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A hydrogen atom is an atom of the chemical element hydrogen.

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Carbon is a chemical element with symbol C and atomic number 6.

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Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane

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Made of Natural Gas.

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It's only composed of carbon and hydrogen atoms.

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Poly(ethene) is made by several methods by addition polymerization of  ethene, which is principally produced by the cracking of ethane and propane, naphtha and gas oil.

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Well over 80 million tonnes of poly(ethene), often known as polyethylene and polythene, is manufactured each year making it the world's most important plastic. This accounts for over 60% of the ethene manufactured each year.

Poly(ethene) is produced in three main forms: low density (LDPE) (< 0.930 g cm⁻³) and linear low density ( LLDPE) ( ca 0.915-0.940 g cm⁻³) and high density (HDPE) ( ca 0.940-0.965 g cm⁻³).

The LDPE or LLDPE form is preferred for film packaging and for electrical insulation. HDPE is blow-moulded to make containers for household chemicals such as washing-up liquids and drums for industrial packaging. It is also extruded as piping.
HDPE LDPE/LLDPE
Figure 1 Uses of poly(ethene).

All forms can be used for injection-moulded products such as buckets, food boxes and washing-up bowls (Table 1).

Table 1 Examples of uses of poly(ethene).

1. Freedonia, 2014
2. US: 17.4 million tonnes in 2014. 2015 Guide to the Business of Chemistry, American Chemistry Council
3. 14.0 million tonnes in 2015, Plastics - the Facts 2016 PlasticsEurope 2016

4. Nexant and ChemVision, 2014
5. 2015 Guide to the Business of Chemistry, American Chemistry Council
6. Plastics- the Facts 2016, PlasticsEurope, 2016
7. LDPE plus LLDPE

* Many plants can produce both forms of poly(ethene) and alter the amount that they produce of each type at short notice. Both use a Ziegler (or Phillips) catalyst. If pure ethene is used, HDPE is formed. LLDPE is produced when a small amount of another alkene, for example but-1-ene, is added to the ethene.

Another form, discussed below, mLLDPE, is, at present, produced in much smaller quantities.

Manufacture of poly(ethene) (polyethylene)

Poly(ethene) is made by several methods by addition polymerization of ethene, which is principally produced by the cracking of ethane and propane, naphtha and gas oil.

A new plant is being constructed in Brazil for the production of poly(ethene), from ethene, that is made from sugar cane via bioethanol. This is sometimes termed bio-based poly(ethene) (bio-based polyethylene).

Low density poly(ethene) (LDPE)

The process is operated under very high pressure (1000-3000 atm) at moderate temperatures (420-570 K) as may be predicted from the reaction equation:

This is a radical polymerization process and an initiator, such as a small amount of oxygen, and/or an organic peroxide is used.

Ethene (purity in excess of 99.9%) is compressed and passed into a reactor together with the initiator. The molten poly(ethene) is removed, extruded and cut into granules. Unreacted ethene is recycled. The average polymer molecule contains 4000-40 000 carbon atoms, with many short branches.

For example,

It can be represented by:

There are about 20 branches per 1000 carbon atoms. The relative molecular mass, and the branching, influence the physical properties of LDPE. The branching affects the degree of crystallinity which in turn affects the density of the material. LDPE is generally amorphous and transparent with about 50% crystallinity. The branches prevent the molecules fitting closely together and so it has low density.

High density poly(ethene) (HDPE)

Two types of catalyst are used principally in the manufacture of HDPE:

a Ziegler-Natta organometallic catalyst (titanium compounds with an aluminium alkyl).

an inorganic compound, known as a Phillips-type catalyst. A well-known example is chromium(VI) oxide on silica, which is prepared by roasting a chromium(III) compound at ca 1000 K in oxygen and then storing prior to use, under nitrogen.

HDPE is produced by three types of process. All operate at relatively low pressures (10-80 atm) in the presence of a Ziegler-Natta or inorganic catalyst. Typical temperatures range between 350-420 K. In all three processes hydrogen is mixed with the ethene to control the chain length of the polymer.

(i) Slurry process (using either (continuous stirred tank reactor) or a )

The Ziegler-Natta catalyst, as granules, is mixed with a liquid hydrocarbon (for example, 2-methylpropane (isobutane) or hexane), which simply acts as a diluent. A mixture of hydrogen and ethene is passed under pressure into the slurry and ethene is polymerized to HDPE. The reaction takes place in a large loop reactor with the mixture constantly stirred (Figure 4). On opening a valve, the product is released and the solvent is evaporated to leave the polymer, still containing the catalyst. Water vapour, on flowing with nitrogen through the polymer, reacts with the catalytic sites, destroying their activity. The residue of the catalyst, titanium(IV) and aluminium oxides, remains mixed, in minute amounts, in the polymer.

