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A new technology of PP thermo-Oxidative Stability
AddTime:2016-01-06

Thermo-Oxidative Stability of Black Pigmented
Polypropylene at Elevated Temperatures
New Technological Advances and Innovations in
Black Pigmentation and Stability of Polyolefins

 

Abstract:

The  dominant  means  for  black  pigmenting  polyolefins  for  over  thirty  years using  polar carbon black has finally been challenged by an alternative non-polar black. While carbon black is known in the art to significantly enhance ultraviolet stability of polyolefins it too has a profound negative effect on the thermo-oxidative stability at elevated temperatures by ASTM D 3012.

The  use  of  biaxial  rotation  at  150℃  during  the  test  increases  the probability that  all specimens will be exposed similarly and the effect of temperature gradients in an oven will be minimized. The test method is used to access automotive under the hood stability and compliments testing done by oxygen induction time at 190℃.

Although  changes  in  the  selection  and  availability  of  carbon  blacks changed  over  time little  improvement  in  elevated  thermo-oxidative  stability has  resulted.  The  reliance  on primary and secondary antioxidants continues to be the only means to enhance long-term heat stability (L.T.H.A). The known limitations of these stabilizers has been the subject of numerous studies over the years and found to decrease with concentration reaching a maximum effectiveness.  In  addition  their  performance  falls  off  dramatically  in  recycled waste streams that are back integrated into automotive applications. These short falls can be  dramatic  and  have  been  reported  and  studied  to  be  as much  as  40%  reduction  in thermo-oxidative stability.

This  limitation  has  finally  been  solved  with  the  development  of  a  new  non-polar  black colorant  without  the  absorptive  and  adsorption  characteristics of known  polar  carbon blacks.

These  characteristics  allow  the  integrity  of  the  primary  and  secondary antioxidants  to stabilize the amorphous zones of the polyolefins without being compromised by known surface effects. These reactive surface chemistries on additives’ start by first being adsorbed and then absorbed by carbon black then deactivating the additives. The new non-polar black is not reliant on co-additives that are typically added to carbon black pigmented polyolefins to coat the black particles reducing their physical chemical effects on LTHA. Nor addition of polymer soluble dyes to enhance Jet of carbon black pigments.

The consequences of this new technology can replace the carbon black entirely and will compliment carbon black. Jet ness of black coloration is not compromised and is instead increased without the use of black dyes traditionally added with carbon black which have led to plate out of the dye on molded surfaces. Profound increases in L.T.H.A at 150℃ followed by equivalent increases in O.I.T (oxygen induction time at 190℃) have resulted in linear increases in thermo-oxidative stability to polypropylene.

The following paper will show the usage of this TOSCA registered substance compared to traditional carbon black systems in polypropylene homopolymer significantly bridge the inferior stability the industry has had to contend with over the last thirty years. In addition to solving an age old problem in our industry the solution is economically viable.

Introduction:
Historically black pigmented polypropylene has evolved very little over the last thirty years with regard to improvements in long term thermo-oxidative stability at elevated temperatures. However, the type of carbon black in polypropylene and other polyolefins have evolved due to Federal Regulations governing the use of select carbon blacks.

Polyolefins are subject to thermal and thermo-oxidative degradation and have significant limitations for long term high temperature applications without proper fortification. Isotactic polypropylene is especially sensitive to oxygen and ozone attack. As a protection against degradation, mixtures of additives consisting of one or more primary antioxidants and a synergist are typically used. However, in filled systems co-additives are typically added to aid in the systems ability to deal with fillers that adsorb and absorb additives from the amorphous zone where they reside.

Thermo-oxidative stability of polypropylene is dependent on many variables that need to be taken into consideration when considering a stabilization system. Besides the obvious factors of test temperature, thickness, oxygen pressure, molding conditions, metallic poisoning, antioxidant levels, ratio of synergist to primary antioxidant, and acid acceptor (buffer) additional details need to be observed. These include the proper handling of the samples prior to testing to prevent oil contamination from the operator which causes sites of premature degradation during high temperature testing and avoidance of contact metals like copper, iron, cobalt, that are known to cause premature contact degradation during testing. This is especially noteworthy when implementing ASTM D 3012 circulating oven testing of suspended samples.

