Thinking Humidity Outside The Box

So there I was, standing in front of an audience, wearing a headset and my voice booming out through large speakers, when suddenly, two minutes into my talk, the screen goes blank! I was giving a presentation called “Fidelity of Moisture Status in the Pharmaceutical Supply Chain” at a conference in Dublin. Nothing else for it, with no slides to show, I switched to talking about “Moisture Matters”. After a few minutes talking about the way materials interact with moisture, I brought my talk back towards the theme of my presentation. With about five minutes left, the screen came back on and I flicked through the key slides at speed, highlighting the points I had just talked about. A further twist in events was that the next speaker, who was talking about getting the right packaging for transporting products, started showing pictures of cardboard boxes and mentioned that magic word to my ears “humidity”.……..

Damaged packaging

Nothing surprising in mentioning humidity when choosing packaging, as we know some products need to be protected from moisture. The really interesting bit for me was the speaker talking about humidity and the problems it causes with “adhesives” used to stick cardboard boxes together. Obviously, if the adhesive doesn’t work, your cardboard box carrying your product falls apart. This does not inspire confidence in the product, especially if you’re the customer.

Storage of cardboard boxes

Damaged cardboard boxes

Making cardboard

Naturally, when I got home, I started researching adhesives used for cardboard boxes and effects of humidity. Some information that popped up is similar to the effects of moisture on paper. I talked about storage of paper and the printing facility environment in my previous blog. Since the raw material for cardboard and paper is wood fibres then it can be expected that both react in much the same way when exposed to changes in humidity.

Ripped cardboard

Exposed layers of cardboard – liner ripped off to show flute

Most of the information offered by my Google search was about how cardboard is made. I never thought about this before, but then, why would you? Cardboard is made using flat sheets of paper as “liner” and a shaped paper sheet called the “flute”. Each layer of cardboard consists of a flute sandwiched between two liners. Of course these layers have to be stuck together and that is done using adhesive. In the picture you’ll see where part of a liner has been removed showing the flute below. Because the liner has been forcibly torn off, the surface of the flute is rough where the adhesive attached the two layers.

Any, or all of these parts (liner, flute and adhesive) can be affected by moisture depending on the humidity at the time of making the cardboard. But, as interesting as this may be, it is a diversion from my search to find out about adhesives and cardboard boxes falling apart.

Adhesives

At this point in my search I was struggling to find a simple report or studies on moisture affecting adhesives or glues used for cardboard boxes. However, there are some detailed published scientific studies and online resources describing moisture reducing the bonding strength of adhesives. Also, people have investigated adhesives used for bonding wood. Two wood adhesives with increasing popularity are polyvinyl acetate (PVAc) and polyurethane (PUR).

Why the popularity of PVAc and PUR? They do not contain formaldehyde which is volatile, banned for use in food containers and best avoided anyway. These two adhesives are also used for gluing the edges of cardboard boxes. If you think about it the fibres in cardboard that are bonded by the adhesive are the same as in wood.

What I discovered

To summarise, what I’ve discovered in my searching, and taking out scientific jargon such as “interfacial bonding”, the effect of humidity and moisture on the adhesive, causing the edges of the cardboard box to become unstuck, is on three levels:

  1. When cardboard is stored at high humidity, moisture uptake happens and this weakens the bonding strength of the adhesive when applied to form the edges of the box.
  2. Even when a strong bond is formed along the edges, high humidity causes moisture to enter the adhesive and can change it chemically and physically weaken its bonding.
  3. High humidity drives moisture between the adhesive and the surface of the cardboard, breaking down the bond.
Humidity and cardboard boxes

From UPS: “Air Freight Packaging Pointers”

There is a fourth level but nothing to do with the glued edges. The courier company UPS have done studies that show a 60% reduction in the strength of cardboard itself at high humidity. This is important because if a cardboard box has been exposed to high humidity it should not be reused but sent for recycling. UPS provide a cold storage transportation service and if you read the first two chapters of my eBook you will understand why the reduced temperature has a big impact on humidity.

If you would like to hear more about moisture and humidity in everyday life, please sign up for email alerts of my blogs.

Welcome to my world of moisture

3 thoughts on “Thinking Humidity Outside The Box

  1. John

    I have a very interesting anomaly for you regarding cardboard boxes and dehumidifiers. Searching for information on the subject it seems this is the place to pass on the information I’ve found.

    I have some lone standing garages that used to be infrequently used. In one garage unit I have had many boxes for many months. Some boxes have various items in such as screws and tools etc, others have home stuff and random kit etc, The units are dry in the sense of no leaks or damp but are not often entered and so the humidity creeps up to 70 – 80 % in the winter according to the digital meters I have placed in there.
    So I moved a number of big and small boxes from one garage to another dry, warm garage. The boxes were dry to the touch, intact and easily stackable.
    While preparing the new garage for long term storage, the relative humidity was around 65 to 70 percent. I decided to get a dehumidifier and bring it down to around 50% over a couple of weeks.
    In the meantime I stacked 3 large boxes on top of each other in the new garage, each box being about 1.5ft wide, 1.5 ft deep and 1.5 feet high . Each box was intact but not very structurally good, just tatty boxes which held the contents but not strong solid boxes. They each contained motorbike gear, gloves helmets and metal and weighed about 8kg each, They stacked and held without a problem for which they had done so elsewhere for many months.
    I then took 3 more small boxes, about 30cmx30cmx15cm containing screws, metal and plastic, perhaps 2kg each and stacked them on top of each other, all solid boxes which had been stored in the previous high humidity garages. Many other boxes were all stacked in the new place but these 6 are of particular interest.

    Over a number of weeks I kept the humidity at around 50 percent with no problem, turning the humidifier on every few days for a day or two at a time until it filled and switched itself off needing emptying.
    Then one day after about 2 months, I returned to the garage and found the bottom box of the 3 big boxes had collapsed on one side and thrown the other boxes on the floor. This may have happened that day, or a week or two before, I’m not sure.
    Studying the crushed box i found to my surprise, the bottom box was totally soaked with water, as if someone had poured a measuring jug of water into the bottom of the box. There was a puddle of water inside bottom of the box, the contents were wet to the touch and small water patches on the floor outside the edges of the crushed box.
    Studying the surrounding area and boxes within a couple of feet nearby, there were no leaks, no other water or watermarks elsewhere and the unit was totally dry. The toppled boxes stacked on top of the crushed wet box were also totally dry.
    A couple of weeks later, in a different area of the large garage, I moved some other boxes that were stacked up against the 3 smaller boxes mentioned earlier. So the 3 smaller boxes were now stacked on top of each other with nothing supporting them either side.

    I came back a few days later, humidifier having been on, and found again, the bottom box of the 3 smaller boxes had collapsed, and the two boxes on top had toppled over.
    Examining the bottom box, it was wet inside and out and had small puddles of water inside the box, soaking the contents and making the metal inside rusty as if it had been that way for a while. You could put your hand in the puddle of water in the box.

    Further investigation showed other boxes stacked on shelves had no problem. No other boxes in the garage were wet, even other bigger stronger boxes placed on the floor in the same area remain dry and their contents dry. Moving them to check, some of these boxes had left very slight darker patches on the floor under them in the square shape of the box, indicating a slight presence of dampness but undetectable when looking and feeling into the boxes and their contents.
    I have since noticed elsewhere that in the same area, I had a sheet of brown paper placed on the floor under a stack of empty cardboard boxes. The paper is now slightly crinkled, showing straight lined water marks surrounding the straight lines of paper, indicating water has been drawn out of the paper and maybe the boxes above, and dried in a straight line next to the paper.

    There has been no flooding or external water coming into the garage at any point. The garage is visited regularly and remains dry at all times.

    I can only conclude that either the de-humidifier had over time, in both instances either drawn all the moisture out of each stack of cardboard boxes, in a downward motion, drawing all the water into the bottom cardboard box, where it then had nowhere else to go being sat on a concrete floor and then collected inside around the contents of the box, or perhaps water has been drawn out of the floor of the garage and been absorbed into the bottom of these two particular boxes.

    Regarding the brown paper. I assume whatever the direction the water came from, above or below, it has since dried out leaving the paper crinkly and a water mark aside.
    Perhaps the water comes both from above and below and the cardboard draws it in like a sponge from both sides. The floor is painted with garage floor paint and so is rubber based but the floor paint is old and so not providing a proper barrier.
    I have since noted that another empty cardboard box, placed on its side, so the smallest surface area was touching the floor, meaning the side edges of lid flaps had been in contact with the floor, all in the vicinity of the dehumidifier had their bottom edges crumpled and the layers of the cardboard separated. Whereas the top edges of the lid on the opposite side of the box were intact. This shows separation of the flute and walls of the edges of the cardboard lid that was touching the floor, protruding a few inches into the cardboard flap. Obviously the cardboard has become damp, disintegrated the glue holding the edges together, then dried again and separated the layers.

    I hope you find this interesting. The fact the water collected only in the bottom box of each of these two particular stacks of boxes in different areas and remained a puddle inside certainly is strange. The last fact I have is that in both stacks of cardboard boxes where the water accumulated, the contents inside the bottom boxes were in plastic bags. Perhaps this made the water vapour harder to escape and so it accumulated between the bags and the floor and so did not dry out.
    The third stack of boxes with the brown paper below remained dry and the single box on it’s side with the separated lower edges both contained no plastic and are dry.
    Perhaps you might like to investigate this yourself and replicate the issue. The use of the humidifier seemed to create a problem where one did not exist before. I’m sure any results you find may help a lot of people to protect their valuable photos and family articles in the future. If the contents of these boxes had been as such, the results would have been devastating. Thankfully a few screws and gloves in plastic bags was not a problem.

    Thanks.

    John

    Reply
    1. PeterMoir Post author

      Hi John, Many aopologies for the delayed reply. I usually get alerts coming through ok but not this time. Facinating and excellent report. I’ll have a think about this and see what comes to mind. Cheers, Peter

      Reply
    2. PeterMoir Post author

      Hello again John, I have thought of a possible explanation. As your dehumidifier removes moisture from the air heat is released due to the latent heat of condensation. This will warm the area around the dehumidifier and the room in which it sits. When the target %RH is reached the dehumidifier stops condensing and depending on the heating, or lack of, in the room, the surrounding air temperature will change. If your boxes are fairly well closed so they do not allow free movement of air, as they cool the %RH within them increases and if the temprature goes through the Dew Point, moisture will condense onto a cool surface such as metal. I envisage a series of heating and cooling cycles close to the Dew Point with the condensed water running off the metal. Once this process begins and liquid water is present within the boxes the moisture of the internal air moves closer towards the Dew Point because of the evaporating water. Without further investigations I can’t say this is a complete explanation but it gives a working hypothesis. Best regards, Peter

      Reply

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