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Vulcanization by dicumyl peroxide and injection molding induced anisotropy of nitrile rubber

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Vulcanization by dicumyl peroxide and injection molding induced anisotropy of nitrile rubber

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Content VULCANIZATION BY DICUMYL PEROXIDE
AND INJECTION MOLDING INDUCED
ANISOTROPY OF NITRILE RUBBER
by
Pin-Huei Yang
A Thesis Presented to the
FACULTY OF THE SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE IN CHEMICAL ENGINEERING
January 1981
UMI Number: EP41805
All rights reserved
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UMI EP41805
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This thesis, w ritte n by
Pm.z..Hu..e.i.YotlQ........
under the guidance of h i s F a cu lty Committee
and approved by a ll its members, has been
presented to and accepted by the School of
E ngineering in p a rtia l fu lfillm e n t o f the re
quirements fo r the degree of
Master o ;f Scl&nce.....
/h Chemical E n ^ in e e n h g
ACKNOWLEDGEMENT
The author is much indebted to professor Ronald
Salovey for his thoughtful guidances and continuous en
couragements, to professor W. V. Chang, for his valuable
opinions, to Parker Hannifin Corp., 0-Seal Division for
providing the materials and the facilities for carrying
out this study, and to Mr. Akbar Naderi for his many
helps.
The financial support for this study was furnished
by the TLARGI Foundation for which the author is thank
ful .
TABLE OF CONTENTS
ACKNOWLEDGEMENT
Page
l i
ABSTRACT iv
LIST OF TABLES V
LIST OF FIGURES vi
NOMENCLATURE vii
INTRODUCTION 1
EXPERIMENTAL 7
A. Rubber Compounding 7
B. Curing Characterization 8
C. Injection Molding of O-ring 9
D. Microswelling
10
E. Bulk Swelling
11
F. Rubber Density
12
RESULTS AND DISCUSSIONS
13
A. Vulcanization Kinetics 13
B. Injection Molding
22
CONCLUSIONS 37
REFERENCES 38
APPENDIX: Method of Finding F^ for Krynac 825
Compound
*
40
iii
Abstract
The vulcanization of a nitrile rubber (Krynac 825)
by dicumyl peroxide was studied by using a Monsanto
oscillation cone rheometer. The vulcanization was found
to be a first order reaction between 300 and 400°F. The
Arrhenius plot of log k versus 1/T led to the rate ex
pression k = 1.017 X 10^e 31140/RT^ anisotropy of
an injection-molded nitrile rubber 0-ring system was
studied by using a microswelling technique. The swelling
profile of the rubber in toluene was determined. The
swelling profile was compared with the flow pattern of
the rubber in the mold and it was found that the rubber
which was subjected to higher deformations had lower
swelling. Based on this fact, it was suggested that the
anisotropy arose from the vulcanization of rubber in
the oriented state, and the orientation of rubber was
mainly determined by the flow pattern of rubber in the
mold.
iv
LIST OF TABLES
Table
1. Reaction rate constant of the vulcanization
of Krynac 825 compound by dicumyl peroxide.
2. Comparison of the reaction rate constant of
dicumyl peroxide in different systems.
