Labor shortages, environmental regulations, depletion of natural resources, insurance regulations and competition are having an impact on the emerging technologies in the industry. All of these regulations and requirements are playing an increasing role in the types of materials and methods of application that will be employed in the near future.
The current material and application changes are the most significant that the industry has seen in decades and parallel changes made after World War II (due to material shortages) and the late 1970s (due to the energy crisis).
Some of the current emerging technologies are:
- Cold-applied modified bitumen.
- Self-adhering modified bitumen.
- Cold process systems.
- Self-adhering TPOs.
Modified BitumenModified bitumen roof systems are increasingly being referred to as the “new” built-up roofs. Their surge in market share in recent years is due to versatility of application methods, performance characteristics and use in hybrid configurations.
Modified bitumen sheets are formulated with modified asphalt (asphalt with a plastic polymer for APP; and asphalt with a rubber polymer for SBS) applied over a reinforcement sheet (polyester, fiberglass or a combination of both). Because the bitumen is built into the sheet, adherence is conducted by heating the bitumen to an adequate softening point. Adhesion was traditionally completed with heated asphalt on SBS sheets or by torch fusion on APP and some SBS sheets. A thin layer of applied SBS is all that is required to properly adhere the sheet. This is similar to the process of torch fusing SBS or APP sheets; the intent is to provide flow of the engineered modified bitumen over the substrate.
Modified bitumen sheets have been used in the United States for over 25 years. Initial application methods included torch fusing of the sheet or the sheets being set in hot asphalt. The technology of these materials has now evolved to include other attachment methods. This is primarily due to the fact that the waterproofing for modified bitumen systems is accomplished by the reinforcement in the sheet, not the bitumen.
The sharp increase in alternative modified bitumen application methods has been propelled by several influencing external factors. Environmental constraints and building owners’ laments regarding fumes have forced manufacturers and contractors to consider alternative attachment methods. The industry is experiencing labor shortages, and it is becoming difficult to properly man hot-applied projects in some regions of the country. Torch-applied systems are jeopardized by insurance carriers who are increasingly denying coverage to contractors that complete more than 10 percent to 25 percent (varies by carriers) of torch-applied applications per year. The regulations against torch use are due to fire and safety hazards associated with these types of applications.
The emerging technologies associated with modified bitumen applications are cold-applied systems and self-adhered systems.
Adhesives are now playing a larger role in roofing applications. The use of adhesives as an attachment for modified bitumen to the substrate (insulation or deck) has increased. This increase is due to the advancement in technology of adhesives.
The science of adhesive bonding has increased to a degree where adhesives must be considered an attractive and practical alternative for modified bitumen attachment. The main function of the adhesive in these configurations is to promote the bonding of the sheet. A properly formulated adhesive should not have an effect on the performance of the sheet. In this respect, it serves the same function as hot mopping or torch fusion.
The most common types of adhesives are solvent-based adhesives. These products are generally asphalt cutbacks, which are manufactured by adding solvents such as mineral spirits and varnishes to asphalt that has been heated to 150° to 200°F. The asphalt is heated to this temperature to make the asphalt fluid while maintaining enough heat to evaporate the solvents. Mastics and roof cements are manufactured by adding asphalt, solvents and organic filler, such as cellotuse.
Adhesives contain solvents that must evaporate to provide a solid or cured state. The solvent evaporation process, or flashing, as it is commonly referred to, depends to a large degree on the rate of material application and ambient weather conditions over time. The system adhesion increases as the solvent evaporates.
The most important aspect of a cold-applied system is that it must properly cure. Once adhesive materials cure, there is no change in their properties. The bonding mechanism occurs as the adhesive dries to a thin film as the solvents evaporate.
There are currently environmental concerns over the use of petroleum solvents in cold adhesives. In some parts of the country, regulations have been imposed on the use of volatile organic compounds (VOCs). This had led to the reduction of solvent content in these adhesives, which can produce a material that is too thick to spread well. Modified sheets require the use of solvent-based adhesives. Waterborne adhesives do not provide proper adhesion for modified bitumen sheets. Some manufacturers currently produce low-solvent content adhesives for use with modified bitumen sheets, and this trend will continue as the technology advances.
Solvent-based products can be applied over a wide range of temperatures including cold or marginal weather conditions. The adhesives cure quickly under conditions of high humidity.
