Q: What is the difference between plastic and elastomer?
A common misconception is that the terms polymer, elastomer (rubber) and plastic are interchangeable. Although they have commonality, they are not necessarily synonymous with each other.
Plastic and elastomer are types of polymers. On a broad scale, polymers are described by molecular characteristics to include the repeat unit composition, chain shape (coiling, twisting, etc.), molecular weight and structure. Plastics and elastomers have overlapping molecular characteristics but are further defined by their response to mechanical stress, temperature and chemical structures.
Polymers are made of repeating units of atoms that are structured in a long, chain-like manner to form a very large molecule. The smallest repeating unit of the polymer chain is a monomer, also referred to as a mer unit. Polymers can be created with a single type of monomer (i.e., polyethylene [PE] or several, such as ethylene propylene diene monomer [EPDM]). The characteristics of the individual monomer will be reflected in the polymer, and the useful properties of multiple monomers in a copolymer can be applied to engineer a more robust material.
The process for making monomers into polymers is known as polymerization. The degree of polymerization (often referred to as molecular weight or MW) is the number of repeating units in the chain. As the degree of polymerization increases, the material properties change. Generally, as MW increases the density, melting and boiling points also increase. Many polymers with a very low MW will be a gas at room temperature. An intermediate MW polymer will consist of a wax-like solid, liquid or grease. At room temperature, most load-bearing applications will require a polymer with a high MW for good mechanical strength. To put MW into perspective, an ultra-high molecular weight polyethylene (UHMWPE) has millions of repeat units.
Polymer classifications were established prior to a thorough understanding of microstructure or intermolecular forces. One classification is based on the polymers’ reaction to temperature change. The two categories of this classification are thermoplastic and thermosetting polymers. Thermoplastics are polymers that will flow under an applied stress when heated. On a molecular level, secondary forces, i.e., relatively weak interatomic and intermolecular bonds diminish as the temperature of the polymer increases. The weakening of these forces results in a softer and eventually liquified material. If the temperature of the material is raised too high, irreversible degradation of the thermoplastic can result. If the temperature is sufficiently lowered, the polymer will return to a solid. Commercially relevant thermo-plastics include polyethylene (PE), polyvinyl chloride (PVC), polypropy-lene (PP) and polystyrene (PS).
Thermosetting polymers, or thermosets, are cured into a permanent shape through an irreversible process. In addition to the secondary bonds mentioned for thermoplastics, thermosets have covalent bonds that crosslink the polymer chains. These covalent bonds are of considerably higher energy and will not break when heated, which in turn, prohibits the polymer chain from moving. Excessive heating of a thermoset will cause thermal degradation. When enough heat is applied to break the crosslink bonds, the backbone of the polymer will also start to break. A few common thermosetting polymers include phenolics, epoxies and polyesters.
A second way to classify polymers is their response to mechanical force. By definition, plastics are polymer materials that have structural integrity (shape) under a load. A plastic material’s ability to maintain its shape under a load depends greatly on the arrangement of its polymer chains. The arrangement of the chain can take on an amorphous shape or varying degrees of crystallinity, which play a role in the properties. To be considered a plastic, the material must operate below the glass transition temperature (if amorphous), below the melting temperature (if crystallin) or have a crosslink density that is sufficient to maintain its shape. Plastics encompass many different polymeric materials including polyamide, polycarbonate, vinyl, among others. Plastics can be a thermoplastic or thermoset.
Elastomers, commonly known as rubber, differ from plastics in the fact they have a great capacity for large elastic deformation under an applied stress. In other words, they can be stretched over 100% of their original length with no permanent deformation. Deformation is accommodated in the elastomer material through the molecular structure and arrangement of the polymer chain. The long polymer chain is highly twisted, coiled and interwoven when at rest. When a stress is applied, the chain will uncoil and become linear. Once the stress is relieved, the chains revert to their original unstressed state. To ensure the chain is in the highly unorganized state when at rest, the molecular structure of the elastomer must not be allowed to align itself and create a crystalline structure.
A common way to prevent crystallinity and regular alignment of the chains is to add side groups on the chain or use more than one monomer. The final component to an elastomer is crosslinking. In crosslinking, adjacent polymer chains are connected by chemical bonds. The bonds restrict movement and increase strength. Generally, elastomers are classified as thermosets. However, there is a subgroup of thermoplastics called thermoplastic elastomers that have rubbery characteristics at ambient temperatures but are still thermoplastic in nature.
Elastomers by themselves have bounded commercial application because of limited properties (depending on use). Compounding the base elastomer by adding different materials can increase strength, decrease degradation, improve processability and lower cost. Selection of the proper elastomer prior to compounding is the key to success. The properties that are inherent to the base elastomer will determine appropriateness for an application. Compounding can improve an elastomer but will not completely overcome its inherent characteristics. The high formability characteristic of elastomers make this material good for sealing components but limit use in applications where maintaining shape is crucial.
Although this article is not a comprehensive review of polymers, a general idea of how polymers are classified can be drawn from their response to the application of heat, mechanical load and molecular structure. A handful of other ways to characterize polymers exist, but this is a good starting point for many professionals in the valve industry.