3. Bulk swelling test result, volumetric swell
ratios of injection-molded Krynac 825 com
pound
Page
24
24
26
LIST OF FIGURES
Figure Page
1. Mold geometry. 10
2. Rheometer trace of Krynac 825 compound. 14
3. Rheometer trace of Krynac 800 compound. 15
4. The degree of vulcanization of Krynac 800 by
dicumyl peroxide vs time, T=350°F. 18
5. The degree of vulcanization of Krynac 825 by
dicumyl peroxide vs time, T= 375 and 400°F. 19
.6. The degree of vulcanization of Krynac 825 by
dicumyl peroxide vs time, T= 330 and 350 F. 20
7. The degree of vulcanization of Krynac 825 by
dicumyl peroxide vs time, T= 300 and 315°F. 21
8. Arrhenius plot of the vulcanization of Krynac
825 by dicumyl peroxide, log k vs 1/T. 23
9. Comparison of vulcanization rates by dicumyl
peroxide in different systems. 25
10. Schematic diagram of the coordinates, sample
positions, and microswelling data. 27
11. Swell ratios in z direction on the center
plane(6=90 ) of section F vs radial position. 28
12. Swell ratios in z direction on the center
plane(0=9O°) of section D vs radial position. 29
13. Swell ratios in z direction on the center
plane of section B vs radial position. 30
14. Amount of rubber injected Q, and pressure inside
the runner near section F vs injection time. 33
vx
NOMENCLATURE
C : concentration
C : initial concentration
o
F : oscillation torque
F : oscillation torque at infinite time
G : dynamic shear modulus
k : reaction rate constant
P : pressure
Q : amount of rubber injected
R : radius
R^ : inner radius
R : outer radius
o
T : temperature
t : t ime
: weight after swelling
NL : weight before swelling
X : crosslink density
Ar : swell ratio in radial direction
A : volumetric swell ratio
v
X : swell ratio in z direction
z
Aq : swell ratio in 0 direction
Pt : density of toluene
Pr : density of rubber
vii
INTRODUCTION
It has long been recognized that rubber products can
exhibit an anisotropy of mechanical behavior with physical
properties dependent on the direction and position of test
specimens in the product. This phenomenon has been studied
under different experimental conditions. Most investiga-
(1-5)
tionsv ■ considered the "two-network” case, where rubber
was crosslinked in the undeformed relaxed state, then
stretched and crosslinked again in the deformed state.
These rubber samples were found to have a higher tensile
modulus and lower degree of swelling in the stretched
direction than in the transverse direction. The anisotropic
behavior of the two-network type rubber has been explained
in theories based on non-Gaussian statistics^^ and
(3 8) (9)
phenomenological theories. ’ Blow and coworkers
studied the rubber samples prepared from a specially
designed compression mold which gave a simple and clearly
defined mold-flow. They reported that the tensile modulus
of the rubber samples in the flow direction are higher than
those in the transverse direction. Using a center-gated
disk shape mold, Tsai^^ found that the rubber prepared
by injection-molding had the similar anisotropic behavior
1
(11)
as that reported by Blow. Recently, Chang et al. used
a microswelling technique to study a compression-molded
rubber packing-unit, which may be the first study reported
on the anisotropy of a commercial rubber product. They
reported that the anisotropy could be related to the flow
of rubber in the compression-mold. It was suggested in
every report that the anisotropy arose from crosslinking
of rubber in the deformed state either during or after
molding that locked in an oriented structure.
An anisotropic product is always stronger in some
directions and weaker in others. The tensile modulus can
vary from 25% to 120%^*^^ in different directions. The
molding shrinkage, which affects the dimensional stabi-
(9)
lity of rubber product very much, can vary up to 1007o .
In an extreme case, the tear energy of a rubber at 300%,
extension was reduced to only about 2.5% of its original
(12)
value in the unstretched state. ' Since the anisotropy
plays such an important role on the physical properties,
it is desirable that every manufacturer be able to detect
the anisotropy of his product and understand the factors
that cause the anisotropy.
Because of the many advantages of injection-molding
over compression-molding and transfer-molding, more and
more people are using injection-molding for the manufacture
2
of rubber products. It is generally recognized that injec
tion-molding will gain a higher degree of acceptance and
play a more important role in the rubber industry.
The most important advantage of injection-molding is
its high production speed which is due to its fast molding
and fast curing. Because of fast molding, rubber is
subjected to high deformation rates. Then because of fast
curing, the deformed rubber will have less chance to relax
before being cured in oriented state. Therefore, we would
expect that injection molding may induce higher anisotropy
than other rubberr-processing methods .
In this study, nitrile rubber O-ring was injection-
molded and the anisotropy of the O-ring system was studied
by using a microswelTing technique. Possible factors that
cause the anisotropy were discussed.
Dicumyl peroxide is selected as the curing agent in
this study because of the following technique interests:
(1) Dicumyl peroxide promotes the C-C bridges between poly
mer chains, thus the vulcanfzates should be more stable
toward oxygen than elastomers crosslinked with sulfur.
(2) Every dicumyl peroxide molecule reacted will produce
exactly one crosslink site in the polymer, thus the
crosslink density may be quantitatively determined by
either determining the residual peroxide concentration
in the rubbet or measuring the elastic properties of
3
the rubber.
