Compositions for Preparing Cement-Adhesive Reinforcing Fibers

This invention relates to a modifier polyolefin composition containing a reactive silicon (IV) group which renders polyolefins highly adherent for silicic surfaces. When compounded with a polyolefin, the modifier composition of the invention provides a polyolefin resin from which cement adherent reinforcing fibers may be prepared which suitably substitute for asbestos fibers as a fibrous reinforcement for cement structures. Alternatively, the modifier composition may be applied as a coextruded surface coating to polyolefin fibers or as a surface layer to a polyolefin film from which fibers are prepared by fibrillation. The invention also relates to the cement adherent polyolefin fibers so prepared and to cementitious articles prepared with such cement adherent polyolefin reinforcing fibers.

Fibrous reinforcement is a well known method for improving the physical properties of cement and concrete structures. Asbestos fiber-reinforced cements have been widely employed as building materials. Asbestos-reinforced cements and concretes have long been used for the production of pipes, corrugated boards, roofing slates, machine foundations, storage tank walls, reactors, aircraft runways, roads, pilings and many other high strength articles. The type of asbestos which is satisfactory as fibrous reinforcement for cement is available in limited quantities. It is probable that the deposits of such workable asbestos will be exhausted relatively soon. Further, asbestos is now known to have carcinogenic effects. The carcinogenic properties of asbestos has lead to governmental regulations controlling and limiting its use.

Of the fibers currently used as an asbestos fiber replacement for cement reinforcement, polyacrylonitrile and polyvinyl alcohol fibers are preferred because they combine high fiber strength with good adhesion to a cement matrix. Unfortunately, both are expensive materials and significantly increase the cost of producing fiber reinforced cement structures.

A variety of other less-expensive materials have been considered for production of cement reinforcement fibers to replace asbestosfibers. Steel fibers have been tried and found inadequate because they suffer from chemical attack by the alkaline cement environment. Various polymeric fibers have also been found to be inadequate. Glass and polyester fibers degrade due to the alkaline environment of the cement matrix. Nylon and cellulose fibers have been found to be too water sensitive to be successfully used.

Ideally, polyolefin fibers could be employed as an asbestos replacement for reinforced cement. Polyolefin fibers possess good inherent properties, such as alkaline resistance, good stiffness and tensile strengths and are relatively inexpensive. Unfortunately, a major obstacle to the use of polyolefins as a cement reinforcing fiber material is their inherent lack of affinity towards an alkaline mineral matrix. Further, polyolefin fibers are less dense than a cement slurry; that low density in combination with polyolefin's lack of affinity for aqueous systems allows polyolefin fibers to float to the slurry surface. Such poor dispersibility of polyolefin fibers results in poor reinforcement of a finished cementitious article. The inclusion in the resin formation of a dense filler such as CaCO.sub.3, talc and the like is sometimes practiced to make the fibers less buoyant and to improve their dispersion.

Extensive efforts have been devoted to preparing polyolefin fibers, particularly polypropylene fibers, in a form which permits them to be successfully used as a replacement for asbestos fibers for reinforcement of cement and concrete structures.

Methods for fabricating polypropylene fibers in a way as to provide physical anchoring sites along the fibers for mechanical attachment of the cement matrix have been tried, as illustrated by U.S. Pat. Nos. 4,261,754; 4,414,030 and 4,477,522. European Patent Application 0 026 581 discloses that such fibers may be prepared by fibrillation of a polyolefin film which has been pretreated with a coupling agent such as an acid anhydride or methacryloxypropyltrimethoxysilane. Other methods involve the addition of certain thickening and clay additives to the cement slurry to hold the polypropylene reinforcing fibers in dispersion until the slurry sets, as illustrated by U.S. Pat. Nos. 4,363,666 and 4,428,775. Still another method to improve flocculation of polypropylene fibers when mixing with cement is to treat the cement-fiber mix with a water-soluble or emulsifiable polymer and a polyvalent salt such as Al.sub.2 (SO.sub.4).sub.3, as illustrated by U.S. Pat. No. 4,339,273.

