matuana2002 (1)

Manufacture of Rigid PVC/Wood-Flour Composite Foams
Using Moisture Contained in Wood as Foaming Agent
LAURENT M. MATUANA* and FATIH MENGELOGLU
School of Forestry and Wood Products
Michigan Technological University
Houghton, Michigan 49931-1295
Relationships between the density of foamed rigid FVC/wood-flour composites
and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all-acrylicfoam modifier, and the extruder die temperature were
determined by using a response surface model based on a four-factor central composite design. The experimental results indicated that there is no synergistic effect
between the CFA content and the moisture content of the wood flour. Wood flour
moisture could be used effectively as foaming agent in the production of rigid
WC/wood-flour composite foams. Foam density as low as 0.4 g/cm3 was produced
without the use of chemical foaming agents. However, successful foaming of rigid
WC/wood-flour composite with moisture contained in wood flour strongly depends
upon the presence of all-acrylic foam modifier in the formulation and the extrusion
die temperature. The lowest densities were achieved when the all-acrylic foam modifier concentration was between 7 phr and 10 phr and extruder die temperature was
as low as 170°C.
INTRODUCTION
E€F
xtruded PVC/wood-fiber composite profile is in
owing demand as a substitute for solid wood
components in the building industry because of the
favorable performance and cost attributes of both wood
fibers and WC matrix (1-5). The higher density, however, at approximately 1.4 g/cm3 for WC/wood-fiber
composites, combined with their brittleness and lower
impact resistance compared to unfilled W C and/or
~ t u r a solid
l
wood (Table 11, may prevent this emerging class of materials from capturing their full market
potential in applications such as decking, siding, and
window and door frames. Fortunately, the foaming of
WC/wood-fiber composites has been proven to overcome some of these shortcomings (6-9).
In the past few years, several investigators have
examined the foamability of WC/wood-fiber composites through batch (6-10) or continuous extrusion processes (11,12).Recently, Einley (12)patented a foamed
composite member comprised of vinyl polymer and
wood fiber. According to his approach, WC/wood-&r
composites containing up to 50% of wood fiber can be
foamed by using physical or chemical foaming agents
(12). The potential of using water as a blowing agent
has also been reported (13). Moisture occurs in wood
*Corresponding author. Current address: Department of Forestry. Michigan
State University. East Lansing, M-i
48824-1222.
264
in three Merent forms: (i)as a free water contained in
the cell lumens, (ii) as a bound water located within
the cell walls, and (iii) as water of constitution, which
is part of the molecular structure of the wood, in the
forms of hydrogen and hydroxyl groups (14-16). According to Rizvi et aL (13),the moisture available in
the wood fibers can be transformed into the gaseous
state at temperatures exceeding 100°C. Bound water
is quickly converted into gas and lost from the extruder’s hopper; the remaining water of constitution (almost 3% based on oven-dry weight of wood) is utilized
as a foaming agent. PS/wood-fiber and HIPS/woodfiber composites were successfully foamed using this
approach (13).However, higher processing temperature
was needed in the extruder barrel (205°C) because of
the high-energy requirement to dislodge water of constitution, which is linked to the cell walls within the
fiber (15.16).In addition, the incorporation of a chemical foaming agent was needed to increase the volume
expansion of foamed samples.
The feasibility of foaming all-acrylic modified rigid
WC/wood-flour composites using wood flour moisture
as foaming agent has also been reported (11).Since
an all-acrylic foam modifier traps the evolving gas and
prevents bubbles from coalescjng during the foaming
process (17-19). it is hypothesized that the bound
water, which requires less energy to remove from fiber
cell wall than water of constitution (15, 16). could effectively be used as a foaming agent (11). In addition,
JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4
M a n u f a e of Rigid WC/Wood-FlourFoam
Table 1. Denlsities of Various Materials.
~~
Table 2. Experimental Formulations Used in Rigid
PVCMlood-Flour Composites.