(3) The curing kinetics of dicumyl peroxide is better
understood than sulfur curing systems, thus we can have
a better understanding of the injection-molding pro
cess.
The kinetics of vulcanization of rubber with dicumyl
peroxide has been reported. T h o m a s , using a chemical
analysis method, studied the reaction kinetics of dicumyl
peroxide in natural rubber between 110°C and 140°C. Rubber
samples were heated in an oil bath at selected temperature,
then the reaction Was quenched at different times by im
mersing the samples in cold water. The reaction products
were extracted immediately and analyzed by infrared and
ultraviolet absorption. The reaction rate was then derived
from the relation of concentration and time. Scheele
studied the vulcanization of natural rubber with dicumyl
peroxide between 120°C and 155°C. He used a similar method
in determining the dicumyl peroxide concentration as Thomas,
and used swelling test in determining the crosslink density
of rubber. It was observed that the disappearance of di
cumyl peroxide and the formation of crosslinks followed
a first order time law with the decomposition of dicumyl
peroxide the rate determining step. Loan^^ studied the
crosslinking by dicumyl peroxide in several unsaturated
4
synthetic rubbers using the same technique that Thomas
used. He found that the crosslinking efficiency was larger
than 967,, almost one crosslink was formed for every reacted
/I £ \
dicumyl peroxide molecule. Thomas reported that the
crosslinking efficiency of dicumyl peroxide in natural rub
ber was 957o to 1007> in the temperature range 110°C to 140°C
. All the experiments mentioned above were conducted in a
temperature range much lower than is usually used in rubber
injection molding, 180°C to 200°C. In this temperature
range, the vulcanization proceeds at a rate that is com
parable to the heat transfer rate, and it is difficult to
quench the reaction. Therefore, the previous methods of
analysis are not suitable. In this study, we used the most
widely used test for cure characteristics,^^ oscillating
rheometer. An oscillating rheometer records the torque that
is required to oscillate the rubber sample in a test cell
during vulcanization. For a fixed cell geometry, the torque
that is required to oscillate the rubber sample is propor
tional to the dynamic shear modulus of the rubber sample.
(18" )
Mussati and Macosko ' have pointed out that from the
rubber elasticity theory, the modulus determined at small
deformation can be related to the crosslink density,
G TX
where, G is the dynamic shear modulus
5
T is the absolute temperature
X is the crosslink density.
As mentioned earlier, since the crosslinking efficien
cy of dicumyl peroxide is very high, we can assume that the
crosslink density is proportional to the amount of dicumyl
peroxide reacted, and the oscillation torque should be
proportional to the amount of dicumyl peroxide reacted.
Therefore, we should be able to investigate the vulcani
zation kinetics from the rheometer trace.
EXPERIMENTAL
A. Rubber Compounding
1. Formula: phr
Krynac 825 100
AgeRite resin 1.5
HAF black 60
Dioctyl phathalate 15
Dicumyl peroxide (40%) 4
Total: 180.5
2: Apparatus:
a. Banbury mixer.
Specification: Size B.
Manufacturer: Farrel-Birmingham Company Inc.
b. Two-roll mill.
ri it
Specification: 6 X 13 flood lubricated laboratory
mill.
Manufacturer: Farrel-Birmingham Company Inc.
* Krynac 825 is the trade name of a acrylonitrile-butadiene
copolymer, a product of Polysar. It was selected because
of its low gel content.
7
3. Batch size: 1100 grains.
4. Procedures:
Nitrile rubber was first broken down in the Ban
bury mixer for one minute. Then all other ingredients,
except dicumyl peroxide, were added into the Banbury
mixer with dioctyl phathalate sandwiched by carbon
black to avoid leakage and contaminating the throat
wall of the Banbury mixer. After being mixed for another
two minutes, the master batch was removed from the Ban
bury mixer to the two-roll mill, where dicumyl peroxide
was added with water cooling to avoid scorch. Then the
mixed rubber compound was stored at 5°C before any
further experiment.
B. Curing Characterization
1. Apparatus: Monsanto Oscillation Cone Rheometer.
Specification: Model 1-C.
Manufacturer: Monsanto Company.
2. Tes>t temperatures: 300, 315, 330, 350, 375, and 400°F.
3. Oscillation arc: 3°.
C. Injection-Molding of O-ring
1. Apparatus: Rubber injection-molding machine.
Specification: Model B-52-K, automatic, ram type,
vertical hold, electric - heating mold,
and water-heating barrel.
