杏吧原创

Greener and greener

Question: When waste paper is recycled the new product is clearly of a poorer
quality than the original paper. Is there a further deterioration if recycled
paper is itself recycled?

Can papers recycled several times be separated from nonrecycled waste paper?
And is there a limit to the number of cycles that constituent materials can go
through?

Answer: Paper-making fibres, which usually originate from wood pulp, can be
used about six times. Each time they are used the fibres deteriorate鈥攖hey
wear down and become shorter. Because of this it is necessary to add virgin
fibres to strengthen and maintain the quality of the finished paper. However,
some papers require special characteristics or are used for purposes best met by
virgin fibre alone.

In the normal course of sorting domestic waste paper, it is practically
impossible to identify and separate those papers made from virgin fibre from
those which contain recycled fibre. It may be easier to separate commercial
waste paper, but only if the supplying source (such as a printer) knows exactly
what the paper is made of and keeps it totally separate from any other waste
paper.

Ruth Mackman

Pulp and Paper Information Centre, Swindon

Answer: Recycling causes paper to deteriorate in two ways. First, the
cellulose fibres which give the paper its strength are shortened, so the
resultant paper is dense, stiff, less opaque, less durable and has low
resistance to tearing. Secondly, impurities, mainly printer鈥檚 ink, make the
paper less white. If the same paper is recycled again and again, both these
problems will increase each time.

The number of cycles that paper can go through is limited by the fact that it
has to support its own weight during the manufacturing process, otherwise it
tears and production time is wasted. The mills avoid this by adding virgin pulp,
which is perfectly legal. The National Association of Paper Merchants allows its
鈥淎pproved Recycled鈥� logo to be used on paper which contains only 75 per cent
鈥済enuine waste fibre鈥�.

Paper that is to be recycled is separated into two main categories:
pre-consumer waste (from the mill itself, for example) and post-consumer waste
(the 鈥渞eal鈥� recycling we do at home and in the office).

Paper can be recycled into products such as packaging materials without
having to be de-inked, or it can be put through a de-inking process and used to
make low- grade materials such as newsprint or paper for directories. This sort
of recycling is universally acknowledged to be a 鈥済ood thing鈥�.

However, top-quality recycled paper, which might be used for company
stationery where colour consistency is required contains up to 25 per cent
virgin pulp, and up to 65 per cent pre-consumer waste. Only 10 per cent comes
from post-consumer waste. Even then the post-consumer waste has to be bleached
after de-inking.

In this way the paper in top-quality products can be recycled indefinitely.
But many people are horrified when they discover the very low post-consumer
waste content of such paper, and the additional bleaching means that the overall
benefit to the environment is questionable.

Jonathan Cooley

Egham, Surrey

Answer: Recycling shortens and weakens the cellulose fibres in paper until
they break.

The resulting pulp has to be used in progressively lower-grade
applications.

When paper gets to the bottom of the reuse chain, in a product such as
corrugated cardboard, a different method of recycling is needed. You can fill an
old cardboard box with corrugated card, urinate on it and leave it to decompose
to compost.

Then you can use the compost to mulch trees and regenerate the cellulose
fibres this way. Food boxes such as cereal packets can also be composted in a
garden compost heap. They provide air spaces, aerating the compost until they
rot down. Card adds useful 鈥渞oughage鈥� to your compost heap, offsetting the
excess nitrogen and general sliminess that results from too many grass
cuttings.

The Centre for Alternative Technology in Machynlleth, Powys, has lots of
information on composting for anybody wanting to take this further.

David Edge

Hatton, Derbyshire

What鈥檚 the crack?

Question: What causes the noise when you crack your knuckles or any other
joint?

Answer: A click or crack is often heard when a joint is moved or stretched.
When the pressure of the synovial fluid in the joint cavity is reduced, this may
create a gas bubble and generate a popping sound. The sound my also be the
result of separating the joint鈥檚 surfaces, which releases the vacuum seal of the
joint.

These noises are sometimes produced during osteopathic treatment, but this
does not prove that the treatment has worked, nor does their absence mean the
treatment has failed. The test of success is whether the joint鈥檚 range and ease
of movement have been improved.

Will Podmore

The British School of Osteopathy, London

Answer: All the soft tissues of the body, including the capsules of joints,
contain dissolved nitrogen. When a vacuum is applied to the joint space by
pulling on the bones, say by flexing the fingers strongly, nitrogen suddenly
comes out of solution and enters the joint space with a slight popping
sound.

Radiologists often see a small crescent of gas between the cartilages of the
shoulder joint on the chest X-rays of children who are held by the arms. This is
due to the force of pulling on the arms causing nitrogen to evaporate into the
joint space. It can infrequently be seen in the hip too.

Small, highly mobile bubbles sometimes appear within the hip joint of a baby
being screened for congenital dislocation of the hip using ultrasound. This
usually happens if the infant is struggling and has to be held firmly. The
bubbles disappear after a short while when the nitrogen dissolves again.

If the fingers were X-rayed immediately after cracking the knuckles a fine
lucency, as a result of thousands of tiny opaque bubbles, would probably be
visible between the ends of the bones.

Tony Lamont

Mater Children鈥檚 Hospital

Brisbane, Queensland

This week鈥檚 question

Space hop : Imagine you鈥檙e sitting in a pub with a pint of real ale. Gravity
keeps the beer in the glass and the bubbles rise through it.

Now imagine that you have a globule of beer floating in a spacecraft in zero
gravity.

What happens to the bubbles? What direction do they move in鈥攊f they
move at all? Are they the same size as on Earth? Would the beer have a frothy
head? Are there likely to be any other unusual effects?

Stephen Stewart

by e-mail, no address supplied