DESIGN IN PLASTICS: PART 2

Physical And Mechanical
Properties of Plastics

This article is the second in a 5-part series that reviews basic concepts for designing and fabricating plastic parts. The first, in August of this year http://www.1rs.com/011df-525 looked at design considerations for injection-molded parts.

By Frank Jaarsma
Ticona Corporation, Summit, NJ

Choosing a plastic for a specific use can be a daunting task. Designers face a seemingly endless variety of resins and a host of properties that define them. Each market usually needs a unique set of properties for the plastics used in it (Table 1). Electronic connectors, for example, are complex, precision components that need good flow in thin section and high dimensional stability. Packaging materials, by contrast, need stiffness, strength and good water vapor barrier properties. These requirements drive the process for selecting plastics. This article reviews plastic materials, and the key physical and mechanical measurements that define them.

Plastics are either thermosets or thermoplastics. Thermosets flow before molding but undergo chemical change during processing, which cures or hardens them to create a complex, interconnected network. If too much heat is added after this, the polymer degrades rather than melts. Thermosetting plastics include phenolic, epoxy, alkyd polyester, polyurethane, urea-formaldehyde and unsaturated polyester resins. Natural and synthetic rubbers, such as latex, nitrile, millable polyurethane, silicone, butyl and neoprene, are also thermosets.

Thermoplastics, by contrast, soften when heated and harden when cooled. Molding does not change their chemical structure. They have a performance-based hierarchy from commodity to engineering grades. Commodity thermoplastics include low and high density polyethylenes and polypropylene. Engineering thermoplastics include acetal, nylon, polycarbonate, polyphenylene sulfide and liquid crystal polymer. Those higher in the hierarchy generally carry greater loads and withstand impact, high temperature and chemical attack better.

Table 1
Key performance characteristics by market

Most thermoplastics are rigid, but some are highly elastic (thermoplastic elastomers, or TPEs), and can be stretched repeatedly to at least twice their original length at room temperature, then return to near their original length. Many injection-moldable TPEs are replacing traditional rubbers. They are often used to modify rigid thermoplastics to aid impact strength. Additionally, most thermoplastics are amorphous, but some have chains packed in an organized way and are considered partly or mostly crystalline. All crystalline plastics have amorphous regions between and connecting their crystalline regions. Liquid crystal polymer, a good example of a semicrystalline resin, has rodlike molecules set in parallel arrays that makes them stiffer, stronger and more resistant to creep, heat and chemicals (Table 2).

Table 2
Comparison of Properties

A plastic's properties depend on its chemistry, structure, chain length and the bonds between chains. Its properties can be altered by combining it mechanically and chemically with one or more polymers. Alloys, or mechanical blends of two or more polymers, improve performance and processability and lower material cost. Their properties usually fall between those of the starting polymers, although some have better properties than either one alone. Copolymers chemically combine two or more repeating units. Copolymers and homopolymers (one repeating unit) in the same plastic family can have different properties.

A plastic's physical and mechanical properties can be modified with additives, fillers and reinforcements. In general, mechanical properties are best increased by adding reinforcing fibers made of glass, metal, carbon or other materials. Particulate fillers like talc or ground calcium carbonate generally increase modulus, while plasticizers decrease modulus and enhance flexibility. Other common additives include flame retardants, oxidation inhibitors, and thermal and UV stabilizers.

Physical properties

Properties can vary through a plastic and with the direction of measurement. Plastics are homogeneous if they have the same makeup throughout as in many unfilled thermoplastics, or they can be heterogeneous and vary from point to point. If properties are the same when measured in any direction, the plastic is isotropic. If not, then it is anisotropic. In molded crystalline and glass-fiber-reinforced resins, properties can differ in the cross-flow and in-flow directions.

Physical properties often evaluated for specifying a plastic include:

* Density and specific gravity -- Density is mass per unit volume (lb/in.³ or g/cm), while specific gravity is the mass of a volume of material divided by the same volume of water (both at 23°C). As a dimensionless number, specific gravity is a good way to compare materials as regards part cost, weight and quality control.

