Crystallizable polyolefin composition having increased crystallization temperature and rate of crystallization
BACKGROUND OF THE INVENTION 
     The present invention relates to crystallizable polyolefin polymers, and more particularly to linear low density polyethylene and copolymers of ethylene with alpha-olefins having 3 to 8 carbons containing a novel nucleating composition, whereupon cooling from a molten state, the temperature of crystallization and rate of crystallization are significantly increased. 
     The use of nucleating agents to modify the crystalline structure of thermoplastic high polymers and to increase the temperature of crystallization and rate of crystallization is well known in the art and is discussed in detail, for example, in U.S. Pat. No. 3,367,926 to John F. Voeks, the disclosure of which is incorporated herein by reference in its entirety. The above-described polymer composition, while in the heat-plastified state, can be fabricated into various articles, such as fibers, filaments, films, tubes or the like, by extrusion or molded by compression or injection or otherwise into molded articles and then cooled to &#34;set up&#34; the shape and induce crystallization. By increasing the temperature of crystallization and rate of crystallization, the cycle time can be reduced and production rates increased. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention is based upon the discovery that the affect of certain known nucleating compounds on increasing the crystallization temperature and rate of crystallization of the herein defined polymers can be significantly enhanced by incorporating in the polymer with said known nucleating compound, a proportion of high isotactic polypropylene. This is particularly surprising since the polypropylene, itself, has been found to have hardly any effect on the crystallization temperature or rate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, the polymers of the present invention are linear low density polyethylene or copolymers of ethylene with at least one alpha-olefin having 3 to 8 carbon atoms. By the term linear low density polyethylene there is meant polyethylene having a density of about 0.91 to about 0.93 gm/ml. The copolymers are those having a preponderance of ethylene units and a melt index of about 0.5 to about 50. The alpha olefins include straight chain and branched chain olefins containing 3-8 carbon atoms and include propylene, butene, 3-methylbutene-1, 4-methyl-pentene-1, octene and the like. 
     The polypropylene additive, also hereinafter termed co-nucleating agent, utilized herein has a melt index of about 1 to 25 and a high iosotactic content of at least about 95%. 
     The nucleating compound, used in conjuction with the polypropylene according to the present invention, is selected from the group consisting of certain phenyl phosphate compounds and dibenzylidene sorbitol. The phenyl phosphate compounds are described in detail in U.S. Pat. No. 4,258,142 in the name of Toshio Ohzeki, the disclosure of which patent is incorporated herein by reference in its entirety. These compounds will be described further hereinafter. The use of dibenzylidene sorbitol as an additive in polyolefin compositions is described in U.S. Pat. No. 4,016,118 in the name of Kenzo Hamada et al, which is also incorporated herein by reference in its entirety. 
     As there disclosed, the dibenzylidene sorbitol is prepared by reacting 1 mol of sorbitol and 2 mols of benzaldehyde in the presence of an acid catalyst at an elevated temperature. 
     In preparing the herein disclosed polymer composition of increased crystallization temperature and increased rate of crystallization, the nucleating compound and the polypropylene co-nucleating agent are incorporated into the molten resin by conventional procedures, such as by using a Brabender Mixer under an inert atmosphere. The polypropylene is utilized in an amount from about 0.1 to about 20% by weight, preferably from about 0.5 to about 5% by weight, when used with dibenzylidene sorbitol or the phenyl phospate compound. The dibenzylidene sorbitol is used in an amount from about 0.05 to about 1.5% by weight, preferably 0.1 to about 0.5%. When utilizing the phenyl phosphate compound as the nucleating compound, it is used in an amount from about 0.05 to about 2% by weight, preferably from about 0.1 to about 1% by weight. All percentages are based on the weight of the total composition. 
     As described in the above-referred to Ohzeki patent, the phenyl phosphate compounds utilized herein have the formula: ##STR1## in which R 1  and R 2  are independently selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, a cycloalkyl group having 5 to 12 carbon atoms, a phenyl group, a phenoxy group, and a group ##STR2## in which R 5  is a hydrogen atom or an alkyl group having 1 to 18 carbon atoms; R 3  is selected from the group consisting of an alkyl group having 1 to 18 carbon atoms, ##STR3## and R 4  ; and R 4  is a hydrogen atom or a metal atom equivalent M 1/a  where a is the valence of the metal atom M. 
     As indicated in the Ohzeki patent, the preferred phenyl phosphate compounds are the metal salts of diphenyl phosphates, and particularly preferred are the calcium, magnesium, potassium and sodium salts of di(p-t-butylphenyl) phosphate. 
     In order to further illustrate the present invention, the following examples are provided. They in no way are intended to limit the scope of the invention. 
     EXAMPLES 1-15 
     The materials used according to the present invention, and used in these examples are shown in Table 1 and identified by designated symbol A, B, C and D. 
     The additives were incorporated into the linear low density polyethylene resin by melt compounding using a Brabender Mixer at 210° C. for 5 minutes under a nitrogen atmosphere. Test specimens were prepared using hot press at a temperature of 180° C. under a pressure of 20,000 psi and allowed to cool to room temperature. The crystallization temperature and rate of crystallization were measured using a Differential Scanning Calorimeter (DSC-2) from Perkin-Elmer. The effect of the additives with various formulations on the crystallization temperature is presented in Tables 2 and 3. 
     
