Abstract

In the last few years high quality 3D printing has become quite inexpensive. Therefore, a look at its possible applications in school chemistry teaching seems worthwhile. This article deals with a project to engage upper school students in the development and production of their own molecular model set using a computer-aided design (CAD) program and a 3D printer. The presented teaching unit can be integrated into regular chemistry classes with six to eight lessons or can be offered as an elective lesson course. Part of this teaching unit is the investigation of different 3D print filaments to elaborate their chemical structure-properties-relations. The filaments are characterized by their melting behavior and by further chemical analysis.

1. Introduction

Fifty-eight years ago the physician and science fiction writer Arthur Clarke hit on the idea ¹ of printing items in three dimensions. Back then, the idea sounded like science fiction but has been reality for over twenty-five years. Since 2018, even the guardsmen of the Swiss Guard have been wearing helmets made by 3D printing: The so-called Morion (Figure 1) is made of polyvinyl chloride (PVA) and weighs only 570 grams ².

Since the development of 3D printing is now advanced and affordable, the use in school education has become more attractive. By now, beginners’ models of 3D printers are offered by electronics retailers starting from 300 € ³. Higher-quality models for further use, like the 3D printer of the Ultimaker series (Figure 2), which was used for the described project in this article, are sold starting from 1.500 € ⁴. The printing accuracy of the beginners’ models is sufficient for the kind of project presented here.

The aim of the project is to engage upper school students in the fabrication of their own molecular model set. Until the 3D printed product is completed (Figure 3) the students undergo the entire process from developing to 3D printing, using a computer-aided design (CAD) program (see also Figure 11). The whole process is under supervision of the students, reflecting their favored ideas regarding color design and plug connection (Figure 4). A convenient advantage in comparison to purchased molecular model sets is that lost "atoms" can be reproduced, if necessary.

The components for the model are printed either directly in school or in the FabLabs (short form for "fabrication laboratory") which are available in many places. FabLabs offer, for a contribution toward costs, the possibility to use 3D printers as well as courses on printing for beginners and non-beginners. These labs are located in all major cities worldwide ⁵.

During the project, the printing filaments are examined in student experiments regarding their melting behavior. In addition, the printing filament ABS (copolymer of acrylonitrile-butadiene-styrene) is chemically analyzed. Based on the findings important printing parameters such as printing temperature can be set for the printing process. Students’ prior knowledge on polymers, the valence shell electron pair repulsion model (VSEPR-Model) and of covalent bonding is required for the presented teaching unit, see paragraph 3.

Until now, 3D printing in chemistry classes has been used by teachers to print larger models of molecules (proteins, DNA, the layered structure of graphite), orbitals or potential energy surfaces ^{6, 7, 8} to provide illustrative material in class. In physics and mathematics school lessons, it is conceivable to use 3D printing for illustrating models of waves or geometric objects ^{9, 10}.

Only in rare cases, students were allowed to use 3D printing on their own responsibility independently. Thus, they seldom receive the chance to use 3D printing for their own projects ⁸. The presented project is different and could be motivating for students, because it fosters valuable insights into an entire process of development and production. In an educational environment like school, interdisciplinary projects may arise, where different subjects like art ¹¹ or physics cooperate. Furthermore, models of molecules for teaching or the illustration of measurement results during experiments, such as printed interference images, can be made permanently available for students by 3D printing. Even children with inclusion requirements such as blind or visually impaired benefit from such haptic models.

2. Principles of 3D Printing

3D printing objects are generated by applying the filament layer-by-layer (additive manufacturing). The most common, and for school teaching relevant printing technique is the "Fused Depositing Modeling" because it’s the only affordable 3D printing technology nowadays. It is based on the printing of a heated thermoplastic applying a print head, the so-called extruder. For that reason, the filament is heated to its specific melting temperature. The print head applies it stepwise on a construction platform that is movable in most cases while the filament sets immediately after printing (Figure 5). A majority of semiprofessional 3D printers are based on this process, which can be explained by the low acquisition and operating costs along with the low maintenance and great flexibility compared to other 3D printing processes. 3D printers can emit fumes and ultrafine particles when printing. To prevent health risks, 3D printers should be used only in fume hoods or well-ventilated rooms.

