Organic oligomers in aqueous environments.

A series of water reducible oligomers were prepared with varying polarities, solubilizer content, and molecular weights. Their behaviors upon reduction in water were observed and compared to each other. Molecular weight and polarity are both shown as important factors in the reduction behavior.

Keywords: Esters, high solids, reactive diluents, urethane, VOC control, waterborne

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This work reports on the first phase of a project conducted to understand the behavior of oligomeric materials in waterborne binder systems. Because the authors have no plans to continue the work, we wish to report these preliminary results that may be worth follow-up work. The initial project intent was to characterize the effect that oligomeric materials of varying polar nature and solubilizer content had on the solids/viscosity behavior of waterborne coatings binder systems. In this reported work, we attempted to prepare a relatively polar set of oligomers based on urethane structures and a relatively nonpolar set based on maleinized linseed oil. Since the sets of oligomers synthesized had several variables (molecular weight, solubilizer content) as well as polarity differences, it is not possible at this point to generate the desired correlations of effects on the solids/viscosity behavior. There are several interesting trends, however, in the data which we believe are of interest for further study.

BACKGROUND

There is a significant history and body of literature describing the advancements in nonlatex-based water-borne coatings materials. The coatings industry has developed materials that perform well. However, many of the functional waterborne coatings for industrial purposes still contain substantial amounts of volatile organic components. The volatile organic materials serve as coalescents and flow aids and may also be present as a means to introduce additives, catalysts, pigments, etc. Some are present for manufacturing and processing purposes. Secondly, waterborne coatings are often lower in application solids than desired for some applications due to the nature of the way polymers behave as they are diluted in water. Oligomeric materials (sometimes referred to as reactive diluents) may provide improvements in waterborne coatings for these situations.

The literature describes the behavior of polymeric materials as they are reduced with water to provide workable solutions or dispersions. (1-4) The usual behavior is a significant rise in viscosity as an organic solution is reduced with water, followed by a precipitous dropoff in viscosity at a low solids content (20-40% solids). Advances regarding the use of oligomeric materials in solventborne coatings have been described, and performance enhancement with these kinds of materials has been demonstrated. (5-7) There is some literature describing reactive diluents in waterborne systems. (8) (Also, urethane oligomers have been a significant ingredient in electrodeposition coatings for over 20 years and polyesters, which may be relatively low in molecular weight, are components of waterborne primer systems.) Less is known (or published) regarding the broader performance of oligomers in waterborne systems for the purpose of improving application solids, reducing volatile organic emissions, and for the enhancement of properties.

EXPERIMENTAL

In this phase of the project, the activities have included: (1) preparation of a series of urethane oligomers with varying molecular weight and carboxylic acid content; (2) preparation of a conventional water reducible polyester; (3) preparation of maleinized linseed oils with varying molecular weight and carboxylic acid content; (4) formulating combinations of polyester with the urethane and maleinized oil oligomers; and (5) generating solids/viscosity curves for the oligomers and formulations.

MATERIALS

The diisocyanates were graciously supplied by the Bayer Polymers Corp. The toluene diisocyanate (TDI) was Mondur[R] TDS, which is 98% of the 2,4 isomer. Other materials were purchased from Aldrich Chemical Co. All these materials were used as supplied. Test methods used were ASTM D 2572-91 (Isocyanate content), ASTM D 465-82 (Acid Number), ASTM D 1259-85 (% Nonvolatile), ASTM D 1296-86 (Brookfield Viscosity).

[FIGURE 1 OMITTED]

Preparation of Urethane Oligomers (Tables 1 and 4)

The urethane oligomers were prepared by a stepwise addition process. The dimethylol propionic acid (DMPA), initial amount of diisocyanate, and acetone were added to a four-necked flask equipped with mechanical stirrer, thermometer, and condenser and held at 65[degrees]C for the indicated period.

(a) #01 -- DMPA, TDI, and acetone were added, heated to 65[degrees], and held for four hours.

