Vulcanized Rubber

Rubber, Vulcanized

(also vulcanizate), the product of the vulcanization of raw rubber. Industrial vulcanized rubber is a composite material that contains as many as 15–20 ingredients, each of which performs a specific function.

The major difference between vulcanized rubber and other polymeric materials is its ability to undergo extensive and reversible high elastic deformations within a broad temperature range, including both room and lower temperatures. The irreversible, or plastic, component of the deformation of vulcanized rubber is much smaller than that of raw rubber, because the macromolecules of raw rubber are linked together in vulcanized rubber by cross-linked chemical bonds, which form a vulcanization network. Vulcanized rubber is superior to raw rubber in strength, heat and frost resistance, and resistance to corrosive mediums.

Table 1. Mechanical properties of vulcanized rubber made from various types of raw rubber¹
Indicators	Natural		Synthetic isoprene		Stereoregular butadiene		Butadiene-α-methylstyrene, oil-filled		Butyl raw rubber		Ethylene-propylene		Butadiene-nitrile		Chloroprene
	I	II	I	II	I	II	I	II	I	II	I	II	I	II	I	II
¹Data for 22° ± 2°C;(l)unfilled vulcanized rubber, (ll) vulcanized rubber filled with active carbon black 21 MN/m² ≈10 kgf/cm²
Stress at 300% elongation² (MN/m²).	2–3	12–14	1.5–3.0	8–13	1.0–1.3	7–11	0.8–1.3	10–11	0.6–1.5	4–7	9–15	11–19	1.5–2.5	11–12	1.0–1.5	6.5–10.5
Tensile strength²(MN/m²).......	25–33	25–35	23–35	23–35	2–5	16–19	2–3	19–25	15–20	15–23	17.5–28.0	20–26	3–4	28–31	21–28	19.5–21.0
Specific elongation (percent).....	800–850	600–850	700–1000	600–800	250–750	400–600	700–800	550–650	800–950	400–850	400–600	370–500	500–700	550–700	750–1100	450–700
Tearing strength (kN/m, or kgf/cm).	50–100	130–150	30–90	110–160	5–7	35–45	7–10	70–90	8–20	50–85	40–55	40–50	—	65–80	25–45	55–70
TM-2 (Shore) hardness .........	35–40	60–75	30–40	60–70	40–52	57–68	32–43	50–60	27–32	60–65	42–68	40–68	—	69–72	37–50	55–60
Rebound resilience (percent).....	68–75	40–55	65–75	37–51	65–78	45–50	50–55	35–46	8–20	20–25	—	55	50–55	28–32	40–42	32–40
Modulus of viscosity (MN/m²).....	0.12–0.26	1.8–2.2	0.13–0.26	2.0–2.4	0.25–0.50	1.6–1.8	0.28–0.35	2.2–2.6	—	—	—	—	—	—	—	—
Wear resistance [cm³/(kW-hr)] ....	—	270–330	—	280–340	—	170–190	—	300–340	—	300–350	—	220–300	—	170–200	—	350–450
Durability upon repeated deformations (thousands of cycles).........	—	170–180	—	130–160	—	100–130	—	60–85	—	—	—	—	—	—	—	—

Classification. The basic groups of vulcanized rubber are distinguished according to the temperatures and conditions of use under which the rubber retains its high elastic properties.

General-purpose vulcanized rubber is used at temperatures between - 50° and 150°C. It is produced from various types of raw rubber (natural, synthetic isoprene, stereoregular butadiene, butadiene-styrene, and chloroprene) and from combinations thereof. Heat-resistant vulcanized rubber is designed for prolonged use at 150^C-200°C. Such rubber is made from ethylene-propylene, organosilicon, and butyl raw rubber. Raw rubber that contains fluorine, as well as rubber-like polymers of the phosphonitrilic chloride type, is utilized to make vulcanized rubber that can be used at temperatures reaching 300°C.

Frost-resistant vulcanized rubber is suitable for prolonged use at temperatures below — 50°C and sometimes even at — 150°C. It is produced from raw rubber that vitrifies at low temperatures, including stereoregular butadiene and organosilicon raw rubber. It can also be made from raw rubber that is not frost-resistant, for example, butadiene-nitrile raw rubber, by introducing such plasticizers as esters of sebacic acid into the rubber mixture.

Oil- and petroleum-resistant vulcanized rubber is suitable for prolonged use in contact with petroleum products and oils. It is made from various types of raw rubber, including butadiene-nitrile, polysulfide, urethane, chloroprene, vinylpyridine, and organosilicon raw rubber and from raw rubber containing fluorine. Corrosion-resistant vulcanized rubber includes rubber that is resistant to acids, alkalies, ozone, and steam. It is made from various types of raw rubber, including butyl, organosilicon, chloroprene, acrylate, and chlorosulfonated polyethylene raw rubber and from raw rubber containing fluorine.

