TMKM98

Engineering Materials-Design

PROJECT ON

Material Selection for Riot Shield

Sl. No.

Name P-Number E-mail ID

1 Arun Manavalan D 850512-3730 arudh797@student.liu.se 2 Ebin Edakkalath Thomas 830107-4236 ebied690@student.liu.se 3 Muhammad Kamrul Islam 831012-T655 muhis240@student.liu.se 4 Tomy Varghese 850420-0539 tomva593@student.liu.se 5 Vinod Nicholas M 841211-T398 vinni006@student.liu.se

Group : A6

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Summary In this project we have done a study on the materials that are currently used for riot shields, and we try to find alternative materials that we can also use. We look into the material factors that would affect the functional characteristics of the shield. The selection of the materials is done with the help of the software CES EduPack 2010. Five polymers are chosen and based on the priority of each material property; a comparative scoring system is used to rank the materials to find a suitable alternative.

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List of Figures Figure 1: Dimensions of shield .................................................................................................... - 6 -

Figure 2: Illustration of Injection Molding .................................................................................. - 18 -

List of Tables Table 1: Calculation of Tensile Strength ....................................................................................... - 8 -

Table 2: Calculation of Impact Strength........................................................................................ - 8 -

Table 3: Material Properties....................................................................................................... - 10 -

Table 4 : Weightage for Material Properties ................................................................................ - 11 -

Table 5 : Weighted Scoring for Material Properties...................................................................... - 13 -

Table 6 : Material Ranking ........................................................................................................ - 16 -

Table 7 : Manufacturing Process Assumptions ............................................................................ - 17 -

Table 8 : Relative Cost Index..................................................................................................... - 17 -

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Table of Contents Summary.................................................................................................................................. - 2 -

List of Figures .......................................................................................................................... - 3 -

List of Tables............................................................................................................................ - 3 -

Table of Contents..................................................................................................................... - 4 -

1. Introduction: ......................................................................................................................... - 5 -

2. Objective: ............................................................................................................................. - 5 -

3. Design of Shield................................................................................................................... - 6 -

3.1 Dimensions of the product ................................................................................................. - 6 -

Figure 1: Dimensions of shield ............................................................................................. - 6 - 3.2 Design Constraints............................................................................................................. - 7 -

3.3 Material Selection ............................................................................................................ - 10 -

3.4 Scoring Method................................................................................................................ - 11 -

4. Discussion.......................................................................................................................... - 14 -

4.1. PEI – Polyetherimide ............................................................................................... - 14 - 4.2. PCTA - Polycyclohexylenedimethylene tere/isophthalate copolyester .................... - 14 - 4.3. PPSU – Polyphenylsulfone...................................................................................... - 15 - 4.4. PC – Polycarbonate................................................................................................. - 15 - 4.5. Polyester Liquid Crystal (PLC):................................................................................ - 16 -

5. Manufacturing Process ...................................................................................................... - 17 -

6. Conclusion ......................................................................................................................... - 18 -

7. References......................................................................................................................... - 19 -

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1. Introduction: Riots are perhaps one of the ugliest facets of human behavior, and often the task of dealing with an agitated crowd is very tough without the right equipment at hand. The modern day ‘riot shield’ has evolved from what was once a tortoise hide that was used to intercept spears and arrows. Over the years, as warfare artillery as progressed improvements in shields have also followed very closely. Today, Kevlar ballistic shields are made to be bullet resistant. In this project, we look at riot shield which are used to protect police personnel from blunt or sharp objects that may be hurled at them during a riot situation. This shield however is not meant to intercept firearms. We explore the possibility of using alternate materials that can be used in constructing a riot shield. Also we look at the alternative processes that can be used to manufacture these shields. It is our interest to compare and conclude on which material and which process would be most suitable, keeping in mind functional constraints, material costs and manufacturability.

2. Objective: Most of the riot shields used by police in developing countries are made of bamboo sticks or Polycarbonate. Bamboo shields are not so effective to withstand high force, temperature and attack from sharp weapons. Our objective of this project is to select a suitable polymer material which can perform well taking into account the mechanical and thermal constraints of a riot shield. The main feature which has to be taken care is the impact strength, Fracture Toughness and high temperature.

