Chloramine effects on elastomer degradation.

As chloramination has been re-introduced, several utilities have reported problems with substantially higher rates of elastomer gasket failure. The failure is disturbing, not only because of the threat to the innumerable gaskets, glands, compression fittings and valve bodies throughout a typical distribution system, but because it is at odds with the long, continuous and successful chloramination history of some utilities (e.g., Denver, Chicago).

Several factors were considered as possibly contributing to the reported elastomer failures:

* The higher operational concentration of combined chlorine, vis-a-vis free chlorine, may accelerate oxidative attack on elastomeric surfaces. This may be especially relevant to the experience in Florida, where high total chlorine doses are often maintained both to disinfect and bleach color from the water.

* Either the mono- or di-chloramine species may be a particularly aggressive oxidant for the materials in question.

* The operators may have incorrectly attributed what would normally be termed routine failure, or cause unknown, to the set of circumstances presented by the changeover to chloramination.

Therefore, a study was undertaken, as reported herein, to evaluate the effects of chloramine on the variety of distribution system elastomers and to determine whether chloramines, relative to other chlorine species, are especially injurious to these materials. The project demonstrated several new or modified techniques for assessing the performance of elastomer materials.

Elastomer failure

The most commonly reported elastomeric failures are related to residential water applications. Anecdotal reports have come from dozens of different communities, though few of these have been formally documented nor deemed serious enough to generate a utility response.

The single largest source of complaint appears to be failure of toilet tank components, particularly tank balls, flapper valves and diaphragms associated with automatic flow control devices. These items are all elastometric components, but the specific type of elastomer is rarely known. The nature of the failure is usually described as a form of swelling (water adsorption) that grossly distorts shape, preventing the piece from performing its function. Surface cracking is sometimes mentioned, along with loss of elastomer filler material.

The following are failure incidents in chloraminated systems for which at least partial documentation and a basis of comparison are available:

* Fort Lauderdale, FL: Rubber parts in toilet tanks are breaking down after only six months. Normally they would be expected to last five to six years (ref. 1).

* Topeka, KS: Four years after changeover to chloramination, city inspectors are finding that gaskets and diaphragms on backflow prevention assemblies are beginning to deteriorate. Because these devices are critical to safe operation and usually difficult to service, the city is working with valve manufacturers to assess the problem. A Topeka hospital reports that maintenance expenses have tripled since chloramination changeover (ref. 2).

* Ramona, CA: O-rings used in chick watering assemblies failed after six months due to severe swelling following chloramination changeover. Prior to changeover, these same assemblies required servicing every third to fourth year (ref. 3).

* Minneapolis, MN: Aerator washers used in airport bathroom faucets swelled shortly after changeover. These same washer assemblies are, however, used in airports in St. Louis and Atlanta without problem (ref. 3).

* Ingleside, TX: 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/l since 1982. Similar valve assemblies on the raw water side show little or no deterioration and have been in service since 1972 (ref. 4).

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. 5).

Research has dealt directly with potential mechanisms of chloramine attack (ref. 3). A comparative study of the effects of chloramines on raw polymers (non-vulcanized and with minimal cross-linkage) and polymer vulcanizates (with cross-linking) demonstrated that the chloramine effects on the raw polymer were minimal, whereas the impact on vulcanizate was substantial, including loss of resilience and extensive swelling. This was true for all three of the base polymers tested (nitrile, butyl and EPDM). Further, the extent of attack was dependent on the form of the cross-linking, the common sulfur-based vulcanizates being more susceptible to attack then elastomers cured in a peroxide process.

Elastomer types

The selected test elastomers were drawn from two groups:

* Commercially available elastomers were obtained as sheet stock from several suppliers. Because of proprietary considerations, complete product formulations or details of the compounding process were not revealed. Thus, little more than the base polymer is known about this group of test materials.

