Applications of Polymers
Sealants are elastomeric materials that can be used for sealing joints against wind and water in construction. Thin curtain wall construction employs highly effective materials to provide the heat installation, but generally there is no cavity for the dispersion of water that may leak through the joints on the outside.
In addition, in the event of air blowing directly to the inside of a joint there must be an effective material to provide heat insulation, consequently a baffling system to provide this must be installed. Therefore, adhesive and elastic sealants are required to enable this type of construction to be used efficiently. The largest variety of sealants fall into the classification of solvent release and are composed of three component parts:
- the basic non-volatile vehicle (the liquid portion of the compound)
- the pigment component
- a solvent or thinner to make application easier.
The non-volatile vehicle can vary from a vegetable oil (e.g. linseed) to a synthetic elastomer. Opacity or colour will be introduced into the material by the pigment component. To enable the sealant to be applied easily and to ensure the correct thickness is achieved a solvent is introduced. The sealant is cured and its required viscosity is reached by the evaporation of the solvent. The butyl rubber solution and the acrylic copolymer solution fall into this category.
Another group of sealants comprises those that are chemically cured. Polysulphide compounds and silicone-based compounds are the main sealants under this heading. The latter, which is a two part sealant, is highly dependent upon the environmental conditions for its rate of cure, thus, if the temperature and humidity are low, the curing period could be very long. The chemically cured compounds require adhesion additives in order to develop bond to a surface, as they do not generally contain much solvent.
The desired properties of a sealant are:
- a good adhesion with the joint
- low rate of hardening
- low rate of shrinkage
- permanent elasticity.
The choice of sealants is a compromise as no one product has all the above mentioned attributes.
Within the context of the construction industry the term adhesive embraces not only those materials that are used to bond together two components of a structure, but also those materials that provide a specific function in themselves (e.g. protection, decoration) and are at the same time self-adhesive to the substrate whose surface they modify. Thus, a mortar that may be used to bond bricks together may also be applied as a self-adhesive protection and often decorative rendering over the finished block work.
In this article adhesive bonded connections are the primary concern. The physical nature of fibre/ matrix composites introduces problems that are not encountered in metals. The fibre type and arrangement, as well as the resin type and fibre volume fraction will influence the behaviour of the joint.
In addition, composites are not generally homogeneous throughout their thickness as many thermosetting polymers have gel coats applied to the laminating resin to protect it against aggressive environmental influences met with in construction; the resin-rich surface layers will thus be brittle and, when overloaded, are liable to display a brittle fracture.
An appropriate resin should therefore be chosen, such as a compliant one that will distribute the applied load over a large area, thus reducing the stress taken by the friable surface of the composite.
There are two particular problems associated with adhesive bonding of fibre-reinforced polymer (FRP) materials:
- the attachment to the surface of a layered material
- the surface may be contaminated with mould-release agents remaining on it from the manufacturing procedure.
As the matrix material in a polymer composite is also an organic adhesive, the polymers that are used to join composite materials together are likely to be similar in terms of chemical composition and mechanical properties.
Currently epoxy- and acrylicbased toughened adhesives are used for general application and have proved over the years to be very versatile and easy to use; they are durable, robust and relatively free from toxic hazards, and the toughened adhesives exhibit high pull strengths.
The basic requirements for the production of a satisfactory joint are:
- The adhesive should exhibit adequate adhesion to the materials involved.
- A two-part epoxy resin with a polyamine-based hardener should be used, which exhibits good moisture resistance and resistance to creep.
- The Tg of the adhesive should generally be greater than 60°C.
- The flexural modulus of the material should fall within the range of 2 to 10 GPa at 20°C.
- The equilibrium water content should not exceed 3% by weight after immersion in distilled water at 20°C. The coefficient of permeability should not exceed 5 × 10−14 m2/s.
- It should possess gap-filling properties, be thixotropic, and be suitable for application to vertical and overhead surfaces.
- It should not be sensitive to the alkaline nature of concrete if this material is the adherent and it should not adversely affect the durability of the joint.
Adhesive formulations are, in general, complex. To the base resin is added one of a range of different types of curing agents (hardener) and additives, such as fillers, toughening agents, plasticizers, surfactants, anti-oxidants and any other required materials.
Curing agents are chosen depending upon whether the cure of the resin is to be at ambient or at elevated temperatures; the rate of a chemical reaction is approximately doubled for every 8°C degree rise in temperature. It will be clear that the properties of the adhesive will be altered with the large variety of additives that can be incorporated into the base resin.
If the adhesive is required to join two dissimilar materials, such as polymer composite and concrete or steel, the mechanical and thermal properties should be considered in relation to these two materials.
