elastomer part with multiple-energy He+ and He2+ ions.
The invention is applicable for example in the biomedical or automotive field, in which it is desired for example to reduce the friction of an elastomer part on a contact surface in order to reduce the resistance forces, abrasive wear or even the noise.
Contact between an elastomer and a rough hard surface takes place by an envelopment of the asperities on the opposing surface. This generates a tangential force which is the result of an adhesive force (due to van der Waals forces) and a deformation force. The deformation force depends on the delay experienced by the elastomer before resuming its initial shape after having followed the asperities of the opposing surface. This delay is called the hysteresis component of the friction and depends on the viscoelastic properties of the elastomer. By increasing the elasticity, this delay time is reduced. The friction force is also the sum of a friction force and a hysteresis force.
The friction coefficient essentially depends on:
The adhesion is an important effect in the case of elastomers, which corresponds to energies of the order of 100 mJ/m2.
Elastomers are defined by their slip G, which is inversely proportional to their friction coefficient μ. The slip is expressed in the following manner:
G=(1/μ)=(1/s)·(m+p)
where s represents the adhesion, m the aptitude of the elastomer to follow the opposing surface and p the contact pressure.
To give an example, the friction coefficient of a natural rubber in static mode varies between 4 and 1.5 for a pressure varying from 0.5 to 3 bar.
In dynamic mode, an increase in the speed produces an adhesion peak in the elastomer on approaching creep speeds and a hysteresis peak at very high speeds.
Elastomers make a particular sound. Under the effect of displacement, appear in the area of contact separation regions between the elastomer and the opposing surface. The surface of the elastomer then undergoes a reptation process, consisting of separation waves propagating in the opposite direction to the friction force. This gives rise to a screaming noise, constituting a real nuisance. To correct this, one approach may consist in reducing the difference that exists between the static friction coefficient and the dynamic friction coefficient. For this purpose, conventional chemical methods of halogenation—fluorination, bromination or even chlorination processes—exist, but these are applicable only to a minority of elastomers. Such processes have great drawbacks: they are very polluting; and they require very large quantities of water to be heated, which then has to be filtered in very expensive reprocessing plants. The effectiveness of these halogenation processes depends greatly on the chemical composition of the elastomer and on its concentration of chemical double bonds capable of undergoing an electrophilic addition. For example, it is very difficult to apply them on an elastomer of the EPDM (ethylene propylene diene monomer) type. In this case, chlorination at very high temperature is recommended.
Apart from friction problems, the elastomer parts must often operate in relatively aggressive chemical environments, with ambient moisture, ambient oxygen, at very low or in contrast very high temperatures, etc., which may result in accelerated ageing.
Certain elastomers are filled with chemical agents for protection against UV or oxidation. The effect of these chemical agents being discharged to the outside is for the elastomer to lose its surface mechanical properties.
Other elastomers, very good from a mechanical standpoint, are however excluded from any medical or pharmaceutical usage because of a minimal risk of toxic discharges—in fact precluding excellent elastomers.
Certain elastomers are insulating and can collect dust, which is retained thereon or even bonded thereto because of electrostatic charges that have built up on their surface during the manufacturing process.
Certain elastomers require the use of talc to avoid parts sticking to one another during the manufacturing process or during assembly.
The object of the invention is to reduce the aforementioned drawbacks and in particular to enable the surface friction of a bulk elastomer part to be reduced, while still keeping its viscoelastic properties in the bulk and avoiding the use of polluting chemical treatments. The invention thus provides a process for treating at least one surface of a bulk elastomer part by helium ions, characterized in that multiple-energy He+ and He2+ ions are implanted simultaneously, in which the ratio RHe, where RHe=He+/He2+ with He+ and He2+ being expressed in at %, is less than or equal to 100, for example less than 20.
The inventors have found that the simultaneous presence of He+ and He2+ ions enables the surface properties of elastomers to be very significantly improved compared with the known treatments in which only He+ or He2+ ions are implanted. They have demonstrated that a significant improvement was obtained for an RHe equal to or less than 100, for example equal to or less than 20.
It should be noted that the invention makes it possible to reduce the adhesion of a bulk elastomer part on an opposing surface and/or to reduce the surface hysteresis component of a bulk elastomer part and/or to reduce the abrasive wear of a bulk elastomer part and/or to reduce the sticking between parts made of the same elastomer and/or to eliminate the bonding of dust.
The invention also makes it possible to increase the chemical resistance of the elastomer, for example by creating a permeation barrier. This barrier can slow down the propagation of ambient oxygen into the elastomer and/or retard the diffusion of chemical protection agents contained in the elastomer to the outside and/or inhibit the leaching of toxic agents contained in the elastomer to the outside.
Advantageously, the invention makes it possible to dispense with the current polluting processes, such as fluorination, bromination, chlorination, and to replace them with a physical process which is applicable to any type of elastomer and is not costly in terms of material and energy consumption.
