February 2000 Interface • 5
Styrene-Ethylene-Butylene-Styrene polymer-modified
mopping bitumen (SEBS•PMB) has emerged from the
shadows of the specialty marketers and is now offered by
many mainstream BUR and modified bitumen manufacturers. At
last count, 21 different manufacturers were marketing or making
available SEBS polymer-modified mopping asphalt as a part of
their warrantied roof systems.
Some of the manufacturers offer multiple grades of SEBS
polymer-modified asphalt so the actual number of different SEBS
polymer-modified bitumen choices exceeds 50. The marketing
and technical information available from the manufacturers for
the wide array of SEBS•PMB choices presents a variety of claims
and conflicting information for the roofing professional to sort
through.
The ASTM D-8.03 subcommittee recognized the market
confusion surrounding SEBS•PMB and sought to clarify the
indiscriminate claims and market disorder by developing a standard
for SEBS•PMB. From ASTM’s unique consensus-forming
committee work, ASTM D 6152, “Standard Specification for
SEBS—Modified Mopping Asphalt Used in Roofing” was
approved in late 1997. It is significant to note that the ASTM
scope states that, “1.3, the specified tests and property values
used to characterize SEBS•PMB modified asphalt are intended to
establish minimum properties” (italics are those of author).
The ASTM standard sets forth nine criteria with minimum,
and where applicable, maximum ranges for SEBS•PMB properties
(as shown in the table on page 8). The properties are to be
measured “as manufactured” unless otherwise noted. Properties
the ASTM standard addresses are:
1. Softening point, (ASTM D-36)
This test procedure measures the temperature at which a
metal ball falls a given distance through a shouldered disk filled
with the tested asphalt (ring and ball). The result provides an
approximation of the transition temperature between a viscous
solid and a thick flowable liquid, the approximate melting temperature.
The higher the softening point temperature, the more
resistant the material is to flow at elevated roof temperatures.
THAT OLD BLACK MAGIC…
MODERNIZED AND
DEMYSTIFIED
BY TIM BARRETT, RRC, CDT
Diagrammatic SEBS illustration, courtesy of Shell Chemical.
2. Softening point change after heat
exposure (ASTM D-36)
This test follows the same protocol as D-36 softening point
test after the SEBS asphalt is heated to 500ºF for 8 hours. The
ability of the material to retain softening point gives a good indication
of the ability of the SEBS•PMB to remain stable and resist
thermal degradation at typical application temperatures.
3. Flash point (ASTM D-92)
Flash point measures the temperature at which fumes above
the asphalt will ignite if exposed to an external ignition source.
Higher flash points indicate lower flammable volatile content.
High flash also indicates the material can be heated to hotter
temperatures with less risk of fire in the kettle. This measure is
generally considered as a safety issue and not a performance
issue. This standard requires a minimum flash
point of 500ºF. ASTM D-312 asphalts have a minimum
flash point of 475ºF.
4. Penetration: before heat
exposure (ASTM D-5)
Penetration measures the depth that a standard
needle weighing 100 grams penetrates a
material at 77ºF in five seconds. The result provides
an indication of the stiffness of a material at
the test temperature. A softer, SEBS polymerized
asphalt (more rubber-like than asphalt-like) will
allow for greater penetration than conventional asphalt.
Penetration at 77ºF does not provide much indication of performance
at hot or cold temperatures.
5. Penetration after heat exposure
(ASTM D-5)
Following the same test procedures as penetration before
heat exposure, penetration is measured after the SEBS•PMB has
been exposed to 500ºF heat for 8 hours. After heat exposure,
most asphalts will become stiffer with less penetration. If a modifier
is used which breaks down during heat exposure, the penetration
will tend to increase significantly. The ASTM D-6152
standard allows a maximum of 12 units change, plus or minus,
after heat exposure.
Electron microscope view of SEBS polymerization network,
courtesy of Shell Chemical.
Schematic drawing of polymerization network, courtesy Momentum Technologies, Inc.
6 • Interface February 2000
February 2000 Interface • 7
6. Solubility in trichloroethylene, %
(ASTM D-2024)
This test requires 99% solubility which shows that virtually
all of the SEBS•PMB components are soluble, and the material
does not contain more than 1% undesirable additives or residues
such as mineral filler, fibers, lime, clay, or refinery impurities.
7. Tensile-Elongation, % at 77°F,
(ASTM D-412)
Tensile indicates the breaking or ultimate strength of a material.
To compare materials, test conditions need to be the same.
