Synthesis of low cost adhesives from pulp & paper industry waste

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1 390 Journal of Scientific & Industrial Research J SCI IND RES VOL 69 MAY 2010 Vol. 69, May 2010, pp Synthesis of low cost adhesives from pulp & paper industry waste R K Gothwal*, M K Mohan and P Ghosh Birla Institute of Scientific Research, Statue Circle, Jaipur , India Received 07 December 2009; revised 01 February 2010; accepted 03 February 2010 This study presents lignin extraction from commercial black liquor and utilizes it as a partial substitute in phenol formaldehyde (PF) resin synthesis. Characteristics of modified resin [non volatile solid content (<44%), alkalinity (<5%), ph (<11.6) and free formaldehyde (<1%)] were found close to control PF resin with a variable viscosity ( cp). Shear test properties showed that phenol (15%) replacement with black liquor lignin produced comparable physical bond properties and could be used to formulate a low cost modified PF resin for plywood panels. Keywords: Black liquor, Lignin extraction, Phenol formaldehyde (PF) resin, Plywood, Wood adhesive Introduction In India, there are around 700 pulp and paper mills producing about 7.0 million tonnes of paper and board 1. Every tonne of pulp produces 1.25 tonnes of black liquor solids as waste 2 (72-75% organics, of which 50% is lignin). Therefore, lignin is produced in large quantities during chemical pulping process of wood for making paper and paper products 3,4. Abundance of different types of lignin as a waste product of paper industry has made it an alternative substrate for preparation of wood adhesives 5. Resins used to bind them are in general phenol-formaldehyde (PF) based adhesives 6. Considerable efforts have been made to substitute PF-based synthetic adhesive by lignin recycled from waste generated from pulping units 7. Bagasse lignin has been studied 8 for preparation of lignin phenol formaldehyde (LPF), which, as wood adhesive, has substituted up to 50% of phenol. Groundnut shell lignin (GNSL) has also been used as an effective phenol replacing agent in LPF wood adhesives 9. Lignin utilization for preparation of different kinds of wood adhesives is extensive but, due to technical and economical reasons, process of partial substitution of PF resins has not reached to an established industrial process 5. Although, agro-based lignin types have been substituted to a greater extent as compared to woody raw materials *Author for correspondence Tel: ; Fax: r_gothwal@yahoo.com but their consistent supply is limited and there are chances of wide variations in quality and types of lignin. Large pulping industries that utilize woody biomass produce lignin in large quantities as a waste product and thus a regular supply of low cost lignin could be maintained along with little variations in lignin types. This study presents extraction, characterization and formaldehyde reactivity of lignin extracted directly from commercial black liquor, and then feasibility as a partial substitute of phenol in wood based PF resins used to bond plywood. Experimental Procurement and Extraction of Lignin from Black Liquor Black liquor (total solids 70%, ph 11) was procured from Orient Paper Mill, Amlai (M P, India) after commercial Kraft pulping of bamboo (60%), eucalyptus (25%) and kneff (15%). Lignin was extracted by acid precipitation and also independently from bamboo and eucalyptus by Kraft delignification process in a laboratory digester (Suwik Eng), and used in further analysis. Characterization of Lignin Characterization of lignin was performed in terms of ash content, moisture content, ph, solubility in different solvents using standard laboratory protocols. Viscosity of lignin was determined by preparing a lignin solution (10% solids) in 0.5 N NaOH with a programmable viscometer (Brookfield Eng Labs Inc, USA) at 25 C.

