Recent advancements in polymer technology have led to the development and use of thermoplastic olefins for roof membranes.

Although polyolefins have been used in a number of waterproofing applications throughout the years, the material's stiff properties initially made them unsuitable for roofing applications. They were primarily used in unexposed applications such as pond liners, waste disposals and open tunnels because they did not posses the proper stabilizers to perform in exposed applications.

Recent advancements in polymer technology have led to the development and use of these polymers for roof membranes. The thermoplastic polymers (polypropylene or polyethylene) are combined with flexibilizing comonomers (elastomeric polymer) in a reactor during the fusion of the polymers. This combination of polymers adds flexibility to the thermoplastics. The recently developed technology has allowed for additives to be integrated with the base polymers, such as ultraviolet stabilizers, pigments (coloring), and fire retardants, to provide the weathering characteristics required in a roof membrane.

Flexibility is achieved through an internal plastification that occurs in the reactor process with the other polymers. No plasticizers are added to thermoplastic olefins; plastification is achieved internally during raw material production through the use of comonomers. In this process, the comonomers create an enlarged free volume that decreases the crystallinity of the polypropylene and polyethylene polymers. Therefore, there is no loss of plasticizing agents from thermoplastic olefins (TPOs).

The TPO polymer is heated and mixed with a custom blend of fire retardants, pigments and ultraviolet stabilizers. Unlike most thermoplastic materials, TPOs do not contain any plasticizers. This ensures that the brittleness point for the TPO membrane is relatively low, creating a consistent product that should retain the same level of flexibility in all temperatures. Depending on the formulated proportions of the polymers, the flexibility of the membranes varies by manufacturer. The most flexible sheets are the easiest to install.

Most membranes supplied in the United States use polypropylene. In polypropylene-based membranes, the polymer is blended with thermoset 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 of ethylene-propylene rubber and approximately 30 percent of 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.

At the present time, there is no specific ASTM standard for the formulation of TPO membranes. This has led to confusion over the classification of membranes within this category. The current ASTM standard - ASTM D 08.18 - states only that the membrane sheet shall contain the "appropriate" polymers. This vague definition opens itself up to an assortment of chemicals that could fall within this standard, and has resulted in an array of chemical formulations. Industry research indicates that there are a number of formulations for the current membranes available in the United States. ASTM is developing a standard specification that, once completed, will be similar to the standards developed for PVC and EPDM.

The products can range from soft and flexible to hard and rigid depending on the distance between the polymer chains. Generally, the closer the polymer chains are the more rigid the material is, the further the polymer chains are apart the more flexible the material becomes. The distance of the polymer chains also affects the ability of the product to absorb oils. As the distance of the polymer chains increases, the product absorbs more oils causing the volume and weight of the material to increase and swell. Flexible membranes have the ability to absorb a high percentage of weight of oils, which will change the physical properties of the membrane (swelling) and have little effect on the physical properties of membrane.

TPOs 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. TPOs do not have the same chemical resistance abilities as CSPE and some PVCs.

TPOs were introduced in the United States in the early 1990s and as with all developing technologies, there were initial problems. The original membranes were manufactured without reinforcements based on the rational 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 TPOs 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 TPOs 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 faired well in these applications.

Another significant problem was the initial membrane's negative reaction to ultraviolet radiation. Some of the early TPO 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.

Membrane Application

The standard thicknesses for TPO in the United States are 0.045 inch (1.1 mm) or 0.060 inch (1.5 mm). One manufacturer produces a sheet that is either 0.048 inch (1.2 mm) or 0.072 inch (1.8 mm). ASTM D 08.18 does allow for thinner membrane thickness than the standard .045 mil and some sheets are within a range of 30-35 mils. However, there are performance liabilities with the thinner sheets. Thin sheets cannot provide sufficient resistance to abrasion from even minimal foot traffic, and are more susceptible to reinforcement separation and deterioration from moisture entry into the sheet. It is also important that there is a consistent polymer compound coating over the reinforcement. As with PVCs, thicker is better.

There are several attachment method for TPOs: ballasted, mechanically attached, fully adhered, or protected membrane systems.

Fire Resistance and Environmental Compliance

One of the primary benefits of TPO is that it can be an environmentally friendly and fully recyclable material. If the membrane sheet is properly formulated it will not pose any environmental hazards and is well suited for landfill disposal, recycling or incineration. There are no environmental concerns with the base polymers and all of the raw materials and base additives are non-hazardous. Furthermore, all of the material components have been on the market for a number of years and there is no expectation of unknown hazards. There are no noxious fumes during installation and no migratory plasticizers that can leach out and into the local water supply. Since all of the components used are in the form of granules or powders, there is no chance of contamination of the environment from accidental spillage.

Unlike PVCs, TPOs do not contain chloride, which also means that they do not have inherent fire resistance capabilities. This is not a concern on ballasted applications because the gravel layer provides the necessary fire protection for the system. This is not the case for TPO membranes used in unprotected or exposed membrane applications, such as on mechanically attached or fully adhered systems, which require fire retardants. Therefore, to meet the stringent standards required to achieve UL Class A fire resistance listing, specially blended fire-retardant chemicals must be added during the compounding process. The selection of the chemical additives is critical from a standpoint of fire resistance and environmental compliance.

For instance, it is reported that some manufacturers have added halogenic materials such as bromine compounds and antimony trioxide to the base polymers to increase the fire resistance capacity of the membrane sheet and to comply with UL requirements. Unfortunately, these chemicals could have negative effects on the UV resistance and thermal stability of the membrane sheets, decreasing the service life of the system. In addition, they could reduce the chances of recycling or incinerating the membrane at the end of its service life, due to environmental concerns with these chemicals.

From an environmental perspective, it is recommended that non-halogenated materials, such as mineral hydrate, be used as flame-retardants. It has also been found that polypropylene-based membranes using bromine flame retardants have substantially lower ultraviolet resistance than the membranes that use mineral hydrate flame retardants. Bromine-containing membranes are subject to dramatic crack formations a short time after initial membrane crazing. This significantly reduces the service life of the membrane as a roof covering.

A properly formulated TPO membrane (with non-halogenic fire retardants) does not require landfill disposal at the end of its service life. These materials can be recycled or incinerated. Most manufacturers currently recycle waste produced during manufacturing for use in other materials. Waste produced in the field during the application process can be saved by the contractor and supplied to the manufacturer for similar recycling. Most manufacturers are currently evaluating means of recycling the membrane once it has reached the end of the service life. At this time, most of this research is being conducted in Europe, which is significantly farther ahead in environmental regulations and product development than the United States.

The membrane can also be incinerated. This is possible because the combustion by-products extracted during incineration are only water and carbon dioxide. Studies have indicated that the inorganic additives of the membrane do not volatize during the incineration process so there is no germane air pollution. TPOs produce less toxic agents and environmental pollution during incineration than typical residential refuse. During incineration the material is transformed into a form of slag, which can be deposited in a landfill with no adverse environmental effects.