Structure of erasers
The first factories manufacturing natural rubber erasers appeared at the end of XIX century in the USA and Germany. Erasers structure, form and color changed since then; the name 'eraser' that means 'elastic' or 'rubber' remained unchanged. With time rubber was replaced by polymers similar to rubber which provided several advantages: the longer keeping time and excellent quality of erasing.
Today erasers are produced from different materials: rubber (natural, synthetic or even crude), plastic and vinyl. But the eminent manufacturers still point out their best erasers' summaries: 'with addition of natural rubber'.
Natural rubber is a connecting substance and its quantity is no more than 10-20 % of the general structure of an eraser. Natural rubber erases graphite lines well, although it has an important shortcoming – it smears graphite. Besides the keeping time of rubber erasers is less and they can cause an allergic reaction.
For improvement of an eraser quality several softeners should be added to natural rubber. They are produced of animal or unsaturated vegetable oils, such as rapeseed oil mixed with sulfur, for example.
In the mid 50s of XX century erasers made of polymer such as polyvinyl chloride first appeared. Since then erasers are made from natural and synthetic raw material both.
Structure of natural erasers
The basic components of natural erasers are:
Factis is extracted from the rapeseed milk and has white-yellow color. It is the most important component of an eraser owing to its ability to adsorb graphite. After a vulcanization (the process of heating and pressing a mass) and addition of sulfur the sticky rubber mass gets elastic. Quartz powder and other stuffs (fillers), cretaceous or pumiceous powder, for example, are added depending on an applicability of an eraser. Dyes included in an eraser define its color. For example, lipoton or zinc are used for white erasers, sulfide of mercury, ferric oxide or sulfide of antimony - for red ones, and so on.
A softness of an eraser is defined by the components proportion. The soft erasers contain more factis, firm erasers – more sulfurs and fillers.
Structure of synthetic erasers
Synthetic (or 'plastic') erasers are made from soft vinyl materials which contain minimum of the abrasive substances.
Often during manufacturing this sort of erasers softeners are added which provides an elasticity and adsorption. The products containing softeners possess an ability to transfer it to other synthetic substances in a close contact. Usually synthetic erasers are packed in polythene to avoid this process. The advantage of synthetic erasers is that their 'shavings' or 'crumbs' can be rolled into a ball and removed easily. As opposed to natural erasers they don’t spoil and don’t stick to a working surface.