Research on the formulation and performance of high-strength resin matrix T1 for wet winding

Aiming at the higher requirements on the performance of composite materials put forward by new specialized equipment, the development of new high-performance winding resin is carried out. First, basing on the epoxy curing system, the selection of the main resin has been completed, we select the appropriate curing agent according to the characteristics of the screened trifunctional glycidylamine epoxy resin and trifunctional alicyclic epoxy resin, the ratio is calculated and optimised, the curing system is established, and the design of the resin formulation system is completed. Second, according to the nature of the resin curing system, the toughening method is screened, and the synergistic enhancement of heat resistance and toughness of the resin is accomplished through the core-shell nano-toughening method. Finally, the glass transition temperature of the new high-performance winding resin T1 is 165.12°C, and the fracture toughness of the resin KIC can reach 1.74MPa-m1/2, and the annular tensile strength of the T1 resin-based T700 carbon fibre composites can reach 3500MPa, which meets the requirements of the design indexes of specialized equipment.


Introduction
Composite materials are multiphase materials prepared by a continuous phase matrix material and one or more dispersed phase reinforcing materials through a composite process [1].As the reinforcing materials have different physical and chemical properties relative to the matrix materials, they can complement each other in performance and produce synergistic effects.The composite materials prepared can make up for the matrix materials' shortcomings by retaining the matrix materials' main characteristics or even producing new properties that the matrix materials do not have, to meet the requirements of various complex working conditions [2][3].
New specialized equipment puts forward higher requirements on the performance of composite materials; and in the process of development, the composite materials are found to have the following problems.Firstly, the composite materials appear to have lamellar interlaminar cracking.Secondly, the composite materials appear to have intralaminar damage along the direction of the fibres.By observing the cracking phenomenon, it is found that most of the cracking of composites occurs in the interlayer.The interlayer cracking and delamination is one of the main reasons leading to the destruction of composites.The performance of the resin matrix plays a role in it, which has always been of great concern.Given the current problems of interlaminar cracking and intra-laminar damage in resin matrix composites, it is hypothesised that resin toughness may be an important influencing factor.In particular, resin fracture toughness characterizes the ability of a material to resist crack extension.Therefore, the choice of resin toughening method is crucial when designing resin formulations.

Test methods
Exothermic curve test: The experiment was carried out using a differential scanning calorimeter model Q200 of TA Instruments for testing the epoxy resin samples.The testing atmosphere was air, the heating rate was 10℃/min, the testing temperature range was 40℃~260℃, and the dynamic heating scan was carried out.Curing degree test: Use DSC method and MDSC method to test the curing exotherm of the prepared adhesive solution and the residual exotherm of the cured casting body, respectively, with a temperature increase rate of 10℃/min and 3℃/min, respectively.
Determination of glass transition temperature: Scan the cured resin by MDSC method to determine the glass transition temperature, the heating rate is 3 ℃ /min, pursuant to GB/T19446.Dynamic mechanical properties (DMA) test: the Q800 dynamic thermo-mechanical analyser is used to test the viscoelasticity of cured resin.The test parameters are size 60cm × 10cm × 4cm, air atmosphere, three-point bending fixture, test frequency 1HZ, heating speed 3℃/min, test temperature range room temperature~200℃.
Resin Fracture Toughness Test: The three-point bending (SENB) method was used to test the fracture toughness of resin materials.According to the requirements of ASTM D 5045-99, the ratio of the height W to the thickness B of the specimen W/B = 2, and the span S = 4 W. The crack length a is the sum of the crack initiation notch length and the prefabricated natural crack length, and a/W is controlled to be between 0.45 and 0.55.
Contact Angle Test: The video optical contact angle tester is used to test the contact angle between resin and composite material according to the shape image analysis method, and the liquid contour fitting calculation method is circle fitting (width and height method), which is applicable to the case where the contact angle is less than 20°.