* Generic elastomers were compounded without any of the wide array of antidegradants that are commonly added to defend against oxidative attack, retard staining and cracking, and prevent deterioration from heat or light exposure. Thus, performance of the generic elastomers would be more indicative of the intrinsic qualifies of the base polymer.

Five types of commercial sheet stock were examined: ethylene-propylene (EPDM), nitrile (NBR), natural rubber (NR), neoprene (CR) and a styrene-butadiene (SBR). Eleven generic elastomer formulations were also tested. These are identified in table 2, along with their trade abbreviation and base polymer ingredient.

Table 1 - elastomer uses in the distribution
            system
Polymer type             Usage               Attributes
Natural rubber (NR)     Flapper valves,    Abrasion resistance
                        large pipeline
                        gaskets
Synthetic rubber        Large gaskets      Abrasion resistance;
(SBR)                                      inexpensive
Buna-N (NBR)            O-rings, valve     Resistance to
                        seats, pump        petroleum solvents
                        impellers, check
                        balls
Ethylene-propylene      O-rings, valve     Chemical resistance;
(EPDM)                  seats, flat gask-  inexpensive
                        ets, pond liners,
                        check balls
Isobutylene-isoprene    Diaphragms          Oxidant resistance
(IIR)
Fluorocarbon (FKN)      O-rings, chemical   Superior chemical
                        feed pumps          resistance
Neoprene (CR)           Pump impellers,     Excellent
                        valve seats,        weathering
                        chemical feed       character;
                        pumps               resistance to
                                            gas and oils
Hypalon (CSM)           Reservoir liners    Ultraviolet
                                            resistance
Silastic (Si)           High-temperature
                        applications

[TABULAR DATA 2 OMITTED]

Elastomeric coupons used in this study conformed to the dumbbell-shaped specimen approved in ASTM D 412-83 (ref. 7). Coupons were cut from sheet using an approved the by Hudson Die Group. The narrow neck of the specimen ensured a consistent breakage pattern, while the splayed ends gave substantially more grip area relative to the neck cross section. The coupons were cut lengthwise along the grain (the direction of extrusion).

Coupon rig

Given the number of elastomer types to be tested, the required replicates for each type, and the substantial number of exposure conditions, it was essential to design a large-capacity coupon rig that was easy to access and adaptable to a variety of materials. Because several rigs were to be operated simultaneously, the system needed to be compact and inexpensive to fabricate. The final system design consisted of a 20-L reservoir mounted on a magnetic stirrer with an electrical resistance immersion heater. A coupon-mounting rig with a maximum capacity of 48 coupons was suspended in the inverted position from the lip of the reservoir. Because of the branch-and-stem appearance of the rig, it came to be called the "inverted Christmas tree." During loading and unloading, the rig could be stood upright on its broad base, which, when inverted, fit snugly on the lip of the reservoir, suspending the rig and providing clearance for the magnetic stir bar at the bottom of the reservoir.

The design addressed three principal concerns:

* Maintenance of fluid motion across the elastomer coupons, preventing development of mass transfer limiting conditions;

* Maintenance of test surface strain during exposure, replicating to a degree the strain placed on most elastomers while in distribution service; and

* Provision for ready inspection and removal of the coupons throughout the exposure sequence.

The tension placed on the coupons also served to ensure that surface cracking was oriented across the neck of the coupon (against the grain). Because the material is strongest in tension with the grain, cracking across the grain produces the greatest degree of performance degradation.

Tensile strength test

The tensile strength of elastomeric coupons was determined per ASTM D 412-83. The coupons were tested to failure. The standard coupon elongation rate was set at one-inch per minute. The load cell accuracy was approximately 0.5 lb-force. Stress was determined based on the original cross-sectional area of the coupon neck. Strain was calculated as percentage of elongation along the neck of the coupon.

