A new ionic cross-linking network type high hysteresis oil resistant ACM/POM TPV preparation method and performance research

Herein, the concept of ionic cross-linking was first introduced into the preparation of thermoplastic vulcanizates (TPV). Oil-resistant TPV was developed based on acrylate rubber (ACM) and polyoxymethylene formaldehyde (POM) using Ca(OH)2 as the cross-linking agent. The physical and mechanical properties, aging resistance, dynamic mechanical properties, micromorphology, and energy loss of the composites under covalent cross-linking networks and ion cross-linking networks with different degrees of cross-linking were systematically studied. The results showed that Ca2+ could form metal ion binding with the -COOH of ACM, playing a cross-linking role for ACM. Compared with the covalent bonding system, the 100% constant tensile stress of TPV in the ionic bonding system can be increased by 31% and the elongation increased by 14.5%. And exhibits better elasticity and lower hysteresis loss below 140 °C. The increase in dispersed phase modulus increases the phase domain of dispersed phase, weakens the binding with matrix, weakens the tensile strength, increases the intrusion of hot oxygen and hot oil, increases the energy storage modulus, and decreases the loss modulus.


Introduction
Thermoplastic vulcanizates (TPVs) are a kind of thermoplastic elastomer with the fastest growth in consumption in recent years [1][2][3][4].TPVs products independently developed by many countries have been published successively, and more and more types of TPVs are gradually involved in multiple fields of production and life.Compared with traditional copolymerization and simple blending thermoplastic elastomers, TPVs have excellent mechanical properties, a lower compression set, fatigue resistance, dielectric resistance, repeatable machining performance, etc. [5][6][7][8][9].However, due to the presence of the resin phase as a continuous phase, it is difficult for TPV to maintain good properties above its softening point, and therefore has poor resistance to high temperatures and hot oils [10,11].Cui et al. [12] used methyl vinyl silicone rubber (MVQ) as the rubber phase and thermoplastic polyurethane (TPU) as the continuous phase, dynamically vulcanized a kind of high temperature-resistant TPVs, and studied the performance changes of TPVs under different vulcanization systems and dosages.They found that the TPVs with 1.5 phr 2, 5-dimethyl-2, 5-di (tert-butylperoxy) hexane (DBPH) as vulcanizing agent have the best heatresistant oxygen aging performance, tensile strength, and better dispersion.Reffai and Naskar [10].prepared a new type of TPV with hydrogenated nitrile butadiene rubber (HNBR) and nylon 12 (PA12) as the substrate, this TPV with the increase in the amount of acrylonitrile (ACN) has an excellent hightemperature resistance, and has a tensile strength of 14 MPa and a tear elongation of 161%, with a retention of tensile strength of 80% or more, after aging in the hot air at 150 ℃ for 72h.
To improve the resistance of TPVs to high temperatures and hot oils, we use acrylate rubber (ACM), which has polar ester groups on the side groups of the molecular chain, and polyoxymethylene (POM), which has saturated C-O bonds on the main chain, as the base materials.And Using ion-crosslinked ACM to prepare TPVs can further improve the stress resistance ability of composite materials under small deformation and improve the problem of insufficient fluidity in TPVs processing caused by large ACM content [13,14].
The research of cross-linking carboxyl rubber can be traced back to the 1950s [15].Brown [15,16] obtained the "ion cross-linking" material by cations complexing the acid ions in it and carried out pioneering research based on the terpolymer of butadiene-acrylonitrile-acrylic acid made by Goodyear Company.In addition to carboxylate ionic ionomers, there are also sulfonic acid ionomers, sulfhydryl ionomers, and phosphoric acid ionomers [17].But at present, the research on carboxylic acid type ionomers is more extensive in the world [18], mainly methacrylic acid type and acrylic acid type.Due to different activation energies, different types and prices of metal oxides, metal hydroxides, or metal complexes will lead to different vulcanization characteristics of rubber and vulcanizing properties.Matsuda and Minoura [19].studied the preparation method of ionic crosslinked rubber by neutralizing the carboxyl terminal liquid rubber with metal oxides and metal carbonate and found that the tensile strength of rubber in bivalent metal ions has a rule of Ba 2+ ≥ Ca 2+ > Mg 2+ > Cd 2+ > Zn 2+ , but the processing property of Ba 2+ crosslinked rubber is poor.And they found that the ion-crosslinked rubber has repeatable properties that conventional rubber does not.Shao et al. [20] synthesized a new type of ion cross-linking agent from bis [3-(triethoxysilyl) propyl] tetrasulfide and zinc methacrylate as raw materials, which was used for cross-linking unsaturated rubber with a reversible ion three-dimensional structure.So that crosslinked styrene butadiene rubber (SBR) with considerable toughness can be recovered by the solution method and resynthesized by simply adding zinc oxide as a cross-linking agent.Gao et al. [21] used sodium and zinc acetate to neutralize the ethylene-acrylic acid copolymer and studied the influence of metal ions on the mechanical properties of the ionomer.The results showed that the mechanical properties of the sodium ionomer increased with the addition of sodium salt, while the mechanical properties of the aluminum ionomer decreased with the addition of aluminum salt.
In this article, a new heat-resistant oil ACM/POM TPVs was prepared by using Ca(OH) 2 as the cross-linking agent of rubber phase.The cross-linking effect of metal ions was confirmed by Fourier transform infrared spectroscopy.Compared with the traditional covalent bond vulcanization system, the effects of the amount of cross-linking agent on the mechanical properties, aging resistance and dynamic mechanical properties of ACM and TPVs were systematically studied.The morphology of dynamically vulcanized ACM/POM blends was studied by scanning electron microscopy (SEM), and the optimal amount of cross-linking agent was finally determined.

