A polyurethane-forming system, a composite comprising the polyurethane-forming system and a fiber-reinforced material, a process for the production of the composite, and the use of the composite TECHNICAL FIELD The present invention belongs to the technical field of the polyurethane composite materials. In particular, the present invention relates to a polyurethane-forming system which includes at least one sorbitol initiated polyol, to a composite comprising the polyurethane-forming system of the present invention and a fiber-reinforcing material, to a process for the production of the composite, and to the use of the composite for making large composite parts used in outdoors. BACKGROUND Traditional thermoset resins like epoxies, unsaturated polyesters, and phenolics are too brittle to realize the ultimate potential of reinforcement. Polyurethanes has good inherent toughness and thus enables better mechanical properties of composites, but its much short gel time brings challenges for the traditional composite manufacturing process. Hereinafter the polyurethane system is mainly based on the reaction between isocyanate and polyol containing hydroxyl and comprises the urethane structure in the polymer chain. A number of efforts and teachings of prior arts therefore are reported to extend the gel time of polyurethane-forming system which makes it suitable for the manufacturing process typically pultrusion, filament winding and vacuum infusion etc. However, those methods to improve gel time usually lead to poor mechanical properties and low heat resistance which in some cases are of prominent importance for successful application. US patent No.7056976 teaches a polyisocyanate-based reaction system of improved processing characteristics i.e. a gel time at 25℃ in the range of from 1000 seconds to 4000 seconds and at 175℃ of less than 120 seconds to produce reinforced composites by pultrusion process. US patent No.9580598 issued to Younes published a polyurethane system having a gel time longer than 60 minutes at room temperature and thus satisfying the manufacture of composites by vacuum infusion process which requires a long operation time. US patent No.10144845 issued to Li et. al discloses a polyurethane system applicable for filament winding process which contains cardanol modified epoxy polyols to realize long gel time and decrease in sensitivity to moisture. All these arts abovementioned focus on gel time extension for better processability while neglect the adverse influence on the heat resistance of the polyurethane resin. Heat resistance of a polymer is characterized by the glass transition temperature Tg. A higher glass transition temperature means better heat resistance. Polyols blend with optional additives is one component of polyurethane system and is normally worked to give a long gel time and meanwhile a good heat resistance. Although it is well known that polyols of rigid chain or of high functionality are favorable to increase Tg, there exists drawback of decrease in gel time and toughness. CN 109265639A teaches a polyurethane system containing a bisphenol A initiated polyol of improved heat resistance for the use in pultrusion process. Sorbitol initiated polyols with a very high molecular weight are reported in US 006103851A for the use in polyurethane elastomer of improved ageing properties but containing no reinforcements. It is often the case, however, that the mechanical properties of the polyurethane at high temperatures are inadequate for individual applications especially in reinforced composites. Therefore, there is a great demand for the polyurethane for use in the reinforced composites, which can realize satisfactory gel time and meanwhile achieve improved heat resistance. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a polyurethane-forming system with long gel time and improved heat resistance for use in the production of fiber-reinforced polyurethane composites. According to the invention, the polyurethane-forming system comprises: (A) an isocyanate-reactive component containing at least one sorbitol initiated polyol; and (B) a di- and/or polyisocyanate component, wherein the sorbitol initiated polyol has an OH number of from 300 to 650 mg KOH/g and a functionality of from 2.5 to 6.0. It is also an object of the present invention to provide a composite comprising the polyurethane-forming system of the present invention and a fiber-reinforcing material. It is another object of the present invention to provide a process for the production of a composite by impregnating a fiber-reinforcing material with a polyurethane-forming system of the present invention, and curing the fiber-reinforced polyurethane-forming system. It is an additional object of the present invention to provide the use of the composite of the present invention for making large composite parts used in outdoors. The invention is based on the finding that the sorbitol initiated polyols comprised in the polyurethane-forming system have slow reactivity and are capable to increase glass transition temperature of polyurethane. It has been found that, surprisingly, the inventive polyurethane-forming system comprising sorbitol initiated polyols not only exhibits long gel time but also has improved heat resistance. Moreover, the reinforced composites obtained by using the polyurethane-forming system of the present invention exhibit good mechanical properties at high temperatures. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described for purposes of illustration and not limitation. Except in the operating examples, or where otherwise indicated, all numbers expressing quantities, percentages, OH numbers, functionalities and so forth in the specification are to be understood as being modified in all instances by the term “about.” Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which the invention belongs. As used herein, the following terms have the meanings ascribed to them below, unless specified otherwise. As used herein, the articles “a” and “an” refer to one or more than one (i.e., at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. As used herein, the term “about” is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result. As used herein, the term “sorbitol initiated polyol” means a polyol that has been produced by using sorbitol as the main initiator. As used herein, the term “bisphenol initiated polyol” means a polyol that has been produced by using bisphenol as the main initiator. All the embodiments and the preferred embodiments disclosed herein can be combined as desired, which are also regarded as being covered within the scope of the present invention. Unless otherwise identified, the temperature refers to room temperature and the pressure refers to ambient pressure. Unless otherwise identified, all percentages (%) are “percent by weight". All values for molecular weight are based on weight average molecular weight, unless indicated otherwise. In one aspect, the present invention provides a polyurethane-forming system for the production of composite materials comprises: (A) an isocyanate-reactive component containing at least one sorbitol initiated polyol; and (B) a di- and/or polyisocyanate component, wherein the sorbitol initiated polyol has an OH number of from 300 to 650 mg KOH/g and a functionality of from 2.5 to 6.0. It has been found that the inclusion of a sorbitol initiated polyol in the isocyanate-reactive component has unexpected benefits. Any of the known sorbitol initiated polyols may be used in the practice of the present invention. The sorbitol initiated polyols employed in the practice of the present invention will generally have an OH number of from 300 to 650 mg KOH/g, preferably, from 350 to 550 mg KOH/g, most preferably, from 400 to 500 mg KOH/g and a functionality of from 2.5 to 6.0, preferably, from 3 to 6, most preferably, from 5 to 6. Preferably, the sorbitol initiated polyol has a molecular weight Mw in the range from 200 to 1000 g/mol, preferably from 300 to 700 g/mol. This sorbitol initiated polyol may constitute up to 50% by weight of the components (A) of the present invention, but will generally be included in the component (A) in an amount of at least 5 wt%, preferably, in an amount of from 10 wt% to 30 wt%. In the production of the sorbitol initiated polyols, it is also preferable to add another initiator together with sorbitol so as to reduce the viscosity of the sorbitol initiated polyols, if possible. Examples of such suitable initiators are ethylene glycol, propylene glycol, 1,4-butanediol, 1,3-butanediol, dipropylene glycol, tripropylene glycol, diethylene glycol, triethylene glycol, glycerol, and mixtures thereof. In a preferred embodiment of the present invention, the isocyanate-reactive component (A) of the polyurethane-forming system further contains bisphenol initiated polyol having a structure of formula (II): wherein R ₁ is C ₁ -C ₅ -alkylene, sulfinyl (-SO-), or sulfonyl (-SO ₂ -); R ₂ and R ₃ are independently C ₁ -C ₆ -alkylene, C ₃ -C ₈ -cycloalkylene, C ₂ -C ₆ -alkenylene, C ₃ -C ₈ -cycloalkenylene, C ₂ -C ₆ -alkynylene, C ₅ -C ₁₆ -arylene; or C ₅ -C ₁₆ -heteroarylene; each of n1 and n2 is independently an integer of 1 to 5. In a further preferred embodiment of the present invention, the bisphenol initiated polyols employed may have an OH number of from 150 to 450 mg KOH/g, preferably, from 200 to 400 mg KOH/g, most preferably, from 250 to 350 mg KOH/g and a functionality of from 2 to 5, preferably, from 2 to 4, most preferably, from 2 to 3. Preferably, the bisphenol initiated polyol has a molecular weight Mw in the range from 200 to 1500 g/mol, preferably from 400 to 800 g/mol. This bisphenol initiated polyol may constitute up to 30% by weight of the component (A) of the present invention, preferably up to 15 wt%. In a preferred embodiment, the sorbitol initiated polyols and the bisphenol initiated polyols may be comprised in the isocyanate-reactive component (A) in a ratio of 1: 0.5-2, preferably in a ratio of 1:0.5-1.5, more preferably in a ratio of 1:1. The isocyanate-reactive component (A) of the present invention may optionally include one or more organic polyols in addition to the above-mentioned sorbitol initiated polyol and bisphenol initiated polyol. These optional organic polyols preferably differ principally in regard to hydroxyl group functionality and molecular weight. These optional organic polyols used in the isocyanate-reactive component (A) are chosen from softblock polyols, rigidblock polyols, polymer polyols, chain extenders, crosslinkers, and combinations of these different types of polyols. Polyols, which furnish softblock segments, are known to those skilled in the art as “softblock” polyols, or as flexible polyols. Such polyols preferably have a number average molecular weight of at least 2000, more preferably from 2500 to 5000, and number average functionality of isocyanate reactive organic –OH groups of preferably from 1.8 to 10 and more preferably from 2 to 4. Such compounds include, for example, aliphatic polyether or aliphatic polyester polyols having primary and/or secondary hydroxyl groups. In the practice of the present invention, it is preferred that such softblock polyols make up from 0 to 20% by weight, preferably from 2 to 16% by weight and more preferably from 2 to 10% by weight of the component (A). Preferred softblock polyols are liquids at 25 °C. A preferred class of polyols that provides structural rigidity in the derived polymer is referred to in the art as rigidblock polyols. Such polyols preferably have number average molecular weights of from 150 to 2,000, more preferably from 200 to less than 1,500; and number average isocyanate reactive group functionalities of preferably from 2 to 10, more preferably 2 to 4, and most preferably 2 to 3. Such compounds include, for example, polyether or polyester polyols having primary and/or secondary hydroxyl groups. In the practice of the present invention, it is preferred that such rigidblock polyols make up from 30 to 90% by weight, more preferably from 40 to 85% by weight, and most preferably from 60 to 80% by weight of the component (A). Preferred rigidblock polyols are also liquids at 25° C. Polymer polyols (“PMPO”s) are