Month: May 2014

0

Sandblast for Rubber Lining and Painting

Posted on by Dan Chamberland

images

General Sandblasting Information for Code Rubber Lining and Painting

Before beginning any kind of blasting you must verify that the humidity and dew point are at acceptable levels. Not respecting the proper dry bulb wet bulb parameters will only lead to flash rust. This is generally not accepted in the coatings industries and can cause failures between the sub straight and the adhesives. There are charts available to understand this phenomenon and will help you find the acceptable ranges.

The second evaluation most codes require you to do, is to perform a blotter test. A blotter test is usually with a white cotton or white cloth testing that your air is free from oil, contamination or water. Given you have a modern, proper blasting or painting system with an in line air water separator and filter system you can be assured that this test is to reassure your customer that your air is good.

Adding oils and water to a blasted surface will negatively affect the adhesives and the overall performance of the liner.

There are other blast tests required for some specification, such as the soluble salts test. Although this test comes up on occasion, for most rubber lining it is not required. We will cover this subject in another post.

For the lining of steel pipe, tanks and other steel sub straights, you must achieve a blast profile of at least SSPC-SP5 classified as a white metal blast.

There are visual charts such as the example below to visually compare. These also have detailed descriptions to evaluate the blast achieved.

Nace

In addition to getting the surface clean you must also provide a profile for the adhesive. You can verify the profile of blast using a Testex strip shown below. This is a common way of evaluating the sandblast profile and proving to your customer the work was performed to specification. This is by far the most important part of the blasting process. Sure, cleaning scale, mill varnish is important, but in order for the adhesive to properly hold  the liner in place, the tooth or profile will give the adhesive a larger surface area to hold onto. Very important.

download

Blast profiles need to be a profile of 3-5 mils for adhesive as it is required for good surface adhesion. For a paint anywhere from 1.5-4.5 mils is acceptable depending on the specification provided. 

If you glass bead a surface you will get a nice finish, looking like SSPC SP-5 but your adhesion will be terrible due to the profile being flat. You can effectively double your adhesion surface by blasting a proper profile.  Achieving the correct  profile has everything to do with your blasting media and once you find the correct blast media and pressure you can achieve this result 100%of the time. Many option of blast media will create this profile.

For painting the exterior of pipe and tanks most specification call for SSPC SP-6 which is classified as a commercial blast.

The profile for paint is important but not as critical as for adhesives.

Once the proper blast profile has been achieved, adhesive needs to be applied before flash rusting can occur. A good rule of thumb is within 20 min, environmental conditions affect this time one way or another dramatically. On a dry sunny hot day with no humidity the piece can stay without adhesive longer with no risk of flash rusting.

If you have followed all of this you will have an excellent base for which the adhesive can be applied.

 

 

0

ASTM specs and what they mean for rubber.

Posted on by Dan Chamberland

industrial-rubber-33plus_sign232-ASTMlogoB

 

There are many standards used to qualify rubber. ASTM, NACE as well as others which attempt to define and standardize the industry.

I will attempt to simplify and clarify some of the standards and usefulness.

Some specifications are valid as a key performance indicators for wear, while others are simply elemental, the physical characteristics.

ASTM D297 or Specific gravity is a good example of an elemental specification. The fact that rubber has a specific gravity less than one is a good indicator that it will float. Floating does not necessarily indicate that the rubber will perform better or worse in a slurry application.

In the old days this was used to determine the actual content of natural rubber. Adders and filler like clay and carbon’s can affect the buoyancy of rubber they can also affect it’s wear characteristics most times in a positive way. Material science has gone a long way since the floating test.

Cut and Chip The best indicator for how rubber sheet will perform in a real world application is the  “Cut and Chip” resistance. This test is used to measure the performance of a rubber in a wear application. This test was originally devised by the tire companies to evaluate service life of tires in various conditions. This is a much better indicator of how a compound will perform in a wear application and much less an elemental test, describing the physical properties.

ASTM D2240 or Hardness Shore A although this is a very popular way of describing rubber. Effectively this is another elemental specification. People will argue that the durometer of rubber aid in different wear characteristics and they would be right. But it is still not a final indicator of how it will wear. It simply defines it’s hardness.

ASTM  D412 This one covers Tensile and Elongation this is an interesting specification because elongation and tensile are very good when describing rubber for track. Let’s take a snowmobile track for example. The moment you spin a track, you want the rubber to have enough elongation to absorb some of the initial  inertia. In a rubber lining application it is somewhat irrelevant. When bonding a rubber sheet to steel sub-straight the elongation will never really come into play for wear.

ASTM D624 Tear Die C another elemental indicator. In rubber lining, the force at which the rubber sheet will tear is not good wear resistance indicator. When building a tire, snowmobile or tank track this is a great indicator. Adhered to steel tearing it is the least of your concerns. Ultimately all these factors only help to describe how this material could perform. The forces required for an adhesive pull test is 25lbs to pass ASTM and Nace pull tests are very low compared to most sheet listed tear specifications.

ASTM D7121 Rebound and Resilience Although rarely listed in a rubber sheet or specification, the ability of a rubber to return to it;s original  shape is important especially when defending against rocks, slurry and wear.

ASTM D5963 Wet and Dry Abrasion loss This is a good indicator of if your rubber will last in a wear application. This test directly measures wear. The best wear resistant rubbers will have good wet and dry abrasion properties mixed with excellent cut and chip resistance.

ASTM D573 Heat Aging Depending on the application this is a good test to evaluate rubber longevity. As rubber is generally a cocktail of materials how they perform over time and temperature will vary. This test monitors the physical changes of rubber with aging. This elemental test will give indication of  how the rubber will perform over time.

Rubber wear characteristics are complex and understanding what the specification mean will help you in the long term choice for the various applications.

 

 

 

 

 

 

0

How to choose rubber for slurry applications.

Posted on by Dan Chamberland

photo_Discharge_CDF_MissionKickOff_010109_sm

 

There are many factors when choosing the right rubber compound for a slurry application.

The first things to consider is the makeup of the slurry.

Is it Acidic or Alkaline? Some rubbers are better for chemical resistance like Chlorobutyls and Bromobutyls but Natural Rubber is better for Hydrochloric Acid

Are there hydrocarbons present? If so then your best bet is Neoprene.

Is it a high temperature? If so your back to Chlorobutyls and Bromobutyls.

Particle sharpness? When you have very sharp particles you want a higher durometer to resist a cut and chip application. Going from a 40 Duro to a 60 Duro gives the rubber a better chance against sharp particles.

The second thing to consider is the design of the pipe tank or chute you need to line.

What are the velocities? The erosion rate of a the liner is direct relation to the increase of velocity. If the slurry moves to fast and  the material is large like 3/4″ minus. Rubber may not even be the proper lining material. In this circumstance a more epensive liner will perform better such as ceramic or chromium carbide.

Are the bend radius and angles good? The biggest problem you can have on a chute, pipe, bin that is lined is the angle the material is hitting the liner.  If the angle is too acute the liner will wear extremely quickly, there are tricks for this you can have the material hit itself using dead bed techniques. Or you can change the angel for example specify a large radius in a bend.

 

So what is the best general liner? Wet sliding abrasion in ambient temperature with velocities that are more or less reasonable with no chemicals that can swell or attack the rubber. Line with than 40 duro natural rubber. For any size of pipe 8″ and below 1/4″ is fine anything above you can use 1/2″.

 

What about thicker liners?

Thicker liners do not mean better performance or longer lasting liners for that matter.  You can pile two inches of rubber in the bottom of a chute. The lining application will be problematic and if erosion start in a spot because of the reasons listed above you are no better off.