The document analyzes the effect of chemical composition on the curing and properties of UV-cured resins made from urethane diacrylate oligomer, N-vinyl pyrrolidone (NVP) monomer, and trimethylolpropane triacrylate (TMPTA) monomer. Mixture experimental design was used to study 10 sample compositions. Key findings: 1) NVP is more effective than TMPTA at reducing viscosity and increasing depth of cure and heat of reaction. 2) TMPTA is more effective at increasing properties like Young's modulus, tensile strength, and thermal stability. 3) The urethane acrylate system exhibits

1. RadTech ‘98 North America UV/EB Conference Proceedings, P.565, Chicago, IL, 4/19-22/1998.
Thermal and Mechanical Properties of Radiation Curable Networks-II:
Composition Effect of Urethane Acrylate/NVP/TMPTA Network
W. Patrick Yang, J. Wijaya, G. Swei
Norton Company
Worcester, MA, USA
ABSTRACT
The effect of chemical composition of reactive mixtures
prepared from urethane diacrylate oligomer, N-vinyl
pyrrolidone (NVP), and trimethylolpropane triacrylate
(TMPTA) on the cure and network properties of UV
cured resins was studied by a variety of thermal analysis
techniques (DPC, DSC, DMA and TGA) and tensile
testing.
NVP monomer is more efficient than TMPTA to reduce
the viscosity of reactive mixture, to enhance the reactivity
of photo-polymerization, to increase tensile elongation of
the cured network, and to attain a higher depth of cure.
However, NVP lowers the thermal degradation
temperature. On the other hand, TMPTA is most
effective to enhance Young’s modulus, tensile strength,
crosslink density and thermal stability. Most of these
findings are similar to a previous study on epoxy
diacrylate oligomer/NVP/TMPTA system1
. However,
there are several important differences between the
urethane acrylate oligomer system versus the epoxy
acrylate oligomer system.
While there is only a single glass transition observed in
the epoxy acrylate/NVP/TMPTA system, there appears to
be two separate glass transitions observed in the urethane
acrylate/NVP/TMPTA system. There is a pronounced
low temperature Tg resulting from the polyol soft
segment of urethane acrylate and a high temperature Tg
resulting from the urethane hard segment domains
reinforced by NVP and TMPTA. In contrast to the epoxy
acrylate/NVP/TMPTA system where a hard and brittle
thermoset network exhibits a low tensile elongation-at-
break and a high Young’s modulus, the urethane
acrylate/NVP/TMPTA system exhibits a wide spectrum
of tensile properties ranging from a hard and strong to a
soft but tough elastomeric behavior.
Mixture experimental design is used to set up the reactive
mixture blends and 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 previous paper, the effect of chemical composition of
reactive mixtures prepared from bisphenol-A epoxy
diacrylate oligomer, N-vinyl pyrrolidone (NVP), and
trimethylolpropane triacrylate (TMPTA) on the cure and
network properties of UV cured resins is studied by
thermal analysis and tensile testing1
. In this paper, we
have extended the work to investigate the effect of
chemical composition of UV reactive mixture on cure and
network properties of another important class of
oligomer, the urethane acrylates.
While epoxy acrylate oligomers are highly reactive, give
hard and chemically resistant films and have been the
workhorse resin in radiation curable applications,
urethane acrylate oligomers offer flexibility, lower
shrinkage, excellent toughness, chemical resistance,
adhesion to difficult substrates, and abrasion resistance.
The diverse variety of isocyanate structures and polyols
provide urethane acrylate oligomers a wide spectrum of
properties.
However, the urethane acrylate oligomers generally are
viscous and have a slower cure rate compared to the
epoxy acrylate oligomers. Acrylate monomers are often
added to the urethane acrylate to reduce viscosity for
application as well as to enhance the cure rate and modify
the final cured network properties. The combination of
NVP and TMPTA monomers has been shown in literature
to provide balanced mechanical properties to the urethane
acrylate formulation2
. This paper will illustrate the use of
mixture experimental design to study the properties of
blend mixture of urethane diacrylate/NVP
monoacrylate/TMPTA triacrylate formulations by thermal
analysis and tensile testing.
EXPERIMENTAL
Sample Preparation
The urethane diacrylate oligomer (Uvithane 782 from
Morton International), NVP (from GAF) and TMPTA
(from Sartomer) were mixed together until homogeneous
according to the required ratio and 3% 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 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

2. 50 ft/min in air through a Fusion UV cure unit (model
DRS120) with D bulb at 300 W/in and H bulb at 400
W/in.
