Thermogravimetric analysis for the characterization of nanomaterials detail.pptx
RADTEC96_final_Corr
1. RadTech ‘96 North America UV/EB Conference Proceedings, P. 675, Vol. 2, Nashville, TN, 4/28-5/2, 1996
Thermal and Mechanical Properties of
Radiation Curable Networks
W. Patrick Yang, C. Wise, J. Wijaya, A. Gaeta, G. Swei
Norton Company
Worcester, MA
ABSTRACT
The effect of chemical composition of reactive mixtures
prepared from bisphenol-A epoxy diacrylate, N-vinyl
pyrrolidone (NVP), and trimethylolpropane triacrylate
(TMPTA) on the cure and network properties of UV cured
resins is studied by a variety of thermal analysis techniques
(DPC, DSC, DMA and TGA) and tensile testing. The glass
transition temperature (Tg) is determined from DSC and
DMA, whereas crosslink density is estimated from DMA
elastic modulus at the rubbery plateau by use of rubber
elasticity theory. NVP is most efficient to reduce the viscosity
of reactive mixture, enhance the reactivity of photo-
polymerization, and increase glass transition temperature (Tg)
and tensile elongation of the cured network. However, being
a monoacrylate, NVP decreases the thermal degradation
temperature. TMPTA is most effective to enhance the tensile
strength, Young’s modulus, crosslink density and thermal
stability. The observed thermal mechanical properties of UV
cured thermoset network can be explained in terms of glass
transition temperature and chemical crosslink density of the
network. Mixture experimental design is demonstrated to be a
very useful tool for formulation. The predictive models from
regression analysis allow for generation of three dimensional
response surface plots for effective overview of trends within
the design space. By graphical or numerical optimization, a
region of formulation compositions with compromised but
balanced properties can be obtained under a given set of
requirements.
INTRODUCTION
In a typical UV formulation, acrylate monomers are often
added to oligomers to reduce viscosity for application as well
as to modify the final cured network properties. Mono-
functional acrylate monomers such as NVP are most effective
to reduce viscosity, increase flexibility and enhance adhesion
by moderating cure shrinkage. Multi-functional acrylate
monomers such as TMPTA, on the other hand, are used to
enhance cure speed, hardness, crosslink density and solvent
resistance. Proper ratio of oligomer, monoacrylate, and multi-
acrylate is often formulated to attain the desired physical
properties. This is very amenable to the statistical mixture
experimental design for systematically screening and
optimizing formulations. The combination of NVP and
TMPTA monomers has been shown in literature to provide
balanced mechanical properties to the urethane acrylate
formulation1
. This paper will illustrate the use of mixture
experimental design to study the blend mixture of bisphenol-A
epoxy diacrylate/NVP monoacrylate/TMPTA triacrylate
formulations.
The properties of thermoset networks depend on glass
transition temperature, crosslink density, and morphology
(e.g., phase separation). These network parameters in turn
depend on the molecular structures, functionality and Tg of
the initial reactive resins and the film formation process. The
film formation process then further hinges on the extent of
cure and efficiency of chemical crosslinking. In a UV curing
process where photo-polymerization occurs in a very short
time scale, efficient network formation to attain the final
network properties in the presence of the effect of
vitrification and frozen stress due to cure shrinkage is even
more challenging.
In the past, simple mechanical tests have been used to measure
coating properties such as pencil hardness, mandrel flexibility,
and MEK rub solvent resistance, etc. to evaluate the
performance of coatings. These tests require inexpensive
equipment and can be conducted with a quick turn-around
time. These practical coating tests, though adequate and
reliable in some degree to provide quality ranking among
similar formulations, does not offer the subtle and more
fundamental structure-property relationship and correlation
between network parameters and thermal-mechanical
properties.
2. There have been increasing efforts in the radiation curing
coating industry to utilize the more sophisticated modern
instrumentation techniques typically used in the polymer
science field. Several papers 2, 3
have provided a good
summary of what is available and the corresponding technical
information that can be attained. In this paper, a variety of
thermal analysis techniques (DPC, DSC, DMA and TGA) and
tensile testing are used to study the effect of chemical
composition of reactive mixtures prepared from bisphenol-A
epoxy diacrylate, N-vinyl pyrrolidone (NVP), and
trimethylolpropane triacrylate (TMPTA) on the cure and
network properties of UV cured coatings.
