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Description  |
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The invention relates to a plastic composition, the polymeric component of
which comprises a thermoplastic polymer, especially a polymer of
.alpha.-olefines, preferably polyethylene or ethylene copolymers, which
composition disintegrates into small particles either under the action of
heat and/or ultraviolet light and/or sunlight and/or under composting
conditions. Since this plastic composition also contains a natural
biodegradable substance, the small plastic particles formed are yet
further degraded by microorganisms such as bacteria, fungi and/or enzymes
which are present in a composting mixture or in the soil. Complete
degradation can thus be achieved under suitable conditions.
The object of the present invention is to provide a plastic composition for
films, sheets or other mouldings, which possess the desired properties of
hitherto known thermoplastic materials, such as, for example, simple
processing, high strength, waterproofness, good resistance to solvents and
other chemicals, and fulfil the requirements to be met during storage and
use, but which can be readily degraded after use under the abovementioned
conditions. Under comparable conditions, the degradation time of the
compositions according to the invention is shortened by at least half,
frequently by 2/3 or even more as compared with hitherto known products of
similar type.
British Patent Specification No. 1.485.833 has disclosed that plastics with
carbon-carbon bonds can be rendered biodegradable by an addition of (a)
starch granules or chemically modified starch granules and (b) an
oxidizable substance such as a fatty acid and/or a fatty acid ester. It is
also mentioned in this printed publication that, in contact with a
transition metal salt contained in the soil, the oxidizable substance is
oxidized to peroxide or hydroperoxide, whereupon splitting of the polymer
chain occurs. However, it has been found that, in the case of a
polyethylene film of this composition, most of the starch granules are
covered by a polyethylene layer and can thus not be attacked by the
micro-organisms.
It has also been found that concentration of transition metal salts under
usual composting conditions is insufficient to cause effective oxidation
of the fatty acid component.
German Offenlegungsschrift 2.224.801 has disclosed that the degradation of
thermoplastic polymers of .alpha.-olefines, especially polyethylene and
polystyrene, under the action of ultraviolet light and/or sunlight can be
accelerated by adding compounds of a transition metal, especially iron
compounds, the effective content being stated as 0.01 to 2.0% by weight.
It has been found, however, that these metal compounds are inert under
normal exterior temperatures (below 35.degree. C.) if light is excluded.
It has now been found, surprisingly, that
[1] a plastic composition which contains
(a) a biodegradable substance, for example starch,
(b) an iron compound which may be a complex and
(c) a fatty acid and/or fatty acid ester, degrades under the action of heat
(preferably >50.degree. C.)and/or ultraviolet light and/or insolation
and/or under composting conditions;
[2] this degradation proceeds significantly faster than that measured
according to the abovementioned patents (see the tables which follow),
that is to say that the simultaneous presence of a biodegradable
substance, an oxidizable substance and an iron compound leads to a
significant synergistic effect; and
[3] the additional presence of a further transition metal compound such as,
for example, copper-(II) stearate, exerts a catalytic effect on this
degradation, which is additionally accelerated.
The present invention and its preferred embodiments are defined in the
patent claims.
Suitable components (a) are biodegradable substances such as, for example,
natural starch, etherified or esterified starch or starch which has been
modified in another way, for example by means of silanes, the content
being in general 2 to 40% by weight, preferably 10 to 16% by weight of the
composition. Other carbohydrates can also be used for the desired purpose.
It has proved to be advantageous to use the biodegradable substance in the
form of granules, which can be completely homogeneously incorporated into
the plastic mass in a known manner.
Component (c) is an oxidizable substance which contains at least one double
bond, this substance being or containing a fatty acid and/or a fatty acid
ester. A very suitable example is natural soya oil. The content of this
oxidizable substance is in general up to 5% by weight, preferably 0.5 to
1.5% by weight, relative to the composition.
