Polymers and their uses

A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass is called a polymer.
A polymer  is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties,both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, viscoelasticity, and a tendency to form glasses and semicrystalline structures rather than crystals.

 

Polyethylene is a thermoplastic polymer with variable crystalline structure and an extremely large range of applications depending on the particular type. It is one of the most widely produced plastics in the world (tens of millions of tons are produced worldwide each year). The commercial process (the Ziegler-Natta catalysts) that made PE such a success was developed in the 1950s by German and Italian scientists Karl Ziegler and Giulio Natta.

There are a vast array of applications for polyethylene in which certain types are more or less well suited. Generally speaking, High Density Polyethylene (HDPE) is much more crystalline, has a much higher density, and is often used in completely different circumstances than Low Density Polyethylene (LDPE). For example, LDPE is widely used in plastic packaging such as for grocery bags or plastic wrap. HDPE by contrast has common applications in construction (for example in its use as a drain pipe). Ultrahigh Molecular Weight Polyethylene (UHMW) has high performance applications in things such as medical devices and bulletproof vests.

What Are The Different Types of Polyethylene?

Polyethylene is commonly categorized into one of several major compounds of which the most common include LDPE, LLDPE, HDPE, and Ultrahigh Molecular Weight Polypropylene. Other variants include Medium Density Polyethylene (MDPE), Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX), High-molecular-weight polyethylene (HMWPE), High-density cross-linked polyethylene (HDXLPE), Cross-linked polyethylene (PEX or XLPE), Very-low-density polyethylene (VLDPE), and Chlorinated polyethylene (CPE).

  • Low Density Polyethylene (LDPE) is a very flexible material with very unique flow properties that makes it particularly suitable to plastic film applications like shopping bags. LDPE has high ductility but low tensile strength which is evident in the real world by its propensity to stretch when strained.
  • Linear Low Density Polyethylene (LLDPE) is very similar to LDPE with the added advantage that the properties of LLDPE can be altered by adjusting the formula constituents and that the overall production process for LLDPE is typically less energy intensive than LDPE.
  • High Density Polyethylene (HDPE) is a strong, high density, moderately stiff plastic with a highly crystalline structure. It is frequently used as a plastic for milk cartons, laundry detergent, garbage bins, and cutting boards.
  • Ultrahigh Molecular Weight Polyethylene (UHMW) is an extremely dense version of polyethylene with molecular weights typically an order of magnitude greater than HDPE. It can be spun into threads with tensile strengths many times greater than steel and is frequently incorporated into high performance equipment like bulletproof vests.

What are the Characteristics of Polyethylene?

Now that we know what it is used for, let’s examine some of the key properties of Polyethylene. PE is classified as a “thermoplastic” (as opposed to “thermoset”), and the name has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (110-130 degrees Celsius in the case of LDPE and HDPE respectively). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like Polyethylene liquefy, which allows them to be easily [injection molded] and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.

Different types of Polyethylene exhibit wide variability in their crystalline structures. The less crystalline (the more amorphous) a plastic is, the more it demonstrates a tendency to gradually soften (i.e. they have a wider range between their glass transition temperature and their melting point). Crystalline plastics, by contrast, exhibit a rather sharp transition from solid to liquid.

Polyethylene is a homopolymer in that it is composed of a single monomer constituent (in this case ethylene: CH2=CH2).

Why is Polyethylene used so often?

Polyethylene is an incredibly useful commodity plastic. Because of the diversity of PE variants it is incorporated into a wide range of applications. Unless it is required for a specific application, we don’t typically use Polyethylene as part of the design process at Creative Mechanisms. For some projects, a part that will eventually be mass produced in PE can be prototyped with other more prototype-friendly materials like ABS.

PE is not available as a 3D printable material. It can be CNC machined or vacuum formed.

How is PE made?

Polyethylene, like other plastics, starts with the distillation of hydrocarbon fuels (ethane in this case) into lighter groups called “fractions” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation). You can read about the process in more depth here.

