By: Matthew Tabassi
Intro
The use of
PVC in the global markets continues to grow. In 2012, total 37.4 million tons of
PVC worldwide was consumed (source:HIS/Vinnolit),
although PVC has special significance for tube, profile and sheet production,
it is also widely used in film manufacturing.
Polyvinyl chloride is the third most widely produced plastic,
after polyethylene and polypropylene. PVC is widely used in construction
because it is cheap, durable, and easy to assemble. PVC production is expected
to exceed 56 million tons by 2017.
Global
Polymer Market (208MMT)
PVC can
be made softer and more flexible by the addition of plasticizers,
the most widely used being phthalates. In this form, it is used in
clothing and upholstery, and to make flexible hoses and tubing, flooring, to
roofing membranes, and electrical cable insulation. It is also commonly used in
figurines and in inflatable products such as waterbeds, pool toys, and
inflatable structures.
In recent
years however, researchers have noted health risks such as reproductive
abnormalities and developmental effects in human. A number of substances have
been identified as alternative plasticizers. These alternatives
include citrates, sebacates, adipates, and phosphates.
They are
being substituted in products that traditionally use phthalates, such as
toys, childcare articles and medical devices.
Plastic food
packaging film, popularly known as cling film, has literally revolutionized the
food industry. It has become a major contributor to food safety, both
protecting and preserving it. At the same time it is now regarded as an
essential and cost-effective tool for food presentation.
Plasticized PVC
films preserve the freshness of meat as they have high oxygen and
water vapor transmission. They are cost effective since they run satisfactorily
on high-speed packing machines and are effective for display as they have good
clarity and are resistant to handling due to their good elastic
recovery and puncture resistance. They have excellent cling and are
easily heat-sealed
For catering
and household use, thinner films with less plasticizers are
supplied. The benefits are the same as with meat wrapping with cling,
clarity and strength being especially important.
Yet for all
its versatility and obvious benefits, there has been media speculation about
its safety. Cling film has been used in the USA and Europe for
decades and scientific research has repeatedly shown it is perfectly safe to
use.
Whatever
material is chosen for packaging food there is always some transfer
from the constituents of the package to foodstuffs. A considerable amount
of experimental work has been carried out to determine the
migration from plasticized PVC into food. This migration
is at levels which are considered totally safe by health authorities
and which fall well within European Union regulations.
PVC
PVC is a
thermoplastic made of 56.5% chlorine, basically derived from industrial grade
salt and 43.5% carbon that derived predominantly from oil / gas via
ethylene. This chlorine gives PVC excellent fire resistance; when
PVC is set on fire, the flames go out as the fire source is
removed due to the material’s self-extinguishing properties making it
number one choice for the cable industry.
PVC is
regarded as perhaps the most versatile thermoplastic resin, due to its
ability to accept an extremely wide variety of additives: plasticizers,
stabilizers, fillers, process aids, impact modifiers, lubricants, foaming
agents, biocides, pigments, reinforcements. Indeed, PVC by itself cannot be
processed! It must have at least a stabilizer, a lubricant, and if flexible,
a plasticizers present.
Plasticized PVC films preserve the freshness of meat as they have
high oxygen and water vapor transmission. They are cost-effective, since they
operate satisfactorily on high-speed packing machines, and are effective for
display, because they have good clarity. Their good elastic recovery and
puncture resistance make them suitable for handling; they have excellent cling
properties and can be easily heat-sealed
Average degree
of polymerization of PVC resin
A good range of
polymerization for PVC is around 1000 to 1300, if `P
< 1000, the film mechanical properties are not good enough. If`P
> 1500, the extrusion processiblity is inferior because the viscosity of
melt is increased, it is easy for degradation by the heat which is generated
during extrusion process. The difference of degree of polymerization preferably
is less than ± 300.
Plasticizer
In cling
film usually manufacturers are using Dioctyl Adipate or DOA as plasticizers.
DOA is an ester of n-octanoland Adipic acid. Its chemical
formula is C22H42O4.
DOA features
flexibility at low temperatures, good electrical properties, good resistance to
weathering, and good stability to heat.
DOA is
used to produce clear films for food packaging applications. In addition,
it is compatible with nitrocellulose, ethyl cellulose, most synthetic rubbers,
and high-butyryl cellulose acetate butyrates.
Short
chain esters are used as high-boiling, biodegradable, low toxicity
solvents and antiperspirants. Long chain esters of
Adipic acid are used as lubricants
for the functions of stability, superior lubricity, corrosion
protection, biodegradability, and excellent performance at both high and
low temperatures.
Adipic acid
esters (C5 - C10) are used as low-temperature-resistant and low
viscosity plasticizers for polymers and cellulose esters.
If the plasticizers is
less than 20 % by weight, the elongation of film, fluid ability, and
heat stability for long run production will be affected defectively,
if the plasticizers is more than 30%, there is weak film
stiffness.
Epoxidized
Soybean oil
Epoxy resins are used as adhesives and structural
materials. A polymer containing unreacted epoxide units is called a polyepoxide or an epoxy.
Polymerization of an epoxide gives a polyether, for example
ethylene oxide polymerizes to give polyethylene
glycol, also known as polyethylene oxide. It will
improve heat stability during extrusion process.
If the epoxidized soybean oil is less than 10% by weight, the heat
stability is not excellent during extrusion process. If the Epoxidized soybean
oil is more than 20% by weight, it might affect the color of film.
Stabilizer
The job of
the stabilizer is to delay heat degradation so that the compound can be formed
into a product before it degrades.
Normally Ca- Zn type stabilizers are using for the food packaging
grade PVC cling film. The performance of more recent developments in calcium/zinc
stabilizers also makes them potential technical alternatives to most other
stabilizing systems, including lead and barium/zinc.
It normally adds in less than 1% by weight, preferably 0.8 –1.5%.
If the stabilizer is less than 0.5, it is not stable thermally during extrusion
process. If the stabilizer is more than 1 %, it will be bleed out on the film
surface by long-term storage. Also it will take more time to gel with PVC resin
and is difficult to mix homogeneously the resins during extrusion because of the
slip property of stabilizer.
Anti-fog
agent
Anti-fog agents, also known as anti-fogging agents and treatments, prevent the condensation of water on a
surface in the form of small
droplets which resemble fog.
Anti-fog treatments are often
used for transparent glass or plastic surfaces
in optics, such as the lenses and mirrors found in glasses, goggles, camera objectives, and binoculars.
Anti-fog treatments work by minimizing surface tension, resulting
in a non-scattering film of water instead of single droplets, an effect called surfactant film or by
creating a hydrophilic surface.
Lubricant
Lubricants are divided into two areas, internal and external lubricants.
The transition between external and internal lubricating effect is fluid,
however, – internal lubricants often also have a certain external lubrication
effect and vice versa. Lubricants having both effects are therefore called
“combined lubricants”.
Internal lubricants reduce the frictional forces
occurring between the PVC molecule chains, thus reducing melt viscosity. They
are polar and thus are highly compatible with PVC. They help achieve excellent
transparency even at high dosages and do not tend to exudates, which helps
optimizing welding, gluing, and printing properties of the final product.
External lubricants reduce the adhesion between
PVC and metal surfaces. They are mostly non-polar such as paraffin and
polyethylene waxes. The external lubrication effect is largely determined by
the length of the hydrocarbon chain, its branching and its functional group. At
high dosages they can lead to cloudiness and exudation.
MIXING AND COOLING
The mixer and coolers are the heart of the compounding system.
This includes the control panels for mixer/coolers, silos, and conveying
systems. These are computer controlled in more modern systems. The following
two types of mixers are used in flexible compounding.
1. Low Intensive Mixer (LIM)
LIMS are ribbon blender-type mixers that are jacketed for heating
and cooling. They have closed barrels and spiral blades which normally run
about 25 to 75 RPM. The blades are designed to move the material to the center
of the barrel providing good mixing. The tip speed is normally about 6 meters per second.
Frictional heating is very minimum, so this causes large heat
gradients within the ribbon blender.
Depending on the hardness of the flexible PVC compound being made,
mixing times can be from one to six hours. Batch size may be up to 5000 pounds.
Heating and cooling must be provided to this type of
mixer.
2. High Intensive Mixer (HIM)
HIMs are like kitchen blenders with very high RPM's (500 to 1500).
They achieve most of their heating from frictional heat. Depending on the
manufacturer, one can have a variety of blades and blade designs. There are 2
to 4 blades in a normal mixer. Three blades are typical for flexible mixing and
four blades for rigid mixing. The blades are designed to give homogenization to
the resin and other ingredients. The tip speed is normally around 30-40 meters
per second. Mixer size ranges from 10 to 1000 pounds. A typical cycle time will
be from 4 to 10 batches per hour.
It is very important to have a good, deep vortex during most of
the mixing cycle. The material must always be turning over and achieve a dry
state before dropping to the cooler. If one looks down into the mixer, the
material goes through several states. As resin is added, there is an uneven
flow.
When adding the plasticizer, this state continues. Around 160°F,
the resin starts to absorb the plasticizer and the vortex decreases. The
material must continue to turn over to get a good mix. At powder peak, all
plasticizer is absorbed and the flow in the mixer is nice and smooth. Mixer
amperage should be observed and/or recorded. HIMs give the most uniform
temperature for the entire batch while achieving uniform temperature the
quickest.
After mixing is complete,
the powder material must be cooled by dropping it into a cooler. Cooling is
usually done in a low intensive mixer. Water is pumped through the water jacket
to speed the cooling process. Coolers may be ribbon blenders, round-like pots,
or barrel type, all of which are closed-type bowls. Some have blades or plows.
The blades have an RPM of 50 to 100 with a tip speed of 6 meters per second.
After cooling, the material should be screened for mixer build-up and foreign
material. Screen size is normally from 10 to 30 mesh depending on the end
product to be extruded.
High Intensive mixer for PVC compounding
Courtesy of Dermak Makina
Extrusion
PVC is non-crystallizable polymer commercially even though there
was reported 5-10% of crystal existed in certain process. The polymer is heated
to a temperature at which it becomes a viscous liquid, and then it is cooled
homogeneously to as near as the glass
transition temperature, is practical for stretching. The material is
stretched mono axially or biaxially either at a constant temperature or under a
falling temperature gradient, and is subsequently quenched to below glass
transition temperature.
Glass transition temperature
of PVC will be vary by adding quantity of plasticizer. PVC glass transition
temperature reported 105 °C
without the plasticizer and 60 °C
by adding 15% plasticizer. The glass transition temperature decrease in
proportionally to the number of polymer molecule of plasticizer as additives. PVC processing
temperature range is from 135ºC to 200ºC
PVC Cling film
Courtesy of Flextrusion Sdn. Bhd.
The
specific environment condition required for PVC cling film winding
Extrusion environment required at absolute humidity 0.014 – 0.024
kg/m³ from die to film passing distance within 1m on the casting roll. If the
absolute humidity is less than 0.014 kg/m³, the quantity of moisture on surface
of film react with antifogging agent is not enough, unwinding distance of film
will be less than 1000mm and slip property is not good enough for packaging
process. If the absolute humidity is more than 0.024 kg/m³, the quantity of
moisture on surface of film react with antifogging agent is excess and
unwinding distance will be more than 1500mm and cling property will be
decreased besides the slip property of film.
The film temperature before winding recommends 30ºC -35ºC for
preventing entrapping air inside film roll. If film temperature is less than 30ºC,
the film will harden and then the air will be entrapped inside of roll,
unwinding distance will increase; cling force of film will decrease. If film
temperature is more than 45 ºC, there is no film stiffness and then easy to
block each film layer, unwinding distance will be decrease.
The film will shrink by the residual stress of winded film during
packing process. In order to release the stress, generally film is heat up
under tension or without tension during film casting process. But it is
difficult to eliminate wrinkles of PVC stretch film.
To release winding stress on the film properties, carry out aging
at least 24 hours to 72 hours at 35-50°C
after winding.
Degradation
When PVC is processed at high temperatures, it is degraded by dehydrochl
ordination, chain scission, and cross-linking of macromolecules.
Free hydrogen chloride (HCl) evolves and discoloration of the
resin occurs along with important changes in physical and chemical properties.
The evolution of HCl takes place by elimination from the polymer
backbone; discoloration results from the formation of conjugated polyene
sequences of 5 to 30 double bonds (primary reactions). Subsequent reactions of
highly reactive conjugated polyenes crosslink or cleave the polymer chain, and
form benzene and condensed and/or alkylated benzenes in trace amounts depending
on temperature and available oxygen (secondary reactions).
The features of PVC cling film for food
packaging
• Excellent Oxygen permeability
• Selective gas permeability
• Good anti-fogging effect
• Optimal stretching property
• Sufficient elastic property
• Excellent cold temperature application
• Excellent cling property
• Safe products for food packaging (all components are for food
grade)
• Transparent (See-through effect)
• Thin gauge
• Optimal machine-ability for automatic packing.
Conclusion
PVC cling wrap, commonly called cling film, is thin flexible PVC
film often 8-10 microns with tear resistant properties.
Uncovered foods are at the risk of contamination from
microorganisms and PVC cling film is a major contributor to food safety during
transportation, distribution and storage of food products. Despite its
versatility and obvious advantages, there has been substantial speculation
about the safety of cling film.
Likely the key message is that all plasticizers are not the same
and regulations statues of various plasticizers vary considerably! PVC cling
film which compounded with an Adipate plasticizer has been safely used in food
packaging for decades.
Food Grade PVC Cling film
References
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Loftus, N.J. Laird, W.J.D.,
Steel, G.T., Wilks, M.F. and Woollen, B.H. (1994) Metabolism and
pharmacokinetics of deuterium labelled di-2-(ethylhexyl) adipate (DEHA) in
humans. Fd Chem. Toxic., 31 (9),
609-614.
2.
Loftus, N.J., Woollen, B.H.,
Steel, G.T., Wilks, M.F. and Castle, L. (1994). An assessment of the dietary
uptake of di-2-(ethylhexyl) adipate (DEHA) in a limited population study. Fd
Chem Toxic., 32 (1),1-5.
3.
www.plastics.com/content/articles/5/1/PVC--Polyvinylchloride-What-is-PVC/Page1.html/print/5
4.
http://www.baerlocher.com/products/lubricants/
5.
Oxy Vinyls, LP,
Mixing Flexible PVC Compound, Technical Report #52
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A. Baruya , D. L. Gerrard , W. F. Maddams Resonance
Raman spectrum of degraded poly(vinyl chloride). 4. Determination of conjugated
polyene sequence lengths
7.
Phthalates and their
Alternatives: Health and Environmental Concerns, by The Lowell Center for
Sustainable Production at the University of Massachusetts
8.
DEHA Fact Sheet DEHA Fact Sheet
2012, South African Vinyls Association.
