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Wear Rate of an Advanced Polymer Coating Experiment

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CHAPTER 1

1.0 INTRODUCTION

The general purpose of coating is to protect the substrate and to be decorative, enhancing the appearance of substrate in terms of gloss, colour, adhesion and wetability. Consequently, in addition, surface properties such as resistance to wear, indentation, and scratching are critical to retain the basic functions [1, 2]. Polymers are common materials which are widely used in industry. Polymer coating acts as a protective covering in corrosive environments. The properties of polymers depend largely on the structures of individual polymer molecules, molecule shape and size as well as how molecules are arranged to form a polymer structure. Polymer molecules are characterised by their very large size - a feature that distinguished them from other most organic chemical compositions.

The wear rate for an advanced polymer materials varies depending on the applied normal load and the sliding speed which is affected by size, shape, the matrix composition and the test conditions [3]. As the load decreases the contact becomes elastic with consequent changes in mechanism of friction.

Epoxy resin are characterized by the presence of three- member cyclic ether group commonly referred to as an epoxy group, epoxies cross-link when a catalyzing agent (hardener) is added, forming a three-dimensional molecular network. Because of their outstanding bonding strength, epoxy resins are used to make coatings, adhesives, composite laminates and have important applications in the aerospace industry. Epoxy powder coating is still widely used today, especially as a functional and tough coating where flexibility, adhesion, toughness and corrosion resistance are needed. Epoxy coatings are based on epoxy resin; they are manufactured in a way which enables the possibility of coating to a certain thickness without defect in the coating after curing [14, 16]. Epoxy power coatings get a good adhesion to aluminium or steel surfaces, and provide a good corrosion protection layer (for indoor use). Most outstanding with powder finish is the excellent resistance against other chemicals. Epoxy powder coatings are suitable especially in aggressive environments. However, they are not suitable for external use where they become discoloured as the pigments are broken down by ultra violet rays when exposed to sunlight. They consequently become chalky and actually decreased in thickness as they are washed away [15].

1.1 AIM

The main aim of this project is to determine the effect of sliding speeds on the wear rate of an advanced Polymer coating.

1.2 RESEARCH OBJECTIVES

The objective of this project is to investigate the wear rate of an advanced powder coating using a reciprocating linear ball-on-flat sliding machine. Due to the nature of the resources required to run the experiments, this investigation included the careful attention taken for wear testing especially in areas of the ball-on-flat sliding machine, data acquisition and wear rate calculation.

The technical approach for the investigation will involved preparing different test specimens cured at temperatures of 140oC, 160oC, 180oC, and 200oC respectively. These test specimens would then be tested for one hour at sliding speeds of 0.02 m/s, 0.03 m/s and 0.04 m/s on a fixed normal load of 0.76 kg. Data from the wear testing would be acquired at once as electrical signals through with the help of electronic data acquisition devices. Also this would involve investigating and analysing the microstructure of the all the tested coated sample to see the level of damage done to it at the above temperatures and sliding speeds.

CHAPTER 2

2.0 LITERATURE REVIEW

2.1 POLYMERS

Modern scientific tools revolutionized the processing of polymers thus available synthetic polymers like useful plastics, rubbers and fibre materials says (Satish V. Kailas) [38]. As with other engineering materials (metals and ceramics), the properties of polymers are related their constituent structural elements and their arrangement. The suffix in polymer ‘mer' is originated from Greek word meros - which means part. The word polymer is thus coined to mean material consisting of many parts/mers. Most of the polymers are basically organic compounds, however they can be inorganic (e.g. silicones based on Si-O network). This chapter introduces classification of polymers, processing and synthesis of polymers, followed by mechanism of deformation and mechanical behaviour of polymers [10, 11, 38].

Polymers are classified in several ways - by how the molecules are synthesized, by their molecular structure, or by their chemical family. For example, linear polymers consist of long molecular chains, while the branched polymers consist of primary long chains and secondary chains that stem from their main chains. However, linear does not mean straight lines. The better way to classify polymers is according to their mechanical and thermal behaviour. Individually, polymers are classified into two main classes - plastics and elastomers [10, 11].

2.1.1 PLASTICS

Plastics are mouldable organic resins. These are either natural or synthetic, and are processed by forming or moulding into shapes. Plastics are important engineering materials for many reasons. They have a wide range of properties, some of which are unattainable from any other materials, and in most cases they are relatively low in cost. Following is the brief list of properties of plastics: light weight, wide range of colours, low thermal and electrical conductivity, less brittle, good toughness, good resistance to acids, bases and moisture, high dielectric strength (use in electrical insulation), etc. Plastics are again classified in two groups depending on their mechanical and thermal behaviour as thermoplasts (thermoplastic polymers) and thermosets (thermosetting polymers) [10, 11].

2.1.2 TYPES OF PLASTICS

2.1.2.1 THERMOPLASTS

These plastics soften when heated and harden when cooled - processes that are totally reversible and may be repeated. These materials are normally fabricated by the simultaneous application of heat and pressure. They are linear polymers without any cross-linking in structure where long molecular chains are bonded to each other by secondary bonds and/or inter-wined. They have the property of increasing plasticity with increasing temperature which breaks the secondary bonds between individual chains. Common thermoplasts are: acrylics, Vinyl resins (PVCs), polyethylene, polypropylene, polystyrene, polyimide, ABS (acrylonitrile butadiene and styrene) etc [10, 11].

Acrylics (poly-methyl-methacrylate)

They are characterised by outstanding light transmission and resistance to weathering; only fair mechanical properties. Their applications are mostly in lenses, transparent aircraft enclosures (aeroplane windows), drafting equipment, and outdoor signs [17].

Polystyrene

They have low specific gravity (1.07), availability in colours form clear to opaque, resistance to water and most chemicals, dimensional stability, insulating ability. Polystyrene is an excellent rubber substitute for electrical insulation. Styrene resin is moulded into battery boxes, dishes, radio parts, lenses, flotation gears etc [17].

Polyethylene

These materials are flexible at room and low temperatures, waterproof, unaffected by most chemicals, capable of being heat-sealed, and can be produced in a variety of colours. Polyethylene, which floats on water, has a density range from 0.91 to 0.96% and is one of the inexpensive plastics, and its moisture-resistant characteristics ensure its use for packing and squeeze bottles [12, 17].

Polyethylene Polymer

Table 2.1 - Properties of polyethylene

Properties of Polyethylene

Tensile Strength

80MPa , for biax film 190-260MPa

Thermal Coefficient of Expansion

100 - 220 x 10-6

Melting Point

110 oC (230 oF)

Glass Transition Temperature

-125 oC (-193 oF)

Density

0.910 - 0.940 g/cm3

Poisson's ratio

0.37-0.44(oriented)

Tensile modulus

2-4 GPa

Surface resistivity

1013 Ohm/sq

Polypropylene

Polypropylene has excellent electrical properties, high impact and tensile strength and is resistant to heat and chemicals. Monofilaments of polypropylene are used in making rope, nets, and textiles.

Polyimide

These thermoplastics are produced in the form of solids, films or solutions. They have unusual heat-resisting properties up to 750oF (400oC), low coefficient of friction, high degree of radiation resistance, and good electrical properties. Products from this include sleeve bearing, valves seats, tubing, and various electrical components. The films, tough and strong, are used for wire insulation, motor insulation, and printed circuit backing [17].

ABS (Acrylonitrile, Butadiene, and Styrene)

This plastic can be compounded to have a degree of hardness or great flexibility and toughness. The ABS plastics are used in applications that requires abuse resistance, colourability, hardness, electrical and moisture properties, and limited heat (2200oF (105oC). These plastics and processed by thermoforming injection, flow, rotational, and extrusion moulding. Applications include household piping, cameras, electrical hand tool housings, telephone handsets, and canoes [12].

Vinyl Resins

These thermoplastic materials can be processed by compression or injection moulding, extrusion, or blow moulding. Vinyl resins are suitable especially for surface coating and flexible and rigid sheeting. The vinyl resins commercially available include polyvinyl chlorides (PVCs), butyrates, and polyvinylidene chloride. Polyvinyl chloride has a high degree of resistance, to many solvents and does not support combustion. It is used for rubberlike products including raincoat, packaging and blow-moulded bottles. Polyvinyl butyrate is a clear tough resin, which is used for interlayers in safety glasses, raincoats, sealing fuel tanks, and flexible moulded products. It has moisture resistance, great adhesiveness, and stability towards light and heat [17].

2.1.2.2 THERMOSETS

These plastics require heat and pressure to mould them into shape. They are formed into a permanent shape and cured or ‘set' by chemical reactions such as extensive cross-linking. They cannot be re-melted or reformed into another shape but decompose upon being heated to too high a temperature. Thus thermosets cannot be recycled, whereas thermoplasts can be recycled. The term thermoset implies that heat is required to permanently set the plastic. Most thermosets composed of long chains that are strongly cross-linked (and/or covalently bonded) to one another to form 3-D network structures to form a rigid solid. Thermosets are generally stronger, but more brittle than thermoplasts [10, 11].

An advantage of thermosets for engineering design applications includes the following: high thermal stability, high dimensional stability, high rigidity, light weight, high electrical and thermal insulating properties and resistance to creep and deformation under load. There are two methods whereby cross-linking reaction can be initiated - cross-linking can be accomplished by heating the resin in a suitable mould (e.g. bakelite), or resins such as epoxies (araldite) are cured at low temperature by the addition of a suitable cross-linking agent, an amine. Epoxies, vulcanized rubbers, phenolics, unsaturated polyester resins, and amino resins (ureas and melamines) are examples of thermosets [10, 11].

Phenolics

These resins for popular for thermosetting applications. The synthetic resign, made by the reactions of phenol with formaldehyde, forms a hard, high-strength, durable material that is capable of being moulded under a variety of conditions. It is characterized by excellent thermal stability to over 150oC (that is, has high heat and water resistance) and can be coloured in a variety of way; may be compounded with a large number of resins, fillers [12]. It is used in manufacturing coating materials, laminated products, grinding wheels, and metals as well as glass bonding agents, and can be cast into moulded cases, bottle caps, knobs, dials, knife handles, electrical appliance cabinets and numerous electrical parts [17].

Epoxy Resins

Epoxy resins, both monomers and oligomers, can be powders or they can be thick and clear or yellow liquids with strong and unpleasant odours. They are known for their excellent adhesion, chemical and heat resistance, excellent mechanical and good electrical insulating properties. Moulding a fibre reinforced epoxy composite is much easier compared to other thermoset resins [19]. The typical applications of epoxy are in adhesives, electrical parts, coating and lamination process, moulds/dies/tools and in military, biomedical and automotive fields.

Epoxy being a thermoset polymer, during the process of curing, when mixed with curing agent or harder, polymerises and cross-links. In other words, this curing agent reacts with epoxy resin monomers to form epoxy product. The curing agent selection will determine to a large extent the performance of the final epoxy composite. Table 2 below shows the properties of epoxy resins.

Table 2.2 - Properties of Epoxy resins

Properties of Epoxy

Ensile Modulus, E

3.5GPa

Poisson's ratio, υ

0.33

Density

1.54 g/cm3

Shear Modulus, G

1.25GPa

Longitudinal Tensile Strength, σ

60MPa

Longitudinal thermal Expansion, α

5.7×10-6K-1

Epoxy resins are the major part of the class of adhesives called ‘structural adhesives'. These high performance adhesives are normally used in the construction of aircraft, automobiles, bicycles, golf clubs, snowboards and other applications where strength bond are required. Also, they are exceptional adhesives for wood, metal, glass, and some plastics. Epoxy resins can be made flexible of rigid, transparent, opaque or coloured and fast setting or extremely slow setting. Furthermore, epoxy adhesives are unmatched in heat and chemical resistance among other adhesives. Usually requires heat curing for maximum performance [22]. Therefore epoxy adhesives cured with heat will be more heat and chemical resistant than those cured at room temperature. Also, the peak adhesion strengths achievable for epoxy/metal interfaces depends greatly on the types and sequences of wet chemicals used to treat the surface. For all these reasons, they are used in high performance and decorative flooring applications too [22].

