Flame-Retarded Thermoplastic Polyester Compositions Having Improved Arc Resistance and Process for Preparing Same

This invention relates to flame-retardant thermoplastic compositions having improved dielectric properties and a process for the preparation thereof. More particularly, it pertains to compositions comprising a high molecular weight linear polyester, a flame-retardant, novaculite, and, optionally, a reinforcing agent and a process for improving the arc track resistance of reinforced, flame-retardant linear high molecular weight polyester compositions.

High molecular weight linear polyesters and polymers of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.

With the development of molecular weight control, use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection modlable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.

One useful family of such compositions comprises those which are reinforced, e.g., with from about 10 to about 40% of filamentous glass, based on the weight of glass and polyester components, and rendered flame-retardant by the incorporation of a flame-retarding amount of a flame-retarding component.

Another useful family of such compositions comprises those containing poly(1,4-butylene terephthalate), a polycarbonate resin, a flame-retardant and, optionally, a reinforcing agent.

Experience has show, however, that, typically, articles molded from these afore-described polyester compositions are characterized with poor arc resistance properties.

For example, the high voltage track rate, as measured according to Underwriter's Laboratory Bulletin 494, of a 30% glass-reinforced, non-flame-retardant poly(1,4-butylene terephthalate) composition is 0.50 to 0.87 inches per minute. The addition of a flame-retardant to this same 30% glass reinforced poly(1,4-butylene terephthalate) composition results in a track rate of 8.2 to 11.6 inches per minute. The higher rate indicates less desirable arc track resistance.

It has now been surprisingly discovered that the incorporation of a particulate silica filler, i.e., novaculite, improves the high voltage arc track resistance of reinforced or unreinforced flame-retarded polyester resins while not adversely affecting the flame-retardancy or the other excellent and desirable physical properties of the composition.

According to this invention, there are provided thermoplastic compositions with improved arc track resistance useful for molding, e.g., injection molding, compression molding, transfer molding, and the like, comprising:

(a) a high molecular weight linear polyester resin;

(b) a flame-retarding amount of a flame retardant;

(c) novaculite; and

(d) optionally, a reinforcing agent.

In a further embodiment of the present invention a process is provided for increasing the arc track resistance of reinforced flame-retarded thermoplastic compositions, said process comprising admixing novaculite with a high molecular weight linear polyester resin, a flame-retardant and, optionally, a reinforcing agent.

The high molecular weight normally flammable linear polyesters used in the practice of the present invention are polymeric glycol esters of terephthalic acid and isophthalic acids. They are available commercially or can be prepared by known techniques such as by the alcoholysis of esters of the phthalic acid with a glycol and subsequent polymerization, by heating glycols with the free acids or with halide derivatives thereof, and similar processes. These are described in U.S. Pat. No. 2,465,319 and U.S. Pat. No. 3,047,539, and elsewhere.

Although the glycol portion of the polyester can contain from two to ten carbon atoms, it is preferred that it contain from two to four carbon atoms in the form of linear methylene chains.

Preferred polyesters will be of the family consisting of high molecular weight, polymeric glycol terephthalates or isophthalates having repeating units of the general formula ##STR1## wherein n is a whole number of from two to four, and mixtures of such esters, including copolyesters of terephthalic and isophthalic acids of up to about 30 mole percent isophthalic units.

Especially preferred polyesters are poly(ethylene terephthalate) and poly(1,4-butylene terephthalate). Special mention is made of the latter because it crystallizes at such a good rate that it may be used for injection molding without the need for nucleating agents or long cycles, as is sometimes necessary with poly(ethylene terephthalate).

Illustratively, high molecular weight polyesters will have an intrinsic viscosity of at least about 0.4 deciliters/gram and preferably, at least 0.7 deciliters/gram as measured in a 60:40 phenol-tetrachloroethane mixture at b 30.degree. C. At intrinsic viscosities of at least about 1.1 decilliters/gram, there is a further enhancement in toughness of the present compositions.

Polyester compositions which are also within the scope of the present invention comprise reinforced and unreinforced poly(1,4-butylene terephthalate) and a polycarbonate resin. It has been found that molded articles made from poly (1,4-butylene terephthalate) can be made substantially resistant to warpage or distortion, among other processing advantages, if a polycarbonate resin is added to the polyester molding composition in amounts, for example, of from about 5 to about 50 parts by weight of the polyester composition.

