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New heat-resistance elastomers.

Introduction

In consideration of higher power, higher efficiency environmental measures and maintenance-free properties of automobile engines, improvement of the heat resistance and extension of the life span are desired for engine peripheral parts, and demand is increasing for improvements in the heat resistance of acrylic elastomers (ACM). On the other hand, due to the active use of amine-type additives for higher performance of engine oils, there is apprehension about negative effects on fluoroelastomers (FKM), such as increased hardness, decreased elongation at break and the occurrence of fine cracks during use. Thus, the need for materials having intermediate properties that supplement the shortcomings of FKM and ACM is increasing with the focus on automobile usage.

Attempts to obtain a cured substance that has intermediate properties by blending FKM and ACM have been carded out for a long time. For example, it was discovered in the 1970s that it is possible to blend FKM and ACM at a 1:1 weight ratio and then cure the mixture with a polymine compound (ref. 1) or cure the same mixture with bisphenol S [bis(4-hydroxyphenyl)sulfone] (ref. 2).

After that, several techniques, such as dynamic curing Ref. 3), co-crosslinking with an ethylene-type elastomer (ref. 4) and mutual crosslinking (ref. 5), were also studied. However, as of today, levels have not yet been achieved that satisfy the demand for heat resistance that includes sealing performance and the demand for cost performance and workability.

Our company is continuing a vigorous study from the above point of view, and after studying the interpenetrating polymer network (IPN) technique (ref. 6), announced a product in 1997, a new heat resistant elastomer with superior cost performance. As shown in figure 1, its basic properties belong to the intermediate area between FKM and ACM that cannot be dealt with by conventional materials.

[Figure 1 ILLUSTRATION OMITTED]

In this article, concepts of material design, basic properties and workability will be introduced with the focus on Dai-El Alloy AG-1530 (this is the brand name, hereafter abbreviated as AG) which is the standard grade of a new family of materials.

Material design

The aim of development of this new family of materials is to fill in the intermediate area between ACM and FKM and exhibit superior cost performance. It was essential to improve the heat resistance in the ACM-rich area in order for it to be realized.

The following two points can be mentioned as key points important for this:

* Selection of polymer combinations with good compatibility;

* selection of heat-resistant crosslinking systems.

Regarding the first, compatibility with ACM varies greatly depending on the fluorine content of FKM. A proper balance is required because if the compatibility is too high, the glass transition temperature [T.sub.g] rises and a phenomenon is observed in which cold resistance deteriorates (ref. 7), and if it is too poor, satisfactory properties cannot be exhibited.

As for the selection of crosslinking systems, metallic soap - sulfur curing systems and amine curing systems, which are normally utilized for ACM, were examined, and for FKM, bisphenol curing systems and peroxide curing systems were examined. In the end, iodine-type peroxide curing, which has the most merits for FKM, was selected. The following can be mentioned as reasons:

* Heat resistance of peroxide-cured ACM is good.

* Curing is fast and the oven curing conditions are mild.

* Metallic hydroxide, which leads to deterioration of ACM, is not required.

* Co-crosslinking is essential in terms of material properties.

In this manner, for AG development, all of FKM, ACM and their co-crosslinking systems have been designed for the use in question, and it is thought that heat resistance can be greatly improved as a result of this, in spite of ACM being the main substance.

The characteristics of the new material are:

* Design flexibility that can cover a wide range of properties (the blend ratio of FKM and ACM, etc.);

* availability of high-filler blends, superior cost performance and lower specific gravity compared with FKM;

* heat resistance, oil resistance and cold resistance that supplement the shortcomings of FKM and ACM; and

* excellent workability (extrusion molding, injection molding, steam curing, etc.)

Basic properties

In table 1, raw rubber properties, standard formulation and mechanical properties are indicated.
Table 1-basic properties of AG

 AG

Properties of raw rubber
Specific gravity at
 23 [degrees] C 1.3
Mooney viscosity
 (ML1+10, 100 [degrees] C) 23
Color tone Milky white

Standard ingredients
 (weight parts)
Polymer 100
MAF carbon black 40
MT carbon black --
Perhexa 25B(3) 1.5
TAIC(4) 1.0
Sulfur --
Sodium stearate --
Potassium stearate --
Stearic acid 1.0
Naugard 445(5) 1.0
Highly active magnesium oxide --
Calcium hydroxide --

Standard curing conditions
Press curing 170 [degrees] C x 10 min.
Oven curing 180 [degrees] C x 4h.

Curing properties
100% tensile stress (MPa) 3.9
Tensile strength (MPa) 9.4
Elongation at break (%) 260
Hardness (Shore A) 74
Specific gravity 1.45

 ACM(7)

Properties of raw rubber
Specific gravity at
 23 [degrees] C 1.1
Mooney viscosity
 (ML1+10, 100 [degrees] C) 42
Color tone Milky white

Standard ingredients
 (weight parts)
Polymer 100
MAF carbon black 60
MT carbon black --
Perhexa 25B(3) --
TAIC(4) --
Sulfur 0.3
Sodium stearate 3.0
Potassium stearate 0.5
Stearic acid 1.0
Naugard 445(5) 2.0
Highly active magnesium oxide --
Calcium hydroxide --

Standard curing conditions
Press curing 170 [degrees] C x 20 min.
Oven curing 180 [degrees] C x 4h.

Curing properties
100% tensile stress (MPa) 5.1
Tensile strength (MPa) 10.4
Elongation at break (%) 210
Hardness (Shore A) 72
Specific gravity 1.27

 FkM(8)

Properties of raw rubber
Specific gravity at
 23 [degrees] C 1.8
Mooney viscosity
 (ML1+10, 100 [degrees] C) 67
Color tone Milky white

Standard ingredients
 (weight parts)
Polymer 100
MAF carbon black --
MT carbon black 20
Perhexa 25B(3) --
TAIC(4) --
Sulfur --
Sodium stearate --
Potassium stearate --
Stearic acid --
Naugard 445(5) --
Highly active magnesium oxide 3.0
Calcium hydroxide 6.0

Standard curing conditions
Press curing 170 [degrees] C x 10 min.
Oven curing 230 [degrees] C x 24h.

Curing properties
100% tensile stress (MPa) 6.4
Tensile strength (MPa) 16.9
Elongation at break (%) 220
Hardness (Shore A) 73
Specific gravity 1.85


(1) Nippol AR-72LS (Nippon Zeon);

(2) Dai-EI G-751 (Daikin);

(3) 2.5 dimethyl-2.5di-(t-butylperoxy)hexane (Nippon Oil & Fat Co.);

(4) Triallylisocyanurate (Nippon Kasei Chem.);

(5) Uniroyal Chem.

The specific gravity is 1.3, which is lower than the 1.8 of FKM, making it advantageous in terms of volume costs.

Moreover, in terms of blending, the Shore A hardness is 74 with 40 weight parts of carbon black, and a higher filler loading is possible compared to FKM.

As for the properties of a cured substance, although the tensile strength is somewhat low due to the influence of ACM, good elongation at break is observed.

Heat resistance

Hot air aging properties

The mechanical properties obtained after hot air aging are indicated in figures 2 and 3, and intermediate heat resistance between FKM and ACM is observed.

[Figures 2-3 ILLUSTRATION OMITTED]

In figure 4, the relationship between the hot air aging test temperature and change in elongation at break are indicated. The continuous use temperature obtained according to SAE J200 from this result is indicated in table 2. The continuous use temperature of new material is higher than that of ACM by approximately 25 [degrees] C.
Table 2 -- continuous use temperature

 Continuous use
Type of rubber temperature ([degrees] C)

H-NBR 145
ACM 150
AEM 160
AG 175
FKM 230


[Figure 4 ILLUSTRATION OMITTED]

Compression set properties

The long-term test results of the compression set properties are indicated in figure 5, and the influence of the test temperature is shown in figure 6. AG has excellent sealing properties after 1,000 hours at 175 [degrees] C or after a short period of time at 200 [degrees] C.