(b) #02 -- DMPA, 1/2 TDI, and acetone were added, heated to 65[degrees], held for one hour. Then neopentyl glycol (NPG) was added and held for one hour, and TDI was added and held for two hours.

(c) #03 -- DMPA, 1/3 TDI, and acetone were added, held for one hour, then 1/2 NPG was added and held for one hour, 1/3 TDI was added and held for one hour, and 1/2 NPG was added and held for one hour, then 1/3 TDI was added and held for two hours.

Dibutyl tin dilaurate (DBTDL) was added, and 1-butanol was added slowly and held for another two hours, until the isocyanate content fell below 0.5%, then cooled. After determining the acid number and % nonvolatile, the materials were neutralized with triethyla-mine to a theoretical neutralization equivalent of 100%. Then they were diluted to 60% solids with acetone.

The isophorone diisocyanate (IPDI) oligomers (#01I, #02I, #03I) were made by the same synthesis steps, except IPDI was substituted for TDI on a molar basis (see Table 4).

Preparation of Maleinized Linseed Oils (Table 2)

The following materials were added to a four-necked flask equipped with mechanical stirrer, thermometer, air purge tube, and condenser: linseed oil, maleic anhydride and xylene. The materials were purged with [N.sub.2], heated to 196-200[degrees]C, and held for four hours. Then the temperature was decreased to 70[degrees]C, and triethyl amine catalyst was added. Next 2-ethyl-1-hexanol was added in 0.5 hr and held at 70[degrees]C for two hours, then cooled to room temperature. The acid number was determined and the material was neutralized to 100% with the theoretical amount of triethylamine and diluted to 60% solids with acetone.

Preparation of Water Reducible Polyester (Table 3)

Adipic acid, phthalic anhydride, and 1,4 dimethylol cyclohexane (CHDM) were charged in a four-necked flask equipped with mechanical stirrer, thermometer, Dean-Stark water trap, purge tube, and condenser. A very slow nitrogen blanket was used during the esterification. The mass was heated slowly, and stirring was started as soon as possible (about 50[degrees]C) to 160[degrees]C. The materials were held for 30 min, then heated to maximum temperature of 220[degrees]C, as the removal of water allowed, then held until the theoretical water was removed. The materials were then cooled to 140[degrees]C, and trimellitic anhydride (TMA) was carefully added. The materials were then heated at 140[degrees]C for 30 min, and allowed to begin cooling. A small amount of acetone was added at 100[degrees]C, and was continually added as the temperature allowed. After determining the acid number, the polyester was neutralized to 100% with the theoretical amount of triethyl amine and diluted to 60% with acetone.

Formulation of Oligomers with Polyester

Each of the oligomers described in Tables 1 and 2 were mixed with the polyester (Table 3) in their 60% solids/100% neutralized form at a 1/1 by weight.

Determination of Viscosities

The viscosities of the solutions, upon consecutive reductions of the 60% solutions with increments of distilled water, were measured with a Brookfield viscometer (Figures 1-5).

[FIGURE 2 OMITTED]

DISCUSSION

Many of the anticipated results of the study to date are difficult to interpret with any confidence because two important parameters vary in each set. The molecular weights and the acid content both change, so that any result could be affected by either one of these parameters. The covariation is due to the synthesis procedures. In order to generate data where only one parameter varies, further synthesis manipulations are needed in each class of "oligomers." Another issue is that the molecular weight of the "polymer" is the same or lower than those of the oligomers. To consider the effect of the oligomer in affecting the properties of a polymer, the polymer molecular weight should be raised significantly. The polyester prepared in this study is already in the oligomer class. (Alternatively, most of the oligomers may be considered to have a molecular weight that is too high.) Desirably, the reductions with water would have been done from an organic solution (acetone in this case) at higher solids content (lower solvent content). The urethane oligo-mers, however, were not easily handled at higher solids content. In order to make valid comparisons, all the materials were reduced to the same 60% solids in acetone.

