Blends of Natural Rubber

Frontmatter

1. Introduction — the book and rubber blends

Abstract

The origins of this book are described in the Preface and are important in defining the form which it takes. It is neither an exhaustive treatment of the subject — rubber blends — nor one of the oft-encountered collection of seminar papers. Whilst the chapters have a number of authors, all have worked as a team tackling a number of related topics in a defined programme of work. Several consequences flow from this. There are common threads and a generality of approach. Only certain blend systems are considered, even where an approach might logically be also applicable to another combination of elastomers. The work described was also constrained by falling within a fixed time frame, and scope for further advances remains in many of the more practical areas.

Andrew J. Tinker

2. Measurement of crosslink density in vulcanized blends

Abstract

Much of the work described in this book has been enabled or assisted by the ability to measure crosslink densities of the individual rubbers in a vulcanized blend. Indeed, for two of the blend systems, NR/NBR and NR/EPDM, the problems were to some extent defined by earlier measurements of crosslink densities. From these earlier studies, it was known that preferential distribution of crosslinks in favour of one of the elastomer components of NR/NBR blends, particularly the NBR, was likely to occur to an extent sufficient to explain the generally disappointing physical properties obtained [1,2]. The failure of EPDM to vulcanize adequately in the presence of the highly unsaturated NR has been known for many years [3–5], but measurements of crosslink densities of blends of NR with chemically modified EPDM showed that good physical properties could be obtained with only modest increases in crosslinking of the EPDM [6].

Andrew J. Tinker

3. Characterization of vulcanized blends by microscopy

Abstract

The primary aims of this chapter are to explain how the images used throughout this book have been formed and how they can be interpreted. In terms of the scope of the material described, good microscopical results can only be obtained by enhancing the contrast between the phases in a blend where very little exists. The following is limited to presenting the techniques applied to obtaining such results and is not intended to be a full and thorough theoretical description of the techniques employed. Other sources should be sought if this is required [1].

Paul Cudby

4. Estimation of crosslink density by solid-state NMR spectroscopy

Abstract

The technique for estimating the extent of crosslinking in the individual phases of a blend by using peak broadening associated with increasing crosslink density, as described in Chapter 2, was developed on an instrument which is designed to obtain spectra of materials in a liquid state. Thus it is necessary to swell vulcanizates for several days before endeavouring to obtain a spectrum. The work described in this chapter has sought to establish whether the technique for estimating crosslink densities in the component rubbers of a vulcanized blend can be translated to a solid-state NMR spectrometer by adopting appropriate operating procedures, and thereby facilitate the estimation of crosslink densities in vulcanized blends.

Aris bin Ahmad, Abu bin Amu

5. NR/NBR blends — basic problems and solutions

Abstract

Nitrile rubber (NBR) is a copolymer of acrylonitrile and butadiene. It is the acrylonitrile component which makes NBR an oil-resisting rubber; the higher its proportion then the greater the oil resistance. Two nitrile rubbers were studied: one containing 41% acrylonitrile, Breon N41 (Zeon Chemicals), and a second containing 34% acrylonitrile, Krynac 34.50 (Bayer). These will be referred to as NBR41 and NBR34 respectively.

Michael V. Lewan

6. Improving the morphology and properties of NR/NBR blends with NR/PMMA graft copolymers

Abstract

Blends of highly incompatible elastomers may sometimes be improved by the addition of small amounts of another polymer. Setua and White [1] applied this technique to improve the homogeneity of binary and ternary blends of CR, NBR and EPM. When a small amount of chlorinated polyethylene is added to NBR/EPM or CR/EPDM blends, they mix more rapidly. The chlorinated polyethylene acts as a compatibilizing agent which appears to form a skin on the EPM particles that helps the larger NBR chunks adhere to them. This increased adhesion and polarity at the EPDM surface increases the compatibility. The presence of block or graft copolymers can also alleviate blending of incompatible elastomers as they can alter interfacial properties [2–6]. Ideally the block or graft component should contain a segment which is chemically identical to one of those in the respective phases, but the desired effect may still be achieved if one polymer of the graft is miscible with, or adhered to, one of the phases.

K. G. Karnika de Silva, Michael V. Lewan

7. Improving the morphology and properties of NR/NBR blends with polychloroprene as the compatibilizing agent

Abstract

Extensive work on compatibilizing NR/NBR blends with Heveaplus MG30 had shown that this graft copolymer of natural rubber with poly(methyl methacrylate) provided improved adhesion between polymer phases with incompatible polarities, which was crucial to stress transfer between these phases (Chapter 6). In addition, the interfacial tension between the phases was lowered by this interfacial activity, and this played an important role in blend morphology, reducing phase sizes and subsequently improving physical properties and crosslink density distribution between the phases of the blend.

Krisna Kongsin, Michael V. Lewan

8. NR/NBR blends — compounding for food contact applications

Abstract

The initial work into preferred formulations for NR/NBR blends produced a series of cure systems suitable for use in blends of NR with NBR containing either 41% or 34% acrylonitrile (Chapter 5). The blends may be further improved by the use of low levels of compatibilizers, either polychloroprene (CR) or a graft copolymer of natural rubber with poly-(methyl methacrylate) (Heveaplus MG30) as described in Chapter 6.

Michael V. Lewan

9. Novel natural rubber/ethylene propylene copolymer (EPM) blends

Abstract

Blends of NR with ethylene propylene copolymer (EPM) which have the excellent ozone resistance of EPM, yet contain a significant amount of NR, should enjoy an economic advantage over current EPM formulations as EPM are relatively expensive elastomers. There may also be some property benefits. The main applications for such materials would be in the cable industry as electrical insulation sheathing. The economic benefits should, of course, be greatest in the natural rubber producing countries of the world where the partial replacement of an imported synthetic polymer with a locally sourced material gives additional financial benefits.

