Materials of construction

Plastics comprise many different materials based on their polymer structure, additives, and so on. Practically all plastics at some stage in their fabrication can be formed into simple to extremely complex shapes that can range from being extremely flexible to extremely strong. Polymers, the basic ingredients in plastics, are high molecular weight organic chemical compounds, synthetic or natural substances consisting of molecules. Practically all of these polymers use virtually an endless array of additives, fillers, and reinforcements to perform properly during product fabrication and/or in service. There are many compounded base polymer combinations so that new materials are always on the horizon to meet new industry requirements that now total over 35,000 plastics worldwide. Table 1.1 provides examples of their manufacturing stages from raw materials to products. Examples of the diversified use of different plastics are shown in Figs. 1.2 and 1.3.

Plastic, polymer, resin, elastomer, and reinforced plastic (RP) are somewhat synonymous terms. The most popular term worldwide is plastics. Polymer denotes the basic material. Whereas plastic pertains to polymers or resins (as well as elastomers, RPs, etc.) containing additives, fillers, and/or reinforcements. An elastomer is a rubber-like material (natural or synthetic). Reinforced plastics (also called plastic composites) are plastics with reinforcing additives such as fibers and whiskers, added principally to increase the product's mechanical properties but also provides other benefits such as increased heat resistance and improved tolerance control.

There are thermoplastics (TPs) that melt (also called curing) during processing. Cure occurs only with thermoset plastics (TSs) or when a TP is converted to a TS plastic and in turn processed. The term curing TPs occurred since at the beginning of the 20th century the term

Table 1.1 Examples of stages in plastic manufacturing Basic Chemicals

Petroleum is converted to petrochemicals such as ethylene, benzene, propylene and Acetylene.

Monomers

Petrochemicals plus other chemicals are converted into monomers such as styrene, ethylene, propylene, vinyl chloride, and acrylontrile.

Polymerization

One or more monomers are polymerized to form polymers or copolymers such as polyethylene, polystyrene, polyvinyl chloride, and polypropylene.

Compounding

Additives, fillers, and/or reinforcements are mixed with polymers (referred to as plastics) providing different properties and/or different fabricating methods for plastics. Hundreds of different materials are used such as heat stabilizers, color pigments, antioxidants, inhibitors, and fire retardants.

Processing

Plastics are formed into different shapes such as sheets, films, pipes, buckets, primary and secondary structures (boats, cars, airplanes, bridges, etc.), toys, housings, and many thousand more products. Basically heat and pressure are used to shape these products that usually are in finished form. Processes used include extrusion, injection molding, blow molding, thermoforming, compression molding, spraying, rotational moldings, reaction injection molding, and filament winding.

Finishing

In certain applications a finishing step is required on the fabricated part such as printing, bonding, machining, etc.

curing was accurately used for TSs. At that time TSs represented practically all the plastic used worldwide. Thus TPs took on the incorrect term curing even though there is no chemical reacdon or curing action.

Appreciate the polymer chemist's ability to literally rearrange the molecular structure of the polymer to provide an almost infinite variety of compositions that differ in form, appearance, properties, cost, and other characteristics. One must also approach the subject with a completely open mind that will accept all the contradictions that could make it difficult to pin common labels on the different families of plastics or even on the many various types within a single family that are reviewed in this book. Each plastic (of the 35,000 available) has specific performance and processing capabilities.

f igure 1 7. Use of plastics in recreational products range from unsophisticated types to high performance types such boats (Courtesy of Plastics FALLO)

Boeing 777 uses different types of plastics that include high performance reinforced plastics

Boeing 777 uses different types of plastics that include high performance reinforced plastics

Rudder Fin torque box Stabilizer torque box

There are many different routes that the starting materials for plastics can take on the way to the user. In this book, we are concerned with those plastics that are supplied to the processor in the form of granules, powder, pellets, flake, or liquids and in turn they are transformed into plastic products. However, the same starting materials used to make these plastics can take other routes and end up in the textile industry (nylon fibers share common roots with a molded nylon gear; acrylic fibers share common roots with acrylic sheet for glazing; etc.), paint industry, adhesives industry, and other industries meeting their special requirements.

