High-strength compound of highly saturated nitrile and its applications.

High-strength compound of highly saturated nitrile and its applications

Highly saturated nitrile elastomer (HSN) is well known for its superior resistance to heat, oil additives, sour fuel and oil, corrosion inhibitors, [H.sub.2]S, [CO.sub.2], etc. It also has excellent physical properties such as very high tensile, tear and abrasion resistance, as well as very good hot dynamic characteristics.

The basic very good physical properties of HSN can be further greatly enhanced over those obtained with conventional reinforcing agents such as carbon black or silicas by a combination of zinc oxide (ZnO) and methacrylic acid (MAA). The basic properties of HSN reinforced with ZnO/MMA are examined and its potential uses discussed.

HSN reinforced with ZnO/MAA

Polyurethanes have been used for many years in applications requiring very high tensile and tear strength coupled with excellent abrasion resistance. Unfortunately polyurethanes soften at high temperatures and are subject to hydrolysis in the presence of moisture. Natural rubber has very good tensile and tear strength, but has poor oil and heat resistance. Ethylene propylene (EPDM) exhibits good heat resistance, but only moderate physicals and is not resistant to hydrocarbons. Fluoroelastomers (FKM) and polyacrylate (ACM) are both resistant to oil and high temperatures but have poor physical characteristics.

Previous studies have been made of various peroxide cured elastomers reinforced with metalic salts of numerous ionomers, such as zinc methacrylate. Good reinforcement was found in the range of 10 to 30 MPa, but not outstanding. The main practical use has been for solid golf balls and rollers.

It has been found, however, that peroxide cured HSN reinforced with ZnO/MAA has excellent modulus, tear and tensile strength, up to 60 MPa, which is combined with its very good heat, oil and chemical resistance.

The reinforcement of HSN with ZnO and MAA has been postulated to be the result of the combined formation of the zinc salt of methacrylic acid polymer particles which graft onto the HSN elastomer chain. This structure follows the theory proposed by Anfimov (ref.3).

The degree of reinforcement depends upon three factors, as illustrated in table 1. These are the affinity of the elastomer to the ZnO/MAA salt, a moderate degree of radical reactivity and the microcrystalline character of the polymer. HSN is an ideal elastomer candidate for enhanced reinforcement.

Table : Table 1 - properties of elastomers

Affinity Moderate Crystal-

Elastomer to MAA-salt radical reactivity lization

HSN             O               O              O
NBR             O               X              X
NR              X               X              O
SBR             X               X              X
EPDM            X               O              X
BR              X               X              X

O - positive or enhanced reactivity X - negative or no reactivity

Basic properties of HSN with ZnO/MAA

The reinforcement obtained with various peroxide cured elastomers using zinc oxide and methacrylic acid are illustrated in figure 1. The tensile strength of the HSN compound was far superior to that obtained with other polymers due to its affinity to methacrylic acid salts, free radical reactivity and microcrystallinity created upon hydrogenation.

Figure 2 shows the effect of the degree of hydrogenation of HSN with ZnO/MAA on its physical properties. It may be noted that both tensile strength and elongation at break increased rapidly between 50 and 80 percent hydrogenation, and at a slower rate thereafter.

The effect of the ratio of ZnO to MAA at various levels on tensile strength may be seen in figure 3. The best reinforcement over the broadest range of loadings was obtained with a ration of 0.75.

Hardnesses obtained varied directly with loading levels irrespective of the ZnO to MAA ratio.

Table 2 illustrates the lack of dependence of hardness and modulus at 100% elongation on temperatures between 25[degrees]C and 150[degrees]C of the ZnO/MAA reinforced HSN at different loadings. The HAF carbon black reinforced HSN used for comparison was more temperature sensitive.

Table : Table 2 - temperature dependence of HSN reinforced by ZnO/MAA and HAF

Compound            ZnO/   ZnO/    ZnO/    HAF
  (phr)             MAA    MAA     MAA
                    15/20  23/30   30/40   50

Hardness, Shore A

  at 25 [degrees]C  76     86      92      92
  at 150 [degrees]C 76     86      91      89

Modulus at 100% elongation, MPa

  at 25 [degrees]C  5      10      15      8
  at 150 [degrees]C 7      10      12      3

The compound viscosity of HSN with varying hardnesses, or loadings of ZnO/MAA, is shown in figure 4. The HSN compounds did not change viscosity as loadings of ZnO/MAA were increased, whereas the HAF compounds used for comparison increased viscosity rapidly.

Figure 5 compares the tensile strength of HSN with either ZnO/MAA or HAF black. The tensile of the ZnO/MAA compounds were over 50 percent higher than those with HAF at all hardnesses or loading levels.

Elongation at break of the ZnO/MAA filled HSN did not decrease nearly as rapidly as those with HAF, as reinforcement quantities were increased. Resilience, as measured by Lupke rebound also was not as sensitive to ZnO/MAA levels as with HAF carbon black.

