FIBER FOR REINFORCING PLASTIC COMPOSITES AND REINFORCED PLASTIC COMPOSITES THEREFROM

Abstract: A high tenacity fiber for reinforcing plastic composites, the fiber being selected from the group consisting of polyester, aliphatic polyamide, and combinations thereof. The fiber is treated with a composition comprising an aqueous solution of carboxyl-terminated, oil-free alkyd re­ sins based on an aliphatic glycol or glycols or a glycol ether or ethers containing 2 to 12 carbon or carbon plus ether oxygens with a combination of aromatic di- or trifunctional carboxylic acids, said resins having a degree of esterificaton below the gel point of the resins to enhance stiffness of the fiber for cutting.

Description: The present invention relates to a high tenacity fiber for reinforcing plastic composites and to the composites themselves wherein the reinforcing fiber, selected from the group consisting of polyester, aliphatic polyamide and combinations thereof, has been treated with a composition comprising an aqueous solution of carboxyl-terminated, oil-free alkyd resins. More particularly, the present invention relates to a high tenacity, high elongation, low shrinkage polyester fiber which has been treated with a composition to enhance its stiffness and interfilament coherency to permit high speed cutting of the fiber, especially for use in reinforcing a composite wherein the resin matrix comprises unsaturated polyester resin or other thermosetting or thermoplastic resins. The reinforcements can be used in bulk (BMC) and sheet molding compounds (SMC), filament winding, pultrusion, spray-up and hand-layup, and are especially useful in sheet molding compounds and spray-up applications due to ease of cutting. A composite consists of basically three major elements: the resin matrix, the reinforcement dispersed in the matrix, and the reinforcement-resin matrix interface. Synthetic fibers in staple or filamentary form, and fabrics produced therefrom, are known for polymer reinforcement. Typical of the fibrous reinforcements are glass, polyester, polyamide (nylon and aramid) and polyolefin fibers. Conventional matrix resins include thermoplastics, such as nylon and polyolefins, and thermosetting materials, such as epoxy and unsaturated polyester resins. Since the primary function of the fiber matrix interface is to transmit stress from the matrix to the reinforcing fibers, the chemical and physical features of the interface are critical to the mechanical properties and end use performance of the composite. The compatibility between the reinforcing fiber and matrix is then a determining factor in the load sharing ability of the composite. Fiber coatings/binders have been used to enhance the compatibility of the reinforcing fibers and resins with which they are to be used. See, for example, U.S. -A- 3 637 417 to Green. It is known to utilize silane coupling agents to bond dissimilar materials such as organic polymer and fibrous glass in the field of reinforced plastics. See, e.g., Green, supra , U.S. -A-4 158 714 to Brichta et al., and U.S. -A- 3 658 748 to Andersen et al., and Marsden and Sterman, HANDBOOK OF ADHESIVES, Second Edition, 40, 640 (1977). Polyester fiber inherently has a low fiber bundle integrity (fiber bundle integrity is the degree to which the individual filaments adhere to each other). The main benefit of a low integrity fiber is that it enables good dispersion of single filaments throughout a resin matrix. This even distribution results in a homogeneous reinforced composite, a direct result of which is an improved cosmetic appearance. However, in operations where high speed cutting, on the order of 500 feet/minute (about 150 m/min) or higher, of the fiber is required, e.g., SMC'S and spray-up, a low integrity fiber tends to fluff or cottonball and jam up the cutter. A high integrity fiber, readily cut at high speed and which does not ball-up within the reinforced composite, is therefore desirable. It is also conventional to substitute organic synthetic fibers in whole or in part for glass fibers in reinforced plastics. Some advantages are pointed out in U.S. -A- 3 639 424 to Gray et al. wherein heatset polyester staple is used to reinforce thermoplastic/thermosetting polymers for improved impact strength. Heatsetting the fiber allegedly permits uniform fiber dispersion in molded products. The use of chemically modified polyester fabric as an auxiliary reinforcing agent with glass fibers for thermosets, including polyester, vinyl ester and epoxy, for improved impact resistance and flexural strength over straight glass reinforcement is disclosed in Plastics World Magazine, November, 1980, Volume 38, No. 11, page 102. The present invention is directed towards improving compatibility between a resin matrix and the reinforcing fibers to thereby enhance the reinforced composite properties. The present invention provides a high tenacity reinforcing fiber with high