Determining when creating composites is the interaction and mutual influence of the components in an elementary volume of the fiber-matrix (binder). The higher the necessary properties of the resulting composite construction purposes, the more complex set of requirements should be maintained in the selection of the starting components, without which it is impossible to produce quality products. These requirements include the following characteristics:
It must be a certain relationship between the mechanical properties of the reinforcing fibers and the matrix (hereinafter, subscripts “c” and “m” refer respectively to the fibers and matrix);
tensile modulus and shear the fibers must be greater than the binder EB> Em; Gv> GM;
strength of the fibers must be greater than the binder SB *> * SM; elongation at break of the fibers to be somewhat less than the binder EB * <* Em;
Poisson’s ratios for the fibers and the matrix is ​​desirable to be close enough so that when the strain of the composite at the interface fiber-matrix is ​​not a voltage, rip them apart and thereby reduce adhesion;
The thermal properties of fibers (melting point or decomposition temperature) should be higher processing temperatures, thermoplastic and thermosetting curing.

The interaction with the matrix fibers must ensure the implementation of the high mechanical properties of the fibers in the reinforced material and its solidity. This requires:
good wettability of the fiber matrix (binder);
high adhesion between the fiber and the matrix, characterized by a shear strength at the interface between the fiber matrix;

· No or minimal change in the properties of the fibers under the influence of matrix components;

· Relaxation of internal stresses in the fiber volume element matrix during heat treatment, or under the influence of the binder components and other factors.

Component selection compositionally fibrous materials carried out taking into account specific properties of the pulp and the polymer binder (polymer matrix), and their mutual effect caused by a number of factors, including the following – a strength, deformation and other properties of the fibers, thermal stability, the length and diameter fiber structure of the fiber material, and the volume fraction of the fiber orientation of the fibrous material; strength, heat resistance, viscosity of the polymer matrix in terms of processing; the ratio of the deformation properties of the components, a change in properties of the fibers under the influence of the components of the polymer matrix, the wetting at the interface, the quantity of adhesion at the interface.

 

2. Preparation of semi-finished reinforcing fiber for composite materials and products

Reinforcing fiber semifinished products (FPA) is an intermediate material containing a predetermined amount of a fibrous filler and a polymer matrix prepared for immediate use. WUAs are a convenient form of discharge of semi-finished for the production of composite materials and products. WUA made the following main types:
based on chopped fibers and thermoplastic matrix in a predetermined ratio – premixes. Final forms – most often pellets;
based on chopped fibers with a predetermined amount of content of the starting components – thermosetting monomers or oligomers, hardeners and other components. They are shaped fibrous lumps of irregular shape – FRP, tablets, granules and dense dough;
based on unidirectional APD (threads, cords, tapes) by using a predetermined number of initial components thermosetting monomers or oligomers, hardeners, and other components – prepregs;
APD based sheet (fabric, non-woven fabrics, papers, etc.) using a thermosetting resin containing a predetermined quantity of the starting components of thermosetting monomers or oligomers, hardeners and other – prepregs. They are in the form of sheets or rolls;
APD based sheet (e.g., tissue) with duplicate thermoplastic film at a predetermined ratio;
based fabrics, nonwoven materials made from mixtures of thermoplastic and reinforcing fibers or filaments at a predetermined ratio.

Fillers may serve a fiber reinforced wood (cellulose), and sometimes other types of vegetable fibers (flax, cotton, etc.), Glass fibers, carbon fibers, asbestos. The length of the fibers is usually in the range of 3 to 20 mm. As the phenol formaldehyde binder commonly used, less melamine, epoxy and other thermosetting resins. The binder content is 40-50% by weight. The fiber reinforced composition may also contain powdery fillers such as talc, silica, mica, antifriction additives (graphite, molybdenum disulfide). FRP have the form of loose mass (flakes) of the adhesive-impregnated short fiber thread pieces or granules, often of irregular shape. Accordingly, referring to the filler referred to as a press FRP (cellulose-based) organovoloknitami, steklovoloknitami, uglevoloknitami, asbovoloknitami etc.

