Imidousilane coupling agents and methods for their preparation and modified glass fibers and composites prepared therefrom

Abstract

This invention provides an imide-based silane coupling agent, its preparation method, and modified glass fibers and composite materials prepared therefrom. The method for preparing the imide-based silane coupling agent involves using a mercapto-containing silane coupling agent and maleimide in a molar ratio of 1:0.5-1:1.5 as raw materials to obtain the imide-based silane coupling agent. This invention also provides the imide-based silane coupling agent prepared by the above method, a method for modifying glass fibers using this imide-based silane coupling agent, and the modified glass fibers and composite materials prepared therefrom. The mercapto-olefin click chemistry reaction used in this invention to prepare the imide-based silane coupling agent has mild conditions, is simple to operate, and has a high conversion rate, which can broaden the types and applications of silane coupling agents.

Description

Technical Field

[0001] This invention relates to the field of polymer composite material preparation technology, and particularly to imide-based silane coupling agents, their preparation methods, and modified glass fibers and composite materials prepared therefrom. Background Technology

[0002] Glass fiber reinforced polyamide 6 (PA6) composites possess excellent mechanical properties, good dimensional stability, and high heat resistance, making them widely used in various fields such as the automotive industry, electronics, and machinery. The interface between the glass fiber and the polymer matrix is ​​a key component of these composites; good interfacial interaction can significantly improve their mechanical properties. Therefore, when glass fiber is used for polymer reinforcement, surface modification is necessary. Common modification methods include impregnation, coupling agent bonding, plasma treatment, acid and alkali etching, and secondary surface grafting. However, plasma and acid / alkali etching treatments can damage the strength of the glass fiber; furthermore, these processes are complex and costly, making them unsuitable for large-scale industrial production. The literature (Polyamide 6 composite with highly improved mechanical properties by PEI-CNT grafted glass fibers through interface wetting, infiltration and crystallization; J Fang, L Zhang, C Li; Polymer; Volume 172, 20 May 2019, Pages 253-264) discloses a method for reinforcing nylon 6 by grafting CNTs onto the surface of glass fibers. However, the reaction process is relatively complex and costly, and the improvement in the mechanical properties of the nylon 6 composite material prepared is not significant. Summary of the Invention

[0003] To address the aforementioned technical problems, the present invention aims to provide a silane coupling agent and its preparation method. An imide-based silane coupling agent is prepared by using a thiol-containing silane coupling agent and maleimide as raw materials, which is then used to modify glass fibers and reinforce polyamide 6.

[0004] To achieve the above objectives, the present invention provides a method for preparing an imide-based silane coupling agent, which is prepared by using a mercapto-containing silane coupling agent and maleimide in a molar ratio of 1:0.5-1:1.5 as raw materials to obtain the imide-based silane coupling agent.

[0005] According to a specific embodiment of the present invention, preferably, in the above preparation method, the molar ratio of the mercapto-containing silane coupling agent to maleimide is 1:1.

[0006] According to a specific embodiment of the present invention, preferably, the preparation method includes the following steps:

[0007] A mercapto-containing silane coupling agent with a molar ratio of 1:0.5-1:1.5 is dissolved in a solvent with maleimide (preferably at a concentration of 0.1-0.3 g / mL), and triethylamine or triphenylphosphine is added as a catalyst. The mixture is reacted at 40-70°C to obtain the imide-based silane coupling agent.

[0008] According to a specific embodiment of the present invention, preferably, in the above preparation method, the molecular formula of the thiol-containing silane coupling agent is:

[0009]

[0010] Wherein, R1 is -OCH3 or -OCH2CH3;

[0011] R2 is -OCH3 or -OCH2CH3;

[0012] R3 is either -OCH3 or -OCH2CH3;

[0013] R4 is -CH2CH2CH2SH.

[0014] According to a specific embodiment of the present invention, preferably, in the above preparation method, the solvent is one or a combination of two or more of acetone, tetrahydrofuran, and toluene.

[0015] According to a specific embodiment of the present invention, preferably, in the above preparation method, the amount of triethylamine used accounts for 1-10% of the sum of the mass of the mercapto-containing silane coupling agent, maleimide and solvent.

[0016] According to a specific embodiment of the present invention, preferably, in the above preparation method, the reaction time is controlled to be 3-8 hours.

