IE41441B1 - Belts - Google Patents

Description

This invention relates to power transmission belts formed from copolyetheresters.

A variety of uses exists for elastomeric materials in applications requiring high-modulus and ultimate strength. Such uses include power transmission belts of the flat and V variety. For such uses there must, of course, remain a slight degree of flexibility within the belt.

In the past, satisfactory power transmission belts have been produced utilizing a variety of different elastomers which have been reinforced with fibers such as nylon. Elastomers which have not been reinforced do not possess sufficient modulus and strength to be used in power transmission belts except in light duty applications.

The use of such reinforced elastomers has presented a problem in that a multi-step operation has been necessary to produce them. The reinforcing cords have to be surrounded by elastomer and this cannot be accomplished expediently.

Thus a need exists for an economical one-step operation for producing a material which will be satisfactory for use as a power transmission belt in heavy duty service.

We have now found that a copolyetherester elastomer can be treated to produce a power-transmission belt having the previously mentioned desired characteristics. We have found that according to our invention, the copolyetherester must be stretched to at least 300% of its original length but not to its breaking point, at a temperature below its melting point by at least 11°C., maintained substantially at that length and brought to or maintained at a heat setting temperature between about 83°C. and 11 °C. below its melting point, and thereafter cooled to a temperature below the heat setting temperature by at least 56°C. in order to be effective as a power transmission belt. The resulting product has outstanding tensile modulus and strength for uses such as in a power belt. Additionally, it still possesses a minor amount of - 2 41441 elasticity so that it may be slightly stretched if necessary. By a minor amount of elasticity it is meant that it has an ultimate elongation of at least 40—60%.

The copolyetherester elastomer which is to be treated by the process of the present invention to form a power-transmission belt comprises a multiplicity of recurring intra-linear long-chain and short-chain units connected head-to-tail by ester linkages, said long-chain ester units being represented by the following structure: 0 IIH —“0 G 0*“--C RC*—— (a) and said short-chain ester units being represented by the following structure: 0 II II —0D0—CRC— (b) , wherein: G is a divalent radical remaining after removal of terminal hydroxyl groups from poly-(alkylene oxide) glycols having a carbon-tooxygen ratio of about 2.0:1 to 4.3:1 and a molecular weight between from 400 and 6000; R is a divalent radical remaining after removal of carboxyl groups from a di carboxylic acid having a molecular weight not greater than 300; and D is a divalent radical remaining after removal of hydroxyl groups from a low molecular weight diol having a molecular weight not greater than 250, with the provisos that the short-chain ester units constitute 15—95% by weight, preferably 25—90% by weight, of the copolyetherester - 3 41441 and ergo, the long-chain ester units constitute 5 to 85% by weight, preferably 10—75% by weight, of the copolyetherester.

The term long-chain ester units as applied to units in a polymer chain refers to the reaction product of a long-chain glycol with a dicarboxylic acid. Such long-chain ester units, which are a repeating unit in the copolyetheresters of this invention, correspond to formula (a) above. The long-chain glycols are polymeric glycols having terminal (or as nearly terminal as possible) hydroxy groups and a molecular weight from 400—6000, The long-chain glycols used to prepare the copolyetheresters of this invention are poly (alkylene oxide) glycols having a carbon-to-oxygen ratio of 2.0:1—4.3:1.

Representative long-chain glycols are polyethylene oxide) glycol, poly(l,2- and 1,3-propylene oxide) glycol, poly(tetramethylene oxide) glycol, random or block copolymers of ethylene oxide and 1,2-propylene oxide, and random or block copolymers of tetrahydrofuran with minor amounts of a second monomer such as 3-methyltetrahydrofuran (used in proportions such that the carbon-to-oxygen mole ratio in the glycol does not exceed about 4.3:1).

The term short-chain ester units as applied to units in a polymer chain refers to low molecular weight compounds or polymer chain units having molecular weights not greater than about 550. They are made by reacting a low molecular weight diol (not greater than 250) with a dicarboxylic acid to form ester units represented by formula (b) above.