Getting fiber reinforced semi-finished products produced by combining fibrous filler to the polymer matrix – a convenient intermediate stage in the process for production of composite materials or products. For WUA use thermoplastic melts or solutions (and dispersion) of liquid oligomers – source components for thermosets. Combination machines are on a batch or continuous, followed by drying (by impregnation with solutions or dispersions) and cooling the impregnated filler.

Preparation of FRP can be performed by various methods:
impregnation of flagellar filaments binder, drying and cutting into pellets of predetermined length. Typically, the size of these beads range in diameter from 1 to 6 mm at a length of 3-30 mm;
of chopped fibers by mixing them with a viscous solution of binder, followed by drying mass and fluff;
based shredded fabric impregnated with a binder, make-textolite crumb.

The quality of the impregnated filler is determined by the uniformity of its composition and distribution of the binder in the pores of the material because of the completeness of interfiber pores depends on the solidity of the resulting composite. To improve the quality of impregnation of the fibrous filler binder it is often carried out with pre-vacuum or pressure; impregnation with continuous use intermediate spin.

WUA thermoplastic matrix – polyethylene, polypropylene, polyamides and others (in her pre-added dyes or other necessary components) can be stored before being processed into composites practically unlimited time.

WUA thermosetting matrices made based on oligomers of thermosetting resins (not fully cured and therefore fluid when heated) phenolformaldehyde, polyester, epoxy and other with the addition of all the necessary components: powdery fillers, colorants, lubricants (to avoid sticking to the molds) and others. WUA thermosetting matrix can be stored for only a limited period of time determined by the technical conditions of their life, because even at room temperature the slow process of curing binder. Often it recommended to store them at a low temperature.

Ready to use as starting materials are molding powders, manufactured on the basis of short-cut fiber filling with thermosetting binders. Their final forms typically tablets or particles of irregular shape. They also contain all the necessary components and ready for processing into articles, normally by hot pressing.

It appears, however, that despite the fundamental suitability of those components to create a fibrous polymeric composites often necessary to modify the surface composition of the fibers or the polymeric matrix to improve wettability and adhesion. For this purpose, used chemically modified fiber processing, etching oxidants, surface hydrolysis, surface application of the adhesive layer. Various methods of treatment in severe physical fields – corona etching, plasma processing, and others.

Getting reinforced thermoplastic fibers, is widely developed.

Prominent among fibrous semifinished products and materials based on them occupy combined or hybrid systems, where the reinforcing component is a blend of two fibers or fiber materials selected so that the negative characteristics of leveled positive properties of another or significantly reduces the cost of materials by introducing more cheap component. For example, the combined use of PCB fabric using carbon fiber materials in combination with glass fiber. This increases the absolute flexural and compressive slightly vary the mass parameters other mechanical properties. Widely recommended as semifinished products based on carbon and sverhvysokomodulnyh organic fibers and filaments and fabrics made from them in various combinations. The resulting composites have high compressive strength, higher fracture toughness and better fatigue properties than the comparable one-component structure.

The reinforcing fibrous semifinished type binder applied are divided into two groups: AVP based on thermoplastic polymers and AVP on thermosetting matrices.

Promising in terms of physical and mechanical properties VPKM and complicated from a technological point of view, are semi-finished products with thermosetting binders, the application of which is usually carried out from solutions in volatile solvents by the following methods:

1 – pulling through a bath with a binder;

2 – dipping the fibrous material;

3 – contact roller;

4 – the centrifugal method;

5 – sucking through the binder layer of fibrous material under a pressure difference;

6 – by capillary saturation through a porous material;

7 – calendering;

8 – spraying;

9 – combined methods.

After the process of impregnation and wringing excess binder excess solvent is removed by drying. The reinforcing fiber semi-finished products based on thermosetting for optimum processing conditions to achieve optimum VPKM and the properties of the latter must satisfy a number of requirements, including the following:
set the binder, a filler and a volatile solvent;
a small degree of cure of the binder and a long term of “life”;
necessary formability and “tack” bond, ie a set of indicators, due to both ductility of the fibrous material, and thermo-mechanical properties of the binder and others.

