Starch-based Surface-sizing Agents in Paper Industry:An Overview

Qiulin Wei,Hao Zheng,Mengni Zhu,Xiao Han,Yao Li,Jinghui Zhou

Liaoning Province Key Laboratory of Pulp and Papermaking Engineering,Dalian Polytechnic University,Dalian,Liaoning Province,116034,China

Abstract:Natural starch is an abundant and inexpensive polysaccharide biopolymer that is widely used as a surface-sizing agent in the paper industry.The surface sizing of paper improves its water and abrasion resistance,as well as its physical strength and printing adaptability.However,natural starch presents some disadvantages,such as high viscosity,poor fluidity,poor filmforming properties,and easy coagulation.Therefore,starch is usually modified and blended with various components to achieve better sizing performance.This article reviews approaches for the surface sizing of paper and modification of starch using enzymes or chemical methods,such as oxidation,cationization,and graft copolymerization.This article also discusses the application of starch-based multiphase systems(obtained by blending starch with various components)as surface-sizing agents.

Keywords:starch;surface sizing of paper;modified starch;starch-based multiphase system

1 lntroduction

The paper industry is one of the ten key manufacturing industries of the China's economy.The magnitude of paper production is an important indicator of the degree of modernization and civilization of a country.The surface sizing of paper is an important step in paper production[1].The surface sizing of paper can effectively improve its water resistance,burst resistance,folding resistance,printing adaptability,physical strength,tensile strength,flat compression strength,delamination strength,and ring-compression strength.Surface sizing is considered to be the most effective technique to modify the surface properties of paper or cardboard[2].

Starch is one of the major storehouses of carbohydrate in nature,which is extensively used in food processing as well as in the textile and paper industries owing to its excellent affinity,low cost,stable supply, good biocompatibility, and biodegradability[3].Natural starch comprises amylose and amylopectin molecules linked by a glucoseα-1,4-glycosidic bond or anα-1,6-glycosidic bond(Fig.1)[4].Starch is the most common surface-sizing agent used in the paper industry owing to its molecular structural similarity with cellulose.However,most natural starches contain many hydroxyl groups,and therefore,form strong intra-and intermolecular hydrogen bonds.Thus,the interaction among starch molecules is greatly enhanced,which poses problems such as poor adhesion and film formation,and considerably limits the applicability of starch as a surface-sizing agent[5].To overcome these problems,starch is usually modified and blended with various components to improve its sizing performance.

Fig.1 Molecular structures of amylose(a)and amylopectin(b)

Numrous studies have reported the modification of starch using physical,chemical,biological,genetic,and compound modification methods[6].Among these methods,chemical modification is widely prevalent.This article discusses chemically modified starch,including oxidized starch,cationic starch,graft copolymer starch,and enzyme-degraded starch[7];and the preparation of surface-sizing agents by blending starch with various components.These surface-sizing agents reduce the production cost and improve the properties of paper[8].

2 Modified starches used for surface sizing

2.1 Oxidized starch

The oxidation of starch refers to the destruction of glycosidic bonds and degradation of starch molecules by oxidants.The hydroxyl group can be oxidized by an oxidizing agent to form a carbonyl group,an aldehyde group,or a carboxyl group;this also leads to changes in the properties of starch[9].Oxidized starch,as a surface-sizing agent for paper,forms a continuous film on the surface of paper,thereby fixing the surface fibers as well as improving the writing and printing properties of the paper[10].

Wang et al[11]used commonly available oxidants,namely ammonium persulfate,hydrogen peroxide,and potassium permanganate,to modify wheat starch,and discovered that the modified wheat starch can be used for the surface sizing of paper.In particular,the ammonium persulfate-modified wheat starch exhibited the optimum sizing effect, contributing to improvements in the compressive strength and breaking length of the paper.Zhang et al[12]employed the TEMPO/NaBr/NaClO system to catalyze the oxidation of starch obtained using enzymatic hydrolysis. The prepared oxidized starch was subsequently used for the surface sizing of paper.The oxidized-starch glue penetrated the paper easily,thereby improving the bond strength between the fibers as well as the properties of the paper.Zhang et al[13]obtained high-performance,oxidized cationic surfacesizing starch via oxidative etherification. They observed that the oxidized cationic starch exhibited enhanced surface strength,smoothness,and water resistance compared to ordinary oxidized starch.And its sizing effect was adequate to meet the requirements of surface sizing in double-adhesive paper production.Kimet al[14]prepared a dextrin polyacrylic acid copolymer(52%solid content)as an additive to be added to oxidized starch for use as a surface-sizing agent.They observed that the addition of the dextrin polyacrylic acid copolymer improved the rheological and gelation properties of starch slurry and prevented the starch from cooling and regenerating.

