Sunday, April 28, 2013

Update on the Demo Part Search

I've updated the Demo Part Search

<CLICK HERE>

I plan to have a series of images posted here this week (hopefully by 5/1) to describe the process used for that demo and images of the results.

Until then...

Thursday, April 25, 2013

Plastics Classification - Chemical Classifications

Types:
 
    So, in the continuing discussion of how we classify polymers, we now need to talk about their chemical classification.  For the purposes of this blog and for the time being I will restrict the discussion of polymer chemistry to thermoplastics, and those most typically used in injection molding; although many of these are also made in specific grades meant for other processes such as Thermoforming, Extrusion, Rotational Molding, etc.
 
    Polymers, being organic compounds created via reactions of other organic monomers and reactants with 2 or more active sites on the molecule, are classified by the chemical groups that make up the repeating segments of the polymer chain.

    The following is a list of polymer chemical classes:
1) Polyolefins
2) Polystyrenes
3) Polyesters
4) Polyethers
5) Polyamides
6) Polyimides
7) Poly-Aldehydes (i.e. Polyoxymethylene - polymerized formaldehyde)
8) Poly-Acrylates


The next several blog posts will be devoted to discussing these and other various polymer chemical groups, the type of polymerization reaction involved in the making of those specific polymers, typical uses and properties for those polymers.


Common Polymerization Reactions:

Polymerization reactions include the following common processes as well as others: Linear Step, Non-Linear Step, Free Radical, Ionic, Cationic, Anionic Polymerization.

Step Polymerization - The combining of a monomer as a whole unit into chains, step-by-step.
 
    Linear step polymerization (i.e. condensation polymerization) is where a monomer will maintain it's structure but become chemically linked into chains, covalently bonded, of varying length.  Polyethylene is one example of this.  Ethane is catalyzed and the molecules link together to form longer hydrocarbon chains.  In this way, octane, heptane, decane, and other hydrocarbons might be looked at as being linked in family with polyethylene.  The term "linear" means that it will yield a thermoplastic material as there 2 and only 2 active sites on the monomer chain.

    Non-Linear Step Polymerization is essentially just like the linear except that there are 3 or more active sites on the monomer and thus a thermoset is formed.  The pint where the first side chain is formed is referenced as the "gel point."  Essentially, these lead to network polymers.

 Living  Polymerization -A Living polymer is one where there is no termination to the polymerization reaction.  These polymers can "self-heal" but the trade-off is that they're not safe for food or drug applications.

Free Radical Polymerization - The monomer has Pi-bonds (double and/or triple bonds) that are broken via the introduction of a source of free radicals.  This then opens the active sites and allows for the formation of the polymer chain and with its creation, the free radicals are recovered like a catalyst.  Typically, free radicals are introduced in the form of peroxides, and can be excited (activated) by heat or UV light.  Polystyrene is the most common example of free radical polymerization.


Ionic Polymerization - Chain polymerization that uses active centers with an ionic charge.  This is done with olyfinic monomers.  There are two types:

    Cationic Polymerization - The active centers are positively charged.  It has a defined termination process.

    Anionic Polymerization - The active centers are negatively charged.  Lacks a termination process. Instead, there is a chain transfer stage.  Polystyrene formed in liquid ammonia was one of the first applications for this process.

Ring Opening Polymerization - Polymers with a structure that looks like:  -[R-Z]n-  (the linking group is Z) can be polymerized by step polymerization.   In the case of a cyclic monomer, the ring can be broken in order to facilitate polymerization.  The primary mechanism that drives this is either the relief of bond angle strain or steric repulsions.
   
Solid State Polymerization - Polymerization of a monomer that exists in the solid and crystals are formed by adding heat or ultraviolet radiation.  These tend to yield highly oriented polymers.


Group Transfer Polymerization - Generally used for acrylic and methacrylic monomers (i.e. methacrylates - PMMA), this type of polymerization propagates by way of reacting a terminal silyl ketene acetal with some monomer by addition thus transferring the group to the monomer and creating a new terminal silyl ketene acetal group on the growing polymer.  It is initiated with monomers containing 2 or more silyl ketene acetal sites, and is catalysed by anions.



Homopolymers versus Copolymers (and Terpolymers):

    The term homopolymer refers to a polymer comprised of a single repeating monomer.  Copolymers have two different monomer links that are connected, perhaps alternating, along the polymer chain.  Examples of co-polymers include Styrene-Acrylonitrile, Butadiene-Styrene (aka High Impact Polystyrene or HIPS).

    Terpolymers are, as one might now imagine, polymers comprised of three monomer units that alternate along the polymer chain.  The most common example of a terpolymer is Acrylonitrile-Butadiene-Styrene (ABS - It's NOT the break system in your car).


Monday, April 8, 2013

Polymer Classification - Morphology

This is a continuation of the discussion started last Wednesday.  Once we determine whether our polymer is thermoplastic or thermoset, then we want to know some things about something called, "morphology."  The term morphology refers to the structure of the molecules as they make up the polymer matrix.  This is more relevant to thermoplastics, but there are some thermosets where the morphology plays a role in the properties of the resin.  The two primary morphological states of interest to us, as engineers, are crystalline and amorphous.  Well, crystalline is a relative term as polymers tend to be more "semi-crystalline" than truly crystalline the way a metal is. 

If one were to imagine a pile of sticks.  Pulling on one of the sticks will lead the puller to the conclusion that the pieces are entangled.  The same is true of amorphous materials such a polycarbonate.  The side-chains on the molecules create entanglements that help add to the strength of the polymer where crystallinity is lacking and would otherwise provide strength.  Also like the stick pile, the molecules in an amorphous material are porous on the molecular level.  This tends to make them especially susceptible to chemical attack.  Clear polymers are generally known for being especially sensitive to solvents when under stress - and more sensitive to tensile stress than compressive.  Some amorphous materials are known for being brittle (i.e. Polystyrene) while others are known for their toughness (i.e. Polycarbonate).  This is generally due to the differences in intermolecular forces (i.e. van der Waals forces). 

Some examples of amorphous materials are:  Polycarbonate, Polystyrene, Polymethylmethacrilate (PMMA - Acrylic), Acrilonitrile Alloys (SAN, ABS, etc), Polyvinylchloride (PVC), Transparent Nylon 12,  and many others.

On the other hand, if one were to imagine the image of a plate of spaghetti, we have an image that's more descriptive of a crystalline material.  The polymer chains are narrow and long.  Now if we imagine that our strands of spaghetti were made of tiny little magnetic balls, then we might get a mental image of how they would want to fold and form crystals as they cool.  When it comes to the flow of a polymer, crystalline polymers tend to align and then flow very freely past one another where amorphous polymers tend to be thicker with a less distinct fluid transition.  The morphology of crystalline polymers is affected by the shear history during flow.  The higher the shear rate, the more nuclei that will form and thus the greater number of crystals of smaller size.  Also, the rate of cooling will secondarily affect the crystalline morphology of polymers.  The slower the cooling, the larger the crystals.  Because of this, crystalline polymers tend to be more dense than amorphous polymers and the appearance of crystalline polymers without colorants will generally be a milky translucent white, and never clear at room temperature as opposed to amorphous polymers which can (as natural, uncolored resins) be clear at room temperature.

Some examples of crystalline polymers are: Polyethylene (PE) - including LDPE, HDPE, & UHMWPE, Polypropylene (PP), Polyoxymethylene (POM - Acetal - Celcon, Celstran, Delrin), Polyamide (Nylon 6, 6/6, etc), Polyethersulphone (PES), Polyesters (PBT, PET, PCT), Syndiotactic Polystyrene, etc.

So when you're thinking about crystalline and amorphous polymers the analogy to a pile of sticks or a plate of spaghetti works well in thinking about the differences in their physical properties and their flow behaviors.



Wednesday, April 3, 2013

Classification of Polymers - Thermoplastics and Thermosets

I know a few students out there who are taking classes in polymer science this term at Kettering University and I thought I would provide some study materials.   Of course I'm happy to take suggestions or answer questions from students as they arise.

On the topic of polymer classification, this is one of the first things that students should be learning about polymers.  This of course assumes that we all are of the understanding that polymers are chemically based materials comprised of long chains of carbon atoms that may also have side-chains that affect the properties of the material as a whole.  Polymers are created via chemical reaction of a monomer or a mix of reactants each with at least two active sites on the molecules.  During the reaction, the chains grow until there is little or no monomer (or reactant) available to continue the reaction.  The size of the chain determines the molecular weight of the polymer.

The first thing everyone should know about any polymer is whether it's a thermoplastic or a thermoset.  Thermoplastics, as the name suggests, are polymers that can be melted and reformed again and again.  This is like candle wax, butter, or chocolate.  Thermosets, as the name suggests, set with heat and do NOT remelt.  Think of an egg; once it's cooked, it's done.  There's no remelting an egg once cooked.  This is because the reaction involves cross-linking between polymer chains.  Heating the set polymer will cause expansion of the molecules but the cross linking will prevent the chains from slipping past one another.  Thermoplastics are polymerized from monomers with two and ONLY two active sites on the molecules.  Thermosets are formed from monomers and reactants with three or more active sites on the molecules.

SO...

When it comes to recycling, Thermoplastics have many uses after recycling where-as thermosets are good for little more than road asphalt filler.