What are the important electrical properties of Plastics?
Commercial plastics are generally very good electrical insulators and offer freedom of design in electrical products. Electrical properties may also be changed by environmental conditions, such as moisture and / or temperature. A basic concept to remember is that electrons must be exchanged between molecules for electric current to flow through a material. Plastic molecules hold on to their electrons and do not permit the electrons to flow easily; thus, plastics are insulators.
Plastics containing oxygen and nitrogen molecules are “polar” which means that they will tend to act like magnets and align themselves in the presence of a voltage or field, the same as a needle in a compass trying to point north. Plastics not containing oxygen and nitrogen molecules such as polyethylene, polypropylene and polystyrene are nonpolar.
Volume resistivity is defined as the ratio between the voltage (DC – direct current), which is like the voltage supplied by a battery, and that portion of current which flows through a specific volume of a sample. Units are generally ohm/cm³. DC electrodes are put on opposite sides of a 1cm cube of a plastic material. When a voltage is applied, some current will flow in time as the molecules align themselves. Ohm’s Law tells us that the voltage (V) divided by the current in amps (I) is equal to the resistance or V/I = R.
The surface resistivity is the ratio between the direct current and current along the surface per unit width. Units are generally ohms. Again referring to Ohm’s Law, the surface resistivity is a measure of how much the surface of the material resists the flow of current.
The dielectric constant is the ratio of the capacitance (AC voltage) of electrodes with the insulating material between them to the capacitance of the same electrodes with a vacuum or dry air in between. The dielectric constant is a measure of how good a material works to separate the plates of a capacitor. Remember that the molecules are like little magnets and are trying to realign themselves every time the voltage (current) changes direction. Some materials do it better than others. The dielectric constant for a vacuum has a value of 1; dry air is very nearly 1 and all other materials have dielectric constants that are greater than 1. The dielectric constant for a plastic material can vary with the presence of moisture, temperature and the frequency of the alternating current across the plates.
Dielectric strength is the voltage difference (DC) between two electrodes at which electrical breakdown occurs and is measured in kilovolts/millimeter of thickness. This is an indicator of how effective an insulator the material is. The test is similar to that used for volume resistivity except the voltage is increased until there is an arc across the plates. This means that the voltage was strong enough to break down the material and allow a large current to flow through it. Again, this property can be affected by the presence of moisture and temperature.
The dissipation factor (AC) is the tangent of the loss angle of the insulating material. It can also be described as the ratio of the true in-phase power to the reactive power, measured with voltage and current 90⁰ out of phase. This is an indication of the energy lost within the material trying to realign the molecules every time the current (voltage) changes direction in alternating current. The property varies with moisture, temperature and frequency.
The arc resistance is the elapsed time in which the surface of the material will resist the formation of a continuous conductive path when subjected to a high voltage (DC), low- current arc under controlled conditions.
How do chemicals and additives modify the properties of Plastics?
Significant advances have been made in the modification of the properties of plastics materials through the incorporation of chemicals and additives to improve performance in different applications. The possibilities are limitless. The following outlines some of the things that are being done;-
Plasticisers usually high boiling point, low volatility esters are incorporated with polymers by heating and mixing to increase flexibility by inserting themselves between the polymer chains and thus reducing the attractions between chains. They are essential in cellulose plastics and include triphenyl phosphate, a common ingredient to reduce flammability. The esters are usually diethyl and dimethoxyl glycol phthalates but in PVC, which accounts for the largest proportion of plasticiser used, phthalate esters of much higher molecular weight are employed because the plasticiser proportion is higher and low volatility is imperative. It is also important that flexibility be maintained at low temperatures. The compound di-2-ethylhexyl phthalate (DOP) is used, and for maximum low temperature flexibility the adipate as well as the azelate are employed. These three plasticisers have the advantage of being food safe in vinyl wrapping film, whereas of the phosphates, only octyl diphenyl phosphate may be used.
Another class of plasticiser, either a complete polyester made from a glycol or an epoxy compound, usually of soya oil, is known as a permanent plasticiser because of its non-volatility; the epoxy types are used with other plasticisers in vinyl compounds and also function as stabilisers.
