Preparation and Aqueous Properties of Starch-grafted Polyacrylamide Copolymers

Mohamed Eutamenea

Abbes Benbakhtia

Mohamed Khodjab

Amane Jadac

a Groupement “API” CorporateUniversity, Boumerdès, Algeria

b Sonatrach/Division Centre deRecherche etDéveloppement,Boumerdès, Algeria

c Institut de Chimie des Surfaceset InterfacesCNRS UPR 9069,Mulhouse, France

Preparation and Aqueous Properties ofStarch-grafted Polyacrylamide Copolymers

In the present work, starch-grafted polyacrylamide copolymers were prepared andtheir rheological properties, either in water or in water-based muds, were investigated.The advantages in using starches as substrates to prepare additives which are suitedfor improving stability and the rheological properties of water-based muds, lie in theirlow cost, and their lower biodegradability than native starches. Thus, various copoly-mer series were prepared by free-radically grafting acrylamide (AM) onto starch usingceric ammonium sulphate (CAS) as initiator. It is shown that the intrinsic viscosity andthe molecular weight of the grafted starches are controlled by the initiator and mono-mer concentrations. Furthermore, to predict the behaviour of the samples under oil-well conditions, the aqueous solution properties of the copolymers such as water sol-ubility, viscosity, shear rate, were determined as function of temperature, salt con-centration and type, and aging time. The knowledge of these properties is a pre-requisite for the use of the copolymers in drilling fluids. The data indicate that starch-grafted polyacrylamide copolymers as compared to the non-modified starch, behaveas shear-thinning, are salt resistant, and their rheological properties are stable withtime. The grafted starches as prepared above were also added to water-based mudand the rheological properties of the resulting muds were determined under oil-wellconditions. Grafted starches, having high AM contents, are more efficient in decreas-ing the filtrate volume, and increasing the plastic viscosity of the muds, when com-pared to PAC-L, a modified cellulosic polymer used in the filtration control of mostwater-based muds. Such improvements in the rheological properties of the muds werefound to result from the behaviour and/or properties of the grafted starches in water.

Keywords: Drilling mud; Fluid loss; Polymer rheology; Starch-grafted polyacrylamide

Starch/Stärke 61 (2009) 81–91 81

1 Introduction

Drilling fluids or drilling muds consist of suspensions ofclay particles in water or in oil [1-3]. They are generally oil-based muds (OBM) or water-based muds (WBM).Whereas bentonites in their raw form are used in WBM,organoclays, which are generally bentonite modified withquaternary ammonium salt surfactant, are used in OBM. Ifthe rheological and filtrate properties of a bentonite areinsufficient, the latter can be activated or treated byextenders such as inorganic salts and/or polymers. Inaddition to bentonite, WBM contains several othersmineral additives such as salts (KCl, NaCl,. . .); barite and/or calcite, which are added to increase density, and lime,KOH, NaOH, etc. are added to rise the pH of the aqueousphase.

In response to environmental requirements, most of thedrilling muds used in oil fields are formulated in water. The

WBM may contain additives such as salts and polymers.The additives used are intended to make mud fulfillingcertain characteristics based on the weight, the viscosity,the fluid loss, and the reactivity of the mud [1, 2]. Thesecharacteristics are in turn a prerequisite for the use of themud in drilling operations to perform specific functions [4]such as carrying the drill cuttings up to the surface, form-ing a filter cake on the walls of the borehole, maintainingthe stability of the wellbore and preventing damage to theproducing formation.

Different chemical and polymeric additives are used indesigning drilling muds to tailor some functional require-ments such as appropriate viscosity, density, activity, andfluid-loss control property of the final product [4, 5],according to the API standard test procedures [6]. How-ever, the selection of appropriate mud additives is com-plex [2], since this selection should take into account bothtechnical and environmental factors [7]. Thus, the, ingeneral polymeric, additives used in aqueous drillingfluids must be water-soluble. Natural, modified-natural,and synthetic polymers are in use [4]. Starches are widelyused in drilling fluids, either in native or modified form.

Correspondence: Amane Jada, Institut de Chimie des Surfaceset Interfaces CNRS UPR 9069, 15 rue Jean Starcky, F-68057,Mulhouse, France. E-mail: [email protected].

