New low-viscosity acrylic-urethane prepolymers and their acrylated oligomers for moisture and UV-curable coatings.

This article describes a new class of urethane acrylate oligomers for weatherable UV-curable systems. Unlike commercial urethane acrylates that are based on polyether or polyester polyols, this new class of urethane oligomers is based on acrylic polyols. These new acrylic polyols are unique in that they produce low-viscosity, nongelling urethane prepolymers with isophorone diisocyanate. These prepolymers can be further treated with hydroxy-functional acrylates such as hydroxyethylacrylate (HEA) or caprolactone acrylate (CA) to yield low-viscosity acrylated urethane acrylic oligomers.

By replacing the polyester and polyether backbone in urethane acrylates with an acrylic backbone, this chemistry opens the door to more weatherable UV-curable coatings, with properties that approach two-component urethane systems. These new urethane acrylates were formulated into sprayable UV-curable coatings with properties that rival two-component urethanes. The influences of diluent monomer type and content, hydroxy acrylate, and solvents on coating viscosity, VOC content, and properties were evaluated.

INTRODUCTION

The majority of commercial urethane prepolymers are based on polyether or polyester polyols and aromatic polyisocyanates, such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI). (1) Polyester prepolymers based on isophorone diisocyanate (IPDI) are often used for weatherable coating applications. Aliphatic isocyanates such as IPDI are less susceptible to yellowing and UV-induced photodegradation than their aromatic counterparts, and polyesters are also more resistant to UV degradation than polyethers.

Reports of urethane prepolymers based on acrylic polyols are virtually non-existent and the authors are aware of no such commercial products. We believe this is in part because, unlike condensation polymers, the functionality of acrylic polyols cannot be readily controlled to 2 or 3. Also, in conventional acrylic polyols, the OH functionality is randomly distributed throughout the polymer and in many cases the OH functionality is primary. Consequently, treating acrylic polyols with diisocyanates usually causes rapid crosslinking of the polymer chains and produces highly viscous or gelled products which are not suitable for high-solids coatings. (2)

However, by using isophorone diisocyanate, allyl-based acrylic polyols with all secondary OH functionality, (3) and carefully controlling the reaction conditions, low-viscosity aliphatic urethane prepolymers were obtained in quantitative yield. The prepolymers were used to formulate moisture-curable coatings and prepare acrylated urethane acrylic oligomers for highly weatherable UV-cured coatings.

EXPERIMENTAL

Raw Materials

ACRYFLOW[TM] acrylic polyols were obtained from Lyondell Chemical Co. Joncryl[R] 920 was obtained from Johnson Polymer. Darocur[R] 4265 was obtained from Ciba Specialty Chemicals. UV monomers were obtained from Sartomer Co. and used as received. Solucote[R] 373 MA was obtained from Soluol, Inc.

Attempted Preparation of an Acrylic Urethane Acrylate Oligomer with a Conventional High-Solids Acrylic Polyol

Joncryl 920 acrylic polyol (850 g, 80% solids in MAK) was charged into a 2-L glass resin kettle equipped with a stirrer, addition funnel, air inlet, thermometer, and heating mantle. Dibutyltin dilaurate (0.88 g) and 2,6-di-t-butyl-4-methylphenol (2.0 g, BHT) were added to the reactor and the reaction temperature increased to 40[degrees]C. Isophorone diisocyanate (125 g) was added to the reactor over a period of one hour under air sparge. The addition rate was controlled to keep the reaction temperature at 40[degrees]C. The reaction contents gelled after 89 g of IPDI had been added.