Although stress and strain data were collected continuously during the tensile test cycle, the most consistent and useful data were derived from the maximum load and elongation at failure. Attempts to interpret the modulus of elasticity at several degrees of elongation (100% and 300%) gave consistent results only for the virgin material. Moduli on exposed material with visible surface cracks were highly variable and difficult to interpret; hence, assessments of mechanical performance focused on the maximum load and elongation parameters which gave a much higher degree of reproducibility between replicates. Mechanical properties of the exposed, virgin and control coupons were established with a minimum of three replicates.

Swelling test

A modification of the ASTM rubber swelling test (ASTM D 412-83) was used for assessing the extent of water adsorption. Prior to exposure, all test and control coupons were cleaned, marked and weighed. Following exposure, surface water was removed by towels, and the coupons were immediately reweighed to determine total water uptake. Since this was non-destructive, all coupons of a particular type and exposure could be tested, giving a total of at least six replicates. Water adsorption is presented as a percentage gain relative to the pre-exposure weight.

Surface characterization

The extent and depth of cracking on the coupons were assessed using a macroscopic optical lens. Using best judgment, a single reviewer subjectively evaluated the surface cracking on all coupon materials under all exposure conditions. At an approximately 10-fold magnification, surfaces were rated in one of five categories:

* No cracking evident;

* minor cracking (minimal surface penetration); moderate cracking (high frequency of surface penetration);

* severe cracking (some crack penetration to a depth of 25% of coupon thickness);

* destructive cracking (crack penetration to a depth of half the coupon thickness).

The surfaces were also rated based on their degree of embrittlement, increase in tack (a sticky and adhesive character on the coupon surface), loss of elastomeric filler and permanent distortion. In each of these categories, a single qualitative assessment was derived from replicate coupons.

Accelerated life cycle testing

Initial experimentation demonstrated that nominal distribution system levels of chloramine and free chlorine (3 - 5 mg/l as [C1.sub.2]) at typical distribution system temperatures would not produce meaningful differences in elastomeric degradation within a convenient laboratory time frame (90-120 days). Accordingly, rubber industry testing procedures that stress elevated temperatures to advance the natural elastomer aging process, and high chemical exposure concentrations to enhance oxidation effects, were adopted for this study. This accelerated life cycle testing attempts to achieve, in a relatively short period of time, the nature and extent of deterioration typical of much longer exposures.

After experimentation to determine an appropriate temperature range, a standard temperature of 100[degrees]F (38[degrees]C) was selected for this exposure protocol. Additional testing defined an appropriate concentration range for the free-chlorine and chloramine oxidants. The standard exposure period was set at 30 days.

Eight different chemical conditions, defined in table 3, served as the basis of the accelerated life cycle testing protocol. To achieve the desired chlorine speciation, exposures were conducted at both high and low pH (pH 8.5 and 5.5) with appropriate controls. Thus, the effects of hypochlorous acid versus the hypochlorite anion could be distinguished for free chlorine; similarly, the dichloramine impacts (low pH) could be assessed relative to the monochloramine (high pH) (ref. 8). [TABULAR DATA 3 OMITTED]

Oxidation reactions on the elastomeric materials consumed substantial amounts of available chlorine in the exposure solutions (both free and combined) and consequently altered solution pH. Exposure conditions in the tank were monitored daily, and solutions changed usually every second day to maintain the test conditions. All test waters were prepared from the same low-mineral tap water having an initial conductivity of 50 [mu]S, a total hardness of approximately 20 mg/l as [CaCO.sub.3] and an alkalinity of 0.5 meq/l. To adjust pH, [H.sub.2][SO.sub.4] or NaOH was added as needed. Chlorine was added to the exposure tanks from a stock 5% solution of sodium hypochlorite. Ammonia was added to the controls and chloramine solutions as ammonium chloride. To provide a uniform excess in the chloramine tanks, the ammonium was added at a [NH.sub.4]:[Cl.sub.2] molar ratio of 2.0.

Tensile performance

Table 4 presents the stress and strain data collected on the 16 elastomer types following the standard accelerated life cycle protocol. The different elastomer types are grouped by their common base polymer. The values shown are averages of the individual replicate coupon values.

