WO2015142156A1

WO2015142156A1 - A method for producing a thermally stable fluid loss reducing agent for water-based drilling fluid

Info

Publication number: WO2015142156A1
Application number: PCT/MY2014/000239
Authority: WO
Inventors: Mohamad Nasir Mohamad IBRAHIM; Fayyadh Faisal Saleh BIN TALEB
Original assignee: Universiti Sains Malaysia
Priority date: 2014-03-17
Filing date: 2014-09-25
Publication date: 2015-09-24
Also published as: WO2015142156A8; MY174937A

Abstract

A method for preparing a lignin grafted polyepoxide (LGPEX) for the use as a fluid loss reducing agent in drilling fluid comprising the steps of preparing a mixture consists of one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted-polyethylene oxide in an alkaline solution; reacting the mixture with the lignin by stirring and heating at a temperature between 88 to 105 °C in the presence of an acid catalyst under a nitrogen atmosphere. The method substantially provides a fluid loss reducing agent which could impart relatively high thermal stability characteristic in association with a water-based drilling fluid. Particularly, the drilling fluid could exhibit thermal stability up to 250 °C.

Description

A METHOD FOR PRODUCING A THERMALLY STABLE FLUID LOSS REDUCING AGENT FOR WATER-BASED DRILLING FLUID FffiLD OF INVENTION

The present invention relates to a method for producing a fluid loss reducing agent to be used in water-based drilling fluid.

BACKGROUND OF THE INVENTION

Formulation of the drilling mud or fluid is one of the most important aspects in geotechnical engineering. The drilling fluid essentially aids the drilling of boreholes into the earth. For instance, it is used during drilling of oil and natural gas wells, water wells or on exploration drilling rigs to cool down the drill bit, to prevent entrance of formation fluids into the wellbore and to suspend the drill cuttings.

In a routine drilling operation, drilling fluid is essentially pumped from the earth's surface to the bottom of the borehole through drilling pipe. The drilling fluid then returns to the surface through an annular zone while carrying drill cuttings such as sand and rocks away from the drill bit. The drilling fluid essentially forms thin layers of filter cake on the wall of the wellbore in order to avoid the loss of fluid into vicinity geological formations. Nevertheless, the drilling fluid could still infiltrate into the formations due to relatively lower hydrostatic pressure in the geological formations (higher hydrostatic pressure in the wellbore). As a result, the amount of drilling fluid decreases in time hence inducing a fluid loss phenomenon. This phenomenon could result in drilling difficulties due to formation damage or stucked drill pipes. Therefore, the use of fluid loss reducing agent in the drilling fluid is being studied. The following description focuses on the type of fluid loss reducing agent which is applicable in water-based or saline-based drilling fluid. To suit different conditions of various boreholes, different types of materials are being introduced as fluid loss reducing agent in drilling fluid formulations. Of interest includes biodegradable materials such as starch and polyanionic cellulose mixture, carboxymethyl starch, hydroxypropyl starch, carboxymethyl cellulose and sodium polyacrylate. However, these materials show limited performance above 120 °C. As deeper wellbore indicates higher underground pressure and temperature, a fluid loss reducing agent with high thermal stability is desired. Commercially available drilling mud having incorporated with fluid loss additive generally has limited thermal stability above 200 °C. For instance, drilling mud having Spersene-CF, Hydro-lig or Resinex Π as additives are thermally stable up to 163 °C but exhibits poor fluid loss reducing ability at 185 °C.

Besides, U.S Patent Application No. 2007/0161516 disclose fluid loss additive comprising carboxymethyl cellulose (CMC) grafted with sodium salt of 2-acrylamido- 2-methylpropane sulfonic acid. Particularly, the disclosed fluid loss additive has a thermal stability from 190 to 193 °C. Nevertheless, the disclosed additive is produced by mixing raw carboxymethyl cellulose with deionized water solution and 2- acrylamido-2-methylpropanesulfonic acid sodium salt. Particularly, a radical polymerisation initiator is required for the production of the mentioned fluid loss additive. Hence, the production of fluid loss additive disclosed in US 20070161516 may not be economically wise and environmental friendly.

