Chlorinated rubber is a rubber derivative obtained by chlorination modification of natural rubber or synthetic polyisoprene rubber.The chlorination process involves complex reaction steps such as addition, substitution, and cyclization, and the final product is an irregular cyclic structure polymer [1], with a chlorine content of approximately 65%, and the empirical formula is (C10H11Cl7)n. Due to the strong polarity and chemical inertness of the C–Cl bond, chlorinated rubber possesses excellent chemical corrosion resistance, wear resistance, flame retardancy, water resistance, and adhesion properties.It has been widely used in the field of anti-corrosion coatings, including ship paint, road marking paint, printing ink, architectural coatings, and offshore oil platform coatings, etc[2]. In fact, chlorinated rubber has excellent adhesion to both metals and polymers. As an adhesive, it also has a broad application prospect. Currently, research on chlorinated rubber adhesives is still limited, and its applications are mainly found in patent literature.

1.Characteristics of Chlorinated Rubber

      The molecular structure of chlorinated rubber is highly polar, yet it has excellent water resistance, with a water absorption rate of only 0.1% to 0.3% [2]. Additionally, the molecular chain contains rigid cyclic structures, which makes movement difficult. This results in a lower permeability to water vapor and oxygen compared to most polar polymers (such as acrylics, epoxies, and alkyds resin), only one-tenth that of alkyd resin[3].Therefore, corrosive media such as water and oxygen are difficult to penetrate through the chlorinated film to corrode the substrat materials like steel.  As an adhesive, it can also prevent the destructive effects of water vapor on the adhesive interface.  

      Due to the high chlorine content, chlorinated rubber possesses a significant cohesive energy, resulting in the formation of a relatively brittle film. Therefore, it is usually necessary to add plasticizers to improve its brittleness. Common plasticizers include chlorinated paraffin, chlorinated biphenyl or phthalates, etc. [4, 5]. For bonding to polar interfaces such as metals, chlorinated rubber has excellent adhesion. However, for some interfaces with weaker polarity (such as natural rubber, styrene-butadiene rubber, etc.), too strong polarity is actually detrimental to adhesion. Therefore, depending on the characteristics of the materials to be bonded, it is often necessary to use a combination of polymers to achieve adhesion between metals and various other materials. Chlorinated rubber can be use in combination with chloroprene rubber, nitrile rubber, phenolic resin, alkyd resin, acrylic resin, and polyurethane, among others [4, 6–10], to prepare various types of adhesives. Chinese Patent CN 101418197 A discloses a neoprene adhesive modified by chlorinated rubber, in which the addition of chlorinated rubber enhances the adhesion to polar surfaces such as metals [6]. According to US patent US 4,256,615 [8], the addition of an appropriate amount of chlorinated rubber to a two-component castor oil-based polyurethane adhesive can significantly improve the initial adhesive strength, with some enhancement in ultimate strength as well.

      Like other chlorinated polyolefins, chlorinated rubber is also prone to releasing hydrogen chloride at high temperatures.Thermal degradation studies have shown that between 160 and 390 °C, chlorinated rubber releases hydrogen chloride, forming conjugated structures, and the color turns yellow; Between 390 and 585 °C, chlorinated rubber mainly undergoes oxidative degradation [11]. In a humid and hot environment, decomposition begins at 60 °C [3]. Therefore, thermal stabilizers must be added to chlorinated rubber products to absorb the trace amounts of hydrogen chloride released during decomposition and prevent it from further catalyzing the dehydrochlorination process. Commonly used thermal stabilizers include lead soaps, epoxy compounds, organic phosphites, and aliphatic polyamine compounds, etc. [4, 12]. With the presence of appropriate stabilizers, chlorinated rubber can be used in heat-resistant adhesives.

2.The Interaction between Chlorinated Rubber and Metal Interface

      The application of chlorinated rubber as adhesive is extensively used in adhesion between metals and rubbers, Therefore, understanding the interaction between chlorinated rubber and metal interfaces is of great significance for designing adhesive formulations. Feliu S et al. studied the interaction of a commercial chlorinated rubber paint (dissolved in xylene, containing alumina and silica fillers, with specific formulation undisclosed) with metal interfaces such as copper, aluminum, stainless steel, cold-rolled steel, and zinc [13].  Research has found that chlorinated rubber has the strongest adhesion to polyvinyl chloride, with no chemical changes observed at the interface. Metallic zinc can degrade C–Cl bonds to form ZnCl2, resulting in the lowest adhesion strength. For copper and cold-rolled steel, a certain amount of Cl⁻ is also formed at the interface, leading to the formation of a weak hydrocarbon interface layer. However, the adhesion strength is higher than that of zinc. Aluminum and stainless steel, due to their tendency of forming a passivation layer (protective layer of Al2O3 or chromium) on their surfaces, produce a very small amount of Cl− through degradation, resulting in higher adhesion strength. Liu XW et al. studied the corrosion protection effect of carbon steel by different coating systems. They found that using a zinc-rich primer with a chlorinated rubber topcoat resulted in more severe corrosion than using a chlorinated rubber coating alone [14]. The likely reason for this is the degradation of chlorinated rubber caused by zinc. Adding an epoxy intermediate coat between the two layers of paint significantly improves the corrosion resistance. This indicates that metallic zinc has a more destructive effect on chlorinated rubber coatings, while chlorinated rubber has better adhesion on inert surfaces such as aluminum and stainless steel.