Thus, these polycarbonate resins which are useful in the practice of this invention are normally slow-burning and non-flame retardant and have recurring units of the formula ##STR2## wherein each --R-- is selected from the group consisting of phenylene, halo-substituted phenylene and alkyl-substituted phenylene, and X and Y are each selected from the group consisting of hydrogen, hydrocarbon radicals free from aliphatic unsaturation and of radicals which, together with the adjoining ##STR3## atom, form a cycloalkane radical, the total number of carbon atoms in X and Y being up to 12.

The preferred polycarbonate resins may be derived from the reaction of bisphenol-A and phosgene. These polycarbonates have from 20-400 recurring units of the formula: ##STR4##

The polycarbonates are described in U.S. Pat. Nos. 3,028,365, 3,334,154 and 3,915,926, all of which are incorporated herein by reference. The polycarbonate should have an intrinsic viscosity between 0.3 and 1.0, preferably from 0.3 to 0.45 as measured at 20.degree. C. in methylene chloride.

The amount of flame-retardant additive used in the polyester composition herein is not critical to the invention, so long as it is present in minor proportion based on said composition--major proportions will detract from physical properties--but at least sufficient to reduce the flammability of the polyester resin. Those skilled in the art are well aware that the amount will vary with the nature of the resin and with the efficiency of the additive. In general, however, the amount of additive will be from 0.5 to 50 parts by weight per 100 parts of resin. A preferred range will be from about 3 to 50 parts and an especially preferred range will be from about 8 to 45 parts of additive per 100 parts of resin. Synergists, e.g., antimony oxide, will be used at about 2 to 10 parts by weight per 100 parts of resin.

Any of the conventional flame-retardants, such as decabromodiphenyl ether, can be employed in the compositions of the present invention. Preferred flame retardants are aromatic carbonate homopolymers having repeating units of the formula: ##STR5## wherein R.sup.1 and R.sup.2 are hydrogen, (lower) alkyl or phenyl, X.sup.1 and X.sup.2 are bromo or chloro or mixtures thereof and m and r are from 1 to 4. These materials may be prepared by techniques well known to those skilled in the art. Also preferred are aromatic carbonate copolymers in which from 25 to 75 weight percent of the repeating units comprise chloro or bromo-substituted dihydric phenol, glycol or dicarboxylic acid units. See, e.g., A. D. Wambach, U.S. Pat. No. 3,915,926, above-mentioned. A particularly preferred flame retardant herein is an aromatic (copoly-) carbonate of 50:50 mole ratio of bisphenol A and terrabromobisphenol A prepared like Procedure A of U.S. Pat. No. 3,915,926.

Moreover, the flame retardants used herein, such as the aromatic polycarbonate flame retardants, can also be used with a synergist, particularly inorganic or organic antimony compounds. Such compounds are widely available or can be made in known ways. In preferred embodiments, the type of antimony compound used is not critical, being a choice primarily based on economics. For example, as inorganic compounds there can be used antimony oxide (Sb.sub.2 O.sub.3); antimony phosphate; KSb(OH).sub.6 ; NH.sub.4 SbF.sub.6 ; SbS.sub.3 ; and the like. A wide variety of organic antimony compounds can also be used, such as antimony esters with organic acids; cyclic alkyl antimonites; aryl antimonic acids and the like. Illustrative of the organic antimony compounds, including inorganic salts of such compounds, are KSb tartrate; Sb caproate; Sb(OCH.sub.2 CH.sub.3).sub.3 ; SB(OCH(CH.sub.3)CH.sub.2 CH.sub.3).sub.3 ; Sb polymethylene glycolate, triphenyl antimony, and the like. Especially preferred is antimony oxide.

The novaculite filler used in the present invention is a variety of natural quartz and is distinguished from other silicas. More particularly, novaculite is a naturally occurring fine-grained platy microcrystalline variety of quartz, i.e., silicon dioxide (SiO.sub.2) and is not to be confused with silicas which are amorphous, such as silica gel, colloidal silica, fumed silica, precipitated silica, etc.

Novaculite is an exceptionally pure microcrystalline form of quartz and is found primarily in and around the Devonian-Mississippian deposits of Hot Springs, Ark. Under the petrographic microscope, the grains of quartz are seen to possess smooth, very slightly curved surfaces. The particle shape of novaculite is generally square or rectangular in outline and in three-dimensional aspect might be designated as pseudo-cubic or thrombohedronic.