* Water absorption -- the percent increase in weight of a specimen (dried for 24 hr.) before and after immersion in 23°C water for various times. This property affects dimensional stability and some mechanical and electrical properties.

* Mold shrinkage -- how much a part's dimension changes as it cools and solidifies in a mold divided by the mold dimension. Molds are sized to allow for shrinkage, which varies with wall thickness, flow direction and other conditions. Reinforced and filled plastics shrink less than neat ones.

* Elasticity -- the plastic's ability to return to its original size and shape after being deformed. This is high in rubber and TPEs. Contrast this with plasticity, the opposite of elasticity, in which a plastic holds the shape to which it is deformed. This occurs when a plastic is stressed beyond its yield point (where permanent deformation begins).

* Ductility -- how well a plastic can be deformed without fracture or how well it can be stretched, pulled or rolled without destroying its integrity. It is measured as a percent elongation.

* Opacity and transparency -- usually measured as haze and luminous transmittance. Haze is the percent of light transmitted through a specimen and scattered more than 2.5 deg from the incident beam. Luminous transmittance is the ratio of transmitted light to incident light.

* Lubricity -- measures load bearing characteristics under relative motion. Good lubricity goes with low coefficient of friction and a tendency not to gall (to be worn away by friction).

Mechanical properties

Mechanical properties are crucial in nearly all plastic applications. Laboratory-generated, short-term data provide ideal values that are useful in selecting a resin. Lab tests usually subject samples to a single, steady force for a limited time. In the real world, many factors occur at once, so parts should be evaluated in actual use to gauge how varying force, temperature and other factors affect them. Note that short-term values do not work for such structural responses as creep, impact and fatigue.

The most significant mechanical properties include:

* Toughness -- the ability to absorb mechanical energy without fracturing. High-impact, unfilled resins generally have excellent toughness. Brittle resins, which lack toughness, often have less impact strength and higher stiffness. Many glass-reinforced and mineral-filled materials are brittle.

* Tensile strength -- the maximum amount of tensile load per unit area a material can withstand.

* Tensile elongation -- how much length increases in response to a tensile load expressed as a percent of the original length. Elongation at break is the maximum elongation the plastic can undergo.

* Flexural strength -- how much of a bending load a plastic can withstand before it ruptures.

* Creep -- a plastic's deformation under load (tension, compression or flexure) over time. It does not include the initial change in dimension when the load is applies. The rate of creep depends on applied stress, temperature and time. It is usually measured at four or more stress levels and plotted as strain vs. log of time. Crystalline resins usually have lower creep rates than amorphous ones. Glass fiber reinforcement improves creep resistance. A plastic part will fail when too high a fixed strain is imposed for too long a time.

* Impact loading -- evaluates how well a part absorbs energy from an impact and is dependent on the shape, size, thickness and type of material. Tests can be done on notched or unnotched samples. Notched tests measure how easily a crack propagates through a material. Impact tests are not analytical, but can be used to compare the relative impact resistance of materials. The main impact tests are:

* Izod impact strength (mainly the U.S.) --a pendulum arm is swung from a height to impact a notched cantilevered specimen. The distance the pendulum travels after fracturing the material indicates the loss of energy, which is the Izod strength in ft-lb/in or J/m. This test also has unnotched or reversed notched (facing away from pendulum) versions.

* Charpy impact (mainly in Europe) -- like the Izod measurement, except the beam is supported at both ends. The pendulum hits the beam at its midpoint.

* Falling dart impact -- involves dropping a weight onto a flat disk. Height of release and dart weight are varied until half the specimens break.

* Fatigue endurance -- evaluates the mechanical deterioration and failure of parts that are cyclically stressed. This test subjects a cantilevered beam to reverse flexural loading cycles at different stress levels.

The next installment of this series will continue to look at plastic properties by reviewing thermal, electrical and chemical properties.

For more information:

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