CA1073581A - Photodegradable polyolefin material - Google Patents

Abstract

PHOTODEGRADABLE POLYOLEFIN MATERIAL
Abstract of the Disclosure A polyolefin material characterized by relatively rapid disintegration when exposed to environmental sunlight and moisture is described. It comprises a polyolefin pigmented with sub-pigmentary anatase titanium dioxide in an amount of 0.5 to 10% by weight based on pigmented polyolefin.
The sub-pigmentary titanium dioxide which must be used is characterized by a particle size in the range of 100 to 1500 Angstroms and a surface area of 10 to 250 square meters per gram.

Description

~0'~358 BACKGROUND OF INVENTIO~
With the current emphasis on ecology and, in parti-cular, the disposal of bulk rubbish, such as bottles, cans and paper, the tremendous increase in the use of plastic containers and plastic films (or sheeting) for packaging foodstuffs, cloth-ing, soft goods and the like not only presents a serious disposal problem but increases unsightly litter in picnic areas, on roadsides and the like. While it is generally recognized that -~
certain polyolefins such as polypropylene and polyethylene are photodegradable when exposed to ultraviolet light, nevertheless, unlike steel containers which will rust away relatively rapidly, the actual decomposition of polyolefins is relatively slow and hence despite some discoloration and/or embrittlement when ex-posed to sunlight these plastic materials tend to remain sub-stantially intact for relatively long periods of time. U.S.
Patent No. 3,705,659 offers one solution to this problem where-in a plastic container is composed of very thin (1.0 mil) in-ner and outer skins of polyethylene with an intermediate layer of a water soluble thermoplastic resinous material. The latter forms the major portion of the composite structure for provi-ding the necessary structural strength. In the act of opening the container the outer layer of polyethylene is torn away thereby exposing the intermediate portion of the container to environmental conditions such as moisture. As a consequence, it quickly disintegrates while the inner and outer layers of polyethylene eventually degrade to a powdery mass. However, despite the use of extremely thin skin of polyethylene comp-lete disintegration due to sunlight and/or moisture is rela-tively slow. ~-~ -10'~581 It is also known to incorporate certain organic compounds in polyolefins to increa.se the rate of photode-gradation and ~hile these are effective the cost of organic additi~es is relatively high ~or use in throw-away plastic materials.
It is desirable, therefore, to provide improved plastic materials for use in production of throw-away contain-ers, packaging films and the like which materials will be relatively inexpensive and will decompose rapidly when ex-posed to environmental conditions including s~mlight andmoisture. Moreover, while the aforementioned organic additi-ves usually make transparent film, films made according to the present invention may range from translucent to opaque depending on the amount of inorganic photodegradable accelera-tor employed - some degree of opacity usually being desired in refuse bags r shopping bags and the like.