[Figures 5-6 ILLUSTRATION OMITTED]

Oil resistance

Engine oil resistance

The engine oil resistance is indicated in figure 7. An excellent resistance is exhibited even under severe conditions in which cracks would occur with FKM.

[Figure 7 ILLUSTRATION OMITTED]

Transmission oil resistance

Transmission oil resistance is shown in figures 8 and 9. AG exhibits superior resistance in terms of the volume increase rate, changes in strength, elongation at break, etc.

[Figures 8-9 ILLUSTRATION OMITTED]

Under these conditions, a resistance that is approximately equivalent to that of tetrafluoroethylene/ propylene-type rubber (TFE-Pr) are exhibited.

Low temperature properties

Low temperature properties measured by different methods are shown in table 3. AG exhibits superior low temperature properties compared to FKM. Moreover, when compared to ACM, its embrittlement properties are superior.
Table 3 - low temperature properties

 AG ACM FKM

Glass transition
 temperature(1), Tg ([degrees] C) -26.0 -28.0 -20.5
Gehman torsional
 test(2), T10 ([degrees] C) -16.8 -21.7 -13.7
TR test(2), TR10 ([degrees] C) -19.5 -27.0 -18.0
Low temperature
 impact test(2),
 brittle temperature ([degrees] C) -35 -22 -28


(1) DSC analysis

(2) Refrigerant: Isopropyl alcohol

Chemical resistance

In table 4, the resistance of AG to different types of chemicals is shown. Although the steam resistance is greatly increased compared to that of ACM, the increase in the resistance to organic solvents is not substantial.
Table 4 - volume change after immersion in chemicals,
[Delta]V (%)

Chemical Conditions AG ACM FKM

Water 40 [degrees] C x 70h. +2 +14 +0
Steam 165 [degrees] C x 70h. +8 +93 +7
30% sulfuric acid 40 [degrees] C x 70h. 0 +2 0
20% sodium hydroxide 40 [degrees] C x 70h. 0 +2 0
Carbon tetra-chloride 40 [degrees] C x 70h. +127 +170 +3
Toluene 40 [degrees] C x 70h. +183 +220 +24
Ethylene glycol 40 [degrees] C x 70h. +1 +8 0
Fuel D 40 [degrees] C x 70h. +122 +120 +8


Summary of the properties

In figure 10, a comparison of the properties with those of FKM and ACM is indicated. The low temperature properties and engine oil resistance are superior to those of FKM, and the compression set properties and heat resistance are superior to those of ACM.

[Figure 10 ILLUSTRATION OMITTED]

In figure 11, a comparison of the properties with those of AEM and H-NBR is indicated. Detailed data have been omitted, but the heat resistance and volume increase rate in engine oil are superior to those of AEM, and the heat resistance and compression set properties are superior to those of H-NBR.

[Figure 11 ILLUSTRATION OMITTED]

Variations of Dai-El alloy

As described earlier, as for new material, by changing ACM, FKM and their blending ratios, "blend materials" having various merits can be designed.

Table 5 indicates examples. AG-1330 is even more superior in cost performance, and AG-1500 has a greatly increased oil resistance. In addition to these, development of high-strength types and cold-resistant types is also being promoted.
Table 5 - basic properties of AG series

Properties of raw rubber AG-1530 AG-1330 AG-1500

Fluorine content (wt%) 34 20 34
Ingredient (weight part)
MAF carbon 40 50 30

Curability
100% tensile strength, M100 (MPa) 3.9 3.6 2.7
Tensile strength, [T.sub.B] (MPa) 9.4 8.9 11.8
Elongation at break, [E.sub.B] (%) 260 240 440
Hardness, [H.sub.s] (Shore A) 74 71 77
Specific gravity 1.45 1.36 1.46
Low temperature properties
 (Gehman torsional test)
[T.sub.2] ([degrees] C) -8.7 -9.2 +5.0
[T.sub.10] ([degrees] C) -17.8 -18.6 -2.5
Off resistance IRM903
 150 [degrees] C x 48h.
[Delta]V (%) +32 +40 +8
Mobile 1 SH/CD 5W-30,
 150 [degrees] C x 168h.
[Delta]V (%) +13 +16 +4
[Delta][T.sub.b] (%) -9 -28 +2
[Delta][E.sub.B] (%) -29 -35 -16
[Delta][H.sub.s] (points) -12 -12 -6


Workability

AG has superior roll workability, scorch stability, extrudability, steam curability and injection moldability, and thus a high cost performance can be expected in terms of forming workability as well.

