EP polymer selection and compound considerations for chloramine resistance.

The failure of rubber parts used in domestic water applications has been reported from Florida, California, Texas and Minnesota. The complaints involved swelling and cracking and are related to nitrile and ethylene-propylene co-polymer type compounds.

Examples of these failures are:

* O-rings used in watering assemblies in Ramona, CA failed after six months due to swelling. Normally, they are expected to last for three years (ref. 1).

* Aerator washers used in the airport in Minneapolis, MN swelled. The same washers are used in St. Louis and Atlanta without any problems (ref. 1).

* Rubber parts used in toilets in Fort Lauderdale, FL are breaking down after six months. Normally, they last four to five years (ref. 2).

* Ingleside, TX had failure of seating materials on large valves in treatment plant filter backwash assemblies. The gasket material is EPDM based and has been exposed to chloramine residuals in the range of 3 to 5 mg./1 since 1982. Since 1982, similar valve assemblies on the raw water side show little or no deterioration and have been in service since 1972 (ref. 3).

Oxidant attack on the elastomeric structure is directed either at the material's polymeric backbone or the cross-linking between polymer groups. Polymeric attack diminishes material strength, while cross-linkage disruption decreases elasticity and resilience (ref. 4).

The failure of the robber parts coincides with the implementation and usage of chloramines as a method of water purification. Prior to chloramination, chlorine gas was added to purify water for domestic use.

Experimental

Designing of aging test

A particularly sensitive and analytically simple methodology for assessing elastomer degradation is the swelling test (water absorption). A variety of elastomers was evaluated using this methodology. The susceptibility to chloramine attack using generic formulation (without antidegradants) is shown in table 1 (ref. 5).

Table 1 - elastomeric susceptibility to chloramine attack(*)

Acutely sensitive   Sensitive           Least sensitive

Nitrile (NBR)       EPDM                Fluorocarbons
SBR                 Isobutyl-isoprene   Silicones
Natural rubbers
Polychloroprenes

(*) Generic formulations (without antidegradants)

Chloramines are almost uniformly harmful to elastomeric compounds. Materials most susceptible are those formulated with natural isoprenes, or synthetic isoprene derivatives. Only the engineered and completely synthetic polymers performed well in the chloramine exposures.

Natural isoprenes and synthetic isoprene derivatives have unsaturation (double bonds) sites for cross-linking in the elastomeric backbone. Chloramines attack these cross-linking sites, micro cracks form on the material surface and water begins to penetrate the elastomer. The cracks seem to be self-propagating, since as they deepen, they expose new material to chloramine attack. The tendency of the elastomer to adsorb water correlates strongly with deterioration of physical parameters (maximum stress/strain and hardness). The water adsorption test is easy to perform and highly reproducible.

EPDM elastomer study

EPDM elastomers are used in numerous applications which require resistance to polar fluids, chemicals, ozone attack and weather resistance. EPDMs are manufactured with varying compositions, i.e., ethylene/propylene ratio weight percent of the third monomer - ENB (unsaturation), molecular weight and molecular weight distribution. This investigation was designed to determine if these composition variables influenced resistance to chloramines. Also evaluated was peroxide vs. sulfur cure systems, the effect of antioxidants and whether practical compounds can be developed for water systems.

Peroxide evaluation

Peroxide cured EPDM elastomers of varying composition were evaluated for chloramine resistance - 60 ppm chloramines at 60 [degrees] C. The results are shown in table 2.

Table 2 - EPDM elastomer evaluation for chloramine resistance - temperature 60 [degrees] C, 60 ppm chloramines peroxide cured

Formulation

Polymer           100
N-990 black         5
HiSil 532 EP       20
Miston vapor       20
Zinc oxide          5
Sunpar 2280 oil    10
Sartomer SR 350     2
DiCup 40KE          7
                  169

Polymer characteristics

            R505    R580HT   R3180   R501    R539    R509

% ENB        8.50      2.6     2.1       4     4.9       8
E/P ratio   57/43    57/43   67/33   59/41   74/26   71/29
Mw            3.9      4.6     4.9     3.4     4.4     4.1
Mw/Mn         2.6      3.1     2.9     2.7     2.6     3.1

Unaged hardness
                     69     63     69     66     78    68

One month

Hardness, Change     -3     -2     -3     -3      1    -3
% volume change    2.98   2.75   2.39   2.76   2.64   2.7

Two months

Hardness, change     -9     -5     -5     -7     -3    -6
% volume change    9.06    8.9   6.23   7.66    5.3   6.8

The EPDM elastomers that have the lowest % volume change after two months exposure in chloramines are R3180, R539 and R509. These elastomers are highest in ethylene content with R539 - 74 weight percent. None of the other elastomer characteristics appear to have a significant influence on the volume change in chloramines.

