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Abstract： Biolatex， a new type of paper coating adhesive that has been developed in recent years， has mainly been used in pre-coating and middle coating， due to its advantages of decreasing the associated cost and pollution by partly replacing petro-latex. In this paper， application of biolatex as a surface sizing agent and effects of various factors， such as titanium dioxide addition， on the properties of the sized paper were compared. The results showed that， addition of titanium dioxide to biolatex could improve the whiteness， internal bond strength， and ink absorption. Furthermore， application of the No.4 biolatex resulted in better surface strength， bond strength， and ink absorption of the sized paper than application of polyvinyl alcohol.
Keywords： biolatex； polyvinyl alcohol （PVA）； titanium dioxide； surface sizing
1 Introduction
Surface sizing agents （SSAs） are used in the paper industry in order to improve the surface physical strength， friction resistance， waterproof and smoothness of paper[1]. The type of SSA has a significant effect on the final product’s properties， and the commonly used SSAs include starch， gelatin， polyvinyl alcohol （PVA）， methyl cellulose， carboxymethyl cellulose， and paraffin wax[2-4]. However， growing environmental awareness triggers the development of new SSAs.
The advantages of low cost and environmental friendliness made biolatex—a new type of paper coating adhesive—a research hotspot of many enterprises[5]. This kind of SSA exhibits many excellent properties such as good cohesiveness and flowing property， low cost， and low environmental impact. Moreover， it can partly replace petro-latex， lowering the costs and abating the pollution， with no influence on the paper properties[6-7].
There are two kinds of additives used in coating formulations to improve the brightness and whiteness of paper： titanium dioxide （TiO2） that enhances light scattering and optical brightening agents （OBAs）[8]. ECOSPERE biolatex， also known as TiO2-biolatex， is a kind of nano-polymer which is treated by TiO2 combined with biolatex， produced by physical and chemical modifications in an extruder[9]. The distance between the nanoparticles in the complex gives better light scattering efficiency， and can improve the brightness and whiteness. The minimal amounts of TiO2 used in the process do not much increase the associated cost.
In view of the excellent performance of biolatex and TiO2， TiO2-biolatex compounds were prepared by microwave-assisted method. The effects of the biolatex and TiO2-biolatex surface sizing compounds on the properties of the sized paper were investigated. 2 Experimental
2.1 Materials
During surface sizing， the reference sample was sized with PVA （1788） from Usolf and the other samples with biolatex adhesives obtained by the research group. Rutile （average particle size 640 nm） from Billionschem was used as the functional additive for biolatex. Electrostatic copy paper （basis weight 70 g/m2） from TANGO was used as base paper.
2.2 Methods
2.2.1 Surface sizing
Surface sizing is a technique of applying a thin layer of adhesive material to the paper’s or paperboard’s surface to make it hydrophobic. This operation not only improves the physical strength of the paper， but also enhances its resistance， durability， handling property， and surface smoothness. It also overcomes the paper’s lint and dusting defects. There are several different surface sizing methods including tub-sizing， roller sizing， calender sizing， and drying cylinder sizing. A rod coater （R303 MULTI COATER） was used in this study because the cylindrical design of this rigid metering device stresses the base sheet less than a blade coater， and the rod can also be rotated around the web during its operation. The SSAs were applied to the coater to cover the paper’s surface， which was then dried in a polishing dryer （TD19-A， Xianyang Tongda Light Industry Equipment Co.， Ltd.） to dryness level above 90%. Sample No.0 was coated only with PVA， No.1～No.5 were coated with biolatex adhesives with TiO2， and samples ranging from No.1-2 to No.5-2 were coated with biolatex adhesives without TiO2， under the same operation conditions. In addition， the different modification methods of the biolatex preparation process resulted in different performance of biolatex No.1～No.5.
