Abstract:
A laminated air brake tubing includes inner and outer nylon layers with an intermediate fiber reinforcing layer. The tubing further includes one and preferably two layers of high density polyethylene positioned adjacent either of said nylon layers with a protective layer separating the fiber reinforcing layer from the high density polyethylene layer. The protective layer is preferably a thin layer of nylon or nylon alloy. Further, the high density polyethylene layer may include compatibilizing agents preventing delamination. The air brake tubing of the present invention meets the current requirements for air brake tubing, however it is less expensive than currently available air brake tubing.

Description:
BACKGROUND OF THE INVENTION 
     Air brake systems are frequently employed for heavy-duty vehicles such as tractor trailers and the like. In such systems, the brake system is activated by pressurized air transported through tubing. Metal tubing, of course, can be employed. However, generally nylon tubing is used and in many applications, reinforced nylon tubing is required. By industry standards, reinforced nylon tubing must include an inner nylon layer and an outer nylon layer with an intermediate polyester or nylon reinforcing layer. This tube is designed to operate at a maximum pressure of 150 psi over a wide temperature range i.e. -40° C. to 90° C. The test requirements are even more strenuous requiring the tubing to withstand 150 psi pressure at temperatures from -40° up to 110C. 
     In the past, such polyester reinforced nylon tubing has performed quite well. It meets the minimum requirements with respect to temperature and pressure and generally exceeds these. In certain applications, it may be desirable to further broaden the temperature characteristics of such tubing. Further nylon itself is relatively expensive. High density polyethylene (HDPE) on the other hand has greater temperature range and is significantly less expensive than nylon. Nylon has been utilized for an extended period of time in automotive and trucking applications and therefore it&#39;s characteristics with respect to fuel stability and the like are well known. Therefore air brake tubing still requires that the inner and outer layers be nylon. 
     If one were to simply utilize a layer of high density polyethylene within the reinforced nylon brake tubing, in turn reducing the amount of nylon, the formed product will tend to delaminate if repeatedly subjected to high pressures. In effect, stress cracking occurs where the polyester strand contacts the HDPE layer. 
     Accordingly, it is an object of the present invention to provide a laminated, reinforced tubing which does not delaminate. Further, it is an object of the present invention to provide such a laminated air brake tubing that has improved low temperature characteristics. Further, it is an object of the present invention to provide such a laminated tubing utilizing layer of a polyolefin preferably high density polyethylene, HDPE. 
     The present invention is premised on the realization that such a tubing can be formed wherein the polyolefin layer is separated from the polyester layer with a protective layer. Preferably, according to the present invention, such a laminated tubing includes an inner layer of nylon, an intermediate HDPE layer, a braided polyester layer, and an outer nylon layer plus a protective layer which separates the HDPE from the polyester braiding. 
     In a preferred embodiment of the present invention, the laminated tubing includes two layers of HDPE, one on either side of the polyester braiding with protective layers separating the polyester from the HDPE. Although a variety of protective layers could be employed, the preferred protective layer is a thin layer of nylon, preferably, nylon 11 or 12. 
    
