Properties of a low viscosity urea-formaldehyde resin prepared through a new synthetic route.

INTRODUCTION

Thermosetting polymers such as urea formaldehyde and melamine formaldehyde are the mostly widely used amino resins (Conner, 1996; Pizzi, et al., 2001 and Updegraff, 1990). 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). These disadvantages limit the rage of uses of amino resins.

Conner (1996) reported that the procedure for the synthesis of urea formaldehyde (UF) resins offers a wide range of conditions that 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. The considerable environmental interest for further reduction of formaldehyde has necessitated intensification of research into modification and improvement of properties of UF 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; Conner, 1996; Achi, 2003) 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.

In our previous work a novel synthetic route (one-step process, OSP) was used to develop and assess the potential of UF resin as a paint binder (Barminas and Osemeahon, 2006). To offer formulators with varied options to tailor performance, the effect of viscosity was examined and used to prepare a low UF resin for possible application in the coating industry.

MATERIALS AND METHODS

Resin Synthesis

The one-step-process used involved preparing trimethylol urea by reacting one mole (6.0 g) of urea with three moles (24.3 ml) of 37% (w/v) formaldehyde using 0.2 g of sodium dihydrogen phosphate (Chen, et al., 2004). 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 thermostatically controlled water bath at 70[degrees]C. The reaction was allowed to proceed for 2 h after which the sample was removed and kept at room temperature (30[degrees]C). The samples with different viscosities used in this experiment were obtained by removing 25 ml of resin from the synthesized resin at 24 h intervals for a period of 120 h and their viscosities determined.

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

Determination of Viscosity and Gel Point

The method reported by Barminas and Osemeahon (2006) was adopted for the determination of viscosity and gel point of the UF resins. Five different readings were taken for each sample and the average value calculated.

Determination of Density, Turbidity, Melting point and Refractive Index 

The above properties were determined according to standard methods (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 (Model, At400) weighing balance. Five readings were made for each sample and average value calculated. The turbidity of the resin samples was determined by using 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 indices of resin samples were determined with Abbe refractometer.

Determination of Moisture Uptake

The moisture uptakes of the different resin films were 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 intake by resin. Triplicate determinations were made for each sample and average value recorded.

Determination of Formaldehyde Emission

Formaldehyde emission test was performed by using the standard 2 h desiccator test as reported by Kim (2001). The evaluation of the absorbed formaldehyde by the 25 ml water was obtained from standard calibration curves derived from refractometric technique using Abbe refractometer.

Tensile Test

Tensile properties (tensile strength and elongation at break were measured as described by Wang and Gen, (2002) using Instron Testing Machine (Model 1026). In brief, resin films of dimension 50 mm long, 10 mm wide and 0.15 mm thick were brought to rupture at a clamp rate of 20 mm/min and a full load of 20 kg. A number of five runs were taken for each sample and the average elongation were taken and expressed as the percentage increase in length.

Dry Time and Water Solubility 

The relative degree of cure was expressed in the form of dry time (dry to touch). This was measured by the qualitative finger-marking test (Ali, et al., 2001). The solubility of methylol urea in water was obtained by mixing 1 ml of the resin with 5ml of distilled water at room temperature (30[degrees]C). A clear transparent solution indicated water solubility while a cloudy solution or white precipitate resulted in the case of insolubility in water.

RESULTS AND DISCUSSION

Melting Point

The melting point of a polymer has a direct bearing to its thermal property (Bindu, et al., 2001). It is related to its molecular weight, degree of crosslinking and the level of rigidity of the polymer (Park, et al., 2001). Figure 1 shows the effect of viscosity on the melting point of methylol urea. The melting point initially increased rapidly with viscosity and slowly approached an asymptotic value at the gel point. This type of behavior agrees with the reports of Ma, et al., (2002) and Markovic, et al., (2001) which was attributed to differences in molecular weight and cross-link density. At the beginning, the molecular weight increased with increases in viscosity until an optimum growth was obtained. At the gel point, the resin may be characterized by molecular rearrangement and crosslinking of resin molecules. This gives an account of the plateau regime seen in Figure 1(Nakason, et al., 2001).

[FIGURE 1 OMITTED]

Refractive Index

Gloss is an important factor of many coating products. The gloss of paint coatings with or without pigments is a function of refractive index of the surface, the angle of incidence of the beam of light, the nature of light and the nature of the material (Trezza and Krochta, 2001). Figure 2 shows the effect of viscosity on the refractive index of urea formaldehyde resin. It can be seen that the refractive index increased rapidly within 2-5 mPa.s viscosity levels. After this the rate decreased with further increase in viscosity of the resin. This observation is due to the differences in molecular weight and crosslink density among the different samples with different viscosities (Trezza and Krochta, 2001; Johnson and Wilke, 2001).