PE for Prototype Development on CNC Machines and 3D Printers:

PE is available in sheet stock, rods, and even specialty shapes in a multitude of variants (LDPE, HDPE etc.), making it a good candidate for subtractive machining processes on a mill or lathe. Colors are usually limited to white and black.

PE is not currently available for FDM or any other 3D printing process (at least not from the two major suppliers: Stratasys and 3D Systems). PE is similar to PP in that it can be difficult to prototype with. You are pretty much stuck with CNC machining or Vacuum forming if you need to use it in your prototype development process.

Is PE toxic?

In solid form, no. In fact, Polyethylene is often used in food handling. It could be toxic if inhaled and/or absorbed into the skin or eyes as a vapor or liquid (i.e. during manufacturing processes). Be careful and closely follow handling instructions for molten polymer in particular.

What are the Disadvantages of Polyethylene?

Polyethylene  is generally more expensive than polypropylene (which can be used in similar part applications). PE is the second best choice for living hinges, behind PP at number one.

Common Uses of Polyethylene

Listed below are the top 5 most common uses for Polyethylene. These are the products containing Polyethylene which you are most likely to find already in your home and at the local supermarket.

Sandwich Bags

At some point in our lives, everyone has used sandwich bags for one reason or another. What you may not know is that Polyethylene is used in the manufacturing process of these bags.

The polyethylene is used to make the plastic film that will eventually become the sandwich bags.

It is also used in the sandwich bags cousin, the freezer bag. More of a heavy duty polyethylene plastic is used in the freezer bags, as they need to be able to with stand extreme cold and still be able to protect the items within.

Cling Wrap

Almost every cling wrap you purchase at the supermarket is made from Polyethylene. The Polyethylene combined with other materials helps to make the cling wrap work and keep its hold.

While the cling wrap itself is actually very thin, low density Polyethylene is not used in its production.

Instead, during the manufacturing process the Polyethylene is stretched multiple times to give the cling wrap its normal, thin appearance.

Moisture Barriers

The moisture barriers they use on construction sites and those that are used in crawl spaces under houses are made from Polyethylene.

Low density Polyethylene is not used in the manufacturing of these items, due to the fact that they need the highest quality of Polyethylene available to make sure that the areas are secured from any moisture getting inside.

You can also see non-moisture barriers made from Polyethylene on construction sites, and those are used to keep people out of certain areas or to help protect areas from damaging winds, dirt, or any other material that the workers want to keep off of the site.

Food Packaging

No doubt you have been in the supermarket and found yourself searching for the best pack of ground beef or chicken.

You may notice that there is plastic wrapped around the meat to help keep it fresh before it can be sold and used.

That plastic wrapping is Polyethylene in a low density form. It is used to help make sure that the meat you are purchasing stays fresh until you can get it home and either freeze it or cook it.

The plastic wrapper is tightly fitted onto the packaging so that no other food particles or bacteria can contaminate the meats you are buying.

It can also be used in bakery wrapping to keep bread and other perishable sweet treats from spoiling before they can be sold and used.

You may notice that the bakery bags are a little different from the plastic wraps over your chicken, and that is due to the quality of Polyethylene that is used to make them. However, they both perform the same duty and keep your food fresh and contaminate free.

Coatings

Polyethylene is also used to make the coatings that you see on fruit juice boxes. Though the container is mostly made of out cardboard type material with a small amount of plastic mixed in, the outer coating that keeps the box from falling apart once the liquid has been introduced is the Polyethylene.

It may look like a shiny coating for the box, but it is actually keeping that box tightly together so that kids can enjoy their juice boxes.

It is also used in the wrapping for the straw, and if you buy the juice boxes wrapped instead of in a large box, Polyethylene is also used to make the wrapping for the box.

Most of the time you will see the juice boxes being wrapped when you purchase them from a larger grocery chain.

Almost anything that is either wrapped in plastic or coated in plastic is made from polyethylene.

Polyethylene is made into millions of products every year, most of which we as consumers use on an everyday basis.