One effect that is clear, however, is that the peak viscosity ("water mountain") was higher as the molecular weight of the class of oligomers increased, no matter what the solubilizer (acid content) level was. Previous literature shows that these two variables cause the peak to move in opposite directions. (2,4) In the urethane series, the acid content was reduced as the molecular weight increased. In the maleinized oil series, the acid content and the molecular weight both increased while the viscosity peak increased. This was perhaps not unexpected, but concentration of the acid salts would also be expected to have a large effect on the height of the reduction curve.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

A second, interesting, observation was seen in comparing urethane oligomer #03 with maleinized oil oligomer #04. The molecular weights and acid contents were very similar and yet the peak viscosity of the lower polarity maleinized oil (#04), was significantly lower than that of the urethane oligomer (#03). The same effect was observed in the combination of oligomers #03 and #04 with the polyester (#10). These observations indicate that there may be significant effects due to polarity of the water reducible material. The more polar urethane caused a much higher increase in viscosity as the system was reduced with water than did the less polar linseed oil-based oligomer. This observation fits the postulate proposed by Hill and Brandenburger (1) that there is a swelling of the polar materials through association with water. The behavior of the polyester (#10) fits between those of the more polar urethane and less polar oil derivative (Table 4).

Relatively small differences in oligomer structure character can make significant differences in the reduction behavior. The urethane oligomers prepared from the aliphatic diisocyanate, IPDI, demonstrated much higher peak viscosities than those prepared from the aromatic diisocyanate, TDI (Table 4). In this comparison, the molecular weights and acid content of the oligomers were similar. This difference could be attributed to either differences in solubility in the organic solvent or in polar interaction with the water during reduction.

With the maleinized oils, as the molecular weights and the acid values increased, the peak viscosities increased substantially and the solids content at peak decreased.

CONCLUSION

Some interesting trends and significant differences were observed as the variables of polarity, molecular weight, and solubilizer content changed. This set of materials did not allow for interpretation of the significance of each of these effects independently. Clearly, the molecular weight trend is significant and expected. The more polar urethane materials would be expected to have more interaction with water through hydrogen bonding, which may explain the higher viscosities as the water content of the diluted materials increased. The much lower peak viscosity of the maleinized oils alone and in combination with the polyester should translate to an easier processing during dilution.

FUTURE WORK

Any future efforts on this project should attempt to fill in the matrix of materials that would represent variations in solubilizer content at similar molecular weights at several molecular weight levels in each class of oligomeric materials. The polyester (#10) could represent a mid-range polarity oligomer and a set of polyesters prepared with variations in molecular weight and solubilizer content. The molecular weights of the maleinized oils are higher than target values. Synthesis of lower molecular weight nonpolar oligomers would lead to more useful materials for this study. Efforts to describe oligomer effects in formulations with higher molecular weight binders need a higher molecular weight binder than the polyester prepared in this work.

Table 1 -- Synthesis of Urethane Oligomers

#01 B-T-D-T-B
#02 B-T-N-T-D-T-N-T-B
#03 B-T-N-T-N-T-D-T-N-T-N-T-B

Name                #01           #02           #03

TDI(T)               132.4         140.8          143.8
DMPA(D)               51.0          27.1           18.5
1-butyl alcohol(B)    56.4          30.0           20.4
NPG(N)                 /            42.1           57.3
Acetone              160           160            160
DBTDL                  6 droplet     6 droplet      6 droplet
Total                400           400            400
T:D:N:B                2:1:0:2       4:1:2:2        6:1:4:2
NV%
  Theory              60            60             60
  Actual              66.26         62.36          63.18
AV
  Theory              88.95         47.25          32.17
  Actual              82.62         40.29          29.79
NCO% remain            0.45          0.55           0.11
Mn                  1011          1954           2483
Mw                  1346          3613           5270
Polydispersity         1.33          1.85           2.12

Table 2 -- Synthesis of Maleinized Linseed Oils

Material               #04        #05     #06

Linseed oil              159.3    147.1    134.9
Maleic anhydride          17.5     22.7     28.0
2-ethyl-1-hexanol         23.2     30.2     37.1
Total                    200.0    200.0    200
Xylene                    10       10       10
Triethyl amine             0.2      0.2      0.2
Acid
  Theory                  50       65       80
Number
  Actual                  34.7     50.2     68.6
Nonvolatile (%)           87.23    91.0     88.68
VIS (cp, 20[degrees]C)   200      7400     6500
Mn                      2038     2262     1961
Mw                      5453     8196     7934
Polydispersity             2.67     3.62     4.04

