What do You Know about Die Casting

Introduction

Die casting is a versatile process for producing engineered metal parts by forcing molten metal under high pressure into reusable steel molds. These molds, called dies, can be designed to produce complex shapes with a high degree of accuracy and repeatability. Parts can be sharply defined, with smooth or textured surfaces, and are suitable for a wide variety of attractive and serviceable finishes.

Die castings are among the highest volume, mass-produced items manufactured by the metalworking industry, and they can be found in thousands of consumer, commercial and industrial products. Die cast parts are important components of products ranging from automobiles to toys. Parts can be as simple as a sink faucet or as complex as a connector housing.

Die cast parts are found in many places around the home. The polished, plated zinc die casting in this kitchen faucet illustrates one of the many finishes possible with die casting.

These connector housings are examples of the durable, highly accurate components that can be produced with today’s modern die casting.

History

The earliest examples of die casting by pressure injection - as opposed to casting by gravity pressure - occurred in the mid-1800s. A patent was awarded to Sturges in 1849 for the first manually operated machine for casting printing type. The process was limited to printer’s type for the next 20 years, but development of other shapes began to increase toward the end of the century. By 1892, commercial applications included parts for phonographs and cash registers, and mass production of many types of parts began in the early 1900s.

The first die casting alloys were various compositions of tin and lead, but their use declined with the introduction of zinc and aluminum alloys in 1914. Magnesium and copper alloys quickly followed, and by the 1930s, many of the modern alloys still in use today became available.

The die casting process has evolved from the original low-pressure injection method to techniques including high-pressure casting — at forces exceeding 4500 pounds per square inch — squeeze casting and semi-solid die casting. These modern processes are capable of producing high integrity, near net-shape castings with excellent surface finishes.

The Future

Refinements continue in both the alloys used in die casting and the process itself, expanding die casting applications into almost every known market. Once limited to simple lead type, today’s die casters can produce castings in a variety of sizes, shapes and wall thicknesses that are strong, durable and dimensionally precise.

A magnesium seat pan shows how complex, lightweight die cast components can improve production by replacing multiple pieces.

The Advantages of Die Casting

Die casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique. Parts have long service life and may be designed to complement the visual appeal of the surrounding part. Designers can gain a number of advantages and benefits by specifying die cast parts.

High-speed production - Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required and thousands of identical castings can be produced before additional tooling is required.

Dimensional accuracy and stability - Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat resistant.

Strength and weight - Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin wall castings are stronger and lighter than those possible with other casting methods. Plus, because die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process.

Multiple finishing techniques - Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation.

Simplified Assembly - Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast.

Die Casting Process

The basic die casting process consists of injecting molten metal under high pressure into a steel mold called a die. Die casting machines are typically rated in clamping tons equal to the amount of pressure they can exert on the die. Machine sizes range from 400 tons to 4000 tons. Regardless of their size, the only fundamental difference in die casting machines is the method used to inject molten metal into a die. The two methods are hot chamber or cold chamber. A complete die casting cycle can vary from less than one second for small components weighing less than an ounce, to two-to-three minutes for a casting of several pounds, making die casting the fastest technique available for producing precise non-ferrous metal parts.

Die Casting vs. Other Processes

Die casting vs. plastic molding - Die casting produces stronger parts with closer tolerances that have greater stability and durability. Die cast parts have greater resistance to temperature extremes and superior electrical properties.

Die casting vs. sand casting - Die casting produces parts with thinner walls, closer dimensional limits and smoother surfaces. Production is faster and labor costs per casting are lower. Finishing costs are also less.

Die casting vs. permanent mold - Die casting offers the same advantages versus permanent molding as it does compared with sand casting.

Die casting vs. forging - Die casting produces more complex shapes with closer tolerances, thinner walls and lower finishing costs. Cast coring holes are not available with forging.

Die casting vs. stamping - Die casting produces complex shapes with variations possible in section thickness. One casting may replace several stampings, resulting in reduced assembly time.

Die casting vs. screw machine products - Die casting produces shapes that are difficult or impossible from bar or tubular stock, while maintaining tolerances without tooling adjustments. Die casting requires fewer operations and reduces waste and scrap.

Choosing the Proper Alloy

Each of the metal alloys available for die casting offer particular advantages for the completed part.

Zinc - The easiest alloy to cast, it offers high ductility, high impact strength and is easily plated. Zinc is economical for small parts, has a low melting point and promotes long die life.

Aluminum - This alloy is lightweight, while possessing high dimensional stability for complex shapes and thin walls. Aluminum has good corrosion resistance and mechanical properties, high thermal and electrical conductivity, as well as strength at high temperatures.

Magnesium - The easiest alloy to machine, magnesium has an excellent strength-to-weight ratio and is the lightest alloy commonly die cast.

Copper - This alloy possesses high hardness, high corrosion resistance and the highest mechanical properties of alloys cast. It offers excellent wear resistance and dimensional stability, with strength approaching that of steel parts.

Lead and Tin - These alloys offer high density and are capable of producing parts with extremely close dimensions. They are also used for special forms of corrosion resistance.

Die Construction

Dies, or die casting tooling, are made of alloy tool steels in at least two sections, the fixed die half, or cover half, and the ejector die half, to permit removal of castings. Modern dies also may have moveable slides, cores or other sections to produce holes, threads and other desired shapes in the casting. Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity. The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant.

When the die casting machine closes, the two die halves are locked and held together by the machine’s hydraulic pressure. The surface where the ejector and fixed halves of the die meet and lock is referred to as the "die parting line." The total projected surface area of the part being cast, measured at the die parting line, and the pressure required of the machine to inject metal into the die cavity governs the clamping force of the machine.

There are four types of dies:

1. Single cavity to produce one component

2. Multiple cavity to produce a number of identical parts

3. Unit die to produce different parts at one time

4. Combination die to produce several different parts for an assembly.

Hot Chamber Machines

Click on the image to see an animation

Hot chamber machines are used primarily for zinc, copper, magnesium, lead and other low melting point alloys that do not readily attack and erode metal pots, cylinders and plungers. The injection mechanism of a hot chamber machine is immersed in the molten metal bath of a metal holding furnace. The furnace is attached to the machine by a metal feed system called a gooseneck. As the injection cylinder plunger rises, a port in the injection cylinder opens, allowing molten metal to fill the cylinder. As the plunger moves downward it seals the port and forces molten metal through the gooseneck and nozzle into the die cavity. After the metal has solidified in the die cavity, the plunger is withdrawn, the die opens and the casting is ejected.

Cold Chamber Machines

Click on the image to see an animation

Cold chamber machines are used for alloys such as aluminum and other alloys with high melting points. The molten metal is poured into a "cold chamber," or cylindrical sleeve, manually by a hand ladle or by an automatic ladle. A hydraulically operated plunger seals the cold chamber port and forces metal into the locked die at high pressures.

High Integrity Die Casting Methods

There are several variations on the basic process that can be used to produce castings for specific applications. These include:

Squeeze casting - A method by which molten alloy is cast without turbulence and gas entrapment at high pressure to yield high quality, dense, heat treatable components.

Click on the image to see an animation

Semi-solid molding - A procedure where semi-solid metal billets are cast to provide dense, heat treatable castings with low porosity.

