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BACKGROUND OF THE INVENTION
Elimination of so-called "hotmelt" coatings from paper mill "broke" and
trimmings or from used consumer items, such as milk cartons, frozen food
boxes and the like is essential to recycling of the high quality cellulose
fibers generally contained in such paperboard.
A number of processes employing various aqueous and nonaqueous solvents,
separately or in combination, have been developed for the purpose of
removing hotmelt coatings. None of these processes employ vapor pools of
non-aqueous solvents. The process presently used commercially is the
so-called "Polysolv" process, described in U.S. Pat. No. 3,058,871. This
patent constitutes the closest known prior art.
In the latter process, shredded "furnish" is washed with progressively
cleaner batches of a super-heated liquid solvent, such as
trichloroethylene, in a rotary extractor or autoclave. The denuded furnish
is then stripped of residual solvent, in the extractor, with steam. The
"dirtiest" solvent is distilled and the bottoms are mixed with fuel oil
and further distilled to remove the last of the solvent. The residual
polymer/fuel oil mixture is incinerated or utilized as boiler fuel.
U.S. patent application Ser. No. 371,836, published in abstract in the Jan.
28, 1975 Official Gazette as part of a Trial Voluntary Protest Program, is
directed to the method of cleaning an article contaminated with organic
acid materials so as to form a biodegradeable waste comprising the steps
of:
inserting the article to be cleaned of organic acid residues into the vapor
section of a vapor degreaser, said vapor degreaser having a two-phase
liquid solvent located in the sump thereof, said two-phase liquid solvent
comprising a first liquid containing an organic solvent and a second
liquid containing water and an inorganic base of low volatility, said
inorganic base being sufficiently strong to neutralize said organic acid
materials, and said first and second liquids being immiscible in one
another; and
heating said mixture to boil said two-phase liquid solvent wherein said
solvent is in a state of turbulence and a vapor of water and organic
solvent is created in said vapor section and refluxing said mixture,
without vaporizing said inorganic base.
U.S. Pat. No. 2,413,144 discloses a method of removing waxes from shotgun
shells (or other coated paper objects) wherein the shells (or objects) are
individually suspended in hot vapors of a suitable solvent. No agitation
is employed.
OBJECTS OF THE PRESENT INVENTION
It is a primary object of the present invention to provide a process in
which coated paper stock can be completely freed of the coating resin by
the flushing action of a hot solvent condensate, i.e., solvent containing
no dissolved polymer.
An additional object is to provide a process in which the treated stock or
furnish is not subjected to steam-stripping and emerges dry.
A further object is to provide for operation of a hotmelt removal process
in such manner that the hotmelt is removed from the furnish largely in the
form of a solvent-swollen gel, rather than as a solution in the solvent,
thereby facilitating economic recovery of both the resin and the solvent
for re-use.
Other objects will be apparent to those skilled in the art.
SUMMARY OF THE INVENTION
Broadly, the present invention is a process for removing a coating resin
from paperstock coated therewith, comprising:
(a) maintaining a pool of hot, saturated vapors of a halogenated, acyclic
hydrocarbon containing from one to three carbons and from two to eight
bromine, chlorine or fluorine atoms, at least two of which are bromine or
chlorine,
said halogenated hydrocarbon melting at a temperature of 60.degree. or less
to form a liquid capable of wetting said resin and said vapors being at a
temperature which is at least as high as the melting or softening point of
the resin and having a density which is at least three times the density
of the ambient air,
(b) placing pieces of the coated paperstock, which are at a temperature
below the condensation temperature of said vapors, in said vapor pool,
thereby causing formation of a continuous, off-running film of condensate
on the resin,
(c) agitating together said pieces of paperstock in the vapor pool until at
least a predominant proportion of said resin has been removed by the
combined actions thereon of said agitation and of the vapors and
condensate.