Epoxy coatings are also widely used as primers to improve the adhesion of automotive and marine paints especially on metal surfaces where corrosion resistance is important. However, they are not used in the outer layer of a boat as they can deteriorate when exposed to ultra violet light. But they are often used during boat repair and assembly and also over-coated with conventional paints or marine varnishes which can provide ultra violet protection [23]. Also, metal cans and containers are often coated with epoxy to prevent rusting especially for foods like tomatoes, which are acidic in nature.

In addition, epoxy resin is an excellent electrical insulator too. It helps to protect electrical components from short-circuiting due to dust and moisture. Hence, epoxy resins are important in the electronic industry, finding application in motors, generators, transformers, switchgears, bushings and insulators. Also, in the electronic industry, epoxy resins are the primary resin used in moulding integrated circuits, transistors and hybrid circuits. The cured epoxy is an insulator and a much better conductor of heat than air. Using epoxy in transformers and inductors greatly reduces hot spots which in turn give the component a stable and longer life than unprotected products [24]. Another interesting property of epoxy is that it does not stick to mould release compounds like paraffin wax, polyethylene sheeting, sandwich bags and the non glued side of packaging tape which is of great use during lay-ups and also during the manufacturing of precision parts.

Though epoxy resins are more expensive than any other resins such as polyester resins, in brief, the purpose of selecting epoxy as the base resin for this research is because of its un-matching high chemical and thermal resistance, good adhesion to various materials, compatibility with various substrates and other additives, low shrinkage, availability of solvent free formulations, light in colour, easy to control viscosity and low vapour pressure besides holding good to excellent mechanical properties and very good electrical insulating properties.

The properties of polymers depend largely on the structures of individual polymer molecules, molecule shape and size as well as how molecules are arranged to form a polymer structure. Polymer molecules are characterised by their very large size - a feature that distinguished them from other most organic chemical compositions.

2.1.2.3 ELASTOMERS

Polymers are long-chain molecules that are formed by polymerization (that is by linking and cross-linking of different monomers. A monomer is the basic building block of a polymer. One of the fascinating properties of the elastomeric materials is their rubber-like elasticity. That is, they have the ability to be deformed to quite large deformations, and then elastically spring back to their original form. This results from the cross-links in the polymer that provides a force to restore the chains to their undeformed conformations. Elastomeric behaviour was probably fist observed in natural rubber; however, the past few years have brought about the synthesis of a large number of elastomers with a wide variety of properties. Typical stress-strain characteristic of elastomeric materials is displayed in figure 2 curve.

Upon stretching, it is immediately noted that there is a flat region in the stress-strain curve. This essentially means that after an initial elongation, there is a region stretching which occurs without increasing strain.

2.2 CRYSTALLIZATION, MELTING AND GLASS TRANSITION

Polymers are known by their high sensitivity of mechanical and/or thermal properties. This section explains their thermal behaviour. During processing of polymers, they are cooled with/ without presence of presence from liquid state to form final product. During cooling, an ordered solid phase may be formed having a highly random molecular structure. This process is called crystallization. The melting occurs when a polymer is heated. If the polymer during cooling retains amorphous or non-crystalline state i.e. disordered molecular structure, rigid solid may be considered as frozen liquid resulting from glass transition. Thus, enhancement of either mechanical and/or thermal properties needs to consider crystallization, melting, and the glass transition.

Crystallization and the mechanism involved play an important role as it influences the properties of plastics. As in solidification of metals, polymer crystallization involves nucleation and growth. Near to solidification temperature at favourable places, nuclei forms, and then nuclei grow by the continued ordering and alignment of additional molecular segments. Extent of crystallization is measured by volume change as there will be a considerable change in volume during solidification of a polymer. Crystallization rate is dependent on crystallization temperature and also on the molecular weight of the polymer. Crystallization rate decreases with increasing molecular weight.

Melting of polymer involves transformation of solid polymer to viscous liquid upon heating at melting temperature, Tm. Polymer melting is distinctive from that of metals in many respects - melting takes place over a temperature range; melting behaviour depends on history of the polymer; melting behaviour is a function of rate of heating, where increasing rate results in an elevation of melting temperature. During melting there occurs rearrangement of the molecules from ordered state to disordered state. This is influenced by molecular chemistry and structure (degree of branching) along with chain stiffness and molecular weight.

2.3 POLYMER COATING

The task of coating technology is to provide surface protection, decorative finish and numerous special functions for commodities and merchandise by means of organic coatings. Many everyday products are only made useable and thus saleable because their surface treatment. To achieve this, relevant coating formulations, their production plant, the coating material and suitable coating processes for product must be available. However, the quality to be achieved by means of coating process is not the only function of the coating material used. The object to be painted or coated itself with its specific material and design and appropriate application process are further variables which play a significant role. Coating itself is a layer of material which is applied to a surface to decorate, preserve, protect, seal, or smooth the substrate; usually applied by brushing, spraying, mopping, or dipping [26].
There are two principal technologies that are the backbone of the coatings industry:

Ø Liquid coating technology (wet), which has been applied for more than two centuries

Ø Powder coating technology (dry), which has been applied on an industrial scale for some 30 years.

2.3.1 WHY POWDER COATING

The global average annual growth for powder coatings has been approximately 7-9% over the last 10 years. From country to country worldwide these figures have varied considerably. This relatively high performance has been achieved by autonomous growth on the one side and by replacing liquid coatings on the other.

