USER'S GUIDE TO PLASTIC

Chapter 2 - Commodities

Polyethylene (PE)

Polyethylene or polyethene is a semi-crystalline commodity, denoted as PE. It is the most common plastic, and more than 60 million tons are manufactured each year worldwide, "Low-in 19s polyethylene (LDPE) was launched on the market by the British chemicals group ICI in 1939.

Classification

Polyethylene can be classified into different groups depending on its density and the lateral branches on the polymer chains:

UHMWPE - Ultrahigh molecular weight

HDPE - High density

MDPE - Medium density

LLDPE - Linear low density

LDPE - Low density

PEX - Cross-linked

Fig 14. When polymerizing ethylene to polyethylene, there are various processes resulting in more or less lateral branches on the molecular chains. A smaller number of lateral branches give a higher crystallinity, molecular weight, and density, since the chains can thus be packed more densely.HDPE has few or no lateral branches and is also called linear polyethylene.

Properties of Polyethylene:

+ Low material price and density   + Excellent wear resistance (UHMWPE)

+ Excellent chemical resistance        + Easy to color

+ Negligible moisture absorption      - Stiffness and tensile strength

+ Food-approved grades are available  -Cannot handle temperatures above 80°C

+ High elasticity down to<- 50°C      -Difficult to paint

The mechanical properties depend largely on the presence of lateral branches, cryalilt,and density, i.e. the type of polyethylene.

Recycling

Polyethylene is one of the most recycled plastic materials. Many of the bags, garbage bags,and dog bags that we use are made from recycled polyethylene. If you use the recycledmaterials in energy production, the energy content is on par with oil.When it comes to recycling, the following coding is used:

Application Areas

1) UHMWPE is processed mainly by extrusion into pipes, film, or sheets.

2)HDPE used for injection molding blow molding. extrusion film blowing, and rotational molding.

3) LDPE is used for film blowing and extrusion.

A large part of all the polyethylene produced is used for film blowing. If the film is soft and flexible, it is either made of LDPE or LLDPE. If it has the rustle of the free bags at the grocery store, it is probably made of HDPE. LLDPE is also used to improve the strength of LDPE film.

4) PEX

Cross-linked polyethylene is mainly used in the extrusion of tubes. The cross-linking provides improved creep resistance and better high-temperature properties.

You can even copolymerize ethylene with polar monomers and get everything from viscous products (e.g. melting glue) to tough films and impact -resistant hard shells such as golf balls.

A common copolymer is EVA (ethylene vinyl-acetate). By varying the concentration of vinyl acetate (VA) from 2.5 to 95%, you can control the properties and produce a range of different types of material. Increased VA content leads to higher transparency and toughness.

Adhesives, carpet underlay, cable insulation carriers of color masterbatches, stretch film, and coating film for cardboard and paper are typical uses of EVA.

Polypropylene (PP)

Polypropylene is a semi-crystalline commodity, denoted by- -and commonly referred to as--PP. It is also known as “polypropylene". It is the second-largest plastic on the market, after LDPE.

Polypropylene was discovered in 1954, almost simultaneously by two independent researchers Ziegler and Natta, who went on to share the Nobel Prize in 1963.

The Italian chemical company Montecatini launched the material on the market in1957.

The polymerization of polypropylene can control both crystallinity and molecule size. One can also copolymerize polypropylene with other monomers (e.g. ethylene).

Polypropylene can occur as a homopolymer, random or block copolymer depending on the polymerization method. Polypropylene can also be mixed with elastomers (e.g. EPDM), filled with talc (chalk), or reinforced with glass fiber. In this way it is possible to obtain more grades with widely differing characteristics than can be achieved for any other plastic. Certain grades of polypropylene can handle a continuous temperature of 100°C plus peaks of up to140°C and can therefore be classified as engineering plastics.

