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UV curable, liquid diacrylate monomers based on (cis, trans)-l,3/l,4-cyclohexanedimethanol.

Abstract Novel, liquid cycloaliphatic diacrylate monomers based on (cis,trans)-1,3/1,4-cyclohexanedimethanol have been synthesized for use in ultraviolet (UV) - and electron beam (EB)-cured coatings, inks, and adhesives. Diacrylate monomers prepared from this unique diol, and containing less than 15 wt% trans-1,4-cyclohexanedimethanol diacrylate, are liquid at room temperature and readily soluble in other acrylate monomers and oligomers (in contrast to diacrylates prepared from 1,4-cyclohexancdimethanol, which are solid). More importantly, UV-cured coatings based on (cis,trans)-l,3/1,4-cyclohexanedimethanol diacrylates (1,3/1,4-CHDMDA) show superior hardness, scratch resistance, and chemical resistance as compared to common diacrylate monomers used in the UV coating industry such as tripropylene glycol diacrylate, hexanediol diacrylate. dipropylene glycol diacrylate, and propoxylated neopentyl glycol diacrylate. Thus, this new monomer appears to be a promising material for enhancing the performance of radiation-cured coatings, inks, and adhesives applied to a variety of substrates, including plastic, paper, wood, metal, and glass. This paper will summarize the synthesis of liquid diacrylates from (cis, trans)-1,3/1,4-cyclohexane-dimethanol, as well as the performance properties of the corresponding UV-cured coatings.

Keywords Cyclohexanedimethanol diacrylate, UV cured coating, Cycloaliphatic, Monomer

Introduction

Ultraviolet (UV) curing refers to the polymerization of coatings, inks, and adhesives by UV radiation, and is widely used because of the significant performance advantages that the technology offers. Since active centers are generated rapidly, UV curing results in extremely fast curing rates and is very energy efficient compared to thermal curing, in which the entire reaction system is raised to an elevated temperature. In addition, UV-cured coatings typically use little to no solvent, thereby reducing volatile organic compound (VOC) emissions in comparison to traditional thermal curing systems. These advantages have led to extensive use of UV curing in commercial applications such as graphic arts, furniture, flooring, optical fibers, adhesives. and electronics. Moreover, the energy-cured sector continues to show significant growth. According to a recent market study, the industry growth rates in 2006 in North America and Europe were ~4%, while rest-of-the-world (ROW) markets grew ~8% (including China, which saw growth of ~12%). (1)

The predominant chemistry used for UV curing involves the photogeneration of radical species that can lead to the polymerization of acrylates. This technology accounts for almost 85% of the UV coatings and inks market. (2) In this case, the major components are:

* Photoinitiators: generate free radicals upon exposure to UV radiation to initiate polymerization. Common classes of free radical photoinitiators are benzophenones, [alpha]-hydroxy ketones, [alpha]-amino ketones, and acylphosphine oxides.

* Acrylated oligomers: typically arc higher molecular weight (MW), viscous prepolymers (800 < MW < 5000 g/mol). The type of oligomer backbone plays a large role in determining the final properties of the coating, such as hardness, flexibility, gloss, adhesion, chemical resistance, and weather resistance. Commonly used oligomers include urethane acrylates (both aliphatic and aromatic), epoxy acrylates, polyester acrylates, and polyether acrylates.

* Acrylate monomers: typically are lower molecular weight (100 < MW < 500 g/mol) reactive diluents. Although monomers are used primarily to reduce the viscosity of a formulation, they can offer additional benefits, such as improved adhesion, reactivity, hardness, or flexibility.

Due to a good balance of properties (viscosity, reactivity, surface tension, and cost), the most commonly used aliphatic diacrylate monomers are tripropylene glycol diacrylate (TPGDA), hexanediol diacrylate (HDDA), dipropylene glycol diacrylate (DPGDA), and propoxylated neopentyl glycol diacrylate (2PO NPGDA). However, aliphatic diacrylates with improved hardness, scratch resistance, chemical resistance, and weatherability are still desired for the next generation of UV-cured coatings, and cyclohexanedimethanol diacrylates are a class of monomers that can potentially offer these benefits.

Overview of commercially available acrylates prepared from 1, 4-cyclohexanedimethanol

Commercially available cyclohexanedimethanol diacrylates arc prepared from 1, 4-cyclohcxanedimethanol, which is a solid diol at room temperature. 1,4-Cyclo-hexanedimethanol diacrylate monomer, as shown in Fig. 1, is touted for its abrasion resistance, hardness, weatherability, and chemical resistance. However, this monomer is a solid at room temperature and insoluble in most acrylates, thus significantly limiting its use in UV/EB curable formulations. (3)

[FIGURE 1 OMITTED]

To alleviate this problem, the solid diol (1,4-cyclo-hexanedimethanol) can be alkoxylated with either ethylene oxide or propylene oxide and then reacted with acrylic acid to form alkoxylated cyclohexanedimethanol diacrylate derivatives, with structures such as those shown in Fig. 2.

[FIGURE 2 OMITTED]

These products are commercially available liquids at room temperature and are readily soluble in acrylates used in UV-curable formulations. According to supplier literature, these monomers exhibit enhanced hardness, scratch resistance, impact strength, and abrasion resistance as compared with industry standard monomers, such as HDDA and TPGDA. (4) However, alkoxylation leads to increased MW and less-defined structures of the monomers, since the length of side chains is difficult to control. Thus, the specific MW of these monomers (ratio of MW to the number of reactive functional groups) is higher than that of the parent 1,4-cyclohexancdimethanol diacrylate, resulting in reduced crosslink density and lower hardness of the cured coating. As a result, the objective of the present work was to synthesize a cycloaliphatic diacrylate monomer that retains or improves upon the valuable properties of 1,4-cyclohexanedimethanol diacrylate, but is liquid at room temperature and free of alkoxylation.