Included among the low molecular weight diols which react to form short-chain ester units are aliphatic, cycloaliphatic and aromatic dihydroxy compounds. Preferred are diols with 2—15 carbon atoms such as ethylene, propylene, tetramethylene, pentamethylene, 2,2-dimethyltrimethylene, hexamethylene, and decamethylene glycols, dihydroxy cyclohexane, cyclohexane dimethanol, resorcinol, hydroquinone, 1,5- 4 41441 dihydroxy naphthalene, etc. Especially preferred are aliphatic diols containing 2—8 carbon atoms. Included among the bis-phenols which can be used are bis(p-hydroxy) diphenyl, bis(p-hydroxyphenyl) methane, and bis(p-hydroxyphenyl) propane. Equivalent ester-forming derivatives of diols are also useful (e.g., ethylene oxide or ethylene carbonate can be used in place of ethylene glycol). The term low molecular weight diols as used herein should be construed to include such equivalent ester-forming derivatives; provided, however, that the molecular weight requirement pertains to the diol only and not to its derivatives.

Dicarboxylic acids which are reacted with diols to produce the copolyesters of this invention are aliphatic, cycloaliphatic, or aromatic dicarboxylic acids of a low molecular weight, i.e., having a molecular weight of not greater than 300. The term dicarboxylic acids as used herein, includes equivalents of dicarboxylic acids having two functional carboxyl groups which perform substantially like dicarboxylic acids in reaction with glycols and diols in forming copolyester polymers. These equivalents include esters and ester forming derivatives, such as acid halides and anhydrides. The molecular weight requirement pertains to the acid and not to its equivalent ester or ester-forming derivative. Thus, an ester of a dicarboxylic acid having a molecular weight greater than 300 or an acid equivalent of a dicarboxylic acid having a molecular weight greater than 300 are included provided the acid has a molecular weight not greater than 300. The dicarboxylic acids can contain any substituent groups or combinations which do not substantially interfere with the copolyester polymer formation and use of the polymer of this invention.

Aliphatic dicarboxylic acids, as the term is used herein, refers to carboxylic acids having two carboxyl groups each attached to a saturated carbon atom. If the carbon atom to which the carboxyl group - 5 41441 is attached is saturated and is in a ring, the acid is cycloaliphatic. Some aliphatic or cycloaliphatic acids having conjugated unsaturation often cannot be used because of homopolymerization. However, some unsaturated acids, such a maleic acid, can be used.

Aromatic dicarboxylic acids, as the term is used herein, are dicarboxyl ic acids having two carboxyl groups attached to a carbon atom in an isolated or fused benzene ring. It is not necessary that both functional carboxyl groups be attached to the same aromatic ring and where more than one ring is present, they can be joined by aliphatic or aromatic divalent radicals or divalent radicals such as —0— or —S02—.

Representative aliphatic and cycloaliphatic acids which can be used for this invention are sebacic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, adipic acid, glutaric acid, succinic acid, oxalic acid, azelaic acid, diethyl-malonic acid, allylmalonic acid, 4 - cyclohexene - 1,2 - dicarboxylic acid, 2-ethylsuberic acid, 2,2,3,3-tetramethylsuccinic acid, cyclopentanedicarboxylic acid, decahydro-1,5-naphthalene dicarboxylic acid, 4,4'-bicyclohexyl dicarboxylic acid, decahydro-2,6-naphthalene dicarboxylic acid, 4,4'-methylenebis(cyclohexane carboxylic acid), 3,4-furan dicarboxylic acid, and 1,1-cyclobutane dicarboxylic acid. Preferred aliphatic acids are cyclohexane-dicarboxylic acids and adipic acid.

Representative aromatic dicarboxylic acids which can be used include terephthalic, phthalic and isophthalic acids, bi-benzoic acid, substituted dicarboxy compounds with two benzene nuclei such as bis(p-carboxyphenyl) methane, p - oxy(p - carboxyphenyl) benzoic acid ethylene - bis(p - oxybenzoic acid), 1,5 - naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, phenanthrene dicarboxylic acid, anthracene dicarboxylic acid, 4,4'-sulfonyl dibenzoic acid, and Ci—C12 alkyl and ring substitution - 6 41441 derivatives thereof, such as halo, alkoxy, and aryl derivatives. Hydroxyl acids such as p(g-hydroxyethoxy) benzoic acid can also be used providing an aromatic di carboxylic acid is also present.

Aromatic di carboxylic acids are an especially preferred class for preparing the copolyetherester polymers of this invention. Among the aromatic acids, those with 8—15 carbon atoms are preferred, particularly the phenylene dicarboxylic acids, i.e., phthalic, terephthalic and isophthalic acids and their dimethyl derivatives.