And if you are like most people, you did not realize the number of items that can be made from Polyethylene, but it is actually the number one used plastic in the world.

You would be surprised if you looked around at the world today and actually noticed everything that is made from Polyethylene.

Companies are even using it to make cable insulators to keep moisture and animals off of the actual cable wires.

Right now, low density Polyethylene is gaining a lot of ground and is being used in more and more places around the world.

Though it is mainly used in the manufacturing of coatings and cable insulators, it is also being considered for another large industry.

Manufacturers of plastic toys have been looking into using low density Polyethylene to create their toys.

Polyethylene is also being considered in the manufacturing of more household goods, though some household goods are already using the plastic.

 

What is Polyvinyl Chloride (PVC), and What is it Used For?

Polyvinyl Chloride (PVC) is one of the most commonly used thermoplastic polymers in the world (next to only a few more widely used plastics like PET and PP). It is a naturally white and very brittle (prior to the additions of plasticizers) plastic. PVC has been around longer than most plastics having been first synthesized in 1872 and commercially produced by B.F. Goodrich Company in the 1920s. By comparison, many other common plastics were first synthesized and became commercially viable only in the the 1940s and 1950s. It is used most commonly in the construction industry but is also used for signs, healthcare applications, and as a fiber for clothing.

PVC is produced in two general forms, first as a rigid or unplasticized polymer (RPVC or uPVC), and second as a flexible plastic. Flexible, plasticized or regular PVC is softer and more amenable to bending than uPVC due to the addition of plasticizers like phthalates (e.g. diisononyl phthalate or DINP). Flexible PVC is commonly used in construction as insulation on electrical wires or in flooring for homes, hospitals, schools, and other areas where a sterile environment is a priority, and in some cases as a replacement for rubber. Rigid PVC is also used in construction as pipe for plumbing and for siding which is commonly referred to by the term “vinyl” in the United States. PVC pipe is often referred to by its “schedule” (e.g. Schedule 40 or Schedule 80). Major differences between the schedules include things like wall thickness, pressure rating, and color.

Some of PVC plastic’s most important characteristics include its relatively low price, its resistance to environmental degradation (as well as to chemicals and alkalies), high hardness, and outstanding tensile strength for a plastic in the case of rigid PVC. It is widely available, commonly used and easily recyclable (categorized by resin identification code “3”).

What are the Characteristics of Polyvinyl Chloride (PVC)?

Some of the most significant properties of Polyvinyl Chloride (PVC) are:

  1. Density: PVC is very dense compared to most plastics (specific gravity around 1.4)
  2. Economics: PVC is readily available and cheap.
  3. Hardness: Rigid PVC is very hard.
  4. Strength: Rigid PVC has extremely good tensile strength.

Polyvinyl Chloride is a “thermoplastic” (as opposed to “thermoset”) material which has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (a range for PVC between the very low 100 degrees Celsius and higher values like 260 degrees Celsius depending on the additives). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polypropylene liquefy, which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.

Why is Polyvinyl Chloride (PVC) used so often?

Rigid PVC in particular has very high density for a plastic making it extremely hard and generally very strong. It is also readily available and very economical which combined with the long-lasting characteristics of most plastics make it an easy choice for many industrial applications like construction.

What Are The Different Types of PVC?

Polyvinyl Chloride is widely available in two broad categories: rigid and flexible.

How is PVC made?

Polyvinyl Chloride is made from one of three emulsion processes:

  1. Suspension polymerization
  2. Emulsion polymerization
  3. Bulk polymerization

Polyvinyl Chloride for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines: There are two main issues working with PVC that make it fairly problematic and not generally recommended for use by non-professionals. The first is the emission of toxic and corrosive gases when melting the material. This happens to some extent or another while 3D printing, CNC machining, and injection molding. We recommend you take a look at the MSDS data sheets for different chlorinated hydrocarbon gases like chlorobenzene and discuss the production process with a professional manufacturer. Second is the corrosive nature of PVC. This is problematic when PVC is repeatedly coming into contact with metal nozzles, cutters, and/or mold tools that are made from a material other than stainless steel or some other similarly corrosion resistant metal.

