Materials - Thermosets, Resins, Reinforcements, & Laminates

Thermoset Plastics

• made differently from thermal plastics

• polymerization (curing) done in 2 stages

  • material is partially polymerized by manufacturer

  • performed by the molder - crosslinking between chains

• polymerization is 3 dimensional crosslinking done during molding

• 3 or more reactive sites are needed for crosslinking

• strong physical bonds of the crosslinks are irreversible, material cannot be remolded after cure

• too much heat, chains back, properties degrade

• in general, thermosets have greater resistance to heat and better dimensional stability than thermoplastics

Thermoset Plastic Types

Rigid Thermosets - short chains, many crosslinks

Flexible Thermosets - longer chains, fewer crosslinks

Thermoset Plastic Curing Reactions

1. Condensation Reaction - 2 or more unlike molecules combined to form larger molecule

   • by product such as water or gas is created in the mold

2. Addition Reaction - no byproduct produced fewer molding problems

   • polyester and epoxy curing

What is a Resin?

• Resin in the term often used for an uncured polymer

• many resins are formed at room temperature by condensation reactions

• many Thermoset are liquid at room temperature

• conversion of monomer to polymer occurs in fabrication process

  • degree of polymerization dependent on heat and pressure

  • liquid changes to solid - increased viscosity

Rate of Polymerization

• some cured by heat

• others use catalysts and promoters

  • catalysts - material that triggers curing process

  • promoter - controls the rate of cure

    • accelerator

    • inhibitor

  • Percent of promotor/inhibitor depends on

    • storage temperature of resin

    • temperature of working area

    • amount of working time

Types of Resins

• polyester

• epoxy

• acrylic

• vinyl ester

Polyester Resins

• versatile

• physical properties - rubber to hard, rigid materials

• no moisture released

• easy to color when used in fabric laminates

• strength to weight ratio is comparable to steel

• good shock resistance

• poor post bonding properties

• curing agents - catalysts, initiators

  • Benzoyl peroxide (0.5 - 3% of paste)

  • Cumene hyrdoperoxide

  • Methyethyl Ketone (MEK) Peroxide

• Rate of polymerization controlled by inhibitors, accelerators (promoters)

  • Cobalt napthenate (0.66-3%) - promoters - allows room temperature cure

  • often already added to the resin

Epoxy Resins

• lo wmolecular weight, syrup-like liquid

• expocy resin and hardener —> curing

• usually 1:1 ration

• properties depend of both

Resin Properties

Typical Polyester Properties

• easy room temperature cure, atmospheric pressure

• low cost

• easy handling

• good mechanical properties

• good dimensional stability

Typical Epoxy Properties

• low MW liquids cured with hardeners

• hard, tough polymers

• hardener becomes part of the finished plastic

• can be room temperature cured - better properties with heat curing

Epoxy vs Polyester Resins Advantages

• increased adhesion, strength, corrosion protection, chemical resistance

• decreased shrinkage

• electrical properties

• increased versatility

• decreased toxicity (when cured)

• increased heat resistance

• increased weather resistance

  • good for protection

• color/odor properties

Epoxy vs Polyester Resins Disadvantages

• increased cost

• decreased ease of handling

• increased toxicity (uncured)

• decreased weathering resistance

Acrylic (PMMA) Resins

• thermop;astic, can be parrtly thermoset

• strong good UV resistance

• transparent

• in sheet form, acrylic is hard to work with, very stiff melt, must be dried

Vinyl Ester Resins

• polyester chain with an epoxy backbone

• compared to polyester resins

  • better bonding properties - less delamination

  • more resistant to water penetration - less blistering

  • less shrinkage on curing

  • tougher, improved durability

  • stronger

• typically, the strength and cost of vinyl ester resins is between polyester and epoxy resins

Reinforcements

• one long axis compared to others

• strength is in long direction

• produced by drawing, induces orientation

Reinforcement Forms

• filament

• strands

• tow

• yarn

• roving tape

• woven fabric

• mat

• knit

• braid

• tape

• cloth

Glass Fiber Reinforcements

• E-glass - most common, good strength, electrical resistance

• S-glass - 40% higher strength

• C-glass - corrosion resistance

• quartz - electrical applications

Glass Fiber Properties

• improves most mechanical properties of plastics by factor of 2 or more

• continuous, long fibers are the strongest

• glass fibers have a chemical size/finish/coupling agent for protection during handling

