Compounding Thermoplastic Polymers

Handbook for the Chemical Analysis of Plastic and Polymer Additives 

In some cases, polymers may be compounded from simpler starting compounds formed with two or more of the raw materials. These are identified as master batches and are often prepared when bulk addition to a single stage process would cause them to separate or to alter the bulk polymer properties during mixing to the point where mixing would be inefficient. 

Thermoplastic compounds are most commonly supplied in the form of pellets. These are formed by extrusion of the polymer through a strand-forming die plate with a pattern of round holes. This is ran under water, in a water-spray environment, or in a chilled airstream. As the hot polymer is extruded, a continuously rotating blade cuts the polymer strands into short segments, thus forming the pellet geometry. With water-cooled pelletizing, the quality of the quench water must be controlled to prevent contamination. 

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Another innovation in the 1990s involved compounding thermoplastic polymers and cellulosic materials along with other ingredients into a feedstock in the form of durable, easy to transport, and durable pellets. 

This type of adhesive is generally useful in the temperature range where the material is either leathery or mbbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mosdy uncompounded state. 

Polyesters are known to be produced by many bacteria as intracellular reserve materials for use as a food source during periods of environmental stress. They have received a great deal of attention since the 1970s because they are biodegradable, can be processed as plastic materials, are produced from renewable resources, and can be produced by many bacteria in a range of compositions. The thermoplastic polymers have properties that vary from soft elastomers to rigid brittie plastics in accordance with the stmcture of the pendent side-chain of the polyester. The general stmcture of this class of compounds is shown by (3), where R = CH3, n = >100, and m = 0-8. 

Performance requirements, environmental issues, and avaUabUity/cost of the material will mainly drive material requirement in the future. In order to face the huge tire wastage problem causing major hazards to the environment, future development in mbbery materials will be focused on development of thermoplastic polymer so that used polymer could be recovered by thermal treatment and separation, biological degradation by radiation/addition of chemical into the mbber compound that could be activated by exposure to radiation and development of biopolymer. 

In the present study, a new way of introducing a non-reactive bromine-containing compound, ammonium bromide, is discussed. The treatment, which can be carried out for many thermoplastic polymers 17,8] in the solid state, is performed in two consecutive stages ... 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Polymers are often divided according to whether they can be melted and reshaped through application of heat and pressure. These materials are called thermoplastics. The second general classification comprises compounds that decompose before they can be melted or reshaped. These polymers are called thermosets. While both thermoset and thermoplastic polymers can be recycled, thermoplastic recycling is easier and more widespread because thermoplastic materials can be reshaped simply by application of heat and pressure. 

First introduced industrially in the 1930s, thermoplastic polymers are today produced and consumed in vast quantities and play a major role in many aspects of our everyday lives. It is estimated that over 16 million tons were consumed in Western Europe alone in 1991 [1]. Mineral fillers have, since the beginning, made an important contribution to the spectacular growth of thermoplastic polymers. The addition of mineral materials was initially seen mainly as a means of extending or reducing the compound cost but, as the relative cost of the polymers decreased, this became less important and attention was more and more focused on the property improvements that could be achieved. 

Continuity can also be reached by polymerizing one of the components within the other. In such a case the blend is called an IPN, an interpenetrating network it is, in most cases formed by a thermoset in a thermoplastic polymer. An example is a compound built-up from 50% of a thermoplast (polycarbonate or polysulphone), and 50% of a cross-linked polymer on the basis of dicyanate bisphenol-A. The skeleton... 

The above thermal analysis studies demonstrated the enhanced thermal stability of POSS materials, and suggested that there is potential to improve the flammability properties of polymers when compounded with these macromers. In a typical example of their application as flame retardants, a U.S. patent39 described the use of preceramic materials, namely, polycarbosilanes (PCS), polysilanes (PS), polysilsesquioxane (PSS) resins, and POSS (structures are shown in Figure 8.6) to improve the flammability properties of thermoplastic polymers such as, polypropylene and thermoplastic elastomers such as Kraton (polystyrene-polybutadiene-polystyrene, SBS) and Pebax (polyether block-polyamide copolymer). 

Inulin etherification products have utility in cosmetics or pharmaceuticals as carriers for water-insoluble substances or to stabilize aqueous solutions of compounds with low water solubility. They may also be used as emulsifiers or as an additive in textiles and paper and as softeners of thermoplastic polymers (Kunz and Begli, 1995). 

Phosphine oxides, phosphonic acids, and phosphinic acids have been found to be flame retardants for various thermoplastic polymers. While there are many reasons for their effectiveness, we postulate that the acidity of the compounds is directly related to their activity and that the formation of polyphosphates (or phosphate glasses) is vital to the mechanism by which they function. 

In our studies we found that phosphonic acids (16), phosphinic acids (25), and phosphine oxides (17) are additives capable of imparting fire retardant properties to thermoplastic polymers. Tables I and II present data for some of these compounds when added to polyethylene or to poly (methyl methacrylate). The concentration reported is not necessarily the lowest effective concentration for the additive in the polymer. These additives also were effective in other thermoplastic polymers such as polystyrene, impact polystyrene, polypropylene and ABS. The compounds were completely compatible with the polymers. 

Melt processing is a common alternative that is particularly useful for dealing with thermoplastic polymers and holds great interest because of the ease with which the process could be scaled up to industrial standards. Thermoplastic polyurethane nanocomposites can be fabricated by melt compounding of CNTs with polymer resin. Melt processing makes use of the fact that thermoplastic polymers soften when heated. Amorphous polymers like elastomer... 

This chapter addresses three basic classes of polymers and the approaches for processing them into compounds. These classes include thermoplastic polymers, and two types of elastomers -crosslinked elastomers, and thermoplastic elastomers. Compounds prepared from each class have a range of achievable properties, and each category of compounds may have overlapping properties. Each category is prepared by different technical approaches with varying controls, energy requirements, and limitations. A brief definition of each class follows. Also included, later in the chapter, is a detailed description of how additives influence the production process. 

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