Stress induced on the molded part versus compression molding of the polyolefin is relevant in all testing at elevated temperatures. Compression molding and testing does not induce stress on the part. The lack of stress typically allows for higher values of lifetime during testing than stress induced by injection molding the samples. Furthermore, the injection molding conditions induce more real life testing of the plastic and provide a heat history and degree of crystallinity not seen in compression molded parts.

Although black pigments like channel black are known to adversely affect the long term elevated thermo-oxidative stabilization of polypropylene so do other pigments. A well-known study by Christopher W. Uzelmeier Ph.D. in S.P.E Journal May 1970, Volume 26 documented the affects of various colorants on the heat stability of stabilized polypropylene. His study clearly showed the catalytic affects of Pigment Red 177, Pigment Red 122, Pigment Blue 15 : 1, Pigment Green 7, Rutile titanium dioxide, Pigment Blue 29, iron oxide tan; etc. This study categorized the antagonisms of these colorants and their relative class on being pro-degradants on polypropylene. In this study Dr. Uzelmeier clearly categorized channel black, Pigment Red 177, Pigment Red 122, Phthalocyanine blue and iron oxide tan as being severe pro-degradants in polypropylene. Those with moderate effects were Pigment Blue 29, Phthalocyanine green, and chromium oxides. While cadmium yellow, Rutile titanium dioxide was considered slightly pro-degradant.

Additional work done by Zenjiro Osawa and Takashi Saito of Guma University, Japan in late 1975 showed the effect of transition metals on the oxidative degradation of isotactic and atactic polypropylene. They were able to rank order the catalytic effects of metallic stearates melt compounded in the resin on thermo-oxidative degradation. This work gave further insight and understanding on the influences of metals on the oxidative stability of polypropylene. It also showed that thermal stability of the polypropylene is dramatically effected by the concentration of metals like copper. In the presence of 0.0032 moles copper stearate the oxidation of the polymer was faster at the early stages of oxidation than of the polymer containing no copper. However, at a high concentration of copper for instance 0.0079 moles, thermal oxidation of the polymer was almost inhibited.

Since the early 1970's very little work has been published on improving the elevated heat stability of carbon black pigmented polypropylene. The known limitations of antioxidant type, and usage levels on solubility, adsorption and absorption effects by the carbon black have compounded the problem. Furthermore, the loss of a historical perspective on the work already done and cut backs on research and development in our industry provides little incentive to work on the problem.

Recent efforts in the area of molecular modeling and in-situ chemical interactions in polymers by computer analysis have led to studies on alternative black substances and non-warping pigments that would either compliment or replace carbon black in polyolefins. These efforts led to the development and patenting of an oxidized, non-cationized, non-silylated, sulfur black pigment. The black pigment is non-polar and has none of the adsorptive and absorptive properties of carbon black. The novelty of this substance in polypropylene is not the black pigmentation imparted to the resin but its unique chemistry with classical primary, secondary antioxidants and proprietary co-additives to significantly enhance long term thermo-oxidative at 150℃ and oxygen induction times at 190℃.

The development and refinement of this new substance which is outlined in U.S Patent application # 20020156165, October 24, 2002 makes for a commercially significant contribution to a thirty year old problem being finally solved. This application is now an official U.S. Patent as of March 25, 2003. The patent number is 6,538,056. Commercial implementation of this technology is now ready for marketing in those areas of concern were elevated long term thermo-oxidative stability of black pigmented polyolefins is required.

The subject contents of this paper provides for the foundation of further innovation and understanding of this technology in other polymeric systems in the future.

Experimental:
The study was accomplished in several phases to capture the full potential of the technology. First stage was to reproduce the years of reported premature failures associated with black pigmented polypropylene. This was done by implementing ASTM procedures for Long Term Heat Aging stability (L.T.H.A), at 150C using a rotating assembly in a circulating air oven and correlating these numbers with Oxygen Induction Times (O.I.T) at 190C. Since stress is a key factor in the determination of practical end use applications like injection molding we melt compounded all systems on a 30 mm APV twin screw extruder to insure good mixing, followed by injection molding on a 30 ton Boy Injection molder. Analytical procedures were implemented to determine final levels of additives present to insure the results we were observing. In addition a combination of GCMS, Ion Chromatography, and other proprietary analytical techniques were utilized to study and insure the level of consistency in black stabilizer used in the study. Additives used in the study included several commercially known products past and present in Carbon Black pigmented polyolefins in the U.S / NAFTA and Europe. These included CAS number 27676-62-6 or chemically 1,3,5-tris (3,5-di-tert butyl-4-hydroxybenzyl)-s-triazine-2,4,6-(1H, 3H, 5H)-trione, CAS number 40601-76-1 or chemically 1,3,5-tris (4-tert butyl-3-hydroxy-2, 6-dimethyl benzyl)-1, 3, 5-triazine-2, 4, 6-(1H, 3H, 5H)-trione, CAS number 693-36-7 chemically distearyl thiodipropionate, CAS number 65447-77-0 chemically dimethyl succinate polymer with 4-hydroxy-2, 2, 6, 6, -tetramethyl-1-piperidine ethanol, CAS number 70624-18-9 chemically Poly [[6-[(1,1,3,3,-tetramethyl butyl) amino]-1,3,5-triazine-2,4-diyl] ]](2,2,6,6,-tetramethyl-4-piperidyl) imino] hexamethylene [(2,2,6,6,-tetramethyl-4-piperidyl) imino]], CAS number 6683-19-8 chemically Tetrakismethylene (3,5-di-tertiary butyl-4-hydroxy hydrocinnamate) methane, CAS numbers 64338-16-5 & 106-89-8 chemically a oligomer obtained by the reaction and subsequent thermal oligomerization of 2,2,4,4-tetramethyl-21-oxo-7-oxa-3, 20-diazadispiro [5.1.11.2] heneicosane with epichlorohydrin (CAS# 106-89-8), CAS Number 32509-66-3 chemically Benzenepropanoic acid, 3-(1,1-dimethylethyl)-beta-[3-(1,1-dimethylethyl)-4-hydroxyphenyl]-4-hydroxy-beta-methyl-1,2-ethanediyl ester. Acid Acceptors used in the study were classic metallic stearates, metallic oxides, hydrotalcite, and fatty acid amides.

Using combined molecular modeling techniques from Accelrys and computational chemistry we were able to follow the course of our progress and make mid course corrections in each of the six phases required to successfully accomplishing this research. All formulations were carried out in Himont 6301 or 6501 polypropylene homopolymer. Pigment Black 6, a finely divided form of carbon black commercially produced by the incomplete combustion or thermal decomposition of natural gas or petroleum oil was used in the study. The CAS number of the carbon black was #1333-86-4 (EINECS: 215-609-9). All melt compounding was done at 225-230℃ melt temperature from powdered mixtures of additives and pigment. Calcium stearate was common to the majority of the formulations unless otherwise mentioned. Melt Extruded pellets were injection molded at similar conditions on a 30-ton Boy Injection molder into 95 mil chips for testing. Four samples from each formulation were suspended in a circulation air oven at 150℃ with one volume change of air per minute. Samples were suspended using a modified ASTM procedure, which call for Teflon tape over the clip followed by aluminum foil. The sample was suspended from a clip putting pressure on the foil. At no time did metal contact the plastic. The presence of Teflon on the plastic supported by 10-mil aluminum foil insulated the clip from the plastic. Failure at 150℃ was determined by first signs of embrittlement and or surface crazing. Oxygen Induction Time (OIT) at 190℃ was done by two independent labs on duplicate samples each keeping sample weight constant for each determination. Sample weights used were 5—6 mg maximum. Induction times in minutes were reported while failure at 150℃ was reported in days. In all instances controls were included based on traditional carbon black using the same stabilization systems.