Further, U.S. Patent Application No. 2013/0079256 Al discloses a method to produce fluid loss reducer for drilling mud. Particularly, the method includes solution polymerization technique in preparing sulfonate polymer as a fluid loss reducing agent from raw materials such as woody and herbal lignin. Nevertheless, this method includes the use of more than 2 monomers selected from sulfonates compounds such as sodium methylallyl sulfonate, ammonium persulfate and sodium sulfate. These materials are substantially toxic in nature. Further, the product has low molecular weight that results in limited thermal stability upon reaching 120 °C after 16 hours of aging. In view of the above drawbacks, this invention provides an economical and environmental friendly method to producing a fluid loss reducing agent which is thermally stable up to 250 °C. Further, the invention provides an effective amount of the fluid loss reducing agent to be used in a water-based drilling fluid.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a simple method for producing a fluid loss reducing agent which exhibits thermal stability at temperature up to 250 °C. Another object of the present invention is to provide a method which produces high yield of fluid loss reducing agent from raw materials used. Particularly, the disclosed method is able to yield more than 80 % of lignin grafted polyepoxide (LGPEX) from raw materials used. Another object of the present invention is to provide a lignin grafted polyepoxide having high molecular weight which ranges from 500,000 to 605050 mg/mol.

Yet another object of the present invention is to provide an environmental friendly fluid loss reducing agent derived from lignocellulosic material.

Further, the present invention produces a fluid loss reducing agent which is substantially compatible with commercially available fluid reduction additives.

At least one of the preceding objects is met, in whole or in part, by the invention, in which one of the embodiments of the invention describes a method for preparing a lignin grafted polyepoxide (LGPEX) for the use as fluid loss reducing agent in the drilling fluid comprising the steps of preparing a mixture consists of one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2- acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted-polyethylene oxide in an alkaline solution; and reacting the mixture with lignin by stirring and heating at a temperature between 88 to 105 °C in the presence of an acid catalyst under a nitrogen atmosphere.

Preferably, the method further comprises a cooling step for the mixture by immersion in an iced bath or water.

Preferably, the method further comprises a drying step to remove 80 to 90% of liquid after the reacting step. Advantageously, the mixture in the alkaline solution is heated to a temperature ranging from 90 to 120 °C for a period of 60 to 90 minutes.

Advantageously, the lignin is any one or a combination of soda lignin, klason lignin, kraft lignin, Milled wood lignin, organosolv lignin, cellulolytic enzyme lignin, steam explosion lignin and acid hydrolysis lignin obtained from lignocellulosic material. Particularly, the lignocellulosic material derives from woody plants including oak, pine, oil palm or date palm. In one of the embodiment where oil palm is preferably used, the source of lignocellulosic material is any one or a combination of empty fruit bunch, oil palm fronds, oil palm trunks and oil palm shell.

Preferably, the alkaline solution is any one or a combination of sodium hydroxide, potassium hydroxide and calcium hydroxide.

Preferably, the acidic catalyst is toluenesulfonic acid or methanesulfonic acid.

Advantageously, the lignin grafted polyepoxide is incorporated into drilling fluid for the use as fluid loss reducing agent. Particularly, the drilling fluid comprises less than 4 % by weight of the lignin grafted polyepoxide. One skilled in the art will readily appreciate that the invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments described herein are not intended as limitations on the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing a lignin grafted polyepoxide (LGPEX). Particularly, the method involves suspension polymerisation of lignin with a mixture having one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted-polyethylene oxide.

Hereinafter, the present invention shall be described according to the preferred embodiments of the present invention and by referring to the accompanying description. But, it is to be understood that limiting the explanation to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise a range of modifications without departing from the scope of the appended claim.

