FIELD OF THE INVENTION

This invention relates generally to plastic compositions, and more particularly, to a ther o- plastic composition which comprises a thermoplastic polymer such as polyetheylene or ethylene copolymers, polypropylene, polystyrene or polyurethane which is photodegradable when exposed to sunlight and/or ultraviolet light.

BACKGROUND OF THE INVENTION

Plastic waste is normally disposed of by recycling, incineration or burying in a landfill. Unfortunately, some materials are improperly dis¬ posed of and become litter. This litter is both esthetically objectionable and poses a threat to wildlife through possible entrapment and/or inges- tion. Therefore, there is a need for a material which will degrade harmlessly when exposed to sun¬ light, ultraviolet light or heat. There are known additive compositions which may be incorporated into thermoplastic materials to enhance degradation of the plastic. For example, U.S. patent 4,931,488 discloses a thermoplastic polymer composition to which has been added a bio- logically degradable substance such as starch, an iron compound, an oxidizable substance selected from fatty acids and/or fatty acid esters and a transi¬ tion metal. This composition demonstrates biodegradation and photodegradation capability when exposed to heat, ultraviolet light, sunlight, or under compositing conditions. But, many states

today require only that a plastic composition only be photodegradable. In this connection, the use of a composition such as that disclosed in patent 4,931,488 may not be advantageous. The starch com- ponent in the composition becomes unnecessary when biodegradation is not required, and the starch may cause the product to become sensitive to moisture pickup during the production process and does little to enhance photodegradation. Therefore, a plastic composition which possesses the desired properties of thermoplastic materials, yet readily degrades when exposed to sun¬ light and/or ultraviolet light, is highly desirable.

SUMMARY AND OBJECTS OF THE INVENTION

Accordingly, it is the primary object of the present invention to provide a plastic composition which readily degrades when exposed to sunlight and/or ultraviolet light and to provide such a plas¬ tic composition for films, sheets or other moldings, which possesses the desired properties of known thermoplastic materials.

In accordance with the present invention, there is provided a thermoplastic composition which is degradable under the action of sunlight and/or ultraviolet light and heat. The thermoplastic com¬ position comprises a thermoplastic polymer, a first transition metal compound which may be a complex, a second transition metal compound and an aromatic ketone. More particularly, the thermoplastic polymer preferably comprises an olefin such as polyethylene, ethylene copolymers, polypropylene, polystyrene or polyurethane. The first transition metal comprises an iron compound such as ferric hydroxy stearate,

which is soluble in the composition and acts as an initiator which promotes further degradation. The second transition metal comprises a copper compound which acts as a catalyst with the iron compound to enhance degradation. The aromatic ketone includes benzophenone. It has been found that by using the composition of the present invention in a thermo¬ plastic material that a significant synergistic deg¬ radation effect occurs on the plastic when exposed to sunlight and/or ultraviolet light.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a graph illustrating loss of elonga¬ tion at break as a function of time in a plastic film product including the composition of the preε- ent invention.

DETAILED DESCRIPTION OF THE INVENTION

The thermoplastic composition in accordance with the present invention facilitates biodegradation of thermoplastic materials under the action of sunlight and/or ultraviolet light and heat. That is, the chemicals in the composition in the presence of the sunlight or ultraviolet light and heat react to form free radicals which break the bonds cf the thermoplastic polymer resulting in a reduction of molecular weight and loss of physical properties. Thereafter, the thermoplastic material becomes weak and brittle which results in a material that is easily disintegrated by exposure to the ele¬ ments. The thermoplastic composition of the present invention comprises as one component thereof a ther¬ moplastic base of any known thermoplastic polymer and preferably polymers of olefins such as

polyethylene, ethylene copolymers, polypoypylene, polystyrene or polyurethane. "Polyethylene" as used herein, comprises any type of polyethylene, such as LDPE, LLDPE, LMDPE, MDPE, HDPE, ULDPE, etc. Exam- pies of suitable ethylene copolymers include EVA,

EBA, EAS, EMAA and ionomers. It is understood that the composition of the present invention may be formed in any of the following manners. The compo¬ sition may be added directly at the final production stage by the manufacturer, formed in a concentrate and supplied to the manufacturer to be added to the base resin or formed with the base resin and supplied to the manufacturer ready for use.