[0017] According to a specific embodiment of the present invention, preferably, in the above preparation method, the reaction temperature is 50-70°C.

[0018] According to a specific embodiment of the present invention, preferably, in the above preparation method, the reaction process is carried out under a protective atmosphere, more preferably, the protective atmosphere used is N2.

[0019] This invention uses a mercapto-olefin click chemistry reaction to prepare imide-based silane coupling agents. The reaction conditions are mild, the operation is simple, and the conversion rate is high, which can broaden the types and application range of silane coupling agents.

[0020] The present invention also provides an imide-based silane coupling agent, which is prepared by the above-described method for preparing an imide-based silane coupling agent.

[0021] The typical structure of the imide-based silane coupling agent prepared by this invention is shown in the following formula:

[0022]

[0023] The present invention also provides a method for modifying glass fibers, which involves modifying glass fibers using an imide-based silane coupling agent provided by the present invention.

[0024] According to a specific embodiment of the present invention, preferably, the above-mentioned method for modifying glass fibers includes the following steps:

[0025] Prepare an ethanol / water solution with a volume ratio of 0.5-2, and adjust the pH of the system to 3-5 (preferably using hydrochloric acid).

[0026] Glass fibers are added to the ethanol / water solution and ultrasonically dispersed under stirring conditions (preferably for 5-7 minutes) to obtain a glass fiber dispersion.

[0027] The imide-based silane coupling agent is added to the glass fiber dispersion, and then ultrasonically dispersed under stirring conditions (preferably for 5-10 min) to obtain an imide-based silane coupling agent / glass fiber dispersion. The amount of the imide-based silane coupling agent is 0.1-4% of the mass of the glass fiber, preferably 0.5-3%.

[0028] The imide-based silane coupling agent / glass fiber dispersion is subjected to hydrolysis and condensation reactions.

[0029] The solvent in the reacted imide-silane coupling agent / glass fiber dispersion was removed by rotary evaporation, and then vacuum dried to obtain modified glass fiber.

[0030] According to a specific embodiment of the present invention, preferably, in the above-mentioned glass fiber modification method, the hydrolysis and condensation reaction is carried out in a shaking state on a shaker, the reaction time is preferably 24-48h, and the shaking speed is preferably 80-110r / min.

[0031] According to a specific embodiment of the present invention, preferably, in the above-mentioned glass fiber modification method, the rotary evaporation temperature is 70-95°C.

[0032] According to a specific embodiment of the present invention, preferably, in the above-mentioned glass fiber modification method, the vacuum drying temperature is 90-100°C.

[0033] The present invention also provides a modified glass fiber, which is prepared by the above-described method for modifying glass fibers.

[0034] The present invention also provides a method for preparing a modified glass fiber and polyamide composite material, which includes the following steps: mixing the modified glass fiber with polyamide 6 resin to obtain the modified glass fiber and polyamide composite material.

[0035] According to a specific embodiment of the present invention, preferably, the amount of modified glass fiber added is 30% based on the total mass of the modified glass fiber and polyamide 6.

[0036] According to a specific embodiment of the present invention, preferably, the preparation method of the above-mentioned modified glass fiber and polyamide composite material includes the following steps:

[0037] Polyamide 6 is vacuum dried (preferably in a vacuum oven at 100°C for 24 hours), and then the dried polyamide 6 is completely melted in a mixer. Modified glass fiber is then added to obtain the composite material of modified glass fiber and polyamide 6. Preferably, the mixing temperature is 230-250°C, the rotor speed is 30-50 r / min, and the total mixing time is 8-13 min.

[0038] The modified glass fiber and polyamide mixture is melt-injection molded to obtain the modified glass fiber and polyamide 6 composite material sample; preferably, the melting temperature is 240-260℃, the injection pressure is 0.6-0.7MPa, and the total injection time is 15-25s.

[0039] According to a specific embodiment of the present invention, preferably, the modified glass fiber and polyamide 6 are mixed by extrusion.

[0040] The present invention also provides a modified glass fiber and polyamide composite material, which is prepared by the above-described method for preparing the modified glass fiber and polyamide composite material.

