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This invention relates to ethylene polymer compositions. Specifically this
invention relates to an ethylene polymer composition which when exposed to
the elements of the environment undergoes degradation.
It has been made dramatically apparent that the huge volume of plastic
products used by industry and the consumer has resulted in a significant
disposal problem. All too often many plastic products, after the use by
the ultimate consumer, become litter.
Over the course of the past several decades, in an effort to meet consumer
demands, the plastics industry has made such plastic products more stable
and as a consequence littered articles have an increased durability. With
the presence of these stable plastic systems and with the advent of the
awareness of the ecological needs of society solutions to the litter
problem are now being sought.
While it was known and disclosed in U.S. Pat. No. 3,454,510 that certain
pro-oxidant metal salts in polyolefin films, specifically mulch films,
would render same environmentally degradable, that disclosure was
inherently limited to opaque films.
It was further disclosed in U.S. Pat. No. 3,320,695 and U.S. Pat. No.
3,341,357 that certain unsaturated hydrocarbons could be added to opaque
polyolefin films to promote degradation. These opaque films, also used in
mulching operations, would require relatively large weight percentages of
certain unsaturated hydrocarbons which resulted in a "soft" product; that
is one not normally considered suitable for consumer product application.
There is described herein an ethylene polymer composition, suitable for
fabrication of translucent to transparent consumer-type plastic products,
which composition retains its structural stability during its normal
useful life and when discarded to the environment the ambient
environmental elements cause the plastic composition to degrade. The
degradation reaction of the ethylene polymer composition occurs at a
significently faster rate after exposure thereof to natural or artificial
actinic light.
The ethylene polymer composition of this invention, comprising the
combination of both an auto-oxidative susceptible organic additive and a
polyvalent transition metal salt in an ethylene polymer, is a product
which has been found to undergo weathering at a greater rate than a
similar product containing the equivalent amounts of only the
auto-oxidative susceptible organic additive or only the polyvalent
transition metal salt. The compositions can include the conventional
additives such as fillers, pigments, slip agents, antioxidants, antistats,
antiblocks, antifogs, or other materials conventionally added to ethylene
polymers.
In certain cases it has been found that the combination of moderate amounts
of both an auto-oxidative susceptible organic additive and a polyvalent
transition metal salt in an ethylene polymer yields faster weathering
rates than equivalent or larger amounts of either one of the individual
components alone added to the same ethylene polymer.
It has been found that products formed with the ethylene polymer
compositions of this invention will, when exposed to weathering, undergo
high levels of multifaceted crazing, followed by cracking and ultimately
resulting in particulate formation. With further passage of time the
crazing continues on the particulates formed resulting in more and finer
particulates. No additional external physical forces are necessary to
cause the particulate formation although such external physical forces can
aid in the "sloughing off" of the outer particulate layers to expose a new
surface to the environment.
Broadly speaking this invention is an environmentally degradable ethylene
polymer composition of (i) an ethylene polymer base resin, and as a
synergistic combination of additives, (ii) an auto-oxidative susceptible
additive as a polymer or low molecular weight organic compound and (iii) a
polyvalent transition metal salt; there can also be present (iv) a
stabilizer or antioxidant for the ethylene polymer. As used in this
specification the term "ethylene polymer composition" has this broad
meaning.
In more specific terms the ethylene polymer composition of this invention
contains (i) an ethylene polymer base resin, and as a synergistic
combination of additives, (ii) a polymer wherein the predominance of the
mer units have, or a low molecular weight organic compound that has, at
least one hydrogen bonded to a carbon atom having an auto-oxidative
susceptibility greater than that of a hydrogen bonded to a normal
secondary carbon atom, (iii) an organic salt of a polyvalent metal wherein
at least one metal is a transition metal wherein electron transfer occurs
in the 3d or 4f sub-shell and (iv) an organic antioxidant for the ethylene
polymer.
In even more specific terms the ethylene polymer composition of this
invention contains polyethylene, polyether or polypropylene, an organic
salt of a polyvalent transition metal wherein the metal can be iron,
manganese, zinc or cobalt, and an antioxidant such as the sterically
hindered phenols, aryl amines, thioureas, thiocarbamates, phosphites and
thioether esters.
