What does not vulcanized rubber look like?



The term rubber (from the Egyptian kami) originally referred to rubber or other rubber-like vegetable juices (milky sap), which harden through polymerization to form plastic-elastic solids when they dry out. They contain a water-soluble part and rubber resins (latex). Today rubber is a versatile material.

Gum arabic is the name for the juice of an African acacia.

As a material rubber is now called vulcanized rubber. rubber is contained in the milky sap (latex) of tropical plants and is mainly derived from the rubber tree (Hevea brasiliensis, a milkweed plant). In nature, it serves to protect the tree, because it seals injured areas from bacterial attack. Vegetable rubber is also used Natural rubber or. Natural latex called.

Today, however, rubber is mainly produced synthetically. Synthetic rubber mostly consists of styrene and butadiene; other raw material bases are styrene acrylate, pure acrylate, vinyl acetate. The first commercially viable one was styrene-butadiene rubber, and another is neoprene.

Natural rubber consists of long polyisoprene chains, which can be crosslinked by adding sulfur under pressure and heat, creating this very elastic material. This process, known as vulcanization, was discovered by Charles Goodyear in 1839. According to archaeologists at MIT, the Central American natives knew how to heat rubber with sulfur 3600 years ago. The specialty of the rubber is that it is extremely elastic (elastomer).

As an industrially manufactured solid, rubber is generally vulcanized and therefore always contains some sulfur. Designations such as "100% natural rubber" or "pure natural latex" (e.g. on disposable gloves) are therefore misleading, because unvulcanized (sulfur-free) rubber has the consistency of liquorice snails (breaks in the cold, sticks and decomposes with a smelly smell in summer heat) and would be practically useless as a material. The name only means that it does not contain any synthetic rubber, but it is often used in advertising to indicate that it does not contain any toxic additives, because unlike most synthetic elastomers, natural rubber is non-toxic. Internationally, in addition to natural plant sap, the rubber made from it is also referred to as latex, although “latex rubber” would be more explicit. However, the terms “rubber” or “latex” are also used by chemists in other contexts for synthetic rubber raw materials (e.g. in words like “silicone rubber” or “latex paint”), which makes things all the more confusing.

Rubber as a material

A large part of industrial rubber production goes into tire manufacture, with different types of rubber being used to achieve optimum load-bearing capacity, abrasion and road grip. Natural rubber is amber in color and is mixed with man-made carbon black for car tires to modify the properties (e.g. abrasion, tear resistance, hardness). This is the reason for the black coloration. For tires that are already available for sale in different colors, however, highly active silicate is used to improve the properties.

To improve durability and processability, industrial rubber is mixed with additives, some of which are toxic. For example, organic solvents, vulcanization accelerators and retarders, dyes, fillers and antioxidants are added.

The simplest form of natural rubber, on the other hand, consists only of natural rubber, water and a little sulfur. Since this mixture would slowly vulcanize by itself even without heating, a little ammonia is added, which keeps the mixture liquid and therefore storable thanks to its basic pH value. When the mixture is exposed to air, the ammonia evaporates, so that vulcanization begins. Heat can accelerate the process, but is not absolutely necessary, so such a mixture is also used as a non-toxic adhesive for latex rubber, as a removable paper adhesive (e.g. Copydex® Rubber Cement) and used as liquid latex for body painting. Pure natural rubber is superior to most synthetic rubbers in terms of its elasticity (7 to 10 times) and, thanks to special vegetable rubber components (e.g. proteins), has an unequaled low internal friction, which increases its speed of contraction and heats up under mechanical stress (e.g. . the rolling of tires). In its pure form it is translucent beige and becomes almost completely transparent when it is strongly stretched through crystallization. However, due to moisture and environmental influences, it darkens brownish. The disadvantage of this type of rubber is its great sensitivity to environmental influences; so it is z. B. decomposed by all oils and fats (except silicone oil) and even permanent wetness.