In a preferred embodiment of the preceding process
(1) said resin is converted to a semi-solid or gel phase as a result of
swelling and softening by the hot vapors,
(2) the combined actions of the condensate and gravity cause said phase to
separate from said paperstock and fall, and
(3) said phase is intercepted and recovered essentially free of any of said
halogenated hydrocarbon present as a separate liquid phase.
In another preferred embodiment of the invention, as above broadly defined,
the halogenated hydrocarbon vapors have a heat of condensation less than
150 BTU/lb.
In a highly preferred embodiment, the only halogen contained in said
halogenated hydrocarbon is chlorine.
In the most preferred embodiment of the process, the liquid halogenated
hydrocarbon is perchloroethylene.
The preferred application of the process is to hot-melt coated paperboard.
DETAILED DESCRIPTION OF THE INVENTION
Apparatus required for the practice of the present invention is generally
as commonly used in conventional vapor degreasing operations, such as
removing grease from metal parts, for example. Modifications, such as
inclusion of means for intercepting resins removed as a gel, for example,
may be readily made and require only ordinary skill in the art of chemical
engineering.
A typical, commercial operation employing the process of the invention will
involve the following steps:
(1) Shredding of the coated paperstock into strips of a width and length
appropriate to the apparatus selected.
(2) Agitating together the strips while exposing them to the hot,
saturated, halogenated hydrocarbon vapors until a desired proportion of
the coating has been removed by the combined actions of the vapors, the
condensate and gravity. Usually, the shredded stock will be kept in the
vapors at least until condensation on the shreds no longer occurs,
resulting in essentially complete removal of the solvent-wettable coating
resin. For example, from 15 seconds to 2 minutes usually suffices when
ordinary hot-melt coated paperboard is immersed in hot vapors of
perchloroethylene.
(3) Interception or separation of any resinous materials flushed from the
paperstock by the condensate as a discrete phase, such as a solution gel
or a melt.
(4) A final rinse of the "degreased" furnish with clean liquid solvent
(usually the same as the solvent of which the hot vapor pool consists but
optionally a different solvent). This step may often be omitted.
(5) Removal of any residual solvent entrained in the stripped paperstock.
(6) Recovery of any dissolved resins carried into the liquid solvent sump
by the condensate, as, for example, by continually removing a bleed which
is distilled (by which means any water present is also removed, usually as
a lower boiling azeotrope). The resin is recovered as a bottoms product.
Alternatively, dissolved resin may be separated by chilling and
centrifuging or filtering or by other conventional separatory procedures.
(7) Recycle of the recovered, water-free solvent (a convenient source of
rinse-solvent for step (4)).
Suitable coated paperstock feed for the present process is any paperstock
which can be sized for the available equipment and which is coated with a
resin upon which hot vapors of a halogenated hydrocarbon will condense to
form a film of liquid capable, in concert with uncondensed hot vapors, of
loosening and flushing off the resin. Thus, the term "paperstock" as used
herein has a broader connotation than "paperboard" and denotes any
substrate consisting predominantly of entangled or woven cellulosic
fibers, including materials which would ordinarily be considered fabrics.
However, the importance of the process resides primarily in its
application to paperboards.
Whether or not a given liquid halogenated hydrocarbon wets a given coating
resin can be determined precisely by wetting angle measurements in
well-known procedures. However, adequacy of wetting can usually be judged
quite satisfactorily by eye. If a liquid appears to wet the resin only
poorly, i.e., will not spread out as a continuous film but tends to
retract or remain as film-like islands or discrete drops, an empirical
laboratory test of the "degreasing" procedure itself may be resorted to.
This will usually be unnecessary. As a general guide, the less polar
halogenated hydrocarbon solvents will be expected to more readily wet
coating surfaces having a low degree of polar character, such as
polyolefins, poly fluoro-olefins, etc. Coatings of more polar polymers,
such as nylons, polyesters, Sarans, polyacrylic acids and sulfonated
polyvinyl aromatics will be expected to be wet more readily by the more
polar of the specified halogenated hydrocarbons.