Automation and Quality Control

Modern die casters use a number of sophisticated methods to automate the die casting process and provide continuous quality control. Automated systems can be used to lubricate dies, ladle metal into cold chamber machines and integrate other functions, such as quenching and trimming castings. Microprocessors obtain metal velocity, shot rod position, hydraulic pressure and other data that is used to adjust the die casting machine process, assuring consistent castings shot after shot. These process control systems also collect machine performance data for statistical analysis in quality control.

Die Casting Design

Die casting is one of the fastest and most cost-effective methods for producing a wide range of components. However, to achieve maximum benefits from this process, it is critical that designers collaborate with the die caster at an early stage of the product design and development. Consulting with the die caster during the design phase will help resolve issues affecting tooling and production, while identifying the various trade-offs that could affect overall costs.

For instance, parts having external undercuts or projections on sidewalls often require dies with slides. Slides increase the cost of the tooling, but may result in reduced metal use, uniform casting wall thickness or other advantages. These savings may offset the cost of tooling, depending upon the production quantities, providing overall economies.

Many sources are available for information on die casting design, including textbooks, technical papers, trade journals and professional associations. While this section is not intended to provide a comprehensive review of all the factors involving die casting design, it will highlight some of the primary considerations. Additional sources of information are listed in the "Resources" section of this brochure.

Alloy Properties One of the first steps in designing a die cast component is choosing the proper alloy. Typical properties for the most commonly used alloys are shown on the linked charts.

Comparing Materials

The cost of materials is another important design consideration. Accurate comparisons require looking beyond the cost per pound or cost per cubic inch to fully analyze the advantages and disadvantages of each competing process. For instance, the relatively greater strength of metals generally allows thinner walls and sections and consequently requires fewer cubic inches of material than plastics for a given application. sumber : http://www.diecasting.org

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Advantages and Disadvantages of Die Casting

Advantages:

• Excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm (0.005 in. for the first inch) and 0.02 mm for each additional centimeter (0.002 in. for each additional inch).
• Smooth cast surfaces (1–2.5 micrometres or 0.04–0.10 thou rms).
• Thinner walls can be cast as compared to sand and permanent mold casting (approximately 0.75 mm or 0.030 in).
• Inserts can be cast-in (such as threaded inserts, heating elements, and high strength bearing surfaces).
• Reduces or eliminates secondary machining operations.
• Rapid production rates.
• Casting tensile strength as high as 415 MPa (60 ksi).
• Castings are made as large as an 8 feet across and 30Lbs in weight. In magnesium

Disadvantages:

• Casting weight must be between 30 grams (1 oz) and 10 kg (20 lb).
• High initial cost.
• Limited to high-fluidity metals.
• A certain amount of porosity is common.
• A large production volume is needed to make this an economical alternative to other processes

sumber : wikipedia

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Equipment of Die Casting

There are two basic types of die casting machines: hot-chamber machines (a.k.a. gooseneck machines) and cold-chamber machines. These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 short tons.

Hot-chamber machines rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the "gooseneck". The gas or oil powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times (approximately 15 cycles a minute) and the convenience of melting the metal in the casting machine. The disadvantages of this system are that high-melting point metals cannot be utilized and aluminium cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.

injection molding machine.

open tooling and injection nozzle.

Complete working cell.

Cold-chamber machines are used when the casting alloy cannot be used in hot-chamber machines; these include aluminium, zinc alloys with a large composition of aluminium, magnesium and copper. This machine works by melting the material, first, in a separate furnace. Then a precise amount of molten metal is transported to the cold-chamber machine where it is fed into an unheated shot chamber (or injection cylinder). This shot is then driven into the die by a hydraulic or mechanical piston. This biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold-chamber machine.^[7]

The dies used in die casting are usually made out of hardened tool steels because cast iron cannot withstand the high pressures involved. Due to this the dies are very expensive, resulting in high start-up costs. Dies may contain only one mold cavity or multiple cavities of the same or different parts. There must be at least two dies to allow for separation and ejection of the finished workpiece, however its not uncommon for there to be more sections that open and close in different directions. Dies also often contain water-cooling passages, retractable cores, ejector pins, and vents along the parting lines. These vents are usually wide and thin (approximately 0.13 mm or 0.005 in) so that when the molten metal starts filling them the metal quickly solidifies and minimizes scrap. No risers are used because the high pressure ensures a continuous feed of metal from the gate. Recently, there's been a trend to incorporate larger gates in the die and to use lower injection pressures to fill the mold, and then increase the pressure after its filled. This system helps reduce porosity and inclusions.

In addition to the dies there may be cores involved to cast features such as undercuts. Sand cores cannot be used because they disintegrate from the high pressures involved with die casting, therefore metal cores are used. If a retractable core is used then provisions must be made for it to be removed either in a straight line or circular arc. Moreover, these cores must have very little clearance between the die and the core to prevent the molten metal from escaping. Loose cores may also be used to cast more intricate features (such as threaded holes). These loose cores are inserted into the die by hand before each cycle and then ejected with the part at the end of the cycle. The core then must be removed by hand. Loose cores are more expensive due to the extra labor and time involved.

A die's life is most prominently limited by wear or erosion, which is strongly dependent on the temperature of the molten metal. Dies for zinc are often made of H13 and only hardened to 29-34 HRC.^[11] Cores are either made of H13 or 440B, so that the wearing parts can be selectively nitrided for hardness, leaving the exposed part soft to resist heat checking.

Typical die temperatures and life for various cast materials

	Zinc	Aluminum	Magnesium	Brass (leaded yellow)
Maximum die life [number of cycles]	1,000,000	100,000	100,000	10,000
Die temperature [C° (F°)]	218 (425)	288 (550)	260 (500)	500 (950)
Casting temperature [C° (F°)]	400 (760)	660 (1220)	760 (1400)	1090 (2000)

Other failure modes for dies are:

• Heat checking: surface cracks occur on the die due to a large temperature change on every cycle
• Thermal fatigue: surface cracks occur on the die due to a large number of cycles

sumber : wikipedia

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History and Process of Die Casting

Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter and tin based alloys, although ferrous metal die castings are possible. The die casting method is especially suited for applications where a large quantity of small to medium sized parts are needed, ensuring precise surface quality and dimensional consistency.

This level of versatility has placed die castings among the highest volume products made in the metalworking industry

History

Die casting equipment was invented in 1838 for the purpose of producing movable type for the printing industry. The first die casting-related patent was granted in 1849 for a small hand operated machine for the purpose of mechanized printing type production. In 1885, Otto Mergenthaler invented the linotype machine, an automated type casting device that became the prominent type of equipment in the publishing industry. Other applications grew rapidly, with die casting facilitating the growth of consumer goods and appliances by making affordable the production of intricate parts in high volumes

Process

There are four major steps in the die casting process. First, the mold is sprayed with lubricant and closed. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. Molten metal is then shot into the die under high pressure; between 10—175 MPa (1,500—25,000 psi). Once the die is filled the pressure is maintained until the casting has solidified. The die is then opened and the shot (shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot) is ejected by the ejector pins. Finally, the scrap, which includes the gate, runners, sprues and flash, must be separated from the casting(s). This is often done using a special trim die in a power press or hydraulic press. An older method is separating by hand or by sawing, which case grinding may be necessary to smooth the scrap marks. A less labor-intensive method is to tumble shots if gates are thin and easily broken; separation of gates from finished parts must follow. This scrap is recycled by remelting it. The yield is approximately 67%.

The high-pressure injection leads to a quick fill of the die, which is required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities are avoided even if the shape requires difficult-to-fill thin sections. This creates the problem of air entrapment, because when the mold is filled quickly there is little time for the air to escape. This problem is minimized by including vents along the parting lines, however, even in a highly refined process there will still be some porosity in the center of the casting.