The effects of environmental and other service conditions on the adhesive material and on the behaviour of bonded joints must be considered carefully. With some bonding surfaces, such as steel, it will generally be necessary to apply an adhesive-compatible primer coat to generate a reliable and reproducible surface.
With concrete surfaces it might be advisable to use a primer to give suitable conditions on which to apply a relatively viscous adhesive. It will not generally be necessary to prime a composite surface.
As with all resins it is necessary to keep a close check on the following items to ensure that the adhesive, when used in the field, is in pristine condition:
- the shelf-life is within the manufacturer’s recommended time limit
- the viscosity and wetting ability are satisfactory
- the curing rate is correct
- the ambient temperature does not fall below the specification value
- post cure is complete before load is applied to the joint.
The elastomer is another member of the polymer family. The material consists of long-chain molecules that are coiled and twisted in a random manner and the molecular flexibility is such that the material is able to undergo very large deformations. The material is cross-linked by a process known as vulcanisation, which prevents the molecules of the elastomer moving irreversibly relative to each other when under load. After a curing process, the molecules are crossedlinked like a thermosetting polymer.
As the vulcanisation process does not change the form of the coiled molecules but merely prevents them from sliding, the elastomeric material will completely recover its original shape after the removal of an external force.
One area in which major advances in polymer science have been made in the last 35 years has been the burgeoning use of these materials in the geotechnical engineering industry. The most common material is the geotextile, a simple definition of which is that it is a textile material used in a soil environment. In the early 1970s these materials were referred to as civil engineering fabrics or filter fabrics, their primary use being as filters.
In the latter part of the decade they became known as geotechnical fabrics, as they were primarily used in geotechnical soil engineering applications. It was in the early 1980s that the term geotextile was suggested as a suitable name for this type of material. At the same time impermeable polymeric membranes were also being used increasingly. These materials became known as geomembranes. Thus in the mid-1980s many types of polymeric-based materials were being used in the geotechnical engineering industry, and many of these could not be classed as either a geotextile or a geomembrane.
To encompass all these polymeric materials the new name ‘geosynthetic’ was derived, which is defined as a synthetic (polymeric) material used in a soil environment. Ingold and Miller (1988) discuss the different materials available for use in civil engineering, their properties and their measurement. Geosynthetics, which are all thermoplastic polymers, can be divided into five broadly based categories:
- Geotextiles: polymeric textile materials used in geotechnical engineering applications. These materials are essentially permeable to the passage of water.
- Geogrids: open, mesh like polymeric structures.
- Geomembranes: polymeric materials in sheet form that are essentially impermeable to the passage of water.
- Geolinear elements: long, slender, polymeric materials normally used as reinforcing tendons in soils and rocks.
- Geocomposites: covers all polymeric materials used in a soil environment not covered by the above four categories.
Each of these is now discussed.
Geotextiles: Geotextiles are usually classified by their method of manufacture and are made in two stages: the manufacture of the linear elements, such as fibres, tapes, etc. and the fabrication of those linear elements into geotextiles. The fibres are the basic load-bearing elements in the material and the forming technique determines the structure and hence the physical and mechanical characteristics of the system. The main fibres used in geotextiles are synthetic ones such as polyethylene, polypropylene, polyester and polyamide.
Geogrids: Geogrids are often grid-like structures of thermoplastic polymeric material, and in conjunction with the soil form a quasi-composite system, where the grid structure is the fibre and the soil is assumed to be the ‘matrix’ and forms an efficient bond with the fibre. Geogrids are of two forms: cross-laid strips and punched thermoplastic polymer sheets. The manufacturing techniques for the strips and polymer sheet are discussed in Hollaway (1993).
Geomembranes: Geomembranes are synthetic materials manufactured in impermeable sheet form from thermoplastic polymers or bituminous materials. Both materials can be reinforced or unreinforced; the former is manufactured on a production line and the latter can be produced on a production line or in situ. The matrix can be reinforced by textiles.
Geolinear elements: Geolinear elements are long, slender strips or bars consisting of a unidirectional filament fibre core that is made from a polyester, aramid or glass fibre in a polymer sheath of a low-density polyethylene or a resin. The components of the system form a composite, in that the fibre provides the strength and extension characteristics and the matrix protects the fibre from internal influences and provides the bonding characteristics with the soil.
Geocomposites: Geocomposites consist of two or more different types of thermoplastic polymer system combined into a hybrid material. Their main function is to form a drainage passage along the side of the water course, with a polymer core as the drainage channel and the geotextile skin as the filter.
As is apparent, many of the materials in each of the above groups are fibre composites of a parent polymer reinforced with polymer fibres.