Paul S. Brown

10. Natural rubber/ENR-25 blends

Abstract

High damping rubber vulcanizates with good physical properties and a small dependence of properties on temperature are candidates for use in bushings and other bearings. Classically high damping may be achieved by either using a rubber with inherent high damping or by adding high levels of fillers and plasticizers to one which has low damping. Both approaches fail to provide the necessary combination of properties; rubbers which are highly damping also show a strong dependence of properties on temperature, whilst the use of high levels of filler and plasticizer, generally used with a low level of curatives, leads to unsatisfactory physical properties.

Sally Groves

11. Partition coefficients for an ester plasticizer in black-filled blends

Abstract

In recent years there has been a requirement for rubber vulcanizates to combine high damping, good physical properties and a low dependence of dynamic properties on temperatures for use in applications such as automotive bushings. Prior research [1, 2] indicated that novel blends of natural rubber (NR) and 25 mole% epoxidized NR (ENR-25), in which the ENR-25 phase contains high levels of filler and plasticizer, can have an appropriate combination of high damping, good physical properties and low dependence of properties on temperature. Incorporating high levels of carbon black into the ENR-25 phase of NR/ENR-25 blends to increase hysteresis necessitates the use of high levels of plasticizer which must remain largely in the ENR-25 phase of the blend. The plasticizer serves key roles: it balances the viscosities of the two rubber masterbatches, thus improving processing and easing the formation of a fine phase morphology, it balances the moduli of the two rubber phases in the vulcanizate, and it reduces the glass transition temperature (T _g) of the ENR-25 to minimize the dependence of properties on temperature.

Jianhe Liao, Sally Groves

12. Improving resistance to low temperature crystallization in NR/ENR-25 blends

Abstract

The automobile rubber products manufacturing industry has expressed a long-standing need for high damping rubbers which combine good physical properties with a low dependence of dynamic properties on temperature. Elastomers with inherently high damping have a high dependence of properties on temperature because the damping derives from relatively high glass transition temperatures (T _g). An alternative approach of using natural rubber (NR) with high levels of carbon black and oil but relatively low crosslink densities limits the degree of damping attainable and gives rather poor physical properties.

Benny George, Jianhe Liao, Sally Groves

13. Compounding NR/ENR-25 and NR-Hv-BR/ENR-25 blends for engineering applications

Abstract

The overall viability of blending normally compounded natural rubber (NR) with highly filled and plasticized 25 mole% epoxidized NR (ENR-25) to attain highly damping materials with a low dependence of properties on temperature is presented in Chapter 10. The aim of this chapter is to show how these novel blends may be processed successfully on a large scale and vulcanized satisfactorily in an injection-moulding process.

Sally Groves

14. Solutions to the basic problems of poor physical properties of NR/EPDM blends

Abstract

The development of blends of natural rubber (NR) with ethylene-pro-pylene-diene monomer (EPDM) with the aim of combining the excellent physical properties of NR with the ozone resistance of EPDM has received much attention over the past three decades [1–7]. The principal target application for such blends has for many years been tyre sidewalls [8] where ozone cracking was seen as a limiting factor on the service life of a tyre. However, extruded weatherseal profiles for vehicles and light-coloured injection-moulded goods for domestic appliances, markets once dominated by NR, but now almost entirely superseded by EPDM, are also attractive areas for NR/EPDM blends. Whilst ozone resistance has been comparatively easy to achieve in NR/EPDM blends [9], the general physical properties of NR/EPDM blends have been less than satisfactory [10]. This is entirely expected as it is generally the case that the physical properties of vulcanized elastomer blends are inferior to those predicted from the properties of the component elastomers [11–13].

Stuart Cook

15. Compounding NR/EPDM blends for tyre sidewalls

Abstract

A tyre sidewall provides the physical connection between a vehicle’s wheel and the tyre tread: it transmits power and braking force to the tyre tread which is itself in contact with the road surface. The sidewall also provides a significant part of a vehicle’s suspension and plays an important role in the general handling of the vehicle on the road. As can be seen in Figure 15.1, the sidewall is a remarkably thin and seemingly fragile part of the tyre considering its function.

Stuart Cook

16. Compounding NR/EPDM blends for light-coloured applications

Abstract

The work described in this chapter, although targeted in particular at gaskets for washing machines and washer/driers, is believed to be applicable to non black-filled applications in general, as the method of manufacture for washing machine gaskets, like many other products for use in the domestic appliance market, is by the process of injection-moulding.

Stuart Cook, Rose Marie Escolar

17. NR/EPDM blends for extruded profile weatherstrip

Abstract

The seals for automotive windows, doors, lights, etc. may all be considered as weatherstrip or weatherseals. The production of weather strip usually involves the extrusion of a complex shaped profile with precisely defined dimensions (see Figure 17.1). Rubber has for many years been used for sealing purposes in a vehicle; it is flexible and resilient and can be extruded relatively easily. In the early 1970s, automotive weatherstrip was made from a single polymer rubber, mainly natural rubber (NR) which has excellent resilience, but relatively poor weathering resistance. However, other elastomers including styrene-butadiene rubber (SBR), polychloroprene (CR) and ethylene-propylene-diene monomer rubber (EPDM) were also used [1]. Towards the end of the 1970s EPDM had become dominant primarily because of its superior weathering and ozone resistance coupled with its high filler acceptance and wide processing latitude. Analysis [2] of a recent, medium-sized, high sales volume vehicle has shown that more than 66% of the non-tyre rubber used in the car is based on EPDM of which some 75% is EPDM weatherstrip of some form. This compares with about 12% of the non-tyre rubber in a car being NR.

Mohamed Aris bin Ahmad, Ridha Arizal, Stuart Cook

Backmatter