A polyurethane elastomer, its preparation and use

Abstract

The application provides a polyurethane elastomer and belongs to the technical field of polyurethane elastomer materials. The polyurethane elastomer is prepared by using polycaprolactone polyol with an average relative molecular mass of 1200-1500 g / mol and hydroxyl full-vulcanized rubber powder as raw materials, and diisocyanate and alcohol chain extender as auxiliary materials. The hydroxyl full-vulcanized rubber powder is in a network-like uniform distribution state in the polyurethane elastomer. When the polyurethane elastomer is subjected to alternating load, the heat generated by the friction of the polyurethane molecular chain is greatly reduced due to the binding effect of the rubber powder, so that the polyurethane elastomer has excellent wet skid resistance and extremely low dynamic heat generation. Moreover, the network-like uniform distribution state of the hydroxyl full-vulcanized rubber powder makes the polyurethane elastomer provided by the application have high tensile strength and excellent wear resistance, and has a good application prospect in medium-high speed wheels or pulleys.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane elastic materials technology, specifically relating to a polyurethane elastomer, its preparation method, and its application. Background Technology

[0002] Thermoplastic polyurethane elastomers possess excellent properties such as high strength, tear resistance, abrasion resistance, oil resistance, and chemical corrosion resistance, making them a widely used polymer material. However, when subjected to alternating stress of a certain frequency and amplitude, polyurethane generally exhibits a hysteresis effect, resulting in a certain degree of dynamic heat generation. When the dynamic heat generation is significant, it can even deteriorate the mechanical properties and abrasion resistance of the polyurethane elastomer, leading to a shortened service life.

[0003] Tires need to possess three important characteristics: high wet skid resistance, low dynamic heat generation, and high wear resistance. Achieving both low dynamic heat generation and high wet skid resistance in polyurethane automotive tires is challenging, representing the primary challenge polyurethane tires face in solving the "magic triangle" problem. Chinese patent CN113278129A discloses a low-dynamic-heat polyurethane elastomer and its preparation method. This method involves reacting waste tire rubber powder with diisocyanate and uniformly dispersing it within a polyurethane matrix, which significantly improves the mechanical strength of the polyurethane elastomer material and reduces its dynamic heat generation. However, while the aforementioned polyurethane elastomer exhibits low dynamic heat generation, it does not possess high wet skid resistance. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a polyurethane elastomer, its preparation method and application. The polyurethane elastomer provided by this invention has low dynamic heat generation and excellent anti-slip properties.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a polyurethane elastomer comprising the following raw materials in parts by weight:

[0007] 98-99 parts of polycaprolactone polyol, 1-2 parts of hydroxyl-modified fully vulcanized rubber powder, 35 parts of diisocyanate, and 5-5.3 parts of alcohol chain extender.

[0008] The average relative molecular mass of the polycaprolactone polyol is 1200–1500 g / mol;

[0009] The hydroxyl-cured rubber powder is uniformly distributed in a network pattern within the polyurethane elastomer.

[0010] Preferably, the hydroxyl-based fully vulcanized rubber powder is obtained by reducing carboxyl-based fully vulcanized rubber powder;

[0011] The reducing agent used in the reduction includes sodium borohydride or a soluble chloride salt;

[0012] The carboxyl-based fully vulcanized rubber powder includes one or more of the following: carboxyl-based fully vulcanized natural rubber powder, carboxyl-based fully vulcanized styrene-butadiene rubber powder, and carboxyl-based fully vulcanized nitrile butadiene rubber powder.

[0013] Preferably, the carboxyl content in the carboxyl-based fully vulcanized rubber powder is 2–10 mmol / g.

[0014] Preferably, the particle size of the hydroxyl-cured rubber powder is 50-300 nm, and the gel content is >60 wt%.

[0015] Preferably, the diisocyanate includes an aromatic diisocyanate;

[0016] The aromatic diisocyanates include 4,4'-diphenylmethane diisocyanate and / or toluene diisocyanate.

[0017] This invention provides a method for preparing the polyurethane elastomer described in the above technical solution, comprising the following steps:

[0018] Hydroxyl-coated rubber powder, polycaprolactone polyol and diisocyanate are mixed and subjected to a prepolymerization reaction to obtain a prepolymer;

[0019] The prepolymer is subjected to degassing treatment to obtain a degassed prepolymer;

[0020] The defoamed prepolymer and alcohol chain extender are mixed and subjected to a curing reaction to obtain a polyurethane elastomer.

[0021] Preferably, the temperature of the prepolymerization reaction is 80-90°C and the time is 2.5-3 hours.

[0022] Preferably, the degassing treatment includes vacuum degassing treatment; the pressure of the degassing treatment is 10-1000 Pa, the temperature is 60-70 °C, and the time is 10-30 min.

[0023] Preferably, the ripening reaction is carried out at a temperature of 100–120°C for 24–36 hours.

[0024] This invention provides the application of the polyurethane elastomer described in the above technical solution or the polyurethane elastomer prepared by the preparation method described in the above technical solution in medium and high speed wheels or pulleys.

[0025] This invention uses polycaprolactone polyol with an average relative molecular mass of 1200-1500 g / mol and hydroxyl-cured rubber powder as raw materials, and diisocyanate and alcohol chain extenders as excipients. The hydroxyl-cured rubber powder exhibits a network-like uniform distribution in the polyurethane elastomer. Under alternating loads, the frictional heat generated by the polyurethane molecular chains is greatly reduced due to the binding effect of the rubber powder, giving the polyurethane elastomer excellent anti-slip properties while exhibiting extremely low dynamic heat generation. Moreover, the network-like uniform distribution of the hydroxyl-cured rubber powder gives the polyurethane elastomer provided by this invention high tensile strength and excellent wear resistance, showing good application prospects in medium- and high-speed wheels or pulleys.

[0026] Furthermore, the nano-scale hydroxyl-cured rubber powder significantly hinders the movement of molecular chain segments, causing the polyurethane elastomer to exhibit higher mechanical losses under alternating loads, thereby improving the elastomer's anti-slip effect.

[0027] This invention provides a method for preparing the polyurethane elastomer described in the above technical solution. The preparation method provided by this invention is simple to operate, has mild reaction conditions, low cost, and is suitable for industrial production. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a tanδ-ω curve of polyurethane elastomer. Detailed Implementation

[0030] This invention provides a polyurethane elastomer comprising the following raw materials in parts by weight:

[0031] 98-99 parts of polycaprolactone polyol, 1-2 parts of hydroxyl-modified fully vulcanized rubber powder, 35 parts of diisocyanate, and 5-5.3 parts of alcohol chain extender.

[0032] The average relative molecular mass of the polycaprolactone polyol is 1200–1500 g / mol;

[0033] The hydroxyl-cured rubber powder is uniformly distributed in a network pattern within the polyurethane elastomer.

[0034] In this invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art.

[0035] The raw materials for preparing the polyurethane elastomer provided by the present invention, by weight, include 98-99 parts of polycaprolactone polyol, preferably 98-98.8 parts, more preferably 98-98.5 parts, and most preferably 98-98.2 parts.

[0036] In this invention, the average relative molecular mass of the polycaprolactone polyol is 1200–1500 g / mol, preferably 1250–1450 g / mol, more preferably 1300–1400 g / mol, and most preferably 1350–1380 g / mol. In this invention, the polycaprolactone polyol is preferably a polycaprolactone polyol with a relative molecular mass of 1250–1500 g / mol, or a polycaprolactone polyol obtained by blending polycaprolactone polyols with relative molecular masses of 1000 g / mol and 2000 g / mol. This invention uses polycaprolactone polyol as a preparation raw material and limits the average relative molecular mass to 1250–1500 g / mol, thereby improving the anti-slip properties of polyurethane elastomers.

[0037] Based on the mass fraction of the polycaprolactone polyol, the raw materials for preparing the polyurethane elastomer provided by the present invention include 1 to 2 parts of hydroxyl-cured rubber powder, preferably 1 to 1.8 parts, more preferably 1 to 1.5 parts, and most preferably 1 to 1.2 parts.

[0038] In this invention, the sum of the mass fractions of the hydroxyl-cured rubber powder and the polycaprolactone polyol is preferably 100 parts. In this invention, the hydroxyl-cured rubber powder is preferably obtained by reducing carboxyl-cured rubber powder; the reducing agent used in the reduction preferably includes sodium borohydride or a soluble chloride salt. In this invention, the soluble chloride salt is preferably one or more of calcium chloride, zinc chloride, and rare earth metal chlorides. In this invention, the carboxyl-cured rubber powder preferably includes one or more of carboxyl-cured natural rubber powder, carboxyl-cured styrene-butadiene rubber powder, and carboxyl-cured nitrile butadiene rubber powder, more preferably carboxyl-cured natural rubber powder. In this invention, the carboxyl content in the carboxyl-cured rubber powder is preferably 2–10 mmol / g, more preferably 4–8 mmol / g, and most preferably 5–6 mmol / g.

[0039] In this invention, the particle size of the hydroxyl-cured rubber powder is preferably 50-300 nm, more preferably 50-200 nm, and most preferably 50-150 nm; the gel content of the hydroxyl-cured rubber powder is preferably >60 wt%, more preferably 65-80 wt%, and most preferably 70-75 wt%.

[0040] This invention uses hydroxyl-modified fully vulcanized rubber powder as the raw material. The hydroxyl-modified fully vulcanized rubber powder exhibits a network-like uniform distribution within the polyurethane matrix, reducing the degree of microphase separation within the polyurethane matrix. Under alternating loads, the frictional heat generation is significantly reduced due to the binding effect of the rubber powder on the polyurethane molecular chains. This results in the polyurethane elastomer exhibiting excellent anti-skid properties while displaying extremely low dynamic heat generation. The nanoscale hydroxyl-modified fully vulcanized rubber powder significantly hinders the movement of molecular chain segments, leading to higher mechanical losses in the polyurethane elastomer under alternating loads, thereby improving its anti-skid effect. Furthermore, compared to waste tire rubber powder, hydroxyl-modified fully vulcanized rubber powder has a smaller particle size and narrower distribution range, simpler microstructure (e.g., spherical) and chemical composition (without complex additive systems such as carbon black), making it easier to control the performance of the polyurethane elastomer compared to waste tire rubber powder.

[0041] Based on the mass fraction of the polycaprolactone polyol, the raw materials for preparing the polyurethane elastomer provided by the present invention include 35 parts of diisocyanate.

[0042] In this invention, the diisocyanate preferably includes an aromatic diisocyanate; the aromatic diisocyanate preferably includes 4,4'-diphenylmethane diisocyanate and / or toluene diisocyanate, more preferably 4,4'-diphenylmethane diisocyanate.

[0043] Based on the mass fraction of the polycaprolactone polyol, the raw materials for preparing the polyurethane elastomer provided by the present invention include 5 to 5.3 parts of alcohol chain extender, preferably 5.09 to 5.24 parts, more preferably 5.09 to 5.15 parts, and most preferably 5.09 to 5.12 parts.

[0044] In this invention, the alcohol chain extender preferably includes lower carbon diols, specifically 1,4-butanediol.

[0045] This invention provides a method for preparing the polyurethane elastomer described in the above technical solution, comprising the following steps:

[0046] Hydroxyl-coated rubber powder, polycaprolactone polyol and diisocyanate are mixed and subjected to a prepolymerization reaction to obtain a prepolymer;

[0047] The prepolymer is subjected to degassing treatment to obtain a degassed prepolymer;

[0048] The defoamed prepolymer and alcohol chain extender are mixed and subjected to a curing reaction to obtain a polyurethane elastomer.

[0049] This invention involves mixing hydroxyl-cured rubber powder, polycaprolactone polyol, and diisocyanate, and then performing a prepolymerization reaction to obtain a prepolymer.

[0050] In this invention, the hydroxyl-cured rubber powder preferably comprises the following steps: mixing carboxyl-based vulcanized rubber powder, a reducing agent, and a metal chloride, and carrying out a reduction reaction to obtain hydroxyl-based vulcanized rubber powder. In this invention, the type of reducing agent is preferably the same as described above and will not be repeated here. In this invention, the metal chloride preferably includes rare earth metal chlorides, specifically preferably one or more of LaCl3, CeCl3, NdCl3, and SmCl3. This invention does not specifically limit the conditions of the reduction reaction, as long as they enable the carboxyl-based vulcanized rubber powder to be reduced to hydroxyl-based vulcanized rubber powder.

[0051] In this invention, the hydroxyl-cured rubber powder is preferably dried before use. The drying process preferably includes vacuum drying; the drying temperature is preferably <150°C, more preferably 90–120°C, and most preferably 100–105°C; the drying pressure is preferably 100–1000 Pa, more preferably 100–300 Pa; and the drying time is preferably 8–12 h, more preferably 9–10 h.

[0052] In this invention, the polycaprolactone polyol is preferably dehydrated before use. Preferably, the dehydration is performed under vacuum; the dehydration temperature is preferably 100–120°C, more preferably 105–115°C, and most preferably 110–112°C; the dehydration pressure is preferably 100–500 Pa, more preferably 100–200 Pa; and the dehydration time is preferably 3–4 h, more preferably 3–3.5 h. Preferably, after dehydration, the dehydrated polycaprolactone polyol is further cooled to 60–70°C, more preferably 60–65°C, and most preferably 62–64°C.

[0053] In this invention, the mixing temperature is preferably 60-70°C, more preferably 60-65°C, and most preferably 62-64°C.

[0054] In this invention, the mixing preferably includes mixing the hydroxyl-cured rubber powder and polycaprolactone polyol to obtain a mixed polyol; and mixing the mixed polyol with diisocyanate.

[0055] In this invention, the temperature of the prepolymerization reaction is preferably 80-90°C, more preferably 82-88°C, and most preferably 84-86°C; the time of the prepolymerization reaction is preferably 2.5-3h, more preferably 2.6-2.9h, and most preferably 2.7-2.8h.

[0056] After obtaining the prepolymer, the present invention performs a degassing treatment on the prepolymer to obtain a degassed prepolymer.

[0057] In this invention, the degassing treatment preferably includes vacuum degassing. In this invention, the pressure of the degassing treatment is preferably 10–1000 Pa, more preferably 100–500 Pa, and most preferably 500–200 Pa; the temperature of the degassing treatment is preferably 60–70°C, more preferably 60–65°C; and the time of the degassing treatment is preferably 10–30 min, more preferably 15–25 min.

[0058] After obtaining the defoamed prepolymer, the present invention mixes the defoamed prepolymer with an alcohol chain extender and carries out a curing reaction to obtain a polyurethane elastomer.

[0059] In this invention, the mixing is preferably agitated mixing; the agitation mixing is preferably carried out in a planetary vacuum mixer. In this invention, the mixing speed is preferably 1000-1200 rpm, more preferably 1050-1150 rpm, and most preferably 1100-1120 rpm; the mixing time is preferably 30-150 s, more preferably 50-120 s, and most preferably 80-100 s.

[0060] In this invention, the temperature of the aging reaction is preferably 100-120°C, more preferably 100-110°C, and most preferably 100-105°C; the aging reaction time is preferably 24-36 h, more preferably 24-32 h, and most preferably 24-28 h.

[0061] This invention provides the application of the polyurethane elastomer described in the above-described technical solution or the polyurethane elastomer prepared by the preparation method described in the above-described technical solution in medium- and high-speed wheels or pulleys. The polyurethane elastomer provided by this invention has low dynamic heat generation, high anti-slip properties, high tensile strength, and excellent wear resistance, and has good application prospects in rollers that are used at high speeds for extended periods.

[0062] To further illustrate the present invention, a polyurethane elastomer, its preparation method, and its application are described in detail below with reference to the accompanying drawings and embodiments. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0063] In all embodiments and comparative examples of the present invention, the hydroxyl-based fully vulcanized rubber powder is obtained by reducing carboxyl-based fully vulcanized rubber powder, wherein the carboxyl-based fully vulcanized rubber powder is an ultrafine fully vulcanized carboxyl butadiene-acrylonitrile powder rubber, purchased from Beijing Yanshan Petrochemical High-Tech Co., Ltd., VP501.