The term "liquid" as used herein with regard to halogenated hydrocarbons is
intended to include such materials which melt at temperatures of about
60.degree. C., or less, although solvents which are liquid at ordinary
ambient temperatures are distinctly preferred.
The invention is not predicated on details of mechanism. Resin loosening
may be due partly or wholly to swelling by imbibed vapors or to softening,
melting or actual dissolution. Preferably, however, a solvent is chosen
which is capable of removing the resin as a discrete, readily separable
semi-solid or gel as a result of swelling and softening by the hot vapors
and by the mechanical flushing action of the condensate. It is a simple
matter to determine whether any given solvent will function this way with
a given coated stock, even if the type of coating resin is not known. A
procedure and laboratory apparatus suitable for this purpose is described
in Example 1 herein.
In general, coating resins consist of inexpensive, readily available
thermoplastics, such as polyethylene or ethylene/propylene copolymers.
However, the process of the invention may be employed to advantage with
any type of resin which can be loosened from and flushed off a paperlike
substrate by the combined actions of the vapor pool and the resultant
condensate (under the influence of a gravitational force).
Suitable halogenated hydrocarbons
Suitable halogenated acyclic hydrocarbons are methanes, ethanes, ethylene,
propanes or propylenes substituted with from two to eight bromines,
chlorines or fluorines, at least two of which are bromine or chlorine. The
compound (or mixture of such compounds) should melt at about 60.degree. C.
or less and the vapor density must be at least three times the density of
the ambient air.
It is also essential that the liquid is capable of wetting the resin to be
removed and boils at or above a temperature at which softening or melting
of the resin occurs. It should be noted that the term softening, as used
herein, includes any softening effect due to imbibing of solvent by the
resin structure. Thus, a resin may exhibit softening in a hot vapor pool
at a temperature lower than its "softening point" as ordinarly defined.
Although solvents suitable for the process can be used to form vapor pools
at temperatures above or below their normal atmospheric boiling points, by
resort to super- or sub-atmospheric pressures, operation at ordinary,
ambient pressures is highly preferable, even in closed systems.
Many coated paperstocks will include water in the cellulosic substrate
which will be volatilized by the action of the hot vapors or condensate.
Accordingly, such stabilizers as may be required to at least minimize the
concentration of hydrolysis products in the halogenated hydrocarbon
solvent will usually be employed. The process is generally operable for
its intended purpose in the absence of such stabilizers, but for reasons
of economy and safety, their use is to be recommended. A wide variety of
stabilizers for halogenated hydrocarbon solvents are known. The majority
of these are various combinations of nitrogen bases, such as amines, with
oxirane compounds, butylene oxide, for example. Proprietary formulations
for commercial degreasing operations, such as the several stabilized
perchloroethylene formulations marketed by The Dow Chemical Company, are
particularly suitable.
The lower the heat of condensation of a given solvent, the greater the
amount of condensate which will result from the cooling action attendant
upon placing a given quantity of coated paperstock in a vapor pool. The
sensible heat of coated paperstock is usually relatively low and
precooling or chilling of it will ordinarily not be practical.
Consequently, it is important to condensation of a sufficient quantity of
the solvent vapors for good flushing action that the latent heat of
condensation be as low as possible. Desirably, this will be below 150
BTU/lb. and preferably less than 100 BTU/lb.
To be suitable for the present process, the hot vapors must have a relative
vapor density of at least 3. Otherwise, a "pool" of the saturated, hot
vapors cannot be established as a body of vapors defined by peripheral
cooling means, even in a closed system, and excessive losses of the
solvent will occur in an open system. This requirement is well known in
the general art of vapor degreasing.
Table I, following, gives boiling points (B. pt.), latent heats of
vaporization (=heat of condensation) and relative vapor densities for
several chlorinated hydrocarbon solvents. With the exception of methylene
chloride and 1,2-dichloroethane, these solvents are well adapted for the
practice of the present invention at ordinary ambient pressures.
TABLE 1
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CHLORINATED C.sub.1 --C.sub.2 HYDROCARBON PROPERTIES
Boiling Latent
Point Heat Relative Vapor
Solvent .degree.C.
