Activity 3.5 - Properties of polymers

1. Through their own exploration, students should explain that polymers are substances of very diverse properties. These properties are connected with a chemical structure of polymer. This can be used for the selection of a proper material for design and production of the given product but also for the identification of unknown samples of polymers.

We will try to explore the properties of some mostly synthetic polymers. The supplied samples of polymers will undergo several tests, from which we try to deduce the properties of these polymers. Try to select the properties that could be studied with polymers, e.g. based on the properties that we would like to find in some of the products.

A range of observed properties can be broad, important for the products can undoubtedly be:

• Weight of product – observed property of the material will be density; it is of great importance e.g. for airspace and automotive industry. E.g. weight saving of 100 kg will restrict fuel consumption in average by 0.4l (http://www.autorevue.cz/automobily-jen-z-plastu-uz-se-to-blizi)
• Wear resistance – observed properties are strength and hardness of material
• Colour of product, possibility of its choice – it is optimal if the product can be coloured as necessary, i.e. a plastic is basically colourless or white and little tinged
• Possibility of processing or production of directly the shape of the given product – possibility of forming through synthesis or processing of synthetic polymer
• Flammability – some products warm up when a synthetic polymer is used for their production; the polymer should be non-flammable and should not be easy to ignite. During its burning or decomposition (depolymerisation or degradation) toxic substances should not be released – the observed property is behaviour in the flame.
• Changes due to heating – the product should not at common operating temperatures change its shape or degrade in any way. On the other hand, formability at reasonably high temperatures is a big advantage through processing (moulding, injection moulding), it reduces the costs for product processing and offers a wide range of possibilities as for the shape and function of the product – this property can be observed through heating on the cooker or the hob.
• Resistance to chemicals – e.g. sewage piping should resist the substances commonly discharged to the sewage system. Important is resistance to chemicals when cleaning the plastic parts of the product or chemical cleaning of textiles made of synthetic polymers not to cause their damage – the test will be solubility in different solvents.
  Some properties of synthetic polymers, such as appearance, density, tip puncture and behaviour in the flame, will be observed in the following experiment; other properties should be discussed.

Students themselves will propose the experiments, which will allow observing the properties given in the discussion and they try to estimate the results.

Gas burner, toluene, chloroform, water, formic acid, sulphuric acid, petrol, ethanol, cyclohexane, phenol, hob or cooker, paper or white board, two beakers of 100-150 ml, needle thermocouple

Samples of plastics (PE – polyethylene, PP - polypropylene, PS - polystyrene, PVAc – polyvinyl acetate, PMMA – polymethylmethacrylate, PC – polycarbonate, PA – polyamide, PUR – polyurethane, PET – polyethylene terephthalate, PVC – polyvinylchloride, PTFE – polytetrafluoroethylene, MF – aminoplastics)

Students can bring the samples of plastics from home; it will be optimal if they find out on the product, according to the abbreviation on the given product, what kind of plastic it is. The following survey refers to the most common plastics and some typical products made of them (in the brackets you can find the number code, which also often appears on the products – it is primarily used for material recycling):

PE – polyethylene (2 a 4) - foil, packaging, plastic greenhouses for growing plants, plastic greenhouses for silage pits, dishes – sieves, strainers, cups, cosmetics packaging

PP – polypropylene (5) – medical aids (e.g. syringes, urinals…), metal tools handles, storage bottles for chemicals, packaging of makeup removers, ointments, drops, packaging material (boxes, yogurt cups, etc.)

PS – polystyrene (6) –softened PS - insulation materials for thermal insulation of houses and structures, mechanical and acoustic insulating packaging materials, protective packaging for electronics, thermos packaging; hardened PS - CD covers, videocassettes, packaging for the so-called “black electronics“ and for pressing the kitchen items – dishes (from yogurt and cheese), graters, hangers, bowls, cheap and durable cladding tiles, model airplanes and boats, toys

PVAc – polyvinyl acetate – painting materials (trade name Latex), adhesives, translucent roofing or dental implants

PMMA – polymethylmethacrylate - shields and goggles and helmets, environment for preservation of preparations, replacements of teeth, joints and cartilages, spectacle glasses, contact lenses, cuvettes, aquariums, etc.

PC – polycarbonate – CDs and DVDs (data area layer), insulator in electronics, polycarbonate plates, instrument covers (mp3 players), lenses, components of cameras, video cameras, flashes, etc. (http://www.koplast.cz/ostatni-termoplasty-popis-termoplastu-0/)

PA – polyamide – sprockets, bearings, covers, colour foils, tights, dental floss, racket strings, parachutes, ropes, synthetic textile fibres (e.g. layer in Gore-Tex)

PUR – polyurethane – insulation (PUR foam),molitan, artificial leather (e.g. barex), textile fibres (lycra), toys, mattresses, upholstery filling

PET – polyethylene therephthalate (1) - synthetic textile fibres, tape foils, packaging for beverages (PET bottles) and foodstuffs and other liquids

PVC – polyvinylchloride (3) – sewage piping, consumer goods, water containers and similar products (cans, etc.)

