Photodegradable plastics containing haloolefins
DESCRIPTION OF THE INVENTION 
     The invention relates to and has among its objects the provision of novel plastic compositions capable of photodegradation. Further objects of the invention will be evident from the following description wherein parts and percentages are by weight unless otherwise specified. 
     Plastics have become an important part of the American way of life. Innumerable articles of manufacture are made of plastics. One of the main uses thereof is in the manufacture of containers for liquids and solids of all kinds, particularly foods. Another important use is the manufacture of plastic sheet materials such as films and foils. For example, plastic films are used in agriculture for covering the soil between plants, thereby to prevent the growth of weeds. 
     One problem with plastics is that they are not easily decomposed. Thus, for example, plastic food containers thrown by the roadside do not decompose but remain until collected, thereby polluting the environment. Similarly, plastic films used as soil coverings must be removed from the fields prior to initiating a new crop. 
     The invention described herein concerns a means for obviating the above problems in that it provides polyolefin plastics which are capable of photodegradation. Containers fabricated from the plastics of the invention when exposed to sunlight will gradually decompose and eventually crumble away. Thus, such containers when thrown along the roadside will eventually become part of the soil. Films prepared from the plastics of the invention when used for agricultural purposes will gradually become friable by the action of sunlight so that they can be readily plowed into the soil. 
     The benefits of the invention are realized by incorporating into a polyolefin any of the compounds described below. 
     Group I. Halo-2-butenes of the structure 
     
         (XH.sub.2 C).sub.n CH.sub.m = CH.sub.m (CH.sub.2 X).sub.n 
    
     wherein: 
     X is chlorine, bromine, or iodine 
     n is 1 or 2 
     m is 2 - n 
     Illustrative examples of compounds included in Group I are: 
     1,4-Dibromo-2-butene 
     1,4-Dichloro-2-butene 
     1,4-Diiodo-2-butene 
     2,3-Bis(bromomethyl)-1,4-dibromo-2-butene 
     2,3-Bis(chloromethyl)-1,4-dichloro-2-butene 
     2,3-Bis(iodomethyl)-1,4-diiodo-2-butene. 
     Group II. Halopropenes of the structure ##EQU1## WHEREIN: X is chlorine, bromine, or iodine 
     Y is hydrogen, chlorine, bromine, or iodine 
     Z is hydrogen, chlorine, bromine, iodine, lower alkyl, or phenyl 
     a is 1, 2, or 3 
     b is 3 - a. 
     Illustrative examples of compounds included in Group II are: 
     Cinnamyl bromide 
     Cinnamyl chloride 
     Cinnamyl iodide 
     Hexachloropropene 
     Hexabromopropene 
     Hexaiodopropene. 
     Group III. Halocyclopentadienes of the structure ##EQU2## wherein: X is chlorine, bromine, or iodine 
     Z is hydrogen, chlorine, bromine, iodine, lower alkyl, or phenyl. 
     Illustrative examples of compounds included in Group III are: 
     Hexachlorocyclopentadiene 
     Hexabromocyclopentadiene 
     Hexaiodocyclopentadiene 
     Pentachlorocyclopentadiene 
     Pentabromocyclopentadiene 
     Pentaiodocyclopentadiene. 
     Polyolefins containing any of the above compounds (or additives as they are often referred to herein) decompose readily when exposed to sunlight. The decomposition, however, is not instantaneous but is gradual, and the rate thereof depends on such factors as the type of polyolefin, the amount of the compound added, and the activity of the latter. The reaction which takes place can be described as a photo-depolymerization in which the polymeric chains are reduced to lower molecular weight under the influence of sunlight. 
     The amount of additive to be incorporated with the polyolefin depends on the activity of the additive, and upon the desired rate of photo-decomposition. In general, one may use about 0.1 to 10% of the additive, based upon the weight of polyolefin. For most purposes about 1 to 5% of the additive is sufficient to obtain a reasonable and useful rate of photo-degradation. 
     The polyolefin to which the invention is applied includes, for example, high and low density polyethylene, polypropylene, polybutylene, polystyrene, mixtures of polyethylene and polypropylene, vinyl acetate/ethylene copolymers, and the like. The incorporation of the additive with the polyolefin may be carried out in any of the ways known in the art of compounding plastics. For example, intimate mixing of the polyolefin and additive may be effected by melting and mixing the polyolefin with the additive by any suitable means such as a mixer of the Banbury or Werner type or in a screw extruder. The compositions of polyolefin and additive can be formed into any desired articles such as films, tubular sheets, foils, bags, bottles, or other containers by application of well-known molding and fabricating techniques. 
     It is within the compass of the invention to use known photo-sensitizing compounds such as dibenzoyl peroxide, azo-bis-isobutyronitrile, and the like in conjunction with the additives of the invention. In some instances such photo-sensitizers increase the activity of the additives of the invention. Thus, polyolefins containing an additive in accordance with the invention and a photo-sensitizer will exhibit an enhanced rate of photodegradation. 
    
