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Cross‐linking of epoxy resins takes place with the participation of oxirane rings present in them. The strained structure of these rings is the reason for their high reactivity and facilitates their opening under the influence of very different factors. Epoxy resin curing generally takes place under the influence and with the participation of multifunctional chemical compounds with active protons. These are mainly polyamines (aliphatic, aromatic, and cycloaliphatic) and carboxylic compounds (acids and anhydrides). Cross‐linking of the epoxy resins can also be accomplished by a mechanism of ring‐opening polymerization using suitable catalysts. This is an important method of cross‐linking, especially coating materials. However, the search for compounds of natural origin, which could replace petrochemical hardeners, is the most important in the case of polyamines and carboxyl compounds that are used for cross‐linking in stoichiometric ratios to the content of epoxy groups in the resin. The consumption of ring‐opening polymerization catalysts is insignificant compared to the polyamine and carboxylic hardeners. Usually, they are used in an amount of up to several percents by weight relative to the weight of the cross‐linked resin. Therefore, new biohardeners should be sought first and foremost among the compounds of natural origin with amine or carboxylic functionalities. The appropriate reactivity and adequate miscibility with epoxy resins are the conditions, which are limiting the use of these new compounds.

Considering the possibilities of using plant oils as raw materials for the synthesis or modification of epoxy resins, an interesting solution would be just to give the plant oils proper functionality so that they can also act as natural hardeners. The use of modified vegetable oils in this role is favored by their very good miscibility with the epoxidized vegetable oils and good miscibility with the epoxy resins based on bisphenol A (despite the differences in the polarity of both groups of materials). For example, hydrolyzed castor oil can serve as a source of dehydrated fatty acids from which reactive polyamides are obtained by reaction with acrylic acid [141] (Figure 1.54) and then with various polyamines (diethylenetriamine, triethylenetetramine, and tetraethylenepentamine) [17].

The obtained polyamides (with amine values from 310 to 389 mg KOH/g) can be used to cross‐link the epoxy resins based on bisphenol A giving the materials with good coating properties.

Figure 1.54 Synthesis of C₂₁ cycloaliphatic dicarboxylic acids and reactive polyamides from them.

Figure 1.55 Scheme of soybean oil functionalization with thioglycolic acid.

Vegetable oil‐based curing agents can be obtained through direct oil modification. The novel bio‐based polyacid hardener is synthesized by the thiol‐ene coupling of soybean oil with thioglycolic acid (Figure 1.55) [142].

The synthesized soybean oil‐based polyacid exhibit a functionality of 3.3 acid functions per triglyceride molecule. This polyacid curing agent is characterized by the high reactivity toward epoxy groups. The low molecular weight bisphenol A‐based epoxy resin cross‐linked with the bio‐polyacid exhibits interesting properties for coating and binders (the Shore A hardness of 52 and E′ = 0.59 MPa). A commercially available mercaptanized soybean oil (prepared by direct addition of H₂S to soybean oil) reacted with allylamine (Figure 1.56) or its salts can also be potentially used as a cross‐linking agent for epoxy resins [143].

Modified lignin derivatives are also applied as curing agents for the epoxy resins. Commercially, various curing agents for epoxy resins are available. The commonly used petroleum‐based curing agents include amines, amides, hydroxyls, acid anhydrides, phenols, and polyphenols. However, recently, a great effort has been put on obtaining new bio‐based hardeners for epoxy systems. Generally, lignin‐based curing agents are prepared by two different methods: (i) the reaction of lignin with ozone in the presence of NaOH to give lignin with unsaturated carboxyl groups (Figure 1.57) or (ii) throughout the reaction of modified lignin (partially depolymerized lignin or polyol solutions of alcoholysis lignin) with anhydrides or trimellitic anhydride chloride [144].

Figure 1.56 Synthesis of a polyamine cross‐linking agent via thiol‐ene reaction of mercaptanized soybean oil with allylamine.

Figure 1.57 Synthesis of carboxylic acid from lignin.

Another interesting approach is using aminated lignin (black powder, amine value: 180–200 mg KOH/g) as a cross‐linker of bisphenol A‐based epoxy resin (epoxy value: 0.48–0.54 mol/100 g) [145]. The obtained aminated lignin contains a large number of primary and secondary amine groups, which successfully cure the epoxy network (Figure 1.58).

The application of aminated lignin has the positive effect at the initial degradation stage of the epoxy resin. Because lignin itself has a good thermal–mechanical performance, samples prepared with its higher content presented accordingly improved properties (Table 1.6).