Based on initial testing a refinement in post synthesis procedures for the black stabilizer were done to remove additional impurities which appeared to antagonize and randomize results. This course of action was based on a thirty-year historical perspective with carbon black in the industry and the relationship between physical chemical properties of carbon black on thermo-oxidative stability. The key worry was pH and the form present. So, in further studies we started modifying systems with various acid acceptors based on current theory past and present. Using modeling and additional computational tools we refined the formulations that provided superior thermal properties over carbon black. In each case duplicate controls with similar formulations using carbon black were added. The results of thermo-oxidative testing were integrated back into the molecular model to make further predictions based on theoretical results. In this way we were able to adjust our model parameters to order the theoretically weighed variables to the actual results. In addition using additional computer structural modeling techniques we were able to determine in-situ side reactions that allowed us to adjust catalyst levels in the system.

To compliment these studies additive adsorption and absorption studies in solution were done using the same experimental design as Chris Uzelmeier Ph. D. From these studies further understanding and guidance was achieved to insure the proper course of our investigation.

In the final phases of our study we were determined to continue to reproduce the results from select systems from former studies. Therefore, we would reformulate and or take retained samples and place them back into the oven for study. This procedure insured and reinforced our confidence in the failure times and mode of failure being observed for each system.

From these experimental results we were able to optimize the process and chemistry for the non-polar black to provide us reliable results for future sampling in the market.

Results:
The most notable observation from injection molded plaques was the repeated Jet-Black Color seen when sulfur black was compared with Carbon Black. It is known in the trades that addition of blue and black dyes are added during compounding of carbon black to enhance the Jet of the pigment. In addition particle size has a strong influence on the Jet color associated with black pigments. Analysis of the particle size of the sulfur black alone was enough to provide for the observed Jet black color.

Figure 1 shows the relationship between carbon black and sulfur black on the thermo-oxidative stability at 150℃. Alone both colorants provide no stability to the polyolefin matrix. However, addition of low levels of an ester of propionic acid alone in the absence of a primary antioxidant gives a significant boost to L.T.H.A (long term heat aging). The increase in concentration of the carbon black typically decreases L.T.H.A in this case and in cases were a primary antioxidant is added. The adsorption and absorption behavior of the carbon black is reportedly responsible for this behavior.

The figure shows that sulfur black when compared to carbon black is more synergistic with the ester than carbon black without the noted decrease with concentration of pigment. The combination of the non-polar characteristic and the low oil absorption relative to carbon black both account for this behavior. Oil absorption of the sulfur black used in this study is 100 times lower than carbon black.

Figure 2 illustrates the influence of four primary antioxidants on L.T.H.A of carbon black pigmented polypropylene stabilized with primary and secondary antioxidants.

Antioxidant AO-1 and AO-3 performance decreases with increasing levels of carbon black. The same is observed with AO-4 to a lesser extent. However, AO-2 shows lower performance overall the influence on carbon black concentration appears negligible.

Figure 3 illustrates the influence of adding catalyst C-1 to the same formulations in Figure 2. The presence of catalyst in these systems increases the L.T.H.A at low levels of carbon black i.e. 0.50% but does little for higher levels of carbon black of 1.0%. This observation appears across all antioxidant chemistries in our study. Again the best performing antioxidant in carbon black pigmented polypropylene is again AO-4.

Figure 4 illustrates alternative catalyst on the performance of antioxidant AO-1. Clearly all three-catalyst types do nothing for the L.T.H.A performance of 1.0% loading of carbon black. At lower levels of 0.50% carbon black only C-3 catalyst provides an increase in performance. Overall, enhancement of the performance of AO-1 falls short of expectations.
Historically this antioxidant shows the same behavior seen in other studies over the last thirty years.

