Method of protecting wooden objects from decay

A method in which a flexible outer barrier layer is applied to at least that part of the object most vulnerable to decay, a bituminous substance is provided between the outer barrier layer and the object, and the barrier layer is heated so as to melt the bituminous substance sufficiently for it to bond intimately to the wooden object. The barrier layer may be a heat shrinkable plastic film and may be a sleeve which is heat shrunk onto the object. The invention is particularly useful for protecting vulnerable parts of elongate wooden objects, such as fence posts or poles, from decay.

BACKGROUND OF THE INVENTION 
This invention relates to a method of protecting wooden objects, 
particularly elongate ground contact objects such as fence posts and 
telephone and electricity poles, from decay. 
Preservative treated fence posts despite advances in preservative 
technology are still prone to ground level decay. Typically, after 20 
years service, 5%-30% of posts will be showing signs of decay at or below 
ground level, although this is to some extent dependent on ground 
conditions and standards of preservative treatment. 
This is unfortunate as the other parts of the posts, i.e. those deep below 
ground and those well above ground, would give a typical life of 40-50 
years or more. 
The top portion of a fence post, i.e. that from approximately 50-75 mm 
above ground level, is exposed to natural air flow and sunlight which will 
ensure a typical moisture content of 15%-70%, dependent on weather 
conditions. These factors combined with preservative treatment should mean 
that decay is very rare. 
The bottom portion of the post, i.e. that from approximately 300 mm below 
ground level, does not decay because of the very high moisture content 
(typically 30%-90%) and the compacted wet soil conditions will ensure 
minimum levels of oxygen which is vital for the growth of rotting 
organisms. 
This leaves the middle portion from about 300 mm below ground level to 
about 75 mm above ground level where conditions are ideal for decay. 
The fence post acts as a wick drawing up water and solids to a band at 
ground level where the water evaporates. This portion of the post is very 
damp, typically 30%-90% moisture content, and is exposed to ground and 
airborne micro-organisms which come into contact with the post and form 
primary moulds which are then followed by soft rots and Basidiomycetes. 
Although preservatives greatly extend the life of fence posts (untreated 
posts typically last only 3-4 years), it is difficult to measure the 
standard of preservative treatment. Consequently, poor treatment can pass 
unnoticed. 
Preservative belts, sleeves and wraps are also known. These relatively 
expensive products are predominantly used for the treatment of posts and 
poles in service, where their main purpose is to provide preservative 
material to replace that which may have been lost due to, inter alia, 
leaching. The sleeve wrap or belt provides additional preservative 
material at the outer face of the pole or post which is then absorbed, the 
objective being to totally penetrate the pole or post with preservative, 
hence eliminating decay. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method of protecting 
wooden objects from decay, the method comprising the step of applying a 
flexible outer barrier layer to at least that part of the object most 
vulnerable to decay, providing a bituminous substance between the barrier 
layer and the object and heating the barrier layer so as to melt the 
bituminous substance sufficiently for it to bond intimately to the wooden 
object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the avoidance of doubt, the term "bituminous substance" used herein 
includes, for example, bitumen, tar, pitch, coal tar, asphalt, gilsonite, 
or any combination thereof in straight or modified form. 
Preferably, the outer barrier layer is in the form of a plastics film or a 
metal foil or a combination thereof. Where a plastics film is used, it may 
be made from recycled plastics material. 
Preferably, the barrier layer is coated with said bituminous substance 
prior to application to the object. 
Preferably, where the barrier layer is in the form of a plastics film, it 
is applied to the object at an elevated temperature. 
In this latter case, the plastics film is, preferably, of heat shrinkable 
plastics material and is formed into, or is in the form of, a sleeve which 
is heat shrunk onto the object. 
Alternatively, the plastics film may be in the form of an elongate strip 
which is wound about the object while applying heat and tension to the 
strip, or may be in the form of a sheet which is wrapped around the object 
whilst applying heat and pressure. 
Typically, the bituminous substance may include wood preservatives and/or 
adhesion promoters. 
In the case of fence posts and ground engageable poles, it is only 
necessary to surround a part of the post with the barrier layer and the 
bituminous substance. This may be a portion of the post intermediate it 
ends or it may be the entire lower portion of the post. It is not 
necessary to surround that portion of the post which will, in use, be 
disposed well above ground level. 
The invention also resides in an object, such as a wooden fence post or 
ground engageable pole, protected by the aforesaid method. 
In one example, a heat shrinkable thermoplastics sheet (typically low or 
medium density U.V. stabilised polyethylene or polyvinylchloride) is hot 
coated on one face with a molten bituminous substance comprising one or 
more of the group comprising bitumen, tar, pitch, coal tar, asphalt or 
gilsonite, in straight or modified form. The bituminous substance should 
be sufficiently fluid to bond to the wooden object at a temperature lower 
than the temperature at which the thermoplastics sheet is heat shrinkable. 
Bitumen has been found to be particularly suitable and typically the 
thickness of the bitumen solution is about 1 mm. The bituminous substance 
may include wood preservatives and/or adhesion promoters. It may also 
include fillers, extenders, plasticisers and/or elastomers. It may also 
include reinforcing fibres. However, the total of any wood preservative, 
adhesion promotors, fillers, extenders, plasticisers and elastomers 
preferably should not exceed 20% by weight. Indeed, if any wood 
preservative is to be used it may well be desirable to overcoat the 
bituminous substance with it. 
The sheet of thermoplastics material is preferably of sufficient thickness 
to be resistant to grass trimmers and animal attack and, typically, has a 
thickness of 100-600 microns, but can be thicker. It may be necessary to 
cool the other face of the sheet during hot coating with the bituminous 
substance to prevent the sheet reaching a temperature at which shrinking 
will occur. Also, it may be desirable to pretreat the sheet using corona 
discharge or a flame to improve adhesion of the bituminous substance to 
the film. 
It has been found that a blend of 50% by weight of 50 PEN grade bitumen 
+50% by weight of 85/40 oxidized grade bitumen (BS3690 Part 2 1994) will 
operate satisfactorily over a temperature range of 0 to 30.degree. C. 
without cracking at low temperatures and without sticking to itself or 
adjacent sleeves at higher temperatures. For a larger temperature range it 
may be necessary to use a very sticky layer of 200-800 PEN grade bitumen 
adjacent to the thermoplastics sheet with a second less sticky top layer 
of 25-50 PEN grade bitumen. Also, in either case, it may be desirable to 
dust over the exposed surface of the bitumen to ensure it does not stick 
to an adjacent thermoplastics sheet during storage and transportation. 
It has subsequently been found that an 85/40 oxidized grade bitumen with 
added temperature dependent viscosity modifier allows the characteristics 
of the bitumen to be maintained to 40.degree. C. whilst giving a rapid 
reduction in viscosity above this temperature thereby ensuring excellent 
bond to the pole or post. A grade of bitumen with a straight viscosity 
modifier has also proved effective. 
The film is then formed into a sleeve (such as by welding) which is of 
large enough diameter (or equivalent dimension) to fit over a fence post 
or pole, but not so large that it cannot be heat shrunk tightly onto the 
post or pole. However, an extruded sleeve can be subsequently coated with 
bitumen. 
In forming the film into a sleeve, the film is preferably orientated such 
that the main direction of shrinkage extends circumferentially around the 
sleeve. 
The sleeve is placed over the fence post (which may or may not be 
previously treated with conventional preservative) and positioned so that, 
when heat shrunk, it will encapsulate the required area of the post or 
pole. 
The sleeve is then heated (typically using hot air or infra red heating or 
possibly even a blow torch) at a temperature of between 
80.degree.-250.degree. C. This temperature will be sufficient to ensure 
that the bituminous substance has melted to a sufficiently liquid state to 
intimately bond to the post prior to shrinkage of the sleeve. Thus, when 
the sleeve shrinks, an airtight sleeve is formed around the post. The 
sleeve is advantageously heated from one end in order to ensure that all 
air is expelled. However, the sleeve could be perforated to allow air to 
escape, the perforations being sealed by the bitumen. 
The sleeve, together with the bituminous substance, will prevent oxygen, 
micro-organisms, nitrogen and termites from the surrounding soil and air 
entering that portion of the post which has been encapsulated and will 
also greatly reduce the leaching of preservatives from the posts into the 
surrounding soil. 
Also, the presence of the sleeve below ground effectively moves the entry 
point for moisture by the length of the sleeve below ground level. Thus, 
in the encapsulated area the moisture content is considerably lower than a 
non-encapsulated area. Depending on sleeve length and ground conditions, 
it is possible to reduce the moisture content below the 20% level needed 
for decay to occur within the encapsulated portion. The overall effect of 
the sleeve is to dramatically reduce the likelihood of decay and resultant 
premature post or pole failure. 
The film may be stored and transported in the form of a coated sleeve or in 
the form of a coated sheet which could be rolled up for ease of storage 
and transportation and which is cut to size and formed into a sleeve by 
overlapping the edges and stapling it to the post or pole when it is to be 
used. It is then heat shrunk as described previously. 
A strap could be tied around the top and bottom of the sleeve where the 
sleeve is most vulnerable and/or the top and bottom of the sleeve could be 
folded into the remainder of the sleeve prior to heat shrinking. This will 
result in a higher clamping force at the top and bottom of the sleeve when 
the sleeve is heat shrunk. 
Electricity and telephone poles are generally treated with creosote. 
Non-cross linked polyethylene can be unsuitable as it can be attacked by 
creosote. This can be overcome by using cross-linked polyethylene for the 
barrier layer, but this is expensive. As an alternative, it would be 
possible to use an intermediate layer, typically of aluminium foil, 
between the polyethylene and the bitumen. The foil and polyethylene could 
be fused together by heat. 
Indeed, it may be possible to replace the thermoplastics sheet with a metal 
foil or sheet and to use this as the barrier layer. However, in this case 
it will not be possible to heat shrink the barrier layer onto the post or 
pole, although heat may still be applied to soften the bitumen on 
application to the pole or post. 
The above method of encapsulation can also be used to protect other wooden 
objects in other applications. 
In another example, the plastic sheet material is in the form of an 
elongate strip which is hot coated on one face with a molten bituminous 
substance and which is then wound about the object in helical manner, 
while applying heat and tension to the strip. 
In yet another example, the plastics film is in the form of a sheet which 
is hot coated on one face with a bituminous substance and which is then 
wrapped around the object while applying heat and pressure. 
In these latter two examples, the plastics material can be of either heat 
shrinkable material or non heat shrinkable material. 
Some preservative materials can be temperature sensitive and their 
effectiveness can be reduced by applying a heat shrinkable sleeve. To 
overcome this, a non-heat shrinkable sleeve can be made with a low melting 
point bituminous substance allowing the sleeve to be applied at lower 
temperatures without neutralising the preservative.