Abstract:
Blends useful as an additive in polyolefin polymers for minimizing the effects of radiation on the physical properties of polymers, which comprises a hindered amine light stabilizer and at least one material selected from the group consisting of: i) amine oxides and ii) hydroxylamines. Various articles of manufacture may be produced using a composition or blend according to the invention, and the physical properties of such articles are less effected by electromagnetic radiation than like-kind compositions of the prior art.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the development of an improved clear, color-stable, and radiation-sterilizable polypropylene for various medical applications, including syringes and other wares normally fabricated from what is recognized in the art as radiation-sterilizable polyolefins.  
       BACKGROUND INFORMATION  
       [0002]     U.S. Pat. No. 4,666,959 teaches the use of a polymeric hindered amine light stabilizer, an alkyl phosphite and a specific phenolic antioxidant as necessary additives to protect polypropylenes from the exposure to high energy “gamma” radiation. U.S. Pat. No. 4,888,369 teaches very similar art as the previous one, except it teaches the use of an additive termed as mobilizer, such as hydrocarbon oil. Although the teachings of these patents are helpful to prevent degradation of polypropylenes from exposure to high energy radiation, the main drawbacks are the yellowing of parts made from the teachings therein due to the presence of hindered phenolic antioxidants. U.S. Pat. No. 6,231,936 claims the radiation tolerant polypropylene composition comprising of polypropylene and polyethylene (1-50%) produced by single-site catalyst along with additives such as hindered amine stabilizers, secondary antioxidants (i.e., phosphites and thioesters), sorbitol type clarifiers. However secondary antioxidants such as phosphites are susceptible to hydrolysis upon exposure to moisture prior to or during extrusion. Also, in the real world case scenario, most of these phosphites are recognized by those skilled in the art as being the cause of black specks in the resin which show up in the molded parts thereafter. This patent also claims the use of thiodipropionate secondary antioxidant selected from the group consisting of distearyl thiopropionate and dilaurylthiopropionate. These sulfur containing additives are known to impart odor. This patent also claims the addition of a sorbitol type clarifying agent (i.e., bis-4-methylbenzylidene sorbitol and bis-3,4-dimethylbenzylidene sorbitol) up to 0.5% to enhance clarity of molded parts. However, sorbitol type clarifying agents impart cherry flavored odor, and in some severe autoclave conditions (per 9 CFR 121 condition A), they form flocculates. U.S. Pat. No. 5,376,716 describes a radiation resistant resin suitable for the manufacture of disposable medical devices comprising of a semicrystalline polypropylene or propylene-ethylene copolymer with 1500-5000 ppm of triallyl trimellitate as well as a phosphite.  
         [0003]     The presentation “New Improvements in Radiation Resistant Polypropylenes” presented at the Fifth International Conference Additives in 1996 showed the formulations consisting of special additives combinations of hindered amine light stabilizers (“HALS”) and phosphites improved the color-stability of polypropylenes after exposure to gamma radiation.  
         [0004]     The polypropylenes commonly contain a hindered phenolic antioxidant along with a secondary antioxidants such phosphites and thioesters as the processing stabilizers. However, upon exposure to gamma radiation, it exhibits undesirable color due to generation of color-bodies from the oxidized hindered phenolic type primary antioxidants. A non-hindered phenolic additive system for such applications is desired. Although addition of a phosphite prevents the discoloration, the phosphite is also the main source of causing black specks in the products during the end-use applications. Although thioesters are good processing as well as thermal stabilizers, they do impart odor, which is not acceptable for most medical uses. Hence, this invention relates to an additive composition that is free of both hindered phenolics, and secondary antioxidants such as phosphites and thioesters. It comprises a combination of a hindered amine light stabilizer and an amine oxide, or a combination of hindered amine light stabilizer and hydroxyl amine compounds that provided excellent color stability after exposure to gamma radiation up to 5 mrads. Also addition of clarifier such as NA-21 (Amfine Chemical Corporation) imparted excellent clarity.  