integrity and selected from the group consisting of polyester, aliphatic polyamide and combinations thereof, for reinforcing composites. The fiber may be staple or continuous filament per se or a knitted, woven or nonwoven fabric or mats. The reinforcing fiber is coated with a composition comprising an aqueous solution of carboxyl-terminated, oil-free alkyd resin based on at least one aliphatic glycol selected from the list set out below, with a combination of aromatic di- or trifunctional carboxylic acids selected from the list set out below, and optionally, unsaturated aliphatic acids. These alkyd resins are polymerized below their gel points, that is the degree of esterification is generally maintained below 90 percent. Bundle integrity is increased while retaining compatibility of the reinforcing fiber to the resin. A sufficient amount of the coating composition is applied to achieve a solids pickup of up to 12 weight percent, more preferably 3 to 12 weight percent, most preferably 3 to 6 weight percent. At about 1 weight percent solids pickup, the stiffness of the fiber bundle for SMC and spray-up applications, which require cutting, is markedly enhanced. The upper limit is dictated by processability and economics, 12 weight percent being a practical limit. The aliphatic glycol is selected from ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, butane diol, butene diol, butyne diol, and combinations thereof. The aromatic carboxylic acid is selected from orthophthalic acid, orthophthalic anhydride, isophthalic acid, terephthalic acid, trimellitic acid, trimellitic anhydride, 2,6-naphthalene dicarboxylic acid, phenylindane dicarboxylic acid, trimesic acid, 4,4'-diphenyl-dicarboxylic acid, 2,6-pyridine dicarboxylic acid, p-hydroxylmethyl benzoic acid, 5-tert-butyl isophthalic acid, bimesitylene-4,4'-dicarboxylic acid, and combinations thereof. When the combination of carboxylic acids also includes an unsaturated aliphatic acid/acids, it is preferably selected from the group consisting of maleic acid, maleic anhydride, fumaric acid and combinations thereof. Preferred solutions are as follows, the most preferred being listed first: (a) a salt, preferably ammonium and sodium in a 50/50 ratio, solution of the condensation polymer consisting essentially of: diethylene glycol, preferably 45 to 55 moles, and most preferably about 49.8 moles; isophthalic acid, preferably 15 to 25 moles, and most preferably about 21.3 moles; terephthalic acid, preferably 15 to 25 moles, and most preferably about 21.4 moles; and trimellitic anhydride, preferably 5 to 10 moles, and most preferably about 7.5 moles. (b) a salt, preferably ammonium, solution of the condensation polymer consisting essentially of: diethylene glycol, preferably 35 to 40 moles, most preferably about 37.4 moles; propylene glycol, preferably 10 to 20 moles, most preferably about 14.6 moles; maleic anhydride, preferably 2.5 to 5 moles, most preferably about 3.8 moles; isophthalic acid, preferably 15 to 20 moles, most preferably about 17.9 moles; terephthalic acid, preferably 15 to 20 moles, most preferably about 16.1 moles; and trimellitic anhydride, preferably 5 to 15 moles, most preferably about 10.2 moles. (c) a solution of the condensation polymer consisting essentially of: diethylene glycol, preferably 5 to 10 moles, most preferably about 7 moles; isophthalic acid, preferably 5 to 10 moles, most preferably about 6 moles; and trimellitic anhydride, preferably 0 to 3 moles, most preferably about 1.05 moles. (d) a solution of the condensation polymer consisting essentially of: propylene glycol, preferably 3 to 8 moles, most preferably about 4 moles; isophthalic acid, preferably 0 to 3 moles, most preferably about 1 mole; trimellitic anhydride, preferably 0 to 3 moles, most preferably about 1 mole; and maleic anhydride, preferably 0 to 3 moles, most preferably about 1 mole. The preferred reinforcing polyesters are the linear terephthalate polyesters, i.e., polyesters of a glycol containing from 2 to 20 carbon atoms and a dicarboxylic acid component containing at least 75 percent terephthalic acid. The remainder, if any, of the dicarboxylic acid component may be any suitable dicarboxylic acid such as sebacic acid, adipic acid, isophthalic acid, sulfonyl-4,4'-dibenzoic acid, 2,8-dibenzofuran-dicarboxylic acid, or 2,6-naphthalene dicarboxylic acid. The glycols may contain more than two carbon atoms in the chain, e.g., diethylene glycol, butylene glycol, decamethylene glycol, and bis-(1,4-hydroxymethyl)cyclohexane. Examples of linear terephthalate polyesters which may be employed include poly(ethylene terephthalate), poly(ethylene terephthalate/5-chloroisophthalate)(85/15), poly(ethylene terephthalate/5-[sodium sulfo]-isophthalate) (97/3), poly(cyclohexane-1,4-dimethylene terephthalate), and poly(cyclohexane-1,4-dimethylene terephthalate/hexahydroterephthalate) (75/25). Suitable reinforcing polyamides include, for example, those