Viscosity
The viscosity of liquid mixture was measured by
Brookfield viscometer (model DCV-II+) at 110°F using
S64 spindle at various rotational speeds of 1, 5, 20 and 50
rpm. All reactive mixes exhibit Newtonian viscosity
behavior.
Depth of Cure
Liquid resin mixture was placed in an aluminum dish and
cured by 2 passes at 50 ft/min in air through a Fusion UV
cure unit with D bulb at 300 W/in and H bulb at 600
W/in. After cure, the solidified sample was taken out of
the dish and wiped clean the residual uncured resin. The
thickness of the solidified samples is designated as the
depth of cure.
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 10
mg. 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 1” and cross-head speed at 0.2 in/min.
Ten 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 the inflection temperature
(Tg, infl) of the step change in heat capacity. The Tg
values are measured from the DSC scan from -100°C to
50°C at 10°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 5°C/min from 30°C to 500°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
5°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 12.5 mm W x 0.35 mm t x 12.5 mm
L. The oscillatory amplitude is 0.12 to 0.15 mm. The
peak temperatures of loss modulus (E”) and tan δ are
reported as Tg (DMA). The network crosslink density is
estimated from the storage modulus (E’) at the rubbery
plateau according to rubber elasticity theory.
RESULTS AND DISCUSSION
1. Mixture Experimental Design
The lower and upper limits of each mixture component
are set as below:
70 ≤ Urethane oligomer (Uvi-782) ≤ 100
0 ≤ NVP ≤ 30
0 ≤ TMPTA ≤ 30
This results in a truncated three-component design space
for urethane diacrylate oligomer, NVP and TMPTA. The
mixture design matrix is listed in Table 1 and the
schematic design points are shown in Figure 1.
Table 1
Composition
DSN Uvithane782 NVP TMPTA
1 100 0 0
2 85 15 0
3 85 0 15
4 70 30 0
5 70 15 15
6 70 0 30
7 90 5 5
8 75 20 5
9 75 5 20
10 80 10 10
1
2 3
4 5 6
7
8 9
10
782/NVP 782/TMPTA
Uvithane 782
TMPTA = 0 NVP = 0
Uvithane 782 = 70(70/30) (70/30)
(100)
Figure 1

3. 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 (#7, #8 and #9)
and the centroid point (#10) are included for model lack
of fit test. These runs of ten design points are adequate
for a comprehensive quadratic mixture experimental
design.
Hence, for example, referring to Figure 1 and Table 1,
design points of #1, #2 and #4 on the left triangular edge
of TMPTA = 0 represent a systematic increase of NVP
amount with respect to Uvi-782 epoxy diacrylate
oligomer in a two-component mixture up to the upper
limit of 30% NVP. On the other hand, design points of
#1, #3 and #6 on the right triangular edge of NVP = 0
allow for an analysis of Uvi-782/TMPTA mixture blend
up to 30% TMPTA. Design points of #4, #5, and #6 on
the lower triangular edge of Uvi-782 = 70 allow for
analysis of the effect of substituting TMPTA for NVP
under a constraint of NVP + TMPTA = 30. Finally, one
can follow the path of design points #1, #7, #10 and #5 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 10 runs
of data points and 3D response surface plots are generated
using Design Expert software from Stat-Ease. The
criteria to include a term in regression equation is at a
significant level of α < 0.05. Overall, log10 viscosity
data, TGA data, crosslink density νe , and ∆Hrxn data
have the best regression results with a R2
ranges from
0.93 to 0.99, whereas the tensile strength, Young’s
modulus and elongation-at-break data have a medium R2
value at 0.88 to 0.91. Most responses can be described by
a reduced quadratic model.
The 3D response surface plots can be generated from the
regression models. These response surface plots provide
a convenient and effective overview of how each
response property varies over the design space.
2. Viscosity
The viscosity data are listed in Table 2 and the 3D
response surface plot is shown in Figure 2. The viscosity
value varies from 3000 cp of 70/30 blend of urethane
oligomer/NVP composition to 122,2000 cp of the pure
urethane oligomer resin. As shown in Figure 2, NVP is a
more efficient viscosity reducing monomer than TMPTA.
At 15% level (cf. DSN #2 vs. #3 in Table 2), NVP is
about 2.7 times more efficient than TMPTA whereas at
30% level it is 5.2 times more efficient (cf. Table 2, DSN
#4 vs. #6).
Figure 2
NVP
TMPTA
Uvithane782
DESIGN-EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (100.00) X2 (0.00)
X3 (30.00)
3.5
3.9
4.3
4.7
5.1
Log10 Viscosity
(cp)
X1 (70.00)
X2 (30.00)
X3 (0.00)
2. Depth of Cure
The depth of cure data are summarized in Table 2 and the
3D response surface plot is shown in Figure 3.
Figure 3
TMPTA
NVP
Uvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (70.00)
X2 (30.00)
X2 (0.00)
X3 (30.00)
X3 (0.00)
2.7
4.0
5.3
6.7
8.0
Depth of Cure
(mm)
X1 (100.00)
Addition of NVP significantly increases the depth of cure
whereas TMPTA does not enhance much of the depth of
cure of urethane acrylate oligomer. The 70/30 Uvi-
782/NVP blend can achieve a high depth of cure as much
as 8 mm in comparison to the 70/30 Uvi-782/TMPTA
blend at 2.94 mm.
3. DPC Data
Heat of reaction, ∆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 4, the heat of

4. 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
Uvi-782 oligomer are 111, 99 and 2500, 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 Table
2). In addition, viscosity reduction due to addition of
monomers also increases the reactivity by enhancing
chain mobility.
Figure 4
TMPTA NVP
Uvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (70.00) X2 (30.00)
X2 (0.00)
X3 (30.00)
X3 (0.00)
38
81
124
167
210
Heat of Reaction (J/g)
X1 (100.00)
It is interesting to notice that in Table 2, DSN #4, #5, #6,
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, as shown in Figure 5, if one plots
∆Ηrxn versus double bond concentration for different
series: Uvi-782/NVP binary blend with TMPTA = 0
(DSN #1, #2, #4), Uvi-782/TMPTA binary blend with
NVP = 0 (DSN #1, #3, #6), and Uvi-782/NVP/TMPTA
tertiary blend with NVP/TMPTA=1 (DSN #1, #7, #10,
#5), 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.
Figure 5
0
50
100
150
200
250
300
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
C=C Concentration (10-3 mole/gm)
HeatofReaction,Hrxn(J/gm)
NVP=0 (DSN #1, 2, 4)
TMPTA/NVP=1 (DSN #1, 7, 10, 5)
TMPTA=0 (DSN #1, 3, 6)
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 the literature3, 4
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 induction time from DPC is an indication of the
photo-cure response of UV reactive mixture. As shown
in Table 2, sample #1, the highly viscous pure urethane
diacrylate oligomer, exhibits a very sluggish cure with a
long induction time of 9.7 sec, whereas addition of
reactive monomers significantly reduces the induction
time to ca. 3.9 to 8.9 sec depending on the composition.
Figure 6 shows the 3D response surface plot of the
induction time data. As shown, NVP is most effective to
reduce the induction time for photo-polymerization.
Figure 6
TMPTANVP
Uvithane782DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (100.00)
X2 (0.00)X3 (0.00)
3.7
5.1
6.4
7.8
9.2
Induction Time
(sec)
X1 (70.00)X2 (30.00) X3 (30.00)
The same trend is observed on the time to reach peak of
photo reaction exotherm (tpeak), where 20.2 sec for pure
oligomer (#1) is reduced to ca. 9.2 to 10 sec with addition

5. of monomers. Note that the two-component mixtures of
oligomer/TMPTA have higher tpeak and tinduct. values
compared to their counterparts of oligomer/NVP blends.
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 urethane
diacrylate oligomer and attains a higher degree of double
bond conversion. This agrees with the finding by Priola
et al.4
where NVP gives a higher rate of unsaturation
disappearance by IR than TMPTA in an epoxy acrylate
system.
The double bond conversion up to the exotherm peak
(αpeak), i.e., the ratio of the partially integrated area up to
the peak relative to the total area, however, does not
exhibit any particular trend and is at ca. 20-30%
conversion.
4. DSC and DMA Glass Transition Temperature (Tg)
While there is only a single glass transition observed in
the epoxy acrylate/NVP/TMPTA system from both DSC
and DMA as discussed in our previous paper1
, there
appears to be two separate glass transitions observed in
the urethane acrylate/NVP/TMPTA system. It has been
reported in the literature5, 6
that some UV cured urethane
acrylates exhibited a phase-separated morphology similar
to the thermoplastic polyurethanes and two transitions
attributed to the soft segment phase and hard segment
domains were observed. This phase separation
phenomenon was observed when the polyol soft segment
M.W. was greater than 1000. Addition of reactive diluent
such as NVP was shown to reinforce the hard segment
domains and increase the hard segment transition
temperature.
In the Uvi-782/NVP/TMPTA resin system studied in this
paper, the DSC curve indicates that there is a pronounced
low temperature glass transition resulting from the polyol
soft segment of urethane acrylate (Tg,s at ca. -37°C) and
there seems to be a very small and not well defined
transition at a higher temperature which may result from
the hard segment domain (Tg,h at ca. 30°C). This
transition becomes a little more definitive in blends with a
high content of NVP and/or TMPTA monomer. Koshiba
et al. 5
and Yu et al. 6
have also reported the difficulty to
detect the hard segment transitions of some UV cured
urethane acrylate systems by DSC. The soft segment
glass transition temperature (Tg,s) data are reported in
Table 3. The relative constancy of the Tg,s suggests that
the soft segment phase is relatively pure without
significant phase mixing of hard segment within the soft
segment phase.
Figure 7 compares the DMA loss modulus (E”) curves of
Uvi-782, 70/30 Uvi-782/NVP and 70/30 Uvi-
782/TMPTA as a function of temperature.
Figure 7
6
7
8
9
-100 -50 0 50 100 150 200
Temperature (C)
LogE''(Pa)
782/TMPTA = 70/30
782/NVP = 70/30
Loss Modulus, E"
Uvithane782
Soft Segment Tg
Hard Segment Tg
There are some minor transitions below -50°C due to the
localized motion of the soft segments7
. The pronounced
and constant position of E” peak at ca. -37°C is due to the
soft segment glass transition. This correlates well with
the Tg,s data from DSC. The Tg,s data from E” peak and
tan δ (= E”/E’) peak in DMA are reported in Table 3.
The 70/30 Uvi-782/NVP and 70/30 Uvi-782/TMPTA
blends exhibit an additional shoulder in the loss modulus
(E”) curve at 25°C and 75°C, respectively, which may
result from the hard segment domain transition. Yu et al.6
has reportrd that in a well phase-separated system, the
reactive diluent such as NVP and HEMA preferentially
segregated to the more polar hard segment phase. It
appears that in Uvi-782/NVP/TMPTA system, the NVP
and TMPTA monomers also segregate preferentially to
hard segment phase and reinforce the hard segment
domains. This reinforcing effect to the hard segment
domains increases the storage modulus (E’), Young’s
modulus, and tensile strength as will be discussed in the
following sections. Also, at the same weight percent,
TMPTA seems to be more effective than NVP to push up
the hard segment transition temperature.
5. DMA Elastic Modulus (E’) and Crosslink Density (νe)
The DMA elastic (storage) modulus (E’) curves of Uvi-
782, 70/30 Uvi-782/NVP and 70/30 Uvi-782/TMPTA as
a function of temperature are shown in Figure 8. Note
that 70/30 Uvi-782/NVP blend, though exhibiting a much
higher modulus than the pure Uvi-782 oligomer in the
leathery region (between the glassy state and the rubbery
plateau), it has practically the same rubbery plateau
modulus as the pure Uvi-782 oligomer. Since the rubbery
plateau modulus is proportional to the chemical crosslink
density, this suggests that the higher elastic modulus of

6. the 70/30 Uvi-782/NVP blend in the leathery region
results from the hard segment domain reinforcing effect
by NVP as discussed in the previous section rather than
the higher chemical crosslink density. On the other hand,
the 70/30 Uvi-782/TMPTA blend has a higher elastic
modulus in both the leathery and rubbery plateau regions.
This indicates that Uvi-782/TMPTA blend exhibits a
higher modulus from a combination of hard segment
domain reinforcing effect and higher chemical crosslink
density.
Figure 8
6
7
8
9
10
-100 -50 0 50 100 150 200
Temperature (C)
LogE'(Pa)
Elastic Modulus, E'
Uvithane 782
782/TMPTA = 70/30
782/NVP = 70/30
Rubbery Plateau
To further quantify the chemical crosslink density in the
urethane acrylate/NVP/TMPTA network, one can utilize
the rubber elasticity theory. Note that the storage
modulus (E’) at rubbery plateau is proportional to the
network crosslink density according to the rubber
elasticity theory8, 9, 10
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 4 and
Figure 9 shows the 3D 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
particularly in DSN #4, #5, #6 in Table 4 and at the plane
of Uvi-782 = 70 in Figure 9, 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 DSN #4, #5 and #6 in
Table 4, 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.