Most die casters perform other secondary operations to produce features not readily castable, such as tapping a hole, polishing, plating, buffing, or painting.

Pore-free casting process

When no porosity is required for a casting then the pore-free casting process is used. It is identical to the standard process except oxygen is injected into the die before each shot. This causes small dispersed oxides to form when the molten metal fills the dies, which virtually eliminates gas porosity. An added advantage to this is greater strength. These castings can still be heat treated and welded. This process can be performed on aluminium, zinc, and lead alloys

Heated-manifold direct-injection die casting

Heated-manifold direct-injection die casting, also known as direct-injection die casting or runnerless die casting, is a zinc die casting process where molten zinc is forced through a heated manifold and then through heated mini-nozzles, which lead into the molding cavity. This process has the advantages of lower cost per part, through the reduction of scrap (by the elimination of sprues, gates and runners) and energy conservation, and better surface quality through slower cooling cycles.

sumber : wikipedia

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Polyester

Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate (PET). Polyesters include naturally-occurring chemicals, such as in the cutin of plant cuticles, as well as synthetics such as polycarbonate and polybutyrate.

Polyesters may be produced in numerous forms such as fibers, sheets and three-dimensional shapes. Polyesters as thermoplastics may change shape after the application of heat. While combustible at high temperatures, polyesters tend to shrink away from flames and self-extinguish upon ignition. Polyester fibers have high tenacity and E-modulus as well as low water absorption and minimal shrinkage in comparison with other industrial fibers.
Applications

Polyesters are the most widely used man-made fiber in the world. Woven polyester fabrics are used in consumer apparel and home furnishings such as bed sheets, bedspreads, curtains and draperies. Similarly, industrial polyesters are used in tyre reinforcements, ropes, fabrics for conveyor belts, safety belts, coated fabrics and plastic reinforcements with high energy absorption. Polyester fiberfills are also used to stuff pillows, comforters and cushion padding.

Polyester fabrics are claimed to have a "less natural" feel when compared to similarly-woven fabrics made from natural fibers (i.e. cotton in textile uses). However, polyester fabrics may exhibit other advantages over natural fabrics, such as improved wrinkle resistance. As a result, polyester fibers are sometimes spun together with natural fibers to produce a cloth with blended properties.

Close-up of a polyester shirt
Polyesters are also used to make bottles, films, tarpaulin, canoes, liquid crystal displays, holograms, filters, dielectric film for capacitors, film insulation for wire and insulating tapes. Liquid crystalline polyesters are among the first industrially-used liquid crystalline polymers. They are used for their mechanical properties and heat-resistance. These traits also important in their application as an abradable seal in jet engines.

Thermosetting polyesters are used as casting materials, and chemosetting polyester resins are used as fiberglass laminating resins and non-metallic auto-body fillers. Fiberglass-reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars.

Polyesters are also widely used as a finish on high-quality wood products such as guitars, pianos and vehicle / yacht interiors. Burns Guitars, Rolls Royce and Sunseeker are a few companies that use polyesters to finish their products. Thixotropic properties of spray-applicable polyesters make them ideal for use on open-grain timbers, as they can quickly fill wood grain, with a high-build film thickness per coat. Cured polyesters can be sanded and polished to a high-gloss, durable finish.

Polyester fiber properties

Mechanical properties
Energy absorption of chemical fiber reinforced plastics (impact, bending and tensile tests) Investigation of the practical requirements for measuring the energy absorption of composite materials, and development of a suitable method for carrying out such measurements. A number of dynamic testing methods for measuring the energy absorption of laminates are reviewed, including animpact bending test, repeated-impact tests, an impact tensile test, and a ram bending test. Also discussed are impact tests on plate laminates. Particular emphasis is placed in these studies on composites with a chemical fiber reinforcement. It is established that a relation exists between the quasi-static energy absorption of the fibers and the dynamic energy absorption of the composite. Composites with commercial polyester and polyamide fibers lead to the highest energy absorptions, in which case the testing apparatus has a significant effect.

Chemical properties

The polyester industry

To get an idea about coverage, importance and complexity of the polyester industry, some basic information about polyester or polyethylene terephthalate (PET) at first:

What is polyester? Polyester is a synthetic polymer made of purified terephthalic acid (PTA) or its dimethyl ester dimethyl terephthalate (DMT) and monoethylene glycol (MEG). It ranges after polyethylene and polypropylene at the third place in terms of market size.
The main raw materials are described as follows:
• Purified Terephthalic Acid – PTA – CAS-No.: 100-21-0
Synonym: 1,4 Dibenzenedicarboxylic acid,
Sum formula; C6H4(COOH)2 , mol weight: 166,13
• Dimethylterephthalate – DMT- CAS-No: 120-61-6
Synonym: 1,4 Dibenzenedicarboxylic acid dimethyl ester
Sum formula C6H4(COOCH3)2 , mol weight: 194,19
• Mono Ethylene Glycol – MEG – CAS No.: 107-21-1
Synonym: 1,2 Ethanediol
Sum formula: C2H6O2 , mol weight: 62,07
More information about polyester raw materials one can find for PTA [1],DMT [2] and MEG [3], at the webpage INCHEM "Chemical Safety Information from Intergovernmental Organizations".
To make finally a polymer of high molecular weight one needs a catalyst. The most common catalyst is antimony trioxide (or antimony tri acetate)
Antimony trioxide – ATO – CAS-No.: 1309-64-4 Synonym: non, mol weight: 291,51 Sum formula: Sb2O3
In 2008 about 10 000 t Sb2O3 are used to produce around 49 Mio t polyethylene terephthalate.
Polyester is described as follows:
Polyethylene Terephthalate CAS-No.: 25038-59-9 Synonym / abbreviations: polyester, PET, PES Sum Formula: H-[C10H8O4]-n=60-120 OH, mol unit weight: 192,17
What are the success factors of the unbroken capacity growth of polyethylene terephthalate?
• The relatively easy accessible raw materials PTA or DMT and MEG
• The very well understood and described simple chemical process of polyester synthesis
• The low toxicity level of all raw materials and side products during production and processing
• The possibility to produce PET in a closed loop at low emissions to the environment
• The outstanding mechanical and chemical properties of polyester
• The recycle ability
• The wide variety of intermediate and final products made of polyester
All these facts are making this polymer one of the key elements of our daily life.
In table 1 we see the estimated world polyester production for textile polyester, bottle polyester resin, film polyester mainly for packaging and specialty polyesters for engineering plastics, which are the main fields of application. According to this table, the world's total polyester production might exceed 50 million tons per annum before the year 2010.

Table 1: World polyester production
Market size per year
Product Type 2002 [Mio t/a] 2008 [Mio t/a]
Textile-PET 20 39
Resin, Bottle/A-PET 9 16
Film-PET 1.2 1.5
Special Polyester 1 2.5
TOTAL 31.2 49

With its production volume and product diversity, polyester ranges after polyethylene (33.5%), polypropylene (19,5%) with a market share of about 18% in third position among all plastic materials produced worldwide. The polyester production chain, and the relative polyester industry chain, will now be explained in greater detail and step by step.

Raw material producer
The raw materials PTA, DMT and MEG are mainly produced by large chemical companies which are sometimes integrated down to the crude oil refinery where p-xylene is the base material to produce PTA and liquefied petroleum gas (LPG) is the base material to produce MEG.