EXPERIMENTAL
Sample Preparation
The epoxy diacrylate oligomer (Ebecryl 3703 from UCB
Radcure Inc.), NVP (from GAF) and TMPTA (from
Sartomer) were mixed together until homogeneous according
to the required ratio and 1% of 2-hydroxy-2-methyl-1-phenyl-
propan-1-one (i.e., HMPP, Darocur 1173 from Ciba Geigy)
was added as the photoinitiator.
The samples for tensile testing, Knoop hardness test, and
thermal analysis were prepared using an open rectangular
aluminum mold (0.5” W x 6.5” L x 1/64” t). The cure
condition was 2 passes at 50 ft/min in air through a Fusion
UV cure unit (model DRS120) with D bulb at 300 W/in and H
bulb at 600 W/in.
Viscosity
The viscosity of liquid mixture was measured by Brookfield
viscometer (model DCV-II+) at 80°F using S64 spindle at
various rotational speeds of 1, 5, 20 and 50 rpm. All reactive
mixes exhibit Newtonian viscosity behavior.
Differential Photocalorimetry (DPC)
The DPC experiments were conducted isothermally at 30°C
for 10 minutes on a TA Instrument 2920 DPC without N2
purge. The typical liquid sample weight is 7.2 to 7.65 mg and
a 7.36 mg of Ebecryl 3703/NVP cured sample was used as the
reference. The heat of reaction (∆Hrxn), induction time
(tinduct.), time to exotherm peak (tpeak), and conversion to
exotherm peak (αpeak) were measured.
Tensile Testing
The tensile testing was conducted on a Tinius-Olsen 1000
tensile tester with a typical specimen thickness of 1/64”,
gauge length of 5” and cross-head speed at 0.5 in/min. Five
specimen were tested for each composition and the averages
of tensile strength (Sb), elongation-at-break (eb), and Young’s
modulus (E) are reported.
Differential Scanning Calorimetry (DSC)
Glass transition temperature, Tg (DSC), of the cured samples
was determined from both the onset temperature (Tg, onset) and
inflection temperature (Tg, infl) of the step change in heat
capacity. The breadth of the transition, ∆Tg, is determined
from the difference between the end point and the onset point
of the step change. All Tg values are taken from the second
DSC heat from -100°C to 250°C at 20°C/min under a N2 gas
purge on a TA Instrument 2920 DSC.
Thermogravimetric Analysis (TGA)
The thermal degradation temperature of the cured samples is
taken as the temperature at which 5% weight loss was
encountered on a TA Instrument 2950 TGA at a heating rate
of 10°C/min from 30°C to 800°C in an air environment.
Dynamic Mechanical Analysis (DMA)
The viscoelastic properties of UV cured samples were
measured at 1 Hz under fixed frequency mode and
temperature from -100°C to 200°C at a heating rate of
4°C/min using a 983 DMA from TA Instrument. Low mass
vertical clamps and quarter size magnets were used to enhance
the signal to noise ratio. Sample dimensions were
approximately 13 mm W x 0.5 mm t x 6 mm L. The
oscillatory amplitude is 0.5 to 1 mm and clamping torque on
sample is 4 to 6 in-lb. The peak temperature of loss modulus
(E”) is reported as Tg (DMA). The network crosslink density
is estimated from the storage modulus (E’) at the rubbery
plateau according to rubber elasticity theory.
Knoop Hardness Test
The hardness reading was obtained from the indentation mark
under a 20x objective lens with a 2 gm load at room
temperature. It is found that the top surface has a higher
hardness reading than the bottom surface. The reading
reported is from the top surface which faced the air
atmosphere during UV cure.
RESULTS AND DISCUSSION
1. Mixture Experimental Design
The lower and upper limits of each mixture component are set
as below:
70 ≤ Epoxy oligomer (Ebecryl 3703) ≤ 100
0 ≤ NVP ≤ 30
0 ≤ TMPTA ≤ 30
This results in a truncated three-component design space for
epoxy diacrylate oligomer, NVP and TMPTA. The mixture
design matrix is listed in Table 1 and the schematic design
points are shown in Figure 1. For a mixture quadratic model
(Scheffe model), six coefficients are needed. These are design
points of #1 to #6. In addition, three axial check points (#13,
#14 and #15) are included for model lack of fit test and
3. triplicate on centroid point (#16) is used to estimate pure
error. These twelve runs of ten design points (#1 - #6, #13 -
#15, and #16) are adequate for a comprehensive quadratic
mixture experimental design.
