A Novel Additive Concept for the Stabilization of ABS

A Novel Additive Concept for the Stabilization of ABS A Novel Additive Concept for the Stabilization of ABS Alex Wegmann Ciba Specialty Chemicals Inc., Plastic Additives Segment, CH-4002 Basel / Switzerland. alex.wegmann@cibasc.com ABSTRACT The grafted rubber phase of emulsion ABS has to be stabilised very efficiently against oxidation by hindered phenols, alone or in combination with secondary antioxidants (e.g. phosphites or thioethers), especially during the drying process. Traditionally, solid products with high melting points were used, but they are not easy to bring into an aqueous formulation. A new liquid antioxidant, besides giving excellent thermal stability to the ABS graft phase, can be dosed with a low margin of error and easily emulsified. Such an emulsion shows very good storage stability, and mixes well with the aqueous ABS latex. The liquid supply form brings evident handling advantages, and the product has a very good toxicological profile, as well as broad food contact approvals in styrenic copolymers. To reduce the initial colour of ABS after processing (extrusion, moulding), new synergistic process stabilisers, based on lactone chemistry, have been developed. These new stabilization concepts for ABS graft phase, and ABS compounds help to improve their technical performance significantly. Presented at the 2 nd International Conference on Polymer Modification, Degradation and Stabilization, 30 June - 4 July, 2002, Budapest, Hungary, and at the ADDCON World 2002 Conference, 22-23 October 2002, Budapest, Hungary. 1. INTRODUCTION ABS (Acrylonitrile-Butadiene-Styrene) is an amorphous, heterophasic polymer with very good mechanical properties, especially high impact resistance. It consists of a polybutadiene (BR) phase, dispersed in the form of small particles in a SAN (Styrene-Acrylonitrile) matrix. The BR particles are grafted with SAN (Figure 1). The grafted SAN chains form a link between the BR particles and the SAN matrix. These SAN anchors are very important for absorbing and dissipating impact energy, and thereby very much responsible for the excellent impact strength of ABS. The size of the rubber particles is also crucial for the performance of ABS. Figure 1 Structure of ABS polymer Polymers & Polymer Composites, Vol. 11, No. 2, 2003 145

Alex Wegmann However, the unsaturated rubber phase is also the weak spot of ABS in terms of thermal and UV lightinduced degradation. The double bonds are prone to oxidation, followed by a chain scission of the polymer, whereas oxidation at the graft site leads to degrafting of SAN. This causes a loss of impact strength and discoloration of the ABS. Peroxidic cross-linking of the BR chains is also possible. The SAN matrix is more stable, although some autoxidation of styrenic and acrylonitrile units can nevertheless take place, as well as a non-oxidative degradation of acrylonitrile. All of this leads to yellowing of the polymer. All these degradation processes can be, if not completely prevented, at least considerably reduced by the proper use of additives. Most ABS is produced by an emulsion process, and the rest by mass polymerization. Mass polymerised ABS is the cleaner product, with better colour, lower rubber content and, therefore, slightly lower impact strength. The mass polymerization is a straightforward but nevertheless rather expensive process. Regarding the need for stabilization during production, moderately performing antioxidants are sufficient. Mass ABS is used predominantly in the automotive industry where the low colour is a big advantage. Emulsion ABS, on the other hand, has a higher rubber content, smaller particles and, therefore, higher impact strength and gloss. Because emulsifiers, coagulation aids, and catalyst residues can end up in the final product, the emulsion process leads to a less clean polymer with a higher yellowness index. Emulsion ABS is used predominantly in the household, toy, and electronics industries. The production process involves several steps (Figure 2): polymerization of butadiene to form a BR latex, grafting of the BR with styrene and acrylonitrile, and stabilization of the formed ABS graft phase, also called high rubber graft (HRG), with an antioxidant emulsion. The stabilised ABS graft latex is then coagulated either with strong acids (e.g. sulfuric acid) or inorganic salts (e.g. magnesium sulfate), and the solid ABS-HRG is separated from water and dried. The resulting powder is compounded with SAN and pelletized to give ABS polymer. The higher the proportion of ABS-HRG, the better the impact strength, but also the softer the polymer. Modern production lines often use dewatering extruders for the drying and compounding with SAN. But still, conventional air drying is mostly used. In this drying step, the small polymer particles of the graft phase, containing a high amount of unsaturated rubber (40-60%), are exposed to oxygen and high temperatures. Spontaneous oxidation of the BR part can lead to discoloration and, in the worst case, to fires in the dryer or the storage silo. Therefore, high performance stabiliser packages are essential. Highly efficient hindered phenol antioxidants have to be added, often in combination with thiosynergists, as emulsion prior to coagulation. Further additives, especially process stabilisers, can be added in compounding. 2. STABILIZATION OF ABS HIGH RUBBER GRAFT There are some critical parameters influencing the thermal stability of the ABS-HRG. As already mentioned above, a high rubber content (many oxidizable carbon double bonds) and small ABS- HRG particles (high internal surface area) lead to lower thermal stability. An efficient stabiliser package Figure 2 Production scheme of ABS emulsion polymerization process 146 Polymers & Polymer Composites, Vol. 11, No. 2, 2003