Recently, a new, liquid cycloaliphatic diol (Fig. 3) consisting of a unique composition of (cis, trans)-1,3-cyclohexancdimethanol and (cis,trans)- 1,4-cyclohexanedimethanol has been introduced. (5) In contrast to 1,4-cyclohexanedimethanol, this new diol (henceforth referred to as 1,3/1,4-CHDM) is a liquid, and can be used to synthesize liquid cyclohexanedimethanol diacrylate monomers that are free of alkoxylation.

[FIGURE 3 OMITTED]

This paper will summarize the synthesis of novel, liquid (cis, trans)-1,3/1,4-cyclohexanedimethanol diacrylates (henceforth, referred to as 1,3/1,4-CHDMDA). as well as the performance properties of the corresponding UV-cured coatings.

Experiment

All of the reagents for synthesis were obtained from Aldrich, while the solvents were obtained from either Aldrich or Fisher Scientific. All of the reactions were conducted under anhydrous conditions. [.sup.1]H and [.sup.13]C NMR spectra were recorded on 300 and 500 MHz NMR spectrometers, and were referenced to residual solvent peaks.

Gas chromatography (GC) analysis

Diol samples were analyzed by FID gas chromatography using the following method (Table 1).
Table 1: GC method for analyzing 1,3/1,4-CHDM

Instrument             HP 5890

Column                 DB-1 60 m x 0.32 mm x 1.0 [micro]m

Injection temperature  300[degrees]C

Detector temperature   300[degrees]C

Oven program           80[degrees]C for 5 min, 5[degrees]C/min for 36
                       min, 260[degrees]C for 12 min

Carrier gas and flow   He carrier @ 1.5 mL/min constant flow

Injector flows         Split ratio 50:1

FID detector flows     [H.sub.2] @ 45 mL/min, Air @ 450 mL/min He
                       makeup @ 30 mL/min

Chromatogram time      53 min

Data system            Beckman-Coulter PeakPro

Sample injection size  0.2 [micro]L injection


Diacrylate samples were analyzed by FID gas chromatography using the following method (Table 2). The purity of the synthesized diacrylates was also supported by gel permeation chromatography (GPC).
Table 2: GC method for analyzing diacrylate samples

Instrument             HP 6890

Column                 ZB-1 60 m x 0.32 mm x 1.0 [micro]m

Injection temperature  300[degrees]C

Detector temperature   300[degrees]C

Oven program           50[degrees]C for 2 min, 300[degrees]C @
                       10[degrees]C/min, hold 6 min

Carrier gas and flow   He carrier @ 1.5 mL/min constant flow

Injector flows         Split ratio 100:1

FID detector flows     [H.sub.2] @ 45 mL/min, Air @ 450 mL/min He
                       makeup @ 30 mL/min

Chromatogram time      33 min

Data system            Beckman-Coulter PeakPro

Sample injection size  0.2 [micro]L injection


Preparation of cyclohexanedimethanol diacrylates from standard 1,3/1,4-CHDM (Sample #1)

A mixture of (cis,trans) 1,3/1,4-cyclohexanedimethanol (100 g; 0.694 mol of UNOXOL[TM] Diol, The Dow Chemical Company) was mixed with toluene (400 mL) and di(isopropyl)ethylamine (252 g; 1.95 mol), and was cooled to 0[degrees]C using an ice bath. Acryloyl chloride (153 g; 1.7 mol) in toluene (200 mL) was slowly added over 1 h with stirring. After the addition, the mixture was stirred for one more hour and then warmed to room temperature. GC analysis revealed that the reaction was complete. The mixture was filtered and the solid residue was washed with toluene (200 mL). The combined filtrate was washed with water (2 x 300 mL), 0.1 M citric acid (5 x 300 mL), saturated [NaHCO.sub.3] (300 mL), saturated NaCl (300 mL), and then dried over [MgSO.sub.4]. Toluene was removed using a rotary evaporator, and the residue was additionally kept in oil pump vacuum (~0.7 mm Hg) at 60[degrees]C for 2 h. The resulting crude product (180 g) was chromatographed on silica gel using hexane-ethyl acetate (from 40:1 to 10:1). The pure fractions were combined, 4-methoxyphenol (MEHQ) polymerization inhibitor (100 ppm) in hexane was added, the solvent was evaporated, and the residue was kept in oil pump vacuum to get the constant weight.

The pure material (138 g, 79% yield) was characterized by GPC and GC (Fig. 4). [.sup.13]C NMR spectrum ([CDCl.sub.3], [delta], ppm): 20.37, 24.84, 25.20, 28.06, 28.73, 29.26, 30.22, 31.82, 32.58, 34.37, 36.60, 36.96 (cyclohexane ring); 67.10, 67.38, 69.27, 69.29 ([CH.sub.2]O); 128.41, 128.42, 130.45, 130.49 (C=C), 166.13, 166.15, 166.16, 166.17 (C=O). GC/MS: 180 ([M.sup.+] - [CH.sub.2]=CHCOOH), 108 ([M.sup.+] - 2[CH.sub.2]=CHCOOH), 93, 79, 67, 55. The diacrylate sample was separated into a solid phase and a liquid phase at room temperature and the GC scans of both of these phases are depicted in Fig. 4. The isomer composition of each phase is highlighted in Table 9 (Sample #1), and the reaction scheme is shown in Fig. 10 in the "Results and discussion" section.