However, six extra one third of edge points (#7 and #8 on the
triangular edge of NVP = 0, #9 & #10 on the edge of TMPTA
= 0, and #11 & #12 on the edge of oligomer = 70) are also
included to allow for more systematic detailed analysis.
Hence, for example, referring to Figure 1 and Table 1, design
points of #1, #9, #5, #10 and #3 on the left triangular edge of
TMPTA = 0 represent a systematic increase of NVP amount
with respect to Ebecryl 3703 epoxy diacrylate oligomer in a
two-component mixture up to the upper limit of 30% NVP.
On the other hand, design points of #1, #7, #4, #8 and #2 on
the right triangular edge of NVP = 0 allow for detailed
analysis of Ebecryl 3703/TMPTA mixture blend up to 30%
TMPTA. Design points of #3, #12, #6, #11, and #2 on the
lower triangular edge of Ebecryl 3703 = 70 allow for analysis
of the effect of substituting TMPTA for NVP under a
constrain of NVP + TMPTA = 30. Finally, one can follow the
path of design points #1, #13, #16 and #6 to evaluate the
effect of increasing the amount of NVP and TMPTA
monomer blend under the constraint of a fixed ratio
NVP/TMPTA = 1.
A global regression analysis is conducted with all 18 runs of
data points and 3D response surface plots are generated using
Design Expert software from Stat-Ease.
Table 2 summarizes the statistical analysis of regression
results. The criteria to include a term in regression equation is
at a significant level of α < 0.05. Overall, log10 viscosity data,
TGA data, Tg data, crosslink density νe , and ∆Hrxn data have
the best regression results with a R2
ranges from 0.78 to 0.99,
whereas the tensile strength, Young’s modulus and
elongation-at-break data have a medium R2
value at 0.59 to
0.71. The Knoop hardness and other DPC data do not yield
good regression results. Most responses can be described by a
reduced quadratic model with the interaction term A*B being
most prevalent. This indicates the significant interaction
Table 1 Mixture Design Matrix Points
DSN ID Ebecryl 3703 NVP TMPTA
1 100 0 0
2 70 0 30
3 70 30 0
4 85 0 15
5 85 15 0
6 70 15 15
7 90 0 10
8 80 0 20
9 90 10 0
10 80 20 0
11 70 10 20
12 70 20 10
13 90 5 5
14 75 5 20
15 75 20 5
16 80 10 10
16 80 10 10
16 80 10 10
Figure 1 Schematic Design Points on Truncated Three-component Mixture
Triangular Diagram
9
10
Oligomer
(100)
Oligomer / NVP
(70/30)
Oligomer / TMPTA
(70/30)
1
7
4
8
2
135
3
12 6 11
1415
16
NVP = 0TMPTA = 0
Oligomer = 70
between the epoxy diacrylate oligomer and NVP reactive
diluent. This will be further discussed in later sections.
The 3D response surface plots based on the regression models
in Table 2 are shown in Figures 2a to 2i. These response
surface plots provide a convenient and effective overview of
how each response property varies over the design space.
Most properties monotonically increase or decrease with
respect to increment of a given component. However, tensile
properties show a non-linear behavior with a local maximum
or minimum on the two-component plane of oligomer and
NVP (i.e., TMPTA = 0). This echoes the earlier notice of the
presence of A*B interaction term between oligomer and NVP
in most of the regression models.
2. Viscosity
The viscosity data are summarized in Table 3a - 3d and are
arranged according to epoxy oligomer/NVP two-component
blend series (i.e., TMPTA = 0 in Table 3a), epoxy
oligomer/TMPTA two-component blend series (i.e., NVP = 0
in Table 3b), NVP + TMPTA = 30 series (Table 3c), and
NVP/TMPTA =1 series (Table 3d). The viscosity value varies
from 738 cp of 70/30 blend of oligomer/NVP composition to
188,400 cp of the pure epoxy oligomer resin. As shown in
Figure 2a, NVP is a more efficient viscosity reducing
4. monomer than TMPTA. At 10% level, NVP is about 3.5
times more efficient than TMPTA whereas at 30% level it is 8
times more efficient (cf. Table 3a versus Table 3b).
3. DPC Data
Figure 3 shows a typical DPC curve, where ∆Hrxn is measured
from the total integrated area above the baseline, αpeak from
the ratio of the partial integrated area up to exotherm peak
relative to total area, tpeak from time to reach the peak of
exotherm, and tinduct. from time to reach 1% conversion. As
shown from the response surface plot in Figure 2b, the heat of
reaction, ∆Hrxn , increases with increasing NVP and TMPTA
content in the reactive mixture. This is expected. The
equivalent weights (E.W.) of NVP, TMPTA, and Ebecryl
3703 oligomer are 111, 99 and 417, respectively, hence the
unsaturation double bond (C=C) concentration increases
steadily with the increasing addition of monomers to oligomer
(cf. Column C=C conc. in Tables 3a, 3b and 3d). In addition,
viscosity reduction due to addition of monomers also
increases the reactivity by enhancing chain mobility. It is
interesting to notice that in Table 3c, where NVP is
systematically replaced with TMPTA, the heat of reaction
(∆Ηrxn) declines accordingly even though the calculated
double bond concentration is increased slightly. Furthermore,
if one plots ∆Ηrxn versus double bond concentration for
different series, i.e., 3703/NVP (Table 3a) , 3703/TMPTA
(Table 3b), and 3703/NVP/TMPTA blend with
NVP/TMPTA=1 (Table 3c) as shown in Figure 4. It is clear
that NVP renders a higher heat of reaction than TMPTA to the
reactive mixture at the same double bond concentration
whereas the blend of NVP/TMPTA stays in between. This
can be explained in terms of the less viscosity reduction
efficiency and higher functionality of TMPTA (F = 3)
compared to NVP (F = 1). It has been well documented in
literature 4, 5
that higher functionality monomers such as
TMPTA, though enhancing the cure speed response, reduce
the final degree of double bond conversion. This is due to the
combined effect of higher viscosity and the early onset of
gelation at a low conversion for high functional monomer
system.
The time to reach exotherm peak (tpeak) is an indication of the
photo-cure response of UV reactive mixture. As shown in
Table 3, sample #1, the highly viscous pure epoxy diacrylate
oligomer, exhibits a very sluggish cure with a long tpeak of 16
sec, whereas addition of reactive monomer significantly
reduces tpeak to ca. 6.6 to 12.8 sec depending on the
composition. The same trend is observed on the induction
time (tinduct.), where 8.3 sec for pure oligomer (#1) is reduced
to ca. 2.4 to 6.9 sec. Note that the two-component mixtures of
oligomer/TMPTA (Table 3b) have higher tpeak and tinduct.
values compared to their counterparts of oligomer/NVP
blends (Table 3a). This correlates well with the lower ∆Hrxn
observed in the oligomer/TMPTA series compared to the
oligomer/NVP series. These data indicate that NVP has a
higher co-polymerization reactivity than TMPTA with the
epoxy diacrylate oligomer and attains a higher degree of
double bond conversion.
The double bond conversion up to the exotherm peak (αpeak),
i.e., the ratio of the partial integrated area up to the peak
relative to the total area, however, does not exhibit any
particular trend and is at ca. 20% conversion.
4. Glass Transition Temperature (Tg)
Figure 5 shows a typical DSC trace for the cured samples. Tg,
onset , Tg, infl. and Tg, end are determined from the onset,
inflection and end of the step change of baseline, respectively.
The breadth of glass transition, ∆Tg = Tg, end - Tg, onset, is also
calculated. The response surface plot of Tg, onset from DSC is
shown in Figure 2c. NVP significantly increases the Tg of
reactive mixture whereas the pure epoxy diacrylate oligomer
has the lowest Tg among all the reactive mixture composition.
