Testing plastics

In Cons DistList Instance 9:12, Patricia Miller asked if there was a
way to test plastics for plasticizers.

In my lectures on selection of plastic storage materials I recommend
that a specimen of the plastic (at least one gram) be immersed in
about 25 ml methanol in a beaker, sealed with a nonvinyl laboratory
wrap or food wrap (such as DuraSeal, Seal-View, Handi-Wrap, not
SaranWrap or aluminum foil) and left for at least 24 hours, checking
to ensure that the methanol has not evaporated.  The specimen should
be finely divided.  Pieces of film or sheet less than 1 mm thick or
slices of solid objects, less than 1 mm thick, are acceptable.
After this period the wrap is removed, the specimen taken out of the
methanol, and the methanol allowed to evaporate to dryness. If
extractable plasticizers were present in the specimen, an oily or
greasy evaporation residue should remain.  If the sample was weighed
before immersion and the amount of evaporation residue weighed, then
a rough estimate (always too low) of the plasticizer content can be
calculated.  For plasticized poly(vinyl chloride) the plasticizer
content is typically 10-25 wt% but may be as high as 50 wt% (see
Williams, R.S. 1987. "Tygon Plastic Tubing: Use With Caution" IIC-CG
Newsletter, Vol. XII, No. 3, March 1987).

Solvent with no evaporation residue must be used for this test.
This can be checked by evaporating an amount of solvent equal to
that use for the test, without the addition of specimen.  There
should be no evaporation residue with the "pure" solvent.  A quick
check for evaporation residue can be done with a few drops.  Put a
few drops of the solvent onto a scrupulously clean glass plate (e.g.
microscope slide), mirror-polished metal plate, or very high gloss
Mylar.  Let solvent evaporate, then examine the plate for tidelines,
residues, smudges, etc.  Anything that disrupts the specular
reflection from this surface indicates an evaporation residue which
disqualifies the solvent for this test.

If the appropriate laboratory glassware is available, a more
stringent or aggressive extraction should be performed using hot
methanol rather than simply immersing the specimen in methanol at
room temperature, as described above.  By attaching a condenser to
the beaker (or more appropriately to an Erlenmeyer or round bottom
flask) the specimen can be boiled in methanol for several hours.  A
more efficient system is to use a Soxhlet extraction apparatus with
methanol to extract plasticizers.  This is one method I use when I
analyzing plastics.

Another method to separate additives from base polymers (remember, a
plastic is composed of the base polymer plus the additives) is the
solution/precipitation method.  The plastic is dissolved in a good
solvent for the plastic, then the polymer portion is precipitated
with a nonsolvent.  For example, poly(vinyl chloride), PVC, plastics
can be dissolved in tetrahydrofuran (THF), then precipitated by the
addition of an excess of methanol to the stirred tetrahydrofuran
solution.  The PVC precipitates as a curdy mass.  This mixture is
filtered or centrifuged to leave the additives in the THF/methanol
solution.  The methanol solution is evaporated to dryness to leave
the additives behind, as described above.  This residue can then be
analyzed easily by a technique such as IR spectroscopy, if further
identification is necessary.  I often use this method with
centrifugation to analyze 1 cm x 1 cm sized specimens (about 0.05 g)
of plastic films.  Although it can be more time consuming and
additional materials are required, it is a better way to separate
high molecular weight additives that are soluble in THF/methanol but
too large to be extracted directly from the intact plastic.

Concerns about plasticizers are usually related to plastic films.
In general, plasticizers are most likely in PVC, poly(vinylidene
chloride) (PVDC, e.g. Saranwrap), and cellulose acetate (CA, e.g.
some photographic film base).  Polyethylene (PE), polypropylene
(PP), polystyrene (PS), poly(ethylene terephthalate) (PET, e.g.
Mylar), and oxygen barrier films seldom, if ever, contain
significant quantities of plasticizers.

A number of simple tests can be used to categorize plastic films
into classes that enable screening of potentially bad or good
products.

