The Backbone Structure Of Polyamide Engineering Essay

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2.1 Introduction

In this chapter, the following areas of study will be introduced:- the addition of carbon black as a filler to cast nylon 6, the method of processing and the polymerisation cycle to be investigated during the casting process using the conductive filler and the effect on the properties of adding carbon black as a filler to cast nylon 6 on antistatic and mechanical properties. It is important that during the casting process of adding the carbon black filler to the monomer (caprolactam) that the carbon black is completely and well dispersed and distributed throughout the nylon 6 upon polymerisation. Therefore, the best type of mixing for dispersion and distribution for achieved, to give the most successful antistatic and mechanical properties on the carbon filled nylon 6. Also, the samples are to be characterised using an analytical based technique either SEM or TEM after the polymerisation of the carbon black filled nylon 6.

2.2 Polyamide

2.2.1 Polyamide 6

Polyamides are also called nylon, as an original trade name [1]. The monomers of polyamides are the single units of amides (-CONH-) are joined together by using repeating uniting units of amide groups by peptide bonds. It is known that nylon is one of the most widely used engineering thermoplastics materials and can also be used to form fibres [2]. The properties of nylon show that it is tough, strong, and abrasion resistant and other properties which make it very useful for particular engineering applications [2].

There are six types of polyamides that are used in engineering applications, and these are:- nylon 6, nylon 6.6, nylon 6.10, nylon 6.12, nylon 11 and nylon 12 [2].

Polyamide 6 (nylon 6) has a repeat unit of C6H11ON, in which the monomer (single unit) used is caprolactam. Caprolactam has 6 carbon atoms. Upon synthesis of caprolactam molecules under heat and an inert atmosphere of nitrogen, long chains of nylon 6 are formed. The structure of polyamide 6 has been shown in figure 1, below [3].

Figure 1. Shows the backbone structure of polyamide 6 [3].

Nylon 6 is a semicrystalline polyamide [3]. Due to the repeating units of -CONH-, these amide groups join together to form polyamides which crystallise with a high intermolecular attraction. Nylon 6 does not form via condensation, however, it is formed by ring-opening polymerisation [3].

2.2.2 Cast process

The casting process is a method by which Nylacast Ltd, a Leicester based engineering polymer solutions company manufacture polyamide 6 (nylon 6) on a regular basis. There are also other companies all around the world who also adopt this particular manufacturing process, e.g. Zell Ltd, Quadrant, Licharz and Scwartz etc. It is a process by which, upon the polymerisation of the caprolactam molecules, cast nylon 6 plastic can be made, under the relevant reaction conditions. The manufacturing of cast nylon 6 uses a different processing procedure than injection moulding, extrusion, blow moulding or rotor moulding etc.

2.2.3 Properties

Nylon 6 is one of the most widely used engineering plastic in the world, in particular as a bearing and wear material. Nylon is used as a replacement for bronze, brass, aluminium, steel and other metals, as well as other plastics, wood and rubber. Nylon 6 offers an exceptionally good wear resistance, coupled with high tensile strength and modulus of elasticity. Also, nylon has a high impact resistance, a high heat distortion temperature (HDT), good wear resistance against abrasion and vibration. In addition, nylon can withstand a sustained contact with a wide variety of chemicals, alkalis, dilute acids or oxidising agents, as tests have shown in the laboratory.

However, in molten caprolactam, lubricants and other additives can be added, allowing the nylon to have other properties in its application in industry. These properties include a low coefficient of friction, high tensile modulus, high compressive strength, high impact resistance, high flexural modulus and a high Shore D (hardness). The addition of certain additives helps in increasing the mechanical and physical properties of the nylon, as well as others.

When taking the above characteristics into consideration, typical nylon 6 applications in industry includes:- rollers, bushes, gears, pulleys, cutting boards, clamps and spacers. The typical industries covered by Nylacast Ltd includes:- rail, quarrying/mining, construction and the petrochemical industries etc [4].

