EVM copolymers: the forgotten rubber.

Copolymerization of ethylene with vinyl acetate gives rise to a unique and often overlooked important class of polymeric materials known simply as ethylene-vinyl acetate copolymers. The radical polymerization performed in solution and under pressure confers a random distribution of the vinyl acetate and ethylene monomers along the chain backbone. Properly compounded, EVM vulcanizates are known for their excellent resistance to aging (maximum service temperature of 175[degrees]C), outstanding weathering/ozone/UV resistance, low compression set at high temperatures and good processability (ML 1+4 @ 100[degrees]C is between 20 and 60 Mooney units). EVM is currently found in many rubber products, including the cable sector, adhesives and in the modification of thermoplastics. It is increasingly being used in the automotive industry for seals and gaskets due to the more stringent high temperature requirements. EVM addition to nitrile robber improves the latter's ozone resistance. Blends of EVM and hydrogenated acrylonitrile butadiene rubber are useful in specific automotive applications where high heat and good oil resistance are required.

Several indispensable reviews describing the attributes of EVM copolymers exist in the current literature and will not be repeated here (refs. 1-3). The current treatise will concentrate on EVM copolymers bearing elastomeric properties, meaning that the vinyl acetate content varies roughly from 40 to 80 weight percent.

Polymerization

The medium pressure solution process, developed solely by Bayer AG over 35 years ago, is the most suitable to produce EVM copolymers in the 40 to 80% vinyl acetate range. The polymerization process takes place at pressures between 200 and 1,000 bars and at temperatures ranging from 50 to 120[degrees]C. Polymerization occurs in an organic solvent possessing a low transfer constant; affording the production of gel-free, random copolymers having Mooney viscosities between 20-35 units and vinyl acetate contents from 40 to 80%. Final polymer conversion rates vary between 50 and 70%. Currently available EVM copolymers prepared by this process are given in table 1.

Properties

Proper compounding of EVM vulcanizates leads to the following general properties:

* Heat resistance up to 175[degrees]C (type E elastomer according to ASTM D2000). The saturated main chain and its known thermal stability are responsible for the excellent thermal oxidation resistance.

* Excellent weathering, ozone and UV resistance due to the absence of unsaturation along the chain backbone.

* Swelling in mineral oil of 0 to 100% by volume. The polar vinyl acetate group provides resistance to non-polar oils and fuels, thus the higher the vinyl acetate concentration, the lower the swelling in these liquids.

* Broad low temperature range (-36[degrees] < Tg < 0[degrees]C) again depending on vinyl acetate content. The ethylene segments, being more mobile, provide the chain backbone with flexibility and contribute to the lowering of the glass transition temperature in the copolymer.

* Good mechanical properties: Incorporation of vinyl acetate breaks up the ethylene sequences capable of crystallization; thus, upon increasing vinyl acetate levels, the amount of crystallinity decreases and becomes 0 at 60% vinyl acetate (completely amorphous). Increasing the ethylene content, however, will increase crystallinity, causing the EVM vulcanizate to become harder and stiffer, as well as becoming less transparent as a material.

* Halogen-free: Upon combustion of EVM, halogen containing gases are not produced.

Main applications (refs. 2 and 4-8)

EVM is primarily known for its use in the wire and cable industry, as an adhesive and for the modification of plastics. In the cable industry, it is seen in heat resistant insulation, as well as the semiconductive layer in medium voltage cable construction due to easy removal by peeling. EVM vulcanizates containing flame-retardant fillers are used in flame-retardant, non-corrosive (FRNC) applications where there is a need for low flammability and combustion gas density, as well as non-halogen containing combustion gases. FRNC qualified compounds are primarily used in security cables, conveyor belts and floor coverings. Hot melt adhesives require EVM copolymers up to 45% in vinyl acetate content, while pressure sensitive adhesive applications make use of the 50 to 80% vinyl acetate EVMs. Poly(vinyl chloride), styrene-acrylonitrile copolymers and cellulose esters are examples of some polymers whose thermoplastic properties are improved with EVM addition. The excellent dynamic properties of EVM for the paper industries.