It is preferred that at least about 50% of the short segments are identical and that the identical segments form a homopolymer in the fiber-forming molecular weight range (molecular weight >5000) having a melting point of at least 150°C. and preferably greater than 200°C. Polymers meeting these requirements exhibit a useful level of properties such as tensile strength and tear strength. Polymer melting points are conveniently determined by differential scanning calorimetry.

The short-chain ester units will constitute about 15—95 weight percent of the copolyetherester. The remainder of the copolyetherester will be the long segments, ergo the long segment will comprise about 5—85 weight percent of the copolyetherester. Copolyetheresters in which the short-chain units comprise 25—90 weight percent with 10— weight percent long-chain units are preferred.

The copolyetherester which is preferred is prepared from the dimethyl ester of terephthalic acid, poly(tetramethylene oxide) glycol having a molecular weight of about 600—2000 and 1,4-butanediol.

In order to form a power transmission belt the copolyetherester is stretched by conventional means at least 300% of its original length and preferably at least 400% at a temperature below its melting point by at least 11°C.; it may be stretched to any point short of its breaking point. Obviously, the exact length needed for a given use will vary but it should be emphasized that to achieve the desired character- 7 1441 istics the copolyetherester must be stretched at least 300$ of its original length. Stretching by less than 300$ or at a temperature below the melting point by less than 11°C, does not produce sufficient orientation of the copolyetherester to give tensile modulus and strength high enough for use in heavy duty applications.

Any temperature from ambient to 11°C. below the melting point of the copolyetherester can he used. Preferably, temperatures ranging up to 33°C. below the melting point may be utilized or more preferably up to 89°C. below the melting point.

The copolyetherester elastomers must then be maintained at substantially this length so that it may be heat set. Again, any conventional method may be used for maintaining it at this length, e.g., clamps.

The stretched copolyetherester elastomer must be brought to a heat setting temperature between 83°C. and 11°C. below its melting point.

If the copolyetherester is stretched at a temperature below this range, heat is applied to raise the temperature to the heat setting temperature. On the other hand, if it is stretched at a temperature within the heat setting range, the stretching and heat setting temperatures may be identical and a separate step to apply heat is not required.

Generally, the heat setting operation requires only instantaneous residence time within the heat setting temperature range, although the length of time at the heat setting temperature is not critical. Longer times may be used but in the interest of economy they are kept as short as possible.

Typically, the melting point will be between 171° and 216°C., preferably between 193° and 210°C. For the preferred copolyetherester mentioned above a heat setting temperature of between about 110° and 199° C. will be utilized.

As a consequence of the orientation resulting from stretching and heat setting the copolyetherester will have much different characteristics - 8 41441 than prior to these operations. For instance, the elongation at break for a given slab of copolyetherester will be between about 30 and 120%, preferably between about 40 and 70%. Prior to the stretching and heat treating the elongation at break would be between about 300 and 900%. preferably between about 500 and 800%.

Physical properties of the oriented copolyetherester are excellent. Both tensile modulus and strength of the unreinforced oriented polymer are similar to those of conventional elastomers which have fiber or fabric reinforcement. Consequently, the oriented copolyetherester is suitable for those applications requiring such reinforcement, such as power transmission belts, but has the advantage that the costly fabrication steps to provide the reinforcement are not required. Another advantage of the oriented polymer relative to its unoriented state is its greater ability to retain its strength at elevated temperature making it more suitable for use at higher temperatures. Furthermore, it is yet an elastic material possessing low hysteresis. This combination of properties is desirable in materials from which power transmission belts are made because it generally results in low heat buildup and therefore a lower operating temperature when the belt is used in heavy duty service. Thus a product has been produced which possesses both high strength and a small degree of flexibility. The flexibility will permit an elongation at break of 40 to 60%.

The cooling of the copolyetherester after heating may be effected by any desired means; the preferred method would be cooling at room temperature but if desired external refrigeration may be utilized to reach ambient conditions. The pressure needed to keep the copolyetherester at the desired elongation may be supplied by any conventional means such as appropriately sized clamps and it is not released until the temperature has been lowered by at least 56°C. below the heat setting temperature. - 9 41441 Power transmission belts of the oriented copolyetherester may be made in a number of ways. For example, a billet can be molded from the polymer in a conventional manner and the billet oriented by stretching, heat setting, and cooling according to the processes of the instant invention. An endless belt is then formed by joining the ends of a desired length of the copolyetherester so oriented using either a cement or the application of heat in the immediate vicinity of the ends to fuse them together. Another method that can be used is to fashion a circular band from the copolyetherester in a conventional manner and then orient the band. The latter method avoids the necessity of having to form a joint in the belt.