Antioxidants for ethylene polymers have been found useful to stabilize the
ethylene polymer compositions so as to provide compositions whereby the
period required before embrittlement occurs may be "built into the
composition." This aspect of the invention is of course valuable insofar
as one knowing the normal useful life (period before disposal) of an
article, could proportion the amounts of antioxidant and additives to give
a structurally stable product during the useful life period but which will
undergo embrittlement within a relatively short time after exposure to the
elements.
The ethylene polymer compositions can be compounded according to any one of
several known techniques, such as, direct addition of all constituents,
master batching wherein any single master batch may contain several
constituents but will not contain both the polyvalent transition metal
compound and the auto-oxidative susceptible organic additive, or any other
compounding procedure.
The production of the compositions by direct addition of all constituents
and blending until a single homogeneous mixture is obtained are well known
techniques. The master batching involves the preparation of two or more
compositions which are subsequently combined into a single homogeneous
mixture. In the master batching procedure the polyvalent transition metal
compound and the auto-oxidative susceptible additive are initially present
in separate master batch compositions. These separate master batch
compositions are then combined or blended in proper proportions at a
future date to produce the ethylene polymer compositions of this
invention. This enables one to prolong the shelf or storage life since the
degradation reaction does not progress to any appreciable extent until
there has been a homogeneous mixing of these two components in the
ethylene polymer composition.
For example, one can produce a first master batch of ethylene polymer plus
the polyvalent transition metal compound plus sufficient antioxidant to
stabilize the first master batch, and a second master batch of
auto-oxidative susceptible additive (e.g. propylene polymer, or alkylene
oxide polymer) with or without ethylene polymer plus sufficient
antioxidant to stabilize the second master batch. In addition, either or
both of the master batches can contain the conventional amounts of the
additives usually known to be useful in ethylene polymers. Further, one
can have more than two so-called master batches, if desired.
During the period in which the first master batch and second master batch
are stored in separate containers the environmental degration discussed
herein will not occur. Likewise, if one were to blend pellets of the two
master batches the blended mixture will not show any signs of
environmental degradation. However, as soon as there has been a
homogeneous fluxing or melting of the two or more master batches such that
the auto-oxidative susceptible additive and the polyvalent transition
metal compound are present together in a single, uniform, homogeneous
ethylene polymer composition then environmental degradation will commence.
This ultimate blending of the multiple master batches can be carried by
any of the known procedures such as solution blending, melt blending,
milling, Banburying, screw driven mixers, and the like. It can also be
carried out in the processing equipment used to produce the ultimate
manufactured product, for example during the film extrusion or spinning
process.
It was surprising to note that in certain instances ethylene polymer
compositions having the same chemical contents produced by the master
batch procedure had longer storage stability properties than those
produced by the direct addition procedure.
The ethylene polymer compositions of this invention can be produced by any
suitable method normally employed in ethylene polymer processing, for
example, extruding, such as blown tubular film extrusion, slot-cast die
sheet extrusion, slot-cast die extrusion coating; molding such as
injection, blow, rotary, transfer and the like; fiber-forming, such as
melt spinning, drawing and the like; and so forth.
BASE RESIN
The base resin is a normally solid thermoplastic ethylene polymer. The
resin may be an ethylene homopolymer or copolymer wherein the ethylene
fraction is predominant or mixtures thereof or with other polymers. Both
high and low density polyethylenes and mixtures thereof can be used.
The high density ethylene polymers useful as the base resins in the present
invention are essentially linear in structure, and are known as "linear
polyethylenes." It is known that high density linear polyethylenes can
contain chain transfer agents, and/or chain terminating agents which are
used to modify the melt viscosity, molecular weight or other properties of
the resins and it is intended to encompass such modified polymers within
the scope of this invention. The high density polyethylenes are generally
characterized by a density that is about equal to or greater than 0.94
g/cc. and is usually in the range of from 0.94 to about 0.97 g/cc. The
high density polyethylenes can have a melt index of from 0.005 to 100 and
preferably from 0.15 to 50 decigrams per minute. (ASTM D-1238). It should
be noted, however, that mixtures of high density polyethylenes can be used
as the base resin in producing the ethylene polymer compositions, and such
mixtures can have a melt index less than 0.005 or greater than 100
decigrams per minute.
