# Performance Analysis of Soluble Bridge Plug Materials for Fracturing in Unconventional Oilfields

Authors: Jianping Wang; Hongjuan Jia; Jiachun You
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

With the continuous in-depth development of oil and gas resources, unconventional oil and gas resources show great potential. In recent years, 70% of the proven oil and gas resources in China are unconventional oil and gas resources. Therefore, the development of unconventional oil and gas resources has a huge impact on the increase in the production capacity of the oil and gas resources in China. Staged fracturing of horizontal wells is the main technical measure to improve unconventional oil and gas production stimulation. In the fracturing process, the soluble bridge plug technology can not only effectively solve the problems of large investment in drilling and grinding of composite bridge plugs, high risks, self-locking of oil, casing deformation, etc., causing the bridge plug to be unable to be drilled and remove and affect subsequent production but also can successfully make up for the problems of subsequent production. The large-diameter bridge plug cannot achieve the full diameter of the wellbore and cannot carry out operations such as postproduction logging and refracturing, which can ensure the integrity of the wellbore to the greatest extent. In this paper, HNBR was used to modify MPU to prepare HNBR/MPU soluble rubber, and the effect of blending ratio on the vulcanization characteristics, mechanical properties, compatibility, thermal stability, and degradation behavior of composites was studied. The results showed that with the increase in the amount of HNBR, the degree of cross-linking of the composite material decreased gradually, and the mechanical properties decreased slightly. With the increase in the amount of HNBR, the thermal stability of the material decreases. The use of HNBR greatly reduced the degradation rate of MPU. When the amount of HNBR was changed from 20 phr to 40 phr, the tensile strength retention rate of the composite after soaking increased from 14.73% to 39.72%.


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## Body

## 1. Introduction

With the deepening of oil and gas exploration and development, unconventional oil and gas resources show great potential under the current economic and technological conditions, and the global oil and gas resources will expand twice. Staged fracturing of horizontal wells is the main technical measure to improve the stimulation and stimulation of unconventional oil and gas reservoirs such as tight oil and gas and low-permeability oil and gas reservoirs. The tool must be flown back out of the wellhead or worn out by drilling and milling tools. The flowback or drilling and milling of the fracturing tool not only increases the complexity of the fracturing process but also prolongs the operation time, which seriously affects the production efficiency of the fracturing operation [1–3]. Cracking tools are of great significance to promoting the development of the petroleum equipment manufacturing industry, increasing oil and gas production, and reducing oil and gas production costs. At present, the research on the main material of soluble bridge plugs at home and abroad has been relatively mature, but the research on rubber sealing materials matched with soluble bridge plugs is still in its infancy [4–6].Soluble bridge plug technology is an emerging technology in the field of staged fracturing of shale gas horizontal wells. Liquid immersion can achieve self-degradation, and the degradation rate is related to the temperature and salt concentration of the immersion liquid. The soluble bridge plug technology not only effectively solves the problems of high investment and high risk in drilling and grinding of composite bridge plugs, self-locking of oil connection, casing deformation, etc., which causes the bridge plug to be unable to be drilled and remove and affect subsequent production but also successfully made up for the shortcomings of large-diameter bridge plug [7]. The inability to realize the full bore diameter of the wellbore and the inability to carry out operations such as postproduction logging and refracturing ensures the integrity of the wellbore to the greatest extent. If the soluble bridge plug encounters abnormal conditions during the construction process, a special liquid can be used for instant dissolving, the treatment method is simple, and it is not easy to cause complicated conditions in the well. Like other staged tools, soluble bridge plugs are the key link in the implementation of staged fracturing of horizontal wells in shale gas reservoirs. Its technical safety and quality reliability directly affect the success or failure of technological measures. Therefore, before the soluble bridge plugs are inserted into the well, it is critical to examine its overall performance.The soluble bridge plug is mainly composed of the bridge plug base, the anchoring mechanism, and the sealing part [5, 8, 9]. The bridge plug base is made of high-strength soluble materials, including the central tube, the cone, the protection ring, and the joint. The anchoring mechanism uses a soluble material as a carrier, and the surface is treated with alloy powder particles, alloy particles, or ceramic particles. Seals are soluble rubber or plastic. The core technology of soluble fracturing bridge plugs has always been monopolized by foreign multinational oil service companies, such as Schlumberger, Halliburton, and Baker Hughes [10–12]. A lot of research on soluble fracturing bridge plugs has also been carried out in related fields in China. Some scholars have developed a fully soluble fracturing bridge plug based on magnesium alloy materials and conducted laboratory experiments on the soluble fracturing bridge plug assembly. The field application results show that the bridge plug can bear a pressure of 70 MPa at 120°C and completely dissolve in 1% potassium chloride solution at 120°C for 10 days [13]. Some researchers processed a new type of dissolvable bridge plug without intervention from degradable powder materials and tested the high-temperature indoor pressure performance of the dissolvable bridge plug. The indoor experimental results showed that the new type of dissolvable bridge plug can meet the pressure sealing performance requirements of 50 MPa under the high-temperature environment of 120°C and 150°C.Some scholars use magnesium-aluminum alloy as matrix material and nickel-copper alloy as cladding layer to prepare dissolvable material and develop a new type of dissolvable fracturing bridge plug without drilling and milling. The higher the immersion temperature, the higher the mass fraction of sodium chloride, the higher the chloride ion concentration, and the faster the degradation rate [7, 14, 15]. The maximum tensile strength of the soluble material is 342 MPa, and the deformation occurs at about 5 mm. The dissolvable bridge plug processed from the dissolving material meets the pressure sealing performance requirements of 70 MPa in a high-temperature environment of 150°C. The new type of dissolvable bridge plug developed by it has been tested in the Changqing Oilfield, and the test effect is good.Aiming at the technical gap of soluble rubber materials used in fracturing tools in China, this paper uses MPU materials that degrade faster as the matrix. This paper studies the use of HNBR with excellent physical and mechanical properties, temperature resistance, oil resistance, and aging resistance to modify MPU to prepare a soluble rubber material with a slower degradation rate. The effects of the HNBR/MPU blending ratio on the vulcanization properties, mechanical properties, compatibility, thermal stability, and dissolution properties of the composites were studied [16, 17].


