Question

Several types of thermoplastic elastomers were described in lecture and the textbook. Briefly describe the two phase model that explains the behavior of these polymers.

Answer 1

Thermoplastic elastomers (TPE) materials, sometimes referred to thermoplastic rubbers (TPR) materials, the definition of thermoplastic elastomers is a physical mixing of polymers, polymer blend or one type of block copolymers usually consist of plastic and rubber without crosslinking occur, which provide both thermoplastic and elastomeric properties and above its melting temperature can be fabrication and recycles to desired shaped, as little as 20 seconds. Not all amorphous polymers are elastomers, but some amorphous polymers are thermoplastics, because of an amorphous polymer is classify a thermoplastic or an elastomer depends on its glass transition temperature (Tg). Above this temperature, which a polymer becomes soft and pliable, and below which it becomes hard and glassy. If an amorphous polymer has a (Tg) below room temperature, it will be an elastomer, because it is soft and rubbery at room temperature. If an amorphous polymer has a (Tg) above room temperature, it will be a thermoplastic, because it is hard and glassy at room temperature. So the elastomer materials have low (Tg) and thermoplastics materials have high (Tg). Thermoplastic elastomer (TPE) materials combine the functional performance and properties of thermoset rubbers with the processability of thermoplastics. Therefore, the thermoplastic elastomers have the properties and performance of rubber but are processing efficiency like a thermoplastic. TPE materials can be stretched to at least twice their original length at room temperature and return to the approximate original shape upon stress release.

The processability at melting temperature of most thermoplastics elastomers makes them easy to use in manufacturing and very suitable for high-volume injection molding and extrusion molding. Thermoplastic elastomers show advantages typical of both rubbery materials and plastic materials. The principal difference between thermosetting elastomers and thermoplastic elastomers is the type of crosslinking bond in their structures. In fact, crosslinking is a critical structural factor which contributes to give high elastic properties.

Molecular Nature of Thermoplastic Elastomers (TPE) Materials

All rubbery materials (thermosetting elastomers) molecule chains having chemical crosslinks is usually non-reversible bonding such as (covalent bonds), between the polymer chains and joining all them into a single networked molecule. When the polymer chains are joined together like these bonds, that is lead to prevent flow under moderate stresses or when heating, therefore it is harder to pull them out of their original positions and back better when stretched. Also, these polymers cannot be recycled, remelted or remolded easily because of these types of crosslinks.
Some of the rubbery materials (thermoplastic elastomers) molecule chains naturally having physical crosslink (physical entangled) is usually weak and reversible bonding, such as (hydrogen bonds and fender walls force) between the polymer chains to bind them together. All these materials will disentangle and flow under moderate stress or when heating at high temperatures, therefore the physical entanglements will act as temporary interlinks.
The physical crosslink (physical entangled) is most important to recycle the hard thermoplastic elastomer, because of when physical crosslink is broken allowing it to take place a new shape, and the crosslinks reform again when it cools.
Therefore these materials can be easily remelted, remolded, recycle and reprocess of the scrap, reject parts and end life products, for this reason, these materials are called thermoplastic elastomer. It is difficult to distinguish between the two types of crosslinks in the elastomer chains that are only physical or chemical bonds. One method to distinguishing the type of crosslinks by the polymer dissolves in a solvent or just swells without dissolving. Covalently bonding does not dissolve in any solvent, therefore this type of bonds is necessary for good solvent resistance and for use at high temperatures.

Structural of Thermoplastic Elastomers (TPE) Material

All structure of thermoplastic elastomers (TPE) materials is composed of both crystalline and amorphous domains along the same polymer chain. The crystalline domains are referred to as the hard phase, and the amorphous domains are referred to as the soft phase. Both phases contribute to mechanical and physical properties of a thermoplastic elastomeric material. Thereby these phases act as a guide in selection or design of thermoplastic elastomers (TPE) compound. The phases of thermoplastic elastomers include:
1- Soft Phase: providing the elastomeric properties which include (hardness, compression strength, flexibility, and lower service temperature.

2- Hard Phase: providing the thermoplastic properties which include (tensile strength, tensile modulus, tear strength, heat resistance, chemical resistance, processing temperature, and adhesion resistance to ink or molding substrates.