Pure natural rubber is very environmentally friendly because it is based on renewable raw materials, is non-toxic and even compostable. Areas are used sustainably for the cultivation of rubber (i.e. not constantly cleared). a. around monoculture and some are located on formerly cleared rainforest areas.

Natural rubber is slightly sticky, especially when freshly made or when it is soaked, and above all tends to stick to itself (which can lead to permanent adhesions due to molecular cross-linking). In contrast, the surface is usually treated with powder or silicone oil or given a solid coating. Talc was often used as a powder, but it damages the lungs when inhaled. Because the vitreous mineral cannot be broken down by the body, phagocytes release acid that primarily attacks its own tissue. Repeated contact can therefore lead to a latex allergy if the immune system (optimized for protein recognition) mistakenly considers latex proteins for the attacker, which adhere to the talc as a harmless rubber component. The use of biodegradable powder (e.g. corn starch) helps to avoid this risk, but toxic additives in rubber can also trigger the allergy if they are used, for example. B. inhaled with powder. By using silicone oil, powder can be dispensed with, but this is also not degradable in the body and must never get into the lungs as a spray.

In order to make technical rubber types insensitive to oil and moisture, they are mixed with stabilizing chemicals or made from synthetic rubber. Because of its adhesive properties and unsurpassed low internal friction, vehicle tires still contain a proportion of natural rubber. Aircraft tires for jumbo jets are even free of synthetic rubber, as its higher internal friction would overheat the tire when landing.

Rubber cannot be stored indefinitely. Erasers, rubber seals and rubber rings become brittle over time and no longer tear or erase. Old car tires are a hazard because they can burst while you are driving. These signs of aging in latex-based rubber are primarily due to environmental influences, which is why antioxidants or surface coatings are usually added for protection. Air pollutants such as ozone, the combination of UV light and oxygen as well as permanent wetness and contact with reactive metals (e.g. copper, silver) lead to the embrittlement of the material, with cracks occurring particularly in mechanically stressed areas. Oils, fats (except silicone oil) and plasticizers (e.g. from contact with soft PVC), on the other hand, break down natural rubber into a sticky mass, whereby oily evaporation can damage it in the long term. The decomposition of rubber gum (especially due to ozone) often discolours its surface grayish and generally causes a characteristically bitter smell very early on, which can be used to detect damage. However, rubber stored airtight, dry, cool and dark hardly decomposes when correctly manufactured and can easily outlast human lives. However, aging leads to hardening due to the progressive crosslinking of the molecular strands; This, too, can lead to embrittlement if mixed incorrectly (excess sulfur), but it usually comes to a standstill beforehand.

The common idea that the rubber would “dry out” when it became brittle is wrong; Latex rubber does not contain any plasticizing liquids that evaporate in the process. However, some types of rubber contain waxes to protect against environmental influences. B. occur in tires due to the flexing movement when rolling to the surface. If tires are exposed to the sun and ozone without moving, the wax that oxidizes on the surface cannot be replaced by newly emerging wax, so that the rubber surface itself is attacked. As a result, tires that are left unmoved outside become more fragile than tires that are regularly used on the vehicle. Winter tires are therefore optimally stored dry, horizontal, protected from light and air in plastic bags made of PE or PP.

In applications in which the elastic properties or the coefficient of friction of rubber-rubber are all important, hardened rubbers can be renewed. I. E. Acetic acid can be used for regeneration wherever the properties of putty are required. Acetic acid breaks the rubber-rubber molecular networks; Briefly moistening with a solution of 1/2 acetic acid essence (38%) and alcohol regenerates the elasticity of the rubber (at the expense of strength) and cleans the surface at the same time. Depending on the duration of exposure, the rubber swells; it swells up to the point of complete loss of all strength. Possible applications are, for example, problems with the paper transport (paper jam / transport jam) with an old printer / ADF scanner / copier, unusable (red) rubber erasers on pencils or leaky bicycle valves. Material-removing processes (sandpaper) change the surface properties. This process only regenerates elasticity; it is based on molecular rather than macroscopic effects. Since the tincture does not contain any harmful intercalation molecules, it only works for about a year. Under no circumstances (!) Should acetic acid be used on mechanically stressed components (rubber rings, rubber seals) or safety-critical components (tires, air springs, hydraulic hoses). Hardened rubber can often be softened again by treatment with boiling water, with restrictions similar to those for acetic acid.

