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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a lightly crosslinked silane functional
vinylidene chloride homopolymer or copolymer suitable for use as a film,
coating, or foam, and more particularly to a polymer composition and
process which is the reaction product of vinylidene chloride, an optional
copolymerizable monomer, and an .alpha.,.beta. ethylenically unsaturated
silane.
Vinylidene chloride polymers are known to possess desirable chemical and
physical properties including resistance to ignition and combustion,
toughness, insolubility in common solvents, and low vapor and gas
transmission rates. Such polymers have heretofore been used in synthetic
fibers, coatings, and films. However, vinylidene chloride polymers, being
essentially linear polymers, are known to have poor melt strengths.
Moreover, polymers of vinylidene chloride have not been readily adaptable
to conventional extrusion techniques used to produce foamed cellular
structures. This has been due to several factors.
Initially, polymers of vinylidene chloride tend to degrade with the
evolution of hydrogen chloride at temperatures only slightly above the
temperatures necessary for melt processing. Additionally, most vinylidene
chloride polymers, being essentially linear in nature, have poor melt
tension. A sharp decrease in melt viscosity occurs at melt processing
temperatures which results in poor foam quality and many open cells.
Finally, vinylidene chloride polymers are insoluble or only somewhat
soluble with many conventionally used blowing agents.
Attempts have been made to produce foams of polyvinylidene chloride. For
example, Suh et al, U.S. Pat. No. 3,983,080, teach forming foams of
normally crystalline vinylidene chloride and copolymerizable monomers
utilizing blowing agents having specified physical properties and
solubility characteristics and utilizing carefully controlled
temperatures. Suh et al also discuss other prior art efforts at producing
vinylidene chloride polymer foams.
Hattori et al, Japanese Kokai No. 78/112,967, have reported producing an
extruded foam utilizing a mixture of polymers containing 60% polyethylene,
20% ethylene/vinyl acetate copolymer, and 20% polyvinylidene chloride
copolymerized with polyolefins or polystyrene. However, it is believed
that as the vinylidene chloride content of the foam is lowered, the
effects of the desirable chemical and physical properties of vinylidene
chloride are lessened.
Because of the poor melt strength of vinylidene chloride polymers, after
heating and melting, some prior art techniques required that the polymer
be cooled to increase melt strength prior to foaming. This required
careful temperature control and very narrow workable temperature ranges.
While the melt strength of such vinylidene chloride polymers may be
increased by the introduction of crosslinking monomers such as diallyl
ether, divinyl benzene, or diacrylates to produce higher molecular weight,
crosslinked vinylidene chloride polymers, such higher molecular weight
polymers would be unacceptable in an extrusion process. Such higher
molecular weights would lead to the generation of shear heat in the
extruder which would cause degradation of the vinylidene chloride polymer.
Accordingly, the need exists in the art for a vinylidene chloride polymeric
composition and process for making it which possesses good melt strength
and melt tension and which can be utilized in conventional melt processing
techniques.
SUMMARY OF THE INVENTION
The present invention provides a polymeric composition which can be made
into either a film, a coating, or a low density closed-cell foam utilizing
conventional techniques such as extrusion and expansion of polymer beads.
The composition comprises a unique reaction product of vinylidene
chloride, an optional copolymerizable monomer, and an .alpha.,.beta.
ethylenically unsaturated silane crosslinking agent. Where a foam is
desired, the composition further includes a volatile blowing agent which,
when activated, expands the polymeric reaction product into a low density
foam. Additional compatible stabilizers, plasticizers, and processing
agents may also be included.
The polymeric reaction product has silane functionality. This functionality
provides the reaction sites for a crosslinking reaction which occurs at an
optimum time during melt processing and thereby minimizes the generation
of shear heat and enables the polymer to be extruded and foamed with
minimum degradation.
The resulting foam can be used as insulation because of its relatively low
density, resistance to chemicals, ignition, and combustion, and has low
thermal conductivity and vapor and gas transmission rates. Further,
because of its toughness, flexibility, and resistance to breakage, the
foam of the present invention can be used as cushioning in packaging or
carpet padding. When used as a film or coating, the polymer of the present
invention possesses the desirable physical and chemical properties of
vinylidene chloride polymers and yet is more readily processable, having
improved melt strength and melt tension.
