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Brentwood Plastics Blog

Homopolymers, Copolymers and Terpolymers - easy as 1, 2, 3

Posted by Joel Longstreth on Thu, Mar 31, 2016 @ 06:16 PM

The terms homopolymer, copolymer and terpolymer may sound like Latin or Greek to you.  In any case, they're not complicated.

Let's start with the common thread "polymer" first.  " Poly " comes from the Greek word for many.  As it pertains to plastic, it means many of the building blocks of monomers such as ethylene, propylene, styrene, etc. strung together in long chains to add up to polymers.

For more in-depth, visit our post on "where does plastic come from ? "

If the polymer is made simply from only one monomer, it is called a HOMOpolymer, or made from only one monomer.  POLYethylene, POLYpropylene, POLYstyrene, etc.  Far as I can tell, 1,2 Syndioatctic Polybutadiene is a homopolymer even thought the word is long.  Butadiene is the only monomer.

You guessed it - if there is another conjoined monomer ( not a mechanical blend ), you got a COpolymer.  Examples:  "ethylene vinyl acetate" or EVA for short - vinyl acetate monomer married to ethylene monomer, "styrene butadiene styrene" or SBS - styrene and butadiene building blocks,

And a TERpolymer is three monomers.  An oldie-but-goodie example is old fashioned phones which were made from "acrylonitrile butadiene styrene" or ABS.  Jump cut to present day - ABS is the default choice for 3 D printing.  Ethyl Methyl Acrylate, or EMA is all around us in upholstery commonly referred to as the "quiet film"

oldphone.jpgphoto courtesy of The Cathedral of Junk in Austin, TX

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Topics: polymers

HMWHD Conundrum

Posted by Joel Longstreth on Thu, Mar 31, 2016 @ 05:20 PM

The acronyms HMWHDPE and  UHMWHDPE are confusing.  This discussion breaks them down and translates them into plain English.


Starting with the MW bit -  MW is short for molecular weight.  MWD, or molecular weight distribution is the full name.  Accoring to Handbook of Polyethylene by Andrew Peacock:

     " The size of a polyethylene molecule is normally described in terms of its molecular weight.  All polyethylene resins consist of a mixture of molecules with a range of molecular weights.. The average molecular weight and the distribution of chain lengths comprising a polyethylene resin profoundly affect is ( sic ) properties.  The molecular weights of molecules found in commercial resins may range from a few hundred up to 10 million... Often it is found that those molecules making up the higher molecular weight fractions also display the lowest levels of branching. "

Without getting deep into the five geeky ways to measure MWD, suffice to say that a high molecular weight resin is concentrated, very crystalline ( little short chain brancing ) and very dense. No surprize that by definition the density is high - high density ( HD ), or over .940 gm / cc.

So HMWHDPE = " High Molecular Weight High Density Polyethylene"

As you might expect, these resins are among the strongest polyethylenes pound for pound.  They are so viscous that the ASTM 1238 test for melt index had to be revised when the original Mitsui 7000F came along in the late 1970's.  Melt index is a measure of viscosity.  The test method consists of a cylindrical specimen in basically a heated can with a weight and a hole at the bottom.  The number of grams which come out of the hole in ten minutes is called the melt index.  This provides a crude predictor of the resin's personality.  Garden variety LDPE resins are about 2 grams or the verbal shorthand " 2 melt".   Mitsui 7000 F, American Hoechst GM9255F2 and Conoco / Dupont 5000 had values of + - 0.05.  So the HMW film resins are now tested with a " high load melt index", or "HLMI" which simply means higher weight to get a more meaningful sample.  Some prefer to make it sound more daunting by saying  "HLMIHMWDPE."  The most cotidien examples are single use grocery bags and star- seal trash bags.


Did you know ?  If there is not adequate haul-off speed, thicker HMWHD films will be weaker.  Why ?
the strength is a function of how well the polymer is "knitted" in the transverse direction ( TD ).  Here's how it works:  HMHD is made with a "stalk" or "wine glass" vis-avis "in the pocket" in LDPE.