Powder coating involves applying a finely grounded resin (powder) to a substrate and subjecting this powder to heat. During the heating process, the powder melts and creates a uniform, continuous coating [26]. The use of powder coating as a finishing process has grown significantly in the past several years. It dates back to the 1950s when powders were flamed-sprayed on metallic surfaces to protect them from corrosion and abrasion. As the process evolved, most powder-coating application involved lowering a heated part (sometimes referred to as a “ware” or a “substrate”) into a bed of fluidized powder. However, this process resulted in inconsistent film thickness. Electrostatic introduced in the early 1960s, enabled powder coatings to be applied to cold substrates, resulting in more uniform, thinner surface application and thus, savings in raw materials [26].

Today, powder-coating processes are employed in many production settings involving protective finishes. Powder formulations can be created to deliver cosmetic, protective, and longevity characteristic, and to achieve maximum hardness, chemical resistance, and gloss retention. More and more companies have turned to powder coating as a way to produce a high-quality finish while increasing production rates, cutting costs, and complying with increasing environmental pressures. Also, ongoing technological breakthroughs are continually knocking down the few barriers that hindered powder coating's ability to grow in the market.

2.4 POWDER COATING MANUFACTURING PROCESS

The process of producing a powder coating is somewhat more complex, the equipments are numerous and the production time is somewhat long. This process can be simplified into 4 basic stages.

2.4.1 PREMIXTURE

This is the first stage in the manufacture of a powder coating and is the most crucial for production. In this stage, the various sizes of resin flake, pigment powder, etc must be transformed into a homogeneous blend of similar sizes before they enter the extrusion stage.

This is for the fact that the extruder is a simple melt mixer and not a very efficient disperser; the premix equipment consists of a mixing bowl with a locking cover. The equipment is usually fitted with a cooling jacket capable of being filled with running water or cryogenic gases. Inside the mixing bowl are mixing-blades placed at different heights and orientation to one another depending on the machine configuration [27].

2.4.2 EXTRUSION

This is the second stage of the powder coat manufacture. The extruder is composed of a horizontally placed barrel. Within the barrel is a cylindrical screw shaft that is slightly smaller than the barrel in diameter. As the shaft turns, the premix moves forward through heated mixing zones. The mixing zones contain attachments called paddles that knead the melted premix and blends the various ingredients.

2.4.3 GRINDING

The third phase (grinding stage) is very important to its performance. This is because the average particle size and distribution are important in the application properties and final surface appearance. In the grinder, grinds the chilled and pressed extruded mix into fine particles. This is usually done using, impact/hammer mill, air jet mill or the air classifier mill.

2.4.4 SIFTING AND CLASSIFYING

The final structuring of the particle size is accomplished by passing the ground materials through sieves or cyclonic separators. This is to sort out the distribution and average particle size of the powder.

2.5 ECONOMIC BENEFITS OF POWDER COATING

Although equipment and materials cost are similar in powder-coating and liquid-coating processes, yet powder coating processes provide a number of advantages over other surface coating methods. These include:

  • Fewer rejects.
  • Less floor space required.
  • Less material waste.
  • Lower energy costs.
  • Lower training and labour costs.
  • Lower waste-disposal costs.
  • More efficient cleaning operations.
  • More uniform finishes.

Powder coating materials are immediately ready for use. They do not have to be mixed with any other ingredients such as solvents or catalysts and are easy to apply, thus labour costs associated with training, setup, and processing are low when compared with liquid-coating processes [26]. Liquid coating usually requires thinning before application, leading to additional material and labour costs. This is not the case with powder coating. Liquid paint requires flash-off time before surface can re recoated which is not applicable to powder, meaning that racks can be spaced closer together and thus more parts per hour can be processed [25]. Powder coating processes results in fewer rejects than liquid coating processes. Since the former is a dry process, air and water associated problems - such as sags, runs, and contaminations are almost eliminated. Blowing off the surface with an air hose and reapplying the powder can easily repair coating rejects in booth or application area.

2.5.1 THERMOSETTING POWDER

Most powder coating materials are thermosetting powders. The greatest technological advances in powder coatings are being made in this area. Thermosetting powders are composed of solid resins higher in molecular weight than resigns found in liquid coatings and lower in molecular weight than those found in thermoplastics.

The solid resins melt and flow chemically, and cross-link within themselves or with other relative components forming a higher molecular weight reaction product. The coating film formed by this reaction is heat stable and will not soften back to a liquid on further exposure to heat (Wick and Veilleux 1985). At these higher temperatures, a coating emerges with different chemical properties than before heating.

Cured coatings have different chemical structures than basic resins. Newly formed cured materials are heat stable and will not re-melt to liquid after further exposure to heat.

These powders are ground from brittle resin systems into fine particles in the range of 0.004 - 0.0016 in (10 - 40 um) or less. ‘Due to the rheology of these resin systems, they can produce thin like paint coatings in the range of 0.001 - 0.003 in (25 - 75 μm) with properties equivalent or superior to coatings produced from liquid-compliance technologies [33].

The types of resins commonly used in thermosetting powder include:

§ Several types of epoxies

§ Hydroxyl and carboxyl types of polyesters

§ Several types of acrylics, and

§ Several types of silicones.

They require lower temperatures for curing than thermoplastic resins.

Table 2.4 - Main properties of different types of thermosetting powder coatings[27]

 

Properties of Thermosetting Powder Coatings

Property

Epoxy

Acrylic

Polyester

Hybrid

Polyurethane

Weatherability

Poor

Excellent

Excellent

Fair

Good

Corrosion Resistance

Excellent

Good

Very Good

Excellent - Very Good

Very Good

Chemical Resistance

Excellent

Very Good

Very Good-Good

Very Good

Very Good

Heat Resistance

Very Good

Good

Good

Very Good - Good

Very Good

Impact Resistance

Excellent- very Good

Good-Fair

Good

Very Good

Very Good

Hardness

HB-5H

HB-4H

HB-4H

HB-2H

HB-3H

Flexibility

Excellent-Very Good

Good-Fair

Very Good

Very Good

Very Good

Adhesion

Excellent

Good-Fair

Excellent

Excellent

Very Good

Thermosetting powders' chemical reaction begins in oven. Ovens produce and maintain heat-the sole cause of chemical reaction needed for in powder coating. Powder-coated parts must be exposed to heat to achieve the user-specified properties. The proper amount of heat at a given time ensures that the decorative, chemical, and mechanical properties are realised.

2.5.2 EPOXY POWDER COATS

Epoxy powders can be formulated to give very high gloss and smooth coatings with excellent adhesion, flexibility and chemical resistance. The film integrity on exterior exposure is excellent. The curing of epoxy powders is an additions mechanism and no volatiles are released during stoving [27]. They are low cost, low maintenance, and provide long lasting protection in chemical aggressive and abrasive environments. Epoxy powders are available in a wide range of formulations, allowing them to be to be applied to as thick films for functional purposes and thin films for decorative purposes.

2.6 ELECTROSTATIC SPRAYING

Electrostatic spray coating (ESC) is promising in addressing the deposition of nanoparticles due to its uniqueness to uniformly charge particles with the same electric polarity, which repels the particles and reduces the chances to form clusters.

In addition, electrostatic spray coating offers the benefits of depositing on large surfaces, coating parts with complex geometries and high deposition rate. Electrostatic spray powder coating uses a powder-air mixture from a small fluidized bed in a powder feed hopper. In some cases, the feed hoppers vibrate to help prevent clogging or clumping of powders prior to entry into the transport lines. The powder is supplied by a hose to the spray gun, which has a charged electrode in the nozzle fed by a high voltage dc power.

In electrostatic spraying, an electrical charge is applied to the dry powder particles or the powder is electrostatically charged while the component to be coated is electrically grounded. The charged powder and grounded workpiece create an electrostatic field that pulls the powder particles to the workpiece. The coating deposited on the workpiece retains its charge, which holds the powder to the workpiece. The coated workpiece is placed in a curing oven.

Electrostatic spray coating is the most common process used for the application of powder coatings on metal finishing. The process consists of four main steps:

  1. Transport the coating powder from a fluidized bed feeder to the spray gun
  2. Charge the particles within the gun
  3. Deposit the powder on the grounded workpiece inside a booth
  4. Collect the overspray and recycle it to the feeder.

After coating, all the workpieces are conveyed to the oven for final curing

2.6.1 ELECTROSTATIC FLUIDIZED BED COATING

This process involves the combination of elements of both fluidized bed and the electrostatic spray processes. Standard fluidized bed equipment is used; however the powder particles are given a negative charge by applying a high voltage direct current. The substrate is electrically 15 grounded and suspended above the fluidized bed, so that the charge particles can be attracted to the part. This process has been found most in coating large complex parts and thin wire gauges.

2.6.2 OTHER BENEFITS OF POWDER COATING

There are several advantages of powder coating over conventional liquid coatings [3]:

  1. Powder coatings emit zero or near zero volatile organic compounds (VOC).
  2. Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.
  3. Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.
  4. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
  5. Powder coating production lines produce less hazardous waste than conventional liquid coatings.
  6. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.
  7. A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes.

2.7 FRICTION

Friction is defined as the contact resistance exerted by one body upon another body when one body moves or tends to move past another body. This force which opposes the movement or tendency of movement is known as frictional resistance or friction. Friction is due to the resistance offered by minute projections at the contact surfaces. Hence, friction is the retarding force, always opposite to the direction of motion.

Frictional resistance has the remarkable property of adjusting itself in magnitude of force producing or tending to produce the motion so that the motion is prevented .

The simplest idea used in these studies of friction is that there are two main components of friction, namely, adhesion and deformation. Such approach is correct for all materials including polymers. The behaviour of polymer has distinguishing features, some of which are described by Briscoe. The main concept of the tribology of polymers consists of three basic elements involved in friction:

  1. Adhesive junctions, their type and strength.
  2. Shear and rupture of the rubbing material in the contact.
  3. Real contact area.

The deformation component of friction results from the resistance of the polymer to ploughing by asperities of the harder counterface. Polymer surface asperities experience elastic, plastic, and visco-elasic deformation depending on the material properties. The adhesion component stems from the adhesive junctions formed on the spots of real contact between the mated surfaces. The adhesion component of friction for polymers is believed to exceed much the deformation. Special consideration is needed for transfer films, being the key factor, which determine the tribological behaviour of polymers and polymer composites.

2.7.1 COEFFICIENT OF FRICTION

Coefficient of friction is the ratio of the lateral force required to slide the surfaces past one another relative to the force holding them in contact. Polymers exhibit two coefficients of friction namely: the static coefficient of friction which is measure of force required to initiate movement, the dynamic coefficient of friction is a measure of force required to sustain movement. In general, the force required to initiate sliding is somewhat greater than that required to maintain a constant rate of movement. The frictional force which is the force required to initiate or maintain motion. If W is the normal reaction of one body on the other, the coefficient o friction is

µ = F/w (2.1)

The coefficients of friction of a polymer depend on many variables, including the chemical composition of material against which is sliding, surface roughness, sliding speed, temperature, and frictional heating. The relationships controlling friction are complex and varied, so it is difficult to generalise with regard to how most of the factors affect the coefficients of friction. Friction generally increases as polymer temperature rises, because it becomes softer and viscous component of its nature plays a greater role. This effect will be seen clearly when comparing coefficients of friction of a polymer measured below and above its glass transmission properties [29].

2.7.2 STATIC AND KINETIC FRICTION

If the force to initiate motion of one of the bodies is Fy and the force to maintain its motion at a given speed is Fk, there is a corresponding coeffient of static friction µs = FS/W and a coefficent of kinematic µk= Fk/W. In some cases, these coefficient are approximately equal: in most cases µs > µk

2.7.3 BASIC LAWS OF FRICTION

The two basic laws of friction, which are valid over a wide range of experimental conditions, states that [28].

  • The frictional force F between solid bodies is proportional to the normal force between the surfaces, that is, µ is independent of W.
  • The frictional force F is independent of the apparent area of contact.

These two laws of friction are reasonably well obeyed for sliding metals whether clean or lubricated. With polymeric solids (plastics) the laws are not so well obeyed, in particular, the coefficient of friction usually decreases with increasing load as a result of detailed way in which polymer deform.

2.8 WEAR

Wear is the progressive damage, involving material loss, which occurs on the surface of a component as a result of its motion relative to adjacent working parts; it is almost inevitable companion of friction. Most tribological pairs are supplied with a lubricant as much to avoid the excessive wear and damage which would be present if the two surfaces were allowed to rub together dry as it is to reduce their frictional resistance to motion. The economic consequences of wear are widespread and pervasive; they involved not only the cost of replacement parts, but also the expenses involved in machine downtime, lost production, and the consequent loss of business opportunities. A further significant factor can be the decreased efficiency of worn plant and equipment which can lead to both inferior performance and increased energy consumption [1].

The wear of polymers is thus imperative to consider for the fact that polymers are more compliant than metals or ceramics, with values of elastic modulus which is one tenth or even less. Their strengths are also much lower, hence material counterface such as metals or ceramics should be taken into consideration when sliding then against polymers to act as rigid body. In addition, polymers exhibit lower coefficient of friction, whether self-malted or sliding against other materials. This affects the wear rate of the polymers. Nearly all deformation due to contact or sliding takes place within the polymer, and the surface finish of a hard counterface ha a strong influence on the mechanism of the resulting wear [2].

2.8.1 CLASSIFICATION OF WEAR

Wear can be categorised as mechanical, mechano-chemical, or thermal. Mechanical wear is defined as “removal of material due to mechanical processes under conditions of sliding, rolling or repeat impact”. Mechanical wear includes adhesive, abrasive, fatigue, and fretting wear. Mechano-chemical wear is one in which both mechanical and chemical factors are important, usually each facilitating the order. Mechano-chemical wear includes fretting corrosion, erosive, and other corrosive wear. Thermal wear is defined as “removal of material due to softening, melting, or evaporation during sliding or rolling. Wear by diffusion of separate atoms from one body to the other, at high temperatures, is sometimes denoted as thermal wear. Also, thermal shock, thermal fatigue, and high temperature erosion may be included in the general description of thermal wear.

Wear can also be categorizes according to the degree or amount of wear, without regard to the specific type of wear process involved. Wear can be normal, mild or severe, with no film delineation from one to the other. Normal wear is the loss of material within the design limits expected for the specific intended application. It also depends upon economic factors, such as the expandability of the worn part. Mild wear is a form of wear characterised by the removal of material in very small fragments. The term “mild wear” is an imprecise term that is frequently contracted with severe wear. Severe wear on the other hand is defined as “a form of wear characterised by removal of material in relatively large fragments [3].

This classification is not really based on any particular numerical value of wear rate but rather on their general observation that, for any pair of materials, increasing the severity of the loading (e.g. by increasing either the normal loading, sliding speed, or bulk temperature) leads at some stage to a comparatively sudden jump in the wear rate. The degree of wear rate can be investigated using the Archard wear equation which states that “wear rate w is directly proportional to the load, W on the contact, but inversely proportional to the surface hardness, Η of the wearing material, so that w ∝W/H, that is,

w = K × (2.2)

The dimensionless constant K is known as the wear coefficient.

Table 2.5 Distinction between mild and severe wear

Mild wear

Severe wear

Results in extremely smooth surfaces - often smoother than original.

Results in rough, deeply torn surfaces - much rougher than original.

Debris extremely small, typically only 100nm diameter.

Large polymeric wear debris, typically up to 0.01mm diameter.

2.8.2 MECHANISMS OF WEAR

The mechanism of wear is very complex and the theoretical treatment without the use of rather sweeping simplifications is not possible. It should be understood that the real area of contact between two solid surfaces compared with the apparent area of contact is invariably very small, being limited to points of contact between surface asperities. The load applied to the surfaces will be transferred through these points of contact and the localised forces can be very large. The material intrinsic surface properties such as hardness, strength, ductility, work hardening etc. are very important factors for wear resistance, but other factors like surface finish, lubrication, load, speed, corrosion, temperature and properties of the opposing surface etc. are equally important [4].

2.8.2.1 ABRASIVE WEAR

Abrasive wear (or abrasion) is wear by displacement of material caused by hard particles or hard protuberances [3]. It could also be said to be the damage to a component surface which arises because of the motion relative to that of either harder asperities or perhaps hard particles trapped at the interface. Such hard surface may have been introduced between the two softer surfaces as a contaminant from the outside environment, or they may have been formed in situ by oxidation or some other chemical or mechanical process. On the other hand, abrasion may take place simply because the counterface is both rough and intrinsically harder than the wearing component [1].

The abrasive wear process is traditionally divided into two groups: two-body and three-body abrasive wear. A distinction is made between two-body abrasion; two-body abrasive wear is one in which wear is caused by hard protuberances on one surface which can only slide over the other and three-body abrasion, in which particles are trapped between two solid surfaces but are free to roll as well as slide. The rate of material removal in three-body abrasion is one order of magnitude lower than that for two-body abrasion, because the loose abrasive particles abrade the solid surfaces between which they are situated only about 10% of the time, while they spend about 90% of time in rolling [4].

Figure 1 above showing Schematics of (a) a rough, hard surface or abrasive particle mounted to the top surface sliding along a softer surface and (b) free rolling or sliding abrasive particles caught between surfaces. At least one of the surfaces is softer than the particles [A.P. Harsha & U.S. Tewari 2002].

Abrasive wear is thus the most important among all the forms of wear because it contributes almost 63% of the total cost of wear [4].

There are a number of factors which influence abrasive wear and hence the manner of material removal. Several different mechanisms have been proposed to describe the manner in which the material is removed. Three commonly identified mechanisms of abrasive wear are:

  1. Plowing
  2. Cutting
  3. Fragmentation

Plowing occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal. The displaced material forms ridges adjacent to grooves, which may be removed by subsequent passage of abrasive particles. Cutting occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves. This mechanism closely resembles conventional machining. Fragmentation on the other hand occurs when material is separated from a surface by a cutting process and the indenting abrasive causes localized fracture of the wear material. These cracks then freely propagate locally around the wear groove, resulting in additional material removal by spalling [8].

2.8.2.2 ADHESIVE WEAR

Adhesive wear is produced by the formation and subsequent shearing of welded junctions between two sliding surfaces. For adhesive wear to occur it is necessary for the surfaces to be in intimate contact with each other. Surfaces which are held apart by lubricating films, oxide films etc. reduce the tendency for adhesion to occur [5].

Adhesive wear also is based on the notion that touching asperities adhere together and that plastic shearing of the junctions so formed ‘plucks' off the tip of the softer asperities leaving them adhering to the harder surface. Subsequently these can become detached giving rise to wear particles or fragments [1].

2.8.2.2.1 EFFECTS OF ADHESION BETWEEN WEARING SURFACES

Strong adhesion between two surfaces generates a large component of frictional force and transfer films. These high friction coefficients could lead to seizure and scuffing. For sliding motions between surfaces, the combined action of adhesion could result into severe plastic deformation of the asperities.

'The formation of transfer films is a characteristic feature of adhesive wear; it distinguishes adhesive wear from most other wear mechanisms [5].

Transfer particles being harder than the substrate material due to severe work hardening are capable of producing grooves in the surface. 'The formation of grooves on the worn surface is frequently observed when adhesive wear occurs and these grooves often occur on the sliding member [5].

2.8.2.3 FRETTING WEAR

Fretting wear is a wear phenomena occurring between two surfaces having oscillatory relative motion of small amplitude over a period of time which will remove material from one or both surfaces in contact [3]. It occurs typically in bearings, although most bearings have their surfaces hardened to resist the problem. Another problem occurs when cracks in either surface are created, known as fretting fatigue. It is the more serious of the two phenomena because it can lead to catastrophic failure of the bearing. An associated problem occurs when the small particles removed by wear are oxidized in air. The oxides are usually harder than the underlying metal, so wear accelerates as the harder particles abrade the metal surfaces further. Fretting corrosion acts in the same way, especially when water is present. Unprotected bearings on large structures like bridges can suffer serious degradation in behaviour, especially when salt is used during winter to deice the highways carried by the bridges.

2.8.2.3 EROSIVE WEAR

Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object. [5]. The impacting particles gradually remove material from the surface through repeated deformations and cutting actions [6]. It is a widely encountered mechanism in industry. A common example is the erosive wear associated with the movement of slurries through piping and pumping equipment.

The rate of erosive wear is dependent upon a number of factors. The material characteristics of the particles, such as their shape, hardness, impact velocity and impingement angle are primary factors along with the properties of the surface being eroded. The impingement angle is one of the most important factors and is widely recognized in literature. For ductile materials the maximum wear rate is found when the impingement angle is approximately 30o, whilst for non ductile materials the maximum wear rate occurs when the impingement angle is normal to the surface .

2.8.3 WEAR RATE

The wear rate, w of a rolling or sliding contact is conventionally defined as the volume lost from the wearing surface per unit sliding length distance; its dimensions are thus those of [length]2. For a particular dry and unlubricated sliding situation, the wear rate depends on normal load, the relative sliding speed, the initial temperature, and the thermal, mechanical, and chemical properties of the material in contact. There are many physical mechanisms that can contribute to wear and certainly no simple and universal model is applicable to all situations. If the interface is contaminated by solid third bodies (for example, by entrained dirt or even just the retained debris from previous wear event) the situation can be much more complex [1].

2.9 ARCHARD'S EQUATION

Sliding wear is commonly treated in terms of the material loss as a function of the hardness, sliding distance, and normal load. The coefficient of friction between the substrate and sliding component is a factor that can be greatly change wear rate. The Archard's equation calculate the wear behaviour by assuming asperity removal, fragments form and contribute to the mass loss based on the assumption that hardness and yield strength of material are proportional. The resulting wear equation is given below.

After running-in, wear rate will be in a steady state and it can be calculated using Archard's equation which is a simple model used to describe sliding wear and is based on the theory of asperity contact [7]. The relation is the most common wear model, based on the principle that the rate of wear for a given material is proportional to the contact pressure and the slid distance, and inversely proportional to the hardness:

Q = KWLH

Where:

Q is the total volume of wear debris produced

W is the total normal load

H is the hardness

K is a dimensionless constant

L is the sliding distance

Instead of relating the wear rate to the volume of wear Q, it can be related to the mass loss m according Q = mρ or the thickness of the layer removed by wear according to:

d = QA = KPLH (2.3)

In which A is the area subjected to wear and P = W/A where A is the contact area. With a constant sliding velocity, the length of sliding L can be replaced by the product of the sliding velocity V and the time. L = Vt so that:

2.10 WEAR OF POLYMERS

Despite the fundamental similarities of wear to both non-metallic and metallic materials, there exist significant differences in the wear mechanisms involved and the level of friction or wear which occurs. ‘These differences can be exploited to produce valuable materials which can change commonly accepted expectations of tribological performance [34]. Thus a careful study of the tribology of polymers is a pre-requisite to their successful adaptation to wear resistant materials. These studies answer questions such as: In what applications should they be used? Can they substitute existing materials which is metals?

These have led to cases where polymers are being used to substitute materials such as porcelain or metals in prosthetics.

Table 2.6 -Triblogical characteristics of typical polymers

POLYMERS

TRIBOLOGICAL CHARACTERISTICS

Polytetrafluoroethylene (PTFE)

Low friction but high wear rate; usually blended with other polymers or reinforced as a composite material. High operating temperature limit

Nylons

Moderate coefficient of friction and low wear rate. Medium performance bearing material. Wear accelerated by water. Relatively low temperature limit.

Polyacetals

Performance similar to nylon. Durable in rolling contacts

Polyetheretherketone (PEEK)

High operating temperature limit. Resistant to most chemical reagents. Suitable for high contact stress. High coefficient of friction in pure form.

Ultra high molecular weight polyethylene (UHMWPE)

Very high wear resistance even when water is present. Moderate coefficient of friction. Good abrasive wear resistance. Relatively low temperature limit.

Polyurethanes

Good resistance to abrasive wear and to wear under rolling conditions. Relatively high coefficient of friction in sliding.

Polyimides

High performance polymers, suitable for high contact stresses and high operating temperatures.

Epoxies and phenolics

Used as binders in composite materials

Polymers were never intended or considered to be utilized as wear resistant materials and are usually unsuitable for this purpose [34], however most research is directed to a few of these polymers that have valuable tribological properties. Thus most polymers used in engineering are blends of different polymers that try to take advantage of individual characteristics of these polymers to make ‘composites' that are suitable for engineering applications. The use of composites has thus extended the range of materials and increased it tribology even further.

2.11 FRACTURE OF POLYMERS

The fracture strengths of polymeric materials are low relative to those of metals and ceramics. As a general rule, the mode of fracture in thermosetting polymers (heavily cross-linked networks) is brittle. In simple term, during the fracture process, cracks form at regions where there is a localised stress concentration (that is, scratched, notches, and sharp flaws) [35]. As with metals, the stress is amplified at the tips of those cracks leading to crack propagation and fracture. Covalent bonds in the network of cross-linked structure are severed during fracture.

For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing ductile-to-brittle transition. Factors that favour brittle fracture are a reduction in temperature, an increase in strain rate, the presence of sharp notch, increased specimen thickness, and any modification of the polymer structure that raises the glass transition temperature (Tg). Glassy thermoplastics are brittle below their glass transition temperatures . However, as the temperature is raised, they become ductile in the vicinity of their and experience plastic yielding prior to fracture.

2.11.1 FATIGUE

Polymers may experience fatigue failure under conditions of cyclic loading. As with metals, fatigue occurs at stress levels that are low relative to the yield strength. Fatigue behaviour of polymers is much more sensitive to loading frequency than for metals [35]. Cycling polymers at high frequencies and/ or relatively large stresses can cause localised heating; consequently, failure may be due to softening of the material rather than as a result typical fatigue processes.

CHAPTER THREE

3.0 METHODOLOGY

3.1 OVERVIEW

The technical approach of this project involves the following: test specimen/substrate preparation; test apparatus preparation; set of conditions required to run the test; data acquisition; measurement and calculation of wear; micrograph studies, analysis of wear and micrograph results.

The information got from journals for electrostatic coating spaying and other texts referring to up-to-date techniques were used for deep understanding and interpretation. With this information, a cognitive idea of the aim of the project and possible future vision/developments for improved technology were gained. The project planning schedule was developed to break down the activities and tasks required to be carried out to achieve the main goal within the specified time. These allow for the monitoring and reviewing the progress by the project supervisor.

3.1 TECHNICAL APPROACH

There was a need to design a sample holder that would accommodate six sample substrates during the electrostatic spray coating without a hole being drilled on them as it used to be. The designed holder consists of steel plate and steel bar with dimensions 480mm x 100mm x 60mm for this experiment. This sample holder was made from a 1mm steel metal sheet and 3mm and a 3-D model of it was drawn as shown in figure 7 below.

3.2 SPRAY POWDER

The powder coating material used to perform this experiment was Epoxy Resin (interpon 100). It was chosen due to its excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and a very good electrical insulating property. The Epoxy Resin powder properties can be summarized below.

Chemical type

Epoxy (Interpon 100)

Particle size

Suitable for electrostatic spray

Specific gravity

1.2-1.7g/cm3 depending on colour

Storage

Dry cool conditions (below)25oC

Shelf life

12 months

Stoving Schedule

20 minutes at 160oC - 10minutes at 180oC - 5 minutes at 200oC (objective temperature)

3.3 TEST SPECIMEN PREPARATION

The materials needed for the preparation of the test specimen were:

  • Steel Substrates
  • Transparent HZG02R Epoxy powder (Interpon 100) manufactured by AKZONOBEL RESICOAT
  • Nordson sure coat electrostatic gun
  • Oven (Heraeus thermo oven made in Germany with Bestell-Nr 51020458 as part no).

3.4 SUBSTRATE PREPARATION

The steel plate of dimension 90mm×50mm×1mm was prepared using guillotine and as shown in Figure 8. Appropriate surface preparation of the substrate was done. The surface of the substrate was cleaned and degrease thoroughly with a soft paper soaked in acetone 99% HPLC grade and then flush out with the use of pressurize air before coating using electrostatic spray. This was done so as to remove dust, oil and grease from the surface of the substrate as well as conditioning the surface to make it suitable for optimal quality coating.

3.5 ELECTROSTATIC SPRAY (COATING)

The electrostatic spray deposition system (Nordson Sure Coat) used in this work consists of a delivery system, the electrostatic power source, feeder hopper, feeding house, a high voltage unit, and spraying gun. The sample was grounded before coating so as to have the powder attracted to the sample since it is electrostatically charged and powered by a voltage of 80kv.

The powder (epoxy) was applied onto substrate by spraying an electrically charged cloud of fine powders onto the grounded sample. The system which uses compressed air for transport, allows the powder to move from the feed hopper to the tip of a spray-gun. Here, the powder passes along an electrode designed to impart an electrostatic charge to the powder material which is quickly charged. Due to electric repulsion between charged powder particles, a cloud of fine powder leaves the spray-gun which is directed towards the workpiece or steel substrate using the electrostatic field and aerodynamic push of the outgoing air flow. The spraying distance to the grounded substrate was 30cm away from the tip of the gun. So, within the gun, the powder is electro-statically charged and directed toward a grounded metal substrate being coated. The figure below shows a simple schematic of the electrostatic-spray powder-coating process.

3.6 CURING

The curing process was done immediately after the coating; the coated substrates were carefully removed from the sample holder and were taken into the oven for cure (Heraeus thermo oven made in Germany with Bestell-Nr 51020458). The ovens have a temperatures range up to 300°C for curing proce


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