Next to the fused deposition modeling (FDM) procedure outlined above other techniques like laser-sintered tools or polymers cured by use of stereo lithography within the additive-assembly method were developed. Furthermore, the filaments become more and more diverse as to their chemical composition. An amount of metals in the filament causes small batches to become firmer and more conductive. Filaments made of polyvinyl alcohol are water-soluble and can be used as a support structure. The Table 1 provides an insight into different 3D printing materials ⁷.

It has become common practice to completely create individual hearing aids and dental implants via 3D printing. In addition to artificial replacements, temporarily aids such as orthosis for leg injuries has become cheaper and easier to print. As another aspect, 3D models are used by Physicians for preoperative training. For example, magnetic resonance imaging (MRI) scans or ultrasound images are uploaded to printing services, e.g. BIOMODEX^®. The printed model will be received within few days. Bones, ligaments, tendons and cartilage are printed in different colors and flexibilities in order to show the diverse properties in such a model. In this way, preoperative training of surgery skills is possible ¹².

Another field of research for 3D printing is the extraction of cartilage, to cultivate and to multiply cells. Following those cells are transferred onto a printed scaffold made of biocompatible filaments. After prolonged cultivation, the cartilage grows over the filament, which undergoes more and more degradation. The cartilage, of an auricle for example, originated this way is then ready to be implanted ¹³.

Spanish scientists succeeded in reprinting the layers of human skin through 3D printing. They extracted fibroblasts and keratinocytes from patients. In combination with blood plasma they were able to build various kinds of bio-ink. The outmost skin layer, the epidermis, was printed with the keratinocyte ink on a special substratum. On top of this, a layer of fibroblasts, that were responsible for the support structure, was set. The skin thus produced can then be transplanted. Following this procedure, patients with burns can be helped much faster since printing 100 cm² skin replacement only takes 35 minutes, whereas in a conventional way the skin tissue had to be grown in a culture dish for weeks ¹⁴.

A worthwhile area of operations seems to be the synthesis of hair follicles. In this field, research aims on the printing of a follicle consisting of 15 different cell types as a viable organ with its surrounding cell layer. If cultivation were successful, the next target would be to transplant these individual hair follicles ¹⁵.

Even in the pharmaceutical industry, 3D printing seems to be a promising method: drugs individualized to the patient and the combination of different active substances in one pill are the targets. This is especially advantageous for patients that cannot swallow many pills or children for whom purchased pills are too highly dosed ¹⁶.

In the food industry, there are manifold opportunities through 3D printing: chocolates, pasta or creatively designed French fries can be printed in small series ¹⁷.

There are even advantages through 3D printing in the building industry. In 2017, a house completely printed of cement was built within 24 hours in additive manufacturing close to Moscow. In this way housing could be provided easily and cheaply ¹⁸.

Due to the large number of available filaments, the features of the printed product can be easily adjusted to particular demands. The three most commonly used filaments are polylactic acid (PLA), acrylonitrile-butadiene-styrene (ABS) and polyvinyl alcohol (PVA) as material for supporting structures.

PLA. Polylactide synthetics, commonly known as polylactic acid, short PLA, are built of intertwined lactic acid molecules (see structural formula) and chemically belong to the polyester group. PLA is made of renewable resources such as corn or sugar cane, it has a melting temperature between 145 °C and 160 °C ¹⁹. It’s suitable as a filament especially for beginners in 3D printing since it keeps its shape during printing. This synthetic is food-safe and used particularly for foils, cartons and thin plastic trays.

ABS. Acrylonitrile-butadiene-styrene was developed in 1946 in the USA and is nowadays one of the most important industrial synthetics worldwide and being used in a large number of applications ²⁰. Since it consists of the three monomers, mentioned below with their structural formula, acrylonitrile, butadiene and styrene, it is a so-called terpolymer. ABS belongs to thermoplastics and is suitable as a filament in 3D printing with extruder technology due to its melting temperature of 220 - 240°C ²¹. ABS can be fabricated in any desired color by adding dyes. The synthetic has a high hardness, a good resistance to chemicals as well as a high scratch resistance; it is food-safe and compatible with other synthetics. Based on these features, this synthetic is widely used in automobiles, electrical and toy industry and is used for the popular LEGO^® bricks. ABS can be coated with metals such as chromium, which is used especially in the automotive industry. ABS is produced by blending, which means the mixture of the two synthetics styrene-acrylonitrile, short SAN, and acrylonitrile-butadiene-rubber, or by a graft copolymerization ²⁰. In the graft copolymerization, monomers of one or more types are attached to the polymer whereby a copolymer with a comb-like structure is built ²².