[FIGURE 4 OMITTED]

[FIGURE 10 OMITTED]

Isolation of pure trans-1,4-cyclohexanedimethanol diacrylate from the diacrylate mixture

The liquid diacrylate product (2.0 g) was mixed with hexane (0.5 mL) and left to crystallize overnight at room temperature. The crystals were filtered off, washed with cold hexane, and dried on the filter to give 0.3 g of the solid material. GC showed one signal (Fig. 11). [.sup.1]H NMR spectrum ([CDCl.sub.3], [delta], ppm, 500 MHz): 1.038 (m, 4H, axial Cy protons), 1.666 (broad s, 2H, H-[CH.sub.2]-O-), 1.834 (m, 4H, equatorial Cy protons), 3.985 (d, 4H, J = 6.5 Hz, -[CH.sub.2]-O-), 5.8.19 (d, 2H, J = 10 Hz, acrylic). 6.118 (dd, 2H, J =10 and 18 Hz, acrylic), 6.394 (d, 2H, J = 18 Hz, acrylic). [.sup.13]C NMR spectrum ([CDCl.sub.3], [delta], ppm): 28.71, 36.94 (cyclohexyl ring); 69.26 (-[CH.sub.2]O-); 128.41. 130.45 (C=C double bond); 166.15 (C=O).

[FIGURE 11 OMITTED]

Small scale, high-efficiency fractional distillation of 1,3/1,4-CHDM

A standard sample of 1,3/1,4-cyclohexanedimethanol (677 g) was fractionally distilled at 123-134[degrees]C and 0.7 mmHg using a 50 cm Oldershaw column (~15 trays). Each fraction was analyzed by GC to determine isomer composition. The residue (12 g) contained only ~15% of the trans-1,4-isomer and was used for the preparation of diacrylate. Sample #2. The compositions of distillation fractions are given in Table 3 and illustrated in Fig. 5.
Table 3: Results of 1,3/1,4-cyclohexanedimethanol fractional
distillation (677 g)

Initial and     Amount (g)  Cis-1,3  Trans-1,4  Trans-1,   C/s-1,4
distilled diol  (% Initial  isomer    isomer    3 isomer  isomer (%)
fractions       material)     (%)       (%)        (%)

Starting        677 (100)    29.4      28.6       24.3       17.7
material

Fraction 1A     123 (18.2)   32.7      34.3       18.5       14.5

Fraction 2A     134 (19.8)   30.5      31.3       21.5       16.6

Fraction 3A     135 (19.9)   29.0      30.7       22.3       18.0

Fraction 4A     183 (27.0)   27.9      28.9       24.3       18.9

Fraction 5A      90 (13.3)   23.1      23.8       29.0       24.1

Residue          12 (1.8)    14.6      15.3       37.9       32.2


[FIGURE 5 OMITTED]

Scale-up of high-efficiency fractional distillation of 1,3/1,4 CHDM

Similar to the previous approach, 2536 g of 1,3/1,4-CHDM was distilled at 123-131[degrees]C and 0.3-0.4 mmHg using the same Oldershaw column (Table 4). The latest two fractions and the residue were used for subsequent acrylation.
Table 4: Scale-up of 1,3/1,4-cyclohexanedimethanol fractional
distillation (2536 g)

  Initial and      Amount (g)   Cis-1,3  Trans-1,4  Trans-1,3  Cis-1,4
distilled diol     (% Initial   isomer    isomer     isomer     isomer
  fractions         material)     (%)       (%)        (%)       (%)

Starting material  2536 (100)    25.3      30.7       29.9      14.1
Fraction 1B         325 (12.8)   29.0      33.8       24.7      12.5
Fraction 2B         551 (21.7)   28.3      32.7       26.2      12.9
Fraction 3B        1078 (42.5)   26.4      30.7       28.4      14.6
Fraction 4B         281 (11.1)   23.6      27.3       32.7      16.4
Fraction 5B         129 (5.1)    21.5      24.6       36.0      18.0
Fraction 6B          65 (2.3)    19.3      22.7       37.8      20.2
Fraction 7B          65 (2.3)    16.6      18.3       43.7      21.4
Residue              40 (1.6)    13.1      14.5       48.2      24.1


Small scale preparation of liquid 1,3/1,4-cyclohexanedimethanol diacrylate

The distillation residue from Table 3 containing (cis, trans) 1,3- and 1,4-cyclohexanedimethanol (12 g) with an altered isomer ratio was acrylated according to the above acrylation procedure. The obtained product (7 g) was a clear, colorless liquid. The isomer ratio for the liquid diacrylate is shown in Table 9 (Sample #2).

The third sample was prepared similarly from a late distillation cut in Table 4 containing 65 g of 1,3/1,4-cyclohexanedimethanol. The cloudy sample was filtered to give 56 g of a clear liquid with the composition presented in Table 9 (Sample #3). GC scans for the liquid diacrylates are depicted in Fig. 6.

[FIGURE 6 OMITTED]

Scale-up preparation of liquid 1,3/1,4-cyclohexanedimethanol diacrylate

A combination of Fraction 6B and residue from Table 4 of 1,3/1,4-cyclohexanedimethanol (105 g; 0.728 mol) were mixed in a 2-L dry flask with toluene (400 mL) and di(isopropyl)ethylamine (264 g; 2.04 mol), and cooled to 5 [degrees]C using an ice bath, forming a two-phase system. Acryloyl chloride (158 g; 1.75 mol) in toluene (30 mL) was slowly added while vigorously stirring. Within 30 min, when approximately 1/3 of the acryloyl chloride was added, the system turned one phase and an exothermic reaction began. The dropwise addition continued for one more hour, and then the ice bath was removed and the mixture was left overnight with stirring. GC analysis revealed complete disappearance of the starting diol and emergence of new peaks with higher retention time. The mixture was filtered and the solid residue was washed with toluene (2 x 100 mL). The combined filtrate was treated with water (500 mL), leading to strong emulsions. Sodium chloride was added to the system to break the emulsion and separate phases, followed by extraction with saturated NaCl solutions (2 x 500 mL).

