Flexible modified epoxy resin and preparation method thereof

Abstract

The invention relates to the technical field of epoxy resin, and discloses flexible modified epoxy resin and a preparation method thereof, polysiloxane of a side chain of a benzoxazine polymer and epoxy resin form the characteristic similar to a sea island structure, and the polysiloxane and the chain-shaped epoxy resin are mutually crosslinked to form a net so as to play a role in toughening; in addition, an active carboxyl functional group in the carboxyl benzoxazine containing the flexible unsaturated alkyl chain and an epoxy group in the epoxy resin can generate a chemical bonding effect during a curing reaction to form a two-phase structure, and crack propagation can be terminated when the epoxy resin is impacted by external force, so that the impact toughness of the epoxy resin is improved; in addition, the introduced flexible alkyl chain can promote the fluidity of the epoxy resin in a curing reaction, the molecular motion constraint is reduced, and the alkyl long chain and siloxane have very strong hydrophobicity and can be bonded into the epoxy resin to improve the water resistance of the epoxy resin.

Description

Technical Field

[0001] The invention relates to the technical field of epoxy resins, in particular to a flexible modified epoxy resin and a preparation method thereof. Background Art

[0002] Since epoxy resin contains active epoxy groups in its molecular structure, it undergoes cross-linking reactions with various types of curing agents to form a three-dimensional network structure polymer, giving it excellent corrosion resistance, bonding strength and chemical resistance, and is therefore widely used in coatings, adhesives, composite materials and other fields. However, the bond energy of CC bonds and CO bonds is small, and the surface energy is large, which causes epoxy resin to have problems such as large internal stress in the cured product, brittleness, and poor impact resistance. Therefore, there is an urgent need to perform flexibility modification on epoxy resin materials to improve the mechanical properties of epoxy resin and expand its application areas.

[0003] Invention No. CN109734923B discloses a method for preparing a bio-based flexible epoxy resin, wherein tung oil is catalytically hydrolyzed to obtain tung oil acid, tung oil is methylated to obtain methyl tung oil, and random carboxyl tung nitrile rubber is obtained by emulsion polymerization with acrylonitrile, and finally maleopimaric acid-type epoxy resin is grafted onto the random carboxyl tung nitrile rubber to obtain a bio-based flexible epoxy resin. The bio-based flexible epoxy resin has low synthesis cost, good flexibility and renewable raw materials, but the maleopimaric acid-type epoxy resin contains a multi-ring structure of rosin, has the disadvantages of high rigidity, brittleness, easy breakage, etc., and the prepared epoxy resin has limited flexibility.

[0004] Patent No. CN108084402B discloses a hemicellulose-based epoxy resin and a preparation method thereof, wherein polyhydroxy hemicellulose recovered from viscose wastewater is used as a raw material, and hemicellulose-based polyether polyol is obtained by hydroxyalkylation modification, and then the hemicellulose-based liquid epoxy resin is prepared by reacting with epichlorohydrin and sodium hydroxide, which has the advantages of low viscosity and good flexibility, etc. However, the cellulose structure is complex, and the dosage is difficult to control in practical applications, which has certain limitations; the present invention utilizes the structural characteristics of two adjacent phenolic hydroxyl groups and unsaturated flexible long chains of urushiol to prepare benzoxazine with flexible long chains, and then undergoes silylation with hydrogen-containing silicone oil to introduce polysiloxane hydrophobic flexible groups, which are cross-linked with bisphenol A epoxy resin to form a three-dimensional network structure, thereby obtaining a benzoxazine-modified epoxy resin with flexibility and hydrophobicity. Summary of the invention

[0005] The technical problem solved by the invention is to provide a flexible modified epoxy resin and a preparation method thereof, thereby solving the problem of poor toughness of the epoxy resin.

[0006] The technical solution of the present invention is: A method for preparing a flexible modified epoxy resin is as follows: Step (1), adding urushiol, p-aminobenzoic acid and polyformaldehyde to a reaction bottle equipped with a condenser, and then adding dioxane as a solvent, stirring for reaction, and performing reduced pressure distillation after the reaction is completed, filtering, washing with acetone, and drying to obtain carboxyl urushiol-based benzoxazine.

[0007] Step (2), adding hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is passed, stirring evenly, adding carboxyl urushiol-based benzoxazine, controlling the temperature to 40-50° C., adding chloroplatinic acid as a catalyst dropwise, cooling after the reaction, filtering, washing with methanol, and drying to obtain benzoxazine containing polysiloxane on the side chain.