The iron compound representing component (b) corresponds to the general
formula X-Fe, wherein X represents one or more ligands, and the compound
can additionally be coupled to a further ligand Y. Fe here designates iron
in any known valency. The ligand X can be an inorganic or organic acid
radical and likewise another ligand bonded in a complex. The following
examples may be mentioned: OH.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
oxalate.sup.-, H-citrate.sup.-, No.sub.2.sup.-, N.sub.3.sup.-, EDTA or a
carbonyl, nitrosyl or porphyrin radical. Examples of suitable ligands Y
are carboxylic acid ions of aromatic or aliphatic monocarboxylic acids or
of dicarboxylic acids, the aliphatic carboxylic acid preferably having 10
to 20 carbon atoms. The ligand Y serves in general to increase the
solubility of the compound X-Fe in the polymer. The content of component
(b) is in general at least 0.01% by weight, preferably 0.05 to 0.5% by
weight, relative to the composition. The content can be 0.02, 0.03 or
0.04% by weight, but it can also exceed 5.0% by weight.
The catalyst which may be added is a transition metal compound, which may
be a complex, of the general formula Z'-Me, wherein Me designates a
transition metal other than Fe nd Z' designates one or more ligands. The
following may be mentioned as examples of the ligand Z': OH.sup.-,
Cl.sup.-, Br.sup.-, I.sup.-, oxalate.sup.-, H-citrate.sup.-, NO.sub.2 --,
N.sub.3 --, EDTA as well as carboxylic acid ions of aromatic or aliphatic
monocarboxylic or dicarboxylic acids, the aliphatic carboxylic acid
preferably having 10 to 20 carbon atoms. Suitable transition metals Me are
mainly the transition metals of the first transition metal row in the
periodic table, such as copper and vanadium. The content of this catalyst
component is at least 0.001% by weight, preferably 0.005 to 1.0% by weight
and especially 0.01 to 0.05% by weight.
The thermoplastic base composition consists essentially of any known
thermoplastic polymer, polymers of .alpha.-olefines, especially
polyethylene or ethylene copolymers, being preferred. "Polyethylene" is
here to be understood as any type of polyethylene, such as LDPE, LLDPE,
LMDPE, MDPE, HDPE, ULDPE etc. Examples of suitable ethylene copolymers are
EVA, EBA, EAA, EMAA and ionomers.
The present invention has the advantage that the degradation can be
controlled depending on the field of application by varying the
concentration of the individual components, without the plastic material
suffering a deterioration in its properties under the normal use
conditions. Particularly interesting fields of application of the
compositions according to the invention are packaging materials, films for
garbage bags for compostable wastes, agricultural films, in particular
those which come into contact with the soil and are intended to
disintegrate after a desired time, films and sheets for carrier bags,
sheeting used on building sites, plastic fibres and plastic tapes,
especially stretched plastic tapes, and the like.
The present invention makes it possible to manufacture products which do
not pollute the environment and which can be degraded without additional
energy consumption and without releasing harmful substances.
The production of the compositions according to the invention and their
processing to give sheets, films, plates or other shapes is carried out by
conventional methods. With advantage, the components are added
individually or as mixtures in the form of so-called master batches.
As far as is known so far or can be assumed as probable, the degradation
proceeds by the following mechanism:
As is known (see A. C. Albertsson, B. Ranby, J. Appl. Polym. Sci: Appl.
Polym. Symp., 35 (1979), p. 423 and the publication of A. C. Albertsson
mentioned therein), plastics with C--C bonds in the main chain are
biodegradable extremely slowly with the formation of CO.sub.2 and H.sub.2
O. The half life of biological degradation of polyethylene was
extrapolated to be about 100 years.
Under the action of ultraviolet light, sunlight or heat or under composting
conditions, free radicals such as, for example, OH.sup..cndot. are formed
due to the presence of the iron ions, and these can react with the
polymers, forming other free radicals. These free polymer radicals are
extremely reactive and can, inter alia, react further with oxygen, with
other chains, with iron ions, with a double bond of the oxidizable
substance, and the like. Polymer chains are thus split, small chains with
or without oxygen-containing groups, such as alcohols, ketones, esters
etc. being formed. During this process, the iron ions act both as an
initiator and as a reaction promoter, whereas the oxidizable substance
acts as a reaction promoter and especially as a chain splitter, since this
substance has a greater tendency than a saturated polymer chain to form
peroxy or hydroperoxy compounds, and starch, because of the large number
of hydroxyl groups in its composition, manifests itself as a promoter and,
in conjunction with the iron ions, as a particularly valuable
co-initiator, since iron-(III) hydroxide complexes are highly reactive.