Preparation of ACM compound and vulcanized rubber
According to the formula in Table 1, prepare the ACM compound on the roll mill.Add stearic acid and silica after the raw ACM roll slowly, and gradually open the roll distance to 2mm.Cut the rubber and make the triangle wrap.Different amounts of Ca(OH) 2 compound were obtained.
ACM compound was added into a 2mm mold and vulcanized in a plate vulcanizing machine.The vulcanizing temperature was 180 ℃, the vulcanizing time was 13min, and the vulcanizing pressure was 10MPa.ACM vulcanizing rubber with different dosage of vulcanizing agent was prepared and named Ca4-Ca8.Comparison with a covalently bonded cross-linked ACM performed using a 1.2phr HMDC, named H1.2.

Preparation of ACM/POM TPVs materials
In ACM/POM TPV, TPU is used to toughen POM, and the mass ratio of ACM/POM/TPU is 65/28/7.The torque rheometer was used to melt ACM compound and POM.The processing process was as follows: The temperature of the torque rheometer was set at 180 ℃ and the rotational speed was 60rpm.Add POM and TPU to the torque rheometer, melt and mix for 5min and then add ACM compound.ACM was shear broken while vulcanized, and dynamic vulcanized for 8min.
The TPV was put into a 2mm mold and formed on a molding machine at 180 ℃.The preheating time was 3min, the molding time was 5min, and the pressure holding pressure was 10MPa.Then the TPV was taken out after cold pressing at 10MPa for 5min and named as TCa4-TCa8.The TPV corresponding to H1.2 is named TH1.2.The preparation process is shown in Fig. 1.

Curing characterization and Torque curves of dynamic vulcanization
The vulcanization characteristics of ACM at the processing temperature were studied by GOTECH M-3000A RPA, determined the scorch time and vulcanization time, roughly judged the variation trend of vulcanization degree.The test temperature was 180 ℃ and the total test time was 20min.
The materials were mixed by HAAKE torque rheometer, and the numerical value of the blend materials' torque changes with time was automatically recorded by computer.Origin graphing software was used to draw the torque-time curve of the blend materials.

