Uses and Performance of Rubber Sealants Uses and ...

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performance with regards to applications and storage ... The most common fundamental property tests for sealants ... Typical application areas for sealants are.
Rohani Abu Bakar

Uses and Performance of Rubber Sealants INTRODUCTION Sealant is a material capable of attaching at least two surfaces together by filling the space between them to eliminate dirt, moisture and chemicals1. Sealants have low strength due to large amounts of inert filler material, mainly for cost reduction and gap filling purposes. Generally, sealants contain a base polymer, filler, plasticisers and additives to improve adhesion1–2. The base polymer is usually supplied in the form of solid rubber, liquid polymer or latex emulsion. Solid rubber is liquefied with a solvent. Liquid polymers with low viscosity such as polysulphide, silicone and polyisobutylene can be used to make sealants with little or no solvent, whilst latex emulsions including acrylic, polyvinylchloride, polyvinylacetate and their blends are easy to formulate since the base material is in a fluid form. Solvents are also required in large amounts to liquefy the base polymer in order to lower its viscosity, facilitate application and help homogenise the various ingredients in sealant formulation. Fillers are added into sealant formulations to increase viscosity and total solid content, resulting in faster drying in the case of water-based sealants. Fillers also provide better gap filling properties while lowering the cost of sealants. In general, a filler does not give much added strength but may affect other performances such as hardness and water resistance. Examples of suitable fillers are calcium carbonate, silica, titanium dioxide and carbon black. Plasticisers may also be added as one of the ingredients in a sealant’s composition. It is used to reduce elastic modulus, attain desirable degree of softness and enhance stress relaxation ability of the sealant composite. Other type of additives may also be included, for instance, coupling agents, primers, antibacterial agents,

UV absorbers and ozone inhibitors. These additives are added to improve adhesion properties and provide special performance with regards to applications and storage properties. A pertinent aspect in the preparation of sealants is testing1,3, which is important to confirm the effectiveness of a bonding process and also to investigate parameters or process variables in the performance of the bonds. There are two general test categories for sealants which comprise of fundamental property test and end-use test. The most common fundamental property tests for sealants are the measurement of viscosity, shelf life, service life, cure rate, hardness and percentage of solids. On the other hand, the end-use tests for sealants are movement capability test, peel adhesion, tear strength, compression set resistance and environmental tests. Uses of Sealants Sealants are often classified according to their enduse or function. The end-use classifications include construction sealers, automobile sealers, insulated glass sealers as well as highway, walkway and airport runway sealers. Typical application areas for sealants are presented in Table 1. Although sealant volume is mostly used in construction and automotive applications, there are other end uses for sealants as niche application in the agricultural sector. For instance, a sealant is used in G-Flex, a latex stimulation technique as well as in the rainguard apparatus, which is as an apparatus to protect latex collection from rain. In the late 1980s6, Rubber Research Institute of Malaysia (RRIM) developed ‘YUSIL’, a type of sealant that can be used with

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TABLE 1. TYPICAL APPLICATION AREAS FOR SEALANTS1,2,4

Type of Sealant

Typical Application



Silicone

solar panel, construction, insulating glass, automotive



Polyurethane

automotive glass bonding/sealing, high performance building joints, insulating glass, waterproofing



Plastisol

body-sealing/automotive



Emulsion

exterior joints and interior use joints on plywood



Butyl

caulking expansion and contraction joints, home sealants, construction, repair of gaskets, tapes, with resin for hot melts on insulating glass



Polysulphide

aircraft fuel tanks, boating, building joints, sealing between dissimilar metals



Silyl Modified

Construction

G-Flex G-Flex

Rainguard Rainguardapplication application

Figure 1. Application of Sealants

corrugated aluminium foil as an apparatus to protect latex collection from rain, commonly termed as a rainguard abbreviated as RRIMGUD. Performance of Butyl Rubber Sealants 1.2 1.2

Malaysian Rubber Board (MRB) in particular has developed a butyl rubber sealant to be used in G-Flex gas stimulation technique. The sealant, made from a compound of butyl rubber, filler and plasticiser, was mixed using an internal mixer followed by the extrusion process. This butyl rubber sealant was further tested according to the American Society for Testing and Materials (ASTM) standard test method. Presently, there are two ASTM test methods adopted in MRB. These methods are Effect of Heat Aging on Weight Loss, Cracking and Chalking of Elastomeric Sealant (ASTM C 792)7 and Adhesion in Peel of Elastomeric Joint Sealants (ASTM C 794)8.

Weight loss (%) Weight loss (%)

Butyl rubber is a copolymer 1.0 1.0 of isobutylene with about 2% of isoprene. It has a relatively long and straight 0.8 0.8 and this regular structure carbon-hydrogen backbone provides butyl low permeability 0.6 0.6 to air, gases and moisture. Approximately 86%5 of worldwide usage of butyl rubbers 0.4 0.4 is found in tyres or inner tubes. In addition to tyre applications, butyl low air 0.2 impermeability, weathering and 0.2 ozone resistances, good flex properties as well as stability The test method for Effect of Heat Aging on Weight 0 make them good material 0for hoses, mechanical goods Loss, Cracking and Chalking of Elastomeric Sealant 2828days days 77days days 2828days days 2828days days 77days days 77days days and construction sealants. (ASTM C 792) is suitable for elastomeric solvent releaseA+B A+B

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AA

BB

Curing Curingperiod periodand andtypes typesofofbutyl butylrubber rubbersealant sealant MRB Rubber Technology Developments

23ºC 23ºC

50ºC 50ºC

70ºC 70ºC

type sealants. The significance of this method is due to weight loss through volatilisation of sealant composition that may affect appearance of the sealant due to shrinkage and sealant performance due to loss of functional sealant compositions. Excessive shrinkage can cause voids in sealant joint and uneven stress distribution. Substantial Rainguard application losses of weight may help predict early failures in durability of sealants. Three types of butyl rubber sealants were developed. The percentages of weight loss of the three sealants were measured and determined according to ASTM C 792. The three sealants were designated as butyl rubber sealant A, butyl rubber sealant B and butyl rubber sealant A+B. Instead of exposing the sealants at 70ºC based on ASTM C 792, the sealants were exposed to 50ºC in an oven to observe temperature dependence. From Figure 3, the percentage of weight loss for butyl rubber sealant B is lower than butyl rubber sealant A and butyl rubber sealant A+B. This indicates that as temperature and time of curing increases, weight loss increased. It can be observed that all sealants lost not more than the minimum requirement of 7% of their original weight or show any cracking and chalking when tested. Low shrinkage is often required after application of sealant since the main objective of sealants is to seal assemblies

and joints. In general, for every 1% weight loss, there is about 2% volume shrinkage. However, a low weight loss and absence of chalking or cracking does not necessarily assure good durability. The G-Flex Adhesion in Peel of Elastomeric Joint Sealant (ASTM C 794) test method provides a good indication of how sealants might perform under severe peel conditions. It gives a valuable measurement of the ability of cured sealants to maintain a bond to the substrate. This is the most common method for determining the relative adhesion of sealant joints. Figure 4 shows that peel strength of butyl rubber sealant B is higher compared to that of butyl rubber sealant A and butyl rubber sealant A+B.

SUMMARY Current demand in construction, transportation, industrial, aerospace, appliances and many other industries could not be met without advances in sealants. Construction and automotive industries represent a very large market for sealants. The basic function of sealants include filling the gap between two or more substrates while maintaining sealing property for the expected lifetime,

1.2

Weight loss (%)

1.0 0.8 0.6 0.4 0.2 0 28 days

7 days

28 days

7 days

A+B

A

7 days

28 days B

Curing period and types of butyl rubber sealant 23ºC

50ºC

70ºC

Figure 3. Weight loss percentages at several curing period of butyl rubber sealants.

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6

Peel strength (N)

5 4 3 2 1

0 A+B

A

B

Types of butyl rubber sealant Figure 4. Peel strength of butyl rubber sealants.

operating conditions and environment. To determine sealant performance, testing is an extremely important aspect which should not be neglected. There are many parameters that can affect performance of sealants. Many of the useful and common tests for sealants are defined in terms of American Society for Testing and Materials (ASTM) specifications and also International Organisation for Standardisation (ISO). They are very useful for comparing and determining the consistency of materials and processes. However, they are less valuable in predicting the actual strength of specific joints under actual operating conditions. The test methods adopted in MRB are selected ASTM standards for sealants. The data obtained from the test methods adopted does not intend to simulate the conditions encountered by a sealant in actual operating condition or field exposure. However, it provides a quantitative approximation in sealant performance.

REFERENCES 1.

EDWARD E. PETRIE (2000) Handbook of Adhesives and Sealants. McGraw-Hill Publications, 451–474.

2.

ADOLFAS DAMUSIS (1967) Sealants. Reinhold Publishing Corporation.

3.

PHILIPPE COGNARD (2005) Adhesives and Sealants: Basic Concepts and High Tech Bonding. Elsevier.

4.

MICHAEL GROWNEY (2013) The Global Sealants Market. www.pcimag.com April 2013, 30–32.

5.

Paintindia (2013) April 2013, 165–166.

6.

M.H. SAKHIBUN, A.A. YUSOF, P.D. ABRAHAM, M.D. MOHD SIDEK, H. ISMAIL, M.T. AHMAD ZARIN, H.G. CHUA (1988) Promising RRIM Rainguard with New Sealant (RRIMGUD), Planters’ Bulletin, 194, 18–23.

7.

ASTM C 792 (1993) Standard Test Method for Effects of Heat Ageing on Weight Loss, Cracking and Chalking of Elastomeric Sealants.

8.

ASTM C 794 (2001) Standard Test Method for Adhesion-inPeel of Elastomeric Joint-Sealants.

ACKNOWLEDGEMENT Assistance rendered by En. Meor Syuhaidy Meor Shapiei is gratefully acknowledged. Acknowledgement is also extended to En Asrul Mustafa, Dr Fauzi Mohd Som, Dr Mazlina Mustafa Kamal, Pn Siti Salina Sarkawi and En Muhammad Akbar Abdul Ghaffar for their support.

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MRB Rubber Technology Developments