Results and discussion

Peroxides in ZnO/MAA reinforced HSN A study of various types of peroxides was made, as seen in table 3, in HSN reinforced with ZnO/MAA. To obtain a Shore A hardness of 75 to 80, a blend of unreinforced HSN 2020 with HSN 2020 reinforced with 15 phr of ZnO and 20 phr of MAA (ZSC 2295) was used. Because the ZnO/MAA was thought to be grafted onto the HSN, it was considered to be 100% elastomer, and was used as such. The amounts of the individual peroxides used were designed to produce the same mole number level of activated oxygen. [Tabular Data Omitted]

The [alpha] [alpha] -bis (t-butyl peroxy-m-isopropyl) benzene (BPIPB) gave the best balance of tensile, elongation and compression set. The t-butyl cumyl peroxide (BCP) also provided good properties. Others may be selected to provide the desired physical properties and cure rate.

The next phase of the study was to examine various levels of BPIPB in the same 55/45 ratio of ZCS 2295/HSN 2020 ZnO/MAA reinforced HSN. As anticipated, the maximum torque increased with increased levels of peroxide.

The physical properties were plotted. Tensile strength and elongation plots in figure 6 show that optimum tensile was obtained at the 3 to 5 phr level of BPIPB. Elongation decreased almost linearly with the amount of peroxide used. The hardness and modulus at 100% elongation both increased in proportion to the peroxide level. The rebound was essentially constant with various amounts of BPIPB, whereas the compression set decreased rapidly between 2 and 4 phr, and at a slower rate at higher peroxide levels.

An examination of the air aged properties indicated that the most severe tensile loss was at the 2 to 4 phr level of peroxide, with improved tensile and elongation retention at higher levels. Please note that these compounds did not contain antioxidants. The hardness change also indicated that higher amounts of BPIPB provided better aging resistance.

Abrasion resistance was also studied using BPIPB peroxide from 1 to 10 phr, as seen in figure 7. At the same time, different amounts of ZnO/MAA to produce hardnesses of 65 to 95 were introduced. The resultant plot of Picco abrasion loss shows that increased reinforcement of ZnO/MAA, as indicated by higher hardness, improved abrasion resistance substantially. Also at higher levels of ZnO/MAA, increased peroxide reduced abrasion loss, but at low levels of reinforcement, there was minimal effect.

Cure times and temperatures of peroxides are important. The tensile strength of HSN with ZnO/MAA reached a high level sooner and plateaued at higher vulcanization temperatures, especially above 170 [degrees]C in figure 8. The elongation results showed that the results plateaued sooner at higher vulcanization temperatures. Hardness and 100% modulus, as expected, also indicated that maximum results were obtained faster at higher cure temperatures. Figure 9, showing compression set data, also confirms the benefit of higher vulcanization temperatures on desired properties.

Fillers in HSN with ZnO/MAA A study of N774 carbon black used in varying ratios with ZnO/MAA is given in table 4. The levels were designed to produce a Shore A hardness of 78. The required levels of ZnO/MAA were obtained by blending ZSC 2295 and HSN 2020. As the N774 carbon black was decreased and ZnO/MAA increased, it was found that:

* Compound viscosity decreased from 138 to 76.

* Tensile strength increased from 24 to 45 MPa.

* Elongation increased from 250 to 430%.

* Modulus at 100% elongation decreased from 8.3 to 4.5 MPa.

* Compression modulus at 30% increased from 4.0 to 5.4 MPa.

* Compression set increased with higher ZnO/MAA ratios.

* De Mattia flex resistance improved by a factor of 20.

* Goodrich flexometer heat build-up decreased.

* Picco abrasion loss dropped nearly in half. [Tabular Data Omitted]

It was readily apparent that, with the exception of compression set, all properties improved as the ratio of ZnO/MAA increased.

The influence of fumed silica also was investigated. The base compound had 4, 8 and 16 phr of fumed silica added. The viscosity increased in proportion to the amount of silica. The results were plotted. Data on HSN with varying levels of ZnO/MAA to produce similar hardnesses were also plotted for comparison.

The desired ZnO/MAA amounts again were obtained by blending ZSC 2295 and HSN 2020. The tensile strength dropped much faster with increased silica as compared to the ZnO/MAA, as seen in figure 10.

Picco abrasion resistance was improved to a greater extent with increased silica in comparison to the ZnO/MAA, although at higher hardnesses they would appear to be the same.

As previously reported, compression set of HSN became poorer at higher loadings of ZnO/MAA, whereas the addition of fumed sillca to increase hardness, gave only a marginal increase in compression set.

Plasticizers in HSN with ZnO/MAA A study of tri isooctyl trimelitate (TOTM) in HSN reinforced with two levels of ZnO/MAA is given in table 5. As expected, tensile strength, hardness and tear resistance decreased with increased TOTM. Although tensile strength did decrease, it was still 25 MPa at a hardness of 45 Shore A. Elongation at break increased as the plasticizer was increased. The compression set of HSN with ZnO/MAA in figure 11 decreased as TOTM was increased to 25 phr, thereafter it began to increase. [Tabular Data Omitted] compression set (aged 70 hours at 120 [degrees]C)

As expected, the Gehman torque properties improved in direct relation to the amount of plasticizer.