The low density ethylene homopolymers have densities of less than 0.94
g/cc. and are usually in the range from 0.91 to 0.93 g/cc. The low density
ethylene homopolymers have melt indices from about 0.05 to about 100
decigrams per minute inclusive, and preferably from 0.5 to 20 decigrams
per minute; mixtures thereof can be used if desired.
The ethylene copolymers useful as base resins are those obtained by the
copolymerization of ethylene with any monomer containing the
##EQU1##
groups which will copolymerize with the ethylene and form thermoplastic
copolymers. Illustrative of such copolymerizable monomers are the alpha
olefins (in minor amounts) containing up to 18 carbon atoms such as
propylene, 1-butene; isobutene, and 1-pentene; halogenated olefins such as
chloroprene, tetrafluoroethylene, chlorotrifluoroethylene,
hexafluoropropylene; vinyl aryls such as styrene, o-methoxystyrene,
p-methoxystyrene, m-methoxystyrene, o-nitrostyrene, p-nitrostyrene,
o-methylstyrene, p-methylstyrene, m-methylstyrene, p-phenylstyrene,
o-phenylstyrene, m-phenylstyrene, vinyl naphthalene, and the like; vinyl
and vinylidene halides, such as vinyl chloride, vinyl fluoride, vinylidene
chloride, vinylidene fluoride, vinylidene bromide, and the like; vinyl
esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl
butyrate, vinyl chloroacetate, vinyl chloropropionate, vinyl benzoate,
vinyl chlorobenzoate, and the like; acrylic and alpha-alkyl acrylic acids,
their alkyl esters, their amides and their nitriles such as acrylic acid,
chloroacrylic acid, methacrylic acid, ethacrylic acid, methyl acrylate,
ethyl acrylate, butyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate,
n-decyl acrylate, methyl methacrylate, butyl methacrylate, methyl
ethacrylate, ethyl ethacrylate, acrylamide, N-methyl acrylamide,
N,N-dimethyl acrylamide, methacrylamide, N-methyl methacrylamide,
N,N-dimethyl methacrylamide, acrylonitrile, chloroacrylonitrile,
methacrylonitrile, ethacrylonitrile, and the like; maleic and fumaric acid
and their anhydrides and alkyl esters such as maleic anhydride, dimethyl
maleate, diethyl maleate and the like; vinyl alkyl ethers and ketones such
as vinyl methyl ether, vinyl ethyl ether, vinyl isobutyl ether,
2-chloroethyl vinyl ether, methyl vinyl ketone, ethyl vinyl ketone,
isobutyl vinyl ketone, and butadiene, isoprene, cyclopentadiene,
hexadiene-1,6 norbornadiene, dicyclopentadiene, and the like; also vinyl
pyridine, N-vinyl carbazole, N-vinyl pyrollidine, acrolein, vinyl alcohol,
vinyl acetal, vinyl butyral, and the like. Other monomers which may be
interpolymerized with ethylene include, carbon monoxide and formaldehyde,
but these are generally not preferred.
These copolymer resins should contain a major amount of ethylene units
polymerized in the copolymer. Preferably the copolymer should contain from
about 50 to about 99 weight percent polymerized ethylene monomer and most
preferably from about 80 to about 99 weight percent polymerized ethylene
monomer, depending upon the particular copolymerizable monomer employed
and the intended end use of the ethylene polymer composition of this
invention.
Other suitable polymers include by way of example
ethylene/ethylidenenorbornene/propene-1 terpolymers and
ethylene/hexadiene/propane-1 terpolymers. In the terpolymers, the ethylene
component is dominant and is present in amounts from about 50 to about 99
percent. The propene-1 component is present in concentrations of from
about 1 to about 50 percent by weight of the terpolymer; the residual
weight percent is of course the third component.
Preferred base resins are the ethylene homopolymers while the preferred
copolymer base resins are ethylene-vinyl acetate; ethylene-ethyl acrylate
and the partially hydrolyzed ionic salt forms thereof; ethylene-acrylic
acid and the ionic salt forms thereof, ethylenepropylene; and
ethylene-styrene. The preferred terpolymer is
ethylene/propylene/ethylidene-norbornene.
The base resin constitutes the major component of the ethylene polymer
composition and is normally present at a concentration of from about 70 to
about 99 percent by weight; the remainder of the composition being the
other additives thereof. Preferably the base resin is present at a
concentration of from about 90 to about 99 weight percent of the ethylene
polymer composition.