3D Printing:

Polyvinyl Chloride is available in filament form as a plastic welding rod (the material used for welding) but it is not presently retrofit for specific use in 3D printing. Although there are a growing number of plastics and plastic substitutes available for 3D printing, by far the two most common are still ABS and PLA. At Creative Mechanisms we typically 3D print with ABS. For a list of reasons why and a comparison of the two most common 3D printing plastics (ABS and PLA) for 3D printing read here.

The biggest issue with PVC for 3D printing is its corrosive nature (potentially compromising the functionality of typical machines if it were used over a longer time period). There was an interesting kickstarter to develop a PVC capable 3D printing nozzle (extruder head) put forward by engineer and entrepreneur Ron Steele that unfortunately closed without enough interest in 2014. You can take a look at the introductory pitch (video) here:

CNC Machining:

Polyvinyl Chloride can be cut on a CNC machine but any machinist who has tried has probably experienced degradation in the cutter depending on the material it is made from. PVC is very corrosive and abrasive and cutters that are not made from stainless steel or a comparably corrosive resistant material are likely to deteriorate over time.

Injection Molding:

Polyvinyl Chloride can be injection molded just like other plastics but the inclusion of chlorine in the material complicates the process. This is because melted PVC can give off a corrosive toxic gas. Accordingly, shops need to be equipped with sufficient ventilation systems. Those that aren’t are likely to be hesitant to work with the material. Additionally, special corrosive resistant materials like stainless steel or a chrome plating are required for the mold tool when injection molding PVC plastic. Shrinkage in PVC tends to be between one and two percent but can vary based on a number of factors including material durometer (hardness), gate size, holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives.

Is PVC Toxic?

PVC can pose a health hazard when it is burned as it emits hydrogen chloride (HCl) fumes. In applications where the likelihood of fire is high, PVC free electrical wire insulation is sometimes preferred. Fumes can also be emitted when melting the material (such as during prototyping and manufacturing processes like 3D printing, CNC machining, and injection molding). We recommend you take a look at the Material Safety Data Sheets (MSDS) for different chlorinated hydrocarbon gases like chlorobenzene and discuss the production process with a professional manufacturer.

What are the Advantages of Polyvinyl Chloride?

  1. Polyvinyl Chloride is readily available and relatively inexpensive.
  2. Polyvinyl Chloride is very dense and thus very hard and resists impact deformation very well relative to other plastics.
  3. Polyvinyl Chloride has very good tensile strength.
  4. Polyvinyl Chloride is very resistant to chemicals and alkalies.

What are the Disadvantages of Polyvinyl Chloride?

  1. Polyvinyl Chloride has very poor heat stability. For this reason additives which stabilize the material at higher temperatures are typically added to the material during production.
  2. Polyvinyl Chloride emits toxic fumes when melted and/or subject to a fire.

Although there are some shortcomings, Polyvinyl Chloride is a great material overall. It has a unique blend of qualities that make it particularly useful for the construction business.

Uses

Building and Construction

About three-quarters of all vinyl produced goes into long-lasting building and construction applications. Life-cycle studies show PVC/vinyl is effective in protecting the environment, in terms of low greenhouse gas emissions and conservation of resources and energy.

Because it is strong and resistant to moisture and abrasion, vinyl is ideal for cladding, windows, roofing, fencing, decking, wallcoverings, and flooring. Vinyl does not corrode like some building materials, does not require frequent painting and can be cleaned with mild cleaning products.

·         Siding and Windows

Vinyl helps produce siding and window frames that are extremely durable, affordable, and help conserve energy when heating and cooling homes. In fact, vinyl windows have three times the heat insulation of aluminum windows.

·         Wiring and Cables

Vinyl is able to withstand tough conditions behind building walls – such as exposure to changing temperatures and dampness – for the life of the building. As a result, it is one of the most prevalent and trusted materials used in electrical wiring and cables.

