recycling

Article
Recycling of Commercial E-glass Reinforced
Thermoset Composites via Two Temperature
Step Pyrolysis to Improve Recovered Fiber
Tensile Strength and Failure Strain
Ryan S. Ginder 1,2, * and Soydan Ozcan 1,2
1 Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville,
TN 37996, USA; ozcans@ornl.gov
2 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
* Correspondence: rginder@vols.utk.edu; Tel.: +1-865-574-9040




Received: 5 April 2019; Accepted: 3 June 2019; Published: 6 June 2019


Abstract: Economic and regulatory pressures on the global composites industry have encouraged the
research and development of technology for the recycling of fiber reinforced polymer composites.
Although significant advancements have been made in the recycling of carbon fiber composites,
more progress is needed in the recovery of glass fibers, which make up the overwhelming volume
of the composites market. In this study, wind turbine blades and automotive sheet moulding
compound (SMC) were subjected to a two temperature step pyrolysis. This multistep process yielded
improvements in the recovered E-glass fiber’s tensile strength, by as much as 19%, and strain to failure,
by as much as 43%, over a single high temperature step pyrolysis. Despite these gains, pre-pyrolysis
fiber measurements indicate that pre-existing damage may inherently limit the quality of glass fiber
recoverable from pyrolysis without any post processing.

Keywords: glass fibers; pyrolysis; mechanical properties; single filament testing

1. Introduction
Over 9 million tons of glass fiber reinforced polymer composites were produced globally in 2015 [1].
Roughly 70% of these composites are made with thermosets [1]. As such, they are quite resilient in
service but do not lend themselves towards recycling like other engineering materials (e.g., metals
and thermoplastics). This issue has not stopped attempts at materials recovery, with approaches
now built around reclaiming the reinforcing fibers through degrading the resin matrix with some
form of mechanical, chemical, or thermal attack [2]. Of the methods developed, one of the simplest
options that has achieved some measure of commercial success is pyrolysis. In a low or no oxygen
environment, composite scrap is heated to thermally break down the resin phase, resulting in recovered
fibers, fillers, char, and hydrocarbon gases and oils [2]. This process has proved fairly successful in the
recovery of carbon fiber through a variety of reactor designs and has reached the point that commercial
quantities are becoming available on the open market [3–5]. The resulting hydrocarbon gases can
contain significant portions of hydrogen, methane, carbon monoxide, and carbon dioxide, while the
resulting oils tend to be rich in aromatic compounds; however, exact compositions can be quite complex
and vary with different resin mixtures, additives, temperatures, and atmospheric compositions [2].
Aside from allowing for recovery of the resin phase as chemical feedstock for reprocessing or energy
conversion, the inert atmosphere used for pyrolysis can limit carbon fiber damage through oxidation
during recovery [2]. At timescales of relevance to recycling, inert nitrogen atmosphere has also been
shown to noticeably reduce damage to glass fiber composites during recovery as well [6].

Recycling 2019, 4, 24; doi:10.3390/recycling4020024 www.mdpi.com/journal/recycling

