Review of common rubber factory problems and published causes and solutions--part I.

Rubber is different from other engineering materials in that it is commonly subject to many unique processing problems that are not normally encountered with the processing of other non-rubber materials. In addition, the literature is somewhat limited in discussing some of these problems and their causes.

This article gives a review of the literature for some of the major factory problems encountered in the rubber production plant, with some suggestions for possible causes and solutions.

It is very important to realize that any change (or changes) applied to a rubber compound or process is certain to also affect, for better or for worse, many other compound properties (either uncured or cured). Therefore, any change that is made to a rubber compound or process should be thoroughly researched beforehand. These changes should first be tried out on a laboratory scale, determining how all the processing and cured physical properties are affected, including all compound specification properties. Also, there should be a limited factory trial performed to determine more clearly what affects this change (or changes) might have on the process and the rubber product. In addition, limited product field studies should also be performed to assure that there are no hidden long-term problems associated with these changes. All appropriate health and safety precautions should be followed. Only those with advanced scientific training and rubber compounding experience should implement such changes.

The following lists the factory problem areas reviewed in this article. This list certainly does not represent all the problem areas encountered when processing rubber. Also, the frequency of occurrence for these problems differs greatly from one production plant to another.

* Plant receiving area

--Cold flow

--Stability of pre-powdered blends

* Mixing

--Quality of mix

--Uncured elasticity (nerviness)

--Viscosity

--Dispersion

--Bloom

--Green strength

--Tackiness

--Stickiness

--Lumps

--Mill bagging

--Mill Back roiling

* Extrusion

--Die swell

--Extrusion rate (melt fracture)

--Appearance (surface smoothness of extrudate)

--Shear thinning

* Calendering 

--Blisters

--Calender release

* Molding

--Mold release

--Mold fouling

--Non-fills

--Porosity

--Shrinkage of cured parts

--Backfinding

Plant receiving area

The following are some common problems associated with the plant receiving area.

Cold flow

Cold flow is the gradual deformation from the force of gravity (or the weight of other bales) on bales of raw rubber. Different grades of rubber possess differing degrees of cold flow. For example, a bale of TSR 10 natural rubber is less likely to cold flow compared to a bale of a highly linear synthetic rubber. In fact, different grades of the same class of rubber can display differing degrees of cold flow. For example, highly linear cis-BR displays more cold flow than cis-BR with long chain branching (ref. 1) or divinyl benzene branching agents (ref. 2). Also, NBR grades with more narrow molecular weight distributions usually possess better cold flow resistance than NBR grades with more broad MWD (ref. 3).

Modern day shipping containers and film wrap have reduced or eliminated many of these cold flow problems; however, cold flow can still be a problem in certain areas.

Stability of pre-weighed powder blends

In the last 20 years, there has been a trend in the rubber industry to use pre-weighed blends of many of the powdered compounding ingredients in special dispersible poly bags, prepared either in-house or from an outside supplier, in order to improve productivity and quality while reducing dust in the factory.

When developing pre-weighed powder blends for factory use, one must take into account powder particle size, differences in density of these additives (to avoid stratification) and possible chemical reactions between different chemical additives after the powder blend has been made and stored. Some of these chemical reactions between the powdered additives can severely limit the useful storage life of the pre-weighed powder blend before it is used in the factory mixing procedure. Some examples of these dry chemical reactions are found with DPG mixed with sulfenamide accelerators (ref. 4) or CTP reacting with sulfenamide accelerators (ref. 5).

Mixing

The following are some common problems associated with the mixing procedure.

Quality of mix

Maintaining a consistent quality of mix (or state-of-mix) is very important to prevent various quality problems downstream. Controlling the total work history (energy at dump) during mixing is very important to maintain low batch-to-batch variation (ref. 6). Also, the order of addition of ingredients, the mixing scheme (single pass, multiple passes, upside down, right side up, etc.), type of mixer, rotor design, rotor speed, even or friction rotor speeds, water temperatures, type of cooling system, tip clearance, fill factor, etc., all have significant effects on the state of mix (ref. 7).

Uncured elasticity (nerviness)

Uncured compound elasticity typically decreases with increased work history during mixing. Different mixed batches can possess differing degrees of uncured elasticity (nerve) which can cause a stock to process differently downstream in the rubber factory. If a stock has received less work history (a poorer state-of-mix), it may be nervier, possess greater die swell during extrusion, or result in non-fills during injection molding (ref. 8).

