Basic Approach for the selection of Engineering Plastics

Plastic! An organic polymer of high molecular weight with certain additives. It has registered its importance in a wide range of applications in this modern world. 

Engineering Plastic :- Engineering plastics are thermoplastics with superior mechanical and/or thermal properties. This specific group of plastics have replaced traditional engineering materials like wood, ferrous metal etc. These are mainly famous for their stability, low cost (as compared to traditional engineering material), repeatability (giving same result in case of mass production) and chemical resistance to ambient condition.

Plastics can be classified as,

• Thermoplastics and Thermosetting plastics
•  Amorphous and Semi - Crystalline plastics

There are so many options in Engineering Plastics, how can we make a correct selection?

1.  Understanding the Morphological Characteristics of plastics

            It basically refers to the arrangement of the constituents after solidification. This can be amorphous or semi-crystalline. Properties associated with these forms are,

 Some common plastics falling under these two categories are as follows,

Amorphous Plastic :- Polyetherimide (PEI), Amorphous Nylon, Acrylic (PPMA), High Impact Polystyrene (HIPS), Polystyrene (PS), Polycarbonate (PC), Acrylonitrile Butadiene Styrene (ABS), Polysulfone (PSU), Polyethersulfone (PES), Styrene Acrylonitrile (SAN) etc.

Semi-Crystalline Plastic :- Polyoxymethylene (POM), Polylactic Acid (PLA), Polyphthalamide (PPA), Polyetheretherketone (PEEK), Polypropylene (PP), Polyethylene (HDPE, LDPE, LLDPE), Polyphenylene Sulfide (PPS), Polyethylene Terephthalate (PET), Polyamide (PA/Nylons), Polybutylene Terephthalate (PBT) etc.

       2. Thermal Stability Requirement based on the point of application

           Thermal stability plays an important role in the selection of particular type of plastic as a Raw Material for a particular product. This property restricts the processing temperature and application to atmosphere which may cause thermal degradation of reinforced fibers, composites and polymer matrix leading to deteriorated mechanical properties.
  Ranking of some common engineering plastics based on their thermal properties are as follows,

         **Thermal stability directly impacts unit price of the plastic grade. So the above table is also showing price comparison for all the products, top row products being costliest. 

Features of some common Engineering Plastics are,

1.  Polycarbonate (PC) :- It is used to manufacture transparent parts with high impact resistance. Moderate cost.
2. Polybutylene Terephthalate (PBT) :- Good Nucleation and Hydrolysis property renders it for easy moulding, dimensional stability against humidity and good electrical resistance. Moderate Cost.
3. Nylon-6 :- It is known for dimensional stability. It is also strong and still but sensitive to humidity. Moderate Cost.
4. Nylon-66 :- Superior to Nylon-6 in terms of thermal properties. Other properties are same.
5. Polyoxymethylene (POM) :- Its use credited to High hardness, Excellent resiliency an Fatigue endurance. Moderate cost.

       So let’s try with some examples, here we will 1^st place our requirement and then select a particular grade of plastic,

How to enhance properties of semi-crystalline plastics?

      1.   Reduction of shrink rate and improvement in dimensional stability.

     If shrinkage in X and Y direction is not same it will lead to warpage. Warpage is quite common for semi-crystalline resins. In order to tackle this, foreign particles are introduced in the lattice structure to ensure uniform solidification and to increase strength. These foreign particles are Glass Fiber and mineral/ beads. The minerals commonly used as fillers in plastic molding compounds include calcium carbonate, talc, silica, wollastonite, clay, calcium sulfate fibers (also known as Franklin fiber), mica, glass beads, and alumina trihydrate.

     Example :- Oil Pan for automobiles are made up of Nylon 66 + 15% Glass Fiber + 25% Minerals.

      2.       To make semi-crystalline plastics as transparent.

     Semi-crystalline plastics are having inferior optical behavior because of partially crystalline matrix reflects wavelength of visible light. Compounding nucleator into semi-crystalline plastics controls crystal size, making it less than wavelength of light, thus renders transparency.

     Example :- Chemical Beakers are made up of PP + Nucleator.

       These are just two cases, there are 1000s of different forms of plastics based on the additives/ alloy. A huge family of plastics is there because of its manufacturing process. Two main processes are used to produce plastics - polymerisation and polycondensation - and they both require specific catalysts. In a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. Each polymer has its own properties, structure and size depending on the various types of basic monomers used.