Figure 5 The slurry process using a loop reactor.
By kind permission of Total.

Figure 4 The manufacture of poly(ethene) using the

slurry process in a loop reactor.

(ii) Solution process

The second method involves passing ethene and hydrogen under pressure into a solution of the Ziegler-Natta catalyst in a hydrocarbon (a C ₁₀ or C ₁₂ alkane). The polymer is obtained in a similar way to the slurry method.

(iii) Gas phase process

Figure 6 Low pressure gas-phase process.

A mixture of ethene and hydrogen is passed over a Phillips catalyst in a fixed bed reactor (Figure 6).

Ethene polymerizes to form grains of HDPE, suspended in the flowing gas, which pass out of the reactor when the valve is released.

Modern plants sometimes use two or more of the individual reactors in series (for example two or more slurry reactors or two gas phase reactors) each of which are under slightly different conditions, so that the properties of different products from the reactors are present in the resulting polymer mixture, leading to a broad or bimodal molecular mass distribution. This provides improved mechanical properties such as stiffness and toughness.

Figure 7 Granules of poly(ethene) which are then used to make film, extruded into pipes or moulded.
By kind permission of Total.

The HDPE powder coming out of any of the reactors discussed above is separated from the diluent or solvent (if used) and is extruded and cut up into granules.

This method gives linear polymer chains with few branches. The poly(ethene) molecules can fit closer together. The polymer chains can be represented thus:

This leads to strong intermolecular bonds, making the material stronger, denser and more rigid than LDPE. The polymer is not transparent.

Linear low density poly(ethene) (LLDPE)

Low density poly(ethene) has many uses but the high pressure method of manufacture by which it is produced has high capital costs. However, an elegant technique has been developed, based on both Ziegler-Natta and inorganic catalysts to produce linear low density poly(ethene) LLDPE, which has even improved properties over LDPE. Any of the three processes, slurry, solution and gas phase, can be used when a Ziegler-Natta catalyst is chosen. The gas phase process is used when the inorganic catalyst is employed.

Small amounts of a co-monomer such as but-1-ene or hex-1-ene are added to the feedstock. The monomers are randomly polymerized and there are small branches made up of a few carbon atoms along the linear chains.

For example, with but-1-ene, CH ₃CH ₂CH=CH ₂, the structure of the polymer is:

The side chains are known as pendant groups, or short chain branching. The molecule can be represented as:

The structure is essentially linear but because of the short chain branching it has a low density. The structure gives the material much better resilience, tear strength and flexibility without the use of plasticisers. This makes linear low density poly(ethene) an ideal material for the manufacture of film products, such as those used in wrappings.

The properties of the polymer, and hence its uses, can be varied by varying the proportion of ethene and co-monomer and by using different co-monomers. All this can be done without shutting down the plant, an enormous advantage.

Metallocene linear low density poly(ethene) (mLLDPE)

Figure 8 Poly(ethene) film is used extensively for wrapping foods.
By kind permission of BP.

This poly(ethene), known as mLLDPE, is produced by a new family of catalysts, the metallocenes. Another name for this family is single site catalyst. The benefit is that the mLLDPE is much more homogenous in terms of molecular structure than classical LLDPE produced by Ziegler-Natta catalysts. Each catalyst is a single site catalyst which produces the same PE chain. Chemists have compared the structure of metallocenes to that of a sandwich. There is a transition metal (often zirconium or titanium) 'filling' a hole between layers of organic compounds.

The catalysts are even more specific than the original Ziegler-Natta and it is possible to control the polymer's molecular mass as well as its configuration. Either the slurry or solution processes are usually used.

Poly(ethene) produced using a metallocene can be used as very thin film which has excellent optical properties and sealing performance, thus making them very effective for wrapping foods. The real plus for the metallocene catalysts are the enhanced mechanical properties of the films made out of mLLDPE.

Co-polymers

Ethene forms co-polymers with propene which have very useful properties.

Date last amended: 27th April 2017

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Well over 80 million tonnes of poly(ethene), often known as polyethylene and polythene, is manufactured each year making it the world's most important plastic. This accounts for over 60% of the ethene manufactured each year.

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(PP) is a thermoplastic “addition polymer” made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic

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Polypropylene (PP) is a thermoplastic "addition polymer" made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges , and textiles.

Polypropylene was first polymerized in 1951 by a pair of Phillips petroleum scientists named Paul Hogan and Robe rt Banks and later by Italian and German scientists Natta and Rehn. It became prominent extremely fast, as commercial production began barely three years after Italian chemist, Professor Giulio Natta, first polymerized it.