The selected weight percentages of each individual additive is of course
dependent on several parameters, including but not necessarily limited to
the desired rate of degradation, molecular weight of the additive,
relative activity of the additive, desired physical properties of the
ethylene polymer composition of this invention being prepared and so
forth.
AUTO-OXIDATIVE SUSCEPTIBLE ADDITIVE
The auto-oxidative susceptible additive can be either a polymer wherein the
predominance of the repeating units have, or a low molecular weight
organic compound that has, at least one hydrogen bonded to a carbon atom
having an auto-oxidative susceptibility greater than that of a hydrogen
bonded to a normal secondary carbon atom. For example the polymer used as
the auto-oxidative susceptible agent has an auto-oxidative susceptibility
that is greater than that of unbranched polyethylene.
Thus, polypropylene, which has hydrogen atoms bonded to tertiary carbon
atoms that are more readily oxidizable than the hydrogen atoms that are
bonded to the normal secondary carbon atoms of polyethylene is a suitable
auto-oxidative susceptible additive in polymer form. Illustrative of other
readily auto-oxidative hydrogen atoms bonded to carbon atoms are the
hydrogen atoms found, for example, in the allylic, benzylic, tertiary
aliphatic, aldehydo, alpha-oxyhydrocarbyl or alpha-halohydrocarbyl groups.
Among the auto-oxidative susceptible polymers one can include the
alpha-olefin polymers which are normally solid at room temperature and
contain the unit:
##EQU2##
wherein R is an alkyl group containing from about 1 to 18 carbon atoms.
Illustrative of such alpha-olefin polymers are polypropylene,
poly(butene-1), poly(pentene-1), poly(4-methylpentene-1), poly(hexene-1),
poly(octene-1), poly(octadecene-1), and the like. It is considered
preferable in this invention that the repeating unit of the auto-oxidative
susceptible alpha-olefin polymers employed possess ratios of tertiary
carbon atoms to secondary carbon atoms in the range of 1:1 to 1:16 and
most preferably 1:1 to 1:6. Other suitable auto-oxidative susceptible
additives include the polyalkylene oxides such as polyethylene oxide,
polypropylene oxide, including the block and random copolymers thereof,
and the like; polyunsaturated hydrocarbons such as polyterpenes and the
like.
The preferred auto-oxidative susceptible additive is polypropylene, atactic
or isotactic, crystalline or amorphous. Polypropylene when employed in the
ethylene polymer composition yields a product having the desired physical
properties for consumer-type applications and furthermore more rapidly
promotes high levels of crazing to form small particulates. Also included
as suitable polymers are block polymers containing a predominant amount of
propylene blocks.
Among the suitable readily auto-oxidative susceptible low molecular weight
organic compounds are those having a molecular weight less than about
5,000, for example, derivatives of aliphatic and cycloaliphatic compounds
containing one or more allylic hydrogens such as myrcene, ocimene,
limononene (dipentene), cyclohexadiene, dicyclopentadiene,
decahydronaphthalene, indene, tetra-hydroindene, ethylidenenorbornene, and
the like; the unsaturated fatty acids such as eleostearic acid, linolenic
acid, linoleic acid, oleic acid, crotonic and sorbic acid as well as
adducts of these and other unsaturated aliphatic and alicyclic compounds
with such as maleic acid, acrylic acid, acrolein, and the like; compounds
with highly reactive benzylic hydrogens such as cumene,
par-isopropylbenzoic acid, and the like.
Preferably the readily auto-oxidative susceptible polymers and low
molecular weight compounds are hydrocarbons but they need not be. The
presence of functional groups is not precluded but neither is it generally
considered desirable.
The auto-oxidative susceptible additives are normally present at
concentrations of from about 0.01 to about 40 weight percent of the
ethylene polymer composition. Preferably the auto-oxidative susceptible
additive is present in amounts of from 0.05 to about 20 percent and most
preferably in amounts of from 0.1 to about 10 percent by weight based on
the total weight of the ethylene polymer composition. Greater or lesser
quantities of auto-oxidative additive may be employed depending upon the
rate of degradation and the physical properties desired in the ethylene
polymer composition.