Figure 5 extends the study of various catalysts with Antioxidant AO-2 that is reported more resistant to acid environments according to its vendor than AO-1. The data shows a more uniform performance increase at carbon black levels of 0.50% with type of catalyst used. Again C-3 with AO-2 gives the best L.T.H.A. However, we see little performance enhancement when higher levels of carbon black are used.

Figure 6 shows the negative reaction of adding any of the three catalysts used in this study. When compared with the control containing primary and secondary antioxidant the catalyst does nothing at either level of carbon black in polypropylene. This antioxidant is known for its performance at very low levels with esters of propionic acid. A further study of this antioxidant at very low levels may be warranted in the future.

Figure 7 represents the last figure showing the relationship between each catalyst on the performance of the various antioxidants in the study. This figure shows that only catalyst C-3 had any positive influence on this antioxidant while catalyst C-2 had a negative influence on L.T.H.A. The slight advantage of C-3 on L.T.H.A versus the control would not warrant the added cost using this antioxidant system.

However, it is interesting to note the relationship between catalyst performance and type of catalyst used in the study on carbon black pigmented polypropylene. Although the level of performance of this antioxidant currently passes the 1,000 hour 150℃ L.T.H.A requirements it shows borderline stability. The statistical error of 20% with oven aging data including variability due to handling, etc. makes this system vulnerable without its ability to extend beyond system requirements.

This same relationship differs with oxidized sulfur black used in this study in both performance and consistency in each relationship with antioxidant. Furthermore, addition of classical acid acceptors used in the plastics industry today provided little or no improvement on L.T.H.A of polypropylene. However, in this study we discovered that instead of using buffers to control acid as is typically practiced with carbon black based systems, the presence of the acid was beneficial. The key was the type of acid formed in-situ with the primary and secondary antioxidants and to control its rate of formation on a consistent and reproducible basis. The discovery of a suitable catalyst was the outcome of our studies with buffers.

 

Figure 8 illustrates the significant improvement over carbon black pigmented polypropyulene with AO-1 and AO-2. Both antioxidants respond well to both concentrations of sulfur black within experimental error of the test. Furthermore, the cushion of stability beyond 1,000 hr requirement is beyond experimental error.

Figure 9 shows that influence of catalyst C-1 on both antioxidants. Clearly AO-1 has a positive influence with catalyst C-1. Antioxidant AO-2 showed little response especially at higher levels of Sulfur Black. These results are in sharp contrast to carbon black data that showed a negative influence on increasing levels of black and only a slight advantage at lower levels of carbon black.

Figure 10 provides us a look at the influence of antioxidant concentration with constant thioester level with 1.0% sulfur black. It is known in the trade that the ratio of AO to Thioester in L.T.H.A at 150℃ is critical with total loading being constant. A relative ration of 1 : 6 exists when using 0.10% AO and 0.6% thioester. A relative ration of 1 :12 exists when using 0.05% AO to 0.6% thioester. We see doubling concentration levels of AO-1 only increases L.T.H.A by 9 days from 43 to 52 (1,032 to 1,248 hrs.). However addition of catalyst C-1 improves the performance of both systems with the higher concentration of AO-1 giving the best results. We see at these levels a 21day increase with only 0.1% catalyst addition from 52 to 73 days (1,248 to 1, 752 hrs.). Clearly superior performance over a carbon black pigmented system using the same antioxidants

Figure 11 extends the date beyond AO-1 to AO-4. In this graphic we are again using 1.0% sulfur black and a low level of primary antioxidant e.g. 0.05% with 0.6% thioester. The effect on AO-1 is positive while the effect of AO-4 is not observed.

