Thermosetting Compositions

This invention relates to a thermosetting composition exhibiting superior physical properties as well as reduced blistering.

Various compositions useful for thermosetting molding have been described in the art. Typical, for example, is U.S. Pat. No. 3,821,158 which teaches the preparation of thermosetting compositions comprising a high vinyl butadiene polymer, polyethylene, an inorganic filler, and a free radical initiator.

To be commercially valuable, thermosetting molding compositions must have good impact resistance, hardness, shrinkage resistance, and both flexural and tensile strength. Furthermore, the physical appearance of thermosetting compositions is important to their commercial value. Such compositions which exhibit blistering or bubbling are highly undesirable. Not only do they detract from the appearance of the composition but they also represent a point of weakness since they typically propagate cracks upon impact.

While the prior art has provided thermosetting compositions with adequate physical properties and reduced blistering, ongoing research is still being conducted to find thermosetting compositions which exhibit both excellent physical properties and little or no blistering. It is therefore an object of this invention to provide a thermosetting composition exhibiting little or no blistering and excellent physical properties.

Other aspects, objects, and the several advantages of the present invention will become apparent from a reading of this specification and the claims.

Specifically, in accordance with the present invention there is provided an improved thermosetting composition consisting essentially of (a) a O.sub.2 or CO.sub.2 terminated high vinyl alkadiene polymer, said alkadiene having 4-8 carbon atoms per molecule with the polymer having at least 50 percent content of 1,2 bonded vinyl units, (b) an organic peroxide compound; (c) a polyolefin modifier; and (d) an inorganic filler selected from the group consisting of Group IIA metal carbonates, talc, mica, clay, wollastonite, aluminum trihydrate, and solid or hollow glass beads. This thermosetting composition exhibits reduced blistering as well as enhanced physical properties such as impact resistance and tensile strength.

The alkadiene polymers used in preparing the thermosetting compositions of this invention can be homopolymers of conjugated alkadienes containing 4-8 carbon atoms such as 1,3-butadiene, isoprene, 2-ethyl-1,3-butadiene, 1,3-pentadiene, and mixtures thereof. Also linear or radially branched copolymers of conjugated alkadienes containing 4-8 carbon atoms with vinyl arene comonomers containing 8-20 carbon atoms, such as styrene, alpha-methylstyrene, 3,5-diethylstyrene, 4-n-propylstyrene, 4-cyclohexylstyrene, 4-phenylstyrene, 3-benzylstyrene, 2,4,6-trimethylstyrene, 1-vinylnaphthalene, 8-phenyl-1-vinyl-napthalene and mixtures thereof can be employed.

The presently preferred alkadiene polymer is a linear homopolymer of 1,3-butadiene.

The above-cited alkadiene homo- and copolymers suitable for this invention have a number average molecular weight (Mn) ranging from about 500 to about 500,000, more preferably from 1,000 to 50,000. The vinyl content, i.e., the proportion of alkadiene units bonded together in the 1,2-mode of addition, thus forming vinyl pendant groups, preferably ranges from about 50 percent to about 100 percent, more preferably from about 60 percent to 100 percent (as quantitatively determined by infrared absorption spectroscopy at 11.0 micron, using a solution of approximately 2.5 grams of polymer in 100 milliliters of anhydrous carbon disulfide).

The alkadiene polymers used in the process of the present invention are terminated with either O.sub.2 or CO.sub.2 or mixtures thereof. In the present invention, a O.sub.2 or CO.sub.2 terminated alkadiene polymer is defined to be one wherein at least 10%, and preferably 25%, of the metallic cations such as Li.sup.+ or Na.sup.+, introduced by polymerization initiators such as n-butyl lithium, are removed from the live polymer by the addition of O.sub.2 or CO.sub.2 and subsequently polymer growth is terminated. In the present invention, gases will be used containing either O.sub.2 and CO.sub.2 or both.

The thermosetting compositions of this invention include an organic peroxide compound as a curing or crosslinking agent for the alkadiene polymer segments. Suitable organic peroxide compounds include diacyl peroxides, ketone peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, hydroperoxides, and peroxyketals, preferably containing up to about 30 carbon atoms per molecule. Exemplary organic peroxy compounds include

alpha,alpha'-bis(tert-butylperoxy)diisopropylbenzene;

2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane,

2,5-dimethyl-2,5-bis(tert-butyl peroxy)hexyne,

n-butyl-4,4-bis(tert-butylperoxy)valerate;

1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, and admixtures thereof. These organic peroxides are effective in crosslinking the composition when the composition is heated beyond the activation temperature, i.e., the decomposition temperature, of said peroxides. Alpha,alpha'-bis(tert-butyl peroxy)diisopropylbenzene is most preferred.

Inorganic fillers are included in the composition of this invention to enhance various properties of the moldings such as flexural modulus, strength, hardness, corrosion resistance, electrical properties, flame retardancy and flow, and to lower or reduce less desirable properties such as warpage, shrinkage and composition cost. Examples of suitable fillers are Group IIA metal carbonates, preferably calcium carbonate, talc, mica, clay, wollastonite, aluminum trihydrate and solid or hollow glass beads.

