INTRODUCTION
Introduction:
The modern plastic industry mainly deals with moldable materials. The term “plastic” is applied to any organic substance of high molecular weight, which can be formed to desired shape by moulding, casting etc. and can be retained its shape, dimension and quality under ordinary conditions of temperature are technically known as “Polymers.
What is a polymer?
Substance in which molecules form long chains usually several thousands of atoms together starting from a single molecule.
Single molecule is called Monomer. Monomer is the start material
A monomer on polymerization becomes a polymer.
Polymerization: is the process of combining molecules of start material (monomer) in to a long chain under favorable conditions of heat, pressure and catalyst
(A Macro molecule, a polymer).
Thus,
Ethylene becomes Poly ethylene
Propylene becomes Poly propylene
Basically, all polymers are formed by the creation of chemical linkages between monomers. The linkages are formed by either one or a combination of two types of reactions namelyaddition and condensation reactions
The behaviour and structure of the polymers are based both on the chemical nature and the way in which the chains fit together
The term “plastic” is derived from the Greek word ‘Plasticos’ means “to form”. Plastics are very large group of synthetic materials whose structures are based on the chemistry of carbon. An important characteristic of plastic is that they can be readily moulded into finished products by the application of heat and pressure.
Advantages & Disadvantages over Metal
Advantages of plastics over metals
- ∙ Low weight. Plastics are many times lighter than metal. The ratio on weight is 1:7 to 1:8. Articles made out of plastics are easier to handle
- ∙ Easy time saving and labour saving in processing. Most of the Plastics are more easily be reshaped or moulded under heat and pressure.
- ∙ Good corrosion and erosion resistance. Plastics are very resistant to chemicals, moisture and totting.
- ∙ Good electrical properties.
- ∙ Plastics have a high specific resistance, breakdown strength and good dielectric properties.
- ∙ Good insulation properties.
- ∙ Good surface properties. Plastics have a smooth even surface and are there fore easy to clean. When required, they can be provided with different surface structure.
- ∙ Economical for production. Even the manufactures of complicated articles require only a few steps.
- ∙ Plastics are also available in the form of syrup or liquid. Paints are thermosetting plastics.
- ∙ Plastics can be casted to shape cold condition. When epoxy resins are mixed with curing agent, it remains solid.
- ∙ Some of the plastics are flexible.
- ∙ Plastic products are cheaper.
- ∙ Most of the Plastics are reusable.
- ∙ Plastics have good optical properties. Plastics are available in opaque, transparent and translucent shades in relation to its capability towards light transmittance.
- ∙ Plastics can be coloured
Disadvantages of Plastics over Metal:
- ∙ Low strength. Only few materials when reinforced will get good strength.
- ∙ Low heat resistance
- ∙ Plastic articles are not dimensionally stable for a long period.
- ∙ Resistance to aging is very poor.
- ∙ After rework, the original surface finish can not be achieved.
- ∙ Plastic articles are difficult or in some case impossible to repair.
- ∙ Poor mechanical properties.
- ∙ Plastics are expensive
- ∙ Plastics have post shrinkage and environmental dimensional changes. They absorb water and moisture and hence change its weight and dimension.
- ∙Destructions of plastic is very difficult. Generally most of the plastic disposable items are insoluble and infusible and it can lead to environmental contaminations.
CLASSIFICATION OF PLASTICS
Plastics are broadly classified in to three groups:
Natural plastics -Milk of Rubber three, Cellulose (No use as it is)
Semi-Synthetic plastics -processed Milk of rubber tree (rubber sheet)
Synthetic plastics - Lab made products (compared with Rasna-Pinapple essence
Plastics are again divided in to two groups based on Chemical behavior and by the way it reacts with heat
Thermoplastic
Thermoset plastics
Thermoplastic resin consists of long molecules each of which may have side chains or groups that are not attached to other molecules (i.e. They are not cross linked). Thus they can be repeatedly softened and hardened by heating and cooling.
Usually thermoplastic materials are in granular form. Heat softens the material so that it can be formed. Subsequent cooling hardens it to the final desired shape. No chemical change takes place during forming.
In thermosetting resins reactive portions of the molecules form cross links during polymerization. Once polymerized or cured the material cannot be softened by heating. Normally they are available in powder form and are cured in the mould at high pressure and temperature.
As technology progresses the line between thermoplastics and thermoplastics has become less distinct. More and bore processes are evolved which make use of the economic processing of thermoplastics and converting the material to a thermoset.
e.g.: Extruded polyethylene wire coating is cross linked after extrusion chemically or by irradiation. The polyethylene is converted to thermoset material which cannot be melted by subsequent heating. On the other hand materials and machinery have become modified to provide the economics of thermoplastic processing to thermosetting materials.
e.g.: Injection moulding of thermosets.