[TABULAR DATA 4 OMITTED]

The pattern evident in the table is that chlorine, and the chloramines in particular, substantially degrade the elasticity of most of the elastomer groups, and that diminished elasticity is closely mirrored by an equal or greater reduction in load-bearing strength. With the exception of the FKM fluorocarbon, the VMQ silicone and the XIIR isobutyl rubber, all materials were seriously degraded; the materials most adversely affected were the natural rubbers and their close polymeric analogue, the neoprenes. For some materials the extent of the attack destroyed structural integrity and elasticity for all practical purposes. The most detrimental exposure condition, by a wide margin, was the dichloramine solution. The monochloramine and hypochlorous acid solutions were roughly equivalent in their respective impacts. The hypochlorite anion solution had a lesser, but nonetheless substantial, effect.

The tensile properties of virgin materials are provided for comparison with the control exposures. Several materials in the control exposures were marginally degraded in their tensile performance relative to the virgin material. This is attributed not to solution contact, but to accelerated general aging at the elevated temperature of their exposure.

Elastomer swelling

Chlorine attack (free and combined) on the interior polymeric structure evidenced itself by substantial water adsorption in almost all of the test exposures. The water adsorption ranged as high as 70% in some chloramine exposures, but generally remained in the range of one percent for all control exposures.

Swelling seems to be a particularly sensitive indicator of elastomer degradation. Tested at selected points during the standard 30-day exposure cycle, the water gain was nearly linear with time. Hence, the process occurs early, as well as late, in the exposure cycle. It is reasonable to hypothesize that the formation of microcracks on the material surface - resulting from a breakdown in the polymeric cross-linking - provides the initial penetration points for water entering the elastomer structure. Since the cracks are probably self propagating, exposing new material to chloramine attack as they deepen, it would appear that water adsorption continues apace until the coupon thickness has been fully penetrated.

Table 5 presents the results of swelling tests on various elastomer types (grouped in the tensile test order). The tendency of the elastomers to adsorb water correlates strongly with deterioration of the physical performance parameters (maximum stress and strain). Whether in the free-chlorine or chloramine exposures, materials which suffered serious degradation of tensile properties showed substantial water adsorption. The tendency to adsorb water was generally higher in the chloramine exposures than in the free chlorine.

[TABULAR DATA 5 OMITTED]

For all materials, water adsorption in the control exposures was minor. As in the tensile testing, the presence of free ammonium or ammonia had no discernable effect on swelling.

For most exposures, there were a minimum of six replicates available for water adsorption measurements. The results demonstrate that this technique is at least as sensitive to elastomer degradation as the tensile tests, and is highly reproducible, as the replicates showed. This type of test is also analytically simple and requires no specialized instrumentation.

Surface characterization

After the standard exposure cycle and subsequent drying, materials were assessed in the following categories related to oxidant attack on the elastomeric structure: Surface cracking, hardness (embrittlement), shape distortion (permanent), loss of filler material and surface tack (resulting from plasticizer leaching) (table 6).

Each assessment represents a qualitative average derived from examining three replicate coupons. An assessment of cracking is given for each type; other characterizations are listed only if degradation exists in that category. Comparisons are made relative to virgin material.

The most immediate conclusion to be drawn is that the control exposures (high and low pH, ammoniated and nonammoniated) suffered no significant surface deterioration. All elastomer types held up well in the control exposures, indicating their relative imperviousness to the pH range typical of most water distribution systems, and reinforcing the earlier observations that ammonium or ammonia by itself is no threat to the integrity of these materials.

Another obvious conclusion (one observed in both the tensile and swelling tests) is that among elastomer types there is a broad range of susceptibilities to both free chlorine and chloramine attack. The more exotic elastomers, such as the FKM fluorocarbon (Viton) and the VMQ (vinyl-methyl-silicone), performed well in all instances, retaining their surface character regardless of the chlorine species and living up to their reputation for chemical imperviousness. The XIIR (isobutylene-isoprene) also performed well, as did the peroxide -cured EPDM. All other materials displayed susceptibility in varying degrees to both the free chlorine and chloramine solutions.

In general, the dichloramine exposure (low pH chloramine) produced the greatest surface deterioration, followed by the monochloramine and low pH free chlorine (hypochlorous acid). Materials that displayed a susceptibility to one chlorine species generally displayed some susceptibility to all.

The NBR materials (acrylonitrile-butadiene), regardless of whether sulfur or peroxide cured, are heavily damaged by chlorine exposure, and appear to be particularly susceptible to chloramines. Substantial NBR cracking was initiated in most chlorine exposures, but the extent of cracking in the chloramine exposures was, in all cases, extreme, oftentimes resulting in complete penetration of the coupon.

EPDM materials (ethylene-propylene-diene) showed varied resistance to chlorine attack. The peroxide-cured generic material was largely impervious, while the sulfur-cured commercial sheetstock showed severe cracking. The sulfur-cured generic material, while it did not crack, swelled and permanently distorted in the presence of the chloramines but remained largely impervious to the free chlorine solutions.

By group, elastomers formulated with natural isoprenes (natural rubber and gum rubber) appeared to show the most degradation, including cracking, embrittlement and substantial loss of filler materials. These outcomes were most pronounced in the chloramine exposures, but were also significant m the free chlorine exposures. The gum rubber was particularly impacted by the chloramines, losing some filler material and appearing to suffer surface dissolution with at least minor loss of polymer.

The neoprenes (chloroprene), close chemical analogues of natural rubber, also display extreme sensitivity to the chloramines, and somewhat lesser impacts relative to free chlorine. Again, cracking and embrittlement predominate.

SBR materials (styrene-butadiene) were markedly more susceptible to chloramines than free chlorine, with both the generic and the commercial materials showing heavy cracking.

The surface characterizations reinforce results of the previous examinations and also add information. As would be expected, materials most adversely affected in tensile properties display the greatest degree of cracking. Many of these materials lose substantial amounts of filler and are severely embrittled. Susceptibility to oxidative attack clearly degrades the intrinsic property of material strength and also manifests itself by altering a variety of extrinsic properties, some of which are readily visible to the unaided eye. Although internal disruption of the polymeric structure is not visible to the naked eye, macroscopic external damage is often visible in the form of deep surface cracks. In many cases the surface cracking is so great as to substantially diminish the cross-sectional area at points along the coupon neck, hence decreasing the area available to bear load.

Elastomer imaging

Given the test coupon surface character, scanning electron microscopy (SEM) was found to give the best image quality. The SEM imaging provided in-depth detail and gave a sense of the three-dimensional character of the surface. To retain a useful image structure, the standard SEM magnification was limited to about 50 times. (At higher magnifications, it becomes difficult to distinguish between the original coupon surface and the crack interior).

At higher magnification SEM images (130 times) of virgin NBR material and control coupon exposures, the control coupon shows a minor degree of swelling and both surfaces are largely unblemished. After only 24-hours exposure to dichloramine, the incipient cracking evident on a test coupon was largely normal to the tensile force exerted along the coupon length.

At the microscopic level the early cracking is evenly dispersed, but by the time crack propagation becomes visible to the unaided eye it will have become localized, concentrating itself in much larger fissures.

Crack propagation appears to favor the formation of a relatively few large cracks over more numerous and dispersed smaller cracks. It is presumed that the growth of a large crack relieves elastomer tension in the immediate vicinity and thus eliminates one of the driving forces for further crack propagation.

Performance ranking

To obtain the relative susceptibility to both free chlorine and chloramine attack, the materials were ranked through an aggregate score compiled from the material's respective performance in the tensile, swelling and surface cracking evaluations. The weighting of the individual test scores composing the aggregate total was approximately equal.