Manufacturer: Rep Machine Company.
2. Experiment conditions:
Barrel temperature: 160°F
Mold temperature : 370°F
Shot size : 11.3 cm^
Injection pressure: 14,000 psi
Holding time : 10 seconds
3. Mold geometry: See Figure 1.
D. Microswelling
1. Apparatus:
a. Microtome.
Specification: Model 820.
Manufacturer: American Oprical Company.
b. Optical Microscope.
Specification: Binocular, magnification 10.
Manufacturer: Bausch & Lomb Optical Company.
9
nozzle
circular
sprue
semi-circular
runner
semi-circular
runner
89.2 mm 27 mm
triangular gate
height: 2.5 mm
gap: 0.4 mm t - *
o
Figure 1. Mold geometry (actual size).
2. Procedures:
Rubber samples cut from sections B, D, and F (see
Figure 1) were frozen with cold nitrogen gas and micro-
tomed into 50]j-thick thin pieces with a microtome. The
thin samples were then trimmed with a blade to a size
about 5 mm X 0.35 mm X 50y with the longest side parallel
to the interested direction. Samples were swelled in
toluene at room temperature for 12 hours. The length
of the samples along the interested direction were
measured with a microscope before and after swelling.
The directional swell ratio is the length ratio of the
sample after and before swelling.
E. Bulk Swelling
Rubber samples cur from sections A, B, C, F, and G
(see Figure 1) were swelled in toluene for 3 days. Samples
were weighed before and after swelling. In order to com
pare the crosslink-density uniformity of the sample,
rubber samples cut from inside the sections were also
tested. The bulk-swell ratio is the volume ratio of the
swelled rubber sample to unswelled rubber sample.
11
Assuming the volume of rubber and toluene in the
swelled rubber are additive, the volumetric swell ratio
was calculated by following equation:
V t
X =
v
w f p r + w . ( p t - Pr )
where is the weight of sample before swelling
is the weight of sample after swelling
3
Pr is the density of rubber = 1.178 g/cm
3
Pt is the density of toluene = 0.866 g/cm .
F. Rubber Density
Rubber density was measured according to ASTM D792-66
(1979) .
12
RESULTS AND DISCUSSIONS
A. Vulcanization Kinetics.
Figure 2 shows a rheometer trace, a plot of the oscil
lation torque versus vulcanization time, of the Krynac 825
compound. The drop of the oscillation torque during, the
initial period of the vulcanization is due to the transient
heating of the material. The decrease of the oscillation
torque after the maximum point is due to the shrinkage of
the material from vulcanization. Figure 3 shows the rheome
ter trace of non-shrinking rubber compound, where Krynac
800 is used instead of Krynac 825.
As mentioned earlier, the oscillation torque is pro
portional to the amount of reacted dicumyl peroxide. So
F = m ( CQ - C ) ( 1 )
where, F is the oscillation torque at time t
C is the dicumyl peroxide concentration at time t
Cq is the initial dicumyl peroxide concentration
m is a proportional constant.
For a first order reaction, the reaction rate is:
13
120
100
w
S3
O '
Pi
o
H
a
o
H
EH
< 1
hJ
hJ
H
U
CO
O
40
400 100 300 0 200
REACTION TIME (second )
Fig. 2. Rheometer trace of Krynac 825 compound, T = 350°F.
h - *
■ p*
rO
I —I
r
a
■H
w
£3
O '
Pd
o
H
S
o
M
H
<
hJ
H
O
CO
O
80
60
40
20
0
0 100 200 300 400 500 600 700
REACTION TIME ( second )
Fig. 3. Rheometer trace of Krynac 800 compound, T = 350°F,
Ln
d C
= - kC ( 2 )
d t
where k is the reaction rate constant.
Integrate equation 2 with the initial condition
C = C @ t = 0
o
we obtain
C = Coe~kt ( 3 )
X*Jhen time approaches infinite, the reaction will be
complete and the reactant concentration becomes zero. Then
from equation 1, we have
F = mC ( 4 )
00 o
where F^ is the oscillation torque at infinite time, i.e.,
when reaction is complete.