         [0005]     The present invention remedies the aforementioned deficiencies by achieving better color stability along with enhanced clarity and impact resistance. A polypropylene random copolymer (nominal MFRs and 9 and ˜25 dg/min @ 230 C/2.16 kg per ASTM D-1238) consisting of: I) a combination of hindered amine light stabilizer (“HALS”) and an amine oxide along with an acid neutralizer (i.e., metallic stearate) or II) a combination of HALS and hydroxyl amine along with an acid neutralizer was exposed to gamma radiation up to 5 mrads. The results indicated that HALS/amine oxide or HALS/hydroxylamine maintained excellent color stability, even showing very little increase in yellowness index after exposure to gamma radiation. Addition of a new clarifier NA-21(Amfine, Allendale, N.J.) imparted better clarity having less effect on tensile and impact properties unlike sorbitol based clarifier such as MILLAD® 3988 (Milliken Chemical Co, Spartanburg, S.C.). Addition of a metallocene catalyzed polyethylene polymer and Ziegler-Natta catalyzed polyethylene containing octene as a comonomer improved the impact strength of the polymer after being exposed to gamma radiation.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a blend useful as an additive in polyolefin polymers for minimizing the effects of radiation on the physical properties of said polymers, which comprises a hindered amine light stabilizer and at least one material selected from the group consisting of: i) amine oxides exemplified by the formula:  
                         
 
 in which R 1  and R 2  are each independently selected from C 10  to C 24  alkyl, aryl, or alkylaryl groups, whether straight-chain, branched, cyclic, saturated, or unsaturated; and ii) hydroxylamines exemplified by the formula:  
                         
 
 in which R 1  and R 2  are each independently selected from C 10  to C 24  alkyl, aryl, or alkylaryl groups, whether straight-chain, branched, cyclic, saturated, or unsaturated. 
 
     
    
     DETAILED DESCRIPTION  
       [0007]     Semi-crystalline polymers such as polypropylene are used in medical devices, and food packaging where these articles are frequently subjected to ionizing radiation for sterilization. It is known that exposure of polymers such as polypropylene to high-energy radiation i.e., electron beam or gamma radiation triggers radiation induced chemical reactions with predominant chain scission mechanism, resulting in loss of physical properties. Such property losses include embrittlement and discoloration, and are not acceptable to end-use applications.  
         [0008]     The present invention provides novel formulations of polypropylene compositions that are radiation resistant, color-stable and clear. In accordance with the present invention, polypropylene and propylene-ethylene copolymer compositions comprise of the following components in amounts equal to about: i) 0.01-0.2 wt % hindered amine light stabilizer (“HALS”); ii) 0.01-0.1 wt % amine oxide; iii) 0.01-0.2 wt % hydroxyl amine; iv) 0.01-0.3 wt % clarifier or nucleator; and v) 0.01-0.2 wt % acid neutralizer. The general chemical formulae for amine oxides and hydroxyl amines are illustrated as:  
                         
 
         [0009]     A combination (1:2) of amine oxide (GENOX® EP) and HALS or 1:1 blend of hydroxyl amine (FS-042) with HALS (CHIMASSORB® 944) more commonly known as IRGASTAB® FS 410 were thoroughly tested, and compared to those of prior art. Individual additives in various radiation-resistant formulations include: Naugard XL-1 (CAS #70331-94-1, 2,2′-oxiamidobisethyl 3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, Crompton Corporation); TINUVIN® 622LD (CAS #65447-77-0, Dimethyl succinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol; Ciba Specialty Chemicals); CHIMASSORB® 944LD (CAS #71878-19-8; N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,6-hexanediamine polymer with 2,4,6-trichloro-1,3,5-triazine and 2,2,4-trimethyl-1,2-pentaamine, Ciba Specialty Chemicals); GENOX® EP (CAS #204933-93-7; Dialkyl methyl amine oxide; Crompton Corporation); IRGASTAB® FS410 (1:1 blend of IRGASTAB® FS042 and CHIMASSORB® 944LD from Ciba Specialty Chemicals); DHT-4A (CAS # 11097-59-9; Synthetic hydrotalcite; Kyowa Chemical); Calcium Stearate (CAS # 1592-23-0; Crompton Corporation); PEP-36 (CAS #80693-00-1; Bis (2,6-di-tert-butyl-4-methylphenyl)pentaerythritol-di-phosphite; Amfine Chemical) ULTRANOX® 641 (CAS #161717-32-4; 2,4,6-tri-tert-butylphenyl 2, butyl 2ethyl 1,3prpane diol phosphite; Crompton Corporation); and WESTON® 619 (CAS #3806-34-6, Distearyl Pentaerythritol Diphosphite; Crompton Corporation).  
         [0010]     Additives useful for improving clarity and nucleation in polyolefin polymers include: MILLAD® 3988 (CAS # 135861-56-1; Bis 3,4,-dimethylbenzylidene sorbitol, from Milliken Chemical); HPN-68 (CAS # 351870-33-2, Proprietary inorganic salt from Milliken Chemical); NA-21 (Proprietary inorganic salt from Amfine Chemical); NA-11 (CAS #85209-91-2, Sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate; from Amfine Chemical); and KM-1500 (CAS #1309-43-4 and 68440-56-2; Magnesium Salt of disproportionated rosin acid; Mitsui&amp;Co).  