2.2.4 Polymerisation

There are two types of polymerisations to make nylon: - step-growth polymerisation and chain-growth polymerisation [5]. The process of polymerising nylon from a diacid and a diamine is by step-growth, also, polymerising nylon from lactams is by chain-growth polymerisation [5]. Using lactams there are two ways of ring-opening polymerisation of ε-caprolactam:- ring-opening can be using a water initiated process or using a strong base such as, sodium hydride (NaH) being used as an initiator as shown in figure 2 [5] and as shown in step one in figure 3 [6].

Figure 2. Due to the hydride anion being an incredibly strong base, the amide hydrogen is pulled off the caprolactam molecule and so the ring opens [5].

The anionic polymerisation of caprolactam to form polyamide 6 is well known. An overview of anionic polymerisation reaction mechanisms is given below in figure 3 [6]:

Figure 3. An overview of anionic polymerisation reaction mechanism [6].

Step 1: Shows the structure of the monomer, caprolactam and the sodium metal with caprolactam leads to the formation of the lactam anion,

Step 2: The lactam anion attacks the carbonyl group on neighbouring caprolactam molecule to form an acyl lactam,

Step 3: Sodium is replaced with a proton and a new lactam anion is formed,

Step 4: Reaction between acyl lactam (initiator) and sodium caprolactamate initiates a fast polymerisation. The reaction mechanism is extremely sensitive to moisture and air.

It is known that the casting of nylon 6 takes place in-situ upon being placed in a mould to polymerise. This process has been used for many years [7]. It is known that in this process the anionic polymerisation is used [7]. The anionic polymerisation process occurs via the steps shown in figure 4. The process of anionic polymerisation is used for the commercial synthesis of polyamide 6. This anionic polymerisation process requires the use of low temperatures compared to the temperatures of other processing techniques for other materials. The melting point of caprolactam is 69-70oC [8] and the processing temperature upon polymerisation takes place in the mould tool is from 140 to 175oC. For polymerisation to occur in the mould tool a catalyst and activator must be placed in situ, prior to pouring the polymeric solution into the mould tool itself.

Wilfong et al, investigated the reaction mechanism of caprolactam which started with a caprolactamate anion such as; lithium-, sodium-, or potassium-caprolactamate, as a catalyst and an acylated caprolactam as an activator [9] to aid the polymerisation process taking place prior to adding the catalyst and activator from situ into the mould tool. A reaction scheme using sodium caprolactamate and adipoyl-bis-caprolactamate (ABC) in a caprolactam monomer to form polyamide 6 [9] is shown in figure 4. In this experiment the rate of polymerisation was being determined, with the levels of ABC, the activator, being added in-situ was held as a variable to produce lower polymerisation cycle times [9]. It was also observed that upon polymerisation followed by crystallisation in the mould tool, the polymerisation temperature in the mould from the heated oven/source was less than the actual crystallisation temperature from the cooling in the melt in the nylon [9].

Figure 4. Shows the reaction scheme for the polymerisation of a caprolactam monomer using a catalyst and activator [9].

2.3 Carbon Black

Carbon black is almost a pure elemental carbon in the form of colloidal particles which is manufactured by an incomplete combustion or thermal decomposition process of gaseous or liquid hydrocarbons under specific controlled conditions [10]. The physical appearance is that of a black, finely divided pellet or powder [10]. Carbon black is well known and is in the top 50 industrial chemicals manufactured worldwide, on an annual tonnage basis [10].

Carbon black is predominantly used as a filler in rubber and elastomers [11]. It is added to rubbers and elastomers to modify their mechanical and electrical properties [11]. Initially, it was first intended that carbon black was used in polymeric materials to improve their resistance to wear [11]. Due to polymers having insulating property adding carbon black into polymeric materials can be a drawback for many applications due to the suppression and presence of carbon black as filler [11]. However, with respect to the electrical conductivity of samples and components in industry, depending on the morphology and the dispersion of the carbon black, if the overall characteristic of a particular system is below a threshold it is an insulator, and above the threshold it becomes a conductor [11]. Polymers are non-conducting and so can be made to be conductive by the addition of fillers, e.g. carbon black. Increasingly, carbon black is chosen in polymers due to its electrical conductivity and/or low cost [12]. In this project the addition of carbon black will be used as a filler to add to cast nylon 6 in order to gain a conductive grade for applications in industry.