In the context of the present invention, the term “bulk” is understood to mean an elastomer part produced by mechanical or physical conversion of a mass of material, for example by extrusion, molding or any other technique suitable for converting a mass of elastomer. Such conversion operations are used to obtain variously shaped bulk parts, for example three-dimensional parts, substantially two-dimensional parts, such as for example profiled strips or sheets, and substantially unidirectional parts, such as threads.
Among elastomer products that may advantageously be treated by the process of the present invention, the following examples may be mentioned: bodywork seals; hydraulic cylinder scraper seals; O-ring seals; lipped seals; ball joint seals; windshield wiper blades; aircraft wing leading edges; nacelle leading edges; and hypodermic syringe piston heads.
Moreover, it goes without saying that the bulk elastomer part may be a portion of a part made of another material, for example attached to this part made of another material.
As examples and among elastomers, the following materials that benefit from treatment according to the invention may be mentioned:
According to one embodiment, the He+ and He2+ ions are produced simultaneously by an electron cyclotron resonance (ECR) ion source.
Using the process of the present invention, it is possible to preserve the original colour of the elastomer, giving it however a glossier appearance.
It is found that the treatment times are not long in relation to industrial requirements.
Furthermore, the process has a low energy requirement, is inexpensive and can be used in an industrial context without any environmental impact.
The treatment of an elastomer part is carried out by simultaneously implanting multiple-energy helium ions. These are in particular obtained by extracting, with one and the same extraction voltage, singly charged or multiply charged ions created in the plasma chamber of an electron cyclotron resonance (ECR) ion source. Each ion produced by said source has an energy proportional to its charge state. It therefore follows that the ions with the highest charge state, and therefore the highest energy, are implanted into the elastomer part at greater depths.
Implantation using an ECR source is rapid and inexpensive since it does not require a high ion source extraction voltage. Indeed, to increase the implantation energy of an ion it is economically preferable to increase its charge state rather than increase its extraction voltage.
It should be noted that the use of a conventional source of He ions, such as in particular the sources for implanting ions by plasma immersion or filament implanters, does not make it possible to obtain a beam suitable for simultaneously implanting multiple-energy He+ and He2+ ions in which the RHe ratio is equal to or less than 100. With such sources, said ratio is at the very most less than or equal to 1000.
The inventors have found that this process enables an elastomer part to be surface-treated without impairing the bulk viscoelastic properties thereof.
According to one embodiment of the present invention, the source is an electron cyclotron resonance source producing multiple-energy ions that are implanted in the part at a temperature below 50° C. and the implantation of the ions of the implantation beam is carried out simultaneously at a controlled depth by the extraction voltage of the source.
Without wishing to be tied by any scientific theory, it is thought that, in the process according to the invention, the ions during their transit excite the electrons of the elastomer, causing a scission of covalent bonds, which immediately recombine to generate, by a crosslinking mechanism, a high density of covalent chemical bonds. This densification of covalent bonds has the effect of increasing, on the surface, the hardness, elasticity and compactness of the elastomer and of increasing its chemical resistance. The crosslinking process is more effective the lighter the ion.
In this regard, helium is an advantageous projectile since:
According to various embodiments of the process of the present invention, which may be combined together:
It has been found that the teaching obtained on a non-elastomer polymer, for example on a polycarbonate, relating to the variations in surface property obtained by implantation of He+ and/or He2+ ions cannot be transposed to the present observations and advantages obtained on elastomers treated according to the process of the invention.
The invention also relates to a part where the depth where the helium is implanted is equal to or greater than 50 nm, for example equal to or greater than 200 nm, and the surface elastic modulus E of which is equal to or greater than 15 MPa, for example equal to or greater than 20 MPa, or even equal to or greater than 25 MPa.
The invention also relates to the use of the above treatment process for treating a bulk elastomer part chosen from the list consisting of a windshield wiper blade, a bodywork seal, an O-ring seal, a lipped seal, a hydraulic cylinder scraper seal, a ball joint seal, an aircraft wing leading edge, an aircraft jet engine nacelle leading edge, a hypodermic syringe piston, or an automobile liner for damping vibrations between contacting parts.
The present invention will now be illustrated by examples of nonlimiting embodiments, especially with reference to the appended drawings in which:
Several methods of characterization have enabled the advantages of the present invention to be established.
In the following examples, the treatment of at least one surface of a bulk elastomer part by implanting He+ and He2+ helium ions was carried out with multiple-energy He+ and He2+ ions produced simultaneously by an ECR source. The treated elastomers were in particular the following: natural rubber (NR), polychloroprene (CR), ethylene propylene diene monomer (EPDM), fluorocarbon rubber (FKM), nitrile rubber (NBR), thermoplastic elastomer (TPE). In all cases, a very significant reduction in the friction coefficient against a glass surface was found.
Comparative tests relating to the measurement of friction coefficient have demonstrated that:
Moreover, other beneficial surface properties may be found:
The relative shiny area represents only 14% of the area of the untreated blade (before treatment according to the invention). The shiny area increases linearly up to 41% for a dose of 3×1015 ions/cm2. Above this, a saturation plateau is observed, the relative shiny area no longer varying but remaining equal to 42% of the area of the blade.
