1, Q: Why the color of acidic crosslinkers is pale or light yellow? Chromaticity of the products on the market is different and with a large fluctuation range, will the transparency of the silicone sealant be affected in the production process?
A: The pale yellow color is caused by the decomposition of few methyl triacetoxy silane during the synthesis of crosslinkers. Since the decomposition point is over 100 degrees, the color will be darker as the reaction temperature higher.
Second, the color range is fluctuated as the rate of decomposition is different in each batch. Since the percentage of crosslinkers added in is small and the color not gets darker at room temperature, the transparency of silicone sealant will not be affected during the production.
2, Q: Some floccules occur occasionally in barreled liquid of acidic crosslinkers. What causes that phenomenon? And how to effectively use in such case?
A: Due to the character of acidic crosslinker is quite active, the floccules caused by the polycondensation reaction after the hydrolysis of the partial acidic crosslinker moleclues during the transportation, storage, and using. Although the quality is not affected, it might impact on the appearance of the downstream products. So, it is better to use after the filtration by filter paper or cloth.
3, Q: When cold season, it will be half or totally crystallized in barrels. What causes that? And how to properly use in that case?
A: The melt point of the purified crosslinker, methyl triacetoxy silane, is 41 Celsius. That is why it gets frozen in cold season. The interesting thing is that it is not solidified at room temperature, 25 Celsius. But it is half or completely crystallized rapidly in barrels while exposed. That phenomenon is call surfusion, which caused by the physical and chemical properties. It more often occurs as the higher purity of the acidic crosslinker. In order to content the special requirements of shelf-life, strength, curing speed and easy use, we decrease the melt point to 5-25 degrees by modifying or compounding the pure methyl triacetoxy silane before selling.
It has to be melted completely before use when the phenomenon occurs, or it will impact the use due to the different contents between the upper and the lower.
There are three main ways to be melted for reference. First, baked 3-5 days until completely melted in drying room at constant temperature in 70-80 Celsius. Second, baked 6-20 hours in water bath with temperature at 70-100 Celsius until completely melted. Third, baked in steam until completely melted, with the acidic crosslinker unexposed.
4, Q: Particles will occur on the surface of new synthesized silicone sealant in 2-3 days when exposed in cold weather. Why that phenomenon happen?
A: That is caused by the crystallization of excess acidic crosslinker in the low temperature. It can be solved mainly in two aspects. First, to control the addition amount of the acidic crosslinkers to let the added acidic crosslinkers completely participate in the producing of sealant. Second, to decrease the melt point of the acidic crosslinker; to use the modified methyl acetoxy silane with lower melt point or to mix acidic crosslinkers in combination to solve that problem.
5, Q: It releases air with heavy smell during the reaction of producing sealant by using acidic crosslinkers, that even smokes eyes. Whether it harms the workers around?Any effective protections?
A: The odor is from a big amount of acetic acid, byproducts of the reaction of methyltriacetoxysilane and 107 glue in producing silicone sealant. Therefore, the workers need to wear labor protection products like protective glasses, respirator, gloves, etc. when operate with acidic crosslinker. The crosslinker itself is non toxic, mainly making irritation on the skin and respiratory. With proper protection, it does not cause any harm to man.
1,Q: It will bubble and blister when using alcoholic type silicone sealant in construction. How to decrease or avoid this phenomenon?
A: There are two main reasons, as one internal and the other external.
1. The external: The construction conditions, mainly due to the high temperature differences between the inner of the rubber and the contact area or the high temperature on the surface of the covered material.
2. The internal: The alcoholic type silicone sealant itself, it results methanol when curing. Due to the boiling of methanol is low, when the temperature of the covered surface is high enough to volatilize the released methanol the rubber will get blisters and bubbles inside as it can not get out to the air timely.
Though it is an unfixed problem till now, there are methods to decrease or reduce generally used in this field according to the curing.
1. To control the addition amount of the crosslinker. Due to the more added, the more methanol resulted which will cause more blisters or bubbles.
2. To slow down the drying process in order to have enough time for the volatilization of the released methanol.
3. To use absorbent for methanol. Till now, there is none one well functional found.
4. To avoid exposing the material on which about to cover.
2, Q: Why the alcoholic type silicone sealant with poor steady in storage?
A: There are many seasons, but mainly two:
1. The resulted methanol molecules will terminate the siloxy of 107 glue which will cause the glue lose reaction in storage.
2. Due to the high reaction of titanium complex, it is easy to hydrolysable or reactive to lose the catalytic function in the storage.
3, Q: Why the color of the sealant made with alcoholic crosslinker change from procelain white to yellow?
A: There are three phenomena for the color change.
1. The whole bottled or barreled get yellow in storage. The quality of the titanium complex is the main reason.
2. The bottom of the bottle or the upper of the barrel partially get change. The main reason is the skinning of the sealant not closed very well in storage. In that case, the color will not change as long as the sealant not get skinning, which can be eliminated by controlling the addition amount of water in the base and the crosslinker, and the improvement in the sealing effect of the bottles or drums.
3. The color change is after the operations. That is caused by the decomposition of the amino which as part of the amino silane(like KH550/792 etc.) in producing sealant under the sunshine.
1,Q: Why the color of the oxime crosslinker gradually get darker in storage?
A: That is because of the nitrogen groups in the crosslinker gradually getting darker, affected by the high temperatures. The vinyl group also can accelerate the color change. So, it happens often in summer, and especially for vinyl trimethylethylketoxime silane, yellowing only after two months.
10, Q: In cold season, why the crystallization, the white powder or precipitation results in the bottom of the drum with combination of the oxime crosslinkers(D-91、D-31)
A: It is because that the solubility of the combination decreases in low temperature. The mixed liquid belongs to tetra methylethylketoxim silane which presents as solid in room temperature, so precipitation occurs in cold season.
It has to heat to dissolve the precipitation before using when the mixed crosslinker with that phenomenon.
2, Q: Why the transparent neutral de-oxime sealant is slow in the internal curing or even not curing at all.
A: That is mainly for the low content of 107 base glue. The oxime crosslinker is less reactive as temperature low, if the content of the base material is not good that may cause the entire inertnesss, causing the partial inside uncuring or curing very slowly. There two main processes to solve that problem.
1. To improve the reaction of the crosslinking agent. To mix the crosslinkers, methyltrimethyethylketoxime silane, vinyltrimethylethylketoxime silane, tetrmethylethylketoxime silane in a certain proportion.
2. To raise the content of the 107 glue.
3. Q: Whether the methyltrimethylethylketoxime silane and viyltrimethylethylketoxime silane need to be used together?
A: Not have to, but in practice we usually use them together. It is mainly because that they can cover well with each other's disadvantages. The disadvantages as below:
Vinyl trimethylethylketoxime silane
1、 Resistant to yellow
2、 Low price
1、 Fast in curing
2、 Not easy to crack
1、slow in curing
1,Q: How to choose a silane coupling agent in application of producing silicone sealant.
A: The selection of the coupling agent silanes impacts a lot in the function of the silicone sealant.
Deacidification type silicone sealant: It is relatively few to use silane coupling agent , now mainly use D-11, di-t-butoxydiacetoxysilane to improve the adhesion to metals.
De-oxime type silicone sealant: The mainly used silane coupling agent are LT-550, LT-560, and LT-792. LT-550 and LT-792 can improve the adhesion, and the stability in storage.
LT-792 is superior to LT-550 on improving the adhesion, and LT-550 is better on the resistance to yellowing. LT-560 can improve the curing speed and strength. But the excess coupling agent will affect the storage stability and resistance to yellowing of the silicone sealant.
Dealcoholization type silicone sealant: The mainly used silane coupling agent are LT-171, LT-550, LT-560 and LT-792. LT-550 and LT-792 can improve the adhesion of the gum while degree the storage stability, meanwhile can cause the yellowing. LT-171 and LT-560 have a good performance on the curing speed and storage stability, while poor on increasing the adhesion.
1,Q: Why the curing speed of two packs silicone sealant in a thin layer is slow, and need to rework?
A: That is a common unfixed trouble, mainly in two reasons.
1. The crosslinker is easy to volatilize when the sealant is sizing in a thin layer, which will cause the crosslinker not enough.
2. The amount of catalyst is quite small used in two components silicone sealant, and the content of the catalyst is even smaller when the sealant sizing in a thin layer, which causes the catalyst ineffective, and need to rework.
There are two way to solve that problem.
1. To increase the content of the crosslinker.
2. To use the crosslinker with less active and heavier molecular weight to limit the volatilization and increase the content of catalyst.