PVC, PVCD, and copolymers of these with each other and with
poly(vinyl acetate (PVAC), contain chlorine and can readily be
recognized by their positive response to the Beilstein test (a green
flame is produced when heated with a copper wire, or penny, in a
propane torch flame).  This test is described in CCI Note 17/1:
Williams, R.S. 1993. "The Beilstein Test: Screening Organic and
Polymeric Materials for the Presence of Chlorine, with Examples of
Products Tested", Ottawa: CCI). Because extractable, exuding, oily
plasticizers may be present, any film or sheet material that tests
positive for chlorine should be avoided, or at least never used in
direct contact with objects, until it is proven that plasticizers
are not present.

PE, PP and copolymers of these with each other and with small
amounts of vinyl acetate (less than about 20% VA, e.g. "EVA" films)
have densities less than 1 and therefore float on water.  No other
commonly encountered plastics in film or sheet form have such low
densities.  A small specimen, such as produced with a paper hole
punch or smaller (down to the limits of visibility), is immersed in
a beaker of water containing 1 drop of dish washing liquid detergent
(or equivalent surfactant) per 100 ml to break the surface tension
of the water.  If the specimen floats (after having carefully
inspected the specimen to ensure that no bubbles are attached), then
the film or sheet is most likely PE, PP, or a copolymer.  These
plastic are very unlikely to have plasticizers.  They may have other
additives (but then so does every plastic).

The presence of mineral fillers increases the density of PE and PP,
but these are generally not present in such high concentrations as
to raise the density above that of water - filler PE and PP will
still float.  Clear, transparent films (colored or not) do not have
mineral additives, so this interference does not occur.

CA, PS and PET (all with no chlorine and denser than water) can be
distinguished by the different solubilities in organic solvents.
PET is insoluble in all organic solvents at room temperature.  PS
dissolves in toluene and acetone.  CA dissolves in acetone but not
toluene.  A single drop of each of toluene and acetone is put on the
surface of the film specimen which is then examined.  Dissolution,
etching of surface, or formation of white deposit with both solvents
indicates PS, with acetone only indicates CA, and with neither
indicates PET.

There are many elaborate, non-instrumental schemes for
identification of plastics and several have been discussed by Coxon
(Coxon, H.C. 1991. "Practical Pitfalls in the Identification of
Plastics" in Saving the Twentieth Century: The Conservation of
Modern Materials"  Ottawa: CCI).  As she mentions the expectation of
absolute identification of all components in any type or form of all
plastics by such non-instrumental tests is totally unrealistic.
However, for specific classes of plastics, such as clear plastic
films, these tests can be very reliable, principally because the
range of possibilities for composition is restricted by the form of
the plastic (e.g., a film of phenol-formaldehyde or epoxy is very
unlikely).

Proposed test for plasticizers

Two observations of the behavior of plasticized PVC on adjacent
materials have prompted me to propose two other tests for
plasticizers in films and sheets.  These are offered for
consideration, I have not done any systematic testing to see if they
work at all, and certainly have not developed strict protocols.

Proposed Test 1: Photocopy toner softening test

Photocopied pages in contact with vinyl three-ring binder covers
often stick to the binder cover.  Analysis of the problem shows that
the phthalate plasticizer from the PVC has exuded to the surface of
the binder cover then, when in contact with the photocopy page, has
been absorbed by the binder of the photocopy printing ink (toner),
thereby softening the toner, increasing its tack and causing it to
stick to the binder cover.  This occurs at room temperature with
slight pressure such as produced with the binder on a book shelf,
after an unknown period (one to several years).

A test is proposed whereby a sheet of high contrast printed
photocopy is placed in contact with the plastic film to be tested
and a weight is applied, the whole assembly then being placed in an
oven at some elevated temperature.  After an incubation time, the
photocopy/plastic assembly is disassembled and the sticking or
transfer of photocopy toner observed.

Transfer is assumed to be due to plasticization and tackifying of
the toner by PVC plasticizer, but simple softening of the toner at
elevated temperature must be ruled out by control tests of a
unplasticized material (maybe a glass microscope slide) against the
photocopy at various temperatures.  It is likely that different
brands of toner will affect the test.  The most susceptible toner
needs to be identified.  Another possibility is to create a simple
but characteristic sensor to replace the variable photocopy. A
colored varnish made of a material such a cellulose nitrate, which
is sensitive to phthalate plasticizer, could be applied to paper and
used in the test to replace the photocopy.

Proposed Test 2: Pressurized exudation of plasticizers onto paper:

In a recent examination of several different types of tubing for use
with organic solvents in an ultrasonic mister, I placed a 1 mm thick
slice of each of several 1/4 inch diameter tubings between sheets of
plain copier paper then pressed these to a gauge pressure of 2000
psi in a Carver press for 10 min.  I have no idea what actual
pressure was exerted on the specimens.  (Note: A "hard" paper like
copier paper rather than a "soft" paper like filter paper was
necessary to prevent crumpling and tearing of the paper as the
tubings squished and extruded during the test.)  After 10 min the
press was opened and the paper/tubing/paper sandwiches removed.  For
some tubings, the paper showed oily stains, which were particularly
noticeable when the paper was held up to the light to observe the
stained area by transmitted light.  The amount of staining was
related to the amount of oily plasticizer content, as determined by
extraction and IR spectroscopy. The following results were obtained.

    Tubing                     Stain   Plasticizer Content

    Tygon PVC                  oily    phthalate, 50 wt% Type 3603

    C-FLEX
    Cole-Parmer 6424-75        oily    mineral oil, 20 wt%

    Masterflex 6411
    Peroxide cured silicone    none    silicone oil, < 2 wt%

Under these conditions, this test seems to detect between 2 and 20
wt% of oily plasticizer.  The sensitivity of the test may be
improved by higher pressure, longer duration under pressure,
increased temperature.  It may be possible to do the test without a
press by using clamps, vises, etc and incubating in the oven.  I
have not examined these variables.

Since performing this test I found ASTM D 3291: Standard Test Method
for Compatibility of Plasticizers in Poly(vinyl chloride) Plastics
Under Compression which "determines the compatibility of
plasticizers in poly(vinyl chloride) plastics by rating the amount
of plasticizer that spews due to compressional stress set up inside
a 180 deg loop bend".  This test applies to sheets that are 1.9 mm
thick and so strictly does not apply to thinner films.  I have not
tried this test so cannot vouch for its performance.

ASTM Summary of Method: "Test specimens of plasticized poly(vinyl
chloride) sheet (1/2 x 1 inch) are bent through an arc of
approximately 180 deg.  The inner radius of the bend is equal to the
thickness of the specimen.  These bent specimens are secured in a
jig designed to hold them in the desired conformation.  At specified
intervals of time (4 h, 24 h, 7 days), a specimen is removed, bent
360 deg in the opposite direction, and the former inside of the loop
(now the outside) is examined for evidence of plasticizer spew."

After taking out of the jig the specimen can be wrapped around the
index finger and the bent area examined for spew by wiping area with
a cigarette paper wrapped around the other DRY index finger.  Spew
can be ranked:

    Description of cigarette paper    Amount of exudate    Grading

    No mark on paper or
    visible evidence in loop          none                    0

    Oily mark on paper very faint
    and discontinuous                 slight                  1

    Wetted area may appear saturated
    in small spots, although much
    of it is not saturated            moderate                2

    Total wetted area is saturated
    by continuous wetted film.
    Large puddle of plasticizer
    over entire wetted area           heavy/dripping          3

Certainly any specimen with grading of 1-3 must be rejected.
Unfortunately, if grading 0 is obtained, we cannot be sure that
plasticizer will not migrate some time in the future as the polymer
degrades and plasticizer compatibility decreases.  This is the
general problem associated with all additives (see Williams, R.S.
1991. "Composition Implications of Plastics Artifacts: A Survey of
Additives and Their Effects on the Longevity of Plastics in Saving
the Twentieth Century: The Conservation of Modern Materials, David
Grattan, Ed. Ottawa: CCI.)

Scott Williams

                                  ***
                  Conservation DistList Instance 9:15
                  Distributed: Friday, August 4, 1995
                        Message Id: cdl-9-15-002
                                  ***