 When the elastic energy dissipates, the bubble is created.  The tear strength and is a function of how fast the melt is oriented in the TD.  Simply slowing the line speed will create weaker, "splitty" film.

UHMWHDPE as you might guess is an even higher protein version with molecular weights starting around 2,000,000.  Those are long chains.  That said, most grades are in the .928 to .941 range which fall into the category of medium density polyethylene ( MDPE ). You can't make blown film out of it.  It has to be cast, or made with a slot die.  It is very stiff and slippery to the point of having a weird self-lubricating effect.  This trait makes it a popular choice for conveyor belts. There are lots of uses - the most popular is pressure-sensitive tape commonly referred to as "poor man's teflon." 

For the basics of PE, click here.

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Topics: HDPE

Packaging Film Blame Game

Posted by Joel Longstreth on Mon, Mar 21, 2016 @ 02:03 PM

A plastic film spec properly crafted by a packaging machine manufacturer ensures that the film can always be blamed  That's what an OEM told me last week.


Over the years, we have tried to partner with packaging machine manufacturers with no success.  Now I know why at the tender young age of 63.  We have even cited success stories in which our film has enabled frustrated plant managers to have consistent performance at speeds which exceed the OEM's claims.


To use a military analogy, the machine manufacturers are like the tank corps.  The tanks take the terriority and the infantry follows.  The machinery makers just figure the customer will figure the film out on their own.  When a machine is placed, the distributors find it first and swarm in.   Another set of problems arises from lack of continuity of supply when the distributor "shops" every order.

The setting of this worn out script which happened last week was the last stage of an installation.  The upstream issues were sorted out.  The last tech who was packing up and leaving said all three laminated film candidates were bad. Last remaining problem: good end seals, inconsistent back seams and occasional pinholes.  " FIrst time it ever happened" didn't do the customer much good.

The panicked customer contacted us.  I explained that if they were getting good end seals, they were hitting the optimum combination of heat / pressure / dwell. (The film is the same throughout, therefore the acid test variable is the sealing jaws.)  The heat and dwell time were both too high, resulting in "overcooking".  Turning the heat down - cooler - solved the problem.  Random pinholes in the end seals were the last problem.  

Turns out the packages were dropped three feet to the conveyor.  The molten film did not have enought time to crystallize.  The impact created the pinholes.  The pinholes disappeared when the conveyor was simply elevated.

We didn't get a sale out of it and consulted for a lot less than minimum wage.   

It's a natural human tendency to blame the party who is farthest away from the scene.  In the packaging business, the Napoleonic logic always blames the film first.  


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Topics: packaging machinery

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 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 plain false.

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


Polyethylene looks like this in English:


Ethylene monomer can also be derived from agricultural sources.  Braskem has been the most successful with their "green" PE and PP derived from sugar cane.  These resins do not break down any faster than resin derived from fossil fuel feedstocks.  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?

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.


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Topics: polyethylene

Why Recycled Plastic Content is not Always Practical

Posted by Joel Longstreth on Tue, Dec 01, 2015 @ 06:47 PM

"Recycled Content" must score well in focus groups.  What else could explain why we are getting more requests for recycled content than ever ? 

 The economics of recycling is an entirely separate discussion.  Here are a few reasons why it is not always practical to recycle plastics.

" ( Post-consumer resins are) very important to some brand owners"   
-  Pedro Morales, KW Plastics


2ndplantbottle.jpg posttotalrecycled.jpgminrecycled.pngrecycsymbols.jpg

Problem:  With very few exeptions, recycled content compromises the properties of the virgin resin.  This has many ramifications.  Taking the exception first, let's look at polyethylene terephthalate a/k/a PETE #1.  It is not melted down and mixed back in with the virgin resin.  Instead, it is repolymerized.  The polymer is broken down into pure monomer and water and reassembled into pure polymer.  PETE is the only recycled resin approved for medical devices.  Not so fun fact:  the general public chooses to recycle only 1 in 3 PET bottles.

