|
For someone who resists change as much as practically possible (I gave up my 8-track
only four years ago and have now advanced to cassettes for most of my music. I do own
three CDs to play in my computer, though!) it is hard for me to admit change is
inevitable. But the world in which we live, and in which we must investigate fires,
is changing. This is planned to be the first in a (short) series of commentaries about
changes in fires, fuels, investigations and investigators.
Part 1: Furnishings
dr. john d. dehaan - The materials that make up the bulk of the fuels involved in most
structure fires have changed dramatically over the last 30 or 40 years, affecting the
way in which fires ignite and spread, the combustion products they create, the heat
release rates and temperatures they produce, and the residues they leave behind. Let's
look at a few examples. Think back to the way your parents' (or your grandparents')
house was furnished in the 1940s or 1950s. It was most likely described as follows:
- Walls were plaster over wood lath or wire mesh, painted or covered with heavy paper or cloth wallpaper.
- Floors were bare wood or linoleum with wool carpets and rugs.
- Window coverings were heavy cotton (brocade) or wool fabrics (or steel-and- wood window blinds).
- Furniture was often bare wood. When it was upholstered, it was usually cotton velour or wool mohair, or leather, over padding of horsehair, or cellulosic (vegetable) fibers (cotton, coir, kapok, Spanish moss, etc.).
- Latex foam rubber was found in some cushions and pillows, while mattresses were stuffed with cotton or other vegetable fiber or even feathers.
- Rooms were usually poorly insulated and glass windows were single-glazed.
Such furnishings were all combustible, but how readily? Most of those materials listed
above were susceptible to smoldering ignition sources such as a dropped cigarette but,
with a few exceptions, would not be readily ignited (to a self-sustaining fire) by a
short-lived flame source such as a common match. Ignition of all but latex foam and
kapok took many seconds or even minutes of exposure, and all produced large quantities
of smoke prior to ignition to flames. Once ignited, they would burn slowly, often
reluctantly, producing small flames, preferring to smolder instead. Heat release rates
would be very low (300400kW being typical for a sofa), so spread to other fuels would
require direct flame contact. Although the localized flame temperatures would be
"normal," i.e., ~1700-1800°F (800900°C), because the heat release rates were so low
(and room insulation so poor), overall hot gas layer temperatures would usually remain
below the 1200°F range, so progression to flashover would be rare, and would take a long
time to develop when it did. Combustion products included CO, soot, acrolein (from wood)
and hydrogen cyanide (from wool). Carpets and floors burned only with reluctance and only
after a lot of radiant heat had been applied. Post-fire residues of liquid accelerants were
relatively easy to identify based on odor, and later, by simple gas chromatography (GC)
because the furnishings offered little in the way of interfering pyrolysis products.
By the 1960s and 1970s, things were changing.
- Wall coverings tended to be paint or vinyl wallpapers over sheetrock (dry wall),
with its better durability under fire exposure).
- Floor-coverings were more often wall- to-wall carpet of nylon or polyester face yarns
with jute fiber backings over a pad of butyl rubber or mixed fiber.
- Furnishings were often polyurethane (PU) foam or cotton padding covered by cotton or
cotton/synthetic upholstery fabric (or vinyl).
- Window coverings were cotton or synthetic, or sometimes fiberglass, fabrics.
- After 1972, mattresses were more commonly combinations of a cotton core with a
polyurethane foam layer to improve resistance to smoldering sources (cigarettes).
- Thermal insulation in rooms became more common and double glazing appeared.
As a result, fire behavior began to change. Synthetic fabrics and PU foam added smolder
resistance to furnishings but made them more susceptible to ignition by even short-lived
open- flame sources. Fires became hotter and heat release rates increased as more
synthetics became involved. Fire growth was more rapid in new style furniture and rooms
could approach flashover more often and in a shorter period of time, but carpets and many
furnishings resisted ignition even by radiant heating. When carpets did ignite, the face
yarns tended to burn off but the heavy jute backings would burn slowly and remain in place
to protect the pad beneath. Vinyl products could resist ignition and slow the progress of
the fire but when finally involved, could contribute hydrogen chloride (HCl) vapors to the
fire gases. Gas chromatography could discriminate flammable liquid residues from all the
pyrolysis patterns thanks to improved chromatography since the peak patterns produced by
these pyrolysis products were easily distinguished from the patterns from common flammable
liquids.
By the late 1980s, floor coverings still consisted of various synthetic face yarns, but
the backing came to be made of polypropylene due to its lower cost, and better resistance
to odors and mildew. Polyurethane foam carpet padding (new or rebond) replaced latex rubber
or fiber padding exclusively. Under severe radiant heating, face yarns could melt and even
ignite, now the backing would also, exposing the combustible PU foam underlay beneath. Such
carpets would tend to resist localized flame and heat sources, but once alight could burn
more completely than before and create a lot more heat and floor-level damage than ever
before. Combustion products now included a wide range of aldehydes, ketones, and toxic
intermediates known as free radicals.
Furnishings became almost exclusively polyurethane foam ( some with flame retardants)
with synthetic coveringvery difficult to ignite with a smoldering cigarette but readily
ignited by even a small flame. Once ignited, flames could spread quickly, and engulf all
of a large chair or sofa in flames in less than 10 minutes. Synthetic upholstery materials
with their low melting points, melt as they burn, producing molten, burning droplets of
materials that fall to the base of the furniture and institute rapidly growing vertical-face
fires on the sides of the furniture, as well as "drop-down" damage to floors and carpets
beneath. Urethane foam pads (particularly rebond) and all-synthetic carpets produced complex
stews of pyrolysis products that could sometimes be quite difficult to distinguish from some
non-distillate type petroleum products. The acquisition of comparison samples of floor-coverings
became more important.
