The Effects Polymerization Inhibitors
Have on Acrylate Monomers and Formulations
Sartomer Company, Inc.
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ABSTRACT Hydroquinone and hydroquinone mono methyl ether
Polymerization inhibitors promote process and shelf are two commonly used inhibitors. Both stabilize chain
stability of acrylate monomer based formulations. radicals before they can react with other unsaturated
Inhibitors scavenge free radicals which can form sites. HQ and MEHQ react with chain radicals by
during the manufacturing process or extreme storage hydride abstraction.
conditions1. Inhibitor concentration is dependent on
the inherent instability of the monomer and its Reaction 1
functionality. The effects inhibitors have on cure rate,
color, and other performance properties are considered OH O*
negligible since only minute amounts (ppm) are used. R-CH-C* + + R-CH2-CH
This paper demonstrates how addition of hydro- OH OH
quinone (HQ) and hydroquinone mono methyl ether
(MEHQ) can affect cure rate and color of This reaction stabilizes the chain radical and halts further
trimethylolpropane triacrylate (SR351) monomer and polymerization. The resulting hydroquinone radical is
its formulations. stable and cannot initiate further polymerization. The
stability of the HQ radical results from delocalization
INTRODUCTION of electron charge density throughout the aromatic
Acrylate monomers are highly reactive. The high structure.
reactivity of acrylates and their analog methacrylates
result from unsaturated terminal sites along their Reaction 2
backbone structures. Due to these unsaturated sites,
acrylates can react to form solid films quickly using OH O*
ultraviolet light in conjunction with photoinitiators. O
Acrylates react to form solid films via free radical OH OH
polymerization. Radicals produced from photo-
initiators or from thermally decomposed peroxides Although the stabilized HQ radical cannot initiate
attack sites of unsaturation which in turn produce polymerization, it can react with additional chain
additional free radicals. As these reactions proceed, radicals and terminate propagation.
the molecular weight of the growing polymer increases
until a solid polymer matrix is formed. Reaction 3
Although it is desirable to avoid incorporating materi- OH O
als which can generate free radicals until usage of a R-CH-C* +
monomer or formulation, complete elimination of these R*
materials cannot be avoided. OH OH
During monomer production, side reactions can occur Hydroquinone radicals also react with each other to
which produce hydroperoxides(2). Hydroperoxides generate both hydroquinone and benzoquinone.
decompose over time (with out ultra-violet light)
forming alkoxy chain radicals and hydroxy radicals Reaction 4
which can initiate polymerization and lead to premature
polymerization. Inhibitors react with chain radicals to O* O* O OH
terminate chain propagation. +
OH OH O OH
Eventually, complete consumption of HQ will occur Sample
which results in the formation of highly conjugated Material A B C D
CN120 30 30 30 30
benzoquinone which is responsible for color
SR306 20 20 20 20
development. SR351 40 40 40 40
BP 5 5 5 5
Both inhibitors readily quench radicals produced from MDEA 5 5 5 5
photoinitiators. HQ 0 0 49 48
MEHQ 165 159 198 198
Viscosity 197 234 199 238
Reaction 5 (cps - 25°C)
These formulations were prepared by first master
+ + batching all materials excluding the SR351. Separate
formulations were then prepared by blending the
master batch with the SR351 monomer in Table 1.
Due to this quenching effect, one cannot merely Cure Equipment
increase the inhibitor concentration to increase shelf Formulations were cured using a standard medium
life. Instead, a balance between shelf life and cure pressure mercury vapor arc lamp operating at 200
speed must be determined. watts per square inch.
EXPERIMENTAL Monomers were cured using Gel-O-Meter model
GP-101*. This instrument utilizes the concept of
Monomers oscillating a liquid inside a micro pipette while exposed
Trimethylolpropane triacrylate (SR351) monomer was to a 20 Watt tungsten lamp. The oscillation of the liquid
used in this study. Properties of this monomer are sample is electronically monitored as exposure
tabulated on Table 1. proceeds. The time required to quench the liquid
oscillation is a measure of cure.
Test Test Value Relative cure efficiency of formulations were
Color (APHA) 30.00 determined using reagent grade methyl ethyl ketone.
HQ (ppm) 0.00 The number of MEK double rub required to disrupt
MEHQ (ppm) 111.00 the coating surface was measured. Testing was
Water (%) 0.03
measured for coatings applied to inked paper stock
Ester Ratio (%) at 3 lbs/ream (3000 sq ft) using an offset proving press.
Difunctional 3.00 Viscosities
Trifunctional 88.37 Viscosity measurement on both monomer and
High Boilers 7.63
formulation samples were conducted using a
Brookfield viscometer model LVT at 25°C.
Testing was carried out on the following formulations:
Monomer and formulation color was measured by
visual comparison using freshly prepared Platinum-
Cobalt standards according to ASTM test method
RESULTS Figure 2
Monomer Color vs Inhibitor Type and
SR351 (TMPTA) monomer (Table 1) was prepared Time to Gelation (seconds)
in the laboratory with extremely low levels of inhibitor.
Low inhibitor levels was confirmed by HPLC analysis.