Large PTA producers are for instance BP, Reliance, Sinopec, SK-Chemicals, Mitsui and Eastman Chemicals. MEG production is in the hand of about 10 global players which are headed by MEGlobal a JV of DOW and PIC Kuweit followed by Sabic.

Let us assume the average production capacity of a single polyester plant is about 200 t/day: we are talking about nearly 500 polyester plants around the globe. Adding to this figure the continuously-growing polyester recycling industry, which is estimated to have processed about 3 million t polyester waste in 2007 alone (5 million T/a in 2010 estimated) and where each plant produces on average about 10 000 t/a, we have another 500 plants. This is 1000 polyester production plants, all needing specific and polyester-dedicated engineering and equipment, machinery, process technology and know-how, producing, processing and recycling polyester.
Among the world's largest polyester producers are the following companies:

Artenius, Advansa, DAK, DuPont, Eastman/Voridian, Hyosung, Huvis, Indorama, Invista, Jiangsu Sanfangxian, M&G Group, Mitsui, Mitsubishi, NanYa Plastics,Reichhold, Reliance, Rongsheng, Sabic, Teijin, Toray, Tonkun, Tuntex, Wellman, Yizheng Sinopec and Sanfanxiang.

One should notice that China's capacity to produce and process polyester in more than 500 plants is nearly half that of the world's polyester capacity meanwhile. More information about polyester in China can be found under the web site of China Chemical Fiber Economic Information Network

Polyester processing
After the first stage of polymer production in the melt phase, the product stream divides into two different application areas which are mainly textile applications and packaging applications. In figure 2 the main applications of textile and packaging polyester are listed.
Table 2: Textile and packaging polyester application list

POLYESTER-BASED POLYMER (MELT or PELLETS)


Staple fiber (PSF) Bottles for CSD, Water, Beer, Juice, Detergents
Filaments POY, DTY, FDY A-PET Film
Technical yarn and tire cord Thermoforming
Non-woven and spunbond BO-PET Biaxial oriented Film
Mono-filament Strapping
Abbreviations: PSF = Polyester Staple Fiber; POY = Partially Oriented Yarn; DTY = Draw Textured Yarn; FDY = Fully Drawn Yarn; CSD = Carbonated Soft Drink; A-PET = Amorphous Polyester Film; BO-PET = Biaxial Oriented Polyester Film; A comparable small market segment (<<> benzene -> PX -> PTA -> PET melt -> fiber / filament or bottle-grade resin. Such integrated processes are meanwhile established in more or less interrupted processes at one production site. Eastman Chemicals introduced at first the idea to close the chain from PX to PET resin with their so-called INTEGREX® process. The capacity of such horizontal, integrated productions sites is >1000 t/d and can easily reach 2500 t/d.

Besides the above mentioned large processing units to produce staple fiber or yarns, there are ten thousands of small and very small processing plants, so that one can estimate that polyester is processed and recycled in more than 10 000 plants around the globe. This is without counting all the companies involved in the supply industry, beginning with engineering and processing machines and ending with special additives, stabilizers and colors. This is a gigantic industry complex and it is still growing by 4–8% per annum, depending on the world region. Useful information about the polyester industry can be found under where a “Who is Producing What in the Polyester Industry” is gradually being developed.

source : wikipedia.org

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Polyphenylethene

Polyphenylethene IPA (IUPAC Polyphenylethene) is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. Polystyrene is a thermoplastic substance, normally existing in solid state at room temperature, but melting if heated (for molding or extrusion), and becoming solid again when cooling off.

Pure solid polystyrene is a colorless, hard plastic with limited flexibility. It can be cast into molds with fine detail. Polystyrene can be transparent or can be made to take on various colours. It is economical and is used for producing plastic model assembly kits, license plate frames, plastic cutlery, CD "jewel" cases, and many other objects where a fairly rigid, economical plastic is desired.

History
Polystyrene was discovered in 1839 by Eduard Simon,[3] an apothecary in Berlin. From storax, the resin of Liquidambar orientalis, he distilled an oily substance, a monomer which he named styrol. Several days later Simon found that the styrol had thickened, presumably from oxidation, into a jelly he dubbed styrol oxide ("Styroloxyd"). By 1845 English chemist John Blyth and German chemist August Wilhelm von Hofmann showed that the same transformation of styrol took place in the absence of oxygen. They called their substance metastyrol. Analysis later showed that it was chemically identical to Styroloxyd. In 1866 Marcelin Berthelot correctly identified the formation of metastyrol from styrol as a polymerization process. About 80 years went by before it was realized that heating of styrol starts a chain reaction which produces macromolecules, following the thesis of German organic chemist Hermann Staudinger (1881–1965). This eventually led to the substance receiving its present name, polystyrene. The I. G. Farben company began manufacturing polystyrene in Ludwigshafen, Germany, about 1931, hoping it would be a suitable replacement for die cast zinc in many applications. Success was achieved when they developed a reactor vessel that extruded polystyrene through a heated tube and cutter, producing polystyrene in pellet form. Polystyrene is about as strong as unalloyed aluminium, but much more flexible.

Structure
The chemical makeup of polystyrene is a long chain hydrocarbon with every other carbon connected to a Phenyl group (the name given to the aromatic ring benzene, when bonded to complex carbon substituents).

A 3-D model would show that each of the chiral backbone carbons lies at the center of a tetrahedron, with its 4 bonds pointing toward the vertices. Say the -C-C- bonds are rotated so that the backbone chain lies entirely in the plane of the diagram. From this flat schematic, it is not evident which of the phenyl (benzene) groups are angled toward us from the plane of the diagram, and which ones are angled away. The isomer where all of them are on the same side is called isotactic polystyrene, which is not produced commercially. Ordinary atactic polystyrene has these large phenyl groups randomly distributed on both sides of the chain. This random positioning prevents the chains from ever aligning with sufficient regularity to achieve any crystallinity, so the plastic has no melting temperature, Tm. But metallocene-catalyzed polymerization can produce an ordered syndiotactic polystyrene with the phenyl groups on alternating sides. This form is highly crystalline with a Tm of 270 °C.

Solid foam

Expanded polysterene tray with tomato seedlings

Expanded polystyrene packaging material
Polystyrene's most common use is as expanded polystyrene (EPS). Expanded polystyrene is produced from a mixture of about 90-95% polystyrene and 5-10% gaseous blowing agent, most commonly pentane or carbon dioxide[4]. The solid plastic is expanded into a foam through the use of heat, usually steam.

Extruded polystyrene (XPS), which is different from expanded polystyrene (EPS), is commonly known by the trade name Styrofoam. The voids filled with trapped air give it low thermal conductivity. This makes it ideal as a construction material and it is therefore sometimes used in structural insulated panel building systems. It is also used as insulation in building structures, as molded packing material for cushioning fragile equipment inside boxes, as packing "peanuts", as non-weight-bearing architectural structures (such as pillars), and also in crafts and model building, particularly architectural models. Foamed between two sheets of paper, it makes a more-uniform substitute for corrugated cardboard, tradenamed Foamcore. A more unexpected use for the material is as a lightweight fill for embankments in the civil engineering industry.
Expanded polystyrene used to contain CFCs, but other, more environmentally-safe blowing agents are now used. Because it is an aromatic hydrocarbon, it burns with an orange-yellow flame, giving off soot, as opposed to non-aromatic hydrocarbon polymers such as polyethylene, which burn with a light yellow flame (often with a blue tinge) and no soot.

