2. Structure, Morphology and Properties

 

Contents: Configuration and conformation of polymers, forces within polymers, molecular weight distribution, determination of molecular weight, polymer solutions: criteria of solubility, thermodynamics of polymer solution, Flory-Huggin’s equation.

Additives and their types: fillers, reinforcements; fibres & resins and their applications; nanocomposites, plasticizers, antioxidants, heat stabilizers, flame retarders, colorants, curing agents, compatibilizers, impact modifiers, lubricants:

Additives: Polymer additives are the chemicals that are added to polymer matrix to improve the process-ability of polymers, enhance the service life of the polymer product or to suit some special end use requirement of the product.

Generally polymer additives can be classified as anti-aging additives, processing additives, optical property modifiers, surface property modifiers and special additives. Typically these additives are added <5% wt of the polymer.

Fillers: 

Filler materials are particles added to resin or binders (plastics, composites, concrete) that can improve specific properties, make the product cheaper, or both.

Worldwide, more than 53 million tons of fillers (with a total sum of approximately US$18 billion) are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants

The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black.

Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity etc. 

A good example of this is the addition of talc to polypropylene

Most of the filler materials used in plastics are mineral or glass based filler materials. 

 

Reinforcements:

Reinforcements are typically non-isotropic (i.e. they have directionality) which results in properties that may be different in the X, Y, and Z directions.

Composites are designed so that a large portion of the load is carried by the reinforcements resulting in high strength to weight ratios.  

The chemical nature of the reinforcement as well as the form of the reinforcement are important composite design parameters.

Types of reinforcements

  • Glass fibers
  • Carbon or graphite Aramid (Kevlar) fibers
  • Ultra High Molecular Weight Polyetheylene (UHMWPE) fibers
  • Exotic fibers (boron)
  • Particulate fillers (ceramic fillers (calcium carbonate, fumed silica), metal fillers)  

Polymer composites:

A polymer matrix composite (PMC) is a composite material composed of a variety of short or continuous fibers bound together by an organic polymer matrix. 

PMCs are designed to transfer loads between fibers of a matrix. 

Some of the advantages with PMCs include their lightweight, high stiffness and their high strength along the direction of their reinforcements. Other advantages are good abrasion resistance and good corrosion resistance.


Polymer nanocomposites (PNC):

Polymer nanocomposites (PNC) consist of a polymer or copolymer having nanoparticles or nanofillers dispersed in the polymer matrix. 

These may be of different shape (e.g., platelets, fibers, spheroids), but at least one dimension must be in the range of 1–50 nm.

The increase in surface area-to-volume ratio, which increases as the particles get smaller, leads to an increasing dominance of the behavior of atoms on the surface area of particle over that of those interior of the particle. 

This affects the properties of the particles when they are reacting with other particles. Because of the higher surface area of the nano-particles, the interaction with the other particles within the mixture is more and this increases the strength, heat resistance, etc. and many factors do change for the mixture.  


Plasticizers: 

Plasticizers are relatively non-volatile organic substances (mainly liquids). When incorporated into a plastic or elastomer, they help improve the polymer's:

  • Flexibility
  • Extensibility and,
  • Processability

Plasticizers increase the flow and thermoplasticity of a polymer by decreasing the viscosity of the polymer melt, the glass transition temperature (Tg), the melting temperature (Tm) and the elastic modulus of the finished product without altering the fundamental chemical character of the plasticized material.


Antioxidants in polymers: Antioxidants inhibit autoxidation that occurs when polymers reacts with atmospheric oxygen. Aerobic degradation occurs gradually at room temperature, but almost all polymers are at risk of thermal-oxidation when they are processed at high temperatures.

Heat stabilizers:

Heat stabilizers are used to prevent degradation of plastics by heat, especially during processing, but also in applications. For example, they are widely used in PVC compounds. Heat stabilizers act by stopping thermal oxidation or by attacking the decomposed products of oxidation. 

ClassificationChemical CompositionApplications
Metallic saltsCalcium-ZincPVC, PS, PE
Organometallic compoundsOrganotinPVC
Nonmetallic organic stabilizersBisphenol type epoxy resinPBT, other thermoplastics
Hydrolyzed polyvinyl alcoholPS

 

Fire retardants:

Fire retardants are chemicals which are added to many materials to increase their fire safety. For example, many plastics are highly flammable and therefore their fire resistance is increased by adding flame retardants in order to reduce the risk of fire.  


Fire retardants:

Fire retardants are chemicals which are added to many materials to increase their fire safety. For example, many plastics are highly flammable and therefore their fire resistance is increased by adding flame retardants in order to reduce the risk of fire. 

Ignition Inhibitors is an important way of inhibiting the ignition of polymeric materials is to increase the formation of carbonaceous ‘chars’ at the expense of combustible fuels. 

Ammonium phosphate has been used for many years as a flame retardant for cotton and is known to work by catalyzing the formation of carbon and water. 

Dyes and Pigments: 

https://www.youtube.com/watch?v=zojamNvb5dQ

Crosslinkers / Curing Agents / Hardeners

Crosslinking is a process of associating polymers through a chemical bond. In most of the cases it is irreversible and can be either intra or inter-molecular. 

Crosslinking boosts the thermal stability and mechanical properties of the polymer. 