~~~
Materials
Density (9/cm3)
HDPE/wood-fiber composite lumber'
PVC/wood-fiber composite lumber'
HDPE'
PVC'
Eastem white pine2
Red maple2
0.70-1 2 0
0.70-1.36
0.95
1.37
0.35
0.54
'Commercial products.
2Values obtained from reference (14)
Ingredients
Concentrations (phr)
PVC (PolyOne C o p )
Tin stabilizer (PlastiStab)
Calcium stearate (Synpro)
Paraffin wax (Gulf Wax)
Processingaid (Paraloid K-120)'
Processingaid (Paraloid K-175)'
Wood flour (American Wood Fibers)
All-acrylic foam modifier (Paraloid K-400)'
AZRV CFA (Uniroyal Chemicals)
100
2
1.2
1
1.2
1
50
0-10
0-1
'Paraloid K-120, K-175 and K-400 were supplied by Rohm B HaasCo.
owing to the lower energV needed to remove bound
water from the cell wall, the processing temperature
could be substantiallly lower during the foaming process. The present study was conducted to verify these
hypotheses.
The main objectives of this investigation are to examine the effects of wood flour moisture content, allacrylic foam modifier content, and extruder die temperature on the faunability of rigid WC/wood-flour
composites. The eOlect of incorporating a chemical
foaming agent (CFA) during foaming process is also
examined to evalu.ate possible synergistic effect between the CFA and the moisture contained in the wood
flour.
Since the number of variables and ranges of interest
are large, the rehti~lonshipsamong the factors and levels cannot be fully understood with simple linear regression models. Fkesponse surface methodology, capable of opthizing a response of interest influenced
by several variables, is required to effectively model
these complex interactions. The central composite design (CCD) is used in this study because it provides
higher level models such as quadratic response models without the reihndancy of a full factorial design
with three levels (20-22).
IEXPERIHENTAL
lmterhb
WC resin with a K value of 57 was used as the polymer matrix. Commercial wood flour from hardwood
maple species (100-mesh size) was utilized as filler.
The moisture from wood flour and modified amdicarbonamide WRV) were used as foaming agents. Allacrylic foam modifier was consumed as a processing
aid. Other ingredients incorporated into the experimental formulations and the suppliers of the materials used in this study are listed in Table 2.
Wood flour with Werent moisture contents was obtained by conditioning wood flour in a walk-in temperature/humidity-controlled environmental chamber for
several days until the desired moisture content was
attained for processing. The temperature and relative
humidity in the room were set at 22°C (72°F)and 65%,
respectively, to yield the equilibrium moisture content
(EMC)of 12%.
~
e
n
t
aDeaign
l
A four-factor central composite design (CCD) was
used to develop a response surface model (RSM) to
study the foamability of rigid WC/wood-flour composites. Wood flour moisture content, CFA content, allacrylic foam modifier content, and extruder die temperature were the four treatments studied: the density
of the foamed composites was the response variable
observed in this investigation. The CCD design matrix
was generated by [email protected] (StatEase Corp., Minnesota). The Design-Expert@software
provides two different methods of displaying the levels
of the factors in a design of experiments: 1) the actual
levels of the factors (i.e., the actual values in the experiment) and 2) the coded factor levels (i.e., as -1 for
low levels, + 1 for high levels, and 0 for centerpoint).
The coded factor levels are defined as (20-23):
coded factor levels =
Acfual value - Factor mean
(Range of thefactorial udue/2)
( 1)
The design levels in terms of actual and coded factor levels and associated response values are given in
Table 3.The average of twenty replicates was used for
each datum used in the RSM.
Compounding and E&nsaion Forming
According to the experimental design, wood flour
with different moisture contents, chemical foaming
agent and all-acrylic foam modifier, at various loading
levels, were dry-blended with WC matrix in a 20-L
high-intensity mixer (papenmeier, "ype TGAHK20) for
15 minutes. The addition level of wood flour in the
mixtures was fixed at 50-phr based on the oven-dry
weight of wood flour. The amount of wood flour incorporated into the formulations was corrected by taking
into account the moisture contained in the wood
flour. This correction was performed by using the following equation ( 16):
w,= w,.