POLYVALENT TRANSITION METAL SALT
This additive may be any metal, organic or inorganic, wherein at least one
metal salt is a polyvalent transition metal, and preferably is an organic
salt of a polyvalent transition metal and most preferably is an organic
salt of a polyvalent transition metal wherein the metal is one wherein
electron transfer occurs in the 3d sub-shell or the 4f sub-shell. The
transition metals referred to are as defined in the Periodic Chart at the
terminal leaf page of the Handbook of Chemistry and Physics. The Chemical
Rubber Co., 49th edition, (1968-69). They are those elements in the Fourth
Period having atomic numbers of 21 to 30, in the Fifth Period having
atomic numbers of 39 to 48, and in the Sixth Period having atomic numbers
of 57 to 71. Among the specific transition metals wherein electron
transfer occurs in the 3d sub-shell one cam mention V, Cr, Mn, Fe, Co, Ni,
Cu Zn, Zr and Ag of the Fourth and Fifth Periods; among the transition
metals wherein electron transfer occurs in the 4f sub-shell are Ce or Pr
in the Sixth Period.
Suitable polyvalent transition metal inorganic salts pursuant to this
invention are by way of example, iron chloride, zinc chloride, mercurous
chloride, chromium trichloride, copper nitrate, copper sulfate, cobalt
chloride, nickel sulfate, iron sulfate, iron bromide, zinc sulfate,
mercuric sulfate, and the like.
Typically the organic salt is the octoate, naphthenate, acetate, stearate
or acetylacetonoate metal salt, but it need not be so limited and other
organic groups may be employed if desired.
Illustrative of suitable organic salts of polyvalent transition metals one
can mention merely by way of examples, cobalt acetate, cobalt octoate,
cobalt naphthenate, iron napthenate, iron octoate, lead stearate, lead
octoate, zirconium stearate, cerium octoate, manganous stearate, manganous
oleate, manganous dodecyl acetoacetate, cobalt acetyl acetonate, cobaltous
acetate, cobaltous oleate, cobaltous stearate, cobaltous dodecyl
acetoacetate, cupric stearate, cupric oleate, ferric acetate, zinc
octoate, zinc naphthenate, iron distearate, potassium permanganate,
potassium trioxalatocobaltate (III), trisethylenediaminecobalt (III)
chloride, sodium hexanitrocobaltate (III), potassium hexacyanocobaltate
(III) and the like.
Polyvalent transition metal salts pursuant to the practice of this
invention may be used individually or in combination. It has been found
that certain combinations of polyvalent transition metal salts promote
degradation more so than the equivalent amount of any one salt of the
combination; this is particularly noticeable with mixtures of iron and
cobalt salts.
The polyvalent transition metal salts are normally present in amounts of
from about 0.002 to about 2.0 weight percent of metal atom, based on the
weight of the total composition. Preferably the metal is present in
amounts of from about 0.005 to about 1.0 and most preferably in amounts of
from about 0.01 to about 0.1 weight percent, based on the weight of the
total composition. The need for only such small amounts of the polyvalent
transition metal salt to give suitable weathering characteristics is an
attractive feature of this invention insofar as the small amounts of salt
generally do not adversely effect the mechanical properties of the base
resin.
ANTIOXIDANT
Any of the antioxidants used with ethylene polymers can be used in the
compositions of this invention. These include the sterically hindered
phenols, the aryl amines, the thioureas, thiocarbamates, thioether esters,
phosphites or mixtures or adducts thereof.
By the term sterically hindered phenol is meant a substituted or
unsubstituted compound containing at least one sterically hindered group
of the structure
##SPC1##
wherein X is hydrogen, alkyl of from 1 to about 10 carbon atoms or a
substituted or unsubstituted phenyl and X.sup.1 is alkyl of from 1 to
about 10 carbon atoms or a substituted or unsbustituted phenyl, said
sterically hindered group being susceptible to proton donation. Generally
the sterically hindered phenol will be one that does not volatilize or
decompose appreciably below temperatures of about 200.degree.C.
