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Best Practices in PET Recycling Click here for printable PDF version Compounding Recycled PET
Issue:Although
PET has an excellent balance of performance properties, there are
some shortcomings that have prevented large scale use of recycled
PET in injection-molded, durable products. The primary drawbacks are
PET's low glass transition temperature, the slow crystallization rate
of copolymer grades of PET, and relatively low impact strength.
The properties of recycled PET must be modified for use in
durable products, such as appliances, electronics, furniture, transportation,
and building and construction. Compounding of recycled PET can correct
these shortcomings, enhance other properties, and tailor performance
properties to meet specifications.
Background: Durable
products are exposed to rigorous and varied environments. Therefore,
the resins used to manufacture durables must have superior mechanical,
thermal, electrical, chemical, and environmental properties to packaging
resins. These so-called engineering
resins possess a balance of properties tailored to each durable application.
In many cases, this tailoring is accomplished by compounding.
This is especially true for post-consumer PET.
An example of rigorous
environmental conditions is the continuous service temperature, which,
for many durable products, is much higher than 175°F. The glass transition temperature at which unmodified bottle-grade
PET resin begins to soften and become rubbery is at 165ºF to 175°F. Therefore, the
direct use (without specific additives) of recycled PET in these products
is not feasible. PET Structure. PET is one of a few plastics whose molecular
structure in a finished part can be amorphous, crystalline, or semi-crystalline
(somewhere in between.) Amorphous
PET tends to be easier to process, tougher, more flexible, and clearer
than crystalline PET, if all else is equal.
Crystallinity is desirable in products that require high temperature
stability, dimensional stability, stiffness, tensile strength, and
barrier properties. The majority of bottle-grade PET being recycled
today is a copolymer and, therefore, is inherently slow to crystallize. The best balance of properties is usually
some combination of these two structures.
Crystalline PET is best
suited for manufacturing processes where its crystallinity is enhanced
by mechanical orientation. Products manufactured this way include fibers,
strapping, film, oriented sheet and stretch blow molded bottles. Crystallinity
in PET products is controlled by orientation, cooling rate, and/or
nucleation. In each case, the formation of strain-induced crystals
is precisely controlled to develop certain targeted properties in
the finished product.
Best Practices: When
molding PET compounds into durable applications, the first step is
to work closely with the customer and develop a comprehensive list
of performance requirements for the product. A technical review of these performance specifications
should be completed to determine if recycled PET, with modification,
could cost-effectively meet the customer's requirements.
The next step is to establish
a partnership with an independent, experienced PET compounder or a
resin company. The key requirement is experience, because
the semi-crystalline nature of PET sets it apart from many other engineering
resins. Successful injection
molding of post-consumer PET is dependent on a full understanding
of the complex relationships between molding conditions and the resin
formulation. There is both art and science involved and
the learning curves for each are quite steep. In addition to experience,
the ideal partner should be capable of compounding different formulations
and injection mold parts on a small scale for evaluation and testing.
Each product will require some formulation development because
of the limited use, to date, of recycled or virgin PET in injection
molding. There are only a few tried and tested PET compounds,
and each is proprietary. The third step is to develop
two or three compounded formulations for trial on commercial molding
equipment. Each formulation should tailor the resin properties
to a "best-fit" of those specified by the customer. The need for variations of the formulation
is to adjust for differences in molding machines, especially for cooling
rates and process controls. Although highly complex
and sensitive to interactions between ingredients, a typical recipe
for a durable product can include some or all the following ingredients:
·
Glass or other
fibers to provide stiffness, strength, and/or electrical properties
·
Nucleator to
control the level of crystallinity
·
Impact modifiers
to impart toughness
·
Flame retardant
additives to meet product safety requirements
·
Mineral fillers
to stiffen, provide dimensional stability and/or reduce cost
·
Heat and/or light
stabilizers
·
Lubricants for
improved processing
·
Antistats to
minimize the development of static charge
·
Liquid color
or color concentrate
An important best practice
is to thoroughly dry the PET blend formulation (e.g., to 100 ppm moisture
or less, per PET Drying Best Practice) just prior to compounding
to avoid hydrolysis of PET and subsequent loss of molecular weight
and properties. Dryness should
be maintained until compounding begins.
The appropriate, dried
formulation is then homogenized, melt filtered, and pelletized in
an extruder. Single-screw
extruders with screw length/diameter ratios of 20-24:1 are generally
acceptable and preferred for the incorporation of several ingredients
at concentrations of a few percent each.
For relatively high loadings of ingredients or glass filling,
a twin screw extruder is preferred.
Controlling crystallinity.
The formation of strain-induced crystals must be precisely controlled
to develop certain targeted properties in the finished product. Crystallinity can be controlled through mechanical
orientation, nucleation, or by controlling the cooling rate. (Also
Refer to the Drying Control Best Practice) [link to pbp3-05-01.htm].
Rapid cooling, such as
in molding of PET bottle preforms, minimizes the formation of crystals.
Slow cooling has the opposite effect allowing crystals to form.
In addition, slow cooling results in lengthy cycle times and undesirable
coarse crystals. Heat-induced
crystals are coarser than strain-induced crystals and can reduce clarity
or impart brittleness. Nucleation is used when
orientation and slow cooling are not practical options. This is certainly the case in injection molding,
where the process is not suited for controlled mechanical orientation.
Small percentages of inorganic minerals, organic compounds,
or melt-compatible crystalline resins are used to control crystallinity
in injection-molded PET parts.
Benefits: Proper blending ensures a homogeneous mixture, which prevents variations
in the processibility of the melted resin, which can cause high scrap
rates and inferior end-products. The durable goods industry represents
a large, relatively untapped, market for recycled PET.
Issue Date / Update: January 1998
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