Binder thermoplastic system

One of the main advantages of thermoplastic systems for ceramic extrusion is the lower abrasivity of the feedstock material relative to other binder systems. The lower abrasivity of a extrusion feedstock prepared with a thermoplastic hinder system compared to one with a solvent-based binder for a given ceramic powder, in this case AI2O3, is shown Fig. 1. In this study a feedstock with 58 vol.-% coarse AI2O3 powder was extruded through a steel die. Looking at the surfaces of the tubes, discoloration of the material stemming from abrasion of the die can he clearly... 

B. D. Nahloosky and G. A. Zimmerman, Thermoplastic Elastomers for S olid Propellant Binders, AERPL-TR-86-069, Aerojet Tactical Systems Co., Sacramento, Calif., Dec. 1986. 

The binder system of a plastic encapsulant consists of an epoxy resin, a hardener or curing agent, and an accelerating catalyst system. The conversion of epoxies from the Hquid (thermoplastic) state to tough, hard, thermoset soHds is accompHshed by the addition of chemically active compounds known as curing agents. Flame retardants (qv), usually in the form of halogens, are added to the epoxy resin backbone because epoxy resins are inherently flammable. 

Wise and Rocchio [32] have discussed the processing techniques for LOVA formulations which invariably depend on the type of binder. The polybutadiene-based formulations referred to as cured systems, are processed through a solventless process whereas formulations based on cellulose derivatives and thermoplastic elastomers as binders are processed by a solvent process similar to standard NC propellants. In conclusion, LOVA formulations offer unique propulsion systems for tanks with the potential to offer high energy and low vulnerability. 

As an additional component, various thermoplastic polymers can be used. As a binder for copper clad laminates, a solution of solid epoxide resin (Epikote 1001), BPA/DC prepolymer, Zn acetate and poly(phenylene sulfide) was used [83], Other binders for reinforced plastics contain polysulfone. Such compositions consist of liquid BPA/ECH epoxide resin, BPA/DC prepolymer, polysulfone and bis(4-hydro-xyphenyl)sulfone [85]. Bis(4-aminophenyl)sulfone can be also added [86]. In such systems the bisphenol reacts with the epoxide resin as a chain extension agent, whereas the diamine crosslinks the diepoxide. The Tg values are close to 200 °C. They can be increased a little, if the BPA/ECH epoxide resin is replaced by the tetra-epoxide A,A,A, A -tetrakis(2,3-epoxypropyl)diaminodiphenylmethane [87]. 

As a binder system polymers are utilized. If the binders contain energy or gas-producing molecular groups (-N02, -N3), one classifies the binders as Active Binders (e.g. polynitropolyphenylene, glycidyl azide polymer, polyvinyl nitrate and nitrocellulose). If these substances are not present, then the binders are classified as inert binders. Depending on available processing methods, binder types such as thermoset material, thermoplast or gelatinizers can be used. They can then be formed and cured by chemical or physical means. 

Sanderson [2] prepared the energetic thermoplastic elastomer poly(3,3-bis (azidomethyl)-oxetane), (II), for use as a binder for a propellant, explosive, or gas generant for a supplemental restraint system in automobiles. Random block copolymers of poly(azidomethyloxirane) and poly(3,3-bis(azidomethyl)oxe-tane), (III), were also prepared by Sanderson [3] using toluene diisocyanate as the coupling agent. 

It should be remembered that the practical energy density Eg in metal-free batteries is often closer to the theoretical value than in the case of conventional systems. This can be rationalized in terms of higher active mass utilizations through thin-layer technology (via thermoplastic binders, for instance), lighter current collectors (at least in bipolar systems), and so on. The lead-acid accumulator has a ratio a = Eg sh)/Es,th 15% thus Eg — 25 Wh/kg. But a metal-free system with s,th = 80 Wh/kg may allow Eg = 40 Wh/kg, if a is 50% in this way. 

In practice, binder system may consist of three or four additives that differ in their volatility and chemical decomposition. Furthermore, interactions between the binder and the particle surfaces may alter the decomposition kinetics of the pure polymer. In view of its complex nature, a detailed analysis of thermal debinding is not useful. Instead, we consider the basic features of the process for a simplified system consisting of a powder compact with a single binder [e.g., a high molecular weight thermoplastic polymer such as poly(methyl methacrylate), poly(propylene), or poly(vinyl butyral)]. 

