Polymers

Biopolymers

As the name suggests, biopolymers are an alternative to oil-based plastics and are developed from living beings. Chemically, biopolymers consist of covalently bonded long chains made of repeating units such as nucleotides, amino acids, monosaccharides, etc. Examples of biopolymers also include proteins, nucleic acids, DNA and RNA that are most important for humans in encoding and decoding the genetic data.
There are four main types of biopolymers-
  • Sugar based– created form lactose obtained from potato, wheat, maize, etc.
  • Starch-based– composed of glucose extracted by melting starch found in corn, tapioca, wheat, etc.
  • Synthetic materials based- biodegradable polymers obtained from petroleum, such as aliphatic-aromatic copolyesters
  • Cellulose-based- composed of glucose and obtained from cotton, corn, wood, etc.

Bioplastics

Every bioplastic is derived in a different way from its respective biopolymer. The two most popular bioplastics are PLA (Polylactic Acid) and Cellophane.

Polylactic Acid (PLA) production

PLA can be produced in two ways: using ring-polymerization and condensation (to produce a comparatively heavier polymer).

In the former process, lactic acid and lactide are the two monomers taking part and forming polymers by ring polymerization (the process in which the terminal end of one monomer forms a binding point for a new one, resulting in further polymer formation). The lactic acid being used is produced from fermenting crops like corn. Metal catalysts are used in order to distribute the different monomers uniformly throughout.

Image: Diagram of polylactic acid(PLA) manufacturing process

Condensation of lactic acid is used to produce heavier species of PLA. This requires complete removal of water so as to get the desired weighing species which is made sure by carrying out this reaction completely (the reaction is reversible if left in-between) in not more than 200°C.

Cellophane Production

Cellophane (or regenerated Cellulose) is produced by treating cellulose films in several baths of carbon disulfide as well as sulfuric acid and sodium sulfate for several interconversions between viscose and cellulose respectively. Also, the cellulose film is bathed in glycerin to prevent its hardening.

(Source: Wikipedia- Polylactic Acid; Cellophane)

You cover your notebooks, wrap your food and pack your other confectionery items with Cellophane. You’d be glad to hear that in doing so, you are helping in saving the environment, or are you? Cellophane (a cellulose-based bioplastic) has an advantage over polyethylene by being biodegradable.

But that’s it.

They just have the tendency to bio-degrade and that too under certain conditions. Their agro-based nature makes them environmentally promising. But from an economic perspective, bioplastics are not that worthwhile. Here’s why.

Which is better – Conventional Plastic or Cellophane?

Bioplastics are attention-bidding and not reliable:

Yellowing of Cellophane

Cellophane can turn yellow after some time due to its tendency of UV Degradation. The major component of light involved in degrading cellophane is the UV radiation which gets absorbed and forms free radicals by disintegrating the structure of the fiber. These free radicals, being highly reactive, get oxidized by the atmospheric oxygen and form carbonyl groups.

Usually, most plastics are known for their stability and reluctance to external structure-disturbing factors. But some (including Cellophane) can be vulnerable to such due to the presence of impurities or catalysts that may respond to the environment. The catalyst (TiO2) present in Cellophane’s structure absorbs the UV rays which cause fiber degradation.

All this absorption of high-energy UV rays leads to the discoloration of the cellophane, making it appear yellow (because of the carbonyl groups formed from oxidation and a change in the chemical composition).

Preventive measure (UV Stabilizers)

The oxidation can be prevented by using UV Stabilizers which work by forming a coating over such unstable polymers and absorbing the UV radiation preventing its further reaction with the actual polymer structure.

Other Major Drawbacks of Cellophane

Ironic to their name, Bioplastics (like Polyhydroxyalkanoates, Cellulose Acetate, Cellophane) are not really boon to the environment. The use of carbon sulfide (capable of brain damage) in the production of Cellophane is a menace to a lot of workers.

Cellophane exhibits immense photic degradation. The components involved are:

High temperatures, which are not suited to Cellophane due to its thermal degradation flaw, and since cellophane contains catalysts like TiO2 responding to the UV radiation, it is highly susceptible to lattice disorientation within a short time span if not protected by UV stabilizers.

What does it mean when a bioplastic is biodegradable?

This shows that a plastic remains a plastic. Bioplastics, however, need controlled conditions to degrade and are different from naturally degradable materials. Even after their complete breakdown into organic by-products, the by-products may still be harmful (as is the case with Cellophane).

Cellophane becomes a dominant producer of greenhouse gases (majorly Methane) when left to decompose without any by-product treatment provision.

Note: Although Methane is one of the best fuels with a very high calorific value, it has the highest heat-trapping capacity as well among the greenhouse gases, enabling it to trap much higher heat than others. Without a check over its production, it can accumulate within the atmosphere and eventually, promote global warming.

(ReferenceWhat is the future of bioplastics?

On the contrary, conventional plastics have an edge over bioplastics (like Cellophane) as they’re better stable and resistant to most environmental vagaries. They are recyclable and economical, balancing their non-biodegradable nature.

However,

Cellophane birefringence makes possible more microscopic observations

Birefringence is the property of a lattice to show different degrees of light-bending depending upon the direction of light or its own orientation. Cellophane also has a birefringent lattice.

Scientists have worked out ways to achieve detailed microscopy by using cellophane sheets in place of expensive commercial mechanisms.

This is done by arranging four parts of cellophane sheet in four different (suitable) orientations, causing the unpolarized light passing through them to polarize and focus at a specific point. This polarization generates Cylindrical Vector Beams.

Read here for a complete description of how it has pioneered highly magnified, deep, clear and cost-effective observations.

Graphene

And this is not it. Any process that needs to prevent air loss can be made better with the use of graphene. As discovered by researchers, graphene can also exhibit natural magnetic properties at atomic level by steering the spinning electrons of its atoms.

To have a look on how it is prepared, watch this video by the science channel.

Material Engineers: Gear up! Let’s find new ways to produce this miracle substance.

Polymethyl Methacrylate

Polymethylmethacrylate or PMMA is another weird yet a very versatile polymer. It is used in making rear lights and spectacle lenses. Another popular use of PMMA is in the Lichtenberg figure sculpture. When a block of PMMA is put into an electron accelerator, a bunch of electrons are fired into the plastic until it has approximately two million volts of charge, then the side of the plastic is brought in contact with a bit of wire, and what follows is magic! (Open this video in youtube and read the description for the science behind it.)

Other Useful Facts

In general, homopolymers offer a high strength to weight ratio making them way stronger and stiffer than copolymers. Why? This is because in copolymers the randomly dispersed comonomer units usually unzip under thermal stress or when exposed to hot water or hot alkaline solutions. However, onset of shear-induced crystallization often makes homopolymers brittle which sometimes acts against their degree of usability.

Therefore, Polystyrene Homopolymer, a transparent commodity-thermoplastic is rigid, relatively hard and an excellent gamma radiation resistor. It is used in making of toys, light diffusers, house wares and packaging materials.

Similarly, Polypropylene Homopolymer also known as PPH is an addition polymer with good chemical resistance and joining ability. Therefore, it is used in many corrosion resistant structures over polypropylene copolymers. It is also used in packaging for consumer products, plastic parts in automotive industry, living hinges, and textiles.