Stabilisers and Antioxidants Polymers are subject, in varying degrees, to degradation by heat, light and oxidation and can be protected by substances that only arrest but do not entirely prevent these processes. PVC in particular requires stabilisation since the manipulation temperatures are high enough to initiate decomposition, but polyolefins and polystyrene are also improved by suitable additives. In PVC the main problem is the emission of hydrochloric acid; this can be reduces by adding lead salts or soaps such as lead stearate. Though these are inexpensive, they cause opacity and are toxic; they are widely used for insulation compounds. Where a degree of transparency is required with freedom from stain, barium- cadmium salts, with or without zinc, are employed. With silicate and asbestos fillers, epoxy resins and phosphites are used, but for rigid transparent PVC processed at high temperature, a different class of stabiliser, that of alkyl compounds of tin, is used.
After 1950, a series of dibutyltin derivatives, such as dilaurate and maleate, were recognized as excellent though expensive stabilisers fir high clarity PVC, a property becoming more important with the increasing use of blown PVC bottles. These stabilisers are now replaced by dioctyltin derivatives containing maleate groups or mercapto- acetates, which are less toxic and cheaper. For plastisols, barium, cadmium, and zinc compounds, together with organic derivatives are preferred. Non toxic stabilisers also include calcium zinc compounds with phosphates and epoxies. Formulations are widely varied according to processing conditions, resin characteristics, and end uses.
Antioxidants fall into five classes: phenol derivatives, amines, esters, organic phosphites, and miscellaneous – the choice depending on the polymer to be protected and the end use. LDPE is an important example of a polymer that must be protected by antioxidants. It is characterised by side chains, and each junction is vulnerable to oxidation, which is dealt with by phenols alone or in conjunction with sulphur containing esters or phosphites. Oxidation caused by light calls for ultraviolet absorbers, the most satisfactory of which is carbon black, the antioxidants in this case being phenols and naphthols containing sulphur.
Because of its structure, polypropylene is extremely susceptible to oxidation, a reaction that becomes important because the substance is handled at high temperatures. Antioxidants similar to those used for LDPE are effective, especially phenols and sulphur compounds, such as distearyl thiodipropionate, used together.
Polystyrene is relatively stable to oxidation but sensitive to light degradation. High impact polystyrene, however, because of its rubber content, requires the use of phenols. Most other polymers are less subject to oxidative degradation, but phosphite esters are often used, especially with polyurethanes, which are likely to discolour.
As already indicated, ultraviolet light combines with oxidation to promote degradation, and combinations of absorbers and antioxidants are common, the ultraviolet absorber being increasingly important as plastics are used in outdoor applications. The two classes of absorbers are screening agents and excited-state quenchers. Excited-state quenchers stabilise the polymer by removing energy from the molecule that has already absorbed ultraviolet light. Screening agents are the most widely used, absorbing most of the incident ultraviolet light and converting it to heat. The most effective of these are the benzophenones, especially their ethers and benzotriazoles. Ultraviolet absorbers also work by converting shortwaves into harmless longer light waves through fluorescence. Protection by fluorescent dyes is particularly applicable to films. Of the excited-state quenchers, nickel compounds are the most effective, especially organo-nickel complexes.
Organic peroxides and hydroperoxides are derivatives of hydrogen peroxide and split readily into free radicals (group of atoms that act as a unit in a chemical reaction) required in addition polymerisation, over a temperature range determined by decomposition. Room temperature splitting can be brought about by activators, such as cobaltous ion or tertiary amine.
Flame Retardants Where nonflammability is especially important, all polymers except rigid PVC and fluorocarbons require flame retardant additives, even though some, such as nylons or polycarbonates, are self extinguishing. The most important flame retardants are bromine, chlorine, antimony, and phosphorus compounds. Antimony compounds can be used only with opaque materials, however. Additives vary with the polymer; various phosphates are used with plasticised PVC but with polyurethanes the phosphates must be brominated, and the same applies to polystyrene foam, where highly brominated phosphates or hydrocarbons are incorporated in the beads as supplied for moulding. An alternative method is to build bromine containing units into the polymer chains, as can be done in the case of polyesters, polyurethane foam and epoxy resins.