Accordingly, it is an object of the present invention to provide a lightly
crosslinked silane functional vinylidene chloride reaction product
suitable for use as a film, coating, or, in the addition of volatile
blowing agent, a foam. This, and other objects and advantages of the
invention will become apparent from the following detailed description,
the accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are graphs of the stress-strain behavior of vinylidene
chloride resins with no crosslinking (FIG. 1) and with silane functional
crosslinking (FIG. 2).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polymeric composition of the present invention is the result of the
reaction between vinylidene chloride and an .alpha.,.beta. ethylenically
unsaturated silane. 0ptionally, a copolymerizable monomer may be included
in the reaction. Such copolymerizable monomers include vinyl functional
monomers such as vinyl chloride, alkyl esters of acrylic and methacrylic
acids such as methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl
methacrylate, and ethylenically unsaturated mono- and dicarboxylic acids
such as acrylic acid, methacrylic acid, and itaconic acid. Mixtures of
such copolymerizable monomers may also be used.
The optional addition of such copolymerizable monomers reduces the degree
of crystallinity in the reaction product and renders it more easily
processable. However, relatively greater amounts of copolymerizable
monomer in the reaction product will lower the serviceability of the foam
at higher temperatures. Accordingly, it is preferred that if a
copolymerizable monomer is utilized, that it be added to the reactants in
an amount of from between 1-99% by weight. As the amount of silane added
is typically quite small, i.e., 0.01 to 5.0%, the vinylidene chloride
component of the reactants may be from 1 to 99.9% by weight, preferably
from 50 to 99.9% by weight.
Additionally, conventional amounts and types of plasticizers, stabilizers,
nucleators, and processing aids may be added to the reaction product. For
example, the addition of from 1-2% by weight of a copolymer of ethylene
and vinyl acetate, a copolymer of ethylene and methyl acrylate, or other
polyolefins may facilitate mixing and impregnation of the volatile blowing
agent or otherwise aid in the melt processability of the reaction product.
As the .alpha.,.beta. ethylenically unsaturated silane reactant, any of a
number of vinyl functional silane compounds may be utilized. For example,
.gamma.-methacryloxypropyltrimethoxy silane,
.gamma.-methacryloxypropyltriethoxy silane, vinyl trimethoxysilane, vinyl
triethoxysilane, vinyl tris(.beta.-methoxy ethoxy)silane, and mixtures
thereof may be reacted with vinylidene chloride to provide a silane
functional polymeric reaction product. Such a reaction may be carried out
utilizing conventional polymerization techniques including suspension
polymerization.
Preferably, the polymeric reaction product has a weight average molecular
weight of between 150,000 and 250,000, and preferably between 180,000 and
220,000. This average molecular weight permits ready melt processing of
the polymer including melting, plasticizing, and mixing of the polymer
with the volatile blowing agent. Higher molecular weight vinylidene
chloride polymers would be subject to shear and polymer degradation during
the initial phases of processing. This molecular weight would, however,
normally be much too low to permit the production of a good quality low
density foam. The melt strength of such a low molecular weight vinylidene
chloride polymer would be inadequate for conventional extrusion foaming
techniques.
However, the unique silane-functional reaction product of the present
invention undergoes a light crosslinking reaction at an optimum point in
the melt processing procedure to improve its melt strength through a chain
extending crosslinking reaction which occurs at the silane functional
sites. It has been found that only a small amount of silane crosslinking
agent is needed, with the range being from about 0.005 to 5.0%, preferably
0.05 to 3.0%, on a mole basis. The optimum amount for improving
processability of the melt is near the lower end of the above range.
However, adding larger amounts of the silane to provide more reaction
sites on the reaction product with vinylidene chloride will produce a more
highly crosslinked structure and result in a cellular foam structure
having a higher heat distortion temperature.
In the presence of trace amounts of water and acid, the polymer melt
undergoes the following crosslinking reaction:
##STR1##
where R is an alkyl group such as methyl or a .beta.-methoxy ethyl group.