prepared by condensation of hexamethylene diamine and adipic acid, condensation of hexamethylene diamine and sebacic acid known as nylon 6,6 and nylon 6,10, respectively, condensation of bis(para-aminocyclohexyl)methane and dodacanedioic acid, or by polymerization of 6-caprolactam, 7-aminoheptanoic acid, 8-caprylactam, 9-aminopelargonic acid, 11-aminoundecanoic acid, and 12-dodecalactam, known as nylon 6, nylon 7, nylon 8, nylon 9, nylon 11, and nylon 12, respectively. The most preferred reinforcing fiber is a polyethylene terephthalate, characterized by a thermal shrinkage of up to 11 percent, preferably 3 percent or less; an elongation of from 10 to 28, preferably 14 to 24 percent; a modulus after cure of at least 60 gram per denier (540 g/tex), preferably 70 to 90 grams per denier (630 to 810 g/tex); and a tenacity of at least 5.5, preferably 7 to 9 grams per denier (63 to 81 g/tex). By modulus after cure is meant the modulus of the fiber after exposure in an unconstrained state to curing temperatures for the composite. The present invention also relates to a fiber reinforced plastic composite comprising a resin matrix and the described reinforcing fiber. The resin matrix may include thermosetting or thermoplastic (including polyolefin) resins. Suitable thermosetting resins include polyester (preferably unsaturated), epoxy, or vinyl ester resin systems. Suitable thermoplastic resin systems include polyvinyl chloride, polyethylene, polypropylene, polystyrene, polyvinyl alcohol, polyamide, polyurethane, etc. - any thermoplastic having a lower melting point than that of the fiber, e.g., less than 230°C for reinforcement with polyester fibers and less than 200°C for reinforcement with nylon fibers. The resin matrix also may include enhancers, mold release agents and fillers, as are known in the composite art, in addition to the treated fibers and resins. It is also preferred that there be other reinforcing fibers, more preferably glass fibers, as will be more fully discussed below. Preliminary research was directed towards engineering a polyethylene terephthalate (hereafter PET) fiber specifically for reinforcement of rigid composites. Research first addressed the question of what type of PET fiber is most suitable as a reinforcing fiber in composites. There are many grades of PET commercially available. However, preliminary screening showed only very specific types to be even potentially useful in composites. With reference to Table I, it can be seen that textile grade PET fiber (Burnet Southern, Inc.) is unsuitable for reinforcement, primarily because of its low tenacity, low modulus, and large thermal shrinkage, resulting in a composite with poor notched impact strength. It was expected that high tenacity industrial PET fibers could provide superior impact strength with adequate tensile strength. Preliminary work showed this to be true; however, a wide range of values was obtained based on differing fiber types. It has been found that tensile properties of the fiber as normally measured, as well as after exposure to the thermal molding conditions encountered in fabricating a reinforced composite article, are important. The latter is more important since the final properties of the fiber in the composite more directly affect composite performance. Example 1 Data for three types of high strength industrial PET fibers, commercially available from Allied Corporation, are shown in Table 1. These are: Type A - a high tenacity, low elongation fiber normally used for tire reinforcement; Type B - a high tenacity, high elongation, low shrinkage fiber normally used for hose reinforcement; and Type C - a high tenacity, high elongation, ultralow shrinkage fiber normally used for coated fabrics. Molded composite specimens were prepared from each of the fibers using a 12 weight percent PET fiber loading, equivalent in volume to 20 weight percent glass. The type B fiber appeared to give the best balance of composite properties. This is unexpected based on initial fiber properties. The fibers were subjected to simulated curing temperatures of from 95-150°C while in an unconstrained state. Fiber physical properties were measured both before (control) and after heat treatment. The effects of thermal exposure on Fibers A, B and C, respectively, are presented in Tables 2-4. As shown in Table 2, Type A underwent a large amount of shrinkage at temperatures as low as 120°C. In contrast, as shown in Tables 3 and 4, the two low shrinkage fibers B and C showed very little shrinkage during thermal exposure, as expected. Since excessive shrinkage during processing would have a detrimental effect on fiber/resin bonding, this might be expected to result in reduced composite properties. The thermal exposure also had greatly differing effects on the moduli of the different fibers tested (Tables 2-4). When tested at room temperature, the high tenacity Type A fiber had a 20-25 percent higher