Natta perfected and synthesized the first polypropylene resin in Spain in 1954, and the ability of polypropylene to crystallize created a lot of excitement. By 1957, its popularity had exploded and widespread commercial production began across Europe. Today it is one of the most commonly produced plastics in the world.

CNC Cut Polypropylene Living Hinge Prototype Child Safe Lid by Creative Mechanisms

According to some reports, the current global demand for the material generates an annual market of about 45 million metric tons and it is estimated that the demand will rise to approximately 62 million metric tons by 2020.

The major end users of polypropylene are the packaging industry, which consumes about 30% of the total, followed by the electrical and equipment manufacturing, which uses about 13% each. Household appliances and automotive industries both consume 10% each and construction materials follows with 5% of the market.

Other applications together make up the rest of the global polypropylene consumption.

Polypropylene has a relatively slippery surface which can make it a possible substitute for plastics like Acetal (POM) in low friction applications like gears or for use as a contact point for furniture.

Perhaps a negative aspect of this quality is that it can be difficult to bond Polypropylene to other surfaces (i.e. it does not adhere well to certain glues that work fine with other plastics and sometimes has to be welded in the event that forming a joint is required).

Although polypropylene is slippery at the molecular level, it does have a relatively high coefficient of friction - which is why acetal, nylon, or PTFE would be used instead. Polypropylene also has a low density relative to other common plastics which translates to weight savings for manufacturers and distributors of injection molded Polypropylene parts.

It has exceptional resistance at room temperature to organic solvents like fats but is subject to oxidation at higher temperatures (a potential issue during injection molding ).

One of the major benefits of Polypropylene is that it can be manufactured (either through CNC or ) into a injection molding, thermoforming, or crimping living hinge . Living hinges are extremely thin pieces of plastic that bend without breaking (even over extreme ranges of motion nearing 360 degrees).

They are not particularly useful for structural applications like holding up a heavy door but are exceptionally useful for non load-bearing applications such as the lid on a bottle of ketchup or shampoo. Polypropylene is uniquely adept for living hinges because it does not break when repeatedly bent.

One of the other advantages is that polypropylene can be CNC machined to include a living hinge which allows for faster prototype development and is less expensive than other prototyping methods. Creative Mechanisms is unique in our ability to machine living hinges from a single piece of polypropylene.

Another advantage of Polypropylene is that it can be easily copolymerized (essentially combined into a composite plastic) with other polymers like polyethylene . Copolymerization changes the material properties significantly, allowing for more robust engineering applications than are possible with pure polypropylene (more of a commodity plastic on its own).

The characteristics mentioned above and below mean that polypropylene is used in a variety of applications: dishwasher safe plates, trays, cups, etc, opaque to-go containers, and many toys.

What are the Characteristics of Polypropylene?

Some of the most significant properties of polypropylene are:

1. Chemical Resistance: Diluted bases and acids don't react readily with polypropylene, which makes it a good choice for containers of such liquids, such as cleaning agents, first-aid products, and more.

2. Elasticity and Toughness: Polypropylene will act with elasticity over a certain range of deflection (like all materials), but it will also experience plastic deformation early on in the deformation process, so it is generally considered a "tough" material. Toughness is an engineering term which is defined as a material's ability to deform (plastically, not elastically) without breaking..

3. Fatigue Resistance: Polypropylene retains its shape after a lot of torsion, bending, and/or flexing. This property is especially valuable for making living hinges.

4. Insulation: polypropylene has a very high resistance to electricity and is very useful for electronic components.

5. Transmissivity: Although Polypropylene can be made transparent, it is normally produced to be naturally opaque in color. Polypropylene can be used for applications where some transfer of light is important or where it is of aesthetic value. If high transmissivity is desired then plastics like Acrylic or Polycarbonate are better choices.

Polypropylene is classified as a "thermoplastic" ( as opposed to "thermoset" ) material which has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (roughly 130 degrees Celsius in the case of polypropylene).

A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polypropylene liquefy, which allows them to be easily injection molded and then subsequently recycled.

By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to h eat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.

Why is Polypropylene used so often?

Polypropylene is used in both household and industrial applications. Its unique properties and ability to adapt to various fabrication techniques make it stand out as an invaluable material for a wide range of uses.

Another invaluable characteristic is polypropylene's ability to function as both a plastic material and as a fiber (like those promotional tote bags that are given away at events, races, etc).

Polypropylene's unique ability to be manufactured through different methods and into different applications meant it soon started to challenge many of the old alternative materials, notably in the packaging, fiber, and injection molding industries. Its growth has been sustained over the years and it remains a major player in the plastic industry worldwide.

