In this post, we will discuss CELLOPHANE, a flexible packaging material. But before, let’s see some easy basics regarding Cellophane
Some information on Packaging Materials:
We see a lot of packages in our daily life, maybe a food packet, a packaged pouch containing portable water, books packed in a type of packaging material, etc.
So, let’s classify packaging materials based on HARDNESS:
In the above flowchart, we can see PMs are divided into 3 parts, that we come in our day to day lives. These are Flexible, Semi-rigid and Rigid Packaging materials.
In the upcoming posts, we will discuss FLEXIBLE PACKAGING
Flexible packaging’s shape can easily be deformed or changed. They have good barrier properties against environmental factors like moisture, oxygen, etc.
Its main types are –Plastic films, paper & Aluminum foils. These are widely used in retail selling like wrapping food products, enclosing potato chips, etc.
Below is a flowchart on Plastic Films, a type of Flexible packaging.
Now keeping the hierarchy narrower and more detailed, this post will discuss CELLOPHANE
It is a very thin wrapping substance derived from CELLULOSE. Since we know that cellulose is a carbohydrate and is a polymer of glucose units, Cellophane is also a polymer of glucose as it is derived from Cellulose only.
So, we know the structure of Glucose i.e.,
It is shown as-
3 figures are shown above.
FIG. 1- Shows Fischer projection of D- glucose, D means it rotates the plane of polarized light to the right
FIG. 2- Shows the FIG. 1- in Cyclic form, also called Haworth projection
FIG. 3 shows a Chair form structure of Glucose- The chair form can also be drawn from the Haworth projection.
Below is additional info on how to convert from Fischer to Haworth, i.e. to make Cyclic form from an open structure just like Fig.1
Also, have a short look at the conversion of cyclic i.e. Haworth projection to Chair form-
So, these were a few basic terms I thought to tell you. We can interchangeably draw any of the structure of Glucose. Since Cellulose is a polymer of thousands of Glucose units, here it is shown how the glucose units in their Haworth projection form, polymerize to form Cellulose. One more thing, each glucose unit will join with others by a GLYCOSIDIC LINKAGE.
See below how glucose units polymerize to form cellulose.
WE WERE TALKING ABOUT CELLOPHANE.
So, How to Make CELLOPHANE from CELLULOSE? (Its very simple)
- STEP 1– Take Cellulose as a Raw material and dissolve in an ALKALI like NaOH. Leave it for several days. It will then be called as a Mercerized pulp.
- STEP 2– Then this mercerized pulp is treated with CS2 (Carbon disulfide) to form an Orange Solution called VISCOSE OR Cellulose Xanthate
- STEP 3– The Viscose solution is then extruded into a bath of H2SO4 and Na2SO4 via a slit, to reconvert the VISCOSE into CELLULOSE. When it is forced through a slit, it is converted into a film.
- STEP 4– Now the film is passed through 3 more baths, first to remove Sulfur, second to bleach the film (bleaching means to make white or colorless) & third to add Glycerin (which is added to become more flexible and to prevent brittleness of the films.
Below figure clearly shows STEP-2, i.e., conversion of Cellulose to Viscose. (Please zoom a little bit:)
- STEP 5– One problem with Cellophane is that it is permeable to water vapor, i.e. outside or inside water vapors get exchanged with the film, so if a food is packed with cellophane, external water vapors may enter into the food via the cellophane film and make the food soggy. One widely used process is to coat the cellophane film with Nitrocellulose. It makes the film impermeable to water vapors. (Nitrocellulose is simply cellulose, only its OH groups are replaced by Nitro groups i.e., NO3, this replacement can be done by adding a mixture of H2SO4 & HNO3 to the Cellulose)
Above 5 steps are the basic steps of Cellophane manufacturing. The more detailed procedure for Cellophane preparation is given below in 3 steps mainly-
STEP 1- VISCOSE PREPARATION
STEP 2- XANTHATION
STEP 3- CELLOPHANE CASTING
STEP 1- VISCOSE PREPARATION
Firstly, we have to take 100g cellulose. Since loose cotton has 90% cellulose, we will take cotton as a sample.