Other procedures designed to render polypropylene fibers suitable as a cement reinforcing material involve the physical modification of the fiber surface by various means. To produce cement adherent fibers U.K. Patent Application No. 2,030,891 teaches a method for embedding inorganic powers in the surface of fibrillated polypropylene. Japanese Patent Publication No. 60 060 960 applies a fine aggregate to the fiber surface by means of a radiation hardenably epoxy binder to produce a cement adherent polypropylene reinforcing fiber.

Still other methods chemically treat the surface of polyolefin fibers to render the fiber surface more adherent to cement. Such methods include treatment of the polyolefin fibers with an aqueous dispersion of colloidal alumina or silica in conjunction with a chlorinated polypropylene, as taught by Japanese Patent Publication No. 7319849; a non-ionic or cationic polymer agglutinating agent such as a polyethylene oxide or poly(alkylaminoacrylate) as shown by Japanese Patent Publication No, 60 081 052; or a solution of an alkali or alkaline earth metal (bi) carbonate as disclosed in Belgium Patent No. 899,810.

Yet others have suggested chemical modifications of the base polyolefin from which the fibers are produced. Hence, U.K. Patent Application No. 2,021,552A states that an inorganic or organic acid group should be incorporated in the base polyolefin, either by copolymerization of acid monomers or by grafting acid groups to a prepared polyolefin, in order to improve the adherence to cement of fibers made of modified polyolefin. Polypropylene grafted with maleic anhydride is illustrated as an example of an improved material for producing cement reinforcing fibers. Japanese Patent Publication No. 49 036 748 also illustrates the use of maleated polypropylene as a material for producing cement reinforcing fibers. Another approach, as discussed in German Offenlegungsschrift DE 3341 462 Al, treats the polyolefin under elevated temperature and pressure with a solution of silane or SiCl.sub.4 together with silicic acid or a metal silicate and thereafter precipitates the reaction product as fibrils by lowering the temperature. The fibrils so produced are useful as asbestos fibers substitutes for reinforced cement structures.

Others have suggested that the surface of polyolfin fibers be catalytically reacted with reactive organic or inorganic Si (IV) compounds to render them adhesive to cement. Among the silanes disclosed as suitable for a method of this type are silicic acid anhydrides, organic halosilanes, and silicate esters as discussed in European Patent Application No. 0 051 256 and German Offenlegungsschrift DE 32 10693 Al.

To date, polyolefin fibers have been made compatible as reinforcement fiber for cement only by incorporating additional compatibilizing agents into the cement slurry into which the fibers are admixed, by the special fibrication of the fibers to provide physical anchoring sites therein for mechanically anchoring to the cement matrix, by the chemical modification of the base polyolefin from which the fibers are then produced, or by the chenical modification of the fiber surfaces by costly and time consuming chemical reactions.

Ideally an agent could be found which, when admixed in small quantities with a polyolefin stock material, would allow the direct production of cement adherent fibers therefrom by conventional fiber production techniques. To date, no such additive has been disclosed by the art.

Compositions which render polyolefins highly adherent to silicic surfaces have been discovered which can be compounded with an olefinic polymer to provide a polyolefin resin from which cementadherent fibers may be directly produced by conventional fiber fabricating techniques. The compounds comprise a polyolefin linked to a highly reactive Si (IV) moiety. The compounds may be produced by melt or solution blending of the components or in some cases by dry blending the components. A preferred modifier composition comprises the reaction product which results upon the high shear dry blending of an alkylamino alkoxysilane with an acid modified polyolefin. A particularly preferred modifier composition is the dry blend reaction product of gamma-aminopropyltriethoxysilane with a maleated polypropylene.

The preferred modifier composition may be dry blended in small amounts with an olefin homopolymer or copolymer to provide a polyolefin resin stock from which cement adherent fibers may be directly produced by conventional fiber extruding, spinning or fibrillation techniques. Alternatively, the modifier composition or a dilution thereof may be coextruded as a surface layer upon an olefin polymer film and cement adherent fibers may be produced from said film by fibrillation techniques.

Fibers produced from a polyolefin resin which incorporates a modifier composition of the invention are adherent to cements and concretes and may be used as a substitute for asbestos fibers for the fibrous reinforcement of cement and concrete articles. Polyolefin reinforcing fibers produced in accordance with the invention impart superior strengths to finished reinforced cement articles as compared to articles reinforced with fibers produced only from an acid modified polyolefin.