## 2. Materials and Methods

### 2.1. Preparation of HNBR/MPU Soluble Rubber Sealing Material

MPU is a linear multiblock polymer composed of polyols such as polyether or polyester as soft segments, and diisocyanate and chain extender as hard segments. The molecules are almost all linear, and there are only hard segments controlled by hydrogen bonding. MPU has excellent physical and mechanical properties such as high wear resistance, oil resistance, chemical corrosion resistance, etc., and is widely used in machinery, light industry, national defense, and other fields. Generally, MPU can be divided into the polyester type and polyether type, and their properties are also different. Polyester MPU has a highly polar ester group, which can form hydrogen bonds between hard segments and can also form hydrogen bonds between soft segments and hard segments. The soft and hard segments are better connected together and play the role of elastic cross-linking points. Because a large number of allophanate, biuret, ester, and other groups in polyester MPU are easy to be decomposed and cannot be used for a long time in a high-temperature water medium. Polyether MPU has high strength, high elasticity, excellent low-temperature performance, and hydrolysis resistance.HNBR is a highly saturated molecular chain rubber containing acrylonitrile, hydrogenated butadiene, and a small number of butadiene segments. The unique structure of the HNBR molecular chain makes it have good mechanical properties, oil resistance, wear resistance, high-temperature thermal oxidation aging resistance, and other properties and is widely used in the field of oil production engineering as a raw material for the preparation of rubber cartridges. Some researchers used polyurethane rubber and hydrogenated nitrile rubber to prepare a soluble rubber composite material that can directly lose mechanical properties in water or completion fluid. The prepared material can be used to prepare a soluble rubber cartridge. The proportion of polyurethane is used to control the dissolution rate of the material. According to the specific example described, when the polyurethane rubber is 68 phr and the hydrogenated nitrile rubber is 32 phr, the prepared composite material is immersed in 90°C water for four days. The fracture has no tensile strength, the surface of the material is not cohesive, and the material degrades into small pieces.In this study, MPU and HNBR with different mixing ratios were wrapped and plasticized, and then, 100 phr of plasticized masterbatch was added to the mini Hack internal mixer, and then, ZnO, SA, and carbon black N220/N550/N774/N990 were added in turn. Banbury for 6 min at 110°C and 80 r/min. Finally, we added DCP and TAIC to the XK-160 open mill in turn. After mixing evenly, we adjusted the roll distance to 2 mm for the lower sheet. The experimental formula is shown in Table1. After 24 hours of natural parking at room temperature, we used the XLB-DQ flat vulcanizer produced by Qingdao Yadong Machinery Group Co. Ltd. to carry out one-stage vulcanization. The vulcanization conditions are temperature of 170°C and pressure of 15 MPa. The vulcanizate was vulcanized in two stages, and the vulcanization condition was 160°C for 4 h, and finally, the HNBR/MPU dissolvable rubber sealing material was obtained. 100/0 represents pure MPU, and 0/100 represents pure HNBR, as shown in Table 1.Table 1
HNBR/MPU dissolvable rubber material formulation.
Chemicals (phr)m (MPU)/m (HNBR)100/00/10080/2070/3060/40ZnO55555SA11111CB6060606060DCP33333TAIC11111Because a large number of allophanate, biuret, ester, and other groups in polyester MPU are easy to be decomposed and cannot be used for a long time in high-temperature water medium. Polyether MPU has high strength, high elasticity, excellent low-temperature performance, and hydrolysis resistance. HNBR is a highly saturated molecular chain rubber containing acrylonitrile, hydrogenated butadiene, and a small number of butadiene segments. The unique structure of the HNBR molecular chain makes it have good mechanical properties, oil resistance, wear resistance, high-temperature thermal oxidation aging resistance, and other properties and is widely used in the field of oil production engineering as a raw material for the preparation of rubber cartridges. So, we choose these two materials for the study.