Figure 12 extends this study into combinations of antioxidants with 1.0% Sulfur Black in polypropylene. We see that AO-1 responds well to catalyst C-1 at an active level of AO-1 at 0.10% final. However, AO-2 is less active in the presence of a catalyst at the same concentration of 0.10% AO and catalyst. When we combine 0.05% each of AO1 : AO2 for a total of 0.10% final we get a 15% increase in L.T.H.A over the AO-1 system and 17% compared to AO-2. Is this within experimental error…. Maybe? When catalyst is added we see virtually the same performance as the AO-1 system but higher if we compare this with the performance of AO-2 system results. Although novel these results alone do not warrant blending of a cheap antioxidant with a more expensive antioxidant for commercialization. A more effective approach from the data is the use of a single antioxidant in the system.

Since performance is measured today in more than one method we looked at Oxygen induction Time at 190℃ on 5-6 mg cut pellets and molded plaques. The relationships between O.I.T and L.T.H.A at 150℃ are not ideally related. Clearly the numbers in minutes at 190℃ which is 30℃ higher than the melting point of polypropylene are lower than for days at 150℃ in a circulating air oven. The two methods measure distinctively different physical chemical processes. One in the molten state for on-set to oxidation or consumption of antioxidant while the other is oxygen diffusion limited and measures degradation in the solid state.

Figure 13 compares two carbon black systems against two sulfur black systems both having 0.50% black in the formulations. Sulfur black and Carbon Black systems SB-1 and CB-1 contain AO-1 and S-1 combinations with 0.10% catalyst C-1. Systems SB-2 and CB-2 containing the same antioxidant systems but without the catalyst present. Clearly the relationship between L.T.H.A and O.I.T is not a 2 : 1 or 1 : 1 Relationship rather something that falls between the two. Both measurements are good indicators of degradation but must be used with good judgement to make sense of the numbers.

To further illustrate the significance of black pigmentation on the thermo-oxidative degradation of polypropylene at 150℃ we see in Figure 14 the differences between non-pigmented and pigmented systems. Without stabilizers natural and black pigmented polypropylene looses physical chemical integrity in less than 24 hours. With minimal stabilization using 0.10% of a typical antioxidant like AO-1 we boost L.T.H.A at 150℃ by 40X while addition of carbon black reduces that same stability by over 75%. Oxidized non-silylated sulfur black also reduces the L.T.H.A by 65%. However, when an ester of propionic acid is used alone in polypropylene it still acts as a secondary antioxidant. With carbon black present there is still a reduction compared with natural non-pigmented resin but with sulfur black present we see at levels higher than 0.5% a linear increase in performance. From 35 days in natural to 21 days with 0.5% CB and 18 days with 1.0% CB. In sulfur black systems we go from 35 days in natural to 23 days at 0.5% and 40 days at 0.10% SB. Just the opposite of carbon black systems.

Furthermore, in combination with AO-1 we see an increase in L.T.H.A to 101 days at 150℃. Addition of 0.5 and 1.0% carbon black reduces thermo-oxidative stability down to 28 and 25 days respectively. This is a drop of 75%. In comparison with Sulfur based technology we go from 101 days in natural to 64 and 78 days using 0.5 and 1.0% SB respectively. This is only a drop from 37 to 23% as we increase sulfur black. The differences in carbon black versus sulfur black again relate to the physical chemical differences in the two pigment chemistries and their reactivity with the primary antioxidant in the system which changes the synergism ratio as reactivity changes in-situ during exposure at elevated temperatures.

As part of our research we also investigated various chemical modifications besides physical chemical changes in the process of producing the product. The most successful outcome of these studies was to for a metallic salt via carboxylation of sulfur black. Figure 15 illustrates this success by comparing standard sulfur black with carbon black and both against a carboxylated metal salt of sulfur black. Again we see the superior performance of Sulfur black versus Carbon Black. However, when the carboxylated salt is compared with standard non-carboxylated sulfur black we see additional improvement in performance. By changing the chemistry we can gain 15 days with AO-1, 33 days with AO-2 or 28% and 64% increase respectively. However, if catalyst C-1 is added we see further improvements of >33 days or more than doubling the performance of SB with AO-1 and 48 days with AO-2 a 109% increase over non-carboxylated catalyzed sulfur black. These improvements are currently being further optimized and cost performance benefits being accessed.

