LON-CAPA OCHem

Polymers and Plastics

Table1

Links in the Chain

Monomer	Common Name	Polymer	Abbreviations	Comments	*Production^, lbs x 10^-9**
	ethylene	polyethylene	PE, LDPE, LLDPE, HDPE	Used in plastic bags, milk jugs	27.136
	propylene	polypropylene	PP, HDPP	Used in indoor-outdoor carpet, clothing	13.320
	styrene	polystyrene	PS	Used in food industry as packaging material.	6.380
	vinyl chloride	poly(vinyl chloride)	PVC	Used in plumbing and vinyl siding, Tygon^™ tubing.	14.084
	vinyl acetate	poly(vinyl acetate)	PVA (polyvinyl alcohol)	PVA, which is made from poly(vinyl acetate) is used as a water-soluble adhesive. It is also used in hair sprays.	......
	methyl methacrylate	poly(methyl methacrylate)	PMM	Commercially known as Plexiglass.	......
	acrylonitrile	polyacrylonitrile	PAN	Commercially known as Orlon.	......
	tetrafluoroethylene (perfluoroethylene)	poly(tetrafluoroethylene)	PFE	Commercially known as Teflon.	......

^* Source = Society of the Plastics Industry

The data in the last column represents the total US production and sales of polyethylene, polypropylene, poly(vinyl chloride), and polystyrene in billions of pounds for 1997.

PE = polyethylene; typical densities average 0.906 g/mL

HDPE = high density polyethylene; typical densities average 0.946 g/mL

LDPE = low density polyethylene; typical densities average 0.922 g/mL

LLDPE = linear low density polyethylene; typical densities average 0.918 g/mL

PP = polypropylene; typical densities average 0.902 g/mL for isotactic material, 0.900 g/mL for syndiotactic.

PVC - poly(vinyl chloride); typical densities average 1.400 g/mL

Additives and Plasticizers

If you look at Table 1, you will notice that poly(vinyl chloride) is used to make both rigid PVC pipe and flexible Tygon^™ tubing. Similarly, the polystyrene in food packaging materials is clear and brittle, while polystyrene "peanuts" are soft and multi-colored. How can the same material have such different properties? In the case of PVC the difference between rigid pipe and flexible tubing stems from the addition of plasticizers. These materials, which are added to the polymer after it has been prepared, are not chemically bonded to the polymer, but rather form a physical mixture with it. One of the most commonly used plasticizers is di-(2-ethylhexyl phthalate):

Exercise 1 What structural feature of di-(2-ethylhexyl) phthalate that confers flexibility on PVC?

This compound is also used in food packaging materials such as Saran Wrap^™. Public safety advocates have recently raised concerns about its use as a packaging material, especially for meats, because it has been shown to be carcinogenic when administered to laboratory animals in very large dosages. It is also suspected to disrupt hormone function in both men and women.

Exercise 2 The argument against using plastic wrap that contains di-(2-ethylhexyl) phthalate to package meat products contends that the fat in the meat leaches the plasticizer from the wrapping material, thereby contaminating the meat. Is this reasonable? Can you think of an adage that suggests the possibility?

In addition to plasticizers, many commercial polymers contain other additives, especially colorants. Two of the most common colorants are charcoal, for black plastics, and titanium dioxide, for white products. Gases are also used as additives. For example, mixing air into polystyrene leads to the formation of the "peanuts" used as packaging material. Air is also used in the manufacture of neoprene wetsuits. In both of these applications gas bubbles become entrapped in the polymer as its viscosity increases during the polymerization process.