Method of making a semi-conductive paint hose

#This invention relates generally to composite tubing and more particularly to a hose for transmitting fluids under pressure in a spraying apparatus such as a paint spray.

#Apparatus for spraying liquids such as paint is provided with a pump and hose assembly for transmitting liquid under pressure from a supply tank to a spray gun nozzle. An electrically conductive ground wire is usually included throughout the length of the hose to drain off any static electricity developed at the spray nozzle of the hose to prevent sparking when it approaches a conductor such as a steel beam or the like. A hose having a flexible polymeric core tube, fibrous reinforcing material wound about the core tube and a protective sheath is used to convey the liquid at a pressure which is sufficently high to produce a spray. The hose must be capable of expanding radially under the pulsation of the pump in order to act as an accumulator which attentuates the pulses so as to minimize pulsating at the nozzle and thereby produce a relatively smooth and uniform spray of liquid. On electric motor operated paint spray systems, the accumulator action also retards the rate of pressure change affecting the motor controlling pressure switch, thereby reducing the number of stops and starts of the motor controlling the fluid pump.

#It has been proposed heretofore to wind a ground wire about the core tube as one of the braids of the reinforcing layer. However, the wire frequently broke under pressure and the abrasion of the braid by the wire sometimes resulted in bursting of the hose.

#Other objects will become apparent from the following description with reference to the accompanying drawing wherein:

#FIG. 1 is a cut-away side elevation, partially in section, of one embodiment of the invention; and

#FIG. 2 is a cross-section taken along the line 2--2 of FIG. 1.

#The foregoing objects and others are accomplished in accordance with this invention, generally speaking, by providing a composite reinforced paint hose having a core tube shaped from two layers of synthetic thermoplastic resin which are in intimate contact with each other with the outer layer of the tube wall being a semi-conductive material, one or more layers of a fibrous reinforcing material disposed about the core tube and a protective synthetic resinous sheath disposed about the fibrous reinforcing material. The composite core tube has an inner wall adapted to resist chemical attack by the fluid conveyed by the hose while the coextensive outer layer of the core tube serves as a means for conducting any static electricity along the length of the hose to a suitable ground wire. The invention thus provides a flexible paint hose having a synthetic resinous core tube, a fibrous reinforcing layer about the core tube, a protective sheath about the fibrous layer and a grounding means forming an integral part of the wall of the core tube. Although the laminated core tube may be formed by spraying the inner layer with a conductive coating or by dipping the inner layer in a conductive resin, the core tube is preferably formed by coaxially extruding two different extrudable synthetic polymers to provide a flexible tube having a wall of two synthetic resinous layers. The two layers may be extruded in two separate extrusion steps or they may be simultaneously coextruded so that the layers are melt fused together. The conductive layer extends throughout the entire length of the hose and is in electrical contact with a grounded hose fitting. The grounding layer is a synthetic polymer which is semiconductive, i.e. it exhibits a volume resistivity of 10.sup.9 ohm-cm or less.

#In airless paint spraying processes, it is undesirable to accumulate more than 1400 static volts at the spray gun of the spraying apparatus because of the danger of igniting the air-solvent mixture about the spray gun. Preferably, a safety factor of 4:1 should be maintained which means that the static voltage on the spray end of the hose should not exceed 350 volts. The hose provided by this invention has a ground which is a semi-conductive layer of polymer which will drain off the static potential so a frightening or dangerous electrical-arc between the spray gun and ground is avoided.

#Any suitable conductive synthetic polymer may be used. Preferably, the conductive synthetic polymer is one which will melt fuse to the polymer of the first layer when the two polymers are coaxially extruded. The conductive polymer must have the required electrical properties. While a semi-conductive polymer having a volume resistivity after application and other processing of as high as 100,000 ohm-cm at ambient temperature may be used, usually the volume resistivity will not be more than about 2000 ohm-cm. It is preferred to use one having a calculated volume resistivity of not more than about 500 ohm-cm. In fact, best results have been obtained so far with a polymer having a volume resistivity of 40 or less ohm-cm. The volume resistivity may be calculated by the method described in "NFPA Journal No. 77, Static Electricity, 1972." Examples of suitable polymers are ethylene-ethyl acrylate, ethylene-vinyl acrylate and thermoplastic rubber having the desired volume resistivity but it is preferred to use a polyurethane of the desired volume resistivity in hoses having a nylon inner layer where kink resistance is desirable because the polyurethane bonds well to the nylon and can be bonded to the reinforcing material.