Introduction to Stabilisers:
Stabilisers:
The PVC molecule is unstable to heat and light. Heating PVC causes breakage of the
Polymer chains, liberating Hydrochloric Acid gas. The HCl catalyses more degradation
and the degradation can become explosive, releasing large quantities of corrosive
HCl and other toxic gases. Exposure to Ultraviolet Radiation also breaks up Polymer
chains but is much slower than Heat degradation.
Heat Stabilisers:
To process PVC, it has to be stabilised against the action of Heat required at processing
temperatures. Most Stabilisers contain metal elements which react with HCl and inhibit
further degradation. They are Metal Salts, Soaps or Complexes. Stabilisation of
PVC is a vast subject, and they are the second most important ingredient in UPVC
Formulations and after Plasticisers in a Flexible PVC formulation. Some of the most
important families are:
|
Type
|
Form
|
PHR used
|
Clarity
|
Toxicity
|
Odour
|
Cost/Kg
|
Leads
|
Powder
|
2 -4
|
Opaque
|
Toxic
|
None
|
~ Rs 120-150
|
Ba- Cd
|
Liquid
|
1.5 – 2.5
|
Clear
|
Very Toxic
|
Slight
|
~ Rs 200
|
Ba - Zn
|
Liquid
|
2 - 3
|
Clear
|
Toxic
|
Slight
|
~ Rs 190
|
Cd - Zn
|
Liquid
|
2 - 3
|
Clear
|
Toxic
|
Slight
|
~ Rs 220
|
Ca - Zn
|
Liquid
|
3 -4
|
Clear
|
Non-Toxic
|
Negl.
|
~ Rs 250
|
Tins
|
Liquid
|
0.5 -1
|
Crystal
|
T & Non
|
Strong
|
~ Rs 350
|
Ba - Zn
|
Liquid
|
2 - 3
|
Clear
|
Toxic
|
Slight
|
~ Rs 190
|
The correct choice of stabilisers for the target application is best done in consultation
with the stabilizer supplier and technical service cells of PVC Resin suppliers,
However, as a guideline the following table would be useful.
Stabiliser application areas:
Type |
Heat Stability |
Main Applications |
Not recommended for |
Leads |
Very Good, esp. long term. |
Cables, Pipes & Fittings, Sleeves, Profiles. Cheapest. |
Clear Application. Can be toxic. |
Tins |
Best |
Transparent tubings and sheeting, High quality Pipes, Medical |
Causes sulphide staining with Leads. Costliest Stabiliser. |
Cd - Zn |
Moderate |
ROHS Cables, Non-Toxic. Pipes and Extrusions. |
For high heat history. |
Ba- Cd |
Good |
Leather Cloth, Calendared products, Footwear. |
Non Toxic applications |
Cd - Zn |
Moderate |
Foamed Leather Cloth |
For high heat history. |
Metallic Stearates |
Low |
Co-stabilisers with lubricating action |
Sole Stabiliser. |
Newer families of heat Stabilisers are being offered with the background of Heavy
Metal environments contamination concerns. Organic Stabilisers and Calcium Zincs
are in the forefront of this trend.
Because of its relatively lower cost and strong Stabilisation action, Lead stabilisers
have historically been the most popular
• Lead Stabilisers: The common Lead Stabilisers are:
o TBLS- Tri Basic Lead Sulphate.
A strong heat stabiliser as it has about 80% PbO, the active ingredient in absorbing
and neutralising the
HCl before
it gets liberated on heating. It is non-lubricating and is opaque, cannot be used
in Transparent/Transluscent applications
o DBLS- Dibasic Lead Stearate. A
Lubricating stabiliser very useful for lubrication at the mixing and metering zone
of the extruder. Translucent.
o LS- Lead Stearate. Weakest Lead
stabiliser. Strong external lubricant especially in the early extrusion stages.
Translucent.
o There are other Lead stabilisers
like Dibasic Lead Phosphite, used when Light Stability is important and Dibasic
Lead Phthalate, used for high speed extrusions, but these are not being covered
in this article, as they have niche usage.
Calcium Stearate and Stearic acid are often used along with Lead stabilisers primarily
to balance the Lubrication. CS is a weak stabiliser but an excellent internal lubricant,
very effective in late stage lubrication at barrel discharge and die head. Stearic
acid is a strong external lubricant, Excessive use can lead to screw slippage and
poor gelation.
Lead Content of Lead Stabilisers.

One disadvantage that Lead stabilisation systems have is that a good, balanced formulations
require a combination of all the Lead stabilisers plus Calcium Stearate and a few
more lubricants. Other systems like Tins and Mixed metals are normally one grade
of Stabiliser and one or two lubricants. With Leads, careful weighing of six or
seven ingredients may be required, which requires skilled operation.
Lubricating Lead Based One Pack Systems
The Stabiliser mix for the high-volume PVC Pressure Pipes which I have used extensively
in the past till the one packs came on the scene, is given in my article on Volume
Costs, referred to in Page 6.
You will note that the dosage recommendations of the regular grades of one packs
for PVC Pipes offered by leading manufacturers start at about 2.2 to 2.4 PHR. This
matches up with the dose of 5-6 ingredients above and is the basis of the convenient
one pack systems.
