Compounding requirements and techniques for rubber covered rolls.

Compounding requirements and techniques for rubber covered rolls

Introduction This article presents a review of compounding techniques for development of rubber compounds for roll applications, focusing primarily, but not exclusively, on EPDM and NBR polymers.

While compounding for rubber covered rolls is inherently similar to that for other rubber applications, rolls have some unique compounding requirements and restrictions. Compounds for rolls are, in many cases, mill-mixed because of quantity and dispersion requirements, and thus must use polymers that mill mix without undue difficulty.

The building of rubber covered rolls involves either calendering or extrusion, so the compounds must process well through these operations.

Compounds must have good scorch safety, flow, and tack or self-adhesion characteristics because of the need for the calendered or extruded layers to flow and bond together into one, homogeneous mass during vulcanization.

Polymer selection for EPDM and NBR, types of fillers, plasticizers, curatives and other additives are reviewed and discussed. Several typical roll compounds are detailed and discussed.

Polymer selection

Selection of polymer type is dependent on the end use parameters required of the roll compound, and is similar to polymer selection for other applications. For good resistance to oils and printing inks, NBR (or NBR/PVC) is a good choice. For high heat resistance, silicone, silicone/EPDM compatibilized blends or EPDM polymers should be used. For general chemical resistance to dilute acids and alkalis, natural rubber, SBR or EPDM can be used. The choice of polymer type is not meant to be a part of this article and for this type of information, the reader is directed elsewhere [refs. 1-3].

When choosing specific polymers with a polymer group, usually there are several options concerning polymer type that are available. For two of the commonly used polymer types used for rolls, EPDM and NBR, the following is given as a guide.

EPDM

EPDM, a terpolymer based upon ethylene, propylene and a non-conjugated diene, is characterized by a saturated backbone, giving it excellent resistance to aging by oxygen/heat, ozone and weathering. It can be used for temperatures up to 175 [degrees] C (350 [degrees] F). It also has good resistance to polar solvents such as ketones, acids and alkalis [refs. 1 and 2].

EPDM types can vary in the following characteristics: Mooney viscosity (a measure of molecular weight), molecular weight distribution, ethylene/propylene ratio, type of termonomer and amount of termonomer. Some high molecular weight types are oil extended to bring the viscosity down to processible levels.

For EPDM polymers, Mooney viscosity typically ranges from 30 to 90+ ML-4/100 [degrees] C. EPDMs of low-to-medium viscosity are usually best for processing and are almost required for mill mixing. Low viscosity polymers also tend to have good uncured tack properties.

Broadness of molecular weights for EPDM can range from very broad to very narrow polymers, with the broader polymers tending to handle better on mills and calenders. They are also suitable for extrusion building.

The ethylene/propylene ratio for EPDMs typically ranges between 50/50 and 75/25. As the ethylene proportion in the polymer increases, the polymer becomes more crystalline and, hence, more difficult to handle on mills. For this reason, low-to-medium ethylene content EPDM polymers are typically used for roll compounds.

There are three types of termonomers commercially used in EPDM: DCPD (dicyclopentadiene), HD (1,4-hexadiene) and ENB (ethylidene norbornene). Their main difference is in rate of cure, with DCPD being the slowest curing and ENB being the fastest curing. Levels of terpolymer can range from 1% to 10%, with the rate and state of cure increasing with increasing termonomer content. A compound's scorching tendencies may be poorer but set properties improved as the termonomer content is increased.

Low durometer roll compounds frequently make use of oil extended grades, which have up to 100 parts of oil incorporated into the polymer. This eliminates the need to add large amounts of free oil to the mix [ref. 4].

For heat resistance beyond the capabilities of standard EPDMs, specially compatibilized blends of EPDM and silicone have found increasing usage [refs. 5 and 6]. They have heat resistance improved over EPDM and improved mechanical and physical properties (e.g., tensile strength, tear strength and abrasion resistance) over silicone rubber.

