Rubber manufacturers have continuously demonstrated that it is possible to replicate the properties of natural rubber in the laboratory. This is done through a process known as polymerization. CR rubber is one of these rubber compounds that have been produced in the laboratory. In this article, we discuss this material’s properties, synthesis, grades, and uses.
What is CR Rubber?
The abbreviation CR stands for chloroprene. Chloroprene is a chemical compound and the common name for 2-chlorobuta-1,3-diene. This compound is derived from acetylene and hydrogen chloride. It is a colorless petrochemical and the base monomer of CR rubber.
Since rubber is a polymer (a compound made of multiple monomers), the chloroprene compound undergoes chemical processes to produce rubber. Chloroprene is not a naturally occurring compound but an artificially synthesized one. Therefore, chloroprene rubber is a type of synthetic elastomer. In other words, a compound that exhibits rubber-like properties.
Chloroprene is made through a two-step process. First, butadiene is chlorinated with the addition of chlorine. The next step involved dehydrochlorination, where the chloride and hydrogen elements are removed from the compound.
Chloroprene rubber is the technical name that is used by the manufacturers. For the commercial and trade front, the term neoprene is used.
Why CR Rubber is also Called Neoprene
The term neoprene was introduced by DuPont Company as the trademark name for polymers of chloroprene. Besides, this is the company that patented the first ever known method of polymerizing chloroprene in the 1930s.
Like most patents, DuPont’s patent expired after a couple of years. This gave other manufacturers the leeway to use this method and improve it to make the polymer. As the commercial implications of chloroprene became more apparent, more manufacturers adopted the synthesis of this compound.
The terms CR rubber and neoprene are used interchangeably. However, you will find that most people use neoprene instead. The reason for this is that neoprene is a more common and easy-to-pronounce word than chloroprene. Moreover, this type of rubber was introduced to the market as neoprene.
Properties of CR Rubber
As a synthetic rubber, you would expect the properties of CR rubber to be quite diverse. For one, these properties can be altered by simply adjusting the agents used in the polymerization process. This can result in a compound with better properties overall. Here are some of the common properties of CR rubber.
Good chemical resistance
The ability to resist degradation caused by chemicals is known as chemical resistance. CR rubber has demonstrated this property after numerous tests.
Rubber is used in conditions where harsh chemicals are likely to exist. These conditions include motor engines, outdoor environments, and so on. Chemicals found in oils and fuels tend to degrade several materials and affect their physical form. CR rubber on the other hand does not succumb to chemical degradation.
High Resistance to Aging and Ozone
For compounds like rubber aging is defined as the degradation in quality over time. This includes loss of physical and chemical properties of the material.
Since natural rubber is made from organic compounds, it tends to degrade at a faster rate compared to synthetic rubber. The monomer of CR rubber, chloroprene, is an inorganic compound. As such, it resists degradation. Aging is characterized by poor tension strength and compression resistance.
Ozone is a freely occurring inorganic chemical compound that exists freely in the air. While the ozone gas is meant to protect us from UV rays, it can become a pollutant when it reaches the surface. CR rubber does not experience cracking, a common thing that happens to natural rubber when exposed to ozone.
Low Risk of Glazing or Abrasion
When exposed to certain environmental conditions, some rubber materials tend to lose their physical characteristics. These conditions include an abrasive environment, high heat, and oxidation. These conditions lead to the formation of a glossy surface on the polymer.
CR rubber does not glaze easily since it has high chemical, heat, and abrasion resistance. These mechanical properties are supported by the molecular composition of the monomers and other vulcanizing agents.
Better Wear Resistance than Natural Rubber
The fortification of CR rubber makes it more resistant to wear and tear than natural rubber. As we said earlier, inorganic compounds are more resilient and durable than organic. These compounds are non-biodegradable which means they cannot be broken down by natural organisms like bacteria. On the other hand, organic compounds are biodegradable.
Wear and tear affects the physical form of rubber. It could include tears, abrasion, discoloration, and so on. The combination of these effects lowers the performance of the product made from the said material. Now that CR rubber has a higher resistance to wear, you can be assured of the prolonged lifespan of the products.
Higher Cost than Natural Rubber
As with anything, the cost of a material is highly influenced by its makeup. Natural rubber exists naturally in trees. Extracting it from the latex is fairly easy and cost-effective. Therefore, it is generally a cheaper material
On the other hand, the polymerization of chloroprene is an expensive process that results in more expensive compounds.
Chloroprene on its own is not easy to manufacture. Most CR rubber manufacturers purchase this petrochemical from a third party and polymerize it. Also, the polymerization process requires resources such as experienced personnel, machinery, and money. Luckily, the worldwide adoption of chloroprene rubber has made this material more affordable.
Decomposes at High Temperatures
When exposed to high temperatures for extended periods, neoprene (CR) melts. Chloroprene is rated to sustain temperatures of between 200 and 500 degrees Fahrenheit. At this range, the material will experience slow degradation.
