Addition of kraft lignin of Eucalyptus sp. on the properties of the lignin-phenol-formaldehyde adhesive

Influência da adição de lignina kraft de Eucalyptus sp. sobre as propriedades do adesivo lignina-fenol-formaldeído

ABSTRACT:

The objective of this research was to extract lignin from Eucalyptus sp. kraft black liquor and to evaluate the influence of its addition on the properties of the lignin-phenol-formaldehyde adhesive. The adhesives were synthesized at different phenol mass percentages by lignin, called lignin-phenol-formaldehyde adhesives in the proportions of 25, 30, 50, 75 and 80%. The parameters evaluated in the adhesives were pH, total solid content, organic compound content, inorganic compound content, gelatinization time and adhesive behavior in the glue line. The pH ranged from 10.63 to 11.65. For the total solid content was found the value of 53 to 69 %. The contents of organic compounds ranged from 51 to 75% and inorganic from 25 to 49%. It was observed that with the addition of macromolecule in the adhesive formulation, the total and organic solid content was reduced proportionally while the inorganic solid was high. The gelatinization time was reduced due to the addition of lignin. The lower proportions of addition presented better penetration and fluidity in interface with the wood. The kraft lignin extracted from eucalyptus kraft black liquor is a great adhesive with potential to partially replace petroleum-derived phenol, with great potential for use and application in the aeronautical and automotive sectors, in the bonding process of structural parts.

Key words:
black liquor; timber industry; phenolic resin

RESUMO:

O objetivo deste trabalho foi extrair lignina de Eucalyptus sp. licor negro kraft e avaliar a influência de sua adição nas propriedades do adesivo lignina-fenol-formaldeído. Os adesivos foram sintetizados em diferentes porcentagens de massa de fenol por lignina, denominados adesivos lignina-fenol-formaldeído nas proporções de 25, 30, 50, 75 e 80%. Os parâmetros avaliados nos adesivos foram pH, teor de sólidos totais, teor de compostos orgânicos, teor de compostos inorgânicos, tempo de gelatinização e comportamento do adesivo na linha de cola. O pH variou de 10,63 a 11,65. Para o teor de sólidos totais foi encontrado o valor de 53 a 69%. Os teores de compostos orgânicos variaram de 51 a 75% e de inorgânicos de 25 a 49%. Observou-se que com a adição da macromolécula na formulação do adesivo, o teor de sólidos totais e orgânicos foi reduzido proporcionalmente enquanto o de sólidos inorgânicos foi elevado. O tempo de gelatinização foi reduzido devido à adição de lignina. As menores proporções de adição apresentaram melhor penetração e fluidez na interface com a madeira. A lignina kraft extraída do licor negro kraft de eucalipto é um ótimo adesivo com potencial para substituir parcialmente o fenol derivado do petróleo, com grande potencial para uso e aplicação nos setores aeronáutico e automotivo, no processo de colagem de peças estruturais.

Palavras-chave:
licor negro; indústria madeireira; resina fenólica

INTRODUCTION

Lignin is one of the most abundant polymers in the globe, the biological activity produces about 150 billion tons / year, it is estimated that 0.082% of solar energy received on Earth is stored in the lignin matrix. This important energy reserve has the potential for a range of high added value byproducts. However, it is basically used for power generation (VITAL et al., 2013).

The pulp and paper industry can add to its industrial plant the generation of by-products from the separation of lignin from cooking liquor, so that the extracted lignin in addition to generating heat and energy would have a range of specific new chemicals (BELGACEM et al., 2003). The extraction of lignin from black liquor also has the advantage of reducing the amount of fossil fuels used in the lime kiln (ARKELL et al., 2014). In this context, industry by-products have received special attention because they optimize resources and safeguard some environmental issues.

Sustainable technologies are the new alternatives for generating products with lower environmental impact. Green chemistry implies in the development of chemical processes and products that lead to a cleaner, healthier and more sustainable environment (DE MARCO, 2018).

Urea-formaldehyde and phenol-formaldehyde phenolic resins dominate the wood adhesives industry. However, formaldehyde is considered one of the main pollutants. Formaldehyde emissions and worker exposure during manufacturing is a major environmental and health safety concern. Significant effort has been made to develop natural adhesives to reduce or replace phenol and part of the formaldehyde content (YU et al., 2018; WANG et al., 2008).

Phenolic rings present in the complex structure of lignin allow its use as a partial substituent of phenol, which is employed in phenolic resin synthesis, since lignin can react with formaldehyde (PAIVA et al., 2000 apud TITA, 2002).