A Novel Additive Concept for the Stabilization of ABS can considerably improve the thermal stability. A very good quality of the antioxidant emulsion (homogeneous, small particle size) is crucial. So far, emulsions of solid stabilisers (hindered phenols, thiosynergists) have been used. Drawbacks of this concept include the necessary handling of solid products, and problems associated with the production, storage and transport of emulsions made from solids, mostly with high melting ranges. The novel concept of using liquid hindered phenols 1 brings substantial advantages: there is higher safety and improved worker s hygiene by handling a liquid instead of solid antioxidants, as well as avoiding hazardous organic solvents. Regarding economical factors, there is an enhancement of plant productivity because preparation of emulsions from liquids is easier, and needs less energy. The emulsions can be stored and pumped at ambient temperature. The waste management is facilitated because boxes and bags, used for solid products, are eliminated. Functional advantages include the accurate dosing of the liquid antioxidant when preparing the emulsion, as well as the excellent thermal stability of ABS-HRG and ABS compounds, achieved with the liquid antioxidant emulsion. DSC (Differential Scanning Calorimetry) can monitor the change in enthalpy associated with the oxidation of the BR. It has proven to be the best method to evaluate the influence of an antioxidant on the thermal stability of the ABS-HRG. As can be seen in Figure 3, it is possible to run the test by either steadily increasing the temperature until the exothermic reaction takes place (dynamic method), or by exposing the sample at a constant temperature and monitoring the time to the exotherm (isothermal method). This paper reports the values at the beginning and/or the maximum of the exothermal reaction. In Figure 4, various stabiliser packages have been Figure 3 DSC measurements of enthalpy (H) Figure 4 Isothermal versus dynamic DSC method Polymers & Polymer Composites, Vol. 11, No. 2, 2003 147

Alex Wegmann evaluated by both the dynamic and the isothermal method. It is obvious that the isothermal method shows bigger differences, and is, therefore, better suited for measuring and comparing the performance of antioxidants. Traditionally, solid hindered phenol antioxidants have been used for the stabilization of ABS-HRG, often in combination with thiosynergists, like dilauryl-thiodipropionate (DLTDP), di-stearylthiodipropionate (DSTDP), or phosphites, like trisnonylphenol-phosphite (TNPP). Some of the chemical structures are shown in Figure 5. A large variety of hindered phenols have been used up to now, including BHT (butylated hydroxy-toluene), and metilox derivatives, like OBP (IRGANOX 1076) or IRGANOX 245, another solid antioxidant. Figure 6 shows clearly that the liquid antioxidant (IRGANOX 1141) is far superior to BHT, OBP, or the solid antioxidant in protecting the ABS-HRG. The two components of the liquid antioxidant form a truly synergistic mixture (Figure 7). Also synergistic is the blend of the liquid antioxidant with DLTDP (Figure 8). This fact can be used to optimise the cost/performance ratio of the stabiliser package, since thiosynergists are less expensive than high performance hindered phenols. Before DSC equipment became readily available, the thermal stability of ABS-HRG was tested by the scorch (Figure 9) or the Metrastat (Figure 11) test. In both cases, the HRG powder is exposed to high temperatures until oxidative discoloration is observed. The two methods are still used today in the industry. The liquid antioxidant has food approval in ABS (FDA, Europe). Figure 5 Chemical structures of antioxidants Figure 6 Comparison of sterically hindered phenol antioxidants in thermal stabilization of ABS-HRG 148 Polymers & Polymer Composites, Vol. 11, No. 2, 2003