As shown in Tables 4a - 4d, Tg ranges from 18.3°C for pure
epoxy diacrylate oligomer to 56.2 °C for 70/30 oligomer/NVP
blend. It is interesting that NVP, though being a mono-
functional acrylate monomer, is very effective in enhancing
the Tg of cured coating. It has been reported in the literature
6, , , , , ,7 8 9 10 11 12
that NVP, in converse to the general trend
expected of mono-functional acrylate monomer, does not give
rise to a flexible network but increases the Tg value, modulus,
and tensile strength of cured coatings. This peculiar behavior
of NVP has been attributed to its high polarity and rigidity of
the resulting polymeric chains. The literature Tg value of
NVP homopolymer is very high at 175°C 16
.
The tri-functional TMPTA also increases Tg of cured coatings
by increasing crosslink density as shown in Table 4b, though
not to the same extent as NVP. In Table 4c, where NVP is
systematically replaced with TMPTA, the Tg value decreases
accordingly even though the TMPTA gives rise to a higher
network crosslink density.
The Tg, infl. as measured from inflection point of the glass
transition, also shows the same trend as Tg, onset. The Tg value
determined from DMA E” peak shows good correlation with
Tg, infl. from DSC as shown in Figure 6. The response surface
plot of DMA Tg (E” peak temperature) is shown in Figure 2d.
It gives the same trend as the DSC Tg, onset response surface
plot in Figure 2c.
The breadth of glass transition, ∆Tg = Tg, end - Tg, onset, is a
measure of the degree of homogeneity of thermoset network
structures. In Table 4a, ∆Tg decreases monotonically with
increasing NVP to epoxy oligomer ratio. This indicates a
more homogeneous network structure with a narrower
relaxation time spectrum is attained with increasing NVP
5. content. This may result from the enhanced molecular
mobility during network formation due to the efficient
viscosity reduction by NVP and the high co-polymerization
reactivity of NVP 8
. In contrast, ∆Tg increases with
increasing TMPTA content as shown in Table 4b. The wide
glass transition is expected from the non-linear and
hetrogeneous network formation due to the high functionality
of TMPTA in a somewhat more viscous medium.
5. Crosslink Density (νe)
Figure 7 shows a typical DMA trace of a UV cured sample.
Note that the storage modulus (E’) at rubbery plateau
increases with respect to temperature according to rubber
elasticity theory 13, ,14 15
, i.e.,
E’ = 3 νe R T,
where
E’= tensile elastic (storage) modulus (dyne/cm2
)
νe = crosslink density (mole/cm3
), number of
moles of elastically effective network
chains per unit volume.
R = gas constant (8.314 x 107
erg/mole °K)
T = temperature (in Kelvin, °K)
Hence, crosslink density can be calculated from the elastic
modulus at the rubbery plateau. The calculated crosslink
density values are summarized in Table 5a - 5d and Figure 2e
shows the response surface plot. As expected, the tri-
functional TMPTA monomer increases the network crosslink
density whereas mono-functional NVP monomer decreases it.
This is clearly demonstrated in Tables 5a and 5b. Particularly,
in Table 5c, where NVP is systematically replaced by
TMPTA, the crosslink density increases accordingly.
Note that some researchers have used the calculated total
double bond (C=C) concentration as a quick estimate of the
crosslink density, which in some cases may lead to wrong
conclusion. For the series in Table 5c, where the double bond
concentration remains roughly the same due to the similar
E.W.’s for NVP and TMPTA (111 versus 99). However, the
crosslink density calculated from DMA data clearly indicates
a significant increase in crosslink density when replacing NVP
with TMPTA. This illustrates the effectiveness of TMPTA
tri-functional monomer as a chemical crosslinker. Even with
a lower degree of double bond conversion during UV cure as
suggested from the discussion on ∆Hrxn in the DPC data
section, TMPTA is still much more efficient in increasing
crosslink density than NVP. Furthermore, for the series listed
in Table 5a, though the double bond concentration is
systematically increased with increasing ratio of NVP to
epoxy diacrylate oligomer, the crosslink density from DMA
actually decreases with respect to an increase of NVP content
except for sample #1. This clearly demonstrates that higher
total double bond concentration does not necessarily
guarantee a network with a higher crosslink density. Instead,
it is the double bond that resides in the multi-functional
monomer that actually contributes to the effective chemical
crosslinking during network formation.