2.3.1 Types of carbon black

The process of producing carbon black is based on the partial oil oxidation of carbochemical and petrochemical origin [13]. There are two major types of manufacturing methods for carbon black; furnace black and thermal black, others include; the channel process and the lampblack process [10]. It is known that the furnace black process is the most common, which uses heavy aromatic oils as feedstock [10]. The thermal black process uses natural gas, which primarily consists of methane or heavy aromatic oils, as feedstock material [10]. The two most common generic terms applied to carbon black is not 'soot' or 'black carbon' due to unwanted by-products that result from the incomplete combustion of carbon containing materials, such as; oils, duel oils or gasoline, coal, paper, rubber, plastics and other waste materials [10].

Commercially, the conductive grades compromise of benzene- and acetylene- based carbon blacks [14]. These processes enable the production of low surface area conductive carbon blacks as well as very high surface area carbon blacks [13]. It is shown that the low surface area materials have a chain-like structure compared to acetylene-black [13]. In these, the high surface area carbon black would belong to an Extra Conductive (EC) family [13]. The EC carbon black grades can combine to an extent in which both the properties of furnace and acetylene black are able to reach an optimal compromise [13].

2.3.2 Properties of carbon black

There are three main properties of carbon black and are as follows [15-16]:-

Particle size:- the smaller the particle size, the higher the blackness of the carbon black can become. However, the problem in dispersion is increased due to a coagulation force.

Structure:- it is also known that the increase in the size of the structure can affect the blackness and dispersibility of carbon black i.e. due to the formation of agglomerates of carbon black particles. So, the bigger the structure of carbon black, the better the conductive property that can be achieved from a particular application.

Surface activity/chemistry:- as there are various functional groups that are on the surface of carbon black, the affinity of this material with inks or paint changes depending on the type and amount of the functional groups.

Others include:- porosity and the physical form of carbon blacks.

Figure 5. Shows an image of the surface chemistry of carbon black that would be taken under microscopy [15].

These are the basic properties of carbon black, and together they are the three main characteristics [15]. It is believed that these three main properties have a large effect on the practical properties such as the blackness and the dispersibility when they are mixed with inks, paints, or resins [15].

2.3.3 Applications of carbon black

It is known that carbon black has a long history in which it has been used as a colouring agent for many years [17]. Carbon black can occur as nano-particles with functions such as ultra-violet absorption and conductivity, and are becoming increasing popular in the field of electronic equipment and devices [17]. There are other applications which simply require that use of carbon blacks which are conventional materials for applications in industry, these include the following [17]:-

Colouring agents for inks and paints - due to a high tinting strength of carbon black as opposed to iron black or other organic pigments, it can be widely used for printing inks, newspaper inks and paints. Also, carbon black can be used as a black pigment for inkjet inks and toners in printers and photocopiers, respectively.

Film and resin colouring agents - due to a high tinting strength of carbon black and being thermally stable it can made available for colouring films and resins that formed by heating. Carbon black can be used for a wide variety of colouring and resin films. Carbon black resins are used in automotive bumpers, wire coverings and steel pipe linings which require resistance to weathering.

Electric conductive agents - due to the carbon particles having graphite-type crystalline structures they are able to provide and excellent electrical conductivity. So, carbon black can be used as conductive filler in plastics, elastomers, paints, adhesives and pastes. In the automotive industry carbon black can be used in fuel caps and fuel introducing pipes such that they are required to prevent a static charge from occurring.

Electronic equipment - as carbon black provides stable resistance it can be used as electronic equipment which relates to material in various magnetic recording materials and displayed components.

Currently, carbon blacks account for most of the production in the reinforcement of elastomers, i.e. the largest amount of carbon black being used is for rubber applications e.g. in automotive tyres [18]. Therefore, due to the tyre industry's specifications and requirements in using carbon black as a reinforcement of elastomers the production of carbon black is dominated throughout its industry [18].

Carbon blacks are used in a wide variety of plastic applications such as the following [19]:-



fibre applications,

EMI Shielding components,

for Electrostatic dissipation,

for colour concentrations in pigmentation,

UV protection/stabilisation

conductive fillers and additives can be used in insulating polymers to achieve surface resistivity in components,

wire & cable compounds.

In extruded nylon 6.6, produced by Zell-Metall Engineering Plastics, nanoparticles are used in POM-C an acetal copolymer to achieve a low surface resistivity of 103 - 104 and a volume resistivity of 104 [20]. This grade of polymer can be used in robotics, material handling, mining, high speed printing, electric, electronic and semiconductor industries and mobile phone production plants [20]. For applications such as:- insulators, relay and transformer housings, bearings, slide pads, integrated circuits, hard disk drives, circuit boards, coil bodies [20].

Conductive carbon black filled cast nylon 6 can be used in the following applications:-

gears can be cast and placed into electrical generators on electrical pylons,

custom made winches and sheaves can be used on porting vessels at the boat yard,

for cable laying,

in load bearing applications.

2.3.4 Previous work

Some work has done previously [21].

One of the projects was to add single walled carbon nanotubes from Bayer as a filler into cast nylon 6 for conductive properties. It was found that added carbon nanotubes (Baytubes) at 1%/wt had:-

a relatively poor dispersion due to a large amount of agglomerates (>100 microns) still remaining as lumps in the final casting.

many of these large agglomerates sank to the bottom of the cast due to an unstabilised pre-dispersion process.

the polymerisation cycle of the carbon nanotubes filled nylon 6 was slow, such that the carbon nanotubes started to settle on the bottom of the casting.

Other methods in this area of work were addressed to improve the effectiveness of Baytubes, such as the following:-

dispersing the Baytubes via a method of ball milling under a high pressure homogeniser whilst in the raw material (caprolactam).

also, stabilising the Baytubes in the pre-dispersion process using a dispersing agent i.e. preventing the settling out of the Baytubes.

Another project concerned the addition of an ENSACO 260G carbon black grade to cast nylon 6 again to achieve conductive properties. This ENSACO 260G carbon black grade was supplied by Timcal Graphite and Carbon based in Willebroek, Belgium. The ENSACO 260G carbon black grade was added as filler to cast nylon 6 at approximately 13-15%/wt. However, it was found that the mechanical properties had deteriorated by adding an increasing amount of ENSACO 260G carbon black to cast nylon 6. However, the conductive properties that were required by the customer were essentially achieved. Due to the decrease in mechanical properties the carbon black filled cast nylon 6 could not be used, as it posed a major risk. Therefore, it was proposed that another ENSCAO grade of carbon black be used at a lower concentration, effectively giving better mechanical properties and the desired conductivity requirements for a customer's application, also, adding another grade of cast nylon 6 produced by Nylacast Ltd.

2.4 Conductive fillers in polymers

As well as improving the conductivity the conductive carbon black grades can have [13]:-

good mechanical properties e.g. impact strength,

low particulate erosion,

high purity in the polymer matrix,

once mixed well upon shearing, good dispersion can be achieved,

low sulphur containing content e.g. in rubbers.

The conductive carbon black filler supplied by Timcal Carbon and Graphite for this project will be used to enhance the conductivity properties of the polymer, as well as, to maintain the mechanical properties to that of a standard cast nylon 6 grade or better, manufactured by Nylacast Ltd.

2.4.1 Percolation theory

The study by Carmona et al states that the percolation theory was mainly developed from the 1970's to 1980's to learn about the communication at a large distance through allowed bond site that were placed at random on a lattice [22]. By using carbon black as filler in the polymer matrix, the size and distribution, the cluster number and the correlation length can be calculated as a function of the bond site on the density of the various lattices in two and three dimensions [22]. This was under the assumption that the allowed bonds between the carbon particles were conducting, however, other polymers without being carbon black filled had properties of insulators which lead to a conductivity threshold identified to the percolation threshold [22].

Timcal Carbon and Graphite have published some percolation curves in their brochure of carbon additives for polymer compounds, as shown in figures 6 and 7 [22]. The percolation curves show the correlation of volume resistivity and the carbon black percentage [22]. The percolation curves enable the usefulness of a comparative tool to predict the conductivity in order to select the most appropriate route to undertake [22]. The curves represented on these graphs allow a sample preparation technique to take place by a process of manipulation to evaluate the levels of carbon black that may be required for loading into a polymer [22].

Figure 6. Shows the volume resistivity ( vs two ENSACO carbon black grades loading in injection moulded polycarbonates (PC) [22].

