High performance polyvinyl acetal and method for its preparation

Abstract

The application discloses a kind of high-performance polyvinyl acetal and its preparation method, is by controlling raw material polyvinyl alcohol molecular chain aggregate structure, to realize the regulation of product polyvinyl acetal molecular chain structure, to obtain high-performance polyvinyl acetal resin and film piece.The process of the application is simple, the cost is lower, the change is smaller than existing production process, the stereoisomer ratio of polyvinyl acetal is large, and the adjustable range of mechanical energy is large, and the existing production equipment does not need to be changed;In addition, the reaction process involved in the application can greatly reduce the washing step, energy saving and environmental protection, and reduce the cost without adding a large amount of emulsifier and other functional additives.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a high-performance polyvinyl acetal and its preparation method, as well as the polyvinyl acetal resin and film prepared by this method. Background Technology

[0002] Polyvinyl acetal is produced by the acetal reaction of polyvinyl alcohol and aldehyde in the presence of a catalyst. It is widely used as a sandwich bonding material in the automotive, construction, and photovoltaic industries, as well as as an adhesive in inks and electronic ceramics. Depending on the application scenario, polyvinyl acetal needs to possess both certain adhesive properties and mechanical strength. Furthermore, the stringent application requirements in the automotive, construction, photovoltaic, and electronic ceramics industries place extremely high demands on the durability and stability of polyvinyl acetal sandwich materials.

[0003] The molecular structure of polyvinyl acetal mainly consists of three structural units: ethylene glycol acetal, ethylene alcohol, and vinyl acetate. The adhesive properties of polyvinyl acetal materials are primarily related to the hydroxyl content and distribution of the ethylene alcohol monomer, while the content and distribution of the ethylene glycol acetal structure largely determine its mechanical properties. Vinyl acetate, on the other hand, determines the flexibility of polyvinyl acetal. However, there is a certain trade-off between adhesive properties and mechanical properties. When the adhesive strength is too high, the polyvinyl acetal adhesive layer cannot effectively dissipate energy through deformation, thus reducing the impact resistance of the polyvinyl acetal. Conversely, if the adhesive properties are too weak, it cannot effectively bond with substrates such as glass. Therefore, precisely controlling the adhesive strength and mechanical strength of polyvinyl acetal is crucial for improving its application performance.

[0004] Patents EP0402213B1 and CN100384887C both report methods to adjust the stereoisomer ratio of polyvinyl acetal molecular chains, which can effectively regulate the modulus and creep resistance of polyvinyl acetal. By adjusting the proportion of meso acetal structures, the modulus of the polyvinyl acetal intermediate film can be increased by two times. However, the above-mentioned regulation of the stereoisomer ratio of polyvinyl acetal is achieved with the addition of a large amount of emulsifier. Due to their amphiphilic properties, emulsifiers are often difficult to remove completely and effectively from polyvinyl acetal products, which has an adverse effect on the high-temperature processing and use of polyvinyl acetal. Patent CN115490792A achieves the regulation of the stereoisomer ratio through a process route of synthesizing high-acetalization polyvinyl butyral followed by deacetalization. Although it does not use a large amount of emulsifier, this method is relatively complex and costly. In large-scale production, it is crucial to find a more efficient and simpler method to regulate the stereoisomer ratio of polyvinyl acetal. Patent US4751266A describes improving the hydroxyl association ability of polyvinyl alcohol by reducing the acetoxy content, thereby controlling the strength of polyvinyl acetal. However, excessively low acetoxy content will cause a decrease in the flexibility of the polyvinyl acetal end product, making it difficult to meet the requirements of high-performance applications. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, the main objective of this invention is to propose a high-performance polyvinyl acetal and its preparation method. By controlling the aggregated state structure of the polyvinyl alcohol molecular chains in the raw material, the molecular chain structure of the polyvinyl acetal product can be regulated, thereby obtaining high-performance polyvinyl acetal resin and films.

[0006] To achieve its objectives, the present invention employs the following technical solution:

[0007] A method for preparing high-performance polyvinyl acetal includes the following steps:

[0008] (1) Polyvinyl alcohol resin raw material is dissolved in deionized water to prepare an associated polyvinyl alcohol aqueous solution with an association degree of 105%-125%. The association degree is defined as the percentage of the rotational viscosity of the treated associated polyvinyl alcohol aqueous solution at 25°C and a concentration of 10wt% to the viscosity of the aqueous solution of the same mass concentration obtained when the same polyvinyl alcohol resin raw material is fully dissolved at 25°C.

[0009] (2) Within a temperature range of 5 to 30°C, the reactant aldehyde solution and acid catalyst are added to the associated polyvinyl alcohol aqueous solution and stirred to react. After the phase change is precipitated, the reaction is continued at this temperature for 0.5 to 2 hours.

[0010] (3) Heat the reaction system to 30-80℃ and keep it warm for 0.5-4 hours; then add an alkaline solution to the reaction system and keep stirring for 5-30 minutes.

[0011] (4) The target product, polyvinyl alcohol acetal resin, can be obtained by washing, filtering and drying the product.

[0012] (5) The polyvinyl acetal resin is mixed with a plasticizer and then extruded and cast to obtain a polyvinyl acetal film.

[0013] The high-performance polyvinyl alcohol acetal has an acetal degree greater than or equal to 75 wt%, an internal / exogenous stereoisomer ratio (m / r) greater than or equal to 3.5, and a mechanical energy at 200% elongation greater than or equal to 1200 KJ / m. 3 Its characteristics.

[0014] The associated polyvinyl alcohol aqueous solution is defined as a polyvinyl alcohol aqueous solution whose viscosity remains relatively stable at a specific temperature after being fully dissolved at high temperature for a long time. The molecular chains in the associated polyvinyl alcohol aqueous solution exhibit a certain degree of association. Alternatively, even when the polyvinyl alcohol molecular chains are in a fully dissolved, free chain state, there are still certain intermolecular interactions, resulting in a higher viscosity.

[0015] The preparation method of the associated polyvinyl alcohol aqueous solution in step (1) includes: directly using polyvinyl alcohol raw materials with a crystallinity of not less than 55%; or improving the crystallinity of polyvinyl alcohol resin through post-treatment methods such as high-temperature and high-humidity annealing (80-100℃, 30-60% RH, annealing for 5-60 min); or improving the associativity of PVA solution by controlling the dissolution temperature process, such as high-temperature dissolution temperature and time (lowering the dissolution temperature or reducing the dissolution time at high temperature can improve the associativity of PVA solution, such as lowering the dissolution temperature from 90℃ to 80℃, and at 80℃ or 90℃). The association degree of polyvinyl alcohol (PVA) aqueous solutions can be regulated through methods such as reducing the dissolution time from 2 hours to 1 hour at high temperatures and adjusting the solution preservation process (after complete dissolution at high temperatures, the solution is cooled below the reaction temperature to enhance intermolecular hydrogen bonding interactions in PVA). Alternatively, other additives with association or thickening effects can be added, such as carbonate or sulfate ions with strong hydration properties (e.g., adding 0.1 wt%-1 wt% sodium carbonate or sodium sulfate aqueous solution of 0.1 mol / L). Furthermore, association can be promoted by applying specific external flow fields such as temperature, magnetic field, and shear rate to a conventionally fully dissolved PVA aqueous solution. It should be noted that increasing the degree of alcoholysis of PVA resin by reducing the acetoxy group content can also enhance the association effect.

[0016] Specifically, the degree of polymerization of the polyvinyl alcohol resin raw material ranges from 800 to 3000, and the degree of alcoholysis ranges from 88 mol% to 99.9 mol%.

[0017] Specifically, in the associated polyvinyl alcohol aqueous solution described in step (1), the solid content of polyvinyl alcohol is 8 wt% to 20 wt%.

[0018] Specifically, the mass of the aldehyde mentioned in step (2) is 50%-60% of the mass of the polyvinyl alcohol resin raw material.

[0019] In particular, in step (2), the order of adding the reactant aldehyde solution and the acid catalyst is to add the aldehyde solution first, then the acid solution, or to add the mixed aldehyde solution and acid solution simultaneously; a more preferred method is to add the aldehyde solution first, then the acid solution.

[0020] Specifically, in step (2), the reactant aldehyde solution and acid catalyst are added slowly and gradually; the addition stage is from the time the associated polyvinyl alcohol aqueous solution stabilizes at 5-30°C to any time before the temperature is raised.

[0021] Specifically, the aldehydes include at least one of formaldehyde, acetaldehyde, butyraldehyde, pentanaldehyde, isovaleraldehyde, hexanal, heptaldehyde, octanaldehyde, nonanaldehyde, decanaldehyde, benzaldehyde, and phenylacetaldehyde.

[0022] Specifically, the acidic catalyst includes at least one of the following: hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, aminosulfonic acid, benzenesulfonic acid, o-(m)toluenesulfonic acid, ethylbenzenesulfonic acid, and isophenylenesulfonic acid.

[0023] Specifically, in step (3), heating the reaction system to 30-80°C means heating the solution obtained in step (2) to 30-80°C over a period of 2-6 hours.

[0024] Specifically, the alkaline solution in step (3) includes at least one of sodium hydroxide, potassium hydroxide, and magnesium hydroxide.

[0025] Polyvinyl alcohol (PVA) is a semi-crystalline polymer with abundant hydroxyl groups on its molecular chain that can form strong hydrogen bonds. The crystal structure of PVA is primarily formed between syndiotactic hydroxyl groups, which contribute to the formation of racemic acetal structures during acetal reactions. Therefore, the racemic ratio of PVA acetals can be controlled by adjusting the strength of the interactions between hydroxyl groups. For example, by using a highly crystalline PVA resin, some of these syndiotactic hydroxyl groups can be retained after solution formation. The reaction rate of these associated PVA acetals participating in the acetalization reaction will be relatively lower, and their competitive advantage will be relatively weaker compared to isotactic hydroxyl groups. This leads to more meso-acetal reactions between isotactic hydroxyl groups, resulting in a higher proportion of racemic acetal structures. Furthermore, after the acetal reaction achieves the target acetal degree, the final product, PVA acetal, will also have a higher hydroxyl blockiness due to the association between hydroxyl groups, thereby improving the modulus and strength of PVA acetals.

[0026] Main test method description:

[0027] Polyvinyl acetal resin can be used for nuclear magnetic resonance (NMR) spectroscopy (1H NMR). 1 H NMR or carbon NMR (H NMR) 13 The degree of acetalization and the ratio of meso to racemic acetal isomers (m / r) can be determined using any one or two techniques in C1NMR. Taking the characterization of polyvinyl butyral (PVB) by 1H NMR as an example, 5–10 mg of PVB can be fully dissolved in 0.6 mL of DMSO-d6, and the sample can be scanned on a 400 MHz NMR spectrometer at least 16 times. The resulting NMR spectrum is integrated using MestReNova software, and the ratio of the mole fraction of hydrogen at 4.5 ppm (meso, m) and 4.75 ppm (racemic, r) is calculated. This ratio is called the ratio of meso to racemic acetal isomers (m / r).

[0028] The acetal degree is the mass percentage of the monomer containing the acetal group in the entire polyvinyl alcohol acetal molecular chain. Similarly, the acetal degree can also be calculated using 1H NMR spectroscopy, specifically as follows:

[0029] Acetal degree (mole fraction): VB m =2 / ((3*A) CH2 / A CH3 )-6).

[0030] Where A CH2 and A CH3 The integral areas are methylene-CH2 at 1.0-1.9 ppm and methyl-CH3 at 0.9 ppm, respectively.

[0031] Among them, acetal degree VB mThis is a relative mole fraction. To better reflect actual industrial processing, this relative mole fraction needs to be converted to a relative mass fraction (VB). w The specific method is as follows:

[0032] Acetal degree (mass fraction): VB w =142*VB m / (142*VB m +(1-VB m )*44).

[0033] Mechanical energy refers to the mechanical energy obtained by integrating the tensile strength over an elongation range of 0 to 200% through a tensile strength test. The test conditions are: stretching a polyvinyl acetal film at 100 mm / s at 20°C; wherein, the polyvinyl acetal film is prepared by mixing polyvinyl acetal resin with 25 wt% of the plasticizer triethylene glycol diisooctanoate, followed by melt extrusion, hot pressing, or solution casting and drying.

[0034] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0035] The process of this invention is simple and low-cost, with minimal changes compared to existing production processes. It offers a wide range of adjustable parameters for the stereoisomer ratio and mechanical energy of polyvinyl acetal, and requires no modification to existing production equipment. Furthermore, the reaction process involved in this invention eliminates the need for large amounts of emulsifiers and other functional additives, significantly reducing the washing steps, saving energy, protecting the environment, and lowering costs. This invention also features simple operation, requires no additional process equipment or expensive raw materials, and possesses strong market competitiveness. Detailed Implementation

[0036] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] The polyvinyl alcohol used in the following examples has a crystallinity of 48.6% (calculated by DSC enthalpy testing), and the viscosity of a 10wt% aqueous solution obtained when fully dissolved is 440cp at 25°C.

[0038] Example 1

[0039] (1) 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% was annealed at 95℃ and 60RH% for 30min, then dissolved in 800mL of pure water and stirred at 90℃ for 2h to obtain a transparent solution. Then the temperature was lowered to 15℃ and stirred at 150rpm for 0.5h to obtain an associative polyvinyl alcohol aqueous solution with an association degree of 124%.

[0040] (2) At 15°C, 57g of butyraldehyde was added dropwise to a polyvinyl alcohol solution at a constant rate over 30 min, followed by the addition of 20g of hydrochloric acid with a mass concentration of 36% to 38% at a constant rate over 30 min; and the reaction was continued at this temperature for 1 h.

[0041] (3) The reaction system was slowly heated to 60°C within 4 hours and stirred at this temperature for 1.5 hours. Then, 40 mL of 15 wt% sodium hydroxide solution was added to the reaction solution and stirred for 15 minutes. The solution was then centrifuged to remove water and a solid product was obtained.

[0042] (4) The solid product was washed and filtered five times with water at 50°C and then filtered by suction. It was placed in a forced-air drying oven and dried at 50°C for 4 hours to obtain polyvinyl alcohol acetal resin product.

[0043] (5) The obtained polyvinyl alcohol acetal resin was mixed with 25wt% plasticizer and then extruded and cast at 180°C by twin screw extrusion to obtain polyvinyl alcohol acetal film.

[0044] Example 2

[0045] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0046] (1) 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% was annealed at 80℃ and 60RH% for 30min, then dissolved in 800mL of pure water and stirred at 90℃ for 2h to obtain a transparent solution. Then the temperature was lowered to 15℃ and stirred at 150rpm for 0.5h to obtain an associative polyvinyl alcohol aqueous solution with an association degree of 115%.

[0047] Example 3

[0048] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0049] (1) 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% was annealed at 95℃ and 60RH% for 60min, then dissolved in 800mL of pure water and stirred at 90℃ for 2h to obtain a transparent solution. Then the temperature was lowered to 15℃ and stirred at 150rpm for 0.5h to obtain an associative polyvinyl alcohol aqueous solution with an association degree of 122%.

[0050] Example 4

[0051] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0052] (1) Dissolve 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% in 800mL of pure water, stir at 90℃ for 2h to obtain a transparent solution, then cool down to 15℃ and stir at 20rpm for 5h to obtain an associative polyvinyl alcohol aqueous solution with an association degree of 111%.

[0053] Example 5

[0054] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0055] (1) Dissolve 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% in 800mL of pure water, stir at 90℃ for 2h to obtain a transparent solution, then cool down to 15℃ and stir at 20rpm for 10h to obtain an associative polyvinyl alcohol aqueous solution with an association degree of 117%.

[0056] Example 6

[0057] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0058] (1) Dissolve 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% in 800mL of pure water, stir at 80℃ for 2h to obtain a transparent solution, then cool down to 15℃ and stir at 20rpm for 5h to obtain an associative polyvinyl alcohol aqueous solution with a degree of association of 108%.

[0059] Example 7

[0060] Except for step (2) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0061] (2) At 15℃, 57g of butyraldehyde and 20g of hydrochloric acid with a mass concentration of 36% to 38% were added dropwise to the polyvinyl alcohol solution at a uniform rate over 60 minutes; and the reaction was continued at this temperature for 1 hour.

[0062] Example 8

[0063] Except for step (2) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0064] (2) At 15°C, 57g of butyraldehyde was added dropwise to a polyvinyl alcohol solution at a constant rate over 30 min, followed by the addition of 20g of hydrochloric acid with a mass concentration of 36% to 38% at a constant rate over 60 min; and the reaction was continued at this temperature for 1 h.

[0065] Example 9

[0066] Except for step (2) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0067] (2) At 15°C, 57g of butyraldehyde was added dropwise to a polyvinyl alcohol solution at a constant rate over 30 min, followed by the addition of 20g of hydrochloric acid with a mass concentration of 36% to 38% at a constant rate over 30 min; and the reaction was continued at this temperature for 2 h.

[0068] Example 10

[0069] Except for step (3) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0070] (3) The reaction system was slowly heated to 60°C within 2 hours and stirred at this temperature for 1.5 hours. Then, 40 mL of 15 wt% sodium hydroxide solution was added to the reaction solution and stirred for 15 minutes. The solution was then centrifuged to remove water and obtain a solid product.

[0071] Example 11

[0072] Except for step (3), which is replaced as follows, the remaining steps and conditions are the same as in Example 1:

[0073] (3) The reaction system was slowly heated to 60°C within 6 hours and stirred at this temperature for 1.5 hours. Then, 40 mL of 15 wt% sodium hydroxide solution was added to the reaction solution and stirred for 15 minutes. The solution was then centrifuged to remove water and obtain a solid product.

[0074] Comparative Example 1

[0075] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1:

[0076] (1) Dissolve 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% in 800mL of pure water. Stir at 90℃ for 2h to obtain a transparent solution. Then cool down to 15℃ and stir steadily at 60rpm for 0.5h to obtain a polyvinyl alcohol aqueous solution with an association degree of 101%.

[0077] Comparative Example 2

[0078] Except for step (1) being replaced as follows, the remaining steps and conditions are the same as in Example 1:

[0079] (1) Dissolve 100g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 98.5mol% in 800mL of pure water. Stir at 90℃ for 5h to obtain a transparent solution. Then cool down to 15℃ and stir steadily at 60rpm for 0.5h to obtain a polyvinyl alcohol aqueous solution with an association degree of 103%.

[0080] Comparative Example 3

[0081] Except for step (2) being replaced as follows, the remaining steps and conditions are the same as those in Comparative Example 1.

[0082] (2) At 15°C, 57g of butyraldehyde was added dropwise to a polyvinyl alcohol solution at a constant rate over 30 min, followed by the addition of 20g of hydrochloric acid with a mass concentration of 36% to 38% at a constant rate over 30 min; and the reaction was continued at this temperature for 2 h.

[0083] Comparative Example 4

[0084] Except for step (3) being replaced as follows, the remaining steps and conditions are the same as in Example 1.

[0085] (3) The reaction system was slowly heated to 60°C within 1 hour and stirred at this temperature for 1.5 hours; then 40 mL of 15 wt% sodium hydroxide solution was added to the reaction solution and stirred for 15 minutes. The solution was then centrifuged to remove water and a solid product was obtained.

[0086] Table 1. Test results of acetal degree, hydroxyl value and mechanical energy of polyvinyl acetal.

[0087] Example 1 78.0％ 3.64 <![CDATA[1668KJ / m 3 ]]> Example 2 77.6％ 3.54 <![CDATA[1520KJ / m 3 ]]> Example 3 77.3％ 3.66 <![CDATA[1651KJ / m 3 ]]> Example 4 77.5％ 3.52 <![CDATA[1482KJ / m 3 ]]> Example 5 77.6％ 3.58 <![CDATA[1566KJ / m 3 ]]> Example 6 77.4％ 3.50 <![CDATA[1369KJ / m 3 ]]> Example 7 78.2％ 3.52 <![CDATA[1380KJ / m 3 ]]> Example 8 78.1％ 3.62 <![CDATA[1528KJ / m 3 ]]> Example 9 78.3％ 3.63 <![CDATA[1624KJ / m 3 ]]> Example 10 77.9％ 3.56 <![CDATA[1542KJ / m 3 ]]> Example 11 78.5％ 3.61 <![CDATA[1530KJ / m 3 ]]> Comparative Example 1 78.2％ 3.36 <![CDATA[1162KJ / m 3 ]]> Comparative Example 2 77.9％ 3.48 <![CDATA[1120KJ / m 3 ]]> Comparative Example 3 78.3％ 3.32 <![CDATA[1288KJ / m 3 ]]> Comparative Example 4 78.0％ 3.43 <![CDATA[1273KJ / m 3 ]]>

[0088] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-performance polyvinyl acetal, characterized in that, Includes the following steps: (1) Dissolve polyvinyl alcohol resin raw material in deionized water to prepare an associated polyvinyl alcohol aqueous solution with an association degree of 108%~125%; the association degree is defined as the percentage of the rotational viscosity of the associated polyvinyl alcohol aqueous solution at 25°C and 10wt% concentration to the viscosity of the aqueous solution of the same mass concentration obtained when the same polyvinyl alcohol resin raw material is fully dissolved at 25°C; the method for controlling the association degree of the associated polyvinyl alcohol aqueous solution includes at least one of the following methods: controlling the crystallinity of polyvinyl alcohol raw material; controlling the dissolution temperature and / or time; controlling the storage conditions of the solution; (2) Within a temperature range of 5~30℃, the reactant aldehyde solution and acid catalyst are added to the associated polyvinyl alcohol aqueous solution and stirred to react. After the phase change is precipitated, the reaction is continued at this temperature for 0.5~2h. (3) Heat the solution obtained in step (2) to 30-80°C within the range of 2-6h and keep it warm for 0.5-4h; then add an alkaline solution to the reaction system and keep stirring for 5-30min; (4) The obtained product is washed, filtered and dried to obtain the target product, polyvinyl alcohol acetal resin. (5) The polyvinyl acetal resin is mixed with a plasticizer and then extruded and cast to obtain a polyvinyl acetal film.

2. The preparation method according to claim 1, characterized in that: In step (2), the order of adding the reactant aldehyde solution and the acid catalyst is to add the aldehyde solution first, followed by the acid solution.

3. A high-performance polyvinyl acetal prepared by the preparation method of claim 1 or 2.

4. The high-performance polyvinyl acetal according to claim 3, characterized in that: The high-performance polyvinyl alcohol acetal has an acetal degree greater than or equal to 75 wt%, an internal / exogenous stereoisomer ratio (m / r) greater than or equal to 3.5, and a mechanical energy at 200% elongation greater than or equal to 1200 KJ / m. 3 Its characteristics.