" We need ( post-consumer) polyethylene to be looked at like recycled PET is...  Recycled PET  is sold at a premium ( to virgin PET ).  ( Post-consumer) polyethylene needs that same kind of attention." 
- John  Picciuto, president of Western Plastics Association

For this to happen, post-consumer PE ( #2  & # 4 ) would have to attain the same FDA-approved status that PETE has.  

Really truly recycled plastic is picked up curbside, sorted, reground, repelletized and extruded again.  Internally generated scrap which is recycled internally does not fit the definition, but nobody cares or checks up.


At every step there is a chance for contamination.  Cross contamination with other polymers is one of the top issues for recyclers.  If the recycler is fortunate enough to have a batch of just one polymer, that's not even half the battle.   There are variations on the theme of each resin.  So contamination from the same polymer can compromise the final product.  With each heat history, the performance of the polymer degrades due to dissipation of the anti-oxidant.  Recycling makes the public feel good.  In reality it is downcycling to a less critical application.

Medical and Clean Room Packaging

Would anyone in their right mind want to use a medical device such as a syringe which might contain even trace amounts of toxins ?   We have made film for ostomy bags ( prolonged direct dermal contact ) for over 15 years. It's tough enough just to get prime virgin resin approved.  The resin has been put through four biotoxicity tests.  We would lose all credibility if we suggested using partially recycled content.  Same goes for surgical drapes which come in brief contact with surgical instruments.

Clean room packaging has a very low parts per million threshold for many chemicals.  Introducing the wild card of recycled content would risk recall of an entire shipment and loss of a customer, not to mention getting charged back for having to repackage.



Food and Water Packaging

PETE is also the only recycled resin approved for prolonged and direct food contact by the FDA.  Same logic applies to food.  Would you feel comfortable with your food being packaged in a container which might contain toxic chemicals ?   Plastics already get enough of a bad rap due to misperceptions and misinformation.
BPA free is an effective greenwash marketing tactic.  Food packaging also gets scrutinized for what is known as leachables and extractables to be sure undesirable flavors are not migrating into food.

cuisinart.jpg images.jpgdreamstime_s_29972885.jpg


Deposit Slip and Coin Packaging


All bags in this category must have a tamper-evident immediate bond.  The three things which prevent tape from sticking are moisture, dirt and grease.  As such, they have a zero tolerance for slip, or erucamides which are commonly added to make bags open easily.  Adding in recycled content which may have trace amounts of slip could prevent the tamper evident tape from sticking.  The center image is fifty pounds of loose coins.  It is very difficult to qualify film made from prime first quality resin.  The bags are put through multiple drop tests.  If the seals fail, the film is always blamed, not the bag maker.  You guessed it - recycled resin would make the bags weaker and possibly keep the tape from sticking.  

There are plenty of other applications, but you get the idea.  The common thread - is it prudent to risk so much just to appease a great idea from the marketing department ?  

If you think so - Mesdames, Messieurs, place your bets !



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Surlyn - sometimes there is no substitute

Posted by Joel Longstreth on Mon, Nov 30, 2015 @ 03:17 PM

Surlyn the most effective, forgiving sealant layer ever invented.  Q.  What makes it so effective ?  Why the search for alternatives?  

A.    Surlyn's properties derive from what is called an ionomer bond.  An ionomer of sodium or zinc is in limbo shared with the orbit of polyethylene molecules.  This delivers unsurpassed tack and toughness and strength.  Surlyn seals through contaminants better than any other resin.  This is very important in the large market for subprimal meat packaging.

A.  part 2

Surlyn has many other applications besides packaging.

Surlyn's  is expensive and difficult to process.  Plastic film plant managers were early adaptors of metallocenes because Surlyn is hydroscopic and corrosive to extruder screws and barrels.  Surlyn has to be dried before extruding.  It is incompatible with other common polyethylene resins.  So blown film managers were early adapters of metallocenes.  