Conclusions

The excellent heat, oil, fuel and chemical resistance of HSN has been previously well established. Also well known are the very good physical and dynamic characteristics of this elastomer.

It has been established in this article that the physical properties of HSN, because of its basic structure, can be greatly enhanced by reinforcing it with zinc oxide and methacrylic acid.

This applies as saturation increases to over 50% and in particular at over 80%. The same improvement was not found in other elastomers.

It was concluded from this study that the addition of ZnO and MAA to HSN polymers resulted in the following:

* Very high tensile strength of 40 to 60 MPa.

* Excellent abrasion resistance, which can be enhanced by the addition of fumed silica.

* Extension modulus decreased and compression modulus increased with higher loadings of ZnO/MAA.

* Hardness and modulus are constant over the temperature range of 25 to 150 [degrees]C.

* Resilience decreased at a much slower rate with ZnO/MAA than with carbon black as loadings were increased.

* The flex resistance improved as ZnO/MAA loadings were increased.

* Although compression set was poorer at higher loadings of ZnO/MAA, this may be compensated by combining lower amounts of ZnO/MAA with conventional reinforcing agents to achieve the desired hardness.

* As with all elastomers, it is important to vulcanize at 170 [degrees]C or higher with peroxides for optimum properties of the HSN.

* Highly plasticized HSN-ZnO/MAA compounds exhibited quite high tensile strength.

* Even at high loadings of ZnO/MAA, the HSN compounds did not increase in viscosity, which would indicate their good flow characteristics.

Applications

The outstanding physical properties of HSN modified with ZnO/MAA make it an ideal candidate to replace polyurethane in many applications which may be beyond its operating temperature.

The combination of high elongation at high modulus and hardness coupled with excellent tensile strength and abrasion resistance are unique properties to be found in a single elastomer.

This is an exciting compound technique for which we anticipate many applications, such as:

* Tubing to replace low pressure hose.

* Base for V-belts, timing and multi-V belts.

* Rollers for can coating, paper mill, laminating and textiles.

* Spinning costs and aprons.

* Wipers.

* Swab cups.

* Drill pipe protectors.

* Mud pump pistons.

* Doffers.

* Vibration isolators.

* Tank track pads.

When considering these, or any other applications, a patent search is recommended.

References

[1.] A.A. Dontsov, V.F. Soldatov, A.N. Kamenskii and B.A. Dogadkin, Kolloidnyi Zhurnal, Vol. 31, No. 3. pp. 370-375 (1969). [2.] A. Dontsov, F. De Candia and L. Amelino, Journal of Applied Polymer Science, Vol. 16, pp. 505-518 (1972). [3.] B. Anfimov, A. Dontosov, E. Ferracini, A. Ferrero, R. Hosemann and F. Riva, Die Markromolekulare Chemie, 176, 2467-2472 (1975). [4.] K. Hashimoto et al., Rubber Division, ACS Meeting, Houston, TX, October 25-28, (1983). [5.] W.J. MacKnight and R.D. Lundberg, Rubber Chemistry and Technology, Vol. 57, No. 3, pp. 652-663 (1984). [6.] K. Hashimoto et al., Rubber Division, ACS Meeting, Cleveland, OH, October 1-4 (1985). [7.] Y. Kubo et al., Rubber Division, ACS Meeting, New York, NY, April 7-11 (1986). [8.] Y. Todani et al., Rubber Division ACS Meeting, Cleveland, OH, October 6-9 (1987). [9.] N. Watanabe et al., Rubber Division, ACS Meeting, Dallas, TX, April 19-22 (1988). [10.] R.C. Klingender et al, Rubber Division, ACS Meeting, Cincinnati, OH, October 18-21 (1988). [11.] Y. Saito, A. Fujino, and A. Ikeda, SAE SP787, "Elastomer developments: Materials, applications, processing and performance, 890359 (1989).

PHOTO : Figure 1 - tensile of elastomers reinforced with ZnO/MAA

PHOTO : Figure 2 - hydrogenation vs. physicals of HSN with ZnO/MAA

PHOTO : Figure 3 - effect of ZnO-MAA ration on textile strength

PHOTO : Figure 4 - compound viscosity vs. hardness of HSN with ZnO/MAA and HAF black

PHOTO : Figure 5 - tensile strength vs. hardness of HSN with ZnO/MAA and HAF black

PHOTO : Figure 6 - tensile strength vs. elongation vs. peroxide

PHOTO : Figure 7 - Picco abrasion

PHOTO : Figure 8 - tensile strength vs. cure conditions

PHOTO : Figure 9 - compression set vs. cure conditions

PHOTO : Figure 10 - silica level vs. tensile strength

PHOTO : Figure 11 - plasticizer level vs. hardness and compression et (aged 70 hours at 120[degrees]C)