2 GOTHWAL et al: SYNTHESIS OF LOW COST ADHESIVES FROM PULP & PAPER INDUSTRY WASTE 391 Table 1 Phenol formaldehyde resin synthesis procedure 14 Step Temp., ºC Material Weight, g Remarks A 25 HCHO (37-41 %) Loaded in a reaction vessel and agitated for 5 min Phenol (99.5 %) B 25 NaOH (40 %) 35.0 Caustic was added in three portions of ~12 ml each at every 5 min interval. C Temperature was increased to 85ºC in increments D Cooked up to desired viscosity at 85ºC E 50 NaOH (40%)Water Temperature was increased to 80ºC F Cooked up to desired viscosity G 65 NaOH (40%) 24.8 Caustic charging was done. Final viscosity should be around 600 cp H Agitated and cooled for storage Table 2 Lignin methylolation procedure Step Temp., ºC Material Weight, g Remarks A 80 WaterNaOH (40%) Loaded in a 250 ml glass beaker in a water bath maintained at 80ºC B 80 Lignin 10.2 Added with continuous stirring C 25 HCHO (37%) 25.0 Added after cooling to 25ºC D Cooked for 1½ h Formaldehyde reactivity of lignin was determined in a reaction kettle taking NaOH (20 g), water (158 g) and % formaldehyde (158 g) at 60 C and sampling was done at different time intervals 13 (0.5, 1, 2, 3 and 4 h). Amount of free formaldehyde in samples was determined by hydroxylamine hydrochloride method and recalculated as equivalents of formaldehyde reacted per 100 g of lignin. Finally, molecular weight of lignin was also identified by gel filtration chromatography (GFC) using Sephacryl S-200 (HR) matrix in a glass column (Pharmacia, 16 mm i.d. x 100 cm l) using standards dextran (MW 9,900, 66,900 and 4,94,000), tannic acid (MW 1,701) and acetone (MW 58.08). Acetone and tannic acid fractions were analyzed spectrophotometrically (280 nm), whereas dextran was monitored by estimating sugars by anthrone method released after complete hydrolysis using 0.1 N HCl. Fractions were eluted with 0.1 N NaOH (flow rate, 1 ml/min) and collected by a Fraction Collector (Gilson, Sweden). Resin Synthesis and Characterization A control (non-modified) PF resin and various lignin-modified PF resins were synthesized in a glass reaction kettle (1 l) equipped with a condenser, temperature control unit and agitator. Detailed procedure for control PF resin cook with a two stage caustic addition was adopted after some modifications 14 (Table 1). Lignin modified PF resins were also prepared in similar fashion as of control except it involved substituting required percentage of phenol (5, 10 and 15%) with lignin material (mass dry wt basis) in a prepolymer synthesis by adding an additional step of lignin methylolation (Table 2). Resins were analyzed in terms of non-volatile solids, free formaldehyde, alkalinity and ph using standard analytical procedures 15. Viscosity of resin samples was determined by a programmable viscometer in a small sample adapter module set at 25 C with spindle No. 18 at 60 rpm. Molecular weight of resin samples was identified by GFC using dextran, tannic acid and acetone as standards as stated above. Adhesive Formulation Formulation of adhesive was performed 16 using soft wheat flour as a proteinaceous extender and coconut shell flour as lignocellulosic filler (both were of commercial grade). In formulation of adhesive, water was used to wet extender and filler followed by addition of a dispersing agent (50% NaOH) to partially dissolve and swell lignocellulosic filler and proteinaceous extender. Prepared resin was added in two increments (first batch was added before addition of NaOH and second after NaOH dispersion).

3 392 J SCI IND RES VOL 69 MAY 2010 Table 3 Properties of lignin extracted from different lignocellulosic biomass Parameters Bamboo Eucalyptus Mixed wood lignin (Black liquor) Ash, % Moisture, % ph of 2 % solution Viscosity of 10 % solids, cp Particle size, µm Solubility at 25ºC, g/l a) Water b) Ethanol c) Methanol d) Acetone e) 0.5 N NaOH Molecular weight, Da Range molecular weight, % a) <100 Da b) Da c) Da d) >5000 Da Plywood Preparation and Testing Pre-conditioned (5-6% moisture) eucalyptus and semal veneers of 2.0 mm thickness were obtained from a commercial plywood industry, stored at 35 C and 25-30% relative humidity (RH) for 2-4 weeks in laboratory. Two days prior to gluing, veneer samples were cut into pieces (30 cm x 30 cm). Material for face (0.4 mm thick) was also obtained from same industry and was cut similarly. For making ply, a temperature controlled hot molding press (used in iron casting) was applied to generate a pressure (750 kpa). Adhesive was applied on veneer surfaces with a hand roller at 220 g/ m 2. All samples were first pressed at room temperature (RT) in between two iron plates for 10 min and then hot pressed for 20 min at 150 C and, finally cooled to RT. Prepared plywood panels were cut precisely into tension shear specimens in accordance with ISI standards 17 and analyzed for various properties. Glue shear strength was determined by using a Tensile Testing Machine (PSI Sales Pvt Ltd, New Delhi) in both dry as well as wet conditions (presoaked in boiling water bath for 72 h; Standard Methods, IS: 1734, Part 6). Wood failure in both cases was examined visually for the area under shear. Test for adhesion of piles was also performed according to ISI standards, in which a specific knife was inserted with