In compound development, different elastomer bases will impart different degrees of nerve to a given compound. On the other hand, different elastomer bases may also break down at different rates during the mix. If a compound breaks down faster during the mixing process, it may ultimately possess less nerve (ref. 9). Lastly, compounds with higher loadings of carbon black and other fillers will tend to have less nerve and die swell (ref. 10).

Viscosity

Viscosity of a rubber compound is simply its resistance to flow. The higher the rubber compound's viscosity, the greater its resistance to flow in downstream processes. Rubber compound viscosity is generally measured by a rotational viscometer (Mooney), a capillary rheometer or a sinusoidally oscillating rheometer (commonly the RPA) (ref. 11).

Just as previously described for the uncured elasticity of a rubber stock, also the final batch viscosity will also decrease with additional work history during mixing. However, many times the compound viscosity does not drop as fast as the uncured elasticity does with increasing mixing time and work history (ref. 12). Also, when the compound viscosity is too high, various down stream problems can result, such as an increased frequency of non-fills in molding or extrusion problems.

Besides modifying various mixing procedures, there are many compounding techniques that are commonly used to lower a rubber compound's viscosity. For example, selecting a base elastomer with a lower average molecular weight (ref. 13), using a small amount of liquid elastomer (ref. 14), using more processing oil (ref. 15), using a lower loading of carbon black or a larger particle size carbon black or a lower structure carbon black (ref. 16), etc., all will result in a lowering of the compound's viscosity. Also, through the proper use of lubricant fillers (ref. 17), organosilanes with silica (ref. 18), surface treatment of fillers (ref. 19), selection of certain coagents for a peroxide cure (ref. 20), etc., can also reduce a compound's viscosity quite well.

Dispersion

Good dispersion of compounding ingredients is usually the outcome from an effective mixing process. Achieving good dispersion can significantly improve important cured compound properties such as wear or abrasion resistance (ref. 21).

Once again, for good carbon black dispersion, when the carbon black is added to the mix is very important to achieve a high level of carbon black dispersion (refs. 22 and 23). Many times, the type of carbon black that is used (high or low structure, fine or large particle size, high or low loading levels) has a lot to do with the ultimate level of dispersion that is practical (ref. 24). Even properties such as pellet hardness can have a great effect on dispersion (ref. 25). Sometimes it is necessary to have multiple mixing passes to achieve the necessary level of carbon black dispersion (ref. 26). The average molecular weight and the molecular weight distribution of the base elastomer(s) can also have an effect (ref. 27). Sometimes the wise use of carbon black masterbatches (such as SBR 1600 series) can help.

Silica is particularly difficult to disperse in many rubber compounds. Sometimes the proper use of some compounding additives may help in achieving a better dispersion (ref. 28).

Since curatives are usually added late in a rubber mix to avoid excessive heat history, they sometimes do not receive sufficient work history for good dispersion. This can hurt the compound's cured physical properties and cause quality problems (ref. 29).

Bloom

The term bloom is commonly used to describe the surface exudation or separation of certain compounding ingredients in either the uncured or cured state causing appearance problems. It is generally recognized that because of the chemical diversity of many compounding ingredients, these ingredients are not completely soluble in the base elastomers and may separate out (bloom) with time (ref. 30). Bloom that occurs with uncured stocks can cause downstream problems in the factory, such as loss of building tack. Bloom that appears on the cured product will usually cause problems with the customers.

The first priority in solving a bloom problem is to identify what compounding ingredient(s) is causing the bloom (the appearance alone may not be sufficient to make this identification). Once this identification is made, various corrective actions might be taken. The source of bloom can come from a large number of places including the in situ formation of zinc stearate (ref. 31), the type and level of sulfur (refs. 32-34), the type and level of accelerators and curatives (refs. 35 and 36), the type of AOs used (refs. 37-39), the type and amount of oil used (refs. 40 and 41), the type of mixing procedures and schemes (ref. 42), etc.

Green strength

This term simply describes the strength of a rubber compound in the uncured state. Sometimes it is important that a rubber compound possess a certain level of green strength in order to prevent a complicated uncured extruded profile from collapsing from the force of gravity or to prevent premature "blow outs" of a green tire on the second-stage tire building machine (ref. 43).

Sometimes poor green strength is caused by excessive mastication of the base rubber during the mixing process (ref. 44). Sometimes using a special phase mixing technique can improve a compound's green strength (ref. 45).

