A Polymer Is Not Defined by an Assay

A certificate for a small-molecule chemical gives you one number — 99.5% minimum — and that is most of what you need. The same document for a bag of ABS resin gives a page of figures, none of them a purity: there is no such thing as 99% pure polymer. The product is a distribution of chain lengths, a ratio of repeat units and an additive package.

Most resin disputes come from two sheets compared on the wrong line, with nobody asking which test produced the figure. Three numbers matter, and each is meaningless without its method.

The Flow Number Means Nothing Without Its Condition

Melt flow index is grams through a defined die in ten minutes, at a defined temperature and load. Every term is a variable: polyethylene at 190 °C under 2.16 kg, polypropylene at 230/2.16, ABS at 220/10, and PMMA at 230 under 3.8. A melt index of 8 for an acrylic and 8 for a styrenic are two different experiments printing the same integer.

Then mass against volume. Flow rate is gravimetric, volume rate volumetric, and melt density links them: polyethylene near 0.76 g/cm³ at 190 °C turns 10 cm³/10 min into about 7.6 g/10 min, a quarter apart. Cross the two conventions in a purchase order and inspection fails while the resin is fine.

The test also sits at one low shear rate. Two resins that agree there can still diverge at the thousand or so a high-speed die applies.

And the number is an inverse proxy for chain length: it rises when chains break. Regrind-loaded lots can hit the specified index with a broader distribution and lower melt strength. Use it to confirm a repeat shipment, not to rank suppliers.

Where the Spec Is Chain Length: IV, K Value, Solution Viscosity

Some families cannot use a melt test: the polymer degrades before it flows, or the application cares about strength rather than flow. The sheet then switches to dilute solution, and solvent and temperature become part of the spec.

For PET polyester it is intrinsic viscosity. Bottle grades sit at 0.78–0.84 dL/g, textile at 0.62–0.64, and whether a preform blows cleanly or stress-cracks in the filler sits in that second decimal. Water destroys IV: dry to under 50 ppm at a dew point near −40 °C, because every 100 ppm at 280 °C comes off the IV and adds acetaldehyde. One ppm against a 3 ppm limit for beverage resin says solid-state polycondensation was run properly.

For polycaprolactone the sheet may quote molecular weight or IV, but in a polyurethane the hydroxyl value is what reacts; the right molecular weight with the wrong OH value misses the stoichiometry. Above its melting point near 60 °C the pellets block in the bag.

Polyvinyl acetate is specified by the viscosity of a solution at fixed solids — typically 10% in ethyl acetate — or the K value derived from it. Both need concentration and solvent to mean anything. Residual vinyl acetate is a separate line, and 1% is a different product from 0.1% for food packaging.

The Composition Ratio Is the Specification

For copolymers the deciding number is how much of the second monomer went in — the line most often left vague.

EVA, NBR and PAN: one ratio, three industries

EVA copolymer is bought by vinyl acetate content: about 9% for flexible film, 18–20% for hot melt, 28% for cable compound and foam, 40% and above for a waxy solid dispersible into emulsions. VA drops crystallinity, so density, stiffness and clarity move with it; it is measured by hydrolysis and back-titration or FTIR, and 18% with no method and no ±1 tolerance cannot be checked on arrival.

Nitrile rubber works the mirror image. Acrylonitrile at 18/28/33/41% sets the trade between oil resistance and low-temperature flexibility, and no additive buys back what you gave up for swelling resistance. Mooney ML(1+4) at 100 °C is what the mixer feels.

Polyacrylonitrile is never a homopolymer in commerce: a few percent methyl acrylate for spinnability, about 1% itaconic acid to start cyclisation lower. In fibre that ratio is dyeability; in carbon precursor it is the stabilisation window.

PLA: the grade is optical purity

Polylactic acid is the clearest case: composition outranks everything else. PLLA with high L content melts near 176 °C and can crystallise; each D unit disrupts the lattice, pulls the melting point down and slows crystallisation, so a heat-resistant cup and a compostable film are the same polymer at different optical purity. Ask for IV and D-content together, plus residual meso-lactide, an impurity that caps molecular weight.

Numbers That Only Appear in the Second Process

The expensive mistakes happen where one material meets another, and the controlling number is rarely the headline one.

C5 petroleum resin: softening point 40–120 °C by ring and ball, Gardner colour 8 or lighter, acid value 1.0 or below, viscosity 200–5000 mPa·s at 25 °C — all quotable. Whether the adhesive holds depends on compatibility with the elastomer, read through cloud point or MMAP. Raise the softening point and shear climbs while tack falls; the base polymer’s polarity sets the limit.

Urea-formaldehyde resin lives on the formaldehyde-to-urea ratio. Near 1.2: fast cure, high strength, more free formaldehyde. At 1.05 and below: E1 and E0 board, longer press, weaker wet bond. Two quotes for “UF resin” can be two emission classes.

Polychloroprene is specified by crystallisation rate more than Mooney. Mercaptan-modified grades crystallise fast and give green strength early, which is what an adhesive formulator wants; sulfur-modified grades, made with xanthogen disulfide, crystallise slowly and store better. Neoprene that hardens in the warehouse is a grade mismatch.

Polyoxymethylene splits into homopolymer and copolymer, as much a processing decision as a property one: the homopolymer is stiffer and more crystalline with a narrower window, the copolymer trades stiffness for tolerance of hot water and alkali.

Brominated polystyrene carries about 68% bromine and must hold stability above 300 °C to survive polyamide processing, where a PBB route is not acceptable; it runs with antimony trioxide at roughly three to one; the spec is bromine plus stability, not the words “flame retardant”. Oxidised polyethylene is bought on acid number — 17 mg KOH/g in the common grade — because that decides emulsification and pigment wetting.

Five Lines to Send Before You Ask for the Price

None of this needs a laboratory, only five questions: which condition the flow number was run under, and whether mass or volume; what IV or K value applies, in which solvent and at which temperature; the comonomer ratio with a tolerance and method; water as packed; and whether the figure is typical of the batch or guaranteed on every shipment. That last one separates a specification from a description.

A longer data sheet is not a better one. Three lines — a flow number with its condition, a chain-length number with its method, a composition ratio with its tolerance — say more about whether the material will run than ten pages of application photographs.