initial modulus and 10 percent higher tenacity than the low shrink fibers. However, after exposure to the elevated temperatures, the Type A fiber showed a more dramatic change than either of the others. Also, and not to be expected from normal measured physical properties, the final modulus of Type A was nearly 30 percent lower than the final modulus of the two low shrink fibers after exposure to 150°C. Therefore, in their effect on composite properties, the two low shrinkage fibers appear to be superior in spite of the fact that tensile properties on virgin yarn appear to be superior for the Type A fiber. The ultralow shrink Type C fibers tested had a lower molecular weight, which results in a lower tensile strength fiber than the Type B low shrink fiber. It was found that the initial tenacity was essentially unchanged by thermal treatment for any of the fibers. Therefore, the slightly better dimensional stability of the Type C ultra low shrink fiber is more than offset by the higher tenacity Type B low shrink fiber. The optimum PET fiber length for staple reinforcement appears to be the longest length which will still allow practical handling and dispersion. Notched impact strength showed a significant response to fiber length. Three fiber lengths, 1/8, 1/4 and 1/2 inch (0.32, 0.64 and 1.3 cm) were used to determine the effect of PET fiber length at several levels of total fiber loading. For each doubling of fiber length the impact strength also doubled. Subsequent testing showed that a 3/4 inch (1.9 cm) PET fiber reinforced composite resulted in still more improved impact strength. There was little difference in composite tensile strength between 1/8 and 1/4 inch (0.32 and 0.64 cm) fiber. However, using 1/2 inch (1.3 cm) fiber resulted in an increase in tensile strength of about 50 percent. These results point out a significant advantage of an organic filament reinforcement. In contrast to glass, which can shatter into shorter lengths during compounding, PET fiber does not break into shorter lengths. This allows PET fiber to retain its maximum advantage for impact strength. Example 2 It is known that the fiber coating is a factor in stress distribution for the composite. In preliminary studies it was observed by scanning electron microscope that a very nonuniform wetout and poor adhesion were achieved between PET fibers and thermoset polyester resins. To improve these features for thermoset polyester resins various types of fiber coatings were used. The properties of these coatings were varied in order to determine the degree of influence of the boundary on the composite performance. The binders applied are listed in Table 5. The yarn utilized was Type B of Table 1. Yarn was removed from a supply package at 1000 ft/min (305 m/min) and passed over a stainless steel lube roll that was partially immersed in the binder/finish composition, after which the yarn was passed through drive rolls and thence to a winder where it was wound. The yarn was removed from the supply package through eyelet guides and two tension gates to flatten the yarn bundle for passage over the roll and to prevent bouncing. The yarn touched the roll tangentially, a minimum contact angle being maintained. The lube roll was 4 inches (10 cm) in diameter and turning at an RPM sufficient to achieve the percent solids on yarn indicated in Table 5. For the composition of the present invention, the roll was turning at 15 r/min. Another mode of applying binder system 4 of the present invention is as follows. Yarn is removed from supply packages at 850 ft/min (about 260 m/min) through eyelet guides and passes through the slot of a slot applicator which is supplied with the binder (30% solids) by a peristaltic pump at 7 cc/minute. This provides a total solids application of 5 percent on the weight of the yarn. After binder application, the yarn passes through a 6 foot (1.8 m) long heat chamber, heated with forced hot air to 215°C. The yarn then passes over two heated drive rolls at 200°C, after which it is taken up on a winder. Several methods to determine the adhesive properties of the PET fiber were evaluated. For comparison and evaluation purposes, the adhesive properties were measured as a function of the interfacial shear strength. The interfacial shear strength was calculated from the pullout load, P using the following equation where τ = interfacial shear strength, D = fiber diameter, and L = embedded fiber length in the matrix. A small portion of an overfinished PET fiber was embedded to a depth of 0.25 inch (0.64 cm) in a compounded thermoset polyester resin of the formulation set forth in Table 1. The fiber was subjected to a tensile force (pullout force) with an Instron in the axial direction of the filaments, causing shear debonding stress on the interface. The Instron has a recorder attached thereto to record the pullout load, P, in pounds (kg). By using this technique, the effect of use of different