Physico-Mechanical and Thermal Properties of Thermoplastic Poly(Vinyl Alcohol) Modified Thermosetting Urea Formaldehyde Resin


 Urea formaldehyde (UF) resins are brittle and to improve their tensile properties poly(vinyl alcohol) (PVA) has been used to modify the UF resin. An easy improved procedure was developed to make PVA modified UF resin on the basis of conventional synthesis of UF resin. Prepolymer of UF was mixed with different weight percentages of PVA (1-5%) to synthesize modified UF resin which can be used to make adhesive for forest products. Both UF and modified UF resins were characterized by FTIR, physico-mechanical and thermal properties analyses. Modified UF resin containing 2 wt. % PVA exhibited better results than the UF.


INTRODUCTION
Urea formaldehyde (UF) resins are thermosetting polymers which are widely used in particleboard, medium density fiber board, interior plywood manufacturing, finishes and molded objects etc [1][2][3][4]. These are amino plastic resins which have a countless variety of condensed structures under various reaction conditions even though they are a chemical product from the reaction of two simple monomers: urea and formaldehyde [1]. They have many advantages, such as high reactivity, colorless glue line, good performance, easy processing and low cost raw materials [2][3][4]. UF resins are important as adhesive for wood industry during the past century due to their attractive properties at reasonable price, accounting for approximately 60% of the wood adhesive market [3]. However, they have drawback such as formaldehyde emissions and the brittleness of cured UF resin during the use of UF-bonded panels. These disadvantages of unmodified resins prevented them from achieving very wide usage which can be reduced by modification of UF resins [3][4]. Bio-oil modified UF resin showed improved performance by reducing formaldehyde emissions and improving thermal stability [3]. Polydimethylsiloxanes with different functional groups were Physico-mechanical and thermal properties of thermoplastic poly(vinyl alcohol) modified thermosetting urea formaldehyde resin adhesives were modified by adding various ratios of resorcinol to make an easy-cured and high-performance phenol-resorcinol-urea-formaldehyde (PRUF) resin adhesive for wood manufacturing [22]. Better internal bond strengths of particleboards above the requirements of the standard EN 312 were obtained by the incorporation of 10% of hydroxymethylated thick spent sulfite liquor (TSSLH) with UF resin [23]. Highly branched polyurea (HBPU) was synthesized to modify traditional low molar ratio UF resin and with the addition of HBPU, the formaldehyde emission level of the plywood panels was decreased by more than 30% [24].Thermoset epoxy resins were modified with thermoplastic polyethersulfone for highperformance applications [25]. Incorporation of plasticizers, like thermoplastic or rubbers to increase the toughness of thermosets is the most promising strategy [26].Thermoplastic PVA is a water soluble hydrophilic synthetic polymer [27]. Thermoset UF resin is also a waterbased polymer resin which has a wide range of raw materials [28]. Considering the miscibility and compatibility between water based thermoset UF resin and water based thermoplastic PVA resin, the present research was undertaken to investigate the effect of PVA resin on UF resin.
In this research, PVA modified UF resin has been synthesized in both liquid and solid form which can be used to make adhesive for plywood and wood products. To improve the tensile strength of the UF resin and to reduce the free formaldehyde content of UF resin, PVA was used as a modifier at prepolymer stage during the synthesis process of UF resin. FTIR, physico-mechanical and thermal characterization of the synthesized resins were investigated and reported.

Synthesis of urea formaldehyde resin by condensation polymerization method
The first reaction was carried out following conventional method [29]. Then the reaction parameters of this work were selected from the series of reactions carried out by observing reaction parameters variation effect on fixed molar ratio of urea and formaldehyde following viscosity and FTIR spectroscopic analyses. Sodium hydroxide solution (NaOH) (1N) was prepared. Starting material urea and formaldehyde (37%) were taken into a round bottom flask at 1:2 molar ratios. At first step, hydroxymethylation of urea was done under basic conditions by adding NaOH solution (1N) to make pH of 7.5-8. The reaction flask was fitted with a condenser and stirred for one hour at room temperature. In the second step, lactic acid solution (1N) was prepared. The reaction mixture in the flask was heated at around 98-100 o C. After 20 minutes, it was made acidic pH 5.5-6.5 by the addition of lactic acid (1N) and the condensation reaction was carried out for 4-5 hours. After completion of the reaction the reaction mixture was cooled and neutralized by adding sodium hydroxide (1N). Water was removed by vacuum distillation and then the solution was casted onto the glass petridish. It was dried in air first and then oven at 60 o C. The dried sheets were taken for further investigation.