By today (the turn of the Century!), wall-to-wall carpets often include low melting point
and readily ignitable polypropylene face yarns with polypropylene backing over polyurethane
underlay (particularly in low- or medium-price-range installations). Except for "high-end"
mattresses where latex foams are being used once again (due to their better breathability
and reduced moisture build-up), polyurethane foams are nearly universal in mattresses and
furniture cushions. Such PU foams are very resistant to smolder ignition but are still
easily ignited with a match. Some improvement is seen today with the inclusion of more FR
(flame resistant) foam rubbers. (In part because the more stringent California Bureau of
Home Furnishings tests are gaining more and more acceptance. Furniture manufacturers are
still fighting improved national standards for flame- and smolder-resistance, but it is
becoming apparent that the simple inclusion of a barrier layer of PVC, aluminized fabric
or even fiberglass can dramatically improve all types of fire resistance at a very modest
increase in cost.) The polypropylene carpets are becoming a widespread problem. They will
pass the Methenamine tablet test (DOC FF 1-70, now known as 16 CFR 1630) (meaning that a
dropped match or similar flame of ~ 50W HRR will not ignite the carpet to a self-sustaining
fire), and are therefore legal for use in residences. Unfortunately, exposure to a slightly
larger fire (say, burning a crumpled sheet of paper) with its higher heat flux, can initiate
a self-sustaining fire that spreads at about one square meter per hour (10 ft2/hr) with very
small flames (2 - 3 inches in height) along the margins (i.e., a very low heat release rate).
If the fire starts in an unoccupied building, it can burn for hours consuming entire rooms of
carpet without running out of oxygen, due to its very low rate of combustion. In addition,
due its low critical incident heat flux, such carpet is more readily ignitable by radiant
heat even at some distance away from a large fire (such as a chair fire). As the face yarn
burns, the polypropylene backing melts and shrinks, exposing the combustible PU foam underlay,
which can now ignite and sustain the spread of fire through and across the carpet. The rate of
spread is dependent on the interaction between the carpet and pad, and can be affected by such
variables as whether the carpet is stretched tightly and securely fastened or lying loosely
atop the pad with plenty of air between the two.
Today's furniture is markedly improved in its resistance to the most common type of
accidental ignition: the dropped cigarette, or other forms of glowing sources (electric
heaters, glowing electrical connections, etc.) but the trade-off is much worse resistance
(read: virtually none) to flaming sources.* Once alight, such furnishings can be completely
involved in 3 to 5 minutes and be reduced to a charred frame in 10 minutes, while producing
very high temperatures (in excess of 2000°F in the flame plume) and enormous heat release
rates (2 to 3 megawatts being common for a sofa or recliner today). Such performance
virtually ensures that the average sized room will be completely engulfed in a post-flashover
fire within 5 to 10 minutes of flame first established in such furniture. The intensity of
such fires can obliterate traces of ignition sources, induce the speed of spread and kind
of damage once thought possible only for accelerated fires, and make the recovery and
identification of possible accelerant traces very difficult. Gas chromatography/mass
spectrometry is now needed to discriminate possible accelerant traces from pyrolysis products.
The best the investigator can do is to be aware of the contributions such furnishings can
make to the ignition and growth of a fire, and to take whatever steps he or she can to
document the kind of furnishings present (New or old? Cellulosic or synthetic fabrics and
padding?), and especially, to collect comparison samples of carpet, pad, upholstery and
padding whenever possible for later identification in the forensic lab (even partially
burned samples are better than no samples at all). This may be the only way to later
reconstruct the events accurately.
*Not long ago, "open flame" meant a dropped match, careless handling of a lighter or
the occasional wayward "spark" from a fireplace but today we are often surrounded by
flaming sources that once were only common during power blackoutscandles. But that is
a topic for another column.
About the Author:
Dr. John DeHaan has been a criminalist for some 32 years. He has worked at county,
State, and Federal forensic labs. He is a native of Chicago and his Bachelor of Science
degree in physics was from the University of Illinois at Chicago. He has been involved
with fire and explosion investigations for over 30 years, and has authored dozens of
papers on fires, explosions, and their investigation and analysis. He is probably best
known as the author of the textbook Kirk's Fire Investigation (now in its Fourth
Edition). His doctorate (in 1995), from the University of Strathclyde in Glasgow,
Scotland, was on the Reconstruction of Fires Involving Flammable Liquids.
He is a member of NFPA, and served on its 921 Technical Committee from 1991-1999. He
is a member of the IAAI and serves on its Forensic Science Committee. He holds a
diploma in Fire Investigation from the Forensic Science Society (United Kingdom) and
one from the Institution of Fie Engineers (U.K.). He is a Fellow of the American
Board of Criminalistics in Fire Debris Analysis and a member of the Institution of
Fire Engineers. He retired from the California Department of Justice in December 1998
and is now the president of his own consulting firm, Fire-Ex Forensics, Inc., Based
in Vallejo, California, where he now serves as a consultant in fire and explosion
cases all over the U.S., Canada and overseas.
Other articles in the Our Changing World Series
|