Substances/ mixture of substances that are added to a plastic composition to assist crosslinking and enhance the curing reaction are called crosslinking/curing/ polymerization additives. 

Some examples of crosslinking/curing/polymerization additives are organic peroxides, amines or amides, silanes, epoxies, free radical & UV cure monomers, isocyanates etc

Compatibilization:

In polymer chemistry, Compatibilization is the addition of a substance to an immiscible blend of polymers that will increase their stability. 

Polymer blends are typically described by coarse (rough or harsh or hairy), unstable phase morphologies which results in poor mechanical properties. 

Compatibilizing the system will make a more stable and better blended phase morphology by creating interactions between the two previously immiscible polymers. 

Not only does this enhance the mechanical properties of the blend, but it often yields properties that are generally not attainable in either single pure component.


 

 PET: Polyethylene terephthalate

PE: Polyethylene

Examples of reactive compatibilizers: acrylic functions grafted on polyolefin, polyethylene and PP, allow compatibilization with PAs, polybutylene terephthalate (PBT), PET. 

Cohesive force:
Ø The cohesive force between chain segments of polymer molecule ,which tightens the polymer coil.
Ø But wh...

 

of reactive compatibilizers: acrylic functions grafted on polyolefin, polyethylene and PP, allow compatibilization with PAs, EVOH, polybutylene terephthalate (PBT), PET.

 

Impact modifiers are added to plastic compounded materials to improve the durability and toughness of a variety of plastic resins.

According to end-use applications and polymer intrinsic resistance, formulators need to achieve a very different level of impact resistance, from general-purpose to super toughness.


The principle is to disperse impact modifiers into the brittle matrix, a dampening phase capable to absorb energy and stop craze propagation.

 

 

Lubricants for Polymers

Lubricants as additives for polymers assist the movement of one object passing another object. Their primary role is to reduce friction, minimize wear and prevent overheating of parts.

While wear and heat cannot be completely eliminated, reducing them to negligible or acceptable levels is must to maintain performance in application. And, the selection and use of right lubricant plays an important role.

Lubricants usually act by modifying the viscosity of the melt, by introducing different surface energies at the interface between the phases. But, simple sticking between the melt and the processing machinery (screws, barrels and dies) can also be a significant slowdown.  

What type of Lubricant do you Need Depending on Load & Speed?

 

A great number of substances have been selected, based on practice, as lubricants, both organic, such as soaps, fats, waxes, certain polymers (PTFE, HDPE), and inorganic ones.

 

Polymer Solution:


The dissolution of a polymer is generally a slow process, which can take even several weeks, depending on the structure and the molecular weight of a given polymer.

When a low molecular weight solute such as sucrose is added to water, the dissolution process takes place immediately. The sugar molecules leave the crystal lattice progressively, disperse in water, and form a solution.

But polymer molecules are rather different. They constitute long chains with a large number of segments, forming tightly folded coils which are even entangled to each other. Numerous cohesive and attractive both intra and intermolecular forces hold these coils together, such a dispersion, dipole-dipole interaction, induction, and hydrogen bonding.

Based on these features, one may expect noticeable differences in the dissolution behavior shown by polymers. Due to their size, coiled shape, and the attraction forces between them, polymer molecules become dissolved quite slowly than low molecular weight molecules. Billmeyer Jr. (1975) points out that there are two stages involved in this process: (i) polymer swelling, and next (ii) the dissolution step itself.

When a polymer is added to a given solvent, attraction as well as dispersion forces begin acting between its segments, according to their polarity, chemical characteristics, and solubility parameter. If the polymer-solvent interactions are higher than the polymer-polymer attraction forces, the chain segment start to absorb solvent molecules, increasing the volume of the polymer matrix, and loosening out from their coiled shape. That is said that the segments are "solvated" instead of "aggregated", as they were in the solid state.

The whole "solvation-unfolding-swelling" process takes a long time, and given it is influenced only by the polymer-solvent interactions.

When crystalline, hydrogen bonded or highly crosslinked substances are involved, where polymer-polymer interactions are strong enough, the process does stop at this first stage, giving a swollen gel as a result.

If on the contrary, the polymer-solvent interactions are still strongly enough, the "solvation-unfolding-swelling" process will continue until all segments are solvated. Thus, the whole loosen coil will diffuse out of the swollen polymer, dispersing into a solution.

However, once all the chain segments have been dispersed in the solvent phase, they still retain their coiled conformation, yet they are now unfolded, fully solvated, and with solvent molecules filling the empty space between the loosen segments. Hence, the polymer coil, along with solvent molecules held within, adopts a spheric or ellipsoid form, occupying a volume known as hydrodynamic volume of the polymer coil.

The particular behavior shown by polymer molecules, explains the high viscosity of polymer solutions. Solvent and low molecular weight solutes have comparable molecular size, and the solute does not swell when dissolving. Since molecular mobility is not restricted, and therefore intermolecular friction does not increase drastically, the viscosity of the solvent and the solution are similar.

But the molecular size of polymer solutes is much bigger than that of the solvent. In the dissolution process such molecules swell appreciably, restricting their mobility, and consequently the intermolecular friction increases. The solution in these cases, becomes highly viscous.









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 https://www.youtube.com/watch?v=l244A_6BJno
 

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