Gel volume fraction (Vr)
The gel volume fraction (Vr) in the rubber was measured by Swelling equilibrium method to characterize the apparent cross-linking density of ACM.The good solvent used for swelling was ethyl acetate and the swelling time was 24h.The formula (1) for calculating Vr is as follows: Vr is the gel volume fraction of rubber, ρ r is the density of rubber before swelling, ρ s is the density of the solvent, here is 0.902 g/cm 3 , δ is the mass fraction of raw rubber in the formula, M 1 is the mass of the sample before swelling, M 2 is the mass of the sample after swelling.

Mechanical characterization
According to GB/T 528-2008 standard, using the I-7000S electronic tension machine produced by Taiwan High-speed Railway Testing Instrument Co. Ltd. to test the dumbbell type 2 sample, the speed was 500mm/min, and the test temperature was room temperature.According to GB/T 531.1-2008 standard test, using Shao A hardness tester for hardness test.

Cyclic stretching and relaxation behavior
The stress-strain process of ACM (300% shape variable) and TPV (100% shape variable) under two tensile cycles was recorded, and the tensile and recovery rates were both 100mm/min.The area enclosed by the stress-strain curve was calculated to characterize the energy loss of the material in the tensile process (W), and the energy loss rate (REL) was calculated by Eq. ( 2).
W is the energy lost in a tensile cycle of the sample, namely, the integral area enclosed by the stress-strain curve; W 0 is the total energy required during the stretching process, i.e. the total area under the stretching curve.

Dynamic mechanical property
The variation of the stored modulus (G') and loss modulus (G") of ACM and TPV with temperature was tested by RPA2000 rubber processing analyzer produced by Alpha Technology Co. LTD.The test method was shear strain temperature scan, strain frequency was 1.7Hz, rotation Angle was 0.5°, and the scanning temperature range was 65 ~ 185 ℃.

Scanning electron microscope (SEM)
ACM/POM TPV samples with different Ca(OH) 2 dosages were brittle with liquid nitrogen.These TPV composites are placed on an aluminum bracket with a double adhesive carbon strip and then sprayed with gold.The fracture morphology was observed by field emission scanning electron microscope (SEM) of JEOL JSM-6700F under 5.0 kV acceleration voltage.

Fourier transform infrared spectrum(FTIR)
In order to determine the feasibility of Ca(OH) 2 curing carboxylic acid type ACM by releasing Ca 2+ , it is necessary to test the cross-linking network before and after the crosslinking to ensure that the vulcanization system is carried out according to the mechanism of metal ion curing carboxylated rubber.The Fourier infrared spectra of ACM raw rubber, compounded rubber and vulcanized rubber were tested respectively.The cross-linking condition was determined by the change in reactivity -COOH on ACM before and after cross-linking, and the results are shown in Fig. 2.
The side group of carboxyl type ACM is composed of a large number of ester groups and carboxyl groups, which is manifested as the carbonyl stretching vibration peak at 1731 cm −1 in the FTIR curve.By calculating the integral area of this peak, it is found that the integral area of ACM raw rubber is the largest, which is 5.19.When Ca(OH) 2 is added and vulcanized, the characteristic absorption peak area here is 3.54, indicating that the carboxyl group of the This peak is the stretching vibration peak of the carbonyl group in metal carboxylate, indicating the formation of the metal carboxylate group [19].In combination with previous studies on ion cross-linking of carboxyl rubber and ionomers, the process of Ca(OH) 2 cross-linking ACM is shown in Fig. 3. Ca(OH) 2 decomposes the metal ion Ca 2+ at high temperature, attacks the hydrogen on the carboxyl group of the ACM side chain, and forms a metal ion combination with it.The bivalent Ca 2+ connects to the main chain of the ACM at both ends to form multiple heavy ion pairs, thus achieving the purpose of cross-linking.Such multiple heavy ion pairs can form an attraction through coordination bonds and van der Waals forces.Multiple ion pairs are attracted together to form ion clusters, further increasing the cross-linking density [22].

