Plastics - a primer

[ZDP-189]

Have you ever looked at a resin or sheet plastic offcut and wondered if it would make a good knife handle? Or why one plastic was well regarded and another only used in cheap working knives? Or why one plastic is generally seen in a production knife and the other in a custom? These are some of my observations from experimenting and reading around the subject over the years.

What's the best plastic to use in a handle and why can't I get a straight answer from anyone?

It's partly 'horses for courses' and partly down to the way knives are made.

Desirable properties in a knife handle include:

• Attractive appearance
• Moisture, chemical and UV stability
• Resilience to impact
• Scratch resistance
• Comfort
• Low thermal conductivity
• Low electrical conductivity
• Tactility/ grip

Desirable commercial production properties include:

• Low material cost
• Rapid production cycle
• Low labour component
• Completed product requires little finishing
• Accuracy and consistency of completed product
• Accuracy and consistency of supplied material
• Mechanical bonding normally preferred

Desirable small-workshop production properties include:

• Low minimum production-run economics
• Flexibility of final shaping
• Top quality appearance; deep lustre, high polish
• Reasonable material cost
• Adhesive bonding may be required

As can be seen from the lists above, commercial production methods and small shop production methods will favour different processes.

Specifically, commercial processes favour thermoplastic injection materials, or materials that can be permanently formed using temperature and/or pressure or U/V light. These materials can be moulded cheaply, quickly and accurately in large production runs in moulds. Utilising only a single material, there are no production complexities due to mixing. Only in some rare cases are thermoset materials such as two-part resins and natural materials such as wood used and these increasingly tend to be sheet materials cut and formed by CNC machinery and these are usually features of high-end ‘mid-tech’ knives.

On the other hand, small workshop processes favour thermoset materials with a high melting point and high tensile strength. Their high temperature stability allows them to be sanded and polished quickly and without special precautions and procedures. Referring to the Wikipedia article attached, thermoset materials also tend to be harder and therefore easier to machine and more durable than thermoplastics. Thermoplastics can be used, however. I have used plenty of jungle bolos that were handled with scales of PVC piping, a notoriously soft and adhesive incompatible plastic. PVC was no doubt chosen by the knife maker because of its plentiful availability, low cost, ease of shaping by hand and because it is a good candidate for mechanical (rivet) fitting. The result was a long lasting, chemically impervious handle that would probably outlast the carbon blade to which it was attached. However, for the mid-sized home workshop, using power tools, PVC is not a material of choice because it melts on the sander, is difficult to polish, hard to adhere to and is less strong and attractive than most other materials.

Not all plastics used in the home workshop must be ground. It may take more time, but the dedicated hobbyist may consider moulding parts. Here, binary plastics such as polyester resin and epoxy are far easier to mould than a thermoplastic such as polyamide (Nylon), because they can be done without expensive dies and injection moulding equipment. Moulding still takes more time and preparation and is generally not suited to hobbyist production, because moulding defects can be hard to overcome, but in some situations, moulding can save labour in small production runs or permit special effects not possible by grinding alone in one-off custom works.

Just like woods, when selecting plastics, the general guide is to select the hardest material that the budget will allow in order to obtain the best finish and durability. However, hard materials (thermosets) tend to be brittle and soft materials (thermoplastics) tend to lack tensile strength. These material defects can be mitigated through reinforcement. Reinforcement may be mechanical (bolsters and rivets, and inlaying) or it may take the form of fibres running throughout the material. Both thermosetting plastics like epoxy (e.g. GRP or carbon fibre construction), and injection thermoplastics like polyaminde (e.g. Zytel) can be reinforced.

There are a few notable exceptions to this guideline. If a rubberised handle is required, a rubber handle can be moulded in the small workshop with little special preparation. The trick when selecting a rubber is to seek one that:

• Does not require vulcanisation
• Is chemically and U/V stable
• Has good wear and tear (meant literally) resistance
• Offers low shrinkage during curing
• Bonds acceptably

These requirements are consistent with a high durometer (hard), high strength elastomer such as a two-part urethane. RTV silicones lack most of the above properties, as does latex. Synthetic rubbers such as Santoprene are usually thermoplastic by design and are not appropriate for a small workshop.