The toluene phase was dried over sodium sulfate. Toluene and di(isopropyl)ethylamine were removed using a rotary evaporator, and the residue was evacuated under vacuum (2 mmHg) for 4 h. The crude product (195 g) was chromatographed on silica gel using hexane-ethyl acetate (20:1), and the collected fractions were analyzed by GC. Almost colorless fractions, containing +95% pure material, were combined, and a hexane solution of the MEHQ inhibitor (~100 ppm) was added. The solvent was stripped off at 40[degrees]C using a rotary evaporator and then, under reduced pressure evacuation (2 mm Hg, 2 h), to give 81 g of product (95% purity) containing 18.4% of the trans-l,4-diacrylate. Precipitation of solid crystals was observed at room temperature after several days and the sample was filtered to give 69.5 g of the liquid diacrylate containing 9% of the trans-l,4-isomer (Sample #4, Table 9). The remaining fractions could be chromatographed for a second time to produce additional amounts of the pure product.

Figure 7 shows the difference in appearance between the liquid diacrylate synthesized from 1,3/1,4-CHDM and the current commercially available solid product: 1,4-cyclohcxanedimethanol diacrylate. The liquid (cis, trans)-1,3/1,4-cyclohexanedimethanol diacrylate (containing less than 15% of the trans- 1,4-isomer) remains completely liquid when stored at room temperature, and does not show any visible signs of crystallization after storage for one year.

[FIGURE 7 OMITTED]

Coating materials

The synthesized liquid 1,3/1,4-cyclohexanedimethanol diacrylate (1,3/1,4-CHDMDA) in Sample #3 was then compared to commonly used diacrylate monomers, as well as alkoxylated cyclohexanedimethanol diacrylates. Three alkoxylated cyclohexanedimethanol diacrylate monomers were evaluated: (i) 3 mole ethoxylated cyclohexanedimethanol diacrylate (3EO CHDMDA), (ii) 5 mole ethoxylated cyclohexanedimethanol diacrylate (5EO CHDMDA), and (iii) 5 mole propoxylated cyclohexanedimethanol diacrylate (5PO CHDMDA). Diacrylate monomers were used as received from the supplier (Sartomer) and without further purification. The chemical structures of the monomers under evaluation are shown in Fig. 8.

[FIGURE 8 OMITTED]

In addition, the performance of the monomers was also evaluated in conjunction with an aliphatic urethane diacrylate (AUDA) oligomer (EBECRYL[TM] 8402, Cytec). To initiate photopolymerization, the photoinitiator, 1-hydroxycyclohexyl phenyl ketone (IRGACURE[R] 184, Ciba), was used. The structure of this photoinitiator is shown in Fig. 9. It is classified as an [alpha]-cleavage (Norrish Type I) initiator because bond cleavage occurs between the carbonyl group and the adjacent [alpha]-carbon atom upon absorption of the proper wavelengths of light. Relative to conventional mercury UV lamps, the principal bands of absorption responsible for initiating polymerization are at 240-250 nm, as well as 320-335 nm.

[FIGURE 9 OMITTED]

UV-curable coating formulations

Coating formulations (15 g) were prepared by mixing the acrylates with a photoinitiator in a FlackTek SpeedMixer[TM] (Model DAC 150 FV-K, FlackTek, Inc.) using a Max 60 Cup and mixing for 5 min at 3000 rpm. Specific compositions are shown in Tables 5 and 6.
Table 5: Composition of UV-curable coatings applied on glass and
polycarbonate

Component                                         Wt% of component

Aliphatic diacrylate monomer                          95.24
1-Hydroxycyclohexyl phenyl ketone photoinitiator       4.76
Total                                                100.00

Table 6: Composition of UV-curable coatings containing urethane
acrylate oligomer and diacrylate monomer applied on polycarbonate

Component                                         Wt% of component

Aliphatic urethane diacrylate (AUDA) oligomer           47.62
Aliphatic diacrylate monomer                            47.62
1-Hydroxycyclohexyl phenyl ketone photoinitiator         4.76
Total                                                  100.00


Coating application and UV curing

Coating formulations were applied on both glass and polycarbonate substrates using wire-wound rods. Coatings were applied on glass substrates using a #8 wire-wound rod to yield ~10 micron (dry film thickness) coatings, whereas coatings were applied on polycarbonate substrates using a #20 wire-wound rod to yield 35-40 micron coatings. Coatings were UV cured in air using a Fusion UV System (Model DRS-10/12QNH with VPS/1600 lamp system) at a conveyor speed of 10 ft/min and a 600 W/inch Fusion H bulb (100% lamp power). Coated samples were exposed to UV radiation for <10 s while passing underneath the lamp; the distance between the coated substrates and the UV lamp (lamp-to-part distance, LPD) was kept constant at 1.5 inches. The corresponding irradiance and dosage levels for these conditions were measured with a PowerPuck[R] radiometer from EIT, Inc. and are shown in Table 7.
Table 7: Irradiance and dosage levels used for UV curing

                     UV A          UV B          UV C          UV V
                 (320-390 nm)  (280-320 nm)  (250-260 nm)  (395-445 nm)

Irradiance           1347          1252          154           900
(mW/[cm.sup.2])

Dosage               1381          1274          151           914
(mJ/[cm.sup.2])


Property testing

Viscosity

Viscosities of the liquid materials were measured with a Brookfield DV III + Rheometer equipped with the Small Sample Adapter (#31 spindle and SC4-35Y sample chamber, Brookfield). For this particular spindle and chamber setup, the required sample volume was 10.5 mL. Viscosities were measured at 25[degrees]C and 20 rpm.

Appearance--initial haze

To characterize the appearance of the coatings, the initial haze of the coatings was measured with a haze meter (Haze-Gard Plus, Byk-Gardner) in accordance with ASTM D1003 and D1044. All initial haze data were acquired in total transmittance mode (wavelength range between 400 and 700 nm) with a port hole size of 1 inch. For coatings on transparent substrates, initial haze values less than 1.0% are desired.