Processing performance and curing characteristics
Crosslink density is an important parameter to be investigated in vulcanization rubber.On the one hand, the different crosslink density of the rubber phase will affect its own performance in TPV; on the other hand, the difference in crosslink density brings about the change of modulus of two phases in the dynamic vulcanization process, which in turn affects the phase distribution when the two phases are comingled and finally changes the performance of TPV.
To determine whether the amount of Ca(OH) 2 would have a large effect on the crosslink density and modulus of the ACM phase, a vulcanizer test was performed on ACM at 180 °C to roughly predict its modulus change during the dynamic vulcanization process.The gel volume fraction of ACM vulcanized rubber was also determined by the Swelling equilibrium method to characterize the apparent crosslink density of the ACM phase.Figure 4(a) shows that with the increase in Ca(OH) 2 usage, the scorch time and vulcanization speed of ACM were slightly accelerated, but the Process curing time (T 90 ) was basically maintained at about 13 min, so the processing method of dynamic vulcanization (8min) + static molding (5min) was chosen for the subsequent TPV preparation.It is more obvious that the increase in Ca(OH) 2 use increases the maximum torque of ACM, but this increase only lasts until Ca7, when the Ca(OH) 2 use continues to increase, causing a decrease in the maximum torque instead.The difference between the highest torque and the lowest torque M H -M L reflects the change of the cross-linking .6% to 12.37%, and when the dosage reached 8 phr, both showed a significant decrease, indicating that too much Ca(OH) 2 does lead to a decrease in ACM crosslink density.From the mechanism of ACM ion cross-linking, it can be seen that when too much hydrogen on the carboxyl group is replaced by metal ions, the carboxyl group is completely blocked and cannot play the role of linking the two molecular chains, so that the cross-linking density is reduced.
The ACM/POM TPV composites were prepared by the dynamic method.Figure 5 shows the torque curves and the variation of the dynamic vulcanization torque peak and equilibrium torque of the composites in the torque rheometer at different Ca(OH) 2 dosages.In the figure, 0 ~ 5 min is the torque peak after POM is added and melted at high temperature; the second peak at 5 ~ 6 min is generated by ACM compound added and softened; and then ACM is vulcanized and the torque rises.At the same time, the effect of heat and shear makes the ACM broken, and the cross-linking and breaking reach balance at 7 ~ 8 min, and the torque curve appears to peak, then the breaking process dominates and the torque decreases slowly, and the torque balance is basically reached at 13 min.With the increase of Ca(OH) 2 dosage, on the one hand, the cross-linking speed of ACM increases, and the degree of cross-linking is higher at the same time, so the torque of ACM/POM blends in 5 ~ 8 min is increased; on the other hand, the faster the modulus of ACM phase increases, the easier it is to be broken under the action of shear force; that is, the equilibrium point of vulcanization and breaking is reached more quickly, and the peak position of dynamic vulcanization is more forward.However, early crushing does not mean that smaller phase domains are formed.ACM with a lower degree of cross-linking, on the contrary, tends to form smaller nano-rubber particles, and a lower degree of cross-linking means that the difference in modulus between the two phases is smaller, which can form a thicker phase interface thickness and a tighter bond between the two phases [23].As the degree of vulcanization increases, ACM becomes more difficult to crush into smaller particles, while the bond between the two phases is weak and prone to interfacial sliding, which also leads to a lower equilibrium torque.