Determination of the host resin
In the formulation of resin matrices, the primary resin refers to the component that plays a principal role when combining two or more materials together within the composite material system.It forms the foundation of the resin adhesive, and the quality of the resin's performance is predominantly determined by this primary resin.The resin matrix developed in this study is intended for use in the carbon fiber winding process.The developmental requirements stipulate it to be compatible with the wet winding processes of two types of fibers, showcasing excellent resin impregnation characteristics.Not only must the resin exhibit appropriate strength and fracture elongation rates, but it also necessitates sufficient adhesive force, along with superior interfacial bonding properties with carbon fibers.
Two trifunctional epoxy resin complexes were finally selected as the main resin of T1, which had the best overall performance, and they each had the following characteristics: Trifunctional Glycidyl amine Epoxy Resin: It is an aerospace grade multifunctional epoxy resin with higher stability, ultra-low viscosity, and higher epoxy value through a special process for the resin.There are several epoxy groups and aromatic rings in the molecular structure of trifunctional glycidyl amine epoxy resin, which can form high cross-linking density and aromatic density during the curing process so that the cured products show good heat resistance, high mechanical strength, low shrinkage rate of curing, and good resistance to radiation, water and chemical corrosion, etc. [4][5] .
Trifunctional alicyclic epoxy resin: It is a few epoxy resins that contain both active glycidyl group and alicyclic ring oxygen in the molecule.Its epoxy value is high, low viscosity, viscosity is only 10-20% of the viscosity of E-51 epoxy resin, so it has both low viscosity, good processing performance, high reactivity, etc., and its cured material bonding, mechanical properties, heat resistance, weatherability and electrical insulation properties are relatively good, making it an ideal matrix resin for high-performance composites [6][7][8] .
The trifunctional group shrinkage glycerol amine epoxy resin is combined with the trifunctional group cycloaliphatic epoxy resin to formulate the primary resin components of T1, fully capitalizing on the strengths of both components to ensure superior comprehensive performance of the resin.In the application research of epoxy resins, the curing agent constitutes a significant component.This is due to the fact that, before curing, the epoxy resin primarily exists in the form of long-chain molecules, exhibiting poor mechanical properties and hence lacking practical value.Only upon the addition of a curing agent into the epoxy resin, under conditions of elevated temperature or other catalytic circumstances, does a reaction occur between the epoxy resin and the curing agent, culminating in a crosslinked cure forming a three-dimensional network structure.This process ultimately facilitates the achievement of high strength and modulus, thereby unveiling its exceptional practical utility [9].
Owing to the selection of primary resins characterized as trifunctional group epoxy resins, a higher functional group density and consequently, elevated reactive activity, are observed.Moreover, in the molecular structure of the trifunctional group shrinkage glycerol amine epoxy resin, the electron cloud density of the entire epoxy group, located at the terminal of the glyceride moiety, decreases due to the influence of the β-position oxygen atom and the γ-position carbonyl group electron-withdrawing effect.This facilitates the assault by the curing agent, rendering it more susceptible to nucleophilic ringopening reactions, thereby enhancing its reactive activity compared to general cycloaliphatic epoxy resins.Amines and imidazole curing agents are commonly utilized in low-temperature curing systems, noted for their inherently high reactive activities.Therefore, to prevent explosive polymerization during curing and to ensure a prolonged pot life that facilitates the completion of the winding process, an organic anhydride type curing agent was ultimately selected.This agent, with its low viscosity and exothermic reaction characteristics, aligns perfectly with the selected high-activity multifunctional group primary resin, thereby bestowing the T1 resin with excellent processing properties.This selection avoids explosive polymerization during curing and guarantees an extended pot life during the winding process.
Having settled on the organic anhydride class of curing agents as the curing agent for the T1 resin, it is imperative to ascertain the requisite quantity of the curing agent.This will be determined in accordance with the formula stipulated for epoxy/anhydride curing systems for calculating the amount of curing agent [10]: • W (anhydride) % = C x anhydride equivalent/epoxy equivalent x 100 Where anhydride equivalent = molecular mass of the anhydride/number of anhydridesgroups; C is a correction factor and different values are used for different occasions.C=0.85, general anhydride.C=0.6, using chlorinated anhydrides, or using organometallic salts such as stannous octanoate.C=1.0, using tertiary amines as accelerators.C = 0.8 when using tertiary amines and M(BF)n.The anhydride equivalent of the organic anhydride curing agent used and the epoxy equivalent of the T1A resin were brought into the formula to obtain an epoxy resin and curing agent dosage of about 100:130.
The DSC isothermal exothermic curves of resin and organic anhydride curing agent in the ratios of 100:110, 100:120, 100:130 and 100:140 were tested respectively, as shown in Figure 1, and the characteristic temperatures and exothermic quantities were calculated for different ratios, as shown in Table 2.  Curing reactions are generally exothermic in nature.By integrating the heat release peak curve to obtain the peak area, one can determine the total enthalpy change indicative of the complete curing of the resin.A larger total enthalpy change signifies a greater number of functional groups participating in the reaction, implying a more reasonable ratio between the epoxy resin and the anhydride curing agent.As can be seen from the total enthalpy changes of different ratios in the table above, the resinto-organic anhydride curing agent ratio of 100:130 results in the maximum total heat release enthalpy change, measured at 363.2 J/g.Hence, a resin to organic anhydride curing agent ratio of 100:130 has been selected.

