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Wear Rate of an Advanced Polymer Coating Experiment

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Published: Wed, 21 Feb 2018

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. P


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