Some plastic materials are even members of both the, groups
e.g.: Thermoplastic polyesters and thermoset poly esters.
Difference between thermoplastics & thermoset plastics
Thermoplastics
|
Thermoset
|
Can be softened and re-softened and reused
|
Cannot be re-softened and reused
|
No chemical changes take place during heating and melting and pressurizing
|
Chemical changes take place during heating , melting and pressurizing
|
In granular form generally
|
Usually in powder form
|
Needle like structure of molecules
|
Cross linked chain molecules
|
Rigid but effected by further heating
|
Rigid and unaffected by further heating
|
Hard but not brittle
|
Hard and brittle
|
THERMOPLASTIC
Thermoplastic in general use for injection moulding include different materials like
ABS-Arylonitride Butadiene Styrene
PMMA- Polymethylmethacrylate
CA -Cellulose Acetate (Perspex)
POM-Polyoxymethylene (Delrin)
HDPE -High Density Polyethylene
POLYESTER-Polyester
LDPE- Low Density Polyethylene
PP- Polypropylene
IONOMER - Ionomer
PS-Polystyrene
PA -Polyamide (Nylon)
PTFE -Teflon (Polytetrafluroethylene)
PC -Polycarbonate
PVC -Polyvinyl Chloride
PEEK - Polyetheretherketone
PUR - Polyurethane
PET - Polyethylene Teraphthalite
SAN -Styrene Acrylonitrile
The thermoplastic moulding materials are normally produced commercially in granular form. The shape and size of the individual granular differ according to the type of material. The particular form of granulate is dictated primarily by processing requirements, but also during manufacture. For optimum processing the most important requirement is a completely clean free running granulate without any tendency to agglomerate. Size and shape of pellet should be such that optimum melts conditions will be facilitated .A constant bulk density of granulate is important on machines where volume feeding is employed. The nature of the surface of the pellet should be such that absorption of moisture will be maintained at tolerable level even during lengthy periods of storage. A high absorption of water leads to the production of steam in the heating cylinder and hence to faulty product. With many materials, optimum quality of product can only be obtained after intensive drying of the granulate before use. Equipment available for drying, range from drying chambers for bulk quantities to hopper with integral drying equipment.
The nature of the material should be such that it can be plasticized at temperatures within a practical range to a homogeneous melt to permit even filling of mould cavity. The material must be stable under temperature changes and not be subject to chemical decomposition during processing. Such decomposition will show up in a number of ways, particularly after overheating. If the materials inside the heating cylinder are exposed to high temperatures or at normal temperatures remain too long in the cylinder due to stoppage in the cycle, thermoplastic of low thermal will quickly show signs of decomposition. This is known as burning and the degraded material may run through the product as streaks or even discolours it through out. These affects are more apparent in transparent or light coloured plastics. Thermal instability in the material may separate out the chemical components in the plastics, which are often corrosive and may attack the interior of the heating cylinder and the mould. Poisonous gases which affect the operator may also be formed.
Ingredients are often added to the material to alter its colour consistency or other properties. These substances include dyes, fillers, plasticizers, dispersers (designed to Increase the adhesion of the pigment dye) and materials to aid free flowing properties. These materials should posses the same thermal stability as the bulk materials and must also be free from constituents which evaporate during processing. For the products to be of uniform size and shape, the materials must exhibit small and even shrinkage on cooling.
Plasticizing the material:
The change in consistency in thermoplastic materials from granulates to a homogeneous plastic state suitable for injection take place in the machine. The materials pass through three different stages. The change from one to the next stage is not sharply defined owing to the differing length of the molecular chains. From the solid state, the materials pass through the thermo elastic state which begins as the softening temperature is reached. It is marked by break down in the intermolecular binding forces. Solidifying decreases due to loosening of the molecular structure. As the thermal effect is increased the intermolecular forces are further reduced until at the melting point (where the material is said to be in thermoplastic state) the molecules are completely mobile within the melt.
Melt flow index (MFI)
MFI indicates the flow capacity of different grades of thermoplastics and is inversely related to the molecular weight. A high MFI grade will flow faster and has a low molecular weight. It is expressed as the amount of material (in grams) extruded in ten minutes through an orifice of standard dimensions under a standard load and at a particular temperature .(MFI of PP is measured at 230 0C under a load of 21.6 N ).
Spiral flow:
It is another way of expressing the flow of a particular grade inside the mould and is determined on a standard mould having a spiral cavity marked in cm. Spiral flow test using different grades of PP.
