Chopped strands

The second main ingredient ia reiaforced plastic is the reinforcement, eg, fibers of glass, carboa, boroa, mineral, cellulose, or polymers. Reinforcements can be configured ia many ways, such as coatiauous or chopped strands, milled fibers, rovings, tows, mats, braids, and woven fabrics. 

Rotational molding is used to form large shells of thermoplastic resin and chopped strands for such appHcations as agricultural tanks and fertilizer hoppers. The resin and chopped glass are placed in the metal mold that is then rotated in an oven where the thermoplastic resin melts and deposits the fiber on the metal surface. When cooled, the mold is opened and the part is removed. 

Fibrous Composites. These composites consist of fibers in a matrix. The fibers may be short or discontinuous and randomly arranged continuous filaments arranged parallel to each other in the form of woven rovings (coUections of bundles of continuous filaments) or braided (8). In the case of chopped strand mat the random arrangement is planar. In whisker (needle-shaped crystals or filaments of carbon and ceramics) reinforced materials the arrangement is usually three-dimensional and the resulting composites are macroscopically homogeneous. 

Fig. 5. Interlaminar fracture toughness, for a number of thermosetting and thermoplastic composites (36,37). Open white bars represent glass-fiber composites shaded bars are for carbon fibers. The materials are A, polyester (unidirectional) B, vinyl ester (CSM = chopped strand mat) C, epoxy (R/BR1424) D, epoxy (T300/914) E, PPS F, PES and G, PEEK. To convert J/m to fdbf/in. multiply by 2100.

This is a material produced from the impregnation of glass fibre-mat (fibreglass, which is in the form of dry sheet, is commonly known as chopped stranded mat (CSM)) or rovings, with a liquid and unsaturated polyester resin, which thickens chemically to a dry sheet form. The total mix is sandwiched between polyethylene films and then roller-pressed to impregnate and consolidate it. 

Chopped strand mat. This consists of chopped strands (bundles of glass filaments) about 2 in long bound togther by a resinous binder. This type of mat is used extensively in glass-reinforeed polyester struetures. 

Needle mat. This is similar to chopped strand mat except that the mat is held together by a loose stitching rather than a binder. 

A number of different binder materials are in use for chopped strand mat and include starch, polyvinyl acetate and polyesters. The binder used depends on the end use of the laminate and the method of fabrication. 

Furane resin-chopped strand mat laminates have tensile strengths in excess of 200001bf/in (140 MPa), a heat distortion temperature of about 218°C and good fire resistance. 

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

In order to get the best out of fibre reinforcement it is not uncommon to try to control within close limits the fibre content which will provide maximum stiffness for a fixed weight of matrix and fibres. In flexure it has been found that optimum stiffness is achieved when the volume fraction is 0.2 for chopped strand mat (CSM) and 0.37 for continuous fibre reinforcement. 

A very wide sheet of grp which is known to contain intrinsic defects 1 mm long, is subjected to a fluctuating stress which varies from 0 to 80 MN/m. How many cycles would the sheet be expected to withstand if it is made from (a) chopped strand mat (CSM) and (b) woven roving (WR) reinforcement. The crack growth parameters C and n, and the critical stress intensity factors, Kc, for these materials are... 

A sheet of chopped strand mat-reinforced polyester is 5 mm thick and 10 mm wide. If its modulus is 8 GN/m calculate its flexural stiffness when subjected to a point load of 200 N midway along a simply supported span of 300 mm. Compare this with the stiffness of a composite beam made up of two 2.5 mm thick layers of this reinforced material separated by a 10 mm thick core of foamed plastic with a modulus of 40 MN/m. ... 

Chopped Strand Mat This consists of strands of long chopped fibres deposited randomly in the form of a mat. The strands are held together by a resinous binder. 

Chondroitin 6-sulfate, 4 706 Chondroitin sulfates, 20 456 Chopped strand mat (CSM), 26 751 Chopper pumps, 21 78 Chop-stx number, for boron hydrides, 4 183 Chorinated trisodium phosphate, 4 52 Christmas tree module design, 15 835 Chromacity diagrams, 7 313-315 Chromaphores, 19 379 Chromate coatings, 9 827 Chromate conversion coatings, 16 218 Chromated copper arsenate, 3 276 ... 

ISO 2558 1974 Textile glass chopped-strand mats for reinforcement of plastics -Determination of time of dissolution of the binder in styrene ISO 3344 1997 Reinforcement products - Determination of moisture content... 

ISO 10122 1995 Reinforcement materials -Tubular braided sleeves - Basis for a specification ISO 10371 1993 Reinforcement materials - Braided tapes - Basis for a specification ISO 12215-1 2000 Small craft - Hull construction and scantlings - Part 1 Materials Thermosetting resins, glass-fibre reinforcement, reference laminate ISO 15100 2000 Plastics - Reinforcement fibres - Chopped strands - Determination of bulk density... 

ISO 3616 2001 Textile glass - Chopped-strand and continuous-filament mats -Determination of average thickness, thickness under load and recovery after compression ISO 4603 1993 Textile glass - Woven fabrics - Determination of thickness ISO 4604 1978 Textile glass - Woven fabrics - Determination of conventional flexural stiffness - Fixed angle flexometer method... 

Because of problems with feeding to the melt and preserving fibre length during compounding, the fibre forms in use in thermoplastics are mostly of short length (chopped strand or milled fibre). Despite this they are still able to produce valuable effects. 

Intuitively, however, the RVE for an amorphous polymer may be expected to be no more than 10 nanometres or so. In an aligned fibre composite with continuous fibres we need to take a sample sufficiently large as to contain perhaps 30 to 50 fibres in a lateral dimension and in this case the coefficient of variation of stress as between fibres would enable a measure to be decided. In a random array of fibres however (chopped strand mat) the RVE must be of the order of a fibre length at least as is also the case for an aligned short fibre composite. 

---