About Chemical Innovation - Subscription Information
May 2001
Vol. 31, No. 5, pp 17—21.
Developing Technology

Table of Contents

Achim Hering

The proof is in the fire

The author looks at the reliability of two international testing standards for passive fire protection products.

opening artIntumescents are substances that swell when exposed to heat. The chemically bound water in intumescents absorbs heat, making them ideal materials for firestopping, fireproofing, and gasketing applications. Some intumescents, however, are susceptible to environmental influences such as humidity, which can reduce or negate their ability to function.

Although somewhat contrary to the true definition, intumescents are categorized as passive fire protection measures because they are used to compartmentalize fire (keep the fire in the location of origin). However, “passive” does not denote an absence of activity. During the intumescing process, physical activity (swelling) and chemical activity (loss of bound water and char formation) are both taking place.

After I wrote an earlier article in The Construction Specifier (1), I received some strong criticism from manufacturers whose products did not meet the pedigree I suggested. In this article, I provide an update from my previous work and a discussion of the two standards—Underwriters Laboratories (UL; Northbrook, IL) Standard 1709 (2) and the German Institute for Building Technology (DIBt; Berlin) Guidelines for Intumescent Building Materials (3)—currently known to address the most critical issues relating to intumescent longevity, which, incidentally, apply to many other products used in passive fire protection.

A great deal of knowledge exists on the topic of intumescents, and there is no shortage of manufacturers and distributors for intumescents worldwide. The trouble is that many intumescent products do not work after they have been installed and exposed to common environmental conditions. An excellent source of information on the history and modeling efforts can be found on the Web (4).

Types and applications
Figure 1. A high-expansion intumescent.
Figure 1. A high-expansion intumescent. This material forms a lightweight carbon foam and is highly endothermic.

Figure 2. High-expansion spray applications. A dual copper piping system for carrying fresh water is firestopped using rockwool packing and a silicone sealant on top. The inner two black pipes have been firestopped with 3M Fire Barrier Mortar.
Figure 2. High-expansion spray applications. A dual copper piping system for carrying fresh water is firestopped using rockwool packing and a silicone sealant on top. The inner two black pipes have been firestopped with 3M Fire Barrier Mortar.

Figure 3. A low-expansion intumescent.
There are two kinds of intumescents: high-expansion and low-expansion. High-expansion intumescents form a lightweight char, or carbon foam (Figure 1 at right). They are highly endothermic, because up to one-third of their mass is chemically bound water. They are unsuitable for plastic pipe penetrations (the char is unable to provide sufficient expansion pressure), yet they are desirable for providing cooling vapors and layers of thermal insulation. One application of high-expansion intumescents is used in structural spray-applied fireproofing (Figure 2 at right).

Low-expansion intumescents tend to have higher strength, partially because they tend to contain less water than the high-expansion intumescents (Figure 3 at right). Depending upon the number of attached waters of hydration, sodium silicates tend to contribute approximately 10% water. Uses of low-expansion intumescents include fire door gaskets, firestops, and closures.

Problems with intumescents
Some intumescents have very limited application because their beneficial properties (and in some circumstances, the applied intumescent itself) can disappear within days of installation. The culprits are various environmental influences, such as humidity (even normal indoor humidity), the UV component of sunlight, and heat generated by normal operations. Intumescents such as ordinary sodium silicates must be protected by epoxy or rubber coatings to ensure operability.

Most vendors of vulnerable intumescents are responsible and clearly state that their products require protection. But not all manufacturers are that responsible and forthcoming. For instance, using commercially available sodium silicates, any moderately skilled entrepreneur conceivably could design a product that will intumesce. The problem is the durability of the product. This is a significant factor, because some regions in Canada, for example, exhibit a wide variety of environmental conditions during a given year. There have been a number of product recalls for faulty firestops in Canada and the United States.

When a car manufacturer recalls a type of car, the owner drives it to the dealer, and the problem is corrected. But it is not quite that simple for passive fire protection products, which are concealed behind the finishes of occupied buildings. I doubt that you will find people digging holes into their drywall or masonry to replace faulty firestops or scraping deficient intumescent fireproofing off structural steel. Building owners are not always well informed of these fireproofing details because they have little to do with the original construction of the building. Thus, in creating the initial design of the formula, the manufacturer should consider the expected environmental exposures of the product and standardized testing and quality control regimes that will assure designers, contractors, and end users about the product’s reliability.

A product may intumesce in the laboratory, but will it perform the same way a week later in a desert, a jungle, or an arctic environment? At present, environmental exposure tests are not required in North America for most intumescent applications. Structural steel coatings are an exception and are discussed later.

Standards for intumescents
In North America, the only standard that adequately addresses serious environmental exposures and the longevity of intumescents is UL1709 (Standard for Safety for Rapid Rise Fire Tests of Protection Materials for Structural Steel) (2). Unfortunately, this standard only covers fire testing of structural steel columns against the hydrocarbon time-versus-temperature curve, which essentially restricts its use to fire protection of exterior steel structures in the oil and petrochemical industries (a minority of applications for intumescent products). In fact, testing against UL1709 can deter manufacturers of intumescents, and for good reasons, which are described later.

UL1709 contains a very tough set of tests, as it should, considering the rigorous applications for exterior hydrocarbon fire protection and the enormous risk potential for refineries and chemical plants. This test regime does nothing for interior products because no one has tested common interior passive fire-protection products to UL1709. One reason is that endothermic coatings such as intumescents generally contain large concentrations of epoxy coatings. Consequently, an unacceptably high amount of smoke is generated when the coating is first exposed to fire, and such a large fuel contribution violates building codes across North America.

Although other methods for testing intumescent materials are available, the best and most scientifically sound bench-scale testing is from DIBt. Under the DIBt approval guidelines, building materials are tested for interior or exterior applications. DIBt-approved firestopping, fireproofing, and gasketing products are available in North America; manufacturers include 3M (intumescent firestops) and Nullifire (a thin-film intumescent spray fireproofing product).

Committees from UL and UL of Canada (Toronto) are currently considering mandating the use of a modified version of the DIBt method for bench-scale testing of intumescent products used in firestopping applications. Neither U.S. nor Canadian manufacturers are enthusiastic about an environmental exposure mandate. However, manufacturer due diligence and providing useful and reliable data to the end user are the real issues because it is the costly environmental exposure testing as well as practical performance that causes some intumescents to fail.

UL test standards
Preceding the fire exposure test is a whole battery of environmental exposures, some of which are mandatory. The “Simulated Environmental Exposures” (2) provide that simulated exposure conditions may include but are not limited to the following:

  • Simulated aging. The sample is placed in a circulating air oven at a temperature of 153–163 °F for 270 days.
  • High humidity. The sample is placed in an environment with the following conditions: 92–98 °F and 97–100% relative humidity for 180 days.
  • Industrial atmosphere. The sample is placed in a chamber in which the gas contains 1% SO2 and 1% CO2. In addition, a small amount of water must be present at the bottom of the chamber, which must be maintained at 92–98 °F. The exposure duration is 30 days.
  • Salt spray. The sample is exposed to salt spray (fog) testing in accordance with the methods described in ASTM B 117-97 (5).
  • Combination moisture, freeze, and dry heat cycling. The sample is exposed to a simulated rainfall of 0.005 mm/s for 72 h, followed by a temperature of –45 to –35 °F for 24 h, followed by a dry atmosphere at 135–145 °F for 72 h. This cycle is repeated 12 times, so the entire test takes 84 days.
  • Acid spray. The sample is exposed to a fog spray consisting of 2 vol % hydrochloric acid. This fog spray must provide 1–2 mL of solution per hour for each 80 cm2 of horizontal sample surface area. The exposure duration is 5 days.
  • Solvent spray. Samples are sprayed with reagent-grade solvents at 65–75 °F. Typical solvents for this test are acetone and toluene. Using a standard spray gun, the solvent spray is applied until the entire surface area of the sample is covered with solvent and excess solvent is observed to run off the sample. One exposure cycle consists of applying the solvent, followed by drying the sample for 6 h, followed by another solvent application, then drying the sample for 18 h. This amounts to 1 day per cycle. The exposure cycle must be repeated 5 times, for a total test time of 5 days.

UL1709 methods are intended to fire-test structural steel columns that have been protected with an exterior grade fireproofing material. Because the standard building elements’ time-versus-temperature curve does not apply to exterior fires involving hydrocarbons, the UL1709 high-intensity or hydrocarbon fire curve is used to measure the much faster heat rises (2). Within the first 5 min of the test, 2000 °F must be reached. The temperature must be maintained for the desired fire-resistance duration, within a prescribed tolerance. The applied intumescent (or any other fire protective material intended to be classified for the same purpose) must keep the 2.4-m-long steel column sample below 1000 °F, on average. None of the eight thermocouples must yield a temperature in excess of 1200 °F.

Running all of the environmental exposure tests takes at least 1.5 years. This duration, along with the cost of testing, is a major factor in the low number of manufacturers vying for this market. Only one vendor that markets an intumescent has qualified its system to all environmental exposure criteria.

DIBt test standards
The DIBt approval guidelines apply generically to all intumescent materials, regardless of what passive fire protection systems or applications they may be used for (however, spray-applied intumescent fireproofing has a unique set of tests [6]). Whereas the UL1709 test exposes the entire test sample to the environmental exposures selected by the manufacturer or submitter, the DIBt test is a bench-scale material test, suited only for intumescents (3).

The DIBt approach contains three sets of tests. Following a set of basic tests that establish sample size, construction, and density, the intumescent qualities of the materials are quantified: how endothermic the product is and the percentage and pressure of its expansion.

The samples are then exposed to a variety of adverse environmental influences. Products are tested only for those environments in which the manufacturer intends them to be marketed. The resulting DIBt approval will clearly spell out what can be used where. The environmental tests include

  • short-term weather exposure,
  • water condensate exposure,
  • heat exposure,
  • effect of coatings on the intumescent (plastic dispersions, alkaline resins, polyurethane acrylics, and epoxy resins), and
  • effect of solvents and oil on the intumescent (butyl alcohol, butyl acetate, heating oil, and benzene).

Finally, the effect of the intumescent on other building materials is tested, specifically poly(vinyl chloride) (PVC) and polyethylene (PE).

Once the samples have been exposed to these adverse environments, the tests of basic characteristics are repeated. The differences in performance, both for foam height and expansion pressure between exposed samples and unexposed samples, are recorded in terms of percentage. All of the testing is performed by governmental laboratories, which are DIBt- accredited.

Within the legal framework of the DIBt standard, all materials must undergo what the Germans call “Baustoffzulassung” (building material approval), whereby the test results from the DIBt-accredited laboratories are communicated immediately to DIBt, which forwards them to an expert committee (SVA) as a request for approval. Within the report, the laboratory must make a recommendation to the SVA on the eligibility of the product for approval. DIBt indicates the reason for this test and certification regime. Prior to the DIBT standard legislation, Germany had many problems with intumescent materials and application; since the establishment of this standard, there has been one update.

The DIBt method is used as a reliability test to detect any serious problem with an intumescent. Additionally, the DIBt standard contains provisions for adapting unique products and applications. If the procedure has to be altered to determine its efficacy more accurately, changes can be made, but they must be approved by the manufacturer, the DIBt-accredited laboratory, and ultimately the SVA expert committee. The same applies to the testing of environments that are not adequately covered by the means established in the standard. For each change to the procedure, three parties have to agree.

Assessing the intumescent market
The largest market for intumescents is the industrial exterior spray fireproofing market. All manner of passive fire protection products compete for this market, including intumescents, other endothermic products, cementitious plasters, fibrous plasters, fibrous wraps, and cast concrete, as well as active fire protection products, such as the type of sprinkler systems used to protect liquefied petroleum gas (LPG) containers.

Let us look at a situation in which problems associated with rival products can be solved by intumescents: petrochemical plants, which contain process pipe bridges (structural steel racks for the purpose of holding up process piping), vessel skirts (round steel sheet structures, which support a vessel above), and spherical or cylindrical LPG containers.

There is a definite demarcation line within petrochemical facilities in terms of the importance given to (and willingness to part with funds to safeguard) pipe bridges and vessel skirts compared with LPG containers. Without vigilant enforcement measures, many above-ground LPG containers remain unprotected and thus subject to fire exposure in case of a flammable hydrocarbon spill. Facility owners who do pay to fireproof their LPG vessels are more likely, if aware of all technical aspects and expenses, to choose an intumescent or other endothermic product, rather than a cementitious or fibrous plaster. There are several reasons for this; one is longevity.

There is considerable variation in reinforcement factors and inherent flexibility between cementitious plaster products used for spray fireproofing (7). Plaster delamination and the corrosion of the steel mesh used to reinforce the plaster have in some cases caused the spray fireproofing to become dislodged and fall off. Common factors known in inorganic chemistry and in the concrete industry contribute to such events. Omitting the exterior waterproofing membrane or the priming layer permits weathering to occur, causing the cement-bound plaster to drop in pH value. This reduces the corrosion protection of the reinforcing mesh, which starts to rust, expand, and thus potentially damage the vulnerable plaster. This effect is especially pronounced in installations near the ocean, where the salt spray accelerates corrosion.

Experience has shown that because of faulty dew-point calculations and insufficient investment in quality materials and installation, fibrous plasters can become soaked with water and then freeze and delaminate. The owner of a petrochemical facility may find that the absolute lowest price is not the best tool for cost effectiveness in the long run.

Intumescent and endothermic products for this application circumvent the problems associated with the cementitious and fibrous plasters, for the most part. There have been cases of misapplication of intumescents (e.g., mixing incorrect proportions of ingredients) leading to the sliding off and total replacement of the product during the initial application. Intumescent coatings have also been known to delaminate; however, these are exceptional cases. One should use caution to choose a competent contractor. Intumescent and endothermic products are supplied in an epoxy paint base, so they are inherently corrosion-inhibiting. They are also much more likely to stretch and move with a sphere as it is being emptied and filled and undergoing weather changes. There can be no water or chloride penetration, and there is no cement to suffer from corrosive effects. Intumescent and endothermic products are significantly more expensive per square meter installed than fibrous and cementitious plasters. In light of the technical aspects, intumescent and endothermic products are worth the extra money, particularly when qualified to the environmental criteria of UL1709 or DIBt.

At present, there is no shortage of vendors for intumescent products, certified or not. If you are a manufacturer of passive fire protection products, it is certainly wise to be up-to-date on DIBt standards. I recommend qualifying your products with this method and obtaining a DIBt approval regardless of where your product may be sold, simply to prove due diligence as a manufacturer. If you are purchasing intumescents for your facility or for resale or you are specifying them for use in someone else’s facility, requesting a current DIBt approval and a system qualified to UL1709 is prudent. Beware that for each standard, not every exposure is mandatory.


  1. Hering, A. Construction Specifier, Sept 1995, pp 80–90.
  2. Underwriters Laboratories. Rapid Rise Fire Tests of Protection Materials for Structural Steel, 2nd ed.; Underwriters Laboratories: Northbrook, IL, 1994; http://ulstandardsinfonet.ul.com/ (accessed May 2001).
  3. German Institute for Building Technology. Approval Guidelines for Intumescent Building Materials, Jan 1990. In DIBt Mitteilungen, Jan 1998; www.dibt.de (English version, www.geocities.com/ghering2000/dibt.html; accessed May 2001).
  4. http://fire.nist.gov/bfrlpubs/fire97/art007.html (accessed May 2001).
  5. American Society for Testing and Materials. Practice B117-97; Standard Practice for Operating Salt Spray (Fog) Apparatus; West Conshohocken, PA, 2001 (supersedes B117-73).
  6. German Institute for Building Technology. DIBt Standard for Testing Reactive Spray Fireproofing of Structural Steel; www.geocities.com/ghering2000/dibt2.html (English version, accessed May 2001).
  7. www.geocities.com/astximw/firestop_terminology.html (accessed May 2001).

Achim Hering is an independent consultant on passive fire protection (PO Box 818, Capreol, ON, Canada P0M 1H0; 705-858-4791; ahering@sympatico.ca; www.geocities.com/achim_hering). He is a member of ULC Task Group 21 on firestops and has written several articles on fire protection. He has been involved in passive fire protection since 1981.

Return to Top || Table of Contents