Conducting vulcanized rubber is made from various types of raw rubber filled with conducting (acetylene) carbon black. Dielectric (cable) vulcanized rubber is characterized by small dielectric losses and high electric strength. It is made from various types of raw rubber, including organosilicon, ethylene-propylene, and isoprene raw rubber, which are filled with white mineral fillers. Radiation-resistant vulcanized rubber, including X-ray protective rubber, is made from various types of raw rubber, such as butadiene-nitrile and butadiene-styrene raw rubber and raw rubber that contains fluorine, and is filled with lead or barium oxides.

Other types of vulcanized rubber include those that are resistant to vibration, light, fire, and water, as well as vacuum and friction types of vulcanized rubber. Vulcanized rubber is also used to make medicinal and food products.

Properties. The properties of vulcanized rubber are determined primarily by the type of raw rubber stock. The filler and the structure and density of the vulcanization network have a significant effect on the mechanical characteristics of vulcanized rubber, including its deformation and strength. The most important deformation property of vulcanized rubber is the modulus, or the ratio between stress and deformation. The modulus depends on the following factors: (1) the static or dynamic conditions of mechanical load capacity, (2) the absolute values of stress and deformation, as well as the type of deformation, including stretching, compression, shearing, or bending, (3) the duration or rate of loading, which is determined by relaxation phenomena, that is, by the change in the reaction of vulcanized rubber to a mechanical action, and (4) the composition of the rubber being compounded.

In a region of relatively small deformations of less than 100 percent, the elasticity modulus of vulcanized rubber is five orders of magnitude less than Young’s modulus for steel—0.5–8.0 and 2 × 10⁵ meganewtons/m² (MN/m²), respectively, or 5–80 and 2 × 10⁶ kilograms-force/cm² (kgf/cm²). In this region of deformations the shear modulus for vulcanized rubber is about three times less than the elasticity modulus. As a result of the practical incompressibility of vulcanized rubber, which has a Poisson ratio of 0.48–0.50 as opposed to the Poisson ratio for metals of 0.28–0.35, the bulk modulus of vulcanized rubber is four orders of magnitude greater than the elasticity modulus.

The dependence of a modulus of vulcanized rubber on the composition of the rubber may sometimes be described by generalized equations, which permit the prediction of both the significance of the modulus and the production of materials with given properties.

Vulcanized rubber that is reinforced with carbon black is

characterized by high viscosity; its deformation results in the transformation of the mechanical energy of deformation into heat energy. This explains why vulcanized rubber is a good shock absorber. Rebound resilience is an indirect indicator of shock-absorption capacity. However, owing to the low heat conductivity of vulcanized rubber, repeated cyclical loading of such large items as tires leads to their self-heating, which is caused by elastic hysteresis. The operational characteristics of these items may deteriorate as a result.

Under actual operating conditions, vulcanized rubber is in a complex state of stress because various deformations act simultaneously. However, the destruction of vulcanized rubber is caused, as a rule, by maximum tensile stress. For this reason, the strength properties of vulcanized rubber are assessed in most cases during stretching deformation.

The industrial characteristics of vulcanized rubber significantly depend on the modes of preparation and vulcanization of the rubber mixture and on the storage conditions for semifinished products and other items. Vulcanized rubber that is made from raw rubber in which the macromolecules contain unsaturated bonds, for example, natural or synthetic isoprene rubber, may deteriorate upon use under conditions of high temperatures, contact with oxygen or ozone, or the action of ultraviolet light.

Uses. The rubber industry is one of the most important suppliers of component parts and manufactured items for many industries. Vulcanized rubber is an irreplaceable material used in the production of tires and various seals and shock absorbers. It is also used in the production of conveyor belts, drive belts, hoses, and various household items and especially in the production of footwear. It is used to make cable insulation, elastic conducting coatings, prostheses (for example, artificial heart valves), anesthesia apparatus, catheters, tubes used for blood transfusions, and many other items.

World production of vulcanized-rubber products exceeded 20 million tons in 1976. The largest consumers of vulcanized rubber are industries producing tires (more than 50 percent) and industrial rubber goods (about 22 percent).

REFERENCES

Koshelev, F. F., and A. E. Kornev, and N. S. Klimov. Obshchaia tekhnologiia reziny, 3rd ed. Moscow, 1968.
Reznikovskii, M. M., and A. I. Lukomskaia. Mekhanicheskie ispytaniia kauchuka i reziny, 2nd ed. Moscow, 1968.
Usilenie elastomerov. Edited by G. Kraus. Moscow, 1968. (Translated from English.)
Spravochnik rezinshchika: Materialy rezinovogo proizvodstva. Moscow, 1971.
Trudy mezhdunarodnoi konferentsii po kauchuku i rezine. Moscow, 1971.
Lukomskaia, A. I., and V. F. Evstratov. Osnovy prognozirovaniia mekhanicheskogo povedeniia kauchukov i rezin. Moscow, 1975.
Entsiklopediia polimerov, vol. 3. Moscow, 1977.

V. F. EVSTRATOV