Abrasion resistance test

All Taber abrasion tests were performed on coatings applied on polycarbonate using a Taber Abraser (Model 5150, Taber Industries, Inc.) equipped with CS-10F abrasive wheels at a total load of 1000 g (500 g on each wheel), in accordance with ASTM D1044. Coated samples were abraded for 100 cycles and the percent change in haze (% [DELTA]Haze) was determined by measuring the difference in haze of the unabraded and abraded areas of the coating using the haze meter. A lower % [DELTA]Haze value indicates better abrasion resistance.

Micro-indentation hardness test

To assess the hardness of the coatings, a FISCHER-SCOPE[R] H100C (Fischer Technology) computer-controlled, ultra-low load dynamic micro indentation system was used, in conjunction with WIN-HCU[R] (Fischer Technology) control software. In this test, a Vickers indenter in the form of a straight diamond pyramid with a square base and opposite sides angled at 136[degrees] was pressed into the surface of the coating with an applied force of 5 mN (rate = 5 mN/20 s). The maximum load is then held for 20 s (creep step) followed by the releasing of the load (rate = 5 mN/20 s). A final creep step of 20 s completes the test cycle. By taking into account the geometry of the indenter and the penetration depth for the applied force, a universal hardness measurement (HU) is obtained. A higher HU number indicates higher coating hardness.

Pencil hardness

To assess the hardness and scratch resistance of the coatings, the pencil hardness was measured according to ASTM D3363. In this test, a number of pencil leads of varying hardness are pressed across the surface of the coating and the hardest pencil lead that fails to scratch the coating down to the substrate is reported. The pencil hardness scale is as follows (softest to hardest):

6B < 5B < 4B < 3B < 2B < B < HB < F < H < 2H < 3H < 4H < 5H < 6H

Adhesion

Adhesion of the UV-cured coatings to polycarbonate was measured according to ASTM D3359 (cross-hatch adhesion). For this test, the coated sample was scribed with a razor, cutting through the coating to form a cross-hatch pattern (typically 10 cuts by 10 cuts, with 2 mm spacing between lines). Double-coated paper tape (3M No. 410) was then applied on the scribed area, pressed down, and then stripped away sharply in a direction perpendicular to the surface of the coated sample. The coating and tape were then visually inspected to see whether any of the coating was removed from the substrate by the tape. If > 5% of the coating is removed, then the coating has failed the adhesion test. Specific ASTM ratings for the adhesion test are shown in Table 8. Based on the ASTM rating system, adhesion ratings of 4B and 5B are desired.
Table 8: ASTM D3359 classification for adhesion

ASTM D3359 rating  Percent of coating removed

        5B              0 (Perfect adhesion)
        4B              <5
        3B              5-15
        2B              15-35
        1B              35-65
        OB              >65


Solvent resistance

The solvent resistance of the UV-cured coatings was tested according to ASTM D5402. using methyl ethyl ketone (MEK). To assess the solvent resistance of the coating, a piece of cotton cheesecloth was attached to a 1.5 lb. hammer with copper wire. The cheesecloth was saturated with MEK and placed on the coating. The hammer was pushed forward and then back in approximately one second (one double rub). Testing was continued over the same test area for a total of 100 or 200 double rubs. The solvent was allowed to dry and the coating was visually inspected for any signs of delamination or damage. The number of double rubs required to damage the coatings and penetrate to the substrate was reported. Higher double rubs indicated better solvent resistance and surface curing.
Table 9: Isomer distribution and physical state of
1,3/1,4-cyclohexanedimethanol diacrylate samples

Sample #  Amount  Physical  Cis-1, 3  Trans-1, 3  Cis-1, 4  Trans-1, 4
          (g)      state     isomer     isomer     isomer   isomer (%)
                               (%)        (%)        (%)

1         138     Solid       18.9       17.4        8.4       52.3
                  (major)

                  Liquid      40.8       37.4        8.4        6.1
                  (minor)

2           7     Liquid      13.4       39.0       33.3       13.4

3          56     Liquid      17.5       50.4       24.9        7.1

4          70     Liquid      17.9       44.1       21.9        9.1

Note: Concentration of the trans-1,4-cyclohexanedimethanol diacrylate
is below 15% in each of the liquid samples


Chemical and stain resistance

The chemical and stain resistance of the UV-cured coatings was tested by exposing the coatings to various chemicals and stains according to ASTM D1308. The list of chemicals included: tap water, ethanol, 4% acetic acid, black Rit[R] dye, 5% sodium hydroxide, yellow mustard, Betadine[R] (10% povidone-iodine), and 4% ammonium hydroxide. Several drops of each chemical or stain were placed on the coating and covered with a watch glass for 24 h. After 24 h, the chemicals and stains were wiped off the coating with water. Coatings were visually inspected for any signs of chemical attack or staining, and were ranked on a scale from 0 (no effect) through 5 (severe chemical attack or staining).

Results and discussion

Synthesis of liquid diacrylates from 1,3/1,4-CHDM

Two options have been explored for synthesizing liquid diacrylates from 1,3/1,4-CHDM. In the first approach, a standard sample of 1,3/1,4-CHDM was acrylated with acryloyl chloride in toluene in the presence of di(isopropyl)ethylamine (Fig. 10). The acrylation was monitored by GC and thin-layer chromatography (TLC) on silica gel to determine reaction progress and completion. Aqueous work-up and purification by column chromatography on silica gel resulted in a mixture of the corresponding diacrylates, consisting of both solid (major) and liquid (minor) phases. The desired liquid product was separated by filtration.

GC analysis revealed that the compositions of the liquid and solid phases were strikingly different, as shown in Fig. 4 (in "Experimental" section) and Table 9 (Sample #1).

Based on the analysis, the trans-1,4-cyclohexanedi-methanol diacrylate is the predominant isomer in the solid phase with the content more than 50% (Table 9). This isomer was actually isolated by crystallization in a pure state as a solid material at ambient temperature (see "Experimental" section).