NBR

Nitriles, polymers based upon acrylonitrile and butadiene, are used in roll applications primarily because of their excellent resistance to hydrocarbon inks, oils and solvents [refs. 1 and 2]. NBRs vary primarily in acrylonitrile (ACN) content and in Mooney viscosity. As the ACN level increases, the oil and solvent resistance improve and the low temperature properties (not usually a factor in roll usage) diminish. Hardness can increase and abrasion resistance can improve, but compression set and tear resistance can both be impaired [ref. 7]. Plasticizer compatibility can be diminished at high ACN levels.

As the polymer viscosity increases, the compound viscosity increases and physical properties generally increase. Mixing may be more difficult. Abrasion and solvent resistance also improves [ref. 7].

Blends of NBR with PVC are used because of their better solvent and oil resistance, better abrasion resistance and greatly improved ozone resistance vs. straight NBRs [refs. 7-9]. The PVC also tends to impart a non-stick surface to the finished roll [ref. 8]. NBR/PVC blends tend to have poorer high temperature set properties than straight NBRs [refs. 7-9].

Blends of NBR with PVC containing large amounts of plasticizer (typically, 100/60/120 NBR/PVC/DOP) are frequently used for very soft roll compounds because the large amount of plasticizer already incorporated minimizes the amount of free plasticizer needed to be incorporated into the soft compound.

Carboxylated NBR is a specialty polymer used primarily for its excellent abrasion resistance [ref. 7].

Other polymers

Other polymers can be shown to vary in many of the same ways that EPDM and NBR do. Polymer selection is a critical parameter in designing a roll compound and each polymer type considered should be investigated thoroughly to determine the best polymer for processing as well as finished roll properties.

Fillers

Filler selection for roll compounds is similar to that for other rubber products. For black compounds, there are numerous grades of carbon blacks to choose from, depending on the end hardness and properties desired in the finished compound. For the best physical properties and abrasion resistance, small particle-sized, high structure blacks are used, such as N351 or N339, although generally the finer particle size black used, the more difficult it is to get good dispersion of the black. This is especially the case when the compound also contains significant amounts of plasticizers. High structure blacks can also give higher hysteresis which can result in high heat buildup in the roll in high pressure applications. Another possible disadvantage is that, to obtain the same hardness, more plasticizer is required, which can later be extracted during use. This could lead to hardening and shrinkage of the roll cover [ref. 8].

For most general purpose black roll compounds, the intermediate reinforcing grades of carbon black are used, such as N550, N650 and N762. These blacks give good physical properties and are easier to process than the more reinforcing blacks noted above. The largest particle sized black, MT (N990), is used in roll compounds for its good processibility and its minimal effect on hardness, compared to other blacks. It also tends to give compounds with good compression set properties [ref. 10].

Roll compounds containing significant amounts of carbon black will be conductive to varying degrees depending on the type and the amount of carbon black used. Although most grades of carbon black will increase the conductivity (reduce the resistivity) of a compound, to obtain a highly conductive compound, special conductive grades of black (e.g., N472) are used. For semi-conductive compounds, other types of black can be used as well, although at higher levels than would be required if N472 were used. An excellent review of conductive rubber rolls is given by Ryder [ref. 11]. Information on the effect of various black types on conductivity is also found elsewhere in the published literature [ref. 12].

Plasticizers

Plasticizers are required in rubber compounds to aid in processing and to control hardness in the final compound. For roll compounds, plasticizers must be chosen with the following factors in mind:

Extractability and volatility - Inks or other media the roll will encounter can extract the plasticizer, or high temperatures can volatilize the plasticizer, resulting in hardening, surface embrittlement and shrinkage [ref. 8].

Bonding - Incompatible plasticizers (or those beyond their compatibility level) may tend to bloom from the compound, resulting in poor bonding to itself (i.e., poor ply-to-ply adhesion) and to the metal core [refs. 13 and 14].

Effect on cure - For peroxide cured compounds, aromatic oils and naphthenic oils are not recommended for use because of their interference with peroxide cures. Paraffinic oils or other plasticizers with minimal aromatic nature should be used with peroxide curing systems [ref. 4].

For many of the general purpose polymers commonly used in rolls (e.g., NR, SBR, CR) aromatic oils are highly compatible and tend to give good tack to the compounds, but they result in staining of the compound and thus their use in light colored compounds is limited [ref. 15]. For light colored rolls using these polymers, non-staining naphthenic oils are suggested [ref. 16].