[0041] According to a specific embodiment of the present invention, preferably, the modified glass fiber and polyamide composite material has a tensile strength of 118-125 MPa, an elongation at break of 3.0-3.1%, and a notched impact strength of 8.6-9.9 kJ / m. 2 .

[0042] Currently, the most commonly used glass fiber modifier is the silane coupling agent. However, the structure of the coupling agent used varies greatly depending on the polymer structure. For polyamide 6, whose molecular chain contains a large number of polar amide groups, constructing a coupling agent similar to the amide groups to modify glass fiber should improve the compatibility between glass fiber and polyamide 6, thereby enhancing their interfacial interaction. Therefore, this invention employs a mild and high-conversion mercapto-olefin click chemistry reaction to react maleimide with γ-mercaptopropyltriethoxysilane to prepare a silane coupling agent with an imide group.

[0043] The solubility parameter, the square root of the cohesive energy density, can be used to evaluate the compatibility between different materials. This invention calculated the solubility parameters of polyamide 6 and imide-based silane coupling agents using all-atom molecular dynamics simulations, yielding results of 23.0 and 22.3 (J·cm⁻¹), respectively. -3 ) 1 / 2 The two are quite similar and have good compatibility, which proves that the present invention, by modifying glass fiber with an imide-based silane coupling agent, can improve the interfacial interaction between glass fiber and polyamide 6 matrix, thus endowing the glass fiber / polyamide 6 composite material with good mechanical properties. At the same time, the synthesis of this coupling agent is significant for expanding the types and application range of silane coupling agents.

[0044] When the mass fraction of glass fiber is 30%, the glass fiber / polyamide 6 composite material obtained by this invention exhibits a tensile strength as high as 124.8 MPa and a notched impact strength as high as 8.9 kJ / m. 2 The modification effect is higher than that of γ-mercaptopropyltriethoxysilane. The preparation of imide-based silane coupling agents using mercapto-olefin click chemistry in this invention is characterized by mild conditions, simple operation, and high conversion rate, which can broaden the types and application range of silane coupling agents. Attached Figure Description

[0045] Figure 1 The image shows the microstructure of the tensile fracture surface of the glass fiber / polyamide 6 composite material modified with 2% imide-based silane coupling agent in Example 2.

[0046] Figure 2 The image shows the microstructure of the tensile fracture surface of the glass fiber / polyamide 6 composite material modified with 2% γ-mercaptopropyltriethoxysilane coupling agent in Comparative Example 1.

[0047] Figure 3 This is a microscopic morphology diagram of the tensile cross-section of the glass fiber precursor / polyamide 6 composite material in Comparative Example 2.

[0048] Figure 4The tensile stress-strain curves are shown for the glass fiber / polyamide 6 composite material modified with 2% imide-based silane coupling agent in Example 2, the glass fiber / polyamide 6 composite material modified with 2% γ-mercaptopropyltriethoxysilane coupling agent in Comparative Example 1, and the glass fiber precursor / polyamide 6 composite material in Comparative Example 2.

[0049] Figure 5 The above are the proton NMR spectra of maleimide, γ-mercaptopropyltriethoxysilane, and imide-based silane coupling agent in Example 1. Detailed Implementation

[0050] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0051] Example 1

[0052] This embodiment provides a modified glass fiber and polyamide composite material, which is prepared through the following steps:

[0053] (1) Weigh out 2g (0.0206mol) of maleimide and 4.912g (0.0206mol) of γ-mercaptopropyltriethoxysilane, respectively, and dissolve them in 50mL of tetrahydrofuran. Then add them to a 250mL three-necked flask, add 0.2g of triethylamine catalyst, start magnetic stirring, and heat the mixture to 55℃ in a nitrogen atmosphere. Maintain the reaction at this temperature for 6h to obtain the product, imide-based silane coupling agent, whose structure is shown in the following formula:

[0054]

[0055] Analysis using hydrogen NMR and infrared spectroscopy revealed that the click chemical reaction was essentially complete when carried out in an equimolar ratio; see the hydrogen NMR spectrum for details. Figure 5 The -SH (1.3 ppm) in γ-mercaptopropyltriethoxysilane and the -CH=CH- (6.69 ppm) in maleimide are completely eliminated in imide-based silane coupling agents.