Weight impregnation matrix (binder) depends on the hydrodynamic conditions of the process and the conditions of the wetting phase boundary fiber-binder. Very important indicator fiber reinforced semifinished products is to achieve a high quality impregnation and absence of air inclusions at the interface as a fiber-binder, and the binder layer. Only under these conditions can be ensured receiving monolithic high fiber composites.

An important issue is to ensure the long-term “life” WUA without losing its technological properties. This is mainly due to the choice of binders, curing rate which storage conditions WUA rather low. However, this requires compliance with the maximum retention period.

The properties of composite materials obtained from semifinished reinforcing fiber, wherein the determined characteristics WUA

3. Laws of molding processes reinforced polymer composites

In the processes of forming articles from composites there are changes in their structure and properties, leading to hardening of the material and the formation of the necessary physical-mechanical properties. The thermoplastics and thermoplastic matrices crystallization proceeds rapidly, and then the subsequent second crystallization (recrystallization), and a non-crystallizing amorphous polymers and copolymers of viscosity increase by many orders of magnitude (hardening) occurs upon cooling below the glass transition temperature. The thermoset curing takes place to form a structural network of chemical bonds – an irreversible hardening.

Crystallization and recrystallization processes to occur crystallizable thermoplastics in time, and their kinetics approximately described by the exponential equation:

Xc = xc, ¥ [1-exp (-Kkr × tq (cr))]

where xc and xcr, ¥ – current limit value and the degree of crystallinity; Kkr – crystallization rate constant; it increases to a certain value when the temperature rises to a temperature of maximum crystallization rate and then decreases. The temperature of maximum crystallization rate of about 0.7-0.8 on the absolute melting temperature (in Kelvin);

q (cr) – coefficient characterizing the crystallization process; – (Kr) is the value for the initial crystallization from 2 to 6 depending on the type of thermoplastic.

Kinetics of curing thermosets includes a complex variety of chemical reactions that also occur at an exponential function of time and temperature increases exponentially accelerated in accordance with the Arrhenius equation. The presence in the binder (thermosetting) reinforcing fibers significantly affects the process of curing. The kinetic equation describing this process is approximated by exponential functions of the form:

X = X ¥ {1-exp [- (Kx × t) q]}

Kx = Z × exp (-Ea / RT),

where X and X ¥ – current and limit the number of chemical bonds are formed; Kx – the reaction rate constant; t – time; q – coefficient characterizing the fractional reaction order; Z – exponential factor; Ea – the activation energy; R – universal gas constant; T – temperature.

However, the temperature rise is limited to the possibility of occurrence of secondary processes – too rapid release of volatile reaction products thermodestructive processes and other factors. Therefore, when molding products from thermosetting commonly used curing step with timed at different temperatures, depending on the type of binder.

Products from a fibrous polymer composite immediately after spinning generally have internal stress (are structurally unstable) and thus can be changed in the following their shape and dimensions, which is extremely undesirable. This is due to several reasons:

· Crystallizable thermoplastics in continuing processes of ordering structure (crystallization), require some time to complete them. Thus there is a seal structure, respectively, a decrease in the volume of the material. In the case of filled and reinforced thermoplastics, this shrinkage is less as a result of lower share of the crystallizing component and due to the presence of the filler, limiting its flow;

· When cured thermosets there is some ordering (seal) of their structure and molecular fragments simultaneously converge network structure due to the formation of chemical bonds shorter than the intermolecular distances. Presence or particulate reinforcing filler and thus a decrease in the proportion of hardener component reduces the amount of shrinkage; presence of volatile components (e.g., solvent) or separation of the reaction products during curing thermosets (e.g., water) requires their removal during molding the product and / or dissolve in the resultant material under pressure during its exposure in the second step molding process;

· Lowering the temperature of the material during cooling the product reduces the amount of which depends on the values ​​of the linear and the volumetric thermal expansion coefficient.

All of these factors lead to relaxation and shrinkage process and the possible change in the size and shape of the product in the process of their formation and during subsequent operation due to ordering of structure (crystallization), the stress relaxation and strain-relaxation shrinkage.