For EPDM, aromatic oils have limited compatibility and are seldom used. Naphthenic oils have excellent compatibility and are commonly used. Paraffinic oils, because of their good compatibility, low volatility and minimal effect on peroxide cures, are good choices for roll compounds, especially the high viscosity types which have excellent heat resistance and very low volatility. Bloom may be a concern if high levels of highly paraffinic oils are used with highly crystalline (i.e., high ethylene) EPDM types [ref. 4].

For NBR, ECO and similar polar polymers, the petroleum-based oils described above have minimal compatibility and are not typically used. Ester plasticizers, such as the phthalates and sebacates, have good compatibility and are commonly used. For very soft compounds, polymeric plasticizers are frequently used because of their low volatility and extractability. Blends of polymeric and monomeric plasticizers are frequently used for easy processing [refs. 13 and 17].

Processing aids

Generally, processing aids used in rolls fall into two categories: tackifiers, which improve tack and bonding, and lubricants, which improve flow, mill and calender release, and processing in general.

Tackifiers are generally solid hydrocarbon resins with melting points between 70 [degrees] C and 100 [degrees] C. Liquid hydrocarbon resins, such as coumarone indene resins, are suggested for improving tack of CSM and NBR [refs. 7 and 16]. Methyl methacrylate was found to improve the tack of EPDM at levels ranging from 5 to 15 parts [ref. 18]. Epoxy resins contribute to the building tack of CSM compounds [ref. 16].

Lubricants are used frequently to assist with release of sticky compounds from mill or calender rolls. For this purpose low molecular weight polyethylenes, paraffin or microcrystalline waxes, petrolatum, metallic stearates, stearic acid or zinc soap of fatty acids can be used. They generally do not impair bonding or ply adhesion if used in small amounts [refs. 16 and 17].

If peroxide cure systems are used, care must be taken to avoid materials with aromatic characteristics to avoid cure problems as described with plasticizers, above. The addition of small quantities of liquid, low molecular weight EPDM polymers has shown to improve the calendering characteristics of EPDM compounds. These materials function as vulcanizable, non-extractable plasticizers [ref. 19].

Curatives

Roll compounds generally contain cure systems similar to those of other rubber products except that they're generally slower curing with longer scorch times. This is because it is important to get good flow of the compound (to get good ply-to-ply adhesion and bonding to the metal core) and to allow any entrapped air within or between plies to dissolve or escape to the surface of the roll before curing takes place.

Peroxide cure systems are frequently used for curing roll compounds because of: the ease of increasing hardness by increasing the level of methacrylate resin coagents; and because they give the best heat aging characteristics. Because these resins are liquid materials, high hardness compounds can be made by using high levels of the liquid methacrylate with a peroxide. The resulting compound has a low viscosity whereas a high hardness, sulfur-cured compound would typically be boardy and of high viscosity because of the high levels of reinforcing fillers required [refs. 14, 17 and 20].

Sulfur cure systems are used for general purpose rolls produced from many different polymers and for high hardness rolls made from NR, SBR and BR. As mentioned earlier, speed of cure is generally of secondary importance to good scorch characteristics. Retarders are frequently used to maintain good scorch and bin stability characteristics.

Varying sulfur level to vary hardness in the general purpose elastomers is commonly used in roll coverings for the paper industry. It provides a reliable, reproducible method of obtaining roll covers of varying hardness without the resulting loss of tack often found in higher hardness compounds with normal sulfur loadings but high filler loadings [ref. 14].

Ebonite roll covers, typically of 80 Shore D hardness, are made from sulfur cures using extremely high levels of sulfur, typically from 40 to 60 phr. These compounds tend to be very sensitive to curing conditions because of the exothermic reaction of sulfur and rubber. Post cure cracking due to the inherent shrinkage of ebonites upon curing can also be a problem [ref. 14].

Special additives

There are numerous examples in the literature of additives employed in roll compounds to achieve special properties in the roll cover compound or on the roll surface.

