Polyurethane Adhesives and Insulation

Use of Polyurethane Adhesives and Insulation in the Roofing Industry

Riku Ylipelkonen, Technical Service Manager, Polyset Roofing Adhesives, ICP Building Solutions Group

Otto Bayer is credited with developing polyurethane in 1937. Original development applications were related to aircraft coatings for World War II and flexible foams. Commercial applications began later in 1952 and grew steadily as polyurethane foams became a viable choice for rigid foam buns and molding parts where the reaction occurred in the mold for the finished part. The advantages of polyurethane foam (PUF) helped grow many applications and uses. High heat resistance and long-term stability due to the polymer being a thermoset plastic led to the development of PUF adhesives in roofing applications.

Polyurethane is a reaction polymer, where isocyanates react with polyols and result in a thermosetting polymer. The main feature of thermosetting polymers is that they do not melt or breakdown with the application of heat. For example, when eggs and flour are combined and cooked, a pancake is made. The pancake cannot be returned to eggs and flour by heating it up. The opposite of thermoset polymers is thermoplastic polymers. Thermoplastic polymers will return to their original state if heated. As an example of this, an ice cube will become water if it is heated. However, if the water is returned to a freezer, it will become an ice cube again.

The other benefit of thermosetting polymers is the long-term stability of the polymer. However, the stability of polyurethane polymers is mainly susceptible to exposure of ultraviolet (UV) light. When PUF is exposed to sunlight, which contains UV light, there is a yellowing discoloration that occurs over time. The higher energy of UV light can also cause reactions that break-up the PUF and flake the exposed foam off as exposure time is prolonged.

Prevalent uses of PUF in the roofing industry have slowly grown over the years. The applications that are specified have used different polyurethane formulations which vary based on the raw materials used. Sprayed polyurethane foam (SPF) roofs are popular and substantial low slope roof applications that use the physical properties of polyurethane more as an insulation than an adhesive. Raw materials are varied to result in a high closed-cell content that create excellent insulation properties. The use of spray equipment, where the reaction is controlled on the rooftop, helps to minimize preparation process to the job site. The top coating is used to minimize sunlight and UV light exposure, to maintain the physical properties and waterproofing qualities of the finished roof.

A growing application for SPF is as an adhesive for roofing materials. On low slope roofing, these adhesives are used to adhere common building materials to make the final roof assembly. On steep slope roofing, SPF adhesives are used for adhering clay and concrete roof tiles. SPF can be advantageous for several reasons. There is no need for high temperature materials like hot asphalt that cost money to heat and transport to the installation site safely (crew safety, property destruction and insurance considerations). SPFs are usually low in volatile organic compounds (VOC), reducing site complaints of fumes during application, environmental considerations for the project and simpler, less costly personal protective equipment (PPE) for installers. SPF adhesives can also minimize

or eliminate the need for mechanical fasteners and roof penetrations, to reduce the likelihood of leaks, energy loss through heat conduction of metal fasteners and the amount of labor required to install materials.

For roofing applications, the PUF options fall into two main categories, one-component (1K, moisture-cured) and two-component (2K, chemical-cured) systems. The advantages of 1K systems are usually ease of use, cost and easier to have multiple systems being used on the roof at the same time. To take full advantage of a 1K product, the isocyanates and polyols are packaged into the same cylinder under pressure and come out of the dispensing equipment as a properly ratioed foam ready to be applied to the materials to be adhered. This eliminates the ratio calibration checks and periodic checks for properly curing foam that most manufacturers require with a 2K product. This allows for simpler operation of the product dispensing system and less packaging than a comparable 2K product. 1K products are usually produced in smaller packaging to allow multiple kits to be used at the same time on a roof with a larger number of qualified applicators in order to get more squares done per day. The tradeoffs for 1K products include that they require ambient moisture to facilitate the curing of the foam, lower values of physical properties than 2K gram per gram and the need for the 1K PUF to be compressed into final place for developing a compatible bond with substrates. 1K will need to be compressed between tile-to-tile applications and cover board-to-insulation layer applications to bite and form a bond to whatever substrate the product is applied. The 1K products also cure from the outside in, which limits how much adhesive can be applied to one location while having the ability for the foam paddy or bead to completely cure to designed physical properties characteristics. The last consideration for 1K components is the limitation of ambient and substrate temperatures. Because the 1K needs moisture and compression to cure, it favors warmer and more humid ambient conditions to facilitate a complete cure.

Two-component systems are favorable when physical properties (like tensile strength) need to be maximized, wider temperature ranges for ambient and substrate temperatures are required and for larger size jobs requiring faster application rates, larger containers of product and more uniform physical property characteristics in the final product without the presence of ambient moisture. 2K products can be fine-tuned to cure quicker, grow more during application and have a specific set of physical properties for the desired application. 2K products are better at expansion to a final size to ensure good contact area under roof tile applications and to account for deck undulations when laying insulation or cover board on an existing roof deck. Since the formulations of 2K products have catalysts in the formula to ensure proper and desired cure, ambient moisture is not a concern and a wider range of ambient and substrate temperatures can be utilized. This also ensures that the 2K products cure from the inside out, so large volumes and increased open times can be taken advantage of if the application benefits from it. This also helps the 2K products to be more uniform in final cured form, possess better cell structure in the body of the foam paddy or bead and they don’t need applied compression to form bonds with the substrate. The trade-off for 2K products also should be considered. Qualified applicators need to make sure the dispensing equipment is set-up properly to prevent crossovers from kit to kit, dispensing equipment must be kept clean, flowing and purged at wait times to prevent any clogs or cured foam within the lines. This could ruin the entire kit if disposable pre-pressurized kits are being used. Depending on the dispensing system, ratio calibration and periodic ratio checks are also required to maintain quality of the job and to catch changing variables like temperature and pressures of the separate components. Also, the chemical must be kept within the recommended temperature range; cold jobsites could ruin productivity if the product is too cold to use. However, if the 2K products are properly set-up, dispensed and applied, the final product will be stronger, faster to apply and more profitable for the roofing contractor.

Product offerings for polyurethane adhesives and insulation are growing constantly in the roofing industry. The list of companies that are offering these products is growing as well. When looking at the options for products and companies, there are benefits to looking out for companies that have knowledge of the roofing applications, have local personnel to support the contractors on the jobsites and provide