Uniquely, novaculite has a minute grain size and individual crystals are usually between about 0.01 and 0.02 millimeter with a maximum of about 0.1 millimeter in size. The index of refraction of novaculite is about 1.550 and it is insoluble in hydrochloric acid.

It has been surprisingly found herein that when minor amounts of novaculite are added to flame-retarded reinforced or unreinforced polyester compositions, the arc resistance of articles molded therefrom is vastly improved. In general, the amount of novaculite required to enhance the arc resistance of the polyester composition is in the range of from about 5 to about 50 percent of the total composition with a preferred range from about 20 to about 30 percent of the total composition.

Moreover, it has been found that both treated and untreated novaculite is capable of improving the arc resistance of the polyester compositions herein. Thus, novaculite, which has been treated with a silane coupling agent such as an organosilane or organotitanate is capable of improving the arc resistance of the polyester compositions herein. Typical organosilane coupling agents include .gamma.-aminopropyltriethoxy-silane, .gamma.-aminopropylphenyldimethoxy-silane, .gamma.-propionamidotriethoxysilane, N-triethoxysilypropyl-N(.beta.-amino ethyl) amine, .gamma.-glycidoxypropyltrimethoxysilane, 3,4-epoxycyclohexyl-ethyltrimethoxysilane, .gamma.-methacryloxypropyltrimethoxysilane, vinyltrichlorosilane, vinyltrimethoxysilane. Organotitanates include isopropyl tri(diisooctylphosphato)titanate.

The novaculite may be pretreated with the coupling agent and then blended with the polyester composition or the coupling agent can be simply blended with the novaculite and polyester and other composition ingredient in the same mixing step. Amounts of coupling agent used, whether the novaculite is pretreated or blended with the coupling agent range from about 0.1% to 5%, preferably 1% by weight of the total composition.

The improved flame-retarded composition of the present invention may also, optionally, contain a reinforcing agent. Although it is only necessary to have at least a reinforcing amount of the reinforcement present, in general the reinforced compositions will comprise from 10 to 80% by weight of the total composition of the reinforcing agent.

The reinforcing agents employed herein are well known but illustratively, they may be selected from the group consisting of metals, such as aluminum, iron or nickel particles and the like, and non-metals, such as carbon filaments, silicates, such as acicular calcium silicate, asbestos, titanium dioxide, potassium titanate and titanate whiskers, wollastonite, glass flakes and fibers.

In particular, preferred reinforcing agents and fillers are of glass, and it is usually preferred to employ fibrous glass filaments comprised of lime-aluminium borosilicate glass that is relatively soda-free. This is known as "E" glass. However, other glasses are useful where electrical properties are not important, e.g., the low soda glass known as "C" glass. The filaments are made by standard processes, e.g., by steam or air blowing, flame blowing and mechanical pulling. The filament diameters range from about 0.00012 to 0.00075 inch, but this is not critical to the present invention. Glass fibers may be surface coated in accordance with standard procedures to improve their reinforcing performances. In general, best properties will be obtained from reinforced compositions that contain from about 15 to about 30 percent by weight of the glass reinforced composition.

The length of glass filaments and whether or not they are bundled into fibers and the fibers bundled, in turn, to yarns, ropes or rovings, or woven into mats, and the like, are also not critical to the practice of the invention. In preparing the present compositions, it is convenient to use the filamentous glass in the form of chopped strands of from about 1/8 inch to about 1 inch long, preferably less than 1/4 inch long. In articles that are molded from the compositions of the invention, even shorter lengths will be encountered because, during compounding, considerable fragmentation will occur. This is desirable, however, because the best properties are exhibited by thermoplastic injection molded articles in which the filament lengths lie between about 0.000005 inch and 0.12 (1/8 inch).

The process herein by which the novaculite is incorporated into the flame-retarded polyester composition to improve the arc resistance of the same can vary among a wide variety of procedures. For example, in one way, the novaculite, the reinforcing agent, if used, and the flame-retardant are all put into an extrusion compounder with the polyester resin to produce molding pellets. The novaculite, the reinforcing agent, and flame-retardant are dispersed in a matrix of the polyester resin in the process. In another procedure, the novaculite, the reinforcing agent, if used, and flame-retardant are mixed with the polyester resin by dry blending, then either fluxed on a mill and comminuted, or they are extruded and chopped. The novaculite, the reinforcing agent, if used, and flame retardant can also be mixed with the powdered or granular polyester and directly molded, e.g., by injection or transfer molding techniques.