binders on the interfacial strength of the composite could be determined. This test is called the button mold pullout test. The interfacial shear strength is the total overall strength of the interface due to physical, mechanical and chemical bonding. The composite physical properties including impact strength, flexural strength, and tensile strength, were determined for the different interfacial shear strength values obtained with the different binders, and the results shown in Table 7. As expected, up to a level of 7.3 kg/cm² (X11.9), composite properties improved proportional to τ ; however, at just over 7.3 kg/cm² (X11.9) the data seem to show that maximum composite properties were achieved and further increases in interfacial shear strength were detrimental. In fact, what was observed is that the type of materials which achieve a high concentration of crosslinking sites and therefore a very high interfacial shear strength showed a poor wetout by the resin. This poor resin wetout created a number of voids and flaws which acted as stress concentrators within the matrix leading to early failure. Thus, fiber/matrix wettability is also a key to better load sharing and load transfer between phases. The wettability of the treated fibers listed in Table 5 for the specified resins was determined as follows. A value of 1 to 10 was assigned to a particular fiber wherein 1 = poor and 10 = excellent based on visual inspection and two tests. The visual examination occurred during BMC compounding of 1/2 inch (1.3 cm) length PET filament (1000 denier, 192 filaments) at 20 percent loading (i.e., volume equivalent 20 weight percent fiberglass) with the resin matrix set forth before. The resin matrix included 1.5 weight percent of a red pigment available as CM-7106 from Plastic Colors to aid the visual inspection. After compounding for 60 seconds, the compounded mass was manually pulled apart, fibers pulled out and felt for wetness and observed for color change. Compounding was restarted and continued for 30 seconds after which the same observations were made. This continued until maximum wetout of the fiber occurred. One of the tests relied upon in obtaining wettability values was scanning electron microscopy (SEM) to evaluate broken composites. The fibers of Table 5 were used to compound a 20 percent load PET fiber reinforced BMC compound. These compounds were molded [320°F (160°C), 1.5 min, 1 ton press] into tensile, flexural and impact specimens, as well as plaques. The specimens were tested in accordance with ASTM procedures: the tensile specimen according to D-638, the flexural according to ASTM D-790 and the impact specimen according to ASTM D-256. The plaques were manually broken. The broken specimens and plaques were taken to an SEM laboratory for examination. The fibers jutting out from the fracture point were observed for residual matrix or matrix cling. If the fiber was clean, adhesion and wetout were ranked 1. The more matrix clinging to the fiber, the higher the ranking, up to a rating of 10. Button test specimens could be similarly evaluated. SEM rankings are presented in Table 6. In another test the contact angle of a polyester molding resin (MR14017 from USS Chemicals) on a unidirectional fabric made with the coated yarns was measured. The unidirectional fabric was made by guiding the coated PET fiber bundles over a winding mechanism so that a fabric could be made from closely packed parallel fiber bundles. A sessile drop of the liquid molding resin was placed on the fabric held under tension (0.2 gram/fil), and the contact angle on the fabric was measured with time. The contact angles measured are presented in Table 6 as well as a ranking of the coated fibers with respect to wettability. The binder systems of the present invention (Numbers 4 and 6-9 of Table 5) were chosen as the high integrity systems because of acceptable adhesion and resin wetout along with enhanced interfilament coherency. The binder systems were ranked as follows with respect to high bundle integrity (subjective test): Example 3 Several thermoset polyester resins were evaluated to determine the best for composite applications where 0.5 inch (1.3 cm) PET fiber coated with 0.2 weight percent solids of binder system 2 was used as reinforcing fiber, alone and in combination with glass. Although binder system 2 does not form part of the present invention, the work with it here and in subsequent examples is deemed applicable to the present invention. A sample with 100 percent glass reinforcement was also run. Total fiber loading was kept constant at the volume equivalent to 20 weight percent glass. The curing temperature used was 300°F (149°C). Typical composite physical properties were measured on the molded samples, and are shown in Table 8. Resin matrix formulation was as previously described, with the resins identified in Table 8. Impact strength is a measure of a composite's resistance to