Sulfur-containing materials and uses thereof

Abstract

A sulfur reaction product is formed by reacting elemental sulfur with an amine or epoxy compound containing a reactive functional group. Such reaction products may be incorporated as modifiers into heat curable resin compositions. When a thermally curable composition containing such a sulfur reaction product is cured, the resulting crosslinked thermoset exhibits improved stress relaxation characteristics consistent with glass polymer-like behavior.

Description

[0001] This application is a divisional application of Chinese patent application No. 202180041903.2, filed on June 11, 2021, entitled "Sulfur-containing materials and their uses".

[0002] This disclosure generally relates to sulfur-containing materials and their applications. Attached Figure Description

[0003] Figure 1 The relative relaxation modulus of thermosetting materials modified with different amounts of diaminodiphenyl sulfide is shown as a function of temperature.

[0004] Figure 2 The relative relaxation modulus of thermosetting materials modified with different amounts of sulfur-MDEA reaction products is shown as a function of temperature.

[0005] Figure 3 The relative relaxation modulus of thermosetting materials modified with different amounts of sulfur-DGEBF (sulfur-PY306) reaction products is shown as a function of temperature. Detailed Implementation

[0006] Composite materials consisting of reinforcing fibers embedded in a thermosetting resin matrix have been used to manufacture load-bearing components suitable for transportation applications (including aerospace, aviation, marine, and land vehicles) and building / construction applications. To form structural parts from composite materials, the material must be shaped and cured. Once cured, thermosetting composites become irreversibly hardened and cannot be reshaped. Recycling the polymer component (or matrix) of cured thermosetting composites is challenging. Conventional recycling methods involve the thermal or chemical degradation of the polymer matrix to produce recyclable elements that can be separated from fiber recycling.

[0007] One attempt to provide reprocessable epoxy composites involves mixing epoxy resins with a crosslinking agent having the formula Ar-SS-Ar, where Ar is a ring system of 5 to 14 carbon atoms (see WO 15181054 A1). The specific crosslinking agent disclosed in WO15181054 is bis(4-aminophenyl)disulfide (AFD). Composites produced from epoxy resins crosslinked with this AFD crosslinking agent have been found to exhibit reprocessability, recyclability, and repairability. A related disadvantage of using AFD crosslinking agents in this way is their high cost, making their large-scale application impractical.

[0008] Elemental sulfur is a widely available, low-cost material, and its polymerization behavior is well-known. S exists at room temperature. 5至8 The sulfur structure undergoes a ring-opening reaction at approximately 110°C-120°C to form a diradical: .

[0009] This type of diradical polymerizes at approximately 150°C to form linear chains with higher molecular weights (Mw); however, the resulting sulfur polymer is unstable and stably reverts to cyclic S over time. 5至8 unit.

[0010] It has been found that when elemental sulfur reacts with certain amines or epoxides possessing reactive functional groups, and the reaction products are introduced into epoxy-based thermosetting materials and cured, the cured modified thermosetting materials exhibit vitrifier-like behavior. Vitrifier-like polymers are a class of polymers that possess the properties of permanently cross-linked thermosetting materials while maintaining processability due to covalent adaptive networks (CANs). When thermally triggered, the CANs can undergo exchange reactions of the cross-links, which promotes rearrangement of the polymer network, enabling macroscopic reshaping. If stress is applied to the cross-links, they can rearrange until stress relaxation occurs, resulting in a new shape. Vitrifier-like behavior can be demonstrated using stress relaxation experiments, in which the material is stretched to a fixed length at isothermal temperatures, and the relaxation modulus is measured over a fixed time period. The stress relaxation behavior can be quantified by comparing the initial relaxation modulus at t = 0 and the relaxation modulus at the end of t = 0. Glass-like polymer behavior is advantageous in thermosetting materials because, theoretically, it allows thermosetting networks containing glass-like polymer functional groups to be recycled by dissociating glass-like polymer bonds.

[0011] Sulfur reaction products can be formed by reacting elemental sulfur with an amine having reactive functional groups, particularly an aromatic diamine having at least one, preferably two, reactive amino groups per molecule. Sulfur reaction products can also be formed by reacting elemental sulfur with an epoxide compound, particularly an epoxide compound having at least one, preferably two, epoxy functional groups per molecule, and a compatible reagent.

[0012] In one embodiment, the sulfur reaction product is formed by reacting elemental sulfur with 4,4'-methylene-bis-(2,6-diethylaniline) (MDEA), hereinafter referred to as the "sulfur-MDEA reaction product".

[0013] In another embodiment, the sulfur reaction product is formed by reacting elemental sulfur with diglycidyl ether (DGEBF) of bisphenol F, hereinafter referred to as the "sulfur-DGEBF" reaction product.

[0014] It has been found that the sulfur-MDEA reaction product is a solid homogeneous material, and the sulfur-DGEBF reaction product is a paste-like homogeneous material. Both reaction products are soluble in epoxy resin at high temperatures. In this context, the term "homogeneous" means that the composition is essentially or largely uniform, without any visual inconsistencies.

[0015] Sulfur reaction products can be incorporated as modifiers into thermosetting resin compositions containing one or more epoxy resins and amine curing agents. When the thermosetting composition containing such sulfur reaction products is cured, the resulting crosslinked thermosetting material exhibits improved stress relaxation characteristics, consistent with glass-like polymer behavior. Due to the formation of glass-like polymer characteristics, the cured material possesses the characteristics of a reprocessable thermosetting material.

[0016] Preparation of reaction products The reaction product of sulfur and amine is prepared by mixing sulfur (in powder form) with an amine, heating the mixture to a temperature higher than the melting temperature of sulfur or amine (whichever is higher), and maintaining the heated mixture for a period of time to ensure that the reactive functional groups present on the amine react with elemental sulfur. In some embodiments, the reaction temperature is in the range of 120°C to 200°C, and in one embodiment it is 140°C. The reaction time is preferably longer than 1 hour.

[0017] For the sulfur-MDEA reaction product, the mass ratio of sulfur to amine can be from 0.01:1 to 1:1, preferably 0.5:1 or 1:1.

[0018] The reaction product of sulfur and epoxy resin is prepared by mixing sulfur (in powder form) with epoxy resin and a compatibility agent, heating the mixture to a temperature above the melting temperature of sulfur, and maintaining the heated mixture for a period of time to ensure that the reactive functional groups present on the epoxide react with elemental sulfur. In some embodiments, the reaction temperature is in the range of 120°C to 200°C, and in one embodiment it is 140°C. The reaction time is preferably longer than 1 hour.

[0019] For the sulfur-DGEBF reaction product, the mass ratio of sulfur to epoxy resin can be from 0.01:1 to 1:1, preferably 0.5:1 or 1:1. The compatibility agent is selected from compounds that show evidence of solubility in a heated sulfur mixture, such as sodium diethyldithiocarbamate (DDC). The amount of accelerator is up to 20 parts by weight per 100 parts by weight of the combined sulfur and DGEBF, and more preferably 5 parts by weight.

[0020] Products and manufacturing methods The sulfur reaction products disclosed herein can be used to manufacture composite materials, such as prepregs or to form polymer articles without fiber reinforcement, or for resin transfer molding or other liquid resin injection or infusion processes.

[0021] According to one embodiment of this disclosure, the prepreg is composed of a reinforcing fiber layer wholly or partially embedded in a resin or polymer matrix containing sulfur reaction products as additives. In another embodiment, the prepreg is composed of a reinforcing fiber layer embedded in sulfur reaction products that form a polymer matrix.

[0022] As used in this disclosure, the term "embedded" means firmly fixed within the surrounding material, and the term "matrix" means a large quantity of material, such as a resin or polymer, in which a substance is encapsulated or embedded. The term "resin" as used herein refers to an uncured or cross-linked monomer, oligomer, or polymer.

[0023] For thermosetting prepregs, the resin matrix contains one or more uncured thermosetting resins and sulfur reaction products as additives. Optionally, a curing agent may be included in the resin matrix to react with the resin and enable crosslinking. The resin matrix of a thermosetting prepreg can be in a partially cured or uncured state. Uncured or partially cured prepregs are flexible or pliable materials ready for layup and shaping into a three-dimensional configuration, subsequently cured to form a hardened composite part. Consolidation by applying pressure (heated or unheated) can occur before curing to prevent voids from forming within the laminate. This type of thermosetting prepreg is particularly suitable for manufacturing load-bearing structural parts, such as aircraft wings and fuselages. Important characteristics of cured thermosetting prepregs are high strength and stiffness, as well as reduced weight.

