Analysis of rubber frosting precipitates and their solutions
I.The main contents of this paper:
1. an overview of the phenomenon of rubber ejection.
2. theoretical analysis and countermeasures of rubber frosting.
3.Theoretical analysis and countermeasures of rubber injection.
4. theoretical analysis and countermeasures of rubber color spraying.
II. Overview of rubber ejection phenomenon
Adverse effects of ejection
Disadvantage
Affect the appearance of the product.
* Reduce the viscosity of rubber compound.
* Reduce the bonding effect between rubber compound and skeleton material.
* Cause pollution to the materials in contact.
* Reduce the physical and mechanical properties and service performance of products.
* Waste of materials, which increases the product cost.
Favorable influence
Advantage
* Ozone aging protection by spraying (microcrystalline wax)
* Improve the appearance of products by spraying (brightener)
* Use spray anti-sticking (wax, vaseline)
* Use spray to prevent frosting (spray inhibitor)
III. The Rubber Blooming
Appearance: at a certain temperature, the compounding agent in the compound migrates from the inside of the compound to the surface, forming a layer of hoarfrost (whitening).
Essence: It is the result that the crystalline compound inside the compound migrates to the surface for recrystallization.
Theoretical analysis and countermeasures of rubber frosting
3.1 Influence of Compatibility between Compounding Agent and Rubber
The ability of compounding agent and rubber to form homogeneous system is called the compatibility of compounding agent and rubber. A homogeneous system is formed, that is, the mixed dispersion at the molecular level is achieved.
The compatibility between compounding agent and rubber determines the solubility and dispersibility of compounding agent in rubber, which is the fundamental reason for compounding agent frosting.
3.1.1 Determination of compatibility between compounding agent and rubber:
It can be regarded as the solubility of compounding agent in rubber, and the thermodynamic equation of its dissolution process is:
Gibbs free energy of solution:△G=△H—T×△S
Enthalpy change of solution: △H= Vm ×（δ1--δ2）2×φ1φ2
Eq:
△G—— gibbs free energy of solution
△H—— change of enthalpy of solution (△H ≥0 in the process of dissolution)
T-dissolution temperature
ΔS - change in entropy of dissolution (dissolution process Δs > 0)
Vm - total volume of the rubber compound
δ1, δ2 - solubility parameters of the compounding agent and rubber
φ1, φ2 - volume fraction of mixing agent and rubber.
3.1.2 Influence of Compatibility of Compounding Agent and Rubber on Ejection
△G > 0, thermodynamically incompatible and not spontaneously dissolved.
When δ1=δ2，△H= 0，△G＜0 , thermodynamically compatible andno ejection at any temperature;
When the difference between δ1、δ2 is small, T increases, ΔG < 0, the process is compatible, and it is sprayed when it drops below a certain temperature;
When the difference between δ1 and δ2 is very large, even if T rises, ΔG > 0, which is completely incompatible, it will be ejected at any temperature; At this time, only φ1 is very small (the dosage is very small), and when the temperature is high, △G＜0, it will be sprayed when it drops below a certain temperature.
3.1.3 Countermeasures against blowout
Choose the cooperating agent with close solubility parameter; reduce the amount of poorly compatible cooperating agent (take and use, replace part of the sulfur yellow with sulfur carrier DTDM); appropriately increase the parking temperature.
Solubility parameters of commonly used rubber and polymer
NR：8.25 IR：7.8-8.0
BR：8.1 SBR：8.5-8.6
EPDM：7.95 IIR：8.4
NBR：8.7-10.3 CR：9.2
CSM：8.9 PU：10.3
MVQ：7.3 (there are great differences among varieties)
PE：7.8 PP：8.1
PVC：9.57 EVA：9.1-9.5
High styrene: 8.5 Nylon: 13.6
Solubility parameters of common additives
Sulfur: 33.73 DTDM:24.31
DCP:19.49 2402 Resin: 25.81
Double 2,5:17.29 M:25.96
DM:28.29 NS:22.81
CZ:23.29 DZ:22.03
TMTD:27.20 TMTM:26.39
TETD:24.04 ZDC:23.67
PX:25.21 BZ:21.08
Promote D:22.45 CTP:24.02
4010NA:20.66 RD:19.80
3.2 Hydrophilicity of Compounding Agent and Influence of Salt and Water
Rubber compound needs water cooling during processing, and rubber products often come into contact with water (fresh water, seawater and water vapor in the air) during use. Salt additives (dithiocarbamates accelerator), diamine additives (antioxidant) and inorganic additives (ZnO, CaCO3, etc.) in the formula are hydrophilic and soluble in water, which will diffuse to the surface of the compound with water when contacting with water for a long time, and crystallize out after drying, resulting in frosting.
With the increase of air humidity in summer, the phenomenon of frost spraying of rubber compound increases.
