Uv Curable Acrylic Coat
Uv Curable Acrylic Coat
Uv Curable Acrylic Coat
UV-Curable Acrylic
Urethane Clearcoats for
Automotive Applications
A
By Daniel B. Pourreau crylic urethane coatings are Automotive OEM clearcoats are also
used in both automotive based on acrylic urethanes, but are
manufacturing and refinishing obtained by crosslinking carbamate-
because of their versatility, durability, functional acrylics with melamine resins.
appearance, superior weatherability and Two advantages of this chemistry are
acid-etch resistance compared to other that the coating is single-component and
resin systems. In automotive refinishing, the melamine crosslinker is less
the most common coating type is the expensive and hazardous than the
two-component (2K) system, where an polyisocyanates used in automotive
acrylic polyol solution is mixed with a refinishing. One disadvantage is that
poly-isocyanate just before it is applied the coatings must be baked.
to the vehicle. The coating then cures by UV-curable coatings are also one-
a combination of solvent evaporation and component (1K) systems that cure by
chemical crosslinking to form a tough photoinitiated polymerization of the
and durable film (Figure 1). acrylated monomers and oligomers in
Figure 1
Conventional 2K acrylic-urethane crosslinking reaction
The cure speed and film properties the formulation (Figure 2). These
can be tailored by varying the hardness typically do not contain VOCs and cure
(Tg) and functionality (EW or OH is instantaneous.
number) of the acrylic polyol, the Compliance and productivity are key
isocyanate and solvents used, and by the benefits of UV-curable coatings over
addition of catalysts and activators. Heat solventborne 1K and 2K urethanes.
is often applied to accelerate cure and However, because of the chemistry of
solvent evaporation. However, compared UV-curable oligomers and the signifi-
to UV-curable coatings, 2K urethanes cant shrinkage that occurs during the
cure much more slowly, and have limited curing process, UV-curable coatings
pot life and higher volatile organic are usually not as tough or weatherable
compounds (VOCs). as 2K urethanes. UV-curable urethane
oligomers, which comprise the majority system, by chemical crosslinking in the 40C. Isophorone diisocyanate (125 g)
of the formulation components, are dark. The final coating appearance and was added to the reactor over a period
usually prepared from polyether or durability were outstanding, rivaling of one hour under air sparge. The
polyester polyols. These resins are the best commercial systems on the addition rate was controlled to keep
more susceptible to UV degradation market today. the reaction temperature at 40C. The
and hydrolysis than acrylic polyols and Replacing polyesters and polyethers reaction contents gelled after 89 grams
thereby reduce the weatherability of in urethane oligomers with acrylics of IPDI had been added.
UV coatings compared to acrylic opens the door to the development of
Preparation of a Low Tg Acrylic-
urethane coatings. more durable UV-curable coatings for
Polyester Urethane Acrylate Oligomer
Reports of urethane prepolymers high-performance applications such as
Isophorone diisocyanate (IPDI)
based on acrylic polyols are virtually automotive topcoats. The synthesis of
(400 g) and dibutyltin dilaurate
non-existent and the author is aware of these new oligomers and their formula-
catalyst (0.42 g) were charged into a
no such commercial products. This is in tion in 1K clearcoats is described. This
1-L resin kettle equipped with a stirrer,
part because, unlike condensation technology is also compared to the 2K
a nitrogen inlet, a thermometer and an
polymers, the functionality of acrylic UV formulations developed earlier.4 addition funnel. Acrylic polyol (780 g)
polyols cannot be limited to two or
was gradually added as a 70% butyl
three. Consequently, treating conven- Experimental
acetate solution to the stirred reactor
tional acrylic polyols with diisocyanates
Raw Materials at 50C over an hour and the reaction
causes rapid crosslinking of the polymer
ACRYFLOW acrylic polyols were mixture stirred for an additional hour.
chains and produces highly viscous or
obtained from Lyondell Chemical Methyl ether hydroquinone (MEHQ,
gelled products, which are not suitable
Company. JONCRYL 920 (acrylic 3 g) was then added to the resulting
for high solids coatings.1
polyol) was obtained from Johnson prepolymer. Hydroxyethyl acrylate
However, by using new acrylic polyols
Polymer. Darocur 4265 (photoinitia- (HEA, 340 g) was gradually added to
with all secondary OH functionality2,3
tor) was obtained from Ciba Specialty the reactor mixture over an hour at
low-viscosity UV-curable acrylic
Chemicals. UV monomers were 50C and the reaction mixture stirred
urethane oligomers in high yield were
obtained from Sartomer Company and for another two hours. The resulting
prepared. These acrylated urethane
used as received. product was a clear solution of
acrylic oligomers were used to
acrylated resin. The reaction yield
formulate tough, 1K UV-curable Attempted Preparation of an Acrylic
was quantitative.
coatings for weatherable applications. Urethane Acrylate Oligomer with a
These same acrylic polyols were also Conventional High Solids Acrylic Polyol Preparation of an Acrylic Urethane
used to formulate 2K UV-curable An acrylic polyol (850 g, 80% solids Acrylate Oligomer for UV-Curable
acrylic urethane coatings by replacing in MAK) was charged into a 2-L glass Coatings
a portion of the solvents and resins in resin kettle equipped with a stirrer, A quart sized round amber bottle
conventional 2K urethane formulations addition funnel, air inlet, and thermom- was charged with 111.0 g of IPDI
with acrylated monomers. Both types eter and heating mantle. Dibutyltin monomer and 0.8 g of DBTDL. To
of formulations were sprayed using dilaurate (DBTDL, 0.88 grams) and this bottle, 325 gram of acrylic polyol
conventional HVLP equipment, 2,6-di-t-butyl-4-methylphenol (2.0 g, (80% solution in n-BuAc) diluted
UV-cured instantly to a tack-free BHT) were added to the reactor and with 121 grams of n-BuAc was
state and, in the case of the dual-cure the reaction temperature increased to added in three parts one hour apart.
Table 1
Relative rate constants of the isocyanate reaction with primary OH groups
The bottle was rolled to mix the Preparation of Acrylic Urethane formulations were reduced to spray
contents between each addition. Acrylate Oligomers for UV-Curable viscosity with acetone and applied to
Hydroxyethyl acrylate (66 g) was then Coatings and Adhesives steel panels coated with Bonderite 1000
added. The mixture was then placed in A reactor was charged under air and white base-coated steel and
a 50C oven for two hours then rolled sparge with 100.0 g of IPDI monomer, aluminum panels. Coatings were
overnight. The resulting mixture was 100 grams of diluent (n-BuAc or irradiated after a 30 minute flash-off
a 70% solids solution of acrylic HDDA), 0.04 grams of MEHQ and 0.8 g period. Physical property tests were
polyol-IPDI-HEA acrylate in n-BuAc of DBTDL. To this mixture was added performed on the coated steel panels.
with a Brookfield viscosity of 2,620 cps. three 146.67 aliquots of acrylic polyol Appearance tests were performed on
Preparation of a Flexible Acrylic- (90% solution in n-BuAc or HDDA, the white base-coated steel panels.
Polyester Urethane Acrylate Oligomer hexanediol diacrylate) one hour apart. Weatherability tests were performed on
for UV-Curable Coatings HEA (57 g) was added over 20 the white base-coated aluminum panels.
A quart-sized round amber bottle minutes. The mixture was then heated UV Equipment and Cure
was charged with 111.0 g of IPDI to 40C and stirred for one hour. The The panels were UV cured using a
monomer and 0.8 g of DBTDL. To this resulting mixture was an 80% solids Fusion UV Model LC-6B Benchtop
bottle, 325 gram of acrylic polyol solution of acrylic polyol-IPDI-acrylate Conveyor equipped with a mercury
(80% solution in n-BuAc) further oligomer with a Brookfield viscosity of vapor lamp (H bulb). The conveyor
diluted with 174 grams of n-BuAc was 3,900 cps (n-BuAc) or 38,800 cps belt speed was set at 16ft/min.
added in three parts one hour apart. (HDDA). The acrylic polyol acrylated Actinometry was performed using a
The bottle was rolled to mix the oligomer solution in n-BuAc was stable PowerPuck from EIT Inc. All coatings
contents between each addition. indefinitely at room temperature were usually, in one pass, tack- and
Caprolactone acrylate (185 g) was whereas the HDDA solution gelled print-free, which corresponds to
then added. The mixture was then after approximately one week. 1.65 J/cm2, and were subjected to
placed in a 50C oven for two hours Preparation of UV-Curable Acrylic 3-11 more passes to ensure full cure.
then rolled overnight. The resulting Urethane Clearcoats The number of passes required to cure
mixture was a 70% solids solution of The acrylated oligomers dissolved the coatings was dependent on the
acrylic polyol-IPDI-caprolactone in acrylate monomers and treated Tg of the oligomers, the monomers
acrylate in N-BuAc with a Brookfield with 4% by weight on total resin solids used and the level of photoinitiator in
viscosity of 1,941 cps. of photoinitiator. The resulting the formulation.
Table 3
Starting formulations and coating properties of UV-curable acrylic urethane coatings
Formulation Components A B C D E F G H
Acrylic Polyol UHA 86 86 86 86
(70% in BuAc)
Acrylic Polyol UHA 75 75 75 75
(80% in BuAc)
IBOA 20 10 20 30
Isooctyl acrylate 10 20 30 20
TMPTA 20 20 30 10
EO6 TMPTA 30 10 20 20
Photoinitiator 4 4 4 4 4 4 4 4
Acetone 20 20 20 20 15 15 15 15
Total grams 150 150 150 150 134 134 134 134
Formulation constants
% solids 69% 69% 69% 69% 78% 78% 78% 78%
grams VOC/liter 218 196 192 199 134 122 124 123
lbs VOC/gallon 1.82 1.63 1.61 1.66 1.12 1.02 1.04 1.03
% photoinitiator on TRS 4% 4% 4% 4% 4% 4% 4% 4%
Brookfield viscosity, cps 235 185 169 130 95 89 52 50
Coating Appearance, 3 mils wet on white basecoat
60 Gloss 94 94 93 92 88 88 88 88
20 Gloss 87 87 87 86 80 80 80 81
DOI 60 50 70 60 90 90 90 100
Yellowness Index 5.1 4.1 6.5 2.3 2.5 2.5 3.1 3.1
Coating Properties, 3 mils wet on Bonderite steel
Dry fim thickness, mils 2.1 2.2 2.2 2.2 2.4 2.5 2.3 2.3
Koenig Hardness, swings 112 121 135 84 90 37 14 80
Impact resistance, in.lbs
Front 50 30 30 70 30 160 160 70
Reverse 10 10 10 60 10 100 100 40
MEK Double Rubs 200 200 200 200 200 200 200 200
Crosshatch Adhesion 0 0 0 0 0 20% 30% 0
preferable so that the flash-off time is solvents are not needed as the acrylate number of viable options. The goal in
relatively short and productivity is not monomers help with flow and leveling. this study was to develop weatherable
significantly affected. In this study, Monomer Selection coatings for spray application. To
n-Butyl acetate was chosen to prepare Choosing acrylate monomer reduce potential respiratory and skin
the acrylated oligomers and acetone to diluents can be confusing, with literally hazards, relatively non-volatile bulky
reduce the coating to spray viscosity. hundreds available. However, careful monomers such as isobornyl and
N-BuAc is non-HAP and acetone has consideration of the intended applica- isooctyl acrylate, trimethylopropane
the added advantage of being VOC tion and the properties of the acrylated triacrylate (TMPTA) and its alkoxylated
exempt in the United States. Tail oligomer can significantly reduce the derivatives were chosen. Although
weatherability data are not yet available yield glossy, solvent-resistant coatings adhesion promoter described by the
on these latest formulations, there is with poor adhesion to pretreated steel. manufacturer as a mono-functional acid
confidence that they will weather as well The coating hardness varied with the ester was added (Table 4). Since acrylic
as earlier 2K/UV-coating formulations.4 hardness of the acrylic backbone and polyol UCA, the caprolactone acrylate
Table 3 lists several starting UV the nature of the reactive diluents. modified urethane prepolymer, gave a
formulations and their coating properties. To improve the impact resistance soft coating without acrylate diluents
The formulations have VOC contents and adhesion of these coatings, HEA (Table 3), isobornyl acrylate (IBOA)
ranging from 120-200 grams VOC/liter was replaced with caprolactone acrylate and trimethylopropane triacrylate
and most can be sprayed with conven- in the oligomer synthesis. Polyesters (TMPTA), two high-Tg monomers, were
tional spray equipment. These formula- such as caprolactone are known to selected as the diluents.
