Polybutylene and Conjugated Diene Butyl Polymer Blends

This invention relates to blends of polybutylene and a conjugated diene butyl polymer. More particularly, this invention relates to blends and cured products thereof formed from an admixture of polyethylene and a conjugated diene butyl rubber.

Products have long been sought which have resistance to set, i.e. are resilient and rebound promptly when compressed or stretched. At the same time these products must also usually exhibit good strength characteristics. It is also desirable that they be re-workable as a plastic, even after curing, so as to be remoldable or reshapable upon heating without destroying the properties of resistance to set and good strength. In short, the desired product would have, in addition to good strength, the resistance to set of a cured rubber and the reworkability of a thermoplastic polymer. Exemplary uses for such products include flexible automotive body and interior components, shoe soles and heels, molded electrical connectors, adhesives, sealants, wire and cable insulation, sporting goods equipment, tubing, hose, and flexible films and sheets.

Unfortunately, cured rubber is not truly a "re-workable thermoplastic" within the accepted meaning of this term since its melt index, to the extent that it exists, is too low to allow any significant reshaping of the product once cured (i.e. having a melt index significantly below about 0.01 gm/10 min.). Likewise, the commonly known olefinic thermoplastics are very low in resistance to set (i.e. exhibit sets of significantly greater than about 80%). Thus, neither, alone, satisfy the need in the art.

Attempts to blend the two types of materials in order to obtain a compromise combination of the desired properties; thus resulting in a product of some limited use, have been less than fully successful. For example, attempts at producing blends of butyl rubber and polypropylene or polybutylene were unsuccessful chemically, although a special process for dynamically forming and curing blends of such a nature is taught by U.S. Pat. No. 3,037,954. Cross-linking (curing) by irradiation was also unsuccessful in that polypropylene, polybutylene and butyl rubber undergo severe chain scissioning when subjected to ionizing radiation. It was known that polybutylene and polypropylene could be cured with various chemical curing agents. It was also known that blends of polyethylene and butyl rubber could be cured with a combination of a bi-functional aromatic compound having two nitrogen containing groups and a free radical catalyst such as a peroxide catalyst. The addition of these curing agents, however, are often less than desirable.

Butyl rubber, as referred to above, is a copolymer of the monomers isoprene and isobutylene and isomers thereof. Such a polymer is usually represented by the repetitive structure: ##STR1##

In U.S. Application Ser. No. 137,079 filed Apr. 3, 1980 (Division of Ser. No. 038,062 filed May 11, 1979), U.S. Pat. No. 4,264,490, entitled "Composition of a Polyethylene and Isobutylene Copolymer" and bearing an overlapping inventive entity herewith, there is disclosed a highly advantageous blend of polyethylene and a conjugated diene butyl polymer. The blend is cross-linkable by low dosage ionizing irradiation, bondable, and is heat recoverable without any significant loss of strength characteristics or seal. As such, it fulfills a very important need in the art. Unfortunately, this unique and otherwise advantageous blend is not re-workable as a thermoplastic once cured. Thus, a void is created in the art for a blend which has good resistance to set, good strength characteristics, and yet is also re-workable after curing.

This invention fills the above-described void or need in the art, as well as other needs apparent to the skilled artisan, once given this disclosure, by providing a composition comprising polybutylene and a conjugated diene butyl polymer. It has been found, quite surprisingly, that the admixing of these two materials act in such a way so as to provide a blend which is curable usually by ionizing radiation, and which when so cured, acts like a cured rubber to resist set, yet can be re-worked as a thermoplastic upon heating without significant degradation of properties.

The blends of this invention exhibit good strength characteristics and, in certain preferred embodiments, after curing by irradiation, have modulus strength (T.sub.100) properties superior to each of the individual components in the blend when measured alone. These blends need not be recured by either ionizing radiation or chemical means after reworking to maintain desirable properties. However, an additional post cure to obtain added strength or other desirable properties by either chemical means or ionizing radiation can be done using methods known in the art. In addition, the blends can be cured by ionizing radiation, usually at relatively low dosage levels (e.g. about 1-20 Mrads, and preferably about 2-4 Mrads), using well known radiation equipment without the use of any additive to enhance radiation response.

Generally speaking, articles formed from the aforesaid blends exhibit to a significant degree, when cured, those desired qualities measured by modulus strength (T.sub.100), tensile strength (U.T.S.), elongation (U.E.), melt index, and set. In the preferred embodiments of this invention these desired qualities fall within the following ranges:

(A) modulus of elongation (100%, T.sub.100, psi) at 72.degree. F. of about 150-1100 (or more);

(B) ultimate tensile strength (U.T.S. psi) at 72.degree. F. of about 450-3000 (or more);

(C) elongation (UE%) at 72.degree. F. of at least about 560;

(D) a melt index (gm/10 min. at 230.degree. C., 5 Kg) of about 1.times.10.sup.-2 -13 (or more); and/or

(E) a percent set of less than about 80% and usually about 25-75% (or less).