[0024] The terms "cure" and "curing" refer to the hardening of prepolymer materials, resins, or monomers by heating at high temperatures. The term "curable" for compositions means that the composition can be cured into a hardened or thermosetting state.

[0025] Thermosetting resins suitable for thermosetting resin matrices include, but are not limited to, epoxy resins, imides (such as polyimide or bismaleimide), vinyl ester resins, cyanate ester resins, isocyanate-modified epoxy resins, phenolic resins, furan resins, benzoxazine, formaldehyde condensation resins (such as those with urea, melamine or phenol), polyesters, acrylic resins, mixtures, blends and combinations thereof.

[0026] This disclosure also relates to a method for manufacturing thermosetting composite materials. According to one embodiment, the method for manufacturing the composite material includes: (a) Adding sulfur reaction products to an uncured thermosetting resin composition; (b) Impregnating the fiber reinforcement layer or injecting the fiber preform with the resin composition of step (a); and (c) Curing the impregnated fiber reinforcement at high temperature, preferably for a period of time, such that the ratio of the curing enthalpy to the uncured enthalpy, as determined by differential scanning calorimetry (DSC), is less than 0.1, and preferably less than 0.05.

[0027] In this embodiment, the sulfur reaction product is used as an additive, which acts as a modifier.

[0028] In an alternative embodiment, the sulfur reaction products are used directly as the polymer matrix in the composite material. In this embodiment, the method for manufacturing the composite material includes; (a) Impregnating fiber reinforcement layers or injecting fiber preforms with sulfur reaction products; and (b) Curing the impregnated fiber reinforcement at high temperature, preferably for a period of time, such that the ratio of curing enthalpy to uncured enthalpy, as determined by DSC, is less than 0.1, and preferably less than 0.05.

[0029] Another aspect of this disclosure relates to a liquid resin infusion method or liquid molding method, particularly resin transfer molding (RTM) and vacuum-assisted RTM (VaRTM). In this resin infusion method, a thermosetting resin composition containing sulfur reaction products, or the sulfur reaction products themselves, is formulated to have a sufficiently low viscosity for infusion / injection into a fiber preform.

[0030] In RTM, a fiber preform is placed in a closed mold and heated to an initial temperature, for example, greater than 25°C, and in some embodiments 90°C to 120°C. A liquid resin composition is then injected into the mold to influence the infusion of the liquid resin into the preform. During the infusion of the fiber preform, the mold can be maintained at a residence temperature of 20°C to 220°C. After infusion is complete, the temperature of the mold is increased to influence the curing of the resin-infused preform, thereby forming a hardened composite article. In VaRTM, a fiber preform is placed in a mold that is closed on one side by a flexible vacuum bag, and a vacuum is applied to draw liquid resin into the preform. The preform consists of one or more layers of reinforcing fibers that are permeable to the liquid resin. When the preform is completely filled with the resin composition, the mold temperature is raised to the curing temperature, for example, for a predetermined period of time within a range of 160°C to 200°C, to allow the resin composition to fully cure. The cured product obtained by this method is a hardened composite article.

[0031] Reinforcing fibers suitable for the purposes disclosed herein include carbon or graphite fibers, glass fibers, and fibers formed from silicon carbide, alumina, boron, quartz, etc., as well as fibers formed from organic polymers (such as polyolefins, poly(benzothiazole), poly(benzimidazole), polyarylates, poly(benzoxazole), aromatic polyamides, polyarylene ethers, etc.), and may include mixtures having two or more of these fibers. Preferably, the fibers are selected from glass fibers, carbon fibers, and aromatic polyamide fibers, such as those marketed by DuPont Company under the trade name KEVLAR. The reinforcing fibers may be in the form of chopped or continuous fibers, as tows consisting of multiple filaments, as continuous unidirectional or multidirectional tapes, or as woven fabrics, non-crimped fabrics, or nonwoven fabrics. The weaving form may be selected from plain weave, satin weave, or twill weave types. Non-crimped fabrics may have multiple layers and fiber orientations.

[0032] Therefore, the present invention provides at least the following: 1. A sulfur reaction product formed by reacting elemental sulfur with an amine selected from an aromatic diamine having at least one, preferably two, amine groups.

[0033] 2. A sulfur reaction product formed by reacting elemental sulfur with 4,4'-methylene-bis-(2,6-diethylaniline) (MDEA).

