Polymers And Dyes

NOTE: This practical is for a teacher demonstration only, and should be performed in a fume cupboard, circumventing any contact to methanal vapour that may be given off during polymerisation. Students should be able to see the demonstration through the fume cupboard. *The details in this practical list outline the quantities and procedure for a single teacher demonstration (RSC, 2011b).

Required Apparatus Fume Cupboard Safety Goggles Lab Coat Disposable Nitrile Gloves 2-3 Disposable Cups (to be used as moulds) Pasteur Pipette (1.5 mL) Glass Rod Tongs Bunsen Burner and Heat Proof Mat Measuring Cylinder (100 cm3) Glass Beaker (150 mL)

Required Materials 20 cm3 Methanal Solution 40% (v/v) TOXIC 1 cm3 Concentrated Sulphuric Acid CORROSIVE 10g Urea (For safety considerations, see notes below)

Health and Safety Considerations This practical should only be performed by a teacher in a fume cupboard within a well-ventilated laboratory. The teacher should wear eye protection (goggles) and nitrile gloves at all times in the preparation and handling of the polymer. The disposable cups are used as a mould but 2-3 should be used (placed inside one another), additionally placed inside a glass beaker, as the reaction is extremely exothermic. The acid catalyst used for the practical should be sulphuric acid, and not substituted with hydrochloric acid due to the possibility of formation of carcinogenic bis-chloromethylether fumes. (Nuffield, 2011b)

Methanal solution gives off toxic fumes, and is toxic particularly if swallowed, can cause severe burns and respiratory irritation (CLEAPSS Hazcard 63) (CLEAPSS, 2010d). Concentrated sulphuric acid is corrosive, reacts violently and may cause burns if in contact with the skin (CLEAPSS Hazcard 98A) (CLEAPSS, 2010e). Urea (carbamide) is reasonably non-toxic (it is used as a fertiliser) and is considered low hazard for this practical (CLEAPSS Hazcard 35) (CLEAPSS, 2010f).

Preparation of the Urea Methanal Polymer

Outside of the fume cupboard: Using 2-3 disposable cups (one inside another), placed inside a glass beaker, 10g of urea should be added into 20cm3 methanal solution. The solutions are then swirled/stirred well using a glass rod. As this part of the reaction is endothermic and requires heat energy, the solution may be warmed slightly using a hair dryer to aid the dissolution of the urea into the methanal. A heat block could be used as an alternative.

Inside of the fume cupboard: With the urea has dissolved in the methanal solution, the containers should be moved into the fume cupboard, and 1cm3 of concentrated sulphuric acid is to be added drop wise into the solution, swirled steadily to mix. The solution will give off a steamy vapour and the reaction rapidly becomes exothermic, changing from a colourless to cloudy solution. The reaction mixture should be left in the fume cupboard for 10-30 minutes to allow polymerisation, at which point the cloudy solution had solidified.

Outside of the fume cupboard: Using gloved hands only, the reaction mixture may be removed from the fume cupboard. After demonstrating that it had solidified, the polymer can be cut free from the cup moulds and washed thoroughly using tap water. Washing ensures that any unreacted chemicals are removed, particularly sulphuric acid and methanal solutions. The solid polymer may only be passed around students to investigate once the solid has been thoroughly washed.

Testing the Urea Methanal Polymer Properties 1. Using the plastic cups as the mould, it is shown that the polymer solid will take on the shape of the container. The deformed plastic cups highlight the exothermic nature of the reaction. 2. The solid polymer may be passed around students, in doing so students are able to see that the polymer is hard and rigid. 3. Force (stress) should be applied to the polymer (using scissors or another tool within the laboratory) to demonstrate the physical properties of the polymer e.g. rigidity and hardness 4. The polymer can be heated, holding it over a lit Bunsen burner using tongs, to further demonstrate the properties of the polymer, and enable students to decide whether it was a thermoplastic or thermosetting polymer. Information adapted from (RSC, 2011b, Nuffield, 2011b)

Polymerisation (cross-linking) of Polyvinyl Alcohol (PVA) and Sodium Tetraborate

NOTE: This practical is considered low hazard and can be performed by students, however care must be taken when using some of the chemicals. Health and safety is therefore addressed in a separate section (Nuffield, 2011a).

To be prepared prior to the practical: A 4% (w/v) solution of PVA must be prepared in advance of the practical, which can be achieved by dissolving 40g of polyvinyl alcohol solid in 960 mL distilled water. This should be prepared on a hot plate using a magnetic stirrer, and should be well cooled in advance of the practical.

A 4% (w/v) solution of Borax (Sodium Tetraboratedecahydrate, Na2B4O7¬.10H2O) should also be prepared prior to the class practical, and can be achieved by dissolving 4g of solid in 96 mL of distilled water.

* The quantities advised should make sufficient for 8-10 groups, and the apparatus and chemicals outlined below highlight the quantities needed per group for one ball of slime (Nuffield, 2011a).