Metallocenes deliver almost the same hot tack characteristics without the aggravation.  Metallocenes are plug and play.  No drying, no corrosion and lower cost.


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EPR Extended Producer Responsibility - You will pay, not producers

Posted by Joel Longstreth on Wed, Nov 18, 2015 @ 06:46 PM

Extended Producer Responsibility ( EPR ) and product stewardship are ideas which are gaining traction globally.  Vilifying corporations may be satisfying to NGO's but the expense of recovery will be passed along to the consumer.  



The logic is that a corporation must be responsible for the entire life cycle of a product all the way to final disposal at end-of-life ( Germany holds the manufacturer legally responsible for the entire life cycle of the product under what is called "comprehensive outcome" ).  This would include re-use, take back and/or recycling. Problem: this costs money which is not presently included in the cost of the item.  Now the consumer pays for only the cost of the item hopefully produced with a small profit and quite possibly at loss. There is simply not enough profit built in to absorb these additional costs.  Packaging does not have a secondary use and cannot be repurposed.  

Container deposit legislation shows the consumer that recycling and recovery costs are extra.

The cost of disposing of toxins is often included in the sale price of some items.  Extending this principle to inert, non-toxic plastics is a bit of a stretch.  Just my opinion.  Container deposit legislation shows the consumer that recycling costs more than the container by itself.

Follwing the logic, the plastic processors are responsible for the behavior of the consumer who chooses to litter instead of recycling.  2 out of 3 containers made from #1 PET, the most recyclable of all polymers, end up as litter or landfill despite the best efforts of large corporations to encourage recycling.

Crafting EPR legislation might get complicated.  How would the cost be allocated ?  How much should the resin manufacturer be responsible for and how much for the processor ?  So-called "triple bottom line" concepts such as environmental full cost accounting ( EFCR ) and true cost accounting ( TCA ) attempt to capture the costs of the entire life cycle. They are open-ended and subjective at best.


If the blame for plastic waste gets shifted to us plastic straw men, reality is the consumer will ultimately pay. Just as corporations pass along the costs of taxes, the plastic industry will pass these costs along.  There are parallels to what is about to happen with electricity costs.  Consumers are going to pay for the higher costs of power generated by non-coal fired plants as dictated by the EPA.


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Visqueen  Construction Film

Posted by Joel Longstreth on Tue, Nov 17, 2015 @ 06:50 PM

Visqueen is another word for construction and agriculture, or "C & A" film.  Visqueen, like "Frigidaire" and "Kleenex" is an example of what linguists call a secondary meaning or what happens when a brand becomes so commonplace that it is substituted for the original meaning.



Click here for a history.

Visqueen is used for non-critical applications not just on construction sites and farms.  It is the lowest end of the blown polyethylene film market. Any PE resin which can be extruded into blown film will do.  Cheating on weights and measures is commonplace.  Full gauge and width is an exception and nobody cares.

Visqueen is often used to make makeshift greenhouses, but it will not last long due to degradation from UV rays.

It is commonly sold in 100 foot lengths in widths from 10 feet and wider in natural and black.


We get lots of inquiries for Visqueen because it is polyethylene film.  We don't make it.  We're set up for large
"mill" rolls.  If you are looking for Visqueen / C & A film, here are links to some US manufacturers:

Did you know ?

The wonderful folks at the Ethyl Corporation who brought you leaded gasoline have never been sued for contaminating the ground with lead 

To create large pond liners, you have to splice big sheets of Visqueen film together.  In the olden days, they used to roll up dynamite fuses in the film and light it to make a seal.

The largest blown film line in North America is located at Poly America in Grand Prairie, TX.  Although Poly America's roots are in C & A film, their mainstay is private label trash bags.

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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 ?

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.  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: .  
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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Odor and Taste / Leachables and Extractables in Plastics

Posted by Joel Longstreth on Thu, Sep 10, 2015 @ 06:47 PM



Leachables & extractables / odor & taste transfer in packaging have been a perennial source of controversy and product rejections.  Nature's penchant for equilibrium results in migration of substances from plastic packaging into the product.