BTU/lb Density at B. Pt.
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Methylene Chloride
40.0 141.7 2.76
Chloroform 61.2 106.2 3.64
Carbon tetrachloride
76.8 93.8 4.48
1,2-Dichloroethane
83.5 139.1 2.82
1,1,1-Trichloroethane
74.0 102.0 3.91
Trichloroethylene
86.9 101.6 3.57
Perchloroethylene
121.0 90.2 4.28
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Various data sources, such as the International Critical Tables, handbooks,
manufacturers brochures, etc., may be consulted for melting points,
boiling points, latent heats and vapor densities of other halogenated
hydrocarbons. In addition, such constants may be determined by well known
physical-chemical methods, such as, for example, the classic methods of
Regnault and Victor Meyer for determining vapor densities.
If a particular solvent is found able to convert a given coating resin to a
semi-solid or gel which falls off the paperstock as a discrete phase which
can be intercepted and thereby kept from dissolving in any liquid body of
the solvent into which it would otherwise fall, that solvent will
generally be highly preferred for removal of the resin.
A representative list of halogenated C.sub.1 -C.sub.3 hydrocarbons and
their boiling points follows. Melting points are also given for compounds
which are not liquids at ordinary ambient temperatures. Those compounds
marked with asterisks would generally be impracticable for use in the
present process.
TABLE II
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Boiling point
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Methanes
Bromo, chloro- 69.degree. C./760 mm Hg
Dibromo- 97
Bromo, chloro, fluoro- 36.1
Bromo, dichloro- 80-90.degree./742 mm
Dibromo, fluoro- 64.9.degree./757 mm
Dibromo, chloro- 118-20.degree./748 mm
Tribromo- 149.5.degree./760 mm
Bromo, trichloro- 104
Tribromo, fluoro- 106
Tribromo, chloro-
mp 55.degree.
158-9.degree.
Ethylenes
1,2-Dichloro- cis 60.3/760 mm
trans 47.5
1,2-Dibromo- cis 112.5
trans 108
Tribromo- 163.4
Ethanes
1-Bromo- 38.4/760 mm
1-Chloro-2-fluoro- 53.2
1-Bromo-2-fluoro- 70.1/745
1-Bromo-2-chloro- 107/760 mm
1,1-Dibromo- 110
1,2-Dibromo- 131
1,1,2-Trichloro- 113
1,1,2-Tribromo- 187-90
2-Bromo-1,1,1-trichloro 129-30
1,2-Dibromo-1,1-dichloro- 176-8
(decomposes)
1,2-Dibromo-1,2-dichloro- 195
1,1,2,2-Tetrabromo- 239-42
(decomposes)
137-8/36
Pentachloro- 162
1,1,1-Trichloro-2,2,2-trifluoro-
45.8
1,1,2-Trichloro-1,2,2-trifluoro-
47.7
1,1-Difluoro-1,2,2,2-tetrachloro
mp 40.6.degree.
91.5
1,2-Difluoro-1,1,2,2-tetrachloro
25 93
* Fluoro, pentachloro-
101 134-6
* Hexachloro- 187 186/777
Propylenes
1-Chloro- cis 32.8/760 mm
trans 37.4
3-Chloro- 45
1-Bromo- 58-60/747 mm
2-Bromo- 48.4/760
3-Bromo- 70
1,1-Dichloro- 78
1,3-Dichloro- cis 112
trans 104.3
3,3-Dichloro- 84.4
2,3-Dibromo 139-40
1,1,2-Trichloro- 118
1,2,3-Trichloro- 142
Propanes
1-Bromo- 70.8
1,1-Dichloro- 88.3
1,2-Dichloro- 96.2
1,3-Dichloro- 120.4
2,2-Dichloro- 69.7
1-Chloro-3-fluoro- 82.5
1-Bromo-3-fluoro- 101.4
1-Bromo-2-chloro- 118
1-Bromo-3-chloro- 141.3-2.3
2-Bromo-1-chloro- 117
1,2-Dibromo- 141.4
1,3-Dibromo- 167.3
1,1,1-Trichloro- 106.5-8.5
1,2,3-Tribromo- 219.2
1,1,1,2-Tetrachloro- 152-3
1,2,2,3-Tetrachloro- 164.degree.
* 1,1,1,2,3-Pentachloro-
mp 179-80 sublimes
1,1,2,3,3-Pentachloro- 198-200
1,1,1,2,3,3,3-Heptachloro- 249
1,1,1,2,2,3,3-Heptachloro-
29.4 247-8
* Octachloro- 160 268-9/734 mm
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It is apparent from the data in Table II that C.sub.2 -C.sub.3 hydrocarbons
which are substituted with more than about four to five bromine or
chlorine atoms tend, with some exceptions, to be so high melting as to be
generally not practicable for use in the present process. On the other
hand, 1,1,1,2,2,3-hexafluoropropane (not included in the table) boils at
1.2.degree. C. Thus, substitution with a total of up to eight halogen
atoms does not render a C.sub.2 -C.sub.3 hydrocarbon unsuitable for the
present process if the additional halogens above about four are fluorines.
In general, however, the higher the relative content of bromine and/or
chlorine, vis-a-vis, hydrogen or fluorine, the higher will be the vapor
density. Since chlorinated hydrocarbons tend to have lower latent heats of
vaporization than comparable brominated hydrocarbons, the halogenated,
acyclic C.sub.1 -C.sub.3 hydrocarbons in which chlorine is the only
halogen present are preferred.
It is to be noted that one of the heptachloropropanes is a high boiling
liquid which should be suitable for removal of resins which soften, melt
or dissolve at temperatures of 249.degree. or less. Ordinarily, however,
much lower temperatures, less conducive to fiber damage, will be
preferred. Vapor temperatures in the range of about
100.degree.-160.degree. are considered optimum for removal of conventional
hotmelt coatings.
Perchloroethylene is a particularly preferred solvent, not only because it
is commercially available in stabilized formulations and is of proven
utility in degreasing operations, but for other reasons as well. The
following advantages may be noted:
1. The boiling point (121.degree./760 mm) of perchloroethylene is high
enough to melt most hotmelt waxes. In contrast, methyl chloroform is not
as effective in rapidly stripping hotmelt coatings because the coating
must be dissolved in order to effect debonding thereof; that is, methyl
chloroform has too low a boiling point to be very efficient;
2. Perchloroethylene is a relatively poor solvent for polymeric hotmelts,
thereby facilitating their recovery as a discrete second phase, such as a
gel;
3. The boiling point of perchloroethylene is low enough not to
deleteriously affect the structure and physical properties of the
cellulose fibers. Measurable properties of handsheets made from repulped
fibers, such as freeness, burst factor and brightness, are unaffected by
contact with this solvent;
4. It has a very low heat of vaporization, the importance of which is
explained above.
EXAMPLES
Laboratory degreaser
A laboratory degreaser was made from a stainless steel beaker about 8" in
diameter and about 12" in height. A disc of 60-mesh, stainless steel
screening, about 6" in diameter, was fastened to three stainless steel,
3/16" legs about 2" long and placed in the bottom of the beaker. A
cylindrical cage, about 3" in diameter and 6" long and having a hinged
door which could be fastened shut, was made from stainless steel, 1/2"
mesh screening and closed at each end with a stainless steel plate 3" in
diameter. An axle rod, about 3/8" in diameter and 1" long, was fastened to
the center of each end plate and two semicylindrical bearing troughs were
mounted diametrically opposite each other on the interior surface of the
beaker at a height of about 6". The cage and axle assembly could be
rotated freely (with a push rod) when positioned with the axle ends in the
bearings. Several coils of 1/4" copper tubing were wound around the
outside of the beaker, to provide water-cooling. The middle coil was
positioned at a height of about 81/2". The unit was placed on a hot-plate
having a controllable heat input and the halogenated hydrocarbon
(perchloroethylene) poured in to form a "sump" layer about 1" deep at the
bottom of the beaker. "Degreasing" operations were carried out in a hood
having a sufficient air velocity to prevent escape therefrom of solvent
vapors.
EXAMPLE 1--Hot-melt Stripping
Heat and cooling inputs were controlled to establish a "pool" of saturated
perchloroethylene vapors having a maximum height of about 9" and a
temperature of about 121.degree.. The cage was charged with about 60 grams
of post-consumer milk carton stock (polyethylene-coated, bleached, Kraft
paperboard which had been cut into strips averaging about 1".times.4" in
size). The cage was placed on the bearings and rotated at a rate of about
1/2 revolution per second. Condensation of vapors on the strips and
run-off of the resulting condensate started immediately upon contact of
the strips with the vapors. The coating rapidly separated into large drops
or blobs and was flushed off (with associated pigments) in about 10-15
seconds. After a total of about one minute no liquid was forming on the
strips and it was observed that portions of the strips which had been
selected together with hotmelt were separated from each other. It was
apparent that contact between the strips due to the agitation (tumbling
action in the rotating cage) was essential to the latter result. The cage
and 60 mesh screen were removed from the beaker.
The paperboard in the cage was found entirely free of the hotmelt coating,
pigments and residual solvent. The screen was found to have intercepted a
quantity of a gel, of which about 85% was liquid perchloroethylene. The
weight of the paper immediately after the vapor treatment was 54 grams. A
total of 2.4 grams of resin (4% of the furnish weight) was recovered. The
resin in the gel intercepted by the screen accounted for 70 wt. % of the
latter recovery. The remaining weight loss was due to water vaporized from
the paper; most of this loss was gained back when the paper was allowed to
equilibrate with the ambient air.
The preceding results make apparent the utility of the process of the
invention for debonding and removing an adhered, solvent-wettable resin
from a cellulosic substrate.
EXAMPLE 2--Solvent and Hot-melt Recovery
A 1% solution of a polyethylene hotmelt formulation in perchloroethylene
was subjected to atmospheric distillation. The apparatus consisted of a
glass round bottom flask heated with an electric mantle and the usual
assembly of thermometers and overhead condenser; no fractionation plates
were provided.
The boiling temperature remained indistinguishable from that of the pure
solvent (121.degree. C.) until the concentration reached 5% (by weight).
The temperature then rose slowly to 150.degree. C. at 33% concentration
and more rapidly to 165.degree. C., at which time the distillation was
terminated. The residue was a yellowish brown, viscous fluid with no
tendency to cling to the glass walls at the terminal temperature. After
rapid quenching, the residue analyzed for 1% perchloroethylene by weight.
Gas chromatographic analysis of the distillate revealed no contamination
of the solvent by hotmelt components.
EXAMPLE 3--Removal of Other Coating Materials
In similar tests to that of Example 1, the present process has been found
effective for rapid removal of STYROFOAM.RTM. fillers from paperboard
substrates. Alsphalt coatings are also removed readily on exposure to hot
halogenated hydrocarbon vapors.
In accordance with common practice in degreasing operations wherein an
article is moved through a hot vapor pool, as on a conveyor, it is
critically important to control the rate of movement such that no gross
disturbance of the air/vapor interface results. A rate of no more than 10
ft. per minute is recommended for typical results. A rate of no more than
10 ft. per minute is recommended for typical operations. No lower limit on
rate of motion, other than is imposed by economic considerations,
prevails.
It is essential to ensure that the pieces of paperstock are agitated
together (and against the walls of any container they are in) in order to
attain efficient removal of their resin coating by the action of the
solvent vapors and/or condensate film.
It is not necessary that the body of boiling solvent from which the vapor
pool is derived be located under the pool, although this will ordinarily
be preferable.
The foregoing examples are only for purposes of illustration and are not to
be construed as limiting the scope of the present invention other than
according to the claims appended herewith.
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