Acrylic rubbers

Acrylic rubber is a relatively low-volume specialty elastomer that is used when very good oil and high-temperature resistance are needed at a reasonable price. ACM is not selected if very good low-temperature properties or water resistance is needed. ACM is weak for these properties. Rubber compounds based on ACM are commonly used with automotive engines and transmissions. Also, ACM does not work well in applications that require good compression set resistance. 

As can be seen, a large number of different types of acrylic elastomers (ACM) can be manufactured from this large number of different reactive monomers. 

Acrylic rubber Polyacrylate rubber Polyacrylate-acrylic acid ester Acrylester rubber Poly (acrylic acid ester rubber) 

There is no standard classification system for ACM. Different grades of ACM polymers are produced from different combinations of alkyl acrylate monomers. 

Rubber compounds based on ACM give excellent hot oil resistance. This makes ACM a good choice for certain automotive engine gaskets, oil seals, automotive packings, and some hose applications. 

This mbber is very tacky in nature and contains acrylic group, which makes it polar in nature. Nanocomposites have been prepared based on this elastomer with a wide range of nanohllers. Layered silicates [53-55] have been used for this preparation. Sol-gel method [56,57], in situ polymerization [58], and nanocomposites based on different clays like bentonite [59] and mica [60] have been described. The mechanical, rheological, and morphological behaviors have been investigated thoroughly. 

Acrylic elastomers have previously been used only in the rubber industry and conventional polymers (EEA or core-shell acrylate polymers) are based on the rubber in hard phases. Based on the soft phase only, however, EniChem s Europrene AR uses original technology for modification of PA 6 with conventional acrylic rubber in granule form. It increases the specific rubber efficiency in the impact resistance characteristics, so differing from other traditional elastomers (EPR, SEES) used in modification of nylon. 

A super-toughness level is obtained with only 17% Europrene AR and impact resistance characteristics are better than those obtained with 20-25% of other elastomers. Both the low level of rubber and intrinsic characteristics of the soft-phase acrylic increase the resistance to high temperature (Vicat B = 170°C) and the flexural modulus of modified nylon. The high thermal/mechanical inertia and the polarity of these rubbers also allow post-treatments to the nylon that were not previously possible. 

Styrenic block copolymers and their compounds have been in widespread commercial use for many years, with many applications. With the latest technology, they have become particularly interesting as impact modifiers for plastics, both thermoplastics and thermosets. Most polymers are thermodynamically incompatible with others polymers and mixtures tend to separate into two phases, even when they are part of the same molecule, as in block copolymers. Poly(styrene-P-elastomer-P-styrene) copolymers, in which the elastomer is the main constituent, give a structure in which the polystyrene end-segments form separate spherical regions ( domains ) dispersed in a continuous phase. 

At room temperature, the polystyrene segments are hard and act as physical cross-links, tying the elastomer chain together in a three-dimensional network, not unlike the network that is formed by cross-linking of thermosetting rubber during vulcanization. 

Well-known materials are the Kraton polymer range, originally developed by Shell, and are produced in several types. The D series has an unsaturated rubber midblock - styrene/butadiene/styrene (SBS), and styrene/isoprene/styrene (SIS) - and the G series has a saturated midblock - styrene ethylene/butylene styrene (SEES) and styrene ethylene/propylene (SEP). The G series has increased resistance to oxidation and weathering, higher service temperature and better processing stability. 

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Acrylic rubbers, as is the case for most specialty elastomers, are characterized by higher price and smaller consumption compared to general-purpose mbbers. The total mbber consumption ia 1991 was forecast (55) at 15.7 million t worldwide with a 66% share for synthetic elastomers (10.4 x 10 t). Acryhc elastomers consumption, as a minor amount of the total synthetic mbbers consumption, can hardly be estimated. As a first approximation, the ACM consumption is estimated to be 7000 t distributed among the United States, Western Europe, and Japan/Far East, where automotive production is significantly present. 

The rubbers may be vulcanised by conventional accelerated sulphur systems and also by peroxides. The vulcanisates are widely used in petrol hose and seal applications. Two limiting factors of the materials as rubbers are the tendency to harden in the presence of sulphur-bearing oils, particularly at elevated temperatures (presumably due to a form of vulcanisation), and the rather limited heat resistance. The latter may be improved somewhat by Judicious compounding to give vulcanisates that may be used up to 150°C. When for the above reasons nitrile rubbers are unsatisfactory it may be necessary to consider acrylic rubbers (Chapter 15), epichlorohydrin rubbers (Chapter 19) and in more extreme conditions fluororubbers (Chapter 13). 

In addition to poly(methyl methacrylate) plastics and polyacrylonitrile fibres, acrylic polymers find widespread use. First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance. Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known super-glues are also made from acrylic polymers. 

Subsequently, several other companies have entered the acrylic rubber market (e.g. Thiokol, American Cyanamid, Goodyear, Polymer Corporation and US Rubber) and this has led to many technical developments. These may be categorised into the three main areas ... 

The changes in acrylic rubber compounds have increased the scope of these materials as heat-and oil-resisting materials able to meet many of the increasingly stringent demands being imposed on rubbers for use in automotive applications. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

The particle size of the dispersed phase depends upon the viscosity of the elastomer-monomer solution. Preferably the molecular weight of the polybutadiene elastomer should be around 2 x 10 and should have reasonable branching to reduce cold flow. Furthermore, the microstructure of the elastomer provides an important contribution toward the low-temperature impact behavior of the final product. It should also be emphasized that the use of EPDM rubber [136] or acrylate rubber [137] may provide improved weatherability. It has been observed that with an increase in agitator speed the mean diameter of the dispersed phase (D) decreases, which subsequently levels out at high shear [138-141]. However, reagglomeration may occur in the case of bulk... 

Acrylated rubber Modified chlorinated rubber Bituminous Vinyl tar... 

Acrylated rubber These are based on styrene butadiene and have become commercially available only relatively recently. They are manufactured in several grades but most have the advantage over other materials in this class of being based on white spirit solvent rather than the stronger and more obnoxious xylol. In other respects, they are similar to chlorinated rubber and cost approximately the same, although they are easier to airless spray and the dried film contains less pores. They are considered to have superior weather resistance to chlorinated rubber and vinyl. 

FIGURE 3.3 (a) Transmission electron microscopic (TEM) image of acrylic rubber (ACM)-siUca hybrid nanocomposite synthesized from 10 wt% of tetraethoxysilane (TEOS). (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl. Polym. Sci., 93, 2579, 2004. Courtesy of Wiley Interscience.) Transmission electron microscopic (TEM) photographs of acrylic rubber (ACM)-silica hybrid nanocomposites prepared from (b) 30 wt% and (c) 50 wt% tetraethoxysilane (TEOS) concentrations. (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl. Polym. Sci., 93, 2579, 2004. Courtesy of Wiley InterScience.)... 

FIGURE 3.16 Morphology and visual appearance of acrylic rubber (ACM)-silica and epoxidized natural rubber (ENR)-silica hybrid composites prepared from different pH ranges (a) transmission electron microscopic (TEM) picture of ACM-siUca in pH 1.0-2.0, (b) scanning electron microscopic (SEM) picture of ACM-siUca in pH 5.0-6.0, (c) SEM image of ACM-siUca in pH 9.0-10.0, (d) TEM picture of ENR-silica in pH... 

FIGURE 3.17 Tensile stress-strain plots for uncross-linked acrylic rubber (ACM)-siUca and epoxidized natural rubber (ENR)-silica hybrid composites synthesized from various pH. Letter p in the legends indicates pH and the numbers following the letter are indicative of the pH ranges e.g., 1 means pH range of 1.0-2.0, similarly 3 means 3.0-4.0. (From Bandyopadhyay, A., De Sarkar, M., and Bhowmick, A.K., J. Mater. Sci., 41, 5981, 2006. Courtesy of Springer.)... 

TPEs prepared from rubber-plastic blends usually show poor high-temperature properties. This problem could be solved by using high-melting plastics like polyamides and polyesters. But, often they impart processing problems to the blends. Jha and Bhowmick [49] and Jha et al. [50] have reported the development and properties of novel heat and oil-resistant TPEs from reactive blends of nylon-6 and acrylate rubber (ACM). The properties of various thermoplastic compositions are shown in Table 5.4. In this kind of blend, the plastic phase forms the continuous phase, whereas... 

Typical Properties of Thermoplastic Elastomers Developed from Nylon-6-Acrylate Rubber Blends... 

Mechanical Properties of Thermoplastic Elastomers Based on Acrylate Rubber-Fluorocarbon Rubber-Polyacrylate Monomer... 

FIGURE 5.16 Viscosity-shear stress relationships for various compositions of nylon-6-acrylate rubber (ACM) blends at 240°C. 

Jha A. and Bhowmick A.K., Thermoplastic elastomeric blends of nylon 6/acrylate rubber Influence of interaction of mechanical and dynamic mechanical thermal properties. Rubber Chem. TechnoL, 70, 798, 1997. 

Jha A., Dutta B., and Bhowmick A.K., Effect of fillers and plasticizers on the performance of novel heat and oil-resistant thermoplastic elastomers from nylon-6 and acrylate rubber blends, J. Appl. Polym. Sci., 74, 1490, 1999. 

Kader M.A., Bhowmick A.K., Inoue T., and Chiba T. Morphology, mechanical and thermal behavior of acrylate rubber/fluorocarbon elastomer/poly aery late blends, J. Mat Sci., 37, 6789, 2002. 

Jha A., Bhowmick A.K., Eujitsuka R., and Inoue T. Interfacial interaction and peel adhesion between polyamide and acrylate rubber in thermoplastic elastomeric blends, J. Adhes. Sci. Technol., 13(6), 649, 1999. 

Wootthikanokkhan and Clythong have studied the effects of additive distribution and its effect on the distribution of cross-link density in NR-acrylic rubber (AR) blends [38]. The formulations of the four blends is given in Table 11.15. 

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