1. Manufacturing label of latex (electrophoretic mobility)
Paper properties of paperboards made of pulp fibers and synthetic latexes. The efect of particle size of latex.
Erika HREHOROVA, Radovan TINO, Stefan SUTY
Department of Chemical Technology of Wood, Pulp and Paper; Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinskeho 9, Bratislava, Slovakia
Introduction
In the production of almost any kind of paper, additives are used because fibers alone cannot impart the properties required the finished product.
A film-forming hydrophobic polymer in the form of latex is a good candidate for an additive, that could simultaneously improve water resistance, liquid penetration, interfiber bonding, tear resistance, and folding endurance. In addition, such a polymer may serve as a retention aid and binder for pigments [5].
The rationale is that latex particles deposited on fibers suspended in water would coalesce into a hydrophobic film as the sheet forms and dries. Thus, the latex would protect the fibers against water. In addition, the properties of the polymer would be reflected in a number of the mechanical properties of the paper. Since the pulp fibers suspended in water are negatively charged, anionic latexes that are commercially available do not deposit on fibers. Consequently, they require cationic aids, usually water-soluble polyelectrolyte, which flocculate the latex particles [3].
How do the latexes influance mechanical and optical properties of paper? Both the tensile strength and the light-scattering coefficient are functions of the bonded area. The unbonded area is the fiber–air interface where the scattering of light takes place.
The illustration (Fig. 1) on the left shows Fibers A and B without polymer. For hydrogen bonding, the surfaces must be in molecular contact, or separated by approximately 0.3 nm. The surfaces are in optical contact even when separated by a greater distance (< 1/2 wavelength of light). The illustration on the right shows polymer filling the voids that are created by surface roughness when Fibers A and B come in contact. The polymer increases the area of molecular contact, but the optical contact remains the same [3].
Sample
Type of latex
Manufacturing label of latex
Particle size [nm] EM
(electrophoretic mobility)
[m2/s.V]
Particle charge
PS1
polystyrene
LYNTRON 2202
200 3,
(-)
PS2
LYNTRON 2503
400 2,
PS3
LYNTRON GA 5705
750
Table 1: Latex characteristics
Fig. 5: Changes of tensile index of tested sheets.
Fig. 7: Changes of bending resistance of tested sheets.
Fig. 2: SEM micrograph of original fiber surface
Fig. 3: SEM micrograph of coverage of fibre by monolayer of latex particles
(PS3 latex)
Fig. 5: SEM micrograph of polymeric film formed when latex particles coalesce
Conclusion
When fibers with deposited latex are formed into a sheet and dried above the film-forming temperature of the polymer, the fiber–fiber bonds are replaced by fiber–polymer–fiber bonds. This sandwich effect could be the case when fibers are fully covered by a monolayer of latex particles. The process is shown in the micrographs of Fig and 4.
Fig. 6: Changes of burst index of tested sheets.
Fig. 8: Changes of reflectance of tested sheets.
The film forming temperature of polystyrene is 108 °C. To compare both the sheets with monolyer of latex particles and the sheets with compact film of latex the two techniques of sheets drying were applied: under the film forming temperature drying (vacuum dried sheets) and above the film forming temperature drying (pressed sheets).
Latex particles deposited on fibers suspended in water form a hydrophobic film after a sheet is dried above the film-forming temperature of the polymer. This film protects the fibers and enhances interfiber bonding. The main contribution of latex incorporated to improve strength is an increase in the bonded area. This assertion is supported by the results of increased measured tensile and bursting strenght of tested sheets (Fig. 5 and Fig. 6).
On the other side of strenght, the formed film is more fragile. The higher partical size of latex, the more fragile film (Fig. 7).
Generally, an increase in tensile strength is accompanied by decreased reflectance. Both the tensile strength and the light-scattering coefficient are functions of the bonded area. Although an increase in bonded area means stronger paper, it also causes a decrease in unbonded area. This statement was indicated by measuring of optical properties of tested sheets (Fig. 8).
Particle size of latex do not markedly influence the air permeability of vacuum dried sheets, but the considerable decrease of air permanence of pressed sheets was observed. The bigger particles of latex, the thicker film and the lower air permeability (Fig. 9).
Fig. 1: Illustration of how latex improves tensile strength without affecting light scattering. Fiber A contacts Fiber B. Hydrogen bonds do not form where fiber surface roughness prevents close contact. However, the fibers are in optical contact. Polymer increases the area of close contact without changing the optical contact.
Experimental
The laboratory sheets (grammage 150 g/m2) were prepared by using bleached sulphate pulp with SR number of 19.
Three types of polystyrene latexes with different particle sizes were used. Particular characteristics of latexes are presented in table 1.
For better determination of influance of partical size of latex on paper properties, the two types of sheets were prepared:
1) laboratory sheets dried in vacuum – vacuum drier is a part of UNGER Laboratory Sheet Former.
2) pressed laboratory sheets – press C.A.M.I.L, temperature 150 °C and pressure 20 Mpa.
Anionic polystyrene latexes required cationic aid to deposit on negatively charged pulp fibers. Positively charged PEI (polyethylenimine) was used to treat the pulp fibers in order to deposition of negative latex particles.
To find the relationship among sheet filled by different types of latexes and unfilled sheet, the following experiments were performed: mechanical properties (tensile strength, bursting strength and bending resistance), optical properties (reflectance) and physical properties (air permeability). The techniques of applied tests are described in [2] and [4].
To observe microscopical structure of pulp fibre the sample PS 3 was chosen, particle size 750 nm.
Fig. 9: Changes of air permeability of tested sheets.
References:
Blažej, A.; Krkoška, P.: Technológia výroby papiera, ALFA, Bratislava, (1989), str. 206, 210
Mišovec P., Krkoška P.: Cvičenia z technológie výroby papiera, SVŠT Bratislava (1989)     
Alince, B: Cationic latex as a multifunctional papermaking wet - end additive, TAPPI J. 82 (3) 175 (1999)     
Souček, M.: Skoušení papíru, SNTL Praha (1977)
Tiňo, R.: Kompozitné materiály z buničinových vlákien a syntetických latexov, Projekt dizertačnej práce, KDCP FChPT STU, Bratislava, (2000)