Rubber tubing is used in flow lines for fluids and gases in a variety of hydraulic, pneumatic, process, medical, and specialty applications. Rubber tubing is flexible and resilient due to its natural or synthetic rubber construction.

Materials

What distinguishes rubber tubing from other types of tubing is its rubber-based construction. Rubbers are elastomers, materials which have high yield strengths which allow them to be stretched much farther than other materials without suffering permanent deformation. Rubbers are known for their flexibility, tear strength, resilience, and thermal stability up to certain temperatures. Rubber materials can be classified as either natural or synthetic.

Natural Rubber

Natural rubbers are based off of naturally occurring compounds such as polyisoprene (also called gum rubber) and latex. They are highly resilient and tear resistant, and also exhibit cold temperature and abrasion resistance. Most natural rubbers have higher elasticity than synthetic rubbers, as well as good elastic memory (resistance to creep, sagging, or stretching). This makes natural rubber tubing an excellent choice for vacuum applications. The drawbacks of natural rubbers include higher cost and poor ozone (weather) resistance and oil resistance.

Synthetic Rubber

Synthetic rubbers are artificial rubbers made primarily from petrochemical feedstocks. They typically have high abrasion resistance, and good fluid and chemical resistance. There are a large number of synthetic rubbers; some of the key types and their properties are listed in the table below.

Specifications

In addition to material selection, rubber tubing must be designed to the correct specifications for the application.

Sizing

Correct sizing of tubes is important for fitting to the needs of the application. Nominal sizes for tubing are based on the outside diameter, which measures across the full cross section of the tube. Wall thickness and inner diameter can then be used to determine the thickness of the tube wall and the flow area (if applicable) for fluid flow inside the tube.

Performance

Tube performance is typically rated based on a few key specifications.

• Pressure rating defines the minimum or maximum pressures the tube is designed to withstand, measured in pounds per square inch (psi). This rating is defined for tubes in fluid flow applications.
• Maximum vacuum defines the maximum vacuum pressure that can be created in the tube, most frequently given in inches or mm of mercury, referenced below one standard atmosphere.
• Minimum bend radius is a rating based on the acceptable amount of deformation the tubing cross-section can undergo, typically given in inches or millimeters. This is particularly important for structural applications where the tubing is subject to loading.
• Temperature range defines the full range of ambient temperatures that the tubing is designed to operate within. Temperature affects other performance properties such as yield strength, oil & chemical resistance, and permeability. Operating outside the specified temperature range of the tubing could cause permanent damage or failure.

Finish

In some applications, it may be important to consider the finish or color of the rubber tubing.

• Clear tubing is completely see-through, which is important for liquid handling applications where the flow must be monitored.
• Solid tubing is opaque and finished with one solid color.
• Translucent tubing allows partial light passage so liquid flow is partly visible and can be monitored.
• Multi-colored tubing is finished with more than one color, either for decorative or indicating purposes.

Features

Rubber tubes may incorporate a number of features based on the application they are designed for.

• Reinforcement - tubing material is reinforced with fibers or banding for added strength.
• Spark resistance - tubing is constructed to maintain integrity in the presence of electrical discharge or sparking. This is a crucial feature for most electrical applications.
• Sterilizability - tubing has the ability to be sterilized for sanitary applications: methods include an autoclave, dry heat, gamma irradiation, and chemical disinfectant. 

Standards

Depending on the application, rubber tubing may require adherence to certain standards. Some standards organizations include DOT, DIN, ISO, SAE, and FDA.

References

Rubber Types - Trelleborg

Synthetic Rubbers - United Chemical Products Ltd.

Image Credit: Grainger Industrial Supply | MISUMI USA | OMEGA Engineering, Inc.