Ebonites

The study of phase size and compatibility requires that the different phases be observed, distinct from one another, and that there is minimum distortion in the polymers. However, multiphase polymers often cannot be stained or sectioned uniformly. There are composite structures that are combinations of soft rubbers, coatings and oriented fibers which caimot be stained with a single staining agent and the sections may be deformed or distorted, limiting both observation and interpretation. The ebonite method was developed by Smith and Andries [111] to stain and uniformly harden polymers. 

Smith and Andries [111] modified a method using molten sulfur to transform rubber to ebonite that could be polished for surface examination. A small block of the sample was held in a molten mixture of sulfenamide accelerator/zinc stearate/sulfur in a 90/5/5 weight ratio at 120°C for 8 h. A control method these authors used was 

A modified ebonite method was developed [113] to study the interfaces associated with polymer tire cords. Tire cords composed of PET, rayon or nylon fibers are generally bonded to rubber with a resorcinol-formaldehyde-latex (RFL) adhesive. The nature of the interfaces are of interest in tire cord studies. OSO4 may be used to stain and harden the RFL, but the soft rubber is not affected by this treatment, and, in fact, it forms a barrier to stain penetration. The ebonite reaction hardens the rubber and hardens and stains the RFL while maintaining the geometrical integrity of the composite. 

The reaction medium consists of molten sulfur/accelerator (iV,iV-dicyclohexyl-2-benzothi-azolylsulfenamide)/zinc stearate in the weight ratio 90/5/5. Small pieces of the cord are cut from the tire carefully trimming some of the rubber but leaving a thin, undisturbed surface layer. Eight hours are required for the reaction at 120°C. Samples are removed, scraped off and placed in a 120°C oven to remove the excess ebonite. The treated cords are embedded and sectioned with a diamond knife to reveal the interfaces that relate to the adhesive coating and 

The recommended method [277] uses molten sulfur to transform rubber to ebonite that can 

The study of phase size and compatibility requires that the different phases be observed, distinct from one another, and that there is 

A modified ebonite method was developed [134] to study the interfaces associated with polymer tire cords. Hre cords composed of PET, rayon or nylon fibers are generally bonded to rubber with a resorcinol-formaldehyde-latex (RFL) adhesive. The nature of the interfaces is of interest in tire 

In summary, the ebonite method can be used routinely for polymer blends or composite specimens, such as tire cords, where hardening. 

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VSEPR theory See valency, theory of. vulcanite See ebonite. 

Cellular ebonite Cellular phones Cellular plastics Cellular polyethylene Cellular polymers... 

Chemical Stabilization Processes. Cellular mbber and ebonite are produced by chemical stabili2ation processes. 

Cellular Ebonite. CeUular ebonite is the oldest rigid ceUular plastic. It was produced in the early 1920s by a process similar to the processes described for making ceUular mbber. The formulation of mbber and vulcani2ing agent is changed to produce an ebonite rather than mbber matrix (114). 

Fig. 3. Effect of density on compressive modulus of rigid cellular polymers. A, extmded polystyrene (131) B, expanded polystyrene (150) C-1, C-2, polyether polyurethane (151) D, phenol—formaldehyde (150) E, ebonite (150) E, urea—formaldehyde (150) G, poly(vinylchloride) (152). To convert...

ISO 75, Plastic and Ebonite Determination of Temperature of Deflection under Eoad, ISO, Geneva, Swit2edand, 1992. 

In extensions of this work on vulcanisation, which normally involved only a few per cent of sulphur, both Goodyear and Hancock found that if rubber was heated with larger quantities of sulphur (about 50 parts per 100 parts of rubber) a hard product was obtained. This subsequently became known variously as ebonite, vulcanite and hard rubber. A patent for producing hard rubber was taken out by Nelson Goodyear in 1851. 

By 1900 the only plastics materials available were shellac, gutta percha, ebonite and celluloid (and the bitumens and amber if they are considered as plastics). Early experiments leading to other materials had, however, been carried out. The... 

In this chapter brief consideration has been given to the major tonnage mbbers. Derivatives of natural mbber such as ebonite are discussed in Chapter 30 and thermoplastic rubbers are reviewed in Chapter 31. Other important speciality mbbers (with their ASTM designations) include ... 

The resins act as a plasticiser during processing but they cross-link while the rubber is vulcanising to give a harder product with improved oxidation resistance, oil resistance and tensile strength. The addition of sufficient resin will lead to an ebonite-like product. 

The detailed structure of ebonite is not known but it is believed that the same structures occur in the rigid material as have been suggested for vulcanised rubber. There will, however, be far more S-containing structures per unit volume and the ratios of the various structures may differ. The curing reaction is highly exothermic. 

Ebonite compositions may be prepared without difficulty either in an internal mixer or on a two-roll mill. In addition to the rubber and sulphur, fillers are invariably present in commercial mixes. These materials have the important function of diluting the rubber phase. Because of this the exotherm will be... 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

Figure SO.3. Variation of internal temperature during cure of ebonite stocks containing 0, 20, 50 and 100 parts of ebonite dust per 100 parts (rubber and sulphur). (After Scott, see bibliography)...

Ebonite, or hard rubber as it is often known, is black in colour and has a specific gravity, in the absence of mineral fillers, of about 1.18. 

Typical properties of a high-quality ebonite are given in Table 30.3. 

On exposure of ebonite to light there is a rapid deterioration in surface resistivity. It is believed that this is due to the formation of sulphuric acid through... 

Table 30.3 Properties of unfilled ebonite vulcanisates (as BS903 where applicable)...

Ebonite has a good resistance to a range of inorganic liquids, including most non-oxidising acids. It is severely swollen by aromatic and chlorinated hydrocarbons. 

Figure 30.4. Influence of exposure to light on the surface resistivity of ebonite, (After Scott)...

The terms ebonite and hard rubber are now extended to cover hard produets made from synthetic rubbers. SBR is now replacing the natural materials in many ebonite applications whilst nitrile rubber ebonites are of interest where oil resistance is required. 

Hard products may also be made by vulcanising rubber (natural or synthetic) using only about two parts of sulphur per 100 parts of rubber. In these cases either the so-called high-styrene resins or phenolie rubber compounding resins are ineorporated into the formulation. These compounds are processed using the methods of rubber technology but, like those of ebonite, the produets are more akin to plastics than to rubbers. Examples of the usage of these materials are to be found in battery boxes, shoe heels and ear washer brushes. 

Ebonite media are manufacmred from partially vulcanized rubber, which is crushed, pressed and vulcanized. These media are resistant to acids, salt solutions and alkalies. They may be used for filtration at temperamres ranging from -10 to + 110 C. 

Soft rubber is obtained by adding 2-4% sulfur by adding extra sulfur (25-40%), the rubber can be made into ebonite, which is a hard, brittle material, having a wider range of chemical resistance than soft rubber. Soft ordinary rubber is chemical and erosion resistant, but its thermal resistance is not high (about 80 C). 

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