Mechanically stressed rubber components cannot be regenerated.

Due to the molecular cross-linking, rubber cannot be melted; if overheated, it decomposes permanently to a sticky mass. Natural rubber is not particularly heat-resistant; Even at just 60 ° C, it loses its strength after a few hours. In general, heat accelerates all decomposition processes of the material. On the other hand, however, it is the ambient heat that gives the rubber the power to contract in the first place, because as an elastomer it draws its energy from it. For example, when it is cold, shriveled balloons only contract when you touch them with a warm hand.

In extremely cold conditions (natural rubber approx. -40 ° C), rubber freezes as hard as glass and breaks when exposed to a brittle layer. But even when it is not so cold, latex rubber with a low sulfur content can become rigid (partially crystallize), making it leathery, stiff and fragile (e.g. black latex cuffs on drysuits in winter). To wake it up, it has to be heated to a significantly higher temperature (some 10 K) than the temperature at which it solidified.

In contrast to metal springs, rubber cannot withstand long periods of strong stretching or point loading, as otherwise it loses its strength and “dies” from exhaustion (material fatigue). The decomposition by pollutants such as ozone is extremely intensified, since the molecular chains, which are strongly strained by stress, tear more easily when the harmful molecules are pulled. This is particularly noticeable in latex balloons, which, depending on the ozone load, can only survive for days to a few weeks when inflated, while they last for many years without being stretched. In order to avoid point loading, objects made of rubber foil must therefore not be stored crossed (i.e. a horizontally folded edge again vertically), otherwise holes will appear at cross-kinked points. (It is correct to fold in only one direction (or not at all) and to roll up in the other.) Sharply creased storage (e.g. due to weights lying on folds) can also damage the material.

The further a piece of rubber has been stretched and the longer the stretching continues, the longer the material needs to recover after relief. Too much stretching leads to permanent deformation, but this can often be reversed by adding heat and movement (e.g. with a warm shower, hairdryer or hand massage).

Torn or cut rubber tears extremely easily when stretched, which can lead to spontaneous total failure of only slightly damaged parts (example: a balloon bursting by a needle prick). In critical applications, it must therefore be constantly checked for tears, or further tearing must be prevented by constructive measures (e.g. textile lamination of inflatable boat material, construction of bungee ropes from many individual strands).

Use of rubber

Rubber can be used in many areas.

