Reinforced Plastics

The term reinforced plastic (RP) refers to composite combinations of plastic, matrix, and reinforcing materials, which predominantly come in chopped and continuous fiber forms as in woven and nonwoven fabrics. Other terms used to identify an RP include: glass fiber reinforced plastic (GFRP), aramid fiber reinforced plastic (AFRP), boron fiber reinforced plastic (BFRP), carbon fiber reinforced plastic (CFRP), graphite fiber reinforced plastic (GFRP), etc.

In addition to fabrics, reinforcements include other forms such as powders, beads, and flakes. Both TP and TS plastics are used in reinforced plastics. At least 90wt% use glass fiber materials. At least 55wt% use TPs. RPs using primarily TS polyester plastics provides significant property and/or cost improvements compared to other composites. Primary benefits of all RPs include high strength, directional strength, lightweight, high strength-to-weight ratio, creep and fatigue endurance, high dielectric strength, corrosion resistance, and long term durability.

Both reinforced TSs (RTSs) and reinforced TPs (RTPs) can be characterized as engineering plastics, competing with engineering unreinforced TPs. When comparing processability of RTSs and RTPs, the RTPs are usually easier to process and permit faster molding cycles with efficient processing such as during injection molding. Higher performing fibers that are used include high performance glass (other than the usual E-glass), aramid, carbon, and graphite. Also available are whisker reinforcements with exceptional high performances (Fig. 1.5).

. . High performance whisker reinforcements compared to other materials

I ~l Ultimate tensile strength, Ibs/sq. in. x 103

100 200 400 600 800 1000 1500 2000 2500 3000

Aluminum alloy Titanium alloy High tensile steel Special glass fibre Epoxy fibreglass Silica Fibre Epoxy carbon fibre Boron fibre Carbon fibre Iron whisker SiC whisker Graphite whisker

5 10 15 20 25 30 35 40 45 Specific modulus, in.'' x 10®

Fiber orientations have improved to the extent that 2-D and 3-D RPs can be used to produce very high strength and stiff products with long service lives. RTPs even with their relatively lower properties compared to RTSs are used in about 55wt% of all RP products. Practically all RTPs with short glass fibers are injection molded at very fast cycles, producing high performance products in highly automated environments.

RPs can be characterized by their ability to be molded into either extremely small to extremely large structurally loaded shapes well beyond the basic capabilities of other materials or processes at little or no pressure. In addition to shape and size, RPs possess other characteristics that make them very desirable in design engineering. The other

I ~l Ultimate tensile strength, Ibs/sq. in. x 103

100 200 400 600 800 1000 1500 2000 2500 3000

5 10 15 20 25 30 35 40 45 Specific modulus, in.'' x 10®

characteristics include cost reduction, ease of fabrication, simplified installation, weight reduction, aesthetic appeal, and the potential to be combined with many other useful qualities.

Their products have gone worldwide into the deep ocean waters, on land, and into the air including landing on the moon and in spacecraft. In USA annual consumption of all forms of RPs is over 3.9 billion lb (1.8 billion kg). Consumption by market in million lb is aerospace at 24, appliances/business machines at 210, building/construction at 775, consumer at 253, corrosion at 442, electrical/electronic at 390, marine at 422, transport at 1268, and others at 116.

The form the RP takes, as with non-reinforced plastics, is determined by the product requirements. It has no inherent form of its own; it must be shaped. This provides an opportunity to select the most efficient forms for the application. Shape can help to overcome limitations that may exist in using a lower-cost material with low stiffness. As an example underground fuel tanks can include ribs to provide added strength and stiffness to the RP orientation in order to meet required stresses at the lowest production cost.

The formability of these products usually leads to one-piece consolidation of construction products to eliminate joints, fasteners, seals, and other potential joining problems. As an example, formed building fascia panels eliminate many fastenings and seals. Examples of design characteristics gained by using RP materials are presented as follows:

Thermal Expansions

Nonreinforced plastics generally have much higher coefficients of linear thermal expansion (CLTE) than conventional metal, wood, concrete, and other materials. CLTEs also vary significandy with temperature changes. There are RPs that do not have these characteristics. With certain types and forms of fillers, such as graphite, RPs can eliminate CLTE or actually shrink when the temperature increases.

Ductilities

Substantial yielding can occur in response to loading beyond the limit of approximate proportionality of stress to strain. This action is referred to as ductility. Most RPs do not exhibit such behavior. However, the absence of ductility does not necessarily result in britdeness or lack of flexibility. For example, glass fiber-TS polyester RPs do not exhibit ductility in their stress-strain behavior, yet they are not britde, have good flexibility, and do not shatter upon impact. TS plastic matrix is britde when unreinforced. However, with the addition of glass or other fibers in any orientation except parallel, unidirectional, the fibers arrest crack propagation. This RP construction results in toughness and the ability to absorb a high amount of energy. Because of the generally high ratio of strength to stiffness of RPs, energy absorption is accomplished by high elastic deflection prior to failure. Thus ductility has been a major factor promoting the use of RPs in many different applications.

Toughness

The generally low-specific gravity and high strength of reinforcement fibers such as glass, aramid, carbon, and graphite can provide additional benefits of toughness. For example, the toughness of these fibers allows them to be molded into very thin constructions. Each fiber has special characteristics. For instance, compared to other fiber reinforcements, aramid fibers can increase wear resistance with exceptionally high strength or modulus to weight.

Tolerances/Shrinkages

TSs combined with all types of reinforcements and/or fillers are generally more suitable for meeting tight dimensional tolerances than are TPs. For injection molded products they can be held to extremely close tolerances of less than a thousandth of an inch (0.0025 cm) effectively down to zero (0.0%). Achievable tolerances range from 5% for 0.020 in. (0.05 cm), to 1% for 0.500 in. (1.27 cm), to 1/2% for 1.000 in. (2.54 cm), to 1/4% for 5.000 in. (12.70 cm), and so on.

Some unreinforced molded plastics change dimensions, shrink, immediately after molding or in a day or a month due to material relaxation and changes in temperature, humidity, and/or load application. RPs can significandy reduce or even eliminate this dimensional change after molding.

When comparing tolerances and shrinkage behaviors of RTSs and RTPs there is a significant difference. Working with crystalline RTPs can be yet more complicated if the fabricator does not understand their behavior. Crystalline plastics generally have different rates of shrinkage in the longitudinal, melt flow direction, and transverse directions. In turn, these directional shrinkages can vary significandy due to changes in processes such as during injection molding (IM). Tolerance and shrinkage behaviors are influenced by factors such as injection pressure, melt heat, mold heat, and part thickness and shape. The amorphous type materials can be easier to balance.

Compounds

Commercial RP compounds are available in several forms: pellets for injection molding or extrusion, unidirectional tape for filament winding and similar applications, sheets for stamping and compression molding, bulk compounds for compression molding, and so on. There are RTP

elastomeric materials that provide special engineered products such as conveyor belts, mechanical belts, high temperature or chemical resistant suits, wire and cable insulation, and architectural designed shapes. Common categories of RP compounds are reviewed.

Prepregs

Preimpregnated materials usually are a compound of a reinforcement and a hot melt or solvent system. Prepreg also includes wet systems without solvent using TS polyester. They are stored for use at a latter time either in-house or to ship to a fabricator. The plastic is partially cured, B-stage, ready-to-mold material in web form that may have a substrate of glass fiber mat, fabric, roving, paper, cotton cloth, and so forth. With proper temperature storage conditions, their shelf life can be controlled to last at least 6 months.

Sheet Molding Compounds

A ready-to-mold material, SMC represent a special form of a prepreg. It is usually a glass fiber-reinforced TS polyester resin compound in sheet form. The sheet can be rolled into coils during its continuous fabricating process. A plastic film covering, usually polyethylene, separates the layers to enable coiling and to prevent contamination, sticking, and monomer evaporation. This film is removed before the SMC is charged into a mold, such as a matched-die or compression mold.

Depending on product performance requirements, the SMC consists of additional ingredients such as low-profile additives, cure initiators, thickeners, and mold-release agents. They are used to enhance the performance or processing of the material. Glass fibers are usually chopped into lengths of 12 mm (0.5 in.) to at least 50 mm (2 in.). The amount can vary from 25 to 50wt%. The usual ratio is based on performance requirements, processability, and cost considerations.

Bulk Molding Compounds

Also called dough molding compounds (DMCs), bulk molding compounds (BMCs) are mixtures usually of short 3 mm to 3 cm (V8 to l]/4in.) glass fibers, plastic, and additives similar to the SMC compound. This mixture, with the consistency of modeling clay, can be produced in bulk form or extruded in rope-like form for easy handling. The extrudate type is called a "log" that is cut to specific lengths such as 0.3 cm (1 ft).

BMC is commercially available in different combinations of resins, predominandy TS polyesters, additives, and reinforcements. They meet a wide variety of end-use requirements in high-volume applications where fine finish, good dimensional stability, part complexity, and good overall mechanical properties are important. The most popular method of molding BMCs is compression. They can also be injection molded in much the same way as other RTS compounds using ram, ram-screw, and, for certain BMC mixes, conventional reciprocating screw.

Commodity Et Engineering Plastics

About 90wt% of plastics can be classified as commodity plastics (CPs), the others being engineering plastics (EPs). The EPs such as polycarbonate (PC) representing at least 50wt% of all EPs, nylon, acetal, etc. are characterized by improved performance in higher mechanical properties, better heat resistance, and so forth (Table 1.4).

' Thermoplastic engineering behaviors

Crystalline

Amorphous

Acetal

Polycarbonate

Best property balance

Good impact resistance

Stiffest unreinforced thermoplastic

Transparent

Low friction

Good electrical properties

Nylon

Modified PPO

High melting point

Hydrolytic stability

High elongation

Good impact resistance

Toughest thermoplastic

Good electrical properties

Absorbs moisture

Polyester (glass-reinforced)

High stiffness

Lowest creep

Excellent electrical properties

The EPs demand a higher price. About a half century ago the price per pound was at 20<t; at the turn of the century it went to $1.00, and now higher. When CPs with certain reinforcements and/or alloys with other plastics are prepared they become EPs. Many TSs and RPs are EPs.

Elastomers/Rubbers

In the past rubber meant a natural thermoset elastomeric (TSE) material obtained from a rubber tree, hevea braziliensis. The term elastomer developed with the advent of rubber-like synthetic materials. Elastomers identify natural or synthetic TS elastomers (TSEs) and thermoplastic elastomers (TPEs). At room temperature all elastomers basically stretch under low stress to at least twice in length and snaps back to approximately the original length on release of the stress, pull, within a specified time period.

The term elastomer is often used interchangeably with the term plastic or rubber; however, certain industries use only one or the other terminology. Different properties identify them such as strength and stiffness, abrasion resistance, oil resistance, chemical resistance, shock and vibration control, electrical and thermal insulation, waterproofing, tear resistance, cost-to-performance, etc.

Natural rubber with over a century's use in many different products and markets will always be required to attain certain desired properties not equaled (to date) by synthetic elastomers. Examples include transportation tires, with their relative heat build-up resistance, and certain types vibrators. However, both synthetic TSE and TPE have made major inroads in product markets previously held only by natural rubber. Worldwide, more synthetic types are used than natural. The basic processing types are conventional, vulcanizable, elastomer, reactive type, and thermoplastic elastomer.

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