Synthesis of PVA modified urea formaldehyde resin by condensation polymerization method
At first stage, hydroxymethylation of urea was done according to the same method mentioned above for UF resin synthesis. Aqueous solutions of PVA (1, 2, 3, 4 and 5 wt. %) on the weight of urea and formaldehyde were prepared separately. At the second stage, after 2 hours of condensation reaction PVA solution was added into the reaction mixture and the condensation reaction was carried out for 2-3 hours at 98-100 0 C. After completion of the reaction, the reaction mixture was cooled, neutralized and water was removed by vacuum distillation. The solution was casted onto the glass petridish and dried in air first and then oven at 60 o C. The resulted sheets were taken for further investigation. PVA modified UF resins were synthesized at different additions of PVA (1-5 wt. %) by following the same procedure described above. Flow chart of manufacturing process of UF and PVA modified UF is presented in Figure 1.

Determination of free formaldehyde content
Free formaldehyde content of synthesized resins was measured by sodium sulphite method [30]. Sodium sulphite solution was prepared and thymolphthalin indicator was added into it and then neutralized by 1 N HCL. Synthesized resins sample was added into the sodium sulphite solution and titrated with 1 N HCL until complete decolourization was obtained. The free formaldehyde (%) content was calculated from the formula given below: Free formaldehyde (%) =V. c. E /10.α Where V is the volume of HCL (ml), c is the concentration of HCL (mol/dm³), E is the equivalent weight of formaldehyde, and α is the weight of samples (gm).

Determination of physical properties
The densities and pH of the synthesized liquid polymers were measured at 24ºC using Density and Refractometer (DMA 5000, Anton Paar, Austria) and Jenway pH & conductivity meter, (model-3540, UK) respectively. Miscibility of synthesized liquid polymers in water Physico-mechanical and thermal properties of thermoplastic poly(vinyl alcohol) modified thermosetting urea formaldehyde resin were observed and presented in result and discussion section. Melting points of the synthesized dried solid polymers were determined using dried solid particles of polymer using Gallenkamp melting point apparatus, made in England. The viscosities of liquid polymers were determined using Rotational Viscometer (Anton Paar, Austria) at 24ºC and presented in result and discussion section.

FTIR spectroscopic characterization of the UF and modified UF resin
The infrared spectra of the UF, PVA and modified UF were recorded on a FT-IR/NIR Spectrometer (Frontier, Perkin Elmer, USA). The sample pellets for FTIR spectroscopy were prepared by dried powdered samples mixing with dried powdered potassium bromide (KBr) in a small agate mortar pestle. The mixture was taken in a die and pellet was made by applying vacuum pressure. FTIR spectra were obtained by printed form and presented in the result and discussion section.

Mechanical properties of the UF and modified UF resin
Mechanical properties of the dried sheet of UF and modified UF were performed by universal testing machine (model-Titan 5, James Heal, UK) for all specimens. Tests were performed according to ASTM D 882-02, (Standard Test Method for Tensile Properties of Thin Plastic Sheeting). The speed was 10 mm/min. The load vs elongation curves were obtained from the instrument and the tensile strength (σUT) is calculated from the formula below: Tensile strength, σUT = W / AT Where W is breaking load and AT is the cross sectional area.

Thermal properties of the UF and modified UF resin
Simultaneous thermal analysis (STA) of the dried UF and modified UF containing 2 wt% PVA were carried out using STA machine (model no-F3 Jupiter, NETZSCH, Germany). The temperature range was 30-600 0 C, at a heating rate of 10 0 C/ min under a constant nitrogen flow rate of 80 ml/min.

Synthesis of urea formaldehyde resin
In this work, UF resin was synthesized to make modification on it. The molar ratio of U/F was 0.5 and hydroxymethylation of urea occurred at the first stage. The first stage of condensation polymerization was to prepare monomethylol urea and dimethylol urea by hydroxymethylation of urea as illustrated below. The condensation reaction of monomethylol urea and dimethylol urea was carried out to prepare urea formaldehyde resin at 98-100 0 C for 4-5 hours as illustrated below. Reaction parameters time, temperature and pH were selected by observing the results of these parameters variation effect on fixed molar ratio of urea and formaldehyde.

FTIR spectroscopic characterization of UF resin
The synthesized UF resin was characterized by infrared spectroscopic analysis. The FTIR spectrum of UF resin is presented in figure 2 (middle part) and described in the table 1. The IR spectrum shows characteristic bands of N-H stretching at the region of 3316.40 cm -1 . The characteristic bands at 1631.14 cm -1 and 1542.31 cm -1 are due to the C=O stretching and N-H bending vibrations. The spectrum is almost similar to the FTIR spectra of the cured UF resin [1,12,30].

