New adhesive, coating attack EPDM seam-weakness nemesis.
These technological advances most directly affect the EPDM roofing industry, one whose 20% annual growth pattern suggests that it might easily become the second-largest application for rubber. In spite of EPDM's remarkable performance in terms of weatherability and ozone resistance, doubts about seam integrity have continued to cast dark shadows over its roofing application. And the stakes are high. Considering the "ten years plus" guarantees of trouble-free service associated with EPDM roofing, premature failure can result in major court battles and even bankruptcy.
Although technological progress toward better adhesives has not been without its contributors, the obstacles to achieving a consistent and acceptable bond are considerable and perplexing. The inherent bonding challenge of EPDM rubber is further complicated by the use of release agents and the myriad of formulas on the market. Although primers can increase adhesion, they do little or nothing to resolve the film integrity issue. The questionable proficiency with which these primers are applied brings into focus another set of uncontrollable variables: the construction-site environment.
This article not only details these bonding difficulties but outlines the performance expectations of an ideal adhesive for the EPDM roofing industry. It goes on to discuss the newly-patented adhesive in terms of solutions to existing problems as well as its ability to meet desired standards. Accompanying test data in terms of peel values and PSI shear strength support the ultimate conclusions drawn about the new adhesive's potential to resolve the dilemmas that continue to plague this burgeoning industry.
EPDM bondability obstacles
The very physical properties of EPDM membranes that make them a miracle material" for roofing applications also render them difficult to bond. This is true of any EPDM "nonpolar rubber" as opposed to "polar" elastomers such as nitrile and chloroprene. Traditional adhesives cannot properly wet the EPDM surface simply because their surface free energy exceeds that of the membrane. The resultant discontinuous adhesive film decreases surface contact and lowers bond strength. No adhesive systems previously developed have been able to satisfactorily resolve this basic compatibility problem.
Further complicating the challenge is a marked difference in bondability among the numerous EPDM formulas each one designed to meet specific economic and property requirements. The chemical composition of the various membranes on today's market are sufficiently different to severely impede the consistent effectiveness of any one adhesive for all EPDM applications. Even EPDM sheets from the same formula but different in age can demonstrate dramatic variations in bondability.
Seam strength vs. film integrity
Innovative steps to increase bondability have included the introduction of a number of adhesive systems, including adhesive tapes. Although these latter products initially promised greatly-improved bond strengths over adhesive systems, their performance history proved less than encouraging - to the point where some were pulled from the market. The greatest increase of bond strength at the membrane-adhesive interface has been achieved so far by the addition of a primer to the bonding process, rendering a more easily-wetted EPDM surface.
Although the advantages afforded by primers are a step in the right direction, they still pose critical limitations beyond their added expense. First of all, the manufacturers involved (EPDM rubber, adhesive or primer) have no way to ensure quality control in regard to end users. Even when the primer application is executed under optimal circumstances and with exacting technique, the fundamental problem of the adhesive's poor film integrity still remains.
Particularly for the roofing industry, seam performance demands are no less stringent than those for the EPDM sheets. The adhesive bond area actually functions as a small membrane that is integral to the reliability of the total roofing system. So, while a few of the currently available adhesive systems (which customarily include a primer) are adequate within limited parameters, the need remains for a more universal adhesive that fits the illusive missing piece of the EPDM puzzle. In other words, the seam must be able to perform on a par with the membrane in every respect. Period.
Packaging and end-user complications
Another obstacle to satisfactory adhesion with EPDM rubber occurs in the prevention of membrane blocking during the packaging process. Although paper liners are available and effective in separating membranes, most manufacturers seem to prefer the less-expensive alternative of dusting with talc or employing a similar type of release agent. In spite of the fact that these release agents also deter optimal bonding later, their use is widespread and will probably continue indefinitely.
Since the responsibility of talc removal falls upon the end user, doubts arise as to whether this essential cleaning process is always taken seriously or performed with conscientious proficiency. Not only do roofing contractors work under a myriad of circumstances, but their employees range from the highly skilled to complete novices. Any steps requiring specialized training or additional time in the contracting business are undesirable.
In fact, the entire environment that surrounds roofing application is subordinate to economics. Contractors are bound to finish the roof in a period of time that allows for a profit. In many locations and during some seasons, this pressure dictates working in variable temperatures and marginal or even inadvisable weather conditions. Workers are not only hurried, but the expertise they bring to the job is unpredictable. Development of any adhesive for this industry must encompass the realities of the application setting, keeping the bonding process as simple and foolproof as possible while achieving optimal performance.