Adhesive bonding presents several distinct advantages over other conventional methods of attachment. One significant advantage is that very little equipment is required in the application of the adhesives. Most of the work can be completed with squeegees, trowels and brooms. Many of the cold adhesives can be applied with spraying equipment, which further reduces labor costs and simplifies the installation process.
Small work crews can be employed; oftentimes crews of three to four workers are sufficient to complete a substantial roof area on a daily basis. The increased rate of application can be attributed to ease of application and the fact that there is no significant downtime. Application rates increase as setup time, equipment takedown time, equipment malfunctions and idle time waiting for proper material heating (in the case of hot-applied systems) are all eliminated.
As with all roofing and waterproofing systems, the success of the system depends to a large extent on proper application methods. The relative ease of application with these systems promotes quick and efficient workman training. Although these materials can be improperly applied, they do not rely on workmanship as much as conventional BUR systems. There is no under-heating or overheating of the materials, and the materials tend to be forgiving of inaccuracies. Due to the curing process, which can be time consuming depending on weather conditions, improperly applied sheets can be adjusted by the applicator during the installation process. This can reduce the cost of maintenance and repair that is required in correcting initial application errors. In contrast, the curing time and the sensitivity of the materials can be burdensome in colder climates.
Some of the advantages of adhesive use in roofing applications include the following items:
- Minimal application equipment.
- Easy delivery and storage at the jobsite.
- Smaller crews that can be easily trained are all that is required, resulting in lower labor costs.
- Reduced setup time and no pre-melting of bitumen.
- Equipment removal is minimal.
- Higher safety rates due to no open flames or hot kettles.
- Repositioning of improperly applied sheets is easy.
- Ease of application in complicated, equipment-laden areas.
- Environmentally safe application.
There are also some disadvantages, which may make adhesive bonding impractical in some roofing applications. Ironically, one the advantages of adhesives may also be a disadvantage. The relative ease of application and the labor savings that may be provided has initiated a growing trend of applications completed by inexperienced applicators. In some instances, building owners, in an effort to save costs, have assembled their maintenance personnel to apply these materials. More often than not, these applications failed due to lack of proper training, supervision and the basic understanding of how to properly apply these materials.
Another disadvantage of these systems is weather constraints. Although all roofing and waterproofing systems have weather constraints, imminent failure is more probable with these materials. The applicator must have knowledge of the weather constraints of the materials and the allotted curing time. It is imperative that the materials are not applied when there is a chance of precipitation or inclement weather. Some adhesives and coatings may wash away if they encounter precipitation prior to proper cure. The temperature restrictions may also prohibit the applicator from working on a day that other systems may be applied.
Although these materials are relatively easy to install, the proper installation procedures must be completed for successful application. Some of the more common application errors include the following items:
- Entrapment of moisture in the insulation or membrane.
- Inadequate or excessive use of adhesives.
- Failure to properly set reinforcement sheets in the adhesive.
- Improper preparation of the substrate or insulation.
- Application of materials in weather conditions that are not suitable for application.
When they are properly applied, self-adhered sheets do not require the cure time typically associated with cold adhesive applications. These membranes can be applied directly to approved substrates and achieve immediate initial peel strengths to provide watertight laps at ambient temperatures. Most manufacturers require that the self-adhered membranes be applied when the ambient outdoor temperatures are above 50°F (10°C). Peel strength at laps is usually achieved in a few days through natural heating methods such as air, sunlight or any other external force.
Best practices require the membrane system to be applied in a multi-ply application (a minimum of two plies). Single-ply applications should not be considered anything other than a short-term solution.
Self-adhered modified sheets offer several initial advantages for contractors. First and foremost, they appear to address the current safety and environmental issues. The application of these sheets is completed with no heat and no fumes, which makes them relatively safe to apply. They are environmentally friendly because there are no volatile organic compounds in the material.
The material is easy to apply - no special equipment is required - and the application is less labor intensive than many other roof systems. Application is completed in a quick three step process. The sheet is rolled out over the properly prepared substrate. Once the sheet has been rolled out and adequately relaxed, the self-adhered sheets are installed by removing a separator sheet from the roll as the material is applied to the substrate.
Subsequent rolls are applied in a similar fashion and adjoined by overlapping end laps and seams in accordance with the manufacturer’s requirements.
The adhesion of the end laps can be reinforced by applying manufacturer approved adhesives or heat welding.
It is also advised that the applied sheet be pressed down with a heavy roller to assist adhesion. A specially formulated primer is traditionally applied over the bottom sheets in multi-ply applications. The primer increases the adhesion capabilities of the membrane sheet.
Like all roofing materials, self-adhered membranes are not without possible disadvantages. First and foremost is negative initial contractor perception. Most roofing contractors have been conditioned (usually by costly failures) to view new materials and systems with a critical eye. This is a wise decision. Many unproven and untested materials have passed through this industry on their way to the landfills. Laboratory and condition testing is regulated and rarely duplicates real-world conditions.
Lack of experience in this country will be an initial deterrent. Although some manufacturers have claimed up to 10 years of experience with these sheets, that has largely been in Europe, where climate conditions, construction methods and application procedures (and applicators) differ from those in this country. Also, to meet the fire codes here in the United States, the formulations have been altered. We have all witnessed the effect this had on TPOs.
As with any new material, the contractors should investigate the manufacturer’s track record with these materials. Visit application sites, have the crews properly trained and start with small projects. Remember, the manufacturers have begun development of these sheets in response to industry and environmental regulations. The day may come sooner rather than later when these regulations mandate changes in the materials and methods we now employ.
In addition to contractor perception, there are some notable concerns that may develop during the application stage.
The success of the membrane is based in large part on the preparation of the substrate. The substrate must be clean, free of all debris and contaminants, and free of any moisture. If the membrane is applied over a wet surface or any points where moisture exists, the membrane will not fully bond. Spots of disbondment can transfer throughout adjacent areas and large-scale delamination can occur. Moisture, including dew, should be evaporated by torch or other drying mechanisms prior to application. Do not apply these sheets when precipitation is present.
Cleanliness of the substrate is also of critical importance. Dust, dirt or any other foreign matter that has collected on the substrate must be removed prior to application. Dirt particles can have an adverse effect on the self-adhered membrane sheets. The particles impede full adhesion and can similarly lead to chronic adhesive and cohesive failures at these points. Proper preparation procedures are required for the success of the membrane.
These constraints may limit the application of the membrane during periods of high winds, particularly on remedial roof projects where debris from tear-offs is present.
As mentioned earlier, there are also temperature restrictions with these membranes. The material should be applied when the ambient temperature is above 50°F (10°C). This obviously restricts application in many parts of the country throughout the year. Temperature restrictions are applied because the membrane does not perform in cold temperatures.
The self-stick blend becomes increasingly stiff and significantly reduces the points of contact. As a result, the membrane does not readily adhere to the substrate, and subsequent attempts to roll and re-roll the membrane into place can trap air between the membrane and the substrate.
Cold Process SystemsCold process systems consist of multiple plies of reinforcement sheets that are set in a cold adhesive (solvent-based or water based). These systems are often referred to as cold process BUR systems because the application procedures are similar to hot-applied BUR systems. The most common reinforcement felts are manufactured from fiberglass or polyester, which is either stitch bound or spoon bound.
One of the primary advantages of cold process roof systems is their relative ease of application. Very little equipment is required, and three or four workers can complete most applications. The systems can be completed as new or remedial construction or can be applied over existing smooth surfaced roof systems in an effort to extend the service life of an existing roof system.
The application procedures vary based on the manufacturer of the material that is used. It is essential that the applicator comply with the material manufacturer’s latest printed specifications during the application procedure.
The waterborne products are permanent roof materials, which are environmentally friendly because they do not contain solvents, organic or inorganic. Waterborne products use water and stabilizing clays other than petroleum or other distilled solvents.
Asphalt emulsions are manufactured by dispersing very small droplets of asphalt bitumen in water. The manufacturing process is to mix the bitumen at a heated temperature at which it will become a free-f lowing liquid (from 320° to 350°F) with a mixture of water (at boiling point 180°F) and an emulsifying agent or clay such as bentonite. The elevated temperature of the water blends with the asphalt and the water cures.
The manufacturing process of modified bitumen adhesive (SEBS) is similar to the emulsion process except polymers, such as rubber, are added to the asphalt. The modified asphalt blend is mixed with the water and clay stabilizers.
Waterborne products cure with heat - the greater the heat, the quicker the cure time. In contrast to solvent based products, humidity slows down the curing process.