SUMMARY OF INVENTION
The present invention is the discovery that by ad-ding a photodegradable accelerator in the form of a uniquebut relatively inexpensive anatase titanium dioxide material to polyolefin materials and, in particular, polypropylene and poyethylene in amounts within a critical range, that these pigmented polyolefins will decompose much more rapidly than unpigmented polyolefins or polyolefins pigmented with commercial grades of TiO2 pigment, when exposed to moisture and sunlight. For purposes of brevity, the term polyethylene, as used herein, will ~e understood to be comprehensive of polymers comprising substantially 100% polyethylene and poly-ethylene copolymers such as ethylene vinyl acetate (EVA) 3'j81 ethylene ethyl acrylate (EEA) and the like.
More particularly, the in~ention provides an improvedphotodegradable polyethylene or polypropylene film of 1 to 4 mils thickness, incorpoxating a photodegrading accelerator comprising extremely finely divided anatase titanium dioxide, referred to hereinafter as sub-pigmentary titanium dioxide -meaning a titanium dioxide material too fine for use in coating compositions such as paints, lacquers, etc., said -sub-pigmentary anatase titanium dioxide having, in general, a particle size in the range of from 100 to 1500 Angstroms, and a surface area in the range of from 10 to 250 square meters per gram, said sub-pigmentary TiO2 being added to said polyolefin in critical amounts in the range of from at least 0.5 to no more than about 10% preferably from 3 to 7.5 of the pigmented polymer.
Polyolefins incorporating the relatively inexpen-sive photodegrading accelerator of this invention in amounts within the critical limits specified herein have been found to decompose from two to three times faster than a clear polyolefin or a polyolefin pigmented with a commercial anatase grade Tio2 and hence are ideally suited for fabrication of throw-away plastic materials, packaging films, sheeting and the like.
PREFERRED EMBODIMENT OF THE INVENTION
.. . _. :
The essence of the invention is the discovery that relatively rapid decomposition of polyolefins can be effected by incorporating a unique type of titanium dioxide material in the polymer the amount of TiO2 material incorporated being within a critical range. One such titanium dioxide material is disclosed in U.S. Patent 3,70-9,984, Dant~o - January 9, 1973. This is a sub-pigmentary titanium dioxide material which is defined as having a particle size in the range of from 250 to 1500A, 1073~81 spectral characteristics in the range of from +4 to +6, and a surface area in the range of from lO to 55 square meters per gram. Another sub-pigmentary titanium dioxide material which has been found to be equally useful for accelerating the photo- - -degradation of polyolefins is disclosed in U.S. Patent ~o.
3,561,968, Dantro February 9, 1971. This material is also sub-pigmentary titanium dioxide having a particle size in the range of from 100 to 500 Angstroms, a surface area of from 60 to 250 square meters per gram and a photoreactivity of at least 5 units in 10 minutes when exposed to ultraviolet light.
The methods used to form these two sub-pigmentary titanium dioxide materials differ in some respects. However, the products are in each case characterized by extremely fine particle size, and high surface area; and while these sub-pig-mentary TiO2 materials are ideally suited for use as accelera-tors according to the instant invention it will be understood that the invention is not limited to these specific materials;
and that sub-pigmentary anatase TiO2 materials having compara-ble particle size and surface area are equally successful as accelerators and are contemplated within the scope of the pre-sent invention. The essence then of the invention is the dis-covery that the aforesaid characteristics of the sub-pigmentary anatase TiO2 material together with additions in the amounts within the critical range specified herein are critical factors in accelerating photodegradation of polyolefins.
The incorporation of these sub-pigmentary TiO2 materi-als in polyolefins is carried out using conventional plastic techniques. Thus, by way of example, clear resin pellets of polypropylene or polyethylene, as the case may be, are dry blended with the sub-pigmentary titanium dioxide material in predetermined proportions and subsequently fused and mixed in conventional processing equipment such as two roll mills, Ban~
bury and Continuous Mixers. In a typical procedure, 20% pig-ment-resin concentrates were prepared on a two roll mill. It will be understood, however, that concentrates comprising dif-ferent amounts of TiO2, as for example 50% TiO2, may be pre-pared depending on the ultimate use of the concentrate.
The above described concentrate is subsequently granu-lated to pellet form, and dry blended with clear resin in pro-portions to yield the desired TiO2 levels in the finished pro-duct - i.e. extruded or blown film of from about 0.5 to 4 mils thickness molded containers, etc. In this manner, plastic items may be made of any selected TiO2 concentration less than that of the original pigment-resin concentrate.
Following this procedure, it has been discovered that polypropylene or polyethylene sheeting incorporating from at - least 0.5 to not more than about 10% sub-pigmentary titanium dioxide material of the type described above exhibits exception-ally rapid decomposition as compared to a clear polymer or a polymer pigmented with a commercial grade~ titanium dioxide pig-ment when subjected to environmental weathering conditions.

TEST
The test used to determine the rate of degradation of the pigmented polyolefins was as follows:

Standard dumbbell-shaped test specimens were died from extruded polyolefin film of about 4 mils thickness pig-mented at various TiO2 concentrations, pigmentation being by one of the ll l ll iO~3581 li sub-pigmentary anatase titanium dioxide materials described above;

2 11 and, for comparison, by commercial grades of anatase titanium 31 dioxide pigment. These specimens, in addition to those obtained 4 from clear, unpigmented film, were evaluated for physicals ~ before and after weathering. Environmental weathering tests 6 included exteTior weathering (New Jerséy) and acceleTated 7 w~athering using FSBL and Fade-0-Meter testing equipment. The 8 physicals, which were determined on an Instron unit, were primaril~
9 Tensile at Break in pounds per square inch (Tg) and percent Elongation at Break tEB). Data obtained after intervals of 11 exposure were reported as percent retained TB or EB. The lower 12 the percent retained values, the more rapid the degradation of the 13 polymer when subjected to environmental weathering conditions.
141 The invention is further illustrated but not limited by the 151 following examples each of which compares the percent retained 16¦ TB and EB values of polyolefin test strips pigmented with the 17~ sub-pigmentary anatase TiO2 materials identified above; and 18'~similar test strips of the clear polyolefin and test strips of 19 ¦!polyolefin pigmented with a commercial grade anatase TiO2 20jlpigment.

22 1~ ~
23 1 EXA~IPLE I
24 I To 40 parts clear commercial grade polyethylene pellets were added 10 parts sub-pigmentary anatase titanium dioxide material of the type hereinabove described and the mix was run 26 Ijthrough a two-roll mill to form a 20% titanium dioxide resin 27 ~concentrate. The concentrate was then let-down by extrusion to 28 orm films varyin~ in TiO2 concentration from 1 to 10%, each film 29 being about ~ mils thick. From these films test strips wcre cut.

3 ~Similar test strips were made from clear polyethylcne and from :' .
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-10~73581 l!lpolyethylene pigmented at similar levels with a commercial grade 21~anatase titanium dioxide pigment. These test strips, identified 3 li in the Table below as Test Strips-A, were then tested for TB and

4 1! EB in the Instron test unit after which the test strips were 51 weathered for 146, 25~ and 353 hours respectively, using the weathering means described above. The test stri~s were then 7 tested again for T~ and EB at the end of each exposure périod 8 and the percent retained TB and E~ values calculated. Data 9 from this first series of tests is shown in Table I.
Further, in order to determine the practical lol~er ll limit of TiO2 concentration for effective photodegradation further 12 polyethylene compositions were prepared as described above from 13 1which clear test strips and test strips of 0.25, 0.5 and l.0~
14 ~1TiO2 concentration were prepared. These test strips, identified l5lias Test Strips-B, were tested both before and after 258 hours 161jexposure in the manner hereinabove described. Test results are 17 ishown in Table I.
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1 1~73581 li From this data it will be a~parent tl1~t the di~ferences 2 in perccnt retained TB of the respec~ive Test Stri~s-A were most 3 marked after 258 and 353 hours exposure respectively; and that in 4 most every instance the polymer ~igmented with the sub-pigmentary

5 titanium dioxide material at levels of 1 3 7.5 and 10% had lower

6 percent retention values than those of the clear ~ol~ner or a

7 polymer pigment~d Wit]1 a commercial grade of anatasè titanium

8 dioxide pigment at comparable pigment levels - which is tantamount

9 to saying that polyethylene pigmented with sub-pigmel1tary titanium ~ -dioxide material at these levels will decompose relatively quickly ll when exposed to environmental sunlight.
~2 These differences appear even more pronounced from the 13 data relating to the retained E~ values. Thus after only 146 14 hours the test specimens pigmented with sub-pigmentary titanium dioxide at levels o 1,3, 7.5 and 10% had substantially lower 16 percent retained EB values as compared to the clear plastic and 17 the plastic pigmented at comparable levels ~ith a commercial 18 anatase titaniu~ dioxide pigment; and after 258 hours exposure l9 the polymer pigmented with sub-pigmentary titanium dioxide had degra~ed even more markedly.
21 It should~also be noted that t~ith regard to tl1e levels 22 of sub-pigmenta~y titanium dioxide used that in every case l.0%
23 sub-pigmentary titanium dioxide concentration produced a marked 24 acceleration in degradation as against the clear plastic; that degradation is optimum at the 3% and 7.5% levels, respectively, 26 but that while degradation at thc 10% level is still good there 27 is, ncvertheless, a reverse trend indicatin~ that pigmenting the 28 polymer Witil TiO2 at levels greater than 10% ~ill reduce rather 29 ¦ than increase the rate of de~radation by weatherin~.
30 ~ Purther Wit]1 rcspcct to Tcst Stril~s-B - it is evident . .

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:1~73~8 that 0.25~ TiO2 concentration produccd relativoly littl~
;2 jimprovement in degradation of the film strips after exposure for 3 1258 hours but that at the 0.5% TiO2 level a significant decrease 4 ¦1 in retained TB and EB takes place and that even greater decreases 51 occur at tlle l.0~ level. It is evident, therefore, that the 6¦ minimum level of TiO2 for effecti~e photodegradation of polyethyle 7 is about 0.~.

9 EX~IPLE 2 The procedure of Example l was repeated except t~lat the ll test specimens ~ere subjected to weathering in a ~ade-O-~leter ' 12 for periods of 75, 125 an~ 200 hours, respectively. As shown in 13 Table II below the polymer pigmented with the sub-pigmentary 14 titanium dioxide ma~erial, at each level, i.e. 1,3, 7.5 ~nd 10%, had lower percent retained TB and EB ~alues than those of the 16 clear plastic and the plastic pigmented at corresponding levels 17 with a commercial grade anatase titanium dioxide pigment. Again, 18¦¦pigmentation as low as 1.0% produced marked acceleration in l9 1ldegradation of the polymer while optimum acceleration occurred 20 ~at the 3 and 7.5~ levels. At ~he 10~ levcl the trend was reversed.

