1. INTRODUCTION:

A thermosetting resin is an adhesive that hardens or sets when heated and cannot be remolded¹. Resol-type phenol-formaldehyde (PF) and urea-formaldehyde (UF) are the most common thermosetting resins used in the wood composites industry. PF resins have a proved excellent performance in producing exterior-grade wood composites, whereas low-cost urea-formaldehyde (UF) resins have performed well in interior applications. Many efforts have been made by researchers to develop a new type of resin due to potential scarcity and high cost of phenolic resins rather than low cost, bad durability and water resistance of UF resins².

Most of the manufacturers of wood products generally make cost effective phenol-urea-formaldehyde (PUF) resin by mechanical blending of resol-type PF resin and UF resin, but it lacks effective co-condensation reaction between phenol and urea during resin manufacturing².

A common method to modify PF resin is the introduction of urea component at the time of resin preparation to synthesize phenol-urea-formaldehyde (PUF) resin, in order to reduce the production cost of PF resin and to introduce co-condensation reaction between phenol and urea^1-3. Several studies in the past have been carried out to prepare low-cost and high-performance PUF resins by changing the molar ratio of different components of PUF resin^2-6.

The objective of the present investigation was to compare the bonding strength of PUF resin synthesized by reacting methylol phenol with urea under acidic condition with commercial synthetic resins like PF, UF and PUF resin synthesized by mechanical blending of PF and UF resin in dry and wet conditions.

2. MATERIALS AND METHODS:

2.1 Materials:

Phenol (90%), formaldehyde aqueous solution (37%), sodium hydroxide (98%), formic acid (85%), sulphuric acid (98%) and commercial grade urea (46% N) were used as raw materials for the preparation of resins. Sal (Shorea robusta) veneers of 1.5 to 2 mm thickness were used as raw materials for making plywood to check the glue shear strength of the synthetic resins.

2.2 Methods:

Four types of synthetic resins viz. PF resin, UF resin, PUF resin by mechanical blending of PF & UF resins and PUF resin by reacting methylol phenol with urea under acidic condition were synthesized in the laboratory.

2.2.1 Synthesis of PF (Phenol-formaldehyde) resin:

The resol type PF resin was synthesized by taking the ratio of formaldehyde to phenol (F/P) as 1.2/1. For this procedure, 250 ml of phenol was charged into a round bottom flask followed by 300 ml of formalin. Stirring was continued throughout the reaction cycle. Required amount of NaOH (5% of the Phenol) was dissolved in twice amount of distilled water. The flask was kept in the boiling water to maintain the temperature range from 85 to 95⁰C and the condenser was kept on the mouth of the flask for 30 minutes. The reaction was stopped when the viscosity of the resin reached to 100 cP and then the resin was cooled at room temperature.

2.2.2 Synthesis of UF (Urea-formaldehyde) resin:

The UF resin was synthesized by taking the ratio of formaldehyde to urea (F/U) as 2.5/1. For this process, 300 gm of formalin was charged into a round bottom flask and pH of the formalin was raised to 7.2 to 7.5 by the addition of NaOH (32-35% solution). Then, 120 gm of urea was added and mixing was carried to get a clear solution. The solution was allowed to boil and the condenser was kept on the mouth of the flask for 20-30 minutes. The reaction was continued till the formation of dimethylol urea. After cooling, the solution was made acidic by addition of 10% of formic acid. Approximately, 25gm of excess water from the resin was removed by distillation till the viscosity of the resin was reached up to 400-500 cP. The resin syrup in the flask was again made alkaline by addition of 30-35% solution of NaOH to store at room temperature.

2.2.3 Synthesis of PUF resin by mechanical blending of PF and UF resin:

PUF resin was synthesized by mechanical blending of PF and UF resins in the ratio of 1:1 prepared by the above processes.

2.2.4 Synthesis of PUF resin by reacting methylol phenol with urea under acidic condition:

For the manufacturing of PUF resin by reacting methylol phenol with urea under acidic condition, the ratio of F/P/U was taken as 2.2/1/1. For this method, 220 ml of formalin was charged in the round bottom flask followed by 100 ml of phenol and stirring was continued throughout the reaction cycle. 11gm of NaOH (5% of the phenol) was dissolved in twice amount of distilled water. The flask was kept in the boiling water and the condenser was kept on the mouth of the flask. The reaction was continued at 90⁰C temperature for 30 minutes then it was cooled at room temperature and reaction pH was brought between 3.5 to 4.5 by adding 10% conc. H₂SO_4. Afterward, 100 gm of urea was dissolved into it and again kept at 90⁰C for 25 minutes till the viscosity was reached up to 100-200 cP. Finally, the resin was made alkaline by adding 30 - 32% of NaOH solution to store at room temperature.