Illustrative of suitable phenol antioxidants one can mention
tetrakis[methylene-3-(3', 5'-di-tert-butyl-4'-hydroxyphenyl
propionate]methane (IRGANOX 1010), stearyl 3-(3',
5'-di-tert-butyl-4'-hydroxyphenyl)propionate (IRGANOX 1076), distearyl
3,5-di-tert-butyl-4-hydroxybenzyl phosphite (IRGANOX 1093),
1,1,3-tris(5'-tert-butyl-4'-hydroxy-2'-methyl-phenyl)butane (TOPANOL CA),
4-methyl-1,-6-di(2'-hydroxy-3'-tert-butyl-5'-methylbenzyl)phenol
(PLASTONOX 80), 2,4-di(3'-5'-di-tert-butyl-4'-hydroxyphenoxy)triazine
(IRGANOX 858), 2,2'-thiobis(4'-methyl-6'-tert-butylphenol)(COA-6),
4,4'-thiobis(3-methyl-6-tert-butylphenol)(SANTONOX R),
3,5-di-tert-butyl-4-hydroxyanisole (TOPANOL 354),
2,6-di-tert-butyl-p-cresol (IONOL).
Among the suitable amine antioxidants one can mention
N-phenyl-beta-naphthylamine, N,N'-diphenyl-p-phenylenediamine,
p-isopropoxy diphenylamine, N,N'-di-beta-naphthyl-p-phenylenediamine,
N,N'-di-(2-octyl)-p-phenylenediamine,
N,N'-di-3(5-methylheptyl)-p-phenylenediamine, aldol-alpha-naphthylamine,
4,4'-dioctyldiphenylamine, 4-octyldiphenylamine, 4-t-butoxydiphenylamine,
the polymer of 1,2-dihydro-2,2,4-trimethylquinoline, and the like.
Among the suitable thioureas are the polyalkyl thioureas having up to about
4 carbon atoms in the alkyl groups such as trimethyl thirouea, 1,3-diethyl
thiourea or ethylene thiourea, and the like. Thiocarbamates include the
alkali metals salts thereof such as sodium dibutyl dithiocarbamate, and
the like. The thioether esters include dilauryl thiodipropionate,
distearyl thiodipropionate, and the like. Among the known phosphites one
can mention the mono-, di- and tri-nonylphenyl phosphites, distearyl
pentaerythritol diphosphite (WESTON 618), the adduct of trinonylphenyl
phosphite with 1,1,3-tris(5-tert-butyl-4-hydroxy-2-methylphenyl)-butane
(ARGUS MARK 1409), and the like.
Other suitable antioxidants include dibutyl-para-cresol,
p-cresol-formaldehyde resins, para-tertiary-alkylphenol formaldehyde
resins in admixture with amino dithioformates, aliphatic polyepoxides,
organic phosphites, thiophosphates, or dithiophosphites, para-tertiary
alkylphenol formaldehyde resins in admixture with mercapto compounds,
2-thiono-2-mercaptodioxaphosphorinane compounds,
tetraphenylsuccinodinitriles or triphenylmethane, or dithiophosphate metal
salts, and the like, as well as combinations thereof; also, chelating
agents, such as for example, carboxylic acids, such as malonic acid,
succinic acid and the like; substituted oxamides such as oxanalide and the
like; amino acids such as glycine, and the like; amino polycarboxylic
acids, such as ethylenediamine tetraacetic acid, diethylenetriamine
pentaacetic acid, hydroxyethyl ethylenediamine triacetic acid,
nitrilotriacetic acid, hydroxyethylimino diacetic acid, diaminocyclohexane
tetraacetic acid, diaminoethyl ether tetraacetic acid, ethylenediamine
di(o-hydroxyphenyl acetic acid); N-phenyl-N'-(p-toluene
sulfonyl)-p-phenylenediamine, N,N-disalicylidene propylenediamine, and the
like; pentaerythritol, sorbitol, resorcinol, and other polyfunctional
alcohols and esters thereof; as well as combinations thereof.
The antioxidant is normally present in quantities sufficient to stabilize
the composition against oxidative degradation for the entire useful life
period desired and is generally from about 0.0025 to about 1 weight
percent preferably from about 0.025 to 0.1 weight percent, based on the
weight of total ethylene polymer composition.
It is of interest to note that almost all ethylene polymers often contain
minute amounts of antioxidant, but this amount is normally not sufficient
to stabilize the ethylene polymer composition of this invention and
additional amounts are often required.