PTFE – polytetrafluorothylene – surfaces of pans; ironing surfaces, ski bases, medical implants (seldom rejection by a human body), protective garments (e.g. for fire-fighters), apparatuses for chemical industry, electrical insulation products, etc.

MF – aminoplasticss – painting materials, adhesives, insulators, for production of consumer goods (e.g. dishes), electro technical material, lining (e.g. Umakart)

Polyisoprene – stoppers, tyres, constructional components of transportation means, condoms, lubricating rubber, etc.

Chloroprene – wetsuits

Testing of materials proposed by the students and corrected by the teacher should include the following tests:

Polymer tests:

Appearance test: polymer can be pre-characterised and also identified by appearance according to the shape of the product (foil, fibres, moulding, …). Describe thoroughly the appearance and shape of the product in the following table. It is possible to tick more than one option. Optical properties shall be determined as follows; in the distance of about 1 cm behind the sample place the text and according to its visibility through the sample determine its transparency:

It is natural that the products can be of different appearance and shape in spite of the fact that they are made from the same polymer. Therefore it is possible to select in the table more than one category (option) for one polymer. Nevertheless this information can provide us with valuable data. E.g. from some polymers it is nearly impossible to make a foil, or foils are not made from them. If, therefore, the product is a foil, it is probably made from (the previously referred polymers) PP, PS, PVC or PE. Some polymers cannot be made transparent or colourless (phenolic plastics) etc.

Density of polymer: Density of polymer can be determined by comparing its interaction with several different liquids of known densities. The more liquids the better; we divide the polymers into three groups based on the comparison with water (density is 1.00 g×cm‑3) and chloroform (density is 1.50 g×cm‑3).

Procedure:

Pour water into one of the beakers and chloroform to the other (work in a fume hood!!!). Throw a sample of plastic gradually into both of the beakers and observe if it is immersed or remains on the surface. Then show, in the following table, the given plastic within the range of densities:

What are the results from the observed data?

From the results it is evident that a number of plastics have a higher density than water, polyolefins (PE a PP) have it even lower. However, only exceptionally, the density of polymers exceeds 1.5. Synthetic polymers are therefore relatively light materials (e.g. of cc. 5-6 x lower density than steel or cc. 2x lower density than alumina). If they meet the other required properties, they are suitable for constructional components in aircraft industry, automotive industry, etc., where every saved kilogram counts.

Determination of hardness by tip puncture: we observe mechanical properties of polymer

(rigid, hard, brittle, and tough) and deformation behaviour. A practically conducted test is undoubtedly more complex; we can only approximately (qualitatively) arrange the studied samples of polymer according to the needle tip penetration into the polymer sample.

Procedure:

Take a needle and insert it into the sample of plastic. Prior to this, insert the needle with its upper end into a rubber stopper or a similar material so that the subsequent pressure on the sample of plastic would be as constant as possible. Qualitatively estimate the penetration of the needle into the sample and arrange the individual samples into the groups according to the ease of needle penetration:

Resistance to chemicals, solubility test:

We are looking for a chemical or solvent in which the given polymer can dissolve.

Procedure:

We sprinkle the sample of plastic with a solvent and observe (after cc. 60 seconds), if the plastic is sticky. Then the test can be considered as positive. On the basis of your exploration, fill in the following table:

Which conclusion can be made from the filled table?

The studied synthetic polymers are materials of diverse chemical properties, according to its structure soluble in different solvents, mostly of non-polar nature. In majority cases these are substances resistant to water and ethanol. Particularly resistant to chemicals are PTFE and MF. The application of suitable material is defined by its properties or, in turn, the properties determine the use of material for the given application. E.g. PE cannot be used for applications where non-polar aromatic solvents occur; however it is suitable for applications with water and polar solvents.

Behaviour in the flame: (according to http://ufmi.ft.utb.cz/texty/kzm/KZM_05.pdf) Based on the polymer composition, the given sample of polymer exerts a typical behaviour manifested through flame colouration, odour, burning (flammable vs. non-flammable), smoke, etc. You can also assess the rest of the sample whether it is charred, brownish, almost unchanged or e.g. swollen. With your samples of polymers you observe their behaviour in the flame and after their removal from the flame according to the following instructions, and record the results in the following table.