Mechanical properties
The mechanical properties of ACM vulcanizates crosslinked by ionic bonding and those cross-linked by covalent bonding showed completely different characteristics.For this reason, a covalent bonding system (HMDC 1.2 phr) specimen with a similar cross-linking density to Ca4 ~ Ca8 was selected for comparison, as shown in Fig. 6.It can be seen that the specimens with HMDC 1.2 phr have a low constant tensile stress at strains from 0 to 300%, and the modulus increases as the strain continues to increase, while the Ca(OH) 2 system has a large constant tensile stress at small deformations, and the modulus decreases slightly after a strain of more than 200%.The difference between the modulus changes of the two systems is mainly due to the different ways of bearing stress in the two types of bonds.The covalent bond system resists stress through the entanglement between molecular chains under small deformation, and when the molecular chains are stretched and oriented, the molecular main chains and cross-linked bonds start to resist stress, and the modulus appears to increase due to the high bond energy and the strong ability to resist stress.In the ionic bonding system in which Ca 2+ forms multiple ions, these ion pairs are attracted to each other to form ion clusters, thus interconnecting the molecular chains.This connection is effective in transferring stress, but once an ion pair in this ion cluster is separated, the attraction between the remaining ion pairs is also reduced, leading to the collapse of the physical network formed by the ion cluster and the reduction of the modulus.Figure 7 shows the stress-strain curve of the ACM/POM TPV.It can be seen from the figure that the reduction of modulus under this large deformation is more obvious in TPV, and the stress is basically no longer elevated after the strain reaches 200%.On the one hand, the TPU toughened and modified POM yields after more than 50% deformation and the constant tensile stress no longer increases, and on the other hand, the ionic clusters and physical attraction in the ACM are destroyed after the deformation continues to increase, and the constant tensile stress basically does not increase with the orientation of the molecular chain.
Figure 8 summarizes the variation of physical and mechanical properties of ACM/POM TPV with different Ca(OH) 2 dosages.The hardness of TPV is increased with the increase in Ca(OH) 2 dosage, which is mainly caused by the ionic clusters formed during the cross-linking of ACM, and the ionic clusters are not destroyed under smaller deformation and can resist stronger stresses.However, contrary to the pattern of hardness, the tensile strength of TPV decreases with increasing Ca(OH) 2 dosage, and it is the phase distribution of the two phases that has a greater effect on the strength at larger deformations.POM is a brittle plastic, and the sensitivity of the gap is very strong.Once the material has a more obvious stress concentration, it will fracture.At a lower cross-linking degree of ACM, the phase of rubber particles formed by dynamic vulcanization is smaller and more uniformly dispersed.The smaller modulus difference also makes the phase interface between the two phases more blurred, which effectively reduces the stress concentration inside the compound and results in higher tensile strength [24].The elongation at break of TPV fluctuates around 300%, and the 3 min permanent deformation after tearing is similar to the fluctuation pattern of the elongation at break.

Tensile retraction performance
In the study of ACM pure rubber, it was found that the tearing permanent deformation of ion-bonded system and covalent bonded system after tensile fracture differed greatly; the tearing permanent deformation of ACM vulcanized by HMDC was 20 ~ 25%, while the tearing permanent deformation of ACM vulcanized by Ca(OH) 2 was as high as 60 ~ 70%.Two repeated tensile curves were tested for two specimens of HMDC 1.2phr and Ca6 with similar crosslink density at 300% deformation variation, as shown in Fig. 9.
The area enclosed by the tensile retraction curve means that the energy lost in the stretching process (W), and the ratio (REL) of the energy required for the stretching process (W 0 ) characterize the resilience; the smaller the REL, the better the resilience.It is obvious that the energy loss of the ACM vulcanized by Ca(OH) 2 in the first stretching cycle is very high, more than double compared to the HMDC system, and the relative value of energy loss REL is also as high as 73.15%, which is higher than 56.8% for the HMDC system.This indicates that the ion clusters formed by the multiple ion pairs are more significantly restricted to the surrounding molecular chains and that the ion clusters are easily destroyed during the stretching process and are difficult to recover in a short period of time, showing a huge energy loss.The cross-linked network without recovery also makes the REL of the second cycle of stretching almost the same for both networks.Also using cyclic stretching, we characterized the energy loss and permanent deformation of TPV during stretching.Figure 10 shows the effect of different cross-linking networks and the degree of cross-linking on the tensile retraction performance of TPV, where the permanent deformation is the remaining strain after each tensile retraction when the stress is 0 MPa.Compared with the tensile retraction curves of ACM, the curves of different cross-linked networks of TPV only have slight differences in the tensile process, which show their respective tensile properties, respectively, and are closer in stress values, while almost no gap arises in the retraction curve curves.After calculation, the REL of TPV with ionic cross-linked network was slightly higher than that of covalent cross-linked network in the first cycle, and slightly lower in the second cycle instead.With the increase of Ca(OH) 2 dosage, the REL of TPV slightly decreased, and the decrease of permanent deformation was more significant; the permanent deformation after the first tensile cycle decreased from 34.57% to 22.53%, and the permanent deformation after the second cycle decreased from 38.80% to 24.84%.The increase of disperse phase cross-linking can effectively improve the TPV plastic phase with large permanent deformation and poor elasticity.The increase in disperse phase cross-linking can effectively improve the problems of large permanent deformation and poor elasticity of TPV plastic phase.

Heat resistant oxygen and hot oil aging properties
The regular structure and high crystallinity of POM can effectively reduce the chain breakage under thermal oxygen conditions, and at the same time, reduce the intrusion of solvents.The presence of ACM containing polar ester groups can form thermoplastic vulcanizates with better heat and oil resistance.Figure 11 shows the physical and mechanical properties of ACM/POM TPV and its rate of change after aging in hot air at 100 °C and 46 # hydraulic oil at 100 °C.Compared with before aging, TPV after both hot air aging and hot oil aging showed an increase in tensile strength and a decrease in elongation at break.At high temperatures, the residual Ca 2+ ions continue to react with the carboxyl groups of ACM, the degree of sulfation continues to increase, the tensile strength rises, and the increase in the modulus of ACM also causes it to form stress concentrations more easily in the matrix.With the increase in Ca(OH) 2 dosage, the change of tensile strength after thermal oxygen aging fluctuates from 20 to 40%, and after hot oil aging, it fluctuates from 5 to 20%, while the tearing elongation is showing a decreasing trend.To confirm this, the mass volume change rates of pure POM and TCa4 ~ TCa8 after aging in hot oil were tested, and the results are shown in Table 2. Pure POM received less erosion in hot oil, TPV showed an increase in mass and volume, and the rate of change of mass and volume gradually increased with increasing Ca(OH) 2 dosage, indicating the intrusion of hot oil, which can be illustrated by combining with the later characterization of two-phase phase domains.Taken together, the TPV has better thermal oxygen and thermal oil aging properties at a Ca(OH) 2 dosage of 4 parts.

Dynamic mechanical property
To analyze the effect of the change of cross-linking network type and degree of cross-linking on the viscoelasticity of ACM and ACM/POM TPV, a shear strain temperature scan was performed, Fig. 12 shows the variation of energy storage modulus (G'), loss modulus (G'') and loss factor (tanδ) of ACM vulcanization with temperature.
The rotation angle of the shear test is 0.5°, the deformation variable is smaller, and at this time the ionic clusters in the ionic crosslinked network are not destroyed and can resist greater shear stress, so the ACM after Ca(OH) 2 vulcanization shows higher G', and with the increase of the amount of vulcanizing agent, the change of G' corresponds to the change law of cross-linked density, the higher cross-linked bond density restricts the movement of molecular chains, so G' is higher.For G'', the loss of the ionic cross-linking network is greater, and the hysteresis of the molecular chain for deformation is more obvious.On the other hand, with the change in temperature, ACM with different cross-linking bonds and cross-linking densities have different sensitivity  to temperature.When covalent bonds are crosslinked, ACM gradually softens with the increase in temperature, and the resistance to shear force is weakened, and G' slightly decreases; meanwhile, the motility of molecular chain segments is enhanced, and the hysteresis of chain segments is weakened, but there is no breakage of crosslinked bonds, so G'' and tanδ decreases, and both changes are more in line with the regularity of conventional vulcanizates.In contrast, the ion-bonded crosslinked ACM vulcanizates exhibit great temperature sensitivity, with G' decreasing by more than 40% from 65 °C to 185 °C, and the vulcanizates also exhibit a more paradoxical G'' increase in the temperature range of 95 °C to 155 °C, and tanδ appears to rise across this temperature range indicating that the ion-cross-linked bonds break down under the action of temperature, leading to slippage between molecular chains and an increase in the viscosity of the material.
Figure 13 shows the dynamic mechanical properties of ACM/POM TPV, where the softening of the POM phase causes a gradual decrease in G' and G'' of the TPV during the temperature increase from 65 °C to 140 °C.The G' of both TPVs with different crosslinked bonds decreases linearly, but since the G'' of the ACM phase from 95 ℃ to 140 °C is on an increasing trend, the G' of TPV in this temperature interval does not decrease linearly, and the rate of decrease is gradually slowing down, showing a higher hysteresis.When the temperature increases from 140 ℃ to 155 ℃, the modulus of TPV shows a sudden drop because the temperature at this time reaches the melting temperature of POM, POM enters the viscous flow state, and the viscosity decreases rapidly.The modulus did not change much after the temperature exceeded 155 °C.On the other hand, the increase in the amount of Ca(OH) 2 increased the G' and decreased the G'' of TPV.The increase in the degree of entanglement of the ACM molecular chain network increases the reversible deformation under shear and makes it more elastic, so G' rises.Above the heat deformation temperature of POM, the degree of ACM cross-linking has almost no effect on the modulus of TPV, indicating that above the heat deformation temperature, the slip between the macromolecular chains of POM starts to occur, and the shear stresses suffered cannot be transferred to the rubber phase, but are all converted into deformation.The rise in tanδ of TPV in the ionic crosslinked system is more pronounced compared to TH1.2, which further confirms the destructive effect of temperature on ionic clusters.

Scanning electron microscope (SEM)
Figure 14 shows the ACM/POM TPV using 4 and 6 parts of Ca(OH) 2 .It can be seen that the different degrees of cross-linking of ACM caused the TPV to form different phase structures.The low degree of cross-linking of ACM resulted in dramatic shear fragmentation during dynamic vulcanization, wanting to form smaller rubber particles with a size of 5-10 μm.And due to the lower viscosity difference between the continuous phase of ACM and POM with low cross-linking degree, the two phases are more tightly bonded, the cross section is rougher, and tensile fracture of ACM is observed.When the degree of ACM cross-linking is larger, the ACM is difficult to break and the island phase phase domains formed are larger [25], and at 8-12 μm, the island phase formed is not obvious.Due to the large modulus difference between the two phases and the thin interfacial layer between the two phases, during the process of brittle fracture, large pieces of ACM are disconnected, while the rubber particles with smaller particle sizes fall off from the surface and form a large number of holes due to the weak bonding with the continuous phase of POM.

Conclusions
ACM was crosslinked with a metal ion vulcanization system completely different from the conventional covalent bonding vulcanization system, and acrylate rubber (ACM)/ polyoxymethylene (POM) TPVs were prepared by dynamic vulcanization technique.The amount of Ca(OH) 2 in ACM was varied, so as to change the modulus of ACM during the dynamic vulcanization of TPVs, and the effect of the difference in modulus between the two phases on the phase state of TPVs was investigated.Comparing with the covalently bonded system after vulcanization of HMDC and analyzing the connection between properties and structure, the following conclusions were obtained: Ca 2+ can form metal ion bonds with -COOH of ACM and form ion clusters with each other to vulcanize ACM.Compared with the conventional covalent bonding system, the TPV under ionic bonding can improve the stretching stress below 100% deformation by up to 31%, the elongation at break by 14.5%, exhibit higher permanent deformation after stretching, higher energy storage

Fig. 1
Fig. 1 Schematic diagram of preparation of the ACM/POM TPV composites

Fig. 2 Fig. 3
Fig. 2 Fourier transform infrared spectrum(FTIR) of ACM a Crude rubber b Mixed rubber c Vulcanized rubber

Fig. 4 Fig. 5 a
Fig.4 The vulcanization characteristics of ACM a Vulcanization curve b Torque difference and degree of cross-linking

Fig. 10
Fig. 10 Tensile retraction performance of ACM/POM TPV a First cycle b Second cycle

Fig. 11
Fig. 11 Changes in physical and mechanical properties of TPVS after aging a Tensile strength and its rate of change b Elongation at break and its rate of change

Fig. 12
Fig. 12 Dynamic mechanical property of ACM vulcanized

Table 1
Master rubber formula of ACM(Units: phr)