Preparation of an Acrylated Urethane Acrylic Oligomer

ACRYFLOW P120 acrylic polyol (680 g, OH# 120 mg KOH/g) was dissolved in acetone (170 g) and charged to the same apparatus as above. Dibutyltin dilaurate catalyst (0.88 g) and 2,6-di-t-butyl-4-methylphenol (2.0 g, BHT) were added to the reactor and the temperature increased to 40[degrees]C. Isophorone diisocyanate (125 g) was added to the reactor over a period of one hour under air sparge. The addition rate was controlled to keep the reaction temperature at 40[degrees]C. Hydroxyethyl acrylate (78.5 g) was added to the reactor over 30 min at 40[degrees]C. The temperature was raised to 60[degrees]C and maintained for four hours. The reaction was monitored by the disappearance of the NCO stretching frequency at 2,265 [cm.sup.-1] in the infrared spectrum. A clear product (1,018 g, 97% yield) with the following properties was collected: OH#: 59.4 mgKOH/g; solids%: 83.0; acrylated P120: 72.2%; IPDI diacrylate: 10.8%; Mn 4370; Mw 24200; viscosity: 4,090 cps; residual NCO content: 0.040 meq/g.

Preparation of an Acrylic Urethane Acrylate Oligomer for UV-Curable Coatings

A reactor was charged under air sparge with 128 g of IPDI monomer, 132 g of diluent (n-BuAc), and 1.02 g of DBTDL. To this mixture were added three 120.53 aliquots of ACRYFLOW M100 acrylic polyol (80% solution in n-BuAc) one hour apart. Hydroxyethylacrylate (HEA), 56.6 g, was added over 20 min. The mixture was then heated to 40[degrees]C and stirred for one hour. The resulting mixture was a 70% solids solution of M100-IPDI-acrylate with a Brookfield viscosity of 19,300 cps (n-BuAc).

Preparation of a Flexible Acrylic-Polyester Urethane Acrylate Oligomer for UV-Curable Coatings

A quart-sized round amber bottle was charged with 111 g of IPDI monomer and 0.8 g of DBTDL. To this bottle were added 325 g of ACRYFLOW M100 acrylic polyol (80% solution in n-BuAc) diluted with 100 g of n-BuAc in three parts, one hour apart. The bottle was rolled to mix the contents between each addition. Caprolactone acrylate (CA) (SR 495 from Sartomer), 182 g, was then added. The mixture was then placed in a 50[degrees]C oven for two hours, then rolled overnight. The resulting mixture was a 70% solids solution of M100-IPDI-caprolactone acrylate in N-BuAc with a Brookfield viscosity of 1,941 cps.

Preparation of Acrylic Urethane Acrylate Oligomers for UV-Curable Coatings and Adhesives

A reactor was charged under air sparge with 100 g of IPDI monomer, 100 g of diluent (n-BuAc or HDDA), 0.04 g of MEHQ and 0.8 g of DBTDL. To this mixture were added three 146.67 aliquots of ACRYFLOW P60 acrylic polyol (90% solution in n-BuAc or HDDA, hexanediol diacrylate), one hour apart. Hydroxyethylacrylate, 57 g, was added over 20 min. The mixture was then heated to 40[degrees]C and stirred for one hour. The resulting mixture was an 80% solids solution of P60-IPDI-acrylate oligomer with a Brookfield viscosity of 3,900 cps (n-BuAc) or 38,800 cps (HDDA). The P60 acrylated oligomer solution in n-BuAc was stable indefinitely at room temperature, whereas the HDDA solution gelled after approximately one week.

Preparation of UV-Curable Acrylic Urethane Clearcoats

The acrylated oligomers were dissolved in acrylate monomers and treated with 4% by weight on total resin solids of Darocur 4265 photoinitiator. The resulting formulations were reduced to spray viscosity with acetone and applied to steel panels coated with Bonderite 1000 and a white basecoat. Coatings were irradiated after a 30-min flash-off period. Physical property tests were performed on the coated steel panels. Appearance tests were performed on the white basecoated panels.

UV Equipment and Cure

The panels were UV-cured using a Fusion UV Model LC-6B Benchtop Conveyor equipped with a mercury vapor lamp ("H" bulb). The conveyor belt speed was set at 16 ft/min. Actinometry was performed using a Power Puck[R] from EIT, Inc. All coatings were tack and print free in one pass which corresponds to 1.65 J/c[m.sup.2] and were subjected to three more passes to insure full cure.

RESULTS AND DISCUSSION

Preparation of Acrylic Urethane Prepolymers

Attempts to prepare acrylic urethane prepolymers with conventional acrylic polyols or with isocyanates other than IPDI produced gels. We attribute this in part to the higher reactivity of conventional polyols that are usually based on hydroxyethyl acrylate (HEA) or methacrylate (HEMA). Primary OH groups react two to four times faster with isocyanates than secondary ones. In addition, isocyanates can have very different reactivities, even in the same molecule (Table 1).

[GRAPHIC OMITTED]

By using IPDI--a slow-reacting isocyanate with a reactivity ratio of 2.7--we were able to produce low-viscosity acrylic aliphatic urethane prepolymers by controlled addition of acrylic polyols to IPDI monomer at ambient temperature (Table 2). The reaction proceeds rapidly at, or slightly above, room temperature, and in quantitative yield. Adding an alcohol at the end of the reaction prevents gel formation and improves storage stability.

UM100 produced fast-drying coatings without amine accelerators compared to a commercial aliphatic urethane prepolymer (Figure 1). This marked increase in cure speed is in part due to the higher functionality of the acrylic polyol and, presumably, to the higher [T.sub.g] of the acrylic polyol. It is not due to a higher NCO content as the commercial product has an NCO content of 7.2% versus 6.1% for the acrylic based prepolymer.

UM100 gives hard films with limited flexibility. Adding a flexible diluent like the uretidione of hexamethylene diisocyanate (HDI dimer) yields coatings with higher solids, improved chemical resistance, and flexibility. Blending UM100 with a polyester-based prepolymer is also expected to improve the coating flexibility and abrasion resistance. The effect of adding these diluents on the cure speed of the coating is illustrated in Figure 2. Weatherability and other performance evaluations of these coatings are underway.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

UP60, on the other hand, cured very slowly in the absence of added isocyanate trimer and yielded soft, tacky films. Adding IPDI trimer to UP60 had a positive effect on both cure speed and final coating hardness (Figure 3). UP60 behaves similarly to a polyester-based prepolymer. This is not surprising, since the OH functionality of the P60 polyol is only 2.4 per polymer chain and its [T.sub.g] is -48[degrees]C. This is in contrast to M100, which has a [T.sub.g] of 5[degrees]C and an average OH functionality of 4.5 per polymer chain. The commercial polyester-based prepolymer also contains 10-15% IPDI homopolymer to improve cure speed and ultimate coating hardness.

Acrylated Acrylic Urethane Oligomers for UV-Cured Coatings

Low-viscosity acrylated acrylic urethane oligomers were prepared by addition of HEA or CA to UM100 and UP60. Oligomer and coating properties are listed in Table 3.

UV-curable coating formulations were obtained by adding 4% of a blend of bisacylphosphine oxide/[alpha]-hydroxyketone photoinitiators. The P60 urethane acrylate (P60 UA) and caprolactone acrylate of UM100 yielded soft films following UV irradiation, whereas the HEA of UM100 yielded a hard film with good adhesion to pretreated metal and good chemical resistance. The diluent had an effect on film properties. P60 UA in HDDA yielded a harder film with greater chemical resistance than P60 UA in n-BuAc. However, its adhesion and impact resistance were reduced.

[FIGURE 3 OMITTED]

Formulation of UV-Curable Coatings

Traditional UV formulations do not contain solvents and are relatively viscous compared to solventborne formulations. That is because acrylate monomers are relatively weak solvents for the acrylated oligomers resins. High coating viscosity makes spray application difficult if not impossible. Hence, UV-curable coatings are usually applied by roll, curtain, or vacuum techniques.

Table 4 lists several starting UV-formulations and their coating properties. The formulations have VOC contents ranging from 120-200 grams VOC/liter and most can be sprayed with conventional spray equipment. These formulations were applied to pretreated and basecoated steel panels and UV-cured to yield glossy, solvent-resistant coatings with poor adhesion to pretreated steel. The coating hardness varied with the hardness of the acrylic backbone and the nature of the reactive diluents.