Grade
|
Thermoset
|
m 1730
|
25.00
|
m 3030
|
32.00
|
m 5630
|
36.00
|
m 0030
|
46.25
|
Flow length to wall thickness ratio (L/T ratio or flow ratio)
The L/T ratio expresses the flow behaviour of the material inside the mould. It is defined as the ratio of the largest flow paths to the thickness of the moulding under optimum moulding conditions. The higher the flow ratio higher is the temperature required to obtain fast mould filling and consequent freedom from stresses). The maximum obtainable flow ratio varies with different grades of plastic. It depends on MFI.
Grade
|
MFI
|
L/T ratio
|
M 1730
|
1.7
|
150
|
M 3030
|
3.0
|
225
|
M 5630
|
5.6
|
275
|
M 0030
|
10
|
350
|
1mportant thermoplastic which are available for injection applications can be broadly divided into two categories - Commodity and engineering plastics
The commodity plastics are used where in no-load or very low load applications, while the engineering plastics can be designed to carry loads for a long period of time
Commodity plastics can be again classified as polyolefin, styrene, vinyl, others.
Polyolefin family contains polyethylene (PE), polypropylene (PP), polybutylene (PB) etc
Styrene family contains polystyrene (PS), styrene-acryonitrile (SAN),
Styrene-butadiene (SB), acryonitrile- butadiene -styrene- (ABS) etc
Vinyl family consists of polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC),
Other commodity thermoplastic contains polymethyl-methacrylate (PMMA), cellulose acetate, cellulose nitrate etc
Engineering plastics contains acetals, fluoro-plastics, polyamides (nylons), polyamide-imides, polyarylates, polycarbonates, polyesters, polyketones, polyphenyleneoxide, polyphenylenesulfide and sulfone etc.
Important thermoplastic which are available for injection applications can be broadly divided again into two categories according to their structure
Crystalline and Amorphous
In crystalline materials, molecules are packed closer together and aligned themselves in some orderly pattern. During processing, they tend to develop higher strength in the direction of the molecules. Since commercially perfect crystalline polymers are not produced, they are identified technically as semi- crystalline thermoplastics. A crystalline material is stiffer and stronger. It is difficult to process crystalline materials due to high melt temperature, melt viscosity, shrinkage
Examples are PP, PE, Nylon
Amorphous materials are having molecules going in all different directions and are normally transparent. They will under go small volumetric changes on processing. They are tougher and more flexible. They will often soften gradually as they are heated but they do not flow easily. Therefore it is easy to process amorphous plastic. A crystalline plastic is heated to a particular temperature and sudden quenching give amorphous structure
Examples are ABS, PMMA, PC, PS, PVC
Properties of Crystalline and Amorphous materials
Crystalline
|
Amorphous
|
Fibers
|
Elastomers
|
Crystalline
|
Amorphous
|
Good mechanical properties
|
Weak mechanical properties
|
Tough
|
Brittle
|
Opaque
|
Transparent
|
Min. shrinkage
|
Max. shrinkage
|
DIFFERENT THERMOPLASTIC MATERIALS COMMONLY USED
ABS - Acrylo Nitrile Butadiene Styrene
Density = 1.05 gm./cc
Shrinkage =0.5 %
A general purpose material
Properties
It is a hard tough material with good resistance to heat and impact. It has low water absorption and good electrical insulation properties. The surface is resistant to scuffing and marking. Pre drying is necessary and is done at 80°c for 4 hours. Its flow properties are similar to PS.
Applications
Safety helmets, automotive instrumental panels and other interior components, pipe fittings, hole security devices and housings for small appliance, heavy duty domestic appliance and fittings, cabinet for radio, TV, typewriter, communication equipment, business machines, automobile grills, wheel covers, mirror housings, refrigeration liners, luggage shells, boat hull, and so on.
Cellulose polymers (CA)
Density = 1.3 gm./cc
Shrinkage =0.3 - 0.8 %
Cellulose is a starch. It is one of the main structural polymers in plant(specially that of wood or cotton). These natural fibers are treated with acids to produce a resin, a process called esterfication. Cellulose acetate (Perspex), Cellulose acetate butyrate (CAB) and Cellulose acetate propionate (CAP) are three common materials
Properties
CA combines toughness, transparency and a natural surface texture. Some cellulose resins are biodegradable allowing their use for envelops with transparent widows that can be disposed of as if they were paper alone.
Applications
Tool handles, panels for illuminated signs, steering wheels, bathroom fittings, decorative trim for cars and consumer durables, drawing stencils, pens, blister packing, laminating with aluminum foil, spectacle frame, goggles, covers for TV screens, cutlery handles
High Density Polyethylene (HDPE)
Density = 0.95 gm./cc
Shrinkage HDPE= 3.0 - 4.0%
One of the groups in Polyethylene, first synthesized in 1933, looks like the simplest of molecules, but the number of ways in which the PE units can be linked in large. It is the first of the polyolefin, the bulk thermoplastic polymers that account for a dominant fraction of all polymer consumptions.
