by Martin Ruch and Jim Collins, PhD, PE
Epoxy types vary widely, and specifiers need to note such attributes as nozzle time, gel time, load time, sag, cure time, and chemical resistance—each property will affect what makes the product ideal for the intended application.
When searching for the proper epoxy to specify, it is important to note there are three general classes of epoxy—pure epoxy, polyester resins, and epoxy acrylates—that break out in different ways with respect to properties and performance.
Pure epoxy is typically just a resin and a hardener. Pure epoxy cures at a slower rate than the other product classes (polyesters and epoxy acrylates) and, as a result, it offers less shrinkage, excellent adhesion, and high strength performance. However, with slow cure times, pure epoxy should not be specified for low-temperature applications (generally limited to a 4-C [40-F] minimum substrate temperature). Further, this material should not be specified for situations where it will be loaded as quickly as the other two epoxy product categories. For instance, in overhead installations, it would not be the first choice.
Pure epoxy components are mixed at relatively close mixing ratios (i.e. 1:1, 2:1, or 3:1). They can usually be identified if the dual cartridges are the same size.
Unlike pure epoxy with its slow cure times, polyester resins cure through polymerization that is relatively fast—this means such products can be specified for lower temperatures (down to as low as 2 C [35 F]), and contractors who use polyester resins will find they can be loaded much sooner after being installed. Polyester resins are ideal to specify in concrete masonry unit (CMU) wall construction.
The third choice, epoxy acrylates, offers specifiers the best features of pure epoxy with those of polyester resins. Epoxy acrylates resins quickly cure, but offer the good chemical resistance properties of pure epoxy.
Epoxy acrylates can also achieve high characteristic loads and they can be specified in applications involving damp substrates or relatively low temperatures. On the other hand, they are a ‘stiffer’ epoxy, making them inappropriate to be specified for applications involving cracked concrete. The components of an epoxy acrylate are mixed in larger mixing ratios (i.e. 10:1), which are easily identified in side-by-side cartridges.
Another epoxy product in product catalogs is ‘vinylester’—a polyester/epoxy acrylate formulation. However, it really is just a term used by marketers, so it is important to check the manufacturer’s literature to know exactly what one is getting.
Color does not matter for any of these epoxy classes. Generally, the industry standard is a white resin and black hardener, but these colors are not important for performance—the resulting gray, without streaks, serves as an indicator the components have been proportionately mixed.
What makes epoxies different?
Epoxy types in each of the three classes named above offer wide-ranging performance traits. If the intended end use is to install overhead bolts in an indoor pool application, for example, specifying a pure epoxy will offer chemical resistance against corrosive chlorine, but its slow cure rate makes it prone to sagging. An epoxy acrylate may be the ideal choice, due to its combination of a fast cure time and chemical resistance.
By comparison, pure epoxy would be ideal to specify for the downward installation of an anchor bolt in dry concrete, yielding best-in-class strength. Need to specify an epoxy into a relatively cold concrete substrate? Polyester resins might be the better product.
When specifying any epoxy product, one must read the manufacturer’s printed installation instructions (MPII) to ensure one is not selecting an anchor or repair product that will have a dangerously compromised bond. Important definitions found in the MPII include those listed in the paragraphs below.
This is the ‘safe harbor’ time between installing the uncured (wet) epoxy and when hardware can be bolted to it. This is not the time that allows for the full load to be applied, just for the hardware installation.
Also expressed as tensile strength, this is the maximum allowable bond stress to which an epoxy can be subjected. ‘Stress’ is expressed in units of psi, which may not be directly helpful in determining what is needed. Most manufacturers also provide sample load tables that translate ‘psi’ into ‘lb’ for a given set of specific conditions.
There are myriad chemicals, even in residential settings, that could act as corrosives on epoxy, such as oil, gasoline, chlorine, salt, or chemicals used in wood preservation. Understanding the chemical resistance of the intended epoxy is essential if such exposure is expected.
Load time is the period from when the resin is extruded until it can be safely loaded to its published allowable load. In other words, it is the time it takes to fully cure. This time greatly depends on the substrate’s temperature. Once a bonded anchor is installed, it should not be fully loaded until the loading time has elapsed.
Typically ranging from three to 15 minutes, nozzle time is dependent on the epoxy type; it is the amount of time the mixing nozzle can remain inactive with epoxy inside of it and still be reusable for the next anchor. Exceeding the working time does not impact the integrity of the epoxy in the cartridges, it just means the installer needs to install a new (empty) mixing nozzle because the epoxy in the nozzle began to cure.
Horizontal and overhead applications require a non-sag product so the installed does not experience epoxy dripping out of a hole. Overhead applications usually require special plugs regardless of the sag properties.
This determines how long the product can sit after being manufactured and still be usable. In most cases, an epoxy that has exceeded its shelf life will be too difficult to extract from the cartridge. It is based on unopened cartridges being properly stored and typically ranges from 12 to 24 months. Installers must check the label for an expiration date before using.
Substrate temperature range
This rates the high and low temperature range of the concrete or CMU wall within which the epoxy will still perform as designated by the manufacturer. It also influences mixing and cure times.
Working time (or gel time)
When an uncured (wet) epoxy is applied, this is the time window when the epoxy can be safely manipulated (as when an inserted bolt is positioned) while still ensuring the epoxy will not be compromised. After the gel time has expired, manipulating the epoxy will compromise the bond.
Jim Collins, PhD, PE, is MiTek USA’s manager of engineering projects. He has worked 14 years in the engineered wood industry and eight in the truss industry. A former director of engineering at International Code Council Evaluation Service (ICC-ES), he has been a licensed professional engineer for 25 years, and is a member of American Society of Civil Engineers (ASCE), the American Concrete Institute (ACI), and Concrete Anchor Manufacturers Association (CAMA). Collins can be contacted at firstname.lastname@example.org.
Marty Ruch is vice-president of retail sales and merchandising for MiTek Builder Products. He has more than a decade of experience in commercial/industrial sales and spent 14 years working in the building materials industry with Gibraltar Industries and MiTek in marketing and product development roles. Ruch can be reached at email@example.com.