Abstract
This report acknowledges the various properties associated with plastics and their behaviour when subjected to different conditions of loading. This experiment investigates the mechanical behaviour of five types of polymers which include Silly putty, six pack rings, nylon cable ties, Ply Doh, and Splat Ball. The properties of plastics can be altered to introduce cross-links and thereby introducing constraints to prevent sliding or rotation. The findings of the experiment concluded that temperatures, speed of loading and chemical composition of plastics are determinants of their mechanical behaviours.
Introduction
Polymers play a critical part in day-to-day life as manifested in the numerous applications. These applications range from toys, adhesives, decorations, packaging, and structures, among others. Polymers are classified into three types which are Thermoplastics, Elastomers, and Thermosetting, depending on the molecular structure and behaviour when subjected to different conditions (Brazel & Rosen, 2012). This experiment investigates the mechanical behaviour of five types of polymers which include Silly putty, six pack rings, nylon cable ties, Ply Doh, and Splat Ball. The mechanical behaviour of thermoplastics is characterized by elastic deformation occasioned by the stretching of covalent bonds within a chain and delayed recovery, unlike in other materials in which recovery is immediate. On the other hand, plastic deformation occurs due to stretching, rotation or disentangling of chains (Ferry, 1980). The properties of plastics can be altered to introduce cross-links and thereby introducing constraints to prevent sliding or rotation.
Experimental
Silly putty
- The putty was formed into a smooth ball and bounced on the table top from a height of 10 cm.
- The height of rebound was measured using a ruler and recorded
- The ball was examined for any permanent deformation and behaviour recorded
- The ball was placed in ice cold water for 5 minutes and step two was repeated
- The putty was formed into a cylinder and the top pulled quickly
- The putty was formed into a cylinder and the top pulled slowly
Plastic Components
- Six pack rings and Nylon cable ties were pulled apart slowly and behaviour recorded
- Six pack rings and Nylon cable ties were pulled apart quickly and behaviour recorded
Play-Doh
- Play-Doh was extruded in a Play-Doh extruder and the force required to extrude a material recorded
- Using one die, a shape was extruded slowly and the subsequent shape was extruded faster
- Two different colours of Ply-Doh were placed side by side with the intersection running parallel to the extrusion axis and extruded
- Two different colours of Ply-Doh were placed side by side with the intersection being perpendicular to the extrusion axis and extruded
- The extrusion was cut open and examined for behaviour
- The diameter of both extrudate and die were measured and recorded
Splat ball
- Splat ball was thrown up against the wall and results were compared to throwing a play-doh against the same wall
Results and Discussion
Silly Putty
In average, the silly putty ball rebounded to a height of 8.37cm as calculated below in three trials.
Rebound height=7.6+8.6+8.93=8.37cm
The silly putty ball became flat following an impact with the table top. The rebound characteristic displayed height of 8.37cm is attributed to the condensation of polymer chains when it was rolled into a ball. When it came in contact with the table top, polymer chains resumed its earlier state by un-condensing, thereby creating the ability to bounce as high as 8.37cm.
When placed in ice cold water, the average height of rebound was 9.4cm which is an average of three trials as obtained below;
Rebound height=9.2+9.6+9.43=9.4cm
As compared to the initial height, an ice-cold silly putty ball bounces 0.93cm higher than that thrown at room temperature. This difference is brought about by the fact that placing the ball in ice cold water condenses the polymer chains and makes them rigid, such that there would be more stored energy to be released for rebouncing. This energy makes the ball to be propelled higher than at room temperature.
When a top part of cylinder shaped silly putty was pulled apart quickly and with a lot of force, the part was detached and the surface at which it was snapped was smooth and clean. This behaviour is attributed to the lack of time required for polymer chains to realign itself following a pulling force. This mechanical behaviour of snapping without prior indication or warning is classified as brittle. On the other hand, slow pulling of the cylinder top and with less force led to notable decrease in cross-sectional area of the cylinder and eventual snapping. The ‘necking’ behaviour prior to snapping is associated to ductile attribute of a material, whereby, failure occurs after a considerable amount of deformation is realized.
Plastic Components
Six pack rings showed a percentage elongation of 7% and 326% when pulled apart slowly and quickly respectively. This material has a linear polymer chain which has ethylene monomers. It structure comprise of amorphous and crystalline regions which explains the high elastic characteristic and capacity to stretch widely. Conversely, the structure of nylon cable ties is characterized by cross-linked polymer chain comprising of amides with a structure of rigid crystalline formation. This formation implies that the strong cross-linking bond prohibits free movement of polymer chains. This attribute make nylon cable ties depict brittle properties and high strength on the other hand. Thus, its capacity to elongate is low prior to fracture (Chanda & Roy, 2012).
Play-doh
A Play-doh with small cross-section area required more force to extrude than the one with larger cross-sectional area. Extruding Play-doh through a smaller die require more force because in this set-up, two processes takes place at the same time. The processes are pushing back of excess play-doh and extruding the rest as per the size of the die.
There was a smooth surface on the extrusion of Play-doh slowly and with little force. In contrary, fast extrusion with large force led to a rough surface of the extrusion. The smooth surface came about because there was enough time for alignment of polymer chains whereas in a quick extrusion polymer chains could not realign itself, thereby yielding a rough surface.
The extrusion of two play-doh with different colours when placed side-by-side yield an extrusion with distinct separated colours, unlike the two play-doh which yielded an extrusion with pink play-doh inside a blue one as shown in the diagrams below.
Figure 2 Extrudates of playdoh
Upon measuring the diameters of the extrudate and the die it was found that the diameter of a die was slightly smaller than the extrudate because of die swell. Die swell phenomenon occurs because when the play-doh is forced through a die, it condenses to fit the diameter and expand soon after exiting the die as it seek to regaining its earlier uncondensed state. This phenomenon should be factored in in the design of extruder such that the die diameter will always be smaller than the actual diameter of extrudate.
Splat-ball
When splat-ball was thrown to the wall, it stuck to the wall whereas play-doh rebounded of the same wall. This sticky behaviour of spat-ball is associated with cross-linking agent that exists between spat-ball polymer chains.
Conclusion
In conclusion, the experiments carried out above have depicted a myriad of behaviour attributed to polymers. In silly putty experiment, it was established that decreasing the temperature leads to higher rebounding because cold temperatures make the polymer chain condensed and store more energy (Budinski & Budinski, 2009). On the plastic components, six pack rings which was pulled apart slowly had a big elongation before fracture because the polymers had time to realign. From the extrusion experiment, it can be concluded that the speed of extrusion affects the exterior surface of extrudate, with a quick and slow extrusions yielding a rough and smooth surfaces respectively. Further, the differences diameters of dies and extrudate highlight a die swell phenomena which requires that the design of dies be of smaller diameter than the actual extrudate to be generated. Finally, splat-ball stuck to the wall unlike silly putty because the cross-linking agents present in it allow it to adhere to surfaces.
References
Brazel, C. S., & Rosen, S. L. (2012). Fundamental principles of polymeric materials. New
York: John Wiley & Sons.
Budinski, K. G., & Budinski, M. K. (2009). Engineering materials. Nature, 25(1), 28.
Chanda, M., & Roy, S. K. (2012). Plastics technology handbook (Vol. 72). London: CRC
Press.
Ferry, J. D. (1980). Viscoelastic properties of polymers. New York: John Wiley & Sons.