2.2.2 Paper properties testing
The sized paper samples were tested using the national ISO standards for whiteness， IGT surface strength， internal bond strength， and ink absorption. For the ISO whiteness measurement， an Intelligent Whiteness Meter WS-SD was used and the test method GB/T 7974—2013 was followed. The IGT surface strength was measured using a Printing Adaptability Tester J-IGT350 and the test method GB/T 22365—2008 was followed. The internal bond strength and the tensile strength were measured using interlayer adhesion strength tester KRK2085-D and tensile strength analyzer 062/969921， respectively， and the GB/T 26203—2010， and GB/T 453—2002 methods were followed， respectively. The ink absorption was measured using an Ink Absorption Meter YM-20 following the GB/T 12911—1991 method. The results were reported as average of the sized paper samples. 3 Results and discussion
3.1 The differences between PVA and biolatex
PVA was commonly used in surface sizing to efficiently improve surface strength of paper. Generally， biolatex was applied specially in pre- and middle coating； however， here it was used for surface sizing. It can be seen in Fig.1 that the biolatex improved the paper properties. Application of the 5 biolatex groups had different effects due to different modification methods used in their preparation. Although the biolatex reduced the paper’s whiteness and surface strength compared with PVA， overall， they remained high （Fig.1a and Fig.1b， respectively）. The paper’s internal bond strength and tensile strength are closely related to adhesive’s migration and its adhesion properties. After an adhesive is applied to the paper’s surface， a part of it is left on the surface to form a film. This film improves the surface strength， friction and water resistance， and other paper properties. The other part of the adhesive， together with the moisture， penetrates the paper sheet’s interior and binds the fibers together to increase the paper’s strength. These factors influenced the internal bond strength （Fig.1c） and the tensile strength （Fig.1d） of the sized papers. The experiment showed that the biolatex could effectively improve the paper’s ink absorption （Fig.1e）. Based on the above performance， it can be concluded that application of the biolatex No.2-2 and No.4-4 had good effects.
3.2 The effects of TiO2 addition to biolatex on surface sizing
TiO2， in the form of rutile， has been used in the coating industry； its excellent performance is due to the stable structure of rutile， good covering power， decorative functions， enhanced chemical stability， and other advantages[10]. Similar to the OBAs， TiO2 is commonly used in coating formulations as a high-whiteness pigment to improve whiteness and brightness of paper and paperboard[9]. TiO2 is used as functional additive to the biolatex in order to obtain TiO2-biolatex compound， which can effectively improve optical properties of sized paper， and have a certain effect on its mechanical strength and adhesion of coating membrane[10]. Below are descriptions of the results on the effects of TiO2 addition on the whiteness， surface strength， internal bond strength， tensile strength and ink absorption of the sized paper.
3.2.1 Whiteness
Fig.2 shows the effect of TiO2 on the ISO whiteness of sized paper. Before the TiO2 addition， the difference among the samples’ whiteness that were at the average level of 102%， was small， and the whiteness increased after the TiO2 addition. The TiO2-biolatex compounds were prepared by microwave-assisted method， which maintained a certain distance between the TiO2 particles. This enhanced the light scattering efficiency and consequently the paper’s optical properties improved. The whiteness of the samples No.3， No.4， and No.5 increased slightly； on the other hand， the whiteness of samples No.1 and No.2 improved significantly by about 9.4% and 5.8%， respectively. TiO2 is a powder with a high degree of whiteness， which improves light scattering efficiency and whiteness of the coating layer when uniformly distributed in biolatex， but the oxide must be in an optimum concentration. That is why the sized paper’s whiteness gradually increased with the addition of TiO2， up to the point where the opposite effects occurred due to crowding of the TiO2 particles. This turning point is the optimum concentration. The effects of TiO2 were diverse due to different surface charges of the biolatex， which caused different distributions of the components； thus， the optimum concentrations of these samples were different. 3.2.2 Surface strength
As can be seen in Fig.3， the effect of TiO2 on the paper’s surface strength was diverse. TiO2 addition reduced the surface strength of sized paper， with the exception of the sample No.4. In previous studies， it was found that the colloidal particles of No.4 biolatex were positively charged and the particles of the other biolatexes were charged negatively. After the surface sizing agent was applied to the paper’s surface， the colloidal particles migrated during the drying process rendering the sizing layer’s uneven distribution. The TiO2 particles were positively charged and formed a TiO2-biolatex conjugate system after they were added to the biolatex. On the other hand， the fiber was negatively charged and when the cationic biolatex was applied to the paper， these cationic colloidal particles were quickly absorbed on it and the migration intensity was relatively low. Furthermore， water-soluble adhesive would form a rough sized surface， while some of the TiO2 particles added to the biolatex should fill the pores to improve the sizing layer’s smoothness， some should migrate with the water to the paper interior， while most of the particles should be retained near the surface. These processes improved both the surface strength and the surface smoothness of sized poper. However， because the negatively charged biolatex that possessed a large number of negatively charged colloidal particles absorbed onto the TiO2 surface， more TiO2 migrated with the colloidal particles to the paper interior under the same dosage； thus， the amount for sizing on the surface was reduced， consequently reducing the surface strength.