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3. Design of Shield

3.1 Dimensions of the product The height of the shield is around 90cm and the width is 50cm. A handle is also provided to hold the shield firmly. The thickness of the shield has been taken as 5 mm.

Figure 1: Dimensions of shield

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3.2 Design Constraints Riot Shields are used by police force personnel while trying to incapacitate a rioting crowd. In order for the shield to be effective, it should satisfy the following constraints.

The properties those are required for the riot shield has been classified into the following groups.

Mechanical Properties

Thermal Properties

Others

Mechanical Properties

1. Tensile Strength: It is the maximum tensile stress the material can withstand before the failure to happen. For the shield, we have done the following calculation to arrive at the required tensile strength of the shield. Suppose a brick of weight 2.4 kgs is thrown at the shield with a speed of 11 m/s. The handle of the shield is being placed at the centre of the shield. The force imparted by the stone has been calculated as shown below and by using the flexure equation; the longitudinal stress has been computed. (Refer Table 1)

Flexure Formula

σ = M*y / I

Where

σ = Longitudinal stress on the cross section, N/m2

M = Bending moment, Nm

y = distance from neutral axis

I = Moment of Inertia of the cross section about transverse axis.

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Table 1: Calculation of Tensile Strength

Force Calculation Parameter Unit Brick Specification

mass of stone 2.4 Kg Dimensions 217x110x66 mm velocity 11 m/s

duration of impact 0.1 Sec Force 264 N kinetic Energy 168 Joules

Shield Specifications Thickness of shield 5 mm Width of the shield 50 cm Bending Length 45 cm Stress 57 MPa

Therefore the material chosen has to have a tensile strength more than 57 MPa.

2. Impact strength :

Impact is a high force subject for a very short duration of time. Even ductile materials tend to fail in a brittle manner when subjected to high energy impact. For a shield to serve its purpose of deployment, it should be able to fend against the impact of high energy projectiles and blows from sticks, and stones without shattering.

To find the impact strength of the shield, following procedure has been followed. We have found out the impact strength required to withstand an impact by a brick.

Kinetic Energy: (1/2) x mass x velocity2 Note: - Impact strength has been calculated based on worst condition i.e. the brick hits normal to shield surface. (Refer Table 2)

Table 2: Calculation of Impact Strength

Brick Length 217 mm Height 66 mm mass 2.45 kg

velocity 7 m/s Kinetic Energy 60 J

Impact Strength Calculation

Circumference 566 mm Thickness 5 mm

Area 2830 mm2 Impact Strength 21 KJ/m2

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3. Fracture toughness:

Fracture toughness is a measure of the ability of a material having a crack in it to resist fracture. It is desirable to have a high value of fracture toughness. The preferred range of fracture toughness is 2-3MPa.m0.5.

4. Young’s Modulus, Stiffness A high Young’s Modulus would imply high stiffness. For ductile materials it is desirable to have a high value of Young’s Modulus whereas in brittle materials it would be a disadvantage since it would mean low fracture toughness. For the shield it is expected to lie in the range of 1-2 GPa.

5. Density/Weight: Since the shield is a hand held device, it should light enough for the personnel to be able to carry it without having his mobility hindered.

Thermal Properties

6. Service Temperature: Often times, rioters resort to torches and fire. It is therefore desirable to have a high service temperature, which would imply a high glass transition temperature. So the shield has to withstand a minimum temperature of 5°C maximum temperature of around 130°C.

Others

7. Optical properties: It is advantageous to have a transparent shield since it would allow the user to see their nemesis while intercepting them.

8. UV stabilization : It is important that the material used doesn’t age too easily or become unstable when exposed to environment like UV rays. This ensures that the shield has a longer service life.

9. Other Consideration: • Material cost • Economy of processing and production • Ability to retard flame • Resistance to Acids and Alkalis.

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3.3 Material Selection With the following constraints, we ran the search in CES EduPack 2010. The following polymers have been short listed for detailed consideration.