[0056] (2) Prepare an ethanol / water (volume ratio of 2) solution and adjust the pH of the system to 4 with hydrochloric acid.

[0057] (3) Weigh 21g of glass fiber precursor and add it to 1575mL of the ethanol / water solution obtained in step (2), and then ultrasonically disperse it under stirring conditions to obtain a glass fiber dispersion. The ultrasonic time is 5min.

[0058] (4) Weigh 0.105g of imide silane coupling agent into the glass fiber dispersion in step (3), and then ultrasonically disperse it under stirring conditions for 5 minutes to obtain imide silane coupling agent / glass fiber dispersion; the amount of imide silane coupling agent is 0.5% of the mass of glass fiber.

[0059] (5) The imide silane coupling agent / glass fiber dispersion in step (4) was subjected to hydrolysis and condensation reaction in a shaker for 48 hours with a shaking speed of 85 r / min.

[0060] (6) The dispersion after the reaction in step (5) is removed by rotary evaporation at 85°C to obtain the modified glass fiber, and then dried in a vacuum oven at 100°C for 24 hours for use in the preparation of polyamide 6 composite material.

[0061] (7) The dried imide-silane coupling agent modified glass fiber is added to the polyamide 6 resin matrix by mixing. Specifically, the polyamide 6 is first dried in a vacuum oven at 100°C for 24 hours, and then 44g of polyamide 6 is completely melted in a mixer. Then, 19g of glass fiber (the mass fraction of glass fiber at this time is 30%) is added. The mixing temperature is 240°C, the rotor speed is 40r / min, and the total mixing time is 10min.

[0062] (8) The glass fiber / polyamide 6 mixture obtained in step (7) was melt-injection molded to obtain dumbbell-shaped specimens and notched impact specimens of the composite material, and the composite material was subjected to relevant mechanical property tests; wherein, the injection temperature was 250℃, the injection pressure was 0.6MPa, and the total injection time was 17s. The tensile specimen shape and size were type 1BA in standard GB / T1040.1-2006, and the tensile rate was 10mm / min. The notched impact specimen was type A notch in standard GB / T 1843-2008, and the impact test was a cantilever beam impact test. The test methods of the following examples and comparative examples are the same.

[0063] After testing, the glass fiber / polyamide 6 composite material prepared in Example 1 showed a tensile strength of 118.0 MPa, an elongation at break of 3.0%, and a notched impact strength of 8.7 kJ / m. 2 .

[0064] Example 2

[0065] This embodiment provides a modified glass fiber and polyamide composite material, which differs from Example 1 in step (4), specifically the amount of imide-silane coupling agent added. 0.21g of the imide-silane coupling agent is weighed into the glass fiber dispersion in step (4), and then ultrasonically dispersed under stirring for 8 minutes. In this case, the amount of imide-silane coupling agent is 1% of the glass fiber mass. The other steps are the same as in Example 1.

[0066] After testing, the glass fiber / polyamide 6 composite material prepared in Example 2 showed a tensile strength of 121.1 MPa, an elongation at break of 3.0%, and a notched impact strength of 8.6 kJ / m. 2 The microstructure of the tensile fracture surface of this glass fiber / polyamide 6 composite material is as follows: Figure 1 As shown.

[0067] Example 3

[0068] This embodiment provides a modified glass fiber and polyamide composite material, which differs from Example 1 in step (4), specifically the amount of imide-silane coupling agent added. 0.42g of the imide-silane coupling agent is weighed into the glass fiber dispersion in step (4), and then ultrasonically dispersed under stirring for 5 minutes. In this case, the amount of imide-silane coupling agent is 2% of the glass fiber mass. The other steps are the same as in Example 1.

[0069] After testing, the glass fiber / polyamide 6 composite material prepared in Example 3 showed a tensile strength of 124.8 MPa, an elongation at break of 3.0%, and a notched impact strength of 8.9 kJ / m. 2 .

[0070] Example 4

[0071] This embodiment provides a modified glass fiber and polyamide composite material, which differs from Example 1 in step (4), specifically the amount of imide-silane coupling agent added. 0.63g of the imide-silane coupling agent is weighed into the glass fiber dispersion in step (4), and then ultrasonically dispersed under stirring for 5 minutes. In this case, the amount of imide-silane coupling agent is 3% of the glass fiber mass. The other steps are the same as in Example 1.