Synthesis of PVA modified UF resin
The modification of UF resin was done during the second stage of UF resin synthesis after 2 hours. PVA was added during the condensation reaction and at this stage prepolymer of UF was reacted with PVA. The condensation reaction between prepolymer of UF and PVA was continued for 2-3 hours to prepare PVA modified UF resin as illustrated below. Modification of UF resins were carried out at different additions of PVA (1-5 wt %) to find out the effect of addition amount of PVA on the properties of resins and the selected addition of PVA was found as 2 wt% by formaldehyde emission and tensile strength. The optimal addition amount of almond shells in UF resin is found as 3 wt% by formaldehyde emission and wet shear strength [13]. in PVA modified UF resin than UF resin which may be due to changes in the polarity of bonds. These observations indicated that structural changes occurred in modified UF resin and PVA structures were introduced into UF resin during condensation reaction.   [3,30].

Mechanical properties of UF and PVA modified UF resin
Mechanical properties such as tensile strength and elongation at break (%) of all synthesized polymers were conducted by UTM. The tensile strength of the UF and modified UF resin with 1-5 wt% PVA are presented in figure 3. Tensile strength of 100% UF resin and 100% PVA is 0.13 MPa and 0.17 MPa respectively. It is found that tensile strengths of the UF resin increase with its modification with PVA. It is also found that the tensile strength of the modified UF is higher at 2 wt% PVA loading and then decreases with increasing PVA content. Similar trend of tensile strength was observed in modified melamine formaldehyde resin and modified epoxy resin [31][32]. The addition of PVA improved the tensile strength performance of the PVA modified UF resin due to the form of physical crosslinking between the resin with flexible chain of PVA, so PVA shared part of the stress uniformly when subjected to external forces that increased the tensile strength of modified resin. At higher loading of PVA after 2 wt%, the tensile strength of modified resin decreases with increasing of PVA due to critical content of PVA. This critical loading is mainly because at higher loading the distance between PVA becomes so small that they tend to stack together due to the van der Waals force that leading to non-uniform stress distribution between the resin and PVA [33]. Elongation at break of the UF resin and modified UF resin are presented in figure  4. Elongation at break of 100% UF resin is 271.69% and 100% PVA is 16.92%. It is observed from the figure that elongation at break of 100% UF resin decreases than modified UF resin. It is also observed that elongation at break of modified UF resin increases with increasing PVA content. The toughness or ductility of the modified UF resins decreases than UF resin caused by uncured state of UF. It is also observed that toughness or ductility property of modified UF resin increases with increasing of PVA content. UF resin was significantly toughened by polyurethane (PU) addition and the elongation at break of PU modified UF resin increased with increasing PU content [34].

Thermal properties of UF resin and PVA modified UF resin
Thermal analyses of UF resin and modified UF resin having 2 wt. % PVA were conducted by STA and the results are presented in the figure 5-6 and table 5 respectively. The thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) results of UF resin and modified UF resin are expressed in table 5. Temperature of initial mass loss (Ti o C) and temperature of maximum mass loss (Tmax o C) with percentages of mass loss and percentage of residue with temperature are shown in table 5. From this comparative observation it is clear that modified UF is thermally more stable than UF. Almost similar TGA is found for neat UF resin but modification of UF with calcium carbonate has no significant effect on the thermal stability of UF resins [14].
The main characteristics parameters such as glass transition temperature (Tg) and melting point (Tm) of UF and modified UF is obtained from DSC analysis (figure 5-6). It is found from the figure 5-6 that the first physically change at differential peak temperature (DTp1) and second physically change at differential peak temperature (DTp2) are almost same for both UF and modified UF resins (

SUMMARY
Synthesis of UF resin and its modification by PVA was successfully carried out. FTIR spectroscopic analyses showed the evidence of the structure of PVA modified UF resin. The amount of free formaldehyde of PVA modified UF resin decreased than neat UF resin and it also decreases with increasing of PVA content in the UF resin.Tensile strength of modified UF resin is higher than that of unmodified UF resin and the highest tensile strength obtained for 2 wt% PVA loaded UF resin. So it can be concluded that addition of PVA can strengthen UF resin by improving its tensile strength and reducing brittleness. Thermo gravimetric analysis also indicated that PVA modified UF is thermally more stable than unmodified UF resin. Therefore, PVA is a potential modifier for UF resin to improve its performance. So, it can be said that PVA modified UF resin will improve the performance of this resin for use as adhesive for plywood and for wood products.