Add 18% solution of NaOH to it and cotton should remain deep in this solution for 1 hour to convert the cellulose into Alkali cellulose.
(C6H10O5)X + xNaOH → (C6H9O5Na)x
Cellulose Alkali Cellulose
Now we have to decant (flow out the liquid from the container) the Wet alkali cellulose and transfer to a Buchner funnel having a suction flask. Please see below fig…
After the suction is complete, alkali cellulose is mostly dried in the Buchner funnel, yet the cake left of alkali cellulose is dried by pressing by hand.
STEP 2- XANTHATION
Xanthation is a simple process of making cellulose xanthate. A cellulose xanthate is a cellulose ester, which we obtain by treating cellulose with a caustic soda (NaOH).
So, after breaking the large lumped pressed cake of Alkali cellulose (from STEP 1) into smaller crumbs, we will transfer it to a wide mouth bottle (1-gallon capacity) and then add CS2 to it and keep at 30℃ for 2 hours.
PROCESS OF XANTHATION-
(C6H9O5Na)x + xCS2 → (C6H9O5CS2Na)x
Alkali Cellulose Sodium Cellulose Xanthate
Now, open the bottle in a hood and add 1kg of Coldwater (5℃) and 200g of 18% NaOH to it. Shake it with an agitator for 2 hours while maintaining 10-15℃.
Finally, we will obtain a thick Orange colored solution called VISCOSE. Store at 0-15℃ for 24 hours. If we want to remove the air bubbles, we can apply a vacuum during storage.
STEP 3- CELLOPHANE CASTING
Now, since we have got VISCOSE, we have to sheets of cellophane. So, spread 0.01-0.02 inch of viscose on a plate and keep in a 40% NH4SO4 bath at 45℃ for 60 seconds. By doing this, coagulation of viscose happens i.e. it changes to a somewhat semi-solid state.
Now to reconvert the coagulated viscose sheet into cellulose, we will put it in a (12% H2SO4 + 18% Na2SO4) bath for 2 minutes. We can write the reaction as-
2(C6H9O5CS2Na)x + x H2SO4 → 2(C6H10O5)X + 2xCS2 +x Na2SO4
Sodium Cellulose Xanthate Reconverted cellulose
Then the film is washed in water for 10 minutes and clamped to an embroidery hoop to avoid shrinking of the hoop during air drying at temperatures <100℃.
If we want more flexible cellophane, we can put the wet sheet into 5% aq. glycerine solution for 15 min before drying. By doing so, the cellophane film will get plasticized i.e. it will become less brittle.
So, these were the detailed 3 steps of Cellophane manufacturing in very detail with the chemical reactions also.
Various types of Cellophanes films are designated specific letter codes on the basis of their properties-
- M means Moisture-Proof,
- C- Colored
- S- Heat Sealable
- T- Transparent
- D- Demi i.e., One side coated
So, you see a cellophane packaging material having code CST, then we can say that this cellophane type is a Colored, heat sealable and Transparent. Heat sealable means two layers can be joined through the application of heat and pressure.
Below pictures show colored cellophane films and film for wrapping meat loaves, that can prevent further moisture or air till consumption.
Applications of Cellophane-
- Cigar packaging- We know that tobacco products are prone to moisture and they become soggy on the absorption of moisture, so, cigars are packaged in nitrocellulose coated cellophane films which makes it impermeable to moisture or water vapor transfer.
- Artwork- Cello Graff is a famous artwork that uses cellophane films to stretch between trees and drawing graffiti arts on them for social or entertainment purposes. (Source-Graffiti artist Evgeny Ches)
- Gift Wrappings- Cellophane films are available in different colors and different attractive colorful designs are printed on them, which are perfect for handing over to others as gifts.
- Adhesive tapes- Sellotape is a world-famous cellophane based adhesive tape which requires a small pressure to stick on joining surface
- CD wrappers- Since CDs are not much of use today, earlier CDs wrappers were used by shoppers to prevent them from becoming dirty before selling.
So, this is a comprehensive and illustrative detailed overview of Cellophane as a packaging material. One of the best things about cellophane is that it is Biodegradable too !!
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