Processing, thermoplastics introduction

As the author pointed out in the first edition of this book, the likelihood of discovering new important general purpose materials was remote but special purpose materials could be expected to continue to be introduced. To date this prediction has proved correct and the 1960s saw the introduction of the polysulphones, the PPO-type materials, aromatic polyesters and polyamides, the ionomers and so on. In the 1970s the new plastics were even more specialised in their uses. On the other hand in the related fields of rubbers and fibres important new materials appeared, such as the aramid fibres and the various thermoplastic rubbers. Indeed the division between rubbers and plastics became more difficult to draw, with rubbery materials being handled on standard thermoplastics-processing equipment. 

Polymers can be modified by the introduction of ionic groups [I]. The ionic polymers, also called ionomers, offer great potential in a variety of applications. Ionic rubbers are mostly prepared by metal ion neutralization of acid functionalized rubbers, such as carboxylated styrene-butadiene rubber, carboxylated polybutadiene rubber, and carboxylated nitrile rubber 12-5]. Ionic rubbers under ambient conditions show moderate to high tensile and tear strength and high elongation. The ionic crosslinks are thermolabile and, thus, the materials can be processed just as thermoplastics are processed [6]. 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

P-plastomers provide a unique combination of ease of processing, such that conventional thermoplastic-processing routines and arid equipment can be adapted to this polymer as weU as for a final fabricated product that is elastic. This combination of properties leads to the easy fabrication of elastic materials such as fibers and films, which traditionally have only been made inelastic by the use of thermoplastics. This advance opens the pathway to the introduction of desirable elastic properties to a host of fabrication processes very different from either the conventional rubber-processing equipment or the conventional rubber products, such as tires. P-plastomers and their fabricated products are not only soft, but also elastic. 

Plastomers represent a major advancement for polyolefins. Their success allows polyolefins to have a continuum of products from amorphous EPR to thermoplastic PE and iPP. This development coincides with the advent of single-site catalysts these are necessary for copolymers of components of widely different reactivity such as ethylene and octene. Their rapid introduction into the mainstream polymer use indicates that this spectrum of properties and the inherent economy, stability and processibility of polyolefins are finding new applications to enter. 

FIRE RETARDANT FILLERS. The next major fire retardant development resulted from the need for an acceptable fire retardant system for such new thermoplastics as polyethylene, polypropylene and nylon. The plasticizer approach of CP or the use of a reactive monomer were not applicable to these polymers because the crystallinity upon which their desirable properties were dependent were reduced or destroyed in the process of adding the fire retardant. Additionally, most halogen additives, such as CP, were thermally unstable at the high molding temperatures required. The introduction of inert fire retardant fillers in 1965 defined two novel approaches to fire retardant polymers. 

The FR properties of polymer-layered silicate nanocomposites have been studied for a wide range of polymers, especially for organomodified montmorillonites (OMMT) in thermoplastics. Depending on the nature of the polymer, the decomposition pathway and decomposition products may change.8 A major consequence of the introduction of modified clays is the formation or the enhancement of charred structure, caused by cross-linking processes possibly catalyzed by the nanoparticles. 

Blends of butadiene-acrylonitrile copolymer rubber (nitrile rubber or NBR) and PVC are among the oldest known examples of commercial elastomer/ thermoplastic blends. The shortage of natural rubber during World War II stimulated research in the USA on the compounding and modification of synthetic polymers to produce rubber-like materials. An outcome of this research was the commercial introduction of NBR/PVC blends by B.F. Goodrich in 1947 under the trade name of Geon Polyblends [Pittenger and Cohan, 1947]. The blend showed improved ozone resistance and melt processability compared to the nitrile rubber (Table 15.12). 

The introduction of gelatinized starch expanded considerably the use of starch as a polymeric material, but this technology based on water processing was limited by the necessity of more than one step, some of which were discontinuous. Nonetheless, these developments showed that it was possible to produce blends of synthetic thermoplastic materials with gelatinized starch, affording new prospects of the utilization of starch as a plastic material in the production of films and injection-moulded goods. 

Introduction of small amounts of ionic groups in hydrocarbon polymers exerts a profound effect on their mechanical properties. These ionic groups neutralized with suitable metal ions act as physical cross-hnks within the polymer matrix. The ionic associations can be thermally relaxed to permit sufficient melt flow at the processing temperature. The ion containing polymer thus behaves as a thermoplastic elastomer having the unique ability to act as cross-linked elastomer at ambient temperature, and to melt and flow at elevated temperatures like thermoplastics [50-53]. 

The most commonly used type of inhibited films is produced by joint processing of thermoplastic polymers and highly dispersed Cl powders. When the melting points, as well as the sublimation and thermal destruction temperatures, of the Cl exceed the viscous flow temperature of the thermoplast, the film acquires the structure of a polymer composite filled with solid inhibitor particles [101]. The particles may partially dissolve in the polymer melt and exert a plasticizing effect on the polymer. Introduction of 1-2% of NBA into PE elevates melt flow by 1.5-1.8 times [10]. Aggregation of solid Cl particles may present essential difficulties for their joint processing with polymer melts and result in structural defects or impair the quality of the film as a whole [4]. 

Introduction of Cl into a thermoplastic composition changes the kinetics of its thermal destruction processes. This is expressed in variations of the thermal resistance temperatures (Tt) and destruction initiation (Td) recorded by DTA. Tt and Td for the PE-based compositions shift to the region of lower temperatures by, correspondingly 1-50°C and 5-20°C. The activation energy (Ta) and macromolecular destruction found by DTA also diminish. Compositions based on HOPE show a reduction of Ta from 266 to 100-150kJ/mole, and the more efficient the antioxidizing effect of Cl towards HDPE the lower the Eg,. This is probably because the lower amount of oxide compounds in the polymer volume retards cross-linking of macromolecules and results in reduction. 

The following formulation illustrates the use of a redox initiator in a batch process. Also, the introduction of A-methylol acrylamide increases the molecular weight and chain cross-linking, minimizing the thermoplastic properties of the adhesive and the tendency to cold flow. 

Figure 33-4. Relationship between the growth, G (in % increase in production per year) and age, A (in years), of an industry since the introduction of the first large-scale technological process. For thermoplasts and thermosets, PL, this is the commercial phenolic resin synthesis, for glass, GL, it is the bottle production by molding process, for aluminum, Al, it is the electrolysis process, for paper, PR, it is the paper machine, and for iron, Fe, it is the use of coke. (After G. R. Snelling.)...

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