[0008] Step (3), adding benzoxazine containing polysiloxane in the side chain and imidazole curing agent to bisphenol A epoxy resin, the curing temperature is 100-150° C., and the curing time is 3-8 hours to obtain a flexible modified epoxy resin.

[0009] Furthermore, the mass ratio of the reactants in step (1) is urushiol: para-aminobenzoic acid: polyformaldehyde = 1: (0.35-0.48): (0.015-0.024).

[0010] Furthermore, in step (1), the reaction temperature is 70-100° C. and the reaction time is 8-16 h.

[0011] Furthermore, the mass ratio of the reactants in step (2) is hydrogenated silicone oil: carboxyl-containing benzoxazine: chloroplatinic acid = 1: (0.24-0.4): (0.0004-0.001).

[0012] Furthermore, in step (2), the reaction temperature is 100-120° C., and the reaction time is 24-48 h.

[0013] Furthermore, in step (3), the imidazole curing agent is 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole or 1-benzyl-2-ethylimidazole.

[0014] Furthermore, the mass ratio of the reactants in step (3) is bisphenol A epoxy resin: benzoxazine containing polysiloxane in the side chain: curing agent = 1: (0.2-0.5): (0.3-0.4).

[0015] The beneficial technical effects of the present invention are: The invention firstly utilizes urushiol containing an unsaturated alkyl chain, p-aminobenzoic acid and polyformaldehyde to react to obtain a carboxyl benzoxazine containing a flexible unsaturated alkyl chain; then, the unsaturated bonds on the flexible long chain of urushiol react with hydrogen-containing silicone oil under the catalytic action of chloroplatinic acid to prepare a benzoxazine polymer containing polysiloxane on the side chain; finally, the benzoxazine containing polysiloxane is blended and modified with a bisphenol A epoxy resin to obtain a benzoxazine-modified flexible epoxy resin.

[0016] The polysiloxane of the benzoxazine polymer side chain forms a feature similar to an "island structure" with the epoxy resin, that is, the polysiloxane phase forms "islands" dispersed in the "ocean" formed by the epoxy resin, and cross-links with the chain epoxy resin to form a network, which plays a toughening role; in addition, the active carboxyl functional group in the carboxyl benzoxazine containing a flexible unsaturated alkyl chain can react with the epoxy group in the epoxy resin to generate a flexible chain block, which can produce a chemical bond with the epoxy resin during the curing reaction to form a two-phase structure of a toughening agent dispersed phase and an epoxy resin continuous phase. When the composite material is impacted, the toughening agent dispersed phase absorbs a large amount of energy due to the stress concentration effect and then stops the crack propagation, thereby improving the impact toughness of the epoxy resin; in addition, the introduced flexible alkyl chain can promote the fluidity of the epoxy resin in the curing reaction, reduce the cross-linking density of the cured product, reduce the constraints on molecular motion, and ensure that the epoxy resin has excellent flexibility under high mechanical strength, and the alkyl long chain and siloxane have strong hydrophobicity, which can improve the water resistance of the epoxy resin when bonded to the epoxy resin. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention belong to the scope of protection of the present invention. Example 1

[0018] Step (1), add 15g urushiol, 6g p-aminobenzoic acid and 0.3g polyformaldehyde in sequence to a reaction bottle equipped with a condenser, then add dioxane as a solvent, stir to react, react at 80°C for 10h, distill under reduced pressure, filter, wash with acetone, and dry to obtain carboxyl urushiol-based benzoxazine.

[0019] Step (2), add 50 g of hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is introduced, stir evenly, add 12 g of carboxyl urushiol-based benzoxazine, control the temperature to 45° C., dropwise add 0.03 g of catalyst chloroplatinic acid, react at 100° C. for 30 h, cool, filter, wash with methanol, and dry to obtain benzoxazine containing polysiloxane on the side chain.

[0020] Step (3), adding 20 g of benzoxazine containing polysiloxane in the side chain and 35 g of 2-ethyl-4-methylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 120° C. for 5 h to obtain a flexible modified epoxy resin. Example 2

[0021] Step (1), add 36g urushiol, 15g p-aminobenzoic acid and 0.7g polyformaldehyde in sequence to a reaction bottle equipped with a condenser, then add dioxane as a solvent, stir to react, react at 90°C for 12h, distill under reduced pressure, filter, wash with acetone, and dry to obtain carboxyl urushiol-based benzoxazine.