Table 3: Material Properties

Sl No Properties PEI (Unfilled) PCTA

(Unfilled)

Polyester Liquid Crystal

(Unfilled)

PPSU (Unfilled)

PC (Copolymer, High Heat)

Unit

Mechanical Properties 1 Impact Strength 3.81-4.2 9.66-17.9 90.5-99.8 11-12.2 6.55-16 KJ/m2

2 Fracture Toughness 1.99-4.03 3.05-9.16 1.89-1.97 2.65-6.08 3.83-4.6 Mpa.m1/2 3 Youngs Modulus 2.89-3.04 1.65-1.96 15-15.4 2.29-2.4 2.21-2.4 GPa 4 Density 1.27 1.21 1.41 1.295 1.16 Tonne/m3

Thermal Properties

5 Minimum Service

Temperature 224-244 219-239 223-243 223-243 226-236 °K

6 Maximum Service

Temperature 434-452 513-533 430-500 441-459 401-417 °K

7 Melting Temperature 582-703 421-431 503-569 580-664 533-623 °K

8 Glass Transition

Temperature 488-490 358-372 389-397 483-503 433-478 °K

Optical Properties 9 Refractive Index 1.65-1.67 1.55-1.57 - 1.66-1.68 1.57-1.58

10 Transparency Transparent Optical Quality Translucent Translucent Transparent

Others

11 Resistance to strong

acid Excellent Limited use Acceptable Acceptable Excellent

12 Resistance to weak

alkali Excellent Acceptable Excellent Excellent Acceptable

13 UV Stabilization Excellent Good Good Good Fair

14 Flammability Self-

Extinguishing Self-

Extinguishing Self-

Extinguishing Self-

Extinguishing Slow Burning

15 Recyclable yes yes yes yes yes

16 Water permeability @

24 hrs .227-.275 .05-.1 .00428-.00473 .336-.407 .15-.2 %

17 Cost 669 255 1375 1400 172.5 INR/Kg Manufacturing Cost

18 Relative Cost Index 1.09E+04 9.23E+03 1.43E+04 1.39E+04 8.89E+03 19 Weight of the shield 2.86 2.7225 3.1725 2.91375 2.61 Kg

20 Approximate cost for

shield material 1911.67 694.24 4362.19 4079.25 450.23 INR

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The materials considered for our study are:

• PEI – Polyetherimide:

• PCTA - Polycyclohexylenedimethylene tere/isophthalate copolyester

• PPSU – Polyphenylsulfone

• PC – Polycarbonate

• Polyester Liquid Crystal

3.4 Scoring Method Once the short listing has been done, the selection of material was done using the procedure of weighted average. Each property was given a weight and the value of each property was allotted a mark based on the maximum value in each property segment. The final selection has been done based on the weighted average of each material.

Table 4 : Weightage for Material Properties

Sl No

Properties Weightage Sl No

Properties Weightage

Mechanical Properties Others 1 Impact Strength 10 11 Resistance to strong acid 10 2 Fracture Toughness 10 12 Resistance to weak alkali 5 3 Young’s Modulus 8 13 UV Stabilization 7 Thermal Properties 14 Flammability 10 5 Minimum Service Temperature 5 15 Recyclable 5

6 Maximum Service Temperature

10 16 Water permeability @ 24 hrs 2

7 Melting Temperature 5 17 Relative Cost Index 10 8 Glass Transition Temperature 5 18 Weight of the shield 10 Optical Properties

10 Transparency 10

19 Cost of shield 10

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Example:

For the material Polycarbonate:

Impact Strength = 16 Kg/m3

Percentile Impact Strength = (Impact strength of PC / Max Impact strength amongst five) x100

= 16 / 99.8 x 100

= 16.03

Weighted Average = ∑ (Percentile for property) x (weight for property) / ∑(weightage of all properties)

∑ (Percentile for property) x (weight for property) for Polycarbonate

= 160.3 + 502.2 + 124.64 + 483.5 + 782 + 443.1 + 475 + 666.6 + 1000 + 333.3 + 233.31 + 500 + 500 + 4.72 + 1000 + 1000+1000

= 9208.67

∑(weightage for all property)

= 10 + 10 + 5 + 5 +10 + 5 + 10 + 5 + 5 + 8 + 10 + 15 + 7 + 10 + 5 + 2 + 10 +10 +10

= 132

Weighted Average = 9208.67/132

= 69.76

NOTE:

1. The property transparency can be opaque, translucent, transparent or of optical quality. The above are given percentile values of 0,25,50,75 and 1 respectively.

