Paper making and fiber liberation – Processes of chemical liberation – recovery or purification...
Reexamination Certificate
2000-05-08
2003-02-25
Wu, David W. (Department: 1713)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
C106S203200, C162S005000, C162S055000, C162S077000
Reexamination Certificate
active
06524435
ABSTRACT:
TECHNICAL FIELD
This invention is directed to a method of dispersing hydrophobic particles in aqueous suspensions using ethylene oxide-propylene oxide block copolymers and to aqueous compositions comprising the ethylene oxide-propylene oxide block copolymers.
BACKGROUND OF THE INVENTION
The deposition of organic contaminants on process equipment, screens, and containment vessels in papermaking significantly reduces process efficiency and paper quality. Deposits on machine wires, felts, foils, headbox surfaces, screens, and instruments require costly downtime for cleaning to avoid the problems associated with poor process control, reduced throughput, and substandard sheet properties. These contaminants are generically referred to in the paper industry as either “pitch” or “stickies”.
Pitch deposits generally originate from natural resins (fatty acids, rosin acids, sterols, etc), resin salts, coating binders, or sizing agents found in the pulp (Journal of Pulp and Paper Science, 22:11,431-439, 1996). Pitch may also contain inorganic materials such as talc, calcium carbonate, or titanium dioxide. Stickies generally refers to the hydrophobic substances used in the manufacture of paper such as pressure sensitive or contact adhesives such as styrene butadiene rubber, hot melt adhesives such as ethylene vinyl acetate and thermoplastics inks (styrene acrylate, polyvinyl butyral) that can form deposits when reintroduced in recycled fiber systems. Other common hydrophobic contaminants found in recycle applications include wax, which originates primarily from wax-coated old corrugated containers, and polyisoprene.
Recycled fiber refers to secondary fibers which are repulped to provide the papermaking furnish with raw material for the production of new papers. The secondary fibers may be either pre-consumer or post-consumer paper material that is suitable for use in the production of paper products. Sources of secondary fiber may include old newspaper (ONP), old corrugated containers (OCC), mixed office waste (MOW), computer printout (CPO), ledger, etc. These once-processed papers contain various types of adhesives (pressure sensitive, hot melts, etc.), inks, and coating binders. Coating binders can include the so-called ‘white pitch’ associated with the repulping of polyvinyl acetate and styrene butadiene coated broke.
Pitch and stickies are hydrophobic in nature and thus quite unstable as colloids in aqueous papermaking environments, thereby facilitating their deposition. The major problems arising from deposition are as follows: (1) reduced throughput due to plugging of forming fabrics and press felts (2) sheet holes or paper breaks due to large deposits breaking loose from the equipment, and (3) reduced sheet quality due to contaminants incorporated in the final sheet.
To deal with the problem of stickies, a number of strategies have been employed by papermakers. These strategies include mechanical and chemical means to either remove or detackify the contaminants. Mechanical means of removing the contaminants include slotted pressure screens, hydrocyclones, and cleaners of various types. Also, thermal/mechanical dispersion units are employed to break the contaminants into micron sized particles, which are then difficult to detect in the final sheet. Despite these techniques 100% removal of contaminants by mechanical means cannot be accomplished.
Chemically, several approaches are taken, including passivation or detackification of the sticky contaminant surfaces. Detackification means the process of reducing the rate of detrimental deposits originating from pitch or stickies. Dispersing is one route of detackification that improves the colloidal stability of contaminant particles in aqueous systems. Common detackification agents include inorganic materials such as talc and zirconium compounds, and organic materials such as polyvinyl alcohol. In addition, various dispersants may be used to prevent the contaminants from agglomerating. The small, dispersed contaminants may be fixed to the paper sheet by the use of cationic polymers. Finally, if the contaminant problem becomes severe, solvents may be used to wash and remove the materials from machine surfaces and clothing.
Previous studies show that a variety of nonionic or anionic polymer/surfactant types can be effective in reducing the surface tension, adhesion tension, and contact angles of aqueous solutions in contact with a polyester surface (Pulp and Paper Canada, 94:10, 17-23, (1993)). These studies, however, relate to the deposition tendency of hydrophobic particles onto surfaces rather than dispersancy of the stickies in aqueous environments.
Similar peel test data has been used extensively to demonstrate the detackification properties of chemical additives such as polyalkylene oxide/vinyl acetate copolymers (U.S. Pat. No. 5,266,166), terphthalate gycol terpolymers (U.S. Pat. No. 5,415,739), methyl cellulose derivatives (U.S. Pat. No. 4,781,794), and polyvinyl alcohol (U.S. Pat. No. 4,956,051).
Peel test data has also been used to show that polyelectrolyte complexes comprised of oppositely charged materials are also useful detackification agents (TAPPI Journal, 81:6, 143-151, (1998), U.S. Pat. No. 5,292,403). While the peel test data has meaningful applications in the paper industry, it fails to accurately predict the relative adhesion properties between aqueous hydrophobic particles in the presence of chemical additives.
More recently, atomic force microscopy (AFM) has been developed as a tool for measuring the microscopic adhesion forces between hydrophobic particles of colloidal dimension in aqueous solutions (J. Colloid and Interface Sci., 185, 363-370, (1997)). This technique has even been applied to adhesion forces between hydrophobic particles and paper (TAPPI J., 82:5, 172-174, (1999)). The AFM measurements are a much better simulation of interparticle collisions in aqueous solution than the peel tester. The industrial standard peel tester applies forces of ~800 kN/m
2
over ~100 cm
2
for 2 minutes compared to the 5 kN/m
2
forces applied over ~30 &mgr;m
2
for milliseconds in AFM measurements. The surface preparation, applied forces, surface areas, and adhesion times are vastly different, with the AFM protocol more closely resembling interparticle collisions in all aspects. Therefore, AFM results will have greater applicability to aqueous dispersancy of hydrophobic particles than the peel tester data, which may have greater applicability in dryer section deposition problems.
International Patent Application No. PCT/US98/05261 discloses a method of removing pitch and stickies from papermaking fiber comprising treating the fiber with a composition comprising ethylene oxide-propylene oxide block copolymers, an organic solvent and a surfactant and then mechanically separating the agglomerated stickies and pitch from the papermaking fiber.
A method of dispersing wax during the recycling of coated paper products using a plasticizer and an ethylene-oxide/propylene oxide block copolymer is disclosed in commonly-assigned Ser. Nos. 09/039,863 and 09/258,627.
SUMMARY OF THE INVENTION
Using AFM adhesion force measurements, we unexpectedly discovered a series of block copolymers of ethylene oxide and propylene oxide (EO/PO block copolymers) that significantly outperform the traditional industry standard chemical treatment programs for detackification.
Although the surface activity of EO/PO block copolymers has been previously reported via contact angle and surface tension measurements, (Pulp and Paper Canada, 94:10, 17-23, (1993)) their effectiveness in reducing adhesion forces between hydrophobic particles in aqueous medium has never been demonstrated until now. This property is extremely valuable for aiding in the dispersion of hydrophobic particle contaminants in recycled paper applications.
Thus, the EO/PO block copolymers described herein are highly effective and efficient additives for dispersing hydrophobic particles in aqueous suspensions. The addition of EO/PO block copolymers to process waters in recycled fiber papermakin
Agarwal Sameer B.
Carter Phillip W.
Coffey Martin J.
Breininger Thomas M.
Egwim Kelechi C.
Martin Michael B.
Ondeo Nalco Company
Wu David W.
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