However, in certain embodiments of this invention the antioxidant need not
be present or need be present in only minute amounts. In these two cases
the ethylene polymer composition will of course degrade very rapidly upon
exposure to the elements. This aspect of rapid degradation is important in
an application where a product prior to use is sealed in a covering which
eliminates action by the elements, particularly sunlight, rain and oxygen.
When sought to be used the particular product is removed from the
covering, used as required within a pre-specified time, and discarded to
the environment for rapid degradation. For such embodiments of this
invention it has been found that concentrations as low as 0.0025 percent
by weight of antioxidant can be used.
As other aspects of this invention it is recognized that certain suitable
antioxidants as aforesaid offer additional benefits which give further
dimension to this invention. For example certain antioxidants, such as
thioureas, are water soluble. Products produced from the ethylene polymer
compositions of this invention containing thioureas may be utilized for
long periods in a relatively dry environment; then after exposure to
aqueous environmental elements such as rain or fog leaching of the
thioureas occurs rendering the product more actively degradable. Another
example is wherein the antioxidant is biodegradable, such as certain
thioether esters, as for example dilauryl thiodipropionate and the like.
In these cases bacteria found in the environment consume the antioxidant
in the product, rendering the product more actively degradable. Still
another example is wherein the antioxidant is volatile, such as
mercaptomalic acid, and certain urea derivatives such as 1,1-diethyl urea.
Because of its volatility such an anitoxidant would generally be applied
to the plastic object after fabrication by immersion in a solution of the
antioxidant or by the use of a roller coater or spray gun or other
suitable application technique. Such volatile antioxidants may also be
used in conjunction with less volatile antioxidants present in minor
concentrations in the ethylene polymer compositions. The volatile
antioxidants upon discard to the environmental elements volatilize
rendering the plastic product more actively degradable. A further example
is wherein the antioxidant is heat stable but light unstable such as
alpha-phenylindole and diphenyl thiourea. A plastic product employing
antioxidants of this nature may be stored in a dark environment and when
exposed to sunlight will readily start to degrade.
In another aspect of this invention it has been found that a preliminary
irradiation of the ethylene polymer composition will greatly enhance the
rate of degradation as compared to non-irradiated ethylene polymer
composition. As previously pointed out an antioxidant may additionally be
included to maintain a more stable composition prior to irradiation.
Normally the requisite level of ionizing radiation to accelerate
degradation is from about 1 to about 20 megareps (MGRPS). Greater or
lesser dosages of radiation may be employed depending upon the particular
desired rate of degradation. Such sources include the van de Graaff
accelerator, cobalt 60, and the like. Other suitable modes of irradiation
are, by way of example, ultraviolet lamp, sunlamp, swirl-flow plasma arc,
mercury lamp, and the like. Any known radiation source can be used.
This irradiation aspect of this invention is important in large volume
plastic waste disposal units wherein the waste plastic is irradiated prior
to exposure to the elements to provide an accelerated rate of degradation.
In the following examples the processing and analytical methods used for
sample preparation are as described immediately hereinbelow. Two
compounding methods were employed. The first and primary method of sample
compounding is by employing the two roll mill (hereinafter called "roll
mill method"). The second method employed a Banbury mixer (hereinafter
called "mixer method").
A 6 .times. 12 inches two-roll mill with heat supplied by full stream at
190.degree.C. and heated for at least 15 minutes is used. With the bit as
close as possible the ethylene base polymer is added and then during a
period of about 1 minute the bite is opened after the ethylene polymer has
begun to flux. The polypropylene or other auto-oxidative susceptible agent
is added. Thereafter antioxidant and other filler (if applicable) are
added. The polyvalent transition metal salt is then slowly added in about
30 seconds. The material is worked for 2 minutes until homogeneous, then
pulled off the rolls and cut into squares about 2 by 2 inches. It is
recognized that any of the other conventional additives usually present,
such as pigment, slip agents, anti-block agents; etc. can be present if
desired. Unless otherwise stated this method was used in the examples.
In the mixer method a 5 lb. Banbury mixer was employed with full steam on
the shell and rotors for 5 minutes to achieve 190.degree.C. The ethylene
base polymer and auto-oxidative susceptible agent, such as polypropylene,
were added. The ram was moved downward at the full pressure of 80 psi and
the Banbury operated at maximum forward speed for 3 minutes or unt | | |