[0022] Step (2), add 30 g of hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is introduced, stir evenly, add 9 g of carboxyl urushiol-based benzoxazine, control the temperature to 50° C., dropwise add 0.022 g of catalyst chloroplatinic acid, react at 100° C. for 28 h, cool, filter, wash with methanol, and dry to obtain benzoxazine containing polysiloxane on the side chain.

[0023] Step (3), adding 30 g of benzoxazine containing polysiloxane in the side chain and 38 g of 2-phenylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 120° C. for 6 h to obtain a flexible modified epoxy resin. Example 3

[0024] Step (1), add 20g urushiol, 9.6g p-aminobenzoic acid and 0.4g polyformaldehyde in sequence to a reaction bottle equipped with a condenser, then add dioxane as a solvent, stir to react, react at 100°C for 16h, distill under reduced pressure, filter, wash with acetone, and dry to obtain carboxyl urushiol-based benzoxazine.

[0025] Step (2), add 100 g of hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is introduced, stir evenly, add 28 g of carboxyl urushiol-based benzoxazine, control the temperature to 45° C., dropwise add 0.08 g of catalyst chloroplatinic acid, react at 120° C. for 32 h, cool, filter, wash with methanol, and dry to obtain benzoxazine containing polysiloxane on the side chain.

[0026] Step (3), adding 40 g of benzoxazine containing polysiloxane in the side chain and 40 g of 2-methylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 110° C. for 8 h to obtain a flexible modified epoxy resin. Example 4

[0027] Step (1), add 80g urushiol, 32g p-aminobenzoic acid and 1.92g polyformaldehyde in sequence to a reaction bottle equipped with a condenser, then add dioxane as a solvent, stir to react, react at 80°C for 15h, distill under reduced pressure, filter, wash with acetone, and dry to obtain carboxyl urushiol-based benzoxazine.

[0028] Step (2), add 70g of hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is introduced, stir evenly, add 21g of carboxyl urushiol-based benzoxazine, control the temperature to 50°C, dropwise add 0.07g of catalyst chloroplatinic acid, react at 120°C for 24h, cool, filter, wash with methanol, and dry to obtain benzoxazine containing polysiloxane on the side chain.

[0029] Step (3), adding 50 g of benzoxazine containing polysiloxane in the side chain and 38 g of 2-ethyl-4-methylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 140° C. for 7 h to obtain a flexible modified epoxy resin.

[0030] Comparative Example 1 Step (1), add 15g urushiol, 6g p-aminobenzoic acid and 0.3g polyformaldehyde in sequence to a reaction bottle equipped with a condenser, then add dioxane as a solvent, stir to react, react at 80°C for 10h, distill under reduced pressure, filter, wash with acetone, and dry to obtain carboxyl urushiol-based benzoxazine.

[0031] Step (2), adding 20 g of carboxyl urushiol-based benzoxazine and 36 g of 1-benzyl-2-ethylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 130° C. for 8 h to obtain a carboxyl urushiol-based modified epoxy resin.

[0032] Comparative Example 2 Step (1), add 70g of hydrogenated silicone oil and solvent toluene into a reaction bottle into which nitrogen is introduced, stir evenly, then add 23g of urushiol, control the temperature to 50°C, dropwise add 0.07g of catalyst chloroplatinic acid, react at 120°C for 24h, cool, filter, wash with methanol, and dry to obtain urushiol containing polysiloxane on the side chain.

[0033] Step (2), adding 20 g of lacquer phenol containing polysiloxane in the side chain and 32 g of 2-ethyl-4-methylimidazole curing agent to 100 g of bisphenol A epoxy resin, and curing at 120° C. for 6 h to obtain a polysiloxane-modified epoxy resin.

[0034] Mechanical properties test: The tensile strength was tested by an electronic universal testing machine with a sample size of 100 mm × 10 mm × 5 mm and a tensile speed of 2 mm / min.

[0035] The bending strength was tested by a universal material testing machine, and the sample size was 100 mm × 10 mm × 5 mm.

[0036] The impact strength was tested by a pendulum impact tester with a sample size of 100 mm × 10 mm × 5 mm.

[0037]

[0038] It can be seen from the above table that with the increase of the content of flexible polysiloxane and long-chain benzoxazine, the tensile strength and flexural strength of the composite material gradually increase. Comparative Example 4 contains 33% polysiloxane and long-chain benzoxazine modifier, and the tensile strength is 32% higher than that of Comparative Example 1 containing only long-chain benzoxazine. This is mainly because the organic silicon oxygen segment contains Si-O bonds with larger bond energy, which can improve the flexibility of the resin. In addition, after the flexible long-chain benzoxazine group is cured with the epoxy resin, the cross-linking density of the system is increased, causing the yield of the epoxy base material, which plays a restraining and closing role on the further expansion of the crack, thereby increasing the tensile strength and flexural strength of the system, and the toughening effect is obvious.