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

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DOI 10.1002/star.200800231

82 M. Eutamene et al. Starch/Stärke 61 (2009) 81–91

Over the past decades, starches have been used as rawmaterials for designing polymeric additives suitable forvarious oilfield applications [8]. Thus, numerous modifiedstarches such as pregelatinized [9], etherified [10] andgrafted [11] starches were used as additives in drillingfluids. Synthetic additives used in drilling fluids includemainly polyacrylate, polyacrylamide, and cationic poly-mers. The advantage in using synthetic additives ascompared to natural ones lies in their simple chemicalstructure and their easy modification to give desiredproperties of the final product.

Various monomers such as amidoethylene [12], mixturesof methacrylonitrile-methacrylate [13], methacrylamide[14], acrylamide [15, 16], and acrylonitrile [17] werepolymerised and grafted onto starch substrates. Thesechemically modified starches with improved propertiesare gaining increasing importance in industry not onlybecause of low cost, but also because they are lessbiodegradable than native starch [5]. For starch-graftpolyacrylamide copolymers, numerous studies deal withtheir synthesis and their oilfield applications mainly inenhanced oil recovery and in drilling fluids [4, 9]. How-ever, such reported works did not study in detail theproperties of the copolymer in aqueous solution, asmeasured under oil-well conditions, and which are aprerequisite for the copolymers application in the drillingfluids.

It should be emphasised that the preferential use ofmodified starch, such as starch-graft polyacrylamidecopolymers, instead of the homopolymer, in drilling fluidsis due to synergistic effects of the former. Hence, it hasbeen observed that grafting synthetic polymers onto thebackbone of natural polymers lead to materials havingcombined rheological behaviours of both natural andsynthetic polymers [18]. Moreover, when graft copoly-mers having longer chains are added to an aqueous dis-persion of clay particles, adsorption of the copolymeronto the clay surface takes place, leading at equilibriumto the formation of a polymeric layer [19–27]. In the poly-meric layer, the natural backbone (less water soluble) is indirect contact with the clay surface, whereas the graftedside chains (more water soluble) stretch into the solutionto build up a brush and to stabilise the clay aqueous dis-persion.

Polyacrylamides, which are usually designated by thegeneric name of partially hydrolysed polyacrylamides(PHPAM), are not biodegradable, and are suitable forcontrolling the viscosity, the filtration properties, and thethermal stability of the drilling fluid. However, some fac-tors such as the presence of salts in the drilling fluid me-dium and the mechanical degradation of the polymer thatmay occur at higher molecular weights, may limit their

use. On the other hand, low cost, renewability, availability,and salt tolerance of starch make it an attractive polymerfor drilling-fluids applications, but its biodegradation andlower thermal stability limit its application. Therefore, thebest-preferred way to overcome these limitations is to usestarch-grafted polyacrylamide copolymer instead of sin-gle polymers.

The purpose of the reported study was: (1) to synthesis aseries of starch-grafted polyacrylamide copolymers bygrafting polyacrylamide onto a starch substrate; (2) tostudy the properties of an aqueous solution of the copo-lymer, under oil-well conditions, as function of tempera-ture, salt, time; and to investigate the effects of the copo-lymer composition on the drilling mud properties; (3) tocompare the properties of the starch-grafted polyacryl-amide copolymers in aqueous solution to the non-mod-ified starch, and water-based muds to a common com-mercial polymer, namely polyanionic cellulose (PAC-L).

2 Materials and Methods

2.1 Materials

Potato starch was purchased from Fluka (Buchs, Switz-erland, viscosity average molecular weight 126104 g/mol, as determined by viscosity measurements). Acryl-amide, hydroquinone and ceric ammonium sulphate werepurchased from Merck, Darmstadt, Germany; whereasMI-gel (bentonite) and PAC–L were obtained from MISwaco Drilling Fluid Company (Hassi Messaoud, Algeria).All chemicals were reagent grade and were used asreceived without further purification.

2.2 Preparation of the starch-graftedpolyacrylamide copolymers

Several initiators are reported in literature for initiatinggraft copolymerisation of natural polymers. For polymersof polyol types, such as polysaccharides, satisfactorygrafting copolymerization results are obtained by usingceric ammonium salts as initiator [12, 18].

