 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to compositions of acrylonitrile polymers and to the
processing and use of the compositions for the formation of films,
particularly barrier films, fibers, co-extruded articles and foams.
BACKGROUND OF THE INVENTION
It has long been recognized that acrylonitrile polymers, and co-polymers
containing acrylonitrile, produce films, fibers and other articles having
desirable properties. Such products of particular interest are barrier
films for use in product packaging. It has also long been recognized that
certain aspects of polyacrylonitrile ("PAN") products need to be improved.
First, there is a need for improved and easier processing. Secondly, there
is a need for improved and more complete removal of the unreacted
acrylonitrile monomer during processing of the polyacrylonitrile, because
the presence of monomer is undesirable in food and drug packaging
products. Thirdly, there is a need to improve the barrier properties of
acrylonitrile polymers and co-polymers. Fourthly, it is desirable that
strength properties, particularly tensile strength, of acrylonitrile
polymers and co-polymers be increased.
Acrylonitrile polymers composed of 90 weight percent or more of
acrylonitrile monomer are generally not melt processable. When heated,
they char before the melt can be processed at temperatures commonly used
in plastics processing equipment.
Shaped articles made from such polymers are presently limited to the
production of fibers by dissolving such polymers in a polar solvent,
forcing the resulting "syrup" through a spinnerette to form fibers,
coagulating them in a heated atmosphere (dry spinning) or in a fluid bath
(wet spinning), tensilizing by stretching, and drying by heat
volatilization. Solvent vapors are condensed, purified and reused. Such
processing is termed "solution" or "solvent" casting.
Experimental acrylonitrile homopolymer and copolymer thin films have been
made by solution casting as described in U.S. Pat. No. 3,437,717 to Isley
et al. A melt extrusion blown film process is disclosed in U.S. Pat. No.
4,536,365 to Zwick, and in U.S. Pat. No. 4,144,299 to Inoue et al. Another
melt reaction-extrusion process utilizing a water leaching system to
remove the solvent component prior to biaxially stretching such formed
film is described in U.S. Pat. No. 4,066,731 to Hungerford. Film property
data taken from such acrylonitrile films show them to have many desirable
mechanical, physical, chemical and electrical values for multipurpose use
applications. Such films are clear, tough, resistant to ultraviolet
radiation, have good temperature resistance, provide high barrier to gas
and moisture transmission, and have good dimensional stability.
It appears that as the level of acrylonitrile monomer in such films is
increased relative to other monomers, property values also increase. This
is especially true of a 99.6 percent acrylonitrile monomer content
polymerized with a polyalkenyl monomer serving as a cross-linking agent,
as in U.S. Pat. No. 3,437,717. It is also apparent that a small amount of
cross-linking agent incorporated into a polyacrylonitrile polymer for
solution spinning filaments, yarns and fibers significantly enhances the
properties of such end products, as disclosed in U.S. Pat. No. 3,268,490
to Sunden, et al. Conversely, the presence of minor amounts, as little as
1 to 2 percent, of comonomers can frequently reduce the desirable
characteristics of the polyacrylonitrile product to a point that little
advantage is gained over competitive polymer products.
Among the undesirable aspects of these polyacrylonitrile films is that the
level of unpolymerized acrylonitrile monomer can exceed the 0.1 parts per
million now allowed under United States Food and Drug Administration
regulations. Other undesirable aspects relate to levels of residual and
possibly toxic solvent, and the implications associated with packaging of
food and drug products, and the higher costs and hazards of recovering,
condensing, purifying and storing the volumes of solvent required.
The acrylonitrile polymer having the highest barrier and other property
values in a biaxially stretched film is the type of polymer found in U.S.
Pat. Nos. 3,437,717 and 3,380,949 to Isley et al., composed of 99.6 weight
percent acrylonitrile monomer, as well as in U.S. Pat. No. 3,268,490 to
Sundun et al., including copolymers composed of 85 weight percent
acrylonitrile monomer. However, the molecular weight of these polymers
increases after polymerization, thus causing decreasing rates of
dissolution from 17 weight percent to less than 5 weight percent in the
solvent system over the course of just 15 days from the date of
polymerization. This indicates polymer instability. It is felt that these
are major factors which have prevented such experimental films from
becoming commercial products.