Successful cold process adhesives (solvent-based and water-based) require a good asphalt bitumen blend to manufacture a good product, provided that the manufacturer also adds a good polymer. The asphalt used is refinery crude oil asphalt that has multiple blends of materials. The adhesive manufacturer can control the type of asphalt obtained from the refinery by specifying the properties the crude oils and blend asphalts must meet. The important physical properties are softening point, penetration, and ductility.
Higher-penetration asphalts and modified asphalts are softer than low-penetration asphalts. Asphalt testing can be completed by conducting ASTM D-36: The Ring and Ball Test.
As noted earlier, the adhesives used in cold-applied systems offer little waterproofing capacities. The waterproofing is provided by the reinforcements.
There are four types of reinforcements that are used in cold process systems:
1. Fiberglass felt plies.
2. Fiberglass scrims.
3. Spun bonded polyester mats.
4. Stitch bonded polyester mats.
Fiberglass felts were introduced in the United States in the late 1940s. They are used in cold process systems in the same manner as in BUR systems in multiple-ply configurations. Fiberglass felts are manufactured with filaments, which are derived from molten glass streams through tiny orifices that are made of precious metals. The molten- glass is taken from batches of sand, limestone, and soda ash, which are continuously deposited into a furnace that is heated above 2,500°F. The fiberglass filaments, which have long cross sections of 12 to 15 inches, are bound with a thermosetting binder, phenol-formaldehyde, urea-formaldehyde, or acrylic resin. Most fiberglass felts are isotropic, meaning they have equivalent properties in longitudinal and transversal strength.
The advantages of fiberglass felts include exceptional tensile strength (in both longitudinal and transversal directions), an inherent resistance to moisture, and a greater resistance to splitting than organic felts. When used in cold-applied applications, the fiberglass felt must be cut from the roll and laid in a relaxed state for approximately a half hour prior to application. The material’s natural memory tendency promotes ridging in these applications if this procedure is not followed. This condition occurs due to the fact that the curing process of cold-applied systems is substantially longer than hot-applied systems, which cure instantaneously.
Polyester has been well accepted as reinforcement in cold process systems. Originally developed in the United States, these reinforcements are made of mats that weigh within 1 to 3 ounces.
These reinforcements can be adhered with solvent-based and waterborne adhesives. Combining the cold process system with the physical attributes of polyester fabrics offer the following advantages:
- Exceptional elongation and recovery.
- Toughness and flexibility.
- Low shrinkage.
- Impressive tear strength.
- Puncture resistance.
- Resistance to moisture, chemicals and ultraviolet light.
- They are lightweight.
There are two common types of polyester mats used as reinforcements in cold process systems: spun bonded polyester mats and stitch bonded polyester mats. Stitch bonded polyester mats are manufactured with fibers that vary in size from 1 to 3 inches, which are formed into non-woven fiber mats.
A chemical binder is added to the fibers to improve the strength of the mat. The result is a non-isotropic (unequal characteristics in all directions) tissue with a ratio between longitudinal and transversal strength of 80:20 to 60:40.
Spun bonded polyester mats are manufactured from endless filaments that are spun and immediately laid into a non-woven mat. The mat can than be stitched together and reinforced by a binder or reinforced by thermally melting the filaments together. The result is an isotropic tissue, which means that the properties in all directions are nearly equal.
The name “cold-applied” is derived from the fact that no torches or hot bitumens are required for application. It does not suggest that the material can be applied in cold temperatures. Cold applied products have their limitations, just as hot-applied BUR systems do. The materials should not be applied in weather that is not suitable for proper installation. The applicator should have sufficient knowledge of the weather constraints of the products they are working with. Particular attention must be paid to ambient weather conditions and the threat of inclement weather, rain or snow, or if there is any trace of precipitation.
The main effect that ambient temperatures will have on the material application will be in dictating cure time. Waterborne products cure faster in heat; solvent-based products cure faster with high humidity. Both types of adhesives provide immediate waterproofing capacities. The cure time determines when the system is capable of providing sufficient wind uplift ratings and prevention of damage from foot traffic. The cure time for both types of adhesives is extended in colder temperatures.
Obviously, the waterborne products are temperature sensitive and should not be stored in freezing climates or applied in temperatures lower than 50°F (10°C). The same limitations generally apply to solvent-based adhesives. The viscosities of the adhesives vary with temperature, and the more the adhesive’s temperature drops, the thicker (more viscous) it becomes. The thickness of the material will not allow proper coverage and will have deteriorating effects to the performance of the system and add cost to the project.