Determination of Toughening Method and Toughening Agent
Utilizing a combination of trisfunctional glycerol etheramine epoxy resin and trisfunctional alicyclic epoxy resin as the principal resin components leads to an increase in functional groups and crosslinking density.This not only augments the strength and rigidity of the resin's cured products but also significantly enhances its brittleness.Concurrently, employing anhydrides with a rigid aromatic ring structure as curing agents can render the cross-linked network within the molecular structure more rigid post-curing, thus bolstering both strength and thermal resistance.However, it also induces increased brittleness and a reduction in toughness.
To ameliorate the compromised toughness, substantial internal stress, and the propensity for crack failure of the T1 resin base, which stems from its high cross-linking density and significant molecular chain rigidity inherent to its curing reaction, it is imperative to select a toughening method that not only exhibits superior properties but also complements the T1 resin.Moreover, it addresses the issues of intra-layer and inter-layer cracking in composite materials.
Comparative analysis of different toughening strategies -utilizing carboxyl-terminated butadiene nitrile liquid rubber (CTBN) in a "sea-island toughening" approach, incorporating flexible functional epoxy resin, integrating polyether-modified bisphenol A difunctional group epoxy resin, and utilizing core-shell nano-toughened epoxy resin -revealed certain insights.The toughening strategies employing flexible functional epoxy resin and polyether-modified bisphenol A difunctional group epoxy resin resulted in premature fracture of the resin, as the toughness agents themselves exhibited lower strength, thereby undermining the structure of the resin before the high toughness could manifest.On the other hand, the use of CTBN for toughening rendered favourable results, but the incomplete phase separation led to a decline in the glass transition temperature and strength, adversely impacting the thermal and mechanical properties of the resin base.Conversely, the core-shell nanotoughened epoxy resin strategy demonstrated distinctly higher strength than the butadiene nitrile rubber, coupled with relatively better toughening effects.This can be attributed to the encapsulation of rubbery cores within thermoplastic polymers with higher glass transition temperatures, typical of nano core-shell particles.A salient advantage of this approach is the minimal structural alterations pre and post-curing of the resin, preventing any detriment to the resin properties due to incomplete rubber phase separation during the curing process [11].Therefore, the core-shell nano-toughening strategy was ultimately chosen as the means to enhance the toughness of T1 resin.A dual-layer core-shell structured polymer has been synthesized utilizing emulsion polymerization technique, with its core composed of lightly cross-linked, elastic nano polymeric rubber elastomers, as depicted in the Figure 2 [12].When subjected to impact, the core acts as a stress concentration entity, capable of inducing both slip bands and shear bands to absorb energy and subsequently halt the progression of slip bands.Concurrently, the core is able to detach from the epoxy resin interface, releasing its elastic strain energy, thereby further enhancing the material's toughness [13].The outer shell is constituted of epoxy resin, a polymer exhibiting excellent compatibility with epoxy resin.During the curing process, it forms a favourable interface with the epoxy resin, consequently improving the system's dielectric properties and surface morphology.This method of constructing core-shell structures using rubber particles is rationally designed, displaying significant toughening effects, and presenting good overall coherence and uniformity, thus representing an exemplary toughening strategy.

