Method of deinking laser printed wastepaper

A process for deinking wastepaper printed with heat fused inks to provide a source of deinked paper which can be reused or repulped and converted into a recycled paper product comprises immersing the wastepaper in a non-aqueous solvent bath containing a solvent capable of dissolving the binder component of the ink, and subjecting the wastepaper to low frequency ultrasound energy to separate the ink particles from the wastepaper. The separated ink particles may be collected magnetically or otherwise and the excess solvent recovered and reintroduced into the solvent bath.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for deinking wastepaper printed 
with heat fusible inks such as laser inks, and more particularly to such a 
process which does not require that the wastepaper be repulped prior to 
treatment. The process uses low frequency ultrasound in conjunction with 
an organic solvent to remove the fused ink particles from the wastepaper. 
There are reports in the literature of ink removal by solvent treatment. 
Likewise, the use of ultrasound in deinking is known. However, the present 
invention is the first instance where the two separately known treatments 
have been combined particularly for deinking laser printed paper. 
Conventional methods of deinking and reclaiming wastepaper used in the past 
involve treating the wastepaper with various deinking chemicals. Such 
methods were reasonably satisfactory and adequate for paper printed with 
standard inks that were readily removed or saponified with chemicals at 
elevated temperatures. However, in recent years, methods of deinking 
involving cooking and the use of chemicals have become increasingly 
unsatisfactory for a number of reasons. 
Ink formulations have become more and more complex and involve the use of a 
wide variety of synthetic resins and plasticizers with each ink 
manufacturer having its own special formulation. Furthermore, multicolored 
printing and advertisements have become more widespread and these involve 
a variety of new ink formulations that include new pigments, dyes and 
toners which cannot be removed or dissolved from wastepaper using 
conventional deinking chemicals. This is particularly true for laser 
printed paper. 
Laser printed paper is one of the most difficult types of wastepaper to 
deink. Laser printing and xerography are variations of the same process 
whereby the inks are fused by heat. In the case of laser printing, both 
heat and pressure are used in the fusion stage. The fusion mechanism and 
the nature of the binder used in toner formulations makes these papers 
most difficult to deink. 
For such wastepaper, the conventional methods of washing and flotation are 
not effective. Washing has been found to be useful only for ink particles 
less than about 25 .mu.m. The fused toner particles are much larger than 
this, on the order of about 40 to 400 .mu.m. Screening is also ineffective 
for the toner particles used in laser printing because of their plate-like 
shape. Laser printing inks contain a polymer binder and carbon black 
pigment. The binder is typically a copolymer of styrene and methyl 
methacrylate (or ethyl methacrylate), with a molecular weight of 70,000 to 
about 100,000, and a melting temperature in the range of 
100.degree.-110.degree. C. The weight ratio of styrene to methacrylate 
ranges from about 60:40 to 70:30. Since the binder is insoluble in water, 
aqueous deinking methods can only dislodge the ink particles from the 
paper fibers. The ink particles must then be agglomerated and separated 
from the paper fibers by flotation techniques or the like. On the other 
hand an appropriate solvent treatment may be used to dissolve the binder 
portion of the laser ink which can then be separated from the paper and 
collected for disposal. In the case of the present invention, a low 
frequency ultrasound is used to separate the toner particles from the 
paper. 
DESCRIPTION OF PRIOR ART 
There are a number of patented methods for deinking xerographic or 
electrostatic printed wastepaper. In U.S. Pat. No. 3,635,789, the deinking 
process involves repulping of the xerographically printed paper and mild 
agitation of the pulp suspension at a temperature of 
150.degree.-205.degree. F., before the addition of an organic solvent 
which is skimmed off with the accumulated toner. U.S. Pat. No. 4,276,118 
discloses a process where the toner is removed from the paper with a dual 
system of a collector such as polystyrene in the form of beads and a 
collecting chemical such as a saturated aliphatic primary alcohol with a 
melting point at or below the approximate softening point of the toner. In 
U.S. Pat. No. 4,561,933, the deinking process is carried out by contacting 
xrographically printed wastepaper pulp with a deinking agent consisting of 
one or more C.sub.5 to C.sub.20 alkanols in an aqueous medium of alkaline 
pH. Finally, in U.S. Pat. No. 5,141,598, the method for deinking dry toner 
electrostatic printed wastepaper comprises adding to a slurry of the 
wastepaper a mixture of petroleum distillates, ethanol and an ethoxylated 
polyoxypropylene glycol. 