Production methods include sheet stamping (PS) and injection molding (both PS and HIPS).
The density of expanded polystyrene varies greatly from around 25 kg/m³ to 200 kg/m³ depending on how much gas was admixed to create the foam. A density of 200 kg/m³ is typical for the expanded polystyrene used in surfboards.

Standard markings
The resin identification code symbol for polystyrene, developed by the Society of the Plastics Industry so that items can be labeled for easy recycling, is . However, the majority of polystyrene products are currently not recycled because of a lack of suitable recycling facilities. Furthermore, when it is "recycled," it is not a closed loop — polystyrene cups and other packaging materials are usually recycled into fillers in other plastics, or other items that cannot themselves be recycled and are thrown away.

Copolymers

Structure of expanded polystyrene (microscope)
Pure polystyrene is brittle, but hard enough that a fairly high-performance product can be made by giving it some of the properties of a stretchier material, such as polybutadiene rubber. The two such materials can never normally be mixed because of the amplified effect of intermolecular forces on polymer insolubility (see plastic recycling), but if polybutadiene is added during polymerization it can become chemically bonded to the polystyrene, forming a graft copolymer which helps to incorporate normal polybutadiene into the final mix, resulting in high-impact polystyrene or HIPS, often called "high-impact plastic" in advertisements. One commercial name for HIPS is Bextrene. Common applications include use in toys and product casings. HIPS is usually injection molded in production. Autoclaving polystyrene can compress and harden the material.

Acrylonitrile butadiene styrene or ABS plastic is similar to HIPS: a copolymer of acrylonitrile and styrene, toughened with polybutadiene. Most electronics cases are made of this form of polystyrene, as are many sewer pipes. ABS pipes may become brittle over time. SAN is a copolymer of styrene with acrylonitrile and SMA one with maleic anhydride. Styrene can be copolymerized with other monomers; for example, divinylbenzene for cross-linking the polystyrene chains.

Cutting and shaping

Expanded polystyrene
Expanded polystyrene is very easily cut with a hot-wire foam cutter, which is easily made by a heated taut length of wire, usually nichrome because of nichrome's resistance to oxidation at high temperatures and its suitable electrical conductivity. The hot wire foam cutter works by heating the wire to the point where it can vaporize foam immediately adjacent to it. The foam gets vaporized before actually touching the heated wire, which yields exceptionally smooth cuts.

Polystyrene, shaped and cut with hot wire foam cutters, is used in architecture models, actual signage, amusement parks, movie sets, airplane construction, and much more. Such cutters may cost just a few dollars (for a completely manual cutter) to tens of thousands of dollars for large CNC machines that can be used in high-volume industrial production.

Polystyrene can also be cut with a traditional cutter. In order to do this without ruining the sides of the blade one must first dip the blade in water and cut with the blade at an angle of about 30º. The procedure has to be repeated multiple times for best results.

Polystyrene can also be cut on 3 and 5-axis routers, enabling large-scale prototyping and model-making. Special polystyrene cutters are available that look more like large cylindrical rasps.

Use in biology
Petri dishes and other containers such as test tubes, made of polystyrene, play an important role in biomedical research and science. For these uses, articles are almost always made by injection molding, and often sterilized post molding, either by irradiation or treatment with ethylene oxide. Post mold surface modification, usually with oxygen rich plasmas, is often done to introduce polar groups. Much of modern biomedical research relies on the use of such products; they therefore play a critical role in pharmaceutical research.

Finishing
In the United States, environmental protection regulations prohibit the use of solvents on polystyrene (which would dissolve the polystyrene and de-foam most of foams anyway).
Some acceptable finishing materials are
• Water-based paint (artists have created paintings on polystyrene with gouache)
• Mortar or acrylic/cement render, often used in the building industry as a weather-hard overcoat that hides the foam completely after finishing the objects.
• Cotton wool or other fabrics used in conjunction with a stapling implement.
[edit] Dangers and fire hazard
Benzene, a material used in the production of polystyrene, is a known human carcinogen. Moreover, butadiene and styrene (in ABS), when combined, become benzene-like in both form and function.[citation needed]

The EPA claims
"Styrene is primarily used in the production of polystyrene plastics and resins. Acute (short-term) exposure to styrene in humans results in mucous membrane and eye irritation, and gastrointestinal effects. Chronic (long-term) exposure to styrene in humans results in effects on the central nervous system (CNS), such as headache, fatigue, weakness, and depression, CSN dysfunction, hearing loss, and peripheral neuropathy. Human studies are inconclusive on the reproductive and developmental effects of styrene; several studies did not report an increase in developmental effects in women who worked in the plastics industry, while an increased frequency of spontaneous abortions and decreased frequency of births were reported in another study. Several epidemiologic studies suggest there may be an association between styrene exposure and an increased risk of leukemia and lymphoma. However, the evidence is inconclusive due to confounding factors. EPA has not given a formal carcinogen classification to styrene."

Polystyrene is classified according to DIN4102 as a "B3" product, meaning highly flammable or "easily ignited". Consequently, though it is an efficient insulator at low temperatures, it is prohibited from being used in any exposed installations in building construction as long the material is not flame retarded e.g. with hexabromocyclododecane. It must be concealed behind drywall, sheet metal or concrete. Foamed plastic materials have been accidentally ignited and caused huge fires and losses. Examples include the Düsseldorf International Airport, the Channel tunnel, where it was inside a railcar and caught on fire, and the Browns Ferry Nuclear Power Plant, where fire reached through a fire retardant, reached the foamed plastic underneath, inside a firestop that had not been tested and certified in accordance with the final installation. In addition to fire hazard, substances that contain acetone (such as most aerosol paint sprays), and cyanoacrylate glues can dissolve polystyrene.

Environmental concerns and bans
Expanded polystyrene is not easily recyclable because of its light weight and low scrap value. It is generally not accepted in curbside programs. Expanded polystyrene foam takes 900 years to decompose in the environment[citation needed] and has been documented to cause starvation in birds and other marine wildlife.[citation needed] According to the California Coastal Commission, it is a principal component of marine debris. Restricting the use of foamed polystyrene takeout food packaging is a priority of many solid waste environmentalist organizations, like Californians Against Waste.

The city of Berkeley, California was one of the first cities in the world to ban polystyrene food packaging (called Styrofoam in the media announcements). It was also banned in Portland, OR, and Suffolk County, NY in 1990. Now, over 20 US cities have banned polystyrene food packaging, including Oakland, CA on Jan 1st 2007. San Francisco introduced a ban on the packaging on June 1 2007

"This is a long time coming. Polystyrene foam products rely on nonrenewable sources for production, are nearly indestructible and leave a legacy of pollution on our urban and natural environments. If McDonald's could see the light and phase out polystyrene foam more than a decade ago, it's about time San Francisco got with the program." Board of Supervisors President, Aaron Peskin

The overall benefits of the ban in Portland have been questioned,as have the general environmental concepts of the use of paper versus polystyrene. A campaign to achieve the first ban of polystyrene foam from the food & beverage industry in Canada has been launched in Toronto as of January 2007, by local non-profit organization NaturoPack.

The California and New York legislatures are currently considering bills which would effectively ban expanded polystyrene in all takeout food packaging state-wide.