[0034] 3. The sulfur reaction product according to item 2, wherein the reaction product is formed by mixing sulfur with MDEA and heating the resulting mixture to a temperature in the range of 100°C to 200°C, preferably for a duration of more than 1 hour.

[0035] 4. The sulfur reaction product according to item 2 or 3, wherein the mass ratio of sulfur to MDEA is from 0.01:1 to 1:1, preferably 0.5:1 or 1:1.

[0036] 5. A sulfur reaction product formed by reacting elemental sulfur with an epoxy compound having at least one, preferably two, epoxy functional groups per molecule and a compatible reagent.

[0037] 6. A sulfur reaction product formed by reacting elemental sulfur with diglycidyl ether (DGEBF) of bisphenol F and a compatible reagent.

[0038] 7. The sulfur reaction product according to item 5 or 6, wherein the compatibility reagent is sodium diethyldithiocarbamate (DDC).

[0039] 8. The sulfur reaction product according to item 6 or 7, wherein the reaction product is formed by mixing sulfur with DGEBF and the compatible reagent, and heating the resulting mixture to a temperature in the range of 100°C to 200°C, preferably for a duration of more than 1 hour.

[0040] 9. The sulfur reaction product according to any one of items 6 to 8, wherein the mass ratio of sulfur to DGEBF is from 0.01:1 to 1:1, preferably 0.5:1 or 1:1, and the amount of the compatibility reagent is up to 20 parts by weight, preferably 5 parts, per 100 parts by weight of the combined sulfur and DGEBF.

[0041] 10. A curable resin composition comprising one or more epoxy resins, at least one amine curing agent, and a sulfur reaction product according to any one of claims 1 to 9.

[0042] 11. A curable composite material comprising reinforcing fibers wholly or partially embedded in a resin matrix comprising one or more epoxy resins and a sulfur reaction product according to any one of claims 1 to 9.

[0043] 12. A composite material comprising reinforcing fibers and a sulfur reaction product according to any one of claims 1 to 9.

[0044] 13. A thermosetting prepreg comprising a unidirectional reinforcing fiber layer embedded in a curable resin matrix comprising one or more uncured epoxy resins and sulfur reaction products according to any one of claims 1 to 9.

[0045] 14. A method for manufacturing a composite material, comprising: (a) Adding the sulfur reaction product according to any one of items 1 to 9 to an uncured thermocurable resin composition comprising one or more thermosetting resins; (b) Impregnating the fiber reinforcement layer or injecting the fiber preform with the thermosetting resin composition formed in step (a); and (c) Curing the impregnated fiber reinforcement at high temperature, preferably for a period of time, such that the ratio of curing enthalpy to uncured enthalpy, as determined by differential scanning calorimetry (DSC), is less than 0.1, preferably less than 0.05.

[0046] 15. The method according to item 14, wherein the thermocurable resin composition of step (a) comprises one or more epoxy resins and at least one amine curing agent.

[0047] 16. A method for manufacturing a composite material, comprising: (a) Impregnating a fiber-reinforced layer or injecting a fiber preform with the sulfur reaction product according to any one of items 1 to 9; and (b) The impregnated fiber reinforcement is cured at a high temperature, preferably for a period of time, such that the ratio of the curing enthalpy to the uncured enthalpy, as determined by differential scanning calorimetry (DSC), is less than 0.1, and preferably less than 0.05.

[0048] 17. Use of the sulfur reaction product according to any one of claims 1 to 9 in a thermosetting resin composition suitable for liquid resin infusion.

[0049] 18. Use of the sulfur reaction product according to any one of items 1 to 9 in the manufacture of fiber-reinforced composite materials.

[0050] Example Example 1 The reaction products of sulfur and MDEA At room temperature, 1 g of elemental sulfur powder and 1 g of MDEA (as a crosslinking agent) were manually mixed in a small glass vial, and then heated to 120 °C for 1 hour on a hot plate with magnetic stirring. After 1 hour at 120 °C, the vial was transferred to an oven and heated at 140 °C for 14 hours. The resulting sulfur-MDEA reaction product (sample A) was found to be a homogeneous solid red / brown product.

[0051] The solubility of the reaction product in MY0510 (triglycidyl ether of p-aminophenol) from Huntsman Advanced Materials was tested at 80 °C by adding 0.1 g of the sulfur-MDEA reaction product to 5 g of MY0510 in an aluminum pan and manually stirring the mixture while heating. The sulfur-MDEA reaction product was found to be completely soluble in MY0510.

