ELASTOMER COMPOSITION AND METHOD OF MANUFACTURING THEROF

Field of the Invention

The present invention relates to an elastomer composition of nitrile and fluoride rubbers and further to a method of manufacturing thereof. Moreover, the invention relates to use of the elastomer composition for sealing of sub-sea installations, and further to a manufactured article obtained by the claimed method consisting of a gasket for a tubing hanger plug.

Background of the Invention

It is known to blend different kinds of elastomers to obtain a combination of beneficial properties that cannot be achieved by one kind alone. However, such blends often result in compositions that are inferior to those of the components because of incompatibility between different pairs of elastomers. Differences in viscosity in the unvulcanized state, in surface energy and in vulcanization rate often produce mixtures that are incompatible technologically. Thus, it is difficult to blend two or more different elastomers to achieve a desired combination of the good properties of each.

Nitrile rubbers have high strength characteristics and satisfactory heat resistance and are also known to have good resistance to hydrocarbons. Fluoride rubbers on the other hand, have satisfactory strength and high heat resistance. Fluoride rubbers also exhibit very good compression set values. Compression set is the ability of rubber to keep elastic properties after aging at pressure at static compression strain.

In US Patent No. 5053450 there is disclosed a method for combining an acrylic rubber with another rubber, which may be a fluoroelastomer or a fluoro silicone rubber. In the method, the mixture is masticated at a vulcanization temperature that will at least partially vulcanize the first component.

In US Patent No. 6306971 there is disclosed a method for obtaining a blend of at least one thermoplastic fluorinated polymer and at least one nitrile elastomer. The proportion of the nitrile elastomer is greater than 30 %. In the method, nitrile elastomers, which optionally have been formulated beforehand with a crosslinking agent, fillers and plasticizers are mixed with the fluorinated polymer at a temperature sufficient to cause the fluorinated polymer to melt. The result is a matrix of nitrile elastomers containing nodules of fluorinated polymer.

US Patent No. 4565614 discloses mixtures of fluoroelastomers and substantially saturated elastomers containing nitrile groups. The mixtures are co- vulcanised by peroxide cross-linking.

Summary and objects of the invention

An object of the present invention is to provide a new elastomer composition which exhibits properties particularly well suited for use as sealants in sub-sea installations. The elastomer compositions comprising a nitrile rubber and a fluoride rubber according to the invention show high heat and stress resistance and longevity in oil and gas applications as well as very good compression set values. Further the invention provides a method for the manufacturing of the elastomer compositions according to the present invention, wherein the sequence of the process steps are essential for the properties of the resulting elastomer composition.

Detailed description of the invention

The elastomer composition according to the invention exhibit high heat and stress resistance and high longevity for oil and gas applications in addition to very good compression set values of 30 % or lower. Two different rubbers are mixed to one compound and cured by one peroxide cross-linking system. The composition is characterized by a plasticized mixture composed of nitrile rubber and fluoride rubber, where said fluoride rubber is mixed to homogeneity with a mineral filler before mixing with the nitrile rubber.

Suitable mineral fillers are for example those comprising ZnO and BaSO ₄ . The ratio of the components (ZnO and BaSO ₄ )of the filler is in the range from about 2:9 to about 4:6. Particularly suitable are mineral fillers where the ZnO: BaSO ₄ ratio is about 3:7, (e.g. Litophone).

The invention further relates to a method for the manufacturing of an elastomer composition. The composition of the invention is prepared by mixing nitrile rubber and fluoride rubber plasticized by using a rubber mill. The steps of the method comprise i) mechanically plasticizing the nitrile rubber in a mill, ii) mechanically plasticizing the fluoride rubber in a mill while adding and mixing to homogeneity with a mineral filler, and iii) mechanically mixing the sheets produced in steps i) and ii) in a mill.

In one aspect of the invention step i) involves the mixing of the nitrile rubber with at least one further component to obtain a first mixture. In yet a further aspect of the invention step iii) involves mixing of the second mixture from step ii) into the first mixture from step i).

The method is performed at a temperature not exceeding 40°C-80°C in steps ii) and iii).