Another notable exception to the guideline is celluloid, which melts, scorches and even cracks when sanded or buffed quickly, but some makers are prepared to put up with its deficiencies due to its beautiful translucent colours. Other clear thermoplastics such as Poly methyl methacrylate “PMMA”/ “Acrylic” (Perspex) and to a lesser extent the harder polycarbonate (“Lexan”) share these problems, but without the glorious and diverse colours of celluloid. Polycarbonate is readily milled, whereas even acrylic tends to chip out and/or melt. Polycarbonate is the transparent material of choice, but if one can spend the time to hand work and finish it, acrylic will often do the job.

Corian too, has its problems. Corian a brand name of Dupont, but here’s we refer to the generic composition of acrylic polymer and alumina trihydrate particles. According to Wikipedia, Corian is said to be a thermosetting plastic, but can be thermoformed by heating it to 300°F (149°C). So it works to a point, but is ultimately still prone to the same problems of thermoplastics.

What considerations are there when designing or using a laminate?

Generally, phenolic resins (Micarta) are cheaper and offer more choice. They can be identified by the dull sound produced when they are tapped. One can buy micartas made of phenolic resin matrix combined with paper, linen, canvas, or other materials. Epoxy resin laminates are harder and stronger and make a 'ping' sound when a billed is tapped. If one wants to do delicate machining of fine parts, an epoxy resin laminate such as G10 is required, but as a replacement for wood, horn or bone a phenolic or polyester based resin would do.

Should one wish to make one’s own laminate, one can purchase large tubs of epoxy or polyester with which to laminate a material of one’s choice. There are at least two good tutorials here on BB, but here are some material considerations to consider. In conventional GRP laminates, the fibre is a critical structural component. Thermosetting plastic resins tend to be very strong in compression, but have relatively weak tensile properties. Thermoplastics are actually weaker, but give by stretching and have elastic properties, so are less likely to snap. By combining a thermosetting resin with a glass fibre mat, the best properties of both materials in the composite are exploited. However, in knifehandles, these tensile properties are less critical, except in the cases of stub-tang knives upon which high bending forces are applied and cases wherein knives are dropped on their handles or the handle is battoned against. Whereas a material selected purely for its structural properties would be limited to high-tensile reinforcement matting such as glass fibre, knife handles are less critical and other aesthetic and practical factors apply. For example, I am told that burlap may have poor tensile properties but the swelling of exposed fibres creates a ‘grippy’ surface when wet. Certainly the effect is noticeable with canvas and to a lesser extent, linen micarta. Paper micarta has long been a mainstay of knifemakers as well.

The conventional engineering application of a composite handle might be a gel-coated monocoque with structural baffles and cross bracing. The matting would be applied in an appropriate weave and laid so as to make the best of the tensile properties of the weave. Of course, a hobbyist maker would not be likely to go to such extremes and normally grinds/sands back from a solid billet of multiply laminate sheet. While it may be more aesthetically pleasing by mimicking woodgrain, it detracts from the desired properties of the laminate, creating a material that is heavier than required and greatly increases the probability of delamination. Poor adhesion of the epoxy matrix material to the reinforcing matting may give rise to fibres fraying at the ground edges (a particular problem with carbon fibre). Voids caused by air bubbles remaining trapped during the lamination process are also possible failure points should the handle receive an impact. Both these effects can result in fraying of drilled holes that is both unsightly and possibly indicative of a poor composite material. A practical remedy for such problems is to use a matrix resin with a high bonding strength, such as epoxy or to select a more porous or easier to adhere reinforcement material. Another approach would be to mix-in the reinforcing material rather than to laminate it. Tufnol has long been available made with cloth fibres mixed in and I understand the former East Germany is still littered with derelict Trabant chassis which are expected to outlive us all.

So where can hobbyists source different types of plastics?

