Mechanical properties of PS injected parts under surface-active substances

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MECHANICAL PROPERTIES OF PS INJECTED PARTS UNDER SURFACEACTIVE SUBSTANCES
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The main objective of this work was to develop the influence of surface-active substances in the mechanical properties of injected plastic parts in presence of cracks, a tension test and a constant load. The material used was Polystyrene and the surface-active substances were olive oil and corporal sweat. This study was conducted because it was observed that the surface-active substances affects the mechanical properties of the resin as well as the presence of cracks, and actually is not reported any methodology for conducting this study.

Experimental Part
For this study, there were injected 110.5*85.5*1 mm plates in a REED-Prentice 100 TE (Clamp Force = 100 ton) injection machine with an injection mold with two similar cavities, but with a different located entrance. The used material was a Polystyrene PS-2820 from Estirenos del Zulia (Table 1). In table 2 are showed the used optimal process conditions. Notches were mechanized in each plate with a saw. The normalized tensile test specimens were cut with a milling machine. It was used a Galdabini 2500 universal test machine to make mechanical tests, and a Starrett Sigma VB300 stereoscopic magnifying glass to verify the crack length. To study the crack surface it was used a Macro Magnifying glass Olympus SZ61. To study the orientation and the stress concentrator in the plaques it was used a polaryscope Photoelastic Inc. 082.

Introduction
Since long time ago, the fracture has been a serious problem for the use of all materials. The fracture may be described how any change of the material properties that make the part functional or structural unacceptable. Nowadays, it is necessary to know and understand why the defects occur and to find the way to avoid them. The polymers are particularly sensitive to its processing. They are affected by environment, time, and temperature storage, shipping and service. Additionally, the plastic parts with geometric changes, holes and cracks in their structures have many stress concentrators that may induce them to an early failure during its use. Polymers failures may occur at low stress levels caused by a creep test, fatigue and liquid environmental agents. When a polymer is exposed to a chemical environment and simultaneously to a stress, it is observed a drastic fracture time reduction. Such agents do not attack the polymers molecules chemistry. They are not also solvents able to dissolve, however, they penetrate superficially the part, allowing the release of freeze stress and superficial rupture. This kind of failure has been called Environmental Stress Cracking (ESC) and it should be under consideration, since 20% of the plastic parts failures are caused by them. That is the reason why, the main purpose of this work is to develop a methodology that allows studying the influence of the surface-active substances over mechanical properties of injected PS parts. Previous works, for example, Kramer

Results and Discussion
This study came from a discovery made by Candal et al [5], where it was found that corporal sweat may act as an surface-active substance when interact with PS, affecting the results of the mechanical tests. Nowadays, there are several plastic parts (CD boxes, table settings, glasses, trays, tool boxes) that may be in contact with corporal sweat. Since there are not previous studies regarding this subject, the following methodology was design: 1. It was chosen a type of specimen to use: it was important to have a plaque able to modify the crack length in order to study the crack’s effect over the mechanical properties of plastic parts. Likewise, it was decided to use a normalized tensile test specimen type IV to study the effect of a surface-active substance without incorporating the crack influence.

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2. Material selection: the material was selected to complete the study and to know the mechanical and rheological properties. In this case it was chosen a PS injection grade. 3. It was established an injection process window of the polymer: this window indicates the temperature and the pressure ranges in which the material may be process, in accordance with the needed design and aesthetic requirements (good surface appeal, complete transparency and no presence of flashes or sink marks). Starting from here, an optimal process condition was chosen. 4. Plaques injection: the most important effect of injection molding over the PS parts was the imposed orientation, causing mechanical properties changes. Moreover, in the post-molding stage, there is a tendency to accumulate residual stress that are the consequence under low cooling conditions, flow and imposed orientation that chains do not relax. These may cause hazes and fragile zones with a tendency of failure by crazes. It is important to underline that in the particular case of PS, when plaques were manipulated; it was necessary to wear latex gloves and due to the direct hand-touch contact with the part may cause an effect that negatively affects the mechanical properties of the PS [5]. From the plaques, the tensile test specimens were die-casted. 5. Qualitative evaluation of stress concentrators: injection molding is a very fast and non isothermal mechanism that has a radical cooling, which solidified the piece. The transparent parts (made by PS) manufactured under this process, when they are studied under polarized light, show a color pattern on the surface where it can be easily recognized. The injection point location, the melted polymer movement direction (meld lines) and the residual stress (Figure 1). These last ones will depend on the melted flow direction, the gate type and gradient temperature used in the mold cycle to make the material injection [6]. 6. Collection and application of surface-active substances methodology: it was decided to develop a methodology that allows collecting the substance. The corporal sweat collection was made fixing a natural latex plaque to another high adherence plaque and putting sterile gauze between latex and skin (Figure 2). The method needs a previous washed with alcohol and water skin area of 24 cm² in each collector plaque. It is possible to put several plaques in the same individual. There were used plaques in both sides of the back, following both scapular lines in order to make the test (Figure 3). The latex plaques were previous cut from non-lubricated preservatives. The corporal sweat collect may be made in two ways: (a) if the individual produce a big quantity of corporal sweat after physical exercise or (b) obtaining corporal sweat from the gaze by centrifugation. Since the obtained surface-active substance needs a lot of preparation and provides small quantities of substance, it was decided to choose an