Sulfur cure evaluation

Sulfur cured EPDM elastomers of varying composition were evaluated for chloramine resistance - 60 ppm chloramines @ 60 [degrees] C. The results are shown in table 3.

Table 3 - EPDM evaluation for chloramine resistance - temperature 60 [degrees] C, 60 ppm chloramines sulfur cured

Formulation

Polymer           100
N-990 black         5
HiSil 210          20
Miston vapor       20
Zinc oxide          5
Stearic acid        1
Sunpar 2280 oil    10
MBT               1.5
TMTM                2
Sulfur            1.5
                  166

Polymer characteristics

            R505    R580HT   R3180   R502    R539

% ENB         8.5     2.6      2.1       4     4.9
E/P ratio   57/43   57/43    67/33   62/38   74/26
Mw            3.9     4.6      4.9     4.0     4.4
Mw/Mn         2.6     3.1      2.9     2.5     2.6

Unaged hardness
                     68     60     70     65     73
168 hours

Hardness, change     -5      4     -4     -3     -5
% volume change      16   11.6   12.6   16.3   10.6

One month

Hardness, change     -8      5     -4     -6     -6
% volume change    24.5   17.4   16.1   28.1     20

Two months

Hardness, change     -9      4     -5
% volume change    31.8   17.5   15.8     na     na

Three months

Hardness, change    -18     na     na    -12     -3
% volume change    60.4     na     na     50   30.3

After two months exposure, R3180, which has a 67 weight percent ethylene, has the lowest % volume change. After three months exposure, R539, a 74 weight percent ethylene has the lowest % volume change; however, the result is much higher than for peroxide.

The weight percent of ethylene in the EPDM elastomer is the predominant elastomer characteristic for resistance to chloramines. The other elastomer parameters may also have an effect on chloramine resistance, but to a lesser degree. In addition to ethylene, it is recommended to select an EPDM elastomer with low to medium weight percent of ENB (ethyldiene norbornene). Even though the unsaturation (ENB) is pendant to the elastomer backbone, it is recommended to minimize any sites which can be attacked by chloramines.

Antioxidant evaluation

EPDM elastomers are inherently resistant to most aging conditions. However, antioxidants enhance heat resistance. An antioxidant used for this purpose was evaluated for resistance to chloramines. The effect of Naugard Q in an EPDM compound based on R580HT is shown in table 4.

Table 4 - evaluation of EPDM R580HT with antioxidant Naugard Q for effect on chloramine resistance

                    A     B    C     D

EPDM               100   100   100   100
N-650 black         40    40    30    30
N-990 black          0     0    20    20
Mistron vapor CB    25    25    25    25
Sunpar 2280 oil     15    15    15    15
Zinc oxide           5     5     5     5
Sartomer SR-350      2     2     2     2
Naugard Q            0     2     0     2
DiCup 40KE           7     7     7     7

Unaged physical properties - cured 10' @ 176 [degrees] C

300% modulus, MPa        8.8      7    8.9    7.1
Tensile strength, MPa   12.2   12.7     14   13.6
% elongation             375    460    420    520
Hardness duro A           63     64     65     64
Tear die "C" KnM          40     41     38     40

% volume change immersed in 60 PPM chloramines @ 60 [degrees] C

30 days                 10.7    4.4    8.6   3.35
60 days                 30.5   11.3   23.7     15

Two parts of the antioxidant were added to both compounds. The % volume change after either 30 or 60 days in chloramine shows the effect of the antioxidant. The result is dramatic, with a 50% reduction in % volume change over the compounds without the antioxidant.

Since these compounds will be used in drinking water systems, all ingredients must meet FDA 177.2600 requirements. Naugard Q is not FDA approved.

Recommended EPDM molding compound

Table 5 shows an EPDM compound for molded baskets, seals, valves and membranes. This compound is cured with dicumyl peroxide and has a low % volume change of 11.3 after 60 days exposure. This value is acceptable for these applications in water systems.