The initial one packs offered were premixed single component stabilisers. The desired
quantity was weighed out into packets according to the amount of Resin charged per
batch. This is in line with the practice followed inhouse, but separate weighing
of 5-6 ingredients was prone to weighing inaccuracies, especially of the minor ingredients.
The stabiliser manufacturers used to mix the one pack recipes in bulk, and the mix
was then meter measured and packed in easy to use packets, say for 25 kgs. PVC,
50 kgs, etc.
There was a lot of resistance to this concept in the early eighties. My personal
experience is that having formulated with single stabilisers for nearly a decade
at Shriram and PRC (DCW), I was introduced to the convenience of One pack systems
when I went to Kuwait. The One packs from Baerlocher GMBH were an eye opener, and
I wanted to introduce the concept in India when I returned to India in 1978 and
joined Waldies, a prominent Lead oxides and PVC stabiliser company.
At that time, PVC Pipe markets was starting to grow rapidly with the leaders like
Wavin, Finolex, Garware and Chemplast using large capacity imported extruders, while
most the others were equipped with the venerable Windsor RC-100. Remember, at that
time, PVC Pipes of the popular sizes up to 110 mm were reserved for small scale,
and only the RC-100 (meaning 100 lbs/hr, about 40 kgs/hr) could be afforded by many
who were entering the field as small-scale industries.
At that time, the PVC recipes for pipes were a closely guarded secret, controlled
by the plant technical heads. Much to the dismay of my friend Mr. Anil Anand of
ALA and myself, who were trying to push the one pack concept, we found that the
technical chiefs were not at all interested in switching to the ease of a preformulated
one pack. It would seriously reduce their importance in their companies!!
There was another factor working against the type of efficient one pack systems
available abroad in the early eighties. The RC-100s with their co-rotating screws
and low outputs and high residence times, required higher dosages, sometimes 4-5
PHR and had to be formulated like single screw extruders recipes. As the PVC pipe
industry had started with small diameter conduits, many single screw extruders were
pressed into service for water pipe. Even Chemplast were using large single screw
extruders as per their American processing DNA. Added to the higher stabilisation
costs, expensive flow promoters were needed for single screw pipes to match the
strength of pipes made on the new imported Counter Rotating twin screw extruders.
The industry rapidly shifted to the imported Twin Screw extruders from companies
like Cincinnati and Kraus Maffei, Battenfeld etc., and as the Pipe market was booming
because of the WHO funded drinking water projects. The extruders were actually airlifted,
such was the demand. Later in the 80s and early nineties, with more number of modern
high output extruders being the norm, the one pack concept started taking hold.
The next step from premixed powders was to safer handling, non-dusting forms. One
big drawback with handling Lead Stabilisers in the factory is susceptibility to
lead poisoning of workmen, if handling, housekeeping and dusting was not kept under
control. Newer one pack delivery systems were developed by heating the mix to about
160C so that the Lubricants, Calcium Stearate, Lead Stearate and Dibasic Lead Stearate
melted and formed a dough with the non-melting TBLS. The dough was then extruded
into pellets or pressed and chipped into flakes. This form was much safer to handle
and gradually found acceptance.
This has been further refined by Stabiliser manufactures who start with Litharge
(PbO). They react this with carefully prepared proportions of Sulfuric Acid, Stearic
acid and some other lubricants like Calcium Stearate and special waxes. On completion
of the reaction, all the PbO is converted to TBLS, DBLS and LS as per the formulation
proportions. The reactor discharges to a rotating drum, where the paste is cooled
and flaked off.
Non-lubricating One packs leads do not have enough lubricants to
coat all the solids in the mix and are sold as powders. They are widely used for
applications like Cables & Wires, Profiles, both Rigid and Flexible sleevings, Injection
Moulded fittings etc.
Special applications like PVC Window Profiles and Injection moulded fittings need
some more ingredients other than Stabilisers and lubricants like Impact Modifiers
and Flow promoters.
Window profiles require 3.5-5PHR impact modifier and 1.5-2PHR Flow Promoter. When
this is sold as a package in a one pack, the dosage must go up by 5-7PHR, that is
2.4+5=7.4 and 2.4+7= 9.4PHR. The price of the one pack goes up as the additives
are quite costly.
In Injection moulded fitting, which are processed on single screw extruders, a flow
promoter needs to be added. At 1.5PHR addition, the One pack dose goes up to 4PHR.
Again, the one pack will be costlier than the normal pipe one packs.
One packs can also be offered with properties like UV Resistance, Antioxidants etc.
if required. The one pack manufacturers invest a lot on developing the right lubricant
mix for an application and offer several grades for the same application of PVC
pipes, like Normal, Filled, Highly Filled, Single or Twin Screw etc. Even refinements
like Conical and Parallel screw suitability can be provided. The price of the One
pack would vary with the special ingredients chosen.