A simple scoring method assigned points based on the percentage degradation relative to control exposures. Hence, for the tensile testing a 50% decrease in the maximum load scored 50 points; similarly, a 40% decrease in elongation at failure scored 40 points. The sum of the stress and strain scores represented the tensile test contribution to the aggregate. The water adsorption (swelling) component was determined by assigning points based on the percentage total weight gain immediately following exposure. A 20% weight gain scored 20 points; a 30% gain, 30 points. The surface cracking score was established by assigning point values (25 points per increment) to the previously designated cracking characterizations: Minor cracking (C1 in table 6) scored 25 points, moderate cracking (C2) scored 50 points and so on. The individual and aggregate scores of the 16 test elastomers, and their relative ranking, are presented in table 7. Admittedly, the scoring is a crude index, intended only to rank the test materials by relative susceptibility to free chlorine and chloramine attack. Materials are presented in rank order from least susceptible to most susceptible, based on dichloramine exposure. Since the seventy of the elastomeric response generally differed between the free chlorine and chloramine exposures, individual scores and ranking are presented for both the dichloramine and hypochlorous acid exposures.

The obvious conclusion from the index score is that chloramine exposure is significantly more destructive to elastomers than the free chlorine. Without exception, each elastomer type scored higher in the dichloramine exposure (about twice as high) than in the hypochlorous acid. This was true for all elastomers, from most susceptible to most resistant.

The dichloramine point scores fall roughly into three groups of materials: those only slightly affected (<100); those degraded but not destroyed (100 - 200); and those with complete (or nearly complete) loss of structural integrity (>200). The bulk of elastomers fall into this last category, including the natural rubbers, SBRs, neoprenes and the nitriles.

The EPDMs, particularly the peroxide vulcanizate, were only marginally degraded by the exposure, and the relatively new synthetic elastomers - specifically engineered for chemical resistance - performed well.

Although the data are not conclusive, commercial sheet-stocks (presumably formulated with antidegradants) were somewhat more resistant to chlorine attack than the generic materials of the same type.

The XIIR material, though it ranked high in both the chloramine and free chlorine exposures, suffered serious plasticizer and process chemical loss in the dichloramine exposure. The leaching of these chemicals produced a substantial increase in surface tack. Rankings, which were based entirely on degradation of tensile properties, swelling and surface cracking, did not consider this aspect of elastomer degradation.

Conclusions

Chloraminated compounds are significantly more aggressive to elastomers than equivalent concentrations of free chlorine. Elastomer failures may, in some cases, be related to a changeover from free-chlorine disinfection to chloramination.

With few exceptions, concentrated solutions of chloramines (either mono- or dichloramines) produce greater materials swelling, deeper and more dense surface cracking, a more rapid loss of elasticity, and greater loss of tensile strength than equivalent concentration of free chlorine. The differential impact between the hypochlorous acid species and the chloramine species was so great as to be conclusive. For most elastomers, the severity of attack was greatest in the dichloramine exposures, followed by the monochloramine, and then the hypochlorous acid and hypochlorite anion exposures.

The mechanism of chloramine attack is paradoxical. Oxidizing agents are, in general, deleterious to rubbers and elastomers; chloramines, however, which are substantially less powerful oxidants than the hypochlor-ous acid species, exert a greater impact on the elastomers than free-chlorine. The nature of the degradation sheds light on the chloramine action. The substantial swelling experienced in almost all the chloramine exposures indicates that the oxidation effect is related to the failure of the elastomer's polymeric cross-linking. In some cases substantial swelling takes place even though the extent of surface cracking is moderate to minimal. The effectiveness of the attack undoubtedly relates to the penetration of the elastomer surface; consequently, it seems probable that the chloramines have greater mobility in the elastomeric structure than the free chlorine species. This may be explained by the charge and relative polarity of the chemical species or by the failure of chloramines, which are weaker oxidants, to be consumed at the elastomer surface, allowing them to penetrate more deeply before exerting their oxidation capacity. Whatever the explanation, the empirical observations suggest that the elastomeric failure occurs in depth and is related to a loss of polymeric integrity.