Generally speaking, any conventionally available polybutylene polymer may be employed in the practice of this invention. Preferably, the polybutylene is one of a relatively high molecular weight (e.g. about 3.times.10.sup.3 to 1.times.10.sup.7). Such preferred polybutylenes are generally synthesized primarily from butene-1 monomer and are flexible, crystalline isotactic polyolefins having a density of about 0.91 or more. The melt index of various commercially available grades of this polymer, acceptable for use in this invention range from about 0.4-20.0 (ASTM D-1238). The term "polybutylene polymer" includes copolymers of a butene monomer and one or more other monomers. However, the amount of such other monomer(s) should not be so large as to destroy the fundamental purpose of this invention (e.g. the basic, desired characteristics of the blend) as described above. The particular choice of a specific polybutylene will depend upon the intended end use of the blend and the characteristics desired thereof, or if overriding, the production or molding parameters of either the blend, the article, or the re-working process.

The radiation curable butyl polymer useful in the practice of this invention is commonly known as a conjugated diene butyl rubber (or polymer). A single conjugated diene butyl polymer or a mixture of two or more such polymers may be used in forming the blend. Of particular use in the practice of this invention are those conjugated diene butyl polymers which comprise a copolymer of isobutylene and a conjugated C.sub.5 diene. Of specific interest are those disclosed in:

1. U.S. Pat. No. 3,968,185

2. U.S. Pat. No. 3,816,371

3. U.S. Pat. No. 3,775,387

4. Baldwin et al. "Graft Curing With a Modified Butyl Rubber". Chemistry and Properties of Cross-linked Polymers, 1977 Academic Press, Inc., pg. 273-287.

5. "Conjugated Diene Butyl" New Products Technical Information, June 1976, Exxon Division, Linden, N.J.

The disclosures of these references are incorporated herein by reference.

Of specific interest in these references and particularly preferred for the purposes of this invention are those conjugated diene butyl polymers called "CDB" wherein the saturated hydrocarbon backbone is isobutylene and the conjugated diene structure is represented by: ##STR2## or mixtures thereof. Preferably the conjugated diene amounts to less than about 5 mole %, still more preferably, about 1-2 mole % of the structure and most preferably about 1.2-1.5 mole %. "CDB" is generally represented by the structures: ##STR3## or mixtures of (A) and (B); wherein N, M, P and R are of a sufficient finite number and total so that the conjugated diene falls within the above ranges.

A particularly preferred CDB is one produced by Exxon Corporation. This CDB is the product of the synthesis of isobutylene and isoprene subsequently halogenated (e.g. Cl or Br) and then dehydrohalogenated. In short, it is the dehydrohalogenation product of chlorobutyl as reported in the above-identified literature reference incorporated herein by reference. This CDB is represented structurally by formulae (A) and (B) above and is usually a mixture thereof. In the most preferred polymers useful in the practice of this invention the structure of formula (A) is primary (i.e. predominates). In addition, N, X, M, P, Y and R have the same values as the halogenated butyl used in the process. A process for preparing these polymers is taught by U.S. Pat. No. 3,968,185 referenced above and incorporated herein by reference.

The percentage of polybutylene and conjugated diene butyl polymer employed may vary over a wide range depending upon the characteristics sought or needed in the final product. Generally speaking, the blends of this invention find advantageous use in forming products which, after curing particularly with ionizing radiation, require good resistance to set (less than about 80%), high strength characteristics as defined hereinabove and re-workability (as exhibited by a melt index of at least about 0.1). The amount of each constituent employed will be varied to achieve the desired end properties. For most compositions contemplated, the blend of polybutylene and conjugated diene butyl polymer will include 90-10% by weight polybutylene and 10-90% by weight conjugated diene butyl polymer. In certain preferred embodiments the blend consists essentially of 50-10% by weight polybutylene and 50-90% by weight conjugated diene butyl polymer.

Other ingredients are optional and may be added to the blend for their known purposes. Such ingredients include, for example, pigments, fillers, antioxidants, plasticizers, and the like, all of which are conventional and known for their intended purposes. Generally speaking, they are added in amounts conventionally recognized, but adjusted where necessary, so as not to degrade or defeat the intended purposes of this invention. Such amounts usually do not exceed 50% by weight of the total blend and usually less than 10% by weight. Carbon black is one specific example of a filler. Others include such compounds as titanium dioxide, other known inorganic opacifiers, organic pigments, and inorganic coloring agents. An example of a useful antioxidant is one manufactured by Ciba Geigy and trademarked IRGANOX 1010. This antioxidant is a hindered phenolic having a molecular weight of 1178. It is, by chemical nomenclature, tetrakis [methylene 3-(3',5',-ditert-butyl-4'-hydroxyphenyl)propionate]methane whose structure is generally recognized as: ##STR4## wherein X is: ##STR5## The amount of this antioxidant use, usually does not exceed about 1%. Other antioxidants, stabilizers, or agents used to impart specific end use properties such as flame retardants or blowing agents to produce foam may also be used. These are employed in amounts well known to those skilled in the art.