Dosage and Pricing
In the extremely competitive field of PVC pipe, where the demand is for cheaper
and cheaper inputs, CaCO3 fillers get added to the one pack to drop the price, However,
to maintain the same stabilising efficiency, the one pack PHR has to be increased
by the amount of inert filler added. This if 1 kg extra of filler is added, no doubt
the price will go down, but the dosage goes up by 2.4+1=3.4. As most processors
clamouring for cheaper one packs are making highly filled pipes, the one pack must
be further boosted to maintain the same % of active PbO as in standard formulations.
The implications of adding fillers to cheapen One packs must be clearly understood,
as Per Kg cost reduction is not the only factor.
Here are some examples:
In this example, I have started with a standard PVC Pipe one pack. If an economical
filler containing One pack is used, the starting point will be 3.2 PHR instead of
2.4PHR, and by the time filler loading reaches 100 PHR, the loading must be nearly
doubled for trouble free operation. A common mistake by processors is to stick to
the same PHR used in BIS Pipes as they increase filler. This drops the active PbO
content and lubrication balance and can lead to severe processing problems.
A study has to be made by multiplying the One pack cost per Kilo and the addition
level required in PHR to arrive at the stabilisation cost per 100 PHR Resin to determine
how effective they are in cost reduction.
The amount of filler addition, especially at higher dosages will bring down the
cost. But what is important is that whether the stabilisation cost is lower with
the Standard or economical One packs.
Another factor which is important is the volume cost, for which the density of each
ingredient is crucial. Anything added with a density more than PVC will increase
the density (Like One Packs, Fillers, TiO2)
For accurate density calculations and assessment, the actual density of all the
ingredients must be known. The one pack density is important as this figure is not
usually given in Technical Data sheets.
For further clarity on the effect of Density and filler loading, please refer to
my article
http://www.plastemart.com/plastic-technical-articles/Volume-Cost-and-its-importance-in-Plastic-applications/1561
Calcium Zinc One Packs.
With the world moving away from Lead stabilisers. Calcium Zinc systems are becoming
prominent for manufacturing Lead Free Pipes and where ROHS requirements are to be
met.
Calcium Stearate is the most common non-Lead stabiliser. However, its stabilising
power is quite low and is more viewed as a very effective Internal Lubricant, playing
a major role in Lubrication at the die head for smooth extrusion and finish.
Calcium Soaps have been combined with Zinc compounds and the resultant Calcium Zinc
Stabilisers have steadily improved in stabilisation efficiency. Driven by the intensive
R&D of the stabiliser industry, Calcium Zincs are rivalling Leads in dosages, 3PHR
vs 2.4PHR.
The lubrication systems are quite complex in Calcium Zinc systems for boosting the
efficiency, so many of the Calcium Zinc grades are available as one packs and not
individual components.
The other major Stabilisation system is with Tins, very popular in the Americas,
as they were very effective with single screw extruders, which was the norm there
before the Americas also switched over to Twin screw.
Tin Stabilisers like, the Mercaptide family, are all liquids. This does not make
it conducive for a one pack delivery system. Normally Tin Stabilised pipes are made
with single components, with the lubricant balance being critical for excellent
performance. Tin stabilisers are more expensive than Leads, but as they are so efficient,
processers with top of the line extruders are reported to have come down to 0.3-0.5
PHR with the powerful methyl tins and being competitive to lead systems.
There are many suppliers of One pack systems, whose numbers have increased as one
packs are now universally used by the huge PVC Pipes and Profiles sector
Siddhartha Roy
|
|
Mr. Siddhartha Roy is a Chemical Engineer from IIT Kharagpur (1968).
He has worked with plastics all throughout his career. He was actively involved
in development of PVC markets and applications, especially Pipes and Fittings. He
worked with Shriram Vinyls, PRC (now DCW) and Chemplast, manufacturers of PVC Resin
& Compounds. He has managed a PVC Pipes & Fittings factory in Kuwait and helped
Jain Pipes (now Jain Irrigation) set up their Pipe production facilities.
He headed R&D at VIP Industries, Nasik, and is well versed in the processing of
Polyolefins, Styrenics, Polyamides and PC.
He has been active in IPI activities and has delivered several Endowment lectures.
He was recently awarded the Fellowship by the Governing council of IPI for his contribution
to the Plastic Industry.
|
|
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11-Jul-23
I'm Formulation chemist OF Pvc compound,Upvc compound,FR,FR LSH, Transparent sheet, profile,pipe Watsapp +917249974815
15-Jun-23
I am a UPVC Plant Lead at Chicason group , would like to further discuss with you more about material formulation and some technical issues we do face in our factory. nzelawrence@gmail.com can also be used ti reach me.
10-Aug-22
dears , i am going to make one pack stabilizer for soft pvc in wire and cable insulation in automotive class ( 125 degree ) . so i have a stabilizer, wax, calcium stearate, stearic acid and anti oxidant. so would you please advise me to compose one pack with these additives . best regards
26-Dec-21
Formulation cansultant PVC Granules, FR, FR-LSH, pvc conduit pipe, HDPE, Pvc Profile extrusion, NBR, TPE, Air Sole, EVA, PVC solvent cement, ZHFR, BIS certification and ISI mark consultant.(R.R Consultancy) 8979684240
13-Jan-21
Yes
01-Aug-19
want to know difference between internal and external lubricants thansk