FIG. 1 is the force-displacement curve obtained upon a bending test is a horizontal configuration of a concrete test bar reinforced with 4 volume percent polypropylene fibers containing a modifier composition in accordance with the invention.

FIG. 2 is the force-displacement curve obtained upon a bending test in a horizontal configuration of a concrete test bar reinforced with 4 volume percent polypropylene fibers not containing any modifying composition of any type.

FIG. 3 is the force-displacement curve obtained upon a bending test in a horizontal configuration of a concrete test bar reinforced with 4 volume percent polypropylene fibers containing an amount of maleated polypropylene equal to the amount of modifier composition contained by the fibers used in reinforce the concrete test bar of FIG. 1.

Any polyolefin may be rendered suitable for production of cement adherent fibers by the addition thereto of a modifying agent of the invention. Examples of suitable polyolefins are polyethylene; polypropylene; polybutene; olefinic copolymers such as ethylene-butene-1 or propylene-ethylene copolymers; olefinic terpolymers such as propylene-ethylene-butene-1 terpolymers; or mixtures and blends of the olefinic polymers. In view of its good intrinsic physical and chemical properties, polypropylene and its copolymers is a preferred material for the production of concrete adhesive reinforcing fibers.

The modifying agent which is compounded with a polyolefin to render it adhesive to cement comprises a compound with the following structure: ##STR1## wherein P comprises a polyolefin segment with degree of polymerization greater than about 50 and desirably in the range of about 50 to about 1,000, preferably in the range of about 100 to about 1,000, and most preferably in the range of about 100 to about 300; is 0 or 1; R.sup.1 is an alkylene group having from about 1 to 10 carbon atoms, arylene or alkyl substituted arylene group having 6 to 20 carbon atoms, oligomeric siloxane group having from 1 to 10 Si atoms, alkylene amide or imide, arylene amide or imide, alkylarylene amide or imide, alkylene or arylene sulfonyl amine, alkylene or arylene ester, alkylene or arylene carbonate with the alkylene or arylene components of the foregoing being in the ranges of 1 to 10 and 6 to 20 carbon atoms, respectively, or other linkages required to affix the silicic moiety to the polyolefin chain. R.sub.2, R.sub.3 and R.sub.4 are each independently hydrogen, halogen, hydroxyl, alkyl or alkoxy groups with the proviso that at least one of R.sub.2, R.sub.3, and R.sub.4 must be either a hydroxy, halogen, or alkoxy group. Preferably R.sub.2, R.sub.3 and R.sub.4 are each a halogen, hydroxy or an alkoxy group. Examples of the effective compositions include: the graft copolymer of vinyl triethoxy silane and polyethylene or polypropylene; the nitrene insertion reaction product of a trimeth- oxyazidosilane with polypropylene or polyethylene; the condensation copolymer of 3-(trimethoxysilyl)-1-propanol and maleated polypro-pylene or polyethylene; reaction product of 3-aminopropyldimethyl-ethoxysilane with an acid modified olefin polymer. A preferred composition of the modifier is the reaction product of an alkylamino alkoxysilane with an acid modified olefinic polymer.

The preferred modifying agent which is compounded with a polyolefin to render it adhesive to cement comprises a dry blend reaction product of an alkylamino alkoxysilane with an acid modified olefinic polymer, copolymer or terpolymer. An alkylamino alkoxysilane is defined herein to mean a Si(IV) compound of the following structure: ##STR2## wherein R.sub.1 is an alkylene having from about 1 to about 10 carbon atoms and preferably from about 3 to about 8 carbon atoms or an alkyl arylene group having from about 6 to about 20 carbon atoms, and R.sub.2, R.sub.3 and R.sub.4 are each independently hydrogen, alkyl or alkoxy groups with the proviso that at least one of R.sub.2, R.sub.3, and R.sub.4 must be an alkoxy group and R.sub.5 is hydrogen, alkyl, aryl or a substituted version thereof. Preferably R.sub.2, R.sub.3 and R.sub.4 are each an alkoxy group. An acid modified polyolefin as defined herein means an olefinic homopolymer, copolymer or terpolymer which contains from about 0.4 wt% to about 8.0 wt% acid functional groups as free carboxylic acid or as carboxylic acid anhydride.