If glass was a new invention, it would be difficult to get FDA approval due to the elements which leach from it.  It's cliche but true - anything in quantity is poison.

The ramifications of leaching and taste transfer can range from annoying and inocuous to toxic.  The metallic taste in tap water resuls from rusting ductile iron pipe.  A faint plastic taste will eventually show up in old bottled water.  Harmless but annoying.  There is no telling what contaminants can enter the recycling stream in post-consumer reprocessed plastic.  For this reason, reprocessed plastic is not an option for food or pharmaceutical packaging.  There is one exception - polyethylene terephthalate ( PET #1 ) which is re-polymerized vis-a-vis reprocessed, 

Commodity plastics such as polyethylene ( PE ), polypropylene (PP) and polystyrene ( PS ) are popular choices for food grade packaging because they are so inert.  For reasons still unknown, some polymers transfer taste to specific products.  For example, anyone who has made institutional milk bags knows there is zero tolerance for EVA.  The bouquet of EVA ( best described as apple vinegar ) transfers to milk.  

The subjective zone often results in vicious disputes.  Years ago, a major manufacturer of cereal detected an oxidized odor in the film yet there was no transfer to the product and there were no consumer complaints.  The film supplier had to make sandwiches out of the returned film because the cereal manufacturer refused to pay the invoice.

After 40 years and millions of pounds of ice bag film, we no longer make film for ice bags due to subjective, specious odor and taste complaints. 

Of course, very real leaching can ruin entire lot of product by the presence of only trace amounts.  Many a lot of circuit boards has been ruined by a few hundred parts per million of slip or silicone. Clean room packaging has zero tolerance for slip and silicone.

The threshold of parts per million has very serious legal implications, mainly about compliance with regulations. California prop 65 is not going away.  The scope is expanding indefinitely.  

Extractables encompass everything from the monomer itself to residual catalysts and additives.  So let's break it down:

monomers  Plastic is made from simple hydrocarbons.  With the exception of PVC, these feedstocks have been shown to be inert and harmless.  Old fashioned HDPE is still a popular choice for distilled water.

catalysts are necessary to facilitate a reaction to create the polymer chains.  The catalyst is either transformed or used up.  Remember the hysteria over BPA ?  Turns out it might have been overblown.  
Like it or not, the catalyst BPA is here to stay.  If BPA is an indisputable carcinogen, we should discontinue the use of ubiquitous 5 gallon polycarbonate carboys today.

Additives is a big subcategory encompassing antioxidants, stabilizers, antiblock, slip and pigments.   Additives each have a specific job to do.  Without antioxidants, plastics would have an odor like the smoke from a candle which was just extinguished.  Consumers would return any food packaged in an "antioxidant free" package.  Most resin antioxidants are tocopherols commonly known as vitamin E.  Only PVC has phthalates for stabilization.  

Antiblock is diatomaceous earth.  It is so inert it is used to encapsulate nitroglycerin to make dynamite.

Slip is used to make plastic slippery as the name implies.  Most slip additives anymore are pure enough that they can be Kosher certified.  

Most US made pigments are HMF, or "heavy metal free" if the end use is food packaging or anything medical.   The yellow does not contain lead and the orange no longer contains molybdenum. But most consumers prefer to eagerly expose themselves daily to dermal exposure to levels of lead which are in direct violation of CA prop 65.  

Quantifying leachables and extractables is done according to ASTM WK43975.  The extraction test is done in two solvents.  The extractables must fall below specific threshold amounts to be considered FDA approved.  

Of course, what is difficult to capture is goodwil and good faith.  If somebody is looking for an excuse to not pay their bill, odor and taste is a handy excuse.  As many injection molders found out ( some were put out of business ), the test method of CA prop 65 was modified to obtain the desired lead threshold results.  Whether or not the thresholds posed a realistic health hazard was irrelevant.  

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