TGA and DMA tests reveal improved thermal–mechanical properties of the bio‐based epoxy resin. In comparison to the epoxy resin cross‐linked with the commercial curing agent based on modified isophorone diamine (W93 curing agent: amine value: 550–600 mg KOH/g), the recorded value of T₁₀, for the material cured with the hardener containing 20 wt% of aminated lignin, increased nearly by 50 °C, while the highest value of T₁₀ was achieved for the epoxy system cured by 100% aminated lignin (T₁₀ = 266 °C for 100% of petrochemical‐based hardener W93, T₁₀ = 315 °C and T₁₀ = 332 °C for aminated lignin hardener, 80%W93 + 20%AL and 100%AL, respectively). Additionally, the mass loss before 300 °C of the epoxy resin cured by W93 is four times higher than the one recorded for the aminated lignin. Moreover, the obtained materials are characterized by improved values of the glass transition temperature and thermal deformation temperature. T_g and T_d of epoxy resin sample cured with the hardener containing 50 wt% of lignin increased by 14 °C compared with the one without lignin.

Figure 1.58 The curing of epoxy resin using aminated lignin as a curing agent.

Table 1.6 TGA, T_g, and T_d values of epoxy resin samples cured with different contents of lignin in the curing agent [145].

Epoxy resin cured by different hardeners	Total mass loss before 300 °C (%)	T10 (°C)	T50 (°C)	Tg (°C)	Td (°C)
100%W93	11.1	266	362	79	70
80%W93 + 20%AL	8.7	315	370	86	74
70%W93 + 30%AL	—	—	—	89	76
60%W93 + 40%AL	7.2	325	364	92	79
50%W93 + 50%AL	—	—	—	93	84
40%W93 + 60%AL	8.4	317	371	—	—
20%W93 + 80%AL	5.5	327	363	—	—
100%AL	3.7	332	372	—	—

Figure 1.59 Preparation of partially depolymerized lignin (PDL) and lignin polycarboxylic acid LPCA) from Kraft lignin.

Another interesting utilization of lignin‐based compounds for the curing purposes of epoxy resin is application of partially depolymerized Kraft lignin [146]. In order to increase its solubility in organic solvents, lignin is subjected to the base‐catalyzed depolymerization in supercritical methanol. The resulting partially depolymerized lignin (PDL) is then converted to lignin‐based polycarboxylic acid (LPCA) by reacting with succinic anhydride (Figure 1.59).

LPCA might be applied as a curing or co‐curing agent for epoxy resins. The curing of a commercial epoxy (DER353, epoxy value: 0.500–0.526 mol/100 g) using LPCA is conducted in the presence of 1 wt% of ethyl‐4‐methyl‐imidazole as a catalyst and at the similar temperature range to the commercial hexahydrophthalic anhydride (HHPA). The obtained cured material exhibits a moderate T_g and comparable storage modulus to that cross‐linked with a commercial anhydride curing agent. Additionally, linear succinate monoester, used in the synthesis of LPCA, enhances the flexibility of the lignin molecules. Therefore, increasing the content of bio‐curing agent in the formulation tends to reduce the T_g of cured resins. For composition of DER353/LPCA with equivalent ratio 1/0.6, 1/0.8, to 1/1, the T_g of the cured resins decreases from 78.5 and 69.4 to 62.3 °C, while the storage moduli at room temperature is comparable (2.4–2.7 GPa). Based on the studies on the application of the solid LPCA together with other liquid curing agents, such as glycerol tris(succinate monoester) and commercial hexahydrophthalic anhydride to cure epoxies, it was observed that using a mixture of LPCA and a liquid curing agent not only adjusts the viscosity of the resin system but also significantly regulates the dynamic mechanical properties and thermal stability of the obtained epoxy materials.

Moreover, interesting two different types of novel cross‐linked epoxy resins from lignin and glycerol are reported [147]. The first one is obtained by mixing the product of sodium lignosulfonate (LS) and glycerol (LSGLYPA, where the content of LSGLYPA varies at 0, 20, 40, 60, 80, and 100%) with polyacid of sodium lignosulfonate and ethylene glycol (LSEGPA) and ethylene glycol diglycidyl ether (EGDGE) at 80 °C (Figure 1.60).

The second one is by mixing LSGLYPA with a mixture of EGDGE/GLYDGE (the content of EGDGE/GLYDGE mixture was 0, 20, 40, 60, 80, and 100%) under similar reaction conditions (Figure 1.61).

The glass transition temperature of the cross‐linked epoxy resins increases with increasing LSGLYPA and GLYDGE contents. The increase of T_g for the product of the first synthesis is due to the increase in cross‐linking density. In turn, for the second sample, an increase in the GLYDGE content increases T_g of the cured epoxy resins (hydrogen bonding became the dominant factor).

Vanillin can be converted into the diamine derivatives (Figure 1.62) [148].