Surprisingly, it has been found that the amino group of an alkylamino alkoxysilane when mixed by high shear dry blending with an acid modified polyolefin will react with the acid functionality of the modified polyolefin without further requirements of a catalyst, solution or extremes of temperature. The resulting modifier composition comprises an alkoxysilane amide or imide adduct of a polyolefin which, in accordance with the invention, may inexpensively be prepared in bulk quantities with readily available dry blend mixing equipment.

A polyolefin resin for production of cement adherent fibers may readily be prepared by blending the modifier composition with an olefinic polymer. The modifier is intimately and homogeneously admixed with the base polyolefin compound to form a resin from which fibers are subsequently produced. Fibers produced from such modified polyolefin resin exhibit a rougher surface texture than fibers produced from a polyolefin not containing the modifier agent. This is believed to be due to the migration of the modifier agent from the polymer matrix of the fiber core to the surface of the fiber during fiber forming or film production from the resin. This migration or blooming phenomena is believed to account for the fact that a small amount of the modifier agent may be incorporated into a polyolefin resin yet significantly increased adhesion of fibers produced therefrom towards cement is still achieved.

The acid modified polyolefins which may be employed to produce the modifier agents of the invention are those acid modified poly alpha-olefins or mixtures thereof; such as acid modified polyethylene, polypropylene, polybutuene-1, and olefinic copolymers. The acid modified polyolefins are known materials and may be prepared by known procedures, as disclosed for example in U.S. Pat. No. 2,973,344. The unsaturated alpha-beta carboxylic acids and acid anhydrides useful for forming the acid modified polyolefins are exemplified by acrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, methacrylic acid, crotonic acid, isocrotonic acid, maleic anhydride, itaconic anhydride, citraconic anhydride, and himic anhydride. Preferably, to insure that the modifier agent produced from such acid modified polyolefins will readily migrate or bloom to the surface of a polyolefin fiber, the polyolefin portion of the acid modified polyolefin used to produce the modifier desirably has a number average molecular weight of from about 2,500 to about 50,000 and desirably 2,500 to about 30,000. If the number average molecular weight of the polyolefin portion of the acid modified polyolefin exceeds about 50,000 there is a reduction in effectiveness probably associated with the reduced mobility of the larger molecule. Of course, acid modified poly-olefins having a number average molecular weight in excess of 50,000 may be used to prepare modifiers of the invention, but in such case it is desirable to incorporate such higher number average molecular weight modifier composition into the base polyolefin as a coextruded layer upon a polyolefin film from which fibers are subsequently produced by fibrillation.

Alternatively, if the acid modified polyolefin is a polyethylene, it may be produced by the copolymerization of ethylene monomer with from about 0.1 to about 8 weight percent of an acid olefin comonomer. Examples of copolymer suitable as the acid modified polyolefin include copolymer of ethylene with acrylic acid, methacrylic acid, itaconic acid, malonoic acid and methylmethacrylic acid.

The acid content of the acid modified polyolefin may range from about 0.4 to about 8.0 weight percent. As noted, the number average molecular weight of the polyolefin component, particularly wherein the polyolefin component is polypropylene, would desirably be from about 2,500 to about 50,000 and desirably 2,500 to about 30,000, and most desirably be from about 5,000 to about 10,000. The content of the acid component should preferably be selected to provide on the average from one to to acid groups per polymer chain. Although an acid modified polyolefin may be used which has an acid content which exceeds on the average two acid groups per polymer chain, generally no significant additional benefit is observed from the excess amount of acid component. The preferred acid modified polyolefin is a maleic anhydride modified polypro-pylene, such as a 5000 number average molecular weight polypropylene with an acid number of about 40, are marketed by Eastman Chemicals as Epolene-43.

Examples of alkylamino alkoxysilanes which may be used to prepare the preferred modifier compositions of the invention included the classes of alkylamino alkyldialkoxysilanes exemplified by a deltaaminoalkyl-methyldimethoxy silane and delta-aminoalkyl-methyldiethoxysilane such as delta-aminobutyl-methyldimethoxysilane; gamma-aminoalkyl trialkoxysilanes as exemplified by gamma-aminopropyl triethoxysilane; N-alkyl substituted gamma-aminoalkyl dialkoxysilanes such as N-ethyl-gamma-aminopropyl methyldimthoxysilane or N-alkyl substituted gamma-amino alkyl trialkoxysilanes such as N-ethyl-gamma-aminopropyltriethoxysilane. The class of aminoalkyl trialkoxysilanes is preferred with gamma-aminopropyltriethoxysilane as the compound of choice.

The preferred modifier composition is prepared by dry blending a suitable acid modified polyolefin, preferably in powder form, with from about 50 to about 150%, and preferably from about 50 to about 100% of the stoichiometric amount of alkylamino alkoxysilane required to react with the acid functionality of the acid modified polyolefin. Wherein the acid modified polyolefin is a maleated polypropylene such as Epolene-43, it is preferably blended with from about 3 to about 5 weight percent of the alkylamino alkoxysilane, preferably gamma-aminopropyl triethoxysilane. Dry blending is preferably performed by a high shear mixing method such as is obtained in a double-arm mixer, or a horizontal stationary cylindrical mixer. Frictional heat produced by such dry blending is sufficient to initiate the occurrence of reaction between the amino functionality of the silane compound and the acid functionality of the acid modified polyolefin. The heat of reaction aids in maintaining the course of the amidation or imidation reaction during dry blending. Upon completion of the dry blending operation, it is preferred to maintain the powder mixture at a slightly elevated temperature from about 30.degree. to about 90.degree. C., and preferably about 60.degree. C., for several hours to ensure the completion of reaction. Another process route to effective compositions is the free radical initiated grafting of a silicon containing vinyl compound onto a polyethylene or polypropylene chain. The peroxide initiated, melt phase reaction of vinyl triethoxy silane with polyethylene or polypropylene is a particular example.

A polyolefin resin for production of cement adherent fibers is prepared by mixing from about 2 to about 50 melt index polyolefin, such as polypropylene, with from about 0.5 to about 5 wt% of the modifier composition. Preferably the polypropylene has a melt index (measured at 230.degree. C.) of from about 2 to about 10. Generally, modifier contents of from about 1 to about 3 wt% provide the resulting polyolefin resin with adequate adhesion toward cement. The modifier may be compounded with the polyolefin in a single-screw or twin-screw extruder of co-rotating or counter-rotating design, extruded to strands and pelletized by a hot die face cutting or a cold cutting system.

The pelletized modified resin may be used as the feed stock for fiber production by any of the conventional fiber forming methods. Filaments of the so modified polyolefin resin may be prepared by melt-spinning techniques or by film slitting.

If desired, the modified resin may be continuously prepared and fed directly to the fiber forming process. The intermediate operation of preparing the resin in pelletized feed stock form may be omitted. Likewise, the modifier composition need not be separately prepared, but may be formed in situ by direct compounding of the requisite quantities of acid modified polyolefin and alkylamino alkoxysilane with the base polyolefin compound.

The cement adherent reinforcing fibers formed from the modified polyolefin resin should be prepared as a 1 to 100 denier fiber, preferably from about 2 to about 80 denier, with the preferred fiber size being dictated by the details of the cementitious matrix. The fibers may be prepared in any length, but lengths of from about 1/8 to 3 inches are preferred, and most preferred are fiber lengths of from about 1/8 to 1 inch.

Cement adherent polyolefin reinforcing fibers prepared in accordance with the invention are suitable for reinforcement of all common cement compositions, such as Portland cements, marble cement, puzzolanic cement, trass cement, blast furnace cement, gypsum cements, calcium silicates and others. In addition to the reinforcing polyolefin fibers of the invention, the cement composition may contain further additives and fillers such as fly ash, limestone, quartz, pearlite, rock wool, cellulose, diatomaceous earth, flue dust, pozzolana or mixtures thereof.

Reinforced cement articles are prepared by mixing polyolefin reinforcing fibers of the invention with the cement slurry in fiber amounts of from about 0.5 to about 20 percent by volume (dry basis), preferably from about 1 to about 10 percent by volume, and most preferably in fiber amounts from about 2 to about 8 percent by volume.

A modifier composition was prepared by mixing 4.5 parts by weight of gamma-aminopropyl tri-ethoxy silane and 95.5 parts by weight of maleated polypropylene (Eastman Chemicals' Epolene-43, MW=5000, acid number about 40) in a lab Waring blender. The resulting mix was then stored at 60.degree. C. for several hours to allow the reaction between the anhydride and the amine to proceed to completion. Next, 1.65 parts by weight of the resulting modifier composition was blended with 98.35 parts by weight of a 3 melt flow rate polypropylene and the blend pelletized in a small single screw extruder.

The pelletized modified polypropylene resin was then extruded into film; the film was oriented, fibrillated and chopped into staple fiber of about 20 denier and 6 mm length. This staple was randomly dispersed in a concrete made by mixing Portland cement type P-40, fine sand type 0000 and water (according to Norm NBN B.12-208; consistency 1.5 as described in Norm NBN B 14-207). The fiber was present in the final formulation to the extent of 4 vol%. Test bars of 15.5 cm length, 2 cm width and 1 cm height were molded from the concrete. The bars were cured in water and then in a wet chamber (as per Norm NBN B.12-208).

Comparison samples were prepared by producing staple fibers in a manner identical to that described above from an unmodified 3 melt flow rate polypropylene and from polypropylene compounded to a resin with 1.65 parts of a maleated polypropylene (Epolene-43). Concrete test bar specimens embodying the unmodified fibers were prepared in exactly the way described above.

The cured bars were then subjected to a bending test in a horizontal configuration with a support near each end of the bar and the load applied from the top in the center of the bar. The force-displacement results are displayed in FIGS. 1, 2 and 3. FIG. 1 gives the results for the bars reinforced with the modified polypropylene fibers of the present invention. FIG. 2 represents the results for the comparison sample using unmodified polypropylene fibers as the reinforcing agent. FIG. 3 represents the result for the comparison sample using a polypropylene fiber containing an amount of maleated polypropylene equivalent to the amount of modifier composition contained in the modified polypropylene fibers of Sample 1.

In a test of this type, the area under the force-displacement curve correlates directly with toughness and extensibility in actual application. Comparing FIGS. 1 and 2, an increase of at least 100% is seen in this critical area measurement in going from the unmodified fiber specimen to the material based on the fibers of the present invention. Comparison of FIG. 2 to FIG. 3 demonstrates that addition of only a maleated polypropylene to the polypropylene fiber resin actually decreases the adherence of such fibers for cement.

A modifier composition was prepared, blended with polypropylene and extruded in accordance with the procedure of Example 1. Prior to extrusion into film/fibers, the resulting modified polypropylene resin was dry blended with a concentrate of CaCO.sub.3 in polypropylene so that the concentration of CaCO.sub.3 in the final blend was about 15 wt%. This increased the density of the resulting fibers and made them more dispersible in the cement slurry.

The dry blend of CaCO.sub.3 concentrate and modified polypropylene resin was extruded into film, oriented, fibillated and chopped into staple in accordance with the procedure of Example 1. Cement articles were fabricated and evaluated as in Example 1. The comparison sample chosen was a 3 MFR polypropylene compounded to a resin with 1.65 wt% of a maleated polypropylene (Epolene-43). Comparison fibers were prepared from this resin in a manner identical to that described above.

The results of the evaluations of these CaCO.sub.3 filled formulations appear in FIGS. 3 and 4. FIG. 3 is the force displacement curve for the CaCO.sub.3 filled version of the modified polypropylene of the present invention. FIG. 4 represents the results for CaCO.sub.3 filled fibers prepared from the comparison resin which contained 1.65 wt% maleated polypropylene (Epolene-43).

As in Example 1, the area under the force-displacement curve is critical. In the case of the cement article reinforced with the fibers of the present invention, this area is seen to be at least 100% greater than for the comparison samples. Thus the presence of CaCO.sub.3 as a densifying agent does not reduce the effect of the modifier composition.

The invention has been described and disclosed with reference to various of its particular and preferred embodiments. Upon reading and understanding this disclosure a person of ordinary skill in the art may appreciate that various modifications and changes may be made in the practice of the invention compared to the particular and preferred embodiments as described herein which do not depart from the scope and spirit of the invention as described above or claimed hereafter.