Combine equations 1, 3, and 4, we have
C F - F
0 0
In ----- = In---------- = - kt ( 5 )
C F
o ° °
So, for a first order reaction, the plot of lnCl-F/F^ )
versus time should be a straight line, and the slope is
equal to the reaction rate constant. If the reaction rate
16
follows Arrhenius law,
k = k0e‘E/RT ( 6 )
where E is the activation energy
R is the gas constant
T is the absolute temperature
kQ is the frequency factor
The plot of In k versus reciprocal temperature should be
a straight line. The slope of the line corresponds to E/R
and the intercept on the ordinate corresponds to kQ.
Figure 4 is the plot of logCl-F/F^) versus time of
the Krynac 800 compound. The plot is a fairly-straight line
except in the transient heating region, indicates that the
vulcanization by dicumyl peroxide is a first order reaction
and the rheometer trace can be used to study the reaction
rate. '
The Krynac 825 compound shrank after some stage of
vulcanization, so we could not find F^ directly from the
rheometer trace. However, the middle part of the trace is
smooth and does fit equation 5 very well. So for Krynac
825 compound, F^ was first found by curve-fitting the
middle part of the rheometer trace with equation 5 (see
appendix for the details of curve fitting), then log
(1-F/F^) was plotted versus vulcanization time in figures
5-7. The data fits equation 5 very well between the
17
o
u
o
! —I
0.01
0 100 200 400 300
REACTION TIME ( second )
Fig. 4. The degree of vulcanization of Krynac 800 compound by dicumyl
peroxide vs time, T = 350°F.
o
o
I I
8
pH
Ph
I
0.8
0.7
run 1
0.6
0.5
0.4
0.3
10 20 30 40 50 60 70 80 90 100
i-1
VQ
REACTION TIME ( second )
Fig. 5. The degree of vulcanization of Krvnac 825 compound b } ? - dicumyl
peroxide vs time, T = 375 and 400 F.
0.9
0.8
0.7
6
.5
.4
.3
run 1
run 2
run 3
0.2
100 50 150 200 250
REACTION TIME ( second )
Fig. 6. The degree of vulcanization of Krynac 825
compound by dicumyl peroxide vs time.
T = 330 and 350°F.
20
0.8
0.7
0.6
0.5
100 200 300 400 500 600 700 800 900 1000
REACTION TIME ( second ) *
Fig. 7. The degree of vulcanization of Krynac 825 compound by dicumyl
peroxide vs time, T = 300 and 315 F.
transient heating region and the shrinking region, so the
vulcanization of Krynac 825 compound by dicumyl peroxide
is a first order reaction in the investigated temperature
range: 300 to 400°F. The reaction rate constant was then
obtained by multiplying the slope by 2.303R. The rate
constant at different temperatures is shown in table 1.
Figure 8 is the Arrhenius plot of the vulcanization reac
tion, log k versus reaction time. The plot is a straight
line, indicates that the reaction can be expressed by
Arrhenius equation in the investigated temperature range.
From the slope and the intercept of the line, the activa
tion energy E was found to be 31.14 Kcal/g-mole and the
13
frequency factor kQ is 1.017 X 10
The reaction rate expression we found is shown in
table 2 with expressions reported by other researchers on
different systems and temperature ranges. The expressions
are also shown schematically in figure 9 \
B. Injection Molding
1. Bulk Swelling:
( 2 1 j
According to the Flory and Huggins theoryv J , the
crosslink density of rubber is related to its bulk-swell
ratio. Bulk swelling data of the injection-molded sample
22
10
2
10
3
10
2.1 2.2 2.3 2.4
1/T X 103 ( °K'1 )
Fig. 8. Arrhenius plot of the vulcanization of Krynac 825 compound by
dicumyl peroxide, log k vs 1/T, T = 300 to 400 F.
