| Seamless and smooth |
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Bodo Kuhnhenn
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Smooth isn t smooth enough: This was the target set by the developers of the new PU hoses. Using a patented new process that combines the positive features of an extruded hose with the special properties of elastomers, seamless smooth-bore hoses can now be manufactured of polyurethane (PU). Developer of the process and the world s only manufacturer of the new hose class Master-PUR Inline is Masterflex AG, Gelsenkirchen, Germany.
Conventional smooth hoses are extruded items that are spirally wound and have a welded seam. However, it is precisely this welded seam that represents a constructional weak point. In general, this is where abrasion of the hose material due to conveyed material is highest, leading to increased wear.
 | | Table 1 |
Special material
Thermoplastic elastomers are mainly used for the extrusion of spiral hoses. This material class combines the elastic properties of elastomers with the processing advantages of thermoplastics. In principle, this unique material combination can be obtained by the following procedures:
■ During mixing of a thermoplastic with an elastomer, the thermoplastic forms a continuous phase, in which the elastomer constituents are finely distributed. Included in this material group are e. g. thermoplastic vulcanizates, thermoplastic olefin elastomers (TPO), etc.
■ If elastic and thermoplastic elements are combined in a molecular chain, the thermoplastic elements create physical linking points, which permit thermoplastic processing above their softening points. Included in this material class are e. g. thermoplastic polyurethanes (TPU), thermoplastic copolyesters (TPC, also TPEE), etc.
When comparing the properties of the most important thermoplastic elastomers, it becomes clear that TPU is best suited for use in spiral hoses (Table 1).Thermoplastic polyurethanes belong to the block copolymers. Their molecular structures are formed by means of poly addition using long-chained dienes, diisocyanates, and short-chained dienes.Hard and soft segments are created, which consist of different chemical structures and must exhibit a certain incompatibility. In this way, TPUs combine high elasticity with low abrasion to provide outstanding mechanical strength in a wide temperature range.
In-depth knowledge about the properties and the behavior of thermoplastic elastomers were the starting point for development of the new, seamless smooth hose class. During the patented manufacturing process, the individual components for the inner and outer wall layers combine permanently into a firm material. Suitable material combinations of the two components permit the hose to be adapted to the most varied application areas.
Alternative to rubber hoses
Previously, rubber hoses were considered as the cheapest and technically best solutions for conveying bulk materials. But now, the seamless conline hose sets new standards in the area of plastic hoses when compared with rubber hoses. The suction and conveying hose waster-PUR Inline 60/4 was specially developed as the first standard product for the bulk goods industry. For these applications, the demands placed on hoses for conveying abrasive and corrosive media are aimed at cutting operating costs by means of longer service life, thus leading to longer inspection intervals and reduced stocks of spare parts. In addition, properties such as low weight and easy handling are required. Tests conducted according to DIN 53516 in the manufacturer's laboratory have shown that under comparable conditions, the Inline hose will last at least 5.5 times longer than the best rubber hose that was tested (Table 2).
 | | Table 2 |
During the DIN 53516 tests, the lowest abrasion factor was achieved with a seamless Inline hose with a hardness of 60 Shore A. Although abrasion increases slightly with lower Shore values, customers have the choice of hardnesses between 50 and 70 Shore A. Compared with rubber hoses, the potential of the Master-PUR Inline hoses was clearly demonstrated by the abrasion tests. For PU hose with an internal diameter of 100 mm (DN 100), abrasion was 11mm3 - the abrasion measured for the best rubber hose was seven times higher.
Another outstanding property is the high pressure resistance that ranges from about 4 bar operating pressure/9,600 mm water gauge (WG) negative pressure with DN 100 down to 2 bar/9,000 mm WG with DN 200, and 1.35 bar/8,500 mm WG with DN 300.
Moreover, the material's high flexibility permits very small bending radii of 300 mm (DN 100), 450 mm (DN 200), and 800 mm (DN 300). The PU Inline hose is so flexible that it can even be tied into a knot.
Due to its low weight, the Inline hose offers a further great advantage compared with a rubber hose, and thanks to its large temperature range (-40℃ up to +125℃) it opens up a wide range of applications. Even continuous high temperatures within the permissible upper temperature range do not affect the hose. Depending on application area or requirements, wall thicknesses between 1 and 10 mm are possible for the new hose.
Application areas
Seamless smooth hoses are particularly suited for high throughput of extremely abrasive material such as sand, gravel, cement, grain and granulate, etc., other typical applications are found in grain silos. When used for conveying grain, the hose contains a lot of air and relatively small amounts of grain. This leads to very high material speeds: The grains fly through the hoses at up to 180 km/h. At every bend they are slowed down abruptly, resulting in higher hose wear at these points. Thanks to their excellent abrasion resistance, elasticity, and wall thickness, PU hoses withstand these loads far better than conventional conveyor hoses.
Another area where the high load rating of Inline hoses is advantageous is railway track construction, where the hoses are already being used successfully in ballast suction machines that remove the stone chippings from the track bed (Fig. 1).
 | Fig.1.Seamless hoses are already being
used in track construction, where their high
load capacity and long service life are
convincing (photo: Masterflex) |
Particularly in locations with restricted access such as turnouts and tunnels, or for clearing drains, the Inline hoses are subjected to very high loads by the vacuum pumps with ratings of more than 1,600m3/h and a maximum vacuum of about 90 %. The abrasive track ballast, but also oil-contaminated and microbe-infested materials, clay, and sand are extremely demanding on the material. Nonetheless, use of the Inline hoses has resulted in a significant increase of machine service life.
New possibilities
Thanks to the properties of thermoplastic polyurethanes and the patented manufacturing process, numerous possibilities for product design are opened up. Special customized hoses can be manufactured precisely for specific applications, simply by introducing additives in the basic polymer materials. This is far more cost effective and easier to implement than with thermoplastics, which require masterbatch manufacturing or compounding. Another promising application field in future is the foodstuffs industry. The hose is easy cleaning and absolute freedom of dead spaces, and so elastic, that residue build-up is hardly possible or flakes off quickly. Future Master-PUR Inline hoses will be free of softeners and halogens. However, these functions will not necessarily be achieved simultaneously, as different properties could be required for outer and inner hose surfaces. However, the numerous combination possibilities will be highly beneficial for many industries.
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