         [0011]     Impact Modifier additives useful for improving impact properties in polyolefin polymers include: ENGAGE® 8200 polyethylene (CAS #026221-73-8); DuPont Dow Chemical); Catalyst LL-1002.09 (CAS #25087-34-7, Ziegler-Natta Catalyzed Linear Low Density polyethylene with butene comonomer; ExxonMobil Chemicals); and Catalyst L8101 (CAS #26221-73-8, Ziegler-Natta Catalyzed Linear Low density polyethylene with octene comonomer; Huntsman Polymers Corporation, Odessa, Tex.)  
         [0012]     An unexpected result of the invention includes improved color stability, enhanced clarity, and improved impact resistance after being exposed to gamma radiation without use of conventional phosphite and thioesters. The present invention is exemplified by what is contained in the examples now presented, and shall not be construed as being limited thereby in any fashion.  
       EXAMPLE 1  
       [0013]     The formulations A1 through N1 are given in Tables 1 and 2. Several formulations had hindered amine light stabilizer (HALS)/phosphites. The phosphites used are ULTRANOX® 641 phosphite (Crompton Corporation, Middlebury, Conn.), WESTON® 619 phosphite (Crompton Corporation, Middlebury, Conn.), DOVERPHOS® S-9228T phosphite (Dover Chemical, Dover, Ohio), PEP-36 (Amfine, Allendale, N.J.). Also, clarifiers/nucleators such as MILLAD® 3988 (Milliken Chemical Spartanburg, SC, HPN-68 (Milliken Chemical, Spartanburg, S.C.), NA-21 (Amfine Chemical, Allendale, N.J.), and KM1500 (Mitsui Chemical) were added at specified levels. The hindered amine light stabilizers are TINUVIN® 622 HALS and CHIMASSORB® 944 HALS (Ciba Specialty Chemicals, Tarrytown, N.Y.). The amine oxide is GENOX® EP amine oxide (Crompton Corporation, Middlebury, Conn.). The impact modifier chosen was ENGAGE® 8200 polyethylene (DuPont-Dow). The specific formulations were pre-blended with un-stabilized polypropylene random copolymer powder (MFR ˜12 g/10 min), and then melt extruded by a Haake TW100 twin screw extruder at a processing temperature of 230° C. Test specimens were molded on a 120-ton Van Dom injection-molding machine under ASTM Conditions. Specimens were irradiated at 2.0 and 4.0 mrads by Isomedix, Whippany, N.J. using a  60 Co source. Control specimens (i.e., non-radiated) were included in each physical test for comparison. Yellowness Index was measured with a Colorgard/05 from BYK Gardner as per ASTM D-1925. Tensile properties were measured on Typ1-I injection molded tensile bars with an Instron 1125 universal testing machine with an initial grip separation of 2.5″ and an extension rate of 5″/min. Melt flow rate (MFR) was measured with a Kayness Galaxy I melt indexer as per ASTM-1238B. Multiaxial impact energy was measured by Dynatup 8250, using velocity of 4.3 m/sec and crosshead weight of 28 lbs (12.7 kg).  
         [0014]     Effect of Gamma Radiation on MFR: The % change in MFR after exposing the samples at 2 and 4 mrads are given in tables 1 and 2. Sample K1 (containing CHIMASSORB® 944 @ 0.15% and ULTRANOX® 641 @ 0.1%) had the least increase of 316.7% in MFR after 2 mrads of exposure, whereas sample F1 (control-containing 0.2% TINUVIN® 622+0.1% WESTON® 619) had the highest increase of 671.7%. At 4.0 mrads of exposure, sample M1 (0.15% CHIMASSORB® 944+0.1% PEP-36+2.5% ENGAGE® 8200) had the least increase of MFR, whereas Sample F1 (Control sample) had the highest increase in MFR of 1704.3%. Sample J1 (containing CHIMASSORB® 944 @ 0.1%, and GENOX® EP @ 0.05%) had the corresponding increase of 325% and 741.7% at 2.0 and 4.0 mrads of exposure, showing that it was relatively more stable compared to the control sample F1.  
         [0015]     In all tables herein, all amounts given in formulations are specified in percentages on a weight basis, based on the total weight of the compositions provided.  