Explosives
Polystyrene is used in some polymer-bonded explosives:
Some Polystyrene PBX Examples
Name Explosive Ingredients Binder Ingredients Usage
PBX-9205 RDX 92%
Polystyrene 6%; DOP 2%
PBX-9007 RDX 90%
Polystyrene 9.1%; DOP 0.5%; resin 0.4%
It is also a component of Napalm and a component of most designs of hydrogen bombs.
[edit] Cleaning
Polystyrene can be dishwashed at 70 °C without deformation since it has a glass transition temperature of 95 °C

References
1. ^ International Labour Organisation chemical safety card for polystyrene
2. ^ A.K. van der Vegt & L.E. Govaert, Polymeren, van keten tot kunstof, ISBN 90-407-2388-5
3. ^ The history of plastics
4. ^ process plastics Moulding Expanded Polystyrene
5. ^ Expanded polystyrene civil engineering products for roads, bridges and culverts: Vencel Resil
6. ^ Jed Norton. "Blue Foam, Pink Foam and Foam Board". Antenociti's Workshop. Retrieved on 2008-01-29.
7. ^ Styrene | Technology Transfer Network Air Toxics Web site | US EPA
8. ^ "Business Gives Styrofoam a Rare Redemption.", Stockton Record (21 September 2007). Retrieved on 2007-10-09.
9. ^ The Berkeley Daily Planet
10. ^ Styrofoam food packaging banned in Oakland
11. ^ Californians Against Waste website
12. ^ San Francisco Chronical article, June 28, 2006
13. ^ San Francisco Chronical article, November 7, 2006
14. ^ San Francisco Chronical Article, June 27, 2006
15. ^ Eckhardt, Angela (November, 1998). "Paper Waste: Why Portland's Ban on Polystyrene Foam Products Has Been a Costly Failure". Cascade Policy Institute. Retrieved on 2007-10-23.
16. ^ Thomas, Robert A. (March 8, 2005). "Where Might We Look for Environmental Heroes?". Center for Environmental Communications, Loyola University, New Orleans. Retrieved on 2007-10-23.
17. ^ Naturopack Campaign Page
18. ^ AB 904

source : wikipedia.org

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Soal dan Jawaban UAS Karet dan Plastik

1. Natural polymers are available in nature. Could you write several examples of natural polymers and their applications (UAS no 1 090824)

Jawab:

Silk: is applied mainly for expensive textiles
Natural rubber: is applied for many different industrial application such as tires, gaskets, belts etc
Wool: is applied mainly for textiles, jackets and winter clothes
Wood: is applied for furniture, timber and construction
Celluloses: are applied for paper, food etc
Starches: are applied for food and recently it is used for fuel as well
Proteins: are mainly for food
etc

2. When thermal energy is applied on polymer materials, they will show two different effects that are thermoplastic and thermosetting polymers. Could you explain what thermoplastic and thermosetting are and what is the different between them? (UAS no 2 090824)
Thermoplastic polymers: They can be softened or plasticized repeatedly on application of thermal energy, without much change in properties if treated with certain precautions, e.g. polyolefins, polystyrene, nylons, linear polyesters and polyethers, poly vinyl chloride, etc. They normally remain soluble and fusible after many cycles of heating and cooling. Thermoplastic polymers can normally be recycled.

Thermosetting polymers: They can be obtained in soluble and fusible forms in early or intermediate stages of theirs synthesis, but they get set or cured and become insoluble and infusible when further heated or thermally treated, the curing or setting process involves chemical reactions leading to further growth and cross linking of the polymer chain molecules and producing giant molecules, e.g. phenolic resins, urea/melamine resins, epoxy resins, diene rubbers, unsaturated polyesthers, etc.. Thermosetting polymers cannot normally be recycled.


3. Could you explain different kind of plastics and their application in the daily life? (UAS no 3 090824)
Polyethylene: is widely used in daily life such as moulded or formed objects, films, sheets, bottles and containers, pipes and tubes, and in wire insulation and cables.
Polypropylene: is used in the form of moulded and formed objects, sheets and films, bristles, monofilaments and fibres, covering such items as luggage, frames containers and different packaging items, ropes, textiles, tows and nets, pipes and tubes, etc
Polystyrene is used in packaging and shock absorbing application, in thermal insulation, and as acoustic improvers in hall and auditoria
Polymethil methacrylate is used for automotive tail lamp and signal light lenses, jewellery, lense of optical equipment and contact lenses
Polyvinylchloride is used in chemical plants and equipments, storage tanks, building items, pipes, sheets, specific moulded objects and containers
Dst


4. Could you explain advantages and disadvantages of both natural and synthetic rubbers? Please, write down several examples of synthetic rubbers? (UAS no 4 090824)
Natural rubber:
Advantages:
• Flexible
• Elastic
• Environmental friendly (biodegradable)
• Raw material is easy to get
Disadvantages:
• Normally weaker then synthetic rubber
• Less consistent due to the season and place
• More expensive
• Easy to react

Synthetic rubber:

Advantages:
• Strong
• Large range of synthetic material
• Consistent
• Cheaper

Disadvantages:
• Less flexible
• less elastic
• Fracture
• Not environmental friendly (non biodegradable material)

Examples of synthetic rubbers:
Styrene Butadiene Rubber (SBR), Polychloroprene Rubber (CR), Nitrile Butadiene Rubber (NBR), Isobutyl rubber (IIR), Ethylene Propylene Diene Terpolymer (EPDM), Polysulphide rubber (PSR) dst

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Polyethylene

Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.

Description

Polyethylene is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). The recommended scientific name polyethene is systematically derived from the scientific name of the monomer. In certain circumstances it is useful to use a structure–based nomenclature. In such cases IUPAC recommends poly(methylene). The difference is due to the opening up of the monomer's double bond upon polymerisation.

In the polymer industry the name is sometimes shortened to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. In the United Kingdom the polymer is commonly called polythene, although this is not recognised scientifically. The ethene molecule (known almost universally by its common name ethylene) C2H4 is CH2=CH2, Two CH2 groups connected by a double bond, thus:

Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. This is because ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.

Classification

Polyethylene is classified into several different categories based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight.
• Ultra high molecular weight polyethylene (UHMWPE)
• Ultra low molecular weight polyethylene (ULMWPE - PE-WAX)
• High molecular weight polyethylene (HMWPE)
• High density polyethylene (HDPE)
• High density cross-linked polyethylene (HDXLPE)
• Cross-linked polyethylene (PEX)
• Medium density polyethylene (MDPE)
• Low density polyethylene (LDPE)
• Linear low density polyethylene (LLDPE)
• Very low density polyethylene (VLDPE)

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight results in less efficient packing of the chains into the crystal structure as evidenced by densities of less than high density polyethylene (for example, 0.930–0.935 g/cm3). The high molecular weight results in a very tough material. UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Because of its outstanding toughness and its cut, wear and excellent chemical resistance, UHMWPE is used in a wide diversity of applications. These include can and bottle handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks and butchers' chopping boards. It competes with Aramid in bulletproof vests, under the tradenames Spectra and Dyneema, and is commonly used for the construction of articular portions of implants used for hip and knee replacements.

HDPE is defined by a density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. The lack of branching is ensured by an appropriate choice of catalyst (for example, chromium catalysts or Ziegler-Natta catalysts) and reaction conditions. HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes.

PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplast into an elastomer. The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced. PEX is used in some potable-water plumbing systems because tubes made of the material can be expanded to fit over a metal nipple and it will slowly return to its original shape, forming a permanent, water-tight, connection.

MDPE is defined by a density range of 0.926–0.940 g/cm3. MDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. MDPE has good shock and drop resistance properties. It also is less notch sensitive than HDPE, stress cracking resistance is better than HDPE. MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags and screw closures.

LLDPE is defined by a density range of 0.915–0.925 g/cm3. LLDPE is a substantially linear polymer with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene and 1-octene). LLDPE has higher tensile strength than LDPE, it exhibits higher impact and puncture resistance than LDPE. Lower thickness (gauge) films can be blown, compared with LDPE, with better environmental stress cracking resistance but is not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. Cable covering, toys, lids, buckets, containers and pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility and relative transparency.

LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

VLDPE is defined by a density range of 0.880–0.915 g/cm3. VLDPE is a substantially linear polymer with high levels of short-chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene and 1-octene). VLDPE is most commonly produced using metallocene catalysts due to the greater co-monomer incorporation exhibited by these catalysts. VLDPEs are used for hose and tubing, ice and frozen food bags, food packaging and stretch wrap as well as impact modifiers when blended with other polymers.

Recently much research activity has focused on the nature and distribution of long chain branches in polyethylene. In HDPE a relatively small number of these branches, perhaps 1 in 100 or 1,000 branches per backbone carbon, can significantly affect the rheological properties of the polymer.

Ethylene copolymers

In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples include vinyl acetate (the resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylates (applications include packaging and sporting goods).

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann who prepared it by accident in 1898 while heating diazomethane. When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy, substance that he had created they recognized that it contained long -CH2- chains and termed it polymethylene.

The first industrially practical polyethylene synthesis was discovered (again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the ICI works in Northwich, England. Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde they again produced a white, waxy, material. Because the reaction had been initiated by trace oxygen contamination in their apparatus the experiment was, at first, difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939.

Subsequent landmarks in polyethylene synthesis have revolved around the development of several types of catalyst that promote ethylene polymerization at more mild temperatures and pressures. The first of these was a chromium trioxide-based catalyst discovered in 1951 by Robert Banks and J. Paul Hogan at Phillips Petroleum. In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminium compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, however, and both methods are used in industrial practice.

By the end of the 1950s both the Phillips- and Ziegler-type catalysts were being used for HDPE production. Phillips initially had difficulties producing a HDPE product of uniform quality and filled warehouses with off-specification plastic. However, financial ruin was unexpectedly averted in 1957 when the hula hoop, a toy consisting of a circular polyethylene tube, became a fad among youth in the United States.

A third type of catalytic system, one based on metallocenes, was discovered in 1976 in Germany by Walter Kaminsky and Hansjörg Sinn. The Ziegler and metallocene catalyst families have since proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including very low-density polyethylene and linear low-density polyethylene. Such resins, in the form of fibers like Dyneema, have (as of 2005) begun to replace aramids in many high-strength applications.

Until recently the metallocenes were the most active single-site catalysts for ethylene polymerisation known—new catalysts are typically compared to zirconocene dichloride. Much effort is currently being exerted on developing new, single-site (so-called post-metallocene) catalysts that may allow greater tuning of the polymer structure than is possible with metallocenes. Recently work by Fujita at the Mitsui corporation (amongst others) has demonstrated that certain salicylaldimine complexes of Group 4 metals show substantially higher activity than the metallocenes.

Physical properties

Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 130 °C ((250 to 265 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (220 to 240 °F).

Most LDPE, MDPE and HDPE grades have excellent chemical resistance and do not dissolve at room temperature because of their crystallinity. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene.

Environmental issues

The wide use of polyethylene makes it an important environmental issue. Though it can be recycled, most of the commercial polyethylene ends up in landfills and in the oceans (notably the Great Pacific Garbage Patch). Polyethylene is not considered biodegradable, as it takes several centuries until it is efficiently degraded. Recently (May 2008) Daniel Burd, a 16 year old Canadian, won the Canada-Wide Science Fair in Ottawa after discovering that Sphingomonas, a type of bacteria, can degrade over 40% of the weight of plastic bags in less than three months. The applicability of this finding is still a matter for the future.


References
1. ^ A Guide to IUPAC Nomenclature of Organic Compounds, Blackwell Scientific Publications, Oxford (1993).
2. ^ a b J. KAHOVEC, R. B. FOX and K. HATADA; “Nomenclature of regular single-strand organic polymers (IUPAC Recommendations 2002);” Pure and Applied Chemistry; IUPAC; 2002; 74 (10): pp. 1921–1956.
3. ^ "Winnington history in the making". This is Cheshire. Retrieved on 2006-12-05.

source : wikipedia.org

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Natural Rubber

Natural rubber is an elastic hydrocarbon polymer that naturally occurs as a milky colloidal suspension, or latex, in the sap of some plants. It can also be synthesized. The entropy model of rubber was developed in 1934 by Werner Kuhn. The scientific name for the rubber tree is Hevea brasiliensis.
Explanation

The major commercial source of natural rubber latex is the Para rubber tree, Hevea brasiliensis (Euphorbiaceae). This is largely because it responds to wounding by producing more latex. Henry Wickham gathered thousands of seeds from Brazil in 1876 and they were germinated in Kew Gardens, England. The seedlings were sent to Ceylon (Sri Lanka), Indonesia, Singapore and British Malaya. Malaya(now Malaysia) was later to become the biggest producer of rubber. Liberia and Nigeria are examples of African rubber-producing countries.

Other plants containing latex include figs (Ficus elastica), Castilla (Panama rubber tree), euphorbias, lettuce, the common dandelion and specially Taraxacum kok-saghyz (Russian dandelion) contains good quantities, Scorzonera tau-saghyz and Guayule. Although these have not been major sources of rubber, Germany attempted to use some of these sources during World War II when it was cut off from rubber supplies[citation needed]. These attempts were later supplanted by the development of synthetic rubber.

Synthetic rubbers are made by the polymerization of a single monomer or a mixture of monomers to produce polymers. These form part of a broad range of products extensively studied by polymer science and rubber technology. Examples are SBR, or styrene-butadiene rubber, BR or butadiene rubber, CR or chloroprene rubber and EPDM (ethylene-propylene-diene rubber).

History

Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736.[1] In 1751 he presented a paper by François Fresneau to the Académie (eventually published in 1755) which described many of the properties of rubber. This has been referred to as the first scientific paper on rubber.

The first European to return to Portugal from Brazil with samples of such water-repellent rubberized cloth so shocked people that he was brought to court on the charge of witchcraft.
When samples of rubber first arrived in England, it was observed by Joseph Priestley, in 1770, that a piece of the material was extremely good for rubbing out pencil marks on paper, hence the name "rubber".

The para rubber tree initially grew in South America, where it was the main source of what limited amount of latex rubber was consumed during much of the 19th century. About 100 years ago, the Congo Free State in Africa was a significant source of natural rubber latex, mostly gathered by forced labor. After repeated efforts (see Henry Wickham) rubber was successfully cultivated in Southeast Asia, where it is now widely grown.

In India commercial cultivation of natural rubber was introduced by the British Planters, although the experimental efforts to grow rubber on a commercial scale in India were initiated as early as 1873 at the Botanical Gardens, Kolkata. The first commercial Hevea plantations in India were established at Thattekadu in Kerala in 1902.

Rubber latex.

Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect, the Payne effect and is often modeled as hyperelastic. Rubber strain crystallizes. Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking

Chemical makeup

Aside from a few natural product impurities, natural rubber is essentially a polymer of isoprene units, a hydrocarbon diene monomer. Synthetic rubber can be made as a polymer of isoprene or various other monomers. The material properties of natural rubber make it an elastomer and a thermoplastic. However it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed.

Elasticity

In most elastic materials, such as metals used in springs, the elastic behavior is caused by bond distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored electrostatically. Rubber is often assumed to behave in the same way, but it turns out this is a poor description. Rubber is a curious material because, unlike metals, strain energy is stored thermally.

In its relaxed state rubber consists of long, coiled-up polymer chains that are interlinked at a few points. Between a pair of links each monomer can rotate freely about its neighbour. This gives each section of chain leeway to assume a large number of geometries, like a very loose rope attached to a pair of fixed points. At room temperature rubber stores enough kinetic energy so that each section of chain oscillates chaotically, like the above piece of rope being shaken violently.

When rubber is stretched the "loose pieces of rope" are taut and thus no longer able to oscillate. Their kinetic energy is given off as excess heat. Therefore, the entropy decreases when going from the relaxed to the stretched state, and it increases during relaxation. This change in entropy can also be explained by the fact that a tight section of chain can fold in fewer ways (W) than a loose section of chain, at a given temperature (nb. entropy is defined as S=k*ln(W)). Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is not electrostatic, rather it is a result of the thermal energy of the material being converted to kinetic energy. Rubber relaxation is endothermic, and for this reason the force exerted by a stretched piece of rubber increases with temperature (metals, for example, become softer as temperature increases). The material undergoes adiabatic cooling during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it.

Stretching of a rubber band is in some ways equivalent to the compression of an ideal gas, and relaxation in equivalent to its expansion. Note that a compressed gas also exhibits "elastic" properties, for instance inside an inflated car tire. The fact that stretching is equivalent to compression may seem somewhat counter-intuitive, but it makes sense if rubber is viewed as a one-dimensional gas. Stretching reduces the "space" available to each section of chain.

Vulcanization of rubber creates more disulfide bonds between chains so it makes each free section of chain shorter. The result is that the chains tighten more quickly for a given length of strain. This increases the elastic force constant and makes rubber harder and less extendable.

When cooled below the glass transition temperature, the quasi-fluid chain segments "freeze" into fixed geometries and the rubber abruptly loses its elastic properties, though the process is reversible. This is a property it shares with most elastomers. At very cold temperatures rubber is actually rather brittle; it will break into shards when struck or stretched. This critical temperature is the reason that winter tires use a softer version of rubber than normal tires. The failing rubber o-ring seals that contributed to the cause of the Challenger disaster were thought to have cooled below their critical temperature. The disaster happened on an unusually cold day.

Current sources

Close to 21 million tons of rubber were produced in 2005 of which around 42% was natural. Since the bulk of the rubber produced is the synthetic variety which is derived from petroleum, the price of even natural rubber is determined to a very large extent by the prevailing global price of crude oil[citation needed]. Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005. The three largest producing countries (Indonesia, Malaysia and Thailand) together account for around 72% of all natural rubber production.

Cultivation

Rubber is generally cultivated in large plantations. See the coconut shell used in collecting latex, in plantations in Kerala. Rubber latex is extracted from Rubber trees. The economic life period of rubber trees in plantations is around 32 years – 7 years of immature phase and about 25 years of productive phase.

The soil requirement of the plant is generally well-drained weathered soil consisting of laterite, lateritic types, sedimentary types, nonlateritic red or alluvial soils.
The climatic conditions for optimum growth of Rubber tree consist of (a) Rainfall of around 250 cm evenly distributed without any marked dry season and with at least 100 rainy days per annum (b) Temperature range of about 20°C to 34°C with a monthly mean of 25°C to 28°C (c) High atmospheric humidity of around 80% (d) Bright sunshine amounting to about 2000 hours per annum at the rate of 6 hours per day throughout the year and (e) Absence of strong winds.
Many high-yielding clones have been developed for commercial planting. These clones yield more than 1,500 Kilogrammes of dry Rubber per hectare (or, over 4 tons per acre), per annum, when grown under ideal conditions

Collection

A tree woman in Sri Lanka in the process of harvesting rubber. In places like Kerala, where coconuts are in abundance, the shell of half a coconut is used as the collection container for the latex. The shells are attached to the tree via a short sharp stick and the latex drips down into it overnight. This usually produces latex up to a level of half to three quarters of the shell.

The latex can be either collected in its liquid state, in which case ammonia solution can be added to the collecting cup prior to tapping in order to prevent natural coagulation, or it can be left in the field to coagulate into a cup lump.

Latex is generally processed into either latex concentrate for manufacture of dipped goods or it can be coagulated under controlled, clean conditions using formic acid. The coagulated latex can then be processed into the higher grade technically specified block rubbers such as TSR3L or TSRCV or used to produce Ribbed Smoke Sheet grades.

Naturally coagulated rubber (cup lump) is used in the manufacture of TSR10 and TSR20 grade rubbers. The processing of the rubber for these grades is basically a size reduction and cleaning process in order to remove contamination and prepare the material for the final stage drying. The dried material is then baled and palletized for shipment.

Uses

The use of rubber is widespread, ranging from household to industrial products, entering the production stream at the intermediate stage or as final products. Tires and tubes are the largest consumers of rubber, accounting for around 56% total consumption in 2005. The remaining 44% are taken up by the general rubber goods (GRG) sector, which includes all products except tires and tubes.

Other significant uses of rubber are door and window profiles, hoses, belts, matting, flooring and dampeners (anti-vibration mounts) for the automotive industry in what is known as the "under the bonnet" products. Gloves (medical, household and industrial) are also large consumers of rubber and toy balloons, although the type of rubber used is that of the concentrated latex. Significant tonnage of rubber is used as adhesives in many manufacturing industries and products, although the two most noticeable are the paper and the carpet industry. Rubber is also commonly used to make rubber bands and pencil erasers.

Additionally, rubber produced as a fiber sometimes called elastic, has significant value for use in the textile industry because of its excellent elongation and recovery properties. For these purposes, manufactured rubber fiber is made as either an extruded round fiber or rectangular fibers that are cut into strips from extruded film. Because of its low dye acceptance, feel and appearance, the rubber fiber is either covered by yarn of another fiber or directly woven with other yarns into the fabric. In the early 1900’s, for example, rubber yarns were used in foundation garments. While rubber is still used in textile manufacturing, its low tenacity limits its use in lightweight garments because latex lacks resistance to oxidizing agents and is damaged by aging, sunlight, oil, and perspiration. Seeking a way to address these shortcomings, the textile industry has turned to Neoprene (polymer form of Chloroprene), a type of synthetic rubber as well as another more commonly used elastomer fiber, spandex (also known as elastane), because of their superiority to rubber in both strength and durability.
Hypoallergenic rubber can be made from Guayule.

Early experiments in the development of synthetic rubber also led to the invention of Silly Putty.
Natural rubber is often vulcanized, a process by which the rubber is heated and sulfur, peroxide or bisphenol are added to improve resilience and elasticity, and to prevent it from perishing. Vulcanization greatly improved the durability and utility of rubber from the 1830s on. The successful development of vulcanization is most closely associated with Charles Goodyear. Carbon black is often used as an additive to rubber to improve its strength, especially in vehicle tires.

References

1. ^ a b http://www.bouncing-balls.com/timeline/people/nr_condamine.htm
• Rubbery Materials and their Compounds by J.A Brydson
• Rubber Technology by Maurice Morton

source : wikipedia.org

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