Worldwide total plastic consumption is over 154 million ton (340 billion lb) with about 90wt% thermoplastics (TPs) and 10% thermoset

(TS) plastics. USA and Europe consumption is about one-third each of the world total.

These two major classifications of thermoplastics (TPs) and thermosets (TSs) in turn have different classifications such as virgin or recycled plastics. Virgin plastics have not been subjected to any fabricating process. NEAT plastics identify plastics with Nothing Else Added To. They are true virgin polymers since they do not contain additives, fillers, etc. However they are rarely used since they do not provide the best performances. Thus the technically correct term to identify the materials is plastics. Of the 35,000 types available worldwide there are about 200 basic types or families that are conunercially recognized with less than 20 that are popularly used. Examples of these plastics are shown in Table 1.2.

Summation of the plastic families with their abbreviations

Acetal (POM) Acrylics Polyacrylonitrile (PAN) Polymethylmethacrylate (PMMA) Acrylonitrile butadiene styrene (ABS)

Ally I diglycol carbonate Alkyd

Diallyl isophthalate (DAIP) Diallyl phthalate (DAP) Aminos Melamine formaldehyde (MF) Urea formaldehyde (UF) Cellulosics Cellulose acetate (CA) Cellulose acetate butyrate (CAB) Cellulose acetate propionate (CAP) Cellulose nitrate Ethyl cellulose (EC) Chlorinated polyether Epoxy (EP)

Ethylene vinyl acetate (EVA) Ethylene vinyl alcohol (EVOH) Fluorocarbons Fluorinated ethylene propylene (FEP)

Polytetrafluoroethylene (FTFE) Polyvinyl fluoride (PVF) Polyvinylidene fluoride (PVDF) lonomer

Liquid crystal polymer (LCP) Aromatic copolyester (TP polyester) Melamine formaldehyde (MF) Nylon (or Polyamide) (PA) Parylene Phenolic Phenolic

Phenol formaldehyde (PF) Polyamide-imide (PAI) Polyarylether Polyaryletherketone (PAEK) Polyaryl sulfone (PAS) Polyarylate (PAR) Polycarbonate Polyester Saturated polyester (TS

polyester) Thermoplastic polyesters (TP polyester)

Polybutylene terephthalate (PBT)

Polyethylene terephthalate (PET)

Table 1.2 continued

Polyetherketone (PEK) Polyetheretherketone (PEEK) Polyetherimide (PEI) Polyimide (PI) Thermoplastic PI Thermoset PI Polymethylmethacrylate (or acrylic) (PMMA) Polyolefin (PO) Chlorinated PE (CPE) Cross-linked PE (XLPE) High-density PE (HDPE) Linear LDPE (LLDPE) Low-density PE (LDPE) Polyallomer Polybutylene (PE) Polyethylene (PE) Polypropylene (PP) Ultra-high molecular weight PE (UHMWPE) Polyurethane (PUR) Silicone (SI)

Styrene

Acrylic styrene acrylonitrile (ASA) Acrylonitrile butadiene styrene (ABS)

General-purpose PS (GPPS) High-impact PS (HIPS) Polystyrene (PS) Styrene acrylonitrile (SAN) Styrene butadiene (SB) Sulfone Polyether sulfone (PES) Polyphenyl sulfone (PPS) Polysulfone (PSU) Urea formaldehyde (UF) Vinyl

Chlorinated PVC (CPVC) Polyvinyl acetate (PVAc) Polyvinyl alcohol (PVA) Polyvinyl butyrate (PVB) Polyvinyl chloride (PVC) Polyvinylidene chloride (PVDC) Polyvinylidene fluoride (PVF)

Within these 20 popular plastics there are five major TP types that consume about two-thirds of all TPs. Approximately 20wt% are low density polyethylenes (LDPEs), 15% polyvinyl chlorides (PVCs), 10% high density polyethylenes (HDPEs), 15% polypropylenes (PPs), 8% polystyrenes (PSs). Each has literally many thousands of different formulated compounds and different processing and performance behaviors. These basic types, with their many modifications of different additives/fillers/reinforcements, catalyst systems, grafting, and/or alloying provide different processing capabilities and/or product performances. As examples there are the relatively new generation of high performance metallocene and elastomeric plastics providing different modifications.