cut ends of panels to grain of outer veneer and worked into. Results of this test are reported as excellent (E), pass (P) or fail (F). All experiments were conducted in triplicate for each experiment. Mean values of each experiment were calculated and presented. Results and Discussion A comparative analysis of properties of lignin from commercial black liquor, bamboo and eucalyptus indicate (Table 3) that all lignins are low in water solubility (<1.5%), moderately soluble in organic solvents (35 g/l in ethanol, 60 g/l in methanol and 200 g/l in acetone), but highly soluble in sodium hydroxide (220 g/l in case of eucalyptus). Determination of Molecular Weight Distribution of molecular weight fractions in lignin greatly affects end use performance of lignin, particularly for lignin substituted wood adhesives. Average molecular weight of lignins was found to be Da (Table 3). Lignin extracted from black liquor comparatively had higher molecular weight than that of eucalyptus. Black liquor lignin had more molecular weight fractions (>5000 Da wt%), which proved its applicability in synthesis of PF-modified resins. Lignin-Formaldehyde Reactivity For synthesis of phenol substituted PF resins, it is essential to achieve maximum reactivity of lignin with formaldehyde during methylolation stage in order to prevent these products from acting only as adhesive fillers 18. Therefore, formaldehyde reactivity of each

4 GOTHWAL et al: SYNTHESIS OF LOW COST ADHESIVES FROM PULP & PAPER INDUSTRY WASTE 393 lignin type was determined at 60 C in presence of NaOH for 4 h (Fig. 1). Reactivity of all lignin towards formaldehyde was found to increase from 0.5 to 4 h of reaction (Fig. 1). Resin Synthesis and Characterization Modified PF resins were synthesized using three different lignin types in various concentrations (5-15%; w/w) and checked for specific properties including nonvolatile solids, alkalinity, ph, free formaldehyde, viscosity and molecular weight distribution (Table 4). Moles HCHO reacted/100g lignin Bamboo lignin Eucalyptus lignin Mixed wood lignin Time, h Fig. 1 Formaldehyde reactivity with different lignin types at 60 C [NaOH, 20 g (50%); HCHO, moles; lignin, 50 g] Comparing molecular weight of control PF resin (9660 Da) with molecular weight of all modified resins ( Da), it has been observed that mixed wood lignin had > 50% fractions of >5000 Da at 15% substitution, which is higher than eucalyptus substituted lignin and slightly lower than bamboo substituted lignin. Non-volatile solid content of modified resins (42-44%) was found similar to that of control (43%). Also, alkalinity (5%) and ph (11.5) of modified resins were close to control (alkalinity, 4.5; ph, 11). Comparing with control (free formaldehyde, <1%; viscosity, 781 cp), free formaldehyde was estimated to be low (<1%) whereas, a variable viscosity values ( cp) were observed in all samples. Adhesive Formulation and Panel Testing PF based adhesives were prepared using a proteinaceous extender (soft wheat flour) and filler (coconut shell flour) in an alkaline dispersion type reaction. Constituents were mixed in presence of water without adding any hardening agent till a certain viscosity for a hand roller application and panels were prepared. Shear test results of plywood bonded panels with a control PF resin and lignin modified PF resins (Table 5) indicated that phenol substituted lignin (5-15%) extracted from black liquor yielded a better glue shear strength as compared to eucalyptus lignin. Glue shear strength was higher for PF modified phenol resin with black liquor lignin at substitution of 15% as compared to bamboo lignin. Table 4 Comparative analysis of control phenol formaldehyde resin and lignin modified phenol formaldehyde resins PF resin by lignin Range molecular weight, % Non- Alkalinity Free Viscosity volatile % ph formaldehyde cp type and with Molecular < >5000 solids, % % lignin substitution weight Control Bamboo 1) 5% ) 10% ) 15% Eucalyptus 1) 5% ) 10% ) 15% Mixed wood 1) 5% ) 10% ) 15%

5 394 J SCI IND RES VOL 69 MAY 2010 Table 5 Properties of plywood panels bonded with control and lignin modified phenol formaldehyde resins PF resin by Glue shear test lignin type and Dry Wet Knife with lignin Shear Wood Shear Wood test* substitution strength failure strength failure kpa % kpa % Control E Bamboo 1) 5% P 2) 10% P 3) 15% F Eucalyptus 1) 5% P 2) 10% P 3) 15% F Mixed wood 1) 5% P 2) 10% E 3) 15% P *E = Excellent, P = Pass, F = Fail PF control adhesive produced significantly higher wood failure than lignin adhesives. Although, lignin adhesives did not produce wood failures comparable to PF control adhesives, shear strength of lignin adhesives bond was found to be as good as that of control. After 72 h soaking in boiling water bath, plywood panels showed lower (10-15%) shear strength and wood failure in all cases but, strength retention after wet test was in general higher (60-90%), thereby indicating suitability of modified adhesives for plywood use where it is subjected to alternate drying and wetting at higher humid conditions. Plywood samples were also tested for adhesion by a knife test, in which a knife was inserted with its cutting edge parallel to grain of outer veneer and worked into, or if possible, along a glue