In contrast, the percentage of the trans-1,4-diacrylate in the liquid phase was the lowest among the isomers (Table 9). Since each diol isomer converts to the same diacrylate isomer (for example, cis-1,3-diol [right arrow] cis-1,3-diacrylate, etc.), it was assumed that decreasing the trans-1,4-diol percentage in the starting 1,3/1,4-CHDM would result in the generation of a single-phase liquid diacrylate.

Therefore, in the second approach, 1,3/1,4-CHDM was first fractionally distilled in a vacuum using a 50 cm Oldershaw column containing about 10 trays to produce a number of cuts with noticeably different isomer ratios and reduced concentrations of the trans-1,4 diol isomer (Tables 3 and 4; Fig. 5). Application of a Vigreaux column proved inefficient for changing isomer ratios in the distillation fractions, leading to almost the same composition as the starting material.

Upon acrylation, the diacrylate mixture showed clear signs of a solid phase when the concentration of the trans-1,4-diacrylate exceeded 15%. The appearance of the solid phase varied from haziness when the trans-1,4-diacrylate content only slightly exceeded 15%, to a substantial crystalline precipitate for a significantly larger percentage of this isomer. However, acrylation of the distillation residue with the percentage of the trans-1,4-diol less than 15% (Table 4), resulted in an exclusively liquid product.

Note that the distillation residue represents only a few percent of the starting diol, and therefore the industrial distillation will bring very little changes to the combined fractions of the distilled 1,3/1,4-CHDM, so it can most likely still be used for regular applications. Thus, sequestering the specific diol composition needed for the liquid diacrylate, which is a new specialty product, should not practically affect consistency of the bulk 1,3/1,4-CHDM, which is a commodity chemical.

Assignment of diacrylate isomers in GC

The diacrylates derived from 1,3/1,4-CHDM show up in GC as a mixture of four components that are presumably cis-1,3-, cis-1,4-, trans-1,3-, and trans-1,4-isomers derived from the corresponding counterparts of the starting diol. The solid component of this mixture has been isolated and characterized by NMR (Figs. 12 and 13). This isomer matches the last diacrylate signal in GC (Fig. 11).

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The presence of six distinct carbon resonances in the [.sup.13]C NMR spectrum (Fig. 12) unequivocally indicates that the isomer has a high level of symmetry with two acrylic groups located at 1- and 4-positions. This compound can be either a cis-1,4- or a trans-1,4-isomer. Further analysis of the isomer geometry was carried out using the [.sup.1]H NMR spectrum (Fig. 13). A typical pattern at 5.8-6.4 ppm evidently corresponds to the acrylic protons. The doublet at 3.98 ppm is assigned to the methylenes connected to the acrylic groups.

Three multiplets in the range of 0.9-2.0 ppm match the three remaining types of protons on the cyclohexane ring. The latter feature provides the key to the structural assignment. At room temperature, ring inversion is fast on the NMR time scale. The trans-1,4-diacrylate and cis-1,4-diacrylate behave differently. For the former, inversion leads to either two equatorial or two axial acrylic groups that render eight cyclohexane methylene protons nonequivalent. Indeed, two multiplets at 1.04 and 1.83 ppm, containing four protons each, can be assigned to the axial and equatorial protons, respectively. The broad two-proton signal at 1.67 ppm is consistent with protons in 1- and 4-positions next to the acrylic groups.

In the case of the cis-1,4-diacrylate, the inversion would lead to the equivalent configuration. Indeed, this isomer would have one equatorial and one axial acrylic group, which would give rise to the identical configuration upon inversion. This will render all the eight methylene protons of the cyclohexane ring equivalent. Thus, the above isomer cannot be the cis- 1,4-diacrylate.

Assignment of other isomers was done by matching GC areas for major and minor isomers of the diols with the known assignment (see Fig. 5; Tables 3 and 4) with that of the generated diacrylates (Samples #1-3, Figs. 4 and 6; Table 9). Thus, the following sequence of elution in GC was determined for the isomeric diacrylates: cis-1,3; trans-1,3; cis-1,4; trans-1,4.

Property testing

Viscosity

Viscosity is a primary consideration in selecting acrylate monomers, since their efficiency as diluents can dictate the rheological behavior of the final formulation. Figure 14 compares the viscosity of 1,3/1,4-cyclo-hexancdimethanol diacrylate (1,3/1,4-CHDMDA) to viscosities reported in supplier literature for commonly used diacrylate monomers.

[FIGURE 14 OMITTED]

As shown in Fig. 14, the viscosity of 1,3/1,4-CHDMDA is ~27 mPa.s, which is higher than HDDA, DPGDA, TPGDA, and 2PO NPGDA, but significantly lower than the alkoxylated cyclohexanedimeth-anol diacrylates (3EO, 5EO, or 5PO CHDMDA). Table 10 illustrates the viscosity reducing power of 1,3/1,4-CHDMDA when blended with a high viscosity aliphatic urethane diacrylate oligomer (50:50 blend by weight).
Table 10: Viscosity reducing power of selected diacrylate monomers
blended with aliphatic urethane diacrylate oligomer (50:50 blend
by weight)

         Formulation                      Viscosity (mPa.s) at
                                              25[degrees]C

Aliphatic urethane diacrylate (AUDA)  15,000 (supplier literature)
AUDA + HDDA                           172.0 [+ or -] 3.0
AUDA + DPGDA                          332.5 [+ or -] 1.5
AUDA + TPGDA                          416.0 [+ or -] 1.0
AUDA+ 1,3/1,4-CHDMDA                  596.5 [+ or -] 1.5
AUDA + 3EO CHDMDA                     970.5 [+ or -] 1.5


As Table 10 shows, 1,3/1,4-CHDMDA can significantly reduce the viscosity of the formulation and has superior reducing power compared to the alkoxylated cyclohexanedimethanol diacrylate. However, it does not reduce the viscosity to the same extent as HDDA, DPGDA, and TPGDA (most likely due to the bulkier cyclohexyl ring).

Performance properties of UV-cured coatings

In conjunction with viscosity, the desired performance properties (i.e., hardness, scratch resistance, chemical resistance, and optical properties) of the final UV-cured coating are key considerations in selecting appropriate monomers for a particular application. Table 11 summarizes the performance properties of 10 micron coatings applied on glass, while Table 12 summarizes the results for 35 micron coatings applied on polycarbonate (PC). These results are for coatings containing only the diacrylate monomer and photoinitiator. Henceforth, each entry in the tables below represents the average of three samples, and the error is indicated by one standard deviation above and below the mean.
Table 11: Properties of 10 micron UV-cured coatings containing
diacrylate monomer and photoinitiator applied on glass

Diacrylate     Universal hardness     Initial   Pencil   MEK double
monomer        (N/[mm.sup.2])         haze     hardness     rubs
                                      (%)

1,3/1,4-CHDMDA  193.44 [+ or -] 1.39   0.15      3H      Pass 100 rubs
HDDA            113.85 [+ or -] 4.81   0.07       H      Pass 100 rubs
TPGDA           116.41 [+ or -] 4.47   0.06       B      Pass 100 rubs
DPGDA           156.40 [+ or -] 5.46   0.05       H      Pass 100 rubs
2PO NPGDA        61.07 [+ or -] 2.88   0.06       B      Fail 50 rubs
3EO CHDMDA       36.38 [+ or -] 3.21   0.04       B      Pass 100 rubs
5EO CHDMDA       18.93 [+ or -] 3.73   0.02      <B      Fail 30 rubs
5PO CHDMDA       16.86 [+ or -] 2.07   0.08       H      Fail 30 rubs

Table 12: Properties of 35 micron UV-cured coatings containing
diacrylate monomer and photoinitiator applied on polycarbonate

Diacrylate monomer   Universal hardness    Initial haze (%)
                       (N/[mm.sup.2])

1,3/1,4-CHDMDA      180.04 [+ or -] 1.80  0.61 [+ or -] 0.02
HDDA                103.82 [+ or -] 1.18  0.67 [+ or -] 0.03
TPGDA               104.77 [+ or -] 2.00  0.68 [+ or -] 0.03
DPGDA               141.83 [+ or -] 2.08  0.62 [+ or -] 0.03
2PO NPGDA            74.29 [+ or -] 3.71  0.63 [+ or -] 0.03
3EO CHDMDA           49.12 [+ or -] 0.58  0.60 [+ or -] 0.03
5EO CHDMDA            9.32 [+ or -] 0.45  0.63 [+ or -] 0.03
5PO CHDMDA            6.80 [+ or -] 0.18  0.60 [+ or -] 0.01

  Diacrylate         Abrasion         Pencil     MEK double   Adhesion
   monomer          resistance       hardness       rubs
                  (%[DELTA]Haze)

1,3/1,4-CHDMDA  18.93 [+ or -] 4.77     HB     Pass 200 rubs     5B
HDDA            19.95 [+ or -] 4.14     HB     Pass 200 rubs     5B
TPGDA           36.77 [+ or -] 6.31     2B     Pass 200 rubs     1B
DPGDA           26.58 [+ or -] 3.97     2B     Pass 200 rubs     0B
2PO NPGDA       47.05 [+ or -] 5.58     2B     Pass 200 rubs     1B
3EO CHDMDA      32.51 [+ or -] 4.15     3B     Pass 200 rubs     0B
5EO CHDMDA      26.30 [+ or -] 0.35     3B     Pass 200 rubs     0B
5PO CHDMDA      50.79 [+ or -] 0.36     3B     Pass 100 rubs     0B


Both Tables 11 and 12 demonstrate that UV-cured coatings derived from 1,3/1,4-CHDMDA have significantly higher hardness (both universal and pencil) as compared to the other diacrylate monomers. The greater hardness of 1,3/1,4-CHDMDA can be attributed to the higher crosslink density, as well as the presence of the cyclohcxyl ring. As expected, the alkoxylated cyclohexanedimethanol diacrylates have significantly less hardness (due to their higher molecular weight and the resulting lower crosslink density of cured coatings) as compared to 1,3/1,4-CHDMDA. Moreover, the abrasion resistance of 1,3/1,4-CHDMDA is significantly better than the other diacrylates (a lower % [DELTA]Haze indicates better abrasion resistance). As a result, the use of 1,3/1,4-CHDMDA results in coatings that are both hard and abrasion resistant. Finally, only the coatings prepared from 1,3/1,4-CHDMDA or HDDA pass the adhesion test on polycarbonate and have 100% adhesion. Coatings based on the other diacrylate monomers show either very poor adhesion or complete adhesion failure to the polycarbonate substrate.

Similar enhancements in performance are seen when 1,3/1,4-CHDMDA is used in conjunction with an AUDA oligomer, as shown in Table 13.
Table 13: Properties of 40 micron UV-cured coatings containing AUDA
oligomer, diacrylate monomer, and photoinitiator applied on
polycarbonate

     Diacrylate         Universal hardness    Initial haze (%)
                          (N/[mm.sup.2])

AUDA                     9.28 [+ or -] 1.08  0.69 [+ or -] 0.03

AUDA + 1,3/1,4-CHDMDA  150.74 [+ or -] 2.14  0.63 [+ or -] 0.05

AUDA + HDDA            101.63 [+ or -] 3.59  0.58 [+ or -] 0.07

AUDA + TPGDA            94.74 [+ or -] 6.36  0.59 [+ or -] 0.01

AUDA + DPGDA           129.61 [+ or -] 2.36  0.60 [+ or -] 0.03

  Diacrylate          Abrasion        Pencil    MEK double    Adhesion
                     resistance      hardness      rubs
                   (%[DELTA]Haze)

AUDA             30.9 [+ or -] 3.23     3B     Pass 100 rubs     2B

AUDA +          14.14 [+ or -] 2.17      B     Pass 100 rubs     5B
1,3/1,4-CHDMDA

AUDA + HDDA     27.12 [+ or -] 2.87     2B     Pass 100 rubs     5B

AUDA + TPGDA    42.68 [+ or -] 0.91     2B     Pass 100 rubs     5B

AUDA + DPGDA    40.01 [+ or -] 0.13     2B     Pass 100 rubs     5B


When used alone, the AUDA is very soft and shows poor adhesion to the polycarbonate substrate. However, when a 50:50 blend of AUDA with 1,3/1,4-CHDMDA is used, the resulting UV-cured coating has significantly improved hardness, abrasion resistance, and adhesion. The other diacrylate monomers also improve the hardness and adhesion of the coatings, but they do not simultaneously improve the abrasion resistance, as does 1,3/1,4-CHDMDA.

