It contains 70 percent of hydrocarbon, 20 percent of resin, 10 percent of insoluble materials, cellulose, liquid etc. It is nicely used in producing beltings, floor tiles, automotive moulded goods, hoses, tyres, seals, gaskets etc. In order to make crude rubber workable, it is kneaded to a plastic mass by passing it through heated rollers which travel towards each other at different speeds. By this process the rubber is subjected to both compression and shearing stresses which cause the rubber to twist, crack or tear and become plastic.
Other ingredients can also be added into the rubber mass during the process. The rubber available in plastic state is compounded with other ingredients to modify its properties.
A number of different substances which can be mixed with rubber to give it definite properties are classified as follows:. These are the substances e. These are essentially required for carrying out the process of vulcanization. These are the materials e. With their use, the quantity of sulphur required for vulcanization is also reduced.
The addition of fillers is made to modify the properties of rubber and to lower the cost of rubber product. Materials like zinc oxide and carbon black serve both as fillers and reinforcing agents. Some other reinforcing agents are magnesium carbonate, calcium carbonate and barium sulphate. A calendering machine is used to obtain rubber in the form of sheets of definite thickness. The rubber mass is made to pass between large steel rollers of the machine, at carefully controlled temperature and pressure.
The rollers press the rubber mass into sheets of uniform thickness and desired finish. The crosslinking process in elastomers is called vulcanization, which is achieved by a non-reversible chemical reaction, ordinarily carried out at an elevated temperature. In most vulcanization reactions, sulphur compounds are added to the heated elastomer; sulphur atoms bond with adjacent chains and crosslinks them.
Useful rubbers result when about 1 to 5 parts by weight of sulphur is added to parts of rubber. Increasing the sulphur content further hardness the rubber and also reduces its extensibility. Unvulcanized rubber is soft and tacky, and has poor resistance to abrasion.
Vulcanization enhances modulus of elasticity, tensile strength and resistance to gradation. The magnitude of modulus of elasticity is directly proportional to the density of the crosslinks.
Vulcanization normally results in increased elasticity and strength, increased durability and increased resistance to adverse effect of weather and chemical agents. Vulcanized rubber has low electrical and thermal conductivity and is resistant to abrasion and action of chemical reagents and weather. The process of vulcanization, in the manufacture of rubber goods, is carried out after calendering and extrusion. When the rubber goods are to be manufactured by the process of moulding, vulcanization is performed simultaneously with moulding.
Fabricating operations and lathe cutting are performed after the vulcanization process. By this process rubber goods of constant cross-section like tubes, insulation on wire etc. In this process the rubber is forced into the mould of heavy metallic construction under high pressure rubber is not melted and poured like metal.
For getting non-porous uniform product, the process of moulding is performed in heated moulds in which the rubber compound is forced under high pressure.
Reclaimed rubber is the rubber recovered from worn-out rubber articles and rubber wastes from factories and then treated suitably for re-use. For reclaiming rubber, the metal and fabric contents of the worn-out rubber are first removed and then it is heated in a closed iron vessel which contains an alkali solution. When you stretch the rubber, the polymers start to elongate or stretch. This requires some force. When you let go, the polymers relax back into their tangled compact shape. Most plastic materials have some elasticity, but the amount the material will stretch depends on the type of polymer and length, etc.
The ability to stretch is what we call elasticity elasticity is the noun, elastic is the verb. Rubber is elastic because the chemical bonds that hold it together can store energy in the form of tension and allow the rubber to stretch, Beyond that I don't think I'm qualified to explain. Why do rubber bands stretch? The elastic nature helps protect the clothes from being stretched out of shape and ruined after only a few uses.
Stretch fabric 4 : Knits - most knits stretch 2 ways with some exceptions that allow it to go 4 ways. This stretching ability makes it a popular fabric to use in making clothes. You can find this fabric in several varieties. There are a lot of choices in this category. The use of elastic is innumerable as so many clothing items can benefit from the added elastic in their construction.
The different styles of elastic have different purposes. There is pajama, underwear and lingerie style which is soft because it comes in direct contact with the skin. Then there is baby elastic that is used primarily in baby clothes. This style is heat resistant and has a lot of stretch to it. Normally it returns the clothes to their normal shape. Also, there is swimwear elastic.
Not only does it resist heat, it can stretch to 3 or 4 times its size. It also resists damage from chlorine, saltwater and sunscreen. Then you have, sports elastic which resists sweat and many washings. It is tough as it has to retain its original shape.
Braided elastic has a long lifespan normally. It is also built for heavy use. Knit elastic is soft and comfortable while cord elastic is used for jewelry and clothing that does not stretch a lot.
Finally, there is buttonhole and drawstring elastic. The former works well with maternity clothes and the latter for expanding waistlines. Elastic fabric 5 : Spandex blends - there is a whole long list of this type of elastic material used in fashion. It is popular because the blends work well.
Like cotton, elastic has a long history. Although that story is not fully known its use has gone back thousands of years and has enjoyed a long life span. The American Indian used rubber long before Columbus appeared. Their word for the material translates into weeping tree.
Then the Mayans used to mix the juice from different vines to create an early form of elastic band that bound different items together. But for modern elastic invention we only have to go back about years. Thomas Hancock is credited with inventing elastic in about His invention started with elastic closures for gloves as well as elastic for shoes, suspenders and socks.
During his experiments he found that he was wasting too much rubber so he invented a machine called the masticator to conserve the rubber material. It is not known who originally invented the rubber band, but it was done in the midth century. In , William Spencer was the first to mass-produce rubber bands. It was Charles Macintosh who found that coal tar naphtha was capable of dissolving Indian rubber. This led to his being the first to make waterproof clothing. The way to weaken elastic is to stretch it far beyond its stretching ability.
Once the material goes beyond that point of no return it is too weak to be of any elastic good. The way to stretch elastic past that point of no return is to stretch it out over an ironing board. Then use a hot iron to apply the heat. As it heats stretch it out, then let the elastic cool. At this point, it may be too stretched to function like it is supposed to.
This same process works for waistband elastic as well.
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