                                                                                                                                                                                                                   TABLE 1                                       Sample ID                A1   B1   C1   D1   E1   F1   G1                    PP Powder (MFR ˜10 dg/min)   99.575   99.525   99.495   99.425   99.495   99.23   99.475       CHIMASSORB ® 944   0.15   0.15   0.15   0.15   0.15       0.15       TINUVIN ® 622                       0.2       ULTRANOX ® 641   0.1   0.1   0.1   0.1   0.1       0.1       DOVERPHOS ® S-9228T       Genox EP       Calcium Stearate   0.05   0.05   0.05   0.05   0.05   0.1   0.05       DHT-4A   0.025   0.025   0.025   0.025   0.025   0.05   0.025       Naugard XL-1                       0.07       Weston 619                       0.1       NA-21   0.1   0.15   0.18   0.1       Millad 3988               0.15   0.18   0.25   0.15       NA-11                           0.05            Physical Properties       at 0.0 Mrads            MFR (g/10 min)   12   11   11   11   11   9.2   12       % Strain at Yield   13.5   13.6   13.9   14   13.9   12.4   14.1       Stress at Yield, psi   4200   4220   4220   4270   4350   4570   4340       % Strain at Break   560   580   570   610   660   650   650       Stress at Break, psi   2760   2770   2780   2770   2850   2910   2860       Multiaxial Impact @ 23° C., in-   250(112)   278(114)   254(73)   52(29)   27(13)   21(4)   157(84)       lbs       Yellowness Index   4.5   4   3.9   4.6   4.8   4.2   4.8       % Haze-25 mil   8.5   7.8   7.3   8.3   8.6   10.7   9.1       % Haze-50 mil   23.2   21.6   20.7   22.5   21   22.6   21.1       Crystallization Temp (° C.)   118.3   119.6   119.4   119.7   119.9   120.9   121            at 2.0 Mrads            MFR (g/10 min)   51   55   56   63   59   71   55       % Strain at Yield   13.6   13.7   13.7   14.1   13.8   11.9   14       Stress at Yield, psi   4170   4190   4190   4230   4320   4570   4350       % Strain at Break   630   620   620   440   510   340   570       Stress at Break, psi   2720   2740   2730   2670   2760   2620   2770       Multiaxial Impact @ 23° C., in-   111   167   131   24   27   22   93       lbs       Yellowness Index   4.6   4.4   4.5   5.2   5.8   6.2   6.5            4.0 Mrads            MFR (g/10 min)   147   118   129   113   127   166   137       % Strain at Yield   14.4   14   14.2   13.6   14.2   13.1   13.9       Stress at Yield, psi   4150   4200   4190   4330   4470   4590   4400       % Strain at Break   690   660   640   330   340   170   270       Stress at Break, psi   2720   2690   2690   2640   2670   3410   2690       Multiaxial Impact @ 23° C., in-   55   85   43   18   15   15   35       lbs       Yellowness Index   5.4   4.8   5.4   5.7   7   6.5   7       % Change in MFR (@ 2 Mrads)   325   400   409.1   472.7   436.4   671.7   358.3       % Change in MFR (@4 Mrads)   1125   972.7   1072.7   927.3   1054.5   1054.5   883.3       Diff. In YI (2.0-0 Mrads)   0.1   0.4   0.6   0.6   1   2   1.7       Diff. In YI (4.0-0 Mrads)   0.9   0.8   1.5   1.1   2.2   2.3   2.2                  
 
         [0016]     Effect of Nucleator/Clarifiers: Since, gamma radiation on clarity of PP has negligible effect, the step-plaques (25/50 mils) were not subjected to gamma radiation. The % haze (for 25 mil plaque) of samples A1, B1, and C1 (containing NA-21 @ 0.1, 0.15, and 0.18% respectively) were 8.5, 7.8, and 7.3 respectively, showing very excellent enhancement in clarity. The corresponding % haze for 50 mil plaques were 23.2, 21.6, and 20.7 respectively, showing excellent clarity. The percentage of haze in samples E1 and L1 (Duplicate run—both containing Millad 3988 @ 0.18%) were 8.6 and 6.3 respectively; showing some variability. However, this might be attributed to the dispersion of MILLAD® 3988 (i.e., poor dispersion may cause higher % haze). Hence in a reactor grade PP, both NA-21 and MILLAD® 3988 imparted similar enhancement in clarity (i.e., reduction in % haze). Samples M1 and N1 (containing 2.5% and 5% ENGAGE® 8200) had the % haze (25 mil) of 8.2 and 9.3 respectively. This indicated that the modifier (i.e., ENGAGE® 8200) had very little effect on clarity. Combination of MILLAD® 3988/HPN-68 (@ 0.15%/0.05%) in samples H1, J1 and I1 imparted % haze values of 9.2, 7.4, and 7.5 units, measured on 25 mil thick plaques. Also, these three samples had crystallization temperatures almost 2° higher than that of sample C1 (containing NA-21 @ 0.18%). Higher crystallization temperature normally results in lower cycle time, and higher productivity.  