Calcium oxide, typically used in the form of calcium oxide/oil dispersion, can be used in small amounts to absorb moisture and other volatiles which can cause porosity in the finished roll [ref. 7].

Zinc oxide, typically used as an activator for sulfur cure systems, is also used as a heat transfer agent because of its high thermal conductivity. High levels in roll compounds, 20 phr or higher, can aid in the uniform curing of thick sections [ref. 13].

Many forms of additives can be used to impart a release surface to the roll surface. Among these are hydrophobic fibers such as nylon [ref. 21], toluene sulfonamide [ref. 22], polytetrafluoroethylene [ref. 23] and polystyrene beads [ref. 16]. Surface chlorination of the cured roll can also provide a releasing surface [ref. 24]. The inclusion of silicone rubber into more conventional elastomers such as EPDM will also add release characteristics to the roll surface [ref. 6].

Ferrous materials such as iron and iron oxide can add magnetic qualities to the roll compound [ref. 25].

Antioxidants are an integral part of roll compounds based upon diene polymers such as NR, SBR, BR, NBR and CR. TMQ (polymerized 1,2-dihydro-2,2,4-trimethyl-quinoline) is a general purpose antioxidant used for most polymers except CR, because of its detrimental effect on scorch.

It is, however, widely used in peroxide cured compounds because of its minimal effect on the cure. ODP (octylated diphenylamine) is another general purpose antioxidant that can be used in most polymers and has minimal effect on the cure rate of CR.

Elastomers without any unsaturation in their backbone, such as EPDM, are inherently resistant to the effects of oxidation and ozone. For extreme heat applications, the addition of antioxidants such as MTI (2-mercaptotolylimidazole) and TMQ to peroxide cured EPDM compounds can be beneficial.

Typical roll compound examples

Roll compounds for several applications are shown in tables 1-6.

Table 1 shows a comparison of NBR with NBR/PVC in a typical roll compound. Factice is used for good processing and grinding characteristics. Coumarone indene resin is used to impart additional building tack to the compound. The NBR/PVC compound, even with 10 additional parts of plasticizer, is 8 points harder and has higher abrasion resistance than the corresponding NBR compound [ref. 27].

Table 2 shows a nitrile roll compound used for rice hulling. The precipitated silica combined with the silane coupling agent give the high strength and abrasion resistance characteristics required in this application. The hexamethylenetetramine is used to cure the phenol formaldehyde resin which, when cured, increases the hardness, strength and abrasion resistance of the compound. The resin acts like a thermoplastic before curing and thus improves processing at milling and calendering temperatures.

Table 3 shows a general purpose EPDM roll compound. A paraffinic oil is used for maximum compatibility with the peroxide cure system. The dicumyl peroxide cure system with the methacrylate resin gives the cured roll excellent compression set and heat aging characteristics. The liquid methacrylate also aids in processing of the uncured compound.

Table 4 shows a typical compound used for a metal coating roller used with a coating system using ketone solvents. EPDM is required for this application because of its excellent resistance to oxygenated solvents such as ketones. The TMQ antioxidant improves the aging characteristics of the compound [ref. 26].

Table 5 shows a heat-resistant roll compound based upon the silicone/EPDM compatibilized blend discussed earlier [ref. 6]. This material offers heat resistance better than EPDM with physical properties (e.g., tensile and abrasion resistance) better than silicone. A peroxide cure system is used for optimum heat resistance and a methacrylate coagent is used for good compression set. Titanium dioxide is added for color.

Table 6 shows a paper mill press roll made from either NR or SBR. These polymers are chosen because the roll hardness can be easily controlled by using varying sulfur levels. The compound contains a very high level of zinc oxide which aids in uniform curing of thick roll sections. Magnesium oxide is used as a cure activator [ref. 26].

Conclusions Compounding for rubber covered rolls is similar to that for other rubber applications, with the exception that processing is a much greater factor because of the need for compounds to handle well during mill mixing, calendering or extrusion, and roll building. Proper selection of polymer, filler, plasticizer, curatives and other additives such as tackifiers, as described in this article, can result in roll compounds that work as well during use as they did in the manufacturing plant. [Tabular Data 1-6 Omitted]

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