Accordingly, in the present work, polyacrylamide (PAM)was grafted by free radical polymerisation of acrylamidemonomer (AM) onto the starch backbone gelled in aque-ous solution by using ceric ammonium sulphate (CAS) asinitiator. Prior to the addition of the initiator and themonomer, the gelled aqueous starch solution wasobtained by dissolving 2.86 g of starch (5.71 g for sam-ples C1 and C2) in 100 mL of distilled water at 807C, fol-lowed by cooling the solutions to 307C. The total volumeof water used for preparing all the samples was 150 mL.


Starch/Stärke 61 (2009) 81–91 Preparation and Properties of Starch-grafted Polyacrylamide 83

The polymerisation was conducted at 307C for 48 h in afour-necked batch reactor (0.5 L), equipped with an elec-tric heater, thermometer, and mechanical stirrer. A nitro-gen atmosphere was maintained throughout the reaction.The reaction was terminated by adding 0.5% (w/w) of asaturated hydroquinone solution. Adding acetone to theaqueous polymer solution precipitated the final product.Subsequently, the product was dried to constant weightin a vacuum oven at 507C for four days.

The effects of the initiator and monomer contents on theproperties of the final product, were studied by synthe-sising a series of starch-grafted acrylamide copolymersat various concentration ranges of acrylamide, [Am] =4.2610-2 – 21.1610-2 mol, and ceric ammonium sul-phate, [CAS] = 1610-4 – 4610-4 mol. It should beemphasised that these concentration ranges are belowthe limit values of 2 and 5610-2 mol for the monomer andthe initiator, respectively, above which the polymerisationof polyacrylamide homopolymer may be initiated [28]. Inorder to ensure that the polyacrylamide homopolymer isnot present in the final product, control reactions werecarried out in the absence of starch under similar experi-mental conditions, and nil yields were obtained indicatingthe absence of polyacrylamide homopolymer in all thecopolymerisation experiments.

2.3 Mud preparation

First, bentonite was analysed for conformity according toAPI specifications. The requirements are based on vis-cometer dial reading at 600 rpm, yield point/plastic vis-cosity ratio, filtrate volume, residue of diameter greaterthan 75 mm. The bentonite used in the present work wasfound to fulfill all these parameters.

A low-solid base mud, containing 4.3% (w/w) pre-hydra-ted bentonite, was prepared by adding 15 g of bentoniteto 350 mL of fresh water and the resulting mixture wasagitated for 20 min at high speed using a Hamilton beachmixer. The pH of the aqueous dispersion was adjustedbetween at 8-9 by adding a small amount of an aqueoussodium hydroxide solution. Before use, the base mud wasaged for 24 h at room temperature to hydrate the bento-nite.

2.4 Mud-starch-grafted polyacrylamidecopolymer mixture

The copolymer was added at varied concentrations to thewater-based mud, and the resulting mixture was agitatedwith a Hamilton beach mixer at moderate speed during 15min for the starch-grafted polyacrylamide copolymer se-

ries, and during 10 min for the PAC-L polymer. In order toattain the chemical equilibrium of the system and toexpose the fluid to thermal conditions similar to what mayoccur in a field application, aging experiments of mud-copolymer mixtures were carried out in a rolling oventhrough hot rolling at 807C for 16 h. The salt effect on themud performance was studied by adding CaCl2, andNaCl to the water-based mud.

2.5 Characterisation techniques

2.5.1 Infrared analysis

Infrared spectra were recorded by using a 560 ESP IRspectrophotometer (Nicolet, Madison, WI, USA). In infraredanalysis, each dried solid sample was mixed with KBr usedas standard reference. A uniformresolution of 3.2 cm-1 and anumber of scans of 30 were maintained in all cases.

2.5.2 Intrinsic viscosity measurements

Viscosity was measured with the help of an Ubbelohdecapillary viscometer (diameter of the capillary tube 0.58mm) at 30 6 0.17C. From the time flow of the aqueouspolymer solution (t) and that of the solvent (to), the relativeviscosity (Zrel= t/to) was obtained. Specific viscosity wascalculated from the relationship Zsp = (t–to)/to. Knowingthe concentration of the polymer solution (C) in g/dL, thereduced viscosity (Zred = Zsp/C), and the inherent viscosity(Zinh = ln Zrel/C) were calculated for a set of four polymerconcentrations. The intrinsic viscosity, [Z], was thendetermined by extrapolating both plots of reduced, Zred,and inherent, Zinh, viscosities to zero polymer concentra-tion (C = 0 g/dL).

