What it is

The plastics industry, like many other industries, is constantly evolving and requires new polymer-based materials. Moreover, one of the main indicators and the requirement for modern plastics is high economic efficiency, that is, the lowest possible cost with satisfactory performance. The material must have, in one combination or another, good thermal and strength characteristics, processability, special properties shown in specific cases, for example, electrical conductivity, and at the same time be relatively cheap.

The issue of cheapening the polymer, as well as giving it some properties, is solved in modern industry fillers. These are simple and complex substances of various chemical nature and physical form, which can be added to a plastic in a sufficiently large amount (several percent or more). Today, hundreds of types of fillers for plastics are used.

The use of these materials to reduce the cost of production does not lead to new important properties of the polymer system. On the contrary, usually the physicomechanical characteristics of plastics are reduced. However, some important properties, especially strength characteristics, can be improved by the introduction of special fillers. A striking example is the filling of plastics with fiberglass. The resulting composite materials are much stronger than the original polymer. However, at the same time, the material, as a rule, becomes more expensive and its ability to process is slightly reduced.

Types of Fillers

There are several types of classification of fillers for plastics. By area of ​​destination, they are divided into:

– reinforcing, for example the previously mentioned glass fiber. These agents improve the physical and mechanical properties of plastic;

– technological, for example DOTF and other plasticizers. Such substances affect the properties of the material during processing and operation, give special properties, such as electrical conductivity, change the coefficient of friction, etc.

– other, including inert fillers to reduce the cost of the material.

By physical form and structure, fillers are classified into:

– fibrous, for example the same fiberglass, carbon fiber, cotton fiber;

– powder or dispersed with different particle sizes, for example chalk, talc, wood flour, etc .;

– other, more rare forms, for example, fabric, paper, etc.

One of the main characteristics of the filler is the morphology and specific surface of the particles. The degree and strength of the interaction of particles with the polymer depends on this. To improve the interaction of high molecular weight compounds and additives are treated with surfactants and other agents.

Consider the main types of fillers in more detail.

Fibrous fillers

The basis for this type of fillers are long or short elementary fibers, both flexible and brittle. As a rule, fibrous composites are obtained to impart special strength properties to a high molecular weight compound. To do this, use high-strength fibers of glass, carbon (including carbon nanofibers or nanotubes), boron, polymer fibers, less often metal, carbides, nitrides, oxides and other inorganic compounds. Organic fibers of plant origin, such as the previously mentioned cotton fiber, are also used.

The fillers used are chopped short and long fiber and continuous fibers. In view of this, the fibrous composite may have properties similar to the material with the use of a dispersed type, and may differ greatly from the latter and have pronounced reinforced or reinforced. When using chopped fibers, the resulting materials are usually easily processed using standard plastic processing methods, such as extrusion and injection molding. When using a long-fiber filler, such methods are not always available. The use of certain types of fibers can increase the mechanical properties of the finished composite in tens and hundreds compared to the original polymer.

The most popular fiber filler in plastics processing is fiberglass. The industry produces many different grades of glass fibers, which differ in geometry, chemical composition and strength characteristics, but for the most part they are quite affordable. Fiberglass is used with almost all large-tonnage thermoplastic plastics, such as polyamide, polyethylene, polypropylene, polycarbonate, polyvinyl chloride, etc. At the same time, fiberglass is also actively used to strengthen thermosetting polymers, for example, materials based on epoxy and phenol-formaldehyde resins, unsaturated polyesters, etc.

Thermoplastics usually fill up to 40% fiberglass, less often up to 70%. Reactoplastics are filled with fiberglass in an amount up to 80%. Glass fiber also has non-deliveries – this is its high fragility and adhesion coatings that reduce the adhesion to the polymer matrix, which are used in the manufacture of fiber.

Dispersed Fillers

This type of additive is a powder of various origin and particle size. They can be either inorganic, such as chalk, talc, powdery glass, or organic, such as wood flour, starch. Dispersed fillers are also distinguished by the size and shape of the particles, they can be scaly, sphere-shaped, lamellar, etc.

The number of types and variety of dispersed fillers is much wider than fibrous. For filling polymers, most of the crushed solids of inorganic and organic origin can be used, for example, in recent years, composites with agricultural and food waste products, for example, with husks of cereals and legumes, fruit seeds, etc., have become widespread.

However, as many years ago, the main dispersed options in the plastics industry are chalk, carbon black and talc. These materials are mainly used in the extrusion of films, pipes, sheets and profiles, as well as in injection molding of technical products.

Chalk is widely used for composites based on PE, PP and PVC. Its main disadvantage is hydrophilicity. Filling with soot increases the service life of some products from PE, PVC, thermosets, increases their light resistance. In the past, asbestos was actively used as a filler-flame retardant for plastics, but in recent years it has been practically taken out of circulation. Calcium or barium sulfates are also used to increase resistance to burning.

In the 21st century, wood-polymer composite, which includes the most important organic filler for plastics – wood flour – gained very great popularity. It represents finely ground and well-dried wood of various species with a particle size of about 100 microns and a bulk density of about 150 kg / m3. Wood-polymer composite is used to produce numerous products for street use and is processed mainly by extrusion. There are also technologies for processing WPC by injection molding and other methods. Advantages of plastics filled with wood flour – low price, excellent appearance, disadvantages – high moisture absorption, and low resistance to heat and chemicals. WPC is produced mainly from polypropylene and PVC.

Other types of fillers

Other types of additives considered for polymers are used less frequently. Woven fillers consist mainly of glass, cotton and carbon fabrics. They are used for the manufacture of high-strength plastics with anisotropic properties. Woven fillers are most often combined with thermosetting polymers. The binder for such plastics can be epoxy oligomers, unsaturated polyesters, but polyamide can also be. The amount of filler in such a composite reaches 40-85%.

Nonwoven fillers are also used that cannot be classified as fibrous or dispersed. These include various nets, cardboard, paper, felt mats, etc. As a rule, these materials are impregnated with binder solutions (most often thermosets). Then, the resulting composite is dried to evaporate the solvent and processed into finished products by cold pressing. Thus, laminated plastics are produced. The method was popular in the 20th century, but in recent years has been inferior to more productive plastics processing technologies such as extrusion.