Effects of Recycling on Impact Strength of High Impact Polystyrene (HIPS)

Mechanical Engineering 1
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Mechanical Engineering 2
Abstract
At the present, several tons of plastic are being disposed every year. Just a fraction of the plastic
is often recycled. Most of the disposed plastics have presented a major challenge in the
management of wastes from several landfill sites. “In order to solve tis menace, it is important to
increase the research levels in order to conduct proper analysis of the variations in the
mechanical features of plastics after being recycled. This will help improve the level of
understanding on the behavior of plastic materials as well as the variety of applications in which
they could be used. High Impact Polystyrene (HIPS) has been largely used in the new era plastic
industry. This is because it is a tough, rigid and equally a material of low cost. It also has a wide
range of uses. Past studies have revealed the impact strength of HIPS degrades whenever it is
recycled. Despite the impact strength, the effects of initial defects on the recycled material have
not been researched into details. This report therefore intends to explore the effects of recycling
on the impact strength oh High Impact Strength (HIPS) as well as the influence that initial
defects have on the plastic. In order to get results on the impact strength of samples, the study
made adopted the use of charpy impact test machine. The samples were for the recycled plastic
up to 4 times through thermoforming into sheets. Testing was also carried out on 3 various defect
paths. The results obtained showed that the impact strength reduces with each cycle of recycling.
The biggest drop is witnessed from the virgin samples to those that have only been recycled
once. Just as expected, the greater defects occur in lower impact strength. (Bile et al, 2011)
Observations also reveal that the behavior of the recycled HIPS is more ductile, hence offers
more areas for future research. Generally, the impact strengths for the recycled HIPS are higher
than the polystyrene for general purpose, implying that it is a very useful and that the carbon
footprint for using HIPS has the possibility of being lowered from using the recycled material.”

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Table of Contents
Abstract........................................................................................................................................2
1. Introduction...........................................................................................................................4
1.2. Aims and objectives..........................................................................................................5
1.3. Rationale...........................................................................................................................5
2. Background study and literature review...............................................................................6
2.1 High impact polystyrene........................................................................................................6
2.2. Degradation of impact strength.........................................................................................6
2.3. Impact testing....................................................................................................................7
2.4. Fracture Mechanics.........................................................................................................10
3. Methodology.......................................................................................................................13
3.1. Charpy Impact Test.............................................................................................................13
3.1.1. Standards..........................................................................................................................15
3.1.2. Objective..........................................................................................................................16
3.2. Requirements..................................................................................................................16
3.3. Charpy impact tests specimens.......................................................................................17
3.4. Test procedure.................................................................................................................19
3.5. Preparation of sample......................................................................................................19
3.6. Impact testing..................................................................................................................21

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3.6.1. Charpy impact testing..................................................................................................22
4. Results.................................................................................................................................22
5. Calculations........................................................................................................................27
6. Graphs.................................................................................................................................27
6.1. Force-time graph for virgin sample....................................................................................27
6.2. Impact energy......................................................................................................................31
6.3. Standard deviation...........................................................................................................34
6.4. Relative impact strength..................................................................................................35
7. Discussions.........................................................................................................................38
8. Professional issues..............................................................................................................40
8.1. Standards.............................................................................................................................40
8.2. Sustainability.......................................................................................................................40
8.3. Risk assessment...............................................................................................................41
9. Conclusions.........................................................................................................................41
9.1. Further work........................................................................................................................42
References..................................................................................................................................44
Appendix 1.................................................................................................................................46
Appendix 2.................................................................................................................................51
Appendix 3.................................................................................................................................54

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Figure 1: impact testing after 6 cycles of processing (Shin et al, 2016)..........................................8
Figure 2: Machine for Izod or Charpy impact testing (Curtius et al, 2012)..................................10
Figure 3: Showing load directions and sample holding methods for izod or charpy impact tests
(Curtius et al, 2012).......................................................................................................................10
Figure 4: drop weight impact test operation (Fatt & Bekar, 2014)...............................................11
Figure 5: showing the plane strain fracture and the fracture toughness (Fatt & Bekar, 2014)......13
Figure 6: Charpy Impact Testing Machine (Horák et al, 2012)....................................................15
Figure 7: Test Specimen (Horák et al, 2012).................................................................................19
Figure 8: Dimensions of a standard specimen for charpy tests (Horák et al, 2012)......................19
Figure 9: showing the test procedure flow diagram (Horák et al, 2012).......................................20
Figure 10: Schematic representation of charpy impact testing (Parres et al, 2011)......................23
Figure 11: Force time curves of initial impacts (Parres et al, 2011)..............................................24
Figure 12: Falling impact of Charpy hammer in degrees (Parres et al, 2011)...............................25
Figure 13: Falling impact of charpy hammer................................................................................25
Figure 14: force-time curves of final impacts (Parres et al, 2011)................................................26
Figure 15: showing relevant data range for virgin sample with no defect....................................26
Figure 16: Showing relevant data range for virgin sample with a defect (Parres et al, 2011)......27
Figure 17: Showing relevant data range for recycled sample with a defect (Parres et al, 2011)...27
Figure 18: for virgin sample with no defect (Parres et al, 2011)...................................................29
Figure 19: virgin sample with a defect of 1.5mm (Parres et al, 2011)..........................................29
Figure 20: with a 0.5mm defect and recycled once (Parres et al, 2011)........................................30
Figure 21: with a 1mm defect and recycled twice (Parres et al, 2011).........................................30