      Gao Shouchao et al. studied the elemental composition of the interface between chlorinated rubber and metal using X-ray photoelectron spectroscopy, It was found that the adhesion between the two relied on van der Waals forces, without the formation of chemical bonds [15]. An increase in surface roughness is beneficial for adhesion as it enlarges the contact area between the two phases [16]. Some studies have shown that the rust layer on the steel surface can degrade chlorinated rubber, forming Cl⁻, which in turn catalyzes the degradation of the coating . In humid environment the degradation will speed up. Humid environments can accelerate degradation [18]. In practical applications, the addition of fillers, such as glass flake, can greatly reduce the moisture permeability of the coating. The research by Berio M et al. showed that the addition of anti-corrosive coatings such as zinc phosphate can actually reduce the water absorption rate of the coating [19]. In addition, many anti-corrosive coatings, such as aluminum tripolyphosphate and zinc phosphate, can form a dense passive film on the surface of steel [3], thereby reducing the degradation effect of metal on chlorinated rubber. However,there is still little research on the interaction between chlorinated rubber and the metal interface in such complex system containing anti-corrosive coatings.

3.Chlorinated Rubber Adhesives
3.1The Applications in Metal-Rubber Bonding
      The metal-rubber bonding has a wide range of applications in aerospace, automotive. Examples include oil seals in engines and flexible joints in solid rocket nozzles, which all involve the bonding of metal and rubber. Due to its excellent chemical inertness, through appropriate formulation design, chlorinated rubber can meet the requirement of oil-resistance and high-temperature-resistance adhesives. 

      Chinese patent CN 101421370 A discloses an oil-resistant and high-temperature-resistant adhesive for bonding acrylic rubber and degreased cold-rolled steel plates, which is mainly used for oil seals in engines and transmission systems [20]. The adhesive is formed by dispersing phenolic resin (45-75% by weight, the same below), chlorinated rubber (5-25%), and metal oxides (zinc oxide and titanium oxide) (10-30%) in a solvent.

      U.S. Patent US 2,459,742 discloses an adhesive for bonding nitrile rubber to metal [21], characterized by the addition of 1-5% by weight of aliphatic polyamines (such as tetraethylenepentamine, diethylenetriamine, triethylenetetramine, etc.) mixed with a chlorinated rubber solution. After curing, its bond strength does not decrease even after soaking in gasoline for more than 10 weeks. The polyamine serves to absorb the degradation product hydrogen chloride and to crosslink the chlorinated rubber adhesive layer.

      U.S. Patent US 3,108,035 discloses a one-component chlorinated rubber solvent-based adhesive, the solid component of which consists of 100 parts of chlorinated rubber, 5-200 parts of lead dioxide (preferably 60 parts), and 2-30 parts of dianisidine (preferably 10 parts) [22]. After curing at 149 to 177 °C, the adhesive strength varies for different rubbers when bonded to 1010 steel. For nitrile rubber and neoprene, a peel strength of 27 kg/cm can be achieved; for natural rubber and styrene-butadiene rubber, a peel strength of 25 kg/cm can be achieved; and for butyl rubber and steel, the bond strength is 16 kg/cm. This variation in bond strength with different rubbers can also be seen in U.S. Patent US 4,994,519, which discloses a rubber-metal adhesive whose main components are chlorinated rubber and brominated poly(dichlorobutadiene). Additionally, para-phenylenediamine and sulfur are added as vulcanizing agents [23]. The bond strength of this adhesive for bonding rubber and steel decreases in the order of nitrile rubber, styrene-butadiene rubber, and natural rubber. Obviously, chlorinated rubber adhesives are very effetive for bonding polar rubbers and metals.

