{"id":7835,"date":"2025-10-26T09:00:00","date_gmt":"2025-10-26T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/silicone-compound-formulation\/"},"modified":"2025-10-26T09:00:00","modified_gmt":"2025-10-26T09:00:00","slug":"silicone-compound-formulation","status":"publish","type":"post","link":"https:\/\/uplastech.com\/id\/silicone-compound-formulation\/","title":{"rendered":"Silicone Compound Formulation: Base, Filler, Additive and Cure"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae\u3002\n   \u6ce8\u610f\uff1aClick to Chat \u7684\u70b9\u51fb\u903b\u8f91\u662f\u300c\u95ee\u5019\u5361\u5143\u7d20\u5b58\u5728\u5c31\u4e0d\u8df3\u8f6c\u300d\uff0c\u53ea\u9760 CSS \u9690\u85cf\u4f1a\u8ba9\u6c14\u6ce1\u53d8\u6210\u6b7b\u94fe\u63a5\u3002\n   Code Snippets #9\u300cUPL single post: WhatsApp bubble jumps straight to chat\u300d\u8d1f\u8d23\u628a\u8be5\u8282\u70b9\u4ece DOM \u79fb\u9664\uff0c\n   \u4e0b\u9762\u8fd9\u6761 CSS \u53ea\u662f\u79fb\u9664\u524d\u7684\u89c6\u89c9\u515c\u5e95\u3002\u5220 #9 \u524d\u5fc5\u987b\u5148\u5220\u8fd9\u6761\u89c4\u5219\u3002 *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/silicone-compound-formulation.jpg\" alt=\"Shore A hardness against fumed silica loading and tear strength against hardness\" width=\"1536\" height=\"909\"><figcaption>Hardness rises almost linearly with silica loading, while tear strength peaks in the middle of the hardness range.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"what-a-compound-is\">What a compound actually contains<\/h2>\n<p>Silicone rubber is not sold as a resin. It is sold as a compound: a high-consistency gum mixed with filler, a structure control additive, a cure system and a short list of minor ingredients, delivered as a soft sheet or a strip that a moulding shop can cut and press.<\/p>\n<p>The gum is a polydimethylsiloxane with a small fraction of vinyl side groups, typically 0.05 to 0.5 mol percent, and a molecular weight around 500,000 to 700,000. That gives a Mooney viscosity of 30 to 80, which is what makes the compound handleable on a mill and mouldable in compression or injection.<\/p>\n<p>Everything else exists to turn that gum into a useful material. Fumed silica raises the strength and the hardness. A structure control additive stops the silica from continuing to thicken the compound on the shelf. The cure system decides whether it is a peroxide or a platinum addition cure, and the post-cure decides how much of the network is finished before the part ships.<\/p>\n<h2 id=\"filler\">Filler: the ingredient that sets hardness<\/h2>\n<p>Fumed silica is the default reinforcing filler, at 20 to 45 parts per hundred of gum. It is made by flame hydrolysis, it has a surface area of 150 to 300 square metres per gram, and its silanol groups hydrogen-bond to the silicone chain. That bonding is the reinforcement.<\/p>\n<p>Hardness tracks the loading almost linearly at about 1.7 Shore A per part of silica, so a 50 Shore A compound carries roughly 32 parts and an 80 Shore A compound roughly 45. Beyond about 50 parts the compound becomes difficult to mix and to mould, and the tear strength has already collapsed.<\/p>\n<p>The reinforcement comes with a side effect. Fumed silica keeps thickening the compound for days after mixing, a process called crepe hardening, until it becomes too stiff to mould. Structure control additives, usually a hydroxyl-terminated siloxane and a small amount of water, block the remaining silanol groups and stabilise the viscosity.<\/p>\n<p>Precipitated silica is cheaper and gives a higher hardness for the same loading, but it is more hydrophilic, harder to disperse and gives a lower tear strength. It appears in cheap general purpose compounds and in oil-resistant grades, not in a seal that has to last.<\/p>\n<h2 id=\"formulation\">A typical 50 Shore A formulation<\/h2>\n<p>The numbers below are a conventional peroxide-cure general purpose compound. They are not a recipe to copy, because the gum and the filler grade determine the result, but they show the proportions.