Recycling 2019, 4, 24 2 of 12
Recycling 2019, 4, x FOR PEER REVIEW 2 of 12

inert nitrogen atmosphere has also been shown to noticeably reduce damage to glass fiber composites
The key to carbon fiber recovery’s success has been both the mechanical quality of the reclaimed
during recovery as well [6].
fibers andThetheirkeyattractive potential
to carbon fiber pricesuccess
recovery’s pointhas
versus virgin
been both thecarbon
mechanical fiber, which
quality is comparatively
of the reclaimed
quite fibers
expensive to produce
and their attractive [2]. Unfortunately,
potential the success
price point versus of carbon
virgin carbon fiber,fiber
whichcomposites
is comparativelyrecycling
has not extended
quite expensive into E-glass [2].
to produce fiber composites,the
Unfortunately, which
successrepresent
of carbonthe overwhelming
fiber volume
composites recycling hasof the
not extended
composites market into E-glass
[1]. The lackfiber composites,
of any which
viable glass representcomposite
reinforced the overwhelming
recyclingvolume
technologyof themeans
composites market [1]. The lack of any viable glass reinforced composite recycling
that almost all composites must still be landfilled for disposal [1]. The problem with E-glass, when technology means
that almost
compared all composites
with carbon, is thatmust stillfibers
glass be landfilled
appearfor to disposal [1]. The problem
be significantly with E-glass,
more degraded bywhen
pyrolysis
compared with carbon, is that glass fibers appear to be significantly more degraded by pyrolysis
processing [1,7,8]. For example, Figure 1a shows E-glass filament strengths, recovered through
processing [1,7,8]. For example, Figure 1a shows E-glass filament strengths, recovered through
pyrolysis by Cunliffe et al., from an unsaturated polyester based sheet molding compound (SMC) that
pyrolysis by Cunliffe et al., from an unsaturated polyester based sheet molding compound (SMC)
indicate an approximately
that indicate 40%–50%
an approximately averageaverage
40%–50% drop indrop
tensile strength
in tensile [7]. Feih
strength et al.etwould
[7]. Feih laterlater
al. would conduct
several in-depth
conduct studies
several on E-glass
in-depth studies on degradation under temperatures
E-glass degradation under temperatures relevant to pyrolysis
relevant [6,9,10].
to pyrolysis
Their [6,9,10].
experiments, summarized
Their experiments, in Figure 1b,
summarized indicate
in Figure that E-glass
1b, indicate filament
that E-glass tensile
filament strength
tensile declines
strength
declines
over time untilover time auntil
hitting hitting
stable a stablevalue
minimum minimum
withvalue
both with both minimum
minimum residualresidual fiber strength
fiber strength and rate of
and rate of strength decay dictated by
strength decay dictated by pyrolysis temperature [6]. pyrolysis temperature [6].

1. (a)1. Average
FigureFigure (a) Average single
singlefilament tensile
filament tensile strength
strength results
results from from pyrolyzed
pyrolyzed thermosetthermoset polyester
polyester SMC
with calcium
SMC with calciumcarbonate
carbonate andand
aluminum
aluminum trihydrate fillers. Redrawn
trihydrate from data from
fillers. Redrawn in Cunliffe
data et
inal. Tables et al.
Cunliffe
Tables88and
and9 9inin[7].
[7].(b)
(b)Minimum
Minimum residual
residualtensile strengths
tensile achieved
strengths in E-glass
achieved fiberfiber
in E-glass at various pyrolysis
at various pyrolysis
temperatures (black circles) versus measured upper bound time limit for reaching said minimum
(red triangles). Redrawn from data in Feih et al. Figure 1(a) in [6].

The data in Figure 1b suggests the potential for higher quality E-glass fiber recovery through
optimization of a two temperature step pyrolysis process. By using a long, lower temperature step
Recycling 2019, 4, x FOR PEER REVIEW 3 of 12

temperatures (black circles) versus measured upper bound time limit for reaching said minimum (red
triangles). Redrawn from data in Feih et al. Figure 1a in [6].