The crosslinking reaction is essentially self-catalyzing because sufficient
trace moisture will almost always be present in the polymer. Likewise, at
the processing temperatures utilized by the invention, there will be some
slight degradation of the vinylidene chloride polymer with accompanying
evolution of trace amounts of hydrochloric acid.
The crosslinking reaction may be further controlled to delay it until an
optimum point in the process by drying the polymer before melt processing.
Trace amounts of water may then be injected into the process at the
appropriate time to initiate the crosslinking reaction. Alternatively,
alcohol may be utilized as a secondary blowing agent and delay the
above-noted reaction until the melt is taken to a zone of lower pressure,
at which point the alcohol will vaporize and permit the crosslinking
reaction shown in Eq. 1 to go to completion. The use of an alcohol in this
manner is more fully explained in commonly assigned copending application
Ser. No. 672,010, filed Nov. 16, 1984, entitled "Alcohol Control of
Lightly Crosslinked Foamed Polymer Production."
The blowing agents utilized in the practice of the present invention may be
any conventional compatible physical blowing agent. Preferred blowing
agents include the group of halogenated hydrocarbon compounds having from
1 to 4 carbon atoms. The compounds include trichlorofluoromethane (FC-11),
dichlorodifluoromethane (FC-12), dichlorotetrafluoroethane (FC-114),
1,1,2-trichlorotrifluoroethane (FC-113), methylene chloride, ethyl
chloride, and mixtures thereof. As mentioned above, an alcohol such as
methanol or ethanol may be utilized as a secondary blowing agent. When
these halogenated hydrocarbon blowing agents are utilized, there can be
from about 0.013 to about 0.50 gram mole of such blowing agent per 100
grams of polymeric reaction product in the polymer melt.
In accordance with a preferred embodiment of the invention, the silane
functional reaction product may be made into foam on conventional melt
processing apparatus such as by continuous extrusion from a screw-type
extruder. Such an extruder typically comprises a series of sequential
zones including a feed zone, melt zone, mixing zone, and cooling zone. The
barrel of the extruder may be provided with conventional electric heaters
for zoned temperature control. Typically, the volatile blowing agent will
be injected into the mixing zone after the polymer had passed through the
melt zone.
However, because of the tendency of vinylidene chloride polymers to degrade
when heated to the necessary temperatures to melt process them, it is
preferred that the blowing agent be incorporated into the polymer prior to
melt processing. This may be accomplished by suspending the silane
functional reaction product, in the form of small pellets or powder, in a
suspending agent such as water and injecting blowing agent into the
suspension while heating and agitating the suspension. In this manner, the
preimpregnated polymer need not undergo substantial mixing in the
extruder, and lower extruder temperatures may be utilized.
After sufficient mixing in the extruder, the hot polymer gel is passed
through a temperature-controlled cooling zone, through a die orifice, and
into a zone of lower pressure (i.e., ambient air atmosphere) where the
blowing agent is activated and the polymer gel expands to a lower density
cellular mass. As the foamed extrusion forms, it is conducted away from
the die and allowed to cool and harden. The density of the foam ranges
from about 9.6 to 400 Kg/m.sup.3 (0.6 pcf to 25.0 pcf).
In practice, the temperature of the feed zone in the extruder is maintained
at 150.degree..+-.5.degree. C., the temperature of the melting and mixing
zones is maintained at 160.degree..+-.5.degree. C., and the temperature in
the cooling and temperature control zone is maintained at
145.degree..+-.6.degree. C. The temperature of the polymer gel as it
expands through the die orifice is preferably just above the temperature
at which solid polymer would crystallize out of the gel and will vary
depending upon the specific polymeric reaction product utilized.
Alternatively, the pelletized reaction product of the present invention may
be impregnated with blowing agent, as discussed above, and may be expanded
in pellet or bead form. The expanded beads may then be used to form molded
foam products. Preferably, the expansion is carried out in steam or hot
air at from 150.degree.-160.degree. C.
In yet another embodiment of the invention, the polymeric reaction product
of the present invention may be formed into a film or coating. Such films
or coatings will possess similar high gas barrier properties as prior art
vinylidene chloride polymers. The lightly crosslinked reaction product of
the present invention, with its enhanced melt strength and melt tension,
would make it more readily processable in the manufacture of films
utilizing conventional blown film technology.