Sample #1, the pure epoxy diacrylate oligomer, has a lower
crosslink density than sample #2. This may result from its
extremely high viscosity impeding the degree of conversion.
6. Thermal Stability (TGA)
Figure 8 shows a typical TGA curve. There is a gradual
weight loss due to thermal degradation with respect to
temperature and an onset of drastic weight loss at ca. 350 °C.
The temperature of 5% weight loss in air environment, T5% loss,
is used as the measure of thermal stability. T5% loss has very
good R2
, adjusted R2
and predicted R2
values as shown in
Table 2. Figure 2f gives the response surface plot for T5% loss.
High TMPTA content in formulation enhances thermal
stability whereas NVP decreases the thermal stability. This is
clearly demonstrated as shown in Table 5a - 5c where T5% loss
increases with increasing TMPTA content but decreases with
increasing NVP content. In particular, in Table 5c, when
TMPTA replaces NVP in a systematic fashion, T5% loss
increases from 193.4 °C to 301.8 °C.
From discussion of previous sections, NVP results in a higher
Tg than TMPTA whereas TMPTA gives a higher network
crosslink density than NVP. Hence, it is evident that crosslink
density of the thermoset network rather than Tg is the
dominant factor in determining the thermal stability of UV
cured coatings of epoxy diacrylate oligomer/NVP/TMPTA
formulations.
7. Tensile Mechanical Properties and Knoop Hardness
All samples in the mixture design exhibit a brittle failure with
a low elongation-at-break of 3.1 to 5.4% and a high Young’s
modulus of 189,600 to 272,000 psi. The tensile strength
(stress-at-break) ranges from 4800 to 7700 psi. Tables 6a - 6d
list the tensile and Knoop hardness data.
The quadratic model can only marginally fit the tensile testing
data including tensile strength (Sb), elongation-at-break (eb),
and Young’s modulus (E) with a relatively low correlation
coefficients (cf. Table 2). It seems difficult to fit the data
obtained from the tensile testing which subjects samples to a
catastrophic failure as the end point of test. This may result
from the inherent high variation of the test and limited
sampling size.
6. However, though a precise predictive model may be out of
reach, the trend from analysis is still valuable and provides
insight into the effects of mixture components on mechanical
properties of cured samples. Figures 2g - 2i give the response
surface plots for tensile strength (Sb), elongation-at-break (eb),
and Young’s modulus (E), respectively. TMPTA increases
both the tensile strength and Young’s modulus of the cured
coatings, whereas NVP gives a higher elongation-at-break.
There is a synergistic and/or antagonistic effect between the
oligomer/NVP and oligomer/TMPTA, respectively. Tensile
strength (Sb) and Young’s modulus (E) go through a local
maximum whereas elongation-at-break (eb) goes through a
local minimum at ca. 80/20 oligomer/NVP blend at the plane
where TMPTA = 0. To verify this, an additional blend of
60/40 oligomer/NVP (#19) was prepared and tested. The data
are included in Table 6a. It shows a continuing decline in
both Sb and E whereas a continuing increase in eb with a NVP
content beyond 20%. Note that a semi-flexible coating can be
obtained from 60/40 oligomer/NVP blend with a relatively
high elongation-at-break of 12.9% in contrast to the typical 3 -
5% of other mixture ratios. Yu, et al.12
also observed
interesting behavior in tensile properties of different urethane
acrylate UV cured coatings modified with 20 to 50% NVP.
Knoop hardness number (KHN) does not show any good
correlation with respect to composition. KHN is more of a
measure of surface hardness and surface cure of coatings. It
then suggests that the surface hardness of all different
compositions is comparable. Since samples were cured in air
without any purge of inert gas, it is possible that oxygen
inhibition or retardation may affect the surface cure and mask
out any difference expected from different compositions. On
the other hand, however, Knoop hardness test may not be as
sensitive as other more sophisticated characterization
techniques such as thermal analysis and mechanical testing,
which also characterize the bulk properties of coatings instead
of surface characteristics. It is not clear whether the method is
not sensitive enough to pick up the subtle difference among
different compositions or the variation of surface hardness for
these samples is minimal.