2.2 Cationic starch

Cationic starches are positively charged ether derivatives that contain amino groups,and are produced by the reaction of starch with amines[15].Cationic starches exhibit high solubility, low gelatinization temperature,high fluidity,and stable viscosity.Considering that many anions are present in the pulp during papermaking,the retention rate of the system can be improved by adding a cationic starch to promote papermaking[16].

An et al[17]investigated the optimum process to prepare cationic starch,and employed it to obtain the best starch paste solution with the optimum degree of substitution and solid content.Paper surfaces sized with a cationic starch exhibited lower water absorption,as well as superior ring-crush index,tensile strength,and printing adaptability,compared with those sized with raw starch.Lee et al[18]prepared several cationic starches and studied the effect of the cationic starch type on paper properties and papermaking.Cationic starches offer a high adsorption rate on fiber and good water-holding capacity,thereby improving the fines retention and paper strength.The base paper sized with cationic starch exhibited better opacity,whiteness,and printing quality.Lu et al[19]mixed dialdehyde chitosan(D-CTS)with cationic starch(CS)to prepare a D-CTS/CS composite surface-sizing agent.After coating the paper surface with the agent,the optimum tensile index and tear index were determined.Simultaneously,the condensation of D-CTS,CS,and cellulose occurred,increasing the degree of cross-linking.The paper surface exhibited a relatively smooth coating and improved mechanical properties.Seo et al[20]added a cationic polyacrylamide to an oxidized starch solution to retain starch molecules on the paper surface during surface sizing.This resulted in better retention rate and stiffness of the starch,as well as improved bending stiffness and whiteness of the paper.

2.3 Grafted copolymer of starch

Grafting has emerged as an essential method to modify polymers[21].The graft copolymerization of starch improves its properties,and therefore has application prospects in green papermaking, environmental protection,as well as in light and textile industries[22].

Chen et al[23]conducted emulsion polymerization to graft a polyurethane prepolymer as a hydrophobic monomer on the degradable cationic starch.A stable and comprehensive polyurethane-graft starch was thus obtained as a surface-sizing agent.The modified starch was suitable for the surface sizing of corrugation base paper.Fu et al[24]employed a styrene(St)-acrylic emulsion to synthesize a soap-free surface-sizing agent via soap-free emulsion polymerization.Starch,the Fenton reagent,and rosin were used as the main chain,initiator, and functional monomer, respectively.Cationic and cross-linking monomers were also added.Surface sizing using the modified starch imparted improved heat resistance and film-forming properties to the paper.Wang et al[25]grafted copolymerized starch with St and butyl acrylate(BA).The synthesized starch-graft polymer exhibited pseudoplasticity and good thermal stability,and was used as a surface-sizing agent.The researchers observed that the waterabsorption rate(WAR)was the lowest when the molar ratio of St/BA was 1:1.Furthermore,the ring-crush index(RCI)and bursting strength(BS)were the highest when the molar ratio of St/BA was 1:2(Table 1).Zhang et al[26]used 4,4-diaminostilbene-2,2-disulfonic acid,N-methylolacryla-mide,and diethanolamine to prepare stilbene-type fluorescent whitening monomers(FBs).The FBs,acrylamide,and St were used as monomers to graft-copolymerize with starch and prepare a new multifunctional starch-graft copolymer fluorescent emulsion(FBs-St)(Fig.2).This emulsion was used in the surface sizing of paper to increase the paper whiteness,inhibit the yellowing of paper,and improve the surface and strength properties of paper.

Fig.2 Synthesis routes of FBs(a)and FBs-St(b)

Table 1 Physicalproperties of paper sized with starchgrafted polymer by varying the St/BA molar ratio

2.4 Enzymatic starch

The modification of starch using amylase is a biocatalytic process and occurs rapidly under mild conditions.Amylase can change the internal structure of starch and destroy the connection between the branch and the main chain.The formation of new hydrogen bonds reduces the viscosity[27].The enzymemodified starch is used for the surface sizing of paper,where the starch penetrates the interior through the pores.The newly generated hydrogen bonds interact with pulp fibers,leading to improved fiber rigidity and physical strength of the paper[28].