• organosilanes - help to bond fibers with matrix material

Carbon Fiber

• long and continuous, short and fragmented, directional or random

• high modulus

• surface treatments - improves ILSS

• most common are pan or pitch based (precursor)

Graphite

• form of pure carbon

• production limited compared compared to carbon fiber

• high modulus, near zero thermal expansion

  • aerospace, nuclear reactors

Carbon Fiber Advantages

• Higher modulus leads to decreased strength (PAN)

• good creep resistance

• good fatigue resistance

• elastic to failure

Carbon Fiber Disadvantages

• brittle

• low impact strength

• low break extension

• low compression strength

• $$

Aramid Fibers

Kevlar

Properties

• lightweight

• strong

• high modulus

• high impact resistance

• tough

• lousy compression strength (1/2 carbon fiber composite)

• chemically resistant - difficult to saturate with resin

Spectra

• Ultra high oriented PE fibers

• properties close to aramid fibers

• specific strength, specific modulus similar to high modulus carbon fibers

Basalt

• made from basalt rock - natural, formed by lava

• drawn, continuous fibers - similar to carbon, aramid and fiberglass

• appearance very similar to carbon fiber

Benefits of Basalt

• extremely durable

• resistant to cracking

• less itch than carbon fiber

• non-carcinogenic and non-toxic

• strong and lightweight

• finished smooth with superior resin saturation

• unique dampening characteristics

• less expensive

Thermoset Composites (Laminates)

Composite - combination of a reinforcement material (such as fibers) in a matrix or binder material

• reinforcement - mechanical strength

• matrix material - rigidity, dimensional stability

Thermoset Composites

High pressure laminates

• 1000-2000 psi

• light weight, strong

• skis, countertops, panels, helmets

Low pressure laminates

• 15-1000 psi

• vacuum bag molding (O and P)

• autoclaving - significantly increases pressure

Contact pressure laminates

• ¼ - 15 psi

• economical

• big structures (boats, etc.)

Structure Styles

1. cellular/foamed - gas bubbles before resin harders

2. sandwich - used to give a high stiffness/weight ratio

• consists of a low-density cellular core between skins of laminate

• core can be - resin-impregnated, balsa wood, cellular plastics, or other materials

3. miscellaneous

• glass and cotton fabrics coated with resins or glass and plastic fibers woven together

• briefly immersed in solvent, can be shaped into a form

  • when solvent evaporates, light, rigid-formed product remains

Carbon Fiber Composites Advantages

• high strength/weight ratio

• high stiffness/weight ration

• excellent fatigue resistance (continuous fibers)

Carbon Fiber Composites Drawbacks

• relatively brittle, low impact strength

• no yield behavior

General Advantages - Composites over Metals

• better corrosion resistance

• lighter weight

• better insulators

• specific strength = 4-6 times of steel or aluminum

• specific modulus = 3.5-5 times that of steel

• good fatigue endurance - better than steel or aluminum

• design flexibility

• can eliminate many joints

• easier processing - lower cost

Composites vs Thermoplastics

• reduced creep or cold flow

• improved crack resistance

• 3 dimensional network - improved machinability, lower shrinkage, better high temperature performance

• poorer impact resistance - TP are better

Fabrication Methods (wet - vaccuum)

Wet layup with vacuum bagging method

• usually done at room temp

• no expensive equipment needed

• resin content usually high (lower mechanical properties)

• uniformity difficult and voids are common

Fabrication Methods - Pre Preg Layup

• controlled resin/reinforcement ratio - better properties

• need to keep in freezer till use - less shelf life

• less health and safety problems than using liquid resins

• often need heat cure - autoclave

Summary

• composite materials consist of a reinforcement materials (such as fibers) in a matrix or binder material

• fibers impart the strength to the composite

  • strength depends on the type and form of the fibers used and direction of the fibers in the composite

• typical matrix materials are polyester, vinyl ester, and epoxy materials

• typical reinforcing fiber materials are fiberglass, carbon, and Kevlar

• composite materials offer much greater strength and mechanical properties than thermoplastics alone, and can have specific strength and specific modulus properties greater than aluminum or steel