Resin heat resistance
The glass transition temperature (Tg) is one of the most fundamental parameters of polymers as structural materials; it is the temperature at which a viscoelastic material changes from a glassy state to a highly elastic state.From the table, it can be seen that the glass transition temperature of T1 resin can reach 165.12°C (DSC method) under the 90°C 8h curing regime, as shown in Figure 3.This is due to the fact that T1 resin adopts compounded multifunctional resin and anhydride with rigid aromatic ring structure to form a multifunctional epoxy/anhydride curing system, and the curing process can form a high crosslinking density and aromatic density, which makes the resin's free volume smaller, and the movement of the chain segments at low temperatures is locked, thus showing good heat resistance.

Mechanical Properties of Resins
As a resin matrix for special equipment composites, the role of the resin is to transfer and distribute the applied stress to each fibre, bonding the fibresas a whole to resist damage and deformation under load.Therefore, the matrix material should have good mechanical properties.T1 resin due to the use of compound multifunctional resin with rigid aromatic ring structure of the anhydride to form a multifunctional epoxy / anhydride curing system, the curing process can be formed with high crosslink density and aromatic density, dense, rigid three-dimensional crosslinked network, in the process of encountering the impact of damage to the T1 resin to provide good mechanical properties, as shown in Table 3. Second for the use of core-shell nano-toughening formed for toughening epoxy resin soft core/hard shell structure, epoxy resin penetrates into the surface layer of the shell, and the particles bonding occurs around the core-shell particles of the epoxy resin due to plastic deformation, the resin damage to the formation of a large number of tough nests and the coreshell rubber particles cavities, so that the structure can impede the expansion of the destruction of the cracks, thereby increasing the destruction of the absorbed energy.E') is directly proportional to the maximum elasticity stored in the sample during each cycle, reflecting the elastic component within the viscoelastic properties of the material and characterizing the material's ability to resist deformation.The larger the storage modulus, the less prone the material is to deformation, indicating greater stiffness.The storage modulus of T1 resin at 55°C is 3090 MPa, as shown in Figure 4, Table 4.Moreover, with a Tg (glass transition temperature) reaching 168.51°C, it exhibits excellent thermal resistance.
Utilizing the inflection temperature of the storage modulus (E') to assess the thermal resistance of polymers in engineering applications is more reasonable.This is because, when the Tg is reached, the failure of the material becomes inevitable.The inflection temperature of the storage modulus is lower than the Tg, indicating an impending risk of a sharp decline in the modulus when reached.However, it can still essentially maintain the rigidity of the structural material without complete failure.This inflection point can be considered as the maximum operating temperature for the resin; exceeding it would result in unstable performance with significant fluctuations.Between the inflection point and the glass transition temperature, the properties decline rapidly, posing a substantial risk for practical applications since it cannot be guaranteed that the operating temperature will remain absolute.Therefore, it is imperative to ensure that the resin material is utilized below its inflection point.The inflection temperature of the storage modulus (E') for the T1 resin can reach up to 153.06°C.

Resin Fracture Toughness
Fracture toughness is the value of the impedance displayed by the material when a crack or crack-like defect in a specimen or component occurs as its starting point and no longer breaks rapidly with increasing load, i.e., when a so-called unstable fracture occurs.The fracture load of the T1 resin is 0.108 KN, and its fracture toughness, denoted as K ⅠC is 1.743MPa-m 1/2 , as shown in Table 5.The T1 resin necessitates substantial critical stress for the unstable propagation of cracks, with a considerable critical dimension being observed when the cracks reach unstable expansion.It demonstrates a strong capability to hinder the progression of cracks, categorizing it as a material with excellent toughness [14].The toughening of T1 resin utilizing core-shell polymers is achieved through the synthesis of a category of polymer composite particles with a unique structure, derived from two or more monomers through seed emulsion polymerization.The internal and external segments of these particles are respectively enriched with distinct components, exhibiting a unique bi-layer or multi-layer structure.The reactive resin polymers serve as a rigid shell, isolating the particles from one another and enhancing the interaction between the core-shell polymers and the base resin through physical or chemical means, hence bestowing them with excellent compatibility and dispersibility.As illustrated in Figure 5 depicting the cross-sectional morphology of the epoxy resin casting after fracture toughness testing, the T1 resin exhibits a rough fracture surface with prominent river-line traces.This also suggests that the T1 resin core-shell particles have good compatibility and dispersibility, complete phase separation in the "sea-island structure", absorbing a considerable amount of energy during fracture.