Defining the ideal adhesive system
Considering the intertwining complexities that would compose an effective and universal adhesive system for the EPDM roofing industry, the research was focused around the following objectives:
* Brush or roller application;
* Roof curable;
* Good initial tack and green strength;
* Resistance to thermal, mechanical, ozone and weathering stresses;
* Effective with all commercial EPDM membranes;
* No primer required;
Very early in research it was recognized that the lack of bondability in EPDM membranes and the poor film integrity of the adhesive might both be resolved by the formulation of an EPDM-based adhesive. Such a formula would not only offer bondability from the perspective of total membrane compatibility, but it could feasibly meet the "ten years plus" service guarantees associated with EPDM roof sheeting. In order to meet the brush/roller and roof-curing objectives, it would need to be applied as a contact adhesive while becoming a thermoset in a cross-linking reaction with the roof s natural solar heat.
Simulating the on-site conditions of a typical roof application during every phase of testing would be critical to the realistic evaluation of results. The performance tests should also duplicate the stresses found in the roofing environment - both peel and shear values. Since one-quarter of the bond area located at the ends of the overlap actually carries three-quarters of the load, lap shear stresses end up in a peel mode prior to failure. (This phenomenon is easily demonstrated during "testing to failure" where initial failure occurs at the ends of the overlap.) All tests should be carried out by an independent agent, and EPDM membrane samples should ideally include 100% of those currently on the market.
Research and testing highlights
After extensive efforts to identify and develop an adhesive formula that approximated what the EPDM roofing industry would consider as "ideal," Syn-Coat's EPDM-based adhesive met all objectives without qualification except for two:
* It was not possible to develop a single adhesive formula that performed equally with all membranes, although a group of three adhesive formulas did achieve acceptable results without the addition of a primer.
* The converse was also true: the addition of a primer permitted the use of one adhesive system for all types of EPDM membranes.
Care was taken to include a comprehensive cross-section of EPDM formulas to increase the universal applicability of results. Although manufacturers provided the majority of samples, a number of varieties were procured indirectly through roofing contractors. Even though considerable effort was applied to realize the 100% goal, sample membranes represented approximately 80% of those conceivably available. Considering that all these formulas consist of varying mixtures of similar properties (i.e. rubber, fillers, curatives and processing aids), test results based on this wide sampling should be valid and highly-credible. All tests were performed by an independent testing facility.
First, all membranes were solvent-wiped with a rag and left to dry prior to adhesive application. Although many of the test specimens still evidenced light streaks of talc after this initial wash, such an undesirable condition is highly likely in practical application. Initial peel values on these test specimens were only 1.1 to 5.1 pounds per lineal inch.
As would be expected, marked contrasts in peel values occurred in relation to he efficiency with which talc release agents were removed from the membranes. TO illustrate the dramatic effect of removing all the talc, EPDM membranes from one manufacturer were meticulously cleaned before the application of the three different adhesive systems. Peel values on these samples pulled around seven pounds per lineal inch.
All adhesives were applied via a paint brush and pressed together after sufficient open time. After samples were rolled with a glass jar to remove any trapped air bubbles, they were cut into one-inch strips and left to dry for two weeks. A companion set was further exposed to heat-curing at 158 degrees F for a period of seven days for verification that crosslinking does occur at moderate temperatures. Peel adhesion was per ASTM D413 Type A (180 degrees ); crosshead speed was two inches per minute in both peel and lap shear tensile tests.
In the course of observing lap shear testing to failure, it appeared that membrane distortion exceeds any imaginable degree that could occur on any roof in any reasonable environment. The adhesion values condensed in table 1 not only contrast the peel and shear results of the three adhesive systems, but they clearly show the results of moderate heat curing. On the left half of the chart are peel and shear values for the 13 EPDM varieties when cured for two weeks at room temperature. The right side of the same chart gives similar data when the samples are aged for one week at 158*F. Table 2 serves as a summary of the top performing adhesive formula for each of the 13 EPDM samples - all without application of a primer.
For each of the EPDM varieties at least one of the adhesive systems achieved acceptable values, and in many cases two formulas performed equally well. A more visual comparison of table l's adhesion values is presented in a graph format in figures 1 and 2, where figure 1 illustrates values at room-temperature aging and figure 2 offers similar data after moderate heat curing. It should be noted here that this data does not make a quality judgment about the EPDM membrane samples but simply reflects the bonding values achieved by using the three adhesive systems.
Primers and the new adhesive system
Once initial tests were complete, the question of how a primer would affect bond strengths (particularly in those cases deemed "marginal") became the focus of further testing. Because the increase in values was so consistent throughout the samples, table 3 offers specific test results for only one representative EPDM membrane. It not only gives "primed" and "unprimed" peel and shear values but includes the differences of room-temperature aging and curing at 158 degrees F. Figures 3 and 4 visually illustrate the same data in graph format.
Based upon test results of the many samples bonded with only a single adhesive system and aged at 158OF for one week, peel strengths quadrupled and shear strengths doubled with the use of a primer. This data concludes that double digit peel values and shear strengths in excess of 40 PSI are attainable with any of the three adhesive formulas in combination with any currently-available EPDM material, as long as a primer is applied.