Chemical and stain resistance

Coatings with improved chemical and stain resistance are required for demanding applications in which there is a high probability of exposure to harsh environments, such as those in automotive, marine, and aerospace applications. Table 14 illustrates the excellent chemical resistance of UV-cured coatings made from 1,3/1,4-CHDMDA (a lower number indicates better chemical resistance). Coatings prepared from 1,3/1,4-CHDMDA were largely unaffected by the tested chemicals, whereas coatings prepared from the other diacrylate monomers were very susceptible to attack by bases, black dye, and mustard. Improved chemical and stain resistance is due to a number of contributing factors, including increased crosslink density, the presence of the cyclohexyl ring, and the hydrophobicity of 1,3/1,4-CHDMDA.
Table 14: Summary of chemical resistance of 35 micron UV-cured coatings
containing diacrylate monomer and photoinitiator applied on
polycarbonate

Chemicals & stains  1,3/1,4-CHDMDA  HDDA  TPGDA  DPGDA  2PO NPGDA

Tap water                 0           0     1      0         2
Ethanol                   1           0     2      1         2

4% Acetic acid            0           0     1      0         0
Black RIT[R] dye          0           2     3      2         4
5% NaOH                   0           0     2      1         1
Yellow mustard            0           1     1      1         1
Betadine[R]               0           0     0      0         0
4% [NH.sup.4]OH           0           1     1      1         4
Total score               1           4    11      6        14

Chemicals & stains  3EO CHDMDA  5EO CHDMDA  5PO CHDMDA

Tap water               0           2           1
Ethanol                 1           1           2
4% Acetic acid          1           0           2
Black RIT[R] dye        4           5           5
5% NaOH                 1           2           1
Yellow mustard          2           2           2
Betadine[R]             2           5           1
4% [NH.sup.4]OH         3           2           3
Total score            14          19          17


Conclusions

Liquid diacrylates derived from (cis, trans)-1,3/1,4-cyclohexanedimethanol are readily soluble in acrylates used in typical UV/EB curable formulations and impart superior hardness, scratch and abrasion resistance, and chemical resistance to the final coatings. This monomer, alone or in combination with acrylated oligomers, appears promising for enhancing the performance of UV/EB coatings, inks, and adhesives for a number of substrates, including plastic, paper, wood, metal, and glass.

Acrylation of (cis, trans)-1,3/1,4-cyclohexanedimethanol (1,3/1,4-CHDM) with acryloyl chloride in toluene, and in the presence of di(isopropyl)ethylamine, resulted in a mixture of diacrylates consisting of both solid and liquid phases. The liquid product was separated by decantation. Alternatively, high efficiency distillation of 1,3/1,4-cyclohexanedimethanol gave late cuts and distillation residue with reduced percentages of the trans-l,4-isomer. Acrylation of the obtained 1,3/1,4-cyclohexanedimethanol compositions (containing less than 15 wt% of the trans-1,4-cyclo-hexane dimethanol isomer) resulted in novel liquid (cis, trans)-1,3/1,4-cyclohexanedimethanol diacrylate (1,3/1,4-CHDMDA) monomers.

Acknowledgments Special thanks go to Heqi Pan for helping with synthesis, Ben Schaefer for helping with property testing of the UV-cured coatings, John Bledsoe for GC analysis of the diacrylates, Don Robinson for recording NMR spectra, and Phil Gaarenstrom and Susanne Chambers for providing GPC data. In addition, the authors are grateful to Scott Bis, Paul Popa, Rodolfo Bayona, and Paul Foley for helpful discussions. Finally, The Dow Chemical Company is appreciated for supporting the external publication of this research.

References

(1.) Wright, T. "Rad-Cure Coatings Market." Coatings World, April 2007 (available at http://www.coatingsworld.com/arti-cles/2007/04/radcure-coatings-markel.php)

(2.) Cohen, G, "North American Market Update." RadTech N.A., p. 11, May 2002

(3.) "Sartomer Application Bulletin: Alkoxylated Cyclohexane Dimethanol Diacrylate for Improved Tensile Strength and Elongation." 4034 06/05 Oaklands Corporate Center, Exton PA 19341 (available at www.sartomer.com)

(4.) "Sartomer Application Bulletin: Alkoxylated Cyclohexane Dimethanol Diacrylate Monomers Offer Low Skin Irritation and Improved Coating Performance in Comparison to Commodity Monomers." 4036 06/05 Oaklands Corporate Center, Exton PA 19341 (available at www.sartomer.com)

(5.) Argyropoulos. JN. Spilman. GE. Franca, M. Hayson, K. "UNOXOL[TM] Diol: A New Liquid Cycloaliphatic Diol for Coatings Applications." PCI Paint Coat. Ind., 22 (6) 60-66 (2006)

[C] FSCT and OCCA 2009

K. K. Baikerikar ([*]), M. L. Tulchinsky, J. Argyropoulos

Core R&D, The Dow Chemical Company, Midland, MI 48674. USA

e-mail: kkbaikerikar@dow.com
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Author:Baikerikar, Kiran K.; Tulchinsky, Michael L.; Argyropoulos, John
Publication:JCT Research
Date:Mar 1, 2010
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