         [0017]     Effect of Gamma Radiation on Yellowness Index: The initial colors of samples A1 through N1 were in the range of 2.1-5.4 units. Sample H1 had the highest initial color of 5.4. The control sample F1 had the initial color of 4.2 units, 6.2 units @ 2 mrads, and 6.5 units @ 4 mrads, thus an increasing trend in yellowness index with increase of dosage of gamma radiation. Sample I1 (0.15% C-944+0.1% DOVERPHOS® S-9228T) had initial color of 4.3 @ 0.0 mrads, 3.9 units @ 2.0 mrads, and 3.9 units @ 4.0 mrads, thus showing slight decrease in yellowness index with increase in dosage of gamma radiation. Sample J1 (0.1% C-944+0.05% GENOX® EP had initial color of 4.6 units, 2.3 units @ 2 mrads, and 2.6 units @ 4 mrads, thus showing decrease in yellowness index with increase in dosage of gamma radiation. All other formulations showed an increase in yellowness index with increase of the dosage of gamma radiation.  
                                                                                                                                                                                                                   TABLE 2                                       Sample ID                H1   I1   J1   K1   L1   M1   N1                    PP Powder   99.475   99.475   99.575   99.475   99.495   99.875   99.375       CHIMASORB 944   0.15   0.15   0.15   0.15   0.15   0.15   0.15       TINUVIN 622       ULTRANOX 641   0.1   —   —   0.01   0.01   0.01   0.01       DOVERPHOS-9228T   —   0.01   —   —   —   —   —       GENOX EP   —   —   0.05   —   —   —   —       Ca stearate   0.05   0.05   0.05   0.05   0.05   0.05   0.05       DHT-4A   0.025   0.025   0.025   0.025   0.025   0.025   0.025       MILLAD 3988   0.15   0.15   0.15   0.15   0.18   0.15   0.15       HPN-68   0.05   0.05   0.05   —   —   0.05   0.05       KM-1500   —   —   —   0.05   —   —   —       PEP-36   —   —   —   —   —   0.1   0.1       EXXACT 8200   —   —   —   —   —   2.5   5            Physical Properties of Samples from above       at 0 megarads            MFR   12   12   12   12   12   11   12       % strain @ yield   14.3   14.2   14.2   14.3   3.9   14.9   15.4       Stress @ yield (psi)   4290   4250   4260   4320   4360   4020   3880       % strain @ break   630   610   620   620   700   590   630       Stress @ Break (psi)   2790   2800   2810   2820   2850   2750   2740       multiaxial impact @   257   301   285   278   318   334   325       23° C., in-lbs       yellowness index   5.4   4.3   4.6   4.6   5.9   3.7   2.1       % haze 25 mil   9.2   7.4   7.5   8.1   6.3   8.2   9.3       % haze 50 mil   21.6   19.6   18.9   20.5   14.3   20.6   23.7       crystallization temp.   121.5   122.5   122.5   120.1   120.9   121.9   122            at 2.0 megarads            MFR (g/10 min)   58   62   51   50   54   47   57       % strain@yield   14.2   14.2   14.2   13.8   14.1   14.5   15.3       stress@yield, psi   4280   4260   4280   43410   4400   4080   3880       % strain@break   480   320   640   530   670   580   580       stress@break (psi)   2700   2680   2800   2770   2870   2720   2690       multiaxial impact   131   106   201   160   155   291   307       @23° C., in-lbs       yellowness index   6.9   3.9   2.3   4.7   5.1   6.7   5.7            at 4.0 megarads            MFR (g/10 min)   118   107   101   129   116   91   117       % strain@yield   13.6   14   14.3   13.7   14 1   4.5   15.1       stress@yield, psi   4300   4290   4270   4320   4380   4060   3880       % strain@break   280   270   290   300   340   370   570       stress@break (psi)   2670   2640   2640   2650   2690   2610   2610       multiaxial impact @   58   45   37   50   31   243   294       23° C., in-lbs       yellowness index   7.7   3.9   2.6   6   6.2   5.1   4.5       % in MFR @ 2.0   383.3   416.7   326   316.7   350   327.3   375       megarads       % in MFR @ 4.0   883.3   791.7   741.7   975   866.7   727.3   875       megarads       in YI (0-2.0   1.5   −0.4   −2.3   0.1   −0.8   3   3.6       megarads)       in YI (2.0-4.0   2.3   −0.4   −2   1.4   0.3   1.4   2.4       megarads)                  
 
         [0018]     Effect of Gamma Radiation on Tensile Properties: % Strain @ yield was unaffected by increase in dosage of gamma radiation. % Strain @ break was affected to some degree with increase in dosage of gamma radiation. At 2.0 mrads of exposure, the losses in % strain at break of various formulations were in the range of 1.7%-47.5%; Sample F1 had the highest loss of % strain at break. M1 (containing 2.5% ENGAGE® 8200) had 1.7% loss of strain at break at 2.0 mrads and 37.3% loss at 4 mrads of exposure respectively. Sample N1 (containing 5% ENGAGE® 8200) had the corresponding loss of 7.9% and 9.5% at 2 and 4 mrads respectively. Hence, it is evident that addition of ENGAGE® 8200 (@ 2.5-5%) helped to maintain the % strain at break.  