Fillers & reinforcements These fall into five main categories: mineral and synthetic inorganic powders, carbon blacks, cellulosics, metal powders, and microspheres. Reinforcements include cotton and asbestos flocks; glass fibres, chopped or in the form of rovings, mats or monofilaments; carbon fibres; and mineral whiskers. A great variety of fillers especially suitable for different polymers and end uses is available- e.g. wood flour, first used for phenolics, which is cheap and reduces mould shrinkage; asbestos flock (tiny fibres of asbestos), which greatly increases impact strength; various fillers that improve polyurethanes; and aluminium, magnesium, and titanium oxides, which give added stiffness to composites. Cotton flock is essential in urea- formaldehyde resin mouldings, and many synthetic fibres are used with different polymers; the great bulk of reinforcement, however, is glass fibre of less than 25 microns in diameter and available as glass cloth; the cloth can be chopped for incorporation in mouldings or used as mats or as a filament for a thin surface mat or for a winding. Carbon fibres made by heating acrylonitrile fibres have enormous tensile strength, and plastics bonded with then can replace metals. Sapphire fibres and single crystal whiskers also provide powerful reinforcement.
Colourants are inorganic or mineral pigments, organic pigments (either toners, insoluble organic salts or metal complexes of dyes), or are lakes, dyes deposited on alumina. Uniform dispersion and standardization of colour are usually accomplished by predispersion in a resin at high temperatures or, especially in the case of epoxy resins, polyesters, and urethane systems by dispersion in the plasticiser, monomer or solvent. Dyes must be used for transparent plastics, either dissolved in solvent or in such fine dispersions, as in the case of basic dyes, that no particles are visible.
Lubricants Plastic materials such as nylon are available in self lubricating grades. These grades are made by the incorporation of materials which are themselves well known for their lubricating properties such as graphite, molybdenum disulphide and in some instances, fluorocarbon polymers.
Foaming Almost all plastics materials can be made in cellular or foam form. Products such as polyurethane, polystyrene and urea formaldehyde in low density foam form have been used in large quantities for many years. The foam forms of plastics are produced when gas, introduced or generated within the plastics, expands and produces a cellular structure. If a mixture of gas and plastics is injection moulded, the gas will expand as the hot plastic enters the mould and a moulding with a dense, cellular core and an integral skin will be produced. The resultant mouldings, which are termed structural foams, offer improved strength weight ratios and rigidity and allow greater density freedom for large structural parts. Finished products can produce the texture and feel of wood and are often seen in furniture applications.
What are the important thermal properties of Plastics?
COEFFICIENT OF EXPANSION
With a change in temperature, plastic materials tend to change size considerably more than other materials, such as steel, ceramics and aluminium. A designer must consider these differences and even the shipping environment which may expose the part to a much greater temperature variation than the part will ever see in use. The measure of how much a part changes size as temperature varies is called the “thermal coefficient of expansion”. The units are usually given in m/m/⁰C.
DEFLECTION TEMPERATURE UNDER LOAD
In addition to changing size, the strength and modulus of elasticity of plastic materials tend to decrease as the ambient temperature increases. The standard test for determining the deflection temperature under load (DTUL) at 0.45MPa and 1.80MPa provides information on the ability of a material to carry a load at higher temperatures. The temperature of the loaded beam is raised until a certain amount of deflection is observed. The temperature when that deflection is reached is called the DTUL.
Many plastics are good thermal insulators; that is, heat does not travel through them easily. The conductivity of plastics is 300 to 2,500 times less than most metals. This is why we can pick up a hot pan by its plastic handle and shows why it takes a long time for a casting or other moulded part to cool down in the middle. Internal stress can be set up in a material because of the differences in the cooling rates between the outside of a part and the core.
What are the important mechanical and physical properties of Plastics?
Load per unit area is called stress. If the loads can be predicted and the part shape is known, then the designer can estimate the worst load per unit of cross sectional area within the part. If the force is in newtons and area is in square metres, then the units for stress are newtons per square metre and 1N/m² equals 1 Pascal (Pa).
The measurement of the stiffness of the material is called the “modulus” or “modulus of elasticity”. If the maximum amount of bending that can be allowed and the shape of the part are known, then the designer can often predict how stiff a material must be. The higher the modulus number the stiffer the material; conversely, the lower the number, the more flexible the material. The modulus also changes as the temperature changes. Modulus numbers are given in MPa.
The measurement of how much the part bends or changes size under load compared to the original dimension or shape is called “strain”. Strain applies to small changes in sizes.
STRAIN = (Final length – Original length) = Change in length or Deformation
Original length Original length
Stress, Strain and modulus are related to each other by the following equation. The modulus or stiffness of a material can be determined when the material is loaded in different ways, such as tension, compression, shear, flexural (bending) or torsion (twisting). They will be called tensile modulus, also known as plain modulus, flexural modulus or torsional modulus.
