Composites - General

Thermoplastic

linear or branched polymers with (no covalent crosslinking ) phenolic rings allowing molecular bending. Chains slide past each other when heated.

Thermoset

polymers that undergo a permanent crosslinking process when heated, forming covalent crosslinks. They do not soften upon reheating and maintain their shape.

Tg

glass transition temperature; where polymer solids change from a glassy state to rubbery; if T > Tg movement of molecules, if T < Tg brittle frozen movement

Sizing

a very thin protective coating applied to reinforcing fibers, reduces abrasion and static friction between filaments.

Contact angle

wettability is improved by decreasing fiber/atmosphere surface tension which reduces the contact angle. If its less than 90 degrees we have good wetting, and if more we have bad wetting.

Interface in composites

A thin but distinct layer between fiber and matrix, which structure and properties are different from the bulk resin matrix due to the presence of the fiber

Anisotropy 

Material Properties are not the same in all directions

Viscoelastic behavior of polymers

Polymer has both elastic and viscous behavior, when an external load is applied an instantaneous elastic deformation followed by a slow viscous deformation; PVC and PE

Linear polymer chains

spaghetti like long chains, held together by weaker van der Waals or hydrogen bonding, typically thermoplastics; PVC and PS

Branched polymer chains

resemble linear chains but with some shorter chains with minimal connecting parts; HDPE and LDPE

Crosslinked polymer chains

resemble ladders, 3D network of chains are much more linked together than branched; tied together by covalent bonding; rubber and elastomer

Networked polymer chains

100% crystallinity and highly corsslinked; typically thermoset; epoxies and phenolics

Differential Scanning Calorimeter (DSC)

a small sample is heated either isothermally or dynamically, to measure the heat generation changes from glassy to rubber; the glass transition temperature is when the heat curve changes

Dynamic mechanical analysis

Gives mechanical properties as a function of temperature. Tg is the peak on the loss/storage modulus

One peak means its amorphous and delta tan corresponds with Tg; two peaks means its semicrystalline and the first peak is Tg and the second its Tm

Griffiths Theory

the strength of brittle fibers decreases as fiber diameter increases. since small fiber diameter gives an increase in surface area for the resin forms a strong composite

Interface mechanisms

Molecular Entanglement interdiffusion

Electrostatic attraction

Cationic group attraction

Chemical reaction

Mechanical Interlocks

Molecular Entanglement interdiffusion

polymer chains interlocking under heat to diffuse into each other

Electrostatic attraction

when materials with opposite electrical charges come in contact they attract via coulomb forces

Cationic group attraction

type of ionic or polar bonding between cations and negatively charged surface sites

Chemical reaction

atoms/molecules form an interface through a new covalent or ionic bond

Mechanical interlocks

physical connection, like resin physically adhering to fibers or like certain adhesives

Gel temperature

the temperature range at which polymer goes from liquid state to a gel state