To improve the impact resistance and adhesion of these coatings, we replaced HEA with caprolactone acrylate in the oligomer synthesis. Polyesters such as caprolactone are known to improve the flexibility of two-component acrylic urethane coatings. We also added a proprietary adhesion promoter described by the manufacturer as a monofunctional acid ester (Table 5). Since M100 UCA, the CA-modified urethane prepolymer, gave a soft coating without acrylate diluents (Table 3), we selected isobornyl acrylate (IBOA) and trimethylopropane triacrylate (TMPTA)--two high-[T.sub.g] monomers--as the diluents.

A couple of trends are evident from these results. Using TMPTA--a trifunctional acrylate monomer--negatively affected the coating's adhesion and color. Reducing the IBOA/oligomer ratio from 40% (1008826) to 25% (1008829) resulted in improvement in impact resistance and abrasion resistance (MEK double rubs). We attribute the improvement in adhesion and flexibility to a reduction in shrinkage during the UV-cure process. The coating with less IBOA also showed improved scratch resistance when subjected to 200 MEK rubs. We attribute this to an increase in the coating's crosslink density and a reduction in hardness as evidenced by the lower Koenig swing value.

Based on these results, we developed optimized formulations based on M100 UHA, an all-acrylic urethane oligomer, and M100 UCA, a caprolactone-modified acrylic urethane resin (Table 6). We added a flow aid to improve gloss and distinctness of image (DOI) and a hindered amine light stabilizer (HAL) and UV-screener package to two of the formulations. The two formulations without HAL and UV-screeners yellowed slightly more than the other two when UV-cured, but otherwise had identical properties.

Film thickness had an effect on impact resistance and surface hardness. The thicker the film, the softer it was and the lower its impact resistance. Overall, film properties were excellent for both formulations, although the caprolactone-modified acrylic urethane has superior impact resistance and DOI. These formulations were sprayable at 66% solids and less than 1.9 lb VOC/gal.

These results also illustrate that using solvents in UV-curable formulations offers several benefits. First, the amount of acrylate monomer used can be substantially reduced while achieving spray viscosity. This has a positive effect on coating adhesion because UV-induced shrinkage is reduced and the solvent cleans the surface. Second, the coating viscosity is reduced, which also has a positive effect on adhesion and appearance. Contrast, for example, the DOI values between the coatings in Tables 4 and 5. Third, coating sag resistance is improved because the solvent evaporates quickly before UV cure, leaving a semi-solid coating with little tendency to flow.

Clearly, fast solvents are preferable so that the flash-off time is relatively short and productivity is not significantly affected. In this study, we chose n-Butyl acetate to prepare the acrylated oligomers and acetone to reduce the coating to spray viscosity. N-BuAc is non-HAP and acetone has the added advantage of being VOC exempt in the United States. Tail solvents are not needed as the acrylate monomers help with flow and leveling.

Choosing acrylate monomer diluents can be confusing, with literally hundreds available. However, careful consideration of the intended application and the properties of the acrylated oligomer can significantly reduce the number of viable options. Our goal in this study was to develop weatherable coatings for spray application. To reduce potential respiratory and skin hazards, we chose relatively nonvolatile monomers such as isobornyl and isooctyl acrylate, and TMPTA and its ethoxylated version. Although weatherability data are not yet available on these latest formulations, we are confident that they will weather as well as our earlier acrylic urethane coating formulations. (4)

CONCLUSIONS

By careful selection of raw materials and reaction conditions, low-viscosity, storage-stable acrylic aliphatic urethane prepolymers were prepared. Addition of hydroxy-functional acrylates such as HEA and caprolactone acrylate to these prepolymers produced acrylated acrylic urethane oligomers suitable for UV-curable coatings and, potentially, adhesives.

Moisture-curable coatings with superior curing speed were prepared from M100, an acrylic polyol with mid-range hardness and OH functionality and IPDI. P60, a liquid acrylic polyol with an average of 2.4 OH groups per polymer chain, produced a slow curing urethane prepolymer. Addition of IPDI trimer to the P60 urethane prepolymer improved the coating's cure speed and chemical resistance properties.

These new acrylic urethane polymers are expected to find use in applications where polyester- or polyether-based prepolymers or acrylated urethanes have insufficient weatherability, hardness, or cure speed. These applications include industrial maintenance, automotive, and wood coatings.