### 2.2. Test Analysis

#### 2.2.1. Vulcanization Characteristics

The vulcanization characteristics of HNBR/MPU composites were tested according to ASTMD-2084-07 using the MDR 2000 rotorless vulcanizer, and the test temperature was 170°C.


#### 2.2.2. Mechanical Properties

MZ-4000D electronic universal testing machine was used to test the tensile stress-strain properties of HNBR/MPU composites according to GB/T 528-2009, and the tensile rate was 500 mm/min; HNBR/MPU was tested according to GB/T 528-2008. The high-temperature mechanical properties are tested with an AI-7000-S high- and low-temperature servo tensile machine to test the tensile strength and elongation at the break of the HNBR/MPU composite material; the LX-A Rubber and Plastic Shore Durometer Test is used to derive the hardness according to GB/T531.1-2008.


#### 2.2.3. Thermal Stability Test

The glass transition temperature (Tg) of HNBR/MPU composites was measured by using a DSC-204F1 differential scanning calorimeter, the test temperature was −60°C–150°C, the heating rate was 10°C/min, and the N2 atmosphere was used. SDT-Q600 thermal decomposition instrument was used, the test temperature was 25°C–700°C, the heating rate was 10°C/min, and the N2 atmosphere was used.


#### 2.2.4. Dissolving Behavior

According to the national standard GB/T 1690-2010, we soak the vulcanizate samples in the Tian DK-98-II water bath (temperature: 100°C, medium: tap water), and take out 5 samples every 24 hours (the first 48 hours, take a sample every 12 h), test their tensile stress-strain performances according to GB/T 528-2009, and test it continuously for 168 h.


## 2.1. Preparation of HNBR/MPU Soluble Rubber Sealing Material

MPU is a linear multiblock polymer composed of polyols such as polyether or polyester as soft segments, and diisocyanate and chain extender as hard segments. The molecules are almost all linear, and there are only hard segments controlled by hydrogen bonding. MPU has excellent physical and mechanical properties such as high wear resistance, oil resistance, chemical corrosion resistance, etc., and is widely used in machinery, light industry, national defense, and other fields. Generally, MPU can be divided into the polyester type and polyether type, and their properties are also different. Polyester MPU has a highly polar ester group, which can form hydrogen bonds between hard segments and can also form hydrogen bonds between soft segments and hard segments. The soft and hard segments are better connected together and play the role of elastic cross-linking points. Because a large number of allophanate, biuret, ester, and other groups in polyester MPU are easy to be decomposed and cannot be used for a long time in a high-temperature water medium. Polyether MPU has high strength, high elasticity, excellent low-temperature performance, and hydrolysis resistance.HNBR is a highly saturated molecular chain rubber containing acrylonitrile, hydrogenated butadiene, and a small number of butadiene segments. The unique structure of the HNBR molecular chain makes it have good mechanical properties, oil resistance, wear resistance, high-temperature thermal oxidation aging resistance, and other properties and is widely used in the field of oil production engineering as a raw material for the preparation of rubber cartridges. Some researchers used polyurethane rubber and hydrogenated nitrile rubber to prepare a soluble rubber composite material that can directly lose mechanical properties in water or completion fluid. The prepared material can be used to prepare a soluble rubber cartridge. The proportion of polyurethane is used to control the dissolution rate of the material. According to the specific example described, when the polyurethane rubber is 68 phr and the hydrogenated nitrile rubber is 32 phr, the prepared composite material is immersed in 90°C water for four days. The fracture has no tensile strength, the surface of the material is not cohesive, and the material degrades into small pieces.In this study, MPU and HNBR with different mixing ratios were wrapped and plasticized, and then, 100 phr of plasticized masterbatch was added to the mini Hack internal mixer, and then, ZnO, SA, and carbon black N220/N550/N774/N990 were added in turn. Banbury for 6 min at 110°C and 80 r/min. Finally, we added DCP and TAIC to the XK-160 open mill in turn. After mixing evenly, we adjusted the roll distance to 2 mm for the lower sheet. The experimental formula is shown in Table1. After 24 hours of natural parking at room temperature, we used the XLB-DQ flat vulcanizer produced by Qingdao Yadong Machinery Group Co. Ltd. to carry out one-stage vulcanization. The vulcanization conditions are temperature of 170°C and pressure of 15 MPa. The vulcanizate was vulcanized in two stages, and the vulcanization condition was 160°C for 4 h, and finally, the HNBR/MPU dissolvable rubber sealing material was obtained. 100/0 represents pure MPU, and 0/100 represents pure HNBR, as shown in Table 1.Table 1
HNBR/MPU dissolvable rubber material formulation.
Chemicals (phr)m (MPU)/m (HNBR)100/00/10080/2070/3060/40ZnO55555SA11111CB6060606060DCP33333TAIC11111Because a large number of allophanate, biuret, ester, and other groups in polyester MPU are easy to be decomposed and cannot be used for a long time in high-temperature water medium. Polyether MPU has high strength, high elasticity, excellent low-temperature performance, and hydrolysis resistance. HNBR is a highly saturated molecular chain rubber containing acrylonitrile, hydrogenated butadiene, and a small number of butadiene segments. The unique structure of the HNBR molecular chain makes it have good mechanical properties, oil resistance, wear resistance, high-temperature thermal oxidation aging resistance, and other properties and is widely used in the field of oil production engineering as a raw material for the preparation of rubber cartridges. So, we choose these two materials for the study.


## 2.2. Test Analysis

### 2.2.1. Vulcanization Characteristics

The vulcanization characteristics of HNBR/MPU composites were tested according to ASTMD-2084-07 using the MDR 2000 rotorless vulcanizer, and the test temperature was 170°C.


### 2.2.2. Mechanical Properties

MZ-4000D electronic universal testing machine was used to test the tensile stress-strain properties of HNBR/MPU composites according to GB/T 528-2009, and the tensile rate was 500 mm/min; HNBR/MPU was tested according to GB/T 528-2008. The high-temperature mechanical properties are tested with an AI-7000-S high- and low-temperature servo tensile machine to test the tensile strength and elongation at the break of the HNBR/MPU composite material; the LX-A Rubber and Plastic Shore Durometer Test is used to derive the hardness according to GB/T531.1-2008.


### 2.2.3. Thermal Stability Test

The glass transition temperature (Tg) of HNBR/MPU composites was measured by using a DSC-204F1 differential scanning calorimeter, the test temperature was −60°C–150°C, the heating rate was 10°C/min, and the N2 atmosphere was used. SDT-Q600 thermal decomposition instrument was used, the test temperature was 25°C–700°C, the heating rate was 10°C/min, and the N2 atmosphere was used.


### 2.2.4. Dissolving Behavior

According to the national standard GB/T 1690-2010, we soak the vulcanizate samples in the Tian DK-98-II water bath (temperature: 100°C, medium: tap water), and take out 5 samples every 24 hours (the first 48 hours, take a sample every 12 h), test their tensile stress-strain performances according to GB/T 528-2009, and test it continuously for 168 h.


## 2.2.1. Vulcanization Characteristics

The vulcanization characteristics of HNBR/MPU composites were tested according to ASTMD-2084-07 using the MDR 2000 rotorless vulcanizer, and the test temperature was 170°C.


## 2.2.2. Mechanical Properties

MZ-4000D electronic universal testing machine was used to test the tensile stress-strain properties of HNBR/MPU composites according to GB/T 528-2009, and the tensile rate was 500 mm/min; HNBR/MPU was tested according to GB/T 528-2008. The high-temperature mechanical properties are tested with an AI-7000-S high- and low-temperature servo tensile machine to test the tensile strength and elongation at the break of the HNBR/MPU composite material; the LX-A Rubber and Plastic Shore Durometer Test is used to derive the hardness according to GB/T531.1-2008.


## 2.2.3. Thermal Stability Test

The glass transition temperature (Tg) of HNBR/MPU composites was measured by using a DSC-204F1 differential scanning calorimeter, the test temperature was −60°C–150°C, the heating rate was 10°C/min, and the N2 atmosphere was used. SDT-Q600 thermal decomposition instrument was used, the test temperature was 25°C–700°C, the heating rate was 10°C/min, and the N2 atmosphere was used.


## 2.2.4. Dissolving Behavior

According to the national standard GB/T 1690-2010, we soak the vulcanizate samples in the Tian DK-98-II water bath (temperature: 100°C, medium: tap water), and take out 5 samples every 24 hours (the first 48 hours, take a sample every 12 h), test their tensile stress-strain performances according to GB/T 528-2009, and test it continuously for 168 h.


## 3. Results and Discussion

### 3.1. Vulcanization Characteristics and Mechanical Properties of HNBR/MPU Composites

In order to improve the hydrolysis resistance of MPU, a sealing material that was dissolved in 100 C water at different times was prepared, HNBR and MPU were blended, and the degree of cross-linking of the two was controlled to prepare HNBR/MPU composites with different HNBR/MPU blending ratios. Firstly, the influence of the HNBR/MPU blending ratio on the vulcanization characteristics and mechanical properties of the materials was investigated.Table2 shows the vulcanization characteristics of HNBR/MPU composites with different blending ratios. It can be found that the scorch time (t10) decreases slightly with the amount of HNBR, while the process positive vulcanization time (t90) decreases more, and the vulcanization index (cure rate index) gradually increases. The difference (MH-ML) between the highest torque (MH) and the lowest torque (ML) is used to characterize the degree of cross-linking of HNBR/MPU composites. It can be seen from Table 2 that with the increase of the amount of HNBR, the difference (MH-ML) is obviously decreased, indicating that the degree of cross-linking of HNBR/MPU composites decreased. When the peroxide DCP vulcanization system is used, the DCP is thermally decomposed to generate a free radical attack, and the free radical attacks the two polymer molecular chains to extract aH, forming macromolecular free radicals, free radical coupling, and further formation of cross-linking bonds. Compared with HNBR, the -CH2 on MPU is more easily attacked by free radicals than the -CH=CH- double bond in HNBR to form MPU macromolecular free radicals, and MPU is more easily cross-linked. Therefore, when the MPU content in the HNBR/MPU composite decreases, the cross-linked degree of connection is reduced.Table 2
Vulcanization characteristics of HNBR/MPU soluble rubber materials.
/m (MPU)/m (HNBR)100/00/10080/2070/3060/40t10/min0.691.120.950.810.76t90/min4.3510.157.236.124.69Cure rate index/min−128.9512.6816.3218.3924.69MH/(dN m)15.015.5120.7815.7710.47ML/(dN m)0.741.081.381.521.56(MH-ML)/(dN m)14.274.4319.414.258.91Figure1 shows the mechanical properties at room temperature and different temperatures of HNBR/MPU composites with different blending ratios. It can be seen from Figure 1 that with the increase in temperature, the tensile strength of all materials decreases greatly. The high-temperature tensile strength of the HNBR/MPU composite material is between the pure MPU and HNBR materials, and with the increase of the relative content of MPU, the high-temperature tensile strength is enhanced, and the tensile strength is the highest when the mass ratio of HNBR/MPU is 80/20. The change of the tensile strength at high temperature is consistent with the law at room temperature, which is between the performance of MPU and HNBR, indicating that the use of two-phase materials in a certain proportion does not affect the high-temperature performance of MPU.Figure 1
The effect of temperature on the tensile strength of HNBR/MPU dissolvable rubber material.


### 3.2. Compatibility and Thermal Stability of HNBR/MPU Composites

The purpose of blending the two rubbers is to improve the deficiencies of the existing rubber properties. Therefore, the glass transition temperature (Tg) of HNBR/MPU composites with different blending ratios was tested by DSC to know the compatibility of the composites. The DSC curve of the HNBR/MPU composite material is shown in Figure2. The Tg of the HNBR/MPU composite material is between the pure MPU and the pure HNBR material and only one Tg appears. In addition, with the increase of the HNBR content, the Tg is biased towards the HNBR side; showing that the HNBR/MPU composite material has good compatibility within a certain blending range. After MPU and a certain amount of HNBR are used together, the two materials do not form aggregates in isolation, and they are cured by peroxide. It can be covulcanized under the system, so a certain amount of HNBR can be used to improve the hydrolysis resistance of MPU.Figure 2
DSC curve of HNBR/MPU dissolvable rubber material.The thermal stability of the composite material can provide technical support when the material is used at a higher temperature. Therefore, we tested the thermogravimetric curve of the composite material as shown in Figure3. The maximum weight loss temperature range of HNBR is 460–550°C, and the total weight loss is 59.81%. The maximum weight loss temperature range of MPU is 260–470°C, and the total weight loss is 54.29%. The final weight loss of the reinforcement systems of the two materials is different because HNBR is a pure carbon chain material, which is completely broken at high temperatures. The content of hard segments in MPU is high, and there are a large number of polar bonds and NH in the urethane group. The high-temperature carbonization phenomenon occurs in the N2 atmosphere and cannot be completely burned at high temperatures. The HNBR/MPU composite has two weight loss processes. The first weight loss step is 260°C–365°C, and the weight loss rate is 23.89%; the second weight loss step is 365 °C∼470°C, and the weight loss rate is 36.36%. The weight loss law of HNBR/MPU composite material reflects the thermal weight loss law of MPU and HNBR at the same time, and it can be clearly seen that the thermal stability of the composite material has been greatly improved.Figure 3
TG curve of HNBR/MPU soluble rubber material.


### 3.3. Dissolution Behavior of HNBR/MPU Composites

Figures4–6 show the change curve of the tensile strength of HNBR/MPU composites with soaking time. It can be seen that after soaking for 24 h, the tensile strength of pure MPU, 80/20, 70/30, and 60/40 decreased sharply, while the tensile strength of pure HNBR has almost no change. The tensile strength of pure MPU is about 3 MPa after soaking for 12 h, which can no longer meet the requirements of fracturing and sealing, and the rate of mechanical properties decline is too fast. After soaking for 48 h in 100°C water, all the dumbbell samples have been broken, and the mechanical properties test cannot be carried out, so pure MPU type materials cannot meet the long-term fracturing production.Figure 4
Effect of soaking time on tensile strength of HNBR/MPU dissolvable rubber material.Figure 5
Effect of soaking time on tensile strain of HNBR/MPU dissolvable rubber material.Figure 6
The effect of soaking time on the hardness of HNBR/MPU dissolvable rubber material.Comparing the three blended materials of 80/20, 70/30, and 60/40 in different proportions, it can be found that the dissolution change of the material is similar to that of pure MPU, indicating that it is the HNBR part that maintains the mechanical properties of the material after 24 h. Because the MPU phase of the composite material dissolves in 100 C water, while the HNBR phase does not change, the carbonyl and carbamate groups in the MPU phase undergo hydrolysis in 100 C water after HNBR/MPU blending. The chain breaks, resulting in the decline of mechanical properties, leaving only the HNBR phase to maintain the mechanical properties of the material. When the HNBR content is increasing, the dissolution rate of the composite material is slower, and the tensile strength retention rate of the composite material is higher. Among the prepared materials, after immersion for 24 h, when the MPU/HNBR blend ratio was changed from 80/20 to 60/40, the tensile strength retention rate increased from 14.73% to 39.72%, and the HNBR/MPU composite material maintained high tensile strength with little change between 48 h and 168 h. Therefore, the blending of MPU and HNBR is used to reduce the dissolution rate of the sealing material, improve the tensile strength of the sealing material after soaking, and maintain the sealing performance of the composite material.After 120, the curves have become steady, so there is no need to extend the time to very long.


### 3.4. Discussion on the Dissolution Mechanism of HNBR/MPU Soluble Rubber Material

Based on the above changes in shape and mechanical properties, the dissolution behavior of the HNBR/MPU dissolvable rubber material during the dissolution process in clean water at 100°C was mainly caused by the degradation of MPU. Therefore, studying the structural evolution of MPU materials during the dissolution process is crucial for understanding the degradation of soluble rubber materials. MPU is a block polyurethane rubber composed of soft segment and hard segment. Since the soft segment of MPU is a degradable polyester molecular chain, when the MPU is immersed in water at 100°C, the polyester in the soft segment is prone to occur in water at 100°C. After hydrolysis, the soft segment is gradually dissolved in water, and the degraded material is mainly composed of the skeleton of the hard segment. Therefore, with the prolongation of the soaking time, the material shows the phenomenon of first softening and then hardening, as shown in Figure7. The initial hardness of the material is 85, when immersed for 24 hours, the hardness is 51, and the hardness decreases by 40% at this time. When the material is soaked for 144 hours, the hardness of the material is 87, which is close to the initial hardness of the material. At this time, the mechanical properties of the material are almost completely lost with no tensile strength and no elongation at break.Figure 7
Effect of soaking time on hardness of MPU material.


## 3.1. Vulcanization Characteristics and Mechanical Properties of HNBR/MPU Composites

In order to improve the hydrolysis resistance of MPU, a sealing material that was dissolved in 100 C water at different times was prepared, HNBR and MPU were blended, and the degree of cross-linking of the two was controlled to prepare HNBR/MPU composites with different HNBR/MPU blending ratios. Firstly, the influence of the HNBR/MPU blending ratio on the vulcanization characteristics and mechanical properties of the materials was investigated.Table2 shows the vulcanization characteristics of HNBR/MPU composites with different blending ratios. It can be found that the scorch time (t10) decreases slightly with the amount of HNBR, while the process positive vulcanization time (t90) decreases more, and the vulcanization index (cure rate index) gradually increases. The difference (MH-ML) between the highest torque (MH) and the lowest torque (ML) is used to characterize the degree of cross-linking of HNBR/MPU composites. It can be seen from Table 2 that with the increase of the amount of HNBR, the difference (MH-ML) is obviously decreased, indicating that the degree of cross-linking of HNBR/MPU composites decreased. When the peroxide DCP vulcanization system is used, the DCP is thermally decomposed to generate a free radical attack, and the free radical attacks the two polymer molecular chains to extract aH, forming macromolecular free radicals, free radical coupling, and further formation of cross-linking bonds. Compared with HNBR, the -CH2 on MPU is more easily attacked by free radicals than the -CH=CH- double bond in HNBR to form MPU macromolecular free radicals, and MPU is more easily cross-linked. Therefore, when the MPU content in the HNBR/MPU composite decreases, the cross-linked degree of connection is reduced.Table 2
Vulcanization characteristics of HNBR/MPU soluble rubber materials.
/m (MPU)/m (HNBR)100/00/10080/2070/3060/40t10/min0.691.120.950.810.76t90/min4.3510.157.236.124.69Cure rate index/min−128.9512.6816.3218.3924.69MH/(dN m)15.015.5120.7815.7710.47ML/(dN m)0.741.081.381.521.56(MH-ML)/(dN m)14.274.4319.414.258.91Figure1 shows the mechanical properties at room temperature and different temperatures of HNBR/MPU composites with different blending ratios. It can be seen from Figure 1 that with the increase in temperature, the tensile strength of all materials decreases greatly. The high-temperature tensile strength of the HNBR/MPU composite material is between the pure MPU and HNBR materials, and with the increase of the relative content of MPU, the high-temperature tensile strength is enhanced, and the tensile strength is the highest when the mass ratio of HNBR/MPU is 80/20. The change of the tensile strength at high temperature is consistent with the law at room temperature, which is between the performance of MPU and HNBR, indicating that the use of two-phase materials in a certain proportion does not affect the high-temperature performance of MPU.Figure 1
The effect of temperature on the tensile strength of HNBR/MPU dissolvable rubber material.


## 3.2. Compatibility and Thermal Stability of HNBR/MPU Composites

The purpose of blending the two rubbers is to improve the deficiencies of the existing rubber properties. Therefore, the glass transition temperature (Tg) of HNBR/MPU composites with different blending ratios was tested by DSC to know the compatibility of the composites. The DSC curve of the HNBR/MPU composite material is shown in Figure2. The Tg of the HNBR/MPU composite material is between the pure MPU and the pure HNBR material and only one Tg appears. In addition, with the increase of the HNBR content, the Tg is biased towards the HNBR side; showing that the HNBR/MPU composite material has good compatibility within a certain blending range. After MPU and a certain amount of HNBR are used together, the two materials do not form aggregates in isolation, and they are cured by peroxide. It can be covulcanized under the system, so a certain amount of HNBR can be used to improve the hydrolysis resistance of MPU.Figure 2
DSC curve of HNBR/MPU dissolvable rubber material.The thermal stability of the composite material can provide technical support when the material is used at a higher temperature. Therefore, we tested the thermogravimetric curve of the composite material as shown in Figure3. The maximum weight loss temperature range of HNBR is 460–550°C, and the total weight loss is 59.81%. The maximum weight loss temperature range of MPU is 260–470°C, and the total weight loss is 54.29%. The final weight loss of the reinforcement systems of the two materials is different because HNBR is a pure carbon chain material, which is completely broken at high temperatures. The content of hard segments in MPU is high, and there are a large number of polar bonds and NH in the urethane group. The high-temperature carbonization phenomenon occurs in the N2 atmosphere and cannot be completely burned at high temperatures. The HNBR/MPU composite has two weight loss processes. The first weight loss step is 260°C–365°C, and the weight loss rate is 23.89%; the second weight loss step is 365 °C∼470°C, and the weight loss rate is 36.36%. The weight loss law of HNBR/MPU composite material reflects the thermal weight loss law of MPU and HNBR at the same time, and it can be clearly seen that the thermal stability of the composite material has been greatly improved.Figure 3
TG curve of HNBR/MPU soluble rubber material.


## 3.3. Dissolution Behavior of HNBR/MPU Composites

Figures4–6 show the change curve of the tensile strength of HNBR/MPU composites with soaking time. It can be seen that after soaking for 24 h, the tensile strength of pure MPU, 80/20, 70/30, and 60/40 decreased sharply, while the tensile strength of pure HNBR has almost no change. The tensile strength of pure MPU is about 3 MPa after soaking for 12 h, which can no longer meet the requirements of fracturing and sealing, and the rate of mechanical properties decline is too fast. After soaking for 48 h in 100°C water, all the dumbbell samples have been broken, and the mechanical properties test cannot be carried out, so pure MPU type materials cannot meet the long-term fracturing production.Figure 4
Effect of soaking time on tensile strength of HNBR/MPU dissolvable rubber material.Figure 5
Effect of soaking time on tensile strain of HNBR/MPU dissolvable rubber material.Figure 6
The effect of soaking time on the hardness of HNBR/MPU dissolvable rubber material.Comparing the three blended materials of 80/20, 70/30, and 60/40 in different proportions, it can be found that the dissolution change of the material is similar to that of pure MPU, indicating that it is the HNBR part that maintains the mechanical properties of the material after 24 h. Because the MPU phase of the composite material dissolves in 100 C water, while the HNBR phase does not change, the carbonyl and carbamate groups in the MPU phase undergo hydrolysis in 100 C water after HNBR/MPU blending. The chain breaks, resulting in the decline of mechanical properties, leaving only the HNBR phase to maintain the mechanical properties of the material. When the HNBR content is increasing, the dissolution rate of the composite material is slower, and the tensile strength retention rate of the composite material is higher. Among the prepared materials, after immersion for 24 h, when the MPU/HNBR blend ratio was changed from 80/20 to 60/40, the tensile strength retention rate increased from 14.73% to 39.72%, and the HNBR/MPU composite material maintained high tensile strength with little change between 48 h and 168 h. Therefore, the blending of MPU and HNBR is used to reduce the dissolution rate of the sealing material, improve the tensile strength of the sealing material after soaking, and maintain the sealing performance of the composite material.After 120, the curves have become steady, so there is no need to extend the time to very long.


## 3.4. Discussion on the Dissolution Mechanism of HNBR/MPU Soluble Rubber Material

Based on the above changes in shape and mechanical properties, the dissolution behavior of the HNBR/MPU dissolvable rubber material during the dissolution process in clean water at 100°C was mainly caused by the degradation of MPU. Therefore, studying the structural evolution of MPU materials during the dissolution process is crucial for understanding the degradation of soluble rubber materials. MPU is a block polyurethane rubber composed of soft segment and hard segment. Since the soft segment of MPU is a degradable polyester molecular chain, when the MPU is immersed in water at 100°C, the polyester in the soft segment is prone to occur in water at 100°C. After hydrolysis, the soft segment is gradually dissolved in water, and the degraded material is mainly composed of the skeleton of the hard segment. Therefore, with the prolongation of the soaking time, the material shows the phenomenon of first softening and then hardening, as shown in Figure7. The initial hardness of the material is 85, when immersed for 24 hours, the hardness is 51, and the hardness decreases by 40% at this time. When the material is soaked for 144 hours, the hardness of the material is 87, which is close to the initial hardness of the material. At this time, the mechanical properties of the material are almost completely lost with no tensile strength and no elongation at break.Figure 7
Effect of soaking time on hardness of MPU material.


## 4. Conclusion

(1)
In this paper, a series of HNBR/MPU dissolvable rubber composites were prepared by blending HNBR and MPU. When the MPU/HNBR blending ratio was changed from 80/20 to 60/40, the degree of cross-linking decreased gradually, and the tensile strength, elongation at break and hardness decreased.(2)
The Tg of HNBR/MPU composite material is between pure MPU and pure HNBR material and only one Tg appears. HNBR/MPU composite material has good compatibility within a certain blending range. When the MPU/HNBR blending ratio changes from 80/20 to 60/40, the thermal stability of the composite material is greatly improved. When the MPU/HNBR blending ratio was changed from 80/20 to 60/40, after soaking in 100 C water for 24 h, the tensile strength retention rate increased from 14.73% to 39.72%, and the composite material was at 48%.(3)
The product has not been applied to practical cases, but it has broad application prospects.



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*Source: 1911173-2022-05-19.xml*