Automotive applications

Rising under-the-hood temperatures in automobiles are increasing the demand for EVM vulcanizates as hose or sealing materials on account of their excellent heat resistance. Rohde (refs. 9 and 10) explored the use of EVM vulcanizates for automotive molded goods (o-rings, bellows, oil pan and gear box gaskets), as well as hoses and tubes (automotive air intake, venting and turbo charger hoses). EVM is suitable for automotive venting hose, which requires good oil and hot air aging resistance, as well as low compression set properties. It has also been used for cylinder head cover seals due to its direct injection moldability into the metal covers and its good compression set achieved without recourse to a post-cure (ref. 11). Mezger (ref. 12) recently presented data showing the effect of vinyl acetate content on dampening properties for vibration torsional dampener type applications. In the work of Meisenheimer (ref. 13), it was shown that EVM copolymers can be a cost-effective alternative to polyacrylate (ACM), polyethylene acrylate (AEM) and silicone rubbers for automotive seals, hoses and vibrational dampeners. Hose cover compounds based on EVM have been proposed for higher operating temperature ranges (-40[degrees] to 140[degrees]C), and have been found suitable for Volkswagen hose air conditioning systems (ref. 14).

Enhanced viscosity grades of EVM elastomers have been developed (table 1) using a free radical based partial crosslinking mechanism in order to generate EVM grades possessing improved hot and cold green strength, thus making them suitable for extruded goods such as hose and profile applications (refs. 15-17). In particular, EVM elastomers were shown to provide excellent resistance to power steering and transmission oil cooler hose fluids. These same enhanced viscosity EVM grades have been shown to overcome blister tendencies during molding, on account of their added ability to expulse air during the molding process.

Improved resistance to thermo-oxidative degradation has been demonstrated in EVM vulcanizates by adding heat stabilizer technology, which is able to essentially double the useful service lifetime of the rubber (ref. 18).

Blending of EVM with other specialty elastomers is widespread (ref. 2). For example, the ozone resistant properties of nitrile rubber are dramatically improved by producing EVM/NBR blends (ref. 19). A 30 phr EVM/70 phr NBR blend combination can be used in light color compounds as an alternative to an amine based antiozonant NBR system that has a tendency to discolor or stain. EVM shows excellent compatibility with hydrogenated (acrylonitrile-co-butadiene) rubber (HNBR), and can be blended into peroxide cured formulations up to 15 phr as a reactive polymeric plasticizer, leading to improved processability and reduced overall compound cost. From the other end of the blending spectrum, the addition of 30 phr of HNBR into EVM elastomers (30/70 ratio) solidifies and also helps to enhance some of the basic characteristics of EVM (refs. 20 and 21). The peroxide cured EVM/HNBR blends were shown to have good compression set and hot air aging resistance, as well as excellent fluid aging resistance (ASTM#1, IRM 903 and SF 105). These unique blends have shown promise for varyious applications, including engine and transmission gaskets, as well as power steering seals.

Automotive seals and gaskets

For static or dynamic seals, the fluid and temperature range of the application often governs the elastomer selection. More often than not, moderate to high modulus compounds are required (ref. 22). Certain silicone based automotive specifications require gasket compounds (60 type A hardness) for use in oil pan, cam or front cover gasket type applications (ref. 23). Depending on the exact specification, a properly formulated EVM compound comprising a vinyl acetate content between 60 to 70 wt. % can meet the demands of such applications. The formulations, rheological characteristics, physical properties, and hot air and fluid resistance of such EVM compounds will be presented at this time. This example will also serve to review some of the main compounding characteristics of EVM type copolymers.

Table 2 lists the recipes of five EVM compounds with varying vinyl acetate concentrations. The blending of EVM 60 and EVM 70 allows for intermediate vinyl acetate contents: 62.5 %, 65% and 67.5%.

In order to fulfill the low temperature requirements of engine gaskets, 15 phr of dioctyl sebacate plasticizer was incorporated into the recipe, and the amount of N550 black was adjusted to attain a 60 hardness level. Given the low Mooney viscosity of the EVM, it is highly recommended to add processing aids (stearates, OTDA) to the formulation to help in mixing. A polycarbodiimide antidegradant (described in table 3) is required since, acting as an anti-hydrolysis agent, it stabilizes and protects the vinyl acetate group during heat aging. EVM is predominantly peroxide cured with the help of a coagent in order to enhance crosslink density and overall compound properties. Metal oxides are generally not necessary, but have been shown to improve the oxidative heat aging resistance. In table 2, it can also be seen that the density of the compounds modestly increases from 60 to 70% vinyl acetate content.

Table 4 presents the rheological characteristics of the series of compounds listed in table 2. All testing was performed according to ASTM procedures. The Mooney scorch is represented by the time required for a 5 MU rise from the minimum Mooney viscosity measured at 135[degrees]C. As seen by the scorch values, premature vulcanization is not an issue for the peroxide cured EVM compounds.

The compound Mooney viscosity data were in the mid to low twenties, signifying excellent compound flow for all types of molding applications. The rheometer (MDR) data illustrate that the maximum torque decreased with increasing vinyl acetate concentration. At the same time, the minimum torque also decreases with increasing vinyl acetate; showing excellent correlation with the compound Mooney data. The delta torque predicts that the overall stiftness will be higher as one goes to lower vinyl acetate levels. The ts1 values indicated that the time to vulcanization was slightly lengthened at higher vinyl acetate contents. The rheometer data also attest to the relatively quick curing behavior of the compounds: time to 90% cure values around five minutes at 180[degrees]C. It is important to remember that these peroxide cured compounds do not require any post vulcanization process to improve physical properties, thus saving labor, equipment and any additional energy costs during part manufacturing.

The unaged hardness and stress strain physical properties of the EVM compounds are given in table 5. Compounds were formulated to possess a final type A2 hardness in the range from 55 to 60 points. As observed by the stress strain data, the ultimate tensile strength for all compounds was above 11 MPa. Lower vinyl acetate content led to a noticeable increase in the tensile strength. Ultimate elongation values ranged from 320 to 370%. The compound stiffness, perhaps best represented by the stress at 100% elongation values, was quite moderate for a soft compound, and varied from 3.1 to 3.9 MPa. Lower vinyl acetate content led to higher modulus compounds, as predicted by the maximum and delta torque data from rheometry (table 4). A similar trend can be ob served with the tear strength, as lower vinyl acetate concentrations provided a better resistance to tear.

The low temperature behavior of the EVM compounds was assessed by temperature retraction and brittle point measurements. Generally speaking, lower vinyl acetate content provided better chain flexibility due to the internal plasticizing effect of the ethylene units. However, between 60 and 70% vinyl acetate levels, coupled with 15 phr of plasticizer, this effect was not particularly strong, as TR10 values of -27[degrees] to -28[degrees]C were measured (figure 1). TR70 data were slightly lower for the 25/75 and EVM 70 compounds compared to EVM 60. According to these data, the low temperature compression set would be expected to be better for the higher vinyl acetate containing compounds. The brittle point graph (figure 2) showed a definite trend as a function of vinyl acetate content. The lower the vinyl acetate content, the lower the brittle point. As observed in figure 2, the brittle point varied from -37 to 48[degrees]C. Certain automotive gasket specifications require brittle points less than -40[degrees]C.

[FIGURE 2 OMITTED]

Figure 3 illustrates the set resistance alter compression of the five EVM compounds for 72 and 168 hours of aging at 150[degrees]C in hot air. All compression set values were in the mid teens after 70 hours and between 20 and 25% after 168 hours, which are quite good for sealing materials. No clear trend of compression set to vinyl acetate content of the elastomer was observed in these data.

[FIGURE 3 OMITTED]

The hot air resistance was determined by measuring the change in hardness and stress strain values after aging the samples for six weeks at 150[degrees]C. The data in figure 4 show that the EVM vulcanizates harden as a function of aging. The tensile change was slightly positive (between 0 and 10%), while the elongation loss did not surpass 40%. Many automotive seal specifications demand no more than a 50% loss in both tensile and elongation under these conditions. EVM based vulcanizates can largely meet this criterium.

[FIGURE 4 OMITTED]

Oil pan gasket specifications, in particular, often require a certain amount of resistance to more aromatic based oils such as IRM 903. The stress strain immersion aging in this oil at 150[degrees]C after one week is shown in figure 5. As expected, all EVM compounds softened in IRM 903 with minimal tensile and elongation loss. Higher vinyl acetate based compounds tend to have better retention of physicals in IRM 903. The effect of vinyl acetate content upon volume swell is readily apparent. A 60% vinyl acetate compound gave about 75% volume swell, whereas at 70% vinyl acetate content, the volume swell was reduced to about 35%. The use of EVM 70 or a blend of the two EVMs may be of interest in the case of satisfactory engine oil resistance.

[FIGURE 5 OMITTED]

Service fluid 105 oil resistance is a standard test in many oil pan and valve cover gasket specifications. The change in physical properties upon immersion in this oil after six weeks aging at 150[degrees]C is presented in figure 6. Softening of all the compounds was again evident in this oil, with the softening effect lessened for higher vinyl acetate contents. Tensile and elongation losses for all the compounds was under 30%, with the best physical property retention depicted by EVM 60 and the 50/50 blend. Gasket specifications often require a loss of no more than half of the tensile and elongation under these testing conditions. As in the IRM 903 oil, the effect of vinyl acetate content played a predominant role in volume swell resistance. The EVM 70 compound displayed a little over 10% volume swell.

[FIGURE 6 OMITTED]

Due to LEV II and PZEV low emission vehicle requirements, the amount of fuel permeation in gasketing materials is of particular current importance. The fuel permeation resistance of the EVM compounds to fuels C and CE10 is illustrated in figure 7. The best fuel C permeation resistance was given by the higher vinyl acetate compounds, led by EVM 70. Fuel CE10 was slightly more permeable than fuel C: with similar trends seen for permeation resistance as a function of vinyl acetate concentration. For comparison's sake, the EVM permeation resistance is much better than a typical silicone compound and at par with currently used AEM elastomers.

[FIGURE 7 OMITTED]

Compressive stress relaxation techniques are routinely used to benchmark different elastomers or even to evaluate formulation differences with the same elastomer for a seal or gasket application. The continuous method of measuring compressive stress relaxation has been proven to be one of the most fortuitous and consistent techniques in obtaining this differentiation. The EVM 60 and EVM 70 compounds of table 2 were slightly modified (60 phr N550 black, 5 phr DOS, 1.5 phr TALC, 5 phr peroxide, with other ingredients unchanged) in order to obtain a 70 type A2 hardness material. A post-cured AEM-G type compound (recipe in table 6) was also included in the testing. Figure 8 illustrates the continuous compressive stress relaxation results, represented as reduced force, as a function of time for the three compounds aged in SF 105 oil at 150[degrees]C for up to 3,000 hours. Oil changes were carried out every 500 hours and washer type specimens were employed. Sealing retention was slowly lost for all three compounds as a function of aging time, with final reduced force values between 0.65 and 0.80. The highest retained force after 3,000 hours of testing was demonstrated by the EVM 60 compound, signifying a better sealing force retention for this compound. Pazur et. al. (ref. 24) have shown that silicone based compounds display poor sealing force retention (less than 0.2% retained force after 1,400 hours aging at 150[degrees]C in motor oil) using continuous CSR as a benchmarking method.

[FIGURE 8 OMITTED]

Vibrational torsional dampeners

Also known as a harmonic balancer, a crankshaft tuner, an engine dampener, a front pulley or a timing wheel, vibrational torsional dampeners (VTD) are devices used to eliminate torsional vibrations inherently produced by the crankshaft in all internal combustion engines (ref. 25). Dampeners are designed to convert mechanical energy into heat, thus the elastomer compound they are composed of must show excellent heat resistance as it dissipates unwanted heat. The higher temperature requirements under-the-hood are displacing natural rubber VTDs by more heat resistant elastomers such as EVM, AEM and EPDM, the latter providing no oil resistance in the case of incident contact. Notwithstanding their peroxide cure, EVM vulcanizates possess excellent dynamic properties for vibrational control applications.

In table 6, two compound formulations based on ethylene vinyl acetate copolymers at 50 and 60% vinyl acetate contents are listed along side a standard post cured ethylene-acrylate elastomer and its recipe.