equivalent environment to develop the method. There were several substances available to choose the most alike environment to corporal sweat and without radically affect PS (margarine, olive oil, and car oil). It was decided to use olive oil. Although olive oil has a completely different composition to corporal sweat, it represented the alternative with most alike viscosity and origin. It was made a test to verify that PS has a low mechanical resistance to olive oil. In this test, there were tested two normalized test specimens. One of them was in contact with the substance but the other one not. Both specimens were under the same pressure to guarantee, that substances were in contact with the specimen. After 30 minutes, it was observed that, the specimen in contact with the substance had a large quantity of small crazes all along the edge of the specimen neck. After 90 minutes, it was also observed that the crazes were located not only on the neck edge, but also in the lengthwise neck (Figure 4). 7. Simple-tensile tests: there were made tensile tests, at room temperature, to the four specimens (complete plaques with one and two notches, medium plaques with one crack and normalized tensile test specimen) (Figure 5). It was possible to evaluate the effect of the crack over the part from one crack plaques. It was decided to evaluate the double crack behavior, but this idea was rejected, because there were putting two stress concentrators in a fragile material, and cracks were propagated at different velocities. At the end, the cracks did not match up in the fracture zone. For that reason, they could not be considered as a valid result. Especially, for the one crack half plaque and the test specimen it is important to underline the place from where the whole plaque was taken (centre or side). That fact will define its behavior in the mechanical property tests. By other hand, the obtained result after studying the whole plaque with one crack was a sum of what happen in the centre of the plaque and in one of its edges, which is different depending on the flow type inside the cavity (extensional, shear, laminar, etc.) in accordance with Castany [7]. 8. The number of stress concentrator influence and the location of the injection point: it was made a study of the chosen injected test specimens from two different locations, in transverse and longitudinal manner to the previous notched plaque (Figure 6). The crack length vs load was charted. It was obtained that the necessary force to break the injected test specimens by the longitudinal flow gate, exceeds at least by five times the requested flow, to break the test specimens with transverse gate (Figure 7). 9. The injection process conditions influence: it was study the effect over the final properties of PS parts. When the melt temperature was modified, there were changes in the fluency of the material and in the stress concentrator patterns of the part.

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10. Behavior of the plaque in presence of different length cracks: for both studied geometries with cracks, it was made a study of the crack length effect over its final properties. It was obtained that the longest the crack length is, the less load is necessary to fracture it. 11. Plaque behavior in presence of a crack and a surfaceactive substance: the used method of surface-active substances application was the following: the specimens impregnated with olive oil selected with a cotton swab were set in a stub. In the small area of the test specimen neck was put a part of cotton to guaranty a surface uniform covering. A dropper that provided the same quantity of oil in each piece was use to try that all the test specimen were under the same conditions. A similar method was followed for the plaques, nevertheless, the central part of the surface was not covered with cotton because it was complicated and the beginning of the crack was left free to achieve better oil spread inside the material. For the corporal sweat case, the test specimens were covered with gazes and kept them inside hermetic containers to guarantee an effective contact and avoid that the substance do not evaporates. When the behavior of the complete plaques was evaluated in presence of a crack in front of surface-active substances at a short time of exposition (4 hours), it was found a failure that mainly occurred by the presence of the crack. While the time of exposition increase to 8 hours, it was possible to evaluate the substance effect. The cracks introduce more severe conditions to the test, because they act as stress concentrators making three-axial stress at the beginning of the crack. 12. Comparisons between olive oil and corporal sweat: it was notice that the corporal sweat generates a smaller decrease of the material properties when it is compare with olive oil. 13. Observation of the test specimen’s surface fractures: the PS behaves fragile, therefore, its fracture involves crack propagation in shorter periods of time, usually under low stress levels, when it is compare to the ductile crack behavior. In the case of a fragile crack, it spreads with little plastic deformation and once the propagation begins, the material is unable to stop it, producing a severe crack.