The present invention is a method for preparing a lignin grafted polyepoxide (LGPEX) for the use as a fluid loss reducing agent in drilling fluid comprising the steps of preparing a mixture of one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted- polyethylene oxide in an alkaline solution; and reacting the mixture with the lignin by stirring and heating at a temperature between 88 to 105 °C in the presence of an acid catalyst under a nitrogen atmosphere.

Any person skilled in the art shall appreciate the fact that the source of lignin is mainly lignocellulosic material of woody plants. All types of woody plants can be used to isolate lignin therefrom. Examples of woody plants include oak, pine, oil palm and date palm. Further, lignin used in the present invention can be isolated from the lignocellulosic material via methods known in the art. Particularly, the known isolation method essentially produce soda lignin, klason lignin, kraft lignin, Milled wood lignin, organosolv lignin, cellulolytic enzyme lignin, steam explosion lignin or acid hydrolysis lignin which is suitable for the use in the present invention.

In one of the preferred embodiment, soda lignin is extracted from oil palm lignocellulosic material from oil palm. Particularly, soda lignin can be extracted from black liquor in pulping or cooking process of paper-making industry. Particularly, the black liquor comprises dissolved oil palm lignocellulosic material such as oil palm empty fruit bunch, oil palm fronds, oil palm trunks and oil palm shell. Any person skilled in the art shall appreciate the fact that black liquor is generated by soaking lignocellulosic wastes and sodium hydroxide in water at high temperature ranged from 140 to 180 °C and pressure ranged from 800 to 1200 kPa (8 to 12 bar), preferably in a steel autoclave. More preferably, the black liquor is generated at a temperature of 160 °C under a pressure of 1000 kPa (10 bar).

Further, the lignin used can be recovered from the black liquor using mineral acid which is capable of precipitating lignin therefrom. Preferably, the mineral acid is sulphuric acid, nitric acid, phosphoric acid or hydrochloric acid. More preferably, sulphuric acid is used.

In the present invention, the method comprises the step of preparing a mixture consists of one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted-polyethylene oxide in an alkaline solution. Preferably, the alkaline solution is any one or a combination of potassium hydroxide, sodium hydroxide and calcium hydroxide.

Pursuant to the preceding description, the mixture in the alkaline solution is preferably heated to a temperature ranging from 90 to 120 °C for a period of 60 to 90 minutes, in an oven, favourably in an oven having a rolling cell. Thereinafter, the method preferably includes a cooling step to reduce the temperature of the mixture using an ice bath or water bath for 5 to 10 minutes.

Preferably, the method includes use of suspension polymerization technique to react the mixture with the lignin to form lignin grafted polyepoxide (LGPEX). In accordance with the preferred embodiment of the present invention, suspension polymerization is initiated via a reacting step conducted in conjunction with heating and stirring. Heating is preferably performed to the mixture in alkaline solution in order to reach a temperature between 88 to 105 °C. Further, an acidic catalyst is preferably used to facilitate the formation and growing of short chain grafted lignin into a high molecular weight grafted lignin. Upon reaching the aforementioned temperature, the lignin is being added into the mixture in the presence of an acidic catalyst. Prior to the reacting step, the lignin is preferably mixed with the acidic catalyst to form an acidic mixture. Stirring essentially facilitates the reactin step in the present invention, in which the stirring process continues for approximately 20 to 30 hours. As a result, the present invention is able to produce lignin grafted polyepoxide having a molecular weight ranges from 500,000 to 605050 mg/mol.

A person skilled in the art shall appreciate the fact that any non-oxidizing strong organic acid can be applied as the acidic catalyst to achieve the desired catalysing effect. In particular, the acidic catalyst can be toluenesulfonic acid or methanesulfonic acid. In accordance with the most preferred embodiment of the present invention, toluenesulfonic acid is used as acidic catalyst. Two different phases of product are obtained from the method of producing LGPEX as set forth in the preceding description, which includes a solid phase containing unreacted lignin and residues of unreacted mixture and a liquid phase containing LGPEX in the alkaline solution. Further, the liquid phase is preferably subjected to a drying step to remove the 80 to 90% of liquid therein. In particular, the drying step can be performed in a vacuum oven or heating oven. Still another preferred embodiment of the present invention is the incorporation of the LGPEX into drilling fluid for the use as fluid loss reducing agent. Particularly, the drilling fluid can be a water-based or saline-based drilling fluid. Preferably, drilling fluid comprises less than 4 % by weight of LGPEX is sufficient to exhibit thermal stability in an environment of temperature up to 250 °C. More preferably, the drilling fluid comprises 1 to 4% by weight of the LGPEX therein.