2. The properties weight and cost of material has to minimized, hence to find their comparative scoring is calculated based on the reciprocal of respective value for each material.

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Table 5 : Weighted Scoring for Material Properties

Sl No Properties

PEI (Unfilled)

PCTA (Unfilled)

Polyester Liquid Crystal

(Unfilled)

PPSU (Unfilled)

PC (Copolymer, High Heat)

Unit Weightage

Mechanical Properties

1 Impact Strength 4.2 17.94 100 12.22 16.03 KJ/m2 10

2 Fracture Toughness 44 100 21.51 66.38 50.22 Mpa.m1/2 10

3 Youngs Modulus 19.74 12.73 100 15.58 15.58 GPa 8

4 Density 1.27 1.21 1.41 1.295 1.16 T/m3 Thermal Properties

5

Minimum Service Temperature 100 97.9 99.5 99.5 96.7 °K 5

6

Maximum Service Temperature 84.8 100 93.8 86.1 78.2 °K 10

7 Melting Temperature 100 61.3 80.9 94.45 88.62 °K 5

8

Glass Transition Temperature 97.4 73.9 78.9 100 95 °K 5

Optical Properties

9 Refractive Index 1.65-1.67 1.55-1.57 - 1.66-1.68 1.57-1.58

10 Transparency 66.66 100 33.33 33.33 66.66 10 Others

11 Resistance to strong acid 100 33.33 66.66 66.66 100 10

12 Resistance to weak alkali 100 66.66 100 100 66.66 5

13 UV Stabilization 100 66.66 66.66 66.66 33.33 7

14 Flammability 100 100 100 100 50 10 15 Recyclable 100 100 100 100 100 5

16

Water permeability @ 24 hrs 1.7 4.7 100 1.16 2.36 % 2

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Manufacturing/Cost

17 Relative Cost Index 81.56 96.32 62.17 63.96 100

INR/Sample 10

18 Weight of the shield 91.34 95.87 82.27 89.58 100.00 Kg 10

19

Approximate cost for shield material 3.77 64.84 10.31 11.03 100.00 INR 10

Weighted Average 69.03 73.18 71.69 63.3 69.76

4. Discussion

4.1. PEI – Polyetherimide:

PEI is an amorphous thermoplastic which is also known commercially as Ultem.

Advantages:

It has excellent heat and environment resistance and it retains a good part of its strength even at elevated temperatures. It is used in re-usable medical devices, in underwater connectors and for structural parts in components used at high temperature.

Transparent

Disadvantages

Impact strength very low

Poor hydrostability.

4.2. PCTA - Polycyclohexylenedimethylene tere/isophthalate copolyester

PCTA is high-performance thermoplastic polyester resin. PCTA are used in the manufacturing of food trays, caps, perfume bottles.

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Advantages:

Best optical quality among the five materials.

Best fracture toughness among the five materials.

Highest service temperature among the five.

Second highest Impact Strength among the five materials.

Comparatively lowest weight among the five.

Disadvantages

Not resistant to strong acid

4.3. PPSU – Polyphenylsulfone

PPSU is a sulphone polymer which goes by the commercial name Radel IR. They withstand prolonged exposure to water, chemicals and heat. PPSU is used in medical equipment because of its ability to withstand repeated steam exposure.

Advantages:

Sulfone polymers in general are high-heat amorphous thermoplastics with more toughness, strength and hydrolytic stability.

Disadvantages

Translucent

Highest material and processing cost among the five materials.

4.4. PC – Polycarbonate

Polycarbonate goes by the commercial name ‘Lexar’. It is the preferred for a lot of applications because of its useful balance of thermal resistance, optical properties and mechanical strength.

Advantages:

Transparent

Lowest cost among the five materials cost.

Disadvantages

Does not have very good water resistance.

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Processing of Polycarbonate is made difficult because it has limited flowability hence higher temperatures are required.

Slow burning, hence vulnerable to catch fire.