Properties
Higher density, rigidity, tensile strength and hardness. Higher chemical resistance while comparing LDPE. Extremely resistant to fresh and salt water, food, and most water-based solutions. Very cheap and particularly easy to mould and fabricate. Accepts a wide range of colours, can be transparent, translucent or opaque
Applications
The property of high stiffness combined with high impact strength and low weight makes it popular material for moulding articles like milk bottle crates, fish boxes, containers used for refrigeration and transport. Pipes from dia 25mm to 800mm for carrying water and chemicals, packing films, woven sacks, monofilament for fishing nets, rope, cane for furniture, coating on paper etc.
Low Density Polyethylene (LDPE)
Density = 0.92 gm./cc
Shrinkage LDPE= 0.8 - 1.5%
One of the groups in Polyethylene, first synthesized in 1933, looks like the simplest of molecules, but the number of ways in which the PE units can be linked in large. It is the first of the polyolefin, the bulk thermoplastic polymers that account for a dominant fraction of all polymer consumptions.
Properties
Crystalline and not available in transparent. Tough and moderate tensile strength. It can be coloured. Oxidation occurs when overheated in barrel. So necessary to add antioxidants
Applications
Shopping baskets, waste paper baskets, artificial flowers and fruits, hair brushes, and blow moulded components, pipes up to 150mm diameter for transporting water, packaging films, coating paper, textiles etc. Linear LDPE is used for overhead water tanks
Ionomer (I)
Density = gm./cc
Shrinkage =0.2 - 0.8%
Introduced by DuPont in 1964, Ionomers are flexible thermoplastics but they have ionic cross-links, from which they derived their name. Their thermoplastic character allows them to be processed by blow moulding injection moulding rotational moulding and thermoforming. But cooled below 40o C, they acquire the characteristics of thermoset
Properties
High strength, good adhesion and chemical stability. It is highly transparent particularly resistant to
Applications
Food packaging, athletic soles with metal inserts, ski boots, ice skate shells, wrestling mats, thermal pipe insulations, license plate holder, golf ball covers, automotive bumpers, snack food packaging, blister packs, bottles.
Nylon (Polyamide-PA)
Density = 1.1 gm./cc
Shrinkage =0.7-1.5%
Nylon is a general term for polyamides
Properties
Tough and can withstand repeated impacts. High rigidity whose properties can be increased by glass reinforcement. Excellent abrasion resistance and exceptionally low co-efficient of friction. High chemical resistance and high temperature resistance. It can be machined. Partially crystalline, therefore cloudy or white and not transparent
Applications
Light duty gears, bushings, sprockets and bearings, electrical equipment housings, lenses, containers, tanks, tubing, furniture casters, plumbing connections, bicycle wheel covers, ketchup bottles, chairs, toothbrush bristles, handles, food packaging, hot melt adhesives for book binding, ropes, fishing nets, carpeting, cables, protective clothing, electrical insulations
Polycarbonate (PC)
Density = 1.2gm./cc
Shrinkage =0.5-0.7%
PC is one of the engineering thermoplastics, meaning that they have better mechanical properties than the cheaper commodity polymers
Properties
Optical transparency and good toughness and rigidity, even at relatively high temperatures. It is possible to co-polymerize the molecules with other monomers to improve the flame retardency, refractive index and resistance to softening, reinforce the PC with glass fibers to give better mechanical properties at high temperatures. Can be electroplated
Applications
Safety shields and goggles, lenses, glazing panels, business machine housings, instrument casings, lighting fittings, safety helmets, electrical switchgears, laminated sheet foe bullet-proof glazing, twin-walled glazing, kitchenware and tableware, microwave cookware, medical (sterilisable) components.
Polyether ether ketone (PEEK)
Density = 1.26 to 1.32 gm./cc
Shrinkage = 0.5%
A high performance thermoplastic. The cost is 50 times more expensive than PP and 10 to 20 times more than nylon. This limits it use to applications in which technical performance
Properties
Among thermoplastics it has exceptionally high stiffness, strength and resistance to heat
Applications
Electrical connectors, hot water meters, F1 engine components, valve and bearing components, wire and cable coatings, film and filament for specialized applications, pump wear rings, bearings bushings.
Polyethylene Teraphalate (PET)
Density =1.38 gm./cc
Shrinkage = 1.5%
The name polyester derives from a combination of polymerization and esterification. Saturated polyesters are thermoplastics - examples are PET & PBT
Properties
Good mechanical properties to temperature as high as 175o C. Pet is crystal clear, impervious to water and carbon dioxide, but little oxygen get through. It is tough, strong, easy to shape, join and sterilize - allowing reuse. When its first life comes to an end, it can be recycled to give fibers and fleece materials for clothing and carpet
Applications
Electrical fittings and connectors, blow moulded bottles, packaging films, films, photographic & x-ray films, audio/video tapes, industrial strapping, drawing office transparencies, metallized balloons, photography tapes, videotapes, carbonated drink containers, oven-proof cookware, credit cards
Polymethyl methacrylate (PMMA - Acrylic)
Density = 1.18 gm./cc
Shrinkage =0.2-0.8%
when you think of PMMA, think transparency. This is a family of acrylic group
Properties
Best transparency and optical properties. Colourless with 92% light transmition. Exceptional resistant to sunlight and outdoor exposure and ultra violet radiation. Unaffected by human tissues
Applications
Lenses of all types, cockpit canopies and aircraft windows, signs, domestic baths, packaging, containers, electrical components, drafting equipments, tool handles, automotive tail lights, chairs, contact lenses, advertising signs, compact discs.
Polyoxymethylene (POM - Acetal)
Density = 1.42gm./cc
Shrinkage =2.5%
POM was first marketed by DuPont in 1959 as DELDRIN. It is rarely used without modification. Most often filled with glass fiber or blended with PTFE or PU. It is used where requirement for good mouldability, fatigue resistance and stiffness justify its high price relative to PE
Properties
Stiffer and has better fatigue and water resistance. POM/PU blend has good toughness.
Costlier than PE
Applications
Seat-belt components, steering columns, window support brackets, shower heads, faucet cartridge and various fittings, quality toys, garden sprayers, stereo cassette tarts, zippers, telephone components, couplings, pump impellers, conveyor plates, gears, sprockets, springs, cams, bushings, clips, door handles, watch components, mechanical pen and pencil parts, milk pumps, food conveyor, TV tuner arms, automotive under hood components
Polyester
Density = 1.2 gm./cc
Shrinkage =0.8%
It is available in both thermoplastics and thermoset. The thermoplastic polyester have wide popularity and application especially in electronic, audio-video, blown bottles and photographic field. Thermoset and thermoplastic polyester is generally known as unsaturated and saturated polyester respectively.
Properties
Highly crystalline with a melting point near 225oC. Hard and strong and extremely tough. High resistance to abrasion and low coefficient of friction. It can be reinforced with glass fiber. It can be extruded, injection moulded or even blow moulded
Applications
Gears, bearings, housing for pumps and appliance , impellers, pulleys, switch parts, packaging materials, blow moulded bottles, etc
Polypropylene (PP)
Density = 0.91 gm./cc
Shrinkage =1.0 - 3.0%
First produced commercially in 1958. A very similar molecules with similar price, processing methods and application as polyethylene. It is produced in large quantities(more than 10 million tones per year I 2000 and 10% increase in every year.
Properties
Flammable and degradable in sunlight. Fire retardants make it slow to burn and stabilizers give it extreme stability, both to UV radiation and to fresh and slat water and most aqueous solutions.
Applications
Ropes, general polymer engineering, automobile air ducting, parcel shelving and air cleaners, garden furniture, washing machine tanks, wet-cell battery cases, pipes and pipe fittings, bear bottle, crates, cable insulations, kitchen kettles, car bumpers, shatter proof glasses, suitcases, thermal underwear.
Polystyrene (PS)
Density = 1.05 gm./cc
Shrinkage =0.2-0.8%
Properties
It is an optically clear, cheap, easily moulded (formed) polymer. In its simplest form, PS is brittle. It’s mechanical properties are dramatically improved by blending with polybutadiene, but with a loss of optical transparency. High impact PS(10% polybutadiene) is much stronger even at low temperature( as low as -12oC.
Applications
The single largest use of PS is form packaging. Other ruses includes toys, light diffusers, beakers, cutlery, general household appliances, video/audio cassette cases, electronic housings, refrigerator liners.
Polytetrafloroethylene (PTFE)
Density =2.15 gm./cc
Shrinkage =5.0 - 10%
It is called Teflon and a member of the fluroplastic family. As cost is very high, it is used in high-value applications.
Properties
Exceptionally low friction, water repellant and extremely stable. Very much chemical inert and best thermal stability and non-wettability- means nothing sticks to it.
Applications
Wire and cable covers, high quality insulating tapes, corrosion resistant lining for pipes and valves, protective clothing, seas and gaskets, low friction bearings, transparent roofing, non-stick cooking products, water repellent fabrics.
Poly vinyl chloride (PVC)
Density = 1.2gm./cc
Shrinkage Flexi=1.5-3.0%
Rigid=0.2-0.4%
One of the cheapest , most versatile and with polyethylene - the most widely used polymer and epitomizes their multi facetted character
Properties
Rigid and not very tough, very low cost , incorporating plasticizers creates flexible PVC a material with leather-like or rubber-like properties and used a substitute for both. Reinforcement with glass fibers gives a material that is sufficiently stiff, strong and light weight structural panels
Applications
Pipes, fittings, road signs, cosmetic packaging, garden hoses, vinyl flooring, windows, vinyl records, dolls, medical tubes, blood storage bags, artificial leather, wire insulation, film, sheet, fabrics, car upholstery.
Polyurethane (PUR)
Density = 1.5gm./cc
Shrinkage =0.8-1.0%
Properties
Very tough, extremely abrasion resistant. Posses good electrical properties and chemical resistance. It can be made in solid moulding or flexible forms.
Applications
Used for PUR springs. Used for making forms
Styrene Acrylonitrile (SAN)
Density = 1.08gm./cc
Shrinkage =0.2 - 0.6%
Copolymer of styrene and acrylonitrile exhibits most of the properties of polystyrene
Properties
Excellent resistance to acids, alkalis, salts and dilutes alcohol. Good resistance to many foods and oils, gasoline, cleaning agents, detergents, cosmetic cream and lotion. Higher tensile strength and better stress corrosion resistance and heat resistance.
Applications
Used for cups, tumblers, dishes, trays, picnic wares, cosmetics and other packaging items
Thermosetting materials
A thermoset are resin that under go chemical change called curing during processing to form cross-linked structure and become permanently insoluble and infusible. Among plastic materials thermosetting materials generally provide one or more of the following advantages
• High thermal stability
• Resistance to creep
• Resistance to deformation under load
• High dimensional stability
• High rigidity and hardness
• Light weight
• High electrical insulation properties
These advantages are coupled with the light weight and excellent electrical insulation properties. The compression and transfer moulding together with the recent evolution of thermo set injection moulding techniques offer low processing cost.
Thermosetting moulding compounds consist of two major ingredients.
1. a resin system which generally contains such components as curing agents, hardeners, Inhibitors and plasticizers.
2. Filler and or reinforcements which may consist of mineral or organic particles, inorganic or organic fibers and or inorganic or organic chopped cloth or paper.
The resin system usually exerts the dominant effect, determining to a great extent cost, dimensional stability, electrical qualities, heat resistance, chemical resistance decorative possibilities and flammability. Fillers and reinforce affect all these properties to varying degrees, but their most dramatic effects are in strength, toughness and sometimes electrical qualities.
The characteristics and properties of eight generic families of thermoset moulding compounds are given below.
Alkyds:
They are primarily electrical materials. They cast less and posses good insulating properties. Their mould ability is excellent; cure time short and pressure are low. They are stable upto a temperature of’ 120 to 150o C.
General purpose grades are normally mineral filled, Compound filled with glass or synthetic fibers improve mechanical strength. Short fiber and mineral fillers give lower cost and good mould ability, longer fibers give optimum strength.
Typical uses for alkyds include circuit breaker insulation, capacitor and resistor encapsulation, cases, housing and switch gear components.
Allelic:
Diallyl phthalate (DAP) is r commonly used moulding material in the allylic family. They are relatively costlier. They posses excellent dimensional stability and a high insulation resistance. They can withstand temperatures of 150 to 175oC. They are normally reinforced with glass, asbestos, acrylic and polyester fibers.
Epoxies:
These materials offer excellent electrical properties and dimensional stability coupled with high strength and moisture absorption. It is used in combination with glass fibers (epoxy glass reinforced plastics) in aircraft components, filament wound rocket motors, casing for missiles, pipes, tank pressure vessels and jigs and fixtures. They are also used In the encapsulation or casting of various electrical and electronic components.
Urea (urea formaldehyde):
Excellent colourabililty, moderately good strength and low cost are the primary attributes of these materials. Dimensional stability and impact strength are poor.
The material is used for producing lighting fixtures, wiring devices, buttons etc.
Phenolics (phenol formaldehyde) :
They are lower in cost, possess good electrical properties, excellent heat resistance and good mechanical properties coupled with excellent mouldability. They are generally limited in colour (usually black or dark brown) and colour stability.
They are generally filled with saw dust or flock. They are suitable for use at temperature upto 150°C. heat resistant grades, usually mineral or glass filled can withstand temperatures in the range of 200 to 260oC They are used for moulding wiring devices, switch gear, relay system, connectors etc.
Melamine:
They posses extreme hardness, excellent and permanent colourability and are resistance and non cracking characteristics, They re self extinguishing. Dishes and other house ware articles and electrical items are manufactured from this plastic.
Polyesters:
Polyester resins can be formulated to be brittle and hard tough and resilient or soft and flexible. When reinforced with glass fibers they. exhibit excellent mechanical properties, high strength to weight ratio and chemical resistance. They are used as liquid resin for
casting furniture part. Polyester glass is used for such parts as automobile body parts, boat hulls, building panels etc.
FILLERS AND ADDITIVES
Plastics are material that are composed not only of large polymeric molecules, but also usually include some additional small molecule materials that have been added to give the plastic some particular characteristics. The characteristics of colour, flexibility, and rigidity, flame resistance, weathering behaviour and process ability can be significantly altered by the use of additives and fillers.
Plastic additives may be grouped into two main categories.
The first comprises of those additives which modify the base polymer’s characteristics by physical means. Additives of this type perform s plasticizer, lubricants, impact modifiers, fillers and pigments.
The second group of additives achieves their effect by chemical reactions. This type includes flame retards, stabilizers, ultraviolet absorbers and antioxidants.
Fillers :
Fillers are classified as inorganic, organic, mineral, natural and synthetic. They are more commonly used with the thermosetting materials such as phenolics, urea and melamine. They fill some thermoplastics as well. Large amounts of fillers are referred as extenders because they increase the bulkiness and decrease the cost of a plastic. Since the properties of an extended plastics often suffer, the use of fillers is often limited to less critical application.
Fillers normally endow plastics with specific mechanical, physical and electrical properties. Filler usually comprises between 10 and 50% of the mass of the mix.
One of the most widely used fillers for the thermoplastic is wood flour. It consists of finely ground powder of hard woods or nut shells. They are cheap, light weight, strong (due to their fibrous nature) and easy to compound. But they have low thermal resistance, low dimensional stability and poor electrical properties.
Mineral fillers used are the purer form of silica, mica, quartz, inorganic pigment, refined metal oxides, sulfates etc. Large amounts of inorganic and mineral fillers increase the brittleness. Asbestos (mineral fiber) combines the advantages of. an inorganic filler with those of fiber. (recently asbestos has
come under stringent safety regulations).
The factors for choosing filler are:
1. Cost. Availability and uniformity
2. Compatibility or meltability with the resin
3. Moisture absorption
4. Physical properties
5. Thermal. stability and mould temperatures
6. Resistance to chemicals
7. Abrasiveness
8. Effect on plastic flow characteristics.
The following chart shows the different fillers used for the different purposes
Plasticizers :
Plastics may need to be plasticized to enhance flexibility, resiliency and resiliency flow. Without the addition of a plasticizer, it would not be possible to make plastics sheeting, tubing, film and other flexible forms of plastics. The plasticizers enable the molecular chains of Polymers to move freely with respect to one another, with minimum entanglement or internal friction. A plasticizer thus acts as an internal lubricant. It is not chemically linked to the plastic.
Nearly 80 % of all plasticizers are used in PVC, but they are used in cellulous, nylon. ABS.
Most plasticizers are liquids; a few are solids that melt at compounding temperature. All must exhibit good compatibility with the resin they modify. They are usually colourless, odourless and posses good thermal stability. But they decrease the strength characteristics, heat resistance, dimensional stability and solvent resistance of resins.
The popular general purpose plasticizers are the phthalates, eporcies, phosphates, adipate diesters, sebacates etc. Plasticizers compose upto 50 to 60% of a final PVC product.
Colourants:
The use of colourants makes it possible to produce a variety of materials in colours varying from pastels to deep hues. This types of colourants are used, dyes and pigments of organic and inorganic nature. Dyes are fairly soluble in plastics while the pigments are not. The choice depends on resin compatibility or the need for solubility. The colourants should be stable. Dyes have lower thermal stability and will be discoloured on overheating. Colour concentrates are also supplied with the plastics by the manufacturer.
Heat stabilizers
Stabilizers are used to prevent the degradation of resins during processing, when melts are subjected to high temperatures. They are also used to extend the life of the end parts. PVC is vulnerable to degradation during processing and is the prime consumer of heat stabilizers. The commercial stabilizers are based on solid barium, cadmium, lead etc.
Antioxidants
Antioxidants are used to protect materials from deterioration through oxidation brought on by heat, light or chemically induced mechanism. Oxidation causes, brittleness, melt flow instability, loss of tensile properties and discolouration.