    
     The objects and advantages of the present invention will be further appreciated in light of the following detailed descriptions and drawing in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary prospective view of air brake tubing made according to the present invention. 
     FIG. 2 is a cross-sectional view taken at lines 2--2 of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings, the present invention is an air brake tubing 11 which has a central inner passageway 12 having a diameter generally from about 0.251 to about 0.556 inches. Tubing 11 has an inner nylon layer 14 and an outer nylon layer 15 with a central layer of fiber reinforcement or braiding 16. 
     Tubing 11 further includes a first layer 18 of polyolefin which has a thickness of about 0.024 to 0.039 inches. Generally for brake tubing the polyolefin will be high density polyethylene. However, for other applications, low density polyethylene, polypropylene or polybutylene may be employed. This is bonded to the outer surface 17 of inner nylon layer 14. Covering the outer surface 19 of polyolefin layer 18 is a first protective layer 21. 
     The outer surface of protective layer 21 in turn is bonded to or embedded in the braiding 16. Braiding 16 is likewise covered by a second protective layer 24. Both protective layers have a thickness of about 0.005 to about 0.010 inches 
     The outer surface of the second protective layer 24 is in turn surrounded and bonded to a second layer 26 of polyolefin again preferably HDPE. The outer surface 27 of layer 26 in turn is bonded to outer nylon layer 15. 
     Both inner and outer nylon layers have a thickness of about 0.005 to about 0.010. Likewise, for brake tubing applications both layers of polyolefin have a thickness of about 0.024 to about 0.039 inches with the two protective layers having a thickness of about 0.005 to 0.010 inch. 
     The nylon layers are generally formed from nylon 11 or nylon 12. Commercially available plasticized polyamide such as plasticized nylon 11 (ELF ATOCHEM Brand RILSAN BNSO E40TL) or plasticized nylon 12 (EMSER brand GRILA MID L25W40NZ) can be utilized in the invention. In addition alloys of nylon 11 and/or nylon 12 can be employed. These alloys, include nylon blended together with less than 50% by weight of a compatible polymer such as high density polyethylene. Hereinafter the term nylon is intended to include both 100% nylon as well as comparably performing nylon blends or alloys. 
     In the preferred embodiment of the present invention, the polyolefin layer will be high density polyethylene as well as comparably performing nylon/polyethylene blends or alloys. The high density polyethylene has a density of about 0.94 to about 0.96 and a melt index of 0.01-3.0 It may include suitable stablizers, processing aids and the like which are typically added to polyethylene. Further, the polyethylene can include a compatibilizing agent to enhance the bonding between the polyamide and the polyethylene layers. A suitable compatibilizing agent is maleic anhydride modified polyolefin. Generally, the nylon content of any polyethylene blend can be up to about 50% by weight. 
     Preferably the high density polyethylene utilized in the present invention includes from about 40 to 98% high density polyethylene from about 1 to 30% compatibilizing agent and from about 1 to about 30% stabilizer. Preferred high density polyethylenes are sold by Exxon Chemical under the name Escorene HD9856B. A suitable compatibilizing agent is Uniroyal&#39;s Polybond brand 3009 which has 10% maleic anhydride grafted onto high density polyethylene. The stabilizer content is about 30 to 99% high density polyethylene combined with 0.5 to 70% of an antioxidant or blend of antioxidants such as Irganox 1010 supplied by CIBA Specialty Chemicals. These components are dry blended together and then extruded to form the high density polyethylene layers. 
     The reinforcing or braiding layer is generally formed from strands of polymeric fibers and preferably polyester fibers. These should have a weight of 500-1000 denier, with 840 denier being preferred. Commercially available polyester fiber is sold by Hoechst Cellanese under the designation 500 denier and 840/70/VAR. 
     The protective layer 21 can be any layer which is compatible with both the high density polyethylene, the braiding and the nylon layers. There are preferably nylon 11 or nylon 12 or alloys of nylon with high density polyethylene. 
     To form the tubing 11 of the present invention, the inner nylon tube 14 is coextruded along with the polyolefin high density polyethylene layer 18 and the first protective layer 21. The melt temperature of the nylon extruder should be 440° F. to about 460° F. preferably 450° F. In the preferred embodiment the protective layer 21 is nylon and is also formed by this extruder. The melt temperature of the polyethylene extruder which is preferably a 21/2 inch single screw extruder is 400° F. to 450° F., preferably 420° F. Again, the first protective layer is coextruded over the high density polyethylene layer at a temperature of about 440° F. to about 460° F. 
     The braiding 16 is then applied over the protective layer 21 by passing the three layer extruded tubing through a braider or fiber reinforcing apparatus. The reinforcing material may be braided, knitted, or spirally wrapped wherein one strand of the material is applied in a pitch to one direction and another strand is applied over the first with a pitch to the opposite direction. The braider is preferably a counter rotating fiber reinforcing device or may be any suitable and known conventional braiders. Preferably the braided layer 16 is applied with 6 bobbins of fiber applied from 2 to 5 pics per inch, preferably 3 pics per inch. 
     Once the braiding is applied, the outer three layers are coextruded over the inner tubing in the same manner as the inner three layers at the same temperatures. This can then be passed through a cooling bath. The resulting extruded product has an outer diameter of about 0.125 to about 0.75 inches and is ready for use. 
     The following detailed example demonstrates the characteristics of the laminated tubing in the present invention and clearly demonstrates that high stress cracking was inhibited utilizing the seven layer tube construction of the present invention. 
     EXAMPLE 
     A seven layer reinforced air brake tube was formed having the following construction: 
     
         ______________________________________Most Inner                 Outer Layer:Layer Nylon12*/HDPE**/Nylon12/Yam/Nylon12/HDPE/Nylon12Thickness   5      18    5    --   5    18  12(mils)______________________________________ *Nylon12 = L25W40NZ Nat. 844 from Emser. **HDPE = Escorene HD9856B from Exxon Chemical Co., Houston, TX. 
    