MODULUS = STRESS or in other words MODULUS = Load ÷ Change in shape when loaded (STIFFNESS)
Choose the type of modulus in the property sheet that most nearly duplicates what the customer expects the major load to be. If the load is unknown, use the lowest moduli value of the two. These numbers can be used for short term loading if the load is to be applied for only a few days at the most. The stress/strain equation is the equation used by designers to predict how a part will distort or change size and shape when loaded. Predicting the stress and strain within an actual part can become very complex. Fortunately, the material suppliers use tests that are easy to understand.
The yield point is that point when a material subjected to a load, tensile or compression gives and will no longer return to its original length or shape when the load is removed. It is a very important concept because a part is usually useless after the material has reached this point. Some materials break before reaching a yield point (e.g. some glass filled nylons or die cast aluminium).
This is the maximum strength of a material without breaking when the load is trying to pull it apart. This is the most common figure given by suppliers to report tensile properties in their advertising. Plastics may demonstrate tensile strengths from 4.8MPa to 345MPa.
Elongation is always associated with tensile strength because it is the increase in the original length at fracture and expressed as a percentage.
Compressive strength is the maximum strength of a material without breaking when the material is loaded. This term becomes less meaningful with some of the softer materials such as P.T.F.E. does not fracture. Consequently, the compressive strength continues to increase as the sample is deforming more and more. A meaningful compressive strength would be the maximum force required to deform a material prior to reaching yield point.
Shear strength is the strength of a material when the material is loaded with the surfaces being pulled in opposite directions. Some examples of items that experience shear loading are the nail holding a picture on the wall, the cleats of an athlete’s shoe and tyre tread as a car accelerates or brakes.
Flexural strength is the strength of a material when a beam of the material is subjected to bending. The material in the top of the beam is in compression (squeezed together), while the bottom of the beam is in tension (stretched). Somewhere in between there is a place with no stress and this is called the neutral plane. Skis, fishing poles, pole vault poles and diving boards are examples of parts needing high flexural strength.
Torsional strength is the strength of a material when a shape is subject to a twisting load. Examples of parts that require a high torsional strength are screws and the drive shaft on a motor vehicle.
Sometimes a designer will need a value for Poisson’s ratio. This ratio occurs in some of the more complex stress / strain equations. It is simply a way of saying how much the material necks down or gets thinner in the middle when it is stretched.
When large weights are hung on bars of different materials, all will experience some initial and immediate deformation or stretching when the load is first applied. As long as the yield point has not been exceeded, a metal sample which acts like a spring will not stretch any more regardless of how long the weight is left on. When the weight is removed, the metal bar will return to its original shape. The length of a plastic bar will continue to slowly increase as long as the load is applied. This “creep” increases as the load and/ or temperature are increased. Some thermoplastics like nylons will creep more when they have been softened because of the presence of moisture.
Plastics, as well as other materials subjected to cyclic loading, will fail at stress levels well below their tensile or compressive strength. The combination of tension and compression is the most severe condition. This information will be presented in S-N curves or tables which stand for Stress – Number of cycles. A part will survive more cycles if the stress is reduced. The stress can be reduced by reducing the deflection and / or decreasing the thickness of the part. Some examples of cyclic loading are a motor valve spring or a washing machine agitator.
Impact strength is the ability to withstand a suddenly applied load. Toughness is usually used to describe the material’s ability to withstand an impact or sudden deformation without breaking. No single test has yet been devised that can predict the impact behavior of a plastic material under the variety of conditions to which a part may be subjected. Many materials display reduced impact strength as the temperature is lowered. Thermosets and reinforced thermoplastics may change less with changes in temperature. Some of the impact tests commonly used in supplier literature are as follows: – Izod Test (a swinging pendulum is suddenly impacted on the material; Tensile impact Test; Gardner Impact Test, Brittleness Temperature Test.
Some plastic materials have exceptional impact performance and very good load carrying capacity. However, the performance of a material can be greatly reduced by having sharp corners on the part either from the design or from machining operations. The Izod impact strength of a tough material like polycarbonate is reduced from 20 to 2 as the radius (R) of the notch is reduced from 0.5mm R to 0.05mm R. The sharp corners not only reduce the impact resistance of a part, but also allow for a stress concentration to occur and encourage the premature failure of a load carrying part. Minimising sharp corners may make the machining operation but it may be critical to the part’s success. Edges of sheet (such as acrylic and polycarbonate) being used in impact applications like glazing must be finished to be free of sharp notches.