tions were applied to pretreated and improve the flexibility of 2K acrylic A couple of trends are evident from
basecoated steel panels and UV cured to urethane coatings. A proprietary these results. First, using TMPTA, a
Table 5
Optimized UV-curable coatings for weatherable applications
Formulation Components M N O P
UHA (70% in BuAc) 86.0 86.0
UCA (70% in BuAc) 86.0 86.0
Isobornyl acrylate 20.0 20.0
Isooctyl acrylate 20.0 20.0
Flow Aid 1.0 1.0 1.0 1.0
Adhesion promoter 5.0 5.0 5.0 5.0
Photoinitiator 3.0 3.0 3.0 3.0
Hydroxyphenyltriazine
UV Screener (50% in acetone) 3.35 3.35 0.00 0.00
Benzylidene Malonate 1.65 1.65 0.00 0.00
UV Screener (50% in acetone)
Acetone 17.5 17.5 20.0 20.0
Total grams 137.5 137.5 135.0 135.0
Formulation constants
% solids 66% 66% 66% 66%
grams VOC/liter 219 215 224 220
lbs VOC/gallon 1.83 1.80 1.87 1.84
% photoinitiator on TRS 3% 3% 3% 3%
Brookfield viscosity, cps 36 130 34 103
Coating Appearance, 3 mils wet on white basecoat
60 Gloss 94 93 94 93
20 Gloss 87 86 87 85
DOI 90 100 95 100
Yellowness Index 1.4 1.4 2.8 3.0
Coating Properties, 3 mils wet on Bonderite steel
Dry fim thickness, mils 2.3 1.7 1.7 2.3
Koenig Hardness, swings 75 95 91 70
Impact resistance, in.lbs
Front 30 160 120 160
Reverse 5 160 160 160
200 MEK Double Rubs pass pass pass pass
Crosshatch Adhesion 100% 100% 100% 100%
tri-functional acrylate monomer, (MEK double rubs). The improvement attributed to an increase in the
negatively affected the coatings in adhesion and flexibility was coatings crosslink density and a
adhesion and color. Second, reducing attributed to a reduction in shrinkage reduction in hardness as evidenced by
the IBOA/oligomer ratio from 40% during the UV-cure process. The the lower Koenig swing value.
(Formulation I) to 25% (Formulation L) coating with less IBOA also showed Based on these results, optimized
resulted in improvement in impact improved scratch resistance when formulations were developed based on
resistance and abrasion resistance subjected to 200 MEK rubs. This was an all-acrylic urethane (UHA) oligomer
Table 6
Optimized UV-curable coatings for weatherable applications
A-side Componentss 2K UV 2K UV-2
Hard Acrylic Polyol (70% in n-BuAc) 16.1 10.7
Liquid Acrylic Polyol 3.75 7.5
IBOA 2.5 2.5
PO6-TMPTA 2.5 2.5
HEA 2.5 2.5
Adhesion Promoter 2.5 2.5
B-side components
HDI Trimer (80% in acetone) 14.5 14.4
9% Photoinitiator (BAPO/AHK) 0.81 0.81
1% DBTDL (tin catalyst) 0.73 0.73
50% Hindered Amine Light Stabilizer (HALS) in acetone 1.1 1.1
50% Hydroxyphenyltriazine UV screener in acetone 1.1 1.1
Flow Modifier 0.5 0.5
Formulation Constants
% solids 80 83
Viscosity, cps <200 <200
VOC content lbs/gal (g/ml) 0.79(95) 0.55(66)
tack & print-free after UV cure yes yes
Film Properties after 7 days
Film thickness (mil) 1.8 1.8
Knig Hardness 158 125
Direct Impact 150 160
Reverse Impact 140 160
Adhesion 5B 5B
MEK Rubs 200 200
involved in the UV-curing reaction, the of the final coatings suffer as is weathering tests under QUVB automo-
acrylate monomers, acrylic polyol and expected from the higher crosslink tive conditions. After 3,300 hours of
isocyanate crosslinker must be carefully density (Figure 5). exposure, both coatings have retained
selected so that a tack-free state is Replacing a portion of the liquid over 80% of their 20 gloss (Figure 6).
achieved after UV cure. polyol with a high-Tg polyol was also
With liquid (low Tg) acrylic polyols effective in achieving a tack-free state Conclusions
as the base polyol and HDI trimer as immediately after UV cure. This By careful selection of raw materials
the isocyanate crosslinker, mono- approach is preferred over the use of and reaction conditions, low-viscosity,
functional acrylates such as IBOA TMPTA because the final coating has storage-stable acrylated acrylic
(isobornyl acrylate) do not yield a better impact resistance and flexibility. urethane oligomers were prepared for
tack-free state after UV cure, unless Formulations sprayable at 80% solids the first time. These new acrylic
the HEA is pre-mixed with the and above were achieved using urethane oligomers UV cure to form
isocyanate, as is illustrated in Figure 4. acetone and acrylate monomers hard, chemically resistant coatings with
This, presumably, generates an (Table 6). VOC contents were below excellent appearance and durability
acrylated polyisocyanate that can now 0.8 lbs/gal or 95 grams VOC/liter. properties. Modification of the urethane
be involved in the UV-curing process. Film appearance and physical prepolymers with caprolactone acrylate
Some tri-functional TMPTA is usually properties were also excellent. instead of HEA improved their impact
required to build enough molecular The cured 2K UV clearcoats resistance. These acrylated oligomers
weight. However, the impact properties were also subjected to accelerated were used to formulate sprayable,