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Brentwood Plastics Blog

What Is Polyethylene?

Posted by Joel Longstreth on Wed, Jan 20, 2016 @ 09:19 AM

What is Polyethylene?

Polyethylene is comprised of long chains of the building block ethylene monomer. Ethylene monomer
C2H4 ( or just "C2" for short ) looks like this in English:

ethylene.jpg

Ethylene monomer occurs naturally in both natural gas and as a component of naptha - the portion of a barrel of oil which cannot be refined into gasoline or oil.  So now you know that correlations between zillions of barrels of oil and plastic grocery bags are just more fake news.

                                      Did you know ?  Climacteric plants produce ethylene to induce ripening.

bananas.jpgtomatoes.jpg

Polyethylene looks like this in English:

polyethylene

How these chains are put together determine the wide variety of resin properties. The fancy name for this is molecular weight distribution, or MWD for short.  MWD influences the 2 main properties which dictate a resins's personaliity - density and melt index.  The variations determine what applications a specific resin will work for.  

Polyethylene is the largest volume thermoplastic resin.  As such, it has a wide variety of uses.  Worldwide, there are probably hundreds of grades just for film alone.

What is Polyethylene?

Polyethylene is very inert.  The prelude to the iconic scene in episode 2 of season 1 of breaking bad illustrates this point.

1x02_-_Cat's_in_the_Bag..._5.png-2.webp

jessetub.jpg

The character Jesse called Walter from the store asking about what kind of plastic bin to buy.  Walter told him to look for the " LDPE 4 polyethylene " symbol on the bottom.  Jesse pours hydrofluoric acid packaged in a #2 HDPE #2 on the body in the bathtub.  After the bathtub falls through two floors, Walter explains that hydrofuoric acid is so aggressive that it eats through ceramic and metal ( but not polyethylene ).

bbadfloorhole.jpg

Melt index (MI) measures viscosity by weighing the amount of plastic resin which extrudes from a cylinder with a hole in the bottom at 300 degrees for 10 minutes. The test method is ASTM D1238. The number of grams is the melt index, or MI.

A low melt index ( under 1 ) is referred to as a fractional melt  or less than a gram / 10 minutes.  Lower MI resins are used for pipe and profile extrusion.   It's common sense that resins with high viscosity (resistance to flow) are inherently strong. In film applications, fractional melt is suitable for shipping sacks and shrink film. Generally there is a trade off between melt index and clarity. The higher the melt index, the clearer the resin.  So a fractional melt is not a good choice when clarity is important and a high clarity "whole melt" or resin with a melt index of over 1.0 is not appropriate for shipping sacks.

By contrast, high melt index resins are used for injection molding.  For injection molding, it is desirable to have a material which flows easily to fill the mold cavity.

This has been a very cursory basic introduction.  To learn about where polyethylene comes from, visit our
post .  Polyethylene is processed by almost every thermoplastic process.

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Topics: polyethylene, LDPE, HDPE

Prime vs. Off Grade Plastic Resins

Posted by Joel Longstreth on Fri, Oct 16, 2015 @ 04:48 PM

Plastic resins are made by major petrochemical companies.  The choicest cuts are sold by their direct sales force or through their "prime" distributors.  So who sells the near miss or transitional resins which are not exactly ready for prime time ?
 
dreamstime_xs_29936287

Starting wiith definitions is difficult because there are no written industry standard definitions of most terms pertaining to resins in this realm.  H L Mencken defined conscience as "that little voice that says somebody somewhere might be watching."  There is no controlling legal authority which oversees the distribution of off grade resins.  Caveat emptor.

A "major" is short for a major petrochemical company which manufactures plastic resin.  A "broker" is an entity which takes title to the resin which the major deems to be not "prime" resin which falls within the target criteria for the pick of the litter.  The majors sell prime through their direct sales force and prime distributors.

A data sheet lists certain key properties which define the characteristics of a resin. The first - melt index or melt flow - describes viscosity.  This tells the processor the most about how the material will behave and how it will affect the outcome of the part or material.  There are other criteria but going into depth would be too inside baseball and geeky.  

Each of these criteria has a target and acceptable range for what is considered to be "prime".  These are not rigid and fixed.  Rather, they are at the discretion of the product manager at the resin producer.  The parameters change depending on the available supply.  When resin is plentiful, the parameters tend to tighten up.  When resin is "tight", they widen.  It's all a function of what the product managers believe they can get away with.

When a product manager gets a report of fresh lots, she decides which ones are prime and which are considered "off grade" or "near prime".  The prime material gets "certs" and is sold through their direct sales force.  What falls outside the parameters is called "off grade" or euphemistically "near prime", "pencil prime" or "excess prime".  Other lots are called "transitional" which means resin made transitioning from one grade to another. 
 

Off grade, or OG for short, is sold through "brokers ".  By selling to a third party, the origin is obfuscated. Brokers  are supposed to be discreet and sell off grade to customers who are not direct customers of the same majors and preferably into a different application than originally intended ( It gets interesting when brokered resin finds it's way to prime customers ).   A business model which relies on somebody's mistakes at first sounds like a scary business model.  When resin is "tight" or when the majors are trying to raise prices, the brokers don't have much to sell.  The truth is there will always be material available which is rejected by processors.  We reject several railcars annually produced by ISO 9000 certified major resin companies.  

The most ethical and truthful brokers graduate to become what is known as "prime" distributors.  They distribute prime to smaller processors not called on by the major's direct sales force.  

It's easy to spot a newbie resin buyer.  They are too smart by half.  They make the rookie mistake of shoving an invoice from a distributor in the face of the direct sales rep.  " I can buy the exact same thing from your off grade broker for 4 cents a pound less !! "  Maybe so for one car.  After that, their reputation precedes them and they get no more deals.  Experienced buyers know the importance of emptying / returning a car quickly so the direct sales rep does not see the car which came from his own plant. 

Fun fact: there are no prerequisites to becoming a resin broker.  

Many brokers misrepresent material while others are ethical and disclose fully the good, bad and ugly about specfic lots.  Processors operating on razor thin margins are tempted to save money with resin that's "not exactly" and brokers are tempted to misrepresent to move their inventory. They hope the processor won't know the difference.  Problem: the processor lives with the material and does know when the raw material is not as advertised.  Here is an example:http://www.plasticsnews.com/article/20150924/NEWS/150929945/suit-alleges-processor-was-sold-the-wrong-resin .  
 
The ramifications go beyond the relationship between the processor and the resin producer.  The finished properties of the plastic film or plastic part within a lot or lot-to-lot are affected with variability of raw material.  FDA approval is meaningless.  Just change a few words in a word document and the broker transforms off grade into FDA approved plastic resin.  It's legal and goes on every day.
 
So how can you use this information ?  When doing the due diligence on a supplier, ask them preferably in person if the resin they use is "prime".  Don't be shy.  Ask to see FDA letters and invoices ( for the resin they use to make your product along with "track and trace" records ) with the price of the resin blocked out. If the letterhead is not from a major or authorized prime distributor, it's circumspect.  There is an exception.  Some brokers distribute "generic prime" which has been tested and sometimes blended to fall within specific ranges.
For non-critical parts, a generic prime will suffice.
 
 
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Topics: LDPE film, LDPE resins, LDPE

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