How to Solve the Heat-Sealing Challenges of PVDC Films?
In the packaging industry, heat sealing is one of the most common and fundamental processes—two plastic films are bonded together simply through heat and pressure. However, PVDC does not always follow conventional processing rules. This is why products such as ham sausages are often sealed with aluminum clips at both ends instead of using direct heat sealing like ordinary plastic bags.
The difficulty lies in the unique thermal characteristics of PVDC. Understanding why PVDC is difficult to heat seal and how to overcome this limitation is essential for developing high-performance PVDC barrier packaging solutions.
1. Why Is PVDC So Difficult to Heat Seal?
The challenge of PVDC heat sealing originates from its thermal sensitivity.
The melting point of PVDC homopolymer is approximately 197–205°C, while its decomposition temperature is around 210°C. The difference between the two temperatures is only a few degrees. As a result, PVDC homopolymer is practically impossible to process through conventional melt processing methods.
In industrial applications, PVDC copolymers are typically used instead. By introducing comonomers such as vinyl chloride (VC) or methyl acrylate (MA), the melting temperature can be reduced to approximately 150–175°C, making melt processing possible.
However, even PVDC copolymers still have a very narrow processing window because their melting temperature remains close to their decomposition temperature. The effective processing range is typically only 20–30°C.
According to industry data, the heat-sealing temperature of PVDC copolymers is approximately 150–160°C, meaning PVDC is not completely incapable of heat sealing. The real challenge is that this temperature range is extremely close to the degradation threshold. A temperature fluctuation of only 2–3°C may trigger thermal degradation.
Therefore, the fundamental contradiction behind PVDC's heat-sealing difficulty is:
PVDC has heat-sealing capability, but almost no processing tolerance.
The early PVDC casing films developed by Dow Chemical provide a typical example. These PVDC sausage casing films could only be sealed through high-frequency dielectric heating and could not be reliably sealed using conventional resistance heating methods.
2. Main Solutions for PVDC Heat-Sealing Challenges
Since PVDC itself is difficult to heat seal, the most direct strategy is to avoid making PVDC perform the sealing function.
Instead, PVDC is used as the middle barrier layer of a multilayer structure, while other easily heat-sealable materials, such as CPP, CPE, and EVA, are responsible for sealing.
A typical multilayer structure includes:
By placing PVDC in the middle, the structure allows the material to deliver its excellent barrier performance while avoiding its weakness in heat sealing.
2.1 Solution One: Solvent-Free Lamination
Solvent-free lamination is currently one of the key technological breakthroughs for solving PVDC heat-sealing challenges.
PVDC film is a highly heat-sensitive material with significant thermal shrinkage behavior. It begins to shrink at ambient temperatures above 30°C.
Taking Dow Chemical's Saran 2032 film as an example, shrinkage begins even at 30°C room temperature. When exposed to 100°C hot water for three minutes, the shrinkage rate can reach:
Therefore, any lamination process involving heat—such as dry lamination, hot-melt lamination, or extrusion lamination—may cause severe PVDC shrinkage, making it difficult to achieve a flat and stable finished film.
The major advantage of solvent-free lamination is that the entire process is carried out at room temperature without additional heating, effectively preventing thermal shrinkage of PVDC films.
However, solvent-free lamination also faces another challenge: after curing, solvent-free adhesives generally provide lower bonding strength compared with some traditional systems and may experience delamination during high-temperature retort processing.
The advanced solution is to combine solvent-free lamination with optimized process technologies, including:
- Precise unwinding tension control
- Proper adhesive system selection
- Optimized curing conditions
These measures ensure that PVDC laminated films maintain strong interlayer adhesion and excellent appearance even after high-temperature treatment.
Currently, PVDC laminated composite films produced through solvent-free lamination have achieved commercial application, and their high-temperature retort resistance has been verified by authoritative testing organizations.
2.2 Solution Two: Multilayer Co-Extrusion
Multilayer co-extrusion is another highly effective approach for addressing PVDC heat-sealing limitations.
Multiple polymers are simultaneously extruded through multiple extruders and a single die to form a tightly bonded multilayer film, with PVDC serving as the central barrier layer.
A five-layer co-extruded heat shrink film with a PVDC core layer is already a mature commercial solution.
A typical structure is:
Patent literature indicates that the PVDC core layer can consist of approximately:
- 98 wt% VDC-MA copolymer resin
- 2 wt% processing additives (such as epoxidized soybean oil)
This formulation can reduce the PVDC melting temperature to approximately 150°C, significantly expanding the processing window and improving heat-sealing performance.
The heat-sealing layer typically uses polyolefin materials such as:
- Metallocene polyethylene (mPE)
- Linear low-density polyethylene (LLDPE)
These materials have melting points as low as 88–96°C, allowing reliable sealing at relatively low temperatures while preventing excessive heat transfer into the PVDC layer and avoiding degradation.
The major advantages of multilayer co-extrusion include:
- No adhesive layer required between polymers
- No solvent residue concerns
- Precise control of individual layer thicknesses
However, when PVDC needs to be co-extruded with high-temperature polymers such as PA, processing becomes more challenging.
PA has a melting point of approximately 220–225°C, creating a temperature difference of more than 60°C compared with PVDC.
The solution is to introduce a thermal insulation bridge structure.
By inserting multiple transition layers made from materials such as EVA between the PVDC layer and high-temperature polymer layers, the temperature difference can be gradually balanced through a gradient structure, allowing successful multilayer processing under the same extrusion conditions.
3. Advanced Techniques for Optimizing PVDC Heat-Sealing Processes
For applications where PVDC coatings themselves must function as the sealing layer, the industry continues to explore advanced processing solutions.
One representative approach is the development of low-temperature heat-sealable barrier coating films.
This type of film typically uses BOPP as the substrate, with:
- A PVDC coating layer (2–3 μm thickness) applied on one side to provide barrier performance
- A low-temperature heat-sealing coating applied on the other side
Experimental results show that under heat-sealing conditions of:
- 70°C
- 0.14 MPa pressure
- 0.75 seconds sealing time
the sealing strength can reach:
Meanwhile:
- Oxygen transmission rate can be reduced to 2.8 cc/m²·24h
- Water vapor transmission rate can be reduced to 1.0 g/m²·24h
This solution is particularly suitable for high-speed packaging of temperature-sensitive products such as chocolate, demonstrating that PVDC can achieve low-temperature heat sealing through coating technology.
4. Frequently Asked Questions
It depends on the selected heat-sealing layer.
When materials such as CPP or CPE are used as the sealing layer, the typical sealing temperature range is approximately 120–150°C.
For PVDC-coated films, the heat-sealing temperature can be reduced to approximately 70°C. Under conditions of 0.14 MPa pressure and 0.75 seconds sealing time, sealing strength can reach 2.0–3.0 N/15 mm.
The key requirement is ensuring that the sealing layer receives enough heat to achieve bonding while preventing heat transfer into the PVDC barrier layer, which could cause degradation.
This is a classic example of the PVDC heat-sealing challenge.
Traditional sausage casings are often made from single-layer PVDC blown film. Because the melting temperature of PVDC homopolymer is extremely close to its decomposition temperature, reliable sealing cannot be achieved through conventional resistance heating.
Therefore, the industry typically uses:
- High-frequency dielectric sealing
- Aluminum clip sealing
For packaging applications requiring bag-making processes, multilayer composite films provide a better solution by using PVDC as the internal barrier layer while assigning the heat-sealing function to other materials.
Both solutions can achieve excellent heat-sealing performance, but they are suitable for different applications.
PVDC multilayer co-extruded films
Advantages:
- No adhesive required between layers
- Barrier and sealing layers are integrated during extrusion
- Suitable for food packaging applications sensitive to solvent residues
- Five-layer co-extruded films can provide heat-sealing layers with thicknesses of approximately 18–25 μm, offering sufficient sealing tolerance
PVDC solvent-free laminated films
Advantages:
- More flexible structural design
- Can incorporate high-temperature-resistant adhesives and CPP sealing layers
- Maintains strong interlayer adhesion after 121°C retort sterilization
The optimal choice depends on:
- Required sterilization temperature
- End-use application
- Packaging performance requirements
Conclusion
The heat-sealing challenge of PVDC films represents a classic trade-off between the material's outstanding barrier properties and its processing limitations.
For packaging manufacturers, the optimal solution depends on product positioning and processing conditions:
- If the application requires high-temperature retort sterilization (121°C), manufacturers should prioritize evaluating solvent-free lamination systems with high-temperature-resistant adhesives and conduct complete retort validation before mass production.
- For conventional low-temperature packaging applications (below 100°C), multilayer co-extrusion offers advantages in production efficiency, cost control, and solvent residue management.
Before selecting a PVDC packaging solution, manufacturers should conduct a comprehensive evaluation based on:
- Existing equipment capabilities (solvent-free lamination lines or multilayer co-extrusion lines)
- Regulatory requirements in target markets (food-contact compliance)
- End-user expectations regarding film flatness and appearance
Through proper material selection, process optimization, and production validation, PVDC's excellent barrier performance can be fully utilized while overcoming its heat-sealing limitations.