Recycling 2019, 4, 24 3 of 12
The data in Figure 1b suggests the potential for higher quality E-glass fiber recovery through
optimization of a two temperature step pyrolysis process. By using a long, lower temperature step to
degrade
to degrademost
mostof of
the composite
the compositeresinresinphase
phaseininconjunction
conjunctionwith
withaabrief,
brief, higher
higher temperature
temperature step
step toto
complete resin removal, it would appear possible to reduce a glass reinforced composite’s
complete resin removal, it would appear possible to reduce a glass reinforced composite’s exposure exposure
to high
to high temperatures
temperatures and and yield
yield aa stronger
stronger reclaimed
reclaimed fiber.
fiber. Obtaining
Obtaining higher
higher strength
strength glass
glass directly
directly
from pyrolysis could aid both efforts to reuse recovered glass fiber material blended
from pyrolysis could aid both efforts to reuse recovered glass fiber material blended with virgin with virgin E-
glass [11,12]
E-glass and
[11,12] andtotoregenerate
regeneratethe theoriginal
originalglass
glassfiber
fiberstrength
strength post-pyrolysis
post-pyrolysis [13].
[13]. Such
Such aa process
process
should also be viable for carbon fiber recovery, allowing for the co-recovery of mixed
should also be viable for carbon fiber recovery, allowing for the co-recovery of mixed recyclates at recyclates at
pilot plant scale with final intermediate mechanical properties between E-glass and carbon
pilot plant scale with final intermediate mechanical properties between E-glass and carbon fiber [14]. fiber [14].
To test
To test this
this concept,
concept, aa case
case study
study was
was designed
designed andand implemented
implemented using
using two
two representative
representative composite
composite
materials used in commerce: E-glass reinforced epoxy wind turbine
materials used in commerce: E-glass reinforced epoxy wind turbine blades and blades and E-glass
E-glass reinforced
reinforced
unsaturated polyester
unsaturated polyester and
and vinyl
vinyl ester
ester automotive
automotive structural
structural SMC.
SMC.

2. Results

2.1. Thermal Testing

To determine
determine the necessary thermal profile needed for pyrolyzing the selected test materials, materials,
each sample material was heated in a TGA at 10 ◦°C/min. C/min. From
From the
the results
results presented
presented inin Figure
Figure 2a,b,
2a,b,
the pyrolysis of the organic compounds in both specimens appears to initiate around 250 °C ◦ C but
proceeds
proceeds very
veryslowly
slowlyuntil
untilthe mid-300s◦ C.
themid-300s °C.Figure
Figure2a2a
shows
showsa distortion in the
a distortion slope
in the to the
slope right
to the of the
right of
main peak in the wt%/ ◦ C derivative. While not as pronounced, there is also a broadening of the right
the main peak in the wt%/°C derivative. While not as pronounced, there is also a broadening of the
side
rightofside
theof the◦wt%/°C
wt%/ C derivative in Figure
derivative 2b. This
in Figure 2b.indicates the presence
This indicates of bothofaboth
the presence higher and lower
a higher and
temperature phase phase
lower temperature being removed
being removedin bothinspecimens. This isThis
both specimens. not altogether surprising
is not altogether as it isas
surprising likely
it is
that the support materials (i.e., balsa wood and PVC foam) and epoxy in the wind blade
likely that the support materials (i.e., balsa wood and PVC foam) and epoxy in the wind blade sample sample and
that
and the
thatunsaturated polyester
the unsaturated and vinyl
polyester and ester
vinylcomponents in the SMC
ester components in thesample
SMC begin
samplebreaking down at
begin breaking
different temperatures.
down at different temperatures.

Figure
Figure 2. Respective TGA
2. Respective TGA results
results for
for each
each of
of the
the tested
tested composite
composite waste
waste materials.
materials. Three
Three samples
samples
scanned at a rate of 10 ◦ C/min for each test material: (a) SMC, (b) wind blade. Three samples during
scanned at a rate of 10 °C/min for each test material: (a) SMC, (b) wind blade. Three samples during
a simulated two temperature step pyrolysis procedure: (c) SMC, (d) wind blade. Data suggests a generic
thermal profile capable of being used in large scale pyrolysis recycling [14].
Recycling 2019, 4, 24 4 of 12

The TGA scan results suggested that a generic thermal profile with a long, low pyrolysis
temperature step at 350 ◦ C followed by a short 450 ◦ C high temperature pyrolysis step would be
capable of completely pyrolyzing all selected sample materials. Based on the Feih et al. results in
Figure 1b, conducting the bulk of the pyrolysis at 350 ◦ C and with limited exposure to 450 ◦ C would be
expected to yield glass fibers with improved tensile strength over a single 450 ◦ C step. The suggested
thermal profile was therefore tested via TGA using the following sequence: (1) sample heated to 200 ◦ C
over 3 min to allow time for the TGA chamber to be fully purged with nitrogen, (2) sample heated over
2 min to 350 ◦ C low temperature pyrolysis setting and held isothermal for 20 min, (3) sample heated to
450 ◦ C high temperature pyrolysis setting over 1 min and then held isothermal for 10 min. The results
in Figure 2c,d show that in both sample materials the low temperature 350 ◦ C step was able to begin
removing material, as indicated by the progressive sample weight loss with time. These results also
show that the 450 ◦ C high temperature step is capable of removing all remaining polymeric materials
within a brief exposure window, as indicated by the narrow spikes in time derivatives followed by
the leveling off of sample weights. Given the significant inhomogeneity in starting fiber to resin to
filler ratio for the shredded specimens tested, assessing whether differences in yield were from starting
composition variation or thermal processing was impractical. Using the known bulk composition
of the SMC sample and the TGA results, an estimate of overall approximate yield for pyrolysis is
~4–12 wt% residual carbonaceous char on top of the 49 wt% fiber and 12 wt% mineral filler. Larger
quantities of material were then processed using this same two temperature step approach in a nitrogen
purged tube furnace to obtain sufficient quantities of sample fiber filaments for mechanical testing.

2.2. Post-Pyrolysis Fiber Characterization

Fibers collected from the single and two-step pyrolysis processes were characterized via single
filament tension testing. Sample fibers were tested without any surface char removal procedure
in order to isolate the effect of the different pyrolysis procedures. Figure 3 indicates that the two
temperature step pyrolysis process yielded a 19% improvement in final Weibull characteristic tensile
strength of the wind turbine blade glass fibers and an around 5% improvement in the SMC recovered
fibers. This suggests that merely adjusting temperature, without using any additional post processing
steps, does have the potential to process glass fiber composite recyclates with higher output mechanical
strength. Of interest though is why there appears to be such a large difference in the effect between
the wind blade and SMC recovered fibers. This can be understood by closer examination of the
Weibull tensile strength plots in Figure 4. While these plots show the expected leftward shift to lower
fiber fracture stresses from virgin to pyrolyzed material, Figure 4b also shows single filament tensile
strength measurements of glass fibers recovered from the shredded SMC, before pyrolysis recovery,
which exhibit reduced strength values as well. This indicates that the glass fibers in the SMC material
had already been damaged with a −27% difference in strength before any recycling attempts were
made. The two temperature step pyrolysis can therefore only be responsible for the remaining −17%
of the total −44% difference in final tensile strength from virgin E-glass. As pyrolysis cannot recover
fiber strength that the input material did not possess, these results suggest that pre-existing fiber
damage may be more of a limiting factor in the recycling of glass fiber composites than damage
introduced during fiber reclamation. This damage may also help explain the trend in Weibull modulus,
a non-dimensional measure of the projected spread in fiber tensile strengths within a specimen with
larger values indicating a narrower distribution. Table 1 shows that the virgin E-glass bobbin specimens
start with the narrowest distribution in tensile strength; however, as some of the fibers are damaged at
random through part production, in-service use, and shredding, the distribution in fiber strengths
widens. As all the fibers become damaged by the pyrolysis heat treatment though, the distribution in
stronger and weaker filaments begins to shrink.
Recycling 2019, 4, 24 5 of 12

Also of note from the mechanical test results presented in Figure 5a is the effect of pyrolysis
processing changes on elastic behavior. Two temperature step pyrolysis yields a relatively minor 2–9%
difference increase in Young’s modulus. This increase climbed to 20–30% with the more aggressive
single temperature pyrolysis, matching similar Young’s modulus increase observations reported by
other authors under much more aggressive pyrolysis conditions [1,15]. The improved residual strength
and closer to virgin fiber Young’s modulus of the two temperature step pyrolyzed fibers means such
Recycling
fibers 2019, 4,axnoticeably
exhibit FOR PEER REVIEW
larger strain to failure of 29–43% over their single step equivalents. 5 of 12

Figure 3. Measured
Figure 3. Measured drops
drops in
in glass
glass filament
filament Weibull
Weibull characteristic
characteristic tensile
tensile strength
strength from
from virgin
virgin fiber
fiber
(represented
(represented by the SMC E-glass bobbin sample) to fibers recovered by pyrolysis: (a) wind blade
by the SMC E-glass bobbin sample) to fibers recovered by pyrolysis: (a) wind blade
fibers,
fibers,(b)
(b)SMC
SMCfibers. Comparison
fibers. against
Comparison schematic
against temperature
schematic process curves
temperature illustrates
process curves correlation
illustrates
between reduced higher temperature exposure and improved recovered filament
correlation between reduced higher temperature exposure and improved recovered filament strength.
strength.
Recycling 2019, 4, 24 6 of 12
Recycling 2019, 4, x FOR PEER REVIEW 6 of 12

Figure 4. Weibull probability plots of tested filament fracture strengths: (a) wind blade results, (b) SMC
Figure 4. Weibull probability plots of tested filament fracture strengths: (a) wind blade results, (b)
results. A leftward shift in sample data indicates a reduction in overall fiber tensile strengths.
SMC results. A leftward shift in sample data indicates a reduction in overall fiber tensile strengths.
Table 1. Tabulated Weibull characteristic tensile strength results from single filament tension tests.
Table 1. Tabulated Weibull characteristic tensile strength results from single filament tension tests.
Name Weibull Strength (MPa) Weibull Modulus
Weibull Strength Weibull
Name
SMC E-glass Bobbin 2346 7.5
Recovered SMC Shreds (MPa)
1701 Modulus
3.0
SMCPyrolysis
SMC 2 Step E-glass 1311 8.5
2346 7.5
SMC 1 StepBobbin
Pyrolysis 1254 4.6
Wind Blade 2 Step
Recovered SMC 1405 4.1
Wind Blade 1 Step 1701
1178 3.03.3
Shreds
SMC 2 Step
1311 8.5
Pyrolysis
SMC 1 Step
1254 4.6
Pyrolysis
Wind Blade 2 Step 1405 4.1
Wind Blade 1 Step 1178 3.3

Also of note from the mechanical test results presented in Figure 5a is the effect of pyrolysis
processing changes on elastic behavior. Two temperature step pyrolysis yields a relatively minor 2–
9% difference increase in Young’s modulus. This increase climbed to 20–30% with the more
 Examining average filament diameters in Figure 5b, there does not appear to be a significant
difference between the fiber diameters from the 1 and 2 step pyrolysis procedures for either sample
material; however, comparison of SMC pre-pyrolysis shreds against pyrolyzed fibers does suggest a
small contraction in fiber diameter. Interestingly, there also appears to be a clear difference in fiber
diameter from the original virgin material and both the pre- and post-pyrolysis fibers. This suggests
Recycling 2019, 4, 24 7 of 12
that further glass fiber structural changes may be occurring earlier in original composite production.

Figure5.5.Average
Figure Average individual
individual filament
filament datadata ± standard
± standard error: error: (a) left/red
(a) left/red columnscolumns representing
representing Young’s
Young’s modulus
modulus and right/purple
and right/purple columns representing
columns representing failure
failure strain for strain for each
each tested tested condition,
condition, (b)
(b) average
individual filament diameter.
average individual filament diameter.