The raw rubber of AG is not adhesive and its roll-windability and removability are both extremely good. As shown in figure 12, AG exhibits good scorch resistance.

[Figure 12 ILLUSTRATION OMITTED]

A comparison of extrusion moldabilities is indicated in table 6. The present materials exhibit the best results among rubbers evaluated in this study. They have smooth extrusion surfaces, are not adhesive, and their die swells are very low. This indicates that thin multi-layer extrusion is also possible.
Table 6 -- extrusion moldability

 AG ACM

Screw rotation (rpm) 20 25
Pressure (MPa) 6.3 6.2
Extruded surface, Excellent Excellent
 visual check
Die swell (%) 8 21
Extruder: Extrusion conditions:
Cylinder diameter = 50mm Barrel temperature = 70 [degrees] C
Cylinder length = 750mm Head temperature = 80 [degrees] C
 Die temperature = 100 [degrees] C
 Extrusion speed = 10L/h

 FKM for extrusion I
 (Dai-EI G-555)

Screw rotation (rpm) 20
Pressure (MPa) 6.1
Extruded surface, Excellent
 visual check
Die swell (%) 44
Extruder:
Cylinder diameter = 50mm
Cylinder length = 750mm


In table 7, the mechanical properties of a steam-cured substance of AG are indicated. This material is a peroxide-cured type and can be fully cured without the steam- contacting area remaining uncured. Al-though the hardness is slightly lower when compared to that of press curing, there is hardly any difference in terms of strength.
Table 7 -- steam curing of new elastomer

 Steam curing

After primary curing 150 [degrees] C x 60 min.
100% tensile stress (MPa) 2.4
Tensile strength (MPa) 8.3
Elongation at break (%) 320
Hardness (Shore A) 62
After oven curing (oven) 180 [degrees] C x 4h.
100% tensile stress (MPa) 2.6
Tensile strength (MPa) 9.3
Elongation at break (%) 320
Hardness (Shore A) 66

 Press curing

After primary curing 160 [degrees] C x 25 min.
100% tensile stress (MPa) 2.5
Tensile strength (MPa) 9.0
Elongation at break (%) 390
Hardness (Shore A) 68
After oven curing (oven) 180 [degrees] C x 4h.
100% tensile stress (MPa) 3.4
Tensile strength (MPa) 9.4
Elongation at break (%) 280
Hardness (Shore A) 71


Application

Because of properties such as superb heat resistance, pressure-resistance compression set properties, oil resistance, etc. and fine workability, AG is expected to be utilized for sealing materials (o-rings, gaskets, shafts, seals, etc.), extruded products (sleeves, hoses, tubes, etc.), valve seats, diaphragms, rolls, belts, coating of electric wires, etc. that are for general and industrial purposes in automotive applications and others.

Conclusion

As described above, AG has high design flexibility. For instance, utilization is considered possible in fields that cannot be dealt with by conventional materials, such as dealing with amine resistance, cold resistance and cost performance at the same time, which cannot be achieved by FKM.

References

(1.) Kometani & Wada (1974) Fluoroelastomer/ silicon rubber, Gousei Gomu Kakou Gijutsu Zensholl. Taiseisha.

(2.) Jap. Pat. 77040553

(3.) Jap. Pat. 01299859

(4.) U.S. Pat. 5578681

(5.) Kimura, H. (1995). Polymer Digest, 6, 58.

(6.) EP 481372.

(7.) IRC '95 Kobe, Full Texts, The Society of Rubber Industry (1995).

Mitsuru Kishine and Tsuyoshi Noguchi, Daikin America
COPYRIGHT 1999 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Noguchi, Tsuyoshi
Publication:Rubber World
Geographic Code:1USA
Date:Feb 1, 1999
Words:2488
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