Different concentrations of HQ and MEHQ was then
dissolved directly in separate 200 gram samples of
the SR351 (TMPTA) monomer. Figure 1 gives the
effects inhibitor type and concentration have on SR351
(TMPTA) liquid monomer color. Inhibitor Concentration (ppm)
Figure 2 shows that increasing the concentration of
inhibitor regardless of type, will increase the time
required to cure monomer.
This relationship is extremely important. Often,
formulators will add excess amounts of inhibitors to a
monomer or formulation with the idea of prolonging
the material’s shelf life. However, as this data demon-
strates, further increases in inhibitor may slow the
conversion process to an unacceptable rate.
Inhibitor Concentration (ppm) Theoretically, excessive additions of inhibitor to
difunctional or monofunctional monomers should
promote even further reductions in cure rate. These
Figure 1 suggests that neither HQ or MEHQ has any monomers will have less unsaturation on a per volume
significant effect on liquid monomer color. basis than a trifunctional monomer.
Monomer Cure vs Inhibitor type and Figure 2 further suggests that HQ is a more efficient
Concentration scavenger of free radicals than MEHQ. Addition of
SR351 (TMPTA) monomer used in this study was excess HQ will have a more detrimental effect on cure
prepared using the same method describe above. In rate than MEHQ. The slower cure response of
addition to dissolving different concentrations of monomer stabilized with HQ may be related to the
inhibitor in the monomer, 2.0 percent by weight fact that HQ has two reactive groups and MEHQ has
photoinitiator (Esacure® KB1, 2,2-dimethoxy-1,2- only one.
diphenylethanone) supplied by Lamberti USA, Inc,
was also added to promote polymerization. Formulation Color vs Inhibitor Type
Figure 3 gives color stability of formulations prepared
Figure 2 gives cure times for SR351 (TMPTA) mono- with the SR351 (TMPTA) monomers described in
mer vs. inhibitor type and concentration. This study Table 1. Formulation color was measured at 24 hours,
was conducted with the Gel-O-Meter model GP 101. three months, and six months while stored at 25 °C.
Figure 3 suggests that addition of hydroquinone
inhibitor dramatically increases formulation color over
time. The same color development was not seen with
MEHQ stabilized formulations.
Figure 3 MEHQ. Viscosity values of 197 centipoise and 199
centipoise were measured for formulation A and C,
respectively. The slight difference in viscosity is
considered within experimental error.
MEK Double Rubs
24 hours 3 month 4 month 4 days/60C
The mechanism causing this color development is
unknown at this time. Hydroquinone reactions can
ultimately produce benzoquinone species which due Cure speed (fpm)
Formulation A Formulation B
to their considerable conjugation are powerful
chromophores. The methoxy group on MEHQ would
reduce conjugation of this inhibitor resulting in less color Figure 4 shows that increasing the inhibitor
development. concentration by 81 ppm resulted in a 150% decrease
in MEK resistance.
This data also suggests that elevated temperatures
(60°C - 4 days) may provide a rapid method for Figure 5
measuring color stability of a formulation.
Formulation Chemical Resistance
Chemical resistance properties of cured formulation
films were used to measure the effect different inhibitor MEK Double Rubs
concentrations have on cure rate. MEK resistance
testing was performed immediately after the samples
were cured. Careful attention to formulation viscosity
was considered in this study. It has been suggested
that viscosity differences between identical
formulations, resulting from different ester ratios, can
influence cure rates. The premise is that initiator radicals Cure speed (fpm)
Formulation C Formulation D
would have greater mobility in low viscosity
formulations. The greater radical mobility would result
in greater chain termination reactions leading to reduced Figure 5 compares MEK resistance of films prepared
molecular weight and ultimately chemical resistance. from formulations B and D. The difference in inhibitor
content between these formulations is 87 ppm. The
Figure 4 compares the MEK resistance of films difference in viscosity was measured at less than 2.0%
prepared with formulations A and C. Formulation A which is within experimental error. Figure 6 shows that
contains 165 ppm MEHQ and zero ppm HQ. a 140% decrease in MEK resistance is measured for
Formulation C contains 48 ppm HQ and 198 ppm increasing inhibitor concentration.
This study shows the following trends: Tim Cauffman, the author of this paper, extends his
thanks to the entire technical staff at Sartomer
MEHQ or HQ between concentrations of 0 Company. Special thanks is extended to Jim Horgen,
to 1000 ppm does not affect color of TMPTA Henry Miller, Tom Hazell, and Susan Hammond for
monomer. their valuable contributes.
Increasing the concentration of either MEHQ
or HQ causes a decrease in the cure rate of REFERENCE
TMPTA. 1 J. Milton Harris & Carl C. Wamser
HQ causes an increase in color development in Fundamentals of Organic Reaction Mechanism
formulations. MEHQ does not cause an p.286 1976
increase in color.
2 Morrison and Boyd Organic Chemistry Furth
Edition p.961 1983
* For more investigative analysis of the Gel-O-meter
GP-101 please refer to Ed Saccocio’s paper A New
Instrument for the Characterization of Radiation Curable
Compositions Radtech Report November/December 1990.
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properties are illustrative only, and not product specifications. SARTOMER Company disclaims any liability in connection with the use of the information, and does not warrant against infringement by
reason of the use of its products in combination with other material or in any process.