Plastic's Micro structure

Understanding a plastic material's microstructure allows the designer to predict its behavior in product form. A plastic's micro structure predicts its physical, chemical, thermal, electrical, and weathering properties. A plastic's microstructure is significantly changed when heat, pressure, and manufacturing processing occur.

A plastic's microstructure implies the particular arrangement of a plastic's molecular chains. The microstructure is controlled by the shape and length of the molecules. However, the micro structure can be stretched, compressed, and changed during processing. Both the length and shape are formed during polymerization by the starting monomer, chemical additives, and process parameters. We will first consider how a molecule shape, and then length, are determined.

Monomers that begin the polymerization process play a primary role in determining a molecule's ultimate shape. Monomers with large side groups produce branched molecules. For example, styrene used for producing polystyrene, or methyl mathalatic used to produce acrylic, are inherently branched. A branched molecule has a side growth of a small molecule. The branch can be long or very short consisting of one carbons and three hydrogens. Figure 2-7 compares a branched and unbranched (called linear) molecules.

During polymerization, the molecules tend to grow side branches. These side branches reduce the physical properties and lower the stability of the plastic. To reduce the number of side branches, and to form more linear molecules during polymerization, time, temperature, and pressures are closely controlled.

The length of the molecule is controlled by chemical end groups. Chemical end groups are molecules that have just one reactive bond. Once an end group links to the growing molecule, it growth is stopped. The greater number of end groups, the shorter the molecule chain. Long molecules increase the strength and stability of the plastic. High pressures, low temperatures, and longer polymerization increase both the length and number of linear molecules. For example, the polymerization of high density polyethylene is three times longer than low density polyethylene.

High density polyethylene is a stiff plastic that can be formed into bottles, containers, and caps. It has molecular chains four times the length of low density polyethylene. Density refers to how the molecules form in the micro structures. Branched molecules form into random patterns call amorphous structures. Linear molecules form into an order called crystal structures. All plastic materials have both structures. The percentage of crystalline to amorphous structure in a plastics material is determine by the degree of branched and linear molecules.

Linear Molecules - Crystalline Micro structures

Linear molecules look like long strings. Plastic molecules bond at angles; without side branches, they tend to twist and then fold into crystals. Figure 2-8 illustrates how linear molecules fold into two crystal forms. One form is a flat crystal shape which forms part of a larger ribbon. The second form is pyramidal shape which is part of a larger crystal network. Both crystal forms are called lamellae. Lamellae are the fundamental building units for the formation of crystalline micro structures.

The flat crystal structures forms into uniform lamella ribbons. Figure 2-9 illustrates how the single molecules stack to form lamella ribbons. The lamella ribbons begin at a center point and build outward into sphere structures. These round three dimensional spheres are the principle form of crystal growth; The crystal growth spheres are called spherulites. Figure 2-10 shows spherulitic structure of polyethylene taken with an electron microscope. Note how it resembles a flat ribbon. Figure 2-11 shows the pyramidal structure in Teflon.

The crystalline areas compress the molecules as close as possible and increase the Van der Waal's forces to their maximum. Crystalline areas with the chains alligned do not allow the transmission of gas molecules or light rays. The number of crystal areas determines the density of the plastic, while the molecular chain length determines its strength. The higher the percentage of crystal areas, and the longer the molecular chains, the more stable the plastic material. Most plastic materials have over 80 percent crystalline structures. These plastics are referred to a crystalline materials.

Branched Molecules - Amorphous Micro structures

Branched molecules are often formed during polymerization. The branching can be formed by either side groups or long molecules that grow out from the side of a chain. The short branches are normally formed by monomer side groups. Side groups are small forms of molecules extending from two to six molecules out to the side. For example, the benzene ring on polystyrene, and the methyl group on polypropylene are examples of side groups. Figure 2-12 illustrates group structures and their characteristics. Also shown are side rings and rings in the molecule. Longer branches are often polymerized short chains that initiate on the side were a hydrogen atom should have bonded. Branched molecules appear similar to the growth of limbs and twigs. Figure 2-13 shows micro structure in cross section of long branched molecules formed around three crystalline areas.

Branched molecules cannot fold. The side groups and branches of the molecule inhibit the chain from compacting into tight crystalline areas. These irregularly branched chains form into random microstructures. These random structures are called amorphous, which mean "without order".

The random pattern of the chains look like cooked spaghetti. The amorphous areas are open with varying distances between molecules. The distances between chains affect the strength of the Van der Waal's force. The greater the distance, the weaker the force. Amorphous areas are more flexible than crystalline areas because of this difference in the strength of the Van der Waal's force.