<\/p>\n<table>\n<thead>\n<tr>\n<th>Ingredient<\/th>\n<th>Parts by weight<\/th>\n<th>What it does<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>VMQ gum, 0.15 mol % vinyl<\/td>\n<td>100<\/td>\n<td>The elastomer<\/td>\n<\/tr>\n<tr>\n<td>Fumed silica, 200 m2\/g<\/td>\n<td>32<\/td>\n<td>Reinforcement and hardness<\/td>\n<\/tr>\n<tr>\n<td>Structure control additive<\/td>\n<td>4<\/td>\n<td>Stops crepe hardening<\/td>\n<\/tr>\n<tr>\n<td>Hydroxyl silicone oil<\/td>\n<td>3<\/td>\n<td>Softener, improves tear<\/td>\n<\/tr>\n<tr>\n<td>Dicumyl peroxide, 40 percent<\/td>\n<td>1.5<\/td>\n<td>Cure<\/td>\n<\/tr>\n<tr>\n<td>Iron oxide, heat stabiliser<\/td>\n<td>1.0<\/td>\n<td>Service life above 200 C<\/td>\n<\/tr>\n<tr>\n<td>Pigment<\/td>\n<td>0.5<\/td>\n<td>Colour<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 - A conventional 50 Shore A peroxide-cure VMQ compound. The functional ingredients are the gum, the silica and the cure; the rest is what makes it last.<\/p>\n<p>Two entries are worth a note. The hydroxyl silicone oil is a softener that reduces hardness without reducing the silica, and it improves tear because it lowers the compound modulus at the crack tip. The iron oxide is a heat stabiliser that scavenges the radicals which would otherwise continue to crosslink the rubber in service at 200 degrees.<\/p>\n<h2 id=\"tear\">Tear strength peaks in the middle of the range<\/h2>\n<p>Tear strength rises from about 22 kN\/m at 30 Shore A to a peak of 42 at 60 Shore A, then falls back to 26 at 80. The shape surprises people who assume harder is tougher, and it has two causes.<\/p>\n<p>At the soft end, the compound has too little filler to deflect a crack, and the network is weak. At the hard end, the filler loading is so high that the material is brittle: the crack runs through the compound rather than being blunted by it. The peak is where there is enough reinforcement to blunt a crack and enough rubber to deform at the tip.<\/p>\n<p>If an application needs tear strength, it is a formulation problem before it is a hardness problem. A 50 Shore A compound with 3 parts of hydroxyl silicone oil and a higher vinyl content will out-tear a 50 Shore A compound with more silica and less oil, at the same hardness on the durometer.<\/p>\n<div class=\"callout\">Hardness is the number on the drawing; tear is the number that decides whether the part survives demoulding. They are not the same axis, and the peak of one is not the peak of the other.<\/div>\n<h2 id=\"cure\">Cure systems: peroxide or platinum<\/h2>\n<p>Peroxide cure is the default for moulded goods. A peroxide decomposes at 150 to 170 degrees, abstracts a hydrogen from the vinyl or the methyl group, and forms a carbon-carbon crosslink. It is cheap, tolerant of fillers and pigments, and it works in thick sections.<\/p>\n<p>Platinum addition cure is the default for liquid silicone and for anything that has to be clear, odourless or food contact. A platinum catalyst adds a silicon-hydrogen bond across a vinyl group, with no by-products. It is faster, cleaner, and far more sensitive to contamination: sulphur, tin, phosphorus and amine compounds poison the catalyst at parts per million.<\/p>\n<table>\n<thead>\n<tr>\n<th><\/th>\n<th>Peroxide cure<\/th>\n<th>Platinum addition<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Cure temperature<\/td>\n<td>150 to 175 C<\/td>\n<td>120 to 160 C<\/td>\n<\/tr>\n<tr>\n<td>By-products<\/td>\n<td>Volatile fragments, smell<\/td>\n<td>None<\/td>\n<\/tr>\n<tr>\n<td>Post-cure needed<\/td>\n<td>Yes, to remove fragments<\/td>\n<td>Less, but still usual<\/td>\n<\/tr>\n<tr>\n<td>Compression set<\/td>\n<td>Bagus.<\/td>\n<td>Better<\/td>\n<\/tr>\n<tr>\n<td>Contamination sensitivity<\/td>\n<td>Rendah<\/td>\n<td>Very high, sulphur and tin poison it<\/td>\n<\/tr>\n<tr>\n<td>Typical use<\/td>\n<td>General moulded goods, thick sections<\/td>\n<td>LSR, medical, food, clear parts<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 - Peroxide against platinum cure. The platinum system is better on every performance measure and worse on every handling measure.<\/p>\n<p>Post-curing at 200 degrees for four hours is not optional on a peroxide compound. It drives off the volatile fragments, completes the crosslink and improves compression set substantially. A part that skips the post-cure meets the drawing as moulded and changes shape in service.