Table 1
Reaction rate constant of the vulcanization of
Krynac 825 compound by dicumyl peroxide.
temperature, °F rate constant k, sec ^
400 0.0412
375 0.0215
350 0.00925
330 0.00307
315 0.00156
300 0.000766
Table 2
Comparison of the reaction rate constant of
dicumyl peroxide in different systems.
system temperature rate constant
(sec )
source
nitrile
rubber
natural
rubber
natural
rubber
cumene
300 - 400°F
120 - 155°C
110 - 140°C
120 - 140°C
1.107 X 1o13e_3ll40/RT
2.0 X io15e"3600°/RT
3.9 X io13e“32500y,RT
4.31 X 101 V 3450°/RT
this work
ref. 15
ref. 16
ref. 20
24
a
a
CO
x
10
10
10
ref. 16
ref. 20
ref. 15
this work
10
2.1 2.2 2.3 2.4 2.5 2.6
1/T X 103 ( °K"1 )
N3
U1
Fig. 9. Comparison of vulcanization rate by dicumyl peroxide in different
systems.
as shown in Table 3. Bulk-swell ratios are very close at
different sections and different portions (inner and outer)
. Therefore we conclude that the crosslink density is very
uniform throughout the sample.
Table 3
Bulk swelling test result: volumetric swell
ratios of injection-molded Krynac 825 compound.
section A B C F G
A
V
A . t .
v, c*
2.18
2.18
2.18
2.18
2.16
2.15
2 .12
2.13
2.16
2.13
* c indicates that the rubber sample was cut
from inside the sections.
2. Microswelling and Swelling anisotropy.
Experiment data, as shown in Figures 10-13, showed
that the swelling is anisotropic at all sections being
investigated. The swell ratio varied not only in different
directions but also at different positions along the same
direction.
Swelling results at sections D and F were very similar
(see Figure 10-12). Swell ratios along the direction
26
7
coordinate s at
positions D and F
^r,e=o°
'Xr, 9=90°
^0,r=O.75R
sample positions at
sections D and F
section
Ar,0=O°
"Ar, 0=90°
Ae,r=0.75R
F
1.31
1.35
1.33
see Fig. 11
coordinates at
position B
A
' X 0,r=O. 75E
sample positions at
section B.
D
1.31
1.36
' 1.33
see Fig. 12
B
1.30-1.36
1.32
see Fig. 13
Fig. 10c Schematic diagram of the coordinates, sample
positions, and microswelling data at sections
B, D, and F.
27
1.31
° 1.29
i i
CD
1.27
1.25
r/R
0
Fig. 11. Swell ratios in z direction on the center plane
(9 = 90 ) of section F vs radial position.
28
o
o
o - \
I I
CD
N
1.32j
1.28 —
1.26
1.24
r/R
Fig. 12. Swell ratios in z direction on the center plane
(0 = 90°) of section D vs radial position.
_22J
1.36
1.34
1.32
r=R. r=R
N )
center plane
1.30
1.28
1.26
0.2 0.4 0 0.6 0.8 1.0
Co
o
(R0 - r)/(R0 - R.)
Fig. 13. Swell ratios in z direction on the center plane of section B vs radial
position.
parallel to the wall (Ar Q=C)o and AQ r=Q 75R) are smaller
than the swell ratios in the direction perpendicular to the
wall (Ar e=9o°)* t^ie center plane of the runner (0=90°),
the swell ratio along the main flow direction (z direction)
was found to be lower in the center part of the runner
than in the wall region (see Figures 11 and 12).
At section B (see Figures 10 and 13), the swell ratio
at r direction was found to vary from 1.30 to 1.36. The
swell ratios along the main flow direction (z direction)
were measured from the inner wall to the outer wall. It
was found that swell ratio in the z direction is higher
in the center region of the duct than that in the wall
region, and a maximum is found located closer to the outer
wall than the inner wall.
At sections D and F, the runners are semi-circular;.