However, temperatures above this range will decompose the material completely. Decomposition destroys the material completely and leaves behind a powdery substance. When recycling CR rubber products, this is one of the methods used in the granulating stage.
The Manufacturing Process of CR Rubber
Essentially, the synthesis of chloroprene rubber depends on the manufacturer. Different manufacturers have different approaches and techniques to make these compounds. However, the major process involves the polymerization of chloroprene monomers.
The manufacturing of CR rubber yields different grades of this compound. This depends on the polymerization modifiers and the vulcanization agents used. In the following sections of the article, you will learn more about the grades and classifications of CR rubber.
Polymerization of chloroprene: The steps in the process
In chemistry, polymerization is a chemical reaction where molecules of monomers are reacted together under specific conditions. This reaction forms a link between the molecules to form a three-dimensional network of monomers. Many forms and approaches of this reaction exist and the one used depends on several factors.
This process is used in the emulsion polymerization of chloroprene.
Step #1 – Producing chloroprene
Acetylene is the starting element in the production of chloroprene rubber. The acetylene gas is turned into chloroprene monomers in chemical plants. The reason this synthesis is made in chemical plants is due to its nature. It requires skilled handling which might not be available in a rubber manufacturing factory.
Still, rubber manufacturers produce this material in some cases. One of the advantages of this approach is to ensure that the molecular composition of the monomer is as desired. It is a quality control practice that is quite helpful.
Step #2 – The emulsion polymerization
This is a 3-step process that results in the formation of latex. It is the most common CR rubber polymerization method. Let us look at the 3 processes or stages that happen during the emulsion polymerization.
- Water dispersion of chloroprene. Chloroprene is added to a solvent, usually water, as the first step. A stabilizing agent (that also prevents coalescing) is also added. It is usually a surfactant. Surfactants are substances that help in reducing the surface tension thereby increasing its spreading and wetting properties.
- Polymerization initiation. Polymerization initiation using a radical initiator – this substance is usually added to the mixture to initiate breaking the double bonds in the chloroprene monomers.
- Polymerization occurs. The initiator kick-starts this process and the chloroprene monomers start bonding. Control is required here to ensure that the end product exhibits the desired properties and standards
Step #3 – Latex formation
Similar to the synthesis of natural rubber, the synthesis of CR rubber involves a latex suspension. Latex is a colloidal suspension. The process yields chloroprene polymers suspended in water in the form of latex. Chloroprene compounds are found suspended in this solution called latex.
Latex is the intermediate product in the synthesis of chloroprene. This means that it is between the first and last two steps in the whole process. It can also not be classified as rubber yet, and still, it is not the monomer that is used at the start. It is however an important step in the process since it shows progress.
Step #4 – Coagulation process
In this step, the latex formed in the previous step is now worked on. Latex in its state is useless unless specialized techniques are used to separate the rubber from the colloidal solution. Coagulation is the process used to separate the compounds in the latex.
On the molecular level, a lot happens. This includes the destabilization of the compounds in the suspension. Then, the repulsive forces that bind the compounds are broken down to allow for separation. It is a rather straightforward process. As long as the technician knows what to do, the technology applied will help them.
Impurities found in the colloidal suspension are also removed. Since chemical reactions can yield unwanted by-products, they have to be removed to ensure a pure material. As you would expect, impurities affect the properties of rubber so they need to be removed.
Step #5 – Processing the material
This is usually the last phase in the manufacturing process of CR rubber. Depending on factors like manufacturing conditions, scale, and material profile, there could be at least three processing processes that happen here.
One, the coagulated substance needs to be tested. Testing accomplishes things like material quality, the presence of chemical and physical characteristics, and so on. If the quality of the rubber is subpar, reinforcement agents are used to improve it. For instance, an agent that improves the tensile strength of rubber can be added to it.
Additionally, CR rubber is usually dried after it has been coagulated. Drying is done either using sunlight or electricity. It is important since it can also reinforce the properties of the material. Other processing includes milling, grading, and so on. Moreover, the grading and classification of CR rubber happens in this step.
Other CR Rubber Polymerization Methods
While emulsion polymerization is the most common method of producing CR rubber, there are other methods that are used as well. The following is a brief overview of three of the other common ones.
Bulk polymerization is a technique used in the manufacturing of CR rubber where the chloroprene monomers are synthesized in the absence of water or in an organic solvent. This method is typically used for producing solid CR rubber. It involves the initiation of polymerization and the formation of long polymer chains in a bulk or solid form.
Removing water from the equation achieves a lot. First, there is more control of the polymerization process. This helps in the determination of essential parameters like the molecular weight of the compounds.
Solvents like Cyclohexane are used in this process. Solution polymerization is the dissolution of the chloroprene monomers in solvents like Cyclohexane. This agent helps to dissolve the monomer compounds and also catalyzes the polymer formation.