<\/p>\n<h2 id=\"additives\">Additives, mixing and maturation<\/h2>\n<h3>The five parts that decide whether it lasts<\/h3>\n<p>Minor ingredients are a small fraction of the compound and most of the difference between a two year part and a twenty year part.<\/p>\n<ul>\n<li>Heat stabilisers: iron oxide, cerium oxide or a titanium compound, at 1 to 3 parts. They scavenge the radicals that would continue to crosslink the rubber at 200 degrees and turn it brittle.<\/li>\n<li>Oil resistance: a fluorosilicone blend or a phenyl group on the chain. Neither is cheap, and both change the cure.<\/li>\n<li>Mould release: a fatty acid ester or a zinc stearate at 0.2 to 0.5 parts. Too much and the part cannot be printed or bonded.<\/li>\n<li>Flame retardants: platinum compounds, zinc borate or a phosphorus additive, at the cost of compression set.<\/li>\n<li>Colour: a pigment masterbatch, which can shift hardness by 1 to 3 Shore A and can affect cure if it carries a sulphur or a tin contaminant.<\/li>\n<\/ul>\n<p>The release agent is the ingredient that causes the most trouble downstream. A compound that demoulds beautifully because it carries 1 part of zinc stearate will not accept a silicone adhesive or a pad print without a primer, and the moulding shop discovers that after the parts are made.<\/p>\n<h3>Mixing: order, temperature and maturation<\/h3>\n<p>High consistency silicone is mixed on a two-roll mill or in an internal mixer, and the order of addition matters more than the machine.<\/p>\n<ol>\n<li>Gum and structure control additive first, until the gum is smooth.<\/li>\n<li>Filler added in thirds, with the band cut and folded between additions. All at once and the compound crumbles.<\/li>\n<li>Softener and process aids after the filler is fully incorporated.<\/li>\n<li>Cure system last, at the lowest possible temperature, because the peroxide will scorch if the mill gets above 60 to 70 degrees.<\/li>\n<\/ol>\n<p>The compound then matures for 24 hours before use. Maturation lets the structure control additive finish its work and the silica network reach equilibrium, which is why a freshly mixed compound moulds differently from the same compound a day later.<\/p>\n<p>Contamination control is the other half of mixing. A few milligrams of a sulphur-cured rubber in a platinum compound stops it curing altogether, and the failure looks like a bad catalyst batch. Mills, knives and trays are dedicated or cleaned to a standard, not wiped.<\/p>\n<h2 id=\"qc\">What to test on the compound, not on the part<\/h2>\n<p>Testing the moulded part tells you what happened. Testing the compound tells you what will happen, and it catches a bad batch before it becomes a bad lot.<\/p>\n<ul>\n<li>Hardness on a 6 mm slab, ISO 7619-1, 1 second reading. The part test is a confirmation, not a measurement.<\/li>\n<li>Specific gravity, which catches a filler loading error faster than hardness does.<\/li>\n<li>Mooney viscosity at 100 degrees, ML 1+4, which catches crepe hardening and a mixing error.<\/li>\n<li>Cure rheometry on a moving die rheometer: scorch time, t90 and torque rise. A flat curve means a dead catalyst or an expired peroxide.<\/li>\n<li>Compression set on a standard button, 22 hours at 175 degrees, which is the single best predictor of seal life.<\/li>\n<\/ul>\n<p>The rheometer curve is the most informative single test and the one most often skipped. A compound that reaches the right torque but takes twice the expected time to get there will mould, and it will have a worse set and a shorter life than the batch that behaved.<\/p>\n<div class=\"faq\">\n<h3>Can I make a 20 Shore A compound?<\/h3>\n<p>Only by reducing the filler and adding softener, and the result has low tear, high set and a tacky surface. Below about 25 Shore A a silicone gel or a foam is usually a better answer than a solid compound.<\/p>\n<h3>Why does my compound get harder on the shelf?<\/h3>\n<p>Crepe hardening: the fumed silica continues to form a network. A structure control additive slows it, and a compound older than its shelf life will need re-milling before use.<\/p>\n<h3>What is the difference between fumed and precipitated silica?