a sudden sharp impact. Replacement of glass fiber by polyester fiber results in a great improvement in this property. Two types of impact tests were run, notched and unnotched. In the notched impact test, impact strength for 100 percent PET fiber reinforced composite increased from 15 to 20 foot pound/inch (800 to 1068 J/m) as the resin elongation was increased from 0.9 percent to 10 percent. For the 100 percent glass reinforced composite and composites reinforced by mixtures of PET fibers and fiberglass, impact strength remained relatively constant with increasing resin elongation. As the proportion of PET fiber was increased, the impact strength was seen to increase at all resin elongations. The increase was greater at the higher resin elongations. Results of the unnotched impact test showed the same general trends observed in the notched test. In this case, however, the composites were reinforced by 100 percent glass fibers, and PET/glass fiber mixtures tended to lose impact strength as resin elongation increased. Again, as the proportion of PET fiber and the reinforcing fiber increased, impact strength increased at all resin elongations with the increase being larger at higher elongations. These results demonstrate that higher elongation resins are preferable for increased impact strength for PET fiber reinforced composites. While PET is stronger than glass on a strength per unit weight basis (9.0 gpd [81 g/tex] versus 6-8 gpd [54-72 g/tex]), the fact that a specific gravity is nearly 50 percent lower than glass means that an equivalent volume of PET fibers will be less strong than glass fibers. Thus, PET reinforcing fibers provide a molded part which is not as strong as that reinforced by 100 percent glass fibers. On an equal volume loading basis, composite strength decreased roughly by a factor of 2 going from 100 percent glass fiber reinforced composite to a 100 percent PET fiber reinforced composite. The change in tensile strength with PET/glass fiber ratio showed a linear relationship in all resin systems, indicating that composite strength followed the rule of mixtures well. Tensile strength did not change dramatically with resin elongation. In general, the maximum value occurred using the 6.1 percent elongation resin, with the tensile strength being 10 to 25 percent higher than for the high and low elongation resin. The PET/glass ratio of the reinforcing fiber used did not appreciably affect the location of this maximum. Therefore, the 6.1 percent elongation resin appears to be the resin of choice to maximize the tensile strength of PET reinforced composites. Flexural strength is another measure of the strength of a composite material, having both tensile and compressive components. The same general trends were seen here that were noted for tensile strength. For 100 percent PET fiber and 100 percent glass fiber reinforced composites the maximum tensile strength was obtained using the 6.1 percent elongation resin. This was also the case for the sample reinforced by a 12.5/87.5 PET/glass fiber mixture. The samples reinforced by 25/75 and 50/50 PET/glass fiber mixtures showed a steady increase in flexural strength with increasing resin elongation. Flexural strength gains of 10 to 30 percent were obtained by using the 6 or 10 percent elongation resins relative to the 0.9 percent elongation resin typically used with 100 percent fiberglass reinforced composites. Therefore, the use of a medium to high elongation resin is preferable to maximize flexural strength. With the exception of Barcol hardness, increasing the resin elongation over that typically used (0.9%) with glass fiber resulted in improved composite physical properties. Tensile and flexural strength had the maximum values using a resin with a 6.1 percent elongation to break. This medium elongation resin thus appeared to be the best for use with the PET fiber coated with a binder system. Example 4 As generally recognized, the mechanical properties of a reinforced composite are strongly dependent on the level of reinforcing fiber used. In general, the properties improve with total fiber loading up to a maximum loading where the mechanical properties level or fall off due to inadequate fiber dispersion or insufficient resin to form a good matrix. In this example BMC formulations were made using 100 percent of PET fiber coated with binder system 2, 100 percent glass fiber, and mixtures of the PET fiber and glass fiber as the reinforcing agent. Fiber loadings were reported as the weight percent glass equivalent to the total fiber volume loading used. Total fiber loadings of 10 to 35 percent were used. Sample specimens were made and tested for tensile strength and impact strength. See Table 9. With each reinforcing system the impact strength increased with increasing fiber loading, as shown in Table 9. The best impact strengths were seen for the composites reinforced with 100 percent PET fiber, having values as high as 21 foot pound/inch (1121 J/m). Impact strengths for the PET/glass fiber mixtures were in between the values for the 100 percent reinforced composites and the 100 percent glass fiber reinforced composites. For this BMC formulation, total fiber loading equivalent to about 25 percent by weight fiberglass appeared to be optimum. Beyond this level, difficulties began to be encountered in compounding, while only small additional gains in impact strength were seen. Tensile strengths also increased with increasing total fiber loading for each of the reinforcing systems studied and as shown in Table 9. PET fiber is as strong as glass fiber on a weight basis, but because of the higher density of glass, glass reinforced composites are stronger on an equal volume reinforcement basis. However, at the 20-25 percent loading level, the composites reinforced by PET/glass fiber mixtures were nearly as strong as the 100 percent glass reinforced composites. Example 5 In this example bulk molding compounds containing 20 weight percent glass loading, glass replaced by PET fiber on equal volume basis, were made and molded into specimens for testing. The BMC resin matrix was as previously described, and the PET fiber had been treated with binder system 2. The composition of the reinforcing fibers was varied, with relative polyester fiber/glass fiber volume ratios of 0/100, 25/75, 50/50, and 100/0 being used. All fiber loading levels reported are given as the weight percent glass equivalent to the particular volume of loading of the fiber used. The same composite properties were measured as reported previously and the results are given in Table 10. These data show that partial replacement of glass fiber by PET fiber in a reinforced composite results in significant gains in impact strength with very little sacrifice in tensile strength, especially at a fiber ratio of 25/75 PET/glass. This results in molded parts with overall properties which are superior to parts reinforced by either fiber used alone. Studies were also run on BMC injection molded composites reinforced by all PET fiber, all glass fiber, and PET/glass fiber mixtures to determine the effect. The results are presented in Table 11. The replacement of glass fibers by PET fibers resulted in greater increases in impact strength than were observed for compression molded samples. At the same time, tensile strength and flexural strength were less effective by replacement of glass with PET fibers than in compression molded samples. This is presumably due to the fact that shear and the injection molding machine broke the glass fibers into shorter lengths without affecting the lengths of the PET fibers. In addition, it may be possible that the presence of the PET fibers protect the glass fibers from damage to some extent. Example 6 PET fiber type B with binder system 4 at 5.00% solids pickup was cut into 1/2 inch (1.3 cm) lengths, and used to compound a 20 percent load reinforced BMC compression compound with the resin matrix formulation of Table 1 (6.1% elongation resin). Composite properties are presented in Table 12. The bundle integrity was high. Example 7 Nylon 6 fiber having a nominal denier of 1260, 204 filaments, modulus of 44, breaking strength (lbs) 24.5 (11.1 Kg), tenacity 8.8 gpd (79.2g/tex), breaking elongation (%) 20.5, commercially available from Allied Corporation, has binder system 4 applied via a kiss roll at a solids pickup of 3 to 6 percent. The fiber is cut into 0.5 inch (1.3 cm) lengths and used to compound a 20 percent load nylon reinforced BMC compression composite with the resin matrix formulaton of Table 1. Composite properties are expected to be acceptable. Bundle integrity is expected to be high. Example 8 PET fiber type B was removed from supply packages at 850 ft/min (about 260 m/min) through eyelet guides and passed across a kiss roll applicator for application of binder system 4 (25% solids solution) to achieve 5 percent solids pickup. The yarn then passed through a 20 kilowatt macrowave oven (Radio Frequency Co.) for about 0.1-0.2 second and was taken up on a winder. The fiber was cut into varying lengths (see Table 13) and used with varying amounts of glass fiber to reinforce BMC injection molded composites. The matrix formulation was a typical automotive formulation having low profile additives for improved surface characteristics (Class A surface), with 43 to 48 percent filler content and 22 to 28 percent polyester resin content. Results of composite testing are presented in Table 13. The composite with longer reinforcing fibers of PET and glass fibers (Sample 9 of Table 13) has an extremely smooth surface as compared with a 100% glass fiber reinforced control with shorter fiber lengths at the same load (Sample 1 of Table 13). This is surprising since longer fiber lengths ordinarily result in poorer surface quality. The shorter PET fiber length samples (2-5) had even better surface appearance.