The contractor will ultimately spend more money to do a bad job. Unlike waterborne adhesives, which cannot be installed in cold temperatures due to the freeze element of the water, solvent based adhesives can be applied in lower temperatures if the product is properly stored in a heated location just prior to application. The application of these products is more difficult in colder temperatures than in warmer temperatures. Adequate curing time is required. As always, the applicator should follow the material manufacturer’s requirements regarding material application in cold weather.
Because cold-applied adhesives have different physical properties than hot-applied bitumen, their application procedures are divergent. Cold adhesives cannot be perceived as a cognate substitution for hot asphalt or torch installations. The differences in application requirements and techniques must be examined carefully to ensure a quality installation.
Applying the adhesive at the proper application rate is the most important criterion of a successful installation. Applying too much or too little of the adhesive material is the most common application problem. When the adhesive is applied at rates below the recommended coverage, the material may cure too quickly. In this case, the adhesion strength of the material is weakened and the performance of the system decreases.
Applying the adhesive at rates above the recommended coverage results in a membrane that may not fully adhere to the substrate because it substantially slows the curing process. In effect, the thicker material will cause the membrane to “float” on the uncured adhesive rather than fully adhering to the substrate. This is essentially the same effect as applying the adhesive at lower than specified temperatures. Solvent-based adhesives become thicker at lower application temperatures. It is imperative that the applicators understand that in these applications more is not better.
It is known that controlling the coverage rate of the adhesive produces the desired thinner, more uniform and continuous adhesive application. Controlling the material’s temperature is also critical in achieving a uniform and proper coverage. For instance, if the required coverage rate of the material is 1.5 gallons per square and the applicator applies the adhesive at a rate of 2 gallons per square, the adhesive use is increased by 25 percent. This will not only add substantial costs to the project, it will add approximately 50 percent more time to the proper cure of the material.
To ensure that the material is applied at the proper coverage rate, the application area should be measured. Calculate the amount of material that is required to cover the area, and then apply that amount of material.
For instance, if the required coverage rate is 1.5 gallons per 100 square feet and the application area is 50 by 20 feet or 1,000 square feet, the amount of material required is 15 gallons.
Application Area:1,000 sq. ft. / 100 sq. ft. = 10
Coverage Rate:1.5 gallons x 10 = 15 gallons
As applicators gain experience with the use of the material, they will obtain a visual appearance of the proper coverage rate. However, until familiarity with the application rates becomes apparent, it is recommended that applicators follow this time-proven application technique. Referring back to the above example, the applicator should divide an area into 3 equal parts, place one 5-gallon pail in each area and apply the full contents of each pail into each area.
The adhesive should be applied in a continuous even application throughout each area leaving no voided areas in the substrate. The membrane is then fully embedded into the adhesive in accordance with the material manufacturers latest printed requirements.
The application of the membrane is also slightly different than in hot mopped or torched systems. The initial adhesion strength - commonly referred to as “green strength” - of the adhesive is not as strong as the initial adhesion strength of hot bitumen. It takes a significantly longer period for the volatiles to f lash off from the cold adhesive than it takes for hot bitumen to cool off and set, which is almost instantaneous. Due to this fact, it is imperative that the applicator avoid walking and trafficking over the newly applied membrane for a significant time period; in some instances it may be a couple of days.
The detrimental effects of trafficking over a wet and uncured cold process system may be displacement of the membrane or displacement of the adhesive, leaving voids within the system.
Ironically, the slow cure rate of the adhesive can be advantageous, as displaced or improperly installed membrane sheets can be easily repositioned to their proper place within the system for a long period after the initial application.
As with all roofing and waterproofing projects, it is recommended that the applicator meet with the project designer and material manufacturer prior to the application procedure. All of the system component materials and application procedures should be reviewed to ensure that all parties are familiar with the system criteria. It is important that all of the parties agree with all application procedures and detail requirements prior to system installation. The differences in material manufacturers’ requirements and specific project constraints warrant that these meetings occur prior to all projects. Unnecessary delays and/ or confrontations during the project can be avoided through this meeting.
Cold-applied adhesives can be applied using spray equipment, squeegees, brushes and trowels. The proper application procedure for the specific manufacturer’s material should be agreed upon at the outset of the project.
If spray equipment is authorized for use, the applicator should train the work crew in the proper use of the equipment. It may be beneficial to have the work crew complete a test area over the existing system in the presence of the required parties. This will give all parties the opportunity to visually inspect proper coverage rates and rectify problems with the equipment.
The work crew should be properly trained in all aspects of cold adhesive system applications. Most importantly, the coverage rates for the project manufacturer’s materials, particularly over different substrates, and methods to control the rates should be reviewed.
The coverage rate recommended varies with the different manufacturers; the coverage rate also varies based on the substrate. Typically, perlite or wood fiber insulation will require more adhesive than a smooth base sheet, polyester or fiberglass felt, or polyisocyanurate insulation.
The work crew should understand the proper method of membrane application. Crew members should be told whether or not the sheets must be cut and allowed to relax prior to the application into the adhesive, how the roll should be set in the adhesive and if the roll must be broomed in place. The work crew must also be trained in proper seam adhesion.
With multiple plies of polyester or fiberglass felts, the adhesive forms a continuous seamless application. Some modified bitumen manufacturers require heat welding or torching to fuse the membrane seams, whereas others only require adhesive application at the seams. Due to the temperature constraints of the materials, the work crews should also know how to properly store and handle the material being used.
During the course of the project, quality control can be conducted by monitoring the daily temperature and the coverage rates of the adhesive. It should be stressed that foot and equipment traffic be avoided at new application areas and strict housekeeping measures should be implemented. In particular, there should be no storage of material or equipment in new areas. When using modified bitumen sheets, all of the seams should be inspected for proper adhesion.
There are two diverse application methods: manual application using a squeegee, roller or brush; and mechanical application using spray equipment. It should be stated that there are systems that allow the adhesion to be completed with hot air welding. Although this is an approved attachment method, it is outside the scope of cold process systems in analysis and will not be included within this text. No matter what type of application procedure is utilized, the preparation process.
Torch-applied systemsare under increasing scrutiny from insurance carriers due to fire and safety hazards associated with these types of applications. is similar. First, establish that the substrate and the adhesive are compatible. This includes substrates in remedial construction (recovers) and new construction.
In new construction, the substrate could be insulation, the structural deck or a base sheet. Make certain that the adhesive is approved for use with these specific materials. The same standards apply for remedial or recover applications. Preparation will be more critical and time consuming on remedial applications.
On smooth surface roof systems, thoroughly brushing or power washing the surface is required to remove all dirt, dust and debris. All loosely adhering material should be detached. Some manufacturers require a surface primer prior to adhesive application. On aggregate surfaced systems, the aggregate has to be fully removed prior to adhesive application. The applied substrate must then be thoroughly cleaned in the manner described above. To avoid any costly mistakes, it is recommended that the applicator clarify all preparation procedures with the material manufacturer prior to application.
Manual application refers to the application of the adhesive with a squeegee, roller, or brush. The first process is to thoroughly mix the adhesive to provide a liquid texture. The adhesive can be mixed by machine mixing or hand mixing. The adhesive is then poured directly in front of the blade or bristles of the spreading equipment and evenly and thoroughly spread throughout the area to the required thickness. The application rate should be in accordance with the material manufacturer’s latest printed specifications.
The adhesive material should be applied in an area slightly wider than the felt roll width. There should not be any voids in the material application over the substrate; 100 percent coverage is required.
Applying the adhesive with a mechanical spraying machine is a relatively faster method of application. The adhesive is evenly applied over the substrate at the required mil coverage. The applicator should use a wet gauge to verify mil thickness. As with manual applications, the adhesive should be applied over the substrate at a 100 percent coverage rate. After the adhesive application, the membrane sheet is rolled out and firmly embedded into the adhesive. The membrane application should concur with the previous application procedures and within the material manufacturers requirements. Again, it is essential that adhesive squeeze out around the end lap; this provides verification that the proper amount of adhesive was applied.
Curing Time Period
In hot-applied roof systems, the curing time is almost instantaneous. With cold-applied systems, the curing period is determined by the type of material composition and weather conditions. The time period can be extensive in some instances. The applied membrane could be in a “delicate” condition prior to full cure. Therefore, the applicator must be cognizant of these conditions. Foot and equipment traffic should be avoided at this time to prevent damage to the membrane surface or misalignment of the membrane.
ThermoplasticsIn the last decade, thermoplastic systems have a gained a significant market share of the U.S. commercial low-slope roof market. The increase is due to advancements in technology and corrections to initial manufacturing and formulation concerns. The other main advantage of Thermoplastics is that they have the inherent ability to be heat welded or fused together at the seams in an easy and efficient manner.
The seams are very dependable because they are as strong as, if not stronger than, the sheet itself. The seams at the tops and bottoms of the sheets are fused together with a homogenous bond that makes these areas stronger than the membrane. This characteristic makes thermoplastics a desirable candidate for roof projects with multiple penetrations where there is a high ratio of lap-seam length to roof area. Properly heat welded thermoplastic seams can provide the same fusion characters as welds in structural steel. This makes these typically vulnerable areas watertight for an extended period of time, increasing the performance life of the roof system. Typical heat welding is completed with hot compressed air at temperatures that range from 500° to 600°F (260° to 315°C).
The primary advantages to contractors are that this process is quick, easy to learn, and reduces labor costs. The fact that hot air welding is less labor intensive than other means of seam application is important because the roofing industry is continually facing a dwindling labor base.
Thermoplastic Olefin (TPO)
The newest entry into the thermoplastic market is thermoplastic olefins (TPOs). These membranes were introduced within the last five years. Thermoplastic Olefin sheets are produced using polypropylene (PP) or polyethylene (PE) polymers. Most membranes supplied in the United States use polypropylene. In polypropylene- based membranes, the polymer is blended with thermo set polymers (elastomeric polymers of EPR) to produce a sheet with characteristics of high flexibility and mechanical strength. The polypropylene-based materials are typically stiff or rigid sheets. The raw materials consist of reactor polymers. A typical formulation for polypropylene-based thermoplastic olefins is approximately 70 percent ethylene-propylene rubber and approximately 30 percent polypropylene.
Propylene-based sheets exhibit better attributes in mechanically attached systems. Polyethylene-based materials consist of special grades of copolymers and terpolymers of polyethylene that are blended together to form a highly flexible polymer alloy. The terpolymers consist of acetates, acrylates or octene. The flexibility of the sheets allows for easier contractor handling.
Thermoplastic olefins have inherent chemical resistance properties and can endure most animal fats, vegetable oils, microbial attack and some acids. They are inorganic materials that do not contain oils or compounds that would support biological growth, thus lowering the possibility of algae or fungus attack. The material contains more than twice the polymer and less than half the filler of EPDM, which makes it chemically resistant to the same types of materials as EPDM. The low filler content ensures very low water transmission and enhances long-term performance. Thermoplastic Olefins do not have the same chemical resistance abilities as CSPE and some PVCs.
Thermoplastic olefins were introduced on the United States roofing market in the early 1990s and, as with all developing technologies, such as PVC, there were initial problems. The original membranes were manufactured without reinforcements based on the rationale that the inherent stability of the sheet eliminated the need for any reinforcing fabrics. While it has been proven that thermoplastic membranes have a high degree of stability and a very low shrinkage factor, there is also a high degree of elongation when the material is heated. The heating of the membrane created two initial problems.
The first problem was that membrane installed in the winter months became severely wrinkled in the summer months when the material heated up. The second problem was that the membrane would become wrinkled during the welding of the seams due to the high temperature of the material at these areas.
The addition of the fabric reinforcement has curtailed a number of these problems. The current thermoplastic olefin membranes are reinforced with glass fiber or polyester mats that provide a number of physical benefits. The benefits include high tensile strength, high puncture and tear resistance, excellent dimensional stability and minimal shrinkage. In comparison to EPDM membranes, reinforced thermoplastic olefin membranes provide up to 40 percent greater tear strength and as much as three times greater breaking strength. The flexibility of the membrane allows for significant structural movement without splitting or cracking. Industry research indicates that polyester reinforcements provide higher tensile strength than glass fiber reinforcement.
Even with the reinforcements, the membranes are prone to dimensional instability. Polypropylene based TPOs have better dimensional stability than the polyethylene based sheets. However, the coefficient of expansion and contraction of both of these polymer-based sheets is limited. The reinforcement sheets improve the puncture resistance and tensile properties of the sheets, but they do not eliminate thermal movement that occurs from temperature changes. Dimensional instability of the membrane may result in difficulty of heat welding the seams at longitudinal edges or loose and/or tight membrane between the fastened seams on mechanically attached systems. This condition makes the membrane susceptible to wind damage.
Thermal expansion also limits the membrane’s ability in adhered membrane systems. The adhesive bond between the membrane and the substrate could be compromised if the adhesive is not fully compatible with the membrane. There has been difficulty in selecting adhesives that provide the adhesion required to bond these membranes. Manufacturers have found that not all common adhesives work. There are also environmental implications when using solvent-based adhesives over water-based adhesives, which have not fared well in these applications.
Another significant problem was the initial membrane’s negative reaction to ultraviolet radiation. Some of the early thermoplastic olefin compounds did not have the proper ultraviolet stabilizers, which resulted in substantial fractures in the membrane, particularly at the seams. Due to the fact that the material resembled PVC, many roofing authorities improperly diagnosed the cracks as conditions created by the loss of plasticizers.
Thermoplastic olefins do not contain plasticizers. Instead, the problem was caused by improper formulation of the material’s 30 percent polypropylene content. For the polypropylene to be effective in this formulation, an ultraviolet stabilizer is required. The current thermoplastic olefin membranes include ultraviolet stabilizers and offer stability superior to other thermoplastic materials.
There have been reports that some of the membranes have failed within five years of application because of polymer degradation and separation of the polymer compounds from the reinforcements in the sheet. In these membranes, the compounds lose elasticity, the sheet hardens, and the reinforcement peels away.
This occurs due to improper lamination during the manufacturing process. Not all of the initial problems were caused by improper material formulations.
Some of the problems were associated with improper workmanship. This could be expected with use of a new product, as there is a learning curve for both the manufacturers and applicators. There were also reported problems with improper use of the materials. For instance, a non-reinforced membrane that was designed for use in a ballasted (covered) application was used in mechanically attached (exposed) applications. Since the material did not provide the proper ultraviolet protection intended for these applications, the membranes ultimately failed.
Self-Adhering TPOsRecently, some thermoplastic manufacturers have begun producing self-adhered TPO membranes. A specially formulated, factory-applied adhesive is set on the bottom side of the membrane during the manufacturing process and covered with a release film. The application concept is similar to self-adhered modified sheets. The substrate is cleaned and prepared for installation; the membrane is then unrolled and positioned for proper placement. The release film is removed from the roll as the membrane is adhered to the substrate and all edges, seams and the field of the roof are rolled for additional adhesion.
The primary advantages to these types of applications are that they are solvent free and there are no volatile organic compounds contained in them. Environmental issues concerning the use of solvents in certain parts of the country, particularly with the use of single-ply bonding adhesives, have forced single-ply manufacturers to investigate the use of alternative attachment methods for fully adhered applications. The other advantage to this attachment method is that it provides for more uniform application rates, eliminating insufficient adhesion and reducing the chance of adhesive-related wind uplift problems.
This application method is also less labor intensive and quicker to install because there is no waiting time associated with adhesive curing. One manufacturer states that these types of applications are up to 50 times faster than taped seam applications, up to five times faster than water-based adhesives, and up to three times faster than solvent-based adhesives. Another advantage is that this method of attachment eliminates the chance of airborne particles contaminating the membrane application, a continual concern with fully adhered single-ply applications.
It should be pointed out that both TPO formulations and self-adhered TPO membranes have less than 10 years of experience in the U.S. market. Although the technology appears sufficient, the long-term capabilities of these products are unknown. These facts should be considered during the selection process.
Meeting Future Challenges
The material and application changes will continue in the roofing industry as we try to meet the challenges of the 21st century. The industry has faced these challenges in the past, and, with resiliency and improved technology, we will meet these challenges again. The new technologies provided here are just a few of the ways that roofing materials and application procedures are changing to conform to current regulations and constraints.
For the best results, the contractor should apply these membrane systems in temperature conditions approved by the manufacturer and closely follow the manufacturer’s application requirements. Time and experience will determine the fate of these membranes and attachment methods in the U.S. commercial roofing market. In my opinion, it is too early to provide any conclusive decisions regarding these systems. These materials and attachment methods promise many advantages, including reduced environmental impact, lower insurance costs, and fewer labor regulations, but these benefits will only prove substantial if these systems can provide long-term waterproofing capabilities. If these materials and attachment methods prove credible over time, they can be beneficial to contractors’ bottom lines.