The steps of the method are performed by using a rubber mill, preferably a two-roll rubber mill. The mixture of step iii) is vulcanized by use of agents and methods well known in the art. In one aspect of the invention the vulcanization is carried out at a temperature higher than 160 ⁰ C, or even at a temperature higher than 170 ⁰ C.

Certain specific properties, for example improvements of the mechanical properties, of the composition according to the invention can be modified by the addition of fillers and plasticizers. Generally, fillers, plasticizers and optionally a crosslinking agent (benzene or peroxide) have been added to the nitrile rubber formulated. However, a part of the fillers may be introduced during the final mixing of the nitrile and fluoride rubbers. The vulcanisation agents, with a modifier such as TAIC (triallyl-isocyanurate), are introduced during the final mixing of the rubbers. Each step of the method is carried out by plasticizing the rubber in a rubber mill in order to disperse the agents into the rubber in a controlled and evenly distributed manner. During the rolling of the rubbers, a part of the sheet formed by the rollers is repeatedly cut and folded over the remaining part. This provides for the best homogeneity of the blend. In one embodiment of the invention the cut is made alternatively from either side, and the cut folded alternatively to the left and to the right. This enables the desired re-orientation of the phase surfaces. This provides more even distribution of the composition ingredients that increases the mechanical properties of the composition. The action mentioned above is carried out during all steps of the method, i.e. the nitrile rubber mixing step, the fluoride rubber mixing step and the step of blending nitrile and fluoride rubbers.

To keep control of the mechanical mixing of the agents, it is important to introduce the various ingredients (fillers, etc.) in an order at a time at which each ingredient is evenly distributed throughout the rubber(s) before the next ingredient is introduced.

The vulcanization agents are introduced during the last step of rolling and by a controlled temperature not exceeding 8O ⁰ C in order to prevent an early vulcanization. If the temperature becomes too high, the fluoride rubber will not mix properly with the nitrile rubber, due to the absence of sufficient tension and sliding velocity in the mill roll opening.

The main initial factor that determines the possibility of rubber usage in the aggressive environment is the degree of its swelling, which should be within defined limits. Swelling is the process typical for the high molecular weight

compounds. Swelling necessarily correlates with the separation of polymer chains, i.e. with the changes of structure that leads to the increasing volume. Thus, the chemical bonds along the polymer chain are not broken and only weak bonds between molecules are destroyed. The aggressive environment influences polymers in two different ways:

- on the one hand, swelling favourably influences strength properties due to more uniform stress distribution, increasing flexibility of macromolecules and easily orientation at stretching (at swelling less than 7-10%).

- on the other hand, swelling reduces strength and lifetime due to decreasing of intermolecular interaction (at swelling more than 15%).

Decrease of rubber lifetime in aggressive environments correlates with the swelling, i.e. the more the rubber is swelled, the more the lifetime is decreased.

Excessive swelling of the dynamic seals increases shear pull, friction coefficient and frictional losses. Excessive swelling of static seals increases extrusions into the openings and jamming.

The nitrile rubber is an acrylonitrile-butadiene rubber which in one aspect of the invention is hydrogenated. Correct formulation of mixture and proper production procedures provide excellent mechanical properties, very good wear resistance, very good compression set, excellent atmosphere- and ozone resistance and high heat resistance, very good longevity in oil and gas applications, even in the presence of hydrogen sulphides and amines. The hydrogenation resulting in a rubber with few double bonds, since this enables the polymer to withstand the stresses encountered during the plasticizing through the roll mill. In one particular aspect of the invention the HNBR is a thermoplastic HNBR. An example of hydrogenated acrylonitrile-butadiene rubber (HNBR) is sold by Bayer under the name of Therban A4307. This rubber contains 41 - 44 % by weight of acrylonitrile, having a Mooney viscosity of 56,0 - 70,0 (ML (1+4) 100° C), and a residual double bond content < 0,9 %, that allows to create rubbers with low swelling in aggressive environments at high temperatures.

