PRINT: Back to Basics

Back to Basics.

Anyone designing an O-ring seal must answer a multitude of questions. In a sense, that’s really what good design work is all about: determining which questions to ask and where to find the answers. Though the sealing industry has a multitude of exotic terms, a clear understanding of a few basic concepts will help you ask the most pertinent questions and find the most productive answers. To that end, here’s a quick review, starting with the building block of all materials, the atom.

AN ATOM IN ACTION
An atom is the smallest unit of an element that 1) retains all the element’s distinctive properties and 2) can enter into a chemical reaction. In other words, anything less than an atom of carbon (C) is no longer carbon. A carbon atom can be split into its component parts (see Figure 7), but the resulting subatomic particles (positively-charged protons, non-charged neutrons, and negatively-charged electrons) do not reflect the properties of carbon. Though their number and arrangement vary from element to element, subatomic particles alone tell you nothing about the atoms from which they came. A proton from a carbon atom is identical to an oxygen (O) proton.

Subatomic particles are important, however, because they determine one of the defining characteristics of any atom: its atomic weight, or the total mass of the protons and neutrons within its nucleus (orbiting electrons are of negligible weight and don’t figure into this total). For example, the nucleus of a hydrogen (H) atom contains one proton and no neutrons, so its atomic weight is 1. Carbon is composed of six protons and six neutrons, for an atomic weight of 12

Each individual atom is also distinguishable by the one or more energy bonds it can form with neighboring atoms. This ability to combine is known as valence, and the amount of valence varies with each element. For example, an atom of hydrogen has a valence of 1, meaning it can form only one such energy bond. Oxygen has a valence of 2 and carbon has a valence of 4, meaning they can form two and four bonds, respectively. To be more precise, atoms need to form these energy bonds in order to be “satisfied” or “stable.” The interaction of differing valences is what allows a group of atoms to join together into a molecule.

MOLECULAR MATCHMAKING
The kind of molecule is determined by the exact type and number of atoms. For example, a water molecule is made up of just three atoms: two of hydrogen and one of oxygen. The components of a water molecule are most simply expressed by the well-known chemical formula “H20” or by the structural diagram: H-O-H (see Figure 8). A water molecule can be considered stable because the valences of each of its atoms are satisfied: the two hydrogen atoms have formed one bond each, and the single oxygen atom has formed the two bonds it needs.

When dissimilar atoms join together (as with water), the resulting molecule is called a compound. There are two major types of compounds: organic and inorganic. Generally speaking, organic compounds contain carbon and inorganic compounds do not, though a handful of carbon-containing compounds (such as metallic cyanides, carbon dioxide, carbides, and carbonates) are studied as part of inorganic chemistry.

The specific way in which a molecule is formed depends on which type of compound it is. Inorganic compounds are formed when an atom gives up, or transfers, one of its orbiting electrons to a nearby atom. Thanks to the rules of valence, this electron transfer can help both the donor atom and the recipient atom become more stable. Because the carbon atom has a compact structure, it is much less inclined to give up an electron. It will, however, share an electron with a nearby atom (such as hydrogen) to attain a more stable compound.

As previously stated, each atom has its own atomic weight. When atoms unite to form a molecule, the sum of these atomic weights is then known as the molecular weight. For example, a methane molecule (CH4) combines the atomic weight of one carbon atom (12) with the atomic weight of four hydrogen atoms (1 x 4), for a total molecular weight of 16. In addition to hydrogen, oxygen, and carbon, there are a handful of other atomic elements that form the basis for the majority of raw materials used in the sealing industry. These include nitrogen (N), fluorine (F), silicon (Si), sulfur (S), and chlorine (Cl). The atomic weights and valences of each of these elements are listed in Table 1.

LINKS IN THE CHAIN
Small, individual molecules are known as mers, or monomers (literally, “single mers”). When conditions are right, these small molecules can chemically “link” together to form long, chainlike structures. The macromolecules (giant molecules) that result may incorporate thousands of the original monomers. These long chain macromolecules are therefore known as polymers (“many mers”). The linking process itself is called polymerization. An example of this process is shown in Figure 9. Methane monomers can combine to form ethane, and eventually, polyethylene. Rubber and plastics are polymer-based materials.

Changes in physical properties as a result of polymerization are largely a factor of molecular weight. When molecules (each with their own total weight) join to form a polymer, the sum of the molecular weights has a huge impact on the polymer’s physical properties. As a general rule, an increase in chain length (and thus molecular weight) also means an increase in strength and viscosity (resistance to flow).

Table 2 shows the effects of increased molecular weight. By adding CH2 groups, the polyethylene molecule goes from a gas (with little or no physical properties) to a liquid, then to a wax. Continued addition of molecules forms a tough solid (of relatively low molecular weight, or LMW) such as is used for plastic grocery bags, then a very tough solid (of high molecular weight, or HMW), and soon an extremely tough, solid plastic (of ultra-high molecular weight, or UHMW) used in abrasion pads and bridge bearings. No matter what its physical state, the molecule is made up of the same two elements, carbon and hydrogen, and is configured the same way. The only difference is the number of molecules (the chain length) and thus the molecular weight. Polymers with higher molecular weights are key in the formulation of high strength materials for extreme applications. In the higher molecular weights, the numbers of carbon and hydrogen atoms are ranges rather than absolute values. The HMW and UHMW polyethylene formulas in Table 2 are averages.

