A new synthetic route for amino resins for paint formulation: effect of sodium dihydrogen phosphate (NaHP) as catalyst.

INTRODUCTION

Thermosetting polymers such as urea formaldehyde and melamine formaldehyde are the mostly widely used amino resins (Conner, 1996) in many indoor applications. However, the acceptance of amino resins as a universal material in many engineering areas such as in the coating industry is impeded by some of its inherent qualities such as brittleness, poor water resistance and formaldehyde emission (Lowel, 1990; Conner, 1996). Conner (1996) reported that the procedure for the synthesis of urea formaldehyde resins offers a wide range of conditions which make the synthesis of these resins with important properties such as gel time, tack and spreadibility of the uncured resin possible. Also formaldehyde emissions and the durability of the cured resin can be controlled and specifically tailored for the final end use of the resins.

At present, the commercial production of urea formaldehyde resins is carried out in two major steps (Conner, 1996). The first step is characterized by methylation reaction in the absence of condensation reactions under basic condition. In the second step, the reaction mixture is brought to acidic side with a pH of about 5 with the condensation reaction being carried out until a desired viscosity is reached. Various reports (Lowel, 1990 and Conner, 1996) have shown that the resins from the above synthetic route are too hard and brittle to be used as a paint binder. This has created interest in an alternative synthetic route that can produce a resin with appropriate properties for application in a coating industry.

Our previous research work evaluated the effect of pH on a one-step-process (OSP) used as a new synthetic route for developing amino resins for paint formulations (Barminas and Osemeahon, 2006). To offer formulators with myriad options to tailor performance, the effect of catalyst will be examined so as to optimize the conditions for the new route.

MATERIALS AND METHODS

Materials

Urea, formaldehyde, sodium dihydrogen phosphate, Sulphuric acid, sodium hydroxide pellets and sucrose were obtained from BDH.

Resin Synthesis

Trimethylol urea was prepared by reacting one mole (6.0g) of urea with three moles (24.3ml) of 37% (w/v) formaldehyde using 0.2g of sodium dihydrogen phosphate (NaHP) (Chen, et al., 2001). The pH of the solution was adjusted to 6 using 0.5 M [H.sub.2] S[O.sub.4] and 1.0 M NaOH solutions. The solution was then heated in a thermostatically controlled water bath at 70[degrees]C. The reaction was allowed to proceed for 2h after which the sample was removed and kept at room temperature (30[degrees]C). The above procedure was repeated at different catalyst concentrations (0.2 - 1.0g). The physical properties of resins were determined at gel point.

Film Preparation

Films of the different resins obtained at various catalyst concentrations were cast on petri dishes by using solution casting method (Mirmohseni and Hassanzadeh, 2000). The resins were then allowed to cure and set for seven days at 30[degrees]C. The physical properties of these films were investigated.

Determination of Viscosity

A 100ml Phywe made graduated glass macrosgringe was utilized for the measurement of viscosity (Barminas and Osemeahon, 2006). The apparatus was standardized with 20% (w/v) sucrose solution whose viscosity is 2.0 x [10.sup.-3] [Nsm.sup.-2] at 30[degrees]C (Lewis, 1987). The viscosity of the resin was evaluated in relation to that of the standard sucrose solution at 30[degrees]C. Five different readings were taken for each sample and the average value calculated. The gel point of the different resins obtained at the different catalyst concentrations were determined by monitoring the viscosity of resins with time until a constant viscosity profile was obtained (Vilas, et al., 2000).

Determination of Density, Turbidity, Melting point and Refractive Index 

The above properties were determined according to AOAC, (2000). The density of the different resins was determined by taking the weight of a known volume of resin inside a density bottle using Metler AT400 weighing balance. The turbidity of resin samples were measured using a Hanna microprocessor turbidity meter Model H193703). The melting point of the different film samples were determined by using Galenkamp melting point apparatus (Model MFB600-010F). The refractive index of resin samples were obtained by using Abbe refractometer.

Determination of Moisture Uptake

The moisture uptake of the different resin films was determined gravimetrically. Known weights of each of the samples were introduced into a desiccator containing a saturated solution of sodium chloride. The wet weights of each sample were then monitored until a constant weight was obtained. The differences between the wet weight and dry weight of each sample were then recorded as the moisture uptake by resin. Triplicate samples were used for each measurement and average value taken.