<\/h3>\n<p>Fumed silica gives higher tear and better dispersion at a higher cost. Precipitated silica gives more hardness per part and is cheaper, but it is more hydrophilic and gives a lower tear strength.<\/p>\n<h3>Does pigment affect cure?<\/h3>\n<p>It can. Some pigments carry sulphur, tin or phosphorus traces that poison a platinum cure, and most shift hardness by 1 to 3 points. A colour change on a critical part needs a cure check, not an assumption.<\/p>\n<h3>Why is post-curing necessary?<\/h3>\n<p>It removes the volatile fragments from a peroxide cure, completes the crosslink and improves compression set. A part that skips it changes dimension and hardness in service.<\/p>\n<h3>Can I blend two compounds to hit a hardness?<\/h3>\n<p>Yes, on a mill, and it is common for a short run. The blend needs its own hardness and cure check, because two compounds with different cure systems do not blend linearly.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Need a compound tuned to a temperature or a chemical?<\/p>\n<p>Tell us the service temperature, the medium and the hardness. We will tell you whether a standard compound will do it or whether the formulation has to change, and what that costs.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/id\/custom-injection-molding\/\">Talk to a silicone engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/id\/silicone-rubber-manufacturing-processes\/\">Silicone Rubber Manufacturing Processes<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/hcr-solid-silicone-rubber\/\">High-Consistency Silicone Rubber<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/silicone-hardness-shore-a\/\">Silicone Hardness: What Shore A 30 and 70 Feel Like<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/liquid-silicone-rubber-lsr\/\">Liquid Silicone Rubber (LSR)<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/id\/silicone-rubber-types-guide\/\">Silicone Rubber Types: A Complete Guide<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A silicone compound is 100 parts gum, 30 parts fumed silica and about five parts of things that decide whether it works. Hardness is bought with filler, and filler costs tear.<\/p>","protected":false},"author":1,"featured_media":7834,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"Silicone Compound Formulation: Base, Filler and Cure","_seopress_titles_desc":"Silicone compound formulation explained: gum, fumed silica loading, structure control, tear strength against hardness, peroxide and platinum cure, additives and","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"39","_seopress_robots_breadcrumbs":"","_seopress_robots_freeze_modified_date":"","_seopress_robots_custom_modified_date":"","_seopress_robots_canonical":"","_seopress_social_fb_title":"","_seopress_social_fb_desc":"","_seopress_social_fb_img":"","_seopress_social_fb_img_attachment_id":0,"_seopress_social_fb_img_width":0,"_seopress_social_fb_img_height":0,"_seopress_social_twitter_title":"","_seopress_social_twitter_desc":"","_seopress_social_twitter_img":"","_seopress_social_twitter_img_attachment_id":0,"_seopress_social_twitter_img_width":0,"_seopress_social_twitter_img_height":0,"_seopress_redirections_value":"","_seopress_redirections_enabled":"","_seopress_redirections_enabled_regex":"","_seopress_redirections_logged_status":"","_seopress_redirections_param":"","_seopress_redirections_type":0,"_seopress_analysis_target_kw":"","footnotes":""},"categories":[39],"tags":[],"class_list":["post-7835","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-silicone-rubber-products"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/posts\/7835","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/comments?post=7835"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/posts\/7835\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/media\/7834"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/media?parent=7835"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/categories?post=7835"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/id\/wp-json\/wp\/v2\/tags?post=7835"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}