Proposed Heat Durability Test for Classifying Adhesives Used in

Transcription

Proposed Heat Durability Test for Classifying Adhesives Used in Cross-Layered Wood Products – An Exploratory Study
Abstract
An exploratory study was conducted to examine the behavior of selected adhesives in the char layer of a
cross-layered wood composite panel. Examples of three adhesive types were evaluated: phenolresorcinol formaldehyde, polyurethane, and polyvinyl acetate. The heat durability test, as described in
CSA Standard O151 and NIST PS 1-07, is a standard test that involves exposing to a Bunsen burner
flame for 10 minutes a plywood specimen bonded with the adhesive. A modified test was also conducted
and preliminary results are discussed.
The standard test has potential as a method for classifying adhesives according to their behavior under
heat exposure. This, in turn, may be correlated to the fire resistance or charring rates of cross-laminated
timber panels. The evaluation would be qualitative in nature relying mainly on the behavior of the
adhesive in the char layer. The occurrence of distinct delamination of the char layer in the test would
provide a means for classifying adhesives that may influence the effective char rate. Fire resistance
model calculations could, for example, be modified to account for adhesives that do not retain char.
ii
Acknowledgements
FPInnovations – Wood Products Division wishes to thank Natural Resources Canada – Canadian Forest
Service for their financial support for this research project. The research team would also like to thank
Momentive (formerly Hexion) Specialty Chemicals Inc., Purbond, and Henkel Corporation for donating
the adhesives and CIPA Lumber Co. Ltd. for donating the veneers used in this study.
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Table of Contents
Abstract .......................................................................................................................................................................... ii Acknowledgements ....................................................................................................................................................... iii List of Tables .................................................................................................................................................................. v List of Figures ................................................................................................................................................................ v 1 Background .............................................................................................................................................................1 2 Objective .................................................................................................................................................................1 3 Study Team ............................................................................................................................................................1 4 Materials and Methods............................................................................................................................................2 4.1 Materials.......................................................................................................................................................2 4.1.1 Adhesive ........................................................................................................................................ 2 4.1.2 Wood ............................................................................................................................................. 2 4.2 Methods .......................................................................................................................................................2 4.2.1 Preparation of Plywood Samples ................................................................................................... 2 4.2.2 Preparation of Test Specimens ...................................................................................................... 3 4.2.3 Testing ........................................................................................................................................... 3 4.2.3.1 Testing in accordance with CSA O151 / NIST PS 1..................................................... 3 4.2.3.2 Modified test ................................................................................................................. 5 5 Results and Discussion...........................................................................................................................................5 5.1 Standard Test...............................................................................................................................................5 4.2 Modified Test.............................................................................................................................................. 10 6 Summary ..............................................................................................................................................................11 7 Literature Cited .....................................................................................................................................................12 ®
© 2010 FPInnovations. All rights reserved.
FPInnovations, its marks and logos are registered trademarks of FPInnovations.
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List of Tables
Table 1 Table 2 Results of the CSA O151 Bunsen burner test (Qualitative) ......................................................................5 Separation of char layer..........................................................................................................................10 List of Figures
Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 CSA O151 heat durability test set-up .......................................................................................................3 Heat durability specimen under test .........................................................................................................4 Progress of heat durability test .................................................................................................................4 Typical char layer characteristics of PRF tested specimen ......................................................................6 Typical char layer characteristics of delaminated PVA tested specimen ..................................................6 Typical char layer characteristics of delaminated PUR tested specimen..................................................7 Other views of char layer characteristics of PUR tested specimen ..........................................................8 Comparison of char layers among three adhesive types under standard test (order from left to right:
PRF, PUR, and PVA)................................................................................................................................9 Comparison of char layers among three adhesive types under modified test ........................................11 ®
© 2010 FPInnovations. All rights reserved.
