Abstract:
A thermoplastic elastomer composition is disclosed, comprising a halobutyl rubber cured by dynamic vulcanization with a multifunctional amine curing agent. The cure is effected in the presence of at least 20 parts of a mixture formed from 10 to 60 parts of a polyolefin and 1 to 90 parts of a thermoplastic elastomer. The parts based on 100 parts by weight of said rubber.

Description:
This application is a CIP of application Ser. No. 100,754 filed Sept. 24, 1987, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a halobutyl-based dynamically vulcanized thermoplastic elastomer which can be used in the pharmaceutical industry for products such as vial stoppers, blood tube stoppers, dropper bulbs and syringe tips. The product does not employ a zinc containing component. 
     BACKGROUND OF THE INVENTION 
     Elastomeric composition in which butyl rubber is dynamically vulcanized in the presence of polyolefins such as polypropylene have resulted in thermoplastic compositions which have been proposed for use for most applications where thermoplastic elastomers would be of advantage. However, many of the products produced by the dynamic vulcanization of butyl rubber in the presence of materials such as polypropylene have not resulted in products which are suitable for vial stoppers, blood tube stoppers and dropper bulb applications, for example, in the pharmaceutical industry. Because of extremely high quality control and concern for the integrity of the product contained in pharmaceutical containers, thermoplastic elastomers have been slow to be accepted as candidates for container closures, vials, stoppers, dropper bulbs and the like. The product must have mechanical strength and set resistance properties, but also must provide moisture and oxygen barriers while not allowing undesirable materials to be extracted from the product. Until the present time, such a material has not been developed which has all of the required properties for use succesfully in the pharmaceutical industry. 
     U.S. Pat. No. 4,130,534 describes a thermoplastic composition containing polyolefin resins and cross-linked butyl rubber. The product is formed by dynamic vulcanization, in which the rubber is the continuous phase prior to cross-linking but becomes the dispersed phase after the cross-linking step. 
     U.S. Pat. No. 4,501,842 describes an adhesive in which a halogenated butyl rubber is combined with a styrene/ethylene butylene/styrene block thermoplastic polymer (SEBS) and other materials to form an effective adhesive. Zinc oxide is used as a strength increaser. 
     U.S. Pat. No. 4,593,062 describes the combination of polyolefins, halogenated butyl rubber and polychloroprene in which these rubbers are dynamically vulcanized in the presence of the polyolefin. This patent contains an extensive description of the dynamic vulcanization process. 
     U.S. Pat. No. 4,607,074 combines polypropylene, a zinc oxide cured halogenated butyl rubber, and a second rubber, preferably EPDM. Various curing agents are suggested for curing the rubber during dynamic vulcanization, including accelerators which assist the zinc oxide cure. 
     U.S. Pat. No. 4,616,052 describes elastomeric compositions which are used as stoppers and syringe plunger tips. The principal component of the product is a dynamically vulcanized ethylene-propylene-diene terpolymer and polypropylene mixed with butyl rubber in the range of perhaps 5 to about 20 percent. A hindered amine is present to act as a stabilizer. 
     U.S. Pat. No. 4,616,064 describes a mixture of polyolefins, SEBS polymers and materials such as polysiloxanes. 
     One particular method for preparing halogenated butyl rubbers through isomerization is described in U.S. Pat. No. 4,634,741. All of the cure processes for these butyl rubbers are based upon a zinc oxide cure system. 
     U.S. Pat. No. 4,639,487 describes a different system using a butyl rubber mixed into an ethylene copolymer resin which is dynamically vulcanized. The preferred curing agent contains zinc oxide. 
     None of the systems described above are capable of providing the ideal composition for use in the pharmaceutical industry. Accordingly, it is an object of this invention to provide a thermoplastic elastomer composition which is suitable for use as a vial stopper, blood tube stopper, dropper bulb or syringe tip in the pharmaceutical industry and which is useful in other appications where low hardness and a good barrier to moisture and oxygen is needed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a thermoplastic elastomer composition in which a halobutyl rubber is cured by dynamic vulcanization. A multifunctional amine curing agent is employed. The cure is effected in the presence of a mixture formed from polyolefins and thermoplastic elastomers. These compositions have properties which are highly desirable for use in the pharmaceutical industry, and are zinc free. 
     Specifically, the present invention comprises a halobutyl rubber which is cured by dynamic vulcanization, using a multifunctional amine curing agent, such that the cure is effected in the presence of at least 20 and preferably from about 20 to about 100 parts of a mixture formed from about 10 parts to 60 parts of a polyolefin and about 1 to about 90 parts of a thermoplastic elastomer. All of the parts are based on 100 parts by weight of the halobutyl rubber. 
     The preferred polyolefins are polypropylene and polyethylene. The preferred thermoplastic elastomers are thermoplastic, elastomeric block copolymers having terminal polystyrene units. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As is noted in U.S. Pat. No. 4,634,741, one of the first olefinically unsaturated synthetic elastomers to be commercially produced ws butyl rubber. The expression butyl rubber is broadly understood to be the product made from a copolymerization of isobutylene and isoprene. For the purpose of this invention, any of the butyl rubbers which are commercially available may be used in the composition of this invention. Halobutyl rubbers are those butyl rubbers which have been halogenated with either chlorine or bromine. Again, the commercially available halogenated butyl rubbers are suitable for use in the present invention. 