2.2.5 Standard characteristics of synthetic resins:

Standard characteristic of synthetic resins like density, flow time, water tolerance, ash content, pH and solid content were evaluated according to the Bureau of Indian Standards.

2.2.6 Testing procedure:

Glue shear strength of plywood was tested in a suitable testing machine in dry and wet conditions by making 3-ply plywood of Shorea robusta veneers glued with each type of synthetic resin^7-8. Totally six samples from each plywood made from different types of synthetic resins were taken to conduct the test in dry and wet conditions. Average glue shear strength and average percentage of glue failure in dry and wet conditions were recorded for comparison.

Figure 1. Dimension of the test specimen of plywood

3. RESULTS AND DISCUSSION:

Four types of thermosetting resins were synthesized in the laboratory according to the technical requirements for the manufacturing of composite wood. The standard characteristic features of these synthetic resins are shown in table 1. The solid content of resins varied from 45 to 50 per cent which is prerequisite for composite wood manufacturing. The ash content of resin is an important parameter to evaluate the knife wear test, low ash content shows low knife wear test and vice-versa⁹. The ash content of resin should not be more than 4 per cent of the oven dry weight of the sample¹⁰. In the present study, the ash content of resins was less than 4 percent. The pH value of synthetic resins was maintained above 7 at room temperature to slow down the polymerization which increases the storage life of resins. Specific gravity of resin is an indication of crystallinity, molecular weight and the presence of voids in the polymers¹¹. In the present analysis, it was found that the specific gravity of resins was more or less same at room temperature. The water tolerance of a resin is an indication of the miscibility of the resin with water. The higher the water tolerance of the resin, the lower is the molecular weight of the resin. A low molecular weight resin has more polar end groups than a more condensed resin¹². It was found that the water tolerance of PF resin was the highest followed by UF resin and PUF resin synthesized by reacting methylol phenol with urea under acidic condition. The PUF resin synthesized by mechanical blending of PF and UF resins showed the lowest water tolerance. It may due to low co-condensation reaction between phenol and urea. The viscosity of synthetic resins was measured in terms of flow time in B4 cup which showed satisfactory results and it varied from 14 to 16 seconds.

Table 1. Standard characteristics of synthetic resins

Characteristics	Synthetic resins
Characteristics	PF	UF	Blending of PF+UF	PUF
Solid content (%) at 102°C, 5 hrs.	50	48.4	48	45
Ash content (%) at 700°C, 5 hrs.	2.3	2.1	2.6	2.9
pH at 25°C	9.5	8.5	9.0	8.5
Specific gravity at 25°C	1.06	1.09	1.05	1.08
Water tolerance at 25°C	4.9 times	3.8 times	0.7 times	2.7 times
Flow time in B4 cup (sec.) at 25°C	16	15	15	14

The comparison of average glue shear strength and glue failure of plywood samples bonded with different types of thermosetting resins in dry and wet conditions are shown in figure 2 and 3 respectively. In dry condition, average glue shear strength of plywood bonded with PF resin was 138 kg and in wet condition, after 8 hours of boiling the glue shear strength was 132 Kg. These values were much larger than the average standard value of BWR grade plywood¹³. The UF bonded plywood in dry condition met the requirement as per IS: 303-1989 and the average glue shear strength was 111Kg, whereas in wet condition after 3hrs of boiling the average glue shear strength was 73 Kg which was lower than the average standard value of MR grade plywood.