In order that the invention may be more readily understood, reference is
made to the following examples, which are intended to illustrate the
invention but is not to be taken as limiting the scope thereof. All parts
and percentages are by weight unless otherwise specified or required by
the context.
EXAMPLE 1
The following monomeric coreactants were polymerized utilizing conventional
suspension polymerization techniques:
6% methyl acrylate
93.87% vinylidene chloride
0.13% .gamma.-methacryloxypropyl trimethoxysilane (available from Dow
Corning under the designation Z-6030)
A polymeric reaction product having a weight average molecular weight of
190,000 was produced.
To 100 parts of reaction product, 1 part of tetrasodium pyrophosphate
stabilizer and 2 parts dibutyl sebacate plasticizer were added. The
reaction product was heated, and in the presence of trace amounts of water
and acid, the silane functional polymeric reaction product underwent a
crosslinking reaction in accordance with Eq. I, above.
The stress-strain behavior of conventional vinylidene chloride (Saran)
molded resin was compared to that of a lightly crosslinked silane
functional molded resin of the present invention. The extensional
viscosity of molded resin samples was determined at 190.degree. C. by
applying a stress force on the sample.
FIG. 1 illustrates the extensional viscosities for three molded Saran
resins at 190.degree. C. The resins were copolymerized utilizing
conventional suspension polymerization techniques using 6% methyl acrylate
and 94% vinylidene chloride monomers. The resins had molecular weights of
109,000, 135,000, and 190,000, respectively.
As shown in FIG. 1, the application of a very low stress force of 1 psi or
less resulted in large extension ratios (.DELTA.L/L.sub.o, where L.sub.o
is the original sample length and .DELTA.L is the change in sample length)
for all three samples. These results are indicative of resins possessing
no melt strength.
FIG. 2 illustrates the extensional viscosities of three lightly crosslinked
molded Saran resins prepared in accordance with the present invention at
190.degree. C. The resins were copolymerized utilizing conventional
suspension polymerization techniques using 6% methyl acrylate, from 93.7
to 93.87% vinylidene chloride, and from 0.13 to 0.3%
.gamma.-methacryloxypropyl trimethoxysilane as a crosslinking agent. The
three resins all had molecular weights of approximately 190,000.
As shown in FIG. 2, these silane-functional resins displayed dramatic
increases in the amount of stress applied to achieve large extensional
ratios. These results are indicative of resins having significantly higher
melt strengths than the uncrosslinked resins of FIG. 1.
EXAMPLE 2
An extruded polymeric foam was prepared as follows. A blend of 90 parts by
weight of a silane-functional vinylidene chloride polymer and 10 parts by
weight of a copolymer of methyl acrylate and vinylidene chloride was melt
processed in an extruder at 180.degree. C. for 1 minute. Pellets of the
blend were produced. The silane-functional vinylidene chloride polymer was
the reaction product of 6% methyl acrylate, 93.87% vinylidene chloride,
and 0.13% .gamma.-methacryloxypropyl trimethoxysilane (Dow Corning
Z-6030). The copolymer component was the reaction product of 6% methyl
acrylate and 94% vinylidene chloride. Both polymers had base molecular
weights of 190,000. The blend also contained about 2 parts/hundred by
weight of Elvax 3180 (trademark), a copolymer of 72% ethylene and 28%
vinyl acetate (25 melt index), 1 part/hundred of tetrasodium pyrophosphate
stabilizer, and 2 parts/hundred dibutyl sebacate plasticizer.
The pellets were impregnated to a level of about 12% trichlorofluoromethane
(FC-11) blowing agent by exposure to the blowing agent at 85.degree. C.
for a period of 24 hours. The impregnated pellets were then melt processed
in an extruder at 135.degree. C. Additional FC-11 blowing agent was
injected into the extruder.
The polymer gel was then expanded through a die orifice in the extruder to
produce a cellular foam structure having a density of about 144 to 160
Kg/m.sup.2 (9 to 10 pcf). The average cell size was less than 1
millimeter.
While the methods and compositions herein described constitute preferred
embodiments of the invention, it is to be understood that the invention is
not limited to these precise methods and compositions, and that changes
may be made in either without departing from the scope of the invention,
which is defined in the appended claims.
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