It is also found by the inventor that the LGPEX substantially seals the gaps and fractures between the formations therefore preventing the influx of contaminants and formation water into the drilling fluid and vice versa.

EXAMPLE

The following example illustrates one embodiment of the present invention in greater detail. This example is not intended in any way to limit the disclosed invention, which is limited only by the claims.

EXAMPLE

Initially, soda lignin is recovered from black liquor of pulping process. Particularly, the black liquor comprises oil palm lignocellulosic material obtained by digesting oil palm empty fruit bunch, oil palm fronds, oil palm trunks, or oil palm shell in sodium hydroxide. The soda lignin is precipitated from the black liquor using sulfuric acid at pH 2.

Hereinafter, 0.60 g of 2-acrylamido-2-methylpropanesulfonic acid and 0.50 g of polyacrylic acid sodium-grafted-polyethylene oxide are mixed in sodium hydroxide in an oven having a rolling cell at a temperature of 110 °C for 90 minutes. Then, the mixture is cooled in an ice bath for 5 to 10 minutes. After that, 1.0 g of recovered soda lignin is added with 0.5 g of toluenesulfonic acid in 100 mL of distilled water to form an acidic mixture at preferred temperature ranged from 88 to 95 °C. Prior to the grafting or reacting process, the mixture containing 2-acryloamido-2- methylpropanesulfonic acid and polyacrylic acid sodium-grafted-polyethylene oxide is heated to 110°C. Then, the acidic mixture with lignin is added to the alkaline mixture and being stirred for 24 hours at 95 °C to allow for grafting reaction. Particularly, the container used for the blending process is continuously supplied with nitrogen gas in order to remove oxygen gas therefrom.

As a result, the product separates into a liquid phase and a solid phase. The liquid phase contains the LGPEX, while the solid phase contains unreacted mixture and lignin residue. Thereinafter, the liquid phase is being dried in a vacuum oven.

Table 1. Fluid loss reducing agent in the present invention and its composition in the drilling mud.

T pe of mud T Ma μ_Ρ τ ,. pH Filtration

(°C) (mPa.s) (mPa.s) (Pa) At 27 °C

Fresh water- 27 25 15 10.22 11.06 15 ML based mud

1 % LGPEX + 27 29 20 9.19 10.30 10 ML water-based mud

1.5 % LGPEX + 27 31.5 22 9.70 10.32 9.5 ML water-based mud

2 % LGPEX + 27 32.5 20 12.77 10.18 9 ML water-based mud A comparison study in regards to fluid loss reduction and rheological properties between the LGPEX in the present invention and commercially available drilling mud additives at temperature 185°C is shown in Table 2. Particularly, the effect of LGPEX and additives are compared before and after 16 hours of heating in a rolling oven. All parameters shown in Table 2 are measured at room temperature (RT) around 26 + 1^°C, where is ₃ (mPa.s) is the apparent viscosity; μ_ρ (mPa.s) is the plastic viscosity and τ _y (Pa) is the yield point of the drilling mud and filtration reduction performance.

Table 2. Comparison study of the fluid loss reducing agent in the present invention with commercially available additives applied in water-based drilling mud. Type of mud μ.: μ_Ρ2 T ,2 pH Filtration Filtrati¬

(mPa (mPa (mPa (mPa (Pa) (Pa) Before At (RT) on after

•s) .s) .s) .s) / After 185°C /

Before 16 hours

Fresh water- 25 45 15 32 10.22 13.28 11.06/ 15 ML 30 ML based mud 10.27

2 % 32.5 39 20 25 12.77 14.30 10.18/ 9 ML 6.5 ML

LGPEX+ 9.30

water-based

mud

2 % Resinex 17.5 30 15 20 2.55 10.22 10.25 / 13 ML 18 ML

II + water- 9.38

based mud

2 % 22.5 32.5 20 25 2.60 7.66 7.67 / 12 ML 22 ML

Spersene-CF 7.20

+ water- based mud

2 % Hydro- 15 47 10 35 5.11 12.78 10.15 / 18 ML 20 ML

Lig + water- 7.75

based mud

Note:

μ_αι indicates apparent viscosity before heating; indicates apparent viscosity after heating; μ_ρ! indicates plastic viscosity before heating; μ_ρ2 indicates plastic viscosity after heating; τ _y! indicates yield point before heating; and τ _y2 indicates yield point after heating.

According to Table 2, LGPEX in the present invention shows a relatively good filtration reducing performance at the same concentration level with commercial available additives such as Resinex Π, Spersene CF and Hydro-Lig.

Table 3 shows a the comparison study between fluid loss reduction and rheological properties of LGPEX and commercially available drilling mud additives in moderate salinity environment at temperature of 185°C after 16 hours of heating in the rolling oven. All parameters shown in Table 3 are measured at room temperature (RT) around 26 + 1°C, where is μ₃ (mPa.s) is the apparent viscosity; μ_ρ (mPa.s) is the plastic viscosity and τ _y (Pa) is the yield point of the drilling fluid and filtration reduction performance. Table 3. Comparison study of fluid loss reducing agent in the present invention with commercially available additives in saline-based drilling mud.

Type of μ.ι M μ_Ρι μ pH Filtration FiltratMud (mPa. (mPa. (mPa. (mPa. (Pa) (Pa) Before/ At (RT) ion s) s) s) s) After after

Before 185° C /

16 hr

2% NaCl 36 45 20 30 15.26 21.33 10.30/ 38 ML 80 ML + Fresh 8.87

water- based mud

3 % 27.5 36 15 27 12.77 10.73 10.27/ 12 ML 10.5

LGPEX + 9.18 ML 2% NaCl+

water- based mud

3 % 15 25 20 10.22 5.11 9.85 / 15 ML 20 ML

Resinex II 8.74

+ 2%

NaCl+

water- based mud

3 % 20 12.5 15 10 5.11 2.55 6.73/ 10 ML 60 ML

Spersene- 7.40

CF + 2%

NaCl+

water- based mud

3 % 20 55 10 25 10.2 30 9.41/ 35 ML 40 ML

Hydro-Lig 7.80

+ 2%

NaCl+

water- based mud

Note:

μ indicates apparent viscosity before heating; indicates apparent viscosity after heating; μ indicates plastic viscosity before heating; μ_ρ2 indicates plastic viscosity after heating; τ indicates yield point before heating; and τ _y2 indicates yield point after heating.

According to Table 3, LGPEX in the present invention shows relatively good filtration loss reduction performance in saline environment at temperature of 185°C at a concentration level of 3 % by weight of LGPEX. Further, Table 4 shows a the comparison study between fluid loss reduction and rheological properties of the LGPEX and commercially available high stability drilling mud additive (Driscal-D polymer) at temperature of 250°C after 16 hours of heating in the rolling oven.

Table 4. Comparison study of fluid loss reducing agent in the present invention with commercially available additives under temperature of 250°C.

Type of μ.ι μ μ_Ρι Mp2 τ pH Filtration Filtrati¬

Mud (mPa. (mPa. (mPa. (mPa (Pa) (Pa) B /A before on after s) s) s) .s) (RT) 250° C /

16 hr

Fresh 25 42.5 15 35 10.22 7.7 11.06 / 15 ML 30 ML water 7.90

base mud

2.5% 40 25 30 22 10.20 3.16 10.60 / 9 ML 15.5

LGPEX 8.53 ML

+ water- based

mud

2.5% 30 22.5 22 15 8.68 7.66 10.25 / 8.5 ML 15

LGPEX 9.50 ML

+ 0.30%

Spersene

-CF+

water- based

mud

2.5% N/A 30 N/A 25 N/A 5.11 10.48/ N/A 16 ML

Driscal (paste (paste (paste 9.42 (paste

D form) form) form) form)

polymer

+ water- based

mud

2.5% N/A 50 N/A 35 N/A 15.33 10.68 / N/A 14 ML

Driscal (paste (paste (paste 8.86 (paste

D form) form) form) form)

polymer

+ 0.30 %

Spersene

-CF+

water- based

mud

Note: μ₃ι indicates apparent viscosity before heating; indicates apparent viscosity after heating; μ_ρ! indicates plastic viscosity before heating; μ_ρ2 indicates plastic viscosity after heating; τ _yi indicates yield point before heating; and τ _y2 indicates yield point after heating. B indicates pH before heating while A indicates pH after heating.

All parameters before shown in Table 4 are measured at room temperature (RT) around 26 ± 1°C, where is μ₂ (πιΡ3.5) is the apparent viscosity; μ_ρ (ΓηΡ3.5) is the plastic viscosity and τ _y (Pa) is the yield point of the drilling mud and filtration reduction performance.

Some formation of drilling mud shown in Table 4 are presented in paste form hence the parameters (μ₂, μ_ρ, τ _y and filtration loss) before heating are not available (shown as N/A in Table 4). According to Table 4, LGPEX in the present invention shows similar performance in fluid loss reduction compared to Driscal-D at a temperature of 250°C.

Further, results from Table 4 shows that the LGPEX is compatible with other commercially available additives such as Spersene-CF in water-based drilling mud. In addition, Table 1 to Table 4 show that the formulations having LGPEX in the present invention and water or saline-based drilling mud possess favourable pH range (8.5 to 10.5).

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims.

Claims

1. A method for preparing a lignin grafted polyepoxide (LGPEX) for the use as fluid loss reducing agent in drilling fluid comprising the steps of preparing a mixture consists of one or more materials selected from acrylic acid, acrylic acid salt, dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, sodium-grafted-polyethylene oxide and polyacrylic acid-sodium-grafted-polyethylene oxide in an alkaline solution; and reacting the mixture with lignin by stirring and heating at a temperature between 88 to 105 °C in the presence of an acid catalyst under a nitrogen atmosphere.

2. A method according to claim 1 further comprising a cooling step on the mixture by immersion in an iced bath or water.

3. A method according to claim 1 further comprising a drying step to remove 80 to 90% of liquid after the reacting step.

4. A method according to claim 1 or 2, wherein the mixture in the alkaline solution is heated to a temperature ranging from 90 to 120 °C for a period of 60 to 90 minutes.

5. A method according to claim 1 or 2, wherein the lignin is any one or a combination of soda lignin, klason lignin, kraft lignin, Milled wood lignin, organosolv lignin, cellulolytic enzyme lignin, steam explosion lignin and acid hydrolysis lignin isolated from lignocellulosic material.

6. A method according to claim 5, wherein the lignocellulosic material derives from woody plants.

7. A method according to claim 6, wherein the woody plants include oak, pine, oil palm or date palm.

8. A method according to claim 6 or 7, wherein the lignocellulosic material derived from oil palm is any one or a combination of empty fruit bunch, oil palm fronds, oil palm trunks and oil palm shell.

9. A method according to claim 1 or 2, wherein the alkaline solution is any one or a combination of sodium hydroxide, potassium hydroxide and calcium hydroxide.

10. A method according to claim 1 or 2, wherein the acidic catalyst is toluenesulfonic acid or methanesulfonic acid.

11. A method according to claim 1 or 2, wherein the lignin grafted polyepoxide is incorporated into the drilling fluid for the use as a fluid loss reducing agent.

12. A method according to claim 11, wherein the drilling fluid comprises less than 4 % by weight of the lignin grafted polyepoxide.

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