The tensile strength will vary depending on temperature, load
rate, and specimen configuration. Higher tensile strengths indicate
a higher strength and greater resistance before breaking.
Tensile strength can be measured using a variety of procedures
including ASTM D-412.
Elongation (the flip side of the stress-strain equation) measures
the amount of extension (in percent) that a material will
permit before breaking. Results will vary depending on temperature,
loading rate, and configuration. The test is somewhat similar
to ductility (with ductility testing, each three cm is equivalent
to approximately 100% elongation). Higher elongations indicate
greater stretching ability prior to breaking. The ASTM D-6152
standard requires a minimum of 750% elongation at 77ºF. Type
III asphalts typically have elongations of 80% or more at 77ºF
and 0% elongation at +20ºF. Polymer-modified materials typically
have high elongation properties across a wide temperature
span. High quality SEBS•PMB can have over 1000% elongation
at both 77ºF and 0ºF.
8. Recovery (ASTM D-412)
Recovery measures the ability of a material to return to its
original dimensions after being extended. Test procedures can
vary significantly. Higher recovery indicates increased elasticity.
The test method specifies temperatures, extension rate and
amount, and the recovery time period. This property test helps
distinguish elastomeric material from non-elastomeric, lowmodulus
material. After 300% extension at 73ºF, a minimum
recovery of 80 percent is required. Polymer-modified materials
experience substantially more elastic recovery than air-blown
roofing asphalt.
9. Cold Bend (ASTM D-3111)
Cold Bend standard procedures can vary somewhat in exact
methodology. This test procedure generally consists of casting a
specimen (.079 inch in thickness), conditioning at the test temperatures
for 24 hours, and then bending the specimen around a
mandrel (1 inch diameter) a specific amount (typically 90º) at a
uniform rate in a specified period (usually 2, 5, and 10 seconds).
A passing result is one in which the specimen does not crack.
This test provides an indication of low-temperature flexibility
characteristics of a material. Passing results at lower temperatures
indicate better low temperature fracture resistance from impact,
expansion, and contraction forces. Type III asphalt will typically
have a fracture temperature (measured using a 1 inch diameter
mandrel and a 90º bend in 10 seconds) of +35ºF to +70ºF,
depending upon the quality and nature of the asphalt.
SEBS•PMB ranges from -30ºF to +20ºF or higher for different
grades of material. The ASTM D-6152 standard requires a minimum
low temperature flexibility of +20ºF.
10. Ductility (ASTM D-113)
Ductility measures the distance in centimeters that a material
can extend at 77ºF at a rate of 5 centimeters per minute. The
ASTM D-312 Type III asphalt specification requires a minimum
of 2.5 cm extension, and Type IV asphalt requires a minimum of
1.5 cm extension. The longer the ductility, the greater the extension
a material can take before fracture at that temperature.
Ductility of polymer-modified roofing asphalts typically ranges
from 25 to 70 cm. Ductility is not a requirement of ASTM
D-6152; however, it is required by the ASTM D-312 asphalt
standard and is noted for comparative reference.
HISTORY
SEBS polymer was developed by Shell Chemical as a “second
generation” of the Styrene-Butadiene-Styrene (SBS) type polymer.
Through a manufacturing process called hydrogenation,
SBS polymer undergoes a chemical transformation, developing
into SEBS. In very simple terms, the Ethylene “E” molecule adds
significant thermal stability and ultraviolet resistance to the SBS
polymer. SEBS polymers have been widely used in the adhesives
industry and the automotive manufacturing industry for more
than two decades. SEBS is the adhesive that holds “Topsider”
boating shoes together during constant wetting and drying
cycles. Gerber uses it for knife handles. There are literally thousands
of unique and demanding applications utilizing SEBS polymer.
SEBS•PMB was first sold in the U.S. roofing industry as a
mopping bitumen by two competing companies in the early
1980s. Both of those suppliers generally had very high quality
product which significantly exceeded the minimum property
requirements set forth in the current ASTM D-6152 standard.
By the mid-1980s, there were five companies marketing
SEBS•PMB in the specialty roof market at the relatively high
prices of $1,400 to $2,300 per ton compared with about $200
per ton for Type III asphalt.
By the early 1990s, after 10 years of in-place field performance,
high quality SEBS•PMB had clearly proven itself as an
engineered enhancement of BUR and mod-bit roofing systems.