Furthermore, for DSN #2 and #4 in Table 4, when the
NVP content in the Uvi-782/NVP binary blend is
increased from 15% to 30%, though the C=C
concentration being significantly increased by 1.8 times,
the crosslink density actually decreases. Note that DSN
#1, the pure urethane oligomer, has a lower crosslink
density than DSN #2, the 85/15 Uvi-782/NVP blend.
This may result from the high viscosity and slow reaction
rate of the pure urethane acrylate which impedes the
degree of conversion and crosslinking. When this sample
was subjected to a second UV exposure in DPC, there
was residual heat of reaction detected. Compared to the
heat of reaction, ∆Hrxn, from the DPC data, the pure
urethane acrylate oligomer cured under the condition
described in the section of sample preparation attains only
70% of the potential double bond conversion.
Figure 9
TMPTA
NVP
Uvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (100.00) X2 (0.00) X3 (30.00)
5.10E-06
1.74E-04
3.43E-04
5.11E-04
6.80E-04
X-link density
(mole/cm3)
X1 (70.00)
X2 (30.00)
X3 (0.00)
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.
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.

7. 6. Thermal Stability (TGA)
The TGA curve of weight loss with respect to
temperature shows an initial gradual weight loss then an
onset of drastic weight loss at ca. 260°C. The
temperature of 5% weight loss in air environment, T5% loss,
is used as the measure of thermal stability. Figure 10
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 4 and Figure 10 where
T5% loss increases with increasing TMPTA content but
decreases with increasing NVP content. In particular,
comparing DSN #4, #5, and #6 in Table 4, when TMPTA
replaces NVP in a systematic fashion, T5% loss increases
drastically from 131°C to 273°C.
Figure 10
TMPTA
NVPUvithane782
TGA Temp.(5% loss)
(degree C)
DESIGN-EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (70.00)
X2 (30.00)
X2 (0.00)
X3 (30.00)
143
179
216
253
289
X1 (100.00) X3 (0.00)
In the previous paper on epoxy acrylate/NVP/TMPTA
system1
, the crosslink density was identified as the
dominant network parameters affecting the thermal
stability. This conclusion, at the first glance of the data,
does not quite apply in this urethane
acrylate/NVP/TMPTA system. Comparing the pure Uvi-
782 oligomer (DSN #1) versus 70/30 Uvi-782/TMPTA
binary blend (DSN #6) in Table 4, the 70/30 Uvi-
782/TMPTA binary blend has only a slightly high
degradation temperature (T5% loss) at 273°C than the pure
Uvi-782 oligomer at 256°C, though its crosslink density
is almost 32 times higher (71.3 x 10-5
vs. 2.2 x 10-5
mole/cm3
). Furthermore, the 70/30 Uvi-782/NVP binary
blend (DSN #4) has a much lower degradation
temperature at 131°C than the pure Uvi-782 oligomer
(DSN #1) at 256°C, whereas both have a comparable
crosslink density (2.2 x 10-5
vs. 2.5 x 10-5
mole/cm3
).
One possible explanation of the unusually high T5%l loss
thermal degradation temperature of this lowly crosslinked
urethane acrylate network may be due to the high
molecular weight of the initial oligomer (M.W. = 5000).
The blown-up portion of the TGA curves in the initial
weight loss region of the Uvi-782, 70/30 Uvi-782/NVP
and 70/30 Uvi-782/ TMPTA is shown in Figure 11.
Figure 11
86
88
90
92
94
96
98
100
102
0 50 100 150 200 250 300 350
Temperature (C)
Weight%
782/NVP = 70/30
782 = 100
782/TMPTA
= 70/30
The very low degradation temperature (131°C) at 5%
weight loss for the 70/30 Uvi-782/NVP binary system
may be due to the early flash-off of some residual NVP
monomer which was not incorporated into the thermoset
network during crosslinking. On the other hand, the high
molecular weight of the Uvi-782 oligomer prevents its
early flash-off at low temperature. Note that the
degradation temperatures at 14% weight loss do follow
more closely to the trend of crosslink density. It appears
that the initial weight loss in TGA of these urethane
acrylate/NVP/TMPTA system may be dominated by the
flash-off of residual low M.W. monomers, whereas the
later stage of degradation is controlled by the crosslink
density.
