Compounding with Reinforcing Fibres

From a mechanical point-of-view, fibre properties that influence these parameters are [67, 68]  

Type and defects such as voids of crystal structure 

Among the reinforcing fibres, glass fibres are the most common reinforcing material. Glass fibres are available as  

Consequently, the compounding process must meet the following requirements  

Because of the extremely abrasive nature of reinforcing materials, they should be added downstream into already melted (plasticised) polymer. The melt can then act as a lubricant and reduce the rate of wear on screws and barrel. In addition, the fibres will be broken down to a lesser extent if they are fed downstream and this improves the product properties. 

---

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

Figure 6.6 Compounding plant for polymer with reinforcing glass fibre...

Ganster J, Pinnow M, Fink H-P (2006) High tenacity man-made cellulose fibre reinforced thermoplastics - injection moulding compounds with polypropylene and alternative matrices. Compos A 37 1796-1804... 

Chemical compounds which contain reactive groups such as the methanol group (-CH2OH) as in methanolamine compounds are able to form stable, covalent bonds with cellulose fibres. This treatment decreases the moisture pick-up and increases the wet strength of reinforced plastics. Isocyanates are also suitable to modify the chemical structure via its reaction with the OH groups of cellulose. The mechanical properties of composites reinforced with wood-fibre and PVC or PS can be improved by an isocyanate treatment of those cellulose fibres or the polymer matrix. The improvement of the properties of the composites can be explained by the reduction in the number of OH groups responsible for moisture uptake and consequently the increase in the hydrophobicity of the fibre s surface... 

The method by which the compound will be moulded or shaped naturally dictates the form of reinforcement. In thermoplastic compounds (which will be predominantly injection moulded), short-length fibre or particulate reinforcement is used, but there has been important development of so-called long -fibre compounds, with a higher ratio of reinforcement to resin matrix and a longer... 

For example, in an ideal PA 66 compound, reinforced with 50% glass fibre and with all fibres aligned along the length of the moulding, the flexural and tensile moduli increase rapidly as the fibre length is increased from 0.1 to 1.0 mm. 

Because of their high crosslink density and very brittle behaviour, phenolic moulding compounds are invariably filled with reinforcing fillers such as wood floui wollastonite, mica, mineral wool flour and glass fibres. Solid moulding compounds are normally novolac based resins for transfer moulding (RTM) and other liquid laminating processes are resols. 

Reinforcement of PBT with carbon nanofibers (CNF) has also been investigated, since CNFs are interesting because of their mechanical, electrical and physical properties which can be imparted to polymer systems [129-131], Many of the general trends seen with glass fibers are also observed with carbon fibers. One important aspect of carbon fibers is that they may bring electrical conductivity to PBT if sufficient fiber connectivity is achieved in the finished parts. Metal fibres and metal-coated carbon fibers are also compounded with PBT, giving improved mechanical properties and enhanced ability... 

PEEK and polytetrafluoroethylene (PTFE) are highly incompatible. However, fine PTFE powder is commonly added to PAEK to act as an internal lubricant in tribiological applications. The PTFE smears across the wear surface and reduces interfacial friction. This reduces interfacial forces and the heat build-up that can lead to failure by melting. PTFE is particularly suitable in applications where there is no external lubricant and the compounds are often reinforced with carbon fibre. PEEK can also be added to PTFE to improve the wear properties of PTFE - although other less expensive polymers can have similar effects. More recently PAEK and PTFE have been blended so as to produce melt-processable PTFE which has a number of interesting properties [24]. This is perhaps the most luilikely example of the use of PAEK to improve the melt-processability of an otherwise hard-to-process material. 

Thermoplastic resins are generally reinforced with short fibres (for use with injection moulding and not considered in this book), with long fibres (thermoplastic sheet compound, Chapter 5) or with glass mat (i.e. glass mat thermoplastics or GMT). Polypropylene is the resin most used as the matrix for GMT and the reinforcement is usually a random mat, primarily chopped strand but it may be continuous filament or needled mat. 

So far when considering carbon fibre composites the matrix has been a thermosetting resin or polymer. The relative merits of thermosetting and thermoplastic polymer matrices were discussed briefly in Chapter 3. Several thermoplastic matrices are compounded with short carbon or glass fibres. In Table 5.22 however the properties of unidirectional, continuous, carbon fibre-reinforced materials are illustrated. 

For moulding compounds the difference between the values for SMC and DMC is due to the presence of longer fibres in the former case. Overall the SMC values compare favourably with those of mat polyester. The materials with continuous fibre reinforcement have much higher impact properties provided a reasonable proportion of the fibres is in the direction in which the main bending stresses are generated in the impact test. 

In December 2002, the average price of PPS was around 7.70-8.50 per kg. Non-reinforced or pure linear grades are at the upper end of the price spectrum, costing aroimd 15-20 per kg. Highly filled compounds with a glass fibre or mineral content are to the lower end of the price range, while high performanee types are more expensive. 

This is a recent but very promising application. In this case, the EPM based compounds are insulating, but they contain hydrated alumina as a filler which gives the compound greater tracking resistance. The compound is used to mould covers for epoxy resin rods reinforced with glass fibre. This type of insulator is very reliable, unbreakable, and weighs 10 times less than its porcelain counterpart. 

---