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Figure 22: with a 0.5mm defect and recycled 3 times (Parres et al, 2011)...................................31
Figure 23 : Comparing the force time graph for all virgin samples without a defect (Parres et al,
2011)..............................................................................................................................................31
Figure 24: bar chart showing average impact energy for cycle of recycling and depth of defect
(Server, 2013)................................................................................................................................34
Figure 25: Bar chart showing average impact energy for the recycling cycles (Server, 2013).....35
Figure 26: Bar chart showing the relative impact strengths for each recycling cycle (Tjong &
Bao, 2015)......................................................................................................................................39
Table 1: numerical result of instrumented charpy impact test (Parres et al, 2011).......................25
Table 2: impact energy results (Server, 2013)...............................................................................33
Table 3: standard deviation results (Server, 2013)........................................................................37
Table 4: Results for the relative impact energy strength (Tjong & Bao, 2015)............................39

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1. Introduction
The currently existing culture of consumers is largely dependent on an unsustainable system.
Over the past 3 decades, the global production has been on a higher rise by up to 500%. From
that impressive statistic, just a tiny fraction of the plastic waste is being recycled, representing a
mere 21.3% of the plastic wastes. Close to half of the plastic waste that is generated from
different sectors in the whole European Union is often sent to various landfills whereas some
proportion is left to pollute the natural environment, some of it always ending up in rivers,
beaches, lakes and oceans. There is no doubt that there is a growing need to recycle a much
higher percentage of the plastics. This is important given that it will help in diverting the wastes
from the landfills as well as lowering the level of crude oil that is used for the production of
virgin plastic, use of energy and the level of released carbon dioxide during the whole process
(Banthia et al, 2010).
The development of High Impact Polystyrene (HIPS) was done to increase the brittle features of
General Purpose Polystyrene. Its concept is currently used widely within the plastics industry.
The HIPS is greatly used for several applications due to several distinct features. Its impact
strength is seven times greater than the GPPS. It is also rigid, tough, low cost and easy to
machine. That makes it largely used in several applications. One shortcoming is that once the
products made using HIPS attain their maximum life; the most common and greatly used option
is the disposal to landfills. This is regardless of the material being highly suitable for recycling.
Several reports and investigations have been conducted on the mechanical features of HIPS
through recycling and the common result is that impact strength is the only property that has a
significant variation. Other studies have also revealed that after 6 cycles of processing, the
impact strength of HIPS was lowered by up to 33% (Barsom & Rolfe, 2013).

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1.2. “Aims and objectives”
The main reason for carrying out this research work is to understand the existing knowledge and
information and equally improve on the current understanding of the recycling effects on the
mechanical properties of HIPS. The major objectives of this study are to;
 “Measure the impact resistance of recycled HIPS through CHARPY impact test
 Understand the effect of the number of cycles of recycling on the impact strength of
HIPS
 Study the effect of size on initial defects on impact resistance and recycling cycles”
1.3. Rationale
From the values collected from the CES, the figures on carbon footprint reveal that in the
primary production of HIPS, up to 3.4kg of carbon dioxide is released for every given kg of the
produced plastic and a mere 1.16 kg of each HIPS kg that is recycled. This implies that 22.4 kg
of the embedded carbon can be saved for each recycled HIPS kg, instead of production of more
virgin plastics that could directly result to global warming. Just a little plastic is currently being
recycled thus implying that large amounts of plastics definitely end up in landfills or lead to
pollution of the environment (Böhme & Kalthoff, 2014). In a bid to increase the amounts of
plastics that are being recycled, it is important to have a better understanding of the reprocessing
effects on mechanical properties. One major basic design parameter that has to be considered in
the manufacture of plastics is the impact strength. This is because the impact strength of plastics
often quantifies how the given plastic will endure abrupt shocks or even greater impulses.