[FIGURE 2 OMITTED]

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). The effect of viscosity on density of urea formaldehyde is shown in Figure 3, where density increased with viscosity until the gel point. This was then followed by a constant regime with further increase viscosity. The increase in density with increase viscosity is due to increase in molecular weight while the constant regime may be attributed to gelation (Sekaran, et al., 2001).

[FIGURE 3 OMITTED]

Moisture Uptake

The interaction of structural network of polymer resins with water is both of fundamental and technical interest (Nogueria, et al., 2001). Water uptake affects vital properties of the polymer material such as the physical, mechanical, thermal and structural properties (Hu, et al., 2001 and Nogueria, et al., 2001). One of the major draw backs of urea formaldehyde resins is their poor water resistant (Conner, 1996). In the paint making industry, the moisture uptake of the paint binder is very crucial because it is responsible for blistering and broominess of paint film (Barminas and Osemeahon, 2006).

Figure 4 shows the effect of viscosity on the moisture uptake of urea formaldehyde resin. It can be observed that the moisture uptake decreases with increase in viscosity and then became constant beyond the gel point. This result can be explained in terms of the differences in crosslink density at different viscosity (Nogueria, et al., 2001 and Hu, et al., 2001). As the viscosity of the resin increases, the molecular weight and hence crosslink density also increases until the gel point is reached after which the crosslink density remain constant.

Formaldehyde emission

One of the major draw backs of urea formaldehyde resin is the emission of the hazardous formaldehyde during cure (Kim, 2001; El-Naggar, et al., 2003; Pizzi, et al., 2001). In the development of paint binder from urea formaldehyde resin serious effort must be made to reduce formaldehyde emission levels to an acceptable level (Barminas and Osemeahon, 2006).

Figure 5 shows the effect of viscosity on formaldehyde emission of urea formaldehyde. It can be observed that the formaldehyde emission increased with increase in viscosity. This result may be ascribed to two reasons: firstly, it may be due to increase in the rate of condensation reactions with increase in viscosity thereby increasing the rate of emission of formaldehyde in the process (Nakason, et al., 2001; Trumbo, et al., 2001 Liang and Wang, 2001). Secondly it may be due to increase in stress during resin cure with increase in viscosity.

[FIGURE 4 OMITTED]

Reduction in stress during resin cure reduces emission (El-Naggar, et al., 2003). Low viscosity gives rise to low molecular weight, which favors molecular chain mobility that enhances flexibility of polymer network; flexibility reduces stress during resin cure and reduction of stress reduces emission (El-Naggar, et al., 2003; Chain and Yi, 2001).

[FIGURE 5 OMITTED]

Tensile Test

The effect of viscosity on the tensile strength and elongation at break are shown in Table 1. The tensile strength decreased while the elongation at break increased with increase in viscosity. This trend is expected and might be due to increase in molecular weight and crosslink density with increase in viscosity (Tai and Li, 2001; Ma, et al., 2002). Viscosities of 2.07 and 3.04 (mPa.s) gave elongation at break of 124.61 and 121.64%, respectively. These values qualify these samples as being ductile having met the minimum requirement of 120% elongation at break (Tai and Li, 2001). This behavior is a typical characteristic for ductile polymers. However, beyond 3.04 (mPa.s) viscosities the polymers became brittle and lose its ductility.

Solubility in Water

In the development of amino resins for emulsion paint formulation, the solubility of the resin in water is paramount. It is important both from the technical and processing point of view. This is more so because the solubility of resins decreases with increase in viscosity (Lowel, 1990, Park, et al., 2001). Table 2 shows the effect of viscosity on the solubility of urea formaldehyde resin in water. Below a viscosity of 11.90 mPa.s, the resin is soluble and beyond this point the resin is insoluble. This result is attributed to differences in molecular weight and crosslink density (Lowel, 1990). Perhaps, the viscosity of 11.90 mP.s seems to represent the gel point of the resin. Thus processing of the amino resin for emulsion paint formulation could be suggested below this viscosity value.

Dry Time

The time it takes for a paint to dry after application is an important factor for the paint formulator. This is because if the paint dries too fast, it will be prone to brittleness and if it dries too slowly, the paint may be subjected to pickup dirt (Trumbo, et al., 2001). Table 3 shows the effect of viscosity on the dry time of urea formaldehyde resin. Results of the study showed that the dry time decreased with increase in viscosity. This can be explained in terms of increase in molecular weight and crosslink density with increase in viscosity (Lowel, 1990).

