PVA Water Soluble Film: How to Improve Adhesion for Sealing Laundry Pods

Have you ever picked up a laundry pod, and watched it burst in just your hands? That is a sealing issue and that occurs more frequently than you would suspect. It does not matter whether you are a manufacturer, a packaging engineer, or simply someone interested in the process of how these little pods are made. The secret of a well-sealed pod is the film itself, namely, PVA water soluble film. Get the adhesion right, your pods will be secure until the point they come into contact with water. Get it wrong, and you have leaks, broken seals and unhappy customers.

In this guide, we’ll break down the precise process of enhancing the adhesion of sealing laundry pods, starting with a grasp of the chemistry of the film itself, and finishing with dialing your machine settings. 

What Is PVA Water Soluble Film and Why Does Adhesion Matter?

PVA film (polyvinyl alcohol film) is the thin, clear material that encases the laundry pods. It goes all the way dissolved in water, leaving nothing synthetic behind in your laundry. But until it is dissolved, it has one very important task: remain closed.

The quality of adhesion is the degree of the bonding between two layers of the film at the point of sealing. With a poor adhesion rate, the pod may tear apart during storage, shipping and handling. When it is excessively strong in the wrong direction, it may tear the film rather than sealing neatly.

Most water soluble plastic film products are made of a polymer that is sensitive to heat, moisture and pressure. That sensitivity is exactly what makes it seal well but also what makes it tricky to work with if conditions aren't right.

Key Factors That Affect Sealing Adhesion

Better adhesion cannot be achieved simply by increasing the heat. It is a matter of knowing what actually governs the bond. Here are the main factors:

1. Temperature

The bonding of PVA film layers is triggered by heat. Too little, and the seal is not going to be formed all the way through. Excessive, you risk molting the film away, or forming a brittle seal that breaks when strained.

2. Pressure

The pressure forces the two layers into each other, and heat is used to bond them. Lack of even pressure causes uneven seals, the most prevalent cause of leaks during the production of the pod.

3. Dwell Time

This is the duration of contact of the sealing bar with the film. Too short, and the bond is not entirely formed. Excessive time is a waste and may harm the material.

4. Film Moisture Content

PVA is hygroscopic. It takes up moisture through the air. Excess moisture would make the film soft and lose the seal. Too little makes it brittle. It is very important to keep the film within a controlled humidity environment.

5. Film Thickness

Thicker films require additional heat and pressure. Thinner films are easier to seal but are easily damaged if the settings are not in the right place, even slightly.

How to Optimize Your Heat Sealing Process

Good sealing is a matter of dialing in your process rather than guessing. The following are steps that can be put into practice:

• Begin with factory specifications. Your film vendor must give a suggested range of sealing temperature. Start with that, not a random number.
• Run a peel test. After sealing, manually pull the seam apart. A good seal tears the film, not the bond. If the bond peels cleanly, it's too weak.
• Check pressure distribution. Place pressure-sensitive film between the sealing bars to identify hot spots or uneven contact zones.
• Adjust one variable at a time. Assuming that pressure and dwell time are held constant, change the temperature. This will assist you in identifying what is making a difference.
• Document your results. Maintain a basic record of what was successful and what was not. Conditions of production vary and a reference will save time in the future.

At POLYVA, the PVA water-soluble film is designed in such a way that it can seal within a range of specific parameters that are reliable and the optimization process becomes highly predictable.

The Role of Humidity in Film Adhesion

This is the single aspect that surprises most people. PVA film acts quite differently when there is moisture in the atmosphere. The film absorbs excessive water in a humid environment (more than 70% RH). This makes the surface soft and decreases the bond strength of the layers between each other. You could end up with a seal that appears excellent but collapses under the light pressure.

With low humidity (less than 40% RH), the film becomes hard and brittle. Seals develop; however, they may crack or break when the pod is pressed or dropped. The sweet spot is generally 45-65% relative humidity. In a place with high seasonal humidity (such as humid summers), the climate-controlled storage and production areas are worth investing in.

Humidity control in the storage and production of films is one of the most neglected aspects of water-soluble packaging quality.

Film Surface Quality

Although you may have ideal settings in your machine, the quality of your seals may be ruined by the quality of the film surface. Here's what to check:

• Contamination. The presence of oil, dust or any residue on the film surface will not allow successful bonding. Film should always be handled with clean gloves and stored in a closed package until use.
• Moisture on the film surface. Condensation (especially when bringing cold film rolls into a warmer production space) leaves water droplets that disrupt sealing. Let rolls acclimate before use.
• Static electricity. PVA film is also susceptible to accumulating static charges, resulting in a sticky or misaligned layer before it is sealed. Anti-static precautions on your machine or in your handling procedure are helpful.
• Film orientation. Some PVA films have a treated side and an untreated side. When the wrong sides are sealed, the adhesion becomes poor. Ask your supplier in case of any doubts.

Machine Settings and Tooling

Your film is as important as your machine. The finest film is not going to stick on the wrong machine or a poorly maintained machine.

• Sealing bar condition: A worn, scratched, or warped sealing bar creates uneven contact. Inspect bars regularly and replace them when worn.
• PTFE coating: To avoid sticking of the film, sealing bars must be coated with a fresh layer of PTFE (Teflon). Worn coatings cause tearing rather than clean seals.
• Cooling zone: After sealing, the film needs a brief cooling phase before it's cut or moved. Rushing this step can distort the seal before it fully sets.
• Calibration: Once it has been sealed, the film must have a cooling period before it is cut or relocated. Going into a rush during this process may cause distortion of the seal before the completion of the seal.

POLYVA's water soluble film packaging machines are designed to work seamlessly with our PVA films, with precise temperature and pressure controls that make consistent sealing much easier to achieve.

Common Sealing Problems and How to Fix Them

Here's a quick-reference troubleshooting guide for the most common adhesion issues:

Why Film Quality Starts Before the Machine

One of the things you should not neglect is the quality of your PVA film before it ever comes into contact with your sealing machine. Storage, handling and sourcing are all contributing factors to the success of your film.

Film that's been stored in a warehouse with swinging temperatures, or left on a loading dock in the sun, won't seal the same way as film stored under proper conditions. Even if the spec sheet says it should.

Here's what proper film storage looks like:

• Temperature: 15-25°C
• Humidity: 45-65% RH
• Away from direct sunlight and UV sources
• Wrapped in original packaging until use
• Stored upright on pallets: not flat-stacked

Where applicable, every roll of plastic film that is water soluble is packed at POLYVA to avoid the ingress of moisture at any point during transit and storage. However, once it is under your roof, it is equally important how you treat it.

Final Thoughts: Better Adhesion, Better Pods

Laundry pods are not very difficult to close, yet one needs to pay attention to details. The temperature, pressure, dwell time, humidity, film quality, and machine condition are all contributing factors.

The good news? After setting your settings and good handling practices, you will have consistent, reliable seals each time.

And when you need a PVA film that provides you with a solid baseline and performance that you can expect, POLYVA's full range of water soluble films will put you in a good starting point. Constancy of film implies constancy of process and constancy of pods.