2.5.3 Elemental analysis

In order to determine the monomer conversion, the meanaverage degree of polymerisation, and the graft content,the nitrogen weight percents of the final products weremeasured using a VarioEL CHNS (Dar El Beida, Algeria)elemental analyser.

The monomer conversion, (XAM), was calculated accord-ing to Equation (1):

XAM ¼Nf

Ni(1)

In Equation (1), Ni is the theoretical nitrogen weight per-cent based on the amount of acrylamide introduced in thereactor, and Nf is the determined nitrogen weight percentof the final product.



Ni was calculated according to Equation (2):

Ni ¼ð0:197� AMðgÞÞðAMðgÞ þ starchðgÞÞ (2)

The mean average degree of polymerisation, DP, wascalculated according to Equation (3):

DP ¼ ðreacted AM ðmolÞÞðmoles of CAS added to the reaction mixtureÞ ¼

XAM � nAM

nCAS(3)

In Equation (3), nAM and nCAS are, respectively, the amountof the monomer, AM, and of the initiator, CAS, initiallyintroduced in the reactor, both expressed in mol.

The number of created sites, Ng, expressed as mol ofPAM formed (XAM nAM/DP) per mol of starch (nstarch), wascalculated according to Equation (4):

Ng ¼XAM

DPnAM

nStarch(4)

2.5.4 Viscometric measurements of aqueoussolutions of copolymer, under oil-wellconditions

In order to mimic the oil-well conditions such as temper-ature gradient, injection and flow rates, time of use (oneday to one week), presence of monovalent and divalentions, the behaviour of the copolymer was followed inaqueous solution as a function of temperature, time,shear rate and salt.

Hence, the effect of temperature was determined by meas-uring the solution viscosity of the grafted starches at varioustemperatures ranging from 247C to 807C and using a low-shear rotational viscometer (Contraves LS 40, Baroid, Celle,Germany) shear rate varying in the range 1–250 s-1). Usingthe same viscometer at low shear rates varying in the range1–5 s-1, the effects of salt concentration and time on the vis-cosity loss of polymer solutions were determined. In theseexperiments, the concentration of the copolymer studiedwas kept constant and equal to 1% (w/v), whereas the salt(NaCl or CaCl2) concentration varied in the range 0–2000,and the time varied from one to twelve days. To describe thevariation of the solution viscosity, we consider the relativeviscosity to time and salt, RVt and RVs, as expressed,respectively, by Equations (5) and (6):

RV t ¼ 100� ZZ0

(5)

RVs ¼ 100� ðZ�ZbrineÞZ

0

(6)

In Equation (5), Z0 refers to the starting salt-free solutionviscosity measured at t0 = 1 day; while in Equation(6) Z0

refers to the starting salt-free solution viscosity measuredat salt concentration, Csalt = 0 ppm. The parameter Z isthe mean average solution viscosity, Z = SZi /Si, where Zi

is the solution viscosity at shear rate gi, measured after atime period, t, or salt concentration, Csalt, and Zbrine is theviscosity of the aqueous brine solution.

The shear effect was studied by using a Rheotest type,Couette rotational viscometer (shear rate range from 0 to1000 s-1). This apparatus is equipped with software thatgives the representative equation and parameters of thepolymer rheological model [29].

2.5.5 Rheological measurements of the mud-copolymer mixture

Mud property tests measurements were performedbefore and after aging according to the American Petro-leum Institute (API) specifications [6].

The mud was prepared as described in section 2.3, andthe amount of the copolymer added to the mud varied inthe range 0–1 g. The rheological properties of the result-ing mud-copolymer mixtures were measured using aFann 35A concentric cylinder rheometer, (Baroid), atshear rate range 5.1–1021.2 s-1. The rheometer used inthis work gives direct reading of PV (plastic viscosity) YP(yield point) and the AV (apparent viscosity). The numer-ical value of PV in mPa s, is given by the difference in theshear stress values F600 and F300 measured, respectively,at shear rates = 600 and 300 s-1:

PV ¼ f600 � f300 (7)

And the numerical value of YP in Pa, is given directly:

YP ¼ 0:511ðf300 � PVÞ (8)

In addition to the PV and YP the apparent viscosity (AV) isdetermined by:

AV ¼ f600

2(9)

The filtration or wall-building property of the mud wasdetermined by means of an API filter press. The filtrationtest consists of determining the rate at which fluid isforced through the filter paper. The test is run under spe-cified conditions of time, temperature, and pressure. Thefilter press being used should meet specifications asdesignated in the API recommended practice and con-ducted in the manner suggested. The API fluid loss isconducted at surface temperature at 100 psi pressure,



and is recorded as the number of millilitres lost in 30 min.The volume of collected liquid VAPI was considered as anindication of the fluid loss properties.