3.2.3 Internal bond strength
Various preparation methods of the biolatex adhesives lead to different colors， adhesion performances， and application potentials. Their application to surface sizing also changed the internal bond strength of the sized samples （Fig.4）. After TiO2 addition， the internal bond strength of the sized paper increased by various degrees. For sample No.4， the increase was by ～26% and the effect was obvious. The main reason for this phenomenon was even distribution of the TiO2 particles in the biolatex； thus， when the surface sizing agent was applied to the paper’s surface， the particles could migrate with the latex adhesives and then fill the internal pores of the paper. The fiber’s surface is usually negatively charged and when the TiO2 and the colloidal particles migrate to the paper interior， TiO2 is absorbed onto the fiber’s surface， which formed an uneven distribution between the fiber layer and the adhesive， what closely bonded the fiber and TiO2 together. Hence， the paper was difficult to peel as the z-direction bond strength increased due to the increased internal bond strength of the sample. 3.2.4 Tensile strength
Fig.5 illustrates the influence of TiO2 on the tensile strength of sized paper. The tensile strength of all the samples decreased due to TiO2 addition. This might be due to the different concentration of TiO2 in the biolatex. At the same sizing agent dosage， the content of the colloid particles in the biolatex adhesives with added TiO2 was lower compared to the sizing agent without TiO2. The strength in the x-y direction is mainly related to the fiber’s strength itself and the internal bond strength between the fiber layer in this direction. Some of the biolatex migrated to paper interior and adhered to the fibers to enhance paper properties. However， after adding TiO2 to the biolatex， the conjugate migration occurred （biolatex and TiO2）， hence there was less adhesive in the paper， at the same applied sizing agent dosage， and the bond strength between its fibers was weakened. All of the phenomena lead to the tensile strength of sized paper decline.
3.2.5 Ink absorption
The effect of TiO2 on the ink absorption of sized paper has been shown in Fig.6. It can be seen from Fig.6 that the ink absorption of the sized paper increased due to the addition of TiO2 similarly for all the samples. The sized surface after application of water-soluble adhesives is rough compared to the latex adhesives. Such surface promotes ink absorption. TiO2 is a pigment characterized with good ink absorption used in papermaking. TiO2 that used in this study （rutile， 625 nm particle size powder） mixed with the biolatex and sized on the paper’s surface and increased its porosity along with the ink absorption increase[11]. For the same sizing agent dosage， TiO2 sized layers were basically the same， although their properties were different due to the biolatex and TiO2 arrangement. Nonetheless， most of the positively charged TiO2 was arranged near the sizing layer’s surface， which had positive effects on the ink absorptivity. There were some differences between the samples due to different arrangement of the sizing layer.
3.3 The effects of biolatex on the sized paper properties
3.3.1 Whiteness
Both surface sizing agents used in this study， the PVA and biolatex， were water-soluble adhesives. They form a rough sizing surface that gives more efficient light scattering and better optical performance. With different color of these five kinds of biolatex， the color of sizing layers was different. Fig.7 shows the whiteness of sized paper. The whiteness of the control sample （sized with PVA） was around 105%， and the whiteness of the samples No.1 and No.2 was 9% and 5% higher than that of the paper sized with PVA， respectively， and the whiteness of sample No.4 was similar to the one of the PVA sample. The main reason for these differences was the different color of the adhesives that coated the paper’s surface with a layer of thin film exhibiting a certain cover effect on the paper’s surface. In conclusion， the biolatex could compete with PVA in terms of whiteness， nonetheless， samples No.1 and No.2 displayed even higher whiteness. 3.3.2 Surface strength
After the adhesive was applied to the paper’s surface， due to the water migration to the interior of the paper， a series of changes in fiber swelling occurred. During the drying process， the adhesive migrated to the sizing surface together with the water， and as the water content was reduced， the adhesive attached to the fiber’s surface， formed a film， and the fiber contracted. With the film’s high strength and good cover effect， the surface attained high strength and its resistance to lint and dust was improved. PVA is a strong cohesion adhesive with good cover effect limiting the lint and dust phenomena. After the printing adaptability measurement of the sized paper， a layer of significant high gloss film on the samples’ surfaces was found. As it was shown in Fig.8， the surface strength of all the samples was over 1.8 m/s， with the sample No.4 having the best score， 5% higher than that of the PVA sized sample.
