# Why are paper straws so bad?

Updated: Aug 31, 2020

Authored by David Walker

We all know the feeling. You’re in the pub after a hard but productive week and you decide to indulge in a cocktail. The experience starts off as you’d expect but then about halfway through your drink you realise something isn’t right. It’s not the taste, temperature or feel but rather the fact you seem to have to sip harder and harder to drink your well earned beverage. You lift your straw up to find that to your annoyance it’s caved in and turned into a soggy mess of gross papery sludge.

Figure 1: An overpriced and probably oversweet cocktail.

Only a couple years ago you would never have had this issue. It is only due to recent mass awareness of the ecological damage caused by plastic straws that has caused so many companies to swap to paper [1]. The truth is however, that even to the most environmentally inclined person, paper straws are mildly irritating at best and rage inducing at their worst. The reason behind this is simple; from an Engineering standpoint paper straws are truly and utterly dreadful.

The Mechanics of Straws

To understand why a straw might buckle we have to first look at how straws work. Straws are essentially thin walled vessels where the person drinking is acting as the mechanical pump. As the person sucks air particles out of the straw we get a pressure difference between the outside atmospheric pressure and the lower internal pressure [2]. The liquid at the base of the straw then moves up the straw (as the internal pressure being exerted on the liquid is lower) until the smaller volume of air inside the straw exerts the same pressure on the liquid as the outside atmosphere (Figure 2).

Figure 2: What happens when you sip on a straw, using the ideal gas equation.

When a person sips hard enough this can only be achieved when the liquid has moved all the way up the straw and thus the device is doing its job. Knowing some basic hydrostatics means we can say:

Where P is the pressure difference, 𝜌 is the density, g is the gravitational constant 9.81, and ∆h is the change in height of the liquid. Assuming a 15cm straw sucking up water, we get a pressure difference of 1.471 kPa. That’s about 50 times less than the pressure difference between the cabin and exterior of an airliner in mid-flight. Now you’re probably thinking at this moment, surely if there is such a small pressure difference the straw should hold up easily? Unfortunately however, you need to take into account the thickness of the material before you can calculate stresses, and that’s where the problems with paper start to unfold (excuse my bad pun).

The Issue with Paper

When you factor in the thickness of the material; straws are under brief but significant mechanical stress. There are two main types of stress that affect thin walled cylinders such as straws: hoop (𝜎_{zz}) and axial (𝜎_{zz}). Hoop is the stress that travels cylindrically around the straw whilst axial travels down the length.

Figure 3: Where pressures and stresses act on a straw.

These are governed by the equations outlined below where p is the internal pressure (or pressure difference from inside to outside), R is the radius and t is the thickness of the straw.

If we plug in some numbers, say an internal pressure of –1.5 kPa as we discovered earlier (negative sign because the pressure is lower inside than out), a radius of 3 mm and a thickness t of 0.3 mm [3], we get a Hoop Stress value of – 15 kPa and an Axial Stress value of – 7.5 kPa. The question is, what does this mean exactly?

To help put some things in perspective of how significant this stress is, 15 kPa is the equivalent of the material stress if a 15 tonne object was placed on a 1 square meter piece of paper. That’s a lot of stress! Comparing the maximum stress the straw experiences with the yield stresses of straw materials in Table 1 [4], we can see where the issues start to lie...

Table 1: Straw Materials and their stresses.

We can see that wet paper at its worst is still supposedly meant to handle the pressure differences of a person drinking. In fact it’s highly likely that straw companies have always put their straws through plenty of testing so that they can advertise that paper straws are actually structurally sound. So what’s the problem?

It doesn’t take long to realise that with a bit of bashing against the side of the glass, a bit of chewing and a bit of bending, we have now pushed the maximum stress well beyond that of

the yield stress of wet paper. Soon the straw is squished up, in half or partially floating in your drink and you think to yourself, surely there’s something better than this?

What makes paper lack water resistance?

The issue with paper straws lies almost entirely with paper’s chemical structure. Paper is mostly made up of long chains of cellulose that are held together by Hydrogen bonding. The issue however, is that water can also form hydrogen bonds to these chains thus weakening the bonds between them. In fact when you tear paper (either wet or dry), you are breaking these hydrogen bonds, thus tearing the cellulose chains apart from one another [5]. Polymers such as Polypropylene, which used to make up the majority of straws on the market, are very non polar and thus do not attract water in the same way between the polymer chains.

Figure 4: The Skeletal structure of Cellulose, showing the hydrogen bonds between the chains.

It is not uncommon for paper straws to have a thin layer of hydrophobic wax coating around them to make them water resistant. However, the truth is if you leave your straw in long enough, this will definitely come off and your straw will become soggy and weak.

Conclusion and Alternatives

It may seem easy after reading this to dismiss biodegradable straws as something irritating and unnecessary. Furthermore, there is also much debate as to the true environmental impact of plastic straws as they only make up 4% of the 8 million tons of plastic floating in the world’s oceans [6]. It can also be argued that the real danger to the ecology of the planet is plastic leachate, which can be toxic to soils, oceans and wildlife [7]. Plastic straws only make up an even smaller percentage of this due to their low mass.

However, it is important to recognise that developing more environmentally friendly devices, however minor they may be, is crucial if we want any chance at ecological sustainability. The truth is that development in this area is still in its infancy and that in the future it is highly likely that eco friendly straws will become better and more practical.

In the mean time, if you’re sick of paper maybe try bringing your own Steel or Bamboo straw? Both are highly reusable and are much better for the environment in the long term. For single use straws that have similar properties to the old plastic ones, look out for straws made of biodegradable polymers such as Polylactic Acid (Figure 5). Having personally used them I can say that they actually work and feel just like the old plastic straws.

Figure 5: Biodegradable Straws in St John’s College.

Unfortunately, due to the higher cost of manufacture compared to paper, it looks as if paper straws are here to stay for now at most shops. Maybe I’ll just stop using straws.

References:

[1] BBC News – Plastic Straws: Which companies are banning them?, https://www.bbc.co.uk/news/newsbeat-43567958, (accessed 17 Oct 2019). [2] Chemistry LibreTexts – Extra Long Straws, https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Introductory_C hemistry_(Tro)/11%3A_Gases/11.01%3A_Extra-Long_Straws, (accessed 17 Oct 2019). [3] Aardark – Paper Straw Dimensions, https://static1.squarespace.com/static/5488af22e4b0eae0eaee7886/t/59c16225d7bdceac0 661e3fe/1505845825338/DISTRIBUTOR+CATALOG.pdf, (accessed 17 Oct 2019). [4] Cambridge University – Materials Data Book, 2003.

[5] Alvaro Tejado, Theo G.M. van de Ven – Why does paper get stronger as it dries?, 2010. [6] Phys Org - Science Says: Amount of straws, plastic pollution is huge https://phys.org/news/2018-04-science-amount-straws-plastic-pollution.html, (accessed 24 Oct 2019).

[7] Sasha G Tetu et al – Plastic leachates impair growth and oxygen production in Prochlorococcus, the ocean’s most abundant photosynthetic bacteria, 2019.

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