The first transition metal component preferably comprises an iron compound generally having the for¬ mula X-Fe, where X represents one or more ligands. The compound may additionally be coupled to a fur¬ ther ligand Y. Fe may designate iron in any known valancy. The ligand X may be an inorganic or organic acid radical or likewise another ligand bonded in a complex. For purposes of illustration, examples or suitable ligands X include OH-, C1-, Br-, I-, oxalate-, H-citrate-, NO- , N-.- EDTA or a carbonyl, nitrosyl or porphyrin radical. Examples of suitable ligands Y include carboxylic acid ions of aromatic or aliphatic monocarboxylic acid or of dicarboxylic acids, the aliphatic carboxylic acid preferably having 10 to 20 carbon atoms. Ligand Y serves in general to increase the solubility of the compound X-Fe in the polymer. The iron compound more preferably is comprised of iron stearate pres¬ ent in the final concentration of the present inven¬ tion in an amount from about 0.01 weight percent to about 0.5 weight percent, and more preferably in an amount from about 0.1 weight percent to about 0.3

weight percent. It is understood that the ranges listed herein are for purposes of illustration. The amount listed for the upper limits is based on eco¬ nomics, and therefore, the components may be present in the concentration in amounts greater than those listed. Although, a saturation limit is reached at which further addition does not provide greater effectiveness. The minimum value listed provides minimum measurable improvements in photodegradation. The second transition metal compound of the thermoplastic composition of the present invention acts as a catalyst with the first transition com¬ pound to enhance degradation of the thermoplastic material. The compound may comprise a complex having the general formula Z'-Me, wherein Me desig¬ nates a transition metal other than iron if iron is the first transition metal and Z ' designates one or more ligands. For purposes of illustration only and, not limitation, examples of ligands include OH-, C1-, Br-, I-, oxalate-, H-citrate-, NO~ ₂ , N,-, 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. The preferred transition metals comprising Me are the transition metals of the first transition metal row in the periodic table, and more preferably copper and vanadium. The most preferred second transition metal compound is copper stearate present in the final concentrate of the present invention in an amount from about 0.005 weight per¬ cent to about 0.1 weight percent, and more pre¬ ferably cupric stearate in an amount from about 0.01 weight percent to about 0.06 weight percent.

The aromatic ketone component of the composi- tion of the present invention has been found to have

a synergistic effect that speeds up the photodegradation of the thermoplastic. This is an unobvious finding since aromatic ketones generally are ultraviolet stabilizers. For purposes of illus- tration only and, not limitation, examples of aro¬ matic ketones include benzophenone, anthraquinone, anthrone or derivatives such as acetylbenzophenone or 4-octyl benzophenone.

The more preferred aromatic ketone is benzophenone which is present in the concentration in an amount of from about 0.01 weight percent to about 0.7 weight percent and more preferably in an amount from about about 0.02 weight percent to about 0.15 weight percent. Utilizing the composition of the present inven¬ tion is further advantageous in that the degradation can be controlled depending on the field of applica¬ tion by varying the concentration of the individual components, without the plastic material suffering a deterioration in its properties under normal use conditions. Particular fields of application of the composition of the present invention include packaging materials, films for garbage bags for co - postable wastes, agricultural films, and in particu- lar those materials which may become litter such as disposable packaging. Furthermore, 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 thermoplastic composition in a concentrate form in accordance with the present invention and its processing to give sheets, films, plates or other shapes is carried out by any conven- tional method. For example, the polyethylene may be

combined with the appropriate amounts of iron stearate, copper stearate and benxophenone in a twin screw compounding extruder or equivalent and extruded and pelletized into a concentrate or mas- terbatch. Extruded film, blow molded parts or injection moulded parts are made by combining this concentrate with polyethylene or other base resins in a film extruder or injection molding extruder, which is normally of the single screw type. As an alternative the chemicals could be added directly at the final production stage in the single screw extruder, this however is not very practical. This technique would require either a premixing step or a number of additional additive feeders on the extrusion machine. In addition, single screw extruders are typically poor mixing devices. By producing a concentrate first the active ingredients are predispersed in the thermoplastic base i.e., polyethylene, making the final dispersal in the sin- gle screw machine much easier.

The degradation process appears to proceed according to the following mechanism, with reference to A.C. Albertsson, B. Randy, J. Appln. Polym. Sci Appl. Polym Sypm. , 35 (1979) p. 423 and the publica- tion of A.C. Albertsson mentioned therein, which is hereby incorporated by reference.

Plastics with C-C bonds in the main chain biodegrade extremely slow with the formation of C0 ₂ and H^O. The half life of biological degradation of polyethylene was extrapolated to be at least 100 years. Under the action of ultraviolet light, sun¬ light or heat free radicals such as, for example OH* are formed due to the presence of 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, other chains, iron ions, and the like. Polymer chains are thus split and small chains with or without oxygen-containing groups, such as alco- hols, ketones, etc. are formed. During this pro¬ cess, iron ions act both as an initiator and as a reaction promoter since iron-(in) hydroxide com¬ plexes are highly reactive. This can be illustrated the by the following equation:

(1) Fe ³ + 0H~ [FeOH] + Fe ²⁺ OH*

The observed catalytic effect of the transition metal compounds, copper or vanadium compounds, is probably attributed to an acceleration of the Fe 3+ Fe 2+ Fe3+ cycle. Without these compounds, the Fe 2+ formed according to equation (1) may be reoxidized by other free radicals or other interme¬ diates at the expense of chain splitting as for example, shown in the following equation:

Fe ²⁺ + ROOH Fe ³⁺ + 0H~ + RO* (2)

In the presence of copper compounds, the Fe 2+ formed is reoxidized faster according to the follow¬ ing equation:

2+ 2+ 3+ + Fe^ + o Fe ^"3 + Cu (3)

and Cu+ ions are reoxidized very fast to Cu 2+ ions by free radicals as follows:

CU ⁺ + RO* Cu ²⁺ + R0 ^~ (4)

This process repeats itself as long as the polymer is exposed to ultraviolet light, sunlight or heat. In this phase, the plastic materials become brittle and fragile and disintegrate into small

particles of about a few mm2 up to about a few cm2.

Depending on the prevailing conditions, this phase takes in general from about 10 to 60 days.

The following table sets forth a preferred for- mulation for the thermoplastic composition of the present invention formed in a concentrate wherein the compositional ranges of the various components are by weight percent of the total concentrate.

TABLE A

a. From about 0.01 percent to about 0.5 per¬ cent first transition metal compound; b. From about 0.005 percent to about 0.1 per¬ cent second transition metal compound; c. From about 0.01 percent to about 0.2 per- cent aromatic ketone; d. Balance thermoplastic polymer.

The following table sets forth a more preferred formulation for the thermoplastic composition of the present invention formed in a concentrate wherein the compositional ranges of the various components are by weight percent of the total concentrate.

TABLE B

a. From about 0.1 percent to about 0.3 per¬ cent iron stearate; b. From about 0.01 percent to about 0.06 per¬ cent copper stearate; c. From about .02 percent to about 0.15 per¬ cent benxophenone; d. Balance thermoplastic polymer.

The following tables set forth a still more preferred formulation for the thermoplastic

composition of the present invention formed in a concentrate wherein the compositional ranges of the various components are by weight percent of the total concentrate.

TABLE C

_) a. About 0.124 percent ferric 12 hydroxy stearate; b. About 0.024 percent cupric stearate; c. About 0.052 percent benxophenone; 10 d. Balance thermoplastic polymer.

TABLE D

a. About 0.31 percent ferric 12 hydroxy stearate; b. About 0.06 percent cupric stearate; ₁₅ c. About 0.13 percent benxophenone; d. Balance thermoplastic polymer.

With reference to Figure 1, there is shown loss of elongation at break using a film product to which the composition of the present invention has been ₀ added, and a control comprised of polyethylene with¬ out the additive composition. The film product indicated by Formulation 1 in Figure 1 has the com¬ position of Table C added thereto. The film product indicated by formulation 2 in Figure 1 has the co - 5 position of Table D added thereto. The control film, Formulations 1 and 2 were exposed in a "QUV" weatherometer, a testing device which uses ultravio¬ let light and moisture to simulate outdoor exposure. It is apparent from Figure 1 that films comprised ₀ with the composition of the present invention photodegrade at a much faster rate than the control film without the composition of the present invention.

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lt will be understood that the foregoing description and illustration is by way of example only and that such modifications and changes as may suggest themselves to those skilled in the art are intended to fall within the scope of the present invention.

What is claimed is: