polymers

Review Recent Progress in Polymer-Containing Soft Matters for Safe Mining of Coal

Hetang Wang 1,2,3,* , Yunhe Du 3, Deming Wang 2,3 and Botao Qin 2,3

1 State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China 2 Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, China; [email protected] (D.W.); [email protected] (B.Q.) 3 School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; [email protected] * Correspondence: [email protected]; Tel.: +86-151-5001-3592; Fax: +86-516-8359-0598



Received: 18 September 2019; Accepted: 13 October 2019; Published: 17 October 2019


Abstract: Safe mining is the premise and guarantee of sustainable development of coal energy. Due to the combination of excellent properties of polymers and traditional soft matters, polymer-containing soft matters are playing an increasingly important role in mine disaster and hazard control. To summarize the valuable work in recent years and provide reference and inspiration for researchers in this field, this paper reviewed the recent research progress in polymer-containing soft matters with respect to mine dust control, mine fire control, mine gas control and mine roadway support. From the perspective role of polymers in a material system, we classify mine polymer-containing soft matters into two categories. The first is polymer additive materials, in which polymers are used as additives to modify fluid-like soft matters, such as dust-reducing agents (surfactant solution) and dust-suppressing foams. The second is polymer-based materials, in which polymers are used as a main component to form high performance solid-like soft matters, such as fire prevention gels, foam gels, gas hole sealing material and resin anchorage agent. The preparation principle, properties and application of these soft matters are comprehensively reviewed. Furthermore, future research directions are also suggested.

Keywords: coal; safe mining; soft matter; polymer additive modification; polymer-based material

1. Introduction Coal is the most abundant and widely distributed fossil energy on earth. It plays a very important role in the worldwide energy supply and is an important support for global economic and social development [1]. For example, in England during the Industrial Revolution, coal became the main energy source to support vigorous development. From 1650 to 1850, per capita coal consumption in England increased 13-fold [2]. Over the past 20 years, coal has accounted for more than 25% of global energy consumption and more than 38% of global electricity comes from coal-fired power plants. Compared to about 48% of oil in the Middle East, 70% of natural gas in the Middle East and the Commonwealth of Independent States [3], the distribution of coal in the world is more disperse. The USA has the largest coal reserves in the world [4]. From 2008–2018, the USA produced 7407.5 Mt (million tons) of coal and consumed 5773.7 Mt of coal. For emerging economies, like China, India and South Africa, coal is the main fuel source for basic industries such as steel manufacturing, cement production and power development [5]. As a country with poor oil reserves, modest natural gas but rich in coal, China’s coal consumption accounts for about 50% of the world’s total [3]. China has made great efforts to reform and optimize its energy structure, the proportion of coal declined from

Polymers 2019, 11, 1706; doi:10.3390/polym11101706 www.mdpi.com/journal/polymers Polymers 2019, 11, 1706 2 of 30

75% in 2010 to 69% in 2015 [6]. However, coal consumption continued to grow and still accounted for 58.2% in 2018 and thus the main energy status of coal has not changed. The demand for energy and the advancement of technology have prompted the vigorous development of the coal industry. Sustainability of coal production is an inevitable choice to maintain a stable world energy supply for many years in the future. Safe coal mining is to the core of sustainable development of coal energy. With the increase in the depth, strength and scale of coal mining [7] and the development of comprehensive mechanization, problems such as mine dust, mine fires, mine gas and roof fall are becoming more significant. Large amounts of dust are generated during coal mining and transportation and dust concentrations at working sites significantly exceed the permissible values in many cases [8,9]. The dust concentration of a fully mechanized mining face can generally reach 2000 mg/m3; the dust concentration of a fully mechanized excavation face can reach 1000 mg/m3, whilst the respirable dust could account for about 20% of the total dust. According to statistics, nearly 28,000 new cases of pneumoconiosis were added in China in 2016, of which coal mine pneumoconiosis accounted for more than 50% [10]. Mine fires can produce toxic and harmful gases, threaten the lives and health of workers and pollute the environment, causing significant economic losses to enterprises [11]. Endogenous fire is the main form of mine fires, in which coal spontaneous combustion in the goaf of longwall faces occurs most frequently [12,13]. Uncontrolled coal fires cause annual coal losses of 10–20 million tones in China, equivalent to monetary losses of 125–250 million dollars [14]. Methane is the main component of mine gas. If the methane content in a coal seam is high or the methane concentration in the air is high, it will seriously threaten the safe production of an underground coal mine [15,16]. In 2012, a major gas explosion accident killed 45 people in Sichuan, China [17]. In the statistics of accidents in 2016, there were four large coal mine roof fall accidents in China, resulting in 40 casualties [18]. Such mine disasters seriously threaten the safety and the health of mine employees. Thus, controlling coal mine disaster/hazard, improving the mine environment and ensuring workers’ safety and health have become the priority for safety and sustainable development of the modern coal industry [19]. Polymer-containing soft matters have important value and broad application prospects in ensuring safe mining of coal. Soft matter refers to substances that are between solid and ideal fluids, including foam, gel and granular matter, for example. Polymers are macromolecule compounds, which are linked by covalent bonds by one or several structural units. The polymers give excellent performance in strength, hardness, heat resistance, corrosion resistance, abrasion resistance along with many other properties. Thus, they are widely used in packaging, plastics, construction, mining and other areas. Polymer materials have become one of the methods to improve the efficiency and safety of coal mine engineering [20]. A series of new, green and efficient polymer-containing soft matters are formed for improving the properties of conventional materials through physical or chemical methods. Such products of polymer-containing soft matters have made an outstanding contribution to dust control, fire prevention, gas drainage, roadway support and repair in coal mines. In general, there are two main directions of development: One is the polymer-surfactant system [21], which depends on the good viscosity, thermal stability and degradability of the polymers, such as environmentally-friendly and high-efficiency dust suppressants [22]. The other is the polymer-inorganic mixture system, which uses the good curing reaction and mechanical stability of the polymer [23], such as for coal and rock crushing reinforcement materials [24]. At present, the cost of dust-suppressing solution materials can be only $1.0–2.0 per kilogram. The characteristics of polymer-containing soft matters, such as safety, high efficiency and reasonable economy, are in line with the concept of modern coal mine safety and sustainable development. In recent years, for the safe and green mining of coal, much research has been undertaken on polymer materials and technologies, with important progress made. However, few summaries have been presented in this regard. Hence, this paper will summarize and review the application and development of polymer-containing soft matters in coal mines in recent years. The aim of the paper is to comprehensively understand the development and application status of polymer-containing soft Polymers 2019, 11, 1706 3 of 30 matters for safe mining of coalmines. In this paper, the main challenges faced by polymer-containing soft matters research in coal mining will be highlighted and suggestions for future development will also be given. This work could provide important reference and inspiration for the research and application of polymer material in mine safety.

2. Classification of Polymer-Containing Soft Matters in Coal Mine Safety Soft matters are widely developed and applied in coal mines. For the safety and greening of coal mining, polymers are typically used as additives or basic materials to improve the original soft matter properties or synthesize new soft matters. These are especially applied in coal mine disaster/hazard control and are mainly divided into four categories: 1. Dust control: Dust suppressant (surfactant solution), dust suppression foam (aqueous foam, foam sol); 2. fire prevention and control: Suspended mortar, composite gel, three-phase foam and coatings slurry; 3. gas drainage: Gas hole sealing material; 4. roadway support and repair: Anchor net shotcrete, liquid accelerator, resin anchorage agent and roadway repair grouting material.

3. Polymer-Containing Soft Matters for Coal Mine Disaster/Hazard Control

3.1. Polymer Additive Soft Matters in Mine Dust Control In relation to the prevention and control of mine dust, compared to ventilation and dust removal fans, the traditional water-spraying dust suppression, coal seam water injection and emerging foam dust suppression are more widely used [25–29]. The core concept is to improve and develop dust suppressant performance and researches on polymer-surfactant system have begun. Polymers have significantly changed the properties of solutions and foam [30]. Spray and water injection mainly reflect the properties of the solution liquid, while foam is the properties of gas–liquid two-phase, which leads to different research directions and development trends.

3.1.1. Surfactant Solution The effect of spray and water injection is related to the wettability and surface tension of the solution. The effect of dust control is related to the adhesion and consolidation ability of the solution. The better the wettability, the smaller the surface tension and thus the better the dust removal effect. The better the adhesion and consolidation performance, the less easy it is to raise dust. Most of the existing wettability studies are determined by contact angle or dust sinking experiments in solution. Dou [31] and others discussed the performance improvement of water-soluble polymer sodium carboxymethyl cellulose (CMC) and laboratory-made superabsorbent polymer (SA) on sodium dodecyl benzene sulfonate (SDBS) solution. The wettability was determined by the settling time of 0.2 g coal dust in 10 mL solution. While both of them reduced the surface tension of the solution, the effect of the absorbent polymer (SA) was more remarkable. It was pointed out that when the mass ratio of SA to SDBS was 1:4, the dust suppression effect was best. Based on the characteristics that the existing surfactants are easy to air dry and volatilize, Xi et al. [32] mixed the thermoplastic powder formed from the industrial-grade polyethylene oxide (g-PEO) (having a melting temperature close to the coal combustion temperatures) with sodium lauryl sulfate (SDS) to obtain a dust suppressant. The coal dust sinking test was used, although the wettability of the g-PEO&SDS solution was weaker than that of the SDS solution, the surface tension was larger, according to Equation (1)

Fc = 2πrγcosθ (1) where Fc is the capillary adhesion, dyn, r is the particle radius, cm, γ is the suppressant surface tension, 1 dyn cm− and θ is the contact angle, ◦. This indicates that the adhesion of g-PEO&SDS solution to particles was enhanced by the larger surface tension and as shown in Figure1, the solidified layer can be formed on the surface of coal dust to reduce the evaporation of moisture in coal dust. PolymersPolymers 20192019,, 1111,, x 1706 FOR PEER REVIEW 44of of 30 30

Figure 1. Principle of solution action and actual permeation: (a) Schematic overview of the Figureindustrial-grade 1. Principle polyethylene of solution oxide action (g-PEO) and and actual sodium permeation: lauryl sulfate (a) (SDS) Schematic solution; overview (b) schematic of the industrialoverview-grade of dust polyethylene control by theoxide g-PEO&SDS (g-PEO) and solution; sodium (c lauryl) penetration sulfate (SDS) test of solution; coal dust (b control) schematic in overviewsolutions containing of dust control SDS, g-PEO by the andg-PEO&SDS g-PEO&SDS solution; [32]. (c) penetration test of coal dust control in solutions containing SDS, g-PEO and g-PEO&SDS [32]. For the control of deposited dust in coal mine, Lai et al. [33] showed that the polymers could effectivelyFor the reduce control the of dispersion deposited of dust dust. in Moreover, coal mine, they Lai indicatedet al. [33] that showed microwaves that the could polymers effectively could effectivelyimprove the reduce reaction the rate dispersion of organic synthesis. of dust. Therefore, Moreover, under they 300 indicated W microwave that irradiationmicrowaves for could two effectivelyminutes, acrylic improve acid the(AA), reaction acrylamide rate (AAM) of organic and oxidized synthesis. starch Therefore, can be polymerized under 300 W to synthesize microwave irradiationpolymer dust for suppressant two minutes, (PSD). acrylic Then acid 10 mL (AA), PDS acrylamide solution (2.0 (AAM) g/L) was and mixed oxidized with 6 starchg pulverized can be polymerizedcoal to prepare to samplessynthesize to determinepolymer dust their suppressant compressive (PSD). strength, Then wind 10 corrosionmL PDS solution resistance (2.0 and g/L) water was mixedcorrosion with resistance. 6 g pulverized The results coal to show prepare that samples the compressive to determine strength their reaches compressive 0.293 MPa,strength, the PDSwind corrosionsolution remains resistance stable and at water12–50 corrosion◦C. The resistance. compressive The strength results ofshow coal that samples the compressive soaked in water strength for − reachesmany times 0.293 remains MPa, the unchanged PDS solution and the remains loss rate stable of coal at − is12 only–50 1.2%°C. The after comp 3 h ofressive strong strength wind erosion. of coal samplesWith soaked the development in water for of many polymer-containing times remains unchanged soft matters, and some the researchers loss rate of have coal broughtis only 1.2% the afterconcept 3 h of of strong green wind environmental erosion. protection into the field of dust control. Zhou et al. [34] used sodiumWith alginate the development as a matrix, of becausepolymer of-containing its excellent soft hydrophilicity, matters, some viscidity,researchers stability, have brought low price, the conceptsafety and of non-toxicity,green environmental then chemically protection modified into by the graft field copolymerization of dust control. technology Zhou et to al synthesize. [34] used sodiuman environmentally alginate as a friendly matrix, dust because suppressant of its excellent under hydrophilicity, certain conditions. viscidity, Caprolactam stability, (CPL) low price, and safetyacrylic and acid (AA) non-toxicity, were selected then as chemically graft monomers. modified The by results graft of acopolyme morphologyrization test, technologycontact angle to synthesizeanalysis, sinking an environmentally test, viscosity test friendly and tensile dust test suppressant show that theunder new certain product conditions. had excellent Caprolactam viscosity, (CPL)wettability, and acrylic strength acid and (AA) deformation were selected ability (Figure as graft2). monomers. The raw material The results is clean, of the a morphology cost is low and test, contactthe preparation angle analysis, is simple. sinking As a natural,test, viscos non-toxicity test andand biodegradable tensile test show polymer that the material, new product Zhang [ 35had] excellentmodified guarviscosity, gum (GG)wettability, with cyanuric strength chloride and deformation(TCT) and sodium ability sulfamate (Figure (SS) 2). to The form raw the material modified is clean,product the GGTCS cost is (Figure low and3). the Thermogravimetric preparation is simple. analysis As showed a natural, that non the- thermaltoxic and stability biodegradable of the polymmodifieder material, GGTCS increasedZhang [35 from] modified 268 ◦C guar to 285 gum◦C. Using(GG) with GGTCS cyanuric as the chloride main material, (TCT) aand new sodium dust sulfamatereducer of (SS) 0.8% to GGTCS form + the1.5% modified glycerol product(GLY) + 0.1% GGTCS sodium (Figure dodecyl 3). Thermogravimetricbenzene sulfonate (SDBS) analysis+ showed0.02% fatty that alcohol the thermal polyoxyethylene stability of etherthe modified (AEO) was GGTCS synthesized. increased Ithad from good 268 water°C to retention285 °C. Using and viscosity. When the coal dust was sprayed, the solidified film formed on the surface of the coal dust GGTCS as the main material, a new dust reducer of 0.8% GGTCS + 1.5% glycerol (GLY) + 0.1% could bear up to 29 kPa of pressure. The solidified film can be buried in soil, with the quality reducing sodium dodecyl benzene sulfonate (SDBS) + 0.02% fatty alcohol polyoxyethylene ether (AEO) was by 60% after one month (Figure4). It also has good degradability characteristics. synthesized. It had good water retention and viscosity. When the coal dust was sprayed, the solidified film formed on the surface of the coal dust could bear up to 29 kPa of pressure. The solidified film can be buried in soil, with the quality reducing by 60% after one month (Figure 4). It also has good degradability characteristics.

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Figure 2. Microcosmic model of product in tension test [34].

Figure 2. Microcosmic model of product in tension test [34]. FigureFigure 2. MicrocosmicMicrocosmic model model of of product product in tension test [ 34]]..

Figure 3. GGTCS synthesis flow chart (modified guar gum (GG) with cyanuric chloride (TCT) and sodiumFigure3. sulfamateGGTCS synthesis(SS) forms flow the modified chart (modified product guar GGTCS gum) [ (GG)35]. with cyanuric chloride (TCT) and Figuresodium 3. sulfamate GGTCS synthesis (SS) forms flow the chart modified (modified product guar GGTCS) gum [(GG)35]. with cyanuric chloride (TCT) and sodiumFigure 3. sulfamate GGTCS synthesis(SS) form sflow the modifiedchart (modified product guar GGTCS gum) [(GG)35]. with cyanuric chloride (TCT) and sodium sulfamate (SS) forms the modified product GGTCS) [35].

Figure 4. Degradability curve of dust suppressant film [35]. Figure 4. Degradability curve of dust suppressant film [35]. For open pit mines, Zhou et al. [36] used sodium lignosulfonate as a surfactant and natural Figure 4. Degradability curve of dust suppressant film [35]. macromoleculeFor open pit to cross-link mines, Zhou with et acrylic al. [36 acid] used to obtain sodium cross-linking lignosulfonate products as a surfactantand subsequently and natural graft Figure 4. Degradability curve of dust suppressant film [35]. macromolecule to cross-link with acrylic acid to obtain cross-linking products and subsequently copolymerizationFor open pit with mines, acrylamide Zhou et to al. form [36] a used gel material. sodium lignosulfonateA new environmental as a surfactant dust suppressant and natural was obtained for after crushing. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction and macromolecule For open pit to mines, cross-link Zhou with et al acrylic. [36] usedacid to sodium obtain lignosulfonate cross-linking productsas a surfactant and subsequently and natural macromolecule to cross-link with acrylic acid to obtain cross-linking products and subsequently

Polymers 2019, 11, x FOR PEER REVIEW 6 of 30 graft copolymerization with acrylamide to form a gel material. A new environmental dust suppressant was obtained for after crushing. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction and Polymers differential2019, 11, 1706 scanning calorimetry (DSC) were used to determine6 of 30 the functional group characteristics, crystallization and thermal stability. Expansion kinetics, viscosity, film-forming hardnessdifferential and scanning peeling calorimetry strength (DSC) were of used spray to determine solution the functional were tested. group characteristics, The results show that it crystallization and thermal stability. Expansion kinetics, viscosity, film-forming hardness and peeling had better windstrength-proof of characteristics, spray solution were tested. film The-forming results show hardness that it had better and wind-proof dust suppression characteristics, ability (Figure 5). film-forming hardness and dust suppression ability (Figure5).

Figure 5. Pictures of the coal pile without and under different treatments: (a) Picture of the primary coal pile; (b) picture of the coal pile after being wetted; (c) picture of the coal pile after being coated Figure 5. Pictureswith of the the coagulant; coal pile (d) picture without of the coal and pile under after the applicationdifferent of coagulanttreatments: [36]. (a) Picture of the primary coal pile; (b) picture of the coal pile after being wetted; (c) picture of the coal pile after being coated 3.1.2. Aqueous Foam with the coagulant; (d) picture of the coal pile after the application of coagulant [36]. The efficiency of foam dust suppression is related to foam properties, which is characterized by stability, adhesion, wettability, initial liquid fraction and bubble size. Wang et al. [37] synthesized six 3.1.2. Aqueous Ftestoam solutions by adding hydroxyethyl cellulose (HEC), polyacrylamide (HPAM) and polyvinyl alcohol (PVA) into two foaming agents, sodium alpha-olefin sulfonate (AOS) and polyoxyethylene octylphenol The efficiencyether-10 of foam (OP-10), dust respectively. suppression The properties is ofrelated the foam (wettability,to foam surfaceproperties, viscosity andwhich stability) is characterized by were characterized by measuring the contact angle, viscosity modulus and foam discharge rate. It stability, adhesion,was wettability, indicated that adding initial polymers liquid to AOS fraction (OP-10) decreased and bubble the wettability size. ofWang the foam, et especially al. [37] synthesized six test solutions bywhen adding more than hydroxyethyl 0.3 wt.%. However, thecellulose polymers increased (HEC), foam polyacrylamide stability and foam surface (HPAM) viscosity. and polyvinyl This was due to the two macromolecular chains, micelles and aggregates which existed in the liquid alcohol (PVA) intophase two when foaming the polymer agents, dissolved in s theodium surfactant alpha solution,-olefin significantly sulfonate increasing (AOS) the viscosity and of polyoxyethylene octylphenol etherthe-10 liquid. (OP The-10), viscous respectively. force increased theThe strength properties of the foam of liquid the film, foam slowing (wettability, down the foam surface viscosity discharge and increasing the stability of the foam (Figure6). With the increase of polymer concentration, and stability) werethe eff ect characterized was more obvious. by Therefore, measuring the low concentration the contact of the polymerangle, (0.1–0.3 viscosity wt.%) has modulus a and foam discharge rate. Itbetter was eff ectindicated on the dust suppressionthat adding ability polymers of the foam. to AOS (OP-10) decreased the wettability of the foam, especially when more than 0.3 wt.‰. However, the polymers increased foam stability and foam surface viscosity. This was due to the two macromolecular chains, micelles and aggregates which existed in the liquid phase when the polymer dissolved in the surfactant solution, significantly increasing the viscosity of the liquid. The viscous force increased the strength of the foam liquid film, slowing down the foam discharge and increasing the stability of the foam (Figure 6). With the increase of polymer concentration, the effect was more obvious. Therefore, the low concentration of the polymer (0.1–0.3 wt.‰) has a better effect on the dust suppression ability of the foam.

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Figure 6. Foam drainage behavior of the surfactant solution with added polymer [37]. Figure 6. Foam drainage behavior of the surfactant solution with added polymer [37]. Figure 6. Foam drainage behavior of the surfactant solution with added polymer [37]. Xu et al. [38] added polymer sodium carboxymethyl cellulose (CMC) to anionic surfactant fatty Xu et al. [38] added polymer sodium carboxymethyl cellulose (CMC) to anionic surfactant fatty alcohol polyoxyethyleneXu et al. [38] added ether polymer sulfate sodium (AES) carboxymethyl solution. Incellulose this i nstance(CMC) to it anionic was shownsurfactant that fatty adding alcohol polyoxyethylene ether sulfate (AES) solution. In this instance it was shown that adding polymeralcohol can polyoxyethylene reduce foaming ether time sulfate at low (AES) gas flow solution. rates In and this improve instance the it wasinitial shown liquid that volume adding of the polymerpolymer can can reduce reduce foaming foaming time time at at low low gasgas flowflow rates and and improve improve the the initial initial liquid liquid volume volume of the of the foam. Subsequently, Xu [39] and others studied the solution and foam properties of AES and foam.foam. Subsequently, Subsequently, Xu [ Xu39] [ and39] and others others studied studied the solution the solution and and foam foam properties properties of AES of AES and and partially partially hydrolyzed polyacrylamide (HPAM) at different concentrations. HPAM increases the hydrolyzedpartially polyacrylamide hydrolyzed polyacrylamide (HPAM) at di (HPAM)fferent concentrations. at different concentrations. HPAM increases HPAM the increasessurface tension the of surface tension of AES solution, but also increases its viscosity. Adding HPAM at low AES AES solution,surface tension but also of increases AES solution, its viscosity. but also Adding increases HPAM its viscosity. at low AES Adding concentration HPAM at could low AES improve concentration could improve foamability and increase bubble size, because the increased bulk foamabilityconcentration and increase could improve bubble foamabilitysize, because and the increase increased bubble bulk size, viscosity because and the the increased hydrogen bulk bonds viscosity and the hydrogen bonds formed between AES molecule and HPAM chain stabilized the formedviscosity between and AES the moleculehydrogen andbonds HPAM formed chain between stabilized AES molecule the bubble and film HPAM and reducedchain stabilized bubble the damage. bubblebubble film film and and reduced reduced bubble bubble damage. damage. The increase increase of of HPAM HPAM concentration concentration can canimprove improve the the The increase of HPAM concentration can improve the liquid carrying capacity of foam and reduce the liquidliquid carrying carrying capacity capacity of offoam foam and and reduce reduce the drainagedrainage rate. rate. These These findings findings indicate indicate that addingthat adding drainage rate. These findings indicate that adding water-soluble polymers to surfactant solutions can waterwater-soluble-soluble polymers polymers to to surfactant surfactant solutions solutions can can improve improve foam foam properties, properties, thereby thereby low low improve foam properties, thereby low concentration surfactants will be applied in coal mines. concentrationconcentration surfactants surfactants will will be be applied applied inin coal mines.mines. WangWang [40] [40 discussed] discussed the the eff effectect of of xanthan xanthan gumgum (XG) and and partially partially hydrolyzed hydrolyzed polyacrylamide polyacrylamide Wang [40] discussed the effect of xanthan gum (XG) and partially hydrolyzed polyacrylamide (HPAM)(HPAM) on the on dust the suppression dust suppression foam foam produced produced by sodium by sodium dodecyl dodecyl benzene benzene sulfonat sulfonat (SDBS) (SDBS) solution. (HPAM) on the dust suppression foam produced by sodium dodecyl benzene sulfonat (SDBS) The foamsolution. properties The foam were properties characterized were characterized from three from aspects: three Foamability, aspects: Foamability, liquid holdup liquid holdup and bubble and size. solution. The foam properties were characterized from three aspects: Foamability, liquid holdup and The resultsbubble size. showed The results that polymers showed that improved polymers foamability improved foamability and liquid and holding liquid holding rate and rate the and e ffitheciency bubble size. The results showed that polymers improved foamability and liquid holding rate and the of XGefficiency was higher of XG than was thathigher of HPAM.than that Itof is HPAM. noted thatIt is noted polymer that XGpolymer has manyXG has branched many branched chains and efficiency of XG was higher than that of HPAM. It is noted that polymer XG has many branched hydrogenchains bonds and hydrogen in its structure. bonds in Hydrogenits structure. bonds Hydrogen can makebonds XGcan curlmake and XG fold,curl and so it fold, has so a largeit has spatiala chainslarge and spatial hydrogen effect bonds(Figure in 7). its In structure. addition, Hydrogen meshes and bonds cavities can can make be formed XG curl to and capture fold, a largeso it has a effect (Figure7). In addition, meshes and cavities can be formed to capture a large number of water largenumber spatial of effect water (Figuremolecules, 7). thereby In addition, increasing meshes the volume and cavities of liquid canin the be foam. formed HPAM to captureis a typical a large molecules, thereby increasing the volume of liquid in the foam. HPAM is a typical linear water-soluble numberlinear of water water-soluble molecules, polymer thereby (Figure increasing 8). Its branches the volume are far ofless liquid complex in the than foam. XG. HPAMTherefore is, athe typical polymer (Figure8). Its branches are far less complex than XG. Therefore, the foamabality, liquid holdup linearfoamabality, water-soluble liquid polymer holdup and (Figure bubble 8). size Its of branches HPAM foam are arefar weakerless complex than those than of XG.XG. Therefore, the and bubble size of HPAM foam are weaker than those of XG. foamabality, liquid holdup and bubble size of HPAM foam are weaker than those of XG.

Figure 7. Structure of xanthan gum (XG) molecular chain [40].

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Figure 7. Structure of xanthan gum (XG) molecular chain [40].

Polymers 2019, 11, x FOR PEER REVIEW 8 of 30 Polymers 2019, 11, x FOR PEER REVIEW 8 of 30 Figure 7. Structure of xanthan gum (XG) molecular chain [40]. Polymers 2019, 11, 1706 Figure 7. Structure of xanthan gum (XG) molecular chain [40]. 8 of 30

Figure 8. Structure of polyacrylamide (HPAM) molecular chain [40].

3.1.3. Foam Sol Figure 8. Structure of polyacrylamide (HPAM) molecular chainchain [[4040].]. For the preventionFigure and 8. control Structure of of dust,polyacrylamide Xi [41] put (HPAM) forward molecular the chainfoam [ 40sol]. with the function of dust 3.1.3.catching, Foam inhibition S Solol and isolation. Initially, foam sol consists of grease, acetate and byproducts 3.1.3. Foam Sol generatedFor in thea slow preventionprevention crosslink and reaction. control ofof A dust, dust,special XiXi [conical41] put forwardnozzle w theas foamdesigned sol withwith for thesprayingthe functionfunction foam ofof sol (Figuredust 9). For catching, Xi the[42 ]prevention also inhibition proposed and and control isolation.a foam of sol Initially, dust, clay Xi mixture, [ foam41] put sol mainly forward consists composed ofthe grease, foam sol acetateof withfly ash and the (FA), byproductsbyproductsfunction surfactant of (n-pentylamine)generateddust catching, in aand slowinhibitionslow polyethylene crosslinkcrosslink and reaction.isolation.reaction. oxide AInitially,(PEO). special When foam conical sol the nozzle consists mass wasw concentrations asof designedgrease, acetate for sprayingwere and 200byproducts foam g/L, sol 35 g/L and (Figure(Figure 1generated g/L, 9 the9).).Xi inXi sol[ [a42 slow had]] also also crosslinkthe proposed proposed highest reaction. a foam foam foam Asol sol halfspecial clay clay-life, mixture, mixture, conical the bestmainly nozzle foaming composed was designed performance of fly fly for ash spraying (FA), and surfactant foam the highestsol stability.(n-pentylamine)(n(Figure-pentylamine) On 9). the Xi [one42 ] and also hand, polyethylene proposed polyethylene a foam oxide sol (PEO).oxide clay mixture, molecularWhen the mainly mass chains composed concentrations are physically of fly were ash (FA),entangled 200 g/L, g surfactant/L, 35 to gg/L/L form hydrogenand(n-pentylamine) 1 1 g bondsg/L,/L, the the solor sol and hadcombine had thepolyethylene the highest with highest foamwater oxide foam half-life, to (PEO). halfform- thelife, When hydrogen best the foaming the best mass foamingbonds, performance concentrations which performance andincreases were the and highest 200 the theg/L, stability.visco highest 35 g/Lsity of Onand the 1 g/L, one hand,the sol polyethylene had the highest oxide foam molecular half-life, chains the bestare physically foaming performance entangled to and form the hydrogen highest the solution.stability. AtOn thethe sameone hand, time, polyethylene polyethylene oxide oxide molecular can adsorb chains fly are ash physically particles entangled and reduce to form particle bondsstability. or combineOn the one with hand, water polyethylene to form hydrogen oxide bonds, molecular which chains increases are the physically viscosity entangled of the solution. to form At dispersionhydrogen to obtainbonds ora stablecombine sol with solution. water Furthermore,to form hydrogen polyethylene bonds, which oxide increases contains the avisco largesity number of thethehydrogen samesolution. time, bonds At polyethylene the or samecombine time, oxide with polyethylene can water adsorb to form flyoxide ash hydrogen can particles adsorb bonds, and fly reduce ash which particles particle increases dispersionand thereduce visco to particle obtainsity of of ether-oxygen units that can form associations with coal. Once the hydrophobic coal adsorbs adispersionthe stable solution. sol to solution. Atobtain the samea Furthermore, stable time, sol polyethylenesolution. polyethylene Furthermore, oxide oxide can contains polyethyleneadsorb afly large ash oxide number particles contains of and ether-oxygen areduce large numberparticle units polyethylenethatofdispersion ether can- formoxygen oxide to associationsobtain aqueous units a stable that withsolution, cansol coal. solution. form Onceits associations hydrophobicity Furthermore, the hydrophobic with polyethylene coal.will coal change Once adsorbs theoxide to polyethylene hydrophobichydrophilicity. contains a oxide large coal The aqueousnumber adsorbs solution can effectivelysolution,polyethyleneof ether-oxygen its permeate hydrophobicity oxide units aqueous the that coal willcansolution, body changeform andits associations tohydrophobicity form hydrophilicity. an witharmor will coal. Thelayer change Oncesolution on the theto canhydrophilicity. surface hydrophobic effectively of the permeate coalThe coal solution adsorbs body the after air-dryincoalcanpolyethylene effectively bodyg. Xi and[43 ]oxide formpermeatethen aqueous antried armor the to coalsolution, layersynthesize body on its theand hydrophobicity surfacea form self- anhardening of armor the coal will layer bodyfoam change on after thesol, to surface air-drying.hydrophilicity.mainly of composed the Xi coal [43 The] body then solutionof triedafterfoaming solutiontoaircan synthesize-dryin effectively(composedg. Xi a[ 43permeate self-hardening ]of then 1 g/L tried the PEO, coalto foam synthesize25 body g/L sol, and sodium mainly aform self - composeddodecylanhardening armor sulfatelayer offoam foaming on sol,100 the mainlyg/L surface solution sodium composed of (composed the silicate coal of body foamingand of 1after g874/L g/L water)PEO,solutionair and-dryin 25 organic g(composedg./L Xi sodium [43 acid.] then dodecylof 1The tried g/L formationPEO, sulfateto synthesize 25 100 g/L mechanism g sodium/L a sodium self -dodecylhardening silicate of foam sulfate and foam sol 874 100 sol,is g /g/LshoL mainly water) sodiumwn incomposed and Figuresilicate organic and 10.of acid. foaming The874 Theg/L results showedformationwater)solution that and (composedwhen mechanism organic the acid.organicof of1 g/L foamThe PEO, acid formation sol 25 isconcentration showng/L mechanismsodium in Figure dodecyl was of 10 10foam. Thesulfate–13 sol resultsg/L, 100is thesho g/L showed wnfoam sodium in thatFiguresol silicate had when 10. the and theThe best organic 874results wettingg/L abilityacidshowedwater), accepted concentration and that organic when loss wasthe ofacid. liquidityorganic 10–13 The gformation /acidL,, theself concentration foam-hardening mechanism sol had was the time of best 10foam –and13 wetting g/L, solthe is the material ability,sho foamwn acceptedinsol hadFigure had goodthe loss 10. best ofThe wind liquidity, wetting results erosion showed that when the organic acid concentration was 10–13 g/L, the foam sol had the best wetting resistance.self-hardeningability , Foraccepted static time lossdust and of suppression, the liquidity material, self had the-hardening good dust wind suppression time erosion and resistance. therate material could For reach had static good 100% dust wind suppression,at 12 erosion m/s wind theability dust, accepted suppression loss rate of could liquidity reach, self 100%-hardening at 12 m /s time wind and speed. the material had good wind erosion speed.resistance. For static dust suppression, the dust suppression rate could reach 100% at 12 m/s wind speed.resistance. For static dust suppression, the dust suppression rate could reach 100% at 12 m/s wind speed.

Figure 9. The detailed structural scheme of special conic nozzle used for spraying foamfoam sol [4141].]. FigureFigure 9. The 9. The detailed detailed structural structural scheme scheme of of special special conic nozzlenozzle used used for for spraying spraying foam foam sol [sol41] .[ 41].

Figure 10. CChemicalhemical mechanism for the formation of self-hardeningself-hardening foamfoam sol [43].]. Figure 10. Chemical mechanism for the formation of self-hardening foam sol [43]. Figure 10. Chemical mechanism for the formation of self-hardening foam sol [43]. 3.2. Polymer-Containing Soft Matters in Mine Fire Control 3.2. Polymer-Containing Soft Matters in Mine Fire Control 3.2. Polymer -Containing Soft Matters in Mine Fire Control

Polymers 2019, 11, 1706 9 of 30

3.2. Polymer-Containing Soft Matters in Mine Fire Control Fire from the spontaneous combustion of coal is one of the most common natural disasters in coal mines. It occurs when coal with spontaneous combustion tendencies encounters oxygen in the air, the heat generated by oxidation is greater than the heat lost to the surrounding environment and the heat accumulates, so that the coal temperature rises to the ignition point and ignites. The effective way to prevent spontaneous combustion of coal is to prevent coal from getting into contact with air [44]. In view of this problem, a variety of fireproof soft matters have been developed: Suspended mortar [45], composite gel [46], three-phase foam [47] and coating slurry [48] and on these bases, the high-quality fireproof polymer-containing soft matters have also been formed. Among them, suspended mortar and three-phase foam can be classified as polymer additive soft matters and composite gel and coating slurry belong to polymer-based soft matters.

3.2.1. Suspended Mortar Grouting, as the first means of fire prevention and control, is not only grouting yellow mud or fly ash, but also improving grouting materials according to the current situation and adapting to the requirements. Xu et al. [49,50] discussed the shortage of water resources in the west and north China and the poor effect of traditional grouting. A sand suspended mortar was developed and a composite additive was synthesized by mineral inorganic gel, organic polymer and dispersant. Inorganic gel can make water become viscous and expansible and colloid uniformly dispersed in water to form charged negative particles. Organic polymers are amylase polymers extracted from algae plants. They can combine with two valence ions such as Ca2+, Cu2+ and Pb2+ in inorganic gel to form a network space structure. The additive improves the quality of sand injection, reduces the need for water resources, increases the viscosity of grouting materials and produces a stable suspended mortar of sand. In the experiment, the suspended mortar of sand increased the Crossing-point temperature and activation energy of lignite and gas-fat coal (Tables1 and2), delayed its oxidation reaction and effectively inhibited the spontaneous combustion of coal.

Table 1. Crossing-point temperature of coal samples.

Crossing-Point Temperature ( C) Coal Samples ◦ Ref Nothing Added Soluble Glass Calcium Chloride Sand-Suspended Colloid Lignite 175.3 184.6 182.8 187.3 [50] Fat-gas coal 166.3 182.7 180.5 178.2

Table 2. Activation energy of coal samples.

Activation Energy (kJ/mol) Coal Samples Ref Nothing Added Soluble Glass Calcium Chloride Sand-Suspended Colloid Lignite 77.00 92.09 74.48 95.37 [50] Fat-gas coal 81.68 82.47 81.30 86.34

In order to improve the stability of fly ash mortar, Shi et al. [51] mixed the guar gum (GG) derivative hydroxypropyl guar gum (HPG) and xanthan gum (XG) at a mass ratio of 1:1 to obtain the mixture (BMS). Subsequently 300 g of fly ash (FA) particles were dispersed into 1 L of BMS solution with different concentrations and stirred to obtain the mixed mortar (FAS). The shear viscosities of BMS and FAS were measured by a rotational rheometer, the viscoelasticity was determined by dynamic angular frequency scanning, the stability was determined by a static sedimentation test, the fiber photographs of FAS samples were obtained using a scanning electron microscope and the flame retardant test was also carried out. The results showed that the viscosity of BMS solution and FAS mortar increased with the increase of HPG/XG concentration. This was due to the interaction between HPG and XG at a low shear rate, forming a gel network structure, supporting FA particles and preventing gravity PolymersPolymers 20192019, 11,, x11 FOR, 1706 PEER REVIEW 10 of10 30 of 30 the interaction between HPG and XG at a low shear rate, forming a gel network structure, supportingsettling, showingFA particles higher and viscosity preventing and excellent gravity water settling, retention showing (Figure higher11). When viscosity the concentration and excellent waterof retention HPG/XG was (Figure more 11). than When 1.6 g/L, the the concentration yield stress of BMSof HPG/XG exceeded was the downwardmore than stress 1.6 g/L, of the the FA yield particles, which kept a stable state for 72 h and FAS mortar delayed the oxidation effect of coal more so stress of BMS exceeded the downward stress of the FA particles, which kept a stable state for 72 h than ordinary fly ash. and FAS mortar delayed the oxidation effect of coal more so than ordinary fly ash.

Figure 11. Schematic diagram of network structures between hydroxypropyl guar gum (HPG) and FigureXG [11.51]. Schematic diagram of network structures between hydroxypropyl guar gum (HPG) and XG [51]. 3.2.2. Composite Gel 3.2.2. CompositeThe inorganic Gel gel has poor water holding capacity and thermal stability. Colloid composed of sodiumThe inorganic silicate and gel ammoniumhas poor water salt is holding prone to producecapacity toxic and and thermal harmful stability. gases and Colloid the cost composed is high. of For this purpose, the composite gels were developed by combining polymer and inorganic gel. sodium silicate and ammonium salt is prone to produce toxic and harmful gases and the cost is high. Ren [52] introduced cationic polyacrylamide (CPAM), anionic polyacrylamide (HPAM) and For thiscarboxymethyl purpose, the cellulose composite (CMC) gels into were sodium developed silicate gel by (WG)combining to obtain polymer three kinds and ofinorganic polymer gel. gels

(CPAMRen [52/WG,] introduced HPAM/WG cationic and CMC polyacrylamide/WG). Subsequently, (CPAM), the crosslinking anionic polyacrylamideagent aluminumcitrate (HPAM) was and carboxymethyladded to three cellulose kinds of (CMC) polymer into gels sodium to obtain silicate polymer-Al gel (WG)3+/WG to gels.obtain The three compressive kinds of properties polymer gels (CPAM/WG,of the gels HPAM/WG were analysed and via aCMC/WG). universal testing Subsequently, machine. The the water-retaining crosslinking agent property aluminum was tested citrate was in added a weathering to three drying kinds experiment. of polymer The gels microstructure to obtain was polymer observed-Al3+/WG using gels. a scanning The compressive electron propertiesmicroscope. of the The gels hydroxyl were analysed and methylene via a universal functional testing groups machine. were analyzed The water by Fourier-retaining transform property was infraredtested in spectroscopy. a weathering Micro drying EDS analysisexperiment. was usedThe tomicrostru determinecture the elementalwas observed analysis. using As showna scanning electronin Figure microscope. 12, the results The hydroxyl showed that and the methylene polymer/WG functional gels form groups a network were gel analyzed system through by Fourier transformintermolecular infrared interactionspectroscopy. and Micro had good EDS water analysis retention was used ability. to determine With the increase the elemental in polymer analysis. concentration, the water retention ability and strength of gels first increased and then decreased, this As shown in Figure 12, the results showed that the polymer/WG gels form a network gel system was related to the swelling degree of the polymers. When a crosslinking agent was added, a more through intermolecular interaction and had good water retention ability. With the increase in stable network structure was formed with the polymer and the water retention ability became stronger. polymerHPAM-Al concentration,3+/WG is an the ideal water fire extinguishing retention ability material and because strength it has of the gels highest first increased water retention and then decreased,abilityand this causes was related significant to inhibitionthe swelling on the degree oxidation of the ofhydroxyl polymers. and When methylene. a crosslinking For Hu et al.agent [53], was added,the a HPAM-aluminum more stable network citrate structure gel system was was formed identified with to the be the polymer most favourable and the water gel system retention for fire ability becamesuppression stronger. in undergroundHPAM-Al3+/WG coal mines. is an ideal Huang fire [54 ]extinguishing and others used m sodiumaterial silicatebecause as ait gelling has the agent, highest waterammonium retention saltability and and sodium causes salt assignificant crosslinking inhibition agents, sodiumon the oxidation bicarbonate of as hydroxyl coagulant and accelerator methylene. For Huand et non-toxic al. [53], sodium the HPAM polyacrylate-aluminum (with citrate strong gel water system absorption) was identified as a polymer to be additive the most to produce favourable gel system for fire suppression in underground coal mines. Huang [54] and others used sodium silicate as a gelling agent, ammonium salt and sodium salt as crosslinking agents, sodium bicarbonate as coagulant accelerator and non-toxic sodium polyacrylate (with strong water absorption) as a polymer additive to produce more environmentally-friendly gel material. Through field inspection (Figure 13), for the three zones of goaf, the heat dissipation zone was advanced by 8

Polymers 2019, 11, 1706 11 of 30

more environmentally-friendly gel material. Through field inspection (Figure 13), for the three zones Polymersof goaf, 2019 the, 11, heat x FOR dissipation PEER REVIEW zone was advanced by 8 meters, the oxidation temperature zone11 was of 30 Polymersshortened 2019, by 11, 20x FOR meters PEER andREVIEW the asphyxiation zone was advanced by 28 meters, showing good11 of fire 30 meters,prevention the oxidation and extinguishing temperature performance. zone was shortened by 20 meters and the asphyxiation zone was advancedmeters, the by oxidation28 meters, temperature showing good zone fire was prevention shortened andby 20 extinguishing meters and the performance. asphyxiation zone was advanced by 28 meters, showing good fire prevention and extinguishing performance.

Figure 12. Schematic diagram of foam gel structure. FigureFigure 12.12. SchematicSchematic diagram of of f oamfoam gel gel structure. structure.

Figure 13. TheThe variety variety of of the the three zones after injection of compound material into the goaf of the Figure 13. The variety of the three zones after injection of compound material into the goaf of the working face: ( aa)) Before Before injection; injection; ( b) a afterfter injection [ 54]].. working face: (a) Before injection; (b) after injection [54]. Based on on the the principles principles of economy, of economy, environmental environmental protection and protection high effi ciency, and highenvironmentally- efficiency, environmentallyfriendlyBased gel on materials the-friendly principles have been gel investigated.materials of economy, have Cheng environmental been et al. investigated. [55] grafted protection 2-acrylamide-2- Cheng etand al . methylpropane high[55] efficiency, grafted environmentally2sulfonic-acrylamide acid- (AMPS)2--methylpropanefriendly and acrylic gel materials sulfonic acid (AA) acid have to form (AMPS) been a polymer, and investigated. acrylic mixed acid it with Cheng (AA) powdered to et form al. corn a[ 55 polymer, straw] grafted to 2-acrylamidemixedsynthesize it with hydrogel-2- methylpropanepowdered and further, corn straw sulfonic added to the synthesize acid foaming (AMPS) agenthydrogel and and expandableand acrylic fur ther, acid graphite added (AA) theto (EG) formfoaming to produce a polymer, agent an mixedandintelligent expandableit with gel powdered (IG) graphite that can corn expand (EG) straw to with to produce thesynthesize environment. an intelligent hydrogel The geland gel was (IG)fur comparedther, that added can with expand the thermosensitive foaming with theagent andenvironment.gel expandable and polyacrylamide The graphite gel was gel. compared (EG) It was to shown with produce tothermosensitive have an good intelligent foaming gel and gel properties, polyacrylamide(IG) that stronger can gel. thermalexpand It was stability shown with the environment.toand have provided good The foaming a widergel was coverageproperties, compared of stronger fire with sources. thermosensitive thermal The stability gel could gel and eandff ectivelyprovided polyacrylamide reduce a wider heat coveragegel. radiation It was of shown andfire to sources.COhave production good The foaminggel and could expanded properties, effectively graphite reducestronger after heat thermal evaporation radiation stability and of water CO and production wouldprovided made and a thewider expanded gel coverage expand graphite again. of fire sources.afterExperiments evaporation The gel showed could of water effectively that would the gel reducemade had the the heat shortestgel radiation expand fire again. extinguishingand CO Experiments production time showed (Table and3 expanded). that Hu the et al.gel graphite [had56] synthesized self-gas foaming hydrogels using biopolymer chitosan (CS), attapulgite (APT), sodium afterthe evaporationshortest fire extinguishingof water would time made (Table the 3). gel Hu expand et al. [ 5again.6] synthesized Experiments self- gasshowed foaming that hydrogels the gel had acrylate (SA) as monomers and Na2CO3 as a foaming agent. Attapulgite (APT) is a type of chain-lamella theusing shortest biopolymer fire extinguishing chitosan (CS), time attapulgite (Table 3). (APT), Hu et sodium al. [56 ]acrylate synthesized (SA) asself monomers-gas foaming and Nahydrogels2CO3 clay mineral containing water and zeopan. It was shown that the swelling ratio (SR) decreased with usingas a biopolymerfoaming agent. chitosan Attapulgite (CS), (APT)attapulgite is a type (APT), of chain sodium-lamella acrylate clay (SA)mineral as monomerscontaining waterand Na and2CO 3 the increase of CS. On the contrary, with the increase of APT, the SR of hydrogels increased initially aszeopan. a foaming It was agent. shown Attapulgite that the swelling (APT) isratio a type (SR) ofdecreased chain-lamella with the clay increase mineral of CS.containing On the contrary, water and and then decreased. The SR of hydrogel increased with the increase in foaming agent. zeopan.with the It was increase shown of that APT, the the swelling SR of hydrogels ratio (SR) increaseddecreased initially with the and increase then decreased. of CS. On the The contrary, SR of withhydrogel the increase increased of with APT, the the increase SR of in hydrogels foaming agent. increased initially and then decreased. The SR of hydrogel increased with the increase in foaming agent. Table 3. Fire-extinguishing times of the four materials.

TableType of3. Firematerials-extinguishing IG TSG times PAMof the fourWater materials Ref .

Type Timeof materials (S) 220IG TSG289 PAM490 Water362 5Ref5 Qin [57] and others tried to apply other field materials to the suppression of spontaneous coal Time (S) 220 289 490 362 55 combustion. For instance, using super absorbent polymers made of polypropylene resin as physical Qin [57] and others tried to apply other field materials to the suppression of spontaneous coal combustion. For instance, using super absorbent polymers made of polypropylene resin as physical

Polymers 2019, 11, 1706 12 of 30

Table 3. Fire-extinguishing times of the four materials.

Type of Materials IG TSG PAM Water Ref Time (S) 220 289 490 362 [55]

PolymersQin 2019 [57, 11], andx FOR others PEER REVIEW tried to apply other field materials to the suppression of spontaneous12 coalof 30 combustion. For instance, using super absorbent polymers made of polypropylene resin as physical inhibitorinhibitor and and ascorbic ascorbic acid acid in in the the food food industry industry as a chemical inhibitor,inhibitor, withwith aa viewview toto formingforming a a new new typetype of of gel. gel. When When the the oxidation oxidation temperature temperature of of coal is belowbelow 100100 ◦°CC,, physicalphysical inhibitorinhibitor cancan absorb absorb waterwater from coal coal and and form form an an inhibitory inhibitory film film on the on surface the surface of coal of that coal prevents that prevents coal oxidation. coal oxidation. Ascorbic Ascorbicacid dispersed acid dispersed in the three-dimensional in the three-dimensional structure ofstructure the polymer of the can polymer be decomposed can be decomposed into hydrogen into hydrogenions and reactiveions and hydroxyl reactive groups.hydroxyl The groups. effect The of physical effect of inhibitor physical is inhibitor weakened is weakened when the oxidation when the oxidationtemperature temperature exceeds 100 exceeds◦C. Hydrogen 100 °C. Hydrogen ions decomposed ions decomposed from chemical from inhibitor chemical can inhibitor react with can reactalkoxyl with and alkoxyl peroxide and radicals peroxide produced radicals by produced oxidation by of aliphaticoxidation coal of moietiesaliphatic to coal form moieties hydroperoxide to form hydroperoxide(R-OOH) and destroy (R-OOH) free and radicals destroy to prevent free radicals further to oxidation prevent offurther aliphatic oxidation groups of to aliphatic CO or CO groups2. These to COhydroperoxides or CO2. These are then hydroperoxides readily decomposed are then into readilyalcohols anddecomposed H2O. Simultaneously, into alcohols the and hydroxyl H2O. Simultaneously,groups of the inhibitor the hydroxyl react with groups the hydroxylof the inhibitor groups react of the with coal the to formhydroxyl highly groups stable of ether the bonds,coal to formwhich highly deactivate stable the ether oxygen bonds, groups. which deactivate the oxygen groups. ZhangZhang [58,59 [58,59]] and and others combinedcombined gel gel and and foam foam to to form form a foam a foam gel. gel. The The effects effects of thickeners of thickeners and andcrosslinking crosslinking agents agents on theon foamthe foam gel weregel were discussed. discussed. The gelationThe gelation time time was foundwas found to be to reduced be reduced and andthe thermalthe thermal stability stability improved improved as compared as compared with conventional with conventional foam. Thickeners foam. and Thickeners crosslinking and crosslinkingagents are polymers. agents are Subsequently, polymers. Subsequently, the self-made foamingthe self- agentmade wasfoaming mixed agent with thewas two mixed polymers with the to twoform polymers foam gel to by form air foaming. foam gel Theby air apparent foaming. viscosity The apparent of the gel viscosity decreased of the with gel the decreased increase inwith shear the increaserate, which in shear is consistent rate, which with theis consistent law of shear with thinning the law of of fluids. shear For thinning high shear of fluids. rates, thickenersFor high s andhear rates,crosslinking thickeners agents and docrosslinking not occur cross-linkingagents do not reactions occur cross in the-linking foam reactions film, which in the can foam satisfy film, the which wide canrange satisfy of flow the andwide di rangeffusion of inflow goaf. and It diffusion was shown in thatgoaf. 0.4% It was foaming shown agent, that 0.4% 0.3% foaming thickener agent, and0.3% 0.3% thickenercrosslinking and agent 0.3% could crosslinking obtain a agent foam couldgel with obt highain a foaming foam gel ratio, with strong high stability foaming and ratio, good strong fire resistance (Figure 14). Subsequently, at this concentration, the effects of the foaming ratio, addition of stability and good fire resistance (Figure 14). Subsequently, at this concentration, the effects of the salts (NaCl, CaCl2 and AlCl3), pH value and temperature on the rheological properties of the foam gel foaming ratio, addition of salts (NaCl, CaCl2 and AlCl3), pH value and temperature on the were discussed [60]. The fire extinguishing time of thickening agent and crosslinking agent mixture at rheological properties of the foam gel were discussed [60]. The fire extinguishing time of thickening different concentrations was also examined [61]. agent and crosslinking agent mixture at different concentrations was also examined [61].

Figure 14. The foamed gel prepared in lab: (a) Foaming; (b) fluidity; (c) film forming ability [59]. Figure 14. The foamed gel prepared in lab: (a) Foaming; (b) fluidity; (c) film forming ability [59]. 3.2.3. Three-Phase Foam 3.2.3. Three-Phase Foam Three-phase foam consists of non-combustible solid matter (loess or fly ash), inert gas (nitrogen) andThree water- andphase has foam good flowability. consists of Compared non-combustible with mortar solid and matter gel, the (loess coverage or of fly fire ash), extinguishing inert gas (nitrogen)is wider [ 62and] and water the and fire-extinguishing has good flowability. methods Compared are diverse. with Qin mortar and others and gel, [63 ]the studied coverage the eofff ectfire extinguishingof fly ash on the is wider performance [62] and of the three-phase fire-extinguishing foam. Figure methods 15 shows are a diverse. three-phase Qin foamand others structure [63] studiedmodel. the It is effect suggested of fly thatash on the the addition performance of a surfactant of three (SDBS)-phase canfoam. promote Figure the15 s hydrophilichows a three groups-phase foamto be structure bound to model.the surface It is of suggested fly ash and that the the hydrophobic addition of tails a surfactant enter into the (SDBS) water. can This promote results the in hydrophilic groups to be bound to the surface of fly ash and the hydrophobic tails enter into the water. This results in the transformation of hydrophilicity of fly ash to hydrophobicity and makes it easier to separate from water and form a three-phase foam stable structure. The addition of fly ash increases the retardation of water flow, slows down the drainage and makes the material exhibit excellent static (half-life of foam volume) and dynamic stability (viscosity changes with the shear rate). In addition, adding polymer carboxymethyl cellulose (CMC) can make the foam more stable.

Polymers 2019, 11, 1706 13 of 30 the transformation of hydrophilicity of fly ash to hydrophobicity and makes it easier to separate from water and form a three-phase foam stable structure. The addition of fly ash increases the retardation of water flow, slows down the drainage and makes the material exhibit excellent static (half-life of foam volume) and dynamic stability (viscosity changes with the shear rate). In addition, adding polymer Polymerscarboxymethyl 2019, 11, x FOR cellulose PEER REVIEW (CMC) can make the foam more stable. 13 of 30

FigureFigure 15. 15. AA structure structure model model of of three three-phase-phase foam to attach flyfly ashash (FA)(FA) particlesparticles atat thethe N2-waterN2-water interfacesinterfaces by by tuning tuning their their surface surface-wetting-wetting properties: (a) SchematicSchematic illustrationillustration ofof thethe stabilization stabilization of of gasgas bubbles bubbles with flyfly ashash particles; particles; (b ()b the) the adsorption adsorption of partiallyof partially lyophobic lyophobic FA particles FA particles at the gas–liquidat the gas– liquidinterface, interface, illustrating illustrating the balance the balance in tension in tension responsible responsible for the attachment for the attachment of particles; of p (carticles;) a possible (c) a possibleapproach approach used to tune used the to wetting tune propertiesthe wetting of originallyproperties hydrophilic of originally particles hydrophilic by the absorbed particles sodium by the absorbeddodecyl benzenesodium sulfonatedodecyl benzene (SDBS) moleculessulfonate ( [S63DB]. S) molecules [63].

OtherOther researchers have have considered considered the the advantages advantages of mortar, of mortar, inert inert gas, gelgas, and gel three-phase and three- foamphase and developed comprehensive fireproofing soft matters. Zhang [64] and others synthesized a type of foam and developed comprehensive fireproofing soft matters. Zhang [64] and others synthesized a multiphase foam gel composite materials using fly ash, a foaming agent, a thickener, a crosslinking type of multiphase foam gel composite materials using fly ash, a foaming agent, a thickener, a agent and nitrogen. The thickener is an anionic polysaccharide produced by the fermentation from crosslinking agent and nitrogen. The thickener is an anionic polysaccharide produced by the xanthomonas campestris. The crosslinker is a galactomannan polysaccharide and belongs to polymers. fermentation from xanthomonas campestris. The crosslinker is a galactomannan polysaccharide and The two reactions form a three-dimensional network structure and build a foam gel rigid skeleton, belongs to polymers. The two reactions form a three-dimensional network structure and build a which has good water absorption, water retention and foam stability. As shown in Figure 16, the foam gel rigid skeleton, which has good water absorption, water retention and foam stability. As bubble distribution of polyphase foam gel without polymers or a single polymer is chaotic, the size shown in Figure 16, the bubble distribution of polyphase foam gel without polymers or a single dispersion is large; the difference between the maximum and minimum foam diameter is almost 10 fold polymer is chaotic, the size dispersion is large; the difference between the maximum and minimum and the foam liquid film is thinner. The addition of thickeners and crosslinking agents has a uniform foam diameter is almost 10 fold and the foam liquid film is thinner. The addition of thickeners and size and the liquid film is thicker than before. When the foam bursts, the three-dimensional structure crosslinking agents has a uniform size and the liquid film is thicker than before. When the foam can still ensure the fly ash and slurry aggregate together form a cladding, with high strength and a bursts, the three-dimensional structure can still ensure the fly ash and slurry aggregate together certain degree of flexibility. It can compactly cover and pack coal, plug cracks and continuously and form a cladding, with high strength and a certain degree of flexibility. It can compactly cover and effectively prevent coal from absorbing oxygen, thus preventing coal oxidation. pack coal, plug cracks and continuously and effectively prevent coal from absorbing oxygen, thus preventing coal oxidation.

Polymers 2019, 11, 1706 14 of 30 Polymers 2019, 11, x FOR PEER REVIEW 14 of 30

FigureFigure 16. The 16. microstructureThe microstructure maps maps of multi-phase of multi-phase foamed foamed gel: ( a gel:) Without (a) Without thickener thickener and crosslinker; and (b) onlycrosslinker; with thickener; (b) only with (c) only thickener; with crosslinker;(c) only with (crosslinker;d) both thickener (d) both andthickener crosslinker and crosslinker [64]. [64].

3.2.4.3.2.4. Coating Coating Slurry Slurry ComparedCompared with with the suspensionthe suspension ability ability of mortar, of mortar, the the thermal thermal stability, stability, expansion expansion property, property, water retentionwater of retention the gel, of the the foamability, gel, the foamability, stability stability and fluidity and fluidity of the three-phaseof the three-phase foam, foam, the coating the coating material focusesmaterial on air focuses tightness on and air tightness mechanical and properties. mechanical Phenol-formaldehyde properties. Phenol-formaldehyde foam [65] is foam widely [65] usedis as an effwidelyective used material as an for effective filling material and blocking for filling air and leakages blocking due air to leakages its heat due resistance, to its heat flame resistance, retardance, goodflame sealing retardance, and construction good sealing convenience. and construction Hu [66] and convenience. others improved Hu [66] phenol-formaldehyde and others improved foam phenol-formaldehyde foam by using 1 mol phenol, 0.5 mol urea, 3 mol paraformaldehyde, 0.05 mol by using 1 mol phenol, 0.5 mol urea, 3 mol paraformaldehyde, 0.05 mol catalyst, for 3 h at 75 ◦C to catalyst, for 3 h at 75 °C to synthesize phenol-urea-formaldehyde (PUF) composite foam sealing synthesize phenol-urea-formaldehyde (PUF) composite foam sealing material. It was shown that material. It was shown that PUF foam had high size stability, foaming ability and oxygen index. At PUF foam had high size stability, foaming ability and oxygen index. At the same time, polyethylene the same time, polyethylene glycol (PEG) was used to improve the toughness and strength of the glycolPUF (PEG) foam was [67]. used PEG to decreased improve the the foam toughness diameter, and slightly strength decreased of the PUF the thermal foam [67 stability]. PEG of decreased the the foamfoam, diameter, but thickened slightly the foam decreased film. Subse thequently, thermal the stability effect of of surfactants the foam, (silicone but thickened oil and Tween the foam-80) film. Subsequently,on PUF foam the was effect considered of surfactants [68]. Hu (silicone [69] then oil optimized and Tween-80) PUF foam on formulation PUF foam and was compared considered to [68]. Hu [69commonly] then optimized used polymer PUF foam foam (polyurethane formulation foam, and phenolic compared foam to and commonly urea formaldehyde used polymer foam). foam (polyurethaneThe composite foam, foam phenolic had the foamgreatest and compressive urea formaldehyde strength, thermal foam). stability, The compositeflame retardance foam and had the greatestminimum compressive foaming strength, temperature thermal and shrinkage stability, flamerate, demonstrating retardance and excellent minimum sealing foaming performance. temperature and shrinkagePolyurethane rate, demonstrating is also an important excellent polymer, sealing with performance. many airtight coating materials having been derived from it. Polyurethane elastomer (PUE) was first used as an air leakage sealant in roadways. Polyurethane is also an important polymer, with many airtight coating materials having been Wu et al. [70] added surface modified TiO2 and SiO2 nanoparticles to obtain reinforced and derived from it. Polyurethane elastomer (PUE) was first used as an air leakage sealant in roadways. toughened PUE nanocomposites. Surface modified nanoparticles were evenly dispersed in the PUE Wu etmatrix, al. [70 which] added improved surface its modified thermal TiOstability2 and and SiO flame2 nanoparticles retardancy. toThe obtain dual action reinforced of the anddispersion toughened PUE nanocomposites.system and hydrogen Surface bond modified in PUE nanoparticles improves the were mechanical evenly propertiesdispersed in of the the PUE material. matrix, The which improvedminimum its thermal elongation stability at break and is flame more retardancy. than 600%. The The minimum dual action tensile of the strength dispersion of the system tested and hydrogenmaterial bond is 2.45 in PUE MPa improves and the maximum the mechanical is 3.21 MPa. properties The leakage of the rate material. decreased The by minimum 90%. Figure elongation 17 at breakillustrates is more a than practical 600%. application The minimum diagram. tensile Hu strength et al. of [ the71] tested also obtained material is flame 2.45-retar MPadant and the maximum is 3.21 MPa. The leakage rate decreased by 90%. Figure 17 illustrates a practical application diagram. Hu et al. [71] also obtained flame-retardant polyisocyanurate–polyurethane foam (PIR–PUR) by adding expandable graphite (EG) and dimethyl methyl phosphonate (DMMP). Polymers 2019, 11, x FOR PEER REVIEW 15 of 30 polyisocyanurate–polyurethane foam (PIR–PUR) by adding expandable graphite (EG) and dimethyl methyl phosphonate (DMMP).

Polymers 2019, 11, x FOR PEER REVIEW 15 of 30

Polymers 2019, 11, 1706 15 of 30 polyisocyanurate–polyurethane foam (PIR–PUR) by adding expandable graphite (EG) and dimethyl methyl phosphonate (DMMP).

Figure 17. Practical application of modified polyurethane elastomer (PUE) in a coal mine [70].

Past literature has also explored polymer emulsions other than polyurethane. Song et al. [72] first mixed styrene-acrylic emulsion with water and then added silica powder, Portland cement and flame retardant solid powder to synthesize a new sealing material. The results showed that the sealing material with 11 wt.% complex flame retardants (composed of aluminum hydroxide and chlorinated paraffin at a ratio of 3:8) had good performance when the retardants were. The tensile strength of this gas sealingFigureFigure 17. 17. material PracticalPractical application application exceeds of of modified modified 2.25 p polyurethaneMPaolyurethane and elastomer the flame (PUE) in resistanta coal mine [ 70 ]property].. conformed to the safety standards of coal mines. Twardowska [73] has long shown that full mixing of fly ash and Past literature has also explored polymer emulsions other than polyurethane. Song Song et et al al.. [ 72]] water has a higherfirstfirst mixed air barrier styrene styrene-acrylic-acrylic effect emulsion than with natural water and and sealing then then added added materials. silica silica powder, powder, Su Portland [74] cement first and mixed cement and flame retardant solid powder to synthesize a new sealing material. The results showed that the ethylene-vinyl acetateflame retardant (EVA) solid copolymer powder to synthesize emulsion a new sealing with material. five The kinds results of showed industrial that the waste powder sealing material material with with 11 11 wt wt.%.% complex complex flame flame retardants retardants ( (composedcomposed of of aluminum aluminum hydroxide and separately and thenchlorinated added paraffin para waterffin at a ratioratioand ofof defoamer 3:8)3:8) hadhad goodgood performanceperformanceto synthesize whenwhen thethe five retardantsretardants kinds were.were. of The fireproof tensile materials. The strength of this gas sealing material exceeds 2.25 MPa and the flame resistant property conformed five types of industrialstrength of thiswastes gas sealing included material exceeds fly ash 2.25 MPa(CFA), and t heconstructio flame resistant nproperty and conformeddemolition to wastes (C&D theto the safety safety standards standards of coal of coal mines. mines. Twardowska Twardowska [73] [has73] haslong long shown shown that thatfull mixing full mixing of fly of ash fly and ash waste), ferronickelwaterand slag water has (FNS),a has higher a higher air steel barrier air barrierslag effect e(SS) ffthanect thannaturaland natural red sealing sealingmud materials. materials.slag Su (RS). [74 Su] first [74In] firstmixedthe mixed solution, cement cement and EVA polymer is and ethylene-vinyl acetate (EVA) copolymer emulsion with five kinds of industrial waste powder adsorbed on the ethylenesurface-vinyl of acetateinorganic (EVA) copolymerparticles. emulsion As cement with five hydration kinds of industrial gradually waste powder consumes interstitial separately and thenthen addedadded water water and and defoamer defoamer to to synthesize synthesize five five kinds kinds of fireproofof fireproof materials. materials. The The five water, the consolidationftypesive types of industrial of ofindustrial polymer wastes wastes included particlesincluded fly ash fly (CFA), ashbegins (CFA), construction toconstructio form and demolitionninterlaced and demolition wastes membranes, wastes (C&D waste),(C&D fill pores and ferronickel slag (FNS), steel slag (SS) and red mud slag (RS). In the solution, EVA polymer is adsorbed cracks and wrapwaste), hydration ferronickel slag products (FNS), steel slag and (SS) aggregatesand red mud slag [(RS).75]. In Thisthe solution, organic EVA polymer network is ensures the adsorbedon the surface on the of surface inorganic of inorganic particles. Asparticles. cement As hydration cement hydration gradually gradually consumes consumes interstitial interstitial water, the compactness andwater,consolidation overall the consolidation microstructure of polymer particlesof polymer begins ofparticles tosealing form begins interlaced materials. to form membranes, interlaced The fill membranes, porespermeability and cracks fill pores and test wrap and and mechanical hydration products and aggregates [75]. This organic network ensures the compactness and overall property test showedcracks andthat wrap the hydrationfive materials products andhave aggregates 3–4 orders [75]. This of organicmagnitude network less ensures permeability the than the compactnessmicrostructure and of overall sealing microstructure materials. The permeabilityof sealing materials. test and The mechanical permeability property test testand showedmechan thatical traditional materialspropertythe five and materials test showedhave have that good 3–4 the orders five tensile materials of magnitude strength. have less3–4 permeabilityorders On of magnitude the than one the less traditional hand, permeability materialsthey than depend andthe on the high have good tensile strength. On the one hand, they depend on the high pozzolanic activity of the waste pozzolanic activitytraditional of the materials waste and materials; have good tensile On the strength. othe Onr thehand one , hand, they they are depend affected on the by high the film-forming pozzolanicmaterials; On activity the other of the hand, waste they materials; are affected On bythe the othe film-formingr hand, they properties are affected of polymersby the film (Figure-forming 18). properties of polymerspropertiesThe air permeability (ofFigure polymers coe 18). (Figurefficient, The 18). tensile airThe strengthairpermeability permeability and elongation coefficient, coefficient, at break tensile of thestrength tensile best flyand ash strengthelongation products and elongation 8 2 were 4.17 10 m /s, 2.14 MPa and 48.90%, respectively.−8 2 at break of the bestat break fly ofash× the −bestproducts fly ash products were were 4.17 4.17 × ×10 10−8 m m/s,2 /s,2.14 2.14 MPa and MPa 48.90% and, respectively. 48.90% , respectively.

Figure 18. SEM image of fireproof material: (a) Hydrated product; (b) polymer film [74].

A. Tosun [76] showed that the polymer was a material with low oxygen permeability, thus the oxygen permeability of fiber reinforced polymer was studied and an epoxy resin/glass fiber composite with very low oxygen permeability and high mechanical resistance was developed. The Figure 18. SEM image2 of fireproof material: (a) Hydrated product; (b) polymer film [74]. Figure cost18. ofSEM this material image (80 of g/m fireproof) was $0.124 material:–0.209. (a) Hydrated product; (b) polymer film [74].

A. Tosun [76] showed that the polymer was a material with low oxygen permeability, thus the oxygen permeability of fiber reinforced polymer was studied and an epoxy resin/glass fiber composite with very low oxygen permeability and high mechanical resistance was developed. The cost of this material (80 g/m2) was $0.124–0.209.

Polymers 2019, 11, 1706 16 of 30

A. Tosun [76] showed that the polymer was a material with low oxygen permeability, thus the oxygen permeability of fiber reinforced polymer was studied and an epoxy resin/glass fiber composite with very low oxygen permeability and high mechanical resistance was developed. The cost of this material (80 g/m2) was $0.124–0.209.

3.3. Polymer-Based Soft Matters in Mine Gas Drainage Gas explosion is one of the main disasters in coal mines, gas outburst has become more frequent in recent years [77]. Gas drainage by drilling is one of the most effective methods to prevent and control gas disasters and the efficiency of gas drainage largely depends on the sealing effect of the drillings. In the past, inorganic sealing materials mainly composed of cement mortar, cement fly ash, cement water glass and other cement-based soft matters were used. While the cement-based soft matters have a low cost and a wide range of sources, it is easy to precipitate during solidification and the volume shrinkage rate is typically more than 15% [78]. Moreover, the poor adhesion of cement-based materials can easily lead to the separation of pore walls and the formation of gas leakage channels [79], so their application is not ideal. In order to improve the sealing quality, polymer-containing sealing soft matters based on cement, fly ash and other basic raw materials have been extensively studied. Xiang et al. [80] synthesized pore-sealing material with fly ash as the basic material, water-retaining agent, expanding agent, cellulose, resin, coupling agent and silicate. Compared with the traditional sealing materials (expansive cement, polyurethane and mucus), new material had good compactness, stability, fluidity and permeability. With the increase of water cement ratio, the stability of the material decreased, and the fluidity and permeability increased. Zhou et al. [81] produced a sealing material: The mass ratios of dispersant, expansive agent, polymer to cement were 1.2%, 0.1% and 6%, respectively. The ratio of water to cement was 1.5:1 in application. The results indicated that with the increase of polymer content, the flexural strength increased, the compressive strength decreased and the density increased after curing. This was due to the polymer being interwoven into a three-dimensional network structure, filled in to the slurry, enhancing the flexibility of the material, while permeating with cement to form a bridge, improving the material structure. Polyurethane is the primary organic sealing material in gas sealing due to its fast curing speed, large expansion rate and excellent adhesion. Therefore, polyurethane-containing soft matters are also of consideration. Zhai [82] and others used cement, expansion agent, polyurethane resin, fibrin and coupling agent to synthesize a sealing material. However, the expanded polyurethane formed a cavity array, the diameter of the cavity was 700–1000 µm and there were pore space connections between the cavities. The overall air tightness of the sealing material was unsatisfactory. Subsequently, Zhai [83] combined polyurethane and fly ash to develop a flexible gel (FG) suitable for borehole deformation and excellent sealing performance. The basic components were fly ash, water retention agent, bulking agent, cellulose, resin, coupling agent and silicate. When the mixed materials began to solidify, the bulking agent fills the gaps between fly ash particles. Cellulose has the ability to retain moisture, bind to the borehole wall and increase the viscosity. The coupling agent was a type of plastic additive, which could improve the interfacial properties between synthetic resin and inorganic filler. The reactive epoxy groups in molecular structure of resin contains can form a three-dimensional polymer mesh structure. The silicate can produce a branched silicone, long-chain, mesh structure in the gel system, which has the synergies for increasing the gel strength. The material had strong adaptability to borehole deformation, good sealing and good water retention (Figure 19). It was found that the material and water mixed well in a low proportion and the sealing property was the best. Polymers 2019, 11, 1706 17 of 30 Polymers 2019, 11, x FOR PEER REVIEW 17 of 30

FigureFigure 19. 19. SealingSealing mechanism mechanism of of the flexibleflexible gel gel (FG) (FG) forfor a aborehole: borehole: (a ()a) Borehole Borehole deformation deformation adaptability;adaptability; ( (bb)) h highigh permeability; permeability; ((c)) preferablepreferable waterwater retention; retention; ( d(d) hydrophobicity) hydrophobicity [83 [83]. ].

As far as polyurethane is concerned, it is not easy to diffuse around boreholes due to its high As far as polyurethane is concerned, it is not easy to diffuse around boreholes due to its high viscosity. The high-pressure injection results in the excessive permeation of mucus into the pores, viscosity. The high-pressure injection results in the excessive permeation of mucus into the pores, which leads to a higher cost. Yang et al. [84] used clay, water retention agent, expansion agent and which leads to a higher cost. Yang et al. [84] used clay, water retention agent, expansion agent and cellulose to obtain sealing material. Compared with cement mortar, it had superior expansibility and cellulose to obtain sealing material. Compared with cement mortar, it had superior expansibility and good water retention ability within 30 h. Li et al. [85] used coal dust as a filler, amino resin as a binder good water retention ability within 30 h. Li et al. [85] used coal dust as a filler, amino resin as a and other additives to synthesize a composite polymer material (CP). Amino resin is a linear polymer, binder and other additives to synthesize a composite polymer material (CP). Amino resin is a linear which connects coal particles to play a role in stress transfer. Active groups such as carbonyl, amino and polymer, which connects coal particles to play a role in stress transfer. Active groups such as subamino groups on resin molecules have strong binding properties, which can enhance the cohesion carbonyl, amino and subamino groups on resin molecules have strong binding properties, which can between coal particles and the resin matrix. In addition, resin molecules contain many hydrophilic enhancegroups, suchthe cohesion as ether bonds between and coal hydroxyl particles groups, and whichthe resin can matrix. be combined In addition, with water resin molecules molecules contain many hydrophilic groups, such as ether bonds and hydroxyl groups, which can be to form a stable dispersion system. –CH2OH, –NH– and other active groups in the resin molecule combinedallow the with polycondensation water molecules reaction to form to forma stable a three-dimensional dispersion system. network –CH2OH structure, –NH– inand the other presence active grouof crosslinkingps in the resin agents. molecule Therefore, allow coal the dust polycondensation particles can be encapsulated reaction to tightly form a in three the resin-dimensional matrix networkwith specific structure mechanical in the strength. presence The of crosslinking toughening agent agents. in the Therefore, additives coal is also dust a linearparticles polymer, can be encapsulatedwhich reacts withtightly the in resin the moleculeresin matrix under with acidic specific conditions mechanical and is stren embeddedgth. The in thetoughening macromolecular agent in thechain additives of the resinis also to a enhance linear polymer, the toughness which and reacts ageing with resistance the resin ofmolecule the material. under The acidic product conditions met andthe requirementsis embedded ofin lowthe pressuremacromolecular grouting chain and the of pulverizedthe resin to coal enhance particles the could toughness fill the and holes ageing and resistancemacro cracks, of the having material. a good The sealing product effect. met Furthermore, the requirements the material of had low strong pressure compressive grouting strength and the pulverizedand plastic coal deformation particles characteristics could fill the and holes could and adapt macro to borehole cracks, deformation having a good to a certain sealing extent effect. Furthermore,(Figure 20). the material had strong compressive strength and plastic deformation characteristics and could adapt to borehole deformation to a certain extent (Figure 20).

Polymers 2019, 11, 1706 18 of 30 Polymers 2019, 11, x FOR PEER REVIEW 18 of 30

FigureFigure 20. Sealing performance performance of of composite composite polymer polymer material material (CP): (CP (a)): High (a) High fluidity fluidity and super and sealing super ability; (b) good combination with coal mass; (c) adapt to the deformation of borehole [85]. sealing ability; (b) good combination with coal mass; (c) adapt to the deformation of borehole [85]. Zhang et al. [86] also used coal dust as basic material and used amino resin as binder, combined Zhang et al. [86] also used coal dust as basic material and used amino resin as binder, combined with other additives to synthesize a new composite sealing material. Its maximum penetration radius with other additives to synthesize a new composite sealing material. Its maximum penetration was 36 mm and it could be closely combined with coal. At the same time, it could also pass through radius was 36 mm and it could be closely combined with coal. At the same time, it could also pass cracks of 10 µm aperture and combine with coal, having a good sealing effect. Ge et al. [87] used through cracks of 10 μm aperture and combine with coal, having a good sealing effect. Ge et al. [87] Portland cement, early strength water reducer and polypropylene fibers to synthesize new hydraulic used Portland cement, early strength water reducer and polypropylene fibers to synthesize new fracturing sealing materials, in which polypropylene fibers were used to increase the density of slurry, hydraulic fracturing sealing materials, in which polypropylene fibers were used to increase the enhance the crack resistance and impact resistance of materials. At the same time, the slurry viscosity density of slurry, enhance the crack resistance and impact resistance of materials. At the same time, was increased, the shrinkage was reduced and the sealing property was enhanced. the slurry viscosity was increased, the shrinkage was reduced and the sealing property was enhanced.3.4. Polymer-Containing Soft Matters in Mine Roadway Support and Repair

3.4. PolymerAnchorage-Containing is one ofSoft the M keyatters technologies in Mine Roadway to ensure Support the safetyand Repair of coal mine and is used to protect from mine roof falls. The application of polymer-containing soft matters in controlling and repairing surroundingAnchorage rocks is one of of roadway the key istechnologies increasing to of ensure late: Stabilizing the safety of weak coal areasmine ofand rock, is used soil to and protect coal; from mine roof falls. The application of polymer-containing soft matters in controlling and repairing stabilizing areas where cracks exist; filling cracks, pores, etc. [88]. Murphy investigated a shale surrounding rocks of roadway is increasing of late: Stabilizing weak areas of rock, soil and coal; broken roof, indicating that polyurethane injection in the roof can chemically bond with rock mass stabilizing areas where cracks exist; filling cracks, pores, etc. [88]. Murphy investigated a shale and strengthen the roof strength [89]. For materials used in roadway support and repair, anchor net broken roof, indicating that polyurethane injection in the roof can chemically bond with rock mass shotcrete and liquid accelerator are classified as Polymer additive soft matters, resin anchorage agent and strengthen the roof strength [89]. For materials used in roadway support and repair, anchor net and repair grouting material are classified as polymer-based soft matters. shotcrete and liquid accelerator are classified as Polymer additive soft matters, resin anchorage agent and repair grouting material are classified as polymer-based soft matters.

PolymersPolymers 20192019, ,1111, ,x 1706 FOR PEER REVIEW 1919 of of 30 30

3.4.1. Anchor Net Shotcrete 3.4.1. Anchor Net Shotcrete For anchoring and shotcreting technology in a bolt-mesh support system, Australian researchersFor anchoring have been and examining shotcreting effective technology materials in a bolt-mesh to replace support anchor system,net support. Australian Lukey researchers et al. [90] indicatedhave been that examining the thin eff ective spray materials lining formed to replace by anchor spraying net support. two commonly Lukey et used al. [90 material] indicated types that (crosslinkingthe thin spray polyurethane lining formed- byor spraying polyurea two-based commonly systems used, and material cement types-reinforced (crosslinking water polyurethane--dispersible systemsor polyurea-based based on ethylene systems,-vinyl and cement-reinforcedacetate copolymer) water-dispersible could provide auxiliary systems basedsupport on for ethylene-vinyl the anchor net.acetate Meanwhile, copolymer) Nemcik could et provide al. [91] auxiliary developed support a powerful for the fiber anchor reinforced net. Meanwhile, polymer Nemcikfor spraying et al. the [91] topdeveloped and surface a powerful of roadways, fiber reinforced indicating polymer that for the spraying material the could top and be surface solidified of roadways, in a few indicating seconds, formingthat the materiala polymer could skin be that solidified could inadhere a few seconds,well to the forming rock/coal a polymer surface skin and that provide could adhereresistance well t too stratumthe rock displacement/coal surface and and provide fracture resistance opening (Figure to stratum 21). displacement In order to prove and fracture that the opening lining formed (Figure by21). fiberglassIn order to polymer prove that is a the suitable lining substitute formed by for fiberglass anchor polymernet, Nemcik is a suitablealso tested substitute the bearing for anchor capacity net, [Nemcik92] and alsoshear tested resistance the bearing [93] of capacity the thin [ 92spray] and polyme shear resistancer lining in [93 2011] of theand thin 2013, spray respectively. polymer lining The resultsin 2011 showed and 2013, that respectively. this kind The of results material showed had high that this compressive kind of material strength had and high displacement compressive resistance,strength and which displacement was not inferior resistance, to the which performance was not of inferior anchor to net the and performance was better ofin anchorsome aspects. net and Shanwas better et al. in[94 some] also aspects. tested andShan compared et al. [94] alsothe testedyield resistance and compared of two the kinds yield resistanceof fiberglass of two polymer kinds commonlyof fiberglass used polymer as thin commonly spraying used lining as thinand sprayingsteel mesh lining to fully and understand steel mesh to their fully failure understand modes their in Australianfailure modes mines. in Australian mines.

FigureFigure 21. 21. ShearingShearing and and dilation dilation mechanisms mechanisms of of fracture fracture sur surfaceface [ [9191]]..

Unlike Australia, in view of the complexity of China’s mines, the metal mesh bolt support is still Unlike Australia, in view of the complexity of China’s mines, the metal mesh bolt support is commonly used in coal mines. Only in stratified mining, some mines will use high-strength polyester still commonly used in coal mines. Only in stratified mining, some mines will use high-strength fiber flexible mesh as false roof instead of the metal mesh [95]. Wet shotcrete, as a synergistic anchor net polyester fiber flexible mesh as false roof instead of the metal mesh [95]. Wet shotcrete, as a support way, is very popular in Chinese mines. In order to improve the low compressive strength, weak synergistic anchor net support way, is very popular in Chinese mines. In order to improve the low ductility and easy cracking of concrete itself, Qi [96] and others discussed the application of inorganic compressive strength, weak ductility and easy cracking of concrete itself, Qi [96] and others fiber concrete in coal mine roadways, which was based on cement slurry, mortar or concrete and was discussed the application of inorganic fiber concrete in coal mine roadways, which was based on synthesized by adding metal materials, inorganic fibers or reinforcing materials. Fiber reinforced cement slurry, mortar or concrete and was synthesized by adding metal materials, inorganic fibers polymer has been used in concrete due to its light weight, high strength, corrosion resistance and or reinforcing materials. Fiber reinforced polymer has been used in concrete due to its light weight, fatigue resistance. A series of cement-polymer composite soft matters have been developed [97]. high strength, corrosion resistance and fatigue resistance. A series of cement-polymer composite soft Xu et al. [98] applied chemical fiber wet shotcrete (Figure 22) in mine field. Polypropylene fibers and matters have been developed [97]. Xu et al. [98] applied chemical fiber wet shotcrete (Figure 22) in steel fibers were added to concrete. The length of polypropylene fibers was 30 mm, the density was mine field. Polypropylene fibers and steel fibers were added to concrete. The length of 0.91 g/cm3, the tensile strength was 500 MPa, the tensile elasticity modulus was 7 GPa, the elongation polypropylene fibers was 30 mm, the density was 0.91 g/cm3, the tensile strength was 500 MPa, the at break was 8% and the equivalent diameter was 0.6 mm. The results showed that chemical fibers tensile elasticity modulus was 7 GPa, the elongation at break was 8% and the equivalent diameter increased the toughness and tensile strength of concrete to a certain extent. After three months of was 0.6 mm. The results showed that chemical fibers increased the toughness and tensile strength of concreteroadway to excavation, a certain extent. there wasAfter no three cracking months of slurryof roadway skin and excavation, the wet sprayingthere was e ffnoect cracking of concrete of slurrywas significant. skin and the wet spraying effect of concrete was significant.

Polymers 2019, 11, 1706 20 of 30 Polymers 2019, 11, x FOR PEER REVIEW 20 of 30

Figure 22. Wet shotcreting process of chemical fiber reinforced concrete. Figure 22. Wet shotcreting process of chemical fiber reinforced concrete. 3.4.2. Liquid Accelerator 3.4.2. Liquid Accelerator In order to accelerate the consolidation of wet shotcrete soft matters, research on liquid accelerators has alsoIn order attracted to much accelerate attention. the consolidation Han et al. [99 ] of developed wet shotcrete a JL-1 low-alkali soft matters, liquid research accelerator, on liquid using acceleratorssulphoaluminate has alsoand neutralattracted sodium much salts attention. as the main Han accelerators. et al. [99] developed Polymer polyacrylamide a JL-1 low-alkali was liquid used accelerator,to optimize the using accelerator sulphoaluminate and improve and its neutral cohesiveness. sodium The salts eff ect as of the the main accelerator accelerators. showed Polymer that the polyacrylamideinitial setting time was was used 2–4 to min optimize and the the final accelerator setting time and was improve 6–10 min.its cohesiveness. The strength The of 28 effect d concrete of the acceleratordoes not decrease showed but that increases the initial slightly. setting Zhoutime was [100 2]– and4 min others and the also final developed setting atime liquid was accelerator 6–10 min.

Theconsisting strength of 55%of 28 aluminium d concrete sulfate does octadecahydrate,not decrease but 4%increases sodium slightly. fluoride, Zhou 2.5%[ triethanolamine,100] and others 0.5%also polyacrylamide,developed a liquid 5% bentoniteaccelerator and consistin 33% water.g of 55% Polymer aluminium polyacrylamide sulfate octadecahydrate, was also used as 4%an additive. sodium Thefluoride, good bonding 2.5% triethanolamine, properties of the 0.5% polymer polyacrylamide, improved the 5% cohesion bentonite of cement and slurry 33% water. and locks Polymer water, polyacrylamidehelping the solidification was also used components as an additive. shorten The the good solidification bonding time. properties of the polymer improved the cohesion of cement slurry and locks water, helping the solidification components shorten the solidification3.4.3. Resin Anchorage time. Agent Bolt support is another important support method. Resin anchorage agent is widely used in bolt 3.4.3. Resin Anchorage Agent support [101]. Every year, resin anchorage is used in approximately 100 million rock anchors installed in minesBolt insupport the United is another States important [102]. Among support them, method. resin Resin materials anchorage are mainly agent unsaturated is widely used polyester in bolt supportresin, epoxy [101 resin]. Every and year,polyurethane resin anchorage resin. Unsaturated is used in polyester approximately resin (UPR) 100 millionanchorage rock material anchors is installeda bonding in material mines in which the United is made States up of [102 UPR,]. Among catalyst them, and marbleresin materials powder inare a mainly specific unsaturated proportion. polyesterHan et al. resin, [103] epoxy subsequently resin and added polyurethane steel sand resin. and steelUnsaturated balls into polyester anchorage resin material. (UPR) anchorag After thee pull-outmaterial test,is a bonding it was shown material that which the size is made and quantity up of UPR, of steel catalyst particles and marble can improve powder the in anchorage a specific performanceproportion. Han of anchorage et al. [103 material] subsequently to a certain added extent. steel Su sand et al. and [104 steel] obtained balls theinto unsaturated anchorage polyestermaterial. Afterresin (UPR)the pull anchorage-out test, materialit was shown by mixing that the UPR size and and CaCO quantity3 powder of steel in aparticles ratio of 5:1can and improve adding the a anchoragesuitable amount performance of accelerator. of anchorage The thermal material stability to ofa certain UPR anchorage extent. Su material et al. in[10 high4] obtained temperature the unsaturatedenvironments polyester facing deep resin mining (UPR) anchorage was studied. material A study by bymixing Wang UPR [105 and] examining CaCO3 powder the diffi inculty a ratio of ofsupporting 5:1 and adding unsaturated a suitable polyester amount resin of accelerator. under water-contained The thermal conditions,stability of UPR synthesized anchorage an ematerialffective water-resistantin high temperature anchorage environments agent by modifying facing deep polyester mining to contain was studied. hydrophobic A study groups by and Wang prevent [105] waterexamining from intruding the difficulty into the of anchorage supporting reaction unsaturated system. Contrafatto polyester et resin al. [106 under] studied water the application-contained condeffectitions, of epoxy synthesized resin anchorage an effective materials water in natural-resistant stone anchorage (basalt, sandstone, agent by limestone) modifying and polyester determined to containthe minimum hydrophobic embedding groups depth and of prevent anchorage. water Compared from intruding with traditional into the concreteanchorage anchorage reaction material,system. Contrafattothe epoxy resin et al anchorage. [106] studied force the was application higher for basalteffect andof epoxy limestone, resin butanchorage lower for materials sandstone. in natural stone (basalt, sandstone, limestone) and determined the minimum embedding depth of anchorage. Compared3.4.4. Roadway with Repairtraditional Grouting concrete anchorage material, the epoxy resin anchorage force was higher for basaltGrouting and islimestone, one of the but most lower important for sandstone. technologies for roadway maintenance and crack repair [107]. Early grouting soft matters were cement, silica sol, water glass, etc. Subsequently, some researchers 3.4.4. Roadway Repair Grouting tried to add a small amount of polymer into these materials. For example, Li used Portland cement, fly ashGrouting and 5 wt.%is one redispersible of the most important polymer totechnologies form emulsion for roadway slurry, thenmaintenance mixed with and thecrack aqueous repair [foam107]. formed Early grouting by twelve soft alkyl matters sulfate were and gelatin cement, and silica added sol, additives water glass, to form etc. an Subsequently, inorganic polymer some researchers tried to add a small amount of polymer into these materials. For example, Li used Portland cement, fly ash and 5 wt.% redispersible polymer to form emulsion slurry, then mixed with

Polymers 2019, 11, x FOR PEER REVIEW 21 of 30 Polymers 2019, 11, 1706 21 of 30 the aqueous foam formed by twelve alkyl sulfate and gelatin and added additives to form an inorganiccuring foam. polymer The foam curing characteristics foam. The foam (stability, characteristics expansion (stability, rate) and expansion mechanical rate) properties and mechanical (elastic propertiescompressive (elastic strength, compressive elastic modulus, strength, crushing elastic engineering modulus, crushing stress, plateau engineering engineering stress, stress plateau and engineeringdense strain) stress were and discussed dense [stra108in),109 were]. discussed [108,109]. In essence, essence, because because inorganic inorganic materials materials such suchas cement as cement are easy are to shrink, easy haveto shrink, poor permeability have poor permeabilityand short grouting and length, short organicgrouting polymer length, grouting organic soft polymer matters such grouting as polyurethane, soft matters epoxy such resin, as phenolicpolyurethane, resin haveepoxy emerged. resin, phenolic Hu et al.resin [110 have] synthesized emerged. six Hu phenolic et al. [1 foaming10] synthesized resins with six phenolic different foamingmolar ratios resins of formaldehydewith different molar to phenol ratios (F /ofP =formaldehyde1.2, 1.4, 1.6, 1.8, to phenol 2.0, 2.2, (F/P 2.4). = The1.2, results1.4, 1.6, showed 1.8, 2.0, that2.2, with2.4). theThe increase results of showed ratio, the that trimer with and the tetramer increase of of the ratio, resins the gradually trimer and increased, tetramer the of crosslinking the resins graduallydegree of the increased, phenolic resinsthe crosslinking increased and degree the chemical of the structurephenolic complexity resins increased of the polymersand the increased chemical structureto form a three-dimensional complexity of the structure. polymers These increased resulted to in forman increase a three in- thedimensional viscosity of structure. the resin. TheseWhile resultedthe foaming in an rate increase decreased, in the the viscosity foam density, of the compressive resin. While strength, the foaming thermal rate stability, decreased, oxygen the index foam density,and flame compressive retardancy increased. strength, thermal When the stability, ratio reached oxygen 1.6, index the size and of flame the foam retardancy cells became increased. more Whenuniform. the Zhang ratio reached [111] and 1.6, others the size used of aminothe foam resin cells as abecame base material more uniform. and added Zhang foaming [111] agent, and others foam usedstabilizer, amino crosslinking resin as a agent,base material toughening and agentadded and foaming coupling agent, agent foam to synthesize stabilizer, a crosslinking polymeric foaming agent, toughening agent and coupling agent to synthesize a polymeric foaming (PF) grouting material. (PF) grouting material. Amino resin has many hydrophilic groups (–CONH2–, –NH2), which are Amino resin has many hydrophilic groups (–CONH2–, –NH2), which are easily bound to water easily bound to water molecules. Carbonyl (–C=O–), hydroxymethyl (–CH2OH) and imino (–NH–), whichmolecules. are active Carbonyl groups, (–C=O have–), strong hydroxymethyl adhesion and (–CH can2OH) enhance and theimino bonding (–NH– strength), which of are materials active withgroups the, have injected strong media. adhesion At the and same can time, enhance the crosslinkingthe bonding agentstrength in theof materials material provideswith the injected enough media.hydrogen At ionsthe same to catalyze time, the the crosslinking condensation agent of resin in the molecules. material Asprovides a result, enough the molecular hydrogen weight ions to of catalyzethe resin theis increased condensation and the of long resin molecular molecules. chain As is a changed result, the into molecular a network weight structure, of whichthe resin has is a increasedcertain mechanical and the long strength. molecular Compared chain with is changed superfine into cement a network and aluminate structure, cement, which the has slurry-coal a certain mechanicalinterface formed strength. by cement Compared grouting with is relatively superfine loose. cement However, and alu someminate chemical cement, grafting the slurry reactions-coal interfaceoccur between formed PF by and cement coal andgrouting a solid is polymerrelatively coating loose. However, is formed some on the chemical surface grafting of coal body. reactions The occurgrouting between reinforcement PF and coal effect and is improveda solid polymer (Figure coating 23). is formed on the surface of coal body. The grouting reinforcement effect is improved (Figure 23).

Figure 23. InterfaceInterface model model schematic schematic diagram diagram of of polymeric foaming (PF) material [ 111111]]..

Due to the high cost of resin, resin, organic organic and and inorganic inorganic composite composite materials materials are are typically typically produced. produced. The excellentexcellent bondingbonding property property and and high high deformability deformability of polyurethaneof polyurethane (PU) (PU) and and the lowthe costlow andcost highand highstability stability of sodium of sodium silicate silicate (WG), (WG), means means it is favorable it is favorable to combine to combine the two the (PU two/WG). (PU/WG). Guan Guan et al. [et112 al]. [first112] added first added 50 g polyether 50 g polyether polyol into polyol 110.5 into g sodium 110.5 g silicatesodium and silicate continued and continued stirring until stirring polyether until polyolspolyether were polyols completely were dispersed completely to form dispersed emulsion. to form Then emulsion. 60.5 g polymeric Then 60.5 aromatic g polymeric diphenylmethane aromatic diphenylmethanediisocyanate (pMDI) diisocyanate was added. (pMDI)The polyurethane was added./sodium The silicate polyurethane/ adhesivesodium solution silicate was obtained adhesive by solutionintense mixing. was obtained The compressive by intense strength mixing. and flexuralThe compressive strength of strength coal/adhesives and flexural composition strength reached of coal/adhesives21.4 MPa and 5.8 composition MPa, respectively. reached It 21.4 is known MPa thatand NCO 5.8 MPa, groups respectively. in pMDI react It is with known OH thatgroups NCO on

Polymers 2019, 11, x FOR PEER REVIEW 22 of 30

Polymers 2019, 11, x FOR PEER REVIEW 22 of 30 groupsPolymers in pMDI2019, 11 react, 1706 with OH groups on coal surface first and then coal particles modified22 by of 30 NCO groups react with polyether polyols and sodium silicate, forming sodium silicate/polyurethane/coal groups in pMDI react with OH groups on coal surface first and then coal particles modified by NCO compound. As shown in Figure 24, the obtained compound is composed of a polymer matrix and groupscoal react surface with first polyether and then coalpolyols particles and modifiedsodium silicate by NCO, forming groups react sodium with silicate polyether/polyurethane/coal polyols and mineral phase. The structure of the polymer matrix is a combination of rigid and flexible blocks, coal compound.sodium silicate,As shown forming in Figure sodium 24, silicate the obtained/polyurethane compound/coal compound. is composed As shown of a polymer in Figure matrix24, the and particles are connected to polymer chains and solid sodium silicate is dispersed in the polymer mineralobtained phase. compound The structure is composed of the of polymer a polymer matrix matrix is and a combination mineral phase. of The rigid structure and flexible of the polymer blocks, coal network structure. particlesmatrix are is a connected combination to of polymer rigid and flexible chains blocks,and solid coal particlessodium are silicate connected is dispersed to polymer in chains the polymerand networksolid structure. sodium silicate is dispersed in the polymer network structure.

Figure 24. Bonding mechanism between sodium silicate/polyurethane adhesive and coal [112]. Figure 24. Bonding mechanism between sodium silicate/polyurethane adhesive and coal [112]. He Figureet al. [24.113 Bonding] indicated mechanism that polyurethane/ between sodiumsodium silicate silicate/polyurethane (PU/WG) adhesive grouting and coalsoft [matter112]. was He et al. [113] indicated that polyurethane/sodium silicate (PU/WG) grouting soft matter was ideal for coal mine reinforcement. The compressive strength, fracture toughness, fracture energy, idealHe et for al coal. [113 mine] indicated reinforcement. that polyurethane/ The compressivesodium strength, silicate fracture (PU/WG) toughness, grouting fracture soft energy,matter was flexural strength and bending die of PU/WG grouting material were increased by 11.65–40.65%, idealflexural for coal strength mine reinforcement. and bending die The of PUcompressive/WG grouting strength, material fracture were increased toughness, by 11.65–40.65%, fracture energy, 9.68%, 21.33%, 6.60% and 15.85%, respectively, by introducing 2.5 wt.% silane coupling agent flexural9.68%, strength 21.33%, and 6.60% bending and 15.85%, die of respectively,PU/WG grouting by introducing material 2.5 were wt.% increased silane coupling by 11.65 agent–40.65%, 3-chloropropyltrimethoxysilane (CTS) (Figure 25). Zhang et al. [114] discussed the effects of 9.68%,3-chloropropyltrimethoxysilane 21.33%, 6.60% and 15.85% (CTS), respectively (Figure 25, ).by Zhang introducing et al. [ 2.5114 ]wt discussed.% silane the coupling effects of agent dimethylbenzydimethylbenzylaminelamine (BDMA) (BDMA) and and dibutyltin dibutyltin dilaurate dilaurate (DBTDL)(DBTDL) catalysts catalysts on on the the PU PU/WG/WG grouting grouting 3-chloropropyltrimethoxysilane (CTS) (Figure 25). Zhang et al. [114] discussed the effects of material.material. DBTDL DBTDL accelerated accelerated the the gel gel and and curing curing reaction reaction ofof thethe materials materials and and BDMA BDMA increased increased the the dimethylbenzylamine (BDMA) and dibutyltin dilaurate (DBTDL) catalysts on the PU/WG grouting fluidityfluidity and andcompressive compressive strength strength of of the the PU/WG. PU/WG. The synergistic catalysis catalysis of of 0.1 0.1 wt.% wt BDMA.% BDMA and and 0.1 0.1 material. DBTDL accelerated the gel and curing reaction of the materials and BDMA increased the wt.%wt.% DBTDL DBTDL can cannot not only only improve improve the the fluidity fluidity of of the materialmaterial but but also also reduce reduce the the curing curing time, time, with with fluidity and compressive strength of the PU/WG. The synergistic catalysis of 0.1 wt.% BDMA and 0.1 the compressivethe compressive strength strength ultimately ultimately reaching reaching 66.1 66.1 MPa. wt.% DBTDL can not only improve the fluidity of the material but also reduce the curing time, with the compressive strength ultimately reaching 66.1 MPa.

Figure 25. Compressive strength of the polyurethane (PU)/sodium silicate (WG)-0.0 wt.% and FigurePU / 25.WG-2.5 Compressive wt.% specimens strength at di ff oferent the curing polyurethane temperatures (PU) for/ threesodium days silicate [113]. (WG)-0.0 wt.% and PU/WG-2.5 wt.% specimens at different curing temperatures for three days [113]. Figure 25. Compressive strength of the polyurethane (PU)/sodium silicate (WG)-0.0 wt.% and APU/WG study-2.5 by wt Gao.% specimens et al. [115 at] different examined curing the temperatures concept that for chemical three days grouting [113]. materials such as polyurethane and urea resin are not suitable for mine environments with more water; thus, a A study by Gao et al. [115] examined the concept that chemical grouting materials such as polyurethane and urea resin are not suitable for mine environments with more water; thus, a

Polymers 2019, 11, 1706 23 of 30

A study by Gao et al. [115] examined the concept that chemical grouting materials such as polyurethane and urea resin are not suitable for mine environments with more water; thus, a new-type polymer grouting material was prepared with catalysts and vinyl epoxy resin, which was made from epoxy resin. This material had low viscosity, good polymerization reaction, accurate and controllable curing time and strong grouting ability. For mine aquifers and drift-sand layers, after solidification and expansion of material, resin and sand are bonded to form a compact structure, which has a better sand fixation effect and stronger bearing capacity (Table4).

Table 4. Main properties of the cured material.

Mechanical Category Ref Characteristics Density 1.6935 g/cm3 Compressive strength 10.38–13.09 MPa Strength of extension 0.32–0.53 MPa Elasticity modulus 401.37–490.64 MPa [115] Poisson’s ratio 0.447–0.846 Cohesion 0.866 MPa Internal friction angle 49.38◦

There are some safety production industry standards of the People’s Republic of China on coal mine polymer materials., such as AQ 1088-2011 (coal mine spraying polymer for sealing ventilation), AQ 1089-2011 (polymer material for consolidating coal and rock in mine) and AQ 1090-2011 (polymer foaming material for filling and sealing in coal mine). The polymer-containing soft matter discussed above generally meets or exceeds the requirements of relevant standards. For example, the tensile strength of PUE Nanocomposites meets the 2.0 MPa standard of AQ 1088-2011 (70). As another example, the minimum requirement for elongation at break is 30% and actual test of the material reaches 20 times the standard. Air tightness and compressive strength of the sealing material meet the standards of AQ 1090-2011 (81). Similarly, the compressive strength of PU/WG grouting material for coal and rock reinforcement exceeds the 60 MPa standard of AQ 1089-2011 (114).

4. Future Research Prospects Regarding the development and application of polymer-containing soft matters in mines, researchers have used sophisticated experimental equipment and rigorous experimental methods to optimize material formulations and test material performance. Surface tension meter measured the tension of the solution. The liquid foam properties were measured by Foamscan. The shear viscosity characteristics of the flowing slurry were measured by rotational rheometer. The microscopic characteristics of the material were determined by a scanning electron microscope (SEM) and fourier transform infrared spectrometer and the mechanical properties of the solidified material were tested by a universal material testing machine. However, most of the materials are only tested by laboratory methods. For the research and application of engineering materials, more attention should be paid to the field effect. The combination of laboratory testing and field verification will better promote the development of polymer-containing soft matters in mines. Overall, the study of polymer-containing soft matters is still in a period of rising development, presenting the research situation of far from adequate. It is reflected in that polymer-surfactant systems mainly focus on common surfactants (SDBS, SDS, AOS, AES, etc.) and polymers (HPAM, CMC, XG, etc.), while polymer-inorganic mixture systems mainly focus on common resin materials, especially polyurethane. Therefore, with the increasing demand for safe and green mining in coal mines and the progress of science and technology, the research of polymer-containing soft matters in mines has very broad prospects. The following are some suggestions on the future development directions. As for the complexity of polymer and surfactant system, the main research directions in the future will expand from common materials to new surfactants and polymers. In the system, not only one Polymers 2019, 11, 1706 24 of 30 kind of polymer is used, but also mixtures of many kinds of polymers are used and in this way, the performance of soft matters can be improved efficiently. Use of single polymer surfactants for research and development of mine materials would achieve the system optimization by two functions of a substance and reduce the cost of soft matters. Modifying polymers or synthesizing ideal polymers by popular graft copolymerization would achieve an economical and practical application mode. In addition, natural degradable green polymers will become the first choice of soft matters. A polymer-inorganic mixture system is based on resin as the core, deepens the use of inexpensive inorganic materials such as fly ash and cement, improves the properties of soft matters and reduces the cost of materials. Facing situations of deep mining with high rock pressure, strong geothermal properties and strong water inrush, high-quality additives or multi-resin composites to enhance the chemical properties (cohesiveness, etc.) and mechanical properties (deformation resistance, etc.) of soft matters should be sought. Other resin polymers will gradually be introduced into the development of mine soft matters. At the same time, synthesizing super resin materials to replace existing materials will be required. In addition, compared with linear polymers, bulk polymers are more likely to bond in the three-dimensional direction because of their large branched chains. It is believed that they will receive more attention and application in the research and development of polymer-containing soft matters in mines.

5. Conclusions In this paper, the research progress of polymer-containing soft matters for safe coal mining is reviewed and the future development trends are suggested. (1) According to the demand characteristics of the solid–liquid state of the soft matters, surfactant solution, aqueous foam, foam sol and three-phase foam are classified as fluid-like soft matters because of their strong liquidity. Suspended mortar, composite gel, coating slurry, gas hole sealing material, anchor net shotcrete and others are classified as solid-like soft matters that depend on the mechanical properties of materials after curing. Polymers are mainly used as additives or basic materials to improve the properties of the original soft matters or synthesize new soft matters, thus forming various high-quality soft matters in the application of different hazard prevention and control in coal mines. (2) For different disaster prevention and control, polymer-containing soft matters have different emphases on property improvement. Surfactant solution has mainly a wetting, viscosity and film forming ability. Besides the above properties, dust suppression foam and foam sol also have good size characteristics and stability. The main matters of suspended mortar are fly ash, loess, sand and so on and mainly improve its cohesiveness and flow characteristics. The chemical gel has higher requirements for thermal stability, expansibility and water retention. A three-phase foam system is based on solid–liquid–gas, which requires foam to have good foaming properties, stability and viscosity. The main concerns of coating materials are mainly air tightness and mechanical properties (tensile strength, etc.). Gas sealing material is required to have good cohesion with the coal seam, permeability, expansibility and considerable deformation resistance. Anchor net shotcrete for roadway support should have certain toughness and deformation resistance (tension and compression resistance). Liquid accelerators need to reduce the consolidation time of the anchor–shotcrete materials. Resin anchorage materials should pay attention to water retention, cohesion with coal, shear and compression resistance. The repair grouting materials should have good permeability, fluidity and compressive strength. (3) There are two aspects in the research of polymer-containing soft matters for mines. One is the polymer–surfactant system. This mainly depends on the three-dimensional structure formed by the polymer itself or when combined with other additives. This structure effectively wraps water, mud, fly ash, etc. The second is the polymer–inorganic mixture system used to achieve effective mixing of the polymers and inorganic substances (cement, mortar, fly ash). In addition to the three-dimensional structure of the polymer itself, it also depends on the chemical bonds formed by the polymer with coal Polymers 2019, 11, 1706 25 of 30 and rock mass and the physical capillary adsorption of coal and rock pores, so that the materials have the ability to resist deformation. (4) With regard to the development and application of polymer-containing soft matters, researchers have paid more attention to the principles of economy and green and environmental protection whilst aiming to maintain high-quality properties. Cheap and easily available inorganic materials and more environmentally friendly and efficient polymers tend to be used to obtain better polymer-containing soft matters.

Author Contributions: Conceptualization, H.W. and Y.D.; methodology, H.W., D.W. and B.Q.; investigation, H.W. and Y.D.; writing—original draft preparation, H.W., Y.D.; writing—review and editing, H.W., D.W. and B.Q.; funding acquisition, H.W. Funding: This work was supported by the Fundamental Research Funds for the Central Universities (2019XKQYMS74). Conflicts of Interest: The authors indicate no potential conflict of interest.

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