[0072] After testing, the glass fiber / polyamide 6 composite material prepared in Example 4 showed a tensile strength of 124.3 MPa, an elongation at break of 3.1%, and a notched impact strength of 8.8 kJ / m. 2 .

[0073] To illustrate the effects of this invention, γ-mercaptopropyltriethoxysilane coupling agent and untreated glass fiber modified from original glass fiber were selected as comparative examples, as detailed below:

[0074] Comparative Example 1

[0075] The difference from Example 1 lies in step (4), where the added coupling agent is γ-mercaptopropyltriethoxysilane. 0.42 g of γ-mercaptopropyltriethoxysilane coupling agent is weighed into the glass fiber dispersion in step (4), and then ultrasonically dispersed under stirring for 8 minutes. In this case, the amount of γ-mercaptopropyltriethoxysilane coupling agent used is 2% of the glass fiber mass. The other steps are the same as in Example 1.

[0076] After testing, the glass fiber / polyamide 6 composite material prepared in Comparative Example 1 showed a tensile strength of 111.2 MPa, an elongation at break of 2.7%, and a notched impact strength of 8.3 kJ / m². 2 The microstructure of the tensile fracture surface of this glass fiber / polyamide 6 composite material is as follows: Figure 2 As shown.

[0077] Comparative Example 2

[0078] Polyamide 6 composite material was prepared using unmodified glass fiber precursor, and its preparation process and testing process were exactly the same as steps (7) and (8) in Example 1.

[0079] After testing, the glass fiber / polyamide 6 composite material prepared in Comparative Example 2 showed a tensile strength of 100.0 MPa, an elongation at break of 2.9%, and a notched impact strength of 6.1 kJ / m². 2 The microstructure of the tensile fracture surface of this glass fiber / polyamide 6 composite material is as follows: Figure 3 As shown.

[0080] As can be seen from the above test examples, the imide-based silane coupling agent synthesized using the present invention has a better reinforcing effect on polyamide 6 than the glass fiber modified with γ-mercaptopropyltriethoxysilane coupling agent.

[0081] The tensile stress-strain curves of glass fiber / polyamide 6 composite material modified with 2% imide-based silane coupling agent in Example 2, glass fiber / polyamide 6 composite material modified with 2% γ-mercaptopropyltriethoxysilane coupling agent in Comparative Example 1, and glass fiber precursor / polyamide 6 composite material in Comparative Example 2 are shown below. Figure 4 As shown.

[0082] As can be seen from the above test examples, the mechanical properties of the polyamide 6 composite material made of glass fiber modified with imide-based silane coupling agent are much higher than those of the polyamide 6 composite material made of glass fiber modified with γ-mercaptopropyltriethoxysilane coupling agent, specifically in terms of tensile properties and impact resistance.

[0083] The above descriptions are merely examples of the technical solutions of this invention. Commonly known structures and / or characteristics of the technical solutions are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of this invention, and these should also be considered within the scope of protection of this invention. These modifications and improvements will not affect the effectiveness of the invention or the practicality of the patent. The scope of protection claimed by this invention should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method of modifying glass fibers, wherein, The modification method comprises the following steps: An ethanol / water solution with a volume ratio of ethanol to water of 0.5-2 is prepared, and the pH value of the system is adjusted to 3-5; Glass fibers are added to the ethanol / water solution, and ultrasonic dispersion is performed under stirring to obtain a glass fiber dispersion liquid; An imide-based silane coupling agent is added to the glass fiber dispersion liquid, and ultrasonic dispersion is performed again under stirring to obtain an imide-based silane coupling agent / glass fiber dispersion liquid, wherein the amount of the imide-based silane coupling agent is 0.1-4% of the mass of the glass fiber; The imide-based silane coupling agent / glass fiber dispersion liquid is subjected to a hydrolysis and condensation reaction; The solvent is removed from the imide-based silane coupling agent / glass fiber dispersion liquid after the reaction by rotary evaporation, and then vacuum drying is performed to obtain modified glass fibers; The imide-based silane coupling agent is prepared from a thiol-containing silane coupling agent and a maleimide in a molar ratio of 1:0.5-1:1.5; and the thiol-containing silane coupling agent has the following molecular formula: wherein R1is or ; R2 is or ; R3 is or ; R4 is .

2. The method of modifying glass fibers according to claim 1, wherein, The imide-based silane coupling agent is prepared from the thiol-containing silane coupling agent and the maleimide in a molar ratio of 1:

1.

3. The method of modifying glass fibers according to claim 1, wherein, The preparation method of the imide-based silane coupling agent comprises the following steps: The thiol-containing silane coupling agent and the maleimide in a molar ratio of 1:0.5-1:1.5 are dissolved in a solvent, and triethylamine or triphenylphosphine is added as a catalyst to react at 40-70°C to obtain the imide-based silane coupling agent.

4. The method of modifying glass fibers according to claim 3, wherein, The thiol-containing silane coupling agent and the maleimide in a molar ratio of 1:0.5-1:1.5 are dissolved in the solvent, and the concentration is 0.1 g / mL-0.3 g / mL.

5. The method of modifying glass fibers according to claim 3, wherein, The solvent is one or a combination of two or more of acetone, tetrahydrofuran and toluene.

6. The method of modifying glass fibers according to claim 3, wherein, The amount of the triethylamine is 1-10% of the sum of the mass of the thiol-containing silane coupling agent, the maleimide and the solvent.

7. The method of modifying glass fibers according to claim 1, wherein, The amount of the imide-based silane coupling agent is 0.5-3% of the mass of the glass fiber.

8. The method of modifying glass fibers according to claim 1, wherein, The hydrolysis and condensation reaction is performed under shaking of a shaking table, the reaction time is 24-48 h, and the shaking speed of the shaking table is 80-110 r / min.

9. The method of modifying glass fibers according to claim 1, wherein, The rotary evaporation temperature is 70-95°C.

10. The method of modifying glass fibers according to claim 1, wherein, The temperature of the vacuum drying is 90-100°C.

11. The method of modifying glass fibers according to claim 1, wherein, The glass fibers are added to the ethanol / water solution, and ultrasonic dispersion is performed under stirring to obtain a glass fiber dispersion liquid, and the ultrasonic time is 5-7 min.

12. The method of modifying glass fibers according to claim 1, wherein, The imide-based silane coupling agent is added to the glass fiber dispersion liquid, and ultrasonic dispersion is performed again under stirring to obtain an imide-based silane coupling agent / glass fiber dispersion liquid, and the ultrasonic time is 5-10 min.

13. Modified glass fibers prepared by the method of any one of claims 1 to 12.

14. A method for preparing a modified glass fiber / polyamide composite material, comprising the following steps: The modified glass fibers of claim 13 are mixed with polyamide 6 resin to obtain the modified glass fiber / polyamide composite material.

15. The method of making a modified glass fiber and polyamide composite of claim 14, wherein, The modified glass fiber is added in an amount of 30% based on the total mass of the modified glass fiber and polyamide 6.

16. The method of making a modified glass fiber and polyamide composite of claim 14, wherein, The method comprises the following steps: The polyamide 6 is vacuum-dried, and then the dried polyamide 6 is completely melted in an internal mixer, and then the modified glass fiber is added to obtain a modified glass fiber and polyamide mixture; The modified glass fiber and polyamide mixture is obtained by melt-injection molding to obtain the modified glass fiber and polyamide composite material.

17. The method of making a modified glass fiber and polyamide composite of claim 16, wherein, The polyamide 6 is dried in a vacuum oven at 100℃ for 24 h.

18. The method of making a modified glass fiber and polyamide composite of claim 16, wherein, The internal mixing temperature is 230-250℃, the rotor speed is 30-50 r / min, and the whole internal mixing time is 8-13 min.

19. The method of making a modified glass fiber and polyamide composite of claim 16, wherein, The melting temperature is 240-260℃, the injection molding pressure is 0.6-0.7 MPa, and the whole injection molding time is 15-25 s.

20. A modified glass fiber and polyamide composite material prepared by the method of any one of claims 14 to 19.

21. The modified glass fiber with polyamide composite of claim 20, wherein, The tensile strength of the modified glass fiber and polyamide composite material is 118-125 MPa, the elongation at break is 3.0-3.1%, and the notched impact strength is 8.6-9.9 kJ / m 2 .

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