Meanwhile, the use of ultrasound in the deinking of wastepaper is disclosed 
in a number of publications, including, an article entitled "Ultrasonic 
Deinking of Wastepaper" by Leslie L. Turai and Chung-Haw Teng, published 
in TAPPI Journal, February 1978, Vol. 61, No. 2, pages 31-34. In this 
article wastepapers heavily printed with high-gloss, polymeric inks were 
subjected to ultrasound to disperse the ink films without the use of 
chemicals. A related pilot plant study by the same authors is disclosed in 
an article also entitled "Ultrasonic Deinking of Wastepaper", published in 
TAPPI Journal, January 1979, Vol. 62, No. 1, pages 45-47. Finally, an 
article entitled "Deinking of Waste Newspaper Under Ultrasonic 
Irradiation", by M Chen, G Tan and G Cao, appeared in China Pulp and 
Paper, 8(2), 34-40 (1989). In this study, optical microscopy of the 
deinked pulp revealed a superiority of deinking by ultrasound over 
chemical and mechanical deinking for newsprint. However, none of the known 
prior art teaches the novel process of the present invention wherein 
sheets or strips of laser printed wastepaper are deinked by suspending the 
printed sheets or strips in a solvent designed to dissolve the binder 
portion of the ink and then subjecting the suspended sheets or strips to 
low frequency ultrasound to separate the ink particles from the paper. 
SUMMARY OF INVENTION 
In accordance with the present invention, a novel process for deinking 
wastepaper printed with heat fusible inks has been discovered and 
successfully reduced to practice. The invention relates to a practical ink 
separation process for laser printed paper which can be carried out 
without first repulping the wastepaper. According to the present 
invention, the wastepaper in sheet or strip form is deinked, and after 
deinking, the sheets or strips of paper may be reused, or, if desired, 
repulped and converted directly into new recycled sheets. The preferred 
practice of the invention relies on the use of a solvent system that is 
designed to dissolve the binders of the heat fused inks and a low 
frequency ultrasound for separating the inks from the wastepaper sheets. 
The solvents found to be effective in the process are represented by the 
following classes of organic compounds, comprising, ketones, cyclic 
ethers, esters, alicylics, aromatic hydrocarbons, terpenes, chlorinated 
hydrocarbons, cyclic amide/cyclic amine, and carbon disulfide. Generally 
less than about ten minutes of exposure to ultrasound is required to 
achieve substantially complete deinking of sheets of wastepaper suspended 
in one or more of the above-identified solvents. A screening sequence 
involving other solvents indicated that ultrasound was only effective when 
used with a solvent capable of dissolving the ink binder. 
The process of the present invention involves first, immersing sheets of 
wastepaper printed with heat fusible inks in a solvent bath to dissolve 
the ink binder; secondly, separating the ink particles from the 
wastepaper; and thirdly, separating the ink particles from the solvent 
bath. An appropriately selected solvent accomplishes the first step. The 
use of low frequency ultrasound satisfies the second step, and, an 
appropriate separation technique is used to complete the process. Sheets 
of laser printed wastepaper deinked by this process have been found to 
retain up to 95% of the brightness of unprinted control sheets. 
Ultrasound refers to sound waves of a frequency above 16 kHz, the audible 
limit of the human ear. The upper limit for ultrasound is about 500 MHz 
for liquids. The use of ultrasound in non-invasive medical diagnosis is 
known, and ultrasound is not alien to the pulp and paper industry. 
However, the only evidence of prior work with ultrasound in deinking 
involves the use of ultrasound for deinking non-fused inks. Likewise the 
use of solvents in deinking is known. However, the mere mechanical 
agitation of laser printed paper soaked in a solvent gave poor deinking 
results. 
Thus it is an object of the present invention to combine two heretofore 
known deinking techniques for conventional inks in the effective deinking 
of wastepaper printed with heat fusible inks. 
It is a further object of the present invention to provide a process for 
deinking wastepaper printed with heat fused inks that is both economical 
and relatively easy to practice. 
It is yet another object to the present invention to provide a process 
which may operate on paper sheets or strips which can be repulped after 
deinking for use in making new recycled sheets. 
