PVC cling films processing

 

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

1.       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

6.       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.