stable dispersions of polymer particles in a polyol and thus are not prone to settling or floating. The polymer particles are chemically grafted to the polyol and act as a better reinforcing filler so that the composition of the polymer may be adjusted to give desired properties. The polymers in polymer polyols generally have a low density in comparison to common fillers such as clays or calcium carbonate. This means that on an equivalent weight percentage, the polymer polyols provide a higher volume fraction. Thus, lower levels of polymer polyols are required to effect a change in properties because polymer polyols can replace the typically more dense resin materials that make up the matrix. In the practice of the present invention, it is preferred that such polymer polyols make up from 0 to 8% by weight and more preferably from 2 to 6% by weight of the component (A). In some embodiments of the present invention it may even be desirable to add a conventional filler along with the polymer polyol(s) because the polymer polyol(s) may help keep the fillers in suspension. Examples of polymer polyols which may be useful in the practice of the present invention include those based on styrene acrylonitrile (“SAN”) copolymers, PHD polyols formed by condensation of amines and isocyanates, and PIPA polyols formed by condensation of alcohol amines with isocyanates. Dispersions based on other monomers may also be used in the practice of the present invention. Dispersions of solids in the polyisocyanate component may also be used. Polyol chain extenders and crosslinkers that may be included in the isocyanate-reactive component (A) will typically have number average molecular weights from 60 to less than 250, preferably from 80 to 150, and isocyanate-reactive group functionalities of from 2 to 4, preferably from 2 to 3. Examples of suitable chain-extenders/crosslinkers are simple glycols and triols, such as ethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, triethanolamine, triisopropanolamine, tripropylene glycol, diethylene glycol, triethylene glycol, glycerol, and mixtures thereof. The most preferred chain-extenders/crosslinkers are liquids at 25° C. Although aliphatic–OH functional compounds, such as those just listed, are the most preferred chain-extenders/crosslinkers, it is also within the scope of the present invention to employ certain polyamines, polyamine derivatives, and/or polyphenols. Examples of suitable amines known in the art include diisopropanolamine, diethanolamine, and 3,5-diethyl-2,4-diaminotoluene, 3, 5-diethyl-2,6-diaminotoluene, and mixtures thereof. Examples of suitable isocyanate reactive amine derivatives include certain imino-functional compounds such as those described in EP 0284253 and EP 0359456 and certain enamino-functional compounds such as those described in EP 0359456 having 2 or more isocyanate-reactive groups per molecule. Reactive amines, especially aliphatic primary amines, are less preferred due to their extremely high reactivity with polyisocyanates, but may optionally be used, if desired, in minor amounts. It is also within the scope of the present invention, albeit less preferred, to include within the isocyanate-reactive component (A) minor amounts of other types of isocyanate reactive species that may not conform to the types described hereinabove. According to the present invention, it is preferred that the isocyanate-reactive component (A) has a hydroxyl number of from 340 to 560 mg KOH/g, more preferably from 400 to 500 mg KOH/g, and most preferably from 410 to 450 mg KOH/g. Hydroxy number indicates the hydroxyl group concentration present in the reaction mixture expressed as milligram of potassium hydroxide (KOH) equivalent for 1 g of the sample (mg KOH/g). Generally, a higher hydroxyl number leads to a shorter gel time. Suitable di- and/or polyisocyanates are known to those skilled in the art and include unmodified isocyanates, modified isocyanates, and isocyanate prepolymers. As used herein, the term “polyisocyanates” refer to those comprising three or more isocyanate groups. Such organic di- and/or polyisocyanates include aliphatic, cycloaliphatic, araliphatic, aromatic, and heterocyclic polyisocyanates of the type described, for example, by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136. Examples of such di- and/or polyisocyanates include those represented by the formula, Q(NCO)n in which n is a number from 2-5, preferably 2-3, and Q is an aliphatic hydrocarbon group containing 2-18, preferably 6-10, carbon atoms; a cycloaliphatic hydrocarbon group containing 4-15, preferably 5-10, carbon atoms; an araliphatic hydrocarbon group containing 8-15, preferably 8-13, carbon atoms; or an aromatic hydrocarbon group containing 6-15, preferably 6-13, carbon atoms. Examples of suitable di- and/or polyisocyanates include: ethylene diisocyanate; 1,4-tetramethylene diisocyanate; 1,6-hexamethylene diisocyanate; 1,12-dodecane diisocyanate; cyclobutane-1,3-diisocyanate; cyclohexane-1,3- and -1,4-diisocyanate, and mixtures of these isomers; 1-isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (isophorone diisocyanate); 2,4- and 2,6-hexahydrotoluene diisocyanate and mixtures of these isomers; dicyclohexylmethane-4,4'-diisocyanate (hydrogenated MDI, or HMDI); 1,3- and 1,4-phenylene diisocyanate; 2,4- and 2,6-toluene diisocyanate and mixtures of these isomers (TDI); diphenylmethane 2,2’-, 2,4’-, and/or 4,4’-diisocyanate (MDI), the mixtures of monomeric diphenylmethane diisocyanates and of diphenylmethane diisocyanate homologs having a greater number of rings (polymeric MDI); naphthylene-1,5-diisocyanate; triphenylmethane-4,4',4"-triisocyanate; polyphenyl-polymethylene-polyisocyanates of the type which may be obtained by condensing aniline with formaldehyde, followed by phosgenation (crude MDI); norbornane diisocyanates; m- and p-isocyanatophenyl sulfonylisocyanates; perchlorinated aryl polyisocyanates; modified polyisocyanates containing carbodiimide groups; modified polyisocyanates containing urethane groups; modified polyisocyanates containing allophanate groups; modified polyisocyanates