The disclosures of the above referenced patents are incorporated herein by
reference.
Thus, it is an object of this invention to provide improved methods of
processing polyacrylontrile and to provide improved and new PAN
compositions and products. This objective, as well as other desirable
objectives, are achieved by the invention described and claimed herein.
SUMMARY OF THE INVENTION
This invention provides a method of preparing acrylonitrile polymers
comprising 90 weight percent or more of acrylonitrile monomer, producing
stabilized polymers in which molecular weight increases are prevented
while in storage after preparation, drying such polymers to a moisture
content of from 5 weight percent to about 30 weight percent under low
temperature conditions which prevent the onset of thermal degradation of
the polymer, and removing substantially all unpolymerized acrylonitrile
monomer.
Another aspect of this invention provides a method of mixing moist
particulate acrylonitrile polymers with a liquid fugitive plasticizing
agent and other desired additives, dehumidifying such mix to a moisture
content of less than 0.5 weight percent, fusing the dry blend in a
compounder/extruder and forming pellets or precursor-shaped articles for
re-extrusion, oriented film, fiber, foam and co-extrusions with other
polymers.
A further aspect of this invention provides a method of extracting about
one-half of the liquid fugitive plasticizing agent from the
polyacrylonitrile extrudate during the initial product forming and
processing steps, then removing the remaining plasticizing agent to the
desired level during subsequent processing steps, including stretching,
biaxial orientation, heat treatment, heat volatilization of the
plasticizer, and the like.
An additional aspect of this invention provides polyacrylonitrile products
having an extremely low unpolymerized acrylonitrile monomer content as a
result of heat volatilization of the fugitive plasticizing agent in the
processing of the formed polymer product, wherein the monomer is carried
out of the polymer by the volatilizing fugitive plasticizing agent.
An additional aspect of this invention provides for the increase of the
polyacrylonitrile molecular weight for increased tensile strength and
other properties. It is believed that biaxial orientation processing steps
and/or heat treatment processing steps provide establishment of mechanical
alignment of the molecular chains within the polymer, extension of chain
lengths within the polymer and/or networking or entanglement of molecular
chains within the polymer, thereby establishing pre-crystalline domains
within the polymer structure.
In another aspect, this invention provides a method of making an extrudable
polymer comprising polyacrylonitrile, which method comprises the steps of
forming a free-flowing particulate polymer comprising polyacrylonitrile
having from about 5 to about 30 percent by weight water or other
absorption aid in the particulate polymer, mixing a liquid fugitive
plasticizer with the particulate polymer whereby the plasticizer is
uniformly absorbed in the particulate polymer particles, then removing
substantially all of the water or other absorption aid from the
particulate polymer mixture, thereby leaving the plasticizer absorbed in
the particulate polymer. The liquid fugitive plasticizer is uniformly,
completely and totally absorbed into the polymer, i.e., excess, unabsorbed
plasticizer is not allowed to remain in the particulate polymer mix.
Instead of or in addition to the water, other absorption aids for the
plasticizer can be used, such as lower alkyl alcohols. In one preferred
aspect, substantially all of the water is removed from the
polymer-plasticizer mix, so that the polymer can be extruded at elevated
temperatures without the evolution of steam. The dried plasticized polymer
can be extruded to form pellets, films, fibers or shaped products. The
pellets can be used for re-extrusion to form desired polyacrylonitrile
products.
In another aspect of this invention, the dried plasticized
polyacrylonitrile is extruded to form thin films or fibers, whereby upon
the initial heating and stretching of the film or fiber, a significant
portion of the fugitive plasticizer exudes from the polyacrylonitrile film
or fiber and is collected from the surface of the film or fiber for
recycle. In a further preferred aspect, the film or fiber is subjected to
further stretching and/or heating during which further exudation and
removal of additional amounts of plasticizer occurs. Thus, the final
polyacrylonitrile film or fiber can be produced according to this
invention having the desired amount of plasticizer remaining in film or
fiber to provide varied physical properties of the final product.