  • Rubber clothing, e.g. B. as rubber boots to keep liquids and gases away from the body. Due to its special properties, such as shine, the special wearing comfort, especially the feeling of being enclosed with this skin-like material, rubber clothing is also used for eroticism and meditation (see sexual fetishism and rubber fetishism).
  • The property of Adhesiveness without sticking is used with the eraser, which removes pencil lines (made of graphite) from paper.
  • The extreme elasticity of natural rubber is used in elastic bands and balloons.
  • Rubber can be foamed by a blowing agent during vulcanization. The resulting Sponge rubber is processed into insulation materials and the like. Foamed rubber is made almost exclusively from synthetic rubber types (e.g. neoprene) and is used as insulation or elastic insulation in a variety of ways, from diving suits to bed mattresses.
  • Synthetic rubber or silicone rubber is also used for special applications; the latter is a polymer based on silicon-oxygen chains with organic side groups (see also: silicone).
  • Rubber is also water-repellent and can therefore be painted on the wall as paint. However, modern latex paint does not contain natural latex.
  • Rubber plays an important role in medicine and hygiene to keep germs out. Because of its non-toxicity, natural latex is mainly used in the manufacture of condoms, disposable gloves, surgical gloves and the like. To protect against more aggressive hazards (e.g. for NBC protective suits), more robust synthetic rubber is usually used. However, due to its wide-meshed molecular structure and tendency to crack, rubber does not offer 100% protection against very small particles (e.g. some viruses). Since rubber can leak through decomposition, it must be checked regularly. To be on the safe side, it is often provided with an expiration date, although this rarely corresponds to the real achievable service life due to the extreme dependence on storage conditions.
  • Rubber is also used in a variety of ways in sports - including: For balls, sports shoes, exercise bands, bungee ropes, gymnastic mats, water sports suits, as a coating for table tennis rackets and as a springy sports field surface.
  • In electronic devices, conductive synthetic rubber has been used for contacts on keyboards and keyboards as well as LCD displays since the 1980s and is very common there. In the case of small and cheap devices, the entire keyboard often consists of it, whereby this is often made of a continuous rubber mat that rests on the circuit board.
  • Rubber is also often found in mining. Mainly to protect the carcass of conveyor belts.
  • Masks and artificial creatures for film tricks and theater are often handcrafted from rubber (often natural latex foam).
  • Gum arabic is used in the production of food, medicines and as a binding agent in painting with tempera and gouache paints.
  • Cherry gum was also used in European painting, but is no longer available today.
  • The raw material for chewing gum used to consist of (unvulcanized) natural latex from certain types of plants; Nowadays, however, it is mostly a synthetic petroleum-based gum.

Colloquial meanings of the term

In addition to the original isoprene-based rubber (latex vulcanized with sulfur) there are numerous other elastomers, which are more or less officially referred to as "rubber".The best-known is probably silicone rubber, but this name is also used for polyurethane (called elastane in lingerie), chloroprene (neoprene) and sometimes even soft PVC, although the latter only becomes stretchable through a lot of plasticizers and is therefore not a real elastomer.

"Rubber" also has special slang meanings:

  • "Give rubber!" = Senseless accelerating - so make sure that a thick black line remains on the road.
  • Rubber as a synonym for condom and for elastic band.
  • Gum as a short form for hair tie
  • "Rubber pants", z. B. as fetish clothing, as trousers for anglers or as waterproof diaper pants, usually also incorrectly used as a synonym for comparable panties made of PVC or PE-coated materials.
  • "Rubber suit" z. B. as fetish clothing, water sports (diving) or protective suit, usually also incorrectly used as a synonym for comparable items of clothing made of neoprene, PVC or PE-coated materials.
  • "Rubber eagle" joke word for tough poultry that has been killed by human hands and grilled for too long.
  • "Rubber boat" Synonym for inflatable boat (although today it is often made of soft PVC).
  • "Rubber doll" Synonym for sex doll (although most of these are made of soft PVC these days).
  • My children “play rubber” in a shortened form when they want to play rubber twists.
  • "Rubber paragraph" for a vaguely defined, easily abusive law.
  • “Rubber” or “office rubber” for office workers, office workers, especially for bureaucrats, often of yielding character traits (Swiss).
  • The gummy bears (and other "gummy candies") are not made from rubber, but from other (edible) substances with rubber-like properties, e.g. B. pectin or gelatin, see also fruit gums.
  • eraser

Cultic significance in American high cultures

Already with the Olmecs, Maya, Aztecs and other South American natives, natural rubber had an important meaning and was used in ritual cult activities. The ritual ball game is particularly well-known, in which some researchers assume that the winning or losing team was sacrificed to the gods after the game. These peoples also burned natural rubber instead of incense (this smell is not nearly as unpleasant as burning rubber, since there is no sulfur).

See also: hard rubber

Categories: Organic Material | plastic