3.3.3 Internal bond strength
Fig.9 provides the information about the internal bond strength of the sized paper. The internal bond strength was closely related to the adhesive migration and the bond property. Part of the surface size agents applied to the papers formed a film on the surface improving the surface strength， rub resistance， waterproof， etc. The other part migrated to paper interior together with the water and cohered the fibers to improve the strength. The bond strength among the samples was diverse due to the different preparation methods of biolatex. Also， the migration was differing from one another with different particle size and their charge. The internal bond strength of the samples No.2～No.4 were similar and 28% higher than the PVA sized sample. At the same time， the result of the sample No.1 was similar to the control one （sample No.0）， while sample No.5 obtained a lower value than sample No.0. Therefore， the treatment applied to the samples No.2～No.4 gave positive effects on the sized paper internal bond strength.
3.3.4 Tensile strength
The surface sizing agent sized on paper’s surface partly migrated to the paper interior and cohered its fibers significantly increasing the paper’s strength. Fig.10 shows the tensile strength of the sized paper. Comparing these samples， it can be seen that the effect of the biolatex on the tensile strength was similar to the PVA’s effect； however， the sample No.5 attained higher tensile strength than the control sample （sample No.0）. The main reason of these changes was the migration of the surface sizing agent between the fibers and the pores in the paper sheet， and the subsequent migration during the drying process. This caused an uneven distribution of the agents inside the paper， both in the z-direction and the x-y plane. The high bond strength and the surface sizing agent content in the x-y plane improved the tensile strength. For example， the surface strength of sample No.3 was lower than the PVA sized one’s. There was high internal bond strength but low tensile strength， and these phenomena showed that the bond strength of sample No.3 was relatively lower， the migrated agent’s volume was larger， hence more colloidal particles were distributed along the z-direction. In the same way， the different bond strengths and various particle migration intensities led to diverse tensile strengths. In summary， the tensile strengths of these samples were similar to the one sized with PVA. 3.3.5 Ink absorption
Paper needs to have a good ink absorption property to insure proper printing quality. As can be seen from Fig.11， the ink absorption of all the biolatex sized samples were higher than that of the PVA sized sample. With starch as raw material， the biolatex was a new paper coating adhesive which shows good cohesiveness and flow ability. As a result， these five kinds of biolatex sized papers presented good ink absorption.
4 Conclusions
By comparing the surface sizing performances of biolatex and PVA， it was found that the biolatex was suitable for surface sizing and could be further processed to improve its performance. TiO2， used as a functional additive for the biolatex， could produce excellent effects on the whiteness， internal bond strength and ink absorption， however it could have adverse influence on the tensile and surface strength.
Among the investigated five kinds of biolatex applied to surface sizing， the surface strength of the sample sized with the No.4 biolatex was the best and higher than that of the control sample sized with PVA， the remaining sizing agents achieved results slightly below the control sample. The internal bond strength was the best after treatment with the biolatexes No.2～No.4 that scored above the control sample， and the result obtained for sample No.1 was similar to the PVA sized one. All the samples had the tensile strength similar to the PVA sized one and higher ink absorption. In conclusion， the surface sizing performance of the biolatex No.4 was the best， appropriate TiO2 concentration was advantageous to surface sizing and the biolatex could be mixed with PVA in order to achieve better properties.
References
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[2] Ning L H. Synthesis and Application of cationic surface sizing agent[D]. Hangzhou： Zhejiang University， 2013.
