Application of Suspension-polymerized Latexes in Surface Sizing and Pigment Coating of Paper

Abstract

최근 펄프가격 상승에 따른 원가상승을 극복하기 위해 펄프 섬유를 충전물로 대체하는 하이로딩 기술의 중요성 및 고도공량의 도공지 생산에 대한 관심이 고조되고 있다. 고도공지 생산 기술의 발달과 더불어 발생되는 핵심적인 문제점의 하나로 도공지의 접힘터짐 (fold crack) 문제가 대두되고 있다. 이를 완화하기 위한 방안으로 본 논문에서는 서스펜션 중합을 통하여 제조된 나노 입경의 라텍스를 표면사이즈액의 첨가제 및 도공액의 보조 바인더로 사용하였으며, 나노 라텍스 첨가에 따른 표면사이즈액 및 도공액의 특성, 종이의 강도적, 표면화학적 특성에 대한 연구를 수행하였다.

표면사이징용 첨가제로 활용할 경우, 일반적인 유화중합법이 아닌 서스펜션 중합방식으로 라텍스를 제조하기 때문에 라텍스 입자를 전분 등의 친수성 고분자로 안정화시킬 수 있었으며, 이는 라텍스입자의 pH 변화에 대한 안정성을 부여함이 구명되었다. 또한 유화중합에 사용되는 계면활성제를 배제함으로써 공정의 거품 발생 억제 및 우수한 파포 특성을 부여 할 수 있었으며, 표면사이징용 전분과의 상용성을 자연스럽게 증대시키고, 전분필름의 신장율을 증가시킴으로 표면사이징된 종이의 접힘터짐 현상 개선이 가능한 것으로 나타났다.

도공액의 보조바인더로 서스펜션 중합 라텍스를 사용할 경우, 친수성 보호콜로이드의 영향으로 도공액의 점도가 상승하며 보수성이 향상되는 결과를 나타내어 도공 작업성 및 도공지의 품질 향상에 기여할 것으로 판단되었다. 하지만 서스펜션 중합 라텍스의 투입량이 5 pph이상으로 과다할 경우 보수성 및 점도가 지나치게 증가하여 적정 투입량을 대체하는 것이 필요하다는 것이 밝혀졌다.

또한 도공지의 제반물성의 경우 서스펜션 중합 라텍스의 투입에 따른 인장강도 개선은 미미하였으나, 도공지의 신장율이 뚜렷이 증가하였으며, 이로 인해 접힘터짐 현상이 개선됨을 밝혔다. 서스펜션 중합된 라텍스가 투입된 도공층의 경우 안료와 바인더 간에 상호 작용이 증가하여 도공층의 공극이 증가하는 경향이 나타났으며, 5 pph 이하의 투입조건에서는 모틀 및 잉크 흡수 성능이 개선되는 결과를 나타냈다.

본 논문에서는 서스펜션 중합 라텍스를 표면 사이징 및 도공에 적용함으로써 최근 펄프원료 절감 및 이를 통한 원가절감을 위한 하이로딩 기술로 유발되는 접힘터짐 문제를 완화시키기 위한 방안을 모색하였으며, 보조바인더의 사용에 따른 도공지의 강도적, 표면화학적 특성 등 다양한 측면에서 효과를 검증함으로써 개발된 라텍스가 도공지의 품질 향상에 큰 효과를 나타냄을 증명하였기에 농학박사학위 논문으로서 충분한 가치가 있다고 판단된다.

Fold cracking of coated paper is one of the most frequent and troublesome problems affecting the use of coated paper and boards. Paper industry is facing more fold cracking problem because substantial amount of filler is being used in papermaking and heavier coatings are applied to substitute the use of expensive fibers with pigment coating.

Fold cracking of coated paper is highly dependent on the properties of coating layer. The coating composition including pigments and binders affects the properties of coating layer. A latex with a good extensional property could improve the flexibility of coating layer and result in a less crack area after folding of coated paper. To minimize fold cracking of coated paper and board, a method to improve the extensional property and tensile strength of the base sheet and coating layer is necessary. Improving the fold crack resistance by optimization of surface sizing or paper coating by use of additives seems a practical method. Hence application of additives which provide both strain and strength might be a useful method to minimize the fold crack.

The physical and chemical properties of latexes depend upon various factors including polymerization methods, and monomers and stabilizer used in the polymerization. In this research, sterically stabilized suspension-polymerized acrylate (SP) latexes were used as additives for surface sizing and pigment coating of paper. A protective shell containing oxidized starch and polyvinyl alcohol (PVA) provided the steric hindrance to the SP latexes.

The property of the suspension-polymerized latex was examined and compared with that of conventional emulsion-polymerized ones. As an approach to decrease the fold cracking of coated paper, SP latexes were tested as a surface sizing additive. By preparation of starch films, it was possible to compare the film properties without variability caused by the base sheet. The strength and stretch properties of starch films without and with SP latex were measured. The effect of using SP latexes as a surface sizing additive on the mechanical properties of the paper was examined.

SP latexes with different acrylic acid content were used as cobinders for paper coating. The physical properties of these latexes including viscosity, particles size and zeta potential as a function of pH were compared. The viscoelastic properties of the coating color containing SP latex as a cobinder were evaluated. Moreover effect of partial substitution of the styrene butadiene (S/B) latex with SP latex on porosity, surface and optical properties of dried coating composite was investigated.

The coating layers containing SP latex as a cobinder were prepared and the tensile strength and strain property of the coating layers were evaluated. Finally the effect of SP latex on fold cracking of low- and high- basis weight coated paper in machine direction (MD) and cross machine direction (CD) was investigated.

The SP latex showed less tendency to generate foam than conventional styrene-butadiene and styrene-acrylate latexes, and they were highly compatible with the conventional surface sizing solution as the strength and stretch of the starch films increased. The SP latex resulted in a greater tensile strength and extensional properties of sized paper than the emulsion-polymerized latexes, which suggested their potential applicability for reducing the fold cracking of coated paper.

Acrylic acid monomer was found to be an important factor affecting viscosity, particle size and zeta potential of SP latexes. As a result of partial substitution of S/B latex with SP latexes in coating color, the viscoelastic properties of coating color was influenced by interaction between coating components. Because of this interaction, a weakly flocculated structure was built up in coating color which affected the formation of coating layer and thereby a more porous coating layer with higher opacity and brightness was formed.

Use of SP latex in coating layer, improved the strength and strain of the coating layer which was attributed to the protective shell and monomer composition of the SP latex. The extensional properties of the coated papers were improved when SP latex was used in coating formulation. Higher tensile strength and extensional properties of coating layer and coated paper, resulted in less crack area after folding. A base paper with higher basis weight gave a larger crack area as did the CD fold. Presence of SP latex in pre- and top-coating reduced the fold crack area, however it was found more effective in top-coating layer. Starch amount and total binder content of pre-coating layer contributed to appearance of larger fold crack area.

Keywords: Suspension-polymerized (SP) latex, surface sizing, paper coating, coating layer, extensional properties, fold cracking, folding direction