Characteristics of commonly used base materials for composite packaging bags (very practical)

I. In order to meet the required packaging requirements, the composite packaging film should have the following properties

A. Mechanical properties: Tensile strength, rigidity, wear resistance, sealing performance, elongation, friction, etc.

B. Physicochemical properties: Moisture and oxygen barrier properties, aroma retention, oil resistance, chemical resistance, light shielding, etc.

C. Durability: Performance at low and high temperatures, stability under high humidity conditions, degradation ability, etc.

D. Processability: Suitable for printing, easy for automated packaging, anti-static, heat shrinkage ability, etc.

E. Commodity display: Transparency, whiteness, gloss, easy opening, easy waste disposal, etc.

II. The packaging requirements for different products are also different

A. For general industrial product packaging, the main considerations are its protection, packaging operability, sales merchantability, and transportability.

B. The packaging of daily chemical products is mostly directly facing consumers, and its protection, aesthetics, convenience, and shelf display should be considered.

C. For the packaging of mechanical parts, its protection, functionality, rust prevention, and transportability should be considered.

D. For electronic product packaging, its protection, shock resistance, anti-static properties, shielding, and barrier properties should be considered.

E. For food and drug packaging, its protection, hygiene and safety, barrier properties, and packaging methods should be considered.

III. The functions of each layer in the composite film are also different. Taking a typical three-layer composite flexible packaging material as an example

Table 1 The function and requirements of the outer substrate (surface layer)

The most important requirement for the outer layer material is high strength, good abrasion resistance, good gloss, heat resistance, good barrier properties, and good printability.

Table 2 The requirements and functions of the middle layer material

The most important function of the middle layer material is to require a certain strength, excellent barrier properties, and double-sided composite properties.

Table 3 The requirements and functions of the inner sealing layer

In view of the above requirements, we must understand the characteristics of various base materials in the production process in order to produce practical films.

IV. Aluminum foil, nylon, polyethylene film, and paper are commonly used in production

1. Aluminum foil

Made of high-purity aluminum through multiple rolling processes, it has good barrier properties against light, ultraviolet rays, water, oxygen, and water vapor, good stiffness, and metallic luster. However, it has no mechanical strength, cannot be heat-sealed, and must be used in combination with other materials. It has a high specific gravity and high cost. The detection of aluminum foil includes the number of pinholes and the degree of oil stains.

2. Polyethylene film

Commonly used PE is divided into the following types:

A. LDPE: Density 0.91～0.925. Many branches, low crystallinity, very soft, very tough, low melting temperature, easy to process and heat seal, good transparency (compared to similar products). General extensibility, poor mechanical strength, used for inner layer heat sealing, tear resistance, impact resistance.

B. MDPE: Density 0.926～0.94. Large heat sealing range, certain rigidity.

C. HDPE: Density 0.941～0.965. The color is milky white, mainly used for shopping bags in supermarkets, and also used in film blowing. Compared with LDPE, HDPE has better water resistance, better strength, and is harder, but its puncture resistance is poor, its melting temperature is high, and it can withstand boiling.

D. LLDPE: The branches are orderly, and the mechanical strength, elongation, and puncture resistance are all good. Under normal circumstances, it has good barrier properties against water and oil, but its transparency and processability are not as good as LDPE. 30%～50% LLDPE is used in film blowing (low cost). Usually C4, C6, C8, among which C8 has the best strength. C4 is widely used as a substitute for LDPE.

E. MLLDPE - Metallocene PE: PE made using metallocene as a catalyst, with a narrow molecular weight distribution, good puncture resistance, higher strength, good anti-pollution properties, can be heat-sealed even with impurities, and has good heat sealing strength, low sealing temperature, long melting range, and heat resistance (can be boiled). Good heat adhesion - that is, even after sealing, the sealed edge still has good heat sealing strength before cooling. However, the better MLLDPE is more difficult to process because of its high viscosity. It is generally C6 because C6 can achieve the strength of ordinary C8. It is added as an auxiliary material during film blowing, and 100% MLLDPE film blowing is not feasible.

3. Polypropylene film

Polypropylene PP has a low density, obvious economic advantages, a melting temperature higher than HDPE, is non-toxic, odorless, tasteless, can be used as food packaging like PE, has good acid, alkali, and salt resistance, stable chemical properties, good waterproof and moisture-proof properties, excellent molding and heat sealing properties, better transparency, oxygen barrier properties, oil resistance, and heat resistance than PE (can withstand 120℃ or even 140℃), but its cold resistance is not as good as PE. It also has the advantage of easy availability.

Copolymer PP: Good transparency, high strength, good toughness, good heat sealing properties, wide melting range.

Homopolymer PP: Stiff, shaped, but not cold-resistant, it is destroyed at 0℃.

BOPP - Biaxially oriented polypropylene, thin, light, and low cost. Good mechanical strength, improved cold resistance (can be frozen), good transparency (can be printed inside), but poor heat sealing properties, generally used as the outer layer, easy to generate static electricity (dust absorption). Poor oxygen barrier properties, lower strength than PET, not suitable for large packaging bags.

CPP - Cast polypropylene, high heat seal strength, suitable for inner layer, tear resistant, high transparency, poor oxygen barrier, less tough than PE, copolymer CPP is generally used as a heat seal layer.

PET - Polyester (Polyethylene terephthalate) has very high mechanical strength (suitable for heavy packaging) and rigidity, high polarity, and high surface tension. It is high temperature resistant, has excellent transparency and gloss, good barrier properties to oxygen and water, good aroma retention, and good printability. It is often used for outer layers. Suitable for making retort pouches that require high barrier properties or need to be sterilized at high temperatures.

4. Polyester Film

Polyester film is a film material made from polyethylene terephthalate as raw material, formed into thick sheets by extrusion, and then biaxially stretched. At the same time, it is a high-molecular plastic film, which is increasingly favored by consumers due to its excellent comprehensive properties. Due to China's current production capacity and technical level still not meeting market demand, some still need to rely on imports. It is a colorless, transparent, glossy film with excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, good chemical resistance, oil resistance, airtightness and aroma retention. It is one of the commonly used barrier composite film substrates. However, polyester film is relatively expensive and is often used as the outer layer material for retort packaging, with good printability. Furthermore, due to the complexity and difficulty in judging polyester film, it brings many difficulties to customs supervision.

According to the raw materials and stretching processes used in the production of polyester film, it can be divided into the following two types:

A. Biaxially Oriented Polyester Film (BOPET), made from bright material (also known as high-bright material, which means adding titanium dioxide to polyester chips, and then drying, melting, extruding, casting, and longitudinally and transversely stretching to form a high-grade film with a wide range of uses. BOPET film has characteristics such as high strength, good rigidity, transparency, and high gloss; it is odorless, tasteless, colorless, non-toxic, and has outstanding toughness; its tensile strength is 3 times that of PC film and nylon film, and its impact strength is 3 to 5 times that of BOPP film, with excellent wear resistance, folding resistance, pinhole resistance, and tear resistance; its heat shrinkage is extremely small, only shrinking 1.25% after 15 minutes; it has good antistatic properties, is easy to vacuum metallize, and can be coated with PVDC to improve its heat sealability, barrier properties, and printing adhesion; BOPET also has good heat resistance, excellent retort resistance, low-temperature freezing resistance, good oil resistance, and chemical resistance. Most chemicals cannot dissolve BOPET film, except for nitrobenzene, chloroform, and benzyl alcohol. However, BOPET can be corroded by strong alkalis, so care should be taken during use. BOPET film has low water absorption and good water resistance, making it suitable for packaging foods with high water content.

B. Cast Polyester Film (CPET), made from semi-dull material (polyester chips without added titanium dioxide), after drying, melting, extruding, casting, and longitudinal stretching. It has the lowest grade and price among polyester films and is mainly used for pharmaceutical tablet packaging. Due to its lower usage, manufacturers rarely produce it on a large scale, accounting for about 5% of the polyester film market. Chinese enterprises also import less of it, with a standard thickness of 150μm.

5. Paper

The characteristics of paper include paper surface, color, gloss, brightness, opacity, basis weight, size, and fiber interweaving:

A. Paper Surface

Generally, paper produced has front and back surfaces with differing properties, including surface smoothness and gloss. During the papermaking process, pulp comes into contact with a wire mesh and felt to form the paper; the metal contact surface is called the Wire Side, and the felt contact surface is called the Felt Side. The felt side is typically whiter and performs better in terms of print ink gloss. When testing paper, both the wire side and felt side must be specified. Surface property tests for paper include smoothness and surface strength.

B. Paper Color

The perceived color of paper is the result of absorbing light at certain wavelengths and reflecting the remaining wavelengths. Therefore, it has a direct impact on the performance of ink on the paper surface.

C. Whiteness, Brightness

Paper mills typically measure the amount of light reflected from the surface of white paper and express the reflection rate as a percentage. The industry uses this rate to grade paper, with higher rates indicating higher quality products.

D. Opacity

This characteristic refers to the phenomenon where printed text and images on one side of the paper do not show through to the other side, even if that side also has text or images printed on it. Good paper has high opacity, allowing readers to focus on the content of the current page in books without being disturbed by faintly visible print from both sides, which can cause eye fatigue. Factors affecting opacity include: a. paper thickness; b. whether it is sized; c. paper surface smoothness and roughness; d. the composition of plant fibers it contains. The testing method involves measuring how much light can penetrate the paper itself, with the unit being a percentage.

E. Grammage, Basis Weight

The basis weight of paper refers to the weight of five hundred sheets of basic size paper. If five hundred sheets of 25x38in. paper weigh 80 pounds, its basis weight is 80 pounds. In the printing industry or among the general public, when considering paper thickness or stiffness and toughness, it is often related to basis weight. Using basis weight to express paper with different degrees of characteristics has been widely adopted.

F. Grain Direction (Fiber Alignment and Interweaving)

During the papermaking process, fibers align along the machine's running direction, and the fiber alignment and interweaving directly affect the paper's flatness and smoothness. Poorly interwoven paper can lead to mottling during printing. To check the fiber interweaving, paper samples are usually held up to light, and the uniformity of the paper's fibers is judged by how light passes through. Currently, scanning electron microscopes (SEM), energy dispersive spectrometers (EDS), and wavelength dispersive spectrometers (WDS) can also be used to test and analyze paper surface structure and observe fiber interweaving.

G. Tensile Strength

The tensile strength of paper indicates the pulling force that fibers can withstand after being combined and interwoven, and then processed through different procedures to become paper, in order to test the quality of the interweaving. This characteristic is particularly important for roll paper used in rotary presses. The test method involves taking a sample with a cutting width of 25 mm and a length of 150-200 mm, applying tensile force to both ends of the paper's length, and measuring the force at which the sample breaks. The unit is Newton Force/mm徑

H. Tearing Strength

The tearing strength of paper refers to applying the same force in different directions to the edge of the paper to tear it. The tearing strength of longitudinal and transverse fibers of paper are not equal; therefore, during testing, the tearing strength of both longitudinal and transverse fibers must be measured to fully analyze the tearing characteristics of the paper. The test method involves cutting a 60 mm x 60 mm paper sample, then clamping the paper onto the instrument's sample plate, pre-cutting a small section, and then activating the pendulum to tear the paper. The unit is Newton-Force徑

6 Nylon

Nylon refers to plastics composed of polyamide resins. Nylon is a crystalline plastic with high mechanical strength, good toughness, high tensile and compressive strength. Products are lightweight, easy to dye, and easy to mold. Due to its low melt viscosity, it flows quickly, is easy to fill molds, and has a high solidification point after filling, allowing for quick setting. Therefore, the molding cycle is short, and production efficiency is high. It is prone to water absorption, and generally requires moisture conditioning and annealing treatment after molding to ensure dimensional and mechanical strength stability. PA burns slowly; if the heat source is removed and the ambient temperature is not too high, it will self-extinguish. The flame is yellow above and blue below, and the surface near the flame will melt and drip. It will also bubble, and after extinguishing, it has an odor similar to burnt wool or fingernails徑

PA - Nylon or NY (Japanese term), very high strength and very soft. It has good puncture resistance, excellent heat and cold resistance, good transparency, and very good gas barrier properties. However, nylon is highly water-absorbent and prone to deformation, so it is used for bags requiring high barrier performance, puncture resistance, and high temperature resistance徑

With the rapid development of technology, the development of environmentally friendly soft packaging materials has become a trend in the packaging industry. In addition, soft packaging materials are gradually developing towards thinning, reducing costs and resources, which is a key issue for new-century packaging. The study of the mechanical properties of composite soft packaging will also become an important indicator for the development of the composite soft packaging industry徑

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