Films made from Plastic Materials – Properties and Uses
1. Low-density Polyethylene 0.915 – 0.93 g. per cubic centimeter:
Low-density polyethylene is primarily used for retail packaging and for the agricultural industry for weights up to 50 kg. The material is semi-transparent with a reasonable tensile strength. It displays good resistance to wear and tear, immunity to fungi and bacteria, low levels of water vapor transmission and is highly durable in water. Printing is possible with proper treatment. The film is also durable at low temperatures reaching down to -60° C. It has high levels of oxygen transmission, as well as for other gases, and excellent welding properties.
- Linear Low-density Polyethylene: LLDPE
LLDPE is the development of a line of polymers with mechanical properties that highly surpass those of LDPE. In order to achieve balanced results and to facilitate the production process, it is common to mix this substance with varying percentages of LDPE.
3. High-density Polyethylene: HDPE – above 0,96 g. per cubic centimeter, and Medium-density Polyethylene: MDPE – 0.935-0.945 g. per cubic centimeter:
HDPE is stronger and more rigid than LDPE, except for its low resistance to tearing, particularly lengthwise. It has poor transparency. The film can be produced at very low levels of thickness (6 microns). All of its mechanical and chemical properties highly surpass those of LDPE and the film can be used at high temperatures, up to 100° C. MDPE is in the middle.
Polypropylene is the technological development of polymers whose basic properties are burned (polymerized) by means of innovative catalysts. Polypropylene has good mechanical properties and excellent optic ones. The material is durable at temperatures up to 135° C, but it loses its flexibility at temperatures below zero. Therefore, it is not suited for cold storage. Its resistance to the transmission of gases has improved but is still relatively low. The film is impermeable to water and fairly sealed against water vapors. Printing requires prior treatment of the film.
The properties of polypropylene depend upon the production method. Films produced by inflation display average properties, with good welding and are intended for simple packaging.
PP – Regular polypropylene is used for simple packaging, like flower bouquets.
OPP – This material has undergone a lengthwise stretching procedure to reinforce its overall physical properties.
BOPP – This excellent material has undergone a stretching procedure in both directions, giving it high mechanical properties and excellent optic ones. It is similar to cellophane but surpasses it in its high durability to water and sealing against water vapors. It cannot be heat-sealed and must be coated with another substance or undergo an extrusion procedure with another polymer in order to use it for welding. There are polymer lines that facilitate problems such as the film’s processing abilities, heat-sealing and better durability at below zero temperatures.
5. Ionomer –Surlyn:
This substance is generally used as a component of double-layered or multi-layered film sheets, rather than as an independent material. It has excellent strength properties, high transparency, is resistant to grease and fuel and has a high welding ability. The secret of the substance is the addition of sodium or zinc atoms (ions), each with its own advantages and disadvantages that must be taken into consideration prior to the production process.
6. P.E.V.A. Polyethylene Vinyl Acetate:
P.E.V.A. is a copolymer of polyethylene with properties similar to rubber, depending on the percentage of vinyl acetate in the polymer. Although the affect is limited at levels of up to 5%, welding properties are improved.
8-12%: There are notable improvements in properties with levels of 8-12%, including flexibility, resistance to tearing, welding and durability at low temperatures.
18-20%: With 18-20% of vinyl acetate, the properties of the material become more similar to those of rubber. It is used as a component in multi-layered films but not on its own.
28% and more: With more than 28%, the material is used as an adhesive or as a connecting layer in multi-layered films.
P.E.V.A. with levels that exceed 10% of vinyl acetate is not suited for coming into contact with food products and the stickiness of the film increases.
7. Other Copolymers of Polyethylene – ESS and Others:
These materials are generally used to achieve specific properties as components of multi-layered films.
8. Very Low-density Polyethylene: VLDPE:
This material has properties that are similar to E.V.A. It is not used an independent substance. It has a density of 0.9 g. per cubic centimeter.
This material is not widely used for films, in spite of the fact that it has a number of outstanding advantages. It has excellent mechanical strength that enables a significant reduction in the thickness of bags for heavy loads. The material also has excellent durability at high temperatures. Its disadvantages include welding difficulties and certain properties that make production problematic.
Efforts are being made to include this polymer as a raw material for films. The material has high transparency and high tensile strength. It has average sealing against the transmission of gases and water vapors. The most outstanding advantage of polycarbonate is its durability at high temperatures. The tensile strength barely changes as temperatures rise.
- Multi-layered Film:
These film sheets are manufactured by lamination or by co-extrusion. When laminated, there is no limit to the number of layers. Layers comprised of different materials can be joined together, including paper or aluminum foil. The films produced by co-extrusion are very popular. These films are used for products with an extensive shelf-life. The demands for resistance to the transfer of gases (oxygen) and water vapors, together with the demands for welding abilities and heat-shaping prevent the use of single-layer film sheets. Therefore, all of the films are comprised of layers with low resistance to transmission
The materials in use include: nylon, polyester, poly vinyl D chloride (Saran), and polyethylene vinyl alcohol (PVAL), used as an important component for packaging preserved goods as it has excellent sealing qualities.
Most of the materials do not stick to each other and an adhesive layer is usually required. This fact adds to the number of layers and the stages of production.
- Poly Vinyl Chloride – P.V.C.:
P.V.C. is rigid and the film has excellent optic properties, excellent mechanical properties and is strong when stretched. Its resistance to the transmission of gases is low but it is very durable against grease. It is also durable against acids and bases, but regarding organic solvents, it has a certain amount of solubility.
- Soft P.V.C.:
All of its properties are dependent upon the amount and the quality of the softening additives (Plasticizers). An increase in the percentage of additives results in the increased flexibility of the material. The materials used for the production of packaging for food products must be checked from a physiological aspect and components without an aroma should be used. Welding is done with a high frequency device, by ultra-wave or ultrasound. The use of soft P.V.C. is on the decline due to environmental issues and food organizations as the chloride component is considered to be a hazard.
- Poly Vinyl D Chloride – P.V.D.C.:
This is an excellent material and it excels in all areas: excellent sealing against the transmission of gases and water vapors, mechanical strength, rigidity, resistance to tearing and welding ability. It is an independent substance used for packaging beef and chicken, as a coating for sealing out gases and for vacuum packaging.
- Poly Vinyl Alcohol - P.V.A.:
This is a special material that dissolves in water. The dissolving temperature can be adjusted to allow for the production of a material that dissolves only at warm temperatures or one that dissolves in cold water. It is used for packaging laundry additives and hospital garbage bags.
Polystyrene film has excellent transparency but its other properties – mechanical strength, resistance to high and low temperatures, sealing against water – are only average. It has good chemical properties regarding acids and bases and is used as a “window” on cardboard boxes. At larger thicknesses, it is used for bubble packaging (Blister).
- Cellulose Acetate:
Cellulose acetate has excellent transparency and gloss. No preliminary treatment is required before printing. It is a rigid material with good mechanical properties. It is sensitive to acids and bases at high concentrations and allows for a high transmission of gases, as well as water vapors. The material includes softeners which must be check to verify suitability for use with food products. Cellulose acetate cannot be welded by heat and adhesives must be used. It is especially used as for laminating books, magazines and posters. Because its transmits oxygen, it is used for packaging fresh food, as a “window” on cardboard boxes and as an adhesive.
- Nylon (Polyamide) :
Nylon represents the group of materials whose properties are influenced by their chemical composition. There are a number of materials, but the primarily ones are nylon 6, nylon 66, nylon 6/10 and nylon 12. Their common properties include high tensile strength and resistance to high and low temperatures. Nylon absorbs water and all of its properties are affected by the water content. From a chemical standpoint, nylon is durable against weak acids and bases, but dissolves in strong, mineral acids. It is resistant to greases and oils, as well as to concentrated bases. Nylon has good transparency and gloss, and no preliminary treatment is required prior to printing. It is widely used as a material for vacuum packaging, cooking and baking bags and is often used together with polyethylene and/or an ionomer.
Thermoplastic polyester is actually polyethylene terephthalate. It has excellent mechanical strength and optic properties. The material also has good sealing against water vapors and is similar to nylon regarding food packaging. It is resistant to greases and oils and has outstanding durability to both low and high temperatures – ranging from -70° C up to 200° C. It is usually used together with polyethylene for MYLAR vacuum packaging. It can be stretched both lengthwise and widthwise and is used for packaging food as well as for cooking bags. The material is good for chrome packaging (Skin), bubble (Blister) and for food containers for both regular and micro-wave ovens.