Figure 6, shows that injection moulded PC was filled with two different grades of E250G and 350G carbon black grades. E250G is a low surface area conductive carbon black grade, in which upon processing in an injection moulding machine would show an advantage on dispersion compared to the high surface area conductive carbon black grade E350G. Also, figure 6 shows that upon a lower loading of the E350G carbon black grade very low volume resistivity can be achieved, compared to the much higher loading level of E250G carbon black grade the lower volume resistivities can be achieved [22]. In summary, of figure 6 shown above, the higher the surface area of the carbon black grade, the lower the percolation threshold [22].

Figure 7. Shows the volume resistivity ( vs three ENSACO carbon black grades loading in High Density Polyethylene (HDPE) [22].

Figure 7, shows that compression moulded HDPE was filled with three different grades of E250G, E260G and E350G carbon black grades [22]. It can be seen in figure 7 that a higher loading level of E250G carbon black is needed to achieve low volume resistivity compared to E260 and E350, respectively, which only require a smaller loading level to achieve the low volume resistivity [22].

Therefore, by looking at the percolation curves shown in figure 6 and 7 a comparison on the tool can be used to predict the volume resistivity on the loading level of carbon black into the polymer matrix [22].

In order to construct a percolation curve, different grades of carbon black must be first experimented by adding into a polymer to gain a result, and tested for the volume resistivity. Thereafter, a graph can be plotted. This enables a person looking at the curve, for example, a polymer manufacturer who was going to use a particular grade will use accordingly to what will suit them the most. For this project, ENSACO 350G will be used as the loading level of the conductive carbon grade needs to be reduced in order to achieve a lower resistivity.

2.4.2 Types of conductive fillers

Other conductive fillers made to enhance electrical properties include the following:-

Jiang et al prepared nickel coated mica as a conductive filler [23], and found that by a dependant of increasing the particle size and the coverage of the mica particles upon a lower loading level an electronically conductive composites could be produced [23].

Krupa et al conducted a study on electrically conductive composites of polyethylene (PE) with polyamide (PA) particles coated with silver [24]. It was found that these composites filled with high filler content were highly electrically conductive, the electrical conductivity had reached the value of 6.8Ã-102 Scm-1 [24].

2.5 Distribution / dispersion of carbon black in Cast Nylon 6

The spatial property of being scattered about over a range, area, or volume is known as distribution [25]. To be able to distribute (particles) evenly throughout a medium is called dispersion [26]. It is important to understand that being able to distribute and disperse a particular filler (organic or inorganic) in a polymer matrix solution is critical for achieving good mechanical properties. Particles which have not completely dispersed or been distributed throughout the polymer matrix may settle to the bottom of the polymer matrix or even form agglomerates in the polymeric matrix. Equipment and techniques are available to enable dispersion to be evaluated relatively easily. The method of distribution of a filler in a polymeric matrix may need to be improved before polymerisation takes place, however, the density of such fillers and additives can now be made to suit the polymer matrix in order to achieve a homogeneous distribution throughout a polymer matrix.

2.5.1 Types

There are number of fillers and additives that can be placed into polymers to gain a wide variety of properties from materials. Fillers and additives come in different shapes and sizes. Material science products that can be introduced to polymer matrices can include the following [27-28]:-

Nanomaterials, such as:- dendrimers, lumidots, nanoclays, nanopowders, nanotubes, fullerenes and silsesquioxanes. Examples include:- graphitised mesoporous carbon, dendrons & hyperbranched polymers, montmorillonate Nanoclays and halloysite nanotubes.

Organic and inorganic fillers, such as:- CaCO3, glass fibre, clay and mica.

Masterbatches containing, pigments useful for colouring, blowing agents to introduce part weight reduction, antioxidants can stabilise the polymer during processing and prolong its useful life in the end application, flame retardants for processing to occur easily and have minimum impact on the physical aspect of finished components, fragrances, antimicrobials, antiblocks, biodegradable, UV stabilisers and many others.

2.5.2 Methods

There are a number of ways in which fillers and additives can be dispersed and distributed homogeneously in a polymeric matrix. This may include:- mixing using a stirring rod, using an extruder to make a masterbatch of the sample or even using a shear mixer to allow correct and efficient mixing in a polymer matrix. It is found that better mixing of a certain filler or additive in a polymer matrix can increase in the mechanical properties.

2.5.3 Processing of carbon black cast filled nylon 6

It is important to consider that adding the E350G carbon black to the melted caprolactam prior to polymerisation, the carbon black must be dispersed and completely distributed throughout out the polymeric solution. By looking at the technical data sheet of E350G carbon black, it has a much higher surface area to volume ratio compared to the other grades of carbon black supplied from Timcal Carbon and Graphite [29]. The technical data sheet suggests that the dispersibility of E350G in polymer matrices is fairly difficult compared to other grades. Therefore, in taking the high surface area to volume ratio and the rate at which this grade of carbon black can absorb water into consideration with caprolactam being hydrophilic to water, major consequences can lie in the polymerisation of carbon black filled cast nylon 6. In order to obtain the suitable properties from the carbon black filled cast nylon 6, the E350G must be added at relatively lower concentration in order to disperse and distribute the filler homogenously throughout the polymer matrix. In order to do this, see Chapter 3 that illustrates the correct procedure in homogenising the carbon black throughout the monomer matrix prior to polymerisation taking place.

2.5.4 Effect of carbon black on surface resistivity properties

Antistatic, electrostatic dissipative (ESD) and conductive additives are increasingly becoming in demand by plastics compounders as they are used in many applications, from the packaging to automotive industries [30]. Additive suppliers are also introducing new migrating antistatic materials, permanent antistatic materials and nanomaterials [30] to develop the properties of polymers either to be electrically conductive or enhance the mechanical properties e.g. impact strength. Figure 8 below shows the range of surface resistivity levels from 1017 to 10-5 ohms/sq [30].

Figure 8. Shows the surface resistivity range from plastics to metals [30].

Figure 8 shows that by adding particular additives into polymers they can enhance the surface resistivity properties, therefore, when a customer is able to make decision of the kind of surface resistivity required, a polymer manufacturer can use the relevant additives to manufacture to the customers needs. An example of this would be when a customer would require longer-term protection for electronics from electrostatic dissipation, therefore, the manufacturer would produce a polymer filled with permanent antistatic or conductive additives such as carbon black, conductive fibres or nanomaterials [30].

Figure 9. Shows the surface resistivity of specific polymers and additives [31].

Conductive carbon blacks can be used for compounding by using masterbatches as well as adding into polymer matrix by using additives. They are used to achieve good electrical properties (as shown in figure 9), at lower loading levels rather than using the conventional carbon black grades.

A study by Cabot Cooperation shows that a method of comparing the performances of the conductive carbon blacks for surface resistivity applications is by the measuring or determining the primary performances by the star diagram represented in figure 10, below [32].

Figure 10. A star diagram representing the best performance for any given parameter is at the perimeter of the diagram [32].

As this is a new method being applied to Nylacast Ltd, Research and Development department the results obtained from testing will be analysed and discussed. In essence, the mechanical and surface resistivity properties in the material should be enhanced upon a loading of 4-5% of E350G carbon black filler. With this loading of filler it is expected that the surface resistivity of 106 to 104 ohms/sq is achieved for conductive properties from the material.

2.5.5 Compounding using conductive ENSACO 350G carbon black grade

Probst et al conductucted a study to improve the conductivity of polymer compounds using lower loading levels of carbon blacks from Timcal Carbon and Graphite grade by compounding to offer good mechanical properties of polymers [33]. Compounding conductive E350G carbon black at lower loadings than a convention carbon black grade will impose the capability of this grade to impart electrical conductivity to a compound depending on its ability to establish and maintaining the conductive network in an insulating polymer matrix [33]. To achieve conductivity from a sample material using a carbon black grade it is understood that these carbon black aggregates have to arrange themselves in a continuous path where the conductive elements are either in direct contact or the electrons can move via hopping or by tunnelling mechanisms [33]. In order to measure this conductivity the carbon black surface must be kept free from any contamination, residue or chemical group which can deteriorate the electrical contact points [33]. Technical data sheets of the E350G carbon black shows that this grade is highly structured and has a high void volume and so a low bulk density [33]. In essence, this grade has a high surface to volume ratio so the powder is of a very fine nature. Therefore, a high filling content must be incorporated into polymer due to its low density. It has been found in this study that incorporating the 350G carbon black grade and applying continuous mixing will tend to strongly increase the viscosity of the polymer matrix [33]. This study found that carbon black agglomerates are mainly based on weak interactions of the aggregates and as such they are sensitive to shear [33]. This means that high shearing which takes place using a mixer, will have to be controlled such that any residues, contamination or chemical groups can remain undisturbed and the agglomerates of carbon black are able to be sheared are dispersed throughout the monomer for the electrons to make contact throughout the polymer matrix to achieve conductivity. Although, this has been proved by the study conducted, in which, the resistivity increases upon mixing as the carbon black dispersion improves to an optimum level [33], consistent mixing would ensure good conductivity and a homogeneous mix.