The blends of this invention are made by mixing polybutylene and one or more types of conjugated diene butyl polymer with an antioxidant and/or processing stabilizer (e.g. IRGANOX 1010) in a conventional rubber mill, Banbury mixer, extruder, kneader or other known type of blending equipment, and blended at elevated temperatures to insure homogeneous mixing, e.g. at about 275.degree.-390.degree. F., preferably 300.degree.-335.degree. F.

The blend may then be cured either before or after forming it into the desired product or ultimate shape. Since the preferred blends contemplated by this invention are curable by ionizing radiation, they may be so cured. In the less preferred blends, curing may be accomplished by the addition of a known and conventional curing agent using recognized curing techniques associated with the agent so used. When ionizing radiation is used, it may be delivered by any conventional equipment such as an electron accelerator. Dosages will be chosen to suit the individual blend, but generally speaking, dosages in the range of about 1-20 Mrads and preferably 2-4 Mrads are used, and have been found adequate to achieve the desired degree of curing without any significant amount of scissioning occurring during the process.

Some typical examples of products formed from the blends of this invention include: athletic shoe soles, electrical connectors, sealing tapes and molded automotive bumper components. These products may be formed using conventional techniques. Where advantageous, curing may be accomplished after the product (article) is formed, rather than curing the blend. However, and as pointed out above, one of the features of this invention is that in many instances working may be done after curing, and without additional curing. Thus, in certain instances, the blend will be cured followed by article formation.

Re-working of the products formed by this invention is possible because of the melt flow properties of the blend. A typical and general procedure for re-working a cured product previously formed is:

1. grinding the molded part in a suitable commercial grinder (e.g. Cumberland) to small chips or particles suitable for easy handling during subsequent steps;

2. re-extruding: re-molding, etc., by conventional techniques; and

3. re-curing if necessary.

The following examples are presented for illustrative purposes:

Mixtures were prepared from 20 wt% polybutylene to 70 wt% polybutylene having a density of 0.910 and a melt index of 1.0 gm/10 min. This is a commercial material sold by Shell Chemical Co., as grade 1600A and 30 wt% conjugated diene butyl rubber to 80 wt% conjugated diene butyl having a diene content of 1.3 mole%, a chlorine content of 0.06 wt% and a molecular weight of 350,000-400,000 MV available commercially from Exxon Chemical Co.

Each of these blends was prepared in a rubber mill at a temperature of 325.degree. F. and pressed by compression mold at 350.degree. F. at 20,000 psi pressure into 6".times.6".times.0.060" pads. These pads were then irradiated using a 3 MeV Dynamitron.RTM. to a dose of 1, 2, 3 and 4 Mrads, an unirradiated pad was also evaluated. The modulus at 100% elongation (T.sub.100); ultimate tensile strength (U.T.S.); elongation, melt index and set are given in Table I.

As can be seen the strength at low elongation as given by T.sub.100 for the blends are unexpectedly higher than either of the components of the blend subjected to the same treatment.

In addition, the data in Table I show that these blends retain some of the beneficial characteristics of both the plastic and the rubber used to make the blend. It is seen from the melt index values that the blends retain plastic flow as the polybutylene phase, while the % set data indicate that the blends do not take a permanent set on stretching as the plastic (H). Hence, one obtains the high strength of the blends along with unique elastoplastic properties not associated with either component.

Mixtures I-L were formulated and tested in the same manner as mixtures A-H of Example I, except polypropylene was used in place of polybutylene.

The following mixture was formulated in the same manner as mixtures A-L in Examples I-II, except that high density polyethylene was employed instead of polybutylene or polypropylene.

The above material has a melt index of 0.1 gm/10 min. (at 230.degree. C.; 5 kg) initially at a dose of 3.0 Mr. Re-working this material by grinding and a subsequent milling for 4 minutes at 320.degree. F. shows that the melt index drops to zero for all practical purposes. An attempt was also made to remold this material by compression molding at 350.degree. F. and 20,000 psi pressure. The particles would not flow to fill the mold and form a 6".times.6".times.0.060" pad.

Once given the above disclosure, many other features, modifications and improvements will become apparent to the skilled artisan. Such other features, modifications, and improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims: