Polyurethane in Textile Machinery

The most importhant application to date in the textile field has been the use of polyurethane elastomers for loom pickers. Here the application is vary arduous, the shuttle weighing about 1 lb traversing a 4 ft loom approximately 150 to 200 times a minute. Previously the materials chiefly used were either raw hide or rubber-fabric composition. On the average polyurethane pickers outlast other types of pickers but they do exhibit a number of unexplained early faileres. The life of the picker is very dependent on the correct setting of the picker in relation to the shuttle, and if badly aligned, the picker will fail rapidly. The search for even better pickers is still proceeding, and the most recent design incorporate a wearing block of high density polyethylene inside a resilient body of a hard thermoplastic polyurethane.

Other application in the textile and allied field include falling bars for selector needles on the knitting machines, cot roller covering, and drive couplings for bobbins.

Polyurethane in Paper-making and Printing

Solid polyurethane are being used successfullyfor the top of vacuum boxed on paper-marking machinery. A gauze holding the paper fibers travels across the vacuum box and excess water is removed by vacuum. The polyurethane is less harsh to the guaze than other materials and is itself resistant to wear. This is an example where polyurethanes are used sucessfully in water at a temperature around 50 *C .

Special grades of very soft polyurethane elastomers have been used for many year for printing rollers. The hardness of these materials vary from around 10 to 30 Shore A and the physical properties are poor compared with what is normally expected from polyurethane elastomers. However, solid polyurethane around a hardness of 60 to 70 Shore A are being used for scraper blades on silk-screen printing due to their solvent resistance and low ink pick up.

Polyurethane Properties : Effect of low temperature

Low temperature environments affect the properties of polyurethane elastomers, but no degradation occurs and effect is completely revesible. The major effect is an increase in Young’ s modulus below 0 *C with a corresponding increase in hardness, tensile strength, tear strength and torsional stiffness and a decrease in resilience.

The two properties that can have an important bearing on the use of polyurethane at low temperature are the decrease in resilience which has already been mentioned and the increase in stiffness. In the use of polyesterurethane at low temperatures another phenomenon can sometimes be observed. This is that of crystallization of the polyurethane when held at moderately low temperature foe some period of time. When crystallized the polymers are considerably harder and their stiffness charecteristics are similar to those normally observed at much lower temperatures. The effect is reversible and can be removed by the application of heat or by the generation of internal heat by flexing. The use of mixed polyesters reduces this tendency to crystallization.

Polyurethane in Shoes and Shoe Machinery

The use of solid polyurethane rubber for ladies’s shoe heel top pieces has been known for several year. Cast grades of medium to high hardness have generally been employed for this purpose, but more recently the thermoplastic polyurethane have been graining ground. The material cost of polyurethane are high compared with more conventional heeling materials and to be competitive the processing cost must be as low as possible. The high cost has so far prevented the use of solid polyurethanes for the soles of shoes but the development of the cellular varieties of the solid materials has reawakened interest in this outlet. By using cellular grades with a specific gravity of 0.4, material cost are reduce to a third of those of the solid. Provided the process can be refined sufficiently to keep the costs low, soles for hard-wearing applications are a distinct possibility. In this case closed cell matreial is necessary to prevent excessive uptake of water.

The process for shoe manufacture is becoming highly automated. The high hardness, toughness and flexibility of the solid polyurethanes make them excellent materials for many items in this application. Replacement of a natural rubber diaphragm by polyurethane increased the diaphragm life from two months to a yeas.

Using polyurethane in mining and quarrying industries.

In the mining and quarrying industries, however, polyurethane is being used for many other applications with advantage and replacing rubber or even hard steel. In general these applications are connected with movement of very abrasive slurries of relatively small particle size such as in the ball and rod mills, flotation plant, hydrocyclones and piping.

Ball or rod mills have generally been lined with manganese steel replaceable linings, but in recent years the use of rubber replaceable linings has been developed. Dependent on the condition existing in a given mill these rubber linings can last several times longer than the manganese steel, and polyurethane linings can last considerably longer than the rubber linings. Further advantages are that the tooling cost with polyurethane are less and the fact that, due to the availability of the harder grades, the polyurethane can be used as a structural material and does not require reinforcement by metals.

Similarly in flotation plant the impellers, rotors, stator and scrapers are made of steel centers often covered in conventional elastomers. Polyurethane again can be used in certain cases and will generally outlive natural rubber. The main precaution to be observed are limitation in temperature and also extremes in acidity or alkalinity of the slurries.

Hydrocyclones used in the separation of ores and cray are also subjected to harsh abrasive condition, and polyurethane have been successfully used in these conditions.

In mines there is often a considerable fire risk, and comprehensive precaution are taken to avoid spark which can start a fire or explosion. For this reason the use of track laying vehicle with metal tracks is prohibited. Covering of the tracks with natural rubber goes only part of the way to solving the problem due to highly abrasive conditions causing relatively rapid breakdown. The polyurethane track pad do not wear so rapidly and are being used successfully. A further advantage of polyurethane as opposed to metal is the increase in traction obtained which enable the vehicle to climb steeper slopes.

Polyurethane in Automotive Industry : Gear change levels

Polyurethane has replaced both rubber and metal in several components used in the modern car. The properties attractive to the design engineer have again been the general wear and tear characteristics coupled with oil and fuel resistance. The remote control gear change mechanism on many car are subject to considerable vibration and can be the source of irritating noise. Wear on the ball joint can also lead to a less positive action. Both these effects can be minimized by the covering of the balls with polyurethane or by the use of polyurethane housings. A further advantage in the case of the ball joint is that no accurate machining of special mating surfaces is required. If the socket and ball are both metal they require accurate machining. The polyurethane housing eliminates one surface completely and the elastic nature of the material overcomes the necessity of the machining the ball itself to highly accurate dimensions. The gear change mechanism shown is for floor-mounted levers. Gear change mounted on the steering column generally involve a highly articulated machanism, and the use of polyurethane housings in all the ball joint can reduce metallic clatter to a minimum and provide a cushioning effect on the system.

Polyurethane tires are used on rotary screen plant

Polyurethane tires are used on rotary screen plant. Rotary screen plant is used extensively in the washing and grading of gravel and sand and also for heat drying of road stones. Other user include the preparation of natural and artificial fertilizers. Where the drum loading and temperature are such that the natural rubber can be used the substitution of polyurethane gives the expected advantages of higher load capacity and longer life. In many case however steel rollers have been in the past used in place of natural rubber due to the high loading required. In these case because of the highly abrasive environment, severe wear can take place both on the steel roller and the steel track on the drum itself. This wear leads to increased vibration in the plant which in turn can result in fractures of welds and rivets. To avoid serious damage the drum tracks have to be remachined at intervals during which the plant is inoperable. More recently these steel rollers have been successfully replaced by polyurethane tires in a number of cases. Taking a typical plant, for example, having an output of 50 to 60 tons per hour of washed gravel the weekly output is equivalent in value to ten times the cost of a set of polyurethane tires to replace the steel tires. The use of polyurethane tires reduces the vibration considerably and substantially reduces the maintenance time required. It also considerably reduces the noise level. In certain case the rotation of the drum can be carried out using polyurethane tires, with a further reduction in vibration.

Polyurethane tires : Overhead cranes

Polyurethane tires are being successfully used on overhead cranes. In these instances the conventional flanged wheel and rail can be replaced by a polyurethane tires which runs directly on the top girder or structure to which the rail would previously be fixed. A polyurethane side rollers is used at the same time to prevent undue sideways movement. The crane is driven by power transmitted througth the tires and the overall result is a noticeable reduction in noise and vibration. Noise elimination is an important factor in modern workshop, and reduction of vibration result in saving on maintenance cost.

A similar use for polyurethane tires is on sewang plant settling tanks. The conventional method is the use of steel wheels running on the circular track, both wheels being driven by a fairly complicate drive mechanism. The substitution of polyurethane tires for the steel tires gives greatly increased traction and allows the replacement of the normal drive mechanism by a single motor on one wheel. This still gives better traction than by driving the two steel wheels separately, especially when the plant is operating in wet condition.

Pneumatic and Hydraulic Polyurethane Seals

One Example of the successful use of polyurethane for sealing is an Oleo-Pneumatic buffer for use on railway rolling stock. The pressures encountered in this application are not particularly high but an efficient seals is of prime importance. In this buffer compressed air or nitrogen is utilized as the spring medium for providing static resistance and recoil, the gas being sealed into the buffer behide the oil by a floating piston. It is therefore very important for the seal carried on the floating piston to be highly efficient throughout the working lift of the piston. Operational experience showed that the original seal in nitrile rubber had inadequate wear resistance and this is one of the main factor in buffer service life. After three years operational experience with polyurethane seals the outstanding properties of polyurethane were quite apparent. Seals examined from these buffers, having operated under the most arduous conditions which would have completely destroyed the nitrile seal, showed virtually no wear.

Polyurethane seals can also be used to advantage in hydraulic pit-props for roof supports and other applications. These props operate at relatively high pressure, but the seal must also prevent leakages at low pressures when the prop is out of operation.

Another use for polyurethane seals in the mining industry is found in the control valve unit of chocks. The seal in this case also acts as a seal for high pressure bleed valve. There is a high velocity flow of hydraulic fluid across the sealing face which rapidly scours nitrile rubber, whereas the polyurethane, being more wear resistant, operates satisfactorily.

Other uses for polyurethane hydraulic seals are found where the equipment is used in dirty condition, such as in the case of earthmoving equipment where the efficient operation of seals is of importance in view of hire charges and cost of down-time arising from failures.

Polyurethanes : Block and Sheet shape

Cast polyurethan elastomers, is the ease with which blocks of different sizes can be manufactured. The tooling costs are very low and due to the simplicity of shape they can be quickly made. Coupled with this is the suitability of most types of polyurethane, in particular the harder grade, for many machining operations, as already discussed in the next article. This enable in certain cases different designs to be turned out quickly fo evaluation and choice of optimum design.

Apart from blocks there is a considerable call for sheeting in various thicknesses for a wide variety of applications. Sheets are usually manufactured in a cast grade in a centrifuge. This method limits the length and width obtainable, and the largest sheets available aer of the order of 15 x 3 ft with thicknesses from 0.030 to 0.50 inches. Longer sheets can noe be made by peeling from a cylindrical block, although the thickness is limited in such cases to around 1/8 inch.

Larger sheets can also be calendered using either the millable or thermoplastic grades and the blown film extrusion of the latter materials gives very thin sheeting in long lengths. Polyurethane sheets are also available with a pressure sensitive adhesive and a non-stick removable backing paper.

Sprayable Polyurethanes

The sprayable types of polyurethane are being used where large or awkwardly shaped surfaces require covering. Before the advent of these materials castable polyurethane were used, but either the moulds required were complicated and therefore expensive. Many of the applications therefore are not entirely new but the use of the sprayable materials has resulted in lower costs and more widespread use.

One example of successful use of sprayable polyurethane is the lining of vibrator finishing barrels. The barrel are filled with an abrasive substance and untrimmed forgings and casting with the object of deburring and polishing the metal parts. The whole action is very abrasive but the sprayed coating is giving a long useful life. A slide valve on a sand hopper is another proven application, as also is the lining of a cement mixing barrel. Other uses includes the covering of wear plates in shot-blast equipment, linings of earth dumper trucks, fertilizer hoppers and even ships’s propellers.

The process is also suitable for the covering of large rollers where the tooling cost for cast polyurethanes would be prohibitive. When spraying, the rollers only need supporting between centres and rotating slowly. Similarly, the sprayable materials offer a quick and economic method for the coating of long lengths of fabric.

Polyurethane elastomer in Aircraft Industry

One problem specific to the aircraft industry is the protection of leading surfaces against rain, dust and stone damage. The main surfaces in question are propellers and helicopter bladed, although leading edges of the mainplane and even the taiplane and fin are now being protected. On propellers de-icing mats are used to prevent icing up, and these are generally covered with a thin sheet of polyurethane. This protects the outer layer of nitrile rubber on the mat from stone and dust damage when landing or taking off and also against rain erosion during flight.

Helicopter blades can be made from either a special alumenium alloy or stainless steel, both of which at high speeds aer susceptible to damage, espectively in desert condition. One report indicates the the lift of the rotor blade increased from 40 to 1,000 hours after installation of protective polyurethane mats. This result in a very considerable saving in cost for replacement and also increases the operational life between changes.

Polyurethane lining : Pulleys

In both coal-mining and quarrying, belt conveyors are used to convey the coal or rock from one location to another. One type of conveyor used is a continuous belt to which a steel rope is attached at either side. This rope runs on pulleys and provided the motive power for belt itself. When using metal pulleys two problem arise, wear on the pulley and wear on the steel rope. The latter problem may prove to be more important, since repair or replacement involves considerable maintenance time and the rope itself is very expensive. Lining the faces of the pulley with polyurethane has a twofold advancetage. Wear on the rope is significantly reduced with a resultant saving on the rope cost. Breakdown time is also less, and since output rates of up to 1,000 tph. Another factor is that the pulley life is increased and there is a very marked drop in noise level, which is very real advantage in coal mines.

Another application for polyurethane lining is on the pulleys employed in the installation of overhead electrical transmission lines. The aluminium sheated transmission line are atthached to a relatively small diameter steel rope during the installation and the pulley has to be capable of taking the high unit load from the steel rope and yet be soft enough not to damage the alumenium sheathing.

Figure 1 Pulley (Polyurethane lining) http://www.indiamart.com

Polyurethane rollers

These rollers are used to deliver case of beer to an automatic palletizing machine and are located at the end of a belt conveyor and at right angles to the conveyor. Polyurethane is also used extensively where rollers are handling sharp-edged or abrasive materials. Examples in this case are rollers for feeding thermoplastic strips to granulators, mills for dehusking rice, and rollers in a glass fibre chopping plant where the cutting of the glass fibre takes place on the rollers itself. Second examples include feed rollers in a plant for the sanding of plywood pannels, computer programme card feed rollers, and guide rollers in a plant for the manufacture of plate glass. The cement conveyor was originally fitted with cast iron rollers witch required replacement in less than 12 week. Substitution of polyurethane for the cast iron showed an immediate improvement and the useful life of the rollers was increased to more than 12 months.

These few example give an indication of the wide use of polyurethane rollers and the very definite advantages to be gained from their use.

Rubber Seals : Use of Polyurethane elastomers

Another large field of application for polyurethane is that of rubber seals .The rubber seals can be static , reciprocating or rotary and can be used in pneumatic or hydraulic system. Since the rubber seals can be in the form of O-ring ,lip seals or simple square section seals.

The economic advantages of polyurethane as a rubber seals are associated with its improved wear resistance and lower friction.Up to pressures of about 1500 psi nitrile rubber sealing are generally quite satisfactory but above this pressure the use of plastic or fabric reinforcement is required to avoid extrusion of the seal. The hard grades of polyurethane are able to seal quite successfully without reinforcement up to pressure of aproximately 6000 psi.

Since the seals are generally required to have the highest possible physical properties the cast polyurethane have been widely used in the past.

When hydraulic fluids are present the main limitation on the use of polyurethane is that of restricted temperature range. Under dry condition , whether pneumatic or hydraulic , an upper temperature limit of around 70 *C for continuous operation should not be exceeded. When the oil is contaminated with water or a water-base hydraulic fluid is used the upper temperature limit is reduced to 40 *C. Certain synthetic oils also attack polyurethane.

Although the abrasion resistance of polyurethane is exceptional at low abrasive speeds, high speeds can cause premature breakdown. For this reason the use of polyurethanes for rotary seals is generally restricted to maximum surface speeds 1 – 2 ft/sec.