Quality control of PVC stabilizers – A kind of Magic?

Quality control of PVC stabilizers - A kind of Magic
Quality control of PVC stabilizers - A kind of Magic

PVC stabilizers are complex mixtures of up to 20-25 single ingredients. Each of it has at least one special function for processing of PVC products like pipe, profiles, cable and flooring. Most the ingredients have more than one function, and in some cases some negative side effects. This results in a “complicated chemical cocktail” inside the mixture which is difficult to explain to people outside the stabilizer business. I used a parable to explain it to our daughter Maria, who is the only medical doctor in our family of chemists.

There are many different types of cakes with different tastes, and different ways to bake these cakes. My grandmother liked to do this and used all the necessary ingredients such as flour, milk, yeast, baking powder, sugar, all kinds of spices, sometimes honey, sometimes chocolate etc, depending on the cake she wanted to bake. These ingredients are mixed into a dough and baked at temperatures of 170-200°C.

In the past, many PVC processors did the same. They bought PVC, PVC superpacks, plasticizers (in some applications), lubricants, acid scavengers, color improvers, pigments etc, and mixed all these materials and manufactured their PVC products from this mixture at temperatures of 170-200°C. If we now compare both processes, the flour corresponds to the PVC, the milk to the plasticizer, the spices etc. to the PVC additives mentioned. The differences are not that big, only that one is very tasty and the other is very practical.

Today there are many ready-made baking mixes that you add to flour and milk, stir into a dough and bake. If you do everything right when baking, you get a cake that always tastes the same. The same applies to PVC products. They should always have the same processing behavior, the same product properties and the same product lifespan. Therefore, a quality assurance system and quality control is essential.

In the past it was different. Stabilizers based on toxic meals like lead were used. (Interestingly, these also taste “sweet” from an application point of view since lead is the best PVC stabilizer).The rule of thumb said that the higher the lead content, the higher the heat stability when processing the blends at temperatures mentioned before. Lead content can be accurately and directly analyzed by several methods. A different method is to burn the lead stabilizer under controlled conditions. The left material is called residue on ignition or ash content. This is when the first inaccuracies creep in, because the presence of other metal compounds results in a higher ash content. Furthermore, different test conditions lead to different analysis results. This type of analysis loses its information content. In addition, recent developments in hybrid stabilizers with lower lead content have shown that the same heat stability can be achieved by using synergists. This means that measuring the lead content is obsolete.

In the case of lead-free stabilizers, such as the Calcium Zinc stabilizer this gets even more complicated because a variety of elements are included: sodium, magnesium, calcium, aluminium, silicon and zinc. For the sake of simplicity, let’s only consider the elements calcium and zinc. On the one hand, they have different effects on heat stability. Zinc reduces it but is essential for some stabilizers because zinc compounds result in whiter color of end product. Unfortunately, this does not apply to all zinc compounds. Calcium, for its part, reduces the whiteness of the end product but improves heat stability in most, but not all cases. On the other hand, neither the calcium nor the zinc content says anything about the “origin” of its presence. Calcium occurs in at least five common components of the blend and zinc in at least four. It is true that all of these elements can be analyzed relatively cheaply and accurately these days, but even an expert cannot get any conclusions about the quality of the stabilizer mixture based on these values. Therefore, the analysis of the metals is actually useless.

In order to get useful information about the quality of stabilizer, ALL the components must be analyzed in detail. This is associated with a very high expenditure of staff, material and time, which very quickly ends up in costs of several thousand Euros. In addition, the investment costs to set up the analytical equipment are very high. Well-trained staff is needed and finally a specialist who can interpret the information. No customer wants to and will pay for this effort.

Another important parameter is the “loss on dry” (LoD) of the stabilizer, because the water inside the blend can increase the frequency of stopping and cleaning the production line. Therefore the sample is heated to 105°C. In the case of lead stabilizers, the result is correct in most cases. The lead-free alternatives may contain substances that thermally degrade or that react with each other or that contain volatile organic substances. As a result, a value higher than  the actual water content is measured.

So far we have only looked at two properties of the PVC product: its color and heat stability. But there are other properties which are difficult to predict based on a full chemical analysis, such as gloss, mechanical or electrical properties, light stability etc.

Therefore, it would theoretically make sense to test the stabilizer, as completely as possible, in an application laboratory. After all, the PVC processor does not buy lead, calcium, zinc or any other element. A processor buys “Performance”, which guarantees problem-free processing and a high-quality end product with a long life cycle. An investment in a high-performance application laboratory is in the range of several 100,000 Euros. A well-trained, experienced staff is againrequired. The cost of materials is 10 to 50 times higher than the amount of heat stabilizer used. Large quantities of PVC waste are produced which can be sold to recyclers, but at a fraction of the cost. In contrast to the above analytical methods, which provide absolute and reproducible results, most performance tests are relative methods. This means that at least one and in some cases several so-called references from previous investigations must investigated again. The costs will go ballistic, due to which all stabilizer manufacturers limit the number of tests as much as possible, and use  methods which are simple and cheap in the application laboratory. However this significantly limits the informative value of such tests.

Conclusion: Actually, the quality control of the stabilizer is completely pointless. In-depth raw material and process control make more sense. These are likely to be cheaper and more effective. And why are these information like lead content, ash residue, bulk density etc still found in the certificates of analysis? – Because customers insist on it. Why do they ask for it? – Because it’s always been like this! A good manufacturer-customer relationship and mutual understanding of quality combined with raw material and process control are, in our opinion, at least in the stabilizer industry, a better approach than creating useless data.

I don’t know how extensive the quality control is on a baking mix. But as a total layman, I can imagine that it might be relatively simple. Mixing dough, baking cakes, touching, smelling and tasting.