Melt index determines the internal flow rate of a plastic through a die at a given temperature and load. In more common terms this test will tell you how fast the plastic will flow when heated and affects how the material will process, fill a mould or flow through a die. Done according to ISO 1133, a sample of plastic is charged into a heated cylinder within the melt index apparatus. A weighted piston is used to push the plastic through the cylinder and through a die at the end. The temperature at which a sample is tested is predetermined by the standard for each given polymer.. After the cylinder is charged and the weighted piston is in place, the piston is blocked and the plastic is heated for 6 – 8 minutes. At that point the block is removed, and the weighted piston forces the molten plastic through the die. A sample is collected for a specific time interval and the extrudate is massed. The melt flow is determined by the following equation;
Grams of extrudate x 10 = melt flow ( g/10 min)
Time in minutes
Melt flow is affected by the molecular structure of a polymer. The more complex the molecule, the melt flow will tend to be lower. If a plastic has polymer molecules of approximately the same length, the melt flow will tend to be higher. Melt index is also affected by fillers and reinforcements in a plastic. Crystallinity will affect the melt flow of a material. The range of melt flows can be from 0.5 g /10 min to as much as 25 g /10 min.
For solids and liquids, specific gravity is the ratio of the density of a material to the density of water at 4⁰C which is taken as 1.00 (since 1cm³ of water weighs 1 g & 1 m³ weighs 1000kg or 1 tonne). If the size of the part is known, specific gravity can be used to determine the weight of that part in a variety of materials.
Water absorption is the degree of penetration of water into the inner structure of another material. A common measure for the degree of absorption is the percentage swell, which measures the change in the surface area of a material. Plastics such as polyethylene have extremely low water absorption, whereas nylon has a relatively high rate of water absorption.
GLASS TRANSITION TEMPERATURE
Glass transition temperature is the temperature above which an amorphous polymer is soft and rubbery. Below this temperature, an amorphous polymer is hard, brittle and glassy.
What are the considerations in the design of a Plastic part?
A designer or engineer will often use design equations that work with metals while a plastic part is being designed. Metals behave like a spring, that is, the force generated by the spring is proportional to its length. A plot of the force as a function of length is a straight line and is called “linear” behaviour. However, plastics do not behave like a spring – that is they are “non-linear”. Temperature changes the behaviour even more so that any designer or engineer needs to be very careful to consult the performance charts for a plastic material to obtain the correct input.
How much load or force will the part be required to carry?
How will the part be loaded?
What are the direction and size of the forces in the part?
Will the part have to withstand impact?
Will the part see cyclic loading?
What temperatures will the part be operating in and for how long?
Will the part be exposed to chemicals or moisture?
Will the material be used in an electrical design?
Will the part be used as a bearing or need to resist wear?
Does the part have to retain its dimensional shape?
Will the material have to stretch and bend a lot?
Will the part have to meet any regulatory requirements?
Does the material or film have to prevent certain gases or liquids from passing through?
Will the part be exposed to radiation?
Does the material need to have a special colour and / or appearance?
Does the part have any optical requirements?
Will the part be used outdoors?
Can any volatiles be given off by the material?
What is the difference between a thermoset and a thermoplastic?
THERMOSETS or THERMPLASTICS
The terms thermoset and thermoplastic have been traditionally used to describe the different types of plastic materials.
A thermoset allows only one chance to liquefy and shape it. With plastic materials in this group, shortly after the viscous melt stage is reached a further chemical reaction occurs which results in adjoining molecules linking together to form a three dimensional network. This process is known as cross-linking. The molecules linked in this manner will no longer flow again under the influence of heat and pressure, even though they are heated to the point where the material chars. These materials can be cured or polymerised using heat and pressure, or as with epoxies, a chemical reaction started by a chemical inhibitor. These types of plastics are generally rigid and are usually brittle and are usually mixed with reinforcing fillers to render them less brittle. Common members of the group are phenol formaldehyde (phenolics); urea formaldehyde (ureas); melamine formaldehyde (Melamines); polyester resins (polyesters); epoxy resins (epoxies) and polyimides.