PVA. Polyvinyl alcohol (see structural formula below) is a thermoplastic made by the hydrolysis of polyvinyl acetate. Due to its solubility in water, this synthetic is used to print support structures on overhangs. Because of its hygroscopic properties the filament is unsuitable for permanent use. To print similarly the PVA as well as the actual print filament a 3D dualextruder printer is necessary.

PLA and ABS compared. PLA is preferable for beginners in 3D printing because of its better printability. This derives from the fact that PLA does not lose its shape when cooling down. This so called “Warp effect” occurs frequently at ABS filament because of different cooling temperatures of each component and every layer that leads to inner tensions in the printed object. It can be controlled by using a heated sample plate.

Since PLA is harder and more brittle than ABS, ABS lends itself better to build plug connections that can be easily solved and plugged together repeatedly.

When performing the experiments described in this article, prior chemical knowledge of polymers is necessary for understanding. For example, some German school curricula contain around ten lessons in the a field of "structure and features of synthetics" in which the "structures and features of thermoplastics, thermosets and elastomers" are explicitly mentioned ²³. The following students’ experiments demonstrate these structure and features.

Student experiment 1: Qualitative determination of filament melting ranges

(See also attached student worksheets)

The three printing filaments PVA, PLA and ABS have different melting temperatures. This fact is important for parameter adjusting of the 3D printer. The melting point of PVA is 200 °C, of PLA 150 °C and ABS 230 °C. To prove this difference, one piece of equal length of each filament are clamped side by side and a weight of 20 g is attached (Figure 7, Figure 8). While the experiment is set up, students can observe the varying flexibility of the different filaments.

Now, the filaments are heated evenly with a heat gun. Since ABS has the least flexibility, it stays in its original bent shape as it has been winded on the round filament coil. Applying the heat gun, PLA becomes liquid first (Figure 8). PVA filament becomes liquid second and the ABS filament at last. In this experiment, the students can easily comprehend why various temperatures of the pressure jet at the 3D printer must be set while using different filaments.

After the melting temperature sequence has been carried in experiment 1 out, the filament ABS will be separated into its chemical components during the following experiment two.

Student experiment 2: Analysis of ABS

(See also attached worksheets)

This experiment is carried out in two steps. Based on made observations during this experiment students’ can conclude that ABS must consist of several components. The practical work relates to the instruction "composition of ABS" ²⁴, was evaluated with students several times and then optimized during this process. The entire practical work is adapted for the stock of chemicals and laboratory equipment that are mostly available at schools. A used drying cabinet can be substituted with a normal oven, which is set to the temperature written in the worksheet. As a replacement for an automatic centrifuge a simple manual centrifuge can be used.

Step 1: ABS is dissolved in acetone and the resulting product is centrifuged. The supernatant solution is drained off and stored for the further experiment. The precipitate is dried completely in a drying cabinet. The dried residue of the centrifugation is polybutadiene and exhibits the expected toughness and elastic features of ABS (Figure 9).

Step 2: The stored solution is added dropwise into water and the resulting precipitate is filtered of and dried in a drying cabinet as well. The resulting product of the precipitation 2 is a styrene-acrylonitrile graft copolymer, which shows the hardness and firmness of ABS while being brittle and chalky (Figure 10).

Within the provided worksheet, two core points are emphasized. On the one hand it contains clear instructions that make the students follow step by step the separation process. On the other hand, in order to develop an understanding of what every sub step is for, the students are asked frequently certain questions to find explanations for their observations.