At Creative Mechanisms , we have used polypropylene in a number of applications across a range of industries. Perhaps the most interesting example includes our ability to CNC machine polypropylene to include a living hinge for prototype living hinge development .

Polypropylene is a very flexible, soft material with a relatively low melting point. These factors have prevented most people from being able to properly machine the material. It gums up. It doesn't cut clean. It starts to melt from the heat of the CNC cutter. It typically needs to be scraped smooth to get anything close to a finished surface.

But we have been able to solve this problem which allows us to create novel prototype living hinges out of polypropylene. Take a look at the video below:

What Are The Different Types of Polypropylene?

There are two main types of polypropylene available: homopolymers and copolymers. The copolymers are further divided into block copolymers and random copolymers.

Each category fits certain applications better than the others. Polypropylene is often called the "steel" of the plastic industry because of the various ways in which it can be modified or customized to best serve a particular purpose.

This is usually achieved by introducing special additives to it or by manufacturing it in a very particular way. This adaptability is a vital property.

Homopolymer polypropylene is a general-purpose grade. You can think of this like the default state of the polypropylene material. Block copolymer polypropylene has co-monomer units arranged in blocks (that is, in a regular pattern) and contain anywhere between 5% to 15% ethylene.

Ethylene improves certain properties, like impact resistance while other additives enhance other properties.

Random copolymer polypropylene - as opposed to block copolymer polypropylene - has the co-monomer units arranged in irregular or random patterns along the polypropylene molecule.

They are usually incorporated with anywhere between 1% to 7% ethylene and are selected for applications where a more malleable, clearer product is desired.

How is Polypropylene made?

Polypropylene, like other plastics, typically starts with the distillation of hydrocarbon fuels into lighter groups called "fractions" some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation).

Polypropylene for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines:

3D Printing Polypropylene:

Polypropylene is not readily available in filament form for 3D printing.

CNC Machining Polypropylene:

Polypropylene is widely used as sheet stock for CNC machine manufacturing. When we prototype a small number of polypropylene parts we typically CNC machine them.

Polypropylene has gained a reputation as a material that cannot be machined. This is because it has a low annealing temperature, which means that it starts to deform under heat. Because it is a very soft material in general, it requires an extremely high skill level to be cut with precision. Creative Mechanisms has been successful in doing so.

Our teams can use a CNC machine and cut the polypropylene cleanly and with extremely great detail. In addition, we are able to create living hinges with polypropylene that have thickness as little as .010 inches. Making living hinges is a difficult endeavor on its own, which makes using a difficult material like polypropylene even more impressive.

Injection Molding Polypropylene:

Polypropylene is a very useful plastic for injection molding and is typically available for this purpose in the form of pellets. Polypropylene is easy to mold despite its semi-crystalline nature, and it flows very well because of its low melt viscosity.

This property significantly enhances the rate at which you can fill up a mold with the material. Shrinkage in polypropylene is about 1-2% but can vary based on a number of factors, including holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives.
Other:
In addition to the conventional plastic applications, polypropylene also lends itself well to fiber applications. This gives it an even wider range of uses that go beyond just injection molding. Those include ropes, carpets, upholstery, clothing, and the like.

Image From AnimatedKnots.com

What are the Advantages of Polypropylene?

1. Polypropylene is readily available and relatively inexpensive.

2. Polypropylene has high flexural strength due to its semi-crystalline nature.

3. Polypropylene has a relatively slippery surface.

4. Polypropylene is very resistant to absorbing moisture.

5. Polypropylene has good chemical resistance over a wide range of bases and acids.

6. Polypropylene possesses good fatigue resistance.

7. Polypropylene has good impact strength.

8. Polypropylene is a good electrical insulator.

What are the Disadvantages of Polypropylene?

1. Polypropylene has a high thermal expansion coefficient which limits its high temperature applications.

2. Polypropylene is susceptible to UV degradation.

3. Polypropylene has poor resistance to chlorinated solvents and aromatics.

4. Polypropylene is known to be difficult to paint as it has poor bonding properties.

5. Polypropylene is highly flammable.

6. Polypropylene is susceptible to oxidation.

Despite its shortcomings, polypropylene is a great material overall. It has a unique blend of qualities that aren't found in any other material which makes it an ideal choice for many projects.

What are the properties of Polypropylene?

Chemical Formula

(C 3 H 6 ) n

Resin Identification Code (Used For Recycling)

*At standard state (at 25 °C (77 °F), 100 kPa) ** *** Source data Source data

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Everything You Need To Know About Polypropylene (PP) Plastic

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Polypropylene (PP) is a thermoplastic "addition polymer" made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges , and textiles.