Cryogenic Neck Tubes: Choosing the Right Material for Maximum Durability and Longevity

The development of materials used for cryogenic systems and parts has progressed since cryogenic research began in the late 19th century. Today, materials that can withstand cryogenic temperatures have become particularly important for preserving biological samples for animal husbandry and invitro fertilization for humans. They’re also critical to store samples for various medical and scientific research applications that require these cryogenic systems. Parts like neck tubes are positioned within what’s known as “dewars” to allow access to these cryogenically frozen samples.

Dewars are essentially aluminum cans surrounding a secondary container that contains these frozen samples. Between the primary and secondary containers is a vacuum chamber that acts as insulation, through which these cryogenic neck tubes provide access. In this internal chamber and the vacuum surrounding it, cryogenically frozen samples are safely suspended. The insulating vacuum protects both cryogenic systems and parts like neck tubes, which must be made from material that can withstand this extreme cold.

Choosing the Best Materials for Cryogenic Neck Tubes

The materials that work best for cryogenic systems and parts depend largely on the application, operating conditions, and system design. Materials for these cryogenic systems, and parts integral to them like neck tubes, require the use of materials that present properties that help them withstand these extreme temperatures.

Material for cryogenic systems and parts should present properties like: 

• High corrosion resistance to protect from leakage of cryogenic chemicals that can result in damage.
• High ductility enables the material to bend or bond without breaking down.
• High strength for withstanding pressure from mechanical loads.
• High toughness to promote fracture and impact resistance.
• Low thermal conductivity minimizes energy loss and heat transfer.
• Low thermal expansion helps prevent thermal cycling and stress that can lead to cracking and deformation of the material.

Cryogenic systems and parts typically need to withstand extraordinarily low temperatures from about -112°F to -320.8°F (-80°C to -196°C). Cryogenic neck tubes allow researchers to access biological samples, including preserved cells, DNA, or tissues, which are analyzed for future research. As neck tubes are in direct contact with samples, the material from which they’re made must also minimize the chance of contamination.

Beyond the ability to tolerate extremely cold temperatures, the composition of cryogenic systems and parts also requires the material to be chemically inert and durable. Cryogenic systems are thus often made with medical-grade materials, especially plastics like polycarbonate and polypropylene. Stainless steel is also used for cryogenic systems and parts, as it’s easily cleaned and nonporous.

Stainless Steel for Cryogenic Systems and Parts

The corrosion resistance, ductility, strength, and toughness of certain stainless steels at very low temperatures would make it ideal for use within cryogenic systems and parts. However, though they have low thermal conductivity, stainless steels tend to have high thermal expansion. At extremely low temperatures, austenitic stainless steels maintain a high ductility, so these steels are typically used for cryogenic equipment.

Used frequently for applications with temperatures down to -452°F (269°C), austenitic stainless steels contain significant amounts of manganese and nickel. These elements within the steel cause a decrease in MS-temperature – the point at which austenitic steels shift to being martensitic – to subzero temperatures. Normally, the  MS-temperature for austenitic steels range from 572°F to 1292°F (300°C to 700°C). Additionally, the tensile strengths of austenitic stainless steels containing chromium and nickel increase as temperatures decrease, while yield strength does as well, but to a lesser extent.

Polypropylene for Cryogenic Systems and Parts

Polypropylene (PP) exhibits very good chemical and temperature stability. It can withstand cryogenic temperatures down to -304.6°F (187°C), which is at the lower end of nitrogen’s gaseous state, which has a dewpoint of -320.44°F (-195.8°C). This, too, is within the temperature range at which certain cryogenic systems and parts operate. For laminates, tapes made from PP present enough ductility to make these thermoplastics useful for certain cryogenic applications.

Testing has also shown that PP-based polymers still absorb over 72 percent of the impact energy when exposed to low-velocity impacts at cryogenic temperatures, so their toughness in cryogenic environments is adequate. Additionally, tests for tensile strength and bending fall within an acceptable range beyond the dewpoint for nitrogen. As such, PP is one of the most cost-effective polymers used in cryogenic systems and parts, while it’s also lightweight and completely recyclable.

Polyethylene for Cryogenic Systems and Parts

Cryogenic systems and parts benefit from the rigidity, strength, and toughness of polyethylene (PE), which combine to give it decent abrasion and wear resistance. Tests performed on high-density PE measured its mechanical and electrical properties, which show how well it withstands lower temperatures. At room temperature and lower, it was shown to retain much of its breakdown strength. In contrast, elastic modulus and mechanical breakdown stress increased directly due to increased draw ratio, which relates to how far it can stretch before breaking.

When exposed to temperatures at which nitrogen becomes liquid – from -320.44°F (-195.8°C) and above – the elastic modulus and mechanical breakdown stress increased for the PE sample. At the same time, strain decreased in these cryogenic conditions. This means that high-density PE’s toughness increases within a certain cryogenic temperature range. Additionally, electrical conductivity was decreased when exposed to cryogenic temperatures.

These properties make it possible to use PE as an insulating material for cryogenic systems and parts, as it stands well in these very low-temperature conditions. While more expensive than PP, PE’s other properties make it a good fit for applications that involve temperatures that range into the cryogenic. Systems and parts made from high-density PE also benefit from its relatively easy fabrication, resistance to solvents, and superior flexibility, which is why it’s often used in medical and scientific laboratories.

Cryogenic Polymers for Cryogenic Systems and Parts 

In the 1990s, researchers began looking at composites that used nanoparticles as fillers to alter their properties to ascertain whether this would make them more useful in cryogenic applications. When mixed correctly, these composites present properties that make them ideal for cryogenic systems and parts. Nanoparticle fillers, in small amounts, increase dimensional stability, modulus, strength, and toughness while also improving resistance to chemicals, permeation of gases, and thermal degradation of polymers.

For example, nanocomposites consisting of PP and montmorillonite (MMT) were tested in room and cryogenic temperatures at which nitrogen becomes liquid. These PP/MMT nanocomposites showed improved tensile strength at both temperatures when compared to PP on its own. However, though tensile strength increased by 3.1 percent at room temperature, it increased by 13.2 percent at cryogenic temperatures.

Though much of this work has focused on thermoplastics like PP and PE, research into composites containing thermosets that can work in cryogenic temperatures is also in the works. Many of these nanocomposites, superconductors, and polymer matrixes are largely being studied for space-related applications. However, their various properties demonstrate that they may someday be used for cryogenic systems and parts.   

Spaulding Composites

Spaulding Composites has been at the forefront of developing materials that can withstand cryogenic temperatures. Widely used in applications involving liquified helium and nitrogen, these composites are used for cryogenic systems and parts that include neck tubes. For more information regarding these materials used in cryogenic storage systems and other composites we make, contact our customer service representatives at Spaulding today.