line and prise upwards the veneer. Black liquor lignin substituted PF resin showed comparable results with control PF resin and is better than other modified PF resins (Table 5). Efficacy of lignin modified phenolic (LMP) resin have been evaluated by replacing phenol with lignin at different weight percentages and found that LMP resins are thermally more stable than pure phenolic resins 19. In LMP resins 20, replacement (up to 80%) of phenol with saw dust isolated lignin is capable of producing a bond quality acceptable for cold water resistant grade plywood. Khan et al 8 found that up to 50% of phenol can be substituted by bagasse lignin. Similarly, GNSL has also been used as an effective phenol replacing agent in LPF wood adhesives and could be substituted up to 50% with comparable mechanical properties 9. In present study, lignin from commercial source black liquor and from bamboo and eucalyptus, has been utilized to substitute phenol (up to 1 %) in PF resin formulation. Plywood panels were prepared with both lignins modified and non-modified PF resins and observed that control PF resin produced higher shear strength which, in general, decreases after substitution of phenol in the formulation. Similar type of reduction in wood failure was also observed after substitution with different lignin types. This reduction might be due to lower pressure generated by hot molding press used in present study for making panels and type of procedure followed. Overall, prepared panels were found to pass through required standard specifications in most of the cases. Conclusions Properties of lignin from black liquor were comparable with lignin from other raw materials. Physical properties of bonded panels showed that up to 15% phenol can be replaced with mixed wood lignin without adversely affecting bond properties. Lignin modified adhesives not only yielded good shear strengths and wood failure of panels to pass relevant standard specifications but also showed sufficient reactivity to yield panels with press times comparable to that of control PF adhesive. Overall, properties of lignin modified PF resin are comparable with control resin under test conditions and thus can be utilized for bonding plywood panels. Acknowledgements Authors acknowledge financial support from Indian Council of Forestry Research and Education, Govt of India, for carrying out this work. Authors also thank Sri S K Bansal, Balaji Industrial Prod Ltd and Sri M C Jain, Swastic Ply Board Pvt Ltd, Jaipur for providing raw materials and testing facilities and extending all help and cooperation in evaluation of adhesives. References 1 Jain R K, Dixit A K, Janbade V A & Mathur R M, Utilization of lignin as a source of industrial by-product to improve economics of agro based mills, IPPTA J, 21 (2009) Gupta A, Rajan N, Mathur R M & Kulkarni A G, Development of lignin by-products for industrial applications, IPPTA Convention Issue, (2001) Dolenko A J & Clarke M R, Resin binders from Kraft lignin, Forest Prod J, 28 (1978)

6 GOTHWAL et al: SYNTHESIS OF LOW COST ADHESIVES FROM PULP & PAPER INDUSTRY WASTE Cetin N S & Ozmen N, Studies on lignin-based adhesives for particleboard panels, Turk J Agric For, 27 (2003) El Mansouri N E, Pizzi A & Salvado J, Lignin-based wood panel adhesives without formaldehyde, Holz Roh Werkst, 65 (2007) Amaral-Labat G A, Pizzi A, Goncalves A R, Celzard A, Rigolet S & Rocha G J M, Environment-friendly soy flour-based resins without formaldehyde, J Appl Poly Sci, 108 (2008) Lei H, Pizzi A & Du G, Environmentally friendly mixed tannin/lignin wood resins, J Appl Poly Sci, 107 (2008) Khan M A, Ashraf S M & Malhotra V P, Development and characterization of a wood adhesive using bagasse lignin, Int J Adhesion Adhesives, 24 (2004) Khan M A & Ashraf S M, Development and characterization of groundnut shell lignin modified phenol formaldehyde wood adhesive, Indian J Chem Technol, 13 (2006) Feldman D, Lignin and its polyblends: a review, in Chemical Modification, Properties and Usages of Lignin, edited by T Q Hu (Kluwer Academics/Plenum Publishers, New York) 2002, Roffael E & Dix B, Lignin and lignin sulphonates in non-conventional bonding An overview, Holz Roh Werkst, 49 (1991) Shiraishi N, Recent progress in wood dissolution and adhesives from kraft lignin. ACS Sym Ser, 397 (1989) Wooten A L, Sellers Jr T & Tahir P M, Reaction of formaldehyde with lignin, For Prod J, 38 (1988) Gardner D J & Sellers Jr T, Formulation of a lignin-based plywood adhesive from steam-exploded mixed hardwood lignin, For Prod J, 36 (1986) Standard Analytical Procedures (Forest Products Laboratory, Mississippi State University, USA) 1989, CR-7, 21, 32, Sellers Jr T, Plywood and Adhesive Technology (Marcel Dekker, Inc., New York) 1985, IS: 1734 (Parts 1-20), Indian Standard Methods of Test for Plywood (Bureau of Indian Standards, New Delhi) Olivares M, Aceituno H, Neiman G, Rivera E & Sellers Jr T, Lignin-modified phenolic adhesives for bonding radiate pine plywood, For Prod J, 45 (1995) Sarkar S & Adhikari B, Lignin-modified phenolic resin: synthesis optimization, adhesive strength and thermal stability, J Adhesion Sci Technol, 14 (2000) Mittal M & Sharma C B, Studies on lignin-based adhesives for plywood panels, Poly Inter, 29 (1992) 7-8.