Table 1 -- Relative Rate Constants of the Isocyanate Reaction with
Primary OH

Isocyanate     k1      k2      k1/k2

TDI            400      33     12.1
MDI            320     110      2.9
IPDI             0.62    0.23   2.7
HDI              1       0.5    2.0
[H.sub.12]MDI    0.57    0.4    1.4

Table 2 -- Physical Properties of Acrylic Aliphatic Urethane Prepolymers

Acrylic urethane prepolymers  UP60        UM100
Nonvolatile content               77%        80%
Viscosity @25[degrees]C cps   11,050      5,000
Viscosity 70% in MAK, cps     --            530
Viscosity 60% in MAK, cps        400      --
% NCO of nonvolatiles              4.9        6.1
% Free IPDI in nonvolatiles        0.22%      0.18%

Table 3 -- Physical Properties of Acrylated Acrylic Urethane Oligomers
and UV-Cured Coatings

Oligomer properties      P60 UHA   P60 UHA  M100 UHA  M100 UCA

Hydroxy acrylate used    HEA       HEA      HEA       CA
Diluent                  HDDA      n-BuAc   n-BuAc    n-BuAc
% Nonvolatile content    100%      80%      70%       70%
Viscosity, cps           38,800    3,900    2,620     1,941
Mn                       800       3,057    2,081     3,442
Pd                       26.4      8.3      8.0       18.8

Coating Properties, 3 mils wet on Bonderite[R] steel

Photoinitiator                   4% Darocur 4265

Koenig hardness           28        18      109        17
Impact resistance
  Front                   60       160       20       160
  Reverse                <20       160      <20       160
MEK rubs                  69        20       90        60
% Crosshatch adhesion      0        20       95        95

Table 4 -- Starting Formulations and Coating Properties of UV-Curable,
Acrylic Urethane Coatings

Formulation Components       1008816  1008817  1008818  1008819  1008820

M100 UHA (70% in BuAc)        86       86       86       86
P60 UHA (80% in BuAc)                                             75
IBOA (SR 506)                                   20                10
Isooctyl acrylate (SR 440)    10       20                30
TMPTA (SR 351)                         20       20                30
E06 TMPTA (SR 499)            30                         10
Darocur 4265                   4        4        4        4        4
Acetone                       20       20       20       20       15
Total grams                  150      150      150      150      134

Formulation Constants
% Solids                      69%      69%      69%      69%      78%
Grams VOC/liter              218      196      192      199      134
Lb VOC/gal                     1.82     1.63     1.61     1.66     1.12
% Photoinitiator on TRS        4%       4%       4%       4%       4%
Brookfield viscosity, cps    235      185      169      130       95

Coating Appearance, 3 mils wet on white basecoat
60[degrees] Gloss             94       94       93       92       88
20[degrees] Gloss             87       87       87       86       80
DOI                           60       50       70       60       90
Yellowness index               5.1      4.1      6.5      2.3      2.5

Coating Properties, 3 mils wet on Bonderite steel
Dry film thickness, mils       2.1      2.2      2.2      2.2      2.4
Koenig hardness, swings      112      121      135       84       90
Impact Resistance in./lb
  Front                       50       30       30       70       30
  Reverse                     10       10       10       60       10
MEK double rubs              200      200      200      200      200
Crosshatch adhesion            0        0        0        0        0

Formulation Components       1008821  1008822  1008823

M100 UHA (70% in BuAc)
P60 UHA (80% in BuAc)         75       75       75
IBOA (SR 506)                 20                30
Isooctyl acrylate (SR 440)             20
TMPTA (SR 351)                                  10
E06 TMPTA (SR 499)            20       20
Darocur 4265                   4        4        4
Acetone                       15       15       15
Total grams                  134      134      134

Formulation Constants
% Solids                      78%      78%      78%
Grams VOC/liter              122      124      123
Lb VOC/gal                     1.02     1.04     1.03
% Photoinitiator on TRS        4%       4%       4%
Brookfield viscosity, cps     89       52       50

Coating Appearance, 3 mils wet on white basecoat
60[degrees] Gloss             88       88       88
20[degrees] Gloss             80       80       81
DOI                           90       90      100
Yellowness index               2.5      3.1      3.1