Amorphous structures have lower melting points than crystal structures and fewer physical properties. Light can easily transmit itself through the amorphous areas; thus plastic materials with a high percentage of amorphous structures are transparent. Acrylic, polyvinyl chloride, cellulose acetate, polyacrylonitrile, polycarbonate, and polystyrene are all transparent and contain over 90 percent amorphous structures.

Thin film made from amorphous plastic transmits small molecules the size of gasoline, ethylene, and oxygen. Thin film polyethylene and polyvinyl chloride are used for translucent produce bags and meat packages. The thin film allows oxygen to transmit to the meat to limit spoiling. The transparency of the wrap allows easy consumer inspection and handling without contaminating the product. The thin plastic film protects the meat from the larger molecules of bacteria and airborne particles.

Side Group Arrangement

The arrangement of the side groups has considerable impact on the plastic properties. The arrangement of individuals side groups is called stereo isomerism. Steroisomers form rapidly into three distinct patterns: syndiotatic, isotactic, and atactic . The type of steroregularity is established during the polymerization reaction. No amount of twisting and turning of the chain about its bonds can convert one steroisomer into another. Figure 2-14 illustrates these three arrangements in polypropylene.

Syndiotactic and isotactic are uniform arrangement of the side groups. Syndiotatic alternates the placement of the side groups on either side of the molecule. Isotactic structures line up all groups on the same side of the molecule. Isotactic polypropylene forms readily into compact crystalline areas. Isotactic forms of polystyrene are opaque. The syndiotatic form of polystyrene is translucent. Atactic is a random arrangements of side groups. The location of the side group is not symmetrical; this lack of irregularity increases the number of amorphous areas in the polypropylene. The atactic form is transparent. Atactic polypropylene has a consistency somewhat like used chewing gum.

Cross Linked Micro structures

Cross linked molecules form networks linked together much like a fish net. A cross linked molecule network is illustrated in Figure 2-15. Cross links can be produced between molecule chains during polymerization, with chemicals, or by radiation.

During polymerization, a monomer with three bonds will form a three dimensional cross linked molecular structure. Chemical cross linking takes place between double bonds on the molecule's chain. Radiation cross linking is produced by eliminating two hydrogen atoms between adjacent molecule chains. The loss of the adjacent hydrogen produces a carbon to carbon cross link. The radiation process is produced by electron beam, gamma ray, or ultra violet light radiation from the sun. All cross linked plastics will not melt and are classified as thermosets.

These cross linked molecular chains resist change. Polyethylene gasoline tanks are chemically cross linked during molding to stabilize their molecular structure. The cross linked polyethylene restricts the transmission of gas fumes that normally would move easily through the amorphous structure of the polyethylene. Polyvinyl chloride wire coating is cross linked by radiation to improve its chemical and weathering resistance. Epoxy glue and polyester resins are chemically cross linked to form rigid bonds in composite materials. The constant exposure to ultraviolet sun light will eventually from cross links in plastic and displace hydrogen atoms; the weathered plastic then becomes brittle.

Processing And Micro Structure

Plastic materials are processed under great pressure and high stress. These pressures distort and compact a plastic's molecule structure. For example, during the process of injection molding, the melted plastic is forced through small restrictions, down channels, and into mold cavities. The melt is pushed from behind. Figure 2-16a shows the flow in an injection mold's channel. The material closest to the side wall of the channel and mold cavity slows due to cooling and friction. The plastic next to the wall hardens then forms a thin skin. Plastic closer to the center, cools slightly, and becomes thicker, forming a second thin layer. The two layers insulate the plastic at the center from the cold mold wall. Figure 2-16b shows the three layers. In the center, the plastic flows rapidly filling forward in the channel eventually filling the mold cavity.

The molecular structure in the outer skin and in the second layer form a distorted structure. The skin is frozen in an amorphous structure to a thickness of approximately 0.002 to 0.006 of an inch. The second layer of molecules begin to form crystal growth that stacks perpendicular to the mold wall. The crystal growth in the second layer is distorted as the transcrystalline layer. The transcrystalline layer has a thickness of approximately 0.034 to 0.038 of an inch. The plastic in the center is insulated and cools slowly to form the true crystalline spherulitic growth. The center portion is the spherulitic core. Figure 2-16c illustrates these three distinct structures in an extruded and molded liquid crystal polymer.

This molecular freezing, distortion, and slow cooling form a unique process structure called skin/core morphology. Morphology is the study of the form and structure of molecules. The injection pressure can severely distort the interior core. The over packing is called molded in stress. Molded in stress will cause the plastic part to distort after molding. The distortion can be immediate or take several months. Low pressures on the interior core will allow the plastic to shrink to the compact crystalline structures. The shrinkage will cause distortion of the plastic part surface. These distortion are blemishes called dimples. The requirement to maintain a balance between high and low pressure contribute to the art of molding plastic parts.