(l+E)
where W, is the moist weight, W, is the oven-dry
weight, and MC is the percent moisture of wood.
JOURNAL OF VINWL 8 ADDITIVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4
265
Lawent M. Matuana and Fatih Mengeloglu
Table 3. Central Composite Design (CCD) Matrix in Terms of Actual and Coded Factor Levels
Generated by the Design-Expert@Software.
Response
Factors
Wood flour
MC
Experiment
number
Types
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Axial
Fact
Fact
Fact
Fact
Fact
Fact
Fact
Fact
Axial
Axial
Axial
Center
Center
Center
Center
Center
Center
Axial
Axial
Axial
Fact
Fact
Fact
Fact
Fact
Fact
Fact
Fact
Axial
(%I1
CFA
content
(PW’
Acrylic
content
(PW
0.5 (0)
0.25(-1)
0.25(- 1)
0.75(+1)
0.75(+1)
0.25(-1)
0.25(-1)
0.75(+1)
0.75(+1)
0.5 (0)
0.0 (-2)
0.5 (0)
0.5 (0)
0.5 (0)
0.5 (0)
0.5 (0)
0.5 (0)
0.5 (0)
0.5 (0)
1.0 (+2)
0.5 (0)
0.25(-1)
0.25(-1)
0.75(+1)
0.75(+1)
0.25(-1)
0.25 (-1)
0.75(+1)
0.75(+ 1)
0.5 (0)
5.0(0)
2.5(-1)
2.5(-1)
2.5(-1)
2.5(-1)
7.5(+1)
7.5(+1)
7.5(+1)
7.5(+1)
0.0(-2)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
5.0(0)
10(+2)
2.5(-1)
2.5(-1)
2.5(-1)
2.5 (-1)
7.5(+1)
7.5(+1)
7.5(+1)
7.5(+1)
5.0(0)
Die
temperature
(“C)’
Density
gkm3
170 (-2)
180 (-1)
180 (-1)
180 (-1)
180 (-1)
180 (-1)
180 (-1)
180 (-1)
180 (-1)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
190 (0)
200 (+1)
200 (+1)
200 (+1)
200 (+1)
200 (+1)
200 (+1)
200 (+1)
200 (+1)
210 (+2)
0.48
0.80
0.70
0.81
0.80
0.60
0.44
0.62
0.41
0.98
0.65
0.82
0.66
0.65
0.64
0.64
0.65
0.66
0.61
0.74
0.55
0.89
0.88
0.96
0.84
0.71
0.57
0.76
0.66
0.88
‘The values in parenthesesrepresentthe coded factors.
After blending, the compounded mixtures were extruded through a 19.1 mm single screw extruder with
a L/D ratio of 30:1 (C.W. Brabender Instruments, Inc.)
driven by 5 hp Prep-Center@(C.W. Brabender Instruments, Inc.) to produce foams. The screw speed was set
at 40 rpm throughout the experiments. The extruder
barrel temperature profile for the first three heating
zones was set at 165/170/180°C. Since the die temperature was one of the independent variables, it was
varied according to the experimental design.
Foam CharactdzOtion
The densities of rigid PVC/wood-flour composite
foams were measured by the water displacement technique (ASTM standard D792). SEM micrographs were
taken from a JEOL JSM-35C scanning electron microscope to determine average cell sizes of foamed samples. Samples were gold coated to provide the necessary
conductivity to the surface for the electron microscope.
the WpOllSe sllrfarrc lyIodel
A central composite design was used to develop a
response surface model (RSM) relating the density
of foamed rigid WC/wood-flour composites to four
266
i
8 Of
+
+
Density = + 0.67 - 0.053A 0.01923 - 0.12C
0.0790 + 0.014A2 0.027C2 - 0.023AC
+
RESULTS AND DISCUSSION
W
d
C
dm
factors: wood flour moisture content, all-acrylic foam
modifier content, CFA content, and extruder die temperature.
A regression analysis was performed on the density
measurements from the extrusion foaming experiment
to obtain the best-fit model equation for the experimental data (Table4). A standard analysis of variance
(ANOVA) showed that the density data was best fit
with a quadratic model (values of ’Rob > F” less than
0.0001). Because the model contained significant and
non-significant terms (Table4). it was simplified by
dropping insignificant terms. This modification did not
affect the adequacy of the model because the R2 and
adjusted R2 for this reduced model were satisfactory
and the predicted R2 value improved (Table5).Therefore, the derived regression equation describing the relationship between the density and the four variables
(in terms of coded factors)was reduced to:
(3)
where A is the wood flour moisture content, B is the
chemical foaming agent content, C is the all-acrylic
foam modifier content, and D is the extruder die temperature.
JOURNAL OF VINYL & ADDITlVE TECHNOLOGY, DECEMBER 2002, Vol. 8, No. 4
M a n u f m e of Rigid WC/Wood-FlourFoam
Table 4. Analysis of Variance (ANOVA) for Responae Surface Quadratic Model.
Source
Sum of
squares
Degrees of
freedom
Mean
square
F-Value
Prob > F1
Quadratic model
0.580
14
0.042
35.78
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.067
8.43E-003
0.320
0.150
7.52E-003
3.66E-003
0.023
1.67E-0.003
5.63E-005
8.56E-0.003
7.56E-0.004
6.25E-006
3.06E-0.004
1.801E-003
57.63
7.24
274.16
127.64
6.45
3.14
19.87
1.44
0.048
7.34
0.65
0.005
0.26
1.55
< o.Ooo1
< o.oO01
A
B
C
D
A2
82
C2
D2
AB
AC
AD
BC
BD
CD
0.067
8.43E-003
0.320
0.150
7.52E-003
3.66E-003
0.023
1.67E-0.003
5.63E-005
8.56E-0.003
7.56E-0.004
6.25E-006
3.06E-0.004
1.801E-003
0.0168
< 0.0001
< o.oO01
0.0226
0.0965
0.0005
0.2494
0.8291
0.0162
0.4332
0.9426
0.6258
0.2324
'Values of "Prob > F' less than I).O500 indicate model terms am significant. In t h s case A, 6,C, D. A2, C2,and AC are the mgnikmi model terms.
Table 5. Values of Coefficient of Regression (Rz)
for the Full arid Reduced Quadratic M o d e l s
Selected From ANOVA Analysis.
Types of R
R*
Adjusted R2
Predicted R2
Full
0.9707
0.9438
0.8354
Models
Reduced
0.9582
0.9449
0.9249
Notice that all the first-order main effects (A, B, C,
and D), most significant second-order main effects (A2
and C2).and the only significant two-factor interaction
term (AC) (Table 4) are included in this equation. The
relative effect of each factor in this equation is expressed by its coeficient and its algebraic sign (23,24).
It is seen from Eq 3 that the density of foamed composites decreases with all-acrylic foam modifier content (factor C) and wood flour moisture content (factor
A) increase (negative algebraic sign in Eq 3).In contrast, the density of foamed composites increases with
the CFA content (factor B) and die temperature (factor
D) increase (positive algebraic sign in Eq 3).
The perturbation plot of density against all four investigated variables (FQ. I) provides supporting evidence of the relative importance of all-acrylic foam
modifier (factor C) and wood flour moisture content
(factor A) on the density of foamed rigid WC/woodflour composites. Figure I shows changes in the density as each factor moves from the chosen reference,
with all other factors held constant at the middle of
the design space (the coded zero level of each factor)
Perturbation
'i
0.9
Hg. 1 . Perturbation,dotof demity
againstfow~~trtedvariables.
A represents the peirent moisture
o~ntainedin w o o d j b ~ :B is the
chemical foaming cgent content;
c is the all-acrylic.foam modLfier
content: and D is the ex&uder die
tempemhrre.
I
-1.o
I
I
I
-0.5
0.0
0.5
I
1.o
Dewation from Reference Point
JOURNAL OF ViNKL & ADDitrVE TECHNOLOGY, DECEMBER 2002, Voi. 8, No. 4
267
Laurent M . Matuana and Fatih Mengeloglu
(22).Once again, the perturbation graph clearly isolates all-auylic foam modifier (factor C) and wood flour
moisture (factorA) as the two most important variables
reducing the density of rigid WC/wood-flour composites. Chemical foaming agent (factorB) produces a relatively small effect whereas the density increases with
the increased die temperature (factorD).
Since the two main factors reducing the density of
composites (A and C) are part of the only significant
interaction (AC) (Eq 3).it would be inappropriate to investigate these factors individually because the effect
of one factor will depend upon the level of the other
(25).
-
'"14
Figure 2 shows the interaction graphs of the variation in density as a function of the interaction between
all-acrylic foam modifier content and wood flour moisture content (interaction AC) for samples produced
without CFA (Figs.2a-b) and by using 1-phr CFA
(Figs. 2c- d ) .The extruder die temperature effect is also
illustrated in these figures (170°C for Figs. 2a and 2c
and 210'C for Figs. 2b and 24.
1.0-
(a)
170°C and no CFA
T
3%
0.8
-
0.8-
0
0
Y
Y
0,
0,
v
v
.b
Effect of Au-amylic foam modifier Contsntl
wood Flour 1yIoimtara content Intarpcuan
(Interadon AC) on the Density of Rigid
PVCIWood-FIour compomites
2
0.6-
In
C
In
n
0
0.6-
C
m
0,
0.5-
0.5-
2 10°C and no CFA
-/
-
0.3
0.3
I
2.5
I
I
I
I
'
3.8
5.0
6.3
7.5
2.5
All-acrylic processing aid content (phr)
3.8
5.0
6.3
7.5
All-acrylic processing aid content (phr)
1.0-
(c)
170°C and 1phr CFA
-
0.8-
0.8
0
Y
0,
v
0.7-
32
w3
tn
C
m
a
I
21OoC and 1 phr CFA
-L
0.51
0.3
4
4
0.3
1
2.5
I
I
I
I
3.8
5.0
6.3
7.5
All-acrylic processing aid content (phr)
'
2.5
3.8
5.O
6.3
7.5
All-acrylic processing aid content (phr)
Fig. 2. Interaction graphs of the uariation in density as afunctionof the interaction between all-acrylicfoam nwd@er content and
luwdflour moisture content (MC)for samples produced without CFA (Figs.2a and 2b) and by using 1 phr of CFA (Figs. 2c and 24.
"he exbuder die temperature eflfect is also illustrated in thesejQures (1 70°Cfor Figs. 2a and 2c and 21 0°C for Figs. 2b and Zd).
268
JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 2002, V d . 8, No. 4
Manufacture of Rigid WC/Wood-FlourFoam
Interaction between all-acrylic foam modifier content and wood flour moisture content has important
effects on the density of rigid WC/wood-flour composite foams (Figs.:?a-d).A strong density dependence on both all-acrylic foam modifier content and
wood flour moisture content is observed. The density
of foamed compositt: decreases with all-acrylic foam
modifier content and wood flour moisture content increase. However, the effect of wood flour moisture
content is strongly d.ependent on the concentration of
all-acrylic foam modifier. Rgures 2a-d illustrate that
the wood moisture content has little effect when the
concentration of all-acrylic foam modifier is a t the
lowest level. The effect of wood flour moisture content
becomes large at higher acrylic processing aid contents. This behavior is clearly seen by comparing the
magnitude of the difference between the lowest (3%)
and highest (9%) wood flour moisture content lines
(Rgs.2a-d).The difference is small at low all-acrylic
foam modaer content whereas opposite trend is observed at higher concentration of all-acrylic foam modifier. These results imply that less amount of all-acrylic
foam modifier is required for a given level of foam density when wood flour with high moisture content is
used in the formulation.
The above-descrilxd interaction between all-acrylic
foam modifier content and wood flour moisture content was expected because the all-acrylic foam modifier improved melt elasticity of the matrix (17-19), so
that evolving gas is trapped and the bubbles are prevented from coalescing during the foaming process.
With the increase of wood flour moisture content, the
amount of gas dissolving in the molten polymer matrix for nucleation ;md cell growth also increases. As a
result, nucleated bubbles can grow more easily and
the foam density is reduced. Moreover, the changes in
the average cell size of foamed composite samples due
to the effect of wood flour moisture content illustrate
this phenomenon. The results listed in Table 6 shows
that foamed samplles with dried wood (sample 1) produced smaller cell :sizeswhen compared to the samples
with 12% moisture content (sample 2).These results
imply that the moiisture present in wood flour acts as
an effective foaming agent, but the addition of allacrylic modifier is needed in the formulation for effective bubble growth.
The effect of incorporating a chemical foaming agent
(CFA) during foaling process was also examined to
Table 6. The Effect of Different Processing Conditions on the
Average Cell Size OF the Rigid PVCMlood-Flour Composites
Foamed Without Using Chemical Foaming Agents.
Average cell size
Sample identification'
(WY
0% MC-5 phr K40(l-170°C (Sample 1)
12% MC-5 phr K400-170°C (Sample 2)
12% MC-5 phr K400-210°C (Sample 3)
60
254
87
'MC IS the moisture content of wood flour; K-400 IS the allacrylic foam modher; and the
temperature descnbes the wtruder die temperature used dunng the expenments
*Average cell sizes were measured directlyfmm SEM mlcmgraphs
evaluate possible synergistic effect between the CFA
and the moisture contained in wood flour. Comparison of Fig. 2a with Fig. 2c shows that rigid WC/woodflour composite can dectively be foamed without using
chemical foaming agent in the formulation, regardless
of the extruder die temperature (Fig. 2b vs. Rg. 2d).
The result suggests that there is no synergistic d e c t
between the CFA and the moisture contained in wood
flour. The amount of gas produced from wood flour
moisture is sufficient to produce rigid WC/wood-flour
composite foams.
However, the density increases with increased die
temperature (Fig.2a vs. 2b and Fig. 2c vs. 2d).independent of the CFA content. It is believed that processing at higher extruder die temperature combined
with the addition of all-acrylic foam modifier in the
formulation lowered the viscosity of the matrix too
much; resulting in an improper cell growth due to the
excessive gas loss during foaming process (7, 9, 11,
23).The results listed in Table 6 (sample 2 vs. sample
3) show that samples foamed at higher temperature
produced smaller average cell size.This result implies
that rigid WC/wood-four composites could effectively
be foamed at lower processing temperature, using
bound water, which requires less energy to remove
from wood cell wall, as foaming agent.
CONCLUSIONS
The dependence of the density of rigid WC/woodflour composite foams on the all-acrylic foam modifier
content, wood flour moisture content, chemical foaming agent content, and extruder die temperature was
investigated using a central composite design. The
complex relationship among the variables was explained with quadratic response model and following
conclusions can be drawn:
i. There is no synergistic effect between the CFA
and the moisture content of the wood flour.
Rgid WC/wood-flour composite can effectively
be foamed without using any chemical foaming
agents in the formulation since the amount of
gas produced from the moisture contained in
the wood flour is sufficient for bubble nucleation
and growth during the foaming process. However, successful foaming of the composite with
moisture content strongly depends on the presence of all-acrylic foam modifier in the formulation.
ii. Less amount of all-acrylic foam modifier is required for a given level of foam density when
wood flour with high moisture content is used in
the formulation. This is due to the trapping of
evolving gas and prevention of bubble coalescence during the foaming process by all-acrylic
foam modifier.
iii. R@d WC/wood-four composites could effectively
be foamed at a lower processing temperature,
suggesting that bound water, which requires less
energy to remove from wood cell wall, can be
used as the foaming agent.
JOURNAL OF WNKL & ADDl77WE TECHhlOLOGY, DECEMBER 2002, V d . 8, No. 4
269
Lament M. Matuana and Fatih Mengeloglu
ACKNOWLEDGMENTS
Partial funding for this project was provided by the
CSREES-USDA-NRI (Agreement No. 99-35103-8601).
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