3. Teaching Unit

A chemistry teaching unit is described in the following section. The unit can be carried out in regular chemistry lessons as well as a part of a project seminar, e.g. on modern materials. Depending on the available time, more or less topics, e.g. printing of a storage box, can be addressed. Core student tasks, which are essential for successfully creating a molecular model set, are listed in Figure 11. Each task should be elaborated by one of the five student groups. The entire program can be carried out in at least six school lessons (Figure 11):

Ÿ Selection of the printing filament: The first group of students has to look up information of advantages and disadvantages, e.g. price, colors, melting temperature, safety aspects, of commercially available printing filaments through research on the internet, available literature and product data sheets. In cooperation with the group ‘development of the plug connection’ and together with the design group a decision on the printing filament should be made collectively covering the specific requirements of the molecule model set.

Ÿ Chemical analysis of the printing filaments: The participants of this group have to determine the melting temperature of the three filaments ABS, PLA and PVA (see student experiment 1 and worksheet 1). The printing temperature of the extruder has to be set exactly to get optimal printing results.

Ÿ Development of the model plug connection: The students in this group develop a plug connection that provides a stable linking and allows several disassembles of "atoms" and “electron pairs”. In order to give students some ideas of already existing products, different molecular sets are offered for viewing (see also Info box).

Ÿ Design of the atom models: This group selects colors and connections of their desired atom models. Additionally, they have to work out the amount and configuration of bonding possibilities of each atom model. In this phase students have to apply their basic chemical knowledge.

Ÿ Creation of the printing files using a CAD software: The group dealing with the CAD program have to work out the designed model. This group works in close cooperation with the groups ‘Development of the model plug connection’ and ‘Design of the atom models’.

4. Practical Evaluation and Outlook

As described the presented project fits well into regular chemistry lessons on the topic "polymers" and in addition it can be linked interdisciplinary to many curriculum contents on the subject of computer applications using the potentially motivating 3D printing technology. The purchase of a 3D printer seems to be a worthwhile investment, especially since 3D printing can be used in school for other projects and courses as mentioned. Offering the project for a longer period of time, further assignments can be involved such as a creation of an appropriate packaging through 3D printing for the molecular model set or the developing of a marketing strategy for the set. Furthermore, the development process of the plug connection can be repeated several times until an optimal result is achieved.

The proposed teaching unit fosters the application of students’ theoretical and experimental knowledge of polymers and was tested with 16 students of an eleventh grade class of a German academic high school. Students were asked about their experiences during the unit. It turned out, that the described project fits into a unit of six lessons, though it might be advisable to give more time to the entire project, as pointed out by the students.

Following some more statements of the students are given:

“I liked the many independent project parts and the attempts to chemically split the plastic.”

“I did not like the sometimes short time and the lack of communication among the groups.”

“Working together with my chemistry class was an interesting successful experience and different from ordinary lessons.”

In addition, first tendencies have been found, showing that the integration of 3D printing has increased the motivation of the students for chemistry class in general as well as for the specific topic of polymers. Further investigations have to prove these first findings, which are based on a very limited number of students.

Additional information: Plug connection

An important step in creating a molecular model set is the development of a suitable plug connection. The main features for such a connection are that it provides a stable link and allows several disassembles of "atoms" and “electron pairs”. The proposed connection (Figure 13) is certainly just one of several possible solutions.

For the linking, the atom models get cylindrical connections with a rounding on the top. The "binding pair of electrons" in form of tubes is attached to these cylinders. For these "binding pair of electrons" silicone hoses of 4 millimeters internal and 6 millimeters external diameter are used which are available for sale in the specialist trade in aquaristics by the meter ²⁵. After a few test prints, the diameter of the cylinder of 4.05 millimeters turned out to be the most favorable one.

Using silicone tube pieces of different lengths, various bond lengths can be displayed. An “atom diameter” of 15-20 millimeters seems to be particularly suitable.

Printing the atom models can be done in two ways. For one thing, the "atoms" can be printed as a whole with the help of supporting structures (Figure 12).

In this way, an almost perfect ball shape arises and the connecting cylinders can be printed with overhangs as well. If no 3D printer with dual extruder is available the "atoms" have to be printed in halves since only overhangs of maximum 60° can be printed (Figure 13).

The cut surface through the center of the sphere serves as a base for 3D printing, the halves are glued together with a plastic adhesive after printing. Both of the variants of printing are shown opposite.

To create the printing files, the free 3D program 123D-Design is well-suited ²⁶. This software is accessible in an English language version, must be installed on computers and llows the concrete setting of angles, which can not be realized in many other 3D programs.

References