In another aspect, the plasticized acrylonitrile polymer containing the
water or other absorption aid can be extruded at conditions appropriate to
form a foamed polyacrylonitrile product. In a preferred aspect of this
method of the invention, the polymer mix containing the plasticizer can be
dried to remove a portion of the water or other absorption aid, but
leaving a sufficient amount in the mix so that when the polymer is
extruded at the elevated temperature, the water or other absorption aid is
vaporized to act as the blowing agent. Alternatively, the plasticized
polymer can be dried to remove substantially all of the water or other
absorption aid. Then, when the polymer is extruded, conventional blowing
agents, such as carbon dioxide, pentane, and the like, can be introduced
into the extruder to provide the desired foamed polyacrylonitrile product.
In another aspect, this invention provides a novel copolymer of
acrylonitrile monomer and a polyalkenyl monomer, wherein the copolymer is
formed by the process described above, and wherein a final heating step is
employed, after the final polymer product has been formed, to increase the
molecular weight of the copolymer in the final polymer product by at least
about 50 percent of its molecular weight when the polymer product is
initially formed by extruding or other means.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawing illustrates various preferred aspects of the methods of this
invention.
DETAILED DESCRIPTION OF THE INVENTION
The principles of the various basic aspects of the present invention can be
understood by reference to certain preferred embodiments of this invention
illustrated in the drawing. In reference to the drawing, a free-flowing,
particulate acrylonitrile polymer having a molecular weight of about
80,000 and a moisture content of about 5 to about 30 percent by weight,
preferably about 10 to 20 percent, is charged via inlet 10 to mixer 14. A
fugitive plasticizer, e.g., ethylene carbonate, is charged to mixer 14 via
inlet 12 in an amount of about 30 to 50 percent by weight bond based on
the weight of the polymer, preferably about 35 to 45 weight percent. A
preferred method of facilitating the mixing of the ethylene carbonate with
the particulate polymer in mixer 14 is to melt the ethylene carbonate and
charge it to mixer 14 and to the particulate polymer mixture in liquid
form through an atomizing nozzle to thereby aid in the even distribution
of the ethylene carbonate throughout the polymer particles. If the
plasticizer is charged to the mixer as a solid or powder, the mixer should
be operated at a temperature sufficient to melt the plasticizer in situ to
enable the plasticizer to be uniformly absorbed throughout the polymer
particles.
After the ethylene carbonate and polymer particles are sufficiently mixed,
the resulting mixture 16 is charged to dryer 22 where the water is removed
at relatively low temperatures. For example, the dryer can operate at
about 150.degree. F. (about 65.degree. C.) at atmospheric pressure or can
operate at about 125.degree. to about 130.degree. F. (about 52.degree. C.
to 54.degree. C.) under a vacuum. It is important to maintain the
operation of the dryer at a sufficiently low temperature so that the
acrylonitrile polymer does not char or degrade. It is generally desirable
for production of film, fiber, etc., to reduce the moisture content of the
acrylonitrile polymer particles to about 0.5 percent by weight moisture,
or less. It is important to note that in the drying operation the ethylene
carbonate does not volatilize with the water but remains absorbed in the
particulate polymer.
The dried, plasticized particulate acrylonitrile polymer 24 is compounded
and processed in extruder 32, which is fitted with a pelletizing die 34 to
produce polymer pellets 35 for storage and re-extrusion into various
products. Alternatively, the dried, plasticized acrylonitrile particulate
polymer 24 may be directly fed to extruder 36, which is fitted with a die
according to the desired end product to be produced. In this embodiment
illustrated in the drawing, extruder 36 is fitted with a slot die 37 to
produce a flat web 44 having a thickness of 16 mils (0.016 inches) (0.04
cm) controlled by the gauge rolls 40. The extruder may be equipped with
vent 38 to allow the removal of residual moisture and vapors. In this
embodiment illustrated in the drawing, the polymer mixture 24 can contain
about 35 to about 45 weight percent ethylene carbonate based on the weight
of the polymer. Whether such polymer mixture 24 is fed into extruder 32
for pelletizing, or into extruder 36 for direct feed to form a continuous
web for film, extruders 32 and 36 should contain staged heat zones ranging
from about 175.degree. F. (80.degree. C.) to about 300.degree. F.