The copolyetheresters described herein can be made conveniently by a conventional ester interchange reaction. A preferred procedure involves heating the dicarboxylic acid, e.g., dimethyl ester of terephthalic acid with a long-chain glycol, e.g., poly(tetramethylene oxide) glycol having a molecular weight of 600—2,000 and a molar excess of diol, e.g., 1,4-butanediol in the presence of a catalyst at about 150°— 260°C. and a pressure of 0.5 to 5 atmospheres, preferably ambient pressure, while distilling off methanol formed by the ester interchange. Depending on temperature, catalyst, glycol excess, and equipment, this reaction can be completed within a few minutes, e.g., 2 minutes to a few hours, e.g., 2 hours.

Concerning the molar ratio of reactants, at least about 1.1 mole of diol should be present for each mole of acid, preferably at least about 1.25 mole of diol for each mole of acid. The long-chain glycol should be present in the amount of 0.0025 to 0.85 mole per mole of dicarboxylic acid, preferably 0.01 to 0.6 mole per mole of acid.

This procedure results in the preparation of a low molecular weight prepolymer which can be carried to the high molecular weight copolyetherester of this invention by the procedure described below. Such - 10 41441 prepolymers can also be prepared by a number of alternate esterification or ester interchange processes; for example, the long-chain glycol can be reacted with a high or low molecular weight short-chain ester homopolymer or copolymer in the presence of catalyst until randomization occurs. The short-chain ester homopolymer or copolymer can be prepared by ester interchange from either the dimethyl esters and low molecular weight diols, as above, or from the free acids with the diol acetates. Alternatively, the short-chain ester copolymer can be prepared by direct esterification from appropriate acids, anhydrides, or acid chlorides, for example, with diols or by other processes such as reaction of the acids with cyclic ethers or carbonates. Obviously, the prepolymer might also be prepared by running these processes in the presence of the long-chain glycol.

The resulting prepolymer is then carried to high molecular weight by distillation of the excess of short-chain diol. This process is known as polycondensation.

Additional ester interchange occurs during this polycondensation which serves to increase the molecular weight and to randomize the arrangement of the copolyetherester units. Best results are usually obtained when this final distillation or polycondensation is run at less than 5 mm. pressure and 200°—270°C. for less than two hours, e.g., 0.5 to 1.5 hours.

Most practical polymerization techniques rely upon ester interchange to complete the polymerization reaction. In order to avoid excessive hold time at high temperatures with possible irreversible thermal degradation, a catalyst for the ester interchange reaction should be employed. While a wide variety of catalysts can be employed, organic titanates such as tetrabutyl titanate used alone or in combination with magnesium or calcium acetates are preferred. Complex titanates, such as MgfHTI(OR)el2> derived from alkali or alkaline earth metal alkoxides - 11 41441 and titanate esters are also very effective. Inorganic titanates, such as lanthanum titanate, calcium acetate/antimony trioxide mixtures and lithium and magnesium alkoxides are representative of other catalysts which can be used.

The catalyst should be present in the amount of 0.005 to 0.2% by weight based on total reactants.

Ester interchange polymerizations are generally run in the melt without added solvent, but inert solvents can be used to facilitate removal of volatile components from the mass at low temperatures. This technique is especially valuable during prepolymer preparation, for example, by direct esterification. However, certain low molecular weight diols, for example, butane diol in terphenyl, are conveniently removed during high polymerization by azeotropic distillation. Both batch and continuous methods can be used for any stage of copolyetherester polymer preparation. Polycondensation of prepolymer can also be accomplished in the solid phase by heating divided solid prepolymer in a vacuum or in a stream of inert gas to remove liberated low molecular weight diol. This method has the advantage of reducing degradation because it must be used at temperatures below the softening point of the prepolymer.