The beneficial enhancements provided by SEBS•PMB were generally
recognized within the industry by this point in time. Some
of the “major” manufacturers took notice of the “specialty” marketers’
success with SEB•PMB and started to compete with the
“specialty” manufacturers, offering their own SEBS•PMB-based
systems.
By the mid 1990s, there were a dozen roofing material manufacturers,
both “major” and “specialty” purveyors, offering SEBS
bitumen as a component of their warranted assemblies.
Polymerization
As the popularity and market share of SEBS bitumen has
increased, so has competition among the various manufacturers.
In classic form, this competition has tended to drive prices
down. The competition also motivated the implementation of
“value-engineering” principles to the formulations used to produce
the SEBS•PMB products. “If it ain’t broke, fix it until it is”
mentality has been used by some SEBS•PMB value engineers,
according to one noted expert in the field. SEBS polymer is relatively
expensive with costs of about $2.50 per pound compared
with an SBS cost of about $.70 per pound. One manufacturer
tried replacing part of the SEBS polymer content with crumb
rubber from reclaimed tires with poor results. Some manufacturers
tried replacing part of the SEBS polymer with the less expensive
SBR, SBS or SIS polymers with poor results, and some
started using filler in the product.
A common response to the increased competition and price
erosion was for manufacturers to reduce their costs by simply
decreasing the amount of SEBS polymer used in the formulation
of the material. All other things being equal, this generally will
result in a corresponding decrease in product quality, particularly
in the areas of elastomeric properties, cold temperature flexibility,
and strength. Perhaps to fight this trend, certain bid specifications
sometimes require a prescribed percentage of SEBS
polymer. Unfortunately, one cannot judge SEBS•PMB solely on
the claimed percentages of SEBS polymer content.
SEBS•PMB must ultimately be judged by the mechanical and
rheological properties of the finished product relative to the
properties desired and the properties published. The ASTM
D-6152 standard sets the protocol and minimum benchmarks to
accomplish that judgment.
The quality determinants of SEBS•PMB are:
• Polymer type selection.
• Quality and quantity of polymer loading.
• Quality and consistency of base asphalt.
• Quality of the mixing process.
“SEBS” is not a specific product but rather a broad class of
polymers which vary in molecular architecture. They are
“designed” for many different applications in addition to asphalt
modification. SEBS polymer can be linear or radial, low molecular
or high molecular weight. Its proportion of styrene to ethylene-
butylene can vary, and its viscosity can vary, among other
possible variations which affect the end product.
In addition to the type and percentage of SEBS polymer content,
the quality of SEBS•PMB will be highly influenced by the
compatibility factor of the base asphalt. The polymers are mixed
with the base asphalt using either a high or low shear mixer with
the appropriate manufacturing techniques. Low shear and high
shear manufacturing processes each produce different results,
although satisfactory results can be obtained from either.
Base Asphalt
Asphalt bitumen is the residue of certain crude oils after
removal of most volatile components, typically by a distillation
process. Crude oil quality is of the first order of importance in
determining the characteristics of the asphalt from which it is
produced. Crude oil composition is not uniform, and it may be
stated that there exist as many kinds of crude oil as there are oil
fields in the world. Each is as unique and individual as a fingerprint.
Depending on the constituents of the individual crude oil,
the asphalts obtained from them will contain various
hydrocarbons and hydrocarbon groups in different
proportions.
At the turn of the century, asphalt was considered
a waste byproduct of the crude oil distillation
process, much the same as coal tar pitch was
the detritus of coal-fired energy production.
Crude oil can have asphaltic content that ranges
from nil to as much as 70%. Much asphaltic-rich
crude oil is used as fuel oil. Current U.S. production
of asphalt is 60 million tons, which is produced
by less than 50 primary asphalt refiners.
The more important constituent groups in
asphalt can be determined by simple testing.
They include:
• Carboids which are insoluble in carbon disulfide.
• Carbenes which are insoluble in carbon tetra
chloride and soluble in carbon disulfide.
• Asphaltines which are insoluble in low boiling
saturated hydrocarbons and soluble in carbon
tetrachloride.
• Malthenes which are soluble in low boiling
saturated hydrocarbons.
The carboid and carbene components of commercial
asphalts are slight and increase only during
cracking. Base asphalt quality is largely
determined by the quantity and nature of the
asphaltene and malthene components.
The hard, rigid asphaltenes in asphalt are con-
4-Color
Garlock
Pick-up from 11/99, page 13
“Fumeguard”
Remove this border.
8 • Interface February 2000
February 2000 Interface • 9
sidered incompatible with the polystyrene domains in SEBS
polymer. As a general guide, if the asphaltene content is over ten
percent by weight, the asphalt will probably be incompatible
with SEBS polymerization. A higher than usual amount of aromatics
in the asphalt can soften the polystyrene domains in the
polymer and make the asphalt more compatible, but it will also
lower the ultimate tensile strength of the SEBS•PMB.
Asphalt characteristics will differ when the asphalt is manufactured
from the same crude by different manufacturing methods,
or with different crudes which are processed by the same
manufacturing method. The most common processes for asphalt
manufacture are distillation and extraction. Blowing of asphalt is
considered further modification of asphalt in the manufacturing
process.
Asphalt produced by distillation is called “straight run”
asphalt or asphalt “flux.” It is commonly used to manufacture
paving grades of asphalt cement, such as AC 10 and AC 20.
Straight run asphalt can be distinguished from “blown” asphalt
by a number of physical properties. Blown roofing asphalt is
asphalt flux which has been “oxidized” with a heat and steam
process to elevate the softening point to conventional ASTM
D-312 roofing asphalt specifications. Unmodified roofing
asphalts usually are “blends” of several different asphalts which
have high asphaltene content by design.
Most modified bitumens, both SBS and SEBS, are made with
straight-run asphalt. Oxidized asphalt has not proven suitable to
most manufacturers for producing polymer-modified asphalts.
Phase separation between the polymer and asphalt has been a
common problem when blown asphalt is used to make
SEBS•PMB.
It is also commonly accepted that using straight-run asphalt
that has not been oxidized will result in slower aging and greater
longevity of the SEBS•PMB as compared with air-blown asphalt.
The oxidizing process of asphalt is an accelerated aging of the
asphalt which raises its softening point and flow resistance to a
temperature where it can be used on a roof without slippage or
displacement. After asphalt is applied on roofs, its softening
point will continue to rise as it ages and becomes increasingly
brittle.
There is a minority view within the industry which claims
that SEBS•PMB can be successfully manufactured with blown
asphalt by changing certain mixing procedures and adding carbon
black compounds. However, many believe the resultant
product will still not provide the extended longevity of unoxidized,
“straight-run” asphalt, and it is doubtful that such a material
would pass the solubility requirements of ASTM D-6152.
When all of the variables of asphalt are evaluated and considered,
it is generally thought that about 80 percent of all asphalt
is incompatible with SEBS polymerization, 12 percent somewhat
compatible, and 8 percent ranging between compatible and very
compatible.
The use of a higher percentage SEBS polymer content with
“somewhat compatible” asphalt can result in a product of lower
quality than a SEBS•PMB made with less polymer and a “very
compatible” base asphalt. The relationship between asphalt compatibility
and polymer loading is intertwined inseparably. Proper
formulation is a balancing act with the objective of blending the
components to be and remain “in phase” with each other for the
service life of the roof.
Market Development
As the competitive battles of the SEBS•PMB manufacturers
continued in the ‘90s, some manufacturers continued to reduce
polymer content to a point where some asphalts marketed as
“SEBS polymer-modified asphalt” had such small amounts of
polymer it was insufficient to form a polymer network, the primary
objective of polymerization (see illustration, page 6).
Some field samples tested by the author have such minuscule
polymer loading, they actually had to have the polymer measured
in parts per million.
Some otherwise reputable manufacturers are currently promoting
SEBS bitumen in their product offering that is manufactured
with only one to two percent polymer loading and
nominally holding them out as the equals to high grade SEBS
polymer-modified bitumen. Two percent polymer loading is
insufficient polymer to form a polymer network with even the
best base asphalt. With present technology, two percent polymer
10 • Interface February 2000
loading is insufficient to meet ASTM D-6152 standards.
In many cases, the price of the low-grade SEBS•PMB product
is commensurate with the quality of the product, which is to say
it is not very expensive and there is some beneficial improvement
in the asphalt, albeit slight. Some contractors, motivated
by economics, will find this type product to be an attractive
option and can be expected to aggressively promote the product
in competition with higher quality and higher priced
SEBS•PMB. If such contractors are successful, it will be at the
expense of the building owner and to the detriment of system
performance.
In a few cases, the price of a low polymer content product is
very high relative to the quality of the product, creating a classic
caveat emptor situation. Sales and marketing obfuscation will
almost always accompany this type product. This can be quite
misleading to the unassuming or uninformed.