·         Water Pipes

PVC helps conserve energy and water by creating virtually leak-free pipes that are not prone to corrosion and resist environmental stress. PVC breakage rates are as low as one percent of the breakage rates of cast metal systems. The lack of build-up in PVC piping improves functionality and increases energy efficiency.

Packaging

Because it is durable, dependable and light weight, flexible PVC helps packaging do its job to maintain the integrity of the products inside, including medicines. Clear vinyl is used in tamper-resistant over-the-counter medications and shrinkwrap for consumer products. Rigid vinyl film is used in blister and clamshell packaging to protect medicines, personal care products and other household goods.

Healthcare

Vinyl plays a critical safety role in dispensing life-saving medicine through IV bags and medical tubing. The advent of the PVC blood-collection bag was a significant breakthrough because blood bags are flexible and unbreakable, enhancing the development of ambulatory medicine and serving as the foundation for modern blood banks.

Household Products

PVC’s affordability, durability and water resistance make it ideal for rain coats, boots and shower curtains.

 

 

Polytetrafluoroethylene or PTFE (more commonly known as Teflon) is a particularly versatile ivory-white and opaque plastic fluoropolymer; it is made by the free-radical polymerisation of many tetrafluoroethene molecules, and is suitable for a wide range of applications in industries as diverse as aerospace, the food and drink industry, pharmaceuticals and telecoms.

Produced by AFT Fluorotec in rods or tubes of any size, or filled with glass, carbon, stainless steel or many other materials to increase wear resistance and strength, whatever your project or build, we are sure to have a material that will work for you.

 

THE MAIN PROPERTIES OF PTFE

If you were trying to invent a highly flexible, chemical resistant, thermal resistant, non-stick and electrically resistant material, and it hadn’t already been done, you’d be hoping you could come up with a material somewhere nearly as good as PTFE is in these areas.

PTFE’s melting point is around 327°C, and pure PTFE is almost totally chemically inert, highly insoluble in most solvents or chemicals, and thermally stable enough to be used between -200 degrees C and +260 degrees C without degrading.

Other useful PTFE properties are its high flexural strength, even in low temperatures, high electrical resistance and dielectric strength, resistance to water (owing to fluorine’s high electronegativity), and low coefficient of friction. PTFE’s density is also very high, at 2200 kg/m3.

In fact, beyond reaction to some chemical agents and solvents (for example, chlorine trifluoride, cobalt(III) fluoride, xenon difluoride or elementary fluorine if at a high pressure and temperature), the only factor to be taken into consideration when using PTFE is that it does not have a good resistance to high energy radiation, which will cause breakdown of the PTFE molecule.

 

MODIFIED PTFE PROPERTIES

In addition to pure PTFE, there are two co-polymers which are equally as useful as PTFE, but with some different properties.

PFA or Perfluoroalkoxy has very similar properties to PTFE in that it is very chemically resistant, flexible and thermally stable (with continuous use up to 260 degrees C), but while PTFE does have some tendency to creep, PFA is creep resistant and is excellent for melt-processing, injection moulding, extrusion, compression moulding, blow moulding, and transfer moulding.

TFM, known as PTFE-TFM, is polytetrafluoroethylene with perfluoropropylvinylether as an additional modifier, giving a denser material which is stiffer, also creep resistant like PFA, and weldable.

FILLED PTFE

Pure or virgin PTFE can deform badly under a load, but the use of fillers can help with this, though it should be noted that not all filled PTFE is suitable for use with food.

Adding a filler to PTFE can increase its strength, improve resistance to abrasion, add electrical conductivity and more; however, adding fillers can also reduce some of the advantageous PTFE properties, such as chemical resistance which will be limited by that of the filler.

Fillers used can range from glass in various percentagesstainless steel, molybdenum disulphide, carbon or graphite, depending on which properties are to be improved.

 

ADVANTAGES AND BENEFITS OF USING PTFE

The biggest advantage of PTFE is its versatility, and the range of applications over so many products and different industries for this material is staggering.