Liu et al[29]adjusted the viscosity of gelatinized starch by controlling the amount of amylase via amylase hydrolysis.The physical strength of paper increased when the modified starch was used for surface sizing.This indicates that more free hydroxyl groups are exposed after enzymatic degradation,and the binding point between the starch and fiber is increased.Xu et al[30]hydrolyzed starch using mediumtemperatureα-amylase,and subsequently used the starch with different viscosities(that is,different molecular weights) in the surface sizing of paperboards.With an enzymatic-starch viscosity of 75 mPa·s and surface-sizing concentration of 12%,the ring-crush index and stiffness of the paperboard improved to varying degrees.Qi et al[31]used liquid bioamylase to degrade starch and determined the optimum degradation process.Subsequently,they applied the degraded starch to the surface of a carbonless copy paper base.Compared with the sizing effect of traditional oxidized starch,the stiffness and surface strength of the base paper improved,while the cost of surface sizing reduced.

3 Starch-based multiphase systems as surface-sizing agents

3.1 Mixing starch with other raw materials

When either starch or modified starch is used as a surface-sizing agent,it is difficult to meet the performance requirements of paper.To achieve wideranging paper properties,starch can be blended with other substances to yield better surface-sizing agents[32].Huang et al[33]used a sodium silicate and starch composite to obtain a starch-sodium silicate composite system that demonstrated high solid content and low viscosity.The ring-crush index and anti-rupture index of the paper improved remarkably after surface sizing.Furthermore,the abovementioned sizing agent also resolved the problems of low stiffness of the adhesive layer and limited increase in the ring-crush strength during the surface-sizing of paper.Moreover,the addition of a certain amount of silicone oil improved the water resistance of the finished paper.Lu et al[34]added a certain amount of aluminum sulfate to corn starch to size the surface of corrugated paper,and observed decreases in the viscosity and pH value of the starch glue solution.When the mass percentage of aluminum sulfate was 3%,the surface-sized corrugated paper exhibited the highest water resistance.The ringcrush strength,breaking resistance,and tensile strength of the paper improved remarkably when the mass percentage of aluminum sulfate was increased to 6%.Peng et al[35]used a starch-sodium silicate composite system as the paper surface-sizing agent.The ringcrush strength and burst strength of the surface-sized paperboard were between those of the papers sized using sodium silicate and starch separately,meeting the requirements of use.Additionally,compared with sodium silicate,the composite system exhibited a high solid content,low viscosity,and good film-forming ability,thus overcoming the limitations of the low coating weight of the slurry and insufficient hardness of the slurry layer.Christophliemk et al[36]prepared a barrier-sizing agent for flexible packaging paper by blending octenylsuccinic acid-modified hydroxypropyl corn and potato starch with polyvinyl alcohol(PVOH)and ethylene-modified polyvinyl alcohol. The hydrogen bonds between starch and hydroxyl groups in PVOH molecules formed a compact,thin layer on the paper surface,thereby improving the film-forming ability and water-vapor barrier performance of the starch and PVOH films.Brodnjak et al[37]used a mixture of chitosan and rice starch to size the surface of packaging paper.After an ink-friction test,they observed that compared with uncoated paper,the coated packaging paper exhibited higher optical density.The packaging paper coated with the mixture of chitosan and starch exhibited optimum printing quality,smoothness,porosity,wear resistance,and other properties.

3.2 Starch-based hybrid emulsion

Gong et al[38]prepared a stable 25%(V/V)emulsion of alkenyl succinic anhydride(ASA)using natural corn starch and bentonite particles as stabilizers.They observed that the size of the emulsion droplets decreased when the mass percentages of bentonite particles and starch in the ASA emulsion equaled 15%and 3%,respectively.Moreover,the stability of the emulsion and the surface-sizing performance of the slurry improved distinctly.Zhang et al[39]added St,BA,and dimethylaminoethyl methacrylate(DM)to the polymer dispersant for soap-free emulsion polymerization to synthesize a cationic copolymer(PSBD).The surface sizing of corrugated cardboard was carried out after blending the emulsion with oxidized starch.The tensile strength and ring-crush index of corrugated cardboard were observed to be higher than those of starch base paper.Zhang et al[40]used St and BA as monomers and oxidized starch as solid-particle stabilizers to prepare a new Pickering Stacrylic polymer emulsion.The experimental results demonstrated that the emulsion had good stability,water resistance,and film formation and thus,could be used as a surface-sizing agent for paper.