The choice of the solvent used during this process depends on several factors. First, the desired properties of CR rubber are considered. It would be catastrophic to use a solvent that affects the overall quality of the material. The people at the manufacturing farm already know what best works to bring out these properties. Therefore, they choose correctly at all times.
Suspension polymerization involves suspending small droplets of chloroprene monomers in a non-miscible solvent. The monomers are then polymerized within these suspended droplets to form CR rubber. This method is less common but has been used for producing chloroprene rubber and can offer unique properties and characteristics.
Classification of CR Rubber
Like other types of synthetic elastomers, CR rubber is classified during its manufacture. Classification is like tagging and identification of the material based on certain aspects. For CR rubber, the polymerization modifier is the most common aspect used to classify them.
Polymerization modifiers of chloroprene
A polymerization modifier is more like a catalyst or accelerator of the polymerization process. It essentially kicks start this process. They are also known as transfer agents. For CR rubber, sulfur and thiols are the two major polymerization modifiers used.
- Sulfur-modified types. As the name suggests, sulfur is used as the modifier to create the polymer compound. CR rubber that results from this process is therefore called sulfur-modified.
- Mercaptan-modified types. Mercaptans are also called thiols. They are mostly sulfur-containing compounds. This tells you that sulfur is probably the single most used polymerization modifier of chloroprene.
Grades of CR Rubber
Chloroprene rubber needs to be graded for various reasons. In grading, the rubber compounds that are arrived at from the polymerization are put in batches according to factors like properties. In other words, the properties of rubber are used to assign grades.
General Purpose Grades
This is the most common grade of CR rubber. N-dodecyl mercaptan is used as the transfer agent during the polymerization process. Xanthogen disulfides can also be used in place of n-dodecyl mercaptan. These grades of CR rubber are used for general-purpose applications such as in the automotive industry.
The approach used here is the fusion of soluble polymers of chloroprene and its crosslinked variety. Cross-linking is the interconnectivity formed by the covalent bonding of polymer chains. In simpler words, it means that the monomers form a three-dimensional network that is quite strong. Besides, crosslinking is what makes rubber so strong yet stretchy.
As we said earlier, sulfur is the most common vulcanizing agent used in the manufacture of CR rubber. As such, rubber made with this compound as the vulcanizing agent is dubbed sulfur-modified grade. The advantage of this agent is that it improves the milling of CR rubber. For example, this rubber is easy to break down during mastication because of sulfur.
Slow Crystallizing Grades
The introduction of irregularities in the polymerization process results in slow crystallization grades of chloroprene rubber. The comonomer (precursor of the polymer) used in the process is 2,3-dichloro-1,3-butadiene. These grades of CR rubber are used for specific applications due to their unique properties.
Quality Control of CR Rubber
Before, during, and after chloroprene rubber is manufactured, it needs to be tested thoroughly. Testing is done to ensure that what is on paper is what is achieved. In other words, it makes sure that the material meets the requirements set out.
CR Rubber Testing Standards
The quality control standards of CR rubber are internationally designated by D3190-06(2021). This is the recognized standard and all manufacturers have to adhere to its directives. The standard dictates the test procedures, benchmark values, and best practices for handling this specific type of rubber.
Test standards are more than just guidelines. They help the manufacturer make the product that you need. This means following the products will satisfy your needs as a customer. If it is the properties, you can be assured that they will be tested properly.
Applications of CR Rubber
There are several applications of CR rubber. Due to its versatility, this material can fit in multiple conditions. However, it is preferred for conditions where its properties are mostly applicable and reliable. Areas saturated with chemicals, UV, and other environmental stressors are great examples. Some of the rubber products that are used in these conditions include;
These are the seals that create a seal between two mating components. Usually found in engine parts, gaskets are commonly made from rubber. Engines are expected to reach high temperatures as the combustion occurs. Therefore, the gaskets are required to have a high thermal resistance to sustain the heat. That is why CR rubber is so useful.
An O-ring acts as a gasket for toric joints. They are mostly used for circular joints hence the name o-rings. These products are found typically in every mechanical part. They are on wheels, engine parts, and essentially other moving parts. With its high compression set and tension force, CR rubber makes a perfect material for these products.
The chemical resistance property of chloroprene is capitalized on here. Gasoline contains several chemicals that can negatively affect many materials. However, this type of rubber does not deform or degrade when used as a hose for gasoline.
Like most rubbers, chloroprene is moisture and water-resistant. Its surface is water-repellant and it does not allow water molecule permeability. Therefore, it makes a perfect choice for the making of wetsuits. These wetsuits can be used for divers, industrial workers, and other conditions in water-prone environments.
The manufacture of rubber compounds is an important process in the rubber industry, it controls the overall quality of the rubber products and more. This article delved into the synthesis of one of the most common synthetic rubber compounds, chloroprene rubber. It covered the methods, classification, grading, and much more.