The flow in that region can be divided mainly into two
types: flow near the advancing front, the fountain flow and
flow behind the advancing front, the fully developed flow
(22)
'. In the front region, the material in the central re
gion just behind the advancing front is approximately
considered to be undergoing a steady biaxial elongational
flow, and moves toward the duct wall just ahead of the
advancing front. So the material stays near the wall re
gion has been the part of the advancing front. In
31
other words, rubber near the duct wall has been extended
biaxially along the plane parallel to the duct wall. It
is expected that the rubber near the duct wall should
have a lower swell ratio in the direction parallel to the
duct wall than in other directions. That is, at sections
D and F, Ar q=qO, which is located very close to the duct
wall should be smaller than Aq which is relatively
closer to the center of the duct and therefore is subjected
to less extension. While A A_Qno, which is measured along
t ,u—yu
the direction perpendicular to the duct wall, should have
a higher swell ratio than those of other directions. This
is what we observed. In the fully developed flow region,
the flow is essentially laminar, with the velocity decreas
ed from the center portion of the duct toward the wall
region. In this region, the material is essentailly sub
jected to shear stresses along the flow direction, with the
maximum shear rate occurs near the wall and minimum sheait
rate at the center portion. Therefore we would expect that
in the z direction, the swell ratio in the center portion
should be higher than that near the wall. However, what we
observed at sections D and F is different. This is due to
the effect of the discharge of rubber after the filling
stage.’ From Figure 14, where the amount of rubber injected
and the pressure inside the runner near section F were
32
CO
CM
00
o
xn
x n
CM
5
discharge
of rubber —
4
3
2
1
0
0
0 1 2 3 4
o
o
3
OJ
TIME ( sec )
Fig. 14. Amount of rubber injected Q, and pressure inside the runner near
section F vs injection time.
oj
plotted versus time, we can see that when the mold was
filled, the injection pressure behind the piston was im
mediately released and the pressure inside the runner became
much higher than the pressure behind the piston. Therefore
the rubber inside the runner started to retract until the
pressure inside the runner decreased to an equilibrium
level. Restricted by the runner geometry, rubber could only
expand along the z direction. The retraction was like a
uni-axial extension along the z direction with the highest
extension occurring in the center portion of the runner.
Therefore we observed that the swelling in z direction is
lower in the center part of the runner than that near the
wall.
Section B is the part of the O-ring that is 90 degrees
from the entrance. The effect of the flow of the advancing
front is similar to that at sections D and F. Rubber near
the wall was extended in the direction parallel to the wall.
However, the flow behind the advancing front is somewhat
(23)
different. Larrain and Bonilla has examined the laminar
flow of fluid in coiled pipes. They found that the velocity
profile of the laminar flow in coiled pipes is not sym
metric about the center of the pipe, and there are second
ary flows in both the upper and lower halves of the pipe.
34
Maximum velocity occurs at a position closer to the outer
wall than the inner wall. Thus the minimum shear rate
occurrs at a position closer to the outer wall, and the
shear rate increases from that position toward both the
outer wall and the inner wall. In other words, rubber near
the wall is stretched to higher deformation along the main
flow direction (z direction) than in the center portion.
So the swell ratio in the z direction should be smaller in
the wall region than that in the center region, and the
maximum swell should occur at a position that is closer to
the outer wall. It is what we observed. The retraction of
rubber at section B should be much less than that in the
runner for the following reasons: (1) Section B is very
close to the far end of the mold; the pressure there is
much lower than the pressure in the runner. (2) There is
a very thin (^0.4 mm) gap around the 0-ring, where the
resistance to flow is very high and the pressure drop
across the gap is also very large. Since the retraction
of rubber at section B is very limited, its effect on the
anisotropy is negligible.
From the investigation discussed above, we find that
the rubber which was subjected to higher deformation has
lower swell ratio. It is reasonable to suggest that the
anisotropy arose from the vulcanization of rubber in the
35
oriented state, and the orientation of rubber is mainly
determined by the flow pattern of rubber in the mold. By
controlling the flow pattern during molding, one should be
able to control the anisotropy of the molded product.
36
CONCLUSIONS
1. The vulcanization rate of a nitrile rubber (Krynac 825)
by dicumyl peroxide at temperatures 300 to 400°F, where
chemical analysis method is not applicable, was found
by using an oscillation rheometer. The vulcanization
was found to be a first order reaction. The Arrhenius
plot of log k against 1/T leads to the rate expression
k = 1.017 X io13e_31140/RT.
2. The anisotropy of an injection-molded nitrile rubber
0-ring system was studied by using a microswelling
technique. The swelling profile of the rubber in to
luene was determined and compared with the flow pattern
of the nitrile rubber in the mold. It was found that
the rubber which was subjected to higher deformation
had lower swelling. It is suggested that anisotropy aro
se from the vulcanization of rubber in the oriented
state, and the orientation of rubber is mainly determin
ed by the flow pattern of rubber in the mold.
37
REFERENCES
1. J. P. Berry, J. Scanlan, and W. F-. Watson, Trans.
Faraday Soc., 52, 1137(1956).