FORCES OF NATURE
Long polymeric chains (such as those in polyethylene) are held in place by intermolecular forces (known as van der Waals forces) and by chain entanglement (as in a bowl of spaghetti). The intermolecular forces are heat-sensitive, so that as a polymer is heated, the molecular motion increases and the attractive forces between the molecules decrease. The polymer chains can then slide past one another.

Some polymers are composed primarily of linear, symmetrical molecules arranged in close proximity to one another. This proximity allows the intermolecular van der Waals forces to be at their strongest, and the polymer will thus be very rigid. These orderly polymers are said to be crystalline in structure (see Figure 10). Polyethylene plastic is a good example of a crystalline polymer.

Other polymers are composed mainly of branched, non-symmetrical molecules that cannot fit closely to one another. Because of this increased distance between the molecules, the van der Waals forces will be at their weakest, resulting in a random mass of twisted and entwined polymer chains. These polymers are said to be amorphous (see Figure 11).

Because their intermolecular forces are not very strong, amorphous polymers can be thought of as very viscous (thick) liquids that appear to be solids. Though it is possible to have an amorphous plastic, most plastics are either crystalline or semi-crystalline. Keep in mind that it is common to see tough polymers in which the numerous crystalline segments are surrounded by a few amorphous areas (as in Figure 12). All rubbers or elastomers are amorphous at room temperature.

Though the term elastomer was initially used to denote a synthetic form of natural rubber, “elastomer” and “rubber” are now more or less synonymous. To be officially considered an elastomer by the American Society for Testing and Materials (ASTM), a polymer must not break when stretched 100%, and it must return to within 10% of its original length within five minutes after being held for five minutes at 100% stretch.

An elastomer is perhaps best described as a visco-elastic material, in that it goes through both a viscous phase and an elastic phase. The visco-elastic behavior of elastomers can be simulated using a spring coupled with a dashpot (damper). The spring illustrates the elastic phase, and the dashpot exemplifies the viscous phase (see Figure 13).

STATE OF ENTANGLEMENT
But why is an elastomer elastic and resilient, able to undergo high strain and yet recover its original shape? Put simply, it’s the tangled nature of its long molecular chains. When pressure (in the form of a compressive load or a stretching force) is applied to the elastomer, the chains rotate around their chemical bonds. This rotation tends to uncoil the entangled mass and straighten the chains. When the pressure is removed, the chains coil up again, reverting to their normal state of entanglement. This tendency to return to its original configuration helps explain an elastomer’s rubbery, resilient nature.

Under certain conditions, a few elastomers will have their molecules align and form crystalline regions. In some cases, this can be advantageous. Natural rubber undergoes strain crystallization, meaning its entangled macromolecular chains will untangle and align to form crystals as a result of a stretching force. This tendency to strain crystallize gives natural rubber inherently good strength and fatigue properties. In other cases, the tendency to crystallize can be a distinct disadvantage. An elastomer that crystallizes due to cold temperatures becomes harder and less able to stretch.

Because many seals will face potentially detrimental service conditions (such as extreme cold or heat), an elastomer alone is seldom an effective seal material. Other ingredients must often be added to make the elastomer easier to process and to augment its physical and/or chemical traits. These other ingredients may include fillers (to reinforce or extend the material), plasticizers (to aid flexibility and processibility), cure activators and accelerators (to initiate and speed processing), inhibitors (to ensure the reaction does not proceed too quickly), anti-degradants (to help resist environmental elements like oxygen or ozone), and pigments (for colorization). The combination of a base elastomer and additives is called an elastomeric compound.

MAKING CONNECTIONS
After a compound has been formulated, it must still be processed into a useful form (such as an O-ring). Under normal conditions, an elastomer’s amorphous chain segments are free to move relative to one another. This is not true only when the chains meet mechanical entanglement (as with the spaghetti effect), or when the separate chains are chemically connected. Vulcanization (also known as cure) is a heat-induced process whereby the long chains of the rubber polymers are permanently cross-linked to one another, thus forming three-dimensional elastic structures (see Figure 14). Aided by curing agents (also known as vulcanizing agents) in the original compound, vulcanization transforms soft, weak, non-cross-linked materials into strong elastic products. In addition to making the compound stronger, the vulcanization process is also generally the point at which the material is molded into a useful shape that it retains due to its memory.

Though every effort has been made to simplify the preceding discussion, it’s important to realize that putting together an elastomeric compound can get quite complex. Decisions made in compounding will ultimately impact the processing and performance of any seals produced from the compound. Depending on the type and degree of additives in use, a single base elastomer can generate hundreds of different compounds, each with unique characteristics. Since choices made during compounding directly determine the properties of an elastomeric seal, let’s look at these physical, thermal, and chemical properties next.

“A clear understanding of a few basic concepts will help you ask the most pertinent questions and find the most productive answers.”

Figure 7