[0052] Example 2 The reaction products of sulfur and DGEBF 1 g of elemental sulfur was mixed with 1 g of Araldite® PY306 (diglycidyl ether of bisphenol F or DGEBF) from Huntsman Advanced Materials, and 0.1 g of sodium diethyldithiocarbamate (DDC) was added as a promoter / compatibility agent. The mixture was manually mixed in a small glass vial at room temperature and then heated to 120 °C with magnetic stirring on a hot plate for 1 hour. After 1 hour at 120 °C, the vial was transferred to an oven and heated at 140 °C for 14 hours. The resulting sulfur-DGEBF reaction product (sample T) was found to be a homogeneous, viscous yellow product.

[0053] The solubility of the reaction product in MY0510 (triglycidyl ether of p-aminophenol) at 80 °C was tested by adding 0.1 g of the sulfur-DGEBF reaction product to 5 g of MY0510 in an aluminum pan and stirring manually while heating the mixture. It was found that the sulfur-DGEBF reaction product was completely soluble in MY0510.

[0054] It was found that when 1 g of elemental sulfur reacted with 1 g of MY0510 (sample G) under the same conditions described above, the reaction produced a dark brown / black two-phase (homogeneous) hard material. The lighter brown areas indicate that unreacted sulfur remains. This reaction was considered unsuccessful. When 0.1 g of DDC was added to the reaction of 1 g of sulfur and 1 g of MY0510 (sample Q), the reaction produced a heterogeneous material with internal bubbles, indicating that the material had decomposed.

[0055] It was found that when 1 g of elemental sulfur reacted with 1 g of MY721 (N,N,N',N'-tetraglycidyl-4,4'-methylenebisphenylamine) from Huntsman Advanced Materials (sample F) under the same reaction conditions, the reaction produced a brownish heterogeneous solid with unreacted sulfur spots throughout the sample. This reaction was considered unsuccessful. When 0.1 g of DDC was added to the reaction of 1 g of sulfur and 1 g of MY721 (sample O), the reaction produced a homogeneous material containing a black solid, indicating that the sample had decomposed. The reaction product was found to be insoluble in MY0510 at 80 °C.

[0056] Example 3 An epoxy resin composition containing MY721, MY0510, and MCDEA (4,4'-methylene-bis-(3-chloro-2,6-diethylaniline)) was prepared according to the formulations shown in Table 1. Amounts are expressed as weight percentages (wt%). Diaminodiphenyl sulfide (AFD), the sulfur-MDEA reaction product (prepared according to Example 1), and the sulfur-DGEBF (or sulfur-PY306) reaction product (prepared according to Example 2) were added as additives to the unmodified epoxy resin composition in the amounts shown in Table 1 to form resin samples. The amount of additives (wt%) is based on the combined weight of the additives and the unmodified resin.

[0057] Table 1

[0058] The resin samples were degassed at 80°C, then cured at a rate of 2°C / min to a target temperature of 180°C for 2 hours. The cured samples were tested using a TA Instruments Q800 DMA to determine their stress relaxation behavior. Stress relaxation test results for samples containing diaminodiphenyl sulfide (AFD) were obtained in... Figure 1 As shown in the image. Figure 1 The data shows that the addition of a higher amount (by weight%) of AFD leads to a decrease in stress relaxation behavior.

[0059] Stress relaxation test results of resin samples containing sulfur-MDEA reaction products (prepared in Example 1) Figure 2 As shown in the image. Figure 2 The data in the middle shows that, with Figure 1 Compared to the addition of a lower amount (wt%) of AFD shown in the figure, the addition of sulfur-MDEA reaction products caused a faster decrease in stress relaxation modulus.

[0060] Stress relaxation test results of resin samples containing sulfur-PY306 reaction products (prepared in Example 2) Figure 3 As shown in the image. Figure 3 The data in the middle shows that, with Figure 1 Compared to the addition of a lower amount (wt%) of AFD shown in the figure, the addition of sulfur-PY306 reaction products caused a faster decrease in stress relaxation modulus.

[0061] Cured samples (experiments 1 to 8 in Table 1) were also subjected to an 8-hour conditioning experiment in benzyl alcohol under reflux (200°C) to understand the effect of stress relaxation on the recycling potential of the cured materials. The results are shown in Table 2 below. These results indicate that samples prepared from thermosetting resins containing sulfur reaction products fractured faster than samples prepared from unmodified thermosetting resins, and also faster than samples prepared from thermosetting resins containing AFD.