         [0019]     Effect of Gamma Radiation on Multiaxial Inpact: Multi-axial impact of samples A1 through N1 were measured by 8250 Dynatup with an impact of 26 lbs under acceleration due to gravity. The initial impact values varied from 21 in-lbs to 334 in-lbs. It was evident that the samples containing MILLAD® 3988 had the least impact values compared to the samples containing NA-21. The loss of impact at 2 mrads were in the range of 0-55.6%. The loss of impact at 4 mrads were in the range of 9.5-89.4%, showing a higher degree of loss at this dosage of gamma radiation. Sample M1 (containing 2.5% ENGAGE® 8200) had a loss of ˜27% in impact; whereas sample N1 (containing 5% ENGAGE® 8200) had only 9.5% loss of impact. Hence, it is evident that addition of ENGAGE® 8200 helped to maintain impact properties at higher dosage of gamma radiation.  
       EXAMPLE 2  
       [0020]     In the second example, some of the formulations of example-1 were repeated. Here the base resin chosen was a random copolymer polypropylene, having melt flow rate of 25 gm/10 min. The formulations (A2-C2) are given in Table-3. Sample B2 contained modifier ENGAGE® 8200 @ 2.5%, and sample C2 contained a linear low density polyethylene available from Huntsman Polymers Corporation of Odessa, Tex. under the tradename of “LLDPE L8101”, which is an octene copolymer. All these formulations were pre-blended with an un-stabilized random copolymer having MFR ˜25 g/10 min, and then were compounded by a 2.5″ Davis Standard single screw extruder at a processing temperature (210° C.). The test specimens (prepared—as described in example-1) were irradiated at 2.5 and 5.0 mrads of gamma radiation by Isomedix, Whippany, N.J. All these samples were tested as described in example 1.  
         [0021]     The yellowness indices of these formulations had very minimal increase at 5 mrads of exposure vs. those of non-radiated samples, showing excellent color stability. Samples C2 had relatively lower increase in MFR at 5.0 mrads. This could be attributed to the presence of a LLDPE (L8101 @ 5 wt %) resulting in crosslinking. Addition of impact modifier such as ENGAGE® 8200 (even @ 2.5%) resulted in better impact energy than the control sample. Addition of L8101 @ 5 wt % did not provide much improvements in impact resistance.  
                                                                                                                                                                   TABLE 3                                       Sample ID                    A2   B2   C2                       PP Powder   99.55   97.05   94.55           CHIMASORB 944   0.15   0.15   0.15           GENOX EP   0.05   0.05   0.05           Ca stearate   0.05   0.05   0.05           DHT-4A   0.025   0.025   0.025           ENGAGE 8200   —   2.5   —           NA-21   0.175   0.175   0.175           L8101   —   —   5                        Physical Properties of Samples from above                Sample                A2   B2   C2                        at 0 megarads                MFR (g/10 min.)   26.8   26.9   23.8           % strain @ yield   13.8   14.2   14           Stress @ yield (psi)   4010   3800   3830           % strain @ break   650   640   370           Stress @ Break (psi)   2590   2500   2490           % haze 25 mil   9.8   12.1   20.5           % haze 50 mil   27.9   33.5   44.1           yellowness index   −0.11   −2.4   −1.44           multi-axial impact @ 23° C.,   60   121   66           in-lbs            at 2.5 megarads                MFR (g/10 min)   127   134   117           % strain@yield   13.9   14.7   14.2           stress@yield, psi   4020   3810   3830           % strain@break   480   640   370           stress @ break (psi)   2480   2480   2490           % haze - 25 mil   9.9   12.1   20.5           % haze - 50 mil   28   33.2   44.3           yellowness index   0.72   −1.25   0.38           multi-axial impact @23° C.,   52   91   42           in-lbs            at 5.0 megarads                MFR (g/10 min)   365   339   230           % strain@yield   14.4   15   15           stress @ yield, psi   3990   3810   3810           % strain @break   360   460   590           stress@break (psi)   2380   2370   2410           % haze - 25 mil   10   12.3   20.6           % haze - 50 mil   27.8   33.6   44.3           yellowness index   1.29   0.49   0.75           multi-axial impact @23° C.,   20   54   26           in-lbs                      
 
       EXAMPLE 3  
       [0022]     In this example, we have repeated some formulations as given in example 1 and also included FS410 as a primary stabilizer system (see Samples F3 and G3). The control formulation was sample D3. The additive formulations (shown in Table-4) with an un-stabilized polypropylene powder having initial MFR ˜25 dg/min were pre-blended and compounded by Haake TW100 twin screw extruder. The test specimens were irradiated at 2.5 and 5.0 mrads as done previously. The test specimens were tested as described in prior example 1.  