The observed changes in Young’s modulus likely stem from the thermal compaction and
3. Discussion
densification of E-glass fibers that is known to occur at elevated temperature ranges [16,17]. Examining
average filament diameters in Figure 5b, there does not appear to be a significant difference between
the fiber diameters from the 1 and 2 step pyrolysis procedures for either sample material; however,
comparison of SMC pre-pyrolysis shreds against pyrolyzed fibers does suggest a small contraction
in fiber diameter. Interestingly, there also appears to be a clear difference in fiber diameter from the
original virgin material and both the pre- and post-pyrolysis fibers. This suggests that further glass
fiber structural changes may be occurring earlier in original composite production.
Recycling 2019, 4, 24 8 of 12

3. Discussion
Perhaps more interesting, though, is the implication of Young’s modulus for the underlying
mechanism behind the pyrolyzed E-glass fibers’ reduced strength. According to the well-established
Griffith criterion, the failure stress of a glass specimen under tension σ f is governed by the relation:
r
2γE
σf = , (1)
πa

where a is the size of an internal, high eccentricity elliptical flaw (modifiable by a constant geometric
factor for other flaw shapes and whether the flaw is internal or located at the surface), E is Young’s
modulus, and γ is fracture surface energy density [18]. Though the exact flaw shape and size
distribution within the surveyed fibers are unknown due to a lack of known method for direct
observation, the responsibility of surface flaws for mechanical failure is clear from Figure 6 where the
observed fiber fracture surface hackle lines lead back to fracture mirror origins at the specimen surfaces.
Examining the SMC material, where the fiber tensile strength right before pyrolysis processing σSMC is
known, it is observed that the two-step process yields 0.77σSMC . For Griffith’s criterion to be satisfied,
the quantity under the square root must also be reduced to 59% of its pre-pyrolysis value. Figure 5a
indicates though that E has only changed about 10%, and so the bulk of the fiber strength loss must
come from changes in a, γ, or both.
Recycling 2019, 4, x FOR PEER REVIEW 9 of 12

Figure 6.
Figure SEM images
6. SEM images ofof example
example fracture
fracture surfaces
surfaces observed
observed from
from recovered
recovered pyrolyzed
pyrolyzed filaments:
filaments:
(a,b) SMC samples; (c,d) wind blade samples. Images captured with a Hitachi S-3400N
(a,b) SMC samples; (c,d) wind blade samples. Images captured with a Hitachi S-3400N SEM.SEM.

In the extended
4. Materials work of Feih et al., virgin E-glass filaments before and after a 450 ◦ C, 30 min
and Methods
heat treatment were artificially notched via focused ion beam (FIB) and pulled in tension to measure
fracture
4.1. toughness
Surveyed Composite From their experiments, a relatively stable value of KIc = 0.91 MPa m0.5 was
KIc . Materials
found, close to the bulk value KIc = 0.93 MPa m0.5 measured by Ghosh et al. [19]. These results indicate
that Composite
any changematerials obtained
in γ, if indeed for isstudy
there any, were
must sourced
be smalldirectly
and so from industrial
fracture manufacturers.
strength changes will
This ensured that the materials characterized would serve as real world representative waste streams
that any composite recycling operation would typically encounter. Before any pyrolysis experiments,
all test materials were shredded down to approximately 50 mm chips to mimic the same size
reduction preprocessing that would be done at a large scale plant. The original E-glass reinforced
automotive SMC parts consisted of 49 wt% fiber, 39 wt% unsaturated polyester/vinyl ester blended
Recycling 2019, 4, 24 9 of 12

be controlled by a. Taking the ratio of Equation (1) for σ f both pre- and post-pyrolysis conditions,
assuming no change in γ and taking the measured changes in E into account, indicates an 86% increase
in flaw size for the SMC fibers following the two-step pyrolysis versus an 145% increase for the
single step. This indicates that, despite the fibers having already been damaged before recycling,
even a basic optimization of temperature processing yielded a −41% difference reduction in final fiber
flaw size. As such, our results not only appear to support the conclusion of Feih et al. that strength
loss from pyrolysis is dominated by temperature driven growth in subcritical surface flaws but that
the relationship between temperature and growth rate can be manipulated to reduce fiber damage
from pyrolysis. However, it is worth noting that our results also contradict their conclusion that
no bulk material property changes occur, as Young’s modulus does appear to have increased more
in agreement with the observations of Thomason et al. [10,15]. Furthermore, our study also shows
that before pyrolysis was even conducted there had already been an 89% growth in surface flaw size
between the pre-pyrolysis shreds and the original virgin fiber. This suggests that glass fiber composite
recycling is more complicated than can be captured with simulated experiments on virgin fiber alone
and that pre-recycling damage can significantly impact final product quality. While the method of
shredding (size reduction) undoubtedly plays some role in the accumulated pre-pyrolysis damage,
ongoing preliminary experiments suggest that shredding is only partially responsible and that other
elements of the original manufacturing process contribute to this degradation.

4. Materials and Methods

4.1. Surveyed Composite Materials

Composite materials obtained for study were sourced directly from industrial manufacturers.
This ensured that the materials characterized would serve as real world representative waste streams
that any composite recycling operation would typically encounter. Before any pyrolysis experiments,
all test materials were shredded down to approximately 50 mm chips to mimic the same size reduction
preprocessing that would be done at a large scale plant. The original E-glass reinforced automotive SMC
parts consisted of 49 wt% fiber, 39 wt% unsaturated polyester/vinyl ester blended matrix, and 12 wt%
calcium carbonate filler. Exact composition percentages for the wind turbine blade specimens are
unknown because the sample came delivered as a pre-shredded mix of large chunks from multiple
wind blades; however, their composition can be estimated from that of the original wind blades being
processed at the disposal site. As such, the sample roughly consisted of 33% fiber, 33% epoxy matrix,
and 33% other blade support materials (primarily PVC foam and balsa wood). Images of the shredded
samples before and after pyrolysis are available in the Supplementary Materials.
For comparison with virgin E-glass fiber properties, samples of the virgin E-glass fiber used
in the automotive SMC were collected straight from the bobbin. While the pedigree of the E-glass
used in the wind turbine blades could not be sourced, the relatively standard behavior of commercial
E-glass means that the SMC bobbin material can also serve as a benchmark for the original wind blade
fiber properties.

4.2. Thermal Treatment Methodology

To determine appropriate temperature conditions for pyrolysis processing, material samples
were probed with a TA Instruments Q500 thermogravimetric analyzer (TGA) under nitrogen.
Three specimens from each test material were heated at a rate of 10 ◦ C/min to determine at what
temperatures the organic phases of the samples begin breaking down. The results of these tests
were used to set conditions for a two temperature step pyrolysis treatment. Before process scale up,
the two-step procedure was simulated with the TGA on three specimens from each material to verify
complete pyrolysis of the samples’ polymer resins.
The TGA detected polymer breakdown temperatures were then used to set the conditions for
pyrolysis of larger sample masses in a tube furnace, continuously purged with flowing nitrogen.
Recycling 2019, 4, 24 10 of 12

To facilitate the two temperature stages of the experimental pyrolysis, the tube furnace was configured
with two independently heated zones, which the samples could be moved between during processing.
Samples were also processed using a single high temperature step pyrolysis for comparison purposes.
Total process time (both temperature ramping and holding) was capped at 40 min for both one and
two step processes. Two-step pyrolysis samples were warmed to the desired process low temperature
and held isothermal for 22 min. At the completion of the low temperature step, samples were further
heated in the high temperature zone and then held isothermal for 11 min. Single step pyrolysis samples
were heated to the desired high temperature over approximately 5 min and then held isothermal for
35 min. The samples were then removed from heating and allowed to rapidly cool under nitrogen
before removal from the furnace.

4.3. Post-Pyrolysis Products Characterization

To assess the effect of the multi-stage pyrolysis processing, the quality of the reclaimed glass fibers
needed to be measured. Individual filaments were extracted from the sample fiber tows by careful hand
separation. In the case of the pre-pyrolysis shred samples, during the shredding process the fiber tows
became sufficiently detached from the adjacent resin matrix to allow manual separation of individual
filaments. Fiber detachment produced during shredding was likely facilitated both by the high fiber
and mineral filler content as well as the fact that during original fiber wet out the liquid resin never
penetrated into the fiber tow. For mechanical performance characterization, single filaments from each
sample material were tested under uniaxial tension using a modified version of ASTM C1557-14 [20].
Individual filament average diameters used for stress calculations were measured via an Olympus
BX50 optical microscope after mounting to paper templates for testing. Tests were performed on
an MTS Alliance RT/5 load frame with an attached 2N load cell. Filaments were tested using a nominal
12.7 mm gauge length and 0.1 mm/s constant displacement velocity. Young’s modulus measurements
were compensated for test system compliance by linear fitting of specimen force normalized filament
length change versus cross-sectional area normalized gauge length [20]. Filament tensile strength
results were treated with a 2 parameter Weibull analysis, as is typical for brittle materials that can
exhibit large scatter in measured results [21]. For the analysis, the filament failure stresses for each
tested fiber condition were ranked using the ASTM C1239-13 recommended probability index formula:

Pi = (i − 0.5)/n, (2)

where Pi is the estimated probability of failure value corresponding to the ith failure stress of n total
measurements [21]. The ranked failure stress datasets were then fitted to the Weibull cumulative
probability distribution function:
m
P σ f = 1 − e−(σ f /σo ) ,
 
(3)
 
where P σ f is the cumulative probability of filament failure, σ f is the measured filament tensile
strength, m is the Weibull modulus (or shape parameter), and σo is the Weibull characteristic tensile
strength (or scale parameter).

5. Conclusions
Two representative materials of common E-glass thermoset composites used in industry were
subjected to one and two temperature step pyrolysis treatments for fiber recovery. Analysis of test
results for each recycled material yielded the following conclusions:

• Adopting a two temperature stage pyrolysis process can achieve polymeric phase removal and
yield recycled glass fibers with improved filament strengths and failure strains.
• Though pyrolysis optimization can improve the quality of recovered fibers, pre-existing damage
from manufacturing, in-service use, and size reduction (shredding) may still act as a limiting
factor for the quality of reclaimed fibers.
Recycling 2019, 4, 24 11 of 12

• Experimental observations coupled with basic fracture mechanics theory indicate that the
improvement in pyrolyzed fiber strength likely comes from an overall reduction in growth
rate of pre-existing surface flaws while at elevated temperature.

Supplementary Materials: The supplementary materials are available online at http://www.mdpi.com/2313-4321/

4/2/24/s1.
Author Contributions: Conceptualization, writing—review and editing R.S.G. and S.O.; methodology, validation,
formal analysis, investigation, writing—original draft preparation, data curation, visualization R.S.G.; resources,
supervision, project administration, funding acquisition S.O.
Funding: The information, data, or work presented herein was funded in part by the Office of Energy Efficiency
and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006926. The information,
data, or work presented herein was funded in part by an agency of the United States Government. Neither the
United States Government nor any agency thereof, nor any of their employees, makes any warranty, express
or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring
by the United States Government or any agency thereof. The views and opinions of the authors expressed herein
do not necessarily state or reflect those of the United States Government or any agency thereof. This manuscript has
been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy.
The United States Government retains and the publisher, by accepting the article for publication, acknowledges
that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or
reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.
The Department of Energy will provide public access to these results of federally sponsored research in accordance
with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).
Acknowledgments: The authors would like to thank the American Composites Manufacturers Association
(ACMA) and its members for technical discussions and supplied test materials. In particular, the authors would
like to thank David Krug at Continental Structural Plastics for providing the automotive SMC materials and
useful feedback.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.

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