A thermoplastic can be melted and shaped several times. When plastics in this group are heated to the viscous melt stage, no cross-linking takes place between adjacent molecules, thus these materials are capable of being softened by heating many times. Thermoplastics range from soft, flexible materials to those which are hard and rigid, depending on the type of polymer and possibly the degree of modification it has received. The thermoplastic materials are either crystalline or amorphous but advances in chemistry have been made to the point where the distinction between them is less clear.
Again the advances in chemistry make it possible for a chemist to construct a material to be either thermoset or thermoplastic. The main difference between the two classes of materials is whether the polymer chains remain linear and separate after moulding or whether they undergo a chemical change and form a network by cross-linking. Due to recent advances in polymer chemistry, the exceptions to the rule are continually growing. These materials are actually cross-linked thermoplastics with the cross-linking occurring either during the processing or during the annealing cycle. The linear materials are thermoplastics and are chemically unchanged during moulding (except for possible degradation) and can be reshaped again and again. Cross-linking can be initiated by heat, chemical agents, irradiation or a combination of these factors. Theoretically, any linear plastic can be made into a cross-linked plastic with some modification to the molecule so that cross-links form in orderly positions to maximize properties. The formulation of a material, cross-linked or linear, will determine the processes that can be used to successfully shape the material. Generally, cross-linked materials (thermosets) demonstrate better properties, such as improved resistance to heat, less creep and better chemical resistance than their linear counterpart; however, they will require a more complex process to produce a part, rod, sheet or tube.
What is the difference between crystalline and amorphous Plastics?
CRYSTALLINE & AMORPHOUS MATERIALS: Polymers are often described as being either “crystalline” or “amorphous” when it is actually more accurate to describe plastics by their “degree of crystallinity”. Polymers cannot be 100% crystalline, otherwise they would not be able to melt due to the highly organizes structure. Therefore, most polymers are considered semi-crystalline materials with a maximum of 80% crystallinity.
AMORPHOUS MATERIALS have no patterned order between the molecules and can be likened to a bowl of wet spaghetti. Amorphous materials include atactic polymers since the molecular structure does not generally result in crystallization. Examples of these types of plastics are polystyrene, PVC and atactic polypropylene. The presence of polar groups, such as a carbonyl group CO in vinyl type polymers, also restricts crystallization. Polyvinyl acetate, all polyacrylates and polymethylacrylates are examples of carbonyl groups being present and the resulting groups being amorphous. Polyacrylonitrile is an exception to this. Even amorphous materials can have a degree of crystallinity with the formation of crystallites throughout their structure. The degree of crystallinity is an inherent characteristic of each polymer but may also be affected or controlled by processes such as polymerisation and moulding.
CRYSTALLINE MATERIALS exhibit areas of highly organized and tightly packed molecules. These areas of crystallinity are called spherulites and can be varied in shape and size with amorphous areas between the crystallites. The length of polymers contributes to their ability to crystallise as the chains pack closely together, as well as overlapping and aligning the atoms of the molecules in a repeating lattice structure. Polymers with a backbone of carbon and oxygen, such as acetals, readily crystallise. Plastic materials, such as nylon and other polyamides, crystallise due to the parallel chains and strong hydrogen bonds of the carbonyl and amine groups. Polyethylene is crystalline because the chains are highly regular and easily aligned. Polytetrafluoroethylene (PTFE) is also highly symmetric with fluorine atoms replacing all the hydrogens along the carbon backbone. It, too, is highly crystalline. Isomer structures also affect the degree of crystallinity. As the atactic stereochemistry resulted in amorphous polymers, those that are isotactic and syndiotactic result in crystalline structures forming as chains align to form crystallites. These stereospecific forms or propylene are those which are preferable for structural applications due to their degree of crystallinity.
The degree of crystallinity affects many polymeric properties. In turn, other characteristics and processes affect the degree of crystallinity. The higher the molecular weight, the lower the degree of crystallinity and the areas of the crystallites are more imperfect. The degree of crystallinity also depends on the time available for crystallization to occur. Processors can use this time to their advantage by quenching or annealing to control the time for crystallization to occur. Highly branched polymers tend to have lower degrees of crystallinity, as is easily seen in the difference between branched low-density polyethylene (LDPE) and the more crystalline high-density polyethylene (HDPE). LDPE is more flexible, less dense and more transparent than HDPE. This is an excellent example that the same polymer can have varied degrees of crystallinity. Stress can also result in crystallinity as polymer chains align orienting the crystallites. Drawing fibres, the direction of extrusion and gate placements will also affect the orientation of polymers and therefore the crystallites of the material. This allows the processor to maximize the effects and benefits of the inherent crystallinity of the polymer being used in the application.
COMMON CHARACTERISTICS OF CRYSTALLINE AND AMORPHOUS PLASTICS
|Higher % Crystalline||Higher % Amorphous|
|Higher heat resistance||Lower heat resistance|
|Sharper melting point||Gradual softening / melting point|
|More opaque||More translucent /transparent|
|Greater shrinkage upon cooling||Lower shrinkage upon cooling|
|Reduced low temperature toughness||Greater low temperature toughness|
|Higher dimensional stability||Lower dimensional stability|
|Lower creep||Higher creep|
What is the chemistry of Plastics?
CHEMISTRY OF PLASTICS Like all chemical compounds, plastics are made up of atoms of various elements linked together to form molecules. Unlike materials such as glass and steel which can, similarly to plastics, be moulded by the application of heat and pressure, the molecules of plastics are relatively long, chain-like structures. The basic chemicals, which provide the building blocks of the various plastics material, are relatively small molecules, and are known as monomers. Under the correct conditions in specially designed plants, these single molecules of the monomers are made to link together to form long chain-like molecules which may or may not have side branches. These materials are known as polymers, whilst the processes whereby they are produced are known as polymerisation. (note – the word “mer” is a Greek word that means “part”; “mono” means “one” and “poly” means “many; hence monomer and polymer). The monomers are held together in a polymer chain by very strong attractive forces between molecules; but much weaker forces hold the polymer chains together.
Plastics encompass a large and varied group of materials consisting of different combinations or formulations of carbon, oxygen, hydrogen, nitrogen and other elements. Most plastics are a solid in a finished form; however, at some stage of their existence, they are made to flow and may be formed into various shapes. The forming is usually done through the application, either single or together, of heat and pressure. There are over 50 different, unique families of plastics in commercial use today and each family may have dozens of variations.
It is becoming more common in modern technology to modify the properties of polymers by linking together, in various proportions, two monomers. This process is known as co-polymerisation.
MONOMERS: Examples of monomers are ethylene, styrene, vinyl chloride and propylene.
HOMOPOLYMERS: Homopolymers are polymers constructed from joining like monomers. Some examples of these are polyethylene, polystyrene, polyvinyl chloride (PVC) and polypropylene (there is also a copolymer version).
COPOLYMERs: Copolymers are polymers constructed from two different monomers. They can be of the alternating type such as ethylene acrylic, and ethylene-ethyl acrylate or of the graft types such as styrene-butadiene, styrene-acrylonitrile and some acetals.
TERPOLYMERS: Terpolymers are polymers constructed from three different materials. An example of a terpolymer is acrylonitrile-butadiene-styrene (ABS).
The two monomers in a copolymer are combined during the chemical reaction of polymerisation. Materials called “alloys” are manufactured by the simple mixing of two or more polymers with a resulting blending of properties which are often better than either individual material. There is no chemical reaction in this process. Some examples of “alloys” are Polyphenylene oxide –high impact styrene, polycarbonate-ABS and ABS-PVC.
MOLECULAR WEIGHT: It is important to know how long the polymer chains are in a material. Changing the length of the chains in a thermoplastic material will change its final properties and how easily it can be shaped. The number of the “repeating units” or molecular group in the homopolymer, the group of molecules in the copolymer and in the terpolymer is called the “degree of polymerisation”. If the repeating unit has a molecular weight of 80 and the chain or polymer has 1,000 repeating units the polymer has an average “molecular weight” of 80,000. The molecular weight is a way of measuring the length of the polymer chains in a given material. The molecular weight of plastics is usually between 10,000 and 1,000,000. It becomes increasingly difficult to form or mould the plastic with the application of heat and pressure as the molecular weight increases. A molecular weight of 200,000 is about the maximum for a polymer to permit reasonable processability. Some higher molecular weight materials, like ultra high molecular weight polyethylene (UHMW-PE), which has a molecular weight from 5,000,000 to 8,000,000, can be cast using processes specially designed to shape it.
Why should I use Plastics?
Why should plastics be considered as either replacement materials or in a new, ground-up design?
Plastics, as a general family of products, share several demonstrable advantages over traditional materials 9metals, wood, glass etc):
Weight Savings: For example, nylon is 1/16th the weight of steel, acrylic is ½ the weight of glass and PVC is 1/16th the weight of copper. The lower weight per cubic metre of plastics can result in power savings (lower energy needs), labour savings (less personnel require to install a product) and other potential benefits.
Corrosion resistance: Most plastics withstand the effects of common chemicals, water and a wide variety of solvents, acids and other corrosive liquids.
Impact Resistance: Plastics can take a punch. Their inherent ability to resist breakage from impact makes them a wise choice in tough applications in both commercial and industrial settings.
Low Friction / Self-Lubricating: There can be considerable cost savings when the need to lubricate is reduced or even eliminated. Plastics can also include additives that improve their inherent good to excellent wear properties. This is especially true in the class of engineering plastics.
Abrasion Resistance: Closely linked to low friction characteristics, products that wear longer reduce maintenance needs, promote longer run times and can result in improved productivity.
Optical Clarity: Other than glass, plastics are the only materials that are transparent. They can combine light transmittance, light weight, strength and impact resistance to achieve truly unique products.
Ease of Machining / Fabrication: Plastics are typically easier to machine than most metals, cause less wear on machine tools, are easier to handle during the machining process, reduce finishing costs, are easily decorated and can exhibit integral colour.
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Environmentally Friendly: Contrary to some misguided opinions, plastics are ‘green”. Plastics require far less energy to produce than metals and glass and many types are recyclable. Their light weight helps fuel efficiency on cars truck and aeroplanes. Plastics are good insulators, saving heating and cooling costs, resulting in more comfortable living spaces. In addition, bio-degradable plastics are becoming more readily available.
What is the history of Plastics?
History of Plastics With the growth of the industry, year by year, to the point that we are now , such that plastics are an essential part of our lives it is startling to realize that the plastics industry first saw the light of day in the year 1862. It was during that year that Alexander Parkes displayed to the public at the Great international Exhibition in London a material called ‘Parkesine”, which was in fact cellulose nitrate. A great future was predicted for this material and the information published at the time read in a similar vein to the data sheets which accompany newly released plastics material today. At more or less the same time, but independently, an American John Wesley Hyatt, experimented with cellulose nitrate as a substitute for ivory in the manufacture of billiard balls and laid the foundations of the technology of celluloid. In spite of its flammability, cellulose nitrate remained a major raw material until the mid 1920’s when, due to the availability of alternative materials, its use declined.
The other person upon whom historians place the honour of founding the industry is Dr Leo Hendrick Baekeland, who took out the first of 119 patents on plastics based on phenol formaldehyde resins in February 1907. These :Bakelite” materials are, even today, important engineering plastics. The development of additional materials continued, and the industry really began to blossom in the late 1930’s. The chemistry for nylons, urethanes and fluorocarbons (Teflon®) plastics were developed; the production of cellulose acetate, melamine and styrene moulding compounds began; equipment to perform the moulding and vacuum forming processes was made commercially available.
Acrylic sheet was widely used in aircraft windows and canopies during World War II as a replacement for glass because of the dangers of glass shattering under attack. A transparent polyester resin (CR-39), vinylidene chloride film (Saran®), polyethylene bottles and cellulose acetate toothpaste tubes were manufactured during this time period.
The postwar era saw the introduction of vinyl resins, the use of vinyl films, the introduction of moulded automotive acrylic taillights and back lit signs, and the development of the first etched circuit boards. The injection moulding process entered production. Due to the newness of the materials, the properties and behaviour of the plastic materials were not completely understood and many products failed, creating a negative impression about plastics in the public’s mind. Occasionally, plastics are still improperly used and draw negative comments. It is frequently forgotten that materials don’t fail, designs do, and the thousands of successful applications that contribute to the quality of our life are seldom noticed and are taken for granted.
Chemists continued the development of materials such as ABS, acetals, polyvinyl fluoride, ionomers and polycarbonate. The injection moulding and casting processes were all improved. This allowed the industry to provide an even greater number of cost effective products suitable for many more demanding engineering applications. The number of variations or formulations possible by combining the many chemical elements is virtually endless. This variety also makes the job of selecting the best material for a given application a challenge. The plastics industry provides a dynamic and exciting opportunity.