containing isocyanurate groups; modified polyisocyanates containing urea groups; polyisocyanates containing biuret groups; polyisocyanates obtained by telomerization reactions; polyisocyanates containing ester groups; reaction products of the above-mentioned isocyanates with acetals; and polyisocyanates containing polymeric fatty acid groups. It is also possible to use the isocyanate-containing distillation residues accumulating in the production of isocyanates on a commercial scale, optionally in solution in one or more of the polyisocyanates mentioned above. Those skilled in the art will recognize that it is also possible to use mixtures of the polyisocyanates described above. Isocyanate-terminated prepolymers may also be employed in the present invention. Prepolymers may be prepared by reacting an excess of organic polyisocyanate or mixtures thereof with a minor amount of an active hydrogen-containing compound as determined by the well-known Zerewitinoff test, as described by Kohler in "Journal of the American Chemical Society," 49, 3181(1927). These compounds and their methods of preparation are well known to those skilled in the art. The use of any one specific active hydrogen compound is not critical; any such compound can be employed in the practice of the present invention. The di- and/or polyisocyanate component preferably contains organic di- and/or polyisocyanates having a number average isocyanate (NCO) functionality of from at least 1.8 to 4.0, more preferably from 2.0 to 3.0, most preferably from 2.3 to 2.9. The NCO functionality of the isocyanate component may be a number ranging between any combination of these values, inclusive of the recited values. The isocyanate component preferably has a free isocyanate group content (NCO content) in the range of from 5% to 50% by weight, more preferably from 8% to 40%, most preferably from 9% to 35% by weight. The free NCO group content of the polyisocyanate component may be an amount ranging between any combination of these values, inclusive of the recited values. The stoichiometry of mixing isocyanate-based polymer forming formulations, containing an organic di- and/or polyisocyanate and a polyfunctional isocyanate reactive component is often expressed by a quantity known in the art as the isocyanate index. The index of such a formulation is simply the ratio of the total number of reactive isocyanate (–NCO) groups present to the total number of isocyanate-reactive groups (that can react with the isocyanate under the conditions employed in the process). This quantity is often multiplied by 100 and expressed as a percent. Preferred isocyanate index values in the mixing activated formulations, which are suitable for use in the practice of the present invention range from 95 to 150. A more preferred range of the isocyanate index values is from 105 to 130. The polyurethane-forming system of the present invention may contain additives, if desired. The additive component of the reaction mixture for forming the polyurethane may be in its entirety or partially mixed with the isocyanate component and/or the isocyanate-reactive component. According to an exemplary embodiment, a portion of the additive component is added to the isocyanate-reactive component before the reaction mixture is formed and another portion is separately added to the reaction mixture. According to another exemplary embodiment, the additive component in its entirety is added to the isocyanate-reactive component before the reaction mixture is formed. Examples of the additives include catalyst, internal mold release agents, fire retardants, smoke suppressants, dyes, pigments, antistatic agents, antioxidants, UV stabilizers, minor amounts of viscosity reducing inert diluents, combinations of these, and any other known additives from the art. Catalyst(s), where used, is/are preferably introduced into the reaction mixture by pre-mixing with the isocyanate-reactive component. Catalysts for the polyurethane-forming reactions of organic polyisocyanates are well known to those skilled in the art. These catalysts are described by way of example in “Kunststoffhandbuch, Band 7, Polyurethane” [Plastics Handbook, volume 7, Polyurethanes] Carl Hanser Verlag, 3rd edition 1993, chapter 3.4.1. Examples of those that can be used here are organometallic compounds, such as complexes of tin, of bismuth, of zinc, of aluminum, of titanium, of zirconium, of iron, or of mercury, preferably organotin compounds, such as stannous salts of organic carboxylic acids, e.g. stannous acetate, stannous octoate, stannous ethylhexanoate, and stannous laurate, and the dialkyltin(IV) salts of carboxylic acids, e.g. dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate, and dioctyltin diacetate, and also phenylmercury neodecanoate, bismuth carboxylates, such as bismuth(III) neodecanoate, bismuth 2-ethylhexanoate, and bismuth octanoate, or a mixture. Other possible catalysts are strongly basic amine catalysts. Examples of these are amidines, such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tertiary amines, such as triethylamine, triethylenediamine, tributylamine, dimethylbenzylamine, N-methyl, N-ethyl, N-cyclohexylmorpholine, N,N,N′,N′-tetramethylethylenediamine, N,N,N′,N′-tetramethylbutanediamine, N,N,N′,N′-tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, bis(dimethylaminopropyl)urea, dimethylpiperazine, 1,2-dimethylimidazole, 1-azabicyclo[3.3.0]octane, and preferably 1,4-diazabicyclo[2.2.2]octane, 1,8-diazabicyclo[5.4.0]-undecen-7-ene, and alkanolamine compounds, such as triethanolamine, triisopropanolamine, N-methyl- and N-ethyldiethanolamine, and dimethylethanolamine. The catalysts can be used individually or in the form of a mixture. Mixtures of metal catalysts and of basic amine catalysts are optionally used as catalysts. The catalyst level required to achieve the needed reactivity profile will vary with the composition of the formulation and must be optimized for each reaction system (formulation). Such optimization is well known within the skill of a person of ordinary skill in the art. In a preferred embodiment, the catalyst is present