Reinforcement performance

Reinforcements can significandy improve the structural characteristics of a TP or TS plastic. They are available in continuous forms (fibers, filaments, woven or non-woven fabrics, tapes, etc.), chopped forms having different lengths (Fig. 3.14), or discontinuous in form (whiskers, flakes, spheres, etc.) to meet different properties and/or processing methods. Glass fiber represents the major material used in RPs worldwide. There are others that provide much higher structural performances, etc. The reinforcements can allow the RP materials to be tailored to the design, or the design tailored to the material.

To be effective, the reinforcement must form a strong adhesive bond with the plastics; for certain reinforcements special cleaning, sizing, finishing, etc. treatments are used to improve bond. Also used alone or in conjunction with fiber surface treatments are bonding additives in the plastic to promote good adhesion of the fiber to the plastic.

FiqiJrr- 3.|4 Fiber strength vs. fiber length (Courtesy of Plastics FALLO)

FiqiJrr- 3.|4 Fiber strength vs. fiber length (Courtesy of Plastics FALLO)

FIBER LENGTHS (mm)

FIBER LENGTHS (mm)

Applicable to RPs is the aspect ratio of fibers. It is the ratio of length to diameter (L/D) of a fiber. In RP fiber L/D will have a dircct influence on the reinforced plasdc performance. High values of 5 to 10 provide for good reinforcements. Theoretically, with proper lay-up the highest performance plastics could be obtained when compared to other materials. To maximize strength and modulus of RPs the long fiber approach is used.

Different types of reinforcement construction are used to meet different RP properties and/or simplify reinforcement layup for certain fabricating processes to meet design performance requirements. They include woven, nonwoven, rovings, and others (Table 3.3). These different constructions are used to provide different processing and directional properties.

Example of E-glass constructions used in TS polyester RPs

Bulk Molding Compound

Sheet Molding Compound

Chopped Strand Mat

Woven Roving

Unidirectional Axial

Unidirectional Transverse

Glass content (wt °/o)

20

30

30

50

70

70

Tensile modulus GPa (Msi)

9(1.3)

13 (1.9)

7.7(1.1)

16 (2.3)

42 (6.1)

12 (1.7)

Tensile strength MPa (Ksi)

45 (6.5)

85 (12)

95(14)

250 (36)

750(110)

50 (7)

There are certain types of so-called nonwoven fabric that are directly formed from short or chopped fiber as well as continuous filaments. They are produced by loosely compressing together fibers, yarns, rovings, etc. with or without a scrim cloth carrier; assembled by mechanical, chemical, thermal, or solvent methods. Products of this type include melted and spun-bonded fabrics. The nonwoven spun-bonded integrates the spinning, lay-down, consolidation, and bonding of continuous filaments to form fabrics. Felt is the term used to describe nonwoven compressed fabrics, mats, and bats prepared from staple fibers without spinning, weaving, or knitting; made up of fibers interlocked mechanically.

A fibrous material extensively used in RPs are the mat constructions. They consist of different randomly and uniformly oriented products: (1) chopped fibers with or without carrier fibers or binder plastics; (2) short fibers with or without a carrier fabric; (3) swirled filaments loosely held together with a plastic binder; (4) chopped or short fiber with long fibers included in any desired pattern to provided addition mechanical properdes in specific direcdons; (5) and so on.

There are reinforcement preform construcdons. A preform is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes (injection, etc.). Oriented patterns can be incorporated in the preforms.

When conventional flat mats are used, they may tear, wrinkle, or give uneven glass distribution when producing complex shapes. To alleviate this problem, it is necessary to take great care in tailoring the mat and in placing it properly in the mold cavity. Otherwise, mats may cause poor products or poor production rates. Preforms are used to overcome these problems. They are slighdy more expensive for short production runs. However they are used when mats are considered impractical, or a relatively high producdon run exists that offsets the higher cost.

Fiber-reinforced plastics differ from many other materials because they combine two essentially different materials of fibers and a plastic into a single plastic composite. In this way they are somewhat analogous to reinforced concrete, that combines concrete and steel. However, in the RPs the fibers are generally much more evenly distributed throughout the mass and the ratio of fibers to plastic is much higher.

In designing fibrous-reinforced plastics it is necessary to take into account the combined acdons of the fiber and the plastic. At times the combination can be considered homogeneous, but in most cases homogeneity cannot be assumed (Chapter 2).

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