Ultra violet light absorbers
Virtually every plastic degrades in sunlight. They lose their colour and physical properties. PS, PVC, ABS, polyesters are affected much. The choice of ultra violet absorber depends on the application, the. basic resin, the ultimate effect and the required durability of the end product. (Ultra violet absorbers are used up in time).
Flame retardants
Flame retardants are used to affect combustion in plastics.
Flame retardants work on four basic principles. When
1. they insulate or
2. create an endothermic cooling reaction or
3. coat the product and exclude oxygen or
4. actually influence combustion through reaction.
The most common flame retardants are antimony, boron, halogens, nitrogen and phosphorus The most common in PVC are phosphate esters.
Antistatic agents
Antistatic agents also called destatisisers are used to reduce the build up of electrostatic charges an the surface of plastics materials by increasing surface conductivity
Plastics susceptible to electrostatic charges are PE, PP, PS, PA, polyesters, urethanes, cellulosies acrylics and acrylonitriles.
The most common antistatic for plastics are amines, ammonia, compounds, phosphate esters, and polyethylene glycol esters.
Antistatic agents are selected on the basis of application, durability, necessary concentration, and effectiveness at low humidity.
Lubricants
Lubricants are additives used to help in the processing of plastics. Internal lubricants act within the plastic to reduce the forces acting between molecules. External lubricants reduce the adhesion of the plastic to the metal surface of the process machinery and mould
Lubricants fall into five categories, metallic stearates, fatty acid amides and esters, fatty acids, hydro carbon waxes and low molecular weight polyethylene.
Reinforced plastics
Reinforced plastic are composite material consisting of a thermoplastic or thermoset matrix combined with another stronger and stiffer material, usually in fiber form which acts as the reinforcement The purpose of the fiber is to carry as much of the load applied to the composite as possible The plastic matrix exists primarily to transfer the load-form point of application into the load-bearing fibers.
The effectiveness of such fibrous reinforcement is dependent on a number of factors. The degree of adhesion between plastic and fiber is fundamental. Other factors of importance are the relative volumes of reinforcement and plastic matrix, the length of’ fiber used and the physical chemical and temperature properties of fiber and plastic.
Reinforcements
Glass filaments are a low cost readily available reinforcement Glass fibers with perfect surface have very high ultimate tensile strength.
Glass fibers are manufactured by extruding molten glass through a die then drawing the glass down to a fine diameter strand (0.01)
Ceramic fibers
To achieve the highest possible strength, stiffness and temperature capabilities of reinforced plastic, ceramic fiber reinforcements may be used. Carbon and boron fibers are presently used.
Fiber to plastic bonding
The effectiveness of fiber reinforcement depends on the bond between matrix and fiber. Glass fibers are generally coated with substances which act as coupling agents to encourage good bounding.
Mould release agents
In moulding any of the reinforcement materials, regardless of the type of reinforcement or the rein used, the tendency of part to stick to the mould surface is a common characteristic. To facilitate the removal of such parts from the mould a release agent ( also called parting agent or mould release ) is used. The release agent forms a barrier film between the mould surface and the part .There are two types of release agent commonly used.
1) External release agent which are applied to the mould surface.
E.g. regenerated cellulose, polyvinyl alcohol (PVA), PVA and nylon solution, pure carnauba wax etc.
2) Internal release agent which are combined with the resin.
E.g. Zinc and calcium stearates, silicone oils, high melting point waxed etc.
Plastic Materials and its abbreviations
Abb
|
Name
|
ABS
|
Acrylonitryle Butadiene styrene
|
BMC
|
Bulk Moulding Compound
|
CAB
|
Cellulose Acetate Butyrate
|
CMC
|
Carboxymethyl Cellulose
|
CP
|
Cellulose Propionate
|
CPVC
|
Chlorinated Polyvinyl Chloride
|
DAP
|
Diallyphthalate
|
EP
|
Epoxy
|
EVA
|
Ethylene vinyl acetate
|
HDPE
|
High Density poly Ethylene
|
LDPE
|
Low Density Poly Ethylene
|
MF
|
Melamine Formaldehyde
|
PA
|
Polyamide (Nylon)
|
PC
|
Polycarbonate
|
PE
|
Polyethylene
|
PEK
|
Poly ether keton
|
PET
|
Polyethylene Teraphthalate
|
PMMA
|
Polymethyl Methacrylate
|
PP
|
Polypropylene
|
PPO
|
Polyphenylene Oxide
|
PTFE
|
Polytetrafluoro Ethylene
|
PVA
|
Polyvinyl acetate
|
PVCC
|
PVC Compound
|
SBR
|
Styrene Butadiene Rubber
|
SP
|
Saturated Polyester
|
UHMW HDPE
|
Ultra High molecular Weight HDPE
|
SI
|
Silicone
|
COMMENTS