The dicarboxylic acids or their derivatives and the polymeric glycol are incorporated into the final product in the same molar proportions as are present in the ester interchange reaction mixture. The amount of low molecular weight diol actually incorporated corresponds to the difference between the moles of diacid and polymeric glycol present in the reaction mixture. When mixtures of low molecular weight diols are employed, the amounts of each diol incorporated is largely a function of the amounts of the diols present, their boiling points, and relative reactivities. The total amount of diol incorporated is still the difference between moles of diacid and polymeric glycol. - 12 41441 Most preferred copolyesters which are stabilized by the process of this invention are those prepared from dimethyl terephthalate, 1,4butanediol, and poly(tetramethylene oxide) glycol having a molecular weight of 600—2000 or poly(ethylene oxide) glycol having a molecular weight of 600—1500. Optionally, up to about 30 mole percent and preferably 5—20 mole percent of the dimethyl terephthalate in these polymers can be replaced by dimethyl phthalate or dimethyl isophthalate.

Other preferred copolyesters are those prepared from dimethyl terephthalate, 1,4-butanediol, and polypropylene oxide) glycol having a molecular weight of 600—1600. Up to 30 mole percent and preferably —25 mole percent of the dimethyl terephthalate can be replaced with dimethyl isophthalate or butanediol can be replaced with neopentyl glycol until up to about 30% and preferably 10—25% of the short-chain ester units are derived from neopentyl glycol in these polypropylene oxide) glycol polymers. The polymers based on poly(tetramethylene oxide) glycol are especially preferred because they are easily prepared, have overall superior physical properties, and are especially resistant to water.

The most preferred copolyetherester compositions may also contain up to 5 weight percent of an antioxidant, e.g., between 0.2 and 5 weight percent, preferably between 0.5 and 3 weight percent. The most preferred antioxidants are diaryl amines such as 4,4'-bis(a,a-dimethylbenzy1) diphenyl amine. The antioxidant may be added during the reaction by means of which the copolyetherester is formed. In fact, it is preferred that an antioxidant be present at any point during the process where the poly(alkylene oxide) glycol is exposed to elevated temperatures, e.g., above 100°C. The antioxidant may, of course, be introduced at any stage in the process and even after preparation of a copolyetherester is complete.

The properties of the copolyetheresters may also be modified by - 13 41441 incorporation of various conventional organic fillers, such as carbon black, silica gel, alumina, clays and chopped fiber glass.

All parts, proportions and percentages disclosed herein are by weight unless otherwise indicated.

The following preparations and Example further illustrate the invention: Preparation 1.

Preparation of Copolyetherester.

A copolyetherester is prepared by placing the following materials in an agitated flask fitted; for distillation.

Polytetramethyleneether glycol; number average molecular weight about 975 38.5 parts 1,4-Butanediol 36.5 parts Dimethyl terephthalate 60.0 parts 4,4' - Bis(alpha, alpha - dimethyl benzyl) di phenyl amine 1.05 parts Catalyst 2.1 parts A stainless steel stirrer with a paddle cut to conform with the internal radius of the flask is positioned about 3.2 mm from the bottom of the flask and agitation is started. The flask is placed in an oil bath at 160°C., agitated for five minutes and then the catalyst is added. Methanol distills from the reaction mixture as the temperature is slowly raised to 250°C. over a period of one hour. When the temperature reaches 250°C., the pressure is gradually reduced to 0.3 ran Hg within 20 minutes. The polymerization mass is agitated at 250°C,/ 0.3 mm Hg for 35 minutes. The resulting viscous molten product is scraped from the flask in a nitrogen (water and oxygen free) atmosphere and allowed to cool.

The polymer has an inherent viscosity of 1.40 at a concentration of 0.1 g/dcl. in m-cresol at 30°C. and a Shore D hardness of 55 and a melt- 14 41441 ing point of 211 °C.

Preparations 2 through 8.

Orientation of Copolyetherester.

A specimen of the copolyetherester prepared in Preparation 1 measuring 2.54 cm x 15.24 cm is cut from a calendered film of 10—13 mils thickness. The specimen is stretched 400$ at one temperature. After being stretched 400$, the specimen has a length five times its original length. While the stretching force is retained to maintain the elongation at 400$, the stretched specimen is adjusted to a second temperature which is the heat setting temperature and then adjusted to a third temperature at which point the specimen is released from the stretching force.

Preparations 2 through 7 in the accompanying Table show the physical properties of the copolyetherester after orientation by use of various stretching, heat setting, and release temperatures. Preparation 8 shows a specimen which is stretched without receiving a heat setting treatment. Comparison of Preparations 2 through 7 with Preparation 8 illustrates the beneficial effects of the heat setting treatment on modulus and tensile strength. - 15 41441 Example Fabrication of a V-Belt.

A 1.52 m, long mold which has been preheated to 149°C. is filled by extruding therein the copolyetherester prepared in Preparation 1.

The mold cavity has a cross section perpendicular to its long axis that is trapezoidal in shape, the longer of the parallel sides having a length of 2.22 cm with both angles adjacent to the longer of the parallel sides being 71° and the distance between the parallel sides being 1.50 cm. The mold is cooled to room temperature and the part removed.

The molded part is stretched at room temperature by 4002! of its original length. In this operation, the 1.52 m. long molded part is stretched to a new length of 7.62m. The stretching force is retained while the stretched part is exposed to hot air for 30 minutes at a temperature of 177°C. and then cooled to room temperature at which point the stretching force is removed from the oriented part.

A V-belt is formed by selecting a 0.91 m. length of the oriented part and cutting the ends at a 45° angle. Both ends of the cut section are brought into contact with a surface heated to 232°C.—288°C. for sufficient length of time to cause melting of the copolyetherester in the immediate vicinity of the ends. The molten ends are removed from the hot surface and immediately joined by bringing them into contact with each other and holding until the molten material has solidified.

When cooled, the joint is trimmed to remove excess material.

Claims (13)

1. WHAT WE CLAIM IS:1. A power transmission belt formed from an oriented copolyetherester elastomer which elastomer comprises a multiplicity of recurring long-chain ester units and short-chain ester units joined head-to-tail 5 by ester linkages, said long-chain ester units being represented by the formula I 0 0 II H —OGO—CRC— and said short-chain units being represented by the formula II 0 0 II II —000—CRC— 10 where G is a divalent radical remaining after the removal of terminal hydroxyl groups from a poly(alkylene oxide) glycol having a molecular weight of 400—6000 and a carbon-to-oxygen ratio of from 2.0:1 to 4.3:1; R is a divalent radical remaining after removal of carboxylic groups from a dicarboxylic acid having a molecular weight not greater than 15 300 and D is a divalent radical remaining after removal of hydroxyl groups from a diol having a molecular weight not greater than 250-, provided said short-chain ester units amount to 15—95 percent by weight of said copolyetherester; wherein said elastomer is oriented by the steps of (A) stretching said elastomer by at least 300% of its original 20 length, (B) bringing said elastomer to a temperature between 83° and 11°C. below the melting point of said copolyetherester, and (C) cooling said elastomer while stretched to a temperature below the temperature of step (B) by at least 56°C.

2. A power-transmission belt as claimed in claim 1 wherein the 25 elastomer is oriented by stretching by at least 400% of its original length at a temperature below its melting point by at least 11 °C. - 18 41441

3. A power-transmission belt as claimed in claim 1 or claim 2 wherein the elastomer is stretched at a temperature up to 33°C. below the melting point.

4. A power-transmission belt as claimed in claim 3 wherein the temperature is up to 89° C. below the melting point.

5. A power-transmission belt as claimed in any of claims 1—4 wherein the melting point of the copolyetherester is from 171° to 216°C.

6. A power-transmission belt as claimed in claim 5 wherein the melting point is from 193° to 210°C.

7. A power-transmission belt as claimed in claim 5 wherein the heat-setting temperature is from 110° to 199°C.

8. A power-transmission belt as claimed in any of claims 1—Ί wherein the elongation at break for a given slab of copolyetherester is from 30 to 120%.

9. A power-transmission belt as claimed in claim 8 wherein the elongation at break is from 40 to 70%.

10. A power-transmission belt as claimed in any of claims 1—9 wherein the group R in the copolyetherester is a 1,4-phenylene group, the group G is derived from a poly(tetramethylene oxide) glycol having a molecular weight of from 600—2,000 and the group D is derived from 1,4-butanediol.

11. A power-transmission belt as claimed in any of claims 1—10 wherein at least 50% of the short segments are identical and the identical segments form a homopolymer in the fibre-forming molecular weight range (>5000) having a melting point of at least 150°C,

12. A power-transmission belt as claimed in claim 1 substantially as hereinbefore described.

13. A power-transmission belt as claimed in claim 1 substantially as hereinbefore described with reference to the Example.