In dry condition, the average glue shear strength of plywood bonded with PUF resin synthesized by mechanical blending of PF and UF resin was less than UF bonded plywood, while in wet condition after 3 hrs heating at 60±2⁰C the glue shear strength was slightly more than UF bonded plywood. The glue failure of PUF resin synthesized by mechanical blending was more than UF resin in both dry and wet conditions which may be due to low bonding strength of PUF resin synthesized at room temperature². In dry condition, the average glue shear strength of plywood bonded with PUF resin synthesized by reacting methylol phenol with urea under acidic condition was slightly less than the UF bonded plywood, however in wet condition after heating at 60±2⁰C for 3hrs; the average glue shear strength was found more than UF resin and PUF resin synthesized by mechanical blending of PF and UF resins. Interestingly, the glue failure of the PUF resin was less than UF resin and mechanical blending of PF and UF resins in both dry and wet conditions.

After comparing the bonding strength of synthetic resins, it was found that PUF resin synthesized by reacting methylol phenol with urea under acidic condition performed better bonding strength than commercial UF resin and mechanical blending of PF and UF resins in both dry and wet conditions. It may be due to high co-condensation reaction between phenol and urea². The performance of PF resin was still better than PUF resin made by reacting methylol phenol with urea under acidic condition.

Figure 2. Comparison of average glue shear strength of test specimens composed of Shorea robusta veneers and different types of synthetic resin in dry and wet conditions

4. CONCLUSION:

The quality of PUF resin synthesized by reacting methylol phenol with urea under acidic condition was found to be intermediate to UF and PF resins and it showed better bonding strength than mechanical blending of PF and UF resins in both dry and wet conditions. Since phenol is costlier than urea therefore, by altering the molar ratio of phenol, urea and formaldehyde the quality of PUF resin can be improved to make it cost effective. It needs further study for better development.

Figure 3. Comparison of average glue failure of test specimens composed of Shorea robusta veneers and different types of synthetic resin in dry and wet conditions

5. ACKNOWLEDGEMENTS:

The present paper is a part of M.Sc. research work of the first author at the Forest Research Institute (Deemed) University, Dehradun. Facilities provided by the FRI (Deemed) University, Dehradun are gratefully acknowledged for carrying out the research work.

6. REFERENCES:

1. Sinha SK. Development of phenol-urea-formaldehyde (PUF) resin, M.Sc. dissertation, FRI Deemed University, Dehradun. 2006: pp.69.

2. Tomita B and Hse CY. Phenol-urea-formaldehyde (PUF) co-condensed wood adhesives. International Journal of Adhesion and Adhesives. 18; 1998: 69-79.

3. Fan DB, Li GY, Qin TF and Chu FX. Synthesis and structure characterization of phenol-urea-formaldehyde resins in presence of magnesium oxide as catalyst. Polymers. 6, 2014: 2221-2231.

4. Fan DB, Chu FX, Qin TF and LI JZ. Effect of synthesis conditions on the structure and curing characteristics of high-urea content PUF resin. Journal of Adhesion. 87; 2011: 1191-1203.

5. Pizzi A, Garcia R and Wang S. On the networking mechanisms of additives-accelerated phenol-formaldehyde polycondensates. Journal of Applied Polymer Science.66; 1997:255-266.

6. He GB and Riedl B. Phenol-urea-formaldehyde co-condensed resol resins: Their synthesis, curing kinetics and network properties. Journal of Applied Polymer Science. 41; 2003: 1929–1938.

7. Bureau of Indian Standards IS 1734 (Part 4). Methods of test for plywood: Part 4 Determination of glue shear strength.1983.

8. Bureau of Indian Standards IS 1734 (Part 6). Methods of test for plywood: Part 6 Determination of water resistance.1983.

9. Sahoo SC, Sill A and Pandey CN. A natural additive approaches to enhance the performance of formaldehyde based adhesive for plywood manufacturing. International Journal of Innovative Science and Modern Engineering. 3(1); 2014: 22-28.

10. Bureau of Indian Standards IS 1508. Specification for extenders for use in synthetic resin adhesive (urea-formaldehyde) for plywood.1972.

11. Anonymous. Synthetic resins technology handbook. Asia Pacific Business Press Inc., Delhi. 2005

12. Fink JK. Reactive polymers fundamentals and applications: A concise guide to industrial polymers. William Andrew, Inc., New York.2005.

13. Bureau of Indian Standards IS 303. Plywood for general purposes-specification.1989.

Received on 13.05.2015 Modified on 10.06.2015

Asian J. Research Chem. 8(7): July- 2015 ; Page 449-452

DOI: 10.5958/0974-4150.2015.00072.3