Table 4 compares some physical properties of low viscosity with those of the conventional urea formaldehyde resins synthesized by different authors. All the properties of the low viscosity urea-formaldehyde resins were much lower than those of the conventional types. These are positive results that could be used to solve the problems of poor water resistance, hardness, and formaldehyde emission associated with the traditional urea formaldehyde resins. The drastic reduction in formaldehyde emission signals the arrival of a better and an environmentally friendly urea formaldehyde resin. Hence low viscosity resin shows potential properties for possible applications in paint coatings.

Some physical properties of the low viscosity urea formaldehyde were compared with some paint binders. The viscosity of the synthesized resin was close to 3.11 mPa.s for rubber seed oil-modified alkyd resins (Aigbodion and Pilla ., 2002), but much lower than 38.00 mPa.s (Gawdzik and Matynia, 2002) and 45.00 mPa.s (Gawdzik et al., 2000) obtained for epoxy-based divinyl ester and epoxy fumerate resins, respectively. The refractive index of our resin (1.4080) is also similar to 1.4838 found for when protein isolate biopolymer (Trezza and Krochta, 2001).

The density of the resin, 0.532 g/[cm.sup.3] is lower than 1.04 g/[cm.sup.3] (Gawdzik and Matynia, 2002), 1.07 g/[cm.sup.3] (Gawdzik et al., 2000) and 0.96 g/[cm.sup.3] (Starostina et al., 2001) reported for epoxy-based divinyl ester, fumerate and aromatic amine-modified polyethylene resins, respectively. The melting point of 128[degrees]C is lower than values determined for resins such as epoxy-based divinyl ester; 197[degrees]C (Gawdzik and Matynia, 2002), styrene-modified epoxy-resin; 200[degrees]C (Yoon and McGrath, 2001), and aromatic amine-modified polyethylene; 133[degrees]C (Starostina et al., 2001). The melting point is however higher than 110[degrees]C and 101[degrees]C evaluated for epoxy fumerate resin (Gawdzik et al., 2000) and epoxy resins (Hu et al., 2001), respectively.

The moisture uptake (3.10 %) of the low viscosity amino resin is much lower than 8.6 % for copolymer latex (Wu et al., 2000) and 17.2% aqueous polyurethane (Lee and Kim, 2001). The elongation at break for the low viscosity resin was 124.6%, which is lower than 170% reported for glycidyl methacrylate and piperazin (Hong et al., 2001). This behavior may be relevant in modifying the creep behaviors of resin composites used in the coating industry.

CONCLUSION

This experiment examined the effect of viscosity on some physical properties of methylol urea synthesized through the OSP synthetic route. It shows that viscosity has a significant influence on the properties of methylol urea derived from OSP. Amino resins with viscosity less than 3.0 mPa.s may be a new class of amino resin composites for application in the coating industry. The properties of the resin when compared with properties of some conventional paint binders revealed that urea formaldehyde resin synthesized with low viscosity using the OSP synthetic route, might meet the requirements of the coating industry.

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S. A. Osemeahon and J.T. Barminas

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

Table 1: Effect of Viscosity on the Tensile Strength and Elongation at
break of methylol urea

Viscosity (mPa.s)     Tensile strength     Elongation at
                      (mN-[m.sup.-2])      break (%)

  2.07                 0.13                 124.61
  3.04                 0.16                 121.64
  4.85                 0.33                  40.83
 11.91                 2.4                   19.20
 85.65                 5.7                    4.45
124.74                10.5                    3.60

Table 2: Effect of Viscosity on the Solubility of Methylol urea in
water at room temperature (30[degrees]C)

Viscosity (mPa.s)     Solubility properties
                      in water

  2.07                Soluble
  3.04                Soluble
  4.85                Soluble
 11.91                Slightly Soluble
 85.65                Insoluble
124.74                Insoluble

Table 3: Effect of viscosity on the dry time of methylol urea

Viscosity (mPa.s)     Dry time (h)

  2.07                  48
  3.04                  36
  4.85                  24
 11.91                  12
 85.65                   6
124.74                   6

Table 4: Comparison of some physical properties of low viscosity urea
formaldehyde (2.07 mPa.s) with convectional urea formaldehyde

                             Low viscosity       Conventional
Property                     urea formaldehyde   urea formaldehyde

Moisture uptake (%)            3.1                18.0

Melting point ([degrees]C)   128                 200

Formaldehyde emission (ppm)    0.08                0.70

Density (g/[cm.sup.3])         1.0532              1.1764

Property                     Reference

Moisture uptake (%)          Ajayi, et al., 2005

Melting point ([degrees]C)   Ajayi, et al., 2005

Formaldehyde emission (ppm)  Kim, 2001

Density (g/[cm.sup.3])       Ajayi, et al., 2005