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  • Subject area(s): Marketing
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  • Published on: 14th September 2019
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Acrylic, also known as poly methyl methacrylate (PMMA) is a synthetic, thermoplastic polymer. Poly methyl methacrylate is the polymer formed from methyl methacrylate. Often enough, acrylic is used for an alternative to glass due to its shatterproof properties. Acrylic in the marketing world is also known as Plexiglas, Perspex and acrylate by trade name.2 It is easy to process and has a low cost. Acrylic is typically used for products that can take advantage of its naturally occurring transparency due to its more amorphous state. Due to the additives that can be easily added to PMMA, properties such as colour can be changed to suit the requirement of the product.

Section One: Properties

Economic properties

Acrylic is a readily available and relatively cheap plastic that is easy to manufacture and available to be produced in various forms such as flat sheets, tubes and rods, moulding powder. 2

The cost for a sheet of PLEXIGLAS Optical Clear that is transparent and 3mm thickness it would cost 46.35 EUR/m2. As more additives such as colour and the opacity of the PLEXIGLAS is altered, then the cost of per m2 increases. 6

Bulk mechanical properties

The mechanical properties of acrylic will vary as the temperature of the material and its surroundings change. Due to this most of the values given below for the mechanical properties are given in ranges rather than an exact value.

Acrylic has a low density value of 1.15 – 1.19g/cm3. 10 From this value it can be assumed that acrylic is relatively lightweight. This is advantageous to the material due to it being used for a replacement to glass at times.

Young's modulus or also known as the elastic modulus measures the stiffness of a material through resistance a material has to deformation. Acrylic PMMA has a low young's modulus; compared to other materials such as metals or ceramics, that is recognised as 3.2GPa or 0.46 x 106 psi.12 This means that in comparison to other materials such as metals and ceramics, acrylic PMMA is more flexible, but in relation to other polymers, acrylic has a relatively high young's modulus14 and is stiff in comparison to those polymers.

In figure 1, it shows the density of some materials, including PMMA and other related polymers, measured against the young's modulus.

Acrylic has an ultimate tensile strength value range of 47 – 79MPa.10 This is a range of where acrylic would rupture as a result of a force loaded in tension at a given cross sectional area.

The hardness of acrylic is measured through the means of Rockwell hardness testing and has a resultant value range of 63 – 97M. 10

The bending strength or also known as the flexural strength of acrylic PMMA is 120 – 148 MPa. 10 The flexural strength is the maximum stress from a load that a material can withstand until rupture.

The Compressive strength is the amount of compressive stress that a material can withstand at the onset of failure. This is described as being the value of 12MPa12 for acrylic.

Acrylic PMMA when put under strain has an Elongation value range of 1 – 30%. 10 Typically, acrylic has a lower elongation value at the point which it breaks compared to other polymers. This means that the material is defined as being more brittle and less ductile.  

Bulk non mechanical properties

The Thermal properties of acrylic are shown through a number of factors such as the temperature the material forms at but also the heat that the material can withstand before it melts and burns. Below is a list of the thermal properties that are associated with acrylic at the minimum and maximum value. 11 With reference to the service temperatures that acrylic can withstand, makes it a good material to use for outdoor objects such as greenhouses.

Property

Value

Glass transition temperature

105oC

Melt temperature

130

Service temperature

-40 to 90

Specific heat capacity

1466 j/kg.K

Thermal conductivity

0.17 to 0.25 W/m.K

Thermal expansion

50 to 90 e-6/K

Thermal diffusivity

0.13 m2/s

Vicat softening temperature 10

90oC

Figure 2

Acrylic is a transparent material with very good optical properties. This is shown through refractive index of 1.510, light transmission, UV transmission and haze. Due to its good optical properties and its degree of computability with human tissue, acrylic can be used as contact lenses or intraocular lenses. In comparison to glass, does not filter ultraviolet light. Instead it transmits UV light down to 300nm and allows infrared light to pass up to 2800nm. 10

Acrylic is an excellent electrical insulator which allows it to be used in the manufacture of products such as lamp covers and switch parts.

Di electric constant or also known as the relative permittivity occurs in an insulating material like acrylic PMMA and is a measure of its capacity to store electrical energy when given a voltage. The given dielectric constant at 1Hz for acrylic is 3.6. 12

The dielectric strength or also known as the electrical strength is the potential electricity that can be conducted per unit thickness of the material. The electrical resistance begins to breakdown and the material will begin to conduct electricity. The given dielectric strength of acrylic PMMA is 30kV/mm (1.2V/mil).12

Surface properties

Oxidation and corrosion through the limiting oxidation index which means that it is the minimum concentration of oxygen that will support combustion of the material. PMMA has a limiting oxidation index of 17%. 12

Acrylic has an oxygen permeability value of 0.5 DK. This is the amount of oxygen that can pass through by diffusion, an important for the design of lenses.

For the purpose of referring to the friction co efficient of acrylic PMMA, it is easier to refer to tables that addressed the material as Plexiglas. Plexiglas is a form of PMMA that is manufactured for trade purposes. When it is rubbed against another piece of Plexiglas the Friction co efficient that is found is 0.80 μs 8 for both dry and lubricated surfaces.

The durability of acrylic is very high which means that is long resisting to wear and tear. Acrylic is one of the hardest thermoplastics in that category and also has a high resistance to scratches on its surface. This adds to the long resistance to wear and tear of the material and keeps the overall appearance looking more professional and high quality for longer. Acrylic is one of few polymers that is most resistant to weather, more importantly sunshine exposure.

Aesthetic properties

Acrylic comes in many forms and variations, but commonly it is available in sheet form. The overall appearance of acrylic is transparent and glossy looking, almost like glass which is a common replacement for acrylic. This is mainly due to the amorphous properties of acrylic.

Section 2: Manufacturing Processes

Intraocular lenses

An intraocular lens is used as an implant for the eye when cataracts or myopia appear. This replaces the natural lens of he human eye. The natural lens of the human eye should be clear but when cataracts appear, it becomes cloudy. When cataracts appear it disturbs the vision of humans by creating a haze and making objects and surroundings appear blurred.5 This is resolved by the implantation of the artificial lens.

The lens works by bending light rays that enter through the eye, which helps humans to see. The implant works by removing the cloudy lens of the natural eye and replacing it with a new and artificial IOL.

Just like the normal human eye, an intraocular lens must be focused in the correct way to suit the measurements of the eye. Intraocular lenses come in different types, the most common being a mono focal IOL. This just means that the distance at which the user can see is set to one type. Other IOLs such as multifocal and accommodative lenses provide the user with a range of distance focuses.13

To find the manufacturing cost of intraocular lenses is difficult due to it being sold to medical clinics and hospitals. The cost of a person to receive the surgery for an intraocular lens is as follows in figure 3, according to Benenden Hospital. 4

Type of Lens

Cost of surgery per eye

Standard Mono-Focal lens

£1,935

Special Lenses

£2,985

Multifocal Lens

£3,205

Multifocal Astigmatism correcting Lens

£3,305

Figure 3

There are both advantages and disadvantages as to why acrylic is a suitable material to use for both intraocular lenses and contact lenses. This is due to its excellent optical clarity and lightweight structure.10 Another good property that makes acrylic a desirable material to use for intraocular lenses is due to its good degree of compatibility with human tissue. 10

Acrylic PMMA lenses have been a continuous material to be used in the making of intraocular lenses for decades and have been stated to have the best safety record in all current biomedical materials in relation to the minimal toxins in the material.7

Due to acrylics good chemical resistance against aqueous solutions, this makes it an excellent material to be used as an intraocular lens as it does not get broken down or damaged by the proteins and other substances in the eye fluid.

Rigid IOLs such as ones made from acrylic are manufactured using a lathe cutting process. 3 The material is placed on a lathe rotating mount whilst cutting tools are used to precisely cut away the excess material. The lathe works through the means of a computer generation which heightens the accuracy and precision levels required for the making of the IOL.1 After the process of cutting away any excess material on the lathe has been completed, the lenses are polished to smooth the surface, placed through a quality control test and packaged to be sent away. 3

The quality control stage of the manufacturing process is especially important due to the lenses being used for medical purposes. Each lens is made individual to fit the intended human. This requires the lens to be checked after each step of the manufacturing process to ensure that no anomalies are encountered. It is equally important that the shape of the lens is correct as proposed for the users needs. 2

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