Molecule Movement

Molecule chain movement is dependent upon changes temperature. Movement of molecule chains reduce a material's density. For example, a small burning piece of polyethylene milk bottle will first expand, become transparent, melt, decompose to gas, and then burn. The process continues until the plastic is completely consumed or the flame is extinguished. The white smoke left after the flame is extinguished will have a similar odor to candle wax. The flame has reduced the long molecules of polyethylene to the short molecules of candle wax.

This change in polyethylene's structures is related to its molecular movement. As shown in Figure 2-22, the four types of movement are: (1) single atom vibration, (2) small groups of 4 to 6 atoms movement, (3) large segments moving in kinks and then unkinking, and (4) molecule sliding. The illustrations show segments of the molecular chain, with the hydrogen atoms removed for clarity.

These four movements predict the physical property of the materials. In single atom vibration, a plastic material becomes brittle. Acrylic plastic exhibits this behavior at room temperature. When groups of 4 to 6 atoms move in unison then produce a cold flow. Cold flow in a plastic is a measurable change in shape over several months to a year. Most plastic materials cold flow at room temperature. Plastics that exhibit the kinking and unkinking motion would be experiencing creep. Plastic creep is an observable change in shape, much like our polyethylene coffee can lid. A plastic material experiencing chain sliding is in a melt condition. A child's silly putty flattens when left on a table. It experiences a melt condition at room temperature. Figure 2-23 graphs these four type of movement relative to temperature and density in a polyethylene plastic.

Temperature Transitions

Plastic materials have two orders of temperature transiftion. The first order is the "freezing" temperature of the amorphous molecular structure. At this temperature the plastic's molecules are below the temperature that would produce movement in a single atom. At and below the glass transition temperature the plastics materials is brittle. The first order transition is called the glass transition temperature. The condition is similar to the freezing of water. All plastic materials have a glass transition temperature. However, the first order transition is only important in amorphous (transparent) plastics such as: acrylic, polycarbonate, PETE, polystyrene, celluslose acetate, polyacrylonitrile, and polyvinyl chloride.

Most plastic micro structures are crystalline and are considered crystalline materials. Only seven plastic materials (with over 90% amorphous structures) are considered amorphous materials. The first order transition, the glass transition temperature affects these materials. The understanding of the first order transition is important in our study of amorphous plastic material behavior.

The second order transition is the melt temperature. This second order transition affects the crystalline areas of the plastics materials. At or below this temperature the crystalline structures unravel and become amorphous structures. Once unraveled, the molecules begin to slide and melt. Second order transitions are important in understanding crystalline plastic material behavior. Table 2-5 lists the first order and second order temperatures for selected plastic materials.

Brittle Plastic

Plastic materials that are at or below their glass transition temperature are rigid and brittle. If we look at the energy in a segment of molecule which is at or below its glass transition temperature, its density is highest. At 32 degree F for polyethylene, atoms do not pull on the covalent bonds of the surrounding atoms. This kind of movement is slow because the amorphous structure is bound closely. If you place a low density polyethylene coffee can lid in the freezer, the temperature drops below its glass transition and the molecules will freeze. The soft and flexible polyethylene lid is now hard and brittle.

All plastics materials have glass transition temperatures. For some plastics like polypropylene - - it is -106 degree F. In contrast, Plexiglas is 200o F. Polypropylene, which looks and feels like polyethylene, would make an excellent ice cube try. In contrast, an ice cube try made of polyethylene would crack when the tray is bent to release the ice cubes. Transparent acrylic, with its high glass transition temperature is brittle at room temperature. An understanding of the glass transition temperature helps the designer to select the best materials for specific functions.

Plastic Cold Flow

A plastic part cold flow is defined as a measurable change in dimension over a long period of time. As illustrated in Figure 2-22, cold flow takes place when the motion in the atoms causes small groups of 4 to 6 atoms to begin moving in tandem. This type of movement takes place in the amorphous area of the plastic material. This movement was referred to earlier in this chapter as being like vines blown by a light breeze.

A plastics product that exhibits cold flow may change dimension three to ten thousandth of a inch over a period of several months to a year. This change can be accelerated if the product undergoes an external force. For example, a plastic screw that secures the continous force of a spring will losen over time.. Plastic products that experience cold flow during their life cycle are not suitable for mechanical parts such a gear, cams, levers, or tape winding hubs. However, they find wide application in snap fit, consumer food containers, and electronic product cases.

Plastic Creep

Plastics creep is defined as an observable change in dimension over a short period of time. Figure 2-22 illustrates plastics creep just above cold flow. As the temperature of the plastic material increases above cold flow, large segments along the molecule move up and down, pulling on the ends of the molecules. The amorphous areas expand and collapse very rapidly. The crystalline areas are being affected slightly as the molecules kink, and then unkink. This motion, of kinking and unkinking, is called plastics creep.

For example, a polyethylene coffee lid exposed to sun light represents a fine example of plastic creep. Plastic products that exhibit creep at room temperature, such as polyethylene, make excellent bags, reusable food containers, and snap fit lids. The plastics bags stretch and expand to hold various sized articles; a plastic lid snaps and fits easily and stretches without breaking.

Plastic Melt

At the second order transition, the melt temperature, one molecule slides past the adjacent molecule. The physical movement of one complete molecule freeing itself from the hold of the Van der Waal's force and sliding past an adjacent molecule is called a melt.

Melting a plastic is complex. The temperature rise must be transmitted through the amorphous structures and into the crystalline structures. The movement of the molecule in the amorphous area transmits energy to the crystalline structures very poorly. The crystalline structures expand, unravel, and become amorphous areas of the plastics. These new amorphous structures must continue to transmit temperature to the remaining crystalline structures. It is at this stage, that plastics enter the melt state. Once the molecules begin to slide into the melt stage, their movement can range from slow to fast. At the lower end of the melt range the plastic becomes soft and flows like taffy; at the upper end of the melt range, the plastic flows like hot pancake syrup.

Plastics, unlike water that has an exact melt temperature, have a melt range. The melting range is dependent upon the degree of crystallinity of the plastic. The higher the crystallinity the narrower the melt range.

In Figure 2-22 the melt range for low density polyethylene begins at 275o and decomposes at 675o F. High density polyethylene, with 25% more crystallinity, begins to melt at 475 degree F but decomposes at the same temperature. The wider the melt range, the easier the plastics materials is to mold and manufacture. Teflon, a plastic material with the highest percentage of crystallinity (about 96%), has a melt range of 30 to 60o F. It cannot be molded with conventional processes.

Plastic's Decomposition

The decomposition temperature for most plastics is approximately 675 to 700o F. At this temperature, the covalent bonds fail. The melt range for plastic materials narrows from the bottom up. The first three molecular movements described as: melt, creep and cold flow occur in amorphous areas. Crystalline areas must become unravelled to experience these molecular movements. It follows that the narrower the melt range the greater percentage of crystalline areas. The more crystalline areas, the less creep and cold flow, therefore, the more stable the product. In contrast, higher crystallinity means higher melt temperatures and the more difficult to mold. Finally, the higher density crystalline structures shrink more from the melt flow than amporphous areas and produce greater shrinkage in molded plastic parts.