     The concept of dynamic vulcanization of butyl rubbers and polyolefins is extensively described in U.S. Pat. No. 4,130,534 to Coran et al, the disclosure of which is incorporated herein. Described therein are definitions of butyl rubber and halogenated butyl rubber. Also described are various thermoplastic olefin resins such as polyethylene, and polypropylene, among others. Polypropylene is the preferred polyolefin of the present invention. Based upon 100 parts of the halobutyl rubber, the amount of polyolefin will range from about 10 to about 60 parts by weight. Most preferred are about 15 to about 30 parts polypropylene per 100 parts of rubber. Also preferred is polyethylene in the same proportions. 
     The thermoplastic elastomer of the present invention, which forms the final component of the thermoplastic elastomer is present in an amount ranging from about 1.0 to about 90 parts of thermoplastic elastomer based upon 100 parts of the halobutyl rubber. Preferred is from about 18 to about 50 parts of thermoplastic elements. A most preferred amount is from about 15 to about 35 parts by weight. The thermoplastic elastomer is combined with the polyolefin to form a mixture which forms the continuous phase during the dynamic vulcanization of the halobutyl rubber. Prior to crosslinking, the butyl rubber is the continuous phase and the polyolefin and thermoplastic elastomers are added. During crosslinking of the halobutyl rubber, it becomes dispersed and, upon vigorous mixing, becomes particulate throughout the continuous phase of the mixture of polyolefin and thermoplastic elastomer. Normally, from about 25 to about 160 parts of mixture is needed per 100 parts of rubber. 
     Various thermoplastic elastomers may be employed with success in the present invention. Particularly preferred are thermoplastic elastomeric block copolymers having terminal polystyrene units. Most preferred are the ethylene, butylene, block copolymers which have terminal polystyrene units and which functional as thermoplastic elastomers. These polymers are commercially available from Shell Chemical Company under the trademark KRATON G. Block copolymers described in U.S. Pat. No. 3,686,364 and U.S. Pat. No. 33,865,776 are typical examples of the block copolymers contemplated by the present invention. In the latter patent, assigned to Shell Oil Company, copolymers are described as having ethylene butylene central blocks of from 50 to 80 percent by weight of the copolymer molecule in approximately equal proportions of ethylene and butylene. The terminal blocks are polystyrene. These ethylene, butylene block copolymers having terminal polystyrene units are referred to by the shorthand designation of SEBS copolymers. SEBS copolymers are the preferred thermoplastic elastomer which is to be used to form a mixture prior to dynamic vulcanization of the halobutyl rubber. Other thermoplastic elastomeric block copolymers with terminal polystyrene units are also preferred. Among these are butylene polymers and isobutylene polymers, referred to by SBS and SIS. These are also available under the trademark KRATON. 
     Another class of thermoplastic elastomers which is useful in the present invention is the group generally known as olefinic thermoplastic elastomers. These products are available commercially. 
     A third class of thermoplastic elastomers which is useful in the present invention is based on a multiplicity of recurring short-chain ester units and long-chain ester units joined through ester linkages. These polyester resins are available, for example, from the DuPont Company under the tradename HYTREL. 
     There are, of course, other thermoplastic elastomers which may be employed in the present invention. All that is necessary is that the thermoplastic elastomer be suitable for forming a continuous, though not necessarily homogeneous, phase in a mixture with the polyolefin during the dynamic vulcanization stage. 
     The dynamic vulcanization of the halobutyl rubber is carried out with a multifunctional amine curing agent. The curing agent, containing a diamine function, is very active, normally considered far too fast for straight vulcanization. Any diamine function which is slow enough to be mixed into the system and fast enough to react during dynamic vulcanization is appropriate. Stated another way, if a diamine curing agent can be incorporated into the system, it can operate to dynamically vulcanize the halobutyl rubber. One preferred curing agent is hexamethylene diamine carbamate, which is blocked diamine having carbon dioxide blocking the amine function to form a carbamate. When heated, carbon dioxide is driven off to immediately cause a reaction during the dynamic vulcanization. Materials of this type are available under the tradename DIAK. DIAK is manufactured by the DuPont Company. 
     The amount of curing agent will be that amount necessary for a complete cure of the halobutyl rubber and will broadly range from less than one part to more than 6 parts of curing agent per 100 parts of halobutyl rubber. For bromobutyl rubber, the diamine can range from 1 phr to 3 phr, with a preferred range of 1.6 to 2.5 phr. For chlorobutyl rubber, 2 to 6 phr has been effective, and 3 phr to 5 phr is preferred. The exact amount of diamine will be sufficient to effect the cure without detrimental effects of overcure. 
     A number of experiments were performed to demonstrate the efficacy of the present invention. In one series of experiments, dynamic vulcanization of bromobutyl rubber was effected using 2 parts per 100 parts of bromobutyl rubber of a hexamethylene diamine carbamate curing agent. The amount of polyolefin and thermoplastic elastomer were varied, and various properties were measured. Specifically, compression set, 100% modulus, tensile strength, and hardness were measured. Presented below in Table 1 are the results of those tests. 
     