Compounds which usually give good green strength are many times based on natural rubber (because of strain crystallization) (ref. 46). However, for elastomers in general, one can achieve better green strength if the base elastomer possesses a higher average molecular weight (ref. 47) or sometimes a narrower molecular weight distribution (ref. 48). Also, with EPDM, the degree of long chain branching and percent ethylene content can have a great effect on compound green strength (ref. 49). Molecular star structures also can improve green strength (ref. 50). Sometimes, block polymers impart some improvement in green strength (ref. 51). Also, labile crosslinks, established from a post-polymerization treatment, have been reported to improve green strength (ref. 52).

Compounding additives can also be used to improve green strength. It is known that small loadings of additives such as trans-polyoctenylene rubber (TOR) can significantly improve a compound's green strength (ref. 53). Also, chemical promoters added to carbon black loaded stocks have been reported to increase compound green strength (ref. 54). It should also be noted that rubber compounds that contain higher loadings of fully reinforcing carbon black will typically have better green strength (refs. 55-57).

Lastly, one effective method of increasing a compound's green strength might be to expose that compound to an electron beam (refs. 58-60).

Tackiness

Good building tack is the ability of two uncured plies of rubber to adhere to each other on contact with only a moderate amount of pressure applied for only a very brief dwell time (ref. 61). Having good tack is very important when constructing a green tire or making a conveyor belt. However, while building tack may be needed for some processes, it is not needed or desired for some molding operations, where too much tack can be a problem.

Compounds based on natural rubber usually give better building tack than those compounds based on other elastomers. In fact, for compounds based on a natural rubber blend, usually those blends that contain the higher natural rubber concentration will most likely have the better building tack (ref. 62). Also, the factory environment (temperature and humidity) can have a large effect on building tack (ref. 63). Sometimes more work history during the mix can improve tack, as well (ref. 64). in addition, the proper selection and use of an appropriate tackifier will also improve tack (ref. 65).

Stickiness

The level of stickiness that a rubber compound has for a metallic surface can be quite important in predicting processability. Also, this stickiness is not the same as tackiness, which we just reviewed. Compounds which are very tacky may or may not be very sticky to a metal surface. Sometimes, a certain level of stickiness is very necessary for good processing. For example, if the compound did not stick to the barrel of an extruder during processing, it would not be extrudable. On the other hand, a rubber compound can be much too sticky to metal surfaces, making it very difficult to process.

Many times, simply making equipment temperature adjustments during processing can help adjust the level of stickiness (ref. 66). However, what is the best temperature for one compound is not necessarily the best for another compound. The best processing temperature is somewhat compound dependent.

Generally, compounds with higher viscosity values tend to stick less than low viscosity compounds (ref. 67). Many times, calender release additives, as well as mill release additives, are available for consideration (ref. 68). Also, blending certain base elastomers, such as polychloroprene, with other synthetic elastomers will sometimes reduce stickiness (ref. 69).

Lumps

Lumps in a batch can have a variety of causes. Sometimes it can be hang up from a previous batch. Other times it can be from mixing very incompatible elastomers in a blend, especially with greatly different Mooney viscosity values (ref. 70). Other times lumps may form due to poor chemical compatibility of fillers (ref. 71), or sometimes lumps are formed from a combination of these factors combined with poor mixing conditions (discussed previously). Sometimes lumps contain undispersed agglomeration. Other times one may find cured lumps where curative dispersion problems resulted, causing excessively high concentrations of undispersed curative building up to form lumps.

Mill bagging

Bagging on a two roll mill is the inability of a rubber stock to form a rolling bank at the mill nip, sagging off the rolls and exhibiting little or no adhesion to the rolls (ref. 72). When there was a movement away from emulsion SBR over to solution SBR in order to improve the tire rolling resistance for a tread, the S-SBR based compounds possessed more bagging characteristics on the mill because of the higher molecular weight and the narrower molecular weight distribution (ref. 73). Also, it has been reported that compounds possessing high loadings of cis-BR may bag (ref. 74). Sometimes powdered milk has reportedly been applied to a hot mill roll to establish a temporary sticky surface (ref. 75).

Sometimes, bagging on the mill can be reduced or eliminated by making the proper adjustments for mill nip distance, mill surface temperatures and/or friction ratio (ref. 76).

Mill back rolling

Just as with mill bagging, discussed above, some stocks will also go to the back roll of a two-roll mill. Usually, adjustments in mill temperature(s), mill nip distance, friction ratio or even changing to another mill size, will commonly correct this problem (ref. 77).

Part II will appear in the September issue.

References

(1.) G. Day, Chapter 7, "General purpose elastomers and blends," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 145.

(2.) R. School, Chapter 6, "Elastomer selection," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 130.

(3.) M. Gozdiff, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 19Z

(4.) J. Sommer, "Stabilized curative blends for rubber," Rubber Chemistry and Technology, vol. 61, p. 149, March-April, 1988.

(5.) Flexsys literature.

(6.) J. Dick, M. Ferraco, K. Immel, T. Mlinar, M. Senskey and J. Sezna "Utilization of the Rubber Process Analyzer in Six Sigma programs," Rubber World, January 2003, p. 32.

(7.) Richard J. Jorkasky, "Improving productivity in the rubber industry," presentation at the Akron Rubber Group, May 13, 2004.

(8.) J. Dick, Chapter 2, "Compound process characteristics and testing," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 30.

(9.) ibid ref 6.

(10.) K. Hale, J. West and C. McCormick, "Contributions of carbon black type to skid and treadwear resistance, "presented at ACS Rubber Div. Meeting, Spring, 1975, paper no. 6, fig. 22.

(11.) J. Dick and M. Gale, "Processing tests," Chapter 8, Handbook of Polymer Testing, edited by R. Brown, pp. 171-223, Marcel Dekker, Inc., New York, NY, 1999.

(12.) ibid ref 6.

(13.) ibid ref 8, p. 23.

(14.) L.L. Outzs, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 210.

(15.) ibid ref 10, fig. 35.

(16.) S. Laube, S. Monthey and M-J. Wang, Chapter 12, "Compounding with carbon black and oil," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 308.

(17.) D. Coulthard and W. Gunter, paper no. 39 presented at the Fall Meeting of the Rubber Division, ACS, 1975.

(18.) L. Evans, J. Dew, L. Hope, T. Krivak and W. Waddell, "Hi-Sil EZ: Easy dispersing precipitated silica," Rubber and Plastics News, July 31, 1995, p. 12.

(19.) R. Grossman, Q & A, Elastomerics, January, 1989.

(20.) M. Wood, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 202.

(21.) ibid ref 16.

(22.) W. Hess, "Characterization of dispersions," Rubber Chemistry and Technology, vol. 64, p. 386, July-August, 1991.

(23.) W. Hacker, Chapter 23, "Rubber mixing," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 513.

(24.) ibid ref 16.

(25.) ibid ref 16, p. 303.

(26.) ibid ref 23, p. 515.

(27.) ibid ref 1, p. 165.

(28.) C. Stone, "Improving the silica 'green tire' tread compound by the use of special process additives," paper no. 77, ACS Rubber Division Meeting, Fall, 1999.

(29.) ibid ref 6.

(30.) J. Dick, Chapter 1, "Rubber compounding: Introduction, definitions and available resources," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 5.

(31.) ibid ref 30, p. 7.

(32.) S. Tobing, "Covulcanization in NR/EPDM blends," Rubber World, February 1988, p. 33.

(33.) M. D. Morris, "Solubility of sulfur and dithiocarbamates in natural rubber," Rubber Chemistry and Technology, vol. 68, p. 794, Nov.-Dec. 1995.

(34.) A.S. Kuzminskii, L.S. Feldshtein and S.A. Reitinger, "The blooming of sulfur and other ingredients form compounded stocks," Rubber Chemistry and Technology, vol. 35, p. 147, Jan-Feb. 1962.

(35.) B.H. To, Chapter 16, "Cures for specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 395.

(36.) R.P. Mastromatteo, J.M. Mitchell and T.J. Brett, "New accelerators for blends of EPDM," Rubber Chemistry and Technology, vol. 44, p. 1,065, Sept.-October, 1971.

(37.) F. Ignatz-Hoover, Chapter 19, "Antidegradants," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 458.

(38.) D.A. Lederer and M.A. Fath, "Effects of wax and substituted p-phenylenediamine antiozonants in rubber," Rubber Chemistry and Technology, vol. 54, p. 415, May-June, 1981.

(39.) Frank Jowett, "The role of petroleum waxes in the protection of rubber," Rubber World, August 1989, p. 36.

(40.) ibid ref 16, p. 312.

(41.) W. Whittington, Chapter 14, "Ester plasticizers and processing additives," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, p. 356 and 363.

(42.) ibid ref 23, p. 514.

(43.) ibid ref 8, p. 41.