The fluorinated polymer could be a thermoplastic fluorinated copolymer, and a suitable fluorinated polymer is a copolymer of vinylidenfluoride (CH ₂ CF ₂ ) and hexafluoropropylene (CF(CF ₃ )CF ₂ ), proportion of components 1 :1 (referred to as FKM-26). An exemplary fluoride rubber is sold by DuPont under the name of Viton A. This rubber has an average molecular weight of 100 - 600.000 and a density of 1.80 - 1.83 g/cm ³ (25 ⁰ C).

These rubbers were chosen partly because they have compatible polarities, although the value differs (Viton has higher polarity). When considering the high polarity of Therban A4307and Viton, a good compatibility of both polymers, increased thermal

stability and low swelling in aggressive environments of the composition is expected.

In one aspect of the invention the weight ratio between the nitrile rubber and the fluoride rubber in the composition is about 3:1. Further the nitril erubber and the fluoride rubber together constitute about 50% by weight of the elastomer composition.

The use of fillers such as carbon black and Litophon (30% ZnS + 70% BaSO ₄ ), vulcanization activators such as magnesium oxide and cadmium oxide and antioxidant agents such as Vulcanox ZMBI and Rhenofit DDA are introduced to add desired properties to the elastomer composition. The weight of additives are generally between 1 and 50 per 100 parts of elastomer composition.

Two different kinds of carbon black are used in the composition, a low active type and an average active type. The low active type has been found to create a structure that will promote increased diffusion rates of hydrocarbon gas out of the rubber at decompression. Low active is carbon black P-803 having parameters such as adsorption of DBE of 83 cm ³ /100g and specific surface area of 14-18 m ² /g. Average active is carbon black No. 550: FEF (Fast Extrusion Furnace) serial nos. 550, 539 and 550.

Some of the agents added to the HNBR in step i) may harm the fluoride polymer chains, and mineral fillers, such as Litophone, improves the fluoride rubber's resistance to some of these other agents added to the HNBR. Litophon was chosen in order to protect the fluoride rubber against destroyed cross-links caused by the interaction of carbon black. Litophon also reduces accumulation of compression set in the fluorinated rubber matrix.

The vulcanization system is well known to the person skilled in the art, and the invention is not limited to one specific type of system. One suitable vulcanization system is a peroxide system, sold under the name Percadox. It has been found that this system will vulcanize equally well both the nitrile and the fluoride component when prepared according to the invention. They enable a composition that is stable to heat and have a higher resistance to aging. Two kinds of Percadox are used. It was found that usage of Percadox 14/40 KPD provides higher tensile strength, while Percadox BC-40 KPD provides higher elongation. The various dosages of these vulcanizers can be used to regulate the parameters of tensile strength and elongation as demanded for the use.

Fluoride systems generally requires long post curing times to obtain low compression set values. In the elastomer composition according to the present invention, a post curing time of only two hours at 200 ⁰ C is needed to achieve excellent results in respect of compression set.

An elastomer composition of the invention may be obtained by mechanically mixing the nitrile rubber and at least a further component to obtain a first mixture, further mechanically mixing the fluoride rubber and at least a further component to obtain a second mixture, and then the second mixture is mixed mechanically into the first mixture before the such obtained mixture is vulcanized.

Further aspects and embodiments of the invention are defined in the claims.

The invention is further illustrated by the following non-limiting examples. The particular materials and amounts thereof recited in these examples, as well as other conditions and details, are only intended to assist one skilled in the art to further understand the invention.

EXAMPLES

An elastomer composition was made according to the invention, referred to as 9-2M-2, having the following ingredients, (Table 1):

Table 1

A desired amount of HNBR rubber (Therban A4307) is cut from a solid block and cut into strands and/or filaments. The cut strands are fed into a two-roll rubber mill to mould and plasticize the rubber into thin sheets, using a gap of 0,5 - 1 mm. During rolling the sheet is cut, and the cut part is folded over the remaining part. In a plasticizing process using rollers, the mechanical forces will tend to orientate the crosslinks in the same direction. This creates weakened parts between the crosslinked strands. By folding the sheet, the direction of crosslinks is changed and results in a more anisotropic structure.

In a preferred embodiment, the cutting is done alternatively from the left and right, and folding is done correspondingly. This ensures a better distribution of the direction of the interphase structures, resulting in a re-orientation of the phase surfaces.