Starting from diglycidyl ethers of 2‐methoxyhydroquinone and vanillyl alcohol through an epoxy ring opening with ammonia, it is possible to obtain two primary amines, which could be applied as epoxy resin cross‐linking agents. Because of β‐hydroxyl groups present in the molecules of diamines, they exhibit the autocatalytic effect on the epoxy–amine reactions. These new β‐hydroxylamines can be used for cross‐linking of diglycidyl ether of methoxyhydroquinone and diglycidyl ether of vanillyl alcohol giving fully bio‐based epoxy systems with good thermomechanical properties and high thermal stability.

Cardanol derivatives are classified as the phenolic curing agents that are cross‐linked with epoxy groups via the phenolic hydroxyl group. Novolac resins (Nov‐I and Nov‐II), containing an amount of unreacted cardanol of 35 and 20 wt%, respectively, are synthesized by the condensation reaction of cardanol and paraformaldehyde using oxalic acid as a catalyst (Figure 1.63) [149].

The cardanol‐based novolacs might be used as curing agents of commercial (the diglycidyl ether of bisphenol A) epoxy resin in the presence of 2‐ethyl‐4‐methyl‐imidazole as a catalyst. The higher cross‐linking density is observed with higher amounts of epoxy resin. Moreover, the resin cured with Nov‐II is characterized by higher T_g and better mechanical properties than Nov‐I‐resin. On the other hand, because of the higher molecular weight and lower unreacted cardanol content, the similar thermal degradation properties are observed for both tested materials.

Figure 1.60 Scheme of preparation cross‐linked epoxy resin.

Figure 1.61 Scheme of epoxy resin by cross‐linking LSGLYPA with a mixture of EGDGE/GLYDGE.

Figure 1.62 Synthesis of dihydroxyaminopropane of 2‐methoxyhydroquinone (a) and (b) vanillyl alcohol.

Figure 1.63 Synthesis of the cardanol‐based novolacs.

The interesting cross‐linking agent is possible to obtain as the diamine derivative of isosorbide (Figure 1.64) and it could be used for curing of diglycidyl ether of isosorbide giving the completely bio‐based epoxy system [150].

The diamine derivative of isosorbide is obtained using microwave assistant thiol‐ene coupling reaction in the aqueous media and with the water‐soluble initiator (NH₄)₂S₂O₈, as the alternative to AIBN. The cured isosorbide‐based resin has good shape fixity, good shape recovery, and satisfied thermal stability. This fully bio‐based resin shows great potential to be used as a candidate for shape memory material. Another synthetic method for obtaining the diamine derivative of isosorbide is the reaction of cyanoethylation of isosorbide, followed by the hydrogenation of di‐cyanoethylated product (Figure 1.65) [151].

Terpene derivatives might also be applied as bio‐based curing agents for epoxy resins. For example, a novel terpene‐based curing agent prepared as an adduct of myrcene, monoterpene containing three double bonds including conjugated diene, and maleic anhydride (MMY) is used to cure bisphenol A‐based epoxy resin [152]. The obtained cured material is characterized by a tensile strength of 48.74 MPa and a storage modulus of 19.06 MPa, but a poor impact property of 8.55 kJ/m² and a very low elongation at break (7.54%). Improvement of properties of cured bio‐based epoxy resins is performed by application of mixture (mixed by weight ratios of 100/0, 75/25, 50/50, 25/75, and 0/100, respectively) of two curing agents: MMY and castor oil‐modified adduct of myrcene and maleic anhydride (CMMY) (Figure 1.66).

Figure 1.64 Synthesis of the isosorbide‐based cross‐linking agent.

Figure 1.65 Synthesis of the isosorbide‐based cross‐linking agent via cyanoethylation step.

Table 1.7 Properties of epoxy resin networks cured with bio‐based curing agents.

Cured samples	Tg (°C)	E at Tg+30°C (MPa)	Tensile strength (MPa)	Elongation at break (%)	Impact strength (kJ/m2)
MMY100	61.59	19.06	48.74	7.5	8.55
MMY75/CMMY25	58.03	19.00	42.11	6.5	13.87
MMY50/CMMY50	54.03	4.18	35.55	6.6	17.29
MMY25/CMMY75	45.09	2.72	11.11	259.4	62.51
CMMY100	15.14	1.11	0.43	565.8	Unbroken

Based on the obtained results (Table 1.7), with the increase of CMMY weight ratio, the tensile strength and T_g of the cross‐linked resin decreases, but the elongation at break and the impact strength increase.

As can be seen from the examples presented above, it is possible to obtain not only epoxy resins from raw materials of natural origin but also cross‐linking agents for them. Interestingly, it is also possible to synthesize completely bio‐derived epoxy systems. Both the epoxy resins based on bisphenol A, as well as cross‐linked with curing agents on the basis of raw materials of natural origin, and the fully epoxy biosystems are characterized by very good final properties.

Figure 1.66 Synthesis of bio‐based epoxy curing agent derived from myrcene and castor oil.