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Why Polypropylene Cannot Be Welded with ABS

March 9, 2017

Why Polypropylene Cannot Be Welded with ABS

In order for two materials to weld, there are three main requirements.

The materials must be chemically compatible (i.e. the polymer chains must be able to form secondary bonds).
Some plastics are compatible this way, however it is mostly different types of amorphous plastics that are compatible with each other. ABS, an amorphous plastic, is compatible with PVC and PMMA. PP, a semi-crystalline plastic, is only compatible with itself.

An amorphous polymer has randomly ordered polymer chains. A semi-crystalline polymer has some amorphous (randomly ordered) regions and some crystalline (ordered) regions.

The materials must have a similar melt flow index.
In order for the polymer chains to mix, the melted plastics must have a similar viscosity (or thickness). You can think of it like trying to mix water with honey. The water pretty well flows around the honey and there is no real mixing. However, if the honey is heated (such as in hot water), it becomes less viscous (thinner) and the two can be mixed. Likewise, room temperature water doesn’t mix with cold butter, but they mix well if the butter is heated.

[image source: Royal Icing Dairies]
[image source: Royal Icing Dairies]

Melt flow index is a measure of how much volume of melted plastic, at a defined temperature, can be pushed through a certain diameter hole within a certain period of time. There are standards used for this test to ensure a comparable measurement. The typical requirement is that the melt flow index must be within 10% in order to mix under pressure.

Per Online Materials Information Resource:
ABS materials have a melt flow range of 0.08-80 g/10 min
PP materials have a melt flow range of 0.2-1200 g/10min

As you can see, this is a very wide range. Even within a the same base polymer, it is possible for some grades of material to be unweldable to each other. This is why it is important to be careful when working with multiple material grades within one assembly.

The materials must have a similar melting temperature.
With most welding methods, the temperature at the joint is the same for both materials. Therefore, both materials must melt at fairly close to the same temperature. Otherwise, one material will melt and the other will not and no mixing of the polymer chains will occur.

One everyday example of this happening is hot glue. Hot glue is a often made from low melting temperature plastic, including polyolefins and polyurethanes. When hot glue is used to make a design on a higher melting point plastics, like Teflon, for example, the glued shape can be easily removed from the plastic surfaces since no melting of the surface occurred.

It is possible to weld materials with dissimilar melting temperatures (and melt flow rates) using hot plate welding. This is done by heating the two materials to different temperatures before pressing them together. However, most welding methods depend on the temperature at the joint being the same for both parts.

Now, amorphous polymers like ABS don’t have a strict melting point. They have a glass transition temperature after which the material gradually softens and becomes less viscous as more heat is added. Semi-crystalline materials like PP, on the other hand, have both a glass transition temperature and a distinct melting point. Typically, the glass transition temperature for semi-crystalline materials is quite low and the melting temperature is quite high.

The glass transition temperature is the temperature when the secondary bonds in the amorphous regions of the polymer break. The melting temperature is when the secondary bonds in the crystalline regions break.

Amorphous polymers need to be heated significantly higher than their glass transistion temperature to weld, about 160°C higher. Semi-Crystalline materials need to be heated to about 60°C higher than their melting point to weld. The welding temperature needs to be within about 20°C for the two materials to be weldable.

Per Online Materials Information Resource:
ABS materials have a glass transition temperature range of 105-109°C
PP materials have a melting temperature range of 61-220°F

Therefore, ABS cannot be welded to PP because:

  1. The polymer chains are unlikely to form secondary bonds
  2. The melt flow ranges are unlikely to match
  3. The welding temperatures are unlikely to match

I use the word unlikely above because there is probably some way to get it to work, just not a way that will be cheap and consistent – both of which are vital to manufacturing.

I have seen applications “welding” two incompatible materials primarily by using a tongue and groove joint where the lower melting temperature and less viscous material makes the tongue which then melts and fills in the groove on the other material to create a mechanical joint through excellent fill. In one case, this worked well enough to provide a hermetic seal.

This article was originally published on the Q&A website Quora.com in response to the question, “Why can’t we weld polypropylene with ABS?”  Miranda Marcus, EWI Applications Engineer, can be reached at [email protected].