Properties of Polypropylene:

+ Low material cost and density               + Fatigue resistance

+ Excellent chemical resistance                - Poor UV resistance (unmodified)

+ Does not absorb moisture                     -Brittle at low temperatures (unmodified)

+ Food-approved grades are available    - Poor scratch resistance

Recycling

Recycling of polypropylene will preferably be made by material recycling and secondly by incineration for energy extraction. The recycling code for PP is the triangular recycling symbol with the number 5 inside or > PP < in terms of technical moldings:

Polyvinylchloride (PVC)

Polyvinylchloride is an amorphous commodity, denoted- and commonly referred to- -as PVC. It is the third largest type of plastic, with more than 20 million tonnes produced each year.

PVC was discovered in the 1800s, but did not come into commercial production until1936 when Union Carbide in the United States launched the material as a substitute for rubber in cable manufacturing.

In the production of PVC, you can use different polymerization methods, and at the compounding stage you can influence the properties more than with any other plastic, ranging from very soft (e.g. garden hose) to rigid and tough (e.g. waste pipes).

PVC is commonly distinguished as one of three different types: rigid, plasticized, and latex.

Properties of PVC:

+ Low material cost and density    + Food-approved grades are available

+ Excellent chemical resistance     + Self-extinguishing (when not plasticized)

+ Does not absorb moisture       + Good UV resistance

+ Resistant to microorganisms     - Hydrochloric acid is formed during

+ Good long-term strength         thermal decomposition (fire/burning)

Recycling

Recycling of PVC will preferably be made by material recycling and secondly by incineration for energy extraction, and the plastics industry is investing heavily to increase the recycled volumes.

The recycling code for PVC is the triangular recycling symbol with the number 3 inside: 

Polystyrene (PS)

Polystyrene is a crystal-clear amorphous commodity denoted by PS. Polystyrene has traditionally been the cheapest plastic to produce and is widely used in disposable products.

Polystyrene was discovered in 1839 but was not produced on a commercial scale until 1931 when it was launched by IG Farben in Germany.

In 1959 expanded styrene was developed, denoted as EPS. Styrofoam by Dow is the best-known brand of expanded styrene.

Classification

The polymerization of styrene results in a transparent, rigid, and hard plastic with a high gloss finish. Unfortunately, it is very brittle.

Sacrificing the transparency and stiffness, it can be mixed with 5- -10% butadiene rubber (BR) to get what is called high-impact polystyrene (HIPS), with an impact strength of up to five times higher than standard polystyrene.

Besides mixing polystyrene with other polymers, styrene can also be copolymerized with otner monomers to improve properties such as heat resistance, impact strength, stiffness, process ability, and chemical resistance. Some, common styrene plastics are:

• Styrene- butadiene plastic (SB)

• Acrylonitrile-styrene -acrylate (ASA)

• Styrene-acrylonitrile (SAN)

• Acrylonitrile-butadiene- styrene (ABS)

Properties of Polystyrene:

+ Low material cost                 -Brittle

+ High transparency (88%)           -Poor chemical resistance

+ Negligible moisture absorption      -Low softening temperature

+ Food-approved grades are available  -Turns yellow if left outdoors

+ High hardness and surface gloss

Recycling

Polystyrene is an easy material to recycle, and is coded:

Application Areas

Polystyrene can be injection molded and extruded. The extruded sheets can be vacuum formed.

Fig 32. Parts of polystyrene foam (EPS)

This material is about 80 times higher in volume than conventional polystyrene and is often used as an insulation material in the construction industry, or for disposable cups, shock absorbing packaging for electronic products etc.as well as floats.

EPS can also be extruded in films. Thicker films can be hot stamped and used for egg cartons, meat trays, and other food packaging.

Styrene -Acrylonitrile (SAN)

SAN is an amorphous plastic belonging to the styrene family. The material has higher strength, significantly improved chemical resistance, e.g. to fats and oils, and is less sensitive to stress-cracking than polystyrene. SAN has a slightly higher operating temperature compared to PS and also has a better outdoor durability.It is used to replace glass in some products and is often used in packaging for the cosmetics industry. Other application areas include household products, toothbrush handles, refrigerator interiors, and disposable medical products. The correct recovery code is > SAN <.

Acrylonitrile-Butadiene Styrene (ABS)

Acrylonitrile-butadiene-styrene is an amorphous copolymer, abbreviated to and commonly referred to as ABS. ABS was introduced onto the market in 1948.

ABS is manufactured by copolymerizing acrylonitrile and styrene in the presence of polybutadiene (latex rubber). Higher levels of acrylonitrile result in higher strength and better chemical resistance, but mean less butadiene particles resulting in lower impact strength.

The styrene contributes with a high surface gloss and good processing properties, and results in ABS fetching an attractive price.

ABS Blends

In addition to being able to control the properties of ABS by varying the concentration of the monomers, its properties can be further improved by blending with certain engineering plastics. Polycarbonate + ABS (PCIABS) or polyester + ABS (PBT/ABS) are standard mixtures, known as “plastic alloys." These mixtures fetch a lower price compared to pure polycarbonate or polyester PBT and can even be made flame-resistant.

PC/ABS blends combine the advantages of both plastics and result in a material with better flow properties and better temperature and UV resistance than that of pure ABS. Furthermore, PBT/ABS blends provide better resistance to chemicals (including gasoline) and dimensional stability at elevated temperatures than pure ABS. In the automotive industry blends of PBT/ABS are replacing ABS, PP, and PCIABS, due to the matte surface providing a better replication of textile surfaces than other plastics, which is much appreciated in interior paneling etc.

Properties of ABS:

+ Combines stiffness, strength, and toughness    + Good electrical insulation

+ Dimensionally stable under stress             + Can be made transparent

+ Does not absorb moisture                   - Heat resistance

+ Good surface gloss                         -Sensitive to stress-cracking

+ Easy to color and to paint                    -Poor UV resistance

+ Excellent for chrome plating                  -Solvent resistance

Recycling

ABS is an ideal material for recycling. The correct recycling code is > ABS < . Sometimes the packaging code is shown as A but this does not indicate the type of polymer used.

Application Areas

ABS can be injection molded and extruded. The extruded sheets can be vacuum formed. ABS is the most suitable plastic to chrome. During the chrome plating process, etching is used to remove small nitrile particles on the surface, resulting in the formation of small craters. The chromium then penetrates down into the craters, and you get an excellent adhesion between the metal surface and the ABS surface. Besides the aesthetic value, the scratch resistance is much improved. Even PC/ABS blends can be chrome plated. Car door handles, among other things, are made of chrome-plated PC/ABS blends.

Polymethyl Methacrylate (PMMA)

Most people don't know the term “PMMA,” but if you say “Plexiglas," which is the most famous brand name, everyone will know what you mean.

PMMA is an amorphous crystal-clear acrylic plastic. It was launched as a replacement for

glass in 1933 by Rohm & Haas in Germany under the name "Plexiglas".

PMMA has a density of 1.15 to 1.19 g/cm³, which is less than half the density of glass. The material made a breakthrough during World War Il in aircraft cockpit canopies.

Usually PMMA is not used in its pure form, but rather with various additives to improve such properties as:

• Heat stability and processability

• Toughness

• Higher operating temperature

• UV stability

Compared with polystyrene, PMMA has better impact resistance and UV resistance.

Compared with polycarbonate, PMMA has lower impact strength but offers greater value for the money. Compared with glass, PMMA is as high in transparency, lighter in weight, and has better impact resistance but lower scratch resistance.

PMMA is supplied as granules for injection molding and extrusion, or in semi-manufactured forms, i.e. sheets, rods, or tubes.

Properties of PMMA:

+ Very high transparency (98%)           -High thermal expansion coefficient

+ High rigidity and surface hardness       -Scratch resistance

+ Very good UV resistance               -Low resistance to stress-cracking

+ Good optical properties                -Solvent resistance

+ Can be used in implants                -High melt viscosity (difficult to fill thin walls)

Recycling

PMMA can be easily recycled, and is denoted by the recycling code > PMMA <.

Application Areas

PMMA can be injection molded and extruded. Semi-finished products in 1 MA can be processed with conventional machining. PMMA is superior to polycarbonate and polystyrene for laser marking.