7. Tensile Mechanical Properties
The representative stress-strain curves of all the urethane
acrylate/NVP/TMPTA blends are shown in Figure 12.
Figure 12
Uvi-782/NVP/TMPTA
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150 200 250 300
STRAIN (%)
STRESS(PSI)
6
1 = 100/0/0
1
2 = 85/15/0
2
3 = 85/0/15
3
4 = 70/30/0
4
5 = 70/15/15
5
6 = 70/0/30
7 = 90/5/5
7
8 = 75/20/5
8 9 = 75/5/20
10 = 80/10/10
10
9

8. In contrast to the epoxy acrylate/NVP/TMPTA system
where a hard and brittle thermoset network exhibits a low
tensile elongation-at-break of 3 to 5% and a high Young’s
modulus of 190 to 270 ksi, the urethane
acrylate/NVP/TMPTA system exhibits a wide spectrum
of tensile properties ranging from a hard and strong to a
soft but tough elastomeric behavior. The tensile strength
varies from 0.9 to 2.6 ksi, the elongation-at-break varies
from 19 to 252% and Young’s modulus varies from 0.4 to
16 ksi.
Figures 13 to 15 give the 3D response surface plots for
tensile strength (Sb), elongation-at-break (eb), and
Young’s modulus (E), respectively. In general, TMPTA
increases Young’s modulus of the cured films, whereas
NVP gives a higher elongation-at-break. The tensile
strength, however, exhibits a local maximum at 80/10/10
Uvi-782/NVP/TMPTA blend.
The tensile properties correlate well with the dynamic
mechanical property characteristics. The NVP monomer
contributes to the tensile properties of urethane acrylate
network primarily through the mechanism of reinforcing
the hard segment domains of the phase-separated
morphology. However, the NVP does not significantly
affects the crosslink density of the urethane acrylate
network. The tensile strength and Young’s modulus
increases but the elastomeric behavior retains in the Uvi-
782/NVP binary blend (cf. stress-strain curves #1, 2 and 4
in Figure 12). On the other hand, TMPTA affects the
urethane acrylate network by both the reinforcing effect
of the hard segment domains and the significant increase
in crosslink density. This enhances the tensile strength
and Young’s modulus significantly but with a reduced
elongation-at-break.
Figure 13
TMPTA
NVPUvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (100.00)
X1 (70.00)X2 (0.00)
X3 (30.00)-0.4
3.9
8.2
12.5
16.8
Young's modulus
(ksi)
X2 (30.00)X3 (0.00)
Figure 14
TMPTA
NVP Uvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (100.00)X2 (30.00) X3 (0.00)
-12
48
108
167
227
Elongation
(%)
X1 (70.00) X2 (0.00)
X3 (30.00)
Figure 15
TMPTA
NVP
Uvithane782
DESIGN EXPERT Plot
Actual Components:
X1 = UV-782
X2 = NVP
X3 = TMPTA
X1 (70.00)
X2 (30.00)
X2 (0.00)
X3 (30.00)
X3 (0.00)
1.00
1.41
1.81
2.22
2.63
Tensile Stength
(ksi)
X1 (100.00)
8. Formulation Optimization
From the discussion of previous sections, it is clear that
NVP and TMPTA contributes different cure and 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. This is illustrated in Figure 16 where the

9. shaded area is bounded by the constraints of the desired
properties. Any compositions within the shaded area will
satisfy the required properties.
Figure 16
UV-782
100.00
NVP
30.00
TMPTA
30.00
0.00 0.00
70.00
Viscosity < 10000cp
Tensile Stength > 1.8 ksi
Young's modulus > 4 ksi
Elongation > 25%
Sb
E
eb
Visc.
The contour plot of formulation 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
urethane diacrylate oligomer, NVP and TMPTA reactive
mixtures have been studied. The observed trends and
effects can be interpreted in terms of network crosslink
density and the reinforcing effect of NVP and TMPTA
monomers to the hard segment domains of the phase-
separated morphology of urethane acrylate network.
The NVP monomer contributes to the physical properties
of urethane acrylate network primarily through the
mechanism of reinforcing the hard segment domains of
the phase-separated morphology. NVP does not
significantly affects the crosslink density of the urethane
acrylate network. The tensile strength and Young’s
modulus increases but the elastomeric behavior retains.