Coating Properties, 3 mils wet on Bonderite steel
Dry film thickness, mils       2.5      2.3      2.3
Koenig hardness, swings       37       14       80
Impact Resistance in./lb
  Front                      160      160       70
  Reverse                    100      100       40
MEK double rubs              200      200      200
Crosshatch adhesion           20%      30%       0

Table 5 -- Starting Formulations and Coating Properties of UV-Curable,
Acrylic-Polyester Urethane Coatings

Formulation Components        1008826  1008827   1008828  1008829

M100 UCA (70% in BuAc)         86       86        86       86
IBOA (SR 506)                  40       20                 20
TMPTA (SR 351)                          20        20
Adhesion promoter (CD 9050)     5        5         5        5
Darocur 4265                    4        4         4        4
Acetone                        10       10        20       20
Total grams                   145      145       135      135

Formulation Constants
% Solids                       75%      75%       66%      66%
Grams VOC/liter               188      192       226      220
Lb VOC/gal                      1.57     1.61      1.88     1.83
% Photoinitiator on TRS         4%       4%        4%       4%
Brookfield viscosity, cps      85      122        54       59

Coating Appearance, 3 mils wet on white basecoat
60[degrees] Gloss              90       93        93       93
20[degrees] Gloss              85       86        86       86
DOI                            90       90        90       90
Yellowness index                3        6.84      9.21     3.33

Coating Properties, 3 mils wet on Bonderite steel
Dry film thickness, mils        2.8      2.8       2.4      2.4
Koenig hardness, swings       115      117       109       95
Impact resistance, in./lb
  Front                        30       40        40      160
  Reverse                      10       10        10      160
200 MEK double rubs           scratch  OK        OK       OK
Crosshatch adhesion           100%      95%       95%     100%

Table 6 -- Optimized UV-Curable Coatings for Weatherable Applications

Formulation Components           1008830  1008831  1008832  1008833

M100 UHA (70% in BuAc)            86.0              86.0
M100 UCA (70% in BuAc)                     86.0              86.0
Isobornyl acrylate (SR 506)                20.0              20.0
Isoctyl acrylate (SR 440)         20.0              20.0
BYK 358 N (Flow Aid)               1.0      1.0      1.0      1.0
Adhesion promoter (CD 9050)        5.0      5.0      5.0      5.0
Darocur 4265 (photoinitiator)      3.0      3.0      3.0      3.0
Hostavin PR-25 (50% in acetone)    1.65     1.65     0.00     0.00
Tinuvin 292 (50% in acetone)       3.35     3.35     0.00     0.00
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
Lb VOC/gal                         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[degrees] Gloss                 94       93       94       93
20[degrees] 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 film thickness, mils           2.3      1.7      1.7      2.3
Koenig hardness, swings           75       95       91       70
Impact resistance, in./lb
  Front                           30      160      120      160
  Reverse                          5      160      160      160
200 MEK double rubs              pass     pass     pass     pass
Crosshatch adhesion              100%     100%     100%     100%

ACKNOWLEDGMENTS

The authors wish to thank Bob Good for expert technical assistance.

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27-29, 2004, in Chicago, IL.

References

(1) Szycher, M., Szycher's Handbook of Polyurethanes, CRC Press, 1999.

(2) Wang, W. and Pourreau, D.B., U.S. Patent 6,696,593, Feb. 24, 2004.

(3) Guo, S.-H., Wang, W., Harris, S.H., Patel, S., Junker, L.J., Blackwell, R., Fadakar, F., and Pourreau, D.B., Paint & Coatings Industry, June 2002.

(4) Arndt, L.W., Junker, L.J., Patel, S., Pourreau, D.B., and Wang, W., Paint & Coatings Industry, page 42, February 2004.

by Daniel B. Pourreau

Lyondell Chemical Company*

and

Scott Smyth

Resin Chemists LLC[dagger]

* 3801 West Chester Pike, Newtown Square, PA 19073. For additional information, email [e-mail] or visit acryflow.com.

([dagger]) Moorestown, NJ.