Low moisture absorption abrasive bristle of polybutylene terephthalate
This application is a continuation of application Ser. No. 514,740 filed July 18, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to bristles having an abrasive filler. More particularly, it relates to abrasive bristles of the type disclosed in U.S. Pat. No. 3,522,342. This patent discloses a method for making a bristle material comprised of a thermoplastic matrix and an abrasive filler material dispersed therein. The thermoplastic materials contemplated, include nylon, and blends of nylon and polyester. While these bristles are generally stiff and abrasive, in applications where they are exposed to hot water their stiffness (expressed as tensile modulus) tends to suffer because of water absorption. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a bristle material having a diameter ranging from about 0.02 to 0.09 inches, and consisting essentially of polybutylene terephthalate (PBT), and an abrasive filler material dispersed therein, said PBT having an intrinsic viscosity (I.V.) of at least about 0.60 in the bristle; and said abrasive filler being present in an amount of about 5 to about 50 weight percent based on the total weight of the bristle. 
     DETAILED DESCRIPTION 
     The bristle material of the present invention consists essentially of a thermoplastic matrix of polybutylene terephthalate having a relatively high intrinsic viscosity (I.V.) in the bristle of at least 0.60. This high viscosity is important because it assures adequate stiffness in the bristles. To achieve this viscosity in the bristle, the polymer fed to the extruder should have a viscosity of at least about 0.80, and the polymer and filler must be kept relatively dry. 
     The polybutylene terephthalate used herein is preferably derived from the polycondensation of 1,4-butane-diol and dimethyl terephthalate or terephthalic acid, but may be derived from other well known raw materials. The polybutylene terephthalate in the bristle has an intrinsic viscosity (I.V.) ranging between about 0.60 and about 1.20, with a preferred I.V. ranging between about 0.7 and about 1.0. Intrinsic viscosity is obtained by extrapolation of viscosity values to zero concentration of solutions of polybutylene terephthalate in a 60 to 40 weight/volume ratio of phenol and tetrachloroethane. The measurements are made at 25° C. The polybutylene terephthalate melts between about 223° C. and about 229° C. The polybutylene terephthalate can contain minor amounts, up to about 5 percent, of other comonomers such as 1,4, cyclohexyldimethyldiol, butyldiol, neopentyldiol, diethylene glycol, or glutaric acid. 
     The abrasive fillers usable include silicon carbide, aluminum oxide, asbestos particles, diamond particles, ceramic particles, etc. The range of filler in the final product is from about 5 to about 50 percent by weight of the bristle, preferably 15-30 percent. 
     The present invention is particularly useful in abrasive filled bristle materials wherein the abrasives have a hardness value, as rated by the Knoop Scale, greater than about 80 and preferably greater than about 1000. The Knoop Scale values are determined by measuring the indentation resulting when a weighted diamond pyramid penetrates the test material. Thus, the hardness value is expressed as the load in kilograms divided by the projected area in square millimeters. 
     In addition to the aforementioned hardness, the abrasives found most useful were those which will pass through a screen size of about 10 to about 1,500 mesh, preferably about 80 to about 500. Accordingly, the abrasive filler may be aluminum oxide or silicon carbide each having a grit size ranging from about 10 to about 1,500 mesh, preferably about 80 to about 500 and a hardness value of about 1,250 to about 1,750 for aluminum oxide and about 2,130 to about 2,500 for silicon carbide, or the abrasive filler may be diamond dust having a grit size of about 80 to about 320 and a hardness value of about 5,500 to about 6,950. Although aluminum oxide, silicon carbide, and diamond dust are the preferred abrasive fillers, other abrasives having the aforementioned hardness and grit sizes can be utilized. Representative examples of other abrasives employable in the present process are boron carbide, tungsten carbide, tantalum carbide, and the like. 
     The extruded bristles may be produced by the process described in U.S. Pat. No. 3,522,342, which is hereby incorporated herein by reference. The total stretch imparted to the bristles is preferably approximately 2 to 4 times their original length. This stretching results in superior tensile modulus and Bend Recovery. 
     The products of the present invention are in the form of a strand having a wide variety of crosssectional configurations and have a diameter from about 0.01 inch to 0.09 inch, preferably 0.02-0.07 inch. 
     The extruded products of the present invention may be cut into unit lengths, such as about 5 to about 10 inches, thereby forming bristles which can be utilized in making abrasive wheels. The unit lengths may be about 2 about 3 feet long for use in making brooms. 
     Suprisingly, it has been found that when a polybutylene terephthalate is used, preferably having an I.V. of about 0.60 to about 1.20, the resulting bristle has stiffness expressed as tensile modulus superior to that of nylon alone and the bend recovery is superior to that of the polyethylene terephthalate, thus providing on balance a product with acceptable bend recovery and superior stiffness relative to nylon. 
     The following examples are examplary of the present invention. 
    
    
     EXAMPLE I 
     Utilizing the two extruder system shown in U.S. Pat. No. 3,522,342, strands consisting essentially of a polybutylene terephthalte matrix containing about 24%, by weight of the total strand, of silicon carbide abrasive were extruded into a water bath, as shown in reference numeral 4 of said patent, being maintained at a temperature of about 20° C. to 40° C. and subsequently taken up over rolls, as shown in reference numberal 54 of said patent, to a take up reel. The strands were subsequently stretched 3.6 times their original length on an Instron Model 1125 Stretcher at a temperature of about 90° C. to obtain bristles having a diameter of about 0.030 inches. The Bend Recovery and the Tensile Modulus, tested dry and Hot Wet, were as follows: 
     
         ______________________________________                                    
DRY                                                                       
20° C. - 65% RH                                                    
                   HOT WET.sup.(1)                                        
TM.sup.(2)                                                                
         Bend.sup.(3)  TM.sup.(2)                                         
                               Bend.sup.(3)                               
(gpd)    Recovery      (gpd)   Recovery                                   
______________________________________                                    
11       70%           11      80%                                        
______________________________________                                    
 .sup.(1) Hot Wet conditions are simulated by immersing samples in boiling
 water for 15 minutes, then testing after removing from the water.        
 .sup.(2) TM = Tensile Modulus which is determined by the method described
 in ASTMD885, immediately after removing from the water, in the case of ho
 wet conditions.                                                          
 .sup.(3) Bend Recovery is a test in which the bristle material is coiled 
 10 times around 0.25 inch diameter mandrel, placed in water for 1 hour at
 room temperature, and then slid off the mandrel and allowed to sit for 1 
 hour during which time the coi ls tend to expand. After the 1 hour period
 the remaining coils are counted. The number of coils remaining (N) is use
 to determine Percent Bend Recovery (BR) according to the following       
 formula: B = (10 - N)10 (In the case of &#34;Hot Wet&#34; c onditions the strand 
 is allowed to cool to room temperature before subjecting it to the Bend  
 Recovery Test.)                                                          
 
    
     EXAMPLE 2 
     Repeat Example 1 but include in the bristle material, 24% by weight of aluminum oxide based on the total weight of the bristle. 
     EXAMPLE 3 
     Repeat Example 1 but use 20 weight precent aluminum oxide based on the total weight of the bristle. 
     While exemplary embodiments of the invention have been described, the true scope of the invention is to be determined from the following claims. 
     While exemplary embodiments of the invention have been described, the true scope of the invention is to be determined from the following claims.