in an amount of from 0.1 to 2% by weight of the isocyanate-reactive component, preferably from 0.3 to 1.8% by weight, more preferably from 0.8 to 1.6% by weight. The catalysts preferably have at least some degree of solubility in the isocyanate-reactive component used, and are most preferably fully soluble in that component at the required use levels. Internal mold release additives are preferably used in the processing of isocyanate-based polyurethane systems to prevent sticking or buildup in the die. Suitable internal mold release agents include, for example, fatty amides such as erucamide or stearamide; fatty acids such as oleic acid; oleic acid amides; fatty esters such as LOXIOL G71S, an inert polyester available from Henkel; carnuba wax; beeswax (natural esters); butyl stearate; octyl stearate; ethylene glycol monostearate; ethylene glycol distearate; glycerin di-oleate; glycerin tri-oleate; esters of polycarboxylic acids with long chain aliphatic monovalent alcohols such as dioctyl sebacate; mixed esters of aliphatic polyols, dicarboxylic acids and long-chained aliphatic monocarboxylic acids; esters of dicarboxylic acids and long-chained aliphatic monofunctional alcohols; esters of long-chained aliphatic monofunctional alcohols and long-chained aliphatic monofunctional carboxylic acids; complete or partial esters of aliphatic polyols and long-chained aliphatic monocarboxylic acids; silicones such as TEGO IMR 412T silicone (from Goldschmidt); KEMESTER 5721 ester (a fatty acid ester product from Witco Corporation); fatty acid metal carboxylates such as zinc stearate and calcium stearate; waxes such as montan wax and chlorinated waxes; fluorine containing compounds such as polytetrafluoroethylene; fatty alkyl phosphates (both acidic and non-acidic types such as ZELEC UN, ZELEC AN, ZELEC MR, ZELEC VM-, ZELEC UN, ZELECLA-1, and ZELEC LA-2 phosphates, which are all commercially available from Stepan Chemical Company); chlorinated-alkyl phosphates; hydrocarbon oils;and combinations of these materials. In the practice of the present invention, it is also preferable to use UV stabilizers. Examples of the UV stabilizers that are suitable here include, but are not limited to, p-aminobenzoic acid derivative(s), salicylic acid derivative(s), benzophenone derivative(s), dibenzoylmethane derivative(s), diphenyl acrylate(s), 3-imidazol-4-yl-acrylic acid and its ester(s), benzofuran derivative(s), polymeric UV absorber(s), cinnamic acid derivative(s), camphor derivative(s), hydroxyphenyltriazine derivative(s), benzotriazole derivatives, trianilino-s-triazine derivatives, 2-phenylbenzimidazole-5-sulfonic acid and salts thereof, menthyl o-aminobenzoates, homosalates, tris-biphenyl-triazine derivatives, TiO2 (partly encapsulated), ZnO and mica, benzylidenemalonates, merocyanine derivatives, phenylene bis diphenyltriazines, imidazoline derivatives, and diarylbutadiene derivatives. More preferably, the UV stabilizers to be used according to the present invention comprise benzophenone-3 (BP3), benzophenone-4 (BP4), 3-benzylidene camphor (3BC), bis-ethylhexyloxyphenol methoxyphenyl triazine (BEMT), butyl methoxydibenzoylmethane (BMBM), diethylhexyl butamido triazone (DBT), drometrizole trisiloxane (DTS), ethylhexyl triazone (EHT), ethylhexyl methoxycinnamate, benzylidenemalonate (BM), diethylamino hydroxy benzoyl hexyl benzoate (DHHB), octocrylene, polysilicone, homosalate, and ethylhexyl salicylate. Other preferred additives for use in the present invention include: moisture scavengers, such as molecular sieves, defoamers, such as polydimethylsiloxanes, coupling agents, such as the mono-oxirane or organo-amine functional trialkoxysilanes, and combinations thereof. Fine particulate fillers, such as clays and fine silicas, are often used as thixotropic additives. Such particulate fillers may also serve as extenders to reduce the amount of resin used. Fire retardants are sometimes desirable as additives in pultruded composites. Examples of preferred fire retardants include, but are not limited to, triaryl phosphates, trialkyl phosphates, especially those bearing halogens, melamine (as filler), melamine resins (in minor amounts), halogenated paraffins and combinations thereof. According to exemplary embodiments, the polyurethane may be formed from the polyurethane-forming system of the present invention. Specifically, the polyurethane may be formed by mixing the di- and/or polyisocyanate component with the isocyanate-reactive component to form a reaction mixture and then curing the reaction mixture. According to exemplary embodiments, the polyurethane may be used in the production of a fiber-reinforced polyurethane composite. It has surprisingly been found that the polyurethane-forming systems of the present invention for the production of the fiber-reinforced polyurethane composite have resulted in substantially improved processing. Higher line speeds, better processing efficiency, and improved part quality have been achieved by using these new polyurethane-forming systems. The reaction systems are characterized by certain gel time ranges under a dry atmosphere. These reaction systems are thermosetting systems, which preferably cure by forming a covalently crosslinked network structure. With respect to gel time of the polyurethane-forming system, a balance may be realized between the need for a quick cure time to form the final composite article, the need for adequate flowability of the polyurethane system during the process of forming the composite material, and the need to prevent excessive wasteful flow of the polyurethane system during the operation. The balance may be achieved by having a gel time that is from 40 minutes to 65 minutes at 25℃. Therefore, the present invention further provides a composite comprising the polyurethane-forming system as mentioned above and a fiber-reinforcing material. The fiber-reinforcing materials as used herein include any fibrous material or materials that can provide long fibers capable of being at least partially wetted by the polyurethane-forming system. The fibrous reinforcing material may be single strands, braided strands, woven or non-woven mat structures and combinations thereof. Mats or veils made of long fibers may be used, in single ply or multi-ply structures. Suitable fibrous materials are known. Examples of suitable fibrous materials include: glass fibers, glass mats, carbon fibers, polyester fibers, aramid fibers, nylon fibers, basalt fibers, and combinations thereof. Particularly preferred in the present invention are long glass fibers. The reinforcing fibers may optionally be pre-treated with sizing agents or adhesion promoters known to those skilled in the art. The weight percentage of the long fiber reinforcement in the composite of the present invention may vary considerably, depending on the fiber type used and on the end use application intended for the composite articles. Reinforcement loadings may be from 30 to 95% by weight of glass, preferably from 40 to 90% by weight of the composite, more preferably from 60 to 90% by weight, and most preferably from 70 to 90% by weight, based on the total weight of the composite. The long fiber reinforcement may be present in the composite of the present invention in an amount ranging between any combination of these values, inclusive of the recited values. Moreover, the present invention further provides a process for the production of the composite by impregnating a fiber-reinforcing material with the polyurethane-forming system of the present invention, and curing the fiber-reinforced polyurethane-forming system. Preferably, the fiber-reinforcing material is impregnated with the polyurethane-forming system, and subsequently the fiber-reinforced polyurethane-forming system is subject to curing. In exemplary embodiments of the present invention, the production of the fiber-reinforced polyurethane composites are performed by using the processes such as pultrusion, filament winding, vacuum infusion, resin transfer molding, compressive molding, or reactive injection molding. These processes are well-known to those skilled in the art. In one aspect, the present invention provides a pultrusion process for preparing a composite material comprising the steps of: 1) pulling continuous fibers through an impregnation die while contacting the fibers with a polyurethane-forming system of the present invention, sufficient to cause substantial polymerization of the polyurethane-forming reaction mixture within the impregnation die to produce a composite of fibers coated by the polyurethane-forming reaction mixture, which is not fully cured, 2) directing the composite of fibers coated by the polyurethane-forming system through a heated curing die to further advance the cure of the polyurethane-forming system so as to produce a solid composite material, and 3) withdrawing the solid composite material from the curing die. The pultrusion apparatus preferably contains at least one impregnation die and at least one curing die. The impregnation die must provide for adequate mixing of the reactive components and adequate impregnation of the fibrous reinforcing material. The impregnation die may preferably be fitted with a mixing apparatus, such as a static mixer, which provides for mixing of the reactive components before the resulting reaction mixture is used to impregnate the fibrous reinforcing structure. Other types of optional mixing devices may be used. They may include, but are not limited to, high-pressure impingement mixing devices or low pressure dynamic mixers such as rotating paddles. In some cases, adequate mixing is provided in the impregnation die itself, without any additional mixing apparatus. The curing die operates at a higher temperature than the impregnation die. The pultrusion apparatus may optionally contain a plurality of curing dies, or zones. Different curing zones may be set at different temperatures, if desired, but all the zones of the curing die should be higher in temperature than the impregnation die. The pultrusion apparatus may optionally contain a plurality of impregnation dies. Preferably, there is just one impregnation die, and this preferably is situated immediately prior to the first curing die (or zone). The impregnation die is set at a temperature that provides for some degree of reaction (polymerization) between the polyisocyanate and the polyisocyanate-reactive components in the reaction mixture before the fibrous reinforcing structure, which has been at least partially impregnated with said reaction mixture, enters the first curing die (or zone). It is highly preferable that the reaction mixture retains some degree of flowability (liquidity) until it enters the first curing die (or zone). It is highly preferred that the wetting of the fibrous reinforcing structure be complete and that there be no dry spots, which would lead to surface defects or voids in the cured composite. Further details about preferred isocyanate-based pultrusion processing methods and apparatus are provided in WO 00/29459. In a further aspect, the present invention provides a filament winding process for preparing a composite material comprising the steps of: 1) impregnating a plurality of aligned continuous fibers by passing the aligned fibers through a channel having a polyurethane-forming system disposed therein, wherein the aligned fibers are contacted with the polyurethane-forming reaction mixture as the fibers pass through the channel, and 2) winding the aligned fibers coated thereby around a mandrel which pulls the aligned fibers through the channel. 3) curing the coated aligned fibers to form a composite material. Filament winding is used for the production of composites, e.g., based on a crosslinking matrix of filaments and polyurethane resin. In a filament winding operation using the polyurethane resin according to embodiments, a filament may be passed through a liquid bath or an injection die and then wound around a mandrel in order to form a hollow cylindrical object. The polyurethane resin may be a one component system (e.g., the isocyanate component and the isocyanate-reactive component are mixed to form the liquid bath and then applied to the filament) or a two-component system (e.g., the isocyanate component and the isocyanate-reactive component are separately applied to the filament such that the liquid bath may include only one of the isocyanate component or the isocyanate-reactive component). The resultant product may be cured (e.g., by the application of heat and/or radiation) in order to form a final composite article. In an exemplary filament winding process, the filament is wetted by the liquid bath and wound around the mandrel, which defines the shape of the final composite article. The wetting of the filament may take place either prior to or concurrently with the winding operation. For example, the filament may be wetted on a continuous basis by a one-component polyurethane resin just before it is wound around the mandrel. The winding operation may be accomplished by rotating the mandrel while the polyurethane resin coated filament is under a controlled amount of tension, and moving the filament up and down the length of the mandrel in any desired pattern. During the winding operation, it is important to minimize the formation of voids or gaps in the filament wound article and to control the degree of wetting of the filament (better wetting is preferred in most applications). For example, the coating of the polyurethane resin on the filament and the winding operation should both take place while the resin is still flowable (e.g., should be homogeneous and separation of solids or gel particles from the liquid bulk of the resin is minimal) In another aspect, the present invention provides a vacuum infusion process for the production of a composite material comprising: 1) de-gassing each of the di- and/or polyisocyanate and isocyanate-reactive components, 2) combining the de-gassed components to obtain a polyurethane-forming reaction mixture, 3) applying vacuum pressure to a dry fiber-reinforcing material in a manner such that the fiber-reinforcing material is infused with the polyurethane-forming reaction mixture. 4) curing the reaction mixture and removing the composite thus formed from the vacuum chamber. More specifically, in producing a composite with the system of the present invention by vacuum infusion, the isocyanate and isocyanate-reactive components are de-gassed and combined to form the reaction mixture. The reinforcing material is placed in a vacuum chamber (typically, one or more bags). The pressure within this vacuum chamber is then drawn down. The pressure difference between the vacuum chamber in which the pressure has been reduced and the atmospheric pressure on the reaction mixture pushes the reaction mixture into the vacuum chamber and into the reinforcing material. The reaction mixture is cured and the composite thus formed is removed from the vacuum chamber. In still another aspect, the present invention provides a process for the production of a composite material from the polyurethane-forming system by resin transfer molding, compressive molding, or reactive injection molding. These molding processes are well-known to those skilled in the art. Further details about these methods and apparatus are provided in US 2020079037A1, US 2019202965A1, US 2010102479A1, KR 20190142819A, CN 110341212A, US 2009292057A1, US 5079084A. The processing characteristics of the polyurethane-forming system of the present invention and the mechanical properties of the fiber-reinforced polyurethane composites obtained by using this system are particularly advantageous for producing large composite parts used in outdoors. Therefore, the present invention further provides the use of the fiber-reinforced polyurethane composite for making large composite parts used in outdoors. These large composite parts include, but are not limited to, bridge parts including bridge decking, beam or girder, body parts of automotive including vehicle exteriors, building parts including strengthening bars, railway parts including railway sleepers, or aerospace parts. The present invention is further illustrated, but is not to be limited, by the following examples. EXAMPLES The following materials were used: Polyols Polyol 1: Glycerol-started polyoxypropylene polyol, with an OH value of 400 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 420 g/mol. Polyol 2: Glycerol-started polyoxyethylene polyoxypropylene polyol, with an OH value of 41 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 2750 g/mol. Polyol 3: Glycerol-started polyoxypropylene polyol, with an OH value of 675 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 250 g/mol. Polyol 4: Glycerol-started polyoxypropylene polyol, with an OH value of 775 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 210 g/mol. Polyol 5: Sucrose/glycerin initiated polyoxypropylene polyol, with an OH value of 490 mg KOH/g, a hydroxyl functionality of about 4.7, and a molecular weight Mw of 540 g/mol. Polyol 6: Trimethylol propane (TMP) initiated polyoxypropylene polyol, with an OH value of 550 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 300 g/mol. Polyol 7: Polyoxypropylene polyol initiated by cardanol, with an OH value of 200 mg KOH/g, a hydroxyl functionality of 4-5, and a molecular weight Mw of 200 g/mol. Polyol 8: Bisphenol A initiated polyether polyol of propylene oxide unit, with an OH value of 280 mg KOH/g, a hydroxyl functionality of about 2, and a molecular weight Mw of 400 g/mol. Polyol 9: Sorbitol-started polyoxypropylene polyol, with an OH value of 490 mg KOH/g, a hydroxyl functionality of about 5.5, and a molecular weight Mw of 630 g/mol. Polyol 10: Sorbitol-initiated polyoxypropylene polyol, with an OH value of 490 mg KOH/g, a hydroxyl functionality of about 5, and a molecular weight Mw of 570 g/mol. Polyol 11: Sorbitol/glycerin started polyoxypropylene polyol, with an OH value of 460 mg KOH/g, a hydroxyl functionality of about 5.5, and a molecular weight Mw of 670 g/mol. Polyol 12: Sorbitol/glycerin started polyoxypropylene polyol, with an OH value of 375 mg KOH/g, a hydroxyl functionality of about 4.5, and a molecular weight Mw of 670 g/mol. Polyol 13: Sorbitol/glycerin started polyoxypropylene polyol, with an OH value of 540 mg KOH/g, a hydroxyl functionality of about 3, and a molecular weight Mw of 310 g/mol. Catalyst: Tin bis(dodecylthio) dioctyl Internal mold release agent (IMR): Blend of organic fatty acids, esters and amine neutralizing agents Isocyanates: diphenylmethane 2,2’-diisocyanate (2,2’-MDI) diphenylmethane 2,4’-diisocyanate (2,4’-MDI) diphenylmethane 4,4’-diisocyanate (4,4’-MDI) Polymeric MDI (PMDI): large molecular weight oligomer of MDI containing 3 or more benzene rings Measuring and test methods For the material characterization, the test methods as follows were used: Gel time: 100 g of the isocyanate-reactive component and the isocyanate component was mixed by normal speedmixer at 2000 rpm for 1 min and then was transferred into the aluminum cup to test gel time by Shyodu gel timer at 25℃ and 50%RH. Tg: DMA, heating rate 5℃/min, test range RT-150℃, frequency 1Hz, tensile mode. Hardness: ISO 7619-1 Tensile test: ISO 527, S2 dumb bell specimen, 2 mm/min grip separation speed Flexural test: ISO 178 Examples 1-18: Preparation of the polyurethane Comparative Examples 1-7 and Inventive Examples 8-18 were prepared according to the polyurethane-forming system as specified in the following tables 1-3. The amounts of the respective materials are given in percent by weight. Each of Comparative Examples 1-7 and Inventive Examples 8-18 include isocyanate-reactive components, which includes therein polyol components and other non-polyol additives, and isocyanate components. The isocyanate-reactive component has an almost constant hydroxyl number. Comparative Examples 1-7 include a polyol system that excludes any of sorbitol-initiated polyols. A total 200 g of the isocyanate-reactive components and the isocyanate components was charged into a speedmixer cup and then mixed by speedmixer at 2000 rpm under a vacuum condition of 20 mbar for 1 min. The mixture was then poured into a pre-heated closed steel mold with dimension of 200*300*4 mm and cured in 70℃ oven for 15 min followed by curing in 150℃ oven for 60 min. The mold was cooled down in natural air and the specimen was cut for mechanics and Tg tests. The results were summarized in the following tables 1-3.

Inventive Examples 8-18 realized similar gel time while exhibited higher Tg improvement efficiency as compared with Comparative Examples 1-7. Inventive Examples 8-13 show that the increase of the content of the sorbitol initiated polyol assists in improving the Tg, and thus the heat resistance. In addition, Inventive Example 18 shows that the use of the combination of sorbitol initiated polyol and Bisphenol A initiated polyol helps to improve the Tg while maintain satisfactory gel time. The heat resistance of the polyurethane Heat resistance is indicated by Tg, which can be expressed as the retention of mechanical properties of the polyurethane at high temperatures. The mechanical properties of the polyurethane at room temperature (RT) and 70℃ are tested according to standard ISO 527. Small dumb bell type specimen S2 with 20mm/min test speed is used for test at RT, while 1A type with 50mm/min test speed is used for test at 70℃. The mechanical properties of the polyurethane at RT and 70℃ are shown in the below table 4. Table 4 Mechanical properties at RT and 70℃ The above results show that the higher Tg of the polyurethane results in higher mechanics retention thereof at high temperatures, and thus better heat resistance. Production of composite materials by using a pultrusion process The general procedure for producing composite materials by using a pultrusion process will be described below. The isocyanate component and the isocyanate-reactive component listed in Tables 1-3 were fed to a mix/metering machine in the amounts specified in Tables 1-3. The resultant reaction mixture was supplied to an impregnation die maintained at a room temperature. The glass fibers were pulled into the impregnation die and then impregnated with the reaction mixture for a time period of 60 sec. The wetted glass fibers were then pulled from the impregnation die through a zoned heating die that was attached directly to the impregnation die. The zoned heating die had a cross section shaped as a flat plank. The wetted fibers were then shaped and cured in the zoned heating die. The zoned heating die had 3 heated zones equipped with electric heated platens individually controlled to maintain the actual temperature at 125°C in the first zone, 180°C in the second zone, and 190°C in the third zone. The entrance to the impregnation die was cooled to prevent premature polymerization. The dynamic force needed to pull the composite through the forming die was supplied by a pulling machine with gripping devices that contacted the cured composite profile (or the glass fibers therein) and gave the traction necessary to pull the composite profile through the die. The pulling machine also had a device that developed a force in the desired direction of pull that gave the impetus necessary to pull the composite profile continuously through the die. The composite profile exiting the pulling machine was then cut to the desired length by an abrasive cut off saw. The amount of glass in the composite part thus produced was 80% by weight, based on total weight of the composite material with the balance being the polyurethane. The mechanical properties of the fiber-reinforced polyurethane composites prepared by using the polyurethane-forming system of example 18 are illustrated in Table 5. Axial direction (along fiber): the direction along the pultruded profile production direction, or axis of the profile, is defined as the axial direction; Transverse direction (across fiber): the perpendicular direction to axial direction; Refer to standard BS EN 13706-2:2002 on the definition of direction.

The foregoing examples of the present invention are offered for the purpose of illustration and not limitation. It will be apparent to those skilled in the art that the embodiments described herein may be modified or revised in various ways without departing from the spirit and scope of the invention. The scope of the invention is to be measured by the appended claims.