LR White™/Gold™ Resin Systems
Some frequently asked questions
This list will forever be a "work in progress". We want to try to anticipate
just about any question one might ask so that we can give them the answer
literally "waiting".
Question 1:
I have heard that LR White has a very low viscosity, is it really like that
of water?
Answer:
Yes, that is correct. That is what makes L. R. White resin to be one of the
most infiltrateable resins available. The only resin monomer that has a
lower viscosity is the SPI-Chem™ Low
Acid GMA.
Question 2:
I have a very difficult to infiltrate system: Seeds. My goal is to do
immunolabelling on them. What would you suggest?
Answer:
See the publication by V. A. Lindley, Microscopy Research and Techniques 21,
355-360 (1992). This protocol uses a pretreatment with gamma-glycidoxypropyl
trimethoxysilane followed by embedding in L R White. Some typical samples
mentioned: weevils, seeds
Question 3:
Just how critical is the exclusion of oxygen for the successful
polymerization of L. R. White resin?
Answer:
Very critical! And there are several different commonly used tricks to make
sure that oxygen is excluded from the system. Some are using plain ordinary
gelatin capsules, the size
of the capsule being determined by the size of the specimen.
Yet others use "flat embedding", using flat
embedding molds, and for UV low temperature polymerization, the SPI clear
UV transparent type really are the best choice for many. To exclude oxygen,
the trick is to "overfill" the cavities, and then place on top another flat
embedding mold. The capillary action tends to seal the interface, excluding
oxygen.
However, whether using gelatin capsules or flat embedding molds, the
recommendation is that the polymerization be not done under vacuum.
Question 4:
I am finding that the resin itself is not the easiest to section, indeed it
seems at times to be quite brittle. What can be suggested to minimize these
problems?
Answer:
Some find that trimming the block first, either with a ready-to-be-
resharpened diamond knife or a glass knife with water in the knife boat, can
minimize damage due to the brittleness of the block, and get the block ready
for serious sectioning with a good diamond knife.
Question 5:
What is the highest cure temperature the LR White resin can withstand
without yellowing or degrading?
Answer:
Polymerization temperatures over 65°C (149°F) will result in a significant
exotherm. The degree of this effect will depend on the size of the block
being cured and of course the higher the temperature the faster the cure
and the more exotherm generated. Eventually, the resin will boil (about
85°C/185°F). The faster the resin cures, the harder and more brittle will
be the final polymerized (cured) block.
Question 6:
Does the LR White need to be gelled prior to being poured into the sample
mold we are using?
Answer:
LR White does not need to be gelled before being cast. It is important
that oxygen is excluded as far as possible to avoid a 'sticky' poorly
polymerized surface.
Question 7:
Can LR White be polymerized at really low temperatures?
Answer:
LR White has really not been formulated to polymerize at low temperature
(and/or with UV) though it will do both. Some customers have reported
polymerization down to -10°C, but we have not seen those results. They
seem to be happy however as evidenced by their ongoing purchases. For low
temperature applications, however, one should be considering
LR Gold.
Question 8:
Can LR White be polymerized under vacuum?
Answer:
For resin system, in our opinion, inherently, is not "compatible" with
vacuum impregnation and curing, mainly because of the loss of the more
volatile components in the vacuum during the process, resulting in a block
of different characteristics. Since the effect would be very difficult to
control from run to run, we would predict that one might have problems with
good reproducibility of the sectioning characteristics of the final blocks.
However, if vacuum process does seem to be indicated, one should keep in
mind that the effect of such processing has to be a function of both the
level of vacuum used and the time under vacuum.
We formulated LR Gold for just these sorts of application, though here we
much prefer blue light and Benzil to UV.
No we don't recommend polymerization under vacuum for just the reasons you
suggest, but of course it does depend how profound the vacuum and for how
long.
Question 9:
I have read your FAQs which leads me to ask, are there ever any instances
where the use of vacuum might be suggested?
Answer:
The reason for limiting the use of vacuum is, as mentioned previously, is to
limit evaporation of the more volatile components of LR White formulation.
You will find mention in the literature of infiltration under vacuum as well
as vacuum embedments. However, in general, we do not recommend or encourage
these protocols simply because we believe that the inherent low viscosity of
LR White enables proper infiltration as long as sufficient infiltration time
is permitted. Most protocols use a graded series of LR White and water (or
other polar solvent such as ethanol) to insure complete infiltration. If
this is achieved, then there is no reason for the use of vacuum. However,
LR White is well-known to cure improperly/incompletely if air/oxygen is not
excluded as much as possible during polymerization. These nuances are often
not apparent in Materials and Methods descriptions, and, for new users of
this resin, it is more important to follow provided protocols with care so
that problems can be addressed with the vendor. Thus, the setup recommended
in Question #3 on the FAQ page. We find this a reasonable and reproducible
solution to the problem of oxygen exclusion. There are perhaps some reasons
to use vacuum (some botanists use it to facilitate infiltration of seeds,
etc.), but in general, LR White is best applied initially with an excess of
time rather than hurried with vacuum assistance. The saying "haste makes
waste" would certainly apply in this case.
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Thursday February 09, 2012
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