[3] Zhang G Y， Wang F， Li J J， et al. Synthesis and application of styrene-acrylate surface sizing agent[J]. Transaction of China Pulp and Paper， 2014， 29（3）： 6-10.
[4] Tang C W， Liu Z L， Cao Y F， et al. The preparation of starch sodium octenyl succinate and its application as surface sizing agent[J]. Materials Science and Engineering Technology， 2014， 936， 992-996.
[5] Zou J， Lin T. Effects of biolatex binders on the properties of color ink-jet printing paper[J]. Transaction of China Pulp and Paper， 2017， 32（2）： 22-27. [6] Du Y F， Liu J G， Wang J F. Study on the properties of starch based bio-latex prepared with different methods[J]. China Pulp & Paper， 2017， 36（1）： 1-8.
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[10] Zhang Y J. Preparation Of papermaking special titanium dioxide[D]. Qingdao： Qingdao University of Science & Technology， 2011.
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Keywords： biolatex； polyvinyl alcohol （PVA）； titanium dioxide； surface sizing
1 Introduction
Surface sizing agents （SSAs） are used in the paper industry in order to improve the surface physical strength， friction resistance， waterproof and smoothness of paper[1]. The type of SSA has a significant effect on the final product’s properties， and the commonly used SSAs include starch， gelatin， polyvinyl alcohol （PVA）， methyl cellulose， carboxymethyl cellulose， and paraffin wax[2-4]. However， growing environmental awareness triggers the development of new SSAs.
The advantages of low cost and environmental friendliness made biolatex—a new type of paper coating adhesive—a research hotspot of many enterprises[5]. This kind of SSA exhibits many excellent properties such as good cohesiveness and flowing property， low cost， and low environmental impact. Moreover， it can partly replace petro-latex， lowering the costs and abating the pollution， with no influence on the paper properties[6-7].
There are two kinds of additives used in coating formulations to improve the brightness and whiteness of paper： titanium dioxide （TiO2） that enhances light scattering and optical brightening agents （OBAs）[8]. ECOSPERE biolatex， also known as TiO2-biolatex， is a kind of nano-polymer which is treated by TiO2 combined with biolatex， produced by physical and chemical modifications in an extruder[9]. The distance between the nanoparticles in the complex gives better light scattering efficiency， and can improve the brightness and whiteness. The minimal amounts of TiO2 used in the process do not much increase the associated cost.
In view of the excellent performance of biolatex and TiO2， TiO2-biolatex compounds were prepared by microwave-assisted method. The effects of the biolatex and TiO2-biolatex surface sizing compounds on the properties of the sized paper were investigated. 2 Experimental
2.1 Materials
During surface sizing， the reference sample was sized with PVA （1788） from Usolf and the other samples with biolatex adhesives obtained by the research group. Rutile （average particle size 640 nm） from Billionschem was used as the functional additive for biolatex. Electrostatic copy paper （basis weight 70 g/m2） from TANGO was used as base paper.
2.2 Methods
2.2.1 Surface sizing
Surface sizing is a technique of applying a thin layer of adhesive material to the paper’s or paperboard’s surface to make it hydrophobic. This operation not only improves the physical strength of the paper， but also enhances its resistance， durability， handling property， and surface smoothness. It also overcomes the paper’s lint and dusting defects. There are several different surface sizing methods including tub-sizing， roller sizing， calender sizing， and drying cylinder sizing. A rod coater （R303 MULTI COATER） was used in this study because the cylindrical design of this rigid metering device stresses the base sheet less than a blade coater， and the rod can also be rotated around the web during its operation. The SSAs were applied to the coater to cover the paper’s surface， which was then dried in a polishing dryer （TD19-A， Xianyang Tongda Light Industry Equipment Co.， Ltd.） to dryness level above 90%. Sample No.0 was coated only with PVA， No.1～No.5 were coated with biolatex adhesives with TiO2， and samples ranging from No.1-2 to No.5-2 were coated with biolatex adhesives without TiO2， under the same operation conditions. In addition， the different modification methods of the biolatex preparation process resulted in different performance of biolatex No.1～No.5.
2.2.2 Paper properties testing
The sized paper samples were tested using the national ISO standards for whiteness， IGT surface strength， internal bond strength， and ink absorption. For the ISO whiteness measurement， an Intelligent Whiteness Meter WS-SD was used and the test method GB/T 7974—2013 was followed. The IGT surface strength was measured using a Printing Adaptability Tester J-IGT350 and the test method GB/T 22365—2008 was followed. The internal bond strength and the tensile strength were measured using interlayer adhesion strength tester KRK2085-D and tensile strength analyzer 062/969921， respectively， and the GB/T 26203—2010， and GB/T 453—2002 methods were followed， respectively. The ink absorption was measured using an Ink Absorption Meter YM-20 following the GB/T 12911—1991 method. The results were reported as average of the sized paper samples. 3 Results and discussion
3.1 The differences between PVA and biolatex
PVA was commonly used in surface sizing to efficiently improve surface strength of paper. Generally， biolatex was applied specially in pre- and middle coating； however， here it was used for surface sizing. It can be seen in Fig.1 that the biolatex improved the paper properties. Application of the 5 biolatex groups had different effects due to different modification methods used in their preparation. Although the biolatex reduced the paper’s whiteness and surface strength compared with PVA， overall， they remained high （Fig.1a and Fig.1b， respectively）. The paper’s internal bond strength and tensile strength are closely related to adhesive’s migration and its adhesion properties. After an adhesive is applied to the paper’s surface， a part of it is left on the surface to form a film. This film improves the surface strength， friction and water resistance， and other paper properties. The other part of the adhesive， together with the moisture， penetrates the paper sheet’s interior and binds the fibers together to increase the paper’s strength. These factors influenced the internal bond strength （Fig.1c） and the tensile strength （Fig.1d） of the sized papers. The experiment showed that the biolatex could effectively improve the paper’s ink absorption （Fig.1e）. Based on the above performance， it can be concluded that application of the biolatex No.2-2 and No.4-4 had good effects.
3.2 The effects of TiO2 addition to biolatex on surface sizing
TiO2， in the form of rutile， has been used in the coating industry； its excellent performance is due to the stable structure of rutile， good covering power， decorative functions， enhanced chemical stability， and other advantages[10]. Similar to the OBAs， TiO2 is commonly used in coating formulations as a high-whiteness pigment to improve whiteness and brightness of paper and paperboard[9]. TiO2 is used as functional additive to the biolatex in order to obtain TiO2-biolatex compound， which can effectively improve optical properties of sized paper， and have a certain effect on its mechanical strength and adhesion of coating membrane[10]. Below are descriptions of the results on the effects of TiO2 addition on the whiteness， surface strength， internal bond strength， tensile strength and ink absorption of the sized paper.
3.2.1 Whiteness
Fig.2 shows the effect of TiO2 on the ISO whiteness of sized paper. Before the TiO2 addition， the difference among the samples’ whiteness that were at the average level of 102%， was small， and the whiteness increased after the TiO2 addition. The TiO2-biolatex compounds were prepared by microwave-assisted method， which maintained a certain distance between the TiO2 particles. This enhanced the light scattering efficiency and consequently the paper’s optical properties improved. The whiteness of the samples No.3， No.4， and No.5 increased slightly； on the other hand， the whiteness of samples No.1 and No.2 improved significantly by about 9.4% and 5.8%， respectively. TiO2 is a powder with a high degree of whiteness， which improves light scattering efficiency and whiteness of the coating layer when uniformly distributed in biolatex， but the oxide must be in an optimum concentration. That is why the sized paper’s whiteness gradually increased with the addition of TiO2， up to the point where the opposite effects occurred due to crowding of the TiO2 particles. This turning point is the optimum concentration. The effects of TiO2 were diverse due to different surface charges of the biolatex， which caused different distributions of the components； thus， the optimum concentrations of these samples were different. 3.2.2 Surface strength
As can be seen in Fig.3， the effect of TiO2 on the paper’s surface strength was diverse. TiO2 addition reduced the surface strength of sized paper， with the exception of the sample No.4. In previous studies， it was found that the colloidal particles of No.4 biolatex were positively charged and the particles of the other biolatexes were charged negatively. After the surface sizing agent was applied to the paper’s surface， the colloidal particles migrated during the drying process rendering the sizing layer’s uneven distribution. The TiO2 particles were positively charged and formed a TiO2-biolatex conjugate system after they were added to the biolatex. On the other hand， the fiber was negatively charged and when the cationic biolatex was applied to the paper， these cationic colloidal particles were quickly absorbed on it and the migration intensity was relatively low. Furthermore， water-soluble adhesive would form a rough sized surface， while some of the TiO2 particles added to the biolatex should fill the pores to improve the sizing layer’s smoothness， some should migrate with the water to the paper interior， while most of the particles should be retained near the surface. These processes improved both the surface strength and the surface smoothness of sized poper. However， because the negatively charged biolatex that possessed a large number of negatively charged colloidal particles absorbed onto the TiO2 surface， more TiO2 migrated with the colloidal particles to the paper interior under the same dosage； thus， the amount for sizing on the surface was reduced， consequently reducing the surface strength.
3.2.3 Internal bond strength
Various preparation methods of the biolatex adhesives lead to different colors， adhesion performances， and application potentials. Their application to surface sizing also changed the internal bond strength of the sized samples （Fig.4）. After TiO2 addition， the internal bond strength of the sized paper increased by various degrees. For sample No.4， the increase was by ～26% and the effect was obvious. The main reason for this phenomenon was even distribution of the TiO2 particles in the biolatex； thus， when the surface sizing agent was applied to the paper’s surface， the particles could migrate with the latex adhesives and then fill the internal pores of the paper. The fiber’s surface is usually negatively charged and when the TiO2 and the colloidal particles migrate to the paper interior， TiO2 is absorbed onto the fiber’s surface， which formed an uneven distribution between the fiber layer and the adhesive， what closely bonded the fiber and TiO2 together. Hence， the paper was difficult to peel as the z-direction bond strength increased due to the increased internal bond strength of the sample. 3.2.4 Tensile strength
Fig.5 illustrates the influence of TiO2 on the tensile strength of sized paper. The tensile strength of all the samples decreased due to TiO2 addition. This might be due to the different concentration of TiO2 in the biolatex. At the same sizing agent dosage， the content of the colloid particles in the biolatex adhesives with added TiO2 was lower compared to the sizing agent without TiO2. The strength in the x-y direction is mainly related to the fiber’s strength itself and the internal bond strength between the fiber layer in this direction. Some of the biolatex migrated to paper interior and adhered to the fibers to enhance paper properties. However， after adding TiO2 to the biolatex， the conjugate migration occurred （biolatex and TiO2）， hence there was less adhesive in the paper， at the same applied sizing agent dosage， and the bond strength between its fibers was weakened. All of the phenomena lead to the tensile strength of sized paper decline.
3.2.5 Ink absorption
The effect of TiO2 on the ink absorption of sized paper has been shown in Fig.6. It can be seen from Fig.6 that the ink absorption of the sized paper increased due to the addition of TiO2 similarly for all the samples. The sized surface after application of water-soluble adhesives is rough compared to the latex adhesives. Such surface promotes ink absorption. TiO2 is a pigment characterized with good ink absorption used in papermaking. TiO2 that used in this study （rutile， 625 nm particle size powder） mixed with the biolatex and sized on the paper’s surface and increased its porosity along with the ink absorption increase[11]. For the same sizing agent dosage， TiO2 sized layers were basically the same， although their properties were different due to the biolatex and TiO2 arrangement. Nonetheless， most of the positively charged TiO2 was arranged near the sizing layer’s surface， which had positive effects on the ink absorptivity. There were some differences between the samples due to different arrangement of the sizing layer.
3.3 The effects of biolatex on the sized paper properties
3.3.1 Whiteness
Both surface sizing agents used in this study， the PVA and biolatex， were water-soluble adhesives. They form a rough sizing surface that gives more efficient light scattering and better optical performance. With different color of these five kinds of biolatex， the color of sizing layers was different. Fig.7 shows the whiteness of sized paper. The whiteness of the control sample （sized with PVA） was around 105%， and the whiteness of the samples No.1 and No.2 was 9% and 5% higher than that of the paper sized with PVA， respectively， and the whiteness of sample No.4 was similar to the one of the PVA sample. The main reason for these differences was the different color of the adhesives that coated the paper’s surface with a layer of thin film exhibiting a certain cover effect on the paper’s surface. In conclusion， the biolatex could compete with PVA in terms of whiteness， nonetheless， samples No.1 and No.2 displayed even higher whiteness. 3.3.2 Surface strength
After the adhesive was applied to the paper’s surface， due to the water migration to the interior of the paper， a series of changes in fiber swelling occurred. During the drying process， the adhesive migrated to the sizing surface together with the water， and as the water content was reduced， the adhesive attached to the fiber’s surface， formed a film， and the fiber contracted. With the film’s high strength and good cover effect， the surface attained high strength and its resistance to lint and dust was improved. PVA is a strong cohesion adhesive with good cover effect limiting the lint and dust phenomena. After the printing adaptability measurement of the sized paper， a layer of significant high gloss film on the samples’ surfaces was found. As it was shown in Fig.8， the surface strength of all the samples was over 1.8 m/s， with the sample No.4 having the best score， 5% higher than that of the PVA sized sample.
3.3.3 Internal bond strength
Fig.9 provides the information about the internal bond strength of the sized paper. The internal bond strength was closely related to the adhesive migration and the bond property. Part of the surface size agents applied to the papers formed a film on the surface improving the surface strength， rub resistance， waterproof， etc. The other part migrated to paper interior together with the water and cohered the fibers to improve the strength. The bond strength among the samples was diverse due to the different preparation methods of biolatex. Also， the migration was differing from one another with different particle size and their charge. The internal bond strength of the samples No.2～No.4 were similar and 28% higher than the PVA sized sample. At the same time， the result of the sample No.1 was similar to the control one （sample No.0）， while sample No.5 obtained a lower value than sample No.0. Therefore， the treatment applied to the samples No.2～No.4 gave positive effects on the sized paper internal bond strength.
3.3.4 Tensile strength
The surface sizing agent sized on paper’s surface partly migrated to the paper interior and cohered its fibers significantly increasing the paper’s strength. Fig.10 shows the tensile strength of the sized paper. Comparing these samples， it can be seen that the effect of the biolatex on the tensile strength was similar to the PVA’s effect； however， the sample No.5 attained higher tensile strength than the control sample （sample No.0）. The main reason of these changes was the migration of the surface sizing agent between the fibers and the pores in the paper sheet， and the subsequent migration during the drying process. This caused an uneven distribution of the agents inside the paper， both in the z-direction and the x-y plane. The high bond strength and the surface sizing agent content in the x-y plane improved the tensile strength. For example， the surface strength of sample No.3 was lower than the PVA sized one’s. There was high internal bond strength but low tensile strength， and these phenomena showed that the bond strength of sample No.3 was relatively lower， the migrated agent’s volume was larger， hence more colloidal particles were distributed along the z-direction. In the same way， the different bond strengths and various particle migration intensities led to diverse tensile strengths. In summary， the tensile strengths of these samples were similar to the one sized with PVA. 3.3.5 Ink absorption
Paper needs to have a good ink absorption property to insure proper printing quality. As can be seen from Fig.11， the ink absorption of all the biolatex sized samples were higher than that of the PVA sized sample. With starch as raw material， the biolatex was a new paper coating adhesive which shows good cohesiveness and flow ability. As a result， these five kinds of biolatex sized papers presented good ink absorption.
4 Conclusions
By comparing the surface sizing performances of biolatex and PVA， it was found that the biolatex was suitable for surface sizing and could be further processed to improve its performance. TiO2， used as a functional additive for the biolatex， could produce excellent effects on the whiteness， internal bond strength and ink absorption， however it could have adverse influence on the tensile and surface strength.
Among the investigated five kinds of biolatex applied to surface sizing， the surface strength of the sample sized with the No.4 biolatex was the best and higher than that of the control sample sized with PVA， the remaining sizing agents achieved results slightly below the control sample. The internal bond strength was the best after treatment with the biolatexes No.2～No.4 that scored above the control sample， and the result obtained for sample No.1 was similar to the PVA sized one. All the samples had the tensile strength similar to the PVA sized one and higher ink absorption. In conclusion， the surface sizing performance of the biolatex No.4 was the best， appropriate TiO2 concentration was advantageous to surface sizing and the biolatex could be mixed with PVA in order to achieve better properties.
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