Ease of ignition of the sample

1. Sample is easy to ignite (ZÁP)
2. Is not easy to ignite (NEZ)

Flammability – sample after ignition and removal from the flame:

1. Continues to burn (HOŘ) until it stops burning,
2. Slowly extinguishes (UHAS) and is not capable of continuous burning and after removal from the flame it extinguishes at different rate
3. Flammable only in the flame but after removal it immediately extinguishes; or non-flammable, it only melts in the flame but does not burn at all (NE)

Colouration of the flame

1. Luminous flame without a blue or green base (SVB)
2. Luminous flame with a blue or green base (SVZ)
3. Non-luminous blue flame (Z)

Smoke – throughout burning the smoke is or is not produced, a character of smoke is also observed; it depends on the chemical structure of polymer and additives (added substances – initiators, plasticizers, etc.) of the polymer. Smoke is observed looking against a sheet of paper or other white mat

1. Thick black sooty smoke (HČS)
2. Not apparent or little apparent smoke (N)
3. Intensive and dark – dark colour of smoke is evident (IT)

E.g. polymers with aromatic nuclei in the chain (PS, PC) produce a thick black sooty smoke. Polymers that do not contain double bonds with single carbonaceous chain (polyolefins, PE, PP) do not release smoke when burning.

Odour of smoke after removal of the sample from the flame: A chemical composition and a structure of polymer influence the nature of substances that are released during burning or depolymerisation or degradation of polymer in the flame. Odour of some of these substances can be characteristic.

Procedure:

Immediately after removal from the flame, we carefully and appropriately sniff and identify odour. It can be:

Paraffin-like (P) - (similar to the smell of burning candle), acid (K), styrene (S), dentacryl (D), honey-like (M), phenolic (F), after the charred horn (R), pungent (Š), amine (A), undefined (N).

Character of charred residue – after removal of sample residue from the flame, the sample exerts a characteristic nature corresponding to its chemical composition. The sample can e.g. only melt or burn out and melt or it leads to its degradation with the occurrence of other coloured substances. E.g. polyolefins are easy to burn and they melt without occurrence of coloured products; the other parts of material have a rough surface due to swelling with releasing gases, sometimes soot occurs, which gives colour to the polymer residue (polymers with aromas).

Procedure:

The burning sample is removed from the flame and carefully extinguished. The charred residue should be explored in terms of colour and further exploration is done by touch (touching by fingers). Subjective observations are recorded.

We can distinguish several degrees of the appearance of charred residue:

1. smooth, no changes or brownish colour (HLH)
2. rough, brownish colour (DH)
3. black or prevailing black (C)
4. smoky (OČ) – soot from the sample rubs against the skin
5. smoulders and leaves ash (DP)

After completing all the experiments in the flame, fill up the following table:

Melting of polymer on the hob and monitoring the transition temperature:

Polymers are often amorphous or only partially crystalline materials. Therefore they cannot be given a concrete and exact melting temperature but rather a range of temperatures. Moreover, and for the exploitation of polymer as a material it is undoubtedly important, before reaching the melting temperature at any temperature (or rather within a range of temperatures), polymers soften or transit into a flexible state. This temperature (range) is called the glass transition temperature and is an important parameter for the given polymer. Let us try to roughly determine this glass transition temperature.

Procedure:

On the hob or heating plate of the cooker, we place a metal plate (which can be damaged) and on this plate we place a sample of the plastic. Temperature of the plate will be measured by a thermocouple followed by observation of the plastic sample. Once the plastic starts to soften, we record the temperature. The experiment will be performed 3x. The results will be recorded in the table.

(data in the table according to http://faculty.uscupstate.edu/llever/Polymer%20Resources/GlassTrans.htm ). The results may quite naturally vary (which can be discussed by the students) since the temperature of glass transition depends on a number of parameters, in particular on the structure of the polymer (LDPE, HDPE – Low Density PE vs. High Density PE, atactic vs. isotactic, polymerisation degree, etc. Nevertheless it is a significant property of the polymer and it is desirable for the students to get an idea of what is in question.

What conclusion results from the conducted experiments?

The target of the conducted experiments is (was) to show that synthetic (and not only synthetic) polymers are substances of many varied properties (with both higher and lower density, flammable and also non-flammable, hard and soft, flexible and non-flexible, of different appearance, colour and resistance to environmental effects. These physical and chemical properties (e.g. products of burning) depend on their chemical composition, structure and other properties of the given polymer (e.g. degree of polymerisation). From this it results that a selection of a proper monomer and reaction conditions (from which the structure of polymer often results) allow a preparation of usually such material that will suit our purpose (kitchenware, covers for electronics, chairs, cars, etc.). It is desirable to note that, though the performed experiments do not support this, other possibilities are offered by a combination of polymeric materials (and the production of the so-called composite materials). Due to this, really only a few products and applications do not allow the use of a proper polymer for their production; therefore the polymers are so widespread and can be found in nearly any product or in majority of them. However a selection of other material can often be necessary, e.g. for applications at high temperatures, etc. it should be noted that another advantage of a number of polymers is often the ease of processing or machining at reasonably high temperatures and also a low price. Along with a high variability of technical options in the use of polymeric materials (colouration, processing, appearance, low demands), this makes the polymers a clear choice for the selection of the material for the given product.

Moreover, the students within the performed experiments learnt the properties of the individual synthetic polymeric substances.

The experiments also show that if the individual polymers differ from one another, the given experiments can also be used as identification tests for determination of the polymers. This is also the subject of the following activity.