On the other hand, TMPTA affects the urethane acrylate
network by both the reinforcing effect of the hard
segment domains and the significant increase in crosslink
density. This enhances the tensile strength and Young’s
modulus significantly but with a reduced elongation-at-
break.
The addition of NVP and/or TMPTA monomers increases
photo cure response by the combination of lowering
viscosity and increasing double bond concentration. NVP
is more effective than TMPTA in viscosity reduction,
increasing depth of cure, enhancing double bond
conversion, and increasing elongation-at-break. On the
other hand, TMPTA is most effective to increases the
tensile strength, Young’s modulus, thermal stability and
crosslink density of the network.
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 possible interactions among reactive
components, and gives rise to predictive models for
property prediction. 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. 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.
Acknowledge
The authors would like to extend their thanks to M.
Pehkonen for collecting some 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, urethane acrylate oligomer,
NVP, TMPTA.

10. Table 2 Viscosity, Depth of Cure and DPC Data
Composition Viscosity Depth of Cure C=C conc. ∆H rxn t induct. t peak α peak
DSN Uvithane782 NVP TMPTA (cp) (mm) ( 10-3
mole/gm) (J/gm) (sec) (sec) (%)
1 100 0 0 122,200 2.98 0.40 34 9.7 20.2 28.8
2 85 15 0 17,280 5.23 1.69 127 4.8 10.2 19.6
3 85 0 15 46,920 2.70 1.86 115 8.9 15.8 24.2
4 70 30 0 3,000 8.03 2.98 219 3.9 10.0 33.4
5 70 15 15 5,520 4.58 3.15 216 4.6 9.2 18.6
6 70 0 30 15,720 2.94 3.31 188 8.4 15.8 25.2
7 90 5 5 41,280 3.39 1.32 100 5.7 10.8 22.9
8 75 20 5 6,000 5.93 2.61 148 4.3 10.0 21.1
9 75 5 20 13,200 3.50 2.77 171 6.1 12.0 24.6
10 80 10 10 14,880 4.03 2.23 155 5.1 10.0 21.1
Table 3 Soft Segment Tg from DSC and DMA
Composition
DSC
Tg
DMA
E" peak
DMA
tan δ peak
DSN Uvithane782 NVP TMPTA (°C) (°C) (°C)
1 100 0 0 -37 -40 -30
2 85 15 0 -37 -36 -26
3 85 0 15 -38 -37 -29
4 70 30 0 -37 -39 -30
5 70 15 15 -38 -35 -24
6 70 0 30 -31 -34 -26
7 90 5 5 -38 -34 -27
8 75 20 5 -37 -37 -28
9 75 5 20 -38 -37 -27
10 80 10 10 -38 -37 -28
Table 4 Crosslink density, TGA and Tensile Data
Composition C=C conc. Crosslink density T5% loss Sb eb E
DSN Uvithane782 NVP TMPTA ( 10-3
mole/gm) (10-5
mole/cm3
) (°C) (ksi) (%) (ksi)
1 100 0 0 0.40 2.2 256 0.9 234 0.4
2 85 15 0 1.69 4.0 203 1.1 200 0.5
3 85 0 15 1.86 15.3 268 1.7 59 2.8
4 70 30 0 2.98 2.5 131 1.3 252 0.7
5 70 15 15 3.15 36.6 244 2.2 39 12.4
6 70 0 30 3.31 71.3 273 1.8 19 16.0
7 90 5 5 1.32 5.4 253 2.1 122 1.2
8 75 20 5 2.61 8.0 171 2.1 103 2.3
9 75 5 20 2.77 27.0 262 2.0 45 6.9
10 80 10 10 2.23 8.4 251 2.6 87 3.0
Sb = Tensile strength, eb = Elongation at break, E = Young's modulus
Uvi-782/NVP binary blend (i.e., TMPTA = 0) series: DSN #1, #2, #4
Uvi-782/TMPTA binary blend (i.e., NVP = 0) series: DSN #1, #3, #6
Uvi-782/NVP/TMPTA tertiary blend with TMPTA+NVP = 30 series (i.e.,Uvi-782 = 70): DSN #4, #5, #6
Uvi-782/NVP/TMPTA tertiary blend with TMPTA/NVP = 1 series: DSN #1, #7, #10, #5

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