Thus a particular advantage of the present invention is that the wastepaper 
does not have to be repulped prior to treatment. Accordingly a significant 
step required in most prior art deinking processes is eliminated by the 
present invention.

DETAILED DESCRIPTION 
A laboratory test method was used to examine the theory behind the present 
invention and to screen for useful solvents. Initially, spent toner from a 
laser jet printer cartridge was extracted with several non-aqueous 
solvents and it was found that the toner binder was readily dissolved in 
methyl ethyl ketone (MEK). Based on this result, paper printed with laser 
ink was immersed in MEK without mixing followed by washing to remove the 
excess ink particles. However, washing the extracted, repulped fibers with 
water did not remove a significant amount of the ink. The same experiment 
was then conducted with simple magnetic stirring. Although this method did 
not enhance the first result, it did show that the ink particles which 
were dislodged were collected on the magnetic stirring device. 
Subsequently the experiment was performed again in the presence of low 
energy ultrasound, with the result that the ink was removed effectively 
and rapidly from laser printed wastepaper in a relatively short time. 
A possible explanation for the effectiveness of ultrasound in deinking 
laser printed wastepaper may be found in its mechanism. Treatment of a 
liquid with ultrasound waves produces a large number of microscopic 
bubbles of about 100 .mu.m in diameter and lifetimes of a few 
milliseconds. Bubble formation is accompanied by pressures of about 500 
atmospheres. Thus, the shock waves generated by cavitation of the bubbles 
are strong enough to dislodge the fine ink particles from the fiber matrix 
of the paper. It is also known that during cavitation, tremendous amounts 
of highly localized heat is evolved. This localized cavitational heat may 
also help to melt the polymer binder of the ink and facilitate its 
dissolution in the organic solvent phase. 
A slightly larger scale deinking experiment was performed next using wide 
mouth bottles containing 100 ml of solvent, into which strips of laser 
printed wastepaper approximately 3 inches by 5 inches were immersed. The 
paper strips were rolled into a cylinder held together by a paper clip. 
Deinking was carried out in two stages. The first stage consisted of two 
minutes treatment with ultrasound at a frequency of about 47 kHz. The 
paper strips were removed from the first bottle and transferred to another 
containing fresh solvent. Ultrasound treatment was continued for another 
five minutes. It was found that the majority of the ink was removed in the 
first stage for all solvents. Collection of the suspended ink particles 
was accelerated by immersing a magnetic stirrer in the bottle. This 
procedure was repeated several times to accumulate enough deinked strips 
for making handsheets. As a control, the same paper that had passed 
through the laser printer but not printed was used. Both the deinked 
samples and controls were soaked in water overnight and made into 
handsheets in the usual manner. 
Table I illustrates the results of the solvent screening study. The 
deinking results recorded for each solvent were achieved with an 
ultrasound treatment time of from about 1-10 minutes at a frequency of 
about 47 kHz. Longer times were used on the solvents which gave poor 
deinking performance. For the best solvents, deinking was substantially 
completed in from about 1-5 minutes. 
TABLE 1 
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Solvent Screening 
SOLVENT DEINKING RESULTS 
______________________________________ 
Ketones: 
Acetone Good 
Acetyl acetone Good 
Acetophenone Excellent 
Diisopropyl ketone Excellent 
Cyclohexanone Excellent 
Methyl ethyl ketone (MEK) 
Excellent 
2-Pentanone Excellent 
3-Pentanone Excellent 
Cyclic Ethers: 
1,4-Dioxane Excellent 
1,3-Dioxolane Excellent 
Tetrahydrofuran (THF) 
Excellent 
Esters: 
n-Butyl acetate Excellent 
Ethyl acetate Excellent 
Dimethyl malonate Fair deinking 
Hexyl acetate Good 
Aromatic hydrocarbons: 
Toluene Good 
Xylene Good 
Terpenes: 
Citral Good 
D-Limonene Good 
.alpha.-Pinene Fair Deinking 
Halogenated hydrocarbons: 
Carbon Tetrachloride 
Excellent 
Chloroform Excellent 
Methylene chloride Good 
Cyclic amide/cyclic amine: 
N-Methyl pyrrolidone 
Excellent 
Morpholine Excellent 
Miscellaneous: Good 
Carbon disulfide 
Alcohols: 
Reagent Alcohol Very little deinking 
Veratryl Alcohol (3,4- 
No deinking 
Dimethoxylbenzyl alcohol) 
Alicyclics: 
Cyclohexane Fair deinking 
Cyclohexene Good 
Aqueous systems: 
5% N-40 dispersant in 0.5 N 
No deinking 
NaOH 
20% Polyvinyl pyrrolidone 
No deinking 
50% 18-crown-6 No deinking 
Aliphatic hydrocarbons: 
Hexane No deinking 
n-Pentadecane No deinking 
Esters: 
Adipic acid monomethyl ester 
Very little deinking 
Dibutyl phthalate Fair deinking 
Dipolar aprotics: 
Acetonitrile Poor deinking 
N,N-Dimethyl formamide 
Good 
Dimethyl sulfoxide (DMSO) 
Poor deinking 
Miscellaneous: 
Silicone Oil No deinking 
VM & P naphtha Very little deinking 
______________________________________ 
Table II presents the results from the handsheet study. A few of the better 
performing solvents were used for this study. The data shows that the tear 
strength of the handsheets prepared from the recycled deinked paper was 
slightly greater in most cases than the controls, while the brightness was 
slightly lower in all cases. These data are believed to demonstrate the 
effectiveness of the process disclosed herein. 
TABLE II 
______________________________________ 
Handsheet Study 
HANDSHEET 
TEAR STRENGTH 
BRIGHTNESS 
CON- CON- 
SOLVENT DEINKED TROL DEINKED TROL 
______________________________________ 
Acetone 73 75.8 (90.6) 
83.7 
MEK 74 80.2 (95.5) 
84.0 
THF 75 67 79.3 (94.6) 
83.8 
Toluene 69 74.4 (88.7) 
83.8 
D-Limonene 
68 67 73.6 (88.5) 
83.2 
______________________________________ 
The handsheet brightness reported in Table II is based on an average of 
eight measurements, four from each side. The numbers in parentheses in the 
column for deinked handsheet brightness represent the percentage of the 
control brightness achieved by each deinked sample for each solvent 
system. 
The Figure of drawing provided herein is intended to illustrate a typical 
embodiment for practicing the present invention on a continuous basis. For 
this purpose, a feedstock of wastepaper printed with heat fusible inks is 
introduced into a treatment vessel 11 containing a solvent bath and 
including an ultrasound generator. The vessel may be of the countercurrent 
flow type, wherein the wastepaper to be deinked is introduced into the 
bottom of the vessel 11, and the deinked product is removed from the top, 
while solvent make-up is introduced at the top of the vessel 11. The 
wastepaper may be introduced into the vessel 11 in any desired form, i.e., 
as whole sheets, strips or the like, but is preferably not introduced into 
the vessel 11 as pulp. After the deinking treatment, the deinked paper 
product is conducted to a solvent stripping tower 12 where excess solvent 
is removed. This solvent is preferably passed through a condenser 13 or 
the like before being reintroduced into vessel 11. Meanwhile, the ink 
particles are removed from the vessel 11 and conducted to a settling tank 
14 or the like. At this point the ink particles may be disposed of by 
burning, deposited in a landfill, or used as recycled raw material for an 
ink manufacturer. Any excess solvent or mixture of solvent and binder from 
the settling tank 14 may also be at least partially recovered with the use 
of a film evaporator 15 or the like which separates the binder from the 
excess solvent. The evaporated solvent from evaporator 15 is passed 
through the solvent condenser 13 and reintroduced into vessel 11 while any 
binder recovered from the film evaporator 15 is burned as fuel or is 
land-filled. 
It should be understood that any number of variations in the exemplary 
process described could be readily made by one skilled in the art. The 
only essential part of the process is the use of an ultrasound treatment 
vessel and an appropriate solvent for the binder of the ink applied to the 
wastepaper. The method used for reclaiming the ink removed from the paper 
is essentially a matter of choice. One advantage of the solvent treatment 
disclosed is that it produces no water effluent that must be treated to 
remove dissolved chemicals. In the example given, the used solvent is 
separated from the ink particles and binder, recovered and recycled to the 
process or to some other use with a minimum of losses and no harm to the 
environment. 
Thus while the invention has been described in terms of a preferred 
embodiment, it is to be understood by those skilled in the art that 
various changes may be made and equivalents may be substituted for 
elements thereof without departing from the true spirit and scope of the 
invention as set forth in the appended claims.