• The design methodology for the study of the crack effect and the surface/active substances over mechanical properties of PS injected parts generates good results and may be use to evaluate other polymers.

References
1. E. Kramer and R. Bubeck, “Growth kinetics of solvents crazes in glassy polymers”, Journal of Polymer Science: Polymer Physics Edition, 16 (7), 1195 (1978). 2.J. Yu, Y. Hu and E. Baer, “Effect of an environmental stress cracking agent on slow crack propagation of Polyethylene”, SPE’s ANTEC Proceedings (2003), 2947. 3. J. Arnold, “The use of flexural tests in the study of environmental stress cracking of polymers”, Polymer Engineering & Science 35 (2), 665 (1994). 4. R. Bubeck and C. Arends, “Environmental stress cracking in impact polystyrene”, Polymer Engineering and Science 21 (10), 624 (1981). 5. M. Candal, R. Morales, H. D´armas and H. Rojas, “Stress Concentration Evaluation in an Injected Commercial Piece using Computational Tools”, SPE´s ANTEC Proceedings (2005), 3385. 6. E. Thompson, "Thermal Stresses", en "Encyclopedia of Polymer Science & Engineering", H. Mark, N. Bikales, C. Overberger, J. Menges, (eds), Vol 16, John Wiley & Sons, Inc., USA, second edition. (1987). 7. J. Castany, “Inyección de termoplásticos amorfos y líneas de soldadura”, Revista de Plásticos Modernos, 75 (504), 596 (1998).

Acknowledges: the authors would like to thank
Inversiones Selva, Estirenos del Zulia, 3M Manufacturera, Laboratorio de Estudios de Superficies of the Universidad Simón Bolívar University (Venezuela) and Centro Catalán del Plástico (Spain) for their cooperation and support. Without them this work would not be possible. Also we would like to thank Fundación Carolina for the financial support to the Engineer Duarte to go Spain. Special thanks to Professor Marco Sabino and Engineer Hector Rojas, both from USB, for their cooperation in the methodology of this work.

Conclusions
• The olive oil and the corporal sweat radically decrease the final PS parts properties. • The one crack plaques and the tensile test specimen produced the best results to verify the surface-active substance effect and cracks over the final properties of PS parts.

Keywords: surface-active substances, crack, mechanical properties, injected plastic parts and load.
Table 1: Properties of PS-2820. Properties Value Unit MFI (200 ºC/ 5 Kg) 22 g/10 min Young Modulus 3500 MPa Rupture Stress 38 MPa Solid Density 1,05 g/cm3

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Table 2 Injection molding process conditions used. Condición Valor Hidraulic Pressure 1,7 MPa Hold Pressure 1,2 MPa Melt Temperature 225 ºC Injection Time 4,0 s Holding pressure time 8,0 s Cooling time 50,0 s Demolding Time 1,0 s Figure 3: Scapularis Lines

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(b) Figure 4: PS normalized test specimens with olive oil.

Figure 1: Qualitative evaluation of stress concentrators.

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(b) Figure 2: Corporal sweat collection (a) Latex plaque and (b) Sterile gauze.

(d) Figure 5: Specimens used for the study (a) completed plaques with one notch; (b) completed plaque with two notches; (C) Half plaques with one notch and (d) tensile test specimen.

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(a) (b) Figure 6: Notch cut in the two types of inject plaques (a) Longitudinal and (b) Transversal position.

Figure 7: Load vs nominal crack length for the complete plaque-