Polymers and composites

Why it matters

The same polymer can be stiff or soft, clear or cloudy, tough or brittle depending on how it was made and processed. A materials scientist in plastics or composites spends much of their time linking processing conditions to structure (chain length, crystallinity, orientation, voids) and structure to properties. When a molded part cracks or warps, the cause is usually in that chain of links.

Thermoplastics, thermosets and elastomers

• Thermoplastics (polyethylene, polypropylene, nylon, PET, polycarbonate) are long chains that soften on heating and can be remelted. They are processed as melts.
• Thermosets (epoxies, phenolics, unsaturated polyesters) cure into a crosslinked network that does not remelt. They are processed as liquids or soft resins that react in the mold.
• Elastomers are lightly crosslinked networks above their Tg, so they stretch and recover.

Molecular weight

Polymers contain chains of many lengths, so we use averages:

number average   Mn = sum(N_i * M_i) / sum(N_i)
weight average   Mw = sum(N_i * M_i^2) / sum(N_i * M_i)
dispersity       D  = Mw / Mn   (>= 1)

Worked example. A blend has 10 chains of 10,000 g/mol and 10 chains of 100,000 g/mol. Mn = 55,000; Mw = (10 × 10^8 + 10 × 10^10) / (10 × 10^4 + 10 × 10^5) = 91,800; D = 1.67. Mw is pulled toward the long chains, which dominate melt viscosity and toughness.

Higher molecular weight usually means higher toughness and strength, but also a much higher melt viscosity, which makes molding harder. Processing itself can cut chains: excessive melt temperature, long residence time, or moisture in nylon and PET (hydrolysis) lower the molecular weight and make parts brittle. That is why these resins are dried before molding.

Crystallinity and cooling

Semicrystalline polymers (PE, PP, nylon, PET) form crystalline lamellae between amorphous regions. Crystallinity raises stiffness, strength, chemical resistance and shrinkage, and lowers clarity and toughness. It depends strongly on cooling rate:

• A hot mold and slow cooling give higher crystallinity (and more shrinkage).
• A cold mold and fast cooling give lower crystallinity; PET quenched fast enough stays amorphous and clear, which is how bottle preforms are made.

You measure the result with DSC (previous module): the melting enthalpy divided by that of a 100% crystalline polymer.

Melt processing

• Extrusion pushes melt through a die continuously: pipe, film, sheet, profiles, fiber. Drawing the extrudate orients chains along the machine direction, so film is stronger in one direction.
• Injection molding fills a closed mold at high pressure, packs it to compensate for shrinkage, then cools it. Key settings are melt temperature, mold temperature, injection speed, pack pressure and time, and cooling time. Results to watch: shrinkage and warpage, weld lines where flow fronts meet (weaker), sink marks at thick sections, residual stress and flow-induced orientation.
• Blow molding and thermoforming shape a parison or sheet with air pressure or vacuum.

Design and processing interact: uniform wall thickness, generous radii and gate placement prevent many defects before any setting is changed.

Composites

A composite combines a reinforcement (glass, carbon or aramid fibers) with a matrix (usually a polymer). Fibers carry load; the matrix transfers load, holds the fibers in place and protects them.

For continuous fibers aligned with the load, the rule of mixtures estimates stiffness:

longitudinal  E1 = Vf * Ef + Vm * Em
transverse    1/E2 = Vf/Ef + Vm/Em   (a rough lower-bound estimate)

Worked example (illustrative values). Carbon fiber with Ef = 230 GPa in epoxy with Em = 3.5 GPa at a fiber volume fraction of 0.60: E1 = 0.6 × 230 + 0.4 × 3.5 = 139 GPa, but E2 is only about 8.6 GPa. The material is very stiff along the fibers and weak across them, which is why laminates stack plies at 0, ±45 and 90 degrees.

Processing routes include hand layup, prepreg cured in an autoclave (heat plus pressure, low voids, aerospace), resin transfer molding (RTM), filament winding and pultrusion. The cure cycle (time, temperature, pressure) sets the degree of cure and Tg, and pressure squeezes out voids. A few percent of voids can cut interlaminar shear strength noticeably, so void content is a routine quality check (by density, burn-off or micro-CT).

Common mistakes

• Blaming the resin for brittle parts when wet resin or overheating degraded the molecular weight.
• Ignoring orientation: testing bars molded in one flow direction and assuming the part behaves the same in every direction.
• Treating composites as isotropic and using one modulus for all directions.
• Under-curing a thermoset to save cycle time, which lowers Tg and strength.

Safety and training

Polymer and composite processing involves hot melts, high pressures, moving machinery, curing agents and fine fibers and dusts. Work on these processes only after your employer's or institution's training, with the required guarding, ventilation and protective equipment.

Recap

• Mn, Mw and dispersity describe chain length; Mw drives viscosity and toughness, and processing can degrade it.
• Cooling rate controls crystallinity, which controls stiffness, shrinkage and clarity.
• Injection molding defects come from fill, pack and cool; composites are anisotropic and sensitive to cure and voids.