Thermoplastics

TPs are plastics that soften when heated and upon cooling harden into products. TPs can be repeatedly softened by reheating. Their morphology, molecular structure, is crystalline or amorphous. Softening temperatures vary. The usual analogy is a block of ice that can be softened (turned back to a liquid), poured into any shape mold or die, then cooled to become a solid again. This cycle repeats. During the heating cycle care must be taken to avoid degrading or decomposition of the plastic. TPs generally offer easier processing and better adaptability to complex designs than do TS plastics.

There are practical limits to the number of heating and cooling cycles before appearance and/or mechanical properties are drastically affected. Certain TPs have no immediate changes while others have immediate changes after the first heating/cooling cycle.

Crystalline & Amorphous Polymers

The overall molecular physical structure of a polymer identifies its morphology. Crystalline molecular structures tend to have their molecules arranged in a relatively regular repeating structure such as acetal (POM), polyethylene (PE), polypropylene (PP), nylon (PA), and polytetrafluoroethylene plastics. The structures tend to form like cooked spaghetti. These crystallized plastics have excellent chemical resistance. They are usually translucent or opaque but they can be made transparent with chemical modification. They generally have higher strength and softening points and require closer temperature/time processing control than the amorphous TPs.

Polymer molecules that can be packed closely together can more easily form crystalline structures in which the molecules align themselves in some orderly pattern. Commercially crystalline polymers have up to 80% crystalline structure and the rest is amorphous. They are identified technically as semicrystalline TPs. Polymers with 100% crystalline structures are not commercially produced.

Amorphous TPs have no crystalline structure. Their molecules form no patterns. These TPs have no sharp melting points. They are usually glassy and transparent, such as acrylontrile-butadiene-styrene (ABS), acrylic (PMMA), polycarbonate (PC), polystyrene (PS), and polyvinyl chloride (PVC). Amorphous plastics soften gradually as they are heated during processing. If they are rigid, they may become brittle unless modified with certain additives.

During processing, all plastics are normally in the amorphous state with no definite order of molecular chains. TPs that normally crystallize need to be properly quenched; that is, the hot melt is cooled to solidify the plastic. If not properly quenched, they become amorphous or partially amorphous solids, usually resulting in inferior properties. Compared to crystalline types, amorphous polymers undergo only small volumetric changes when melting or solidifying during processing. This action influences the dimensional tolerances that can be met after accounting for the heating/cooling process and the design of molds or dies.

Crystalline plastics require tighter process control during fabrication. They tend to shrink and warp more than amorphous types due to their higher melting temperatures, with their relatively sharp melting point, they do not soften gradually with increasing temperature but remain hard until a given quantity of heat has been absorbed, then change rapidly into a low-viscosity liquid. If the correct amount of heat is not applied properly during processing, product performance can be drastically reduced and/or an increase in processing cost occurs. With proper process control this is not a problem.

During the melting process as the symmetrical molecules approach each other within a critical distance, crystals begin to form. They form first in the areas where they are the most densely packed. This crystallized area becomes stiff and strong. The noncrystallized, amorphous, area is tougher and more flexible. With increased crystallinity, other effects occur such as with polyethylene (crystalline plastic) there is increased resistance to creep.

Liquid Crystalline Polymers

A special classification of TPs are liquid crystalline polymers. They are self-reinforcing because of densely packed fibrous polymer chains. Their molecules are stiff, rod-like structures organized in large parallel arrays in both the melt and solid states. They resist most chemicals, weathers oxidation, and can provide flame resistance, making them excellent replacements for metals, ceramics, and other plastics in many product designs. They are exceptionally inert and resist stress cracking in the presence of most chemicals at elevated temperatures, including the aromatic and halogenated hydrocarbons as well as strong acids, bases, ketones, and other aggressive industrial products. Regarding flammability, LCPs have an oxygen index ranging from 35 to 50% (ASTM). When exposed to an open flame, they form an intumescent char that prevents dripping.

When injection molded or extruded the molecules align into long, rigid chains that in turn align in the direction of flow. Thus the molecules act like reinforcing fibers giving LCPs both very high strength and stiffness. LPCs with their high strength-to-weight ratios are particularly useful for weight-sensitive products (Table 1.3). They have outstanding strength at extreme temperatures, excellent mechanical property retention after exposure to weathering and radiation, good dielectric strength as well as arc resistance and dimensional stability, low coefficient of thermal expansion, excellent flame resistance, and easy processability.

Liquid crystal polymer properties compared to other thermoplastics

Property

Crystalline

Amorphous

Liquid crystalline

Specific gravity

Higher

Lower

Higher

Tensile strength

Higher

Lower

Highest

Tensile modulus

Higher

Lower

Highest

Ductility, elongation

Lower

Higher

Lowest

Resistance to creep

Higher

Lower

High

Maximum usage temperature

Higher

Lower

High

Shrinkage and warpage

Higher

Lower

Lowest

Chemical resistance

Higher

Lower

Highest

Their UL (Underwriters Laboratory) continuous-use rating for electrical properties is as high as 240°C (464°F), and for mechanical properties it is 220°C (428°F) permiting products to be exposed to intermittent temperatures as high as 315°C (600°F) without affecting performance properties. Their resistance to high-temperature flexural creep is excellent, as are their fracture-toughness characteristics.

Because of their structure they provide special properties such as greater resistance to most solvents and heat. They have the lowest warpage and shrinkage of all the TPs. Unlike many high-temperature plastics, LCPs have a low melt viscosity and are thus more easily processed resulting in faster cycle times than those with a high melt viscosity thus reducing processing costs.

Thermosets

Outstanding properties of TS plastic products are their substantially infusible and insoluble characteristic along with resistance to high temperatures, greater dimensional stability, and strength. TSs undergo a crosslinking chemical reaction by techniques such as the action of heat (exothermic reaction), oxidation, radiation, and/or other means often in the presence of curing agents and catalysts. However, if excessive heat is applied, degradation rather than melting will occur.

TSs are not recyclable because they do not melt when reheated, although they can be granulated and used as filler in other TSs as well as TPs. An analogy of TSs is that of a hard-boiled egg that has turned from a liquid to a solid and cannot be converted back to a liquid. As shown in Fig. 1.4, TSs are identified by A-B-C-stages during the curing process. A-stage is uncured, B-stage is partially cured, and C-stage is fully cured. Typical B-stage is TS molding compounds and prepregs,

Figure 1.4 Thermoset A-B-C stages from melt to solidification

Figure 1.4 Thermoset A-B-C stages from melt to solidification

MELTING CROSS LINKING TIME-►

which in turn are processed to produce C-stage fully, cured plastic material products.

TSs generally cannot be used alone in primary or secondary structural applications; they must be filled with additives and/or reinforcements such as glass or wood fibers, etc. These compounds provide dimensional product precision and certain other desirable properties for use in certain products. There are TSs particularly suitable as substitutes for metals in products that have to meet severe demands such as high temperature with the added advantage of offering a very good cost reduction. Applications include kitchen appliances, heat-shield for an electric iron, collectors and a wide variety of circuit breaker housings in electrical devices, and automotive parts including headlamp reflectors, brake servo units, brake pistons, pump housings, valve caps, pulleys, and so on. Compression and transfer molding (CM and TM) are the two main methods used to produce molded products from TSs.

Within the TS family there are natural and synthetic rubbers, elastomers, such as styrene-butadiene, nitrile rubber, millable polyurethanes, silicone, butyl, and neoprene. They attain their properties through the process of vulcanization. Vulcanization is the process by which a natural rubber or certain plastic elastomer undergoes a change in its chemical structure brought about by the irreversible process of reacting the materials with sulfur and/or other suitable agents. The crosslinking action results in property changes such as decreased plastic flow, reduced surface tackiness, increased elasticity, greater tensile strength, and considerably less solubility.

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