These are a few of my favourite sources:

• Tufnol (a brown phenolic resin laminate), natural yellow green epoxy fibreglass G10 nylon and occasionally carbon fibre formed into sheets, rods and blocks are available from electrician supply outlets. Electricians use them as insulators. These places are often a good source of cheap offcuts.
• Electricians’ supply outlets also stock vulcanised fibre sheets in large rolls and it is surprisingly cheap.
• One can also buy black two-part potting compound from electronics hobby supply outlets. It’s cheap and a dense opaque black, and while it lacks the mechanical strength of epoxy, it flows very readily and remains fluid for longer, making it good for filling mosaic pins.
• Acrylic and polycarbonate sheet, rods, tubes in various colours, plus sheets and rods of slate grey PVC sheet are available at plastic supply merchants and offcuts are both plentiful and very cheap. I know one village smith that handles his bolos from medium diameter scrap PVC piping, but PVC sheet is much easier to fit to a tang.
• Materials like bakelite and similar sawdust-based, pressure and heat-set are commercially outdated and unsuitable for pre-forming into sheets and so are unavailable to the hobbyist.
• Laminated/composite thermoset plastics (other than Tufnol and colourless G10) are best purchased from knifemaking suppliers such as Texasknife or Sheffield Supply, in my opinion. Masecraft, in particular, specialise in laminates. By buying from a supplier, one has a greater selection of colours and laminate materials and one can be assured of receiving a uniform, void free material of high quality, with flat surfaces that are easy to fit to a tang. If the material one is looking for is not available online, one of the ‘custom’ makers should be able to supply what one is looking for. Hobbyist makers operate under different economics and so can produce very small runs sufficient only to make one pair of scales.
• I have found a local industrial resin supplier that can supply a vast range of resins for home and industrial casting. Volumes sold range from 1kg to a 50 gallon drum. These suppliers offer RTV silicones, polyurethane resin, polyurethane foam, two-part casting silicone putty, epoxy, black loaded epoxy, acrylic and polyester resins, fibreglass matting, and essentially all other types of thermosetting resin. It is always cheaper to buy in bulk, as long as one can use it all within a short timeframe, as air can initiate bonding, especially in polyurethanes. For example, 2kg of Araldite brand 90 minute epoxy costs £32 (£16/kg), whereas 25 ml a syringe of 90 minute Versachem wholesales for £1.50 (£30 per kg) and retails for around 30% more.
• As a matter of convenience, a DIY, motoring or yachting superstore would be a good source of glassfibre matting, epoxy and polyester gelcoat, but paying retail prices for materials one may need by the kilogram should be an exceptional occurrence.
• The only place I have found celluloid is online from one of the major knifemaking supply houses.

References and Reading:

Thermosetting plastics

Thermosetting plastics (thermosets) are polymer materials that irreversibly cure, to a stronger form. The cure may be done through heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation.

Thermoset materials are usually liquid or malleable prior to curing, and designed to be molded into their final form, or used as adhesives.

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, when the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.

Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled, except as filler material.

Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material.

Some examples of thermosets are:

• Vulcanized rubber
• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plastic wear)
• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)
• Melamine resin (used on worktop surfaces)
• Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)
• Polyimides (used in printed circuit boards and in body parts of modern airplanes)

Some methods of molding thermosets are:

• Reactive injection molding (used for objects like milk bottle crates)
• Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables
• Compression molding (used to shape most thermosetting plastics)
• Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)

Wikipedia on Thermosetting Plastics

Other Wikipedia Entries:
Plastic
GRP
Epoxy
Micarta

Other Links:
Tufnol
Adhesives
Micarta

Polymerplastics' excellent guide
Composites
Mechanical Plastics
Performance Plastics
Flouroplastics
Transparent Plastics
Forming Grade Plastics
Corrosion Resistant Plastics
Film & Graphic Plastics

BB Turorials:
joelbolden's inspirational Micarta tutorial
One of fod's many great micarta threads

PS/

Tuffnol, Nylon, Micarta, Perspex, Lexan, Zytel and many of the other terms herein are registered tradenames and the properties of their respective trademark holders, but their use is commonly applied to generics of their type and I have used them as such.


PPS/

This thread is intended as a primer for new makers and people that are forming first opinions about what they want on a knife. The comments here represent only my opinion and my current opinion, at that. I have done my best to get my facts straight and to think this through properly, but I am no chemist, nor an engineer and have been known to get things totally wrong. If I have done so, or you disagree, or have other observations please say so and don't be shy in contradicting me. Debate and dissent makes us all stronger. United we stand; divided we move ahead