Science Behind Slime: What Does Borax Do In Making Slime

Slime is a popular toy made with simple ingredients: glue, borax, and water. So, what does borax do in making slime? In this article, we explain the science behind slime, what borax does, and why we need to find alternatives.

The Slime-Making Process: What Happens?

Mix glue, borax, and water in the correct proportions. After kneading for a while, the mixture will turn into slime. During the kneading process, the liquid glue gradually becomes solid and non-sticky.

Obviously, it is the borax that makes the glue elastic. So how does it do it?

The Chemistry of Cross-Linking: How Slime Works

In fact, when making slime, the glue and borax will undergo a chemical reaction called “cross-linking”.

First, the glue we use contains polyvinyl acetate / polyvinyl alcohol (PVA). It is a long-chain molecule that can be imagined as scattered wool or spaghetti. Most of the time, these long-chain molecules will be entangled, making it difficult for them to move, so PVA glue is a sticky liquid.

Borax (sodium borate) is a solid inorganic salt (powdered) that forms an alkaline solution when dissolved in water. As we add borax and water to PVA glue, the borate ions in the borax solution will connect the long-chain molecules, turning them from disordered yarns into the three-dimensional net. Thanks to the borate ions, they support the long-chain PVA molecules like bones, which makes slime soft and elastic.

Is Borax the Best Slime Activator

From the chemical reaction above, we know that the main function of borax is to connect PVA molecules, and we call this ingredient borax a cross-linking agent or activator.

So, is borax the best slime activator? For a long time, it was.

The borax and PVA reaction is easy and controllable, and its low cost has made it a staple in slime making for a long time. However, in recent years, the safety of borax has become a controversial topic. 

Although there are no clear regulations prohibiting the use of borax, the popularity of borax-free slime is gradually increasing, which shows that people are looking for safer alternatives.

Is Borax Safe to Touch in Slime

Despite the safety concerns of borax, its content in slime is tiny. EU EN71 requires that the boron content in toys should not exceed 1200 ppm (0.12%) or 300 ppm (0.03%). Under strict safety testing requirements, slime that meets the standards is safe enough to touch.

Of course, it is not safe to swallow slime or let it come into contact with your eyes.

What Happens When You Put Too Much Activator in Slime

In the lab, technicians calculate the optimal mixing ratio of PVA and activator, since too much or too little activator will make a difference in the slime textures.

Generally speaking, activators increase the viscosity of glue, making it hard and reasonably elastic. So, when we add too much activator, PVA will be over-crosslinked, making the finished slime too solid to use.

Does Borax Make Slime Fluffy

When making fluffy slime, we may use borax, but it is not the cause of the fluffiness. The simplest recipe uses only PVA glue, borax, and water to make non-fluffy slime.

The fluffiness of slime comes from other foaming substances, in the homemade recipe, we use shaving cream.

Borax-free: Which Activator is Best for Slime Now

From the previous introduction, we know that it is the borate ions, not borax, that play a role in the cross-linking reaction. Thus, the activator that replaces borax needs to contain borate ions and be harmless to the human body.

At present, the common activator substitutes used to make borax-free slime are mainly liquid starch, baking soda with contact lens solution and cornstarch.

Liquid Starch

Liquid starch, common in detergents, forms borate ions when mixed with water and can replace borax in reacting with PVA molecules.

The texture of slime made with liquid starch is typically the same as that of borax-based slime, but it is not safe enough.

Cornstarch

Cornstarch is an edible ingredient, so it is undoubtedly safe to make slime. However, cornstarch is not an activator here. When making slime, cornstarch does not chemically react or cross-link with glue.

In fact, making slime with cornstarch is a physical change, that is, the interaction between starch particles and water forms a non-Newtonian fluid. Therefore, the texture of cornstarch slime will be more like dough or oobleck. Moreover, it is almost impossible to make transparent slime with it.

Baking Soda with Contact Solution

Baking soda is easy to find in home kitchens, so you’ll often see it in recipes for homemade borax-free slime. But be aware that baking soda (sodium bicarbonate) does not form borate ions when dissolved in water.

It must be mixed with contact solution to be an effective activator.

Science Behind Slime With Contact Solution

Similar to borax, contact solution and PVA glue will also undergo cross-linking reactions. Because contact solution contains boric acid and sodium borate, they exist in the solution in ion form. That is, there are a lot of borate ions in the contact solution.

When we drop contact solution into glue, borate ions will combine with PVA long-chain molecules to form a cross-linked network.

This is a simple recipe for making slime:

• 1/2 cup PVA glue
• 1/2 cup water
• 1/2 teaspoon baking soda
• 1-2 tablespoons contact lens solution

However, in actual production, we do not use contact lens solution since it is costly.

How Long does Borax Slime Last

Compared with other alternatives, borax slime has a longer shelf life and duration (usually more than half a year), which is why people have always used borax as a cross-linking activator.

The main signs of slime deterioration are mold/smelly, discoloration/fading, and drying/hardening. Using the correct storage methods, we can extend the life of the slime as much as possible.

Moldy/Smelly

Polyvinyl acetate is a biodegradable green material, and many microorganisms will decompose it. If you store slime in the open air, bacteria and microorganisms will settle on it. 

Moldy and smelly slime can only be thrown away, and sometimes it will become sticky. So, to avoid this situation, we should use sealed containers or ziplock bags to store it when not playing.

Discoloration/Fading

The color in slime mainly comes from pigments or food colorings, which are usually not resistant to ultraviolet rays or high temperatures. Ultraviolet rays and high temperatures will decompose pigments and change the color of slime. 

This discoloration or fading is irreversible, so the slime should be stored in a cool place away from direct sunlight.

However, there is a special color-changing putty that changes color when exposed to ultraviolet light or temperature changes, but can recover afterwards. When storing this putty, avoid sunlight and high temperatures to make its color-changing properties last longer.

Drying/Hardening

Frequent play can cause slime to harden due to material fatigue, but letting it rest for a while usually restores it. 

Another case of hardening is that the slime has dried out. During the cross-linking reaction, PVA molecules capture water, so the slime also contains water. When we play with slime, this water may decrease. In this case, you can spray some water on the surface of the slime to soften it.

Conclusion

The magic of slime lies in the chemical reaction of polymers, specifically the cross-linking of PVA long-chain molecules and borate ions. Understanding this helps us find borax alternatives, like liquid starch and baking soda with contact solution. Cornstarch as a substitute for slime is a physical reaction, and the slime made with it is a non-Newtonian fluid.

From the science of slime making, to preserve slime, seal it and avoid direct sunlight, extending its shelf life. This will help us extend the shelf life of slime toys.

References

1. The Science of Slime!
2. slime_activity.pdf
3. Cross-link – Wikipedia
4. Polyvinyl acetate – Wikipedia