FPInnovations, its marks and logos are registered trademarks of FPInnovations.
v
1
Background
Cross-laminated timber (CLT) is a new structural wood product being considered for production in North
America. Because panels are likely to be greater than four inches in thickness, it is tempting to assume
they possess the same good fire performance of heavy timber construction. As the types of CLT to be
manufactured in North America are yet to be determined, there is a need to understand how the various
components in a CLT panel contribute to its fire resistance. The performance of the adhesive under fire
conditions and its expected behavior in a CLT panel is a fundamental area that requires study.
In a recent study (Frangi et al., 2009), it was found that the fire behavior of CLT panels was strongly
influenced by the behavior of the adhesive used for bonding the individual layers. For “temperaturesensitive” adhesives, charred layers were observed to separate from the panel at bond lines in the charred
zone. Because char is a good insulator, the char layer that develops around thick wood members
normally serves as a protective layer to reduce the temperature that the wood is exposed to. This, in turn,
reduces the char rate and allows the member to retain a portion of its original strength during a fire.
However, if the char layer were to fall off, the un-charred surface is directly exposed to high temperature
and the char rate increases again. For less temperature-sensitive adhesives, the study noted that charred
layers almost remained in place until the end of the test. For example, for specimens manufactured with
temperature-sensitive adhesives (i.e., some types of polyurethanes), in which falling off of charred layers
was observed, the fire resistance of the CLT with five layers ranged from 60 to 73% that of solid timber
of the same thickness with an assumed fire resistance of 92 minutes. On the other hand, for specimens
manufactured with a less temperature-sensitive adhesive (i.e., melamine-urea formaldehyde), in which the
charred layers were observed to almost remain in place, the fire resistance ranged from 107 to 113% that
of solid timber. Fire resistance was defined as the time until the CLT test specimen was completely
charred, i.e., when the temperature between the CLT specimen and the timber panel glued on the
unexposed side of the CLT reached 300ºC.
The above observations indicate that the properties of the bonding adhesive at high temperature can
influence the effective charring rate of CLT panels. Thus, it appears that the adhesive plays an important
role in determining the fire resistance of CLT panels.
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Objective
The objective of the study was to examine the effect of adhesive type on the charring behavior of
plywood (a cross-layered panel). The test used was the Bunsen burner test as described in CSA Standard
O151 (2009) using plywood panels to simulate the adhesive effect in CLT. This is the same test found in
NIST PS 1-07 (2007).
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Study Team
Romulo Casilla
Conroy Lum
Ciprian Pirvu
Research Consultant (formerly Senior Research Scientist with Canfor Research
and Development
Group Leader and Senior Scientist, Building Systems, FPInnovations, Wood
Products Division
Project Leader and Wood Engineering Scientist, Building Systems,
FPInnovations, Wood Products Division
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Paul Symons
Isaac Chiu
Axel Andersen
Structures Systems Engineer, Building Systems, FPInnovations, Wood
Products Division
Technologist, Building Systems, FPInnovations, Wood Products Division
Technologist, Wood Composites, FPInnovations, Wood Products Division
4
Materials and Methods
4.1
Materials
4.1.1
Adhesive
The adhesives evaluated were the following:
•
•
•
Phenol-resorcinol formaldehyde (PRF). This is a well-known performing adhesive for structural use.
Polyurethane (PUR). This is a structural adhesive that is relatively new compared with PRF.
Polyvinyl acetate (PVA). This is considered a non-performing adhesive and generally not suitable for
structural applications where the bond line may be subjected to a sustained shear load. It is
commonly used in the fingerjoint industry for the production of “vertical stud use only” fingerjoined
lumber.
4.1.2
Wood
The wood material used was 1/8-inch thick rotary-cut Douglas-fir veneer, 4 x 8-foot sheets (full-size),
obtained from a local plywood mill. Sixteen full sheets were obtained from the mill. Three sheets were
selected at random, and cut into 12 x 12-inch veneer substrates. The substrates were selected such that
they were free of knots and other defects. They were randomized in the preparation of the plywood
samples.
4.2
Methods
4.2.1
Preparation of Plywood Samples
The veneer substrates were conditioned at 20°C and 65% relative humidity to yield equilibrium moisture
content (EMC) of approximately 12%.