    
     EXAMPLES 
     The invention is further demonstrated by the following illustrative examples. 
     Example 1 -- Photodegradable Polystyrene Films 
     A. Commercial polystyrene powder was first purified as follows: The powder (90 g.) was placed in a 2-liter Erlenmeyer flask together with 510 ml. of chloroform and the mixture was shaken until dissolved. The solution was poured slowly with vigorous stirring into a one-gallon Waring Blendor containing 2 liters of methanol. The finely precipitated powder was filtered, washed with methanol, air-dried, and finally dried in a vacuum oven at 52° C. and 30 p.s.i. This procedure was repeated for a total of 3 times and a polystyrene containing no styrene odor was obtained. 
     B. Incorporation of additive: A wide-mouth jar was charged with 3 g. of the purified polystyrene powder, 0.15 g. of additive, and 17 ml. of chloroform, then shaken on a wrist-action shaker until solution was obtained. The solution was allowed to stand for a few minutes to remove entrapped air bubbles. Afterward, the solution was spread on a 4 × 8 inch glass plate with a film-casting knife with a setting of 0.038 in. The plate was suspended above chloroform in a covered tray to retard evaporation of the solvent. After the plate had dried overnight, it was placed in a tray containing distilled water, which floated the film away from the glass. This film of polystyrene plus the incorporated additive was about 0.004-0.005 inch thick. A piece, 3/4 × 13/8 inches was cut from the film and its infrared spectrum was taken. 
     C. Test procedure: The said piece of film was then irradiated for 66 hours by maintaining it on a revolving table 9 in. in diameter with the film sample 6 in. from a 275-watt RS sunlamp. After irradiation, an infrared spectrum of the sample was again determined. The extent of photo-oxidation was taken as a measure of the photodegradability of the irradiated material. Photo-oxidation was determined by measuring the increase in the carbonyl absorption band of the irradiated sample over that of an irradiated sample containing no additive. 
     D. Specific additives used: The sequence described above in parts A, B, and C was performed with the following additives, each in the amount of 5%, based on the weight of polystyrene: ##EQU3## 
     The results are tabulated below: 
     
                       Table 1                                                     
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Polystyrene and 5% Additive                                               
Run  Additive           Increase   Additive                               
                        in carbonyl,                                      
                                   effective-                             
                        absorbance ness                                   
                        units      ratio                                  
______________________________________                                    
A    1,4-Dibromo-2-butene                                                 
                        0.939      3.8                                    
B    2,3-Bis(bromomethyl)-1,4-                                            
      dibromo-2-butene  1.335      5.4                                    
C    Cinnamyl bromide   0.997      4.1                                    
D    Cinnamyl chloride  0.598      2.4                                    
E    Hexachloropropene  0.534      2.2                                    
Control                                                                   
       None used        0.246      1.0                                    
______________________________________                                    
 *Additive effectiveness ratio is equal to the increase in carbonyl for a 
 particular additive divided by the increase in carbonyl obtained without 
 additive (control). Thus, for example, polystyrene containing            
 1,4-dibromo-2-butene is oxidized photochemically 0.939/0.246 or 3.8 times
 more than polystyrene without an additive.                               
 
    
     Example 2 -- Photodegradable Polypropylene Films 
     To 0.6 gram of powdered polypropylene resin was added a solution of 0.006 gram of additive in 0.6 ml. of acetone. The mixture was stirred to evenly distribute the additive solution over the particles of polypropylene. The mixture was spread as a thin layer on a Mylar sheet supported by a ferrotype chrome plate. After allowing the acetone to evaporate, the said layer was covered with another Mylar sheet and chrome plate. This assembly was heated for 30 seconds at 350° F. and then pressed at 370 psi. for 30 sec. The assembly was then transferred to an unheated press and pressed at 4,000 psi. while cooling. A film of polypropylene and 1% additive having a thickness of 0.003 to 0.004 inch was thus obtained. 
     A similar procedure was employed for the preparation of polypropylene films containing other additives. The temperature of the heated press was varied from 350°-412° F., depending on the melting point of the additive. In general, the temperature of the press was about 10° F. above the melting point of the additive. 
     Samples of the polypropylene-additive film and control polypropylene film without any additive were tested for photodegradability as described in Example 1. 
     The additives used were: 
     A. 1,4-dibromo-2-butene 
     B. 2,3-bis(bromomethyl)-1,4-dibromo-2-butene 
     C. hexachloropropene 
     D. hexachlorocyclopentadiene ##EQU4## 
     The results are tabulated below: 
     
                       Table 2                                                     
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Polypropylene plus Additives                                              
                              Increase                                    
                              in                                          
Run   Additive       Amount   carbonyl,                                   
                                      Additive                            
                     (%)      absorbance                                  
                                      effective-                          
                              units   ness ratio                          
______________________________________                                    
A   1,4-Dibromo-2-butene                                                  
                     5        0.582   1.3                                 
B   2,3-Bis(bromomethyl)-1,4-                                             
     dibromo-2-butene                                                     
                     5        0.720   1.6                                 
C   Hexachloropropene                                                     
                     1        0.577   1.3                                 
D   Hexachlorocyclopentadiene                                             
                     3        0.688   1.5                                 
Control                                                                   
       None used     0        0.446   1.0                                 
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     Example 3 -- Photodegradable Polyethylene Films 
     Polyethylene films containing 5% of 2,3-bis(bromomethyl)-1,4-dibromo-2-butene were prepared by the same procedure described in Example 2. Photodegradability of the resulting films was determined as described in Example 1, but using an irradiation time of 100 hours. 
     It was found that the film containing the additive was photodegraded 1.032/0.212 or 2.6 times more than the control film (polyethylene with no additive).