[0064] The preparation method of hydroxyl-modified fully vulcanized rubber powder is as follows: Carboxyl-modified fully vulcanized rubber powder is dispersed in methanol to obtain a methanol dispersion of the rubber powder; NaBH4 and the methanol dispersion of the rubber powder are sequentially added to a 500 mL reaction vessel placed in an ice-water bath, and the mixture is stirred for 1.5 h; CaCl2 is added to the reaction vessel and mixed evenly, the temperature is raised to 80℃, and the mixture is stirred for 3 h; after the reaction is completed, the mixture is allowed to cool and then hydrochloric acid solution is added to quench the reaction; finally, the crude rubber powder product is repeatedly washed with deionized water and vacuum dried at 100℃ for 10 h to obtain hydroxyl-modified fully vulcanized rubber powder. The mass ratio of carboxyl-modified fully vulcanized rubber powder to NaBH4 is 1:8.5, and the mass ratio of carboxyl-modified fully vulcanized rubber powder to CaCl2 is 1:1.5.

[0065] Examples 1-4 and Comparative Examples 1-5

[0066] Under conditions of <150℃, hydroxyl-cured rubber powder was vacuum dried for 10 hours. Polycaprolactone polyol was vacuum dehydrated at 120℃ for 3 hours and then cooled to 60℃. The hydroxyl-cured rubber powder was then added and mixed evenly to obtain a mixed polyol.

[0067] The mixed polyol and 4,4'-diphenylmethane diisocyanate were mixed evenly at 60°C, and the mixture was heated to 80-90°C for 2 hours to obtain a prepolymer. The prepolymer was then subjected to degassing treatment under reduced pressure to obtain a degassed prepolymer, which was then sealed for later use. The degassing treatment was carried out at a pressure of 300 Pa, a temperature of 60°C, and a time of 15 minutes.

[0068] 1,4-Butanediol was added to the degassed prepolymer and mixed thoroughly. The mixture was then aged at 100°C for 24 hours to obtain a polyurethane elastomer. The mixing was carried out in a planetary vacuum mixer at a speed of 1200 rpm for 600 seconds.

[0069] The formulations of the raw materials used in Examples 1-4 and Comparative Examples 1-5 are shown in Table 1. The amounts of each raw material used are expressed in parts by mass. PCL-1k, PCL-2k, and PCL-1.25k represent polycaprolactone polyols with relative molecular masses of 1000 g / mol, 2000 g / mol, and 1250 g / mol, respectively. MDI is 4,4'-diphenylmethane diisocyanate. BDO is 1,4-butanediol.

[0070] Table 1. Proportions of raw materials used in Examples 1-4 and Comparative Examples 1-5

[0071]

[0072] The loss tangent (0℃, 60℃, and 140℃), tensile strength, DIN volumetric wear rate, and hardness of the polyurethane elastomers prepared in Examples 1-4 and Comparative Examples 1-5 were tested, and the results are shown in Table 2. Dynamic mechanical properties were tested using a DMA instrument at a frequency of 10Hz and a heating rate of 3K / min. Tensile strength was tested according to GB / T 528-2009 at a tensile speed of 500mm / min. DIN volumetric wear rate was tested according to GB / T 9867-2008 with a loading force of 10N. Hardness (Shore A) was tested according to GB / T531.1-2008.

[0073] Table 2 Comparison of the performance of polyurethane elastomers prepared in Examples 1-4 and Comparative Examples 1-5

[0074]

[0075] As shown in Table 2, the polyurethane elastomer provided by this invention has a higher tanδ at 0℃, resulting in better wet skid resistance. When applied to tires, it provides a larger contact area with the ground, greater friction, and improved safety. At 60℃ and 140℃, its tanδ is lower, leading to less internal heat generation under cyclic loads. When applied to tires, it results in even lower dynamic heat generation during high-speed rolling (heat generated by tire-ground friction, reaching temperatures of 60-120℃). Furthermore, the polyurethane elastomer provided by this invention exhibits high tensile strength and hardness, low DIN volumetric wear rate, and good wear resistance.

[0076] Figure 1 The graph shows the tanδ-ω curve of polyurethane elastomer. Figure 1 It is known that polyurethane elastomers possess a microphase separation structure, consisting of hard and soft segments. When subjected to shear force, they exhibit a significant loss peak phenomenon with increasing shear frequency (as shown in the curve of Comparative Example 1). However, the introduction of hydroxyl-cured rubber powder significantly reduces the degree of microphase separation in the polyurethane matrix, resulting in a weakening or even disappearance of the loss peak (as shown in the curve of Example 3). Furthermore, the hydroxyl-cured rubber powder, through chemical bonds and strong electrostatic interactions, greatly restricts the movement of polyurethane molecular chains, thereby reducing the frictional heat generation of the polyurethane elastomer under dynamic load and causing the polyurethane elastomer to exhibit lower dynamic heat generation.

[0077] In summary, this invention provides a polyurethane elastomer with high anti-slip properties and low dynamic heat generation. Compared with existing technologies, this invention, through rational formulation design, introduces hydroxyl-containing fully vulcanized nano-rubber powder in a network-like uniform distribution within the polyurethane matrix, while simultaneously reducing the degree of microphase separation in the polyurethane matrix. This results in the polyurethane elastomer exhibiting high mechanical strength under uniaxial tensile loads, and under alternating loads, its frictional heat generation is significantly reduced due to the binding effect of the rubber powder on the polyurethane molecular chains, exhibiting extremely low dynamic heat generation. By rationally controlling the relative molecular mass of the polyol, this invention enables the polyurethane elastomer to possess both extremely low dynamic heat generation and high anti-slip properties, giving it significant market competitiveness in high-speed roller applications.

[0078] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A polyurethane elastomer, characterized in that, The preparation raw materials include the following parts by weight: 98-99 parts of polycaprolactone polyol, 1-2 parts of hydroxyl-cured rubber powder, 35 parts of diisocyanate and 5-5.3 parts of alcohol chain extender; The average relative molecular mass of the polycaprolactone polyol is 1200~1500 g / mol; The hydroxyl-cured rubber powder is uniformly distributed in a network-like state within the polyurethane elastomer. The hydroxyl-based fully vulcanized rubber powder is obtained by reducing carboxyl-based fully vulcanized rubber powder; The reducing agent used in the reduction includes sodium borohydride or a soluble chloride salt; The carboxyl-based fully vulcanized rubber powder includes one or more of the following: carboxyl-based fully vulcanized natural rubber powder, carboxyl-based fully vulcanized styrene-butadiene rubber powder, and carboxyl-based fully vulcanized nitrile butadiene rubber powder. The carboxyl group content in the carboxyl-based fully vulcanized rubber powder is 2~10 mmol / g; The hydroxyl-cured rubber powder has a particle size of 50~300nm and a gel content of >60wt%.

2. The polyurethane elastomer according to claim 1, characterized in that, The diisocyanate includes aromatic diisocyanates; The aromatic diisocyanates include 4,4'-diphenylmethane diisocyanate and / or toluene diisocyanate.

3. The method for preparing the polyurethane elastomer according to any one of claims 1 to 2, characterized in that, Includes the following steps: Hydroxyl-coated rubber powder, polycaprolactone polyol and diisocyanate are mixed and subjected to a prepolymerization reaction to obtain a prepolymer; The prepolymer is subjected to degassing treatment to obtain a degassed prepolymer; The defoamed prepolymer and alcohol chain extender are mixed and subjected to a curing reaction to obtain a polyurethane elastomer.

4. The preparation method according to claim 3, characterized in that, The prepolymerization reaction is carried out at a temperature of 80-90°C for 2.5-3 hours.

5. The preparation method according to claim 3, characterized in that, The degassing treatment includes vacuum degassing treatment; the pressure of the degassing treatment is 10~1000Pa, the temperature is 60~70℃, and the time is 10~30min.

6. The preparation method according to claim 3, characterized in that, The ripening reaction is carried out at a temperature of 100-120°C for 24-36 hours.

7. The application of the polyurethane elastomer according to any one of claims 1 to 2 or the polyurethane elastomer prepared by the preparation method according to any one of claims 3 to 6 in medium- and high-speed wheels or pulleys.

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