Five-ply plywood samples were prepared. As indicated above, the veneer substrates were randomized.
The substrates were laid up in accordance with section 7.1.2 of CSA O151, e.g., the “tight” sides of the
outermost plies were exposed. In addition, the veneers were laid up in such a way that the outermost
bond lines had a “tight-loose” side interface. This way both outer bond lines would have similar
characteristics and either face of the plywood sample could be used for the fire exposure test.
The plywood samples were pressed in accordance with the adhesive manufacturer’s recommendations.
All samples were pressed at a specific pressure of 125 psi. The PRF samples were pressed at a platen
temperature of 150ºC until a bond line temperature of 85°C (185ºF) as measured by a thermocouple
inserted in one of the innermost bond lines was reached. The PUR samples were pressed for 2 hours at
ambient room temperature, which at the time of specimen preparation was at least 20°C. The PVA
samples were also pressed at ambient room temperature for 1 hour at a bond line temperature of about
20ºC as measured by a thermocouple.
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Nine plywood samples were prepared, three for each adhesive type. The samples were conditioned to
12% EMC.
4.2.2
Preparation of Test Specimens
Each plywood sample was cut into two test specimens, 140 x 203 mm (5.5 x 8.0 inches), with the longer
dimension along the grain direction of the outer plies, in accordance with CSA O151. This provided a
total of 18 specimens, six for each adhesive type. The 2 x 5.5-inch (51 x 140-mm) trimmings across the
width were saved for the modified test described in Section 4.2.3.2. The specimens were conditioned to
12% EMC prior to testing.
4.2.3
Testing
Testing was performed at the CertiWood Technical Centre in North Vancouver, B.C. CertiWood is
accredited to carry out the Bunsen burner test by the International Accreditation Service, Inc.
4.2.3.1 Testing in accordance with CSA O151 / NIST PS 1
The test set-up, consisting of a lighted Bunsen burner and specimen stand, is shown in Figure 1. The
temperature of the flame measured by a thermocouple held close to the top end of the flame was about
821°C (1510ºF). This was within the temperature range specified in CSA O151, which is 850 ± 50°C.
For the two specimens prepared from each plywood sample, one was tested on the face (i.e., the side
exposed to the flame) and the other on the back side. Figure 2 shows a specimen being tested, and Figure
3 shows the progress of the test. The test was carried out for 10 minutes. The specimen was removed
from the stand and the charred ply was examined for delamination (separation) from the bond line.
Figure 1
CSA O151 heat durability test set-up
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Figure 2
Heat durability specimen under test
Figure 3
Progress of heat durability test
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4.2.3.2 Modified test
An exploratory test was also conducted using the 2 x 5.5-inch (51 x 140-mm) trimmings from the
plywood samples. The same set-up described above was used in this test, but the edge of the specimen
was exposed directly to the flame. Since the depth of the specimen was only 2 inches (51 mm), the flame
wrapped around opposite edges of the specimen. The test was also carried out for 10 minutes.
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Results and Discussion
5.1
Standard Test
The results of the test, by visual observation, are summarized in Table 1. All of the PRF specimens
showed no visible char delamination, about 60% of the PVA specimens exhibited delamination, while all
of the PUR specimens showed very distinct delamination. The typical char layer characteristics of the
PRF, delaminated PVA, and PUR tested specimens are shown in Figures 4, 5, and 6, respectively.
Figures 7a and b show side views of the char layer of the PUR tested specimen. Figure 8 shows a
comparison of the char layers among the three adhesive types tested. The performance observed for the
PVA tested specimens, which was between that of PRF and PUR, was probably because the PVA used
was a modified adhesive. This was confirmed by the adhesive supplier.