This can be illustrated by the following equation (1):
Fe.sup.3+ OH.sup.- .fwdarw.[FeOH].sup.2+ .fwdarw.Fe.sup.2+ +OH.sup..cndot.(
1)
The observed catalytic effect of the transition metal compounds, for
example copper or vanadium compounds, is probably to be attributed to an
acceleration of the Fe.sup.3+ .fwdarw.Fe.sup.2+ .fwdarw.Fe.sup.3+ cycle.
Without these compounds, the Fe.sup.2+ formed according to equation (1) is
reoxidized by other free radicals or other intermediates at the expense of
chain splitting as, for example, shown in equation (2):
Fe.sup.2+ +ROOH.fwdarw.Fe.sup.3+ +OH.sup.- +RO.sup..cndot. ( 2)
In the presence of copper compounds, the Fe.sup.2+ formed is reoxidized
faster according to equation (3):
Fe.sup.2+ +Cu.sup.2+ .fwdarw.Fe.sup.3+ +Cu.sup.+ ( 3)
and Cu.sup.+ ions are reoxidized very fast to CU.sup.2+ ions by free
radicals:
Cu.sup.+ +RO.sup..cndot. .fwdarw.Cu.sup.2+ +RO.sup.- ( 4)
This process repeats itself as long as the polymer is exposed to the
ultraviolet light, sunlight or heat. In this phase, to be described as the
first phase, the plastic materials become brittle and fragile and
disintegrate into small particles of a few mm.sup.2 up to few cm.sup.2.
Depending on the prevailing conditions, this phase takes in general 10 to
60 days.
In the subsequent second stage, the following can be observed:
[A] Under the action of ultraviolett light, sunlight or heat, the
degradation process continues as in the first stage. The small particules
disintegrate further into smaller and smaller particles until they
disappear.
[B] In the presence of microorganisms, that is to say bacteria, fungi
and/or enzymes, such as occur under composting conditions or in contact
with the soil, a further degradation stage follows. Due to the
disintegration into small particles, the area of the starch subject to
attack by the microorganisms is enlarged several times. The starch is
completely biodegraded, whereas the oxygen-containing, split polymer
chains are degraded at least partially. Depending on the prevailing
conditions, the degradation processes of the first stage can still
continue, leading to even shorter oxygen-containing polymer chains which,
due to the close contact with the microorganisms or enzymes, are in turn
partially degraded further. In this way, complete biodegradation at the
end of the second stage can be achieved. In general, this takes place, for
example, under usual composting conditions which comprise temperatures of
up to 75.degree.-80.degree. C. and gradually adjust to the exterior
temperature in the course of 6 to 8 months.
Such a two-stage degradation is advisable especially in the case of
agricultural sheets which are in contact with the soil, or of scattered
wastes. After the first stage, the plastic particles are then so small
that they can penetrate under exterior influences, e.g. rain, into the
soil. They are then not accessible to light anymore so that a biological
degradation of starch can take place which would not occur in the case of
conventional photodegradable plastic composition.
EXAMPLES
The films A-Q were produced by the blown film extrusion process in the
conventional manner, with the use of master batches. They all had
comparable thickness. Film A did not contain any degradation-promoting
additive. Films B and C, which did not contain all the required additives,
serve as comparative tests. The varying contents of silicone-modified
starch, iron hydroxide stearate, soya oil and copper stearate can be seen
from Table 1. The change in tensile strength and elongation at break at
different temperatures, under composting conditions and under UV
conditions was measured for each composition as a function of time. At an
elongation at break of less than 5% in the transverse direction, the
product is so fragile and brittle that measurements are no longer
possible, so that the film can be regarded as degraded.
The results of the investigations can be seen from Tables 2 to 7 which
follow.
TABLE 1
______________________________________
Film compositions
Additives in % by weight
Compo- FeOH Film
sition (steara- soya Cu (stea-
thickness
No. starch te).sub.2 oil rate).sub.2
in .mu.m
______________________________________
A -- -- -- -- 55
B 10 -- 0,5 -- 60
C -- 0,05 -- -- 55
D 10 0,05 0,5 -- 58
E 10 0,1 0,5 -- 62
F 16 0,05 0,8 -- 55
G 10 0,15 0,5 -- 56
H 10 0,05 1,0 -- 58
I 10 0,05 0,5 0,025 57
______________________________________
Plastic: LDPE, melt index 1,2
Change in tensile strength and elongation at break at 65.degree. C.
TABLE 2a
______________________________________
A B C D
______________________________________
Tensile strength in N
Original longitudinal
30,1 19,6 29,3 19,2
transverse 28,5 17,6 27,3 19,0
6 days longitudinal
29,3 18,6 18,7 17,6
transverse 28,8 17,9 19,6 13,7
10 days longitudinal
30,7 17,7 16,0 15,1
transverse 27,9 16,6 13,7 12,2
15 days longitudinal
30,5 19,1 15,5 14,0
transverse 28,9 16,1 14,0 12,5
20 days longitudinal
29,2 18,1 14,3 13,5
transverse 27,9 15,5 14,4 13,0
30 days longitudinal
30,0 18,5 13,6 12,5
transverse 28,5 15,8 14,0 12,1
Elongation at break in %
original longitudinal
390 233 258 232
transverse 515 530 520 531
6 days longitudinal
402 239 217 167
transverse 505 539 437 346
10 days longitudinal
388 226 95 86
transverse 508 518 109 27
15 days longitudinal
395 195 67 22
transverse 520 500 47 11
20 days longitudinal
375 203 54 15
transverse 495 462 41 9
30 days longitudinal
385 185 44 10
transverse 495 430 37 5,8
______________________________________
E F G H I
______________________________________
Tensile strength in N
Original
longitudinal
13,8 14,0 14,2 19,5 19,8
transverse 9,3 8,9 9,8 19,1 18,9
10 days longitudinal
11,8 10,7 9,6 15,5 14,5
transverse 8,7 8,0 9,5 13,0 13,5
20 days longitudinal
10,5 9,8 9,1 12,5 13,2
transverse 9,2 7,9 8,1 12,0 12,2
25 days longitudinal
9,5 8,8 8,3 12,0 12,5
transverse 8,2 7,1 7,7 11,2 11,7
30 days longitudinal
9,2 8,7 7,5 11,0 12,0
transverse 9,2 7,8 7,7 10,5 11,2
Elongation at break in %
Original
longitudinal
162 132 163 241 237
transverse 434 304 400 525 521
10 days longitudinal
109 71 12 88 75
transverse 35 7,4 6,7 25 15
20 days longitudinal
38 11 6,3 14 13
transverse 9,2 4,7 4,2 8,2 6,3
25 days longitudinal
9,4 7,7 5,4 9,1 7,5
transverse 6,2 4,0 3,9 6,2 4,9
30 days longitudinal
7,1 5,4 4,3 7,4 5,0
transverse 5,1 3,8 3,7 4,3 4,2
______________________________________
Change in tensile strength and elongation at break of film D at 70.degree.
C. and 75.degree. C.
TABLE 2b
______________________________________
70.degree. C.
75.degree. C.
______________________________________
Tensile strength in N
Original longitudinal
19,2 19,2
transverse 19,0 19,0
6 days longitudinal
15,4 15,2
transverse 12,4 13,6
10 days longitudinal
13,6 14,4
transverse 12,9 13,8
15 days longitudinal
14,1 11,7
transverse 12,9 12,1
20 days longitudinal
14,1 8,6
transverse 13,7 10,8
Elongation at break in %
original longitudinal
232 232
transverse 531 531
6 days longitudinal
103 42
transverse 86 12
10 days longitudinal
41 9
transverse 14 8
15 days longitudinal
18 4,4
transverse 10 4,3
20 days longitudinal
11 3,4
transverse 7 3,6
______________________________________
Degradation under composting conditions
TABLE 3
______________________________________
A B C D
______________________________________
Tensile Strength in N
Original longitudinal
30,1 19,4 26,7 20,6
transverse 28,5 15,7 26,8 16,0
3 weeks longitudinal
29,7 19,2 25,1 19,9
transverse 28,0 15,4 15,4 15,5
7 weeks longitudinal
29,2 20,8 21,3 20,1
transverse 28,3 14,3 22,2 12,6
13 weeks longitudinal
30,2 18,5 25,0 19,2
transverse 27,6 14,0 21,7 11,5
20 weeks longitudinal
29,5 18,4 22,0 18,9
transverse 27,7 13,2 16,1 12,7
Elongation at break in %
Original longitudinal
390 282 286 253
transverse 515 752 638 667
3 weeks longitudinal
385 231 247 190
transverse 500 476 206 437
7 weeks longitudinal
370 170 107 128
transverse 480 314 534 143
13 weeks longitudinal
400 252 198 205
transverse 510 537 534 114
20 weeks longitudinal
385 152 153 174
transverse 495 263 570 300
______________________________________
Degradation under composting conditions
TABLE 4
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Film composition
Additives in % by weight
Composi- FeOH (stea-
Soya Cu (stea-
Film thick-
tion No.
Starch rate).sub.2
oil rate).sub.2
ness in .mu.m
______________________________________
J 11 0,10 1,3 -- 55
K 11 0,15 1,3 -- 58
L 11 0,15 1,3 0,025 56
______________________________________
Plastic: LDPE, melt index 0,8
TABLE 5
______________________________________
Evaluation of tensile strength and of the
elongation under composting conditions
J K L
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Tensile strength in N
Original longitudinal
16,8 16,5 16,1
transverse 9,8 10,3 16,0
1 week longitudinal
-- -- 16,0
transverse -- -- 10,3
4 weeks longitudinal
16,5 16,1 15,9
transverse 9,1 9,0 8,2*
7 weeks longitudinal
16,6 16,4 15,2
transverse 8,9 8,8* 7,8*
10 weeks longitudinal
16,3 16,1 15,1
transverse 8,7* 8,3* 7,7*
15 weeks longitudinal
15,9 15,6 --
transverse 8,5* 8,3* --
20 weeks longitudinal
15,8 15,1 --
transverse 8,6* 7,8* --
Elongation in %
Original longitudinal
154 126 185
transverse 422 436 361
1 week longitudinal
-- -- 153
transverse -- -- 121
4 weeks longitudinal
135 106 145
transverse 143 43 71*
7 weeks longitudinal
130 107 132
transverse 95 36* 30*
10 weeks longitudinal
121 103 129
transverse 86* 42* 25*
15 weeks longitudinal
115 101 --
transverse 84* 40* --
20 weeks longitudinal
114 85 --
transverse 84* 35* --
______________________________________
*Holes/Fractures in the film are observed in a number increasing with
time. Measurements are done on samples without any hole.
Degradation under ultraviolet light
TABLE 6
______________________________________
Film composition
Additives in % by weight
Composi- FeOH (stea-
Soya Cu (stea-
Film thick-
tion No.
Starch rate).sub.2
oil rate).sub.2
ness in .mu.m
______________________________________
M -- -- -- -- 100
N -- 0,15 -- -- 58
O 10 -- 1,3 -- 56
P 10 0,15 1,3 -- 54
Q 10 0,15 1,3 0,025 56
______________________________________
Plastic: LDPE, melt index 0,8
TABLE 7
______________________________________
Evalution in the time of the tensile strength and of the elongation
under ultraviolet light [XENOTEST 250]
M N O P Q
______________________________________
Tensile strength in
transverse direction in N
Original 32,7 17,7 13,4 9,4 10,3
104 hrs 27,4 -- 12,5 7,3 8,1*
150 hrs -- 11,8 11,6 7,2* 6,7*
209 hrs 26,6 10,6 11,5 5,3* --
250 hrs -- 11,4 -- -- --
305 hrs 27,8 11,5 -- -- --
Elongation in transverse
direction in %
Original 734 630 527 385 361
104 hrs 634 -- 369 6,7 4,5*
150 hrs -- 30 367 4,0* 3,5*
209 hrs 672 17 231 2,9* --
250 hrs -- 10 -- -- --
305 hrs 669 6,8 -- -- --
______________________________________
*brittle
The results of the above tables clearly show the synergistic effect of
components (a), (b) and (c) on the degradation of polyethylene polymers.
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