The initial hardness and physical properties for the three formulations are shown in table 7. The EVM compounds were around 60 in hardness, while the AEM-G was about five points harder. Tensile strengths and elongations were similar for all three compounds, however, property differences can be seen between the 50 and 60% vinyl acetate levels. Besides the hardness, the tensile, elongation and tear strength were higher for the lower vinyl acetate compound, due in part to the added strength imparted by its crystalline domains.

An MTS dynamic tester operating at 60[degrees]C with a 1[degrees] static strain, coupled with a 3[degrees] dynamic strain applied to the test specimen, was used to measure dynamic mechanical properties. Figures 9 and 10, respectively, illustrate the effect of frequency on tan delta and transmissibility for the three vulcanizates. All three tan delta curves increased quickly with frequency and reached a plateau or a maximum in a range from 30 to 50 Hz. The two EVM vulcanizates reached a plateau at their maximum tan delta values and retained their constant elastic behavior up to 500 Hz. On the other hand, AEM-G showed more variation with frequency and slowly descended after peaking in tan delta. It can be observed that the higher vinyl acetate content (60%) compound provided the best dampening behavior between 100 to 500 Hz. The transmissibility curves in figure 10 are typical of rubber materials. The natural frequency of the three samples was situated around 70 Hz, thereafter transmissibility decreased rapidly as a function of frequency. The transmissibility behavior of AEM-G was situated in between that of a 50 and 60% EVM, with the latter compound demonstrating the best isolation behavior in the high frequency region.

[FIGURE 10 OMITTED]

The DeMattia flex test is commonly used to assess the fatigue characteristics of parts found in constant dynamic motion. It is quantified as the number of kilocycles necessary in order to reach a certain level, either 300 or 600% of crack growth length. Figure 11 illustrates the unaged results of the three samples. It can be readily observed that the crack growth behavior was similar for all samples up to 300% in length; however, AEM-G displayed somewhat better resistance up to 600% in crack growth length. On the other hand, when the samples were air aged for one week at 150[degrees]C and run on the DeMattia flex tester, the superiority of the EVM vulcanizates became apparent. Whether it be until 300 or 600% crack growth length, both of the EVM vulcanizates showed excellent resistance to crack growth despite the effects of heat aging. Although it is often difficult to correlate these tests to real life conditions, these test results would suggest a longer service time life for an EVM vulcanizate used as a VTD.

[FIGURE 11 OMITTED]

Conclusions

Two main uses of EVM type elastomers in automotive applications have been covered in this investigation. It has been shown how EVM compounds can be used to meet the requirements of a typical seal or gasket (oil pan, rocker or valve cover) compound. The excellent dynamical properties of EVM elastomers lend themselves nicely for use in applications such as vibrational torsional dampeners. Furthermore, EVM is readily extrudable and can be fabricated into automotive hose or tube type compounds. It can be concluded overall that EVM type elastomers possess the right combination of properties (physical as well as heat and oil resistance) for sealing or vibration control in under-the-hood applications. Although currently under-used in automotive applications, the future of EVM for engine type materials has never looked brighter.

Table 1--commercially available EVM copolymers prepared by
the medium pressure solution process (HV = high viscosity,
VP KA = trial product)

EVM copolymer   Vinyl acetate (wt. %)   ML (1+4 @ 100[degrees]C)

400                      40                        20
450                      45                        20
500 HV                   50                        27
600 HV                   60                        27
700 HV                   70                        27
800 HV                   80                        28
VP KA 8939               91                        38
VP KA 8857               50                        55
VP KA 8815               60                        55
VP KA 8784               70                        60
VP KA 8936               80                        55

Table 2--compound formulationsm

Ingredient           EVM 60   75/25   50/50   25/75   EVM 70

EVM 60                  100      75      50      25        0
EVM 70                    0      25      50      75      100
Carbon black N 550       55      55      55      55       55
OTDA                    0.5     0.5     0.5     0.5      0.5
MgO                       3       3       3       3        3
SPDA                    1.5     1.5     1.5     1.5      1.5
Antidegradant             3       3       3       3        3
Stearic acid            0.5     0.5     0.5     0.5      0.5
ZnO                       2       2       2       2        2
DOS                      15      15      15      15       15
TAIC                      1       1       1       1        1
Peroxide                  4       4       4       4        4
Phr                   185.5   185.5   185.5   185.5    185.5
Specific gravity       1.21    1.22    1.23    1.23     1.24
  (g/[cm.sup.3])