According to one embodiment, it is estimated that the surface properties of an elastomer, especially the friction properties, are significantly improved using a dose of 1015 ions/cm2, which represents a treatment rate of about 30 cm2/s for a helium beam consisting of 4.5 mA of He+ ions and 0.5 mA of He2+ ions.
The simultaneous implantation of helium ions may take place at variable depths, depending on the requirements and the shape of the part to be treated. These depths depend in particular on the implantation energies of the ions of the implantation beam. For example, they may vary from 0.1 to about 3 μm for an elastomer. For applications in which the mechanical stresses are high, such as those relating to bodywork seals rubbing on a glass pane, treatment depths of around 1 micron will for example be used. For applications in which for example anti-sticking properties are desired, a depth of less than one micron may for example be sufficient, thereby reducing the treatment time accordingly.
According to one embodiment, the He+ and He2+ ion implantation conditions are chosen so that the elastomer part retains its bulk viscoelastic properties due to keeping the part at treatment temperatures below 50° C. This result may especially be achieved for a beam of 4 mm diameter delivering a total current of 60 microamps with an extraction voltage of 40 kV, which is moved at 40 mm/s over displacement amplitudes of 100 mm. This beam has a power per unit area of 20 W/cm2. To use beams of higher current with the same extraction voltage and the same power per unit area, and to maintain the bulk viscoelastic properties, a scale rule may be suggested that consists of increasing the diameter of the beam, of increasing the rate of displacement and of increasing the amplitudes of displacement in a ratio corresponding to the square root of (desired current/60 microamps). For example for a current of 6 milliamps (i.e. 100 times 60 microamps), the beam may have a diameter of 40 mm in order to maintain a surface power of 20 W/cm2. It is necessary under these conditions to increase the speed by a factor of 10 and the amplitudes of displacement by a factor of 10, thereby giving a speed of 40 cm/s and displacement amplitudes of 1 m. The number of passes should also be increased by this same factor in order in the end to have the same treatment dose expressed in ions/cm2. In the case of the continuously running treatment, the number of micro accelerators placed for example along the path of a strip may be increased in the same ratio.
It has also been found that other surface properties are very significantly improved by virtue of a treatment according to the invention and that performance levels apparently unable to be achieved with other techniques have been demonstrated.
The surface elastic modulus may be measured in particular using an instrumented nano indentation technique. This technique is used for mechanically characterizing the surfaces of materials at depths of the order of a few tenths to a few tens of nanometers. The principle consists in applying a load, via an indenter, on a surface. The instrument measures the penetration and quantities (stiffness, phase, etc.) corresponding to the response of the material to the stress. The surface elastic modulus may thus be measured as a function of the depth. In the case of an elastomer material, loading is followed by unloading, which has a reversible character in which the unloading behavior as a function of time is analyzed so as to determine the viscoelastic properties of the material and to deduce the surface elastic modulus. The measurement may be carried out statically or dynamically.
The following publications serve to illustrate metrological methods of this type so as to determine the surface elastic modulus of an elastomer:
In
In
It may be seen that elastomer parts having a surface modulus E equal to or greater than 15 MPa, for example equal to or greater than 20 MPa or even equal to or greater than 25 MPa, may be obtained. These surface elastic modulus values are remarkable and have not been found for elastomers. Surprisingly, it may be seen that the surface elastic modulus E varies differently in three consecutive He ion dose ranges with a substantially linear behavior in each of these three regions: from 0 to about 3×1015 ions/cm2, the surface elastic modulus increases very substantially; on about 3×1015 ions/cm2 to about 8×1015 ions/cm2, the surface elastic modulus increases more slowly; and it increases more rapidly above about 8×1015 ions/cm2. This observation is noteworthy as it is commonly accepted that ion implantation can make it possible to improve a property characteristic of the behavior of the surface of an organic material but that this improvement reaches a plateau after which there is in general a degradation in said property when the implanted ion dose increases.
In the present case, it may be seen that above a second region, lying between about 3×1015 ions/cm2 and about 8×1015 ions/cm2, which may be termed the plateau region, a property characteristic of the behavior of the surface of an elastomer may be greatly improved.
According to one embodiment, when it is desired to improve a surface property of an elastomer very significantly, an ion dose range is determined in which the variation of the chosen characteristic property is advantageous and behaves differently in three consecutive ion dose regions forming said ion dose range, with a substantially linear behavior in each of these three regions and in which the absolute value of the slope of the variation in the first region and that of the third region are greater than the absolute value of the slope of the variation in the second region, and in which the multiple-energy dose of He+ and He2+ ions is chosen to be in the third ion dose region in order to treat the bulk elastomer part.
The invention is not limited to these types of embodiment and must be interpreted non-limitingly, as encompassing the treatment of any type of elastomer.
Likewise, the process according to the invention is not limited to the use of an ECR source, and even though it might be thought that other sources would be less advantageous, the process according to the invention may be implemented with mono-ion sources or with other multiple-ion sources provided that these sources are configured so as to allow simultaneous implantation of multiple-energy He+ and He2+ ions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09 01002 | Mar 2009 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2010/050379 | 3/5/2010 | WO | 00 | 9/2/2011 |
