Processes

Polymer processing overview

The processing of polymeric materials -plastics, elastomers and composites- is characterized by a wide variety of distinct methods or techniques. Techniques involving the continuous manufacture of a product basically have uniform cross section, which include extrusion, extrusion covering, film blowing and calendering; techniques involving the shaping of a deformable polymer perform against a mold surface, which involve coating and rotational molding; and, finally, techniques which involve the complete filling of a mold cavity, and include casting, compression molding, transfer molding, injection molding and reaction injection molding. 

Fundamental to the choice of polymer processing technique is the question of whether to use a high molecular mass starting material or a system that polymerizes in the mold. 

In the liquid state, most monomers and low molecular mass polymers flow in much the same way as molten metals in that the shear stress needed to make them flow is directly proportional to the shear strain rate- they are Newtonian fluids. As their molecular masses increase their viscosities increase but at some point the long thin chains begin to rearrange themselves under the applied shear stressed to line up in the direction of flow, and the proportionality between stress and strain rate starts to change- the polymer has become non-Newtonian. 

The consequences of the much higher pressures needed to cast high molecular mass polymers are not difficult to appreciate. In addition, the arrangements for keeping the mold closed will need to be more robust since the pressures applied to the mold tend to force the two mold halves open during filling and feeding. And the molds themselves must be made from stronger materials to withstand being repeatedly exposed to these pressures.

Reducing the viscosity of the polymer will clearly allow higher flow rates at the same applied pressures, or permit the use of substantial machinery and tooling. The polymer chains tangle around each other and form so-called mechanical cross links, effectively strengthening the material in the solid state but making it more difficult to cast in the fluid state. So the grade polymer, which is easy to cast, is going to give inferior performance in the end product and the best performance will be obtained from a material that is more difficult to cast. 

Some History

Mankind has used natural polymeric materials such as wood, leather and wool sincethe beginning of history, but synthetic polymers only became possible after the developmentof rubber technology in the 1800’s. The first synthetic polymer material, celluloid, wasinvented by John Wesley Hyatt in 1869, from cellulose nitrate and camphor. A majorbreakthrough in synthetic polymers was the invention of Bakelite by Leo Hendrik Baekelandin 1907. Hermann Staudinger’s work in the 1920’s clearly demonstrated the macromolecularnature of long chains of repeating units1. The word “polymer” comes from the Greek and itmeans “many parts”. The rapid growth of the polymer industry started shortly before theSecond World War, with the development of acrylic polymers, polystyrene, Nylon,polyurethanes and the subsequent introduction of polyethylene, polyethylene terephthalate,polypropylene and other polymers in the 1940’s and 1950’s. While only about 1 million tonswere produced in 1945, production of plastics in volume surpassed that of steel in 1981, andthe gap has been continuously growing ever since.

The world production of polymers increased from 27 million tons in 1975 to about 200 million tons per year in 2000 and is still growing. According to a recent report4,shipments of plastics products in the USA in 2000 amounted to $330 billion, and upstream supplying industries had sales of $90 billion, bringing the annual total to $420 billion. Total employment was estimated to be 2.4 million – about 2% of the U.S. workforce. The growth of the polymer industry is due to the unique combination of properties of plastic products  that include easy shaping and fabrication, low densities, resistance to corrosion, electricaland thermal insulation, and often favorable rigidity and toughness per unit weight

Current trend and future

The polymer industry experienced exponential growth during the last half of the 20th Century. There are about 50 resin producers around the world. The well known big chemicalc ompanies contributing most of the production volume of 200 million tons per year. Theprocessing industry, on the other hand, is fragmented to tens of thousands of small andmedium sized enterprises around the world. For example, Germany has 2500 plasticsprocessing companies. Most of the manufacturers of extruders, injection molding machines,and other types of equipment are also small or medium sized enterprises having less than500 employees. The growth of the plastics industry is likely to continue, especially indeveloping countries. Plastics consumption is likely to increase as more people around theworld try to satisfy their needs in transportation, food packaging, housing and electricalappliances. However, this industry is considered to have reached a stage of maturity.

Research and development efforts by the major resin producers have been severelycurtailed in recent years. The plastics processors and original equipment manufacturers arenot big enough to sustain major R&D programs that could lead to “quantum jumps” intechnology.

At a recent workshop of university and industry experts48, it was concluded thatfuture efforts should go beyond machinery design and process analysis and optimization.The focus should be on predicting and improving the product properties of polymer-basedproducts. The term “macromolecular engineering” was introduced as being more descriptiveof the future developments in the transformation of monomers into long chain molecules andtheir subsequent shaping or molding into numerous useful products.

The prediction of end-use properties of polymeric products is faced with some hugechallenges. The current process simulation approach, which is based on the continuummechanics of non-Newtonian fluids, must be combined with models describingmacromolecular conformations, relaxation and polycrystalline morphologies. The varioustypes of constitutive models, whether continuum, reptation, or pom-pom have had verylimited successes in predicting the unusual rheological phenomena exhibited by polymericliquids, even under isothermal conditions. Determination of heat transfer coefficients and modeling of flow-induced crystallization are necessary for the eventual prediction ofproperties of films and other extruded products. Numerous other problems remainunresolved in other polymer processes, such as the prediction of shrinkage, warpage andstress cracking in injection molding. The goal of precise property prediction is likely toremain a challenge for a considerable length of time. However, new technologies, evenwithout detailed scientific understanding, are likely to play a significant role in the field o fpolymers. These include: nanocomposites with exceptional properties, conductive plasticsfor electronics, self-assembly processes for the creation of special polymeric structures andfabrication of biomaterials and polymer-based tissue engineering.

Plastics are perceived as long lasting pollutants in the environment, because of theirdominant role is disposable items. Societal and legislative pressures for reuse and recyclingare likely to increase in the years to come. Plastics waste collection, reprocessing andburning for energy recovery are some technologies of current and future development