         [0023]     The control sample D3 had YI colors of −1.79 (@ 0.0 mrads), 5.39(@2.5 mrads), and 5.91 (@5.0 mrads). The samples B3 (containing CHIMASSORB® 944 and GENOX® EP along with ENGAGE® 8200) had the least YI value of −1.67 after being exposed to 5 mrads of gamma radiation. Also the samples (F3 and G3) containing FS410 exhibited much lower YI (1.09 and 0.54 respectively) at 5 mrads&#39; exposure. Hence, it is apparent that additive formulations consisting of either a combination of amine oxide and a HALS (i.e., CHIMASSORB® 994) or FS410 exhibited excellent color stability after being exposed to gamma radiation up to 5 mrads.  
                                                                                                                                                                                                                   TABLE 4                                       Sample ID                A3   B3   C3   D3   E3   F3   G3                        PP Powder   99.625   94.625   99.475   94.23   99.575   99.575   99.625       CHIMASORB 944   0.15   0.15   0.15   —   0.15   —   —       GENOX EP   0.05   0.05   0.05   —   0.05   —   —       FS 410   —   —   —   —   —   0.2   0.2       Ca stearate   0.05   0.05   0.05   0.1   0.05   0.05   0.05       DHT-4A   0.025   0.025   0.025   0.05   0.025   0.025   0.025       HPN-68   0.1   0.1   —   —   0.05   0.05   0.1       Millad 3988   —   —   —   0.25   —   —   —       NA-21   —   —   —   —   0.1   0.1   —       ENGAGE 8200   —   5   —   —   —   —   —       TINUVIN 622 LD   —   —   —   0.2   —   —   —       NAUGARD XL-1   —   —   —   0.07   —   —   —       WESTON 619   —   —   —   0.1   —   —   —       LL-1002.09   —   —   —   5   —   —   —            Physical Properties of Samples from above       at 0 megarads            MFR (g/10 min)   29   28   30   27   30   30   31       crystallization temp   125.8   125.1   121.9   122.3   125.8   125.1   121.9       (° C.)       % strain @ yield   13   14.3   13.7   12   14.1   13.9   12.9       Stress @ yield (psi)   4300   3890   4540   4440   4270   4240   4380       % strain @ break   630   440   730   370   720   770   690       Stress @ Break (psi)   2670   2560   2790   2650   2680   2770   2850       multiaxial impact @   42   225   29   80   61   40   29       23° C., in-lbs       yellowness index   −0.98   −3.67   0.36   −1.79   −0/34   0   −0.73       % haze 25 mil   10.7   15.2   5.8   16.8   8.9   9.2   11.4       % haze 50 mil   24.7   35.8   13.6   32.8   22.4   23   24.3            at 2.5 megarads            MFR (g/10 min)   127   123   129   152   129   112   107       crystallization temp   125.7   125.4   121.7   122.7   123.8   122.8   125.6       (° C.)       % strain@yield   13.4   14.3   13.6   11.9   13.8   13.2   12.6       stress@yield, psi   4280   3830   4540   4470   4270   4340   4350       % strain@break   200   300   460   320   610   600   610       stress@break (psi)   2500   2470   2650   2550   2570   2540   2540       multiaxial impact   26   77   22   32   33   30   30       @23° C., in-lbs       yellowness index   −0.13   −2.15   1.24   5.39   0.57   0.46   −0.27            at 5.0 megarads            MFR (g/10 min)   223   221   234   321   228   225   242       crystallization temp   125.4   124.9   121.8   121.2   123.1   121.6   124.9       (° C.)       % strain@yield   12.6   14.1   13.5   11.7   13.9   13.7   13.5       stress@yield, psi   4400   3820   4540   4460   4270   4250   4240       % strain@break   520   320   240   340   210   200   190       stress@break (psi)   2540   2440   2560   2540   2420   2450   2470       multiaxial impact @   16   50   16   24   24   21   28       23° C., in-lbs       yellowness index   0.38   −1.67   1.77   5.91   1.09   1.09   0.54                  
 
       EXAMPLE 4  
       [0024]     In this example, we have repeated some formulations as given in example 1 using a polypropylene copolymer having MFR ˜10 dg/min and also included FS410 as a primary stabilizer system (see Sample F4 in Table 4). The additive formulations with un-stabilized polypropylene powder having initial MFR ˜10 dg/min were pre-blended and compounded by Haake TW100 twin screw extruder. The test specimens were irradiated at 2.5 and 5.0 mrads of gamma radiation as done previously. The test specimens were tested as described in prior example 1.  
         [0025]     The properties of these formulations (sample A4-F4) are given in Table 5. The sample B4 (containing CHIMASSORB® 944/GENOX® EP plus NA-21) had YI of 0.47 units (@ 0.0 mrads), 1.3 units (@2.5 mrads) and 1.76 units at (@5.0 mrads). Sample F4 (containing FS410 and NA-21) had the corresponding YI of 1.34, 2.44 and 2.52 units. Thus it is again evident that CHIMASSORB® 944 with either GENOX® EP (amine oxide) or FS042 (a hydroxyl amine) provided excellent color stability at 5 mrads of gamma radiation. Also, addition of MILLAD® 3988 (a sorbitol based nucleator/clarifier in sample C4) showed slight increase in YI compared to other samples (i.e., sample A4 with HPN-68, Sample B4 with NA-21). Note that the specific concentrations of nucleators/clarifiers were chosen, because they are known to improve clarity/nucleation at these levels. It was also evident that addition of 10% ENGAGE ® 8200 (i.e., sample D4) resulted in higher multiaxial energy, compared to other formulations.  
                                                                                                                                                                                               TABLE 5                                       Sample ID                A4   B4   C4   D4   E4   F4                        PP Powder   99.625   99.575   99.55   89.525   99.5   99.575       MFR 9 dg/min.       Chimassorb 944, wt %   0.15   0.15   0.15   0.15   0.15   —       Genox EP, wt %   0.05   0.05   0.05   0.05   0.05   —       FS410, wt %   —   —   —   —   —   0.2       Calcium Stearate, wt %   0.05   0.05   0.05   0.05   0.05   0.05       HPN-68, %   0.125   —   —   0.075   0.1   —       NA-21, wt %   —   0.175   —   —   —   0.175       Millad 3988, wt %   —   —   0.2   0.15   0.15   —       Engage 8200, wt %   —   —   —   10%   —   —            Physical Properties       at 0 Mrads            MFR (g/10 min)   11   11   11   12   11   11       % Strain @ Yield   13.3   14.3   14.4   16.6   13.9   14.3       Stress @ Yield, psi   3990   4060   4110   3360   4240   4240       % Strain @ Break   670   660   620   830   660   650       Stress @Break, psi   2650   2680   2700   2500   2710   2710       Multi-axial Impact @23° C.,   117   346   190   319   140   96       (in-lbs)       Yellowness Index   0.47   0.5   1.52   −1.65   1.13   1.34       % Haze - 25 mil   24.7   7.9   5.4   19.5   11.5   6.3       % Haze - 50 mil   47.4   19.4   10.6   46.2   23.5   13.1            at 2.5 Mrads            MFR (g/10 min)   63   66   71   56   55   54       % Strain @ Yield   12.7   13.9   14.8   16.6   14.3   14.3       Stress @Yield, psi   4050   4150   4040   3370   4040   4170       % Strain @ Break   610   580   530   810   660   670       Stress @ Break, psi   2550   2600   2630   2430   2630   2620       Multiaxial Impact @ 23° C.,   81   215   136   311   80   82       in-lbs       Yellowness Index   1.3   1.52   2.55   0.69   2.35   2.44            at 5 Mrads            MFR (g/10 min)   72   115   95   99   103   103       % Strain @ Yield   13.56   14.3   14.3   16.1   13.8   14.8       Stress @ Yield, psi   3930   4040   4180   3520   4180   4080       % Strain @ Break   460   650   630   880   660   540       Stress at Break, psi   2540   2440   2560   2430   2580   2560       Multiaxial Impact @23° C., in-   29   128   36   301   30   42       lbs       Yellowness Index   1.76   1.86   2.87   0.59   2.21   2.52                  
 
         [0026]     Consideration must be given to the fact that although this invention has been described and disclosed in relation to certain preferred embodiments, obvious equivalent modifications and alterations thereof will become apparent to one of ordinary skill in this art upon reading and understanding this specification and the claims appended hereto. The present disclosure includes the subject matter defined by any combination of any one of the various claims appended hereto with any one or more of the remaining claims, including the incorporation of the features and/or limitations of any dependent claim, singly or in combination with features and/or limitations of any one or more of the other dependent claims, with features and/or limitations of any one or more of the independent claims, with the remaining dependent claims in their original text being read and applied to any independent claim so modified. This also includes combination of the features and/or limitations of one or more of the independent claims with the features and/or limitations of another independent claim to arrive at a modified independent claim, with the remaining dependent claims in their original text being read and applied to any independent claim so modified. Accordingly, the presently disclosed invention is intended to cover all such modifications and alterations, and is limited only by the scope of the claims which follow, in view of the foregoing and other contents of this specification.