Table 3

Abbreviation     Chemical name

EVM 50           50% ethylene vinyl acetate copolymer
EVM 60           60% ethylene vinyl acetate copolymer
EVM 70           70% ethylene vinyl acetate copolymer
AEM-G            terpolymer of ethylene, methyl acrylate
                   and organic acid cure site
OTDA             Octadecyl amine (Armeen 18D)
SPDA             4,4' bis-(([alpha]-dimethylbenzyl) diphenylamine
  (Naug. 445)
Antidegradent    Polycarbodiimide (Rhenogran GE1777)
DOS              Dioctyl sebacate
TAIC             Triallyl isocyanurate (Diak #7)
Peroxide         Bis(t-butyl-peroxy) disopropylbenzene
                   (Vulcup 40KE - 40% on Burgess clay)
OPEA             Organic phosphate ester free acid
  (Vanfre VAN)
Plasticizer      Ether/ester plasticizer
DOTG             Di-o-tolyl guanidine
HMDC (Diak #1)   Hexamethylene diamine carbamate

Table 4--general rheological characteristics of EVM and its blends

Property                      EVM 60   75/25   50/50   25/75   EVM 70

t05 (min.) 135[derees]C          >30     >30     >30     >30      >30
ML 1+4 @ 100[degrees]C (MU)       26      25      24      23       22

MDR at 180[degrees]C, 1.7 Hz, 1[degrees] arc, 30 min. run time

MH (dN.m)                       17.4    16.4    16.1    15.3     14.2
ML (dN.m)                        0.6     0.6     0.6     0.6      0.5
Delta MH-ML                     16.8    15.8    15.5    14.7     13.8
ts1 (min.)                      0.66    0.72    0.72    0.72     0.75
t'90 (min.)                     5.10    5.20    5.10    5.20     5.20

Table 5--unaged hardness and stress strain properties

Property                  EVM 60   75/25   50/50   25/75   EVM 70

Type A2 hardness (pts.)       58      56      57      58       56
Ultimate tensile (MPa)      13.4    13.1    12.9    12.5     11.5
Ultimate elongation (%)      340     330     320     340      370
Stress @ 25% (MPa)           0.9     1.0     0.9     0.9      0.8
Stress @ 50% (MPa)           1.6     1.7     1.5     1.5      1.4
Stress @ 100% (MPa)          3.9     3.9     3.7     3.6      3.1
Stress @ 200% (MPa)          9.4     9.6     9.2     8.9      7.8
Stress @ 300% (MPa)         12.7    12.8    12.6    11.9     10.8
Tear str., die C (kN/m)     33.4    31.9    31.2    30.4     30.6

Table 6--compound formulations used for VTD study

Ingredient                        EVM 50   EVM 60   AEM-G

EVM 50                               100        0       0
EVM 60                                 0      100       0
AEM-G                                  0        0     100
Carbon black N550                     50       50      60
OTDA                                 0.5      0.5     0.5
OPEA                                   0        0       1
MgO                                    3        3       0
SPDA                                 1.5      1.5       2
Antidegradant                          3        3       0
Stearic acid                         0.5      0.5     1.5
ZnO                                    2        2       0
DOS                                   10       10       0
Plasticizer                            0        0       7
DOTG                                   0        0       4
Diak 1                                 0        0     1.5
TAIC                                   1        1       0
Peroxide                               4        4       0
Phr                                177.4    177.4   177.5
Specific gravity (g/[cm.sup.3])     1.18     1.21    1.18

Table 7--unaged hardness and stress strain properties

Property                  EVM 50   EVM 60   AEM-G

Type A2 hardness (pts.)       62       57      67
Ultimate tensile (MPa)      16.2     14.5    15.6
Ultimate elongation (%)      280      240     250
Stress @ 25% (MPa)           1.3      1.1     1.8
Stress @ 50% (MPa)           2.2      2.0     3.3
Stress @ 100% (MPa)          5.1      5.1     7.4
Stress @ 200% (MPa)         12.5     12.6    13.8
Tear Str., die C (kN/m)     34.1     29.5    34.3

References

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