Ask the Extrusion Expert: Why does PC cost so much? How do I determine if the plastic I bought is really the plastic I bought? What determines tensile strength?

Will he answer every question sent? No, but he will answer many. Our aim is that this column becomes the leading source of extrusion information on the web. Have a plastics extrusion question for Allan? Send it to him at [email protected]. We will not publish your name or company name unless you expressly ask us to do so, but also will not accept anonymous questions as this opens the opportunity for a supplier to send a "baiting" question for which the answer might highlight his products.

Below are some attendees' questions plus Allan's answers from our latest Extrusion Expert webinar on "Plastics Chemistry for Non-Chemists." If you were not one of the nearly 400 processors who attended, here is a link to the archived Extrusion Expert webinar (and the topic and presentation was one for the entire industry, not only for extruders).

To the Q&A:

Q: How would a supplier confirm for a customer that a product is exactly the chemistry desired on the purchase order? Specify a spectrochemical analysis method?

A: For better or worse, people usually buy on properties, rather than chemistry. They trust the resin maker to use appropriate chemistry to get the results claimed in the property data sheet, or such basics as "food grade per FDA rules," or "degradable as per ASTM D6400." Sometimes the resin maker will "certify" certain properties like melt index or density, but seldom mention the allowed variation (standard deviations) nor how frequently they test, unless asked. Spectra and other lab tests may be useful to resolve any disagreements.

As for the buyers/processors, if they run tests on incoming materials at all, it is likely for contamination or flow properties (usually melt index). I have long been a proponent of testing such materials by running them though a small extruder or torque rheometer, which will give flow properties more meaningfully than melt index, and the extruder will also show up color and contamination, which may be important. Other tests of incoming materials may include density (especially for PE) and lubricity (slippery pellets have lower in-push/rpm which may lead to overheating).

Q: How are polymer lengths and branching controlled?

A: The answers seem simple - time, temperature, pressure, co-monomers and catalysts/other active chemicals - but their application can be quite complicated. Regarding length, the longer the mass is at reaction conditions, the more of it will react, but there is a diminishing-returns principle acting when the mass is, say, 95% polymerized, and the remaining 5% of monomer is looking for a loose end to hook up with. Some polymerization, notably PVC, can be done in water suspension, which enables more free movement of monomer and short chains looking for others, but requires a draining and drying step at the end of the line.

The role of co-monomers is shown with linear-low-density PE. To get short branches, a small amount of another monomer with a double bond such as butene (4 carbons instead of the two of ethylene) is included. The butene goes into the chain with its 2-carbon double bond just like ethylene, but that leaves 2 carbons in a short chain (the branch) dangling from the main chain. Similarly, hexene (6 carbons) makes 4-carbon branches and octene (8 carbons) makes 6-carbon branches.

There are many more complications but this addresses the basics of your question.

Q: Can PP be mixed with nylon in injection molding?

A: Not as a direct mixture, as the two materials are incompatible. However, you could use an additive that adheres to both molecules (a compatibilizer) to bond the two together. I am most familiar with this technique to make multilayer films, but it may work for molding, too.

There is an interesting product from DuPont called Selar RB, in which nylon flakes are dispersed in HDPE to provide a better gas barrier. The idea should work for PP as well, but I haven't heard about this product, even with HDPE, for a long time. I did get a positive response to Googling and see that it may have some use in small fuel tanks. Maybe DuPont can tell you more.

Q: What plastics can be used (friendly) in medical devices?

A: The answer is virtually all of them. Plastics are essentially inert and harmless, despite loud objections from plastophobes. In deciding what plastic to use, a key question is whether the product is in short-term or long-term contact with the body or with medications.

Implants will have a different set of requirements than tubing, and a prefilled syringe that is expected to remain useful for years (some military uses demand this much) will have different requirements than a specimen cup or test tube. The additives are at least as important as the base plastic, and some may in fact be unsuited to medical applications. It may be primary that the plastic is not harmful, but it is also important that the plastic is not affected in a way that compromises its function. Resistance to sterilization and heat/cold requirements are also critical, and will favor certain plastics above others.

Q: What determines the price of a plastic? Why does polycarbonate cost so much?

A: Price depends on a variety of factors, including the market demand, the total volume sold, the size of the specific order, the investment needed to make the polymer, the cost of necessary additives, and sometimes the quality control demanded by the application (e.g., medical items). It does depend on raw material costs, too, but not as much as we may think. The fluctuation in crude oil price may be used to "justify" price increases, but this is as much a sales tactic as a necessary constraint. Remember that even though the basic feedstock for most plastics is still petroleum or natural gas, the "spot" prices that we see published are not necessarily what the producer is paying for that petroleum - in fact, many of the plastics' suppliers are part of the petroleum companies themselves. I covered this in more detail in my June 2011 webinar, Economics of Extrusion.

As for polycarbonate, it is a more complicated molecule than the simple polyethylene I used in my webinar example, and needs the synthesis of two separate monomers, one quite dangerous to handle (phosgene). Ultimately, the market price depends on what the producers can get for it, considering its remarkable properties and the costs of competitive plastics.

Q: What does it mean to have a "tin" stabilizer in PVC?

A: All PVC needs a stabilizer to oppose molecular breakdown when heated to extrusion temperatures. Typical stabilizer content is between 1% and 3 %. Most of these compounds react with the hydrochloric acid (hydrogen chloride = HCl) produced when a molecule breaks apart, thus preventing that HCl from breaking another molecule.

Stabilizers cost more than the PVC resin itself, so there is always pressure to use as little as necessary. Thus, since more heat means more danger of degradation, the ability to run cooler is really an economic advantage, and is the underlying rationale for the use of twin-screw extruders for PVC products.

Organic tin compounds are an important group of stabilizers, and are commonly used for pipe and profiles as well as food packaging, where their clarity may be needed. Some of the best tin stabilizers are also strong smelling (mercaptides) and may therefore be restricted to outdoor or other appropriate uses.

Q: What determines the tensile strength of a plastic?

A: The basic polymer has a tensile strength which depends on the arrangement of the atoms (including partial crystallinity), the length (weight) of the molecules, their distribution (all similar versus more variation) and the branching. It also depends on temperature (warmer = less tensile strength) and on additives (fibrous ones may increase it, softer ones may decrease it, but may have other advantages).

Still another dependency is on the test specimen and the way the molecules are oriented along lines of flow. That is why an injection-molded specimen or one cut from oriented (stretched) film or sheet will show substantially higher tensile strength that one cut from an extruded non-stretched sample.