                       TABLE 1                                                     
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Desig-                                                                    
nated                                                                     
Sym-                                                                      
bol   Chemical             Specification                                  
______________________________________                                    
A     Linear Low Density Polyethylene                                     
                           MI.sup.(1) = 2,d.sup.(2) =                     
                           0.9204 g/cc                                    
B     97% Isotactic Polypropylene (PP)                                    
                           MI.sup.(1) = 4.5,d.sup.(2) =                   
                           0.9100 g/cc                                    
C     Phenyl Phosphate Compound.sup.(a)                                   
                           White Powder,d =                               
                           1.24 g/cc                                      
D     Dibenzylidene Sorbtoil.sup.(b)                                      
                           White Powder,d =                               
                           1.047 g/cc                                     
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                       TABLE 2                                                     
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                      Crystallization                                     
                      Temperature                                         
                      (°C.)                                        
Example #                                                                 
         Material           Tc,i.sup.(1)                                  
                                    Tc,p.sup.(2)                          
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1        A                  101     97                                    
2        99.7% A + 0.3% D   104     99                                    
3        99% A + 1% B       101     97                                    
4        98.7% A + 1% B + 0.3% D                                          
                            107     104                                   
5        97.5% A + 2.5% B   101     97                                    
6        97.2% A + 2.5% B + 0.3% D                                        
                            107     104                                   
 6a      97.45% A + 2.5% B + 0.5% D                                       
 6b      96.5% A + 2.5% B + 1% D                                          
7        95% A + 5% B       101     97                                    
8        94.7% A + 5% B + 0.3% D                                          
                            110     106                                   
9        79.7% A + 20% B + 0.3% D                                         
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 .sup.(1) Tc,i: Initial Crystallization Temperature                       
 .sup.(2) Tc,p: Peak Crystallization Temperature                          
 
    
     
                       TABLE 3                                                     
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                      Crystallization                                     
                      Temperature                                         
                      (°C.)                                        
Example #                                                                 
         Material           Tc,i.sup.(1)                                  
                                    Tc,p.sup.(2)                          
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10       A                  101      97                                   
11       99.7% A + 0.3% C.  105     101                                   
12       95% A + 5% B       101      97                                   
13       94.7% A + 5% B + 0.3% C                                          
                            107     103                                   
14       94.95% A + 5% B + 0.05% C                                        
15       94% A + 5% B + 1% C                                              
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     From the above, it is seen that the addition of polypropylene has hardly any effect on the crystallization temperature of the linear low density polyethylene. By incorporating the nucleating compound benzylidene sorbitol or the phenyl phosphate compound, Example 2 in Table 2 and Example 11 in Table 3, respectively, it is seen that the crystallization temperature has been raised to some degree. However, by incorporating both the nucleating compound and polypropylene into the polymer, it is seen that the crystallization temperature has been increased significantly. In Table 2, comparing Example 4 with Example 3 and Example 6 with Examples 5 and 2, Example 8 with Example 7, the unexpected effect is clearly demonstrated. Likewise, comparing Example 13 to Examples 11 and 12 in Table 3, the unexpected enhancement of crystallization is also readily seen. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The single drawing figure is a graph illustrating increase in crystallization rate brought about by the present invention, where the curve to the left represents linear low density polyethylene, the middle curve represents linear low density polyethylene containing dibenzylidene sorbitol and the curve to the right indicates the linear low density polyethylene containing dibenzylidene sorbitol and polypropylene. The above-referred to designated symbols are indicated. 
     The effect of the additives on crystallization is further illustrated in the Figure, wherein crystallization half time is plotted against crystallization temperature. The crystallization half time, t 1  /2, is defined as the time to reach the half completion of crystallization at the specified isothermal crystallization temperature. The results show that the nucelating composition of this invention functions to shorten the crystallization time, in other words, to increase the crystallization rate. Similar results are obtained when sodium di(b-tert-butyl-phenyl)phosphate is substituted for the dibenzylidene sorbitol. 
     Although the examples disclosed and/or exemplified are the preferred embodiments of this invention, variations or modifications apparent to one skilled in the art are intended to be within the scope of this invention.