Research on lignin-based wood adhesives has had little practical success; although, it offers advantages over conventional adhesives. One issue that seems to have hampered research regarding the use of lignin with binder was the lack of reproducibility and characterization of lignin. This presents a barrier to commercial development, as different mixtures and source of lignin will lead to products with heterogeneous properties (HAN et al., 2018; WANG et al., 2019).

With advances in genetic improvement and cloning, research around lignin for use as a green product has the potential to leverage again. Among the researches in this area, the ones that seek the application of natural resources in the preparation of the materials have been growing, and the use of kraft lignin can be highlighted. Thusobjective of this research to observed the influence of the addition of kraft lignin on the properties of lignin-phenol-formaldehyde adhesive (ASTM, 1993).

MATERIALS AND METHODS

Obtaining and characterization of kraft black liquor

The kraft black liquor was fed by cooking Eucalyptus sp. wood using the 7 liter capacity M & K digester with forced circulation and heat exchange. For a physical characterization of black liquor, determined the total solids content according to TAPPI Methods - T650 om-89. Organic and inorganic solids were determined according to TAPPI Methods - T625 cm-85.

Kraft lignin extraction

The extraction of lignin present in black liquor followed the methodology described by Öhman et al. (2008), with adaptation in the concentration of black liquor. A 15% black liquor solution was prepared and concentrated to 27% by evaporation. CO₂ (carbonation) was added for 15 minutes at 70 °C to pH 8.7, then acidified with 20% v / v H₂SO₄ to pH 2. The precipitated lignin was filtered and washed with 80 °C water previously acidified with H₂SO₄ to pH 2 and then with distilled water at 80 °C, centrifuged and oven dried at 40 °C, ground and sieved (100 mesh).

Kraft lignin characterization

After extraction, kraft lignin was characterized according to the standards described in table 1.

Table 1
Characterization of lignin obtained from Eucalyptus sp.

Description	Methodology
Carbohydrate Content *	WALLIS et al. ( 1996)
Soluble Lignin	GOLDSCHMID (1971)
Insoluble lignin	GOMIDE & DEMUNER (1986)
Content of inorganic compounds	ABNT NBR 6293 (2015)
Lignin syringyl/guaiacyl	LIN & DENCE (1992)

• (^*) Glycan, xylan, galactan, mannan and arabinan.

Adhesive synthesis

For the preparation of the five different lignin-phenol-formaldehyde adhesives the formulations were performed according to table 2. In all proportions the amount of formaldehyde (H₂CO), 50% sodium hydroxide (NaOH) v/v and methanol (CH₃OH), but the proportion of phenol (C₆H₅OH) and kraft lignin varied according to table 2.

Table 2
Proportion of each reagent used in the adhesives as a function of phenol.

Treatments	Kraft lignin (g)	C6H5OH (mL)	H2CO (mL)	CH3OH (mL)	NaOH (mL)
T1	25	75	6.0	10.0	1.0
T2	30	70	6.0	10.0	1.0
T3	50	50	6.0	10.0	1.0
T4	75	25	6.0	10.0	1.0
T5	80	20	6.0	10.0	1.0

For the preparation of each formulation, kraft lignin was mixed in a flat bottom flask with two inlets, one coupled to a reflux condenser and the other to a thermometer. Methanol (CH₃OH) was added to the lignin and stirred with a glass rod until complete dissolution of the mixture as the kraft lignin was in powder form. After solubilization, phenol (C₆H₅OH) and formaldehyde (H₂CO) were added. After an hour of initial reaction sodium hydroxide (NaOH) was added in order to catalyze the process. The reaction remained on the heating blanket at 75 °C under constant stirring for 4 hours with the aid of a magnetic stirrer.

Adhesive properties

The pH, solids content (%), organic and inorganic content (%) and gel time (min) were analyzed for all proportions, being performed in duplicate. The pH was measured directly with the aid of digital pH meter. The solids content of the adhesives was calculated according to TAPPI Methods - T625 cm-85 (2015). The content of organic and inorganic solids were determined according to TAPPI Methods - T413 om-11 (2015). The gelatinization time was obtained with 3 g samples of the lignin-phenol-formaldehyde adhesive, which was placed in 15 cm high and 2 cm diameter test tubes, in which a glass rod was dipped inside. The tube-stick assembly was heated to 135 °C in glycerin solution, timing the time taken for polymerization of the adhesive.

Evaluation of adhesive flow behavior

With the aid of an automatic pipette, 0.05 mL (microns) of adhesive samples were added to a pine wood sample on the transverse, tangential longitudinal and radial longitudinal faces. Adhesive samples were only added to the initial wood on the three faces of the wood. The material was kept in an air-conditioned room (25 ºC) until complete polymerization.

Statistical analysis

A triplicate was performed for all evaluated properties and simple linear regression analysis to estimate the equations that best represented the behavior of dependent variables as a function of the percentage of lignin added in the preparation of the lignin-phenol-formaldehyde adhesive. The analysis was performed by stepwise procedure. For gelatinization time variable, the means comparison test was performed by Tukey test, at 5% significance.

RESULTS AND DISCUSSION

Kraft black liquor characterization

Table 3 shows the characterization data of the kraft black liquor used to obtain lignin. The results showed that kraft black liquor presented 15% of total solids, of which 51% is composed of organic matter and 49% inorganic. Therefore, the organic / inorganic ratio was 1.04. In general, the organic fraction is larger than the inorganic; however, it varies with the cooking yield and the alkaline charge demand for wood cooking; ie, if the yield is lower, a higher organic fraction content will be dissolved in the liquor.

Table 3
Evaluation of the average solids content of Eucalyptus sp.

Sample	Solid content (%)
Kraft black liquor	Total solids	Organic	Inorganic
	15	51	49

Extraction and characterization of kraft lignin

For lignin extraction kraft black liquor was concentrated to 30% of total solids and then acidified to obtain kraft lignin. Table 4 presents the characterization of the lignin obtained.

Table 4
Chemical constitution of kraft lignin extracted from black liquor Eucalyptus sp.

Sample	Carbohydrates (%)	Total Lignin (%)	S/G (mol.mol-1)
Kraft lignin	5.7	92.9	1.60

The kraft lignin presented 5.7% carbohydrates. It is known that during kraft cooking, xylans are dissolved by the process chemist (white liquor) and precipitate in an acid medium (DALTON & COLODETTE, 2011). Kraft lignin presented 93% yield, and the amount of carbohydrates dissolved in the kraft black liquor was the component that most contributes to its impurity during the extraction process.

The S/G ratio is a very important parameter in the selection of clones for cellulosic pulp production, facilitating the delignification process. This ease in delignification contributes to the reduction of chemicals in the cellulose pulp production process to the same kappa number, thus contributing to the reduction in carbohydrate dissolution (cellulose and hemicelluloses), and consequent reduction of impurities in the kraft black liquor for extraction purposes of kraft lignin (GOMES et al., 2008).

The kraft lignin presented an S/G (syringyl (S) and guaiacyl (G)) ratio of 1.60, lignin macromolecule has different reactive stability, and guaicil (G) lignin is more resistant to delignification due to the absence of the methoxyl group in carbon 5 of the aromatic ring, while syringyl type (S) is easily solubilized. The higher reactivity of this type of lignin means that a higher S/G ratio is desired to decrease process chemical costs (LOURENÇO et al., 2012). The higher proportion of type S kraft lignin may be due to its reactivity, imply better phenolation during the adhesive synthesis process due to the group available for access, thereby increasing the substitution of lignin for phenol in the adhesive synthesis process (GOUVÊA, 2015).

Adhesive properties

The figure 1 shows the pH values as a function of the proportion ofkraft lignin in the adhesive. Increasing the proportion of kraft lignin from 20 to 80%, there was a reduction in the pH value from 10.6 to 11.6, which was expected, since kraft lignin has acid character. Alkaline pH in phenolic adhesives for wood bonding has two main functions, being the cleaning of the wood surface by dissolving certain contaminants that eventually form part of the cured film and the swelling of the wood to open the cell wall structure to improve the surface. penetration and anchorage of the adhesive (DIAS, 2014).

Figure 1
Influence of the addition of kraft lignin onthe pH of the adhesives.

According to IWAKIRI et al. (2005) the ideal pH of an adhesive should be between 2.5 and 11, as extreme pH can degrade wood fibers. The proportions T3, T4 and T5 are within this limit. The figure 2 shows the total solids values as a function of the kraft lignin proportions in the adhesives.

Figure 2
Influence of the addition of kraft lignin on the total solids content of the adhesives.

Increasing the proportion of kraft lignin from 20 to 80% resulted in a reduction in the pH value from 10.6 to 11.6. In this research the solids content reported was much higher compared to the literature. DIAS (2014), evaluating different adhesive formulations, found values below 50%, except for the 40% proportion of phenol substitution, which presented solids content higher than 50%. In the present research was reported 53% of total solids in the proportion of 80% replacement of phenol, a possible explanation would be the purity of reagents, including lignin which was 93% pure. In the other proportions were found values greater than 60% of total solids, this is due to the high reactivity of lignin to formaldehyde, highlighting the 25% substitution which reached the highest percentage of solids content. Figure 3 shows the values of sustained overviews as a function of kraft lignin proportions in the adhesives.The figure 4 presents the average values of inorganic content as a function of the proportions of kraft lignin in the adhesives.

Figure 3
Influence of kraft lignin addition on organic solids content.

Figure 4
Influence of kraft lignin addition on inorganic solids content.

The organic/inorganic ratio (O/I) ranged from 1 to 3. It was observed that in the 25% substitution ratio it was 3, while for the 80% it was 1.03, which is close to the original found for the obtained kraft lignin. Assuming that inorganic materials are inert, that is, in these compounds the proportions of ions are such that the electrical charge is canceled, causing the compound as a whole to be electrically neutral, consequently the proportion 80% would not bring good results for adhesive glue line due to the low ratio O/I (VITAL et al., 2013). The table 5 shows the gelatinization values of the adhesives as a function of the kraft lignin proportions in the adhesives.

Table 5
Influence of kraft lignin addition on gelatinization time of adhesives.

Treatments	Kraft lignin (%)	135 ⁰C (min.)
T1	25	8.20b
T2	30	8.40b
T3	50	9.30b
T4	75	15.0a
T5	80	17.0a

• ^*Means followed by the same letters within a column do not differ statistically by Tukey's t-test (P > 0.05).

It was observed that the cure time of the adhesives maintained an increasing order in the degree of polymerization according to the higher the substitution rate, with the proportion of 25% reaching a time 8.20 minutes, while at 80% 17.0 minutes. The curing time of the glue line or working time is directly related to the total solids contents, as observed in figure 1, where the total solids contents were reduced with increasing substitution. Thus, the higher the proportion of substitution, the lower the total solids content and the longer the time required to cure the adhesive.

Evaluation of adhesive penetration behavior in wood

According to IWAKIRI (2005) the movements of the adhesive in bond formation vary in magnitude, according to its composition and bonding conditions. The formed glue line can be classified as: hungry, normal, ungrounded and pre-hardened. According to figure 5, it can be observed in the 2^nd and 3^rd phase that the proportions 4 and 5 showed starchy glue line, ie, very fluid, causing excessive penetration and disappearance of the adhesive through the porous structure of the wood resulting in a situation insufficient amount of adhesive on the glue line.

Figure 5
Adhesive-wood interaction on transverse (A), longitudinal radial (B), tangential longitudinal (C) face in three phases of wood-adhesive interaction (1^a, 2^a and 3^a) until complete polymerization.

The proportion 30% was normal, where there was sufficient fluidity to anchor and still remained sufficient to be transferred to another surface of the substrate (wood), thus, it is shown as the best proportion of lignin to replace phenol. While the proportions of 25.50 and 75% did not flow through the porous structure of the wood, presenting low mobility.

CONCLUSION

The pH of the adhesive remained alkaline with the addition of kraft lignin, even in high proportions. However, with an increase in the percentage of lignin in the formulation, the pH has to decrease. The total solids content influenced the polymerization time of the proportions, being inversely proportional. The inorganic solids content increased with the addition of kraft lignin. Gelatinization time was shorter for higher proportions of phenol replacement by lignin. The replacement of phenol by kraft lignin directly influenced the properties of the adhesive, where high proportions were negative for total, organic and inorganic solids and positive for gelatinization time. According to the properties evaluated, the recommended proportion of kraft lignin to replace phenol in the lignin-phenol-formaldehyde adhesive formulation is 30% considering an extraction purity above 92%.

ACKNOWLEDGEMENTS

Our thanks to Universidade Estadual de Mato Grosso do Sul (UEMS) and Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the financial support

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• CR-2023-0347.R2

Edited by

Editors: Leandro Souza da Silva (0000-0002-1636-6643) Rômulo Trevisan (0000-0002-8535-0119)

Publication Dates

• Publication in this collection
  04 Oct 2024
• Date of issue
  2025

History

• Received
  28 June 2023
• Accepted
  13 June 2024
• Reviewed
  25 Aug 2024