Storage, handling, blending, and coal blend plastification Figures 403(a) to (e) show the layout of a typical coal briquetting system for a production capacity of 60-70 metric tons per hour with two roller presses. A plant using pitch or similar thermoplastic binder is depicted. 

Plastisols and organosols are a special case of physically drying coatings systems in which the binders consist of finely dispersed poly( vinyl chloride) or thermoplastic poly(meth)acrylates suspended in plasticizers. Organosols also contain some solvent. On drying at elevated temperatures, the polymer particles are swollen by the plasticizer, a process known as gelation. 

The increasing importance of environmental considerations places new requirements on paint resins and has broadened the range of paint systems. Paints are now required that have a low solvent content (medium-solids, high-solids coatings) or are solvent-free (powder coatings), that can be adjusted by dilution with water (waterborne paints), and that are thermoplastic or capable of undergoing cross-linking. All of these properties must be obtained via the polymer structure of the binders. Important parameters are described below. 

Only a limited amount of resin can be added to white paints due to its intrinsic color. Addition of these hard resins increases the hardness and the gloss of the paint films, accelerates and improves the drying of oxidatively cross-linking alkyd resins, and optimises sanding properties and corrosion protection in putties. Modified phenolic resins have lost much of their importance because they have been replaced by more efficient binder systems (e.g., thermoplastic and cross-linkable acrylic resins, polyurethane systems). Rosin-modified phenolic resins are, however, still extremely important in the production of resins for printing inks. 

Table 3.4 summarizes the consumption of thermosetting binders used in various regions. The epoxy-polyester systems predominate over polyester-trisglycidyl iso-cyanurate and epoxy systems. The consumption of thermoplastic coating powders, particularly those used in the fluidized-bed and flocking processes, is substantially lower than that of thermosetting powders (Table 3.5). Their production does not show a comparably significant growth rate.

Binders. Thermosetting and thermoplastic binders are listed, together with their abbreviations, in Table 3.6. A broad range of raw materials is now available and it has become difficult to systematically classify binder properties. Outstanding properties are obtained with suitably selected systems and have contributed to increased... 

The so-called solvent-based binder systems contain polymers which solvate or swell in a solvent (e.g. water, alcohol). Typical polymers used in extrusion are PEG, PVA, agar agar and cellulose. Thermoplastic materials are polymers which when heated, soften, melt or become more pliable, and harden during cooling in a reversible physical process. Materials in this class which are used quite often for ceramic processing are PE, PP, EVA, POM and PMMA. Thermosetting materials are polymers which can be melted only once and which, after melting, harden as more heat is added. Thermoset plastics which are used in the ceramic industry are phenolic resins and different silicon resins like polysiloxane. 

Another advantage of thermoplastic binder systems over solvent-based ones is the contour accuracy of the extruded material which permits easy fabrication of fine structures (e.g. thin-walled tubes, micro-tools). For such fine structures with very small cross-sectional areas, extrusion pressures can be as high as 700 or 800 bar, and at such high pressures phase separation in feedstocks with solvent-based binder systems can occur. Even for higher pressures such phase separation effects are generally not observed when using thermoplastic binder systems. 

A thermoplastic binder system generally consists of two or more different organic components. These components can be classified into one of four categories [Qua 82] ... 

In prachce a given organic component can be classified into more then one of these categories, e.g. stearic acid is a surfactant, but when used in conjunction with thermoplastic binders like polyolefins, it will also act as a plasticizer. Accordingly it is possible that a thermoplastic binder system will consist of only two organic components which each perform two or more of the functions described above. 

In the following, different binder systems which have been used for thermoplastic extrusion of ceramic bodies will be discussed [Lenk 97, Weg 98, Hoy 98, Cle 00, Schub 00, Bee 02, Gri 02, Tru 04, Schey 04, Pou 04, Yoo 05, Cle 05]. 

In 1997, Lenk et. al. described a thermoplastic binder system consisting of polyethylene, paraffin, wax and surfactants p enk 97]. Various binder components for injection molding, extrusion and hot molding were tested for the fabrication of bars, tubes, discs, rings and balls of SiC-platelet-reinforced SiC composites. Good alignment of the SiC platelets in the matrix made it possible to improve the densification of the composite. 

---