Even though the trouble and expense of adding a primer to the roof-application process is unpopular with contractors, it will probably remain a common industry practice. The only realistic solution for omitting primers entirely would be standardized adhesion specifications for EPDM membranes - which is not likely to occur.
Testing qualifications and considerations
Several qualifications must preface any conclusions drawn from test results for this new EPDM-based adhesive system. No doubt, the only truly valid assessment of seam integrity is through observation of a trial roof over a period of several years - not a realistic option for new product development. Without industry standards for testing seam strength, any random comparison of adhesives solely on the basis of test reports is meaningless. Nevertheless, observation of laboratory peel and shear results is the most practical method for comparing a number of different adhesives, provided that testing is executed with verifiable consistency.
In evaluating this new EPDM-based adhesive, great care was taken to simulate on-site application conditions and include values for the entire spectrum of membrane samples - not just those closest to desired results. Although a few of the adhesives that were on the market or are currently being manufactured claim excellent peel and shear values, none of those tested under the same controlled conditions either matched or out-performed the new adhesive system.
Since the EPDM membranes themselves are not required to meet any performance standards for adhesion, the whole issue of seam adhesion strength is one that must be addressed by each manufacturer or supplier. Certainly, some EPDM formulas are extremely difficult to bond under any circumstances; while others offer considerable compatibility to specific adhesive formulas. Thorough testing of this new adhesive product is encouraged to more fully explore its potential in combination with specific prime-user requirements.
Judging from extensive laboratory testing, the ultimate licensing and marketing of this newly patented adhesive system and white EPDM coating will enable the EPDM roofing to hurdle its greatest difficulties. The following points emerge as valid in assessing the potential for these two products to shape a brighter future for EPDM technology in general and the roofing industry in particular:
* This new EPDM-based adhesive offers unprecedented compatibility with EPDM rubber sheets, resolving a simple but crucial bonding problem. Although changes in the EPDM rubber formulation could also alleviate this situation, it is unlikely that the EPDM industry will develop any bondability standards at this point in its evolution. Innovations in the adhesive formula remain the most realistic approach.
* The EPDM-based adhesive was able to duplicate the membrane's resistance to thermal, mechanical, ozone and weathering stresses - with film strength that sometimes surpassed that of the EPDM rubber. In an cases it exhibited greater film strength than that of the many other adhesive systems and adhesive/primer combinations tested.
* The unique formulation of the new adhesive system requires no specialized training, tools or procedures. It is applicable with brush or roller and thermally cross-links at ordinary roof temperatures. The elimination of a primer from the bonding process is possible with the new adhesive, dependent upon the basic bondability of the EPDM membrane and desired peel and shear values.
* A simple restructuring of the basic adhesive formula elements produces a satisfactory bond regardless of the EPDM formula. Only three adhesive systems were necessary to satisfactorily bond the wide variety of EPDM rubber accumulated for laboratory testing.
* Excellent proportions of double-digit peel values are attainable with this new EPDM-based adhesive without the addition of a primer for several EPDM varieties. To these select EPDM manufacturers, the new adhesive would mean high-margin pricing by eliminating expensive primers and labor costs.
* For inherently less-bondable EPDM membranes, the addition of a primer allows any formulation of the new EPDM-based adhesive to perform at highly-acceptable levels. Within this context, the goal of universal effectiveness within the industry becomes a reality.
* As a solution for seam-weakness doubts on existing roofs, Syn-Coat is also ready to license its energy-saving white EPDM coating. Equally suited for application on existing or new rubber-roof installations, it seals and protects seams from water penetration as it dries to a thin rubber film with similar expansion characteristics of the roof.
* The white EPDM coating can also be applied as a tandem product with its EPDM-based adhesive system to guarantee the most reliable and energy efficient roof available with modem technology.
European Rubber Journal, Volume 167, Number 5. May, 1985. "The sky's the limit" by Robert Grace, pp. 19-21.
European Rubber Journal, Volume 167, Number 5. May, 1985. "A three-cornered battle" by Dr. Alan Strong, p. 22.
Review on curing and crosslinking of elastomers, " by Robert P. Quirk. Paper #53 presented at the 132nd meeting of Rubber Division, ACS; Cleveland, Ohio. October 6-9, 1987.
Elastomerics, Volume 116, Number 5. May, 1984. "Seam adhesion of EPDM roofing membranes" by S. A. Westley, p.13. Elastomerics, Volume 116, Number 5. May, 1984.
"Future looks bright for single-ply roofing. " Elastomerics, Volume 11 7, Number 5. May, 1985. Bonding EPDM single-ply roofing with ChemLok TXL adhesive" by SA. Westley and E.L. Polaski, p. 23.
(Tabular Data and Other Figures Omitted)
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|Title Annotation:||ethylene-propylene rubber|
|Date:||Nov 1, 1990|
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