2.6 Characterisation of carbon black filled cast nylon 6

In this section, to introduce the principles of Transmission Electron Microscopy TEM, Differential Scanning Calorimetry (DSC) and the mechanical properties, such as:- tensile strength, flexural strength, compression strength, impact strength and hardness, as well as the coefficient of friction to measure the properties and detect the structural features of carbon filled cast nylon 6. These properties and structural features of the polymeric material will help to determine the ability and characteristics of the carbon filled cast nylon 6. The main property in this project would be to measure the electrical resistivity of the cast samples by using an Electrometer. In principle, by carrying out the above characterisation technique on the carbon filled cast nylon 6 using TEM it can be determined that if agglomerates are formed they can be visualised using this technique and also, if there is any settling out of the carbon particles i.e. to the bottom of the cast nylon 6, then this can also be visualised. The mechanical properties are also to be determined.

2.6.1 Transmission Electron Microscopy (TEM)

TEM provides detailed and structural information at levels down to atomic dimensions [34]. This technique is now being used to characterise materials down to these dimensions, as part of identifying particular structures in polymeric materials. By using TEM the range of information obtainable from the technique is 1-100nm [34].

The TEM samples require special preparation. TEM requires the samples under investigation to be less than 1µm thick [34]. TEM plays a pivotal role in investigating the fine - scale structure of polymeric materials, for both industries and research purposes [34]. It will be important to characterise and determine at the highest levels of resolutions so that the behaviour of the carbon black filled cast nylon 6 material can be maximised in giving it excellent properties that need to be achieved. Using this technique the dispersion and the distribution of the carbon black in cast filled nylon 6 sample pieces can be visually assessed.

2.6.2 Differential Scanning Calorimetry (DSC)

DSC is a thermoanalytical technique which is used to measure the heat flow in a sample material [35]. The DSC instrument contains two aluminium pans, in which, they contain a reference sample and the other one contains the sample itself. The melting point and weight fraction crystallinity can be determined by using the DSC [35]. During the characterisation process the sample in the aluminium pan undergoes a physical transformation, in which, due to the constant rate of heating the temperature in the sample is changed as opposed to the reference pan. The data provided from the DSC instrument is used to determine the melting point and enthalpy of fusion from which the weight fractional crystallinity is calculated.

2.6.3 Mechanical properties

Mechanical testing will include:- tensile, flexural, compressive, impact and the coefficient of friction on each of the samples made in the experimental on the carbon black filled cast nylon 6 samples. In particular, the tensile strength will focus on the elongational modulus on each of the cast nylon 6 samples.

2.6.4 Electrical resistivity properties

It is important to measure the electrical resistivity of the carbon black filled cast nylon 6 samples as it will determine the conductivity properties that will be achieved. The electrical resistivity measurements will be based on measuring the volume resistivity on the samples of carbon black filled cast nylon 6. There are two basic ways of measuring the volume resistivities on the specimens using an electrode arrangement [13]. One of which is a rectangular or cylindrical block with electrodes on both ends [13]. The other is like that used for dielectric measurements where the electrodes are applied to each side of the sample, in which the samples in this case are of a disc shape [13]. A two point or four point contact method will be used at the Timcal Carbon and Graphite facility in Belgium to determine the electrical resistivities on each of the polymer samples prepared. The four point probe method has proven to be used on a more regular basis, for measuring the volume resistivity on each of the polymer material samples [13].