8. Formulation Optimization
From the discussion of previous sections, it is clear that NVP
and TMPTA contributes different physical properties to the
resulting cured coatings. Hence, a compromise in properties
may be necessary in formulation. With the predictive models
attained from regression analysis of various response
properties, it is possible to optimize the reactive mixture
composition to achieve the desirable combination of physical
properties. This can be accomplished either by graphical
optimization or numerical optimization.
In graphical optimization, by superimposing response
contours, one searches for a “compromise” optimum that
meets the simultaneous requirements of multiple properties.
The contour plot of cost can also be calculated from the cost
of individual component and the corresponding mixture ratio.
This contour of cost can then be overlapped with the region
which meets the performance requirement to
pick the oligomer/NVP/TMPTA composition which provides
the required balanced properties and offers the best
performance to cost ratio.
Graphical optimization works great for three factors, but may
become tedious as factors increase to more than three.
Numerical optimization is more efficient to explore multiple
factors and multiple responses and find the optimization
solution quicker. The Design-Expert software from Stat-Ease
utilizes Derringer and Suich’s optimization method to search
for the greatest overall desirability. One can assign
desirability indices to each response by setting parameters of
goal, low and high values and also assign additional weights
to emphasize the importance of a target value.
CONCLUSION
The thermal and mechanical properties of UV cured thermoset
networks from different compositions of epoxy diacrylate
oligomer, NVP and TMPTA reactive mixtures have been
studied. The observed trends and effects can be interpreted in
terms of the resulting network parameters of Tg and crosslink
density. The addition of NVP and/or TMPTA monomers
increases photo cure response by the combination of lowering
viscosity and increasing double bond concentration. Both
NVP and TMPTA increase Tg of the cured coating. The
increase in Tg is accomplished either by means of the rigidity
and polarity from NVP or the chemical crosslinking from
TMPTA. NVP is more effective than TMPTA in viscosity
reduction, enhancing double bond conversion, raising Tg of
cured coatings, and increasing elongation-at-break. On the
other hand, TMPTA increases tensile strength, Young’s
modulus, thermal stability and crosslink density of network.
Thermal stability of these epoxy acrylate/NVP/TMPTA UV
cured coatings is dominated mainly by the crosslink density
rather than Tg.
The mixture experimental design is a very useful tool to
facilitate the formulation of UV curable resins. The statistical
regression analysis provides a quantitative ranking of the
effects of individual mixture components, identifies the
interactions among reactive components, and gives rise to
predictive models for property prediction. The prevalent
interaction term if exists is the one between epoxy acrylate
oligomer and NVP. The predictive quadratic models allow for
estimation of response properties at any given composition
within the design range limits. They can also be used to
generate 3D response surface plots to provide an effective
overview of the trend of response within the design space.
7. By graphical or numerical optimization, a region of
formulation compositions with compromised but balanced
properties and cost can be obtained under a given set of
requirements.
The methodology and analysis used in this study is not limited
only to the epoxy acrylate/NVP/TMPTA reactive mixtures but
can be extended to other chemical systems.
16. Figure 6 Correlation between Tg (DSC) and Tg (DMA)
DSC Tg,inflec. vs. DMA Tg,E" peak
y = 0.5674x + 15.94
R2
= 0.7198
25
35
45
55
65
20 30 40 50 60 70 80
DSC Tg (C)
DMATg(C)
17.
18. REFERENCES
1
Saha, T.K., Khan, M. A., Ali, K.M. I., “ Physical and
Mechanical Properties of Ultraviolet (UV) Cured Films”,
Radiat. Phys. Chem. Vol. 44, No 4, pp. 409-414 (1994).
2
Julian, J. M., “Analyzing Radiation Curable Coating
Properties”, Proceedings of RadTech ‘94, pp. 495-500 (1994).
3
Rabek, J.F.,”Experimental and Analytical Methods for the
Investigation of Radiation Curing”, Chapter 7 in “Radiation
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Acknowledgement
The authors would like to extend their thanks to J. Haacker
and M. Pehkonen for collecting most of the data used in this
study.
Key Words
UV, thermoset network, crosslink density, glass transition
temperature, thermal stability, mechanical properties, thermal
analysis, mixture experimental design, formulation
optimization, epoxy acrylate oligomer, NVP, TMPTA.