Required Apparatus Goggles and Lab Coats Disposable Gloves (non nitrile) 100cm3 glass beaker Small Petri Dish Glass Rod 20/50cm3 measuring cylinder

Required Materials 40cm3 Polyvinyl Alcohol (PVA) Solution (4% w/v) 10cm3 Borax Solution (4% w/v) Food Colouring 0.5mol dm-3 Hydrochloric Acid (HCl(aq)) 0.5mol dm-3 Sodium Hydroxide (NaOH(aq))

Health and Safety Considerations Disposable plastic gloves and eye protection should be worn throughout. The gloves should be non-nitrile, as they stick to the 'slime'. Gloves prevent staining of clothes and hands, although the slime should be kept away from clothing, as it contains food colouring that may stain. Safety glasses should be worn, as students will be using hydrochloric acid and sodium hydroxide solutions.

Borax solution is low hazard, although may cause irritation to skin and eyes (CLEAPSS Hazcard 14), and there are no associated hazards with PVA, it is non-toxic and is used within schools as an adhesive (CLEAPSS, 2010a). Hydrochloric acid (CLEAPSS Hazcard 74A) and sodium hydroxide solution (CLEAPSS Hazcard 91) are dilute (0.5mol dm-3), and are considered low hazard at this concentration, but are best supplied in small glass bottles with droppers, as they are irritant (CLEAPSS, 2010b, CLEAPSS, 2010c).

Preparation of the Slime Polymer Using the 100cm3 beaker for measuring, approximately 40cm3 of PVA should be measured out and several drops of food colouring should be added and stirred using a glass rod. The PVA solution will be a viscous liquid. Using the measuring cylinder, approximately 10cm3 of borax solution should be measured and added to the PVA/food colouring. The borax solution is a colourless thin liquid, which pours easily.

The solutions should be stirred vigorously using a glass rod until they form a thick gel, at which point the 'slime' may be removed from the glass beaker, kneaded for several minutes to remove trapped air bubbles, and kept out for testing. During all of these stages, students should be encouraged to wear their disposable gloves (RSC, 2011a, Nuffield, 2011a, SeeK, 2011).

Testing the 'Slime' The properties of the slime may be tested under the following conditions Information adapted from (RSC, 2011a, Nuffield, 2011a, SeeK, 2011) 1. The slime should be pulled apart slowly under low force - students are to record what happens to the slime as it is pulled slowly. 2. The slime should be pulled apart quickly with more force - students are to record what happens to the slime when it is pulled quickly, and note how this is different to test 1. 3. The slime should be rolled into a ball and dropped onto the table - students are to record what happens to the slime 4. The slime should be hit with high force (whilst on the table) - students should note what happens i.e. does the slime splash like a liquid or does something different happen to the slime 5. A small piece of the slime should be placed into the petri dish - students should add dilute hydrochloric acid drop wise into the dish and stir with the glass rod - students should note what happens to the slime, what change in texture is there, and how many drops were required for a change in texture 6. To the petri dish, students should add dilute sodium hydroxide solution drop wise and note any changes in texture, and how many drops were required for this to occur

Rigidity and Hardness: When tested for rigidity and hardness, the urea formaldehyde was hit/cut using force. It was demonstrated that although it is a solid lump, it is brittle and the polymer crumbles easily under force forming chalky lumps. The urea formaldehyde is therefore rigid and not flexible, and is not easily bent out of shape, but is not particularly hard, as although it is solid, it is not resistant to pressure, is brittle and crumbles easily under force.

Resistance to Heat: When testing for reaction to heating using a Bunsen burner, if held into the flame, the solid appears to be fairly fire resistant. It does not soften under the heat of the Bunsen burner, instead blackens and chars and becomes more crumbly. When held under the flame for a longer time, an unpleasant smell is given off and the polymer becomes increasingly more crumbly.

The slime forms a viscous/elastic gel, which appears to act as both a liquid and a solid depending on the test conditions.

Pulled apart slowly When pulled apart slowly, the slime becomes a very viscous liquid, almost gel like. The slime when pulled slowly flows and can be pulled apart to form a thin film. If pulled too slowly, the slime becomes less viscous, acting increasingly more liquid like making it difficult to pull apart. The slower it is pulled, the less viscous it becomes.

Pulled apart quickly When the slime is pulled rapidly, it solidifies and becomes brittle. Due to this, if it is pulled quickly, rather than forming a thin sheet (as above), it breaks into two separate pieces.

Dropping on the bench When the slime is rolled into a ball and dropped onto the bench, it bounces like a ball and recoils some of the way in the ball shape. When left on the bench too long after bouncing, it starts to lose shape and form a liquid pool.

Hitting the slime When the slime is hit, it does not splash like a liquid. If the slime is rolled into a ball and hit, it solidifies and fractures into little pieces. If a pool of slime is hit, the pool solidifies on impact and does not squash or splash.

Addition of Hydrochloric Acid When drops of hydrochloric acid are added, the slime dissolves and becomes a thin, non-viscous liquid. The more hydrochloric acid added, the less viscous the slime becomes.

Addition of Sodium Hydroxide When drops of sodium hydroxide are added to the dissolved slime, it regenerates and turns the liquid back into a gel. However, it never reaches the same gel like state (not as viscous) as it was before the hydrochloric acid was added. Notably, it takes more sodium hydroxide drops to regenerate the slime than it did to dissolve it initially.