Oxygen Induction Time measurements at 190℃ differ dramatically from Long Term Thermo-Oxidative Stability measurements in a circulating air oven at 150℃. Although both measure degradation one is more dependant on solid phase degradation while the other measures degradation in the molten state at 190℃. Since O.I.T is a requirement by some customers as an accelerated technique and an official A.S.T.M we spent the time and effort to generate the data for comparative purposes to oven testing at 150℃.

Figure 16 illustrates the relationship between antioxidant type with a set concentration of 0.10% and 0.6% thioester in formulations containing either 0.5% or 1.0% carbon black. In this chart we see that regardless of the additive type and level of carbon black the level of performance rarely goes above 25 minutes at 190℃. Therefore, the relationship between this chart and a chart showing L.T.H.A at 150C do not correlate very well with each others perception of the additive systems used.

Figure 17 shows that using a catalyst with the antioxidant systems illustrated only reduces the performance or lowers induction times. Again these results are within experimental error of the test. However, they show a trend downward rather than enhanced performance and compliment earlier slides of oven testing which also show that catalyst C-1 did little to enhance long term oven aging performance a 150℃.

Figure 18 clearly shows the enhanced performance of standard and catalyzed sulfur black (B298-299) pigmented systems versus comparable carbon black pigmented systems in both concentration of additives and pigment. These results also compliment the enhanced oven testing results seen at 150℃. They also fall outside the range of experimental error especially when independent labs provide similar results.

Besides this data we continue to perform additional testing on other antioxidants to broaden the scope of understanding and to continue to increase the envelope of stability.

Conclusions:
Carbon Black pigmented polypropylene continue to perform as they have for over thirty years. Theses studies continue to confirm the reproducibility of oven aging experiments at 150℃. We continue to see same trends and level of performance in days to failure. We continue to reproduce the performance relationships between various antioxidants and rank order of performance in both concentration and ratio of thioester and concentration of carbon black in the plastic. Mode of failure is consistent with embrittlement and filled plastic at 150℃.

We have extended the poor relationships between oxygen induction times at 190℃ and Thermo-Oxidative Testing at 150℃ established by others in the field. Again these results prove that O.I.T at 190C and L.T.H.A at 150℃ for carbon black pigmented polypropylene show a poor correlation.

We have reproduced the work of Christopher Uzelmeier; Ph.D., which shows that carbon black, has adsorptive and absorptive characteristics that chemically compromise the antioxidant in the polyolefin resin. Thereby reducing the overall performance as a long-term heat stabilizer at elevated temperatures.

We have shown that oxidized sulfur black as practiced in U.S. Patent 6,538,056 produces a significant advancement in long term elevated temperature stability of polypropylene both in solid state but also in the molten state. Elevated stability at 150℃ has been extended far beyond the 1,000 hour 150℃ requirement set forth by Auto Industry. Today we are able to double the lifetime at this temperature. By adjusting the proper catalyst and antioxidant type we can increase the lifetimes beyond 2,000 hours at 150℃.

We have shown we can impart excellent Jet-Black to the plastic without the utility of dyes, which are typically added to carbon black to change shade of black.

We have shown that oxygen induction times at 190℃ come closer to correlating with long term thermo-oxidative stability at 150℃ than with carbon black pigmented systems. This added performance over carbon black is in part related to the non-polar character of sulfur black and its lower oil absorption Character compared with carbon black.

We have also shown that further treatment of oxidized sulfur black in U.S. Patent 6,538,056 by conversion to a carboxylated metal salt will extend the utility of the chemistry. This has exciting and far-reaching possibilities for the future of sulfur black type chemistries for the treatment of other plastics in our industry.

Although the full extent of the chemistry and the mechanism of action is not fully understood the work established in this first definitive publication lays the foundation for future work beyond black to other colorants from the same class.

 


 

 

 

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