Determination of Formaldehyde Emission

Formaldehyde emission test was performed by using the standard 2h desiccator test reported by Kim, (2001). The evaluation of the absorbed formaldehyde by the 25.0ml water was obtained from standard calibration curves derived from refractometric technique using Abbe refractometer.

A mold made from aluminum foil with a dimension of 69.6mm x 126.5mm (Kim, 2001) and thickness of 1.2mm (Wang and Gen, 2002) was used in this experiment. Triplicate samples were used for each determination and the average value taken.

RESULT AND DISCUSSIONS

Gel Viscosity and Gel Time

From our previous study the entire process of polymer network formation can be divided into two parts separated by the point of gel formation (Barminas and Osemeahon, 2006). In the first phase, before the gel point, the reaction mixture showed remarkable viscosity changes. A great increase in the modulus which occurred after the gel point might be the formation of a rubbery or glassy state, characteristic of the second phase. Hence the formation of a polymer network can be evaluated from dynamic measurements by monitoring rheological parameters such as viscosity, storage modulus etc., as function of time (Markovic, et al., 2001).

Rheological characteristics of polymer resins are important both for the processing and final product quality evaluation (Petrovan, et al., 2000). Reheological properties such as the viscosity and the dynamic moduli can be directly correlated to the envolving physical and mechanical properties during resin cure (Hu, et al., 2001). Viscosity of a paint binder affects flow properties such as leveling and sagging (Lowel, 1990). Viscosity also affects fire resistance, drying rate of a paint film and its thermal and mechanical properties (Duguesne, et al., 2001). Film forming property, adhesion and gel point of paints are functions of viscosity (Barminas and Osemeahon, 2006). Thus an understanding of the various synthetic parameters such as the amount of catalyst on the viscosity of a paint binder is not only important but crucial.

[FIGURE 1 OMITTED]

Figure 1 shows the effect of NaHP on the gel viscosity of methylol urea. Viscosity was found to increase with increase in NaHP concentration. This result may be attributed to increase in molecular weight and crosslink density (Markovic, et al., 2001 and Park, et al., 2001). Increase in NaHP concentration gave rise to an increase in the degree of methylolation which in turn increase the level of condensation reactions, molecular weight and hence the viscosity build up (Park, et al., 2001).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Figure 2, showed a decrease in gel time with increase in concentration of the catalyst due to the increase in reaction rate. Therefore resin obtained may cure faster when developed with higher catalyst concentration. This result agrees with similar work on phenol-formaldehyde resol resin (Park, et al., 2001).

Refractive Index

Paint coatings are known to have varying degrees of opacity or transparency depending on the amount of light transmitted through or reflected from the surface of the paint. Therefore the gloss of the paint coatings is a function of refractive index of the surface and particle size (Trezza and Krochta, 2001). Figure 3 represents the effect of temperature on the refractive index of methylol urea resins. The refractive index increases with increased in NaHP concentration which may be attributed to the differences in molecular weight in the resin obtained with different concentration of catalyst (Trezza and Krochta, 2001). This study therefore demonstrates that gloss can be increased by increasing the concentration of the catalyst in resin composites in paint formulations.

Density

In the coating industry, the density of the paint binder has a profound influence on factors such as pigment dispersion, brushability of paint, flow, leveling and sagging (Lowel, 1990). Figure 4 shows the influence of catalyst on the density of methylol urea. The density decreased with increase in NaHP concentration. This phenomenon may be attributed to the packing nature of the resin molecules (Chain and Yi, 2001). Density depends on free volume and packing efficiency of molecular chains. The reduction in density with increase in catalyst concentration indicates inefficient molecular chain packing (Chain and Yi, 2001).

[FIGURE 4 OMITTED]

Melting Point

Figure 5 shows the effect of NaHP on the melting point of methylol urea. The figure showed a gradual increase in melting point with increase in NaHP concentration. This result agrees with earlier report on the studies of thermal behavior of phenol-formaldehyde resol resins Park, et al., (2001). The increase in methylolation rate which lead to increase in molecular weight and cross link density was explained to be responsible for this observation (Park, et al., 2001).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Moisture Uptake

Water uptake affects vital properties of polymer materials such as the physical, thermal and structural properties (Hu, et al., 2001). In the coating industry, the moisture uptake of the binder is very crucial because it is responsible for blistering and brominess of paint film. Figure 6 shows the effect of NaHP on the moisture uptake of methylol urea. The moisture uptake recorded increased with increase in NaHP concentration. This trend may be due to differences in the chain topology which is related to the molecular size holes in the polymer structure which also depends on morphology and crosslink density (Kim, 2001). Hence the formulation of the resin with concentrations below 0.2g could be useful in coatings where low water uptake is required.