The phenomenon of frost spraying is aggravated after the rubber compound is soaked in salt water.
Frosting is more likely to occur where the rubber material is touched by human hands.
Countermeasures for blowout prevention: Try to minimize contact with water (low humidity storage, drying treatment of ingredients, using surfactants, sticking plastic film on the surface of rubber compound, coating or coating the surface of products, etc.).
3.3 Influence of molecular weight and dosage of complexing agent
The complexing agent has large molecular weight, large side groups, high melting point, slow migration and difficult frosting.
When the dosage of complexing agent is large, the solubility enthalpy change △H is large, and it is more difficult to dissolve △ G < 0, and it is easier to spray when cooling.
The critical dissolving amount of compounding agent in rubber is about 1%. When the dissolving amount of compounding agent in rubber exceeds 1%, frosting will occur when the temperature is lowered. The amount of complexing agent dissolved in rubber compound is affected by temperature.
Blowout prevention countermeasures:
The compounding agent is a variety with large molecular weight (such as polysulfide), large side groups and high melting point;
Use a combination method to reduce the dosage of compounding ingredients; Fully sulfurized to avoid sulfur deficiency.
Adding additives that can dissolve or adsorb additives that are easy to spray into the rubber compound, such as Ninas paraffin oil, naphthenic oil coumarone, coal tar, rubber powder, blowout preventer, activated carbon, etc.).
Low-temperature processing (such as low-temperature mixing, low-temperature long-time vulcanization, etc.)
Improve the dispersibility of additives (adding dispersant, changing mixing process and conditions)
3.4 The influence of temperature
For most compounding agents, the solubility in rubber increases with the increase of temperature, and the amount of dissolution increases. When the temperature decreases, it is easy to reach supersaturated state, thus crystallization precipitates and frosting occurs. Therefore, the higher the processing temperature, the more prone to frosting. High temperature will also transform polymerized sulfur into soluble sulfur, which will aggravate frosting.
When the temperature drops quickly (such as the air velocity is fast and the rubber compound is thin), the solubility of the compounding agent drops quickly, and it is easy to reach supersaturation and crystallize out, resulting in frosting.
The parking temperature is low, the solubility of the compounding agent is low, and it is easy to achieve supersaturation and crystallize out, resulting in frosting.
Countermeasures for blowout prevention: low-temperature processing, slow cooling and proper increase of parking temperature.
3.5 Matters needing attention in formula design:
(1) Among the curing systems, sulfur curing system is the most easy to spray, followed by resin curing system, and peroxide curing system is relatively light. In sulfur curing system, insoluble sulfur is preferred as curing agent, or sulfur carrier is used to replace part of sulfur; The accelerator is preferably sulfenamide such as NS, DZ and CZ, thiazole is preferably M, and Qiu Lan is preferably TBzTD and TETD；; Dithiocarbamates is easy to spray in humid environment, BZ is preferred, followed by ZDC；The ratio of ZnO to SA is appropriate.
(2) The reinforcing filling system, such as carbon black and nano kaolin, is difficult to spray, which has an indirect effect on frost spraying. Carbon black with small particle size and high structure tends to promote frost spraying, and the frost spraying is aggravated with the increase of carbon black dosage; The adsorption of nano-kaolin is beneficial to reduce the phenomenon of frost spraying. Because of its high porosity and large specific surface area, the flake structure of Snobrite75 is very effective in inhibiting frost spraying.
(3) RD, AW, BLE, 4020, 4010, DNP, H, etc. in the protection system are not easy to be ejected; 4010NA will migrate with water; Prevent D, 4030, MB and wax from being sprayed easily; Phenolic antioxidants are easy to spray out in humid environment because of the hydrophilicity of -OH. Pentaerythritol 1010 is easy to spray.
(4) Plasticizer: paraffin oil, naphthenic oil, coumarone resin, coal tar, black ointment, etc. from heavy hydrogenation process can dissolve part of sulfur and reduce sulfur frosting.
(5) The anti-coking agent CTP is easy to spray, and the dosage should be small, not exceeding 0.5phr.
(6) Organic or inorganic pigments or dyes are easy to spray, and the dosage is less.
(7) Decabromodiphenyl ether, chlorinated paraffin, phosphate, Sb2O3 and Mg(OH)2 in flame retardants are easy to spray out.
(8) Use blowout preventer.
Design principle:
Try to choose additives that are not easy to spray, and add additives that can hinder spraying; If it is necessary to choose additives that are easy to spray because of performance needs, it is recommended to use them in combination to reduce the dosage. For the rubber compound containing hydrophilic additives, try to avoid contact with water (including water vapor).
(2) For thin films or products, avoid strong wind blowing, cool slowly, and park at a higher temperature;
(3) the surface of the rubber compound sheet or the calendered semi-finished product clings to the plastic film;
(4) Surface treatment of products, such as halogenation, dipping polyurethane film, hydrogel, etc.