(44.) S. Monthey, "The influence of carbon blacks on the extrusion operation for hose production," Rubber World, May 2000, p. 38.

(45.) W. Hess, C. Herd and P. Vegvari, "Characterization of immiscible elastomer blends'," Rubber Chemistry and Technology, vol. 66, p. 329, July-August 1993; patent no. 4,455,399.

(46.) S. Kawahara, Y. Isono, T. Kakubo, Y. Tanaka and E. Aik-Hwee, "Crystallization behavior and strength of natural rubber isolated from different hevea clone," Rubber Chemistry and Technology, vol. 73, p. 39, Mar.-Apr. 2000.

(47.) G. Hamed, "Tack and green strength of NR, SBR and NR/ SBR blends," Rubber Chemistry and Technology, vol. 54, p. 403, May-June 1981.

(48.) ibid ref 3.

(49.) S. Brignac and H. Young, "EPDM with better low-temperature performance," Rubber & Plastics News, August 11, 1997, p. 14.

(50.) G. Jones, D. Tracey and A. Tesler, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 182.

(51.) ibid ref 1, p. 182.

(52.) E. Buckler, G. Briggs, J. Dunn, E. Lasis and Y. Wei, "Green strength in emulsion SBR," Rubber Chemistry and Technology, vol. 51, p. 872, Nov.-Dec. 1978.

(53.) J. Sommer, Elastomer Molding Technology, p. 180, Elastech, Hudson, OH, 2003; and A. Draxler, "A new rubber: Trans-polyoctenamer," Chemische Werke Huels AG, Germany.

(54.) L. Ramos de Valle and M. Montelongo, "Cohesive strength in guayule rubber and its improvement through chemical promotion," Rubber Chemistry and Technology, Vol. 51, p. 863, Nov.-Dec. 1978.

(55.) C. Cable, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 219.

(56.) P.L. Cho and G.R. Hamed, "Green strength of carbon-black filled styrene-butadiene rubber," Rubber Chemistry and Technology, vol. 65, p. 475, May-June 1992.

(57.) S. Monthey, B. Duddleston amt J. Podobnik, "New blacks address compounding challenges," Rubber World, June 1994, p. 17.

(58.) S. Mohammed, J. Timar and J. Walker, "Green strength development by electron beam irradiation of halobutyl rubber," Rubber Chemistry and Technology, vol. 56, p. 276, March-April 1983.

(59.) S. Mohammed and J. Walker, "Application of electron beam radiation technology in tire manufacturing," Rubber Chemistry and Technology, vol. 59, p. 482, July-Aug. 1986.

(60.) B. Thorburn and Y. Hoshi, "Electron beam tire processing equipment," Rubber World, July 1992, p. 17.

(61.) C.K. Rhee and J. Andries, "Factors which influence auto-adhesion of elastomers, " Rubber Chemistry and Technology, vol. 54, p.101, March-April 1981.

(62.) E. McDonel, K. Baranwal and J. Andries, Polymer Blends, vol. 2, p. 281, Chapter 19, "Elastomer blends in tires, "Academic Press, 1978.

(63.) ibid ref 61.

(64.) Polysar Halobutyl Innerliner Problem Solving Guide, Processing Problem No. 2.

(65.) B. Stuck, Chapter 18, "Tackifying, curing and reinforcing resins," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 438.

(66.) ibid ref 64, Processing Problem No. 4.

(67.) ibid ref 64, Processing Problem No. 3.

(68.) J. Dick, How to Improve Rubber Compounds, 1500 Experimental Ideas for Problem Solving, Hanser Publishers, 2004, Section 4.5, "Reducing stickiness to metal surfaces."

(69.) ibid ref 68.

(70.) M.H. Walters and D.N. Keyte, "Heterogeneous structure in blends of rubber polymers," Rubber Chemistry and Technology, vol. 38, p. 62, March 1965.

(71.) C.A. Carlton, "Chemical compatibility as a factor in the dispersion of fillers in rubber," Rubber Chemistry and Technology, vol. 35, p. 881, Oct.-Nov. 1962.

(72.) Harry Long, Basic Compounding and Processing of Rubber, Rubber Division, ACS, 1985, p. 223.

(73.) ibid ref 1, p. 153.

(74.) ibid ref 1, p. 145.

(75.) ibid ref 68.

(76.) Noboru Tokita, "Analysis of band formation in mill operation," Rubber Chemistry and Technology, vol. 52, p. 387, May-June 1979.

(77.) ibid ref 76.