3 Results and Discussions

The experimental conditions and the yields for the syn-thesis of the starch-grafted polyacrylamide copolymerseries (samples A1-A6, B1-B3, C1-C2) are presented inTab. 1. In the same table the details of the control experi-ments are also indicated, i.e., details for the preparationunder similar experimental conditions as the grafted star-ches series but in the absence of starch of two PAMhomopolymers, referenced as samples D1 and D2.

3.1 Effect of initiator and monomer contents onintrinsic viscosity of the copolymer

The effect of CAS, at constant amount of AM (8.44610-2

mol), on the intrinsic viscosity (data not shown), indicatesthat the intrinsic viscosity, [Z], increases with the increasenumber of moles of CAS until it reaches a maximum valueof 7.5 dL/g, at an optimal CAS amount of 2610-4 mol, andthen decreases thereafter. The increase of [Z] up to 7.5dL/g by increasing the amount of CAS, at constantamount of AM (8.44610-2 mol), can be explained by theincrease in molecular weight of the PAM side chains. Thedata indicate that CAS is an efficient initiator for acryl-amide polymerisation when it is used at its optimumamount, resulting in high values of intrinsic viscosity, [Z],and molecular weight of the copolymer. The obtained

results indicate also that at CAS amount above the opti-mal value, a decrease in [Z] occurs. Such decrease in [Z]is due to a reduction in the molecular weight of the PAMside chain and/or the PAM-g-starch copolymer [28], asresulting from the increase in the number of active siteswith increasing the CAS amount. The decrease of PAMmolecular weights may also result from termination reac-tions of growing chains by the initiator [30]. On the otherhand, the effect of AM, at constant amount of CAS(2610-4 mol), on the intrinsic viscosity, shows an increasein intrinsic viscosity with increasing the amount of AM.Further, a slight increase in intrinsic viscosity wasobserved at monomer contents above 8.44610-2 mol.The increase in [Z] with increased amount of AM resultsfrom the increase in the molecular weight of the PAM sidechain, as was observed by others authors [31].

The results of nitrogen analysis, the percents of monomerconversion, XAM, the degree of polymerisation, DP, andthe intrinsic viscosity [Z] values for various samples arepresented in Tab. 1. As can be observed in the table, XAM

is greater than 56%, while Xgrafted ranges from 2.1% to16.8%. Further, Tab. 1 indicates that the highest CASamount, 4610-4 mol, gives the highest grafting,Xgrafted = 16.8%, while the highest monomer amount,AM = 21.1610-2 mol gives the highest degree of polym-erisation, DP = 728 and the highest intrinsic viscosity,[Z] = 11.23 dL/g. Such results show clearly that cericammonium sulphate (CAS) generates free radicals on thestarch molecules from which polyacrylamide grafts grow.Regarding the variation of DP with the concentrations ofmonomer, AM, and initiator, CAS, it appears from Tab. 1that DP increases either by increasing AM at constant

Tab. 1. Experimental conditions for the synthesis and properties of starch-graft-polyacrylamide copolymers.

Sample Starch[mol610-5]

AM[mol610-2]

CAS[mol610-4]

Yield[%, w/w]

Ni

[%, w/w]Nf

[%, w/w]XAM

[%]Ng DP [Z]

[dL/g]

A1 2.38 8.44 1.0 75 13.1 - - 2.1 - 2.4A2 2.38 8.44 1.5 80 13.1 7.7 59 6.3 332 5.5A3 2.38 8.44 2.0 90 13.1 9.3 70 8.4 295 7.5A4 2.38 8.44 2.5 100 13.1 8.7 66 10.5 123 6.1A5 2.38 8.44 3.0 82 13.1 7.4 56 12.6 158 4.8A6 2.38 8.44 4.0 100 13.1 - - 16.8 - 3.5B1 2.38 4.22 2.0 79 9.8 8.5 87 8.4 184 4.5B2 2.38 14.07 2.0 88 15.0 10.4 69 8.4 485 8.6B3 2.38 21.10 2.0 92 16.4 9.8 60 8.4 633 9.5C1 4.76 14.07 2.0 89 12.3 9.2 75 4.2 527 10.5C2 4.76 21.10 2.0 94 14.7 10.1 69 4.2 728 11.2aD1 - 8.44 1.0 Nil - - - - - -aD2 - 8.44 1.5 Nil - - - - - -

(a) Samples D1 and D2 are PAM homopolymers prepared in the absence of starch and under similar experimental condi-tions than starch-grafted polyacrylamide copolymer series.



CAS, or by decreasing CAS at constant AM. The poly-mer’s weight-average molecular weight, Mw, can be esti-mated from the intrinsic viscosity [Z]. The Mark-Houwinkequation [Z] = KMa is generally used to estimate Mw forlinear polymers, where K and a are constants dependingon the polymer, the solvent and the temperature. For iso-lated PAM the values of K and a were determined [32] andare given in Equation (10):

[Z]= 6.31610-5 Mw0.80 (10)

Thus, taking into account the fact that the percentage ofstarch is small, between 0.01 and 0.06, in comparison tothe polyacrylamide, it is then very likely that the increaseof [Z] with the amount of AM is likely due to an increase ofthe PAM side chain molecular weight.

3.2 Infrared spectra of starch-graftedpolyacrylamide copolymers

In all cases, the infrared spectra of the grafted starchesshowed characteristic absorption bands at 1020 cm-1,1080 cm-1 and 1150 cm-1, which are due to carbonylstretching vibrations of the starch substrate. The infrareddata showed others characteristic absorption bandsoccurring at 1670 cm-1 and 1620 cm-1, which correspondto carbonyl and (hydrogen bonded) carbonyl absorptions,of the polyacrylamide CONH2 groups, respectively,. Suchstretching vibrations occurring at 1670cm-1 and 1620 cm-1

were not observed in the infrared spectrum of the non-modified starch. Thus, the obtained infrared spectra of allgrafted starches samples were consistent with the pres-ence of PAM grafting onto starch substrate [28].

3.3 Effect of monomer and initiator contents oncopolymer water solubility

Since we are dealing with water-based mud, the watersolubility of the grafted starches is of prime importance.Further, in oil recovery processes, solubility of polymers inwater is one of the most important and critical require-ments [33]. Various methods [34, 35] were described inliterature to determine the solubility of water-solublepolymers in aqueous media. However, such methods arelimited to very diluted solutions or may be influenced bythe polymer particle size, temperature, or stirring speed.In the present work, we used a quantitative method basedon weighting the insoluble fraction of the polymer inwater. Thus, to determine the water solubility of the syn-thesised polyacrylamide-g-starch copolymers, 1 g of thesample was added to 100 mL of bidistilled water and theresulted mixture was agitated for a given time. The insol-uble fraction of the sample was recovered by filtration,

dried, and weighed. The percent water solubility wasdetermined according to Equation (11):

Water-solubility in % ðw=wÞ ¼ W0�W insW0

(11)

Where W0 is the initial weight of the copolymer (1 g) andWins is the weight of the copolymer-insoluble fraction. Inall experiments dealing with water solubility, the amountof starch was kept constant and equal to 2.381610-5 mol.

For all the starch-grafted polyacrylamide samples syn-thesised in the present work, the water solubility wasfound to decrease from 92% to 82% as the number ofmoles of CAS increases from the optimal value, 2610-4

mol to 4610-4 mol. This decrease in water solubilityresults from an increase in the number of active sites,leading to copolymers having shorter PAM side chains.However, water solubility was found to increase from 82%to 96% with the increase in AM content from 4.2 to21.1610-2 mol at optimal CAS amount. This increase inwater solubility with increasing monomer content resultsfrom grafted starches having fewer but longer and moresolvated PAM side chains leading to less aggregated andstable grafted starches aqueous solutions.

3.4 Effect of temperature, time and salt oncopolymer-aqueous solution viscosity

The effect of temperature on the solution viscosity ofsample C2, at shear rate varying in the range 1-250 s-1, isshown in Fig. 1. It should be emphasised that the tem-perature range of 24-807C was chosen in order to mimicthe well conditions. Further, the sample C2 under investi-gation was prepared with the optimal initiator and thehighest monomer amounts. It is then expected that thesample C2 will have the highest molecular weight and/orthe highest intrinsic viscosity. On the other hand, the vis-cosity of the polymer solution is related to the size of thepolymer chain, which is determined by chain conforma-tions [36, 37]. In general, the polymer dimension, the intra-and inter- molecular association of polymer chains, poly-mer-solvent interaction and external factors (tempera-ture, salinity), control the observed macroscopic beha-viours of polymers, such as water solubility and solutionviscosity [35-37]. Therefore, the highest solution viscosityvalue, , 100 mPa s, as observed in Fig. 1 at 247C, at lowshear rate, may result either from the larger size of theexpanded PAM chain and/or from PAM chains over-lapping. However, as observed in Fig. 1, the copolymersolution viscosity decreases either, at given temperature,by increasing the shear rate, or at given shear rate, byincreasing the temperature. In all cases, shear-thinning(non-Newtonian, pseudo-plastic) behaviours of the



Fig. 1. Effect of temperature on aqueous solution viscos-ity. Sample C2 = 1% (w/v), shear rate range = 1-250 s-1.

starch-grafted polyacrylamide copolymers are observedin Fig. 1. It is likely that shearing grafted starches aqueoussolutions may induce contraction and/or collapse of PAMchains. Similar shear-thinning behaviours in water werealso observed at 207C for polyacrylamide solutions [38],and for dextran-grafted polyacrylamide copolymers [39].According to the reported work [38], the pseudo-plasticbehaviours of polyacrylamide solutions are characterisedby the power-law model, m = K gn-1, where m, is the vis-cosity, g is the shear rate, K and n are, respectively, theconsistency and the flow behaviour indexes.

The decrease of shear viscosity at increasing temperaturecan be explained by an increase in water-PAM interactionenergy, which is expressed by an increase of the Floryinteraction parameter, w [36] leading to contraction orcollapse of grafted polyacrylamide side chains. In gen-eral, the values of the dimensionless solvent parameter ware in the range 0-1, and quantify the polymer-solventinteraction. Good solvents for polymers have low wvalues, while poor solvents have high w values. It is alsoimportant to note that w increases with temperature foralmost polymer-solvent systems. Our results are inagreement with reported work [37] dealing with tempera-ture effect on molecular dimension and interaction pa-rameters of isolated PAM in water-DMSO. In that work, ithas been shown that the increase of temperature (30-507C) leads to the decrease of the dimension and therigidity PAM chains.

The variation of solution viscosity of the copolymer C2and non-modified starch 1% (w/v) with time for a period of12 days is presented in Fig. 2. As can be seen, while thecopolymer viscosity remains constant, within an experi-mental error range of63%, by increasing number of days,starch viscosity starts to decrease after a period of one

Fig. 2. Relative viscosity as function of time. SamplesC2 = 1% (w/v) and non-modified starch = 1% (w/v), shearrate range = 1-5 s-1.

day. Thus, grafting polyacrylamide onto starch substrategives copolymers having enhanced and stable viscosity.In other words, grafting polyacrylamide, which is not bio-degradable onto starch, a biodegradable polymer, greatlyimparts starch resistance to biodegradation.

Fig. 3 shows the variation of the solution viscosity of thecopolymer C2 1% (w/v) in the presence of increasingamounts of monovalent and divalent salts (NaCl andCaCl2). As can be seen, the effect of divalent salts on vis-cosity is more pronounced and PAM side chains becomeless expanded by increasing salt concentration. However,in all cases the viscosity loss of the aqueous copolymersolution with increasing salt concentration is within theexperimental error range of 63%, and is less than 8%.Such behaviours indicate weak electrostatic interactionbetween the salt and the PAM chains owing to non-ioniccharacter of the starch-grafted polyacrylamide copoly-mers.

3.5 Effect of the shear rate on copolymer-aqueous solution viscosity

Fig. 4 shows the solution viscosities for grafted starchesC1, C2, and non-modified starch, at the same sampleconcentration of 1% (w/v). The rheological data shown inFig. 4 were obtained after aging the samples and using ashear rate varying in the range 1–1000 s-1. Hence, thecomparison of the data in Fig. 1 and in Fig. 4, indicate thatwhatever is the shear rate range used, the grafted star-ches are shear-thinning, i.e. having pseudoplastic beha-viours, while that of the non-modified starch is New-tonian. Other features can be observed in Fig. 4 and con-cern the viscosity values, which are several time higher for



Fig. 3. Relative viscosity as function of salt nature andconcentration. Sample C2 = 1% (w/v), shear rate range =1-5 s-1.

Fig. 4. Aqueous solution viscosity as function of shearrate (1-1000 s-1), after aging. Samples C2 = 1% (w/v),C1 = 1% (w/v), and non-modified starch = 1% (w/v).

the grafted starches than for the non-modified starch, inthe whole shear rate range investigated. Therefore, graft-ing PAM chains on the starch backbone leads to shear-stable and viscosity-increasing grafted starches copoly-mers.

3.6 Effect of the amount of starch-graftedpolyacrylamide copolymer on the filtratevolume and the plastic viscosity

The effects of grafted starches on the filtrate volume ofthe WBM, before and after hot rolling, are shown,respectively, in Figs. 5 and 6. As can be seen the filtratevolume decreases with increasing polymer concentration.

Fig. 5. Effect of grafting and polymer amount on the fil-trate volume of fresh water-based mud polymer mixture,before aging, and comparison of the data to the PAC-Lpolymer.

Fig. 6. Effect of grafting and polymer amount on the fil-trate volume of water-based mud polymer mixture, afteraging, and comparison of the data to the PAC-L polymer.

Such decrease in filtrate volume may result from anincrease in the viscosity of the aqueous dispersion and/or an increase of adsorbed amount of polymer on themud clay particles. It is likely that in the polymeric layeradsorbed on the bentonite particles, the starch back-bone is in direct contact with the clay surface, whereasthe grafted polyacrylamide side chains stretch into thesolution to build up a brush and to stabilise the clayaqueous dispersion. Thus, Figs. 5 and 6 indicate thatgrafted starch sample C2 having the highest DP(DP = 728) is a very effective filtrate reducer. Further-more, the data indicate also that performing an agingtest to the WBM, leads to an increase of filtrate volumethat may result from an increase of the filter cake per-



meability [40]. It is concluded from the collected datathat grafted starches copolymers and PAC-L polymer,may be classified according to their efficiency in reduc-ing the filtrate volume, as follows C2.PAC-L.C1.Figs. 7 and 8 show the variation of plastic viscosity ofthe mud versus the polymer concentration, respectively,before and after aging. As can be observed in thesefigures, the presence of starch-grafted polyacrylamidein the aqueous dispersion medium, at polymer amount�0.5 g, leads to mud plastic viscosities values higherthan that obtained with the PAC-L. The present workindicates that grafted starches having higher DP valuesare suitable in decreasing the filtrate volume andincreasing the plastic viscosity of the water muds.

Fig. 7. Effect of grafting and polymer amount on theplastic viscosity of fresh water-based mud polymer mix-ture, before aging, and comparison of the data to thePAC-L polymer.

Fig. 8. Effect of grafting and polymer amount on theplastic viscosity of water-based mud polymer mixture,after aging, and comparison of the data to the PAC-Lpolymer.



4 Conclusions

Starch graft-polyacrylamide copolymers were synthesisedby free-radical polymerisation and were used to improve thestability and the rheological properties of water-basedmuds. The concentrations of the reactants in the synthesismedium play a major role in controlling both the properties ofthe copolymer aqueous solutions and the rheological prop-erties of water-based muds. Based on these investigations,the main conclusions are the following:

. the intrinsic viscosity, the molecular weight, and thesolubility in water of grafted starches may be controlledby the initiator and monomer concentrations.

. the grafted starches are salt resistant and theirrheological properties are stable with time as com-pared to the non-modified starch.

. aqueous solutions of grafted starches behave as shear-thinning, while aqueous solutions of non-modifiedstarch behave as Newtonian fluids.

. the grafted starches may be designed to control thestability and rheological properties of the mud. Hence,under certain experimental conditions, the graftedstarches are more efficient filtrate reducers and givehigher plastic viscosity than the commercial PAC-Lpolymer.

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(Received: March 18, 2008)(Revised: September 12/September 28, 2008)(Accepted: September 29, 2008)


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Preparation and Aqueous Properties of Starch-grafted Polyacrylamide Copolymers