Biaxially oriented polypropylene (BOPP) film is a product made by stretching a thick film or cast sheet longitudinally and transversely at a temperature below the melting point of the film material and above the glass transition temperature (Tg), followed by appropriate cooling or heat setting treatment or special processing (such as corona treatment, coating, etc.) under tension. BOPP film is made using this method. BOPP film is an important product in the packaging industry, possessing advantages such as light weight, transparency, non-toxicity, moisture resistance, low air permeability, and high mechanical strength. It is widely used in the packaging of food, pharmaceuticals, daily light industrial products, cigarettes, and other products, and is extensively used as a substrate for composite films, earning it the nickname "Queen of Packaging." The biaxial stretching process is a highly technical plastic molding and processing method. Besides requiring high-performance processing equipment, it is crucial that production personnel have a thorough understanding of the properties of PP and the impact of processing conditions on product performance, enabling them to promptly resolve problems encountered in production.
1. Main production methods and processes of BOPP film
Currently, the main production methods for BOPP film are the tubular film method and the flat film method. The tubular film method is a biaxial one-step stretching method; the flat film method is further divided into biaxial one-step stretching and biaxial two-step stretching methods. The tubular film method has advantages such as simple equipment, low investment, small footprint, no edge material loss, and simple operation. However, due to disadvantages such as low production efficiency and large product thickness tolerance, it has seen almost no development since the 1980s and is currently only used for producing special varieties such as BOPP heat shrink film. The biaxial one-step stretching method produces products with balanced longitudinal and transverse properties and almost no film breakage during the stretching process. However, due to problems such as complex equipment, difficult manufacturing, high price, large edge material loss, difficulty in high-speed production, and limited product thickness, it has not yet been widely adopted. In contrast, the biaxial two-step stretching method has mature equipment, high production efficiency, and is suitable for mass production, and is used by the vast majority of enterprises.
2. Factors affecting the physical and mechanical properties of BOPP film
2.1 Raw material properties
The main component of BOPP film used in industrial production is polypropylene (PP). PP is a typical stereoregular polymer, and based on the distribution of hydrocarbon groups on both sides of the molecular plane, it can be divided into isotactic PP, syndiotactic PP, and atactic PP. Isotactic and syndiotactic PP have different crystal structures. Isotactic PP crystallizes through a homogeneous nucleation three-dimensional growth process, while syndiotactic PP mainly crystallizes through a homogeneous nucleation two-dimensional process, forming small crystallites with irregular dimensions. Furthermore, due to the lower regularity of the syndiotactic PP molecular structure, it exhibits a lower crystallization rate and crystallinity. Studies have shown that a higher degree of isotacticity leads to a faster crystallization rate, resulting in a significant increase in the yield strength and surface hardness of the film product. Atactic PP acts as an internal lubricant within the polymer and facilitates polymer orientation, thus contributing to improved optical properties of the film.
Currently, there are many varieties of BOPP film with significant differences in performance. This is mainly due to the different raw materials and production processes used. Practice has shown that only PP with a mass fraction of 95%–97% isotactic PP and 3%–5% atactic PP is suitable for producing BOPP film, and generally, PP with a melt flow rate of 2–4 g/10 min is selected. Furthermore, by co-extruding one or more layers of low-melting-point copolymers onto the surface of the PP film, the application range of BOPP film in the packaging industry can be expanded.
2.2 Longitudinal and transverse tensile ratio
The draw ratio is a crucial process parameter, significantly impacting the physical and mechanical properties of BOPP film, both longitudinally and transversely. At a given temperature, a higher draw ratio results in a greater degree of orientation of the PP molecular chains. This leads to improved mechanical strength, increased modulus, decreased elongation at break, increased impact strength and folding endurance, and better air permeability and gloss. Orientation in BOPP film production primarily occurs during longitudinal and transverse stretching. After longitudinal stretching, the polymer chains exhibit uniaxial longitudinal orientation, greatly enhancing the longitudinal mechanical properties of the cast film, while transverse properties deteriorate. Further transverse stretching results in a biaxial orientation of the polymer chains.
As the orientation of molecular chains increases, the number of straight segments in the film increases, the number of folded segments decreases accordingly, and the connecting segments between the wafers gradually increase. This leads to a corresponding increase in the material's density and strength, while reducing the elongation at break. Therefore, biaxial stretching can comprehensively improve the performance of PP films.
The difference between the longitudinal and transverse stretch ratios ultimately determines the differences in the physical and mechanical properties of BOPP films in the longitudinal and transverse directions. If the longitudinal and transverse stretch ratios are not significantly different, there is no significant difference in molecular orientation in the two directions, and the BOPP film exhibits isotropy. To produce BOPP films with higher longitudinal properties than transverse properties, the selection of the longitudinal and transverse stretch ratios is crucial. Generally, the longitudinal stretch ratio (4.5–5.5) is smaller than the transverse stretch ratio (7.5–9.0). Transverse stretching of BOPP films is an important and complex process, conducted entirely in a continuous thermal environment. The transverse stretching process has multiple stretching initiation points, mainly caused by certain weak points in the transverse direction, a high transverse stretching rate, and factors such as impurities, bubbles, and appearance defects in the film. Multiple stretching initiation points easily lead to uneven product thickness. Simultaneously, during transverse stretching, there are issues of "step stretching" and "inherent stretch ratio." That is, during the transverse stretching process, there are several "step" points in the transverse direction where the film is suddenly stretched to its maximum stretch ratio. As the stretching process continues, the "steps" gradually expand to both sides until the entire film is stretched. In BOPP film production, the degree of stretching must reach the "inherent stretch ratio," meaning the product of the longitudinal stretch ratio and the transverse stretch ratio must be around 40. If the longitudinal stretch ratio is insufficient, many "zebra stripes" or thick streaks will appear in the transverse direction of the stretched film; if the transverse stretch ratio is insufficient, thick streaks will appear on both edges.
2.3 Temperature distribution in different zones during temperature stretching is a key factor affecting the stretching orientation and crystallization of BOPP film. Temperature influences the orientation process through the effects of polymer viscosity and relaxation time. As temperature increases, polymer viscosity decreases. Under constant stress, both elastic deformation and viscous deformation increase. While the increase in elastic deformation is limited, the development of viscous deformation is rapid, which is beneficial for polymer orientation.
(1) When stretched above the viscosity flow temperature Tf or melting point (Tm), the polymer macromolecules have strong mobility. Under very small external stress, molecular chains will untangle, slip and orient. However, under high temperature, the rate of molecular disorientation will also accelerate, reducing the effective orientation degree.
(2) When the temperature gradually rises above Tg, the polymer becomes elastic, and the energy of thermal motion overcomes the restraint of some physical cross-linking points, causing the chain segments to move, but the entire molecular chain cannot move.
(3) When stretched below Tg, external force can only cause minor changes in molecular chain stretching, vibration, and bond angle. The stretching temperature of plastic films is generally between Tg and Tm (or Tf), and the specific temperature depends on the properties of the polymer.
Practice has shown that using relatively low preheating and stretching temperatures, or cooling immediately after stretching, are effective methods to improve the orientation degree and reduce the crystallinity of BOPP film. Excessively high preheating temperatures can cause PP to form spherulites, reducing film transparency; while excessively high stretching temperatures can easily cause PP segments to deorient, leading not only to the sticking of the heat-sealing surface material to the rollers but also significantly reducing the physical and mechanical properties of the BOPP film. The temperature distribution in the transverse stretching zone should be as uniform and stable as possible; otherwise, it will affect the uniformity of the transverse thickness of the BOPP film and the continuity of stretching.
PP is a crystalline polymer, and its maximum crystallization rate occurs at temperatures approximately 0.80 to 0.85 times the crystallization temperature (Tm). Higher temperatures (near Tm) or lower temperatures (near Tg) make crystallization more difficult. To prevent a sharp increase in PP crystallinity during preheating and stretching, it is best to avoid selecting a stretching temperature within the range of its maximum crystallization rate. Instead, choose a temperature where crystallization begins to melt and the molecular chains can move, i.e., stretching within a temperature range approximately 25°C below Tm.
3. Common problems and solutions in BOPP film production
The main objectives in BOPP film production are, firstly, to achieve continuous production at the highest possible speed; secondly, to improve BOPP film performance and ensure quality; and thirdly, to reduce energy consumption. However, in actual production, various problems arise with BOPP film due to multiple factors, making it difficult to achieve these production goals. This paper proposes solutions to common production problems, based on the 8.2m BOPP film production line introduced from the French company DMT.
3.1 Common Defects and Solutions for Castings
3.1.1 Horizontal stripes
(1) The main causes of large-spacing transverse stripes are unstable extrusion melt pressure, uneven speed or temperature of the quench roll, and excessive fluctuations in air knife volume. Among these, the first point is more common. There are many factors that cause unstable pressure. The most important one is that the production line speed is increased too quickly, causing the metering pump speed to increase rapidly. On the other hand, the main extruder screw speed increases relatively slowly, resulting in insufficient material discharge from the die head and unstable pressure. In such cases, it is best to appropriately extend the speed-up time. After the linear speed stabilizes, the transverse stripes will disappear naturally. Another common situation is raw material factors. When all process conditions are well controlled and there is no significant improvement after multiple adjustments, it is necessary to consider changing the raw material.
(2) Small-pitch horizontal stripes are not common in actual production. There are four reasons for their occurrence: unsuitable machine head angle, unsuitable air knife angle or air volume, airflow influence near the machine head, and unstable speed of the quenching roller. These four aspects can be addressed.
3.1.2 Longitudinal Stripes: During the casting process, continuous longitudinal stripes may sometimes be observed in localized or fixed locations on the extruded sheet. Using such sheets to produce BOPP film will result in uneven transverse film thickness; and obvious protrusions (ripples) or longitudinal stripes will appear on the film's appearance during winding and slitting. Common measures to eliminate longitudinal stripes include: ① Selecting a well-structured, high-quality die head, ensuring a smooth lip without any mechanical damage. ② Enhancing melt filtration. ③ Promptly removing debris from the lip and maintaining the die head. ④ Improving the uniformity of air knife blowing. ⑤ Properly controlling the temperature of each extrusion stage. ⑥ Adjusting the position of the die head relative to the quenching roller.
3.1.3 Warping on both sides
This phenomenon is mainly caused by poor sheet attachment and excessive temperature difference between the two sides during the casting process. Cast sheet warping will affect the film's smoothness. In the case of PP, uneven cooling and crystallization of the cast sheet directly affect the film's film-forming properties. The edges of the cast sheet usually warp towards the cooler side. Therefore, during production, the warping of the cast sheet can be used to determine the temperature difference between the quench roller surface and the cooling water in the water tank, and then solutions can be considered.
3.1.4 Bubbles appear
If the melt contains impurities, the raw material moisture content is too high, the extrusion temperature is too high, the material heating time is too long, or air or degradation products accumulate in the extruder or filter, bubbles may appear in the cast sheet. If bubbles appear during normal production, carefully observe their shape and color, analyze the cause, and resolve it. If the extruder is started with an empty screw or after replacing the melt filter and restarting, air or degradation products may remain in the extruder or filter, and bubbles will generally appear in the cast sheet. This problem can usually be solved by ensuring adequate material discharge.
3.1.5 Irregular edges
Uneven edges on the cast sheet may be caused by damage to the seals at both ends of the die lip, resulting in material leakage at the edges; it could also be due to a malfunction in the edge clamping system or unstable extrusion melt pressure. Once the cause is identified, appropriate solutions should be implemented promptly; otherwise, it can easily lead to lateral pull-out and disengagement.
3.1.6 Other defects
During the casting process, defects such as crystal points, coke, unmelted material, unsuitable crystallinity, poor gloss, and sharkskin-like appearance may occur in the cast sheets. These defects generally do not appear on production lines with mature processes and high technical levels.
3.2 Causes and solutions for membrane rupture during stretching
During the production process, the film may break at any time, from casting to winding. Generally, based on the location of the breakage, film breakage is categorized as pre-stretch breakage, post-stretch breakage, and lateral stretch breakage. 3.2.1 Pre-stretch breakage
When production conditions change significantly during casting or longitudinal stretching, the longitudinal thickness of the film varies greatly, or the casting has significant defects, the local tensile stress during stretching may exceed the material's allowable stress, leading to transverse film breakage. However, film breakage before transverse stretching is rare in normal production.
3.2.2 Transverse stretching to break the membrane
Transverse stretching is the most common type of film breakage during production. Films are most prone to rupture when stretched laterally at high speed. Transverse stretching is generally classified into three types: transverse film breakage, longitudinal film breakage, and snap-off film breakage.
(1) Transverse membrane rupture: There are many reasons for transverse membrane rupture, which can be specifically divided into:
① The raw materials contain impurities with significant differences in properties (low molecular weight substances, oil stains, etc.).
②The casting sheet has obvious horizontal stripes and bubbles.
③ Various subtle pre-stretching film-breaking factors amplify (longitudinal thickness fluctuations, etc.), leading to excessive strain in localized areas. ④ Excessive deviations in the crystallization and orientation of the cast film. ⑤ Damaged filters, resulting in high impurity content in the cast film. ⑥ Material leakage at the die head. ⑦ Roller surface damage. ⑧ Scratches on the film from waste materials and equipment. ⑨ Improper extrusion and longitudinal stretching temperature settings. ⑩ Various volatiles accumulating on the top of the drying oven and ductwork fall onto the film.
In addition, damaged chain clamps are also a significant cause.
(2) Longitudinal membrane rupture
If longitudinal membrane rupture occurs, it can be analyzed from the following aspects:
① The transverse thickness deviation of the film is too large.
② The longitudinal and transverse stretch ratios are too large.
③ The edge temperature is too high during longitudinal stretching.
④ The longitudinal stretching and setting temperature is too high, resulting in poor crystal orientation of the cast sheet.
⑤ The chain clamp temperature is too high.
⑥ Waste material inside the horizontal drying oven scratched the film.
(3) Decoction and membrane rupture
The process of detachment is mainly analyzed from three aspects: diaphragm, fixture, and process.
First, if the edges of the casting are poor or the thickness deviation is large, it can easily cause the casting to come loose. In this case, the casting process should be adjusted in time to eliminate the defects.
Secondly, if the chain clamp comes off during normal production and remains off after manual repositioning, the problem should be considered a potential equipment issue. This could be due to a damaged chain clamp preventing closure, waste film caught on the chain clamp, or a malfunctioning inlet guide. In such cases, the machine should be stopped immediately and a thorough inspection conducted.
In addition, preheating during transverse stretching of the film, excessively low stretching temperature, and unsuitable inlet tension can also cause the film to come off the clamp.
3.2.3 Film Breakage After Lateral Stretch (Traction/Rewinding Breakage) Film breakage after lateral stretching is mainly caused by equipment malfunction or improper operation. It can be summarized as follows: ① Excessive traction/rewinding tension. ② Insufficient distance between the corona electrode and the corona roller, causing abrasion to the film. ③ Sharp edge cutter, resulting in uneven film edges. ④ Delayed edge suction. ⑤ Film wrapping around the roller. ⑥ Inappropriate flying knife, preventing normal roll changing, etc.
3.2.4 Basic Principles for Preventing Membrane Rupture
(1) Select and use raw materials strictly in accordance with requirements and regulations.
(2) Regularly inspect the equipment to ensure it is in normal working condition.
(3) Develop a reasonable production process.
(4) Improve the technical skills and sense of responsibility of staff.
(5) Find out the cause of membrane rupture in time and develop a reasonable solution.
4. Conclusion BOPP film production technology has only been introduced to my country for a few decades. However, due to the active introduction of advanced production technologies and equipment from industrialized countries worldwide, coupled with the dedicated research and development efforts of Chinese professional technicians, my country's BOPP film production has secured a significant position in the global BOPP film market. Currently, BOPP film production processes are becoming increasingly mature, and the BOPP film market remains relatively stable. Therefore, promptly addressing problems during production and striving to improve product quality and grade have become common concerns for all BOPP film manufacturers.