The polyolefins employed in the composition of this invention as shrink control agents have a molecular weight in the range of about 1,000 to 50,000. Examples of such modifiers are atactic polypropylene, polyethylene, polypropylene, polyisobutylene, polybutylene, ethylene-propylene copolymers, ethylene-butylene copolymers, ethylene-isobutylene copolymers, propylene-butylene copolymers and propylene-isobutylene copolymers, preferably atactic polypropylene and isobutylene.

The composition of this invention preferably contains an internal mold release agent to facilitate the removal of the cured composition from the mold. The materials commonly employed for this purpose are metal salts of long chain C.sub.10 to C.sub.20 fatty acids such as zinc stearate and calcium stearate.

Additionally, the composition may contain a coupling agent to enhance the properties of molded articles by promoting bonding and adhesion of the polymer to the filler and/or glass fiber reinforcement. The organic silanes and titanates which can be employed as coupling agents in accordance with the present invention include vinyltriethoxysilane, gamma-methacryloxypropyltrimethoxysilane, isopropyl trimethacryl titanate, isopropyl dimethacryl titanate, and the like, and mixtures thereof.

The composition of this invention optionally contains cross-linkable plasticizers such as the trimethyacrylate of trimethylolpropane, pentaerythritol acrylate, or other high boiling, low volatility, liquid hydrocarbons containing double bonds, which reduce viscosity during compounding but crosslink during curing. These cross-linkable plasticizers have the added advantage that they will not be lost by evaporation; thus, the compositions need not be stored in an air tight or sealed container. Although it is possible to use said plasticizers in compositions containing a liquid monomer, it is presently preferred to use said plasticizers or liquid monomers separately. It is presently preferred to use cross-linkable plasticizers, comprising polymethacrylates of polyglycols, to minimize viscosity while mixing compositions containing little or no liquid monomer.

The most preferred thermosetting compositions of this invention, having improved physical properties, and no or little blistering, will vary depending on the methods of compounding and their end uses but will generally be within the ranges of ingredients stated in the following table:

The ingredients of the thermosetting compositions of our invention can be mixed by any suitable mixing means such as a Brabender Plasticorder at a relatively low speed for a time sufficient to accomplish complete mixing and wetting of fillers and reinforcing fibers. Preferably the filler is added in two increments at a speed of about 100 r.p.m., whereas the fibers are added thereafter at a much lower speed, e.g., 20 r.p.m., until fully "wet out".

The thermosetting compositions of this invention are molded at an elevated temperature of about 150.degree.-200.degree. C. and a pressure of about 800-1500 psi for about 2-10 minutes, and preferably a temperature of about 180.degree. C., a pressure of about 1100 psi, for about 5 minutes.

The following examples further illustrate the present invention.

In this example the general procedure for preparing high-vinyl polybutadiene, terminated with various agents, is described. A jacketed, stirred reactor was charged with variable amounts of cyclohexane, tetrahydrofuran (THF; used as randomizer), 1,4-butadiene and, finally, n-butyllithium (NBL; used as initiator). The reactor was stirred for about 30-45 minutes at about 30.degree. C. Thereafter variable amounts of a terminating agent (2-propanol, ethylene oxide and propylene oxide in control runs; O.sub.2 (air) and CO.sub.2 in invention runs) were introduced so as to stop the polymerization reaction.

The terminated reaction mixture was then blown into a dry flask containing about 200 milliliters of cyclohexane, about 2 milliliters of isopropanol and a small amount of an antioxidant (usually BHT= 2,6-di-t-butyl-p-cresol). The polymer solution was finally desolventized by heating it in a rotating, titled film evaporator flask.

Polymerization conditions for preparing high-vinyl butadiene polymers terminated with different agents and pertinent polymer properties are summarized in Table II.

In this example the compounding of the high-vinyl butadiene homopolymers of Example I to form bulk molding compounds, conditions for molding them, and pertinent physical properties of these compounds after molding are described.

Compounding was carried out using a CW Brabender Plasticorder fitted with an unheated sigma-bladed mixing head. The high-vinyl polybutadiene, polymeric modifiers, and approximately 50% of the filler (CaCO.sub.3) were added initially, followed by the other additives. After 1-2 minutes, at a mixer speed of about 100 r.p.m., the remainder of the filler was added, and mixing was continued for about 5 minutes. The mixer speed was then reduced to about 10 r.p.m., and the glass fiber reinforcement was added. Mixing was continued only long enough (usually about 3 minutes) to wet out and disperse the glass fibers. Extended mixing at this stage caused attrition of the individual glass fibers and a resulting reduction in mechanical properties. Compounding recipes are summarized in Table III.

Molding was done using a Pasadena press fitted with a 6 inch.sup.2 polished positive pressure mold at about 177.degree. C./1100 psi for 5 minutes. Test specimens, were prepared by sawing molded plaques into 6.times.1/2.times.1/8 inch strips. Pertinent physical properties of molded compounds are summarized in Table IV.

Data in Table IV clearly show that molded compounds containing high-vinyl polybutadiene terminated with O.sub.2 (air) or CO.sub.2 exhibited no or very few surface blemishes (blisters), whereas the termination of the polybutadiene with an alcohol (isopropanol) or two epoxides (ethylene oxide or propanol oxide) resulted in severely blistered molded compounds. In addition, flexural and tensile properties of inventive compounds (runs 4 and 6) were higher than two control compounds (runs 1 and 3).

Reasonable variations and modifications are possible from the disclosure of the present invention without departing from the spirit and scope thereof.