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:loq3ssi ¦ EXA~IPLE 3 1 A third test run was on test strips prepare~ as 2 ~described above wllich were exposed to outdoor weathering ~or a 3 Iperiod of 24 montlls. Pigmentation of t}le test strips was at the 41 3~ level. Data of the physicals are shown below.

7 Polyethylene-Physicals 8 Outdoor Weathering _gment % of Original TB and T Values Retained - - E

11 TB EB

12 Ti02** 33.91,9.0

13 Ti2*** 78.597.0

14 Clear 62.2130.0 16 **Sub-pi~mentary anatase Ti02 17, ***Commercial-grade pigmentary anatase Ti02 1811 ..
1911 Polyethylene test strips pigmented at 3% level with the 20 ¦I sub-pigmentary anatase Ti02 hereinabove described degraded far more rapidly than compa~rable unpigmented (clear) test strips or 22 ¦test strips pigmented with a pigmentary gra~e Ti02 at comparable 23 ¦jlevels - when exposed to outdoor weathering. .
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EXA~IPLE 4 1 ¦ Another set o~ test strips wer~ prop~red as d~scribad 2¦¦above except in this case the polyolefin used was polypropylene.
3 ¦1 In these tests only one level cf TiO2 (3.0~) was used in the 4 ~)igmented test strips. Weathering was carried out by exposure in c FSB~ Ullit for 70 hours. The resulting ~hysi~als, i.e. TB and E~
6 and percent retained TB and EB are sho~ in Table IY below.
7 - .

Polypropylene-Physicals 11 70 Hours PSBL
12 ~ nt TB_ EB ; ~ ~ Retained 13 Clear 0 8085 555 89 98 14 TiO2** 3 3460 50 44 10 TiO2*** 3 4715 415 54 74 161 .
17 li **Sub-pigmentary anatase TiO2 18 ¦ ***Commercial-grade ~igmentary anatase TiO2.
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:10~3581 ~ From the above data it is evident that polypropylene 2 ll test strips pigmented at the 3% level with the sub-pigmentary 3 ¦l anatase titanium dioxide material of the type identified above 41 had much faster degradation rates than test strips of clear polymer or of a polymer pigmented at corresponding levels with a 6 commercial grade anatase titanium dioxi.de pigment.
7 The instant invention thus provides an improvcd photo-8 degradable polyolefin and in particular polypropylene and poly-9 ethylene materials which are especially s~ited to the production of relatively inexpensive throw-away articles of commerce such ll pàckaging-~ilms, sheet;.ng, coatings and the likei- c]laracteri~ed 12 by accelerated photodegradation when exposed to environm~ntal ~.
13 weathering conditions such that said articles will disintegrate 14 relatively rapidly, thçse improved polyolefin materials cmbodying an inorganic photodegradation accelerator in the form of a unique 161 type of sub-pigmentary anatase titanium dioxide in critical 17 llamounts in the range from 0.5 to 10~.
18 ¦I While the illustrations have been of thin polymer l9 llsheeting of approxi.mately 4 mils thickness it will be understood 20 ~that the invention is also applicabls to throw-away articles 21 1 of commerce of heavier cross-section such as polyethylene or poly-2~ 1 propylene bottles, cartons and the like.
23 l The invention may be carried out in other specific ways 2~ ¦ than those herein set forth without departing from the spirit and 25 ~,essential characteristics o~ the invention, the present embodiments 26 ~lbeing illustrative and not restrictive and all changes coming ~7 j within the meaning and equivalency range of the appended claims ~8 bein~ embraced therein.
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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A photodegradable polypropylene or polyethylene film having a thickness in the range of from 1 to 4 mils, said film being pigmented with subpigmentary anatase TiO2 in an amount in the range of from at least 0.5% to no more than about 10 percent on a pigmented polyolefin basis, said sub-pigmentary TiO2 characterized by a partlcle siee in the range of from 10 to 1500 Angstroms and a surface area of 10 to 250 square meters per gram.

2. A photodegradable film according to claim 1 com-prising polyethylene pigmented with from 3 to 7.5% sub-pigmentar,v anatase TiO2.

3. A photodegradable film according to claim 2 com-prising polypropylene pigmented with from 3 to 7.5% sub-pigmentary anatase TiO2.