Table 1
Results of the CSA O151 Bunsen burner test (Qualitative)
Specimen No.*
Visual Observation
PVA1-1
PVA1-2
PVA2-1
PVA2-2
PVA3-1
PVA3-2
Char layer scraped to examine inner layer of specimen
Some delamination
Some delamination
Some delamination
Charring, no delamination
PRF1-1
PRF1-2
PRF2-1
PRF2-2
PRF3-1
PRF3-2
PUR1-1
PUR1-2
PUR2-1
PUR2-2
PUR3-1
PUR3-2
Char layer delaminated
Some delamination
Some delamination
* PVA - Polyvinyl acetate
PRF - Phenol-resorcinol formaldehyde
PUR - Polyurethane
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Figure 4
Typical char layer characteristics of PRF tested specimen
Figure 5
Typical char layer characteristics of delaminated PVA tested specimen
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Figure 6
Typical char layer characteristics of delaminated PUR tested specimen
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(a)
(b)
Figure 7
Other views of char layer characteristics of PUR tested specimen
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Figure 8 Comparison of char layers among three adhesive types under standard test (order from left
to right: PRF, PUR, and PVA)
The char layer of the PVA tested specimen consisted of larger pieces which were loose to the touch and
with less checking along the length, as compared to that of the PRF tested specimen in which the char
layer consisted of much smaller pieces that appeared to be intact and with more frequent checking.
Because the bond line remained intact, it likely caused the char layer to form more checks to relieve the
stresses, instead of delaminating. By relying just on visual observation, the difference in the appearance
of the char layer between the PRF and the PVA tested specimens may not be quite distinct. However, this
should not be much of a concern because the PVA adhesive used can be screened out by one or more of
the tests included in the structural wood adhesive standard, CSA O112.10, which specifies the minimum
requirements for adhesives used for CLT.
On the other hand, the char layer of the PUR tested specimen was distinctly raised above the specimen
surface consisting of much bigger pieces and clearly showing delamination with openings (four out of the
six specimens). These results corroborated the observations made earlier by Frangi et al., (2009). While
the above authors observed in their study that the char layers of the specimens bonded with PUR fell off,
the char layer of the PUR-bonded specimens in the present study did not fall off because it was thin and
restrained by the un-charred portion of the surface which was not exposed to the flame. However, the
openings in the char layer would allow faster spread of the flame into the next un-charred layer resulting
in an increase in the char rate, which would reduce the fire resistance of the material as observed by
Frangi et al., (2009). However, it should be noted that the purpose of the current study is not to measure
the char rate but to understand the adhesive behavior during charring, i.e., bond line delamination in the
char layer.
With the above observation, the char layer separation (i.e., height of the char from the un-charred wood
surface) may be used as a possible evaluation for the fire resistance of the adhesive or the likelihood that a
char layer would fall off during a fire test. Table 2 gives a comparison of the char layer separation
between the PUR and PRF tested specimens. Measurements were made using a caliper. The results
showed that the char layer separation for the PUR was up to about 10 mm, while that of the PRF was less
than 0.4 mm. In fact, half of the PRF tested specimens exhibited char layer surface receding below the
un-charred wood surface indicating that the char layer actually contracted as a result of the bond line
remaining intact, as indicated above.
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Table 2
Separation of char layer
Specimen
No.
PUR1-1
PUR1-2
PUR2-1
PUR2-2
PUR3-1
PUR3-2
Charred + Plywood
Thickness (mm)
Scraped off
22.96
25.98
18.81
17.57
22.88
PRF1-1
Scraped off
PRF1-2
15.93
PRF2-1
15.42
PRF2-2
below thickness of board
PRF3-1
PRF3-2
* Not available
4.2
Plywood
Thickness
(mm)
N/A*
15.89
15.84
15.87
15.76
15.78
Char Layer
Separation
(mm)
N/A
7.07
10.14
2.94
1.81
7.10
N/A
15.57
15.37
N/A
0.36
0.05
Modified Test
The results are shown in Figure 9. Visible delamination was observed in the bond lines of the PVA and
PUR tested specimens, but not in the PRF tested specimen. The total bond line delamination measured
was 86 mm for PVA, and 146 mm for PUR. The length of the charred region was 102 mm for PRF
giving a total char bond line length (CBL) of 408 mm for the four bond lines, 106 mm for PVA for a total
CBL of 424 mm, and 105 mm for PUR for a total CBL of 420 mm. Based on the total CBL, the
percentage delamination was calculated to be 20.3% for PVA and 34.8% for PUR.
The average original thicknesses of the specimens were PRF (15.13 mm), PVA (15.90 mm), and PUR
(15.78 mm), and those of the tested specimens were PRF (14.68 mm), PVA (15.59 mm), and PUR
(16.25 mm). These data show that the thickness of the PRF tested specimen contracted by about 3%, that
of the PVA contracted by about 2%, while that of the PUR expanded by about 3%. Since no
delamination was observed in the PRF tested specimen, the contraction was probably caused by the
shrinkage of the wood during the test. It is reasonable then to assume that the same amount of wood
shrinkage also occurred in the PVA and PUR tested specimens. Based on this assumption, the
delamination observed in the PVA tested specimen caused a thickness expansion of about 1%, while that
in the PUR tested specimen caused an expansion of about 6%. These results corroborated the char layer
separation results presented in Table 2 above for the PUR and PRF.
It is to be noted that the results of the modified test are not required by the standard.
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Figure 9
6
Comparison of char layers among three adhesive types under modified test
Summary
The Bunsen burner test described in CSA O151 / NIST PS 1, using five-ply plywood made of 1/8-inch
veneer, has potential as a method for classifying adhesives that might impact differently the fire resistance
of CLT panels. The evaluation would be qualitative in nature relying mainly on the behavior of the char
layer. The occurrence of distinct delamination or separation of the char layer from the un-charred wood
surface, particularly with wide openings, would provide a means for classifying adhesives that may
influence the effective char rate. As pointed out by some fire code authorities, it is essential to recognize
the impact of the type of adhesive used in bonding CLT products. Fire resistance model calculations can
be modified to account for adhesives that do not retain char.
The modified test described above may also have potential for classifying adhesives. The evaluation
would be based on the magnitude of delamination of the bond lines and / or the movement (contraction or
expansion) of the cross section of the flame-exposed zone with exposed bond lines. The former would
probably be preferred as it is a more common method of evaluation. However, only a small number of
specimens were used with this test, thus further testing is recommended. So for now, the attention should
be focused more on the standard test.
In both tests, PRF exhibited behavior suggesting that the bonded specimen would behave similar to solid
wood above charring temperatures. PVA showed slightly different, and PUR significantly different
behavior to solid wood. These observations apply only to the adhesive formulations tested in this study.
They may not apply to other formulations of PRF, PVA, and PUR adhesives. In addition, these results
apply only to rotary-cut veneer, which contains lathe checks. Consequently, the results may not be
replicated when using other types of veneer that do not have lathe checks. It is suggested that further
studies be conducted using sliced veneer or sawn veneer to assess the effect of lathe checks on the
observations.
It is suggested that further tests be conducted using CSA O151 / NIST PS 1 procedure to include
adhesives that have passed CSA O112.9 / O112.10, as well as adhesives that are well-known to be nonperformers. It is probably meaningful to carry out the standard Bunsen burner test to evaluate the
adhesive performance in addition to full-size CLT panel fire tests. Although PVA will not be used for
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CLT production, it would be useful to carry out full-size fire tests on PVA bonded CLT panels to help
interpret the results from the small-scale test. Given the modest cost of this test compared to most fire test
on full-scale CLT panels, it is also recommended that this test be carried out alongside such tests.
7
Literature Cited
1. CSA Standard O151-09. 2009. Canadian softwood plywood. Canadian Standards Association,
5060 Spectrum Way, Suite 100, Mississauga, ON.
2. Frangi, A., M. Fontana, E. Hugi, and R. Jobstl. 2009. Experimental analysis of cross-laminated
timber panels in fire. Fire Safety Journal 44 (2009)1078-1087.
3. NIST PS 1-07. 2007. Structural Plywood. Voluntary Product Standard, National Institute of
Standards and Technology, Technology Administration, U.S. Department of Commerce.
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Proposed Heat Durability Test for Classifying Adhesives Used in

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