In a related development, the NJ State Commission of
Investiga tion (SCI) held public hearings on December 8 and 15,
1999. The SCI alleged that a roofing material manufacturer and
its contractors substituted “cheaper” materials for those specified
on certain school reroof projects, defrauding the school boards
of more than $10 million dollars. Some of the substitutions
involved SEBS-based materials. (Editor’s note: See article, page 40).
Conclusions
High quality SEBS•PMB has stood the test of two decades of
field experience and clearly provides an opportunity to improve
the performance and presumably the longevity of BUR and mopapplied
SBS mod-bit systems.
SEBS•PMB is offered by many manufacturers in many grades
in a wide spectrum of prices.
The experiential history of SEBS•PMB was forged with relatively
high quality material from the various pioneers of the
industry. Some newer, less polymerized formulas are marketed
and promoted by some manufacturers as though they are the
same product as the higher polymerized product which developed
the successful track record. They are not the same.
The roofing system performance enhancements provided by
SEBS•PMB, meeting or exceeding ASTM D-6152 standards, are
worthy of consideration by specifiers, consultants, and owners
who desire roofing systems with performance characteristics
which are demonstrably greater than those of conventional
ASTM D-312 asphalt based systems.
The performance enhancements provided by SEBS•PMB
which does not reach the minimal levels of ASTM D-6152 standards
will not provide significant enhancement to the roof system
performance. Such products should not be confused with
the proven performance record established by high quality
SEBS•PMB.
If low performance, low polymer content SEBS•PMB is purchased
at a corresponding low price, the nominal improvement
of system performance may be worth evaluating by a seasoned
roofing professional.
Recommendations
ASTM has published minimum standards of performance for
SEBS•PMB which approximate a basic minimum level of asphalt
polymerization or a “network” structure. Specifiers and users can
and should use the ASTM standard as a guide to:
• Accept ASTM minimum standards for SEBS•PMB for significant
performance increases.
• Accept less than ASTM minimal standards for SEBS•PMB
at a lower cost for nominal performance improvement.
• Require SEBS•PMB standards of performance higher than
ASTM standards, as is encouraged and promoted by some
manufacturers, for substantial performance increases.
As is usual for our industry, an intelligent and informed roofing
professional should be able to sort through available information,
ask the right questions, examine the evidence, and
ultimately choose wisely.
References and Further Study:
ASTM, Section 4, Construction, Vol. 04.04 Roofing, Waterproofing
and Bituminous Materials.
Chernotowich, Kenneth A., et. al., “Comparison of Standard and
SEBS Modified Asphalts in BUR Membrane,” Roofing Research
and Standards Development: 4th Volume, ASTM STP 1349,
American Society for Testing and Materials, 1999.
Diebold, T., “Choosing Bitumens and Polymers when
Waterproofing with Prefabricated Sheets,” Proceedings of the
International Symposium on Roofing Technology; 1985: NRCA.
Dregger, Philip D., “Comparison of SBS and SEBS Polymers:
Factual Statements Can Still Be Misleading,” Interface, Vol.
XVI, No.10, October 1998.
Dupuis, René M., “Strain Energy and Elongation Behavior of
Field Samples taken from Polyester Reinforced Built-up Roofs,”
Roofing Research and Standards Development: 2nd Volume, ASTM
STP 1088, American Society for Testing and Materials, 1990.
Gooswilligen, V.G. and Vonk, W.C., “The Role of Bitumen in
Blends with Thermoplastic Rubbers for Roofing
Applications,” Internal Journal of Roofing Technology, Spring
1989, Vol. 1, Issue 1, NRCA.
Kuszewski, JR., Gorman, W.B., Kane, E.G., “Characterization of
Asphalt and Volatility using TGA and Introscan Analyses,”
Proceedings of the Fourth International Symposium on Roofing
Technology; 1997: NRCA.
Loden, Tony, “Selection of SBS Modified Asphalt Membranes: A
Comparison by Evaluation,” Interface, Vol. VI, June 1994.
Shell Chemical literature.
Various manufacturers’ literature.
Zakar, Pal, “Asphalt,” Chemical Publishing Co., Inc. 1971.
Tim Barrett, is President of the
Barrett Company, Millington, NJ
(www.barrettroofs.com) and has
been involved with the manufacture,
sales, and installation of polymermodified
asphalts since 1975. Tim is
a Registered Roof Consultant and an
active member of the Continuing
Education Committee of the Roof
Consultants Institute.
ABOUT THE AUTHOR
TIM BARRETT
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