Formaldehyde Emission

One of the major drawbacks of urea formaldehyde resins is the emission of the hazardous formaldehyde during cure (Kim, 2001). In the development of paint binder from urea formaldehyde therefore, serious efforts must be made to reduce formaldehyde emission levels. The effect of NaHP on formaldehyde emission is shown is Figure 7. There was a gradual increase in emission level with increase in NaHP concentration. This result can be attributed to increase in the rate of condensation reactions with increase in the concentration of NaHP catalyst (Park, et al., 2001). At high concentration of NaHP, the rate of condensation reactions may be high leading to increase in formaldehyde emission. We therefore suggest that use of lower concentration of NaHP catalyst (<0.20) be encouraged so as to reduce the volume of formaldehyde emitted into the environment when the resin is used in paint formulation.

[FIGURE 7 OMITTED]

CONCLUSION

This experiment studies the effect of NaHP catalyst on some physical properties of methylol urea synthesized through the OSP. The result revealed that catalyst has a significant influence on the physical properties of methylol urea synthesized through the new OSP synthetic route. Low amount of NaHP ([less than or equal to] 2.0g) per three moles and one mole of formaldehyde and urea respectively) may be optimal to produce a new class of amino resin composites for application in the coating industry with minimum amount of moisture uptake, hardness and formaldehyde emission level. The result of this experiment therefore, will contribute towards optimization of the new synthetic route.

REFERENCES

(1.) AOAC. 2000. Official Method of Analysis International (Horwitz, W., Editor). Gaithersburg Mongland, USA (17th edition) Vol. 1,

(2.) Barminas, J. T. and Osemeahon, S. A. 2006. European Journal of Scientific Research. (in press).

(3.) Chain, K. S. and Yi, S. 2001. J. Appl. Polym. Sci., 82: 879-888.

(4.) Chen, C., Lue, J. and Yen, C. 2001. J. Appl. Polym. Sci., 83: 1242-1251.

(5.) Conner, H. A. 1996. Urea--Formaldehyde Adhesive Resins. In: Encyclopedia of Polymeric Material (ed Joseph C., Salamone, J., Clapol A., Demby A. and M. Aller), Vol. 2, pp. 8495-8500.

(6.) Duquesne, S., Bras, M. L. Bourbigot, S., Delobel, R., Camino, G., Eling, B., Lindsay, C., Roels T. and Veszin H. 2001. J. Appl. Polym. Sci., 82: 3262-3274.

(7.) Hu, X., Fan, J. and Yue, C. Y. 2001. J. Appl. Polym. Sci., 80: 2437-2445.

(8.) Kim, G. M. 2001. J. Appl. Polym. Sci., 809: 2800-2814.

(9.) Lewis, M. J. 1987. Physical Properties of Food and Food Processing Systems. Camelot Press, Southampton Great Britain :111.

(10.) Lowel, H. J. 1987. Coating. In: Encyclopedian of Polymer Science and Engineering (Ed. Jacqueline J. Kroschwitz. H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges) Vol. 8

(11.) Markovic, S., Dunfic B., Zlatanic, A. and Djon, L. J. 2001. J. Appl. Polym. Sci., 79: 1062 -1065.

(12.) Mirmohseni, A. and Hassazadeh, V. 2001. J. Appl. Polym. Sci., 79 :1062-1065.

(13.) Park, S., Seo, M. and Lee, J. 2001. J. Appl. Polym. Sci., 79 : 2229-2308.

(14.) Petrovan, S., Collier, J. R. and Negulescu, J. I. 2000. J. Appl. Polym. Sci., 79:396-405.

(15.) Trezza, A. T. and Krochta, J. M. 2001. J. Appl. Polym. Sci., 79: 2221-2229.

(16.) Wang, H. and Gen, C. 2002. J. Appl. Polym. Sci., 84: 797-805.

(17.) Vilas, L. J., Laza, J. M., Garay, M. T., Rodrigue, M. Leon, L. M. 2000. J. Appl. Polym. Sci., 79: 447 457.

S.A. Osemeahon and J.T. Barminas

Department of Chemistry, Federal University of Technology, Yola, P. M. B. 2076 Yola, Nigeria * email:[e-mail]