The use of PTFE can have massive benefits in manufacturing and engineering, not just in making tubes or liners for handling or storing corrosive chemicals, but by coating parts such as bearings or screws to increase the lifetime of both the parts themselves and the machinery they are part of.

A PTFE-coated screw will be resistant to corrosion, due to PTFE’s ability to repel water and oil, and lubricated by the material to smoothly drive into whatever surface you are fastening to, with reduced friction, resulting in less wear on both the screw and the surface, and a longer-lasting, more secure finish.

Friction and wear can also be factors with bearings, and a PTFE coat can give the same benefits as with coating screws, with the additional advantage that the coating will also be heat-resistant.

It’s clear that longer lasting, higher-performance parts can add to the efficiency of any machinery, reduce the need to constantly acquire replacement parts, both saving money and the time needed to fit the replacements, as well as reducing waste. This will also reduce maintenance needs as there are less likely to be faults with the equipment, and also greatly reduce, or even eliminate, any expensive manufacturing downtime due to faults or repairs.

Cleaning of equipment can also be reduced in some cases as a PTFE coat is non-wetting, facilitating self-cleaning of parts.

And Teflon textile finishes can even help the environment, because, when applied to fabric, the finish will repel water and oil stains, reducing the need to use dry cleaning, and fabrics will also dry more quickly, using less energy with tumble drying, and last longer due to reduced wear.

With the added advantages that PTFE is non-toxic, has only a minor contraindication for humans from polymer fume fever (only if the temperature of any Teflon-coated pans reaches 260 degrees C) and is FDA approved and food-safe, this material really is of great benefit in many different areas.

USES OF PTFE

Most people have never heard of PTFE industrial coating, but when you mention Teflon, a look of understanding passes easily on their faces. PTFE (Polytetrafluoro Ethylene) is the technical name of the material, and it’s commonly sold under the Teflon brand name, which is manufactured by DuPontDr. Roy Plunkett, a researcher who worked at DuPont, is credited with developing PTFE industrial coating in the late 1930s.

At the time of his discovery, he was actually trying to create a new refrigerant. During the course of development, he noticed that the gas inside the bottle he was using actually stopped flowing out before the bottle should have been empty. He sawed the bottle open and discovered that the inside was coated with the non-stick material we now know as Teflon. His contribution has changed the face of plastic manufacturing forever.

Teflon is probably best-known for its role as the non-stick surface inside cookware. This is because PTFE industrial coating is one of the most slippery materials that’s in existence today. In addition to being slippery, the material also brings a number of other features to the table, offering high temperature resistance, little reaction to most chemicals, and reduced stress cracking and corrosion. These features make Teflon perfect for numerous applications, including:

  • Cookware– As already mentioned, the slippery surface created by Teflon makes it perfect for cookware. Many brands offer lines of cookware that are coated with PTFE in order to prevent food from sticking to the pots and pans. This reduces the need for cooking oil because these pots and pans are naturally non-stick.
  • Nail polish– That smooth surface that doesn’t crack is often achieved through the use of PTFE industrial coating.
  • Hair styling tools– Hair straighteners and curling irons are often coated with Teflon because of the high temperatures emitted by these tools.
  • Windshield wiper blades– There are numerous applications for PTFE industrial coating within the automotive industry as well. The blades of windshield wipers are the most notable because the smooth surface enables them to glide smoothly across the windshield.
  • Fabric and carpet protection– Stains are less likely to stick to carpets or fabrics that have been treated with PTFE industrial.
  • Chemical and steel industries– Hoses and other machine parts commonly handle some highly corrosive substances that sometimes are transferred at extremely high temperatures. PTFE industrial coating is one of the best materials to handle this type of use because it addresses all of the problems that are otherwise caused by working with chemicals or steel. Every type of hose will deteriorate over time, but those that are made of PTFE industrial coating will do so much more slowly than those made of other materials because of the many features of the material.

 

 

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