Rubbers density

Foams can be made from thermoplastics, thermosets and rubbers. Densities can be obtained from nearly solid down to 200 times diluted. Structural (or integral) foams have a solid skin. The best known foam materials are polystyrene foam, polyurethane foam and polyether foam. 

Problem 3.21 A neoprene rubber vulcanizate containing 90% (by wt.) chloroprene has 1% of the latter units crosslinked. Given that the interaction parameter ix) value of the polymer in n-hexane is 1.13, calculate an estimate of the maximum swelling expressed as volume of solvent imbibed per unit volume of the vulcanizate. [Data Rubber density = 1.23 g/cm molar volume of n-hexane at 25°C= 131.6 cm /mol.]... 

Natural rubber of Mn = 2.23x10 was vulcanized with 2% sulfur. Calculate an estimate of the extent of swelling of the vulcanizate in benzene at equilibrium, given that the interaction parameter of rubber and benzene is 0.41. Assume that all sulfur is used in crosslinldng and that there is, on the average, one crosslink for every eight sulfur atoms. [Data Rubber density = 0.91 g/cm molar volume of benzene = 89.4 cm- /moL]... 

The strength properties of the composite are defined by belt width (Wb), cord pitch (Pc), cord diameter (Dc) and strength coefficient x strength grade (Kf-Rr). The weight properties of the composite are defined by the rubber density, belt width (Wb), carry cover thickness (Cc), pulley cover thickness (Cp), cord pitch (Pc), cord diameter (Dc) and mass coefficient (Km). 

The actuality of researches of multi-layer objects with high density of materials for airspace technics is substantiate by several firms. The distribution of the heavier in longerons and elements of the screw of the helicopter was determined with the help of RCT. In the filler made of graphite or rubbers the layers of heavier from leaden alloys and the air stratification 1 up to 5 mm wide are revealed with the ratio signal / noise more than 3, even at presence of an external steel cover. 

An important application of Eq. (3.39) is the evaluation of M, . Flory et al.t measured the tensile force required for 100% elongation of synthetic rubber with variable crosslinking at 25°C. The molecular weight of the un-cross-linked polymer was 225,000, its density was 0.92 g cm , and the average molecular weight of a repeat unit was 68. Use Eq. (3.39) to estimate M. for each of the following samples and compare the calculated value with that obtained from the known fraction of repeat units cross-linked ... 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Mobile-Bed Scrubbers Mobile-bed scrubbers (Fig. 17-51) are constructed with one or more beds of low-density spheres that are free to move between upper and lower retaining grids. The spheres are commonly 1.0 in (2.5 cm) or more in diameter and made from rubber or a plastic such as polypropylene. The plastic spheres may be solid or hollow. Gas and liquid flows are countercurrent, and the spherical packings are flmdized by the upward-flowing gas. The movement of... 

Polymer-Fluid Equilibria and the Glass Transition Most polymer systems fall in the Class HI or Class V phase diagrams, and the same system can often change from one class into the other as the polymer s molecular weight changes. Most polymers are insoluble in CO9 below 100°C, yet CO9 can be quite sohible in the polymer. For example, the sorption of CO9 into silicone rubber is highly dependent upon temperature and pressure, since these properties have a large influence on the density and activity of CO9. 

Fig. 6.2. How Young s modulus increases witl) increasing density of covalent cross-links in polymers, including rubbers above tbe glass temperature. Below To, be modulus of rubbers increases markedly because tbe Van der Waals bonds take hold. Above Tq they melt, and the modulus drops.

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

Following the considerable commercial success of Ziegler-Natta polymerisation systems which made possible high density polyethylene, polypropylene, ethylene-propylene rubbers and a number of speciality materials, a considerable... 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

A somewhat different approach to the production of thermoplastic polyolefin rubbers has been adopted by Allied Chemical with their ET polymers. With these materials butyl rubber is grafted on to polyethylene chains using a phenolic material such as brominated hydroxymethyl phenol. The initial grades of these polymers, which were introduced commercially towards the end of the 1970s, had polyethylene butyl rubber ratios of 50 50 and 75 25. Both low-density and high-density polyethylene-based varieties were produced. 

Today polyether foam with a density of less than half that of rubber latex foam is widely used as a cushioning material. Polyester foams, although tending to be more expensive, continue to have a number of outlets, particularly where a high initial modulus is desirable. In addition to miscellaneous upholstery applications... 

If natural rubber is treated with proton donors a product is formed which has the same empirical formula. (CjHjj), and is soluble in hydrocarbon solvents but which has a higher density, is inelastic and whose unsaturation is only 51% that of natural rubber. It is believed that intramolecular ring formation occurs to give products containing the segments shown in Figure 30.5. Known as cyclised rubber it may be prepared by treating rubber, on a mill, in solvent or in a latex with materials such as sulphuric acid or stannic chloride. 

Withdrawn) 1974 AMD 1 Laboratory methods of test for determination of smoke generation charactenstics of cellular plastics and cellular rubber materials. Part 1 Method for testing a 25 mm cube test specimen of low density material (up to 130 kg/metres cubed) to continuous flaming conditions (AMD 7688) dated 15 July 1993. Withdrawn, superseded by BS ISO 5659-2 1994... 

Both side groups and carbon-carbon double bonds can be incorporated into the polymer structure to produce highly resilient rubbers. Two typical examples are polyisoprene and polychloroprene rubbers. On the other hand, the incorporation of polar side groups into the rubber structure imparts a dipolar nature which provides oil resistance to these rubbers. Oil resistance is not found in rubber containing only carbon and hydrogen atoms (e.g. natural rubber). Increasing the number of polar substituents in the rubber usually increases density, reduces gas permeability, increases oil resistance and gives poorer low-temperature properties. 

Density. Most fillers added in rubber base formulation have a density between 2 and 2.7 g/cm-, except barium sulphate (4-4.9 g/cm- ) and zinc oxide (5.6 g/cm ). Addition of filler increases the free volume of the polymer and, in general, there is a critical concentration of filler at which the density of the formulation increases. The method of incorporation of filler in the adhesive formulation is important because air voids may appear when a poor dispersion is produced. 

---