     As shown below, the seven layer embodiment fulfills the requirements of SAE J844 and DOT-106. 
     
         __________________________________________________________________________7 Layered ProductSAE J844Requirements  SPEC     Nylon Control                            Seven Layer AirbrakeTEST DEFINITION         REQUIREMENT                  Test Value                            Tubing__________________________________________________________________________                  3/8&#34; 1/2&#34; 3/8&#34; 1/2&#34;9.2 Moisture Absorption         2% Max.  1.40%                       1.35%                            0.91%                                 0.91%9.3 UV Resistance 300 hrs         80% min burst                  Pass Pass Pass PassQuv tester with UVA-340 bulb         after ambient                  1720 psi                       980 psi                            1832 psi                                 1525 psi         impact9.4 Cold Temperature         No Fractures                  Pass Pass Pass PassFlexibility            1451 psi                       1062 psi                            1617 psi                                 1392 psi9.5 Heat Aging Phase I         80% min burst                  Pass Pass Pass PassPhase II      after ambient                  1279 psi                       1135 psi                            1416 psi                                 1391 psiPhase III     impact   1280 psi                       1141 psi                            1493 psi                                 1340 psi9.6 Resistance to Zn         No visible                  Pass Pass Pass PassChloride      cracks9.7 Resistance to Methanol         No visible                  Pass Pass Pass Pass         cracks8 lbstiffness 3/8 &#34;         3.7 lbs  8.9 lbs                       6.7 lbs                            16.5 lbs20 lbs        1/2 &#34;9.9 Boiling Water Burst         80% rated burst                  1739 psi                       945 psi                            1334 psi                                 1112 psi9.10 Room Temp Burst1400 psi      3/8 &#34;         1457 psi 1113 psi                       1647 psi                            1471 psi950 psi       1/2 &#34;9.11 Cold Temperature         80% rated burst                  Pass Pass Pass PassImpact        after -40 F                  1558 psi                       1252 psi                            1665 psi                                 1427 psi         impact9.12 Adhesion Test         inseperatable                  Pass Pass Pass Pass9.13 Heat Age Adhesion         inseperatable                  Pass Pass Pass Pass9.14 Collapse Resistance         20% Max Collapse                  Pass Pass Pass Pass__________________________________________________________________________ 
    
     
         __________________________________________________________________________DOT-106requirements  SPEC      Nylon Control                           Seven Layer AirbrakeTEST DEFINITION         REQUIREMENT                   Test Value                           Tubing__________________________________________________________________________                   3/8&#34;                       1/2&#34;                           3/8&#34; 1/2&#34;S7.3.2        No Visible Cracks                   Pass                       Pass                           Pass PassHigh Temperature Resistance -5&#34; dia.       3/8&#34;condition around a mandrel7&#34; dia.       1/2&#34;for 70 hrs at 212 F Aftercooling straightenSk7.3.3       No Visible Cracks                   Pass                       Pass                           Pass PassLow Temperature Resistance -5&#34; dia.       3/8&#34;After conditioning for 70 hrs7&#34; dia.       1/2&#34;at 40 F Bend around a mandrelS7.3.4 Oil Resistance - 70         100% Maximum Swell                   Pass                       Pass                           Pass Passhrs at 212 F in ASTM #3 oil                   -3.0%                       -3.7%                           2.8% 3.1%S7.3.5 Ozone Resistance         No cracks in Outer                   Pass                       Pass                           Pass Pass         CoverS7.3.6 Length Change         -7% to +5% at 200                   0%  0%  0%   0%         psiS7.3.7 Adhesion         8 PLI min Pass                       Pass                           Pass PassS7.3.8 Air Pressure         5 psi lost in 5                   Pass                       Pass                           Pass Pass         min with 200 psi         in hoseS7.3.9 Burst Strength         800 psi min.                   1229                       1031                           1526 1240S7.3.10 Tensile Strength150           3/8&#34; or 1/2&#34;         197       265 194 285compressino fittings         lbsS7.3.11 Water Absorption and         requirements as                   225 325 212  281tensile strength         aboveS7.3.12 Zinc Chloride         No cracks with                   Pass                       Pass                           Pass Passresistance    power         magnification__________________________________________________________________________ 
    
     As shown by this example the seven layer embodiment of the present invention meets the technical specifications of the DOT and the SAE and tubing of the present invention is less expensive than reinforced nylon tubing. Thus, utilizing the present invention one achieves savings without reduction in required performance of the air brake tubing.