3.2The Applicatons in the Field of fiber-reinforced rubber
      Fiber-reinforced rubber is mainly used in transmission systems, such as timing belts and conveyor belts in automobile engines. The working environment temperature of these applications is is relatively high (above 100 ° C), requires that the fiber-rubber adhesive must be able to withstand high temperature. In recent years, heat-resistant and oil-resistant rubbers such as hydrogenated nitrile rubber and chlorosulfonated polyethylene have been widely used in the manufacture of conveyor belts. The reinforcing fibers mainly include glass fiber, polyester, nylon, and others. To enhance the adhesion between fibers and rubber, fibers typically undergo a three-layer treatment: first, the fiber surface is activated with an epoxy compound or isocyanate; second, a resin emulsion containing resorcinol-formaldehyde-latex (RFL) is applied; and finally, a chlorinated rubber adhesive is coated. The third layer of adhesive serves as the bonding agent.

      Chinese Patent CN 101671959 A discloses a treatment agent for preparing glass fiber strands used for reinforcing rubber(The fiber shall be pretreated with RFL lotion before using the treatment agent) [24]. This treatment agent enhances the adhesion between chlorinated polyethylene, hydrogenated nitrile rubber, and glass fiber, which can improve the service life and heat resistance of automotive engine timing belts. The treatment agent contains chlorosulfonated polyethylene and chlorinated rubber, as well as components such as isocyanates. The specific formula is shown in Table 1. As can be seen from the table, the treatment agent containing a combination of chlorinated rubber and chlorosulfonated polyethylene has improved the bonding strength and hot water resistance, performing better than using either one alone.

 Table 1. Adhesive formulation and properties for the interface between glass fiber and hydrogenated nitrile rubber[24 ]

Table 2. Adhesive Formulation and Properties for the Interface between Polyester Fiber and Hydrogenated Nitrile Rubber[ 25]

      U.S. Patent US 5,219,902 also discloses an adhesive for hydrogenated nitrile rubber reinforced with fibers (such as polyester, aramid, nylon, etc.) [25]. This adhesive contains (hydrogenated) nitrile rubber and chlorinated rubber (generally used in equal weight ratio) along with toluene solvent (with a solid content of 10%). Before using this adhesive, the fibers need to be activated with isocyanate and coated with an RFL emulsion. Table 2 presents the properties of polyester-hydrogenated nitrile rubber composites prepared using this adhesive. After treatment at 140°C for 7 days, the adhesive strength is well maintained, which is better than the results obtained when using nitrile rubber or chlorinated rubber alone as the adhesive. The conveyor belt made of this composite material appeared cracks in the rubber after 220 hours of use in an environment at 120°C. In contrast, the reference sample exhibited fiber stripping (Table 2), indicating that the adhesive strength of the reference sample was insufficient.
      Japanese Patent JP 02258653 discloses an adhesive composed of hydrogenated nitrile rubber, chlorinated rubber, and isocyanate, used to enhance the adhesion between glass fibers and rubber for the preparation of high-performance rubber products [26].

3.3Applicatons in Other Fields

      Chinese Patent CN 102153965 A discloses an adhesive for high-temperature resistant conveyor belt splicing that cures at room temperature, as well as its preparation method [27]. This adhesive uses chloroprene rubber, chlorinated rubber, and isocyanate as the main film-forming substances. It can be used to bond the overlapping surfaces of conveyor belts at room temperature, with a surface drying time of 1 to 10 seconds, an average shear strength of 3 MPa, and an average tensile strength of 2.5 MPa. The adhesive has excellent high-temperature resistance. After aging at 200°C for 12 hours, it still maintains a shear strength of 1.55 MPa and a tensile strength of 1.45 MPa. This adhesive can increase the heat resistance of the conveyor belt splice to 180°C.

      John N et al. studied rubber adhesives for bonding between wood chips [28], which basically consist of neoprene and chlorinated rubber. When the usage of chlorinated rubber is between 0~30%, the adhesive strength increases almost linearly. However, when the content of chlorinated rubber exceeds 30%, the adhesive strength decreases instead (see Figure 1). The adhesive strength is the lowest when chlorinated rubber is used in combination with natural rubber, and increasing the amount of chlorinated rubber does not lead to significant changes in strength.

4.Conclusion
      Chlorinated rubber possesses water resistance, strong polarity, and chemical inertness, making it compatible with a variety of polar polymers. After adding appropriate heat stabilizers (to prevent the release of hydrogen chloride), the prepared adhesive exhibits high bonding strength, oil resistance, and high-temperature resistance. Metal-rubber products and fiber-reinforced rubber products prepared using chlorinated rubber adhesives have been well applied in areas such as automotive oil seals and engine drive belts. Currently, application data are mainly found in patent literature. The properties of chlorinated rubber itself (such as molecular weight, molecular weight distribution, etc.) have not yet been systematically studied for their impact on adhesive performance. The emergence of high-viscosity chlorinated rubber (i.e., high molecular weight chlorinated rubber) is very likely to further enhance its adhesion, resulting in chlorinated rubber adhesives with superior performance.