Xu et al[41]used St,BA,and methyl methacrylate as the main raw materials,low-viscosity cationic starch(CS-8)as an emulsifier and dispersant,and rosin as a functional monomer to prepare a styrene acrylate emulsion(SAE).When the amount of CS-8 was 8 wt%,ammonium persulfate(APS)was 0.5 wt%,rosin was 2 wt%,an excellent sizing effect could be obtained by using SAE for paper surface sizing.Compared with unsized paper,the water resistance of the paper sized with the rosin-modified SAE increased remarkably.

3.3 Starch-based bio-nanocomposite materials

The application of nanotechnology and nanostructured materials in the field of papermaking has provided new opportunities for creating new types of papers with enhanced performance[42].In recent decades,various starch-based bio-nanocomposites that impart functionality to paper and paperboard have been developed.Starch and its derivatives can be used as nanofillers as well as matrixes owing to their versatility[43].

Salam et al[44]modified starch nanoparticles(SNs)using diethylenetriamine pentaacetic acid (X)and subsequently conjugated the modified starch nanoparticles(SNX)with chitosan(C)to synthesize nano-starch derivatives(SNX/C).The tensile strength and breakage index of waste corrugated board coated with 1%SNX/C solution increased by 120%and 70%,respectively. Furthermore, the gloss improved remarkably,whereas the roughness and tearing strength of the board decreased.The improvements in the tensile strength and breaking index indicate an enhanced adhesive force between the synthetic SNX/C and the pulp fiber.

Nanocrystalline cellulose(NCC)is an emerging renewable nanomaterial.Cellulose nanomaterials are extensively used to reinforce starch-based biological nanocomposites owing to the similar polysaccharide structure of cellulose nanomaterials and the starch matrix,which is conducive to the interaction between nanofillers and the matrix.Yang et al[45]prepared 0.1 wt%and 0.5 wt%NCC-reinforced starch-based composite suspensions and used them as surface-sizing agents for cellulose paper.When the mass percentage of NCC was 0.3 wt%,the tensile index,tearing index,folding strength,and burst index of the surface-sized paper increased by 6%,9%,23%,and 4%,respectively,compared to those of the paper sized using a starchbased composite suspension not containing NCC.The experimental results revealed that NCC effectively improved the mechanical properties and air permeability of the surface-sized paper.In addition,starch exhibited good compatibility with nano-zinc oxide particles.Ni et al[46]introduced nano-zinc oxide into a starch solution using magnetic stirring and ultrasound to prepare a hydrophobic starch solution containing nanoparticle-coated starch particles.This was followed by the process of film formation and surface sizing of paper.Nano-zinc oxide particles form a hierarchical structure on the starch-film surface and a complete hierarchical structure on the coated paper,thereby improving the hydrophobic properties of the film and paper.

4 Summary and Prospects

This study discusses several chemically modified starches as well as the application of starch-based multiphase systems as surface-sizing agents.In recent decades,many starch modification and blending methods have been developed to achieve the desired surface-sizing properties suitable for various applications in the paper industry.

Oxidized starch and cationic starch are the most common types of starch used in the paper industry.Moreover,many grafted copolymer starches and enzymatic hydrolysis-modified starches have been developed.Enzymatic hydrolysis-modified starch is environment-friendly and more economical than other modified starches.The quality of starch can be effectively controlled by replacing oxidized starch with enzymatically hydrolyzed-modified starch.Although starch and its derivatives are extensively used as surface-sizing agents, meeting the performance requirements of paper using starch alone is difficult.To achieve specific functional requirements,starch-based multiphase systems (composites or blends) are attracting increased research interest.Numerous studies have been conducted on starch-based bionanocomposites;however,their large-scale application as paper surface-sizing agents has not yet been realized.Therefore,further research is necessary to realize the industrial applications of these surfacesizing agents.

Acknowledgments

This study was supported by National Natural Science Foundation of China(Nos.31800498,22078035),Natural Science Foundation of Liaoning Province of China(No.2019-BS-16),Open Fund of Jiangsu Provincial Key Laboratory of Pulp and Paper(No.KL201903).