                       TABLE I______________________________________Experi- Polypro-         Com-  100%ment  pylene   SEBS    pres- modulus                               Tensile                                     HardnessNo.   phr      phr     sion set                        psi    psi   Shore A______________________________________1     25       33      25    282    1285  552     22       33      16    280     725  533     18       33      18    253     615  524     10       10      15    348     480  505     20       20      22    575    1070  656     40       40      31    645    1700  777     60       60      37    770    2080  858     80       80      47    720    2250  869     50       30      41    730    1580  8210    50       10      33    845    1830  8611    30       50      51    490     810  6712    10       50      70    225     390  4913    50       --      33    840    1900  86______________________________________ 
    
     Various other components making up the continuous phase mixture were evaluated and results are summarized in Table II below. Specifically, polyethylene and polypropylene were both evaluated as the polyolefin, based on 100 parts of bromobutyl rubber and 2 parts of DIAK 1 curing agent. Various thermoplastic elastomers were also used at a 40 parts per 100 parts of rubber level. SEBS was used with both polypropylene and polyethylene. Also, tested were two polyesters. 
     
                       TABLE II______________________________________Experi-  Polyolefin   TPE        Compres-                                 Tensilement No.  phr          phr        sion set                                 psi______________________________________14     polypropylene, 40               SEBS, 40   31     170015     polyethylene, 40               SEBS, 40   40     80016     polypropylene 40               Polyester, 40                          39     85017     polypropylene, 40               Polyester, 40                          38     900______________________________________ 
    
     The experiments shown below in Table III demonstrate that both bromobutyl and chlorobutyl rubber may effectively be employed in the process of this invention. In both cases, a mixture of 40 parts of polypropylene and 40 parts of SEBS were employed as a continuous phase. Dynamic vulcanization was effected using 2 parts of DIAK 1 for bromobutyl and 5 parts DIAK 1 for the chlorobutyl experiment. 
     
                       TABLE III______________________________________                              Ten-Experi-  Rubber    Polyolefin,                      Hardness,                              sile,                                   Compres-ment No.  100 parts and TPE   Shore A psi  sion set______________________________________18     bromobutyl            80 parts  77      1700 3119     chlorobutyl            80 parts  70      1100 51______________________________________ 
    
     As shown in Table IV below, in which the amount of hexamethylene diamine carbamate was varied. For bromobutyl rubber, a preferred range is 1.6 to 2.5 parts of the multifunctional amine such as shown in Table IV. 
     
                       TABLE IV______________________________________Experi-  DIAK 1   Tensile   Compres-                              Hardnessment No. phr      psi       sion set                              Shore A______________________________________20       1.0      520       65     6221       1.5      520       76     5622        1.67    970       23     6623       2.0      1250      22     7024       2.2      1010      23     66______________________________________ 
    
     Shown below in Table V are experiments using 2 parts per 100 parts of rubber of 4 curing agents, except for experiment 28 which used 2.5 phr. 
     
                       TABLE V______________________________________Experi- Curing agent              Tensile   Compres-                               Hardnessment No.   at 2 phr   psi       sion set                               Shore A______________________________________25      DIAK 1     1700      31     7726      DYTEK A    1400      37     7427      Tetraethylene              1600      40     75   pentamine28      Polyethylene              1850      37     76   polyamine   (2.5 phr)______________________________________ 
    
     Finally, presented below in Table VI are various pharmaceutical properties and functionally of a preferred embodiment of the present invention which was shown in Experiment 1 in Table I. This product compares favorably with a standard commercially available butyl based thermoset product manufactured by The West Company. Thermosets, of course, cannot be reprocessed. 
     
                       TABLE VI______________________________________Pharmaceutical Closure Properties Comparison              Invention                      Standard______________________________________Compression set, %   25        13Autoclave set, %     80        14Moisture vapor transmission (g/m.sup.2 /day)                0.4       0.65Oxygen transmission (cc/m.sup.2 /day)                287       214pH shift             -0.3      -0.3Turbidity (Nephelos units)                7.0       2.0Reducing agents (ml of 0.01 N I)                0.01      0.01Total extractables (mg)                1.4       0.6Heavy metals, ppm lead and zinc                0.0       0.0Toxicity, acute systemic                non       nonToxicity, intracutaneous                non       nonToxicity, cytotoxicity                non       nonCoefficient of friction                47        65Needle penetration, mg                364       342Reseal               acceptable                          acceptableFreezer leaks        none      noneVacuum retention, in Hg                21        21Swelling, sesame oil, %                5         16Swelling, cotton oil, %                5         10Swelling, mineral oil, %                91        82______________________________________ 
    
     Presented in Table VII are additional experiments on which other thermoplastic elastomeric block copolymers having terminal polystyrene units are shown to be very effective embodiments of the present invention. Both SBS and SIS demonstrated effectiveness in preparations which are useful thermoplastic elastomer composition. All of these products had 100 parts of bromobutyl rubber and 2 parts of DIAK 1 curing agent. 
     
                       TABLE VII______________________________________Experi- Polypro-          Com-  100%   Ten-ment  pylene   TPE      pres- modulus                                sile HardnessNo.   phr      phr      sion set                         psi    psi  Shore A______________________________________29    40       SBS, 40  56    570    1400 7530    40       SIS, 40  59    590    1360 75______________________________________ 
    
     As can be seen, a variety of useful compositions can be prepared according to the present invention, particularly for use in the pharmaceutical industry. The above formulations have been made into pharmaceutical products such as vial stoppers, blood tube stoppers, dropper bulbs, syringe tips and the like.