Table of Contents
Introduction
Blown film extrusion is a making process used to produce thin plastics film. Highlight its applications in packaging, agriculture, and consumer goods. Mechanical properties like elongation and stiffness are crucial. They determine the film’s usability, durability, and suitability for specific applications.
1. What is elongation?
- Elongation is the ability of a material to stretch under stress without breaking. It is measured as the percentage increase in length relative to the original length.
- Describe the method for determining elongation in tensile tests, where the material is stretched until it breaks and the elongation percentage is calculated.
- High elongation ensures flexibility and prevents breakage during stretching or handling.
How to Calculate Elongation in Blown Film
Elongation is typically determined through tensile testing, which involves stretching a sample of the blown film under controlled conditions until it breaks. The calculation is derived as follows:
- ΔL: The change in length of the film (elongation).
- L0: The original length of the film sample.
Key Points in Blown Film:
- High Elongation: Films with higher elongation can stretch significantly without breaking, making them suitable for flexible applications like stretch wraps.
Factors Affecting Elongation:
- Material Composition: Polymers like LDPE and LLDPE offer higher elongation compared to HDPE.
- Processing Conditions: Blow-up ratio, cooling rate, and die design influence the molecular orientation and elongation of the film.
Types of Elongation:
Elongation can be categorized based on the direction of testing:- Elongation at Break: The percentage strain a material can withstand before failure.
- Machine Direction (MD) Elongation: Elongation measured along the direction of extrusion.
- Transverse Direction (TD) Elongation: Elongation measured perpendicular to the extrusion direction.
2. What is stiffness?
Stiffness refers to a material’s ability to withstand stress without breaking. In blown film, it is often described using the modulus of elasticity (Young’s modulus) or flexural modulus.
Types of Stiffness:
- Tensile Stiffness: Resistance to elongation under tensile stress.
- Flexural Stiffness: Resistance to bending or flexing.
- Impact Stiffness: Resistance to deformation under sudden force (impact).
Importance of Stiffness:
- High stiffness leads to excellent rigidity, stable dimensions, and ease of handling.
- Important for applications like lamination films, shopping bags, and rigid packaging.
Stiffness Testing and Calculation
1. Tensile stiffness:
Measured during the tensile test using the modulus of elasticity: E=σ/ε
Where:
E: Tensile stiffness (Young’s modulus).
σ: Stress (force per unit area, in MPa).
ε: Strain (relative elongation, dimensionless).
σ: Stress (force per unit area, in MPa).
ε: Strain (relative elongation, dimensionless).
2. Flexural Stiffness:
3. Impact Stiffness:
Tested using an impact tester like a dart drop test (ASTM D1709), which measures resistance to impact force.Comparison of Elongation and Stiffness
Factors Influencing Elongation and Stiffness
Polymer Type:
LDPE and LLDPE provide high elongation, while HDPE and nylon increase stiffness.
Processing Parameters:
- Blow-Up Ratio (BUR): Higher BUR improves orientation, affecting elongation and stiffness.
- Cooling Rate: Rapid cooling increases stiffness; slow cooling enhances elongation.
- Thickness: Thicker films tend to be stiffer.
Conclusion: Elongation and Stiffness in Blown Film Extrusion
Elongation and stiffness are two critical mechanical properties that define the performance and usability of blown films in various applications. Elongation ensures flexibility, stretchability, and durability, making it ideal for products like stretch wraps and agricultural films.On the other hand, stiffness provides rigidity, dimensional stability, and handling ease, which are crucial for lamination films, rigid packaging, and shopping bags.
As one property increases, the other tends to decrease, and vice versa, requiring careful optimization to achieve the desired balance for specific applications. Factors such as polymer selection, extrusion parameters, cooling rates, and film thickness significantly influence elongation and stiffness.
Question, answer, and explanation of elongation and stiffness:
Que: Which of the following factors primarily influences the elongation strength of a blown film?
a) Film thickness
b) Blow-up ratio (BUR)
c) Melt temperature
d) all of the above
Ans: d) all of the above
Explanation:
- Thicker films generally exhibit higher elongation strength due to a larger cross-sectional area.
- Higher BUR leads to thinner films, which can reduce elongatin strength.
- Most efficient melt temperature is crucial. Too high can degrade the polymer , while too low can lead to insufficient molecular chain orientation, both affecting elongation
Que: How does increasing the cooling air temperature generally affect film elongation strength?
a) Increase elongation strength
b) Decreases elongation strength
c) Has no significant effect
d) Can either increase or decrease
Ans: a) Increase elongation strength
Explanation:
- Faster cooling promotes fast solidification and molecular orientation, leading to improved tensile properties like elongation strength.
Que: Which polymer type typically presents higher elongation strength? high-density polyethylene (HDPE) OR low-density polyethylene (LDPE)?
a) HDPE
b) LDPE
c) They are generally similar
d) Depends on the specific grade and processing conditions
Ans: b) LDPE
Explanation:
- LDPE’s branched structure provides greater chain flexibility, leading to better elongation properties compared to the more linear HDPE.
Que: What is the primary role of additives like slip agents in influencing film elongation strength?
a) Directly increase elongation strength
b) Improve processability, indirectly aiding in better elongation
c) Reduce friction, leading to higher output but not affecting elongation
d) Primarily affect film gloss, not elongation
Ans: b) Improve processability, indirectly aiding in better elongation
Explanation: Slip agents facilitate smoother film bubble formation and reduce die increase, contributing to more uniform thickness and better overall mechanical properties.
Que: How does increasing the die gap ( distance between the die lips) typically affect film elongation ?
a) Increase elongation
b) Decrease elongation
c) Has nosignificant effect
d) Can either increase or decrease depending on the polymer
Ans: a) Increase elongation
Explanation: A wider die gap allows for increased bubble growth and stretching, leading to higher elongation in the machine direction.
Que: What is the effect of extreme air ring pressure on film elongation?
a) Increased elongation
b) Decrease elongation
c) Has no significant effect
d) Can either increase or decrease depending on the polymer
Ans: b) Decrease elongation
Elongation: High air pressure can overcool the film too quickly, block molecular orientation, and reduce elongation.
Que: Which of the following is NOT a common method to improve film elongation strength?
a) Increase die lip land length
b) Reducing melt temperature
c) Optimizing cooling air temperature and pressure
d) Using a higher blow-up ratio
Ans: b) Reducing melt temperature
Elongation: Lower melt temperatures can block molecular chain mobility, negatively impacting elongation.
Que: How does increasing the screw speed generally affect film elongation?
a) Increase elongation
b) Decrease elongation
c) Has no significant effect
d) Can either increase or decrease depending on the polymer and processing conditions
Ans: d) Can either increase or decrease depending on the polymer and processing condition
Explanation: Higher screw speeds can increase shear, which can either aid in molecular orientation (improving elongation) or cause excessive degrdation ( reducing elongation).
Que: What is the relationship between film orientation and elongation strength?
a) No direct relation
b) Higher orientation leads to lower elongation
c) Higher orientation leads to higher elongation
d) The relation ship is complex and depends on the specific polymer
Ans: c) Higher orientation leads to higher elongation
Explanation: Molecular orientation aligns polymer chains in the machine direction, enhancing tensile properties like elongation strength.
Que: Which of the following analytical techniques is commonly used to measure film elongation strenth?
a) Tensile testing
b) Melt flow index (MFI)
c) Differential scanning calorimetry (DSC)
d) Gel permeation chromatography (GPC)
Ans: a) Tensile testing
Explanation: Tensile testing directly measure a material’s ability to withstand tensile stress before failure, providing numerical data on elongation at break.