Table 3 -- Polyester Preparation

Name                         W         %     Mol

Phthalic anhydride           51.6     20.66  1.00
Adipic acid                  50.9     20.38  1.00
1,4-cyclohexanedimethanol   130.7     52.26  2.60
Trimellitic anhydride        16.8      6.70  0.25
Total                       250.0    100.00  --
Theory water of reaction     18.8      7.53  --
Theoretical product         231.2     92.47  --
Triethyl amine               17.6      7.06  --
Acetone                      35.9     14.38  --
AV
  Theory                     42.3      --    --
  Actual                     47        --    --
Mn                         1588        --    --
Mw                         3040        --    --
Polydispersity                1.92     --    --

Table 4 -- Molecular Weight/Acid Content/Viscosity Comparisons

Urethane
Oligomers
Number              Mw/Mn    Acid #    Peak     %NV at  Visc. @ 30%NV
                                     Viscosity   Peak

  01              1300/1000    83         60      35          60
  02              3600/2000    40        650      45          20
  03              5300/2500    30       2200      45          50
  01 1            1800/1000    73        100      35         100
  02 1            2000/5300    41      17400      35        4200
  03 1            2300/5800    27      34500      40         225
Maleinized Oils
  04              5500/2000    35        130      45          20
  05              8200/2300    50        580      35         320
  06              8000/2000    69        800      30         140
  07             18000/2300    30        670      40          60
  08             33000/2500    40       4500      35        2050
  09             39000/2500    53      31500      30       31500
Polyester
  10              3000/1600    47        320      45          10
Combinations
  01 and 10                               80      45          40
  02 and 10                              525      45          30
  03 and 10                              550      45          70
  04 and 10                              200      50          10
  05 and 10                              250      45          30
  06 and 10                              400      45          70

ACKNOWLEDGMENTS

This work was supported in part by the EPA, National Risk Management Research Laboratory, and the Air Pollution and Prevention Control Division through the National Science Foundation Industry University Cooperative Research Center [NSF I/UCRC] in Coatings Research. Support for Shoubing Liu was also provided by the Personnel Department of Hunan Province, China.

References

(1) Hill, L.W. and Brandenburger, L.B., Prog. Org. Coat., 3, 361-379 (1975).

(2) Hill, L.W. and Richards, B.M., "Viscosity of Cosolvent/Water Solutions of Amine Neutralized Acrylic Copolymers," JOURNAL OF COATINGS TECHNOLOGY, 51, No. 654, 59 (1979).

(3) Hill, L.W. and Wicks, Z.W., Prog. Org. Coat., 8, 161-379 (1980).

(4) Wicks, Z.W., Anderson, E.A., and Culhane, W.J., "Morphology of Water-Soluble," JOURNAL OF COATINGS TECHNOLOGY, 54, No. 688, 57 (1982).

(5) Jones, F.N., "Toward Solventless Liquid Coatings," JOURNAL OF COATINGS TECHNOLOGY, 68, No. 825, 25 (1996).

(6) Barsotti, R.J., Hazan, I., and Neff, B.L., U.S. Patent 6,551,712 (April 22, 2003).

(7) Adamsons, K., Blackman, G., Gregorovich, B., Lin, L., and Matheson, R., Proc. of the XXIII Athens Conference on Organic Coatings, 23, 151 (1997).

(8) Blank, W.J., "Polyurethanes as Reactive Cosolvents in Waterborne Coatings," JOURNAL OF COATINGS TECHNOLOGY, 61, No. 777, 119 (1989).

J. David Nordstrom and Shoubing Liu--Coatings Research Institute, Eastern Michigan University*

*College of Technology, 430 W. Forest Ave., Ypsilanti, MI 48197.