(5) adopting appropriate rubber mixing process conditions, such as mixing rubber at low speed for a long time, to improve the dispersibility of compounding ingredients;
(6) Full vulcanization to avoid sulfur deficiency. (Due to low heat dissipation of the mold temperature, no time compensation was made, resulting in sulfur deficiency).
(7) Vulcanization temperature should not be too high. It is suggested that thick products should be vulcanized at low temperature for a long time or gradually increased in temperature.
IV. Rubber injection
4.1 overview
Appearance: at a certain temperature, the liquid compounding agent in the compound migrates from the inside of the compound to the surface, forming oil drops or oil films.
Essence: the result of liquid compounding agent migrating to the surface inside the rubber compound.
Reasons for fuel injection:
The compatibility between liquid compounding agent and rubber is poor, the polarity difference is large and the dosage is large;
The liquid compounding agent has small molecular weight and low viscosity;
The rubber compound contacts the medium with better compatibility with the liquid compounding agent.
Blowout prevention countermeasures:
Select liquid compounding agent with similar solubility parameters, polarity and saturation, large molecular weight and high viscosity;
Appropriately increase the crosslinking density of rubber compound;
Coating or coating on the surface of rubber compound.
4.2 Solubility parameters of common liquid compounding agents
Aromatic oil has a high aromatic hydrocarbon content, and its solubility parameter should be close to that of toluene and xylene (about 8.8), while paraffin oil has a high linear paraffin content, and its solubility parameter is close to that of n-hexane (about 7.3). The solubility parameter of naphthenic oil is about 7.2.
V. Rubber spray color
5.1 Overview
Appearance: Different colors (flooding) appear on the surface of mixed rubber or rubber products during parking. There are mainly red, blue and colorful phenomena.
Essence: the result of molecules with chromophores migrating to and adhering to the surface.
A substance containing groups such as carbon-carbon double bond, carbonyl group, aldehyde group, carboxyl group, azo group, nitroso group and -C=S in its molecule absorbs light of a certain wavelength, but does not absorb light of another wavelength, and the reflected light shows different colors after being irradiated by sunlight. Therefore, these groups are called "chromophores/chromophores". When one or more substances containing chromophoric groups migrate to the surface of the rubber compound, the surface of the rubber compound will have one or more colors, resulting in color spraying.
Most of these molecules with chromophores come from the small-molecule volatiles in raw rubber and compounding agents and the intermediate products that react with each other, or the small-molecule volatiles in release agent, which have good compatibility with the rubber compound, and migrate to the surface of the rubber compound due to heat volatilization during processing, and are miscible with or adsorbed by the rubber surface.
5.2 Source of surface chromophore molecules:
(1) Residual monomers (acrylonitrile, styrene, ENB) or filler oil in raw rubber
(2) Volatiles adsorbed on the surface of carbon black, such as NO2, N2O4, and polycyclic aromatic hydrocarbons (mainly) that have not been completely cracked.
(3) Polycyclic aromatic hydrocarbons in liquid plasticizer (mainly)
(4) A complexing agent containing carboxyl, heterocyclic, azo and azide components.
(5) The nitrogen-containing components or additives in the rubber compound are oxidized at high temperature to form nitroso compounds.
(6) Rubber molecular chains are oxidized at high temperature to form carbonyl compounds, such as aldehydes, ketones and esters (aging discoloration).
(7) Aromatic acid ester or salt surfactant component in the release agent (full color under the light of soap bubbles)
(8) Migration and spraying of amine antioxidant in rubber compound.
-Pan-color is common, and there are many reasons. Find out the source, can be completely removed.
5.3 BOP countermeasures
(1) Reduce the residual monomer content in raw rubber as much as possible (control during synthesis or bake the rubber before processing).
(2) Choose carbon black with high toluene transmittance. Generally, carbon black with small particle size and high structure has strong adsorption, and there are many residual oils, which are easy to flood; The surface carboxyl group content is high and blue, such as 2 series and 3 series carbon black; Large-size carbon black has low coloring strength and is mostly red, such as N774. Medium and low particle size carbon black is used together. Because of the special process, Nitinon carbon black has less color development factor and can reduce color spraying by controlling the dosage.
(3) Choose the oil with less polycyclic aromatic hydrocarbons, high flash point and high viscosity, and the oil consumption should be as little as possible.
(4) As far as possible, no complexing agent containing -N=N-, -CN-, -C=S, -C=O and -COOH is needed.
(5) Use less antioxidant 4010NA, petroleum resin and coumarone resin.
(6) Use surfactant (dispersant) or release agent containing carboxyl or benzene sulfonate as little as possible.
(7) Reduce the processing temperature (mixing and vulcanization) to ensure that the rubber compound is vulcanized.
(8) Adding high-absorbability materials such as porous materials, anti-spray agents, nano-kaolin, etc. into the formula.