A Novel Additive Concept for the Stabilization of ABS Figure 7 Synergism of the liquid antioxidant Figure 8 Boosting effect of thiosynergists Figure 9 Scorch test for ABS-HRG Polymers & Polymer Composites, Vol. 11, No. 2, 2003 149

Alex Wegmann There are two standard methods for coagulating an ABS graft latex. One uses a strong acid (sulfuric acid), the other an inorganic salt (magnesium sulfate, or calcium chloride) in combination with a moderately strong acid (acetic acid). It was found that the thermal stability of ABS-HRG coagulated with magnesium sulfate is strongly dependent on the ph after coagulation (Figure 10). There is a minimal thermal stability at ph 7, whereas at lower and higher ph the thermal stability is good. However, when sulfuric acid is used as coagulant, then the thermal stability is more or less independent of the ph. This behaviour is not only apparent in DSC measurements, but can also be seen in Metrastat oven tests (Figure 11). A possible reason could be that magnesium salts reduce the thermal stability. Under strong basic or acidic conditions, magnesium salts can be solubilised and washed out, whereas under neutral conditions the salts remain in the ABS. 3. STABILIZATION OF ABS COMPOUNDS The stabiliser package should not only protect the HRG but also the ABS pellets after compounding. Figure 10 Influence of ph on the thermal stability of ABS-HRG Figure 11 Stability of ABS-HRG in a Metrastat oven ph 3.7 ph 5.0 ph 7.0 residence time in the oven (180 C) 150 Polymers & Polymer Composites, Vol. 11, No. 2, 2003

A Novel Additive Concept for the Stabilization of ABS DSC tests (Figure 12) at 180 C in air show that an exothermic reaction is hardly noticeable, because an ABS compound is less sensitive to oxidation than ABS-HRG. The reason is that, compared to the ABS- HRG, the amount of BR is lower (only about 15-25%), and the pellets are much larger than the particles in the HRG powder. Only at higher temperatures, and in oxygen, exothermic reactions are detectable. In these cases, however, the time to the exotherm gets very short. This means that it is difficult to observe performance differences between different antioxidants. Therefore, DSC does not seem to be a suitable method to test ABS pellets. A better method is oven ageing at 80 C of ABS plaques, made by compression or injection moulding. The long-term thermal stability gives an indication about the behavior of the polymer during the lifetime of the finished article. Figure 13 shows that the liquid antioxidant also gives the best performance in this respect. However, the differences between various antioxidants are much smaller than in the HRG. This means also that it is very difficult to further improve the colour of an ABS compound by changing the stabiliser package in the ABS-HRG. To improve the initial colour of moulded ABS plaques, it is better to add process stabilisers in the compounding step. A Figure 12 DSC of ABS compounds Figure 13 Thermal stability of ABS compounds (oven aging of 2 mm thick ABS plaques) Polymers & Polymer Composites, Vol. 11, No. 2, 2003 151

Alex Wegmann good indication for the efficiency of a process stabiliser is the difference of the yellowness index at two different moulding temperatures. The lower the difference, the better the stabiliser. Sophisticated, synergistic blends of process stabilisers based on lactone chemistry can give better cost/performance at low concentrations compared to traditional process stabilisers, like phosphites or phenol/phosphites (Figure 14). 4. SUMMARY AND CONCLUSIONS A liquid sterically hindered phenolic antioxidant gives excellent thermal stability to an ABS-HRG, besides being easily emulsifiable, and having broad food approvals. Emulsions based on liquid antioxidants have also good storage stability at ambient temperature. Isothermal DSC is the best method to evaluate the thermal stability of an ABS-HRG. The influence of the ph can be considerable, depending on the coagulation method. DSC is not a suitable technique to analyze the thermal stability of ABS pellets. Discoloration of moulded plaques after oven aging is a more reliable test. The difference of various antioxidants in thermal stability of ABS pellets is smaller than in thermal stability of ABS-HRG. This means the possibility of improving the colour of ABS pellets and/or finished goods by changing the stabiliser package in the HRG is limited. However, by using a synergistic process stabiliser blend to reduce the initial colour of moulded articles, it is possible to reduce the concentration compared to traditional process stabilisers and achieve reduced formulation costs. 5. REFERENCES 1. N. J. Earhart, New Developments in the Stabilization of Styrenic Polymers, SPE PMAD RETEC Conference, October 20 22, 1997, Ft. Mitchell, KY, USA Figure 14 Process stabilizers for ABS compounds 152 Polymers & Polymer Composites, Vol. 11, No. 2, 2003