[0062] Table 2

[0063] RT refers to room temperature.

Claims

1. A sulfur reaction product formed by mixing elemental sulfur with an epoxy compound having at least one epoxy functional group per molecule and a compatible reagent, and heating the resulting mixture to a temperature in the range of 100°C to 200°C for a duration of more than 1 hour.

2. A sulfur reaction product formed by reacting elemental sulfur with diglycidyl ether of bisphenol F and a compatible reagent.

3. The sulfur reaction product according to claim 1 or 2, wherein, The compatibility reagent is sodium diethyldithiocarbamate.

4. The sulfur reaction product according to claim 2, wherein, The reaction product is formed by mixing sulfur with diglycidyl ether of bisphenol F and the compatibility reagent, and heating the resulting mixture to a temperature in the range of 100°C to 200°C.

5. The sulfur reaction product according to claim 2, wherein, The reaction product is formed by mixing sulfur with diglycidyl ether of bisphenol F and the compatibility reagent, and heating the resulting mixture to a temperature in the range of 100°C to 200°C for a duration of more than 1 hour.

6. The sulfur reaction product according to claim 2, wherein, The mass ratio of sulfur to diglycidyl ether of bisphenol F is from 0.01:1 to 1:1, and the amount of compatibility reagent is up to 20 parts by weight per 100 parts by weight of the combination of sulfur and diglycidyl ether of bisphenol F.

7. The sulfur reaction product according to claim 6, wherein, The mass ratio of sulfur to diglycidyl ether of bisphenol F is 0.5:1 or 1:

1.

8. The sulfur reaction product according to claim 6, wherein, The amount of compatibility reagent is up to 5 parts by weight per 100 parts by weight of the combination of sulfur and bisphenol F diglycidyl ether.

9. A curable resin composition comprising one or more epoxy resins, at least one amine curing agent, and a sulfur reaction product according to any one of claims 1 to 8.

10. A curable composite material comprising reinforcing fibers wholly or partially embedded in a resin matrix comprising one or more epoxy resins and sulfur reaction products according to any one of claims 1 to 8.

11. A composite material comprising reinforcing fibers and a sulfur reaction product according to any one of claims 1 to 8.

12. A thermosetting prepreg comprising a unidirectional reinforcing fiber layer embedded in a curable resin matrix comprising one or more uncured epoxy resins and sulfur reaction products according to any one of claims 1 to 8.

13. A method for manufacturing a composite material, comprising: (a) Adding the sulfur reaction product according to any one of claims 1 to 8 to an uncured thermosetting resin composition comprising one or more thermosetting resins; (b) Impregnate the fiber reinforcement layer or inject the fiber preform with the thermosetting resin composition formed in step (a); as well as (c) The impregnated fiber reinforcement is cured at high temperature such that the ratio of the curing enthalpy to the uncured enthalpy, as determined by differential scanning calorimetry, is less than 0.

1.

14. The method of claim 13, wherein, The thermocurable resin composition of step (a) comprises one or more epoxy resins and at least one amine curing agent.

15. The method according to claim 13 or 14, wherein, Step (c) involves curing the impregnated fiber reinforcement at a high temperature for a period of time, such that the ratio of the curing enthalpy to the uncured enthalpy, as determined by differential scanning calorimetry, is less than 0.

05.

16. A method for manufacturing a composite material, comprising: (a) Impregnating a fiber reinforcement layer or injecting a fiber preform with the sulfur reaction product according to any one of claims 1 to 8; as well as (b) The impregnated fiber reinforcement is cured at a high temperature such that the ratio of the curing enthalpy to the uncured enthalpy, as determined by differential scanning calorimetry, is less than 0.

1.

17. The method of claim 16, wherein, Step (b) Curing the impregnated fiber reinforcement at high temperature for a period of time, such that the ratio of curing enthalpy to uncured enthalpy, as determined by differential scanning calorimetry, is less than 0.

05.

18. Use of the sulfur reaction product according to any one of claims 1 to 8 in a thermosetting resin composition suitable for liquid resin infusion.

19. Use of the sulfur reaction product according to any one of claims 1 to 8 in the manufacture of fiber-reinforced composite materials.

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