During rolling of HNBR, the temperature in the mill is kept below 15O ⁰ C ₅ preferably below 120 ⁰ C, and most preferably below 100 ⁰ C. The agents (fillers) are introduced between the rollers to intimately mix and disperse the fillers with the HNBR. This action is carried out for approximately 20 minutes. The time spent rolling the sheet must be sufficient in order to distribute well the fillers added in the rubber matrix. The HNBR is now allowed to cool to room temperature.

In a separate step, the FKM is plasticized in a two-roll rubber mill in a similar manner. However, the mill is water cooled to keep the temperature below 4O ⁰ C-

80 ⁰ C.

A mineral filler (Litophone) is introduced and mixed with the FKM matrix at this step. Litophone improves the resistance of fluoride rubber to some of the agents added in the first step to the HNBR, that may harm the fluoride polymer chains. Litophon was chosen in order to protect the fluoride rubber against degradation of cross-links caused by the interaction of carbon black. Litophon also reduces accumulation of compression set in FKM matrix.

In the third step, the sheets of HNBR and fluoride rubber are introduced into the roller mill. The gap between the rollers are increased to 1 - 1,5 mm. The resulting sheets are cut and folded as explained above for a period of approximately 5 minutes. At this step, the vulcanization agents are introduced. This step should be completed in the shortest possible time, while still allowing the rubbers to be completely mixed, to avoid heating of the fluoride. If the matrix becomes too hot, the fluoride and nitrile rubbers begin to scorch. In an alternative embodiment, about ³ A of the carbon black with agents are introduced during HNBR milling and the remaining ¹ A introduced during the mixing with FKM. The great value is due to the order of carbon black incorporation. In this case, rubber combining occurs at more intensive shift efforts that result in the best dispersion of fluoride in HNBR matrix. Injection of carbon black in HNBR allows to receive vulcanizes possessing higher physical properties and oil resistance.

The finished sheets were vulcanized for 20 minutes at 17O ⁰ C and then post-cured for 2 hours at 200 ⁰ C. The following parameters were achieved (Table 2):

Table 2

Vulcanizing properties shown in Table 3 :

Rheometer Monsanto 100, Temperature 18O ⁰ C, Chart motor -24 minutes

Table 3

The rubber was then tested at different temperatures and in different conditions, achieving the following results.

Swelling in composition of 70% heptane + 30% toluene and in ASTM Oil #3

Swelling: weight change, %

Composition of 70% heptane + 30% toluene (24 hours at room temperature)

Elastomer compositions indicated in tables 4- 6 refer to the reference elastomer composition E50159 and elastomer compositions 9-2M-2 according to the invention.

Table 4

Table 5

ASTM Oil #3 15O ⁰ C

Table 6

Swelling: tensile properties change.

Composition of 70% heptane + 30% toluene

Aging in air. Changes in compression set.

Aging temperatures: 130, 150, 180 ⁰ C. Compression 30 %.

Table 7

Compression set at 13O ⁰ C ₅ 15O ⁰ C and 18O ⁰ C

Results of data of rubbers is produced in accordance with the ISO 11346 after aging in air at 18O ⁰ C.

Aging in ASTM Oil #3. Changes in tensile strength and elongation. Aging temperatures: 100, 130, 150 ⁰ C. Dumbbells Die C.

Table 8

Tensile strength after aging in ASTM Oil #3

Table 9

Elongation, % after aging in ASTM Oil #3

Rubber extrapolation property changes at thermal aging in oil at 18O ⁰ C has been produced according to ISO 11346. All results of extrapolation for E50159 and 9- 2m-2 at T=I 8O ⁰ C on air and in oil are listed in Table 10.

Table 10

Analysis of the results presented in tab e 10 shows very good results with respect to change of tensile strength, elongation and compression set for rubber 9-2m-2 after aging on air and in oil.

σ _f2 o and S _f20 are the original values for new (unaged) rubber at 20°C, σ _f r/ σ _f2 o and ε _f τ/ε _f2 o — changes in properties, where σfr and S _f r - values after aging (T - aging temperature during marked aging interval).

The graphics below present the extrapolations of σ _f r/ O _f20 and S _f r/S _f20 .

« /' f20