Claims: A high tenacity reinforcing fiber selected from the group consisting of polyester, aliphatic polyamide, and combinations thereof, for reinforcing plastic composites, said fiber being coated with a composition to enhance stiffness of the fiber for cutting, characterised in that said composition has been deposited from an aqueous solution and comprises a carboxyl-terminated, oil-free alkyd resin based on at least one aliphatic glycol selected from ethylene glycol, diethylene glycol, triethylene glycol propylene glycol, tetraethylene glycol, butane diol, butene diol, butyne diol, and combinations thereof; with a combination of aromatic di- or trifunctional carboxylic acids, selected from orthophthalic acid, orthophthalic anhydride, isophthalic acid, terephthalic acid, trimellitic acid, trimellitic anhydride, 2,6-naphthalene dicarboxylic acid, phenylindane, dicarboxylic acid, trimesic acid, 4,4'-diphenyldicarboxylic acid, 2,6-pyridine dicarboxylic acid, p-hydroxymethyl benzoic acid, 5-tert-butyl isophthalic acid, bimesitylene-4,4'-dicarboxylic acid, and combinations thereof; said resins having a degree of esterification below 90%. A fiber according to Claim 1 characterised in that the combination of carboxylic acids includes an unsaturated aliphatic acid selected from maleic acid, maleic anhydride, fumaric acid, and combinations thereof. A fiber according to Claim 1 or 2 characterised in that the fiber is treated with a sufficient amount of the composition to achieve at least 1 percent solids pickup, wherein the aliphatic glycol is selected from diethylene glycol, propylene glycol, and combinations thereof; and the aromatic carboxylic acids are selected from isophthalic acid, terephthalic acid and trimellitic anhydride, and combinations thereof. A fiber according to Claim 3 characterised in that the solids pickup is 3 to 6 weight percent, and the solution is a salt solution of the condensation polymer of 45 to 55 moles of diethylene glycol in conjunction with 15 to 25 moles of isophthalic acid, 15 to 25 moles of terephthalic acid and 5 to 10 moles of trimellitic anhydride. A fiber according to Claim 3 characterised in that the solids pickup is 3 to 6 weight percent, and the solution is a salt solution of the condensation polymer of 35 to 40 moles of diethylene glycol, 10 to 20 moles of propylene glycol, 2.5 to 5 moles of maleic anhydride, 15 to 20 moles of isophthalic acid, 15 to 20 moles of terephthalic acid and 5 to 15 moles of trimellitic anhydride. A fiber according to Claim 3 characterised in that the solids pickup is 3 to 6 weight percent, and the solution comprises a condensation polymer of 5 to 10 moles of diethylene glycol, 5 to 10 moles of isophthalic acid, and 0 to 3 moles of trimellitic anhydride. A fiber according to Claim 3 characterised in that the solids pickup is 3 to 6 weight percent, and the solution comprises a condensation polymer of 3 to 8 moles of propylene glycol, 0 to 3 moles of isophthalic acid, 0 to 3 moles of trimellitic anhydride and 0 to 3 moles of maleic anhydride. A fiber according to any one of Claims 1 to 7 characterised in that it is a polyester fiber having a thermal shrinkage of at most 3 percent. A fiber according to any one of Claims 1 to 7 characterised in that the fiber is polyester which is treated with a sufficient amount of the composition to achieve 3 to 6 weight percent solids pickup and said polyester fiber has a thermal shrinkage of up to 11 percent, an elongation of from 10 to 28 percent, and a modulus after cure of at least 60 grams per denier (540 g/tex). Use of a fiber according to any one of Claims 1 to 9 as reinforcing fiber in a fiber reinforced plastic composite comprising: (a) a resin matrix; and (b) the reinforcing fiber. Use of the fiber in a composite according to Claim 10 characterised in that the resin matrix comprises a thermosetting polyester, epoxy or vinyl ester. Use of the fiber in a composite according to Claim 10 characterised in that the resin matrix comprises an unsaturated polyester resin. Use of the fiber in a composite according to Claim 12 characterised in that it additionally comprises glass reinforcing fibers. Use of the fiber in a composite according to Claim 13 characterised in that the polyester fiber has a tenacity of 7 to 9 grams per denier (63 to 81 g/tex), a thermal shrinkage of at most 3 percent, an elongation of 14 to 24 percent and a modulus after cure of 70 to 90 grams per denier (630 to 810 g/tex).

Inventors: COFFIN, DAVID RICHARD, LAZARUS, STANLEY DAVID, SOLARES CORDOVA, DAVID, YOUNG, STEVEN ANTHONY
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1985
Main classification: D06M15
IPC codes: B32B17, D06M101, D06M15, C08J5, D06M13