2. A. Greene and A. Ciferri, Kolloid-Z., 186, 1(1962).
3. A. Greene, K. J. Smith, Jr., and A. Ciferri, Trans.
Faraday Soc., 61, 2772(1965).
4. 0. Krammer, R. L. Carpenter, V. Ty, and J. D. Ferry,
Macromolecules, 7(1), 79(1974).
5. R. L. Carpenter, 0. Krammer, and J. D. Ferry, Macro-
moledules, 10(1), 117(1977).
6. K. J. Smith, Jr., A. Ciferri, and J. J. Hermans, J.
Polymer Sci., A, 2, 1025(1964).
7. K. J. Smith, Jr., and R. J. ' Gaylord, J. Polymer Sci.,
A, 10, 283(1972).
8. W. V. Chang, R. Bloch, and N. W. Tchoegl, in "Chemistry
and Properties of Crosslinked Polymers," p. 431, Aca
demic Press, N . Y. (1977).
9. C. M. Blow, H. B. Demirli, and D. W. Southwart, J. IRI,
8, 244(1974).
10. B. C. Tsai, Rubber Chem. Tech., 51, 26(1978).
11. W. V. Chang, P. Yang, and R. Salovey, presented at a
meeting of the Rubber Division, American Chemical So
ciety, Las Vegas, Nevada, May 1980.
38
12. A. N. Gent and H. J. Kim, Rubber Chem. Tech., 51, 35
(1978).
13. J. D. Byam and G. R. Colbert, presented at a meeting of
Rubber Division, American Chemical Society, Chicago,
111, May 1977.
14. W. A. Wheelans, "Injection Moulding of Rubber," p .16,
Halsted Press, N. Y.(1974).
15. W. Scheele, Rubber Chem. Tech., 34, 1306(1961).
16. D. K. Thomas, J. Appl. Polymer Sci., 6(24), 613(.1962) .
17. L. D. Loan, J. Appl. Polymer Sci., 7, 2259(1963).
18. D. A. Hill, "Heat Transfer and Vulcanization of Rubber,
p.63, Elsevier Publishing Co. LTD, London, England
(1971) .
19. F. G. Mussatti and C. W'. Macosko, Polymer Eng. Sci.,
13(3), 236(1973).
20. G. W. Godin and H . C. Bailey, Trans. Faraday Soc., 52,
68(1956).
21. L. R. G. Treloar, Rep. Prog. Physics, 36, 755(1973).
22. Z. Tadmor and C. G. Gogos, "Principles of Polymer
Processing," p.59..1, Wiley, N.Y. (1978) .
23. J. Larrin and C. F. Bonilla, Trans. Soc. Rheology,
14(2), 135(1970).
APPENDIX
Method of Finding F for Krynac 825 Compound
F and t values were taken at three different points
on the smooth middle portion of the rheometer trace, say
(Ff, t-^), (F2 , t2 ), and (F^, t^) . Substituted these values
into equation 5, we have
ln(l-F1/F00> = kt1 ( Al )
ln(l-F2/Fco) - kt2 ( A2 )
In (l-F^/Foo) = kt^ ( A3 )
Combine equations Al and A2, we have
ln(F00-F1) - ln(Foo-F2)
k = -------------------- : --- — ( A4 )
fcl - fc2
Similarly, from equations Al and A3 we have
ln(F -F-.) - ln(F -F,)
00 X co . 3
k = ( A5 )
t. l " t3
40
Equating equations A4 and A5, we have
ln(F„-Fi) - lnCF^-Fj) tx- t2
= ( A6 )
ln(Fco-F1) - 1„(F„ F3) t1- t3
Equation A6 was solved by trial and error to give F^.
The advantage of using equation A6 is that only the time
differences between different points were involved, thus
the effect of transient heating on reaction time was
avoided.
41

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Asset Metadata

Creator Yang, Pin-Huei (author)

Core Title Vulcanization by dicumyl peroxide and injection molding induced anisotropy of nitrile rubber

Degree Master of Science

Degree Program Chemical Engineering

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag engineering, chemical,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Salovey, Ronald (committee chair), Chang, Wenji Victor (committee member), Goddard, Joe D. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c20-312957

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

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