SUPERABSORBENTS – POLYMERS OF AMAZING STRUCTURE AND QUALITIES: PROPOSAL OF CHEMISTRY CLASS MINI-PROJECT

ABSTRACTThe proposal of the class project deals with studying structure, qualities and use of special polymers, so called „superabsorbent polymers (SAP)“, which can be found e.g. in diapers (irreversible SAP), toys „Growing Beasts“ (reversible SAP) or other humidity absorbers. The aim of the project is to innovate students´ laboratory activities and form their attitude to currently used materials and technologies. Activities with a molecule simulating set are followed by several experiments with SAP and toys „Growing Beasts“.

Key words: Superabsorbent Polymers, Toys „Growing Beasts“, Experimental Activity, Searching and Critical Assessment of Information.

1 INTRODUCTION

Since the beginning of their development the synthetic polymers have been widely used generally in all fields of human activities, especially because of their low price and long lifetime period. The most often used are packaging materials films, fibres etc. In the last 25 years new polymers are appearing in the market in growing extend and their production is steadily increasing. The „Superabsorbent polymers (SAP – see the notice below)“ are an example of the polymers. They are found in diapers (irreversible SAP), toys „Growing Beasts“(reversible SAP) and other humidity absorbents. This type of polymers also includes so called hydroabsorbents, which retain liquid in the ground and are used in oil industry and agriculture.

2 THE PROPOSAL OF PROJECT

2.1 AIM OF THE PROJECT

The project aims at:- Getting information on polymers via active working – on the functioning, history, positive and negative

influences when they are used in various fields of human activities, - Innovating laboratory activities with SAP as products of everyday life, - Contributing to gaining critical view on getting information and work with it, especially searching for

information in professional literature, on the Internet etc.,- Supporting creative students´ activities, ability to present results of their work and active teamwork.

2.2 THEORETICAL PART

The introductory part of the students´ project (about one hour long) will be devoted to creating teams (two teams), planning schedule of the tasks and discussions with the teacher. Teams are expected to be heterogeneous (according to the sex, school results, interest, etc.).

2.2.1 Team One – Searching for Information – Polymers and World of MaterialsThe task is to define basic macromolecular chemistry terms and structure them. The necessary information will be gained from the library (studying professional literature), Internet and professional magazines. The questions below may work as a clue:

What is it a macromolecular stuff, polymer?

What are other types of polymers?

What is the difference among polymer, oligomer and monomer?

What we can say about the reaction course in macromolecular stuff?

2.2.2 Team Two – Searching for Information – Special PolymersThe task is to study basic structure of superabsorbent polymers and describe them. Sources for searching are the same as in the team one. The questions below may work as a clue:

How can we structure special polymers?

What is the definition of superabsorbent polymers (SAP) and what are their features?

What is the widest and last field of the SAP use?

What are the advantages and disadvantages of the SAP use?

How is influenced the environment by the SAP?

Students prepare detailed materials on the topics mentioned above, partly during lessons; partly they study on their own. Having finished this phase, the results will be presented in lessons where members of both teams will participate. Preparing and presenting posters or editing down the results on the flipchart might be suitable ways of presentation.

2.3 PART OF PRACTICE

The lessons following the theoretical part will be devoted to activities with molecular simulating set, i.e. building the SAP structure (set Molecular models III [1]). Then laboratory activities with superabsorbent polymers, i.e. with powdered SAP and toys on the basis of reversible superabsorbent polymers „Growing beasts“ will follow in two teams, it enables division of tasks.

2.3.1 Setting of Part of SAP Molecular Model Students will work in two groups with a building set for simulating molecular structure. All types of schools in the former Czechoslovakia were given the set in 1986. Students can either build a model of the superabsorbent structure before and after the liquid absorption with the help of formulas or complete the single constitutional unit according to the structures prepared by teacher.

Setting the SAP Molecular Structure before the Liquid Absorption Students come from structures (see Fig. 1, 2 and 4). First they set constitutional units of polyacrylic acid, sodium polyacrylate and cross-linked stuff MBA (N,N – methylene-bis-acrylamide). Then they try to complete the polymer chain from single parts (with the teachers´ help, if needed).

Fig. 1 Structure formula of polyacrylic acid [5]

Fig. 2 MBA (N,N – methylene-bis-acrylamide) [3]

Fig. 3 Structure of “superabsorbent-crystal“ before and after adding water [6]

Fig. 4 Chemical structure of superabsorbent – cross-linked sodium salt of polyacrylic acid (sodium polyacrylate; polymer of sodium propenamide) before and after adding water [6]

Setting the SAP Molecular Structure after the Liquid AbsorptionStudents set both constitutional units of SAP and modules of H2O molecules. They try to put single parts together by binding a H2O molecule at certain positions of chain of polyacrylic acid or sodium polyacrylate. Doing this it is necessary to pay attention to standardised terminology of colours used for various atoms and specifying standardised large of single types of bonds (in this case there were used the following colours: carbon – black, hydrogen – white, oxygen – red, nitrogen – blue, sodium – green).

Fig. 5 Stick structure model of SAP before and after water absorption (the size is given by the number of used pieces from the building set) [1]

2.3.2 Experiments with Powdered Superabsorbent and Toys „Growing Beasts“ After the explanatory demonstration of rough SAP structure students can try experiments with stuffs of different absorbency and stuffs containing SAP (diapers).

Absorbency ComparisonStudents cut similar size pieces of SAP diaper, tissue, toilet paper, dish sponge etc. They put these „absorbent materials“ into beakers or on Petri-scales. They add the same content of water (e.g. 15 ml) and observe the speed of absorption for a one-minute period. According different weight materials of the best and worst absorbency result from the experiment.

Gaining Superabsorbents from a Diaper and Their Absorption of WaterStudents rub a piece of a diaper in fingers over a colour sheet of paper to see tiny white crystals loosing from it. They mark the SAP volume in the test tube. They successively drop water from the pipette and count the absorbed content.

Interdependence of Absorption on Temperature of the LiquidFirst students prepare a piece of SAP diaper. They drop drinking water of different temperatures (15 0C, 21 0C, 37 0C, 50 0C, 65 0C) for a three-minute period. According to the absorbed content of liquid they set the water of the best absorbency temperature.

Independence of SAP Absorption on pH and Ion-content of NaCl in the Absorbed Liquid Students carefully put a soft white powdered superabsorbent (e.g. using a scuttle) into the test tube and mark the SAP content. They successively drop low acid, low base or neutral solutions or solutions with different concentration of NaCl. They set the conditions of the best absorbency. Then student run experiments with „Growing beasts“ – i.e. growing animals which are sold in toy shops. Seal, whale and salamander are the most suitable for the experiments.

Distilled and Drinking Water Absorption First students weigh a selected animal on laboratory scale and record its weight. Then they put it to the distilled water for four-hour period. At the end they weight the animal once more and calculate the content of absorbed water. They repeat the experiment in drinking water. It is necessary to use another animal or dry it completely. Finally they count the difference in the content of absorbed liquid.

Independence of Absorption of the „Growing beast“ Animals on pH of Absorbed LiquidStudents prepare low acid, low base or neutral solutions in 1000 ml beakers. First they weigh the animal and then put it into the prepared solutions for a 3-hour period. According to the difference in the weight before and after the experiment they decide the solution in which the absorption run best.

Independence of Absorption of the „Growing beast“ Animals on Absorbed Ion-content LiquidStudents prepare water solutions with different concentration of NaCl (f. e. 0,2 %, 0,5 %, 0,9 %, 1,5 %) in 1000 ml beakers. First they weigh the animal on laboratory scale and then put it into prepared solutions for a 3-hour period. According to the difference in the weight before and after the experiment they decide the solution in which the absorption run in the most effective way.

The „Growing beast“ Behaviour under the Pressure (in Comparison to other Absorbent Materials)First students weigh the animal on laboratory scale. Then they prepare pieces of different absorbents of the same weight (SAP diaper, tissue, dish sponge etc.). They immerse single pieces of materials into beaker with water so that all parts were under the water. After 30 minutes they take them out and put water away. They discuss the qualities of various materials according to the difference in weight and content of water in them.

Fig. 6 Growing Beasts – „gekon“ – before and after 24-hour period of drinking water absorption

2.4 PART OF RESULTS PRESENTATION

In the following lesson students each other present the results of their work. First of all the function of superabsorbent in relation to the human organism should be emphasized (the independence of the absorption speed on temperature, pH a NaCl water solution concentration). The presentation and interpretation of results in graphic should be necessary part.Final task is an Internet survey on searching for the firms in the Czech Republic, which deal with SAP production and in what extend, e.g. Degussa Krefeld, Kimberly-Clark etc., supported by photos from the firms and educational excursion.

3 CONCLUSION

The suggested students project aims at presenting interesting and currently used SAP materials in everyday life. With the reference is displayed their negative influence on the environment because of their problematic biodegradability and recycling.

NOTICEWhat are the Superabsorbents?The superabsorbents are polymers with strong ability to swelling up. They absorb water liquid solutions and create fixed consistence gels. The liquid content depends on the ion-content and pH of the given solution. All the plastics consist of molecules, which link with other compounds, so-called „cross-linkers“ into space, 3-D nets. The liquid is fixed in the net and cannot be release back even if pressed (Fig. 3). Superabsorbents in hygienic products are usually cross-linked sodium salts of polyacrylic acid (sodium polyacrylate; polymer of sodium propenamide) where water can diffuse until the cross-linked chains cannot part aside (Fig. 4).

Published:

Bílek, M. Superabsorbents - Polymers of Amazing Structure and Qualities: Proposal of Chemistry Class Mini-project. Dydaktyka chemii w dobie reformy edukacji. Poznań : Sowa, 2012, pp. 121 – 128. ISBN 978-83-89723-93-X

AUTHOR ADDRESS Prof. PhDr. Martin Bilek, Ph.D. Department of Chemistry, Faculty of Science, University of Hradec KraloveRokitanského 62, CZ – 500 03 Hradec Kralove, Czech Republice-mail: martin.bilek@uhk.cz