NIST Combinatorial Methods Center Focused Project
High Throughput Flammability Test Methods for Compositionally
Graded Samples
We are very excited about the potential this program has to
impact the speed of R&D in the flame retardants and polymer additive areas.
The Fire Research Division at NIST has allocated over $650K of internal funding
for 2002 for High Throughput flammability research. Membership in this Focused
Project on High Throughput Flammability Test Methods affords members an
excellent opportunity to leverage their R&D funds for the purpose of gaining
information, knowledge and skills in the high throughput field. This is also an
opportunity for the members to help set the direction for the development of
these new R&D tools.
Project Leader: Jeffrey W. Gilman
Phone: 301-975-6573
Email: jeffrey.gilman@nist.gov
Introduction:
Flammability performance standards for new materials are most
often met through the use of additives. To ensure compliance, complex mixtures
containing the polymer resin, stabilizers, processing agents, pigments, and
flame retardant additives are formulated, characterized, and tested one at a
time. While it is in the interests of the public that flammability testing
continues to be a critical component of materials research and development
(R&D), there are no guarantees that innovation cycle times can be
sufficiently reduced using this approach to respond to the competitive pressures
of the global marketplace. The recognition that industry needs more efficient
tools for materials R&D has prompted the Building and Fire Research
Laboratory (BFRL) at the National Institute of Standards and Technology (NIST)
to initiate a program of research directed at the development of high-throughput
(or combinatorial) methods for materials flammability research. In this context,
we use the term "high-throughput" to refer to a research strategy
characterized by conducting many experiments at the same time. By virtue of its
inherent efficiency, this approach will provide us with the capability to
explore compositional space and thereby develop a better understanding of the
interactions between components and their effects on the ultimate performance of
these materials. This knowledge can be used by industry to determine optimal
compositions with respect to materials flammability and other properties of
interest.
Technical Approach:
A high-throughput method for evaluating the flammability
performance of experimental formulations under realistic processing conditions
is needed to complement the work we are doing in developing micro-scale
platforms for lead discovery motivated R&D. Our approach is embodied in the
device, known as the continuous gradient extruder (CGE), depicted in the Figure
1. The CGE consists of a series of programmable gravimetric feeders capable of
producing a constant compositional gradient in an extruded polymer strip and
sensors that monitor both the concentrations (by quantitative infrared
spectroscopy) and degree of dispersion of the additives (by changes in the
dielectric response). In principle, each sample contains a wealth of information
about the effects of composition on the flammability of the base polymer. This
obvious extension of existing technology will enable researchers to screen
hundreds of possibilities, as determined by variations in the relative amounts
of critical additives, in the same time it now takes to test just a couple of
formulations.
Figure 1. Prototype extruder capable of producing polymer
samples with multiple additives having concentrations that vary linearly along
the length of the strip.
Preliminary Results:
A compositionally graded sample containing ammonium
polyphosphate (APP) and pentaerythritol (PER) in polystyrene was produced in the
CGE and burned in our horizontal ignition flammability test (HIFT) device under
a constant flux of (16.8 ± 0.4) kW/m2. The sample was a strip
(approximately 1.5 m long, 7 mm wide and 2 mm thick) consisting of PS blended
with varying amounts of a 3:1 mixture of APP and PER. We choose the upper limit
of concentration at 30 % since this formulation has a V-0 rating in the UL94 V
test. We attempted to create a linear concentration gradient (from C = 0 % to C
= 30 % additive by mass) by increasing the rate of feed from the hopper
containing the mixture of APP and PER linearly with time.
Figure 3. Apparatus for measuring flame spread over gradient
samples.
The flame velocities were measured by pulling the extruded strip
through a 300 mm heating zone (Fig 3.) such that the flame front remained
at a fixed position. The results are plotted in Figure 4. The data (t) was
collected at 50 mm intervals, which were marked off on the strip, with the
initial point (at x = 0) corresponding to the pure polymer (C = 0 % at x = 0).
The solid line was obtained by fitting the experimental data to a hypothetical
function (exponential) that was derived by assuming a linear dependence between
flame velocity and additive concentration, which was also assumed to decrease
linearly with distance. The validity of these assumptions is supported by the
fact that this function does a good job of representing the experimental data.
The derivative of the function in Figure 4 is a linear function of x (or C = ax)
with a negative slope (a) (e.g. Fig 6) indicating a reduction in the flame
velocity with increasing concentration of additive as expected.
Figure 4. The progression of the flame front as a function of
time measured for the PS/APP/PER gradient. The solid line was obtained by
fitting the experimental data (circles) to a hypothetical function derived on
the basis of assumptions stated in the text.
Figure 5. The progression of
the flame front as a function of time measured for PS/clay gradient. The solid
line was obtained by fitting the experimental data (circles) to a hypothetical
function derived on the basis of assumptions stated in the text.
Figure 6. Plot of flame speed versus composition.
Comparable data was collected for a PS/clay gradient sample with
an approximately linear concentration gradient of clay varying from 0 % to about
10 % by mass. In this case, the flame velocities were observed to accelerate
with increasing clay concentration (Figure 5), suggesting that the presence of
the clay actually enhances the spread of flames over the surface of this
material. This observation is consistent with our previous observation that
polymer/clay nanocomposites ignite at lower temperatures (even though, once
ignited, they tend to burn with a lower HRR) than the pure polymer, since lower
ignition temperatures facilitate flame spread. Eventually, we would like to make
simultaneous measurements of both flame velocity and heat release rate (HRR) to
determine the optimal composition, which results in the best compromise between
flame spread and HRR.
This approach will also allow us to evaluate the large variety
of combinations and parameters associated with the flame retarded polymers.
Project Deliverables:
The project team will assemble, calibrate and demonstrate a
prototype High Throughput Flammability screening system for making simultaneous
measurements of the HRR and flame spread velocities of compositionally graded
samples as a function of the incident flux. This prototype system will initially
consist of the apparatus described above. A test method will be developed and
validated by correlating the results with conventional measurements, such as
cone calorimetry and UL-94, or with other fire tests, which the members desire
studied. This High Throughput sample preparation, characterization and
flammability screening will be complimented with High Throughput physical
property characterization (modulus and elastomeric properties) of the gradient
samples (before burning and after) using an automated Nanoindentor. An
Annual Report will be delivered to the members within 2 months of the end of the
project.
Project Meetings and Web-site:
We will have 3 meetings during the Project. One at the 1 month
point, for the purpose of finalizing the research plans, and the second and
third meetings will occur at 6 and 11 months after the start of the project,
respectively. The purpose of these later two meeting is to share results and
plan future experiments with Project members. A web-site will be available to
members to facilitate communication of Project results and administrative
information.
Project Milestones:
-
Assemble and demonstrate the prototype measurement device
using known formulations.
-
Develop a test protocol
-
Validate the high-throughput flammability screen by
comparing the relative performance of test samples with results obtained on
the cone calorimeter and UL-94 or other fire tests of interest to members.
-
Validate the use of HT Nanoindentor for characterizing
physical properties of gradient polymer samples..
These milestone will be completed by the end of the first full
year of the Project. Membership is for one year at a time, therefore we have
only included plans for year one in this document. Plans for a second year will
be discussed at the third Project meeting.
Membership Fee:
The membership fee is $20,000 per year.
Upon execution of the attached Project Agreement the fee and
agreement should be sent to:
NIST
Attention Sharon Rinehart
100 Bureau Drive
Gaithersburg, MD USA
Mail Stop 8602
sharon.reinhart@nist.gov
301-975-5876