It is always important to thoroughly free all of the ingredients, novaculite, resin, reinforcement, flame-retardant and any other optional, conventional additives from as much water as possible.

In addition, compounding should be carried out to ensure that the residence time in the machine is short; the temperature is carefully controlled, the friction heat is utilized; and an intimate blend between the novaculite, the resin, and the flame-retardant is obtained.

Although it is not essential, best results are obtained if the ingredients are pre-compounded, pelletized and then molded. Pre-compounding can be carried out in conventional equipment. For example, after carefully pre-drying the polyester resin and flame retardant, e.g., under vacuum at 125.degree. C. for 4 hours, a single screw extruder is fed with a dry blend of the polyester resin, flame retardant, novaculite and reinforcing agent, if used, the screw employed having a long transition metering section to ensure proper melting. On the other hand, a twin screw extrusion machine, e.g., a 28 mm Werner Pfleiderer machine can be fed with resin and additives (e.g., novaculite and flame-retardant) at the feed port and the reinforcement downstream. In either case, a generally suitable machine temperature will be about 450.degree. to 570.degree. F.

The pre-compounded composition can be extruded and cut up into molding compounds such as conventional granules, etc., by standard techniques.

The compositions of this invention can be molded in any equipment conventionally used for glass-filled thermoplastic compositions. For example, with poly(1,4-butylene terephthalate), good results will be obtained in an injection molding machine, e.g., of the Newbury type with conventional cylinder temperature, e.g., 450.degree. F. and conventional mold temperatures, e.g., 150.degree. F. On the other hand, with poly(ethyleneterephthalate), because of the lack of uniformity of crystallization from interior to exterior of thick pieces, somewhat less conventional but still well-known techniques can be used. For example, a nucleating agent such as graphite or a metal oxide, e.g., ZnO or MgO can be included and standard mold temperature of at least 230.degree. F. will be used.

In order that those skilled in the art may better understand how to practice the present invention, the following examples are given by way of illustration and not by way of limitation.

A dry blend of 41.8% by weight poly(1,4butylene terephthalate), about 0.9 intrinsic viscosity, 13% by weight aromatic (copoly-) carbonate of 50:50 mole ratio of bisphenol-A and tetrabromobisphenol-A, 5% by weight antimony oxide, 20% by weight fibrous glass, 20% by weight untreated novaculite (Novacite, sold by Malvern Minerals Company), 0.15% by weight Irganox 1093 (Ciba-Geigy, tetrakis (3-(3',5'-di-t-butyl-4-hydroxyphenyl) propionyloxymethyl)methane) and 0.05% Ferro 904 (diphenyl decylphosphite) is compounded and extruded. The extrudate is pelletized and injection molded. The molded sample is treated for electrical arc resistance, as measured by the ASTM D-495 test procedure and found to be about 80 seconds. Other physical properties are:

Notched Izod Impact, ft. lbs./in.--0.75

Unnotched Izod Impact, ft. lbs./in.--6.7

Flexural Strength, psi--21,000

Tensile Strength, psi--12,000

Flame-retardancy--V-O

A similar composition but not containing the novaculite filler exhibits an arc resistance of about 15 seconds.

Four compositions, the same as that prepared in Example 1, except that treated novaculite is substituted for the untreated novaculite, are prepared and molded. The composition containing a sulfonylazide silane-treated novaculite, namely, Novakup 3076 (sold by Malvern Minerals Company) exhibits an arc resistance of about 90 seconds. The second composition containing gamma-glycidoxy propyltrimethoxysilane treated novaculite, namely, Novakup 187 (sold by Malvern Minerals Company), exhibits an arc resistance of about 75 seconds. The third compositions containing gamma-aminopropyltriethoxysilane treated novaculite, namely, Novakup 1100 (sold by Malvern Minerals Company), exhibits an arc resistance of about 85 seconds. The fourth composition containing isopropyltri(diisooctylphosphato)titanate treated novaculite, namely, Novakup TTOP-12 (sold by Malvern Minerals Company), exhibits an arc resistance of about 80 seconds. Other physical properties of these compositions are:

Notched Izod Impact, ft.lbs./inch--0.8

Unnotched Izod Impact, ft.lbs/inch--17.5

Flexural Strength, psi--23,000

Tensile Strength, psi--14,000

Flame-retardancy--V-O

It is too be understood that changes may be made in the particular embodiments of the invention described which are within the full intended scope of the invention as defined by the appended claims.