(149.degree. C.). The polymer melt rheology may be adjusted by altering
the ratio of plasticizer to polymer, until the required rheology is
achieved at the most desired temperature at the exit die 34 for pellets,
or slot die 37 to form web 44. The pellet or web product is quench cooled
upon exiting die 34 or die 37, respectively, to avoid any degradation or
undesired molecular weight increase of the polymer.
Web 44 then enters the machine direction orientation (MDO) machine 50,
where the web is stretched. Web 44 first passes between warm-up rolls 41,
having temperatures of from about 180.degree. F. (82.degree. C.) to about
220.degree. F. (104.degree. C.) to facilitate the machine direction
stretching in the series of stretch rolls 43. As the web 44 is heated to
its proper temperature on rolls 41, about one-half of the fugitive
plasticizer exudes as a liquid from the surface of the web, and is removed
from the web surface by suitable means, such as air knives 42, which
direct the flow of such plasticizer into receptacle 46 for recovery and
recycle with appropriate purification. If the plasticizer cools and
solidifies on the surface of the web, it can be removed therefrom as a
powder and similarly collected for recycle. In some instances, additional
plasticizer exudes during stretching on rolls 43 and can be similarly
removed and collected by air knives 42. Typically the MDO stretch is about
4 to 1.
The machine direction stretched web 48 then enters the transverse direction
orientation (TDO) machine and oven 52, which is divided into separate
heating and treatment zones 52a, 52b, 52c and 52d. Each heating zone
temperature is set to accomplish its specific task. Since web 48 has been
reduced in thickness of web 44 to about one-fourth of the original
extruded thickness due to machine direction stretching in 50, i.e., about
4 mils (0.004 in. or 0.01 cm) complete heat penetration to the core of the
film web occurs quickly and efficiently in preheat oven zone 52a. After
the web is preheated in zone 52a, the web enters tenter frame zone 52b
where its edge areas are gripped by clips fastened to two opposing
continuously moving chains which travel apart from each other to effect a
cross-wise stretching action on the web to the degree desired. Such chains
are a part of a tentering frame apparatus inside zone 52b. As the film is
stretched transversely, it becomes thinner. In stretch zone 52b, final
desired film thickness is produced. A typical TDO stretch ratio is about 4
to 1, which in this case produces a film of about 1 mil (0.001 in. or
0.0025 cm). Plasticizer within the film volatilizes from the film with the
increasing temperature of heat zone 52c at 350.degree. F. (177.degree.
C.), at which temperature substantially all remaining plasticizer in the
film is volatilized as vapor in the oven.
A still higher temperature is used in heat zone 52d to fully volatilize any
remaining plasticizer from the film. In such volatilization of plasticizer
from the film, any unpolymerized acrylonitrile monomer which may have
remained in the polymer up to this point, will have been dissolved by the
plasticizer and volatilized with the vaporized plasticizer. In TDO oven 52
all volatiles are collected through manifold vapor duct 53, then collected
in a vapor condenser/recovery unit for subsequent purification and recycle
to mixer 14. This aspect of the present invention enables the production
of PAN film or fiber with extremely low or negligible unpolymerized
monomer content. It is possible to remove essentially all traces of
unpolymerized acrylonitrile monomer using this process, thereby producing
a high purity PAN film acceptable for the most stringent packaging
requirements.
Upon exiting TDO oven 52, the biaxially stretched film 55 is quickly quench
cooled, edge trimmed, and rolled up onto standard roll cores.
Alternatively, time-temperature histories in oven heat zones 52c and 52d
may be adjusted so that some residual plasticizer is left in the film, to
provide desired properties for secondary thermoforming, for ultrasonic
welding, or for other use applications that do not require high barrier or
low monomer properties. In other uses, such as information storage media
(computer tapes and floppy discs), audio tapes, photographic media, and
the like, it may be desired to have a plasticizer level in the film of
about 0.5 to 5 percent, preferably about 1 or 2 percent, to provide
appropriate mechanical properties and surface properties, such as bonding
sites for the respective media coating binder.
In the embodiment shown in the drawing, the preferred acrylonitrile polymer
used is one which undergoes molecular weight increase through chain
extension, whereby a lattice structure is formed and crystalline domains
are established within the polymer structure in the finished film 55.
Molecular weight increases from the original 80,000 to over 230,000 have
been observed, which impart increased tensile and dimensional stability
properties, improved gas and moisture barrier properties, and elevated
flex crack resistance.
In understanding the basic operation of the preferred embodiment of the
present invention, it is important to note that the plasticizer is mixed
with the polyacrylonitrile containing the above mentioned amount of
moisture, whereby the moisture functions as an absorption aid to cause the
plasticizer to be absorbed totally and uniformly into each polymer
particle thereby providing a uniform distribution of such plasticizer
throughout the plasticizer/polymer mixture. It is also essential to
thereafter remove all but about 0.5 percent or less of the moisture from
such plasticizer/polymer mixture, to eliminate the formation of excess
steam and consequent bubbles or defects within the extrudate pellets 35 or
web 44 during their formation from extruders 32 or 36, respectively.
In another aspect of this invention, a polyacrylonitrile foam product 84 is
produced as illustrated in the drawing. The plasticizer/polymer mixture 16
can be processed directly in extruder 82, without drying such mixture. In
this embodiment, the temperature in extruder 82 generates steam from the
moisture present, which acts as a blowing agent for the production of PAN
foam 84. As the polymer melt exits extruder 82 at the elevated temperature
of about 300.degree. F. (149.degree. C.), the steam pressure causes the
acrylonitrile polymer melt to expand, thereby resulting in the formation
of a cellular structure with biaxially oriented cell walls, thus forming a
cellular PAN foam. In this aspect of this invention, it will be apparent
to one skilled in the art that the moisture content and the plasticizer
content of the polyacrylonitrile mixture 16 entering extruder 82 can be
adjusted over a wide range to provide the polyacrylonitrile foam product
84 having the final physical properties desired. It will also be apparent
that the foam product can be made in various thicknesses and dimensions as
is conventionally practiced in producing extruded polymeric foams.
Alternatively, the moisture can be removed from the plasticized polymer 16
before use in extruder 82, i.e., polymer 24 from dryer 22 and/or polymer
35 from pelletizer 34 can be used in extruder 82. In such embodiments, a
conventional blowing agent, such as CO.sub.2, pentane, etc., can be added
to extruder 82 through inlet 83 to produce PAN foam 84. It is equally
apparent that a portion of the moisture can be removed from polymer mix 16
and the steam supplemented with a conventional blowing agent added through
port 83 to produce a variety of PAN foams having desired characteristics
and properties.
As used herein, the term "particulate polymer comprising polyacrylonitrile"
is to be understood to include homopolymer polyacrylonitrile, copolymers
of acrylonitrile, block copolymers, graft copolymers, etc., which contain
a significant portion of acrylonitrile polymerized or copolymerized in the
polymer system. Monomers which can be included in the polymers useful as
chain extenders and cross-linking agents are polyalkenyl monomers having
at least two vinyl groups per molecule, including butanediol-1,4-divinyl
ether, divinyl benzene, and the like. Monomers which can be employed to
form copolymers of acrylonitrile useful in this invention include acrylate
esters, such as methyl acrylate, ethyl acrylate, butyl acrylate, and the
like; and methacrylate esters such as methyl methacrylate, ethyl
methacrylate, butyl methacrylate, and the like.
The "absorption aid" used to facilitate the mixing and absorption of the
liquid fugitive plasticizer into the particulate polymer is usually water.
However, other absorption aids include the lower aliphatic alcohols such
as methyl alcohol and ethyl alcohol, which may be used alone or in
combination with water. Other materials may be used as absorption aids so
long as the material properly functions to facilitate the absorption of
the plasticizer into the polymer particles completely and uniformly. In
addition, the absorption aid must then be capable of essentially complete
(at least for most film and fiber production) removal from the plasticized
polymer at temperatures and conditions which do not degrade the polymer or
remove the plasticizer from the polymer.
The amounts of absorption aid used in the polymer should be between about 3
percent and about 30 percent by weight based on the weight of the polymer.
Preferably, the range is about 5 percent to about 20 percent by weight,
and more preferably about 10 to about 15 percent by weight based on the
weight of the polymer. The amount will vary depending on the particular
polymer or copolymer, absorption aid, plasticizer and the desired end
product being produced.
The plasticizer employed in this invention is referred to as a "liquid
fugitive plasticizer" because its function is to readily absorb uniformly
into the polymer, remain in the polymer during processing, then readily
separate from the extruded product which is formed, such as film, sheet,
web, fibrils and co-extruded layers or laminates with other polymers. Such
separation is accomplished by the elevated temperatures used throughout
the product forming processes. It is believed that stretching to some
extent also causes or assists in such separation. The plasticizers useful
in the polymers mentioned above include ethylene carbonate and propylene
carbonate either alone or in combination with each other. In addition,
acetonitrile, used with either ethylene carbonate or propylene carbonate
or a mixture thereof, is useful as a low boiling plasticizer which rapidly
volatilizes at relatively low process temperatures, and is used with such
carbonates at from about 10 to about 50 weight percent based on the weight
of the carbonate(s), preferably about 20 to about 30 weight percent.
The liquid fugitive plasticizer must be one which can be uniformly,
completely and intimately absorbed throughout the particulate
acrylonitrile polymer with the assistance of the absorption aid. Thus, the
plasticizer must be in liquid form at mixing temperatures to enable the
plasticizer to penetrate uniformly through the particulate polymer for
absorption therein. A plasticizer which is not liquid at mixing
temperatures may be used if it can be placed in fluid form in a solvent or
co-solvent for mixing. The solvent or co-solvent can then be removed with
the absorption aid. Preferably, the plasticizer is liquid while in use in
the polymer, i.e., at processing temperatures, and is liquid when
separated from the polymer product. The plasticizer must be capable of
remaining in the polymer when the absorption aid (water and/or alcohol) is
removed from the plasticized polymer mix. In some instances, such as with
acetonitrile, it may be appropriate to mix in one plasticizer, such as a
carbonate, remove the absorption aid, such as water, then mix in a second,
more volatile plasticizer, such as acetonitrile. This avoids the problem
of the acetonitrile being removed with the absorption aid, such as water.
The acetonitrile will mix with and be absorbed into the dry polymer due to
the presence of the first plasticizer. The plasticized polymer can then be
processed according to this invention, where the more volatile
plasticizer, such as acetonitrile is quickly removed from the extruded
film or fiber, then the less volatile plasticizer, such as a carbonate, is
then removed with further heating and/or mechanical action.
Finally, the plasticizer should be readily separable from the final formed
polymer product to the desired degree by heat treatment and/or mechanical
treatment, such as disclosed herein. Thus, the plasticizer should not
volatilize at the temperatures and conditions for absorption aid removal,
but should exude, liquify or volatilize at temperatures and conditions for
processing and treatment of the formed PAN products. Of course, a more
volatile second plasticizer which is mixed in the polymer mix after
removal of the absorption aid will generally be easily separable from the
final polymer product to the degree desired.
One skilled in the art will be able to select appropriate liquid fugitive
plasticizers for use in this invention based on the criteria described
above and following the embodiments and examples disclosed herein.
The amount of plasticizer(s) used can generally range from about 20 percent
to about 60 percent by weight based on the weight of the polymer,
preferably about 30 percent to about 50 percent and more preferably about
35 to about 45 percent. It has been found that with ethylene carbonate a
wide range of polymer melt viscosity can be obtained by small adjustments
of the amount of plasticizer within the preferred 35 to 45 percent range.
Having illustrated the present invention in the drawing described above,
other more general aspects and some more particular aspects of this
invention can now be described.
Polymers useful | | |