Elastomeric surfaces in chloraminated distribution systems are exposed on a continuous basis to levels of chloramines (measured as total chlorine) four to five-fold higher than systems using free chlorination (ref. 9). Thus, ample opportunity exists for long-term deterioration of system components, given the uniquely aggressive nature of chloramines.

Excess ammonia addition during the chloramination process, and/or the presence of free ammonia in the distribution system, is not responsible for observed elastomeric deterioration. Control exposures utilizing ammoniated solutions at both high and low pH were run in parallel with the other controls and chloraminated test solutions. In all cases, coupon exposures in the ammoniated solutions showed only the extent of deterioration typical of the dechlorinated tap water controls at the respective pH. This was true of solutions at both room temperature and elevated temperatures. Elastomer formulations vary widely m their resistance to chlorine attack. With exception of the fluorocarbon and silicone-based materials, all elastomers tested showed substantial susceptibility to chloramine attack.

Chloramines are almost uniformly injurious to elastomeric compounds. Materials most susceptible to attack are those formulated with natural isoprenes, or synthetic isoprene derivatives. Only the newly engineered and completely synthetic polymers - developed specifically for their chemical resistance - performed well in the chloramine exposures.

The limited testing done on commercial sheetstocks with antidegradants (proprietary formulations) showed a wide variation in response to chloramine attack. In general, they appear to be more resistant to chlorine attack than the corresponding generic formulations, but results on this point are not conclusive.

A particularly sensitive and analytically simple methodology for assessing elastomer degradation is the swelling test (water adsorption). As chloramine attack breaks down the polymeric cross-linking, microcracks form on the material surface and water begins to penetrate the elastomer. The cracks appear 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 the physical performance parameters (maximum stress and strain). The water adsorption test is fast and requires a minimum of laboratory equipment. It is also easy to perform, highly reproducible and more sensitive of material degradation. Accelerated life cycle testing can produce measurable water adsorption in only a few days.

Stress and strain testing of the elastomeric material is sensitive to chloramine degradation, but other widely used physical tests are not. Flexure testing under load was laborious and time consuming and yielded little useful information. Durometer hardness testing, while easy to perform, gave highly variable results that correlated poorly with other physical parameters. Temperature plays a critical role in determining the rate of chloramine attack. Even modest changes in temperature produce substantial variation in the extent and speed of material degradation from chloramine attack. Accelerated life cycle testing showed that an increase of 30'F over nominal distribution system conditions produced uniformly greater deterioration of all performance parameters.

Distribution systems in the southern United States, with relatively warmer waters, reportedly have the greatest number of elastomer failures. The geographical variation in ambient water system temperatures probably accounts for much of the distribution of chloramine-related problems.

References

[1.] Wilkes, J.F., "Accelerated failure of rubber components in Florida potable water supplies," proceedings - Environmental Chemistry Division ACS, Miami, (1989). [2.] Panel discussion, proceedings - ABPA Conference, Backflow Prevention (1986). [3.] Simmons, C.L., Evanson, P.P., "Effects of additives in domestic water systems on rubber vulcanizates," proceedings - Rubber Division, ACS, Dallas, 1988. [4.] Naismith, J., Manager, San Patricio Municipal Water District, personal communication (1990). [5.] Ward, I.M., Mechanical properties of solid polymers, 2nd ed., John Wiley and Sons, New York, (1983) [6.] Simmons, C.L. and Evanson, P.P., "Effects of domestic water systems additives on rubber vulcanizates," Rubber World 32:1:16 (1988) [7.] ASTM Standards, Volume 9.01: Rubber, natural and synthetic - general test methods, ASTM, Philadelphia, (1986). [8.] Weber, W.J., Physiochemical processes for water quality control - chapter nine, Wiley Interscience, New York (1972). [9.] Haas, C, "WIDB chloramination review," proceedings - Chlorination Seminar, AWWA National Conference, Philadelphia, (1991).