[0039] The impact strength increases with the increase of the content of flexible polysiloxane and long-chain benzoxazine. The proportion of the flexible modifier in Comparative Example 3 is 28%, which is nearly twice the composite material prepared from the polysiloxane without carboxyl group and epoxy resin in Comparative Example 2. The reason is that the carboxyl-containing polysiloxane can undergo a chemical grafting reaction with the epoxy group of the epoxy resin to form a strong chemical bonding effect, which makes the modifier in the system compatible with the epoxy resin, increases the flexibility of the system, and retains the rigidity of the epoxy resin itself, forming a "rigid and flexible" three-dimensional network structure. Therefore, when subjected to external force, the flexible cured network structure can resist cracking, so that the system exhibits higher impact toughness, thereby obtaining a flexible epoxy resin.

[0040] Contact angle test: The contact angle was tested by a contact angle meter. The prepared modified epoxy resin was placed on a glass slide and cured to obtain an epoxy resin film with a thickness of 2 mm.

[0041]

[0042] It can be seen from the above table that as the content of polysiloxane increases, the surface contact angle of the composite material gradually increases. In Example 1, only 16% of polysiloxane and long-chain benzoxazine modifier are added, but the surface contact angle is 19° higher than that in Comparative Example 1, showing stronger hydrophobicity and water resistance. The main reason is that benzoxazine itself has a low water absorption rate and a hydrophobic long-chain structure, so that the prepared long-chain benzoxazine has a certain enhancement effect on the hydrophobicity. In addition, it is chemically bonded with the hydrophobic polysiloxane group, so that part of the silane chain segment is enriched on the surface after curing with the epoxy resin, which increases the contact angle and thus improves the water resistance of the epoxy resin. With the addition of the flexible modifier, the organic silicon segment on the surface of the composite material tends to be saturated, so that the growth rate of the surface contact angle slows down, but at this time the water resistance of the modified epoxy resin has been greatly improved.

[0043] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for preparing a flexible modified epoxy resin, characterized in that: The preparation method is as follows: Step (1), adding urushiol, p-aminobenzoic acid and paraformaldehyde in sequence to a reaction bottle equipped with a condenser, then adding dioxane as a solvent, stirring for reaction, and after the reaction is completed, distilling under reduced pressure, filtering, washing with acetone, and drying to obtain carboxylurushiol-based benzoxazine; Step (2), adding hydrogenated silicone oil and solvent toluene to a reaction bottle into which nitrogen is introduced, stirring evenly, adding carboxyl urushiol-based benzoxazine, controlling the temperature to 40-50° C., adding chloroplatinic acid as a catalyst dropwise, cooling after the reaction, filtering, washing with methanol, and drying to obtain benzoxazine containing polysiloxane on the side chain; Step (3), adding benzoxazine containing polysiloxane in the side chain and imidazole curing agent to the bisphenol A epoxy resin, and curing to obtain a flexible modified epoxy resin; The mass ratio of the reactants in step (1) is urushiol: para-aminobenzoic acid: paraformaldehyde = 1: (0.35-0.48): (0.015-0.024); The mass ratio of the reactants in step (2) is hydrogen silicone oil: carboxyl-containing benzoxazine: chloroplatinic acid = 1: (0.24-0.4): (0.0004-0.001); The mass ratio of the reactants in step (3) is bisphenol A epoxy resin: benzoxazine containing polysiloxane in the side chain: imidazole curing agent = 1: (0.2-0.5): (0.3-0.4).

2. The method for preparing a flexible modified epoxy resin according to claim 1, characterized in that: In the step (1), the reaction temperature is 70-100° C. and the reaction time is 8-16 h.

3. The method for preparing a flexible modified epoxy resin according to claim 1, characterized in that: In the step (2), the reaction temperature is 100-120° C. and the reaction time is 24-48 hours.

4. The method for preparing a flexible modified epoxy resin according to claim 1, characterized in that: In the step (3), the imidazole curing agent is 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole or 1-benzyl-2-ethylimidazole.

5. The method for preparing a flexible modified epoxy resin according to claim 1, characterized in that: In the step (3), the curing temperature is 100-150° C. and the reaction time is 3-8 hours.

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