Carbon fibre composite properties 3.3.1. Interfacial Wetting Properties
The interface of a composite material is a tiny area where there is a significant change in the chemical composition between the matrix and the reinforcement of the composite material, which constitutes a combination with each other, and where the load can be transferred.It is a layer with a certain thickness (nanometre or more), the structure varies with the matrix and reinforcement, and there is a significant difference with the matrix of the new phase -interface phase (or interface layer).The structure and properties of the interfacial phase of composites are crucial to the overall performance of composites.In order to improve the performance of composites, interface design and control must be considered.And two phases with good wettability is a prerequisite for good interfacial properties.The resin liquid is dropped on the fibre surface and the liquid either spreads out and covers the fibre surface or forms droplets resting on it, depending on the nature of the system.The shape of the formed droplets can be described in terms of contact angle.From the above figure, it can be seen that the T1 resin has completed the wetting, wettability and spreading by itself, i.e., the wetting work Wa, the wettability Wi and the spreading coefficient S of the T1 resin are all greater than zero.
The T1 resin changed the contact angle θ from the initial 39.26° to 8.07° within the range of 7.23 s that could be detected by the contact joint tester.The final equilibrium contact angle was close to 0 and the spreading was completed, as shown in Figure 6.T1 resin has a small contact angle with T700 carbon fiber, fast wetting speed, good interface wettability and winding process performance.The good interfacial wettability of the fiber can effectively reduce the porosity of the composite material.

Mechanical properties of composite materials
Carbon fiber composite materials constitute the primary raw material for specialized equipment.Consequently, attaining the designated mechanical performance benchmarks is a prerequisite to ensuring the long-term safe and reliable operation of such equipment.By employing T1 resin in accordance with the winding process to fabricate composite materials, a comprehensive evaluation of the various attributes of the resultant composites can be conducted, as shown in Table 6.T1 resin performance indicators meet the requirements of special equipment design indicators.

Conclusions
• The formulation system of winding resin T1 is to compound two trifunctional epoxy resins as the main body of the resin, organic anhydride as the curing agent, and core-shell nanomaterials as the toughening agent.• T1 resin glass transition temperature is 165.12℃,E' inflection temperature is 153.06℃;resin strength is 91.4 MPa; resin elongation at break is 6.8%, resin fracture toughness KⅠ C is 1.74 MPa-m 1/2 .Core-shell nanomaterials can be used in the case of small impact on the strength and heat-resistant property The core-shell nanomaterials can be toughened with little effect on strength and heat resistance.• The interlaminar shear strength of T1 resin-based T700 carbon fibre composites is 69.3MPa; the circumferential tensile strength of the composite material can reach 3500MPa.T1 resins satisfy the wet winding process, and at the same time meet the high performance requirements of resins for specialized equipment.

10th
Global Conference on Polymer and Composite Materials (PCM 2023) Journal of Physics: Conference Series 2652 (2023) 012008

Figure 2 .
Figure 2. Structural model of the core-shell of PB/DGEBA composites.

10thFigure 3 .
Figure 3. Glass transition temperature of T1 resin under DSC test method.

Figure 5 .
Figure 5.SEM photograph of cross section of T1 resin casting body.

Figure 6 .
Figure 6.Wetting process of T1 resin on the surface of T700 carbon fibre.

Table 1 .
Main instruments and models used.
Longitudinal performance test of composite materials: longitudinal strength and longitudinal modulus of composite materials are in accordance with GB/T15385-1994 "Bursting Method for Hydrostatic Test of Gas Cylinders". 3

Table 6 .
Interlayer properties of T1 resin-based T700 carbon composite flat plate specimens .T1 unidirectional composite cylinders were prepared and subjected to hydraulic burst test and strip specimen tensile test with the following results, as shown in Table7: