5 Reasons Why Your Business Needs PVC Cazn Stabiliizer for Rigid Foaming Product？

1 Introduction

Poly(vinyl chloride) (PVC) is one of the most worldwide used thermoplastics with a vast spectrum of properties including low cost, simple processing, and flammability resistance. The rigid PVC form exhibited good market growth in major PVC product applications, i.e., pipes, due to its magnificent functional properties and cost competitiveness [1,2].

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Because of the structural defect in the PVC structure due to the presence of allylic chloride atoms, it is extremely sensitive to heat. There have been many attempts to improve the thermal stability of this polymer because almost all the shaping processes encompass high-temperature usage. Even at lower processing temperatures, PVC undergoes decomposition and loses hydrogen chloride (HCl), and as a result, there is deterioration in its physical and mechanical properties. The HCl elimination is a chain reaction with a special mechanism called “zipper elimination.” The chemical breakdown is usually accelerated by heat, releasing HCl, which leads to discoloration and polymer degradation. It is possible to inhibit the released HCl and stabilize PVC, or suppress the catalytic effect of HCl in this reaction by adding different heat stabilizers.

Heat stabilizers are usually used as a major additive for PVC in order to protect it against high temperatures during processing and the high heat it may encounter during its shelf time and storage. Thermal stabilizers of PVC must have the ability to neutralize HCl evolved during the degradation and to interrupt the conjugated polyene formation [2,3,4].

As reported in the literature, PVC stabilizers could be classified either as primary or secondary, organic or inorganic, and many other classifications. Various thermal stabilizer types have been widely used in a large range of PVC commercial formulations to enhance their thermal stability, especially through the shaping process. Lead and tin stabilizers were the first ones to be used successfully to stabilize PVC. Until recently, these materials still accounted for the major portion of the stabilization system used, with lead being used in industrial pipe compounds and alkyl tin mercaptide in portable water pipes, profiles, etc. But a wide new variety of PVC stabilizers are used currently, including salts of calcium, barium, cadmium, lead, or zinc of the carbonic, phosphoric, benzoic, and stearic acids as well as organic compounds based on tin, organic phosphates and using heavy metal carboxylates and organo- tin mercaptides [6,7].

The mechanism of degradation is complex, and many attempts were made to reveal this confusing mystery. Starnes [5] presented a detailed review of the proposed mechanism of PVC thermal dehydrochlorination and stated that the detailed mechanism of polyene termination seems to be difficult to track down. Folarin and Sadiku [6] reported that the mechanisms of stabilization include the replacement of labile chlorine atoms, scavenging chlorine radicals and HCl, and addition across polyene sequences and aromatic alkylation. Chabira and Sebaa [7] reported that differential scanning calorimetry and thermogravimetry are two powerful tools providing useful information for understanding the mechanism of degradation of PVC. They implemented dibasic lead stearate and a mixture of (Ba/Cd/Zn) metal salt carboxylate, as stabilizers, and investigated the changes using differential scanning calorimetry and thermogravimetry. They found that during processing, the dibasic lead stearate is only relatively consumed, while almost all of the mixture of (Ba/Cd/Zn) metal salt carboxylate reacts.

There are many approaches for determining the extent of thermal degradation depending on the physical or chemical changes that occur in the polymer. Many instruments were used depending on the properties to be studied. Wu and Wicks [8] developed an experimental procedure using liquid sampling and fluorescence measurement to determine the performance of stabilizers based on the change in fluorescence emission after thermal aging. Bodecchi et al. [9] used Fourier transform infrared spectroscopy for understanding the behavior of calcium and zinc carboxylate PVC heat stabilizers.

Amar et al. [10] used lead-based stabilizers to study the thermal aging of PVC, which was performed in an oven at 60°C. The RPVC showed a growing carbonyl band in the IR spectrum with the aging time. Borukaev et al. [11] used a complex stabilizer based on a mixture of calcium and zinc stearates and investigated their thermal stability by determining its effect on the time of the induction period and determining some mechanical and rheological properties of PVC used for cable production. Jubsilp et al. [12] compared the effects of organic-based stabilizers with traditional lead and calcium zinc heat stabilizers in rigid PVC pipe applications. They proved that the organic-based stabilizer provided a good impact on preventing the autocatalytic thermal degradation of the PVC samples due to the lower gelation speed and the longer fusion time. Tomaszewska et al. [13] studied the thermal stability of PVC by using visual color changes and thermogravimetric tests. The melt flow index was also recorded to identify the degradation-induced changes in processing properties. Fechter et al. [14] developed and proposed a method for simulating the torque–temperature curves by using Python programming language from a thermal stability test on PVC using a thermal rheometer.

The principles for selecting the most suitable stabilizer systems depend on the cost, toxicity, and processing technique besides some critical factors, namely the induction period, rheological stability time, and the value of minimum torque in the plateau. In this article, we test and evaluate the activity of 12 common PVC thermal stabilizer systems by implementing a Brabender torque roll mill as a vital method for detecting the aforementioned critical factors.

3 Results and discussion

The fusion rate plays an important role in the processing of rigid PVC, especially in the extrusion technique. The fusion time is defined as the time required to convert a cold formulation of PVC when introduced in an extruder to a melt.

The experimental results indicated that the chemical structure of the heat stabilizer has a notable effect on the fusion rate of PVC systems. The control for comparison was pure PVC with no added stabilizer.

Table 1 sorts the induction period of the studied stabilizer systems in increasing order. It can be seen that the fusion time obtained varied between 30 and 50 s, except the fusion time of the PVC/lead stearate system, which has a value of 180 s. This may be due to the higher melting point of this stabilizer besides its strong lubrication properties. Therefore, this stabilizer is not used alone in practice but it is usually mixed with other stabilizers. It is worth mentioning that the fusion rate of any particular stabilizer system can be easily controlled by using lubricants, resins, and other formulation components.

Table 1 Stabilizer systems Fusion time (s) Induction period (min) Increase in the induction period (%) Without stabilizer 30 3.0 — Irgastab BC 103 40 5.0 66.6 Irgastab A 70 35 5.2 73.3 Thermolite 25 30 6.5 116.6 Dibasic lead phosphite 40 7.0 133.3 Thermolite 813 50 9.0 200.0 Mixture of lead stabilizers 50 9.0 200.0 Mark 235 35 10.7 256.6 Dibasic lead stearate 180 11.0 266.6 Mark A 40 11.5 276.6 Advastab 188 35 13.0 333.3 Mark 292 50 14.0 366.6 Irgastab 15 MS 45 15.5 416.6

On the other hand, it was found that the four systems based on organo-tin mercaptide were far superior to other stabilizers, while the fusion time measurements indicate that most of the stabilizers used were equivalent under the experimental conditions.

The induction period can be expressed in terms of chemical changes, and it is defined (in the case of PVC) as the time required for the evolution of hydrogen chloride gas. The experimental results obtained show that the used stabilizers have different effects on the induction period of hydrogen chloride evolution. Table 1 illustrates the induction periods obtained under the experimental conditions mentioned above. The results confirm the occurrence of interaction between the PVC chain and the stabilizer; several articles imply this fact [8,15,16,17].

These results showed that three stabilizer groups having different efficiencies are obtained. These stabilizers are classified as high-, medium-, and low-efficiency stabilizers. The high-efficiency stabilizers are the organo-tin mercaptide, and the increase in the induction period ranges between 276 and 416% compared to non-stabilized PVC. The comparison of the efficiency of the four types of organo-tin mercaptide stabilizers used showed that dibutyl tin mercaptide (Irgastab 15MS) has the highest efficiency. The sulfur-free organo-tin stabilizers were found to be less efficient than the organo-tin mercaptides and their ability for stabilization varies from each other. The induction period increased by 116% when dibutyl tin monoalkyl maleate was used versus 200% di-n-octyl tin maleate was used.

From the above data, it is clear that the stabilizers containing dibutyl groups are less efficient than di-n-octyl groups. Lead and Ba/Cd stabilizers could be classified as medium stabilizers for PVC. The mixture of lead stearate/lead phosphite (1:2) part stabilizers is considered a medium-low stabilizer. In spite of its lead content, this type of stabilizer is still in use for the production of potable water pipes in local plants.

The torque–time and temperature–time data obtained from Brabender measurements for various PVC dry blends are represented in Figures 1–13. The rheological curves (torque–time curves) showed that non-stabilized PVC has the highest minimum torque in the plateau and reaches 1.8 m kp (17.652 N m), as shown in Figure 3. The time of constant torque is defined in this study as the rheological stability period, where the torque can vary within 5%.

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13

The minimum torque value and the rheological stability time are shown in Table 2. The results showed that the different stabilizer systems used decreased the torque and tend to increase the time of rheological stability, except Ba/Cd/Zn stabilizer (Irgastab BC 103) as shown in Figure 3. It can be concluded that the decrease in torque has a positive effect on the flow properties and consequently on the production rate. Thus, a general rule for selecting the suitable stabilizer system is to prefer those that reveal the lowest torque in the plateau.

Table 2 Stabilizer systems Minimum torque (m kp) % Decrease Rheological stability period (min) Without stabilizer 1.80 — 2.0 Irgastab BC 103 1.68 6.6 1.5 Irgastab A 70 1.63 9.4 2.5 Thermolite 25 1.52 15.5 3.0 Dibasic lead phosphite 1.74 3.3 2.0 Thermolite 813 1.52 15.5 4.5 Mixture of lead stabilizers 1.67 7.2 4.0 Mark 235 1.63 9.4 7.0 Dibasic lead stearate 1.50 16.6 8.0 Mark A 1.45 19.4 7.5 Advastab 188 1.62 10.0 9.0 Mark 292 1.47 18.3 9.0 Irgastab 15 MS 1.60 11.1 12.0

The results indicate that both dibutyl tin-bis-(lauryl mercaptide) stabilizer (Mark A) and dibutyl S,S-bis-(isooctyl thio glycolate) (Mark 292) showed the greatest decrease in torque while, in contrast, dibasic lead phosphite (Dyphos) showed the lowest decrease. This indicates that the following stabilizers cannot be applied with PVC formulations: Ba/Cd/Zn stabilizer (Irgastab BC103), thio-di-ethylene-glycol-β amine stabilizer (Irgastab A70), dibutyl tin monoalkyl maleate (Thermolite 25), and dibasic lead phosphite (Dyphos).

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The rheological stability period of the final plastic product is affected by the processing time, i.e., the time of the exposure of the polymer system to heat at different stages of the processing. This study suggests that the time of heat exposure during processing should not exceed 50% of the rheological stability time.

The remaining 50% or more of the stability time is expected to be enough to prevent the degradation of the polymer during its use. If we excluded the four previous stabilizer systems mentioned above, it can be seen from Table 2 that the rheological stability time of the mixture of lead stabilizers is about 4 min, which represents the lowest rheological stability period among the rest efficiently used stabilizers according to their application in PVC.

The measured temperature of the polymer melt in the rheological stability period was between 190 and 200°C, which is about 10–20°C higher than the test chamber temperature. This difference is very likely due to pressure and friction effects resulting from shear stresses in the test chamber.

In recent years, considerable effort has been expended toward the development of new stabilization systems for PVC processing, driven by a desire to move away from stabilizers based on heavy metals. Although there are now numerous alternatives to lead and cadmium stabilizers--including calcium/zinc, organotin, and barium/zinc systems--each has its drawbacks for the flexible vinyl formulator.

For example, Ca/Zn stabilizers are preferred for use in critical applications because of their low toxicity, but they are generally the least effective option in terms of thermal performance. Organotin mercaptides are widely accepted as the most efficient thermal stabilizers. However, the odor properties of the sulfur-based ligands prevent their use in most flexible PVC processes. Although Ba/Zn products have proven to be a suitable compromise between Ca/Zn and organotins from a thermal perspective, many believe that barium could be the next metal targeted by regulators.

Morton International, previously a supplier of stabilizers only for rigid PVC, recently launched a new all-organic-based heat-stabilizer technology for flexible calendering, extrusion, plastisols, injection molding, and rotomolding. Called Advaflex, these stabilizer systems address the limitations of currently used stabilizers. Presently being field tested, these systems are also cost-effective and free of cadmium, lead, barium, phosphorus, phenolics, and other components that raise environmental and health concerns. End-use applications initially targeted include pond liners, flooring, roofing, footwear, calendered film, wire, and cable.

The impetus for the development of this novel technology is the identification of new activated organosulfide compounds. Such compounds are chemically prepared as blocked or protected thiols, which are designed to decompose under vinyl processing conditions and to generate highly active stabilizing mercaptide species. These "latent mercaptides" offer unique performance attributes relative to conventional mixed-metal systems or organotin mercaptides.

Because the sulfur-bearing compounds are bound to organic protecting groups, they do not exhibit any adverse interactions that would occur with conventional sulfur ligands. As a result, the Advaflex systems have very low odor and volatility. They are hydrolytically and oxidatively stable, and are compatible with a broad range of additives for PVC. Compared with Ba/Cd, Ba/Zn, and Ca/Zn, the stabilizers also show equivalent or even superior performance in both clear and pigmented systems.

Very low metal content

A totally organic product, Advaflex LM-300 can be utilized in various ways. Most significantly, it can be combined with low levels of conventional metal carboxylates to provide highly efficient stabilizer systems.

The simplicity of these two-component systems makes it convenient for processors of flexible PVC to design their own unique stabilizers depending on their varied processing needs. This approach can potentially reduce material inventories and give the user complete control over stabilizer composition and performance, making it especially desirable for those processors and compounders who currently use multiple stabilizers for different processes and applications.

Formulation of the two-component stabilizers simply entails combining Advaflex LM-300 with any of several metallic species, most preferably zinc salts.

Although any number of zinc salts can serve as the metal source, zinc octoate or zinc stearate are the most convenient. Advaflex LM-300 allows very low levels of metal to be employed. In fact, certain systems have been developed that utilize as little as 0.5% zinc as their sole source of metal (versus 10-12% Zn with standard mixed-metal stabilizers).

Ultimately, the optimal ratio of metal to Advaflex LM-300 depends on the PVC formulation, application, and heat-stabilization requirement. As demonstrated in Figs. 1 & 2, systems can be optimized to meet or exceed the performance of current Ba/Zn/P and Ca/Zn technologies at a competitive cost. These thermal stability trends have been observed in both dynamic and static testing.

The formulation used for the dynamic tests shown here is as follows:

Dioctyl phthalate, 55.0 phr
CaCO3, 10.0 phr
TiO2, 5.0 phr
Epoxy soybean oil, 3.0 phr
Stearic acid, 0.2 phr.

Alternative uses

For processors that do not require multiple stabilization systems or do not care to utilize the two-component approach, Morton also offers a drop-in replacement. Available as the Advaflex LM- series, these are uniquely formulated stabilizers containing Advaflex LM-300 and the desired amount of zinc component.

Another potential way to use Advaflex LM-300 is as an adjunct to the primary metal stabilizer. Under this scenario, reductions of up to 75% in the use level of the primary stabilizer have been observed. This approach is effective with both Ba/Zn/P and Ca/Zn. The performance of Advaflex LM-300 with a reduced level of Ca/Zn is shown in Fig. 3.

This new approach to PVC stabilization is consistent with many laws in the U.S. mandating reduced use of toxic heavy metals. Toxicity screenings on this new chemistry are now complete. Based on the results, Morton International is optimistic about the future use of Advaflex in FDA-regulated applications.

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