Table 5 - EPDM R580HT compound recommended for molded gaskets, seals, valves and membranes

EPDM R580HT                            100
N-650 black                             40
Mistron vapor CB                        25
Sunpar 2280 oil                         15
Zinc oxide                               5
Naugard Q                                2
Sartomer SR 350                          2
DiCup 40KE                               7
                                       196
Mooney viscosity,
ML 1+4 @ 100 [degrees] C                65

Unaged physical properties - cured 10' @ 176 [degrees] C

300% modulus, MPa                        7
Tensile strength, MPa                 12.7
% elongation                           460
Hardness duro A                         64
Tear die "C" KnM                        41

Compression set D395 method B cured 20' @ 176 [degrees] C

70 hours @ 125 [degrees] C (button)     21

% volume change in 60 ppm chloramines @ 60 [degrees] C

30 days                                4.4
60 days                               11.3

Highly loaded EPDM compound for extrusion

EPDM R539, a high ethylene elastomer, is generally used for highly loaded extruded applications. This EPDM has previously demonstrated excellent resistance to chloramines. Extruded compounds are commonly cured via microwave or hot air systems. Therefore, a dessicant was added to prevent porosity. The effect on chloramine resistance on the dessicant was determined in this evaluation.

Also, sulfur cured systems are used when curing EPDMs in hot air, because peroxides oxidize the surface, causing tackiness when they decompose. As previously shown, the sulfur cure has higher % volume swell after both 30 and 60 days than the peroxide cure. Apparently, the chloramines attack the weaker sulfur bonds (s-s-s) more readily than the peroxide c-c bonds. The sulfur cure values are acceptable for many low risk applications.

The addition of the dessicant did not affect the % volume change in the peroxide cured compounds.

Conclusions

With the proper choice of EPDM elastomer, high ethylene content, antioxidant and a peroxide cure, 5% volume change after 60 days can be achieved. The test conditions were designed to accelerate the effect of chloramines - 60 ppm @ 60 [degrees] C. The actual exposure condition will vary in both concentration of chloramine and temperature, depending upon location. Peroxide cures with the stronger c-c crosslinks provide low % volume change after chloramine exposure than s-s-s crosslinks. The addition of an FDA approved antioxidant enhances the resistance to chloramines. An antioxidant such as Aminox is recommended

There is a wide variation in susceptibility between EPDM elastomer types and formulations. The potential for serious performance degradation is application specific, with those applications having only a minor degree of wetted surface exposure being at minimal risks of swell, cracking or deterioration. Examples of applications at minimal risk include most pipe gaskets, o-rings, valve packing and any other application where the ratio of wetted surface to total surface area is low. Applications that are at the greatest risk are installations with a high wettted to total surface area ratio, including pump and valve diaphragms, tank liners and valve seats.

Table 6 - evaluate highly loaded EPDM compounds for resistance to chloramine - extruded applications

                          A      B      C

EPDM R539                100    100    100
N-650 black              100    100    100
Mistron CB                40     40     40
Zinc oxide                 5      5      5
Zinc stearate            1.5      0      0
Rhenosorb C/GW             8      8      0
Britol 50 T oil           75     75     75
Paraffin wax               5      5      5
TBBS                     1.5      0      0
TMTD                     0.8      0      0
DTDM                       2      0      0
Sulfur                   0.8      0      0
DiCup 40KE                 0      8      8
Sartomer SR 350            0      2      2
Aminox                     2      2      2

Unaged physical properties 10' - cured @ 176 [degrees] C

100% modulus, MPa        3.8    6.1    5.5
Tensile strength, MPa   13.7   12.2   12.9
% elongation             320    320    430
Hardness duro A           79     76     75

% volume and hardness change immersed in chloramines 60
PPM @ 60 [degrees] C

30 days                  9.3    2.6    2.7
Hardness change           -7     -2     -4
60 days                 13.2    4.7    4.7
Hardness change           -7      2      1

References

(1.) Simmons, C.L., Evanson, P.P., "Effects of additives in domestic water systems on rubber vulcanizates," proceedings - Rubber Division, ACS, Dallas, 1988.

(2.) Wilkes, J.F., "Accelerated failure of rubber components in Florida potable water supplies," proceedings - Environmental Chemistry Division, ACS, Miami, 1989.

(3.) Naismith, J., Manager, San Patricio Municipal Water District, Personal communication (1990).

(4.) Ward, I.M., Mechanical Properties of Solid Polymers, 2nd ed., John Wiley and Sons, New York, (1983).

(5.) Reiber, Steve, "Chloramine effects on elastomer degradation, "presented at Rubber Division Meeting, Orlando, FL.3