4.5. Polyester Liquid Crystal (PLC):

Polyester Liquid Crystals are class of aromatic polyester polymers which have very good mechanical strength as well as chemical properties. They are made of liner copolymer of hydroxybenzoic acid and hydroxynaphthoic acid. The partial crystalline structure of PLC renders them with good mechanical properties like toughness. Also PLC is extremely inert to reaction with acids, alkali and has good weatherability and flame retardant properties.

Advantages:

Highest Impact strength

Highest Young’s modulus.

Disadvantages

Very high material cost.

Cannot be processed by extrusion.

Opaque.

Anisotropy.

Material chosen after study:

The Table 6 shows comparative scores and respective ranks of chosen materials.

Table 6 : Material Ranking

From the above table we can infer that PCTA is the most suitable material for the shield.

Rank Material Score

1 PCTA 73.18

2 PLC 71.69

3 PC 69.76

4 PEI 69.03

5 PPSU 63.30

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5. Manufacturing Process The process which has been selected for the manufacturing the shield is Injection Molding based on the following requirements.

a) The injection molding is convenient for getting the curved shape of the shield in a single manufacturing step. If an alternative process like extrusion were chosen, we would require another thermoforming or bending to achieve the required sheet curvature. Since the number of manufacturing step is one, the production cost can be minimized for a bulk.

b) The cost of the process is comparatively low.

The comparisons of cost for injection molding of the different material have been done based on the relative cost index. The calculation of relative cost index was done using EduPack 2010 and the same has been included in the Table 7 & 8. The following assumptions have been taken for the calculation of relative cost index.

Table 7 : Manufacturing Process Assumptions

Overhead Rate 4.96E+03 INR/hr Load Factor 0.5 Capital Write off time 1.57E+08 Sec Batch Size 1000

Table 8 : Relative Cost Index

Polyetherimide

(Unfilled) PCTA

Polyester Liquid Crystal

(Unfilled) PPSU (Unfilled)

PC (Copolymer, High Heat)

Relative Cost Index

(/unit) 1.09E+04 9.23E+03 1.43E+04 1.39E+04 8.89E+03

The relative cost index is based on the material cost as well as weight of single unit. From the calculations, it’s seen that the relative cost index is highest for Polyester liquid Crystals. PCTA has low relative cost index which is almost 65% the manufacturing cost index for PLC.

The schematic of the injection molding is shown below. The die is fabricated to suit the shape of shield with the required tolerances. The heated polymer is injected into the die under high pressure and temperature through a single nozzle point. Then the product is cooled and the unwanted projection is chipped off and machined to get the final product.

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Figure 2: Illustration of Injection Molding

6. Conclusion In the present scenario, the material most commonly used for riot shields are Polycarbonates and Plexiglas. Poly methyl methacrylate (PMMA) also known ‘Plexiglas’ is often used in application that requires shatterproof glass. However the mechanical properties of PMMA, especially impact properties, are inferior to those of polycarbonates and polyesters. Hence PMMA is only considered as an economical alternative to Polycarbonates when they are only required for low load applications. Polycarbonate is preferred over other materials for this application mostly because of their superior impact strength and also it is comparatively light weight and shatter / splinter proof material.

The alternative material that we have chosen for the shield is PCTA. PCTA is a co-polyester of Terephthalic acid and Isophthalic acid with cyclohexanedimethanol. It is known by the trade name ‘Thermx’. It is used currently in electrical, automotive and other industrial and consumer products. It has a maximum tensile strength of 77MPa which is greater than our requirement of 57MPa. It has the highest fracture toughness value among the short-listed materials and hence can resist propagation of cracks. It also holds the advantage of being light weight and good optical quality. Even though the material cost of PCTA is slightly more than PC, we recommend PCTA over PC since it has superior mechanical properties and overcomes the manufacturing difficulties of PC.

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7. References http://www.wikipedia.com

CES EduPack 2010

http://www.ptsllc.com/polcarb_intro.htm

http://www.cranfield.ac.uk/cds/departments/deas/pdfs/mre-shields.pdf

http://www.hpwhite.com/uploads/file/402-01.pdf

http://www2.dupont.com/Automotive/en_US/news_events/article20030305f.html