More Events:

November 18, 2015

Infrequently Asked Questions: Why do disposable coffee cups still leak?

Coffee Design
111616_IFQCoffee_Carroll.jpg Photo Illustration by Thom Carro/PhillyVoice

Coffee drips from the seam of a paper cup.

The world is full of questions we all want answers to but are either too embarrassed, time-crunched or intimidated to actually ask. In the spirit of that shared experience, we've embarked on a journey to answer all of the questions that burn in the minds of Philadelphians -- everything from universal curiosities (Can rice really fix a water-damaged phone?) to Philly-specific musings (How does one clean the Liberty Bell?). 

Breathe in, breathe out: You've just lived every coffee drinker's moment of horror.

You're snaking through the crowd to get to work on time -- coffee splashing about in your paper cup all the while -- and finally sit down to realize your brand-new white shirt or sweater is dotted with specks of brown. Suddenly, as you watch the culprit drip from the seam of the cup or underbelly of its lid, making that pit stop for coffee doesn't seem worth it.

Consider it a problem that, despite all the mainstream innovations that strip us of these sorts of everyday mood-killers, remains prevalent. What's the deal?

We asked Drexel University's Product Design Program Director Michael Glaser for an answer.

Why does coffee tend to drip from the seam of paper cups?

The issues with the paper cup or 'seam' on a cup is, by its very nature, a flawed design. It is a sandwich. We know through the universal law of sandwiches if you put a layer of something on top of another layer of something, then something else must go between those two layers. It is why the Oreo cookie is so good. Liquid is also very good at wicking and finding its way through very small gaps -- a principle of fluid dynamics known as capillary action, where liquid is literally drawn through the crack by intermolecular forces between the liquid and surrounding solid surfaces. It is a miracle of modern man that more cups don’t leak. 

Most failures are failures in manufacturing, such as poor adhesion of glue, not enough pressure during manufacturing, a mismatch in overlap, degradation of the seam over time and shipping wear, etc. Now consider that 58 billion cups are produced each year. Even if manufacturers could maintain a 99.9 percent success rate in assembly, that still leaves approximately 58,000 cups with leaks. Fundamentally, the seam is actually more reliable than my nemesis the plastic lid. 

For me, the lid is a bigger cause of failure because it also has the element of user error and is a problem on both paper and Styrofoam cups alike. 

What qualities would an ideal lid have, so as to not drip the coffee?

If I may be allowed to dream 'pie in the sky,' I’m going to suggest an all-in-one cup with an integrated lid. This would eliminate the two main problems with having to put on a lid after being filled, which are a second seam or having to create a seal around the top, and the fact that each lid has to be put on by hand. My cup would be a one-piece coffee container that would be filled through the drinking hole, or filled from the bottom with a leak-free valve.  

Filling would be automated, cream and sugar could be added as part of the process to everyone's individual taste. The only thing that would be lost is seeing the lovely milk pattern on the top of your latte. This new cup-and-lid combo could be manufactured like soda bottles but in biodegradable plastic made of corn starch called [polyactic acid] ...

Styrofoam seems to be a good solution for the dripping problem. For example, I never have troubles with a medium- or large-sized cup at Dunkin’ Donuts. Why don’t more shops use those? Environmental reasons?

While Styrofoam is a huge environmental issue, it does solve the seam issue and the sleeve issue. Styrofoam, we all know, is also a good insulator. In several studies testing the eco-friendliness of paper versus Styrofoam, researchers found a paper cup requires 12 times the amount of water, 36 times the amount of electricity and costs double the amount of money to produce. If you want to get real geeky, the carbon footprint of transporting paper cups is much worse because of its weight than transporting the lighter Styrofoam. We have been duped to believe paper is much more recyclable, but it's not really -- especially if it is coated with wax that protects the paper from breaking down and being biodegradable. 

Some companies, like Viora, have attempted to come up with cup and lid solutions. And Sheetz just switched over to Versalite cups. Are these real fixes to the drip problem? 

There is not really a great solution other than making everyone bring their own mug to the coffee shop. There was a design competition a few years ago to have designers try to solve this issue.

Why do you think there hasn’t been a universal solution to a problem all coffee drinkers experience on the regular?

Well, I’m sure inventors and entrepreneurs have tried making a better cup and lid and either failed or did not get enough investment. It is just my speculation, but I think it is a two-fold problem. 

One, the cost of change in a system that is already standardized on a paper cup is huge. It means convincing large manufacturers to stop doing what they have already invested millions in and are very efficient at, to reinvest and retrain for a totally new system. This is too much disruption and inefficiencies for most companies to consider a redesign. For a commodity like a cup, it is really a large ship to turn. Innovation will have to come from a startup, like the Uber of coffee cups. So, unless the system is forced to change either by disruption or by law, it won’t change. 

Two, I think it's the coffee drinker who is not motivated enough to demand change to the system. While it sucks to have your blouse or tie ruined from a leaky cup, it is happening at a low enough frequency to each individual user that no one is compelled to change the design. Sadly, most users often think they have little control over the product that are offered to them. I call it the 'Why Me' syndrome -- we accept that we have bad luck and that crappy things are going to happen, but it's just me. It is a rare few that see past the 'just me' and see this as an opportunity. 

Have a question you're dying to have answered? Send an email to, and we'll find an expert who can give you the answer you're craving.